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THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge;
Superintendent of the University !\Iuseum of Zoology

A. E. SHIPLEY, M.A., P\R.S., Fellow of Christ's College, Cambridge;
University Lecturer on the Morphology of Invertebrates

VOLUME VII

UIVtSiONUttl^tU

1>

HEMICHORDATA

By S. F. Harmer, Sc.D., F.R.S., Fellow of King's
College, Cambridge.

ASCIDIANS AND AMPHIOXUS

By W.'A. Herdman, D.Sc. (Edinb.), F.R.S., Professor of
Natural History in the University of Liverpool.

FISHES

(Exclusive of the Systematic Account of Teleostei)

By T. W. Bridge. Sc.D., F.R.S., Trinity College, Cam-
bridge ; Mason Professor of Zoology and Comparative
Anatomy in the University of Birmingham.

FISHES

i' .Systematic Account of Teleostei)

By G. A. Boulenger, F.R.S., of the British Museum
(Natural History).

H\

L

ilontion
MACMILLAN AND CO., Limited

NEW YORK: THE MACMILL.\N CO.MP.^XV
1904

All rishts resofed

Third Fisherman. — Master, I marvel how tbe fishes live in the sea.
First Fisherman. — Why, as men do a-lami, — the great ones eat up the little
ones. Pericles, Act II. Scene i.

PEEFACE

Owing to unforeset'ii circuinstcinces, nut uiicoiiiiected with the
t'uuudatiou of a new University, the puhlieation of this volume
has been unduly delayed. Some parts of the work have actually
been in type tor more than four years ; and although the autliors
have made every effort to keep them up to date, the arrangement
is naturally not quite what it might have been if the articles
had been written immediately before publication.

In view of the novelty of Mr. Boulenger's classification of
the Teleosteans, and of the fact that several independent workers
have been occupying themselves witli the subject during the
last year or two, it is fair to state that this part of the volume
was completed in 1902. Professor Herdman's account of the
Ascidians was ready for publication two years earlier.

Professor Bridge wishes to express his best thanks to Dr.
E. H. Traquair, F.E.S., for his kindness in reading the proofs of
the pages which deal with the fossil Crossopterygii, Chondrostei,
Holostei and Dipnoi, and for much helpful advice and criticism ;
to Mr. G. A. l)Oulenger, FR.S., for his valuable and suggestive
criticism on certain points; and to Mr. Edwin Wilson, for the
care which he has taken in the preparation of the figures.

July 1904.

CONTENTS

Pkefack . .

Scheme of thk Classification adopted in this Book ....

HEMICHOEDATA

CHAPTER I

Chordata and Vertebrata — Hemichoi:uata — Enteropneusta — Ex-
ternal Characters and Habits — Structure — Genera — Develop-
ment — Pterobranchia — Cephalodiscus and Rhabdoplevra — Phoro-

NIDEA — PHORONIS AND AcTINOTROCHA — AFFINITIES OF THE HeMI-
CHORDATA

TUNICATA

CHAPTER II

Introduction — Outline of History — Structure of a typical Ascidian

—Embryology and Life-history .35

CHAPTER III

Classification : Larvacea — Appendicularians — Structure, etc. —

ASCIDIACEA — SiMPLK ASCIDIANS — SPECIFIC CHARACTERS — COMPOUND

Ascidians — Gemmation — M erosomata — Holosomata — Pyrosoma-
tidae—Thaliacea — DoLioi.iDAE— Salpidak — General Conclusions—
Phylooeny t)3

CEPHALOCHORDATA

CHAPTER IV

Introduction — General Characters — Anatomy of Amphioxus —
Embryology and Life-history— Classification of Cephalochordata

— Species and Distribution 112

vii

CONTENTS

CYCLOSTOMATA AND FISHES
CHAPTER V

PAOE

The Systematic Position and Classification of Fishes . . 141

CHAPTER VI

External Characters of Cyclostomata and of Fishes : External
Characters — Coloration — Poison Glands and Poison Spines —
Phosphorescent Organs 150

CHAPTER VII
The Skin and Scales 182

CHAPTER VIII
The Skeleton 193

CHAPTER IX

The Dentition, Alimentary Canal, and Dujestive Glands . . . 247

CHAPTER X

The Respiratory Organs 277

CHAPTER XI
The Air-bladder 297

CHAPTER XII
The Vascular System, the Lymphatics, and the Blood-glands . . 313

CHAPTER XIII

Muscular System — Locomotion — Sound - producing Organs — Electric

Organs 349

CHAPTER XIV
Nervous System and Organs ok Spe<ial Sense 367

CONTENTS

CHAPTER XY

I'AOE

TiiK Kidneys and the REi'iiODurrivE ( )kgax.s— Breeding . . . 397

CHAPTER XVI
Cyclostiimata (Systematic) 421

CHAPTER XVII
Elasmobranchii : General Chai;acters — Pleuuopterygii — Ichthyotomi

— ACANTHODEI — PlAGIOSTOMI — SeLACHII — BaTOIDEI — PIOLOCEPHALI . 431

CHAPTER XVIII

Teleostomi : General Chai;aiters — Crossopterygii — Chondrostei —

holostei 475

CHAPTER XIX
DiPNEUSTI .... 505

CHAPTER XX
Appendix to the Fishes: Palaeospondylidae — OsTRAtODERMi— Hetero-

STRACI— O.STEOSTRACI — AXASPIDA — AnTIARCHI— ArTH RODIRA . . 521

CHAPTER XXI
Teleo.stei : General Characters— Malacoptekygh — Ostariophysi . 541

CHAPTER XXII

Teleostei (coyTiyuED) : Symbranchii— Apudes — Haplomi — Hetkromi— •
C.vrosTEoMi— Perce.soces — Anacanthini 597

CHAPTER XXIII

Teleostei {coyTixvED): AcANTHOPTKRYoir — Opisthomi — Pediculati—

Plectognathi *^-'0

INDEX 729

SCHEME OF THE CLASSIFICATION ADOPTED
IN THIS BOOK

Order.
ENTERO-
PNEUSTA

(p. ro

PTERO-

BRANCHIA

(p. 21)

PHORONIDEA

11'. ^7)

The names of extinct groujys are jirinted in italics.

CHORD AT A (p. 3).

I. HEMICHORDATA (p. 3).

Family.

(Glandicipitidae(p. 17).
Ptychoderidae (j). 17).
Harrimaniidae ([>. 17).

II. UROCHORDATA = TUNICATA (pp. 4, 35, 63).

LARVACEA

(p. 64)

ASCIDI-
ACEA

(p. 70)

THALI-
ACEA

(p. 95)

Sub-(.)raer.

Ascidiae
Simplices

(l>. 71)

Ascidiae
Compositae

d'. SO)

Ascidiae
Luciae

(p. 90)

C Cyclomyaria

I (p. 95)
j Hemimyaria

I (p. 101.

Family.
( Kowaleviskiidae (p. 68).
-j Appendiculariidae
( (p. 68).

Clavelinidae (p. 71j.

Sub-Family.

Ascidiidae (^p. 72j.

I

Merosomata

'p. S5)

Holosomata

(p. 88)

Cyntliiidae (p. 71).

, Molgulidai! (p. 77).
f Uistoniatidae (]). 85).

Coelocorniidae (p. 86).
- Didemnidae (p. 86).

Diplosoniatidae (p. 87).
I I'olycliniilae (p. 87).
I Botryllidae (p. 88).
I, Polystyelidae (p. 89).

Pyiosomatidae (]>. 91).

Doliolidae (p. 96).

/ Salpidae (p. 101).

( Oc'taciiemidae (p. 108).

j' H3'pobytliiinae

(p. 72).
I Ascidiinae (p. 72).
[ Corelliiiae (p. 73).

Styelinae (p. 74).

Cynthiinae (p. 75).

Bolteiiiiiae (p. 75).

SCHEME OF CLASSIFICATION

Sub-Class.

III. CEPHALOCHORDATA (pp. i, 112).

Family.
Brancliiostoraatidae
(p. 137).
IV. CRANIATAfpp. 4, Ul).

Class — Cyclostomata (pp. 145, 150, 421).

or.ii-r. Sub-Order.

Myxinoides (p. 421)

Petroniyzontidae (p. 426)

Painily.
( Myxinidae (p. 422).
\ Bdellostomatidae (p. 423).

I Petromyzontes

I (p. 425)

Class — Pisces (pp. 145, 431).

ELASMO-
BRANCHII

>_ (p. 431)

Pleuropterygji

(p. 436)

ICHTHYOTOill

(p. 438)

ACAXTHODEI

(p. 440)

Plagiostomi (p. 442

TELEOSTOMI

Holocepbali (p. 466)

Crossopterygii

(p. 476)

Chondrostei

-p. 485)

Cladoselachidae (p. 438).

Pleurchcantldclae (p. 440).

f Di-placanthidae (p. 441).
\ Acanthodidae (p. 441).

Notidanidae (p. 442).

Chlamydosclachidae (p. 443).

Heterodoutidae [\i. 444).

Cocliliodontidac (]). 445).

Psaiiiiiiodonfidae (p. 446).

Pctalodontixlac (j). 446).

Scylliidae (p. 446).
' Selachii (p. 442) -| Carchariidae (p. 448).

Spliyrnidae (p. 449).

Lamnidae (p. 450).

Cetorhinidae (p. 453).

Rhinodoiitidae (p. 454).

Spiuacidae (p. 454).

Rliinidae (p. 456).

Pristiophoiidae (p. 457).
' Pi-istidae (p. 459).

Rhinobatidae (p. 460).

Raiidae (p. 461).
Batoidei (p. 457) \ Tamiohatidae (p. 462).

Torpedinidae (p. 462).

Trygonidae (p. 464).
, Myliobatidae (p. 465).
C Ptyctodontidac (p. 468).
I Squaloraiidae (p. 468).
i Mjiria&tnthidrir. (p. 468).
\ Chimaeridae (p. 468).

OSTEOLEPIDA

(P- -177)
Cladistia (p. 481)

f Usfcolrpidiic (p. 477).
I P/ikodo)

nitidac (p. 478).
j Huloptychidae (p. 479).
V Codacanthidac (p. 480).

Polypteridae (p. 4S1).
' Palaeoniscidne (p. 486).

P/a/ysomidac (p. 487).

Belonorhynchidac (p. 48f

Catopteridae (p. 488).

C/wndrostcidae (p. 489).

Polyodontidae (p. 491).
I, Acipeuseridae (p. 492).

{Continued on the next page.)

SCHEME OF CLASSIFICATION

Sub-Class.

Order.

Holostei

(p. 495)

Sub-Onler. Family.

S'cmionotidue (p. 497).
Macrosemiidne (p. 498).
Pycnodontidac (p. 498).
EugnatJiidac (p. 498).
"^ Amiidae (p. 499).

Fachijcormidae (p. 501).
AspidorhynrUidde (ji. 502)
Lepidosteidae (p. 502).
PlLolidoijhoridue (p. 545).
Avcliacomaenidac (p. 545).
Olii/oplcuridac (p. 545).
Lcpiolepididafl (p. 546).
Elopidae (p. 546).
Albulidae (p. 547).

Sub-Family.

TELEO-
STOMI

{contd. )

Teleostei

(]>P- 504,
541)

Malaco-
pterygii -

(p. 543)

IMormyridae (p. 549)

Hyodontidae (p. 552).
Notoptei'idae (p. 554).
Osteoglossidae (p. 555).
Pantodontidae (p. 558).
CtciinfJti-hsidae (]>. 559).
Phractolaemidae (p. 560).
Saarodinitidac (p. 561).
Chiroceutridae (p. 561).

Clupeidae (p. 562)

Salraonidae (p. 565).
\_Pachyrhi~odont'idac

(p. 569).]
Alepoceplialidae (p. 569).

Stomiatidae (p. 570)

Morniyrinae (p. 551).
Gymnarchinae(p. 551)

Ostario-

physi ^
(p. 573)

Characinidae (p. 575)

Gymnotidae (p. 579).
Cyprinidae (p. 581)

Thrissopatrinae

(p. 562).
Engraulinae (p. 563).
Clupeiiiae (p. 563).
Cliauinae (p. 563).

Gonorliyncliidae (p. 572).
. Cromeriidae (p. 573).

Chauliodontinae

(p. 571).
Sternoptychinae

(p. 571).
Stoniiatiuae (p. 571).

Erythiininae (p. 575).
Hydrocyoninae(p. 575).
Serrasalmoninae

(p. 576).
Ichthyoboriuae

(p. 576).
Xiphostoniinae

(p. 576).
Anostoniinae (p. 576).
Hemiodontinae

(p. 576).
Distichodontinae

(p. 576).
, Citharininae (p. 576).

Catostominae (p. 581).
Cyprininae (p. 582).
Cobitidinae (p. 582).
Honialopterinae
(p. 582).

[Continued on the next pxifje.)

SCHEME OF CLASSIFICATION

S»ib-Class.

Older.

Sub-Order.

Ostario-
pbysi

{fontd.)

TELEO-
STOMI

{contd. )

Sym-
branchii

(p. 597)

Apodes

(p. 599)

Teleostei

(contd.)

Haplomi

(p. 605)

Family.

Sihiridae (p. 5?

CDS').

Heteromi

(p. 621)

Catosteomi

(p. (J'iti)

Percesoces

(p. 636)

Loricariidae (p. 594)

\^ Aspredinidae (p. 596).

Symbranchidae (p. 597).
1 Ainphipnoidae (p. 598).

r Anguillidae (p. 600).

Nemichthyidae (]•. 603)
I Synaphobranchidae
I • (p. 603).

Saccopharyngidae (
\ Mnraenidae (p. 604).
Galaxiidae (p. 607).
Haplochitoiiidae (p. 608).
Eiichodontidac (ji. 608).
Esocidae (p. 609).
Dalliidae (p. 610).
Si-Dpi'liilae (p. 611).
Alepidosanridae (p. 614).
Cetomiiiiidae (p. 614).
Cliiroth i-icidae (p. 615).
Kneriidae (p. 615).
Cyprinodontidae (616).
Ainblyopsidae (p. 618).
St"plianob(.Tycidae (p. 619).
Percopsidae (p. 620).
Dcrcetidae (]). 623).
Halosauridae (p. 623).
Lipogenyidae (p. 624).
Notacaiithidae (p. 624).
Fierasferidac (p. 625).
Lamprididae (p. 628)
Gastvosteidae (p. 629)
Aulorhynchidae (p. 631)
ProfosiingncUh/idnc (p. 631)
Aulostoiuatidae (p. 632)
Fistulariidae (p. 632)
Ceiitriscidae (]>. 633)
Amphisilidae (633)
Solenostoiiudae (]>. 633)
Syngnatbidae (p. 634)
Pegasidae (p. 635)
Si'onibrcsocidae (p. 637).
Aiiiiiiodytidae (p. 639).
Atliorimdae (p. 639).
MugiU(bic (p. 640).
Polyueniidae (p. 640).
, Cbiasiiiodoiitidac ()). 641)

Sub-Family.
'Clariinae (p. 588).
Sihiriiiae (p. 588).
Pjagrinae (p. 588).
Doradinae (]). 588).
Malopteruriiiae

(p. 588).
Callielitbyiuae

(p. 588).
Hypoplitlialniiiiae

(p. 589).
Tricboiiiycterinae
(p. 589).
) Arginae (p. 595).
(Loncariinae(p. 595).

= Selenicbthyes

(p. 627).

:=H('iiiibranchii

(p. 627).

") =Lo]ibobraiu'hii
/ (p. 628).
= Hy]ioRtomides

(p. 628)

{Coiilinued Oil the nc:d page.)

SCHEME OF CLASSIFICATION

Sub-Class. Order. Sub-Order.

Perce -
soces

[could. )

TELEO- Tele-
STOMI ^ ostei

(cQiifd. ]

(contd. )

Anacan-
thini

a-, tiio)

Acantho-
pterygii -

(1). or.o)

Division. Katiiily.

Siiliyraciiidae (p. 642).

Tetrtigomiridae (p. 642).

Strouuiteidae (p. 643).

Icosteidae (p. 644).

Opliio('eplialidae(p. 644).

Anal)aiiti(lae (p. 645).
( Macruridae (p. 647).
I Gadidae (j). 647).
I Miiraenolepididae
I (p. 649).

Bcrycidae (ji. 655).

Monocentridae (p. 656).

Penijilieridae (p. 656).

Ceiitrarcliidae (p. 657).

Cyphosidae (p. 657).

Lobotidae (p. 658).

To.Notidae (p. 658).

Nandidae (p. 658).

Percidae (p. 658).

Acropomatidae (p. 659).

Sub-Family.

Perci-
formes

(p. 652)

Serranidae (p. 659)

Anomalopidae (p. 660).
Pseudochroniididae

(p. 661).
Cepolidae (p. 661).
Hoplognatliidae (p. 662).
Sillaginidae (p. 662).
Sciaenidae (p. 663).
Gerridae (]). 663).
Lactariidae {[>. 663).
Trichodontidae (p. 665).
Latrididae (j). 663).
Haplodactylidae (p. 664).
Pristipomatidae (p. 664).
Sparidae (p. 664 "i.
Mullidae (p. 665).
Scorpididae (p. 666).
Gaproidae (p. 666).
Chaetodontidae (]). 667).
Drepanidae (p. 668).
. Acanthuridae (p. 668).

Serraninae

(p. 659).
Gianimistinae

(p. 660).
Priacanthinae

(p. 660).
Centro])oniinae

(p. 660).
Pomatoniinae

(p. 660).
Anibassiuae

(p. 660).
Chilodipterinae

(p. 660).
Lutjaninae

(p. 660).
Cirrhitinae

(p. 660).
Pentacerotiiiae

(p. 660).

{Continued on the next page.)

XVI

SCHEME OF CLASSIFICATION

Sub-Class. Order. Sub-Order. Division.

r f

"^^cm^^T i Teleostei

{contd.)

Acantho-
pterygii

{contd. )

Opisthomi

(p. 716)

Pediculati

(p. 717)

Perciformes

{contimied)

Scombriformes

(p. (575)

Zeorhombi

(p. 682)

Kurtiformes

(p. 687)
Gobiiformes

(p. 688)
Discocephali

IP- 691)

Scleroparei

(p. 692)

Jugulares

(p. 702)

Taeniosomi

(p. 714)

Familj-.
Teuthididae (p. 668).
0«[)hi'onieiiidae (p. 669).
Enibiotocidae (p. 670).
Cichlidae (j). 670).
Poniacenlridae (p. 672).
Labridae (p. 673)
vScaridae (p. 674).
Carangidae (p. 677).
Kliachiceiitridae (p. 677).
Scorabridae (p. 678).
Tricliiuridae (p. 679).
Histi<iplioridae (p. 679).
I'dhdorlnjnchidae (p. 680).
Xiphiidae (p. 681).
Luvaridae (p. 681).
Corypliaenidae (p. 681).
Bramidae (p. 682).
Zeidae (p. 683).
A Dvphistiidae (p. 684).
Pleui'onectidae (p 684).

i

\ Kurtidae (p. 687).

- Goliiidae (p. 689).

Echeneididae (p. 691).

f Scorpaenidae (p. 694).

Hexagramniidae (p. 696).

Coniephoridae (p. 696).

Rhamphocottidae (p. 697).

Cottidae (p. 697).

Cyclopteridae (p. 698).

Platycephalidae (p. 699).

Hoplichthyidae (p 699).

Agonidae (p. 700).

Triglidae (p. 700).
1^ Dactylopteridae (p. 701).

Trachiiiidae (p. 704).

Percophiidae (p. 705).

Leptoscopidae (p. 705).

Nototheniidae (p. 705).

Uranoscopidae (j). 706).

Trichonotidaa (p. 706).

Callionymidae (p. 706).

Gobiesocidae (p. 707).

Bleuniidae (p. 709).

Batracliidae (p. 710).

Pholididae (p. 711).

Zoarcidae (p. 712).

Coiigrogadidae(p. 713).

Ophidiidae (p. 713).
J^idatelidae (p. 713).
/ Trachyptevidae (p. 715).
\ T>oiihotidao (]). 716).

Mastaceiiibelidae (i). 716).

(Lopliiidae (p. 718).
Ceratiidae (p. 719).
Anteiinariidafi (p. 720).
Gigantactinidae (p. 720).
Malthidae (p. 720).

{Continued on the. next 2>a(te.)

SCHEME OF CLASSIFICATION

Sub-Class.

TELEO-
STOMI

{CO lit ill Mil)

Order.

Teleostei

{contin ucd)

DIPNEUSTI {
= DIPNOI

(p. 505) y

S^nb-Order.

Plectognathi

(1>. 721)

Sclerodermi

(p. 722)

Gymnodontes

(p. 725)

Family.
f TiiMcanthi<l:ie (p. 722).
I Tviodontidue (p. 723).
\ Balistidae (p. 723).
I, Ostraoiontidae (p. 724).
i Tctroddiitidae (p. 726).

Diodontidae (]>. 726).
I IVlolidae (p. 726).
l' Ctcnodontidae (p. 505).
I Uroiicmidae (p. 507).
\ Ceratodontidae (p. 507).
V Lepidosirenidae (p. 511).

OF UNCERTAIN POSITION

P.lLAEO^POXDYLIDAE (p. 521).

OSTRACODERMI (p. 522

A XT I ARC HI (p. 532)
Arthrodira (p. 535)

Order.
Heterostraci {]). 524)

OSTEOSTRACI (p. 527)

\ Anaspida (p. 531)

Family.

■ CncJofrpidrir (p. 524).
DnjHimisiHilae (p. 525).
Pstnininixtiidac (p. 526).
Plcra&pidae (p. 527).
Atcleaspidae (p. 528).
Cfpluihtsjudiie (p. 528).
Trc„n,l„spi,lac (p. 530).
■Birkaiildac (p. 531).
Asterolepidae (p. 534).
Coccosteidae (p. 536).

HEMICHORDATA

SIDNEY r. HAEMEE, Sc.D., F.E.S.

Fellow of King's College, Cambridge.

VOL. VII

CHAPTER 1

HEMICHORDATA

CHOEDATA AND YERTEBRATA HEMICHORDATA ENTEROPNEUSTA

EXTERNAL CHARACTERS AND HABITS STRUCTURE GENERA

DEVELOPMENT PTEROBRANCHIA CEPHALODISCUS AND

RHABDOPLEURA PHORONIDEA PHOROXJS AND ACTINO-

TROCHA AFFINITIES OF THE HEMICHORDATA.

The Hemichorclata, a inariue group which includes the worm-like
Balanoglossus, owe much of their interest to the fact that they
are believed by many zoologists to be related to the lower Verte-
brates. This view is one of a number of mutually exclusive
hypotheses, which seek to derive Vertebrate animals from various
Invertebrate ancestors. It is supported by many striking re-
semblances between Balanoglossus and the lowest forms which
are by common consent regarded as belonging to the A^ertebrate
alliance ; but it must be distinctly understood that Balanoglossus
is at most the much modified modern representative of extinct
forms which were also the ancestors of Vertebrates.

The axis of the backljone of all Vertebrates is formed by an
elastic rod known as the " notochord " (Figs. 72, 115), which lasts
throughout life in some of the lowest forms, but in the higher
forms appears only in the embryo. The universal occurrence
of this structure has been regarded as the most important
characteristic of the Vertebrata and their allies, which are
accordingly grouped together in the Phylum CHOEDATA.
The members of this Phylum are further distinguished from
other animals by several important features. Of these one of
the most important appears to be the existence of lateral out-

3

DIVISIONS OF CHORDATA

growths of the pharynx, which unite with the skin of the neck
and form a series of perforations leading to the exterior. These
structures are the gill-slits, and in the Fishes their walls give
rise to vascular folds or gills. "With the assumption of a terres-
trial life, the higher Vertebrates lost their gills as functional
organs, respiration being then performed by entirely different
organs, the lungs. But even in these cases, the gill-slits appear
in the embryo ; and remains of one pair can usually be recognised
in the adult state of even the highest Vertebrates. Another
fundamental characteristic of the Chordata is given by the
central nervous system, which lies entirely above the alimentary
canal, just dorsal to the notochord. Not only does this position
of the nerve-centres distinguish the Chordata from Invertebrates,
but a further point of difference is found in the development.
While in Invertebrates the ventral nerve-cord is formed as a
thickening of the ectoderm or outermost layer of the embryo,
in the Chordata the nervous system is usually formed as a longi-
tudinal groove running medianly along the back of the embryo.
This groove closes to form a tube of nervous matter, the cavity
of which always persists throughout life as the " central canal "
of the spinal chord and its anterior prolongation which con-
stitutes the " ventricles " of the brain.

Although the animals which are considered in this chapter are
not admitted by all zoologists to be related to the Vertebrates,
there can be no question that their respiratory organs closely
resemble typical gill-slits. Since, moreover, they possess struc-
tures which can be regarded, with a fair amount of probability,
as agreeing in essential respects with the notochord and the
tubular dorsal nervous system of Vertebrates, it appears justi-
fiable to include them in the Chordata, which are then sub-
divided into (1) Hemichordata, in which a "notochord" occurs
in the anterior end of the body only; (2) Urochordata (Tunicata
or Ascidians), in which the notochord is restricted to the tail ; (3)
' Cephalochordata (Amphioxus), in which the notochord extends
the entire length of the body and of the head ; (4) Craniata, in
which a brain is developed as an enlargement of the central
nervous system, the notochord does not extend farther forward
than the middle of tlie brain, and a vertebral column is present.
These last are thus usually known as Vertebrata, although in dis-
tinguishing an " Invertebrate " from a " Vertebrate " it is more

HEMICIIORDATA — ENTEROPNEUSTA

logical to regard all Chordata as Vertebrates, since the Inverte-
brata are in no sense a natural group with common charac-
teristics, their union under one name merely implying that they
have no close affinity to the Vertebrates. It is often convenient
in practice to divide animals into Vertebrates and Invertebrates,
but from a zoological point of view a division of the animal
kingdom into Molluscs and Non-Molluscs would have as much
or as little significance.

The sub-phylum Hemichordata ^ consists of the Orders : — (I.)
ENTEROPNEUSTA,^ including Balanoglossus (Fig. 1); (II.) Ptero-
BRANCHIA,^ represented by the genera Cejyhalodiscvs (Fig. 9) and
Wiahdopleura (Fig. 12). To these should possildy be added
(III.) Phokonidea, for the reception of Phoronis (Fig. 13).

Order I. Enteropneusta.

Worm-like Hemichordata, with numerous gill-slits, a straight
intestine, and a terminal anus. Proboscis separated hy a narrow
stalk frorn the simple ring-sharped, collar, which is succeeded Jjy an
elongated trunk.

The structure of Balanoglossus, formerly the sole genus
belonging to this Order, but now divided * into the genera
Ptychodera, Balanoglossus, Glossohalanus, Glandiceps, Spengelia,
Schizocardium, Harrimania, Dolichoglossus, and Stereobalanus,
has of recent years formed the subject of elaborate investigations
by Spengel,^ Bateson,*" and Willey." More than thirty species
are known, ranging in length from 25 mm.*^ (Pt. hahamensis) to
2500 mm. (i?. gigas), and for the most part inhabiting shallow
water ; Glossohalanus sarniensis .occurring between tide-marks in
the Channel Islands. Glandiceps tcdaloti has, however, been
dredged near Marseilles from as much as 190 fathoms, while

^ Bateson, Quart. J. Mlcr. Sei. xxv. Suppl. 1SS5, p. 111.

- Gegenbaur, Grundzilge vergl. Anat. 2 ed. 1870, p. 158.

3 Lankester, Quart. J. Micr. Sci. xvii. 1877, p. 448 ( = Aspidophoiia, Allman,
J. Linn. Sac. xiv. 1879, pp. 490 n., 586).

■» Spengel, Zool. Jahrb. Syst. xv. 1902, p. 209.

^ Fauna u. Flora G.v. Neapcl^ 18 Mouogr. 1893 (reviewed by MacBride in Quart.
J. Ificr. Sci. xxxvi. 1894, p. 385) ; Zool. Jahrb. Anat. xviii. Pt. ii. 1903, p. 271.

« Quart. J. Micr. Sci. xxiv. 1884, p. 208 ; xxv. Suppl. 1885, p. 81 ; xxvi. 1886,
pp. 511, 535.

" Zool. Results, Part iii. Cambridge, 1899, p. 223. » =1 inch.

HEMICHORDATA

G. ahyssicola was found by the " Challenger" at a depth of 2500
fathoms, off the West Coast of Africa. Balanoglossus, the largest
genus now recognised by Spengel, appears to be practically
world-wide in its distribution ; ScMzocardium is recorded from

Fig. 1. — Forms of Balanoglossus. A, Balanoglossus daviijcrus, Eschsch., Naples, x h\
B, (j!landiceps hacksi. Mar. (incomplete), Japan, x 1 ; C, Schizocardiumhrasiliense,
Speng., Rio de Janeiro, x 1 ; D, JJolichoglossus kowalevskii, A.Ag., Chesapeake
Bay, X 1. «, Anus ; ab, abdominal and caudal regions ; b, branchial region ; f,collar ;
g, genital region ; g.p, gill-pore ; g.w, genital wing ; h, hepatic region ; vi, position
of mouth ; j), proboscis ; t, trunlc. (A, B, and C from Spengel ; D from Batesou.)

both sides of S. America ; Glandice'jis from the Atlantic, the
Mediterranean, Japan, and the Indian Ocean ; Spengelia from
the South Pacific ; and other species from the White Sea to
New Zealand. The habitat is usually sand or gravelly sand,
in which the animal forms a kind of tube by means of the
abundant mucus secreted by its skin. Dolichoglossus koioalevskii
(Fig. 1, D), according to Bateson,^ lives between tide-marks at a
1 Quart. J. Alicr. Sci. xxiv. 1884, p. 209.

DISTRIBUTION COLOUR HABITS

depth of about eight inches. The greater part of the body is
coiled ill an even, cork-screw-like spiral, while the anterior end,
including the front part of the branchial region, is maintained
in a vertical position. The posterior end is also kept upright,
and can be moved up and down in a vertical shaft opening on
the surface, thus enabling the animal to eject the undigested
sand from its anus.

The coloration of Balanoglossus is often brilliant. That of
D. koiualcvshii ^ is as follows : — ^The " proboscis " (cf. Fig. 1 , B, j:?)
is yellowish white ; the " collar " {<:■) is brilliant red-orange
(especially in males), with a white ring posteriorly ; the " trunk "
{t), the subdivision of which into " branchial," " genital," " hepatic,"
" abdominal," and " caudal " regions is better indicated in other
species (Fig. 1, A, h, g, h, ah), is orange-yellowy shading to green-
yellow in the semi-transparent caudal region. The genital region
is grey in females and yellow in males, a sexual difference in
colour being common in Enteropneusta. The hepatic papillae of
other species may be bright green.

The odour of D. koicalevskii resembles that of " chloride of lime
with a faecal admixture," while that of Balanoglossus aurantiacus
suggests iodoform. All Enteropneusta are said to have a more
or less offensive smell. A species of Balanoglossus is known to
be intensely phosphorescent.^

The mouth (Fig. 7, m) is situated on the ventral side, at the
base of the proboscis, and is concealed by the free anterior edge of
the collar, which encircles the thin "proboscis-stalk" (Fig. o,2).s).
The animal has the singular peculiarity of being unable to close
its mouth ; ^ and thus, as it b^^rrows through the ground, the
sand which passes into the alimentary canal leaves it in a
continuous column through the terminal anus."' The large coiled
" castings " formed in this way lietween tide-marks enable the
experienced collector to infer the presence of Balanoglossus ; and
in a West Indian species described by Willey^ they are so large
as to form " an important feature in the landscape at low tide."

The principal agents in burrowing are the proboscis and collar.
An animal observed by Spengel pushed the tip of its proboscis
into the sand, waves of muscular contraction meanwhile passing

^ Quart. J. Micr. Sci. xxv. Siq^pl. 1885, p. 91.

- Pouchet, C. R. Ac. Sci. cii. 1886, p. 272.

^ Kowalevsky, M&m. Ac. St. Petersb. (7) x. No. 3, 1866, p. 7.

^ Spengel, Monogr. y. 474. ^ Zool. Res. Pt. iii. 1899, p. 256.

HEMICHORDATA

CHAP.

over the surface of the proboscis. At first the animal made slow
progress ; but the collar, becoming surrounded by sand, soon
became a point of resistance by means of which the proboscis
could bury itself yet more deeply. The animal quickly disap-
peared as soon as the first two regions of its body were engaged
in the task of burrowing.^

This action is due partly to the muscles of the body-wall, but
largely to the power possessed by the proboscis and collar of
becoming swollen and turgid. Spengel has observed that these
parts become flaccid when the animal is taken out of water, and
can only swell again when it is replaced therein ; and it may thus
fairly be concluded that the enlargement is due to the taking in
of water. This is probably in fact the most important function
of the " proboscis-pore " and of the " collar-pores " which are de-
scribed below.

Body- Cavities. — The existence of five separate body-cavities
(Fig. 2) is one of the most fundamental facts in the anatomy of
Balanoglossus. The first body-cavity, or
cavity of the proboscis (ft.c^), is single
and unpaired ; the second body-cavities
(h.c") are paired spaces, one belonging to
each side of the collar ; the third body-
cavities (b.c^) are similarly paired, and
correspond with the trunk. While there
is no connexion between successive body-
cavities, there are in certain regions com-
munications between the two cavities of
the same pair. Each of the paired
cavities is at one time a closed lateral
space between the skin and the alimentary
canal. As the two spaces which con-
stitute the pair grow towards one another,
both al)Ove and below the alimentary
canal, they come into such close apposi-
tion that they remain separated only by their conjoined walls.
In this way are formed the dorsal and ventral mesenteries (Fig.
4, d.m, v), the former being the only one to persist in the higher
Vertebrates. The body-cavities of the adult become to a large, ex-
tent disguised by being traversed hj connective tissue and muscles.
^ See also Ritter, Biol. Bull. iii. 1902, j.. 255.

'■■1.03

Fig. 2. — Diagram of a dorsal
view of a Balaiioglossiis-
embryo, after the forma-
tion of the body-cavities.
«, Alimentary canal ; b.c^,
body-cavity of the pro-
boscis ; h.c^, of the collar ;
b.(^, of the trunk. (From
Bateson.)

BODY-CAVITIES NERVOUS SYSTEM

The hinder part of the proboscis-ciivity is divided by the
forward growth of the notoehord (Fig. ."5, n) into dorsal and ventral
portions. The dorsal cavity in extending backwards becomes
further subdivided into right and left
halves, the latter typically opening dors-
ally to the exterior on the proboscis-
stalk by an asymmetrical " proboscis-pore "
{2^.p.). Two symmetrical proboscis-pores
may, however, occur, or a median pore
connected with the left division of the
proboscis-cavity. These may be indi-
vidual variations within the linuts of a
single species, or may occur as a normal
feature of a species.

The collar-cavities open by two " collar-
pores " (Fig. 3, c.jh), situated at the
posterior end of the collar, into the first
pair of gill-pouches, near their external
opening. Willey has recently described ^
vestigial pores in relation with the " peri-
haemal spaces," a pair of dorsally situated
outgrowths of the third body -cavities
into the collar-region. Narrow " peri-
pharyno-eal spaces," also a forward o-rowth ^^^- ^-"T^"^"*^^ Y';*:^^,'^^, ^^^^

j: '^ o r ' ^ ^ o ^ anterior enil of the body of

of the third body-cavities, closely invest DoUchogiossus koivaievskU,
the ])harynx in some species.

Body-Wall and Nervous System. —
The body-wall (Fig. 4) consists externally
of a thick ciliated epidermis {c), con-
taining numerous gland-cells which secrete
an abundant mucus. Beneath the epi-
dermis is a basement-membrane, while
more internally are layers of muscles,
whose arrangement differs in different
parts of the body and in different species.

The nervous system consists of a plexus of cells and fibres

which lie in the basal part of the epidermis of all parts of the

animal, outside the basement- membrane ; the thicker portions of

the plexus forming definite nerve-tracts. This intimate connexion

1 Zool. Ecs. iii. 1899, pp. 273, 280.

S'

"v

X 3. c, Collar ; c.n, cir-
cular nerve ; c.p, collar-
pore ; d, dorsal nerve ; g,
gill-])ore ; n, notocliord ;
n.s, central nervous sys-
tem, showing the anterior
and posterior neuropores ;
2), proboscis ; 2)-l>t probos-
cis-pore ; 2).s, proboscis-
stalk ; t, trunk ; r, ventral
nerve. Tlie nerve-plexus
of the proboscis is repre-
sented as a black line.
(After Bateson.)

lO

HEiMICHORDATA

between the epidermis and the nervous system is usually restricted
to embryonic life in other animals.

The main nerves of Balanoglossus are a dorsal and a ventral
tract in the trunk region (Fig. 4, d.n, v.n), a circular tract
(Fig. 3, c.n) connecting these two at the posterior edge of the
collar, and a strong concentration of nerve-tissue round the whole
of the proboscis-stalk, and of the posterior end of the proboscis
(Fig. 3). In the region of the collar the nervous system attains

its highest develop-
^^1 ? ff^^^s^ ment, taking the

form of a median
cord passing above
the alimentary

■gP canal. This cord,
known as the cen-
tral nervous system
(Fig. 7, n.s), runs
tlirough the cavity
of the collar, but is
connected with the
epidermis at each
end. It thus be-

comes continuous

Fig. 4. — Pli/chodem bahamensis, Bahama, Is. Transverse -j,, frnnl- witli fl-io
section through the branchial region, b, Branchial ^ ^'^ ^'^^

part of pharynx ; b.c'\ third body-cavity ; d.m, dorsal lierve-laycr On the

mesentery; d.n, dorsal nerve; d.r, dorsal vessel; e, ,^vn1->nonio .<- 11

epidermis, with nerve-layer (black ) at its base ; </, genital P^'JIJO^'-l'' " SLailv,

wing ; g.p, gill-pore, encroached on by the tongue-bar while posteriorly it
{t) ; I, lateral septum ; vi, longitudinal muscles ; o, ' j- i■^ A

oesophageal or alimentary part of pharynx ; r, repro- P^'-SSCS intO tlie QOr-

ductive organ ; t, tongue-bar ; v, ventral mesentery and sal and the circular
ventral vessel ; v.w, ventral nerve. (After Spengel.) , _

nerve - tracts. In
nearly all cases the epidermis is pushed into the cord at the
points where it passes into the skin, in the form of an anterior
and a posterior " neuropore " (Fig. 3). A transverse section
through the extreme front or Iiind end of the collar accord-
ingly sliows a tubular nervous system. In certain species,
as in Glossolalanus sarniensis and Ptychodera flmia, a central
canal, opening in front and behind, exists throughout the entire
length of the central nervous system, while in G: minutus a
canal of this kind occurs in the young animal, but not in the
adult. The central nervous system is developed as a longitudinal

I ALIMENTARY CANAL GILL-SLITS I I

dorsal groove in the larva/ and in a similar manner in the collar
which is formed as the result of regeneration after injury."
Balanoglossus is thus typically provided with a dorsal, tubular,
central nervous system, and although this arrangement does not
extend beyond the limits of the collar, it shows a noteworthy
resemblance to Vertebrate animals.

In some cases the central nervous system is connected with
the dorsal epidermis by a varying number (1-17) of median
"roots," which have been compared by Bateson with the dorsal
roots of the spinal nerves of Amphioxus, and are probably
remains of the embryonic connexion of the collar nervous system
with the dorsal epidermis.

Alimentary Canal. — The mouth (Fig. 7, m) leads widely into
the alimentary canal, which, passing through the collar, enters
the branchial region, where it is characterised by the existence
of communications with the exterior. These, the gill-slits, are
developed, as in Vertebrates, as paired outgrowths of the
alimentary canal, and new gill-slits are constantly being formed
at the posterior end of the branchial region with advancing age.
The maximum number of the gill-slits, and the extent of the
branchial region, are by no means uniform throughout the
Enteropneusta. Thus Dolichoglossus otagoensis is said to have
no more than 12 pairs, Glossohalanus minutus only 40 pairs,
while Balanoglossus aurantiacus may have as many as 700 pairs.
In Ptycliodera Jlava the variation is so great that Willey dis-
tincjuishes ^ two extreme conditions as " macrobranchiate " and
" brachybranchiate " respectively, although intermediate con-
ditions are also found. It should be noted that Balanoglossus
agrees with Amphioxus in the indefinite number of the gill-
slits.

The gill-slits usually have the form of the so-called " branchial
pouches " or " gill-sacs " (Figs. 5, 6, g.s). Each ordinarily opens
to the exterior by a small pore (Fig. 1, D, o,g.2i) or slit, sitviated
on the dorsal side, in a shallow longitudinal groove not far from
the middle line. The gill-sac has a complete wall of its own,
and lies between the alimentary canal and the body-wall, com-
municating with the former by a U-shaped slit. AVhile a dorsal

1 Morgan, J. Morphol. v. 1891, p. 422 ; ix. 1894, pp. 44, 48, 72.

~ Willey, Zool. Res. Pt. iii. 1899, p. 245.

3 Zuol. Ecs. Pt. iii. p. 228.

12

HEMICHORDATA

view of the animal thus shows a linear series of simple pores, a
view of the pharynx from the inside appears as in Fig. 5.

At the hind end of the pharynx the inner opening of the
developing gill -sac is circular. Slightly further forward the
dorsal side of the pore is indented into a crescent, which grows
longer in a dorso-ventral direction, and becomes a U, whose two
limbs are nearly separated by a mass of tissue, the so-called
" tongue-bar " (Fig. 5, t). The special interest of this mode of
development is that it is identical with what occurs in Amphioxus
(p. 120), which is universally admitted to belong to the Chordata.

The gill-sacs of Balanoglossus follow one another closely,
the hind wall of one being in contact with the front wall of
the next, and constituting a " branchial septum " (l).s). Both

septa and tongue -bars are

'^ supported by chitinous rods,

which are special thicken-
ings of the membrane at
the base of their epithe-
lium. Two rods occur in
each tongue-bar, separated
by an interval of body-
cavity (Figs. 5, 6), and only
one rod in each septum.
Originally of this form
— nn nn — the rods have
joined in pairs, the united
limbs forming the single
rod of each branchial sep-

FiG. 5. — Diagram of two gill-sacs of Balano- tuiu. In this rCSpCCt again
glossus, seen from the inside of the pharynx. -, " '1 "f 1
h, Branchial skeleton, consisting of a single ^^'^ ^^^^'^ ^ SUnilarity DC-
forked bar in each branchial septum (b.s), twCCn BalanOgloSSUS and
and of two bars in each tongne-bar; g.p, i -i • i. +.1, <. •
gill-jwre, opening on the dorsal surface of AmplllOXUS, except ttiat m
the trunk ; g.s, gill-sac ; s, synapticulum the latter the COncreSCCnCC
(only one or two shown) ; t, tongue-bar. The p i
arrows indicate the communications of the proCCedS One Step lartlier,
gill -sacs with the exterior and with the ^^^^ ^hc twO rods of the
pharynx.

tongue-bar unite, like those
of the branchial septum. The latter, the so-called " primary "
skeletal rods of Amphioxus, are forked ventrally as in Balano-
glossus (Fig. 5).

In Amphioxus, as in most Enteropneusta, adjacent rods are

ALIMENTARY CANAL

13

connected at intervals by chitinous " sjnapticula " (Fig. 5, s),
which traverse one or the other of the halves of the gill-slit. In
Dolichoglossus, where no synapticula occur, the tongue-bars may
be turned inside out by slight pressure, and then project to the
exterior through the gill-pores.

The subdivision of tlie branchial region of the alimentary
canal into two parts, as shown in Fig. 4, is characteristic of
Glossobalaiius and its allies. In Dolichoglossus and Glandiceps
there is no such constriction, the region occupied by the gill-slits
being merely the dorsal half of a tube with a simple circular,
section. Scliizocardium (Fig.
6) agrees with Amphioxus
in the fact that the gill-slits
occupy nearly the whole of
the wall of the pharynx ;
the only parts not perfor-
ated by gill-slits being the
small dorsal and ventral
portions.

In Ptychodera (Fig. 4),
the gill-sacs are practically
absent. The U-shaped slits

of the pharyngeal wall thus pj^^ G.—Schizocardium hrasiliense; transverse

section through the branchial region, show-
ing tlie great extent of the branchial part
(b) of the jiharynx ; the oesophageal jiart
(0) is reduced to a mere groove ; g, gill-
pore ; f/.s, gill-sac ; ?■, reproductive organ ;
*, synapticula (cf. Fig. 5) ; t, tongue-bar.
The muscles of the body-wall are not indi-
cated : in other respects the figure corre-
sponds with Fig. 4. except for the absence
of genital wings in this region of the body.
(After Spengel.)

open directly to the exterior,^

and can be seen from the

outside. In species which

have this arrangement, the

genital wings are greatly

developed, so as to arch over

the back of the branchial

region. The gill-slits thus

open into a kind of "atrium," resembling that of Amphioxus

in its relation to the gill-slits, and in having the generative

organs on its outer side, but differing from it in being dorsal to

the pharynx.

At a certain distance behind the branchial region, the

alimentary canal in Balanoglossus and Scliizocardium is produced

into a series of outgrowths, into which food does not pass.

These "liver-sacs" give rise to corresponding folds (Fig. 1, A, li)

1 Spengel, Monngr. pp. 179, ]87 ; Willcj', Zool. Res. iii. p. 236.

14

HEMICHORDATA

,. C.TTV.

of the dorsal body-wall, a conspicuous external feature of the
species in which they are present. The most interesting-
peculiarity of the digestive

tract in this region is the
existence, in certain species,
of pores, possibly vestigial gill-
slits, leading from it to the
exterior.

Notochord and Skeleton.
— The structure compared by
Bateson with the Vertebrate
notochord is a hollow dorsal
outgrowth of the alimentary
canal of the collar-region (Fig.
7, n). Near its origin it is
slender, but in the proboscis
it dilates into a comparatively
large organ, which in most
cases retains its cavity. Its
cells have a vacuolated appear-
ance, which recalls the fine
structure of the Vertebrate
notochord. In Schizocardium
and Glaiidiceps, the organ is
produced into a slender " ver-
miform process " (;'), w^hich
extends nearly to the tip of
the proboscis.

The main support of the

Fi(j. 7. — Schizocardium brasilienne ; longi-
tudinal, median section through the pro-
boscis, the collar, and the first part of the T^^obospis ^t'llk is " thp " i^rn
trunk; h, main blood-space of tlie pro- prODOSClS-Staik IS tne pro-
boscis ; J.c\ h.(?, h.c^, first, second and boscis - skeleton " (s), a A-
third body-cavities ; cm, circular muscles it i -i •

of proboscis; e, epidermis; i.m, longi- shaped Organ whose median
tudinai muscles of proboscis ; m, mouth ; part lies beneath the base of

w, notochord ; h..s, central nervous system, , i. \^ ^ u A'

continuous witli the subepidermic nerve- tlie notOCUOrcl, itS diverging

plexus (black) of tiie proboscis, and witii lecrg extending backwards

the dorsal nerve [d) ; p.c, pericardium ; , . ,

^..9, proboscis -stalk ; s, proboscis -skeh- along the outer Side ot the
ton; ^vermiform process of notochord. alimentary canal of the collar.

(After Spengel.) "^

The proboscis -skeleton, like
the branchial skeleton, is a special development of the structure-
less membrane which is found at the base of the layers of cells

SKELETON — VASCULAR SYSTEM

of Balanoglossus, and in most species it grows merely hy the
deposition of laminae of cliitin from the notochord, and from the
ventral epidermis of the proboscis-stalk.

In some species, however, and particularly in Balanoglossus
auraniiacus and Glandiceps, the primary skeleton becomes sur-
rounded by an extensive development of a secondary cartilaginoid
skeleton, consisting of a structureless substance into which tlie
adjacent body-cavities of the proboscis and collar send cellular
outgrowths. The possibility of a relation between this tissue,
more or less surrounding a part of the notochord, and the carti-
lage of Vertebrates cannot be overlooked.

The caudal region may be stiffened (?) by a " pygochord " ^
which is a median derivative of the alimentary canal on its
ventral side.

Vascular System and Proboscis-Gland. — The main vessels
are a dorsal and a ventral vessel (Fig. 4, d.v, r), lying in their
respective mesenteries. The details of the vascvdar system are
complicated, and ha\'e not been thoroughly made out, the nearly
colourless character of the blood making their investigation a
difiicult matter. The following points may, however, be noted.
The blood is said to pass forwards in the dorsal vessel, which,
like the ventral vessel and a pair of lateral vessels in the hepatic
region, is contractile. In the collar the dorsal vessel lies be-
tween the two perihaemal spaces, on the dorsal side of the base
of the notochord. The principal blood-space in the proboscis
(Fig. 7, h) lies between the notochord (n) and an organ known
as the " heart-vesicle " or " pericardium " (2^.c). The latter has
muscular walls and it contracts rliythmically in the larva. Its
behaviour in the adult is not so easily made out, but it is prob-
able that, although it does not communicate with the vascular
system, its contractions propel the blood contained in the space
immediately beneath it. The blood, after passing to a glandular
organ, the " proboscis-gland " or " glomerulus," which lies at the
sides and in front of the notochord, appears to pass round the
collar to the ventral vessel. A'arious systems of vessels are con-
nected with the skin, the gills, the alimentary canal and the
generative organs.

The function of the proboscis-gland is possibly excretory.
In this case it is probable that the proboscis-pore eliminates the

1 Willev.

1 6 HEMICHORDATA

waste products discharged by the gland into the anterior body-
cavity, though this view is not favoured by Willey.

Reproductive Organs. — The sexes are separate, the repro-
ductive organs consisting of a series of simple or branched
glands which occur along the dol'so-lateral lines of the anterior
part of the body ; being usually found throughout the branchial
a,nd generative regions and ending at the beginning of the
liepatic region. The reproductive organs may pass into great
extensions of the body - wall known as the " genital wings,"
specially developed in some species oi Balanoglossus and Ptycliodera
(Figs. 1 A, 4).

Stereohalanus canadensis, a species with long slit-like external
gill -pores, is interesting in possessing a well -developed genital
wing botli dorsally and ventrally to the series of gill-pores of
each side.

Each reproductive gland opens by its own pore or pores
directly to the exterior. Several glands and pores may occur in
the same transverse section.

According to Spengel there is no definite relation between
the number of the reproductive organs and that of either the
gill-sacs or the liver-outgrowths. The only definite segmenta-
tion exhibited by Balanoglossus is thus the division into three
regions which is so distinctly sliown by the arrangement of the
body-cavities; though the gill -sacs may indicate an incipient
further segmentation of the major part of the body. In this
connexion it is interesting to notice MacBride's statement ^ that
the body-cavity of Amphioxus develops in the embryo as five
cavities, just as in Balanoglossus ; the segmented part of the
body being formed by a secondary segmentation of the third
body-cavities.

Regeneration. — Balanoglossus, like Fhoronis (p. 30), possesses
great powers of regenerating lost parts. The posterior part of
the body is readily re-formed, while Spengel has shown " that
even the proboscis, collar and branchial region can be regenerated,
apparently from a fragment of the l;)ody.

Genera of Enteropneusta. — ^ Spengel, whose Monograph
is indispensable to every student of the Enteropneusta, formerly

1 Quart. J. Micr. Set. xl. 1898, \). 601 ; xliii. 1900, ]>. 351.

2 Monogr. p. 684, PI. xxvi. Figs. 14-18 ; see also Willey, Zool. Res. iii. p. 245,
and DawydofT, Zoolog. Anz. xxv. 1902, p. 551.

I ENTEROPNEUSTA — GENERA I 7

in-oposed to divide the old genus Balanoglossus into four ; but he
now recognises no less than nine.^ Some of the more important
characters are given below, l)ut for the arrangement of the
muscles, important from a systematic point of view, reference
must be made to the original sources.

A. Xotochord with a vermiform process (Fig. 7, v) ; pericardium ■with
anterior diverticula more or less developed. . Glandicipitidae
(a) Liver-sacs and synapticula present ; gill-slits almost equalling the

pharynx in dejath, so that the ventral, non-branchial part of the
pharynx is reduced to a mere groove (Fig. 6) ; nerve-roots absent ;
23ericardial diverticula long. . . Schizocardium, Speng.

(h) Liver-sacs absent ; - ventral part of pharynx well developed ; peri-
cardial diverticula short.

(i.) Synapticula and nerve-roots absent. . Glandicefs, Speng.

(ii.) SjTiapticula present ; nerve-roots present or absent ; genital

region with dermal pits. . . Spengelia, Willey.

B. Xotochord with no vermiform ]>rocess ; j^ericardium simple ; ventral
jiart of jjharynx large, and sometimes more or less sejaarated from the
branchial part (Fig. 4).

(a) Liver-sacs,'^ synapticula and nerve-roots jiresent. . Ptychoderidae

(i.) Genital wings well developed.

(a) Gill-sacs opening by long slits. . Ptychodera, Eschsch.

(/?) Gill-sacs opening by small pores. Balanoglossus, Delle Chiaje
(ii.) Genital wings hardly developed. . Glossobalanus, Speng.

(b) Liver-sacs, synapticula and nerve-roots absent. . Harrimaniidae

(i.) Proboscis long ; one proboscis-pore. . Dolichoglossus, Sj)eng.
(ii.) Proboscis short ; two jDroboscis-pores.

(a) Two pairs of genital wings. Stereohalanus canadensis, Speirg.

(fi) No genital wings . . . Harrimania, Eitter.

The name Balcmoglossiis was introduced by Delle Chiaje in
1829 for B. davigerus (Fig. 1, A), from the neighbourhood of
Naples. As Spengel has shown, its etymology has been much
misunderstood. The second half of the name refers to a fancied
resemblance between the Balanoglossus, with its largely developed
genital wings, and the tongue of an ox. /3d\avo<i means " acorn,"
and it has usually been supposed that this name was suggested
by the resemblance of the proboscis, projecting from the collar,
to an acorn in its cup, a view which finds its expression in the

' Zool. Jalirb. Syst. xv. 1902, p. 209. Tlie Harrimaniidae = Balanoglossus of
the Monograph (1893): Glossobalanus = Ptychodera, s.str., 1893: Balanoglossus =
Tauroglossus, 1893: Ptychodera= Chlamydothorax, 1893.

- Punnett ("Enteropneusta," in Gardiner's Fauna and Geogr. Maldire and
Laccadive Arch. ii. Pt. ii. 1903) finds small liver-sacs in Spengelia, and describes
Willeyia, a new genus of Glandicipitidae. ^ Exc. G. ruficollis, "Willey.

VOL, VII C

HEMICHORDATA

CHAP.

name " Eichelwurin " used by German zoologists. But the idea
expressed by Delle Chiaje was really a similarity between the
collar oi Balcnioglossus and the outer shell oi Balanus, the barnacle
or " acorn-shell " found everywhere on rocks between tide-marks.

e Fig. 8. — Metamorphosis of

Balanoglossus, probably
of Balanoglossus bimini-
ensis Willey, Bahama
Islands. All the figures
are magnified to the same
scale ( X 14). A, fully
developed free-swimming
larva, or Tornaria, side
view ; B, commencement
of metamorphosis, side
view ; C, later stage, dor-
sal view. Increase in size
takes place after this
stage ; a, anus ; bx', hody-
cavity of proboscis ; c,
collar ; c.r, transverse
ciliated ring ; cl.21 (in A),
dorsal pore ( = proboscis-
pore), seen also in C on
the left side, just behind
the reference line ^;.c ; e,
eyes and sensory thicken-
ing of skin (in A) ; g,
gill-pore ; g.s, gill-sacs,
developing as outgrowths
of the alimentary canal :
three are already present
in B, Init are better seen
in C, in which they are
still without openings to
the exterior ; /, postor:il
part of the longitudinal
band of cilia ; V, its jn-ae-
oral part ; Ijotli I and l'
are produced (in A) into
tentacles, over which the
l)and of cilia is looped ;
the groove in the nuddle
of the figure, between I
and /', conilucts the food
by the transverse groove
to the mouth [m] ; 2'-<^-
blood-space of proboscis
and pericardium ("heart "
of larva) ; s, stomach.
(After Morgan).

larval stage of Balano-
Several distinct forms

Development. — The free-swimming,
glossus is known as Tornaria (Fig. 8, A)
of the larva are known/ although it is not yet possible to refer
them with certainty to their respective adults.
^ Spengel, Monogr. p. 370 f.

DEVELOPMENT I 9

Tornaria was described and named by Johannes Mliller, who
regarded it as the larva of a Starfish,^ in spite of his intimate
knowledge of the development of these animals. Its correct syste-
matic position was first demonstrated by Metschnikoff in 1869.

The larva agrees with many other pelagic forms in being
excessively transparent. The form described by Spengel as
7'. grenacheri attains the remarkable length of 9 mm. (nearly
|th inch).

The full-grown larva is usually ovoid, and a complicated
" longitudinal " band of cilia runs in several loops over its
anterior two-thirds. In side view, part of the surface limited by
the ciliated band appears like a T with a double outline, the cross
piece being bent downwards on each side, so as to form an anchor-
like curve, the middle of which is at the anterior pole of the
larva. In T. krohni, which occurs on our south coast,' the
ciliated band has a w'avy course. In the West Indian larva ^
shown in Fig. 8 A, the ciliated band is produced into numerous
tentacles, which fringe the sides of the T-shaped areas or grooves
of the surface. These grooves and the cilia which border them
are used for conveying food to the mouth.'* At the apex of the
larva is a thickening (e) of the ectoderm, bearing two eye-spots.
The main locomotor organ is a simple transverse band (c.r) of
" membranellae," vibratile structures composed of fused cilia.
The mouth (rn), on the ventral side, leads into the oesophagus,
and this into the stomach (s). The latter is separated by a
nuirked constriction from the intestine, which opens by the
anus (a) at the posterior pole.

On the dorsal side is a pore, the " dorsal pore " (d.j).), which
leads into a thin-walled sac (b.c'^) destined to become the pro-
boscis-cavity of the adult. To the right of the dorsal pore lies
the pulsating " heart," which apparently becomes the pericardium
of the adult. Bourne and Spengel regard it as a right proboscis-
cavity. In the older larvae, the second and third body-cavities
appear as paired thin-walled sacs in close contact with the
hinder part of the stomach. The skin is very thin, and the
five body-cavities do not nearly till the space between it and the

1 Cf. Spengel, 2foHu>jr. p. .36-3 f.

- Bourne, J. Mar. Biol. Ass. (N.S.), i. 1889-90, p. 63.

^ This closely resembles T. grenacheri, but see Willey, op. cit. p. 285.

■^ Haldeman, Johns Hopkins Univ. Circ. vi. Xo. 54, 1886, p. 45.

20 HEMICHORDATA

alimentary canal. This space becomes obliterated for the most
part by the enlargement of the body -cavities, and its last remains
persist, as in many other animals,^ as the vascular spaces of the
adult.

In Dolichoglossus hoivahvskii, and probably in other species
with large eggs,^ development proceeds by gradual stages to the
adult form, and no Tornaria-stage is passed through. The opaque
young animal, on being hatched, creeps about in the muddy sand
in which the adult is found, later moving in a leech-like manner,
by alternately attaching itself by its two ends. The young
stages were ingeniously obtained by Bateson, to whom our
knowledge of the development of this species is due,^ by allowing
a large quantity of the mud to settle after being stirred up, the
layer of the specific gravity corresponding with that of the young
Balanoglossus being then separated by means of a siphon. The
young stages previously contained in several hundredweight of
mud were thus easily collected into a pint of water. Morgan
recommends treating the layer obtained by a similar process with
picric acid, which stains the young Balanoglossus yellow.

The embryo early becomes a " blastospbere " or hollow vesicle
formed of a single layer of cells. One half of this is invaginated,
or pushed into the other half, and a " gastrula " is thus developed,
the cavity of which is the " archenteron," and the two cell-
layers respectively " ectoderm " and " endoderm." The " blasto-
pore," or orifice of invagination, is at the posterior pole of the
larva, where it narrows and closes, the locomotor, transverse band
of cilia developing round it. No other bands of cilia appear in
this form of development. The proboscis becomes marked out
externally by the appearance of a circular groove, near the
middle ; and behind this groove a second one appears, which
forms the posterior boundary of the collar. The larva, which
now resembles Fig. 8 C, is usually liatched at this stage. Two
gill-slits make their appearance, and the mouth and anus are
perforated ; the anus being in the position of the blastopore.

^ For Vertebrates see Shipley, Quart. J. Micr. Set. xxvii. 1887, p. 340.

^ The largest known eggs are those of Harrimania kupfferi (1'3 ram.). The
€ggs of Dolichoglossus koivalevskii measure *37 ram., while the youngest Tornaria
Ibund by Morgan, already transparent and with their tissues distended by water,
were only about two-thirds that size.

2 Quart. J. Micr. Sci. xxiv. 1884, p. 208 ; xxv. Suppl. 1885, p. 81 ; xxvi. 1886,
pp. 511, 535.

PTEROBRANCHIA 2 1

The body -cavities are formed as five derivatives of the
archenteron. One of these is unpaired, and becomes the
proboscis-cavity ; while the others are the paired cavities of
the colhir and trunk (cf. Fig. 2). There is some uncertainty
about the origin of the body -cavities of the free -swimming
Tornaria, although it seems most probaljle that they are developed
either from the wall of the stomach or intestine/ or from scattered
mesoderm cells " which lie in the segmentation-cavity.

The metamorphosis of Tornaria is accompanied l)y a great
diminution in size,^ probably due to the loss of water ; by this
cause and by the simultaneous thickening of the skin, the larva
loses its transparency.

The external features of the metamorphosis are sufficiently
indicated by Fig. 8, the ciliated bands finally disappearing.
The dorsal pore persists as the proboscis-pore ; the notochord
and numerous gill -slits are developed as outgrowths of the
alimentary canal, the reproductive organs make their appearance,
probably from the mesoderm,'* the trunk meanwhile elongating
so that the proportions of the adult are acquired.

Order II. Pterobranchia.

Tubicolous Hcmichordata, iciih one i^ctir of giU-sUts or none, a
U-sliapecl alimentary eanal, and a. dorsed anus situated near
the mouth. Frohoscis flattened ventrally into a large " buceal
disc," its base covered dorsally hy the collar, ivhich is
IJToduced into tiro or more tcntaculiferoiis arms. TrunJc
short, 2^'>^olonged into a stall: Reproduction hy hudding
occurs.

This group consists of the two genera Ccphalodiscus (Fig. 9)
and PJicthdo'pleura (Fig 12). The latter, first dredged by G.O.
Sars, in 18G6, from 120 fathoms off the Lofoten Islands, was
included in a catalogue of deep-sea animals published by his
father, M. Sars, in 1868 as Hcdilophus mirahilis, a name which

^ Agassiz, Bourne, Spengel, Morgan (in T. agassizii).

- Morgan (in Balanoglossus himiniensis).

^ A similar shrinkage takes place in the metamorphosis of the larva {Lepto-
cephalus) of Eels, as has been shown by Grassi, Quart. J. Micr. Sci. xxxix. 1897,
p. 374.

* Schimkewitsch, Znol Anz. xi. 18S8. p. 283 ; IM organ, J. Morjjhol. ix. 1894,
p. 60 ; Punnett {op. ci(. p. 661) believes that they are ectodermal.

22

HEMICHORDATA

has been superseded by Bhahdopleura normani, Allman/ based

i /

■ ^«''

y

Fig. 9. — Cephalodiscns dodecalophus, M'lntosh, Sti-aits of Magellan ; A. small portion of
the common "house," x 1 ; a , a, single individual, shown also as B, -X 65 ; six of
the tentacular arms, belonging to the collar, are seen sprirging from hehind the
proboscis or "buccal disc." This has a crescentic band of pigment parallel with its
posterior border, which conceals the mouth. The .stalk, bearing a bud, which
already shows the beginning of two tentacular arms, is seen to tlie right. (After
M'lntosh, B from Parker and Haswell.)

■• Allman's name {Quart. J. Micr. Sci. ix. 1869, p. 57 f. ) replaces that given
by Sars, because the latter gave no description by which the organism could be
recognised.

PTEROBRANCHIA 23

Oil specimens dredged by Canon Norman in 90 fathoms, off the
Shetland Ishinds. The structure of Rhcibdopleura has been
described by Sars/ Lankester,^ and Fowler.^ R. normani is
common in certain Xorwegian Fjords, at depths of 40 fathoms
or more, and has been recorded by Fowler from the Tristan
d'Acunha group in the S. Atlantic ; R. compacta has been found
off the N.E. coast of Ireland * and near Koscoff, on the N. coast of
Brittany ; while forms described by Jullien ^ as R. grimaldii and
R. manubialis have been dredged off the Azores. I have recently
found a fragment of Rhabdopleura from South Australia. It is
doubtful how far these species are distinct.

Ccphalodiscus dodeccdoplius '^ was found in the Straits of
Magellan, during the " Challenger " voyage, at a depth of 245
fathoms, and has recently been rediscovered in shallower water in
the same neighbourhood by the Swedish Antarctic Expedition.
Another Cepludodiscus, at present undescribed, has been obtained
by Dr. Levinsen from 100 fathoms off the coast of Japan ; while
the Dutch expedition carried out by the " Siboga " has resulted in
the discovery of two other specimens, one from a reef close to low-
tide mark on the coast of Borneo, the other from 41-52 fathoms
off Celebes. These three specimens differ markedly from one
another and from the '•' Challenger " specimen of C. dodecalophiis,
and it is probable that they all belong to new species. The
occurrence of a deep-sea animal at a great distance from the
locality at which it was first found is not in itself a matter for
great surprise ; but in the present instance two of the newly
discovered forms are from shallow water, and one of them is
actually littoral. The occurrence of so many species of Ceplia-
lodiscus in Oriental waters sugcrests that the Pacific or the
Indian Ocean may be the headquarters of the genus, which
may prove to be far less of a rarity than has hitherto been

^ "Remarkable forms of Animal Life," i. Christiania Univ. -Program for the
Jirst half-year, 1869 ; and Quart. J. Micr. Sei. xiv. 1874, p. 23.

2 Quart. J. Micr. Sci. -xxiv. 1884, p. 622.

^ Proc. Roy. Sac. lii. 1893, p. 132 ; Festschr. 70 te^i. Gcburtstage P. Leuckarts,
4to, Leipzig, 1892, p. 293.

■* Hiiicks, Hist. Brit. Marine Polyzoa, vol. i. 1880, p. f>81.

•^ Pts. Caitq]. Sci. Prince de Monaco, Bryozoaires, 1903, p. 23.

" Challenger Reports, Part Ixii. 1887. See also Masterman in Quart. J. Micr.
Sci. xl. 1898, p. 340 ; xlvi. 1903, p. 715 ; Reih Brit. Ass. (1898), 1899, p. 914 ; Tr.
R. Soc. Ed-inh. xxxix. 1900, p. 507 ; and the notes in the Zool. Anz. xx. 1897,
pp. 342, 443, 505 ; xxii. 1899, pp. 359, 361 ; and xxvi. 1903, pp. 368, 593.

24

HEMICHORDATA

supposed. There is evidence derived from the results of the
" Siboga " expedition that abyssal animals may migrate into
comparatively sliallow water in the Malay Archipelago.

Oephalodiscus and Rhabdoplevra are remarkable for their
power of producing buds. In the former these arise from the

apex of a stalk which
/^^/-Vy ^-^^^X , is given oft' on the
ventral side of the
body, and they break
off when they reach
a certain age ; in the
latter they do not be-
come free, and a colony
results, wdiicli consists
of a creeping " stolon "
from which vertical
branches are given off
at intervals, each end-
ing in an individual
of the colony. Ceiilia-
lodiscus forms a gela-
tinous " house " (Fig.
9, A), in the passages
of which are found
laro:e numbers of the
individuals, to-
gether with their
eggs and embryos.
Eh ahdopleu ra (Fig.
12) is protected by
cylindrical tubes, one

Fig. 10. — Longitudinal median section of Cephalodiscus free
dodecalo2]hi(s. a, Anns ; 6.c\ b.c"^, b.c^, first, second,
and third liody-cavities ; int, intestine ; jn, mouth ;
nch, notochord ; n.s, central nervous system ; oes,
oesophagus ; oj), operculum, the ventro-lateral part
of the collar ; ov, ovary ; ovd, pigmented oviduct ;
2)h, pharynx ; 2^-2^< proboscis-pore ; ps, proboscis ; st,
stomach ; stk, stalk.

of which corresponds with each individual.

Cej)halodiscus, though no more than two or three millimetres
in length, is provided with practically all the important organs
possessed by Balanoglossus. Its prol)oscis or " buccal shield "
(Fig. 10, ^s) is a large flattened structure, which overhangs and
entirely conceals the mouth. The anterior body-cavity opens to
the exterior by two symmetrically placed proboscis-pores {p).p),
just in front of the tip of the notochord {nch). The collar,
which has paired body-cavities, is produced dorsally into 4-6

STRUCTURE OF CEPHALODISCUS

25

pairs of plume-like arms, which bear an immense number of
pinnately-arranged tentacles. The arms, which may end in a
swollen bulb,^ have ventral grooves along which food doubtless
travels to the moutli liy ciliary currents. The anterior edge of the
ventral half of the collar is di'awn out into a narrow flap or oper-
culum (Fig. 11, op), in front of which
is the mouth, and behind it the gill-
slits {rj) and collar - pores (c). The
central nervous system {ii.s) is a thick
mass of nerve - tissue in the dorsal
epidermis of the collar ; it is not sunk
beneath the skin as in Balanoglossus.
The details of the nervous and vascular
systems, and the development of the
Inids, have 1;)een described by Master-
man. In the dorsal region of the collar
the alimentary canal has a slender
diverticulum, the notochord, which
passes into the base of the proboscis ;
it is believed by ^lasterman to have a
function similar to that of the neural
gland (cf. p. 52) of Tunicates.

The next part of the alimentary
canal, the pharynx," has a single pair
of simple gill -slits opening to the
exterior immediately behind the collar-
pores. The short oesophagus (Fig.
10, oes) is followed by the wide
stomach {sf), and this by the intes-
tine {int^, which opens by the anus
(rt) near the front end of the body.

The trunk contains paired third body -cavities (li.c ^), the
septum between which and the collar-cavities is slightly liehind
the line of origin of the operculum. Two ovaiies {pv) are
situated between the pharynx and the last part of the intestine,
each opening to the exterior dorsally between the central nervous

Fig. 11. — Longitudinal section
through Cephalodiscus dode-
calophus, passing through the
two sides of the body ; a,
tentacnlar arm ; b.c ^, collar-
cavity ; b.c •', trunk - cavity ;
c, collar-pore ; g, gill-slit ; t,
intestine ; n.s, central nervous
system ; 0, oesojihagns ; op,
operculum ; j:;, pharyn.x ; s,
stomach.

1 Cf. Cole, /. Linn. Soc. xxvii. 1899-1900, p. 256.

- Two dorsal portions of this region, which are regarded by ilasterman as
lateral notochords, appear to me to represent the dorsal part of the pharynx of
Ptxjchodera.

26

HEMICHORDATA

system and the anus. Each oviduct {ovd) contains dark pigment,
which is seen through the dorsal skin on removintf the tentacular

arms. Eggs, each enclosed in a
stalked membrane, occur in num-
bers in the cavities of the gelatin-
ous house. The early stages of the
development are passed through
inside the tubes ; but there is at
present little other information
with regard to the embryonic de-
velopment of the Pterobranchia.
The specimen obtained by the
" Siboga " from Celebes is a male
colony with dimorphic individuals,
the reproductive organs being con-
fined to two-armed zooids with
vestigial alimentary canal.

Bhahdoph'ura differs from
Ceplialodiscus in its much smaller
size,^ and it is perhaps due to
its minuteness that it does not
possess certain organs found in
the latter. The stalk is repre-
sented by a long muscular cord,
which is merely a narrow part
of the body. Basally the stalk
of each individual passes into a
common axis, which is for the
most part attached to the sub-
stance on which the colony is
Fig. 12.— Small portion of colony of growing, and is to some extent
Rhahdophura normani, Aiiman, branched. The muscular stalk

Lofoten Islands, x 16. «, Anns ; . .

j3, proboscis (= buccal disc) ;?•, rod- caii be contracted luto a spiral,
like axis of the adherent part of the thereby retracting the animal

colony, prolonged into s, the stalks / °

of the individuals ; st, stomach ; t, into itS tube. The stalks and

(IVersarfP'''''""''*^^^'''''""'' ^lie youiiger parts of the axis

which connects them are soft, but
the older parts secrete a dark ].»rown cuticle, forming a narrow

^ The diameter of a single individual removed from its tube is given by Fowler
as •123 mm.

RHABDOPLEURA — niORONIDEA 27

tube which becomes embedded in the adherent wall of the outer
tube. The thin dark axis, to which the name Rltahdojyleura
refers, is the feature by which the animal can most readily be
recognised without magnification.

The outer transparent tube is constructed by the proboscis, or
buccal shield, the secretion of which appears to lie intermittent,
so that the tube consists of a series of rings piled on one another.
The animal crawls up the inside of its tube by means of its
proboscis, while it is retracted by means of the muscles of its
stalk.

The growing axis ends in a row of young buds, the buccal
shields of which early reach a relatively large size. The
terminal bud gives rise to tube-rincps, so that the axis is sur-
rounded by a cylindrical outer tube, which becomes interrupted
by transverse septa, each bud, except the end one, thus lying in
a closed chamber. The wall of each chamber becomes perforated,
and the buccal shield then prolongs this perforation by adding
tube-rings, the formation of which continues till the tube reaches
a considerable length. The bud remains connected with the
axis by means of its narrow proximal region, which forms its
stalk. The adherent part of the adult colony thus consists of
a row of short tubes, traversed by the common axis of the
colony. Each tube is produced laterally into the upright tube
of an individual.

The general anatomy closely resembles that of Cephalodiscus}
Tliere are five body-cavities and a notochord. Collar -pores
exist, but proboscis-pores and gill-slits have not been described.
The dorsal region of the collar bears only a single pair of
arms.

Order III. Phoronidea.

The structure and development of Fho7'onis (Fig. 13), have
already been described in Vol. II."' of this series ; and Master-
man's investigations, then published in a preliminary form
only, are there alluded to. Since then this author has pub-
lished fuller accounts ^ of his results, which, if substantiated,

' See, however, Conte and Vaney, C. 11. Ac. Sei. 135, 1902, pp. 63, 748.

- Pp. 450-462.

'■^ Quart. J. Micr. Sci. xl. 1898, p. 281 ; xliii. 1900, p. 375 ; xlv. 1902, p. 485.

28

HEMICHORDATA

would indicate a near relationship between CephalocUscus and
Fhoronis.

Fhoronis is a small tubicolous animal, of gregarious habits,
which has usually been regarded as related to the Gephyrea, Its
body ends in a plume of ciliated tentacles, which can be pro-
truded from its tube, and the anus is on the dorsal side, not
far from the mouth. In both these respects it agrees with
Ceyhalodiscus, but a more striking simi-
^vt\th;s;^K'J,Ji?ft,. larity is asserted by Masterman to exist
between the latter and Actinotrocha, the
larval stage of Fhoronis. The prae-oral
ciliated hood (Fig. 14) of Actinotrocha is
regarded as the proboscis, and it contains^
a median cavity, traversed, like that of
Balanoglossus, by muscular fibres. The
collar is the region between the con-
stricted neck and an ol^lique line, parallel
to and immediately behind the series of
tentacles, which thus belong to the collar.
This division has a collar-cavity which is
said to be distinct from the prae-oral
cavity, and is separated by a septum from
the posterior body - cavity. Its dorsal
epidermis contains the central nervous
system (n.s), which is connected with
a system of nerves resembling those
of Balanoglossus. A median diverti-
culum of the alimentary canal of this
part may be compared with the noto-
chord of that animal, but there are no

Fig. is.— Phnronis buslii, -ii i-^
M'lutosh, Philippine g^^^"^^^''^-
Islands, x about 2.
(After M'Intosli, from
Shipley.)

The remainder of the body of Actino-
trocha corresponds with the trunk of
Balanoglossus. Its body-cavity is distinct
from that of the collar, and is divided by a ventral mesentery,
though not by a dorsal mesentery. A noteworthy fact is that
both Actinotrocha and Tornaria swim by means of a ring of
strong cilia or membranellae ^ which surrounds the anus.

Important memoirs on the structure of Actinotrocha have

1 Cf. p. 19.

PHOKONIS AND ACTINOTROCIIA

29

recently been published by Ikeda/ de Selys Longchamps,- Good-
rich,^ and Schultz/ who criticise many of Masterman's state-
ments. While it is admitted on all sides that an olilique septum
following the line of the bases of the tentacles completely sub-
divides the Ijody-cavity, Masterman's account of the anterior
cavities is not confirmed, the spaces indicated by h.c^ and l.c^ in
Fig. 14 being stated to be really continuous with one another,
while the " subneural sinus " {s.s) is regarded as a part of this

Fig. 14. — Adinotrocha-laxva. of Phoronii. a. Anus;
b.c^, b.c", b.(?, first, second and third body-cavities ;
c, circular nerve, running in the posterior boundary
of the collar, immediately behind the ring of ten-
tacles ; (■./■, ciliated ring ; d, diverticulum (paired) of
alimentary canal ; vi. mouth ; n.s, central nervous
system ; Pi nerve running round the ventral border
of the proboscis; s, sense-organ; s.s, subneural
sinus, a vascular space whose hind wall is con-
stituted by the front boundary of b.c^, its front
wall being formed by the hind wall of b.c^ ; in this
region is seen a median outgrowth of the alimentary
canal, which may be compared with tlie notochord
of Cephalodiscus, or of the young Tornaria (cf.
Morgan, J. Morphol. v. 1891, Plate xxvi. Fig. 40.)
(After Mastermau.)

space. It appears, however, from the account given by Ikeda,
and followed by Goodrich, that the old Actinotrocha has two
distinct spaces in front of the septum. The first of these corre-
sponds with h.c^ + most of h.c' in Fig. 14, and is continuous
with the cavities of the larval tentacles. Into it project the
blind ends of the larval excretory organs, which, according to
Goodrich, bear numerous " solenocytes " similar to those described
by the same author in Amphioxus and in Polychaet worms (Fig.
79, p. 127). The second cavity is a relatively small crescent
(not shown in Fig. 14), lying on the anterior face of the septum,

1 J. Coll, Japan, xiii. Pt. iv. 1901, p. 507.

^ Arch. Biol, sviii. 1902, p. 495 ; Wiss. Meeresimtersueh. vi. Abt. Helgoland,
Heft 1, 1903.

3 Quart. J. Micr. Set. xlvii. Pt. i, 1903, p. 103.
^ Zeiischr. wiss. Zool. Ixxv. 1903, pp. 391, 473.

30 HEMICHORDATA

the tips of the crescent nearly meeting dorsally, so as to con-
stitute an ahnost complete ring following the bases of the
tentacles, into each of which it gives off a blind outgrowth. At
the metamorphosis, the crescentic space becomes the prae-septal
body-cavity and the cavities of tlie tentacles of the adult, the
circular blood-vessel of wdiich is formed from the remains of the
large prae-septal space of the larva. Schultz, in calling atten-
tion to the fact that both Phorouis and its larva have a striking-
power of regenerating lost parts, confirms the conclusion that
this animal belongs to the Hemichordata. He gives reasons,
however, for believing that it is in the adult Phoi'onis rather
than in the larval Actinotrocha that it is possible to discover
the most satisfactory evidence of this affinity.

The metamorphosis ^ of Actinotrocha is very remarkable, and
is accompanied by the e version of a ventral ingrowth of the
body- wall. A loop of the alimentary canal passes into this
eversion, which becomes the main part of the body of the adult ;
and the anus is thereby brought relatively nearer the moutli
than in the larva. The occurrence of this process may help to
explain the position of tlie anus in the Pterobranchia.

Affinities of the Hemichordata. — There can be no doubt
that some of the resemblances, in structure and in development,
between Balanogiossus and certain Vertebrates are extremely
striking. The view that Balanogiossus is related to the ancestors
of Vertebrates ^ appears to exclude other views ^ which have Ijeen
suggested with regard to the same question. The Balanoglossus-
theory does not explain the similarity between the segmentation
and the excretory systems of Vertebrates and Chaetopods ; but,
on the contrary, there are important characters which Vertebrates
share with Balanogiossus but with no other " Invertebrates."
Of these the most important appear to be the resemblances
between the gill-slits and gill-bars of Balanogiossus and
Amphioxus ; the position, structure and mode of development of
the central nervous system ; and the presence of a structure in
the Hemichordata, which may be regarded as the notochord.

' Vol. II. 1). 4f.9.

- Huxley, in 1877 {Man. Anat. Invert. Animals, p. 674), proposed to unite the
Enteropneusta with the Tiinicata as Pharyngopneusta, in allusion to the gill-slits
connected with the pharynx ; but the view was first defended in detail by
Bateson.

^ See, for example, Minot, Amcr. Nat. xxxi. 1897, p. 927.

AFFINITIES 31

There are other points in which Balanoglossus specially resembles
Amphioxus, such as the early development, the mode of formation
of the body-cavities/ and the presence of numerous generative
organs.

All these, taken together, make it necessary to consider
carefully the claims of Balanoglossus to relationship with the
ancestors of Vertebrates in making any speculations on this
interesting problem.

However improbable it may appear at first sight, it is
possible to hold the view that Balanoglossus is related at the
same time to Vertebrates and to Starfishes and other Echino-
derms. The similarity lietween a young Tornaria and a young
Bipinnaria-larva of a Starfish is so great as to have misled even
Johannes Muller. The more obvious resemblances are the almost
identical course of the longitudinal ciliated band in the young
stages, and the presence of a dorsal pore. The Echinoderm-
larva is not, however, provided with eye-spots, nor has it the
posterior, or transverse, ciliated band of Tornaria.

Eecent studies on the development of Echinoderms " have
made it probable that the five body-cavities of Balanoglossus are
represented in the larvae of those animals ; and this materially
strengthens the probability of the view that the respective adults
are also allied.^ It may be added that the relationship which
appears to be indicated is between Balanoglossus and the bilateral
ancestors from which the radially-symmetrical Echinoderms are
probably descended.

In comparing the Enteropneusta with the Pterobranchia, the
disproportionate size of the trunk of Balanoglossus may perhaps
be explained by assuming that the region of the third body-
cavities has been enlarged since Balanoglossus branched off from
the ancestral stock."* The approximation of the anus to the
mouth in Pterobranchia is perhaps the result of their tubicolous
habits."^ In the position of the central nervous system in the
skin of the collar, Cephalodiscus appears to be more primitive

1 See .MacBride, Quart. J. Micr. Sci. xl. 1898, p. 580 : xliii. 1900, p. 351.

- Bury, Quart. J. Micr. Sci. xxix. 1889, p. 409; xxxviii. 1896, p. 125 ; MacBride,
ibid, xxxviii. p. 395 ; Masterman, Tr. H. Soc. Edinh. xl. Pt. ii. No. 19, 1902, p. 403.

^ This view was definitely formulated by Metschnikoft" in 1881 (Zool. Anz. iv.
1881, pp. 139, 153).

^ Cf. Morgan, J. Morphol. v. 1891, p. 445; ix. 1894, pji. 64-66.

^ Cf. Lang, Jena. Zeitschr. xxv. 1S91, p. 1.

32 HEMICHORDATA

than Balanoglossus, as has been pointed out by Morgan.^ It is
not impossible that the presence of one pair of gill-slits in
Cephalodiscus indicates that this animal diverged from the
ancestors of Balanoglossus before the gill-slits were metamerically
repeated.

^ J. Morphol. ix. p. 72.

ASCIDIANS AND AMPHIOXUS

AY. A. HEEDMAN, D.Sc. (Edinb.), F.RS.

Professor of Natural History in the University of Liverpool

VOL. VII

CHAPTER 11

TUNICATA (ASCIDIANS AND THEIR ALLIES)

INTRODUCTION OUTLINE OF HISTORY STRUCTURE OF A

TYPICAL ASCIDIAN EMBRYOLOGY AND LIFE-HISTORY

The Tunicata are marine animals found in practically all parts
of the sea, and at all depths. They extend from the Arctic and
Antarctic regions to the tropical waters, and from the littoral zone
down to the abyssal depths of over three miles. They are
abundant in British seas. They vary greatly in shape and
colour, and range in size from an almost invisible hundredth of
an inch to large masses a foot or more in diameter. And yet
most Tunicata have a characteristic appearance by which they
can be readily distinguished from other animals. They form a
well-defined group, with definite anatomical characters, and
there are no known forms intermediate between them and other
groups. The Tunicata were formerly regarded as constituting,
along with the Polyzoa and the Brachiopoda, the Invertebrate
Class " Molluscoidea." They are now known to be a degenerate'
branch of the lower Chordata, and to be more nearly related to
the Vertebrata than to any group of Invertebrates.

Tunicata occur either fixed or free, solitary, aggregated or in
colonies (see Fig. 27, p. 64). Tlie fixed forms, found on the
sea -bottom, are usually termed " Ascidians," those that are
solitary or merely aggregated being " Simple Ascidians " or
Monascidiae, and those that are organically united into a
colony being " Compound Ascidians " or Synascidiae. The
colonies have been pi'oduced by budding, a process which is
very general in the group, and the members of the colony
are conveniently known as '' Ascidiozooids." Some exhibit

35

36 ASCIDIANS

alternation of generations, and all pass through remarkable
changes in their life -history, nearly all of them undergoing
a retrogressive metamorphosis.

Outline of History.

More than two thousand years ago Aristotle gave a short
account of a Simple Ascidian under the name of Tethyum. He
described the appearance and some of the more important points
in the anatomy of the animal. From that time onwards com-
paratively little advance was made until Schlosser and Ellis, in
a paper on Botryllus, published in the Philosophical Transactions of
the Eoyal Society for 1756, first brought the Compound Ascidians
into notice. It was not, however, until the commencement of the
nineteenth century, as a result of the careful anatomical investiga-
tions of Cuvier ^ upon the Simple Ascidians, and of Savigny - upon
the Compound Ascidians, that the relationship between these
two groups of Tunicata w^as conclusively demonstrated. Up to
1816, the date of publication of Savigny 's great work, the
few Compound Ascidians previously known had been generally
regarded as Alcyonaria or as Sponges ; and although many new
Simple Ascidians had been described by 0. F. Miiller ^ and
others, their internal structure had not been investigated.
Lamarck* in 1816, chiefly as the result of the anatomical
discoveries of Savigny and Cuvier, instituted the class Tunicata,
which he placed between the Eadiata and the Vermes in his
system of classification. The Tunicata included at that time,
besides the Simple and the Compound Ascidians, the pelagic
forms Pyrosoma, which had been first made known by Peron in
1804, and Salpa described by Forskfil in 1775.

Chamisso, in 1819, made the important discovery that Salpa
in its life- history passes through the series of changes which
were afterwards more fully described by Steenstrup in 1842 as
" alternation of generations " ; and a few years later Kuhl and
Van Hasselt's investigations upon the same animal resulted in
the discovery of the alternation in the directions in which the
wave of contraction passes along the heart, and in which the

^ Mini. Mus. Paris, ii. 1815. - J/tO;i. s. I. Anim. s. Vert. Pt. ii. Paris, 1816.

^ Zoologia Danica, iv. 1806.

* Hist. Nat. d. Anim. sans Vert. Paris, 1815-1822, t. iii.

II OUTLINE OF HISTORY 37

blood circulates through the body. It has since been found that
this observation holds good for all groups of the Tunicata. In
182G, H. Milne-Edwards ^ and Audouiu made a series of oljserva-
tions on living Compound Ascidians, and amongst other discoveries
they found the free-swimming tailed larva and traced its develop-
ment into the young Ascidian.

In 1845, Carl Schmidt" first announced the presence in
the test of some Ascidians of " tunicine," a substance very similar
to cellulose ; and in the following year Liiwig and XiJlliker ^
confirmed the discovery, and made some additional observations
upon this substance and upon the structure of the test in general.
Huxley,'* in an important series of papers published in the
Transactions of the Eoyal and Linnean Societies of London from
1851 onwards, discussed the structure, embryology, and affinities
of the pelagic Tunicates, Pyrosoma, Salpa, Doliolum and Appendi-
cidaria. These im])ortant forms were also investigated about the
same time by Gegenbaur, Vogt, H. Milller, Krohn, and Leuckart.

The most important epoch in the history of the Tunicata is
the date of the publication of Kowalevsky's celebrated memoir'^
upon the development of a Simple Ascidian. The tailed larva had
been previously discovered and investigated by several naturalists,
notably by H. Milne -Edwards,'' V. J. van Beneden, and
Krohn ; but its minute structure had not been sufficiently
examined, and the meaning of what was known of it had not
been understood. It was reserved for Kowalevsky in 1866 to
demonstrate the striking similarity in structure and in develop-
ment between the larval Ascidian and the Vertebrate endiryo.
He showed that the relations between the nervous system, the
notochord, and the alimentary canal are practically the same in
the two forms, and have been brought about by a very similar
course of embryonic development. This discovery clearly indicated
that the Tunicata are closely allied to Amphioxus and the
Vertebrata, and that the tailed larva represents tlie primitive or
ancestral form from which the adult Ascidian has been evolved
by degeneration. This led naturally to the view usually
accepted at the present day, that the group is a degenerate side-

^ M6m. Instit. Paris, xviii. 1842.

^ Zur vergl. Physiol. WirheUos. Thiere, Brunswick.

' Comptes Pe7idus, Paris, xxii ; and Ann. Sci. Nat. ser. 3(Zool.) v.

•• Phil. Trans. 1851 ; Trans. Linn. Soc. xxiii. 1860.

^ Mem. Acad. St. Peterslourg (7), x. 1866. " Mem. Instit. Paris, xviii. 1842.

38 ASCIDIANS

branch from the lower end of the phylum Choedata, which
includes the Tunicata (or Urochordata), Balanoglossus and its
allies (Hemichordata), Amphioxus (Cephalochordata), and the
Vertelirata (or Craniata). Kowalevsky's great discovery has
since been confirmed and extended to all other groups of the
Tunicata by Kupffer/ Giard, and others.

In 1872 Fol ^ added largely to the knowledge of the
Appendiculariidae, and Giard ^ to that of the Compound
Ascidians. The latter author described a number of new forms
and remodelled the classification of the group. The most
important additions which have been made to the Compound
Ascidians since Giard's work have been the species described by
von Drasche,'* from the Adriatic, and those discovered by the
" Challenger " expedition.^ The structure and the systematic
arrangement of the Simple Ascidians have been discussed of
recent years mainly by Alder '' and Hancock, Heller,' Lacaze-
Duthiers,^ Traustedt,^ Eoule, Hartmeyer, Sluiter ^^ and Herd-
man.^^ In 1874 Ussoff investigated the minute structure of
the nervous system and of the underlying gland, which was first
discovered by Hancock, and showed that the gland has a duct
which communicates with the front of the branchial sac or
pharynx by an aperture in the dorsal (or " olfactory ") tubercle.
In an important paper published in 1880, Julin ^"•^ drew atten-
tion to the similarity in structiire and relations between this
gland and the " hypophysis cerebri " of tlie Vertebrate brain, and
insisted upon their homology. Metcalf has recently added
further to our knowledge on this and related matters.

The Thaliacea or pelagic Tunicata have of late years been
the subject of several very important memoirs. The researches

^ Arch, iidkr. Anat. vi. 1872.

- M6m. ,Soc. Phys. Hist. Nat. Geneve, xxi. 1872.

^ Arch. Zool. Expir. i. 1872.

^ Synaseidien der Hucht von Rovigno, ^Vieii, 1883.

5 Challenger Reiwrts, Tunicata, Part i. \ol. vi. 1882 ; Part ii. vol. xiv. 1886 ;
Part iii. vol. xxvii. 1888.

^ Ann. Mag. Nat. Hist. (3) xi. 1863, p. 153 ; Journ. Linn. Sue. 1868, etc.

■^ Denksclir. Akad. JViss. Wien, 1875 and 1877.

* Arch. Zool. Exper. iii. 1874, and vi. 1877 ; J/e'wi. Instit. Paris, xiv. 1892.

3 Vid. Medd. Nat. For. Copenliagi-n, 1880, 1882, 1884, etc.

" Nat. Tijdschr. Ned. -Indie, 1885, etc.

^^ Journ. Linn. Soc. Zool. xv. xxiii. and xxiv. ; Cat. of Tunicata in Australian
Museum, 1899 ; also Cltallenger Re2wrts (see note 5).

^"^ Arch, de Biol. ii.

II STRUCTURE OF ASCIDIA 39

of Todaro, Brooks/ Salensky,' Seeliger;' Korotneff,'* and others
have ehicidated the embryology, the gemmation and the life-
history of the Salpidae : and (Irobben, Barrois;^ and more
especially Uljanin," have elaborately worked out the structure
and the details of the complicated life-history of the Doliolidae.
Finally we owe to the labours of Metschnikoff, Kowalevsky,
Giard, Hjort, Seeliger, Eitter, Van Beneden and Julin, much
detailed information as to development and life -history, the
process of gemmation and the formation of colonies, wliich has
added greatly to our knowledge of the position and aftinities of
the Tunicata and of their natural classification.

Structure of a Typical Ascidian.

If a typical " Simple Ascidian," such as the common British
Ascidia vientula (Fig. lb),ov Ascidia virginea, be examined alive
and expanded in sea-water it will be seen to bear on the upper
surface two short projections, each terminated by a wide tubular
opening, througli which the animal, when touched, can emit jets
of water with considerable force — thus accounting for the
popular name " sea-squirts." The rest of the body is covered by
the dull grey tough cuticular outer " test " or " tunic " (hence
Tunicata) by means of which the animal is attached to a rock
or other foreign body. One of the tulnilar openings, the mouth or
" branchial aperture," is terminal, and indicates the morphological
anterior end ; it is surrounded by eight lobes. The other opening,
the cloaca or " atrial aperture," is on the dorsal edge, from one-
third to one-half way down the body, and is bounded by six lobes
only ; consequently the two apertures, and so the ends of the
body, can be distinguished externally by the number of lobes —
an important matter. The area of attachment is usually the
posterior part of the left side ; in Fig. 1 5 the animal is seen
from the right hand side.

If a little carmine-powder, or some other insoluble particles
be scattered in the water in which the Ascidian is living, the

^ "The Genus Saljm," Mem. J. Hopkins Univ. 1S93.

- Zeits. wiss. Zool. 1876, 1878 ; Mitth. Zool. Stat. Xcapel, 1883, etc.

^ Jen. Zeitschr. 1886, 1888, ett;. ; also Bronn's Thier-Reich.

■' Mitth. Zool. Stat. Neapel, 1893 and 1897 ; and Zeits. wiss. Zool. 1895 and 1896.

'' Jo^irn. Anat. Phys. Paris, xxi. 1885.

" Fauna and Flora G. r. Neapel, Monogr. x. 1884.

40

ASCIDIANS

Br.\

particles will be seen to converge to the braueliial aperture and
be sucked in by the inhalent current entering the body. After

a short interval a certain proportion
of tlie particles will be shot out from
the atrial aperture with tlie exhalent
current.

These particles have passed through
the pharyngeal portion of the ali-
mentary canal and the cloacal passages,
with the water used in respiration,
but a considerable amount of such
particles taken in with the water
do not reappear, as they are retained
l)y the nutritive organs and pass along
the remainder of the alimentary
canal with the food. The current
of water passing in at the branchial
and out at the atrial aperture is of
})rimary importance in the life of
the Ascidiaii. Besides serving for
respiratory purposes it conveys all
the food into the body and removes
, .,. , , T- waste matters both intestinal and

Y\Qi. 1.5. — Ascidia mentula Linn.

from the right side (natural renal, and also cxpcls the rcproduC"
.size), Loch Fyne, N B. ; /;,• ^-^^ products from the body.

Branchial aperture ; At, atrial J^ -^

aperture. Arrows show the The TeSt. The test is notable

direction of the water currents. , • i j- j. r

amongst animal structures tor con-
taining " tunicine," a substance which appears to be identical
in composition, and in behaviour under treatment with various
reagents, with cellulose. It is cartilaginous in appearance and
consistency, and to some extent in structure, as it consists
of a clear (or in some cases fibrillated) matrix in wliich are
embedded many corpuscles or cells. It is tlie matrix that
contains the cellulose, wliich may form over sixty per cent by
weight of the entire test. As the test is morphologically a
cuticle, being a secretion on the outer surface of the ectoderm
(Fig. 16, ec), the cells it contains have iminigTated to it from the
body, and it has recently been shown that many of these are
mesodermal cells (leucocytes or connective tissue wandering cells,
amoebocytes, and in some cases embryonic " kalymmocytes," or

STRUCTURE TIIK TEST

41

egg-follicle cells, see below, p. 56), which have jiassed through
thb ectoderm. This process commences in the laivjil state witli
the migration of mesenchyme cells from the Llastocoele through
the epiblast. Ectoderm cells, and possibly also some i)riniitive
endoderm cells, also take part in forming the test. Many of
these cells in the test remain small and simple, as the fusiform
and stellate test-cells ; some become pigment-cells, while others
enlarge and become vacuolated to form the large (up to 0'15
mm. in diameter) vesicular or " bladder " cells — this is especially
the case in the outer layer of the test in Ascidia mentula (see

TEST

a\®

o

o

■O

Qq o

Q

^ (S>

blc.

Fig. 16. — Diagrammatic section through test and mantle of Ascidia to show the rela-
tions of ectodenii to body-wall and cnticle. bl.c., Bladder-cells ; bl.s, blood-sinus ;
c.t.c, connective tissue cells ; ec, ectoderm ; nies.c. wandering mesoblast cells : ?«./,
muscle fibres ; t.c, test-cells ; t.r, " vessel " of the test."

Fig. 17, hi) where there are innumerable clear vesicles, each sur-
rounded by a thin film of protoplasm and having the nucleus
still visible at one point of the surface. In some of the Tunicata
the test-cells produce calcareous spicules of various shapes (see
below, p. 86).

The test also becomes organised hj the growth into it of the
so-called "vessels." These are outgrowths of the body-wall
covered by ectoderm and containing prolongations of blood-
channels from the connective tissue of the " mantle " (body-wall).
Fig. 16, t.v shows such an outgrowth, and exhibits the general
relations of test (cuticle), ectoderm, and mesoderm. It also
explains how it is that the blood-channel being pushed out as a

42

ASCIDIANS

loop gives rise to the double or paired " vessels " seen branching
through the test (see Fig. 17, v). The two vessels of a pair are
■one blood-channel imperfectly divided by a connective-tissue
septum. The l)lood courses out along one side, round the com-
munication in a " terminal knob " at the end, and back down the
other side. The " terminal knol)S " are very numerous, and form
a marked feature in the outer layer of the test (Fig. 17, tJc);
in some cases {Culeolus murrdyi), they probaljly form an accessory
<jrgan of respiration, while in others (Botryllidae), they pulsate
and aid in keeping up the circulation.

The ectoderm is a simple epithelial layer (Fig. 16, ec\ It is
turned in for a short distance at the branchial aperture (mouth),

^•'"•■•r'.---.c>ov

Fig. 17. — Section through the surface layer of test oi Ascidia 77ie7Ut(hi, x 50. W. Bladder
cells ; t.c, test cell ; t.k, tenuiiial knobs of vessels ; v, vessels of test.

and atrial aperture (cloaca), as a short stomodaeum and procto-
daeum respectively, lined in each case by a delicate prolongation
of the test.

Fig. 24, A, p. 52, shows the relations of ectoderm, mesoderm,
and endoderm in a section through the antero-dorsal part of the
body. The cavity marked p.&r is a portion of the atrial cavity
lined by ectoderm, and must not be confounded with a coelom.
The absence of a true coelom in tlie mesoderm will be noticed
in this and other figures, and yet the Tunicata are Coelomata,
although it is very doubtful whether the enterocoel which has
been described in the development of some is ever found. The
coelom is in any case largely suppressed later, and is only re-
presented in the adult by the pericardium and by small cavities
in the renal and reproductive organs and ducts.

Body-Wall and Cavities of the Body. — The name " mantle "
is given to the ectoderm with tlie parietal mesoderm which form
the body-wall inside the test. It is largely formed of connective

STRUCTURE BODY-WALL, ETC.

43

tissues — both homogeneous and fibrous — with cells, blood-
sinuses, and many muscle - bundles large and small running
circularly, longitudinally, and obliquely, and interlacing in all
directions (i'ig. 18,

■m). The muscles
are all formed of
very long fusiform
non- striped fibres.
The mantle in some
Ascidians is often
brilliantly pig-
mented — red, yel-
low and opaque
white, the coloured
cells being exactly
like those found in
the blood.

The mantle
forms two well-
marked siph ons or
short wide tubes,
which lead in from
the branchial and
atrial apertures.
These are sur-
rounded by strong
sphincter muscles,^
and are lined by the
invaginated ecto-
derm and test. The
one leads into the
branchial sac or
modified pharynx,
and the other into

-Sr

br.s'

end.

p.br.c.--

FiQ. 18. — Dissection of Ascidia, from right side, to show
anatomy, a, Anns ; At, atrial aperture ; £r. branchial
aperture ; hr.s, br.s', branchial sac ; end, endostyle ; g.d,
genital dncts ; yon, ovary; h!/p, neural gland; hyp.d,
the duct leading to dorsal tubercle ; m, mantle ; n.g,
ganglion ; oes, oesophagus ; p.br.c, peribranchial cavity;
ren, renal vesicles ; st, stomach ; t, test ; tn, tentacles ;
ty, typhlosole.

the atrial or peribranchial cavity (see Fig. 18, and Fig. l^,p.lir).
Figs. 18 and 19 show the relations of the branchial and
peribranchial cavities to one another. The peribranchial cavity

^ These sphincters close the only openings through the tough test so effectually
that when collectors are preserving Ascidians in alcohol it is advisable to make one
or more slits in the test to allow tlie sea-water to escape and the sjiirit to enter.

44 ASCIDIANS

opens to the exterior dorsally by the atrial aperture, forms the
cloaca along the dorsal edge of the body, and has extensions
laterally on each side of the branchial sac, with the interior of
which it is placed in communication by the secondary gill-slits
or " stigmata " (Fig 19, sg). Along the ventral edge the mantle
is united to the wall of the branchial sac, and it is only this
union (Fig. 19, end) that prevents the peribranchial cavity from
completely surrounding the branchial sac.

The following list of the cavities present in the body of the
adult Ascidia may be useful at this point : —

1. The alimentary canal, including the branchial sac. This is
derived from the archenteron of the embryo, is lined throughout
by endoderm, and the system of cavities of the intestinal gland is
to be regarded merely as an outgrowth from the alimentary
canal.

2. The peribranchial (atrial) cavity, derived from two lateral
ectodermal invaginations which join dorsally to form the cloaca
and open to the exterior by the atrial aperture.

3. The original embryonic segmentation cavity (blastocoele)
remains, where not obliterated by the development of the meso-
dermal connective tissue, as the irregular system of blood spaces,
with its outgrowths in test and branchial sac. The heart, which
has differentiated muscular walls, becomes secondarily connected
at its ends with these blood spaces.

4. The pericardium and epicardium (see p. 83) originate as
outgrowths from the archenteron. They may therefore be re-
garded as enterocoelic spaces. The pericardium becomes com-
pletely closed off and separated from the alimentary canal. The
epicardium may form paired tubes of great length, and may
remain permanently connected with the branchial sac.

5. The cavities of the renal vesicles and of the gonads and
ducts are spaces formed in the mesoblast. They have been
variously interpreted : —

(«) As of the same nature as the blood spaces (blastocoelic), or
{h) As formed by a splitting of the mesoblast (coelomic).

6. The cavity of the neural gland and of its duct opening at
the dorsal tubercle is derived from the primitive dorsal neural tube
of the embryo, and so may be regarded as a part of the lumen of
the cerebro-spinal nervous system.

Tentacles, etc. — The branchial aperture leads through the

STRUCTURE TENTACLES, ETC.

45

branchial siphon into the branchial sac. At the base of the
siphon, just about the line of junction of the ectoderm of the
stoniodaeuni with the endoderm of the mesenteron, is placed a
circle of simple hair-like tentacles (Fig. 18, tn) which stand out at
right angles to the wall, and more or less completely meet in the
centre to form a delicate, sensory grid or sieve through which all
the water entering the body has to pass. These tentacles not only
act mechanically, but are also sensitive although only scattered
sensory cells, and no specially differentiated sense-organs are
found upon them. Behind the tentacles lies the plain, or papil-

^9' ren

At.
Dorsal.

l-'.y- "m.

Fig. 19. — Semi-diagrammatic transverse section of Ascidia, passing through the atrial
aperture, seen from anterior surface, left side uppermost. At, Atrial aperture ; at.l,
atrial lobe ; Br.s, branchial sac ; d, cloaca ; con, connective ; d.bl.s, dorsal blood-
sinus : d.l, dorsal lamina ; end, endostyle ; g.d, genital ducts ; i, /', intestine ; l.v,
interstigmatic vessel ; m, mantle ; vi.b, muscle-bundles ; ov, ovary ; ^J.fir, peri-
branchial cavity ; r, rectum ; ren, renal vesicles ; sg, stigmata ; sph, atrial sphincter ;
t, test ; tr, transverse vessel ; ty, typhlosole ; r.bl.s, ventral blood-sinus.

lated, prebranchial zone (Fig. 21, jj.&r.^;), bounded beliind by a
pair of parallel and closely placed ciliated ridges with a groove
between — the peripharyngeal bands — which encircle the anterior
end of the branchial sac.

The branchial sac is very large — -much the largest organ of
the body — and extends almost to the posterior end of the body,
while the rest of the alimentary canal lies upon its left side.
The food particles, consisting of microscopic plants and animals,
are carried in through the branchial aperture by the current of
water, but most of them do not pass out through the gill-slits to
the atrium, being entangled in the viscid mucus which passes
by ciliary action along the groove between the peripharyngeal
bands.

46

ASCIDIANS

Endostyle. — ^The mucus just referred to is Y)roduced iu the
long canal-shaped gland called the endostyle or hypohranchial
groove, which runs along the entire ventral edge of the
branchial sac (Fig. 18, end). The sides, and especially the floor
of the endostyle, are richly ciliated, while there are four (or six)
strongly -marked, peculiarly -shaped glandular tracts, two (or
three) on each side (Fig. 20, gl) running along its length, and
separated by areas of closely-packed fusiform cells with short
cilia, amongst which are found some bipolar sensory cells.

This organ corresponds to the hypopharyngeal groove of

Fig. 20. — Transverse section of the endoRiyle oi Ascidia mentula, x 350. W. 5, Blood -
sinus ; end. I, lips of the endostyle ; gl, glandular tracts ; i.l. internal longitudinal
bar ; l.v, interstigniatic vessels ; m, mantle ; p.br, peribranchial cavity ; sg, stigmata ;
r.hl.s, ventral blood -sinus.

Amphioxus and the median part of the thyroid gland of Verte-
brata. It is interesting to notice that the (at least) four longitudinal
tracts of gland-cells are of remarkable constancy, being found
not only in all groups of Tunicata, including even the pelagic,
tailed Appendicularians, but also in Amphioxus and in the
young thyroid gland of the Ammocoete. When, in Ascidians, a
third marginal glandular tract is added it has a different appear-
ance from the two characteristic tracts. The mucus is carried
forward by the action of the large floor-cilia of the endostyle
(Fig. 20) to the groove between the peripharyngeal bands, and
after encircling the anterior end of the branchial sac and collect-
ing the food particles, it passes backwards along the dorsal edge

STRUCTURE — BRANCHIAL SAC

47

of the branchial sac to the oesophagus, guided by a membranous
fold, the dorsal lamina (Fig. 21, d.l), which is more or less ridged
or corrugated, and may be armed with marginal tags or even
replaced by larger processes (the " languets ") in some species
of Ascidians. In the living animal the lamina has its free
edge curved to the right hand side in such a manner as to
constitute a fairly perfect tube along which the train of food
passes.

Branchial Sac. — Thus we have the dorsal lamina (or the
languets) along the dorsal edge, the endostyle along the ventral
edge, and the peripharyngeal bands around the anterior end. The
wall of the branchial sac itself is penetrated by a large number
of channels through

which blood flows. Some
of these run in one direc-
tion and some in an-
other, so as to form
complicated networks,
which difter greatly in
their arrangement in
different Ascidians. Be-
tween these blood-
channels there are clefts
(" stigmata "), the
secondary or subdivided
gill-slits, by means of
which the current of
water passes from the
branchial sac to the
large external peribran-
chial or atrial cavity.
All the stigmata (of
which there may be several hundred thousand) in the wall of
the branchial sac are bounded by cubical or columnar epithelial
cells, which are ciliated. These cilia, so long as the animal is
alive, are in constant motion, so as to drive the water onwards,
and it is this constant ciliary action in the walls of the ]:)ranchial
sac that gives rise to the all-important current of water stream-
ing through the body. In addition to tbe stigmata there are
generally one or two much larger elongated slits (Garstang's-

-~'br.s,.

Fig. 21. — Antero-dorsal part of pharynx in Ascidia
vientula, x 15. br.s, Part of branchial sac ; d.l,
dorsal lamina ; d.t, dorsal tubercle ; p.hr.z, pre-
branchial zone ; /*./), peripharyngeal bands ; sjih,.
sphincter of branchial aperture ; in, tentacle.

48

ASCIDIANS

phuryngo-eloacal slits) i)lciced close to the dorsal lamina and
leading direct to the cloaca.

Fig. 22 shows a small part of the wall of the branchial sac, in
which it may be seen that the bars containing the blood-
channels are arranged in three regular series: — (1) The "trans-
verse vessels " which run horizontally round the wall and open
at their dorsal and ventral ends into large median longitudinally
running tul)es, the dorsal blood-sinus (or " dorsal aorta ") behind
the dorsal lamina, and the ventral blood-sinus (or " brancliial
aorta ") beneath the endostyle ; (2) the fine longitudinal or

" interstigmatic vessels " which
run vertically between adjacent
transverse vessels and open into
them, and which therefore bound
the stigmata ; and (3) the " in-
ternal longitudinal bars " which
run vertically, in a plane internal
to that of the transverse and
fine longitudinal vessels. These
A bars (Fig. 22, i.l) communicate
with the transverse vessels by
short side branches where they
cross, and at these points are

Fig. 22.-Amesh of the hrancliial sac of p^.QlQj^cred into the Cavity of
Ascldia, soeu A, from inside ; B, in ^ ^ .

horizontal section. c.d. Connecting the SaC m the form of hollow

duct ; A.m, horizontal membrane ; 1./, •i^.^g^ l^ g^j^^g AscidianS

internal longitudinal bars ; l.v, inter- ^ ^

stigmatic vessels ; }), p', papillae ; sg, (e.g. CoreUci and most of the

stigmata ; tr, transverse vessels. Molgulidac) the intCrstigmatic

vessels are curved so that the stigmata form more or less com-
plete spirals (see Figs. 35 and 41). In some species of Ascldia,
and other Ascidians, the interstigmatic vessels are inserted into
the transverse vessel in an undulating course in place of the
straii^ht line seen in Fio-. 22, P>, l.v, the result being that the
stigmatic part of the wall of the branchial sac seems to be folded
or thrown into microscopic crests and troughs. This is known as
'' minute plication." In some cases, again (Cynthiidae), the
whole wall of the sac is pushed inwards at intervals to form
large folds visible to the eye (see Fig. 36, A and ]>). The
intersections of the internal longitudinal bars with the trans-
verse vessels divide up the inner surface of the branchial

STRUCTURI'. IIIOART AND BLOOD 49

sac wall into rectangular areas called " meslies." One such mesh,
containing eight stigmata in a row, is seen in Fig. 22, A. The
internal longitudinal bars hear papillae at the angles of the
meshes, and occasionally in intermediate positions. Tliere are
frequently liorizontal membranes (Fig. 22, B, li.rn) attaclied to
tlie transverse vessels between the papillae. There are many
" connectives " running from the outer wall of the branchial sac
to the mantle outside, and allowing the l)lood in the transverse
vessels to communicate with that in the sinuses of the mantle
(see Fig. 19, con).

Heart and Circulation.^ — It is one of the notable features of
the Tunicata that the circulation is not constant in direction,
but is periodically reversed.

The blood of Ascidians is in the main transparent, but usually
contains certain pigmented corpuscles in addition to many
ordinary leucocytes or colourless amoeboid cells. Tlie pigment in
the coloured cells may lie red, yellow, brown, or in some cases
blue or opaque white. The blood may reach the branchial sac
either from the dorsal or from the ventral median sinus according
to the direction in whicli the lieart is beating at the moment (see
below) ; and it is a most interesting and beautiful sight to see
tlie circulation of the variously coloured corpuscles through the
transparent vessels, and the lashing of tlie cilia along the edges
of the neiu'hbouring stigmata in a small Ascidian under the
microscope.

In Ascidia (Fig. 23) the heart is an elongated fusiform tube
placed on the ventral and posterior edge of the stomach, project-
ing into a space (the pericardium) which is a part of the original
coelom, the remainder of which is represented in the adult by
the reproductive and renal cavities. The wall of the heart is
continuous along one edge with that of the pericardium, and the
heart is to be regarded as a tubular invagination of the pericardial
wall, shutting in a portion of the surrounding space (the blastocoel
of the embryo), and having open ends which communicate with
the large blood sinuses leading to the branchial sac, to the viscera,
and to the body-wall and test. The cavity of the heart is not
divided and there are no valves. Its wall is formed of a single
layer of epithelio-muscular cells, the inner, muscular, ends of
which are cross-striated fibres running round the heart — the only
striated muscular tissue found in the body. Waves of contrac-
VOL. VII E

50 ASCIDIANS

tioii pass along the heart from end to end, first for a certain
number of beats in one direction, and then, after an interval, in
the other. If a small or young Ascidia be placed alive, left
side uppermost, in a watch-glass or small trough of sea-water,
and examined with a low power of the microscope, the heart will
be readily seen near the posterior end of the transparent body.
It will be noticed that the " beating " looks like successive waves
of blood pressed through the tubular heart from one end to the
other by its contractions. After watching the waves passing, let
us say, from the right liand end of tlie heart to the left for about
a minute and a half (perhaps 60 or 80 to 100 beats), it will
be seen that they gradually become slower and then stop
altogether. But after seven or eight seconds a faint wave of
contraction will start from the lefi end of the heart and pass
over it to the right ; and this will be followed by larger ones
for a minute and a half, and then again a pause will occur and
the direction change. It has been suggested that the cause of
this remarkable reversal may possibly be that the heart being on
tlie ventral vessel, which is wider than the corresponding dorsal
trunk, pumps the blood into either the lacunae of the branchial
sac or those of the viscera in greater volume than can possibly
get out through the smaller branchio- visceral vessel in the
same time, the result being that the lacunae in question soon
become engorged, and by back pressure cause the stoppage, and
then reversal of the beat. The absence of any valves in the
heart to regulate the direction of flow obviously facilitates this
alternation of the current.

The larger channels through which the blood flows may be
lined with a delicate endothelium, but the smaller passages are
merely spaces in the connective tissue. The heart, although
anatomically a " ventral vessel," runs in tlie main dorso-ventrally.
The blood-channel leaving the ventral end of the heart is the
" branchio-cardiac vessel " (Fig. 23, h.c). This gives off a branch
which, along with a corresponding branch from the " cardio-visceral "
vessel (c.v) at the other end of the heart, goes to the test, and then
runs along the ventral edge of the branchial sac as the branchial
aorta (b.a), external to the endostyle, communicating laterally
with the ventral ends of all the transverse vessels of the branchial
sac. The cardio- visceral vessel (Fig. 23, c.r) after giving off its
branch to the test breaks up into a number of sinuses which

STRUCTURE — CIRCULATION

51

ramify over the alimentary canal and the otlier viscera. These
visceral lacunae iinally communicate with a third great sinus, the
" hranchio-visceral " vessel (h.v) which runs forward along the
dorsal edge of the branchial sac as the dorsal aorta (d.((),
externally to the dorsal lamina, and joins the dorsal ends of all
the transverse vessels of the branchial sac. Besides these three
chief systems — the branchio-cardiac, the cardio-visceral, and the
branchio-visceral — (see Fig. 23), there are numerous lacunae in
all parts of the body by means of which anastomoses are established
between the different currents of blood.

When the heart contracts ventro-dorsally the course of tlie

Fig. 23. — Diagrammatic dissection of Ascidia, from left side, to show course of circula-
tion. Front part of branchial sac opened, back part covered by viscera, h.a,
Branchial (ventral) aorta ; b.c, branchio-cardiac vessel ; b.i\ branchio-visceral vessel ;
c.v, cardio-visceral vessel ; d.a, dorsal aorta ; ht, heart. A, anterior ; P, posterior;
D, dorsal ; T', ventral. ■

circulation is as follows : — the blood which is flowing through
the vessels of the branchial sac is collected in an oxygenated
condition in the branchio-cardiac vessel, and after receiving a
stream of blood from the test enters the ventral end of the heart.
It is then propelled from the dorsal end into the cardio-visceral
vessels, and so reaches the test and the digestive and other
viscera ; then, after circulating in the visceral lacunae it passes
into the branchio-visceral vessel in an impure condition, and is
distril^uted to the branchial vessels to be purified again. When
the heart, on the other hand, contracts dorso-ventrally, this course
of the circulation is reversed, the " veins " and " arteries " exchange
functions, and what a minute before was a " systemic," is now a
" respiratory " lieart. This is a phenomenon without parallel in
tlie animal kingdom.

52

ASCIDIANS

All the lilooil-spHces and lacunae are probably derived, like
the cavity of the heart, from the blastocoel of the einljryo, aud
are not, like the cavity of the pericardium, a part of the coelom
(of endodermal origin).

Neural Gland and Dorsal Tubercle. — In the dorsal median
line near the anterior end of tlie l)ody, aud imbedded in the
mantle on the ventral ^ surface of the nerve-ganglion, there lies a
small glandular mass — the neural gland — which, as Julin first

Peribranchial
cavity
e. p.br

Fig. 24. — Antero-dorsal part of Ascidia showing the relations of the layers of the body,
and of the nervous sj'stem. A, in sagittal section ; B, in transverse section, d.bl.s.
Dorsal blood-sinus: d.l, dorsal lamina; d.n, dorsal nerve; d.t, dorsal tubeicle ;
ect, ectoderm ; en, endoderm ; e.p.br, epithelium of peribranchial cavity ; gl.d, duct
of subueural gland ; l.v points to the ciliated epithelium covering a longitudinal
vessel of branchial sac; m, mantle; n, nerve; n.g, ganglion ; 11. gl, neural gland ;
p.hr, peribranchial cavity ; pp.h, ijeripharyugeal bands ; sph, branchial sphincter ;
t, t' , test ; tn, tentacle.

showed, there is some reason to regard as the homologue of the
hypophysis cerebri of the Vertebrate brain. Metcalf has recently
shown that the neural gland may be a double structure — partly
cereljral and partly stomodaeal — as in Vertebrates.

The function of this gland is still somewhat mysterious. It
may merely form the viscid secretion which is carried along the
peripharyngeal bands and down the dorsal lamina. On the other
hand, it has been suggested that the function of the organ may
possibly be renal, for the remo^■al of nitrogenous waste matters in
the neighbourhood of the nervous system. Finally, it may be a
lymph gland.

^ Except in Cynthiidae and Botryllidae where it is dorsal.

II STRUCTURE NERVOUS SYSTEM 53

The neural gland, which was first noticed by Hancock, may
1)0 continued backwards along with the dorsal nerve, and it
coninmuicates anteriorly by means of a narrow duct witli the
front of the l)ranchial sac (pharynx). The opening of the duct
is enlarged to form a funnel-shaped cavity (Fig. 24, A), which
may l»e folded iqwm itself, convoluted, or even broken up into a
number of smaller openings (see Fig. 43, p. 79), so as to form a
complicated projection called the dorsal tubercle, situated in the
dorsal part of the prel)ranchial zone. The dorsal tuliercle in
Ascidia mentula is somewhat horse-shoe shaped (Fig. 11,d.f)] it
varies in most Ascidians (see Fig, 43) according to the genus and
species, and in some cases in the individual also. Sensory cells
are found in the epithelium, and so it is liighly probable that
besides being the opening of the duct from the neural gland, this
convoluted ciliated ridge may be a sense-organ for testing the
quality of the water entering the l)ranchial sac.

Nervous System and Sense-Organs. — Tlie single elongated
ganglion (Fig. 24, n.y), in the median dorsal line of the mantle,
between the branchial and atrial siphons, is the only nerve-
centre in Ascidia and most other Tunicata. It is the degenerate
remains of the dorsal wall of the tubular cerebro-spinal ner\ous
system of tlie trunk -region of the tailed larval Ascidian — the
ventral wall opposite having given rise to the subneural gland.
The more posterior or spinal ])art of tlie larva has almost entirely
disappeared in most adult Tunicata. It persists, however, in the
Appendiculariidae, and traces of it have been found in the dorsal
nerve running backwards towards the oesophagus in some Ascidians
(e.g. Clavelina). It may be ganglionated in Molgulidae.

The ganglion has small rounded nerve-cells on its surface,
and interlacing nerve -fibres inside. It gives off distributory
nerves at both ends (Fig. 24, A), which run through the
mantle to the neighbourhood of the apertures, where they divide
up to supply the lobes and the sphincter muscles. The
only sense-organs are the pigment spots (" ocelli," formed of
modified ectoderm cells imbedded in red and yellow pigment),
between the branchial and atrial loljes, tlie tentacles at the base
of the l)ranchial siphon, and probably the dorsal tubercle and
the languets or dorsal lamina, in all of which, as well as in
the endostyle and peri])haryngeal bands and in pa])illae on the
ectoderm and in the branchial sac, sensory cells have been found.

54 ASCIDIANS

Tliese, considered as sense-organs, are all in a lowly-developed
condition. The lar^'al Ascidians, on the other hand, have well-
developed intra-cerebral optic and otic sense-organs (see Fig. 26,
p. 60), and in some of the pelagic Tunicata, otocysts and pigment-
spots are found in connexion with the ganglion.

Alimentary Canal. — The month and pharynx (branchial sac)
have already been described. The remainder of the alimentary
canal is a bent tube, which in A. mentula and most other
Ascidians lies imbedded in the mantle on the left side of the
body, and projects into the peribranchial cavity (see Figs. 18 and
19). The oesophagus leaves the branchial sac in the dorsal
middle line, near the posterior end of the dorsal lamina. It is a
short cur^'ed tube which leads ventrally to the large fusiform
thick-walled stomach, ridged internally. The intestine emerges
from the ventral end of the stomach and soon turns anteriorly,
then dorsally, and then posteriorly, so as to form a cur\'e, the
intestinal loop, in which the o^'ary lies, open posteriorly. The
intestine now curves anteriorly again, and from this point runs
nearly straight forward as the rectum, thus completing a second
curve, the rectal loop, in which the renal vesicles lie, open
anteriorly. The wall of the intestine is thickened internally to
form the typhlosole (Fig. 18, fy), a pad which runs along its
entire length, so as to reduce the lumen of the tube to a crescentic
slit. The anus opens into the dorsal or cloacal part of the
peribranchial cavity near the atrial aperture. The walls of the
stomach are glandular, and most of the endoderm cells lining the
tube are ciliated. A system of delicate, microscopic, branched
tubules with dilated ends (the " refringent organ '''), which
ramifies over the outer wall of the intestine, and communicates
with the cavity of the stomach at the pyloric end by means of a
duct is probably a digestive gland. There is in Ascidia no
separate large gland to which the name " li^-er " can be applied,
as in some other Tunicata.

Renal Organ. — A mass of large clear-walled vesicles which
occupies the rectal loop (Figs. 18 and 19, rcn), and may extend
over the adjacent walls of the intestine, is a renal organ without
a duct. Each vesicle is the modified remains of a part of the
primitive coelom or body-cavity, and is formed of cells which
eliminate nitrogenous waste matters from the blood circulating in
the neighbouring blood-lacunae, and deposit them in the cavity of

II STRUCTURE RENAL AND REPRODUCTIVE ORGANS 55

the vesicle, where they form one or more concentrically laminated
concretions of a yellowish or brownish colour, sometimes coated
with a chalky deposit. Tliese concretions contain uric acid, and
in a large Ascidian are very numerous. The nitrogenous waste
products are thus deposited and stored up in tlie renal vesicles in
place of being excreted from the body. In other Ascidians the
renal organs may differ from the above in position and structure :
but in no case have they any excretory duct, unless the neural
"land is to be regarded as one of the renal organs — which has not
yet been proved.

Reproductive Organs. — Ascidia mentula is hermaphrodite,
and the reproductive organs lie with the alimentary canal, on the
left side of the body (Fig. 19, or). The ovary is a ramified gland
which occupies the greater part of the intestinal loop. It con-
tains a cavity which, along with the cavities of the testis, is derived
from an embryonic coelom ; the ova are formed from its walls,
and fall when mature into the cavity. The oviduct is continuous
with the cavity of the ovary, and leads forward alongside the
rectum, finally opening near the anus into the peribranchial
cavity (Fig. 18, gxl). The testis is composed of a great number
of delicate, branched tubules, which ramify over the ovary and
the adjacent parts of the intestinal wall. These tubules terminate
in ovate swellings. Near the commencement of the rectum the
larger tubules unite to form the vas deferens, a tube of consider-
able size, which runs forward alongside the rectum, and, like the
oviduct, terminates by opening into the peribranchial cavity close
to the anus. The lumen of the tubules of the testis, like the
cavity of the ovary, is a part of the embryonic mesoblastic space,
and the spermatozoa are formed from the cells lining the wall. In
some Ascidians (certain Molgulidae and Cynthiidae), reproductive
organs are present on both sides of the body, and in others,
as in Polycarpa, there are many complete sets of both male and
female systems attached to the inner surface of the mantle on
both sides of the body and projecting into the peribranchial
cavity.

Embryology and Life-History of a Typical Ascidian.

The eggs of Tunicata are for the most part of small size,
nearly colourless and transparent, and with little or no food-yolk.

5 6 ASCIDIANS

In some, however (such as some of the Cynthiidae, and some
Compound Aseidians), the eggs are larger, more opaque, and have
a fair amount of food-yolk. Ova of this type are not expelled
from bhe body of the parent as ova, but are fertilised, and remain
in the atrial cavity or in a special diverticulum thereof — the
incubatory pouch — until they are far advanced in development ;
and usually leave the body as tailed larvae. In many species, the
ova and spermatozoa matm^e at different times in the life-history,
and so self-fertilisation is prevented. Some species (such as
many Botryllidae and Distomatidae) are protogynous, the ova
being produced and shed before the testes have matured, while
other species (Coelocormus huxleyi) are protancbous, being male
while young and female later. But there is no doubt that in
other cases (e.g. Ascidia mentida) self-fertilisation is not only
possible, but does take place. After maturation certain of the
follicle-cells which invest the ovum in the ovary migrate into the
egg and proliferate so as to form a layer in the superficial part
of the egg, where they appear as the so-called " testa-cells " or
" kalymmocytes " (Fig. 25, A, t.e). The remaining follicle-cells
may form two or more layers, usually one of large cubical cells,
which may become greatly vacuolated, next to the ovum, and an
external flattened layer which is cast off when the egg escapes
from the ovary.

Segmentation is complete and results in the formation of a
spherical blastula with a small segmentation-cavity (Fig. 25, C).
The blastula grows larger and begins to differentiate.^ There
are slightly smaller cells which divide more rapidly at one end of
this embryo, the future ectoderm, and slightly larger and more
granular cells at the other, which become chiefly endoderm
(hypol)last). Invagination of the larger cells then takes place
(Fig. 25, D), resulting in the formation of a gastrula witli an
archenteron. The hypol)last cells lining the archenteron become
columnar (Jnj). The curving and more rapid growth at the
anterior end of the embryo narrow the primitively wide open
blastopore, and carry it to the posterior end of the future dorsal
surface (Fig. 25, E). The orientation of the Ijody is now clear.
The embryo is elongated antero-posteriorly, the dorsal surface is

^ The early stages of Ciona, of which Castle has given a very complete account
(Bull. Mus. Comp. Zool. xxvii. No. 7, 1896), differ in some jioints from those of
Ascidia described here.

EMBRYONIC DEVELOPMENT

57

flattened, and the blastopore indicates its posterior end. Around
the blastopore the large ectoderm cells form a medullai'}' })late,
along which a groove (the medullary groove), runs forwards,
bounded at the sides by medullary folds which meet behind the
blastopore. Underneath the posterior part of the medullary groove

Fig. 25. — Embryology of Ascidian. A, mature ovum : foil, follicle-cell : m, membrane ;
?!, nucleus; ^;, protoplasm ; /.c. test-cell ; B, mature spermatozoon ; C, segmentation-
stage in section to show blastocoel ; D, early gastrula-stage ; E, later gastrula- stage ;
F, later embryo showing rudiments of notochord and neural tube ; G, transverse
section of body of embryo showing mesoblast and formation of neural canal ; H, late
embryo showing body and tail, notochord, neural canal, and mesenteron ; I, young
larva ready to be hatched ; K, transverse section of tail of larva, ar, Archenterou ;
at, atrial invagination; au, otocyst ; h.c, blastocoel ; h.p, blastopore ; ch, notochord ;
ep, epiblast ; /, tail-fin ; hy, hypoblast ; m.h, mesoblast ; mes, mesenteron ; muse,
muscle-cell; n.c, neural canal; ne.c, neurenteric canal; /i.r, neural vesicle; oc,
ocellus. (Modified from Kowalevsky and others.)

certain of the hypoblast cells from the dorsal wall of the archen-
teron, in the median line, form a band extending forwards (Fig.
25, E, ch). This band separates off from the hypoblast, which
closes in beneath it, and thus gives rise to the notochord (Fig.
25, F). The more lateral and posterior cells become mesoblast,
and separate off as lateral plates, which show no trace of metameric
segmentation (Fig. 25, G). The remainder of the archenteron

58 ASCIDIANS

becomes the branchial sac, and by further growth buds off the rest
of the alimentary canal.

The medullary groove now becomes converted into the closed
neural canal by the growing up and arching inwards (Fig. 25, G,
n.c) of the medullary folds, which unite with one another from
behind forwards in such a way that the blastopore now opens
from the enteron into the lioor of the neural canal, forminti; the
neurenteric passage (Fig. 25, F, n.e.c). For a time the anterior
end of the neural canal remains open as a neuropore. By this
time the posterior end is elongating to form a tail, and the
embryo is acquiring the tadpole-shape (Fig. 25, H) characteristic
of the free larva. The tail grows rapidly, curves round the body,
and also undergoes torsion, so that its dorsal surface comes to lie
on the left side. It contains ectoderm cells on its surface, noto-
chordal cells (in single file) up the centre (see Fig. 25, H, ch), a
neural canal dorsally, and a row of endoderm cells representing
the enteron ventrally to the notochord. Later on the mesoblast
also is prolonged into the tail, where it forms a band of striated
muscle-cells at each side of the notochord. When the ectoderm
cells begin to secrete the cuticular test this forms two delicate
transparent longitudinal (dorsal and ventral) fins in the tail (Fig.
25, K,/), and especially at its extremity where radial thickenings
form striae resembling fin-rays. The ectoderm on the anterior end
of the body grows out into three adliering papillae (Fig. 26, A).

The neural canal now differentiates into a tubular dorsal
nervous system. The anterior end dilates to form the thin-
walled cerebral vesicle (see Figs. 25, I, and 26, A), containing later
the intra-cerebral, dorsal, pigmented eye (oc), and the ventral
otolith {au) of the larva. The next part of the canal thickens
to form the trunk -ganglion, and behind that is the more slender
" spinal cord," which runs to the extremity of the tail. A ciliated
diverticulum of the anterior end of the enteric cavity (future
pharynx) which enters into close relations with the front of the
cerebral vesicle,^ and later opens into the ectodermic invagination
which forms the mouth at that spot, is evidently the rudiment of
tiie neural duct or hypophysial canal. The future branchial sac
(pharynx), with a ventral median thickening which will be the
endostyle, is by this time clearly distinguishable by its large size

^ Possibly the diverticulum may be wholly derived from the neural tube (see
Willey, Quart. J. Micr. Sci. 1893).

LARVAL DFAELOPMENT 59

from the much narrower posterior part of the enteron, whicli
grows out to become the oesophagus, stomacli, and intestine. Tlie
notochord does not extend forward into the pharyngeal region,
but is confined to the posterior or caudal part of the embryo. It
now shows lenticular pieces of a gelatinous intercellular substance
secreted by the cells and lying between them (Fig. 25, I). "J'he
mouth forms as a stoinodaeum, or ectodermal invagination,
antero-dorsally in tlie region where the neuropore has closed, and
about the same time two lateral ectodermal involutions form
(Fig. 26, A, at), which become the atrial or peribranchial
pouches, at first distinct, afterwards united in the mid-dorsal line
to form the adult cloaca and atrial aperture. Ingrowths from
the atrial pouches and outgrowths from the wall of the pharynx
coalesce to form the proto-stigmata (primary gill-slits) by which
the cavity of the branchial sac is first placed in communication
with the exterior through the atrial apertures. Opinions differ
as to whether only one or a few pairs of true gill-clefts are repre-
sented in the young Ascidian ; and the actual details of their
formation and subdivision, to form the stigmata of the adult,
differ considerably in different forms. In Clavelina the stigmata
are formed as independent perforations of the pharyngeal wall :
in Ascidia two pairs of protostigmata increase to six pairs, which
are subdivided into stigmata ; Botrylhis and other forms are
intermediate in some respects. No doubt the subdivision of proto-
stigmata is primitive, but has been lost from the ontogeny in
some cases. To what precise extent the walls of the atrial or
peribranchial cavities are formed of ectoderm, or of endoderm, is
still doubtful.

The embryo is hatched about two or three days after fertilisa-
tion, as a larva or Ascidian tadpole (Fig. 26, A) which leads a
free-swimming existence for a short time, during which it develops
its nervous system and cerebral sense-organs, and the powerful
mesoblastic muscle-bands lying at the sides of the notochord
(now a cylindrical rod of gelatinous nature surrounded by the
remains of the original cells) in the tail which form the
locomotory apparatus. Fig. 26, A, shows this stage, the highest
in its chordate organisation, when the larva swims actively
through the sea by vibrating its long tail with the dorsal and
ventral fins.

In addition to the structures already mentioned, the mesoderm

6o

ASCIDIANS

has formed the beginning of the muscular body-wall, the con-
nective tissue around the organs, and the blood; the endostyle has
developed as a thick-walled groove along the ventral edge of the
pharynx, which has become the branchial sac ; and the peri-
cardial sac and its invagination the heart have formed in the
mesoblast between the endostyle and stomach. The " epicardiac
tubes " grow out from the posterior end of the endostyle to join the
pericardium. They play an important part in the formation of

Fig. 26. — Metamorphosis of an Ascidiaii. A, free - swimming tailed larva ; B, the
metamorphosis — larva attached ; C, tail and nervous system of larva degenerating ;
D, further degeneration and metamorphosis of larva into E, the young fixed Ascidian.
at, Atrial invagination ; ch, notochord ; hy, hypoblast cells ; i, intestine ; m, mouth ;
mes, mesenteron ; n.c, neural canal ; n.r, neural vesicle with sense-organs. (Modified
from Kowalevsky and others.)

buds in the colonial Tunicata. The heart acquires a connexion
with blastocoelic blood -spaces at its two ends. The heart
and pericardium show the same relations in Tunicata as in
Enteropneusta, but it is very doubtful whether these organs are
genetically related to the Vertebrate heart.

The unpaired optic organ in the cerebral vesicle when
fully formed has a retina, pigment layer, lens and cornea ; while
the ventral median organ is a large, spherical, partially-
pigmented otolith attached by delicate hair-like processes to
the summit of a hollow " crista acustica " (Fig. 26, A). After
a few hours, or at most a dav or so, the larva attaches itself

METAMORI'IIOSIS

by one or more of the three anterior ectodermal glandular
papillae (one dorsal ami two lateral) to some foreign body, and
commences the retrogressive metamorphosis which leads to the
adult state. The adhering papillae, having performed their
function, begin to atrophy, and their place is taken by the rapidly
increasing test. The tail which at first vibrates rapidly is parth-
withdrawn from the test and absorbed, and partly cast off in
shreds (Fig. 26, B, C, D). The notochord, nerve-tube, muscles,
etc., are withcbawn into the body, where they break down and
are absorl)ed Ijy phagocytes. The posterior part of the nerve
cord and its anterior end with the large sense-organs disappear,
and the middle part or trunk -ganglion is reduced to form
the relatively small ganglion of the adult, underneath which
the hypophysial tube gives rise to the neural gland. While the
locomotory, nervous and sensory organs are thus disappearing, or
being reduced, the alimentary canal and reproductive viscera are
growing largely. The branchial sac enlarges, its walls become
penetrated by blood-channels, and grow out to form bars and
papillae, and the number of openings greatly increases by the
primary gill-slits being broken up into the transverse rows of
stigmata. The stomach and intestine, which developed as an out-
growth from the back of the branchial sac at the right side,
become longer and curve, so that the end of the intestine acquires
an opening into at first the left hand side, and eventually the
cloacal or median part of the atrial cavity. The adhering
papillae have now disappeared, and are replaced functionally by a
growth of the test over neighl)ouring objects ; and at the same
time the region of the body between the point of fixation and the
mouth (branchial aperture) increases rapidl}' in extent, so as to
cause the body of the Ascidian to rotate through about 180°, and
thus the branchial siphon is carried to the opposite end from the
area of attachment (see Fig. 26, B, C, D, E). Finally the gonads
and their ducts form in the mesoderm between the stomach and
intestine. We thus reach the sedentary degenerate fixed adult
Ascidian with little or no trace of the Chordate characteristics so
marked in the earlier larval stage (see E and A, Fig. 26). The
free -.swimming tailed larva shows the Ascidian at the highest
level of its organisation, and is the stage that indicates the
genetic relationship of the Tunicata with the Vertebrata.

In some Ascidiaus with more food-yolk in the egg, or in whicli

62 • ASCIDIANS

the (levelopinent takes place within the hody of tlie parent, the life-
history as given above is more or less modified and abbreviated,
and in some few forms the tailed larval stage is missing. Some
exceptional cases of development will be noted below under the
groups to wliich tliey belong.

Tlie remarkable life-liistory of the typical Ascidian, of which
the outlines are given above, is of importance from two points of
view : — •

1. It is an excellent example of degeneration. Tlie free-
swimming larva is a more liighly developed animal tlian tlie
adult Ascidian. The larva is, as we have seen, comparable with
a larval fish or a young tadpole, and is thus a Chordate animal
showing evident relationship to the Verteljrata ; while the adult
is in its structure non-Chordate, and is on a level with some of the
w^orms, or with the lower Mollusca, in its organisation, althouiih of
an entirely different type.

2. It shows us the true position of the Ascidians (Tunicata)
in the animal series. If we knew only the adult forms we
might regartl them as being an aberrant group of Worms, or
po.ssil)ly as occupying a position between worms and the lower
Mollusca, or we might place them as an independent group ; but
we should certainly have to class them as Invertebrate animals.
But when we know the whole life-history, and consider it in the
liglit of " recapitulation " and " evolutionary " views we recognise
that the Ascidians are evidently related to the Vertebrata, and
were at one time free-swimming Chordata occupying a position
somewhere below the lowest Fishes.

CHAPTER III

TUNIC ATA {COSTINUED)

CLASSIFICATION : LARVACEA APPENDICULAR! ANS STRUCTURE, ETC.

ASCIDIACEA SIMPLE ASCIDIANS SPECIFIC CHARACTERS

COMPOUND ASCIDIANS GEMMATION MEROSOMATA llOLO-

SOMATA PYROSOMATIDAE THALIACEA DOLIOLIDAE

SALPIDAE GENERAL CONCLUSIONS PHYLOGENY.

We now turn to the systematic classification of the group ; and
further details of structure or function, points of interest in the
life-history such as budding and the formation of colonies, the
habits and occurrence, and other peculiarities such as phos-
phorescence, will all be noted under the orders, sub-orders, families
and genera in which they occur.

CLASS TUNICATA.

The Tunicata or Urochordata are hermaphrodite marine
Chordate animals, which show in their development the essential
Vertebrate characters, but in wdiich the notochord is restricted to the
posterior part of the liody, and is in most cases present only during
the free-swimming larval stages. The adult animals are usually
sessile and degenerate, and may be either solitary or colonial, fixed or
free. The nervous system is, in the larva, of the elongated, tubular,
dorsal. Vertebrate type, but in most cases it degenerates in the
atlult to form a small ganglion placed above the pharynx. The
l)ody is completely covered with a thick cuticular test (" tunic ")
which contains a substance similar to cellulose. The alimentary
canal has a greatly enlarged respiratory pharynx or bTanchial sac,
which is perforated by two or many more or less modified gill-
slits opening into a peribranchial or atrial cavity, which communi-
cates w^ith the exterior by a single dorsal exhalent aperture (rarely

63

64

ASCIDIANS APPENDICULARIANS

two ventral apertures). The ventral heart is simple and tubular,
and periodically reverses the direction of the blood-current.

" Aacidiid.s

MoJgulids

CyntKiids

J CoiupoimoL
l^Ascidians

Fig. 27. — Sketch of the cbief kinds of Tunicata found in the sea.

This Class is divided into three Orders : — The Appendicularians,
the Ascidians, and the Salpians (see Fig. 27).

Order I. Larvacea (Appendicularians).

Free-swininiing pelagic forms, in which the posterior part of
the body takes the form of a large locomotory appendage, the

Ill CHARACTERS AND HABITS 65

" tail," in which there is a skeletal axis, the urochord. A
relatively large cuticular test, the " house," may be formed with
great rapidity (in an hour or so) as a secretion from a part of the
ectoderm ; it is, however, merely a temporary structure which is
soon cast off and replaced by another. The Ijranchial sac is simply
an enlarged pharynx with two ventral ciliated openings (stigmata)
leading to the exterior. These may be regarded as the repre-
sentati^■es of the primary gill-slits (undivided) of the Ascidian.
There are thus a single pair. There is no separate peribranchial,
atrial, or cloacal cavity. The nervous system consists of a large
dorsally placed ganglion and a long nerve-cord, which stretches
l)ackwards over the aliyieutary canal to reach the tail, along
which it runs on the left side (morphological dorsal edge) of
the urochord. The anus opens ventrally on the surface of the
l)ody, usually in front of the stigmata. No reproduction by
gemmation or metamorphosis is known in the life-history.

Structure and Mode of Life. — Tliis is one of the most
interesting groups of the Tunicata, as it shows more completely
than any of the rest the probable characters of the ancestral
forms. It has undergone little or no degeneration, and con-
sequently corresponds more nearly to the tailed, larval condition
than to the adult forms of the other groups. It retains, in fact,
the originally posterior, chordate, part of the body which is
lost in the metamorphosis of all the other Tunicata. Hence the
Appendicularians have been described as permanent, or sexually
mature, larval forms, and hence also the adult Ascidia may be
said to correspond to the trunk alone of the Appendicularian.
The Order includes a single group, the Appendiculariida, all
the members of which are minute (usually about 5 mm. in
total length) and free-swimming (Fig. 28). They occur near
the surface of the sea (and exceptionally in deeper water) in most
parts of the world, moving in a characteristic vibratory manner
by the contractions of the powerful tail (see Fig. 27). They
possess the power of forming with great rapidity, from tracts of
specially large glandular ectoderm cells, the " oikoplasts," an
enormously large (many times the size of the body) investing
gelatinous layer, which probably corresponds to the test of otlier
groups, although it is doubtful whether it contains cellulose, and
it differs also in having no immigrated cells and in its temporary
nature. This structure (Fig. 28) was first described by Yon

VOL. VII F

66

TUNICATA APPENDICULARIANS

Mertens, and by him named " Haus " ; it has recently V)een more
minutely investigated by Lohmann. It is only loosely attached
to the body, and is frequently thrown off soon after its formation.
Its function is probably protective, and possibly to some extent
hydrostatic, and it may also be of use in straining the nutritive
particles from the large volumes of water which filter through its
complicated passages and perforated folds.^ The long, laterally
ccmipressed " tail " in the Appendiculariida is attached to the
ventral surface of the body (Fig. 30), and is bent downwards and
forwards, so that it usually points more or less anteriorly ; and is
twisted throuo-h an angle of 90°, so that the dorsal edge lies to

Fig. 28. — Appendiculaiiida. A, Appendicularia sicula, Fol, ■with house ; B, Megalo-
cercus abi/ssorum, Chun, nat. size ; C, Oikopleura cophocerca, Gegenb., with house ;
D, Fritillaria mcfjachile, Fol, with vesicle ; E, Appemiicularian in its house :
F and G, two stages in the formation of the house. (A to D from Seeliger ; E to G
from Lohmann.)

the left. It shows what have been interpreted as traces of meta-
meric segmentation, having its lateral muscle-bands broken up
into successive pieces (supposed myotomes, probably only cells),
while the nerve-cord presents a series of enlargements formed of
groups of nerve-cells from which distributory nerves are given off.
In Oikopleura the muscle-band in the tail is formed of ten cells
fused on each side. Near the base of the tail there is a distinctly
larger elon crated gano-lion. The urochord in the tail consists of
a homogeneous rod surrounded by a sheath containing nuclei.

The anterior (cerebral) ganglion has connected with it an
otocyst (Fig. 29), a pigment spot, and a tubular richly ciliated
process opening into the branchial sac, and representing the dorsal
tubercle and associated parts of an ordinary Ascidian. The tube
ends in a plain or coiled cellular mass lying to the right of the
1 See Lohmann, Sckrift. Naturiv. Ver. Schlesw. -Hoist, xi. 1899, 347.

STRUCTURE

67

ganglion. Xo neural gland is found. The branchial aperture or
mouth leads into the simple branchial sac or pharynx (Fig. 30,
hr.s). There are no tentacles. The endostyle is short, is a closed
tube both anteriorly and posteriorly (Fig. 29), and has about four
longitudinal rows of gland-cells. There is no dorsal lamina,
and the peripharyngeal bands run dorsally and posteriorly to
unite close in front of the oesophageal opening. The wall of the
branchial sac does not show the complex structure usual in Tuni-
cata, and has only two ciliated apertures (Figs. 30, 31, 32, sr/).
These are homologous with the primary stigmata of the typical
Ascidians, and with a pair of the gill -clefts of Vertebrates.
They are placed far
l)ack on the ventral
surface, one on each
side of the middle
line, and lead into
short funnel-shaped
tul:)es wliich open on
the surface of the
body behind the
anus (Fig. 30, at).
These tubes corre-
spond to the right
and left atrial in-
volutions, which in
an ordinary Asci-
dian fuse to form
the peribranchial
cavity. The remainder of the alimentary canal consists of
oesophagus, stomach (which may have a glandular diverticulum),
intestine and rectum (Fig. 30). The heart, surrounded ventrally
by a delicate pericardial membrane, lies below and in front of the
stomach, and is formed by the differentiation of the outer ends
of epithelial cells into muscular fibrillae. Two specially large
glandular cells are placed at the opposite ends of the heart.
Tliere are no blood-vessels except the remains of the primary
body-cavity (blastocoel). No heart can be seen in some of the
smaller species of Oikophura. Nearly all the species are herma-
phrodite, and the large ovary and testis are placed at the posterior
end of the body. There is no proper oviduct, the genital pro-

FiG. 29. — Transverse section through anterior ])art of Oiko-
pleura to show ganglion, sense-organs, endostyle, etc.
X 300. br.s, Branchial sac ; c.f, ciliated funnel ; ec,
dorsal ectoderm ; end, closed anterior end of endo-
style ; h>/, hypobranchial groove in Hoor of branchial
sac ; n.g, nerve -ganglion ; or.gl, oral gland ; ot, otocyst ;
X, opening of ciliated funnel into pharynx.

68

TUNICATA APPENDICULARIANS

ducts merely breaking through to the exterior at the point
marked g.d in Fig. 30. The spermatozoa are generally matured
and shed before the ova, and thus self-fertilisation is prevented.
The ova are very small, and little is known of the develop-
ment.

S^ -...

Fig. 30. — Longitudinal optical section of Oikopleura. Part of the tail is cut off. «,
Anus ; at, atrial opening ; hr.s, branchial sac ; c.f, ciliated funnel ; ec, ectoderm ;
end, endostyle ; vp.p, epipharyngeal ridge ; g.d, opening of gonads to exterior ;
ht, heart ; hy.p, hypo^jharyngeal ridge ; i, intestine ; m, mouth ; mxis, muscle-
bands in tail ; n, nerve-cord ; n', nerve in tail ; iL.ch, urochord ; n.g, nerve-
ganglion ; n.g' , ganglion in tail ; oes, oesophagus ; <yr.gl, oral gland ; ot, otocyst ;
oi\ ovary ; sg, stigmata ; so, sense-organ ; sjj, testis ; st, stomach ; t, test. (After
Herdman.)

Classification. — There are two Families of Larvacea : First,
the KoWALEVSKiiDAE, including only the remarkable genus
Koivalevskia, Fol, in which the heart and endostyle are absent,
and the branchial sac is provided with four rows of ciliated tooth-
like processes. The two known species have been found in the
Mediterranean and in the Atlantic.

The second family Appendiculakiidae comprises about eight
genera, amongst which may be mentioned: — (1) Oikopleura,
Mertens, and (2) Appendicular ia, Fol, in both of which the body
is short (1 or 2 mm. in length) and compact (Fig. 30), and the
tail relatively long, while the endostyle is straight. (3) Megalo-
ceixus, Chun, from deep water in the Mediterranean ; M. abyssorum
is the largest Appeudicularian known, having a total length of

OCCURRENCE

69

hy.j)

tall

3 cm. — it is of a bright red colour. (4) Fritillaria, Q. and G., in
which the liody is elongated (Fig. 32) and composed of anterior
and posterior regions, tlie tail relatively short, the endostyle

Fig. 31. — • Transverse
m.s- section of body and

tail of Oikopleura
flubellnm (?) cd,
Atrial tube ; hl.s,
71. blood - space ; br.s,

cavity of pharynx
or branchial sac ;
^"^ ec, ectoderm ; en,

endoderm ; fJ>-J},
epipharyngeal cili-
c/eZ. ^ted bauds ; gel,

gelatinous layer be-
tween ectoderm and
endoderm ; hi/.p,
hl.s. liypopharyngeal

V " S \^-^\ \ ciliated band ; ?«M.s,

V y Xi.Y' muscular tissue on

^~-^^ ^^-- --r. inner surface of

ectoderm of tail ;
ij n, nerve-cord ; n' ,

.- _ its continuation in

the tail : n.ch, noto-
y chord in tail ; r,

^f. rectum ; sij, one of

the stigmata or cili-
ated openings from
■ the branchial sac to

"•c^- ^*^- the atrial tube ; t,

test (= young "house"); x, bridge of gelatinous tissue in front of stigma closing
liranchial sac oft" from atrial tube. (After Herdman. )

recurved, the stigmata opening far in front of the anus, and an
ectodermal hood is formed over the front of the body.

In all nearly forty species of Larvacea are known.

Occurrence. — Although for the most part transparent, and
usually almost invisible in sea-water, some Appendicularians may
have certain parts of the body (alimentary canal, endostyle,
gonads, etc.) brilliantly pigmented (orange, violet, etc.), and may
under exceptional circumstances be present in such profusion as
to colour tracts of the sea. Appendicularians are widely dis-
tributed, having been found in all seas from the Arctic to the
Antarctic, Ijoth round coasts and in the open ocean. Although a
few species have been found at considerable depths in the
Mediterranean, still in the Atlantic they are not deep - water
animals, and as a group must be regarded as surface-forms.
They are fairly abundant to a depth of 100 fathoms, and some
few reach 1500. Species of Oilcopleura and Fritillaria are

70

ASCIDIANS

frequent round the British coasts, our commonest species being
probably 0. dioica, Fol, and F. furcata^ Moss. Young specimens
appear in the plankton about February and March, and larger
forms are as a rule found later in the summer. Several instances
have been recorded of swarms of especially large forms, provided

hood

tuii

Fia, 02. — Diagram of Fritillaria seen from the right side to show the elongated body,
the hood, and the relative positions of anus, atrial opening, and gonads. (Compare
with Oikopleura, Fig. 30.) «, Anus ; at, opening of atrial tube ; hr.s, branchial sac ;
end, endostyle ; ht, heart ; m, mouth ; n.ch, notochord ; n.g, nerve-ganglion ; ves,
oesophagus ; ov, ovary ; sg, stigma ; sp, testis ; st, stomach.

with massive tests (the " house "), having appeared suddenly on
our coast in such abundance as to form an important element in
the surface life of the sea.

Order II. Ascidiacea (Ascidians).

Fixed or free-swimming Simple or Compound Ascidians, which
in the adult are never provided with a locomotory appendage or
tail, and have no trace of a notochord. The free-swinnning
forms are colonies, the Simple Ascidians being always sedentary
and usually iixed. The test is permanent and well developed,
and becomes organised by the immigration of cells from the body ;
as a rule it increases in size with the age of the individual. The
branchial sac is large and well developed. Its walls are perforated
by numerous slits (stigmata) opening into the peril)ranchial
cavity, which communicates with the exterior by the single atrial
aperture. Many of the Ascidiacea, both jfixed and free, reproduce
by gemmation to form colonies, and in most of them the sexually
produced embryo develops into a tailed larva.

The Ascidiacea includes three groups, the Simple Ascidians, the
Compound Ascidians, and the free-swimming colonial Pyrosoma,
which in some respects connects this Order with the Thaliacea.

ASCIDIAE SLMPLICES CLAVELINIDAE

71

Sub-Order 1. Ascidiae Simplices.

Fixed Ascidians, which are solitary, and very rarely reproduce
by gemmation ; it", as in a few cases, small colorues are formed,
the members are not buried in a connuon investing- mass, but each
has a, tlistinct test of its own. No strict line of demarcation can
be drawn between the Simple and Com])ound Ascidians ; and one
of the families of the former group, the Clavelinidae (the " Social "
Ascidians of ]\Iilne-Edwards), forms a transition from the typical
Simple forms which never re])roduce by gemmation, to the Com-
pound forms which always do. Over 500 species of Ascidiae
Simplices are now known, but there are probably very many more
still undescribed. The sub-order may be divided into the follow-
ing families : —

Fam. 1. Clavelinidae. — Simple Ascidians which reproduce
by gemmation to form small colonies (Fig. 33), in which each
member, or ascidiozooid, has a distinct test, but all are connected
by a common blood-system, and Ijy a prolongation of the " epicardiac
tubes " (see p. 83) from the
branchial sac. Buds are formed
on the stolons (Fig. 33), which
are vascular outgrowths from
the posterior end of the body,
containing prolongations from
the ectoderm, mesoderm, and
endoderm (the epicardium) of
the Ascidiozooid. Branchial sac
not folded: internal longitudinal
bars usually aljsent ; stigmata
straight ; tentacles simple. The
Clavelinidae are the simplest of
the Ascidiae Simplices. They are the forms that come nearest
to the Compound Ascidians, and are closely related to the
Distomatidae. They are probably the nearest representatives
now existing of the ancestral forms from which both Simple and
Compound Ascidians are descended.

This family contains amongst others the following three
genera : — Uctemascidia, Herdman, with internal longitudinal bars
in the branchial sac ; Clavclina, Savigny, with a long body and
intestine extending behind the branchial sac (Fig. 33) ; and

Fig. 33. — Colony of C/avelina lejpadiformis
(nat. size).

72

ASCIDIANS

Perophora, Wiegmann, with a short coiupact body and intestine
alongside the branchial sac. ClaveJi'na lejKtdiformis and Feroj^hora
listcri are common British species found at a few fathoms depth
off various parts of our coast. Both occur round the south end
of the Isle of Man. In autumn Clavelina accumulates reser^'e-
material in the ectoderm cells of parts of the stolon, which remain
when the rest of the colony dies away, and then form new buds
in spring.

Fam. 2. Ascidiidae. — Solitary fixed Ascidians, never forming
colonies : with gelatinous or cartilaginous test ; Ijranchial aperture
usually eight-lobed, atrial aperture usually six-lobed ; branchial
sac not folded ; internal longitudinal bars usually present ; stig-
mata straight or curved ; tentacles simple : gonads in or around
the intestinal loo]). This family is divided into three sections : —

Sub-Fam. 1. Hypobythiinae. — Branchial sac wdth no internal
longitudinal Ijars, test strengthened witli curious symmetrically
placed nodules.

The one genus Hyj^ohythius, Moseley, contains two stalked
deep-water forms found by the " Challenger ; " H. ca/ycodes (Fig.
34, A), from the North Bacitic, 2900 fathoms, and If. moseleyi
from the South Atlantic, 600 fathoms.

Fig. -34. — A, Ili/pohfjthius falycodes, Moseley ; B, Cheh/osoma ntadeainoinm. Brofl. and
Sowl). ; C, ('oryiiufiddia suhvii, Herdnian ; D, Rhodosnnia calleiise, Lac.-Dutli.

Sub-Fam. 2. Ascidiinae. — Internal longitudinnl l)ars present;
stio;mata straiuht. Maiiy uenera, of which the foUowinu- fire the
more important: — Ciona, Fleming, dorsal languets present;
Ascidia, Linnaeus (in part Phallvsia, Savigny), dorsal lamina

ASCIDIAE SIMPLICES ASCIDIIDAE

73

present (Fig. 15, p. 40) ; Rhodosoma, Ehrenberg, anterior part
of test modified to form operculum (Fig. 34, D) ; Abyssascidia,
Herdman, intestine on right side of branchial sac. The type
genus of this section, Ascidia, has been described in detail above
(Chapter II. p. 39), and Figs. 15 to 26 illustrate its structure
and life-history. There are many species. Ciona intestinalis, Linn.
(Fig. 40, B), is one of the commonest of British Ascidians, and
lives readily in a(iuaria.

Sub-Fam. 3. Corellinae. — Stigmata curved and forming
spirals (Fig. 35). Three genera: — Co?T//a, Alder and Hancock,
test gelatinous, body sessile ; Corynascidia, Herdman, test

Fig. 35. — A, branchial sac of Corynascidia suhmi, Herdman ; B, branchial sac of
Corella japonica, Herdman. i.l, Internal longitudinal bars ; tr, transverse vessels.
(After Herdman.)

gelatinous, body pedunculated (Fig. 34, C), a remarkable deep-
sea form with very delicate spirally-coiled vessels in the branchial
sac (Fig. 35, A), found in the Pacific (2160 faths.) and the
Southern Ocean ; Chelyosoma, Brod. and Sovvb., upper part of
test modified into horny plates (Fig. 34, B).

CoreJla contains several British species, one of which, p.
imrcdlelogramma, O. F. Mlill., is one of the commonest and most
handsome Ascidians in our coralline zone (about 20 faths.).
Through its clear crystalline test the lemon-yellow and carmine
pigmentation of the mantle, and even (with a lens) the working of
the cilia along the spiral stigmata of the branchial sac (compare
Fig. 35, B), can readily be seen. The beating of the heart can
be seen just in front of the viscera upon the riglit side of the
branchial sac (compare with Ascidia, Fig. 23).

In the family Ascidiidae the eggs are minute and contain

74

ASCIDIANS

CHAP.

little or no food-yolk, and the tailed larvae (Figs. 26, 42, A) are
of the typical form and structure described in Chapter II.

Fam. 3. Cynthiidae. — Solitary fixed Ascidians (Fig. 39),
sometimes occurring in aggregations, but never forming colonies ;
usually with leathery or fibrous, opaque test, which is sometimes
encrusted with sand ; branchial and atrial apertures usually both
four-lobed. Branchial sac longitudinally folded (Fig. 36, A);
stigmata straight; tentacles simple or compound (Fig. 37);

•-■end'-''

-end.'-'

Fig. 36. — Diagrammatic transverse sections of branchial sacs of Cynthiidae. A, Cynthia ,
B, Styela : C, Stye/opsis ; D, Pelonaia. Br.f 1-7, First to seventh brauchial fold
d.l, dorsal lamina ; end, endostyle ; mh, meshes.

neural gland dorsal to ganglion ; gonads attached to body-wall.
This family is divided into three sections :- — -

Sub-Fam. 1. Styelinae. — Xot more than four folds (Fig. 36,
B) on each side of branchial sac; tentacles simple (Fig. 37, A).
The more important genera are — Styela, Macleay, and Polycarpa,
Heller (Fig. 39), with stigmata normal; and Bathyoncus, Herd-
man, with stigmata absent or modified. There are a very large
number of species of both Styela and Polycarpa, from all parts of
the world, including our own seas. A ^■ery abundant British
littoral form has been placed in an allied genus under the name
Styelopsis grossularia (Fig. 39, A). It is known in some places
round our coasts as " the red-currant squirter." This species has
only one well-marked fold in the branchial sac (Fig. 36, C).
Another exceptional British Styelid is Pelonaia corrugata, Forb.
and Goods. (Fig. 39, I), with no branchial folds (Fig. 36, D).

ASCIDIAE SIMPLICES CYNTIIIIDAE

75

Sub-Fam. 2. Cynthiinae. — More than eight folds in branchial

sac (Fig. 36, A); tentacles compound (Fig. 37, B);'body sessile

or with a short stalk (Fig.

39, F). The chief genus

is Cynthia, Savigny, with

a large number of species,

so)ne of which are British.

Rhahdocynthia has echin-

ated calcareous spicules in

the mantle (see Fig. 50, 1),

p. 87).

Forhesella fessdlata is a
remarkable British species, having the test marked out into plates
(Fig. 39, B). It is intermediate in some characters between
Styelinae and Cynthiinae.

Sub-Fam. 3. Bolteninae. — More than eight folds in branchial
sac; tentacles compound; body pedunculated (Fig. 38, A). The
chief genera are — BoUenia, Savigny, with the branchial aperture

Ah-

FiG. 37. — Tentacles of Cyntliiidae. A, Simple, in
Styelinae ; B, Compound, in Cynthiinae.

tr

'if

tr. '

■ a.

Fig. 38. — Cultolus wi/riUe-thomsoni, Herdman. A, from left side (half-nat. size) ; B,
part of branchial sac. At, Atrial aperture : Br, branchial aperture ; br.f, liranchial
fold; i.l, internal bar : sp, spiciiles ; tr, transverse vessel. (After Herdman.)

four-lobed, and the stigmata normal ; and Culeolus, Herdman
(Fig. 38), with branchial aperture having less than four lobes,
and the stigmata absent or modified (Fig. 38, B), the branchial
sac showing a wide mesh-work of vessels stiffened by branched
calcareous spicules. Culeolus is a deep-sea genus discovered by the

76

ASCIDIANS

" Challenger " expedition ; eight or nine species are now known
from various parts of the world, ranging in depth from G30 to
2425 fathoms. Most of the species are from the Pacific; only
one from the North Atlantic. The curiously curved type of
spicule found in the branchial sac and other organs is shown at
Fig. 50, C (p. 87).

Amongst the Cynthiidae are found most varied conditions of
the reproductive organs. The gonads are sometimes on both, some-
times on only one side of the body, sometimes in one or several

Fig. 39. — Various Cynthiidae. A, two forms of Sti/elojisis grossularia, Van Ben. ; B.
Forhesella teasellata, Forb. ; C, I'olycarpa aurata, Q. and G. ; D, Styela clava,
Herdman ; E, Polycarpa thidor, Q. and G. ; F, Cynthia formnsa, Herdman ; G,
Pohjc irpa comata, Alder ; H, I'olycarpa pedata, Herdman ; I, Pelonaia corrugata,
Forb. and Goods. (After Herdman.)

branched masses, and sometimes distributed as a large number
of minute " polycarps " over the inner surface of the mantle.

The family Cynthiidae is the largest section of the Simple
Ascidians. The species range from the size of a pea to that of a
large cocoa-nut. They are for the most part opaque, and often
riclily coloured — reds, yellows and rich browns predominating —
and so look very different to the grey gelatinous Ascidiidae, and to
the sand-encrusted Molgulidae. They extend from between tide-
marks (Sti/elo])sis grossularia), down to the abysses (^Styela hythia
and aS'. squamosa at 2600 fathoms). Some genera {Styela and tlie
closely related Dendrodoa), extend far into Arctic seas, but many
allied forms {Styela and Folycarpa) are also found in the tropics.

ASCIDIAE SIMPLICES MOLGULIDAE

77

Fam. 4. Molgulidae. — Solitary sessile Ascidians, soinetiiues
not fixed ; liraiicliial ajiertun; six-Iubed, atrial four-lobed. Test
usually encrusted witli sand, whieli is generally attached to

Fig. 40. — Three simple Ascidians with vascular adhering processes from the test (nat.
size). A, Ascidiella aspersa, 0. F. Miiller ; B, Ciona iniesiinalis, Linn. ; C,
Molgula ocidata, Forb.

branched hair-like processes from the test (Fig. 40, C). Branchial
sac longitudinally folded ; stigmata more or less curved, usually
arranged in spirals (Fig. 41); tentacles compound. The chief

'd't.

' -'\;

i:2v

'W\V

wmm

A/

B

Fig. 41. — Branchial sacs of Molgulidae showing ciir^'ed stigmata. A, Ascopera ijiganiea,
Herdnian ; B, Molyulu pyriformis, Herdman ; C, Euijyra kerguelenensis, Herd-
man.

genera are— Ifolgida, Forbes (Fig. 40, C), with distinct folds in
the branchial sac (Fig. 41, B), and Eugyra, Aid. and Hanc.,.
with no distinct folds, but merely broad internal longitudinal
bars in the branchial sac (Fig. 41, C). In some of the Molgulidae

78

ASCIDIANS

(genus Anurella, Lacaze-Duthiers), the embryo does not become
converted into a tailed larva, the development being direct
without metamorphosis (see Fig. 42, C). The embryo when
hatched gradually assumes the adult structure, and never shows
the features characteristic of larval Ascidians, such as the
urochord and the median sense - organs. Fiy;. 42 shows an
Ascidiid (A), a Cynthiid (B), and this exceptional Molgulid (C),
type of larva, and three forms of Compound Ascidian larvae, the
Distomatid (D), the Botryllid (E), and the Diplosomatid (F).

Fig. 42. — Larvae of various Ascidians. A, Ascidia mentula, Linn. ; B, Polycarpa
glomerata, Alder; C, Anurella rosconla, Lac.-Diith. ; D, Distaplia magjiilarva,
Delia Valle ; E, Polycyclus renieri, Larak. ; F, Diplosomoides lacazii, Giartl.
(Mostly after Lahille.)

In the Molgulidae the viscera are characteristic in position
and appearance. The alimentary canal lies on the left side of
the branchial sac, and the intestine forms a long narrow loop
directed in the main transversely. The pericardium and heart
are on the middle of the right side, and behind them is placed
the single sac-like ductless renal organ, generally occupied by
one or more concretions. The gonads are in most ca.ses on both
sides of the l;»ody, in front of the intestine on the left, and in
front of the lieart on the right ; l)ut in Eugyra tliere is no gonad
on the right side, and in some other forms the gonad on the left
side is alxsent. (For Oligotrema, see p. Ill, note.)

There are a number of British JMolgulidae, the two commonest

ASCIDIAE SIMPLICES SPECIFIC CHARACTERS

79

of which are — Mohjtda omdata, Forljes, thickly covered with gravel
or broken shells, and Ibrniing an ovate mass as large as a walnut ;.
and Eugyra glutinans, Moller, a smaller more globular body, the
size of an acorn, and covered with fine sand, except at one circular
area near the posterior end, where the leaden grey test shows
through. Both these species are obtained by dredging in from
10 to 30 fathoms, and lie freely on the bottom. A rather rarer
littoral species Molgula citrina, Hancock, found on some parts of
our coast (e.g. in the Firth of Forth, at Arran, and at Port Erin),
is exceptional in having the test free from sand, and in being-
fixed like an Ascidia, generally to the lower surfaces of large
stones near low tide.

Specific Characters and Dorsal Tubercle. — The chief points
in which the various genera and species of Simple Ascidians
differ are the details of the branchial sac (see Figs. 22, 35, 36,
38, and 41), the condition of the tentacles (Fig. 37), the dorsal
lamina or languets, and the dorsal tubercle, in addition to form,
colour, and other external features.

O Q^ ^ O

Fig. 43. — Various forms of dorsal tubercle iu Simple Ascidians. 1. Mohjula py'i'ifd'nnis ;
2. ForbeseUa tessellata ; 3. Ascidia meridional is ; 4. Cynthia forimsa ; b. Cynthia
papietensis ; 6. Ascidia challenyeri ; 7. Polycarpa tinctor ; 8. Cynthia cerebri-
formis ; 9. Asccpera fjif/antea : 10. Boltenia tuhcmdata ; 11. Ascidia translucida ;
12. Culeolus moseleyi ; 13. Ascidia 2}yriformis ; 1i. Boltenia pachyder7nati»a ; 15.
Microcosimts draschii ; 16. Styela ethcridgii; 17. Styela whiteleggii ; 18. Foly-
carpa aurata. (After Herdman.)

Fig. 43 shows some of the more remarkable forms of dorsal
tubercle. Starting with a simple circular opening (1) surrounded
by a thickened ciliated ring, the anterior border becomes pushed
in to form a crescentic slit (2 and 3). The horns of the crescent
then grow longer and may be turned in (4 and 5) or out (6 and
7), and so give rise to the many varieties of horse-shoe (such as
6), perhaps the commonest form of dorsal tubercle in Simple
Ascidians. In many Cynthiidae the central part of the horse-

8o

ASCIDIANS

shoe remains small, while the horns become long and much coiled
so as to constitute two prominent spirals (8, 9, 10). In other
exceptional forms again the curved slit becomes straightened out,
undulating (11), irregularly bent (12 and 13), elaborately folded
(14 and 15), or broken up into pieces (16), so that there come
to be several or even a large number (17 and 18) of minute
openings in place of the original single aperture.

It cannot be said that any form of dorsal tubercle is charac-
teristic of any of the families or genera of Ascidians, and in the
case of some species the organ is. liable to great individual
variation ; but still in most species there is found to be a char-
acteristic shape or appearance of tubercle which is a useful
diao-nostic feature.

Sub-Order 2. Ascidiae Compositae.

Fixed Ascidians which reproduce by gemmation so as to form
colonies (Fig. 44) in which the ascidiozooids are buried in a

Fig. 44. — Colonies of Coinpouml Ascidians (nat. size). A, Cvlella quoyi, Hrdn.
Autarct. ; B, Leptoclbiuui neijlcdum, Hrdn. ; C, Fharynrioclidyon inirabilc, Hrdu.
Southern Ocean ; D, Botryllns schlosseri, Sav. Europe. (After Herdman.)

common investing mass (Fig. 45) and have no separate tests —
hence " Synascidiae," a name they often receive from foreign writers.
This is probably a somewhat artificial assemblage formed of
those two or three groups of Ascidians which produce colonies,
in which the ascidiozooids are so intimately united that they
possess a common test or investing mass. This is the only char-
acter which distinguishes them from the Clavelinidae, but
the property of reproducing by gemmation separates them from
the rest of the Ascidiae Simplices. In some cases the atrial
apertures of several neighbouring ascidiozooids join to open to
the exterior by a common cloacal apertm^e (Fig. 45, c.c). Such

ASCIDIAE COMPOSITAE

8i

groups of the ascidiozooids of a colony are known as " systems "
or coenobia (see Fig. 44, D ; also Fig. 53, p. 89).

The Ascidiae Compositae may be divided into seven families,
which seem to fall into two well-marked sets: — (1) Mekosomata,
in which the heart and alimentary and reproductive viscera are
placed behind the branchial sac, so as to constitute a more or
less extended body divided into at least two regions (Fig. 46, B),
and sometimes three (Fig. 46, C) — thorax, abdomen, and post-

FiQ. 45. — Vertical section through a small part of a compound Ascidian colony. Asc. 1
and Asc. 2, Parts of two ascidiozooids whose cloacas (d) open into the common
cloacal cavity {c.c) of the colony ; at J, atrial lobes ; t, t, t, common test of the
colony. The structure of the posterior parts of the ascidiozooids would depend upon
the family (see Fig. 46). The arrows show the direction of the water currents.

abdomen ; and (2) Holosomata, in which the body of the
ascidiozooid is short, compact, and not divided into regions (Fig.
46, A). The latter group comprises the two families Botryllidae
and Polystyeliclae, which agree both in points of structure and in
having the same type of budding, and are probably derived from
ancestral Cynthiidae amongst Simple Ascidians ; while the Mero-
somata seem more nearly related to the Clavelinidae.

Gemmation takes place in the Compound Ascidians in a
variety of ways, being sometimes very different in its details in
closely allied forms. There are, however, two main types of
budding, to one or other of which most of the described methods
may be refen-ed. These are : —

1. The Stolonial, or " epicardiac " type — seen in the Mero-
somata, typically in Distomatidae and Polyclinidae, and compar-
aljle with the gemmation in Clavelinidae, Pyrosomatidae, and
Thaliacea outside this group.

VOL. VII G

82

ASCIDIANS

2. The Parietal, or " peribranchial " type — seen in tbe Holo-
somata, typically in the Botryllidae.

The remarkable process of gemmation seen in the families
Didemnidae and Diplosomatidae, where the bud arises from at least
two rudiments, the one stolonial or epicardiac in origin, and the

Fig. 46. —A, Ascidiozooid from a Bot-
ryllid colony ; B, ascidiozooid from
a Distomid colony ; C. ascidiozooid
from a Polycliuid colony. «,
Anus ; at, atrial aperture ; at. I,
atrial languet ; br, branchial aper-
ture ; cl, cloaca ; d.l, dorsal languet ; ee, ectoderm ;
end, endostyle, ep.c, epicardiac tube ; gl, intestinal
land ; h, heart ; i, intestine ; n.g, nerve-ganglion ;
oes, oesophagus ; ov, ovary ; p.c, pericardium ; r,
rectum ; s(f, stigmata of branchial .«ac ; sjj, spermatic sacs ;
sjih, sphincter ; st, stomach ; t, tentacle ; t.k, terminal
ampullae of vessels in test ; v, colonial vessels ; v.apj),
" vascular appendage " (stolon).

other formed by one or more oesophageal or intestinal outgrowths,
has been called " entero-epicardiac," but it may probably be re-
garded as a modification of the stolonial type.

The marked differences in the appearance of the colonies of
Compound Ascidians is largely due to the methods of budding ;
and even in those of the stolonial type, where the budding is
practically the same in essential nature, the results may be very
different in superficial appearance, according as the buds are

GEMMATION 83

formed on a short stolon close to the parent body, or from the
extremity of the Y)Ost-abdomen (as in the Polyclinidae), or from
a long epicardiac tube (as in Colella, Fig. 47), wliieh may extend
for some inches from the ascidio/ooid. The post-abdomen of the
Polyclinidae may be regarded as a stolon invaded by the gonads
and the heart (see Fig. 46, C), and traversed by the epicardium
in the form of a flattened tube dividing a dorsal blood-sinus con-
taining the gonads from a ventral sinus which has merely the one
extremity of the tapering pericardium. The whole of this post-
abdomen segments to form the buds, the heart at the extremity
being absorbed, and a new one formed from the anterior end of
the pericardium.

The epicardium, which supplies the endodermal element to
each bud, was first described by E. van Beneden and Julin in the
development of Clavelinn} as a structure concerned in the forma-
tion of the pericardium and heart — hence its unfortunate name.
It grows backwards in the larva, from the posterior wall of the
branchial sac, close to the endostyle, as a tube which usually
divides into two lateral branches to be imited again eventually
so as to form the single tubular flattened partition of the stolon
in Polyclinidae, Distomatidae, Clavelinidae, etc. In some Com-
pound Ascidians the epicardium is, from its origin, two distinct
lateral tubes, which grow back from the inner vesicle of the
embryo (later the branchial sac). These unite in the post-
abdomen to form the flattened tube, which in its turn forms the
inner vesicle of the futm-e buds, and so the endodermal element
is handed on from generation to generation. In addition to the
epicardium, the stolon contains also a x>rolongation of the ovary
of the parent, or at least a string of migrating germ cells, so
that the reproductive elements are also handed on.

It is clear from the recent researches of Hjort, Eitter, Lefevre,"
and others, that the development of the bud (blastozooid) and
that of the embryo (oozooid) do not proceed along parallel lines.
It is evidently impossible to harmonise the facts of gemmation
with the germ-layer theory ; and attempts to explain budding in
Ascidians solely as a process of regeneration by which the organs
of the parent or their germ-layers give rise to the corresponding
organs in the bud have in many cases failed.

^ Arch, dc Biolmjic, vi. 1887. U. ' ' ^

- SiGQ Journ. of Morphology, xii. -xiv. 1896-1898.

84 ASCIDIANS

The rudiment of the bud is in typical cases composed of two
vesicles, an outer derived from tlie ectoderm of the parent and
enclosing free blood-cells (mesodermal) between its wall and that
of the inner vesicle — which is usually of endodermal origin, but
in Botryllidae is derived from the peribranchial sac, an ecto-
dermal structure. The inner vesicle, derived in the two cases
from different germ-layers, forms the same organs of the bud,
and these organs may ]m of widely different origin in the
larva. Moreover, free cells of the blood may play in the bud a
very important part, and give rise {Peropliora) to such important
systems as pericardium and heart, neural tube and ganglion, the
gonads and their ducts, some of which are of ectodermal and
others of endodermal origin in the larva.

In some cases of precocious budding (blastogenetic accelera-
tion) the young buds begin to appear during the tailed larval
stage. The larva may even contain a first blastozooid (bud)
with a branchial sac as large as that of the oozooid (derived from
the egg); and in the Diplosomatidae the larva (see Fig. 42, F),
when it settles down, may be already a small colony of three
young ascidiozooids.

The larvae in most Compound Ascidians, in place of adhering
papillae, have several or even a considerable number of ecto-
dermal tuljes or prolongations from the body (see Fig. 42, E and
F) into the surrounding test. These apparently aid in the
formation of the common test of the young colony, which grows
over and adheres to foreign objects.

There are many irregularities in the larval development of
Compound Ascidians, due to the very different amount of food-yolk
present in the ova in different genera. In some cases there is even
dimorphism, two forms of larvae being found in the same colony.

Compound Ascidians are amongst the most varied and
brilliant of sessile animals seen at low tide on our own and most
other coasts. Some are stalked and form club-shaped or knob-
like outgrowths. Others again form flat gelatinous expansions
attached to sea-weeds or stones, and are synnnetrically marked
with bright spots of colour in the form of circles, meandering
lines, or star-like patterns. In such colonies each spot of colour
or ray of a star represents an ascidiozooid or member of the
colony, equivalent to the whole animal in the case of the solitary
Simple Ascidian.

ASCIUIAE COMPOSITAE DISTOMATIDAE

85

incp.

fyenib

Group A. 3fER0S0MATA.

Viscera posterior to liranchial sac ; budding stolonial.

Fam. 1. Distomatidae. — Ascidiozooids divided into two
regions, a thorax, containing the branchial sac, and an abdomen,
with the remaining viscera (Fig. 47, B); testes nnmerous ; vas
deferens not spirally coiled. The chief genera are — Distoma,
Gaertner, with some British species ; Chondrostachys, Macdonald,
Cystodytcs, v. Drasche, with calcareous plate-like spicviles in the test

Fig. 47. — A, Colony
of Colella peduncu-
lata, Q. and G., nat.
size: a, zone of buds;
h, zone of young
ascidiozooids ; c,
zone of reproduc-
ing adults ; d, old
decaying adults and
incubatory pouches
with larvae. B,
Ascidiozooid, with
incubatory pouch
enlarged : At, atrial
aperture ; Br, bran-
chial aperture ; emh,
embryos ; end, en-
dostyle ; ep.c, epi-
cardium ; incp,
incubator}^ pouch ;
od, oviduct ; od',
its prolongation
into incp ; od", its
termination at tip
of incp; or, ovary ;
p).hr, ]ieribranchial opening of incp ; st, stomach.

(Fig. 50, A); Distaplia, Delia Yalle, and Colella, Herdman, forming
a pedunculated colony (Fig. 4*7, A), in which the ascidiozooids (Fig.
47, B) are provided with large incubatory pouches, opening from
the peribranchial cavity, but also connected, as Bancroft ^ has re-
cently shown, with the end of the oviduct (see Fig. 47, B). In
these pouches the embryos undergo their development, and are
set free Ijy the decay of the top of the colony. The stolons pass
from the ascidiozooids in the upper part of the colony down into
the stalk, and there produce buds which gradually work up to
the top of the stalk, where they take their places as young ascidio-
zooids. At the top of the colony the old ascidiozooids die
and are removed (see Fig. 47, A). CauUery has shown that iu

1 Bull. Mus. Comp. Zool. x.\.\v. No. 4, 1899, p. 59.

86

ASCIDIANS

od.

St..

this genus there may be dimorphism in the buds, some of them

placed deeply in the stalk
having a large amount of
reserve food-matter in their
ectoderm, and remaining
dormant until required to
regenerate the " head " or
upper part of the colony
when it is lost. This genus
was made known by the
" Challenger " expedition.
The species are mostly trop-
ical, or from southern seas.
Fam. 2. Coelocormidae.

Distoniid. hl.s. Blood-sinus ; ec, ectoderm ; Colonv not hxed luivin""

cp.c, epicardium ; gl, intestinal glands ; A, heart ; . ' . ^^

i, intestine ; /. w, longitudinal muscles ; mes, 9- large axial CaVlty With

mesoderm ; od, oviduct ; ^ac pericardium ; st, ^ terminal aperture. Bran-

stomach ; v.d, vas deferens. (After Seeliger. ) . ■■•

chial apertures five-lolied.
This includes one species, Coclocormus hiLxleyi, Herdman, which is
in some respects a transition-form between the ordinary Com-
pound Ascidians {e.g. Distomatidae) and the Ascidiae Luciae
{Pyrosoma, see p. 90).

Cp.f.

JJ.C.

Fig. 4S. — Transverse section of the abdomen of

br.s.-

vd.

Fig. 49. — Section of Leptodinum colony, showing the distribution of spicules and parts
of the ascidiozooids. h. Base of cnlouy ; hi\ branchial aperture ; br.s, branchial sac ;
sp, spicules ; st, stomach ; ies, testis ; v.d, vas deferens.

Fam. 3. Didemnidae. — Colony usually tliin and incrusting.
Test containing stellate calcareous spicules (Figs. 49 and 50, B).

ASCIDIAE COMPOSITAE DIDEMNIDAE, ETC.

^7

Testis single, large ; vas deferens spirally coiled (Fig. 49).
The chief genera are — Didemnum, Savigny, in which the colony
is thick and fleshy, and there are only three rows of stigmata on
each side of the branchial sac ; and Leptoclinum, Milne-Edwards,
in which the colony is thin and incrusting (Fig. 49), and there

Fig. 50. — Calcareous spicules of the Tunicata, enlarged. A, From Cystodytes ; B, from
Leptoclinum; C, from Culeolus ; D, from Rhabdocynthia.

are four rows of stigmata. Colonies of Leptoclinura, forming
thin white, grey, or yellow crusts under stones at low water, are
amongst the commonest of British Compound Ascidians.

Fam. 4. Diplosomatidae. — Test reduced in amount (Fig. 51),
rarely containing spicules. Vas deferens not spirally coiled. In

c.cl.

Fig. 51.-

-Section of a colony of iJiplosoma, (enlarged) to show the small amount of test
present, hr. Branchial aperture ; c.d, common cloaca ; t, test.

Diplosoma, Macdonald, and other allied genera (Fig. 51), the
larva is gemmiparous (Fig. 42, F). Some species are common
British forms, especially on Zostera-he^h and amongst seaweeds.

Fam. 5. Polyclinidae. — Ascidiozooids divided into three
regions — thorax, al)domen, and post-abdomen (Fig. 46, C).
Testes numerous ; vas deferens not spirally coiled. The chief
genera are — Pharyngodictyon, Herdman, with stigmata absent or
modified, containing one species, Ph. mirahile (Fig. 44, C), the

88

ASCIDIANS

only Compound Ascidian known from a depth of 1000 fathoms ;
Polyclinum, Savigny, with a smooth -walled stomach (Fig. 52,
A) ; Aplidiuvi, Savigny, with the stomach-wall longitudinally
folded (Fig. 52, B) ; Morchellium, Giard, with an " areolated "
stomach (Fig. 52, D), bearing knobs on the outside ; and Aviarou-
cium, Milne-Edwards, in which the ascidiozooid has a long post-
abdomen and a large atrial languet, and wdiere the stomach-wall
shows longitudinal ridges breaking up into knobs (pseudo-

A B

Fig. 52. — Various conditions of stomach ia Polyclinidae. A, Polydinum, molle, Herd-
man ; B, Aplidium zostericola, Giard ; C, Amaroucium proli/erum, M.-Edw. ;
D, Morchellium argus, M. -Edw.

areolated, Fig. 52, C). The last four genera contain many common
British species.

Many of the Compound Ascidians die down in winter; but
amongst Polyclinidae, as in Clavelina, a form of hibernation is
found, the old ascidiozooids dying, but some of the buds in
the basal part of the colony accumulating a large store of reserve-
material in their ectoderm, and lying dormant until spring, when
they regenerate the colony.

Group B. HOLOSOMATA.

Body short, compact, with viscera by the side of branchial sac ;
budding parietal.

Fam. 6. Botryllidae. — Ascidiozooids grouped in systems
round common cloacal apertures (Fig. 53). Ascidiozooids having
the intestine and reproductive organs by the side of the branchial
sac (Fig. 46, A, p. 82). Dorsal lamina and internal longitudinal
bars present in the branchial sac. Neural gland, as in Cyn-
thiidae, dorsal to the ganglion in place of ventral as in the
majority of Tunicata, The chief genera are — Botryllus, Gaertn.
and Pall., with simple stellate systems (Fig. 53), and Botrylloides,
Milne-Edwards, with elongated or ramified systems. There are

ASCIDIAE COM POSIT AE BOTRVLLIDAE, ETC.

89

many species of both these genera, which form brilliantly coloured
fleshy crusts under stones and on sea-weeds at low tide. They
are amongst the commonest and the most beautiful of British
Ascidians. Both genera contain species remarkable for the rich
profusion of ectodermal " vessels " which ramify and anastomose
in the colonial test. On the margins of the colony these vessels
end in knob-like dilatations, the ampullae (Fig. 46, A, t.l^, which
are said by Bancroft to pulsate rhythmically, and so aid in keep-

FlG.

"systems" from a Fig. 54. — Goodsiria placenta, Herdmau.

colony of Botryllus violaceus,
M.-Edw. c/, Common cloaca of a
system ; or, branchial apertures of
ascidiozooids, magnified. (After H.
Milne-Edwards.)

A, Colony (half nat. size) ; B, section
of colony showing ascidiozooids. (After
Herdman, from Challenger Rejjorts.)

ing up the colonial circulation. They are also storage reservoirs
for the blood, doubtless help in respiration, and are organs for
the secretion of the test-matrix.

Fam. 7. Polystyelidae. — Ascidiozooids not grouped in
systems ; branchial and atrial apertures four-lobed ; branchial sac
may be folded ; internal longitudinal bars present. The chief
genera are — Thylaciuvi, Carus, with the ascidiozooids projecting
above the general surface of the colony ; Goodsiria, Cunningham,
with the ascidiozooids completely imbedded in the investing mass
(Fig. 54); and Chorizocormus, Herdman, with the ascidiozooids

90 ASCIDIANS

united in little groups which are connected by stolons. The last
genus contains one species, Ch. reticulahis, in some respects a
transition-fonn between the other Polystyelidae and the Styelinae
amongst Simple Ascidiaus.

Budding- in Holosomata — In the Polystyelidae, according to
Eitter,^ the budding is of the same type as in Botryllidae, the
bud arising in each case from the lateral liody-wall of the parent.
In Boty-yUus " the oozooid formed from the larva gives rise at
a very early period to the first blastozooid of the future colony.
This then forms the two buds of the second generation on its
sides (see Fig. 55), and these in their turn form the third, and
these the fourth generation, in which there are thus eight blasto-
zooids ; aud so the process goes on, the buds of each generation

arranging themselves in a circle to form
a system. As each new generation
makes its appearance, the preceding one
undergoes degeneration, and is eventu-

(^3, j-v ,M ; , ally absorbed. Consequently, in a system

^^ ..-Y / ^ there can usually be seen, in addition
to the adult members, certain older
ones in various stages of degeneration
^*^ and removal, and certain younger ones

Fig. 55.— Diagram to illustrate arising as buds on the sides of their pre-
the budding and formation deccssors, or just Separated from them,

of a system in Botri/Uus. .

Ooz, oozooid ; Bl 1, first and ready to take their places as young
Sstr^lLL'sclbUr" ascidiozooids ia the system. Three dis-
tinct generations are thus commonly
seen in a system. Now aud again one or two young ascidio-
zooids become squeezed by the pressure of their neighbours out
of a system into the surrounding test, and so give rise to new
systems which add to the extent of the colony.

Sub-Order 3. Ascidiae Luciae.

Free-swimming pelagic colonies having the form of a hollow
cylinder closed at one end (Fig. 56). The ascidiozooids forming
the colony are imbedded in the common test in such a manner
that the branchial apertures open on the outer surface and the

^ Journ. Morph. xii. 1896, p. 149.
2 See Pizon, Ann. des Sci. Nat. 7^ ser. Zool. xiv. 1892.

<

ASCIDIAE LUCIAE PYROSOMATIDAE

91

atrial apertures on the inner surface next to the central cavity of
tlie colony. They are placed with their ventral surfaces towards
the closed end (Fig. 56, C). The first ascidiozooids of a colony
are produced by gemmation from a stolonic prolongation of an
imperfect oozooid or rudimentary larva (the " cyathozooid "),
developed sexually. The subsequent ascidiozooids are formed
from these as buds on a ventral stolon.

This sub-order includes a single family, the Pyrosomatidae,
containing one well-marked genus Pyrosoma, Peron, with about six
species. They are found swimming near the surface of the sea.

Fig. 56. — Pyrosoma. A, lateral view (nat. size) ; B, end view ; C, diagram of longitudinal
section, at, Atrial apertures ; hr, branchial apertures ; c.cl, common cloaca ; end,
eudostyle ; t, test ; r, velum or diaphragm at terminal opening.

chiefly in tropical latitudes, and are brilliantly phosphorescent.
A fully developed Pyrosoma colony may be from an inch or two
to upwards of twelve feet in length.

The Colony. — The shape of the colony is seen in Pig. 56, A.
It tapers slightly towards the closed end, which is rounded. The
opening at the opposite end may be reduced in size (see B and C),
by the presence of a membranous prolongation of the common
test, which can be contracted or expanded by means of the
muscle-bands it receives from the atrial siphons of neighbouring
zooids. The branchial apertures of the ascidiozooids are mostly
placed upon short (in some cases longer) papillae projecting from
the general surface, and many of the ascidiozooids have long conical
processes of the test extending outwards beyond their branchial

92

ASCIDIANS

apertures (Fig. 57, t'). There is only a single layer of adult
ascidiozooids in the wall of the Pyi'osoma colony, as all the fully
developed ascidiozooids are placed with their antero -posterior
axes at right angles to the surface and communicate by their
atrial apertures with the central cavity (Fig. 56, C). Their
dorsal surfaces are turned towards the open end of the colony,
and the buds are given off from their ventral edges (Fig. 57).

Anatomy. — The more important points in the structure of
the ascidiozooid oi Pyrosoma are shown in Fig. 57. A circle of

Fig. 57. — Ascidiozooid of
Pyrosoma from the right
side. «, Amis ; A t, atrial
aperture ; at.m, atrial
mitscles ; Br, branchial
aperture ; hr.s, braiicliial
sac ; d, cloaca ; d.l, dor-
sal lamina ; d.t, dorsal
tubercle ; ec, ectoderm ;
en, eudoderm : end, endo-
style ; Ht, heart ; l.o,
luminous organ ; mes,
mass of mesoderm cells ;
m.f, muscle fibre ; n.g,
nerve-ganglion ; oes, oeso-
phagus ; sg, stigmata ; st,
stomach ; stol, stolon ;
t, test ; f , projection of
test near branchial aper-
ture ; tes, testis ; tn, ten-
tacle ; 1, 2, 3, buds.

tentacles, of which one, placed ventrally (tn), is larger than the
rest, is found just inside the circular branchial aperture. From
this point a wide cavity, with a few circularly placed muscle-
bands running round its walls, leads back to the large branchial
sac {hr.s.), which occupies the greater part of the body. The
large stigmata are elongated transversely (dorso-ventrally), and
are crossed by internal longitudinal bars running antero-posteriorly.
The dorsal lamina is re^Dresented by a series of eight or ten

Ill PYKOSOMA STRUCTURE, DEVELOPMKNT 93

lauguets. The nerve-ganglion (on which is placed a small pigmented
sense-organ, the impaired " eye "), the neural gland, the dorsal
tubercle, the peripharyngeal bands and the endostyle are placed
in the usual positions. On each side of the anterior end of the
branchial sac, close to the peripharyngeal bands is a mass of
rounded mesodermal gland-cells il.o), which are the source of
the phosphorescence. They are apparently modified leucocytes
lying in blood-sinuses. The alimentary canal is placed posteriorly
to the branchial sac, and the anus opens into a large peribranchial
or atrial cavity, of which only the median posterior part icl), is
shown in Fig. 57. The heart [Ht) lies between the posterior
end of the branchial sac and the intestine, close to where the
endostyle is prolonged outwards to form the inner tube of the
ventral stolon The reproductive organs are developed from a cord
of germinal tissue which forms a part of every budding stolon, and
so establishes a continuity of origin between the ova of successive
generations of Pyrosoma. On the ventral edge of the body,
immediately behind the stolon, with part of which it is continuous,
a portion of this germinal tissue gives rise to a lobed testis {tes),
and to a single ovum surrounded by indifferent or follicle-cells.

Development and Life-History. — The development takes
place within the body of the parent, in a part of the peribranchial
cavity. It is a " direct " development, the tailed larval stage being
omitted. The segmentation is incomplete or " meroblastic," and
an elongated embryo is formed on the surface of a mass of food-
yolk. Follicle-cells, or kalymmocytes, migrate into the embryo,
where they aid in its nutrition. The embryo (or young oozooid),^
after the formation of an alimentary cavity, a tubular nervous
system, and a pair of laterally placed atrial tubes, divides into
an anterior and a posterior part (see Fig. 58). The anterior
and ventral part, or stolon, then segments into four pieces (the
tetrazooids or first blastozooids),^ which afterwards develop into
the first ascidiozooids of the colony, while the posterior part
remains in a rudimentary condition, and is what was called by
Huxley the " cyathozooid " (Fig. 58, cy). This is really the
degenerate oozooid, and eventually atrophies without having

^ "Oozooid" and " blastozooid " have not always been used in the same sense.
It is best to regard as oozooid the first member of a new colony derived from
an embryo fonned by the fertilisation of an ovum, and to call the remaining
ascidiozooids produced by gemmation the blastozooids.

94

ASCIDIANS

completed its development, but having precociously given rise to
the budding stolon.

As the four ascidiozooids increase in size, they grow round
the cyathozooid and soon encircle it (Fig. 58, B). In this con-
dition the young colony leaves the body of the parent and becomes

free. The cyathozooid ab-

A,

sorbs the nourishing yolk
upon which it lies, and dis-
tributes it to the ascidio-
zooids by means of a heart
and system of vessels which
have been meanwhile
formed. When the cyatho-
zooid atrophies and is
absorbed, its original atrial
aperture remains and

Fig. 58. — Development of Pyrosoma colony.

young stage showing oozooid or cyathozooid,
cy, with stolon divided into four blastozooids

(I. -IV.) : V, viteiius. B, older stage showing deepens to bccome the ccn-

the four blastozooids in a ring around the , -i •, i j? fi

remains of the cyathozooid. (After Salensky.) ^^^^ cavity 01 Uie yOUng

colony, which now consists
of four ascidiozooids placed in a ring, around where the cyatho-
zooid was, and enveloped in a common test. The test is at first
formed by the ectoderm cells of the cyathozooid. Later it
becomes invaded by mesoblast cells from the ascidiozooids in the
usual manner. The colony gradually increases by the formation
of buds from these four original ascidiozooids. The young colony
is, in some species, at first male, and only becomes hermaphrodite
when it has attained to some size.

Occurrence. — The half-dozen known species of Pyrosoma
are widely distributed over the great oceans, although they are
probably most abundant in tropical waters, Pyrosoma atlanticum,
Peron, and P. giganteum, Lesueur, are the commonest forms.
Although sometimes abundant in the Mediterranean and the North
Atlantic they have apparently not been found in British seas.
P. elegans, Lesueur, is a Mediterranean form allied to the last two ;
and P. minatum and P. aherniosum, Seeliger, were discovered
during the German " Plankton " expedition in the tropical Atlantic.
Finally, the enormous P. spinosum, Herdman, was found by the
"Challenger" in both North and South Atlantic in 1873 ; and

' According to Kowalevsky. Salensky, however, considers that the atrial
aperture closes, and that a new surface depression appears later.

Ill TIIALIACEA (SALPIANS) 95

some years later (Perrier's P. excelsior) by the French " Talisman "
expedition in the tropical Atlantic. The late Professor Moseley
said of this (" Challenger ") species, " I wrote my name with my
linger on the surface of the giant Pyrosoma as it lay on deck in
a tub at night, and my name came out in a few seconds in letters
of lire." Bonnier and Perez have recently recorded that they
saw an enormous profusion of a large Pyrosoma (up to four
metres in length) in the Arabian part of the Indian Ocean.

Order III. Thaliacea (Salpians).

Free-swimming pelagic forms of moderate size, w^hich may be
either simple or compound, and in which the adult is never
provided with a tail or notochord. Consequently the whole body
here corresponds to the trunk only of the Appendicularian with-
out the tail. The test is permanent, and may be either well
developed or very slight. In all cases it is clear and trans-
parent. The musculature of the body -wall is in the form
of more or less complete circular bands, by the contraction of
which water is ejected from the body, and so locomotion is
effected. The branchial sac has either two large, or many small,
stigmata, leading to a single peribranchial cavity, into which
the anus also opens. Blastogenesis takes place from a ventral,
endostylar stolon. Alternation of generations occurs in the
life-history, and may be complicated by polymorphism. The
Order Thaliacea comprises two groups, Cyclomyaria (such as
Doliolum) and Hemimyaria (such as Salpa).

Sub-Order 1. Cyclomyaria.

Free-swimming pelagic forms wliich exhibit alternation of
generations in their life-history, but never form permanent
colonies. The body is cask-shaped, with the brancliial and atrial
apertures at the opposite ends. The test is moderately well
developed, never much thickened. The musculature is mostly in
the form of complete circular bands surrounding the body. The
branchial sac is fairly large, occupying the anterior half or more
of the body. Stigmata are usually present in its posterior part
only. The peribranchial cavity is mainly posterior to the
branchial sac. The alimentary canal is placed ventrally, close to

96

TUNICATA SALPIANS

the posterior end of the branchial sac. Hermaphrodite repro-
ductive organs lie ventrally near the intestine.

This group is clearly distinguished from the second sub-order,
the Hemimyaria, by the condition of the muscle-bands and of the
branchial sac, and by the life-history. The muscle-bands are
complete rings (except in Arichinia), while in the Hemimyaria
they are always more or less incomplete. The branchial sac
in the Cyclomyaria is a distinct cavity, and communicates with
the peribranchial cavity only by small slits or stigmata. The
life-history is also very characteristic, as the sexual generation in
the Cyclomyaria is always polymorphic, while in the Hemimyaria
it consists of one form only.

Structure of Doliolum. — The single family Doliolidae in-
cludes three genera, Doliolum, Quoy and Gaimard, Dolchinia,
Korotneff. and AncJiinia, Eschscholtz. Doliolum, of which about

a dozen species
pbr are known, from

various seas, has
a cask - shaped
body (Fig. 59),
usually from 1
to 2 cm. in
length. The ter-
minal branchial
and atrial aper-
tures are lobed,
and the lobes are
with

br.I

Fig. 59. — Sexual generation of Dolioluni tritonis, Herdnian,
from left side, x 10. at. Atrial aperture ; at.l, atrial
lobes ; at.m, wall of atrium ; br, branchial aperture ; br.I, provided
branchial lobes ; br.s, branchial sac ; d.t, dorsal tubercle ; _ ™r^

end, endostyle ; h, heart ; i, intestine ; m, mantle ; m^-m?, SenSC-OrganS. inc
circular muscle-bands ; m, nerve ; n.g, nerve-ganglion ; ov, {^ggl^ jg g, thin but
ovary ; p.hr, peribranchial cavity ; p.p, peripharyngeal ■■

bands ; sg, stigmata ; s.gr/, neural gland ; s.o, sense-organ ; tOUgll transparent
st, stomach ; t, test ; tes, testis ; z, prebranchial zone.
(After Herdman.)

and

layer, ana con-
tains no " test "
cells. It is merely a cuticle covering the surface of the squamous
ectoderm. The body-wall has eight or nine circular muscle-bands
surrounding the body. The most anterior and posterior of these
form the branchial and atrial sphincters. The wide branchial and
atrial apertures lead respectively into branchial and peribranchial
cavities separated by the posterior and postero-lateral walls of
the branchial sac which are pierced by a considerable number of

Ill DOLIOLUM — STRUCTURE 97

small stigmata ; consequently there is a free passage for the
water through the body along its long axis, and the animal
swims by contracting its ring-like muscle-bands so as to force out
the contained water posteriorly. When stigmata are found on
the lateral walls of the branchial sac (see Fig. 59) there are
corresponding anteriorly directed diverticula of the peribranchial
cavity. There is a distinct endostyle on the ventral edge of the
branchial sac and a peripharyngeal band surrounding its anterior
end, but there is no representative of the dorsal lamina along its
dorsal edge ; and there are neither branchial nor atrial tentacles.
The oesophagus commences rather on the ventral edge of the
posterior end of the branchial sac, and runs backwards to open
into the stomach, which is followed by a curved intestine opening
into the peribranchial cavity. The alimentary canal as a whole
is to the right of the middle line. The hermaphrodite repro-
ductive organs are to the left of the middle line alongside the
alimentary canal. They open into the peribranchial cavity.
The ovary is nearly spherical, while the testis is elongated, and
may be continiied anteriorly for a long distance. The heart is
placed in the middle line ventrally, between the posterior end of
the endostyle and the oesophageal aperture. The nerve-ganglion
lies about the middle of the dorsal edge of the body, and gives
off many nerves. Under it is placed the neural gland, the
duct of which runs forward and opens into the anterior end of
the branchial sac by a simple aperture surrounded by the spirally
twisted dorsal ends of the peripharyngeal bands.

Life - History. — The ova produced by the Doliolum of the
sexual generation, after a complete or " holoblastic " segmenta-
tion, and normal invagination, produce tailed larvae with a
relatively small caudal appendage, and a large body in which
the characteristic musculature begins to appear (Fig. 60, A).
These larvae after metamorphosis lose their tails and develop
into oozooids, known as " nurses," which are asexual, and are
characterised (Fig. 60, B) by the possession of nine muscle-bands,
by the stigmata being few in number and confined to the posterior
end of the branchial sac, by an otocyst on the left side of the body,
by a ventrally-placed complex stolon or " rosette organ " near the
heart, from which primary buds are produced by constriction,
and by a dorsal outgrowth (" the cadophore") near the posterior end
of the body. The buds (blastozooids) give rise eventually, after

VOL. VII II

98

TUNICATA SALPIANS

further division, to the sexual generation, which is polymorphic
— having tliree distinct forms, in two of which the repro-
ductive organs remain undeveloped.

The primary buds are constricted off while still very young
and undeveloped (Fig. 60, D, B, and E) ; they migrate from their
place of origin on the stolon, over the surface (aided by large
amoeboid test-cells which become attached to the buds) (Fig. 60,
B), multiply by fission, and become attached (again by the help
of amoeboid test-cells and ectoderm cells which form a slight

FiQ. 60. — Life -history of Dolioluni. A, tailed larval stage; B, "nurse" or oozooid,
showing buds (blastozooids) migrating from the ventral stolon to the dorsal process ;
C, posterior jiart of much later oozooid to show buds arranged in three rows on
dorsal process ; D, stolon segmenting ; E, young migrating bud ; F, trophozooid
developed from one of the buds of a lateral row. At, Atrial aperture ; b, buds ; Br,
branchial aperture ; cl, cloaca ; d.p, dorsal process ; end, endostyle ; lit, heart ; Lb,
lateral buds; m.b, median buds; n.g, nerve - ganglion ; ot, otocyst ; jj.c, peri-
cardium ; sk, stalk ; sto, stolon. (After Uljanin and Barrois.)

" placenta ") in three rows — a median and two lateral — to
the dorsal outgrowth (Fig. 60, C) of the body of the nurse.
This parent - form by this time has become greatly modified,
and its structure is largely sacrificed for the good of the buds or
growing zooids, for which it really forms a locomotory organ.
Its muscle-bands become greatly developed in width (Fig. 60,
C), and the branchial meshwork, endostyle, and alimentary canal
disappear.

The three forms produced in the second generation are as
follows : — (1) Nutritive forms (" trophozooids ") derived from the
lateral rows of buds, which remain permanently attached to the

Ill DOLIOLUM — LIFE-HISTORY 99

oozooid, and are sacrificed for the benefit of the rest of the
colony. They serve merely to aid in respiration, and to provide
the food for the nurse and the median buds. Their development
is arrested ; they have the body elongated dorso-ventrally with a
large funnel - like branchial apertm-e (Fig. 60, F), and the
musculature is very slightly developed.

(2) Some of the median buds become foster forms (" phoro-
zooids "), which, like the preceding trophozooids, do not become
sexually mature, but, unlike them, are eventually set free as
cask-shaped bodies having the Dolioluvi appearance, with eight
encircling muscle-bands, and having, moreover, a ventral out-
growth (not a stolon), which is formed of the stalk by which
the body was formerly attached to the dorsal process of the
oozooid. On this ventral outgrowth the " gonozooids " (3) are
attached while still very young buds, and after the phorozooids
are set free these reproductive forms gradually attain their com-
plete development, become sexually mature, and are eventually
separated off, finally losing all trace of their temporary connexion
with the foster-forms. They resemble the foster-forms in having
a cask-shaped body with eight muscle-bands, but differ in the
absence of a ventral process, and in having the sexual repro-
ductive organs fully developed.

Occurrence. — The best -known member of the genus is
Doliolmn tritonis, Herdman, which was captured in the tow-nets
in thousands by Sir John Murray during the cruise of H.M.S.
" Triton" in the summer of 1882 in the North Atlantic. Since
then that species, or the closely allied D. nationalis, Borgert, have
been found on more than one occasion in the English Channel
and other parts of our south-west coast, and so Doliohim may
be regarded as an occasional member of the British svirface
fauna.

It is probable that the occasional phenomenal swarms ^f
Doliolum which have been met with in summer in the North
Atlantic are a result of the curious life-history which, under
favourable circumstances, allows of a small number of oozooids
producing from minute buds an enormous number of phorozooids
and gonozooids.

As the result of the careful quantitative work of the German
" Plankton " expedition, Borgert thinks that the temperature of
the water has more to do with both the horizontal and the

lOO TUNIC AT A SALPIANS chap.

vertical distributiou of these Thaliacea in the sea than any other
factor.

Other Genera, — Anchinia, of which only one species is
known, A. rubra, Vogt, from the Mediterranean, has the
sexual forms permanently attached to portions of the dorsal
outgrowth from the body of the unknown oozooid (" nurse ").
The stolon is probably much longer than in Doliolum, and curves
round so as to reach and lie along the dorsal outgrowth, upon
which it places the buds.

The body of the adult is elongated dorso-ventrally. The
test is well developed and contains branched cells. The mus-
culature is not so well developed as in Doliolum. There are
two circular bands at the anterior end, two at the posterior, and
two muscles on the middle of the body, which unite to form the
characteristic S-shaped lateral bands. The stigmata are confined
to the obliquely-placed posterior end of the branchial sac. The
alimentary canal forms a U-shaped curve. The reproductive
organs are placed on the right side of the body. The life-history
is still imperfectly known. As in the case of Doliolum, the
sexual generation is polymorphic, and has three forms, two
of which remain in a rudimentary condition so far as the
reproductive organs are concerned. They are known as the
first and second sterile forms, or " trophozooids." In Anchinia,
however, the three forms do not occur, so far as we know,
together at the same time on the one outgrowth, but are
produced successively, or in different regions, the reproduc-
tive forms of the sexual generation being independent of the
" foster- forms." ^

The third genus, Dolchinia, contains also only a single species,
D. mirahilis, found by Korotneff ^ in the Gulf of Naples. It
must have three different forms in its life-history — oozooid,
phorozooid, and gonozooid, but the first of these is still unknown.
On what must be body processes detached from the oozooid
are found phorozooids somewhat like those of Doliolum, bearing
sexual forms attached to ventral stalks. Dolchinia is inter-
mediate on the whole between Anchinia, the most simple member
of the family, and Doliolum the most complex ; and may eventu-
ally come to be united with the latter genus.

^ See Barrois, Journ. d'Anat. et Physiol. 1885.
2 Mitth. Z. Stat. Neapel, x. 1891.

SALPIDAE 10 1

Sub-Order 2. Hemimyaria.

Free-swimming pelagic forms which exhibit alternation of
generations in their life-history, and in the sexual condition form
colonies. The body is more or less fusiform, with the long axis
antero-posterior, and the branchial and atrial apertures nearly
terminal and opposite. The test is well developed but trans-
parent. The musculature of the body-wall is in the form of
a series of transversely -running bands which do not usually form
complete independent rings as in the Cyclomyaeia. These
partially-encircling muscles in the Salpidae (see Fig. 61, m.&)
are probably to be regarded as modified branchial and atrial
sphincters which have spread over the intervening body. The
branchial and peribranchial (cloacal) cavities form a continuous
space in the interior of the body, opening externally at the
ends by the branchial and atrial apertures, and traversed
obliquely from the dorsal and anterior to the ventral and
posterior end by a long narrow vascular ciliated band, which
represents the dorsal lamina, the dorsal blood- sinus, and the
neighbouring parts of the dorsal edge of the branchial sac of an
ordinary Ascidian. The alimentary canal is placed ventrally.
It may either be stretched out so as to extend for some distance
anteriorly, or, as is more usual, be concentrated to form along
with the testis a rounded opaque mass near the posterior
end of the 'body, known as the visceral mass or " nucleus."
The embryonic development is direct, no tailed larva being
formed. The embryo is united to the parent for a time by
a " placenta."

This sub -order contains, in addition to its typical members,
the Salpidae, another still somewhat problematical family the
OcTACNEMiDAE, including a single very remarkable deep-water
genus (Octacnemus), which in some respects does not conform
with the characters given above, and exhibits a certain amount
of affinity with the primitive fixed forms from wliich Salpidae
have been derived.

Occurrence and Reproduction. — The family Salpidae^ in-
cludes the single genus Salpa, Forsk^l, which, however, may be

1 The most useful works on the Salpidae are Traustedt, Vid. Se/sk. Skr. ii. 8,
1885, Copenhagen; and Brooks's "The genus Salpa," Johns HojyJcins Biolog.
Memoirs, ii. 1893.

I02

TUNICATA — SALPIANS

divided into two well-marked groups of species — (1) those such
as S. {Cyclosal'pa) j'jmwa^fa, in which the alimentary canal is
stretched out (" ortho-enteric " condition) along the ventral surface

Fig. 61. — Salpa runci-
nata -/nsiformis. A,
aggregated or "chain "
form ; B, solitary form.
At, Atrial aperture ;
at.m, atrial muscle.s ;
Br, branchial aper-
ture ; br.m, branchial
muscle.s ; d.l, dorsal
lamina or "gill" ; d.t,
dorsal tubercle ; e)nb,
embryo ; end, endo-
style ; m, mantle ;
m.b, muscle - bands ;
n.g, nerve - ganglion ;
p.p, peripharyngeal
bands ; sf, stolon ; st",
' of buds ; (,
visceral " uu-

Gt.m .

" chain '
test ; V,
cleus."

of the body, and (2) those such as S. ruminata-f nsiformis, in
which the alimentary canal forms a compact globular mass (Fig.
61, v), the "nucleus" (" caryo- enteric " condition), near the

Fig. 62. — Diagram to .show the arrange-
ment and connexion of the aggregated
zooids in a young chain of Salps. 1, 3, 5,
zooids on the right ; 2, 4, 6, zooids on
the left. At, Atrial aperture of a zooid ;
Br, branchial aperture of another; c.t
at the top of the figure points to three
pairs of connecting tubes ; c.t at the
foot, to two pairs. Each zooid is united
to each of the four neighbours it touches
by a pair of connecting tubes, and so has
eight such tubes in all.

posterior end of the body. About fifteen species altogether are
known ; they are all pelagic in habit, and are found in nearly
all seas. Each species occurs in two forms (Fig. 61, A and B),
the solitary asexual {i^roles solitaria), and the aggregated sexual
{;proles gregaria), which are in most species quite unlike one

SALPA REPRODUCTION

103

another, the aggregated form being nsiially more rounded, ovoid,
or fusit'orni (Fig. Gl, A), and the solitary more quadrangular,
and often provided with conical processes- or projecting points.

The solitary form gives rise, by gemmation at the posterior
end of the endostyle (Fig. 63), to a complex tubular stolon, con-

end-l

lion ; 01; ovum ; s, stolon ; st, stomach ; I, II, III, groups of buds

Fig. 63. — Diagram to
show the relations
of the groups of
young Imds, when
first formed on the
stolon of Salpa. at,
Atrial aperture ; l/i;
branchial aperture ;
el, elaeoblast ; end,
endostyle ; h, heart ;
n.(/, nerve - gang-
( After Brooks.)

taining processes from the more important organs of the parent-
body, which give rise to an endodermal tvibe, two peribranchial
tubes, a neural tube, two blood-sinuses and mesoblast cells, a
genital cord, and over all the ectodermal covering (see Fig. 64).
This stolon becomes segmented (Fig. 63) into a series of buds or

Fig. 64. — Transverse
section through
endostyle and
young stolon of
iSaljM pinnata. ec,
Ectoderm of parent
reflected at ec' to
cover base of
stolon ; ec", ecto-
derm of stolon ;
end, endoderm of
stolon ; g, ovary ;
vies, mesoderm
cells ; 71, nerve-
tube of stolon ;
2i.br, peribranchial
tubes of stolon.
(After Brooks.)

7'-^rV^>^

young " chain " individuals, of which there may be several
hundreds. As the stolon elongates (Fig. 61, B, st^'), the buds

I04 TUNICATA SALPIANS chap.

undergo lateral shifting, and rotation round their longitudinal
axis, so as to acquire the relations seen in the " chain," which
then emerges from the tube in the test through which it has been
growing, so as to project to the exterior near the atrial aperture.
The buds at its free end which have now become far advanced
in their development are set free in groups, which remain attached
together by processes of the test, each enclosing a diverticulum
from the body-wall (Fig. 62), so as to form "chains." Each
member of the chain is a Salpa of the sexual or aggregated form,
and when mature may — either still attached to its neighbom's or
separated from them — produce one or several embryos (Fig. 61,
A, emh), which develop into the solitary form of Saljja. Thus
the two forms, different in appearance and structure and different
in mode of origin, alternate regularly in the life-history of Salpa.
Structure. — The more important points in the structure of a
typical Salpa are shown in Fig. 65. The branchial and atrial
apertures are at opposite ends of the body, and lead into large
cavities, the branchial and peribranchial sac respectively, which are
in free communication at the sides of the obliquely-running dorsal
lamina or " gill " (d.l). The transparent test is usually thick,
and varies from a gelatinous to a stiff cartilaginous condition ; it
adheres closely to the surface of the mantle (ectoderm and body-
wall). The muscle -bands (from 4 to about 20 — usually 8
or 10) of the mantle do not in most cases completely encircle
the body. They are present dorsally (Fig. 65, mus.hds) and
laterally, but the majority do not reach the ventral surface. In
many cases neighbouring bands join in the median dorsal line
(Fig. 61). The muscle fibres are striated, and have rows of
large equidistant nuclei. The anterior end of the dorsal lamina
is in some cases prolonged to form a prominent tentacular organ,
the languet or dorsal tentacle, projecting into the branchial sac,
while near this opens a ciliated funnel corresponding to the
dorsal tubercle, but having no connexion in the adult with
either ganglion or subneural gland. The conjoined ganglion and
subneural gland, the dorsal lamina, the peripharyngeal bands and
the endostyle are placed in the usual positions. Eyes in the
form either of a continuous horse-shoe-shaped pigmented ridge
on the dorsal surface of the ganglion immediately below the ecto-
derm, or of one larger median and several smaller lateral ocelli
are found in the various species of Salpa. These eyes have in

SALPA STRUCTURE

105

most cases a retina formed of elongated cells, and a pigment-layer
placed upon the ganglion.

The so-called otocysts of Saljja have been shown by Metcalf
to be really glandular organs. They have been called lateral
neural glands ; they do not open at the dorsal tubercle, but
separately into the pharynx. These lateral neural tubular glands
have also been regarded as nephridia.

The large spaces at the sides of the dorsal lamina (often

mus. ids

Fig. 65. — Diagrammatic sagittal section of a " chain " SaJpa. an. Anus ; at, atrial
aperture; at.m, muscles of atrial aperture; atr.cav, atrial cavity; hr, brancliial
aperture ; l/r.in, muscles of branchial aperture ; hr.s, branchial sac ; d.l, dorsal
lamina or "gill" ; d.t, dorsal tubercle ; end, endostyle ; ht, heart; int, intestine ;
I, sensory languet ; mus.hds, muscle-bands ; n.g, nerve- ganglion ; oc, eye-spot ; oe,
oesophagus ; ov, ovary ; p.p.h, peripharyngeal band ; s.gl, neural gland ; stom,
stomach ; t,t', test ; tes, testis ; z, prebranchial zone. (After Herdman.)

called the gill or branchia of Salpa), by means of which the
cavity of the branchial sac is placed in free communication with
the peribranchial cavity, are to be regarded as gigantic gill-slits
formed by the suppression of the lateral walls and small stigmata
of the branchial sac. The alimentary canal at the posterior end
of the " gill " consists of oesophagus, stomach, and intestine, with
a pair of lateral gastric glands or caeca. The^e viscera along
with the reproductive organs, when present, make up the
"nucleus" (Fig. 66, v).

Alternation of Generations. — Fig. 6 6 represents an
aggregated or sexual t:>alpa, which was once a member of a chain,
since it shows a testis and a developing embryo. The ova
(always few in number, usually only one) appear at a very early
period in the developing chain Salpa, while it is still a part of
the gemmiparous stolon in the body of the solitary Salpa. This
gave rise to the view put forward first by Brooks that the ovary

io6

TUNICATA SALPIANS

really belongs to the solitary stolon-bearing Salpa, which is
therefore a female producing a series of males by asexual
gemmation, and depositing in each of these an ovum, which will
afterwards, when fertilised, develop in the body of the male into
a solitary or female Salpa. This idea, if adopted, would pro-
foundly modify our conception of Salpa as an example of a life-
history showing alternation of generations, but it seems to me to

give a distorted view of the
sequence of events. The
fact that the stolon while
in the solitary Salpa con-
tains, along with representa-
tives of other important
systems of the body, a row
of germinal cells, does not
constitute that solitary
Salpa the parent of the ova
which these germinal cells
will afterwards become in
the body of an independent
bud. We must regard as
the parent the body in
which the ova become
mature and fulfil their func-
tion. The sexual or chain
Salpa, although really her-

jyLcnd

m.b7---ix W^&

Fig. 66. — Salpa hexagona, Q. and G. Chain
form dissected from the left side. «, Anus ;

tubercle ; emb, embryos ; end, endostyle
m.b 2, m.h 7, second and seventh muscle-
bands ; 71.IJ, nerve - ganglion ; v, visceral
" nucleus." (After Traustedt. )

at, atrial aperture ; b>; branchial aperture ; maphroditc in itS lifc-llis-
d.l, dorsal lamina ("gill"): d.f, dorsal . . n i j_

- - - . ' ^ ' . - - tory, IS usually proto-

gynous, i.e. the ova mature
at an earlier period than
the male organ or testis.
This prevents self-fertilisation. The ovum is presumably fertil-
ised by the spermatozoa of an older Salpa belonging to another
chain, and the embryo is far advanced in its development before
the testis is formed. The development takes place inside the
body of the parent, and is " direct " — no tailed larval form being
produced.

Development and Life-history. — The segmentation of the
egg is holoblastic, and gives rise to a number of blastomeres,
^ According to Metcalf, Salim cylindrica is protandrous.

SALPA LIFE-HISTORY

107

which are for a time masked by the phenomenal activity of
certain cells of extraneous origin, the "kalymmocytes," derived
fi'om the follicular epithelium surrounding the ovum. These
follicular kalymmocytes migrate into the ovum, surround groups
of blastomeres, and arrange themselves so as to reproduce the
essential structure of the future embryo for which they form
wliat may be termed a scaffolding or temporary support. After
a time the blastomeres become active, proliferate rapidly, and
finally press upon and absorb the kalymmocytes, and so eventu-
ally take their proper place in building up the organs. Some
observers regard the kalymmocytes as being passive and nutritive
only in function.

ainap bf

rect

Fig. 67. — Young solitary
Salpa democratica-mucro-
nata attached to the
parent by the placenta.
atr.ap, Atrial aperture ;
hr, dorsal lamina ; cil.gr,
dorsal tubercle; M,
elaeoblast ; end, endo-
style ; n.gn, nerve-gang-
lion ; oes, oesophagus ;
or.ap, branchial aperture ;
peric, pericardium ; pi,
placenta ; rect, intestine ;
s<oZ, stolon ; stovi, stomach.
(From Parker and Has-
well, after Salensky.)

At an early period in the development a part of the surface
of the embryo, on its ventral edge, becomes separated off, along
with a part of the wall of the cavity (" oviduct " — a diverticulum
from the atrium) in which it lies, to form the " placenta " (Fig.
67, pi) in which the embryonic and maternal blood-streams
circulate in close proximity, and so allow of the conveyance of
nutriment to the developing embryo by means of large migrating
placental cells. At a somewhat later stage a number of cells
placed at the posterior end of the body alongside the future
nucleus become filled up with oil-globules to form a mass of
nutrient material — the "elaeoblast" (Fig. 67, ell) — which is
used up later in the development. Many suggestions have
been made as to the homology and meaning of the elaeoblast ;
but it may now be regarded as most probable that it is reserve
food-material associated with the disappearing rudiment of the

I08 TUNICATA SALPIANS chap.

tail found in the larval condition of most Ascidians. The develop-
ment is direct ; and it may be said, then, that this young asexual
(solitary) Sal^oa differs from the corresponding form in the life-
history of Doliolum (Fig. 60, A) in that its tail is no longer a
locomotory organ, but is represented by a nutritive mass, the
elaeoblast, while the body, in place of being free, is attached by its
ventral surface to a special organ of nutrition — the " placenta "
— in connexion with the blood-stream of the parent.

This embryo sexually produced inside the body of an aggre-
gated form becomes a solitary Salpa (such as Fig. 61, B), which
differs in appearance, structure, and habits from its parent, and
has no reproductive organs. After swimming for a time, how-
ever, it develops the ventral stolon on which buds form which are
eventually sexual Salpae. These are set free from the solitary
form in sets, still connected together, and they may swim about
together for a time as a chain of aggregated Salpae before separating
to become the adult sexual individuals (such as Fig. 61, A).

Classification. — Salpa may be divided into the following sub-
genera : ^ — Cyclosalpa, Blainville, in which the alimentary canal is
ortho-enteric, and the " chain " consists of individuals united in a
circle ; lasis, Savigny, with several embryos formed at a time ; and
Pegea, Sav., Thalia, Blumeubach, and Salpa, Forskal, all with
one embryo only, and differing from one another in the condition
of the " gill " and other details : all except Cyclosalpa have the
alimentary canal caryo-euteric. Cyclosalpa has three species, the
best known of which is C. pinnata of the Mediterranean, a form
possessing light-producing organs like those of Fyrosoma, but
placed along the sides of the body. Salpa has four or five species,
one of which, S. runcinata-fusiformis (Fig. 61), has occasionally
been found in British seas ; Thalia includes the species T.
democrcUica-mucronata, which has been sometimes obtained in
swarms in the Hebridean seas, or cast ashore on our southern or
western coasts ; Pegea has the species P. scutigera-confoederata ;
and lasis contains the remaining half-dozen species, the best
known of which is /. cordiformis-zonaria, the only other Salpian
which has been found in British seas.

The family Octacnemidae includes the single remarkable

' For a more detailed accoiuit of these subdivisions of the Salpidae, and other
groups, see Herdraan's "Revised Classification of Tunicata, " J'oMni. Linn. Soc,
Zool., x.xiii. 1891, p. 558.

OCTACNEMUS

109

genus Octacnemiis, now known in a solitary and an aggregated
form. It was found during the " Challenger " expedition, and was
first described by Moseley. It is apparently a deep-sea repre-
sentative of the pelagic Salpidae, and may possil)ly be fixed at
the bottom. The body in the solitary form is somewhat discoid,
with its margin prolonged to form eight tapering processes, on to
which the muscle-bands of the mantle are continued. The
alimentary canal forms a compact nucleus, whicli is attached to
an apparently imperforate membrane which stretches across the
body, separating the branchial from the atrial cavities. The

end br'

FlQ. 68, — A, solitary form of Octacnemus h/thius (after Moseley) ; B, diagram of struc-
ture of Octacnemus (after Herdmaii) ; C, aggregated form of 0. patagoniensis (after
Metcalf). 1, from outside ; 2, with test removed ; and 3, with mantle removed.
a, Anus ; adh, area of attachment ; at, atrial, and hr, branchial aperture ; br.s,
branchial sac; eiic?, endostyle ; g.s, gill - slits ; i, intestine; n.y, nerve-ganglion;
oe, oesophagus ; ov, ovary ; p.br, peribranchial cavity ; st, stomach ; stol, stolon.

endostyle is very short, and the dorsal lamina is also much
reduced. The reproduction and life-history are entirely unknown.
The aggregated form consists of a small number of individuals
united by a slender cord composed of test, body -wall, and endo-
dermal tissue. Octacnemus has been found ^ in the South Pacific
from depths of 1070 and 2160 fathoms, and off the Patagonian
coast from 1050 fathoms. Two species have been described : 0.
hythius, Moseley, and 0. patagoniensis, Metcalf Metcalf, who
has recently investigated the aggregated form {0. patagoniensis),
considers that the genus is more nearly related to the Clavelinidae
than to the Salpidae. Possibly its position might be best

^ See Herdman, Challenger Report on Tunicata, part iii. 1888, p. 88 ; and Met-
calf, Johns Hopkins Univ. Circ. No. 106, 1893, and Zool. Jahrb. Abih. Anat.
xiii. 1900, p. 572.

IIO TUNICATA CHAP.

indicated by a line diverging from near the point (3) in the
phylogenetic diagram below.

General Conclusions.

The following diagram is a graphic representation of the
genetic aftiuities, or what is now generally supposed to have been
the probable course of phylogeny of the Tunicata. It will be
noticed that it shows (1) the Proto-Tunicates arising from Proto-
Chordata, not far from the ancestors of Amphioxus (see also, this
vol. p. 112); (2) that the Larvacea are regarded as the most

Molgulidae
Ascidiidae I
Amphioxus (5) \ | Cynthiidae.

Larvacea.

(1) (2) I

Proto-Tunicata

Doliolidae — :
Salpidae

(31

(6) Polystyelidae
Botryllidae
Distomatidae

j Dideranidae and
/ ,, j Diplosomatidae

1 ^ irolycliuidae

Pyrosomatidae

primitive section of the group; (3) that the Thaliacea (Dolio-
lidae and Salpidae) are supposed to be derived not directly from
primitive pelagic forms, but through the early fixed Ascidians,
not far from (4) the ancestral compound Ascidians, which gave
rise to the Pyrosomatidae; (5) that the Ascidiidae and other
higher Simple Ascidians are derived, like the Compound Ascidians,
from ancestral Clavelinidae; and (6), that the Ascidiae Compositae
are polyphyletic, the Holosomata (Botryllidae and Polystyelidae)
being derived from ancestral Simple Ascidians independently
of the Merosomatous families.

The Tunicata are remarkable for the variety in appearance,
structure, and life-history which they present. Ko group illus-
trates in a more instructive manner so large a number of
important biological principles and phenomena. They show
solitary and colonial forms, fixed and free, pelagic and abyssal.
The development is in some cases larval and with metamorphosis,
in others abbreviated and direct. Persistent traces of ancestral
characters are seen in the embryonic and larval stages, while the
adults present the most varied secondary adaptations to littoral,

CONCLUSIONS III

pelagic, and deep-sea, free-swimming and sessile modes of exist-
ence. In the details of their classitication they demonstrate Loth
stable and variable species, monophyletic and polyphyletic groups.
They exhibit the phenomena of gemmation and of embryonic
fission, of polymorphism, hibernation, alternation of generations,
and cliange of function. They have long been known as a stock
example of degeneration ; but in fact they lend themselves
admirably to the exposition of more than one " Chapter of
Darwinism."

Note to P. 78. — Oligotrema, Bourne {Quart. J. Micr. Sci. xlvii. Pt. ii.
1903, p. 233), a Molgulid from the Loyalty Islands, has a reduced branchial
sac and greatly developed pinnate, muscular branchial lobes, probably used
in cajituring food.

CHAPTER IV

CEPHALOCHORDATA

INTEODUCTION GENERAL CHARACTERS ANATOMY OF AMPHIOXUS

EMBRYOLOGY AND LIFE- HISTORY CLASSIFICATION OF

CEPHALOCHORDATA SPECIES AND DISTRIBUTION

The CEPHALOCHORDATA Comprise only a small group of little
fish - like forms, the Lancelets, usually known as " Amphi-
oxus," and referable to about a dozen species arranged in
several closely allied genera under the single family Branchiosto-
matidae. The best known form is Branchiostoma lanceolatum
(Pallas), the common Amphioxus or Lancelet, which has been
found in British seas, and even as far north as the coast of
Norway, but is much more common in warmer waters, such as the
Mediterranean, and is also found in the Indian Ocean. It is
abundant in the Bay of Naples, and lives and breeds in great
numbers in a salt lagoon, the " Pantano," near Messina, and from
these localities most of the specimens have been obtained for the
numerous recent researches upon its structure and development.

Amphioxus was first discovered and described (1778) by
Pallas, who regarded it as a Mollusc,, aud named it Limax
lanceolatus. It was first correctly diagnosed as a low Vertebrate,
and named Branchiostoma, by Costa, in 1834. The term
Amj)hioxus, under which it has become -so well known, was
applied to it a couple of years later by Yarrell.

The anatomy was for the first time fully investigated by
Johannes Miiller in 1841, and this important memoir has been
supplemented in regard to special systems and histological
details by numerous papers by many leading zoologists, such as
those by Huxley in 1874, Langerhans in 1876, Laukester in

CHAP. IV AMPHIOXUS HISTORY I I 3

1875 and in 1889, Eetziiis in 1890, and Boveri and Hatschek,
both in 1892. Important papers on special points have also
been written by Eolph, Eohde, Benliani, Andrews, Goodrich, and
others. The development was first elucidated by Kowalevsky in
1867, at about the same time when he studied the development
of the Ascidians, and later again in 1877. Further papers on
the development and metamorphosis we owe to Hatschek in
1881, Lankester and Willey in 1890 and 1891, Wilson in 1893,
and quite recently to MacBride. Dr. Willey 's book, AmpMoxus
and the Ancestry of the Vertehrata (1894), contains a summary
of investigations on structure and development, an interesting
discussion of the relations of Amphioxus to the otlier Chordata,
and a full bibliography.

In addition to such original researches, Amphioxus is
studied in more or less detail every year by countless senior and
junior students in zoological laboratories and marine stations
throughout the civilised world. The value of this primitive
form as an object of biological education depends upon the fact
that it shows the essential Vertebrate characters, and their
mode of formation, in a very simple and instructive condition.
Although no doubt somewhat modified, and possibly degenerate
in some details of structure, in its general morphology it
presents us with a persistent type probably not far removed
from the ancestral line of early Chordata. There are no sufficient
grounds for the view that Amphioxus is a very degenerate re-
presentative of fish-like Yertebrata.

General Characters. — The Cephalochordata (or Acrania, in
contradistinction to the Craniata or Vertehrata) are marine,
non- colonial Chordata, in which the notochord extends the
entire length of the body, running forward into the snout beyond
the nervous system. There is no skull, and the notochord is
not surrounded by any vertebral column. There are no limbs
nor paired fins. There is no exoskeleton, and the ectoderm is
a single layer of non -ciliated columnar cells. The mouth
is ventral and anterior, tlie anus is ventral, posterior, and
asymmetrically placed on the left side. The pharynx is a large
branchial sac, having its sides perforated by many gill-slits, and
is surrounded by an ectodermal enclosure, the atrium, which
opens to the exterior by a median ventral atriopore. The
stomach gives off a simple saccular pouch, the liver, which has
VOL. VII I

114 CEPHALOCHORDATA

connected with it a simple hepatic portal blood system. There
is a respiratory circulation, the contractile ventral vessel which
represents the heart sending the colourless blood forward to the
respiratory pharynx to be purified. Tlie body-wall is segmented
into over fifty myotomes. There are numerous separate nephridia
which develop from the mesoderm and open into the atrium.
The brain remains undeveloped, being scarcely distinct from the
spinal cord. There are two pairs of cerebral nerves, and many
spinal, in which the dorsal and ventral roots or nerves do not
unite. The sense-organs are simple ; there are no paired eyes
and no auditory organs. The sexes are separate ; the gonads are
metamerically arranged on the body-wall, and have no ducts :
they burst into the atrium. In tlie development the segmenta-
tion is complete, a gastrula is formed by invagination, the
nervous system is formed from the dorsal epiblast, the noto-
chord from the hypoblast, and the mesoderm arises from meta-
meric coelomic pouches. The body -cavity is an enterocoele.
The gill-slits are at first perforations of the body-wall opening
from the pharynx to the exterior, which later become enclosed
l>y the development of the atrium.

Anatomy.

External Characters. — Amphioxus^ is about 1|- to 2^
inches in length, slender, somewbat translucent, and pointed at
both ends (Fig. 69). It lives in shallow water and burrows in
the sand, head first, with great rapidity. It frequently remains
with the anterior end protruding from the sand. When on the
surface it lies on one side. It is said to swim freely at night.
The head end is rather the tliicker, and the anterior two-thirds
of the ventral surface are flattened (Fig. 70, A), and may be
slightly ridged longitudinally. The lateral edges of this flat
area project as metapleural folds (Fig. 70, mt.pl), which begin
anteriorly at tlie edges of the external mouth, and die away in
tbe middle line posteriorly behind a median opening, the
atriopore (Fig. 70, atrj/). From this point a ventral median'
fin (veyit.f) extends backwards around the pointed posterior end

' Althougli the correct systematic name of the commonest species is Branchio-
stoma lanceolatum (Pallas), it is convenient in non- systematic usage to employ
the term " Amphioxus," which is iu general use in zoological laboratories.

EXTERNAL CHARACTERS

115

(caudal fin, cd.f), and then forwards along the upper surface
(dorsal fin, dors.f) to the anterior end of the body. These fins

'^i.vi:--.,.

Fig. 69. — Amphioxus (Branchiostoma lanceolaiivm) in the Pantano at Messina.
(After Willey. )

thus constitute a continuous median fold around a great part of
the animal (Fig. 70, B, and Fig. 71).

orhd.

jfon

mtpl

atrp

Fic. 70. — Branchiostoma lanceolatnm. A, ventral; B, side view ol' the entire .aiiimaL
an, Anus ; atrp, atriopore ; cd.f, caudal fin ; cir, cirri ; dors.f, dorsal fin ; dors.f.r,
dorsal fin -rays; gon, gonads; vitj)!, nietapleure ; myom, myomeres; nch, noto-
chord ; or.hd. oral hood; vent.f, ventral fin; ventf.r, ventral fin -rays. (After
Kirkaldy.)

The surface is soft all over, there being no exoskeleton.
The epidermis or ectoderm is formed by a single layer of

ii6

CEPHALOCHORDATA

epithelial cells (see Fig. 72, p. 118), some of which bear sensory
processes, while others have a striated cuticular border. There
is no general ciliation of the surface in the adult.

The true mouth is a small pore at the bottom of a large
vestibule (the stomodaeum), placed at the anterior end of the

ccnt.c

Fig. 71. — Diagram of the anatomy of Amphioxus. A, anterior; B, posterior part.
an. Anus ; atr, atrium ; atr , its posterior prolongation ; uh-p, atriopore ; hr, brain ;
ir.d, branchial clefts ; hr.f, browu funnel ; hr.scp.X, primary, hr.sep.'I, secondary
branchial lamella ; br.r.l, primary, br.r.2, secondary branchial rod ; cand.f, caudal
fin ; cent.c, central canal ; cir, cirri ; coel, coelom ; dors./, dorsal fin ; durs./.r, dorsal
fin-ray ; en.coe, cerebral vesicle ; e.sp, eye-spot ; gon, gonad : int, intestine ; Ir,
liver ; vith, mouth ; myom, myotomes ; nch, notochord ; rqih, nepbridia ; ol/.p.
olfactory pit ; or.f.hd, oral hood ; ph, pharynx ; sl% skeleton of oral hood and cirri
(dotted) ; sp.cd, spinal cord ; vent.f, ventral fin ; vent.f.r, ventral fin-ray ; rl, velum ;
vl.t, velar tentacles. (From Parker and Haswell.)

ventral surface (Figs. 70 and 71), and formed by the " oral hood,"
which may be a prolongation forwards of the atrial or meta-
pleural folds at each side. The edges of the oral hood bear 12 to
20 pairs of cirri (Fig. 70, cir) or ciliated tentacles (strengthened
by skeletal rods), which form a sensory fringe around the open-
ing. The anus (Figs. 70 and 71, an), is asymmetrical, being

STRUCTURE 11/

placed on the left side of the ventral fin, some distance behind
the atriopore, and not far from the posterior end of the body.
Tlie short region behind the anus and surrounded l)y the caudal
fin may properly be called " tail." The current of water for
respiratory and nutritive purposes, and which may carry the ova
and spermatozoa to the exterior, usually passes in at the mouth
and out at the atriopore, as in the Tunicata. On occasions,
however, it is said to be reversed.

General Structure. — The general plan of organisation of
the body (see Yig. 71) is that a longitudinal skeletal axis, the
notocliord (nch), separates a dorsal nervous system {sp.rd) from a
ventral reduced coelom (coel), in which lie the alimentary canal
{int), the gonads {gon), and other organs. Thus a transverse
section of the body (see Fig. 72) shows the typical Chordate
arrangement of parts, and is comparable with a transverse section
of a tadpole, a young fish, or a larval Ascidian. A peribranchial
{citr) or atrial cavity (which is morphologically a part of the
external world shut in) lies external to the coelom and body-
wall around the pharynx and the greater part of the alimentary
canal, and opens to the exterior by the atriopore. As in the
Tunicata, the perforations (gill-slits) in the wall of the pharynx
{hr.cl) open into the atrial cavity and so indirectly to the
exterior.

Musculature. — The thick body -wall is largely formed by
muscular tissue metamerically segmented into about 60 myotomes
(Fig. 71, myom). These muscle-masses, which (as is usual in
Vertebrata) are thickest dorsally at the sides of the notochord
and spinal chord (Fig. 72, m), are so arranged as to present the
appearance in a lateral view of the body of a series of shallow
cones (« ) fitting into one another and with their apices
directed forwards. The muscle fibres are striated, and run longi-
tudinally along the body from the anterior to the posterior edge
of each myotome, so as to be attached at their ends to the two
septa of connective tissue which form the boundaries of the
myotomes. These septa, the myocommas, are conspicuous features
in the external appearance of the l)ody (Fig. 70, B). They aTe
not arranged so as to be opposite one another on the two sides,
but the myotomes on the right and left sides alternate, as can be
seen in a transverse section (Fig. 74, A, p. 121). It is by
means of these lateral muscle-bundles that the rapid vibration

ii8

CEPHALOCHORDATA

or alternate bending of the body from side to side in swimming
or burrowing can be performed. There are usually, on each side,

Fig. 72. — BrancJtiostoma lanceolatum. Diagrammatic transverse section of tlie pharyn-
geal region, passing on the right through a primary, on the left through a secondary
branchial lamella, ao. Dorsal aorta ; c, dermis ; ec, endostylar portion of coelom :
f, fascia, or investing layer of myotome ; fh, compartment containing fin-ray ; g,
gonad ; gl, glomerulus ; k, branchial artery ; kd, pharynx ; Id, combined atrial and
coelomic wall (ligamentum denticulatum) ; //;, myotome ; mi, transverse muscle ;
n, nephridium ; n.ch, notochord ; of, metapleural lympli space ; ^A atrium ; sc,
coelom ; si, ventral aorta ; sk, sheath of notochord and spinal cord {sp.cd) ; ;//'.
spaces in ventral wall. (From Korsehelt and Heider, after Boveri and Hatschek.)

35 myotomes in front of the atriopore, 14 between the atriopore
and the anus, and 11 postanal, making 60 in all: some species
have only about 50 myotomes, and some as many as 85. (See

MUSCLES — SKELETON

119

Classification, p. 137, where a list of the species with the number
of myotomes in each is given.)

There are also transverse muscles (Fig. 72, wt) extending
across the ventral surface in tlie region of the body enclosed by
the metapleural folds, and serving to compress the atrial cavity,
and so aid in the expulsion of its contents.

Outside the muscular layer of the liody-wall the thin in-
tegument is formed of a dermal layer of soft connective tissue,
covered by the epidermis, a single layer of columnar cells, many
of which, especially on the oral cirri, have sensory bristles.

Skeleton. — The endoskeleton consists of the notochord and
some tracts of modified connective tissue which support various
parts of the body.

The notochord of this animal is noteworthy amongst Chordata
for extending practically the entire length of the body, including
the head, from snout to tip of tail (Fig. 71). It lies in the
median plane, but nearer the dorsal than the ventral surface
(Fig. 72), and has the myotomes at its sides, the nervous system
above and the alimentary canal below. It is elliptical in
section, and tapers to the two ends. The nuclei of the original
notochordal cells are displaced to the dorsal and ventral edges,
and the greater parts of the

cells, in the adult, are occu- ^i'"^7ir~^^^TTW^?1^rrT( 7F^/^
pied by large vacuoles filled
with a fluid secretion, so as
to form by their distended
condition a stiff elastic struc-
ture. This state of the cells,
and the appearance it gives
rise to (Fig. 73), seen best
in young specimens, is very
characteristic of notochordal
tissue. Around the notochord lies a sheath of connective tissue
which is continuous with the similar sheath around the nervous
system and with the septa between the myotomes.

In addition to these skeletal layers of connective tissue there
is a cartilage-like tract in the oral hood. This is jointed, or
made up of separate rod-like pieces, one at the base of each
cirrus, into which it sends a prolongation (Fig. 71, sk). The
dorsal and ventral fins are supported by single and double rows

Fig. 7-3. — Median sagittal section of notochord
of an Amphioxus of 32 mm.

I20 CEPHALOCHORDATA chap.

respectively of what have been called " fin-rays." They are short
rods of gelatinous connective tissue, each enclosed in a lymph
space. Finally, the bars constituting the walls of the pharynx
between the gill-slits contain slender skeletal rods which run
obliquely dorso-ventrally, and are of a stiff, gelatinous nature (see
Fig. 75, p. 122). This skeletal connective tissue consists in all
cases of a fibrous deposit or matrix produced by the layer of
epithelium (ectodermal, endodermal, or mesodermal) which adjoins
the tissue.

Alimentary Canal. — This has, as its most noteworthy feature,
the Chordate characteristic that the pharynx gives rise to the
respiratory organ (see Figs. 71 and 74, A) ; and in size and pro-
minence, both in side view and in sections, the modified pharynx
of Amphioxus is fairly comparable with the branchial sac
(pharynx) of many Tunicata (see Fig. 23, p. 51), and might be
called by the same name.

The small primitive mouth, at the bottom of the cavity
bounded by the oral hood (stomodaeum), has a membranous
border, the velum (Fig. 71, vl), the edges of which are prolonged
into a circle of 10 or 12 (up to 16 in some species) simple oral
tentacles turned inwards towards the pharynx (compare tentacles
of Ascidians, p. 45).

The pharynx, by far the largest part of the alimentary canal,
and extending nearly half-way along the body, is more important
as a respiratory than as a nutritive organ. Its walls over nearly
the whole extent are perforated by a large, and indefinite,
number (100 or more on each side) of gill-slits which run on
the whole dorso-ventrally, but in the contracted condition seen
in preserved specimens have their lower ends directed obliquely
backwards, so that a vertical transverse section may cut through
a number of such slits and the intervening branchial bars (Fig.
74, A, kh). These bars, and therefore the slits between them,
are of two orders, primary and secondary, the latter being devel-
oped later in larval life as downgrowths or " tongue-bars," one
from the top of each primary gill-slit, so as to divide it into
two secondaries. The primary and the secondary (or tongue-)
bars can be distinguished from one another by their structure in
the adult animal (Fig. 75, A and B).

It must be rememljered that tliese branchial bars, or septa
between the gill-slits, are not merely portions of the wall of the

PHARYNX AND GILL-CLEFTS

12 I

pharynx, but are iu a sense portions of the hody-wall as well,
and correspond in nature, though not in number, to the visceral
arches in a Vertebrate lying between the visceral clefts whicli
open on the exterior. In the adult Ani])hioxus the clefts in
the wall of the pharynx do not open directly to the exterior,
but into the peribranchial cavity or atrium, which, however, is
onlv formed at a late larval period as an invagination or enclosure

CO '^^om-

cc&Z ^,

s.inl

Fig. 74. — Branchiostoma lanceolatuin. A, transverse section of the pharyngeal region.
a. Dorsal aorta ; b, atrium ; c, uotochord ; co, coelom ; e, endostyle ; g, gonad
(ovary) ; kb, branchial septa ; kd, pharynx ; I, liver ; my, myotome ; n, neph-
ridium ; ?•, spinal cord ; sn, sn, dorsal and ventral spinal nerves. B, Transverse
section of the intestinal region, atr, Atrium ; coel, coelom ; d.ao, dorsal aorta ;
int, intestine ; myom, myotome ; nch, notochord ; wew, spinal cord ; s.int.v, sub-
intestinal vein. (From Parker and Haswell's Zoology. A. From Hertwig, after
Lankester and Boveri ; B, partly after Rolph.)

of ectoderm. Previous to that the first formed gill-slits opened
to the exterior in Amphioxus (see larva, Fig. 8G, p. 134), just as
they do in a fish or a young tadpole. The atrial cavity is there-
fore, from its origin, lined by ectoderm, and the outer surface of
a branchial bar is virtually a part of the outer surface of the
body. It is only natural then to find that each bar contains a
small section of the coelom in its interior, communicating dorsally
and ventrally with other parts of that cavity (see Figs. 75 and
76). There are also blood-vessels which run in the branchial
bars and their junctions. The greater part of the epithelium
covering a branchial bar is pharyngeal epithelium or endoderm

12 2

CEPHALOCHORDATA

(Fif. 75, Jir.cji), but the external, wider, non-ciliated cells (Fig.
75, at.ep) are ectodermal cells lining the atrium. The gelatinous
skeletal rods in the primary bars are forked ventrally, while those
in the secondary bars are simyjle ; and there are other points of de-
tail in which the two kinds (if bar differ. These bars are obviously
more numerous in the adult than the myotomes, l)ut in the
young larva the first formed gill-clefts are metamerically arranged,

and then later they
increase greatly in
number. It is the
cilia covering the
pharyngeal epithe-
lium on the branchial
bars, possibly aided
l)y the ciliated tracts
of the oral hood,
which cause the cur-
rent of water already
alluded to.

Transverse bran-
chial junctions (syn-
apticula) run across
the l)ranchial bars,
connecting them at

B

Fly. 75. — Transverse sections tlirongh primary (A) and frequent intervals,

secondary (B) branchial bars of Amphioxus. at.ep, r^^-^J these tranSVerSC
Atrial eiiitheliuni ; bl.s, blood spaces or "vessels";

hr.ep, branchial epithelium ; rod, coelomic cavity in Connexions, like the

primary bar; sk, skeletal rods.' (From Willey, after b^-anchial barS, are
Benliam. ) '

supported by skeletal
rods. Along the ventral median line of the pharynx runs a
groove, the endostyle or hypopharyngeal groove, comparable
with the similar structure in the branchial sac of Tunicata.
This longitudinal groove (Fig. 76, ///) is lined by ciliated epithe-
lium containing four tracts of gland cells (compare endostyle
in Ascidians, Fig. 20, p. 4G). There is reason to believe
that this organ is the homologue of the thyroid gland of Verte-
brata. As in the case of Tunicata the endostyle secretes mucus,
which is carried forwards by the cilia to constitute a train with
entangled food particles which pass back dorsally to the stomach.
At the anterior end the ciliated lips of the endostyle diverge to

ALIMENTARY CANAL — COELOM

123

the right and left to encircle the front of the pharynx as the
peripharyngeal hands. These nnite again dorsally to form the
epipharyngeal (or hyperpharyngeal) groove whicli leads backwards,
corresponding to the hypopharyngeal groove below (see Fig. 74, .V),
till the posterior end of the pharynx is reached.

The remainder of the simple alimentary canal is straight, and
is scarcely differentiated into regions. A slight narrowing of the
tube behind the pharynx has been called the oesophagus, and a

Fig. 76. — Transverse sec-
tion of tlie ventral
part of tlie pharynx
of Ainphioxus. c,
Coelom ; e, endostyle ;
gl, eudostj'lar glands ;
m.b.a, median hran-
cliial artery ; p.b,
primary bar ; sk, en-
dostylar and branchial
rods and skeletal
plates ; t.h, tongue-
bar. (After Lankes-
ter.)

slight enlargement which follows, the stomach. From this point
the intestine tapers backwards to the anus (Fig. 71, p. 116). The
ventral edge of the stomach gives off a blind pouch, the hepatic
caecum or saccular liver, which runs forwards on the right-hand
side of the pharynx TFig. 74, A, I). This is a digestive gland, is
lined with glandular epithelium, and apparently corresponds with
the liver of Yertebrata. There are no other digestive glands in
connexion with the alimentary canal of Amphioxus.

Coelom. — In the young larva there are at first (as in Balano-
glossus) five coelomic spaces, a median anterior " head-cavity," a
pair of antero-lateral " collar-cavities," and a pair of more posterior
long lateral grooves from which arise, in the later larva, the seg-
mented myotomes and ventrally a large coelomic space surrounding
the alimentary canal and separating it frr)m the body- wall. In the
adult animal, however, the coelom has been so much displaced by
the formation of the spacious atrium that in front of the atriopore
it can only be recognised as a series of canals and crevices. The
relations of coelom to atrium in the region of the intestine are

124 CEPHALOCHORDATA

seen in Fig. 74, B, and in the region of the pharynx in Fig. 74,
A. Fig. 72 shows the distribution of the spaces more in detail
(see also Fig. 71). Beginning anteriorly, along the dorsal sur-
face of the pharynx and beneath the notochord run a pair of
dorsal coelomic canals, one at each side of the epipharyngeal
groove ; these give off ventral diverticula which pass down
the primary branchial bars of the pharyngeal wall and unite
ventrally in a median tube, the endostylar coelom (see Fig. 72, ec).
At the posterior end of the pharynx these dorsal and ventral
canals unite in a narrow coelomic space encircling the stomach,
inside the wall of the atrium, and sending an extension forwards
around the liver (Fig. 74, A, /). In the region of the intestine,
behind the atriopore, the coeiom is allowed to expand to its
primitive condition on the left-hand side (Fig. 74, B), but is
still reduced on the right side, where there is a prolongation of
the atrial cavity reaching nearly to the anus. All these coelomic
spaces are lined by a coelomic epithelium.

The Blood System of Amphioxus, although as simple as that
of a Chaetopod worm, is undouljtedly laid down on the Vertebrate
plan — even though there is no distinct heart and the vessels are
few and of simple structure. Capillary networks are formed in
some places, but the colourless blood also extends into many
lacunae or lymph spaces, such as those around the fin-rays and
in the metapleura. As in a typical lower Vertebrate, there is a
contractile ventral vessel (the ventral or branchial aorta, Fig. 77,
v.ao) running forwards under the alimentary canal to the pharynx,
and giving off on each side afferent branchial vessels, which pass
up the primary branchial l)ars and give off branches joining the
vessels in tlie secondary Ijars. These latter do not communicate
directly with the ventral aorta, but the vessels in all the branchial
bars open dorsally by efferent branchial vessels into the paired
dorsal aortae (Fig. 77, d.ao), which run backwards along the top
of the pharynx, one at each side of the epipharyngeal groove.
In the vessels of the branchial bars and their connectives the
blood is aerated by the current of water passing through the
gill-slits, and so reaches tlie dorsal aortae in a purified condition.
The right-hand dorsal aorta is continued forward further into
the snout than its fellow of the other side, and is dilated at
its extremity (Fig. 77). At the posterior end of the pharynx
the paired dorsal aortae unite to form the median dorsal aorta

BLOOD SYSTEM — RENAL ORGANS

1:25

which runs backwards, lying between notochord and alimentary
canal. Tliis vessel gives off branches to the wall of the intestine,
and these break up into capillary networks (Fig. 77, ('}>'), from
which the blood is collected by the median sub-intestinal vein.
This then flows forwards to pass by the hepatic portal vein to
the ventral edge of the saccular liver, in the wall of which it
is distributed in a capillary network. The blood is collected
on the dorsal edge of the liver by the hepatic vein, which
runs posteriorly and then turns downwards and forwards to

el.ao

cL a,o

of bra' ^'^ a/^bra. i-^

'h-€.p port V

Fig. 77. — Diagram of the vascular system of Amphioxus. af.hr.a, Afferent branchial
arteries ; af.br. a', similar vessels of the secondary (tongue) bars ; br.cl, gill-slits ;
cp, intestinal capillaries ; d.ao, paired dorsal aortae ; d.ao', median dorsal aorta ;
ef.br. a, efferent branchial arteries ; fiep.jjort.v, hepatic portal vein ; Iiejj.v, hepatic
vein; int, intestine; /r, liver; ph. pharynx; s.int.v, sub -intestinal vein; v.ao,
ventral aorta. (From Parker and Haswell.)

become continuous with the posterior end of the ventral aorta
or " heart."

It is clear that this course of the circulation agrees with tliat
of a typical lower Vertebrate in all essential points: — (1) in
having the main artery a dorsal aorta in which the blood flows
backwards; (2) in having a ventral vessel representing the heart,
and sending impure blood forwards to the respiratory region of
the alimentary canal to be aerated ; and (3) in having a hepatic
portal system consisting of the capillaries of the liver, through
which the blood from the intestinal wall has to pass before reach-
ing the ventral vessel (heart).

Renal Excretory functions have been attributed to various
organs in Amphioxus, and it is quite possible that, iji addition to
the true nephridia which are now known, other tr&cts of tissue
in the body may be able to eliminate nitrogenous waste matters.
Such are certain clumps of columnar epithelial cells on the floor
of the atrium, and the single pair of large brown atrio-coelomic
funnels lying on the dorsal edge of the posterior end of the

I 26

CEPHALOCHORDATA

pharynx (Fig. 71, l^T.f). There are, however, a large number
(about 100 pah's) of minute nephridia, discovered (1890) by
Weiss and by Boveri independently, lying at the sides of the
dorsal coelomic canals aljove the pharynx, which must be regarded
as the chief functional renal organs. These are bent tubules,
partly glandular and partly ciliated, each giving off several caecal

Mi

Fig. 78. — Branch iostonm lanceolatum. A nephridium of the left side with part of the
wall of the pharynx, as seen alive, liighly magnified. (From Willey, after Boveri.)

knolis (at first sup2:)osed to be open nephrostomes, one shown at
each end of the tubule and three along its upper surface in Fig.
78), which project into the coelom, and opening by one nephridio-
pore (on the lower surface, and opposite a tongue bar of the
pharynx) into the atrial cavity. The knol)s, or closed nephro-
stomes, are surrounded hy peculiar, slender, club-shaped tubular
and flagellated cells — which Goodrich ^ has shown to correspond
to the " solenocytes " in the nephridia of Polychaete worms
(see Fig. 79).

1 Quart. Journ. Mkr. Sci. -\lv. I\Iarcli 1902, p. 493.

NERVOUS SYSTEM

127

Tlie Central Nervous System is dorsal and tubular as in
Vertebrates, and lies in a eouneetive-tissue sbeath immediately
above tlie notochord (Figs. 71, ete., and 80, A). Posteriorly it
tapers to a fine point a little in front of the end of the noto-
chord, but antei'iorly it ends abruptly some distance behind the
anterior extremity of the notochord. The central canal is
connected with the dorsal surface by a median longitudinal
cleft (Fig. 80, C), and at the anterior end it dilates to form the
cerebral vesicle (c.v) with which two simple sense-organs, an eye-
spot (f) and an olfactory pit (olf), are connected. A patch of
ciliated epithelium in the floor of the vesicle has been described

Fig. 79. — Nephridia. A, por-
tion of a iiephridium of
Phyllodoce, a marine Poly-
cliaete, for comparison
with B, portion of a
nephridium of Anipliio.xns.
These figures show the
solenocytes with their
fiagella projecting throngli
the long tubes into tlie
Imnen of the excretory
organ, and demonstrate
the essential similarity of
the nephridia of Ani-
phioxus with those of
Polychaete worms (after
Goodrich).

as an " infundibular - organ." There is also a surface dilata-
tion of the dorsal cleft behind the cerebral vesicle {dil). The
nervous system as far back as this point may be regarded
as the brain, though scarcely distinguishable externally (Figs.
71 and 80, A) from the spinal chord behind. From this
" brain " arise two pairs of " cranial " nerves, the first (I.) from
the anterior end, and the second (II.) from the dorsal surface of
the cerebral vesicle ; both are in front of the first myotonies of
the body, and supply the pre-oral snout with nerves.

The spinal cord gives oft' a large number of spinal nerves
segmentally arranged, but, like the myotomes, not opposite and
symmetrical on the two sides, but placed alternately (Fig. S 1 ).
Moreover, the spinal nerves arise on each side at two levels,
there being a more dorsal series each arising Ijy a single root and

128

CEPHALOCHORDATA

supplying the integument as well as the transverse muscles, so
as to be sensory as well as motor, and a ventral series arising
each by a number of roots (Fig. 81) and wholly motor in
function, as they supply only the myotomes. These two series
may be compared to the dorsal and ventral roots which in the
Vertebrata join to form a mixed spinal nerve.

In addition to ordinary small nerve cells the central nervous
system contains certain large nerve cells with very long processes.

Fig. so. — Branchiostoma lanceolatum. A, liraiii and cerebral iierve.s of a young speci-
men ; B, transverse section through neuropore ; C, behind cerebral vesicle : D
through dorsal dilatation, ch, Notochord ; cr, cerebral vesicle ; dil, dorsal <lila-
tation ; e, eye-spot ; ■/yj, neuropore ; olf, olfactory pit ; / and II, cranial ner\es.
(From Willey, after Hatschek.)

the " giant fibres," wiiich extend through the greater part of the
length of the spinal cord. No trace of a sympathetic nervous
system has been found.

The Sense-Organs connected with the nervous system are
few and simple. There are sensory cells in the ectoderm, on the
margin of the velum, on the velar tentacles, and especially in
clumps on papillae of the cirri around the mouth, which are
probably tactile. In the roof of the oral hood there is a sensory
structure, the " groove of Hatschek," which is supposed to be an
organ of taste. The olfactory pit alluded to above opens ex-
ternally on the left-hand side of the snout. It is ciliated
internally and leads to the so-called olfactory lobe, an antero-

REPRODUCTION

129

dorsal hollow outgrowth from the brain. In the young animal
the olfactory pit opens by the neuropore into the central canal
(Fig. 80, A), but that passage is closed in the adult. Possibly
the olfactory pit is homologous with the hypophysis or pituitary
body of Vertebrates, the homologue of which in Tunicata has a
ciliated funnel. Finally, the median cere-
bral eye (Figs. 80 and 81) is a mere pigment
spot in the anterior wall of the cerebral
vesicle, and a series of somewhat similar
pigment spots occurs along the floor of the
central canal in the spinal cord.^ There is
no known auditory organ. On the under
surface of the oral hood patches of ciliated
epithelium drawn out into rounded lobes
were called by Johannes Mliller the " Eader-
organ." This is probably of use in drawing
water inwards to the pharynx, but it may
also be a sense-organ.

The Gonads are segmentally arranged
along the sides of the body, projecting into
the atrial cavity at the sides of the pharynx
and intestine. In some species the gonads
are paired, but in others belonging to the
genus Asymvietron (p. 137) only a single
series, that of the right side, is present. In
the common Amphioxus {Br anchio stoma
lanceolafum) there are about 26 pairs (Fig. 70,
B), lying in somites 25 to 51; and ovaries Fig, 81
and testes are found in separate individuals
in all other respects. Each gonad is sur-
rounded by a layer of coelomic epithelium.
The gonad must therefore be regarded as hav-
ing grown down from a myotome of the body-
wall into a coelomic pouch, carrying before it the coelomic and
then the atrial epithelium (Figs. 72, and 74, A,^). Eventually the
gonads, when ripe, burst through the layers of epithelium, and
the ova and sperms are shed into the atrium and escape to the
exterior by the atriopore, or it may be in some cases by the mouth.

^ The cerebral eye and the pigment spots of the spinal cord are especially
prominent in the oceanic species B r anchio sto ma 2Hlagic%im, Giinther.

VOL. VII K

Branchiostoma
lanceolatum. Anterior
portion of central nerv-
ous system from ahove,
showing dorsal and
ventral spinal nerves.
(From Willey, after
Schneider.)

I30

CEPHALOCHORDATA

Embryology and Life-History.

Development takes place in the sea-water where the egg is
fertilised — apparently always about sunset, the embryonic stages
being passed through during the night, and the larva hatched in
the early morning.

The egg is small (0'105 mm. in diameter when shed) and
contains very little food-yolk. Segmentation is complete (Fig.
8 2, A), is nearly regular, and results in the formation of a hollow

c

A B

Fia. 82. — Stages in the segmentation of Amphioxus. A represents the eight-celled stage :
B, tlie sixteen-celled ; D, vertical section of C ; F, vertical section of the blasto-
sphere or blastula stage (E). (From Korsclielt and Heider, after Hatschek.)

V)lastosphere (Fig. 82, E, F), the wall of which is one cell thick.
The lower cells (Fig. 82, B, C, D) are slightly larger than the
upper. Invagination of the lower cells then takes place (Fig. 83,
A), resulting in the suppression of the blastocoele or segmentation
cavity and the formation of an archenteron, at first shallow and
opening widely to the exterior (Fig. 83, B), and then deeper and
with the opening narrowed to a small posterior blastopore (Fig.
83, C). This " gastrula " stage differs from the blastosphere in
having a mouth or blastopore, and in being two cell-layers thick
— epiblast (ectoderm) on the outside and hypoblast (endoderm)
within. It soon shows the future aspects of the body by its

EARLY EMBRYONIC DEVELOPMENT

131

elongation and shape (Fig. 83, C), as the dorsal surface becomes
flat aud the ventral convex, while the blastopore is at the
posterior end of the dorsal surface. The blastopore soon closes,
and the mouth and anus are formed independently later.

The epiblast cells become ciliated all over the surface,

so that the embryo rotates within the thin covering which still

surrounds it. And now all the chief systems of the body begin

to be marked out. The tubular nervous system develops from

A B

c

y\c,o

o|oibrooo|o|o'o/c

Ao\o

m^m

Fig. 83. — Three stages in the formation of tlie gastrula of Amphioxus. In A the nuclei
of the endoderm have been omitted ; C has tlie dorsal surface uppermost, and the
posterior end to the right. (From Korschelt aud Heider, after Hatsehek.)

a depression of the epiblast (the medullary plate) in the middle
line of the flattened dorsal surface (Fig. 84, A, 7«^j). The edges
of the depressed area grow inwards and unite over the deeper
layer of epiblast, which becomes the wall of the neural canal or
embryonic nervous system (Fig. 84, D, 7t) ; and further back
these edges of the medullary plate join one another behind the
Ijlastopore, so that the latter comes to open into the floor of the
neural canal, thus forming the neurenteric canal (Fig. 85, A,
C7i). Anteriorly the neural canal (7^) opens to the exterior for
some time by the neuropore.

The hypoblastic walls of the archenteron give off a long
median dorsal groove which becomes the notochord (Fig. 84,
C and D, ch) ; and also an anterior pouch and certain lateral pairs

132

CEPHALOCHORDATA

of diverticula which are the enterocoeles or coelomic pouches,
and give rise to the mesoblastic somites (Fig. 84, B and C, 7nk).
The notochord (Fig, 84, D, ch) is at first a longitudinal cellular
ridge, which becomes segmented off from the hypoblast as a rod
lying below the neural canal. It is seen in various stages of
A

~ wk

Fig. 84. — Four stages in the development of the notochord, nervous system, and meso-
derm of Amphioxus. ak, Ectoderm ; ch, notochord ; dh, cavity of archenteron ;
hb, ridge of ectoderm growing over medullary plate ; ik, endoderm ; Ih, coelom ;
mk, coelomic pouch ; mk'^, parietal layer of mesoderm ; 771k", visceral layer ; mp,
medullary plate ; n, neural canal ; ns, protovertebra. (From Korschelt and
Heider, after Hatschek.)

development in Figs. 84 and 86, leading to the vacuolated con-
dition of the adult.

The coelomic pouches are five in number — (1) one median,
anterior, which gives rise to the two head cavities, the left-hand one
of which opens to the exterior by means of the pre-oral pit ; (2)
a pair of small lateral pouches, placed anteriorly and dorsally, which
do not divide but give rise to the first pair of myotomes only and
their outgrowths which extend back into the metapleural folds,
where, however, they are later replaced by lymph spaces ; and

LATER EMBRYONIC DEVELOPMENT

133

(3) a second pair of diverticula, more posteriorly placed, which
continue to grow back tow^ards the blastopore, and have paired
niesoblastic somites, the cavities in which are the beginnings of the
coelom in the body, constricted off from them successively from
before backwards (Fig. 85, A, ush) to form all the remaining
myotomes.^ This is the first sign of segmentation in the animal,

Fig. 85. — Embryo of Amphioxus. A, iu vertical section, slightly to the left of the
middle line. B, in horizontal section, ak, Ectoderm ; en, neurenteric canal ; dk
and «rf, archenteron ; ik, endoderm ; ink, mesodermal folds ; n, medullary canal ;
us, first coelomic pouch ; ush, coelomic cavity ; T', anterior, //, posterior, end.
(From Korschelt and Heider, after Hatschek.)

and at this stage, wlien it has about five pairs of niesoblastic
somites, it breaks out of its covering and becomes a free-swimming
larva.

The mouth now appears, and soon grows to a large opening
on the left side of the now pointed anterior end (Fig. 86, A, m),
and the first gill-slit {ks) forms as a direct communication from the
front of the mesenteron (pharynx) to the exterior. It is ventral
at first, and then shifts over to the right side.

The anus forms posteriorly, and the neurenteric canal closes

* The mesoblastic somites in Figs. 84 and 8.^) are all derivatives of the larger
posterior pair of coelomic pouches, the smaller more anterior ones not being shown.
For further details in regard to the coelomic pouches see MacBride, Quart. Journ.
Micr. Sci. xliii. p. 351, 1900.

134

CEPHALOCIIORDATA

up. A depression on the floor of tlie enteron close to tlie mouth
k TTjr cli

B

y

k V

mr
■ch

d
sr

l^s

Fig. 86. — A, young larva of Amphioxus. B, anterior end enlarged, c. Provisional
tail-fin ; ch, notochord ; en, nenrenteric canal ; d, enteron ; h, coelom of snout ; /•,
• club-shaped gland; k' its external aperture; ks, first gill-slit; ?», mouth; mr,
nerve -tube ; up, neuropore ; sv, sub-intestinal vein ; w, pre-oral pit, (After
Hatschek. )

aa^ ^P oe

cm c

Fig. 87. — More advanced larva of Amphioxus. an. Anus ; au, eye-spot ; c, larval tail-
fin ; ch, notochord ; d, enteron ; Jf, rudiment of endostyle ; /.', club-shaped gland ;
k', its external aperture ; in, nioiith ; np, neuropore ; ic, pre-oral pit ; x, provisional
nephridium ; 1-4, gill -slits. (From Korschelt and Heider, after Lankester and

Willey.)

gives rise to the "dub-shaped gland" (Fig. 86, B, /.), which is
probably a gill-cleft in its nature.

The walls of the coelomic pouches, which have been extend-
ing both dorsally and ventrally (Fig. 84, I)), become the meso-

LARVAL STAGES I 3 5

derm, the outer tlie somatic and the inner the splanchnic layer ;
and the ventral parts of their cavities unite to form the coelom.
The cells of the dorsal parts become muscle fibres, and constitute
tlie myotomes internally and the connective tissue of the skin
externally.

The larva (Fig. 87) is now long and narrow with many
segments, pointed ends, and a caudal lin. The gill-slits all
appear first in the mid-ventral line and then shift over to
the right side (Fig. 87, 1-4) : they are metamerically arranged.
After fourteen have been so formed a series of eight appear

rf If I

Flo. 88. — Ventral asjiect of three larvae of Aniphioxus, showing the nietapleural folds
and the formation of the atrium, ap, Atriopore ; k, gill-slits ; //' and /•/', left and'
right nietapleural folds ; m, mouth ; to, pre-oral pit. (From Korschelt and Heider,
after Lankester and Willey.)

dorsally to those on the right side, and then the first set,
originally ventral, move over to the left side, and by the sup-
pression of some they become equal in number and segmentally
arranged on the two sides of the body. This is perhaps the
stage at whicli Amphioxus shows the nearest approach to the
typical embryo of a higher Vertebrate. The gill-slits are liere
seven to nine on each side, and the Vertebrate embryo has usually
five to seven on each side. These first gill-slits in Amphioxus
are later subdivided by the downgrowth of the tongue-bar from
the dorsal edge.

The atrium is an ingrowtli of the external space between
the two ventral nietapleural or atrial folds (Figs. 88 and 89),
paired lateral ridges of the body-wall, and so is lined by ecto-
derm. This ingrowth is shut off from the exterior by the

136

CEPHALOCHORDATA

growth towards each other of sub-atrial ridges on the inner sides
of the metapleural folds (see I'ig. 89, A, si), and then becomes
greatly enlarged hj the increased relative growth of the ventro-
lateral part of the body -wall (Fig. 89, B, C). The posterior
opening between tlie metapleural folds remains as the atriopore
(Fig. 88, C, aj)); while the anterior end (Fig. 88) also remains
open for some time, but eventually closes. As the metapleural
folds lie outside the gill-slits (Fig. 88, A) when tliese folds close

Fig. 89. — Diagrammatic transverse sections of three larvae of Ampliioxus to show the
development of the atrium, ao, Aorta ; c, dermis ; ch, notochord ; d, intestine ;
/, connective tissue ; fh, cavity of dorsal fin-ray ; m, myotome ; n, nerve-tube ;
p, atrium ; sf, metapleural folds ; s/h, lymph space in metapleural folds ; si, sub-
intestinal vein ; sk, sheaths of notochord and nerve-tube ; si, sul)-atrial ridge ; ,«/*,
coelom. (From Korschelt and Heider, after Lankester and AVilley. )

in (B and C), it comes about that the gill-slits which formerly
opened freely to the exterior now open into the cavity of the
atrium (compare Figs. 87 and 88).

The mouth now becomes median and ventral, and is reduced
in size, the oral hood (stomodaeum) is formed in front of it, the
gill-slits become more numerous and vertically elongated, the
endostyle forms along the floor of the pharynx, and tlie gonads
grow as paired pouches from the body-wall. Tliis brings the
animal to the young adult condition, reached at a period of prob-
ably about three months after the fertilisation of tlie egg.

The development as a whole shows a very marked resemblance

CLASSIFICATION 1 37

to that of the Timicata (see p. 55), but lends no support to
the view that Amphioxus has degenerated from a higher group
of the Yertebrata.

Classification of the Cephalochordata.

The known species of Amphioxus may be classified as
follows ^ : —

Family Branchiostomatidak.

Genus 1. Branchiosfoma (Coi^ta, 1834).

Having biserial gonads and symmetrical metajileura.
B. lance.olatum (Pallas) — Myotomes 36 + 14 + 12, gonads 23-29 jiairs :

Mediterranean, N.W. Euroi^e, Ceylon, E. of United States.
[B. belcheri, Gray — Myotomes 38 + 17 + 9: Torres Straits, Singapore,

Borneo, Ceylon.
[B. nakagaivae, Jord. and S. — Myotomes 37 + 16 + 11 : Japan.
[B. caribbaeitm, Sundevall — Myotonies 37 + 14 + 9 : West Indies, Atlantic,

N. and S. America.
B. cajjense, Gilchrist — Myotomes 47 + 19 + 9 : S. Africa.
B. californiense, J. G. Cooper — Myotomes 45 + 17 + 9 : California.
B. (Dolichorhynchus) indicum (Willey) — Myotomes 42 + 14 + 15: India
and Ceylon.
(?) B. elongatum, Sundevall^Myotomes 49 + 18+12: Pern.
(?) B. i)elagicum, Giinther — Myotomes 36 + 16 + 15: Honolulu, Gulf of
Manaar, South Indian Ocean.

Genus 2. Asymmetron (Andrews, 1893).
With uniserial (right) gonads and asymmetrical metapleura.

A. lucayanum, Andrews — Myotonies 44 + 9 + 13: Bahamas, Maldives,

Zanzibar.
A. caudatum (Willey) — Myotomes 40 + 9 + 11 : Louisiade Archipelago.
A. (Heteropleuron) bassanuvi (Giinther) — Myotomes 45 + 16 + 14: Bass
Straits, Australia.
„ cingalense (Kirkaldy) — Myotomes 39 + 16 + 8: Ceylon.

„ cultellum (Peters) — Myotomes 32 + 10+10: Torres

Straits, Australia, Ceylon.
„ maldivense (F. Cooper) — Myotomes 45 + 16 + 12 : Mal-

dive Archipelago, Zanzibar.
„ hectori (Benham)^ — Myotomes 53+ 19 + 12 : New Zea-

land.

^ I have to thank Mr. Walter Tattersall, B.Sc, working in my laboratory, for
a detailed summary and discussion of the various published schemes from which
this table has been drawn up. He has also filled up for me the map (Fig. 90)
showing the geographical distribution of the species. (See also Trans. Biol. Soc.
Liveiyool, vol. xvii. 1903, p. 269.)

138

CEPHALOCHORDATA

Thus sixteen species have been described, of which the three
under Branchiostoma placed after square brackets, seem to be
merely varieties of B. lanceolatum, and B. nakagau-ae is probably
identical with B. helcheri ; while it is a question whether Asym-
metron caudatum is more than a variety of A. lucayanum, thus
leaving eleven or twelve species that seem fairly well character-
ised. The exact positions of the two marked (?), viz. B. elonga-
tum and B. pelagicum, cannot be determined in the absence of
fuller descriptions of these species.

160 120 80 JO O W 80 120 IBO

■Sioji/crds Oacy* £^ud* Is/uiar*

Fig. 90. — Sketch-map showing geographical distribution of the Cephalochordata.
+ indicates Branchiostoma; o indicates Asymnietron.

The list above, and the map (Fig. 90), give some indication
of the geographical distribution of the group, and "show that,
although the few species are widely distributed over the shallow
waters of the globe, most of the records lie between 40^ N. and
40° S. latitudes. In flict the group is mainly a tropical one,
and is most abundant in the Indo-Pacific region. The crosses
indicate records of species of Branchiostoma, and the circles those
of Asymmetron (including HderojjJeuroii) ; the latter are confined
to the Indo-Pacific seas, with the exception of A. lucayanum
from the Bahamas — one of tlie numerous cases of interesting
similarity between the marine faunas of the East and West
Indies.

FISHES

(EXCLUSIVE OF THE SYSTEMATIC ACCOUNT OF TELEOSTEI)

BY

T. W. BEIDGE, ScD, F.E.S.

Trinity College, Cambridge ; Mason Professor of Zoology and Comparative
Anatomy in the University of Birmingham

CHAP TEE V

THE SYSTEMATIC POSITION AND CLASSIFICATION OF FISHES

In the first chapter of this vohmie it was pointed out that the
Craniata, of which tlie Fishes form a subordinate group, is
the hxst of the four principal divisions into which the Chordata
are divided. The animals included in the first three, viz. the
Hemichordata, the Urochordata, and the Cephalochordata, have
already been dealt with in the earlier chapters, and it now
remains for us briefly to consider the diagnostic characters of
the Craniata, and then, more in detail, the organisation of the
Fishes.

The Craniata, often termed Vertebrata, form one of the best
defined and most easily recognisable divisions of the animal
kingdom. As the name implies, they are distinguished from the
more primitive Chordata by the formation of a definite " head,"
as the result of the modification of the anterior portion of the
central nervous system to form a complex brain, round which are
concentrated the chief organs of special sense. This is combined
with the evolution of a skull, which, in addition to providing a
" cranium " for the enclosure and protection of the brain, and partial
or complete capsules for the sense-organs, is connected behind with
a system of bony or cartilaginous visceral arches, which loop
round the pharynx between the gill- clefts. Besides supporting
the breathing organs (gills) in the lower aquatic Craniata, or
existing as embryonic vestiges in the higher lung-breathing forms,
these arches usually form the basis of jaws for the mouth. The
epidermal portion of the superficial skin is always composed of
several layers of cells. The notochord, which is always present
in the embryo, and in a few Craniates, both living and extinct,
may even be retained in its entirety in the adult, fails to reach

141

142 FISHES CHAP.

the anterior end of the brain. In most Craniates, however, the
notochord becomes more or less completely replaced in the adult
by the development round it of a series of vertebrae, forming the
backbone or vertebral column. Two pairs of limbs, and cartila-
ginous or bony limb-girdles for their support, are very generally
present.

The segmentation, or serial repetition of certain organs of the
body, which is so marked a feature in tlie Cephalochordata, is
also characteristic of the Craniata. Examples of this may be
seen in the division of the lateral longitudinal muscles of the
body wall into muscle-segments or myotomes by a series of
transverse fibrous septa ; in the formation of the vertebral
column by a series of successive joints or vertebrae ; in a similar
serial repetition of the cranial and spinal nerves, the gill-clefts
and branchial arches, certain blood-vessels, and the renal tubules.
There is sometimes, however, no precise regional or numerical
correspondence between the different organs which are successively
repeated in this way, and hence it is probable that, in at least
some of the organs of the Craniate body, the segmentation has
been independently evolved in each case.

The pharynx is relatively much shorter than in other
Chordata. The gill -clefts are few in nvmiber, whether, as
in the lower Craniata, they are retained as the functional
breathing organs, or are present, as vestiges only, in the embryos
of the higher members of the group. In no instance are they
subdivided by the growth of " tongue-bars " or " synapticula,"
nor do they open externally into an atrial or peribranchial
cavity. The liver is a massive compound tubular gland, never,
in the adult at all events, a simple caecal sac ; and usually there
is a pancreas and a spleen.

A spacious epithelium-lined body cavity or coelom, which, as
regards its origin, may be regarded as a "syncoelom," ^ surrounds
the alimentary canal and separates it from the body wall. From
the epithelial walls of the coelom are derived the gonads (ovaries
and testes), which in the adult are limited to a single pair ; while
paired and often segmentally-arranged lateral tubular outgrowths
from it (renal tubuli) acquire a glandular character and form
the basis of the excretory or kidney system. A special portion

^ A coelom formed liy the union of one or more pairs of in-imitivelj' distinct coelomic
cavities.

GENERAL CHARACTERS 1 45

of the coeloiu also surrounds the heart uud forms a pericardial
cavity, and in some Craniata the genital ducts may be formed
from its linintr membrane.

There is always a muscular heart, consisting of at least three
chambers, a sinus vehosus, an auricle and a ventricle, and formed
l)y a modification of the initial portion of the ventral or cardiac
aorta of the Oephalochordata. The disposition of the great blood-
vessels is based on a common plan in all Craniata, and the blood
which circulates in thein is red in colour owing to the presence of
red, haemoglobin-containing corpuscles in addition to the colour-
less leucocytes which alone are present in the Oephalochordata.
Ductless blood-glands of various kinds (spleen, tliyroid, thymus,
inter- and ad-renal bodies) are very generally present, and
modify in different w^ays the character of the blood as it
circulates through them. Besides blood-vessels there is also
a somewhat similar system of lymphatic vessels distributed
throughout the organs and tissues of the body, which serves the
purpose of re-collecting the fluid portion of the blood that has
diffused from the blood-vessels for the nutrition of the tissues,
and conveying it back to the blood vascular system. These lymph-
atics contain lymph, a fluid comparal)le to dilute blood plasma,
in which leucocytes float. In addition to their continuity with
the blood-vessels at certain points, the lymphatic vessels may
also communicate with the coelom, and hence the Craniata must
be included among those somewhat rare exceptions to the general
rule that no connexion exists between the series of blood-con-
taining channels and the coelom.

In the excretory system the renal tubuli in the aelult Craniata
rarely retain their primitive embryonic communication with
the coelom, and in no instance have they separate and in-
dependent external apertures ; on the contrary, by the union of
their outer or distal extremities, common efferent ducts are formed,
which either open into a " cloaca," or directly on to the exterior of
the body near the anus.

In all Craniates the dorsally-placed and tubular central nervous
system has its anterior portion enlarged and otherwise modified
to form a " brain," while the remaining portion, retaining a
simpler and more uniform structure, forms the spinal cord. In
the embryo the brain always consists of three successive sac-like
enlargements known as the fore-, mid-, and hind-brain, and from

144 FISHES CHAP.

these are developed the various parts of the complex adult brain,
which in the disposition and mutual relations of its parts con-
forms to a common plan in all the members of the group. There
are at least ten pairs of cranial nerves having their origin from
the brain, and, in addition, a varying number of spinal nerves
arising from the spinal cord, and as a rule formed in each case
by the union of a mainly sensory, ganglionated, dorsal root with
a mainly motor, non-ganglionated, ventral root.

The median and usually vestigial, parietal, or pineal eye may
sometimes be retained as a functional organ, but there exist in all
Craniates, in addition, paired eyes, the sensory portion of which,
the retina, is derived as an outgrowth from the first of the
primary embryonic brain -vesicles. To these organs of special
sense are added a pair of auditory organs, and a pair of olfactory
organs, besides, in the lower aquatic Craniates, the peculiar
sensory organs of the " lateral line."

The gonads are reduced to a single pair in the adult, although
it is possible that they may have a multiple origin in the embryo.
Gonoducts for the discharge of the sex-cells are almost invariably
present, and may owe their origin either to a change of function
on the part of certain kidney-ducts, or to independent evolution
from the lining membrane of the coelom. The ova are generally
provided with a large amount of nutritive reserve in the shape
of food-yolk, and hence the process of segmentation is frequently
partial or " nieroblastic," but in some groups, in wdiich the ova
have less food-yolk, it is complete or " holoblastic." The typical
invagiuate gastrula stage, which is so striking a feature in the
embryonic history of the lower Chordata, occurs also in a few of
the lower Craniates, but in most of them it is apt to become
masked or modified in various ways by the presence of a super-
abundant amount of food-yolk.

Functional hermaphroditism is of very rare occurrence in
Craniates, and, as in the Cephalochordata, reproduction by
budding and the formation of colonies are unknown.

Thus distinguished from other Chordata, the Craniata are
divided into six " classes," which may be variously grouped, as
the following table shows : —

SYSTEMATIC POSITION

145

f I. Cyclostomata. >j

Agnathostomata.

ICHTHYOPSIDA.

Lampreys and Hag-
Fishes. )

"1

Without biting
jaws.

Breathing by gills at some
period of Hie.

II. TlSCES.

Anamniota.

True Fishes.

No embryonic covering
or amnion.

Anallantoidea.

III. Amphibia.

Newts, Frogs, and
Toads.

No embryo)
organ 01

lie respiratory
allantois.

SArROPSIDA. -

f IV. Reptilia.

Lizards, Snakes,
Turtles, and
Crocodiles.

1 Gnathostomata.
With biting jaws.

Amniota.

Amnion present.

V. AvES.
Birds.

Allaxtoidea.

AUantois present.

_

VI. Mammalia.
Hairy Quadrupeds.

Apart from the distinctive characters of the six " classes " into
which the Craniata are divided, two or three of these classes may
possess important structural features in common by which they
are distinguished from others. Thus, Cyclostomata, Fishes and
Amphibia agree with one another, and differ from all the remaining
groups in breathing by gills and in possessing lateral line sensory
organs during part, or the whole, of life. Their embryos have no
investinu; amnion, neither does the sac-like outgrowth from the
hind-gut, which is known as the allantois, if present at all, ever
extend beyond the coelom to form an embryonic investment or
to act as a primitive breathing organ. Hence, therefore, the
terms Ichthyopsida, Anamniota, and Anallantoidea have been
applied to these three classes. Similarly, the term Sauropsida, as
applied to Eeptiles and Birds, is a convenient means of giving
expression to the fact that, underlying the most striking diversity
of outward form and habits, there is a community of inward
structure which justifies the conclusion that these animals are
more closely related to one another than either group is to
any other class of Craniates. And again, the application
of the terms Agnathostomata and Gnathostomata brings into
sharp relief the fundamental distinction between the Cyclo-
stomata and all the remaining groups of Craniata which is

VOL. VII L

146 FISHES CHAP.

only partially illustrated by the presence or absence of biting
jaws.

In a general and popular sense the Cyclostomata are usually
regarded as " Fishes," but this usage rests on no better founda-
tion than a certain agreement between the Cyclostomata and the
true Fishes in outward form and habits, and in their method
of respiration by gills. On the other hand, it has been
maintained that the distinctive features of the Cyclostomata
are of sufficient importance not merely to separate them
from the true Fishes, but possibly even (as is to some extent
expressed by the use of the terms Agnathostomata and Gnatho-
stomata) to warrant their elevation to a group equal in taxonomic
value to all the remaining living Craniata taken collectively.
The organisms included in the Cyclostomata, the Lampreys, and
especially the Hag-Fishes, exhibit in many respects an extremely
low grade of Craniate structiu-e ; but how far the simplicity or
archaic nature of some of their organs is primitive, or has been
acquired through degeneration, it is difficult, and is sometimes
impossible, to determine with any degree of satisfaction. In
other respects, such as the presence of a rasping " tongue," it
is obvious that the Cyclostomata have attained a high degree
of specialisation. As one of several illustrations which might
be given of difficulties of this kind, it may be mentioned that
it is by no means certain that the Cyclostomata are not the
degenerate descendants of primitive but now extinct Gnatiio-
stomata. At all events the presence of paired cartilages in
the skull of the Lamprey, which, with some show of reason,
may be regarded as representatives of the primitive upper
and lower jaws of the latter group, would seem to suggest this
conclusion. If this be correct, we must regard the formation
of a suctorial buccal funnel, with its complex system of sup-
porting cartilages — one of the most striking features in the
structure of this animal — as a secondary and adaptive special-
isation of a mouth originally provided witli biting jaws. But
in spite of such difficulties there can be no question that the
Cyclostomata are the most primitive of all existing Craniates,
and so far differ from the true Fishes and from all other classes
of Craniate animals, that their inclusion in a class by themselves
is the least that can be done to give graphic expression to their
isolated position, even if we do not fully accept the dictum of

V SYSTEMATIC POSITION 1 47

Haeckel that " they are further removed from Fislies than Fishes
from Man."

Briefly stated, the Cyclostomata or Agnathostomata are
distinguished from " Fishes " and all the remaining Craniata
(Gnathostomita) by the following characters : —

The mouth is either nearly terminal, as in the Hag-Fishes
{Mijxine) ; or, as in the Lampreys {Pctroviyzon), it opens out
of a spacious, pre-oral, suctorial, Ijuccal funnel, which, in its rela-
tions to the hypophysis or pituitary body, recalls the pre-oral
buccal cavity of the Cephalochordata. As in Amphioxus, the
hypophysis ^ is displaced dorsally l)y the forward growth of the
pre-oral portion of the head in the embryo, and consequently it
only attains its normal relations to the infundibular dow^ngrowth ^
from the ventral surface of the fore-brain by perforating the
floor of the skull from above instead of from below as in all
other Craniates. In one section of the group (e.g. Myxinc) the
hypophysis opens into the oral cavity, and serves as a tubular
passage for the inspiratory water-current to the gill-sacs, a feature
in which these Cyclostomes are unique. The apparently median
olfactory organ is carried inwards with the hypophysial involution,
and communicates with the latter throughout life. A primitive
upper jaw (palato-(piadrate cartilages or sub-ocular arches) is
present, and in at least some Cyclostomes (e.g. the Lampreys),
and possibly in all, there are structures which very probably
represent a primitive lower jaw (Meckel's cartilages) ; but such
structures are always non-biting, and merely form skeletal supports
for other portions of the skull. In place of Ijiting jaws the
mouth is provided with a complex rasping lingual apparatus
supported by special cartilages, the so-called tongue, which bears
horny teeth and is capaljle of protrusion and retraction. Paired
limbs are entirely wanting.

In the Grnathostomata, on the contrary, there is no buccal
funnel, and the mouth, whether terminal or ventral in position,
opens directly outwards. The hypophysis is usually carried
inwards witli the stomatodaeal invagination which in the embryo
gives rise to the mouth, and is therefore from the first in relation
with the ventral surface of the brain. Biting jaws (palato-
quadrate and Meckelian cartilages), formed by the modification
of an anterior and primitively gill-ijearing visceral arch, are
1 Cf. p. 129. - Cf. p. 391.

148 FISHES CHAP.

invariably present. The olfactory organs are obviously paired, and
they are distinct from the hypophysis. Paired limbs are present.

As previously stated, the true Fishes form the second of the
six " classes " into which the Craniata are divided. As compared
with the higher Craniata, their distinctive characters may be
concisely stated as follows : —

Fresh water or marine Gnathostomata, which in their shape
and in method of breathing are adapted for an aquatic life.
Throughout life their respiratory organs are in the form of
vascular processes (gills) derived from the walls of the branchial
clefts, and supported by a series of branchial arches. The
principal organ of locomotion is the powerful muscular tail ; in
addition, however, there are paired fins, pectoral and pelvic,
corresponding to the fore- and hind-limbs of the terrestrial
Craniata, and possessing a supporting cartilaginous or bony
skeleton (" ichthyopterygium ") which cannot readily be com-
pared with the limb-skeleton of the latter. Fishes also possess
a system of median fins, supported by a special skeleton of their
own. An exoskeleton of dermal spines or denticles, scales or bony
plates, is usually present. Except in one group, the Dipnoi, the
heart has but one auricle, and receives only venous blood, which
it forces, first, through the blood-vessels of the gills, and thence,
as arterial blood, through the vessels of the body generally. An
air-bladder is frequently present, and serves as a hydrostatic organ
or float, but in a few cases it may act as a lung, and helps the
gills in the work of respiration. The paired olfactory organs rarely
communicate with the oral cavity by internal nostrils. Peculiar
cutaneous sense-organs are disposed in linear tracts along the sides
of the body (lateral line sensory organs), and on the head, and
appear to be specially associated with a life in water.

. Fishes may be divided into the following " sub-classes," and
these in turn may be subdivided into various " orders " and
" sub-orders " : —

(i.) Elasmobrakchii ; e.cj. Sharks, Dog-Fishes, Skates, and Rays.

(1) Pleuropteiygii f ; e.g. Cladoselache.

(2) Iclithyotomi t ; e.g. Pleuracanthus.

(3) Acanthodei f ; e.g. Acanthodes.

(4) Plagiostomi.

(a) Selachii ; e.g. many extinct and all living Sharks

and Dog-Fishes.

(b) Batoidei ; e.g. Skates and Eays.

(5) Holoccphali ; e.g. Chimaera and Callorhynchus.

CLASSIFICATION 1 49

(ii.) Teleostomi ; e.g. such well-known Fishes as the Perch, Cod, Salmon,
and Herring, and also the less familiar " Ganoids," living and
extinct.

(1) Crossoj^terygii ; e.g. Polypterus.

(2) Chondrostei ; e.g. the Sturgeons {Acipem^er').

(3) Holostei ; e.g. the Bow-fin {Amiu), and the Gar Pike

{Lepidosteus).

(4) Teleostei ; e.g. the Perch, Cod, Salmon, etc.
(iii.) Dipnoi ; e.g. Neoceratodus, Protoi)terus, and Lepidosiren.

A'ppendi.x. to the Glass Pisces.
(i.) PalaeospondylidaeI ; e.g. Palaeospondyhhs.

(ii.) OSTRACODERMlf.

(1) Heterostraci ; e.g. Ptcraspis.

(2) Osteostraci ; e.g. Gephalaspis.

(3) Anaspida ; e.g. Birkenia.
(iii.) AntiarchiI ; e.g. Pterichthys.

(iv.) ArthrodiraI ; e.g. Coccostcus, Dinichthys.

The Fishes included in the Teleostomi were formerly arranged
in two groups : the Ganoidei, including the Crossopteiygii,
Chondrostei, and the Holostei, with their numerous fossil allies ;
and the Teleostei. Living Ganoids agree with one another, and
differ from Teleosts in possessing an intestinal spiral valve and
a conus arteriosus. It is difficult, however, to separate the two
groups, inasmuch as in each group there are living forms which
tend to approximate to the other ; and numerous fossil genera,
of whose soft parts nothing is known, are in many respects
intermediate between the two. The position and relationships
of the Palaeospondylidae, Ostracodermi, Antiarchi, and Arthro-
dira are very uncertain. The Palaeospondylidae have been in-
cluded in the Cyclostomata, or at all events have been regarded
as more or less closely related to that group, while the absence
of paired Jfins and the apparent want of jaws have suggested
that the Ostracodermi occupy an intermediate position between
the Cyclostomata and the Gnathostomata.^ On the other hand,
the Arthrodira are either regarded as an independent group of
Fishes, or are included amongst the Dipnoi. In the latter case,
the Dipnoi are divided into the Arthrodira and the Sirenoidei,
the last mentioned group including Neoceratodus, Proto2)terus and
Lepidosiren, and their extinct allies.

t Entirely extinct. ^ Gadow, A Classification of Vertebrata, 1898, p. 4.

CHAPTER VI

EXTERNAL CHARACTEKS OF CYCLOSTOMATA AND OF FISHES

EXTERNAL CHARACTERS COLORATION POISON GLANDS AND

POISON SPINES PHOSPHORESCENT ORGANS.

In all the Cyclostomata the body is Eel-like in shape, the head
and trunk being nearly cylindrical, and the tail somewhat flat-
tened from side to side. In Pctromyzon the head terminates in a

br.cl.i

pn.

Fig. 'd\.~Petromyzon murinus. A, ventral ; B, lateral ; and C, dorsal, view of the head.
hr.clA, First liranchial cleft ; huc.f, buccal funnel ; eye, the eve ; vitiu mouth ; nu.np,
nasal aperture ; j), papillae ; pn, pineal area ; ?\ f-, (\ teeth of buccal funnel ; i\
teeth on the tongue. (From Parker and Haswell, after W. K. Parker.)

ventrally-directed, funnel-like cavity — the buccal funnel — in the
roof of wliieh tlie relatively small mouth is situated (Fig. 91, A.).
The margin of the funnel is fringed by a series of short papillae,

150

CHAP. VI

EXTERNAL CHARACTERS

151

Imt in the Hag-Fishes {Myxine and Bdcllostoma), where a buccal
funnel is not developed, longer tentacle-like structures are present
on each side of the mouth. On the upper surface of the head
is the single median nostril, or naso-pituitary aperture, placed
between the eyes in the Lampreys (Fig. 91, B, C), but at the
anterior margin of the

br.cl.

head in Myxine and
its allies (Fig. 92). In
tlie living Lampreys a
semi-transparent area of
skin may be noticed
behind the nasal organ,
which coincides with
the position of the more
deeply - seated parietal
eye. On each side of
the body, commencing a
short distance behind the
eye, is a series of small
and almost circular bran-
chial clefts {Petromyzon,
Bdellostoma). In Myxine,
however, the clefts of
each side have a single
common external aper-
ture, situated on the
ventral side of the body
and some distance be-
hind the head (Fig.
9 2, A). At the junc-
tion of the trunk with
the tail is the anus,
behind which is the
papilla which carries the urino-genital aperture at its extremity.
There are no paired limbs or vestiges of such organs. Median
fins are represented in the Lampreys by an anterior dorsal fin
and a posterior dorsal fin, the latter being continuous with the
caudal fin which fringes the upper and lower margins of the
protocercal tail. In Ifyxine a caudal fin only is present, surround-
ing the extremity of the tail.

\

oes.ct.d

Fig. 92. — Head of Myxine glutinosa (A), and of
Bdellostiiina forsteri (B), from beueath. br.cq^.
Left external branchial aperture ; br.cLl, first
branchial cleft ; mth, mouth ; na.ap, nasal aper-
ture ; oes.ct.d, oesophageo - cutaneous duct. The
smaller openings in A are those of mucous glands.
(From Parker and Haswell, after W. K. Parker.)

152

FISHES

In Fishes the characteristic shape of the body is more or less
that of a spindle, tapering at each end and somewhat flattened
from side to side ; and, as a rule, the three regions of the body —
head, trunk, and tail — pass almost imperceptibly into one another
(Fig. 93, A). Nevertheless, there is great diversity of form in
different Fishes. Compare, for example, the elongated, cylindrical
shape of the Eels (which is perhaps associated with their habit of

^■f' a,. '■' '■"■

t.S.

i^.-g.f.

Fig.

93. — Tilapia dolloL To sliow the external characters of an Acanthopterygian
Teleost. A, side view ; B, the first branchial arch, a,/, Spinose part of the anal
fin ; a.f^, soft rays ; c.f, caudal fin ; d.f, spinose portion of the dorsal fin ; d.f^,
soft rays ; g.f, gill filaments ; (j.r, gill rakers ; ■;'././, inferior lateral line ; n, nostril ;
p.f, pelvic fin ; p.op, preoperculum ; pt.f, pectoral fin ; s.l.l, superior lateral line ;
t.S, transverse row of scales. (From Boulenger.)

insinuating themselves into holes and crevices, and their undula-
tory, snake-like movements when swimming); the compressed, band-
like shape of the Ptibbon-Fishes (Trachypteridae) ; the flattened
bodies of those Fishes which habitually live and move on the
bottom, like the Skates and Eays ; the thin, laterally -compressed
bodies, often nearly as high as long, of the Fiat-Fishes (I'leuro-
nectidae), which always swim and rest on either the right or left
side ; the almost spherical Globe-Fishes {Tetrodon) which often
float passively in the water ; and the singular rectangular,
coflfin-like Coffer- Fishes (Ostracion). There is also much differ-
ence in the relative proportions of tlie three regions of tlie
body in different Fishes, as witness the enormous size and

VI EXTERNAL CHARACTERS I 5 3

grotesque appearance of the head of the Augler-Fish {Lophius) ;
the huge high trunk and abbreviated tail of the Sun -Fish
(Orfhagoriscus) ; and the short high trunk and long tail of
Kotopterus (Fig. 334).

In its external appearance the head perhaps differs more in
different Fishes than any other part of the body. Long and
flattened in the Skates and Eays, the head becomes short and
high in most Holocephali and in many Teleosts, or is shaped like
a blunt cone, as in such Dipnoi as Frotoptems and Lepidosiren ;
or becomes long and pointed, as. in the North American " Gar
Pike " (^Lcpidosteus) ; or, finally, as in the Hammer-head Shark
{Spliyrna), the head may be produced into great lateral exten-
sions, carrying the eyes at their extremities (Fig. 256, B). Apart
from its relative shape and size, the appearance of the head may be
further modified by the thinness of the investing scaleless skin,
wliich readily allows the surface and contour lines of the bones
of the skull to be seen through it, as in the Crossopterygii, and
in such Teleosts as the Siluroid genera Clarias and Callichthys ;
or the skin, even if devoid of scales, may be so thick that scarcely
any of the bones are visible externally. The exoskeleton, whether
in the form of scales or bony plates, may extend to a varying
degree on to the surface of the head in different Teleosts, or may
even invest nearly the whole of the head. When, as is not infre-
quently the case {e.g. many Scorpaenidae) certain of the bones of
the skull are produced into projecting spines, the head assumes a
singularly formidable appearance (Fig. 424).

The mouth differs greatly in size and position. In existing
Elasmobranchs it is generally crescentic in shape and always
ventral in position, but in certain primitive fossil members of the
group, as in the Palaeozoic Cladoselache, it is anterior and terminal.
The Sturgeon and other living Chondrostei have the mouth
ventral. In the Dipnoi also the mouth is ventral, but is near the
extremity of the snout. As a rule, the mouth is terminal or
nearly so in the living Crossopterygii and Holostei, and in the
great majority of Teleosts, although in the latter group it is
occasionally distinctly ventral, especially when a snout is de-
veloped, and it may sometimes look upwards by reason of the
projection of the lower jaw in front of the upper. A pronounced
" beak " is sometimes formed by the forward prolongation of both
jaws, as in the Gar Pike {Lcpidostcus), with the result that the

154 FISHES CHAP.

vertical gape of the mouth is greatly increased, but in a few
Teleosts a beak may result from a forward extension of one jaw
only, the upper in the Sword-Fish {Xiphias) and the lower in
the " Half-Beak " {Hemirlianvphus). A further modification is to
be noted in many Teleosts, in which, owing to the forward pro-
longation and inclination of the skeletal supports of the jaws,
the mouth is at the extremity of a longer or shorter spout-like
beak, and is then usually very small. This is the case in the
" Sea-Horse " {Hippocaminis), the Pipe-Fishes {Symjnatlms), the
" Flute-mouths " {Fist id aria), and the Trumpet-Fish (Centriscus),
and especially in certain species of the African lamily Mormy-
ridae, where the pore-like mouth is at the extremity of a long,
tapering, downwardly-curved proboscis (Fig. 330). In many
Teleosts the mouth can be protruded and withdrawn at will by a
sliding motion of the bones of the upper jaw (premaxillae) on the
anterior skull Ijones by which they are supported. From this
point of view the toothless mouth of the Sturgeon is even more
remarkable. By a forward or a Ijackward swing of the elements
which form the upper half of the hyoid arch (hyomandibular and
symplectic) the mouth can be thrust downwards from the under
side of the head like a spout, when the Fish is feeding, and subse-
quently retracted. In not a few Fishes the forepart of the head
is prolonged forwards over the mouth and jaws in the form of a
rostrum or " snout " ; it is, in fact, to the growth of a snout that
the ventral position of the mouth in Fishes is generally due. This
feature is more or less characteristic of most Elasmobranchs, in
which the snout forms a cut-water overhanging the mouth. In
the Holocephali the snout is short and blunt, except in Harriotta,
where it is pointed and unusually long. Among the Chondrostei
the Sturgeon has an exceptionally massive snout, the length and
shape of which differs in different species. In the allied Polyodon
the thin, flattened, spoon-like snout is scarcely less than one-
fourth the length of the body (Fig. 289).

Simple or branched tactile filaments or " barbels " are present
on different parts of the head in many Teleostomi, sometimes at
or near the chin, as in certain Gadidae, like the Haddock and
Cod, or on the under surface of the snout, in front of the mouth,
as in the Sturgeon. In the Siluridae (Fig. 356), where they are
found in relation with the upper and lower jaws, and even between
the nostrils, these structures are often remarkably developed.

VI EXTERNAL CHARACTERS - I 5 5

The eyes of Fishes fire usually very large. They are generally
situated on the sides of the head, but in the " Star-gazers "
(Unoioscopus) they are on the upper surface and close together.
In the goggle-eyed FeriophfJudmus the eyes seem to protrude
from their orbits, and in a variety of a species of Carp, the G old-
Fish {Cijiwinus auratus), the protrusion is so marked that the eyes
seem as if on stalks. In a few species, which live either in caves
or at very great oceanic depths, the eyes become vestigial, and are
hidden beneath the skin, or are even covered by scales (Fig. 430).

In the Elasmobranchs and Dipnoi the olfactory organs retain
their primitive position as pit-like sacs on the ventral surface of
the snout, just in front of the mouth. In the Dipnoi (e.g. Proto-
vterus) eacli olfactory sac has two apertures, of which one, the
external nostril, is placed on the vmder surface of the snout, while
the other, the internal nostril, opens within the upper lip into
the oral cavity — a feature which is unique among Fishes. In
nearly all Teleostomi, also, each sac has two nostrils, which, how-
ever, are situated either on the upper surface or on the sides of
the fore-part of the head, and have no communication with the
mouth.

Directly behind the head in Elasmobranchs, or Ijeneath its
hinder part in all other Fishes, are placed the external apertures
of the branchial clefts. In the former group tliese apertures
are visible externally in the form of a series of narrow vertical
slits, but in the latter they communicate with the exterior by
opening on each side into a common branchial cavity, tlie outer
wall of which is formed by a movable flap-like fold with a free
hinder margin and a special internal skeleton of cartilaginous
rays or of bony plates and rods, the gill -cover or operculum
(Fig. 161, B). Behind the free margin of the operculum there
is a slit-like orifice, the gill-opening or external branchial aper-
ture, which curves from above downward and forward toward
the chin, and places the branchial cavity in conniumication with
the exterior. Througli this aperture the water, which has
entered through the mouth, traversed the gill-clefts, and bathed
the gills, finds its exit from the body. The space on the ventral
side of the head between the two halves of the lower jaw, and
between the two external branchial apertures, is termed the
" isthnnis." The size of the external branchial aperture differs
greatly in different Fishes, according to the extent to which the

156 FISHES CHAP.

free opercular margin fuses below with the isthmus, or behind
with the side of the head. Thus the aperture may extend from
the chin in front upward and backward to near the dorsal sur-
face of the head, or it may be reduced to little more than a mere
pore situated on any part of the opercular edge (e.g. Hippocampus);
or, as in Syiiibranchus, the reduced pores of opposite sides may
coalesce in the floor of the throat in a common median opening.

In the Elasraobranchs and in the Dipnoi the cloacal aperture
is always situated at the junction of the trunk with the tail. In
the Teleostomi, however, where the intestine has a separate ex-
ternal orifice or anus, distinct from, and placed in front of, the
separate or combined urino-genital ducts, the anus may either
retain its primitive position near the union of the trunk and
tail, or occupy almost any intermediate position between this
point and the throat.

Most Fishes possess both median and paired fins (Fig. 93, A).
From an evolutionary point of view the median fins have a far
greater antiquity than the paired fins. They appear before the
latter in embryonic development, and in the Cephalochordata, and
such lower Craniates as the Cyclostomata, they are the only fins
which exist. The isolated median fins of most Fishes are discon-
tinuous remnants of a primitively continuous structure, which,
extending like a fringe along the median line of the back, was
thence continued round the end of the tail and forward along
the ventral surface as far as the cloacal or anal orifice. This
primitive condition, which, as we have seen, is characteristic of
Ampliioxus, is also very general in the embryos and larvae of
Fishes (Figs. 238 and 309), and is more or less completely
retained in the Dipnoi and in many adult Teleosts, notably in
those species in which the body is greatly elongated and locomo-
tion is effected by serpentine lateral undulations, as in the Eels
(Anguillidae), and in others which, eitlier through their quasi-
parasitic or commensal habit (e.g. Ficrasfcr acus), or by reason of
a peculiar environment, as in certain deep-sea Fishes (Fig. 430)
are distinguished by the retention of many larval features. More
generally, however, the continuity of the fin becomes inter-
rupted, and that portion of it which surrounds the extremity
of the tail is the first to become separated from the rest as
a caudal fin (Fig. 429). By further interruptions the remaining
dorsal portion may become divided into two or three isolated

VI EXTERNAL CHARACTERS I 5/

dorsal fins (Fig. 398), or even into a series of isolated finlets ; and
similarly also with the ventral portion or anal fin ; or, without
undergoing subdivision, both fins may become reduced in length
to an extent which differs greatly in different Fishes, and persist as
single dorsal or anal fins. But e^'en when a median fin is reduced
in length by atrophy, or becomes subdivided by breaches in its
continuity, the externally invisible supporting radial elements
frequently remain to prove the originally greater length of the
fin, or the continuity of its detached remnants.

Like the median fins, the paired fins may also be regarded
as discontinuous remnants of an originally continuous lateral fin
which extended along each side of the body from the head to
the vent, and of which only the anterior and posterior portions
now remain as the pectoral and pelvic fins. Pectoral fins are
rarely absent in existing Fishes, and when present they are
always situated just behind the branchial clefts, where, as in
most Teleostomi, the outline of their supporting pectoral girdle
can often be seen. They vary greatly in form and size in
different Fishes, and in the Elasmobranchs are larger than
in most others. In certain members of the latter group,
the Skates and Kays, in which the feebly -developed tail is
probably useless as a locomotor organ, the pectoral fins are
exceptionally large, forming broad triangular lobes, the broad
bases of which are continuous with the sides of the body from
the anterior part of the head to near the origin of the pelvic
fins, and thus in outward form, if not in inward structure,
simulate re-acquired continuous lateral fins. Except in a few
instances, the Teleostomi have relativel^^ small fan-shaped or
paddle-like pectoral fins, and usually only tliat portion of each
fin which is supported by the dermal fin -rays is visible exter-
nally. In the Crossopterygii, however, each fin appears to con-
sist of a central lobe invested by scales and encircled by a
peripheral fringe of fin-rays, and is hence described as a " lobate "
fin (Fig. 279). When the central lobe is much increased in
length but reduced in width the fin becomes acutely lobate. A
similar type of fin is present in the Dipnoi, but in Proto2:)terus
and Lejndosiren, owing to the length and narrowness of the
central lobe, and the reduction or suppression of the marginal
fringe, the pectoral members assume the condition of long
tapering filaments (Fig. 304).

I5S FISHES CHAi'.

Although as a rule smaller in size, the pelvic fins bear a
general resemblance to the pectoral fins, Ijut in certain groups,
especially in Teleosts, they are liable to undergo extraordinary
changes in position, and, as will be seen presently, are much more
prone to exhibit the effects of adaptive modification and degenera-
tion. They are present in all existing Fishes, with the exception
of the Crossopterygian CaUimichthys and some Teleosts, and,
except in the Teleostei, they invariably retain their primitive
position near the junction of the trunk with the tail, and directly
in front of the cloacal or the anal aperture ; in this position
they are said to be " abdominal." In other Teleostei the fins
undergo forward displacement and come to lie directly beneath
the pectorals (Fig. 415), when they are said to be " thoracic,"
as in the Mackerels (Scombridae) and the Horse -Mackerels
(Carangidae) ; or even in front of the pectoral fins on the under
surface of the throat, when they are described as "jugular," as
in the Cod and other Gadidae (Fig. 398).

Both the median and the paired fins are supported by an
internal skeleton, consisting (i.) of a series of cartilaginous or
bony, rod-like radial elements or pterygiophores, for the support
of the inner or proximal portion of the fins, and (ii.) of a series
of horny fibres, or bony dermal fin-rays, which fulfil a like
function for the outer or distal portion. The radial elements,
however, are never visible externally, even when, as in most
Elasmobranchs, they support the greater part of the fins,
inasmuch as they are invested by the fin-muscles and the skin ;
and in the same group, where horny fibres complete the fin-
skeleton, they too are 'covered by the spinose skin, and hence
offer no external evidence of their existence. In the Teleostomi
a marked reduction in the number and length of the radial
elements of the paired fins, and the insinking of those pertaining
to the median fins into the adjacent muscles of the body-wall,
leaves the dermal fin-rays, with their thin covering of transparent
and usually scaleless skin, as obvious features in the external appear-
ance of the Fish, and apparently as the sole support of the fins.

The dermal fin-rays of the Teleostomi exhibit an obvious dis-
tinction into spines and soft rays (Fig. 93, A). The former are
stout, rigid, and unbranched structures, pointed at their free distal
ends, which, in numbers differing in different genera and species,
support the anterior portions of the dorsal, anal, and pelvic fins.

VI EXTERNAL CHARACTERS 159

Soft rays are flexible, branched distally, and generally exhibit a
transversely-jointed structure ; when present in conjunction with
s])ines they invariably lie behind the latter. The presence of
both kinds of fin-rays, or of soft rays only, is one of the more
obvious distinctions between the Teleostean groups of the Acan-
thopterygii and the Malacopterygii, of which the Perch and the
Salmon respectively are well-known examples. Powerful sjiines
are frequently developed in front of the dorsal fin in many living
and extinct Elasmobranchs, and, under the general term of
" ichthyodorulites," constitute the sole fossil remains of many
extinct Devonian and Carboniferous genera.

The caudal fin and the terminal portion of the tail exhibit
interesting modifications which are highly characteristic of par-
ticular groups of Fishes. In the embryonic and early larval
stages of most Fishes the tapering caudal extremity retains its
coincidence with the axis of the body, and divides the caudal
fin into two equal portions, a dorsal and a ventral lobe, the two
being continuous round the tip of the tail ; and this condition,
which is certainly the most primitive, is termed " protocercal "
or " diphycercal " (Figs. 238 and 309). Such a symmetrical
tail, as we have seen, is retained in the Cyclostomata, and was
also present in certain extinct palaeozoic Sharks (e.g. Plcura-
canthus), but it may be doubted if any existing Fish has a
tail which is truly and primitively diphycercal. The Dipnoi
(Fig. 304) and the Crossopterygii, including fossil representa-
tives of both groups, and perhaps a few Teleosts, seem to
approach this condition ; but it is by no means certain that the
apparent symmetry is primitive, and has not been secondarily
acquired. In other Fishes the terminal part of the tail,
including also its section of the vertebral column, is bent
upwards, and is fringed along its upper border by the reduced
dorsal lobe of the caudal fin, which, nevertheless, retains its con-
tinuity with the ventral lobe round the tip of the tail. The
latter, or rather its hinder portion, is strongly developed, but,
owing to the prolongation of the up-tilted caudal axis beyond it,
the dorsal lobe appears longer than the ventral, and hence there
is a marked want of symmetry between the upper and lower
division of the caudal fin (Fig. 253, A). The Ostracodermi, all
living and nearly all extinct Elasmobranchs, the Acanthodei,
Holocephali, some extinct Dipnoi, and amongst the Teleostomi,

l6o FISHES CHAP.

the living Chondrostei and certain extinct Crossopterygii, afford
examples of this unsymmetrical or heterocercal type of tail. A
third type is the " homocercal." In this type the caudal fin
appears externally as if perfectly symmetrical, the supporting
fin-rays radiating from the blunt extremity of the tail in such a
way that a prolongation of the axis of the body appears to divide
the fin into equal-sized and continuous upper and lower lobes
(Fig. 343). Dissection, however, reveals the fact that the
terminal portion of the vertebral column is bent upwards as in
the heterocercal tail, and that while the dorsal lobe is almost
vestigial,' the ventral lobe is enormously developed, and its
supporting rays so inclined backwards parallel to the axis of the
body as to form practically the whole of the caudal fin, with the
exception of the dorsal border, which is formed by the few
remaining fin-rays of the dorsal lobe (Fig. 140). A homocercal
tail, therefore, is a disguised or masked heterocercal tail. It is
specially characteristic of Teleosts, and is closely approached in
the Holostean genera Lejndustetis (Fig. 299) and Amia, which
offer an interesting transition from the heterocercal to the homo-
cercal types ; and, singularly enough, even the heterocercal tail of
the Palaeozoic Shark Cladoselachc (Fig. 249), seems as if it had
imdergone some degree of independent specialisation in the same
direction. The homocercal tail exhibits much diversity of form in
different Teleosts, sometimes being rounded or lancet-shaped, and
sometimes having a deeply-forked hinder margin. One of the
Eibbon-Fishes, Trachypterus taenia, is singular in having the
caudal fin on the dorsal side of the tip of the tail, and directed
upwards like a fan. In some f Teleosts, again, there is no recog-
nisable upward deflection of th6 terminal portion of the vertebral
axis, and the caudal fin-rays seem to be derived in equal propor-
tions from the dorsal and ventral lobes of the fin (Fig. 414).
This apparently diphycercal tail is probably a secondary acquisi-
tion, and may be considered due to the atrophy of the terminal
portion of the vertebral column, and the subsequent coalescence of
the dorsal and ventral lobes of the caudal fin round the extremity
of a more or less abbreviated tail. It is even possible that in
some Fishes the proper caudal fin has completely atrophied,
and that the apparent caudal fin has really been formed by a
similar modification affecting the hinder portions of the dorsal and
anal fins. In the extinct Crossopterygian genera, Coelacanthus,

VI EXTERNAL CHARACTERS l6l

Dii^lurus, and Unclina (Fig. 278), there is evidence that the latter
modification has actually taken place, for the atrophying terminal
part of the tail, with a vestige of the original caudal fin, is still
retained as an axial prolongation between and even beyond the
secondarily formed caudal fin. To this secondary diphycercal tail
the term " gephyrocercal " has been applied. The apparent diphy-
cercal tail of many Fishes, and especially of Teleosts, is really a
gephyrocercal structure. The ancestral evolution of the different
types of caudal fin is recapitulated in the embryonic histories of
their possessors. The heterocercal condition of an adult Fish is
always preceded by a transitory embryonic diphycercal stage :
from the same starting - point the homocercal condition is
attained after passing through a heterocercal stage ; while the
gephyrocercal may perhaps be derived by degeneration from any
one of the others.

The normal function of the fins, both median and paired, has
reference to locomotion in the form of progression, steering or
balancing, but in not a few Fishes the fins may be variously
modified and adapted for quite different purposes ; and especially
is this the case in the dominant group of existing Fishes — the
Teleostei. Thus, to quote a few examples, the first dorsal fin of
the Sucker-Fishes (JRemora, Echencis) forms a cephalic sucker,
by means of which the Fish attaches itself to Sharks and
Turtles (Fig. 421); or, as in the Angler-Fish {Lophius), its
anterior rays are much elongated, and terminate in lobes which
serve as a bait to attract the prey on which the animal feeds ;
again, in some of the deep-sea Fishes the dorsal fin, like the
pectoral and caudal fins in others of a similar habitat, is pro-
duced into long trailing filaments whose use is probably tactile.
The pelagic young of many Teleosts, such as some of the
Eibbon- Fishes and the Horse -Mackerels (JJaranx), also have
certain of their fin-rays prolonged into similar filaments.
The pectoral fins are enormously elongated and wing-like in
the Flying-Fishes (Fxocoefiis), and, after the fashion of a para-
chute, serve to sustain the Fish in its flying leaps through the
air. They are also similarly modified for a like purpose in the
so-called Flying-Gurnard {Dactylopterus volitans). The pectoral
fins may also be used for progression on land, as in the African
and East Indian Goby (Periophthalmtis), where the fins are large
and muscular and are applied to the ground like feet, enabling
VOL. VII M

1 62 FISHES CHAP.

the Fish to hop about the muddy or sandj fiats left bare by the
retreating tide, in pursuit of the small Crustaceans on which it
feeds. In other Teleosts certain of the rays of the pectoral fin
separate from the rest and from one another, and form free
tentacle-like structures the use of which is probably tactile. In
the Gurnards these organs are relatively short and stout, but in
other Fishes they may form long slender filaments twice as long
as tlte animal, and capable of being moved independently of the
fin, as in the West African and West Indian species of Poly-
nemidae (Pe7itanemus quinquarius). Similar free rays are also
present in some deep-sea Scopelidae, as in Bathypterois dubius,
where they are nearly as long as the Fish itself (Fig. 371, B). A
familiar modification of the pelvic fins in several Teleosts is their
coalescence and more or less complete conversion into a ventrally-
placed sucker-like organ of attachment, as in the common Lump-
Sucker {Cyclopterus) and the Gobies (Gohiiis). In the gaudy
Chilian Fish, Sicyases sanguineus (Fig. 428), the anterior part
of a huge ventral sucker is supported by the jugular pelvic
fins, and the hinder part by prolongations from the pectoral
girdle. Certain Cyprinidae (e.g. Gastromyzon, which frequents
the rapidly-flowing mountain streams of Borneo), have the whole
ventral surface of the trunk, in conjunction with the outwardly
and horizontally directed pectoral and pelvic fins, modified to
form an efficient adhesive surface for attaching the Fish to
the stones and rocks of the river bottom ^ (Fig. 355). In the
males of Elasmobranchs, except in the Palaeozoic Shark Clado-
selache, and of Holocephali, the hinder portions of the pelvic fins
are modified to form copulatory organs, the claspers, mixipterygia,
or pterygopodia. Lastly, it may be mentioned that the spines,
often long, pointed, and sometimes serrated, with which the paired
and median fins of many Fishes are provided, furnish formidable
offensive or defensive organs, especially when they are associated
with poison glands, and also that in by no means an inconsider-
able number of Teleosts the spines may form part of a stridulating
vocal mechanism.

In different Fishes the pectoral and pelvic fins and the

^ Sucker-like modifications of the ventral surface of the body, in which the
paired fins take no part, are present on the throat in many Fishes which frequent
liill-streams, as in some small African and Asiatic Cyprinidae (e.g. Discognathus)
and a few Siluridae (e.g. Eiujlyptosternum).

VI EXTERNAL CHARACTERS 1 63

median fins may, individually, all be absent througb atrophy.
The pectoral fins are rarely absent : nevertheless, in certain
species of Syngnathidae, and in most Muraenidae, for example,
these fins are entirely wanting. The pelvic fins are much less
constant and are often absent in entire families, as in the Pipe-
Fishes (Syngnathidae), the " Electric Eels " (Gymnotidae), and
the true Eels (Anguillidae), and in the Globe-Fishes and Porcu-
pine-Fishes (Tetrodon, Diodon), as well as in certain genera of
ftimilies where they are usually present, as in some of the
Blennies (Blenniidae) and in the Ophidiidae. Even when present
the pelvic fins are often reduced to mere vestiges in the shape of
filaments, as in some of the Gadoids (Gadidae) and Eibbon-
Fishes, or are represented only by a pair of defensive spines, as
in some Sticklebacks {Gastrostcus), or even by a single spine
(Balistidae). Complete suppression of the pelvic fins, or their
reduction to vestigial remnants, seems to be of frequent occur-
rence in Fishes which live in the mud, or are able to pass a
longer or shorter time in soil periodically dried during the hot
season, as in some Cypriuodontidae, and in species of such tropical
Teleostean families as the Ophiocephalidae, Galaxiidae, and
Siluridae. Suppression of the dorsal fin is apparent in the
Gymnotidae, and of the anal fin in the Eibbon- Fishes. In
some of the latter family, as in the rare British visitor the
Oar-Fish {Regcdecus hanksii), and in the Sea-Horse {Hippocampus),
where the tail becomes a prehensile organ for coiling round sea-
weeds when the animal is not swimming, the otherwise remark-
ably constant caudal fin is absent.

An initial stage in the degeneration of median fins is to be
seen in many of the Salmonidae and Siluridae, in which a
posterior division of the dorsal fin becomes reduced in size, loses
its fin rays, acquires much fat in its substance, and becomes an
" adipose fin."

The " lateral line " is a notable feature in the external appear-
ance of most Fishes. Originally developed in the superficial
epidermis of the skin in the form of linear tracts of isolated
and often segmentally arranged masses of sense-cells, these organs
subsequently become imbedded for protection in the epidermic
lining of either an open groove or a closed canal extending along
each side of the trunk and tail, and prolonged on to the more
exposed parts of each side of the head in the shape of a more

1 64 FISHES CHAP.

complex system of branching grooves or of deeply-seated and
externally inconspicuous canals. The course of the lateral line
can, as a rule, readily be detected by the naked eye, and, even
when not otherwise distinguishable, may be traced by the series
of simple or multiple pores through which, at intervals, the
canal communicates with the exterior (Fig. 93, A), and often also,
in the trunk and tail, by a band of coloration different to that of
the rest of the body.

Coloration.

Contrary to popular opinion, it may be doubted if any
animals, even Insects or Birds, can vie with living Fishes in the
brilliancy and changeability of their colours. The nature of
their habitat, the rapid fading of the natural tints after death,
and the fact that museum specimens, however carefully pre-
served, afford but a ghostly resemblance to the colours of the living-
animal, account, no doubt, for much of the prevalent ignorance
of the extraordinary extent to which colour-development may
proceed in a considerable number of Fishes. Like the generality
of northern forms of life, the Fishes of our own seas, rivers and
lakes, are less conspicuous for vivid and striking coloration than
those of tropical or subtropical climes, although such familiar
Teleostean Fishes of our seas and fresh waters as the Mackerel, the
Salmon and Trout, the males of the Stickleback and Dragonet,
some of the Grurnards (Triglidae) and Wrasses (Labridae), the
Opah or King-Fish (Lampris luna), and many others, are notable
exceptions. Brilliancy of coloration is most conspicuous in the
Teleostei : in nearly all other Fishes the colours are more uniform,
usually sober and often sombre, with no more variety than is
afforded by the presence of dark spots or bands on a lighter
ground, or vice versd, or by the lighter colour of the ventral as
compared with the dorsal surface. In Teleosts all the resources
of colour -formation, pigmentation, reflection, and iridescence
through optical interference, in diverse combinations, are employed
in the production of the various tints, while the dominant ground
colour is often diversified by the presence of stripes, bands or
bars, longitudinal or transverse, or of spots of different hues,
frequently arranged in striking and intricate patterns.

The possibilities of coloration in these Fishes may be briefly
illustrated by a few erxamples : —

COLORATION 1 65

In an Australian Fish (Plcetrujwma richardsoni) the pre-
valent ground colour of the body is a brilliant carmine, with a
tendency to yellow beneath, and diversified on the back and sides
with ultramarine spots of almost sapphire-like intensity.^ Certain
Australian species of JBcnjx {B. affinis and B. milllcri) ^ have a
similar ground-colour when freshly caught, but with various
opalescent tints, chiefly blue and lilac reflections. In Polynenvus
vcrekcr'^ the ground colour is chrome yellow, with darker markings,
the pectoral and caudal fins are bright orange, the remaining fins
being a lighter shade of the same tint, and by contrast the long
free filaments of the pectoral fins are a bright vermilion red. The
Velvet-Fish (Holoxemis cutanetis), also a denizen of Australian
seas, has a dominant colour of brilliant scarlet vermilion, or a
mixture of vermilion and orange. The skin has no scales and
presents a singular pilose or velvety appearance.* It is, however,
in some of the Pacific Trigger-Fishes (e.g. Monacanthus) and
Coffer -Fishes (species of Ostracion) that the eccentricities of
coloration are perhaps most strikingly manifest, for not only are
the prevailing colours of the most brilliant description, but the
presence of differently coloured bands or stripes, often arranged
in complex patterns, adds greatly to the gorgeous and singularly
bizarre appearance of these Fishes. To quote one illustration,
the male of the Tasmanian Coffer-Fish (Ostracion ornatus) ^ has
the back and sides of its body grass-green and its belly pale
lemon : the caudal fin is orange-yellow, and the remaining fins a
neutral transparent tint. The sides of the trunk and head are
traversed by broad, irregular, and somewhat interrupted bands
of the most brilliant ultramarine blue, the edges of which are
sliarply defined by dark chocolate-brown lines. Two or three of
the blue body -bauds are continued on to the caudal fin, where
they curl into characteristic loop -like patterns. The lemon-
yellow of the belly is further variegated by a reticulated pattern
in pale l)lue. In the female, formerly regarded as a distinct
species, the ground colour is not green but a pale pinkish-grey,
or dove-colour, with local flushes of a more decided pink, and the
belly is a pure yellow. The blue stripes of the male are repre-
sented in the female by comparatively unbroken bands of a rich
reddish-brown which, at the bases of the pectoral and dorsal fins,

^ Saville Kent, The Naturalist in Australia, London, 1897, p. 150.
- Hid. p. 167 ^ Ibid. p. 168. ■• Ibid. p. 173. = Ibid. p. 188.

1 66 FISHES CHAP.

form an irregular spiral pattern. In both sexes the pattern of
the longitudinal bands is never precisely the same in any two
individuals. Scarcely less brilliant is the coloration of those
Teleosts, notably species of Pomacentridae and Chaetodontidae,
which frequent the coral reefs of the East Indian Archipelago
and the Pacific and feed on the coral polypes, and of many
of the Wrasses (Labridae). Many other groups, such, for
example, as the Percidae, Cirrhitinae, and the Pipe-Fishes
(Syngnathidae), include species in which the coloration is vivid
and often beautiful, although less striking than is the case with
the Fishes mentioned above. As illustrating the opposite
extreme in the scale of coloration, between which and the
brilliant tints just described every conceivable gradation exists,
mention may be made of the colourless appearance of those Fishes
which, like the Kentuckian Blind-Fish {Amblyopsis spelaea), are
denizens of subterranean rivers ; and, omitting a few species in
which the coloration is almost brilliant, the prevalent sombre
tints, dark brown or black, rarely relieved by spots, bands, or
other distinctive markings, of the Fishes inhabiting the abyssal
waters of the deep sea.

The coloration of Fishes is due to the presence in the dermic
portion of the skin of (a) special pigment-containing cells (colour-
sacs, chromoblasts or chromatophores), and (h) a peculiar reflecting
tissue composed of iridocytes.^ Chromatophores are probably
branched connective-tissue cells in which pigments of various
colours are deposited. The colouring matter present in different
chromatophores is red, orange, and yellow, all of which belong to
the lipochrome group of pigments, or black (melanin group), but
by the combination or blending of differently -coloured chromato-
phores other colours may be produced. Thus, green results from
the mixing of yellow and black in suitable proportions ; brown
from the blending of yellow and black ; and other shades or
tints from an appropriate mixture of chromatophores of various
colours. As a rule the muscles of Fishes contain but little
haemoglobin, but, when visible through the skin, the occasional
presence of this substance in localised patches may contribute
a few red spots to the general coloration, as is the case in the
British Fiat-Fish ZejndorJw^nhus megastoraa.

^ Cunningham and MacMuun, Phil. Trans. 184, 1893, p. 765, Avhere references
to many other jiapers are given.

COLORATION 1 6/

Iridocytes consist of guanin, which, in its chemical reactions,
closely resembles the guanin obtained from guano, and therefore
is to be regarded as a further illustration of the utilisation of
waste excretion products for the production of colour in animals.
In forming iridocytes the guanin is deposited in the shape of
granules, or of rounded, polygonal, or stellate bodies, or in
flattened plates. Considered ,-a

as an agent in the produc- '-■#t"''%'%''*-*^1k^l^,

tion of colour, the chief li* ffr'P" ^

feature in the iridocytes is
their opacity and great re-
flecting power ; and accord-
ing to the way in which
light is reflected from them,
the result may be a chalky
white or a bright silvery
appearance. By interfer-
ence these colour elements

are also responsible for the Fig. 94. — The coloration elements in the skin of
. . 1 1 the upper side of a freshly -killed normal
prismatic colours ana Flounder (P/wrrwprft-s/mM), seen by trans-
brilliant iridescence which mitted light. The stellate black bodies are
_,. . 1 -1 '4- ^^^® black chromatophores ; the grey bodies
so many TlSlieS exniOlt. of similar shape rejiresent the yellow chro-
The optical properties of niatophores ; and the small grey plates the
• 1 II- • iridocytes. (From Cunningham and Mac-

guanin has led to its use m Munn.)
the manufacture of artificial

pearls. " Essence d'orient," or " blanc d'ablette," ^ from which
these pearls are made, principally in Paris, is obtained from the
scales of the Bleak {Alhurrms lucidus), and is really the guanin
of which the iridocytes of this Cyprinoid are composed. It is
also to the presence of crystals of guanin that the beautiful
metallic lustre of the iris in many Fishes is due."

The chromatophores and iridocytes are chiefly disposed in two
layers in tlie skin, one outside the scales and the other on tlie
inner surface of the scales, between the latter and the under-
lying muscles ; and although the two kinds of coloration
elements may be present in both layers, their relative abund-

^ Ahlctte is the French name for the Bleak.

"^ Either singly or in combination with lime (Guaninkalk), guanin is often present
in the tissues of Fishes (air-bladder, gall-bladder, subcutaneous connective tissue,
muscle-fasciae, peritoneum, and the retinal epithelium and tapetum of the eye).
For references see Cunningham and MacMunn, op. cit, p. 781 et seq.

1 68 FISHES CHAP.

ance varies in different Fishes, and in different parts of the
surface of the same Fish. Where chromatophores are most
abundant, usually on the back, the reflecting tissue is relatively
scanty, and vice versd. On the sides and belly of a Fish the
place of the inner layer of the dorsal surface may be taken by
the " argenteum." This layer is devoid of chromatophores, and
consists of reflecting tissue in which the iridocytes form a con-
tinuous stratum, either in the form of granules, or as a close
network of rod-like bodies or of polygonal plates in contact with
one another, instead of being less numerous and more scattered
as on the back. When iridescence is produced, it is due to
the iridocytes of the outer layer of the skin ; the dead whiteness
and silvery lustre, on the other hand, have their origin in the
different ways in which incident light is reflected from the inner
layer or argenteum.

To the relative abundance of chromatophores, the kind of
pigment they contain, and the manner in which they are dis-
tributed and blended, combined with the different reflecting
properties, or the iridescence, of the iridocytes, are due the extra-
ordinary wealth and variety of colour in Fishes.

The part played by the different coloration elements in the
production of the characteristic colours of different Fishes may
be illustrated by two examples.^

In the common Whiting (Gadus merlangus) the back of the
Fish is a dark bluish-grey ; the sides have a beautiful iridescence
and silvery glitter, while the belly is very nearly a dead white.
Briefly, these appearances are due to the fact that chromato-
phores (black and deep yellow) are most abundant on the back,
less numerous on the sides, and wanting altogether on the belly ;
while the iridescence and silvery appearance of the sides are due
to the iridescence of the iridocytes external to the scales, com-
bined with the non-iridescent but highly reflective property of a
layer of iridocytes internal to the scales ; and the dead white of
the belly to the different reflecting power of the argenteum, and
the absence of chromatophores in that region.

In the Mackerel (Scomber scomhrus) the distribution of colora-
tion elements is different, inasmuch as they are mainly situated
in the deeper part of the skin, internal to the deciduous scales.
The back is marked by the well-known alternating wavy bands

^ Cunuingham and MacMunn, oj}. cit. pp. 768 and 771.

COLORATION 169

of black and green ; th6 sides gleam with the most brilliant
iridescence, changing from silver to yellow or red gold, according
to the angle at which the Fish is viewed. The black bands of
the back are produced by the crowding together of black chromato-
phores and the diminished number of yellow ; the green bands by
an equal blending of yellov/ and black. Over the dorsal surface
and sides of the Fish, where the coloured bands extend, there
is also a reflecting layer external to the chroniatophores, and
to this layer is due the silvery reflection and iridescence. On
the belly the disappearance of the chromatophores and the
greater thickness and opacity of the argenteum account for the
lighter colour and the diminished iridescence and silvery glitter
of this part of the skin.

Many Fishes are known to have the power of changing their
colours, and in some the change is rapid. Such changes are due
to incident light reflected from surrounding surfaces, acting
through the visual organs and the nervous system on the
differently coloured chromatophores. The latter are capable of
contraction and expansion. Expansion of any particular kind of
chromatophores is accompanied by a diffusion of their pigment —
black, red, orange, or other colour as the case may be — and,
according to the number and distribution of the chromatophores
affected, the prevailing tint or tints of the whole body will be
intensified, or only spots, bands, patches, or flushes of colour will
be produced. Conversely, when chromatophores contract, they
may shrink up to mere dots and bring about a diminution in
the vividness of their respective colours, or even an alteration of
colour, seeing that yellow chromatophores become orange when
contracted, while orange or red appear brown or black. Colour
changes of this kind may be artificially brought about. Experi-
ments with Sticklebacks (Gastrosteusf , kept in glass dishes with
a bottom of black or white tiles, have shown that the Fishes
over the white tiles became partially bleached, while others with
a background of black tiles retained their original coloration.
Bleached Fishes exposed to the white tiles for a relatively short
period (three to ten days) tend to regain their original colour
when subsequently removed to a background of black tiles, but
prolonged exposure to the former conditions (five to six weeks)
seems to render the acquired light colour more or less permanent.

^ A. Agassiz, Bull. Mas, Comp. Zool., Camb. U.S.A., xxiii. 1892, p. 189.

1 70 FISHES CHAP.

The interior of a Minnow-can is sometimes painted white, so
that the bait may assume a lighter colour, and thus become more
conspicuous in the deeper and darker water where Perch and
Pike abound. Hence the colour of a Fish may vary with its
surroundings ; and, as will shortly be shown, the capacity for
producing such changes under natural conditions is of the
greatest importance to Pishes in various ways.

Change of coloration may take place through the develop-
ment of new chromatophores under the influence of new condi-
tions, and is then comparatively slow. Artificial illumination of
the unpigmented white side of a Flounder {Pleuronectcs Jlesus),
by means of a mirror, induces the formation of chromatophores,
and produces a coloration more or less closely resembling the
upper pigmented side.^ A similar change sometimes occurs as a
natural variation, and is then usually associated with structural
deformity in other respects.

Coloration also varies with age, sex, ill-health, and even with the
emotions. Young or immature Fishes are often marked by bands,
bars, or blotches of colour (e.g. the Parr-marks of young Sal-
monidae), which, as they disappear when the Fish approaches the
adult state, are perhaps residual traces of ancestral coloration ;
although, no doubt, change of habits and surroundings are some-
times responsible for such colour changes as are observable during
growth. Conspicuous coloration is one of the most frequent of
secondary sexual characters, the colours of the male being brighter
than those of the female, particularly during the breeding season.
A diminution of colour has been noticed in the artificially-induced
pigmentation of the lower side of a Flounder when the Fish was
suffering from partial suffocation owing to the temporary failure
in the supply of fresh water, the normal colour returning when
the deficiency had been remedied. A similar pallor was caused
by fright when the same Fish was disturbed.^ A nocturnal colour-
change has been recorded in the Tasmanian Trumpeter {Latris
hecateia).^ In addition to the olive-green longitudinal bands
which are always apparent, there are visible at night five broad,
transversely-arranged, blackish bands. As illustrating the im-
portance of vision in colour-changes, it may be mentioned that in
a specimen of this Fish, living in a tank, which had been blinded,

^ Cunningham and MacMunn, op. cit. p. 791, et seq.
2 Ibid. p. 800. 2 Saville Kent, Kat. Austr. p. 163.

COLORATION I 7 I

probably by a rat or a cat, the dark bands were permanently
retained.

Changes of coloration sometimes take place, which either have
no discernible relation to age, condition, or surroundings, or are
brought al)0ut by domestication; and in individuals of the same
species there is often a wide range of colour- variation, which is
sometimes, but not always, associated with particular localities.
In some fresh-water Fishes a yellow colour may replace the
original tint (xanthochroism). The usually dull greenish Tench
{Tinea vulgaris) occasionally becomes a bright orange -yellow.
Another Cyprinoid, the common Gold Fish {Cyi^Tinus auratus), in
its wild state in China is also a dull brown or green, but, when
domesticated, assumes in the first year of its life a black colour
(melanism), then a silvery hue, and finally the vivid ruddy golden
colour of the adult ; occasionally, but rarely, the Fish is an
albino.

The value of a particular coloration in Fishes, either as an aid
to concealment and protection from enemies, or Ijy enabling them
to secure their prey, may now be illustrated by a few examples.

As previously shown, the colours of Fishes may be artificially
varied according to their surroundings. Changes of a similar
kind occur naturally, and when they tend to assimilate the tints
of the Fish to the prevalent hues of its surroundings, and con-
sequently aid concealment, we have examples of what has been
termed variable protective resemblance. Individuals of the same
species vary in colour according to the opacity of the water they
live in, becoming darker in muddy or peaty water, and brighter and
lighter in shallower or clearer water. Trout caught in a stream
with a gravelly or sandy bottom are lighter in colour than those
obtained from a muddy stream, and it is well known that the
same Fish changes colour as it passes from the one background to
the other.^ In a lake in County Monaghan, Ireland, the Trout are
darker on that side which is bounded by a bog, but are of the
beautiful and sprightly variety generally inhabiting rapid and
sandy streams on the opposite side where the bottom is gravelly ;
and narrow as the lake is, the two kinds of Trout appear to con-
fine themselves to their respective areas.^ Trout obtained from a

1 Poulton, The Colours of Animals, Internat. Scientific Series, London, 1890,
p. 82.

- Percy St. John, quoted by Day, Fishes of Great Britain and Ireland, London,
1880-84, ii. p. 58.

1/2 FISHES CHAP.

stream near Ivy Bridge have become much lighter since the pollu-
tion of the water by white china clay.^ As an illustration of the
necessity of vision to such colour-changes, it may be mentioned
that blind Fishes cannot vary their tint in this protective fashion.
A blind Turbot living upon a light sandy bottom differed from
its fellows in being much darker and more conspicuous. Dark
Trout have been observed among their light-coloured brethren in
a chalk stream in Hampshire, but the former were invariably
blind, probably, as their larger size indicated, through age.^

Of other forms of protective resemblance, reference may be
made to the bottom-feeding Flat-Fishes (Pleuronectidae), many of
which have the upper surface of the body coloured with various
shades of brown, speckled with black or light specks or blotches,
in harmony with the prevailing tints of the sandy banks which
usually form their feeding-ground. When disturbed these Fishes
court concealment, and render themselves still less conspicuous by
partially burying themselves in the sand. Many of the Skates
and Eays, which have a white ventral surface, have the back
mottled and coloured in accordance with the colour of the sea-
bottom, but in this case it is possible that the advantage lies
in enabling the Fish to secure passing prey by concealing its own
whereabouts.

Striking examples of protective coloration occur among the
Pipe-Fishes and Sea-Horses (Syngnathidae),which usually frequent
groves of Zostcra, Fucoids, and other sea-weeds. A British
species of Pipe-Fish (Siphonostoma typhle)^ which lives among the
blades of the sea-grass. Zoster a, is olive-green in colour, and is a
typical example of protective resemblance both in colour and in
the slender elongated shape of the body. Similar protective re-
semblances are noticeable among the Sea-Horses, the coloration
varying with the general hue of their envu'onment of sea-weed; but
the climax is certainly reached by the singular Australian species,
PhylloiiUryx eqiies (Fig. 388).^ In this Fish the skin is produced
into numerous long, flattened, branched filaments, which are pro-
longed from the extremities of spine-like outgrowths of the dermal
skeleton, and marked by alternate bands of brown and orange,^

1 Poiilton, op. cit. p. 82. - Ibid. p. 86.

' Cunningham and MacMiinn, 011. cit. p. 773.
* C. Stewart, quoted by Poulton, 07). cit. p. 67.

^ Saville Kent, op. cit. p. 186, describes the colours of the living Fish as " various
shades of light crimson and lilac."

COLORATION 1 73

thus resembling both in shape and colour the fronds of the sur-
rounding fueoids and other marine algae amongst which the Fish
lives.

Many of the Fishes frequenting the coral reefs of the East
Indicin and Pacific areas, especially those belonging to the Teleo-
stean families Chaetodontidae and Pomacentridae, have a most
brilliant and vivid coloration, frequently marked by bands or
stripes of different tint. So fjir from rendering these Fishes
unduly conspicuous, there can be little doubt that, by harmonising
with the striking and varied coloiu's of the anemone-like coral
polypes, their coloration is distinctly protective ; and it is inter-
esting to note that similar colour-patterns have been indepen-
dently reproduced in both families.^ Even the reef-frequenting
Fiat-Fishes (Pleuronectidae) have the usually sombre upper surface
ornamented by vivid colours and striking patterns.

Pelagic Fishes, like the Herring, Mackerel, Flying-Fish {Exo-
coetus), and many others, often have the belly and sides silvery or
white, and the back dark green, black, or steely blue. Seen from
below against the light sky, or viewed from above against the
background of the dark water, these Fishes would seem to be
practically invisible to their predatory foes, whether Fishes or
Birds, or at all events not easily detected.

Coloration may not only be protective, but also aggressive, by
helping to conceal the proximity of an animal from its prey ; add
to this some device for deceiving and attracting the prey, and we
have an example of " alluring " coloration."'

As an example of coloration which is both aggressive and
alluring, the Angler-Fish or Fishing-Frog {Lo'phius ■piscatorius) of
our own coasts may be quoted. Naturally sluggish and inactive
in its habits, and often using its muscular pectoral fins for crawl-
ing about the sea-bottom, the Angler-Fish usually hides itself in
the sand or amongst sea-weeds, which it closely reseinbles in
general colour. Curious branched tag-like processes of soft skin
fringe the sides of the head and body, and in appearance and
colour resemble the smaller fronds of the surrounding sea-weed.
So far the coloration is simply aggressive, and helps to conceal
the Fish from its prey, l;)ut in addition the animal is provided
with a special device for luring its prey within the reach of
its capacious and Frog-like mouth. The first three spines
^ Gunther, Study of Fishes, London, 1880, p. 524. - Poulton, u^j. cit. p. 72.

174 FISHES CHAP.

of the dorsal fin are detached from one another and greatly
elongated, and moreover extend along the middle of the dorsal
surface of the head. The first, which is the longest, terminates
in lobes or lappets of skin, and can be freely moved in every
direction by the muscles inserted into its base. By the agita-
tion of this lure or bait smaller Fishes, probably mistaking the
disturbance for the presence of a wriggling worm, are tempted to
their fate, and soon find themselves engulfed in the enormous
mouth of the artful angler.^ In some allied forms (e.g. Ceratias
hispinosus and Oneirodes eschrichtii) '^ which live in the abyssal
darkness of the deep sea, use is made of the attraction which
light has to aquatic animals, and the fishing-rod spine termi-
nates in a phosphorescent organ, which is probably used for
enticing smaller Fishes within the reach of the jaws of these
singularly modified Angler-Fishes.^

It is by no means improbable that examples of " warning "
coloration occur amongst Fishes. The brilliant colours of some
of the Trigger-Fishes (Balistes, Monctcanthus), Goffer -Fishes
(Ostracion), and Globe -Fishes {Tetrodon) are perhaps of this
nature. They are often associated with the presence of strong
spines, defensive and often erectile, either in connexion with the
dorsal fin or on the general surface of the body, and may tlierefore
serve the purpose of a danger signal to such predatory foes of
these Fishes as might otherwise be tempted to attack them —
to the mutual advantage of the Fishes themselves and their
would-be enemies. The British Weever-Fish {Trachinus) may
perhaps offer another example of warning coloration.^ The Fish
is armed with poisonous spines on its opercula, and, in addition,
has a conspicuous black dorsal fin. When the body of the Fish
is buried in the sand, only its projecting dorsal fin remains to
indicate its whereabouts to predatory Gurnards, which might
otherwise mistake the Weever for harmless Fishes of similar
size and habits. The existence of " recognition " colours or
markings peculiar to the species, to enable individuals of the
same species to recognise one another and to keep together in
shoals, has not yet been proved. It is probable that the relatively

^ For another view of the use of the "lure," see Cunningham, Marketable
British Marine Fishes, London, 1896, p. 338.

^ Giinther, Chall. Heports, Zool. vol. xxii. 1887, p. 50.

' Suggested by Liitken ; Giinther, I.e.

* Garstang, quoted by Poulton, op. cit. p. 165.

COLORATION 1 75

limited range of vision, even in the clearest water, would render
coloration unsuitable for this purpose. Kecognition sounds are
likelj to be far more effective, and there is evidence of their
production by a special vocal mechanism in many Fishes.^

The examples given above show how natural selection may
lead to the evolution of distinctive forms of coloration which are
advantageous to the Fish either for concealment, aggression, or
protection, and in conclusion it may be pointed out that by the
same cause colour may be eliminated or its development checked
if in any way harmful to the animal ; and further, that if a par-
ticular coloration becomes useless to the Fish by reason of a
change in its habits or environment, natural selection ceasing to
act where its intervention is no longer necessary to maintain the
coloration, the latter will sooner or later tend to disappear.

The absence of pigment is sometimes protective. The surface-
swimming larvae of many Teleosts have no chromatophores, and
therefore no obvious pigmentary colours. Their bodies are so
translucent that they can be seen through, and hence are visible
only with difficulty. The transparency of the body may even be
increased by the absence of the red haemoglobin of the blood, as is
the case with the pelagic Zejyiocejjhctlus-larYeLe of the Eel.^ The
iridocytes of the reflecting tissue may also disappear under the
influence of changed surroundings. The larvae of various species
of Onus (Gadidae) are silvery in hue during their pelagic
career, owing to the presence of iridocytes in the skin, but on
becoming mature they change to a dull dark colour, and live under
stones or in holes and crevices in the rocks. During the change
of habit the reflecting tissue (argenteum) is lost, and the needful
chromatophores are acquired.^

Instances of the loss of pigmentary colours, owing to the
cessation of the controlling influence of natural selection, are to be
found in the absence of chromatophores on the white under surface
of the Fiat-Fishes, where such colours are useless but not
necessarily harmful, and in the colourless, cave-inhabiting Fishes,
of which the Blind-Fish {AmUyopsis) of North America may be
taken as an example.

1 See p. 364. - E. Ray Lankester, Proc. Roy. Soc. 1873, p. 70.

^ Cunningham and MacMunn, op. cit. p. 781.

176

FISHES

Poison Glands of Fishes.

A few Teleosts are provided with weapons of offence or
defence in the shape of poison-glands, probably derived from the
epidermis, and associated with spines on the gill-covers, or in
connexion with the dorsal fin, or with both.

The two British species of "Weever" (Trachinus draco and T.
vi^era) are both provided with poison-organs in connexion with
a spine on the operculum and with the five or six spiny rays of
the anterior dorsal fin.^ The first of these spines is a structure
projecting backwards from the hinder margin of the opercular
bone of the gill-cover, and is traversed along both its upper and
lower margins, from base to point, by a deep groove. Except at

-opm.

Fig. 95. — The opercular spine of Trachinus draco and its poison-glands, ai; Articula-
tion of the opercular bone with the hi'omandibular ; gl.gl, the two poison-
glands ; o/J.OT, opercular membrane ; op.s, opercular spine ; r, outer ridge of the
spine ; sh, sheath of the spine. (From W. Newton Parker.)

its protruding naked point the spine is ensheathed in an exten-
sion of the external skin. Along each of the grooves there
extends a solid pear-shaped mass of gland-cells, the broad base of
which coincides with the base of the spine, w^hile the gradually
tapering, narrower portion is continued as far as the sharp point.
The glands enclose no cavity, and there is no duct, so that what-
ever poisonous fluid their cells secrete is probably set free by the
rupture of the cells and discharged into the grooves, along which
it passes to the point of the spine, somewhat after the fashion of
a hypodermic syringe. The origin of the gland-cells from an in-
pushing of the epidermis is indicated by the continuity of the
two structures near the point of the spine. Both in structure
and in their relation to poison-glands each of the spines of the
1 W. Newton Parker, F.Z.S. 1888, p. 359.

POISON GLANDS 177

dorsal fin is almost precisely similar to the opercular spine.
There is no evidence as to how the poison is ejected into a
wound, and it can only be conjectured that it may be caused by
the pressure exerted on the gland when the spine is forcibly
thrust for some distance into the flesh. Certain it is that these
structures are capable of inflicting painful and troublesome
wounds when the Fish is incautiously handled and the skin
accidentally punctured, and no doubt they can be used with
great effect as offensive organs.

A similar poison apparatus exists in certain species of Batra-
chidae, such as Thalassopliryne reticulata} which is by no means
uncommon at Panama. This apparatus is formed by a spinous
outgrowth from the opercular bone and by the first two dorsal
spines. Instead, however, of having two grooves, the opercular
spine resembles the fang of a venomous snake, and is perforated
by a complete canal which is only open at the base and point of
the spine. A poison-sac at the base of the spine discharges its
contents into the canal. The nature of the glands which secrete
the poison has yet to be discovered, but it is probable either that
there are glands in connexion with the poison-sac, or that the
latter is lined by a glandular epithelium. The structure of the
dorsal spines is similar. In some species of the Scorpaenoid
genus Synancia ^ (e.g. S. verrucosa, from the Indian Ocean), the
terminal portions of the dorsal spines are deeply grooved on each
side, and at the origin of each groove there is a pear-shaped bag
containing a milky poison. The bag is prolonged into a duct which,
after traversing the groove, opens at the extremity of the spine.

Many Siluridae are armed with powerful and often serrated
dorsal and pectoral spines which are certainly capable of inflict-
ing dangerous wounds, and not a few of them possess a sac-like
organ with an external opening in the axilla of the pectoral fin.
It is possible that the sac secretes a poison for anointing the
spine, but at present there is no evidence that such is the case,
or that the sac produces any poisonous secretion at all.^

Among the Elasmobranchs the Eagle-Rays (Aetobatis)* and
the Sting-Eays {Trygori), have barbed or serrated spines on the
tail, which inflict wounds far more severe than those caused by

1 Giinther, Trans. Zool. Soc. vi. 1869, p. 437.
- Ibid., Stuchj of Fishes, Edinburgh, 1880, p. 191.
3 Ibid. p. 192. ■» Ibid. p. 190.

VOL. VII N

1/8 FISHES CHAP.

mere mechanical laceration ; but, except the mucus secreted by
the gland cells of the skin, which may possess venomous pro-
perties, no special poison-forming glands in connexion with the
spines are at present known.

Phosphorescent Organs.^

In common with many other animals of similar habitat,
phosphorescent organs (photophores) are highly characteristic
structures in many deep-sea Teleosts belonging to widely different
families (e.g. Stovviatidae, Scopelidae, Halosauridae, and Anoma-
lopidae). These organs probably had their origin in local aggre-
gations of the gland cells of the epidermis, which had acquired
the power of secreting a luminous slime. Luminous organs vary
greatly in number and in their mode of distribution in the skin.
Usually they are found on the sides and ventral surface of the
body and head, very rarely on the dorsal surface, and they often
present the appearance of brightly glistening jewels set in the
skin. A very frequent method of arrangement is in one or two
longitudinal lines along the lateral and ventral surfaces, sometimes
extending continuously from the head to the end of the tail (Fig.
371, A, and Fig. 379), but occasionally interrupted and limited to
portions of the body and tail ; and in a few a distinctly meta-
meric disposition is obvious. On the other hand, the very numerous
and simple organs of Opostomias are disposed in many transverse
bands along the sides of the Fish. In addition to these organs,
which are usually numerous, and whose arrangement is linear,
specially large and often structurally complex luminous organs are
present on different parts of the head and body. In Opostomias
micripnus there is a phosphorescent organ on a median barbel
depending from the chin. Sternoptyx diaphana has one on the
lower jaw. The presence of one or two organs beneath the eyes (Fig.
96) is characteristic of several species (e.g. Opostomias viicripnus,
Astronesthes niger, Pachystomias microdon, Scopelus benoitii, Mala-
costeus indicus). Opostomias micripnus has a luminous organ on
the isolated and elongated first fin ray of the pectoral fin, while in
certain deep-sea Angler-Fishes (e.g. Ceratias) there is one on the
anterior cephalic fin-ray of the dorsal fin. The Scopelid Jpnops

' Lendenfeld, Chall. Eeports, Zool. x.\ii. 1887, p. 277. For references to papers
by LfjyJig, Ussow, Emery, and others, see Lendenfeld, op. cit.

PHOSPHORESCENT ORGANS

179

murrayi^ Q^ig. 371, C) has a singular organ, probably luminous,
beneath the transparent superficial bones of each side of the roof of
the skull. Another member of the same family (Scoj^elns henoitii)
is interesting in having a phosphorescent organ in the middle of
the back, which is directed backwards. An American genus of
Batrachidae {PoricJitliys) has about 350 photophores in relation
with the lateral sense-organs of each side of the head and body.^

Fig. 96. — Pachystomias microdon, showing the two rows of phosphorescent organs
along the side of the body, and the anterior and posterior suborbital luminous organs.
(After Giinther.)

The existence of luminous organs has also been noticed in the
Haddock {Gadidae)? A primitive form of photophore, distributed
in considerable numbers on the head and trunk, either in lines or
diffused over the surface, exists in eleven species of Selachii
{Spinacidae), of which some are known to be luminous.*

Diversity of structure is equally marked. The essential part
of each luminous organ is always a collection of gland cells,
usually disposed so as to form the lining of a series of radially
arranged gland-tubules in the deeper part of the organ, which
also contains ganglion cells, and is supplied with nerves from
contiguous spinal or cranial nerves. The simplest form of phos-
phorescent organ consists of little more than these essential
elements. In the more complex organs an investing pigment-
sheath, reflecting
and lens-like struc-
tures, and an iris
diaphragm, either
singly or in com-
bination, may be Fig. 97. — OjMstomias mkrijmus. Median section of a simple
111 T-i • rv ^T phosphorescent organ, a. Radial gland tubes. (After

added. iiw ^^ ^ * o j.

y

97

Lendenfeld.)

represents one of

the simplest types of phosphorescent organ, wdiich, in groups of

^ Moseley, Challenger Reports, Zool. xxii. 1887, p. 267.

'- C. ^\. Wilson, Journ. Morph. xv. 1899, p. 667.

' Burckhardt, Ann. Mag. Nat. Hist. (7), vi. 1900, p. 568. ^ Ibid. op. cit. p. 558.

I So

FISHES

50 to 100, are arranged in transverse bands on the sides of
Opostomias micripnus, and appear as small white spots on the
otherwise black skin of this Fish.

Each organ has the shape of a biconvex lens, sunk to about
half its thickness in the skin. The inner half is formed of
radially-arranged gland tubes filled with small granular cells,
and converging towards the centre of the organ. Into the con-
nective-tissue walls of the tubes extend blood-vessels and nerves.
External to the gland tubes there is a layer of long slender cells
arranged perpendicularly to the surface, and more externally
still a layer of ganglion cells. There is evidence that these
organs multiply by division. Such simple phosphorescent organs
as these differ little from the groups of epidermic gland cells,
which probably formed the evolutionary starting-point in the
development of these singular structures.

A much more complex type of luminous organ is to be found
in the suborbital organs of Pachystomias microdon, of which
there are two on each side, appearing as conspicuous white
masses, one in front of the other, and situated just below the
eye. The more anterior of the two organs is somewliat pouch -

shaped in section, its walls
consisting of several con-
centric layers (Fig. 98).
Externally there is a layer
of black pigment, within
which is a stratum of
irregular gland tubes. More
internally still there is a
thick layer of light-reflect-
ing spicules, probably de-
rived from an inverted and
modified dermal scale. The
axial part of the organ is
occupied by a number of
radial -disposed structures,
probably similar to the
gland tubes of the simple organs of Ojyosto^nias, and continuous
with a lens-like structure which, as it were, closes the expanded
mouth of the pouch. The superficial skin which forms the
margin of the aperture partially projects over the outer sm-face

Section of
g, Irregular

Fig. 98. — Pachystomias microdon
the anterior suborbital organ,
gland tubes ; (j^, radial gland tubes ; /, iris
like diaphragm ; /, lens-like body ; ;j.5, pig
ment sheath : s. layer of light - reflecting
spicules. (After Lendenfeld.)

VI PHOSPHORESCENT ORGANS I 8 I

of the lens-like body, somewhat after the fashion of an iris-
diaphragm. The organ is supplied by a branch of the fifth
cranial nerve. Between such simple and complex organs as
those above described there are various other types which are
more or less intermediate in character.

A particular type of phosphorescent organ is not necessarily
restricted to the same species ; both the simplest and one or more
of the more complex types may be represented in the same Fish.
Thus, Ojjostojnias 77iicripnus, \Yhich. frequents depths of over 2000
fathoms, has not only the simple organs described above, but also
others differing from the former in having an external pigmentary
sheath, which are scattered all over the body at intervals of 1 to
3 mm. There are also larger and still more complex organs which
are disposed in two parallel rows along each side of the body ;
and finally, the same species has special luminous organs on a
median chin-barbel, and also on an elongated fin-ray pertaining
to the pectoral fin.

The light emitted by phosphorescent organs is probably of use
to deep-sea Fishes in enabling them to seek and detect their
prey in the sunless depths which they frequent. The position
of the organs on the sides and ventral surface of the body, and
the frequent presence of special luminous organs in the vicinity
of the mouth, render them admirably adapted to light up the
water in front of and beneath the Fish, while the existence of
optical accessories for intensifying the luminous beams, and for
regulating their distribution, combined with an abundant nervous
supply, suggests that the emission of light is under the control of
the Fish, and may be varied as the occasion requires. That
these organs may also be defensive, in some instances at all
events, seems not improbable. A flash-light from the dorsal
luminous organ or " stern-chaser " of Scopelus henoitii would
probably dazzle and frighten an enemy in hot pursuit of the
Scopelus. The use of phosphorescent organs as baits or lures
for enticing prey has already been alluded to. There is some
evidence that the colour of the emitted light differs in different
Fishes ; and as there is considerable variety in the precise dis-
position of the organs, it seems probable that in deep-sea Fishes
recognition lights may take the place of the recognition colours
and sounds of those whose lot is cast in a sunnier habitat.

CHAPTER VII

THE SKIN AND SCALES

The skin of the Cyclostomata and Fishes consists (1) of the
epidermis, formed of several layers of epidermic cells, which are
constantly being recruited by the division of the cells of the basal
layer ; and (2) of a stratum of connective tissue with intermingled
unstriped muscle -fibres, blood-vessels and nerves, which con-
stitutes the deeper layer or dermis. From the epidermis are
formed the various unicellular or multicellular glands with which
the skin is provided ; and from one or both of the skin layers
originate the different calcareous structures which constitute the
hard exoskeletou.

In the Cyclostomata the epidermis is particularly rich in
goblet-shaped, mucus-secreting, gland-cells. The Myxinoids also
possess numerous pockets of so-called " thread-cells." In each of
these cells the protoplasm secretes a long spirally-coiled thread,
and under the influence of appropriate stimuli the thread is shot
out and unwound to a g-reat length. The threads and the mucus
are so abundant that one of these animals will convert a bucket
of water into a thick mass of jelly. No scales or other hard
exoskeletal structures are present in any of the Cyclostomata.

In Fishes mucus-glands are also abundant in the epidermis,
and to their activity is due the slimy mucus which lubricates the
surface of the body. They are specially numerous in the Dipnoi
(e.g. Protopterus), where, in addition, there are many simple multi-
cellular glands which secrete the " cocoon " or capsule in which
the Fish is enclosed during the dry season. From the epidermis
are derived the poison-glands of some Teleosts, and also the
" glandula pterygopodia " in relation with the claspers of the
male Elasmobranchs. The glandular structures in connexion with

182

DERMAL SriNES I 83

the phosphorescent organs of the deep-sea Fishes will no doubt
be traced to the same source.

lu the great majority of Fishes the skin becomes the seat of
calcareous deposit, and gives rise to such diverse exoskeletal
structures as the varied forms of spines and scales with which
the surface of a Fish is invested.^ These structures, probably
the most ancient form of Vertebrate skeleton owing its existence
to the presence of lime salts in the tissues of the body, present
higlily characteristic modifications in the different groups.

Exoskeletal structures are of two kinds: (1) those which
owe their formation to the secretory activity of cells belonging both
to the epidermis and the dermis, and (2) those which are derived
solely from the dermis. To the first belong the dermal denticles
or so-called placoid scales of most Elasmobranchs, and to the
second the scales which form the skin-skeleton of living and
extinct Teleostomi and Dipnoi. With the exception of enamel,
which is always formed by the cells of the epidermis, the hard
exoskeletal tissues owe their existence to the secretion of certain
cells of the dermis (scleroblasts),^ the inclusion of which in a
growing calcifying tissue is the cause of whatever cellular struc-
ture the tissue may present. It will shortly be apparent that
the dermic scleroblasts are by no means uniform in their pro-
ducts, and that in different Fishes they give rise to widely
different hard tissues.

The dermal denticles or " shagreen " of the ordinary Sharks
and Dog-Fishes (Elasmobranchii) probably represent the most
primitive form of exoskeleton. In the development of a dermal
denticle a papilla of the dermis grows up into the overlying
epidermis, pushing before it the basal layer of epidermic cells,
which forms an investment to the papilla and constitutes the
so-called "enamel organ" (Fig. 100). The papilla itself sub-
sequently becomes converted into dentine, leaving, however, a
central pulp-cavity, while the apex of the papilla is invested by
a cap of enamel formed by the enamel organ. Ultimately the
base of the papilla widens out into a more or less rhomboidal
basal plate formed of bone. In this way there is formed a

^ "Williamson, Phil. Trans, cxxxix. 1849, p. 435 ; Hertwig, Morph. Jahrb. ii.
1S7C, p. 328 ; v. 1879, p. 1 ; vii. 1882, p. 1 ; Klaatscli, ib. xvi. 1890, p. 97 et seq.,
p. 209 et seq.

^ Klaatsch has since affirmed the epidermic origin of the scleroblasts, ibid. xxi.
1894, p. 1.53.

1 84

FISHES

pointed, enamel-tipped spine of dentine which protrudes through
the epidermis, and projects backwards on the surface of the body,
but is firmly fixed in the skin by the basal plate with which it
is continuous. The centre of the under surface of the basal plate
is perforated for the entrance of the blood-vessels which pass to
the cellular pulp in the axis of the spine. In the adult Fish
the denticles form a fairly close-set covering to the whole body,
including the head and even the surfaces of the fins, and are
larger on the dorsal than on the ventral surface (Fig. 99). In
the Eays {Raia) they are more sparsely scattered, and in different
parts of the body may form spines of considerable size for offen-

FiG. 99. — Surface view
of the dermal den-
ticles of ScyUium
sp., showing their
arrangement in
oblique transverse
rows, h. Basal plate ;
c, canal which per-
forates the basal
plate and becomes
the axial pulp-cavity
of the spine ; f.b,
intersecting fibrous
bands of the dermis ;
s, spine ; in the
spine of one scale the
dentinal tubules are
shown. The smaller
denticles are those
most recently
formed. (After
Klaatsch.)

sive or defensive purposes. The spines vary greatly in shape in
different members of the group, sometimes being acutely pointed,
and sometimes flattened or depressed, and often they are furnished
with smaller accessory spines developed at their bases or from
the surface of the basal plate. An arrangement of the denticles
in oblique transverse rows is observable in some genera (e.g.
ScyUium). In the Saw- Fishes (e.g. Fo'istis) the denticles
which fringe the lateral margins of the long flattened rostrum
are not only enormously enlarged, but are implanted in sockets
and form the teeth of the saw (Fig. 2G2). In the Holocephali
the smooth skin is almost entirely devoid of exoskeletal struc-
tures, but dermal denticles are present on the frontal and anterior
claspers, and in the young there may be a double row of small
denticles alonu; the back.

f.l.-^.

f.b.-

SCALES

185

In the remaining groups of Fishes, the Teleostomi and the
Dipnoi, the spine of the primitive dermal denticle is either
evanescent or entirely wanting, while the equivalent of the basal
plate remains to form the unit of a scaly armature. Evidence
of this may be found in the presence of transitory evanescent
spines, provided with an enamel-cap, secreted by the basal epi-
dermis, on the developing rhomboidal scales, as in the young
Lepidosteus^ (Fig. 101); while the entrance of blood-vessels into
the scales through perforations on their inner surfaces, as in
Polypterus and Lepidosteus, obviously recalls the perforated base
of a dermal denticle (Fig. 99). The epidermis now ceases to

Fio. 100. — Vertical section through the skin of an embryo Shark. C, Dermis ; c.c.c.d,
laj'ers of the dermis ; E, epidermis ; e, enamel organ ; o, enamel layer ; p, papilla of
the dermis. (From W^iedersheim, after Gegenbaur.)

take any part in the formation of the scales, and hence enamel
no longer enters into their structure. A more regular and definite
arrangement of the scales is noticeable, and whether distinct, or
articulating with one another, or overlapping like the slates on
the roof of a house, they are usually disposed in a series of
successive oblique transverse rows. In some of these Fishes the
embryonic epidermic covering of the scales becomes lost, and
their outer surfaces are naked. More frequently, as in the
generality of Teleosts, and in the Dipnoi, the reverse is the case,
and the scales are more or less completely invested both by the
dermis and the epidermis. As regards their shape, size, and
minute structure there is much variation. In some Teleostomi

' .Klaatsch, Murph. Jahrh. xvi. 1890, p. 125 ; Nickerson, Bull. Mus. Comp. Zool.
Harvard, xxiv. 1893, p. 115.

i86

FISHES

the primitive rhomboidal shape of the dermal denticle is retained ;
in others a rounded or cycloid scale supplants the earlier rhombic
type. Within the limits of the same group {e.g. Crossopterygii)
there are examples of the independent evolution of a cycloid from
a pre-existing rhombic squamation ; and with the introduction of
the cycloid type an overlapping or imbricated disposition of the
scales always takes the place of the marginal articulation of the
rhombic type.

As to the causes which may have determined the shape and
mutual relations of scales interesting suggestions have been made.^
Scales bear a segmental relation to the subjacent muscle-segments
or myotomes, sometimes being disposed in oblique transverse rows

Fig. 101. — Development
of a scale in Lepi-
dosteus osseus x 330.
h.jj, Basal plate, with
included bone cells,
at first distinct from
the spine ; e, enamel ;
e.o, enamel organ ;
ep, epidermis, with
large gland cells ;
2), dermic papilla
which forms the ves-
tigial spine ; Sd,
scleroblasts. (From
Klaatsch. )

Scl.

coinciding with the latter, or the rows may be so far increased as
to be multiples of the myotomes. From mechanical considerations
depending on the sigmoid shape and interdigitating relations of
the myotomes and their separating fibrous septa or myocommata,
and the attachment of the myocommata to the dermis, the con-
traction of the myotomes during the lateral flexions of the trunk
in swimming has a tendency to wrinkle the skin into definitely
circumscribed rhombic areas, thus determining the shape, limits,
and disposition of the scales which are developed in those areas.
The rhombic was probably the primitive shape of scales, and is
certainly characteristic of the palaeontologically older types of
scaly Fishes. Generally the rhombic condition is associated with
a peg-and-socket articulation between the upper and lower margins
of adjacent scales. But a rhombic squamation is not without

^ Ryder, Proc. Acad. A'at. Sci. Philadelphia, 1892, p. 219 ; Smith Woodward,
Nat. Sci. iii. 1893, p. 448.

SCALES

187

disadvantages, and would certainly impose some restriction on the
lateral tlexures of the body in swimming, and hence in the different
groups of Fishes it may happen that, in the more specialised forms,
an imbricated cycloid squamation supersedes a rhombic condition,
and with the change the Fish acquires greater lateral mobility.
Even in the same Fish the gradual substitution of the cycloid for
the rhombic type may be observed. In the Australian Aetheoleiyis}
a fossil genus related to the European Liassic Dapedius, there is a
gradual transition along the sides of the body between the articulated
rhombic scales of the relatively immobile trunk and the cycloid
overlapping scales of the flexible tail ; and it may be mentioned
that, even where a typical rhombic squamation exists, the peg-and-

Z.-i

Fig. 102. — Acipenser rnih-
eniis. A, Side view of
the trunk of a speci-
men 30 cm. in length
(nat. size) ; d, dorsal
row of plates ; I, I',
lateral rows ; between
the rows of large
scutes may be seen the
numerons small den-
ticles which are repre-
sented ( X 10) in B ; C,
one of the large scutes
( X 10). (From Hert-
wig. )

socket articulation may be wanting in the caudal region, so as to
ensure greater freedom of movement. Mechanical considerations
may also explain the overlapping of cycloid scales. From the
mode of attachment of the myocommata to the dermis, the con-
tractions of the myotomes, through the pull which they exert on
the former, tend to deflect or depress the scale-areas, particularly
at their anterior margins.

In the surviving Crossopterygii, as in Polypterus, the scales
are rhomboidal and thick, and they only slightly overlap. They
articulate with one another by means of marginal peg-and-socket
articulations (Fig. 106, B). A thick layer of hard, glistening,
enamel-like substance or " ganoin " forms the outer layer of the
scale ; the inner layer consisting of bone in which dentinal
tubules as \9'ell as bone-cells are present. In the numerous fossil
members of the group the scales are either rhomboidal or cycloid.

^ Smith AVoodward, op. cit. p. 449.

i88

FISHES

The oldest representatives of the Chondrostei, the Palaeonis-
cidae (Fig. 283) possessed a complete armature of rhombic
scales, but in all the surviving members of the group the scales
have undergone considerable modification in some respects, and
in others are degenerate. In the Sturgeon (Acipenser) ^ the
primitive rhombic squamation is retained only on the sides of
the terminal part of the tail, and there they are in close apposi-
tion in oblique rows. The rest of the body is traversed by five
widely-separated longitudinal rows of large bony scutes, which,
like the rhombic scales, are furnished with ridges and projecting

Fig. 103.— Surface view of
the rhombic scales of
a youug Lepidosteus.
In two scales the parts
which are overlapped
by adjacent scales are
shaded, c, Position of
the central canal which
perforates the inner
surface of each scale ;
f.h, intersecting fibrous
bands of the dermis ; s,
vestigial spines. (After
Klaatsch.)

spines (Fig. 102). Between the rows of large scales there are
numerous denticle-like structures arranged in oblique rows. Each
of these consists of a basal plate imbedded in the dermis, and of
one or more projecting spines which perforate the epidermis.
All the scales have the same minute structure, consisting mainly
of bone ; but the surface layer and the spines seem to be composed
of a hard laminated substance from which bone-cells are absent
(ganoin). In Polyodon the scutes are wanting, but vestigial
denticles are retained.

Among the Holostei the scales are very different in the two
surviving members of the group. In Lepidosteus (Fig. 103) the

1 0. Hertwig, Morph. Jahrb. ii, 1876, p. 374 ; Klaatsch, xvi. 1890, p. 146.

VII SCALES 189

scales are rhombic, and both in arrangement and structure, as
well as in their method of articulating with one another, they
closely resemble those of Polypterus. In A'mia} <y^ the contrary,
the relatively thin scales are cycloid in shape, and in their im-
bricated arrangement, in their enclosure in pouches of the dermis,
and in tlie absence of any superficial covering of ganoin, they
are very similar to the scales of the more typical Teleosts (Fig.
295). The resemblance extends even to histological structure,
for each scale consists of an outer layer of bone, which gradually
passes into an inner fibrous stratum.

In Teleosts the usually thin and flexible scales are primarily
developed from dermic papillae, but subsequently they come
to lie in pockets or pouches in the dermis. As a rule no
spines are developed, and so far no trace of an enamel organ
has been detected during their development. The scales in their

Fig. 104. — Diagrammatic
lougitudinal section
through the skin of a
Teleost to show the
position of the scales.
(/, Dermis ; ?/>, epi-
dermis ; s, scale.
(After Boas.)

dermal pouches are disposed obliquely to the surface of the
body, so that the hinder part of one scale overlaps the anterior
portion of the scale next behind it (Fig. 104). Only the free
hinder part of each scale has an epidermic investment (Fig. 105).
In minute structure each scale consists of an outer layer of bone,
which, like the bone of the endoskeleton, may either be homo-
geneous except for a feeble lamination, or it may contain bone-
cells arranged in successive layers, parallel to the surface of the
scale. In addition, there is an inner fibrous stratum in which
the fibrous bundles in any one plane cross those in planes above
or below them. The scales are either " cycloid," that is, they
have smooth, unbroken margins (Fig. 105), or the free margin of
each scale is produced into a series of tooth-like spines, and the
scale is said to be "ctenoid" (Fig. 106, A).

Some Teleosts, however, have scales which are neither cycloid
nor ctenoid, and in certain features seem to be intermediate
between ordinary Teleostean scales and dermal denticles. Thus,
on certain parts of the body of Gentriscusf each scale consists

^ Klaatsch, op. cit. p. 178. "^ 0. Hertwig, Morph, Jahrb. vii. p. 15.

190

FISHES

of a rhombic basal plate, produced into a curved, backwardly-
inclined spine, the axis of which contains a pulp-cavity opening
on the inner surface of tlie basal plate (Fig. 107). Some Mal-

^^>;V

Fig. 105. — Cycloid scale of Salmo fario.
a. Anterior portion covered by overlap
of preceding scales ; b, free portion
covered only by pigmented epidermis.
(From Parlcer and Haswell.)

Fig. 106.— a, Ctenoid scale; B, "Ganoid"
scale. (After Giiiither ; from Parker and
Haswell.)

thidae (e.g. Malthe ^) have similar scales, but with round basal

plates and solid spines (Fig. 108, B). Similar scales (Fig. 109),

sometimes rhombic in shape, with' one or more spines, which may

be simple or branched, are also found in the

Sclerodermi (e.g. Balistcs, MonacantJms, Tria-

canthtis)?

Amongst some of the usually scaleless
Siluroid Fishes the scales assume a very
peculiar structure. In Hypostoma^ (Pleco-
stomus) the sides and back of the Fish are
covered by large bony plates, but on the
under surface and on the fins these are
replaced by much smaller ones. Both kinds,
however, carry numerous small movable spines
implanted in sockets (Fig. 110), a fact which
suggests comparison with a stage in the de-
velopment of the scales oiZcjyidosteiis, when the
independently formed and evanescent spines
have not yet fused with the basal plates.

In other Teleosts, as in the Agonidae
and some Triglidae, the body is completely
cuirassed with large keeled bony plates.
The singular appearance of many of the Plectognathi is largely
^ 0. Hertwig, Morph. Jahrh. vii. p. 7. " Ibid. vii. p. 29. ^ Ibid. ii. p. .334.

Fig. 107. — Centriscus sco-
lopax. A, Scale from
the orbital region, x
50 ; B, scale from the
base of the pectoral
fin, X 100. (From
Hertwig.)

SCALES

191

Fig. 108. — A, Scale ot Antennarius hispidus, x 100 ; B,
scale of a young Malthe vespertilio, x 100. (After
Hertwig.)

the result of the curious modifications which their scales
undergo. In some of
the Coffer-Fishes (Os-
tracion) these struc-
tures assume the form
of polygonal bony
plates which suturally
articulate w^ith one
another and enclose
the trunk in a rigid
cuirass, from which the
scaleless tail protrudes
behind (Fig. 438);
while in some Globe-
Fishes and Porcupine-
Fishes (e.g. Tetrodon,
Diodon) the prolongation of the scales into strong erectile spines

equally well serves the pur-
pose of protection (Fig. 439).
Most Teleostomi have
the scales along the " lateral
line " perforated by single or
multiple apertures, through
which the sensory canal
communicates with the
exterior.

In a few Teleosts scales
are entirely absent, as in
most Siluridae ; or they
exist only as microscopic
vestiges hidden in the skin,
as in Eels ; or, as in such
naked forms as Antennarius
inarmoratus and Leimdo-
gaster, and in some Silu-
ridae, they become reduced
to mere papillae of the
dermis.

(Fig. 105) on the surface
index to the a^e of the

Fig. 109. — A, Scale of Batistes capriscus, x 20 ;
B, scale of Monacanthiis scopas, x 20. (After
Hertwig.)

The concentric rings observable
of many Teleostean scales are. an

192 FISHES CHAP, vn

Fish.^ The formation of these rings depends on the fact that
the lines of growth on the surface of the scale are more widely
separated from one another on that portion of the scale formed
during summer, and relatively closer together on that part which
is formed during the winter ; the more rapid growth in the warmer
season probably being due to favourable conditions as to food
and temperature, and the retarded growth of the colder season to
the reverse. Hence, by counting the alternating zones of close-
set winter lines and less closely approximated summer lines of
growth, a reliable clue may be gained as to the age of the Fish.

In the Dipnoi," as in Teleosts, the scales are enclosed in
dermal pockets, and exhibit a regular, imbricated disposition in
oblique rows (Fig. 304, A). In shape they
are nearly cycloid, or slightly oval, with
the long axis coinciding with that of the
body. Structurally, also, they bear some
resemblance to Teleostean scales, although
^ ,,„ „ differing in details. On the outer surface

lIG. 110. — Hrjpnstovia com- n ■, ^ ^

mersonii. A scale from ol the scales there are numerous small
the periphery of the conical spines. No significance, other than

caudal fin, x 50 ; one of ^ .

the spines (s) is im- as an example of evolutionary convergence,
fFroifnertwig^f '*^''^" ^an be attached to the resemblance between
the scales of Fishes so widely separated as
the Dipnoi and the Teleosts.

All known fossil Dipnoi had scales of a similar character,
although differing greatly in size in different genera. In some
(e.g. Dipterus) a layer of enamel -like substance invests the
exposed portions of the scales.

^ Hoffbauer, "Die Altersbestimmung des Karpfen an seiner Schuppe." Jahresh.
des Schlesischen Fischcrei-Vcreins, 1899 ; J. Stewart Thomson, Joihrn. Marine Biol.
Assoc, vi. No. 3, 1902, p. 373.

"^ Giinther, Phil. Trans, clxi. 1871, p. 516 ; Klaatsch, o^;. cit. p. 209.

CHAPTER VIII

THE SKELETON

All Fishes possess an internal skeleton which, in order that it
may be distinguished from the more superficial scaly exoskeleton
described in Chapter VII., is termed the encloskeleton. The
latter consists (i.) of an axial part, including the vertebral column
and the skull ; and (ii.) of an appendicular portion, consisting of
the skeleton of the limbs and their supporting pectoral and
pelvic girdles.

The Vertebral Column.^ — The individual segments or vertebrae
which, arranged in a linear series, collectively form the vertebral
column, are highly complex structures, each being formed by a
number of vertebral elements, the sum total of which constitutes
a vertebra. Perhaps the best conception of the nature of vertebral
elements is to be gleaned from the study of such primitive Pishes
as the Elasmobranchs, in which not only are all the vertebral
components present, but they are less modified by suppression
and fusion than in most other Fishes, and on this account they
afford a convenient introduction to the study of the puzzling
eccentricities of vertebral structure in other groups. Selecting
any common Dog-Fish, such as Scyllium canicula, and starting
with an early embryonic stage, it may be stated that the first
indication of a vertebral column is the formation of the notochord,
which, invested by its chordal sheath, extends from the tip of
the tail to a point on the under surface of the brain just behind
the hypophysis or pituitary body. Subsequently, a number of
cartilaginous pieces are developed in connexion with the dorsal

^ This portion of the cliapter is mainly based on tlie important researches of Dr.
Oadow and Miss Abbott. See Pliil. Trans. 186, 1S95, p. 163 ct scq. where copious
references to the work of other writei's are given.

VOL. VII 193

194

FISHES

b.d. i.d-

s.d.

and ventral siu^faces of the notochord, which, as they form
portions of a system of dorsal and ventral arches, are termed
'•' arcualia" (Fig. 111). On the dorsal side there are : (i.) a series
of paired hasi-dorsal cartilages (neurapophyses or neural arches),
the two elements of each pair contributing to form the side walls
of the neural canal in which the spinal cord is lodged (Fig. 112,
A) ; (ii.) a series of inter-dorsal cartilages (intercalary neural

arches), regularly alternating with
the preceding, and completing
the walls of the neural canal by
filling up the intervals between
the basi-dorsals ; and (iii.) a series
of supra-dorsal elements, typi-
cally also in pairs, but in the
Dog-Fish fused to form single
median cartilages. Of the latter
there are two sets — one the
supra, -hasi -dorsals, or neural
spines, are situated over the basi-
dorsals ; and the other, supra-
V?j^f?*-ffors«/s, alternating with the
former, lie over the inter-dorsals,
the two series forming the key-
stones of the dorsal arches, and

indicate the limits of neuromeres and ^J^^ completing the rOof of the
myotomes. The small circles represent ■> -i ^ ^ i • j

the exits of the dorsal and ventral roots neural caual. On the ventral Side
of spinal nerves. (After Eidewood. j ^f ^^g uotochord this arrange-
ment is substantially repeated by a series of ventral arcualia,
which, however, are somewhat differently arranged in the
trimk and tail. Thiis, in the trunk there are : (i.) a series of
hasi-ventral or haemal cartilages, corresponding with the basi-
dorsals above, which grow out laterally into short processes,
the 2^^'''^^P'^P^^W^ o^ transverse processes, and terminate in
(ii.) short, slender cartilages — the costal elements or ribs — which
may perhaps be regarded as the ventral equivalents of supra-basi-
dorsals. The ribs project outwards into the dorsal wall of the coelom
and end in the myocommata separating the myotomes of the body-
wall. In the tail the basi-ventrals lose their ribs and, gTOwing down-
wards into ventral prolongations, they unite in pairs beneath the
caudal artery and vein, and so form a series of inverted arches

Fig. 111. — A, side view of precaudal verte-
brae of ScylUum canicida ; B, similar
Yiew of caudal vertebrae, h.d, Basi-
dorsal ; c, centrum ; h, basi- ventral ;
h.s, haemal spine : i.d, inter-dorsal ;
}}, parapophysis ; r, rib ; s.d, supra-
dorsals. The vertical dotted lines

VERTEBRAL COLUMN

195

{haemal arches) enclosing a haemal canal (Fig. 112, B). The apex
of each arch is prolonged into a median process or haemal spine.
Although not recognisable in the Dog-Fish, paired inter -ventral
cartilages, corresponding with the inter-dorsals above, are present
in some Elasmobranchs and alternate with the basi-ventrals. In the
caudal region of others {e.g. Skates and Eays) ventral counterparts
of the supra-interdorsals are present, and are termed infra-ventral
cartilages. ]\Iuch in the same way that their dorsal equivalents
enclose a neural canal, so the ventral arcualia partially surround the
viscera-containing coelom in the trunk: and in the tail, but more
completely, the

vestigial coelom ^ ^

of that region or
the haemal canal.
The different
vertebral compon-
ents are by no
means of equal
morph ological
value. The basi-
dorsals and basi-
ventrals, and the
inter-dorsals and
inter-ventrals, are
the primary elements and the most important. The supra -dorsals
are merely cartilages segmented off from the basi-dorsals and inter-
dorsals, while the ribs and the infra-ventrals are similarly derived
from the basi-ventrals and inter-ventrals respectively. As to the
vertebral elements which collectively form a vertebra in the Dog-
Fish, it would seem from evidence afforded Ijy the neuromeres.^ and
more especially by the facts of development, that each complete
skeletal segment or vertebra consists of a pair of basi-dorsals with
the preceding pair of inter-dorsals, and of a pair of basi-ventrals
with the next succeeding pair of inter-ventrals. It must be
emphasised, however, that, considered as a joint or segment in a
flexible liack-bone, a vertebra is a physiological unit, the morpho-
logical value of which may chffer widely in different Fishes.
Hence, in other Fishes, the gTouping of vertebral components to

^ Neuromeres are body-segments defined and limited by the exits of the suc-
cessive pairs of spinal nerves from the neural canal.

Fig. 112. — A, transverse section of a precaudal vertebra: B,
similar section of a caudal vertebra, h.a, Haemal arcli
(basi-ventrals) ; li.c, haemal canal : h.s, haemal spine ; n.c,
neural canal. Other reference letters as in Fis. 111.

196 FISHES CHAP.

form individual vertebrae may be quite different to that whieli
takes place in the Dog-Fish, and may even be accompanied l)y
their more or less complete fusion.

In the more primitive types of vertebral column, such as
are characteristic of many fossil and not a few existing Fishes,
arcualia alone are present, and remain associated with a per-
sistent notochord which has grown with the growth of the
animal. In the more specialised Fishes, on the contrary, the
need of an axial support for the body, which, while retaining
the necessary flexibility, must possess greater strength, has
resulted in the development of a series of solid cartilaginous,
calcified or bony, discoidal joints or segments, the centra, which
surround and more or less completely replace the notochord, and,
while supporting, form also a bond of coimexion between the
dorsal and ventral arches. Notwithstanding their superficial
resemblance, an important developmental distinction is to be
noted in the mode of formation of centra in different Fishes,
which enables one kind to l)e distinguished as "chorda-centra"
and another as " arch-centra." ^ Chorda-centra are centra formed
by the conversion of the chordal sheath into a series of ring-like
cartilaginous segments, which subsequently, by a process of inward
thickening, become biconcave, disc-like structures, and more or less
completely replace the notochord, except in the spaces between
them. Arch-centra, on the other hand, owe their formation to
the growth of the bases of the primary arcualia round the noto-
chord, external to the chordal sheath, and their subsequent fusion
to form annular segments, wdiich, later, become biconcave centra.
Of Fishes which possess vertebral centra the Elasmobranchs alone
have chorda-centra ; the Holostei and the Teleostei, and very
probably the Crossopterygii also, having arch-centra. The Dipnoi
and the Holocephali, and the Chondrostean Teleostomi are acent-
rous— that is, they are devoid of vertebral centra and possess a
persistent notochord. Neither in their embryonic development nor
in their evolution in time are the different vertebral components
synchronous in their appearance. Developmentally, the arcualia
are the first to be formed, and of these those on the dorsal aspect
of the notochord appear earlier than their representatives on the
ventral side, while the centra are the last of all ; and in a general
way the palaeontological sequence agrees with the embryological.

^ Gadow, op. cit. p. 190.

VIII VERTEBRAL COLUMN 1 97

The independent evolution of a more specialised vertebral
column from a more primitive one may often be traced within
the limits of the same group of Fishes when the more ancient
genera are compared with the more recent. In the Elasmo-
branclis and the Crossopterygii, for example, the oldest knowai
types were acentrous, while the more recent have acquired calci-
fied or bony centra, and altogether they have reached a more
advanced stage of vertebral evolution. Some Fishes, like the
Chondrostei and the Dipnoi, seem, however, to exhibit com-
paratively little advance in vertebral structure, since both the
Palaeozoic and the living representatives of these groups agree
in being acentrous.

Some of the more notable features in the structure of the
vertebral column in the Cyclostomata and Fishes will now be
briefly considered.

In the Cyclostomata the acentrous vertebral column is more
primitive than in any other Craniates, and in the Lamprey it
consists of a persistent notochord, supporting a series of isolated
cartilages on each side of the spinal cord.^ As two pairs of
these cartilages are included in each neuromere it is possible
that they represent alternating basi-dorsals and inter-dorsals.
There are no ventral arcualia in the trunk and no ribs. In the
Hag-Fisli {2Iyxine) the dorsal cartilages are restricted to the
tail.

The description of the vertebral column of the Dog-Fish may
be taken as fairly applicable to Elasmobranchs in general, and
hence only certain notaljle features in some other members of
the group need be referred to here. The most primitive Elasmo-
Ijranchs, the Palaeozoic genera Cladoselache and Fleur acanthus
were acentrous, although calcified rings have been observed in a
Permian species of the latter genus and scattered calcifications
in others. Some of the earlier Mesozoic genera (e.g. Hylodvs)
were also devoid of centra, at least in the trunk-region. The
first indication of complete centra occurs in the Lower Lias
Cestraciont, Palaeospinaxr' All the later extinct, as well as all
existing forms, liave more or less well-developed centra, hardened
by the deposit of lime salts in their primitively cartilaginous

^ Schneider, Beitr. z. vcrrjl. Anat. u. EntivicH. TFirbcUh., Berlin, 1879, p. 51 ; also
Gadow, op. cit. p. 196.

2 Smith Woodward, Brit. Mus. Cat. Fossil Fishes, Pt. i. 1S89, p. xvii.

198

FISHES

substance, but never in the form of true bone. The mode in
which the lime is deposited is marked by certain peculiarities
which are characteristic of particular families^ (l^ig- 113). In
some genera, as in the extinct Palaeospinax and the living
Acanthias and Scymnus, the calcified portion of each centrum
takes the form of a cylinder constricted across the middle, like
two cones joined apex to apex (cyclospondylic). This condi-
tion is probably the most primitive, but it may be modified in
other genera by the further addition of calcic salts in two
different ways. Thus, the deposit may take place by the simple
addition of concentric layers to tlie original constricted cylinder

h.a--

FiG. 11-3. — Schematic transverse section through the midille of a Cyclospondylic (A), a
Tectospondylic (B). and an Asterospondylic vertebra (C). </, Middle portion of tlie
calcified double cone ; d', additional concentric calcitied layers ; </", double cone
with radiating calcified layers ; cx.m, external elastic membrane : h.a, haemal arch ;
71. a, neural arch ; n.c, notochordal cavity. (From Zittel, after Hasse. )

(tectospondylic), as in the Skates and Rays ; or it may take the
form of a series of longitudinal plates radiating outwards from
the cylinder, and giving rise to a star-like pattern in cross-section
(asterospondylic), as in Scyllinm and Lamna. In most living
Elasmobranchs (e.g. Scylliuni), but not in such genera as Axdi-
danus, Heterodontns, and Squatina, the bases of the dorsal and
ventral arches grow round the centra and meet, or even fuse, so that
the hitter become surrounded Ijy rings of cartilage which, after a
fashion, suggest incipient arch-centra (Fig. 112, A). The caudal
portion of the vertebral column is often described as " diplo-
spondylic," that is, there are two centra, two pairs of basi-
dorsals, two pairs of inter-dorsals, and two pairs of basi- ventral s,

' Hasse, Das naturlichc Syst. d. Elas)nohranchicr, etc., Jena, 1879, p. 30, ct. seq.

VERTEBRAL COLUMN

199

or in other words, two vertebrae to each neuromere ^ (Fig.
Ill, ]'.)•

The Holocephali have a vertebral column essentially similar
to tluit of other Elasmol)ranchs, but of a more primitive type (Fig.
114). The notochord is persistent and there are no centra ; but ring-
like calcifications, four or five

to each neuromere, occur in the
ehordal sheath in Chimaera,
although not in Callorhynchus.
Eibs are absent. In the whip-
like terminal portion of the
tail the arcualia and the
notochord become replaced by
a slender continuous filament
of cartilage.

In the more obvious fea-
tures of vertebral structure
the Dipnoi - have much in
common with the Elasmo-
branchs, especially with cer-
tain of the acentrous Palaeo-
zoic representatives of that
group. The notochord is per-
sistent, centra are wanting,
and the different vertebral

n.sp

h.r

B

components continue to retain Fig. 114. — A, transverse section of the verte-
bral column of Chimaera monstrosa ; B,
lateral view. c.r. Calcified ring ; A.r, basi-
ventral ; /)ii, inter-dorsal ; ■?(.«, neural arch
(basi - dorsal) ; nch, no'tochord ; nch.sh,
ehordal sheath ; n.sp, neural spine (supra-

dorsal).
Hasse.)

(From Parker and Has well, after

their primitive distinctness.
On the other hand, the basi-
dorsals are much better devel-
oped than the inter-dorsals,
which are either vestigial or
absent. The basi-dorsals unite in pairs over the spinal cord to
form complete neural arches, and each arch supports dorsally
the legs of a A -shaped, gable-like element or neural spine, which
probably represents a pair of fused supra-basidorsals. Ventrally,
there are basi- ventral cartilages, fused in pairs beneath the noto-
chord, and supporting well - developed, bone - ensheathed ribs.

^ Gadow, op. cit. p. 194 ; Ridewood, Jourii. Linn. Soc. Zool. xxvii. 1899, p. 46.
- Giinther, Phil. Trans. 161, 1872, p. 526 ; Wiedersheim, Morph. Studien, Jena,
1S80, Pi. i. p. 65 ; Gadow, op. cit. p. 198.

200

FISHES

luter-veutrals appear to be absent. Each neuromere corresponds
with a pair of basi-ventrals, of basi-dorsals and of inter-dorsals.
The haemal arches and spines are formed partly by the basi-
ventrals, but mainly by the ventral union of the successive pairs
of ribs. As in the Holocephali, the terminal arcualia of the tail
become fused into a straight axial cartilaginous filament, trans-
versely divided into segments, which replaces the notochord.

s.n

Fia. 115. — Side view of the preeauJal vertebrae of a Sturgeon {Acipenser sturio). a.c,
Aortic canal, formed by the median union of ingrowths from the basi-ventrals and
inter- ventrals of opposite sides ; h.d, basi-dorsals ; h.v, basi-ventral ; i.d, inter-
dorsal ; i.v, inter - ventral ; w, notochord ; w.c, neural canal ; n.sp, neural spine ;
nt.s, cuticular sheath of the notochord ; p, parapophysis ; r, rib ; s.?;,- aperture for
the root of a spinal nerve.

Each segment supports a variable number of dorsal and ventral
gable-pieces, or neural and haemal spines. Certain of the verte-
bral components, such as the ribs, and the neural and haemal
spines, are ensheathed by membrane bone.

With certain modifications in details the preceding descrip-
tion will also apply to the vertebral column of the Chondrostei
(Fig. 115). It will be noted, however, that the inter -dorsals
are much better developed than in the Dipnoi, although when

VERTEBRAL COLUMN

20I

compared with the basi-dorsals they take but a small share in
forming the walls of the neural canal. Well -developed but
somewhat fragmentary inter-ventrals are present. The haemal
arches and spines are formed by the downgrowth and ventral
union of the basi-ventrals as in the Dog-Fish, and apparently
without the aid of costal elements. In Polyodon the ribs are
vestigial/ but in Aclpenscr they are well developed. The neural
arches and spines, and their haemal representatives in the tail,
and also the ribs, are partially ossified, or ensheathed by bone.
In the existing Crossopterygii, Holostei, and Teleostei, popularly

Fig. 110. — Diagram to illus-
trate the grouping of
vertebral elements to form
vertebrae. A, in au Elasmo-
braucli, B, in Amia, and
C, ill Leijidosteus. B.D,
Basi-dorsals; B.V, basi-
ventrals ; I.D, inter-
dorsals ; /. V, inter-vent-
rals ; in.v.c, inter- verte-
bral cartilage divided by
a coiicavo - convex cleft ;
p.c, precentrum ; ^^i.c,
postcentrum. The square
blocks represent indi-
vidual vertebrae, and the
olilique lines, the attach-
ments of the myocomniata.

known as the " bony Fishes," the vertebral column assumes a more
familiar character, and at the same time we meet with interesting
illustrations of the different methods by which the separate com-
ponent vertebral elements of the more primitive types of " back-
bone " are concentrated together in groups, and fused to form that
complex physiological product, the complete bony vertebra." In
most of these Fishes each vertebra is formed by the aggregation
and fusion of a pair of basi-dorsals and a pair of basi-ventrals,
and includes, in addition, a pair of inter-dorsals, which may
either be the pair in front of the basi-dorsals or the pair behind,
and also a pair of inter-ventrals, which, similarly, may be the

1 Bridge, P.Z.S. 1897, p. 722. - Gadow, oj). cit. p. 201, ct. seq.

\

P;""

ptc

\

\ BD

ID

\ BD

ID

\ BD

ID

Bv\

iV

Bv\

IV

Bv\

IV

I —

202 FISHES CHAP.

pair in front or behind the basi-ventrals (Fig. 116). The product
of this fusion is a series of bony vertebrae, each consisting of a
biconcave arch-centrum, which includes the fused basal portions
of a pair of basi-dorsals and a pair of basi-ventrals. The distal
portions of the basi-dorsals form tlie neural arch, while the rib-
bearing parapophyses are lateral outgrowths from the basi-
ventrals which otherwise have become merged in tlie centrum.
Finally, the centrum is completed by its fusion with a pair of
inter-dorsals and a pair of inter- ventrals. Supra-dorsal elements
may also be included as minor contributory factors. The supra-
l:)asi-dorsals co-ossify with their basi-dorsals and then unite to
form the ordinary unpaired neural spine of most bony Fishes, or,
as in Amia, they remain distinct from each other, and are obvious
as a double spine. In Zepidosteiis these elements co-ossify with
the neural arches and fm^m the post-zygapophyses. Supra-inter-
dorsals have been identitied in the embryo as distinct elements,
l)ut their eventual fate is not always known. In Lepidostevs they
persist as distinct cartilages in the adult (Fig. 118, A). Well-
developed bony ribs are usually present. The haemal arches ot
the tail are formed by the downgrowth of the parapophyses and
their ribs, or by the latter alone, and by their ventral union to
form haemal spines ; consequently, each arch always includes a
pair of costal elements. With such general features in common
there are certain notable variations in some of these Fishes, to
which brief reference may be made.

Little is at present known of the development of the
vertebral column in either of the only two existing genera of
Crossopterygii, PolyiHcrus^ and Calaniichtliys, and hence the
precise mode of grouping of their vertebral components to form
vertebrae is unknown. The condition of the vertebral column
in the fossil forms varies greatly in different families, but in
none is it so specialised as in the surviving members of the
group. In the Devonian Holoptychidae, and even in genera so
comparatively recent as the Upper Cretaceous Coelacauth Macro-
poma, the persistence of the notochord and the absence of centra
indicate a very primitive grade of vertebral evolution. The
Devonian and Carboniferous Rhizodontidae (e.g. IJusthciiopteron
and Rhizodus), on the contrary, seem to have had well-ossified
ring-like vertebrae.

' See Budgett, Trans. Zool. Soc. .\vi. Pt. vii. 1902, p. 315.

VERTEBRAL COLUMN

203

In the caudal region of Amia the basi- dorsals and basi-
ventrals, and the inter-dorsals and inter-ventrals, form separate
arch -centra which remain distinct; hence each vertebra is
double, and there is a regular alternation of arch-bearing " pre-
centra " and arch -less "post-centra" (Fig. 117, D). In the
trunk-region the pre- and post-centra have fused, and in this
region the vertebrae are single.

A very primitive type of vertebral column occurs in some of
the Jurassic allies of Amia, in which certain of the vertebral
components, confluent in the adult Amia, retain some measure
of their primitive distinctness.^ Thus, in the precaudal region
of Eurijcormus (Fig. 117,B) there is a series of alternating dorsal

7J.Sp ■-

Fig. 117. — A, precamlal verte-
brae of Caturus f ureal us ;
B, similar vertebrae of
Eurycormus speciosus ; C,
caudal vertebrae of the
latter species ; D, caudal
vertebrae of Amia calva.
h.a, Haemal arch ; h.sp,
haemal spine ; hi/.c, hypo-
centrum ; n.a, neural arch ;
n.sp, neural sjiine ; p, para-
pophysis ;p).c, pre-centrum ;
pl.c, pleuro - centrum ; jjf.c,
post-centrum ; r, rib. (After
Zittel.)

and ventral half-rings of bone, which between them completely
invest the persistent notochord. Each ventral half- ring or
'■' hypocentrum " represents a pair of fused and ossified basi-
ventrals, and possibly also a pair of included inter-ventrals, and
supports dorsally a pair of basi-dorsals, forming a neural arch,
and laterally a pair of ribs. The dorsal semi-rings, or " pleuro-
centra," similarly represent fused and ossified pairs of inter-
dorsals. In the tail, modifications approximating to what is
seen in the caudal region of A7nia are to be noticed (C). By
the upgrowth of the ventral arch-bearing semi-rings, and their
conversion into complete rings encircling the notochord, incipi-
ent pre-centra are formed, and by a similar modification of the

' Zittel, Handb. d. Palaeontoloyie, iii. 1SS7-1890, p. 137 et scq. ; Gadow, op.
cit. p. 208.

204

FISHES

down-growing, archless, dorsal half-rings, structures comparable

to post-centra are produced. In brief, Eurycormus, as well as

such other extinct Aniioid genera as Caturus (Fig. 117, A), Cal-

lo2)teriis, and Euthynotus, retain in the adult a stage of vertebral

evolution which is closely paralleled by transitory stages in the

embryonic and young forms of Amia.

Lcpidosteus ^ is unique amongst existing Fishes in having

opisthocoelous vertebrae ; that is, the centra are convex in front

and concave behind,
is ... ^

and therefore articu-
late with one an-
other by ball-and-
socket joints (Fig.
118). This condi-
tion is due to the
development of a
series of interver-
tebral rings of car-
tilage round the
notochord. The
subsequent inward
growth of each of
these rings leads to
the constriction, and
ultimately to the
complete oblitera-
tion, of the noto-

FiG. 118. — A, two vertebrae from the trunk-regiou of chord, inuch ill the
Lepidosieus ; B, anterior face of a vertebra. en,
Anterior convex face of the centrum ; cm', posterior
concave face ; h.a, ]iarapophysis, with its articular facet
for a rib ; i.c, median cartilage, representing a pair of
fused supra-interdorsals ; i.s, radial element of the dorsal
fin ; U, superior longitudinal ligament ; n.a, neural arch.
(From Wiedersheim, after Balfour and W. N. Parker.)

same way as l)y
the growth of ordi-
nary centra. Later,
this solid mass of
cartilage becomes
transversely divided by a cleft which is convex anteriorly and
concave behind (Fig. 116, C), and of the two portions one fuses
and co-ossifies with the centrum of the vertebra in front, and the
other with the one pertaining to the vertebra behind. Reference
to Fig. 116 will show that the grouping of the vertebral elements
to form the individual vertebrae is not the same as in Aviia.
^ ¥. M. Balfour and W. N. Parker, Phil. Trails. 173, 1882, li. 388.

VERTEBRAL COLUMN

205

111 the dominant group of existing Fishes, the Teleostei, the
centra are almost invariably biconcave, although in the Eels they
may be flat or even slightly convex in front. Eibs are absent in
the Syngnathidae and in the Plectognuthi. In addition to the
usual articulation between the centra, the vertebrae often articu-
late with one another by means of paired processes arising from
the anterior margin of each neural arch, or from the centrum
at the base of the arch (pre-zygapophyses), and meeting similar
processes which project either from the hinder margin of the arch
of the verteljra in front, or from the adjacent portion of its centrum
^post-zygapophyses). The haemal arches may have similar pro-

Fir,. 119. — A, side view of precaudal vertebrae of a Cod {Gadus morrhiia) without the
ribs ; B, similar view of caudal vertebrae of the same Teleost. c, Centrum ; h.a,
haemal arch ; h.sp, haemal spine ; 7i.a, neural arch ; 7i.sp, neural spine ; ^Jj para-
pophysis ; jj.z, pre-zygapoph\-sis ; jj^s, post-zygajjojihysis.

cesses (Fig. 119). One, two, or in some Teleosts, three pairs of
slender intermuscular bones radiate outw^ards from the centra
into the inyocommata (epicentrals), or from the neural arch
(epineurals), or from the ribs (epipleurals).

The Ribs. — It is doubtful if the structures termed "ribs"
are homologous in the different groups of Fishes. There appear
to be two kinds, distinguishable as dorsal and ventral ribs (Fig.
15 6). Dorsal ribs are situated in the fibrous tissue separating
the epiaxial from the hypaxial muscles of tlie body wall, and they
take no part in forming the haemal arches of the caudal region.
Ventral ribs, on the other hand, always lie internal to the
hypaxial muscles, and directly external to the peritoneal lining
of the coelom, and they usually contribute to the formation of

206 FISHES CHAP.

the haemal arches. To the former belong the ribs of the Elas-
mobranchs, and to the latter the ribs of the Teleostomi and
Dipnoi. Pohjptcras alone has both kinds of ribs.

The Skull.

The skull is a highly complex structure, the various com-
ponents of which are as different physiologically as they are
morphologically. It consists (i.) of the cranium, for the
enclosure and protection of the brain ; (ii.) of sense cwpsides,
which fulfil a like function for the auditory, visual, and olfactory
organs ; (iii.) of certain vertebrae or vertebral elements fused with
the hinder part of the cranium ; (iv.) of a series of visceral
arches ; and (v.) of a series of paired or median cartilages developed
in relation with the mouth and nostrils, which may be collectively
spoken of as " labial " cartilages.

The cranium is formed in the embryo from two pairs of
cartilaginous rods or plates, developed in the mesoblast of the
head. Of these the posterior pair, or paracliordals, underlie the
hinder part of the brain, and are situated one on each side of the
cranial portion of the notochord. The anterior pair or traheculae
are pre-notochordal, and lie beneath the anterior portion of the
brain.^ Between their hinder extremities, and in front of the
anterior termination of the notochord, is the pituitary body. As
development proceeds the parachordals blend with each other
and with the traheculae, while the latter fuse in front to form
a median plate — the mesethmoid cartilage. The hinder portions
of the two traheculae remain distinct for some time, and enclose
between them the pituitary fontanelle, but later they fuse
beneath the pituitary body, leaving, how^ever, a pit for its
reception — the pituitary fossa. Cartilaginous capsules are formed
round the cranial sense organs. The auditory or periotic capsides
fuse on each side with the parachordals. The oiAic cajJSidcs,
either fibrous or cartilaginous, remain free, and do not fuse with
the adjacent trabecular region. The olfactory capsules alone are
not developed independently, but are formed as lateral out-

^ As additional primary cranial elements mention may be made of a pair of
independently developed "alisphenoid " cartilages, which lie in front of the
j)arachordals between the brain and the ej^es, and above the traheculae, and form
a considerable part of the inter-orbital region of the cranium. See Sewertzotf,
Anat. Anz. xiii. 1897, p. 413 ; ibid., Kupffcr Festschrift, Jena, 1899, p. 281.

VIII SKULL 207

growths from the mesethmoid plate. Later, the parachordals and
trabeeulae grow upwards on each side round the brain, and to a
greater or less extent they meet and fuse on its dorsal surface,
thus enclosing the latter organ in a cranial cavity, leaving,
nevertheless, a large foramen behind (foramen magnum) through
which the In-ain is continuous with the spinal cord. In this
condition the primitive cartilaginous cranium, with its included
sense-capsules, has reached a stage wdiich is permanently retained
in such Fishes as the Elasmobranchs.

The visceral arches consist of a number of pairs of curved
rods of cartilage, at first simple, but subsequently segmented, and
developed in the splanchnic mesoblastic walls of the oral cavity
and pliarynx. Each rod is connected with its fellow by a
median cartilage in the floor of the pharynx, so that the whole
form a series of dorsally incomplete hoops encircling the anterior
portion of the alimentary canal. No doubt all the visceral
arches were originally branchial arches, and were so disposed
between the successive gill-clefts as to support their walls and
the vascular folds or gill-lamellae to which they gave rise. In
Fishes most of the arches still retain their primitive gill-
supporting function, but the first or mandibular arch has become
modified to form upper and lower jaws, altliough in the Sharks
and Dog-Fishes it may lie in front of a gill-cleft and still be
associated with vestigial gills. The second or by old arch is less
removed from the condition of a branchial arch, and generally
supports either a functional or a vestigial gill, but in most
Fishes it has acquired the secondary function of forming a
suspensorium for the attachment of the jaws to the cranium.

The skull of the common Dog-Fish, Scyllinni canicvia (Fig.
120),-' may be studied as a type which in the adult remains carti-
lao-inous, and has no secondary addition of cartilae;e- or membrane-
bones. In this Fish the chondrocranium, or primary cartilaginous
cranium, presents the appearance of a somewhat depressed oblong
Ijox, W'hich has a complete roof, side-walls, and floor, but is open
in front {anterior cranial foiitanelle) and also behind {forcnnen
rnagmuii). Tlie hinder, or parachordal portion of the cranium
surrounds the foramen magnum, and there forms the occipital
region. At the ventral margin of the foramen there are two
^jrominences, or occijntal condyles, for articulation with the first
1 W. K. Parker. Trans. Zool. Soc. x. 1S7S, i>. 189.

208

FISHES

vertebra, and between them the remains of the notochord are
traceable into the cranial floor. In front of the occipital region
two lateral bulgings indicate the j^^^'^otic capsules, and more
anteriorly still, in the trabecular region, the sides of the cranium
are modified to form two spacious lateral recesses, the orbits, each of
which is bounded above and below by supra-orbital and infra-orbital
ridges respectively, behind by an outgrowth from the periotic
capsule {post-orhital pivocess), and in front by a similar projection
from the hinder wall of the olfactory capsule {lateral ethmoidal

ATv.S

etUcLcji

'hz/.cfi'

^br.a.i

Fig

120. — Side view of the slaill of the common Dog-Fish {SojUimn canicula). aud.cji,
Auditory capsule ; hr.a 1, 5, branchial arches ; br.r, br.r', cartilaginous rays
attached to the hyoid arch and the first four branchial arches ; Cr, cranium ; ex.br,
extra-branchial cartilages ; hy.cn, cerato-hyal ; hy.m, hyomandibular ; Ih, labial
cartilages ; Ig, ligaments passing from the jaws to the cranium and to the distal end
of the hyomandibular ; Ig', ethmo-palatine ligament ; l.j, lower jaw or Meckel's
cartilage ; ^v. 2, optic foramen ; Xi\ 5, foramen for the Vth and part of the Vllth
cranial nerves ; olfxp, olfactory capsule ; ar, orbit ; vp.j, upper jaw or palato-
quadrate cartilage. (From Wiedersheim, alter W. K. Parker.)

■process). In front of the cranial cavity and the orbits may be
seen the laterally-pkced dome-like olfactory cajmdes, which are
open below, where the nasal sacs communicate with the exterior.
Between the two capsules an anterior extension of the cranial
floor forms a flattened mesethmoidcd plate, behind which is the
large, membrane-closed, anterior cranicd fontanelle. The lateral
walls of the cranium are perforated by numerous apertures, some
of which serve for the entrance or exit of blood-vessels, and
others, mostly pertaining to the inner walls of the orbits, for the
transmission of the different cranial nerves from the brain to

VIII SKULL 209

various parts of the head. In many Elasmobranchs the roots
of certain of the anterior spinal nerves perforate the side-walls
of the occipital region, and indicate the fusion of vertebral
components with the cranium. In the cranial roof between the
two periotic capsules there are two small apertures at the bottom
of a common median depression : through each aperture the
ductus endolymphaticus (aqueductus vestihuli) passes from the
vestibular part of the auditory organ to the exterior of the skull.

Three cartilaginous rods, one from the roof of each olfactory
capsule, and one, the prenascd or rostral process, from the ethmoid
cartilage, converge and meet, or nearly meet, in front to form
the rostrum or support for the preoral or " cut-water " portion
of the head.

The visceral arches are seven in number. The first or 7nan-
dihular arch consists on each side of an upper portion, the palato-
'pteryyo-qiiadrate or palato-quadrate cartilage, which passes forwards
in the side-wall of the oral cavity, along the upper margin of the
mouth, its anterior or palatine part curving inwards to a liga-
mentous connexion with its fellow beneath the cranial floor.
Each cartilage has an upwardly directed process (cthmo-palatine
jjrocess) which is connected by a suspensory ethmo-palatine ligament
with the lateral wall of the cranium behind the lateral ethmoid
process. The lower or ventral half of the mandibular arch
(Meckel's cartilage) is similar in shape to the upper ; it articu-
lates behind with the quadrate portion of the latter by a movable
joint, and is thence prolonged forwards and downwards in relation
with the lower margin of the mouth to a median ligamentous
union with its fellow of the opposite side. The palato-pterygo-
quadrate and Meckel's cartilages together form the primitive
upper and lower jaws, and support the teeth. The hyoid arch
also consists of a dorsal and a ventral half on each side. The
dorsal half or hyomandilndar element articulates above with the
periotic capsule. The ventral portion, or cerato-hyal, passes
downwards and is connected with its fellow by a median copula
or hasi-hyal cartilage situated in the floor of the oral cavity. A
series of simple cartilaginous rays (branchial rays) are attached
to the hinder margins of the hyomandibular and cerato-hyal
elements. The distal end of the hyomandibular is connected
by strong ligaments with the hinder portions of both the
palato-pterygo-quadrate cartilage and Meckel's cartilage ; in fact,
VOL. VII P

2IO FISHES CHAP.

the hyomandibular is the effective suspensorium by which the
upper and lower jaws are connected with the skull, and all
Fishes in which this arrangement exists are said to be hyostylic}
Behind the hyoid arch follow five hranchial arches. Each of
these is segmented into a dorsal or pharyngo-hranchial element,
followed by an ejji-, a cerato-, and a hypo-hranchial piece, but the
later element is absent in the fifth arch. The lateral halves of
the last three arches are connected ventrally by a large median
basi-branchial cartilage, but in the first and second arches by the
median apposition of their respective hypo-branchial elements.
Like the hyomandibular and cerato-hyal segments of the hyoid
arch, the epi- and cerato-branchial elements of all the branchial
arches except the fifth are fringed along their outer convex
margins by a series of hranchial rays, and, in addition, there
are three pairs of slender, curved, cartilaginous rods, or extra-
hranchials, in relation with the distal extremities of the branchial
rays of the second, third, and fourth branchial arches. The
function of the branchial arches, and their branchial rays, and
extra-branchial cartilages, is to support the inter-branchial septa
which separate the gill-clefts and carry the vascular gill lamellae.
All the arches lie near the inner margins of the septa, close to
the hypoblastic epithelium of tlie pharynx, while the outer por-
tions of the septa are supported by the branchial rays and the
extra-branchials, the latter lying directly Ijeneath the external
skin. The segments of the arches are movably connected with
one another by ligaments ; and by the contraction of the branchial
muscles the arches may be separated or approximated so as to
enlarge or diminish the size of the intervening clefts.

The labial cartilages are represented by a pair of slender rods
in relation with the outer surfaces of the palato-pterygo-quadrate
cartilages, and a similar pair in connexion with the Meckelian
cartilages. There is also a pair of small cartilages in relation
with the nostrils. It is probable that the rods which constitute
the lateral elements of the rostrum belong to the same category.

In the Cyclostomes and the Elasmobranchs the skull is entirely
cartilaginous, although it may often be superficially calcified in
Elasmobranchs, and although there may even be definitely and
symmetrically arranged calcified plates in Fleur acanthus, true bone
is never present. In many Fishes, and notably in the Teleostomi,
1 Huxley, P.Z.S. 1876, p. 40.

VIII SKULL 211

the embryonic cartilaginous cranium becomes complicated by the
addition of an extensive series of investing membrane bones, formed
by the ossification of the connective tissue external to the cartilage,
so that a secondary bony cranium is formed external to the primary
cranium much in the same way that a secondary pectoral girdle
is formed in connexion with the primary girdle. Such bones
probably owe their primary origin to the fusion and insinking of
exoskeletal structures (scales or dermal spines). To these invest-
ing bones there may also be added a series of bones formed by
the actual conversion of the cranial cartilage into osseous tissue
(cartilage bones), which to a greater or less extent in different
Fishes replaces the original cartilage. The bones of the skull
may conveniently be classified as follows : — (i.) Dermal or mem-
brane hones. Under this head are included — (a) the ordinary
investing bones of the skull. (b) Tooth -hones, that is, bones
formed by the fusion of the bases of teeth and developed in
relation with the walls of the oral cavity. Probably all tooth-
bearing bones are of this nature, (c) Sensori/ canal bones, that
is, tubular bones developed round the sensory canals of the head.
Certain of these bones may secondarily acquire the shape and
character of investing bones while still retaining protective
relations to their sensory canals, (ii.) Cartilage bones.

As an easily obtainable example of a skull which has acquired
a fairly complete series of both cartilage- and membrane-bones,
while retaining a well-developed primary cranium, the skull of
the Salmon (Salmo salar) may be described.^ At an early stage
of development, even so late as the second week of hatching, the
primary cranium is still entirely cartilaginous, and in this con-
dition the Salmon's skull is comparable with that of an adult
Dog -Fish. As development proceeds the primary cranium
becomes supplemented by the addition of numerous investing
dermal bones which form the secondary cranium, and later
cartilage bones appear and, to a considerable extent, replace the
original cartilage. The Salmon's skull is interesting in this
respect, that the primary cranium grows with the growth of the
Fish, so that in the adult the nasal, ethmoidal, and prenasal
regions are entirely cartilaginous, and in the hinder part of the
cranium cartilage is largely persistent between the cartilage bones.

Dealing first with the cartilage bones of the primary cranium,

1 W. K. Parker, Phil. Trans. 163, 1873, p. 95.

21 2

FISHES

it may be stated that there are formed in that part of the
parachordal cartilage surrounding the foramen magnum a median
hasioccipital below, which is concave behind where it articulates
with the centrum of the first vertebra, a suj^raoccipital above,
and two laterally-placed exoccijntal bones (Figs. 121, 122).
Each periotic capsule is ossified by the formation of five bones
in the primitively cartilaginous mass, the prootic, sphenotic, opisth-
otic, epiotic, and the pterotic. The inner walls of the capsules
have atrophied in the adult, and hence the cavities which contain
the auditory organs appear as open lateral recesses of the cranial
cavity. In front of the periotic capsules there are various bones
which are formed in the cartilage of the trabecular part of the

ps ?^°

Fig. 121. — Side view of the cranium of a Salmon {Salmo salar). Most of the membrane
bones and the jaws have been removed. The cartilage is dotted, al.s, Alispheuoid:
ho, basioccipital ; bs, basisphenoid ; eo, exoccipital ; ejj, epiotic ; l.eth, lateral
ethmoid ; ol, olfactory capsula ; o}), opisthotic ; o.s, orbito-sphenoid ; pr.o, pro-
otic ; ps, f)arasphenoid ; pt.o, pterotic ; so, supraoccipital ; sj^.o, sphenotic ; t.c,
trabecular coruu ; u.l.c, u.l.c', first and second upper labial cartilages ; v, vomer ;
II, foramen for the optic nerve. (From W. K. Parker.)

cranium. Thus, in front of the basi-occipital, and developed in
the cartilage of the cranial floor, there is a median Y-shaped hasi-
sphenoid, and, at some distance al:)ove it on each side, an ali-
sphenoid helps to form the lateral wall of the cranial cavity.
Between the eyes the side walls of the cranium fuse to form a
vertical inter-orbital septum, and, in consequence, two orhito-
sphenoid l)ones, which normally form the lateral cranial walls in
this region, become partially confluent in the median line and
close the cranial cavity in front. The only cartilage bones
found in the massive persistent portion of the primary cranium
which forms the pre-orbital region are the projecting latcrcd
ethmoids, forming the posterior boundaries of the recesses for the
olfactory organs, and separating the latter from the orbits.

The roof and floor of the primary cranium is completed by

SKULL

213

certain investing dermal bones (Fig. 123, A). A pair of large
frontal bones form the cranial roof, and also help to roof in the
orbital cavities. Behind the frontals, and separated from each
other by the supraoccipital, there is a pair of small parietals, and
anterior to the frontals a median dermal mesethmuid. A small
nascd bone overlies each olfactory recess. Ventrally, the base of
the cranium, from the basi-occipital to the prenasal region, is
strengthened by a large parasphenoid behind, and a much smaller
vomer in front, both of which lie in the roof of the mouth. The
vomer is a tooth-bone, and probably the parasphenoid also.

The mandibular arch (Fig. 123,B) is more modified than that
of the Dog-Fish. The palato-pterygo-quadrate bars, or primitive
upper jaw, no longer meet in front beneath the cranial floor, but
each separately articulates in front with the lateral ethmoid of its

Fig. 122. — Vertical aud
longitudinal section of
the cranium of Salmo
salar, showing the right
half of the cranial
cavity. Cartilage is
dotted. /, Frontal ; v',
fat-containing cavity in
the mesethnioid carti-
lage ; V, VII, IX, X,
foramina for the fifth,
seventh, ninth, and
tenth cranial nerves. Remaining reference-letters as in Fig. 121. (From W. K. Parker.)

'lie —

j>s

JDS is pr.D in

side. Although still partly cartilaginous each bar is largely replaced
either by cartilage bones, or by bones which begin as membrane
bones or as tooth-bones and complete their growth by invading
the cartilage and becoming in part cartilage bones. Its anterior
portion is formed by a loalatine bone which articulates with the
lateral ethmoid, and the middle portion by a pterygoid and a
mesopterygoid bone, while the hinder part is ossified above as a
metapterygoid and below as a quadrate. The latter articulates with
the lower jaw. Functionally, however, the primitive upper jaw
is now replaced by a secondary upper jaw, formed on each side
by a series of tooth-bones, situated external to the former, and
meeting in front of the prenasal region of the primary cranium
(Fig. 123, A). The series includes a dentigerous piremaxilla and
maxilla, and a small toothless, scale-like jugal bone. Each half
of the lower jaw (Fig. 123, A, B) consists of a rod-like Meckel's
cartilage or primary lower jaw. The hinder part of this is

214

FISHES

ossified to form the articular, which has a deeply concave surface

- ep o.

mks ar

Fig. 123. — A, view of the left side of the skull of a Salmon ; B, the left half of the primary
upper and lower jaws, and the hyoid arch. The cartilage is dotted, an. Angular ;
ar, articular ; h.liy, basi-hyal ; hr.r, bianchiostegal rays ; c, cranium ; c.h, cerato-
hyal ; cor, circum-orbital bones ; d, dentary ; d.eth, dermal niesethnioid ; ep.h, epi-
hyal ; ep.o, epiotic ; etli.p, ethmo-palatine process ; /, frontal ; hJnj, hypo-hyal ;
hym, hyomandibular ; i.np, inter-operculum ; j, jugal ; mks, Meckel's cartilage ;
mpg, mesopterygoid ; mt.pg, metapterygoid ; mx, maxilla ; n, nasal ; op, operculum ;
op', condyle on the hyomandibular for the operculum ; orb, orbit ; 2h parietal ; ^ja,
palatine ; p.vix, premaxilla ; p.op, pre-operculum ; p)t, pterygoid ; ji:»i!.o, pterotic ;
q, quadrate ; so, supra-occipital ; s.ojy, suboperculum ; sjj.o, sphenotic ; s.t, supra-
temporal (or squamosal) ; st.hy, stylo-hyal ; sy, symplectic ; u.l.c, u.l.c', upper
labial cartilages ; ^i.l.c^, second upper labial. (From W. K. Parker.)

for articulation with the quadrate ; and below this there is a small
membrane bone, the angular. The rest of the cartilage is partially

VIII SKULL 2 I 5

ensheathed on its outer side by a large tooth-bone, the denti-
gerous dentary. The hyoid arch is similar to that of the Dog-
Fish, except that its primitively cartilaginous segments are almost
completely ossified (Fig. 123, B). The large vipper segment or
hyomandihular bone articulates mainly with the pterotic, but
partly also with the sphenotic element of the periotic capsule ;
below it is connected with a slender symplectic bone, and from
the cartilage connecting the two depends the rest of the hyoid
arch, consisting in succession of stylo-hyal, epi-hy(d, cerato-liyal,
and hypo-hyal bones, with a median teeth-bearing hasi-hyal. The
palato-pterygo-quadrate bar has no direct connexion with the
skull, except anteriorly where its palatine element articulates
with the lateral ethmoid. The real suspensorium is formed by
the hyomandil^ular and symplectic bones, to wdiich the hinder
margins of the quadrate and metapterygoid bones are rigidly
attached by suture, hence, as in the Dog-Fish, the skull is
hyostylic. Behind the hyoid arch there are five branchial
arches, which generally reseml)le those of the Dog-Fish, except
that their component segments are ossified as cartilage l:)ones.

Connected with the hyomandihular and cerato-hyal elements
of the hyoid arch there is, on each side, a series of membrane
l)ones for the support of the movable operculum or gill-cover.
These consist of an opercidum above, which articulates with a
backwardly projecting process from the hyomandihular, followed
in succession below by a suh-operculuiri and an inter-ojjercul'um,
the latter being connected by ligament with the angle of the
lower jaw. The series is completed by ten sabre-shaped hranchio-
steijal rays, which are attached to the cerato-hyal and support the
lower margin of the gill-cover.

Sensory canal bones are represented in the Salmon by a ring
of small bony plates which encircle the orbit (Fig. 123, A), and
by one or two small bones situated above and on the outer side
of each periotic capsule {squamosals). To these may be added
the prc-02)erculum situated external to the hinder margins of the
hyomandihular and quadrate bones, firmly clamping these bones
together, and also the ijost-temporals, by which the secondary
pectoral girdle is attached to the skull. The nasal bones may
also be regarded as pertaining to the same series.

In other Fishes with a more or less complete bony skull there
are certain additional cartilaire- and membrane-bones which are not

2l6 FISHES CHAP.

present in the Salmon. There is usually a median ossification of the
ethmoid cartilage, the mcsethmoid. An entoptcrygoid is sometimes
added to the palato-pterygo-quadrate series of bones. An ossifica-
tion of the anterior extremity of each Meckelian cartilage may form
a mento-Meckelian bone. Certain additional membrane bones are
sometimes developed in relation with the lower jaw, such as splcnial
and coronary bones on the inner side, and a supra-angular bone at
the angle of the jaw, above the angular element. To these there may
be added the singular series oiinfra-dentaries, which in some fossil
Crossopterygii (e.g. Rhizodopsis) fringe the outer margin of the jaw
beneath the true dentary (Fig. 274, A). A system oi jugidar plates
may also form a characteristic armature for the throat betvyeen the
lateral halves of the lower jaw (Fig. 274, C). Besides those already
mentioned, additional sensory canal bones are present in some
Fishes. A transverse row of plates {sup)ra-temporals) sometimes
crosses the occipital region behind the parietals. There are also
other canal-ossicles which lose their identity by fusing with certain
cranial or periotic bones. Thus, each of the pterotic and sphenotic
bones often includes a superficial dermal bone transmitting a section
of a sensory canal, which has fused with it ; and as the frontal
bone is often similarly perforated, it may be taken that it also
includes a canal-ossicle ; and the same can often be said of the
articular and dentary bones of the lower jaw.^

Having now considered the general structure of a primitive
cartilaginous type of skull, and the nature, disposition, and
terminology of the various membrane- and cartilage-bones which
may be added to, or more or less completely replace the former,
reference will now be made to the more important features in
the structure of the skull in the Cyclostomata and the Fishes.

In the Cyclostomata the skull presents a remarkable combina-
tion of characters, in some of which it is more primitive than in
any other Craniates, while in others it has evidently attained a
very high degree of specialisation on lines peculiar to the group,
but differing in the two subdivisions. In the Lamprey ^ (Fig.
124) the paired parachordals and trabeculae together form a
trough -like chondrocranium, which has only a fibrous roof,

1 M'Murrich, Proc. Canadian Inst. (N.S.) ii. Toronto, 1S84, p. 278; Cole,
Trans. Linn. Soc. vii. Pt. v. 1898, p. 131.

2 W. K. Parker, PJdl. Trans. 174, Pt. ii. 1883, p. 411 ; Huxley, Joiirji. Anat.
and Phys. x. 1876, p. 412 ; Howes, Trans. Biol. Soc. Livcrjwol, vi. 1891, p. 122.

SKULL

17

except where a slender synotic band of cartilage extends
between the two periotic capsules. The floor is also incom-
plete, a large pituitary fontanelle remaining to indicate the
original separation of the trabeculae while transmitting the hypo-
physial or pituitary caecum. The notochord traverses the floor
of the parachordal portion of the cranium as far as the pituitary
fontanelle, and from the sides of the synotic ring the auditory
capsules project in the shape of conspicuous lateral prominences.
In front the otherwise open end of the cranial cavity is closed
by the dorsally-placed and unpaired olfactory capsule, which is

br.cl.7

Fig. 124. — Skull, with branchial basket and anterior j^art of the vertebral column, of
Petromyzon marinus. a.d.c, Anterior dorsal cartilage ; a.lat.c, anterior lateral
cartilage; an.c, annular cartilage ; au.c, auditory capsule ; br.b.1-9, vertical bars of
the branchial basket ; br.cl. 1-7, external branchial clefts ; cn.c, cornual cartilage ;
cr.r, cranial roof; l.c.1-4, longitudinal bars of branchial basket; l(/.c, lingual car-
tilage ; m.v.c, median ventral cartilage ; 7i.a, neural arches ; na.ap, nasal aperture ;
n.ch, notochord ; ^Vc^, foramen for optic nerve ; olf.c, olfactory capsule ; ^jc.c,
cartilage surrounding pericardial cavity ; p.d.c, posterior dorsal cartilage ; p.Iat.c,
posterior lateral cartilage ; sb.oc.a, subocular arch ; st.}), styloid process ; sty.c,
styliform cartilage ; t, teeth. (From Parker and Haswell, after W. K. Parker.)

perforated behind by two apertures for the olfactory nerves, and
has only a fibrous connexion with the cranial walls. Anteriorly
to the olfactory capsule the cranial floor is prolonged forwards
over the roof of the mouth as a large laterally-expanded plate,
formed hj the united anterior portions of the trabeculae, and no
doubt representing the mesethmoid cartilage of the Dog-Fish. So
far the cranium presents no special difficulty, and in its general
features may be readily compared with that of an embryonic
Elasmobranch. As for the rest of the skull, it is obvious that it
has been greatly modified, partly to form and to support the
skeletal framework of the remarkable suctorial buccal funnel, and
partly to form the singular rasping lingual apparatus. Hence it
is always difficult and sometimes impossible to identify with

21 8 FISHES CHAP.

certainty the component parts as being represented in other
Craniates. On each side of the cranium, beneath the eye, there is
a characteristic V-shaped suhocular arch. Of its two legs the
hinder one is continuous above with the periotic region of the
cranium, and the other with tlie anterior trabecular region, while
the pointed apex ,is directed oljliquely downward and forward.
From the hinder margin of the posterior limb a slender styloid
'process passes downward in the side wall of the pharynx, and
terminates below in a forwardly directed cornual cartilage. A
velum, fringed along its free margin with a series of tentacles,
projects forwards into the oral cavity from between the oral
apertures of the oesophagus and the branchial canal, and probably
serves to prevent the entrance of foreign particles to the gill-sacs.
This valve-like velum is supported by a t-elar skeleton, consisting
of two lateral cartilages which are prolonged into the tentacles,
and extend transversely between the inner surfaces of the two
styloid processes. The apex of each subocular arch is connected
with a small and somewhat triangular cartilage {iiostero-lateral
cartitaf/e), which is directed upward and forward, and lies in the
side wall of the oral cavity. With some degree of probability
the subocular arch may be compared to the palato-quadrate
cartilage of a skull which has become " autostylic " in order to
form a rigid support for the skeleton of the buccal funnel ;
the styloid processes and cornual cartilages to the hyoid arch;
while the relations of the posterior lateral cartilages to the sub-
ocular arches suggest that they may possibly be regarded as
Meckelian cartilages which have lost their primitive function of
forming biting jaws. In the median line lielow, and projecting
backward for some distance beneath the branchial canal, there is
a long and stout lingual cartilage, carrying a small median and a
still smaller pair of lateral cartilages at its anterior extremity,
where it supports the lingual teeth and projects into the buccal
funnel beneath the mouth. In front of the lingual cartilage,
and connected Ijy fibrous tissue with the inferior and hinder
margin of the annular cartilage, there is a median T-shaped
element, the median ventral cartilage. It has been conjectured
that the lingual cartilage is a free basi-hyal element, and the
median ventral cartilage the equivalent, elsewhere unknown, of
the corresponding element of the mandiljular arch.^
^ Huxley, o^;. cit. p. 421.

VIII SKULL 219

The remaining anterior skull elements are principally skeletal
supports for the roof and walls of the buccal funnel. The roof
is supported by an extended anterior dorsal cartilage, which is
overlapped behind by the ethmoid cartilage, while the circular
margin of the funnel is strengthened by a large ring-like annular
cartilage. On each side of the latter there is a slender, rod-like,
styloid cartilage, and above the latter a small anterior lateral
cartilage. All these cartilages are usually termed labial cartilages,
and it is at least possible that they possess representatives in the
similarly named structures of the Dog-Fish and the larvae of
some of the tailless Amphibia. It must not be forgotten, how-
ever, that the annular cartilage bears some resemblance to the
ring of cartilage which encircles the lips of the buccal cavity in
Amphioxus.

The complex supporting skeleton of the gill-sacs forms a basket-
like structure. It consists on each side of nine unsegmented,
irregularly curved, and slightly branched cartilaginous rods,
situated in the outer margins of the inter -branchial septa,
directly internal to the skin. The first lies directly behind the
styloid process (hyoid arch), the second and third in front of and
behind the first gill-sac, and of the remainder one lies just behind
each of the six succeeding gill-openings ; above and below each
gill-aperture the rods are connected by longitudinal bars, and also
in the median ventral line by a pair of similar partially united
bars. The dorsal ends of the rods are also connected on each side
by another longitudinal bar, which runs alongside the notochord
and in front blends with the chondrocranium. The rods forming
the last pair are continuous with a cup-like cartilage, supporting
the lateral and hinder walls of the pericardium.

This singular branchial basket undoubtedly bears a superficial
resemblance to the branchial arches of Fishes, but in any comparison
of the two structures it is well to bear in mind that the branchial
rods of the Lamprey are situated along the outer edges of the
inter-branchial septa, and are therefore external to the gill-sacs
and branchial arteries, and further, that they are developed in
the somatic mesoblast of the embryonic protovertebrae, whereas
true branchial arches are situated at the inner margins of the
septa, internal to the gill-clefts and branchial arteries, and have
their origin from the splanchnic layer of the mesoblast. So far
as their position is concerned, the rods agree rather with the

220

FISHES

extra -branchial cartilages of an Elasmobranch than with the
more deeply -seated branchial arches.

While the skull of the Myxinoid Cyclostomes ^ is constructed
on the same general lines as that of the Lamprey, it is in some
respects more primitive. It is also clear that in other features
the skull has undergone marked specialisation on lines of its
own, and in some points again it seems to deviate less from the
more normal Craniate type. Of the more obvious differences, as
illustrated by the skull of Bclellostoma (Figs. 125-127), it will

Fig. 125. — Side view of the skull of Bclellostoma ; the gill-apertures ami their cartilages
have been omitted. A, Auditory capsule ; B, B', B", the anterior, middle, and
posterior segments of the lingual bar ; 6^, cartilage connecting the hyoid arch with
the second branchial arch ; hr^, br^, first and second branchial arches ; c.c, coronal
cartilage ; Or, cranium ; D, dental plate ; dt, median dorsal tooth ; Ex.n.c, ex-
ternal part of the naso-pituitary canal ; Up, hypophysial plate ; Hy, hyoid arch ;
^V. subnasal cartilage ; mc, neural canal ; iN7, notochord ; OC, olfactory capsule ;
PL, palatine portion of the palato - quadrate cartilage FQ ; S, supra -pharyngeal
plate supporting the velum ; t, tendon of the retractor mandibuli muscle ; <^ t-, fi,
tentacular cartilages ; t\ cartilage supporting mouth lobe ; tr, trabecula ; T'^, rod
connecting S with the inner surface of the hyoid arch of its side ; V, outer lateral
rod which joins V^ ; 1, 2, 3, fenestrae. (Modified from Ayers and Jackson.)

be sufficient here to mention the following : (i.) The more primitive
condition of the chondrocranium, the roof and side walls of the
cranial cavity being entirely membranous, (ii.) The non-develop-
ment of a suctorial buccal funnel and the presence of oral tentacles,
associated with the absence of the complex system of labial
cartilages and the substitution of a special tentacular skeleton,
(iii.) The special modifications induced by the length and physio-
logical importance of the naso-pituitary canal and by its com-
munication with the pharynx after perforating the pituitary
fontanelle in the cranial floor. Under this head may be included

1 W. K. Parker, Phil. Trans. 174, 1883, pp. 376-405 ; Ayers and Jackson, Journ.
Morph. xvii. 1901, p. 193.

SKULL

22 I

the depression of the mesethnioid or hypophysial plate for the
support of the naso-pituitary canal, the forward prolongation and
median union of the palato-
quadrate cartilages of oppo-
site sides beneath the external
portion of the canal, appar-
ently for the support of the
latter,and the encircling of the
canal by supporting annular
rings of cartilage, (iv.) The
presence of two branchial
arches, connected, as in
Fishes, with a median basi-
branchial segment which
forms the middle one of the
three divisions of the lingual
apparatus, (v.) The reduction
of the complicated extra-

FiG. 126. — View of the upper surface Fig. 127. — Dorsal view of the skull of Bdelhi-

of the dental plate of Bdellostoma. stoma. Reference letters as in Fig. 125.

^.Tendon of retractor muscle. (From (Alter Ayers and Jackson.)
Ayers and Jackson.)

branchial basket to small isolated cartilages in relation with the
gill-apertures and the cesophago-cutaneous duct, (vi.) The extra-
ordinary development of the lingual apparatus, of which it has

222

FISHES

been remarked that it " dominates the whole body, everything
else yields to it." Meckel's cartilages are represented either
by the cornual cartilages, as seems most probable, or hj the
dental plate (Fig. 125, ex. and D).

In the generality of Elasmobranchs the skull resembles that
of the Dog -Fish in essential structure. The more important
modifications within the limits of the group relate to differences
in the mode of attachment of the primitive upper jaw to the
skull, and the number of branchial arches. In most Elasmo-
branchs the skull is hyostylic, as in Scy Ilium, but there are

/f. orb

Fig. 128. — Lateral view of the skull of Notidanus {Heptanchus) cinereus ; vick, Meckel's
cartilage, or primitive lower jaw; j^aZ.g'if, palato - quadrate cartilage or primitive
upper jaw ; pt.orb, post-orbital process of the cranium with which the pcst-orbital
process of the palato - quadrate articulates. (From Parker and Haswell, alter
Gegenbaur. )

two genera which, in different ways, are exceptions to this rule.
In Notidanus the hinder part of each palato-quadrate cartilage
grows upwards into a strong post-orbital process, which articulates
with the suitably modified post-orbital process of the periotic
capsule (Fig. 128); hence the primitive upper jaw acquires a direct
dorsal connexion with the cranium, and, as the hyoid arch is now
relieved from taking any part in its support, the hyomandibular
is reduced to the condition of a relatively slender rod of cartilage.
By this arrangement both the mandibular and hyoid arches have
their own separate and independent connexions with the cranium,
and the skull is said to be amjjhistT/lic} The Port Jackson Shark

1 Huxley, P.Z.S. 1876, p. 40, et seq.

SKULL

223

{Heterodontus) exhibits another and quite different modification.
In this Fish the dorsal border of each palato-quadrate cartilage
fits into a deep groove along the infero- lateral surface of the
cranium, and is firmly attached thereto by ligament. Thus the
first step is taken towards that more complete fusion of the two
structures which is so characteristic a feature in the more typically
autostylic Fishes like the Holocephali and the Dipnoi. Auto-
stylism, whether incipient, as in Heterodontus, or complete, is to be
regarded as a secondary modification, which may be independently
acquired in widely different groups of Fishes, and is usually
associated with the need of a firm and rigid support for an ex-
ceptionally massive dentition.^

Fig. 129. — Lateral view
of skull of Chimaera
monstrosa. a.s. c. Posi-
tion of anterior semi-
circular canal ; c.hy,
cerato - hyal ; e.hy,
epi-liyal ;fr.cl, frontal
clasper ; h.s.c, posi-
tion of horizontal
semicircular canal ;
i.o.s, inter - orbital
septum ; Ib.l, lh.2,
lb.3, labial cartilages ;
Mck.C, mandible ;
Nv.2, optic foramen ;
Nv. 10, vagus fora-
men ; olf.cp, olfactory
capsule ; op.r, oper-
cular rays ; pal.qu,
palato - quadrate ;
ph.h)/, pliaryngo-hyal,
or hyoniandibular ;
qu, quadrate region ; r, rostrum. (From

p.s.c

p.s.c, position of posterior semicircular canal
Parker and Haswell, after Hubrecht.)

In the Holocephali {e.g. Chimaera "^) the cranium retains its
primitively cartilaginous condition, and assumes a somewhat
peculiar appearance owing to the lateral compression and vertical
growth of its inter-orbital and nasal regions (Fig. 129). There
is a complicated series of labial cartilages in relation with the
ventrally -placed nostrils and the upper and lower jaws. In the
males of Chimaera and CaUorhynchus, but not in Harriotta, a
movable cartilage is attached to the cranial roof, and supports
the frontal clasper. The skull is typically autostylic. Along

1 Dollo, Bull. Soc. Beige G6ol. etc. ix. 1895, p. 110.

^ Hubrecht, Niederland. Archivf. Zool. iii. 1877, p. 255.

224

FISHES

the whole length of its dorsal border the palato-quadrate carti-
lage is fused with the inferior lateral margin of the cranium from
the periotic to the olfactory region, thus forming a triangular
plate of cartilage, the base of which is continuous with the
cranium, while the downwardly directed apex provides an articu-
lar surface for the lower jaw. The hyoid arch is little l^etter
developed than the succeeding branchial arches, and includes a
vestigial hyomandibular, an epi-hyal, and a cerato-hyal. As in
other autostylic skulls the hyomandibular element is attached by
ligament to the hinder margin of the palato-quadrate, instead

It JVif Orb FF

m

Fig. 130. — Side view of the skull of a Sturgeon, with the investing membrane bones re-
moved, a, Pharyngo-branchial ; AF, antorbital or lateral ethmoid cartilage ; AR,
articular; h, epi-branchial ; c, cerato-branchial ; C, notochord ; Coj), basi-branchials ;
d, hypo-branchial ; De, dentary ; GK, auditory capsule ; Hm, hyomandibular ; hy,
cerato-hyal ; Ih, inter-hyal ; Aid, lower jaw ; Xa, nasal capsule ; Ob, neural arches ;
Orb, Orbit ; PF, post-orbital process ; PQ, palato-quadrate ; Ps, Ps, Ps", para-
sphenoid ; Psj), neural spines; Qu, quadrate; R, rostrum; Ri, ribs; S2).X,
foramina for spinal nerves ; Sy, symplectic ; WS, vertebral column ; x, foramen
for the vagus nerve ; I-V, branchial arches ; II-V, foramina for the optic and the
fifth cranial nerves. (From Parker and Haswell, after Wiedersheim. )

of being directly connected with the periotic capsule, and
obviously takes no part in supporting the jaws. Branchial
rays for the support of the operculum are attached to the
cerato-hyal, and some of them have their bases fused together.
The five branchial arches resemble those of the Dog-Fish, except
that they tend to become concentrated beneath the skull.

The existing Chondrostei,^ and especially the Sturgeon, are
remarkable for the persistence and continuous growth of the chon-
drocranium, and the absence of true cartilage bones. Numerous

1 W. K. Parker, Phil. Trans. 173, 1882, p. 139 ; Bridge, Phil. Trans. 169,
1878, p. 683.

SKULL

2 2 C

dermal bones invest the dorsal surface of the chondrocraniuni,
and only to a limited extent correspond witli the less numerous
membrane bones of the Salmon. To these are added a series of
circum-orbital bones and a large parasphenoid. Undoubtedly the
most striking feature in these Fishes is the primitive character
of the upper jaw. In Polyodon (Fig. 131) the palato-quad rates
are wholly cartilaginous, and, as in the Dog-Fish, they meet
in front beneath the basis cranii, where the two are connected
l)y ligament. The secondary upper jaw is but feebly developed,
and is represented on each side l)y a thin splint-like maxilla in
relation with the outer surface of each palato-quadrate cartilage.

b.br

mk.c

Fig. 13L — Lateral view of the primp,ryand secondary upper ami lower jaws ai Polyodon.
b.br', First basi-branchial ; ch, cerato-hyal ; d, dentary ; hy.h, hypo-hyal ; hy.vi,
hyomandibular ; i.hy, inter-liyal ; i.op, inter-operculum ; Igs, ligaments connecting
the palato-quadrate cartilage with the hyomandilnilar ; mk.c, Mfckel's cartilage ;
mx, maxilla ; op, operculum ; ^j«, palatine ; pa.q, palato-quadrate ; ps.l, pre-
spiracular ligament ; q, quadrate cartilage ; sym, symplectic. (From Bridge.)

which meets its fellow in front. There are no premaxillae. The
lower jaw is also very primitive. Meckel's cartilages are per-
sistent, and except for a mento-Meckelian bone on each side, they
are unossified, although membrane bones representing dentary and
splenial elements are present. The skull is hyostylic. The hyoid
and branchial arches are only partially ossified. Each opercular
fold is supported by an operculum and an iuteroperculum, and both
of these retain somewhat the shape of the cartilaginous hyoidean
rays which they have replaced. In the Sturgeon (Fig. 130) the
upper jaw is greatly modified in relation with the singular mouth
of this Fish. The palato-quadrate cartilages meet not only in
front, but also along their dorsal margins, and, with the help of
the similarly opposed and somewhat fragmentary metapterygoid
VOL VII Q

2 26 FISHES CHAP.

cartilages, they form a complete concave roof for the protrusible
spout-like mouth. Palatine, mesopterygoid, and pterygoid bones
invest, and in some measure replace these cartilages. In brief,
the skull of the Chondrostei occupies an interesting intermediate
position between the purely cartilaginous and mainly bony types.
While retaining a well-developed and unossified priniary cranium,
it has acquired a complete secondary cranium of dermal bones.
Equally notable is the condition of the jaws. Unique among
the Teleostomi in possessing the typical Elasmobranch union of
the palato-quadrate cartilages beneath the basis cranii, the Chon-
drostei are so far specialised that they have acquired certain of
the membrane bones which constitute the secondary jaws of the
more typical bony Fishes.

As regards the general structure of the skull and the nature
and disposition of its cartilage- and membrane-bones, the remain-
ing living Teleostomi have much in common with the Salmon.
In all the skull is hyostylic, and, unlike the Chondrostei, each
half of the primitive upper jaw remains distinct from its fellow,
and is separately articulated in front with the lateral ethmoid
of the same side by its palatine element. The palato-quadrate
cartilage is always more or less completely replaced l^y bones
similar to tliose of the Salmon, and although they often carry
teeth, as a rule they do little more than constitute a rigid
buttress for the fixation of the cj[uadrate condyle for the lower
jaw. The secondary upper jaw is nearly always well developed,
and includes a premaxilla as well as a maxilla on each side.
There are, however, certain features in each of the minor groups
which are either distinctive or highly characteristic.

In the surviving Crossopterygii (e.g. PohjiAerus ^) the chondro-
cranium is complete in the ethmoidal and post-orbital regions,
except where it has been partially replaced by cartilage bones, but
in the inter-orbital region the continuity of the roof is interrupted
by a large fontanelle, which is only closed by the investing frontal
bones (Fig. 132, C). There is also a large basi-cranial fontanelle
in the sphenethmoid, closed, however, by the underlying para-
sphenoid. A large " occipital " bone continuously ossifies in the
occipital cartilage and completely surrounds the foramen magnum.
Prootics and pterotics are absent, and the opisthotics seem to be

^ Traquair, Journ. Anat. and Phys. v. 1871, p. 166 ; Bridge, Proc. Birm. Phil.
Soc. vi. 1888, p. 118 ; Budgett, Trans. Zool. Soc. xvi. Pt. vii. 1902, p. 315.

SKULL

! 2 7

confluent with their respective epiotics. The floor and side walls
of the inter-orbital section of the cranium are formed by a remark-
able " sphenethmoid " bone which occupies the position of the
paired ali- and orbito-sphenoids in other bony Fishes ; and in one
species, P. hipradei} it forms in front distinct tubular investments
round tln> olfactory nerves. In many respects this bone is singu-

Fiu. 132. — A, side view
of the skull of Pulyp-
tents ; B, dorsal view,
showing the chief
dermal bones ; C,
similar view of the
choiidro-craniuni after
the removal of the
dermal hones. An,
Angular ; Ar, articu-
lar ; Z>, deutary ; A',
mesethmoid ; /.m,
foramen magnum ;
I'r, frontal ; l.e, lateral
ethmoid ; J/.r, max-
illa ; JVa, Na , nasal
and accessory nasal
bones ; occ, occijntal ;
ol, nasal aperture ;
Op, operculum ; op.o,
opisthotic ; O.t, os
termiuale ; Pa, pari-
etal ; Pm.a\ pre-
maxilla ; P.t, post-
temporal ; Ptf, post-
frontal ; Qu, quad-
rate ; tS.b, I'^.J', civ-
cum-orhital ossicles ;
S.Oji, sub- operculum ;
Sp, splenial ; f:2).eth,
sphenethmoid ; sji.o,
sphenotic ; -^jyi',

spiracular ossicles, between which is the spiracle ; S.t, supra-temporals ; Y, cheek-plate
(pre-operculum) : }". >'", smaller cheek -plates ; z, z, z, z, post -spiracular ossicles ;
z', z' , prespiracular ossicles. In C the cartilage is dotted. (From Traquair.)

larly like the sphenethmoid bone of the Frog and other tailless
Amphibia. A median ethmoid as well as lateral ethmoids are
present. In addition to the ordinary dermal bones which invest
the cranial roof there is a transverse row of supra-temporal
]ilates crossing the cranial roof behind the paired parietals
(Fig. 132, A). Fringing the outer margins of the frontals
and parietals a row of pre- and post-spiracular ossicles extends
nearly to the orbits, and between two of them, which form a

1 Budgett, Trans. Zool. Soc. xv. 1900, p. 334.

228 FISHES CHAP.

valve, is the spiracular aperture itself. There is a dentigerovis
splenial on the inner surface of the lower jaw. The hyoid arch
has no separate symplectic bone. An operculum and a suboper-
culum are present, but no inter-operculum ; and unless the hinder
part of the large cheek-plate, which is traversed by the mandibulo-
hyoid sensory canal, represents a pre-operculuni, the latter is
wanting. Branchiostegal rays are absent, but there is a single
pair of large jugular plates.

Very little is certainly known al)0ut the cranial cartilage-
bones in the fossil members of the group, but the investing
dermal bones, which bear a general resemblance to those of
Polypterus, are often somewhat more numerous, and they form
a very complete dermal armature for the entire head. Tliere is
a very complete ring of cireum-orbital bones, and very often a
ring of sclerotic plates. Two large cheek-plates are often present.
Nothing comparable to pre- and post-spiracular ossicles is known,
but squamosal and supra-temporals can often be identified. To
the ordinary bones of the lower jaw there may be added a series
of infra-dentary plates, and besides the paired principal jugular
plates there may also be present a small anterior median plate and
a series of small lateral jugular plates on each side, as in the Car-
boniferous lihizodopsis (Fig. 274). Most of the superficial dermal
bones, both in the living and extinct Crossopterygii, are invested
externally by a granulated or rugose layer of enamel-like ganoin.

In the Holostei, and especially in Aviia, the skull approxi-
mates more closely to the normal Teleostean type as represented
by the Salmon's skull. In Amia ^ all the occipital cartilage-
bones are present — a basi-occipital, two exoccipitals, and a supra-
occipital ; and, except for the absence of a pterotic, the periotic
series of bones is also complete. Paired ali- and orbito-sphenoids
form the lateral walls of the inter-orbital portion of the cranial
cavity. Above, the complete cartilaginous roof of the cranial
cavity is invested by a shield of suturally united and ganoin-
covered dermal plates. The hyomandibular element has a
symplectic Ijone at its distal extremity. There is a complete
series of opercular bones, and the branchiostegal rays are
numerous. A single median jugular plate is present. The lower
jaw has on each side five dentigerous splenial bones in addition
to dentary and angular bones, while cartilage-bones are repre-
^ Sagemehl, Murph. Jahrh. ix. 1884, p. 177.

VIII SKULL 229

sented by articular and inento-Meckelian elements. In its
essential structure tlie skull of Lcpidostcus ^ resembles that of
Amii(, but it lias obviously undergone much specialisation. In
some s})ecies {e.g. L, osscus) its appearance is greatly modified by
the exceptional length and tapering shape of the beak, due to
the elongation of tliat part of the skull which lies between the
orbital and nasal regions; but in L. plKtyceplialvs the reduced
length and greater width of the beak, combined with its some-
what flattened condition, impart an almost Crocodilian aspect
to the head. Amongst other points of difference it may be
mentioned that in Lepidost&us the continuity of the chondro-
cranial roof is interrupted by a large superior fontanelle. There
is no supra-occipital, and there are no lateral ethmoids, at all
events in the usual position. The inter-orbital portion of the
cranial cavity is largely obliterated by the formation of an inter-
orbital septum, consisting of a thin vertical plate of bone, which
eitlier represents a pair of fused orbito-sphenoids or a pair of
similarly modified lateral ethmoids. In addition to the ordinary
investing dermal bones, including circum-orbitals, squamosal, and
supra-temporals, there are numerous scale -like ossicles which
take the place of tlie cheek-plates of Polypterus. The maxillae
are segmented into numerous dentigerous bones fringing the
margins of the upper jaw. The lower jaw has no mento-Meckelian
bones, but there is a very complete series of dermal elements,
including dentary, coronary, splenial, angular, and supra-angular
bones in addition to an articular cartilage -bone. One of the
most remarkable features in the skull of Lepidosteus is the exist-
ence of a secondary articulation between the metapterygoid bones
and a pair of transversely elongated condyles formed on each
side by a lateral outgrowth from the parasphenoid and ali-
sphenoid bones. By a horizontal sliding movement of the
former on the latter, provision is made for the lateral expansion
and contraction of the walls of the oral cavity and the separation
and approximation of the lateral halves of the upper jaw."

The generality of Teleosts ^ more or less closely agree with
Amia in the main features of their cranial structure. There are,
however, certain minor features which are characteristic if not

1 W. K. Parker, Phil. Trans. 173, 1882, p. 443. 2 Bridge, P.Z.S. 1895, p. 30:^.
' Sagemeh], Morph. Jahrb. x. 1885, p. 1 ; xxvii. 1891, p. 489. Swinnerton,
Quart. J. Micr. Sci. xlv. 1902, p. 503.

230 FISHES CHAP.

always distinctive of the group. As a rule, to which, nevertheless,
there are notable exceptions, there is little of the primary carti-
laginous cranium in the adult, nearly the whole of it having become
absorbed or converted into cartilage-bones. A supraoccipital is
invariably present, and usually a mesethmoid and a basisphenoid.
An additional bone is added to the periotic series, viz, a pterotic.
Supra-temporal bones and jugular plates are always absent, and
it may be doubted if mento-Meckelian bones and dentigerous
splenials are ever developed in the lower jaw. Within the
group itself the skull exhibits many notable modifications, of
which only a few can here be mentioned. The shape, size,
and character of the mouth and jaws, the extent to which they
can be protruded and retracted, and the nature of the denti-
tion, are the source of many characteristic modifications in the
structure and appearance of the fore-part of the skull, and these
again largely depend upon differences of habit and food. A
protrusible mouth, or a mouth which is projected forwards, is
usually associated with a suspensorium (hyomandibular) of con-
siderable length, and so greatly inclined forwards as to make a
more or less acute angle with the forepart of the cranium.

The presence or absence of an inter-orbital septum is also a
feature in which considerable variation occurs. In some Teleosts
there is no septum, and the cranial cavity is prolonged forwards
between the orbits, where its lateral walls are formed by well-
developed, paired ali- and orbito-sphenoid bones, as, for example, in
the Carp and other Cyprinidae. In others the fusion of the cranial
walls is accompanied by the median union of the orbito-sphenoids,
so that a partly bony and partly cartilaginous inter-orbital septum
is found, and the cranial cavity becomes largely obliterated in
this region, as in the Salmon ; or the orbito-sphenoids may be
non-existent, the cartilage may undergo absorption, and the inter-
orbital septum may become reduced to a vertical fibrous sheath
extending between the frontals above and the parasphenoid below,
as is the case in the Cod (Gadus).

An interesting modification of certain of the bones of the
primary and secondary upper jaw occurs in the Siluridae. In
these Fishes the maxillae are very small and edentulous, and
serve no other purpose than forming basal supports for the
maxillary barbels, while the rod-like palatine bone, losing its
connexion with the pterygoid portion of the primitive upper jaw.

VIII SKULL 23 I

but retaining its articulation with the lateral ethmoid, serves to
support the maxilla, and at the same time receives the insertion
of the muscles by which the barl)el is moved in various directions.

In the Plectognathi the premaxillae are co-ossified witli the
maxillae. Many other interesting cranial modifications occur in
Teleosts, and to some of them reference is nuide in subsequent
chapters.

In some respects the skull of Dipnoi ^ is remarkably like that
of the Holocephali, especially in its typical autostylism ; but in
possessing both cartilage- and membrane -l)ones it in some
measure approaches the Teleostome skull. The investing dermal
bones are not always easy to identify with those of other Fishes.
In Neoceratodus an anterior median membrane -bone or dermal
mesethmoid covers the ethmo-nasal region, and, on each side of
it, forming the anterior boundary of the orbit, there is situated
a pre-orbital or dermal lateral ethmoid. Behind the mesethmoid
there is a much larger posterior median bone, and on each side a
singular backward prolongation of the dermal lateral ethmoid
separates it from a squamosal element. The latter bone
descends on the outer surface of the quadrate portion of the
palato-quadrate cartilage as far as the condyle for the lower
jaw. Collectively, these bones form a fairly complete invest-
ment to the upper surface of the cranium, bvit the posterior
median bone and the adjacent portions of the dermal lateral
ethmoid and the squamosal are widely separated from the
underlying chondrocranium by the powerful jaw muscles, and
in this respect they differ from the ordinary roofing bones of
other Fishes.

In Protopterus (Fig. 133) and Lepidosiren (Fig, 134) the
posterior median bone is non-existent, and its place is taken by
a large fronto- parietal, which forms the greater part of the
cranial roof, internal to the jaw muscles, and is much larger
in the latter Dipnoid than in the former. Circum-orbital bones
are present only in Neoceratodus. A large parasphenoid supports
the cranial floor. Vomers are absent, although there are two
small vomerine teeth. Relatively small opercular and inter-
opercular bones are present, and on the inner surface of each

1 Giinther, Phil. Trans. ]6], 187L P- 521; Huxlt-y, P.Z.S. 1876, p. 31;
Wiederslieiiii, Morpli. Stud. i. Jena, 1880, p. 46 ; Bridge, Trans. Zool. Soc. xiv.
1898, p. 350.

232

FISHES

may be found vestigial remains of cartilaginous hyoidean rays.
The chondrocranium is complete in Neoceratodus, but in
the remaining genera it has undergone considerable absorption
in the inter-orbital region, so that the roof and floor, and, in
part, even the side walls of the cranial cavity, are formed by
the fronto-parietal and parasphenoid bones. Two exoccipitals
are present in all Dipnoi. There are small labial cartilages

(lie. P'^^ ir^

Fig. 133. — Side view of the skull of Protopte.rus, with the pectoral girdle and liii. an.
Angular; an.c, antorbital cartilage ; c.c, coracoid cartilage (epi-coracoid) ; c.hy, cerato-
hyal ; cl, clavicle ; c.r, cranial rib ; c.si\ coraco-scapular cartilage ; d.e, dermal
etlimoid ; d.l.e, dermal lateral ethmoid ; e.g.f, external gills ; eo, exoccipital ; f.p,
fronto-parietal ; mk.c, Meckel's cartilage ; n.a, neural arches ; ol.c, fenestrated roof
of the olfactory capsule ; p.f, skeleton of the pectoral fin ; p.pt, palato-pterygoid
bone; p.q, palato-quadrate cartilage; s.d, supra-clavicle ; spi, splenial ; sq, squa-
mosal ; 1-6, the branchial arches ; the segmentation of the second and third arches
is not shown. (From Wiedersheim.)

in relation with the ventrally-placed nostrils, and large lateral
outgrowths from the ethmoid cartilage furnish the olfsictory
organs with conspicuous lattice-like roofs. A pair of strong
palato-pterygoid bones fringe the lower margins of the palato-
quadrate cartilage, and meeting in front beneath the ethmoid
region their symphysial extremities support the large palatal
teeth. The Meckelian cartilages are persistent in all Dipnoi. In
Neoceratodus each is flanked by a dentary and an angular externally,
and internally by a splenial ; ])ut in Protopteri(s and Lepidosiren
distinct dentary bones are wanting. The hyoid arcli is best

SKULL

233

die

fp

developed in Neoceratodus,^ and includes a small hyoniandibular car-
tilage, a partially bony cerato-hyal and cartilaginous hypo-hyal and
basi-hyal element. In the other genera (Fig. 13o) only a cerato-
jiyal is retained. The branchial arches are but feebly developed
in the Dipnoi. Neoceratodus has five, of which the first four are
divitled into epi- branchial
and cerato- branchial seg-
juents, while the fifth is
undivided. Frotopterus has
six, but only the second and
third are segmented as in
Neoceratodusr In Lepido-
sireii all the arches are
simple undivided rods.

In all three genera the
skull conforms to the same
general type of structure,
Init it is much more primi-
tive in Neoceratodus than
in the other tw^o genera.

With reference to the
fossil Dipnoi, it may be
stated that, so far as they
are known, the cranial
roofing bones are more
numerous than in the exist-
ing genera, and they cannot
readily be compared with

those of the latter, or with ^^'I«^^3^•-^''^■'^^ TY °L\^'^'^'."".?*'.^.f f^^^
the numerically reduced and
more definitely arranged
bones of most Teleostomi.
There is also evidence that in some fossil Dipnoi (e.g. Dipterns)
the chondrocranium and the mandibular suspensorium (palato-
quadrate) must have been replaced by cartilage bones to an
extent which has no parallel in any of the surviving types.^
Jugular bones were present in Dipterus and P]ianerop)lenron.

^ Ridewooil, P.Z.S. 1894, p. 632. ^ Ibid. p. 638.

■• Traquair, Ann. Marj. Kat. Hist. (5), ii. 1878, p. 1.

siren, o.n.c, .Condyle on the quadrate cartilage
for the lower jaw ; n.sp, neural spine ; op,
0])ercidum. For other reference letters see
Fig. 133. (From Bri.lge.)

234

FISHES

Median Fins and Appendicular Skeleton

The Median Fins. — Whether existing in the form of a
continuous fin, or as discontinuous isolated fins, the median
fins are provided with skeletal supports, and also with muscles,
primitively formed from intrusive clusters of cells derived
from a varialile number of the neighbouring myotomes, for
their varied movements. The skeletal structures of the dorsal
and anal fins consist of a series of bony or c.irtilaginous, rod-
like, and typically tri-segmented radial elements or pterygio-
phores,'" supporting distally a series of dermal structures in
tlie shape of numerous slender horny fibres or ceratotrichia,
as in the Elasmobranchii and Holocephali, or a smaller numl)er

of bony dermal fin-
rays, which are prob-
ably modified scales
or lepidotrichia," as in
the Teleostomi. The
typical tri-segmented
character of the radi-
alia is often retained
in many existing Elas-
mobranchs (Fig. 135)
and in Pleuracan-
thns, in Neoceratodus
amongst the Dipnoi,
in the Chondrostei,
in existing Holostei
(Fig. 136), and to a
greater or less extent
in several families of Teleosts {e.g. Salmonidae, Esocidae,
Cyprinidae, and some Acanthopterygii) ; but in the latter
group the radialia are greatly prone to reduction, and hence
they are more generally bi-segmented, and sometimes consist
of a single proximal segment only (e.g. Gymnotus). In
all these Fishes the proximal segments are the longest
and the most persistent, and when reduction occurs it is at

^ Thacker, Trans. Connecticut Acad. iii. 1877, p. 281 ; Mivart, Trans. Zool.
Soc. X. 1879, p. 439 ; Bridge, Linn. Soc. Journ. Zool. xxv. 1896, p. 530.
2 Goodrich, Quart. Journ. 3Iicr. Sci. 47, 1903-1904, p. 465.

Fig. 135. — The cartilaginous radialia of tlie first dorsal
tin of Mustdus antarctictL<s. (From Mivart.)

MEDIAN FINS

235

the expense of the middle aud distal segments. The cause of
this reduction is often, but not always, to be found in the
fact that, whenever the dermal tin-rays take the form of stout
spines, as in the anterior dorsal fin in many Acanthopterygian
Teleostei, the segmentation of their radialia would obviously
detract from their value as skeletal su])ports, and hence tliey
rarely consist of more than their proximal segments, although
the radialia which in the same Fish support soft rays may be
bi-seg-mented or tri-sey;niented. The radialia are, however, uu-
segmented, even slightly branched, cartilaginous rods in the
Cyclostomata ; short simple rods in the Holocephali ; and equally
simple bony rods in the dorsal fin of Fohjpterus, where they sup-

FiG. 136. — The tri-segniented radialia and the liu-
rays of part of tlie dorsal fin of Ainia calm.
p.s, m.s, and d.s, The proximal, middle, and
distal segments of a radial ; f.r, fin-rays. (From
Bridge. )

Fig. 137. — The first four radialia

of the dorsal fin of Mesoprion
ijemhra, showing the chain-
links for the ring-like bases of
the fin-rays. r.e^, r.e^. First
aud fourth proximal radialia.

port the strong spines of the numerous finlets ; Init they are bi-
segmented in the soft-rayed anal fin. As previously mentioned,
the proportional share taken by the radialia and the horny fibres
or the dermal fin -rays in the support of the fins differs greatly
in different Fishes. In the Cyclostomata radialia are the sole,
and in Elasmobranchs the main supports, and they may extend
Uvsarly to the free margin of the fin. In the more specialised
Fishes, as in most Teleostomi, the reverse is the case. The
radialia sink into the muscles of the body-wall and leave the
strongly developed fin-rays as the sole support of the visible
portions of the fins. In not a few Fishes there is an obvious
segmental correspondence between the radialia and tlie vertel)ral
neural or haemal spines, to the extent that the former equal the
latter in number and articulate with their distal extremities, as,
for example, in the caudal region of Plcuracanthus and in existing

236

FISHES

Dipnoi
segment

In others again, as in most Teleostomi, there is no sneh
al relation, and the radialia are more numerous than tlie
vertebrae wlienever the two are co-ex-
tensive. The exoskeletal iin-supports
exhibit similar relations to their radialia,
but in inverse order. ]\Iuch more numer-
ous than tlie radialia in the Elasmo-
branchs, Holocepliali, and the Dipnoi, the
former become gradually reduced in the
Teleostomi, until in the Holostei and Tele-
ostei they correspond in number with the
supporting radialia. Complete numerical
correspondence between the neural and
haemal spines
and the radi-
aliii and fin-

rays IS very
rare, and has
only been ob-
served in the
caudal region
of certain
Crossopterygii
{e.g. the Coelacantliidae).^

In not a few Fishes the radialia of
the median fins undergo modifications
which offer an interesting parallel to
an early stage in the evolution of the
paired fins from primitively continu-
ous lateral fins. Tlie concentration of
radialia which occurs in isolated median
fins often results, tlirough growth pres-
sure, in the complete fusion of tlie
proximal segments of more or fewer of
the radialia into two or three basal

Fig. 138.— The posterioi- dorsal
fin of Holoptychius lepto-
pterus from the old Red Sand-
stone of Nairnshire. Traces
of dermal fin -rays may be
seen at the distal margin of
the fin. (After Smith Wood-
ward. )

PTG.1

Fig. 139. — A dermal fin-ray and
its supporting radial or
pterygiophore in the Trout
(Saimofariu). D.F.R, Dermal
tin-ray; PTG.l, PTG.2,i;<sr.3,
the proximal, nuddle, and
distal segments of which the
tri-segmented radial consists ;
ptg.^ is cartilaginous ; the
other two are bony. (From
Parker and Hasvvell.)

supports, or even into a single basal
piece. Examples of such basal fusion are frequent in the
dorsal fins of Elasmobranclis, and the same modification may also
be seen in the anal fin of Flcuracanthus, and especially in the
' Smith Woodward, Kat. Sc. i. 1892, p. 29.

CAUDAL FIN

'^Z7

dorsal fin of the Devoniun Oossopterygian, Holophjchius^ (^ig-
138), where several radialia, which are free distally, have their
bases united into a single basal piece, or basipterygiuni. In
most Teleostoiai elevator and depressor muscles arise from the
radialia, and are inserted into different points on the bases of the
fin-rays, and by their contraction the latter may either be elevated
into an erect position, or folded back like a fan along the middle
line of the body, where, as in some Teleosts, there is a groove for
their reception. When fin-rays are only capable of simple eleva-
tion or depression, the connexion between a radial element and
its fin-ray is usually by ^

some form of a hinge-joint,
the cleft base of the ray
clipping the distal segment
of the radial (Fig. 139).
In some Teleosts the articu-
lation of the two is by
means of a kind of chain-
link (Fig. 137). In those
Fishes in which the median

fins are capable of lateral Fl«- 140.— Caudal end of the vertehral column of

■A'Y\ovX{Salv)i> fario). CN, Centrum ; D.F.R,

Undulatory movements the .lenual fin-rays ; H.SP, haemal spine ; H.ZYG,

articulation is of a more i;f;'":ii zygapophysis ; N.SP, neural spine ;

IS.ZiG, neural zygapophysis ; LIST, the up-

mobile character. tilted, partly ossified, and unsegmented ter-

Tn the different types "i*"''^^ ^'f ' °Vi' "°^,°f°"^' °^ ^'■°^*>'^^-

•^ ^ (From Parker and Haswell.)

of caudal fin, diphycercal,

heteroeercal, and homocercal, the supporting elements of the
ventral lobe are formed by the haemal spines of the terminal
caudal vertebrae which are inclined backwards, and are often
greatly expanded for the purpose (Fig. 140). The dorsal lobe
may be supported either by the adjacent neural spines, or by
radialia, or by both.

The Appendicular Skeleton." — It is probable that the skeleton
of the paired fins and the pectoral and pelvic girdles have been
formed from the supporting radialia of the isolated and enlarged
anterior and posterior portions of primitively continuous lateral

^ Smith Woodward, Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 335.

^ W. K. Parker, Hhoulder-fjirdle and Sternum of Vertehrata, Ray Soc. 1868 ;
Gegenbaur, Untersuch. Vergl. Anat. Wirbelth. Ft. ii. Leipzig, 1865 ; Wiedersheiiu,
Das Gliedmassenskelet d. JFirbclth. Jena, 1892.

2 38 FISHES CHAP.

fins, by a sequeuce of structural modifications in the same direc-
tion as in the median fins. The initial stage was probably
marked by the fusion of the proximal portions of the radialia
to form a basal support or basipterygium for the free distal
portions. Subsequently, it may be, a rudiment of the future
limb-girdle became segmented off from the inner extremity
of the basipterygium, and by its dorsal and ventral growth
in the body- wall the lateral half of a girdle was developed.
The subsequent union of the two halves across the mid-ventral
line resulted in the evolution of the dorsally incomplete hoop
of cartilage wliich is the primary form of the complete limb-
girdle in Craniates. The primitive fin skeleton or " archi-
pterygium " was formed from the residue of the basipterygium
in conjunction with the free distal radialia which it carried.
The precise structure of the archipterygium is purely hypothetical.
Possibly it was a biserial fin of the PleuracantJins or Neoceratodus
type, consisting of a cartilaginous segmented axis, fringed along
its anterior and posterior, or pre-axial and post-axial margins, by a
series of slender, simple, or jointed radialia (Fig. 14V); or it
may have been a uniserial structure, somewhat resembling the
pelvic fin of Pleuracantluis, or the pectoral and pelvic fins of
existing Elasmobranchs (Figs. 250, 141), in which an axis
formed by the residue of the basipterygium or metapterygium
had a fringe of radialia on its anterior or preaxi.il side only.
If the archipterygium was biserial then the uniserial fin was
proljably derived from it by the sul)sequent siippression of all
the post-axial radialia ; or, if uniserial, the biserial fin was evolved
by a later extension of radialia on to the post-axial margin. The
evidence of comparative anatomy is not conclusive as to tlie
nature of the archipterygium, and palaeontology seems to support
either view with puzzling impartiality.^ It may be admitted
that the lateral fin theory offers the best solution of the problem
of the origin of the paired fins, but it must be borne in mind
that no Fish, living or fossil, is known to possess fins of this
nature, unless the singular lateral lobes of some Ostracodermi
{e.g. the Coelolepidae) are kindred organs " ; neither do continuous
lateral fins ever exist as vestiges, unless, indeed, the bilateral
series of spines, which extend between the pectoral and pelvic

^ Tra(}iiair, Nature, 62, 1900, p. 502.
- Traquair, Trans. Roy. Soc. Edin. xxxix. 1899, p. 843.

TECTORAL AND 1M-:LVIC GIRDLES

239

fins, in some of the Lower Devonian Acanthodei (e.g. Climatius),
may lie regarded in that light.

IVie Pectoral and relvic Girdles. — The pectoral girdle is more
l)rimitive in Cladoselache and Fleuracanthiis than in any other
Elasmobranch. In the former (Fig. 145, A) it may be doubted
if the girdle has passed beyond the basipterygial stage, and
although a definite girdle is present in the latter genus (Fig.
250) its lateral
halves retain their
primitive distinct-
ness. Existing
Elasmobranchs, in-
cluding the Holo-
cephali, have a
pectoral girdle in
the form of a
dorsally incomplete
hoop of cartilage
imbedded in the
muscles of t li e
body-wall, close
behind the last
branchial arch (Fig.
141). The upper
or dorsal portion
of each half is the
scapula, antl the
ventral is the cora-
coid. Between
these two portions
of the girdle, and defining their limits, there are articular surfaces
for the basal cartilages of the pectoral fin.

Cladoselache (Fig. 145, B) had no pelvic girdle, nor does it
appear that this primitive Elasmobranch had acquired even
a basipterygium. Pleur acanthus, on the contrary, had a pair of
pelvic rudiments distinct from well-developed basipterygia. In
other Elasmobranchs there is a distinct girdle, formed by the
median union of primitively distinct lateral rudiments, consisting
of a simple transverse bar of cartilage, imbedded in the ventral
abdominal wall, just in front of the cloacal aperture, and having

Fig. 141. — The right half of the pectoral gh\lle and the fiu
of au Elasiuobraiicli [Chiloscyllium). d.r. Dermal horny
hbres ; mesu, niesopterygium ; meta, metapterygiuni :
2-)ect, pectoral girdle ; pro, propterygium, (From Parker
and Haswell.)

240

FISHES

articulated to each of

its outer extremities the liasal cartilage
(metapterygium) of the pelvic
fin. Sometimes there is a rudi-
ment of a dorsally-directed " iliac "
l)rocess at each extremity of the
U'irdle, but in no Fish do these
processes ever acquire a dorsal
connexion with the vertebral
column. In the Holocephali the
iliac processes are better developed
than in any other Fishes, but
ventrally the lateral halves of the
girdle are united l;)y ligament
alone. In the Teleostomi im-
portant differences are observable
in both girdles. The primary
cartilaginous pectoral girdle now
consists of distinct lateral halves
which have no ventral connexion
other. In addition, there is developed on the outer
each half a series of membrane bones, which form

Fig. 142.— The left half of the pelvic
ginlle and the right pelvic fiii of
Qhiloscyllum. vieta, Metapterygium ;
jyelv, pelvic girdle. (From Parker and
Haswell.)

with each
surface of

'3.AV2 /'^{'^ PTG.l

Fig. 143.— Left half of
the pectoral girdle of
a Trout {Salmofariv),
seen from the inner
surface. CL, Clavicle
(cleitlirum) ; COR,
coracoid ; D.F.li,
dermal tin - rays ;
MS. COR, meso-cora-
coid ; F. CL, P. CL',
p o s t - c 1 a V i c 1 c s ;
PTG.l, proximal;
2^tg.3, distal pterygio-
p ho res ; P. I'M,
pnst-teniporal ; S.CL,
supra-clavicle ; SCP,
snapula. (From

Parker and Haswell.)

a secondary girdle (Fig. 143). From above downward the series
includes a supraclavicle and a cleithrum (clavicle of Teleosts)
which are always present, and to these may be added in the

PECTORAL AND PELVIC GIRDLES

241

Crossopteiygii and Chondrostei au infraclavicle or clavicle proper,
while one or two " post-clavicles " may be present in relation with
the hinder margin of the cleithrum. The infraclavicles, or in
their al)sence the cleithra (e.g. Holostei and most Teleostei),
usually meet in a median ventral symphysis, so that the secondary
girdle tends to acquire the characteristic hoop-like arrangement
of its parts which has been lost in the primary girdle. With
the development of a bony secondary girdle, the primary girdle
(scapula and coracoid) becomes much reduced, and, as a rule,
does little more than connect the tins with the cleithra.
The secondary girdle acquires a dorsal connexion with the skull
on each side by means of the
post -temporal bone, which is
attached below to the supra-
clavicle and al:»ove to the periotic
capsule. In the Chondrostei
and the Dipnoi the primary
girdle retains its primitive carti-
laginous condition, but in the
Crossopterygii, Holostei, and in
all Teleosts it is ossified as dis-
tinct scapulae and coracoids. To
these may be added in some
Teleosts a mesocoracoid formed
by a separate ossification of the
coracoid cartilage (Fig. 143).^

AVith the possible exception
of small paired or median carti-
lages inserted between the inner
extremities of the basipterygia
in Polyptcrus and a few other
Teleostomi, the pelvic girdle is absent in all the existing members
of this group, having either become completely suppressed, or
remaining unseparated from the basipterygia of the pelvic fins."
In the Dipnoi (Fig. 144) there is a true pelvic girdle which has
some points of resemblance to that of certain of the caudate Am-
phibia. It is represented by a median, lozenge-shape, cartilaginous
^ It is more probable that in most existing Teleostomi the pelvic girdle has
undergone complete suppression, in which case these cartilages are vestiges and

Or—

Fig. 144. — Ventral view of the pelvic girdle
of Protoptcrus. «, I'rejiubic process ;
6, lateral process for the fin ; c, epipubic
process ; <.ir, ridge for the origin of the
tin muscles ; HE, skeleton of the
fin ; M, myotomes ; M', niyoconmiata.
(From Wiedersheim. )

not rudiments.

- See, however, Goodrich, Quart. Journ. Micr. Set. xlv. 1901,
VOL. VII

311.

242

FISHES

plate, produced in front into a long tapering epipubic process,
and on each side of this into a forwardly inclined prepubic pro-
cess. The hinder part of the plate l)ears two short processes for
the basal cartilages of the pelvic tins. There is no trace, how-
ever, of iliac processes.

The Pectoral Fins. — The skeleton of the pectoral fins exhibits
reniarkal)le structural variations in different Elasmobrauchs. In
the existing members of the group two large basal cartilages, the
propterygium and the mesopterjgium, are formed hj the concen-
tration and fusion of the proximal portions of certain of the
preaxial radialia, and they, with the metapterygium, articulate

with the pectoral girdle ;
hence the fin is tribasal
as well as uniserial (Figs.
141 and 146, A, B). In
striking contrast to all
other Elasmobranchs the
pectoral fin of CladoselacJic
(Fig. 145, A) is far more
primitive than in any other
Fish. Each fin is supported
1 ly a distal series of slender,
more or less parallel, un-
jointed, cartilaginous
radialia, and basally by a
similar series of shorter,
stouter, and less numerous
cartilages, which apparently
were iml jedded in the l:)ody-
wall, the entire fin skeleton
presenting a striking re-
semblance to an isolated
median fin in which the supporting radialia have concentrated
V)y growth pressure, and their proximal portions have been
reduced
other hi

supporting fan-like clusters of horny fibres at their distal ends
(Fig. 250).

The broadly lobate pectoral fin of the existing Crossopterygii

^ Bashford Dean, Anat. Anz. xi. 1896, p. 673.

Fig. 145. — A, Pectoral fin, ami B, pelvic fin of
Cladosdache. (From Bashford Dean.)

)wi}n pressure, ana ineir proximal portions nave ueen
1 in numl)er liy partial fusion.^ Pleuracantlnis, on the I
land, had a biserial fin, the preaxial and postaxial radialia

PECTORAL FINS

243

(Fig. 146, G) is uniserial, closely resembling that of the more
typical Elasmobranchs.^ There are three basal elements, a pro-
]>terygium, a mesopterygium, and a metapterygium, each of
which supports a series of partially ossified radialia. Little is

Fig. 146. — Pectoral fins of various Fishes. A, Acanthias %-ulgaris ; B, Raia sp. ;
C, Chhnaera monstrosa, ; T), Acipenser rhyyidmeus ; E, Amia calva ; F, Lepidosteus
platyrhynchus ; G, Pohjpterus bichir ; H, Salmo salvelinus. The preaxial side of
each fin is to the left and the postaxial to the right, f.r, Dermal fin-ray ; ms,
mesopterygium ; mt, metapterygium ; p, propterygium ; r, free radialia ; 1, 5, the
preaxial and postaxial basal elements in a Teleost, which may be mesopterygial and
metapterygial pieces respectively, the three remaining basal pieces probal)ly being
intrusive metapterygial radialia dii-ectly articulating with the pectoral girdle. In
B, D, E, and F, similar intrusive radialia are shown. (From Gegeubaur.)

known of the endoskeletal elements of the broadly or acutely
lobate fins of the fossil Crossopterygii, but it seems probable
that their disposition was uniserial and abbreviate in obtusely
lobate fins and biserial in acutely lobate fins. In the remaining
Teleostomi (Actinopterygii) the endoskeletal elements become

1 Budgett, Trans. Zool. Soc. xvi. Part vii. 1902, p. 328.

244

FISHES

gradually reduced in number and importance, their place as
fin - supports being usurped by the dermal fin - rays. In
addition, more than three, usually several, basal elements
articulate directly with the pectoral girdle, and hence the
fins become multi-basal. In the Chondrostei and the Holostei
a metapterygium is always recognisable, supporting several
radialia along its preaxial border, as in Acvpenscr (Fig. 146,

D) and Amia (Fig. 146, E), or only
a single one, as in Lrpidostcus (Fig.
146, F). The anterior part of the fin
is supported by a variable number of
cartilaginous or bony radialia, which,
with the metapterygium, articulate with
the limb-girdle. In Teleosts the pro-
cess of reduction reaches its maximum.
Usually there is but a single row of
short, hour-glass-shaped ossicles, of which
the postaxial one may represent a ves-
tigial metapterygium, and sometimes
there is also a distal row of small
cartilages or ossicles, partially hidden
in the cleft bases of the dermal fin-
rays (Fig. 146, H). In all these Fishes
the fin is a much reduced uniserial fin,
in which more or fewer of the preaxial
radialia have acquired a direct secondary
connexion with the pectoral girdle.
Fig. I47^ie left pectoral fin Of living Dipnoids Neoceratoclus has a
of Xeoceratodus. a, b. First nearly typical biscrial fin, but, as seems

two segments of the axis ; i^.S, , , ,, . ^^ n x> i.i • 4. j.

preaxiai horny fii^res ; t, f, to be the casc in all fins ot this type at
pre- anil post-axial radialia. present kuowu, there is a marked ab-

( After Wiedersheim.) ^ , • ,1 i j

sence of symmetry m the number and
disposition of the radialia on the two sides of the axis. There
is also much individual variation. No two fins are precisely
alike, and the radialia may sometimes divide. In the very
acutely lobate fins of the remaining Dipnoids it is evident that
great reduction has taken place. Protopterus has lost all trace
of postaxial radialia, and in Lepidosiren even the preaxial have
atrophied, leaving only the long jointed axis to represent the
originally biserial fin.

PELVIC FINS

245

The Pelvic Fins. — In the simplicity of their endoskeletal
su|)purts the pelvic lins of Cladoselache are the most primitive
type of paired fins at present known (Fig. 145, B). In general
structure they resemble the pectorals, but the radialia are fewer
iu number, less modified by concentration, and exhibit little,
if any, trace of basal fusion. Add to such features as these
the apparent absence of any trace of pelvic rudiments, or of
basipterygia, and it will be obvious that the pelvic fins differ
but little from the median fins of the same I'ish except that
they are paired. In Pleuracanthus the pelvic fins differ from
the corresponding pectorals iu being uniserial instead of biserial
(Fig. 250). All other
Elasmobranchs, including
the Holocephali, have uni-
serial fins, which consist
of a large metapterygiiim,
supporting a preaxial
fringe of segmented
radialia. A proptery-
gium is sometimes present,
notably in some of the ^^'^"^l^^
Skates and Eays, and, like
the metapterygium, it is
directly connected with
the pelvic girdle.

Tlie skeleton of the
pelvic fins of the Teleostomi is often extremely degenerate.
It is perhaps best developed in the Chondrostei,^ where each
fin is supported by numerous segmented radialia, more or
fewer of which fuse towards the base of the fin, and those
form a large and slightly ossified basipterygium (Fig. 148).
In the living Crossopterygii, Holostei, and Teleostei, the pelvic
fins are similar in essential structure, but are very degenerate.
The basipterygium is usually well developed and is always bony
(Fig. 149), and in many Teleosts it acquires so extensive a
sutural connexion with its fellow that, physiologically, it supplies
the place of a true pelvic girdle. At its distal end there may
be a single row of small cartilaginous or bony nodules., repre-
senting vestigial radialia, as in the Crossopterygii, Holostei, and

1 Thacker, Trans. Cunncdicut Acad. iv. 1877, p. 233.

SkeJeton of a pelvic tin of Polyodon
fdUum, ventral view, vvitli the anterior margin
of the fin to the right ; to show the partial
fusion of the proximal portions of primitively
distinct radialia to form a basipterygium. b,
Inner or mesial extremity of the l>asipterygium ;
d.j), dorsally directed, rudimentary iliac process ;
11, foramen for nerves. (After Rauteufeld.)

246

FISHES

CHAP. VIII

Teleostei, but even these may be absent, and the dermal fin-rays
then articulate directly with the basipterygium. Little is known of
the skeleton of the pelvic fins in the fossil Crossopterygii, but there
is evidence of the existence of a higher
grade of structure than in their surviving
allies. In Eusthcnopteron} for example,
the fin is supported by an axis of at least
three bony segments, with at least three
ossified preaxial radialia ; hence, it has
obviously undergone less degeneration than
in Polypterus, where the fin -skeleton is
essentially Teleostean. In the Dipnoi the
pelvic fins are similar to the correspond-
ing pectoral fins, but individual variation
is more marked and even the central
axis may divide.' In the males of all
existing Elasmobranchs, including the
Holocephali, certain of the more distally
situated metapterygial radialia become
modified to form a supporting skeleton
for the copulatory organs, the claspers,
or mixipterygia. In the latter group
the anterior claspers are also provided
with cartilaginous supports articulatincf
with the pelvic girdle directly in front of the pelvic fins.

Fig. 149. —Skeleton of the
left pelvic fin of a Trout
{Salmo fario), seen from
the dorsal surface. B.PTG.
Basipterygium ; D.F.R,
dermal fin rays ; PTG,
distal radialia. (From
Parker and Haswell.)

1 Traquair, Gcol. Maij. vii. 1890, p. If) ; Goodrich, I.e.
- Haswell, Proc. Linn. Soc. X.S. JF. ix. 1881, p. 71 ; Howes, F.Z.S. 1887, p. 3.

CHAPTER IX

THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS

The alimentary canal is a muscular tube with an epithelial
lining, formed for the reception and the digestion of the food.
It begins with a mouth, and from thence it extends backwards
through the coelom, finally communicating with the exterior
either by a cloacal or by an anal orifice. The oral or buccal
cavity into wiiich the mouth leads is a stomodaeum, and is lined
by inpushed epidermis, while the hinder portion of the cloaca
and the anus are lined by a somewhat similar inpushing of the
epidermis which forms the proctodaeum. The rest of the
alimentary canal, consisting in succession of a pharynx, an
oesophagus, a stomach, and an intestine, constitutes the mesen-
teron, and is lined by endoderm. Teeth are developed from the
walls of the stomodaeum, and glands for the secretion of digestive
fluids from the endoderm of the mesenteron.

Dentition.

In the Lampreys among the Cyclostomata teeth are developed
in the form of yellow conical structures on the inner surface of
the l)uccal funnel, and on the extremity of the rasping "tongue"
(Fig. 91, A). Each tooth consists of an axial papilla of the
dermis, sometimes enclosing a pulp -cavity, and invested by
the epidermis, and also by a stratified horny cone which forms
the projecting hard part of the tooth. The dermal papilla with
its ectodermal investment bears a .superficial resemblance to the
germ of a true calcified tooth, but no odontoblasts are formed,
nor any calcic deposit, the laminated horny teeth being formed
by the gradual conversion of the successive strata of the

247

248

FISHES

epidermic cells into horny layers.^ The old teeth are verticallj'
replaced hj new teeth developed beneath the functional teeth.
With the exception of a median tooth above the oral aperture,
Myxine and its allies have only lingual teeth. These are comb-
like, and they are formed by the basal fusion of primitively
distinct tooth -germs. The structure and development of the
teeth of the Cyclostomes lend no support to the view that the

.•^0

~ /l6'

-dp.

Fig. 150. — Vei'tical section of developing tooth in Petronti/zon marinus, showing a
successional tooth, which is just beginning to cornify at its apex beneath the
functional tooth, d. Dermis ; d.}), dermal papilhie ; ep, epidermis lining buccal
funnel ; ep^, epidermis which has formed the liorny functional tooth ht ; ep-,
epidermis forming the horny cone of the successional tooth ht^. (From Warren.)

teeth are degenerate calcified structures. AVith greater prob-
ability they represent a stage in the evolution of teeth and
dermal spines, which has been succeeded by a later stage in
which calcification superseded cornification as a method of
hardening.

True calcified teeth first make their appearance in Fishes,
where they assume the form of modifications of exoskeletal
structures." The teeth of Elasmobranchs are identical in essential
structure, as well as in the manner of their development, with
the ordinary dermal spines of the skin, and in the embryo the

1 Warren, Quart. Journ. Micr. Sd. xlv. 1902, p. 631.

- See Ridewood, Kat. Sci. viii. 1896, p. 391, for references.

DENTITION

249

dermal spines form a continuous series with those wiiich invest
the jaws and eventually become teeth (Fig. 151). It is only
later, when lips become apparent, that the continuity of the
teeth and dermal spines is interrupted, and the two structures
assume their distinc-
tive characters.

The tissues of wliich
the teeth of Fishes are
composed are (1) den-
tine, wbich is a non-
vascular, calcified
tissue, traversed by
numerous radiating,
branched, dentinal
tubuli, into which ex-
tend protoplasmic pro-
longations from the
cells (scleroblasts) by
which the dentine is
secreted. Dentine
forms the greater part
of the body of a tooth.
(2) vasodentine and
(.')) osteodentine are
modifications of ordi-
nary dentine, the
former containing
blood - vessels ramify-
ing in its substance
but no dentinal tubules, and the latter more closely resembling
bone. (4) enccmel, an exceptionally dense, non- vascular, non-
tubular tissue, which may or may not exhilut traces of the
prismatic structure so characteristic of this tissue in the higher
Vertebrates, forms the outer investment of the teeth.

As regards their fixation, the more primitive forms of teeth,
such as those of Elasmobranchs, are simply embedded in the
gums, and are only connected with the jaws by fibrous tissue ;
but in some of the older fossil Sharks the fixation of the teeth
is effected by the mutual articulation of the basal plates of the
teeth with one another. The Chondrostean rolyodon, so shark-

FiG. 151. — Transverse section through the lower jaw of
an emhryo Scyllmm, to show the gi-adual transition
from dermal spines {d, d, d) on the outer surface of
the jaw to teeth {t, t, t) on the oral surface, c, Car-
tilage of the lower jaw. (From Gegenbaur.)

250

FISHES

like in many other respects, also lias teeth implanted basally in
the gums, and quite free from any special connexion with the
jaw-bones. In some Teleosts with movable teeth, the latter
are merely attached to the jaws by fibrous, and often elastic,
ligaments, as in the Pike (Esox) and the Angler- Fish (Zoj^hius).
As a rule, however, the teeth are directly ankylosed to the
bones developed in relation with the jaws. Very rarely, as, for
example, in some Characinidae, are the teeth implanted in
sockets.

Nearly all Fishes are polyphyodont, that is, the old teeth are
constantly replaced by new teeth as fast as they become worn
down or fall out. In the Sharks and Dog-Fishes, for example,
where the teeth are arranged in rows parallel to the axis of each
jaw, the functional teeth along the upper edge of the jaw are

usually erect, while those in the
rows more internally situated point
inwards towards the oral cavity ;
and behind these again there are
rows of developing teeth in different
stages of growth, and partially hidden
beneath a projecting fold of the oral
mucous membrane (Fig. 152). As
the teeth in use become lost they
are successively replaced by the
inner rows, which, with the mucous
Fig. 152.— Transverse section through membrane in whicli they are eni-

the jaw of a Shark (Carcharias), ■, -, t ^ n nj.j.i ^

showing how the teeth .are re- bedded, movc forwards to the edge
jjaced. c, Cartilage of the jaw ; of the jaw, whcre they bcconie erect

t, functional tooth ; t', its im- , „ . , „, - . i p xi

mediate successor; t", t'\ still and iuuctional. Ihc teeth ol the
younger teeth, covered by the piolocephali and of the Dipnoi are

fold ot mucous membrane, m. m. i i t i i • i

(From Ridewood.) not shed, but the loss which they

sustain through wear and tear is
made good by persistent growth at their bases. In the Teleo-
stomi the succession is less regular, new teeth being formed between
or at the bases of the old teeth. In the case of socketed teeth
the succession is usually vertical, the new teeth being formed at the
sides of the old ones ; and by the absorption of the bases of the
latter, the former come to lie directly below them, and eventually
they occupy the same sockets.

As might be expected from the remarkaljle diversity in the

-^--m.m.

DENTITION 251

habits and in the food of different Fishes, the teeth exhibit an
equally striking diversity in form, size, and structure. The
most primitive type of tooth resembles an ordinary dermal spine,
and is little more than a simple pointed cone. A few Elasmo-
branchs and many Teleostomi possess teeth of this kind. By
the flattening of the cone parallel to the axis of the jaw, the
tooth becomes triangular, and then the margins may either
remain smooth and trenchant, or they may become complicated
by the formation of marginal serrations or of accessory basal
cusps, and by such modifications the characteristic teeth of
most Elasmobranchs are formed. The simple cone may also
be modified to form crushing teeth — short, blunt, more or less
hemispherical teeth — or even transformed into a mosaic of hexa-
gonal plates, as in the Myliobatidae amongst Elasmobranchs.
Massive, flattened, scroll-like crushing teeth are also formed by
the fusion of adjacent teeth, or of several successional teeth, and
of such composite teeth we have examples in the Heterodontidae
and in the Palaeozoic Cochliodontidae. By a somewhat similar
process of concrescence the anomalous composite teeth of such
Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish
(Scarus), have been evolved. The singular dental structures
of the Holocephali are probably composite teeth, and it is
certain that the highly characteristic teeth of the Dipnoi have
resulted from the basal fusion of primitively distinct simple
conical denticles. The dentition is often heterodont. In Hetero-
dontus {Cestracion), for example, the anterior teeth in each jaw
are pointed and prehensile, while the hinder ones are scroll-like
and crushing. Prehensile and crushing molar-like teeth are also
present in such Teleosts as many of the Sparidae, and in the
Wolf- Fish {AnarrMchas). The existence of sexual differences
in the dentition is illustrated in the Skates and Eays {Raia),
where teeth which are simple and pointed in the male become
flattened and plate-like in the female. A few Teleosts, like the
Syngnathidae, Cyprinidae, and some Siluridae, are entirely devoid
of jaw-teeth.

In addition to jaw-teeth, many Teleosts possess pharyngeal
or gill-teeth, developed in connexion with the inner margins of
the branchial arches, to which they are usually firmly ankylosed
(Figs. 35 2, 412 and 413). As a rule "the pharyngeal denti-
tion is inversely proportional to the extent of tooth development

252 FISHES CHAP.

on the jaws." ^ Pharyngeal teeth dift'er greatly in size and
structure in different Teleosts, and, like the jaw-teeth, they are
capable of replacement by vertical succession. The teeth are
sometimes restricted to the inferior pharyngeal bones (cerato-
branchials of the last branchial arch), and then, as in the Carp
{Cyiyrinus), they may bite against a callous pad on the under
surface of the basioccipital bone ; or, as in some of the Wrasses
(Lahrus), the inferior teeth are opposed to superior teeth on the
upper pharyngeal bones (pharyngo-brauchials of more or fewer
of the branchial arches). "When pharyngeal teeth are present
it is probable that they are the principal masticatory organs, the
jaw-teeth being used for seizing or holding the prey.

Alimentary Canal.

A protrusible tongue is never developed in Fishes. A rudi-
ment of that organ is present in the Elasmobranchs (Fig. 153)
and Dipnoi, and also in the Crossopterygii, and usually consists
of an elevated area of mucous membrane provided with free
lateral edges and a forwardly projecting apex ; it is supported by
the basi-hyal element of the hyoid arch. In the Crossopterygii
(e.g. PolyiJterus) the tongue contains muscle fibres, and in the
Dipnoi, where the organ is better developed than in any other
Fishes, special lingual muscles are present.

The pharynx succeeds the oral cavity, and is perforated on
each side by the branchial clefts (Figs. 153, 154). The rest of
the alimentary canal differs considerably in various Fishes in
the degree of distinctness of its several regions, and in the
extent to which it is convoluted. As a rule the pharynx is
followed in succession by an oesophagus, a stomach, and an
intestine (Fig. 153), the latter terminating in a portion usually
termed the " rectum." The boundaries of these regions are not
always very obvious, but are indicated by variations in calibre,
by changes in the character of the lining epithelium, by special
valves or sphincter muscles, or by the entrance of the ducts of
certain glands like the pancreas and liver. The oesophagus is
occasionally separated from the stomach by a slight constriction,
but more frequently the replacement of the squamous epithelium
of the oesophagus by the columnar epithelium of the stomach
^ Ridewood, op. cit. p. 390.

ALIMENTARY CANAL

53

and the appearance of gastric glands in the wall of the latter
cavity afford the only distinction between the two regions. The

pituitary body ; j^r.rt, pelvic arch ; pijl.st, pyloric
lobe of liver ; rct.gl, rectal gland ; s}}, spiracle ;
sac ; s/j.i7, spiral valve ; s.v, sinus venosus ; t7i(/,
v.j, upper jaw ; vr, metanephric duct ; v, venti
or mesonepliric duct ; vs.sem, vesicula seminalis.

Fig. 153. — Dissection of a
male Dog - Fish (Sci/l-
Uum). The left side of
the body is cut away
to the median plane so
as to expose the ab-
dominal and pericardial
cavities and the neural
canal in their whole
length. The alimentary
canal and the liver have
been drawn downwards,
and the oral cavity, the
pharynx, part of the
intestine, and the cloaca
have been opened. The
cartilaginous parts of the
skeleton are dotted, and
the calcified portions of
the vertebral centra are
bl;ick. abcl.cav, Abdo-
minal cavity ; au, au-
ricle ; li.hr. basi- bran-
chial ; h.hy, basi-hyal ;
c.nrt, couus arteriosus :
cd.a, caudal artery ;
cd. sf, cardiac jiart of the
stomach ; cd.v, caudal
vein ; cl, cloaca ; en,
centrum ; cr, cranium ;
crb, cerebellum ; d.cto,
dorsal aorta ; die7i, tha-
lamencephalon ; epid,
epididymis ; /on, fouta-
nelle ; gul, oesophagus ;
h.a, haemal arch :
i.br.a^ — i.br.a,^ internal
gill -clefts ; int, intes-
tine ; kd, kidney ; l.J,
lower jaw ; l.lr, left lobe
of liver ; med.obl, me-
dulla oblongata ; mes,
mesentery ; 11. a, neural
arch ; n.cav, neural
canal ; olf.l, olfactory
lobes ; ojit.l, optic lobes ;
pan, pancreas ; pcd.car,
pericardial cavity :2'cf.(U
pectoral arch ; jih,
pharynx ; pin, pineal
body ; p.n.d, vestigial
Miillerian duct ; prs,
prosencephalon ; pty,
portion of the stomach ; y, rostrum ; r./r, right
sp.cd, spinal cord ; sjjl, spleen ; sjJ.s, sperm
tongue ; ts, testis ; u.r/.s, urino-genital sinus ;
icle"; i-.rto, ventral aorta ; v.def, vas deferens
(From Wiedersheim, after T. J. Parker.)

2 54 FISHES CHAP.

commencement of the intestine is usually indicated by a pyloric
"valve" (Fig. 155, A, B), in the form of a ring-like, inwardly
projecting thickening of the circularly-disposed muscle fibres of
the terminal extremity of the stomach, and usually also by the
entrance of the distinct or united ducts of the liver and pancreas ;
sometimes, as in certain Elasmobranchs and in the Dipnoi, by a
special dilatation or " Bursa Entiana " (Fig. 155, A). The rectum,
or terminal portion of tlie intestine, is distinguished from the rest
of the gut by its straight course to the cloacal aperture or the
anus, and sometimes by an increase in calibre. In Box vulgaris
and a few other Teleosts ^ a caecal diverticulum indicates the
commencement of the rectum, while in a few cases the pre-
rectal portion of the intestine communicates with the enlarged
rectal segment by a much constricted valvular orifice which is
suggestive of the ileo-colic valve of the higher Vertebrates," as
in the Teleosts Amiurus caius,^ Trigla gurnardus, and Cydo-
pterus lump us.

The relation of the regional divisions of the intestine in
Fishes to those of other Vertebrates are somewhat difficult to
determine. If we may regard the " rectal " gland of Elasmo-
branchs and the intestinal caecum of certain Teleosts as homo-
logous with each other, and with the caecum coli of the higher
Vertebrates, then it would seem that by far the greater part of
the intestine of Fishes, including that portion in which a spiral
valve may be developed, is homologous witli the pre-caecal
segment of the gut or small intestine in other Vertebrates, and
that the post-caecal section, or large intestine, of the latter is
represented in Fishes only by that relatively short portion of the
gut which lies posterior to the rectal gland or its homologue in
Teleosts, the equivalent of the colon of Mammalia being, as in
Amphibia, Eeptiles, and Birds, practically undifferentiated."*

In the Cyclostomata the alimentary canal retains much of its
primitive simplicity. It pursues a straight course from mouth
to anus, and the usual regions are very obscurely indicated. The
same remarks apply also to the Holocephali and a few Teleosts,
although in these Fishes the limits of the different regions are

1 For references see Howes, Limi. Soc. Journ. Zool. xxiii. 1890, p. 381.
- Howes, oip. cit.

^ jMacallum. Reprinted from Proc. Canadian Instit. N.S. ii. 1884, p. 387.
* Plowes, op. cit.

ALIMENTARY CANAL

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256 FISHES CHAP.

somewhat more clearly defined. In the Dipnoi (Fig. 155, A), a
contracted sigmoid curve between the somewhat dilated stomach
and the spacious intestine is the only departure from the straight
course of the preceding groups.

In the remaining Fishes the degree of convolution varies
within rather wide limits. The oesophagus is usually straight
and wide, but in Lutodeira, among Teleosts, it is long and even
convoluted, and in the Plectognath Teleosts it gives off a large
sac-like outgrowth (" air-sac "), which extends anteriorly as far as
the head, and posteriorly to the beginning of the tail, and
communicates with the oesophagus by two apertures. The
stomach may be U-shaped with the concavity directed forwards,
and consisting of a right limb passing backwards from the
oesophagus, and a left limb curving forwards to its junction
with the intestine (Fig. 153). In such instances as these the
stomach and the adjacent section of the intestine describe a
characteristic siphonal curve. In certain other Fishes (Fig. 160),
the oesophageal portion of the stomach terminates behind in a
tubular or sac-like dilatation at some distance posterior to the
laterally situated pylorus, which indicates the origin of the
intestine. The intestine is straight, or nearly so, in Elasmo-
branchs, Crossopterygii, and Dipnoi, and also in a few Teleosts ;
but sometimes, and very generally in Teleosts, it is more
or less convoluted, notably in some of the Mugilidae, and in
the Loricariidae, where, as in Plecostoinus, it is disposed in
numerous spiral coils like a watch - spring. The terminal
portion of the intestine or rectum either opens into a cloaca,
which also receives the urinary and genital ducts, as in Elasmo-
branchs (Fig. 153), and Dipnoi (Fig. 155, A), or opens externally
by an anus, situated in front of the separate or united urino-
genital ducts, as is the case with all the remaining groups
of Fishes (Fig. 154). The cloacal aperture is invariably
situated near the junction of the caudal and trunk regions, and
as a rule is median in position, rarely, as in tlie Dipnoi, displaced
to the right or left of the middle line ; but the anus differs
greatly in position, sometimes retaining its primitive position at
the hinder end of the trunk, as in the Holocephali, Choudrostei,
Crossopterygii, Holostei, and many Teleosts, or occupying almost
any position between that point and, as in the " Electric Eels "
(Gymnotidae), the ventral surface of the throat (Fig. 351.)

ALIMENTARY CANAL

257

Id-

sp.v. <----iM

Fig. 155. — A, alimentary
canal and liver of a
female Prntnptems,fr om
the left side. Part of
the left wall of the
stomach and intestine,
and the peritoneal in-
vestment of the spleen
have been removed, a.j).
Abdominal pore ; b.d,
bile-duct ; h.e^it, Bursa
Entiana ; d, cloaca ;
cl.cq}, cloacal aperture ;
cl.c, caecum cloacae ;
c.m.a, coeliaco - mesen-
teric artery ; cy.d, bile
duct ; k.d, kidney duct ;
m.a, mesenteric ar-
teries ; od, oviduct ;
pt.G, post - caval vein
or inferior vena cava ;
21.V, portal vein ; the
other reference letters
as in B. (From

Newton Parker.) B,
viscera of an adult
female Lepidosfeus, ven-
tral view. The oeso-
phagus, the commence-
ment of the intestine
and the rectum have
been laid open. ab,
air-bladder ; an, anus ;
b.d, intestinal aperture
of the bile-duct ; g.b,
gall-bladder ; gl, oeso-
phageal ajjerture of the
air-bladder ; h.d, hep-
atic duct ; I, liver ; oes,
oesophagus ; py, py-
lorus ; P1/-C, pyloric
caeca ; j)y.c' , the four
intestinal orifices of the
pyloric caeca ; r, rec-
tum ; s, spleen ; sp.v,
spiral valve ; st,
stomach. (From Bal-
four and Ne^vton
Parker.)

— an.

VOL. VII

258

FISHES

The whole length of the alimentary canal from the oeso-
phagus to the rectum is invested externally by the visceral
layer of the peritoneum (Fig. 156), which histologically consists
of a stratum of connective tissue, supporting on its free sur-
face an epithelial stratum (coelomic epithelium). Primarily,
the investing peritoneum is continued both dorsally and
ventrally into bilaminar suspensory folds, the dorsal and ventral
mesenteries {d.ms, v.ms), which extend to the mid-dorsal or mid-
ventral line of the abdominal cavity. The two layers then
separate and become continuous with the parietal layer of the
peritoneum lining the whole of the inner surface of the

CCX.I:-

v.msr-
F-

verse process ;
and Haswell.)

Fig. 156. — Transverse section
of a Fish, diagrammatic.
en, Centrum ; cod, coel-
ome ; d.a, dorsal •aorta ;
d.f, dorsal fin ; d.vi, dorsal
muscles ; d.vis, dorsal
mesentery ; f.r, fin ray ;
gon, gonad ; int, intestine ;
l.v, lateral vein ; msn,
mesonephros ; msn.d,
mesonepliric duct ; «.«,
neural arch ; p, parietal
layer of the peritoneum ;
p', visceral layer ; p.c.i\
posterior cardinal vein ;
pji.d, Miillerian duct ; r,
ventral rib ; r', dorsal rib ;
sp.c, spinal cord ; t.p, trans-
ventral muscles ; v.ms, ventral mesentery. (Modified, after Parker

body -wall. Embryologically, the two mesenteries owe their
formation to the fusion above and below the mesenteron of the
contiguous walls of two laterally situated and primitively distinct
coelomic cavities. The dorsal mesentery in the adult is occa-
sionally complete, as in the Myxinoid Cyclostomata and in
the Elasmobranch Hi/pfws subnigruni} and also in some Dipnoi
and in a few Teleosts, but much more frequently it is reduced
by absorption to anterior and posterior remnants, or to a series
of isolated bands, or even, as in the Lamprey {PetromyzoTi), to
a few filaments accompanying the intestinal blood-vessels. The
ventral mesentery, on the contrary, is rarely present, and if
present is never complete. In Lepidosteus ^ a ventral mesentery

1 Howes, P.Z.S. 1890, p. 669.

- Balfour and jSTewton Parker, Phil. Trans. 173, 1882, p. 425.

ALIMENTARY CANAL

259

is said to be present in connexion with that part of the
intestine which contains the spiral valve. In Protopterus} and
also in JSfeoeeratodus^ there is a well-developed ventral mesentery
in relation with the greater part of the length of the intestine,
although in the former Dipnoid its continuity is interrupted by

Fig. 157. — Transverse section through a portion of the wall of the intestine, combined
from the condition seen in both the higher and the lower Vertebrata. Semi-
diagrammatic, a.c, Epithelial cells in the amoeboid state ; h.v, blood-vessels ; cm,
circular muscular layer ; g, one of Lieberktihn's glands in the higher Vertebrates ; Lej),
intestinal epithelium ; I, leucocytes ; V, leucocytes in the intestinal epithelium ; l.f,
lymph follicles; I.m, longitudinal muscular layer; lym, lymphatic vessels ; p,
visceral layer of the peritoneum ; sm, the submucosa ; v, villi of the higher Verte-
brates. (From Wiedersheim.)

one or two vacuities, and in the latter the mesentery is incom-
plete posteriorly. A ventral mesentery is also present in the
intestinal region of some of the Muraenidae among Teleosts.^

Internal to its peritoneal investment the wall of the alimentary
canal consists in succession from without inwards of (1), a

^ Newton Parker, Trans. Roy. Irish Acad. xxx. 1892, p. 140.

2 Giinther, Phil. Trans. 161, 1S71, pp. 542-543.

^ Owen, Anat. Pliys. Vertebrates, London, 1866, i. p. 424.

26o FISHES CHAP.

muscular coat, (2) the submucosa, and (3) an epithelial stratum
or mucous membrane, the first two of these layers, with the
addition of the peritoneum, being derivatives of the inner or
splanchnic portion of the embryonic mesoblast.^

Excluding the oesophagus, where the muscular coat is mainly
composed of striated fibres, the musculature of the alimentary
canal usually consists solely of non-striated, spindle-shaped fibres
disposed in two layers, an external stratum of longitudinally
arranged fibres, and an inner stratum of circularly disposed fibres
(Fig. 1 5 7), with the addition, in the stomach, of an oblique layer
between the two. In the oesophagus the reverse arrangement
may exist, the circular layer being external and the longitudinal
internal. The muscular coat varies considerably in thickness in
different regions and in different Fishes, and in the Cyclostomata,
the Holocephali, some Teleosts, and the Dipnoi may be very
feebly developed, or even entirely absent, as in the intestine of the
Hag-Fish (^Myxine). In the Gillaroo Trout (Sclmo stomachicus),^
on the contrary, the distal section of the siphonal stomach has its
musculature unusually thickened, so as to form an incipient
o-izzard for the crushing of the shells of the freshwater Molluscs
on which the Fish feeds. In some of the Mullets (Mugilidae),^
a true gizzard is developed by the enormous thickening of the
muscular coat of the caecal stomach, the cavity of which, in
consequence, is reduced to a mere vertical fissure, and is lined Ijy
an exceptionally thick, horny epithelium.

There are a few exceptions to the rule that the muscular
fibres are of the non-striated variety. Thus in some Teleosts, as in
the Tench (Tinea vulgaris), striated fibres are continued from the
oesophagus into the walls of the stomach and intestine, and there
form an outer longitudinal and an inner circular layer, situated
externally to the corresponding layers of the non-striated stratum.

^ For the histology of the alimentary canal and its glands in Fishes, see Leydig,
Lehrb. d. Histol. d. Alenschen ti. d. Tiere, 1857 ; Id. Beitr. zu mikrosk. Anat. u.
EnhvicM. d. Hochen u. Haie, Leipzig, 1852 ; Id. Anat. -histol. Untersuch. iib. Fische
u. Rcptilicn, Berlin, 1853 ; Molin, Sitz. d. k. Akad. d. Wiss. zu IFien, v. 1850,
p. 416 ; Jlacallum, Proc. Canadian Inst. N.S. ii. 1884, p. 387 ; Id. Journ. Anat.
and Phrjs. xx. 1886, p. 604 ; N. Parker, Trans. Roy. Irish Acad. xxx. 1893, p. 109 ;
Ayers, Jen. Zeitsch. xviii. 1885, p. 479 ; Edinger, Archivf. mikr. Anat. xiii. 1876,
p. 651 ; Trinkler, Archivf. mikr. Anat. xxiv. 1884, p. 174. Also Oppel, Lehrb. d.
vcrgl. mikrosk. Anat. d. Wirbeltiere, i.-ii. Jena, 1896-97, where numerous other
references are given.

2 Owen, 0}'). cit. p. 418. ^ Owen, I.e.

IX ALIMENTARY CANAL 261

In the Siluroid, Ammrus, the striated fibres of tlie outer circular
layer of the oesophagus are continued, although but sparsely, into
the inner circular layer of the stomach.

The subniucosa (Fig. 157) lies between the muscular layer
externally and the epithelial lining internally, and is charac-
teristically developed in the stomach, and even more so in the
intestine. Histologically, it consists of a framework of connective
tissue, enclosing in its meshes masses of leucocytes (lymphoid
tissue), some of which are amoeboid and migratory, and may
even be found between the cells of the intestinal epithelium
(including in some instances the cloacal epithelium), probably
actively participating in the transnussion of food material
from the alimentary canal to the lymphatics and blood-vessels ;
while other and somewhat similar, but larger, leucocytes (phago-
cytes), are concerned with the elimination of waste substances or
noxious micro-organisms. In addition to the diffused lymphoid
tissue of the submucosa, special rounded or oval, and sometimes
encapsuled, masses of this tissue (lymph follicles) are common in
the intestinal wall (Fig. 157) of Acipenser, the Dipnoi and some
Elasmobranchs, and are perhaps the only representatives in Fishes
of the solitary follicles or " Peyer's patches " of the higher
Vertebrates. A .mass of lymphoid tissue exists in the axis of
the spiral valve of Acijjenser, which has been compared with a
similarly situated structure in Zepidosiren} In some Elas-
mobranchs a large lymphoid organ is imbedded in the submucosa
of the oesophageal wall, while a local thickening of the tissue is
met with in the pyloric sphincter. Protopterus is remarkable
among Vertebrates for the extraordinary development of lymphoid
tissue,^ which, apart from its distribution in the submucosa, is
abundantly present between the longitudinal and circular muscle
layers, and the peritoneal and muscular coats of the intestine.

In addition to the lymphoid tissue the submucosa contains non-
striated muscle cells and plexuses of capillary blood-vessels, which
in certain Loaches (e.g. Misgurnus), where intestinal respiration
occurs, extend between the cells of the intestinal epithelium. A
network of lymphatic spaces or vessels surrounds the blood-vessels.
In some Elasmobranchs the small arteries of the submucosa of
the stomach are provided with singular sphincter muscles, which

1 Hyrtl, Lcpidosiren paradoxa. Abhand. d. luhm. Gescll. d. Wiss. 1845, p. 629.

2 Newton Parker, op. cit.

262 FISHES CHAP

occasionally encircle both the artery and the corresponding

1

vein.

The lining epithelium differs considerably in character in
different portions of the alimentary canal. The epithelium of the
mouth, pharynx, and anterior section of the oesophagus is often
squamous and is succeeded in the hinder part of the oesophagus,
and in the stomach and intestine, by a columnar epithelium.
As a rule the epithelium of the rectuni is also columnar, but in
Elasmobranchs it may become squamous. Goblet cells are of very
frequent occurrence throughout the whole length of the alimentary
canal, from the mouth to the rectum inclusive, interspersed
between the superficial epithelial cells ; in the same position
in the intestine migratory leucocytes have been found. The
primitive ciliation of the Vertebrate alimentary canal is retained
to a greater or less extent in many Fishes, and is sometimes, but
not always, associated with a feeble development of the muscula-
ture. In the larval form of Petromyzon (Ammocoetes), the whole
canal is ciliated except the pharynx and rectum ; but in the
adult ciliation is retained only in places which gradually become
fewer as the rectum is approached. In the Myxinoids, however,
cilia are said to be absent.

In the Dipnoi (e.g. Protopterus) the epithelium of the stomach
and intestine is largely ciliated, but in Elasmobranchs, ciliation
is usually restricted to the posterior portion of the oesophagus
and the edge of the spiral valve. Among the more generalised
Teleostomi (e.g. Acipenser, Lepidosteus, Amia), the oesophagus,
stomach, and intestine may be ciliated, but to an extent which
varies in different genera. The pyloric appendages, when present,
are also more or less extensively ciliated. In Teleosts, however,
the recorded instances of ciliation are relatively rare. Neverthe-
less, ciliated epithelium has been found in the intestine of a few
species (e.g. Bhomhus aculeatus and Syngnathus acus), and also in
the pyloric appendages; in the stomach (e.g. Perca and Esox),
and even in the oesophagus (e.g. Perca).

The mucous membrane, including the submucosa, is frequently
developed into variously arranged ingrowths projecting into the
lumen of the alimentary canal ; these are generally of the nature
of longitudinal or transverse ridges, or a combination of the
two, giving rise to retiform structures. The simple longitudinal
^ Paul Mayer, 3Iitt. zool. Stat, zu Neapd, viii. 1888, p. 307.

IX

ALIMENTARY CANAL

263

folds, which are sometimes found iu the oesophagus, stomach, and
rectum, often disappear on distension, and probably merely
provide for the enlargement of these cavities during the degluti-
tion of relatively large prey, or for the accumulation of faeces.
On the other hand, the permanent and often complicated folds of
the intestinal mucous membrane are probably related to an
increase in the secretive or absorptive area of this portion of the
alimentary canal. In the stomach the mucous membrane is
usually smooth, rarely, as in the " Electric Eel " {Gymnotus),
reticulate. In the intestine the folds assume a highly character-
istic and often complicated disposition.^ In the Cyclostomata

Fig. 158. — The intestinal mucous membrane of different Fishes, to show the transition
from simple longitudinal and transverse folds to crypts. A, Of an Elasmobranch ;
B. C, and D, of various Teleosts. (After Wiedersheim.)

the folds are simple and longitudinally arranged. In Elasmo-
branchs (Fig. 158, A), obliquely transverse folds are present in
addition, and, uniting with the longitudinal ridges, bound linear
depressions.

In various Teleostomi (Fig. 158, B, C, D), the union of the
two series of folds becomes more or less retiform, and the network
of intersecting ridges bounds a series of deep tubular crypts which
appear to penetrate to a considerable distance into the intestinal
wall, and possibly foreshadow the characteristic Lieberkiihn's
glands of Mammalia. Crypts may also be found in the stomach,
where they receive the apertures of the gastric glands, as in
Amiurus, but more usually they are restricted to the intestine.
In the Dipnoi (e.g. Protoioterus) the mucous membrane of the

^ Wiedersheim, Lekrb. d. vergl. Anat. d. JVirbelthiere, ed. ii. Jeua, 1886, p.
576.

264 FISHES CHAP.

stomach, and — excluding the Bursa Entiana where a number of
oblique folds are present — of the intestine also, is, on the con-
trary, perfectly smooth.

In addition to transverse and longitudinal folds the mucous
membrane of the various sections of the alimentary canal is often
developed into outgrowths which are more or less linear.^ In
the oesophagus these may be papilliform, as in Box and Caesio ;
obtuse in Acipenser, hard and almost spine-like in species of
Hhomhus ; or in the form of pyramidal retroverted processes
with jagged or fringed edges, as in the Spiny Dog -Fish
(Acanthias vulgaris). In the Basking Shark (Selache) similar
processes are present, which, near the stomach, become unusually
long and branched, so that the entrance to that cavity is
surrounded by a series of backwardly-directed arborescent tufts.
Peculiar papillose or tag-like processes of the mucous membrane
are frequently present on the spiral valve of Elasmobranchs, in
the intestine of such Teleosts as Balistes, Mugil and some
Pleuronectidae, and also in the rectum of JRhomhus maximus.

Of all the outgrowths from the mucous membrane of the
alimentary canal the so-called " spiral valve " of the Cyclostomata,
Elasmobranchs, Holocephali, Chondrostei, Crossopterygii, Amiidae,
Lepidosteidae and Dipnoi is the most characteristic. The first
appearance of this structure was probably in the form of a straight
longitudinal fold or ridge projecting into the cavity of the
intestine, similar, perhaps, to the typhlosole of many Inverte-
brata. This primitive condition is not retained in any existing
Fishes, although it may be closely approached in the larval
Cyclostome {Ammocoetes), and is perhaps also indicated in the
straight anterior portion of the spiral valve of Polyptcrus.
Absent altogether in the Myxinoids, the valve is represented in
its simplest condition, as in certain other Cyclostomata (e.g.
PetroynyzoTh), by a ridge of mucous membrane which commences
anteriorly on the dorsal side, and, after describing a partial
spiral as it passes backwards, terminates posteriorly on "the
ventral side, the width of the valve not exceeding half the
diameter of the intestine. This simple type of valve is repeated
in embryo Elasmobranchs, but in the adults of these Fishes the
valve becomes much more complicated, and. exhibits a wide range
of structural variation. The increased complexity of the valve

^ Owen, op. cit. p. 415.

SPIRAL VALVE 265

seems to depend on several factors, the effect of which, in diflercut
Elasmobranchs, is best studied in a series of valves of progressively
higher differentiation.^

In a hypothetical simple type of valve, easily derivable from
tlie more primitive type of Petromyzon, it may be conceived that,
while not exceeding in width the semi-diameter of the intestine,
the valve becomes disposed in several complete and more or less
closely approximated spiral turns, the free edge of the valve being
on the same level as its attached margin, and leaving an open
axial canal along the centre of the gut. The nearest approach
to this hypothetical type, which has been compared, not inaptly,
to ?/7i escalier tournant sans noyaii, is perhaps to be found in the
Thresher-Shark (Alojjecias vulpes).

The structure of the more complicated spiral valves of other
Elasmobranchs are well illustrated within the limits of the single
genus Haia.

In one specimen of Raia sp. (Fig. 159, A) the last four coils
of the valve are similar to those of the hypothetical type, but the
more anterior ones, owing to the greater width of the valve,
which here exceeds the semi-diameter of the intestine, have their
free margins deflected downwards, while that portion of the valve
which forms the first half turn is coiled inwards upon itself, so as
to form a hollow cone, open dorsally, and having its apex directed
forwards. In other examples a further modification is intro-
duced by the increasing width of the valve, which now, through-
out its whole length, equals the semi-diameter of the intestine ;
and by the formation of an axial columella by the thickened free
edge of the valve, which is traversed by a central band of
unstriped muscle, as well as by the intra-intestinal artery and
vein, and takes the place of the central canal of the preceding
types. The valve is, however, still regular, and its free margin
remains on the same level as the corresponding portion of the
attached edge. In other specimens, again, additional complica-
tions are introduced by a still further increase in the width of
the valve, which now exceeds, often considerably, the semi-
diameter of the intestine, and the consequent deflection of the
free edge of the valve either forwards or backwards (C and D),
As shown in C the valve, in consequence of the backward deflec-
tion of its free margin, presents the appearance of a nest of

1 T. Jeffeiy Parker, Trans. Zool. Soc. xi. 1879, p. 49.

266

FISHES

imperfect truncated cones with their apices directed backwards,
the successive cones adhering so closely to one another that they
conihine to form a central conical chamber with a spirally disposed
cavity winding round it. In D, on the contrary, the free edge
of the valve is deflected forwards, so that, as in C, a nest of cones

Fig. 159. — Examples of various types of the spiral valve in Elasmobranchs. A, B, C,
and D in specimens oi Rata spp. ; E, in Sphyrna malleus. A, B, and D represents
longitudinal sections of the intestine, the ventral portion of the valve being re-
moved. In C successive portions of the ventral wall of the intestine have been cut
out. In E the intestine has l)een opened along the mid-ventral line and its wall
reflected to the right and left ; the ventral portion of each coil of the "scroll " valve
has been removed. In most of the figures the pylorus is shown in the upper part,
and the "rectal" gland in the lower. (From T. Jeflery Parker.)

is formed, but the apices of the successive cones are directed
forwards instead of backwards. Notwithstanding these variations
in tlie structure of the valve as a whole, the first coil or half coil
nearly always resembles that described in A.

It is obvious that the structure of the valve varies consider-
ably within the limits of the genus, and it may be added that
various intermediate types of structure occur between A and B,

SPIRAL VALVE 267

A and C, and A and D. The individual variations are perhaps
even more remarkable, and appear to be quite independent of age
and sex. By way of example it may be mentioned that valves
approximating to one or other of those represented by C and D
occur in different individuals of Raia maculata of the same sex
and similar in size, even in yovmg specimens not more than three
inches in length.

As regards other Elasmobranchs, the common Dog-Fish
(Scy Ilium caniculaY has a well-developed spiral valve disposed
in twelve coils, which structurally represents a more highly
developed example of the type D. The existence of considerable
individual variation is nevertheless indicated by the fact that
in one specimen examined the valve was intermediate between
C and D, five of the eight cones projecting forwards and three
backwards. In a specimen of Notidanus sp.'^ there were as many
as twenty coils, which in disposition were intermediate between
B and C, approximating, however, more nearly to B. In a
specimen of the Port Jackson Shark {HeUrodontusf the valve
had eight coils, and in structure was also intermediate between
B and C, but approached more nearly to C. Some of the
Hammer-headed Sharks (e.g. Sphyrna malleusy possess a type of
spiral valve which differs considerably from any of those hitherto
described, and is termed a "scroll" valve (Fig. 159, E). The
attached edge of the valve pursues a straight longitudinal course,
or at any rate only describes a half turn and back again in
passing from the pyloric to the cloacal extremity of the gut. In
the middle of its course the width of the valve is about equal to
two-thirds of its length, but towards either extremity it gradually
diminishes until the free and attached margins meet. The valve
thus constituted is rolled upon itself from left to right, the
successive coils being comparable to a series of cylinders placed
one inside the other, and becoming gradually larger both in
length and diameter from within outwards. A similar valve is
present in some of the Carchariidae.

In the Holocephali (e.g. Chimaera monstrosa)^ the valve
describes only three and a half coils, and is further remarkable
in that the attached margin, for a considerable portion of its

^ Jeffery Parker, op. cit. pi. xi. Fig. 5. - Ibid. p. 58.

3 Ibid. p. 58. * Ibid. p. 59.

5 Ibid. p. 58, pi. xi. Fig. 6.

268 FISHES CHAP.

extent, does not form a regular spiral but describes only a slightly
sinuous course. Posteriorly, the valve is more normal, and con-
sists of about two cones with their apices directed forwards.

In the Dipnoi the spiral valve is well developed, and in Neo-
ceratodus^ describes nine coils, and in Protopterus"^ six or seven.
The structure of the valve in the latter Dipnoid resembles that
of ScylliiLm canicula, except for the smaller number of cones.

In the more generalised Teleostomi the valve is best developed
in the Sturgeon (Acipenser) and in Polypterus. In the former^ the
valve is restricted to the posterior half of the total length of the
intestine, often extending to within an inch of the anal aperture,
and describing in its backward course about seven or eight coils.
The width of the valve is about equal to the semi-diameter of the
intestine, and the thickened free margin forms a well-marked
axial columella, round which the cavity of the gut winds, as in
the type B, except that the spiral is a more open one. In
Polypterus the valve begins close to the solitary pyloric caecum,
and for some distance pursues a straight longitudinal course, but
eventually forms a few spiral coils, ceasing, however, at a con-
siderable distance from the anus. The evidence afforded by
petrified faeces or " coprolites " proves that certain extinct Cross-
opterygii (e.g. Macropoma, Megalichthys), like their living repre-
sentative, Polypterus, possessed a spiral valve."^ In Amia and
Lepidosteus^ the valve is almost vestigial, being restricted to the
terminal portion of the intestine, and is somewhat variable as to
the precise number of its coils. In Amia there are nearly four
coils, extending over 3 cm., that is less than a tenth of the total
length of the intestine, but in some specimens the coils do not
exceed two and a half or three in number. Lepidosteus *" has a
still shorter valve which, in specimens of 7-10 cm. in length,
may not consist of more than three and a half coils, and in much
larger specimens .may be reduced to less than two coils, a
variation which suggests that a reduction takes place in the
number of coils as the fish increases in age and size. The
structure of the valve in the three last-mentioned genera
resembles that described in Acipenser, and in none of them does

^ Gunther, op. dt. p. 544. ^ Newton Parker, op. cit. p. 141.

^ Macallum, Journ. Anat. and Phys. xx. 1886, pp. 618, 619.
* Owen, 0}i. cit. p. 424. ^ Macallum, I.e.

® Balfour and Newton Parker, op. cit. p. 425.

SPIRAL VALVE 269

the width of the valve so far exceed the semi-diameter of the
intestine as, by forward or backward deflection, to give rise to
the highly characteristic cones of Elasmobranchs and Dipnoi.

In the more specialised Teleostomi (Teleostei) the spiral valve
is wholly wanting, except perhaps as a vestigial structure in
certain Clupeoids, as, for example, Chirocentrus} and possibly
also in some Salmonidae.^

From wdiat has been said as to the structure of the spiral
valve in the different groups of Fishes, it may be concluded that
the valve most nearly retains its primitive condition in the
Cyclostomata ; attains its maximum development in tlie Elasmo-
branchs, especially in the Notidanidae, and shows no indication
of degeneration in the Dipnoi. In the Holocephali and the lower
Teleostomi, on the other hand, the valve exhibits various stages
of retrogressive modification, and in the Teleosts is either absent
altogether or persists only as a vestigial structure in a very few
species.

From a physiological point of view the object of the spiral
valve is to increase the absorptive inner surface of the intestine,^
but, from what has been said as to the structural variability of
the valve, it is obvious that its efticacy from a functional stand-
point must be equally variable. The value of the valve as an
absorptive mechanism necessarily depends on thearea of absorption-
surface which it provides, as well as on the degree of resistance
which it offers to the passage of food material along the cavity
of the intestine. These factors will in turn depend on the number
of coils, on the width of the valve, and on the extent to which its
free margin is deflected in forming the series of cones, but these
again are precisely the structural features which are most liable
to variation. The total absorption area in the four types of
valve characteristic of the genus Fuiia has been calculated, and
maybe expressed in square centimetres as follows: — A, 136'64;
B, 143-82; C, 254-3; and D, 276-7.'' Hence as regards mere
absorption area a spiral valve of the type D has twice the extent
of a valve of the type A, and if, in addition, account be taken
of the retardation of the food due to the increased obstruction
offered by the columella and cones in D, it is clear that the

^ Cuvier and Valenciennes, Hist. Nat. d. Poiss. xix. 1846, p. 15L

2 Rathke, tJb. d. Darmkanalu, d. Zeucjungsorgane d. Fische, Halle, 1824, pp. 62 f.

3 Edinger, ojj. cit. p. 678. * T. Jetfery Parker, 02). cit. p. 55.

270 FISHES CHAP.

difference in physiological value between the two types must be
far more considerable than is indicated by a comparison of their
relative superficial areas alone.

The evolution of the spiral valve was probably due to the
necessity of increasing the absorptive area of an almost straight
unconvoluted intestine, a result which in other animals is often
obtained by an increase in the length and concurrent convolution
of the intestine itself. Any attempt to correlate the variations in
the degree of perfection or imperfection of the valve considered as
an absorptive mechanism with any special variations in the nature
or quality of the food is, however, a very diflicult problem, and a
satisfactory explanation has yet to be found. The difficulty,
moreover, is increased by the fact that the majority of Fishes
with a spiral valve are mainly carnivorous ; the Elasmobranchs,
in which this structure is at the same time most highly developed
and most variable, exclusively so. On the other hand, the term
" carnivorous " covers a multiplicity of minor differences in the
nature and relative digestibility of different forms of animal food,
and it is quite possible that it is with differences of this kind
that the specific or individual variations in the development of
the spiral valve are associated. The absence of the valve in the
variously nourished Teleosts, save perhaps as a vestige in one or
two, is also difficult to account for, although it is not improbable
that compensating structural modifications exist in this group.
As a rule, the intestine is much more convoluted in these Fishes,
but to an extent which varies greatly in different species, while
the characteristic pyloric caeca and the spiral valve appear to a
certain extent to be developed in inverse proportion to one
another.

The Glands.

The glands associated with the alimentary canal in different
Fishes are (1) the gastric glands, (2) the liver, (3) the pancreas,
(4) the pyloric appendages, and (5) the "rectal" gland.

Oral salivary glands are wanting in all Fishes, the only
secretory structures in the mouth being numerous mucus-secreting
goblet cells, which here, as elsewhere throughout the alimentary
canal, are intermixed with the ordinary epithelial cells.

The Gastric Glands. — The Cyclostomata and Dipnoi do
not possess any specially differentiated gastric glands, and it is

DIGESTIVE GLANDS 2/1

probable that in these Tishes the secretion of the digestive fluids
is effected by the ordinary lining epitlieliinn of the stomach or
intestine, or both. In the remaining groups gastric glands are
generally present in the form of simple caecal structures em-
bedded in the submucosa and opening on the surface of the
mucous membrane into the cavity of the stomach. The glands
differ in different Fishes in the character of their lining epi-
thelium and in the extent to which their component cells are
differentiated from the epitlielium of the stomach. There does
not appear, however, to be any distinction into " central " (pepsin-
forming) and " parietal " (acid-secreting) cells, as is the case in
the higher Vertebrata. Towards the pyloric end of the stomach
the true gastric glands are often replaced by mucous glands.
There are, nevertheless, not a few Teleosts in which special
gastric glands are absent, as, for example, Syngnathus acus, and
several species of Cyprinidae, Labridae, and Blenniidae, etc. In
at least two genera (Gastrosteus and Cobitis), belonging to
widely different families, gastric glands are present in certain
species but absent in others. As suggested by Edinger,^ the
absence of these glands may possibly be due to degeneration.

It may be remarked that the formation of such digestive
ferments as pepsin and trypsin, which are associated with the
stomach and pancreas respectively, in the higher Vertebrates, is
not nearly so strictly localised in Cyclostomes and Fishes. So
far from peptic digestion being limited to tlie stomach, it may
take place in the pharynx, stomach, and intestine of Ammocoetes,
and in some Elasmobranchs (e.g. Scyllium), and in such Teleosts
as the Pike, Eel, and Carp, the peptic region extends from the
stomach for some distance along the intestine, while trypsin has
been obtained from the mucous membrane of the stomach, intes-
tine and pyloric caeca, as well as from the pancreas.'

Intestinal glands analogous to the glands of Lieberkiihn in
the higher Vertebrates seem to be entirely wanting in Fishes,
unless represented by the sac-like or tubular crypts which are so
generally present in the Teleostomi.

The Liver. — Phylogenetically the oldest gland in connexion
with the Vertebrate alimentary canal, and in size by far the

^ Ardiiv f. mikr. Anat. xiii. 1876.

"^ Krukenberg, quoted by Miss Alcock, Joum. A7iat. and Phys. xiii. (N.S. ),
1899, p. 613.

272 FISHES CHAP.

largest, the liver arises as a caecal outgrowth from the embryonic
mesenteron, and in this primitive stage recapitulates a condition
which is retained throughout life in Amphioxus. By the sub-
sequent division and branching of this outgrowth the massive
compound tubular gland of the adult Fish is eventually formed.

The liver of Fishes (Figs. 153, 154) is very variable in size,
shape, colour, and degree of lobulation. Anteriorly, it is usually
moulded to the posterior face of the transverse septum between
the pericardial and abdominal portions of the coelom, and from
thence extends Ijackwards in the abdominal cavity to a varying
distance, in some Sharks as far as the cloaca. Externally, the
gland is invested by the peritoneum, which extends on to it from
the pericardial septum and forms a suspensory fold, and also from
the oesophagus and stomach. The shape of the liver usually
bears some relation to that of the body, being, for example,
longest in the Eels and broadest in the Eays. In the great
majority of Fishes the liver is bilobed, consisting of two sub-
equal lateral lobes, disposed longitudinally and confluent anteriorly
for a portion of their extent. From this normal type there are
a few minor variations.^ In Petromyzon, Lepidosteus (Fig. 155,
B), and a few Teleosts (e.g. the Gymnodontes, Lophobranchii, and
some Salmonidae) the liver is unilobed. In the Myxinoids and
in the Dipnoi (e.g. Protopterus), the organ is bilobed, but the
small anterior lobe lies immediately in front of the much larger
posterior lobe, with the gall-bladder between the two (Fig. 155,
A). In some Teleosts (e.g. Scomber), the liver is trilobed. A,
gall-bladder is invariably present in either the larval or adult
Cyclostomata, in the Chrondrostei, Holostei, Crossopterygii and
Dipnoi, and generally also in Elasmobranchs and Teleosts. In
the Elasmobranchs it is rarely entirely wanting, as in Sphi/rna
and Pristis, and in the Teleosts in some of the Gurnards (Trigla).
The gall-bladder and bile-duct of Petromyzon fiuviatilis atrophy
after the metamorphosis which follows the larval Ammocoetes stage,
but in Petromyzon marimis the duct, although usually absent, is
sometimes retained. In the Ammocoetes the epithelium lining
the gall-bladder is ciliated. In some Fishes, as, for example, in
many Elasmobranchs, the gall-bladder is more or less completely
embedded in the substance of the liver ; in others, as in most
Teleostomi, the organ is quite distinct from the gland (Fig. 154).
^ Stannius, Handbk. d. ZooL, Berlin, 1854, ii. jj. 201 ; Owen, op, cit. p, 425.

DIGESTIVE GLANDS 273

A simple arrangement of the ducts from the liver and gall-
bladder is that found in the common Dog-Fish {IScyUium ainicula).
In this Elasmobranch a cystic duct leaves the gall-bladder, and,
after receiving several hepatic ducts from the lobes of the liver,
becomes the bile-duct and opens into the commencement of the
intestine. In the Myxinoids and in the Dipnoi (e.g. Protopteriis),
there are but two hepatic ducts, one from each lobe of tlie liver ;
these unite and then meet the cystic duct to form the bile-duct
(Fig. 155, A). The nmnl)er of hepatic ducts may, however, be
consideral )ly increased, as, for exam})le, in the Siluroid Ainiurus}
where 8-10 separate ducts join the cystic duct. In a few
instances one of the hepatic ducts opens directly into the
intestine, independently of that wliich unites with the cystic
duct in forming the bile-duct. In the Dipnoi (e.g. Fruto-pterns)^
and in some Teleostomi (e.g. Lepidosteus)^ the bile-duct receives
the duct from the pancreas before opening into the intestine.

The Pancreas. — In the Cyclostomes (e.g. Petromyzon, BdcUo-
stoma, Myxinr') a rudimentary pancreas is apparently present,
but the evidence as to its identity is not wholly conclusive. A
well-developed pancreas occurs in Elasmoljranclis, in at least one
of the Dipnoi, and probably in most Teleostomi."^

In Elasmobranchs the pancreas is a compact structure, uni-
or bi-lobed, and entirely distinct from the li^'er. In Scyllium
canicula (Fig. 153), the bilobed gland lies in the angle between
the distal lind) of the stomach and the adjacent portion of the
intestine, and from the smaller of its two lobes the duct issues
to pass to its intestinal aperture near the commencement of the
spiral vah'e. In most of the Teleostond in which its existence
has hitherto .been recorded, the pancreas is a singularly diffuse
gland ; and usually a considerable portion, or even the whole of
it, is endjedded in the substance of the liver, its lobules accom-
panying the ramifications of the hepatic artery and duct, and
the portal vein. The pancreatic duct usually opens into the
intestine near the aperture of the bile duct (e.g. Amiurus) ;
sometimes the two ducts open on the apex of a common papilla
(e.g. Acipenser and Amia), or by their union form a conanon

^ Macallum, reprinted from Proc. Canadian Institute, N.S. ii. 1884, p. 407,
- Xewton Parker, op. cit. p. 138.

' Macallum, Journ. Anat. and Fhys. xx. 1886, p. 632.

•* Legouis, A7in. Sci. Nat. (5), xvii. 1873, Art. 8 ; and r.vm. 1873, Art. 3.
Also Macallum, op. cit. p. 629.

VOL. VII T

2/4 FISHES CHAP.

duct (e.g. Zepidosteus). Among the Dipnoi a well-developed
pancreas is present in Protoptcrus} embedded in the wall of the
stomach and intestine, internal to the peritoneal investment of
these organs, and extending even into the first fold of the spiral
valve. The gland is traversed l)y fine ductules which unite
together and open into the bile-duct just before the latter enters
the intestine. In the remaining Dipnoi the existence of a
pancreas has yet to be ascertained. Developmentally, the pan-
creas resembles the liver, and, histologically, is very similar to
tliat of the higher Vertebrates, consisting of terminal glandular
alveoli continuous with intermediary tubular portions, and eventu-
ally with the liner ductules, which, by their union, form the
main efferent duct.

The Pyloric Caeca. — These structures are caecal outgrowths
from the intestine, and are situated close to the pyloric extremity
of the stomach and the intestinal apertures of the bile and
pancreatic ducts. Wliolly wanting in the Cyclostomata and
Dipnoi, and, unless represented by a pair of caeca opening into
the long, tubular, non-valvate anterior portion of the intestine in
the Greenland Shark {Laemargus horecdis)'^ in the Elasmobranchs
also, they are very generally present in the Teleostomi, although
extremely variable both in numljer and arrangement in different
families. In Amia there is no trace of pyloric caeca. Polypteriis
has a single short caecum with a thick muscular wall. In
Acipenser, Folyodon, and Lejndosteus, on the contrary, pyloric
caeca are unusually well developed. In Acipenser the caeca are
not only numerous, but are so connected together by connective
tissue and blood-vessels, and so invested externally by the peri-
toneum, as to form a large, compact, gland-like mass, communicat-
ing with the intestine by a single wide duct. In Polyodon the
organ is essentiall}' similar, but is lobed externally. In Zepidostevs
(Fig. 155, B,^>?/.c), the caeca are also very numerous, but relatively
short, and, although united into a compact mass, open by four
pit-like orifices into the intestinal cavity. In Teleosts the caeca
are subject to extraordinary variations in nundjer, size, and
arrangement.^ In some families, and even in groups of higher
taxonomic value, they are entirely absent, as is the case with the

^ Newton Parker, op. cit. pp. 138-139.
'■^ Turner, Journ. Anat. and Phys. vii. 1873, p. 233.
3 Stannius, op. cit. pp. 197, 198 ; Owen, op. cit. p. 428, ct scq.

PYLORIC CAECA

275

Siluridae, Esocidae, Cjprinodontidae, Labridae, Plectognathi, and
Lopliobrancliii. The " Sand-eel " {Avimodytes) has hut a single
caecum ; the Turbot (lihomhvs maximns) two, and other I*leiiro-
neetidae three to five; and the Perch (Percr/), three (Fig. IGO,
■jrij.c).

In other Teleosts, on the contrary, tliese structures are much
more numerous. In Lahrus lahrax there are ahout GO, in the
Whiting {Gadas mcrlcaiqvs) 120, while in the Mackerel (f^comhcr
sromhrus) there are no fewer than 191. If few in nundjer the
caeca open separately into the intestine, but when numerous,
more or fewer of them may unite to form a smaller number of
efferent ducts, as in the Whiting, where four such ducts are
formed. In some instances, as in the Tunny {Tliunims), the
union of the caeca by connective tissue leads to the formation of
a compact mass. As regards their arrangement, the caeca may
either be disposed in a whorl round the intestine, as in the
AVhiting, or in a linear series, as in the
Salmon {Salino) and in some of the
Olupeidae.

The mucous membrane lining the
anterior pyloric caeca is often developed
into a network of ridges, limiting crypt-
like or tubular depressions ; and not in-
frequently the epithelium is ciliated.

The precise function of these organs,
whether digestive or absorptive, is still
uncertain.^ That they may be dig-estive
is suggested by tlie presence of certain
amylolytic and proteolytic enzymes, l)ut
this obvious conclusion is to some extent
A'itiated by the close proximity of these
organs to the stomach, and more especially
to the intestinal orifice of the pancreatic
duct. It is by no means improbable,
however, that the caeca are both digestive
and absorptive organs. An attempt has
been made to sliow that the pyloric
caeca and the spiral valve vary inversely
as regards the extent of their development in different groups of

^ For references, see ]\[acallum, Journ. Ana/,, and Pliys. xx. [1. 624 ct scq.

—an.

Fig. 160. — Tlie aliineiitavy
canal of a Perch (Perca).
o,n, Anus ; in, intestine ;
oes, oesophagus ; ^jy,
pylorus ; p]/.c, pyloric
caeca ; st, stomach. (Aftei'
Wiedersheini. )

2-j6 FISHES CHAP. IX

Fishes.^ To some extent the reciprocal variation of these
structures supports this view, but it is also evident that there
are obvious objections to its unqualified acceptance. Thus, in
some Teleostomi (e.g. Acqjcnscr, Polyodon), exceptionally well-
developed and numerous caeca and a spiral ^'alve are both
present. Amia with an almost vestigial spiral valve has no
trace of pyloric caeca, and in Teleosts the absence of a spiral
valve is associated with the complete suppression of the caeca in
many large and important groups.

The Rectal Gland. — The " rectal " gland, or appendix digiti-
formis, is a small organ of unknown function with complex
glandular walls, and a central duct opening dorsally into the
terminal portion of the intestine.- The organ is generally
present in Elasmobranchs (Fig. 153, rct.gl), in which group the
intestinal orifice of its duct may either be close to the termina-
tion of the spiral valve, or, as in Cklamydosclachus^ near the
cloacal outlet of the gut. An apparent representative of tlie
gland, the " caecum cloacae," is also present in the Dipnoi,* but
communicates directly with the cloaca (Fig. 155, A, cl.c). The
" rectal " gland is perhaps homologous with the intestinal caecum
which is to be found in some Teleosts (e.g. Box vulgaris), and
possibly also with the " caecum " (caecum coli), and its vermiform
appendix in the higher Vertebrata.° The caecum cloacae, on the
contrary, is morphologically a urogenital sinus, formed as a
dilatation of the fused hinder portions of the mesonephric ducts,
and probably comparable with the sperm sacs of male Elasmo-
branchs, and also with the urinary bladder of Teleostonies."

^ Wiedersheim, op. ci/. [<. 556.
^ Howes, ojh cit. p. 393.

■'' Giinther, Challenger Reports, "Zool." xxii. 1887, p. 3; Garman, Bull. Mus.
Comp. Zool. Camb. Mass. xii. 1885, p. 20.

■* Giinther, op. cit. p. 545 ; Newton Parker, op. cit. p. 137.

' Howes, op. cit. p. 393 et seq.

« Graham Kirr, P.Z.S., 1901, ii. p. 484.

CHAPTER X

THE RESPIUATORY ORGANS

The principal respiratory organs consist of a series of pairs of
branchial clefts in the form of perforations in the side walls
of the throat, which place the pharynx in free communication
with the exterior. The first and most anterior of these clefts,
the mandibulo-hyoid cleft or "spiracle," is situated between
the mandibular and hyoid arches ; the second, the hyo-branchial
or hyoidean cleft, between the hyoid arch and the first branchial
arch ; and the remaining clefts between the succeeding branchial
arches. On the anterior and posterior walls of more or fewer of
the clefts highly Aascular plate-like, or variously shaped fila-
mentous outgrowths of their lining membrane are developed,
which subserve the purpose of exposing the blood to the influence
of the oxygen - containing water, and are termed branchial
lamellae or " gills." In addition to their usual respiratory organs,
the gills, a few Fishes utilise the air-bladder either as a functional
lung or as an oxygen reservoir, and in others accessory breathing-
organs of various kinds are developed.

The arrangement of the branchial clefts and the gills may
be conveniently studied first in the Elasmobranchs. Excluding
the spiracles, there are usually in this group (Fig. 161, A), five
pairs of branchial clefts, but in certain primitive members of the
group the number may be larger. Thus, in Notidanus griseus
(Rexanchus) and in Chlamydoselachus there are six, and in Koti-
danus cinereus (ffejJtanchus), seven clefts. The pharyngeal aper-
tures of the clefts are relatively wide, but their external openings,
which are freely exposed on the lateral surface of the head between
the eye and the pectoral fin, are usually narrow and slit-like.

The successive clefts are separated from one anotlier by a

277

278

FISHES

series of inter-branchial septa, each of which consists of the lining
membrane of two contiguous clefts and a median fibrous sheet ;
it is further strengthened on its pharyngeal margin by a
branchial arch, and more externally by the fringe of cartilaginous
rods (branchial rays) with which the outer convex edge of each
arch is provided. The anterior and posterior walls of each
septum are produced into a number of outwardly -radiating
vascular plates or folds (branchial lamellae or " gills "), which by
their free edges project into the cavity of tlie cleft (Fig. 161, A).

Fig. 161. — A, Horizontal section through the head of an Elasmohranch ; B, similar
section of a Teleost (diagrammatic), b.c, Branchial cavity ; b.l, branchial lamellae ;
c, coelom ; c.h.a, external branchial aperture ; hy.a, hyoid arch ; hy.c, hyo-branchial
cleft ; l.s, interbranchial septum ; n, nasal organ ; ot-s, oesophagus ; o}), operculum ;
j').q, palato-quadrate cartilage ; I'h, pharynx ; s}\ spiracle ; s.ps, spiracular jJseudo-
branch ; 1-5, 1st to 5th branchial arches. (From Boas, slightly altered.)

Although slightly free at their outer extremities, the lamellae do
not extend so far as the external margin of the septum to which
they are attached (Fig. 164, B). Each series of lamellae is termed
a " hemiljranch," and, from what has been said, it is obvious tliat
each iuter-ljrancliial septum and its supporting brancliial arch carry
two hemibranchs, an anterior and a posterior, the two forming a
complete biserial gill or " holobranch." Tlie hyoid arch, how-
ever, has only a single hemiln-anch, viz. that pertaining to the
anterior wall of the hyo-branchial cleft, and as the fifth or last
cleft has a hemibranch only on its anterior wall, the fifth arch is

X RESriKATORV ORGANS 279

gill-less.^ The spiracle is a vestigial cleft. At an early stage
of embryonic growth it differs but little from its fellows, but
subsequently degenerating it is represented 'in the adult by a
tubular passage between the oral cavity and the exterior, which,
however, is often complicated by the development of caecal out-
growths." The anterior widl of the spiracle often retains a
rudiment of a hemibranch in the shape of more or fewer vascular
lamellae, which, as they are supplied with arterial blood, and not
with venous blood like the ordinary gills, are said to form a man-
dibular or spiracular " pseudobranch." The spiracle varies greatly
in size in different families, being largest in the Trygons and
Torpedos, and very small, or even absent in the Lamnidae. Its
pseudobranch is best developed in the Notidanidae, where it has
the essential structure of a true hemibranch, and, as in other
Elasmobranchs, but to a greater extent, probably aids in the
additional aeration of the blood which is distributed to the eye
and brain. The characteristic opercular covering of the external
apertures of the gill-clefts in the Teleostomi and Dipnoi is
wanting in Elasmobranchs. It is interesting to note, however,
that in Chlamydoselachus ^ curious frilled cutaneous folds are
developed as extensions of the outer edges of the inter-branchial
septa, as well as of the hyoid region, and, like a series of
incipient opercula, project backwards over the successive branchial
clefts rFig. 252).

While in many respects more primitive than in Elasmobranchs
the branchial system of the Cyclostomata presents certain special
and peculiar features. The branchial clefts assume the form of
oval, antcro-posteriorly flattened pouches or sacs, varying, how-
ever, in number, and in their mode of communicating with
the exterior, in different genera. In the Lamprey (Fetro-
myzon) there are seven pairs of obliquely- disposed gill-sacs
opening externally by small rounded orifices, and by similar
apertures, not directly into the pharynx, but into a branchial
canal (Fig. 162, r.t), which underlies the oesophagus, and, while
ending blindly behind the last pair of sacs, communicating in

^ In those Elasmobranchs which have more tlian five branchial clefts there is a
corresponding increase in the number of branchial arches and hemibrauchs.

- Ridewood, Anat. Anz. xi. 1895, p. 425.

^ Garman, Bull. Mus. Comp. Zool. Harvard, xii. 1SS5, p. 1 ; Giinther, Challewjer
Reports, "Zool." xxii. 1887, p. 2.

28o

FISHES

f;tmij

front witli the oral cavity.^ The first of the series of gill-sacs
corresponds to the liyo-branchial or hyoidean cleft of Elasnio-
branchs and other Fishes. Spiracles are absent in the adidt, Ijut
in the enibr}X) are represented by pouch-like outgrowths of
the hypoV)last of the oral cavity, which subsequently undergo
singular changes.'-'' Thus, the outgrowths become converted

into tlie lateral halves of a
complete ciliated circum-oral
groove, which is retained
even in the Ammocoetes
stage, and recalls the ciliated
peripharyngeal ring of Asci-
dians. Another archaic
feature is also to be noted
in the continuity of the
groove with a ciliated mid-
dorsal pharyngeal ridge,
which has been compared
to the " dorsal lamina " of
Ascidians, and to the
equally characteristic hyper-
branchial groove of Am'plii-
oxus? Ventrally also, the

v^. i.j.v T.m.t

Fig. 162. — Petromyzon marinus. Transverse
section through the branchial j-egion (semi-
diagrammatic), br.m, Branchial membrane ;
d.ao, dorsal aorta ; d.c, dorsal cartilage of
the branchial basket ; d.m, dorsal muscles ;
e.a, external aperture of a gill -sac ; f.t,
tibrous tissue enclosing neural canal ; /;, ;',

lateral longitudiual cartihiges of the bran

chial basket; i.« internal apertiire of a l^^^^^l l^^lvCS of the groOVe
gill - sac ; I.JU, inferior jugular v6in ; _;(', _ o _

jugular vein (anterior cardinal) ; viy, spinal unite tO form a single

cord; nc, notochord ; n.ca, neural canal; „„„„„„ vvhiVh after vpppiv

«.j^, neural process ; oes, oesophagus ; ^j.6^ grOOVC, wniCH, aitCl ICCei^-

peri-branchial lymph sinus ; r.m.t, retractor ing the median aperture of
muscle of the tongue ; r.t, respiratory tube , , ■ -, ■,. f 4 •

or brauchial canal; s, circum- oesophageal ^'^^^ tliyrOlCl rUCliment, IS

lymph sinus ; v.ao, ventral . aorta ; v.c, continued backwards in the
ventral cartilage of branchial basket; v.m, ., i. i t i- 4-i

ventral muscles. (From T. J. Parker.) mid - ventral line Ot the

pharyngeal wall as far as
the last branchial arch. No trace of these ciliated structures is,
however, to be met with in the adult.

The branchial lamellae are represented by a series of vascular
horizontal and parallel ridges radiating outvvtirds along the roof,
floor, and lateral walls of each gill -sac, and invested by an

^ In the Ammocoetes stage the gill-sacs open directly into tlie larval pharynx,
which is retained as tlie branchial canal, the oesophagus of the adult being an
independent and later formation.

- Dohrn, Mitth. Zool. Stat. Neapcl, vi. 1886, p. 49.

2 Shipley, Quart. J. Mia: Sci. xxvii. 1887, p. 350. ^ Cf. p. 343.

RESPIRATORY ORGANS

Si

epitlieliiini which is partially
ciliated. The iuter-Lriinehial
septa are much thicker than in
Elasiuobraiichs, and include not
only the walls of adjacent sacs
and the hranchial muscles, Init
also contain cavernous peri-
branchial lymph-sinuses. The
cartilaginous branchial skeleton
is situated wholly external to
the gill - sacs, the so - called
branchial arches lying between
the external apertures of the
sacs, and directly beneath the
superficial skin, or, in other
words, on the outer margins of
the inter -branchial septa, and
not on the inner, as is invari-
ably the case with the branchial
arches of Fishes.

In the Hag -Fish {Myxine)
(Fig. 163), there are usually
six, very rarely seven, pairs
of gill-sacs, all of which open
directly into the pharynx, and
not into a branchial canal as in
the Lampreys. On the other
hand, Myxine is unif|ue in
having the outer extremities
of its gill-sacs produced into
a corresponding number of
tubular canals which, after a
longer or shorter course ob-
li(|uely backwards and out-
wards, unite to form on each
side a ventrally - situated ex-
ternal aperture (Fig. 103).
In the same genus a short
canal, or oesophageo -cutaneous
duct, passes from the pharynx

— — CJ

282 FISHES CHAP.

behind the last gill-sac of the left side, and opens externally
with the common external branchial aperture of that side.

In Bdellostoma there are usually six or seven pairs of gill-
sacs, but some species have ten or even fourteen pairs.^ I'hey
agree with those of the Lamprey in having independent external
apertures, but resemble the corresponding organs in Myxine in
opening directly into the pharynx. An oesophageo -cutaneous
duct is also present."'

In the Holocephali there are but four branchial clefts, the
fifth cleft being closed. Spiracles are absent in the adult,
although present in the young of Chimaera. The branchial
lamellae resemble those of Elasmobranchs, but the inter-
branchial septa are somewhat shorter, so that the lamellae
project slightly beyond their outer margins (Fig. 164, B). A
hyoidean hemibranch is present. A noteworthy feature is the
development of a cutaneous fold from the outer surface of the
hyoid arch, which grows backwards over the gill -clefts, and,
uniting above and below with the body-wall, terminates in a free
posterior margin, just behind the last gill-cleft. By the growth
of this opercular fold the gills become enclosed in a spacious
branchial cavity, and the clefts communicate with the exterior
through a slit-like opening between the free margin of the fold
and the body-wall.

The reduction in the extent of the inter-branchial septa which
is initiated in the Holocephali is carried to a still further extent
in the Teleostomi. Commencing with the Chondrostei, and
passing thence to the more specialised Teleostei, the septa become
gradually reduced in length, and the branchial lamellae project
freely beyond their outer margins to an increasing extent.

This modification, least marked in Acipenser (Fig. 164, C)
and Polyodon, attains its maximum in the Teleosts (Fig. 164,
1) and E), where the branchial lamellae take the form of a
double series of free filaments disposed along the convex outer
margin of each branchial arch, and attached by their bases only
to the reduced and inconspicuous septa. As a general rule each
of the first four arches supports two hemibranchs,^ forming a

1 See p. 423.

- Howes {P.Z.S. 1893, p. 730) has described certain remarkable variations in
the respiratory organs of Pctromyzon and Mijxinc.

^ In certain Teleosts more or fewer of the branchial arches may lose their gills.
This reduction attains its maximum in the singular Indian amjihibious Fish,

RESPIRATORY ORGANS

283

l)iserial gill or Iiolobrancli. Tii shape the brancliial filaments
are usually somewhat triaiinular, and consist of an axial su])port-
ing cartilage or V)oiie, invested sui)erti(.'ially l»y a highly vascular
mucous membrane. As in most of the preceding groups the
fifth branchial arch is gill-less. All Teleostomi possess a well-
devel()])ed moval)le operculum, su})i!orted l)y a more or less
c(im])lete series of opercular bones, with or without the addition
of liranchiostegal rays (Fig. 161, B). The size of the external
branchial aperture varies consideralily. Usually the hinder and
lower margins of the operculum are free, and then the aperture
is spacious. Xot infrequently, however, the moi"e or less exten-
sive fusion of the ventral and hinder edges of the operculum
with the body-wall reduces the aY)erture to a narrow slit, as in

Fig. 164. — Trausversc sections of brancliinl arches in different Fishes. A, Elasniobraiich :
B, Chiinaera; C, Acipenser ; D and E, Teleosts. h.a, Branchial arch; g.l, gill-
lamellae ; <ji; gill-raker ; i.s, inter-liranchial septum. (From Boas.)

the Eels and some Siluridae, or to a small upwardly directed pore,
as in the "Sea-Hor.se" {Hippocanvpus). In the Symbranchidae
the branchial apertures close dorsally, but fuse ventrally, leaving
a single median orifice on the under side of the throat.

Open spiracles are wanting in most adult Teleostomi, but are,
nevertheless, retained in the Crossopterygii (Polyptcrus), and in
the Chondrostei {Acipenscr and Folyodon). They have Ijeen
observed, however, in the embryos of some Teleosts, as in the
Salmon (Salmo),^ and even in the adults of Amia^ Lejnclosieus,

Amphipnous cuchia, where only the second arch has a biserial gill, the renuiiuing
arches being wholly devoid of gills (rf. p. 598).

1 Balfour, Couip. Embryol. ii. 1881, p. 62.

- Ramsay 'Wright, Journ. Anat. and rinjs. xix. 1885, p. 476.

284 FISHES CHAP.

and a few Teleosts ^ are represented by pouch-like recesses of the
oral cavity. A few vestigial branchial lamellae may be developed
on the anterior wall of each spiracle in Acipcnser and Polyodon,
but are wanting in PoJypterus, and, as in Elasmobranchs, repre-
sent a mandibular or spiracular pseudobi^anch.

The structure usually regarded as a hyoidean hemibranch in
tlie Teleostomi diifers greatly in its development in different
members of the group. In Acipcnscr it is undoubtedly the
liemibranch of the hyoid arch and is a true gill, receiving venous
blood from the ventral aorta and returning arterial blood to the
dorsal aorta, as in Elasmobranchs. In Polyodon and in Polypterus
the hemibranch is suppressed. Zejjidosteus," on the other hand,
has two series of lamellae on the inner surface of the operculum,
a dorsal and a ventral series meeting at an angle (Fig. 197).
The ventral lamellae are supplied with venous blood, the dorsal
with arterial,^ so that while the former retain their primitive
character as a functional hyoidean hemibranch, the latter is a
pseudobranch. It is interesting to note, however, that the
development of this pseudobranch and its blood-vessels proves
that it does not represent any portion of a true hyoidean hemi-
branch, but is really a spiracular pseudobranch.* In most other
Teleostomi a degenerate hemibranch occupies a similar position.
In Amia^ it is very feebly developed, and is lodged in a canal
communicating with the branchial cavity by a small aperture,
and situated directly anterior to the dorsal end of the first
branchial arch. Its blood supply is arterial, and tl'ie organ is
therefore a pseudobranch. In Teleosts the hemibranch is invari-
ably a pseudobranch ; nevertheless, its primitive condition as a
gill is indicated either by its structure or by its embryonic
history. In some genera tlie pseudobranch consists of short
free lamellae, as in some Pleuronectidae ; or it is partly free and
partly concealed, as in some of the Horse Mackerels (Caranx)
and in Sal mo ; or it may be completely hidden beneath the oral
epithelium, as in the Cod (Gadus), where the organ is very
degenerate, and is little more than a " rete mirabile " of blood-
vessels. The nature of the Teleostean pseudobranch is not in

^ Sagemehl, Morph. Jahrh. i.x. 1884, p. 213.

- Ramsay Wright, o^j. cit. p. 482. ^ See p. 335.

■• F. W. Miiller, Arch. Mikrosk. Anat. xlix. 1897, p. 403.

^ Rainsay Wright, op. cit. p. 492.

RESPIRATORY ORGANS

285

all cases quite clear. In f^nhno it is said that tliere is no

hyoidean lieniibraneli, and that the psendol)i'anch is really a

persistent spiraeular })seud()hraiich ;^ hence it is pr()l)alile that a

like siy-niticance must he attached to this singular structure in

otlier Teleosts. The evidence of the cranial nerves on this point

is contlictiny. If the pseudobrancli ])ertains

to the spiraeular cleft its nerve supply

should be derived from the nerve of tliat

cleft — viz. the seventh or facial nerve ; l)ut

if it represents a hyoidean hemibranch,

then one would expect it to be innervated

by the ninth or glossopharyngeal nerve.

As a matter of fact, however, the organ is

said to be supplied by the seventh in some

Teleosts, and in others by the ninth

nerve.

In the Dipnoi the branchial system is
best developed in Xeuccratodus, the increas-
ing impoitance of the lungs as respiratory
organs in Frotopterus and Lcpidosiren being-
associated with a corresponding reduction
in the structural and functional develop-
ment of the gills. There is no trace of
spiracles in the adult.

In Ncoccratodus'^ there are five branchial
clefts, including the hyobranchial. Each
of the first four lirancliial arches carries a yig. 165.— Transverse sec-

pair of liemibranchs, and, as in the Holo-
cephali, the gill-lamellfje are attached along
nearly their whole length to a well-
developed interbranchial septum (Fig. 165).
A peculiarity of Neoceratodus, wliich has no
counterpart in any other Fishes, is the
extension of the branchial lamellae on to
the dorsal and ventral walls of the branchial clefts, so that the
hemiliranchs on opposite sides of each cleft are continuous Ixith
dorsally and ventrally (Fig. 166). The fifth arch is gill-less.

tion tlirough a branchial
arch of 2\'corcra(oi(iis
(semi - (liagraniTiiatic).
a.b.a. Afferent branchial
artery ; b.a, branchial
arch ; b.f, branchial
filaments ; e.b.v, ellerent
branchial vessel ; (/.r,
gill-rakers. (From Bald-
win S[)encer. )

1 F. Maurer, Morph. Jahrb. ix. 1884, p. 229 ; xiv. 1888, p. 175.

- Giinther, Phil. Trans, clxi. 1871, p. 511 ; Baldwin Spencer, Maclcay Memorial

Volume, 1892, p. 1.

286

FISHES

^^=3,-7^0

In addition to the normal gills there is also a hyuidean pseudo-
Ijraiieh. As in other Dipnoi, an operculum forms the outer
^vall of the branchial cavity, and leaves but a narrow, slit-like
external branchial aperture.

In Protoptcrus^ the number of branchial arches is increased
to six, but, in consequence of the closure of the hyobranchial
cleft, there are but live open clefts. The first, second, and third

arches are wholly devoid of l^ranchial
filaments: the fourth and fifth support
each a biserial gill, while the sixth
arch retains only an anterior liemi-
branch, which, however, as the source
of its blood supply seems to indicate,
may consist of "emigrant" gill -fila-
ments from the posterior hemibranch
of the fifth arch.^ Interbranchial septa
are practically non-existent, the flat-
tened leaf-like gill-lamellae being free
except at their attached bases, and thus
repeating a characteristic Teleostean
feature. A " hyoidean " hemibranch or
Fig. 166.— The secomi branciiial pseudobraucli. Supplied from the ventral
cleft of Neoceraiodu% to show aorta, is present, but as the hyobran-

ths dorsal and ventral con- ^ ■ ^ ^ n • ^ -< •

tinuity of two hemibranchs on chial clcft IS cloSCd it prOJCCtS luto the

oppasite sides of the same cleft, branchial cavity immediatelv in front of

o.c, Branchial clefc ; o./, bran- ''

chial filaments; g'.r, gill- the clcft between the first and second
rakers. (From Baldwin branchial archcs. lu Lepidosiren^ the

Spencer.) -^

branchial arches are reduced to five and
the clefts to four, the hyobranchial and fifth clefts being closed.
There is a " hyoidean " hemibranch resembling that of Protopterus.
The facts furnished by the study of the numerical and
structural variations in the gill-clefts, gills, and gill-arches of
different groups of Fishes prove that atrophy of tliese structures
takes place at opposite ends of the series. We have examples of
this anteriorly in the suppression of the hyo-mandibular cleft and
its hemibranch, and of the hyoidean hemibranch, as the result of

^ Newton Parker, Trans. Boy. Irish Acad. xxx. 1892, ji. 161 ; Bridge, Trans.
Zool. Soc. xiv. 1898, p. 361.

- Boas, Morj}h. Jahrb. vi. 1880, p. 345. See Fig. 201, p. 340.

" Bischoff, Lc2)idosircn jniradoxa, Leipzig, 1840 ; HjTtl, Ahhand. d. bohm.
Gesellsch. 1845, p. 637 ; also Bridge, o}). cit. pp. 344, 345.

RESPIRATORY ORGANS 2 8/

the conversion of the man(lil)ular and hvoid arches into jaws, or
into skeletal supports for the jaws ; and posteriorly, in the
reduction which is evident wlicn the generality of Fishes are
compared with such primitive Elusmohrauchs as Clilamydosdadtvs
and Xotidunvs.

In most Fishes the concave pharyngeal margins of the branchial
arches are fringed with a double series of either cartilaginous or
l>ony tubercles or filaments, the "gill-rakers" (Figs. 1()1 and
164). The anterior row of gill -rakers on each arch usually
interdigitate with those of the posterior row on the preceding-
arch, and in this way the two rows form a sieve-like mechanism
to prevent any solid particles, which may enter the pharynx
with the respiratory current of water, from passing into the gill
clefts and clogging or otherwise injuring the branchial filaments.

In a few Fishes the gill-rakers are enormously developed, and
subserve a function similar to that of the baleen plates of the
Whalebone Whales in acting as a filter for straining from the
water the small pelagic organisms on which the Fish feeds.
This is notably the case in the great Basking Shark {Sclaclic
maxima) ^ in which the closely-set, flattened, tapering gill-rakers
may be so long as four or five inches, and, while somewhat
resembling " whalebone " in appearance, have the histological
structure of vascular dentine. Tlie nature of the food, which in
the stomach of one specimen examined consisted solely of an
immense quantity of plankton, including Copepods and the larvae
of other Crustaceans," affords clear evidence of the great value of
such a filtering mechanism to this Shark, and, at the same time
offers an explanation of the striking and significant reduction in
the size of the teeth, which, relatively to the dimensions of the
Fish, are so small as to be almost vestigial. A similar filter has
been observed in an extinct Selache (S. aurata) ^ from the
Antwerp Crag, and also in an existing South African Shark
(Ehinodon typicvs) ; ■* and in the latter, as in the Basking Shark,
is associated with a marked reduction in the importance of the
dentition. The lonu' slender o-iH -rakers of the Chondrostean

' Turner, Journ. Anat. and Plnjs. .\iv. 1879, p. 273. For references to otlicr
writers see Turner, op. cit.

2 For this information, which was based on an e.xaniination of a specimen,
parts of which are now in the Cambridge University Museum, I am indebted to
Dr. Harmer. " Van Beneden, quoted by Turner, o;;. cit. p. 282.

•* Andrew Smith, also quoted by Turner, ojt. cit. p. 281.

2 88 FISHES CHAP.

Polyoclon also constitute an efficient filter, and the same may be
said of several plank ton -eating Teleosts.

The Mechanism of Respiration. — The aeration of the blood
is ett'eett^'d by tliL' rhythmical suction of water into the oral
cavity, and its suljsequent expulsion through the gill-clefts,
bathing the highly vascular gill-lamellae in its course. In any
single act of inspiration the mouth is opened, and the oral cavity
enlarged by the lateral expansion of its walls. When the oral
cavity is filled with water, the mouth is closed and the expiratory
process begins. By the lateral contraction of the oral walls the
water is driven, outwards through the gill-clefts, and over tlie
gill-lamellae. During this process the branchial arches become
widely separated by the contraction of their muscles, the oper-
culum is elevated, and the oesophagus is closed by the contrac-
tion of its muscular wall. In many Fishes the course of the
expiratory water-current is controlled by special valve-like folds
of the oral mucous membrane, the maxillary and mandibular
" breathing- valves." ^

The rate of " breatliing " varies considerably in different
Fishes, even in allied species." In the Blue "Wrasse {Lahrus),
and the Eockling {Motella), the number of respirations per
minute is 15, in the Minnow (Lcuciscus), and Stickleback
(Gastrosteus), as many as 150. A deficiency of oxygen in the
water accelerates the respiratory movements, and the Fish appears
to " pant " or breathe hurriedly. In the Lampreys, both insY)ira-
tion and expiration may take place through the external gill-
apertures by the alternate expansion and contraction of the
gill-sacs, more especially when the suctorial buccal funnel is used
for the attachment of the animal. On the other hand, the singular
habits of the Myxinoids involve a further modification of the
respiratory process. In these Cyclostomata the inspiratory current
enters the external naso-pituitary aperture and reaches the pharynx
through the naso-pituitary canal, and thence, as an expiratory
stream, traverses the gill-sacs on its way outwards. The pharynx
is closed behind the last pair of gill-sacs by a constrictor muscle,
which prevents the entrance of the water into the oesophagus,
and converts the pharynx into a respiratory tube for the time

» Dahlgren, Zoul. Bull. ii. -3, Boston, 1898 ; AUis, Anat. Anz. xviii. 1900,
r. lir,7.

- -M'Keiidrick, Journ. Anat. and Pltys. xiv. 1879, p. 461.

RESPIRATORY ORGANS

289

Ixnng ; but, when food is being swallowed, the pharyngeal con-
strictor is relaxed and the internal apertures of the gill-sacs
are closed by the contraction of their own sphincter muscles.

In addition to the usual respiratory organs it is probable that
in not a few Fishes the superficial skin may share with the gills
the function of breathing. In this connexion may be mentioned

eZ.o

Fig. 167. — Embryos, of the Electric Torpedo {Torpedo ocellata). A, dorsal view ;
B, veutral view of a slightly younger specimen, c/, Cloaca ; el.o, electric organ ;
ex.b, external gills ; p.f, pectoral fin ; pv.f, pelvic fin ; sjp, spiracle ; y.s, stalk of
yolk-sac.

the fact that in Feriophthalmus the tail is used for respiration.
Hickson ^ observed that a species of this genus, frequenting the
extensive sandy shores of the Island of Celebes, often rests with
its tail in the water, the head and trunk being exposed. Under
such circumstances the gills are probably of little use, and the
tail is utilised as a breathing organ, principally, as Haddon "
subsequently pointed out, through the agency of its extremely
vascular caudal fin.

Some Fishes possess larval breathing organs ; others, even
when provided with gills, either utilise the air-bladder, or develop
special accessory organs, for aquatic or, more usually, for aerial
respiration.

1 Naturalist in Celebes, London, 1889, p. 30. - Nature, xxxi.x. 1889, p. 285.

VOL. VII U

290

FISHES

Larval Gills. — In early life many Fishes acquire larval gills,
either as the result of the precocious growth of the normal
gills, or by reason of the development of evanescent structures.
In the embryos of Elasmobranchs " external gills," in the
form of long filiform processes invested by hypoblast, are
developed from the walls of all the branchial clefts, including
the spiracles, and protrude outwards for some distance through
the external apertures of the clefts (Fig. 167, B). They perhaps
facilitate respiration within the egg, as they completely disappear
after hatching ; but there is also reason for believing that they
aid in the absorption of nutriment. Similar gills are present in
young Holocephali. In some larval Teleosts, as in certain

^9

Fig. 168. — Head of young Polyptci-ns. e.x.<j, External gill of the left side.
(From Steindaclnier. )

genera of the Osteoglossidae and Mormyridae (e.g. Heterotis and

Gyninarchus) ^ these structures are remarkably developed (Fig.

239). The young of the Loach {Misgurnus) and of the Salmon

(Sahno) also have the ordinary gill-filaments prolonged externally

as filiform structures, which subsequently become reduced to their

normal size.^ In its larval state Pohjiiterns ^ has a pair of

pinnately-fringed ectodermal or cutaneous gills projecting from

the lateral surfaces of the head behind and above the external

branchial apertures (Figs. 168 and 281). Apparently as an

individual peculiarity the right gill has been retained in a

specimen of P. congicus so large as 22 cm. in length, although

the left one had entirely disappeared.* Each gill is supplied

with blood from the ventral aorta by a vessel which ascends the

1 Budgett, Trans. Zool. Soc. xvi. Pt. ii. 1901, p. 126.
^ Gotte, quoted by Balfour, Com}). Emhryol. ii. 1881, p. 62,
^ Steindachner, Sitz. d. k. Akad. d. Wiss. i. 1869, p. 103 ; Hyrtl, ibid. p. 109 ;
Bud,i,'ett, op. eit. p. 118.

•» Boulenger, P.Z.S. 1899, p. 554.

X RESPIRATORY ORGANS 29 1

hyoid arch, and is apparently the representative of the artery
supplying the hyoidean hemibranch in Elasmobranchs. The
efferent vessel of each gill joins the common trunk formed by the
union of the efferent vessels of the normal gills of the same side.

The cutaneous gills of the Dipnoid Protoptertis may also be
included in the category of larval breathing organs. They
consist of three simple unbranched filaments on each side of the
head, and, as in Polypterus, are situated at the dorsal extremity
of the external gill aperture (Fig. 309). Although usually
represented in the relatively young or half grown specimens
which, so far, have reached Europe, it is extremely probable
that these organs atrophy in older individuals. Similar gills are
present in the larval Lepidosiren (Fig. 311), but disappear at a
much earlier stage. At no period of its development are larval
gills present in Neoceratodus}

The Air -Bladder as a respiratory Organ. — In certain
Fislies the air-bladder may become subservient to the function of
respiration. In Amia and Lepidosteus the internally sacculated
and vascular air-bladder is obviously adapted for air-breathing,
and there are not wanting observations ^ which suggest that the
organ is actually used for this purpose after the fashion of a
lung;. Accordinsf to Jobert,^ this is also the case with the
sacculated air-bladder of certain Brazilian Teleosts, viz. Sudis ,. - - ■/T"
gigas, Erythrinus taeniatus and E. hraziliensis, since these Fishes
die of asphyxia when the organ is cut off from communication
with the exterior by the ligature of its ductus pneumaticus. It
is in the Dipnoi, however, that the air-bladder becomes most
completely a true lung. In Neoceratodus * the lung is probably of
the greatest use to the Fish when the rivers are low during the hot
season and the water is charged with foul gases from decompos-
ing vegetable matter, and possibly also when the water is filled
with sediment in the rainy season. In Protopterus, and more
especially in Lepidosiren, the partial atrophy of the gills renders
it highly probable that the lungs are the principal breathing
organs at all times. Nevertheless, it must be emphasised that
in all these Fishes respiration by means of the air-bladder

' Semon, Zoologischc Forschungsrcisen in Australien, Pt. i. p. 44, and Atlas.
- Burt G. Wilder, Proc. Amcr. Ass. Adv. Sci. 1875, p. 151 ; ibid. 1877, p. 306.
' Ajui. d. Sci. Nat. sei'. 6, vii. 1878, Art. 5.
* Baldwin Spencer, op. cit, p. 3.

292 FISHES CHAP.

necessarily involves a transit of air to and from that organ
through the ductus pneumaticus, and at present nothing is
known as to the method by which such inspiratory and expiratory
currents can be produced.^

There is also some experimental evidence for the belief that
the air-bladder of some Teleosts may be subsidiary to respiration
by acting as a reservoir for the superabundance of oxygen which
is taken into the blood through the gills, and subsequently re-
absorbed into the blood when the Fish is in water containino-
relatively little oxygen.^ It is clear, however, that the conditions
under which the air-bladder can be used in this way are by no
means fully understood, for, under experiment, such Fishes died
of asphyxia even though after death the air-bladder still contained
upwards of fifty per cent of oxygen.

Accessory Organs of Respiration. — In certain Fishes of
peculiar habits, or living under special external conditions, acces-
sory respiratory organs are developed.

Although in this particvilar instance no special organs are
formed, mention may first be made of the singular method of
intestinal respiration in vogue in some Teleosts. In one of the
Loaches {Misgurnus fossilis)^ air is swallowed and passed along
the alimentary canal until it is finally voided at the anus. The
mucous membrane of the intestine is extremely vascular, and
hence the blood comes into sufficiently intimate relations with
the swallowed air to admit of it exchanging carbon dioxide for
oxygen. Intestinal respiration also occurs in species of the
South American freshwater genera of Siluridae and Loricariidae,
Callichthys, Doras, Loricaria, and Flecostom'us ; * and in some
cases the area of respiratory surface is considerably increased by
the development of folds and processes of the intestinal mucous
membrane.

In a few tropical Teleosts curious labyrintliiform organs are
developed in connexion with certain of the branchial arches, and
serve as accessory breathing organs. In the Indian " Climbing
Perch " (Anahas scandens),^ of the family Anabantidae, the organ
(Fig. 169) consists of three or more concentrically-arranged bony

^ Sorensen, Jov,rn. Anat. and Phys. 1894, p. 127-138.

^ Moreau, Ann. d. Sci. Nat. ser. 6, Zool. iv. 1876, Art. 8, p. 62.

^ Erman, Gilbert Ann. d. Physik. xxx. 1808, p. 113.

^ Jobert, op. cit. ; ibid. v. 1877, Art. 8.

' Zograff, Quart. J. Micr. Sci. xxviii. 1888, p. 501.

RESPIRATORY ORGANS

293

laminae, ^vith wavy, crenulated margins, attached liy a common
bony base to the upper extremity of the fourth branchial arch,
and enclosed in a special dorsal enlargement of the branchial
cavity. The vascular membrane which invests the laminae is
abundantly supplied with venous blood by a branch of the fourth
afferent branchial artery, the equivalent efferent vessel joining
the dorsal aorta. Essentially similar organs are found in several

Fig. 169. — Labyrinthiform organ oiAnabas
scandens, exposed by the removal of
the greater part of the operculum, ^''^im
b.a', First branchial arch ; l.o, laby-
rinthiform organ ; op, operculum ; sb.c,
supra-branchial cavity.

I-'

cephalus. Ventral view, as seen after
the removal of the ventral halves of the
branchial arches, h.a}'^, The first four
brancliial arches ; o.c, roof of oral cavity ;
oes, oesophagus ; -p.t, pharyngeal teeth ;
sb.c, left supra - branchial cavity ; v.f,
folds of the lining membrane of the
cavity.

genera of Osphromenidae (e.g.
Polyacanthiis, Osphromenus, and
Trichogaster). A simpler form

of respiratory organ of some- Fj^_i7o__g^^r^.t,j.^„^j^i^^l,^^iyg3„f Oi>/»o.
what the same type occurs in
the Indian family Ophiocepha-
lidae.^ In these Fishes there
is, on each side, an accessory
branchial cavity, situated above
that which contains the gills,
but freely communicating with it (Fig. 170). The cavity is
lined by a thickened and puckered vascular membrane, but other-
wise contains no special respiratory structures.

In the Siluroid genera Clarias and Heterohranchus the accessory
organ takes the form of branched, arborescent and highly vascular
structures, developed as outgrowths from the dorsal extremities
of one or two branchial arches, and enclosed within a posterior
and dorsal expansion of the proper branchial cavity (Fig. 171).

' Hyrtl, Sitz. d. k. Akad. Wiss. .\. 1853, p. 148.

294

FISHES

Another example of these interesting structures occurs in
Chanos salmoneus and a few other Clupeidae ^ in the shape of a
coiled gill-like organ (" gill-helix "), which is supported by the
dorsal segment of the fourth branchial arch, and enclosed in a
similarly curved caecal extension of the branchial cavity. Each
gill derives its blood from the fourth afferent branchial artery,
the corresponding efferent vessel joining the fourth efferent
branchial artery. A similar spirally-coiled " gill-helix " is found

/>

^■"^ S'cv? '^a'^

Fig. 171. — Accessory respiratory organ of Clarias, as seen after the removal of the left
operculum, a, Anterior a,rborescent organ ; b.a^"^, the first four branchial arches
and their holobranchs ; d.b.c, dorsal extension of the left branchial cavity ; /,
modified gill - filaments ; op, base of the operculum ; p, posterior arborescent
organ.

also in Hderotis ehrenhergii^ amongst the Osteoglossidae, and in
several species of Characinidae.^

In other Teleosts the accessory breathing organ assumes the
condition of paired lung-like outgrowths of the branchial cavity.
Thus, in one of the Symbranchidae, the Indian " Cuchia Eel "
(A7n2yhi2mous cuchia),* there is a pair of small bladder-like sacs,
with membranous and vascular walls, each of which opens into
the branchial cavity above the first gill-cleft, and is supplied
with blood by the afferent branchial artery of the gill-less first
branchial arch. An extreme modification in the same direction

' Hyrtl, Denksch. k. Akad. IViss. Wien. xxiii. 1863, p. i. ; ibid. x. 1855, p. 48.
2 Hyrtl, ibid. viii. 1854, p. 185.

2 Hyrtl, ibid. xxi. 1863, p. 7 ; Sagemchl, 3IorpJi. Jalirb. xii. 1887, p. 307.
* Taylor, Edin. Journ. 8ci. v. 1831, p. 33 ; Hyrtl, Denksch. k. Akad. Wiss.
Wien. xiv. 1858, p. 39.

RESPIRATORY ORGANS

295

c a.~-

/

'^•l.av.

is presented l^y the Indian Siluroid SaccohrancJnfs} In this
Fish a long caecal diverticulum of the branchial cavity extends
backwards on each side from the dorsal region of the first
In-anchial cleft to the tail, and in its course is situated internally
to the lateral trunk musculature, and close to the vertebral
column (Fig. 172). The walls of the caeca are vascular, but no
special respiratory structures are developed within their cavities,
which, during life, only contain air.
In S. singio the right caecum is
supplied with blood by an exten-
sion backwards of the dorsal portion
of the first afferent branchial artery
of that side ; the left, on the con-
trary, being supplied by the corre-
sponding portion of the fourth
afferent artery of the same side.
In S. fossilis ^ both air-sacs are sup-
plied by the fourth afferent branchial
artery. The efferent vessels join the
fourth efferent branchial artery,
right or left as the case may be.

With perhaps one or two excep-
tions, the accessory respiratory organs
of Fishes seem to exist for the pur-
pose of enabling their possessors to
breathe in air. This is certainly
the case with the labyrinthiform
organs of Anahas and its allies, and also in such Fishes as
Amphipnous, Saccohranchtcs, and the Ophiocephalidae, and
probably in others. Nearly all these Fishes are tropical in
geographical distribution, more or less amphibious in their
habits, and usually possess a remarkable capacity for sustaining
life out of water, under conditions which are promptly fatal to
ordinary Fishes. Thus, Anahas scandens may be kept alive for
days in earthen pots without water, and when free is able to
travel short distances on land, especially in the early morning
when the dew is on the ground, while AmpJiipnous frequents

Ftg, 172. — Air-sacs of Saccobmnchus
singio. a.b, The air-bladder en-
closed in its bony capsule ; a.c,
right air -sac ; a.s, left air -sac ;
c.a, biilbns aortae ; l.a.v, afferent
vessel of the left air -sac; r.a.v,
afferent vessel of the right air-sac ;
r.e.v, efferent vessel of the right
sac. (After Hyrtl, altered by
Hubrecht.)

^ Hyrtl, SB. AJcad. TVias. IVien. xi. 1S53, p. 302 ; Day, Linn. Soc. Joxhrn. Zool.
xiii. p. 198.

- Burne, Journ. Linn. Soc. Zool. xxv. 1894, p. 48.

296 FISHES CHAP. X

marshes, lurking in holes in the grass and about the sides of
ponds. In fact, even when in the water, access to air, which is
probably swallowed and passed over their accessory breathing
organs, is indispensable to their existence. Experiments con-
clusively prove that if the Fish is artificially prevented from
obtaining air in this way asphyxiation speedily ensues.^

In addition to breathing air through the agency of special
organs evolved for the purpose, there are many freshwater Fishes
which, like those just mentioned, periodically rise to the surface
and swallow air in order to saturate the water which bathes the
gills with oxygen.^

1 For much interesting information on aerial respiration in Fishes, see Day, op.
cit. ; also P.Z.S. 1868, p. 274 ; and Dobson, ihid. 1874,, p. 312.

^ Semper, Animal Life, Intern. Sci. Series, London, 1881, p. 172.

CHAPTER XI

THE AIR-BLADDEK

v.c

\-\-pp

Ix the Crossopterjgii, Chondrostei, and Holostei, in the Dipnoi,
and in the great majority of Teleosts, 'there is situated on the
dorsal side of the coelom, between the alimentary canal below
and the kidneys and vertebral
column abov'e, a more or less elong-
ated sac with membranous walls, an
internal epithelial lining and gase-
ous contents — the air-bladder (Figs.
154 and 173). Usually developed
in the embryo as a caecal outgrowth
from the dorsal surface of the
oesophagus, the air-bladder grows
anteriorly and posteriorly, and may
either retain throughout life its
primitive connexion with the
alimentary canal by means of a
longer or shorter tubular canal, the
ductus pneumaticus, or become
completely separated therefrom in
the adult by the atrophy of the
duct. Its walls sometimes, but
rarely, contain muscle-fibres, as in
Lepidosteus, Amia, and the Dipnoi,
and are always more or less vascular, while laterally and ventrally
the organ is invested externally by the peritoneum (Fig. 173). In
addition to the muscle-fibres distributed in its walls, the bladder
is often provided with powerful extrinsic muscles, more especially
in those Fishes in which it is used as an oro;au for sound-

up

Fig. 173. — Transverse section of the
body of a Teleost, to show the
position of the air-bladder (dia-
grammatic). «.&, The air-bladder ;
c, coelom ; d.p, ductus pneuma-
ticus ; k, the kidneys ; o?s, oeso-
phagus ; p.p and v.p, parietal and
visceral layers of the peritoneum ;
r, rib ; v.c, vertebral column.

297

298

FISHES

al. — t

production. In the different groups of Fishes in which it is pre-
sent the air-bladder frequently undergoes remarkable structural
modifications and becomes adapted for various distinct functions.
In the Cyclostomata there is no trace of an air-bladder, and,
unless represented in certain Sharks (e.g. Mustelus, Galeus, and
Acanthias),'^ by a small caecum embedded in the dorsal wall of
the oesophagus and communicating with its cavity, it is also
absent in all Elasmobranchs. In the Crosso-
pterygii (e.g. Polypteriis)^ the air-bladder is
double, but while the right sac is long and
somewhat tubular, the left is much smaller
and oval in shape (Fig. 174). Near their
anterior extremities the two sacs fuse into a
single unpaired chamber, beyond which they
again project in the form of two short caeca.
The median chamber opens into the oesophagus
on the ventral side by an orifice {gV) bounded
by prominent lips and furnished with a mus-
cular sphincter. The organ is devoid of internal
sacculations. In the Cliondrostei (e.g. Acipenser)
the air-bladder is oval in shape, with a smooth,
non-sacculated, inner surface, and a lining of
ciliated epithelium, and it communicates with
the oesophagus by means of a relatively wide,
dorsally placed, funnel-like orifice.

In the Lepidosteidae the single air-bladder

\ '/ extends tlie whole length of the abdominal

v,/ cavity, and, as in Folypterus, communicates with

the exterior through a larynx-like vestibule

Fig. 174. — Air-blad- .. ^ ., ..,■,..■,

der of Pohjpterits. provided witli a glottis,"^ which, however, opens

fir/ glottis. (From dorsally into the oesophagus (Fig. 175). A

Wiedersheim.) "^ . .

strong fibrous band runs along the median line
of the inner surface of its dorsal wall, from which extends
ventrally on each side a series of transverse fibro- muscular
ridges, forming the boundaries of a double row of regularly
arranged alveoli (Fig. 176). The bottom of each alveolus

^ Miklueho-lMaclay, Jen. Zeitscli. iii. 1867, p. 448.

^ Wiedersheim, Lehrh. d. vergl. Anat. d. Wirlclth. ed. 2, Jena 1886, p. 616.
^ The glottis is furnished M'ith a structure analogous to the epiglottis-like plate
oi Protopterus (Wiedersheim, op, cit. p. 616).

AIR-BLADDER

299

is still fiirtlier sacculated by finer branches of the principal

fibrous bands/ In the Amiidae the bladder is very laro-e and

except that a short median cleft divides

it in front into two short caeca, it is

unpaired. Internally, its walls are

much sacculated, but the alyeoli are

smaller and arranged less regularly

than in Lepidosteus. The aperture of

communication with the oesophagus is

dorsally situated.

It may be mentioned that in all

the preceding Teleostomi the ductus Fig. 175. — Portion of the air-
, ■ . Ill 1 ... .1 1 (ladder, with the ventral

pneumaticus is remarkably short, the wall removed, and the glottis,

of Lepidosteus. a.h. Air-
bladder ; gl, glottis ; s, bulg-
ing of the hinder wall of the
vestibule into the cavity of
the air-bladder ; r, cleft
leading from the air-bladder
into the vestibule. (From
Wiedersheira.)

tth.-l'-

retia mirabilia," " red

connexion between the air-bladder and

the oesophagus being almost direct by

means of a larger or smaller orifice,

which, except in Acipenscr, is more

anteriorly placed than in most other

Teleostomi ; and further that, unlike

many Teleosts, there are no special

bodies," or " red glands."

In the Dipnoi the structural resemblance of the air-bladder

to a true lung, %vhich to some
extent is indicated in Poly-
pterus, Amia, and Lejndosteus,
becomes still more marked.

In Neoceratodus^ the organ
is not unlike that of Lepi-
dosteus, and takes the form
of a spacious unpaired sac,
extending from one end of
the abdominal cayity to the

Fig. 176.— Portion of the air-bladder of Lejyi- q^I^q^^ Qn its inner surface
dosteus, opened along the mid-ventral Ime

to show the alveoli, av, Alveolus ; f.b, tWO fibrOUS bands, One of
medio-dorsal fibro-muscnlar band. (From ^.j^-^j^ -^ ^^^^^.^^ ^^j ^.|_,g ^^^j^g^.

Wiedersheim.)

ventral, traverse the whole
length of the bladder, and project slightly into its cavity.

^ Balfour and Newton Parker, Phil. Trans. 173, Part ii. 1883, ix 425.
- Giinther, Fhil. Trans. 161, 1871, p. 511 ; Baldwin Spencer, Zoolofjisclie
Forschunijsreisen in Australien (Semen), i. Jena 1898, p. 53.

300

FISHES

Between these median ridges extend a nvimber of transverse
septa, forming the Ijoundaries of a series of pairs of bilaterally
symmetrical oval alveoli, the walls of which are still further
sacculated by a network of finer ridges (Fig. 177). The short
ductus pneumaticus seems to be an anterior continuation of
the right half of the bladder, and opens into the oesophagus by

a small glottis, sitviated on the
ventral side, a little to the right
of the median line.

The more complicated and
much more lung -like air-bladder
of Frotopterus (Fig. 178) ^ is essen-
tially double, consisting of an
anterior unpaired portion, and of
two sac-like prolongations which
extend backwards the whole
length of the coelom, gradually
tapering towards the cloaca. An-
teriorly, the unpaired portion of
the organ is continued into a
vestibule or pneumatic duct, which,
after passing ventrally on the
right side of tlie oesophagus, opens
into the latter by a ventrally-
situated, slit -like glottis, imme-
interior of a portion of the diatcly behind the kst pair of
gill -clefts. The margins of the
glottis are provided with radially-
arranged dilator muscles, and in
connexion with its anterior border there is an epiglottis-like
fibro-cartilaginous plate.^ The central cavity of each lung (Figs.
178 and 179) communicates with a series of larger or smaller
alveoli in the lung-wall, and each of the latter opens in succession
into smaller tubular cavities, and then into still smaller terminal
caecal sacculi. Hence, much more than in Neoceratodus, the
lungs approximate in structure to those of the higher terrestrial

Fig. 177

air-bladder oi Xeoceratodtis. av, Al
veolus ; f.r, the two fibrous ridges
(From Giiuther.)

^ Newton Parker, Travis. Roy. Irish Acad. xxx. 1892, p. 109 ; Baldwin Spencer,
op. cit. p. 54.

- Henle, quoted by Howes, P.Z.S. 1887, p. 501 ; also Wiedersheira, oji. cit. p.
622 and Fig. 483.

AIR-BLADDER

301

Yertebrata. Non-striated muscle cells, pigment cells, and blood
capillaries are abundantly present in the connective tissue
external to the lining epithelium of the lung-cavities.

The air-bladder of Zcpidosircn closely resembles that of

'/.•"

%

-II

Fig. 178.— a, the air-bladder
of Frotojjterus, viewed
from the ventral side.
Portions of the ventral
walls of the pharynx and
bladder have been re-
moved, gl, Glottis ; ///,
undivided portion of the
lung; l.l, left lung; ocs,
oesophagns ; p.a^, j).a-,
the left and right piil-
monary arteries ; ^jA,
pharynx ; p.r, pulmonary
vein ; r.l, right lung ; vb,
vestibule. (From Newton
Parker.) B, portion of
one lung of Protoptervs,
opened from the dorsal
side to show the alveoli.
«Z, Alveolus. (From Bald-
win Spencer.)

ProtopteriLS, and, as in the latter Dipnoid, the
glottis seems to be furnished with an epiglottis.^
In all the Dipnoi the air-bladder is highly
vascular, but nevertheless presents no trace of
" red bodies " or " red glands."

The most striking features in the remark-
ably polymorphic air-bladder of Teleosts relate
"^ to («) its presence or absence ; (&) differences

in shape and relative size ; (c) the development of caecal out-
growths ; {d) the subdivision of its cavity by the formation of
internal septa ; (e) the retention or suppression of the ductus
pneumaticus, and the occasional development of secondary ducts
communicating directly with the exterior ; (/) the presence of
" red glands " or " red bodies " ; {g) its connexion with the audi-
tory organ ; (A) its adaptation as an organ for sound-production.

1 Bischoff, Ann. d. Sci. Nat. (2) Zool. xiv. 1840, p. 136.

302

FISHES

(a) The air-bladder is by no means universally present in
Teleosts. It is absent in several entire families/ such as, for
example, the Flat Fishes or Pleuronectidae, the Scopelidae, and
the " Lump-suckers " (Cyclopteridae). In a few families, as in
the Mackerels (Scombridae), the " Blennies " (Blenniidae) and the
Polynemidae, the organ is present in most genera, but absent in
a few, or even present or absent in different species of the same
genus. Thus, of the three British species of Mackerel, viz. the
Spanish Mackerel {Scomher colias), S. 2^neiijnatopho7-us, and the
//A ^ common Mackerel (S. scombrus), an

W// ' I \ ^!^ air-bladder is present in the first
two, but absent in the third.^

(h) As might be anticipated,
the shape of the air-bladder is
extremely different in various
Teleosts, and usually conforms
to the shape of the body, while
Fig. 179. -ShoNNdng the structure of oue (differences in relative size are

of the larger alveoli of the air-bladder

of Protopterus. 1, Ceutrai cavity of of frequent Occurrence, even in
the lung ; 2, alveolus ; 3 tubular ^.1^^^;^^ related species. Some-

cavities communicating with 4, the _ " ^

small terminal sacculi. (From Bald- times the Orgail is more Or leSS

win Spencer.) tubukr, fusifomi, ovoid, or hcart-

shaped ; occasionally it is shaped like a " dumb-bell," consisting
of two lateral sacs connected by a median tubular portion, as
in the Siluroids Glarias and Callichthys ; or it may be
horse-shoe- shaped, as in the Silurid Ailia? Not unfrequently
a transverse constriction divides the air-bladder into two inter-
communicating sacs, as in most of the Carp family (Cyprinidae),
or three such sacs may be formed by two constrictions (e.g.
Ophidium). In the " Electric Eels " (Gymnotidae) there are two
sacs, connected by a slender canal, from which the ductus
pneumaticus takes its origin.'^

The air-bladder is either more or less free in the abdominal
cavity, or firmly attached to the vertebral centra and their rib-
bearing processes by fibrous extensions passing between the two
structures. Xot rarely the organ extends from the abdominal

^ Stannius, Handb. d. Zool. Berlin ii. 1854, jj. 220.
- Giinther, Sfudy of Fishes. Edinburgh, 1880, p. 457.
3 Bridge and Haddon, Phil. Trans. B, 184, 1893, p. 209.
^ Reiuhardt, quoted by Stannius, o^j. cit. p. 225.

AIR-BLADDER 303

cavity into the tail, sometimes penetrating for a short distance
into the expanded haemal canal of the anterior caudal vertebrae,
or extending unsymmetrically along either the right or left side
of the tail. ]\Iore frequently, perhaps, where the air-bladder is
prolonged into the tail, it assumes the form of two bilaterally
arranged and symmetrical caeca, which extend backwards for
a variable distance internal to the caudal muscles and in contact
with the adjacent skeletal elements, as in Notopteridae, and in
some Sparidae, Carangidae, and Scombridae. The extension of
the air-bladder into the tail is often associated with a short,
laterally-compressed trunk, wdiich, if the bladder is to attain its
normal degree of development, necessitates its prolongation into
the caudal region.

(c) A characteristic feature in the air-bladder of many
Teleosts belonging to widely different families is the develop-
ment of a more or less complex system of simple, or vari-
ously branched, caecal outgrowths, wiiich, like the internal septa,
are specially characteristic of those Fishes in which the bladder
is used as a vocal organ without, however, being peculiar to
them.

In some of the Gadidae, as in the Cod (Gadus morrJma), the
air-bladder divides anteriorly into a pair of caecal prolongations
which extend forwards to the head, and are often curiously
coiled. Somewhat similar caeca are also present in species of
Berycidae, Sparidae, Siluridae, Clupeidae, and Notopteridae.
Caecal prolongations may also be developed from the hinder end
of the bladder, and, as already mentioned, extend into the tail ;
or even from both ends in the same species (e.g. NotoiJterus)}
In the Silurid, Rita crucigcra;' a long tubular caecum is
developed from each side of the heart-shaped bladder, and
thence is prolonged backwards to the anus. In certain species
of Doras of the same family (e.g. D. maculatus)^ an elegant
series of variously sized branched caeca fringe each of the lateral
margins of the bladder. It is, however, in the Physoclist family
of the Sciaenidae ^ that the branching of the air-bladder attains
its greatest development in extent and variety.

^ Cuvier and Valenciennes, Hist. Kat. d. Poissons, xxi. 1848, p. 139 ; Bridge,
Journ. Linn. Soc. Zool. xxvii. 1900, p. 503. ^ Day, P.Z.S. 1871, p. 703.

^ Sorensen, " Lydorganer hos Fisko," Copenhagen, 1884, p. 85; Kuer, SB. k.
Akad. TViss. JFien, xi. 1853, p. 138.

* Cuvier and Valenciennes, op. cit. v.

304

FISHES

In Otolith us (Fig. 180) two short tubular canals are given
off from the antero- lateral angles of the bladder, each sub-
sequently dividing into two elongated,
tapering sacs, of which one is directed
forwards and the other backwards. In
Corrina lohata (Fig. 181) the lateral
margins of the bladder are everywhere
fringed with a series of tufts of caeca,
each tuft being connected by a short
common canal with the cavity of the
organ. In the " Drum " {Pogonias chromis)
(Fig. 182) each side of tlie anterior third
of the air-bladder has a series of digitately
branched caecal appendages, the most
posterior of which on. each side are con-
nected by a tubular canal, also bearing
branched caeca, with the corresponding
postero-lateral extremity of the bladder.
..,,,, „ CoUichthys^ has a still more remark-

FiG. 180. — Air - bladder of -^

Otolith us. (From Cuvier able arrangement. In this Sciaenoid
and Valenciennes.) ^_p-g_ -^g3^J twenty-five tubular branches

are given off from each side of the bladder, all of which soon
subdivide into a dorsal and a ventral
division. These still further divide, and
their branches either end blindly or are
prolonged into a series of arches to the
mid-dorsal or mid -ventral line as the
case may be, where they become con-
tinuous with the corresponding branches
of the opposite side. The series of dorsal
branches, enveloped in their peritoneal
investment, extend between the body of
the air-bladder and the roof of the body-
cavity, while the corresponding ventral
branches, similarly invested, surround
that part of the coelom which contains j,j^_ isi.-Air- bladder of
the stomach, intestine, and liver.

(d) In addition to the subdivision of
the cavity of the air-bladder by the externally obvious, trans-
1 Giinther, Brit. Mus. Cat. Fishes, ii. 1860, p. 313.

Corvina lohata. (From
Cxivier and Valenciennes.)

AIR-BLADDER

305

verse, or longitudinal constrictions already described, or by the
growth of simple or branched prolongations, the organ is often
chambered or sacculated by the. development of internal septa
or partitions.

In many of the Gurnards (Trigla)^ the cavity of the bladder
is divided into two intercommunicating compartments by a
t r a n s V e r s e 1 y - d isposed

and centrally-perforated r«( '\i i ^m),-^ .7J\ ^—r-^

diaphragm. The large
air-bladder of some
species of Erythrinus ^
is subdivided internally
into numerous alveoli or
sacculi. In Koto2)tcrns
a longitudinal septum
divides the cavity of
the abdominal portion
of the bladder into two
lateral chambers, which,
however, freely inter-
communicate anteriorly.
In the great majority of
the Siluridae ^ the cavity
of the organ is divided

1 )y a characteristic
T - shaped arrangement
of a primary transverse
and a longitudinal
septum into three com-
municating chambers, of which one is anterior and transversely
disposed, and two are posterior and longitudinally arranged (Fig.

2 2 2). The posterior compartments in many genera are still further
divided by the growth of secondary transverse septa, extending
outwards from the median longitudinal septum, without, how-
ever, reaching the external lateral walls of the chambers. In a
few genera, as in certain species of Pangasins^ additional fibrous
bauds and ridges passing between the primary and secondary

1 Moreau, Comiit. r.end. lix. 1864, p. 436.
- J. Miiller, £cr. d. k. Akad. d. Wiss. Berlin, 1842, p. 177.
3 Bridge and Haddon, op. cit. p. 234, PI. II. Fig. IS.
VOL. YII

Fig. 182. — Air-Lladder oi Pogonias chromis.
(From Cuvier and Valenciennes.)

•» Ibid. p. 216.
X

3o6

FISHES

CHAP.

V.C.

r--r(ib.

v.h.-

'-J — iW--f-nt:

I —

septa give to the cavities of the lateral compartments the appear-
ance of being occupied by a coarse spongy network.

(e) In its relations to the oesophagus and to the air-bladder
the ductus pneumaticus exhibits striking modifications in
different Teleosts. With very rare exceptions, an open ductus
is wanting in the Heteromi, Catosteomi, Acanthopterygii, Opis-
thomi, Pediculati, Jugulares, and the Plectognathi, for which
reason the term " Physoclisti " has often been used as a collec-
tive name for these Fishes. On the other hand, a permanently

open ductus is generally pre-
sent in the Malacopterygii,
Ostariophysi, Apodes, and the
Haplomi, which, in conse-
quence, have been designated
" Pliysostomi." It must be
emphasised, however, that all
Teleosts are Physostomous in
the embryonic condition, and
whether they eventually be-
come Physoclistous or remain
Physostomous depends entirely
on the abortion or retention
of the primitive communica-
tion between the air-bladder
and the alimentary canal.
When present in Teleosts, the
ductus pneumaticus, with a
few exceptions (e.g. Noto-
2)terus), where it is both short
and relatively wide, is almost
invariably much longer and narrower than in the other orders
of Teleostomi and in the Dipnoi, sometimes passing directly from
the air-bladder to the oesophagus, but not infrequently describing
a sigmoid curve, as in some Cyprinidae, or an even more tortuous
course. The opening into the alimentary canal is, with perhaps
a single exception, dorsal, but may vary from the commence-
ment of the oesophagus to the hinder end of the stomach. In
Erythrinus the oesophageal aperture is lateral. In two instances
the air-bladder has acquired secondary openings to the exterior,
and of these one occurs among the Physostomi and the other

Fio. 183. — Transverse section through the
abdominal region of CoJlichthijs lucida.
a.b, Air-Uladder ; d.b and v.b, the dorsal
and ventral branches of the air-bladder ;
I, liver ; m, mesentery ; s, stomach ; v.c,
vertebral column. The dotted and broken
lines surrounding the bladder and its
branches represent the peritoneal invest-
ment of these parts. (From Giinther.)

AIR-BLADDER 307

ill the Physoclisti. In the Herring {Clwpea harengus)} in
addition to the proper ductus, which is connected with the
distal end of the caecal stomach, a tubular canal leaves the
hinder extremity of the bladder and opens externally on the left
side of the genital aperture ; consequently, in this Fish the air-
bladder has a secondary and direct connexion witli the exterior
in addition to the primary and indirect communication by means
of its proper duct. The Horse-Mackerel {Caranx trachurus) '" is
even more peculiar. This Teleost has no true pneumatic duct,
but instead a special duct which passes from the bladder to open
into the right branchial cavity by a very minute aperture. In
neither case is anything known of the mode of origin or morpho-
logical nature of the secondarily acquired duct.

(/) The air-bladder differs greatly in its degree of vascularity
in various Teleosts, as well as in the extent to w^hich its capil-
lary blood - vessels accumulate at special points on the inner
surface to form the so-called " red bodies " or " red glands."
In some Teleosts the distribution of capillaries is uniform or
nearly so ; in others, as in the Carp {Cyprinus carpio) the vessels
are arranged in fan-like, radiating tufts over almost the whole
extent of the inner surface ; in others again, as in the Pike
{Usox lucius) the tufts are larger and more definitely localised.
A more extreme modification occurs in some of the I'hysostomi,
in which a remarkable concentration of capillaries takes place
at one or more points on the inner surface of the bladder, which
project into the cavity of the organ in the form of variously
shaped blood -red masses. These "red bodies" are essentially
retia mirabilia, consisting of masses of interlacing, tightly-
packed capillaries with their afferent arteries and efferent veins.
The flattened lining epithelium of the bladder is continued over
these bodies without undergoing any special modification. In the
common Eel {Anguilla vulgaris) there are several of these bodies,
of which the largest are near the entrance of the pneumatic duct.

In the Physoclisti the " red bodies " seem to be replaced by
true glands,^ which nevertheless in appearance closely resemble
the former. Some of the Gadidae, such as the Cod (Gadus

^ Weber, Le aure ct auditu Hominis et Animallum, Leipzig, 1820, p. 73.
- Moreau, Compt. Rend. lxx.\. 1875, p. 1247.

2 Coggi, Mifth. Zool. Stat. Kcapel, vii. 1887, p. 381 ; Swale Vincent and Stanley
Barnes, Journ. Anaf. and Pliys. xxx. 1896, p. 545.

;o8

FISHES

morrJiua), the Haddock {G. aeglcfiniis), and the Hake (Merluccius
vulgaris), have a single large " red gland " projecting into the
interior of the bladder from its dorsal or ventral wall (Fig. 184, A).
Tiie John Dory {Zeus faber) has five such glands, worm-like and
curved in shape, with their concavities facing a central point
between them (Fig. 184, B). In these Fishes a "rete mirabile "
of blood-vessels forms the vascular basis of the glands. The
ordinary pavement epithelium of the bladder becomes replaced

FlQ. 184. — Red glands, A, of the Cod [Gndus morrhua), and B, of the John Dory {Zeus
faber), seen from the interior of the air-bladder, bv, Blood-vessels ; r.g, red glands.
(From Swale Vincent and Stanley Barnes.)

by faintly granular, columnar, and evidently glandular cells as
it passes over the retia mirabilia, and at the same time becomes
invagiuated into the mass of capillaries in the form of a number
of simple caecal glands (Fig. 185). So far as is at present
known, the " red glands " are only found in those Teleosts in
which the air-bladder has no ductus pneumaticus, whereas in
those Fishes which retain the ductus throughout life there are
either no special retia mirabilia, or, as in the Eel, only the
so-called " red bodies." ^

^ For the blood-supply of the air-bladder see Chap. XII.

AIR-BLADDER

309

{g) and {h) The structural modifications involved in the con-
nexion of the air-bladder with the auditory organ, and its
adaptation for sound-production, as well as its use in respiration,
are considered elsewliere.^

The Gases of the Air-Bladder. — The gaseous contents of the
air-bladder consist of oxygen and nitrogen, but the relative
proportions of the two
gases differ in different
Fishes, and even in
the same Fish, under
different conditions.
Normally the propor-
tion of oxygen is con-
siderably less in fresh-
water than in marine
Fishes, and amongst
the latter the propor-
tion of oxygen is often
enormously greater,
amounting in some
cases to 87 per cent.,
in deep-sea species as
compared with their
shallow water con-
geners. A trace of
carbondioxide is also
usually present. The
gases are derived from
the blood as the latter circulates through the capillaries in the
walls of the bladder, and it is highly probable that the "red
glands " take an important part in the process ; at all events, ex-
perimental research has shown that the " secretion " or diffusion
of gas into the air-bladder, as well as the absorption of gas
from the bladder into the blood, take place most rapidly in those
Fishes in which '•' red glands " or " red bodies " are present."

The Functions of the Air-Bladder. — Probably no single organ
in any group of Vertebrata is associated with the performance of
a greater variety of functions than the air-bladder of Fishes.
Originally evolved, it may be, as a glandular caecum in certain

1 See Chaps. XIV. XIII. and X. - IMore.an, Ann. d. Sci. Nat. (6) iv. 1876, Art.

Fig. 185. — Vertical sectiou through a "red gland"
(diagrammatic). c. Capillary blood - vessels ; g,
tubular glands. (From Vincent and Barnes.)

310 FISHES CHAP.

Sharks, the air-bladder in the Dipnoi, and some of the more
generalised Teleostomi (e.g. Amin and Lepidosteus), and perhaps
also in a few of the more specialised members of the latter
group {e.g. certain Teleosts), is to a greater or less extent an
accessory respiratorj organ. In not a few Teleosts it is an organ
for sound-production, and in others again it is sometimes regarded
as having an important relation to the sense of hearing. But
omitting such subordinate functions which, as it were, have been
grafted on to the air-bladder, there can be no doubt that in the
great majority of Fishes its primary use is to act as a hydro-
static organ or " float." From this point of view experimental
investigations ^ seem to justify the following conclusions : —

The function of the air-bladder is to render the Fish, bulk
for bulk, of the same weight as the water in which it lives. In
this condition of equilibrium, or plane of least effort, the Fish floats
ill the water, and therefore it is able to swim with a minimum
of muscular effort. It is obvious, however, that as a Fish rises
or sinks it becomes exposed to an increase or a diminution of
hydrostatic pressure, which will necessarily bring about the
expansion or contraction of the volume of gas in the air-bladder,
and, therefore, by decreasing or increasing the specific gravity of
the animal, will tend to remove the Fish from its plane of least
effort. To counteract this, and to restore the Fish to a plane of
equilibrium at the new level, gas is either absorbed from the air-
bladder, or more gas is secreted into the bladder, as the case may
be. According to Moreau, by this process of automatic adjust-
ment a Fish will always find, sooner or later, a plane of least
effort, whatever may be its depth in the water ; and further,
this process takes place much more readily in those Fishes which
possess " red glands " or " red bodies, and with extreme slow-
ness in those in which these organs are absent. Nevertheless,
it seems doubtful if this process of adjustment can be of much
use to a Fish in ordinary vertical movements, inasmuch as
gaseous secretion and absorption are comparatively slow processes,
the length of which in different Fishes, and under different
conditions, varies from a few hours to several days. On the whole
it seems more probable that adjustment to the varying pressures
of different depths by such means is far more likely to be useful
during such slow and gradual changes of level as are encountered

^ Moreau, op. cit.

AIR-BLADDER 3 I I

in the course of diurnal, seasonal, or other periodic migrations
than during the rapid changes of level which may take place in
ordinary vertical locomotion/ In the generality of Fishes, and
more especially in the Physoclisti, it may he concluded that the
possession of an air-hladder restricts freedom of movement in the
vertical direction, and confines ordinary locomotion within more
or less well-defined vertical limits ahove or below the plane of
least effort for the time being. As illustrating this point, and as
a proof of the danger incurred by a too rapid rise in the
water, the following remarks with reference to the " Kilch," a
small Salmonoid {Coregonus) inhabiting the Lake of Constance, and
a favourite article of food, may be quoted : - — The Fish " are caught
in nets, and brought to the surface of the water ; they come up
invariably with the belly much distended, the air in the swimming-
bladder, being relieved from the pressure of the column of water,
has expanded greatly and occasioned this unnatural distension,
which renders the Fish quite incapable of swimming. Under
these conditions the Fish is naturally unable to live for any length
of time. But the fishermen of the lake have a very simple
remedy ; they prick into the air-bladder with a fine needle ; the
air escapes with some force, the distension subsides, and the fishes
are enabled to live under totally changed conditions as to pressure,
even in quite shallow water and at the sm'face, swimming quite as
freely as their companions, the natives of the surface water.
Hence the Kilch is confined to a certain depth, because it is not
capable of accommodating the tension of its swimming-bladder to
the change of pressure in the column of superincumbent water."

It is not improbable that the Physostomi, or at any rate
most of them, are somewhat more advantageously placed in this
respect. From the general absence of " red glands " in this group,
it may be inferred that whatever capacity for gaseous secretion or
absorption they possess must be exercised with exceptional slow-
ness, and, therefore, as a means of pressure-adjustment may be
neglected. On the other hand, they seem to possess the com-
pensating advantage of being able to substitute for absorption the
mechanical liberation of gas through the ductus pneumaticus. It
would seem, therefore, that the Physostomi have a distinct advan-
tage over the Physoclisti in that during ascent in the w^ater they

' Bridge and Haddon, op. cit. p. 286.
- Semper, Animal Life, Internat. Sci. Series, London, 1881, p. 321.

3 1 2 FISHES CHAP. XI

can more readily adapt themselves to the diminished pressure of a
higher level by ejecting the needful amount of gas than by relying
upon the process of gaseous absorption/ This conclusion is in
harmony with the results of experiment and with much that is
known of the habits of these Fishes and their greater freedom of
locomotion in the vertical direction.

These briefly summarised conclusions as to the hydrostatic
function of the air-bladder must, however, be accepted only in a
general sense. There are many structural anomalies in the air-
bladder of Fishes which are very difficult to explain, or to corre-
late with any variations in the habits or in the locomotor activities
of its possessor.

In this connexion it may be mentioned that the presence or
absence of an air-bladder in different Fishes seems to some extent
to be governed by two causes. First, whenever the requirements
of a Fish necessitate exceptional freedom of locomotion in all
directions the restrictions imposed by the presence of an air-
bladder are removed by its partial or complete suppression ;
a result produced, secondly, by the assumption of a bottom feeding
or ground habit on the part of the Fish. Fishes like the Flat
Fishes or Pleuronectidae, when not in motion by the exercise of
their fins, habitually rest on the sea-bottom, and, as an air-bladder
is useless under such conditions, it has, in consequence, undergone
complete atrophy. Not a few Siluridae, and some Cyprinidae,
inhabit the comparatively shallow waters of rapidly flowing moun-
tain torrents, and are often provided with suckers for attachment
to stones or rocks. To such Fishes as these a hydrostatic organ
is obviously useless, and it has hence become greatly reduced in
size, and in other respects approaches the condition of a vestigial
organ.

^ Moreau, oj?. cit. pp. 3, 4.

CHAPTER XII

THE VASCULAR SYSTEM, THE LYIMPHATICS, AND THE
BLOOD-GLANDS

The Cyclostomata and Fishes possess a closed vascular system,
consisting of a heart, arteries, capillaries, and veins, the whole
forming a continuous series of hlood-containing channels provided
with definite limiting walls, through which the hlood is propelled
in a constant direction by the rhythmical contractions of the
heart. In the course of the circulation the blood flows from the
heart through a single large trunk, the ventral aorta, to the capil-
laries of the gills. From the gills the arterialised blood is collected
into a large dorsally-situated vessel, the dorsal aorta, and thence
is distributed through a system of arteries to the capillaries of
the various organs of the body. Finally, the blood is collected
from the capillaries and returned to the heart by the veins.

Although in most instances the organs of the body are supplied
with arterialised blood conveyed to them by arteries, there are
nevertheless cases in whicli an organ may receive venous blood by
a vein in addition to arterial blood supplied by an artery. For
example, the capillaries of the liver not only receive blood from
the hepatic artery, but also venous blood by a large vein (hepatic
portal vein), formed by the union of a number of smaller veins
by which venous blood is collected from the capillaries of the
stomach, intestine, spleen, and pancreas. In this and similar
instances, where a vein formed by the union of the capillaries of
an organ, or of a series of organs, instead of uniting with other
veins and proceeding towards the heart, becomes continuous with
a second set of capillaries in some other organ, a " portal " system
is said to be formed, and in the particular example of the liver
it is termed the " hepatic portal " system. A similar, or " renal

313

3 1 4 FISHES CHAP.

portal," system also exists in connexion with the kidneys in the
majority of Fishes.

There is little doubt that, primarily, the vascular system of
Vertebrate animals consisted of a dorsal artery (dorsal aorta),
running along the median dorsal line of the alimentary canal, and
a ventral or subintestinal vein similarly related to the ventral
surface of the digestive tube. The two vessels were connected by
a series of pairs of lateral branches, which had their origins from
the dorsal vessel, and, by their subdivision, formed a capillary
network in the walls of the alimentary canal. From these
networks paired veins issued and opened into the subintestinal
vein. The simplicity of this primitive arrangement was some-
what disturbed in the region of the pharynx by the development
of gill-clefts, in the walls of which the blood circulated for
respiratory purposes from the ventral to the dorsal vessel ; and
also by the development of a hepatic portal circulation in con-
nexion with the liver. In -the latter instance the subintestinal
vein entered the liver and subdivided into capillaries in the
substance of that organ, the corresponding efferent vessel, or
hepatic vein, becoming continuous with the anterior or pharyngeal
section of the subintestinal vein, or, as it is usually termed, the
ventral aorta. In this low grade of vascular system, which is
perhaps most completely retained in Amphioxus, the circulation
of the blood was probably effected by the wave-like contractions
of more or fewer of the larger vessels ; but subsequently a definite
chambered heart was developed at the origin of the ventral
aorta.

Of Fishes in general it may be said that the primitive dorsal
and ventral vessels are present in the embryo, and for a time
retain their original relations and physiological importance. To
a very unequal extent they may also be retained in the adult,
where, however, they co-exist with numerous other vessels, which
the increasing differentiation of the body has called into existence.
Thus, at a later j^eriod of embryonic life, the subintestinal vein
becomes somewhat fragmentary. Its caudal section (caudal vein)
ceases to be continuous with the precaudal portion, and the blood
collected from the muscles and other structures of the tail is con-
veyed to the heart by a pair of posterior cardinal veins, which
are either directly continuous with the caudal vein, or indirectly
through the intervention of a renal portal system in the kidneys.

VASCULAR SYSTEM 3 I 5

The precaudal portion of the subintestinal vein is represented by
a vein which runs forwards in the intestinal wall, and is one
of the minor affluents of the hepatic portal vein, while its
prehepatic section is represented in succession by the hepatic
vein, the heart, and the ventral aorta. Of the additional veins
which supplement these remnants of a primitively continuous
subintestinal vein, the largest and most constant are (a) the
posterior cardinal veins which, commencing in the kidneys and
receiving the blood from those organs, pass forwards to the heart;
(h) a pair of anterior cardinal veins, formed by the union of
smaller veins from the head, including the brain, and passing
backwards towards the heart. At the level of the latter organ
each anterior cardinal vein joins the posterior cardinal of the
same side of the body to form a short but wide transverse vessel,
the Cuvierian duct or precaval vein, which opens into the hinder-
most of the cavities of the heart, viz. the sinus venosus ; (c) a
pair of inferior jugular veins by which the nutrient blood of the
branchial apparatus is returned to the right and left Cuvierian
ducts. In addition to these principal veins there may also be a
pair of lateral veins collecting the blood from the lateral walls of
the trunk, and also opening into the Cuvierian ducts ; and sub-
clavian and femoral veins from the pectoral and pelvic fins.

On the other hand, the primitive dorsal vessel (dorsal aorta),
retains not only its original position and relations, but also its
primary function as the main channel for the distribution of
arterialised blood to all parts of the body. Tlie system of
lateral and probably segmentally arranged vessels, by which the
dorsal and subintestinal vessels were connected in the primitive
Vertebrata, have undergone considerable modification in all exist-
ing Fishes, but nevertheless retain much of their original disposi-
tion and relations in the pharyngeal region of the alimentary
canal, where they are represented by the afferent and efferent
vessels of the gills.

A more detailed account of the condition of the vascular
system in the Cyclostomata and Fishes will now be given.

The Venous System. — The Cyclostomata,^ as might be ex-
pected, exhibit a more primitive condition of the venous system

^ J. Muller, Vergl. Anat. d. Myxinoiden, Pt. iii. (1839), Berlin, 1841, p. 186.
For an account of the vascular sj'stem oi Bdellostoma see Jackson, Journ. Cincinnati
JSoc. Nat. Hist. xx. 1901, p. 13.

3l6 FISHES CHAP.

in certain features than is the case in any other group. The pre-
cauclal portion of the subintestinal vein retains much of its original
importance and runs in tlie rudimentary intestinal spiral valve as
far as the liver, where it becomes the hepatic portal vein. From
the liver the blood is collected into a single hepatic vein, and by it
is conveyed to the sinus venosus. The caudal section of the sub-
intestinal vein, now known as the caudal vein, bifurcates near the
anus, and its two branches become directly continuous with the
right and left posterior cardinals, without forming a renal portal
system. In their forward course to the heart the posterior cardinals
are situated directly beneath the notochord, and after receiving
the blood from the kidneys and gonads, and from the numerous
pairs of segmental veins of the body-wall, join the corresponding
anterior cardinal veins, and form on each side a short transverse
Cuvierian duct which opens into the sinus venosus. There is
also a pair of inferior jugular veins which, however, unite opposite
the fifth pair of gill-sacs to form a single trunk ; this vessel is
continued backwards, externally to the medio-ventral cartilage of
the branchial basket, and finally opens directly into the sinus
venosus.

In Elasmoljranchs (e.g. Mitstelus antardicus) ^ the caudal vein
(Fig. 186) lies in the haemal canal of the caudal portion of the
vertebral column. On reaching the kidneys the vein divides into
two renal portal veins, which, however, are not directly continuous
with the posterior cardinal veins as in the Cyclostomata, but,
on the contrary, after receiving the posterior segmental and
oviducal veins, become continuous with the capillaries of the
kidneys.

From the latter organs the blood is collected by a series of
renal veins, and by them conveyed to the posterior cardinals, and
thence to the Cuvierian ducts. In the adult, therefore, there is
a well-developed renal portal system, but it is worthy of note,
nevertheless, that this system is developed comparatively late in
embryonic life, and that at an earlier stage the caudal vein is
directly continuous with the two posterior cardinals, precisely as
is the case in the Cyclostomata throughout life. The posterior
cardinal veins are situated in the dorsal wall of the coelom (Fig.
187), beneath the vertebral colunui. For the hinder portion of

' T. Jeffery Parker, Phil. Trans. 177, Pt. ii. 18S6, p. 702. For references to
Elasmobranchs in general, see Parker, o}^, cit, p. 725.

VASCULAR SYSTEM

17

11 md.v-

IL ft.s

their extent they are embedded in the kidneys (Fig. 186); and in

this region the two veins are in close relation in the median line,

and here and there freely communicate uith each other. More

anteriorly, they enlarge so much

that they present the appearance

of cavernous sinuses. In addition

to the anterior segmental and ovi-

ducal veins, the posterior cardinals

receive the spermatic or the ovarian

vein from the male or female

gonad.

The precaudal section of the
primitive subintestinal vein, now
termed the internal intestinal vein
(Figs. 186 and 187), traverses the
spiral valve as it passes forwards
to the liver, but from a physio-
logical point of view is now merely
one of the factors of the great
hepatic portal vein, the principal
tributaries of which are the veins
from the stomach and intestine,
including the rectal gland, and the
pancreas and spleen. On entering

spv-

Fig. 186. — Venous system of Mustelus avt-
arcticus. a. Auricle ; ax, anterior cardinal ;
h.a, conus arteriosus ; h\v, brachial vein ;
c.d, Cuvierian duct or precaval vein ; c.v,
caudal vein ; cl.v, cloacal vein ; f.v, femoral
vein ; h.s, hyoidean sinus ; h.v, hepatic vein ;
i.i.v, internal intestinal vein ; i.j, inferior
jugular ; k, kidney ; /, liver ; l.i; lateral,
vein ; md.v, mandibular vein ; n.h.v, nutrient
hyoidean veins ; o.s, orbital sinus ;i>.c, pos-
terior cardinal ; p.v, hepatic portal vein ;
rp.v, renal portal vein ; sc.v, subscapular
vein ; sp.v, spermatic vein ; s.v, sinus
venosus ; v, ventricle ; v.a, ventral aorta.
(After T. J. Parker.)

l.v-

the liver the hepatic portal vein divides into two principal
branches for the right and left halves of the gland. From the
liver the blood is conveyed by two hepatic veins to the sinus
venosus.

The lateral veins (Fig. 186) are situated in the lateral walls

3i8

FISHES

...sp.c.

da.

spi/.

of the abdomen, immediately external to the peritoneum (Fig.
187). Each vein begins near the pelvic fin, where it is con-
nected with its fellow across the dorsal face of the ischio-pubic
cartilage, and thence runs forward towards the pectoral fin. At
its origin the lateral vein receives a femoral vein from the pelvic
fin and a cloacal vein, and also, near its anterior end, a brachial
vein from the pectoral fin, finally joining the Cuvierian duct of
its side.^

The anterior cardinal vein is situated directly above the gill-
arches of its side of the head, and extends forwards from its
junction behind with the Cuvierian duct to the outer side of the

Fig. 187. — Diagrammatic trans-
verse sectiou of an Elasmo-
brancli, showiug the position
of the principal longitudinal
blood-vessels, c, Coelom ;
d.a, dorsal aorta ; d.c.v,
dorsal cutaneous vein ; d'i.r,
dorsal intestinal vein ; -i,
intestine ; i.i.v, internal in-
testinal vein ; Lev, lateral
cutaneous vein ; l.tf, lateral
vein ; m.v.a, niyelonic vein
and artery ; j).ca\ posterior
cardinal vein ; sp.c, spinal
cord ; njy.v, spiral valve ; v,
vertebral centrum ; v.c.i\
ventral cutaneous vein ;
t^-i-v. \^ v.i.v, ventral intestinal vein.

^'"^ ' (From T. J. Parker.)

auditory capsule, where it communicates by a valvular orifice with
a large sinus surrounding the eye-muscles (orbital sinus), and
ventrally, l)y means of a similar aperture, with another large
sinus, the hyoidean sinus, which lies on the outer face of the
corresponding by oid arch, and is continuous ventrally with its fellow
of the opposite side. Into the orbital sinus open the anterior
facial vein from the anterior and external regions of the head, and
the anterior cerebral vein from the lateral half of the brain, and,
into the hyoidean sinus, the nutrient veins from the hyoidean
hemibranch.

The inferior jugular veins are situated beneath the branchial
apparatus. Each vein begins anteriorly by communicating with

' In the common Dog-Fish {Scyllium canicula) each lateral vein joins the posterior
cardinal near the junction of the latter with the Cuvierian duct, the subclavian
vein from the pectoral fin opening directly into the corresponding Cuvierian
duct.

VASCULAR SYSTEM 319

the liyoidean sinus of its side, and, after receiving the nutrient
veins from the holobranchs of the iirst four branchial arches, opens
into the corresponding Cuvierian duct.

The venous blood from the heart itself is collected into twO'
coronary veins, which open into the sinus venosus.

In addition to the more important veins already described,
there is also a series of median and lateral cutaneous veins com-
municating at different points with certain of the more deeply
seated veins (Fig. 187).

Characteristic features in the venous system of Mitstclus, as
also of Elasmobranchs in general, are the development of trans-
verse connexions between certain of the principal paired veins,
and the tendency of many of the main veins to enlarge into more
or less irregularly-shaped sinuses.

In its broad outlines the venous system of the Teleostomi
agrees with that of Elasmobranchs, but is nevertheless character-
ised by several more or less important modifications, while at
the same time exhibiting many differences in minor details.

A renal portal system is usually present, but is singularly
variable in the source of its tributary veins, even in closely allied
forms.^ In the Sturgeon (Aci2Jenser) and in some Teleosts, as in
the Siluroid, Amiurus catus, it resembles that of Elasmobranchs.
In other Teleosts, on the contrary, the renal portal system presents-
various grades of degeneration, or, possibly, of imperfect evolution,
as will be seen from the following illustrations of its condition
in different genera.

In Amiurus the caudal vein, after giving off right and left
renal portal veins to the renal capillaries, emerges from the ventral
surface of the kidneys, and is then continued forwards between
the gonads, the veins from which it receives, as the radicle of the
hepatic portal vein.

In the Eel {Anguilla vulgaris) the caudal vein (Fig. 188)
traverses the fused hinder portions of the kidneys, receiving
several segmental veins from the body-wall and also giving off
from each side numerous renal portal branches. More anteriorly,
where the two kidneys become distinct, the caudal vein also-
divides into two renal portal veins and, as each vein extends

1 Jourdain, Ann. Sci. Nat. (4), xii. 1859, p. 321 ; M'Kenzie, Reprint from the
Proc. Canadian Institute (N.S.) ii. 1884, p. 428. For references to Hyrtl and
other writers, see Jourdain, op. cit.

320

FISHES

^7J.i/

forwards along the outer border of the kidney of its side, it re-
ceives a number of segmental veins, and, at the same time, gives
off branches to the renal capillaries. In addition, each renal
portal vein is connected with the hepatic portal vein by a series

of singular arch-like vessels into
which the ovarian or spermatic
veins open.

It is obvious, therefore, that in
both Amiurus and Anguilla the
primitive direct continuity of the
caudal and posterior cardinal
veins has been interrupted by the
formation of a well -developed
renal portal system, and further,
that t]ie residue of the caudal
venous blood finds its way to
the liver through the hepatic
portal vein ; hence it follows that,
as in so many of the lower air-
breathing Vertebrates, the whole
of the venous blood from the tail
is distributed either to the kidneys
or liver in the course of its return
journey to the heart.

The Tench {Tinea mdr/aris)
Eel {Anguilla vulgaris), c.v, Caudal exhibits the interesting anomaly

vein ; i.v, intestinal vein ; l.p.c, r.p.c, . i i •

left and right posterior cardinal ot pOSSCSSnig tWO Caudal veins,

veins; IMS hepatic portal vein; R ^ doYsal and a Ventral (Tig. 189).

kidney ; r.jJ.v, r.jj.v , renal portal . . .

veins ; sg.v, segmental veins ; x, arch- The dorsal Vein is directly COn-

like anastomoses between the renal tinUOUS with the right posterior

])ortal and hepatic portal veins ; y, _ _ or

vein from the urinary bladder. (From Cardinal, while the Ventral 0116

°"'^' '""■ ' divides into three branches, two

forming right and left renal portal veins and receiving numerous
segmental veins, and the third becoming one of the affluents of
the hepatic portal vein. In this Teleost it is clear that a
portion of the caudal blood passes directly to the heart through
the right posterior cardinal without traversing either the renal
portal or hepatic portal system.

In the Cod (Gadus morrJuia) the caudal vein divides into two
branches. The larger right vein retains its direct continuity

Fig. 1S8. — Renal portal circulation in the

VASCULAR SYSTEM

321

:-sffV

rV yrpXK

with the corresponding posterior cardinal ; the left, on the con-
trary, has ceased to be continuous with the greatly reduced left
posterior cardinal and forms a renal portal vein, the distribution
of which is, however, restricted to the hinder portion of the left
kidney (Fig. 190). As in Amiurus, a branch of the caudal vein
forms one of the tributaries of
the hepatic portal vein. In the
Cod it would therefore seem
that only a relatively small
proportion of the caudal blood
flows through the imperfectly
developed renal portal system,
the bulk of it traversing the
right posterior cardinal and
passing directly to the heart,
leaving, nevertheless, a modi-
cum for transmission to the
liver. Finally, it may be
mentioned that in some Teleosts
the caudal vein retains its em-
bryonic continuity with one,
usually the right, posterior
cardinal, without giving off a
renal portal affluent, as in the
Perch (Pc7xa fiuviatilis) ; or,
after division, with both pos-
terior cardinals, as in the
Lump - sucker {Cydopterus
hmqms). In such instances as
these no portion of the caudal
blood traverses the kidneys,
and if a renal portal system
exists at all, the only true
renal portal veins are the ad-
jacent segmental veins, which transmit venous blood directly to
the kidneys, instead of first uniting with renal portal branches of
the caudal vein as in the Tench and the Eel.

Whatever may be the condition of the renal portal system, all
the renal blood is eventually collected by renal veins and conveyed
to the posterior cardinals, which are often connected by one or
VOL. VII Y

Fig. 189. — Eenal portal system in the Tench
{Tinea vulgaris), d.c.v, v.c.v, Dorsal and
ventral caudal veins ; k, kidney ; l.p.c.
7:p.c, left and right posterior cardinal
veins ; p.v, hepatic portal vein ; r.2}.v,
renal portal vein ; sg.v, sg.v', segmental
veins. (From Jourdain.)

122

FISHES

by several transverse anastomoses (Fig. 190). In the region of

the heart each posterior
'Y cardinal joins the corre-

sponding anterior cardinal
to form a short bnt wide
Cuvierian duct, which
finally opens into the
sinus venosus.

A subintestinal vein
is present in the embryo
(e.g. Lepidosteus, Acipenser,
and some Teleosts)/ but
in the adult Teleostome
its precaudal section is
usually absorbed, or at
all events ceases to be
recognisable except, per-
haps, as one of the minor
tributaries of the hepatic
portal vein.^

The hepatic portal vein
is formed as in Elasmo-
branchs, but in different
Teleostomi it may also
receive the veins from
the pyloric caeca, from a
portion of the air-bladder,
the gonads, and, as pre-
viously mentioned, a tri-
butary from the caudal
vein. There are usually
two hepatic veins open-
ing into the sinus venosus,
and generally of equal
size (Fig. 190).
Most of the veins from the air-bladder join the hepatic portal

-r—rjiT/.

Fig. 190. — Venous system of a Teleost (diagram-
matic). A, Auricle ; ah.v, vein from the air-
bladder ; a.c, anterior cardinal ; c.d, Cuvierian
duct ; cp.c, transverse anastomoses between the
two posterior cardinals ; c.v, caudal vein ; li.v,
hepatic vein ; i.j, inferior jugular ; ^,kidney ; I,
liver ; jp.c, left posterior cardinal ; p.v, hepatic
portal vein ; r.p.c, right posterior cardinal ; r.jy.v,
renal portal vein ; sc.v,, subclavian vein ; sg.v,
segmental vein ; sjp.v, spermatic vein ; s.v, sinus
venosus.

^ Balfour, Comparative Embryology, London, ii. 1881, pp. 66, 91, and 96,
■■^ A subintestinal vein is also present in adult Holocepliali (e.g. Callorhynchus
antardicus), T. Jeffery Parker, op. cit. p. 706. The persistence of this vein in
adult Fishes is associated with the presence of a ■well-developed spiral valve.

VASCULAR SYSTEM 323

vein, as already mentioned (Fig. 190), but more or fewer of them,
especially those from the dorsal wall of the organ, open into the
posterior cardinals. They may, as in Folypterus, even join the
hepatic veins.-^

The veins from the gonads are very variable in their destina-
tion, sometimes joining the posterior cardinals, as in the Salmon
{Salmo salar) ; or the hepatic portal vein, as in Amiurus ; or, as
in the Perch (Fcrca JluviatUis), forming by their union a single
trunk, which communicates directly with the left Cuvierian
duct.

Eepresentatives of the great lateral veins of Elasmobranchs
appear to be absent in the Teleostomi, the veins from the
pectoral and pelvic limbs joining the Cuvierian duct and the
posterior cardinal veins respectively.

The two large anterior cardinal veins, which collect the blood
from the head and brain, occupy their usual position directly
above the branchial apparatus, and are sometimes connected by
transverse anastomoses as they pass backwards to join the
Cuvierian ducts. The inferior jugular vein is either single (e.g.
Gadus); or paired, as in Pcrca (Fig. 190).

In the Dipnoi the venous system is distinguished by an
interesting combination of characters, some of which are either
primitive or peculiar to the group, while others exhibit a distinct
transition to the embryonic or the adult condition of the lower
air-breathing Vertebrates.

In Neoceratodus " (Fig. 191) the renal portal system is unusually
complex, the veins distributing venous blood to the kidneys being
derived from several sources, as follows: (1) from each of the
two branches into which the caudal vein divides on its exit from
the haemal canal (a/.r.v) ; (2) from a common trunk (pt.v) which,
on each side, is formed by the union of segmental veins from
certain of the post-cloacal myotomes and is united with its fellow
by a transverse anastomosis; (3) from more anteriorly situated
intercostal or segmental veins (i.c.v) which enter each kidney
directly ; and (4) from a vein on each side corresponding to the
renal portal vein of Amphibia. The latter vein (ly.v) is formed
by one of the two branches of the iliac or femoral vein, and joins

1 Budgett, Tram. Zool. Soc. xiv. Pt. vii. 1901, p. 332.

2 Giinther, Phil. Trans. 161, 1872, p. 535 ; Baldwin Spencer, Madcay Memorial
Volume, 1894, p. 17.

324

FISHES

cxL.

the corresponding vein from the caudal myotomes ; from the
common trunk numerous branches enter the kidney.

In the derivation of renal
portal veins from each of the
two veins into which the caudal
vein divides, Neoccratodus ap-
proaches the Elasmobranchs.
On the other hand, the utilisa-
tion of ordinary segmental veins
from the caudal and pre-caudal
myotomes, some of which
directly enter the kidney, is a
feature which has already been
remarked in some Teleosts ;
while the formation of a renal
portal affluent by a branch of
the femoral vein is an even
more striking Amphibian char-
acteristic.

The efferent renal veins ^
join the root of the left pos-
terior cardinal and the adjacent
portion of the caudal vein.

Of the two great venous
trunks into which the caudal
vein divides, the right is much
the larger and behaves some-
FiG. 191 -Venous system of mocemtodus. ^^i^^ differently to the left.

a.ao, Anterior abdommal ; af.r.v, aflerent _*'

renal veins ; h.v, brachial ; c.d, Cuvieriau The former {i.V.c) paSSeS for-

MheSi""^;, (.feSitr'ny wards in relation with the right

inferior jugular ; iZ.t', iliac ; t.uc, inferior kidney, receiving in itS COUrse
vena cava or postcaval ; k, kidney ; l, ,^ ,•

liver; ?.«, left auricle ; /.?;.c, left posterior the spermatic or ovarian vsins

cardinal ; l.v, lateral cutaneous vein ;
pt.v, vein from postcloacal mj'otomes ;
p.v, pulmonary vein ; pv.v, pelvic ; r.a,
rp.v, renal portal ; s.c.v,

right auricle

from the gonad of its side, and
then traverses the liver, finally
opening into the median portion

subscapular ; s.j, superior jugular or ^J; . .

anterior cardinal ; t, testis ; v, ventricle ; ot the SinuS VenOSUS, between

vv, right vertebral vein. (After Baldwin ^Jjg orificeS of the tWO hepatic

Spencer.) '■

veins. The left branch of the
caudal vein {l.'p.c) also passes forwards in relation with the left
* Baldwin Spencer, o^j. cit, pp. 24, 30-31. Not represented in Fig. 191.

VASCULAR SYSTEM 325

kidney and receives veins from the corresponding gonad ; but,
instead of traversing the liver, it passes above that organ, and
finally opens into the left Cuvierian duct. The course of the
left vein, and the relations of the vessel to the caudal vein and
the left Cuvierian duct, point to the conclusion that it represents
the left posterior cardinal of other Fishes. From its continuity
witli the caudal vein it is also obvious that the hinder or renal
portion of the right trunk is a remnant of the right posterior
cardinal ; but the more anterior section so closely resembles the
postcaval vein, or inferior vena cava of the higher Vertebrates,
in its relations to the liver, the hepatic veins, and the sinus
venosus, that its identity as such seems beyond doubt, and this
interpretation is supported by well-known observations ^ on the
mode of origin of the inferior vena cava in Amphibia, and
especially the union of the independently formed inferior vena
cava with the posterior or inter-renal portion of the embryonic
right posterior cardinal vein, combined with the atrophy of the
anterior portion of the latter vein." The singular connexions
and relations of these two great veins afford an additional
illustration of the significant transitional condition of the venous
system in the Dipnoi. On the other hand, the direct continuity
of the caudal vein with vessels which, wholly or in part, repre-
sent the two posterior cardinals, is a feature alike characteristic of
the adult Cyclostome and the embryonic Elasmobranch, Teleost,
and Amphibian.

As in the Cyclostomes and Elasmobranchs, the precaudal
section of the embryonic subintestinal vein is represented in the
adult by an intra-intestinal vein which traverses the spiral valve
near its free edge and is a tributary of the hepatic portal
vein.

The two veins from the undivided air-bladder unite to form a
single vessel, which, instead of joining the hepatic portal or
posterior cardinal veins as in other Fishes, opens into the left
auricle, like the pulmonary veins of the Amphibia.

A further resemblance to the Amphibia is to be found in the
presence of an anterior abdominal vein. After leaving the pelvic

1 Hochstetter, Morphol. Jahrh. xiii. 1888, p. 153.

- The vertebral vein, which is present only on the right side, may represent the
reduced anterior portion of the right posterior cardinal, as Baldwin Spencer [op. cit.)
has suggested.

326

.. '.

FISHES

h.p.v.

Fig. 192. — Venous system of Proto-
pterns. a, Auricle ; a.c, anterior
cardinal ; an.v, anastomotic veins ;
c, intestine ; f.v, femoral or iliac
vein; g.h, gall-bladder; li.ji.v, hep-
atic portal vein ; i.j.v, inferior
jugular ; ov.v, ovarian veins ; p,
pericardium ; p.c.v, left posterior
cardinal ; ^).v', parietal or segmental
veins ; ,s, stomach ; sh.v, subclavian.
Other reference letters as in Fig.
191. (From Newton Parker.)

limb each femoral vein divides
into two branches ; one of these
forms a renal portal vein as previ-
ously described ; the other, which
may rightly be termed a pelvic
vein {pv.v), unites with its fellow
to form a median anterior ab-
dominal vein (a.ah). Pursuing its
course forwards in the ventral ab-
dominal wall, the vein eventually
reaches the heart and opens into
the sinus venosus. The direct
connexion of the anterior abdo-
minal vein with the heart is yet
another example of the retention
in the adult Kcoceratodus of a
transitory embryonic feature in
the developing Amphibian.^

As in other Fishes, the blood
from the head is conveyed to the
Cuvierian ducts by an anterior
cardinal and an inferior jugular
on each side. There are no
lateral veins, the blood from the
pelvic fins flowing into the renal
portal system or into the anterior
abdominal vein, and that from
the pectoral fin through sub-
scapular and brachial veins into
the Cuvierian ducts. Lateral
cutaneous veins are, however,
present ; and, as in Elasmobranchs
(e.g. Mustclus antarcticus), anas-
tomose anteriorly with the sub-
scapular vein and behind with the
caudal vein.

^ As an abnormality the adult Frog may
retain the embryonic connexion of the
right anterior abdominal vein witli the
heart (Buller, Jo%irn. Anat. 'and Phys. iii.
1896, p. 211).

VASCULAR SYSTEM 327

Less is known of the venous system of Protoj)tcrus} but it is
certain, nevertheless, that it presents a more advanced grade of
evolution than in Neoceratodus, and, except for the doubt as to the
existence of an anterior abdominal vein, it is essentially similar
to that of a Urodele Amphibian in which the right posterior
cardinal vein has aborted.

The caudal vein (Fig. 192) divides into right and left renal
portal branches, neither of which, however, is directly continuous
with the inferior vena cava or the left posterior cardinal ; on the
contrary, each renal portal vein is joined by the corresponding
iliac or femoral vein, and also by numerous segmental veins, and
then distributes the whole of its venous blood to the kidney.
The radicles of the inferior vena cava and the left posterior
cardinal are formed by the renal veins from the two kidneys, and
in their forward course to the heart both veins receive in addition
genital and segmental veins. In its course through the liver the
inferior vena cava receives several hepatic veins, and finally
opens into the sinus venosus, while the left posterior cardinal
vein joins the corresponding Cuvierian duct, which also receives
anterior cardinal, inferior jugular, and subclavian veins. There
is an intra-intestinal vein as in Neoceratodus, but an anterior
abdominal vein has yet to be discovered. The two pulmonary
veins from the double air-bladder form a single trunk before
communicating with the left auricle.

With the exception of certain doubtful details which need
further investigation, the venous system of Zepidosiren ^ seems to
resemble that of Protoptcrus.

The Heart. — The heart is more anteriorly placed than in
other Vertebrates, being situated directly behind and beneath the
last pair of branchial clefts and internal to the ventral portion of
the pectoral girdle. The organ is enclosed in a pericardial cavity,
which, in the adult, is separated from the abdominal portion of
the coelom by a transverse pericardio-peritoneal septum, and in
the Lamprey {Pctroviyzon) is partially enclosed within a cartila-
ginous, cup-like modification of the hinder part of the branchial
basket. In the Ammocoetes-stage of the Lamprey the peri-
cardium is in communication behind with the general coelom,
but the connexion is lost in the adult. In Elasmobranchs the

^ Newton Pnrker, Trans. Boy. Irish Acad. xxx. 1892, p. 179.
2 Hyrtl, Ahhand. d. liohm. Gescllsch. 1845, p. 643.

328

FISHES

two cavities are connected bj a single pericardio-peritoneal canal,
or by two such canals ; and in Chimaera, and in the Sturgeon
{Acipenser) and Pohjodon, by a single canal.

The heart consists of at least three chambers, a sinus venosus
which receives the venous blood from the body, an auricle and
a ventricle, to which is added a conus arteriosus in the Elasmo-
branchs, certain Teleostomi (Crossopterygii, Chondrostei, and
Holostei), and in the Dipnoi. Through these cardiac chambers

the blood is forced in
the order mentioned. In
the Dipnoi the auricle is
subdivided by a more
or less complete inter-
auricular septum into a
right and left auricle,^
the former receiving the
venous blood from the
sinus venosus, and the
latter the aerated blood
from the lung-like air-
T. ,^o X.. bladder.

I'IG. 19.3. — Diagram of the structure of the heart in .

different Fishes. A, In an Elasmobranch ; B, in Ihe SmUS VCnOSUS and

Aniia ; and C, in a Teleost. a, Auricle; b.a, fV.g auricle have Verv
bulbus aortae ; c.a, conus arteriosus ; s.v, sinus . ;

venosus ; v, semi-lunar valves ; v', auricido-ventri- thin Walls \ the VCntri-

Boas.r^''' '''■''' '''''*'^^''°'*''''"^''''''*"'^'' ^^'°''' """^^^^ ^^'^^^' ^^ ^^^ ^^^'

trary, are very thick and

in great measure are composed of a sponge-like network of mus-
cular bundles which generally encroaches considerably on the ven-
tricular cavity. Membranous valves, the sinu-auricular, and the
auriculo-ventricular valves, are developed at the junctions of the
sinus venosus with the auricle, and the auricle with the ventricle
respectively. The conus arteriosus is muscular and contractile,
and is interposed between the ventricle and the root of the
ventral aorta. Internally, the conus is provided with several
transverse rows of pocket -shaped or semilunar valves. In
Teleosts the conus is non-muscular and vestigial, and has but
a single row of valves, corresponding to the most anterior
of the multiple rows of valves in the Elasmobranchs. In these

' There is an incomplete auricular sejitum in the Holocephali (e.g. Chimaera
monstrosa), see Ray Lankester, Trans. Zool. Soc. x. 1879, p. 502.

VASCULAR SYSTEM 329

Fishes the vestigial conus is succeeded by a non-contractile, bulb-
like dilatation, or bulbus aortae, of the root of the ventral aorta.
In only a single Teleost, viz. Alhula, one of the Albulidae, is the
vestigial conus muscular, and at the same time provided witli
two rows of valves.^ In the Cyclostomata there is a bulbus
with a single row of two valves, but no true conus.

In the Dipnoi (e.g. Protopterus) the heart, like the rest of
the vascular system, exhibits certain interesting resemblances to
the Amphibian heart. In addition to a more or less complete
interauricular septum separating right and left auricles, there is
a median longitudinal ridge, partly muscular and partly fibrous,
which incompletely subdivides the cavity of the ventricle. The
spirally-twisted conus arteriosus is furnished with several trans-
verse rows of valves, certain of which coalesce longitudinally to
form a complete septum dividing the cavity of the conus into
two distinct lateral channels : with this septum there coalesces
another septum, which cuts off the origins of the anterior two pairs
from the remaining afferent branchial arteries. The formation of
these septa has the physiological effect of subdividing the series of
cardiac cavities into two parallel channels, of which one has its
origin behind in the sinus venosus and transmits venous blood
to the posterior afferent branchial vessels; while the other, com-
mencing with the left auricle, conveys arterial blood to the first
two pairs of afferent branchial arteries." In Neoceratodus, however,
the longitudinal septum in the conus is incomplete, and hence
the l:)lood which is sent to the anterior afferent vessels is mixed.^
The Arterial System. — The ventral aorta is a median artery
situated beneath the floor of the pharynx, and having its origin,
behind, either directly from the ventricle or from the conus
arteriosus.

In the Cyclostomata'* (e.g. Petromyzon) the ventral aorta
(Fig. 194) is continued forwards from the heart as a single
vessel to the fourth pair of gill-sacs, where it divides into right
and left branches which extend as far as the anterior walls of
the first pair of gill-sacs. Eight pairs of afferent branchial
arteries arise from the ventral aorta and its two branches, of

^ Stannius, Handh. d. Anat. d. Wirhelth. Berlin, ii. 18o4, p. 235 ; Boas, Moiyhol.
Jahrb. vi. 1880, p. 527.

- Boas, Morphol. Jahrb. vi. 1880, p. 321. ^ Ibid. o-p. cit.

* J. MuUer, Fcrgl. Anat. d. Myxinoiden, Pt. iii. (ISSO') Berlin 1841 p. 179.

330

FISHES

V.C.

which the first and last supply the anterior walls of the first pair
of sacs and the posterior walls of the last pair respectively. Each
of the remaining afferent vessels extends into an interbranchial

septum, and supplies the gill-lamellae
of the posterior wall of one sac and
those of the anterior wall of the
next sac behind. The corresponding
-ex.c. efferent branchial vessels have a
similar distribution, and unite dors-
ally to form a median dorsal aorta.
Beneath the base of the skull the
latter vessel divides into two branches
^^"Zw^ which, after receiving the first pair
of efferent branchial vessels, pursue
a divergent course forwards, but
subsequently converge and unite to
form a " circulus cephalicus," as in
Teleosts. From the cephalic circle
are given off on each side (1) an
" internal carotid " artery for the
brain and eye ; (2) an " external
carotid " for the lateral and ventral
walls of the head ; and (3) a large
ventral branch which supplies the

i.o.

efl.cL

cm. a

Fio. 194. — Branchial arterial system
of the Lamprey {Petromi/zon flu-

viatilis). The ventral aorta and i m j^

the afferent branchial arteries are Imgual apparatus ; while Irom the

seen on the left si.ie and the effe- abdominal portion of the dorsal aorta

rent branchial and dorsal aorta on ■•-

the right (diagrammatic), af.h.a, are derived, first, a coeliaco-meseii-

efh.a, Afferent and efferent bran- ^ • ^^^^ ^^^ ^j^^ j-^^^^. ^^^^ ^|-_

dual arteries ; a.o, auditory organ ; •'

h.c, branchial canal ; ex, cephalic meutaiy canal, and Subsequently

circle; cm.a, coeliaco - mesenteric ■, i rj-i^^^j-^^^i-i

artery; c^.«, dorsal aorta ; e, eye ; branches for the myotomes, kidncys,
ea-.c, "external carotid "; A-, heart; and the gonad. The terminal por-

i.6.s, interbranchial septum ; in.c, .. f, ., , ,i , 2.^ j. •^

internal carotid ; oph.a, ophthai- tion of the aorta then enters the, tail
mic artery ; r.v.a, i.v.a, right and and forms the caudal artery.

left ventral aorta ; v. a, median i i / t^ j 7

ventral aorta ; v.c, "ventral caro- In Llasmobranchs ^ (e.g. MusteluS

tid"; 1-7, gill-sacs. (Modified antavcHcus) the undivided ventral
from Vogt and Yung.) j_ • £c n • e &> t.

aorta gives on nve pairs oi anerent
branchial arteries which, on each side, ascend in succession the
outer convex sides of the hyoid and first four branchial arches

1 T. Jeffery Parker, Phil. Trans. 177, Pt. ii. 1886, p. 686 ; of. H. Ayers, Bull.
Mus. Comp. Zool. Harvard, xvii. No. 5, 1889, p. 191.

VASCULAR SYSTEM

331

(Fig. 195). The first or most anterior of these arteries supplies
the hyoidean heinibranch, while the succeeding four supply tlie
holobranchs of the four branchial arches. The blood is collected
from the capillaries of the brancliial lamellae l)y a series of
efferent branchial vessels, a pair for the two hemibranchs of each
branchial arch and a single vessel for the hyoidean hemibranch,
which unite with one another in a somewhat singular fashion.
The efferent arteries from the anterior and posterior hemibranchs
of each branchial cleft unite above and below each cleft in such

p.e.a.

7iba.

Fig. 195. — Tlie brancliial arterial system of Mustelus antarcticus. Left lateral view.
The ventral aorta and afferent brancliial vessels are in solid black, the efferent
arteries and their branches liave double contours. The branchial clefts have fringed
borders to indicate their hemibranchs, and the arches are iu simple outline, a.c.a,
Anterior carotid; a.d.a, anterior dorsal aorta; a/.b.a, afferent branchial artery;
br.a, brachial artery ; c.ni.a, coeliaco-niesenteric ; d.a, dorsal aorta ; E, ej"e ; e/j.«,
epihranchial artery ; //, heart ; h.h.a, hypobranchial artery ; hy.a, afferent pseudo-
branchial or hyoidean artery ; md.a, mandibular artery ; op.a, ophthalmic artery ;
p.c.a, posterior carotid ; sh.a, subclavian ; sp, spiracle ; v.a, ventral aorta ; 1-5,
the hyobranchial and four succeeding branchial clefts. The hypobrAuchial
artery is seen immediately beneath the ventral aorta. (After T. Jeffery Parker,
diagrammatic. )

a way as to form a series of complete vascular loops round the
hyoidean cleft and the three succeeding branchial clefts, which
are connected by short longitudinal trunks in each arch and also
by a longitudinal commissural vessel between their ventral
extremities. As the fifth arch is gill-less, there is no complete
loop round the fifth cleft, the blood collected by the efferent
vessel of the posterior hemibranch of the fourth arch Ijeing
conveyed to the corresponding vessel of the anterior hemibranch
of the same arch by one of the short longitudinal vessels above
mentioned. Dorsally, each arterial loop is continuous with an
epihranchial artery ; and by the dorsal union of the four epi-

332 FISHES CHAP.

branchial arteries of the two sides the dorsal aorta is formed. It
may be pointed out that the anterior efferent vessel of each arch,
which is usually larger than the posterior one, is to be regarded
as the primary efferent artery of the corresponding holobranch,
and as such is directly continuous with an epibranchial artery,
the posterior efferent artery being a secondary vessel which opens
not into the primary trunk of its own branchial arch, but into
that of the succeeding arcli.^ The principal arteries which
supply the various parts of the head with blood are derived
from the first efferent branchial vessel. From the ventral end
of this artery a mandibular artery is given off, which subdivides
into branches for the muscles of the lower jaw as well as into
nutrient vessels for the hyoidean hemibranch. At about the middle
of its length the same artery gives off an afferent pseudobranchial
or hyoidean artery, to the spiracular or mandibular pseudobranch.
From the latter organ the blood is collected by an anterior
carotid artery which, after giving off an ophthalmic branch to
the eye, perforates the orbital wall and enters the cranial cavity,
where it is joined by an anastomotic trunk from the posterior
carotid of the opposite side ; finally, the anterior carotid divides
into anterior and posterior cerebral arteries for the brain. The
third and last of the cephalic arteries is the posterior carotid ;
this artery arises from the dorsal extremity of the first efferent
branchial vessel, and, on entering the orbit, gives off the anosto-
motic trunk previously mentioned. The latter vessel enters the
cranial cavity, and, after crossing its fellow, joins the anterior
carotid of the opposite side, as described above. The main trunk
is then continued forwards in the orbit, and its various branches
eventually supply the eye-muscles, the mandibular adductor muscle,
and some other parts of the head.

It is worthy of note that the median dorsal aorta is prolonged
forwards in front of the first pair of epibranchial arteries as a
slender median vessel (a.d.a), which ultimately divides into two
branches, each branch uniting with the posterior- carotid of its
side.

A remarkable system of arteries for the supply of nutrient
blood to the gills and heart has its origin in the following

^ Chlamydoselachus is more primitive in this respect, and has but a single
efferent vessel for the two hemibranchs of each arch, which corresponds with the
more anterior of the two in Mustclus (Ayers, op. cit.).

VASCULAR SYSTEM 333

manner. On each side, the longitudinal commissural vessel,
which connects the ventral ends of the arterial loops surrounding
the different gill-clefts, gives origin to a series of pairs of short
transverse vessels, and by their union these combine to form a
median longitudinal hypobrauchial artery which lies beneath the
ventral aorta. From the hypobranchial artery are derived the
coronary arteries for the heart ; and from the same artery, or
from its lateral connexions with the longitudinal commissural
artery, and, in the case of the hyoidean hemibranch, from the
mandibular artery, are derived the various nutrient vessels for
the gills.

The arteries for the trunk, and for the pectoral and pelvic limbs,
arise in succession from the dorsal aorta. The first of the series
is the subclavian artery, which has its origin from the aorta close
to the dorsal extremities of the fourth pair of epibranchial arteries.
Each subclavian artery gives off a brachial artery to the pectoral
fin, and is then continued forwards as a lateral hypobranchial
artery, which, with its fellow of the opposite side, eventually
becomes continuous with the hinder end of the median hypo-
branchial artery. Behind the subclavian artery there is a
median coeliaco-mesenteric artery, the various branches of which
are distributed to the liver, stomach, and intestine. A lieno-
gastric artery supplies the pancreas and spleen, and also sends
branches to the stomach. In addition, there are also arteries for
the gonads, numerous segmental arteries for the myotomes, and
renal arteries for the kidneys. Finally, the aorta gives off a
pair of iliac arteries for the pelvic fins, and then enters the haemal
canal as the caudal artery.

The more important differences in the arterial system of the
Holocephali and the Teleostomi relate to (1) the absence of the
posterior efferent branchial artery in each branchial arch ; (2)
modifications dependent on the condition of the spiracular and
hyoidean hemibranchs, and the mode of origin and the course of
their afferent and efferent vessels ; and (3) the source from
whence the air-bladder derives its blood when that organ is
present.

(1) The branchial arterial system is somewhat more primitive
than in the generality of Elasmobranchs.-^ There are no complete
vascular loops round the gill-clefts, and the blood from the two

1 Cf. footnote to p. 332.

334

FISHES

Ml/, a.

p.c.

d.a.

a.c}).a<\.

md.a

hemibranchs of each branchial arch is conveyed to the dorsal
aorta by a single efferent vessel which corresponds to the more
anterior of the two in Mustehis antarcticus}

(2) In Callorhynelius^ among the Holocephali, where the
spiracle is absent but the hyoidean hemibranchi is still a true
gill, the latter organ is supplied with venous blood by a branch
from the ventral aorta, the corresponding efferent vessel joining
the dorsal aorta (Fig. 196). In the absence of a spiracular

pse vidobranch
the anterior
carotid may be
regarded as con-
tinuous with the
hyoidean artery,^
and as having its
origin directly
from the efferent
artery of the
hyoidean hemi-
branch (Fig.
196). At its
origin the an-
terior carotid
anastomoses with
the mandibular
artery.

The Sturgeon more closely resembles the Elasmobranchs.
The hyoidean gill is supplied by an afferent branchial artery
from the ventral aorta, and its efferent vessel joins the corre-
sponding trunk from the holobranch of the first branchial arch.
A hyoidean artery supplies the spiracular pseudobranch, the
efferent vessel of which contributes to the blood-supply of the
brain and the eye, and probably represents an anterior carotid.
Lepidosteus^ offers a singularly interesting transition from the

' Note, however, that in the young Lcpidosteus there are two efferent vessels
in each arch, which, nevertheless, differ from those of Mustehis in uniting to form
an epibranchial artery before joining the dorsal aorta (F. W. Miiller, Arch. Mikr.
Anat. xlix. 1897, p. 463).

■- T. Jeffery Parker, op. cit. p. 691. ^ cf. Figs. 195 and 196.

* Ramsay Wright, Journ. Anat. and Phys. xix. 1885, p. 482 ; F. W. Miiller,
op. cit.

Fig. 196. — Portion of the efferent branchial system of Callo-
■rhynchus. a.c, Anterior carotid ; a.cb.a, anterior cerebral
arteries ; d.a, dorsal aorta ; ef.h.a, 1-4, efferent branchial
arteries ; ef.hy, efferent artery from the hyoidean henii-
branch ; hij.c, hyobranchial cleft ; md.a, mandibular artery ;
my. a, niyelonal artery ; p.c, posterior carotid ; p.ch.a, pos-
terior cerebral artery. (From T. Jeffery Parker.)

VASCULAR SYSTEM

335

Elasmobranch to the Teleost. As indicated in the preceding
chapter, this Fish possesses both a hyoidean gill and a spiracular
pseudobranch (Figs. 197 and 198). The hyoidean gill is sup-
plied by an afferent artery direct from the ventral aorta, but the
proper efferent vessel of the gill, which primitively joined the
dorsal aorta, is suppressed, and the blood is collected into a
vessel, which, like the hyoidean artery in Elasmobranchs, becomes
the afferent artery of the spiracular pseudobranch. The latter

ef.cc.

Fig. 197. — Blood-vessels of the spiraeular pseudobranch and the hyoidean gill in Lepi-
dosteus. af.a, ef.a, Afferent and effei'ent vessels of the hyoidean gill ; a/.i^.a,
ef.ps.a, afferent and efferent vessels of the spiraeular pseudobranch; ca, carotid
(posterior) ; d.a, dorsal aorta ; ef.h.a^-*, efferent branchial vessels ; hy.a, hyoidean
artery ; hy.h, hyoidean gill ; hy.ps, spiracular pseudobranch ; v.a, ventral aorta.
(From F. W. Mtiller, after Johannes Miiller.)

artery unites, however, with a second hyoidean artery derived
from the efferent branchial vessel of the first branchial arch, and
represents the artery termed " hyoidean " in Teleosts. The
efferent vessel from the spiracular pseudobranch joins an internal
branch from the carotid artery, and then distributes its blood
both to the eye and the brain.

In Teleosts, as already mentioned in a preceding chapter,
it is probable that the hyoidean hemibranch is suppressed, the
so-called hyoidean pseudobranch being a spiracular pseudo-
branch. The latter is now supplied by a " hyoidean " artery,
which has its origin from the ventral end of the efferent

336

FISHES

branchial artery of the first branchial arch, the corresponding
efferent trunk forming an ophthalmic artery, and passing to the
choroid gland of the eye (Fig. 199). Both the proper afferent
and efferent arteries of the hyoidean hemibranch either dis-
appear or, as in the Cod (Gadus morrhiia), the efferent artery
may be represented on each side by an anastomosis between the
hyoidean artery and the cephalic circle. Hence, the " hyoidean "
artery of Teleosts corresponds to the one which has a similar
origin in Lepidosteus.

A brief description of the remaining efferent branchial arteries

Fig. 198. — The branchial circulation in Lepidosteus (diagrammatic). «, a, Afferent
branchial arteries ; c, carotid ; d.a, dorsal aorta ; e, e, efferent branchial arteries ;
e/.a, efferent vessel from the hyoidean gill which, after its union with the hj'oidean
artery, becomes the afferent vessel of the spiracular pseudobranch ; e/.a', efferent
vessel of the spiracular pseudobranch ; hy.a, hyoidean artery ; hi/.g, hyoidean
gill ; sp.ps, spiracular pseudobranch ; v. a, ventral aorta ; 1-5, the hyo-branchial
and succeeding gill-clefts. (After F. W. Miiller and Ramsay Wright.)

and their derivatives in the Cod (Gadus morrhua) will illustrate
the condition of these structures in a well-known Teleost.

In this Fish the efferent branchial vessels open dorsally into
right and left suprabranchial arteries,^ which unite behind to
form a median dorsal aorta (Fig. 199). Anteriorly, the paired
suprabranchial arteries extend towards the base of the skull as
the so-called " carotid " arteries. The two carotids enter the
cranial cavity, and there unite in the median line, as in the
Cyclostomes. By the union of these arteries in front, and of the

^ These vessels are not to be regarded as homologous with tlie primitive paired
aortae of Amphioxus and the embryos of higher Vertebrates. The true dorsal
aorta sometimes persists as a median vestigial vessel -which traverses the circulus
cephalicus.

VASCULAR SYSTEM

337

right and left suprabrancliial arteries behind, the characteristic
" circulus cephalicus " of Teleosts is completed.^ From the
anterior part of the cephalic circle are derived two internal
carotid arteries ^ for the brain, and also a pair of orbito-nasal
arteries for the eye- muscles and the nasal sacs, while more
posteriorly an external carotid has its origin from each supra-
brancliial artery.

l.da.

Fig. 199. — Branchial arterial system of the Cod {Gadus morrhua). Lateral view.
af.b.a. First afferent branchial artery ; cl.a, coeliac artery ; d.a, median dorsal
aorta ; ef.h.a, first etferent branchial artery ; ex.c, external carotid ; //, heart ;
Inj.a, hyoidean artery ; Ili/.b.a, hypobranchial artery for the heart and pelvic
fins; hi/.ps, spiraciilar pseudobranch ; in.c, internal carotid;^ l.d.a, left supra-
branchial artery ; vi.a, mesenteric artery ; on, orbito-nasal artery ; oph.a,
ophthalmic artery ; r.d.a, right suprabrancliial artery ; sb.a, subclavian artery ;
v.a, ventral aorta ; 1-5, hyobranchial and succeeding gill-clefts. (Altered from T.
Jeffery Parker.)

(3) In most Teleostomi the air-bladder is supplied with blood
by branches of the coeliac artery, with the addition of small
branches arising directly from the dorsal aorta. PohjiJterns and
Arnia ^ are, however, exceptional, inasmuch as the arteries for the
air-bladder are derived from the last or fourth pair of efferent
branchial vessels, and in this respect, but not in the destination

^ For the relations of the efferent branchial vessels to the cephalic circle and
the median dorsal aorta in different Teleosts, see Ridewood, P.Z.S. 1899, p. 939.

" Only one of the two internal carotid arteries is shown in Fig. 199.

3 J. Midler, U. d. Bau u. d. Grenzen d. Ganoiden, Berlin, 1846, p. 43 ; Ramsay
Wright, Standard Nat. Hist. iii. pp. 48, 49.

VOL. VII Z

338

FISHES

of the corresponding veins, the two genera exhibit a significant
resemblance to the Dipnoi.

In the Dipnoi the ventral aorta is so short that the afferent
branchial arteries arise almost directly from the conns arteriosus
with their roots in close contiguity to one another (Fig. 200).

In Neoceratodus (Fig. 200),^ there are two efferent vessels to
each gill-bearing branchial arch, which unite above to form an
epibranchial artery, and by the successive union of the four

a/.ci.a.

-p.c'b.co.

ep.a.

T.eLOL.

af.ict'.

Fig. 200. — Branchial arterial system of Xeoceratodus. Lateral view. The conns arteriosus
and the afferent branchial vessels are represented in solid black, the efferent vessels
and their derivatives with double contours, a, Auricle ; a.c.u, anterior carotid ;
a.ch.a, anterior cerebral artery ; af.b.a', first afl'erent branchial artery ; br.a,
brachial artery ; c.a, coronary artery ; car, conus arteriosus ; c.m.a, coeliaco-
mesenteric ; ep.a, epibranchial artery; hb.a, hypobranchial artery; hy.a, hyoid
artery ; hy.ar, hyoidean arch ; La, lingual ; l.d.a, r.d.a, left and right dorsal aortae ;
oca, occipital artery ; oes.a, oesophageal artery ; p, pericardium ; 7>.«, pulmonary
artery ; px.a, posterior carotid ; p.cb.a, posterior cerebral artery ; s.v, sinus
venosus ; r, ventricle ; 1, hyobranchial cleft ; 2-5, branchial clefts. (After Baldwin
Spencer, diagrammatic.)

epibranchial arteries a short common trunk is formed on each
side. Posteriorly, the two trunks unite to form a median
dorsal aorta. Immediately above the gill-clefts each efferent
vessel gives off a branch which, passing either forwards or back-
wards, unites with the corresponding branch of the efferent vessel
in front or behind as the case may be. A hyoidean artery
arises from the ventral extremity of the anterior efferent artery
of the first branchial arch, and, after giving off a lingual artery,
ascends the hyoid arch and supplies the hyoidean pseudobranch.
The efferent vessel of the pseudobranch (a.c.a) or anterior

^ Baldwin Spencer, Macleay Memorial Volume, 1892, p. 1.

VASCULAR SYSTEM 339

carotid artery, eventually enters the cranial cavity and sub-
divides into anterior and posterior cerebral arteries for the brain,
also giving off a branch which unites with its fellow of the
opposite side directly behind the infundibulum. A posterior
carotid springs from the epibranchial of the first branchial arch
and divides into palatine, orbital, and ocular branches ; and from
the ventral end of the anterior efferent vessel of the second
branchial arch is derived a hypobranchial artery for the heart
and pericardium. The pulmonary arteries for the lung-like air-
bladder have their origin from the fourth pair of epibranchial
arteries.

As in so many other details of its anatomy, Neoceratodus
exhibits in its arterial system abundant evidence of the wide-
spreading affinities of the group to which it belongs. In its
branchial arterial system Neoceratodus presents a singular com-
bination of features which, individually, are characteristic of
Amphibia and Elasmobranchs. Special Amphibian features may
be noted in the origin of the afferent branchial arteries almost
simultaneously from the anterior end of the conus arteriosus ;
in the mode of union of the epibranchial arteries to form
the dorsal aortae ; in the origin of a lingual artery from the
efferent vessel of the first branchial arch ; and in the derivation
on either side of a pulmonary artery from the fourth epibranchial
artery. Agreement with Elasmobranchs is to be found in the
presence of two efferent branchial vessels in each branchial arch,
although the relations of these arteries are more primitive than
in most adult Elasmobranchs, inasmuch as the two efferent vessels
of the same arch unite to form an epibranchial artery ; and also
in the origin and distribution of the anterior and posterior
carotids. Lastly may be mentioned the fact that Neoceratodus
agrees not only with the Amphibia but also with those generalised
Teleostomi, Polypterus and Amia, in the mode of origin of the
great arteries for the air-bladder.

Of the two remaining Dipnoi, the arterial system of Proto-
pterus ^ is better known than that of Zepidosiren, but in both
cases further research is needed before a satisfactory comparison
can be made with Neoceratodus and other Vertebrates. It is
evident, nevertheless, that both genera differ from Neoceratodus
in approximating more closely to the Amphibia than to the

^ Newton Parker, Trans. Hoy. Irish Acad. xxx. 1892, p. 173.

340

FISHES

lower Fishes, in so far as the branchial part of tlie arterial system
is concerned.

In their origin from the conns the four afferent branchial
arteries of Protopterus resemble those of Ncoceratodii s, but their
relations to the branchial clefts are somewhat different (Fig.
201). The first or hyoidean cleft is closed, and the first afferent
vessel lies between the second cleft and the third, and is there-
fore in relation with the second branchial arch. The remaining
afferent arteries are disposed between the succeeding clefts and

- ^ pa.

a±:

ci£6.a.:-*

Fig. 201. — Branchial arterial system ol Protopterus (diagram matic). a, Auricle ; a.c.a,
carotid artery ; af.h.a ^~'*, afferent branchial arteries ; af, ef, afferent and efferent
vessels of the hyoidean pseudobranch ; h.ci?, second branchial arch, the vestigial
first arch being omitted ; c.a, coinis arteriosus ; e.g, external or cutaneous gill ;
ejs.a, epibranchial artery ; hy.ar, hyoid arch ; hy.ps, hyoidean pseudobranch ; I. a,
lingual artery ; l.d.a and r.d.a, right and left dorsal aortae ; Z./i.a, left pulmonary
artery ; s.v. sinosus venosus ; v, ventricle ; 2-6, tlie second branchial and succeeding
clefts, the hyobranchial cleft being closed. The vestigial first branchial arch is not
shown. The epibranchial arteries unite to form the right or left dorsal aorta at the
same point and not in succession as in the figure. (Altered from Newton Parker.)

are related to the corresponding arches. As the second and third
arches, like the vestigial first arch, bear no gill-lamellae, their
afferent arteries are directly continuous with the corresponding
efferent vessels, as in those Teleosts in which certain arches are gill-
less, as well as in the Tadpole-stage of the tailless Amphibia when
the internal gills begin to degenerate ; and they apparently transmit
arterial blood directly to the dorsal aorta.^ The third and fourth
afferent arteries, on the contrary, supply venous blood to the two
hemibranchs which are borne by each of the two corresponding
arches, viz.: the fourth and fifth, and from each pair of hemibranchs
the blood is collected into two efferent vessels which unite dorsally

^ According to Boas ; for reference, see p. 329.

VASCULAR SYSTEM 34 I

to form an epibrauchial artery. From the dorsal end of the fourth
afferent artery there arises a recurrent branch which curves round
the upper margin of the sixth cleft and supplies the gill-lamellae on
the posterior margin of that cleft, a fact which lends support to
the view that these lamellae are " emigrants " from the anterior
margin of the cleft ; the efferent vessel from the " emigrant "
lamellae joins the fourth epibrauchial artery. The blood-sup])ly
of the external or cutaneous gills is derived from the dorsal
extremities of the second, third, and fourth afferent arteries,
while the efferent vessels from these organs join the correspond-
ing epibrauchial arteries ; in this respect there is a close resem-
blance between Protopterus and those larval Amphibians which
possess similar cutaneous gills. All four epibrauchial arteries
unite together at about the same point to form a short common
trunk, the right or left dorsal aorta, which subsequently unites
with its fellow to form the median dorsal aorta.

There is a so-called " hyoidean " artery, which, however, has
its origin, not from an anterior efferent branchial vessel as in
Neoceratodus, but from the first afferent branchial artery. After
giving oft' a submaxillary or lingual artery, the " hyoidean "
artery {cif) becomes the afferent vessel for the " opercular gill "
or " hyoidean pseudobrauch," ^ and supplies the latter with arterial
blood. The efferent vessel {ef) from the pseudobrauch unites with
the four epibrauchial arteries in forming the right or left dorsal
aorta, A " carotid " artery arises from the efferent vessel of the
" hyoidean pseudobrauch," and a pulmonary artery has its origin
from the root of the dorsal aorta of its side, and not from the
fourth epiljranchial artery as in Neoceratodus.

The Blood. — The blood consists of a nutritive fluid plasma in
which float red corpuscles and leucocytes. In the Cyclostomata
(e.g. Petromyzon) the red corpuscles are circular, but in Myxine
they have the usual oval shape. In Fishes the red corpuscles
are almost invariably flat, oval, biconvex, and nucleated, and owe
their colour to the presence of the characteristic oxygen-absorb-
ing, iron-containing pigment, haemoglobin. They are unusually
large in the Dipnoi and are only exceeded in size by those
of certain Urodele Amphibians. The leucocytes are much less
numerous than the red corpuscles, although their relative propor-
tions are very variable, even in the same species under different

^ This structure may prove to be a heniibrauch of the first branchial arch.

342 FISHES CHAP.

conditions. They appear to be more numerous in the Dipnoi
(e.g. Protopterus) than in any other Vertebrates, except under
pathological conditions.-^

The Lymphatic System. — In addition to blood-vessels, Fishes
possess a lymphatic system, consisting of smaller vessels, lymph-
capillaries or lymph-spaces, distributed in the connective tissue of
different parts of the body, and by their union ultimately forming
larger lymph-vessels or sinuses which communicate with certain of
the principal veins, the whole forming a series of channels for the
collection of the blood -plasma which has exuded from the blood-
capillaries for the nutrition of the tissues, and for its conveyance
to the general venous system. The fluid in the lymphatics, or
lymph, consists of dilute blood-plasma containing leucocytes but
devoid of red corpuscles. At the points where the larger lymph-
atics open into the veins, lymph-hearts may be developed. In the
Eel {Anguilla vulgaris) there is a lymph-heart in the tail, which
communicates by a valvular orifice with the smaller of the two
caudal veins, and by its rhythmical pulsations propels the lymph
into the vein. In Silurus there are two caudal lymph-hearts.
Apart from the lymphoid tissue, which is so abundantly present
in certain parts of the body. Fishes appear to be devoid of the
special " lymphatic glands " of the higher Vertebrates.

The Ductless or Blood-Glands. — All the important blood-
glands of other Vertebrates have their representatives in Fishes.
Nothing is certainly known of the function of these organs in
Fishes, but from the general structural resemblance which they
present to their equivalents in the higher Vertebrates, it is
perhaps not unreasonable to infer that they are similar in func-
tion. If this be so, the blood-glands of Fishes are organs for
leucocyte-formation and phagocytosis, involving the destruction
and removal of effete red blood- corpuscles ; in addition, they
may also be concerned with certain obscure chemical changes in"
the composition of the blood, which have an important relation
to general or local nutrition.

The Spleen. — This lymphoid organ is the largest of all the
blood -glands, and, in the form of a compact or more or less
lobulated body, is present in all Fishes, and possibly in Cyclo-
stomes. In position the spleen is usually in close proximity
to the stomach, to which it is attached by an extension round it
^ Newton Parker, op. cit. p. 167.

BLOOD GLANDS 343

of the peritoneal investment of that organ. Thus, in the Dog-
Fish {ScylliiLm), the spleen is a large reddish body attached to
the convexity of the U-shaped stomach, and, in addition, sends
a long narrow lobe between the distal limb and the valvate
portion of the intestine (Fig. 153, spl). In the Sturgeon
(Acipenser), the organ is also large, but is attached to the left
side of the commencement of the intestine. In the Cod (Gadiis)
among Teleosts the spleen is much elongated and is situated
on tlie dorsal side of the stomach. In the Dipnoi (e.g. Proto-
pterus) ^ the organ is probably represented by a large compact
lymphoid mass, closely connected with the dorsal and lateral
walls of the stomach (Fig. 154, A, s).

The Thyroid Gland. — This organ " usually arises in the form
of a small median evagination of the hypoblastic epithelium of
the ventral wall of the pharynx, in the region of the second
visceral arch. Later it becomes detached from the place of
origin and converted into a solid spherical body. Eventually
the component cells form the limiting epithelium of a series of
follicles or vesicles embedded in a matrix of connective tissue and
blood-vessels, and the characteristic adult structure is attained.

Among the Cyclostomata the evagination is relatively large
in the young Lamprey {Petromyzon fluviatilis), as also is the
orifice of communication with the pharynx (Fig. 202, ^A).^ The
aperture soon becomes reduced to a mere pore, and finally
disappears. During the larval or Ammocoetes-stage the organ
consists of a median cilated portion, communicating with a pair
of laterally placed glandular sacs, but in the adult it is much
smaller, and acquires the usual follicular struct are. In adult
Elasmobranchs the thyroid is represented by a moderately large
compact organ, situated near the anterior end of the ventral
aorta. In Teleostomi the organ may be paired, or, as in the
Perch {Perca), more diffuse, consisting of masses of reddish
lobules lying beneath the aorta, and also scattered for a variable
distance along the course of the afferent branchial arteries.

In the Dipnoi (e.g. ProtopterusY the thyroid is small, con-

^ Newton Parker, op. cit. p. 138.

"^ De Meuron, Becherches sur Ic d^velo2ypement die Thymus et de la (jlande thyrco'ide,
Inaug. Dissert. Geneve, 1886 ; Maurer, Morpli. Jahrh. xi. 1886, p. 129 ; W. Miiller,
Jen. Zeitsch. vi. 1871, p. 428 ; vii. 1873, p. 327 ; Dolirn, Mitth. Zool. Stat. Neapel,
vi. 1886, p. 49 ; vii. 1887, p. 301.

3 Cf. p. 280. ■* Newton Parker, op. cit. p. 135.

344

FISHES

sisting of two lateral lobes connected by a constricted median
portion, and situated beneath the epithelium of the tongue,
immediately above the hyoidean symphysis. A similar structure
has been described by Bischoff ^ in Lepidosiren, and was regarded
by him as a salivary gland.

As in Reptiles, Birds, and Mammals, paired or accessory
thyroid bodies (" supra-pericardial organs")" are present in many
Fishes, and appear to be similar in structure to the median
thyroid. In Elasmobranchs these bodies originate as a pair of

sp.c.

©

2%.

•5°- brc: tJi.

Fig. 202. — A, a vertical section through a just- hatched larva of Petromyzon. a.v.
Auditory vesicle ; br.c, branchial cleft ; h, heart ; m, mouth ; n, notochord ; ol,
olfactory pit ; ^/i, pharynx ; sp, septum or velum between the stomodaeum and
the mesenteron ; sp.c, spina! cord ; th, thyroid outgrowth from the floor of the
pharynx. (From Gegenbaur, after Calberla.) B, diagram illustrating the develop-
ment of the thyroid, the thymus, and the accessory thyroids, and their relations to
the branchial clefts, a.th. Accessory thyroids; g.2), gill-pouches; Ph, pharynx;
t, thymus ; th, median thyroid. (From Hertwig, after de Meuron.)

outgrowths from the epithelium of the pharynx behind the last
pair of branchial arches (Fig. 202, B, a.th). Subsequently they
become detached from the pharynx, and in the adult are situated
on the dorsal side of the pericardium, remote from the median
thyroid.

According to Dohrn the median thyroid is to be regarded as
the vestige of a gill-cleft which primitively existed between the
hyomandibular cartilage and the hyoidean arch. This conclusion
seems, however, to be less in harmony with the facts of develop-
ment than the view ^ that the organ is derived from the
characteristic hypobranchial groove or " endostyle " of Ascidians

^ Quoted by N. Parker, I.e.

'^ Vau Bemmeleu, Anat. Anz. iv. 1889, p. 400.

^ W. Miiller, op. cit.

BLOOD GLANDS 345

and Amphioxus, which has undergone a change of function from
a mucus-conveying groove to a blood-gland. On the other hand,
the mode of origin of the paired thyroids certainly favours the
suggestion that they represent a posterior pair of vestigial gill-
clefts, a view which derives some support from the fact that in
Notidanus, where additional branchial arches and clefts are
present, the paired thyroids are absent.

The Thymus. — In the embryo Elasmobranch and Teleost'
the thymus has a multiple origin, being derived from a series of
distinct epithelial thickenings, one of which is developed at the
dorsal extremity of each of the gill-clefts except of the spiracle.
These rudiments subsequently detach themselves from the epithelial
surface and sink inwards, eventually fusing together on each side
to form a single independent structure. Later, the epithelial mass
thus formed becomes invaded by connective tissue, and by leucocytes
which form lymph follicles, and the thymus gradually assumes
the structure of a lymphoid organ. From its mode of develop-
ment it has been suggested that the thymus owes its evolution
to the metamorphosis and ingrowth of branchial filaments,' but
it is also noteworthy that each embryonic rudiment of the organ
closely resembles, both in position and origin, one of the develop-
ing branchial tongue-bars of Amphioxus.^ The abundance of
leucocytes which it contains has also prompted the further
suggestion that the origin of the thymus may be due to the
necessity of providing for the phagocytic protection of the gills
themselves from the ravages of harmful micro-organisms, fungoid
spores, etc., as well as to aid in the removal of such portions of
the gills as may have been injured.*

A thymus is probably present in all Fishes, if not in the adult
at all events in the embryo, but is always relatively small in size.
In Elasmobranchs the organ lies on each side above the branchial
arches and beneath the dorsal musculature ; and in Teleostomi at
the dorsal extremity of the last branchial arch, in close proximity
to the mucous membrane of the branchial cavity. In a similar
position in the Dipnoi (e.g. Proto2:iterusY there are, on each side,

^ Dohrn, Mitth. Zool. Stat. Neapel. v. 1884, ]}]}. 141-151 ; see also the pre-
viously cited works of De Meuron and Maurer.

^ Dohrn, op. cit.

^ See pp. 120 and 135. AVilley, Amphioxus and the Ancestrtj of the Vertebrates,
New York, 1894, pp. 30, 31.

■* Beard, A}iat. Anz. ix. 1894, p. 485. '' Newton Parker, op. cit. p. 135.

346

FISHES

two contiguous lobes of lymphoid tissue which apparently
represent a thymus.

The Supra-renal Bodies. — The supra-renal bodies are organs

of problematic function, which are present in the Cyclostomata,

Qgg and probably in all

Fishes, and situated
in close proximity
to the kidneys.

In the Cyclosto-
mata {Petromyzon)
these bodies are re-
presented by lobules
of cells along the
posterior cardinal
veins, and also by
masses of peculiar
cells (" chromaffin
cells") along the sides
of the aorta and seg-
mental arteries.^ In
Elasmobranchs there
are two distinct
structures, the paired
supra-renals and the
inter -renals (Fig-
203, A). The former
are a series of pairs of
segmentally arranged
bodies, situ.ated on
the successive pairs
of segmental arteries
given off from the
dorsal aorta. The
two bodies which
form the first pair are much larger than any of the others, and were
formerly spoken of as " axillary hearts." The inter-renal is usually
a thin elongated " ochre-yellow " body, from which one or two lobes
may be detached in front, and extends for a variable distance in

^ Giacoiniiii, quoted by Swale Vincent, Journ. Anat. and Phys. xxxviii. 1903,
p. 41.

Fia. 203. — Supra-renal and inter-renal bodies of Fishes.
A, oi Scyllmm catulus ; B, of A cipenser sturio ; C, of
Pagellus centrodontus. d.a, Dorsal aorta ; ir, inter-
renal body ; l.vi, lymphoid portion of the mesonepliros ;
in, mesonephros ; mtn, metanephros ; ues, oesophagus ;
sg.a, segmental arteries ; sr, supra-renal bodies ; sy.n,
sympathetic nerves. (From Swale Vincent.)

BLOOD GLANDS 347

tlie median line between the two kidneys, or is unsymmetrically
placed on the ventral surface of either kidney.-^ Sometimes (e.g.
in Rata) the inter-renals are paired, in w^hich case they are applied
to the inner and hinder margins of the kidneys. In the Sturgeon
(Acipenser sturio) the " supra- renals " appear as numerous " ochre-
yellow " bodies, variable in size and distribution (Fig. 203, B).
Some of them are visible on the su.rface of the kidneys, while
others are scattered about in their substance, but on the whole
are more anteriorly placed than in Teleosts. In the latter group
the " supra-renals " are usually two in number (Fig. 203, C), but
may be as many as five or reduced to one. They are disposed
either on the ventral or the dorsal surface of the kidneys,
generally near their hinder extremities, or more or less deeply
embedded in their substance. Besides these bodies there are
also chromaffin cells in the walls of the anterior cardinal veins.^
Histologically, the paired segmentally arranged bodies of
Elasmobranchs differ considerably in structure from the inter-
renal bodies, the former resembling the " medulla," while the
inter-renals, as well as the so-called supra-renals of Aci'penser,
exhibit a striking resemblance to the alveolar " cortical " sub-
stance of the Mammalian supra-renals.^ In Cyclostomes the
cortex is apparently represented by the lobules of cells along the
posterior cardinal veins and the medulla by the " chromaffin "
cells, while in Teleosts the cortex and the medulla have their
respective counterparts in the supra-renals and the " chromaffin "
cells in the walls of the anterior cardinal veins. It may be
concluded, therefore, that Elasmobranchs, Cyclostomes, and
Teleosts possess anatomically distinct representatives of both the
" medulla " and " cortex " of Mammalia, although the Sturgeon
is at present only known to possess the equivalent of the
" cortex." In Amphibia, Eeptilia, and Aves both " cortex " and
" medulla " are present, and in the varying intimacy of their
relations offer a transition to the Mammalian arrangement of a
central medulla closely invested by a sheath of cortical substance.
A more or less intimate connexion exists between the paired
supra-renals of Elasmobranchs and the sympathetic nervous

' Vincent, Trans. Zool. Hoc. xiv. Part iii. 1897, p. 4L For bibliography see
Vincent, Internat. Monatsschr. f. Anat. u. Phys. xv. 1898, p. 319.

- Giacomini, quoted by Swale Vincent, Journ. Anat. and I'hija. xxxviii. 1903,
p. 41.

3 Vincent-, op. cit. pp. 32, 33.

348 FISHES

CHAP. XII

system. The former are usually well supplied with sympathetic
nerve fibres, and contain ganglion-cells in their substance.

Tlie primitive origin of these organs is very obscure, and as
regards their development there is much diversity of opinion.
It seems certain, however, that the cortex and medulla of the
higher Vertebrates, including their equivalents in the Elasmo-
branchs, have independent origins, and the balance of opinion
seems to point to the derivation of the cortex from some portion
of the germinal coelomic epithelium, while the medulla is derived
from the embryonic nerve cells of the sympathetic ganglia.

Lymphoid Tissue. — In addition to certain of the ductless
glands, and the local or diffused masses of their characteristic
tissue already mentioned in connexion with the alimentary canal,
lymphoid tissue is often abundantly present in other parts of the
body. There is, for example, a mass of this tissue on the heart
of the Sturgeon (Acipenser). The anterior enlarged portion of
the mesonephros, commonly termed the " head-kidney " of the
Teleostomi (Fig. 203, B, C), is almost entirely composed of
lymphoid tissue,^ which has replaced, wholly or partially,
the proper renal structure ; and from the presence of free red
blood-corpuscles and of crystals of oxy-haemoglobin and other
derivatives of haemoglobin, it may be inferred that the " head-
kidney," in common with the more orthodox blood-glands, per-
forms a blood -destroying function.- On the other hand, the
example of the spleen, which is alike the seat of leucocyte-
formation and of blood- destruction, renders it unnecessary to
reject the view that the " head-kidney " is an organ in which
leucocytes or blood-corpuscles are formed. In but few Teleostomi
is a purely lymphoid " head-kidney " entirely wanting, as, for
"1 - [ example, in the Sun-Fish (Orthagoriscus mola)? As previously

mentioned the Dipnoi are remarkable for the extraordinary
development of lymphoid tissue, inasmuch as it forms a thick
investing mass round the kidneys and gonads in addition to its
exceptional abundance in the walls of the alimentary canal.

The absence of ordinary lympliatic glands in Fishes is well
known, and it is at least probable that, functionally, the want of
these lymphoid organs may be compensated for by the super-
abundance of lymphoid tissue in other parts of the body.'*

' Balfour, Quart. J. Micr. Sci. xxii. 1882, p. 12.

^ Swale Vincent, op. cit. p. 78. ^ Ibid. pp. 77, 78. * Balfour, op. cit. p. 16.

CHAPTER XIII

MUSCULAR SYSTEM LOCOMOTION SOUND-PRODUCING ORGANS

ELECTRIC ORGANS

Muscular System. — The various muscles of the body may be
arranged in two systems : (i.) the somatic or ■parietal, composed of
striated or voluntary muscle-fibres ; and (ii.) the splanchnic or
visceral, consisting for the most part of unstriated or involuntary
fibres. Somatic muscles form the great lateral longitudinal
muscles of the trunk and tail, which retain the primitive
embryonic metamerism to a greater extent in Fishes than in
any other Vertebrates, and are the principal muscles associated
with locomotion. The lateral muscles are composed of a series
of transverse muscle-segments or myotomes, which are >-shaped,
or S-shaped, or they even take a zigzag course from above
downward. The myotomes are disposed in pairs, and they are
separated from one another by fibrous septa or myocommata.
Each myotome is divided into a dorsal or epiaxial portion, and a
ventral or hypaxial portion, by a longitudinal, horizontal, fibrous
septum extending outwards from the vertebral centra to the skin.
The muscles of the pectoral and pelvic fins are derivatives from
more or fewer of the adjacent myotomes. The splanchnic muscles
include the musculature of the walls of the alimentary canal, as
well as those specialised portions of the visceral system which
are represented by the muscles of the branchial arches and the
jaws, and are composed of striated fibres.

Locomotion. — A Fish and a Bird are equally remarkable for
the many and various ways in which they are adapted for locomo-
tion in the particular medium in which they live. In its shape
the Fish is admirably adapted for cleaving the water. Spindle-
like in shape, but thicker in front than behind, a Fish resembles

349

3 50 FISHES CHAP.

a double wedge, the thick part of which is represented by the head
and one of the thin edges by the free hinder margin of the caudal
fin. The body is bounded by smooth flowing contour lines, unbroken
by any sharp separation of the body regions from one another, and
with no points of resistance to its forward motion through the
water. The body being thicker in front than behind, and, as seen
in transverse section, broader aljove than below, it follows that its
centre of gravity will be nearer the head than the tail, and nearer
the dorsal than the ventral surface. The dorsal position of the
centre of gravity necessarily renders the equilibrium of the body
unstable, and were it not for the balancing action of the paired
fins the Fish would float belly upwards, as is always the ease
after death. Most Fishes are provided with a membranous gas-
containing sac, the air-bladder, the principal function of which is
to render the Fish, bulk for bulk, of the same weight as the
water, so that in this position of equilibrium, or plane of least
effort, the animal can execute its various locomotor movements
with a minimum expenditure of muscular effort — an advantage
which no other animal possesses.-^ To give stability to the body,
and to steady its course when swimming, the Fish has a dorsal
and a ventral keel, formed by the anal and dorsal fins, which,
like the sliding keel of a yacht, can be raised or lowered as
occasion requires. When these fins are removed the course of
the Fish becomes zigzag, and the animal wobbles.

The organs more directly concerned with swimming are the
tail and the caudal fin, and the pectoral and pelvic fins, but the
relative share which these structures take in the actual pro-
pulsion of the Fish differs greatly. The principal organ of loco-
motion in the typical Fish is the powerful muscular tail, which,
in swimming, is lashed from side to side by the alternating con-
traction of the great longitudinal muscles on opposite sides of the
vertebral column.^ In such movements the tail is first flexed or
bent, say to the right side : this stroke has been termed the non-
effective or back stroke. By a stroke in the reverse direction the
tail is then extended and straightened, that is to say, the Fish
makes the forward or effective stroke. By a rapid succession of
such strokes to the right and left sides alternately the Fish is

' See Chapter XI.

- Pettigrew, Animal Locomotion, Internat. Sci. Series, London, 1874, p. 64 ;
Gadow, Science for All (Cassell), v. p. 302.

LOCOMOTION

351

forced through the water. It is obvious, however, that tlie
extension or effective stroke must have a considerable surphis of
power over the flexion or non-effective stroke, and how this result
is achieved will now be briefly considered. Experiment, and the
observation of Fishes like the Sturgeon, which habitually move
with sutticient slowness to allow the phases of their swimming
movements to be followed without much difficulty, show that in
swimming a Fish throws its body into two opposite and comple-
mentary curves, a cephalic curve formed
by the anterior half of the body and a
caudal curve by the tail. The double
curve enables the Fish always to present
a convex, less resisting or non-bitinsj sur-
face to the water during the flexion of
the tail to the right or left as the case
may be, and a concave or biting surface
during extension, that is when the tail is
straightening itself during the effective
stroke.

Fig. 204, which represents a Fish in
two successive positions while swimming,
will serve to illustrate these conclusions.
A Fish in the position A has its body
thrown into a cephalic concavity directed
to the right and a caudal concave surface
facing the left. The tail is bent to the
right of the line a h, which corresponds
to the axis of the Fish when at rest and
to the course pursued by the animal
when swimming, and is in the position
which it assumes during a flexion stroke,
with its convex non-biting surface directed
outwards and its concave biting surface inwards. The tail is
now ready for an extension stroke, and while this is in
progress it is clear that the concave biting surface of the tail
will meet the water, while at the conclusion of the stroke
the tail will be in a line with a h. At the same time the
cephalic curve has so far diminished that the long axis of
the body for a momentary period will also coincide with a h,
and the Fish is free to advance without impediment. The tail,

Fig. 204.— To illustrate the
mode in which the tail of
an ordinary Fish is used
in swimming. See the
text for the lettering.
(Slightly altered from
Pettigrew.)

35 2 FISHES " CHAP.

however, continues its movement to the left, but now as a flexion
stroke, and assumes the curvature and position indicated in B,
with a reversal in the direction of both the cephalic and caudal
curves, but in the meantime the force of the preceding extension
stroke has forced the Fish along the line a h to the new position
indicated by B. By a rapid succession of alternating flexions
and extensions, during which the tail describes figure - of - 8
curves, the Fish travels in an undulating forward course with a
maximum of propelling power and a minimum of " slip." In
short, the action of the tail precisely resembles the action of the
stern-oar in the operation of sculling a boat.

There are also other considerations which add to the surplus
power of the extension stroke by lessening the resistance of the
water to the flexion or non-effective stroke. During the flexion
stroke the tail fin is less expanded and its area diminished, and
by the rotation of the Fish on its long axis the surface of the
tail strikes the water obliquely, and further, the tail moves with
less rapidity. On the contrary, when the extension stroke is
made these conditions are reversed. The caudal fin is expanded,
the stroke is more rapid, and by the reverse rotation of the Fish
the tail now strikes the water with its flat surface. In other
words, the action of the tail during the two strokes may be com-
pared to the " feathering " of an oar in rowing. Nor is this
all. A Fish in motion through the water produces a suction
current behind it. The current offers but little resistance to the
flexion stroke, inasmuch as the direction of the two coincide,
but during the extension stroke the tail meets the full force of
the current, and consequently its grip and propelling power are
greatly enhanced. There is a striking analogy between the
movements of a Fish's tail in swimming and the action of the
screw of a steamer, but as a propelling organ the former is far
superior to the latter. As we have seen, the tail of a living
Fish can so adjust its shape and surface that it alternately eludes
and grips the water in accordance with the needs of particular
strokes.

The curves into which the body of a Fish is thrown when
swimming are never less than two, but in long-bodied Fishes,
such as the Eels, the number may be increased, and in every
case the curves occur in pairs and are complementary to one
another.

LOCOMOTION 353

Many Fishes can jump out of the water, either in pursuit of
insect food, like the Trout, or to enable them to escape the pur-
suit of their foes, like the Flying-Fish (Uxocoetus), by means of
a single forcible stroke of the tail, when the Fish is in a nearly
vertical position close to the surface of the water. It is thus
that the Salmon executes its remarkable leaps over weirs or up
salmon -ladders when ascending rivers for spawning.

The tail is also used for steering. If kept bent to one side when
the Fish is moving the tail acts like a rudder, and the course of
the Fish is deflected to that side ; or the direction may be altered
by single strokes of the tail to the right or left, according to the
course which the Fish desires to pursue.

In the majority of Fishes the paired fins are probably of
little use for propulsion, and their action in this as in other
functions is not always clear. In the Sharks and Dog-Fishes as
well as in some Teleosts their planes are nearly horizontal when the
fins are extended from the body ; in others they are more oblique,
so that the surfaces of the fins look upwards and backwards, and
downwards and forwards ; and in others again their surfaces are
so nearly vertical that their strokes will be backwards and for-
wards. The pectoral fins also vary in their position on the sides
of the body, being much more dorsal in some Fishes than in
others. The paired fins may act as lateral keels in steadying the
course of the Fish especially when the fins are extended and
their planes are horizontal. They certainly seem to act as
balancers in keeping the Fish on an even keel, and in counter-
acting the tendency of the Fish to turn belly upwards — a result
which is attained by a slight upward and downward movement
of the fins, and particularly of the pectoral fins. A Fish deprived
of its pectoral members sinks downwards at the head and assumes
an oblique position in the water. Eemoval of both the pectoral
and pelvic fins of one side causes the Fish to roll over to that
side ; and if the fins are removed from both sides the animal
turns belly upwards like a dead Fish. The pectoral fins may
also be used for steering : a backward stroke of one fin while the
other is kept folded back against the body will wheel the Fish
round to the opposite side. From the ventral position of its
mouth a Shark is forced to turn over to one side in order to
seize its prey, and this movement of rotation is probably pro-
duced by the down strokes of the pectoral fin of one side. In

VOL. VII 2 A

3 54 FISHES CHAP.

some Fishes it would seem that the pectoral fins may assist loco-
motion by acting as paddles. The 15-spined Stickleback (Gast-
rosieus spinosus) frequently progresses by their aid alone ; and, as
their action can be reversed at pleasure, it is not unusual to see
this Fish move backwards. The fins appear to be rotated or
twisted in spiral movements like the tail when used for swim-
ming, or like the wings of Insects in flying.

It has been mentioned that the function of the median fins
(dorsal and anal) is to give stability to the Fish by acting as
dorsal and ventral keels. This is certainly the case in the
generality of Fishes. Nevertheless, there are exceptional in-
stances in which one, or even both, of these fins are important
swimming organs, acting either as a substitute for a tail which
has become adapted for other uses, or as supplementary to that
organ. Thus, in some of the Syngnathidae (Pipe-Fishes and Sea-
Horses) the small size or absence of the caudal fin, and its use as
a prehensile organ, renders the tail of little or no value as a pro-
pelling organ : hence it is that these Fishes swim by a lateral
undulating movement of the dorsal fin. To enable them to do
this the supporting skeleton presents certain interesting modifica-
tions. In the majority of Teleosts the arrangement of the fin-
muscles, and the nature of the articulation between the dermal
fin-rays and their basal radial supports, which is generally some
form of a hinge-joint, are such as to limit the motion of the rays
to simple elevation or depression in the vertical plane, and no
lateral motion of the fin is possible. But in the Syngnathidae,
as in the Pipe -Fish {Siphonostoma typlile), there is an excep-
tionally mobile articulation between the dermal fin-rays and
the distal radial nodules which their cleft bases embrace and the
bony proximal or basal radials, so that the fin can be flexed or
bent to the right or to the left. In addition to this, by a
change in the insertion of their tendons, the muscles correspond-
ing to the ordinary elevator and depressor muscles of the fin-rays
in other Fishes are capable of producing extensive lateral move-
ments of the fin, or, by contracting in orderly sequence, of bringing
about the characteristic undulating motion of the fin. A similar
mechanism exists in many Plectognathi {e.g. species of Balistes,
Monacantluis, Diodon, Tetrodon and Orthagoriscus) ^ in connexion
with both the dorsal and anal fins, but in these Fishes the

^ Bridge, Journ. Linn. Soc. (ZooL), xxv. 1896, p. 530.

LOCOMOTION 355

action of the median fins in swimming must be regarded as
supplementary to that of the tail.

Swimming is by no means the only form of locomotion in
vogue amongst Fishes. A few, like the Angler-Fishes (Zoj^Jiins),
halntually use the pectoral fins for crawling about the sea-bottom.
The East Indian Goby, Periophthalmus, uses its pectoral fins,
which are bent at an angle like an elbow-joint, for hopping over
sandy fiats left bare by the retreating tide. The Flying- Fish
{Exocoetus), when projected from the water by a stroke of its
powerful tail, expands its large pectoral fins, and, using them
after the fashion of a parachute, floats through the air for con-
siderable distances before returning to its natural medium. The
" Flying Gurnards " {Dactyhpterus) are also capable of short
aerial excursions in a similar fashion. Nor is tree-climbing
beyond the province of a Fish, if credit be given to the assertion
that the Indian " Climbing-Perch " {Anabas scandens) uses its
opercular spines for ascending trees. Many freshwater Fishes
are known to migrate across land from one pool or river to
another, usually during the night. Eels do so by a serpentine
or wriggling motion of their long bodies, but in others the
pectoral fins seem to be the principal organs used for the
purpose, aided, it may be, by a perverted use of the tail.

Sound-producing Organs. — Contrary to popular belief sound-
producing or vocal organs are by no means uncommon in Fishes,
especially in certain families of Teleosts. It is not always easy,
however, to discriminate between involuntary, abnormal, or acci-
dental sounds, and those due to the action of special vocal organs.
There are, moreover, some Fishes which observations have shown
to utter highly characteristic sounds, although the precise nature
of the sound-producing mechanism is at present unknown ; while
other Fishes appear to possess organs which, on anatomical
grounds, are perhaps vocal in function, although nothing is
known of the nature of the sounds they emit. Here those
organs only will be considered which, either with certainty or
with some degree of probability, may be regarded as vocal struc-
tures. For most of our knowledge of these interesting structures
we are indebted to the researches of Sorensen and Dufosse.-*

^ Siirensen, Om Lydorganer hos Fiskc, Copenhagen, 1884 ; Dufosse, Ann. d.
Sci. Nat. Ser. 5, xix. Art. 5, 1874, and xx. Art. 3, 1874. For references to earlier
papers see Sorensen, op. cit.

356

FISHES

(a) Stridulation. — Stridulation as a method of sound-produc-
tion has been recorded in many Teleosts, and one of the most
interesting examples occurs in the singular Indian Siluroid,
(CaUomystax gagata)} In this Fish (Fig. 205) the first five
vertebrae are rigidly connected with one another and with the
skull, mainly through the union of the neural spines of the third,
fourth, and fifth vertebrae, and their articulation with the supra-
occipital 1)one. The united spines together form a high, laterally-
compressed lamina of bone, the hinder jjortion of wdiich is

vertically cleft into two thin
plates separated by an in-
terval sufficiently wide to
receive the first interspinous
bone of the dorsal fin. The
inner surface of each of the
two plates is traversed by a
series of about thirty par-
allel, close-set, vertical ridges,
while the first interspinous
bone is similarly ridged on
both its faces like a double
file. Lastly, it may be men-
tioned that owing to the
width of the intervertebral
ligament between them the
fifth and sixth vertebral
centra are articulated by a joint of unusual mobility. The
action of the mechanism is simple. By the vertical movements
of the sixth and succeeding trunk vertebrae, with the inter-
spinous bones which they support, on the rigid structure formed
by the head and first five vertebrae, the file-like first interspinous
bone moves backwards and forwards, and, by scraping against
the ridges on the inner surfaces of the cleft neural spines, gives
rise to a harsh grating noise, which is particularly unpleasant
when artificially produced. The lateral movements of the trunk
in ordinary locomotion do not affect the mechanism : it is only
when the trunk is alternately flexed and extended in the vertical
plane that the mechanism comes into play and a noise is pro-

^ Haddon, Journ. Anat. and Phys. xv. 1881, p. 322; Bridge and Haddon, Phil.
Trans. 184, 1893, p. 168.

Fig. 205. — Stridulating apparatus of Callo-
mystax gagata. is^, The first iuterspiuous
bone, the lower part of which forms the
double file and fits iuto the interval between
the cleft neural spines «*■■' and ns^ ; is'-, is^,
second and third interspinous bones ; 7is%
ns\ ns^, neural spines of the third, fourth,
and fifth vertebrae ; s^, s", spine-like rays
of the dorsal fin ; so, supra-occipital. (After
Haddon. )

XIII SOUND-PRODUCING ORGANS 357

duced. In the Bull-head (Cottus scorpius) the preoperculum is
modified for stridulation, and in Dndyloptcrus the hyomandiljular
bone ; in other Fishes, as in some Siluroids {e.g. species of Doras),
stridulation takes place between a basal process from the great
spine of the pectoral fin and the wall of a socket in the cleithrum
into which the process is received, or between the small first
spine of the dorsal fin and a roof-like process at the upper ex-
tremity of the first interspinous bone ; also, in a somewhat similar
fashion in the anterior dorsal fin of such widely different Fishes
as certain Trigger-Fishes (Sclerodermi) pertaining to the genera
Balistcs, Monacanfhus, and Triacanthus, Acanthurus chiruryus
(Acanthuridae), the Boar-Fish (Capros aper), Centrisciis scolopax
(Centriscidae), and the Three -spined Stickleback (Gasirosteus
aculeati's) ; and even between the spinose ray of the pelvic fin
and the basipterygium in Triacanthus, Capros, and Gastrostcus.

In the "Drumming" Trigger-Fish {Batistes aculeatus),^ which
frequents the coral-reefs off the Island of Mauritius, stridulation
takes place between the postclavicles and a longitudinally grooved
area on the inner surface of each cleithrum. Both the cleithra
and postclavicles are in intimate relation with the air-bladder,
and the sound produced by friction is apparently strengthened
by the transference of the vibrations to the walls and gaseous
contents of that organ. The passage of the sound -vibrations
to the surrounding medium is facilitated by the fact that for a
portion of their extent the lateral walls of the air-bladder are
in contact with the superficial skin, which visibly shares in
the vibratory movement of the bladder when the characteristic
drumming sounds of Balistes are being emitted.

Stridulating sounds may also be produced by the friction of
the upper and lower pharyngeal teeth, as in a species of Mackerel
{Scor/iher hrachyurus). By the grating of its teeth the Sun-Fish
{Ortliagoriscus mola) is said to emit sounds similar to those pro-
duced by the grinding of the teeth in Pigs and Piuminants ; and
Moseley '^ has remarked of a species of Balistes that the " living
Fish when held in the hand makes a curious metallic clicking
noise by grating its teeth."

(b) Breathing sounds. — Characteristic breathing or murmur-
ing sounds, or " bruits de souffle " as Dufosse terms them, are

1 Mobius, Sitz. d. Berlin. Akad. d. Wiss. 1889, p. 999.
" Xotcs bij a Xaturalist on H.M.S. " Challenger," London, 1879, p. 51.

358

FISHES

produced by a few Teleosts, among which may be mentioned the
Eels, certain Cyprinidae, as, for example, the Carp (Cyprinus carpio),
several species of Loaches (e.g. Misgurnus fossilis and Colitis
taeni(i), and the European Siluroid, Silurus glanis. According
to Dufosse these sounds originate in some cases from the expul-
sion of gas from the air-bladder through the ductus pneumaticus
and mouth, and in others, as in Misgurnus fossilis, they are pro-
duced by the rapid ejection through the anus of bubbles of air
previously taken in at the mouth.

(c) Sounds produced through the agency of muscles connected
with the air-bladder. — In addition to its iisual function as a

Fig. 206. — The air-bladder and elastic-spring-mechanism in .iKf/^cjii^j^eri^iwf/os?**. A,
Cavity of the bladder exposed by the removal of its ventral wall : a.c, anterior
chamber ; cl, clavicle ; c. tr, crescentic process of the tripiis ; I. c, left lateral
chamber ; l.s, longitudinal septum separating the two lateral chambers ; oex,
oesophagus ; ^'••5, pectoral spine ; t.s, the narrow transverse septum which partially
separates the anterior from the two lateral chambers. B, Ventral view of the
anterior vertebrae, to show the elastic springs : es, the oval bony plates in whicli the
elastic springs terminate ; r^, first rib ; t.}/', transverse process of the fifth vertebra ;
v^, fir-st vertebral centrum ; cl, oes, and_^w, as in A. (From Bridge and Haddon.)

hydrostatic organ or " float " the air-bladder is often modified in
various ways in different Teleosts, and adapted for use as a sound-
producing organ.

In the South American Siluroid, Auchenipterus nodosus, the
transverse processes of the fourth vertebra are bent downwards
and backwards, and at the same time become converted into
flexible and highly elastic springs (Fig. 206, B). Their distal
extremities expand into oval bony plates which are imbedded in
the anterior wall of the air-bladder, and often cause the latter
to bulge inwards (Fig. 206, A). From the occipital region of
the skull arise two powerful muscles which pass backwards to

XIII SOUND-PRODUCING ORGANS 359

their insertion into the anterior faces of the two springs. liy
the contraction of these muscles the springs, and consequently
also the front wall of the Ijladder, are drawn forwards ; hut
directly the muscles relax, the elasticity of the springs causes
them to move backwards to their former position, carrying with
them the wall of the air-bladder. Hence it follows that the
rapid alternating contraction and relaxation of the muscles will
impart a vibratory movement to the anterior wall of the bladder
and to the gaseous contents of that organ, with the result that a
sound is produced. As a rule, those Fishes in which an elastic-
spring-mechanism is present have the air-bladder subdivided by
internal septa into a series of chambers freely communicating
with one another ; and no doubt the intensity of the sound is
greatly increased by the vibratory movements of the gases across
the free edges of the septa, and from one chamber to another. The
elastic-spring type of vocal organ is apparently restricted to the
Siluridae,^ and besides occurring in Auc]icni2')teTus is found also
in the South American genera Boras, Oxydoras, Bhinodoras, and
Euanemus ; in the African genera Synodontis and MalojJterurus ;
and in at least four species of the Indian genus Pangasius?
There are also a few Teleosts in which the air-bladder is provided
with special muscles, but, instead of being connected with elastic
springs, the muscles extend from the skull, and are inserted
directly into the wall of the bladder (Fig. 207) ; or, without being
in any way attached to the skeleton, the muscles simply invest
some portion of the surface of the air-bladder. In other Fishes
the air-bladder, without possessing special muscles of its own,
may, nevertheless, be partially invested by tendinous, or partly
muscular and partly tendinous, extensions from the muscles of
the body-wall (Fig. 208), or may be intimately related to certain
muscles connected with the pectoral girdle. Whatever the
precise relation of the air-bladder to its muscles it is probable
that the physiological effect is in most cases the same. By the
rapid alternating contraction and relaxation of the muscles, some
part of the wall of the bladder becomes alternately compressed

^ The elastic -spring -mechanism has been described by several writers, who
liad assigned to it various functions, but Sbrensen {op. cit. pp. 85-91) was the
first to discover its vocal function by observations and experiments on Doras
macuJahis.

'^ The mechanism is apparently absent in one species otPangasius {F. micronema).
Bridge and Haddon, ojj. cit. p. 220.

36o

FISHES

and relaxed in such a way as to initiate a series of vibratory-
movements in the gases of that organ, and so produce definite
sounds. In not a few of the Fishes the cavity of the bladder
is subdivided by external constrictions or by internal septa, or
is complicated by the development of lateral, tubular, caecal
branches ; and hence the ^^ibratory movements of the gases will be
greatly strengthened by their passage across the edges of the
septa, or the apertures of the caeca, and the intensity of the

pti lo m^

Fig. 207. — Ventral view of the air-liladder and
its extrinsic muscle in Platystoma. a.b,
Air-bladder ; a.l.c, left antero-lateral caecum
of the bladder ; b.o, basioccipital ; h.ir, body-
wall in contact with the lateral wall of the
bladder ; c^, centrum of the first vertebra ;
d, clavicle ; d.p, ductus pneuuiaticiTS ; m^
and )/r, extrinsic muscles of the bladder ;
pf.i, post-temporal. (From Bridge and
Haddon.)

Fig. 208. — Air-bladder and its
muscles in Micropogon un-
dulatus. a.b, Air-bladder;
Lb, right lateral caecum ;
ni, m, musculo-tendinous ex-
tensions from the muscles of
the body-wall, which partially
invest the surface of the air-
bladder. (From Sorensen. )

resultant sounds also increased. It will be readily understood
that the nature and quality of the sounds emitted by different
Fishes will necessarily vary with the shape of the air-bladder,
the number and arrangement of the internal septa and the caeca,
and the strength and disposition of the contracting muscles.
In a few Teleosts (Triglidae and Zeidae) sounds are said to be
produced by the rapid vibration of an annular, or centrally-per-
forated, muscular diaphragm, which stretches across the cavity of
the air-bladder.^ Nevertheless, it must be strongly emphasised
that, while in some Fishes the air-bladder and its muscles

1 Moreau, Compt. Ecndus, lix. 1864, p. 436 ; Ann. d. Sci. Nat. (6) iv. 1876,
p. 65.

SOUND-PRODUCING ORGANS 36 I

uiidoul )tedly constitute a vocal organ, there are many others
in wliich the bladder can only be inferred to be sound-producing
from its general agreement in anatomical structure with the
same organ in Fishes where its vocal function has been clearly
proved.

By one or other of these various methods the air-bladder is
either known to be sound-producing, or is believed with good
reason to be such, in the following Teleosts,^ and many others : —
Certain species of the South-American genera of Siluridae,
Pimelodiis, Soruhim, Platy stoma, Piratinga, Cenfro7noch/us, and
Trachelyiypterus ; species of the South-American family Chara-
cinidae ; Aml)Iyo2:)sis sjyelaea, the blind Fish from the Mammoth
Cave of Kentucky (Amblyopsidae) ; among the Syngnathidae,
the short-snouted Sea -Horse {Hippocanvpus hrerirostris) of the
British Coasts ; certain Sclerodermi, such as the Trigger-Fishes,
Balistes vetula, Triacanthus hrevirostris, T. hiaculeatus, and
Monacanthus pardalis, and also some " Coffer Fishes " (e.g.
species of Ostracion) ; some Gymnodontes (species of Diodo?i and
Tetrodon) ; a few Serranidae {e.g. species of Therapon and
Pristipoma) ; species of Holacanthus (Chaetodontidae) and in
Holocentrum sogho (Berycidae) ; such Sciaenidae as the "Drum"
{Pogonias chromis), the " Maigre " {Sciaena aquila), which has
sometimes been taken in British waters, Umhrvna cirrhosa,
Otolithus regalis, and Micropogon undidatvs, and, with more or
less probability, many other species of the same family ; one
species of Zeidae, the John Dory {Zeus ftd^er) ; Batraclius tav
among the Batrachidae ; several species of Gurnards (Triglidae)
belonging to the genera Prionotvs and Trigla\ the so-called
Flying Gurnard, Bactylopterus volitans (Dactylopteridae) ; the
Indian species Ophiocephalus maridius and 0. gaehua (Ophio-
cephalidae) ; amongst the Gadidae, the Cod {Gadus morrhuct)
and the Haddock {G. aeghfinus) ; in such Zoarcidae as the blind
Fish (Zucifuga snhterranea) from the subterranean waters of
the caves of Cuba, and also in some Ophidiidae {e.g. species of
Opiiidium).

In Fishes other than Teleosts, instances of normal sound-
production by special vocal structures are rare. No recorded
instances are known in the Cyclostomes or the Elasmobranchs,^

^ Sorensen, Lydorganer, p. 82, et. seq.
2 Cf. Mettenheimer, Arch. f. Anat. to. Physiol. 1858, p. 302.

362 FISHES CHAP.

but there is evidence that sounds are emitted by Polypteriis
among the Crossopterygii, and by the Dipnoids JVeoccratodus,^
Protoptcrus, and Lepidosiren, although it is not certainly known
how they are produced, or that they may not be the accidental
concomitants of the inspiratory or expiratory action of the lungs
in breathing.

As to the nature of the sounds produced by the air-bladder
and its muscles in different Teleosts, a few examples may be
given.

The sound produced by the elastic -spring -apparatus of a
recently caught Doras maculatus, has been described as a " deep
growling tone," which may be distinctly heard at a distance of
100 feet when the Fish is out of the water. Under like condi-
tions the air-bladder and its muscles, in a species of Platystoma,
emit a similar sound. On the other hand, the sound produced
by the elastic springs of the Electric Siluroid {Mcdopterurus
dectricus) has been compared to the hissing of a cat. The Sea-
Horse {Hippoco.mpus hrevirostris) utters a monotonous sound
analogous to that of a tambour, which is characteristic of both
sexes, but is more intense and frequent in the breeding season.
The " Coffer Fish " {Ostracion trigonus) emits a growling sound,
as also does the "Globe Fish" {Tetrodon Jionckenii) when taken
out of the water.^ The air-bladder and its muscles in the
" Drum " (Pogonias chromis), constitute the most powerful
sound -producing organ yet found in any Fish. The sounds
emitted by the " Drum " are better expressed by the word
drumming than by any other, and have frequently been heard by
persons in vessels lying at anchor on the coasts of the United
States, where these Fishes abound.^ The " Drum " begins its
drumming noise in the spawning season in April, but is rarely
heard afterwards. The " Maigre " (Sciaena aquila), whose musical
performances aue perhaps responsible for the Homeric fable of the
song of the Sirens, is remarkable among Fishes for the variety
of its sounds, which have been compared to bellowing, purring,
buzzing, and whistling.'* The sound is often so intense that it
may be heard when the Fish is at a depth of 18 metres, and the

^ Gtinther, Phil. Trans. 161, 1871, p. 542.

2 Pappe, Synopsis of the Edible Fishes at the Oa2)e of Good Hope, Capetown,
1853, p. 8.

=* Giinther, Study of Fishes, Edinburgh, 1880, p. 427.

'' Day, Fishes of Great Britain and Ireland, London, i. 1880-1884, p. 151.

XIII SOUND-PRODUCING ORGANS 363

ear of the observer two metres above the water ; and it has been
recorded that bj listening for these sounds, shoals of Maigres have
been successfully netted. They rarely emit sounds when isolated ;
but in shoals, during the breeding season, they do not cease to
make sounds with a vigour and a persistency which apparently
must soon wear out their strength. One of the Indian Horse-
Mackerels {Caranx hippos) grunts like a young Pig w^hen captured,
and the sound is repeated whenever it is moved, as long as vitality
remains. A West Indian species of the same family {Argyriosus
vomer) has been observed to produce a like sound, while an
Egyptian Caranx (C. rhonchus) is known to the Arabs as the
" Chakoura " or " Snorter." ^ The sounds produced by the
different British Gurnards, such as the Grey Gurnard {Trigla
gurnardus), the Piper [2\ lyra), the Elleck or Cuckoo Gurnard
{T. cnc'idus), and the Tub-Fish {T. Mrundo), have been compared
to snoring, a sonorous and prolonged grunting, crooning (whence,
perhaps, the term "crooner," l:)y which the Grey Gurnard is known
in Ireland), and croaking. The John Dory {Zeus faher) " also
utters sounds analogous to those of the Gurnards. Among the
Dipnoi Lepidosiren is said to make a growling sound, and Neo-
ceratodus a grunting noise which may be heard at night for
some distance.

"Whatever the nature of the vocal mechanism, it is highly
probable that the sounds produced by Fishes travel to considerable
distances in the water, inasmuch as the latter medium is a far
better conductor of sound than air, and, moreover, the transmission
of sound- vibrations from the air-bladder to the water is facilitated
in many Fishes by the fact that, for a portion of its extent on
each side the Ijladder is in direct contact wdth the superficial skin
behind the pectoral girdle.

From the by no means exhaustive list of examples given above,
it is obvious that in some form or other vocal organs are present
in a considerable number of Fishes, both freshwater and marine,
belonging to widely different groups ; and further, that even in the
same species (e.g. Doras maculatus and other Siluridae), both
stridulation and the action of extrinsic muscles on the air-bladder
may be utilised as a means of sound-production. Certain
Teleostean families like the Siluridae, the Sciaenidae, and the
Triglidae, seem to be distinguished above all others by the pre-

^ Sorensen, oj). cit. - Moreau, 0;;. cil.

364 FISHES CHAP.

valence of some form of vocal organ. Accordino^ to Sorensen, the
first mentioned of the three families includes no less than 68
species, which utilise the air-bladder alone as a sound-producing
organ. Nevertheless, there still remain many Teleostean families,
rich in genera and species, and with an almost world-wide
geographical distribution, in which such organs have not yet
been found.

The advantages which Fishes derive from the possession of
sound-producing organs are sufficiently obvious.

A characteristic feature in the reproduction of most Fishes is
the general absence of any process of conjugation between the sexes,
the eggs being fertilised in the water after their extrusion from
the body of the female, and, consequently, any device which will
facilitate the formation of shoals during the breeding season must
be of great advantage to the species by largely increasing the
chances that the ova will be fertilised, and thus secure the more
successful propagation of the race. Hence it may be concluded
that the vocal organs of Fishes are a means to this end, and that
the sounds they produce are in fact recognition -sounds which
enable Fishes of the same species to congregate together at
periods when reproductive activity is greatest. This view is
in harmony with much that is known of the habits of these
Fishes, especially with the fact that particular sounds are often
characteristic of particular species, and that the sounds are pro-
duced most frequently and with greater intensity during the
breeding season than at any other time. While useful to all
Fishes that possess them, vocal organs are, no doubt, specially
serviceable to those Fishes which, from the nature of their habitat,
can make but little use of their eyes ; and this fact may perhaps
explain the prevalence of such organs in the Siluridae, which
are frequently bottom- or ground-feeding Fishes, and often live in
muddy waters.

The sounds emitted by Fishes may also, in some instances at
least, be warning sounds. Many of the sound-producing Fishes
are provided with exceptionally strong spines either in connexion
with the median and paired fins, as in many Siluridae, or on the
general surface of the body, as in Diodon hystrix. Such spines
are very effective weapons for offensive or defensive purposes,
and are capable of inflicting very severe wounds. The natural
enemies of these Fishes learn by experience or instinct to

ELECTRIC ORGANS

365

associate particular sounds with the possession of dangerous
spines, and warned by the sounds, they refrain from attacking
the owner of tlie spines, to the mutual advantage of Loth.

Electric Organs.
— Electric organs y ^ f° ^

capable of generat-
ing more or less
powerful electric
discharges are pre-
sent in certain
Fishes, both marine
and freshwater.

They occur in a
few Elasmobranchs
(species of Eaia,
Torpedo, and

JTi/pnos), in such
Teleosts as the
African Silurid
Maloptcrurus, the
"Electric Eel"
(Gymnotus), and in
species of Mormy-
ridae (e.g. J/or-
tnyrus). With one
exception electric
organs are com-
posed of metamor-
phosed mus-
cular fibres, and
their nerve-endings
motor end-

FlG. 209.— An Electric Ray {Torpedo) dissected to show its
electric organs. On the left the nerves supplying the
organ are dissected out. The prismatic areas on the
surface of the organ indicate the vertical columns of
electric jilates, of which there may be 500,000 in each
organ. The dorsal surface of the brain is exposed, hr.
Gills ; / spiracle ; 0, eye ; o.e, electric organs ; U mucus
canals ; ;■/•, tri-geminal nerve ; tr\ its electric branch ;
r, vagus ; /, fore-brain ; //, mid-brain ; ///, cerabellum ;
IV, electric lobe of the medulla oblongata. (From Parker
ami Haswell, after Gegenbaur.)

or

plates. The species

of Baia have two

small electric

organs, one on each

side of the terminal portion of the tail.

In Gymnotus ' tlie

1 Ewart, Phil. Trans. 179 (b), 1888, pp. 399, -110, and 5:i9 ; 183 (b), 1893, p. 389.

2 Ballowitz, Arc/t. Mikr. Anat. 1. 1897, p. 686 ; Carl Sachs, Untersuclmngen am
Zitteraal, Leipzig, 1881.

366 FISHES CHAP. XIII

organs are much larger, and extend the whole length of the tail,
which is fully four-fifths of the total length of the Fish. The
Mormyridae also have their feeble electric organs in the caudal
region. In all these Fishes the electric organs are modified
portions of the caudal muscles. In the Torpedo, however, these
organs are two large oval masses, one on each side of the head,
between the gills and the cephalic prolongation of the pectoral
fin (Fig. 209). Malopterurus^ is exceptional in possessing an
electric organ derived from the epidermis and not from the
muscular system. In this Fish the organ envelops nearly the
whole body like a mantle, between the skin and the subjacent
muscles of the trunk and tail. An electric organ is composed
of an immense number of " electric plates " (modified motor end-
plates), abundantly supplied with nerves on one of their surfaces,
and disposed in a series of vertical {Torpedo) or longitudinal
{Gymnotus) columns, separated by septa of connective tissue.
In the active state of the organ in the Torpedo ^ the ventral
surfaces of the plates, on which the nerves are distributed, become
negative to the dorsal, and " the effect in all the plates of a
column when summed up is, therefore, such that the dorsal end
of a column becomes positive to the ventral end." ^ Hence the
current in the form of a succession of shocks passes from the
ventral to the dorsal surface of the head. In Gymnotus, where
the columns are longitudinally arranged, it is the anterior and
posterior surfaces which become oppositely electrified, and the
current passes from the tail to the head. The shock imparted
by an electric discharge is most powerful in GymnotxLsf Malo-
pterurus, and Torpedo, in the order named, and relatively weak in
the remaining genera. The strength of the shock increases with
the number of electric plates included in the circuit. Thus in
Gymnotus the maximum shock is given when the body of the
Fish is so curved that the head and the tail are in contact with
different points on the surface of some other Fish. The discharge
may be reflex or voluntary. Eepeated discharges induce fatigue
and weaken the shocks. Electric organs are powerful offensive
or defensive structures, enabling the Fish to repel the attacks of
enemies, or to stun or kill their prey.

' Ballowitz, Das Electrische Organ des Africanischen Zitterwelses, Jena, 1899.
2 Gotch, Phil. Trans. 178, 1888, p. 487. » Id. op. cit. p. 535. ^ Gf. p. 580.

CHAPTER XIV

NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE

The nervous system consists of the brain and the spinal cord,
and of the cranial and spinal nerves. The rudiment of the
future brain and spinal cord first appears in the embryos of
some Cyclostomes (e.g. Bdellostoma), of Elasmobranchs, and of
Chondrostei (e.g. Acipmiser), and of Neoceratodus among the Dipnoi,
in the form of a tubular medullary canal pinched off from the
epiblast of the dorsal surface of the body. By a somewhat
ditferent method, but with the same final result, a medullary
canal is formed in other Cyclostomes (e.g. Petromyzo7i), in the
Holostei and Teleostei, and in Lepidosiren} from a solid in-
growing keel of epiblast which subsequently becomes tubular.
Later, the medullary canal in the head enlarges, and becomes
divided by two transverse constrictions into three vesicles, the
primary fore-, mid-, and hind-brain, leaving the rest of the canal
to form the spinal cord.

The Spinal Cord. — This portion of the medullary canal
retains a simpler and more uniform cylindrical structure. Its
walls thicken and their component cells become converted into
nerve cells and nerve fibres, but a remnant of the original
cavity remains in the adult as a minute axial canal, with a
ciliated epithelial lining, the central canal of the spinal cord or
myelocoele. In most Fishes the spinal cord extends the whole
length of the body, but in some Teleosts, especially in certain
Plectognathi, it is remarkably short. In a Sun-Fish {Ortliago-
riscus), 2^- metres long, and weighing about a ton and a half, the
cord was only 15 mm. in length, or shorter than the brain.

The Brain. — At an early stage in its embryonic history the

1 Graham Kerr, Quart. Journ. Micr. Sci. -xlvi. 1902, p. 417.
367

368 FISHES CHAP.

brain consists of three simple vesicles, the fore-, the mid-, and
the hind-brain, the first of which lies in front of the anterior
end of the notochord and is therefore pre-chordal in position.
As development proceeds the walls of the vesicles undergo local
thickenings, or they give rise to hollow paired or median out-
growths, and by one or other of these methods the different parts
of the complex adult brain are evolved, while the original
cavities of the vesicles or of their outgrowths persist as a con-
tinuous system of epithelium-lined spaces or " ventricles." ^ The
fore-brain is remarkable for the number and importance of the
parts to which it gives rise. First, it bulges out in front into a
hollow vesicle, the prosencephalon, leaving the rest of the fore-
Ijrain as the thalamenceijhalon or diencephalon (Fig. 210). The
cavity of the prosencephalon is the 2^'^^osocoele, and a pair of
thickenings in its floor form two basal ganglia or corjwra striata.
In many Fishes the prosencephalon retains this simple vesicular
condition, in wiiich case the roof or 7:)a//ii^??i is usually epithelial
and non-nervous ; but in others two hollow lobes grow out from
it in front and give rise to two cerebral hemispheres or paren-
cephala? Both contain extensions of the prosocoele, the para-
coeles or lateral ventricles, from the floor of which the corpora
striata now project. The prolongation of the pallium forming
the roof of the lateral ventricles either remains partially epi-
thelial, or it may acquire a wholly nervous structure and
thicken to an extent which differs greatly in different Fishes.
With the formation of the hemispheres the prosencephalon
and its prosocoele become of secondary importance, and may
cease to be recognisable as distinct from the thalamencephalon
and its ventricle. The lateral ventricles then appear to com-
municate directly with the third ventricle by two apertures, the
foramina of 3Iunro. The forward growth of the brain is com-
pleted by the development of two hollow lobes, the olfactory lobes
or rhinencephcda, each of which contains a ventricle or rhinoeoele
communicating behind with the prosocoele, or, if hemispheres
are present, with the corresponding lateral ventricle. Scarcely

^ For the nomenclature of the brain and its cavities see T. J. Parker, Nature,
XXXV. 1886, p 208 ; and Parker and Haswell, Text-Book of Zoology, London, 1897,
ii. p. 94.

"^ It is iiossible tliat the prosencephalon is merely the bulging anterior part of
the thalamencephalon ; if this be so the hemispheres are really paired outgrowths
from the thalamencephalon.

NERVOUS SYSTEM

369

loss complicated, and perhaps even more interesting from a
morphological standpoint, are the structures arising out of the
thalamencephalon. By thickenings of its lateral walls two large
ganglia, the optic tJialami,ave formed, and on the inner or dorsal
aspect of each of these a ganglion kcibenulae is developed. From

F.B

M.B

210. — Diagram of the general structure of the hrain in Craniates. A, vertical
longitudinal section ; B, dorsal view showing the brain cavities on the right side.
c. Cerebellum ; c.c, central canal of the spinal cord ; c.h, cerebral hemispheres ; c.s,
corpus striatum ; F.B, fore-brain ; f.rn, foramen of Munro ; H.B, hind-lirain ; in,
infundibulum ; l.r, lateral ventricle ; m, mesocoele ; M.B, mid-brain ; m.o, medulla
oblongata; o.l, olfactory lobe; op.l, optic lobe; op.t, optic thalamus; jj, jiara-
jihysis ; pc, prosocoele ; pn.o, pineal organ ; p.o, parietal organ ; pr, prosen-
cephalon ; j)t, pituitary body ; rh, rhinocoele ; sp.c, spinal cord ; s.v, saccus vascu-
losus ; th, thalamencephalon ; iii, iv, third and fourth ventricles. (After Parker
and Haswell.)

the sides of the thalamencephalon the primary optic vesicles are
derived, which later become transformed into the retinal parts
of the paired eyes and the optic nerves. Besides the optic
vesicles there is a second pair of embryonic outgrowths which
arise from the roof of the thalamencephalon. These outgrowths
form stalked vesicles and represent a pair of degenerate visual
VOL. VII 2 B

370

FISHES

orgaus. Usually they become so displaced that the left one lies
iu front of the right, and they appear as if median. The subse-
quent fate of the vesicles differs greatly in different Craniates.
Both persist in the Lamprey, the right vesicle to some extent
retaining its primitive visual function as a imrietal eye and
directly overlying the left oxijineal vesicle. In Elasmobranchs the
two unite to form a glandular organ, the so-called 'jjinecd hocly of
the adult, and in Teleosts the left vesicle disappears, leaving the
right as a jjineal body.^ There is also an embryonic median
outgrowth from the roof of the prosencephalon, the ^^arcq^liysis,
which soon disappears and whose significance is not known. A
median hollow downgrowth from the floor of the thalamencephalon
forms the infundibulum, which becomes attached to a caecal
diverticulum from the roof of the mouth. With rare exceptions
the diverticulum loses all connexion with the mouth, and, as the
lyituitary hocly or hypojdiysis, it appears as an appendage to the
extremity of the infundibulum. In the Crossopterygii the con-
nexion is retained even in the adult by means of a slender canal
extending from the pituitary body and opening into the oral
cavity. Laterally, the base of the infundibulum grows out into
a pair of rounded lobes, the lohi inferiores, and distally into a
thin-walled glandular sac, the saccus vasculosus, which lies just
behind the pituitary body. The cavity of the thalamencephalon
persists as the third ventricle or diacoele. The parts of the brain
developed from the mid-brain and the hind-brain are much less
complicated, and, except for variations in size, they present a
fairly uniform character in most Fishes.

In the mid-brain the roof bulges out into a pair of ojjtic
lobes, and by the growth of lateral thickenings in its floor two
thick strands of longitudinally disposed nerve fibres, the crura
cerebri, are formed. The cavity of the mid-brain remains as the
mesocoele, and from it an extension may be prolonged into each
optic lobe.

From the hind-brain are formed the cerebellum or epen-
cephalon and the medulla oblongata or metencephalon, the former
as a dorsal bulging, the latter as a ventral thickening. Except
where the cerebellum is developed the dorsal wall remains
epithelial, and forms the roof of the persistent cavity of the

^ In Lizards either of the two vesicles may become a parietal eye (Dendy,
Quart. Journ. Micr. Sci. xlii. 1899, p. 111).

XIV

NERVOUS SYSTEM

371

hind-lirain, the fourth ventricle or Tiietacoele, which retains its
primitive continuity with the central canal of the spinal cord.
Lateral lobe-like outgrowths from the dorsal colunms of tlie

■medobl —

Nv.8

med-obi

Fig. 211. — Dorsal (A) and ventral (B) views of the brain of Pcfronn/zon mnriin/s.
ch.2>f-1. Anterior choroid plexus forming the roof of the prosencephalon and thalim-
encephalon ; ch.pl.2, aperture in the roof of the mid-brain exposed by the removal
of the middle choroid plexus ; ch.pl.'Z, the fourth ventricle exposed by the removal
of the posterior plexus ; cr.crh, crura cerebri ; crh, cerebelhini ; crh.h, cerebral hemi-
spheres ; dien, thalamenoephalon ; i)if. infundibulum ; l.gn.hb, left ganglion
habenulae ; med.oM, medulla oblongata ; nv.\, olfactory ; nv.2, optic ; nv.Z, oculo-
motor ; WW, 5, trigeminal ; and nv.%, auditory nerves ; olf.l, olfactory lobes ; opt. I,
optic lobes ; pn, pineal organ ; r.gn.hb, right ganglion habenulae. (From Parker
and Haswell, after Ahlborn. )

medulla are conspicuous structures in some Fishes, and are
known as coiyora restiformia. The paired portions of the brain
are connected across the middle line by a series of transverse
commissures. The more important modifications of the brain in
Cyclostomes and Fishes will now be briefly dealt with.

FISHES

In the Cyclostorae Petromyzon there is a small prosencephalon
with an undivided prosocoele, and on each side of it a small
cerebral hemisphere which appears as a mere appendage to the
much larger olftictory lobe (Fig. 211). The prosocoele divides
in front into two outwardly directed branches, and of the two
diverticula into which each branch divides one extends as a

lateral ventricle into the hemisphere
of its side, and the other as a rhino-
coele into the corresponding olfactory
lobe. The ganglia habenulae are
unusually large, the right one being
larger than the left. The optic
lobes are large, but not obviously
double. So small is the cerebellum
that it seems to be little more than
a narrow transverse band crossing
the fore-part of the fourth ventricle.
The roof of the brain is largely
epithelial, especially in the prosen-
cephalon, the thalamencephalon, and
the hind-brain. Over these epi-
thelial areas the pia mater is un-
usually vascular and forms a series
of " choroid plexuses." The ventri-
cular system is complete and con-
tinuous. By contrast with the

^'^I'xinc^ is

Fia. 212. — Dorsal view of the brain
of Myxine. c.r, Corpora resti-
forniia ; m.o, medulla oblongata ;
n.p, naso- pituitary canal; uLo,
olfactory organ enclosed in its
fenestrated cartilaginous capsule ;
op.l, optic loVies ; pr, prosen-
cephalon ; s, s, dorsal roots of
sjiinal nerves ; sji.c, spinal cord ;

th thalamencephalon. (From Lamprcy the brain of

Wiedersheini, alter Retzius. ) j. ./

very primitive, more so perhaps
than in any other Craniate (Fig. 212). In a dorsal view the
brain is divided into four pairs of laterally expanded and longi-
tudinally compressed lobes by a median longitudinal fissure and
three transverse fissures. The two anterior lobes are little more
than the thickened anterior wall of the thalamencephalon,
although, judging from their histological structure, they represent a
very imperfectly differentiated prosencephalon and olfactory lobes.
The second and largest pair constitute the thalamencephalon.
The last two pairs of lobes represent a transversely divided pair
of optic lobes, or " corpora quadrigemina." There is a large
medulla oblongata with a pair of corpora restiformia, but the
^ Holm, Morph. Jahrl. x.\i.\. 1901, p. 365.

NERVOUS SYSTEM

373

cerebellum is entirely absent. The ventricles are siilijeet to
some individual variation. Third and fourth ventricles are
generally recognisal)le, either as isolated cavities or connected
by a remnant of the mesocoele. In the feeble development of
the prosencephalon, in the striking preponderance of the mid-brain

Fig. 213. — The brain of a Dog- Fish (ScyUium canieuln). A, dorsal view ; B, ventral
view. The choroid plexuses covering the roof of tlie third and fourth ventricles
have been removed, h.o, Olfactory lobe ; ep, origin of the stalk of the pineal body ;
f.b (in A), prosencephalon ; f.b (in B), cerebral hemispheres ; /r, fourth ventricle ;
^. 6, cerebellum ; h.p, pituitary body; i.f, lobi inferiores ; m.h, optic lobes; m.d,
medulla olilongata ; sc, saccus vasculostis ; th, thalaniencephalon ; t.o (i) olfactory
peduncle ; i.-x. cranial nerves. (From Wiedersheim.)

over the rest of the brain, and in the absence of a cerebellum,
Myxine is iinicpie amongst Craniates.

In Elasmobranchs among Fishes the brain attains a much
higher grade of structure. In Scyllium (Fig. 213) there is
a large prosencephalon, and directly in front of it a pair of
imperfectly differentiated cerebral hemispheres, while from its
aiitero-lateral regions the large olfactory lobes arise. The proso-
coele divides in front into four diverticula, of which the two

FISHES

inner ones extend into the hemispheres as lateral ventricles,
and the two outer as rhinocoeles into the olfactory lobes (Fig.
214). In connexion with the infundil)nluni there is a pair of
sacci vasculosi, consisting mainly of gland-tubules, opening into
the infundibular cavity.^ The cerebellum is exceptionally large,

but it does not form a " valvula cere-
belli." Large ear-like corpora resti-
formia are present. The third and
fourth ventricles alone retain an epi-
thelial roof in relation with choroid
plexuses.

In all essentials the brain of the
Holocephali is a repetition of the
Elasmobranch type, more especially of
the elongated form seen in Notidanus.
Indications of a higher grade of struc-
ture are, however, to be seen in the
reduction of the prosencephalon which,
with its prosocoele, is now scarcely
distinguishable from the thalamen-
FiG 914 — Hciriznntai 1 o-"t ccphalon and its ventricle ; and in the
dinai section of brain of c%i7o- more complete differentiation of the

scyl/inm, to show the ventri- i i i ■ i r i.i

cies; semi-diagrammatic, cer, Cerebral hemispheres from one another
Origin of cerebellar ventricle and from the rest of the brain. Large

or epicoele : dia, third ven- n -n i , •<•

tricie ; Her, mesocoeie ; vieta, inHed corpora restifomna are con-
fourth ventricle ; opt, opto- spicuous structures ou cach side of the

coele, or cavity of an optic t n i i , -n • i i i

lobe ; para, lateral ventricles ; medulla oblongata. LeSltleS the USUal

^w«. prosocoele ;rA,rhinocoele. intra-crauial pituitary body, there is

(From Parker and Haswell.) . .

also a separate extra-cranial portion
lodged in a pit on the ventral surface of the basis cranii : in the
embryo the two are continuous.

In the Teleostomi the brain is distinctly of a more primitive
type than in any other Fishes (Fig. 215)."^ The most striking
feature is the absence of cerebral hemispheres, the evolution of
the primary fore-brain proceeding no farther than the formation
of an undivided prosencephalon with a non-nervous roof, and
a prosocoele which forms a continuous cavity with the tliird
ventricle, or at the most is only separated from it by an infolding

^ The sacci jirobably .secrete the liuid contents of the ventricles.
- Haller, 3Io'iph. Ja.hrh. xxvi. 1898, p. 345.

NERVOUS SYSTEM

375

of the epitlieliul roof or velum transversuni. Amongst other
diagnostic characters may he mentioned tlie predominance of the

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mid-brain over the other divisions, the anterior extension of
the large cerebellum into the mesocoele as a " valvula cerebelli,"

376

FISHES

and the absence of corpora restiformia. This type of brain is
most strongly marked in the Teleostei, but in other Teleo-
btomes some, like Acipenser} are typically Teleostean in this
respect (Fig. 216), while others, such as Lepidosteus, have
small cerebral hemispheres with lateral ventricles as well as a
prosencephalon.

The most obvious feature in the brain of the Dipnoi is the
great development of the cerebral hemispheres. In this respect
these Fishes approach the Amphibia, but in other features of brain-

pn.o.

Fig. 216. — Vertical longitudinal section of the brain of a Sturgeon {Acijjoiser nUhenus.)
c.p. Posterior commissure ; c.r, cranial roof; vie, mesocoele ; op.ch, optic cliiasma ;
ja.ch.p, posterior choroid plexus; v.c, valvula cerebelli ; v.t, velum transversum ;
r.iii, r.iv., the third and fourth ventricles ; other lettering as in Fig. 210. (From
Gorouowitsch.)

structure they present points of agreement with most other
groups of Fishes without being closely related to any one of them.
In Protopiterus^ (Fig. 217) the hemispheres are quite distinct
except behind, and the walls of their spacious lateral ventricles
are entirely nervous. Olfactory lobes are sessile on their anterior
extremities, and behind and below they enlarge into ventral lobes
which probably represent the hippocampal lobes of the higher
Vertebrates. A vesicular pineal body at the end of a slender
stalk overlies a singular conical projection from the roof of the
thalamencephalon or " pineal pillow." The optic lobes form a
single oval body, and, as in Petromyzon and the Amphibia, the

^ Goronowitsch, Morph. Jahrh. xiii. 1888, p. 427.

- Burckhardt, Das Central-Nervensystcm v. ProtoiJtcrus anncdens. Berlin,
1892.

XIV

NERVOUS SYSTEM

177

cerebellum is very small. A posterior choroid plexus covers
the roof of the fourth ventricle, and an anterior plexus in con-
nexion with the roof of the thalamencephalon projects downwards

Fig. 217. — Dorsal (A), ventral (B), and lateral (C) views of the brain of P i-otoiitcms
aniieciens. C, Cereljellum ; C.II, cereliral hemisphere; D.S.K, brandies of tlie
sinus endolyniphaticus ; //(, iiifundibulum ; L.I, lobi iuferiores ; J/.O, medulla
oblongata ; O.L, olfactory lolje ; Op.L, optic lobe ; P, pituitary body ; P.B, "pineal
pillow" ; S.E, sinus endolyniphaticus ; Sp.c, spinal cord ; Sp.n, spinal nerve ; Yel,
velum transversum; Z, pineal body ; IV. V., fourth ventricle ; ii., iii., iv., v., vi., vii.,
viii.l, viii.2, viii. 3.4, i.x., and x., roots of the cranial nerves. (From Burckhardt.)

into the third ventricle, and is also prolonged forwards into each
lateral ventricle. lu Neoceratodus ^ the brain is certainly more
primitive and distinctly less Amphibian. As compared with
Protopterus the olfactory lobes and the cerebellum are larger, and
the optic lobes are paired. The smaller hemispheres are nou-

^ Sanders, Ann. Xat. Hist. (6; ill. 1889, p. 157.

378 FISHES CHAP.

nervous dorsally and mediaulj, the roof and inner wall of each
being formed by an extension of the thick, glandidar choroid
plexus which forms the roof of the thalamencephalon.

The Spinal Nerves. — Tlie spinal nerves of Cyclostomes (e.g.
Petromyzoii) consist of a series of dorsal nerves arising on each
side from the dorsal surface of the spinal cord, and of a similar
double series arising from the ventral surface, the dorsal nerves
regularly alternating with the ventral nerves. Each myotome is
supplied by a dorsal and a ventral nerve which pass separately to
their peripheral distribution in the skin and muscles. In Fishes,
as in the higher Vertebrates, each dorsal nerve, now termed a
dorsal root, enlaro-es into a crancrlion and then unites, either before
or directly after issuing from the neural canal, with the next
ventral nerve or ventral root in front to form a main spinal
nerve. At the same time the spinal nerves of opposite sides
tend to form pairs in the same transverse plane. After the
union of the two roots the spinal nerve divides into three typical
branches : a dorsal nerve (ramus dorsalis), and a ventral nerve
(ramus ventralis), both of which include somatic sensory or
afferent fibres, and somatic motor or efferent fibres, for the
innervation of the skin and muscles of the dorsal and lateral
portions of a myotome ; and a visceral branch (ramus visceralis),
composed of afferent and eflerent visceral fibres, which supplies
the adjacent viscera (alimentary canal and its glands and blood-
vessels), and helps to form the sympathetic nervous system.^ The
somatic afferent and the visceral afferent fibres enter the spinal
cord by the dorsal roots, the somatic efferent leaving the cord
through the ventral roots, although the visceral efferent fibres
traverse both roots. In the region of the paired fins more or fewer
of the rami ventrales unite to form a plexus, the brachial or the
pelvic plexus, from which the nerves to the fins take their
origin.

The Cranial Nerves. — It is usual to describe the cranial
nerves of Cyclostomes and Fishes as consisting of ten serially
disposed pairs, viz. : the olfactory (i.), oi^tic (ii.), oculomotor (iii.),
trochlear (iv.), trigeminal (v.), ahducens (vi.), facial (vii.), auditory
(viii.), glossopharyngeal (ix.), and the vagus (x.) Like the spinal
nerves, the cranial nerves collectively include somatic sensory
(general cutaneous) and motor fibres, and also visceral sensory

^ See Gaskell's important paper, Journ. Physiol, vii. 1886, p. 1.

CRANIAL NERVES 37

ami motor fibres, all of which have their own special centres in
the brain, but the proportions of these nerve components dilfcr
greatly in different nerves. Certain preoral nerves (iii., iv., and
vi.) are exclusively somatic motor; others (i. and ii.) are .special
sensory nerves for the olfactory and visual organs ; but most uf
the other cranial nerves include several components, and are
therefore " mixed " nerves. Besides these components some
cranial nerves include also a quasi-independent system of nerve-
fibres, which converge from certain cutaneous sense-organs to an
independent centre in the medulla oblongata, the Uibei' acusticinn}
and is probably derived from the general cutaneous system of
nerve components. Such nerve fibres, including also the auditory
nerve, which has its origin from the same centre, constitute the
lateralis system. Perhaps the most striking feature in the post-
oral cranial nerves is the predominance of the visceralis or
sympathetic system over the somatic. Omitting the lateralis
fibres and a relatively few somatic sensory fibres, visceral fibres,
sensory and motor, are the principal components of all these
nerves, includincr v. but excluding viii. The reason for this is
to be found in the fact that splanchnic or visceral muscles in
relation with the jaws and branchial arches have usurped the
place of somatic muscles in the muscular system of the head.
For developmental and other reasons the olfactory and optic
nerves stand in a category of their own, and the same may be
said of the third, fourth, and sixth nerves, which innervate the
muscles of the eyel:)all. The remaining nerves, all of which
have their origin in the medulla oblongata, possess certain
features in common, and as they are related to the gill-clefts in
such a w"ay that each forks over a cleft, they may be conveniently
distinguished as " branchial " or " hranchiomeric nerves." A typical
branchial nerve consists of (1) a po^incipal ganglion near the
origin of the nerve from the Ijrain ; (2) a main trnnk wdiich
gives off' (3) a somatic sensory branch or dorsal nerve to the
skin ; (4) a imlcdinc nerve (visceral sensoiy) to the oral or
pharyngeal mucous membrane ; (5) an epihranchial ganglion
which is associated with a transitory embryonic e2nhranchial
sensory organ at the dorsal border of a brancliial cleft; (G) a

1 Herrick, Journ. Xcnr. ix. p. 153 ; Cole, Trans. J!oy. Soc. Eciinb. .\x.\viii.
1S96, p. C.-Jl : Id. Trans. Linn. Soc. vii. 1898, p. 115, to which an excellent

Inbliograpliy is appended.

38o

FISHES

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CRANIAL NERVES ^8 I

prc-hranchial nerve (visceral sensory), skirting the anterior
margin of a cleft in its ventral course; and (7) a 2^osi-hranchial
liraneh (visceral motor) similarly related to the liinder margin.

The first six cranial nerves resemble those of the higher
Craniates in their mode of origin from the brain, in the pliysio-
logical nature of their component fibres, and in their peripheral
distribution, and therefore they need not be specially referred to
here. The principal branches of tlie fifth or trigeminal nerve
are shown in Fig. 218. Comparing this nerve with a ty])ical
branchial nerve it would seem that the profmidus and si'jjer-
jicialis oj^hthali/iic nerves are dorsal nerves; the maxillaris and
inandihularis, pre- and post-branchial branches, respectively, in
relation with the modified gill-cleft which forms the mouth,
while the branch to the oral surface represents a ^?rt/o^i7ie nerve.
The most important of the distinctive features in the cranial
nerves of Fishes are to be found in the relations of nerves vii., ix.,
and X. to branchial clefts, and in the lateralis system of nerve
components and its association with the lateral line sensory
organs. The seventh or facial nerve is an exceptionally interest-
ing nerve. Besides the usual components of a typical branchial
ner^'e certain of its so-called branches are wliolly or largely
derived from the lateralis system. For tliis reason the nerve
may be said to consist of two portions, the facial 'pro'per, or
those fibres which constitute the facial nerve in air-})reathing
Craniates, and the lateralis hranches which solely innervate
lateral line sense-organs, and are therefore peculiar to aquatic
forms. The facial proper has a ganglion {tlie facial or geni-
culate ganglion) on its root, and on entering the orbit, after
traversing the cranial wall it gives off a ]jalatine nerve. Just
over the spiracle a pre-lranchial nerve, the representative of the
chorda tympani of ]\Iammals, leaves the main trunk, and passes
ventrally in relation with the anterior wall of the spiracle to
its ultimate distribution in the walls of the mouth cavity.
The main trunk, now called the ramu>> Jiyomcmdihiilaris,
then pursues a ventral course beliind tlie spiracle as a
post-branchial nerve, and certain of its mainly motor branches
which pass downwards in connexion with the hyoid arch
supply the muscles of that arch, and, if an operculum
is present, the opercular muscles as well. The lateralis
portion of the facial includes the following principal branches,

382 FISHES CHAP.

each of ^vhicll may liave a ganglion on its root: (1) an
ophtlialmicus supe7i/icialis ; (2) a huccalis nerve with its ramus
oticus ; and (3) external mandibular nerves which course in the
ramus hyomandibularis. The addition of the great lateralis
nerve, which is usually described as the lateral branch of the
tenth nerve, and of the eighth or auditory nerve which supplies
the auditory organ, completes the enumeration of the main
factors of the lateralis system. The ninth or glosso-pharyngeal
nerve, perhaps the -most typical of all the branchial nerves,
has pre- and post -branchial branches which enclose the hyo-
branchial cleft. Its palatine nerve usually extends forwards
and anastomoses with the corresponding branch of the seventh,
thus forming a connexion (Jacohson's anastomosis) between the
two cranial nerves. In some Elasmobranchs and Teleosts fibres
derived from the dorsal branch of the ninth nerve innervate a
few sense-organs of the lateral sensory canal of the head, and
hence that nerve sometimes contains lateralis fibres. The tenth
or vagus is a compound nerve. Besides the great lateralis nerve
generally associated with it, the vagus includes as many
typical branchial nerves as there are branchial clefts behind the
hyo-branchial cleft, and in Elasmobranchs and in Chimaera these
nerves have independent origins from the medulla oblongata.
Each nerve has the typical structure, a ganglionated trunk which
forks over a gill-cleft into the usual pre- and post-branchial
branches, and palatine branches to the pharyngeal walls. In
the Dipnoi the lateralis nerve is connected with the superficial
ophthalmic branch of the seventh nerve by a commisural nerve
which curves across the outer face of the auditory capsule. A
somewhat similar anastomosis is also present in Petromyzon. The
vagus also includes a large raimts intestinalis, which in Elasmo-
branchs, at all events, has a distinct ganglionated root. The
nerve forms characteristic plexuses on the oesophagus and stomach,
and in Cyclostomes its branches may extend nearly the whole length
of the intestine. In Ganoids and Teleosts there is an interesting
nerve known as the " lateralis accessorius." It is a compound
nerve, and owes its formation to the union of somatic sensory
fibres derived in succession from dorsal branches of the v., vii.,
ix., and x. nerves, and also from the corresponding branches
of a varialile number of spinal nerves. The finer branches of the
nerve are distributed to the skin of one or more of the fins, or

SENSE-ORGANS 38,

even, as in Gadus, to all the fins, especially to the numerous
"end -buds" which are present on those organs. In many
Fishes a variable numljer of the anterior spinal nerves (sjnno-
occijntcd) perforate the occipital region of the skull. They
probably represent the ventral roots only of the ordinary spinal
nerves of this region.

Sense-Organs

The Cutaneous Sense-Organs. — These organs, the most re-
markable and certainly the most characteristic of the sense-organs
of Cyclostomes and Fishes, are Ijud-like groups of epidermic cells in
relation with the ends of sensory nerve fibres. Each consists of a
central core of sensory cells, provided with terminal cuticular sensory
hairs, and surrounded by a zone of supporting and mucus-secret-
ing cells which lea^'e the hairs exposed at the apex of the bud.
Two kinds of these organs can be distinguished, which differ in
their innervation and in their position in the skin. Of the two,
the so-called end-huds are the more primitive. They occupy a
superficial position in the epidermis, and their sense-cells are as
long as the supporting cells. They are present in Cyclostomes
and Elasmobranchs, and especially in Teleosts, where they are
irregularly distributed over the surface of the body, on the fins,
lips, and barbels, and also in the epithelium of the mouth and
pharynx. In the Dipnoi they are limited to the oral cavity, and
in the higher Craniates they become taste-buds.^ Their somatic
sensory nerves'- are derived from the vii., ix., and x. cranial nerves,
and the lateralis accessorius. In the second type, usually
called " nerve-eminences," the sensory cells are shorter than the
supporting cells, and they are always innervated by the lateralis
system. AVhen first developed in the emlu-yo they are quite
superficial, like end -buds, but later the epidermis in which
they lie sinks inwards so as to line a series of pits, closed sacs,
tubes, open grooves, or closed canals. Pit-organs, so abundant on
the head and trunk of Teleosts (Fig. 220), are simple epidermic
pits with insunken nerve-eminences, disposed in groups or in

1 For a discussion of the relations of "end -buds" to the sense of taste in
Fishes, see Bateson, Journ. Marine Biol. Ass. i. (N.S.) 1890, p. 225 ; and Herrick,
U.S. Fish Commiss. Bull 1902, p. 237. In the latter paper a bibliography of the
subject is given.

- These fibres are included in the visceral sensory or "communis" system by
Herrick.

384 FISHES CHAP.

lines (accessory lateral lines) or irregularly distributed. The
" Spalt-pajnllen " of Elasmobranchs are pit-organs in which the
orifice of the pit is reduced to a slit. The more deeply-seated
Savi's vesicles on the ventral surface of the Torpedo, and the
newc-sacs of Ganoids, are similar organs converted into closed
sacs and pinched off from the rest of the epidermis. Lorenzmis
ampullae or mucus canals, which are found in definitely located
groups on the lateral and upper surfaces of the head in Elasmo-
branchs, may perhaps be compared to pit-organs prolonged inwards
to form subcutaneous tubes, each of which terminates in a
radially -septate, chambered dilatation or ampulla, containing
groups of sensory cells.

Besides the more diffusely scattered sense-organs there are
others which become disposed in definite lines along the sides
of the body and on the head, and, enclosed in grooves or closed
canals, constitute the highly characteristic lateral line system of
Cyclostomes and Fishes.^ The auditory organ must also be included
as a specialised portion of this system. Both organs are inner-
vated by the lateralis system, and both arise from a common
rudiment in the embryonic epidermis in the position of the
future auditory organ. This rudiment grows backwards along
the side of the body in the form of a cord of cells differentiated
from the epidermis, and also forwards, where it soon divides
into the rudiments of future supra-orbital and infra-orbital
canals. Sense-organs are differentiated at intervals along the
line of the cord ; and in the body, but not on the head, they
frequently exhibit a segmental disposition. Each sensory organ
then sinks down into a short epidermic groove, which by the
subsequent meeting of its lips becomes a canal detaclied from
the epidermis. The short canals then become continuous, leaving,
however, an externally opening primary pore between every two
consecvitive canals, and the result is a continuous canal having
sense-organs imbedded in its epidermic lining and connected with
the exterior by pores at intervals (Fig. 219).^ The' enclosure of
the canals in the scales of the lateral line of the trunk or in
special drain-pipe ossicles on the head, and the dichotomous

^ See previously cited papers by Herrick and Cole ; also Ewart, Trans. lioy. Soc.
Edinb. xxxvi. 1892, p. 59 ; Collinge, Quart. Journ. Micr. Sci. xxxvi. 1894, p. 499 ;
and Herrick, Journ. Coinp. Keurology, xi. 1901, p. 177.

- Allis, Journ. Moryh. ii. 1889, p. 463.

SENSE-ORGANS

385

subdivision of tlie primary pores into groups of surface-pores,
complete the evolution (jf the system in its more advanced
condition. Typically, the lateral line system consists of certain
canals or grooves, usually Init not invariably continuous, and
defined by their innervation, (i). a lateral canal extending along
the side of the body and the hinder part of the l)ead, and liaviiig
its sensory organs supplied by the great lateralis nerve (Fig.
220); {n.) -A supra-orhital canal passing forwards over the eye
and innervated by the superficial ophthalmic branch of the
facial nerve; (iii.) an infrct-orhifal canal supplied liy the buccalis

Fig. 219. — Vertical lougitiuliual section through the lateral caual oi' Am ia calra. l.n,
Lateralis nerve with its branches, n, n, to the sensory organs, s.o, s.o ; p,p,2h external
jiores ; s.c, sensory canal ; s, s, scales of the lateral line. (From Wiedersheim, alter
Allis.)

and otic branches of tlie same nerve ; and (iv.) a liyo-mandihular
or opcrcvlo-niandihular canal, situated on the outer side of the
hyoid region, and thence prolonged downward and forward in
relation with the lower jaw, and innervated by the external
mandibular branches of the facial nerve. The hyo-mandi-
Ijular canal is sometimes distinct from the other canals, as in
Elasmobranchs and some Teleosts (Fig. 220); and in certain
North American Siluroids the same may be said of the supra-
orbital. But, as a rule, the infra- orbital is continuous behind
both with tlie lateral and the supra-orbital canals, while the
hyo-mandiljular canal joins the infra-orbital, or, exceptionally,
VOL. VII 2 c

386

FISHES

the supra-orbital canal. Transverse commissural canals often
connect the lateral and supra-orbital canals of opposite sides
across the dorsal surface of the head, and the corresponding
infra-orbital and hyo-mandibular canals may also be continuous
at the extremity of the snout or at the mandibular symphysis.

Throughout their extent the canals communicate with the
exterior by pores, or short canals terminating in pores, or by
branched canals ending in groups of pores. In Cyclostomes ^ the

lateral line system
is represented by
pit-organs disposed
as in Fishes, and
innervated by a
true lateralis nerve.
Some Elasmo-
branchs have the
lateral canal of the
trunk represented
by an open groove
protected by mar-
ginal denticles.
Chimaera is more

Fig. 220. — Sensory canals of the left side of the head of iji-imitive still in

(rculus virens. e, Eye ; i.o, infra-orbital canal (dotted) ; •'■ .

l.r, lateral canal (oblique shading) ; n, nasal apertures ; this respect, for OU

nj), operculum ; ojt.m, operculo-niandibular canal (longi- f Ug liead 'IS well
tudiual shading) ; p.o, pit-organs ; s.o, supra-orliital canal

(cross-hatched); s.o.c, supra - orbital commissure; sJ, aS Oil the body the

supra-temporal branch ; U, tubuli by which the canals „p..„r,,.„ nro-nn« nvp

communicate with the exterior. (From Cole.) seilSOl} Olgdlisaie

in open grooves.
Amongst Fishes these organs are most primitive in the Dipnoi,
where they retain their superficial position in the epidermis. In
Teleostomes the lateral canals perforate the scales of the lateral line,
and at intervals they open externally by simple or multiple pores
which perforate the scales. On the head they are more or less
completely enclosed in special ossicles which either remain distinct
or fuse with certain of the adjacent dermal or cartilage bones of
the skull. The use of the lateral line organs is not certainly
known. They occur only in Fishes and Amphibia, and as blind
Fislies are able to avoid obstacles with the greatest ease when
swimming, it is possible that these organs enable their possessors
^ Jolinston, Journ. Comp. Neurology, xii. 1902, p. 2.

SENSE-ORGANS 387

to appreciate undulatory inoveinents in water in the shape of
reflex waves from contiguous surfaces or objects.^ Their great
antiquity is shown by their existence in most of the Hetero-
straci, and in the Antiarchi and Artlu'odira, although they have
not yet been discovered in the Osteostraci.

The Auditory Organs,- — In its more typical condition each
autlitory organ consists of a memljranous sac or vestibide,
partially constricted into an upper portion or utriculus and a
lower or sacculus (Fig. 221, A). Three semicircular canals are
connected with the utriculus, of which two are vertical and at
riglit angles to one another, and the tliird is horizontal. One end
of each canal is dilated into an ampulla. A slender tulje, tlie ductus
end(ilymY»haticus, leaves the sacculus, and ends in a sac-like
swelling, the sinus endolymphaticus, which apparently represents a
portion of the embryonic epidernuc involution from which the
auditory organ is formed. A smaller sac-like outgrowth from
the sacculus, the lagena, corresponds to the cochlea of the
higher Vertebrates. The epidermic lining of this system of
cavities is differentiated into patches or ridges of sense-cells
Cmaculae or cristae), separated by supporting cells and innervated
1 )\ the terminal branches of the auditory nerve. There is a crista
acustica in each ampulla ; and maculae acusticae are present in
the utriculus, sacculus, and lagena. A fluid, the endolymiih, fills
all the cavities, and a similar fluid or lyerilyvvph occupies the
spaces in the periotic capsule in which the various chambers are
lodged. Among the more notable deviations from this type of
auditory organ the Cyclostome Myxine, apparently, has but a
single semicircular canal with an ampulla at each end, and the
vestibule is a simple sac (Fig. 221, B). Petromyzon lias two
canals, but lacks the horizontal canal. In Elasmobranclis, in-
cluding Chimaera (C), the ductus endolymphaticus retains its
primitive connexion with the exterior by means of a pore on the
dorsal surface of the head. In the Dipnoi (e.g. FrotopfcrHs) the
paired endolymphatic sinuses divide into a number of caecal
branches containing otoliths, wliieh meet and interlace over the
fourth ventricle (Fig. 217).''^ Otoliths, either in the form of fine,

1 Fuchs {Arcliivf. d. gcs. Physiol, lix. 1895, p. 45-1) has suggested that these
organs may he concerned with the perception of pressure variations. It has also
been argued that they are concerned with equilibration and the co-ordination of
the movements of the fins. (See American Journ. Physiol, i. ]\ 128.)

- Curckhardt, Das Central- Nervensy stem v. Protoptcrus, Berlin, 1892, p. 32.

388

FISHES

inucus-coniiected, calciiieous particles, as in Elasmobranchs, or as
massive solid concretions in Teleosts, are present in relation with
the sensory areas of the utriculns, sacculus, and lagena.

In a few marine and in a large number of freshwater Teleosts

■S- ms.

Fig. 221. — Auditory organs of Fishes. A, of a typical Fish ; B, of Myxine ; C, of
Chimaera ; and D, of Perm, a.r, Anterior canal ; am', am", am"', ampullae ; am.n,
nerves to ampullae ; c, semicircular canal (in Myxine) ; d.e, ductus endo-lymphaticus ;
h.c. horizontal canal ; /, lagena ; vie, macula acnstica ; m.s, macula acustica of
the sacculus ; n, nerves to ampullae ; o, external aperture of the ductus endo-
lymphaticus ; ^j.c, posterior canal ; s, sacculus ; s.e, sinus endo-lymphaticus ; sk,
superficial skin ; .s-..s, sinus superior ; u, utriculus ; viii, auditory nerve. (From
Wiedersheini, after Ketziiis.)

the auditory organ enters into a more or less intimate connexion
with the air-bladder by one of three different methods. The
first and simplest is by the apposition of the extremities of a
pair of caecal tubular prolongations from the air-ljladder to the

SENSE-ORGANS

389

outer surfaces of the fibrous nienihranes which close a pair of
vacuities iu the outer l>oiiy walls of the periotic capsules, the
iuuer surfaces beiug bathed by the perilymph surrounding
the auditory oro-;uis. This method is characteristic of certain
Serranidae, Berycidae, Sparidae, Gadidae, and Notopteridae,^ and
probably iu the Hyodontidae. In the second method, of which
several Clupeidae {e.g. Herring, Pilchard, etc.) furnisli examples,
the periotic vacuities are open in-
stead of closed, and the sac-like
ends of the tubular extensions
from the air-bladder are in actual
contact with protruding outgrowths
from the utriculus."^ The third
method, by far the most elaborate,
is by the intervention of a series
of movably connected " Weber-
ian " ossicles, of wliich the most
posterior on each side (the tripus)
is inserted into the dorsal wall of
the air-bladder (Fig. 223), while
the anterior one (scaphium) forms
the outer Avail of a median back-
ward prolongation (sinus impar)
of the perilymph-containiug spaces
surrounding the two auditory
organs. This in turn encloses a
similar median prolongation (sinus
endolymphaticus) from the two sub-cerebrally imited endo-
lymphatic ducts (Fig. 223).^ This complex mechanism is present in
the Cyprinidae, Siluridae, Characinidae, and Gymnotidae; and hence
the term " Ostariophysi '' '^ as a collective name for these families.^

^ Bridge, Journ. Linn. Soc. xxvii. 1900, p. 503.

- Ridewood, Journ. Anat. and Phys. xxvi. 1892, p. 26.

^ E. H. Weber, De aure ct auclitu Hominis et Animalium. Pars i. De aure
Animalium Aquatilium, Leipzig, 1820 ; Bridge and Haddon, Phil. Trans. 184,
1893, p. 65.

•* Sagemehl, Morph. Juhrh. x. 1885, p. 22.

* The Weberiau ossicles are modified components of certain of the anterior
vertebrae. The scaphium represents the neural arch of the first vertebra ; the inter-
calarium is the arch of the second vertebra ; while the tripus is i)robably the rib
of the third vertebra. In the Characinidae and the Cyprinidae an additional
ossicle, the "claustrum" is present.

Fig. 222. — Cavity of tlie air-'oladder of
a Siluroid [Macrones nemiirxs) ex-
posed by the removal of its ventral
wall, a.c. Anterior chamber ; b.o,
basioccipital ; b.u\ body wall, here
reduced to the external skin ;
cl, clavicle ; Lr, lateral chamber ;
/.s, longitudinal .septum , 2)f, ])ost-
tempoval ; trjt, anterior portion of
the tripus ; tr.c, crescentic portion
of the tripus ; t.s, transverse sep-
tum ; t.s', shorter transverse sep-
tum. (From Bridge and Haddon.)

190

P^ISHES

The physiological raison fVetre of the connexion between the
air-bladder and the auditory organ cannot yet be regarded as
satisfactorily determined. It is possible, as Weber thought, that

it may be an auxiliary to tlie
function of hearing by trans-
mitting to the ear sound-waves
impinging on the surface of the
body and affecting the gases in
the air-bladder.^ On the other
hand, it may be m-ged with
perhaps greater probability that
the connexion exists for the
purpose of conveying to the
ear stimuli due to the varying
degrees of distension of the
air-bladder, such as, it may
be presumed, are naturally
brought about by the varia-

Fig, 223. — Diagram to show the Weljerian
ossicles and their relations to the ear
and the air-bladder, at. Atrium, an
extension of the sinus impar ; a.v.c,

anteriorverticalcanal;J.«- bony wall of ^^^^^g ^f hydrostatic pressure
the penotie capsule ; d.e, the medianly- -^ i

united endolymphatic ducts ; h.c, hori-
zontal canal ; in, intercalarium, a third
ossicle imbedded in the ligament {Ug)
connecting the scaphium with the
tripus ; n, bony nodules on the sides of
the complex vertebral centrum ; p.v.c,
posterior vertical canal ; s. sacculus ;
sc, scaphium ; s.e, sinus endolymph-
aticus ; s.i, sinus impar ; tr.a, tr.c. the

which a Fish encounters in
the course of its ascent or
descent in the water.^ Whether
regarded as an accessory to
hearino;, or as a means of regu-
lating the distension of the

anterior and crescentic parts of the air-bladder, the physiological

tripus ; ut, utriculus. The radial lines pi i ./ o

represent the fibres of the dorsal wall of Value ot the connexion must

the air-bladder. (From Bridge and ^^ considerable, and Oil this
Haddon.)

point it is at least significant
that the Weberian mechanism is characteristic of tlie dominant
families of freshwater Teleosts at the present day.^

The Olfactory Organs. — These organs are essentially a pair
of pit-like inpushings of the skin of tlie ventral side of the head
in front of the mouth, with their lining epidermis differentiated
into sensory cells separated l)y supporting cells, and connected
with the olfactory lobes of the brain by olfactory nerves. The

^ See also Sbrensen, Journ. Anat. and Plnjs. xxix. 1895, p. 399 ; and Bridge,
Jotirn. Linn. Soc. xxvii. 1900, p. 531.
^ Bridge and Haddon, oj). cU. p. 261.
3 Id. Proc. Roy. Soc. lii. 1892, p. 139.

SENSE-ORGANS

391

Cyclostomata are unique amongst Craniates in the apparently
unpaired condition of the olfaetory organ, and in its remarkable
relation to the pituitary involution. In the emlayo I.amprey
the median and ventral olfactory pit is carried inwards with the
pituitary invagination, so that the former appears as a dorsal out-
growth from the latter, and the two have a common external
opening, the naso-pituitary aperture (Fig. 224). Later the ex-
traordinary forward growth of the upper lip to form the roof
of the buccal funnel has the effect of shifting the naso-pituitary
involution and its aperture to a final position on the dorsal side
•of the head. It is due to this dorsal displacement that, as we

u.lp. lip.

Fig. 224. — Two stages in the development of the olfactory orgau and the pituitary
invohitioa in Petroinyzon. A is the earlier, B a much later stage, hr, Brain ;
in, iufundibulum ; Lip, lower lip ; ms, mesenteron ; «, notochord ; ol.o, olfactory
organ : pn, pineal body ; pit.s, pituitary sac ; .s-^, stomodaeiini ; «.//;, upper lip.
(From Parlver and Haswell, after Dohru.)

shall see, the pituitary caecum reaches the ventral surface of the
brain by perforating the basis cranii from above, instead of from
below as in all other Craniates. The pituitary body is pinched
off from the dorsal side of the naso-pituitary involution. In
the adult. Lamprey the olfactory organ appears as a round sac
divided by a median septum into two lateral chambers (Fig.
225), the lining epithelium of which is raised into prominent
ridges. Behind the sac the pituitary involution is prolonged
backwards beneath the brain, and, after traversing the basi-
cranial fontanelle, it widens out into a spacious cul-de-sac and
terminates on the dorsal side of the pharynx, beneath the an-
terior end of the notochord. In Myxine the pituitary involution
ends by opening into the pharynx. The apparently monorhinal

39:

FISHES

condition of the Cyclostomes is probably a secondary acquisition.
At the earliest embryonic stage at which any trace of an olfactory
organ is apparent, there is a median thickening of the epidermis,
possibly a vestige of some older sensory organ comparal >le, it may
be, to the so-called olfactory organ of Amphioxus ; on each side
of it there is a lateral thickening, the rudiments of the paired
organs.^ The three thickeninsrs, or " plakodes," then sink inwards
to form an olfactory pit. The partial subdivision of the adult
organ by a vertical septum, and the presence of two olfactory

cl/ m, m

pty.p

Fig. 22.5. — Side view of the brain of Petrnmyzon , witli the olfactorj^ organ and the
pituitary caecum in section, chlm, Cerebellum ; rrh.h, cerebral liemisphere ; dien.
thalamencephalon ; /, fold in the nasal tube ; ,'//, nasal glands ; inf, infundibulum ;
l.gn.hh, left ganglion habenulae ; med,.obU medulla oblongata : na.cq:), naso-pituitary
aperture ; n.ch, notochord ; Nv^-nv^'^, cranial nerves ; Xv^-, first ventral spinal
nerve; olf.cp, olfactory capsule; olf.l, olfactory lobe; olf.in.riu olfactory mucous
membrane; opt.l, optic lobe; j^n, pineal body;'^>»', inferior pineal body ; ^j?i.e,
parietal eye ; ptij.b, pituitary body ; j^^iZ-P- pituitary cul-de-sac ; sp. median septum
of the olfactory sac ; sp^, first dorsal spinal nerve. (From Parker and Haswell, after
Ahlborn and Kaenische.)

nerves, point to the same conclusion.^ All Fishes possess olfactory
organs wliich are obviously paired. In Elasmobranchs and
Dipnoi they retain tlieir primitive ventral position. Many
Sharks and Dog-Fislies possess an oro-nasal groove leading from
each olfactory organ to the corresponding angle of tlie mouth.
The Dipnoi proceed a stage farther, and, Ijy the conversion of
the grooves into short canals, the olfactory pits communicate
with the mouth by true internal nostrils, as in the higher

^ Kupiler, St7tcl. rercjl. EntvncM. d. Kopfes d. Kraniaten, iii. 1895, p. 6.
- In Bdellostoma the olfactory organ arises as a ^)«zV of outgrowths from the
jiituitary involution (Bashford Dean, Kuptfer's Festsclmfl, Jena, 1899, p. 269).

SENSE-ORGANS

393

Vertebrates. Tu the adults of existing Teleostomi the orifice
of each organ is usually divided into two l)y tlic growth of a
fold of skin across it, and tlie two apertures rotate outwards and
upwards on to the lateral or the upper surface of the snout. Of
the two nostrils the posterior one prol)ably corresponds to an
external nostril, and the anterior one to the internal nostril.
Occasionally each olfactory organ has only a single orifice. In
the Crossopterygii and in some Teleostei the nostrils l)econie
tubular. The lining epithelium of the olfactory pits is usuidly
produced into ridges, disposed longitudinally or transversely, or
in the form of radii from a central point in the roof. Many
Teleosts have each olfactory organ prolonged backwards into one
or two sacs, the nasal sacs, which are either simple reservoirs, or
glandular and mucus-secreting. In a species of Chinese Sole
(Cynoglossus semilaevis) the t\vo sacs, one from each olfactory
organ, unite over the roof of the mouth in a common median
sac, and in one unique specimen the latter communicated with
the mouth by a large naso-pharyngeal aperture.^

The Eyes. — In essential structure the eyes of Cyclostomes
and Fishes resemble those of the higher Craniates. As a rule,
in Fishes they are relatively larger, however, and the lens is
globular and the cornea somewhat flatter. Ciliary processes
and ciliary muscles are absent. As the eyes are nearly always
lateral in position it is probable that monocular vision is
the rule. In Teleosts and in Amia a " choroid gland," consist-
ing of a mass of capillary blood-vessels, surrounds the optic nerve
externally to the retina, and derives its blood from the efferent
artery of the pseudobranch (Fig. 226). In most Teleostomi, but
not in Cyclostomes, Elasmobranchs, and Dipnoi, there is a
singular prolongation of the choroid coat, known as the " ]»r()-
cessus falciformis," which extends across the vitreous humour to
the inner face of the lens, where it ends in an expansion, the
"campanula Halleri " (Fig. 226). Accommodation to vision at
different distances is not effected by alterations in the convexity
of the lens, but by a change in its position with regard to the
retina, apparently brought about by the contraction of a special
retractor muscle.^ Some oceanic pelagic Teleosts are remarkable
for their curious telescopic eyes in the shape of short protruding

^ Kyle, Joiii-n. Linn. Soc. (Zool.). xxvii. 1900, p. i>Al.
- Beer, jrien. klia. JI'ochc7ischr., No. xlii. 1898, \>. 11.

394

FISHES

GTl

Z-

tr

ch.cfLtJL

cylinders, each terminating in a strongly convex cornea (Fig.
227).^ The eyes are directed either upwards or forwards,
and, as their long axes are parallel in either position, it
is probable that these Fishes are capable of binocular vision.
In the young of certain Teleosts occurring in the Antarctic
and Indian Oceans the large eyes are situated at the ex-
tremities of extraordinary long stalks extending from the sides
of the head.

In the quasi -parasitic Cyclostome, Myxinc, and in many
Teleosts belonging to widely different families, which live at

great depths in the sea or
scl cp.haZ inhabit subterranean waters,

the eyes suffer from disuse
and degenerate in structure.
The influence of a deep-sea
habitat on tlie eyes of Fishes
is somewhat varied. The
eyes are often small. A few
abyssal Fishes are totally
blind, and no external trace
of eyes can be seen (Fig.
430). In such Fishes com-
pensation is often afforded
by an extraordinary develop-
ment of tactile organs in
the form of long barbels, or
of trailing filaments derived
from the median or the paired
fins (Fig. 371, B). Many deep-sea forms possess eyes of the
normal siz3, or even exceptionally large eyes, probably because
either they occasionally migrate towards the surface, or else they
possess phosphorescent organs and are able to see by the aid of
the light they themselves emit. A blind Siluroid (Amiiirus
nigrilabris) frequents the cave streams of Pennsylvania, and
many members of the same family which live in muddy waters
have very small or even minute eyes. One of the Gobies {Typhlo-
fjohhis)'^ which buries itself in the sand, or is found under stones in
the holes of a burrowing Crab on the coast of California, is also

' Chun, Aus den Ticfen ch'S JFeUmeeres, Jena, 1900, p. 534.
- Ritter, Bull. 3fus. Com}). Zool. xxiv. ]893, p. 51.

PS

Fig. 226. — Vertical section of tlie eye of Salmo
fario (semi-diagrammatic). «/v/, Argentea ;
cli, choroid ; clujld, choroid gland ; en.
cornea ; cp.hal, campanula Halleri ; //•, iris ;
/, lens ; opt.nv. optic nerve ; x>9^ pigmentary
layer ; pr.fl, processus falciformis ; rt, retina ;
scl, sclerotic. (From Parker and Haswell. )

SENSE-ORGANS

395

bliud. Amongst other blind Fishes Amllyopsis and TpphlichtJnjs
(Amblyopsidae) ^ and Lucifuga (Zoarcidae) may be mentioned, the
first two inhabiting the cave streams of North America, while the
third has a similar habitat in Cuba. AVhen the eyes degenerate
they dwindle in size and recede from the sm-face. The lens and
the iris wholly or partially disappear, and although it is generally
recognisable the retina loses certain of its characteristic layers, or
the latter are but imperfectly
formed. In Myxine even the
eye-muscles are absent.

The eyelids of Fishes are
little more than marginal folds
of skin, capable of little if
any movement, and leave the
eyes largely uncovered. Some
Sharks have a tliird eyelid or
" membrana nictitaus " at the
anterior corner of the eye.
Lachrymal glands are im-
known.

The Parietal Eye. — It is

only in the CyclostomeS that Fig. 227.-The telescopic eyes of O^/.s//,,,-

/ '' proctus soleatus, \ aiU. (A), and ot a

this structure can have any species of a new family of Teleosts from

claim to be regarded as a visual ^1' ^'f^'' ^^^^^ (^)- ' ^^^- ''''■ (^™"^

o _ Chun.)

organ. In the Lamprey (Fig.

228) the parietal eye is a slightly flattened vesicle lying
directly over the pineal vesicle, and connected by a slender
stalk or nerve with the rio'ht oanglion habenulae. The dorsal
or more external half of the vesicle is l)i-convex, and forms the
"pellucida," while the inner half or retina is said to consist
of supporting cells with interspersed deeply pigmented sense-
cells and ganglion cells."-^ The external skin over the parietal
eye is partially transparent in the living animal.

In many of the oldest known Fishes, such as the Ostracodermi,
the Antiarchi, and the Crossopterygian Osteolepida, there are
indications of the existence either of one or of two median sense-
organs on the upper surface of the skull, in the shape of one or
two foramina, or hollow protuberances, or pit -like grooves or

^ Eigenmann. Arch. f. EnUrickelungsimcli. viii. 1899, p. 545.
- Studnicka, Sitzhcr. l\ bohm. Gcs. IViss.. 1899, No. xxxvii.

396

FISHES

CHAP. XIV

depressions, but, as a rule, when one of them is present the
other is absent. It is probable that both these structures
were associated with sensory organs, of which one may have

Fig. 228. — Vertical section through the parietal eye and the pineal vesicle of Petro-
myzon marinus. c.t, Connective tissue ; jj. iiellucida ; p.o, pineal organ ; 2^i-"i
parietal eye ; r, retina ; iii v, third ventricle. (From Wiedersheini, after Stud-
ui6ka.)

been a parietal eye and the other a pineal eye. Some Teleosts
{e.g. many deep-sea Scopelidae) have a transparent, convex,
cornea-like prominence on the upper surface of the head which
may be related to one of these singular organs.^

^ Chun, loc. cit. p. 536,

CHAPTER XV

THE KIDNEYS AND THE llEPKODUCTIVE ORGANS BREEDING

The kidneys and the reproductive organs are so intimately con-
nected that it is necessary to deal with them together. Both
organs are specialised portions of the coelom and its epithelial
lining. The kidneys are essentially a series of tubular and at
first segmentally -disposed outgrowths from the coelom (urocoeles)
which acquire a connexion with the exterior, while the gonads
have their origin from local modifications of the coelomic epi-
thelium. At a very early embryonic stage each lateral half
of the coelom presents three well-marked divisions: (1) a series
of dorsal portions (" myocoeles "), the cavities of the myotomes or
muscle-segments; (2) a longitudinally continuous unsegmented
portion extending round the alimentary canal, the " ventral
coelom " ; and (3) a series of intermediate tubular portions or
" nephrotomes," each of which leads from a myocoele to the ventral
coelom (Fig. 229, A). The essential components of the kidneys,
the urocoeles or renal tubules, are derived from the nephrotomes.
In its typical condition each kidney consists of three portions,
which, in accordance with their embryological and evolutionary
sequence, are termed the " pronephros," the " mesonephros," and the
" metanephros." The pronephros, the larval or provisional kidney,
is formed from a limited number of the nephrotomes immediately
behind the head. From each nephrotome a hollow tubular out-
growth is formed, which grows towards the lateral surface of the
body, and then unites with its fellows of the same side to form
a main longitudinal duct — the " archinephric " or " pronephric
duct" (Fig. 229, A, Fig. 230, A). This duct grows backwards
until it opens into the cloaca.^ At the same time the nephro-
' It is probable that the archinephric duct is derived from the embryonic

397

398

FISHES

tomes lose their connexion with the myocoeles, although they
still retain their " nephrostomes " or apertures through which they
communicate with the ventral coelom. When fully developed the
pronephros consists of a few tubules, more or less convoluted, open-
ing at their inner extremities into the coelom by means of their
ciliated nephrostomes, and at their outer ends communicating
with the exterior through the archinephric duct. In relation

mnf

Fig. 229. — Diagrammatic transverse sections through an embryo Craniate to show the
mode of development of tlie pronephros (A) and of tlie mesonepliros (B). The right
side of eacli figure shows an earlier stage than the left. In B (left side) the con-
nexion of a vas efterens with a mesonephric tubule, and the division of the archi-
nephric duct into Miillerian and mesonephric ducts are shown, a, Aorta ; a.c,
alimentary canal ; a.d, archinejihric duct ; (/, glomus ; gl, glomerulus ; i.7i, inner
nephrostome ; vib, Malpighian body ; md, Miillerian duct ; mud, mesonephric
duct ; nmt, mesonephric tubule ; myc, myocoele ; vit/t, myotome ; m, notoohord ;
np, nephrotome ; nt, nephrostome ; o.n, outer nephrostome ; 2'n-t, pronephric
tubule ; s.c, spinal cord ; t, testes ; v.c, ventral coelom ; v.ef, vas efferens. (After
Kingsley and Semen. )

with the pronephros a branch from tlie dorsal aorta forms a tuft
of capillary blood-vessels or " glomus," opposite the nephrostomes,
which projects into the ventral coelom on eacli side. Later, a
second series of much more numerous tubules is formed behind
the pronephros, which constitute tlie mesonephros. In forming
mesonephric tuludes the nephrotomes become disconnected from
the myotomes and their myocoeles, and curving outwards they

epiblast ; hence tlie suggestion that in the primitive Vertebrates the duct was a
longitudinal groove in the superficial skin into which the pronephric tubules
opened externally.

XV KIDNEYS 399

come to open into the archinephric duct, although they do not in
any way contribute to its formation (Fig. 229, B). Segmentally-
arranged twigs from the dorsal aorta end in tufts of capillaries
or glomeruli, each of which projects into a small sac-like enlarge-
ment of a mesonephric tubule, pushing before it the wall of the
sac. In this way a double- walled " Alalpighian body," contain-
ing a " glomerulus," is formed in connexion with each tubule.
Subsequently, the mesonephric tubules increase in numljer by
budding. New nephrostomes and Malpighian bodies are developed
on the secondary branches, and the original segmental arrange-
ment of the tubules becomes obscured. With the growth of
new tubules, and the formation of blood-vessels and of connective
and lymphoid tissues between them, each mesonephros finally
assumes the condition of a compact gland imbedded in the dorsal
wall of the coelom, with its ventral surface invested liy the
peritoneum. A " metanephros," wliich in the higlier Vertebrates
replaces the mesonephros as the functional kidney, is perhaps
not represented in Fishes.

A more or less well-developed pronephros is present in the
embryos or larvae of the Cyclostomes and of all Fishes, but as a
rule it completely disappears at an early period and is replaced
])j the mesonephros. It is retained throughout life, however,
in the Myxinoid Cyclostomes (Fig. 230, B), and has its persistent
nephrostomes opening into the pericardial cavity.^ In a few
Teleosts the pronephros is also persistent, as in Fierasfer and
Dactylopterus, and in others the organ may not completely dis-
appear until the approach of sexual maturity. I^)Ut with these
exceptions the mesonephros is the sole functional kidney in the
adults of the Cyclostomes and of all Fishes. As regards the
nature of the duct by wliich the excretion of the mesonephros is
conveyed outw^ards, there are notable differences in different
Craniates. The Cyclostomes and the Teleostomi retain that part
of the archinephric duct into which the mesonepliric tubules open,
and which remains after the atrophy of the pronephros (Fig. 230,
B, E, F). In Elasmobranchs, and probably also in the Dipnoi,
a special mesonephric dupt is developed in a way which will
be described later (Fig. 2-30, C, D). In the males of Elasmo-
branchs some of the hinder mesonephric tubules unite to form

1 W. Miiller, Jen. Zeitsch. ix. 1875, p. 107 : Senion, Carl Gegenhaur" s Festschrift ,.

Leipzig, 1S96, iii. p. 169.

400

FISHES

Fig. 230. — Showing the principal modifications of the kidneys and reproductive organs
in Cyclostomes and Fishes. A, The ])rone2ihros and its duct in the embryo ;
B, the kidneys and genital pores in Pctromi/zon, the vestigial jironephros repre-
sented as in Myxine ; C and D, the urinogeuital organs of a male and female
Elasmobranch ; E, of a male or female Teleost, or a male Le^ndosteus ; F, of a
female Polypterus, Aripcnser, Amia, or Osmerus. a, Anus ; a.d, archinephric
duct ; c, cloaca ; c.«, the coelomie aperture of the Milllerian duct ; c.p, cutaneous
pit ; g, gonad ; gd, gonoduct ; g.p, genital pore ; i, intestine ; j«, Malpighian liody ;
m.d, Miillerian duct ; mn, mesonephros ; mn^, vestigial mesonephros ; mn'-, ex-
cretory portion of the mesonephros ("metanephros ") ; vm^, genital portion of meso-
nephros ; mn.d, niesonephric duct; mtn.d, metanephric duct; ?(, nejihrostome ;
ov, ovary ; p.a, abdominal pore ; ^j.*^, peritoneal funnel ; pn, pronephros ; 2-*"',
vestigial pronephros ; s.g, shell gland ; s.s, sperm sac ; /, testis ; vg.s, n.s, urino-
geuital sinus ; v.ef, vasa efferentia ; v.s, vesicula seminalis.

ABDOMINAL TORES

401

a single main duct opening into the terminal part of the meso-
nepliric duct, and these tubules and their separate duct are
sometimes regarded as a metanephros and a metanephric duct.
The mesonephric nephrostomes are persistent throughout life in
a few Elasmobranchs {e.g. Notidanidae, Heterodontid.ae, lihinidae,
and some Scylliidae), and also in Amia : ^ in all other Fishes
as well as in the Cyclostomes they become closed in early
life.

In many Fishes the hinder extremity of the coelom com-
municates directly with the exterior through " abdominal pores,"
of which there is usually a
pair, rarely a single pore,
situated close to the cloacal
or the anal aperture.^ Elasmo-
Ijranchs usually have a pair,
often at the extremities of a
pair of cloacal papillae (Fig.
231), but they are absent in
some families {e.g. Hetero-
dontidae and Ehinidae) ; and
in some Scylliidae (e.g. Scyl-
liuvi canicula) they are very
variable, being either present
or absent on both sides, or an

open pore is present on one Fig. 231.— Diagrammatic horizontal section

side only. Pores are present

a.p ^

U.S -

and paired in the Crosso-
pterygii, the Chondrostei, and
the Holostei. Amongst the
Dipnoi Neoceratodus has a pair
of pores. Froto'ptcrus some-
times has two pores opening

tliroiigh the abdominal pores and cloaca
of an Elasniobranch. a.^>, Abdominal
pore ; c, coelom ; cl, cloaca ; d.p, cloacal
papilla ; c.p, cloacal pit ; od, oviducal
apertures in the female ; r, rectum ;
u.x, cloacal aperture of the urinary sinus
(female), or the urogenital sinus (male).
In some Elasniobranihs the abdominal
pore opens at the base of the cloacal
papilla, as shown at a.p^. (Modified from
Bles.)

into the cloaca, but as a rule

the two become continent and have a single external aperture.
In Leiyidosiren pores are wanting. Abdominal pores are rarely
present in Teleostei. They exist, however, in the Mormyridae
{Gymnarclms and several species of Mormyrus), and also in the

1 Jungersen, Zool. Anz. xxiii. 1900, p. 328.

- Bridge, Journ. Anat. and Phys. xiv. 1879, p. 81 ; Bles, ih. xxxii. 1898, p.
484 ; Proc. Roy. Soc. Ixii. 1898, p. 232.

VOL. VII 2 D

402 FISHES CHAP.

Saliiionidae/ where they are as singularly variable in different
species and individuals as in the Elasmobranch Scylliidae. The
use of abdominal pores is not certainly known, unless the coeloni of
those Fishes which possess them continues to retain some measure
of its primitive excretory function, and the pores act as excretory
ducts. That the nephrostomes are excretory organs has been
shown by experiment, and it is worthy of note that there exists a
reciprocal relation between these structures and abdominal pores,
to the extent that while there are a few Fishes {e.g. certain
Elasmobranchs and Amia) in which both coexist, there are
many others in which the presence of nephrostomes is correlated
with the absence of pores and vice versd.

The male and female gonads, testes and ovaries, are derived
from the coelomic epithelium near the inner or median aspect of
the nephrotomes (Fig. 229, B). Here the epithelium remains
columnar, and soon projects into the ventral coelom as a con-
tinuous longitudinal ridge. It is probable that at first the modified
epithelium is segmented as a series of " gonotomes," but if so, the
latter must soon coalesce into a continuous ridge. Some of the
epithelial cells enlarge to form the primitive sex-cells. In the
development of an ovary, portions of the epithelium sink inwards,
carrying with them the primitive ova. Certain of the cells form
the epithelial walls of a number of ovisacs, each of which encloses
an ovum. As the ovisacs increase in number and size the
germinal ridges project more and more into the coelom until,
as ripe ovaries, they become suspended from its dorsal wall by
a double peritoneal fold, the " mesovarium " (Fig. 1 5 G). The testes
develop in a similar fashion except that the primitive sex-cells,
which later give rise to spermatozoa, form the lining of a number
of simple or ampulla-like tubules, tlie seminiferous tubules, and
the suspensory fold is termed the " mesorchium."

The Cyclostomes have gonads in the shape of unpaired
organs extending nearly the whole length of the coelom, but
in all Fishes the organs are primarily paired, although by fusion,
or by the absorption of one gonad, the ovaries or the testes
sometimes appear as if single. The ovaries may either be naked,
as in Elasmobranchs, Dipnoi, Crossopterygii, and Chondrostei,
and in Amia amongst the Holostei ; or, as in Zepidosteus and
most Teleosts, they become enclosed in coelomic sacs. The
1 Ma.K Weber, Jlorph. Jahrh. xii. 1886, p. 336.

REPRODUCTIVE ORGANS

403

former, or " g-ynmoarian," coiulition is primitive ; the latter, or
" cystoarian," is secondary, and is brought aliout by the growth
of two peritoneal folds round the ovary and the union of their
margins. Into these coelomic sacs the euu-bearimr or real ovarian
tissue projects eitlier in the form of processes or of trans-
versely- or longitudinally-arranged plates or folds (Fig. 232, B).
The testes are composed of seminal ampullae, as in Elasmo-
Iiranchs, or of radially-arranged and sometimes plexiform tubules

Fig. 232. — Diagrams to sliow the structure of the testes (A) and of the ovaries (B)
ill a Herring. (From Cunningham.)

opening into the gonoduct, as in nearly all other Fishes (Fig.
232, A).

In the Cyclostomes (e.g. Petromyzon) the eggs and sperma-
tozoa are discharged from the gonads into the coelom, whence
they reach the exterior through a pair of " genital pores " leading
from the hinder end of the coelom into a urinogenital sinus
formed by the united extremities of the two archinephric ducts.^
Myxine has, however, but a single median pore, opening into an
integumentary cloaca, which also receives the rectal and urinary
orifices. Bdellostoma has two such pores communicating with
a similar cloaca.^

' Ewart, Joiini. Anat. and F/ii/s. x. 1876, p. 488.
^ Burne, Linn. Soc. Journ. Zooh xxvi. 1898, p. 487.

404 FISHES CHAP.

The nature and homologies of the genital ducts in the
different groups of Fishes are amongst the most puzzling of the
many prol)lems which vex the soul of the Vertebrate morpho-
logist, and although tliere is a fairly general agreement on some
points, there are others of great importance of which it may be
said quot homines, tot sententiae.

Broadly speaking, there are two types of genital ducts in
Fishes: (1) those which are obviously derived from some part
of the kidney system ; and (2) those which are special ducts and
appear to have no connexion with kidney-ducts.

The Elasmobranchs offer a typical example of gonoducts of
the first kind. At an early embryonic period in both sexes
each archinephric duct becomes longitudinally split into two
ducts, of which one continues to receive tlie openings of the
mesonephric tubules and remains as a mesonephric duct (Fig.

229, B).^ The other, which has no connexion w^th the meso-
nephros, opens anteriorly into the coelom by means of the
united nephrostomes of the pronephros, and is known as the
"Miillerian duct" (Fig. 230, C and D). In the adult male
the Miillerian ducts are useless vestiges, but in the female they
persist and act as oviducts, receiving the eggs set free from
the ovarian ovisacs through their coelomic apertures, and thence
conveying them to the cloaca. In the male, certain of the
anterior mesonephric tubules become connected with the testi-
cular ampullae by means of a network of slender tubules, the
" vasa efferentia " or testicular network, and through the latter the
spermatozoa pass from the testes to the mesonephric duct (Fig.

230, C). Consequently, the mesonephric duct conveys both
spermatozoa and the kidney excretion to the cloaca. It is
obvious, therefore, that both the male and female gonoducts are
derived from kidney-ducts.

The Teleostei afford an equally typical illustration of the
second type. Each female gonoduct (oviduct) is formed by a
backward growth of the same two peritoneal folds which enclose
the ovary ; these are converted into a " peritoneal tube " or
canal by the union of their margins. The male gonoducts
are also formed in continuity with the testes, that is, as
backward prolongations from the latter. Each duct, male or

^ Semper, Cnitralhlatt f. Med. Wiss. 1875, No. 29 ; F. M. Balfour, Journ. Anat.
and Phys. x. 1875, p. 17 ; Id. Comparative Embryology, London, 1881, ii. p. 568.

XV REPRODUCTIVE ORGANS 405

female, seems to be a duct sui generis and to have no con-
nexion whatever with the kidney system (Fig. 230, E). In the
Salmonidae, Anguillidae, Galaxiidae, Hyodontidae, Notopteridae,
and Osteoglossidae, and also in Misgtiryius, the oviducts lose
their continuity with ovaries and degenerate to an extent which
differs greatly in different families. Thus in some Salmonidae,
as in the Smelt (Osmenis ejjerlanus),^ the oviducts end anteriorly
in wide funnel -like coelomic apertures after the fashion of
Miillerian ducts, and do not embrace the ovaries : hence the
ovaries are naked and not cystoarian, and their ducts are
not peritoneal tubes but "peritoneal funnels" (Fig. 230, F).
In other Salmonidae and in the Anguillidae the oviducts appear
to have so far degenerated that they are represented either by a
pair of very short funnels or by a pair of genital pores, which, as
in the Salmon, have a common external aperture behind the anus
and in front of the single orilice of the united archinephric ducts
(Fig. 233, A). In all such instances the eggs are set free from
the ovaries into the coelom, from whence they escape through
the peritoneal funnels or genital pores. In the Eels the male
gonoducts also degenerate, and, losing all connexion with the
testes, they become reduced to genital pores as in the female.

The Holocephali and probably tlie Dipnoi conform to the
Elasmobranch type in the nature of their male and female gono-
ducts. In the Crossopterygii ^ each testis has its own proper duct,
which has no connexion with the kidney system and apparently
belongs to the Teleostean type, while the oviduct, which is almost
certainly not a Miillerian duct, is probably a peritoneal funnel.
On the other hand, the Chondrostei and the Holostei are in the
interesting transitional condition of possessing male ducts of
the Elasmobranch type and female ducts of the Teleostean type,
the latter being either ducts directly continuous with the ovaries,
as in Lepidosteus, or of tlie nature of peritoneal funnels, as in
Acipenser, Polyodon, and Amia (Fig. 230, E and F).

How far the distinction between the two types of gonoduct
holds good in the case of the male is not quite clear, and it has
recently been argued that the Dipnoi off*er a connecting liid<
between the two.^

1 Huxley, P.Z.S. 1883, p. 132.

- Budgett, Trans. Zool. Soc. xv. 1901, p. 323 ; xvi. 1902, p. 315.
3 Graham Kerr, P.Z.^. 1901, p. 484 ; Proc. Phil. Soc. Cambridge, xi. Tt. v.
1902, p. 329.

4o6

FISHES

In Protopterus each testis is divided into an anterior sperm-
producing part and a posterior tuljular portion which has lost
the capacity of producing sex-cells. The testicular network is
greatly reduced, and forms hut a limited connexion between the
tubular portion of the testes and the mesonephric duct (Fig.
233, B). If it be supposed that the testicular network became
still further reduced so that the connexion between the testes
and the kidney-duct took place directly througli a single channel
instead of through several, the result would be a gonoduct
essentially similar to the male duct of an ordinary Teleost.

ov

\J

mn

Fig. 233. — Diagram to show the kidneys and gonoducts of a I'eiuale Salmon (A), and of
a male Proto2'>terus (B). md^ and md-, Anterior and posterior vestiges of the
Miillerian duct ; t.t, tubular posterior portion of the testis (t). Otlier reference
letters as in Fig. 230. (B, after Graham Kerr.)

Should this view prove to be correct, it will follow that tlie
male gonoducts of all Fishes are differently-modified examples
of the Elasmobranch type. But there will still remain the
female gonoducts of Ganoids and Teleosts, which must be
regarded as distinct from Miillerian ducts unless it can be shown
that their different methods of development are not necessarily
fatal to their homology with Miillerian ducts, or that l)oth types
of gonoduct can be derived from some intermediate type.
Assuming that some Fishes do possess male or female ducts
which have not been derived from the kidney system, but have
been independently acquired, there is still the question, which
of the two types is the more primitive, or, in other words,
has the Elasmobranch type superseded the Teleostean, or vice

XV REPRODUCTIVE ORGANS 407

versa ? To this question no decisive answer can at present be
given.

Tlie terminal relations oi' the kidney-ducts and the gonoducts,
and the presence of accessory or of vestigial organs in connexion
with them, will now be briefly dealt with. In the males of the
Elasmobranchs the mesonephric ducts which, as already pointed
out, act both as kidney-ducts and gonoducts, dilate posteriorly
to form a pair of vesiculae seminales, and then unite to form
a urinogenital sinus, opening into the cloaca at the extiemity
of a median papilla (Fig. 230, C). The sinus also receives ducts
from the hinder part of the mesonephros, either separately, as in
the female, or by a common duct on each side — the so-called
metanephric duct — as in the male. Two tubular caecal out-
growths from the sinus form two sperm sacs. Only the anterior
portions of the Miillerian ducts with their coelomic apertures are
retained in the adult. In the female the mesonephric ducts are
purely excretory, but otherwise they are similar, and the oviducts
(Miillerian ducts) open into the cloaca separately or by a
conmion orifice (Fig. 230, D). A glandular dilatation of each
oviduct forms the oviducal or shell gland by which the horny
egg-cases are secreted. In the males of the Holocephali the
gonoducts open into a urinogenital sinus with an external orifice
distinct from and behind the anus ; but the female has separate
apertures for the rectum, tlie conjoined oviducts, and the
united mesonephric ducts. Both sexes have complete Miillerian
ducts communicating with the coelom in front, and behind with
the exterior. The Dipnoi of both sexes essentially resemble
the Elasmobranchs in the general relations of their ducts, but the
Miillerian ducts of the. male exhibit marked differences in the
three genera.^ In Neoceratodus the ducts are as complete as their
functional representatives in the female. Protopterus retains
anterior vestiges and the coelomic apertures, and also vestiges of the
hinder portions which unite and end blindly in the urinogenital
papilla, but the middle sections of the two ducts are suppressed
(Fig. 233, B). In the Teleostomi there is a general similarity
in the terminal relations of the gonoducts and kidney-ducts. In
the Ganoids the archinephric ducts unite and then expand into
a urinary sinus or bladder, and the gonoducts of the female, or
of both sexes in Lepidosteus, open either into the archinephric

^ Graham Kerr, op. cit.

408 FISHES CHAP.

ducts or into the common sinus, and therefore both ducts
communicate with the exterior hy a urinogenital orifice behind
the anus. Peritoneal funnels, similar to the functional ovi-
ducts of the female, are present in tlie males of the Chrondrostei
and of Amia. In Teleosts the terminal connexions of the ducts
tend to become less intimate. The archinephric ducts often dilate
into a urinary bladder either before or after their union, and
the common duct joins the united gonoducts to form a short
urinogenital sinus which opens externally, or the confluent
gonoducts have an independent genital orifice between the anus
and the urinary aperture. Not rarely the genital or the urino-
genital orifice is prolonged into a tubular papilla, which in the
male acts as an intromittent organ, or, as in the females
of the Cyprinoid Rhodeus amarus, the long oviducal tube
serves the purpose of an ovipositor. The males and females of
the Siluroid Plotosiis have a remarkable vascular and glandular
arborescent appendage just behind the urinogenital papilla, the
use of which is unknown.^

The egg's of different Fishes '^ exhibit considerable diversity
in size and shape as well as in the nature of their external
coverings and their mode of deposition.^ The size of the eggs
largely depends on the quantity of food-yolk stored up in their
substance for the nutrition of the embryo : hence the eggs of
Elasmobranchs, which resemble Fowls' eggs in the superabund-
ance of their yolk, are by far the largest. Teleostomi liave
much smaller eggs. The largest Teleostean ova are those which
are heavy and sink (demersal ova) ; the smallest, those which
are buoyant and float (pelagic ova). Of the former, the eggs of
Gymnarchus are about 10 mm. in diameter; those of the Salmon
about 5 mm.; and those of some species of Ariits, 5 to 10 mm.
The eggs of the Wolf-Fish (Anarrhichas luptis) are about 6 nun.
Smaller demersal ova are those of the Lump-sucker {Cycloi^erus)
and Heterotis, which are 2*6 and 2"5 mm. respectively. Pelagic

' Hirota, Jonrn. Coll. Sci. Imp. Univ. Japmi, vii. 1895, p. 367.

- For the eggs of Cyclostomes see Chapter XVI.

' For a description of the eggs and breeding habits, and the larval develop-
ment and migrations of British Marine Fishes, see M'Intosh and Prince, Trans.
Hoy. Soc. Eclin. 1890 ; M'Intosh, Ann. Report FisherT/ Board for Scotland, 1892 ;
Cunningham, Markctahle iFarine Fishes of the British Islands, London, 1396 ;
M'Intosh and Masternian, Life -Histories of the British Marine Food -Fishes,
London, 1897 ; also numerous papers by Cunningham, Holt, Garstang, and Allen,
in the Journ. Marine Biol. Assoc. Plymouth, vols, i.-vi.

EGGS

409

eggs are very small, those of the I'laice, which are exceptionally
large, varying from 1'65 to 1'95 mm.

An egg-cell consists of living protoplasm and a nucleus, a
variable quantity of non-living food-yolk, and of certain envelop-
ing and protective egg -membranes. The ova of Fishes differ
principally in the amount and disposition of the food-yolk, in the

Fig. 234. — Diflerent types of egg-segmentation in Fishes. A, a typical telolecitlial egg.
Holoblastic and unequal segmentation in Amia (B) and in Lejyidosteus (C). D, tlie
nieroblastic segmentation of a Teleost. a.p, Animal pole; e.m, egg-menihrane ;
/iia, macromeres ; 7ni, micronieres ; n, nucleus ; o.fj, oil globule ; p^ protopla.sm ;
r.p, vegetative pole; i/, yolk. (From Ziegler : A, after Hertvvig ; B, after Whit-
man and Eycleshymer ; C, after Eycleshymer.)

character of the egg-membranes, and in the presence or al)sence
of special perforations in the egg-membranes for the entrance of
spermatozoa into the eggs. In the small ova of some of the
lower Chordata (c.r/. Amphioxus), where the very small quantity
of food-yolk is uniformly distrilmted, and its presence affects all
parts of the egg alike, tlie process of segmentation which follows
fertilisation results in tlje transformation of the entire egg into
ii mass of approximately equal-sized cells or blastomeres (Fig. 82).

4IO FISHES CHAP.

The eggs are therefore described as " alecithal," and the segmenta-
tion as being " holoblastic " and " equal." On the other hand, all
Fishes possess " telolecithal " eggs, tliat is, ova in which the food-
yolk is more or less abundant, and tends to accumulate at one
pole of the egg (" vegetative pole "), while the opposite or " animal
pole " consists of protoplasm, comparatively free from yolk
granules and containing the nucleus (Fig. 234, A) The term
telolecithal is, however, a somewhat comprehensive one, and
covers important variations in the relations of the inert food-
yolk and the living protoplasm in different Fishes, which greatly
modify the process of segmentation. Thus there are some Fishes
in which the amount of food-yolk at the vegetative pole is
sufficient to retard segmentation in that part of the egg without
actually preventing it, and consequently segmentation begins in
the animal pole, and takes place more rapidly there than it does
when it extends into the vegetative pole. Hence it follows that
although the entire egg is segmented the blastomeres are of
unequal size, the animal pole giving rise to a large number of
small cells or micromeres, and the vegetative pole to a smaller
number of much larger cells or macromeres. The segmentation
of such an egg is said to be holoblastic but unequal (Fig. 234,
B and C). This type of egg is characteristic of the Chondrostei,
the Holostei, and the Dipnoi. In other Fishes, like the Elasmo-
branchs and the Teleostei, the food-yolk so greatly preponderates
that it entirely prevents segmentation in the vegetative part
of the egg, and segmentation is restricted to the small mass of
protoplasm (germinal disc) at the animal pole, in which the
nucleus is situated (Fig. 234, D). Eggs undergoing partial
segmentation in this way are termed " meroblastic." No hard
and fast line can be drawn between the two types, and in the
Chondrostei and Holostei an interesting transition between the
holoblastic and meroblastic ova may be observed. The egg-
membranes are formed either by the egg itself or Ijy the
epithelium of the ovarian ovisacs, and, as will shortly be seen,
the character of the outer egg-membrane greatly influences the
luode of deposition of the eggs and their location afterwards.
In Elasmobranchs the egg is enclosed in a stout horny egg-shell,
secreted by the oviducal shell gland.^ In many Fishes, as in
the Chondrostei, Holostei, and Teleostei, tlie egg- membranes

1 See ChapttT XVII.

XV EGGS 411

are perforated at the animal pole of the egg by a small aper-
ture or " micropyle," which is only large enough to admit of
the entrance of a single spermatozoon at a time (Fig. 235).
Generally, there is only a single micropyle, but, according to
Salensky, the Sturgeon (A. sturio) has from 3 to 0, and the
Sterlet (A. ruthenus) from 5 to 13.

An important distinction may be made between the ova of
different Teleostomi as regards their location after extrusion from
the female. From this point of view two types of ova can be
distinguished, demersal and inlagic ova. Demersal eggs are
characterised by their larger size and greater weight, so that they
always sink after extrusion ; and by their opacity. They may
either have an outer egg-membrane wdiich is viscid and adhesive,
so that the eggs readily adhere to one another or to foreign objects,
or the membrane is smooth and non-adhesive. The Salmonidae,
for example, produce non-adhesive demersal eggs, which remain
separate after being deposited on the gravelly bed of a stream.
Most freshwater and many marine shore Fishes have adhesive
demersal eggs, which are deposited at the bottom of the water,
generally adhering to one another in larger or smaller clumps,
masses, or sheets, and attached to rocks, stones, or empty shells,
like the eggs of many shore Fishes, or to aquatic plants after the
fashion of the eggs of the Carp, Perch, and Pike, or even to
branching zoophytes, as is the case with the eggs of the Sea-snail
(Liparis). In some adhesive eggs the external egg-membrane
forms threads for their attachment. The eggs of the Gar-Fish
(Belone), and those of the Saury Pike {Sco7iibresox) and of the
Flying Fishes {Exocoetus), have viscid threads developed from
opposite points on the surface, which are either attached to
foreign objects or they become entangled with those of other
eggs of the same species. The oval eggs of some of the Gobies
have a bunch of fibres at one pole which serves to attach them.
In the Smelt (Osmerus eperlanus) a portion of the outer egg-
menil)rane breaks away from the rest and becomes turned back,
inside out, but remains attached to the egg at one point. Py
means of this membrane the egg is attached to I'ocks or stones.
Pelagic eggs are distinguished by their lightness and buoyancy,
so that they always float near the surface of the water, and by
their smaller size and remarkable transparency (Fig. 235). A
conspicuous feature in many of them is the presence of a single

412

FISHES

large oil globule on the surface of the yolk, and not infrequently
the yolk becomes partially or completely broken up into small
masses. Pelagic eggs are always non-adhesive and free, and they
invariably belong to marine Fishes. Amongst the British food
Fishes which produce pelagic ova may be mentioned the Gadidae
{e.g. Cod, Whiting, Hake, Ling), the Pleuronectidae {e.g. Turbot,
Brill, Sole, Plaice), Scombridae {e.g. Mackerel), Triglidae {e.g. the
Gurnards), Percidae {e.g. the Bass), and Clupeidae like the Pil-
chard and Sprat, but not tlie Herring, whose adhesive demersal
eggs are deposited in clumps on shingly banks in the sea at
varying distances from the shore.

The eggs of Elasmobranchs are deposited singly or in pairs at

Fig. 23.5. — Diagrams of the pelagic ova of a Cod or a Plaice (A) and of a Ling (Molva).
O, Germinal disc ; M, micropyle ; O.G, oil globule ; V, yolk. (From Cunningham.)

considerable intervals, and the period of egg-laying is prolonged
over a considerable part of the year. In most other Fishes, as in
Teleosts, the period of spawning is limited to a few months,
usually in the spring and summer in temperate latitudes ; and in
the case of a single Fish it may last only a few days or weeks,
but the number of eggs produced is often enormous. Thus, in a
Ling 61 inches long and weighing 54 pounds the ovaries con-
tained 28,361,000 eggs. A Turbot, 17 pounds in weight, had
9,161,000 eggs; and a Cod of 21^ pounds 6,652,000. The
least prolific of the British food Fishes is the Herring, in which
the number of ovarian eggs varied from 21,000 to 47,000 in four
specimens examined.^ The extraordinary fecundity of many Fishes
seems to bear no relation to the relative abundance of the Fishes
themselves, but rather it is to be associated with certain disad-

^ Cunningham, oj). cit. p. 69.

BREEDING 413

vantaoes attendant on the sexual relations of Tislies, involvino' a
considerable waste of the sex-cells, while in many Fishes it no
doulit helps to compensate for any subsequent mortality among
the larvae, which may result from an uncertain and precarious
food supply and from the attacks of enemies. "Whenever internal
fertilisation is the rule, or when, as in nest-buildino' and mar-
su})ial Fishes, the propinquity of the sexes in the breeding season
ensures the fertilisation of a larger proportion of the eggs and the
protection of the young, the number of eggs produced is small.

The male sex-cells or spermatozoa are essentially similar to
those of other Vertebrates, although in different Fishes they maj^
vary in such details as length, and the shape and size of the
head, which may l)e rod-like and wavy, elliptical or globular.

As a rule, in Fishes females are more numerous than males,
and generally they are larger, but to both statements there are
notable exceptions. The relations of the sexes in the breeding-
season are usually very promiscuous, especially in those Teleosts
which discharge their sex -cells while swimmino; together in
shoals. A female may, however, consort with several males
{polyandry), or a male with several females (polygamy) ; or, as in
some of the nest-building Fishes (e.g. Gastrostevs), there are not
wanting examples of the pairing of one male with one female
{mo7ioga77iy).

Fishes often migrate at the commencement of the breeding-
season to localities most suitable for the deposition of the eggs.
Many marine species seek banks or shallower water near
the shore, and some, like the Salmon and the Sturgeon, are
anadromous, and ascend rivers for long distances to deposit their
spawn.

In all Fishes except the Elasmobranchs and a few Teleosts
the fertilisation of the eggs takes place in the water after their
extrusion, the male depositing its seminal fluid over the eggs or
in their neighbourhood. The waste of the sex-cells is often, no
doubt, very considerable, especially when the eggs are adhesive
and fixed, and the seminal fluid is liable to drift at the mercy of
tides and currents. With pelagic ova tlie waste is perhai)s not
so great, inasmuch as the eggs as well as the spermatozoa
would proliably drift at the same rate and in the same direction.
Liability to waste must also be greatly diminished in many
Fishes by their habit of living in shoals, or of congregating

414 FISHES CHAP.

together in the breeding season, in which they are sometimes
aided by their power of emitting characteristic sounds, and in tlie
case of nest-building Fishes by the still more intimate relations
of the sexes. Even when the liability to waste is very great,
compensation may be afforded by exceptional fecundity. The
copulation of the sexes and the internal fertilisation of the eggs
occur only in Elasmobranchs and some Teleosts. The copulatory
organs of Elasmobranchs are the so-called " claspers " with which
the males are provided. Some form of copulation is probably the
rule in the viviparous Teleosts, where the eggs are fertilised in
the oviducts, or even while they are still in the ovaries, and the
young are born alive. As mentioned above, an intromittent
organ is often formed by the prolongation of the genital or the
urinogenital orifice into a papilla, or a longer or shorter tube.^
Some Cyprinodontidae ^ (e.g. AnaMeps) have the anterior part of
the anal fin modified in the male to form an intromittent organ
along which the urinogenital canal runs (Fig. 374). In the
females the genital aperture is covered by a special scale, which is
free on one side and not on the other. " The male organ in some
individuals is turned to the right, in others to the left, and in
some females the opening beneath the special scale is to the right,
in others to the left. Copulation thus takes place sideways, a
left-sided male pairing with a right-sided female, and vice versa." ^
The anal fin also forms an intromittent organ in the " Half-beak "
{Hemirhamphus). In a genus {Girardinus) of the same family
the anal fin is modified to form an apparatus for holding the
female during sexual congress.'' The singular method of fertilisa-
tion practised by the males and females of Callichthys 2'>aleatus is
referred to elsewhere.^

With the exception of the pelagic Antennarius, which builds
its nest in the Sargasso weed in mid-ocean, nest-building and
parental solicitude for the young are confined to freshwater
Fishes and to marine forms with demersal ova. Pelagic ova
must necessarily be beyond the scope of parental care. As a
rule it is the male which acts as guardian nurse, the female
troubling herself but little about the fate of her eggs or her

1 Guitel, Arch. Zool. Expir. ct Gin. (3), i. 1893, p. 611.

- Garman, Mem. Mus. Comp. Zool. xix. 1895, No. 1, p. 11.

•'' Cunningham, o;;. cit. p. 358.

•• H. V. Jhering, Zeitschr. wiss. Zool. x.\xviii. 1883, p. 468. ^ See p. 592.

BREEDING 415

olisi)rin(j;. Perliaps the more primitive form of parental i'ore-
sight is exliibited by those Fishes which, like the females of the
Salmouidae, make a furrow in the gravelly bottom of a ruiniiiig
stream for the reception of the eggs, and then cover them over
with a layer of gravel, or like the Siluroicl Arias australis, of the
Burnett river in Queensland, which deposits its eggs in circular
excavations in the sandy bed of the river and covers them with
layers of large stones. But in neither case does it appear that
either the male or the female takes any further interest in the eggs
or in the young wiien hatched. Without actual nest-building,
or even tlie preparation of a place for their reception, the eggs
may be protected in various ways by the male. The common
British Gunnel or Butter-Fish (Pholis gunncUus) rolls its eggs

.^!|gSg ^«w»— ■-■

Pig 2n6. — Tlie Butter-Fish [PhoHs gnnnelhis) coiling roinul a mass of eggs.
(From Cumiiiigliam, after Holt.)

into a rounded mass by coiling its body round them, the male
and female taking possession of them alternately. The little
clumps of eggs are then deposited in holes made by the boring
Mollusc, PJiolas. Some British Blennies attach their eggs in a
single layer to the sides of cavities in rocks, or between stones,
where tliey are watched over by the male parent. The eggs of
the Lump-Sucker (C^/clopterus lumpus) are attached in masses to
rocks or to piles and guarded by the male, who aerates them hj
keeping up a flow of water over the spawn through the action
of his pectoral fins. When hatched, the young fry cling to
the body of their watchful parent by their suckers. A more
decided approach to nest -building is exhibited by the Sand
Goby (Gohius minutus). In this species the male scoops out
the sand from Ijeneath an empty shell, generally that of a
Pecten, and the female deposits her adhesive eggs on the imdcr

4i6

FISHES

surface of the shell. The male remains on guard, and by tlie
movements of its pectoral fins promotes the aeration of this rude
form of nest. References to some of the more striking examples
of true nest-building in Fishes will be found in the systematic
part of this volume, especially in those chapters treating of the
Dipnoi and Amiidae, and such Teleosts as the Mormyridae,
Osteoglossidae, Siluridae, Gastrosteidae, Centrarchidae, Osphro-
menidae, Labridae, and Antennariidae. Other illustrations of

9- -

■U.j

Fig. 237. — Showing the embryos of Rfiodens amarus in the gill-cavities of Unio. e, Em-
bryos ; g, inter-laniellar cavities ; i.l.j, an inter-lamellar junction. (From Olt. )

parental care are to be found in the development of mar-
supial pouches or grooves for the reception of the eggs in the
males of the Syngnathidae (Fig. 387) and the females of the
Solenostomidae, and the use of the oral cavity for a similar
purpose in the males, rarely in the females, of some Siluridae,
and tlie males or females, according to the species, of the
Cichlidae. The singular method by which the female Aspredo
safeguards both her eggs and her progeny is referred to on
p. 596. The Cyprinoid, Rhodeus amarus (the " Bitterling " of
Central Europe), is unique in the means which it adopts to

BREEDING 417

secure the same result.^ By means of its long ovipositor the
female Fish deposits its eggs in the mantle cavity of a Unio,
or of an Anodon. Here they are fertilised by spermatozoa
carried in through the inhalent siphon of the Mollusc with the
inspiratory water current, and they complete their development
in the gill-cavities (Fig. 237).'^

The time which elapses between the fertilisation of the egg
and the hatching out of the young Fish varies greatly in
different Teleosts. The eggs of some Clupeidae hatch in a very
short time, two to three days in the Anchovy, and three to four
days in the Sprat. In most of the British marine food Fishes the
period rarely exceeds twelve to fourteen days. The larger
demersal eggs with much food-yolk are longer in hatching ; in
the Salmon the time ranging from thirty-five to one hundred
and forty-eight days. A low temperature lengthens the time.
The eggs of the Herring which hatched in eight to nine days at
a temperature of 52° to 58° F. took forty-seven days in water at
32° F.

The extent to which the development of the embryo proceeds
while it is still enclosed in the egg-membranes, and consequently
the condition of the embryo when hatched, depends largely but
not exclusively on the quantity of food-yolk which is present in
the egg and available for the nutrition of the embryo during its
earlier stages. Embryos hatched from pelagic ova are very
small and imperfectly developed. The mouth is usually not yet
formed. The median fins, which later become isolated, are
continuous, and the caudal fin is diphycercal, although it sub-
sequently becomes homocercal after passing through a hetero-
cercal stage. The blood is colourless, and even the gill-clefts
may at first be lacking. In this condition the newly-hatched
Fish is nourished at the expense of the residual food-yolk, which
is enclosed in a yolk-sac projecting from the ventral surface of
the body (Fig. 238). As the yolk is gradually used up the mouth
is formed, and the young Fisli feeds on the minute organisms of
various kinds living in the sea, and by degrees the form, propor-
tions, and structure of the more mature Fish are acquired. In
the case of tlie larger demersal eggs the young are not only
longer in hatching, but when hatched they are larger and more
advanced in development. The young of many Fishes are

1 Olt, Zcitschr. idss. Zool. Iv. 1893, p. 543. - Cf. p. 584.

VOL. VII 2 E

4 1 8 FISHES CHAP.

provided with larval or provisional organs, and they may be so
unlike the adult in other respects that their subsequent develop-
ment assumes the form of a more or less striking metamorphosis.
As examples of larval organs, mention may be made of the adhesive
or cement organs of the larval Cho.ndrostei and Holostei, and of
the Dipnoi (e.g. ProtoiJtenis and Zejndosiren), and also of a
Teleost, probably the Mormyrid {Hyperopisus hebe, Lacep) ; ^ the
cutaneous gills of the Crossopterygii and some Dipnoi ; the so-
called external gills of such Teleosts as Colitis, Gymnarchus
(Fig. 239), and Seterotis, which are singularly like those of
Elasmobranchs ; and the defensive spines which are developed
on the scales or scutes of the trunk, and the dermal bones of
the skull, in the young of some Plectognathi The most striking

Fig. 238. — Newly-tatched embryo Teleost from a pelagic egg. A, Auditory organ;
E, eye ; FJSI, coDtiuuous median fiu ; Ht, heart ; 1, intestine ; X, nostril ; Yk,
yolk-sac. (From Cunningham.)

metamorphosis to be found in Fishes occurs in the Fiat-Fishes
and in the Eels, an account of which will be found in other
parts of this volume (pp. 685, 602).

The only examples of viviparous Fishes occur in certain families
of Elasmobranchs,^ and in five families of Teleosts, viz. the Blen-
niidae, the Cyprinodontidae, the Scorpaenidae, the Comephoridae,
and the Embiotocidae.^ In the Teleosts mentioned the eggs are
fertilised while they are still either in the ovarian ovisacs or in
the cavity of the ovary, and their development may take place in
either position. In such Cyprinodonts as Gamhusia and AnaUeps
the embryos are developed in the ovisacs, but as a rule both
fertilisation and development occur in the ovarian cavity. During
a prolonged gestation the yonng are nourished partly by the food-

1 Budgett, Trans. Zool. Soc. xvL Ft. ii. 1901, p. 130. ^ See Chap. XVII, p. 434.
^ Eigenmann, Bull. Fish Comm. (U.S.), 1892, p. 381 ; Arch. Entwickclungsmech.
iv. 1896, p. 125 ; Cunniughani, op. cit. p. 356, et scq.

BREEDING

419

yolk present in the eggs, and partly by a nutritive secretion
derived from the ovarian walls or from the epithelial wall of the
ovisacs as the case may be. In AnaUeps the secretion of the
walls of the ovisacs is absorbed by papillae developed on
the surface of the yolk-sac of the embryo along the course of its
blood-vessels. The eggs of the Embiotocidae have little food-
yolk, and the embryos are mainly nourished by the secretion of
the ovarian walls, which is swallowed by the embryo and
absorbed by villi on the inner surface of the intestine. The
number of young produced varies considerably. In the Embi-

FiG. 239. — Young Gymnarchus niloticus, with its large yolk-sac {ij.s) and its long
external gills (e.g). (From Budgett.)

otocidae the ovarian cavity contains 40 to 50 young. The
viviparous Scorpaeuid, Sebastes norvegicus of Northern Europe,
produces, it has been estimated, about 1000 young, while the
Blenny (Zoarces vivijmrus), the only other European viviparous
Teleost, produces from 20 to 300 or more, according to the size
of the female. In the Blenny the eggs are hatched in about
twenty days, but the young are not born until about four
months after fertilisation, when they are about an inch and a
half long, and in every outward respect similar to the adult
Fish.

Besides the distinction between the sexes resulting from the
different nature of their gonads and sex-cells, the males and
females are often distinguished by secondary sexual characters

420 FISHES CHAP. XV

(" sexual dimorphism " ^). As mentioned above, females are usually
larger as well as more numerous than the males, although in one or
both respects the reverse may be the case. Secondary sexual char-
acters are best marked in Teleosts, where they are generally related
to the special role which each sex takes in the deposition and
fertilisation of the eggs, and the nurture and protection of the
young, of which, examples have already been given. To a more
limited extent they may be associated with the struggle of the
males for the females, and in at least a few Teleosts the
exuberant coloration of the males in the breeding season suggests
that instances of courtship and sexual selection are not altogether
wanting.^

Although the vast majority of Fishes are dioecious, instances
of functional hermaphroditism are not unknown in a few Teleosts.^
Species of the Percoid genus Serranus (e.g. >S'. cahrilla, S. hepatus,
and aS^. scriha) are invariably hermaphrodite and self-fertilising.
Chrysophrys auratus is an example of successive hermaphroditism,
the male and female sex-cells ripening alternately. As an
occasional variation hermaphroditism has been recorded in several
other Teleosts, including amongst others such well-known Fishes
as the Cod, the Mackerel, and the Herring. The relations of
the gonads in hermaphrodites is subject to much variation.
In the Cod, for example, the testes may be double, each being
continuous with the hinder end of the ovary of its side, or there
may be only a single testis confluent with the anterior or the
posterior portion, or with some other part of the surface, of either
the right or left ovary. In other Teleosts individuals occasionally
present themselves with a testis and an ovary on opposite sides.

^ For a general account of Sexual Dimorphism in Fishes, see Cunningham's
Sexual Dimorphism in the Animal Kingdom , London, 1900, pp. 178-227. Some of
the more striking examples of Sexual Dimorphism are mentioned in the chapters
dealing with the different families of Fishes.

^ Holt, "On the Breeding of the Dragonet (Oalliomjmiis lyra)," P.Z.S. 1898,
p. 281.

^ Howes, Lin7i. Soc. Journ. Zool. xxiii. 1891, p. 539, where references are given
to the literature of the subject.

CHAPTER XVI

CYCLOSTOMATA (SYSTEMATIC)

CLASS I. CYCLOSTOMATA

The Cyclostomata, or, as they are sometimes called, the
Marsipobranchii, from the pouch-like, or rather sac -like, shape
of their branchial clefts, are divided into two orders, the jBrst
comprising the " Hag-Fishes " or " Borers," and the second the
Lampreys.

Order I. Myxinoides.

The Hag-Fishes are probably the most primitive of all exist-
ing Craniates. The mouth is nearly terminal, and there is no
buccal funnel. The naso-pituitary involution communicates be-
hind with the oral cavity and functions as a channel for the
in-streaming water-current to the gilis. Four pairs of short
tentacles, supported by a special tentacular skeleton, are present
in relation with the mouth and the terminally -placed naso-
pituitary orifice. The gill-sacs open directly into the pharynx.
The branchial basket is but feebly developed, and at the most it
is only represented by small isolated cartilages in relation with
the external branchial apertures. The lingual apparatus is
remarkably developed. Besides the lingual teeth there is only a
single dorsal tooth in the roof of the mouth. The dorsal arcualia
are restricted to the tail, or they extend for a short distance only
into the trunk. A spiral valve is absent. There is a row of
mucus-secreting sacs along each side of the body. The brain has
no obvious cerebral hemispheres, nor a cerebellum. Only one
semicircular canal is present in the auditory organ. The eyes
are degenerate, and the usual eye-muscles with the cranial nerves

421

422

FISHES

supplying them have atrophied. The embryonic pronephros is
retained in the adult. The eggs are large ; segmentation is
meroblastic ; and development is direct, without a larval meta-
morphosis. Two families can be distinguished.

Fam. 1. Myxinidae. — Gill-sacs not exceeding six pairs, with
a common external aperture on each side of the body.

The family includes a single genus, Myxine, of which the
common Hag {M. glutinosa) from the North Atlantic is the best
known species (Figs. 92, A, and 240). This Hag-Fish occurs
off the coasts of Northern Europe, including the British Isles, as
w^ell as on the Atlantic sea-board of North America,-^ southwards
to Cape Cod. Other species are found off the coasts of Chili

Fig. 240. — Myxine glutinosa. A, lateral view; B, view of the ventral surface of the
head, showing the mouth and tentacles, l.l.p, Lateral pore-like apertures of the

mucus-sacs ; v, anus.

and Japan. Myxine is quasi-parasitic in its habits, boring its
way into the bodies of large Fishes. By means of its rasping
" tongue " it devours all the soft parts of its prey, leaving little
more than a mere shell of skin and bones. The Fishes usually
attacked are the Cod and other Gadoids, but the Sturgeon is not
immune, and the presence of a Hag in the abdominal cavity of a
Shark {Lamna cornuhica) has been recorded. Myxine has the
reputation of being very destructive to Fishes caught on lines,
and it is said that whole " catches " have been destroyed by its
depredations, so that North Sea fishermen have been forced to
change their fishing-ground. To what extent the Hags attack
Fishes which are living and free is somewhat uncertain, but the
little evidence obtainable seems to point to the conclusion that,
as a rule, they only prey on Fishes when the latter are hooked or
netted, or injured or dead. When not seeking food the Hag lives

^ The American Hags probably belong to a distinct species, M. limosa Girard ;
Bashford Dean, Science (KS.), xvii. 1903, p. 433.

CYCLOSTOiM ATA 423

in the mud of the sea-bottom at depths ranging to nearly 350
fathoms. They are able to swim very rapidly in an undulatory
eel-like fashion. M. ghctinosa may grow to a length of nearly
two feet. The Hag has been described as a protandrous herma-
phrodite, that is, it is first a male and then a female, the gonad
of the young first producing spermatozoa, and at a later period
becoming an ovary and giving rise to eggs. This view has
hitherto met with general acceptance, but it has recently been
urged with some force that the presence of the two kinds of sex-
cells in a young animal is no proof of functional hermaphroditism,
since it is not uncommon " to find immature eggs in the testis
of many Vertebrates (Teleosts, Petromyzon, Amphibia), where the
assumption of hermaphroditism, to say nothing of its protandric
form, is entirely unwarranted."-^ Myxine produces eggs similar
to those of Bdellostoma. Nothing is known of its breeding
habits, or of its embryology.

Fam. 2. Bdellostomatidae. — Gill-sacs 6-14 pairs, all with
separate external orifices. Bdellostoma (Fig. 92, B) is found on
the Pacific sea-board of both North and South America, at the
Cape of Good Hope, and on the coasts of New Zealand. The
numerical variation of the gill-sacs in different species, and in
different individuals of the same species, and even on opposite
sides of the same individual, is very remarkable. Out of 354
examples of the Californian species {B. stouti) examined by
Dr. Ayres," 101 had 1 1 gill-sacs on each side ; 2 6 had 1 1 on
one side and 12 on the other; 208 had 12 on each side;
1 1 had 1 2 on one side and 1 3 on the other ; and 8 had
13 on each side. Occasional specimens may have 14 gill-sacs
on each side. The variations are apparently quite independent
of size, age, or sex ; and when the gill-sacs are asymmetrically
developed, the additional sac may be either on the right side or
on the left. In the Chilian species there are 1 gill-sacs on each
side, but in the species from the Cape of Good Hope the number
is reduced to 6 or 7. Bdellostoma closely resembles Myxine in its
habits and mode of feeding. The Californian species attaches
itself to the gills or to the isthmus of large Fishes, and then
rapidly bores its way into the body, devouring the viscera and
muscles but leaving the skin intact. It usually attacks large

1 Bashford Dean, Kupffer's "Festschrift," Jena, 1899, p. 227 et seq.
2 Journ. Morph. xvii. 1898, p. 213.

424

FISHES

Flounders and species of Sehastodes, and it is especially destruc-
tive to Fishes taken in gill-nets. At Monterey every net in
the summer contains the empty shells of eviscerated Fishes, and
when these are taken out of the water the Hag scrambles out
with great alacrity. Large fishes of even 30 pounds weight are
often captured without either flesh or viscera, and it cannot be
supposed that they entered the net in this condition.^ The
species lives on the sea-bottom most abundantly at a depth of
10-20 fathoms, but becomes rarer as the water deepens or
becomes shallower.

Fig. 241. — A, Cluster of the eggs oi BdeUostoma stonti, connected by the interlocking of
their anchor-shaped filaments ; B, the animal pole of an egg, showing the polar
" anchors " and the opercular ring. (From Bashford Dean.)

The eggs of the Calif ornian Bdellostoma are large, varying in
size from 14"3-29 mm. in length, and from 6"8— 10"5 mm. in
width, *and each egg is enclosed in a horny egg-case secreted
by the epithelium of its ovarian ovisac " (Fig. 241). At each
pole of the egg-case there is a tuft of numerous horny filaments
which end in 2- 3- or 4-hooked, anchor-like extremities. In
the centre of the tuft of filaments at the animal pole of the egg
the egg-case is perforated by a micropyle, and a little below this

^ Jordan and Everraaiin, Biul. U.S. Nat. Ihis. No. 47
and Middle America, Pt. i. 1896, p. 6.
^ B. Dean, oj). cit. p. 230 et seq.

The Fishes of North

XVI CYCLOSTOMATA 425

point the case is encircled by an opercular groove, which enables
the polar portion to be thrown off like a cap at the time of
luitching, so as to allow the young Bdellostoma to make its
escape. The large size of the egg, which almost completely fills
the cavity of the egg-case, is due to the fact that it consists
mainly of food yolk, the germinal protoplasm containing the
nucleus forming only a small hillock near the inner extremity of
the micropyle. Bdellostoma spawns during the greater part of

Fig. 242. — Embryo of Bdellostoma stoiiti near the time of hatching.
(From Bashfonl Deau. )

the year, but chiefly in the early summer, and probably about
20 eggs are deposited at one time, generally on a shelly or
rocky bottom. After deposition the eggs become connected
together in long chains or clusters by the interlocking of their
polar hooks. Fertilisation takes place after extrusion, and the
segmentation is meroblastic and discoidal, much as in Teleosts.
The embryo completes its development within the egg, and when
hatched it is a miniature of the adult.

Order II. Petromyzontes.

In the Lampreys there is a large suctorial buccal funnel
leading behind and above into the mouth, which is supported
by special cartilages, and furnished with a marginal fringe of
small cirri. Numerous horny teeth are present on the inner
surface of the funnel as well as on the tongue. The naso-pitui-
tary involution forms a caecum and does not communicate with
the mouth. The gill-sacs, seven in number, open externally by
separate orifices, but internally they open into a median branchial
canal, situated below the oesophagus and opening into the mouth
in front. There is a well-developed branchial basket. Dorsal
arcualia are present throughout the precaudal as well as in the
caudal region. A rudimentary spiral valve is present. The
brain consists of parts usually present in other Craniates, includ-
ing cerebral hemispheres and a cerebellum. Tlie auditory organ

426 FISHES CHAP.

has two semicircular canals, and the eyes are not degenerate.
The pronephros is suppressed in the adult. The eggs are small ;
the segmentation is holoblastic ; and there is a larval meta-
morphosis. There is but one family.

Fam. 1. Petromyzontidae. — The family has a nearly world-
wide distribution. Most Lampreys are marine, although to a
greater extent in some species than in others, but all of them
seem to ascend rivers for spawning. The genus Petromyzon is
characteiistic of the northern hemisphere, where it is repre-
sented by various species on the coasts and in the rivers of
Europe, West Africa, Japan, and N'orth America. Three species,
widely distributed in Europe, occur in the British Isles, viz. :
— the Sea-Lamprey {Petromyzon marinus), which may reach or
even exceed three feet in length, and is also found on the west
coast of Africa and on the Atlantic coast of North America ;
the " Lampern " or fresh-water Lamprey (P. fiuviatilis), about 1 8
inches long ; and the Sand-Pride, Sand-Piper, or lesser fresh-
water Lamprey (P. 'planeri), usually less than a foot in length.
Ichthyomyzon, Bathymyzon, Entersphenv.s, and Lam'petra are also
northern forms, collectively distributed along the Atlantic and
Pacific coasts and in the rivers and great lakes of North
America.^ Other Lampreys occur only in the southern hemi-
sphere. Geotria is common in the rivers of Chili, Australia,
and New Zealand ; and another genus, Mordacia, has a parallel
distribution, being found on the coasts of Chili and Tasmania.
A new genus and species from Chili has been recently
described under the name of Ilacrophthalmia chilensis?
This Lamprey, which is only 107 mm. in length, has re-
markably large eyes (2*5 mm. in diameter), vertically com-
pressed gill -clefts, and a simple dentition resembling that of
Myxine. All Lampreys are carnivorous. They feed by attach-
ing themselves to the bodies of Fishes by their suctoral buccal
funnels, and then rasping off the flesh with their lingual teeth.
While thus engaged they are carried about by their victims.
Salmon have been captured in the Ehone with the marine
Lamprey attached to them. The Lamprey usually keeps near
the bottom, either swimming with a graceful serpentine move-

^ Jordan and Evermann, op. cit. p. 9 et scq.

2 Plate, Sitzungsb. d. Gcsellsch. Naturforsch. Freunde Berlin, No. 8, 1897,
p. 137.

CYCLOSTOMATA

427

ment, or attached to stones by the buccal funnel. In the spring
the Sea-Lamprey ascends the rivers to spawn, and, after deposit-
ing its egcrs in furrows which it excavates in the river-bottom,
it returns to the sea. The river-Lampreys spawn in the smaller
streams and brooks. The Xorth American Brook -Lamprey,

Fig. 243. — Spawning of the Brook- Lamprey (P. icilderi). On the right side of the figure
a male is attached to the head of a female. (From Bashford Dean and F. B.
Sumner. )

Petromyzon {Lamiddra) wilderi, which is found in the neighbour-
hood of New York, deposits its eggs on the gravelly bottom of
a brook, in a small gravel-filled hole lying between a number
of large rounded stones^ (Fig. 243). In the vicinity of the
" nest " some ten to twelve Lampreys congregate, the males,
however, being much more numerous (five to one) than the

1 Bashford Dean and F. B. Sumner, Trans. X. T. Acad. Sci. xvi. 1897, p. 321.

428

FISHES

females. Much energy is spent by both sexes in moving stones
by lifting them with the buccal funnel, but it is not always
clear that this is done to circumscribe the nest, or to remove
impeding obstacles. Eventually, a male attaches himself to the
back of the head of a female, who at the same time is holding
fast to a stone. The male then rotates its body so that the
urino-genital papilla is brought near the genital orifice of the
female, and the simultaneous extrusion of eggs and spermatozoa

at once follows. Owing to
the small amount of food-
yolk which they contain
the eggs of the Lamprey
(e.g. P. planeri) are small,
measuring about 1 ' 1 — 1 • 2
mm. in length, and from
0"9 — 1"0 mm. in width.
There is a micropyle at
the animal pole of the egg,
but the characteristic horny
egg-case and the polar
hooks of the Myxinoids
are both wanting. The
embryo hatches out as a
larva known as the " Am-
mocoetes." At this stage
of its development the larva
lacks several of the most
striking features which
characterise the adult, and it is highly probable that the Ammo-
coetes represents a stage in the evolution of Vertebrates in some
respects intermediate between Amphioxus and a very primitive
Craniate. The mouth of Ammocoetes is bounded laterally and in
front by a curious hood-like upper lip, and behind by a short
transverse lower lip (Fig. 244). The eyes are deeply seated and
rudimentary, and as visual organs they are useless, but the parietal
eye is well developed. As in the adult, there are seven pairg of
gill-sacs, but they open internally into a pharynx, directly con-
tinuous behind with the rest of the alimentary canal, and there
is no dorsal oesophagus. Like the skull, the branchial basket is
still very rudimentary. The dorsal and caudal fins are con-

FiG. 244. — Head of the Ammocoetes of P.J/uvia-
tilis. A, ventral view ; B, side view. br. 1,
First iDranclual aperture ; eye, eye ; LI, lower
lip ; na.ap, naso-pituitaiy aperture ; n.l, upjier
lip. (From Parker aud Haswell, after W. K.
Parker. )

CVCLOSTOMATA 429

tinuous. A gall-bladder is present, and also a bile duct open-
ing into the gut. In its mode of life, and especially in the
manner in which it obtains its food, Ammocoetes presents a
most remarkable resemblance to Amphioxus and the Ascidians.
In the median line of the pharyngeal floor there is an open
groove, the hypopharyngeal groove or endostyle, and a tract of
ciliated cells along the dorsal wall represents a hyperpharyngeal
groove : connecting the two in front there is a peripharyngeal
cilated groove.^ The Ammocoetes feeds on small food particles
carried through the mouth into the pharynx by currents of
water produced by ciliary action. The food becomes entangled
in strings of mucus probably secreted by the cells lining the
endostylar groove. The mucus is then swept upwards in^the
pharyngeal groove, and finally wafted backwards to the stomach
and intestine by the cilia of the hyperpharyngeal band. The
skin exhibits the remarkable peculiarity of containing a peptic
ferment capable of digesting proteids in a "2 per cent solution
of hydrochloric acid. As the larva lives buried in the mud, the
epidermic secretion probably helps to keep the skin free from
bacteria, microscopic spores, and fungoid, or other parasitic
growths.^ The young Lamprey lives as an Ammocoetes from 3-4
years, and then in the course of a few weeks in the winter it under-
goes a metamorphosis, losing its larval characters and acquiring the
structure and habits of the adult. During this period the buccal
funnel is completed and teeth are developed. The eyes approach
the surface and become functional. The continuity of the median
fins becomes interrupted. The endostylar groove becomes trans-
formed into a thyroid gland, the gall-bladder disappears, and the
bile duct becomes obliterated and changed into a mass of small
follicles. The skull and branchial basket complete their develop-
ment. At the same time the pharynx loses its connection with the
rest of the alimentary canal and remains as the branchial canal.
The so-called oesophagus of the adult is apparently a new formation
which grows forwards and acquires a connection with the mouth.
It is probable that it represents a hyperpharyngeal groove con-
stricted off from the dorsal wall of the pharynx.

Both the marine Lamprey and the " Lampern " are captured

1 Dohi-n, Mitth. Zool. Stat. Neapel, vi. 1886, p. 59 ; Shipley, Quart. Journ.
Microsc. Sci. xxvii. 1887, p. 325.

2 R. Alcock, Journ. Anat. and Phys. xiii. (N.S.), 1899, p. 623.

430 FISHES CHAP. XVI

for food, either by nets or wicker traps. Formerly the Lampern
was taken in enormous numbers in several British and Irish
rivers, especially in the Severn from February to May, and in
the Thames during May and June, but for various reasons the
supply has much diminished in recent years. The Lampern
makes excellent bait for Cod and Turbot, and for this purpose
large numbers used to be taken in the Trent and Thames for
despatch to Grimsby and other fishing ports.^

^ Day, Fishes of Great Britain and Ireland, Load. ii. 1SS0-S4, p, 360.

CHAPTER XVII

ELASMOBEANCniI : GENEEAL CHAEACTEKS PLEUEOPTERYGII

ICHTHYOTOMI ACANTHODEI ■ PLAGIOSTOMI SELACHII

BATOIDEI HOLOCEPHALI

CLASS II. PISCES.

Sub-Class 1. Elasmobranchii.

Ix both the ancient and the modern Sharks, Dog-Fishes, and
Eays, the exoskeleton takes the form of a more or less uniform
investment of dermal denticles or " shagreen." The endo-
skeieton is wholly cartilaginous or partially calcified, and there
are neither cartilage- nor membrane-bones. The vertebral colunm
is acentrous or chordacentrous, generally with alternating basi-
and inter-dorsal elements, and terminating in a heterocercal tail.
The skull is usually hyostylic, very rarely amphistylic or auto-
stylic, and the lateral halves of the primary upper jaw (palato-
quadrate cartilages) usually meet in a highly characteristic
median symphysis beneath the base of the skull. Branchial
arches and clefts are five to seven in number, and the clefts are
separated by complete interbranchial septa, which, as a rule, are
continuous externally with the skin. An operculum is developed
only in the Holocephali. A pelvic girdle is present. "With rare
exceptions the pectoral fin is uniserial. The pelvic fin is in-
variably uniserial. The exoskeletal supports of all the fins
consist of ceratotrichia, and, when present, the fin-spines are
invested by enamel. Claspers are generally present in the males.
In the surviving members of the group the nostrils retain
their primitive ventral position. There is a conus arteaiosus
with several rows of valves. A spiracle, often furnished with a
spiracular pseudobranch, is generally present, and, as a rule,

431

432 FISHES CHAP.

there is a hyoidean hemibranch supplied with venous blood from
the ventral aorta. The gill-filaments are attached throughout
their length to the interbranchial septa. There is an optic
chiasma. An air-bladder is not developed. The intestine has a
spiral valve, and there is a cloaca. The gonoducts in both sexes
are derived from the kidney system. The ova are large, few in
number, and enclosed in horny egg-cases, and they are fertilised
before extrusion. The segmentation is meroblastic, and the
embryo is furnished with long external gills.

The Elasniobranchs are for the most part active predaceous
Fishes, living at different depths in the sea, from the surface to
nearly a thousand fathoms, and ranging from mid-ocean to the
shallower waters round the coasts in almost every part of the
world. Although typically marine, they sometimes ascend rivers
beyond the reach of tides, and a few are permanent inhabitants
of fresh water. They are most abundant in tropical and sub-
tropical areas, where they also attain their greatest size, and are
numerous in temperate regions, but there are some species which
are typically Arctic. None of them are small, and some of the
Sharks are the largest of living Fishes. All are carnivorous, but
so diversified is their food that in different species it may range
from other Fishes of no mean size to Molluscs, Crustaceans and
other Invertebrates, or even to plankton. In their breeding habits
the Sharks and Dog-Fishes present many interesting features.
Unlike the generality of Fishes, the eggs are fertilised internally
as a sequel to the copulation of the sexes. For this purpose the
males are furnished with special intromittent organs, the myxo-
pterygia or so-called claspers, which are developed as modifica-
tions of the hinder portions of the pelvic fins.^ Each clasper is
supported by an internal skeleton, consisting of several cartilages
derived from the radialia of the fins, and is traversed along its
inner aspect by a groove. When sexual congress takes place the
claspers are thrust through the cloaca of the female into the ovi-
ducal orifices, and in some instances it is probable that they are
retained in this position by hook-like denticles developed at
their free extremities. The seminal fluid then flows along these
conduits into the oviducts, in the upper portions of which it meets
and impregnates the eggs. After fertilisation the egg is enclosed
in a dark brown horny egg-case, secreted by the oviducal gland.

' Den Danskc Ingolf-Expedition, ii. No. 2, Copenhagen, 1898.

ELASIMOBRANCHII

433

As a rule each egg-case has but a single egg, but in Bhinohatus
and Trygonorliina (Batoidei), both of which are viviparous, each
case contains three to four eggs. Generally the egg-cases are
somewhat quadrangular in shape, with the four angles, two at
each end, prolonged either into short horns, or into long tapering
tendrils (Fig. 246). The oval egg-cases of the Heterodontidae
are remarkable not only for their size, but also for the presence
of a broad spiral lamina winding
round the exterior of the case
from one pole to the other (Fig.
245). The majority of the
Sharks, Dog-Fishes, and Eays are
viviparous, that is, the young are
born alive ; the rest, like the
Scylliidae (e.g. the common British
Dog-Fishes, Scyllium canicula and
>S'. catidus), the Heterodontidae,
and tlie Eaiidae are oviparous,
that is, tlie young are hatched out
after the extrusion of the eggs.
In the oviparous species the eggs
are extruded either singly or in
pairs, and generally deposited on
the sea-bottom. When, however,
the egg-cases are provided with
tendrils, as, for example, in the
two British Dog-Fishes just men-
tioned, these organs act as anchor-
incr filaments. When extruding
an egg, the female swims round

and round some piece of upright seaweed, and the curling tendrils
Ijecome entwined round it in such a w^ay that the egg becomes
securely attached thereto (Fig. 246).^ The embryos are long in
developing, and in Scyllium it may be several months after
fertilisation (200 to 275 days) before they are hatched, the
young Fish finally escaping through a rupture in the egg-case.
In the oviparous species the nutritive food-yolk stored up, first
in the egg and subsequently in the yolk-sac (Fig. 248), suffices
for tlie nourishment of the embryo until the period of hatching,

Flo. 245. — Egg-case of Hetcrodontns
[Cestracion) galeat'us. (From Parker
and Haswell, after Waite. )

VOL. VII

Cunningham, Marketable Marine Fishes, London, 1896, p. 64.

2 F

434 FISHES CHAP.

but in viviparous forms, whose embryonic development is
completed within special uterine dilatations of the oviducts,
additional means of nutrition are provided for the young. Such
Elasmobranchs as Spinax, Acanthias, Centrina, Scymnv.s, Trygon,
Torpedo, and MijUoliatis have long filaments (villi or trophone-
niata) developed from the inner surface of the uterus, wliich
secrete a nutritive fluid, and this fluid is eitlier absorbed by the
blood-vessels of the embryonic yolk-sac, or it is taken up by
the embryo in some more dii-ect manner. In some of the
Trygonidae and Myliobatidae of the Indian Ocean it seems prob-
able that the secretion is taken into the alimentary canal of
the embryo either through the mouth or through the open

Fig. 246. — Egg of the Spotted Dog-Fish {Scyllium canicida), showing its mode of
attachment after extrusion. (From Hertwig, after Kopsch. )

spiracles.^ One species, Pteroplatea micrura, has its long and
highly vascular and glandular trophonemata gathered into two
bundles, which are thrust through the huge spiracles into the
pliarynx of the embryos, of which there may be from one to three,
and the nutritive secretion is apparently digested in the
alimentary canal of the embryo and absorbed by the foetal
blood-vessels (Fig. 247). A few Sharks, like most species of
Mustelus, develop a placenta when the food-yolk in the yolk-
sac of the embryo is nearly used up. Folds or projections from
the highly vascular wall of the yolk-sac interlock with similar
vascular folds of the lining membrane of the uterus, and a
diffusion of nutrient material takes place from the maternal
blood in the uterine blood-vessels to the foetal blood in the
^ Wood-Mason and Alcock, Proc. Roy. Soc. 49, 1891, p. 359.

ELASMOURANCIIII

435

vessels of the yolk-sac.^ Each embryo has its own placenta, and
iu Ifustelus aritarcticus the uterine portion of the oviduct is
divided by septa into several chambers, each containing a single
embryo.^ It is worthy of note that in the viviparous species a
distinct l;)ut very thin, delicate egg-case is formed, occasionally
even with tlie rudiments of tendrils, which may either be
retained or thrown off' in the
uterus. The Greenland Shark
(Laemarijiis horealis) is unique
amongst Elasmobranchs. Its
eggs are small and unprotected
by egg-cases, and their fertilisa-
tion is said to be effected in the
water after deposition, as iu the
generality of Fishes.

Fossil remains of Elasmo-
liranchs in the shape of fin-spines
(ichthyodorulites) and dermal
denticles, associated with various
Ostracodermi (Coelolepidae, Pter-
aspidae, and Cephalaspidae), are
amongst the earliest undoubted
indications of Vertebrate life.
They first appear in the Upper
Ludlow Eone Bed and in Silurian F^o. 247.
rocks in other parts of Europe,
and in North America ; and in
greater or less abundance the
group is represented in almost
every subsequent geological period,
group shows signs of decadence, for Elasmobranchs still survive
in apparently undiminished numbers and variety in the marine
fauna of the present day.

The Elasmobranchs are certainly a very prindti\'e race of
Fishes. Their earliest representatives of whose structure we
have any precise knowledge (e.g. Cladoselache and Fl cur acanthus)
are in many respects the most archaic of known gnathostomatous

-Embryo of an Indian Sting
Ray {Ptenqjlatea micrura) as seen
when the litems is laid open. t. t.
Two bundles of trophonemata in-
serted into the spiracles, s}i, sj).
(From Wood- Mason and Alcock.)

It cannot be said that the

^ Leydig, Mlkrosl: Anat. u. Entwick. d. Rochen u. Haic, Leipzig, 1852, p. 90
" T. J. Parker, Trans. Xeiu Zealand Inslit. x.xii. 1889 (1890), p. 3G1.

436

FISHES

y.S-

FiG. 248.

Craniates, and from such types as these, amongst others, we may
reasonably look for the ancestors of all or most of the remaining
groups of Fishes. It has been well said of Pleuracanthus that
" it is a form of Fish which might with little modification become
either a Selachian, Dipnoan, or Crossopterygian," ^ while the con-
dition of the primary

upper jaw in the Chon-
drostean Pohjodon sug-
gests that even the more
primitive Actinopterygii
had an Elasmobranch
origin. The important
researches of Dr. Tra-
quair render it also
highly probable that the
ancient Ostracodermi
may claim kinship
through their Coelolepid
ancestors with some primitive type of Elasmobranch ; and within
tlie limits of the group there is ample evidence that differentia-
tion has taken place on many divergent lines, of which we have
notable examples in such specialised offshoots as the Acanthodei
and the Holocephali, to say nothing of several highly specialised
families which became extinct at successive periods in the history
of the group.

-An embryo Shark, with its yolk-sac
{y.s) ; sp, spiracle.

Order I. Pleuropterygii.

The only certain representative of this group is the Palaeozoic
form Cladoselache, probably the most primitive Elasmobranch at
present known (Fig. 249). Elongated and somewhat cylindrical
in shape, Cladoselache " has a terminal mouth, five or possibly
seven pairs of branchial clefts, and a pair of olfactory organs,
lateral in position near the extremity of the snout. AVide-based,
triangular pectoral and pelvic fins, a low anterior and a posterior
dorsal fin, devoid of spines, and a heterocercal caudal fin with
homocercal tendencies, are present, but no anal fin has yet been

' Smith W'oodward, Vertebrate Palaeontology, Cambridge, 1S98, p. 32.
^ B. Dean, Journ. Morjih. ix. 1894, p. 87. Trans. New YorkAcud. Sci. xiii. 1894,
p. llf).

ELASMOBRANCIIII I'LEUROPTEKYGII

437

detected. The exoskeleton consists of minute lozenge -shaped
denticles, wliich invest the body und extend on to the surfaces
of the fins, and there is also a circumorbital ring of several con-
centric rows of small square plates. A lateral line, in the form
of a groove between two rows of denticles, extends along each
side of the l)ody. The notochord is persistent. Calcified neural
and haemal arches (basidorsals and basiventrals) have been ob-
served in the caudal region, where they correspond numerically
with the remains of the myotomes, but interdorsal or inter-
calary arcualia seem to be absent. The upper and lower jaws,

Fig. 249. — Restoration of Cladoselarhefyleri. Lateral and ventral views.
(From Parker and Haswell, after Dean.)

similar in size and shape, are apparently supported by a hyo-
mandibular cartilage ; hence the skull is hyostylic. The endo-
skeletal supports of the pectoral, and especially those of the
pelvic fins, exhibit a more primitive disposition than in any
other Fishes. They extend nearly to the distal margins of the
fins, where they seem to interdigitate with the proximal ends of
feebly-developed ceratotrichia (Fig. 145). The extension of the
fins in the horizontal plane, the gradual shading off of their
broad bases into the sides of the body, and the resemblance
between their radialia and those supporting median fins, are very
suggestive of the origin of the paired fins from continuous lateral
fin-folds. Claspers are absent. The dentition is well developed,
and several rows of teeth seem to be functional at the same time.

43S FISHES CHAP.

EacH tooth consists of a broad base, supporting a long pointed central
cusp and a variable number of similarly shaped but much shorter
lateral cusps. The teeth in the various transverse rows from
without inwards are closely wedged together by the interlocking
or overlapping of their bases.

Fam. 1. Cladoselachidae. — Several species of Cladosclaclie,
varying from 2 to 5 feet in length, have been found in the
Cleveland Sliale (Upper Devonian) of Ohio. Isolated teeth
similar to those of Cladosclaclie occur in the Lower Carboniferous
of Europe, India, and North America, and have been referred to
various species of the genus Cladodus, but with one exception
nothing more is known of the structure of these Fishes, and
conse(|uently their relationship to Cladosclache is doubtful.
C. nellsoni} from the Lower Carboniferous (Calciferous Sand-
stones) of Kilbride in Scotland, has a very different type of
pectoral fin, which appears to be distinctly uniserial, but inter-
mediate in structure between the biserial fin of Pleuracanthus
and that of the modern sharks. There are several other genera
from the Devonian and Lower Carboniferous whose claims to
inclusion in this group rest on no better foundation.

Order II. Ichthyotomi.

While more specialised than the Pleuropterygii the Fishes
included in this group represent an extremely generalised type
of Elasmobranch, which, as already indicated, may easily have
been the ancestor of more than one group of Fishes. In the
typical genus Pleitrcicanthvs" (Fig. 250)^ the body is elongate,
but slightly depressed, with a terminal mouth, and a tapering
diphycercal tail fringed above and below by a continuous caudal
fin. A long dorsal fin, two small anal fins, and well-developed
paired fins with contracted bases, are present. The head is
armed with a prominent, serrated, dorsal spine, but it is doubtful
if dermal denticles (shagreen) are present. The vertebral column

1 Tra(nuiir, Gcol. Maij. (3), v. 1888, p. 81 ; Trans. Gcol. Soc. Glusgoiv, xi. 1897,
p. 41.

- For references see Zittel's Text-Book of Palaeontology (Eng. trans, ed. by C. R.
Eastman), London and New York, ii. 1902, pp. 22-23.

^ See also restoration of Pleuracanthus gaudryi from the Coal - Measures of
Commentry, Allier, France, by C. Brongniart ; Zittel, o^). cit. p. 23.

ELASMOBRANCHII — ICHTHYOTOMI

439

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is acentroiis, and the persistent notocliord supports a series of
^asidorsal cartilages, which alter-
nate with small interdorsals, a
series of hasi^'entrals supporting-
small ribs, and in the caudal
region well - developed haemal
arches. The dorsal fin is sup-
ported by slender, tri-segmented
radialia, which appear to be
twice as numerous as the neural
arches in the trunk, but in the
dorsal lobe of the caudal fin
the two structures agree in
number. Ventrally - prolonged
haemal spines are the sole endo-
skeletal supports of the inferior
lobe of the caudal. The coraco-
scapular cartilages of opposite
sides remain distinct, and each
supports a biserial fin. The
pelvic girdle is represented by
a pair of small cartilages sup-
porting the basipterygia. The
pelvic fins are uniserial, with
post-axial skeletal supports for
claspers in the males. Both
the median and the paired fins
are provided with marginal
ceratotrichia. The skuU is
probably amphistylic. Five,
possibly six or seven, branchial
arches, bearing clusters of
minute denticles, are present.
Circumorbital plates are want-
ing. All the endoskeletal struc-
tures are partially calcified. The
teeth are tricuspid, each with
two long divergent lateral cusps
and a minute median cusp ; the broad bases of the teeth overlap
and articulate with one another by means of facets.

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440 FISHES • CHAP.

Fam. 1. Pleuracanthidae. — The single family included in
the group ranges from the Lower Carboniferous to the Lower
Permian. Within these limits the family is widely distributed
in different formations in Great Britain, Continental Europe,
Xew South Wales (Lower Hawkesbury Formation), and North
America. PI eur acanthus, of which complete skeletons and skulls
have been found, is the best known genus.

Order III. Acanthodei.

The Fishes comprising the Acanthodei ^ may be regarded as a
highly specialised and terminal offshoot from some primitive race
of early Elasmobranchs. The Elasmobranch kinship of the Acan-
thodei is indicated by their exoskeleton of shagreen tubercles ;
the completely heterocercal tail ; the absence of an operculum,
the external gill-clefts apparently being exposed ; the position
of the lateral line of the trunk between two rows of shagreen
denticles ; the nature of the powerful spines in connexion with
the dorsal and anal, and the pectoral and pelvic fins ; and the
formation of the hard parts of the skeleton, not by ossification
involving the presence of bone -cells, but by the calcification of
cartilage, or of more superficial membranous or fibrous tracts.
On the other hand, it may be noted that the Acanthodei appear
to have undergone much specialisation on lines in some respects
parallel to those which have marked the evolution of the
Teleostomi, but by methods which are simply an exaggeration of
features normally characteristic of Elasmobranchs. Perhaps the
most striking illustration of this is to be seen in the develop-
ment of a species of secondary skull by an extension of a process
of calcification as distinguished from ossification. Hence the
presence of membrane-calcifications in relation with the upper
and lower jaws, whose development is proportional to the size of
the teeth they support, and of smaller investing plates of the
cranial roof. Similar exoskeletal calcifications, when most com-
pletely developed (e.g. Diplacanthus), form a dorsally incomplete
arch, apparently corresponding to a secondary pectoral girdle
for the support of the stout pectoral spines, in which elements

^ A. Fritsch, Fauna clcr Gaskohlc in Bblnncn, ii. Prague, 1889 ; Kncr, SB.
Akad. IViss. Wien Math.-Naturw. CI. Ivii. Ft. i. 1868, p. 290 ; Traijuair, Geol.
Mag. (3), v. 1888, p. 511, and (4) i. 1894, p. 254.

XVII ELASMOBRANCHII ACANTHODEI ' 44 1

analogous to clavicles or cleitlira and infra- clavicles can be
recognised. Each pectorjil spine forms the preaxial margin of
the fin, and behind it there is a series of ceratotrichia. Nothing-
is known of the endoskeletal supports, but having regard to the
nature and proportions of the pectoral spines it may be inferred
that the exoskeletal elements of the fins predominate over the
former to an extent which is only paralleled elsewhere in the
Teleostei.

Apparently the notochord is persistent, and there are long
and slender neural and haemal arches, but no ribs. The dermal
denticles are uniform in size, and so small as to give a granular
appearance to the skin. In structure they are thick, with a
fiat, enamelled, often sculptured, external surface, quadrate or
rhombic in shape, and fitting closely together. Teeth are either
absent or very minute, but sometimes (e.g. Acanthodo2)sis and
IschnacantJius) they are few in number and large, conical in
shape, occasionally with minute cusps between the larger teeth.
Claspers are absent. The Acanthodei are small Fishes, most of them
being less than '3 m. in length, and ranging from the Upper Silu-
rian to the Lower Permian inclusive. Two families are recognised.

Fam. 1. Diplacanthidae. — Two dorsal fins are present.
Usually there is a row of lateral spines extending along each
side of the body between the pectoral and pelvic fins. Exclusively
Upper Silurian and Devonian.

The genera Diplacanthus, Climatius, Parexus, Euthacantlius,
and IschnacantJius are all found in the Lower Old Eed Sandstone
of Scotland. Climatius and Dijjlaccmthus are also represented
in the Devonian of Canada.

Fig. 251. — Restoration of Aconthodes wardi. Carboniferous of England and Scotland.
(From Smith Woodward.)

Fam. 2. Acanthodidae. — A single dorsal fin; lateral spines
vestiu'ial or absent. Lower Devonian to the Lower rermian.

442 FISHES CHAP.

The widely-distributed genus Acanthodes (Fig. 251) is repre-
sented in the Lower Old Eed of Scotland, the Devonian of Siberia
and Canada, the Carboniferous of England and Scotland, and the
Lower Permian of France, Germany, and Bohemia. AcanthodojJsis
(Coal Measures), and Mesacanthus and Cheiracanthus (Lower Old
Eed) are the remaining genera.

Order IV. Plagiostomi.

Head prolonged in front of the ventrally-situated mouth as a
more or less prominent preoral rostrum, vertebral column consist-
ing of alternating basi- and inter-dorsal cartilages, generally
supported by more or less well-developed chorda-centra. Pectoral
and pelvic fins uniserial. Pelvic girdle and claspers present.
Except in two families the branchial arches and clefts are
invariably five in numbei*. An operculum is not developed.^

Sub-Order 1. Selachii.

Body elongate or fusiform, shading imperceptibly into a
powerful swimming tail. Pectoral fins of moderate size, with
contracted bases ; not confluent with the sides of the head.
Branchial clefts lateral in position. Vertebral centra generally
asterospondylic or cyclospondylic.

This sub-order includes such typical Elasmobranchs as the
modern Sharks and Dog-Fishes as well as numerous fossil repre-
sentatives ranging from the Carboniferous, and probably from
still earlier periods, to the present day.

Fam. 1. Notidanidae. — Body moderately elongate, the spine-
less dorsal fin opposite the anal. Mouth ventral ; nostrils ventral,
near the extremity of the snout, without oro-nasal grooves.
Branchial arches and clefts six or seven. Interbranchial septa
devoid of marginal frills. Notochord persistent and continuous,
partially constricted by simple chorda-centra, each consisting of two
distinct rings, without either concentric or radial lamellae, except

' Giinther, Study of Fishes, Edin. 1880 ; British Mvs. Cat. Fishes, viii. 1870 ;
Miiller and Henle, Syst. Beschr. d. Plagiost. Berlin, 1841. Hasse, Natiirl. Syst.
d. Elasmohr. Jena, 1879. Goode and Bean, Oceanic Ichthyology, W'ashington,

1895. Jordan and Evermann, Fishes of North and Middle America, 'Washington,

1896, Pt. i. Smith Woodward, Vertebrate Palaeontology, Cambridge, 1898 ; id.
Brit. Mus. Cat. Fuss. Fishes, i. 1889, ii. 1891 ; Zittel, op. cit.

XVII ELASMOBRANCHII PLAGIOSTOMI 443

ill one species (Xotidanus cinerevs), which exhibits a feeble
asterospondylisni in the caudal vertebrae. Skull aiuphistylic.
Teeth unlike in the two jaws ; those in the upper jaw usually
witli a large central cusp and smaller lateral cusps ; those in the
lower jaw comb -like, each consisting of numerous graduated
pointed cusps inclining in the same direction, and supported on
a long basal plate.

The very few species included in this family are widely
distributed in the tropical and subtropical regions of the Atlantic
and Pacific Oceans. JSfotidanus (ffe^^tanchics) cinereiis, which has
seven branchial arches and clefts, inhabits the Mediterranean
and Atlantic. JV. {Hexanchus) griseiis, with six branchial arches
and clefts, has a similar distribution, but besides being an
occasional visitant to the British coasts, it is not uncommon at
Cuba in the "West Indies. It is said to grow to a length of
26 feet.

Fossil remains of Kotidanus, principally teeth, occur in the
]\Iiddle and Upper Jurassic, in the Cretaceous, and in the Eocene
and Pliocene of England and the Continent.

Fam. 2. Chlamydoselachidae (Frilled Sharks). — Body much
elongate. Median tins as in Xotidanus. Mouth nearly terminal.
Xostrils lateral, nearly terminal, and without oro-nasal grooves.
Branchial arches and clefts six. The outer margins of the inter-
branchial septa are produced into overlapping cutaneous frills,
the first of which is developed from the hyoid arch and overlaps
the hyobranchial cleft, like a rudimentary operculum. Vertebral
column as in the preceding family, but in the hinder part of the
trunk the notochord is unconstricted and uniform in diameter,
centra being absent. Skull hyostylic. Lateral line an open
groove. Teeth alike in both jaws, each consisting of a broad
basal plate supporting three slender curved cusps, separated by a
pair of much smaller cusps.

The only living species known is Clilamydoselaclms anguineus
(Fig. 252),^ which occurs in the Pacific near Japan, in deep water
off Madeira, and also off the Azores and the coast of Xorway.
It reaches a length of -4 to 5 feet. Teeth from the Pliocene
deposits of Tuscany have been referred to an extinct species,
C. lawleyi.

^ Garman, Bull. Mus. Comp. Zool. Harvard, xii. No. 1, 18S5, p. 1 ; Giintber,
Chall. Ecp. Zool. xxii. 18S7, p. 2.

444 FISHES CHAP.

)Scarcely anything is known of the habits of the Notidanidae
and the Chlamydoselachidae. It is evident that they are closely-
related forms, and from the unusual number of their gill-clefts
and branchial arches, and the condition of the vertebral column,
it is also obvious that they are the most archaic of modern
Selachians.

Fig. 252. — Chhunydoselachus anguineus. (From Giiuther.)

Fam. 3. Heterodontidae (Bullhead Sliarks). — Head large and
high, with a blunt snout projecting but little in front of the
small and almost terminal mouth, and with prominent supraorbital
crests. Trunk thick -set and somewhat trihedral, covered with
fine shagreen. Nostrils ventral but nearly terminal, with oro-
nasal grooves. Spiracles small, beneath the eyes. Two dorsal
fins, each with a spine in front, the first opposite the interval
between the pectorals and pelvics, the second in front of the
anal. Vertebral centra asterospondylic when fully developed.
Palato-quadrate cartilages with an extensive articulation with
the sides of the preorbital regions of the cranium, the normal sus-
pensoria of a hyostylic skull (hyomandibular cartilages) taking
little share in their support. Dentition similar in both jaws.
Teeth at the symphyses numerous, small, and conical, furnished
with three to five cusps in the young; those behind broad and
pad-like, arranged in oblique rows, the teeth forming the two
middle rows being much larger than those in the front or behind.
Living species, oviparous. Egg -cases large, w'lih. an external
spiral lamina (Fig. 245).

About four species belonging to one genus, Hetcrodontus
( = Cestracion) (Fig. 253), or possibly to two, represent this
dwindling family. All are inhabitants of the Pacific Ocean (Japan,
Amboyna, Australia, the Galapagos, and the Californian coast of
North America). Little is known of their haljits. They feed

ELASMOBRANCIIII SKLACIIII

445

principally on Molluscs, the shells of which are crushed by their
massive grinding teeth. The dilferent species vary in size from
2 to 5 feet.

The Heterodontidae were the most characteristic and abundant
Sharks of the Mesozoic period. Amongst extinct genera Hyhodus
ranges from the Middle Trias to the Lower Cretaceous (Wealden);
an allied genus, Acrodus, from the Middle Trias to the Upper
Cretaceous (Gault). Pcdaeospinax occurs in the Lias and
possibly in the Upper Trias. Synechodus is a Cretaceous genus,
and Asteracanthns, which has large hooked spines on the head, is

Fig. 253. — Port Jackson Shark (Heferodontus philippi). A, lateral view ; B, mouth
and nostrils. d, Clasper. (From a specimen in the Cambridge University
Museum.)

characteristic of the Middle and Upper Jurassic. An even
greater antiquity may be claimed for the Heterodontidae if, as
is not improbable, such Palaeozoic Sharks as Orodus, Sphen-
acanthus, Tristychius (Carboniferous), and Wod7iiha (Permian)
belong to this family. Many ichthyodorulites are probably the
spines of various extinct Heterodontidae.

Fam. 4. Cochliodontidae.^ — This Palaeozoic family includes
a numljer of Sharks probably related to the Heterodontidae, but
of which little is known except their dentition. The teeth are
in some respects similar to those of Heterodontus, except that
those which appear to correspond to one or both of the middle
rows of the latter genus tend to fuse and form a few large,
convex, and often scroll-like plates. The typical Cochliodonts
are exclusively Carboniferous (Europe and North America).
Fsephodus, Fleuroplax, Deltodus, Foecilodus, Cochliodus, Deltopty-
chivs, Helodus, and Menaspis (Permian) are characteristic genera.
J Smith Woodward, Nat. Science, i. 1892, p. 671.

44^ FISHES CHAP.

Probably some ichthyodorulites described under various generic
names belong to this family.

Fam. 5. Psammodontidae. — ■ Teeth large, flat or slightly
arched, oblong or quadrate, and arranged in one, two, or more
longitudinal rows. Only the teeth are known, and from differ-
ences in their shape, size, and surface markings, the genera
Fsammudi(s, Archeobatis, and Copodns have been recognised.
The family is confined to the Lower Carl)oniferous of Great
Britain and Ireland, Eussia, Belgium, and North America.

Fam. 6. Petalodontidae. — Teeth transversely elongated, with
a blunt or a sharply-ridged crown, separated from a single or
multiple root by a constricted neck, and disposed in transverse
and longitudinal pavement-like rows ; exoskeleton of smooth, oval,
rounded or cjuadrate shagreen denticles. Only the teeth, and in
some genera the dermal denticles, are known, except in Janessa,
which has a Eay-shaped body, with large pectoral fins prolonged
towards the head. The family is mainly confined to the Car-
boniferous formations of Great Britain, Europe, and North
America. Petalodus, Janessa (also represented in the Permian),
Glossodus, Pohjrhizodus, and Calloj^risiodus are characteristic
genera.

Fam. 7. Scylliidae (Dog -Fishes). — Dorsal fins two in
number, small, and without spines, the first above or behind
the pelvic fins, the second usually behind the anal. Tail
not bent upwards or but slightly so, without lateral keels.
Spiracles present. Nictitating membranes absent. Vertebrae
asterospondylic. Teeth small, each with a median cusp, and
one to four small cusps on each side. Oviparous. Egg -cases
(Fig. 24G) large, quadrate, with long twining tendrils at the
angles for attachment.

The genus Scyllium includes the true Dog-Fishes (Fig. 254).
The species are coast Fishes of small or moderate size, and are
widely distributed in temperate and tropical seas, at depths not
as a rule exceeding 400 fathoms. Two species, S. canicnla and
S. catul'us, are common on the British coasts, living near the
bottom and feeding on Crustaceans and Molluscs. An allied
form, Pristhirus, is also common in European and British waters.
Chiloscyllntm is a widely -distributed genus ranging from the
Cape of Good Hope thi-ough the Indian Ocean to the coasts
of Australia, China, and Japan. Stegostoma tigrinum of the

ELASMOBRANCIIII SELACMII 447

Indian Ocean attains a length of 10 to 15 feet, and is renuirk-
able for its liandsome coloration of dark Lands on a yellow
ground, whicli lias suggested the name of Tiger- or Zehra-Shark.
The pelagic genus GingJymostoma has the terminal portion of the
tail bent upwards, and grows to a length of G to 12 feet. It is
represented by species in the Indian Ocean and the tropical
parts of the Atlantic (West Indies and the west coast of ]\Iexico).
Crossorhiiius includes species of large size, some of which are
10 feet long. They are ground-sharks, frequenting the coasts of
Australia aiid Japan, which lie on the bottom watcliing for their
prey, and in accordance with this liabit their coloration closely
resembles that of their surroundino-s.^ A laro-e North Atlantic
Shark {Pscudotriakis microdon), of which only two specimens are

Fig. 254. — A female Dog-Fisli {Sci/liium canescens), from the south-western coast of
Soutli America. (From Giiutlier. )

known, one taken on the Portuguese coast, and the other, 10 feet
in length, off Long Island, on the Atlantic coast of North
America, has tlie general characters of the Scylliidae, except that
the first dorsal fin is opposite the interval between the pectoral
and pelvic fins. Some Scylliidae live at great depths, Scyllium
{Scylliorhimis) profundorum having been obtained from a depth
of 816 fathoms in the North Atlantic."'

Most of the fossil Scylliidae belong to existing genera. Tlie
earliest known representatives of the family occur in the Upper
Jurassic (Lithographic Stone of Bavaria), where the extinct genus
Falaeoscy Ilium, a near ally of the existing Scyllimn, and Pris-
tiurus, are found, nearly complete. Scyllium itself ranges fi'oni
the Cretaceous through the different Tertiary formations. A
species of Chiloscyllium has been recorded from the Miocene
Tertiaries, and detached teeth of Ginglymostoma from the Eocene
of Belgium and North America. An extinct genus (Mesiteia),
which is found in the Upper Chalk of Mount Lebanon and the
Upper Eocene of Monte Bolca, is remarkable for the enclosure of

' Giinther, Study of Fishes, p. 328. ^ Goode and Bean, op, cit. p. 23.

448 FISHES CHAP.

its lateral sensory canals in a series of incomplete calcified rings,
as in the Holocephali.

Fam. 8. Carchariidae. — Sharks with two dorsal fins,
the first in front of the pelvic fins and the second opposite the
anal fin, both devoid of spines. Tail without lateral keels.
Preoral rostrum elongated. Mouth crescentic. Eyes with
nictitating membranes. Spiracles small or absent. Vertebrae
asterospondylic. Teeth usually consisting of a single triangular
cusp, with smooth, trenchant, or serrated margins, rarely with
basal cusps; generally with an axial cavity when fully developed.
Viviparous. The family comprises about twenty genera, and
approximately sixty species ; found in all seas, often in mid-ocean.
Amongst the more important genera may be mentioned Carcliarias
(CarcIinrJii.ius), Galeocerdo, Triahis, Thalassorliinus, Galeus, Mus-
telus and Scylliogcdeus.

Fig. 255. — The Blue Shark {Carcharias glaucus). (From Miiller and Henle.)

Species of Carcharias are found in nearly all tropical and sub-
tropical seas. The genus is a somewhat comprehensive one, and
groups of its species have been distinguished as sub-genera under
the names of Prionodon, Hy^toprion, Scoliodon, A^yrionodon} etc.
One of the most widely distributed of the thirty to forty species is
the Blue Shark, C. {Prionodon) glaucus (Fig. 255), of the Atlantic
and Pacific Oceans, which may grow to a length of 25 feet, although
the young forms not infrequently captured in British waters do
not exceed 6 to 8 feet. It is a slender, swift, pelagic Shark, of a
slaty-blue colour above and white underneath, and a voracious
hunter of other Fishes. C. nicaraguensis, a Shark about 7 feet
long, is confined to Lake Nicaragua and its outlet the Eio San
Juan, and is one of the very rare strictly freshwater Sharks.
Galeocerdo is a large Shark found in temperate and tropical
waters, but one species, G. arcticus, is confined to Arctic seas.
The variegated G. tigrinus, or West Indian Tiger-Shark, is said

^ Miiller and Henle, op. cit.

XVII ELAS.MOBRANXHII — SELACHII 449

to reach a length of 15 to 20 feet. The genus Galeus includes
the small Sharks commonly known as " Topes/' which are
common in nearly all tropical and temperate seas. The British
species, G. canis, which ranges from 4 to 6 feet in length, is a
bottom-feeding Fish, preying on Molluscs, Crustacea, Star-Fish,
and small Fishes. The various species of Mustelus, or " Hounds,"
resemble the Topes in their habits and distribution. Living
principally on Molluscs and Crustaceans, the dentition has lost
the trenchant, unicuspidate type characteristic of njost other
Carchariidae, and is adapted for crushing and grinding, the teeth
being flat, without cusps, and arranged in pavement-like rows.
Two species, M. vulgaris and M. laevis, are abundant on the
coasts of Europe and the British Isles. Scyllio galeus, which com-
bines the general characters of Mustelus with nostrils similar to
those of a Scyllium, is known only from a single specimen from
the coast of Natal.^

The Carchariidae are comparatively modern Sharks. Xo
undoubted remains are known earlier than the Eocene, in which,
as in the succeeding Miocene and Pliocene deposits, they are
represented principally by their characteristic teeth. The extinct
fossil genera are few in number, and so far as their dentition is
concerned they differ but little from their living allies.

Fam. 9. Sphyrnidae (Hammer - head Sharks). — In their
general characters the Hammer - head Sharks agree with the
Carchariidae. They are distinguished, however, by the remark-
able shape of the head, which is prolonged into two conspicuous
lateral lobes, supported internally by corresponding cartilaginous
outgrowths from the post-orbital and the lateral ethmoidal or
nasal regions of the skull, with the eyes at their distal extremities,
and the nostrils in relation with their anterior margins. One
genus and five species.

The Sphyrnidae are denizens of nearly all tropical and sub-
tropical seas. Sphyrna {Zygaena) tucles occurs in the Mediter-
ranean, and >S'. zygaena is a very rare visitant to the British coasts.
A specimen over 13 feet in length was captured at Ilfracombe
in 1865, and other examples have been taken off Banffshire, at
Xewlyn in Cornwall, at Yarmouth, and in Carmarthen Bay."
The shape of the head differs in different species, and in young

1 Boulenger, Ann. Mag. Nat. Hist. (7), x. 1902, p. 51.
- Day, British Fishes. London, 1880-8-1, ii. p. 291.
VOL. VII 2 G

450

FISHES

forms the peculiarities of the adult are less marked. In the
Bonnet Shark (>S'. tiburo) (Fig. 256, A), the head is crescentic or
kidney-shaped, with prominent postero-lateral angles, and between
this type of head and the more pronounced " hammer " of
aS*. zygaena (Fig. 256, B) an almost perfect gradation is supplied
by other species. The Hammer - heads are voracious Sharks,

Fig. 256. — Ventral view of the head and trunk (A) of a young Bonnet Shark (Sphyrna
tiburo), and (B) of a young male Hammer-head [S. zygaena). c, Clasper ; d,
cloacal aperture ; e, eye ; n, nostril ; «', nasal groove.

usually living in deep water, and they may grow to a length of
1 5 feet. As many as thirty-seven embryos have been taken from
the oviducts of a female nearly 1 1 feet in length.^

Teeth assigned with more or less probability to SiJhyrna are
found in the Miocene of Europe and North America.

Fam. 10. Lamnidae (Porbeagle Sharks). — Large, stout-bodied
Sharks with two dorsal fins, the first just behind the pectoral
fins, the second, which is small, opposite the small anal fin ; both

' Cantor, (quoted by Giinther, op. cit. p. 318.

ELASMOBRANCHII SELACHII 45 I

without spines. Tail with a prominent lateral keel on each
side. Nictitating membranes absent. Spiracles minute or
wanting. Branchial clefts very wide. No oro- nasal grooves.
Vertebrae asterospondylic. When fully developed the teeth are.
solid.

In the genus Lamna, which includes the Porbeagle Sharks,
the teeth are large, each consisting of a long narrow central cusp,
usually with smaller cusps at the base. The common Porbeagle
{L. cornvMca), a fierce pelagic Shark, which may reach a length of
10 feet, frequents the North Atlantic and the North Pacific
(Fig. 257). It has often been captured off the coasts of
Great Britain and Ireland in Mackerel or Salmon nets, or
by lines laid for food Fishes. An allied genus, Isurus, is

Fig. 257. — The Common Porbeagle {Lamna cornuhica). (From Parker and Haswell,
after Bashford Dean.)

represented by species on the Atlantic coast of North America,
in the Mediterranean and the neighbouring parts of the
Atlantic, and also in Asiatic seas. Carcharodon rondeldii ^ is a
pelagic Shark with large, triangular, finely-serrated teeth, without
basal cusps, and is found in all tropical and subtropical seas from
the Mediterranean to Australia and New Zealand. It is one
of the largest and most formidable of Sharks, and it is said to
grow to a length of 40 feet. Nothing is known of its breeding
habits. Odontasjns, which has minute pore-like spiracles, but
no lateral caudal keels, is a Shark of moderate size, chiefly
inhabiting the Atlantic, but found also in the Mediterranean
and the Southern Pacific. Its teeth are long and awl-like, with
small basal cusps.

The Thresher or Fox Shark {Alopecias tndpes) is remarkable
for the extraordinary length of the upper lobe of the caudal fin,
1 T. J. Parker, P.Z.S. 1887, p. 27.

452

FISHES

which is as long as the rest of the body (Fig. 258). Its teeth
are of moderate size, triangular in shape, and without serrations.
The " Thresher " has a wide distribution, being abundant in the
Atlantic and Pacific Oceans, besides being the commonest of the
larger Sharks frequenting the British coasts. It grows to a
length of 15 feet, of which the tail forms at least one-half.
Quite inoffensive to man, the Thresher feeds on the shoals of
smaller Teleosts, such as Pilchards, Herrings, and Sprats. When
feeding it swims in gradually diminishing circles round the
shoal, splashing the water with its long tail, and keeping its
victims so crowded together that they become an easy prey. A
remarkable Lamnoid Shark {Mitsukurina owstoni)} which has
the snout produced into a " long, flat, flexible, leaf- like blade,"

Fig. 258. — The Thresher Shark {Alopecias vuljjcs). (From Jordan and Evermanu.)

somewhat resembling that of Polyodon, but narrower and more
pointed, and has protractile jaws and large spiracles, is found
in deep water near Yokohama, and may prove to be generically
identical with the Cretaceous Shark Scapanorhynchusr

Lamnoid Sharks are not certainly known to have existed
until the Upper Cretaceous formations, in which, as w^ell as in
different Tertiary deposits, teeth indistinguishable from those of
the existing genera Lamna, Odontaspis, and Carcharodon are
found. The interesting genus Carcharodon has one extinct
species in the Cretaceous and several others distributed in
Tertiary formations in nearly every part of the world. The
teeth of some of the Tertiary species measure 5 inches along
the margin and 4 inches across the base, and it is evident that
they belonged to Sharks so gigantic as completely to dwarf the
existing species. That these giant Lamnidae have only recently

' D. S. Jordan, California Acad. Sci. (3), Zool. i. 1898 ; Bashford Dean, Science
(N.S.), xvii. 1903, p. 630.

- Smith Woodward, Ann. Mag. Nat. Hist. (7), iii. 1899, p. 487.

ELASMOBRANCHII SELACHII

453

become extinct is proved by the fact that similar teeth have been
dredged from the bottom of the Pacific. Teeth and detached
vertebrae from various Tertiary deposits have lieen referred to
species of AloiKcias. Entire Fishes, with an elongated rostrum
and an extensive anal fin, from the Cretaceous of Mount
Lebanon, have been assigned to an extinct genus, Scapanorhynchus.

Fam. 11. Cetorhinidae (Basking Sharks). — Two dorsal fins,
without spines, the anterior midway between the pectoral and
pelvic fins. Tail without lateral keels. Nictitating nieml)ranes
absent. Spiracles small, situated just above the angles of the
mouth. Branchial clefts wide and of great vertical extent,
extending from the dorsal to the ventral surface. Teeth small,
very numerous, conical in shape, without serrations. Claspers of
the male provided with horn-like denticles.

The single species included in this family, the Basking Shark,
(Cetorhinus (Selache) maxim us), is one of the largest of living
Fishes, reaching a length of 40 feet (Fig. 259). It is a pelagic

Fig. 259. — The Basking Shark (Cetorh inns (Selache) maximus).
(From Goode and Bean.)

Shark, inhabiting the Arctic seas, but wandering as far south on
opposite sides of the Atlantic as the Mediterranean, the coasts of
Portugal and Virginia, and in the Pacific to the Californian
coast. Although generally described as a northern form, Ceto-
rhinus is known to occur in Australian waters.^ It is fairly
common off the coasts of Scotland, and it has been seen or
captured at various points on the western coast of Ireland, and

^ Kershaw, Victorian Natural, xix. 1901, p. 62 ; "Waite, lice. Austral. Mus. iv.
1901, p. 263.

454 FISHES CHAP.

the eastern and southern coasts of England. The Fish is
gregarious in its habits, often swimming in shoals near the
surface. The name " Basking Shark " has been suggested by
its habit of lying motionless on the surface in warm or calm
weather, as if basking in the sun, with its dorsal fin protruding
from the water. Unless attacked, this Shark is quiet and
inoffensive. It derives its food-supply from small pelagic Fishes,
and also from marine Invertebrates, which are strained from the
water by the fringes of long, slender gill-rakers with which
the branchial arches are provided. At one time harpooned
and caught off the Irish, Scotch, and Norwegian coasts for the
sake of the oil obtained from its liver, the Fish is now of little
economic importance. Nothing is known of its mode of repro-
duction.

Extinct species of Cetorliinus have been founded on detaclied
vertebrae and isolated teeth from deposits of Pliocene age in
Belgium and Italy, and possibly from still earlier Tertiary
formations. Dermal spines similar to those found on the
claspers of the males in the existing species occur in the Antwerp
Crag, and in the Eed Crag of Suffolk.

Fam. 12. Rhinodontidae. — Two dorsal fins, without spines,
the anterior a little in front of tlie pelvic fins, the second opposite
the anal. Tail with lateral keels and a pit at its root. Spiracles
small. Nictitating membranes absent. Mouth and nostrils
nearly terminal. Teeth very minute, numerous, and conical in
shape.

One genus, Bhinodon, with one or two species, is known. These
Sharks are very widely distributed, specimens having been seen
or captured in the neighbourhood of Ceylon, at the Seychelles,
the Cape of Good Hope, Callao on the Peruvian coast, in the
Gulf of California, and off the coast of Florida. Bhinodon is
probably the largest known Shark. It is stated to exceed 50
feet in length, but to be quite harmless. Scarcely anything is
known of its habits, but the small size of the teeth, and the
length of the gill-rakers, which resemble those of tlie Basking
Shark, suggest a similar kind of food.

Fam. 13. Spinacidae. — Two dorsal fins, the first in advance
of the pelvic fins. Anal fin absent. Nictitating membrane
absent. Spiracles rather large. Vertebrae cyclospondylic. Teeth
variously modified in different genera.

XVII ELASMOl'.RANCHII SELACHII 455

The more typical representatives of this family are the Spiny
Dog-Fishes, which are distinguished by the presence of a strong
spine in front of each dorsal fin. They are more abundant in
temperate regions than in the intervening tropics. The more
important genera are Acanthias,Centrina,Centro2)horus, Sinnax, Siin\.
Centroscyllium. Aca7ithias vulgaris, the Picked or Piked Dog-
Fish, is a gregarious, voracious Shark, about 3 to 4 feet in
length, and is frequently seen in huge shoals all round the
I'ritish coasts, especially during the summer months. It is very
destructive to food Fishes, and its ravages result in serious loss
to fishermen. Acanthias is viviparous. Centrina salviani is a
much smaller Shark, which frequents the Mediterranean and the
Bay of Biscay ; on rare occasions it has been taken off the
southern coast of England. Centrophorus occurs in deep water
in the Mediterranean and adjacent portions of the Atlantic, and
off the coasts of Japan. Centroscyllium is found on opposite
sides of the North Atlantic (Greenland and Massachusetts), and
in the opposite hemisphere at the Falkland Isles. A deep-water
form, Paracentroscyllium, has been obtained in the Bay of Bengal
at depths from 285 to 405 fathoms.^

Three remaining genera {Scymnus, Laeniargus, and Echino-
rhimis) differ from the preceding in the absence of dorsal spines.
Scymnus lichia is common in the Mediterranean and the neigh-
bouring parts of the Atlantic. The Greenland Shark {Laemargus

Flu. '260. — The Greenland Shark (Laemargiis borealis). (From Goode and Bean.)

horealis) (Fig. 260) is an inhabitant of the Arctic regions, wander-
ing as far southwards on opposite sides of the Atlantic as
the French coast and Cape Cod. It is a huge, clumsy shark,
reaching a length of 26 feet. Numerous instances are recorded
of its capture off the coasts of Great Britain, especially in northern
waters. The Greenland Shark is said to be a determined foe to

' Alcock, A}m. Mag. Nat. Hist. (6), iv. 1889, p. 379.

456

FISHES

the Eight Whale, which it attacks, biting pieces out of its body.
Scymnus is viviparous, Laemargus oviparous, and the latter is
unique among Sharks in producing eggs devoid of a horny shell,
which are deposited on the sea-bottom. Echinorliiniis has dermal
denticles in the form of relatively large rounded tubercles, each
surmounted by a tuft of fine spines. One species only is known,
E. spinosus, a large Shark attaining a length of 10 feet, and
frequenting deep water off the Atlantic coasts of Europe and
Africa from tlie North Sea to the Cape of Good Hope. A single
specimen has been taken at Cape Cod on the eastern coast of
the United States, and another off Dunedin, New Zealand.
The capture of thirty examples in British waters since 1828
has been recorded,^ the largest a female 9 feet in length.

Most of the existing genera of Spinacidae are represented by
teeth or detached spines in the later Tertiary deposits, but none
are certainly known to occur earlier than the I'liocene.

Fam. 14. Rhinidae (Angel-Sharks). — Ray-like Sharks with a
flattened head and body, and nearly terminal mouth and nostrils.
Pectoral fins very large, horizontally expanded, but constricted
at the base and not adherent to the sides of the head or trunk.

sp

ft i>

^■

<r'

nf " >».^

Fig. 261. — The Angel-Shark (llliind sqiiatina). A, dorsal view ; B, view of the month
and nasal Ijarbels. ^j.f. Pectoral tin ; ^.it'./, pelvic tin ; s^j, spiracle.

Two dorsal fins, both small, without spines, and situated on the
tail behind the pelvic fins. Anal fin absent. Spiracles large

^ Day, oj). cit. ]). 324. See also Stead, Journ. Mar. Biol. Ass. iv. 1895-97,
p. 264.

XVII ELASMOBRANCHII — SELACHII — BATOIDEI 457

and cresceiitic. Vertebrae tectospoiidylic. Teeth conical iuid
pointed. A single sp(3cies only is known.

liJiina sqnatina, the Angel-Shark or Monk-Fish (Fig. 201), is
intermediate between the ordinary Sharks and the Skates and
Eays, both in external appearance and internal structure, l)ut
is more liay-like tlian Shark-like in its habits. Witliin the
temperate and tropical regions of both hemispheres it is almost
cosmopolitan in its distribution, frequenting the coasts of Europe,
including the British Isles, the Atlantic and Pacific coasts of
North America, and the shores of South Australia and Japan.
The Angel-Shark is viviparous, producing about twenty young at
a time. Not rarely it grows to a length of 5 feet.

The family ranges from the Upper Jurassic to the present
time. Species of Bhina are represented by more or less com-
plete skeletons in the Lithographic Stone of ]^>avaria, and in the
Upper Cretaceous of Westphalia and Mount Lebanon, and 1jy
teeth and vertebrae in the English Chalk, as well as in different
European Tertiary formations.

Fam. 15. Pristiophoridae. — Prenasal portion of the head
and cranium produced into a long flattened rostrum, furnished
with a pair of long tentacles on its under surface, and, as in
Saw-Fishes, w4th a series of large, tooth-like, dermal denticles, of
equal or unequal size, along each of its lateral margins. Two
dorsal fins, without spines, the first in front of the pelvics. No
anal fin. Pectoral fins large, distinct from the head and trunk,
with a contracted base. Spiracles large and crescentic. Teeth
small, witli a conical cusp and a broad base.

These singular Sharks closely resemble the true Saw-Fishes
(Pristidae), but they differ in the lateral position of their gill-
clefts, the presence of rostral tentacles, and their smaller size.
The few species known belong to the genus Fr'istio2yhorus, and
are confined to the Australian and Japanese seas.

Pristiophorus is represented in the Upper Cretaceous of
Mount Lebanon, and in the Miocene deposits.

Sub-Order 2. Batoidei.

Body generally discoidal or rhoml)ic in shape, the axial
portion being formed by the flattened head and trunk, and the
lateral portions by the enormously expanded pectoral fins, which

458 FISHES CHAP.

are usually confluent with the sides of the head. Tail slender,
sharply marked otf from the trunk, to which it usually appears
as a mere appendage. Dorsal tins, when present, on the tail.
Anal fin absent. Branchial clefts ventral in position. Spiracles
large, usually crescentic. Vertebrae tectospondylic.

For the most part the Batoidei are sluggish ground -Fishes,
slowly moving over the sea-bottom by the gentle undulatory
vibrations of the margins of their huge pectoral tins, the tail
being of little use in locomotion. They feed principally on
Crustacea, Molluscs, and the smaller Teleosts. As with other
Fishes of similar habits, the coloration of the dorsal surface
harmonises with that of the sea-bottom, while the ventral surface
is either deficient in pigment or white. The majority of them
are coast Fishes, rarely descending to a greater depth than 500
fathoms, but some are pelagic. The Batoidei are a relatively
modern race, first appearing towards the middle of the Mesozoic
period, and evidently representing an assemblage of specialised
Elasmobranchs adapted for a bottom-living existence. As re-
marked by Smith Woodward, the three families, Rhinobatidae,
Eaiidae, and Trygonidae, are not so clearly differentiated before
the close of the Cretaceous period as they subsequently become.^

The first two families, the Pristidae and the PJiinobatidae, are
interesting connectincr-links between such Selachii as the Ehinidae
and the Pristiophoridae and the more specialised Batoidei like the
Skates, Eays, and Trygons. While they agree with the latter in
the ventral position of the gill-clefts, the absence of an anal
fin, and the caudal position of the dorsal fins, the body still
retains an elongated and somewhat Shark-like shape, and shades
off imperceptibly into a powerful swimming tail, and in the
Pristidae at all events the pectoral fins are of moderate size
and free from any fusion with the sides of the head. It must be
admitted that the institution of the two sub-orders introduces
a somewhat arbitrary distinction between certain families of
Plagiostonies which has little to recommend it except custom
and some measure of convenience. The two series of Fishes shade
almost imperceptibly into one another, and the importance of
the ventral position of the gill-clefts has probably been over-
estimated. Primitively, the gill-clefts are lateral, and lie wholly
in front of the pectoral fins, a position which is retained in many
' Vertebrate Palaeontology, Cambridge, 1898, p. 32.

XVII ELASMOBRANCHII BATOIDEI 459

Selachii. In others, liowever, the hinder gill-clefts tend to
extend backwards above the base of the pectoral fins, while in
some the clefts assume a more ventral position, and extend
beneath the pectoral fin ; hence, even within the limits of the
Selachii the position of the gill-clefts varies to the extent that
these structures may be lateral, or they may tend to become
either dorsal or ventral.^ On the score of coin^enience the
customary usage is adopted here.

Fam. 1. Pristidae (True Saw-Fishes). — Although somewhat
depressed, the body is still elongate and Shark-like, with a well-
developed tail terminating in a heterocercal caudal fin. Dorsal
fins large, the first opposite the pelvic fins. Head and skull pro-
longed into a long flattened rostrum, the lateral margins of which
are armed with a series of strong tooth-like denticles, firmly im-
planted in sockets in the calcified rostral cartilage. No rostral

Fig. 262. — The Savz-Fisli (Pristis antiquontm). (From Cuvier.)

tentacles. Teeth in the jaw^s minute and obtuse. One genus
and about four or five species are known, all inhabitants of
tropical and subtropical seas.

Some of the true Saw-Fishes attain a considerable size, 10
to 20 feet or even longer, and "saws" 6 feet long and a foot
in width across the base are not uncommon. By means of
powerful lateral strokes of its saw the Fish is capable of
lacerating the bodies of other animals and tearing off pieces
of flesh, which it then devours. Indian species are known to
ascend rivers beyond tidal influence, and an American species,
ranging northwards to the West Indies and the Gulf of Mexico,
where it is abundant, enters the lower Mississippi. F. anti-
quorum occurs in the Mediterranean and the Atlantic, but does
not extend so far northward as the British coasts.

The earliest known representative of the family is the

^ I am indebted to Mr. Bouleiifrer for these observations.

460

FISHES

extinct genus SclerorhyncJius from the Upper Chalk of Mount
Lebanon, in which the smaller size and more superficial position
of the rostral " teeth," and the absence of sockets in the rostral
cartilage, prove that the " teeth " approximate more to ordinary
dermal spines in this genus than in any of the more recent Saw-
Fishes. An extinct genus Projnistis, from the Upper Eocene of
Egypt, with non-socketed teeth, and species of the existing genus
Pristis from the English Middle Eocene, are also known.

Fam. 2. Rhinobatidae. — Owing to the increased expansion
of the pectoral fins and the forward growth of their anterior
cutaneous portions along the sides of the head, as well as back-
wards along the trunk, the body now assumes a sub -rhombic
shape, and approximates to the disc of the more typical Batoidei,

Fig. 263. — Rhinohatus granidatus. (From Miiller and Heiile.

but the tail with its dorsal and caudal fins is still strongly
developed, and blends imperceptibly with the trunk in front.
Teeth very obtuse. No electric organs. About five genera and
twenty species are known, distributed in most tropical and sub-
tropical seas.

The cosmopolitan Rhinohatus is represented by species from
the Mediterranean, the Eed Sea, the west coast of Africa, the
Indian Ocean, Australia and China, as well as from the Atlantic
and Pacific coasts of America, and the Galapagos. Rhyncliohatus
ranges from the Eed Sea through the Indian Ocean to China,
Zwptenjx occurs at San Diego and Panama, and Flati/rhinoidis
on the Californian coast. Trygonorhina is an Australian
genus.

The family dates from the Upper Jurassic. Rhinohatus is-

ELASMOBRANCIIII — BATOIDEI

461

represented by complete skeletons in the Lithographic Stone of
Bavaria, the Upper Cretaceous of Mount Lebanon, and the Upper
Eocene of Monte Bolca. Trygonorliina occurs in the Eocene.

Fam. 3. Raiidae (Skates or Eays). — The endoskeletally
supported portions of the large pectoral fins extend along the
lateral margins of the trunk and head fi-om the pelvic fins to
the snout, and are confluent therewith, forming the lateral por-
tions of a large rhombic disc. The tail is slender, and sharply
marked off from the tnmk. Usually two small dorsal fins on
the tail. Caudal fin small or absent. No serrated spine on the

"Ni^

Flu. 264. — Roia murrayi, from Kerguelen Islaiu
(From Giintlier.)

A, male ; B, female.

tail. Caiidal electric organs are often present. Larger or
smaller denticles or spines are generally present on the skin.
Oviparous. Egg -cases four -horned, without tendrils. Four
genera and from thirty to forty species. Found in all temperate
seas, a few ranging into deep water.

The great majority of the species belong to the genus Raia
(Fig. 264), which chiefly inhabits temperate seas, but is more
abundant in the northern than in the southern hemisphere, and
approaches nearer to the Arctic and Antarctic regions than any
other Batoidei. The colour of the upper surftice of the body is
closely assimilated to that of the sandy or gravelly bottom on

462 FISHES CHAP.

which they live, and thus concealed, small Fishes, Crustaceans,
and other organisms are lured unsuspectingly within the reach of
the comparatively inactive and sluggish Eay. From the ventral
position of the mouth the Eay cannot at once seize its prey, but
the Fish darts over its victim and covers it with its body, and
then readily devours it. The sexes are usually distinguished by
secondary sexual characters, which take the form of differences in
size and coloration, in the dentition, and also in the presence and
position of patches or rows of specially modified dermal spines on
the dorsal surface (Fig. 264). Some of the larger species reach
a great size, the disc measuring 7 to 8 feet in width. A few
species range into deep water. B. mamillideiis, a uniformly jet-
black species, has been obtained from a depth of 597 fathoms in
the Bay of Bengal,^ and B. abyssicola from 1588 fathoms off
Queen Charlotte Islands, British Columbia." Tlie following are
British species : the Thornlxick {B. clavata) ; the Spotted Eay
{B. maculata) ; the Painted Eay (B. microcellata) ; the Starry
Eay {B. radiata) ; the Cuckoo or Sandy Eay (B. circularis) ;
the Skate {B. hatis) ; the Flapper Skate (B. macrorhynchus) ; the
White Skate {B. alha) ; the Long-nosed Skate {B. oxyrhynchus) ;
and the Shagreen Eay {B. fullo7iica)^ Most of the species are
of some economic value as food Fishes. Bsammohatis, with a
circular disc, frequents the southern coasts of South America,
and Platyrhina the coasts of India, China, and Japan.

The family ranges from the Upper Cretaceous, in which, as
well as in different Tertiary deposits, it is represented by species
of Baia. An extinct genus, Cyclohatis, with a circular or oval
disc, occurs in the Upper Cretaceous of Mount Lebanon.

Fam. 4. Tamiobatidae. — The systematic position of the only
representative of this family, Ttmtiohatis vetustus,^ from the
Devonian or Lower Carboniferous of Kentucky, is very uncertain,
but in some respects this unique type seems to be intermediate
between the modern Sharks and the Eays.

Fam. 5. Torpedinidae (Electric Eays). — A disc is formed as in
the Eaiidae, Ijut it is sub-circular in shape rather than rhombic,
and in the nature of its endoskeletal supports it is in some
respects unique. Its semicircular anterior margin is supported

1 Alcock, Ann. Mag. Nat. Hist. (6), iv. 1889, p. 380.

- Jordan and E%'ermaun, o}?. cit. p. 76.

■* Day, op. cit. p. 336. * Zittel, 0|). cit. p. 41.

ELASMOBRANCHII BATOIDEI

463

in the centre by a branched prenasal rostrum, and laterally by
the curiously branched preorbital cartilages, each of which
radiates outwards and forwards from a common basal articulation
with the lateral ethmoid regions of the skull. Tail relatively
short and thick, with two dorsal fins, a caudal fin, and two
lateral longitudinal folds. Skin smooth, without denticles.
]\Iouth transverse and ventral. A characteristic quadrangular

Fig. 265.— The Electric Ray {Torpedo ocellata). Dorsal (A) and ventral (B) views.
p.f. Pectoral tin ; pv.f, pelvic fin ; sjj, spiracle.

naso-frontal lobe, with a free hinder margin, which forms the
anterior lip, is enclosed by the two nasal organs and the oro-
nasal grooves leading from them to the corresponding angles of
the mouth. A pair of large electric organs between the pectoral
fins and the head. Seven genera and about fifteen species.
Inhabitants of most warm seas.

The well-known genus Torpedo (Fig. 265) is represented by
species in the Mediterranean {T. marmorata, T. narcc, I', hebetans),
the Eed Sea, and the Atlantic and Pacific Oceans. T. hebetans

4^4 FISHES CHAP.

has been taken at several places in British waters. An American
Torpedo {Tetronarce) is represented by species on the Atlantic
and Pacific coasts. Narcine is a very widely distributed genus,
species having been recorded from the East Indies, Tasmania,
China, Japan, South Africa, and the Atlantic coasts of North
and South America. Discopyge is an eastern Pacific genus (Peru
and Panama). Jffi/jnws frequents the Australian seas.

The family seems to be exclusively Tertiary, and its earliest
fossil representatives are from the Upper Eocene of Monte Bolca.

Fam. 6. Trygonidae (Sting- or Whip-tailed Pays). — Disc
sub-rhombic, broader than long. Pectoral fins confluent with
the sides of the head, tlieir preaxial endoskeletal radialia meet-
ing in front of the skull along the lateral margins of a slender
prenasal rostral cartilage. Tail usually whip-like, terminating
in a small caudal fin, and generally armed with a sharp, serrated
spine, which takes the place of a dorsal fin. Skin smooth or
spinose. A rectangular naso-frontal flap in front of the mouth.
About ten genera and fifty species. Found in nearly all
tropical and subtropical seas.

Of the more important genera, Trygon {Dasyatis) is represented
Ijy numerous species in the tropical parts of the Atlantic and
Pacific Oceans, including the Pacific coasts of North and South
America. Two species occur in the Mediterranean, and one of
them {T. 'p'-'-stinaca), ranges from the coasts of Norway and
the British Isles through the Atlantic and Indian Oceans
to Japan. Urogymnus frequents the Eed Sea and the Indian
Ocean. Uroloplms includes a few species of small size, distributed
along the Atlantic and Pacific coasts of Central and North
America, and in Australian seas. Fteroplatea comprises rather
large species, and is almost cosmopolitan in its distribution, being
represented by species on the Atlantic and Pacific coasts of North
and South America, in the Mediterranean and the Eed Sea, the
Indian Ocean, the Malay Archipelago, and on the coasts of China
and Japan. The caudal spines, which may be 8 to 9 inches
long in some of the larger species, are capable of inflicting very
severe wounds, the danger of whicli is greatly increased by the
apparently poisonous cutaneous mucus introduced into the wound.
As the spines become lost they are replaced by others developed
from behind. Some Trygonidae live in fresh waters. Trygon
{Dasyatis) sahina frequents the streams and estuaries of Florida

XVII ELASMOBRANCHII BATOIDEI 465

as well as on the adjacent coasts, and specimens have been
obtained from Lake Munroe at some distance from salt water.^
Ellipesurus and Paratrygon are freshwater genera, found in
Colombia, Venezuela, and Guiana.

Fossil remains of undoubted Trygonidae appear to bo confined
to the Tertiary period.

Fam. 7. Myliobatidae (Eagle-Eays). — Disc much broader than
long, and rhombic in shape. The huge pectoral tins are not
continued to the extremity of the snout, but cease on the sides
of the head, and reappear in front of the snout as a pair of
distinct folds, the so-called cephalic fins. The head projects
above the level of the disc, and consequently the eyes and
spiracles are lateral in position. Tail long, slender, and whip-
like, with a single dorsal fin near the root, and usually one or two
serrated spines behind the fin. A rectangular naso-frontal fold is
present. The dentition consists of flat, hexagonal, pavement-like
crushino; teeth arrano;ed from before backward in arched rows in
both jaws, and there is either a single median row of large teeth,
with (e.g. j\fi/liohcdis) or without (e.g. Ai'tohatis) the addition of
several rows of much smaller teeth on each side, or there are
numerous rows, the teeth then decreasing in size from the middle
line laterally (e.g. Rhiyioptera). Skin smooth. Sexes similar.
Five genera and about twenty -seven species are known; all
inhabitants of tropical and subtropical seas.

Myliohatis is represented in the Mediterranean by two species,
and one of them, the almost cosmopolitan M. aquila (Fig. 26G),
has been taken at various points on the eastern and southern
coasts of England. Aetohatis is also widely distributed in
tropical seas, but is unknown in European waters. Bhinoj^tera
has one species in the Mediterranean, while others have been
recorded from Brazil, the Atlantic and Pacific coasts of iSTorth
America, and the East Indies. The two tropical genera
Dicerolxitis and Ceratoptera have the cephalic fins prolonged
anteriorly into a pair of horn-like appendages, which are said
to be used in conveying food to the mouth. The teeth are
small, flat or tubercular, and are arranged in numerous rows. In
Ceratoptera they are wanting in the upper jaw. The Eagle-
Rays feed principally on Molluscs, the shells of which they
crush with their large grinding-teeth. Some of them attain

^ Jordan and Evermann, 07). cit. p. 8-5.
VOL. VII 2 H

466

FISHES

CHAP.

an enormous size, and are among the largest of Fishes.
Cerato'ptera vampyrns of the West Indies, for example, grows to
a width of 20 feet, and an embryo extracted from the oviduct
of a gravid female 15 feet wide, and from 3 to 4 feet in thick-
ness, measured 5 feet across the disc and weighed twenty-
pounds.^ This Fish is much dreaded by the divers engaged in

Fig. 266 —The Eagle-Ray, Myliohatis aqmla.

the pearl fisheries near Panama, whom it is said to devour after
enveloping them with its vast wings. ^'

The family is exclusively Tertiary, and with the exception
of an extinct genus, Promyliohdis, from the Eocene of Monte
Bolca, all the fossil species belong to the existing genera
Myliohatis, Hhinoptera, and Aeiohatis.

Order V. Holocephali.

The propriety of including the Holocephali in the sub-class
Elasmobranchii is scarcely open to doubt. Like the Acanthodei

' Ciinthcr, op. cit. p. 348.
^ Duiueril, (quoted by Jordan and Evermaiiii, np. cit. ]>. 92.

XVII ELASMOBRANCIIII IIOLOCEFIIALI 467

they seem to represent a divergent and specialised oftshoot from
some primitive Elasmobrauch type, and while retaining most of
the essentially distinctive features of their ancestors, they have
acquired, perhaps independently, certain characters distinctive
of the Teleostomi, combined with others peculiar to themselves.
In the few surviving genera agreement with the Elasmobranchs
is to be seen in the wholly cartilaginous condition of the endo-
skeleton and the complete absence of cartilage- and membrane-
l)ones. The vertebral column is acentrous and ribless, and the
notochord is persistent ; the dorsal arcualia include supradorsals
and regularly alternating basi- and inter -dorsals. The limbs
and limb-girdles are essentially Elasmobranch. Dermal denticles
are present, either locally, or, as in some of the fossil types, in
the form of a general investment. The brain and the reproduc-
tive organs agree more closely with the corresponding structures
in the Elasmobranchs than with those of any other Fishes, and
the agreement extends to the large size of the eggs and their
enclosure in horny egg-cases. In both groups the nostrils are
connected with the mouth by oro-nasal grooves ; the hyoidean
hemibranch is a true gill, and there is no air-bladder. The
Holocephali also agree with the Elasmobranchs in retaining such
primitive features as an intestinal spiral valve and a conus
arteriosus. On the other hand, indications of specialisation in
the Teleostome direction are to be noticed in the tendency to
the concentration of the branchial arches towards and beneatli
tlie skull ; the reduction of the interbranchial septa to the
extent that tliey are no longer continuous with the skin, and
the gill- filaments project beyond their outer margins ; the
presence of an operculum ; the suppression of tlie spiracles ;
and the absence of a. cloaca, the rectum opening externally by
an anus in front of the urino-genital apertures. Among the
more notable features evolved within the limits of the group
mention may be made of the autostylic condition of the skull,
])robably an adaptive modification induced by the large size of
the crushing dental plates which have taken the place of
ordinary teeth ; and the singular development of anterior and
frontal " claspers."

The group is one of great antiquity. Apart from the isolated
spines or " ichthyodorulites " common in Devonian and Carboni-
ferous strata, some of wliicli are probably the frontal or the

468 FISHES CHAP.

fin-spines of ancient Holocephali, dental plates, closely resembling
those of modern Chimaeroids and referred to the Ptychodontidae,
are probably the earliest indications of the existence of the
group. The Holocephali become more abundant in the Mesozoic
period, but of the four families usually recognised, only one, the
Chimaeridae, has survived.

Fam. 1. Ptyctodontidae. — This Palaeozoic family is known
only by the dental plates, of which there is a single pair in each
jaw, meeting at the symphysis. Ptyctodvs ^ and Rhynchodus occur
in the Devonian of either Eussia or Germany, and in N'orth
America, and Palaeomylus only in the Devonian of North
America.

Fam. 2. Squaloraiidae. — General shape of the body similar
to the existing Harriotta. There is a long, depressed, preoral
rostrum, and in the male the head carries a long slender frontal
spine. Conical denticles are sparsely present on the head and
body. No dorsal fin-spine. Dental plates similar to those of
the living Chimaeroids, but thinner, the tritoral areas being less
well defined. The only genus is Squaloraia from the English
Lias, of which nearly complete skeletons are known. ^

Fam. 3. Myriacanthidae.^ — Body elongate, but less depressed.
A dorsal fin-spine is present, and in the males a frontal spine.
The dentition consists of a median incisor-like tooth at the
symphysis of the lower jaw, in addition to dental plates similar
to those of Squaloraia. There is a symmetrical series of
tuberculated dermal plates on the lateral surfaces of the head,
which probably represent groups of fused denticles. One species
(Ilyriacanthus granulatus) ^has its rostrum terminating in a
cutaneous flap, as in CaUorhynchus. Myriacanthus, from the
Lower Lias of Lyme Eegis, and Chimaeropsis, from the Litho-
graphic Stone of Bavaria, are the only two genera.

Fam. 4. Chimaeridae. — Body elongate and shark-like in
form, but the head is compressed and the mouth is small.
Pectoral and pelvic fins large, especially the former, which are
somewhat ventrally placed. Two dorsal fins, the anterior over
the pectorals, with a stout spine in front ; and a small anal fin.
Dermal denticles restricted to the claspers, and to localised areas

^ Rohon, Verhandl. k. Min. Gcs. Petershurg, xxxiii. 1895, p. 1.

- Smith Woodward, Proc. Zool. Soc. 1886, p. 527 ; and 1887, p. 481.

^ Id., Ann. Mag. Nat. Hist. iv. (6), 1889, p. 275.

ELASMOBRANCHII HOLOCEPIIALI

469

on the dorsal surfoce in young forms. Dental plates large and
thick, including a single pair in the lower jaw and two pairs,
vomerine and palatine teeth, above, which combine trenchant
edges with well-marked grinding areas. Three genera are known.
In Chimarra (Fig. 267) the mouth and nostrils are ventral,
posterior to a l)luntly conical snout. ■ Head surmounted in the
males by a club-shaped appendage armed with a pad of recurved
denticles, the frontal clasper ; there is also an anterior clasper
armed with similar denticles and retractile into a shallow
glandular pouch in front of each pelvic fin, in addition to the
ordinary clasper behind the fin. The caudal fin consists of nearly
equal-sized dorsal and ventral lobes, between which the slightly

Fig. 267. — OJiimaera monstrosa (male), m, Moutli ; n.p, frontal clasper ;
0/), operculum.

up-tilted caudal axis is prolonged as a long tapering filament :
hence the tail appears to be nearly diphycercal. C. monstrosa
occurs off the coasts of Europe from Norway to Portugal, includ-
ing the Mediterranean, and also in the neighbourhood of the
Azores, as far south as the Cape of Good Hope, and eastwards
off the coast of Japan. It is the largest of the living
species, reaching a length of 3 feet. C affinis was first
taken off the coast of Portugal, and subsequently on the
North American side of the Atlantic, at depths ranging from
200 to 1200 fathoms. C. {Hydrolagus) colliei is restricted to
the North Pacific, and is especially plentiful off South-eastern
Alaska, and about the wharves at Esquimalt. Unlike most
otlier Chimaeroids this species swims at the surface, and there
is no evidence that it is a deep-sea form. In its breeding
habits, and in the mode in which its eggs are fertilised, Chi-
maera probably resembles the oviparous Sharks and Dog-Fishes.

470

FISHES

The eggs appear to be deposited on the sea-bottom in deep water,
but they are very rarely obtained. An egg-case dredged up off
the south-west coast of Ireland, at a depth of 315 fathoms, and
about 6^ inches in length, is shown in Fig. 268.^ It consisted

Fig. 268. — Egg-case of a species of Chimaera. a, Trausverse section across the case
at .r, showing the lateral valvular slits ; h, similar section across a-', showing the
vertical ridges. (From Giinther.)

of a broad, somewliat oval, flattened portion which contained the
egg, and terminated at one end in a truncated margin, while at
the other it was produced into a long tapering styliform process,
traversed by dorsal, ventral, and lateral ridges. The cavity of

Fig. 269. — CallorhynchKs antarcticus. Male. A, lateral view ; B, front view of the
mouth ; C, front view of nasal process, a.c, Anterior clasper ; b.a, external
branchial aperture ; f.c, frontal clasper ; p.c, posterior clasper. (From a specimen
in the CamVmdge Museum.)

the egg-case was open in front, and also along each side, where
linear, slit-like valvular apertures freely admitted sea-water into
the central cavity. A similar egg-case from Japan, measuring
9 inches in length, had its surface traversed by longitudinal and
^ Giinther,' Ann. Maij. Nat. Hist. (6) iv. 1889, p. 415.

ELASMOBRANCHII — HOLOCKPHALI

471

»'!'

ft.

tmnsverse ridges, and no doubt belonged to a Japanese Cliimaera}
In neither egg-case was there any trace of tendrils. The eggs
probably lie on the sea-bottom, or, when the cases have styliform
prolongations, it is possible that they are implanted in the ooze.
CaUorliynclms (Fig. 269) is distinguished by a singular pro-
longation of the rostrum, which terminates in a downwardly-
directed cutaneous flap, evidently from its abundant nerve-
sup{)ly an important tactile organ.
A frontal clasper is present in
the male. The prolonged caudal
axis is up-tilted, and the tail is
more distinctly heterocercal than
in Chimaera. The only species, C.
cmtarcticus, is confined to the Ant-
arctic basin and the South Pacific.
The egg -cases of Callorhynchus
differ considerably from those of
Chimaera, and so large are they
that one may measure 25 cm. in
length, or nearly- as long as the
abdominal cavity of the Fish. Each
case is ovoid in shape, surrounded
by a wide flat margin which is
covered on one side with yellow
hair-like fibres, thus giving to the
case a protective resemblance to a

mass of seaweed (Fig. 270). In Fig. 270.— Egg-case of CaUorhynclms

the central part of the case there «Mte'-c</c«,9 kid open to show the

^ _ emDryo and its lobed yolk - sac

is a pear-shaped cavity in which {y.s) ; s, dorsal spine. (Cambridge

the egg or the embryo is contained. useum.)

From one end of this cavity a passage, guarded by a valve, leads

to the exterior, and provides for the escape of the young. While

in the egg-case the nearly ripe embryo has long external gills,

and its body is nearly sessile on a large and singularly lobed

yolk-sac.

The third genus, Harriotta (Fig. 271),' is remarkable for its

^ See also an account of the egg-case of a Chimaeroid dredged from a depth of
516 fathoms in the Bay of Bengal (Wood-Mason and Alcock, Ann. Mag. Nat.
Hist. (6) viii. 1891, p. 21).

"^ Goode and Bean, 02>. cit. p. 32.

472

FISHES

elongated, tapering, and depressed rostrum, and for the large size

Fio. 271. — Ilarriotta rcdeighana. A, lateral view ; B, ventral view of a male.
(From Gooile and Bean.)

and wing-like appearance of the pectoral fins. There is no frontal
clasper, and the ordinary claspers in the young male examined

ELASMOBRANCHII HOLOCEPIIALl

473

were very small and simple. Tlie caudal filament, which is
longer in older specimens than in the younger, and is not
developed at all in the youngest examples at present known
(Fig. 27:^, A), is not uptilted, although the lower lobe of the
caudal fin is much larger than the upper. Young forms have a
double row of stout spine-like denticles in front of the second
dorsal fin, and also in the interval between the latter and the
upper caudal lobe. Similar denticles are also present on the
upper surface of the head between the orbits (Fig. 272). //.
raleighana is found in the Xorth Atlantic. Individuals varying

Fig. 272. — Young example of Harriotta raleighaiut, 4 inches in length. A, side view ;
B, dorsal view. (From Goode and Bean.)

in length from 4 to 25 inches have been taken at depths
ranging from 707 to 1081 fathoms. A species of Harriotta
has also been recorded as occurring in Japanese waters.^

With the prolmble exception of Chimaera colliei the surviving
Holocephali are denizens of deep water ; hence their comparative
rarity and our almost complete ignorance of their habits. Young
forms of C. vionstrosa, 1^ to 5 inches in length, have been
dredged in the Faroe Channel at depths from 505 to 555
fathoms ; " and the youngest specimen of Harriotta was obtained
from 991 fathoms. Egg-cases are rarely obtained, and then
only from considerable depths. It is therefore reasonable to

' Mitsiikuri, Zool. Mag. Tokyo, 1895, quoted in Nat. Sci. viii. 1896, p. 10.
- Gunther, Chall. Reports, Zool xxii. 1887, p. 12.

474 FISHES CHAP. XVII

infer that these Fishes breed in deep water. As might he
expected, little is known of the embryology of any of the Holo-
cephali, but that little adds further proof of the Elasmobranch
relationship of the group. The segmentation of the egg of
Chimaera and the overgrowth of the yolk by a cir(;ular blasto-
derm are essentially as in Elasmobranchs. The early embryos
are said to be shark-like, and to possess both spiracles and
" external gills," and the primary upper jaw is less completely
confluent with the skull than in the adult. It is also said that
the palatine dental plates are represented at an eaily stage by
series of small, more or less conical elements, which, outwardly at
least, resemble the rudiments of the grinding teeth of the
Cestraciont Sharks.*

The Chimaeridae first appear in the Lower Oolites, and attain
their maximum development in the Cretaceous and the Eocene."
Ganodus is an Oolitic genus. Ischyodus ranges from the Lower
Oolites to the Lower Cretaceou.s. Udaphodon is Cretaceous and
Eocene, extending, however, into the Miocene, and Elasmodus
ranges from the Upper Cretaceous into the Eocene. Teeth of
the existing genus Ccdlorhynchus occur in the Cretaceous of
New Zealand, and of Chimaera in the Upper Tertiary of
Europe and Java. The fossil Holocephali afford little evidence
of the origin of the group from more typical or more primitive
Elasmobranchs. So far as their structure is known, they all
possess the essentially distinctive features of their modern repre-
sentatives, and offer little evidence of transitional forms. The
surviving Chimaeroids seem to have acquired a more specialised
dentition, but in other respects they are either more primitive,
or possibly somewhat degenerate.

1 Bashford Dean, Mem. New York Acad. Sci. ii. Pt. i. 1899, p. 28 ; L'iol. Bull.
iv. 1903, p. 270.

- E. T. Newton, Mem. Geol. Surv. Monoyr. iv. 1878 : Riess, Palacontogr.
xxxiv. 1887, p. 1 ; Smith "Woodward, Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 52 ;
Zittel, Text-Book of Palaeontology, English ed., London and New York, ii. 1902,
p. 46.

CHAPTER XYIII

TELEOSTOMI : GENERAL CHAUACTEKS CKOSSOPTEllYGIl

CHONDROSTEI HOLOSTEI

Sub-Class II. Teleostomi.

In this group of Fishes the primary upper and lower jaws (palato-
(|uadrate and Meckelian cartilages) are supplemented by the
addition of certain tooth-hearing membrane bones which form
secondary jaws corresponding to the functional jaws of the higher
Craniates.^ The chondrocranium and the primary jaws are
usually more or less completely ossified by cartilage bones, and
there is always a secondary cranium of dermal hones, of which
paired parietals and frontals above, and a median vomer and a
parasphenoid below, are amongst the most constant. The skull
is hyostylic. An operculum covering the gill-clefts and supported
by a special opercular skeleton is a constant feature. Tlie verte-
bral column is often acentrous, and when centra are present they
are invariably arch-centra. There is a well-developed secondary
pectoral girdle, connected dorsally with the hinder part of the
skull. , As a rule the pelvic girdle is absent altogether, and
when present it is rarely more than a rudiment or a vestige.
The endoskeletal supports of the paired fins are uniserial. The
dermal fin-rays of the paired and median fins are probably modified
scales or lepidotrichia. In the median fins the fin -rays are at first
more numerous than their supporting radials, but in the more
specialised Teleostomes they ultimately equal them in number.
The body is usually invested by an exoskeleton of articulated
rhombic or imbricated cycloid scales. Claspers are unknown.
In the surviving members of the group there is usually an air-

' Hence tlie name "Teleostomi" or " perfect-moutlieil " Fishes.

475

4/6 FISHES CHAP.

bladder. The gill-filaments project freely beyond the outer edges
of the greatly reduced interbranchial septa. The external open-
ing of each nasal sac is usually divided into two distinct aper-
tures, and there is no oro-nasal groove leading from the sac to
the mouth. The brain has no proper cerebral hemispheres, but
retains an undivided prosencephalon with a non-nervous roof.
A cloaca is not developed, the rectum opening externally by an
anus in front of, and distinct from, the separate or united urino-
genital apertures. The ova are small and numerous, and the
segmentation is either holoblastic and unequal, or meroblastic.
Besides a large number of fossil forms the group includes the
vast majority of living Fishes.

The Teleostomi include four " Orders," the Ceossoptekygii, the
Ch(jndrostei, the Holostei, and the Teleostei. Of these the
Crossopterygii occupy a remarkably central position. Eemotely
connected with the Elasmobranchs on the one hand, and more
intimately related to the Holostei and Teleostei on the other,
they also probably represent the ancestral stock from which
the Stegocephalan Amphibia and the Dipneusti have had their
origin. Of the three remaining groups, often collectively spoken
of as " Actinopterygii," the Chondrostei are the oldest and
most primitive. Like the Crossopterygii, they are not without
evidence of a remote kinship with the Elasmobranchs, but in a
broad general sense they also represent the initial stages in a
sequence of structural modifications, of which the Teleostei, the
dominant Fishes of the present day, are the final outcome.

Order I. Crossopterygii.

Pectoral fins obtusely lobate and probably uniserial, or acutely
lobate and probably biserial. Pelvic fins abdominal in position,
uniserial, non-lobate, or obtusely lobate. Scales rhombic or cycloid,
and, like the dermal cranial bones, they are generally invested by
a layer of enamel-like ganoin. Tail heterocercal, or apparently
diphycercal or gephyrocercal. Vertebral column acentrous, or with
ring-like centra, or even with complete bony amphicoelous centra.
Lower jaw with dentigerous splenials. As a rule, the opercular
series includes an operculum and a suboperculum. Branchio-
stegal rays absent, their place being taken by a remarkable
armature of jugular plates (Fig. 274). Secondary pectoral girdle

XVIII TELEOSTOMI CROSSOPTERVGII 47/

complete, including a pair of infra-clavicles. With rare excep-
tions the fin-rays of the median fins retain their numerical pre-
ponderance over the supporting radials. The group is divisible
into two " sub-orders," the Osteolepida and the Cladistia.^

Sub-Order 1. Osteolepida.

The obtusely or acutely lobate pectoral fins articulate with the
pectoral girdle by a single basal endoskeletal element. Nostrils
on the ventral surface of the snout. Two dorsal fins and an
anal fin. Dermal bones of the ethmoid region often fused witli
one another and with the premaxillae in front and the frontals
behind to form a continuous rostral shield. Infra-dentary bones
may be present. A series of lateral jugular plates often present
in addition to the pair of principal plates. The Osteolepida
first make their appearance in the Old Eed Sandstone and
Devonian formations, where they become abundant. They are
also well represented in the Carboniferous, but only one family
survived to the Mesozoic period, finally becoming extinct in
the Upper Cretaceous. The following are the more important
families : —

Fam. 1. Osteolepidae. — Scales rhombic and thickly en-
amelled. Pectoral and pelvic fins obtusely lobate. Tail hetero-
cercal. Teeth simple, not complicated by surface infoldings

Fig. 273.— Restoration of Osteolepis macroUpidola. Old Red Sandstone.
(From Traquair.)

except quite at the base. Genera: — Osteoleiris (Fig. 2-73),
Thursius, Diploi^terus (Middle Old Eed Sandstone, Scotland),
Ghjftopomus (Upper Old Eed Sandstone, Scotland), Megalick-

1 Boulenger, Foissons du Bassin du Congo, Bnixelles, 1901, p. 2. Smith
Woodward {Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 317 ; and Fcrt. Palaeont.
Cambridge, 1898, p. 78), following Cope, recognises four sub-orders, the Haplistia,
Rhipidistia, Actinistia, and Cladistia. The first sub -order is reserved for tlie
Tarrasiidae, a family which includes only the little known Tarrasius problematicus
from the Lower Carboniferous of Scotland.

4/8

FISHES

thys (Carboniferous and Lower Permian of Europe and North
America).

Fam. 2. Rhizodontidae. — Scales cycloid and overlapping.
Paired fins ol)tusely loljate. Tail lieterocercal, sometimes appar-
ently gephyrocercal. Teeth with the external enamelled layer
of dentine infolded towards the axis in the form of radially

Fro. 274. — Skull of a Rliizoilont (R/nr.oifo/).sis sauroidcs). Lower Carlwiiiferous. A,
lateral view ; B, the dorsal surface ; and C, ventral view. ««,, Angular ; d,
deutary ; /, frontal ; i.d, infra-deutary ; ,/, principal jugular plates ; l.j, lateral
jugulars ; m, mandible ; ?«..;, median jugular ; 7nx, maxilla ; o, orbit ; op, oper-
culum ; p, parietal ; p.f, post-frontal ; p.mx, premaxilla ; p. op, preoperculum ; .so,
suborbital ; s.op, suboperculum ; sq, squamosal ; st, supra-temjioral ; x, x , cheek
plates. (After Traquair.)

arranged folds. In some genera ring -like vertebral centra
have been recognised and also a preoperculum. Genera : —
Jihizodus, Lower Carboniferous of Scotland and Northumberland ;
TrisiicJwpterus^ (Fig. 275), Old Eed Sandstone of Scotland;
Musthenopteron ^ (Fig. 27())> Upper Devonian of Scaumenac

' Traquair, Trans. Hoy. Soc. Edinb. xxvii. 1875, p. 383.
- Wliiteaves, Trans. Hoy. Soc. Canada, vi. 1888, p. 77.

CROSSOPTERVGII

479

Bay, Canada; Gymptyclnus, Old Eed Sandstone, Scotland;
Rhizodo2ysis^ (Fig. 274), Carboniferous of England, Scotland,
Silesia, and North America ; Strepsodus, Carboniferous of Great
Britain, Ireland, and Xorth America.

PiQ_ 275. Restoration of Tristichnpterus alatus. Old Eed Sandstone, d, Clavicle.

Remaining reference letters as in Fig. 274. (After Traquair.)

Fig. 276. — Restoration oi Eusthenopteron foordi. Upper Devonian of f^canmenac Bay,
Canada. The scales have been omitted in the hinder part of the body to show the
vertebral column and the radials of the median fins, d, Clavicle ; /.c/, infra-
clavicle ; s.d, siipra-clavicle ; for other reference letters see Fig. 274. x about |.
(After Whiteaves.)

Fam. 3. Holoptychidae (Dendrodontidae). — Scales cycloid.
Pectoral fins acutely lobate ; pelvic fins short and somewhat
obtusely lobate. Tail heterocercal. Teeth similar to those of

Fio. 277. — Restoration of llolnptijdiivsfleminfji.

(From Traquair.

the Ehizodontidae but more specialised, the enamelled dentine
infoldings being much more complicated, presenting a radiating

' Traijuair, I'rans. Hoy. Soc. Edinh. xxx. 1881, p. 1G9.

48o

FISHES

arborescent appearance in transverse sections. Vertebral column
acentrous. Genera: — Holoptychius^ (Fig. 277), Old Eed Sand-
stone of Scotland ; Devonian of Belgium, Eussia, North America,
and East Greenland. Glyptolepis has a similar range.

Fam. 4. Coelacanthidae." — Scales cycloid. Paired fins ob-
tusely lobate. Tail symmetrical but apparently gephyrocercal,
usually with a protruding axial vestige of the disappearing
terminal part of the tail and of the proper caudal fin. Kadialia
of the functional caudal lobes agree in number with the con-
tiguous neural and haemal arches and dermal fin-rays, the
diagnostic feature of Smith Woodward's Actiiiistia. Proximal

Fig. 278. — Restoration of Undina gulo. Lower Lias of Dorset. Scales and supra-
clavicle omitted. The ossified air-bladder is shown beneath the anterior part of
the vertebral column. The facial bones in front of the orbit are unknown, and
the cheek-plates are supposed to be arranged as in other Coelacanths. x about f.
(From Snuth Woodward.)

radials of the dorsal and anal fins fused into a single, internally-
forked basipterygium in each fin. Teeth simple. Vertebral
column acentrous. The skull presents several interesting
features. The hyomandibular and the palato-quadrate bar,
for example, are fused on each side into a continuous tri-
angular bone, articulating with the cranium above and with
the lower jaw below. The opercular skeleton is reduced to an
operculum and two jugular plates. A very singular feature in
these Pishes is the ossification of the walls of the air-bladder
(Fig. 278), a structural modification which has no parallel in
Pishes, except in certain Teleosts (Siluridae and Cyprinidae)^

' Traquair, Proc. Roy. Sue. EcUnb, xvii. p. 388.

" Reiss, Die Coelacanthinen, Palaeontogr. xxxi. 1888, p. 1 ; Smith Woodward,
Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 394.
* See also Kurtus indicus, p. 688.

CROSSOPTERYGII 48 1

in which the organ becomes encapsuled by bone owing to the
partial ossification of its walls.

From their first appearance in the Lower Carboniferous the
Coelacanthidae range, practically unchanged, through the inter-
vening formations to the Upper Cretaceous. Cuclacanthus itself
occurs in the Carboniferous and Permian of England, Scotland, and
Germany, and in the Carboniferous of North America. Unclina ^
(Fig. 278) is a Jurassic genus. IHplurus is found in the Trias
of Xorth America, and Macropoma is a well-known form from
the Middle and Upper Cretaceous beds of England, and other
parts of Em'ope.

Sub-Order 2. Cladistia.

Pectoral fins imiserial and abbreviate, with three basal endo-
skeletal elements. Nostrils on the upper surface of the snout.
Entire skeleton well ossified. Notochord replaced by bony,
amphicoelous vertebral centra. Bones of the ethmoid region
not fused to form a rostral shield. Infra-dentary bones absent.
Jugular plates reduced to a single pair of large plates. As this
group includes the only Crossopterygii which have survived to
the present day, it is noteworthy that they retain certain primi-
tive features indicative of their remote origin. The spiracles
are persistent ; the intestine has a spiral valve ; and the conus
arteriosus is furnished with several rows of valves. Amongst
other characters of contrary significance, the air-bladder is double ;
its oesophageal aperture is ventral ; and its afferent arteries are
pulmonary arteries derived from a posterior aortic arch.

Fam. 5. Polypteridae.^ — Pectoral fins obtusely lobate. Pelvic
fins non-lobate. Scales rhombic and thickly enamelled. Dorsal
fin in the form of a series of isolated finlets, each consisting of a
stout spine-like ^ fulcral scale supporting a single soft ray, or a
fringe of several rays, along its hinder margin. Tail symmetrical,
apparently gephyrocercal. Teeth simple. Nostrils tubular.

The only representatives of the sub -order and the sole

^ Smith "Woodward, op. eit. p. 412.

- Boulenger, Puiss. Bass. Congo, p. 10. For a list of the more important
papers, see pp. 18-19 of that work.

^ Mr. Boulenger informs me that he regards these spines as modified ridge
scales or fulcra. The latter are median spine-like or A-shaped scales in relation
with the anterior margins of the median fins in some Crossopterygii {e.g. Osteo-
lepidae) and in many Chondi'ostei and Holostei.

VOL. VII 2 I

482

FISHES

surviving family of Crossopterygii, the Polypteridae, are restricted
to the Nile and to the river basins of tropical Africa which drain
into the Atlantic (Fig. 280). Only two genera are known,
Folypterus and Calamichthys, neither of which has yet been dis-
covered in any geological deposits, ancient or recent.

In Polypterus each of the spines of the dorsal fin supports
several soft rays. Pelvic fins and a suboperculum are present.
Ten species are known, of which six pertain to the Congo and
its tributaries.^ P. hichir is said to attain a length of four
feet.

Until recently little was known of the habits of Polypterus,
but the observations of Budgett " on the widely distributed P.
senegalus and those of Harrington ^ on P. hichir, have brought to
light many interesting facts about these most interesting Fishes.

Fig. 279. — Polypterus senegalus. From a specimen in the Cambridge University
Museum. The arrow points to the position of the left spiracle, x ^.

P. hichir haunts the deeper holes and depressions of the
muddy bed of the Nile, although it is " not essentially a bottom-
• liver or a mud-fish." It is most active at night when in search
of food, and then it may readily be taken by trawl lines. The
lobate pectoral fins are used for progression, but their primary
function is to act as balancers, and they exhibit the characteristic
trembling movements so often seen in the balancing fins of
Teleosts. Polypterus does not readily live out of water, rarely
longer than three to four hours, and then only when covered
with damp grass or weeds. P. hichir is said to feed on small
Teleosts, which it swallows whole, and to these there may be
added in other species, Batrachians and Crustaceans. The
observations of Budgett show that in captivity Polypterus often
remains motionless for a long time at the bottom of the water,
the anterior part of the body resting upon the tips of the

^ Boulengcr, op. cit. p. 20 ct seq. ; id. u4m?i. Afiis. Congo, Zool. (1), i. Fasc. 4,
Bru.xclles, 1899, p. 61 ; ii. Fasc. 2, 1902, p. 23.

• Proc. Camb. Phil. Soe. x. 1900, p. 236 ; Trans. Zool. Soc. xvi. Pt. ii. 1901,
p. 115.

3 Amer. Nat. xxxiii. 1899, p. 721 ; Science (2), ix. 1899, p. 314.

CROSSOPTERYGII

483

pectoral fins. According to the Siime observer, the air-bladder
is an accessory respiratory organ, supplementary to the gills,
rather than a hydrostatic organ.

In P. hichir the eggs ripen from June to September, inclusive,
and, as in most other Nile Fishes, the breeding season is during
or just after the period of inundation. F. senegahcs and P.
lajrradci spawn during the rainy season in the months of July,
August, and September, but nothing is certainly known as
to the place or mode of deposition of the eggs. During the
breeding season Pohji^terus is unusually active and excitable,
and at this period the anal fin of the male becomes greatly
thickened and enlarged, and has its surface thrown into deep

i'lG. 2S0. — Map showing the distribution of the Polypteridae.

folds between the successive fin-rays.^ The use of the modified
fin is not known. During his stay at McCarthy Island, about
160 miles up the Kiver Gambia, Budgett ^ was fortunate in
securing a larva of P. senegalus, 1 to 1^ inches in length, or
only about one-third the length of any larval Polypterus previ-
ously known (Fig. 281). The larva is described as a most
beautiful object, " marked with black stripes on a golden ground,
with a conspicuous golden stripe on each side above the eye,
across the spiracle, and along the dorsal surface of the external
gill." The pinnate external or cutaneous gills were relatively of
much greater size than in the considerably more advanced stage
figured elsewhere,^ and reached half-way to the tail. The dorsal
fin is not divided into finlets, and behind it is continuous with
the caudal, while the anal fin is scarcely distinct from the

1 Budge tt, Trans. Zool. Soe. xv. Pt. vii. 1901, p. 330.
2 Trans. Zool. Soc. xvi. Pt. ii. 1901, p. 118 ; also footnote on j). 317. ^ p. 290.

484

FISHES

CHAP.

lower lobe of the caudal. The fin -rays which support the
ventral portion of the caudal fin are more numerous and longer
than those in relation with the dorsal lobe, and hence at this
stage the tail is really heterocercal.

FiQ. 281. — Larva of Polypterus scnrgalus. x 4. Showing its cbaracteristio attitude
when resting on the bottom of an aquarium, and the large size of the cutaneous
gills. (From Budgett.)

In the genus Calamichthys the body is greatly elongate and
Eel-like in shape. Pelvic fins are absent, and normally there is
no suboperculum. The dorsal finlets are more isolated than in
Polyi^terus, and each spine supports but a single soft ray. Only
a single species is known, C. calaharicus^ (Fig. 282).

Fig. 282. — Calamichthys calabar icus. x ^. (From a specimen in the Cambridge
University Museum.)

Calamichthys has a more restricted distribution than Poly-
pterus, and is confined to certain rivers of West Africa. First
obtained at Creek Town on the Old Calabar river, it is now
known to occur in the delta of the Niger, on the coast of
Cameroon, and as far south as the river Chiloango, frequenting
the smaller muddy rivers opening into the estuaries." It is a

1 Traquair, Journ. Gcol. Soc. Ireland (2), 1871, p. 249.
'^ Boulengcr, Les Poisso7is du Bassin du Congo, Bruxelles, 1901, p. 27.

CHONDROSTEI 485

very agile Fish, swimming like a snake, and subsisting on
insects and crustaceans. The anal fin is enlarged in the male,
and the young are provided with cutaneous gills. Calamichthys
may attain a length of nearly 40 cm.

In the remaining Teleostomi (Actinopterygii) the paired fins
are invariably non-lobate, with abbreviate, multibasal endoskeletal
supports. Fin-rays are the main support of both the median
and paired fins. Jugular plates are usually replaced by branchio-
stegal rays, but both may co-exist. The Actinopterygii are the
successors of the Crossopterygii in palaeontological sequence,
and when the latter began to decline in Carboniferous and
Permian times, the former, mainly represented by the earlier
Chondrostei, had already become the dominant Fishes of the
period.

Order II. Chondrostei (Acipenseroidei).

In these Fishes, the oldest and the most primitive of the
Actinopterygii, the fin-rays of the median fins still continue to
retain their primitive numerical superiority over the radials, and
the tail is heterocercal. There is a single dorsal and an anal
fin, which, like the upper lobe of the caudal fin, are generally
provided with fulcra. Pelvic fins abdominal. Squamation
typically rhombic and ganoid. Vertebral column acentrous.
So far as is known the chondrocranivun is but little ossified,
and the cranial bones are mainly dermal. The secondary pectoral
girdle still includes a pair of infra-clavicles.

The Chondrostei are first represented in the Lower Devonian
by the solitary Palaeoniscid genus Cheirolepis, a contemporary of
the earliest Crossopterj^gii. They occur throughout the Mesozoic
period, except in the Cretaceous, and also in the Eocene, and
while steadily diminishing in number and variety they gradually
approximate to their degenerate and in some respects highly
specialised descendants, the Sturgeons and Paddle-Fishes of the
existing Fish fauna. Of the seven families included in the
group the Palaeoniscidae are the oldest and the most generalised.
The Platysomidae are a specialised offshoot from the Palaeoniscidae,
and, if they are rightly to be considered as Chondrostei, perhaps
the same may be said of the problematic Belonorhynchidae. On

486

FISHES

the other hand, there are certain features in the Catopteridae
which indicate an approach to Fishes of an altogether more modern

^^.

F]G. 283. — Palaeoniscus macropomus. Restoration, nearly one-half nat. size.
(From Traquair. )

tjpe. Finally, the Chondrosteidae represent a stage in a career

of degeneration, the climax
of which is reached by the
modern Polyodontidae and
Acipenseridae.

Fam, 1. Palaeoniscidae.^
— Fishes with fusiform bodies,
short dorsal and anal fins,
and usually with a complete
investment of articulating
rhombic, rarely cycloid, ganoid
scales (Fig. 283). Fulcra
generally present at the bases
of the median fins, and especi-
ally along the dorsal border of
the upper caudal lobe. Eibs
are not Icnown to be present.
Skull invested by a very com-
plete series of jDaired dermal
bones, which in number and
conform to the

Fig. 284. — Outline restoration of the skull and
secondary pectoral girdle of Palaeonisciis
iruicropoiiiHS. an. Angular ; br.r, branchio-
stegal rays ; cl, clavicle (cleithrum) ; d,
dentary ; d.ect, dermal lateral ethmoid ; /,
frontal ; i.cl, infra-clavicle ; i.op, suboper-
culum ; mx, maxilla ; n, nostril ; op, oper-
culum ; or, orbit ; p, parietal ; p)-A pectoral
fin ; p.mx, premaxilla ; p. op, preoperculum ;
?).<, })0.st-temporal ; s.cZ, supra-clavicle ; s.o,
circumorbitals ; sq, squamosal ; the single
median bone overlying the short rostrum
is probably a dermal niesethmoid, and the
one intercalated between the squamosal
and post-temporal a supra-temporal. The
dotted lines indicate sensory canals. (From disposition

normal Teleostome type (Fig.
284). The secondary upper jaw includes both premaxillae and
and, as a rule, both the dentary and splenial bones

large maxillae

^ Traquair, Monogr. Palaeont. Soc. 1877 ; Quart. Journ. Geol. Soe. xxxiii. 1877 ;
Trans. Roy. Soc. Edinh. xxx. 1883, p. 22 ; Ann. Mag. Nat. Hist. (4) xv. 1875,
p. 237 ; Smith Woodward, Mem. Gcol. Surv. N. S. Wales, Palaeont. No. 4, 1890,
and No. 9, 1895.

CHONDROSTEI

487

of the lower jaw are dentigerous. Except for the absence of an
interoperculum, the opercular series of bones is complete, includ-
ing numerous brauchiostegal rays. There is a single small
median jugular plate.

The Palaeoniscidae are remarkable both for their individual
and specific abundance and for their extensive range in time.
Eepresented only by Cheirolejns in the Middle Old Eed
Sandstone and Devonian, the family attained its maximum
development in the later Palaeozoic rocks (Carboniferous and
Lower Permian), became rare in the Mesozoic, finally dwindling
away at the close of the Jurassic period. Their geographical
distribution in the past is hardly less remarkable. In various
geological formations they have been found in Great Britain and
Ireland, in widely remote parts of continental Europe, and in
North America, South Africa, and Australia. Cheirolepis, Ambly-
jpterus, Canohius, Phanerosteon, Eloniclithys, Crypliiolepis, Pcdaeo-
niscus, and Trissolepis are Palaeozoic genera. Gyrolepis, Uroleins,
Coccolepis, Oxygnathus, and Centrolepis are characteristic Meso-
zoic forms.

Fam. 2. Platysomidae.^ — More or less deep-bodied Fishes,
with elongated dorsal and anal fins, a high head, short jaws.

Fig. 285. — Restoration of Enrynotus crenatus. in.cl, lufra-clavicle ; U, lateral line ;
orb, orbit ; other reference letters as in Fig. 284. (From Traquair.)

usually armed with bluntly conical tritoral teeth, and a complete
investment of high, narrow, rhombic scales. They agree with
the Palaeoniscidae in their osteology and in most other essential

1 Trat^uair, Trans, lioij. Soc. Edinh. xxix. 1879, p. SIS.

488

FISHES

features, and they flourished in large numbers during the Carbo-
niferous and Permian periods. Platysomus ranges from the
Lower Carboniferous to the Upper Permian in Great Britain
and continental Europe, and also occurs in the Carboniferous
of North America. Eurynotus (Fig. 285), and the singularly
deep -bodied Cheirodus (Fig. 286), in which pelvic fins are
unknown, are British Carboniferous genera.

Fig. 286. — Restoration oi Cheirodus granulosus, d.ect, Dermal lateral ethmoid ; d.eth,
dermal mesetlimoid ; d.s}), either a dermal sphenotic or a post-orbital bone ; l.l,
lateral line ; orh, orbit. The pectoral fin is indicated in dotted outline. Other
reference letters as in Fig. 284. (From Traquair.)

Fam. 3. Belonorhynchidae. — The systematic position of
these Triassic forms is very doubtful, and it is by no means
clear that they are Chondrostei at all.

Fam. 4. Catopteridae. — It is very probable that this widely-
distributed Triassic family is an offshoot from the Palaeoniscidae.
It agrees with the latter in the general character of the head
and pectoral girdle and in the rhombic squamation, but differs
from its progenitors and approaches the more modern Holostei
in the semi-heterocercal condition of the tail, and in the approxi-
mate numerical agreement between the fin-rays and radialia of
the dorsal and anal fius.^

^ Smith Woodward, Brit. Mus, Cat. Foss. Fishes, iii. 1875, p. 7.

CHONDROSTEI 489

Fam. 5. Chondrosteidae. — This family affords an interesting
annectant link between the Palaeoniseidae and their degenerate
living representatives the Polyodontidae and Acipenseridae.
They agree with the latter in the general shape of the body, the
growth of a preoral rostrum, and in the relatively small size
of their ventrally-placed and probably protrusible mouth (Fig
287). The skin is entirely scaleless, except on the upper lobe of
the caudal fin, where, as in Polyoclon and Acipenser, the primitive
rhombic squamation and a series of fulcra are retained.

On the other hand, their relationship to the Palaeoniscidae
is indicated by the general disposition of the dermal bones of

^-s^

Fig. 2S7. — Restoration of the skeleton of Chrondrosteus acipenseroides. a./, Anal fin ;
c.h, cerato-hyal ; e, eye ; h.a, haemal arches ; hym, hyomandibular ; j, jugal ;
n.a, neural arches ; n.c, notochord ; n.s, neural spines ; pc.f, pectoral fin ; p./,
pelvic fin ; s.o, suborbital ; s.op, suboperculuni ; other reference letters as in Fig.
284. (After Smith W^oodward.)

the cranial roof, and the presence of a transverse row of supra-
temporals and of an extensive series of branchiostegal rays
(Fig. 288). The family is represented by C'hondrosteus ^ from
the Lower Lias of Dorset and Leicestershire, and GyrosteiLS from
the Upper Lias of Yorkshire. From an evolutionary point of
view it is significant that the Chondrosteidae do not make their
appearance until the Palaeoniscidae are approaching extinction.

The two remaining families, the Polyodontidae and the
Acipenseridae, agree in presenting a remarkable leaven of char-
acters otherwise distinctive of the typical Elasmobranch, asso-
ciated with certain primitive features which they have doubtless
inherited from some remote ancestral stock common both to
existing Elasmobranchs and to the other primary groups of

1 Traquair, Gcol. Mag. (3) iv. 1887, p. 248 ; Smith Woodward, Brit. Mus. Cat.
Foss. Fishes, iii. 1895, p. 23.

490

FISHES

Fishes, and also with others obviously due to degeneration. The
most interesting illustration of the first point is to be found in
the condition of the primitive upper jaw which, especially
in the Polyodontidae, is typically Elasmobranch in the median
union of the palato-quadrate bars beneath the basis cranii, but
Teleostome in the presence of a secondary upper jaw formed by
two maxillae. Both families also agree in possessing an acentrous
vertebral column which, if it does so far resemble that of Teleo-
stomes in being potentially arco-centrous, nevertheless has a
better developed series of distinct inter-dorsal and inter- ventral
cartilages, regularly alternating with only partially bony basi-

Fia. 288. — Lateral view of a restored skull and pectoral girdle of Chondrosteus acipen-
seroides. a, Angular ; br, branchiostegal rays ; c.h, cerato-hyal ; h.m, hyomandi-
bular ; /, jugal ; ^y./, post-frontal ; s.op, suboperculum ; s.U supra-temporal ; other
reference letters as in Fig. 284. (After Traquair.)

dorsals and basi-ventrals, than is to be met with in any other adult
Fishes except Elasmobranch s. Primitive features are apparent
in the presence of spiracles, sometimes associated with pseudo-
branchs ; the presence in one family (Acipenseridae) of a
hyoidean hemibranch supplied with blood directly from the
ventral aorta, and the existence of a multi-valvular conns
arteriosus and an intestinal spiral valve. Finally, the massive
growth of the chondrocranium wholly devoid of cartilage bones,
except in so far as they may be represented by splint-like
membrane bones, the fragmentation of the investing dermal
bones, the degeneration of the opercular skeleton and the loss of
branchiostegal rays, and the almost complete disappearance of
the primitive rhombic squamation, are probably to be regarded

CIIONDROSTEI 491

as the outcome of a long-continued career of degeneration from
some remote PaLieoniscid ancestor.

Fam. 6. Polyodontidae. — The Polyodontidae are more
generalised, and in some features decidedly more Selachioid
tlian the Acipenseridae. Body fusiform and apparently scaleless,
l)ut the primitive squamation is still represented by isolated
vestigial scales imbedded in the otherwise soft skin, and by a
continuous series of rhombic scales on the upper caudal lobe,
which also has a dorsal fringe of large fulcra.^ Eostrum excep-
tionally long, spatulate or. somewhat conical, with a rigid axis
and thinner and more flexible mai-gins. Barbels absent. Mouth
wide, not spout-like. Pectoral fins devoid of spines. Two pairs
of membrane-closed vacuities separate the paired dermal bones of
the cranial roof (possibly parietals and frontals) from the more

Fig. 289. — Polyodon folium, a. Anus ; /, fulcra ; n, nostrils ; op, operculum ;
sc, rhombic scales on the upper caudal lobe ; sp, left spiracle.

laterally-placed post-temporals and squamosals, and there are no
median plates posterior to the orbits, nor any representatives of
supra-temporals. A feeble suboperculum is retained in addition
to a small rayed operculum. Hyoidean hemibranch completely sup-
pressed. Two genera only are known, each with a single species.
The Paddle-Fish or Spoon-Bill, Polyodon folium (Fig. 289)
inhabits the rivers of the Southern States of North America, the
Mississippi, Ohio, and Missouri, and their numerous tributary
rivers and streams. A Fish of sluggish habits, Polyodon feeds
chiefly on mud and the minute organisms it contains, the excep-
tionally long gill-rakers probably forming an efficient filter to
prevent the food particles escaping through the gill-clefts with
the expiratory water current. The singular rostrum is appar-
ently used for stirring up the mud when feeding, but in view of
the muddy waters the Fish frequents, and the very small size of

^ Jordan and Evermann, "Fishes of North and Middle America," Bull. U.S.
Nat. Mus. No. 47, Pt. i. 1896, p. 101.

492 . FISHES CHAP.

the eyes, its value as a tactile organ must not be overlooked.
Polyodon may attain a length of 5 to 6 feet. The time of
spawning varies, according to locality, from March to June.
Nothing is known of the development of Polyodon. Young less
than G to 8 inehes in length are unknown, and specimens of this
size are very rarely seen. The jaws are furnished with minute teeth
until the Fish is about half-grown, when they become edeutalous.
Caviare is made from the eggs, and the centres at which this
industry is carried on are chiefly situated along the course of the
Mississippi. The second species, Psephurus gladius, inhabits the
Yang-tse-Kiang and Hoangho rivers of China, and differs from
Polyodon in the conical shape of its rostrum and the smaller
number and larger size of its fulcra. Psephurus is stated to
reach a length of 20 feet. The family is represented in the
Eocene of Wyoming by the genus Crossopholis, which is note-
worthy for the retention of trunk scales in the form of small,
somewhat quadrate denticulated discs, arranged in oblique rows.

Fam. 7. Acipenseridae. — In the Sturgeon family the body
is elongate, cylindrical, and somewliat bulky. Kostrum well
developed and often massive, with a transverse row of simple or
branched preoral barbels on its ventral surface. Mouth small and
remarkably protrusible. Jaws devoid of teeth except in the larvae.
As in the preceding family, the primitive rhombic squamation
is confined to the upper lobe of the tail, which, like the dorsal
and anal fins, is furnished with fulcra. Elsewhere the scales are
represented by five longitudinal rows of large bony scutes and
by intervening small scattered ossifications. The anterior dermal
ray of the pectoral fin is stout and spine-like. The dermal
bones of the cranial roof suturally articulate with one another
to form a continuous shield, uninterrupted by lateral vacuities.
A median dermal bone in the occipital region transmits the
occipital sensory canal. The opercular series is represented only
by an opercular bone.

The family includes but two genera, Acipenser (Fig. 290) and
Scaphirhynchus, and about twenty species, confined to the seas,
estuaries, and rivers of the temperate and north temperate regions
of the northern hemisphere. Acipenser includes the more typical
Sturgeons, and is distinguished by tlie presence of spiracles, and
by the fact that the longitudinal rows of scutes remain distinct
to the base of the caudal fin. There are probably about fifteen

CHONDROSTEI

493

Species, but the exact number is uncertain. Sturgeons are abundant
in the Black Sea, the Sea of Azov, the Caspian, and their tribu-
tary rivers, notably the Danube, Don, Dnieper, Ural, and Volga.
They are also present in the rivers and on the coasts of Northern
Europe and of China. Five species occur in North America, on
the Atlantic and Pacific coasts, and in the rivers of these regions
as well as in the Great Lakes.^ One or two species are almost
exclusively fresh-water, but most Sturgeons are migratory Fishes,
living in the sea, but ascending rivers for spawning. Their food
consists of worms, molluscs, the smaller Fishes and aquatic
plants ; and in feeding the mouth is protruded downwards in
the form of a cylindrical, spout-like structure and thrust into the
mud. The only species certainly known to frequent the British

pctj-

Fig. 290. — The Sterlet {Acipense)- ruthenus). o, Barbels ; c.f, caudal fin ; cl.f, dorsal
fill MKt.f, pectoral fin ;pv.f, pelvic fin ; sc, scutes ; v.f, ventral or anal fin. (From
Parker and Haswell, after Cuvier.)

coasts is the common Sturgeon {A. shirio), which is also found
in the Black Sea and the Mediterranean, and is abundant on the
Atlantic coast of North America from Maine to South Carolina.
The species occurs all round our coasts, more plentifully, perhaps,
on the northern and eastern shores. In the spring and summer
the Fish ascends the rivers, often to a considerable distance. Its
presence has been recorded in the Severn, near Shrewsbury ; in
the Trent at Nottingham, and also,