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I

A STUDENT'S TEXT-BOOK OF ZOOLOGY

VOLUME ONE

PROTOZOA TO CHAETOGNATHA

The Protozoa section has been entirely re-written
by Professor J. S. Dunkerly

Second Edition 2^s. net

VOLUME THREE

THE INTRODUCTION TO
ARTHROPODA, THE CRUSTACEA,

AND XIPHOSURA.
THE INSECTA AND ARACHNIDA

Second Impression 2^. net

A STUDENT'S TEXT-
BOOK OF ZOOLOGY

by

ADAM SEDGWICK, M.A., F.R.S.

VOLUME II

GHORDATA TO MAMMALIA

LONDON: GEORGE ALLEN & UNWIN LTD
NEW YORK: THE MACMILLAN COMPANY

FIRST PUBLISHED IN IQOS
SECOND IMPRESSION 1932

AU rights reserved

PRINTED IN GREAT BRITAIN BY
UNWIN BROTHERS LTD., WOKING

PREFACE

In presenting the second part of my work on Zoology to the
public I must apologise for the delay in its appearance and
for the fact that I am not keeping to the undertaking which
I gave in the preface to my first volume that the work would
be completed in two volumes The delay in pubUcation
has been caused in part at least by the fact that the Verte-
brata compel a lengthier and more detailed treatment than
the other groups. Not only is more known about them,
but they excite greater interest, and their palaeontological
history has been more completely worked out than in the
case of any other phylum. The result has been the present
bulky volume which deals only with them and with Amphi-
oxus.

Embryology is of course excluded, except in the case of
Amphioxus, but I have endeavoured to deal fairly fully
with anatomy, habits, and classification. In the systematic
portions I have probably been too ambitious, but the
usefulness of a book of this kind depends largely upon its
completeness in this respect, and in cases of doubt I have
generally included rather than excluded. In Aves alone
have I made a selection ; for there are many excellent
works devoted to them and it would be impossible to give
anything like a complete list of their genera.

In judging the anatomical portions I would ask the reader
to remember that this is not exclusively a work on Compara-
tive Anatomy, and limitations of space forbid an exhaustive
treatment. This branch of the subject has not, however,
been neglected, and questions of general interest have usually
been at least touched upon.

Considerable space has been given to extinct forms.

LimmfmmiEaimtt,%matQk'W

Wdict^c gffiat wcKk en tiie > .

a WHwif raWe iwmilifr age faom dans' Xdbr&adb^ but
oi n^iAl have liem hwumUIj^H to n^iie -y the

eoortecf ai Ihe sadhor and. jjuMa ii g n ^-nfth

WoodvanTs VertdrnxU PalaumtblogK ^
Skdekm, Wlomet and IjdeUKi^s i/ ^

^^iilMKS^, Tlovcf^s C>«feofo9y o/ Jtfima^
<i/ YertdnraU AmmaU, Skapkj sa^
ZftteTii GnmshS^ der Ptdaeomklogie, Y.

AwtJU/mifi d*ir Wnbdikiert, Yfiederr:

AnakmU der WnhdOtiere, Pctricr'ic 7 d^ Z

PREFACE VU

Balfour's Comparative Embryology, Gadow's Amphibia and
Reptiles.

The third volume deals with the Tunicata, Enteropneusta,
Echinodermata and Arthropoda,

It has been pointed out to me by friends who have read
the proofs that I have made statements which without a
fuller treatment may give rise to the view that I am unortho-
dox on the great question of organic evolution. This is not
the place to give that fuller treatment, but in order to pre-
vent misunderstandings I may say that any such view would
be erroneous. I am and always have been a convinced
evolutionist. I hold, that is to say, that matter is constantly
undergoing change, and that natural selection, taking Eidvan-
age of its endless diversity in form and properties, has played
and is playing an important part in determining what form
of it (whether living or non-Hving) shall exist and what shall
cease to exist. I hold further that the forms of Uving matter,
as well as those of non-Hving matter, owe their existence and
their properties to the operation of natural laws, though
here we are treading on more uncertain ground, for we
know nothing of the origin of living matter or of the sources
of its properties. The chemist has made many forms of
matter which have, at present at least, no existence in
nature apart from organisms, but he has not yet succeeded
in making Uving matter. Whether he will ever be able to
do so is a question which may fairly be asked, but is one
which cannot now be answered. The view that Hving
matter arose in response to the operations of natural laws
cannot be either proved or disproved. It must remain a
matter of beUef for which there is much to be said. As to
the origin of the manifold properties of Uving matter we
know nothing. The Darwinian theory did not account for
properties ; it left their origin to an imperfectly understood
interaction between the organism and the environment,
and further than this we cannot at present go. It may,
however, be pointed out that there are two ways in which

VUl PREFACE

this great question may be attacked. One of these is by
the method of experiment — a method which is being
pursued with increasing vigour by more than one school
of Biologists ; the other is the careful and thorough exam-
ination of living and extinct organisms, particularly in their
relations to one another. It is the second of these methods
which comes under our notice in the three volumes of the
first part of this work dealing with systematic zoology. It
is unquestionable that this study does shed light, if only a
dim light, on the course of organic evolution and indirectly
on the origin of the properties of living matter, and it is
most important that the light so obtained should be brought
to bear upon the problem. To discover this we must
approach the subject with unbiassed minds, for it is one
of immense complexity and it is extremely unlikely that
any particular solution which commends itself to us
will turn out to be final. I would therefore ask for
lenient judgment if in some pages of this work I have
seemed to take up an unduly critical position with regard
to views widely prevalent at the present time on some
aspects of organic evolution. That does not mean that I
am unsound on the great question itself, but only that
I am sceptical as to the value of some hypotheses widely
held as to the course of organic evolution. It is true that
working hypotheses are necessary in constructive work, but
in a subject of the complexity of the present one, they can only
be provisional and as such are legitimately open to criticism.
It may be urged that I have said too much or too little, that
I ought not to have touched upon the matter unless I was
prepared to state fully my own views. While allowing
that there would be some justice in such a criticism, I do
not admit its complete vahdity. In deference to it, how-
ever, I have materially altered in proof what I had written
in manuscript, but it was not possible to remove all refer-
ence to the subject. It was necessary to note the facts in
passing. In the final volume on the Principles of Zoology,
I return to it and endeavour to justify, in the fuller
treatment which is there, the criticisms which are only

PREFACE IX

hinted at here. At the same time I camiot hope to
build. That is the task of the great band of workers in
many departments of Biology, who, undeterred by failure
and urged on by the fire, enthusiasm, and generous aspira-
tions of youth, return time after time, generation after
generation, to the assault of the fortresses of nature, well
knowing that their material reward will be small, that
defeat means the world's neglect and that success, except
the greatest, brings but a pittance of its esteem. To them
I inscribe this book in the hope that it may serve if only to
a small extent to smooth over the difficulties of part of the
road which at first they have to traverse.

A. SEDGWICK.

Trinity College, Cambridge,
February, 1905.

CORRIGENDA

Page 17. In the description of flg. 6, for " spina " read " spinal."

117. Line 5. For " branchia " read " branchial."

125. Line 8. For " Myliobates " read " Myliobatis."

146. Lines. For "somactido " read "somactids."

210. Line 15 from bottom. For " Gastrosteus " read " Oasterosteiu."

219. Line 4. 'Foi " vulggaris" iea,d. " vulgaris."

219. Line 20. For " E. Ind. Oreinus; McCIell " read " E. Ind. ; Oreinus McClell."

220. Line 12. For " RHODENIA " read " Rhodeina."

230. Line 13. Insert a comma after " Raf ."

235. Line 18. For " specie " read " species."

242. Line 27. Insert comma after Elanura, and another after Melletes.

242. Line 11-12 from bottom. The 5th letter of the word Stellerina is indistinct.

244. Lme 23. For " Thallasso- " read " Thalasso-".

255. Line 14 from bottom. For " succus " read " saccus."

273. In the second footnote, for " Paratoids " read " Parotoids."

276. Line ISfrom bottom. Insert after the words "in other Amphibia." "In iJana the
ductus endolymphaticus which passes off from the saccule enters the cranial
cavity through the foramen endolymphaticum and there dilates into the saccus
endolymphaticus. This extends back into the neural canal of the vertebral
column, where it lies in close apposition to its fellow along the dorsal side of
the spinal cord and gives off transverse diverticula which end la small dilata-
tions over the posterior root ganglia. The fluid contained in the saccus and
its extensions is milk-white, the milkiness being due to the presence of crystals
of carbonate of lime (otoliths). The calcareous glands are the terminal
dilatations of the transverse diverticula above referred to (see Gaup, op. cit.,
p. 261). The function of this peculiar extension of the membranous labyrinth
is imknown (for a somewhat similar condition in the Ascalabota.see p. 323)."

316. Line 10 from bottom. For " Trimatosaurzis " read " Trematosaurus."

334. Line 21. For " phenodon " read " Sphenodon."

393. Line 6. For " Ion ifrons " read " longifrons."

400. In description of Fig. 221. For " (Pareiasaurus) " read " {Pariasaurus)."

459. The footnote is referred to on line 1 of p. 460.

465. Line 25. For " M. gallopavo " read " Meleagris gallopavo."

476. Line 21. For " M. superba " read " Menura superba."

682. Line 7 from bottom. For " H " read " Hippopotamus."

655. Line 16. For " pectora " read " pectoral."

TABLE OF CONTENTS

An asterisk signifies that the group is extinct.

PAGE

THE CHORDATA .... 1
PHYtUM CEPHALOCHORDA . 10

Phylum VERTEBRATA

45

Class I. PISCES .... 51

Sub-class 1. Marsipobranchh 95

„ 2. Elasmobranchh 118

Order 1. Pleuropterygii* . 145

„ 2. Acanthodii* . .146

„ 3. Ichthyotoml* . . 147

„ 4. Selachii (Plagio-

stomi) . . .148

Sub-order 1. Notidani . .149

2. Squali . . 150

3. Baji . . .153
Order 5. Holocephali . . 155

Sub-class 3. Ganoidei . . 159

Order 1. Chondrostei . .167

Order 2. Crossopterygii . .171

Sub-order 1. Ostedepida* . 175

„ 2. Cladistia . .176

Order 3. Lepidostei . . .176

„ 4. Amioidei . . .180

Sub-class 4. Teleostei

183

Sub-order 1. Malacopterygii
(Salmoniclwpei-
formcs) . . 213

„ 2. Ostariophysi
{Cyprinisiluri-
forines] . .216

Sub-class Teleostei— conW.

FAOK

Sub-order 3. Symbranehii

{Syvibranchi-

formes) .

222

>>

4. Apodes {An-

guilliformee)

223

99

5. Haplomi

{Esociformes)

225

)J

6. Hcieromi{Der-

cetiformes) .

227

»

7. Cateostomi
{Gastrostei-

formes) .

228

Tribe A. Selenichthyes .

228

>»

B. Hemibranchii .

228

ft

C. Lophobranchii .

220

»»

D. Hypostomides .

230

Sub-order 8. Perce^oces

(Mugiliformes) 230

»j

9. Anacanthini

{Gadiformes)

232

J9

10. AcarUhoptery-

>

gii . . .

233

Tribe

A. Perciformes

238

t.

[B. Scombriformes

238

99

C. Zeorhombi

239

If

D. Kurtiformes

240

,^

E. Gobiifonnes

241

ft

F. DLscocephali

241

,t

G. Sclerop.irei

241

,,

IJ. Juculares

243

»»

I. Taeniosomi

244

Sub-order 11. Opisthomi .

245

»

12. Pediadaii

{Lophiiformea)

245

»»

13. Pltciognathi

(Balisiiformes)

246

Tribe

A. Sclerodenni

24«

ft

B. GymnodoDtea .

•

247

Zl

60275

Zll

TABLE OF CONTENTS

Sub-class 5. Dipnoi . .

A. Ctenodipterini*

B. Sirenoidei
Aethrodira*

A. Heterostraci* .

B. Osteostraci* .

C. Antiarcha*

Icthyodorulites'
Conodonts* .

PAGE '

. 248

. 259
. 259

. 260

. 261

. 261

. 261

. 262

. 262

. 263

Sub-class DINOSAUEIA* — co}Ud.
Tribe 1. Ornithopoda*
„ 2. Stegosauria*
„ 3. Ceratopsia*

Oass II. AMPHIBIA . .

Order 1. Gymnophiona

(Apoda) 300

2. Urodela(Caudata) . 304

3. Anura (Batrachia) . 307
Sub-order 1. Aglossa . .309

2. Phaneroglossa 310

Order 4. StegocephaU* . . 313

Sub-order 1. Branchiosauri* 5W

2. Aistopoda* . 315

3. Lahyrintho-

dontia* 315

Microsauria* 315

aass III. REPTILIA . . .316

Sub-class 1. Rhyncocephalia 329

2. Lepidosaitkia . 334

Order 1. Dolichosauria* . 334

2. Mosasauria* . . 334

3. Lacertilia . . -335
Sub-order 1. Lacertilia vera 348

2. RUptoglossa . 354
Order 4. Ophidia

Sub-class 3. Crocodilia

Order 1. Parasuchia*
2. Pseudosuchia*
„ 3. Eusuchia .

Sub-class 4. Dinosauria*

Order 1. Theropoda*
„ 2. Sauropoda*
.. 3. Predentata*

355
372

381
382
382

383

384

385
386

PAQB
, 386
, 387
. 387

Sub-class 5. Pterosatjria* . 388

„ 6. ichthyosauria* 391

7. Plesiosauria* . 395

„ 8. Anomodontia*

Order 1. Pareiasauria* .
„ 2. Theriodontia* .
„ 3. Dicynodontia* .

Sub-class 9. Chelonia .

Sub-order 1. Athecae .
„ 2. Thecophora

A. Cryptodira .

B. Pleurodira .

C. Trionychoidea

aass IV. AVES . . .

Order 1. Archaeornithes*

„ 2. Neornithes . .

Sub-order 1. Ratitae .

„ 2. Odontolcae*

„ 3. Carinatae

Tribe 1. Ichthyornithes*

„ 2. Colymbiformes

„ 3. Sphenisciformcs
4. Procellariiformes

„ 5. Ciconliformes .

„ 6. Anseriformes .

„ 7. Falconiformes .

„ 8. Tinamiformes .

„ 9. Galliformes

„ 10. Gruiformes

„ 11. Charadriiformes

„ 12. CuciUiformes .

„ 13. Coraciilormes .

.. 14. Passeriformes. .

398
399
400
401

402

412
412

412
414
415

. 416

. 454
. 456
. 456
. 458
. 460

. 466

. 466

. 466
461

. 461

. 462

. 463

. 464

. 464

. 466

. 466

. 469

. 471

. 474

. 479

Class V. MAMMALIA. .

Order 1. Monotremata . . 524

„ 2. Marsupialia . . 529

Sub- order 1. Diprofodontia 534

2. Polyprotodontia5^8

3. Allotheria* . 541

TABLE OF CONTEXTS

XIU

PAGE

Orders. Edentata . . .542

Xenarthra . . . 543

Nomarthra . . 548

„ 4. Sirenia . . . -549

„ 5. Cetacea . • • 553

Sub-Older 1. Mystacoccti . 560

„ 2. Odontoceti . 561

„ 3. Zeuglodonta* . 564

Order 6. Hyracoidea . • 565

„ 7. Proboscidea . • 567

„ 8. Ungulata . . .573

Sub-order 1. Artiodactyla . 576

„ 2. Perissodactyla 592

3. Lipoterna* . 602

Order 9. Amblypoda* . . 603

„ 10. Toxodontia* . . 605

„ 11. Typotheria* 606

PAGE

Order 12. Tillodontia* . • 607

„ 13. Ancylopoda* . • 609

,, 14. Condylarthra* . . 609

„ 15. Creodonta* . .611

„ 16. Carnivora . . .612

Aeluroidea . .618

Cynoidca . . .621

Arctoidea . . . 622

„ 17. Pinnipedia . . . 624

„ 18. Rodentia . . .627

Simplicidentata . 632

Duplicidentata . 636

„ 19. Insectivora . . .636

„ 20. Chiroptera . . .641

„ 21. Prosimiae . . . 649
. 22. Primates ... 663

j»»i

TABLE OF GEOLOGICAL PERIODS
AND FORMATIONS

TERTIARY OR CAINOZOIC PERIOD.

Pleistocene

Pliocene .

Miocene

OUgocene .

E«cene

Recent Deposits.

Valley and Cave Deposits

Glacial Deposits.

Cromer Beds.

Norwich Crag.
, Red Crag.

Coralline Crag.
vin India, the Siwalik Formation.

Not known in Britain but wide-spread on the Continent
of Europe.

/Loup Fork.
Lacustrine deposits of North America ] Deep River.

Ijohn Day.
The Santa Cruz Beds of Patagonia are referred to the
Miocene.

Hamstead and Bembridge Beds.
Headon and Osborne Beds.

In N. America the freshwater White River Beds belong
to the Ohgocene.

Upper: Barton Beds; In N. America, Uinta Group.

Middle : Bracklesham Beds. In N. America, Bridger Group.

Lower : Bagshot Sands, London Clay, Woolwich and

Reading Beds.

rl. Wind River Group.

T -NT * • 12. Wasatch Group.
In N. America-' — - ^

1 3. Torrejon Group.
1.4. Puerco Group.

XIV

TABLE OF GEOLOGICAL PERIODS AND FORMATIONS

XV

Cretaceous

Jurassic .

Triassic

SECONDARY OR MESOZOIC PERIOD.

Upper Chalk (with flints).

:Middle Chalk (with few flints).
-, Lower Chalk.

Upper Greensand.
l^Gault.

f Lower Greensand.
I Wealden.

Upper

i Lower

( /Purbeck and Portland Beds.

Upper \ Kimmeridge Clay (Solenhofen Slates in Bavaria.)
iCorallian and Oxford Clay.

J Tif. J 11 ( Great Oolite.

•< Middle I Inferior OoUte (Stonesfield Slate belongs here}.

Upper Lias.

Lower Middle Lias.

Lower Lias.

/' Yj (■ RhaeticBeds.

P" \ Keuper Marls and Sandstone.
Middle : Muschelkalk, absent in Britain.
Lower : Buuter Sandstone, Pebble Beds, Karoo of S
Africa.

Permian .

Carboniferous .

Devonian

Silurian

Upper Cambrian"

Lower Cambrian

Precambrian

PRIMARY OR PALAEOZOIC PERIOD.

/ Upper : Magnesian Limestone,
l Lower : Red Sandstones.

TT ( Coal Measures.

Upper I Millstone Grit.
Lower : Carboniferous Limestone and Shales.

Upper Old Red Sandstone.
Limestones (marine).
I Lower Old Red Sandstone.

(Ludlow.
Wenlock,
May Hill Sandstone (Llandovery).

Bala.
Llandilo,

Arenig.

/•Tremadoc Series.
I Lingula Flags.
' Menevian Series.

Harlech Series.
I Olenellus Beds.

L. Silurian of Murohison, Ordovician of Lapworth-

CHAPTER I.

CHORDATA.

Animals with a rwtochord, a hollow dorsally placed nervous
system, and a jjharynx opening to the exterior by lateral pcissages.

The group Chordata is a division of the animal liingdom
superior to a phylum. It includes four phyla and is to be
compared in its rank to such groups as the Jletazoa and Codo-
mata, both of which are phylum-including di\'isions. The four
phyla into which the group Chordata is divided are, stating them
in the order in which they are dealt with in this work, the
Cephalochorda, which includes but a single genus, Amphioxus ; the
Vertebrata, which is by far the largest and most important
division of the group ; the Tunicata, which includes a con-
siderable number of marine forms of low organization ; and
lastly the Enteropneusta, which has but a small number of
genera mainly of vermiform appearance and is the most out-
lying phylum of the group. Indeed, by some higlily competent
authorities the Enteropneusta are placed altogether outside the
Chordata, largely on account of their early development, which
differs in important particulars from that of other Chordata
and approaches that of Echinodermata ; and because it is not
certain that they possess that typically chordate organ, the
notochord. While not presuming to pronounce an opinion
on the latter point beyond saying that if the notochord is present
in Enteropneusta, its development, structure and relations to
other organs differ considerably from those of the notochord in
the other phyla, we desire to emphasise quite distinctly our
opinion that the Enteropneusta are Chordates. They present
most clearly the other characteristic features of that group, viz.,
the hollow central nervous sj^stem and the perforated pharjTigeal
wall — features of organization found in no other group of the

z— n B

CMORDATA.

animal kingdom ; and in the arrangement of their coelom they
come close to the Cephalochorda and Vertebrata.

The notochord itself is a rod-like structure in all cases
developed from the dorsomedian endoderm of the embryonic
enteron. This streak of tissue undergoes a modification of
structm'e almost identical with that presented by the axial endo-
derm of the tentacles of many Coelenterata. The modification,
which may be described as being of a skeletal nature, consists
in both cases of a a vacuolisation of the protoplasm of the

endodermal tissue (Fig. 1) and of a
considerable development within this
tissue of cuticular structures (vide
Vol. i, p. 101). Indeed the function
of the notochord, like that of the ten-
tacular endoderm referred to, is a sup-
porting one : it supports the axis of
the body and particularly the central
nervous system beneath which it lies.
In the Cephalochorda this function is
discharged by the notochord during
the whole of life ; in the Vertebrata
and Tunicata however it is purely
embrj^onic or larval in its duration.
In the Vertebrata the notochord,
though it may in some forms, e.g.
Pisces, persist throughout the whole of
life, becomes surrounded by a stiff
sheath, which takes over its function of
axial support and becomes, especially in those forms in which
the endoskeleton acquires rigid texture, divided up into segments
corresponding with those of the embryonic muscular system.
The central nervous system develops from the ectoderm of
what is usually called the dorsal surface, and at first nearly
always has the form of a groove, which, excepting in the Entero-
pneusta, extends along the whole of the dorsal surface and closes
completely to form a canal — the central canal of the nervous
system. It is characteristic of Cephalochorda, Vertebrata and
Tunicata that this canal opens in the embryo for a shorter or
longer period unto the enteron (neurenteric canal ). This
neurenteric communication is however never maintained in

FlQ.

-'; lausverse section
through notochord and spinal
cord of the larva of Bombina-
lor igneus (aftei Gotte, jrom
Claus). ChS notochordal
sheath ; Ch notochord ; Sk
skeletogenous layer ; N spinal
cord.

MOUTH AND ANUS. 8

the adult, and its transitory existence is a highly remarkable
fact for which no satisfactory explanation has ever been offered.
In the Enteropneusta alone is the central nervous system
confined to a short portion only of the dorsal surface (so-called
collar region), and m them alone does the central canal remain
permanently open and never acquire a communication with
the enteron.

We have said that the central nervous system arises on the
dorsal surface. Now it is quite clear that this surface corresponds
to the ventral surface of other Coelomata, so that it would
be convenient to excliange the term dorsal for a term which would
include the same surface in all Coelomata. Such a term is
afforded by the term neural surface, which implies, and cor-
rectly implies, that the central nervous system is developed
upon it. Another term, blastoporal, having reference to the
position of the embrj^onic blastopore might also be used. In
all the Coelomata the blastopore is not only placed on the neural
surface of the body, but actually perforates the embryonic
rudiment of the central nervous system. This is seen most
clearly in tlie embryonic history of the Cephalochorda, the Verte-
brata, the Annelida, Arthropoda and Mollusca. In the Entero-
pneusta and Echinodermata this relation is masked, and by
many morphologists would be held not to occur at all. But that
it does exist we are convinced, and is a most important morpho-
logical fact appertaining to all Coelomata. Now in some Coelo-
mata it has been definitely proved that the mouth and anus
of the adult animal are directly derived from the embryonic
blastopore, and it becomes a question whether this deriv^ation,
though not embryonically manifested in all forms, does not
also hold throughout the Coelomata. Believing as we do
in the homology of the mouth and anus, at least in the
phyla Annelida, Arthropoda and Mollusca, it follows that
this relation holds for them. In Peripatus the mouth
and anus are not only derived from the elongated blasto-
pore by its constriction into two openmgs, but remain
throughout life included within the nerve ring derived
from the neural rudiments of the embryo.* If in other

* Sedgwick, " Monograph of the Development of Peripatus capensis,"
Studies from the Morphological Laboratory of the University of Camhridfje,
A, 1889, p. 1.

4 CHORDATA.

Artliropoda, in Annelida and in Mollusca we find, as we do, that
the nerve ring referred to is, in the adult, incomplete behind the
anus, and that the mouth and anus, though obviously referable
to the blastopore, are not actually both derived from it, must we
on this account deny this most obvious relation and maintain
that the mouth or anus, as the case may be, in these forms is
not homologous with that of Peripatus ?

To maintain such a position appears to us impossible, and we
entirely accept the doctrine that the mouth and anus of
the Annelida, Artliropoda and Mollusca are both perfor-
ations of the embryonic neural surface and are specialisations
of parts of one original opening which is represented in most
embryos by the blastopore. When however we come to apply
this doctrine to the Chordata we stand upon more debatable
ground. Placing the Enteropneusta on one side as not ob-
viously conforming to our plan, we find that it is a fact of
observation that in the Chordata the blastopore perforates the
embryonic nerve rudiment, and that in some of them the anus
is directly derived from it (many Pisces, some Amphibia,
e.g. newt), whereas in others, not at all remote from these,
the blastopore closes entirely and the anus is a new formation
(some Pisces and Amphibia, e.g. frog, Amniota), Here also
we think it may fairly be maintained that notwithstanding
the diversity in the mode of development of the anus it is, in all
Vertebrata at least, a derivate of the blastopore. The non-
inclusion of the anus within the nerve rudiment in the adult,
and its shift on to the ventral surface, caimot be brought against
this view, because these facts apply both to animals in which
the anus is a persistent part of the blastopore, as well as to
those in which it is a new formation. Here again, as in the
invertebrate phyla already dealt with in this connection, the
anus escapes in the adult from the embryonic nerve rudiment ;
or to put it in another way the part of the nerve rudiment behind
the anus never attains full development, but early undergoes
atrophy.* So far then all is plain sailing, in the Vertebrata at
least : the anus is a persistent portion — not the whole, as is clear
from a consideration of the development of Elasmobranchs and
some Amphibia — of the blastopore, as it is in the invertebrate

* See especially Lepidosiren, in which the medullary folds of the embryo
include the blastopore which becomes the anus.

MOUTH. O

Coelomata ; and as in most of the latter the part of the nerve
rudiment behind it (in the primitive position, anterior in the
position which the anus secondarily acquires on the ventral
surface) undergoes atrophy.

We now come to the question of the chordate mouth, a much
vexed question, and one about which much of a highly specu-
lative character has been written. We may at once concede

9 ■

Fio. 2.— Heads of young Elasmobranch embryos (Scyllium caniciUa) (after Sedgwick).

A. Ventral view of head of embryo, 7 mm. in length, with two open pharyngeal clefts.
The mouth is present as a longitudinal groove in tlie ectoderm of the buccal depression.

B. Same view of a slightly older embryo ; the buccal groove has become a longitudinal
slit. C. Side view of head of embryo, 9 mm. in length, with three open slits. D. Side
view of head of embryo, 11 mm. in length ; rudiments of e.xternal gills have appeared on
the hyoid and on the first and second branchial arches. E. Side view of head of embryo
of 16 mm. ; external gills have appeared on mandibular arch and the angle of the jaw is
marked. 1 mandibular arch ; 2 angle of jaw ; 3 second pharyngeal cleft ; i nasal pit ;
5 eye ; 6, midbrain ; 7, auditory sac ; 8 hyoid arch ; 9 spiracle.

the point that the chordate mouth has never been brought
into developmental relation with the blastopore. Even if it be
allowed that the chordate blastopore really extends to the
front end of the nerve rudiment (medullary plate), which is in
itself a disputed point, no morphologist has ever brought to
light any embryological fact which is at all in favour of the
view that the mouth was originally within the nerve rudiment,

6 CHOKDATA.

and that its present position outside it and on the ventral surface
is a secondary one, due to shifting and to atrophy of the part of
the nerve rudiment in front of it. There are a number of
features of vertebrate morphology of the highest interest in
connection with this point : such are the cranial flexure, the
close relation of the infundibulum, which there is good reason
to believe is the real front end of the nerve axis, to the anterior
end of the mouth, the slit-like form * (Fig. 2), which at first
characterizes the buccal opening, and its extension into the rudi-
ment of the pituitary body ; but there is no actually ascertained
fact which tends to show that the mouth is a derivate of the
blastopore, as it must be conceded to be in most other coelomate
phyla, t

The last chordate character to be considered is the posses-
sion of lateral pharyngeal apertures. These are often used
for respiration and are in consequence generally termed gill-
slits. They are not however always respiratory in function
— indeed in the majority of the Vertebrata in which they
form a very conspicuous feature they are not respkatory at
all, but are entirely functionless, being found only in the
embryo.

An attempt has been made in some quarters to refer the
chordate mouth to a modification of a pair of these structures.
We can see no fact in favour of such a view, and we are not
prepared to give up the homology of the chordate mouth with
that of other Coelomata. We have already stated the case with
regard to its relation to the blastopore, and we have seen that
there is no good embryological evidence in favour of its being
so related, but we do not consider that this absence of evidence
is sufficient to put out of court the view that it is the homologue
of the mouth of other Coelomata. In many of these, too, no
relation can be shown between the mouth and the blastopore in
development, but yet we well know that in them the mouth
is homologous with the mouth of forms in which it is directly
derived from a part of the blastopore.

Finally there is one point in the morphology of the Chordata,

* Sedgwick, " Notes on Elasmobranch Development," Q.J.M.S., 33,
1892, p. 559.

t For a fuller discussion of these questions, the reader is referred to the
article "Embryology" in the recently issued supplement of the Encyclo-
paedia Britannica.

COELOM.

- -1

-4vs^--a

-3

6/- — -4

which though not referred to in the definition, is of considerable
importance, and must be shortly dealt with here, and that is the
form and development of the Coelom.^ In the Cephalochorda
and Enteropneusta the coelom originates as outgrowths of the
primitive gut (archenteron). In Vertebrata, though there is
no actual outgrowth of the enteron, the walls of the coelom
originate from tissue which is derived from the wall of the
enteric space, and there can be but little doubt that the mode of
development is referable to the enterocoelic type, found in
the two other phyla, and is indeed a modification of it.

If then we leave out of consideration the Tunicata, which
in this respect cannot at present be brought into line with the
other chordate phyla,
we may assert that
an enterocoeUc origin
of the coelom, or a
modification of it, is
characteristic of the
Chordata. Ou t s i d e
the Chordata a similar
mode of origin of the
coelom is found m the
Chaetognatha (vol. i,
p. 590), in the Bra-
chiopoda (vol. i, p.
580), probably in the
Phoronidea (vol. i, p.

546), and as will be shown further on in the Echinodermata.
But the coelomic resemblances between these animals go farther
than this. In the Enteropneusta the archenteric outgrowths
are five in numl^er — two pairs and an anterior unpaired out-
growth (Fig. 3). These, following the nomenclature of Bateson,
have been named according to the position they occupy in the
adult : the anterior unpaired sac is called the proboscis cavity ;
the sacs of the anterior pair are the collar cavities ; and the
posterior sacs are the trunk cavities. In the Enteropneusta
they undergo no further division, but remaining in the parts
of the body indicated by their names, they give rise to the
coelomic spaces of the adult.

In Amphioxus the anterior unpaired sac is called the preoral

Fio. 3. — Diagrams showing the oriy;in and primitive
relations of tlie coelomic sacs A in Balanoglosstis, B
in Amphioxus (after MacBride). 1 proboscis cavity
in A, preoral cavity in S ; 2 collar cavity ; 3 anterior
somite of trunk ; 4 trunk cavity.

8 CHORDATA.

or head cavity ; it obviously corresponds to the proboscis cavity
and remains in the head region of the animal. The sacs of the
second pair are called the collar cavities, because they correspond
to those cavities of the Enteropneusta. They are in reality the
anterior pair of somites, and give rise dorsally to the first pair
of myotomes. Their exact disposition in the adult is not quite
certain, but they appear to get some backward extension. The
posterior sacs which come off as one pair from the enteron and
correspond to the trunk cavities of Enteropneusta undergo in
subsequent growth a segmentation and give rise to the whole of
the mesoblastic somites of the trunk from the second pair back-
wards.* In the development and arrangement of its coelomic
sacs Am'phioxus resembles in a remarkable manner the Entero-
pneusta, the difference between them consisting in the segment-
ation which the trunk cavities undergo in Am'phioxus.

In the Vertebrata, though it is not possible to point to such
close resemblances as those which we have just described,
there is a remarkable similarity in the embryonic arrangement.
The first coelomic sac is preoral and unpaired ; the second is
paired and large, extending backwards in the mandibular arch,
so as to overlap the following somites. These mandibular
cavities are clearly homologous with the collar somites of the
other types. Following them we fuid on each side one large
cavity, the dorsal parts of which are divided up into segments and
become the myocoeles, and their walls the myotomes of the later
embryo. These posterior cavities clearly correspond to the trunk
cavities of the other types : as in them they are extensive, and
occupy the whole trunk region, and as in Am'phioxus they are
metamerically segmented.

In Amphioxus it is said that the preoral somite does not give rise to
striated muscles ; in Vertebrata it gives rise to a considerable number of
the eye muscles.

With regard to the nonchordate phyla with enterocoelic
coelom, we have only space to say this, that in the Echinoderms
the Enteropneust plan of an unpaired anterior cavity and two
pairs of posterior cavities can, according, to MacBride's researches,
generally be made out ; that in the Chaetognatha there is an ap-
proximation to the Enteropneust arrangement, but the unpaired

* MacBride, Q.J. M.S., 40, 1898, p. 589

CHORDATA.

cavity is at the hind end ; in the Phoronidea there are indications
that the Enteropneust arrangement or a modification of it exists,
but the indications are not very clear ; while in the Brachiopoda
according to our present knowledge no resemblance to the
Enteropneust plan exists save in the enterocoelic origin of the
coelom.

The formation of the coelom in the other non-chordate phyla,
the Annelida,the Mollusca, and the Artliropoda, must be regarded
as a modification of the enterocoelic method, but it is never
possible in them to trace the arrangement into an unpaired
chamber and two pairs of chambers which is so characteristic
of the Chordata.

CHAPTER II.

PHYLUM CEPHALOCHORDA.*

With dorsal tubular nerve-cord, and persistent notochord extend-
ing forwards in front of the nerve-cord. The muscular system and
(jonads are segmented, and the pharynx possesses a large number of
branchial slits which open into an atrial cavity and are provided
with tongue bars. Without paired fins, jaws, brain, vertebrae
and generative ducts. The larval life is prolonged and the larva
is remarkably asymmetrical.

The phylum Cephalochorda contains but the single genus,
Amphioxus Yarrell. It was discovered by Pallas in 1778, who
took it for a slug and named it Limax lanceolatus. Its true
position in the animal kingdom was first recognized in 1834 by
Costa, by whom it was named Branchiostoma. Two years later
it was described by Yarrell, who called it Amphioxus, by which

* J. Miiller, TJeber den Bau und die Lebenserscheinungen des Branchi-
ostoma luhricum [Amphioxus lanceolatus), Berlin, 1844. Quatrefages,
" Sur le systeiBe nerveux et siir I'histologie du Branchiostome," Ann. des
Sci. Nat. (3), 2, 1845. Kovvalevsky, " Entwick, v. Amphioxus lanceolatus,"
Mem. Acad. Imp ^-c, St. Petersboiirg (7), 2, 1867. Id. " Weitere Studien,
etc ," Arch. f. mic. Anat. 13, 1877. Stieda, " Ueb. d. Amphioxus lanceo-
latus," Mem. Acad. Imp. Sc, Petersbom-g, (7), 19, 1873. Rolph, "Ueb.
d. Bau d. Amphioxus," Morph. Jahrb. 2, 1876. Langerhans, " Zvu- Anat.
d. Amphioxus," Arch. mic. Anat., 12, 1876. A. Schneider, Anat. u. Ent-
wick. der Wirbelthiere, Berhn, 1879. Hatscliek, " Ueb. d. Entwick. Am-
phioxus," Arb. a. d. Zool. Inst. Wien, 4, 1881, also Zool. Anz., 7, 1884,
p. 517, and Anat. Anz., 3, 1888, p. 662. Rohon, " Ueb. Amphioxus lanceo-
latus," Denksch. k. Akcid. d. Wissenschaft, Wien, 45, 1882. Lankester,
" Contributions to the knowledge of Amphioxus lanceolatus,'''' Q.J. M.S., 29,
1889, p. 364. MacBride, " The Early Development of Amphioxus,"
Q.J M S., 40, 1898, p. 589. A. Willey, Amphioxus, etc., New York, 1894,
and Q.J.M.S.. 31, 1890, p. 445, and 32, 1891, p. 183. v. Wijhe, " Beitr. z.
Anat. des Kopf region des Amphioxus," Petrus Camper, 1901, and Anat.
Anz., 8, 1893. C. F. Cooper, " Cephalochorda," in J. S. Gardiner's Fauna
etc. of Maldive and Laccadive Archipelago, 1, 1903, p. 347. R. C. Punnett,
" Meristic Variation in Cephalochorda." Ibid., p. 361.

HABITS. 11

name it has since been known. According to the strict rules of
zoological nomenclature this is incorrect, the generic nams
Branchiostoma having two years' precedence over Amphiozus.
But, as so often happens in human alf airs, the unwritten law has
triumphed over the written, and the almost universal custom
of zoologists has been to call the genus Amphioxus. From this
custom we shall not venture to depart in this work.
<< Having thus entered our protest against a breach of con-
ventional rule which is made knowingly, we had almost said
wantonly, by all zoologists, we may proceed to consider the
actual position in the system of this remarkable creature.
Here fortunately there is no conflict between preaching and
practice,, between a pedantic conformity to rule and a lawless
adhesion to custom. For law and custom alike agree that the
position of an animal in the system shall be determined by its
natural affinities as revealed by a study of its structure and
development Judged by this test there can be no question
that Amphioxus is closely allied to the Vertebrata and must be
placed either within that group or in close juxtaposition to it.
As our readers know we have adopted the latter course and
have placed Amphioxus in a special phylum of its own, equal in
morphological importance but very inferior in the number of
its members to the great phylum Vertebrata, and have applied
to it, out of a number of claimants,* the name Cephalochorda,
in allusion to the extension of its notochord into the anterior
part of the cephahc region.

Amphioxus^i is a small, semi-transparent, colourless animal.
Its body is elongated, laterally compressed, and pointed at each
end ; and it may attain a length of two inches. It is entirely
marine, and is found at moderate depths in many parts of the
world. It has a remarkable power of moving in sand, in which
it is usually partially buried, its mouth alone protruding. But
it is capable of swimming, and when removed from the sand
bends its body with great activity from side to side. The
mouth is an elongated oval aperture on the ventral surface
immediately behind the anterior end of the body. It is sur-
rounded by a number of dehcate ciliated processes, the oral cirri.

* PharyngohrancMi, Acrania, Leptocardii, etc.

t The anatomical description refers, unless otherwise stated, to A.
lanceolatua.

12 PHYLUM CEPHALOCHORDA.

The anus is also ventral and is placed slightly to the left of
the middle line at some little distance from the posterior end
of the body (Fig. 5). Extending from the mouth backwards
along the ventral surface for about two-thirds of the length of
the animal is a wide median groove, bounded by lateral folds
and perforated at its hind end by a pore (Fig. 5). The folds are
called the metapleural folds and the pore the atrial pore.

There are no paired fins, but there is a continuous median fin
consisting of a fold of skin extending along the whole length of the
dorsal surface (dorsal fin), and round the hind end of the body
on to the ventral surface as far forwards as the ventral groove
(Fig. 4). Anteriorly it is also continued on the ventral surface,
reaching as far as the mouth, with the right side of which it is
continuous (Fig. 5). The portion between the ventral groove
and the anus may be called the anal fin, and that between the
anus and the hind end of the body the ventral part of the caudal
fin.

Amphioxus is a segmented animal. The segmentation is
marked externally by a number of V-shaped grooves, placed
one behind the other on each side of the body, the apex of the
V being directed forwards (Fig. 4). These markings are caused
by the insertion into the skin of a number of transverse septa
of connective tissue, which divide the great lateral longitudinal
muscles of the body into a series of successive segments, placed
one behind the other and called myotomes. The grooves of the
two sides of the body alternate with one another. The seg-
mentation is also exhibited by the gonads which consist of a
series of saccular bodies extending throughout the greater part
of the pharyngeal region as far back as the atrial pore (Fig. 4).
They correspond in number with the myotomes of that part of
the body in which they occur and alternate with those of the
opposite side of the body.

The body of Amphioxus is traversed throughout almost
its entire length by a fiexible skeletal rod — the notochord.
The notochord is pointed at either end and is placed in the
centre of the body, but nearer to the dorsal than the ventral
surface (Fig. 4). Lying immediately on the dorsal side of the
notochord is a cord of nervous matter which may be called the
cerebrospinal cord and constitutes the central nervous system.
Behind, this nervous cord tapers and ends in a point, or a small

PHYLUM CEPHALOCHORDA.

13

<0-

iO

Tj- -

' H

14 PHYLUM CEPHALOCHORDA.

swelling, immediately over the hind end of the notochord ; in
front it tapers very slightly, and possesses a somewhat blunt
termmation placed some little distance behind the front end of
the notochord> On the ventral side of the notochord is the
alimentary canal, which has the form of a straight tube extending
between the mouth and the anus. The central nervous system
therefore, lies entirely dorsal to and the alimentary canal
entirely ventral to the notochord.

The alimentary canal consists of three parts : —

1. The buccal cavity. This is a short chamber opening to
the exterior by the mouth and behind by a somewhat constricted
opening into the pharynx.

2. The pharynx is the widest and longest portion of the
alimentary canal, extending nearly half the length of the body.
Its walls are perforated on each side by a number of obliquely
directed slits (from above and in front ventralwards and back-
wards) which place its cavity in communication with a space
lying immediately outside it, and called the atrial or 'peripharyn-
geal chamber. The atrial chamber entirely surrounds the
pharynx except along the dorsal middle line (vide Fig. 10 and
explanation). It opens to the exterior by the atrial pore —
already mentioned — which is found at the hind end of the ventral
groove (Fig. 5). The pharynx is mainly a respiratory organ,
inasmuch as the blood which circulates in its walls and in the
walls of the atrial cavity is aerated by the water which is con-
tinually being taken in by the mouth and driven by the action
of cUia through the pharyngeal slits — or gill slits as they
may be called — into the atrial cavity.

3. The intestine which extends as a straight tube from the
hind end of the pharynx to the anus. The anterior part of the
intestine is slightly dilated and receives ventrally a simple
caecalsac, which, pushing the body wall before it, extends for-
wards in the atrial cavity on the right side of the pharjmx and
is called the liver.

Detailed Description of the Organs.

The ectoderm consists of a single layer of columnar or in some
places cubical cells, which cover the whole external surface of
the animal, are prolonged for a short distance into the buccal
cavity and line the whole of the atrial cavity (Fig. 10). They

SKELETON.

15

are without cilia except on the cirri, in the mouth and in the
atrial cavity, and their outer surface is covered by a porous
cuticle. Immediately beneath the ectoderm is a layer of fibril-
lated tissue called the cutis. Beneath this comes the subcu-
taneous tissue which consists of a gelatinous matrix containing
sinuous fibres. The tissue within this has a similar form and
extends between the myotomes, as the inter-muscular septa,
to become continuous with the sheath of the notochord. In
fact all the connective tissues of the body may be said to form
a continuous framework which supports the organs and is on the
whole of very similar structure tliroughout. In some parts
it is firmer than in others and in some places it contains fibres,
but it never presents a modification of a cartilaginous or osseous
nature and never, except at the ventral ends of the primary
pharyngeal bars, contains cells other than the epithelial cells
which bound the spaces contained within it. These spaces are
in some cases vascular and in others coelomic, but it is not possible
in every case to be certain as to which of these two organs they
belong. This absence of what we may call mesenchymatous
elements from the connective and supporting tissues is one of
the most remarkable peculiarities of Amphioxus.

As skeletal tissue we may rank the notochord, the supporting
tissue of the buccal ring and the axial tissue of the buccal cirri,
possibly also the axial tissue of the pharyngeal bars. The
notochord is made up of a number of discs placed verti-
cally, and transversely to the long axis, and consisting of
gelatinous tissue. It is surrounded by a tough sheath of
connective tissue, which is continuous with the rest of the
connective tissue framework of the body. Nuclei are present
on the dorsal and ventral sides in the neighbourhood of two
spacer, the so-caUed dorsal and ventral Ipnph canals of the
notochord.

The edges of the mouth contain a ring of skeletal tissue the
buccal ring, resembling the notochord in structure. It consists
of about twelve pi'eces on each side, and each piece gives attach-
ment to a rod of the same substance, which occupies the axis
and forms the support of one of the oral cirri.

The tissue of this buccal skeleton consists of a number of gelatinous
discs surrounded by a fibrous sheath. By some observers it is claimed
as cartilage, each disc being a cell and the smrounding membrane the

16 PHYLUM CEPHALOCHOEDA.

cartilaginous matrix. The tissue of the axial rods of the pharyngeal bars is
sometimes described as skeletal. It consists of a clear chitin-like substance
devoid of cellular structures except in the ventral bifurcated parts of the
primary bars, which contain branched cells. Excluding the nuclei of the
notochord, which is an endodcrmal structure, this is the only instance of
a tissue containing mesenchymatous elements.

The nerve-cord is surrounded by a tough sheath, which is
continuous with the sheath of the notochord.

The fin-rays are found in the greater part of the dorsal
fin in a single series, and in the anal fin in a double series (Figs.
4 and 5). They are absent from the cephalic fin, and from the
anterior and posterior portions of the dorsal fin, and from the
ventral part of the caudal fin. They consist of small cubical
pieces of a tough fibrous connective tissue, which in the dorsal
fin are continuous with the fibrous investment of the nerve-
cord. They are more numerous than the metameres, there
being four or five to each muscle segment.

The fins contain a longitudinal canal lined by epithelium and divided
by septa into compartments. In the region of the fin-rays each of these
compartments contains a fin-ray which pushes in its ventral wall and
projects into it. The nature of these fin spaces is not known, but it is
stated by Hatschek and others that they are, in the dorsal fin at least,
coelomic in origin, being derived from the coelom of the muscle-plate (see
below).

The muscular system consists of striated and unstriated
muscular tissue. The striated muscles are composed of fibrillated
rhombic plates, and are devoid of sarcolemma. They con-
stitute the lateral muscles, the transverse muscles, the muscles
of the lips and cirri, and the sphincter muscle of the velum and
anus.

The great lateral muscle of the body is divided up into a
number of successive segments, the myotomes (myomeres)
by septa of connective tissue. These septa have a peculiar
V-shaped course, and their insertion into the skin causes the
V-shaped external markings already referred to (Fig. 4). The
myotomes of opposite sides alternate with one another, i.e.
the intermuscular septa of one side are opposite the middle
of the myotomes of the other side. The full number of
myotomes is laid down in the embryo. In Amphioxus lanceolalus
there are about sixty -two on each side. The plate-like fibres
of which these muscles are composed extend the whole length

NEEVE CORD.

17

of the myotome from septum to septum. The other striated
muscles are very similar to the lateral muscle in structure, but
the cross-striation is less marked, and they are not segmentally
arranged, nor derived from the myotomes of the embryo. The
transverse muscles extend from the ventral end of the lateral
muscles to the middle Ime of the floor of the ventral groove,
where they are inserted into a median connective tissue septum
(Fig. 10). The unstriped muscle confers contractility on the
walls of the intestine and larger blood-vessels. It is exceedingly
inconspicuous and thin. It is doubtful indeed if it really exists
as a distinct tissue.

The nervous system. The
nerve-cord (cerebro - spinal
cord) contains in its ventral
portion a small circular cen-
tral canal, which extends as a
narrow fissure to the dorsal
summit of the cord (Fig. 6).
This canal is lined by colum-
nar epithelial cells, some of
which are continued into the
substance of the cord as sup-
porting fibres, whQe others
may have the form of nerve
ceUs. The cells linmg the
dorsal part of the canal are in
close contact, so that the
cavity here is virtual. The

nerve cells are for the most part placed m the central part of
the cord, and some of them are of giant size,* and extend
right across the fissure-like part of the central canal. On the
ventral side of the canal, at short (metameric) mtervals along
the whole length of the cord behmd the second myotome, are
small groups of black pigmented cells, f These structures are
probably sensitive to light. There is also a pigment spot,
commonly called the eye, and placed at the front end of the
cord in the anterior wall of the cerebral vesicle.

* For the arrangement of these giant nerve-cells and of the giant fibres
which issue from them, we refer the reader to Rohde, in Zool. Beitrage, 2,
1888, p. 1G9.

t Hesse, Z. f. w. Z., 63, 1898, p. 456.

Z. II. c

</'

Fig. 6. — Trausvcrse section of the spinal
cord of Amphioxus (after Rohde). gz
nerve cells ; stf supporting fibres ; ck cen-
tral canal.

18 PHYLUM CEPHALOCHORDA,

Anteriorly, though the cord itself tapers slightly, the central
canal widens out into a spacious vesicle, tlie cerebral vesicle.
This is the only representative of the brain of the Vertebrata.
It gives off from its front dorsal wall a small hollow diverticulum,
which ends blindly against the inner side of a small asymmetrical
pit of ectoderm called the olfactory pit. This pit is placed
on the left side of the body, is lined by ciliated cells, and is
supposed to be oKactory in function. In the larva the process
of the cerebral vesicle and the pit are in communication by a
pore, which appears to be the persistent neuropore of the
embiyo. The cerebral vesicle is also said to possess a ventral
diverticulum in the liinder part of its floor, w^hich has been com-
pared with the infundibulum of the vertebrate brain, but it is
doubtful if it is always present. Posteriorly the cord tapers
considerably and ends, usually in a small swelling, just short
of the hind end of the notochord. In some cases it is said to
extend as a filament rovnd the hind end of the notochord on
to the ventral surface.

The nerves issuing from this cord, except in the case of
the first two, are dorsal and ventral in their origin and
correspond with the dorsal and ventral roots of the spinal
nerves of the Vertebrata. In Amphioxus they do not join
and there are no ganglia on the dorsal roots. The dorsal
nerves arise from the dorsal part of the cord. The nerves
of the first two pairs (so-called cranial nerves) arise opposite
one another, altogether in front of the myotomes. Tliey are
entirely sensory, and supply the preoral part of the body. Their
finer branches possess nerve cells not far from their terminations.
The remainder of the dorsal nerves alternate in their origin
on the two sides, and contain motor as well as sensory fibres.
They pass out behind the myotome to which they belong, and
divide in the subcutaneous connective tissue into dorsal and
ventral branches. The ventral of these supplies nerves to the
transverse muscle of the sub-atrial floor. The dorsal nerves
also supply the muscles of the lips and velum. The sides of
the mouth (oral hood) and the cirri are supplied by the third
to seventh dorsal nerves, and the nerves to the inner side of
the lips of both sides are derived from the dorsal nerves which
arise on the left side of the cord (reminiscence of larval con-
dition, see below). The velar supply comes from dorsal nerves
four to seven.

ALIMENTARY CANAL.

19

ol..-

The ventral nerves (Fig. 7) are not united

in bundles nor surrounded by a sheath, but

issue as linear groups of fine fibres, which pass

immediately to the adjacent myotome. They

arise slightly anterior to the dorsal nerve of

the segment to which they belong, and are

exclusively motor for the muscles of the myo-
tomes.

Sense organs. Scattered amongst the ecto-
derm cells are cells bearing short hairs. They

are specially numerous at the front end of the

body and round the mouth. They may be

supposed to be tactile organs. Tliey are also

found in the mouth and on the velum.

Organs which are supposed to be visual and

olfactory have already been described.

The most striking peculiarities of the ner-
vous system of Amphioxus as compared with
that of the Vertehrata are the absence of an
anterior cerebral enlargement ; the peculiar form
of the central canal, the dorsal fissure-like por-
tion of which is probably represented by the fiq. 7.— Anterior en i
dorsal fissure of the vertebrate spinal cord ; ?4,„pAlox«r^s*j?owin"
the absence of a junction between the dorsal fljstlwrplt^s^^ni ^^2^
and ventral nerve roots and of a ganglion on anrp"Lse'o*p''potitro°il^
the dorsal roots ; and, lastly, the imperfect ^^^^^^'^^^ o?'ihe'brln-
condition of the organs of special sense. ^^^^.^ "arises^indepTn'^

The alimentary canal. The mouth is a ;n"-th/™preS-at^'n
large, somewhat circular opening, placed on in'l'^„^g'°made^ uftlr
the ventral side of the body, a little distance ti'y^i>^ole,,fnf%
from the front end. It leads into a spacious S'iail f /!°//T/F
cavity, the buccal cavity, which is bounded by ieKt'^onr'^miTi *o^f
a fold of the integument called the oral hood ro'tTohhe^saJI"*'''''
(Fig. 4). The free edge of the oral hood,
which may be called the lip, contains the skeletal framework
already described, and bears a number (from twenty to thirty,
increasing with age) of delicate ciliated processes, the oral
cirri. The right side of this oral hood is, as has already been
mentioned, continuous with the preoral ventral part of the
median fin, which is in accordance with what might be expected

:^^

s^//

^Z7

"TV

20

PHYLUM CEPHALOCHORDA.

from the development of the parts (see below). Tiie buccal
cavity is bounded behind, at the level of the anterior angle
of the seventh myotome, by a muscular membrane, the velum,
which separates it from the phaiynx and is perforated by an
aperture, which has been sometimes called the true mouth
and is the actua.1 opening which formed the mouth in the larva.
The edges of the velum around this opening bear twelve delicate
tentacles, the velar tentacles, which project backwards into
the pharjmx. The buccal cavity is lined by a ciliated epithelium,
and bears on its roof slightly to the right of the median line
a pit called Hatschek's pit, or simply the preoral pit (Figs. 8, 9).
This organ is lined by a columnar epitlielium, the cells of which
bear stiff sensory hairs. It opens in a groove of columnar
finely ciliated epithelium, which extends a short distance in
front of it in the roof of the mouth and behind bifurcates
into two ciliated grooves ; these pass obliquely backwards and
outwards to the velum, and then pass ventrally along the
junction of the velum and sides of the mxOuth. They terminate
either by simply coming to an end, or by running into one
another on the floor of the buccal cavity. These two diverging

J, ciliated furrows

{ give off access -

^:^i^S^^ ory furrows,

which are
formed by folds
of the antero-
lateral walls of
the main fur-
rows. The ac-
cessory ciliated
furrows pass for-
wards for a
short distance
on the roof and
sides of the
mouth. The
wliole structm-e
constitutes the
" Mheel - organ "
of J. Miiller. It

5—

7 8 9 '

FlO. 8. — View of the roof of thu uucciil cavity of Amphioxits
lanceolaUis from below (after v. Wijhe). 1 posterior wall
of buccal cavity ; 2 ciliated groove of left side ; 3 left side
of preoral hood ; 5 notochord ; 6 second myotome ;
7 Hatschek's (preoral) pit ; 8 ciliated pit ; 9 right si( e of
preoral hood ; 10 third myotome ; 11 ciliated groove of
right side ; 12 accessory ciliated groove ; 13 sphincter
muscle of velum ; 14 velar tentacles.

PHAEYNX.

21

is an organ for creating currents in the mouth back to the
pharjTix. The preoral pit which opens into its front part
has been variously interpreted as a sensory organ and a gland.

If the preoral (Hatschek's) pit be regarded as a gland, it has been sug-
gested by V. Wijhe that it is comparable to the neural gland of the Tunicata,
and that the ciliated groove is comparable to the opening of that gland,
the dorsal tubercle, the edges of which are frequently drawn out in a manner

-20

-19.

-17

Fig. 9. — Transverse section through the middle of the buccal cavity of A mphtaxtislanceolatus
to show the preoral (Hatschek's) pitaudtlie ciliated furrow (after v. Wijhe). 1 Hatschek's
pit ; 2 ri;?ht side of preoral hood ; 3 outer Up-cavity ; 4 inner lip cavity (3 and 4 are parts
of the left collar somite of the embryo) ; 5 labial nerve ; 6 coelom (dorsal buccal, a portion
of the collar somite of the embryo) ; 7 contorted blood vessel (continuation of right aorta) ;
9 aorta ; 10 Hatschek's nephridium (a process from the pharynx) ; 11 buccal cavity ; 12
internal labial muscle ; 13 skeleton of cirrus ; 14 cavity (lymph space) of cirrus ; 15 exter-
nal labial muscle ; 17 left side of preoral hood ; 18 second, 19 third, 20 fourth myotome.

very similar to the course of the ciliated groove of Am-phioxus. Against
this interpretation we must set the fact that the preoral pit, whatever
its origin (see below), has no relation to the central nervous system.
Moreover the interpretation of it as a gland is a very doubtful one. Van
Wijhe describes a special part of the ciliated furrow just behind and in
close connection with Hatschek's pit as the ciliated 'pit (Fig, 8, 8).

The pharynx is a large chamber tapering slightly posteriorly
on account of the dorsal inclination of its ventral wall. Its side

22 PHYLUM CEPHALOCHORDA.

walls are perforated throughout their whole dorso- ventral
extent (except for a short distance behind the velum) by a
number of vertically directed slits, which have a slight inclination
ventralwards and backwards (more marked in the preserved
than in the living animal), and which open into the atrial cavity.
The dorsal and ventral parts of the pharyngeal wall are not
perforated and constitute the hyperpharyngeal groove and the
endostyle (hypopharyngeal groove) respectively. These termi-
nate independently of one another posteriorly, but anteriorly
they are connected by the peripharyngeal ciliated bands which
arch round the pharynx immediately in front of the gill-slits.
In front of the peripharyngeal bands there is a small portion
of the pharnyx adjacent to the velum without gill-slits. ^

The pharynx is lined by a cUiated epithelium, which is con-
tinuous through the slits with the ectodermal epithelium lining
the atrial cavity. Along the endostyle there are four bands of
specially glandular cells, which, like the remaining pharyngeal
cells, bear cUia, and secrete the mucus, which passing forwards
along the endostyle is driven upwards by the peripharyngeal
bands into the front end of the hyperpharyngeal groove. Along
this it is carried by ciliary action into the stomach. The food,
consisting of small floating organic bodies brought into the
pharynx by the ciliary currents, is entangled in this mucus and
so separated from the water which passes through the gill-slits
into the atrial cavity and out by the atrial pore.

New gill-slits continue to be formed long after the development
has ceased, during the growth of the animal. They are conse-
quently more numerous in large than in small specimens. In
large specimens there may be as many as 180 secondary (see
below) or 90 primary gill-slits on each side.

The new slits are formed at the hind end of the pharynx close
to the jmiction with the stomach, as small circular perforations
(see Fig. 24 in the account of the development). These soon
become partly divided into two by the growth ventralwards of a
process from the dorsal wall of the aperture (Fig. 25). This
downward projection, from its resemblance to the tongue of a
Jew's harp, is called the tongue bar, ^ It eventually fuses with the
ventral wall of the slit, so that the primary slit becomes completely
divided into two secondary slits separated by the tongue bar.
In correspondence with this we may call the parts of the pharyn-

GILL-SLITS. 23

geal wall intervening between two primary slits the 'primary

^i„r i^'"^'^°l* transverse section through Amphioxus in the hinder part of the

pharyngeal region showing the brown canals (after Lankester, from Perrier). The division
^Ln*"?'"";- ^-eptral portion of the myotomes into two groups of fibres separated by a
connective tascia is shown ; a atrium ; arf root of dorsal aorta ; 6 primary bar • 6' secondlrv
bar; c hver (caecum); ca lymph space; cd notochord ; c« tube of' atriocoe°omic
funnel ; en connective tissue framework of the body ; c p somatic wall coveZa
caecum; et skeletal plate of endostyle ; e ectoderm; / branchial slit VhS
pharyngeal groove ; gv endostyle ; H ventral canal of notochord ; h dok^l ca^^Tot
the notochord ; m transverse muscle of atrial floor ; M mvotomes ; T^rmetapleure
iV spinal cord ; «a ventra. np dorsal root of a spinal nerve ; o gonadTp poS of a
fold in dorsal wall of atrium ; ph pharynx ; r dorsal fin ray ; s so-called lymph-spaces
In the case of the hepatic caecum, s points to the blood-vessels ^ ^

24 PHYLUM CEPHALOCHORDA.

bar. The primary gill-slits of opposite sides alternate with one
another, as do the myotomes. In the adult they are more
numerous than the n\yotomes, though when they first make their
appearance, they corresj)ond with them. The anterior primary
slit on each side is not divided by a tongue bar.

On account of the obliquity of their direction a great many,
both of primary and tongue bars, are cut in transverse section
(Fig. 10). The primary bars differ from the tongue bars in
structure (Fig. \\, A, B). In both there is an axial rod of a
clear chitin-like substance (msoluble in potash), placed nearer
the atrial than the pharyngeal side of the bar. In the primary-
bar this rod is double and without a cavity, while in the tongue
bar it is single and has a cavity. These rods are continued inwards
towards the pharynx as a thin membrane called the septal
membrane (Fig. 11). The character of epithelium covering
the bars may be gathered by an inspection of Fig. 11. On the
outer side the ectoderm of the atrium is found. It is said to be
non-ciliated and to be separated from the more extensive pharyn-
geal endoderm by some pigmented cells. The cilia of the endo-
derm vary in character on different portions of the bar, recalling
the condition found in the Lamellibranch gill. The primary
bar contains between the ectoderm and the skeletal rod a chamber
which is a portion of the coelom, being continued dorsally into
the dorso-pharyngeal coelom, and ventrally into the endostylar
coelom. In addition tlu-ee blood-vessels can be seen in the
primary bar in the positions and with the names indicated in the
figure. In the tongue-bar the visceral (10) and somatic (7)
vessels are present, and in addition there is a space in the
skeletal rod (3). This is interpreted by Lankester and Benham
as coelomic, by others as vascular. According to Benham this
space contains a blood-vessel (omitted in the figure) corresponding
to the external blood-vessel of the primary bar (4). Successive
primary bars are connected by transverse bars (synapticula),
which thus pass across the primary slits, internally to the tongue
bars, with which, however, they are comiected (Fig. 12, s). The
primary bars may thus be distinguished from the tongue bars
in transverse section ; and they may be distinguished by in-
spection of the pharynx as a whole for the skeletal rod of the
primary bars bifurcates ventrally on reaching the level of the en-
dostyle, whereas that of the tongue bars does not bifurcate. Dor-

INTESTINE.

25

sally the skeletal rods of both bars bifurcate and arch over the
clefts to join the branches of the adjacent bars. Below the
endostyle are some flat plates of skeletal tissue, which partially
overlap one another. They correspond in number with the
primary slits at the lower end of which they are placed.

Fig. 11.— Transverse sections at right angles to the length of the pharyngeal bars of Amphioxus
lanceolaius, A of a tongue bar, B of a primary bar (after Benham slightly altered). The
relative sizes of the two bars is maintained. 1 endoderm epithelium on the pharyngeal end
of a bar ; 2 pigment cells ; 3 coelom of the primary bar in B, coelom or external blood-
vessel contained in the skeletal rod of the tongue bar in A (coelom according to Lankester
and Benham) ; 4 external blood-vessel of the primary bar ; 5 atrial epithelium ; 6 skeletal
rod ; 7 somatic blood-vessel ; 8, 9 septal membrane ; 10 visceral blood-vessel.

The pharynx opens posteriorly into the mtestine which runs
straight back to open by the anus placed a little to the left
of the median line at the level of the septum between the 51st
and 52nd myotome. The anus is provided with a sphuicter of
striated muscular fibres. The intestine is lined by a columnar
ciliated epithelium. The anterior part of the uitestine is slightly
dilated and called the stomach. It gives ofif a forwardly directed

26 PHYLUM CEPHALOCHORDA.

diverticulum, the liver, which pushes before itself the ventral
body wall, the whole projection lying in the atrial cavity along-
side the pharynx on its right side (Fig. 10). This process is
attached to the dorsal wall of the atrium in front. The linmg
cells of the hepatic caecum are coloured green in the living
animal, as are the cells of the stomach from which it is given off.
Outside the intestinal epithelium is a thin connective tissue
layer which may contain unstriated muscular fibres.
f The atrial cavity is a space lined by ectoderm and surround-
ing the pharynx and anterior part of the intestine ventrally and
laterally. It opens to the exterior at the hind end of the ventvAl
groove at the level of the 36th myotome. It extends back on
the right side behind the atrial pore almost as far as the anus.
Its lining cells are in part ciliated and pigmented with a brown
pigment. The dorsal wall of the atrium is folded, in consequence
of the fact that it is reflected on to each of the primary
bars at a point considerably ventral to that at which
it joins the secondary bars (Fig. 12, U). This gives rise
to somewhat puzzlmg features in transverse sections in
which the dorsal regions of the primary bars appear to
be connected to the side walls of the atrium by strands
of tissue traversmg the atrium (Fig. 10). The ligamentum
denticulatum of J. Miiller is this folded roof of the atrium
cut longitudinally. It follows from this arrangement that
the atrium is prolonged further dorsally in the region of the
tongue bars than it is over the primary bars.

The hrown canals are two tubes lined by a pigmented epithe-
lium, and projecting into the dorso-pharyngeal coelom (Fig. 10).
They lie parallel to the long axis of the body and probably
end blindly in front at the 27th myotome. Posteriorly at the
level of the junction of the pharynx and intestuie they open by
funnel-shaped apertures into the dorsal part of the atrial cavity,
one on each side. They are to be regarded as forwardly directed
diverticula of the atrium, of unknown function.

These structures are often called the atrio-coelomic funnels. They
were discovered by Lankester. It is doubtful whether they end blindly in
front or open into the dorso-pharyngeal coelom.

There is a well-developed coelom with which the gonads are
in relation. A complete comprehension of the coelom cannot

EXCRETORY ORGANS. 27

be obtained until the development is studied, but the main
features in its tojoographical arrangement seem fairly clear, and
may be described at this point. In the adult there are many
spaces in the tissues the exact nature and origin of which is not
understood. Such will be referred to by the general term lymph
spaces, f It has been stated by some observers (Scluieider,
Lankester and others) that the coelom and vascular system are
in certain parts of the body continuous. But having regard to
the doubtful character of many of the body-spaces above i-eferred
to and to the difficulties to investigation presented by the
vascular system, this statement cannot be accepted without
further evidence.

In the region of the intestine there is a perivisceral cavity which
is coelomic. It entirely surrounds the intestine except dorsally,
where it is interrupted by the mesentery. In the region of the
pharymx the same cavity is found, but it is broken up by the
gill-slits into a number of parts all continuous with each other.
There are two dorsolateral chambers, one on each side of the
hyperpharyngeal groove. These extend a little way outward
in the lateral walls of the atrium, and dip down into the folds
of its roof along the primary bars. They constitute the dorso-
pharyngeal coelom, and are continuous through the above-
mentioned folds with the coelom present on the outer side of
each primary bar. Ventrally the coelom of the primary bars
opens into a median ventral chamber below the endostyle,
called the endostylar coelom.

The arrangement of the coelom about the mouth, which has been
described by v. Wijhe, is too complicated for description in this work :
it is, however, referred to in the section on development.

Excretory organs. In the dorsal wall of the atrium lying between the
atrial epithelium and the epithelium of the dorso-pharyngeal coelom are
a number of tubes with a ciliated lining, which have been supposed to be
renal in function (Figs. 12, 13). They correspond in number and position with
the primary gill-slits and do not extend behind the region of the pharynx.
They open into the atrium opposite the dorsal end of a tongue bar and at
the summit of one of the dorsal pouches of the atrium found at that point
(p. 26). They divide into two canals close to the opening ; one of these
passes forward and then turns round to travel for a short distance ventral-
wards ; the other passes backwards. They possess a variable number of
branches (from 1-5), the number being least at the two ends of the series
and greatest in the middle of it. Curious fibres ending in small knobs,
each of which contains a nucleus, pass off from the ends of these branches
and project into the coelom. These fibres are fine tubes ending blindly
internally and opening into the secretory tube. They contain a long

28

PHYLUM CEPHALOCHORDA.

vibratile flagellum, arising from the protoplasm around the nucleus at
the internal knobbed end and extending along their whole length so as to
project into the excretory tube (Fig. 13). They have been termed soleno-
cytes from their resemblance to the fibres found on the excretory tubes
of some invertebrates (e.g. polychaetous annelids). The tubes them-
selves are lined by a ciliated epithelium and a tuft of specially long
cilia projects through the renal opening into the atrium. They receive
a special vascular supply from the dorsal ends of the pharyngeal vessels,
the blood being returned into the adjacent aorta. That these organs are
excretory is inferred from their structure, which, as stated, closely resembles
that of the excretory organs of some polychaetous annelids, etc., among
the invertebrates, and on account of Weiss' experiments. He fed the

ms

■m

VlQ. 12. — Dorsal portion of the left pharyngeal wall of AmpMoxus, showing three renal
canals, on one of which the solenocytes are siiown ; seen from the side, diagrammatic (from
Korsclielt and Heider after Boveri). Id optical section of the folded ventral wall of the
dorso-pharyngeal coelom (ligamentum denticulatum) ; m myotome ; ms intermuscular
septum ; nc termination of the branches of the renal tube with the solenocyte? removed ;
nk renal canal ; np opening of the renad canal into the atrium ; s synapticulum ; I primary
gill-bar ; II tongue bar.

animal with finely-divided carmine, and then found that the cells of the
main tubes contained carmine particles. But he also found carmine in
the lining cells of the atrium including those of Miiller's papillae. It is
possible, of course, that Weiss' interpretation of these facts is correct,
and that the carmine found in these cells was in the act of being excreted
from the system into which it had been taken by the intestinal epithelium,
but on the other hand it may well be that the carmine entered the cells
concerned from the atrial cavity directly, in the same way that, according
to Weiss' view it must have entered the intestinal walls.*

* Vide Weiss, Q.J. M.S., 31, 1890, p. 497 ; and Boveri, Zool. Jahrb., 5,
1892, p. 429. Goodrich, Q.J.M.S., 45, 1902, p. 493.

VASCULAR SYSTEM.

29

The vascular system. There is no heart, but the larger
vessels are peristaltically contractile. Lying on the ventral
side of the endostyle in the pharyngeal wall there is a
longitudinal sub -pharyngeal vessel, which corresponds to
the heart and ventral aorta of the Vertebrata. x\nteriorly it
terminates by branching to the lips. Laterally all along
its course it gives off branches,
which have on their bases small
contractile swellings called bul-
bils, to the primary bars. These
ascend dorsalwards and open
into the aortic root of their own
side. The secondary bars receive
their blood supply from the
primary through the transverse
bars. The roots of the aorta lie
on each side of the hjrperpharyn-
geal groove (Fig. 10). In the
intestinal region they unite to
form the single dorsal aorta,
which gives off branches to the
intestine and lateral body walls.
Both the aortic roots are con-
tinued forward as the carotid
arteries. These are joined to-
gether by a transverse anasto-
mosis, and the right vessel gives
off a large much convoluted
branch which passes ventral -
wards at the level of the velum
and ends blindly (Fig. 9). The
anterior part of the right carotid
has the form of a plexus, and
gives off branches to the oral cirri of both sides.

The intestinal blood is collected into a sub-intestinal vein lying
in the ventral wall of the intestine. This vein is not a simple
vessel, but consists of a plexus of vessels, frequently communi-
cating and lying side by side. Anteriorly the sub-intestinal vein
appears to break up into a capillary system on the wall of the
hepatic caecum. The blood of the caecum is collected into the

Fig. 13. — Portion of the excretory canal
of a young Amphioxus with its soleno-
cytes, from tlie living animal (after
Goodrich). 1 solenocyte ; 2 tube of
solenocyte ; 3 excretory canal.; 4
flagellum of solenocyte.

30 PHYLUM CEPHALOCHORDA.

vein, the hepatic vein, which hke the sub-intestinal vein consists
of a plexus of vessels (Fig. 10). These commence in front and
behind unite to form a single vein which is continued into the
hind end of the sub-pharyngeal vessel. The hepatic vessels are
said to communicate at the front end of the caecum with the
dorsopharyngeal coelom, but this statement must be accepted
with caution. There is undoubtedly a connection between the
anterior end of the caecum and the lateral wall of the atrium.
The blood is colourless ; it contains amoeboid cells, and accord-
ing to some observers a few red oval corpuscles.

In the larva the hind end of the subintestinal vein is continued directly
into the siib-pharyngeal vessel. The direction of the flow of blood is
forward in the subintestinal vein and subpharyngeal vessels, both of which
are, according to J. Miiller, contractile. It follows from this that
the flow must be dorsalwards in the pharyngeal bars, backwards in the
dorsal aorta and ventralwards in the peri-intestinal vessels. There is a
considerable vascular development in the lateral walls of the atrium and
a longitudinal vessel runs along the line of the gonads, but how these
and the body- wall vessels generally are related to the main trunks described
above is not known.

The other spaces of the body may be classed as lymph-spaces. They
are lined by an epithelium and contain a coagulable fluid. Their origin
and relations are not certainly known. Some of them, e.g. the lymph
canals in the fins and certain spaces within the myotomes are said to be
coelomic and derived from the mesoblastic somites of the embryo. Others
may possibly be purely vascular ; e.g. the large canal found in each meta-
pleure — the metapleural lymph canals.

Generative organs. The sexes are separate. There are no
external sexual differences. Generative ducts are absent,
and the generative organs, which are segmented in cor-
respondence with the myotomes and are placed in the
lateral wall of the atrium at the ventral ends of the myo-
tomes, dehisce their products into the atrial cavity by
rupture of their walls. From the atrium the generative
products pass to the exterior usually through the atrial
pore, but in some cases, according to the observations of
Kowalevsky and Hatschek on the living, and of Marshall on the
preserved animal, they occasionally pass from the atrium through
the gill-slits into the pharynx and are spawned by the mouth.
Spawning takes place at sundown only and fertilization is effected
either in the sea or in the atrium.

In a fully-developed Amphioxus lanceolatus the gonads are
Bomewliat cubical bodies, twenty-six in number on each side.

THE EGG. 31

The first of them appears to be placed at the ventral end of
myotome 10, and the last at the ventral end of myotome 35 just in
front of the atriopore. They are contained in coelomic sacs,
which in development are derived from the ventral ends of the
myotomes, and to the wall of which they are attached.

The phylum possesses only a single genus* A7)iphioxus Yarrell (i?ra/ic/t-
iostoma Costa). It is found in all seas. About ten species are known.
They differ in the number of myotomes, the presence or absence of a
caudal expansion of the median fin, the presence of gonadial sacs on one
or on both sides of the body, the continuity of the right metapleur witli
the anal fin or the cessation of both right and left metapleur behind the
atrial pore ; the presence or absence of fin-rays and fin-ray spaces in the anal
fin. There appears to be a considerable range of meristic variation in some
of the species (Punnett). ^4. lanceolntus Pallas, Europe and most seas ;
A. bassanum Giinther, right metapleur continuous with anal fin, gonads
on right side only, anal fin with fin-rays and fin-ray spaces, Bass Straits ;
^4. cultdlum Peters, like the last, anal lin with chambers but without rays,
Torres Straits ; A. lucayanum Andrews, like the last, but without caudal
fin, hind end of body being a urostyle-like process without myotomes, and
fin-rays and chambers absent from anal fin, Bahamas.

It has been suggested that one or two species are pelagic, but this la
uncertain.

Development. The development of Amphioxus presents some
remarkable features, and contrary to our usual custom we have
decided to give a full account of it in this work. Tliough
strangely similar in many of its features to the type of develop-
ment found in the Vertebrata, it presents some very marked
features of difference. Of these we may at once mention the
small size of the ovum, the archenteric origin of the coelom, the
absence of any nephridial apparatus comparable to that of
the Vertebrata, the origin of the gonads from the myocoel, and
the extraordinary asymmetry of the larva.

Our account is based upon the important researches of
Hatschek and Willy, who, as is well known, worked at the
species found in the Pantano at Faro in Sicily.

The egg is small ('1 mm. in diameter), is surrounded by a vitel-
line membrane and contains but a small quantity of yolk, which
is uniformly distributed. Only one polar body is attached to the
ovum after deposition ; it is probable that this is the second,

* By some zoologists the phylum has been broken up into genera and
sub-genera (Kirkaldy, Q.J. M.S., 37, 1895, p. 303), but this, considering
the small nimaber of species and points of difference, seems hardly necessary
at present.

32

PHYLUM CEPHALOCHORDA.

the first having been formed in the ovary and rubbed ofiE during
the dehiscence. The segmentation is complete and almost equal,
the segments of the lower pole being slightly larger than those
of the upper. It leads to the formation of a hollow blasto-

G

H

KiG. 14. — Cleavage of AmpMorns (after Salensky from Korsdielt anrl Heider). A egg before
cleavage, with polar body ; B two-cell stage ; C four-cell stage ; D the same seen from the
upper surface ; iS eight-cell stage ; F sixteen-cell stage ; 6' stage showing more rapid division
at the animal pole ; H the same in section ; / surface view of lilastosphere.

sphere (Fig. 14). This becomes invaginated to form a cup-
shaped gastrula (Fig. 15). The blastopore, at first wide, soon
narrows to a small opening placed at the hind end of the future
dorsal surface. The embryo now elongates in the direction of
the antero-posterior axis (Fig.16); and the ectoderm of the dorsal

MEDULLARY CANAL.

83

surface becomes more columnar to
form the medullary plate (Fig. 17).
As the medullaiy plate extends to the
hind end of the dorsal surface, the
blastopore is included in it. The
lateral part of the ectoderm now be-
comes detached from the medullary
plate, and grows over it (Fig. 17).
This overgrowth begins at the hind
end of the medullary plate, so that
tlie blastopore is covered over and
comes to open into the space between
the overgrown ectoderm and the
medullary plate (Fig. 18). Later the
medullary plate curves over dorsally
(Fig. 17), and by the junction of its
two lateral edges forms the walls of the
medullary canal (Fig. 26). From what
has been said it is clear that tlie
medullary canal, which is gradually
developed from ])ehind forwards, opens
posteriorly into the archenteron by
the blastopore and anteriorly to the
exterior by the neuropore. As the
medullaiy canal becomes the central
canal of the nervous system, the
blastopore is henceforward known as
the neur enteric canal. It closes soon
after the commencement of larval life.
The anterior neuropore persists

Via

15. — Fonnatiou of the
<iastriila of Amphioxus (from
Claus, after Hatschek). A blas-
tosphere ; B commencing in-
vagination of tlie lower surface
of the blastosphere to form
the endoderm ; C later gas-
trula ; all in optical section ;
O anterior lip of blastopore.
The cilia of the ectoderm are
omitted.

throughout larval life and only
closes on the attamment of the
adult state. It marks the site of
the olfactory pit.

Meanwhile two pairs of dorso-
lateral outgrowths of the archen-
teron are formed (Figs. 17, 18).
The anterior of these ]'etain their
^'witi^nTr^rot^SlLtolor^Zdtt! Communication with the archen-
fr^rnVSchetrn^Serf '"' teron for some time and give rise

Z — II D

34

PHYLUM CEPHALOCHORDA.

to the somites of the first pair. They are the " collar " somites
of MacBride (Fig. 19, 6). The posterior outgrowths constitute
the coelomic grooves of MacBride. They remain open to the
gut for a considerable time behind, but as growth progresses
they are continually constricting off sacs anteriorly (Fig. 19, 5).
Eventually when about fourteen pairs of somites have been
formed they become separate from the endoderm of the arch-
enteron and form a solid plate on each side from which the
remainder of the somites are successively developed. In add'tion
to these two pairs of archenteric outgrowths there is a median
anterior outgrowth (Fig. 19, 7). This grows back on each side

Fio. 17.— Triinpverse section through tw. tmhrycs of Anrhicxvs to show the enclosure of
tlie metlullarv plate and the formation of the coelomic pouches. .1 section through an em-
hrvo with the rudiment of one pouch and of the notochord. B section through a slightly
oilier embryo, showing the complete sn^ratioo of a coelomic pouch from the archenternn
(from Korschelt and Heider after Hatschek) : ak ectoderm ; ch rudiment of notochord ;
hb lateral ectoderm growing over the medullary plate ; tk-, il endoderm ; Ih enteron ;
Ik future coelom ; tnk coelomic pouch (future somite) ; tnp medullary plate.

and becomes separated from the gut as a single cavity with two
backwardly projecting horns. This unpaired anterior sac gives
rise to the head cavities ; it becomes divided into two, of whicli
the right shifts ventrally, becomes thin-walled, and forms the
cavity of the snout in the larva (Fig. 20). It becomes largely
obliterated in the adult. The left head-cavity, on the other
hand, becomes transversely placed beneath the notochord and
opens to the exterior on the left side in front of the mouth
(Fig. 20, w). It becomes the preoral (Hatschek's) pit, and gives
rise by the extension of its lips to the wheel organ (p. 20). The
collar-somites (so-called first pair) and the somites developed
from the coelomic groove give rise to the mesoderm, body-cavity

HATCHING.

85

— TTrr

^

Fia. 18. — Longitudinal-vertical
section through an embryo of
Amphioxus with the "rudi-
ments of two somites (after
Hatschek). mp pole-cells, tlie
existence of which is now
denied ; mr uncovered part of
medullary plate ; mr^ space
between the medullary plate
plate and overgrown ectoderm
(future medullary canal );
«»', MS' mesoblastic somites.

and generative organs in a manner de-
scribed below. The collar-somites send
forward a process dorsal to the head
cavities on each side, the walls of
which give rise to the mesodermal
structures of the preoral region.

Wliile these changes have been tak-
ing place the notochord is developed.
It arises as a groove of the dorso-
median endoderm (Fig. 17 B), which
is constricted off so as to form a solid
rod of cells lying between the dorsal
endoderm and the medullary plate. It
develops from before backwards, ex-
cept the front portion, which separates
from the endoderm later than the
rest and extends to the anterior end
of the body.

The embryo becomes hatched and
begins to swim freely in the sea by

means of the cilia of the ectoderm cells at about the stage of
Fig. 18, about twelve hours after fertilization. But it remains
opaque and, being incapable of taking in food, is generally spoken
of as an embryo untU about the thirty-sixth hour, when the yolk
is sufficiently absorbed to leave the tissues transparent, and the
mouth, anus, and first gill-sht are formed (Fig. 20).

The mouth is
formed on the
left side ; the
first gill-slit on
the ventral
middle line, soon
moving on to
the right side,
and the anus at
the hind end of

FiQ. 19.— Diagrammatic longitudinal section through an embryo ^^^^ body slightly
of Amphioxus, to show the formation of the coelomic sacs, i ai, i rx e i-\
The figure combines features which in reality would only be ^^ tne leiu 01 tllC
shown by two sections. 1 neuropore ; 2 neural canal ; 3 neuren- • i ji i •
teric canal ; 4 coelomic groove ; 6 somite constricted off from midClie line.
anterior end of coelomic groove ; 6 cavity of first mesoblastic rp i i i

somite (so-called collar cavity) ; 7 head cavity ; 8 archenteron. -Lnc larval

36 PHYLUM CEPHALOCHORDA.

stage lasts for about three months, during which the larva
swims freely usually at a considerable depth by tne contrac-
tion of its body, and acquires the adult form and habits. The
principal changes which take place relate to the formation of
the gill-slits, of the preoral hood, and symmetrical adult mouth,
and of the atrial cavity.

The mouth acquires a gigantic size and forms a most con-
spicuous object on the left side of the body (Figs. 21, 22). The
gill-slits are formed successively on the ventral middle line to the
number of fourteen.* All of these except the last few shift
soon after their formation on to the right side. They corre-
spond in number to the myotomes in the part of the body

Fig. 20. — AmpJiwxus larva of about thirty-six hours from the left side, when the preoral pit-
mouth, first gill-slit and anus are established (from Korschelt and Haider after Hatschek);
c larval caudal fin ; ch notochord ; en neurenteric canal ; (I alimentary canal ; k right preoral
sac ; k club-shaped gland, which has acquired an opening to the exterior on the left side
ventral to the mouth ; ks first gill-slit ; m mouth ; mr nerve tube ; np neuropore ; sv sub-
intestinal vein ; w preoral pit (left head cavity^

in which they occur, but this relation is ultimately lost,
wlien they become more closely packed and the myotomes
increase in size. Of these fourteen first-formed gill-
slits the first and the last five close up, so that eight only are
left.

The atrial folds arise as ridges of the skin. Posteriorly they
lie close together in the middle line enclosing between them a
small groove (Fig. 23). Anteriorly they pass on to the right
side, one on either side of the gill-clefts. By the union of these
ridges, which begins in their posterior region and gradually
extends forwards, the groove becomes converted into a canal,

* These first formed gill-slits are often called the primary gill-slits, in
contradistinction to the later formed secondary slits which, when the
animal becomes symmetrical, are placed on the right side, the primary
slits having passed over to the left side. Forster Cooper (op. cit.) describes
a larva taken in the open ocean in the Maldive Archipelago containing as
many as thirty-one primary slits.

\ ' '-§

a:

m-

\m

\m

w,

v~fl*

M.\

&:..

^.

<u iT ,■-

a5§

(.--'X*

J= cS

■3 -a

o

n;^

•ftj

-^1^

rn^

X

*v

(Ti^

&'

§

\^

*^1'

Hfes

^'e-a

g^-

•>v.

a

37

38 PHYLUM CEPHALOCHORDA.

the atrial cavity (Fig. 23). At their hind end they remain
separate, thus giving rise to the atriopore. At first the atrial
cavity is a small canal restricted to the ventral side of the
pharynx. Later it becomes larger and acquires the adult
relations.

Meanwhile a row of eight or nine gill-clefts appear on the right
side of the body dorsal to those first formed (Figs. 24, 25). Both
sets of gill-clefts acquire the U-shaped form, the tongue bar being
developed (except in the first cleft (Fig. 25) which remains
simple). The first-formed clefts then shift on to the left side of
the body, and become the gill-clefts of the left side. At the
same time the mouth shifts to the middle line, and the preoral

Fig. 23.— Ventral view of three larval stages of Amphioxus (after Lankester and Willoy,
from Korschelt and Heider) ; A, the atrium is still entirely open ; B, the atrium is partially
closed behind ; C, the atrium is alnjost completely closed ; ap atriopore ; * giil-slits ;
If left atrial foldj m mouth ;i rf right atrial fold ; to preoral pit.

hood which had made its first appearance while the mouth was
still on the left side becomes developed. The apertures of the
club-shaped gland (see below) and of the ciliated pit are both
enclosed by the preoral hood. The original mouth opening
shifts to the back of the oral cavity and persists as the aperture
in the velum.

The principal phases of the development are now accom-
plished and the larva, in the mam symmetrical, assumes the
sand-burrowing habits characteristic of the adult.

The mesoblastic somites, after then separation from the
archenteron, which after that event is termed simply enteron,
extend ventralwards on each side (Fig. 26), till they meet on the

SOMITES.

89

ventral side of the alimentary canal. The septum between them
(ventral mesentery) breaks down and the somites of the two
sides become contmuous.

i^:fc;si^ve--^-^^T|^ - >

1_-

dc is 1

'°- f*-~''-'"'o ''"■val stages of ilmp/itoTKi from the right side, showing the origin of the cill-
clefis of tlie rigtit side of the a.Iult (from Korschelt and Heider. after Wiiley) 1 2
9, 12, 14 first, etc., to fourteenth gUl-cleft of future left side • /, VH first and seventh
of the later formed set which eventually belong to the right side ; au eye-spot • ch noto-
chord ; dr club-shaped glani ; es rudiment of endostvle ; fh dorsal, fh' ventral fln spaces •
^- rudiments of the later formed gill-clefts ; m margin of mouth ; mf edge of right

metapleure :
preoral pit.

n nerve cord ; p atrial cavity ; «i sub-pharyngeal vessal ; v velum

Fig. 25.— Ventral view of Amphioxus larva, rather later than Fig. 24 (from Korschelt and
Heidpr, after Wiiley). 2 second, 12 vestige of twelfth cleft of the first-formed row, now
passed on to the left side ; /, F/// first and eighth of the later formed clefts of the right
side ; be buccal cirri ; ch notochord ; es endostvle ; m mouth (larval) ; v velum.

At the same time a septum is formed, dividing off the dorsal
part of the somites from the ventral. The dorsal part becomes

40

PHYLUM CEPHALOCHORDA.

the myotome ; it retains its original segmentation and the septa
separating the successive somites here persist as the myosepta
(intermuscular septa). In the ventral portion (comparable to
the lateral mesoblastic plate of the Vertebrata) the transverse
septa, formed by the adjacent walls of the successive somites of
the same side, break down and the cavities of the somite become
continuous to form the splanclmocoele or body-cavity of the
adult. The cells of the inner wall of the myotome become con-
verted into muscles, and consti-
tute the lateral muscles of the
adult, while the outer wall
which is applied to the ectoderm

Fio. 26. — Transverse section through
the middle of tlie body of an Amj'M-
oTiis embryo with 11 somites. On the
right side the section has managed to
cut two somites (from Korschelt and
Heider). aX- ectoderm ; c7i notochord ;
dh enteron ; tie endoderm : Ih coelom ;
mk' somatic, mk" splanchnic layer
of mesoderm : n nerve tube ; us meso-
blastic somite.

Fia. 27. — Transverse section of
the middle of the body of an
Amphioxus larva with five
gill-slits, diagrammatic (from
Korschelt and Heider). 1
outer wall of myotome (cutis
layer) ; 2 inner or muscular
wall of myotome ; 3 com-
mencement of the sclerotome ;
4 septum between myocoek
and splanclmocoele : 5 somatic
mesoderm ; 6 splanchnic
mesoderm ; I myocoele ; //
splanchnocoele. The sub-in-
teatinal vein is shown in the
splanchnic mesoderm.

remains thin (Fig. 27). The sclerotome is an outgrowth
from the ventral and imier wall of the myocoele (Fig. 27). It
acquires a considerable development extending dorsalwards
between the muscles and the notochord and spmal chord (Fig. 28).
Its inner wall gives rise to the sheath of the notochord and of the
nerve cord, while its outer wall forms the so-called fascia-layer,
or mternal sheath (Fig. 29).

The dorsal part of the myocoele is said to give rise to the
dorsal fin canal ; and a ventral extension of the same space to
the ventral fin canal. The myocoele appears to abort in the
adult, but the sclerocoele probably in part persists as the lymph
spaces on the internal sides of the lateral muscles.

GONADS.

41

In the region of the pharynx, the atrial cavity extends dorsal-
wards between the splanchnocoele and the ventral extension of
the myocoele (Fig. 29), and the splanclmocoele becomes cut up
by the gill-clefts into the sections of the coelom already described
as occurring in the primary pharyngeal bars (p. 24), The dorsal
and ventral regions of this part of the splanchnocoele furnish
the dorso-pharyngeal {sc) and endostylar coelom (ec) respect-
ively.

The first myotome is developed from
tlie somites of the anterior pair, the so-
called collar somites (p. 34), which retain
their communication with the enteron
longer than the others. This communi-
cation on the left side becomes elongated
and gives rise to Hatschek's nephridium
(Fig. 9, 10). These somites send back ven-
tral extensions which lie in the developing
atrial folds. It has been suggested that
these give rise to the metapleural lympli
canals, but this has been denied. The
origin of the metapleural lymph canals
is not certainly known.

Van Wijhe in his recent important
paper (op. cit.) states that the walls of
the collar cavity give rise to several
myotomes. It is not quite clear to us
whether this statement is based on em-
bryological study or not. He further
states that the second myotome of the
body is the anterior of these myotonies
which come from the collar-somite ; thus
implying that the walls of the head
cavities (preoral somite) give rise to a
myotome. So far as we know, the head
cavities do not give rise to myotomes.
The collar-coelom appears to give rise to

Fig. 28. — Transverse section through
a young Amphioxus immediately
after metamorphosis, between the
atriopore and the anus, diagram-
matic (from Korschelt and Heider,
after Hatschek). 1 outer wall of
myotome (cutis layer) ; 2 muscles ;
3 fascia layer (outer wall of sclero-
tome ; 4 skeletogenous (inner)
wall of sclerotome ; 5 u 6 ventral
continuation of skeletogenous
layer and somatic wall of splanch-
nocoele ; 7 splanchnic ditto ; /
myocoele ; /' dorsal /' ventral fln-
space ; // splanchnocoele.

the stomocoel and cavum epipterygium
of van Wijhe, who states that the cavum epipterygium communicates with
the metapleural canals. This confirms Mac Bride's statement that the
metapleural lymph canals are parts of the collar coelom. The cavum
epipterygium is also stated to communicate with the enlostylar coelom
through the coelom in the first branchial arch, which, being part of the
splanchnocoele, it might reasonably be expected to do. The arrangement
of the parts of the coelom about the mouth as explained by van Wijhe is
complicated.

The gonads are segmented in their origin. The generative
cells are first seen as the thickenings of the coelomic epithelium
at the ventral ends of the myotomes, on the anterior wall of the

42

PHYLUM CEPHALOCHORDA.

myocoele. They soon come to project into the cavity of the
somite in front, pushing the myosepta before them (Fig. 30).

£2^

-ft

FIG. 29. — Transverse section through the branchial region of AmpMoxus showing on the left
the condition of a secondary, and on the right that of a primary gill-bar, diagrammatic
(after Boveri and Hatschek, from Korschelt and Heider). oo aorta ; c cutis layer of
myocoele ; ec endostylar coelom ; / fascia layer ; fh dorsal fin canal ; g genital sac : gl renal
vessels ; jc vessel in primary bar ; kd pharynx ; Id ligamentum deuticulatum ; m muscle-
plate ; mt transverse muscle ; n renal canal ; of metapleural lymph space ; p atrial cavity ;
sc dorsopharyngeal coelom ; si sub-pharyngeal vessel ; sk skeletogenous layer of sclerotome ;
uf lymph canals of the atrial floor.

They lie therefore as small sacs in the preceding myocoele
attached to its hind wall by a pedicle (Fig-^ 30 C, 31). Later, the
part of the myotome in which they lie becomes separated from

ENDOSTYLE.

43

the rest and forms the perigonadial coelom which lies in the
outer wall of the atrium (Fig. 10).
The club-shaped gland and endostyle. The club-shaped gland

A B G

Fig. SD. — A, B, C. — Three side views of tfie vcutral end of a myotome of a young Amphioxus,
showing the development of thp genital rudiment and its projection into the cavity of
the preceding somite (from Korschelt and Heider, after Boveri).

is developed as a transverse groove on the floor of the pharynx
and continued on to the right and left walls. It becomes con-
stricted off from the pharynx, and acquires an opening to the
exterior on the left side of the body just in front of the mouth.
Later on the right end of it acquires an opening into the pharynx.
The club-shaped gland is of unknown function and eventually
atrophies. It has been suggested
without any obvious justification
that it is the metamorphosed an-
terior gUl of the right side.

The endostyle is a ciliated tract
of columnar epithelium just an-
terior to the club-shaped gland
on the right side (Fig. 24). It
subsequently becomes bent on
itself in a V-shaped manner
(Fig. 25), and grows backwards
between the two rows of giU-
slits. It is at first, therefore,' on
the right side of the body, but
when the larva becomes sym-
metrical, it takes up its position ^^rhe'g7nitarr"lILVnTra^"
in the ventral middle line. it7^.fSonS ^codo^tm

From the above account it is flSZ^i;'P/iottT^

clear that in the young larva ^0^^000"^"^' '"" '"'"

44 PHYLUM CEPHALOCHORDA.

the future ventral middle line of the phaiyngeal region
is on the right side of the body as shown by the first
trace of the subpharyngeal vessel, the endostyle and the gill-
slits ; whereas in the buccal region later median structures are
on the left side. It results from this that, at the so-called
metamorphosis when the larva becomes symmetrical, the buccal
region of the body and the pharyngeal regions must rotate, so
to speak, in opposite directions.

No satisfactory explanation of this extraordinary larval
asymmetry has even been suggested. Though largely rectified
in the adult a trace of it persists in the slightly asymmetrical
position of the oKactory pit, the anus, and the continuity of the
cephalic fin with the right side of the preoral hood, and in the
innervation of the preoral hood (p. 18).

CHAPTER III.

PHYLUM VERTEBRATA* (CRANIATA).

Chorda t a in which the dorsal nerve cord extends some distance
in front of the notochord, and is expanded at its anterior end into
a brain. The axial skeleton is divided into an unsegmented
cranial fmtion, which surrounds the brain, and a segmented
vertebral portion which forms the axis of the body and protects
the spinal cord.

The various animals included in this phylum were first put
together by Aristotle, who called them " animals with blood " ;
he also recognized the possession of a bony or cartilaginous
skeletal axis as a common characteristic. But it was Lamarck
who first adduced tlie presence of a vertebral column, as a most
important character, and introduced before Cuvier the name
of Vertebrata into the science. This term, however, is not
entirely appropriate, for in some Pisces the sheath of the noto-
chord is not segmented, and there are no vertebrae (Marsi-
pobranchii, Dipnoi, some Ganoidei). Nevertheless, the term
may fairly be retained, for not only has it the sanction of long
usage, but the cases in which the vertebral column is not jointed
are few in number and unimportant in character. As already
pointed out, the segmentation of the vertebral column is corre-

* Stann'ni'ifHandhuchder Anatomie der Wirbelthiere, 2nd ed., Berlin,
1854. Rathke, Beitrdge zur Bildunga und Entwickelungsgeschichte des
Menschen und der T/iiere, Leipzig, 1833. Owen, The Anatomy of Vertebrates,
3 vols., London, 1866-G8. 'Rn's.ley, A Mamuilof the Anatomy of Vertehrated
Animals, London, 1871. Gegenbaur, Vergleichenie Anatomie der Wirbelthiere,
Leipzig, 1898, 1901. Zittel, Handbuch der Palaeontologie, Munich, vols,
iii., iv., 1887-93 ; and Grundziige der Palaeontologie, Munich, 189.5.
(English translation, Macmillan and Co., IHOO). A. S. Woodward, Out-
lines of Vertebrate Palaeontology, Cambridge, 1898. Balfour, Comparative
Embryology, vol. ii., 1882. C. S. Minot, Human Embryology, New York,
1892.

46 PHYLUM VERTEBRATA (CRANIATA).

lated with its rigidity, and is therefore best developed in
those animals which have to support the body on land.

The integument consists of two distinct layers, the epidermis
externally and the cutis internally. The epidermis is composed
of many layers of cells, of which the upper and older layers are
worn off, while the lower layer {stratum malpighi) is actively
growing, and serves as a matrix for the continual renewal of
the upper layers, and sometimes contains pigment. The cutis
is principally formed of fibrous connective tissue, with which
muscular elements — striped and unstriped — come into relation
without however forming a dermo-muscular envelope, as in the
Annelids. Some of the appendages of the skin are epidermal
structures (hairs and feathers). Some are derived from ossi-
fications of dermal papillae, which sometimes may even give
rise to a hard and complete dermal armour (scales of fishes and
reptiles, carapace of armadillos and tortoises). The epidermis
is derived from the ectoderm of the embryo, the cutis or dermis
being mainly a mesodermal product.

The endo-skeletal tissue of the lower Vertebrata and of all
vertebrate embryos consists solely of cartilage {Marsipobranchii ,
Elasmohranchii), but in most groups osseous tissue, supple-
menting or, in the higher forms, largely replacing the cartilage,
is present in the adult.

The muscular tissue may be divided into two categories :
They are (1) the somatic or myotome muscles, which are derived
from the epithelial wall of the myotomes or dorsal segmented
parts of the mesoblast of the embryo, and (2) the mesenchyma-
tous * muscles, which are developed from the ventral part of
the mesoblast (wall of the splanclmocoel). The myotome
muscles are innervated exclusively by the ventral roots of the
spinal nerves, and by the third, fourth and sixth cranial nerves,
which are the only ventral nerve roots found in the brain. The
mesenchymatous (visceral) muscles, which appear to be derived
from mesenchymatous mesoderm, are iimervated by the ventral
roots in the trunk, but in the head by the fifth, seventh, ninth
and tenth cranial nerves, which are usually regarded as dorsal
nerve-roots, and which contain also afferent nerve fibres (see
account of nerves under Pisces). The somatic muscles are cross-

* Sometimes called visceral, but this is a bad name, as many of them
lie in the body-wall.

NERVOUS SYSTEM. 47

striped and voluntary. The mesenchymatous muscles for the
most part consist of unstriped fibres, but some of them are
cross-striped, and even voluntary. The muscles of the heart
and oesophagus are examples of cross-striped mesenchymatous
muscles in the trunk ; they are not under the control of the will.
In the head many of the mesenchymatous muscles are cross-
striped and voluntary ; e.g. the facial muscles, tlie mandibular
and the branchial muscles. The eye-muscles are myotome
muscles, and supplied, as stated above, by ventral roots ; but
they differ from the muscles of the great lateral sheet of myotome
trunk muscles in the fact that their fibres are directed dorso-
ventrally, and not longitudinally, as in the latter.

The dorsal nerve-cord extends in front of the notochord, and
is enlarged in front to form the brain, which is constructed on
the same fundamental plan in all classes. The posterior part
constitutes the spinal cord. The skeletal investment of the
brain is unsegmented, and constitutes the skull, while the spinal
cord lies in a tube of the vertebral column, which always shows
some sign of segmentation and is usually completely segmented.
The spinal nerves, which are segmentally arranged, possess
two roots, a dorsal and a ventral, which join. The dorsal of
these roots carries a ganglion and contains afferent nerve fibres ;
the ventral contains efferent fibres only. The brain possesses
ten pairs of nerves, which are very similarly arranged in all
Vertebrata. They differ from the spinal nerves in the fact\|
that except in the case of three of them, they have the dorsal
roots only. The third and sixth nerves may be regarded as
the ventral roots of the fifth and seventh nerves respectively,
and the fourth nerve must also be regarded as a ventral root,
though it arises from the dorsal surface of the brain. The ninth
and tenth nerves appear to be altogether without ventral roots.
In the higher Vertebrata there are two additional pairs of cranial
nerves, the eleventh and twelfth. Of the cranial nerves, the
fifth, seventh, eighth, ninth and tenth are usually regarded
as being serially homologous with the posterior roots of spinal
nerves, and are supposed to be related to a vanished segmenta-
tion of this part of the body.* They resemble these in having

* A short account of the modern views on the nature of cranial nerves
and of nerves in general, and of cranial segmentation is given in the
chapter on Pisces.

48 PHTLUJl VERTEBRAJA (CRAXIATaX

a ganglion, but. with the exception of the eighth, they differ
from them in containing a fair proportion of efferent nerve
fibres. The first and second cranial nerves, which supply the
special sense organs of smell and sight respectively, appear to
differ fimdamentally from the other cranial nerves.

In all Vertebrata there are three organs of special sense on
the head, the olfactory and visual, the innervation of which
has just been referred to. and the auditory, which is supphed
by the eighth cranial nerve. The connection of these oi^ans
with the head has profoundly modified the structure of both
skull and brain.

A special visceral nervous system, known as the sym-
pathetic, is nearly always present.

The alimentary canal presents very similar features through-
out the series. It consists of a stomodaeum. pharynx, oeso-
phagus, stomach, intestine. The stomodaeum contains the
teeth and the openings of the salivary glands if present, and
passes without any line of demarcation iuto the pharynx, which
in all Vertebrata is at some time of life connected with the ex-
terior by lateral apertures, the phort/ngeal apertures or visceral
chfls. These are never more (usually less) than eight in number
on each side.* In Fishes and Amphibia the first of these (spiracle)
is always smaller than the others and may be completely absent :
in the Amniota (Reptiha. Aves, and Mammalia) it is always
present, and not smaller than the others. In Fishes and
Amphibia the visceral clefts are used for respiratory purposes
as in other Chordata, but they are never put to this use in the
Amniota. where they appear to have no function at all. In
such cases the respiratory organ of the adult is the lung, which
is developed as a median outgrowth of the ventral waU of the
pharynx. There are two other nearly constant features
of the vertebrate alimentary canal, viz. (1) the connection of
two large glands, the liver and pancreas, with the anterior part
of the intestine, and (2) the connection of the generative and
renal organs with the hind end of the intestine, which is com-
monly called the cloaca. The junction of the endoderm and
ectoderm appears to take place at the anus, and there is prac-
tically no proctodaeiun in the Vertebrata.

* Except Id some Maisipobraiich&:

PHYLUil VERTEBKATA (CRAXIATA). 49

The vascular system is well developed, consists of arteries,
capillaries and veins, and contains a red blood. There is a
median ventral subpharyngeal vessel, the hinder end of which
is especially muscular and contractile and differentiated as
the heart. The lymphatic system consists of vein-like
vessels containing a colourless fluid — the lymph — in which
float numerous amoeboid cells (lymph coi-puscles). These
vessels commence by blindly-ending fine tubes or sinuses in
the tissues, which gradually imite with one another to form
the main lymph vessels, which open into the venous system.
Special gland-like bodies, the so-called lymphatic glands, in
which the tymph corpuscles are produced, are mserted in the
course of the h-mphatic vessels. The lymphatic system is a
draining system, for the purpose of carrying away from the
tissues the fluid which has exuded into them through the walls
of the blood-capillaries, and is undoubtedly a specially differ-
entiated part of the vascular system.

The body-cavity is a coelom, and has the usual relations
of that organ to the urinary and reproductive organs. It is
laid down early, makmg its appearance eis a split (schizocoel)
in the mesoblast ; and in the Elasmobranchii, at any rate, a
certain resemblance between it at its first appearance and that
of Amphioxus can be detected (p. 33). But it differs from
that of Arnphioxtis in that the ventral portions of the trimk
somites are never distinct from one another, but form from
the first a continuous splanclinocoel. In the adult the
body-cavity is always divided more or less completely into
a pericardial division m front and a peritoneal division or general
body-cavity beliind. In the mammals the latter is still furtlier
subdivided, in that two anterior horns are cut off from it to
form the plem-al cavities. There is no coelom in the head of
adult vertebrata.

The urinary organs consist typically, in theii- origin at
least, of segmentally arranged nephiidia, which open internally
into the body-cavity. Externally they open mto a longitudinal
duct which leads into the hind end of the intestine in almost
aU cases. Both nephridia and ducts develop as special portions
of the coelom.

The generative organs develop from the lining of the
unsegmented ventral part of the coelom (splanclinocoel), and

z — n E

50 PHYLUM VERTEBRATA (CRANIATA).

never present any trace of segmentation.* In the female they
retain this relation throughout life (except in Teleostei), but
in the male the generative part of the coelomic epithelium
always (except in Marsipobranchii) loses its relation with the
general body-cavity in the adult.

The ovum varies considerably in character in the different
classes. In Pisces (except Elasmobranchii) and in Amphibia
it is comparatively small and holoblastic,f and the young are
always hatched out in an immature condition as larvae. In
Elasmobranchii, Reptilia, and Aves the ovum is large and
meroblastic, and the young when hatched resemble the adult,
a larval stage being absent. In Mammalia the egg is smaller
than in any other Vertebrate, and except in one class under-
goes almost the whole of its development in the oviduct, the
young being born in a condition closely resembling the adult.
An amnion, allantois and primitive streak are found in the
embryos of all Reptiles, Birds and Mammals, but are absent
from all Pisces and Amphibia.

The division of the Vertebrata into the four classes — Pisces,
Amphibia, Aves, and Mammalia was first established by
Linnaeus, though it had already been indicated in the system
of Aristotle. The Pisces and Amphibia are cold-blooded
animals (i.e. animals with a varying temperature) ; Aves and
Mammalia are warm-blooded. Since Linnaeus' day, his group
Amphibia has been split up into the naked Amphibia and into
the scaly animals or Reptilia. Pisces and naked Amphibia
have many characters in common, e.g. the branchial respiration,
the frequent persistence of the notochord, the absence of an
amnion and allantois, etc. On these grounds and in con-
sideration of the many relations between Reptiles and Birds,
Huxley has distinguished three principal groups of Vertebrata
— the Ichthyopsida (Pisces and Amphibia), the Sauropsida
(Reptilia and Aves), and the Mammalia.

•

* See note on p. 88.

t In Teleostei the ovum though small is meroblastic.

CHAPTER IV.

CLASS PISCES.*

Aquatic vertebrata ivhich breathe by means of pharyngeal gills
and possess typically two pairs of appendages which never present
any trace of a pentadactyle structure. Median fins supported, except
in Marsipobranchs, by dermal fin-rays (dermotrichia) are always
present. There are ten pairs of cranial nerves and paired posterior
cardinal veins.

Fishes are sharply marked off from all the other classes of
Vertebrata by the form of their pectoral and pelvic appendages.
These, which must be regarded as homologous with the limbs
of the higher groups, are cutaneous expansions supported by
skeletal structures, which, though presenting in their fan-like
arrangement some distant resemblance to the skeletal structures
of the pentadactyle limb, are yet never arranged on the penta-

* Lacepede, Histoire naturelle des Poissons, 6 vols. Paris, 1798-1803.
G. Cuvier et Valenciennes, Histoire naturelle des Poissons, 22 vols.,
Paris, 1828-1849. Baer, Entwickelungsgeschichte der Fische, Leipzig,
1835. J. Miiller, Vergleichende Anatomie der Myxinoiden, Berlin, 1835-
45. Id. Ueber den Bau u. die Grenzen der Ganoiden u. d. natiirliche
System der Fische, Abh. d. Berlin, Akad, 1846. L. Agassiz, Eecherches
8ur les Poissons fossiles, 5 vols. Neuchatel, 1833-44. Stannius, op.
cit. Heckel and I^er, Die SOsswasserfische von der osterreischischen
Monarchie, Leipzig, 1858. A Dumeril, Ichthyologie, etc., 2 vols., Paris,
1866. Siebold, Die SOsswasserfische von Mitteleuropa, Leipzig, 1863.
Blanchard, Les Poissons des eaux deuces de la France, ParLs, 1866. Cope,
" Classification of Fishes," Trans. Amer. Phil. Soc, 1870, and Proc.
Amer. Ass. for Adv. of Science, 1871. A. Gunther, Introduction to the Study
of Fishes, Edinburgh, 1880; and Catalogue of Fishes in the British
Museum, vols, i-viii., 1859-1870. A. S. Woodward, Catalogue of Fossil
Fishes in the British Musewn, 3 vols. London, 1889-95. F. Day, The
Fishes of Great Britain and Ireland, London, 1881-83. Jordan and Ever-
man. Fishes of North and INIiddle America, Bull. U. S. National
Museum, no. 47, Pts. 1-4, 1896-1900. T. W. Bridge, Fishes in the Cam-
bridge Natural History, 1904. G. A. Boulenger, Poissons du Bassin du
Congo, 1901.

52 CLASS PISCES.

dactyle plan. The ichthyopterygium, though clearly homolo-
gous with the cheiropterygium, is sharplj^ marked off from it,
and there are no intermediate forms connecting the two.

Though the structure of the limb skeleton, and possibly the
possession throughout life of the paired posterior cardinal veins,
are the only absolutely characteristic features of fishes, there is a
number of features which, while not distinctive, are highly
characteristic. We may enumerate some of these : —

1. Median unpaired folds of the integument, constituting
the unpaired fins and supported by dermal structures called
fin-rays or dermotrichia, would be absolutely characteristic
were it not for the fact that the fin-rays are absent from the
median fins of Marsipobranchii.

2. The absence of an internal opening of the nasal sac would
be absolutely characteristic of the class were it not for the
presence of internal narial openings in Dipnoi and of the pharyngo-
nasal duct in Myxinoids.

3. Respiration by means of lateral pharyngeal apertures and
gills is found in the adult in no other group excepting in a few
genera of the Amphibia.

4. The absence of a cloacal bladder might perhaps be cited
as a distinctive character, for it is present at least in the embryos
in all the higher Vertebrata.

5. Excepting in the Dipnoi the auricle is undivided, and the
ventral aorta is a tube of considerable extent.

6. Excepting in the Dipnoi there is nothing corresponding
to the median inferior vena cava of the other classes.

7. The absence of a tympanic cavity and membrane, and of
anything corresponding to the auditory ossicles of the higher
types may, we think, be cited as a distinctive character, for
these structures are very rarely, if ever, completely absent in
the other classes.

8. The presence of the peculiar sense organ known as the
lateral line might almost be regarded as a piscine character,
were it not for the fact that it is not clearly present in all
Marsipobranchii, and that it is found in some Amphibia.

9. The permanent division of the great lateral longitudinal
muscles of the body into segments (myotomes) corresponding
in number to the vertebral segments is only found outside
Pisces in some Amphibia, and in the tail of some Repbelia.

SCALES. 53

The epidermis contains large mucous cells which discharge
their contents on the sui'face. It may also contain pigment
cells and leucocytes. In many fishes the slime which is excreted
by the skin is poisonous.

The skin is seldom completely without skeletal structures
{Marsipobranchii). As a rule scales, formed as ossifications
of dermal papillae which are typically completely covered by
the epidermis, are embedded in it. Fish scales * are of three
princij)al kinds : (1) Placoid scales which consist of small
plates of bone in the dermis carrying an upstanding spine which
projects freely, and is formed of dentine capped Avith enamel.
These are found in Elasmobrancliii and some Ganoids, (2)

FlQ. 32. — Perca fiuviatUis (Regne animal).

Ganoid scales are bony plates covered with a smooth layer of
a substance called ganoin. Ganoin f is a dermal product
allied to vitro-dentine. These scales are entirely dermal, and
if their surface is exposed, it is owing to the fact that the epidermis
has been rubbed off. Such scales are found in most Ganoids.
(3) Scales of varying thickness consisting of bone only, without
ganoin. They are found in most Teleosteans, and are there
called cycloid and ctenoid scales according to the nature of their
edges.

The unpaired fins arise as a continuous fold of skin extending

♦ A fuller account of the scales is given with the accounts of the orders.
For principal recent literature, see Klaatsch, Morph. Jahrb. 16, p. 258,
and 21, 1894, p. 153; F. Maurer Die Epidermis u. ihre Abkommlinge, Leip-
zig, 1895. O. Hertwig, Alorph. Jahrb., ii. and vii. C. Rose, Anai. Anz.,
14, 1897 ; pp. 21 and 33. Nickerson, "The development of the scales of
Lepidosteus," Bull. Mus. Harvard, 24, 1893.

t It was formerly supposed to be enamel and to be epidermal in origin,
but this has been proved to be erroneous.

54 CLASS PISCES.

along the middle dorsal line of the trunk, and continued round
the tail on to the ventral surface as far as the anus. It may
persist in this form, but as a rule it becomes broken up into a
variable number of dorsal fins, a caudal fin which consists of a
dorsal and ventral part, and an anal fin between the ventro-
caudal and the anus (Fig. 32). The unpaired fins are almost
always supported by the so-called fin-rays or dermotrichia.
These are horny fibres of the dermis {Elasmohranchii), or bony
rods {Teleostei, Ganoids, Dipnoi) which may be segmented, and
more or less soft and flexible (Malacopterygians) or stout and un-
segmented(Acanthopterygians). These dermotrichia are absent
only in Marsipohranchii. They are composed of two closely
approximated halves, and are carried except in the case of the
ventro-caudal fin, by the somactids or radialia. These are carti-
laginous or bony rods, placed as a rule in the basal part of the
fin-fold, and between the muscles of the back. They do not
necessarily correspond in number with the vertebrae. They
are usually segmented into two or three pieces, to the distal of
which the dermotrichia are attached. The basal piece is some-
times called the axonost ; in the Teleostei it is known as the
interspinous bone, because it occurs between the spines (neural
or haemal) of the vertebrae. The second piece is sometimes
known as the haseost. In a few fishes (e.g. Dipnoi), the
somactids articulate with the spines of the vertebrae.

The strong spine-like anterior fin-ray often found in Teleosteans and
bony Ganoids is formed of bone. In Elasmohranchii, the strong spines
which are sometimes found in connection with the fins are tooth-like in
structure.

The dermotrichia are of three kinds. *

1. In Elasmohranchii and Holocephali they are unjointed, occasionally
branched, fibrous rays of a horny consistency and without osseous tissue :
these are called ceratotrichia. Similar dermotrichia are found in the
larval fins and at the edges of the adult fins of Teleosteans and Ganoids :
in this case they are called actinotrichia. They are more numerous than
the somactids.

2. In adult Teleosteans and Ganoids the fins have jointed, branched,
bony dermotrichia developed between the actinotrichia and the skin.
They are supposed to be modified scales, which they sometimes resemble,
and are called hpidotrichia. They correspond in number with the
somactids except in the cartilaginous Ganoids, in which they are more
numerous.

3. In the Dipnoi the dermotrichia have been called kamptotrichia

* Goodrich, " Dermal fin-rays of Fishes," Q. J. M. S., 1904.

CAUDAL FIN. 55

They are branched, jointed bony rays, and appear to be raerelj' modified
lepidotrichia. They are more numerous than the somactids.

In all fishes the ventral part of the caudal differs from the
other median fins in the fact that the dermotrichia (fin-rays)
are supported directly by the haemal arches. These are fre-
quently imperfectly segmented from one another, and may,
in the adult, have the form of two or three bony plates, or even
of a single plate. They are sometimes called, when ossified, the
hypural bones.

A few Teleostei (eel-hke forms, some Gadidae, etc.) and the Dipnoi
constitute apparent exceptions to this rvile as to the structure of the
caudal fin, but in the former of these it is probable that the anal fin
has fused with the ventral part of the caudal fin, for in a small por-
tion of the ventro-caudal fin a few dermotrichia are carried directly by
haemal arches. In the Dipnoi on the other hand, and in some Teleostei,
the caudal fin appears to be entirely unrepresented, for the tail gradually
tapers to its termination. What appears at first sight to be the ventro-
caudal fin is really the anal fin, and in no part of it are the dermotrichia
supported directly by the haemal arches. In the crossopterygian Ganoids,
in which there is a diphycercal tail, the ventral dermotricliia of the caudal
fin are clearly supported by haemal arches and not by somactids.

Considerable importance has been attached to the form of the
tail and to the structure of the caudal fin in fishes. In the
simplest cases the vertebral column is continued straight to
its termination, and the dorsal and ventral part of the caudal
fin are equal and symmetrical with each other. This type of
caudal fin is called diphycercal or protocercal. In many fishes,
however, the posterior part of the vertebral column is bent
dorsalwards, and a special enlargement of the ventro-caudal
fin is formed at a short distance from the end of the tail. This
type of caudal fin is called hetewcercal (Fig. 64), and is character-
istic of Elasmobranchs and chondrostean Ganoids. In such fishes
the tail may be said to be bifid, presenting a dorsal lobe and a
ventral lobe. The dorsal lobe consists of the real hind end of
the tail with the dorso-caudal (if present at all) and part of the
ventro-caudal fin, M'hile the ventral lobe is the specially enlarged
part of the ventro-caudal fin above described.

In Teleostei, and bony Ganoids, and some Selachians, the
dorsal lobe thus defined shrinks and almost disappears relatively
to the greatly enlarged ventral lobe, which now forms the whole
of the tail fin, and becomes svmmetrical in itself. Such a tail

56 CLASS PISCES.

is called homocercal. In many Teleostei the tail fin of the larva
begins diphycercal then becomes heterocercal and finally assumes
the homocercal form. Th's correspondence between the develop-
mental history of the tail and the three forms of tail fin found
in living fishes is supposed to be highly significant from an
evolutionary point of view, for it is supposed that the diphy-
cercal tail is the most primitive, and that the homocercal is the
most specialised, the heterocercal tail intervening between
the two. This supposition is to a certain extent borne out by
palaeontology, which seems to show that Teleosteans are the
most modern group of fishes. Unfortunately for the theory,
however, the oldest fishes known to us had heterocercal tails and
not diphycercal, as the theory requires.

In addition to these tliree types of tail-fin, intermediate conditions have
been named. For instance, the terra heterodiphycercal has been applied
to slightly heterocercal tails in which the fin is much less developed on
the dorsal side than on the ventral (some Crossopterygians, Fig. 104),
while tails, in which the tail fin is externally symmetrical, but the
hind end of the vertebral column is bent and extends some way
into the dorsal lobe of the fin (Amia, etc.), are called hemiheterocercal
(Figs. 107, 109). The true homocercal tail is distinguished from the
hemiheterocercal by the fact that the vertebral column, which is bent
dorsalwards, does not extend into the fin, but terminates in front of it.

As has already been mentioned, the dermotrichia of the ventro-caudal
fin of all Pisces are attached directly to the haemal arches (for apparent
exceptions to this see p. 55). In the homocercal tail of the Teleostean
these haemal arches are called the hyptiral bones and are frequently fused
together to form a single broad plate of bone. In Ganoids with hetero-
cercal tails, when the upper lobe of the caudal fin (dorso-caudal) disappears
it is replaced by a series of ridge scales : the " fulcra " of palaeontologists :
in Elasmobranclis, when absent, it leaves no trace.

The i^ectoral and pelvic fins also possess dermotrichia (fin-rays)
and somactids (radialia). A certain number of the somactids
are directly articulated to the limb girdles, and are then called
basalia. There are usually three of these, which are then called
pro- meso- and meta-pterygium, but their number varies con-
siderably. The important point to notice is the arrangement of
the peripheral somactids.* In CladoselacJie, a Palaeozoic fish,
these are parallel to one another (Fig. 83), and the fin-skeleton
may be termed orthostichous.-f In most fishes, and notably in

* Wiedersheim, Das GUedmassenshelet der Wirbelthiere, Jena, 1892.
t The same feature is found in the pelvic fins of the Ganoid Psephurus
(Regan, Ann. and Maq. Nat. Hist (7), 13, 1904, p, 333.)

ORIGIN OF LIJIBS. 67

Elasmobranclis they are arranged in a fan-like manner, and the
fin may be described as rhipidostichous. In Dipnoi the somactids
of the fin are represented by a basal piece, followed by a row of
them occupying the axis of the fin, with or without pre- and post-
axial pieces placed like the barbs of a feather (Fig. 138). Such
an arrangement may be termed rachiostichous and mesorachic.

In some sharks and in the extinct Pleuracanthidae there is a succession
of somactids forming a rachis, but the rachis is placed on one side of the
fiQ and carries peripheral somactids mainly on that side. * Such an arrange-
ment may be called rachiostichous and pleiirorachic (Fig. 76).

A similar reduction in the number of basal somactids is sometimes
found in the median fins of extinct fishes, e.g. in the anal fins of Pleura-
canthus (Fig. 87).

In the discussion of the vexed question of the origin of the
vertebrate paired limbs, much attention is paid to the arrange-
ment of these somactids {radialia). According to Gegenbaur
the Ceratodus type (Fig. 138) of fin skeleton (rachiostichous) is
the most primitive, and this fin constitutes what he calls the
archipterygium. On this view the skeleton of the paired fin^
and their girdles have originated from a branchial arch and its
branchial rays ; the girdle being derived from the branchial arch
and the somactids from the branchial rays. One important
objection (among others) to this view is that tlie branchial arches
are in the gut-wall, whereas the limb girdles lie hi the body wall.

On another and perhaps more acceptable view, if any view on
these insoluble questions can be regarded as acceptable, the
pectoral and pelvic fins are to be regarded as local specialisations
of a once continuous lateral fold of the body wall, containing
skeletal structures comparable to those of the unpaired fins, viz.
basal segmented somactids (radialia) and peripheral dermo-
trichia. This view was first suggested by Balfour. According
to it the fin-skeleton of Cladoselache would be appealed to with
its parallel somactids as being an obvious local specialisation of
a once continuous fold, with parallel somactids all along its
course.

It would be useless to study the skeleton except in detail,

* Tliere appears to be some dispute as to whether this side is pre- or post-
axial. According to Wiedersheim and Fritsch, the side on which the
majority of the rays are placed is post-axial, but according to the more
generally receiv'ed opinion it is pre-axial, the limb in the specimen from
which Fig. 87 is taken having been displaced.

58

CLASS PISCES.

and for that we refer the reader to the account of the different
sub-classes. We must content ourselves here with an account
of its more general features, to which it is desirable to call
attention.

The notochord forms the
basis of the axial skeleton. It
always persists in the adult,
though it is generally consider-
ably reduced. Its longitudinal
extent is from the pituitary
fossa of the skull in front to
the end of the vertebral column
behind. The notochord itself
rarely forms an important
element of the axial skeleton
of the adult. Its supporting
function — so conspicuous a
feature in Amphioxus — is taken
over by its sheath and by
cartilaginous structures de-
veloped around its sheatn.
In the trunk these structures
are, with few exceptions, seg-
mented, and constitute the
vertebral column ; in the skull
they are not segmented, and
are known in the embryo as
tlie parachordal cartilages.

At an early stage in the embryo

a well defined structureless sheath

is formed round the notochord.

This is called the memhrana elastica

interna. A little later a second

sheath is formed round this. This

outer sheath, also structureless, is

knowTi as the memhrana elastica

externa. Cells of the skeletogenous mesoblastic layer, which surrounds

the notochord, appear now to penetrate the elastica externa and invade

the elastica interna (Fig. ,S3), which thus becomes nucleated.

r

The chordal sheath sometimes remains as a continuous
structure (Sturgeon, Dipnoi, etc.), but as a rule becomes seg-

Fro. 33. — Transverse Sectioa tlirough
the vertebral column of an advanced
embryo of Scyllium in the caudal
region ; na skeletogenous tissue of
neural arch, ha of haemal arch ; ch
notochord ; sh notochordal sheath,
which has acquired nuclei (elastica
interna) ; ne outer cliordal sheath
(elastica externa) (After Balfour).

VERTEBRAL COLUMN.

&d

FlO

a Inugitudiua

mented ; in other words it becomes differentiated into alter-
nately short fibrous and longer cartilaginous portions (Fig. 34).
The fibrous portions become the intervertebral ligaments, the
cartilaginous , portions the
bodies or centra of the verte-
brae. The sheath thickens in
the central part of the verte-
bral regions and constricts the
notochord, so that the noto-
chord assumes a beaded form,
being narrowest in the middle
of the vertebral regions and
widest in the intervertebral
(Fig. 34). In this way a bi-
concave or amphicoelous cen-
trum — a form eminently
characteristic of fishes — arises.
When the centra are formed
entirely or mainly from the
chordal sheath they are called
cJiordo-centrous {Dipnoi, Elas-
mobranchii). But it frequently
happens that they are rein-
forced by cartilage derived
from the arch-tissue. The arch-tissue arises from the meso-
blastic tissue (skeletogenous layer), which surrounds the noto-
chord, and is continued dorsally round the spinal cord. Four

special concentrations of this tissue are
formed adjoining the notochord, two dor-
sal and two ventral. In these the
chondrifications which give rise to the
neural and haemal arches begin. The
neural arches do not always completely
enclose the spinal canal, but are supple-
mented by the intercalated pieces (Fig.
terior' trvmk-verte- 35) which are placed between them, i.e.
(after" Hasse"^ from intervertebrally. The haemal arches may
areh°w^ithVoramen"for ^Iso be Supplemented by intercalated

anterior root; in in- „,'^„„„ rni i-i • i

tercaiated piece witii picces. Ihcse cartilagmous arches may
roSTAhaemaT^ch.' Spread out round the notochord outside

34. — a Diagram of
section of the vertebral column of a
Teleostean with vertebral constriction of
the notochord (from Claus). b vertebrae
of a bony fish, ch notochord ; D neural
spine ; i)' haemal spine ; J interverte-
bral ligament ; k body of vertebra ; 06
neural arch ; iJ rib ; Wk vertebral body.

60 CLASS PISCES.

the membrana elastica externa and unite with each other, and
so reinforce the vertebral centra. When the chordal sheath is
inconspicuous, and the centra appear to be mainly derived
from the arch tissue, the vertebral column is said to be arci-
centrous {Ganoidei, Teleostei).*

In this way a segmented vertebral column is established. In
Teleosteans and bony Ganoids a further complication is added
in the replacement of the cartilage by osseous tissue. The
centra in the trunk (Fig. 34) carry as a rule short transverse
processes, which may be called haemal arches, though they do
not meet ventrally except in the caudal region, where they en-
close a space containing the caudal artery and vein. The ribs
are never more than short pieces of cartilage or bone attached
to the outer ends of the transverse processes in the trunk
region. There is no sternum in fishes.

The primitive craniumf consists of a cylinder of continuous
cartilage, to which are attached anteriorly the nasal capsules
widely open below, and posteriorly the auditory capsules. It is
thus divided into four regions ; the occipital surrounding the
foramen magnum, the wide auditory region, the narrow sphen-
oidal or interorbital region, and the wide nasal or ethmoidal region.
The junction of the parachordal with the trabecular region of
the skull is marked externally by the foramen in the median
floor, which transmits the internal carotid arteries (Fig. 36, 13),
and internally by the posterior clinoid ridge which forms the
hinder wall of the fossa for the lodgment of the pituitary body
(Fig. 36, 6). In the embryo two elongated cartilages — the para-
chordal cartilages — are developed on each side of the cranial part
of the notochord. They unite with each other around the
notochord and form the basilar plate which gives rise to the
occipital and part of the sphenoid regions. The auditory capsules
which are developed round the membranous labyrinth become
fused with this part of the skull. The anterior end of the skull
in front of the pituitary fossa is formed by a second pair of
embryonic cartilages, the trabeculae. To the front end of these
the nasal capsules become attached, thus giving rise to the
ethmoidal region.

* Vide Gadow, Phil. Trans., 186, 1895, p. 165.

t C. Gegenbaur, Untersuchungen z. vergl. Anat. d. Wirbellhiere,
Heft 3. Leipzig, 1872. Id., " Ueb. d. Occipitalregion, etc. der Fische."
Kolliker'a Festschrift, Leipzig, 1887

CKANIUM.

61

The hinder ends of the trabeculae embrace the front end of the notoehord.
so that the posterior cUnoid ridge must be regarded as being formed by
the hind end of the trabeculae. Moreover it must not be forgotten that the
internal carotid artery enters the skull in the embryo through the space
between the trabeculae before they fuse, so that the carotid canal also
belongs to the hind end of the trabecular region.

Though the roof of the cranium is largely cartilaginous in fishes
even when membrane bones are present on it, there is always a
considerable fontanelle in which cartilage is absent. The carti-
laginous cranium so constituted becomes in the Teleosts,
Ganoids and Dipnoi replaced by bone to a varying extent, and
reinforced by the development of osseous tissue in the adjacent
connective tissue. The membrane hones, formed in the latter

17-

Fio. 36. — Median section of the cranium of Heranchus, inner view (after Gegenbaur). 1 Fora-
men for vagus, 2 glossopharyngeal, 3 audit iry, 4 facial, 5 trigeminal nerves ; 6 posterior
clinoid ridge ; 7 foramen for oculomotor, 8 trochlear, 9 optic nerves ; 10 fontanelle ;
11 rostrum ; 12 lateral process of ethmoid region ; 13 foramen for carotid ; 14 transverse
canal in skull base ; 15 notoehord ; 16 foramina for spino-occipital nerves ; 17 neural
arch of the first vertebra with nerve foramina.

manner, apply themselves to the subjacent cartilage and help in
forming the cranial wall. The occipital region of the cranium
is attached to the anterior end of the vertebral column, usually
mthout any special articulation (except in Batoidei and Chima-
era, etc.), the basioccipital region having the conical depression
and form of a vertebral body. The cranial part of the noto-
ehord persists in the adult in some forms, but it more usually
undergoes atrophy. It occasionally happens, as will be men-
tioned in the special accounts, that a few of the anterior
vertebrae are fused with the occipital region of the cranium.

Visceral Skeleton. — The walls of the anterior part of the
alimentary canal (mouth and pharynx) are supported and

62 CLASS PISCES.

strengthened by incomplete cartilaginous rings, analogous to the
cartilaginous rings found in the trachea of the higher Vertebrata
Like the tracheal rings, they serve to keep open the tube — in this
case the mouth and pharynx — through which the respiratory
medium passes to the respiratory organ, but they differ from
them in being divided up into segments which are movable upon
one another by means of muscles. The Visceral Arches, as
these structures are called, are placed in the splanchnic meso-
derm, as shown by embryology, and therefore have nothing to
do with ribs, with which they have sometimes erroneously been
compared. They may be described as consisting of a series of
cartilaginous rods on each side, joining one another ventrally,
but usually (except in the case of the first two) ending freely
dorsally without connection with other skeletal structures.

The first arch is called the Mandibular ; its skeleton lies near
the lips and constitutes the jaws. The second is called the
Hyoid Arch : it lies in the pharynx wall between the spiracle
and the first branchial cleft. The remainder, of which there are
usually five,- are the Branchial Arches : they lie in the pharynx
wall between the branchial clefts, the last always occurring
behind the last cleft. In Hepfanchus there are seven branchial
arclies and seven clefts ; in Hexanchus and Chlamydoselachus
there are six.

The mandibular arch becomes closely associated with the
cranium. It always becomes divided into two pieces : of these
the dorsal piece forms the skeleton of the upper jaw and is called
the palato-quadrate bar, while the ventral piece constitutes the
cartilage of Meckel. The dorsal piece is longitudinally directed
beneath the skull from the auditory to the ethmoidal region ; it
gives articulation at its posterior (quadrate) end to Meckel's
cartilage. This upper segment of the mandibular arch presents
two principal arrangements in fishes. In the one of these, that
which is generally called the hyostylic, its hind end is not attached
to the skull directly but is held up by the stout dorsal segment
of the hyoid arch, which is for this reason called the hyo-mandib-
ular. This is the arrangement found in most fishes. In
Chimaera and Dipnoi, however, a different arrangement is found.
In these and in some extinct fishes the palato-quadrate bar is
fused with the skull not only posteriorly in the auditory region,
but anteriorly in the ethmoid region and in the intermediate

ALIMENTARY CANAL. 63

sphenoid region. It is indeed fused all along with the side of
the cranium and has the form of a laterally projecting tri-
angular shelf, the projecting angle of which is the quadrate
region and gives articulation to Meckel's cartilage. This
arrangement is called Autostylic ; * the hyoid arch taking no
part in the suspension of the upper jaw.f It has been proposed
to divide fishesi nto two great groups based upon the condition
of the primitive upper jaw skeleton — the Autostylici and the
Hyostylici ; but as we shall explain in the sequel, there appear to
be good reasons for adopting a different arrangement.

In the Teleostei, Ganoidei and Dipnoi cartilage bones are
developed in both the mandibular and hyoid arches to a varying
extent and membrane bones may come to overlie them, largely
supplementing them and even replacing them.

The digestive organs vary much in structure. The mouth,
which is placed at, or near, the anterior end of the head, usually
has the form of a transverse slit, and can sometimes be extended
forward by means of the movable supporting bones of the upper
and lower jaws. The buccal cavity is distinguished by its width,
and by the great number of teeth it contains, which are developed
from the papillae of the mucous membrane by dentinal ossifica-
tion. There are often two curved parallel rows of teeth on the
upper jaw ; an outer row on the premaxilla, and an inner row
on the palatine, and there may also be a median unpaired row on
the vomer. On the lower jaw there is only one curved row of
teeth. There may also be teetli on the hyoid arch and on the
maxillae, pterygoids, and parasphenoid, and, as a rule, on the
branchial arches also, especially on the upper and lower pharyn-
geal bones. The teeth may be distinguished according to their
shape into pointed conical prehensile teeth, and grinding teeth.
They are developed in the mucous membrane and are attached to
the skeletal structures by ligament or by ankylosis. In a few
cases only are they implanted in sockets.

A small, hardly movable tongue is developed on the floor of the

* The so-called amphistylic arrangement which is found in a few Elasmo-
hranchs (see below) would seem to be a variety of the autostyhc.

t The suspension of the mandibular arch, found in the skulls of Am-
phibia and Sauropsida, in which the palato -quadrate bar is not attached
along its whole length, but only in the auditory and ethmoid region,
must be regarded as a more typical form of the autostylic arrangement
than that found in Chimaera and the Dipnoi.

64

CLASS PISCES.

buccal cavity, and the lateral walls of the pharynx are pierced by
the gill-slits. Following the pharyngeal cavity, there is a usually
short funnel-shaped oesophagus, and a large stomach, which is
frequently drawn out into a caecum of considerable size (Fig. 37).
Caecal appendages (pyloric appendages) are not unfrequently
met with at the entrance of the lower mid-gut (small intestine)
which is marked off by a valve ; they probably serve the purpose
of increasing the extent of the secreting surface of the alimentary
canal. The intestine is usually several times coiled, and its
internal surface is remarkable for the longitudinal folds of the
mucous membrane ; villi such as are found in the higher Verte-
brates are only rarely present ; but in the Selachians, Ganoids,
and Dipnoi there is a peculiar spirally-coiled longitudinal fold —

Fig. 37. — Alimentary canal and generative organs of Clupea harengus (after Brandt). A
anus : Ap pyloric appendages ; Br gills ; £> intestine ; Dp pneumatic duct ; Gp genital
pore ; Oe oesophagus ; S spleen ; T testis ; V stomach ; Vd vas deferens ; Fn swimming
bladder.

the so-called spiral valve — which contributes essentially to the
enlargement of the absorbent surfaces. A rectum is not always
clearly marked off, and when present is always short, and in the
Selachians it is furnished with a caecal appendage. The anus is
usually situated far back, and is always ventral, and in front of
the urinary and generative openings, when the latter do not
lead unto the rectum (cloaca). In fishes with jugular pehic
fins, and in some Teleosteans without pelvic fins, it is situated
very far forward, and may even be on the throat.

Salivary glands are absent in fishes, but there is a large liver
which is rich in fat and is usually provided with a gall-bladder ;
there is also usually a pancreas, which is by no means replaced
in Teleosteans by the pyloric appendages as was formerly be-
lieved.

AIR-BLADDER. RESPIRATION. 65

In many fishes the swimming bladder, an organ which by its
mode of origin corresponds to the lungs, is developed as a diver-
ticulum of the alimentary canal : it is sometimes closed, but
sometimes remains in communication with the interior of the
alimentary canal by the pneumatic duct (Physostomi) (Fig. 37,
Dp). Its walls are formed of an external elastic membrane
which is sometimes invested with, muscles, and an internal
mucous membrane. Glandular structures are sometimes pre-
sent in the internal coat, and these may exert an influence
on the enclosed air. The internal surface is usually smooth,
but is sometimes provided with reticulated projections w^hich
lead to the origin of cellular cavities (some Ganoidei). Physio-
logically the swimming bladder is a hydrostatic apparatus,
the function of which seems to consist essentially in rendering
the specific weight of the fish variable. Wlien it is present
the fish must have the power of compressing it, partly by
the muscles in its walls and partly by the muscles of the
body, thus rendering the body specifically heavier so that
it sinks. When the compression of the muscles is removed
the compressed air will again expand, the specific gravity di-
minish and the fish will rise. If the anterior and posterior parts
are separated and the pressure on them is unequal, then that
half of the fish which is rendered specifically heavier will sink.
Still more complicated relations, however, seem to exist.*

Respiration is in all cases effected by gills, which may be
supplemented by other structures, e.g. the lungs in the Dipnoi,
and in Teleostei by vascular folds found in cavities (Fig. 38) in
connection wdth the gill passages themselves or with the
cavity beneath the operculum into which the gill-slits open.
For a description of these we refer the reader to the special
account of the Teleostei. The gills themselves are folds, con-
taining many blood-vessels, of the mucous membrane of the
passages which lead outward between the branchial arches to
open on the side of the head. These passages, which may be
short and slit-like, or long and tubular, open either directly to
the exterior {Elasmohranchii) or their outer openings are covered
by a fold of skin generally containing cartilaginous or bony
supports and called the operculum. In this case they may be

said to open into a branchial cavity which itself opens to the

* See eiccount of Teleostei.
Z— II. F

06

CLASS PISCES.

exterior. The gills are either lamelliform {ElasmdbrancJiii) and
attached along their whole length to the interbranchial septa,
or filiform and projecting (so-called pectinate gills of Teleostei,
etc.). They are arranged in a row on each side of the branchial
arch, so that each branchial arch carries two rows of gills [holo-
hranch), one on its anterior and one on its posterior face.
Sometimes there is only one row {hemibranch), and some-
times gills are absent on each side, or present only as a vestigial
structure called a fseudobranch.

The general arrangement is as follows. The branchial passage
between the mandibular and liyoid arches is called the spiracle.
It is always reduced and is present only in most Elasmobranchs
and some Ganoids, Behind this there follow typically five

branchial passages or gill-
clefts. The mandibular arch
never carries more than a
vestige of a gill, which is called
the mandibular pseudobranch
or pseudobranch of the spir-
acle. The hyoid arch never
carries more than a demibranch
and that on its posterior face.
The first four branchial arches
carry typically holobranchs,
while the last branchial arch
is ahvays without a gill. It
thus happens that, if the hyoid arch carries a hemibranch
on its hinder surface, the first four branchial passages have
gills on both anterior and posterior walls, while the fifth
branchial passage has a gill only on its anterior wall, the fifth
branchial arch being always gill -less. In most fishes, however,
the hyoid demibranch is reduced to a vestige, and is then known
as the hyoidean or opercular (because the hyoid arch carries
the operculum) pseudobranch. Externally projecting gills are
found in the embryos of Elasmobranchs and a few Teleostei. They
are not true external gills, but are much elongated internal gills.
In the young Polypterus and some Dipnoi there apj)ear to be
true external gills. ^

The brain of fishes is small and does not fill the cranial
acvity. It presents all the parts of the typical vertebrate brain.

Fig. 38. — Head of Anabas scandens (Itfegne
animal). The operculum has been re-
moved to show the excavated superior
pharyngeal bones (pharyngobranchials).

BRAIN. 67

It is perhaps chiefly characterised by the small development of
the cerebral or prosencephalic part of the fore-brain. The anterior
end of the medullary tube becomes at an early embryonic stage,
when its walls are still epithehal, differentiated by two constric-
tions into three vesicles, the fore, mid, and hind cerebral vesicles.
Of these the posterior vesicle or hind-brain gradually tapers
behind into the spinal cord, and the portion of the medullary
canal contained in it gives rise to the fourth ventricle of the adult.
Its walls become transformed into the medulla oblongata, which
is a development of the floor and sides of the hind-brain and is
frequently called the myelencephalon. The cerebellum {tneten-
ce'phalon) is a special development of the anterior part of the
dorsal wall of the hmd-brain. The greater part of the dorsal
wall remains throughout life at the epithelial stage and never
develops nervous tissue. The mesoblast {pia mater) overlyin»
this permanently epithelial wall becomes especially vascular
and gives rise to the choroid plexus of the fourth ventricle.

The middle vesicle or mid-brain {mesencephalon) gives rise by
its roof and sides to the optic lobes or corpora bigemina, and by
its floor to a stout nervous mass consisting largely of strong
bundles of nerve fibres which in the mammalian brain constitute
the crura cerebri or peduncles of the cerebrum. The portion of the
medullary canal m the mid-brain is the iter a tertio ad quartum
ventriculum or aqueductus sylvii.

The anterior vesicle or fore-brain becomes early differentiated
into three parts ; a posterior part, the thalamencephalon, the
central canal of which constitutes the third ventricle ; a ventral
part usually described as part of the thalamencephalon, the
infundibulum ; and an anterior part the prosencephalon or cere-
brum, the ventricle of which is the second ventricle. The cere-
brum is usually divided into a right and left lobe by a longitu-
dinal vertical constriction, but this hai^pens rarely (Dipnoi,
Marsipobranchii) in fishes, though there are sometimes indica-
tions of this division in the form of a longitudinal surface
groove, and in Elasmobranchs the contamed ventricle is ac-
tually divided into a right and left ventricle which oj)en be-
hind into the third ventricle by the foramen of Munro and
are termed the lateral ventricles.

The anterior end of the cerebrum is always marked off as two
lobes of varying size and shape into which the second ventricle

68 CLASS nscES.

is continued (as the first ventricle) and which give off from their
anterior ends the olfactory nerve fibres : they are called the
olfactory lobes or the rhinencephala. In the embryonic brain
before the differentiation into thalamencephalon and prosen-
cephalon has been effected, the fore-brain becomes bent ventrally,
forming an angle with the posterior part of the basi-cerebral
axis. This bend in the cerebral axis constitutes the cranial
flexure ; it takes place, roughly speaking, at the junction of the
fore-brain and mid-brain and a short distance behind the front
end of the notochord, the anterior end of which is under the pos-
terior part of the fore-brain. The notochord is therefore in-
volved in the cranial flexure and its front end becomes hook-
shaped. The other organs of the anterior end of the head
are also affected as is shown by the somewhat longi-
tudinal disposition of the anterior gill-slits and arches (Fig. 39)
as compared with the transverse disposition of those behind. In
consequence of this bend in the nerve axis the anterior end of the
neural tube becomes eventually directed ventralwards and, by a
posterior outgrowth, backwards beneath the floor of the mid-
brain : it constitutes the infundihulum, which has already been
mentioned as one of the three divisions into which the fore-brain is
differentiated. In addition to the infundibulum, a fourth divi-
sion of the fore-brain has to be distinguished. At an early em-
bryonic stage, before the prosencephalon is marked off, the
anterior cerebral vesicle gives off a right and left lateral
outgrowth : these are the optic vesicles. They at once become
applied externally to the lateral skin of the head, and their con-
nection with the brain becomes constricted to form a stalk-like
structure which eventually becomes solid and forms the optic
nerve. At the same time the cavity of the optic vesicle becomes
obliterated by the invagination of its outer wall next the skin
upon the inner wall on the brain side. This collapse of the optic
vesicle, if not caused by, takes place in connection with the
formation of the lens from the outer ectoderm at the point where
the optic vesicle before its collapse touched the skin ; it gives
rise to the formation of a two-walled optic cup, the mouth of
which is occupied by the lens and the double wall of which
becomes the retina of the eye.

To return to the fore-brain. When the cranial flexure is
established, the posterior part of its dorsal wall looks forward.

CRANIAL FLEXURE. FOKE-BRAIN. 69

This part becomes greatly developed and produced forwards
into a large vesicle, the front part of which soon becomes
marked off as the rudiment of the cerebrum or prosencephalon.
This forward growth is shown by the subsequent relations of the
optic nerve to have taken place behind (in the original position of
the parts) the attachment of that structure {optic chiasma) to the
cerebral roof (Fig. 113). If this interpretation of the complex em-
bryonic growths is correct, it would appear that the cerebrum is
derived from a dorsal extension of the original fore-brain just behind
the point of origin of the optic nerves, and that the olfactory nerves
which are developed from the front end of the cerebrum and are
usually described as the first pair of cranial nerves, are in reality
the second, the optic nerves being anterior to them in position.
The optic nerves then are attached to the roof of the original
fore-brain at about halfway between its front and hind ends.
A ganglionic mass is formed on each side at this point in the
side walls of the fore-brain ; these great ganglia are the optic
thalami and lie in the adult brain at the side of the third ven-
tricle, constituting the chief bulk of the thalamencephalon.
Immediately in front of the optic thalami two great ganglionic
developments are formed in the ventral wall of the cerebral out-
growth : these are the corpora striata found in the adult on the
floor of the second ventricle or, if it is divided, of the lateral
ventricles.

The front end of the cerebral outgrowth also gives rise to
nervous tissue of the olfactory lobes.

We now come to the roof of the reconstituted fore-bratn, after
the cerebral outgrowth has been formed.* Tliis roof is divided
into two parts by the velum transversum (see below). Of these
the posterior part, the part overlying the third ventricle, remains
in all Vertebrates almost entirely in an epithelial condition. It
gives rise by its posterior part to the epiphysis or pineal body.
To this we shall return shortly. The anterior part, the part
belonging to the cerebral rudiment, is called the pallium. It is
marked off from the posterior part by a transversely directed
fold of the epithelial roof. This fold dips down into the ventricle
at the junction of the thalamencephalon and prosencephalon
and encloses between its two laminae a vascular development
of the pia mater, which is always present and gives rise in the
* Minot, American Journal of Anatomy, 1, 1901, p. 81.

70 CLASS PISCES.

higher brains to the choroid plexuses of the lateral ventricles.
It is called the velum transversiim.

This folded-in part of the roof is generally regarded as belong-
ing to the cerebrum : in all Vertebrata it retains throughout
life its epithelial condition. In front of it, the roof of the brain
(pallium) behaves in a different way in different animals. In
Elasmobranchs, Marsipobranchs, Dipnoi, and in all Vertebrata
above fishes, it loses its simple epithelial condition and develops
nervous tissue, forming the dorso -lateral part of the cerebral
hemispheres above the lateral ventricles. In most other fishes,
Teleosteans, and Ganoids, the pallium retains its epithelial
condition throughout life, so that in these groups the roof of
the lateral ventricle remains permanently thin and epithelial,
as does the roof of the third ventricle and that of the posterior
part of the fourth ventricle, and in Lampreys of the aqueductus
sylvii as well.

Curiously enough — for what reason it is difficult to under-
stand — the condition in which the cerebral pallium consists of a
thin epithelial layer is regarded as secondary. By all the or-
dinary tests which are applied in speculations of this kind — viz.,
embryonic development and general diffusion of the character
in the lower Vertebrata and absence in the higher, it should surely
be regarded as a primitive character. Indeed, if we may be
allowed to indulge in a little speculation of this kind, it would
appear from development that the whole medullary canal at
one time had purely epithelial walls, and there appears to be a
tendency to the retention of this character along the middle
dorsal line throughout life in all Vertebrata.

To return to the pineal body. It is developed as a divert-
iculum of the hinder part of the roof of the thalamencephalon.
Its terminal portion becomes the pineal body or epiphysis ; its
proximal part is the pineal stalk. The terminal part sometimes
gives rise on its anterior wall to an outgrowth which is called
the 'parietal organ. The parietal organ may be developed
separately from the brain roof just in front of or by the side
of the epiphysis.* It is not always formed, and usually van-
ishes with later growth ; but in Lampreys and Lizards it

* On account of this fact it has been suggested that the epiphysis is
really a paired organ, one of the pair becoming the actual epiphysis
(pineal body) of the adult, and the other either degenerating or becoming

PARIETAL ORGAN.

71

persists and assumes a peculiar structure resembling that of the
retina of the eye. For this reason it has been called the pineal
eye. In liampreys the pineal body also assumes the same
structure. A great deal of significance has been attached to
the curious eye-like structure which is assumed by the parietal
organ. It has been regarded as the vestige of an unpaired eye.
In our opinion the resemblance to an eye is accidental, but for a
discussion of the question we refer the reader to the account of
the parietal organ in the section devoted to Reptilia.

Fio. 39. — Heads of young ElasmobrancN embryos (Scyllitim canictda) (after Sedgwick).

A. Ventral view of head of embryo, 7 mm. in length, with two open pharyngeal clefts.
The mouth is present as a longitudinal groove in ttie ectoderm of the buccal depression.

B. Same view of a slightly older embryo ; the buccal groove has become a longitudinal
slit. C. Side view of head of embryo, 9 mm. in length, with three open slits. D. Side
view of head of embryo, 11 mm. in length ; rudiments of external gills have appeared on
the hyoid and on the first and second branchial arches. E. Side view of head of embryo
of 16 mm. ; external giUs have appeared on mandibular arch and the angle of the jaw is
marked. 1 mandibular arch ; 2 angle of jaw ; 3 second pharyngeal cleft ; 4 nasal pit ;
5 eye ; 6, midbrain ; 7, auditory sac ; 8 hyoid arch ; 9 spiracle.

The superior commissure is a small nervous development in
the otherwise epithelial roof of the third ventricle just in front
of the attachment of the pineal stalk. The posterior commissure,

the so-called parietal organ (pineal eye). See Dendy, Q. J. M. S., 42,
1899, p. 111. This view is supported to a certain extent by the arrange-
ment in the lamprey (see p. 106).

72 CLASS PISCES.

which probably belongs to the mesencephalon, is just behind the
attachment of the pineal stalk.

The paraphysis is the recess in the roof of the cerebrum caused
by and just in front of the velum transversum. By some mor-
phologists it is regarded as a special glandular organ, the
secretion of which passes into the ventricle. It is not always
present as a distinct structure.

The pituitary body or hypophysis develops as an evagination
of the front part of the buccal cavity. It is indeed the anterior
part of this cavity. In Elasmobranchs the original buccal slit —
for the vertebrate mouth perforation has at first the form of a
longitudinally extended slit (Fig. 39) — is continued into it.
It is applied to the infundibulum and eventually becomes cut
off from the mouth, except in Polypterus and Calamoichthys in
which the buccal opening is retained throughout life. At the
point in the embryo where the pituitary rudiment meets the
infundibulum there is a close approximation and partial fusion
of three other organs, viz., the front end of the gut, the anterior
end of the notochord, and the median part of the premandibular
somite (preoral coelom). The lobi inferiores and saccus vas-
culosus are parts of the infundibulum, very generally present
in fishes. The former are lateral diverticula or thickenings of
the infundibulum ; while the saccus vasculosus, or infundibular
gland, is a glandular dilation of its end, where it is in contact
with the pituitary body.

With regard to the cranial nerves, it ought to be noticed that
they all arise from the walls of the mid- and hind-brain, except
the olfactory and optic nerves. These come off from the pre-
notochordal part of the fore-brain, and it is doubtless to them
that the fore-brain owes its relatively enormous development in
all Vertebrata. The other cranial nerves, from the third nerve
onwards, probably all belong to the series of nerves which is
continued along the spinal cord as the spinal nerves. Indications
of this are shown by a careful study of the early stages of their
growth, particularly in Elasmobranch embryos, in which they
appear to be associated with the cephalic segments of the coelom.

These segments, for a knowledge of which we are indebted to Balfour*
and to the later researches of Van Wijhe.f are as follows : The first cranial

* A Monographof the Development of Elasmobranch Fishes, 'London, 1878.

t *' Ueber die Mesodermsegmente und die Entwickelung der Nerven des

Solachierkopfes," Verhandl. der k.Acad. d. Wissensch. zu Amsterdam, \8H2.

CRANIAL SEGMENTS 73

segment is represented by the premandibular somite* — an unpaired sac
with epithelial walls, immediately in front of the notochord (preoral
head cavity, vide p. 8). The walls of this sac give rise to all the eye-
muscles except the superior obUque and external rectus, and to mesen-
chyme. Its cavity vanishes, as do the cavities of all the cranial segments.
The nerves are the ramus ophthalmicus profundus, which develops from
the nerve crest immediately in front of the trigeminal and represents
the dorsal root, and the third nerve which represents the ventral root.
These two roots are both connected to the ciliary ganglion (see account of
cranial nerves under Elasmobranchii).

The second segment is the mandibular somite which is dilated in its
dorsal muscle-plate region and extends ventrally to the lower end of the
mandibular arch (collar-somite, vide p. 7). The walls of this sac give
rise dorsally to the superior oblique mascle and ventrally to the mesen-
chyme and muscles of the mandibidar arch. The nerves are the trigeminal
and the fourth, the latter being regarded as an abnormally situated anterior
root. The first two cranial somites were discovered by Balfour.

The third and following segments are represented only by their dorsal
muscle-plate sections, the ventral portions being merged in the continuous
splanchnocoel (pericardial division). These segments may be regarded as
the anterior of the trunk series of Amphioxus. They do not apparentlv
communicate with the ventral splanchnocoel, which in this region imder-
goes a pseudo-segmentation in consequence of the formation of the gill-
pouches. These pseudo-segments, or hyoid- and branchial-arch cavities,
open ventrally into the pericardium, of which they are a part, and were
taken by Balfour for the posterior cranial segments. The first of these
muscle-plates, which may be called, from its position, the hyoid myotome,
is better developed than the rest and gives rise to the external rectus
mascle. It was observed by Balfour. The next three, which were
discovered by v. Wijhe, are very faintly marked and transient and give
rise to no muscles, in correspondence with which fact may be noted the
absence of ventral roots. The next three segments (seventh-ninth) are
represented by well-developed muscle plates which persist and give rise to
muscles. The nerves of the hyoid segment (third) are the facial (dorsal
root) and sixth (ventral root). The nerves of the next three segments
are supposed to be represented by the auditory, glossopharyngeal and
vagus and are without ventral roots in correspondence with the absence of
myotome muscles. In the last tliree (or sometimes more) cranial seg-
ments, dorsal roots are present only in the embryo for a short time, but
ventral roots are developed, supplying presimaably the myotome muscles
of this region and called the occipito-spinal nerves. These were mistaken
by Gegenbam-, who did not study the embryo and observe the transient
dorsal roots belonging to them, for ventral roots of the vagus. The
following table represents in brief the view of cranial segmentation
which has just been described.!

* There is in some forms a pair of head cavities in front of the 'pre-
mandibular somite. These are sometimes in communication with and
developed as diverticula of the premandibular somite, but in Acanthias
they are said to be independent of it (.1. B. Piatt, Journal of Morvholom
5, 1891, p. 79). J 1- yj'

t The view here given takes no accoimt of the scheme given on p. 77,
according to which the cranial nerves were originally tripartite, consisting
of dorsal, lateral and ventral roots. It was formulated before the modern

74

CLASS PISCES.

Coelomic Sac

Nerve

Dorsal Root

Ventral Root

Cranial segment

1

Premandibular so-
mite. Its walls
give rise to all the
eye muscles sup-
plied by the third
nerve.

Ramus ophthal-
micus profun-
dus.

Third nerve.

»» »»

2

Mandibular somite.
The walls of its
dorsal part give
rise to the superior
oblique muscle.

Trigeminal.

Foiu'th nerve.

»» »>

3

Hyoid muscle plate
giving rise to ex-
ternal rectus mus-
cle.

Facial.

Sixth nerve.

»> "

4

Muscle plate (tran-
sient).

Auditory.

None.

»» »»

5

Muscle plate (tran-
sient).

Glossopharyn-
geal.

None.

yf )9

6

Muscle plate (tran-
sient).

Vagus.

None.

>f J»

7

^ Muscle plate per-

/-None.

J

"j Spino-occipital

»> »•

8

sistent and giving

nerves (so-

5J J?

9

1 rise to muscles.

called ventral

and sometimes more

J

j vagus roots).

The spinal nerves have two roots, which unite, and the dorsal
of which has a ganglion. The ganglion may, however, be placed
at the junction of the two roots. The cranial nerves are in ten
pairs, but there are often some small nerves — the spino-
occipital nerves (miscalled ventral vagus roots) — arising from
the ventral side at the hind end of the medulla. They pass
out through foramina in the skull, but are perhaps better
regarded as anterior spinal nerves (see above) the dorsal roots
of which are not developed.

analysis of the cranial nerves, which is due to Gaskell and is there referred
to, was fully developed. The later work, which is still incomplete, may
very possi]>ly necessitate a new scheme of cranial segmentation, and the
groups of cranial nerves expressed by the terms fifth, seventh, ninth and
tenth may possibly be found to be connected with the ventral segmenta-
tion (pseudo-segmentation of the text) of the branchial pouches, and to be
independent of the mesoblastic segmentation which is so conspicuous
a feature in the trunk.

CfeANIAL NERVES. SYMPATHEMIC. <0

For an account of a typical arrangement of the cranial nerves
the reader is referred to the section on Elasmobranchii. Their
arrangement in fishes differs from that in higher t}^es, mainly
on account of the presence of the lateral hne sense organs. The
nerves to these appear to arise from a special part of
the brain, the tuherculum acustiaim, from which the auditory
nerves also arise. They are associated in their course to the
periphery with the seventh and tenth nerves, and constitute
the acustico-lateralis system. The fibres of this sj'stem which
run with the fiftli and ninth are derived from these two nerves.
The nerves which pass from the facial roots to the fifth nerve
cause an intermingling of the roots of these two nerves, which
is not easy to unravel, and which is characteristic of fishes.

A sympathetic nervous system appears to be present. In
Marsi'pohranchii, in which all the nerves are without a medullary
sheath, it cannot be fully traced, but the spinal nerves give off
branches which pass to the viscera, where small ganglia are
found. In other Pisces there is a series of sjonpathetic ganglia
which develop as outgrowths of the spinal nerves, becoming
detached from the rudiments of the spinal ganglia at an early
stage. These ganglia are usually connected by longitudinal
commissures, but though regularly developed, their arrange-
ment is not easy to trace in the adult. In Elasmobranchii *
the system tends to take a plexiform structure, and lies in the
neighbourhood of the cardinal veins. There is an especially
large ganglion at about the level of the ductus cuvieri ; this is
supplied by a number of spinal nerves, and gives off several
branches, which are distributed to the viscera with the coeliac
artery. The system appears not to extend into the head. In
Teleostei there is a definite chain of small ganglia on each side
of the vertebral column. In these forms it is continued into
the head, where it is connected with the trigeminal nerve and
ciliary ganglion, and into the taU, where it runs in the caudal
canal.

The analysis of the nerves, which is the outcome of the recent work f
of morphologists and physiologists, is beyond the scope of this work,
but the following points may be noted here :

* R. Chevrel, " Surl' anatomic dusystemenerveuxgrandesympathetique
des Elasmobranches et des poissons osseux." Arch. Zool. Exp. (2) 5,
supplement.

t W. H. Gaskell, "The structure and function of the nerves which iimer-

76 CLASS PISCES.

Five kinds of nerve fibres, characterised by their structure, function
and distribution, may be distinguished.

1. The system of the somatic sensory (afferent) fibres. These include
the largest heavily meduUated fibres which terminate in the skin and
myotome muscles. They pass out by the dorsal roots in the cord, and by
the roots of the trigeminal in the brain ; * their ganglia being the posterior
root ganglia (spinal), and the gasserian.

2. The somatic motor (efferent) system. The fibres of this system are
also large and heavily meduUated : they terminate in the myotome
(somatic) striated muscles ; i.e. the muscles derived from the muscle-
plates, including those derived from the dorsal part of the mandibular
and from the preraandibular somites. They pass out by the anterior
roots in the cord, and by the third, fourth, and sixth cranial nerves, and
are without peripheral ganglia..

3. The visceral sensory (afferent) system {communis system). The
fibres of this system are smaller, and they are distributed to the internal
mucous surfaces. They leave the cord by the posterior roots, their ganglia
here being posterior root ganglia. The cranial fibres of this system are
present in the roots of the fifth, seventh, ninth and tenth nerves ; the
ganglia being the gasserian (in part), geniculate, glossopharyngeal and
jugular ganglia. Their destination is mainly the mucous surfaces of the
anterior part of the alimentary canal.

4. The visceral motor (efferent) system. The fibres of this system may
be subdivided into (a) those which innervate the striated voluntary
muscles (mesenchymatous) of the anterior part of the alimentary canal
(mandibular, branchial, and facial muscles), are fairly large, and are non-
ganglionated, and (6) those which supply the unstriped muscles through-
out the body (blood-vessels, gut-wall, skin, etc.). The latter are small
fibres, all of which pass through peripheral ganglia. The true motor-
fibres to the muscles of the small intestine and anterior part of the alimen-
tary canal and its appendages (lungs, etc. ) are derived from cranial nerves,
whereas the fibres to blood vessels, skin, walls of Miillerian and kidney
ducts come from the spinal cord by anterior roots. The cranial nerves
contain no vaso-motor fibres.

5. The acustico -lateral system (see p. 75). This system consists of
large fibres and passes out exclusively in the roots of the seventh, eighth,
tenth and possibly ninth cranial nerve. It is absent in the trtmk and in
higher Vertebrates, except in the auditory nerve, and is distributed only
to the membranous lab5Tinth and the lateral line sense organs (lateral
line, ampullae and pit organs).

From this account it will be gathered that in the head the visceral
motor (efferent) fibres travel out with the visceral sensory fibres and in
the case of the fifth with the somatic sensory fibres as well, the somatic
motor fibres being distinct ; whereas in the trunk they leave the cord
with the somatic motor fibres.

vate the visceral and vascular systems." Journal of Physiology, 7, 1886;
and "On the cranial nerves" in Journal of Physiology, 10, 1889. O. S.
Strong, " The cranial nerves of Amphibia," Journal of Morphology, 10, 1895;
C. J. Herrick, " The cranial and first spinal nerves of Amphibia," Journal of
Gomp. Neurology, 9, 1899, p. 157.

* The third nerve appears to contain fibres belonging to this system
(muscular sense), and it is possible that a few of them may be contained
in the vagus (Arnold's nerve).

SENSE-ORGANS. 77

To summarise the matter Gaskell has suggested that in the primitive
condition, both in brain and spinal cord, there were three rows of nerve
roots : (1) a dorsal containing somatic sensory fibres, (2) a ventral with
somatic motor fibres, and (3) a lateral row containing both visceral sensory
and visceral motor. This condition is modified in all existing forms in the
cord by the splitting of the lateral roots in such a way that the visceral
sensory roots have joined (1) and the visceral motor roots (2) ; whereas in
the brain the roots of the lateral row have persisted and the somatic sen-
sory roots (restricted to one) have joined them, the somatic motor roots
(three in number) remaining distinct. This scheme does not. however,
take account of the acustico-lateral system.

The Eyes have a flat cornea, and a large almost spherical
lens, the anterior part of which projects far out of the pupil.
Movable eyelids are present in Elasmobranchii, but are absent
from most other fishes. There are no laciymal glands. The
usual eye muscles are present. There is frequently a rete
mirabile, the choroid gland, on the ophthalmic artery as it enters
the eye near the entrance of the optic nerve. The processus
falciformis and campanula halleri are described below under
Teleostei. The eyes are much reduced and functionless in most
adult Marsipobranchs and some cave-dwelling and abyssal
Teleosteans. The Auditory Organ consists of the otocyst
or membranous labyrinth, which is embedded in the side walls
of the auditory region of the skull. It lies in a cavity which
is closed from the cranial cavity in most Elasmobranchs, but
communicates with the latter in Chimaera, and Teleosteans,
Ganoids and Dipnoi. It consists (Fig. 40) of a central
chamber, the vestibule, and of three semicircular canals opening
into the vestibule. The vestibule is divided into two parts
by a constriction ; of these the upper is the utricle, the lower
the saccule. The semicircular canals open into the utricle
while the saccule in some fishes gives off from its posterior end
a process called the lagena, which is an incipient cochlea. In
Chimaera and the Squali the ductus endolymphaticus which
is given off by the saccule opens on the surface of the head.*
Both saccule and utricle contain a chalky mass of otoliths. When
the lagena is well marked its papilla acustica (p/) becomes
separate from the macula acustica sacculi and receives a separate
nerve (vide account of membranous labyrinth under Teleostei).
The membranous labyrinth enters into peculiar relations with

* This is the remains of the aperture of invagination of the embryo.

78

CLASS PISCES.

the air bladder in some Teleostei, which are fully described in

the account of that sub-class.

The Olfactory Organs are a pair of simple pits or sacs, in the

lining of which the
fibres of the olfactory
nerve terminate. In
the 3Iarsipobranchii
the olfactory organ
is partly single and
presents peculiar re-
lations (see account
of Marsipobranchii).
In other fishes each
sac usually has two
openings, both of
which are external
except in Dipnoi.
In Elasmobra n c h s
there is usually only
one opening. The
internal surface of
the sacs is generally
increased by folds of
the mucous mem-
brane.

We know practi-
cally nothing about
the sense of taste.
The tactile sense is
no doubt specially
served by the lips
and their appendages.

aa

Fig. 40. — Right membranous labyrinth of Chimaera, seen
from the median side (from Wiedersheim, after Retzius).
aa ampulla of anterior vertical canal ; ac auditory nerve ;
ade opening of ductus endolymphaticus ; ae ampulla of
horizontal canal ; ap ampuJla of posterior vertical canal ;
ass process of the sinus utriculi ; ca anterior, ci> posterior
vertical canal ; ce horizontal canal ; cr crista acustica
ampullae ; de ductus endolymphaticus, which opens at ade
through the skin ha ; mn macula acustica neglecta ; ms
macula acustica sacculi (the macula ac. utr. rec is on the
other side and not properly visible) ; pi papilla acustica aUQ by SpCCial parts
lagenae (the lagena, however, can hardly be said to be

present in this form) ; branches of auditory nerve raa to 01 tllC appendages
anterior ampulla, rae to horizontal ampulla, rap to pos- • i i

terior ampulla, rec to macula acustica utriculi, rs to wlllCll are ricniy
macula sacculi and lagenae, ru to macula recessus utriculi ; . j. j /

s saccule ; sp sinus utriculi posterior, »s sinus utriculi innervated (e.g.
superior; « utricle. THgla.)

The system of embedded epidermal* sense organs which are
found in all fishes requires a detailed description.

* Le.ydig, Lehrbtich d. Histologie des Menschen u. d. Thiere, 1857. Solger,

LATEEAL LINE.

79

In Elasmobranchs four kinds of organs are included under
this head : (1) the lateral line proper, or mucous canals , with its
cephaHc ramifications ; (2) the ampullary canals, or Lorenzini's
ampullae ; (3) Savins vesicles ; (4) pit organs. The essential

r'.

CSO sot orn.

opr Th Th Gp ! y^^, 7

Fla. 41.— Diagram illustrating the distribution of the dorsal branches of the cranial nerves
and of the lateral line canals, and the position of the groups of ampullae in an Elasmobranch
(:ifter Ewart, from Gegenbaur). A auditory nerve with labyrinth ; it also points to the
groups of Lorenzini's ampullae ; Bu buccal branch of facial ; bu inner branch to part of
infraorbital canal, and to the inner buccal group of ampullae, bu' its outer branch which
supplies part of the infraorbital canal, and the outer buccal group of ampullae ; ch post-
branchial branch of facial to mucous membrane, and giving off motor branches to some
jaw muscles ; CSO, CSO supraorbital canal ; CJO, CJO infraorbital canal ; Fa, Fa'
roots of facial nerve ; Gp glossopharyngeal, arising under cover of the lateralis branch of
the vagus nerve ; Bm hyomandibular canal arising from the infraorbital, and giving off
the mandibular canal, the mandibular group of ampullae is in the angle between these two ;
Hm' branch of the hyomandibular nerve to the hyoid group of ampullae ; in intestinal
branch of vagus with ganglion, where it separates from fourth branchial branch ; In lateralis
branch of vagus nerve ; m mouth ; N nasal sac ; om deep branch of oculomotor giving off
short root of ciliary ganglion (shown, but not marked), the long root is also shown, as
are the short ciliary nerves to the eye ; opr root of ophthalmicus profundus ; opv dorsal
branch of same, giving off long ciliary nerves ; pol second branch of lateralis supplying some
lateral line sense organs and a row of pit organs, the first branch supplies the commissure
connecting the two lateral canals, and some sense organs of the main canal ; sof ophthalmicus
superflcialis facialis, which supplies the supraorbital canal, and the superficial ophthalmic
group of ampullae ; sot ophthalmicus superficialis trigemini ; it arises from the gasserian
ganglion ; sp spiracle ; Tr trigeminus ; F', V^, V^ the first three branchial branches of
the vagus nerve, each with a ganglion and with pharyngeal, prebranchial and post-branchial
branches ; F* the united fourth branchial branch of vagus and intestinal branch ; 1-5
gill-slits.

part of these organs seems in all cases to be sensory patches
of the epidermis, consistmg of sensory cells, bearing short
sensory hairs, and of supporting cells.

Neue Untersvichungen zur Anatomie der Seitenorgane der Fische, Arch
f. mic. Anat., 1879-80. Allis, Anatomy and Development of Lateral
Line system in Amia, Journ. Morphology, 2, 1889. Fritsch, Die electrischen
Fische, Leipzig, 1890. Ewart, Tlie sensory canals of Laemargus, Trans.
Roy. Soc. Edinburgh, 37, p. 59, 1891 ; and The sensory canals of the
skate, Ibid. Pollard, The lateral line system in Siluroids, Zool. Jahrb.
5, 1892. Cole, On tlie cranial nerves and lateral sense organs of fishes,
Trans. Linnean Soc, 1898.

80 CLASS PISCES,

In the lateral line system these sensory patches are modifi-
cations of the lining epithelium of a canal, which extends the
whole length of the body and on to the head, where it branches
in a somewhat complicated manner. The canals lie in the dermis
or deeper in the subcutaneous tissue, and their walls contain
either stiff connective tissue or cartilage (skates) for the purpose
of keeping them permanently open. They communicate at
intervals with the exterior by tubules. The trunk section of the
canal usually lies at the junction of the dorsal and ventral
divisions of the lateral muscles. The sense organs and the
tubules seem to be usually metamerically arranged in the trunk,
and the sense organs and tubules correspond, but in the head
the metameric arrangement is of course out of the question,
and the sense organs appear to be more numerous than the
tubules.

That this system has originated from a skin groove is indicated
by its development and by the fact that in some Elasmobranchs
it has the form of an open groove throughout life. In Chlamy-
doselachus it has the form of a groove guarded by overlapping
scales. In Chimaera it is also an open groove, though in the
head the lips of the groove tend to approximate over the sense
organs (Fig. 42.) In Heptanchus it is a groove in the greater
part of the trunk, but closes into a canal in front and on the
head. The course of tlie cephalic portion in a typical case is
shown in Fig. 41. The lateral canal on reaching the head is
connected with its fellow of the opposite side by a cross canal —
the commissural canal — which may pass in front of or belimd
the openings of the otocysts. A short distance in front of this
it branches into a canal passing above the eye — the supraorbital
canal {CSO) and one passing below the eye, the infraorbital
canal {GJO). The supraorbital canal extends to the front end
of the snout and then passes back to join the infraorbital canal.
The infraorbital canal gives off a branch back to the hyoid region,
called the hyomandibular canal {Hm), which itself gives off a
branch to the mandible. In Chimaera (Fig. 42) the arrangement
is very similar.

In skates the hyomandibular canal is enormously extended backwards
in a loop which lies partly on the dorsal and partly on the ventral surface
of the pectoral fin, and communicates with the exterior by rather long
tubules. In the same animal the lateral line canal near the head gives of?

LORENZINl'S AMPULLAE.

81

two long canals which pass backwards and outwards on the dorsal side of
the fin ; the anterior of these anastomoses with the dorsal part of the
above described extension of the hyomandibvilar canal.

The whole of this system of canals is in Elasmobranchs
supplied by the facial nerve and the lateralis branch of the vagus,
which probably belongs to the facial system (see account of
cranial nerves under Elasmobranchii and Fig. 41).

The ampullary canals or Lorenzini's ampullae, are un-
branched canals (Fig. 43), opening, usually in groups, on the
surface of the head and ending internally in vesicles — the
ampullae — which are beset with radial dilatations (Fig. 44).
The ampullae are placed in groups, the position of which in
a typical case is shown in Fig. 41.

Fio. 42. — Cephalic lateral line of Chimaera (from Gegenbaur). a lateral
groove of trunk, 6, c" infraorbital, c supraorbital groove, c' supraorbital
grooves ; passing back to join infraorbital ; x frontal appendage.

The sensory epithelium is confined to the ampullse to which the
nerves, in all cases branches of the facial, are distributed. The
tubes and ampullae contain a gelatinous matter.

Savi's vesicles are found in Torpedo round the electrical organs
They are completely closed.

The pit organs, found in many Elasmobranchs are sense
organs sunk in pits on different parts of the head and trunk, and
are supplied by the facial nerve, the lateralis of the vagus, and
the trigeminal.

In Teleosteans, Ganoids and Dipnoi the lateral line system
and the pit organs alone are present. The lateral line has an
arrangement very similar to that described for Elasmobranchs,

Z— II G

82

CLASS PISCES.

Fig. 43. — A portion of the snout of Scyllium in section, show-
ing ampullary tubes (from Gegenbaur). N nerve ; a ampullae ;
c epidermis ; t tubes ; c' dermis ; o openings of the tubes ;
o' passage of a tube through the dermis.

but the canal wall is sometimes ossified, especially on the head,
and the ossifications may be fused with the dermal and cranial
bones. Very often the canal traverses the scales and bones, and

the sense organs
are contained in
the osseous tissue.
In such cases the
lateral tubules,
which are in some
cases branched,
their openings
forming so-called
cluster pores, per-
forate the bone,
as does the nerve
going to the sense
organs. In this way certain scales on the body and bones of the
head may acquire a special relation to these organs. Pit organs
are present both on the trunk and head and often lie along the
course of the main canals. In a few cases {Esox, Gobius, Liparis,
etc.) the cephalic canals are alone present, the sense organs in
the trunk being isolated and not cormected by a longitudinal
canal. In a few cases the longitudinal canal
may have the form of a groove for a part
of its extent. The openings of the lateral
tubules may occur between the scales as
well as upon them.

In addition to the innervation found
in Elasmobranchs the glossopharyngeal
frequently sends a branch to a few of the
posterior cephalic sense organs ; and it has
been stated that the ophthalmicus super-
ficialis trigemini also takes part, but this
must be regarded as doubtful. In any
case the nerves innervating this system of
lateral line sense organs can always be
traced to the special centre in the brain
from which the auditory nerve arises. The
pit organs are innervated by the trigemi-
nal as well as by the facial and lateralis of the vagus

Fig

44. — Lorenzini's am-
pulla. A from the side
with nerve n and portion
of tube c ; B in section
(from Gegenbaur).

ELECTRICAL ORGANS.

83

Electrical organs,* the function of which is to develop con-
siderable quantities of electricity, are found in some fishes.
They occur in different parts of the body and in fishes belong-
ing to quite different groups (e.g. Torpedo and Hypnos among
Elasmobranchs, and Gymnotus and Malapterurus among
Teleoste a n s ) . „

They d i ff e r
both in struc-
ture and po-
sition in the
body, but they
always consist
of peculiarly
modified cross-
striped muscu-
lar tissue.

In Torpedo
(Fig. 45) they
are placed be-
tween the bran-
chial pouches
and the anterior
cartilages of tlie
pectoral fins,
and occupy the
whole space be-
tween the dor-
sal and ventral
integum ent.
They consist of
vertically ar-
ran g e d col-
umns, s u p -
ported by walls

of connective tissue, and divided by horizontal septa of the
same material into a number of compartments placed one

Fig. 45. — Torpedo with electric organ EO and brain exposed (aftei
Gegenbaur), dorsal view. On tlie right side the dorsal surface
only of the organ is exposed ; on the left the nerves which supply
it are shown. 6r branchial sacs ; Gr sensory canal tubes of the
skin ; Le electric lobe of the brain ; o eye ; Tr trigeminal nerve ;
V vagus nerve.

* Fritsch, Die electr. Fische, Abt. 1 and 2, Leipzig, 1890. Ballowitz,
Electr. Organ v. Torpedo, Arch. f. niic. Anat., 42, 1893. Sanderson
and Goteh, Elect. Organ of Skate, Journ. Physiology, 10, 1889. Ewart,
Electric Organ of Skate, Phil. Trans., 1888 -fe 1892.

84

CLASS PISCES.

above the other. Each compartment is filled with gelatin-
ous tissue, through the middle of which runs a horizontal plate
composed of a finely granular nucleated substance and of numer-
ous nerve-endings. This is the electrical plate. The electrical
plates correspond to a certain degree to the copper and zinc ele-
ments of the voltaic pile, the gelatinous matter representing the
moist intermediate layers ; while the connective tissue frame-
work serves to hold the parts together and to carry the blood-
vessels and nerves. The face of the plate on which the nerves
ramify is the same in all the columns of the same organ, and is
always electro-negative, the other surface being positive. In
Torpedo the nerve enters on the lower surface of the plates, the

upper surface is therefore electro-
positive. The organ is supplied
by five strong nerves, of which
the anterior is a branch of the
facial, the four posterior being
branches of the vagus group.

In the electric Teleostei the
electric organs are placed in the
trunk and tail, and are supplied
by spinal nerves. They are simi-
larlv constituted, but the col-
umns are horizontally placed. In
Malapterurus they lie along the
body beneath the skin, and the
posterior surface of the plates, the surface on which the nerve
enters, is electro-positive. This apparent exception is explained
by the fact that the nerves pass through the plate and
are distributed on the anterior surface, which is electro-nega-
tive. In the electric eel [Gymnotus electricus) the electric
organ lies at the side of the tail, and consists of long horizontal
columns (Fig. 46).

The so-called pseudo-electric organs found in the tail of Baja
and of Mormyrus have a similar structure, but manifest only
feeble electric phenomena. They constitute a very good example
of an organ which is practically of no use to its possessor, and
which we should entirely fail to understand the meanmg of were
it not for the cases in which the electric organ is fully developed.
Vascular System. — The blood is generally red ; it is white

Fio. 46. — Longitudinal section through
two columns of the electric organ of
Oymnotus. a horizontal partition ;
I transverse partition walls, convex
headwards ; e electric plates (from
Gegenbaur, after Max Schultze).

VASCULAR SYSTEM.

85

only in the Leptoceplialidae (larvae of the eels) ; it circulates in a
3losed vascular system, in which a muscular pulsating region or
heart is present. The heart (Fig. 47) is placed far forward on
the throat, ventral to the branchial
framework, and is enclosed in a
pericardium, the cavity of which
communicates with the body cavity
in some Plagiostomes, Chimaera.
Acipenser, etc. It is a simple
venous branchial heart, and is
composed of a sinus venosus, a
thin-walled large auricle and a very
powerful muscular ventricle. The
sinus venosus receives the venous
blood returning from the body, and
the ventricle forces it through the
ventral aorta to the respiratory
organs. The aorta begins with a
bulbous swelling [hulbus arteriosus)
which in the Ganoids, Plagiostomes
and Dipnoi is replaced by an inde-
pendently pulsating part of the
heart, with rows of semi-lunar
valves (conus arteriosus). While
the fishes with a simple non-mus-
cular bulbus arteriosus have but
two semi-lunar valves at its origin,
the above-mentioned orders usually
have two or four, or rarely five,
rows of three, four, or more valves
each, in the conus
The aorta at once divides into a
number of paired vascular arches,
corresponding to the embryonic
aortic arches. These are the bran-
chial arteries ; they pass into the
branchial arches and give off branches to form the capillary net-
works of the gills. From the capillary networks small vessels
pass out which unite to form a larger vessel in each branchial
arch (epibranchial or efferent branchial artery). The arrange-

nr|-prin«jns! ^^°- ^"^ ■ — Diagram of the circulation of
aiLciiu;5u._. ^ Teleostean. Ab arterial irches ;
Ao aorta desceudens into which the
epibranchial arteries passing out from
the gills unite ; Ba ventral aorta with
the arterial arches which carry the
blood to the gills ; D intestine ; Lk
portal circulation ; N kidneys ; V
ventricle. The branchial capillary
system is omitted.

8G CLASS PISCES.

ment of these vessels corresponds to that of the afferent
branchial arteries ; they unite to form the large aorta descendens
or dorsal aorta. Before they unite the cephalic arteries pass off
from the efferent vessels of the anterior arch.

The arrangement of the principal venous trunks in fishes is
most nearly related to the embryonic condition. Corresponding
to the four cardinal veins of the embryo, two anterior and two
posterior cardinal veins bring back the blood from the anterior
and posterior part of the body respectively. These veins unite on
each side to form two transverse veins — the ductus Cuvieri —
which enter the sinus venosus of the heart. The course of the
returning venous blood is complicated by the insertion of a double
portal circulation. The caudal vein does not pass directly into
the posterior cardinal veins, but breaks up into capillaries in the
kidneys, from which the blood passes into the posterior cardinal
veins. There is thus a renal-portal circulation. For the hepatic
portal circulation on the other hand the venous blood of the
intestine is used ; this blood after passing through the capillaries
of the liver is returned to the heart by one or more veins, which
open into the sinus venosus between the two ductus Cuvieri.
Such capillary systems must be a considerable hindrance to the
circulation of the blood and explain the development of the so-
called accessory hearts on the caudal vein of the eel and on the
portal vein of Myxine.

The urinogenital organs are described under the different sub-
classes. With regard to them the following general remarks may
be made. A pronephros is present and functional in the larva of
all fishes except Elasmobranchii, in which there is no larval stage.
It has been maintained, and a great deal has been written on the
subject, that there is a vestige of a pronephros in the embryos of
Elasmobranchs ; but if there is it is very feebly developed and
never possesses a glomerulus.

The pronephros is the anterior and first developed portion of a
longitudinal gland, which extends, in the embryo at least, the
whole length of the body cavity from the pericardium to the
hind end. This extended excretory organ consists of nephridia,
which in Elasmobranchs are developed, as was first shown by
Sedgwick,* from the portions of the body cavity which connect
the lower ends of the muscle plate cavities with the general body
* Q. J. M. S., 20. 1880, p. 164.

EXCRETORY ORGAXS. 87

cavity. These portions of the body cavity are called nephro-
tomes, and accurately correspond at first with the segments of
the embryonic muscular system. The pronephros is the anterior
end of this excretory organ, which is developed before the rest
to meet larval needs. The serial homology between the prone-
phros and the hinder part of the excretory system was for many
years denied, partly because of a certain difiference in structure
and partly because there is usually a gap between it and the
front end of the rest of the organ. But the differences in struc-
ture are very small, in some cases indeed (e.g. Lepidosteus) do
not exist ; and vestigial nephridia have been found in the gap
between the two organs (cf. especially Price's researches on the
development of the excretory organs of Bdellostoma). Finally,
Brauer's recent researches on the development of the excretory
organs of Gymnophiona remove all doubt on the point.

The hinder part of the excretory system differs from the
pronephros mainly in the fact that the glomerulus— the vas-
cular tuft which secretes the fluid part of the urinary excre-
tion — is segmented into portions, one for each nephridium (or
kidney tubule), instead of forming a continuous structure as in
the proneplu-os ; and the portion of the body cavity containing
each of these is partly shut off from the rest to form the
malpighian body of the kidney tubule. This malpighian capsule,
however, frequently, though not always, retains its connection
with the rest of the body cavity by the so-called nephrostome.
The mternal opening is retained m most Elasmobranchs, but
is lost in Teleostei, Dipnoi, Ganoidei and Marsipobranchii.
This hinder part of the excretory system becomes in
Elasmobranchii much reduced in front and largely developed
behind. In consequence of this it is described as consisting
of the m&sonephros in front and the metanephros behind ; but
this differentiation is not found in other fishes.

As in all Vertebrates, the longitudinal duct (archinephric duct)
is the first part of the excretory organs to appear. The prone-
phros is developed in connection with the front end of this duct,
so that the duct is at first the pronephric duct. The Miillerian
duct is found in all Pisces with the probable exception of the
Marsipobranchii and the Teleostei, and in all cases it becomes
the oviduct in the female, but is reduced in the adult male. In
Ganoids the longitudinal duct joins the oviduct (Miillerian)

88 CLASS PISCES.

before opening externally, but in Elasmobranchii the two ducts
open separately into the cloaca. The development of the Mijl-
lerian duct is known only in Elasmobranchii. It there arises
in connection with the first establishment of the longitudinal
duct as an evagination of the parietal mesoderm of one of the
anterior nephrotomes, so that it at first consists simply of a
funnel-shaped opening of the longitudinal duct into the body
cavity. It soon, however, by a process of gradual shifting,
comes to open further and further back into that duct until it
acquires an independent opening into the cloaca.

The female genital glands, which are, as is usual in Vertebrates,
specialised patches of the lining of the coelom, and of the unseg-
mented * portion of it called the splanchnocoel which persists
as the general body-cavity, dehisce their ova into the body
cavity, whence they escape by the Miillerian ducts — except
in Marsipobranchii and Teleostei. These exceptions however
are doubtful. In Marsipobranchs the genital pores by which
they escape may be Mullerian ducts, though it must be confessed
that there is not much to be said for so regarding them. In
Teleostei the ovaries are generally saccular and continued directly
into their ducts, but in some families they discharge into the body
cavity and the eggs are taken up by two funnel-shaped structures
which join each other and open behind the anus. It is quite
possible, though not definitely proved, that these funnels are
short Miillerian ducts, and that the ducts in the more usual con-
dition, in which they are continuous with the walls of the ovary,
are also Miillerian ducts, which have spread round the ovary or
fused with the edges of a peritoneal recess into which the ovary
has sunk.

The male gonads also are specialised patches of the coelo-
mic lining, but the Marsipobranchii alone retain the primitive
condition of testis dehiscing into the general body cavity,
escape being made by genital pores of unknown homology.

In all others the testis is continuous with its duct. In Teleos-
teans this continuity is very like the continuity found in the
female between the ovary and its duct, and the homology of the
male duct in these animals is not understood. It may be a per-

♦ The contention which has been put forward in some quarters that
the gonads of Elasmobranchs arise from the segmented part of the coelom
cannot be seriously maintained.

GENERATIVE ORGANS. 89

sistent Miillerian duct which has fused with the testis or it may
be something else. In other Pisces, with the apparent exception
of the Ganoid Polypterus, the testis has come to consist of tubules
which are connected by means of a network of tubes, called the
testicular network, with some of the kidney tubules. The con-
nection may take place along the greater part of the length of the
kidney, as in Lepidosteus and Acipenser, or it may be confined to
the anterior region (mesonepliros), as in Elasmobranchs, or
finally, as in Dipnoi, it may only occur through the hind end of
the kidney. The connection is usually through the malpighian
bodies of the renal tubules, but in Amia the tubes from the testis
join the renal tubes beyond the malpighian bodies. In Poly-
pterus alone is there no connection with the kidney, the testis duct
passing directly back from the testis to join the longitudinal duct
near th3 cloaca. This condition in Polypterus is not understood
any more than is the condition of the male Teleostean, though
theories have been put forward to account for it. It may be
that in these forms the Miillerian duct has acquired a connection
with the male gonad and persisted ; or it may be that the con-
nection is really effected through a part of the kidney which has
lost all kidney structure, as has happened in some male Amphibia
and in the higher Vertebrata. A study of development can only
settle the question, and that has not yet successfully been
made.

To return to the longitudinal duct. This, as explained above,
is called at first the pronephric duct, except in Elasmobranchs,
in which it is called the segtnental duct, there being no functional
pronephros. Later, when the kidney is formed and the prone-
phros has atrophied, it becomes the kidney duct. In Elasmo-
branchs, in which the kidney differentiates into meso- and meta-
nephros distinguished, not by any break in continuity, but by size
and by the course of the so-called collecting tubes of the nephridia
(see below), it is called the mesonephric duct, because it appears to
be related more especially to the mesonephric portion of the
kidney. Inasmuch as in the male Elasmobranch this meso-
nephric duct is chiefly concerned with carrying off the spermatozoa
which pass, as has been described above, through a part of the
mesonephros, it is also called the vas deferens. In the higher
classes of Vertebrata the mesonephric duct is called the Wolffian
duct in the embryo, and persists in the male adult as the vas

90 CLASS PISCES.

deferens, but disappears or is reduced to a slight vestige in the
female.

In Elasmobranchs the longitudinal duct is at first called the
segmental duct on the view that the Miillerian duct is segmented
off it. As we have seen, this is not a good description of what
happens, and the name is not a happy one. After the Miillerian
duct has become distinct from it, it becomes the duct of the per-
sisting kidney, and eventually, owing to the shifting back of the
point of opening of the metanephric tubules, the mesonephric
duct.

The nephridia typically open directly by the so-called collect-
ing tubules into the part of the longitudinal duct opposite to
them, but with the differentiation of the metanephros the collect-
ing tubes of the posterior nephridia shift their point of opening
into the longitudinal duct backwards, so that they all come to
open close together into the longitudinal duct — now called
mesonephric duct — close to the cloaca. They are usually re-
ferred to as ureters.

The development of the nephridia of the part of the kidney
behind the pronephros, as direct transformations of a portion
of the coelom occurs only in Elasmobranchs. In other fishes
the development of these tubules is delayed until the myotomes
and adjacent tissues have become functional, and have lost their
primitive relations. The consequence is that the development is
modified and the nephridia (except of the pronephros) are de-
veloped from small nodules of growing tissue, which make
their appearance during larval life in the proper positions.

Abdominal pores, as distinct from generative pores, are pre-
sent in most Elasmobranchs, some Teleostei and in Ganoids,
but they are strangely variable in their occurrence. They never
act as generative outlets, and their function would appear to be
for the outlet of excretory substances of the body-cavity itself.
AsBles has pointed out, they are rarely present in forms in which
the nephrostomes of the kidneys are persistent.

Generative Organs. — Excepting in certain forms, such as
Serranus and Chrysophrys, which are hermaphrodite, fishes
are of separate sexes ; the two sexes sometimes present
external differences. The male and female reproductive
organs often resemble one another so closely in form and position
that it is necessary to investigate their contents in order to dis-

HABITS.

91

I

tinguish the sex, especially as external sexual differences are
frequently absent.

Copulatory organs are only found in male Elasmobranchs, in
the form of long grooved cartilaginous appendages (claspers) of
the pelvic fins.

Most fishes are oviparous ; only a few Teleosteans, as Ditrema,
Zoarces, the Cyprinodonta, etc., and a great number of the sharks,
bear living offspring, which for the most part undergo their em-
bryonic development in a dilated part of the oviduct, which
serves as an uterus. Reproduction usually takes place only once
in the year, most frequently in spring, more rarely in the summer,
and exceptionally, as in many of the Salmonidae, in winter.
Many fishes, especially the males, undergo changes of colour and
develop growths of skin at the spawning time. The two sexes
often assemble in great shoals and seek out shallow places
near the banks of rivers or near the sea coast (Herrings) for spawn-
ing. Some make more extended migrations and pass in large
shoals over great distances along the sea-coast (Tunny-fish).
Others leave the sea and pass up the mouths of rivers, and over-
coming great obstacles (Salmon leaps) make their way up into the
smaller streams, in which they deposit their spa\vn in sheltered
places where the food is plentiful (anadromous, as the Salmon,
Sturgeon, etc.). The Eels, on the other hand, migrate from the
rivers into the sea, and in the following spring the young Eels
enter the fresh waters by millions and pass up stream (kata-
dromous). The spawn is as a rule fertilized in the water, and
thus artificial fertilization and pisciculture are rendered possible.
In the viviparous fish, and in the Rays, Chimaera, and Dogfishes,
which lay large eggs enclosed in a horny shell, a true copulation,
and an mternal fertilization of the egg takes place. It is worthy
of note that in a few exceptional cases the male undertakes the
charge of the brood {Hippocampus, Cottus, Gasterosteus).

The embryonic development of fishes is principally distin-
guished from that of most higher Vertebrates by the fact that
neither amnion nor allantois are developed. Both the small eggs
of the Teleosteans, which are provided with a micropyle, and the
large eggs of the Elasmobranchs, which are surrounded by a hard
horny case, contain a large quantity of food yolk, and undergo a
partial segmentation. The eggs of Cyclostomes, Ganoids and
Dipnoi, however, undergo a total segmentation. As a rule

92 CLASS PISCES.

the young fishes leave the egg-membranes tolerably early, with
more or less distinct remains of the yolk-sac, which still projects
externally, like a hernia. Although the body form of the just-
hatched fish differs essentially from that of the adult animal, no
sudden metamorphosis takes place save in a few exceptional cases.

Most fishes live in the sea, and the number of their species and
genera increases as we approach the equator. But they are not
all exclusively confined to fresh or salt water. Many, as the
Plagiostomes, live almost entirely in the sea ; others, as the
Cyprinidae and Esocidae, are confined to fresh water, but there
are also fish which periodically change their habitat, especially
at spawning time. Some fish live in subterranean waters, and
are blind, like the inhabitants of caves {AmUyopsis spelaeus).
Few fish are able to live any length of time out of water ; as a
rule the wider the gill-slits the quicker does the fish die on dry
land. Fishes with narrow gill-slits (Eels) possess an uncommon
tenacity of life out of water. According to Hancock, a species of
Doras migrates in great shoals over the surface of the ground
from one piece of water to another. Except the Dipnoi, certain
East Indian freshwater fish, the upper pharyngeal bones of
which are hollowed out into the form of a labyrinth (Fig. 38)
and form a multicellular reservoir for water, are capable of living
the longest time out of water {Anabas scandens). There are
even fishes which can float through the air {Exocoetus,
Dactylopterus).

Marine fishes may be distinguished into shore fishes, pelagic
fishes, and deep-sea fishes, which, as in the case of marine and
fresh-water fishes, graduate into one another. Shore fishes live
near the surface, and do not descend to any great depth ; they
are comparatively restricted in range. Pelagic fishes inhabit
the surface waters of the ocean, where they usually spawn,
though some visit the shores for this purpose ; they are usually
strong swimmers and wide ranging, but a few (e.g. Hippocampus,
Antennarius, etc.) are poor swimmers, and infest floating sea-
weed, or drift on the surface. Some pelagic fishes come to the
surface at night only, descending in the daytime to a consider-
able depth [Brama, Sternoptychidae, Scopelus, Astronesthes).
The largest fishes belong to the pelagic fauna, e.g. Rhinodon,
Selache, Carcharodon, Myliobatidae, Thynnus, Xiphiidae, Ortha-
goriscus. The features of deep-sea fishes are referred to below.

DEEP-SEA FISHES. 93

Owing to the uniformity of the conditions of Hfe in the abyss in
different parts of the world, they are probably for the most part
wide ranging.

The greatest depth at which fishes are known to exist is 2,900
fathoms. Many littoral fish descend periodically within the
limits of the deep-sea fauna, but these are not conspicuously
modified. Fishes which habitually live at a depth of 80-120
fathoms, have a black lining to the pharynx and large eyes.
Fishes which belong to the real deep-sea fauna all present very
similar characters, those from 300 fathoms being as much
modified as fishes from 2,000 fathoms. The principal changes
in external conditions to which deep-sea fishes are subjected
as compared with surface fomisare (1) absence of light, (2) still-
ness of the water, (8) constant low temperature, and (4) increase
of pressure. With regard to the latter, it may be stated that
pressure increases by one ton on the square inch for every 1,000
fathoms of depth. The principal bodily characters are as
follows : The eyes are largely developed and luminous organs,
or, to speak more correctly, organs the function of which is
probably to supply luminosity are present. When the supposed
luminous organs are not present the lateral line canals are much
dilated, sometimes into wide cavities, and full of mucus. The
eyes are, however, in some cases reduced or absent. The osseous
and muscular systems are feebly developed ; the bones being
light and provided with little calcareous matter, and the muscles
thin. When the fish are brought to the surface the bones are
found to be but loosely bound together, and the body easily
falls to pieces. This is probably due to the expansion of gases
within the body. The air-bladder presents no special modifica-
tions, and appears to be always without a pneumatic duct, even
in Physostomous forms. It is generally ruptured in fishes brought
up from the deep sea, and in fishes from 80 fathoms it is much
distended, and the eyes protrude and the stomach is everted.
Deep-sea fishes are sometimes found floating on the surface in
a dead or dying condition, and often with the stomach distended
with recently swallowed prey. It is conjectured that such fishes
have accidentally ascended too far above their normal depth,
possibly during the struggle of swallowing their prey which
may be as large as themselves, and then owing to the expansion
of gases consequent on the diminished pressure have been

94 CLASS PISCES.

carried to the surface. Sharks, Rays and flat-fish (with one
exception in each case) ceass below 500 fathoms. Twelve
hundred fathoms is the limit for Holocephali. The eggs of some
deep-sea fishes ascend to and develop at the surface, but in
other cases the development undoubtedly takes place in the
abyss.

Fishes are of great importance to our knowledge of the develop-
ment of animal life on the earth, owing to the frequent appearance
of their fossil remains in all geological periods. In the palaeozoic
formations very singular fish-forms, as the Cephalaspidae
{Cephalaspis, Coccosteus, Pterichthys), constitute the oldest
representatives of the Vertebrata. From the palaeozoic forma-
tions to the chalk we find almost exclusively cartilaginous fishes
and Ganoids, amongst which the forms with persistent notochord
and cartilaginous skull predominate. Ganoids, with a fully
developed bony skeleton, round scales and an externally homo-
cereal caudal fin, appear for the first time in the Jurassic rocks,
where we also find the first Teleosteans. From the chalk
onwards, in the more recent formations, the Teleosteans increase
in number and variety of forms the nearer we approach to
the fauna of the present time.

The class Pisces is divided into the five sub-classes,
Marsipobranchii, Elasmobranchii, Ganoidei, Dipnoi, and
Teleostei.

CHAPTER V.

SUB-CLASS MARSIPOBRANCHII
(CYCLOSTOMATA).*

Vermiform fishes with smooth scaleless skin, cartilaginous
skeleton and persistent notochord ; with suctorial mouth, single
nasal organ, and straight intestine ; without jaws, 'paired appen-
dages, generative ducts, sympathetic system, and conu3 arteriosus.
The unpaired fins are witlwut actinotrichia {dermal fin-rays).

The Marsipohranchii are vermiform in appearance, varying in
length from two feet or more {Bdellostoma) to a few inches
[Petromyzon fluviatilis). The skin is smooth and without scales,
and the skeleton is cartilaginous and notochordal. They are
without paired fins, but possess an unpaired caudal fin {Myxi-
nidae. Fig. 62), to which may be added a dorsal unpaired fin
in the posterior region {Petromyzontidae, Fig. 48). In the

* J. MiJller, Verglcichende Anatomic der Myxinoiden, Berlin, 1835-45.
A. Giinther, Catalogue of the fishes in the British Museum, London, 1870.
C. Kupffer, Die Entwickelung der Petromyzon planeri, Arch. /. mic. Anal.,
35, 1890. P. Fiirbringer, XJnters z. vergl. Anat. d. Muskulatiir. d.
Kopfskelets d. Cyclostomen, Jena. Zeitsch, 9, 1875. W. K. Parker,
Skeleton of Petromyzon and Myxine, Phil. Trans., 1883, p. 373. P.
Langerhans, Unters. iib. Petromyzon planeri, Ber. d. naturf. Gesellsch.
zu Freiburg, 1873. A Schneider,' BeVtrdg-e, z. vergl. Anat. etc. d. Wirbel-
thiere, Berlin, 1879. T. H. Huxley " On the Cranio-facial apparatus
of Petromyzon." Journal of Anat. and Physiology, 10, 1876, pp. 412-28.
F. Ahlborn, Das Gehirn v. Petromyzonten. Z. f. w. Z., 39, 1883,
pp. 191-295, and Hirnnerven v. Petromyzon Z. f. w. Z., 40, 1884, pp.
286-308. G. C. Price, Ontogenie d. Myxinoiden Bdellostoma stouti, Sitz. her.
Math.-phys. klasse d. k. buyer. Akad". d. Wiss, 26, 1896, Munich. .. 7i.
Development of excretory organs of Bdellostoma stouti, Zool. JahrbiKh.
Anat., 10, 1897, p. 207. W. F. R. Weldon, The head-kidney of BdeUos-
toma, Q. J. M. S., 24, 1884, p. 171-182. J. W. Spengel, Die Excretions-
organe von Myxine, Anat. Anz., 13, 1897, p. 49-60. F. C. Studnicka,
Sur les organes parietaux de Petromyzon planeri, Vestnik Ceske Spol. Nauk
Prog, p. 1-50, 1893. J. D. Ogilby, "A Monograph of the Australian
Marsipohranchii ;' Proc. Lin. Soc. N. S. W., 21, 1896, p. 388-426.

96

SUB-CLASS MARSIPOBRANOHII (CYOLOSTOMATA).

Petromyzontidae and in the tail of myxinoids the fins are sup-
ported by cartilaginous somactids. They possess a suctorial
mouth, which is without jaws, but is provided with horny teeth.
By means of it, with the assistance of a suctorial tongue-like
structure they attach themselves to and suck their prey.
Myxine indeed bores its way into the body cavity of other fishes,
and is truly parasitic. The nasal aperture is single, and leads
into an unpaired nasal sac. In this and in other features of
their anatomy, which will be described later, they are unique
amongst Vertebrates. Nevertheless, we shall not follow the
example of some zoologists who liave established the Marsi'po-
hranchii as a separate class of the Vertebrata, distinct from the
class Pisces. We hold them, in spite of the remarkable and
unique features of their organization to be true Pisces, not only

Fio. 48. — a. Pelromyzon fluviatilis (after Heckel and Kner). b, c, d, stages in the transforma'
tion of Ammocoetes branchialis into Petromyzon planeri (after v. Siebold) ; 6 head of an eye-
less larva, side view ; j the same, ventral view ; d later stage with small eyes, side view.

by their aquatic habit of life, but by the characters of their
respiratory and vascular organs. They possess a simple tubular
heart, which distributes the blood by means of a ventral aorta
to the walls of the gill pouches ; and these open to the exterior
on the sides of the body in the ordinary piscine manner. In
the structure of their mouth parts they present some resemblance
to the larvae of anurous Amphibia, but the resemblance is too
vague to permit of any definite approximation to that group
in classification.

The Marsipobranchii fall into two main groups which present
marked points of difference from one another. These are the
Petromyzontidae or lampreys, and the Myxinidae or hag-fishes.
In the Petromyzontidae the nasal sac does not communicate with
the mouth, the eyes are normally developed, and possess eye-
muscles with their corresponding nerves, the pericardium does

LATERAL LINE. MUSCLES. 97

not communicate in the adult with the abdominal part of the
body cavity, and the pronephros does not persist. In the
Myxinidae the nasal sac does communicate with the alimentary
canal by an aperture which perforates the roof of the mouth, the
eyes are much reduced and without the muscles and the cor-
responding cranial nerves, the pericardium communicates with
the general body cavity by a wide opening on the right side,
and the pronephros is persistent in the adult. Moreover, the
Myxinidae possess a contractile dilatation on the portal vein
(portal heart) which is not present in the lampreys.

The skin is slimy, and has the usual vertebrate structure. It
possesses unicellular glands which secrete the mucus. In the
Myxinidae there is in addition on each side of the body and
embedded in the subcutaneous tissue, a row of segmentally
arranged slime-glands, which open on
the surface, and pour out a mucus
containing an immense number of
threads. These threads arise in special
cells of the gland and unwind them-
selves when the mucus is discharged.
They were discovered by Retzius and a/w/'^^^
described and figured by Miiller. C

Nothing of the nature of lateral line
sense-organs has been observed in Fia. 49.— Thread-ceii ofA/yi-me

Ti«- . . 7 1 , • ,1 1 11 glutinosa with unwinding

Myxinidae, but m the lampreys small thread (after Muiier).

sensory eminences, partially sunk in pits,

are found on the head and in two double rows on the body.*

The great lateral muscles are divided up by septa, which have
a zig-zag course, into myomeres of the usual piscine t3rpe. The
myomeres extend on to the head to just behind the eyes. In
the Myxinidae there is in addition a ventral sheet of obliquely
directed muscle-fibres which is unsegmented. There is a
complicated system of muscular bands connected with the
mouth, tongue, and pharynx.

In Petromyzon the portion of the lateral muscles dorsal to the gill-sacs
is continued to just behind the eye and contains a greater number of seg-
ments than the corresponding ventral portion. The ventral part reaches
to just in front of the first gill opening. In Ammocoetea there is one
myomere anterior to the first gill apertvire; this in the adult divides up
into nine or ten myomeres (Schneider).

* Langerhans, op. cit.
Z— II H

98

SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA).

The skeleton consists of cartilaginous, notocliordal, and mem-
branous tissue ; there is no bone. There are two kinds of
cartilage at least, the hard with considerable intercellular
matrix, and the soft with but little.

The vertebral column consists of a persistent notochord with a
tough sheath, which is formed of two layers, an inner somewhat
fibrillated chordal sheath (or memhrana elastica interna) and an
outer thin elastic coat {memhrana elastica externa) ; both are
devoid of nuclei. The notochord so constituted is surrounded
by a nucleated membranous sheath (the so-called skeletogenous
tissue, or memhrana reuniens), which extends dorsalwards on
each side so as to enclose the spinal cord. Small cartilaginous

! / /

Fig. 50. — Cartilaginous skeleton of the anterior part of the body of Petromyzon ftuviatilis ;
side view (after A. Schneider). 1 Foramen for sensory, 2 for motor root of spinal nerve ;
3 eleventh dorsal arcualium ; 4 first '.dorsal arcualium, pierced by the first anterior root,
which passes into the ligament between the fourth and fifth myomere ; 5 foramen for
vagus ; 6 auditory capsule ; 7 foramen for trigeminal ; 8 foramen for optic ; 9 nasal
capsule ; 10 posterior dorsal cartilage ; 11 anterior dorsal cartilage ; 12 annular lip carti-
lage ; 13 anterior lateral cartilage ; 14 styliform cartilage of 12 ; 15 unpaired lingual
cartilage; 16 posterior lateral cartilage; 17 subocular arch; 18 styloid process; 19
cornual cartilage ; 20 branchial basket-work ; 21 the seventh gill aperture (the first and
sixth gill apertures are omitted) ; 22 pericardial cartilage.

pieces are developed on the membrana reuniens on each side :
these, the dorsal arcualia {dorsalia), are roughly segmentally
arranged.

In Petromyzon there are in the branchial and trunk region two pairs of
arcuaha in each segment, while in the tail they are fused to form on each
side a continuous ridge, with which the cartilaginovis fin-rays (somactids)
here present are continuous. Ventraha, fused to a continuous ridge, are
also present in the caudal region, and are continuous ventrally with ventral
somactids. In myxinoids cartilaginous elements are restricted to dorsaUa
and somactids in the caudal region, and to some somactids in the trunk.

Anteriorly the notochord extends into the base of the skull,
ending just behind the pituitary body.

The skull consists of cartilage and membrane. The roof is
entirely membranous in myxinoids, but in Petromyzon there is a
narrow bar of cartilage passing across the posterior part of the

SKULL.

99

otherwise membranous roof. In the floor there is a basi-cranial
fontanelle (Fig. 52, 9) in the anterior (trabecular) region just in
front of the anterior end of the notochord ; this transmits the
pituitary pouch or posterior nasal canal, which, passing back
from the nasal capsule, ends blindly in the Petromyzotitidae
(Fig. 51, 1), but opens into the mouth in the Myxinidae. This
canal lies between the basilar plate and the roof of the mouth.
The olfactory capsule is single and attached to the anterior part
of the cranium by fibrous tissue (Fig. 50, 9). The auditory

Fig. 51. — Longitudinal vertical section through tlie anterior end of Petrotnyzon fluviatilis
(after Huxley) , 1 Blind end of posterior nasal canal ; 2 hinder margin of hard palate (inter-
trabecula) ; 3 cartilaginous roof of skull ; 4 brain ; 7 nasal capsule ; 8 posterior wall of
nasal capsule ; 9 the anterior portion of the subocular arcade ; 10 postero-lateral carti-
lage ; 11 postero-dorsal cartilage (ethmovomerine plate) ; 12 tongue ; 13 anterolateral
cartilage ; 14 anterodorsal cartilage ; 15 annular lip cartilage ; 16 median ventral carti-
lage ; 17 lingual cartilage; 18 ventral division of fifth nerve coming tlirough the sub-
ocular foramen ; 19 cornual cartilage ; 20 posterior part of subocular arch ; 21 styloid
process (hyoid) ; 22 tentaculate branchial valve ; 23 pharyngeal velum ; 24 internal
opening of first Ijranchial pouch ; 25 ditto of second branchial pouch ; 26 suboesophageal
tube ; 27 oesophagus ; 28 notochord ; 29 spinal cord.

y.

capsules (Fig. 50, 6) are attached laterally in the posterior region
on each side of the basilar plate. The subocular arch (Fig. 50, 17)
is a ventro-lateral continuation of the basilar plate and trabe-
cular region on each side ; it contains a fenestra and is supposed
to correspond to the subocular or palato-quadrate arcade of the
Amphibian skull. At the point where the posterior part of this
arch joins the basilar plate, there is given off ventralwards the
styloid process (Fig. 50, 18, and Fig. 51, 21), the end of which in
Petromyzon extends horizontally as the cornual cartilage (Fig.
50, 19). The styloid process and cornual cartilage have been

100

SUB-GLASS MAKSIPOBRAXCHII (CYCLOSTOMATA).

O..A<*^X*^44

C^^A*^JU.—^

compared to the hyoid arch of the higher forms. In the
Petromyzontidae the following additional cartilages are present.
Attached to the anterior end of the base of the skull is a large
median plate of cartilage — the posterior dorsal cartilage
(Fig. 50, 10, and Fig. 51, 11). Immediately in front of this,

and overlapped by it, is the anterior
dorsal cartilage (Fig. 50, 11, and Fig.
51, IJ/)- Just below the latter there
is an annular cartilage (Fig. 50, 12,
and Fig. 51, 15), which lies within the
lips, and from which there projects
back on each side the so-called styli-
form cartilages (Fig. 50, 14)- The
anterior lateral cartilages are paired
rods (Figs. 50 and 51, 13) in relation
with the anterior dorsal cartilage, and
the posterior lateral (Fig. 50, 16, and
Fig. 51, 10) are similarly in relation
with the posterior dorsal.

Finally, in the tongue in the floor

of the mouth there is a median

petromyzon marinus fafter""w! Hngual Cartilage (Fig. 50, 15, and Fig.

K. Parker). 1 posterior dorsal pri ^^\ j -r^pTitrnl fn tTii« tViA en

cartilage (ethmovomerine platp), o 1. , 1 / ) , A;l\U. venuai tO l-nis tne SO-

Sc'uiaS%assil,T be^indTnto called median ventral cartilage (Fig.

;VSioVS^n°™?raKre; 51^ ^^ ; ^ot shown in Fig. 50). The

Lfa'defl*Sid^'o?e'sf'^6 lingual Cartilage is also present in

Its ^rtfoVinvesS'ma'J Myxinoids, in which it is Very largely

(formed by union of parachorial /-l^^-iTplrii-x^rl

cartilages), showing contained ^leveiupeu.

notochord ; 9 basi-cranial fonta-

nelle, or space between the t, ■ uiij.ii.ii j. • i i

unfused posterior portion of ^^ ^^ probable that the posterior dorsal

trabeculae ; 10 cartilage formed cartilage is derived from the fused anterior
by fusion of middle portion of j r j.i. j. i, i j -j. i i

trabeculae (so-called hard palate end of the trabeculae and it has been

of J. Miiller) ; it lies between suggested that the posterior lateral cartil-
mouth and posterior nasal pouch, j.i, i, i r -nr i i> ^-i

and corresponds to the posterior ages are the homologues of Meckel s cartil

intertrabecula of myxinoids. age. The anterior dorsal, and the anterior

laterals, and the annular cartilages are
generally regarded as labials, while the lingual cartilage has been compared
to the basi-hyal ; it is connected with the styloid process (supposed hyoid)
in Myxinoids (Fig. 53).

*io. 52.— Ventral view of skull of

The foramen for the optic nerve is in the side wall of the skull
above the subocular arch (Fig. 50, 8), that for the fifth nerve
just in front of the auditory capsule (Fig. 50, 7), while the seventh

SKULL OF MYXINE.

101

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102 SUB-CLASS MAKSIPOBKANCHII (CYCLOSTOMATA).

nerve passes out through the auditory capsules. Tlie vagus
nerve passes out behind the skull, between it and the first dorsal
piece of the vertebral column (Fig. 50, 6).

In its general features the chondro-cranium of Marsipohranchii resembles
that of other fishes, but is more largely supplemented by membranous
structures, nearly the whole of the roof, the entire occipital region and
the basicranial fontanelle being membranous. From what is known of
the development of the lamprey's skull, it would appear that the basilar
plate is formed by two parachordals between which the notochord hes
(Fig. 52), that these become continuous above and below the notochord
(below only in Myxine), and that to their outer sides the auditory capsules
become attached. In front of the parachordals and continuous with them
are the trabeculae cranii which always remain separate posteriorly, leaving
the basicranial fontanelle, but imite in front to form in Petromyzon (Fig. 52)
the hard palate, the ethmoid and the posterior dorsal plate (ethmovomerine
plate), and in Myxine (Fig. 53, 13) a small median piece in front of the
large basicranial fontanelle. In front of the latter there is in Myxinoids a
pair of cartilaginous horns (Fig. 53, 17) which may be regarded as the
homologues of the posterior lateral cartilages of the lamprey. Moreover
in Myxinoids there are two median cartilages, called intertrabecular car-
tilages not present as separate structures in Petromyzon ; one of these —
the posterior intertrabecular — is a spoon-shaped cartilage lying in the basi-
cranial fontanelle and underlying the naso-palatine canal ; the other,
the anterior intertrabecular, extends in front of this and lies beneath the
nasal canal. In all Marsipobranchs there is a ventre -lateral process of
the hinder part of the trabecular region (anterior lateral process) which
meets and fuses with a corresponding process of the anterior end of the
basilar plate or auditory cartilage (posterior lateral process, said to be
comparable to the pedicle of the Amphibian suspensoriimi. Fig. 53, 7) ;
thus forming the so-called subocular arcade. The subocular arcade closely
resembles the same structure in the Amphibian skull, but against the
comparison of the two is the fact that the ventral division of the fifth
nerve passes dorsal to the arcade in Amphibians, whereas in Marsipo-
hranchii it passes through the fenestra (Fig. 51, 18). In Myxinoids the
hinder part of the subocular arcade contains two fenestrae (Fig. 53,6, 21),
which are not present in lampreys. The supposed hyoid arch arises in
all Marsipobranchs from the hind end of the subocular arcade (Fig. 50, 18 ;
Fig. 53, 23). In lampreys it ends in the expansion of the cornual cartil-
age (Fig. 50, 19) and is not connected with the lingual {15) which is the
supposed median element of the hyoid arch. In Myxinoids it joins the
great lingual cartilage (Fig. 53, 18) which consists of several parts. In
the same group the hinder part of the subocular arcade also gives off
close to the point of origin of the hyoid a bar of cartilage which passes
ventralwards just within the hypoblastic epithelium to join the subocular
arch lower down (Fig. 53, 24:). This structure is not represented in
lampreys and is supposed to be the first branchial arch. The velum in
Myxine is supported by some pieces of cartilage which are in connection
with the upper end of this supposed branchial arch (Fig. 53, 2, 3).
' In Myxinoids the brain lies entirely above the cartilaginous skull, which
is a mere floor, the side walls and roof being entirely formed of membrane.
Moreover in Myxinoids the angle of the subocular arcade is posterior (Fig.

BRANCHIAL SKELETON. 103

53), i.e. the palatine process of the trabecular region is directed backwards
and not merely outwards as in lampreys (Fig. 50). Further the labial
cartilages of the lamprey, viz. the anterior dorsal, the anterior laterals and
the annular are not present in Myxinoids. On the other hand the oral
barbules of the Myxinoids contain a cartilaginous axis and the lingual cartil-
age is enormously developed (Fig. 53), and connected with the styloid
process (supposed hyoid arch).

In the Petromyzontidae there is a branchial basketwork of
cartilage placed superficially, near the skin and supporting the
outer parts of the branchial passages. It consists (Fig. 50) of
eight irregularly curved bars of cartilage placed between the
successive gill sacs.* They are connected dorsally by a longi-
tudinal band of cartilage, which lies along the notochord sheath
and is con inuous with the hind end of the skull, and by three
other longitudinal bands, of which two are lateral, one being
above and one below the branchial apertures, and one is ventral
and partly fused with its fellow. The branchial basket is con-
tinuous posteriorly with a cartilaginous cup which supports the
wall of the pericardium (Fig. 50, 22). The first branchial aper-
ture is behind the first bar, and the seventh or posterior in front
of the last bar.

The branchial basketwork is supposed to be developed in the somato-
pleure and not to be homologous with the branchial arches of other fishes
which lie in the gutwall. The only representative of these structures in
Myxinoids (in addition to the supposed traces of mandibular and hyoid
arches) is the structure described above as the first branchial arch (Fig. 53,

2J:).

The alimentary canal. The mouth, or buccal fumiel (Fig.
54), is suctorial and armed with horny epidermic teeth ; in the
lamprey it is surrounded by a lip carrying short papillae, in
Myxinidae by eight barbules (Fig. 61). On the ventral side of
the mouth is the tongue, which, moving backwards and forw^ards
like a piston, enables the animal to attach itself by its mouth as
by a sucker. The tongue carries teeth (two rows in the myxinoids
on the supralingual cartilages), which enable it to inflict con-
siderable wounds upon its prey. The buccal funnel leads into a
tube, which is supposed to be stomodeal, and may be called the
buccal cavity. This is continued behind into the oesophagus,

* There appears to be some variation in the details of the arrangement
of the branchial basketwork in the different species (cf. W. K. Parker's
account with Schneider's figure).

104

SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA).

which passes back through the pericardial cavity to become
continuous by a valvular aperture with the straight intestine,
which opens at the anus. The anterior end of tlie intestine is
slightly dilated, and receives the opening of the bile-duct, and
in Petromyzon the intestine is provided with a longitudinal fold
or valve, which takes a slightly spiral course,

In Myxinoids the naso-palatine canal (pituitary pouch) opens into the
hinder part of the buccal cavity. This may be taken to mark the junction
of mouth and pharynx. The opening is guarded by an epiglottis-like
valve and directly behind there is a velar membrane of a peculiar form
hanging from the dorsal wall and supported by the so-called pharyngo-
branchial cartilages (Fig. 53, 2, 3). The part of the alimentary canal
immediately succeeding the mouth and receiving the internal gill aper-
tures should be called pharynx, though it is
commonly termed oesophagus. In Petromyzon
a velar fold marks the junction of thesuboeso-
phageal tube or bronchus with the hinder part
of the mouth.

There is a gall bladder, but in the
adult Petromyzon the bile-ducts and gall-
bladder atrophy and the liver cells be-
come filled with fat (Schneider). A
pancreas and spleen appear to be absent,
and the mesentery is very imperfectly
developed. The anus is placed in the
median ventral line in a shallow pit im-
mediately in front of the urogenital

Fig. 54. — Head of Petromyzon oDPninO'
marinus, seen from below "P^ &•

?eethofthe\uifcaf?uJ:r" The respiratory _ Organs consist of a

number of branchial sacs, in which the
branchial lamellae are contained. In Petromyzon there are
seven pairs of these sacs, and each of them opens independ-
ently to the exterior in the anterior region of the body, by
a short external branchial passage (Fig. 48), but internally
they open into a longitudinal suboesophageal tube (bronchus),
which opens into the buccal cavity in front (Fig. 51), and ends
blindly behind. The anterior opening of the suboesophageal
tube is guarded by a membranous velar fold.

In Myxine there are six pairs of branchial sacs (seven have
been observed in rare cases). In Bdellostoma the number is more
variable ; seven pairs appear to be the usual number, but

RESPIRATION. 105

there may be six pairs, or seven on one side and six on the
other (heterotrema), or there may be more than seven (up to
fourteen pairs). In both genera the sacs are connected directly
with the oesophagus (pliarynx) by internal branchial tubes, and
with the exterior by external branchial tubes, but whereas in
Bdellostoma each of the external branchial passages opens separ-
ately by a small aperture (Fig. 61), in Myxine all the external
branchial tubes of the same side are directed backwards, and
unite together before opening to the exterior by a common
opening at the hind end of the branchial region. In both genera
there is on the left side a tube, the oesophageo-cutaneous duct,
which leads directly from the oesophagus behind the last gill
sac, to open with the external branchial tube of the last left
gill sac in Bdellostoma, and with the left gill aperture of
Myxine. The oesophageo-cutaneous duct is much wider than
the external branchial tubes.

In all Marsipobranchs respiration can be effected while the
animal is adhering to foreign objects by the suctorial mouth.
In Petromyzon respiration is effected by taking in water through
the external branchial openings into the branchial sacs, and
then expelling it again by the same way. In Myxinoids the
water is said to enter through the nasal tube, which communi-
cates with the mouth through the posterior nasal passage, and
passes out by the branchial sacs ; but the nasal passage is a
narrow one, and perhaps hardly sufficient to supply all the
respiratory water. Moreover, it would not be available when
the animal's head is buried in the tissues of its prey. It would
appear more probable that some at least of the inspiratory water
enters through the oesphageo-cutaneous duct.

In Petromyzon the branchial basket plays an important part
in respiration. In expiration it is compressed by the trans-
verse muscles ; in inspiration it recovers by its own elasticity.
In Myxinidae it is possible that the huge lingual apparatus may
play some part in bringing about inspiratory dilatation of the
pharynx, but it has been asserted that the inflow of water in
these animals is effected through the nasal canal by ciliary
action.

In the respiration of Ammocoetes water is taken in through the
mouth and passed out by the clefts. The expulsion of the water
is effected by muscular constriction of the branchial region ; the

106

SUB-CLASS MARSIPOBRANOHII (CYCLOSTOMATA).

-IS

inspiration by the dilatation of the branchial region caused
by the elasticity of the branchial basketwork. The double
valve (velar fold) at the junction of the mouth and branchial
portion of the alimentary canal prevents the regurgitation of
water in expiration.

In Petromyzon it has been ob-
served that in every inspiratory
and expiratory movement of the
muscles of the branchial region
water is at the same time taken
in and expelled from the nasal
opening.

The central nervous system
is constructed on the usual
vertebrate type. The brain of
Petromyzon (Fig. 55) is unique
amongst Vertebrata for the fact
that the median part of the roof
of the sylvian aqueduct (iter)
is epithelial and covered by a
choroid plexus. Moreover, the
cerebellum is very small, and
the thalamencephalon of some
length. The third ventricle di-
vides in front into a right and
left canal, each of which, passing
laterally, gives off a branch back-
wards into the cerebrum and
forwards into the olfactory lobe.
The pineal body, or epiphysis,
consists of two vesicles, lying,
the one dorsal to the other, over
the anterior part of the thala-
mencephalon. The larger dorsal
vesicle is the pineal body proper, the ventral smaller one being
called the parietal organ. The dorsal vesicle lies close beneath
the skull-wall, and is the so-called pineal eye. The ventral
part of its walls contains a white, sometimes a black, pigment,
and presents a structure which recalls that of the retina. It is
connected by a solid stalk (pineal stalk) containing nerve-

Fio. 55. — Dorsal view of the brain of
Petromyzon fluviatilis (after Ahlborn).
1 olfactory nerves : 2 left ganglion
habenulae (the two pineal bodies
have been removed) ; 3 continuation
of 2 along the roof of the third ven-
tricle ; 4 swollen termination of 3
which is connected with the ventral
of the two pineal bodies ; 6 fourth
ventricle ; 6 edge of thin roof of fourth
ventricle ; 7 cerebellum ; 8 optic
lobes ; 9 edge of thin roof of iter ;
10 posterior commissure ; 11 right
ganglion habenulae ; 12 cerebral
hemisphere ; 13 olfactory lobe.

BRAIN. CRANIAL NERVES.

107

fibres with the right of two thickenings on the superior com-
missure, called the right ganglion habenulae. The ventral and
smaller vesicle also presents in its lower wall, though not so
markedly, features which recall retinal structure. It is hollow
and is connected with a small solid body on which it lies, and
which is the anterior part of the small left ganglion habenulae
(Fig. 55, 3). These two bodies, though in contact, are apparently
not connected. The pineal stalk is connected to the roof of the
brain just in front of the posterior commissure, while the
parietal organ is attached just anterior to the superior com-
missure. The pineal body lies close to the roof of the skull,
and the skin above it is not pigmented (Fig.
56).

The hypophysis or pituitary body is dorso-
ventrally flattened and follicular in structure.
It lies beneath the infundibulum.

In Myxinoids the corpora bigemina are present in
the normal form and there is no thin place in the
roof of the iter. The thalamencephalon is not nearly
so prominent, the optic lobes being approximated to
the cerebral lobes. The anterior part of the brain is
solid, the central canal not extending beyond the mid-
brain. A small space in the region of the thalamen-
cephalon may be made out but it is quite isolated
from the iter. A pineal body, has, so far, not been
found in the myxinoid brain.

,-Na.

FlO. 56. — Dorsal view
of the head of Pe-
tromyzon planeri

(after Ahlborn). Na
external nasal aper-
ture ; Ep position of
the epiphysis (the
non-pigmented char-
acter of the skin at
this spot is not clearly
shown).

The cranial nerves are fairly normal in
their arrangement. There are ten pairs, but
in Myxinoids the third, fourth and sixth
appear to be entirely absent, in correspond-
ence with the absence of eye-muscles. The optic nerves, which
are very small in Myxinoids, appear not to cross, the chiasma
being hidden in the substance of the brain. In Petromyzon the
sixth nerve arises close to the fifth, and supplies the inferior
rectus as well as the external rectus. There is said to be no
lateral line branch of the vagus in Myxinoids, but in the lam-
preys this nerve is well developed and reaches to the end of
the tail. It is, however, in an unusual position, being placed
far from the skin close to the neural sheath of the vertebral
column, and it appears to be connected with the posterior
roots of the spinal nerves dorsal to which it passes.

108 SUB-CLASS MARISPOBEANGHII (CYCLOSTOMATA).

Tlie fifth nerve divides into two branches, the ophthalmic * which is
purely sensory and passes to the skin on the lioad, and the ventral
' ■' branch, which is both motor

and sensory. The ventral
branch divides into an ex-
ternal and internal branch,
which do not correspond to
the superior and inferior
maxillary branches of other
fishes, for they both supply
muscles which in Selachians
are supplied by the inferior
maxillary branch, t T^®
seventh nerve is in Petromy-
zon a purely sensory nerve ;
in Myxinidae it is mainly
sensory.

The vagus arises by eight
roots, of which the four an-
terior group themselves to-
gether as a nerve which by
its distribution to the tissue
between the first and second
branchial pouches must ob-
viously be compared to the
glossopharyngeal (Fig. 57,
Br^). The other four vagus
roots unite in a ganglion
which is joined by a com-
missural branch from the
seventh nerve (vii.-x.) and
gives off dorsally the lateralis
(lateral line branch) and ven-
trally the visceral branch.
The latter supplies the bran-
chial region and the whole
length of the intestine, in the
wall of which it lies. In the
branchial region in Petromy-
zon, and possibly in the intes-
tinal region as well, the vis-
ceral branch of the vagus is
connected with the posterior
roots of the spinal nerves.
As suggested by Miiller, it
very possibly represents the
sympathetic which is other-
wise absent in Marsipo-
branchs.

Some ventral roots arise

Fig 57.— Diagrammatic dorsal view of the pos-
terior cranial nerves of Petromyzon (after
Ahlborn). o Sensory root of hypoglossal
from the glossopharyngeal ; Au auditory
capsule; Br^ glossopharyngeal; G. sp. 1,
O.SP 2 etc., ganglia on postrior roots of spinal
nerves ; Lai. lateral branch of vagus ; N.X.
vagus roots (including those of the glosso-
pharyngeal) ; N. sp. spinal nerves ; OpMh.
ophthalmic branch of trigeminal ; Pn vagus ;
rd dorsal, r.v ventral ramus of ventral root
of spinal nerve ; vm motor, vs sensory root
of trigeminal ; w.d. dorsal, w.v. ventral roots
of spinal nerves ; VII facial nerve ; Vll-X
branch connecting facial and vagus ;F//i
auditory nerve ; XII hypoglossal ; XII rd.
dorsal branch of XII to muscles of head.

behind the vagus (so-called ventral vagus roots) and imite to form a nerve
* A motor branch is mentioned by some authors.
•]• Furbringer, loc. cit.

SPINAL CORD. NOSE. EYES. 109

(xii.) which supplies the tongue muscles. It is called the hypoglossal, and
gives off near its origin a branch which supplies the anterior part of the
dorsal muscles.

The spinal cord is flattened ; it has neither dorsal nor ventral
fissure, though traces of a dorsal fissure may be represented by
a fine tract of connective tissue which passes from the dorsal
side of the central canal to the dorsal surface of the cord.

The spinal nerves have dorsal and ventral roots which unite
in Myxinoids, but not in Petromyzon. The posterior roots
possess a ganglion, which lies just outside the skeletogenous
wall of the neural canal. All the nerves are Avithout the medul-
lary sheath and the motor fibres are larger than the sensory.

In Petromyzon the dorsal root of the first spinal nerve enters the septum
between the fourth and fifth myomeres,the ventral root divides and supplies
the fourth and fifth myomeres. The motor root of the second spinal
nerve supplies the fifth and sixth myomeres, while the third and subse-
quent spinal nerves each supply one myomere only.

Sense Organs. The external nostril and the nasal sac are
single and median, though the olfactory nerves are double.
From the ventral side of the nasal sac a tube — the nasopalatine
canal — is continued backwards between the brain and the skull
floor, passes through the basicranial fontanelle and ends
blindly on the ventral side of the anterior end of the notochord
in Petromyzontidae, whereas in Myxinidae the same tube opens
posteriorly into the mouth. The palatal opening of this
canal has nothing to do with the posterior nares of higher
vertebrates. It appears to be derived from the pituitary
invagination of the embryo, which arises in Marsipobranchs ,
not as in most Vertebrates from the mouth, but as an
ectodermal invagination in front of the mouth, which secondarily
becomes connected with the nasal pit. It is for this reasoji
sometimes called the pituitary pouch.

The eyes are normal in Petromyzon, and possess the usual
eye muscles.

In the Myxinoids they are extremely reduced and without
eye-muscles. In Bdellostoma they are embedded in a spherical
fatty mass, and placed beneath the skin which is without
pigment immediately over them. In Myxine they lie deeper
within the muscles close to the skull wall.

no SUB-CLASS jSIARSirOBRANCHII (CYCLOSTOMATA).

The eye in Myxine is without any trace of lens and appears to have but
Uttle if any pigment.* It consists of Uttle more than the much degener-
ated optic cup. According to Miiller the optic nerve in Myxine passes
dorsal to the ophthalmic branch of the fifth.

The auditory organ differs from that of other Vertebrata.
In Petromyzontidae it consists of a vestibule and two semi-circular
canals, in Myxinidae of a single semicircular canal only.

The body cavity is divided into two parts, the pericardial
cavity and the general body cavity. These two cavities com-
municate by a wide opening on the right hand side in Myxinidae ;
in Petromyzontidae they communicate in the larva, but not in
the adult. The general body cavity opens into the urinogenital
sinus by two genital pores (one in Myxine), through which
the generative products escape.

In the anterior part of the body there is a system of spacious
venous sinuses. These are specially developed round the
branchial sacs, and in Myxinoids round the ventral aorta, thus
constituting a kind of haemocoelic body cavity for these parts.

The vascular system is arranged essentially in the manner
usually found in fishes. The heart consists of sinus venosus,
large auricle, and ventricle. There are only two valves at the
junction of the ventricle and ventral aorta, and the base of the
ventral aorta [bulbus) is much swollen owing to the large amount
of elastic tissue in its walls. This bulbus is without any muscular
tissue. There is no conus arteriosus. The branches of the
ventral aorta are distributed one to each branchial sac, and tire
efferent branchial vessels are collected into two aortic roots
which are continued forward as carotids, and unite behind to
form the dorsal aorta. The dorsal aorta is also continued
forward in the middle line as a carotid. In Myxinidae the
genital vein and some veins from the anterior part of the body
wall fall into the portal. Moreover, in Myxinidae the portal
vein is dilated into a contractile sinus, which, contracting about
as rapidly as the heart, drives the blood through the liver. It
is a remarkable fact that no muscular fibres can be found in the
walls of this portal sinus (J. Miiller). The portal vein extends
for some distance in the intestinal wall and has been called the
sub intestinal vein. r-

* C. Kohl, " Rudimentare Wirbelthieraugen," Bibl. Zool. (Leuckart&
Chun.), 4, heft 13, 1892.

KIDNEYS.

Ill

The kidneys are not divided into meso- and meta-nephros.
They are placed along the dorsal side of the body cavity for the
middle of its length, being absent at the anterior and posterior
ends. The longitudinal ducts (pronephric ducts) extend along
their whole length and join together posteriorly in Petromyzon
to open by a single opening into the
urogenital sinus. The urogenital sinus
which also receives the two genital pores
opens at the end of a papilla just be-
hind the anus into a depression of the
skin into which the anus also opens.
In Ammocoetes the kidney ducts open
into the hind end of the intestine
(cloaca). The separation of the uro-
genital sinus and the formation of the
genital pores takes place just before
the metamorphosis.

In Myxinidae * there is a shallow
cloaca which receives the opening of the
intestine in front, the wide genital pores
(single in Myxine) on the dorsal side of
the anus, and the two urinary ducts,
opening close together at the end of a
papilla behind.

In Myxine the kidneys have an ex-
ceedingly simple structure. The longi-
tudinal ducts give off at segmental
intervals short lateral tubes which open
into large malpighian bodies. The
glomeruli are multipolar, i.e., are con-
nected at several places with the wall
of the capsule. In Petromyzon the
structure is very similar but more
complicated, though the malpigliian
capsules of successive tubules are separate, the vascular tissue
of the glomeruli is continuous.

The tubules of the kidney do not open into the body cavity
by nephrostomata.

Fig. 58. — Portions of the kidney
of Myxine (after J. Muller).
B represents a portion of
A highly magnified ; a pro-
nephric (longitudinal) duct ;
b kidney tubule ; c glome-
rulus ; d afferent, e efferent
artery.

* R. H. Burne, " Porus genitatis in Myxinidae,''^ Journ, Linn. Soc, 26,
1898, p. 487.

112 SUB-CLAS SMARSIPOBRANCHII (CYCLOSTOMATA).

Abdominal pores as distinct from the genital pores appear
to be absent.

In Myxinidae the pronephros persists as a lobulated organ
in the pericardial cavity of the adult, and was called by Miiller
the suprarenal body. In Petromyzon it is quite absent in the
adult through present in the larva.

The pronephros of Petromyzon is developed in the embryo and has three
or four body-cavity openings and a continuous glomerulus. It is in rela-
tion with the pericardium and atrophies during the Ammocoetes stage.
The kidney is developed in the young larva posteriorly, its anterior
end being a short distance behind the pronephros. The tubules arise
as excavations in the mesoblastic tissues. This larval kidney of the
Ammocoetes atrophies after the metamorphosis and is replaced by an
exactly similar structure placed further back.

The pronephros of the adult Myxinoid consists of a large number of
nephrostomes which end blindly internally in a mass of lymphoid tissue,
or possibly in some cases, perhaps in young specimens, open internally
into an isolated anterior portion of the pronephric (segmental) duct. There
is also at the hind end of this organ a glomerulus of some size projecting
into an open recess of the pericardial cavity.

The whole excretory system of Bdellostoma * appears to develop in the
same way in which the pronephros does in other types, that is to say the
longitudinal dvict (pronephric) and the excretory tubules (segmental
tubules) arise in continuity with each other from the body-cavity epithe-
lium. The parts of the body-cavity into which the segmental tubes open
soon become separate from the rest and form a series of small vesicles each
communicating with a segmental tube. These vesicles become the malpi-
ghian bodies and the segmental tubes become the renal tubules. It is
not known how the pronephric part of the system acquires the peculiar
structure which it exhibits in the adult.

As stated above the persistent kidneys of Marsipobranchs show no
differentiation into meso- and meta-nephros and the testes are not con-
nected with them.

The generative organs are unpaired. They are attached to
the dorsal wall of the body cavity by a broad membrane, and
the generative products, both male and female, are shed into the
body cavity, whence they escape by the genital pores. Myxine
is hermaphrodite, and the reproductive gland produces sper-
matozoa before ova f (protandrous). Petromyzon is dioecious,
but ova have been observed in the testis.

* Price, op. cit.

t J. T. Curm^ingham, *' Reproductive elements in Myxine ghitinosa"
(Q. J. M. S., 27, 1887), and " Spermatogenesis in Myxine " {Q.J. M. S., 33,
1891). The hermaphroditism of Myxine is denied by Dean {Journ. Coll
Sci., Tokyo, 19, 1904).

DEVELOPMENT 113

The development has been fully worked out in Petromyzon planeri.*
The egg is small, about 1 mm. in diameter. It is enclosed in a membrane
formed of an inner perforated and an outer structureless layer. Outside
there is a mucous envelope which causes the egg to adhere to foreign objects.
The male adheres to the female during oviposition and the ova are depos-
ited in a hole previously made and subsequently covered up, the fish moving
stones for this purpose by means of their suctorial mouths. The eggs are
laid in April and May. Segmentation is unequal but complete ; the gastrula
is formed by a combination of invagination and epibole, and the blasto-
pore persists as the anus. The central nervous system is formed by a solid
keel-like ectodermal ingrowth, in which the central canal arises by excava-
tion, and there is a solid cord of cells connecting the hind end of it with the
dorsal hypoblast. The pituitary body is formed from an invagination of
the ectoderm and subsequently becomes connected with the nasal pit,
with which its proximal part remains in communication throughout life,
as the naso-palatine canal or pituitary pouch.

The yovmg are hatched as larvae which soon become Ammocoetes. These
live for three or four years, developing and increasing in size until they
become as large as or larger than the adult. They then undergo a sudden
(in three or four days) metamorphosis (from August to January) and
become adult (Fig. 48). The adult possesses ripe generative organs and
spawas in April-May. After spawning the lamprey (in the case of P. fltwi-
atilis at least) dies.

Ainmocoetes was formerly regarded as a distinct genus of
animal, and separate species were distinguished. The fact that
it becomes metamorphosed into Petromyzon was discovered two
hundred years ago by L. Baldner, a fisherman of Strasburg,
and rediscovered by Aug. Miiller.t

In Ammocoetes the buccal cavity is without the annular lip,
but possesses a semicircular upper lip (Fig. 59), and a small
separate lower lip. There are no teeth, but several fringed
barb<^ls surround the mouth. The eyes are imperfect and
hidden beneath the skin. The gill openings are placed in a
groove on each side. The median fin extends all along the back,
as a continuous structure. The branchial pouches open into
the pharynx directly, and there is no suboesophageal tube or
bronchus distinct from the pharynx. In Ammocoetes there is
a gall bladder and bile duct, which opens into the intestine.
In the lamprey both these structures are absorbed, and the
intestine itself undergoes partial atrophy. The eye in Ammo-

* F. M. Balfour, A Treatise on Comparative Embryology, vol. ii. 1881
(with literature to date). A. E. Shipley, Q. J. M. S., 27, 1887. C. Kupffer,
Arch. mic. Anat., 35, 1890. P. Bujor, " Metamorphose de V Ammocoetes
hranchialis,^^ Rev. Biol. Nord. France, iv. 1891, p. 41.

t Miiller's Arch., 1856, p. 325.

z — II. I

114

SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA).

.-;;|iiipj;jtt|i)^|^:.^
^^'!»iii*p||Sil<(.-i!:

Flo. 59.— A. Ammo-
coetes of Petromy-
zon planeri, 2 ins.
long, side view
(after W. K. Park-
er) brl first, br7
seventli branchial
aperture ; na na-
sal aperture; «eye.

coetes is only partly developed and lies be-
neath the skin. In the lamprey it becomes
fully developed and travels to the surface. The
pericardium of Ammocoetes opens into the
general body cavity, but the two become
completely separate in the adult. The an-
terior part of the kidney which had been
developed in the Ammocoetes disappears, and
a fresh posterior part is formed. The prone-
phros had already begun to disappear dur-
ing the Ammocoetes stage, and the portion
of the cloaca into which the urinary ducts
open becomes separated ofif as a urogenital
sinus shortly before the metamorphosis.

The skeleton undergoes very considerable
change at the metamorphosis. The Ammo-
coetes is without the cartilaginous dorsalia or
neural arches in the trunk region. These
appear at the metamorphosis as do cartilages
of the mouth, and the side walls and roof of
the skull. The spinal cord, which is nearly
round in section in Ammocoetes, becomes
flattened at the metamorphosis.

The head muscles of Ammocoetes are entirely
destroyed and reformed at
the metamorphosis.

The thyroid body arises in
the embryo as a groove in
the branchial region of the
g pharynx. The opening soon
becomes narrowed to a pore
placed between the second
and third permanent* branch-
ial pouches (Fig. 60). In Am-
mocoetes the tube so formed
becomes divided and assumes

B Ventral view of
the head of the
same larva.

* There is said to be a trace of an
eighth pouch, in front of the first
permanent one, in the embryo and young larva. It is supposed to repre-
sent the hyomandibular cleft of other fishes, but it never acquires gill folds
or an external opening.

HABITS. AFFINITIES.

115

a very complicated glandular form. After the metamorphosis
it is less conspicuous and appears to be without the opening
into the throat.

In Myo:inidae the eggs are very much larger (19 mm. x 7 mm.
in Myxine, 31 mm. x 95 mm. in Bdellostoma) and contain a
considerable quantity of yolk. They are enclosed in a horny
case with hooked processes proceeding in tufts from each end.
The egg case appears to be the vitelline membrane.

The Ammocoete lives buried in mud and sand and likes
dark places. It lives on small aquatic organisms {hifusoriaf
Daphnia, Rotifers, etc.). The marine lamprey ascends rivers
at spawning time, sometimes carried by the salmon or shad
{Alausa vulgaris), to spawn. They eat worms and small aquatic
animals. The Myxinidae live exclusively on other fishes. They
are able by their
formidable dental
armature and
powerful lingual
muscles to make
their way into the
body cavity of
their prey, in which
they are sometimes
found embedded.

The Marsipo-
branchii are sometimes spoken of as a degenerate group.
We do not think that there is any evidence of degenera-
tion. The most important points in which they differ from other
fishes relate to the skeleton, and to the nasal organ. But these
are precisely the organs which show the greatest amount of
variation within the group as at present constituted. This
seems to point to the fact that they separated off from other
fishes at a time when these two organs were in a highly indeter-
minate condition, and had not attained to that fixitj^ of structure
which characterises on the whole the general arrangement of the
skeleton, and nasal and pituitary sacs in other fishes.

The condition of the eyes in Myxinidae might be held to be evidence of
degeneration, but we should rather be inclined to regard it again as the
survival from a time when the visual organ was more variable and had
not obtained that fixity of character which it has at the present day. No

Fio. 60. — Diagrammatic longitudinal sjction through the
head of a larva of Petromyzon (after Balfour). Ab optic
vesicle ; C heart ; Ch notochord ; H thyroid involution ;
Ks branchial pouches ; N nervous system ; O mouth ;
01 olfactory pit ; Ot auditory vesicle (represented as
visible) ; Ve velum. ■

IK)

SUB-CLASS MAKSIPOBRANCHII (CYCLOSTOMATA).

doubt many individuals were then produced with imperfect visual organs.
Most of these naturally died out in competition with their more highly
endowed brethren, but in some cases compensating advantages in other
organ? enabled them to hold their own in spite of their defective sight.
To hold that a free-living animal, and a myxinoid must after all be regarded
as such, can lose its eyes through disuse would seem to be an impossible
position. The absence cannot be considered as other than a disadvantage
to it.

Fam. 1. Petromyzontidae {Hy-
peroartia), lampreys, nine-eyes.
With seven external gill apertures
on each side which lead into a sub-
pharyngeal tube opening anteriorly
into the pharynx and ending
blindly behind, and with a com-
plete branchial basket-work, i The
suctorial mouth is surrounded by
a circular lip and is provided with
horny, simple or multicuspid teeth,
without barbules. Tlie single nasal
opening is in the middle of the up-
per side of the head, and the nasal
duct (pituitary sac) ends blindly
behind. Eyes are present. With
two dorsal fins, and a spiral valve in
the intestine ; gall-bladder absent.
The eggs are small, and there is a
prolonged larval stage in which
the young are known as Ammo-
coetes. Fresh waters and coasts
of the temperate regions of both
hemispheres. Petromyzon Art.
coasts and fresh-waters of the
northern hemisphere ; P. marinus
L., sea-lamprey ; P. fluviatilis L.,
river-lamprey. Ichthyomyzon Gir-
ard, west coast of North America ;
Mordacia Gray, without gular
pouch, coasts of Chile, south-east
AustraUa and Tasmania, entering
fresh-water to breed ; Geotria, Gray,
with gular pouch, rivers of Chile,
south and south-east Australia and
New Zealand ; Velasia Gray, with-
out gular pouch, is an immature
stage of Geotria.
Nasal aperture single, at the anterior

Fig. 61. — Ventral view
of anterior end of
Bdellostoma forsteri
(after W. K. Parker)
br p. external aper-
tures of branchial
sacs ; d. oe c opening
of ductus oesopha-
geus cutaneus.

l^'iG. 62.—Myx-
ine glutinosa
(Regne lAni-
mal).

Fam. 2. Myxinidae (Hyperotreta).

end of the body ; the nasal duct (pituitary sac) opens posteriorly into the
pharynx and has cartilaginous rings ; mouth suctorial, without lips, with
barbules, with one median palatal tooth and two rows of lingual teeth ;
branchial openings far behind the head, lead directly into pharynx ; branch-
ial basket-work not present ; a series of mucous sacs on each side of the
bodj' ; eyes hidden under the skin, and very imperfect, without lens or

MYXINIDAE.

117

muscles ; intestine without spiral valve ; gall bladder present ; egg large
with horny case provided with threads for adhesion ; marine in temperate
regions of both hemispheres. Myxine L., hag-fish, with six pairs of bran-
chial sacs opening by one external opening on each side. Bdellostoma
J. Miill. {Homea Fleming), with six or more (up to fourteen) branchia,

Fig. 63. — Palaeospondylus gunni ventral view of head and side
view of vertebral column. tp trabeculo-palatine part of
skull ; pa parachordal part of skull ; d c, Ic, vc oral cirri ; a, 6, c,
n markings of more uncertain significance ; x post occipital
plates (from S. Woodward).

apertures on each side, each leading to a branchial sac ; the number of
branchial sac3 may be different on the two sides of the body ; southern
hemisphere. Palaeospondylus Traquair, from the old Red Sandstone of
Scotland, is supposed to be a fossil Marsipobranch with calcified cartila-
ginous endoskeleton (Fig. 63). The notochordal sheath appears to have

contained rings.

CHAPTER VI.

SUB-CLASS ELASMOBRANCHII.*

Fishes with a cartilaginous endosJceleton, 'placoid scales, and
abdominal pelvic fins provided with claspers in the male. There
is a conus arteriosus, an optic chiasma and a spiral valve in the
intestine. There is no air-hladder. The eggs are large, and, except
in Laemargus, provided with a horny case. In the embryo the
gills project from the gill clefts as filaments.

The Elasmobranchii or cartilaginous fishes include the sharks
and rays. With the exception of one or two sharks and a few
rays they are entirely marine forms. They are remarkable for
possessing more features which are embryonic in the higher
purely terrestrial Vertebrata than any other group of fishes.
Of such may be mentioned the oro-nasal groove, the opening
between the membranous labyrinth and the exterior, the un-
covered gill apertures, the open spiracle, the cartilaginous
skeleton, the opening between the pericardium and the body
cavity. Lastly they are the only fishes which possess eggs
containing so much yolk that the whole development is embryonic.

Only two species of shark are known to be exclusively
inhabitants of fresh-water {Carcharias nicaraguensis and gan-
geticus), but several ascend large rivers, e.g., the Tigris and
Ganges, to a considerable distance. Most Selachians are pelagic
or shore forms, and some descend to great depths [Scyllium has
been taken at 700, Chlamydoselachus at 100-150, Centroscyllium
at 245, Pnstiurus at 500, and Centrophorus at 345-500

* See Giinther, Day, Jordan and Evermann, Bridge, Boulenger, cited
under Pisces ; J. Miiller and J. Henle, Systematische Beschreibung der
Plagiostomen, Berlin, 1839. F. M. Balfour, Development of Elasmohranch
Fishes, London, 1878.

EXTERNAL FEATURES. 119

fathoms). Their flesh is not usually esteemed as food, but some
of them are eaten by poor people.

The body is elongated and spindle-shaped in the Squah, the
anterior part being somewhat broad and depressed dorso-ven-
trally as compared with the narrower posterior region ; in the
Raji it is strongly compressed dorso-ventrally. In some forms,
mostly in the Raji, the snout is prolonged to a greater or less
extent. This is most marke.dly the case in the saw-fish shark
Pristiophorus, and in the saw-fish Pristis. In the hammer-
heads the anterior part of the head is elongated transversely,
the eyes being placed at the ends of the prolongations. The
median fins are typically two dorsal, a caudal, the ventral
part of which is divided by a notch into two parts, and an anal
placed between the caudal and the anus. The paired fins are
well-developed ; the pelvic being smaller than the pectoral,

Fig. 64. — Acatithias vulgaris, spl spiracle ; it* gill slits (from Claus).

and abdominal in position. In the males the pelvic fins are
provided, each with a copulatory appendage — the clasper
{pterygopodium, mixipterygium), which is grooved on its dorsal
side, the groove leading into a cavity at the base of the appendage.
In the Raji the pectoral fins are very large and their line of
attachment to the body has a considerable antero-posterior
extension. The muscular system is on the usual piscine type.
The great lateral muscle is divided into a dorsal and ventral half,
the myomeres of which alternate. There is the usual system
of branchial and mandibular muscles.

The gill-clefts are tubes usually five on each side (in Chlamy-
doselachus and Hexanchus there are six, in Heplanchus seven),
and their external openings which are placed laterally in Squali,
ventrally in Raji, are not covered by an operculum (Fig. 64).
Internally they open into the pharynx and their walls are pro-

120 SUB-CLASS ELASMOBRANCHII.

vided with a number of lamellate folds of the mucous mem-
brane, which are placed on their anterior and posterior walls
(except in the last tube, which has no branchial lamellae
on its posterior wall), and are attached through their whole
length (Fig. 118), not projecting freely as do the pectinate gill
processes of Teleosiei. In addition there is usually an anterior
tube leading outwards from the pharynx and opening exter-
nally on the dorsal surface close behind the eye. This opening
is called the spiracle and the tube itself must be regarded as
belonging to the series of brancliial tubes of which it is the first.
It differs from these, however, in never possessing branchial
lamellae, though it often has traces of these as a few small folds
of the lining of its anterior wall, which constitute the fseudo-
hranch or mandibular gill of these fishes. In the embryo long
filaments — the so-called external gills — project from all these
openings including the spiracle ; they are in reality externally
projecting internal gills.

In Raji the spiracle is much larger than in Squali and it doubtless allows
of the entrance of water into the pharynx when the animal is lying flat upon
the ground or partly buried in sand. In Squali, in which it is very variable,
being sometimes absent and nearly always small, its function is not clear.
In some species in which it is very small it may be present or absent in
different individuals. It is sometimes present in embryos of forms in which
it is absent in the adult (Carcharias), but whether this is always the case
is not known. From the fact that it is smaller than the posterior branchial
apertures even at its first appearance (which is subsequent to that of the
others) it may be presumed that it is usually absent in such cases, but the
matter wants looking into. When it is absent in the adult and present in
the embryo, it is without projecting gill filaments in the embryo (Miiller);
In Scyllium, Pristiurus, Mustelus, etc., the spiracle gives off a diverticulum
to the auditory cartilage of the skull.

The nasal apertures and mouth are almost always placed
on the ventral surface of the head (in Chlamydoselachus the mouth
is anterior and the nasal apertures are dorsal), usually at a
considerable distance from the front end. The nasal apertures
are frequently connected with the mouth by a groove, the oro-
nasal groove, and sometimes they are so close that their open-
ings are confluent with the mouth. In other cases they are at
some distance from the mouth and there is no oro-nasal groove.
The anus (cloacal opening) is placed between the pelvic fins,
and there are two abdominal pores, one on either side of the
anal opening, which lead into the body-cavity.

SENSE ORGANS. SKIN.

121

Abdominal pores vary considerably.* In some species they are absent
altogether, in others they are present in the adult, while in yet others
they are present in some individuals and not in others. Their external
opening is always on an ectodermal surface, either just outside the
cloacal boundary, or into a cloacal pouch, which is a diverticulum of
the proctodeal part of the cloaca.

The eyes are usually provided with upper and lower cutaneous
folds which represent eyelids, and in some forms there is a third
inner eyelid or nictitating membrane which can be dra^vn over
the eye.

The otocysts retain their communication with the exterior
by means of a canal, the
aqueductiLS vestibnli, which
opens on the dorsal surface
throughout life (p. 77). The
lateral line is a canal which
extends in the skin from the
very hind end of the body to
the head, where it branches
out to different parts m the
usual piscine manner (p. 80).
It opens to the exterior at
intervals. In addition to the
system of the lateral line,
there are the openings of the
so-called ampullary canals.
These are placed in groups in
the head (Lorenzini's ampul-
lae, p. 81). Luminous or-
gans t irregularly scattered

over the body are found in many pelagic members of the
Spinacidae (e.g., Spinax, Laemargus, Isistius) in the form
of minute cutaneous patches which probably secrete a
luminous mucus.

The skin is tough and rough owing to the presence of a vast
number of placoid scales. These are rhombic bony plates
embedded in the cutis and carrying a small spine, which

* Bles, " Correlated Distribution of Abdominal Pores and Nephrostomes,"
Journ. Anat. and Phys., 32, 1838, p. 484,

t R. Burckhardt, "Luminous Organs of Selachians, Anyi. and Mag. Nat.
Uist. (7) 6, 1900, p. 558-568.

Fig. 65. — Placoid scales of an adult SeyUium
in surface view (after Klaatsch). The
anterior end of the figure is uppermost.
The spines are omitted from some of the
scales. Ck the central canal (pulp cavity)
of the spine as it perforates the basa!
plate Sb of the scale ; Sa spine of the
scale.

122 SUB-CLASS ELASMOBRANCHII.

projects freely on the surface in a backward direction and
consists of dentine containing a pulp cavity and capped by
enamel. The placoid scales though numerous are not in con-
tact, and fresh scales are continually being developed between
them, to replace those worn off. The presence of these spines
enables the skin of Plagiostomes to be used by polishers
(shagreen). The spines are sometimes much enlarged, e.g. the
peculiar spines on male skates, the caudal spine of the
sting ray {Trygon), the large spines often present on the
dorsal fins, etc. The teeth in the mouth are special modifi-
cations of placoid scales.

The endoskeleton is entirely cartilaginous, but the cartilage

is frequently more or less calcified.

It is possible that
perichondria! ossifi-
cation takes place
in some forms in
the bodies of the
vertebrae between
the arches (see p.
124), bnt the tissue
resulting is without
cells. This does
not however pre-

„ „ ^ elude its being

Fia. 66. — Longitudinal section through the vertebral column , t ' T ^

of Squat.ina vulgaris (after Hasse from Gegenbaur). a verte- bone, lOr m leieos-

bral body, showing concentric calcified lamellae (cyclospondy- tei, etc., undoubt-

lous) ; iv intervertebral ligament ; ch notochord ; fc attenu- rl v. '

ated part of notochord. The double calcified cones d are ®^ Done occurs

shaded dark. without any bone

cells, or haversian
canals. Anatomists are divided on the point, but the preponder-
ance of opinion is in favour of the absence of osseous tissue in Elasmo-
branchs. KoUiker * takes this view. Gotte f on the other hand holds
that the calcified cartilage is true bone. There can be little doubt that
bone is quite distinct from cartilage and always arises from elements. out-
side it, possibly as dermal plates. These elements may invade the cartilage
and bring about so-called cartilaginous ossification or they may always lie
outside it, giving rise to membrane bones proper, f

The vertebral column § (p. 58) presents the most remarkable

* Ueb. d. Wirbel der Selachier, Abh. Senckenb. Ges. 5.

t Arch. f. mic. Anat., 1878.

j Stephan, Bull. Sci. France et Belgique, 1900, p. 281.

§ For a detailed account of the vertebral column of Plagiostomes "^see
C. Hasse, Das natiirliche System der Elasmobranchier, etc., Jena, 1879, 1882
and 1885, and especially A. Kolliker " Uber die Beziehung der Chorda zur
Bildung der Wirbel der Selachier, " etc., Verhandl. der physik. medic.
Oesellsch. zu Wiirzburg, 10, 1860, and " Weitere Beobachtungen," etc.,
Abhdlg. der Senckenherg. Oesellsch. zu Frankfurt, 5.

VERTEBRAL COLUMN. 123

variations which would, in any other group, be regarded
as of great morphological importance. It varies in the
extent and uniformity of its segmentation, in the arrangement
and number of the cartilaginous pieces which enclose the spinal
canal, and their mode of attachment to the vertebral central
and in the extent of the calcification of the cartilage. It con-
sists typically of a series of amphicoelous vertebrae, through
the centra and intervertebral ligaments of which runs the per-
sistent but reduced notochord. The neural arches of each
vertebra always consist of more than one piece of cartilage on
each side ; and the haemal arches extend outwards in the trunk
region where they carry short ribs, and downwards, meeting
each other ventrally below the caudal artery and vein, in the
caudal region. The vertebral column is

formed of hyaline and fibro-cartilage, |g4 ;> •,*.... .-^^-''f

which tissues pass into one another
quite gradually.

A tough fibrous membrane, containing
cells and surrounded by the elastica externa,
is formed (see p. 58) at a comparatively
early stage, round the notochord and is

called the chordal sheath (Fig. 33). This in pio, 67.— Three pos-

most Plagiostomes becomes differentiated terior trunk verte-

• ■ ,, ., liz'i ji brae of Centrophorus

mto alternately short fibrous and longer (after Hasse from Geg-

cartilaginous portions, i.e. segmented (Fig. enbaur). n neural arch,

cc\ rrv. cu i- u iu • i. with foramen for an-

bb). Ihe fibrous portions become the inter- terior root • m inter-

vertebral ligaments, while the cartilaginous calated piece with

portions form the bodies or centra of the rootT^^ iTaemalarch.

vertebrae ; so that the vertebrae are described

as being chordo-centroiia. The centra are, however, frequently reinforced
by the arch tissue in the manner described on p. 59.

The cartilaginous arches (neural and haemal), which appear in the neural
and haemal ridges of the skeletogenous tissue, may, as stated above, spread
out round the notochord outside the elastica externa (which may persist or
disappear) and unite with each other and so reinforce the vertebral centra;
or they may remain separate from one another. In the latter case the
neural and haemal arches are separate from the centra ; in the former
they are continuous with the centra. KoUiker states that in some cases
the centra are reinforced by calcified fibro-cartilage which proceeds from
the perichondrium of the centra between the insertion of the neural and
haemal arches {Carchariidae, etc.). In this case we get the cruciform
figure in section described below.

In Chlamydoselachus, Echinorhinus, Hexanchus and Heptan-
chus, the cartilaginous notochordal sheath is not definitely seg-
mented : it consists of continuous cartilage, though in the centre

124

SUB-CLASS ELASMOBRANCHII.

of the vertebral regions (as shown by the arches) it has thickened,
become firmer, and has encroached upon the notochord (Fig. 68).
(In Chlamydoselachus this only happens in front, the notochord
is unconstricted behind, and in Heptanchus the chordal sheath
has segmented in the caudal region.)

In other Plagiostomes the notochordal sheath is segmented
into alternately cartilaginous portions which constitute; the bodies
of the vertebrae and fibrous parts which are the intervertebral
ligaments (Fig. 66). The sheath thickens in the centre of the
vertebral portions and constricts the notochord ; moreover its
tissue consists of three layers, forming the inner, middle and
outer zones. The middle zone is fibrous and is nearly always
calcified, forming the so-called double calcareous cone (Fig. 66, c).

''J'^vy^^^ln Hexanchus and Laem-
e ■^:^^:7Xa^ orgus borealis the verte-
,, .,_.. ,,,„ „ „ ,. „ brae are entirely uncalci-

^6 e

I'lG. 68. — Longitudinal section through tlie hinder
part of tlie vertebral column of Heptanchus, show-
ing incipient vertebral bodies (e), and extensive
intervertebral I'gaments (n). a fibro-cartilage
of chorda-sheath ; b notociiord ; c calcified double
cone; d external calcification; e hyaline cartilage
of the incipient centrum external to the double
cone (after Kolliker).

Centra (Fig. 69) in which
the internal calcareous mat-
ter is confined to the middle
zone (double cone) have been
called cyclospondylous. In
some, however, there are
additional concentric calcar-
eous rings outside the double
cone ; such vertebral centra
are called tectospondylous.
In others again there are radiating calcareous lamellae proceeding from
the double cone through the outer zone of the chordal cartilage. Such
are termed asterospondylous.

The amoimt of calcification varies from the cyclospondylous condition
of the simple double cone with soft tissue outside and inside, to the condi-
tion found in many Raji and in the Lamnidse in which the whole of the
chordal sheath is calcified both inside and outside the double cone. The
first calcification is always in the chordal sheath in the fibrous tissue of the
middle zone (double cone). Later arise the concentric rings of theTecto-
spondyli or the radiating lamellae of the Asterospondyli ; these are calcifica-
tions of the hyaline cartilage of the chordal sheath. When skeletogenous
tissue is developed outside the elastica externa either by an extension
of the cartilage of the arches over the centra (many Cyclospondyli), or
{Carchariidae, etc.) by development on the inside of the perichondrium of
the centrvun, it frequently becomes calcified as four wedges, which extend a
certain distance into the body between the insertion of the arches. These
hard wedges may or may not reach the double cone (sometimes they do
so at the end and not at the centre of a vertebral body) ; they give rise to
the cruciform radiating figure seen in the section of some vertebrae. In
such cases the uncalcified cartilaginous arches appear to be continued like

VERTEBRAL COLUMN'.

125

wedges into the centrum. These calcified wedges are formed in a fibrous
cartilage or fibrous tissue and contain blood vessels. Calcifications are
sometimes found on the surface of the arches or even in their tissue.

It is often difficult to determine in any given vertebra whether the arches
grow round and reinforce the centrum unless the elastica externa persists,
and there seems to be considerable variation in this respect in different
animals and even in different parts of the vertebral column of the same
animal. In Hexanchus, Heptanchus (anterior), Myliohates (posterior part),
Rhinohatus (posterior part), Trygon and Squatina the arches remain separate
and do not grow round the chordal sheath. In Heptanchus (posterior),
Centrophorus, some Spinacidae and Scymnidae, Squatina, many Rajidae
the arches grow round and reinforce the centrum. In the Carchariidae
and Lamnidae the centrum is strongly reinforced by perichondria! calcifi-
cation and the cruciform appearance in section is very marked, but it is
not clear whether the arches contribute to the centrum.

The neural and haemal arches are always segmented. The
neural arch is
generally incom-
plete, being sup-
plemented by the
intercalated pieces
(Fig. 67), and there
are often some
special supra- dorsal
pieces which form
the actual roof. In
some cases (Alo-
pecias) both neural
arches and intercalated pieces extend across the roof
of the spinal canal (Fig. 70). The anterior root of a spinal
nerve frequently perforates the neural arch, the posterior
root the intercalated piece, or the nerves may pass out
between the arch and the intercalated piece. In the trunk
region the vertebrae carry short dorsal spmous processes, but
in the caudal region these may attain some length. There is a
longitudinal elastic ligament extending along the ridge of the
upper arches ; it may be embedded in the cartilage.

Sometimes (Fig. 71) more than one pair of intercalated pieces may occur
in each vertebra, and the niunber may vary in different parts of the verte-
bral column of the same animal. In Zygaena there are two. It frequently
happens that in the caudal region there are two complete vertebrae (cen-
trum, arch, intercalated pieces, etc. ) for every spinal nerve (diplospondyly).*

B

C

Fio. 69. — Diagrammatic transverse sections of vertebrae, to
illustrate A the cyclospondylous ; B the tectospondylous ;
and C the asterospondylous condition. C notochord ;
I) central calcareous ring ; £ elastic externa ; N neural,
U haemal arch (from Hasse).

* W. Ridewood,
Soc, 27.

Caudal diplospondyly of Sharks," J own. Linn.

126

SUB-CLASS ELASMOBRANCHII.

In the middle part of the trunk region in the Notidanidae, each centrum
carries two sets of neural arches and intercalated pieces, and corresponds
to two pairs of spinal nerves, and therefore to two segments.

The haemal arches, which are sometimes supplemented by

intercalated pieces, are in the trunk directed outwards, and

carry very short ribs ; in the tail they are directed ventralwards,

and meet below the caudal artery and vein.

In the Raji the anterior vertebrae are not separate, but form a continuous
cartilaginous structure into which the notochord extends only for a short
distance and which articulates with the occipital region of the skull. In
Notidanidae the anterior part of vertebral column has fused to the skull.

The cranium, of which a general description is given on p. 60,
consists of continuous cartilage, which may be partially calcified.

7K-

Fig. 70. — Three trunk vertebrae of
Alopecias vulpes (from Gegen-
baur, after Hasse). iv inter-
vertebral ligament ; n neural
arch ; in intercalcated piece of
neural arch, m of haemal arch

/l~

---in

-7o

Fig. 71. — Lateral view of a portion of the
caudal vertebral column of Rhinoptera
(Zygobates) (after Hasse), showing large
number of extra dorsal pieces above the
Intercalated pieces in and the neural
arches n. The neural arches are con-
tinued into the haemal h round th° centra.

The notochord may be entirely atrophied in the parachordal
region (Raji, etc.) or it may persist to a greater or less extent
(HeiJlanchus, Gentrophorus, Acanthias, Squatina, Prionodon).
In Raji there is a distinct articulation between the occipital
region and the anterior vertebral plate of the vertebral column
by means of two occipital condyles and an ' odontoid process ' of
the vertebral plate. In the Squali there are also usually two
occipital condyles, but no regular joint between the first vertebra
and the skull. In the Notidanidae, in which the anterior end of
the notochordal sheath is not segmented, the latter structure
runs continuously into the cartilaginous region of the skull.

CRANIUM. 127

The occipital region is more sharply marked off from the vertebral
colvmm in the Raji than in the Squah. In correspondence with this the
occipito-spinal nerves (p. 138) are reduced or absent in Raji. In a few
cases the occipital cartilage extends back over the anterior vertebrae and
envelopes them {Carcharias), while in Mtistelus one vertebra is fused with
the skull. O

The greater part of the roof of the skull is formed of cartilage,
but there is a well-marked median fontanelle in the roof of the
ethmoidal region. The space in the auditory cartilage in which
the otocyst lies communicates with a canal which contains the
aqueductus vestibuli and opens on the dorsal surface, either into a
depression in the cartilage which receives the opening of the

Fig. 72. — Median section of the cranium of Heianchus, inner view (after Gegenbaur). Fora-
men for 1 vagus, 2 glossopfiaryngeal, 3 auditory, 4 facial, 5 trigeminal nerve ; 6 pos-
terior clinoid ridge ; foramen for 7 oculomotor, 8 trochlear, 9 optic nerve ; 10 fontanelle ;
11 rostrum ; 12 lateral process of ethmoid region ; 13 foramen for carotid ; 14 transverse
canal in skull base ; 15 notochord ; 16 foramina for spino-occipital nerves ; 17 neural
arch of the first vertebra with nerve foramina.

corresponding canal of the other side (Squali) or on the surface
separately from the latter (Raji).

The ethmoidal region is frequently prolonged in front of the
nasal capsules. In the Raji and a few Squali this continuation
is a well-marked process, the rostrum (very largely developed in
Pristis) ; in Squali it is, if present, usually confined to a slender
process, the prenasal process, with which two processes of the
nasal capsules may be connected.

The labial cartilages are small cartilaginous rods in folds of
skin at the sides of the mouth, near the angle ; there are usually
two pairs above and one pair below. They are less developed
and less constant in the Raji.

Visceral Afches (p. 61). — The mandibular arch is always

128

SUB-CLASS ELASM0C13RANCH1I.

divided into two pieces ; of these the doi'sal forms the skeleton
of the upper jaw, and is called the palato-quadrate bar, while the
ventral constitutes the lower jaw and is called the cartilage of
Meckel. The dorsal posterior end of Meckel's cartilage articulates
with the hind and ventral end (quadrate portion) of the palato-
quadrate bar, while the ventral anterior end meets its fellow at
the symphysis of the lower jaw. Anteriorly the palato-quad-
rate bar joins its fellow beneath the ethmoid region but its rela-
tion to the skull varies in the most remarkable manner throughout

'^"-^ Jlot^ .

fuJuXoy^-o-'

{^.L^ '

Md

/^ fjCt./O-iJ^^t'^

FlO.^73. — Skull of Hexanchus with mandibular and hyoid arch (after Gegen-
baur). P-Q palato-quadrate ; Um hyomandibular ; Md Meckel's cartilage ;
C basihyal, L, L' labial cartilages ; p palatine process of palato-quadrate ;
M lateral process of etlimoid ; N nasal sac ; Po postorbital process ; Gp fora-
men for glossopharyngeal nerve.

the group. In the Notidanidae (Fig. 73) it articulates with
the skull at two points ; anteriorly it has a process, the palatine
process, which is connected with the skull between the exit of the
optic and trigeminal nerves, while posteriorly it articulates with
the postorbital process of the auditory cartilage, i.e. anterior to
the spiracle. In these skulls the hyoid arch, which bears
branchial rays, is attached to the auditory region and is
segmented into two pieces on each side and a median piece,
the hypohyal or copula.

In other Selachians the palatine process is present, though the

VISCERAL ARCHES.

129

connection is further forwards with the ethmoid region (ethmo-
palatine ligament), but the posterior articulation, viz. with
the postorbital process is not present. On the other hand the
posterior end of the bar is attached to the dorsal element of
the hyoid arch by ligaments.

In such Selachii the condition of the hyoid is very similar to
that in the Notidanidae (already described), excepting that the
dorsal piece is much stouter and assists to a marked extent by
means of ligamentous bands, attaching it to both the quadrate
end of the palato-quadrate bar and to Meckel's cartilage, in
supporting the mandibular arch. This dorsal piece is for this
reason called the hyomandibular, the other piece being called
the ceratoliyal. In Raji the palatoquadrate bar is without

Fio. 74.— Diagram showing the principal modiflcations in the arrangement of the hyOid arch
in Plaaiostomes (after Gegenbaur). A in Notidanus ; £ in pentanchal Selachians ; C in
Torped^ ; D in Raja. Hm hyomandibular ; p process of fl m ; hy lower part of hyoid
arch ; r branchial rays.

the palatine process, and the mandibular arch is entirely sup-
ported by the hyomandibular. This structure is without gill-
rays and has developed a process for the support of the mandibu-
lar arch (Fig. 74, p). It has almost lost its relation to the
lower part of the hyoid arch, which is connected only to its
dorsal end, or is attached dorsally direct to the auditory cartilage
(many Rajidae, etc.). The hyoid arch of such forms is only
equivalent to the ventral part of the hyoid of the Notidanidae
and Squali generally. It is a comparatively slender, jointed
structure, closely resembling the posterior branchial arches,
like which it bears branchial rays. There are frequently two
or three small cartilages, the prespiracular cartilages, in the
anterior wall of the spiracle.

Z-II K

130 SUB-CLASS ELASMOBRANCHII.

In the Holocephali the autostylic arrangement (p. 63) prevails,
the palato-quadrate bar being continuous with the skull.

The branchial arches are t5rpically and usually segmented
into four pieces on each side and a median piece, the basibran-
chial or copula. The dorsal of the four pieces is called the
pharyngobranchial, the second the epibranchial, the third the
ceratobranchial, while the ventral and smallest piece is called
the hypobranchial.

The hypo- and basi-branchial piecea are somewhat variable, the hypo-
branchial of the anterior arches being often undeveloped (some Raji) and
the basibranchial being often absent from the anterior arches. In the
latter case the last basibranchial (cardiobranchial) which is always larger
than the others, is much developed and has several arches attached to it.
The hypobranchial of the first arch is frequently attached to the basihyal.
The last arch is always smaller tlian the others ; its pharyngobranchial
is fused with the pharyngobranchial of the preceding arch and its hypo-
branchial is not developed. In Raji the last arch articulates with the
shoulder girdle. In Squali the dorsal elements are not closely attached
to the vertebral column, but in Raji there is a firm attachment. All the
branchial arches except the last bear branchial rays on the epi- and cerato-
branchial segments ; and in most Squali cartilaginous rods called extra-
branchials are placed close beneath the skin near some of the external
branchial apertures ; they are absent in Raji.

The supporting structures of the median fins are, as in the
case of the paired fins, of two kinds ; the cartilaginous somactids
and the horny dermotrichia. In the dorsal fins, in the
dorsal part of the caudal, and in the anal fin the dermotrichia
are carried by somactids which are usually imbedded in the
muscles and not attached to the neural or haemal spines. In
the ventral lobe of the caudal fin there are no somactids and the
dermotrichia are carried by the haemal spines.

The pectoral girdle consists of two dorso-ventrally directed
cartilaginous rods, placed one on each side of the body, and
each divided by the surface (glenoid) for the articulation of
the skeleton of the pectoral fin into a dorsal portion, the scapula,
and a ventral portion, the coracoid. The two coracoids are
continuous with one another ventrally, beneath the pericardium,
but the scapulas end freely dorsally, except in Raji, in which
the dorsal portion, sometimes partly marked off as a supra-
scapula, is attached to the anterior vertebral plate (p. 126) of the
spinal column.

The skeleton of the pectoral fin consists of a number of

PECTORAL FIN.

131

somactids, which carry peripherally the dermotrichia. * There
are generally three basal somactids which articulate directly
with the pectoral girdle (Fig. 75, B) : these are called respectively
the pro- meso- and meta-pterygium. They carry the peripheral
radialia. The metapterygium is the largest of the basal pieces
and carries the greatest number of peripheral somactids. These
are placed mainly on its preaxial side (Fig. 75), there being
few, if any, on its postaxial side.

A

Fig. 75. — Skeleton of pectoral fln A of Scymnus, . B of Acanthias vulgaris (after Gegen-
baur). p pro-, rws meso-, mt meta-pterygium ; B postaxial (median) side of fin. (Tlie
line drawn through mt in B indicates what some anatomists regard as the axis of the
so-called archipterygium ; on the same view the dotted lines R, R indicate the prea.xial
radii, ii' the postaxial radii).

The metapterygium appears to be the most important basal, and when
there is only one basal somactid, as in Scymnus (Fig. 75, .4), it is supposed
to be the metapterygium.

In Uvdng forms the fin-skeleton is always rhipidostichous (p. 57), but
in some extinct forms {Cladoselache, Fig. 83) it is orthostichous, and in
others [Pleur acanthus. Fig. 76) it is rachiostichous (and unibasal) and on
the whole pleurorachic (p. 57). In Raji the propterygium and mesoptery-
gium are elongated and segmented, and the propterygiima is attached to
the olfactory region of the skuU. Moreover in the Raji there are sometimes
additional basals inserted between the meso- and meta-pterygivun.

The pelvic girdle is not attached to the vertebral column ;
it consists of a transverse bar of cartilage placed just ventral to

* Dermotrichia are absent from the paired fins of some Raji (e.g. Tor-
pedo, etc.).

r

132

sub-claSS elasmobra\( nil.

the cloaca and carrying at its outer end the skeleton of the
fin. The main portion may be called the ischio-pubic portion,
the short process external and anterior to the limb being all
that can be compared to an ilium.

The fin presents two basal cartilages of which the larger
is the metapterygium and carries most of the peripheral somac-
tids ; the propterygium is small. In the male the metaptery-
gium is continued to form the skeleton of the clasper.

Here as in the fore-limb there is some variation in the number of basal
cartilages, mainly manifested in extinct forms.

The brain * (Figs. 77, 78) of
Plagiostomes presents great varia-
tions but is always characterised by
the following features : (1) The
olfactory lobes, which may be
placed some distance in front of
the cerebrum and connected with
the latter by a long stalk, or may
be placed quite close to the cere-
brum, are connected with the cere-
brum laterally and somewhat
dorsally ; (2) the cerebrum is
unpaired though internally it
possesses two lateral ventricles, each
of which is usually continued into
its olfactory lobe ; it is not sharply
marked off from the thalamencepha-
lon ; (3) the epiphysis is elongated,
its front end lying over the cerebrum either beneath or
just behind the cranial fontanelle (in Baja in the sub-
cutaneous tissue above the fontanelle) ; (4) the optic
nerves form a chiasma, and the infundibulum has lobi
inferiores and an infundibular gland (saccus vasculosus) ;
(5) the cerebellum is large and its surface is frequently con-
voluted. There is said to be a paraphysis, but there is

* N. V. Miklucho-Maclay, Beitrdge zur vergl. Neurologic d. Wirhelthiere,
Leipzig, 1870. J. V. Rohon, " Das Centralorgan des Nervensystems der
Selachier," Denkschr. d. Wiener Akad. Math., etc., 38, 1877. L. Edinger,
" Unters. lib. d. vergl. Anat. des Gehirns, Abh. d. Senckenbergischen naturf.
Gesellschaft, i. and ii. 1888 and 1892.

FIG. 76. — Skeleton of right pectoral
fin of Pleuracaiithus [Xenacan-
thus) decheni (after A. Fritsch,
from Gegenbaur).

BRAIN.

133

no pineal eye (parietal organ). The pituitary body lies along
the ventral side of the infundibulum ; it is said to open in the
adult into the cranial cavity within the dura mater (Haller),
but this must be regarded as doubtful.

muscle of eye ; P sp nil nerved Prpalatoauadr^t'^'^^ ' °* '-^'"°'" ''"'

internal, fls superior rectus m^cle of eve ^S«nirtU ,f°'!l',''' ""^^'^ = ^' external, Ri
ophthalmicus superficial trig^Sii- rr'luDerfor^TK'-tn'fiH^ nerve (trochlear) ; Tr'

nerve ; V, tenth nerve (vag^.T^A ^^reS'KbraST'^TrSfal^^^^^^^^^^^ '""^ ^"'^

The cerebrum is generally marked by longitudinal grooves
mdicating the mternal division, but in Carcharias (Fig 78)
Sphyrna, etc., there are no traces of these. Plagiostomes

134

SUB-CLASS ELASMOBRANCHII.

differ considerably as to the length of the thalamencephalon.*
In some, this part of the brain is much elongated, being com-
pletely exposed in its dorsal part and passing without break
into the anteriorly-placed cerebrum {Notidanidae, Spinacidae,
Scymnidae and most Plagiostomes). In others the thala-
mencephalon is very short and its dorsal surface is almost com-
pletely covered by the cerebrum, the hinder part of which
almost, if not quite, touches the optic lobes {Carcharias, Sphyrna,

Oxyrrhina, Galeus, Trygon). It would
appear that the latter condition is develop-
mentally the most primitive, for brains with
a long thalamencephalon in the adult, e.g.
Scyllium, have in the later embryonic
stages a short thalamencephalon with the
cerebrum and optic lobes in contact over it.

The cerebellum in some forms is very
large and may reach forward to the cere-
brum. The restiform tracts, or sides of
the medulla are much developed, and folded
or even convoluted. This is ascribed by
Burckhardt to the enormous development
of the sensorial centres in the dorso-lateral
parts of the spinal cord and brain in conse-
quence of the insertion of the nerves of
the lateral line organs.

The dorso-median tract of non-nervoiis tissue
which is so often found along the central nervous
system of the Vertebrata and was first mentioned
by the physiologist Haller {Opera Minora, 1768,
bd. 3) has been detected as a septum in the cere-
bellum of many Plagiostomes. It is of course also present, as in all Ver-
tebrates, over the third and fourth ventricles and on the posterior wall
of the lateral ventricle. Very possibly the very thin dorsal wall — at
the bottom of the dorsal fissure — of the central canal of the spinal cord is
to be regarded as part of it, though there is here a little nervous
tissue.

Cranial nerves t (see p. 72). There is nothing special to

* This and other features of the brain seem to be influenced by the size
and position of the eyes, vide Burckhardt, British Association Reports, 1900.

t H. Stannius, Das peripherifiche Nervensystem der Fische, Rostock, 1849.
J. C. Ewart, " On the Cranial Nerves of Elasmobranch Fishes," Proc. Roy.
Soc, 45, 1889, p. 524. Id., Supplementary note on the cranial nerves of
Elasmohranchs, Edinburgh, 1892. A. M. Marshall and W. B. Spencer, " On

Fig. 78.— Brain of Car-
charias from above
(after Miklucho-Mac-
lay). SI olfactory lobe ;
Vh cerebrum ; Mh
optic lobe ; Uh cere-
bellum ; N restiform
tract ; Vag vagus.

CKANIAL NERVES. 135

note about the first, second, third, fourth, and sixth cranial
nerves.

The ophthalmicus profundus (Fig. 79, opv) is a well marked
and distinct nerve in most Elasmobranchs, but in some (e.g.
ScyUium) it is less marked. It arises either with or just in
front of {Laemargus) the main root of the trigeminal. After
emerging from the skull wall with the trigeminal it presents a
ganglion — the 'profundus ganglion, and passes forwards in the
orbit, dorsal to the external and internal recti, but ventral to
the superior rectus and superior oblique. It penetrates the
anterior wall of the orbit, and is distributed to the skin of the
front of the snout. In the orbit it gives off long ciliary nerves
to the eyeball, and one or more nerves which anastomose with one
or more filaments from the ventral branch of the oculomotor
(om), a small' ganglionic swelling being found at the point of
junction. This small ganglion is the ciliary or oculo-motor
(lenticular, ophthalmic) ganglion ; it gives off some ciUary
nerves (short ciliary) to the eyeball. The filaments connecting
this ciliary ganglion with the ophthalmicus profundus (some-
times with the profundus ganglion) constitute the radix longa,
while those passing to the oculomotor represent the radix
brevis. The filaments passing from the ciliary ganglio" to the
eyeball represent the short ciliary nerves.

The ciliary ganglion of Plagiostomes seems to be variable in its size and
position ; it is sometimes absent, sometimes close to the profundus nerv^e,
sometimes approximated to the ventral branch of the third nerve. In the
embryo the oculomotor nerve passes directlj' from its origin to the pro-
fundus ganglion ; later it becomes detached, the connecting cord being
the radix brevis. The ciliary ganglion is probably a detached portion of
the profundus ganglion, in which case its relation to this ganglion is exactly
that of a sympathetic gangUon to the ganglion of a posterior root of a
spinal nerve. The ophthalmicus profundus itself probably corresponds
to the nasal nerve of the Mammalia. The third nerve must be regarded
as the ventral root of a nerve of which the ophthalmicus profundus
is the dorsal (see p. 73).

The trigeminal nerve arises usually by a single root from the
sides of the medulla. It swells either inside or outside the
cranium into the gasserian ganglion and there gives o£f the

the cranial nerves of ScyUium," Q. J. M. S., 21, 1881, p. 469. C Gegen-
baur, " Die Kopfnerven von Hexanchus," Jena. Zeitsch., 6, 1871, p. 497.
F. J. Cole, " On the cranial nerves of Chimaera, " Trans. Roy. Soc. of Edin-
burgh, 38, 1896, p. 631. Also literature cited on p. 75.

136

SUB-CLASS ELASJIOBRANCHII.

.superficial ophtliaimic {ramus ophthalmicus super ficialis, sut).
This nerve, which is quite inconspicuous in Raji and in forms
in which the ophtnalmicus profundus is well developed (at once
joining the ophthalmic branch of the facial), runs close to or in
conjunction with the superficial ophthalmic of the facial ; it
is distributed to the subcutaneous tissue and skin of the snout.
The main nerve passes on and divides into the superior
and inferior maxillary branches. Of these the inferior maxillary
is a mixed nerve ; the superior being mainly sensory but con-
taining motor fibres in some forms (e.g. Chimaera).

CSO sot om spf

cpr Tr ra^. flJjjJL^^^-

4--^....^>-!'-. l'> Mllllim-^?^' •'—^J-^

Fig. 79. — Diagram illustrating the distribution of tlie dorsal branches of the cranial nerves
of the lateral line canals, and the position of the groups of ampullae in an Elasmobranch
(after Ewart, from Gegenbaur). A auditory nerve with labyrinth ; it also points to the
groups of Lorenzini's ampullae ; Bu buccal branch of facial, bit inner branch to part of
infraorbital canal, and to the inner buccal group of ampullae ; bu' its outer branch wliich
supplies the part of the infraorbital canal, and the outer buccal group of ampullae ;
cli postbranchial branch of facial to mucous membrane, and giving off motor branches
to some jaw muscles ; CSO, CSO supraorbital canal ; CJO, CJO infraorbital canal ;
Fa, Fa' roots of facial nerve ; Gp glossopharyngeal, arising under cover of the lateralis
branch of the vagus nerve ; Hm hyomandibular canal arising from the infraorbital and
giving off the mandibular canal ; the mandibular group of ampullae is in the angle
between these two ; Hm' branch of the hyomandibular nerve to the hyoid group of
ampullae ; in intestinal branch of vagus with ganglion, where it separates from fourth
brancliial branch ; In lateralis brancli of vagus nerve ; m mouth ; N nasal sac ; om
deep branch of oculomotor giving off short root of ciliary ganglion (shown, but not
marked), the long root is also shown, as are the short ciliary nerves to the eye ; 0T>r
root of ophthalmicus profundus ; opv dorsal branch of same, giving off long ciliary nerves ;
pol second branch of lateralis supplying some lateral line sense organs, and a row of pit
organs, the first branch supplies the commissure connecting the two laternl canals, and
some sense organs of the main canal ; sof ophthalmicus superflcialis facialis, which
supplies the supraorbital canal, and the superficial ophthalmic group of ampullae ; sot
ophthalmicus superflcialis trigemini ; it arises from the gasserian ganglion ; sp spiracle ;
Tr trigeminus ; F', F^, F^ the first three branchial branches of the vagus nerve, each
with a ganglion and with pharyngeal, prebrancliial and po.stbranchial branches ; F* the
united fourth branchial branch of vagus and intestinal branch; 1-5 gill-slits.

The facial nerve with its ganglion, the geniculate ganglion,
has three or four roots which after communicating with one
another separate into three nerves, the ophthalmicus superficialis

4

CRANIAL NERVES. 137

portio facialis {sof), the buccal (Bu), and the hyomandihular nerves
{Fa', ch, Hm'). The two first of these and the external mandi-
bular branch of the last innervate the sense organs of the sensory
tubes. The hyomandihular * excluding the external mandi-
bular branch may be called the facial proper {ch) ; it is a mixed
nerve for the hyoid arch with branches to the roof of the mouth
{palatine) and mandibular arch {prespiracular).

llie three roots of the facial are as follows : a dorsal root {Fa) , from
which probably most of the lateral-sense organ nerves are derived, and
two ventral roots, of which the anterior is just behind the root of the
5th and is possibly double, wliile the posterior is just in front of the root
of the auditory.

The ophthalmicus superficialis freely communicates with the
buccal nerve as it passes through the cranial wall which it does
by a foramen dorsal to that for the trigeminal. Outside the
cranium it presents a ganglion and passing along the dorsal side
of ihe orbit is distributed to the supraorbital branch of the ceph-
alic lateral line and to the ampullae of the ampullary canals of
the snout.

The buccal nerve {bu, bu') after passing through the cranial
wall with the trigeminus becomes closely applied to the gas-
serian ganglion and swells into a ganglion. It runs along the
floor of the orbit close to the maxillary nerve. It supplies the
inner and outer buccal groups of ampullae, and the sense organs
of the orbital and suborbital lateral line.

The hyomandihular nerve, after leaving the skull by a foramen
which is generally through the auditory cartilage, behind and
distinct from the trigeminal, dilates into a ganglion where it gives
off forward the palatine nerve (indicated but not marked in the
figure). The palatine nerve at once gives off the prespiracular
nerves which are supposed to be homologous with the chorda
tympani of mammals, and is continued to supply the roof of the
mouth. It is homologised with the great superficial petrosal
of mammals. The hyomandihular then gives off a branch to
the mandibular and hyoid group of ampullae and lateral line —
the external mandibular— and is continued to the muscles, etc.,
of the hyoid arch (internal mandibular of Stannius, ch).

The auditory nerve arises by a root immediately behind

* In the figiu-e the hyomandihular nerve (ch) is smaller than the external
mandibular which runs with it, so that it appears as a branch of the latter.

138 SUB-CLASS ELASMOBRANOHII.

those of the trigemino-facial group and is distributed to tlie
walls of the otocyst.

The glossopharyngeal (Gp) arises just in front of the vagus by
three or four rootlets. It passes out by a canal below the audi-
tory capsule, dilates into a ganglion and gives off a small dorsal
nerve which in some cases is said to supply a part of the cephalic
lateral line. The nerve continues to the first branchial arch
giving off a prebranchial branch to the hyoid and a pharyngeal
branch.

In the vagus the root of the lateral line nerve {lateralis) must
be distinguished from the roots of the rest of the nerve. The
lateralis nerve {In) arises by a root dorsal to and slightly in front
of the glossopharyngeal. It leaves the skull with the rest of the
vagus to which it is closely applied and dilates into a ganglion.
It is continued to the end of the body and supplies the trunk
portion of the lateral line canal, and a small portion of the cranial
lateral line. The remaining vagus roots, which are numerous,
unite to form five nerves each of which has a ganglion. These
are at first closely connected together and are distributed in the
typical manner, the first four to the four hinder branchial arches
and the last is continued as the intestinal branch {in).

A variable number of anterior roots of spinal nerves * of which
the posterior roots though present in the embryo are absent in
the adult, arise from the medulla ventral to the vagus roots.
They were formerly mistaken by Gegenbaur for ventral roots of
the vagus and were called ventral vagus roots. They are absent
in some Raji. They leave the skull wall by special foramina in
the occipital region and innervate some muscles of the fore-limb
and some ventral branchial muscles. They are called the spino-
occipital nerves.

It has been shown in Amphibia that the superficial ophthalmic
of the facial, the buccal and external mandibular of the facial
and the lateralis of the vagus arise from the acustico-lateralis
nucleus in the brain (see p. 75). All these nerves are developed
from the surface ectoderm and sink inwards to their adult posi-
tions and when in Amphibia the lateral line sense organs dis-
appear the whole of these nerves disappear also.

* M. Furbringer, Ueb. d. Spino-occipitalen N erven der Selachie^, etc.,
Leipzig, 1897.

ALIMENTARY CANAL. 139

In Chimaera a small portion of the supraorbital canal is supplied by
the ophthalmicus profundus, and in some if not all Plagiostomes {Mus-
telus, Laemargus), Teleosts, Ganoids, the dorsal branch of the glosso-
pharyngeal innervates a small portion of the cephalic lateral line.

For an account of the sense organs the reader is referred
to p. 121, and to the section deahng with Pisces (pp. 77-82).
For the sympathetic, see p. 75.

The alimentary canal is fairty similar throughout the order.
The mouth is usually ventral. Teeth varying considerably in
shape are present in several rows on the palato-quadrate bar and
cartilage of Meckel. As those in the row next the mouth
opening are worn away, those of the next row advance, and a
new row is added internally. The basihyal projects in a tongue-
like manner from the floor of the mouth. There are no salivary
glands. The hinder part of the mouth passes without demarca-
tion into the pharynx which receives the internal openings of
the spiracle and gill-slits.

The oesophagus leads into the stomach, which is U-shaped,
the pyloric limb of the U being narrow and opening into the short
somewhat swollen anterior end of the intestine (duodenum, bursa
entiana). Into this open the bile duct and the pancreatic duct.
It is followed by the rest of the intestine which is provided with
a spirally disposed longitudinal valve. The intestine ends behind
in a narrow rectum, which receives a dorsal gland, the rectal
gland, and opens into the cloaca. The cloaca opens externally
between the pelvic fins. The alimentary canal is supported by
a mesentery which is defective in part. The liver is well devel-
oped and usually provided with a gall bladder. The pancreas
is also large. There is a well-developed spleen in the neighbour-
hood of the stomach.

In Laemargus borealis two large caeca open into the commencement
of the intestine. In some forms (Zijgaena, Carcharias) the longitudinal
valve is not spirally arranged, but is rolled upon itself.

There is no air-bladder.

A thyroid is present between the rami of the mandibles. It
frequently has a pyriform shape and lies over the bifurcation
of the ventral aorta.

The thymus is represented by an elongated lobed gland placed
over the dorsal ends of the branchial arches and derived, as usual,

140 SUB-GLASS ELASMOBRANCHII.

from epithelial outgrowths of the embryonic branchial passages.

Body cavity. The pericardial cavity communicates with
the general body-cavity by a canal which leaves it dorsal to the
sinus venosus and passing along the ventral side of the oesophagus
opens behind by a slit-like aperture into the body-cavity. This
canal may divide in a Y-shaped manner behind, but usually only
one of the limbs is complete.

The body-cavity may communicate with the exterior by abdo-
minal pores, with the kidney-tubules by persistent nephrostomes
and with the cloaca by the oviducts.

Abdominal pores are usually present, though there is consider-
able variability as to these openings in closely allied species or
even in the same species at different periods of life. They are
placed on each side of the cloacal opening, either on the surface
or at the bottom of a pouch (cloacal pouch). They lead into
that part of the body cavity which is placed on each side of the
cloaca (peritoneal canals).

The vascular system is arranged in the usual piscine manner.
The pericardial cavity is placed dorsal to the median union of
the coracoid cartilages of the shoulder girdle. Its walls are
stiffened by that skeletal structure and by the basibranchial
plate which lies dorsal to it. It thus happens that its walls are
unyielding and when the ventricle contracts, blood flows from
the great venous sinuses outside the pericardium into the large
auricle. The ventricle is provided with a muscular conus arteri-
osus in which there are from two to five or even more rows of
semilunar valves. The ventral aorta sends branches to the hyoid
arch, which has a demibranch on its hinder face, and to all the
branchial arches except the last. The carotids arise from the
dorsal system and the internal carotids anastomose as they enter
the cranial cavity. The efferent vessel coming from the hyoid
demibranch gives off near its ventral end an artery which passes
forward ventrally to the spiracle on to the mandibular arch. It
passes near the front wall of the spiracle, giving off vessels to the
pseudo branch and then pierces the cranial wall to join the internal
carotid artery within the skull.

The great veins are much dilated into sinuses as they approach
the heart. This applies to the anterior and posterior cardinals
and to the hepatic veins. Moreover the anterior cardinal is
dilated round the eye-ball forming the space (a kind of hfemo-

URINOGENITAL ORGANS. 141

coelic body-cavity) in which the movements of the eye occur.
There is a renal-portal system furnished by the caudal vein which
brandies when it reaches the kidney, and the posterior cardinals
arise in the kidney.

The kidney (Fig. 81) of each side is a single gland but by the
arrangement of the collecting tubules may be divided into an
anterior thin part, the mesonephros (almost absent in the adult
female), and a posterior thick well-developed portion the meta-
nephros. The longitudinal duct extends along the whole length
of the gland and posteriorly joins its fellow to form an unpaired
tube — the urinary sinus — which opens into the cloaca. This
duct has various names none of which are entirely satisfactory.
It is called the pronephric, the mesonephric, and the wolffian
duct (p. 89). We shall call it usually the longitudinal duct.
The collecting tubes of the mesonephros are directed transversely
and open at once into the longitudinal duct ; the collecting tubules
of the metanephros, the so-called ureters, are directed back-
wards and after a certain amount of union amongst themselves
open into the hinder part of the longitudinal duct close to the
urinary sinus by one or more openings.

The ovaries and testes are slung to the dorsal wall of the body-
cavity by the mesovarium and mesorchium respectively, mem-
branes which are either attached close to the mesentery or to
the mesentery itself. In some few cases the ovaries of the two
sides are united into one body which is placed slightly on the
right side. The testes are connected with the anterior end of the
mesonephros by a network of tubules — the testicular network
(Fig. 80), which is not visible (Fig. 81) without special prepara-
tion, and which typically consists of the longitudinal canal of
the testis, the longitudinal canal of the mesonephros (wolffian
body) and the vasa efferentia connecting these two. The longi-
tudinal canal of the mesonephros (wolffian body) is connected
with the malpighian bodies of a number of the anterior meso-
nephric tubules. The number of tubules implicated varies very
considerably in different species, and very possibly in individuals
of the same species. In Scyllium canicula it appears frequently
to be four, in Squatina vulgaris five. The sperm therefore passes
through anterior kidney tubules into the longitudinal duct which
is in the male much convoluted and functions as the vas deferens.
The hind end of the longitudinal duct is dilated to form the vesi-

142

SUB-CLASS ELASMOBRANCHII.

cula seminalis (Fig. 81, vs). The two vesicula? seminales join
to form the urogenital sinus and just before their union each
receives the opening of the seminal bladder (ss), which is a
pouch lying on the ventral side of the vesicula seminalis, and
the openings of the ureters, usually four or five in number in
Scyllium canicula. The urinary sinus which in the male is
common to the urinary and generative organs opens into the
cloaca through its dorsal wall by a median papilla — the urino-
genital papilla {ug.p.).

In the female the hind end of the longitudinal duct is dilated
(ub) and receives not far from its union with its fellow the ureters

(ur) by one oi
more openings.
The oviducts
open close to-
gether through
the dorsal wall
of the cloaca
in front of the
urinary papilla
They extend for
ward to the front
end of the body
ca V ity into
which they open
close together
on the ventral
side of the
anterior end of the liver {fl.t'). Not far from their front end their
walls are much thickened owing to the presence of glandular
tissue constituting the oviducal gland {od.g).

Traces of the oviducts are often present in the male, particu-
larly near the abdominal openings.

The oviduct in the viviparous forms presents a uterine dilata-
tion and the oviducal gland is much reduced.

In many Selachii the nephrostomes of a certain number of
the primary kidney tubules persist into the adult as ciliated
openings. These are minute in Scyllium, but in some forms they
attain a c( nsiderable size.

The egg is large and heavily yolked. It receives a coat of

Fig. 80. — Testis and cuterior part of mesonephros (Wolffian
body) of an embryo of Squatina vulgaris (after Balfour) : to
show the testicular network. I'here are live vasa efferentia
connecting the longitudinal canal in the base of the testis
with a longitudinal canal in the mesonephros. From the
latter there pass off four ducts to as many malpighian bodies.
1 vasa efferentia ; 2 malpighian bodies ; 3 mesonephros ;
4 longitudinal or mesonephric duct ; 5 longitudinal canal of
the testis ; 6 testis.

THE EGG.

143

albumen during its passage down the oviduct, and, in the ovi-
parous forms, a shell.* The shell is horny and generally oblong
in shape with the angles prolonged into four long coiled tendril -
like fibres (absent in the Skates), by which it is attached to
seaweed and other bodies. In Cestracion it is ovoid with a
spirally wound flange. In some viviparous Rays {Trygonorhina,
Rhinobatus) more than one ovum is enclosed in the same shell,
and the same fact has been observed in other forms.

Many sharks and the Torpedoes and one or two other Raji

'MP

B

Fig. 81. — A mile, B female uriuogenital organs of .'Jj/a 6a<is (from Wiedersheim, after T. J.
Parser), up abdominal pore; cl cloaca; epd convoluted anterior end of mesonephric duct
(vas deferera) of male lying on and concealing the epididymis ; fit anterior end of ovi-
duct ; fit' abdominal opening of the two oviducts ; ir interrenal organ (part of suprarenal
body) ; k metanephros ; mn.d mesonephric (longitudinal) duct of female ; od.g oviducal
gland ; oes oesophagus ; ov right ovary ; pn.d remains of oviduct in male ; ss sperm
sac, ««' its opening into the urinogenital sinus ; t left testis ; m6 dilated lower end
of mesonephric duct in the female ; ug.p urinogenital papilla in the male ; ug.s urino-
genital sinus ; u.p urinary papilla in female ; ur collecting tubes (ureters) of meta-
nephros, ut" their openings into the urinogenital (urinary) sinus ; ut dilated part of left
oviduct, ut' its opening into the cloaca ; vd lower end of mesonephric duct of male
(vas deferens) ; vs vesicula seminalis, vs' its opening into the sperm-sac.

* The shell is formed round the ovum and its albumen in the lower
dilated part of the oviduct, but the material of which it is composed is
secreted by the oviducal gland.

144

SUB-GLASS ELASMOBKANCHII.

are viviparous. The rest are oviparous. In the viviparous
forms a thin inconspicuous shell may be formed, but in many
there is no shell, and there is a placenta-like interdigitation
of folds of the yolk-sac, with corresponding depressions in the
uterine walls {Mustelus, Carcharias).

The mucous membrane of the uterine portion of the oviduct is glandular
and in many viviparous forms is raised into vilU. In Pteroplataea micrura,*
some of these villi are especially long, and dip down into the wide spiracles.
Moreover, they possess glands which secrete into the pharynx of the
embryo an albuminous fluid, which passes into the intestine and serves
as nutriment. In other viviparous forms {e.g. Myliobatis nieuhofii, Try-
gon, etc.) the glandular mucosa of the uterias secretes an albuminous fluid,
which would appear to enter the alimentary canal of the embryo by the
spiracles, and to be absorbed in the intestine. It has been suggested

that the long ex-
ternally project-
ing gill-filaments
of the embryo,
which are highly
vascular, may
assist in absorb-
i n g nutriment
from the uterine
fluids in the vivi-
!)arous forms.

In Laemar-
gus the eggs
are said to be
laid without
a shell, and to
b e fertilised

externally (Turner). In other cases sperm is introduced
into the female ducts by means of the claspers — though
the copulation has rarely been observed. The period of
gestation is of considerable length (seven to ten months, or
even more), and the young are born or hatched fully developed
and of considerable size. The cleavage is partial, closely re-
sembling that of Sauropsida, and there is no larval stage. The
development has been largely studied, and throws much light
on many morphological problems (e.g. the origin and relations
of the excretoiy organs, of nerves and muscles, cranial seg-
mentation, etc.).

Fig. 82.— Embryo of Mustelus luens connected witli the wall Of the
oviduct by the yolk-sac (from Glaus, after J. Muller).

* Akock, P.R.S., 49, 1891, p. 359, and 50, p. -202

PLEUROPTERYGir.

145

The Elasmobranchii are the most ancient of all kno^\^^ fishes
They make their appearance in the Upper Silurian. They are
almost entirely active, carnivorous, predatory fishes and with
very few exceptions exclusively marine.

The following is the classification adopted in this work : —
Order 1. Pleuropterygii (extinct).

„ 2. acanthodi „

„ 3. jchthyotomi „

„ 4. Selachii (Plagiostomi).
Suborder 1. Notidani.
„ 2. Squall.

3. Raji (Batoidei).

„ 5. HOLOCEPHALI.

Fig. 83. — Cfadosehiche. A iiuctwiil, B pelvic fins x J ; B basal suiuactRls within the body-
wall, D dermal fin membrane, li peripheral somactids. Left border preaxial (after Dean,
from Woodward).

Order 1. Pletjeopterygii.*

With unconstricted notochord and heterocercal caudal fin. Paired find
with unsegmented parallel radials, reaching to the edge of the fin. Eyes
with a circle of thin dermal plates. Male icithout claspers on the pelvic
fins.

The skull is unknowTi, but the jaws are suspended by a slender hyo-
mandibular. The teeth have a principal cusp and several accessory
lateral cusps. They resemble teeth which have long been known from
the Carboniferous under the generic name Cladodus. Tliere were certainly

* B. Dean, Contributions to the INIorpliology of Cladoselache, J our v.
Morph., 9, 1894. Jael^el, Ueber Cladodus, Sitzungsb. d. Gesellsch. naturf.
Freunde, Berlin, 1892. R. Traquair, Geol. Magazine, 1888, p. 83.

Z — II L

146

SUB-CLASS ELASMOBRANCHII.

five gill-arches but there may have been more. Two dorsal fins have
been seen (one only shown in Fig. 84), but no anal. The caudal fin
is strongly heterocercal ; the neural arches are continued to the end
of the tail, and carry stout somactids which extend to the edge of the
fin. The paired fins are horizontal expansions of the integument.
The peripheral somactids are parallel, vmsegmented, and extend to the
margin ; between their distal ends are slender cartilages which are pos-
sibly displaced somactido. Thebasals are also parallel, and are contained

Fio. 84. — Bestoratioa of Cladosdache newberryi Dean (from Woodward, after Dean),

in the body wall. The skin is covered by minute denticles, not enam-
elled. Cladoselache Dean, Lower Carboniferous of Ohio ; Cladodus Ag.
for some time known only by teeth ; Devonian, Carboniferous, and Per-
mian.

Order 2. Acanthodii.*

With dermal calcareous plates on the skidl and pectoral arch, and with
a mosaic of quadratic dermal scales on the body. All the fins except the
caudal, with a powerful dentine spine on their anterior margin. Without
claspers. There are no cranial bones, nor fnembrane boties connecting the
pectoral arch with the cranium.

This group, which was formerly placed with the Ganoids, is now placed
with the Elasmobranchs. The endoskeleton contains granular calcifica-
tions, and the dermal plates placed on the head, body and pectoral girdle

Fig. 85. — Acanthoden Wanlixi (after Woodward).
gill-£rills are hypothetical.

The orbit is made too small and the

seem to have consisted of vaso-dentine or of structureless lamellae without
bone-cells. The most marked characteristic of the group is the large

* Huxley, Geological Survey of the United Kingdom, 10, 18G1. Fritsch,
Fauna der Oaskohle in Bohmen, 2, 1889. Reis, Zur Kenntniss des Skelets
der Acanthodinen, Geognost. Jahreshefte, Miinchen, 1890, 1S94. Traquair,
Oeol. Mag., 1888, p. 511 ; 1889, p. 17.

ICHTHTOTOMI. 147

spines on the front of the fins. These appear to have consisted of dentine,
and are doubtless comparable to the spines found in similar positions in
the fins of Elasmobranchs. It is probable that a number of isolated
spines which have received special generic names (Onchus, Byssacanthus,
Homacanthus, etc.) may have belonged to fishes of this group, and that
a number of quadratic scales, e.g. Thelolepis, Coelolepis, etc., from the
Upper Silurian, were part of the dermal armature of similar fishes. Pec-
toral and pelvic fins are always found.

Fia. 86. — Climatius scuHger, outline of fish with spines shaded. The pectoral fins pines pet
are the two large spines next the head ; then follows a double row of smaller spines i, the
last of which are the pelvic fin-spines plv. The large fln with spine a between the paired
spines and the caudal flu is the anal ; d\ d^ dorsal fins.

The eye is surrounded by dermal plates ; the notochord must have been
persistent ; the supports of the fins are not preserved ; the tail is hetero-
cercal and the caudal fin without any trace of upper lobe. Comparatively
small fishes. Acanthodea Ag. (Fig. 85), Lower Devonian to Lower
Permian ; Diplacanthus Ag., and Climatius Ag., Upper Silurian and
Lower Devonian, without teeth, with four or five pairs of spines between
the pectoral and pelvic fins (Fig. 86). Most of the sub-order do not
show teeth, but there is a powerful dental armature in lachnacanthua

Order 3. Ichthyotomi.*

The cartilaginous endoskeleton is permeated by granular calcifications ;
notochord unconstricted, with slight calcifications in its sheath ; neural and
haemal arches calcified, with long spinous processes ; tail diphycercal ;
pectoral and pelvic fins with long segmented axis and biserial radii ; pelvic
fins with claspers in the male. The teeth have two large lateral cusps, with
one small median cusp. Lower Carboniferous to Lower Permian.

Pleuracanthus Ag. {Xenacanthua Beyr.) (Fig. 87). Body elongated,
to half a metre ; skin probably naked, with a long spine attached to the
occipital region of the cranium ; five, possibly seven, branchial arches ;
all the fins with dermotrichia ; pectoral girdle arch-Uke, united with its
fellow, composed of two pieces ; pectoral fin (Fig. 76) with segmented
axis ; rachiostichous and pleurorachic, or nearly so (p. 57) ; pelvic fin
similar, but with no postaxial somactids, with claspers ; pelvic girdle of
two separate arches. The somactids of the long dorsal fin are segmented

* Fritsch, loc. cit. Davis, On the Fossil Fish Remains of the CoEd
Measures in the Brit. Islands, I. Pleuracanthidae, Trans. Roy. Dublin
Soc, 4, 1892.

us

SUB-CLASS ELASMOBRANCHII.

rm

%

li^

m

'j'^/'"

m

'.^i

s

mi

viafc.

Fig. 87 .—Pleuracanthus deckeni,
restored by A. Fritsch, x i ;
Lower Permian, Bohemia (from
Woodward). A^, A^ anal fins;
C dorsal part of caudal fin ; D
dorsal fin. The specimen from
which this figure was taken has
been crushed in such a way
that the paired fins appear to
have their postaxial sides
turned forward.

intu tliree pieces, and are twice as numer-
ous as the neural arches, those of the
dorsal part of the caudal fin are similar,
but equal in nximber to the neiu-al arches ;
the anal fin is double, and its supports
are partly fused, and branch peripherally.
Didymodiis Cope, Permian of Texas, skull
shows symmetrical fissm-ing, to which the
name of the sub-order is due.

Order 4. Selachii (Plagiostomi),

Elasmobranchii with characters of
the soft parts as defined for the sub-
class, with hyostylic skull [except
Notidanidae) and heterocercal tail.
The notochordal sheath is always
segmented, though sometimes imper-
fectly ; the pectoral fins with three
basal cartilages, and the pelvic fins of
the male with claspers.

The mouth is placed on the under
surface of the head, except in
Chlamydoselachus in which it is sub-
terminal, and Rhinodon and Rhina
in which it is terminal. The skin
has detached placoid scales only.
The body is either fusiform or flat-
tened dorsoventrally, and there is
usually a spiracle, but the pseu-
dobranch is absent in the Seym-
nidae, Lamnidae, Myliobatis, Trygon,
etc. In Carcharias and Zygaena, in
which the spiracles are absent, a
pseudobranch is present buried in
the flesh or placed on the front wall
of a recess of the mouth. They are
almost all marine, but a few ascend
American and Asiatic rivers, and a
few are confined to freshwater (some
Trygons, two species of Carcharias).
They have existed since Palaeozoic
times

NOTIDANI.

149

Sub-order 1,

NOTIDANI.

With six or
seven branchial
apertures and a
small spiracle
with one dorsal
fin without spine.
Vertebral col-
umn imperfectly
segmented. Cau-
dal fin without a
pit at its root ;
without labial
fold and nictitat-
ing membrane.

Fam. 1. Chlamy-
doselachidae.*

15 od y eel-like;
mouth anterior ;
nasal opening
divided and on side
of head ; lateral
line as an open
iiroove on the body,
but closed (with
openings left) on
the head ; with
six gill openings
and six branchial
arches ; opercular
fold (first gill-
cover) free across
the isthmus ; the
palatoquadrate is
orbital process of
hyomandibular ;

Fig. 88. — A. CMamydoselachus angui-
neus Garman ; B. side view of the
head of the same (after Garman).

not articulated with the post-
the skull and there is a large
notochord unconstricted pos-
teriorly ; teeth similar in both jaws, each with
three slender, curved cusps separated by a pair
of rudimentary denticles on a broad base ; vivi-
parous. Chlamydoselachus Garman, from the deep
sea, 5-6 feet. Japanese seas, Atlantic and
Arctic.

* Garman, Bull. Mus. Zool. Harvard Colleye. 12,
1S85.

150 SUB-CLASS ELASMOBRANCHII.

Fam. 2. Notidanidae. Mouth sub-inferior ; nostrik on lower side,
nearer snout than mouth ; dentition unequal in the two jaws ; in the upper
jaw one or two pairs of awl-shaped teeth, the next six teeth broader and
each provided with several cusps, one of which is the strongest ; lower
jaw with six large, comb-like teeth on each side, beside the smaller pos-
terior teeth ; viviparous ; sometimes reach a large size. The palato-
quadrate articulates with the postorbital process of the skull and the hyo-
mandibular is comparatively slender ; each segment of the vertebral
column of the middle part of the trunk region carries two neural arches
and corresponds to two pairs of spinal nerves (p. 126); the pseudobranch
is very large and has several well-developed laminae. Temperate and
warm seas. Hexanchus Raf., with six pairs gill-apertures ; vertebrae
without calcification ; H. griseus Gmelin, 8-26 feet, B't^editerranean,
W. coast Scotland ; Heptanchus Raf., with seven pairs gill- apertures,
vertebrae asterospondylous.

Sub-order 2. SQUALL

Vertebral column well segmented, vertebrae amphicoelous
with a double cone of calcified cartilage, outside which and
springing from it there may be radiating calcareous lamellae
(asterospondylous) or additional concentric calcified rings (cyclo-
spondylous) ; with two dorsal fins, and with or without anal fins.
With five gill apertures laterally placed, spiracle present or
absent, never large. The palatoquadrate is not articulated
directly to the skull except in Cestraciontidae.

This sub-order includes the great body of living sharks, and has
existed since the Silurian period. Some of the living genera have
existed since early times ; e.g. Cestracion, Upper Jurassic ;
ScylUum, and Scapanorhynchus {Mitsukurina) Cretaceous ;
Pristiurus, Upper Jurassic. Most of them are active predatory
creatures, and some attain a considerable size. The largest
are however harmless creatures, which like the whalebone
whales exist on small marine organisms which are detained on
their prolonged gill-rakers {Selache, Rhinodon).

Fam. 3. Cestraciontidae. Bull-head sharks ; asterospondylous ; the
palato-quadrate articulates by an extensive surface with the preorbital
region of the skull ; two dorsal fins with spines, the first dorsal opposite
the space between pectorals and pelvics, the second in advance of the
anal ; upper lip divided into seven lobes, the lower with fold ; spiracle
small, below posterior part of eye ; without nictitating membrane ; den-
tition similar in both jaws, viz. small obtuse teeth in front, pointed and
provided in young individuals with three to five cusps ; lateral teeth
large, pad-like, twice as long as broad ; Pacific and East Indian Archi-
pelago ; size small ; oviparous, egg-case spirally twisted. Cestracioti
Cuv. {Heterodontus Blainv.), C. phillipi Blainv., Port- Jackson shark.

SQUALL 151

Extinct genera are Orodus Ag., Campodus de Kon., Sphenacanthus Ag. ,
Carboniferous Limestone ; Hyhodus Ag., Trias to Cretaceous ; Palaeo-
spinax Eg., Lias ; Acrodus Ag., Trias to Cretaceous, etc.

Fam. 4. Scylliidae. Dog-fishes ; asterospondylovis, dorsal fins
without spine ; first dorsal above or behind the pelvic ; an anal fin ; no
membrana nictitans ; spiracle distinct ; mouth inferior ; teeth small ;
nostrils near the mouth, sometimes confluent with it, sometimes ^vith cirri.
ScylUum Cuv. {Scyllicrhinns Blainv.), upper edge of caudal fin smooth ;
Sc. canicula Cuv., small-spotted dog, single nasal flap, pelvic fins separated ;
eggs laid in April, hatched in December ; Sc. catulus Cuv., large-spotted
dog, nurse hound, nasal flap divided, pelvic fins almost conjoined ; Pris-
tiurus Bon., snout much produced, small flat spines along upper edge of
caudal fin, P. melanostomus Bon. ; Ginglymostoma il. and H., large sharks
of the warm seas, nostrils confluent with mouth ; Stegostoma ^L and H.,
tiger-shark to 15 feet, India ; Parascyllium Gill, Tasmania, 2i feet ;
Chiloscyllium M. and H., nasal and buccal cavities confluent, Indian and
Austrahan, 2|- feet ; Crossorhinus M. and H., Australia and Japan to 10
feet, mouth nearly anterior, nasal and buccal cavities confluent.
Extinct genera : Palaeoscyllhim Wagn., L^pper Jurassic ; Mesiteia
Kramb., Cretaceous.

Fam. 5. Carchariidae. Asterospondylous ; first dorsal opposite space
between pectoral and pelvic, witliout spine ; an anal ; with nictitat-
ing membrane ; mouth crescentic, inferior ; spiracles present or absent.
Carcharias M. and H. (sub-genera, Scoliodon ]M. and H., Physodon M. and
H., Aprionodon Gill, Hypoprion M. and H., Prionodon M. and H.) com-
prises the true sharks ; no spiracle ; teeth with a single cusp, snout pro-
duced, pit at root of tail ; temperate and tropical ; C. glaucus Cuv.,
blue-shark, 25 ft. ; C. gangeticus of the Ganges and inland lakes of the Fiji
Islands, and C. nicaraguensis G. and B. , Lake Nicaragua, only fresh-
water sharks known, 7 ft. ; Hemigaleus Bleek, East-Ind. Archipelago ; Loxo-
don JI. and H., Ind. Ocean ; Galeocerdo M. and H., arctic, temp,
and >rop. seas ; Thalassorhinus M. and H., Med. and Atl. ; Galetis
Cuv., small spiracle, teeth with single cusp, snout elongated, no pit at
root of tail, temp, and trop., viviparous ; G. canis Bond., tope,
whithound, penny dog, miller's dog, 7 ft. ; Zygaena Cuv. (Sphyrna
Raf.), temp, and trop., no spiracles, hammer-headed sharks, eyes at
extremity of head lobes ; Z. malleus Shaw, hammer-shaped head, vivi-
parous, balance fish ; Triaenodon JI. and H., no spiracles. Red Sea,
Ind. Ocean ; Leptocarcharias Smith, no spiracles, S. Africa ; Triads
M. and H., Ind. and Pac. Oceans ; Mustelus Cuv., viviparous, teeth flat
and paved, temp, and trop. seas, spiracles small, no pit at root of tail,
bottom-fish ; 31. laevis Risso, embryo attached to uterus by placenta ;
M. vulgaris I\I. and H., no placenta, 6 ft., smooth hound, skate-toothed
dog. Scylliogaleus Blgr., Natal.

Fam. 6. Lamnidae. Mackerel sharks. Large sharks with large
teeth ; first dorsal opposite space between pectoral and pelvic, without
spine ; asterospond ylous ; an anal fin ; no nictitating membrane ; mouth
crescentric, inferior ; spiracles absent or minute, varying even in the same
species ; pelagic, attain large size. Lamna Cuvier, Porbeagles, teeth
lanceolate, large, with smooth edges, keel at side of tail, temperate and
tropical seas ; L. cornubica Gmelin, Porbeagle (porpoise and beagle) or
Beaumaris shark, viviparous, 10 ft. ; Carcharodon M. and H., C. rondeletii
M. and H., great blue-shark, man-eater, spiracle minute or absent, Med. to

152 SUB-CLASS ELASMOBRANCHII.

Australia, 30 ft. ; Odontaspis L. Ag., toiiip. and trup. seas ; Aloperidn
(Alopias) M. and H., teeth triangular, flat, smooth edges, caudal fin
long, no keel at side of tail, temp, and trop. seas ; A. vulpes Gmelin,
Mu-asher-shark, fox ; Selache Cuv. (CetorJiinus Blainv.), teeth small,
aumerous, conical and smooth, keel at side of tail, whalebone-like gill-
rakers consisting of dentine on the gill-arches, Arctic to IMed. ; S. maxi-
mus Gunner, baslcing-shark, siui-fish, one of the largest of livmg fishes,
to 40 ft., large gill apertures, vertebrae appear to be tectospondylous,
owing to presence of a number of concentric lamellae in the adult ; in-
offensive, of great strength, has been known to tow a 70-ton boat against
a fresh gale ; a large fish yields li tons of oil ; Pseudotriacis Capello.
Mitsukurina Jordan {Scapanorhynchus S. Wood), Japan. Extinct
genera : Orthacodus S. Wood, Jurassic and Cretaceous.

Fam. 7. Rhinodontidae. Whale-sharks ; asterospondylous ; origin
of fii'st dorsal fin in front of pelvic ; the second small, opposite the anal,
both without spines ; a pit at root of caudal ; side of tail with keel ;
spiracle small ; membrana nictitans absent ; mouth and nostril near the
front of snout ; teeth small ; gill openings wide, with gill-rakers. Cape of
Good Hope, Seychelles, Japan. Bhinodon Smith ; a gigantic shark known
to exceed 50 ft., said to attain 70 ft.

Fam. 8. Spinacidae. Cyclospondylous ; spiracles present ; gill
openings narrow ; without nictitating membrane ; a deep gi'oove along
either side of the mouth ; a spine on front side of each dorsal fin ; without
anal fin. Centrina Cuv. [Oxynotus Raf.), body somewhat three-sided
with a fold of skin at each angle, teeth in lower jaw triangular, erect and
with finely serrated edges, no membrana nict., Med. and adjacent At-
lantic ; C. Salviani Risso, attains to 4—5 ft. ; Acanthiaa Risso (Squalus),
teeth rather small, their points placed so obUquely that their inner margin
which is smooth forms the cutting edge, no membrana nict., temp,
seas of both hemispheres ; A. vulgaris Risso, picked dog-fish, spur-, spear-,
or bone-dog, hoe, skittle-dog; viviparous; Centroplwrus M. and H.,
Evu". seas, Moluccas; some species five at a great depth (400-500
fathoms) ; Scyinnodon Boo and Cap. ; Spinax Cuv. {Etmopterus Raf.),
Eur. Seas, W. Indies ; Centroscy Ilium M. and H., Greenland, has been
taken 300-400 fathoms.

Fam. 9. Scymnldae. Like Spinacidae, but no spines on dorsal fins ;
Scymnus Cuv., Med. and Atlantic ; Laemargus M. and H. (Somniosus
Le Sueur), teeth in upper jaw small and conical, those in lower jaw in several
rows, their points placed so obMquely that their inner margin, which is
smooth, forms the cutting edge ; L. microcephalus Kroyer {borealis M. and
H.), Greenland shark ; notochordal sheath imperfectly segmented, un-
calcified (calcified and segmented in L. rostratus) ; attains 25 ft., bites
pieces out of whales ; with two pyloric caeca ; eggs large, soft, globular,
without shell, dropped in the ooze on the sea bottom, said to be fertilised
externally; they breed at considerable depths (100 fathoms); Euproto-
micrus Gill, Ind. Ocean ; Echinorhinu^ Blainv., skin with irregularly
placed rotuid osseous tubercles, teeth large, oblique, with several small
cusps on each side of the main one, Med. and Atl., E. spinosus Blainv.,
to 8 ft. ; Isistius Gill.

Fam. 10. Rhinidae. Ray-like sharks. Tectospondylous ; spiracles
large, gill openings wide, lateral, and partly concealed from above by
pectoral fins; body flat ; mouth anterior ; nostrils at front end of snout

BATOIDEI. 15B

witli skiiuiy valvular coverings ; pectoral fins large, laterally expanded,
but not attached to head ; dorsal fins spineless, in caudal region, no anal ;
males with claspers ; temperate and tropical seas ; intermediate between
the sharks and rays. Rhina Klein [Squatina Dum.), angel-fish, monk-
fish, viviparous, to 8 ft.

Fam. 11. Pristlophoridae probably here. Shark saw-fish. Rostral
cartilage produced into long flat lamina, armed along each edge with teeth ;
Pristiophorus M. and H., Japan, Australia.

Sub-order 3. RAJI (BATOIDEI).

Gill openings ventral, five in number ; spiracle always present,
without anal fin ; dorsal fins, if present, on the tail ; vertebrae
tectospondylous. Skates and Rays.

The body is much flattened dorso-ventrally and the pectoral
fins are enormously expanded in an antero-posterior direction.
The five branchial apertures are entirely on the ventral surface
of the body. The spiracles are dorsally placed behind the eyes ;
they are wide and can sometimes be closed by a valve. It is
probable that they are used for the intake of ths respiratory
water when the fish is lying on the ground. The caudal region
is usually slender, and in some forms very much so. There is no
anal fin, and the dorsal fins when present are placed on the tail.

The Pristidae and Rhiiiobatidae, which have a well-developed
caudal region and are intermediate in the form of their body
between the sharks and rays, are powerful swimmers, but most
of the Raji lead a more sedentary life on the bottom, rarely
coming to the surface. They feed chiefly on Mollusca and
Crustacea. A few deep-water forms are known, but they are
rarely taken below 100 fathoms. Most are shore-forms.
The Myliobatidae, which include the largest forms in the sub-
order, are however met with in the open sea. Some species
are confined to fresh water. They are for the most part
oviparous. The flesh of many of the species is eaten. Some
of the living families have existed since the Jurassic.

Fam. 1. Pristidae. Saw-fishes. Snout much produced (rostral
process of cranium) with lateral saw-like teeth ; body somewhat shark-
like, the disc-like body gradually passing into the tail, which is com-
paratively thick, with two dorsal fins and a caudal fin, without serrated
caudal spine. Pristis Latham, tropical and sub-tropical, attain a con-
siderable size, with a saw 6 ft. long and 1 ft. broad at base.

Fam. 2. Rhinobatidae. Tail long and strong with two dorsal fins,
a caudal and a longitudinal fold on each side, without serrated caudal
spine ; rayed portion of pectoral fins not continued to snout ; no electric

I f) i SUB-CI.ASS ELASMOBRANCHII.

organ ; viviparous. Rhinobalus Bl. and Schn., guitar-fishes, tropical and
sub-tropical, and fossil from the upper Jurassic ; Rhynchohatus M. and
H., Ind. Ocean to China ; Trygonorhina M. and H., Australia ; Zapteryx
Jor. and Gilb., Peru ; Platyrhinoidis Gar., California. Extinct genera :
Asterodermus Ag., Belemnobatis Thiol., Upper Jurassic.

Fam. 3. Torpedinidae. Trunk a broad, smooth disc ; tail with
rayed dorsal (absent in Temera) and caudal fins and a longitudinal fold
on each side, without serrated dorsal spine ; anterior nasal valves con-
fluent into a quadrangular lobe. An electric organ between the pectoral
fins and the head. Eocene to the present time. Torpedo Dum. (Narco-
batia Blainv.), large specimens (width from 2 to 3 ft.), can disable
a man. Med., Atl., Ind. Oceans ; T. nobiliana Bon., spiracles not
fringed at their margins ; on flat sands or mud, 40-50 fathoms ; T. mar-
morata Risso, spiracles fringed ; Narcine Henle, trop. and sub-trop. ;
Hypnos Dum., Australian ; Discopyge Tschudi, Peru ; Astrape M. and H.,
Ind., S. Afr. ; Temera Gray, E. Ind.

Fam. 4. Rajidae. Skates. Disc broad, rhombic, generally with
asperities or spines ; tail with longitudinal fold on each side ; pectorals
extend to snout ; no electric organ or serrated caudal spine ; oviparous ;
sexual differences are frequently observable, in colour, form of teeth and
arrangement of spines. Cretaceous to the present time. Raja Art., tail
distinct from disc, pectoral fiins not extended to front end of snout, caudal
fin rudimentary ; may attain width of 6-7 ft. ; in some species the teeth
of the male are sharper than in the female, and in all species the males
are armed with patches of claw-like retractile spines on the upper side
of the pectoral fin ; seas of both hemispheres ; R. batis L., skate, oviposi-
tion from May to September, to 6-7 ft. ; R. macrorhynchus Raf., flapper
skate ; R. alba Lacep., white skate, to 8 ft. ; R. oxyrhynchus L., long-
nosed skate ; R. fullonica L., Fialler's ray, shagreen skate ; R. clavata L.,
thornback ; R. maculata Montagu, homelyn ray, spotted ray ; R. micro-
cellata Montagu ; R. radiata, starry ray ; R. circularis Couch, sandy ray.
Psammobatis Giinth., South America ; Sympterygia M. and H. ; Platy-
rhina M. and H. Extinct genus : Cyclobatis Eg., Cretaceous.

Fam. 5. Trygonidae. Sting-Rays. Pectorals continued to and
confluent at end of snout ; tail long and slender, without lateral folds ;
vertical fins none or imperfect, often replaced by strong serrated spine.
Tertiaries to present time. Urogymnus M. and H., Ind. Oc. ; Ellipe-
surus Schomburgk ; Trygon Adanson {Dasyatis Raf.), tail with long
serrated spine, temp, and trop. ; T. pastinaca, sting ray, sandy ground
near land, caudal spine causes severe wounds ; Taeniura M. and
H., Indian seas, fresh-waters of trop. America ; Urolophus M. and H.,
Australian and Caribbean seas ; Pteroplatea M. and H., temp, and
trop. seas.

Fam. 6. Myllobatidae. Eagle-rays. Disc broad with large pectoral
fins which are not present at the sides of the head, but reappear as at the
extremity of the snout as a pair of detached fins ; teeth hexagonal, flat,
tesselated ; tail long, thin, whip-like ; viviparous. Myliobatis Cuv.,
snout with a soft prolongation with fin rays ; temp, and trop. seas ;
M. aquila L., mill-skate, whip-ray, eagle-ray ; Aetobatis M. and H.,
tropical seas ; Rhinoptera Kuhl, trop. and sub-trop. seas ; Dicero-
batis Blainv. {Aodon Lac.) (Cephaloptera Dum.), head with a forwardly-
pointing horn-like projection on either side ; attain great size ; temp,
and trop. seas ; D. giornae. Ceratoptera M. and H. {Manta Bancroft),

HOLOCEPHALI. 155

temp, and trop. seas, attain great size, 20 ft. wide. Last two genera
often called sea-devils. Extinct genera : Ptychodus Ag., Cretaceous ;
Promyliobatis Jaekel, Eocene.

The following extinct Palaeozoic families are placed here : —
CoCHLiODONTiDAE, with Several genera, from the Carboniferous Lime-
stone ; PsAJMMODONTiDAE, from the Carboniferous Limestone ; and
Petaxodontidae, also from the Carboniferous.

Order 5 — Holocephali.*

Without spiracle, with four clefts covered by an opercular fold
which contains a cartilaginous plate. The skull is autostylic, and
the notochordal sheath is unsegmented. There are two dorsal fins
and an anal.

The Holocephali differ from the Plagiostomi in the fact that
there are only four gill clefts (though there are five branchial

FlQ. 89. — Chimaera monslrosa (E6gne Animal).

arches). Moreover the gill apertures are covered by an opercular
fold, and the palatoquadrate bar is continuous with the skull
in its whole extent. They have a cartilaginous skeleton and
claspers on the pelvic fins of the male.

The mouth is small, ventral, and bounded by lip-like folds
supported by labial cartilages. The nostrils are confluent with
the mouth. The urogenital part of the cloaca is separate from
the rectum and opens behind the anus. The anterior dorsal fin
has a strong spme, on its front border, which is attached to the
fused neural spines of the anterior vertebrae. The tail is hetero-
cercal and prolonged in Chimaera into a long filament. There

* G. Good and T. H. Bean, Oceanic Ichthyology, Memoirs of the Museum
of Comp. Anat. Harvard College, 22, 1896. A. A. W. Hubrecht, Kent-
niss des Kopfskelet d. Holocephalen, Niederldnd Arch. Zool, 3, 1877.
S. Garman, The Chimeroids, Bull. Mus. Harvard Coll., 43, 1904.

J 56 SUB-CLASS ELASMOBRANCHII.

is on each side in tlie male a peculiar structure consisting of
a plate carrying teeth and sunk in a pit just in front of the
pelvic fin.

The head of the male is provided with an erectile hook-like
process projecting forwards over a groove and armed on its lower
surface with small spines. The skin is usually naked except
in the young, in which small placoid spines are found principally
in a double row on the back. The lateral line may be an open
groove (Chimaera) or a closed canal [Callorhynchus). In
Chimaera the lips of the groove are approximated on the
head (Fig. 42), but remain apart at intervals giving the appear-
ance of openings. AmpuUary canals are present as in Plagio-
stomes. The eyes are without lids.

The notochordal sheath is thick, cartilaginous and unsegmented.
It contains in Chimaera numerous calcified rings (four or five to
each segment). The arch tissue is segmented, except in the
front and in the whip-like tail, and does not meet round the
notochord sheath, except again at the front end and in the tail.
The neural arches consist of two pairs of pieces and a dorsal
piece for each pair of spinal nerves. The neural spine of the
anterior fused arch tissue is large and carries the anterior dorsal
fin, the basals of which are fused into one piece.

The skull is autostylic (p. G3), has rostral continuations and is
without the prefrontal fontanelle. It has a well marked mem-
branous interorbital septum, which is placed dorsal to the brain.
It articulates with the vertebral column by two condyles. The
auditory capsule is incomplete internally so that the space for
the membranous labyrinth is open to the cranial cavity. There
are three pairs of labial cartilages and the hyoid arch which
carries branchial rays is attached by ligament to the skull.
There are five branchial arches. The hyoid arch carries a
demibranch (uniserial), the first three branchials each have a
holobranch (biserial), and the fourth branchial carries a demi-
branch. There is no gill-cleft between the fourth and fifth
branchial arches. The gill filaments are attached as in Plagio-
stomes and do not project.

The paired fins and their girdles are formed on the Plagiostome
type, except that the two halves of the pelvic girdle are united
only by ligament.

The teeth are large and few in number consisting of strong

HOLOCEPHALI. 157

plates with cutting edges. There are two pairs in the uj^ptr
and one pair in the lower Jaw. Tlie intestine has a spiral
valve and the anus is in front of the urogenital aperture. There
is a conus arteriosus with three rows of valves.

The brain is characterised by the great length of the thala-
mencephalon, and the cerebrum is very small. The olfactory
peduncles are long and there is an optic chiasma. The pineal body
is as in Plagiostomes, and there is an extracranial part of the
pituitary body lodged in a pit on the base of the skull. The
cranial nerves* are arranged in the usual manner ; the roots of
the fifth and seventh are more distinct than in most fishes.

There is a pericardio-peritoneal canal.

The urogenital organs appear to be similar to those of Plagio-
stomes. In the female the shell gland is large. In the male
there is a large vesicula seminalis, and the miillerian ducts are
complete tubes opening at each end.

They are oviparous and have large eggs. The eggshell is
covered with hair-like processes, and may attain a great size.
They are probably laid in deep water, where the young are for
the most part found.

They have existed since the Jurassic period.

In many points of structure these animals depart from other
Elasmobranchs, and they have by some authorities been removed
from that group and raised to the rank of a sub-class with.
affinities to the Dipnoi by the characters of their skull and teeth.
There is much to be said for this view, for they present affinities
to more than one piscine sub-class : to Elasmobranchs by their
placoid scales, cartilaginous skeleton, absence of membrane bones,
their gill-laminae, the open otocyst, the ampullary canals, the
form of the brain, the structure of the urinogenital organs, their
fin skeleton and claspers, and by their large eggs and develop-
ment : to the Ganoids by the separation between the urinogenital
sinus, and the ahmentary canal, and by the incomplete internal
wall of the auditory capsule : and to the Dipnoi by the last-
named feature, by their autostylic skull, their peculiar teeth, and
their vertebral column. They differ from Elasmobranchs and
resemble Ganoids and Dipnoi in having an operculum, but they
stand by themselves in having only four branchial clefts and a

*r. J. Cole, Cranial Nerves oi Chimaera. Trans. Roy. Soc. Edinburgh,
38, 1S96, p. 631.

158 SUB-CLASS ELASMOBRANCHII,

demibranch on the fourth branchial arch. It is clear from this
that the Elasmobranch characters strongly predominate, and
in our opinion they may fairly be retained as an order of that
subclass.

Chimaera L., snout soft, prominent, without appendage ; tail produced
into a fine filament ; deep water (200 to 1,200 fathoms) of coasts of
Europe, N. Pacific, Cape ; C. monstrosa L., King of the Herrings ;
attains 3 or 4 ft.

Hydrolagus Gill, like Chimaera, but three dorsals and caudal, and tail
without filament ; surface waters, N. Pacific.

Callorhynchus Gronov. Snout with a cartilaginous prominence ending
in a cutaneous flap ; S. Pacific, Cape ; egg-case 9 or 10 in. x 3 in.

Hariotta Goode and Bean. Snout elongated, no frontal clasper, anal
fin as cutaneous fold, deep water 700 to 1,200 fathoms.

Extinct genera*: Ischyodus Eg., Upper Jurassic and Lower Cretaceous ;
Oanodus Ag. ; Edaphodon Buckl., Cretaceous, Eocene, Oligocene ; Pachy-
mylus Smith, Upper Jurassic, etc.

The extinct families Ptyctodontidae, known by teeth from the De-
vonian ; Sqctalorajidae, knowm by its skeleton, from the Lias ; Myria-
CANTHiDAE, also knowH by skeletons from the Lias and Upper Jurassic,
are placed here.

* E. T. Newton, Chimaeroid Fishes of the British Cretaceous Rocks,
Mem. Geol. Soc. U. Kingdom, 1878.

CHAPTER V]T.

SUB-CLASS GANOIDEI. *

Fishes with a conus arteriosus, optic chiasma, free pectinate gills
a7id an operculum, abdominal pelvic fins, a spiral valve in the intes-
tine, an air -bladder, and without a processus falciformis and
choroid gland. The oviducts and urinary ducts unite and open by
a common urogenital aperture behind the anus. The skull is hyo-
stylic and is without a supraoccipital bone. The segmentation of
the ovum is complete, a pronephros is present in the larva and
abdominal pores are always found.

Very few of the characters mentioned in the definition are pecu-
liar to Ganoids : they are ahnost all found in Teleostei or Elas-
mobranchii. This fact coupled with the great variations of
structure found in the group points to the conclusion that the
sub-classes, Elasmobranchii, Teleostei, Ganoidei and Dipnoi are
the survival of a once great and continuous group of animals, a
large number of which have become extinct, leaving three groups,
Elasmobranchii, Teleostei and Dipnoi, each fairly compact and
showing but little variety of organization, and one, the Ganoidei,
loose and heterogeneous with large gaps between the individual
members. Although it may fairly be held that by such forms
as Lepidosteus and Amia the Ganoids more nearly approach the
Teleostei than the Chondrostei do the Elasmobranchii ; we cannot

* J. Miiller, Ueb. d. Bau u. d. Grenzen der Ganoiden, Ahh. d. Berlin
Akad. d. Wiss. 1844 ; id. Myxinoiden, op. n'f. J. Hyrtl, Ueb. den Harn-
werkzeuge bei den Ganoiden, Wien Denkschriften, 8, 1855. Liitken, Ueb. d.
Begrenziing imd Eintheilung d. Ganoiden, Palaeontographica, 22, 1872.
Huxley, ' The systematic arrangement of the fishes of the Devonian
Epoch,' Mem. Geolog. Survey. London, 10, 1861 ; and 12, 1866. Tra-
quair. Ganoid fishes of the British Carboniferous Formations, Palaeontogr.
Soc. 1877. V. Wijhe, Visceral Skel. u. Nerven des Kopfes der Ganoiden u.
Ceratodus, Nied. Arch. Zool., 5, 1882, p. 207. Zittel, Grundzuge d. Palaeon-
tologie, Leipzig, 1895 (English Translation, Macmillan, 1902). Smith-
Woodward, Outlines of Vert. Palaeontology, Cambridge, 1898.

160 SUB-CLASS GANOIDEI.

agree with those zoologists who wish to unite the Ganoids and
Teleosteans into a single group, distinct from other piscine orders.
In this opinion we are in company with two of the greatest ana-
tomists of the last century, J. Miiller and F. M. Balfour. The
latter has expressed his view in words, with which we are in
entire agreement and which we quote here, because they appear
to express in the most judicial form the state of the question.
He says, " We do not recommend such an arrangement (union
of the Ganoids and Teleosteans) which in view of the great pre-
ponderance of the Teleostei amongst living fishes would be highly
inconvenient, but the step from Amia to the Teleostei is certainly
not so great as that from the Chondrostei to Amia, and is un-
doubtedly less than that from the Selachii to the Holgcephali.^^

The scales present some variation in arrangement and struc-
ture. In the living Chondrostei they may be almost absent on
the body as in Polyodon, or arranged in rows as in Acipenser ;
not, however, forming a continuous cuirass except in the caudal
region. They frequently carry bony spines, which are without
any enamel cap. In some extinct Chondrostei they form a con-
tinuous cuirass, and have often the rhombic form tjrpical of the
order In Crossopterygii they form a continuous armour and
are cither rhombic or cycloidal ; in the living forms it can be
shown that they are coated externally by ganoin and that in
some cases they carry spines which consist of cones of dentine
capped with enamel. In Lepidostei there is also a continuous
armour of rhombic or cycloid scales, and in the living Lepidosteus
it has been shown that these scales are coated with ganoin
{yai'o? sheen) and may, especially in the young state, carry one
or a number of small spines having exactly the structure of
the spine of a placoid scale.

In Amia the scales, which form a complete armour, are
Teleostean in character and consist of bony plates without ganoin.
Moreover it has been shown from a study of living forms that
the scales save for the toothlike projections which occur in Poly-
pterus and Lepidosteus are purely mesodermal products, and that
the ganoin which was formerly thought to be enamel is really
of the nature of vitrodentin and is formed by the scleroblasts *
of the dermis.

* Scleroblasts are cells which secrete a hard skeletal substance.

VERTEBRAL COLUMN.

161

3

Although the scales are dermal structures, they are said to be
frequently exposed on the surface in the adult. This, if true,
must be due to the fact that the overlying epidermis and dermis
have been rubbed off. This may frequently happen in the hand-
ling of animals during their preservation.

Vertebral column. In the Chondrostei the notochord is
persistent and its sheath is stout but unsegmented and unossified.
The neural and haemal arches on the other hand are segmented ;
they are attached to the sheath but do not extend round it. In
other living Ganoids (for the vertebral column of extinct genera
the reader is referred to the special accounts), vertebral bodies
are formed by the extension of the arch tissue round the sheath,
its chondrification, segmentation and ossification. In Poly-
pterus and Amia the
vertebral bodies are
amphicoelous as in
Teleosteans ; in Lepi-
dosteus they are con-
cave behind and con-
vex in front (opistho-
coelous).

In some extinct Ganoids
(Fig. 90a) bony plates
are found, corresponding
to each arch, on the
ventral side of the noto-
chord. They are called hypocentra, and carry the haemal arches.
A corresponding plate, which may be composed of two pieces, is
found on the dorsal side, and called pleurocentrum. Such incipient
vertebrae are called half-vertebrae. They may, each of them, extend
completely round the notochord (Fig. 90b), as in the tail of the
living Amia, in which case one of them only carries the neural and
haemal arches. Ring-vertebra is the term used when the pleurocent-
rum and hypocentrum are joined to form a ring round the noto-
chord, as in the amphicoelous vertebrae of Polypterus, Teleostei, etc.,
in which the bony ring has thickened so as to constrict the notochord
in the middle of each vertebra. In Lepidosteus the arches are continuous
with the bony centrum. In all other Ganoids with bony centra the
arches are separated from the centra by persistent cartilage.

A B

Fig. 90. — A. Vtitt'bra of Calurus fuicaius. B. Caudal
vertebrae of Eunjcormus rpeciosus (after Zittel). 1
neiiral arch ; 2 the bifurcated neural spine ; 3 hypo-
centrum ; 4 pleurocentrum ; 5 rib.

The caudal fin is diphycercal in Polypterus. In other living
genera it is heterocercal. In Amia it is hemiheterocercal (ex-
ternally homocercal, internally heterocercal). and the dorsal
lobe of the caddal fin is reduced to the covering of fulcra.

z.-n.

M

162 SUB-CLASS GANOIDEI.

The structure of the skull is very varied, but the cartila-
ginous cranium always contains cartilage bones and is invested
by membrane bones. The dorsal membrane bones are dermal
structures and are frequently in the adult exposed on the surface
in consequence of the superjacent epidermis and dermis having
been rubbed off (see p. 161). The suspensorium is always
hyostylic.

The shoulder girdle and pectoral fin skeleton presents great
variation, but on the whole inclines to the Teleostean type of
structure. In the Chondrostei and in Amia the pectoral girdle
is cartilaginous. In the other orders it is ossified very much as in
Teleosts.

In the Chondrostei and Crossopterygii there are three separate
membrane bones in relation with the shoulder-girdle, the supra-
clavicle, clavicle and infra-clavicle,* while in the other orders the
infra-clavicle is absent. In the pectoral fin the skeleton of
Polypterus is on the Elasmobranch type (rhipidostichous), in
extinct Crossopterygians on the Dipnoan type (rachiostichous
and mesorachic). In other orders the arrangement on the
whole is Teleostean.

The other anatomical characters of the Ganoids are (1) the
possession of a conus arteriosus with more than one row of
valves ; (2) the very general presence of a gill on the hyoid
arch supplied by a branch of the ventral aorta, or of a spiracle
(see below) ; (3) the presence of an air-bladder with pneumatic
duct ; (4) the union of the urinary and Miillerian ducts and their
opening by a median urogenital pore behind the anus ; (5) the
testis appears to be connected except in Polypterus with the
kidney by a testicular network ; (6) a spiral valve is present in
the intestine (small in Lepidosteus and Amia) ; (7) the optic
nerves form a chiasma ; (8) the openings of the nasal pits are
double as in Teleosteans ; (9) the processus falciformis and choroid
glands are absent in Polypterus, Lepidosteus, and possibly in
other Ganoids ; (10) the scales are bony plates embedded in the
dermis, and frequently covered by a layer of peculiar substance
called ganoin. Ganoin is probably vitro-dentin : it used to be

* These bones are now often called supracleithrum, cleithrum and
clavicle respectively, on the view that the last-named alone is homo-
logous with the clavicle in the Amphibia and Amniota, the cleitliral
elements not being represented in these groups.

BRAIN. GILI^S,

163

regarded as enamel. These scales in the young state and some-
times throughout life bear spines which project through the
epidermis and are formed of dentine capped by enamel {Poly-
pterus, Lepidosteus). (11) The fins frequently, not always, possess
a single or double row of spine-like scales, called fulcra, on their
anterior edge (absent in Polypterus, Polyodon and Amia). The
dermotrichia are soft and segmented, and the pelvic fins are
abdominal. (12) They lay relatively small eggs which undergo
complete cleavage, and the young are hatched out as larvae
which differ in many respects from the adult and possess a prone-
phros as in Teleosteans. In Lepidosteus the medullary canal
arises as a solid keel-like projection of the ectoderm which
subsequently

becomes hoi- „ ^P^

low. P^'

The brain
is on the
whole Teleos-
tean - like .
There is a
thin pallium
(Fig. 91) and
a val vul a
cerebelli and
the cerebrum
is undivided ;
though it

may be grooved externally so as to suggest a division into two
lobes.

The branchial apparatus presents remarkable variation. In
Spatularia {Planirostra edentula) there is no hj^oid gill, but a
pseudobranch in the spiracle as in Acipenser. In Polypterus
there is no hyoid gill, nor spiracular pseudobranch. The spiracle
is present in Acipenser, Polyodon and Polypterus, but absent in
Scaphirhynchus, Lepidosteus and Amia. The following table
summarizes the matter : —

Hyoid gill, pseudobranch and spiracle . Acipenser.

Hyoid gill, pseudobranch* but no spiracle Lepidosteus.

* It is doubtful if the struct\ire identified by Miiller as pseudobranch in
Lepidosteus is really such.

Fig. 91. — Median section tlirough tlie brain ot Acipenser ruthenus
(from fiegenbaur, after Coronowitsch). Cb cerebellum ; ch
optic nhiasn?a ; cp pineal body ; hy pituitary body ; pi pal-
lium with choroid plexus projecting in between the" third ven-
tricle z and the ventricle of the cerebrum V ; p/' roof of fourth
ventricle ; Sv saccus vasculosus.

164 SUB-CLASS GANOIDEI.

Hyoid gill, but no pseudob ranch or

spiracle Sca'phirhynchus.

Hyoid gill absent, pseudobranch and

spiracle present Polyodon

{Spatularia).

Hyoid gill and pseudobranch absent,

spiracle present Polypterus.

Hyoid gill absent ; pseudobranch and

spiracle also absent (4 double gills) . Amia.

In those forms {Polypterus and Polyodon) without hyoid gill
the ventral aorta still gives a branch to the hyoid arch.

Abdominal pores* are present in all living Ganoids. Their
external openings are placed on each side of the anus and they
open internally into the body cavity.

Pericardio-peritoneal canals are present in the Sturgeons as
single unbranched tubes ; they appear to be absent in other
Ganoids.

Of these characters which on the whole suggest Elasmobranch
more than Teleostean affinities, it will be convenient to treat the
urinogenital organs more fully at this point.

The urinogenital organs present many features of interest,
and as they differ in important points in the different members
of the group it will be necessary to describe them at some length.
In all of those, the development of which is known, there is a
pronephros in the larva, constructed on the Teleostean type, i.e.
it consists of one large malpighian body (Fig. 92, v) connected
by a convoluted tube (pr.w) with the anterior end of the longi-
tudinal duct {sd) of the future kidney. In Lepidosteus and
possibly in others, this body is connected with the peritoneal
cavity by a ciliated canal {/). In none of them does the kidney
show any differentiation into a meso- and meta-nephros, and
in all the oviduct (miillerian) joins the kidney duct of its side
posteriorly. Moreover miillerian ducts appear to be in all classes
present, generally in both sexes, and they present the peculiarity
ot being short and opening into the body cavity at a point much
nearer the anus than in other groups in which they are found.

We have full knowledge of the urinogenital organs in two

* Bles, " Correlated distribution of Abdominal Pores and Nephro
stomes in Fiphes," Journ. Anat. and Phys., 32, 1898, p. 484.

URINOaENITAL ORGANS,

165

Ganoids ; one of these is Lepidosteus of which we have a
description from the pen of Balfour,* and the other Polypterus
in which their structure has been recently elucidated by
Budgett.f

In Lepidosteus the kidneys extend forward from the anus
about three-fifths of the length of the body cavity (Fig. 93, k).
Anteriorly they are continuous with a band of lymphatic tissue
of a very similar appearance. The ureters (sg), which lie on
their ventral side and receive the openings of numerous collecting
tubes, enlarge posteriorly, and approaching each other coalesce
to form an unpaired vesicle (bl) which opens by a median pore

Fia. 92. — Diagrammatic view of the pronephros of Lepidosteus (from Baliour),
isolated and seen from the side, pr.n Coiled tube ; sd longitudinal duct of kidney ;
V malpighian body of pronephros ; / tube leading from v to the body cavity (peri-
toneal funnel) ; bv blood vessel of glomerulus of v.

on a papilla {ug) behind the anus. The ovary is a hollow sac,
attached about its middle to the oviduct and continued back-
wards and forwards from this attachment into a blind process.
The oviduct is a thin-walled tube, continuous in front with the
ovarian sac and opening behind (od) into the dilated part of the
kidney duct of its side. In the male the testis is lobulated and
the vasa efferentia pass in the mesorchium to the kidney, where
they open into a longitudinal canal. From the longitudinal
canal pass off tubules which open into the kidney tubules them-
selves. Near the testis the vasa efferentia are united into an

* Phil. Trans., 1882.

t Trans. Zool. Soc, 15, 1901, p. 323.

166 SUB-CLASS GANOIDEI.

irregular network. No trace of the oviducts has been observed
in the male.

In Amia* there is a testicular network, and the posterior part of
the kidney is provided with nephrostomes. In the female the oviduct
opens into the body cavity.

In Polypterus the oviduct is short and opens anteriorly into
the body cavity, and posteriorly into the kidney duct just before
its union with its fellow. In the male the kidney duct dilates
behind and joins its fellow to form a considerable sinus which
receives the testis duct of each side and opens to the exterior
behind the anus. The testis is an elongated body almost as long
as the kidney, to the ventral side of which it is attached. Two
parts may be distinguished in it — an anterior dilated portion
which forms spermatozoa and is the functional testis, and a
posterior streak-like portion — the testis-ridge — in which the

tig

Fig. 93 — Diagram of the urinogonital organs of an adult female of Lepidosteus (from Balfour
and Parker), od Risht oviduct ; the od which is placed on the upper side of the fisuie
points a little to the right of the opening of the oviduct into the dilated lower end of the
kidney duct ; ov ovary ; bl urogenital sinus ; tuj urogenital aperture ; sg kidney duct ;
the reference line goes a little too far in the figure ; k kidney, ly lymphatic organ. The
organs of the right side only are shown.

testicular tubules are few in number and do not form sperma-
tozoa. The testicular tubules, both of testis and testis-ridge,
open by numerous short ducts into the testis duct which is a
longitudinal canal extending the whole length of the organ close
to the ureter. In Polypterus then the testis tubules do not
communicate with the kidney tubules, but there is a special testis
duct which opens behind into the kidney duct at the point at
which the oviduct opens in the female. No trace of oviducts has
been seen in the male and there are no nephrostomes in the adult.

In the Sturgeon there is a testicular network in the male, as was first
sho^vn by Rathke, and there are short miillerian ducts in both sexes open-
ing widely into the body cavity and behind into the kidney duct. In
the female the miillerian duct is of course the oviduct. Open miillerian
ducts have been seen in both sexes of Scaphirhynchus and Polyodon, but
a testicular network has not yet been seen in those genera.

* Jungersen, Zool. Anzeiger, 23, 1900, p. 328.

CHONDROSTEI. 167

The Ganoids reached their greatest development in the Palae-
ozoic, Triassic and Jurassic periods. From the Cretaceous
epoch onwards they have gradually become less numerous, until
at the present day they are represented by a few widely scattered,
extremely isolated, and for the most freshwater genera. The
Ganoids are here divided into four orders — Chondrostei, Cross-
opterygii, Lepidostei and Amioidei.

Order 1. Chondrostei.

Endo-skeleton largely cartilaginous ; head covered with hony
plates. Body naked or with rows of bony plates, or ivith rhomboid,
rarely cycloid scales. Operculum weakly developed, branchio-
stegals numerous, feiv, or absent; teeth small or absent. Infra-
clavicle present. Caudal fin usually heterocercal. One dorsal and
one anM fin, with fulcra. Paired fins non-lobate. Pelvic fins
ivith a series of basal cartilages. Lower Devonian to present time.

So far as the facts which can be made out from a study of
the extinct forms are concerned, this order agrees with the
Crossopterygians in the feebleness of the ossification round the
notochordal sheath (except Polypterus), and in the presence of
infra- clavicles ; it differs from them in the absence or absorption
into the body of the basal lobe of the paired fins, in the
heterocercal tail, and in the absence of jugular plates between
the rami of the mandible.

There are two living families, the Acipenseridae and the Spa-
tulariidae. The following remarks apply to them : —

The tail is heterocercal. The notochord is persistent and un-
constricted. Its sheath is thick, unsegmented, and entirely within
the elastica externa. The neural and haemal arches are seg-
mented (except in front) and placed upon the sheath, but are not
continuous with one another round it (fi.g. 94). There are inter-
calated pieces both in the dorsal and ventral arches, and there is
a longitudinal ligament on the neural arches. The haemal arches
of the two sides meet ventrally and enclose the dorsal aorta. In
addition they carry lateral projections which must be regarded
as the transverse processes. These meet ventrally in the tail
to enclose the caudal vein, so that in the caudal region there are
two canals, one for the dorsal aorta and a ventral one for the
caudal vein. The anterior part of the column is continuous
with the skull, and here the neural and haemal arches are not

168

SUB-CLASS GANOIDEI.

segmented. The notocliord is continued into tlie base of the
skull to the pituitary fossa in Polyodon, but not in Acipeiiser.

In the Acipenseridae there are five rows of large scales with
projecting spines, * one dorso-median and two on each side
(Fig. 96). Between these are numerous small scale-like plates,
which frequently carry one or more spines. In the caudal
region these smaller plates, also carrying spines, are rhombic,
in contact, and regularly arranged. In Polyodon {Spatularia),
which is often described 9.s naked, there are also small spine-
bearing scale-like structures. The scales are placed in the sub-
cutaneous tissue and are composed of bone (with bone corpuscles
and branched processes, and in the larger scales haversian canals).

The spines are composed of a
similar substance in layers,
either with bone cells in theii"
lower portions or without them.
There is no enamel and appar-
ently no ganoin.

The cranium-j- is massive and
largely cartilaginous, there being
but slight traces of ossification
(in the periotic region). As in
other Ganoids and in Teleostei
the inner wall of the otic capsule
is not developed. The roof con-
sists of almost continuous cartilage, but the fontanelles though
not conspicuous are not altogether absent. It is completely
covered by membrane bones which lie in the dermis. There
is a large parasphenoid and a single or paired vomer in the
floor.

There is a partially ossified hyomandibular and a symplectic
cartilage (Fig. 95) which suspend the mandibular arch. The
anterior ends of the pterygoids meet in a symphysis. The
visceral arches are partially ossified but the meckelian cartilages
are unossified. The hyomandibular carries an opercular plate
and the hyoid arch a branchiostegal ray. Membrane bones (a

Fig. 94. — Lateral view of the vertebral
column of Spatularia (from Wieders-
heim). Cs slieath of notochord ;
Ic intercalated piece ; Ob neural
arch ; Ub haemal arch ; Ps neural
spine.

* H. Klaatsch, Morph. Jahrb., 16, 1890, p. 146. O. Hertwig, ihid., 2,
1876, p. 373.

f Bridge, Osteology of Polyodon. Phil. Trans. 1878, p. 683.

CHONDROSTEI.

169

maxilla and dentary) are developed in connection with the jaws.
Teeth are entirely absent in the adult, but small teeth are present
in the young Polyodon and possibly other genera. There are
five branchial arches.

In the unpaired fins the fin rays (dermotrichia) are more
numerous than the interspinous supports (somactids), and the
caudal fin has fulcra in a single row. The pectoral arch is un-
ossified and the two halves are not united ventrally. They are
covered by membrane bones; supra-clavicles, clavicles, and infra-
clavicles being present. The pelvic arch is absent, its place being
taken by enlarged basal cartilages of the fins as in Teleostei.
The paired fins are without a basal lobe (non-lobate), their whole
free portion being supported by dermal fin-rays. These are
carried by a row of cartilages of which the posterior is possibly

Fig. 95. — Cephalic skeleton oi the sturgeon (from Glaus after Wiedersheim). Ro Rostrum ;
Cn nasal pit; O orbit; Ilm hyomandibular ; S symplectic ; Pq palatoquadrate ; ild
lower jaw ; Hy hyoid ; T' foramen for vagus ; B. ribs.

a basal somactid (metapterygium) and the others peripheral
somactids which have become articulated to the pectoral arch.
The anterior dermal fin-ray is enlarged and directly articulates
with the pectoral girdle.

The branchial apparatus presents remarkable variability. In
Acipenser there is a spiracle with a pseudobranch, and a hyoid
demibranch ; in Scaphirhynchus there is no spiracle or pseudo-
branch, but a hyoid demibranch is present. In Polyodon spiracle
and pseudobranch are present, but there is no hyoid gill. Each
of the first four branchial arches carries a double row of gill-
filaments and there is a cleft behind the fourth gill arch. In
Acipenser the inter-branchial septa are fairly broad. In
the conus there are three longitudinal rows of valves with three
or four valves in each row.

170 SUB-CLASS GANOID EI.

Fam. 1. Acipenseridae. Sturgeons. With five rows of keeled plates
in the skin, elongated snout, small mouth without teeth, -with four barbels
in front of the mouth, opercular and four double gills. Stomach without
blind sac. With fulcra on the dorsal lobe of the caudal tin. Dorsal and
anal fin with two rows of supports (axonosts and baseosts). Large fishes
in the seas and rivers of the northern hemisphere, feeding on small
animals and plants. Most are migratory (anadromous). Caviare is the
ovary of the sturgeon. Acipenser L. with spiracle. Scaphirhynchus
Heckel, without spiracles. Fossil remains rare ; an Eocene species
(A. toUapicus Ag.) is known, and isolated remains of scales from the
Upper Lias of Whitby {Oyrostevs) and from the Upper Chalk of Kent
[Pholidurus).

Fam. 2. Spatulariidae. Skin with small isolated scales, tail scaled as
in sturgeons, snout prolonged into a thin flat blade ; without barbels ;
with spiracle, without hyoid giU ; gills, 4 ; gill-rakers long, in a double
series on each arch, except on the fifth, which has only one series.
Air-bladder cellular. Jaws with fine teeth in young indi\dduals. Fresh-

FlQ. 96. — Acipenser ridhenv^ (after Heckel and Kner).

waters of N. America and China. Polyodon Lac, Mississippi ; Pse-
phurus Giinther, Chinese rivers ; a fossil genus CrossophoUs Cope, from
the Eocene of W5^oming.

EXTINCT FAMILIES.

Fam. 1. Chondrosteidae. Parietals and frontals paired ; near the
parietal a great squamosal. Jaw edentulous. Branchiostegal rays
present. Operculum small, sub-operculum large. Body naked ; the
dorsal lobe of the caudal fin with fulcra and covered with rhombic ganoid
scales. Chondroslcus Egerton, Lower Lias, England.

Fam. 2. Belonorhynchidae. Trias, Lias, may be placed here.

Fam. 3. Palaeoniscidae. Body elongated with rhombic, rarely cycloid
scales ; with opercukun, sub-operculum and branchiostegal rays. In no
single genios are the osteological characters well known. Devonian to
Jurassic. Cheirolepis Ag., Devonian ; Palaeoniscus Blv., Upper Per-
mian ; Coccolepis Ag., Jurassic.

Fam. 4. Platysomidae. Deep-bodied fishes from the Carboniferous
to the Permian ; very similar to the Palaeoniscidae. Cheirodus M'Coy,
Eurynotus Ag., Platysomus Ag., Benedeniiis Traquair.

The Catopteridae, Triassic, may be placed here.

I

CROSSOPTER YGI I .

Order 2. CKOSSOPTERYon.*

Vertebral column ossified, or unossi-
fied. Tail diphycercal or hetero-diphy-
C6rcal {upper lobe of caudal fin weaker
than the lower lobe, Fig. 103). Paired
fins with scaled basal lobe which is either
pointed or rounded. In the former
case it is unibasal, rachiostichous and
mesorachic, in the latter case, found only
in living forms, the pectoral fin is
tribasal and rhipidostichous. In place
of the branchiostegal rays are two large
jugular plates between the rami of the
lower jaw, near which there are in many
palaeozoic forms a number of smaller
lateral plates and a median anterior
plate. Ganoid scales, rhombic or
cycloidal, cover the whole body and
tail. Dorsal fin either two in number ;
or if single, long or multifid. Devonian
to present time.

This order which was established by
Huxleyt in 1861, is mainly based on
the form of the fin, which is fringed
with dermotrichia on both sides. It
is difficult to state any other charac-
ters peculiar to the living and extinct
forms, unless it be the two jugular
plates between the rami of the lower
jaw.

The following remarks apply to the
living Polypteridae : —

The scales of Polypterus are very
similar to those of Lepidosteus (see

171

Fig. 97. — Clieirolepis trailli ; re-
stored (except margin of tins)
by Xraquair (from Smith-
Woodwara), quarter natural
size.

p. 177). They are

* Traquair, Cranial Osteology of Polypterus, Journ. Anat. and
Physiology, 4, 1871. Pollard, " Anatomy of Polypterus," Zool. Jahrb., 5,
1892.

t Preliminary Essay upon the systematic arrangement of the fishes
of the Devonian Epoch, Mem. Geol. Survey United Kingdom, 1861.

172

SUB-CLASS GANOIDEI.

4 _

attached to one another in much the same way and are
arranged in oblique rows. They are rhombic in form
and similarly covered by ganoin. The canals in the
deeper part of them, containing blood vessels, open on the
surface, but differ from those of Lepidosteus in being branched.
They are without spines and the remains of spines, except at
the bases and on the hinder surface of the pectoral fins and on
the plates overlying the shoulder girdle. They are covered by
soft skin in the adult, but this is frequently rubbed off in pre-
served specimens, leading to the view
.2 /"^ that the scales are freely exposed in

the adult.

The vertebral column is completely
ossified ; the vertebrae are amphi
coelous, as in Teleosteans, and carry
in the body two pairs of ribs, of which
the shorter and ventral pass between
the muscles and the peritoneum and
correspond to the ribs of other forms,
while the dorsal and larger pair lie
between the dorsal and ventral lateral
muscles. The body of the first verte-
bra is united to the skull, but the
arch and ribs are separate. The
cranium consists mainly of persistent
cartilage (Fig. 98) with a large fon-
tanelle in the roof and in the floor
(pituitary) and a few cartilage bones, of
v/hicli the most important are exoc-
cipitals which completely surround
the foramen magnum, opisthothic {7'),
sphenoid (? sphenethmoid), postfrontal {S), prefrontal (9) and
a median ethmoid (-?). There is a slender arcade of car-
tilage passing forward from the postfrontal to the anterior part
of the sphenoid cartilage.

The whole is invested dorsally by membrane bones, while
ventrally there is a parasphenoid which extends back beneath
the body of the first vertebra, and two vomers (as in Lepidos-
teus). The mandibular arch is very like that of Teleosteans : in
the upper jaw arcade (Fig 100) are palatine, ectopterygoid,

Kia. 98.— Dorsal view of the
i"nrtil:ii;inous cr;anium of
Poli/pterus with the mem-
brane bones removed (after
Traqnair). 1 Ethmoid, 3
nasal opening, 3 splienoid,
4 optic foramen, 5 occipital
bone, ft foramen magnum,
7 opisthotic, 8 post-frontal,
9 pre-frontal.

CROSSOPTERi^GII.

173

Fig. 99. — Side view of skull of Polyptenis with membrane bones
(after Traquair). 1 Nasal opening, a nremaxilla, 3 ethmoid, 4
accessory nasal, 5 os terminale. t> nasal, 7 anterior suborbital,
S posterior suborbital, 9 postirontal, 10 frontal, 11 prespira-
cular, la spiracular bones, 13 parietal, 14 post-spiracular, ir>
supratemporal, 16 post-temporal, 17 operculum, 18 sub-oper-
culum ; 19 preoperculum, an quadrate, 2i jugals, 22, articular,
2.'! angular, 24 dentary, 2.5 maxilla.

entopterygoid, metapterygoid and quadrate ; the lower jaw has

an articular and mento-meckelian cartilage bone and an angular,

splenial and dentary, the two latter being dentigerous. On the

upper jaw

are premax-

illae, max-
illae and

jugals (two).

The hyo-

m andibular

is ossified

and bent ;

there is no

sy mplectic.

There is an

ope rculum,

s u b o p e r-

culum and

a large bone

called preoperculum. There are also circumorbital bones and

pre- and post-spiracular bones (Fig. 99), as well as two bones

{12) — the spiracular bones — in the flap which covers up the

spiracle.

There are also three supra- temporals (13) and a post-temporal

{16) on each side. The membrane bones are in the skin and their

outer surfaces are smooth and tuberculated, and not exposed in

uninjured speci-
mens. The pre-
maxillae and
maxillae bear
teeth, and the
vomers, ptery-
goid and para-
sphenoid are
covered with
fine closely set
teeth.
The cartilaginous shoulder girdle is small and contains a scapular

and coracoid ossification ; it is overlaid by a series of clavicular

bones, of which the supraclavicle is attached to the post- temporal;

Fig

[~;;j-^;^.7^.»>.^;^-^

palatognadrate, hyomandibular and
opercular bones of Polyptenis (after Traquair). 1 vomer,
2 palatine (smr.ll and not appearing in palate), 3 e'^toptery-
eoid, 4 entopterygoid, 5 quadrate, 6 subopcrculum, 7
operculum, 8 hyomandibular, 9 metapteiygoid.

174

SUB-CLASS GANOIDEI.

there is an infraclavicle (clavicle, see note, p. 162). The skeleton

of the pectoral fin is tribasal as in
Elasmobranchs ; it consists of three
basal somactids, carrying two rows
of peripheral somactids {r, r") to
which the dermotrichia are attached
(Fig. 101). The pro- and meta-
pterygium are ossified, the meso-
pterygium is mainly cartilaginous
but contains an ossification (o).

The pelvic girdle is represented
by a small piece of cartilage (Fig.
102) to which the large ossified
basal somactid is attached. The
latter carries a row of ossified peri-
pheral somactids to which the der-
motrichia are attached. There are
no fulcra. There is a number of
dorsal fins in Polypterus, each
with an anterior spine ; and the
tail is diphycercal.

In Polypterus there
are three main longi-
tudinal rows of valves
in the conus arteriosus with nine valves in each
row, and between these there are three incom-
plete rows of smaller valves (about forty-five in
all). Both ductus Cuvieri open into the
auricle. The spiracle is present, but the hyoid
gill and pseudobranch are absent. The ventral
aorta sends off a branch to the hyoid arch which
supplies the external gill of the larva. The
fourth branchial arch bears only one row of
filaments and has no slit behind it. The
stomach has a caecum and there is one pyloric
appendage. The air bladder is double and cel-
lular and its duct opens into the ventral wall of
the pharj^nx ; its blood supply is from the last
efferent branchial vessel and its vein joins
the hepatic. According to Budgett Polypterus is capable of

Fia. 101.— Skeleton of pectoral fin
of Polypterus (from Gegenbaur).
R propterygium, R' metaptery-
gium ; ossification in meso-
pterygium ; r', r primary radials
(basal somactids) ; r" secondary
radials ; s fin rays.

P

Fig. 102. —
Pelvic girdle
p and skele-
ton of pelvic
fin of Poly-
pterus (from
Gegenbaur,
after v.
Davidoff). 6
basal somac-
tid ; r peri-
pheral somac-
tids ; p trace
of pelvic
girdle.

CROSSOPTERYGII. 175

breathing air. The pituitary body retains its opening into the
buccal cavity through life.* The young Polypterus has an
external gill, which is attached to the operculum.

Classification of Crossopterygii, living and extinct.

Sub-order 1. OSTEOLEPIDA.

Notochord more or less persistent. Pectoral fins rounded or pointed,
vinibasal. Nares on tlie lower surface of snout.

Fam. 1 . Tarrasiidae. Axonosts and baseosts of median fins in simple
regular series, much fewer in number than the dermotrichia. Tarrasius
Traquair, Calciferous Sandstones (Lower Carboniferous) of Dumfriesshire.

Fam. 2. Holoptychiidae. Body covered with overlapping cycloid
ganoid scales ; vertebral column unossified ; pectoral fin with long pointed
scaled axis (mesorachic) ; tail hetero-diphycercal. Axonosts of each of
the dorsal and anal fins fused into a single piece with a broad distal
end, bearing three to six rod-Hke baseosts, which are much fewer than
and overlapped by the dermotrichia in all the median fins. Lateral jugular

Flo. 103.— Iloloflty<:.hiuK fiemingi, Devonian, Scotland, after restoratiuu by I'raquair, from
Woodward.

plates, clavicles and infra-clavicles present ; the teeth have a compli-
cated folded structure. Holoptychitis Ag. (Glypfolepis, Platygnathus Ag.)
Devonian ; 'solated teeth have been described as Dendrodtis, Lamnodus,
Apedodus.

Fam. 3. Rhizodontidae. Like the preceding but with shorter pec-
toral and pelvic fuis. Devonian, Carboniferous. Rhizodua, Strepsodus
Rhizodopsis, etc.

Fam. 4. Osteolepidae. Ring vertebrae in the caudal region, Devonian.
Osteolepis, Thursius, Diploptertis, Megalichthys.

Fam. 5. Onychodontidae, known only by fragments from the Devonian,
is placed here.

Fam. 6. Coelacanthidae. Notochord persistent, vertebral column im-
ossified. Axonosts of each of the dorsal and anal fins fused into a single
piece ; a series of axonosts, equal hi number to the neural and haemal
spines present in the caudal fin above and below, each axonost directly
connected with a single dermal fin ray. Caudal fin diphycercal ;
air-bladder with ossified walls ; paired fins with short, obtuse axis ; onl^-
one opercular bone. Lower Carboniferous to the Upper Chalk. Coela-
canthus Ag., Undina Miinst., Macropoma Ag., etc.

* Bickford, Anat. Am., 10, 1895.

176 SUB-CLASS GANOIDEI.

Sub-order 2. CLADISTIA.

Notochord more or less constricted and replaced by ossified vertebrae.
Baseosts in median fins rudimentary or absent ; axonosts of dorsal fins
equal in number to the apposed dermotrichia. Pectoral fins di- or tri-
basal. Nares on the upper surface of the snout.

Fam. 7. Polypteridae. Body covered -with rhombic ganoid scales ;
vertebral column ossified ; tail diphycercal ; pectoral fins tribasal ; two
jugular plates only ; dorsal fins numerous ; pulp cavity of teeth simple.
Polypterus Geoffr. (Fig. 104), with pelvic fins, rivers of North and
Equatorial West Africa , Calamoichthys Smith, elongated and without
pelvic fins, rivers of Old Calabar and the Cameroons.

Fio. 104. — Polypterus bichir (from C'laus).

Older 3. Lepidostei.

Body covered with rhombic or rhomhoidal scales arranged in
oblique rows and articulated together. Caudal fin hemi-hetero-
cercal. Vertebral column in the most different degrees of
ossification. Unpaired, and sometimes paired fins with fulcra.
Branchiostegal rays numerous, and often a median jugular plate
Always four opercular bones ; between preoperculum and orbit
at least one row of postorbitals. Infraclavicle absent. Somactids
of the unpaired fins as numerous as the dermotrichia. Teeth
pointed or conical. The pelvic fins are without baseosts.

This order, which appears to be closely allied to the Palaeonis-
cidae on the one hand and to the Amioidei on the other, in-
cludes the living genus Lepidosteus. With the exception of
the Permian genus Acenfrophorus, the extinct members are
found in the Lias, Jurassic, Lower Cretaceous, and Tertiaries.

The following remarks apply to the living Lepidosteidae.
The body is covered by rhombic scales articulated together
(see p. 177). The tail is hemi-heterocercal. The paired fins
are non-lobate, and all the fins bear paired fulcra. The jaws
are much elongated, forming a snout ; the premaxillae form most
of the upper jaw. Both jaws bear teeth, small and large, and
there are fine close-set teeth on the palatines and vomers. The
vertebrae are well developed and ossified, and have opisthocoelous
centra. The chondrocranium is large and cartilaginous witli

LEPIDOSTEI. n**

embedded cartilage bones much as in the salmon, and is com-
pletely invested by membrane bones. The vomer is double.
There is a symplectic in the suspensorum, several bony elements
are present in the mandible, and the maxilla is divided trans-
versely into many bones.

The pectoral girdle is as in Teleosteans, with scapula and
coracoid ossifications and overlying clavicle. The pectoral fin
has one row of basal elements carrying the dermotrichia. The
pelvic girdle is absent.

In Lepidosteiis * the scales of the trunk are arranged in oblique
rows which pass from above and in front backwards and ventral -
wards. The scales of a row are more closely connected with each
other than with those of neighbouring rows, in consequence of a
peg and socket articulation. Each scale has its anterior and
dorsal angle produced into a pro-
cess which fits into an excavation
under the next dorsal scale of the
same row, where it is attached
by a ligamentous band. The
centre of the scale is bored ^7 ^ A

one or several canals which pass j.^^ 105.-.4 portion oi tue armour 01

.■i"rrl-if flirr-.nrrli if nnfl trflTT^TTlit, Lepidosteus ; B single scale showiag

ngnt tniOUgn it ana iranbmil antero-dorsal process which fits below

Klnnrl x-o«qp1« Tlip qfalp COH- ^he next dorsal scale (after O. Hert-

UlOOa vessels. Xne bcaie cun ^^,jg^ . ^ foramina for blood vessel?.

sists in its deeper j)ortions of

bone with bone corpuscles and vertically directed fibres, which
are prolongations of large scleroblast cells at the surface (so-
called odontoblasts, they do not become enclosed) and of a
superficial layer of structureless dentine or ganoin, as it was
called by Williamson, which is only found on the exposed part
of the scale, and was formerly taken for enamel. In some
parts of the body (ventral side of head, investing bones of skull
and shoulder girdle, the fin scales) the scales carry one or a
number of small teeth, which consist of enamel, dentine with
fibres, and a pulp-cavity. In the young animal all the scales
carry teeth, with the dentine of which the ganoin of the scale
is continuous. The scales are developed as plates in the dermis,

* O. Hertwig and Klaatsch, op. cit. ; see also W. C. Williamson, On
the microscop. structure of scales, etc., of fishes, Phil. Trans., 1849 and
1851. Nickerson, Scales of Lepidosteus, Bull. Mus. Comp. Zoology,
Harvard, 24, 1893.

z.-ii. N

178

SUB-CLASS GANOIDEI

/'. th

the scleroblasts of which become successively included in the
scale to form the bone corpuscles. The ganoin is formed later
(in a fish of fifty-two months) by the cells on the upper surface of
the plate. The teeth are developed as in Selachians on a papilla
of dermis projecting into the epidermis. The cells of the papilla
form the dentine and those of the epidermis the small cap of
enamel. Some of the scales possess processes which indicate
where teeth were formerly attached.

There is no spiracle in the
embryo or adult, though there
is a pouch-like trace of one in the
embryo ; but the hyoid bears a gill,
dorsal to which is a structure called
by J. Miiller the pseudobranch.
This so-called pseudobranch is not
found in the larva. There are
four double gills, and a cleft behind
the fourth branchial arch.

The conus arteriosus possesses
eight * equally developed longi-
tudinal rows of valves, five in
each row. The ventral aorta gives
off on each side posteriorly two
branches, one to the third and
fourth branchial arches, and one to

Flo. 106.— Dorsal view of the brain fl^p cppnnrl • nnfprinrlv if al«n
of Lepidosteut (after Balfour and ^^^^ beconu , anteiioriy It aiSO

fe'rio^nnd%^osS"^o=be"of"ce^rr. g^VCS off twO, the posterior of

S'lkcTory Z^'t tn^^ whi^^h goes to the first branchial
Shaion."'* """'"^^ °* thaiamen- ^nd the anterior to the hyoid.

The so-called pseudobranch ap-
pears to be supplied by arterial blood from the efferent
vessel of the first branchial arch, as in other fishes.
The left ductus Cuvieri opens into the sinus, the right
into the auricle. The stomach is without a blind sac and
there is a number of pyloric caeca and a small but distinct
pancreas. The intestine is coiled and has a spiral valve pos-

* Boas stato?, Morph. Jahrb. 6, p. 323, that the valves of four of these
rows are smaller than those of the otlier four. There seems to be some
variation in the valves of the conus of Ganoids.

LEPIDOSTEI.

179

teriorly. The air-bladder is single and opens into the pharynx
dorsally. It has a continuous central cavity opening on each
side into lateral chambers which are placed in the thickness
of the wall.

In Lepidosteus the cerebrum (Fig. 106) is divided into two
parts, an anterior and a posterior. The anterior part tapers
in front into the olfactory lobes and is double ; the posterior
part is single and its ventricle possesses a thin roof like that
of the thalamencephalon, with which it is continuous. The
dorsal part of its side walls, where they pass into the roof are
everted and thickened and form the prominent posterior cerebral
lobes (Fig. 106). The anterior part of the roof of the thalamen-
cephalon is produced dorsalwards into a large tliin-walled
vesicle which projects just in front of the pineal body. The
cerebellum is of medium size and has a forwardly projecting
lobe.

pio. 107. — Dapediiii po/t<u«, restored with scales removed, quarter natural si; e, Lower Lias
(from British Museum Catalogue).

Fam. 1. Lepidosteidae. Body covered with tliick, rhombic, ganoid
scales, vertebral column completely ossified, vertebrae opisthocoelous ;
tail heterocercal ; the snout is much elongated. Lepidosteus Lac, gar-
pikes. Fresh-waters of N. America and Cuba, one species is known
from China. Sluggish habit, voracious, valueless as food.

Extinct families.

Fam. 1. Stylodontidae. All fins with fulcra. Jaws and vomer with
several rows of teeth. Verlebral column composed of half vertebrae or
of ring-vertebrae. Upper Permian to Cretaceous. Acentrophortis Tra-
quair. Semionotus Ag., Dapedius de la Beche, a deep-bodied fish
(Fig. 107).

180

SUB-CLASS GANOIDEI.

P^ani. 2. Sphaerodontidae. Very similar to preceding ; with obtvasely
cDiiical or chisel-shaped teeth. Trias to Chalk. Colobodus Ag., Lepi-
dotus Ag. (Fig. 108).

Fam. 3. Eugnathidae. Very similar to preceding ; elongated bodies ;
caudal fin homocercal or hemi-heterocercal ; Trias to Cretaceous.
Eugnathus Ag., Ptycholepis Ag., Gaturus Ar Strohilodus Way.

Fam. 4. Macrosemiidae. Macrosemius Ag., Ophiopsis Ag., Prop-
erus Ag., Notagogus Ag.

Fam. 5. Pholidophoridae. Pholidopleurus Broim.

Fam. 6. Pycnodontidae. Body laterally compressed, high, oval ;
covered by rhomboidal scales, articulated together and strengthened
by a vertical ridge ; scales sometimes absent in the caudal region, rarely
absent altogether. Notochord persistent, without ossifications in its
sheath, but ribs, arches and spinous processes ossified. Caudal fin inter-
nally hemi-heterocercal ; fulcra absent ; pelvics small, anal and dorsal
long ; fulcra absent ; somactids of the unpaired fins equal in number to

1 10. 108.— Lepi dot Ks iimior, restored, one-flfth natural size, PurbecK Beds (from British
Museum Catalogue).

the segmented dermotrichia ; opercular apparatus reduced, often only
one opercular bone present. Dentition of oval, crushing teeth. The
chondrocraniimi often well ossified. Infraclavicles absent. Jurassic,
Cretaceous, Eocene. Gyrodus Ag., Mesturus Wagn., Mesodon Wagn.
(Fig. 109), Pycnodus Ag., etc.

Fam. 7. Aspidorhynchidae. Very similai- to preceding, but with
ring-shaped or complete and biconcave vertebrae (pleurocentra and hj^o-
centra never distinct). Caudal fin homocercal ; snout elongated and
pointed ; scales rhomboidal, unequal ; lower jaw with movable pre-
mandibular ; fi.ilcra weak. Lower Oolite to Upper Chalk. Aspidorhynchus
Ag., Belonostomus Ag.

Fam. 8. Lepidosteidae. See p. 179.

Order 4. Amioidei.

Body covered with thin cycloid or rhombic scales, overlapping,
hut not articulating, without ganoin. Caudal fin internally
heterocercal, externally symmetrical (hemi-heterocercal). Vertebral
column with either half -vertebrae or completely ossified amphi-
coelous vertebrae, or without vertebrae, ossification extending from

AMIOIDET.

181

the arches into the notochordal sheath. Fulcra present or absent.
Branch iostegal rays flattened ; a median jugular plate. Teeth
pointed, conical. From the Lias onwards.

This order, which is close to the Lepidostei, contains the
single living genus Amia. It approaches more closely to the
Teleostei than any other ganoid fish, but it differs from them

pynz.

Fia. 109. — Mefodon niacropteriis, restored, with cheek-plates removed, two-thirds
natural size, Uppor Juras^;;c (after Smith Woodward), /r frontal, meth mesethmoid,
md mandible, op operculum, orb orbit, p.op preoperculura, pa parietal, pmx
premaxiila, socc 3upraoc!-ipital, sg squamosjil, v vomer. (The caadcl region is
destitute of scales in this fish.)

in important features of the urinogenital organs and structure
of the heart, and alimentary canal.

Fam. 1. Pachycormidae. Extinct. Ethmoid region more or less
produced in front of the mouth ; vertebral column variable as in the
order. Fulcra present, branchiostegal rays nimaerous. Lias to Lower
Cretaceous. Pachycormus Ag., Euthynotua Wagn., Hypsocormus Wagn.

Fam. 2. Amiidae. Vertebral column ossified with complete amphi-
coelous vertebrae. The caudal region consisting of vertically divided

\^2 SUB-CLASS GANOIDEI

lialf- vertebrae, of which one half (variously described as the anterior and
posterior) carries the arches. Fulcra absent. Tail somewhat heterocercal,
more so in the young.

The scales of Amia, which are thin, elastic and cycloidal, consist of a
superficial layer containing bone corpuscles, and a deeper layer containing
fibres. They thus closely resemble the scales of Teleostei, and are without
a superficial layer of ganoin (Klaatsch, op. cit., p. 179).

The skull* is decidedly Teleostean in its structure. The chondrocranium
is well developed and contains a few widely separate cartilage bones.
As in Lepidosteus, it is without fenestrae in the roof. From the fact that
two neural arches are attached to the basioccipital bone, it would appear
that the centra of the two first vertebrae are fused to the skull. The
investing bones of the dorsal surface, though closely applied to the carti-
lage, are dermal structures, and are for the most part not covered externally
by soft skin in the adult. The vomer is double and bears teeth. The
suspensorial apparatus is almost exactly similar to that typical of Teleos-
teans. The premaxillae, maxillae, palatines, and pterygoids also bear
teeth. There are four opercular bones, numerous branchiostegal rays, and
a median jugular plate. The shoulder girdle is cartilaginous and is over-
laid by the large clavicle. It carries one basal cartilage (metapterygium),
to one side of which the somactids, which carry the dermotrichia, are
attached. Spiracle, pseudobranch and hyoid gill are absent. There are
four double gills and a slit behind the fourth arch. The conus arteriosus
has three transverse rows of valves with four or five valves in each row.
The base of the ventral aorta is slightly swollen into a kind of bulbus. The
air-bladder is cellular and lung-like and opens by a duct into the dorsal
wall of the pharynx. Stomach with a blind sac, pyloric caeca absent.
The intestine contains a spiral valve. For urinogenital organs, see p. 166.
A single living genus Amia L., with one species, in the freshwaters of the
United States ; fiesh valueless as food ; species are known in the Lower
Tertiaries of Europe and N. America. Megalurus Ag., with fulcra, from
the Upper .Jurassic may be placed here.

Fam. 3. Oligopleuridae. Upper Jurassic to Upper Cretaceous. Oenos-
copua Costa, Spathiurus Davis.

* Bridge, Cranial Osteology of Amia calva, Journ. Anat. and Phys.,
[., 1877, p. 605.

CHAPTER VIII.
SUB-CLASS (AND ORDER) TELEOSTEI*

Fishes with a hony endoskeleton, distinct, usually amphicoelous,
vertebrae, asupraoccipital bone, pectinate gills, a branchial oper-.
culum and usually a hyoidean pseudobranch. Without spiracle,
conus arteriosus, optic chiasma, and intestinal spiral valve. An
air-bladder is often present. The gonads are usually continuous
with their ducts, and the testes are not connected with the kidney.
The eggs are heavily yolked, but usually small, and the young are,
with a jew exceptions, hatched in an immature condition and
undergo a larval development.

The Teleostei include the vast majority of living fishes. They
are found in freshwater as well as in the sea, and in some cases
they possess organs which enable them to exist in or to breathe
air {Anabas, Periophthalmus, fishes which live in foul or muddy
water, e.g. many marsh fishes).

The form of the body is exceedingly variable, most often it is
typically piscme, but it may be elongated and snake-like as in
the eels, strongly compressed laterally in the ribbon-fishes
(Trichiurus, etc.) and in the flat-fishes [Pleuronectidae), or the
vertical axis may equal or even exceed the longitudinal in
length {Orthagoriscus). The tail which is usually the principal
organ of locomotion is in the last-named modification so much
reduced that it appears to be absent, and in the sea-horses
{Hippocampus) it is without caudal fin and is used as a pre-
hensile organ.

The body is divided into head, trunk, and tail ; the gill-
opening usually marks the boundary between the head and

* See, besides the works of J. Miiller, Giinther, Day, Jordan and Ever-
mann, Boulenger, Bridge, already cited, G. B. Goode and T. H. Bean,
" Oceanic Ichthyology," Memoirs of the Museum of Comparative Anatomy
at Harvard College, 22, 1896.

184 SUB-CLASS (AND ttKDEH) TKLF.osTEI

trunk, and the vent that Vjetween the trunk and tail. The mouth
is usually at the anterior end of the head,but it varies considerably
in form and position ; it may be anterior or superior (on the upper
surface), or inferior or lateral (extending along each side of the
head). The upper jaw may be formed by the premaxillaries
(intermaxillaries) and maxillaries or by the premaxillaries only,
and both jaws may be provided with tactile appendages called
barbels. The nostrils are usually double on each side, and in a few
cases (some eels) the anterior or lower opening perforates the
upper lip. The eyes are large, are usually without lids, and the
cornea is flat. In cave-fishes and in abyssal forms they may be
much reduced and even hidden beneath the skin, but in some
deep-sea forms they are enormously enlarged. The branchial
slits are hidden by the gill-cover, which is a fold of skin, con-
taining dorsally four flat bones, the opercular bones or opercles,
and ventrally a number of bony rods called the branchiostegal
rays. The branchial aperture or aperture of the space bounded
by the operculum is usually a slit of considerable dorso-ventral
extent, but in some cases it is much restricted, and in Sym-
branchus, the openings of the two sides coalesce in the middle
line. The space on the throat between the gill apertures is called
the isthmus. There is no spiracle. The actual gill-openings
themselves, i.e. the openings leading from the pharynx into the
space below the gill-cover are not tubular but simply slits,
separated by the rod-like branchial arches. These carry the gills
which are usually filiform and project freely. There are usually
two rows of them (holobrancks) on each branchial arch, projecting
like the prongs of a comb ; hence they are said to be pectinate
(Fig, 118). There are usually five gill-slits, but the last is always
smaller than the others, and is sometimes absent, in which case
the fourth branchial arch bears only one row of gill-filaments
(hemibranch). They may be still further reduced. There are
five branchial arches, but the last never bears filaments.
The hyoid never carries true gill-filaments, but it generally
carries them in a reduced form as a pseudobranch. In some
cases the pseudobranch is without a trace of filaments and has
the form of a red glandular patch. The vent is usually at the
junction of the trunk and tail, but it may be shifted far back on
the tail or far forward on the trunk. It consists of at least two
openings placed close together, the foremost being the anus

FINS. 185

and the hinder one the uiinogenital aperture. The genital
aperture may, however, in some forms be separate from and in
front of the urinary.

The median fins are subject to considerable variation.
There are usually two dorsal fins, a caudal and an anal fin, but
there may be a single dorsal fin continuous round the tail with
the anal, or there may be a series of finlets in place of the dorsal
fin, and the anal may be multiple or absent. The pakedfins are m
two pairs, the 'pectoral and pelvic (sometimes called ventral).
Of these the pelvics are small, vary considerably m position
and are sometimes absent (fishes which live in mud) ; they may
be behind the pectoral on the abdominal surface in which case
they are said to be ahdominal in position, or they may be below
the pectorals {thoracic) or in front of them {jugular). In some
cases they coalesce and form a suctorial organ {Gobies) and in
some Blennies they are adapted for walking. The pectorals
are usually close behind the gill-opening and vary much in size.
They assist the fish in balancing and enable it to execute back-
ward movements. In some forms {Periophthalmus, Trigla
Lophiu^, etc.) the pectoral fins are used for walking, and in the
flying fish {Exocoetus) they are enlarged to form parachute-like
organs. All the fins are supported by osseous fin-rays
(dermotrichia) which are for the most part jointed and fiexible
(sometimes they branch peripherally), but in some cases
(Acanthopterygian fishes) more or fewer of the anterior der-
motrichia in the dorsal (anterior dorsal if there are two), anal,
and pelvic fins are unjointed and generally stiffened. Such
dermotrichia are called spines. The caudal dermotrichia are
always jointed, as are all the dermotrichia in the Malacoptery-
gian fishes. In some Malacopterygian fishes the posterior
dorsal fin is without jointed dermotrichia, but contains adi-
pose tissue only and delicate unjointed horny dermotrichia
resembling the embryonic dermotrichia ( actinotrichia) :
this is the adipose fin of certain Siluridae, Salmonidae, etc.
The dermotrichia of the ventral part of the caudal fin are
carried by the haemal arches, those of the other unpaired fins
by special skeletal rods — the somactids (p. 54) — lying in the
median fibrous septum separating the dorsal parts of the muscles.
These somactids of the Teleostean unpaired fin are often called
interspinous bones.

186 SUB-CLASS (AND ORDER) TELEOSTEI.

The spines of the dorsal fmof Acanthopterygians can be raised or de-
pressed at will. In the depressed position the spines may cover one
another completely {Jiotnacanth), or they may be turned slightly to one
side or the other alternately {heteracanth).

The skin usually contains pigment and may be very brightly
coloured. Many fishes possess the power of changing their
colour in a protective manner according to their surroundings,
and in almost all the dorsal surface is darker in colour than the
ventral. The body is usually covered with scales, overlapping
one another in such a way that the posterior part of the scale is
free and covers the anterior part of the next scale behind. The
scales are thin plates of bone imbedded in the dermis, and are
more frequently absent from the head and fins than elsewhere.
They are absent in most eels and in fishes with electric organs.
The epidermis is soft and contams many mucous and pigment
cells.

The scales have a concentric striation and are of two kinds, cycloid
and ctenoid. In ctenoid scales the posterior free margin possesses denti-
culations which may extend on to the surface, whereas in cycloid scales
such denticulations are absent. In some cases the scales are enlarged
into great scutes {Siluridae, Lophobranchii, Plectognathi) and in Balis-
tidae they have the form of ossified papillae which project in a shagreen-
like manner. In Siluridae the scales may carry movably articulated
dermal teeth.

The scales of most Teleostean fishes are thin calcified lamellae, without
bone corpuscles, lying in a dermal sac. They are surrounded by sclero-
blasts which, as in Lcpidosteus, form them and add to them during growth.
The outer portion of the scales is structureless, the inner contains fibres.
In some forms, scleroblasts of the upper surface are included and become
bone corpuscles, so that the scales consist of an outer part showing ordin-
ary bone structure and an inner consisting of connective tissue impreg-
nated with calcareous matter by the scleroblasts.

The spines which are present on the scales of some Teleostei, e.g. some
Acanthopterygians and Plectognaths, frequently contain a cavity which
suggests a pulp cavity. They may possibly be regarded as homologous
with the spines of Elasmobranchs, but without the enamel cap. The
spines of the hinder part of the ctenoid scales are due to the sculpture of
the surface and are not denticles.

According to the recent and as yet (March 1904) unpublished researches
of Marett Tims, the scales of Gadus consist of a number of small plates of
structureless bone (vitrodentine) lying close together on a fibrous basis.
In the j'oung state each plate carries a small spine which does not reach
the epidermis.

The sense organs * (nerve eminences) of the lateral line are

* F. Leydig, Integument u. Hautsinnesorgane der Knochenfische, Zool.
Jahrb. Anat.^ 8, 1894, p. 1-152. W. CoUinge, Sensory Canal System of
Teleostei, Proc. Zool. Soc, 189.5, pp. 274-299. B. McDonnell, Lateral
line in Fishes, Trans. R. Irish Acad., 129, 1862, pp. 161-187.

PHOSPHORESCENT ORGANS. 187

usually enclosed in canals, but they may be unenclosed and only
protected by flaps of skin {Batrachus, Lophius, etc). Their
distribution, whether they are enclosed or not, resembles that
found in Elasmobranchs and is indicated on Fig. 42. When
they are enclosed in canals, the tubes which leave the canal and
open on the surface are simple, ending in a single external pore ;
or they branch considerably before reaching the surface and open
by several pores. The trunk part of the lateral line canal is
placed in the dermis and is usually without osseous supports,
but on the head the canals are either enclosed in small bones
which lie outside the skull bones, or they burrow in and are
protected by the bones of the skull and visceral arches themselves.
There may be accessory lateral lines in the trunk, placed on
the sides of the body near the dorsal and ventral middle lines.
There are no ampullary canals.

Many deep-sea fishes possess numerous shining bodies in the
skin resembling in their general features eyes.* It appears
probable that they are phosphorescent organs. They are
found either on the head near the eyes, on the lower
jaw, at the end of barbels, under the gill-cover, or in
rows in which they may be segmentally arranged along
the sides of the body, and sometimes in connection with
the lateral line. They vary considerably in structure from
being simply glandular patches of the skm which are supposed
to secrete a phosphorescent mucus to a state in which they
are more eyelike in appearance and possess a lens-like body
which, it is suggested, acts like the lens of a bull's-eye lantern in
concentrating the rays proceeding from the internal parts of the
organ. In the latter case a kind of tapetum can often be made
out at the back of the organ which appears to act as a reflector.
They are probably in all cases modified skin glands. It appears
probable that in many cases these organs are for the purpose
of enabling then- possessors, which are generally provided with
large eyes, to see in the dark abysses of the ocean, but in some
cases no doubt they act as lures (when placed at the end of
barbels or far back on the body). In the case of Ipnops in which

* F. Leydig, Die augendhnlichen Organe der Fische. Bonn, 1881.
M. Ussow, Ueber d. Bau der sogennanten augenahnlichen Flecken etc.,
Bull. Soc. Nat. de Moscou, 1879. A. Giinther, Deep-Sea Fishes, in Chal-
lenger Reports, 22, 1887, and appendices by H. N. Moseley and R. v.
Lendenfeld.

\fiS SUB-CLASS (AXU OKDEK) TELEOSTEI.

there is no trace of eyes, optic nerve or olfactory nerve, and in
which the supposed luminous organs have the form of two
broad laminae on the upper surface of the head, and in other
deep sea forms in which the eyes are imperfect (e.g. the
Pediculati) they can only be of use as lures.

In the endoskeleton the primitive cartilage is largely replaced
by bone, but some cartilage, varying in amount in the different

forms, may persist.

The vertebral column is usually completely ossified and consists
of amphicoelous vertebrae (Fig. 34). The vertebrae are con-
nected by articulating processes placed on the neural arches.
In the trunk the centra carry transverse processes, which are
directed outwards, and ribs which are articulated to the centra
or to the base of the transverse processes (Fig. 34). In the tail the
centra carry complete haemal arches, which enclose a canal
containing the caudal artery and vein and are prolonged like
the neural arches into a median spine. In some forms a pair of
small bony rods — the inter-muscular bones, are attached to the
centra near the neural arches.

The vertebrae are arcicentrous, the iiotochordal sheath remainmg
thin, but the skeletogenous tissue develops very httle cartilage being
rapidly replaced by membrane bone in the centra as well as in the arches.

In most Teleosteans the end of the vertebral column is bent
dorsalwards, and is unsegmented, though the notochordal
sheath is ossified to form a bony urostyle. The haemal arches
of this part of the vertebral column persist in a modified form
as the hypural bones, which carry the dermotrichia of the ven-
tral part of the caudal fin (p. 55). In such cases the tail
though symmetrical externally, is internally asymmetrical,
and is said to be homocercal (see pp. 55, 56). In a few forms
{Gadidae, etc.), the end of the vertebral column is not bent
dorsalwards, and the tail-fin is symmetrical internally as well as
externally (diphycercal). In these fishes the dermotrichia of
the ventral part of the caudal fin are carried by interspinous
bones, and it seems highly probable that the true tail-fin has
atrophied completely, as it has in some Heteromi, Syngnathidae,
etc., in which the tail tapers to a point and is without any trace
of a caudal fin, and has been secondarily replaced by a backward
extension of the dorsal and anal fins (p. 55). Such tails are
therefore secondarily diphycercal.

SKULL.

189

\

The skull is always hyostylic and possesses both mem-
brane and cartilage bones. It differs considerably in the
extent to which the primitive cartilage persists. In
many forms, e.g. the salmon, pike (particularly in the less
specialised, more ganoid-like fishes), a considerable amount
of cartilage persists and the cartilage bones are separated

Os

FlG. 110. — Cephalic skeleton of Perca fluviatUv) (R6gae animal). Ac post-clavicles; Ala
alisphenoid ; An angular ; Ar articular ; Brs branrhio-stegal rays ; CI clavicle ; Cor fora-
coid ; D rtentary ; Ekp erto pterygoid ; J?np entopterygoid ; Ethi median ethmoid ; EtU
lateral ethmoid (prefrontal) ; Fr frontal ; Frp postfroutal (sphenotic) ; Um hyomandibular ;
lit, hyoid arch ; Jm premaxilla ; JUp intcroperculum ; Mtp metapterygoid ; Mx maxilla ;
Oex epiotic ; Op operculum ; Os supraoccipital ; Pal palatine ; Par parietal ; POf> i)re-
operculum ; PrO prootic ; Ps parasphonoid ; Q quadrate ; .S symplectic ; Sc scapula ;
SOp suboperculum ; Sq pterotic ; Ssc supraclavicle ; Vo vomer.

by wide tracts of intervening cartilage. In others, e.g. the cod,
the cartilage is almost entirely ossified. The cartilage is usually
deficient in the roof of the skull except in the occipital region,
in which a basi-, tv/o ex- and a suiDra-occipital are developed.
The auditory region usually presents five separate cartilage bones,
the epiotic, opisthotic, prootic, the pterotic and the sphenotic
(postfrontal). The sphenoid region is feebly ossiiied : there
is always a small basisphenoid and sometimes an alisphenoid

100 suB-ri,ASS (and order) teleostei.

and orbitosplienoid, and the anterior part of it generally acquires
a considerable vertical extension forming an interorbital septum
(absent in Siluroids, Cyprinoids, etc.). The ethmoid region
remains unossified, or at most has two bones — the lateral eth-
moids or prefrontals (Fig. 110 Ethl). The membrane bones of the
roof are parietals {Par), large frontals {Fr) and a bone over the
ethmoid (supraethmoidal or median ethmoid, Eihi). There
may be other bones, called nasals, over the ethmoidal region.
The parietals may touch in the middle line between the frontals
and supraoccipital, or be pushed apart and separated by the
junction of the frontals and the supraoccipital. In the floor
there is a large and important parasphenoid strengthening
the base of the skull and a vomer {Vo) underlying the
ethmoid region. The orbit is surrounded by a ring of
circumorbital membrane bones (not shown in Fig, 110),
of which the anterior is called the lacrymal. Premaxillae
{Jm) and maxillae [Mx) are present, and there may be a
jugal, but the maxillae are usually toothless and frequently take
no part in the formation of the edge of the mouth. The palatine
bar of the mandibular arch always presents osseous palatine,
pterygoid and quadrate elements : in front is the palatine
(Pal) often dentigerous ; then follows the pterygoid region
usually presenting three elements — the pterygoid (ectoptery-
goid), the mesopterygoid (entopterygoid Enp) and the meta-
pterygoid [Mtji) ; lastly comes the quadrate {Q) which gives
articulation to the lower jaw. The front (palatine) end of this
bar is attached to the ethmoid region, while the quadrate is not
attached to the cranium directly, but is supported by the
strong dorsal element (hyomandibular) of the hyoid arch. In
the lower jaws Meckel's cartilage persists, being ossified proxi-
mally as the articular {Ar), and ensheathed by the dentary
bone (D) distally : in addition there is often an angular element
{An). In the hyoid arch there is a powerful dorsal hyomandi-
bular element which presents two bones, the hyomandibular
{Hm) anH the symplectic {8). The hyomandibular bone
articulates with the auditory region of the cranium and passes
ventralwards behind the metapterygoid, while the symplectic
lies distally and is closely applied to the quadrate. The rest
of the hyoid arch consists of three ossified pieces on each side
(Fig. Ill) — the epihyal (c), ceratohyal (6) and hypohyal. The

VISCERAL SKELETON.

191

epihyal is joined to the cartilage interval in the hyomandibular
element between the hyomandibular and the symplectic
bones by a small osseous piece the interhyal (d), while ventrally
the hypohyal joins the large median, sometimes toothed
glossohyal. In connection with the hyoid arch is a number of
membrane bones — the opercular bones supporting the opercu-
lum, and the branchiostegal rays in the branchiostegal mem-
brane. The opercular bones are in cormection with the dorsal,
hyomandibular element and consist of the operculum (Fig. 110,
Op) and preoperculum (POp), and sometimes of a suboperculum

Via. 111. Hyoid apparatus and branchi&l arches of Perea /tutiatilis (Rdgne animal).
a, b e, i segments of the branchial arches ; the upper Joints Opt are the superior
pharyngeal bones (pharyngobranchials) ; VI, Opt the inferior pharyngeal bones (reduced
flfth branohials) ; Cop median pieces (copulae) ; Rb branchiostegal rays; I=Zbg
hyoid arch ; // — F branchial arches.

(SOp) and interoperculum {J Op). The branchiostegal rays
{Brs) are attached to the lower part of the hyoid arch, partly to
the inner and partly to the outer side (Fig. Ill Rb). There are
five pairs of branchial arches. Of these the anterior four are
usually segmented into four pieces (Fig 111), the pharyngo-
{Ops=d), epi-(c), cerato-(6) and h3rpo-(a) branchials. More or
fewer of the pharyngo-branchials, which are not joined to the
skull or vertebral column, are united with one another to form
the so-called superior pharyngeal hone which generally bears

192

.SUB-CLASS (AND ORDER) TELEOSTEI

teeth (Ops). The hypobranchials, which may be wanting
in the fourth arch, are attached to a varying number of median
elements, the copulae or basibranchia's {Cop). The fifth bran-
chial arch is reduced to a single rod on each side which is usually
strongly toothed, and the pair are called the inferior pharyngeal
bones {Opi) ; they are sometimes ankylosed to form a single
bone. The four anterior branchial arches bear small tooth-like
projections, in one or two rows, which act as strainers ; these are
the gill-rakers.

Pectoral and pelvic* paired fins are present, but one or both of

them may be absent.

In the pectoral girdle,
which is usually present
even when the fin is
absent, the primitive
cartilaginous c o r a c o-
scapular elements are
but slightly developed
and relatively unim-
portant, while the
membrane bones (clavi-
cles t) are largely de-
veloped.

Fio. 112. — Right pectoral girdle and fin ot Vadus
(after Gegenbaur). c clavicle (rlcithium) ; b
supraclavicle (supracleithrum) ; a post-temporal ;
d post-clavicle ; / scapula ; e coracoid ; g basal
Bomaotids of the fln ; h bony dermotrichla.

The coraco-scapular arches do not join ventrally and are
attached to the inner sides of the clavicles. They present two
bony elements — the scapula and coracoid (by some regarded as
precoracoid) with persistent intervening cartilage. The scapula
usually has a foramen, and there is sometimes a third bony
element placed dorsal to the coracoid and in front of the scapula
and called the mesocoracoid. The clavicle (clei thrum) is a
large membrane bone meeting its fellow ventrally under the
throat. To its dorsal end there is usually attached a smaller
supraclavicle (supracleithrum) which is connected dorsally with
a forked bone the post- temporal. This bone is attached to the
auditory region of the skull, by one prong to the epiotic and
by the other to the pterotic bone. Projecting back from the

* The pelvic paired fins are usually called ventrals.

■'\ Cnlled cleithra by some anatomists, on the view that they are not
homologous with the clavicles of higher Vertebrates (see notes, p. 162).

BRAIN.

193

upper end of the clavicle is a bony rod, the post-clavicle. There
is no infraclavicle. The skeleton of the fin consists of usually
five basal ossified somactids which are articulated with the
coraco-scapula, and of a row of small cartilaginous pieces
representing distal somactids. These are followed by the
dermotrichia, the anterior of wliich is continuous with the an-
terior of the basal somactids.

The pelvic girdle is always absent, its place being taken by a
large osseous basal somactid, commonly called the basiptery-
gium ; to this are attached a few small, partly bony, distal
somactids, which carry the dermotrichia.

The brain * of Teleosteans presents the following features.

Fig. 113. — Median longitudinal vertical section through the brain of the trout (from Gegen-
banr, after Rab-Kiickhard). Aq aqueductus sylvii; Bo olfactory lobe; Cbl cere-
bellum; Cc central canal of spinal cord; Ccn anterior commissure; Cho. optic ner\-es;
C'i inferior commissure; G';> pineal body; Hi/, Sy- hj^pophysis; J infundibulum; Nol
olfactory nerve; Pa pallium; pf vehmi transversum; Sv saccus vasculosus; Tco pia
mater on the dorsal side of the mid-brain ; Tl dorsal wall of mid-brain between the two
optic lobes; /r crossing of the fibres of the fourth nerve ; Fc vah-ula cerebelli; Fcm ven-
tricle of the cerebnim ; Fg fourth ventricle; T'< third ventricle.

(1) The olfactory lobes are usually much elongated and
slender; they are swollen at their free ends against the nasal
capsules and at their origin. The cerebral ventricle is continued
into the swollen base.f

* Rabl-Riickhard, Das Grosshini der Knochenfische, etc., Arch, f,
Anat. und Phys., Anat. Abt., 1883, p. 279-322. E. Baudelot, Recherches
sur le systeme nerveux des Poissons, Paris, 1883. A. Schafer, Die morphol.
u. hist. Entwick. d. Kleinhirns der Teleostei, Morph. Jahrb., 21.

t There seems to be some difference of opinion as to whether these basal
swellings alone constitute the olfactory lobes, the slender prolongations
being only olfactory nerves. It has been suggested that in some fishes,
e.g. Salmon, this is the case, whereas in others, e.g. Cj^rinoids, the
whole elongated structure is also part of the olfactory lobe.

Z— II o

194

SUB-CLASS (AND ORDER) TELEOSTEI.

ce

(2) The cerebrum is not clearly marked off from the thalamen-
•ephalon, and its roof is entirely composed of a thin pallium.
The latero-ventral parts are thickened into the great corpora
striata, which were formerly taken for the cerebral hemispheres
themselves. The ventricle of the cerebrum is entirely un-
divided, and the pallium or dorsal wall of it is not marked by a

groove (Fig. 114).

(3) The thalamencephalon is very inconspicuous and the optic
thalami are hardly if at all developed. The anterior part of the
thin roof is folded inwards in the usual way (Fig. 113 pf.). The
pineal body lies in the skull over the pallium ; it has folded walls
and appears to open by a narrow pore at its point of attachment
to the roof just in front of the posterior commissure. There is

no parietal organ. The floor
presents the usual structure;
in front is thethickenmg caused
by the optic nerves, which sim-
ply cross after leaving the brain
and do not form a chiasma.
The infundibulum possesses
lateral lobi inferiores, and is
provided posteriorly with a
glandular sac, the saccus vas-
culosus, opening into it by a
minute pore. The pituitary-
body is solid and is attached
to the infundibulum in front
of the saccus vasculosus.

(4) The mid-brain presents the two optic lobes (corpora
bigemina) dorsally and contains a projection formed by the
wall of the brain at the junction of the optic lobes and cerebellum.
This is the valvula cerebelli (Vc), or fornix of Gottsche.

(5) The hind-brain is in the usual form, the cerebellum being
large and containing a prolongation of the fourth ventricle. It
projects back over the medulla oblongata. In some Teleosteans
(e.g. Gymnarcfius, Mormyridae) the brain attains a very large
size, the cerebellum and sometimes the optic lobes being
especially well developed.

The spinal cord frequently ends in an oval or spherical swell-

Fio. 114. — Transverse section through the
cerebrum of the trout, through the hue
XX in Fig. 113. Vcm aiiQ Vt Ventricle of
the cerebrum ; C4 corpus striatum ; Sm
pallium ; Gp pineal body ; PI choroid
plexus (after Kabl-Riiokhard from
Gegenbaur).

mg.

In some forms (Plectognathi) it is much reduced in length ;

NERVES. SENSE ORGANS. 195

e.g. in Orthagoriscus mola it is barely as long as the brain and
hardly reaches beyond the skull. In such cases it ends in a
sJender hlum terminale.

The central canal usually contains a fibre * (Reissner's fibre), which
extends trom the anterior end of the optic lobes, with which it is continuous,
backwards along the whole length of the central canal. It consists of a
biuidle ot nerve tibres, and communicates with the tissues of the spinal
cord throughout its course. It appears to be absent in blind fishes.

In some Teleosteans (Ctenolabrus, Pleuronectes, etc.) giant nerve cells f
are tound in the posterior fissure of the spinal cord, their neurites passing
into the substance of the cord.

The cranial nerves t resemble in their general arrangement
those of other fishes.

The ramua ophthalmicus profundus if present is
much reduced. Tiiere is a dorsal branch of the fifth
nerve — the ramuts lateralis accessorius or r. recurrens
irigemini — which receiving branches from the facial
and vagus passes dorsalwards in the cranium to
perforate the frontal bone. It then travels backwards
near the skin to supply the skin of the trunk near the
dorsal fin (sense-buds and pit-organs). It appears
to be composed partly of so-called communis (afferent-
visceral) fibres (ramus lateralis accessorius), and partly FigT 115. — Horl-
of lateralis fibres supplying pit-organs and derived from ^^^^ v. th^^*'°°

the facial. of Esox lurius

(from Claus). Co
cornea ; L Ions ;

Sense organs. The olfactory organ is Pf rroressus fai-

*= .7 6 ciformis ; CH

usually provided with two OpenmgS. campanula Hal-

mi 1- 111! . •e''' : -^'^ °P*'<=

The eye is distinguished by the possession nerve; sc ossifl-

n 1 1 . 1 • 1 cations in the

of a flat cornea and a sclerotic which is sclerotic.
frequently more or less ossified. The lens
is closely approximated to the cornea, the anterior chamber
of the eye being small. Traversing the vitreous humour, some-
what on the lower side of the eyeball, and extending from the
entrance of the optic nerve to the eye, is a band of tissue (a
process of the choroid coat) containing blood vessels and smooth
muscular fibres ; this is the processus falciformis. At its point
of attachment to the lens it is swollen into the so-called cam-
panula halleri. One of the functions of this structure is said to
be that it assists in accommodation for vision of distant objects,

• Sargant, Anat. Anzeiger, 17, 1900, p. 33. A similar fibre has been
described in Petromyzon.

t Sargant, Anat. Anzeiger, 15, 189S, p. 212.

j Stannius, op. cit. Cole, Gadus, Trans. Lin. Soc, (2), 7. Herrick,
Menidia, Journ. Comp. Neurol., 9, 1899.

196 SUB-CLASS (AND ORDER) TELEOSTEI.

by drawing the lens nearer to the retina. The eyes of fishes
when at rest are accommodated for vision of near objects, i.e
the opposite of the condition in the eyes of the terrestrial
Vertebrates. There is a layer of tissue between the choroid
and sclerotic which contains crystals, the argentea ; and round
the entrance of the optic nerve between the same coats there
is in many Teleosts (those with a pseudobranch) a vascular
plexus of unknown function, called the choroid gland.

The eye-miiscles are the
usual four recti and two
obUqui ; the former often
arise from a subcranial bony
canal the floor of which is
formed by the parasphenoid.
Movable eyelids are not
present, though there may
be a circular fold of skin round
the ej^e. In Anableps the
cornea is crossed by a hori-
zontal stripe which divides
the pupil, so that there
appear to be two pupils one
above the other.

The auditory organ *
consists of the membra-
nous labyrinth and is
without accessory struc-
tures except in those
forms (Ostariophysi) in
which a chain of small
bones connects it with
the air bladder (see
p. 202). The membra-

Fio. 116.— Membranous labyrinth of Perca fiuvia-
tilin, inner view (from Wiederslieim). aa anterior
ampulla ; ac auditory nerve ; ae external, ap
posterior ampulla ; ass apex sinus supenons ;
ca anterior, ce horizontal, cp posterior semi-
circular canal ; de ductus cndolymphaticus ; I
lagena cochleaeti mn macula acustica neglecta ;
ms macula acasca sacculi ; mu macula ac. re-
cessus utriculi ; o otoliths of the rccessus utriculi,
the saccule and the lagena ; pi papilla acustica
lagenae ; raa, rar, rl, rs, nerves to the am-
pullae of the anterior, and posterior semi-
circular canals, to the lagena, and to the
saccule ; rec reccssus utriculi ; s saccule ; ss
sinus utriculi superior ; u utricle.

nous labyrinth is con-
tained in the auditory region of the skull wall, but the cavity
in which it is placed is not shut off from the cranal cavity by
bone or cartilage. It is constructed on the usual plan, con-
sisting of a central chamber or vestibule and three semicircular
canals (Fig. 116). The vestibule is divided by a constriction
into two parts, an upper, the utricle {u) into which the semi-

* G, Retzius, Das Gehororgan der Wirbelthiere, Bd. 1., Stockholm, 1881.

ALIMENTARY CANAL. 197

circular canals open, and a lower the saccule (s), which possesses
a small posteriorly directed process (lagena) representing the
cochlea. The otoliths vary much in form in different fishes.
They occur in the utricle, saccule and lagena. That in the
saccule (the sagitta) is generally the largest and of a crystalline
structure. There is another small one {asteriscus) in the lagena,
and a third in the utricle close to the ampullae of the anterior
and horizontal canals (o). The ductus endolymphaticus is
present as a process of the saccule, but does not open externally.

The lateral line has already been described (p. 187). Its
sense organs are probably innervated by branches of the facial
and lateral line branch of the vagus as in other fishes, but this
has not been shown in all cases.

The alimentary canal is distinguished by the very general
presence of an air bladder, which must be regarded as an appen-
dage of the oesophagus, though it is often in the adult separate
from this ; by the presence of more or less numerous appendages
— the pyloric caeca, opening into the first part of the intestine ;
by the inconspicuous character of the pancreas, which in some
cases is even said to be absent ; by the absence of a spiral valve
in the intestine ; and by the absence of a cloaca common to the
alimentary canal and urinogenital organs.

The teeth * are usually well-developed, but in some cases are al-
together absent (some Lophobranchii, Coregonus). They may be
borne by the maxillary, premaxillary, palatine, vomer, dentary
bones, by the glosso-hyal and by the branchial arches, and rarely
by the pterygoid and parasphenoid. The maxilla is usually with-
out teeth, and does not always form part of the edge of the mouth.
The teeth are generally ankylosed to the subjacent bony
structures, but in some cases there is a ligamentous connection
of such a kmd that they can be bent inwards when food is being
swallowed, but not in the reverse direction (some Gadidae,
Lophius, Esox). In a few cases they are implanted in sockets
{Sphyraena, etc.). As a rule teeth continue to be developed
throughout life from new germs (not from pre-existing germs),
placed behind the functional teeth. These come into function
and position as the old teeth are worn down and cast off. When
the teeth are implanted in sockets, the new tooth, though

♦ R, Owen, Odontography, London, 1840-45. C. S. Tomes, Dental
Anatomy, London, 1898.

198

SUB-CLASS (AND ORDER) TELEOSTEI.

developed behind the old one, comes to lie beneath it, so that
the succession is vertical.

The teeth are generally conical, and may be minute, slender and sharp-
pointed (villiform, e.g. Perch), or longer but very fine (ciliiform, setiform,
as in Chaetodonts). Larger conical teeth are termed card-like (rasp-teeth,
raduliform). In Goniodonts the teeth are bent on themselves like a
tenterhook. They may vary in shape in different parts of the mouth,
the anterior teeth being conical, and the posterior broad and molar-hke
for grinding the food, as in the wolf -fish (Fig. 117), and many Sparidae.
In Sargus indeed the anterior teeth are incisor-Uke, and in Dentex there
are canine-like teeth. Small molar -like teeth are called granular.

Ill Labrus crushing teeth are borne by the upper and lower pharyngeal
bones. Compound teeth, which are found in the Gymnodonts and the
Scari* are made up of a number of teeth which are developed successively
but are joined by cement when full grown and functional. They thus
present the appearance of teeth which continue to grow throughout life.
Pharyngeal teeth may be present on the superior and inferior pharyn-
geal bones. In the Cyprinoids the mouth is edentulous, and teeth are
only found on the inferior pharyngeal bones, which bite against a tubercle

on the basi-occipital. They
are also present on the edges
of the branchial arches, but
except in a few cases, e.g.
Orthagoriscus, in which they
are long and sharp, these gill-
teeth are little more than
horny excrescences, which
however are sometimes elon-
gated into setiform horny
processes — the gill-rakers.

Fia. 117.— Teeth of the Wolf fish, Anarrhichas lupus

(after Ounther). _, . „ ,

There is usually on the
floor of the mouth a small non-muscular elevation which
represents the tongue ; it is supported by the glosso-hyal,
and is sometimes toothed. The oesophagus is a wide
tube, hardly if at all marked off from the stomach. The
stomach, which varies in form, is usually but slightly dilated
and is either U-shaped as in Elasmdbranchs, or is provided with
a caecal prolongation of its cardiac portion and a short pyloric
region placed near the junction with the oesophagus (Fig. 37).
The intestine is usually slightly convoluted, is without a spiral
valve (though a trace of one may be made out in some genera),
and opens to the exterior by the anus. In some forms (e.g.
Tinea, Cobitis) the striped muscles of the oesophagus are con-
tinued over the stomach and intestine outside the smooth

• J. E. Boas, Die Zahne der Scaroiden, Z. /. w. Z., 32, 1879, p. 189-216.

RESPIRATORY ORGANS.

199

muscles. The pyloric caeca are tubular structures opening

into the first part of the intestine just beyond the pylorus ;

they vary in number from one to two hundred, and are very

generally present. The liver is provided with a gall bladder

which opens just beyond the pylorus. It generally contains

much oily matter, but in some forms the oil occurs in all

the tissues of the body and is not only a feature of the liver.

The pancreas,* the presence of which in Teleostei used to be

denied, is not usually a conspicuous

structure, though functionally of great

importance, especially in those Teleosts

in which the stomach is without gastric

glands. It is either embedded in the

liver or diffused in the mesentery, and its

duct opens in close connection with the

hepatic duct. In some forms [Salmo,

Gadus, Perca, Platessa, Brama, etc.) there

is a small gland opening with the bile

duct, which is possibly pancreatic.

The Thyroid f body is represented by
small reddish masses lying ventral to the
ventral aorta, and the thymus f is placed
at the dorsal ends of the last pair of
branchial arches.

The respiratory organs. — There is no
spiracle. The branchial apertures are
narrow slits and the tissue between them
has the form of an arch, not of a sep-
tum. In consequence of this the gills
themselves are filamentous, not lamellar
(Fig. 118). The external openings of the
clefts are covered by the operculum.
The gill-filaments are borne in a double

row (holobranch) by the four anterior branchial arches, the
last gill-aperture is smaller than the others, and the fifth
branchial arch never bears gill-filaments.

Fio. 118. — Transverse sec-
tion through 1 branchial
aroh of Zygaena {right
han'l) and of Gadus (left)
(after R. Hertwig, from
Sviedersheim). a and v
afferent and efferent
blood vessels, 6 skeletal
branchial arch, U^ and
feoi^posterior and anterior
demibranch together con-
stituting a holobranch,
h septum between two
branchial apertures in
Zygaena, r cartilaainous
branchial ray supporting
the same, z small tooth-
likc tubercle (sometimes
elongated as a gill-raker)
in a double row on the
branchial arch of Oadus.

E . Laguesse, Structure, etc., dvi pancreas d'apres les travaux recants,
Journ. Anat. Phys., Paris, 30, 1894, p. 591 and 731. E. Goeppert, Die
Entwick. d. Pancreas Teleost., Morph. Jahrb., 20, 1893.

t F. Maurer, Schilddriise u. Thymus in Teleostier, Morph. Jahrb.,
xi., 1886, p. 128-75.

200 SUB-CLASS (and order) teleostei.

A pseudobranch * is generally present on the posterior side of
the hyoid arch. It contains a rete mirabile and usually has
the form of short filaments or ridges. In some cases it is con-
cealed below the mucous membrane, and the organ may have
the form of a red, lobed swelling (so-called glandular pseudo-
branch or vaso-ganglion). Sometimes it lies so far from the
surface that it is quite hidden : indeed it may be covered by
fat and muscles and even by bone. It is sometimes absent,
and its absence appears to be very generally correlated with
that of the choroid gland. The function of the pseudobranch
is unknown ; it lies in the course of the greater part of the blood
supply to the eye (see below), and it is generally regarded as a
vestige of a hyoid gill.

Certain fishes, e.g. eels, can exist for some time out of water,
but those with large gill- apertures usually perish rapidly. In
some active fishes, e.g. the Scomhridae, the temperature of the
blood is considerably higher than that of the water ; probably
it is always slightly higher, but we know very little on this
subject.

The Teleosts have usually five gill-clefts, but the fifth is always smaller
than the rest and is sometimes absent. In this case the fourth branchial
arch bears one row of filaments only (demibranch) or may be gill-less.
In some forms the gill-apparatus, both arches and gills, may be still more
reduced (SymbrancMdae, Malthe, etc. ; in Amphipnous cuchia the second
branchial arch alone bears gill-filaments). In some Lophobranchii the gilLs
have the form of curious tufted processes.

Accessory respiratory structures are met with, especially in cases in
which the gill-filaments are reduced. Tlius in Amphipnous there is a
lung-like vascular-lined sac, opening into the first gill-cleft, for breathing
air. In Saccobranchus there is a very similar sac. In Anabas scandens,
in which the gill-apparatus is complete, the superior pharyngeal bones are
honeycombed so as to form a laminated organ covered with vascular
mvicous membrane to enable it to breathe out of water (Fig. 38). Accessory
respiratory organs are also found in the Ophiocephalidae, certain Siluridae
(Clarias, Heterobranchus, Heterotis), and in Chanos.

The air-bladder is present in most but not in all Teleostei.
It presents great variety of structure throughout the group.
In the Malacopterygii, Ostariophysi, Apodes, and Haplomi it
usually, but not always, opens into the alimentary canal (usually
into the oesophagus) on its dorsal side by the pneumatic dud
(lajte rally in Erythrinus), which may however be partly or

* Joh. Miiller, Vergl. Anat. der Myxinoiden, loc. cit. Maurer, Morph.
Jahrbuch, 9, 1883, p. 229.

AIR-BLADDER. 201

entirely occluded. In other Teleostei there is rarely * a ductus
pneumaticus in the adult, though such may be present in the
young form ; and the air-bladder is a closed sac. Inasmuch as
it develops in the embryo as a diverticulum of the oesophagus,
this closed condition must be regarded as secondary. In the
Clupeidae the ductus pneumaticus opens into the fundus of the
stomach and in the Herring there is a second duct opening to
the exterior on the left side of the reproductive aperture.
The air-bladder always contains gas which consists of nitrogen,
oxygen and a trace of carbonic acid. It lies dorsal to the ali-
mentary canal, and is usually closely adherent to the ventral
surface of the kidneys, lying between those organs and the
peritoneum, and in many Siluroids it is partly enclosed in
osseous capsules formed by the vertebrae. In some cases
however it projects into the body cavity, lying more or less
loosely. Its walls consist of connective and elastic tissue and
yield isinglass. Tufts of blood vessels in the form of retia
mirabiUa (red bodies) covered by a glandular epithelium are
often present on its walls, and sometimes project into it, like
huge vascular glomeruli. These vascular bodies are absent
from the Ostariophysi and from most fishes which have a
pneumatic duct. They are however present in some of the
latter, e.g. the eel. The air-bladder may be coextensive in length
with the body-cavity, but it frequently extends as a single or
double prolongation some distance beyond into the caudal region
beneath the caudal vertebrae {Gymnotus, Ophiocephalus, etc.),
or forwards into (see below) or towards the head {Gadus, etc.).
Sometimes it is much restricted (some Siluridae, Pediculati,
Plectognathi, etc.). In some cases it is partially or completely
divided transversely into two or even more compartments
{Cyprinidae, Characinidae) ; more rarely it is divided longi-
tudinally (Arius). In some forms it is so much reduced in size
that it almost escapes notice (some Siluridae and Cyprinidae).
It frequently gives off diverticula, which in the Sciaenidae and
Polynemidae are numerous and branched. As a general rule the
internal cavity is unbroken except for the partitions complete or

* An open pneumatic duct is said to be present in Holocentrum, Pria-
canthus, Caesio, etc. See Kner, Einiges lib. die Thj^raus bei Fischen u. d.
Schwiramblase der Stachelflosser, Sitz. Mat. Nat. Classe Akad. Wiss.,
Wien, 49, 1864, p. 455.

202 SUB-CLASS (AND ORDER) TELEOSTEI.

incomplete just referred to, and the lining may be smooth or
cellular {Clupeidae, etc.), but sometimes it is much broken up
and has spongy, lung-like structure {Heterotis, Gymnarchus,
and other forms).

In the Ostariophysi the anterior end of the air-bladder is
connected by a chain of small bones, which are probably de-
tached portions of some of the anterior vertebrae and are
called the Weberian ossicles,* with the wall of a chamber in
the skull wall enclosing a diverticulum of the membranous
labyrinth ; and in other cases anterior prolongations of the ah-
bladder reach the skull and come into immediate contact with
the wall of the space in which the membranous labyrmth is
contained.

In the simplest cases (Myripristis, Holocentrum, Sparus, Sargus, etc.)
the two anterior horns of the air-bladder apply themselves to membranous
spaces in the bony wall of the occipital region containin'j the membranous
labyrinth. In many Clupeidae the slender anterior end of the air-bladder
enters a canal in the basi-occipital and divides into two narrow branches.
Each of these dilates within the bone and divides again into two, each of
which forms a spherical swelling. A process from the vestibule (utricle)
of the membranous labyrinth comes into contact with these vesicles ;
moreover, the vestibules of the two sides are connected by a transverse
canal. In the Ostariophysi, in which the connection is effected by a chain
of ossicles, a few of the anterior vertebrae are ankylosed together and
modified in certain of their parts, some of which are partially detached
to form the auditory or Weberian ossicles. These are foui- in number,
of which three, the tripus (malleus, Fig. 119, H), intercalarium (incus, 9)
and scaphiiun (stapes 8) form a chain connecting the air-bladder with
the membranoiis labjTinth. In addition there is a fourth — the claus-
triun — partly dorsal to and partly in front of the scaphium, which, how-
ever does not form part of the chain, but simply Ues in the wall of the
atrium (see below).

The first vertebra is represented by the centrum only which is distinct
from but firmly connected to the skull and the next centrum. The second
centrum (10) although it shows no marked sign of being composite, con-
sists of the completely fused contra of vertebrae 2, 3 and 4. It is
therefore called the complex centrum. To the hind end of the complex
centrum the three next centra may be united, but these remain distinguish-
able. The saccule {15) of the membranous labyrinth gives off a process,
the ductus endolymphaticus (4), which unites with its fellow in the middle
line, and gives off posteriorly from the point of union a median sac, the
saccus endolymphaticus [5). The saccule and saccus lie in excavations
of the basioccipital bone, called respectively the foveae sacculi (16) and
the cavum sinus imparls (14). These are partly separated from one

* E. H. Weber, De aureetauditu hominis etanimalium,Pars I.,De aure
animalium aquatilum, Leipzig, 1820. T. W. Bridge and A. C. Haddon,
The air-bladder and Weberian ossicles in the Siluroid Fishes, Phil. Trans.,
184, 1893, p. 65-333.

AIR-BLADDER.

203

another and from the cranial cavity by bony plates, but they open into
the cranial cavity in front. The foveae sacculi (16) end blindly behind,
but the cavum sinus imparls opens behind into two laterally-placed
chambers the floor of which is formed by the basioccipital, the sides and
roof by thick fibrous walls. These chambers are the atria sinus imparis
(13). To the outer wall of these is attached a process of the scaphium (8),
the anterior of the tliree ossicles. Of these, the posterior or tripus (11)
is by far the largest, and is inserted behind into the fibres of the anterior
chamber of the air-bladder ; laterally it has a process articulating with
the complex centrum, and anteriorly it is connected by a strong ligament
— the inierossicular ligament (12) — to the scaphium. In this ligament
and between the
tripus and scaph-
ium, is the inter-
calarium (S). The
tripus and inter-
calarium are partly
enclosed in a thin-
walled fibrous sac,
containing a deli-
cate fibrous net-
work and called
the saccus para-
vertebralis. The
air-bladder is
divided into a n
anterior and pos-
terior chamber, of
which the anterior
is usually especially
distensible, and in
the Siluridae comes
into close contact
with the skin on
each side. The pos-
terior division is
generally divided
into two by a
longitudinal septimi
which frequently
gives off incomplete

fia. 119.— A view from above of the cranial floor and anterior

vertebrae of Macrones nenuirus, semidiagrammatic (after
Bridge and Haddon). The brain has been removed, and the
bone cut awav, so as to expose more completely the mem-
branous labjTuith. 1 Bphenotic, 2 prootic, 3 pterotic,
4 ductus endolymphaticus, 5 saccus endolymphaticus,
6 epiotic, 7 exoccipital, 8 scaphium, 9 intercalariura, 10
complex centrum, 11 tripus, 12 interossicular ligament,
i3 atrium sinus imparis, 14 cavum sinus imparis, /5 sacculus,
16 fovea sacculi, 17 ductus sacculo-utricularis, 18 utricle.

transverse septa. Tlae ductus pneumaticus opens

into the anterior chamber.

Striped muscles are frequently present in the wall of the air-
bladder (species of Trigla, Batrachus, Pogonias, Zeus, and others).
Sometimes there are extrinsic muscles passing from the ventral
surface of the vertebral column on to the air-bladder (species of
Gadus, Diodon, Tetrodon, etc.).

The blood supply of the air-bladder is arterial from the system
of the dorsal aorta, either from the efferent branchial vessels,
from the coeliac artery or from the dorsal aorta. In Gym-

204 SUB-CLASS (AND ORDER) TELEOSTEI.

narchus the efferent vessels of the third and fourth branchial
arches go exclusively to the air bladder (Hyrtl). The veins
join either the system of the posterior cardinal, or the hepatic,
or the portal.

The air-bladder is extraordinarily variable in its occurrence.
It is entirely absent in some families, e.g. Bhnnidae, Pleuro-
nectidae, Symbranchidae. It may be present in some genera of
a family and absent in others, or even in different species of
the same genus.*

Several functions f have been ascribed to the air-bladder ; it
has been said to be hydrostatic, a resonator, sound producing,
and respiratory. There can be but little doubt that it is a
hydrostatic organ : % its function appears to be to keep the
weight of the fish equal to the weight of the volume of water it
displaces. Thus if the fish smks, its body is compressed and
the specific gravity is increased. To meet this the air-bladder
slowly secretes gas, which distends the bladder and so restores
the specific gravity of the fish to its former point. Further,
when the fish rises, its air-bladder becomes distended and its
specific gravity diminishes. The fish consequently has some
difiiculty m preventing its body rising to the surface. To meet
this, the superfluous gas is slowly absorbed and the air-bladder
becomes reduced in bulk so that the specific gravity of the fish
returns to its normal point. In the Ostariophysi the reduction
of pressure causes the fish to expel the gas through its pneu-
matic duct, but this does not always occur in other fishes with
pneumatic duct, though possibly it does so in some cases. In
fishes without pneumatic duct the only way in which the super-
fluous gas can be removed is by absorption. With regard to
the process of secretion, it takes place so slowly that it would
not be worth while for a flsh to change its depth unless it meant
the change to be of some duration. Moreover, Biot,§ and more
recently Moreau, have shown that the gas secreted is mostly
oxygen. The gas in the air-bladder of fishes taken near the
surface contains nitrogen, oxygen, and a trace of carbonic acid
(not more than 1 or 2 per cent.). The nitrogen in such cases is

* See Stannius, Handbuch, 2nd edit. he. cit., p. 221.

t Vide W. Sorensen, in Journal of Anato^ny and Physiology, 29, 1895,
p. 109, et acq.

X A. Moreau, Recherches exp. s. 1. functions de la vessie natatoire,
Ann. d. Sci. Nat., 4, 1876. '

§ Mimoires d. I. Societe d'Arcueil, 1, 1807.

AIR-BLADDER. 205

considerably in excess of the oxygen, which amounts to from
9 to 20 per cent. In fishes taken from deeper water the per-
centage of oxygen increases, to as much as 87 to 90 per cent, in
fishes taken from a great depth. Further, if the air-bladder
be artificially emptied, the fish sinks to the bottom, but it
slowly recovers by gas-secretion ; the gas so secreted is richer in
oxygen than air. At the same time nitrogen must also be
secreted, and sometimes appears to be the only gas secreted.*
Recently Bolir f has shown that section of the vagus prevents
the secretion of gas into the air-bladder.

The evidence that it acts as a respiratory organ is very slight.
Many fishes swallow air, but there is nothing to show that the
air is taken into the air-bladder. In some cases however it
has been shown (e.g. by Budgett, op. cit., in young Gymnarchus)
that the fish dies if it is prevented from coming to the surface
to take in air. It has however been suggested with more
probability that the oxygen secreted into the bladder may
serve as a store when the fish enters water in which the supply
of oxygen is too small.

In some fishes, e.g. the Ostariophysi, in which the posterior
chamber is non-distensible and often enclosed by bone, while the
anterior chamber is distensible and connected by ossicles to the
membranous labyrinth, and in other fishes (see above) in which
the air-bladder is in connection with the ear, it has been surmised,
though not in any way proved, that the air-bladder acts as a re-
sonator in intensifying sound vibrations and transmitting them to
the auditory apparatus. On this view the Weberian apparatus
may be of use in increasing the acuteness of hearing. It has
also been suggested that it acts as a sound-producing organ, as
a consequence of the incomplete septa and membranes which
project into it being set in vibration by a movement of the con-
tained air, caused by contraction of the extrinsic and intrinsic
muscles which are contained in its walls. This suggestion rests
on observation, for many fishes possess the power of producing
sound (grunting, drumming, hissing, etc.), as many Sciaenidae,
some Siluridae {Doras, Platy stoma, etc.), Trigla gurnardus,
Dactylopterus volitans, Malapterurus electricus, and many others ;
and in some cases the sound has actually been detected

* See Hiifner, in Arch. f. Anat. und Physiologic, 1892, p. 54.
t C. Bohr, Influence of section of vagus on gas secretion in air-bladder,
Journ. of Physiology, 15, 1894, p. 494-500.

206 bUB-CLASis (AND URDfiifl) TELEOSTfcx.

proceeding from the air-bladder in tishes just removed from the
water and opened. It appears that sounds may be produced
in some fishes without special air-bladder muscles by the activity
of muscles, with the fascia of which the air-bladder is in close
connection {Peristedion cataphractum, Triglu lyra, Sciaena
aquila, etc.).

Vascular System. The heart is without a conus arteriosus,
and is usually separated from the ventral aorta by two semilunar
valves only, though there is sometimes a small third valve. In
some Clupeids it is said * that a trace of a small conus provided
with striped muscles may be made out, and in Butirinus [Albula)
there are actuaUy two rows of valves (two large and two small
in the proximal, and two large in the distal row).

The ventral aorta presents at its ventriculai end a swelling,
the bulbus arteriosus, due to the thickness (elastic tissue and
smooth muscular fibres) of its walls at this pomt. it gives off
branches to the four anterior branchial arches, wliich usually
bear gills.

The ventral aorta does not as a rule give off branches to arches which
are without gills, but in some cases with deficient posterior gills (Sym-
branchus, Amphipnoiis, etc.) the afferent vessel from the ventral aorta
is present and passes round directly into the efferent vessel, so that
venous blood is conveyed into the dorsal aorta. As an example, we may
mention Amphipnous, the first branchial of which has no gills ; the second
has a few filaments ; the third has a transparent fringed membrane, and
the fourth has no gills. The breathing organs are two sacs filled with
atmospheric air and placed over the upper ends of the branchial arches ;
they open into the branchial cavity between the dorsal end of the hyoid
and first branchial arches. The ventral aorta gives off a branch on each
side, which passes to the fourth branchial arch and joins its fellow to form
the dorsal aorta ; it then gives off small branches to the second and third
brancliial arches and to the air-cavities, the blood from which is returned
in two trunks which join the dorsal aorta.

The blood after passing through the gills is collected by the
efferent branchial arteries, of which one leaves each gill-bearing
arch. These fall into the two roots of the dorsal aorta, wliich
anastomose in front dorsal to the parasphenoid bone and con-
stitute the so-called cir cuius cephalicus (Fig. 120, cc). The
circulus cephalicus gives off anteriorly the internal (at a) and
external {b) carotid arteries, and receives an anastomosing
branch from the hj'^oidean artery {vh) ; posteriorly it gives off the

* J. E. V. Boas, Morph. Jahrh., vi., 1880, p. 527.

I

VASCULAR SYSTEM.

207

two subclavian arteries, and on the right side the large coeliac
artery (r). The hyoidean artery (vh) is a continuation of the
ventral end of the efferent vessel of the first branchial arch on to

the hyoid. up which it runs in a dorsal direction to supply the
pseudobrancn (fb). The efferent vessel of the pseudobranch

208 SUB-CLASS (AND ORDER) TELEOSTEI.

(c/) anastomoses with its fellow of the opposite side and then
passes round the external carotid to the choroid gland, a rete
mirabile in the choroid coat round the entrance of the optic nerve
into the eye. When the pseudobranch is not present, there is
no choroid gland. In Gymnarchus the efferent vessels of the
third and fourth branchial arches do not join the dorsal aorta
but pass to the air-bladder.

The blood of the choroid gland supplies the choroid coat. The iris
and sclerotic are supplied by the external carotid. The efferent bran-
chial vessels give off small vessels for the nutrition of the branchial arch
tissues, and near their ventral ends they give off vessels for the ventra
part of the body, the neighbouring parts of the head, and even in some
cases the heart. The hyoidean artery which supplies the pseudobranch
is an example of this system of arteries.

The dorsal aorta is frequently closely adherent to the ventral wall of
the vertebral cokrnin, so that the latter appears to form part of its wall.
It may be swollen at intervals, and in some forms (Esox, Clitpea, Salmo,
Silurus, etc.) a fibrous elastic band projects into its cavity. The principal
branches are subclavian, which may come off from the circulus cephalicus,
the coeliaco -mesenteric which frequently gives off the air-bladder vessel
and a posterior mesenteric.

The veins are arranged in the usual piscine fashion. The left
posterior cardinal is often smaller than the right and appears as
a small branch of the latter coming from the anterior part of the
kidney ; or the right vein may alone be present, l^dng almost in
the middle line and receiving branchlets from each side. A
renal-portal system appears to be present in most Teleosteans,
The hepatic -portal vein may receive tributaries from the air-
bladder, the gonads {Perca, Blennius, Cyjyrinus, Osmerus, etc.),
and the ventral body wall {Salmo, Alosa, Clupea, etc.), though
these veins more generally open into the posterior cardinals.

The body-cavity has the usual piscine arrangement. The
pericardium is completely separated from the general body-
cavity. Paired abdominal pores opening at the sides of the
anus are absent in most Teleosteans, but they are found in the
Salmonidae and Mormyridae, though not universally. They
must not be confused with the pore-like oviducts of female
Salmonidae, etc. (see below).

The urinary organs are paired streaks of kidney substance

* M. Weber, Morph. Jahrbuch, 12, 1886, p. 336. Jungersen, Arb. a. d.
Zool. Inst. Wiirzburg, 9, p. 93.

URINOGENITAL ORGANS.

209

placed on the ventral side of the vertebral column between it
and the air-bladder.

They have a great longitudinal extension, frequently reaching
from the head to the end of the body- cavity, or even in some cases
extending into the caudal region. Their front ends are enlarged
into the so-called head-kidneys. The head-kidney, as was
shown by Balfour, consists of lymphatic tissue which occupies
the place of the pronephros of the larva. There are two
longitudinal urinary ducts which unite
posteriorly to form the single ureter.
This structure, which frequently has a
bladder-like dilatation, passes ventral-
wards on one side of the air-bladder
to open externally behind the anus, or
into the rectum, into which the genera-
tive duct may also open (some Sym-
branchii, Pledor/nathi, Pediculati), or, in
the Pleuronectidae, on a papilla placed
asymmetrically on the coloured side of
the body. Nephrostomata are never
present.

The generative and urinary open-
ings, whether separate or united, fre-
quently open on a papilla which may bo
of some length {Blenniidae, Gobiidae,
etc.). In Ehodeus the opening of the
oviduct is prolonged in the breeding
season into a tube, by means of which
the female deposits her ova in the
shells of living bivalves (Fig. 123). The
ovaries are usually double, rarely single,
saccular bodies the walls of which are continued into the short
median oviduct which opens between the anus and the urinary
opening, or with the latter. In some Teleosteans the ovaries are
separate from their ducts, and the ova are dehisced into the body-
cavity whence they escape by two funnels which join to form a
short tube which opens to the exterior usually between the anus
and the ureter (Salmonidae, Muraenidae, etc.). In viviparous
forms development takes place in the ovaries or in the oviduct.
The testes are paired saccular bodies, and are apparently always
z-n. P

FlO. 121.— Kidneys of Salmo
fario (after Hyrtl). D ductus
Cuvieri ; R kidneys ; U
ureter ; Vr efferent duct of
bladder ; Ve bladder-like
dilation ; Vs subclavian vein.

210 SUB-CLASS (and ORDER) TELEOSTEI.

continuous with the short duct which either opens in the same
position as in the female, or joins the ureter, so that there
is a median 'poriis urogenitalis behind the anus.

In the viviparous forms fertihsation is effected by an in-
tromittent organ, which is usually formed by the urogenital
papilla. A few Teleosteans (Serranus, etc.) are hermaphrodite.

The ova fall into the body-cavity and escape by porelike oviducts in the
Oalaxiidae, Hyodontidae, Notopteridae, Muraenidae and Salmonidae.

In Fierasfer there is said to be a pronephros in the adult, and the pos-
terior part of the kidney is not developed.

The ova are always provided with soft shells and vary con-
siderably m size ; amongst the largest are those of Gymnarchus
(10 mm.) and of Arius (5 to 18 mm.). They may be deposited
singly (salmon, trout, etc.), or they may be agglutinated to-
gether by a substance secreted by the walls of the oviduct. In
freshwater forms they either adhere to some foreign body or are
deposited in nests ; in marine forms they are either attached to
foreign bodies or float freely in the surface waters of the ocean,
or sink to the bottom as in the herring. Most fishes breed once a
year at a definite period, but some breed more than once, and
in some the breeding period is much prolonged — as in the cod
and herring.

Care of the brood by the female is often found {Asjwedo,
Solenostoma, Cichlidae, see systematic part) ; in tlie male it is
more frequent (by nests in Gymnarchits* Heterotis, Cottus,
Gastrosteus, Cyclopterus, Antennarius, Ophiocephalus, Callichthys,
etc. ; in Arius the eggs are carried in the pharynx of the
male, in Lophobranchs in a pouch on the abdomen).

The segmentation is meroblastic and the germinal layers arise
by delamination. The cerebro-spinal cord is formed as a solid
keel-like thickening of the ectoderm,which subsequently becomes
hollow. The young are hatched at an early stage and undergo
the remainder of their development as larvae. The larvae have
a pronephros and considerable remains of the yolk-sac. The
Teleostean pronephros is characteristic, in that the portion of
the body-cavity containing the glomerulus is quite cut off from
the rest and is in relation with the pronephric duct by one body-
cavity opening only.

* Budgett, Breeding Habits of some W. African Fishes, etc.. Trans,
Zool. Soc., 16, 1901, p. 115.

HABITS. 211

Teleosteans frequently undergo remarkable changes of form
in their growth. This is a marked feature of the group, and
leads to some difficulty in the recognition of species. As ex-
amples may be mentioned the Pleuronedidae , Cyttidae,
Muraenidae, Xiphiidae, Plectognaihi. In many cases the young
are so different that they have been described as distinct genera.

Moreover, Teleostean fishes are often highly variable under
the influence of changed conditions (variation in acquired char-
acters), particularly with regard to colour, both of skin and flesh.
The change in the colour of the skin is due to the pigment cells
(chromatophors) .

Secondary sexual differences are usually present. The male
is generally smaller than the female, and some of its fin-rays or
fins may be prolonged. The male is often more brightly coloured
in the breeding season, or its skin may become warty.

Hybridism is also known to occur {Serranus, Pleuronedidae,
Cyprinidae, Salmonidae, etc.).

Some fishes are very long-lived (carp and pike to beyond 100
years), and growth frequently appears to be somewhat indefinite
and to continue for a long time. Fishes which rapidly a,ttain to
their full size (e.g. sticklebacks) are said to be shortlived.

A few fishes have been domesticated and transported to
different parts of the globe (carp. Crucian carp, tench, goramy),
and certain species of salmon and trout have been acclimatised
in countries in which they are not indigenous (see accounts of
families).

Marine fishes are usually extremely sensitive to changes of
temperature, freshwater fishes much less so. It is said that the
carp will survive after being frozen in a block of ice. A modified
hibernation has been observed in some Cyprinoids and Muraen-
oids in cold weather, and many tropical fishes (Siluroids,
Lahyrinthici, Ophiocephaloids, etc.) pass the dry season in a
torpid state in hardened mud.

The flesh of many fishes is poisonous, and in unknown waters,
especially in tropical seas, great care must be exercised in select-
ing fish for food.

The wounds caused by the spines of many fishes are poisonous.
This is generally due to the poisonous nature of the mucus which
covers the body, but it may be caused by special poison glands,
as in Synanceia, Tlialassophryne.

212

SUB-CLASS (and ORDER) TELEOSTEI.

There is a considerable number of marine fishes which occasionally
wander into freshwater and ascend rivers (e.g. Sciaenidae, Pleuronectes,
species of Clupeidae), and a smaller number of freshwater fishes which
occasionally descend into the sea (some species of Salmo, of Siluroids,
of Coregonus, and pre-eminently the Oastrosteidae and Cyprinodontidae) ;
but most of these are inhabitants of the brackish water. They must be dis-
tinguished from those fishes which migrate for the purpose of spawning.
Such are of two kinds ; there are the anadromous fishes which ascend
rivers to spawn in freshwater, as the salmon and the salmon-trout, some
Clupeids, etc., and Jcatadromous fishes, like the freshwater eel, which
descend to the sea to spawn. There are many clear cases of marine fish
which by geological changes have been retained in freshwater basins ;
such are Cottus quadricornis, in the large lakes of Scandinavia ; species
of Gobius, Blennius and Atherina in the lakes of N. Italy ; Comephorus
in the depths of Lake Baikal.

The classification of the Teleostei adopted here is essentially
that of Mr. G. A Boulenger, E.R.S., to whom I am greatly in-
debted for having allowed me to see proofs of his work before
its publication. It is as follows :* —

MALACOPTERYGn (SAI.MONICLUPEIPORMES).t
OSTARIOPHYSI (CyPRINISILURIFORMES).
SYMBRANCHn (SYMBRANCHirORMES).

Apodes (Anguilliformes).
Haplomi (Esociformes).
Heteromi (Dercetitgrmes).
Catosteomi (Gastrosteiformes).
Tribe A, Selenichthyes.
„ B. Hemihranchii.
„ C. Lophobranchii.
„ D. Hypostomides.
Percesoces (Mugiliformes).
Anacanthesti (Gadiformes).
Acanthopterygh.
Tribe A. Perciformes.

Scombri formes.

Zeorhombi {Zeirhomhiformes),

Kurtiformes.

Gobiiformes.

Discocephali (Echineiformes).

Scleroparii {Trigliformes).

Jug-ulares {Blenniiformes).

Sub-order

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

B.

C.

D.

E.

F.

G.

H.

* Boulenger, Ann. and Mag. Nat. Hist. (7), 13, 1904, p. 161.
t The names in brackets are those used in the fish-gallery of
British Museum.

the

MALACOPTERYGH. 213

Tribe I. Taeniosomi {Lophotijorm.es).
Sub-order 11. Opisthomi.

„ 12. Pediculati (Lophiiformes).
„ 13. Plectognathi (Balistiformes).
Tribe A. Sclerodermi.
„ B. Gymnodontes.
The old group Physostomi (with a ductus pneumaticus to the air-
bladder), which is sometimes referred to in the preceding pages,
included, rouglily speaking, the MalawpUrygii, Ostariophysi,
Symbranchii, Apodes, Haplomi, Heteromi (in part), and Perce-
soces (in part), of the above classification.

Sub-order 1. MALACOPTERYGH (SALMONI-CLUPEIFORMES).

Soft-rayed fishes with the anterior vertebrae simple, unmodi-
fied, and without auditory ossicles ; symplectic present or absent ;
opercular bones distinct ; pharyngeal bones simple above and
below, the lower not falciform. Pectoral arch suspended from the
skull ; mesocoracoid always well developed. Maxillary bone
forming part of margin of upper jaw ; no barbels. Supra-
occipital sometimes separated from the frontals by the parietals.
Gills 4, a slit behind the fourth. Air-bladder if present with
a pneumatic duct. Dorsal and anal fins without true spines.
Pelvic fins abdominal, sometimes absent ; scales usually cycloid,
sometimes ctenoid ; occasionally absent. No developed photo-
phores. Adipose fin present or absent. This sub-order of
Teleostei is nearest to the Ganoids.

Fam. 1. Leptolepidae. Extinct. Upper Lias to Lower Cretaceous;
vertebral centra nearly complete, pierced by the notochord ; without
fulcra ; scales cycloid. Leptolepis Ag., Thrissops Ag.

The Pholidophoridae (p. 180), Oligopleuridae (p. 182), and the Archaeo-
maenidae, all extinct, are placed here by Smith Woodward and Boulenger.

Fam. 2. Motmyridae.* Body and tail scaly ; head scaleless ; upper
jaw formed by the two premaxillaries which are fused, and by the maxil-
laries. Sub- and very small inter-operculum present ; supraoccipital
separated from frontals by parietals. On each side of the skull there is a
large cavity leading into the interior and covered by a thin bony
lamella. They are without pharjiigeal teeth. All the fins are well
developed in Mormyrus, caudal, anal and pelvic fins are absent in Gym-
narchus. No adipose fin. Pectorals directed upwards. Pseudobranch
absent, gill-apertures reduced to a short slit. Air-bladder simple, com-
mvmicating with the ear. Two pyloric caeca. A series of pores along

* KolUker, Bericht v. d. zootom. Anstalt zu Wiirzburg, 1849. Hyrtl,
Denkschr. Akad. Wiss. Wien, 1856, xii. p. 1. Erdl., Miinchner Gelehrte
Anzeigen. Boulenger, Poissons du Bassin du Congo, 1901.

214 SUB-CLASS (and order) teleostei.

the base of the dorsal and anal (if present) fins. f. w. of tropical
Afr. They possess an electric organ on each side of the tail with feeble
electric functions, consisting of modified muscle-tissue. The snout fre-
quently of strange shape ; eyes often reduced. The brain is remarkable
for its size ; 10 genera. Mormyrua L., teeth in rows along
the middle of the palate and the tongue ; M. oxyrhynchus GeoS., venerated
by the ancient Egyptians ; Hyperopisus, Mormyrops ; Gymnarchus Cuv.,
Nile and W. Afr. rivers, eel-like, each jaw with incisor-Mke teeth ;
air-bladder cellular, very extensible, duct with sphincter at oesophageal
opening ; lays very large eggs ; the gills of the embryo project beyond
the gill-openings. Gnathonenms.

Fam. 3. Clupeidae. Body covered with scales, head naked ; supra-
occipital in contact with frontal. Abdomen frequently compressed into
a serrated edge. Maxillaries (of three pieces) and premaxillaries both
enter into upper jaw. Opercular apparatus complete. Adipose fin
absent, dorsal not elongate, anal sometimes very long. Stomach with
blind sac, pyloric caeca nvunerous. Gill openings usually wide. Pseudo-
branch usually present. Air-bladder simple, large, communicating with
the ear. Principally coast fishes ; none from the deep sea ; may enter
f. ws. communicating with sea ; temp, and trop. zones. Many
fossU forms. Engraulis C. et V., anchovies, upper jaw prominent ;
mouth with a very deep cleft ; eyes covered by skin ; E. encrasicholus L.
{Anchovia J, and E.), the anchovy, abvmdant in Med., also
taken in E. Channel ; Cetengraulis Gthr. ; Stolephorus Lac. ; Coilia
Gray; Dussumieria ; Etrumeus ; Chatoessus C. et V. (Dorosoma Raf.),
C. Amer., Aust., E. Ind., Japan. Clupea Cuv., herrings, upper
jaw not projecting, eyes with free lateral adipose Uds, more than
60 species, most used as food, but some trop. species poisonous ;
G. harengus L., the herring, incredibly prolific, whitebait consists
chiefly of the young of the herring (and sprat), the air-bladder opens
into the stomach, and also on the left side near the anus,* the eggs are
attached to stones, etc.; C. pilchardus Walb. (Clupanodon Lac), the
pilchard (the yoiing is the sardine), equally abundant in Brit. Channel,
on coast off Portugal and in Med. ; C. sprattus L., the sprat,
in Norfolk sold as anchovies ; C. alosa L. (Alausa, Alosa), the shad or
allice-shad, coasts of Eur. ascending rivers ; C. finta, the twaite-shad.
Other Clupeoid genera are Clupeoides, Pellonula, Clupeichthys, Pellona,
{Ilisha), Pristigaster, Chirocentrodon, Pomolohus, Sardinella, Opisthoiiema,
Brevoortia, Opisthopterus, Odontognaihus, Pristigaster ; Chanos Lacep.
with accessory branchial organf in a cavity behind the gill-cavity, Indo-
Pac, 4 ft., edible.

The following genera may be placed here : Elope, Megalops (M. atlan-
ticus, the tarpon), Alhula (Butirinus) with a trace of the conus (with two
rows of valves) in the heart ; Pterothrissus (Bathythrissa), deep sea, Japan.

Fam. 3a. Hyodontidae (moon eyes), f. w. fish of N. America,
no oviducts ; Uiodon, Le Su.

Fam. 4. Alepocephalidae. Deep-sea fishes approaching the Sal-
manoids ; without adipose fin or air-bladder. Phosphorescent spots
none or small. Stomach ciorved, without blind sac ; pyloric caeca in
moderate number. Pseudobranch present. Alepocephalus, Mitchillina,
Bathytroctes, Talismania, Conocara, Platytroctes, Aleposomus.

* Weber, " De aure et auditu," 1, 1820, vii., 63.
t J. Miiller, Bau u. Grenzen d. Ganoiden, p. 75.

MALACOPTERYGII. 215

Fam. 5. Notopteridae, with one genus Notopterus, f. w. of E. Ind.
and W. Afr.

Fam. 6. Osteoglossidae. Body covered with large mosaic-like scales ;
head scaleless, its integument confluent with the bone ; dorsal fin on tail
and opposite anal ; gill openings wide, pseudobranch absent ; air-
bladder simple or cellular, stomach without caecal sac, pyloric append-
ages two. Eggs fall into body cavity. Large f. w. fishes of the
tropics. 4 genera. Oateoglossum VandelU, S. Amer. ; Ara-
paima Miill., Brazil and Guyanas ; Heterotis Ehr., trop. Afr.; Sclero-
pagea, Australia, E. Ind. Arch. Excluding the E. Ind. Archipelago,
the distribution of this family is the same as that of the Dipnoi.
Heterotis niloticus forms a nest and the young larvae have external gills.

Fam. 7. Pantodontidae. Onegenus,f. w.,W. Afr., pectorals very large.

Fam. 8. Ctenothrissidae. Extinct, Cretaceous.

Fam. 9. Phractolaemidae. One genus, W. Afr.

Fam. 10. SaurodoQtidae (Ichthyodectidae). Extinct, Cretaceous ;
Portheus, Ichthyodectes.

Fam. 11. Chirocentridae. One genus, Ind. Ocean and Seas of China
and Japan.

Fam. 12. Salmonidae. Body generally covered with scales, head
scaleless ; margin of upper jaw formed by maxillaries and premaxillaries ;
a small adipose fin behind the dorsal ; pyloric caeca generally present
and numerous ; air-bladder large and simple with a pneumatic duct ;
pseudobranch present ; no oviducts. Inliabitants of sea and f. w. ;
most of the mar. genera are from the deep sea ; most of the f. w.
forms are peculiar to the temperate and arctic region of the Northern
Hemisphere, one occurring in New Zealand ; many f. w. species are
anadromous ; no fossils of f. w. species known. Osmerua and other
genera from the Miocene.

Salmo Art., trout, salmon and charr, inliabitants of f. w., many
species descending to the sea after spawning (anadromous), the young
of all are barred, the bars vanishing in adult except in small varieties ;
many of the species are highly variable and capable of considerable adapta-
tion to their surroundings ; the marine forms usually silvery with or
without black spots, the f. w. forms more or less speckled with
black and red. Some individuals of full size are sterile, but tliis is pro-
bably only a temporary condition ; overgrown individuals are sterile ;
anadromous fish, generally return to their native river. River and sea
trout have been acchmatised in Tasmania and N. Zealand, and appar-
ently in India. As these species are highly variable in response to change
of condition the observation of these acclimatised races will afford an
extremely interesting study.

S. solar L., salmon, N. Hemisphere between latitudes 45° and
75° ; does not occur in rivers opening into Med.,* the last Thames salmon
was caught in 1833 ; a marine fish ascending rivers to spawn (Sep. to Jan.
in Britain), the nest or redd is dug out by the female in gravel
and the eggs are buried ; young salmon of the first and second
year are called parr or pink (4 to 6 in.) ; they then become smolts,
which descend to sea, and reascend the rivers as grilse, which having
spawned go to the sea and return as salmon ; a salmon which has spawned
is a kelt, kelts go to the sea, and probably reascend next year ; kipper
is a male kelt, or a salmon which has been detained in f. w. and got

* Not even in those of Macedonia, notwithstanding Fluellen (Henry
v.. Act 4, Sc. 7) 1

216 SUB-CLASS (and ORDER) TELEOSTEI.

lean ; male parrs may become sexually mature and fertilise the eggs of a
full-grown female ; will hybridise (artificially) with the trout and charr ;
not a highly variable species and change of conditions is fatal. The
various kinds of British trout are probably all varieties of the same species,
as they will freely cross, but three species may for convenience be dis-
tingviished : S. trutta Flem., sea- or sahnon-trout, phinok, sewin, a migra-
tory species ; S. fario L., the brook trout ; and S. levenensis Walker,
Loch Leven trout. To these the bull-trout, S. eriox, and great lake-
trout, S. ferox, may possibly be added.

S. alpinus L., the charr, breed Nov. to Dec. ; redd usually in
gravelly shallows in the lakes (the Windermere charr is known as S.
wilhighbii); the charrs are migratory or non-migratory and inhabit
the deep waters of lakes ; there appears to be one British species
with several varieties. S. jontinalis Mitchell, f. w. of Brit. N.
Amer., acclimatised in Britain. Oncorhynchus Suckley, anadromous fish
in American and Asiatic rivers flowing into the Pacific ; 0. tschmvytscha,
the Cahfornian salmon ; Brachymystax Gthr., Siberian rivers ; Lucio-
trutta Gthr. (Stenodus Rich.), Arctic N. Amer. ; Plecoglossus Schley.,
f. w. of Jap. and Formosa. Osmerus L., smelts, migratory,
ascending rivers to spawn and frequently becoming resident in them,
Atl. coasts of N. Eur. and N. Amer. ; O. eperlanus L., the
smelt, sparling, irregular in its migrations, spawning in rivers near
high-water mark, generally found in rivers from Aug. to May, spawns
about March or April, when fresh exceedingly good eating, but deteriorate
in a few hours ; allied genera are Hypomesus and Thaleichthys from the
Pacific coast of N. Amer., the latter, known as Oulachan, has so
much oil it will burn hke a candle ; Mallotus Cuv. ; Coregonus Art. {Argy-
rosomus Ag.), whitefish, mostly lacustrine, a few anadromous, northern
parts of temp. Eur., Asia, and N. Amer. ; C. oxyrhynchus L.,
houting, marine entering f. w. Holland, Germany, Demnark ; C.
clupeoides Lac, schelly, f. w. of Lake District and Wales ; C. vande-
aivs Rich., vendace, f. w. lochs of Scotland ; C. pollan, Thomps.,
pollan, f. w. of Ireland ; Thymallus Cuv., graylings, clear streams
of Eur., Asia, and N. Amer.; Th. vulgaris Nilss., gi-ayUng, flesh
good, in best condition Oct. and Nov. ; Salanx Cuv. ; deep-
sea genera are Argentina, Microstoma, Bathylagus.

Fam. 13. Stomiatidae. Scales absent or thin ; a hyoid barbel ; eyes
large ; Ivuninous spots more or less developed ; no pseudobranch ; ovi-
ducts present. Deep-sea fishes descending to the greatest depths and
distingviished by their barbel and formidable dentition. Astronesthes
Rich., Stomias Cuv., Echiostoma Lowe, Grammatostomias G. and B.,
Photonectes Gthr., Malacosteus Ayres, Bathyophis, Chauliodus Bloch
and Schneider, Bathylaco, Maurolicus, Sternoptyx, etc.

Fam. 14. Gonorhynchidae, one genus, Aust. and Japanese seas.

Fam. 15. Cromeriidae, with one genus Cromeria, recently discovered
in the White Nile.

Sub-order 2. OSTARIOPHYSI (CYPRINI-SILURIFORMES).

The anterior vertebrae are co-ossified and have some of their
lateral elements detached to form a chain of small bones, the
Weberian ossicles, which connect the air-bladder with the ear
(p. 202). The air-bladder is probably always present though

I

OSTARIOPHYSI. 217

it may be very small. When well developed it has a pneumatic
duct. Pectoral arch suspended from the skull, mesocoracoid
present. The great majority of freshwater fishes are included
in this sub- order.

Fam. 16. Characinidae. Body scaly, head naked ; barbels absent,
margin of upper jaw usually formed by the premaxillaries and maxillaries,
rarely by the premaxillaries only ; jaws usually toothed ; parietals
distinct from supraoccipital ; symplectic present ; generally a small
adipose fin behind the dorsal ; pelvics abdominal ; pyloric appendages
more or less numerous ; air-bladder divided into two portions ; pseudo-
branch absent or much reduced. Freshwaters of Africa and of tropical
America. In America they replace the Cyprinoids ; unknown as fossils.
Erythrinina. Adipose fin absent ; trop. Amer. The genera
are Macrodon, Erythrinus, Lehiasina, Nannostomua, Pyrrhulina,
Corynopoma.

Curimatina. A short dorsal and an adipose fin ; dentition im-
perfect ; trop. Amer. ; Prochilodus, Caenotropus, Hemiodus,
Saccodon, Parodon.

Citharinina. A rather long dorsal and an adipose fin ; minute
labial teeth ; trop. Afr. Citharinus Cuv., attaining to 3 ft.

Anastomatina. Short dorsal and an adipose fin ; teeth in both
jaws well developed ; the gill membranes grown to the isthmus ;
nasal openings remote from each other ; trop. Amer. Lepo-
rinus, Anastomus, Rhytiodus.

N anno char acina. Like the last, except that incisors are notched, and
nostrils close together. Nannocharax.

Tetragonopterina. Short dorsal and an adipose fin ; teeth well
developed, notched or denticulated ; gill-membranes free from the
isthmvis ; nasal openings close together ; S. Amer. and trop.
Afr. Alestes M. and T., trop. Afr. ; Tetragonopterus Cuv.,
trop. Amer. Of the other genera Nannaethiops and Bryconae-
ihiops are African, the rest are S. American, viz., Chirodon,
Megalobrycon, Gastropelecus, Piabucina, Scissor, Pseudochalceus,
Aphyocharax, Chalceus, Brycon, Chalcinopsis, Bryconops, Creagrutus,
Chalcinus, Piabuca, Paragoniates, Agoniates.

Hydrocyonina. Short dorsal and adipose fin ; teeth well developed
and conical ; gill-membranes free from the istlimus ; nasal openings
close together. S. Amer. and trop. Afr. Fishes of prey.
Hydrocyon Cuv., trop. Afr., and Cynodon Spix., S. Amer.,
both to 4 ft. Except Sarcod<ices from W. Afr. the other genera
are trop. Amer., e.g., Anacyrtus, Hystricodon, Salminus, Oligo-
sarcus, Xiphoi-hamphus, Xiphostoma, etc.

Distichodontina. Dorsal fin rather elongate, adipose fin present ;
gill-membranes attached to the isthmus ; belly rovmded. Trop.
Afr. Distichodus M. and T.

Ichthyborina. An adipose fin ; dorsal rays 12 to 17 ; giU-mem-
branes free from the isthmus ; belly rounded ; canine teeth ; trop.
Afr. Ichthyhorus Giinth., Nile ; Eugnathichthys, Phago, W. Afr.

Crenuchina. Dorsal fin rather elongate, an adipose fin ; gill-
membranes free from the isthmus ; belly rovmded ; without canine
teeth. Crenuchiis Giinth., Brit. Guiana ; Xenocharax, W. Afr.

218

SUB-CLASS (and ORDER) TELEOSTEI.

Serrasalmonina. The caribe. Dorsal fin rather elongate ; an adipose
fin ; gill-membranes free from the isthmus ; belly serrated ; trop.
Amer. ; exceedingly voracious, they assail persons entering the
water. Mylesinus, Serrasalmo, Myletes, Catoprion.
Fam. 17. Gymnotidae (71'M''6s naked, vwtos back). Head scale-
less ; barbels nonq ; body eel-shaped ; scales small or absent ;
margin of upper jaw formed by premaxillaries and by maxillaries ; an-
terior vertebrae united, modified, with Weberian ossicles ; dorsal fiji
absent or reduced to adipose strip, caudal generally absent ; tail ending
in point, can be regenerated ; anal long, pelvics absent ; anus on or
near the throat ; shoulder girdle attached to skull ; ribs well developed ;
gill-openings narrow ; air-bladder double ; stomach with caecal sac ;
pyloric caeca present ; ovaries with oviducts. Eel-like f. w.
fishes from S. America. Sternarchus Cuv., Bhamphichthys M. and
T., Sternopygus M. and T., Carapus M. and T., Gymnotus Cuv. {Electro-
phorus), electric eel, Brazil and Guyanas, electric organ along each side
of the tail ; Giton Kaup, Eigenmannia J. and E.

Fam. 18. Cyprinidae. Body generally covered with scales ; head
naked. Anterior 4 vertebrae modified and joined, margin of the upper

jaw formed by the premaxillaries. Belly
rounded or if trenchant without ossifi-
cations. No adipose fin ; a dorsal and
anal fin, pelvic fins abdominal. Stomach
without blind sac. Pyloric appendages
absent. Mouth toothless ; lower pharyn-
geal bones well developed, falciform,
sub-parallel to the branchial arches, pro-
vided with teeth in one, two, or three
series. Air-bladder large, divided into an
anterior and posterior portion by a con-
striction, or into a right and left portion,
enclosed in an osseous capsule. Ovarian
sacs closed. About 200 genera and 1,200
species ; freshwaters of the Old World
The fossil forms can be referred mostly to living

Fm. 122. — Lower pharyngeal bones
of a carp (after Heckel and ICner,
from Claus).

and N. America,
genera.

I. Catastomina. Pharyngeal teeth in a single series, numerous ;
dorsal fin long, anal short ; barbels none. Lakes and rivers of N.
Amer., 2 spec, from N.-E. Asia, generally known as
suckers. Ictiohius Raf., Carpiodes Raf., Cycleptus Raf., Panto-
steus Cope, Catostomus Le Sueur, Chamistes Jordan, Xyrauchen,
Eig. and Kirsch, Erimyzon Jordan, Minytrema Jordan, Moxo-
atoma Raf., Placopharynx Cope, Lagochila Jord. and Bray.

II. Cyprlnina. Anal fin short with not more than 5 or 6,
rarely 7, branched rays. Abdomen not much compressed. Barbels
often present, never more than 4. Three branchiostegals. Air-
bladder without osseous covering.

Gyprinus Art., carps ; large scales ; dorsal fin long with its last un-
divided ray osseous and serrated ; pharyngeal teeth in three rows,
molar-like (Fig. 122); fovu- barbels. C. carpio the carp, indigenous
in Persia and China, introduced into Europe (known 1258 a.d.), into
England (known 1496); food vegetable and animal ; bury themselves in
mud in winter, will live for some time out of water, may attain a large

OSTAEIOPHYSI. 219

size (20-50 lb.)» and great age (50-100 years, Gesner, Bufion), very
prolific, spawn on weeds about May, said to form hybrids with
the Crucian carp with the tench and the bream. Carassitis NUsson,
without barbels ; C. vulggaris Nilss., the Crucian carp, Prussian
carp, Eur. and Siberia ; C. auraius L., gold-fish, China and Japan,
introduced into Eur. and Amer. as an aquarimn fish and natural-
ised in many streams ; very variable under domestication in
colotu' and otherwise, brilhancy generally decreases when turned
into the open, in the wild state greenish ; so-called telescope-fish
is a variety ; breeds in May and June. Catla C. and V., E. Ind. ;
Labeo Cuv., Afr. and E. Ind. ; Discognathus Heck., Ind.,
Ceylon, S.-W. Asia, Air.; Capoeta C. and V., W. Asia; Barbus
Cuv., barbels, 200 species, Eur., Asia, Afr., dorsal fin with the
(third) longest simple ray sometimes enlarged and serrated
only exceptionally with more than nine branched rays commencing
opposite or nearly opposite the root of the pelvic fin ; eyes without
adipose eyehd ; mouth arched without inner folds ; hps without
horny covering, barbels 4, 2 or ; B. vulgaris Fleming,
Europe, to 50 lb., as food coarse, roe sometimes poisonous. Thynn-
ichthya Bleek, E. Ind. Oreinus ; McClell, Himalajas ; from
same region Ptychobarbus , Gy7nnocypris, Schizopygopsis, Diptychus ;
Gobio Cuv., Eur. a small maxillary barbel; dorsal fin with few
rays, without spino ; G. fluviatilis Flem., the gudgeon. Allied are
Ladislavia and Pseudogobio, E. Asia; Ceratickthys Baird and Gerard,
N. Amer., called chub in the U. S. ; similar" genera of
N. Amer., and generally called "minnows," are Pimephales
(black head), Hyborhynchus, Hybognathus, Campostoma (stone-
lugger), Ericymba, Cochlognathus, Exoglossum (stone-toter or cut-
lips), Rhinichthys (long-nosed dace). Other Old World genera are
Cirrhina, Dangila, Osteochilus, Barynotus, Tylognathus, Abrostomus,
Crossochihis , Epalzeorhynchtis, Barbichthys, Amblyrhynchichthys,
Albulichthys, Aulopyge, Bungia, Pseudorasbora.

III. Rohteichthyina. Anal fin very short, with not more than
six branched rays ; dorsal fin behind pelvic ; abdomen compressed ;
no barbels ; pharyngeal teeth in triple series. Rohteichthya Bleek,
East Ind. Arch,

IV. Leptobarbina. Anal fin as in last ; dorsal opposite pelvic ;
abd. not compressed ; barbels present, not more than 4 ; phar.
teeth in triple series. Leptobarbua Bleek, E. Ind. Arch.

V. Rasborina with Raabora, from E. Ind. Cont. and Arch, and
E. Afr. ; Amblypharyngodon, Luciosoma, Nuria and Aphyocypris,
from E. Ind. Cont.

VI. Semiplotina with Cyprinion from Syria, Persia, Semi-
plotua from Assam.

VII. Xenocypridina with Xenocypria and Paracanthobrama from
China, Mystacoleucua from Stimatra.

VIII. Leuciscina. Anal fin of short or moderate length, with
8-11 branched rays, not extending forwards below the dorsal,
which is short and without osseous ray. Barbels generally ;
pharyngeal teeth in a single or double series. Leuciacua Klein,
white-fish, north temperate zone of both hemispheres ; species
found in England are L. rutilua Flem., the roach, said to form hybrids
with the bream and rudd ; L. cephalus Flem., the chub ; L. vul-

220

SUB-CLASS (and v^>RDEB) TELEOSTEI.

garis Flem., the dace ; L. erythrophthalmus Flem., the rudd or red-
eye ; L. phoxinus Flem., the minnow ; L. idus, the id or nerfling,
foimd in Europe, is domesticated in Gen^any, assuming the golden
hue of semialbinism like a goldfish. Tinea Cuv., Eur. and Asia
Minor, has been acclimatised ra India, T. vulgaris Cuv., the tench,
golden tench as a variety, due to albinism, as in the id and gold-
fish. Leucosomus Heck., N. Amer., L. pulchellus (fall-fish, dace
or roach), L. corporalis (chub), Chondrostoma Ag , Eur. and W.
Asia; other Old World genera are Myloleucus, Ctenopharyngodon,
Paraphoxinus ; N. American are Mylopharodon, Meda, Orthodon,

Acrochilus.

IX. Rhodenia with genera Achilognathus, Acanthorhodeus,
Rhodeus, Pseudoperilampus, roach-lik