(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Text-book of comparative anatomy"

BOUGHT WITH THE INCOMB 
FROM THB 

SAGE ENDOWMENT FUND 

THE GIFT OF 

Munt^ W. Sage 

X891 



'/j-?^/S6 ^.LL±(1±. 



TEXT-BOOK 

OF 

COMPAEATIVE ANATOMY 



TEXT-BOOK 



OP 



COMPAEATIVE ANATOMY 



BY 

DR ARNOLD LANG 

PROFESSOR OP ZOOLOGY IN THE UNIVERSITY OP ZURICH 
FORMERLY RITTER PROFESSOR OF PHYLOQENY IN THE UNIVERSITY OF JENA 



TRANSLATED INTO ENGLISH BY 

HENRY M. BEENAED, M.A. Cantab. 

AND 

MATILDA BEENARD 



PART II. 



fLonion 

MACMILLAN AND CO., Ltd. 
NEW YORK : MACMILLAN" & CO. 

1896 



TRANSLATORS' PREFACE 

The fact that this second volume of the translation appears four years 
after the first is due partly to the delay in the issue of the third and 
fourth German parts of which it is composed, and partly to the 
increased difficulty in the work of translation. A comparison of the 
two volumes will show at a glance that the work has developed 
under the hands of the author : the treatment has become more 
elaborate. The two " chapters " which practically fill this volume are 
in reality more like comprehensive treatises on the groups with 
which they deal, and as such could only be adequately translated from 
the German by some one with a very special knowledge of both 
groups. There are probably few zoologists who have attempted to 
make a special study of two such heterogeneous phyla as the Mollusca 
and the Echi-nodermata. In addition, therefore, to frequent references 
to the original literature and to constant applications to kind friends, 
the whole of the text relating to the two chief groups was submitted 
to specialists for revision. The translators beg to tender their 
warmest thanks to their friends who kindly undertook this laborious 
task. Mr. B. B. Woodward read the text of the chapter dealing 
with the Mollusca, revising the terminology, and suggesting slight 
alterations, which have been either adopted without comment in the 
text or else placed in short footnotes. Mr. W. Percy Sladen and Mr. 
F. A. Bather revised the text dealing with the Echinodermata, each 
with special reference to the group with which his name is most asso- 
ciated. Thanks are also due to Professor Jeflfery Bell for his kind 
assistance in the solution of difficulties. We have no hesitation in 
saying that it is to the generous help of these gentlemen that 



vi COMPARATIVE ANATOMY 

our translation owes mucli of the value it may possess for the English 
student. 

In the use of certain technical terms we have given the English 
or the Latin form indifferently, e.g. pinnule or pinnula, auricle or 
auricula, with deliberate inconsistency. On the other hand, we have 
throughout used the terms madreporite, madreporitic, and Echinoder- 
mata, although some authorities are more in favour of madrepore, 
madreporic, and Echinoderma. We feel it our duty to call the atten- 
tion of students to these points. 

The following author's preface is a free translation of the 
Xachwort which appeared at the end of the fourth German part. 
In it the author answers the only serious charge against the work 
as a test-book which has been brought to our notice. It finds its 
most appropriate place as a preface to the second volume of the 
translation. 

H. & M. BERNAED. 



AUTHOE'S PEEFACE TO THE SECOND VOLUME 

With the publication of the last two chapters, dealing with the 
Echinodermata and the Enteropneusta — that is of the fourth German 
portion — I bring this text-book to a close for the time being, as a 
comparative anatomy of the Invertebrata. 

I feel that some excuse is necessary for the tardy appearance of 
the separate parts, especially of the third (Mollusca). This was 
mainly due to my call to the iJniversity of Zurich, where official 
duties left only the holidays and vacations for my own work. When 
I add that the greater number of the illustrations were drawn by 
my own hand, the reader will, I trust, pardon the lapse of time. 
Indeed, if he be a trained zoologist, he will be specially sympathetic 
and indulgent, and will be able to realise my feelings as I watched 
the fresh relays of books piling up before me at the commencement 
of each new chapter. Original sources alone have been relied upon 
for the subject matter of the work. 

In spite of the imperfections and deficiencies of which I am 
only too conscious, the book appears to have been found useful, 
judging from the favourable reception almost universally given to 
it, and from the circumstances that, even during its appearance, 
it was translated into foreign languages. 

I am fully aware that the matter is unequally worked up. The 
divisions treated in the first volume are too briefly dealt with, a 
defect which must be remedied in a new edition. Any criticisms 
or advice with which my colleagues may favour me will be gladly 
accepted in the spirit in which they are intended. 

I have been blamed by many for not mentioning the names of 



viii COMPARATIVE ANATOMY 

authors in the text. From the very first this question caused me 
much perplexity, and I made repeated attempts to indite single 
chapters so as to bring in the historical development of the branch 
dealt with, together with the names of the most important authors. 
I then found that if this course were pursued the book would 
attain twice its present dimensions, that is, if strict impartiality 
were to be invariably observed. This latter I was resolved on no 
account to renounce, and I therefore determined to exclude from 
the text the names of all authors without distinction. Any one who 
is interested in knowing how a special question stands, can easily 
find his bearings by careful comparison of the text with the illustra- 
tions (the origin of which is everywhere given), and by consulting 
the literature. I have convinced myself of this among my own 
students. 

I must here express my thanks to my honoured and dear friend, 
Mr. Gustav Fischer, for the care and patience he has exercised in 

connection with this work. 

AENOLD LANG. 

Zurich, Jidy 1894. 



CONTENTS 



CHAPTER VII 
MOLLUSCA 

PAGE 

Systematic JReiiieio . 2 

Class I. Amphinetjka .... 2 

II. Gastropoda (Cephaloehoba) . . 3 

III. SCAPHOPODA . . . .13 

IV. Lamellieranchia (Pelecypoda, Bivalta, Aoephala, 

Aglossa) . .14 

V. Cephalopoda . 21 

I. Organisation of the Primitive Mollusc . . 26 

II. Review of the Outer Organisation characterising the 

Chief Groups of the Mollusoa . . .28 

A. PlAOOPHORA or PoLTPLACOPHORA (CHITONID.fi) . 29 

B. Aplacophora, Solbnogastres . . 29 

C. Gastropoda (Cephalophora) . 80 

D. Scaphopoda . 34 

E. Lamellieranohia . 34 

F. Cephalopoda . "' . .36 

III. The Integument, the Mantle, and the Visceral Dome . 39 

A. Placophora . 39 

B. Solenogastres . • 41 

C. Gastropoda . 42 

D. Scaphopoda . 49 

E. Lamellieranohia . 49 

F. Cephalopoda 53 

IV. The Shell . 55 

A. Amphineura 68 

B. Gastropoda 58 



s COMPARATIVE ANATOMY 

PAGE 

C. Lamellibranchia . . . .61 

D. Cephalopoda . . 67 

V. Arrangement of the Organs in the Mantle Cavity, and 

OE the Outlets of Inner Organs in that Cavity . 71 

A. Gastropoda . . .71 

B. SCAPHOPODA . 80 

C. Lamellibranchia . 81 

D. Cephalopoda . . 81 

VI. The Respiratory Organs . 84 

The True Gills or Ctenidia 84 

A. Amphineuka . 86 

B. Gastropoda 88 

C. Lamellibranchia . , 91 

D. Cephalopoda 96 
Adaptive Gills 97 
Lungs . . 99 

VII. The Hypobranohial Gland 101 

VIII. The Head . 101 

A. Gastropoda 102 

B. Scaphopoda 104 

C. Cephalopoda 105 

IX. The Oral Lobes of the Lamellibranchia 105 

X. The Foot and the Pedal Glands 106 

A. Amphineura . 106 

B. Gastropoda 107 

C. Scaphopoda 112 

D. Lamellibranchia . 112 

E. Cephalopoda . . I15 

XI. Swelling of the Foot (Turgescence) . Hg 

XII. Musculature and Endoskeleton II9 

A. Amphineura . . ]^20 

B. Gastropoda 220 

C. Scaphopoda . _ j.^3 

D. Lamellibranchia _ 124 

E. Cephalopoda . . _ ^26 

XIII. The jSTbrvous System j28 

A. Amphineura . . . -[28 

B. Gastropoda . j32 



CONTENTS xi 

PAGE 

1. The Aeeas of Innervation of the taeious Ganglia . 133 

2. Origin of the Crossing of the Pleuroviscbral Con- 

nective (Chiastoneury) . . . 135 

3. Special Remarks on the Nervous System of the Gas- 

tropoda . . . . 137 
0. scaphopoda . .142 

D. Lamellibranchia . . . 143 

E. Cephalopoda . . .146 

XIV. An Attempt to Explain the Asymmetry of the Gastropoda 149 

XT. The Sensory Organs . 162 

A. Integumental Sensory Organs 162 

1. Tactile Organs . .162 

2. Olfactory Organs . . 162 

3. The "Lateral Organs" of the Diotooardia 165 

4. Gustatory- Organs . . 166 

5. Subraddlar Sensory Organ of Chiton . . 166 

6. The Sensory Organs on the Shell of Chiton . 166 

B. Auditory Organs 167 

C. Visual Organs . 169 

1. Optic Pits 169 

2. Optic Vesicles or Vesicular Eyes . 170 

3. The Eye of the Dibranchiate Cephalopoda . . 170 

4. The Dorsal Eyes of Oncidium and the Eyes at the 

Edge of the Mantle in Pecten 173 

5. The Eyes on the Shell of Chiton . 175 

6. The Compound Eyes op Arca and Pectunoulus 175 

7. Degeneration of the Cephalic Eyes . 176 

XVI. The Alimentary Canal . . 176 

A. Buccal Cavity, Snout, Proboscis 178 

B. The Pharynx and Jaws, the Tongue and Salivary 

Glands .... • 180 

Formation of the Radula 183 

C. The (Esophagus . 187 

D. The Mid-gut with the Stomach and Dige.stive Gland 

(Liver) . . 190 

1. Amphineura . • 191 

2. Gastropoda 192 

3. ScAPHOPODA . . 193 

4. Lamellibranchia 194 

5. Cephalopoda ISA 

E. Hind-gut (Rectum) • 195 

XVII. The Circulatory System . 198 

A. General . 198 



xii COMPARATIVE ANATOMY 



B. Special 

1. Amphineuka 

2. c4a.ste0p0da 

3. soaphopoda 

4. Lamellibeanchia 



PAGE 

201 
201 
201 
206 
206 



5. Cephalopoda ■ 208 

XVIII. The Body Cavity 211 

XIX. The Nepheidia 215 

A. Amphineuka 216 

B. Ga.stkopoda , 217 

C. Scaphopoda 221 

D. Lamellibeanchia . 221 

E. Cephalopoda 222 

XX. Genital Oegass 225 

A. Genekal 225 

B. Special . 227 

XXI. PARA.S1TIC Gasteopoda . 244 

XXII. Attached Gasteopoda 248 

XXIII. Ontogeny . 248 

A. Amphineuea 248 

B. Gasteopoda 252 

XXIV. Phylogeny . 268 

R'jvi'no of the most Iniportmit Literniv rr 269 

Appendage, — Rhodope Vekanii 281 



CHAPTER VIII 

ECHINODERMATA 

Systematic Review . . 285 

Class I. Holothurioidea 285 

II. ECHINOIDEA . 288 

III. ASTEEOIDEA . 295 

IV. Ophiueoidea 299 
V. Pelmatozoa . 302 

1. Crinoidea 302 

2. Cystidea 313 

3. Blastoidea . 314 

I. General Moephology of the Echinodeem Body 315 

II. Moephology of the Skeletal System . 317 



CONTENTS xiii 

PAGE 

Introduction . . 317 

A. The Apical System (Calyx) . 319 

1. echinoidea . 319 

2. astekoidea . . . 326 

3. Ophiiteoidea . 327 

4. Pelmatozoa 328 
(a) Ceinoidea . 328 
(6) Blastoidea . . 330 
(c) Ctstidea . . .332 

B. The Oeal System of Plates . . 333 

C. The Perisomatio Skeleton . . 337 

1. Holothueioidea ... . 337 

2. eohinoidea . ..... 338 

(a) The Numeee of the Vertical Rows of Pla'jes . 339 

(6) The Poees of the Ameulaceal System . 340 

(c) The Symmetey of the Eohinoid Shell 340 
{d) The Relation of the Ambulaoral and Inteeameu- 

laceal Plates to the Peristome . . . 344 
(e) Manner in which the Skeletal Plates are Con- 

^ NEOTED ... ... 345 

(/) Special Modifications of the Ambulacra 346 

{g) Special Modifications of the Inteeeadii . . 348 

(A) FOEM OF THE Peeistome . 349 

(i) Oenamentation . . 349 

{k) Maeginal Incisions oe Peeforations . 349 

{I) The Perignathio Apophysial Girdle 350 

3. Asteeoidea ... . . 351 
(a) The Ambulackal Skeleton . 351 
(6) The Inteeambulacral Skeleton . 353 
(c) The Accessory Skeletal System . . 354 
{d) Comparison of the Perisomatic Skeleton of the 

Asteeoidea with that of the Echinoidea . . 355 

4. Ophiueoidea . ... . 355 
(a) Skeleton of the Aems . 355 
(J) The Oeal Skeleton . . . 358 

5. Ceinoidea . . 362 

(a) The Peeisomatic Skeleton of the Calyx . 362 

a. The Apical Capsule or Dorsal Cup 367 

i. The Tegmen Caly'cis . . 369 

(b) The Brachial Skeleton . 370 

(c) The Stem (Columna) . ... 373 
{d) The Manner of Connection between the Skeletal 

Pieces ....... 376 

(e) The Nerve Canals of the Arms and of the Apical 

Capsule .... . 377 



dv COMPARATIVE ANATOMY 

PAGE 

{/) The Water Pores 377 

6. Blastoidea 379 
(a) The Ambulacral Skeleton 379 
(S) The Stem 384 

7. Cystidea 384 

D. The Spixes and their Derivatives — The SPHiERiDiA and 

the Pedioellarke . . 387 

E. The Masticatory Apparatus of the Eohinoidea. (Aris- 

totle's Lantern) . . 400 

F. The Calcareous Ring ob- the Holothurioidea 403 

G. Further Deposits op Calcareous Matter . 405 
H. Concluding Remarks on the Skeleton 405 

III. The Outer Morphology' of the Holothurioidea 406 

IV. The Position and Arrangement of the most Important 

Organs in the Radii 409 

V. The Integument 414 

VI. The "Water Vascular System . 416 

A. The Madreporite and Stone Canal 417 

B. The Water Vascular Ring . . 423 

C. The Radial Canals, the Canals of the Tentacles and 

Tube-feet, etc. . 426 

D. The Ambulacral Appendages 431 

VII. The Ccelom . 436 

A. The Body Cavity 437 

B. The Brachial Cavities 440 

C. The Peeicesophageal Sinus 441 

D. The Perianal Sinus . 444 

E. The Axial Sinus . 444 

F. The Axial Organ 445 

G. The Chambered Sinus 446 

VIII. The Pseudoh^mal System , 447 

IX. The Epineural System 443 

X. The Blood Vascular or Lacunar System 449 

XI. The Nervous System . 453 

A. The Superficial Oral System . , 454 

B. The Deeper Oral Nervous System 458 

C. The Apical or Aboral Nervous System 459 

D. The Third Nervous System of the Crinoidea . 46I 



CONTENTS XV 

PAGE 

XII. The Sensory Organs . 462 

A. The Ambulaoral Appendages as Sensory Organs 462 

B. Nerve Endings in the Integument . 466 

C. Auditory Organs, Organs op Orientation . 468 

D. Eyes . . .468 

XIII. The Body Musculature 470 

A. HOLOTHURIOIDEA . 471 

B. EOHINOIDEA 471 

C. ASTEROIDEA 472 

D. OPHIUEOIDEA 474 

E. Crinoidea . 474 

XIV. The Alimentary Canal 474 

A. General Review 474 

B. HOLOTHUBIOIDEA 476 
0. EOHINOIDEA 479 

D. Crinoidea . 481 

E. Asteeoidea 483 

F. Ophiueoidea 485 

XV. Respiratory Organs 485 

A. The (inner) Respiratory Trees of the Holothurioidea . 487 

B. Eeview of the Respiratory Organs of the Echinoder- 

mata . . 487 

XVI. The Cuvibeian Organs of the Holothurioidea . 488 

XVII. Excretion- . . 489 

XVIII. The Sacculi of the Crinoidea . 489 

XIX. Genital Organs . 490 

A. General Morphology 490 

B. Holothurioidea 491 

C. Asteroidea 492 

D. Ophiueoidea 494 

1. The Burs^ . . 494 

2. The Genital Apparatus . 495 

E. Echinoidea . 498 

F. Crinoidea . • 500 

G. Origin of the Sexual Products 501 
H. Hermaphroditism in Echinoderms . 501 

1. Care of the Brood and Sexual Dimorphism . 502 

XX. Capacity foe Regeneeation and Asexual Repeoduction 504 

XXI. Ontogeny ..... 506 

A. The Various Larval Forms of the Eohinodeemata 506 



xvi COMPARATIVE ANATOMY 

PAGE 

B. Ontogeny of the Holothubioidea . 510 

C. Ontogeny of the Echinoidea 519 

D. Ontogeny of the Astekoidea 524 

E. Ontogeny of the Ophiueoidea . 532 

F. Ontogeny of the Crinoidea 533 

XXII. Phylogeny . 545 

Review of the most Important Literature . 551 



CHAPTER IX 

ENTEROPNEUSTA 

I. Outer Organisation . 562 

II. The Body' Epithelium , 563 

III. The Nervous System 564 

IV. The Sensory Organs 565 
V. The Alimentary Canal . . 565 

VI. The Ccelomic Sacs and the Body Musculature . 571 

VII. The "Heart Vesicle" . . . 578 

VIII. The Limiting JIembranes, the Probosoidal Skeleton, and the 

Branchial Skeleton . . , 579 

IX. The Blood Vascular System 581 

X. The Gonads . 585 

XI. Ontogeny' 58g 

XII. Phylogeny' 591 

Literature 595 

Appendage to the Enteropneusta 
I. Cephalodisous 

II. Rhabdopleura 

Literature 



Index 



596 

600 

602 

603 



CHAPTER VII 
SIXTH RACE Oil PHYLUM OF THE ANIMAL KINGDOM 

MOLLDSCA. 

The Mollusca are essentially bilaterally symmetrical animals with 
unsegmented bodies. The ventral wall is thick and muscular, and 
forms a foot which is used for locomotion, and assumes the most 
varied shapes. A fold of the body wall forms a circular mantle, which 
hangs down round the body, enclosing a space which is called the 
mantle or pallial cavity. This cavity is originally deepest and 
most spacious posteriorly, and contains, at the sides of the median 
anus, symmetrically grouped, the two gills and the renal and 
genital apertures. The dorsal portion of the animal is generally 
developed into a visceral dome or sac, and is protected down 
to the edge of the mantle by a shell. The mouth lies at the 
anterior end of the body and leads into a pharynx, which is usually 
provided with jaws and a rasp-like organ called the radula. The 
mesenteron or mid-gut is supplied with a large digestive gland (liver). 
The secondary ccelom (enclosed by its own walls) is reduced, but 
always persists as a pericardium. The blood vascular system is open, 
and generally to a great extent lacunar. The heart is dorsal and 
arterial, and was primitively provided with two symmetrical auricles. 
The nephridia were originally paired, and in open communication 
with the pericardium. The central nervous system consists of paired 
cere))ral, pleural, pedal, and visceral ganglia. The Mollusca are either 
sexually separate or hermaphrodite. The gonads are usually single, 
with paired or unpaired ducts. In the course of development a 
modified Trochophora arises from the gastrula; this is the Veliger 
larva, typical of the Mollusca. 

These general characteristics of the Jlolluscau body have to be modified for each 

class. In each class there are series of forms which deviate from the typical 

oro-anisation iu some one important point, or in several. The shell may disappear, 

and so may the mantle. Either one or both of the gills or ctenidia may be lost, 

VOL. II B 

^ 



COMPARATIVE ANATOMY 



CHAP. 



and new, morphologically different respiratory organs may be substituted. The 
visceral dome may be flattened down, and the foot become rudimentary or disappear. 
Teeth of all kinds may be wanting. The complex of the sub-pallial organs may be 
so displaced as to lie anteriorly, thereby causing a very pronounced asymmetry of 
the whole organism. But the typical MoUuscan characteristics are never so entirely 
obscured that the members of the race cannot be recognised, on the one hand by 
means of transition forms leading to well-known Molluscan types, and on the other 
by their developmental history. 

The Molluscs are divided into the five following classes : — 



I. Amphineura. 
III. Scaphopoda. 



II. Gastropoda. 
IV. Lamellibranchia. 
V. Cephalopoda. 



Systematic Review. 

CLASS I. Amphineura. 

Bilaterally-symmetrical Molluscs. The nervous system consists of two lateral 
and two ventral nerve trunks, bound together by numerous commissures, and 




Fig. 1.— Chiton, from life (after Pr6tre, in the Voyage de V Astrolahc). 

provided with ganglion cells throughout their whole length ; these pass anteriorly 
into the cerebral ganglion. Special sensory organs are reduced. Marine. 



Okdek 1. Placophora (Polyplaoophora) sive Chitonidae. 

On the dorsal side there are eight consecutive shelly plates overlapping like the 
tiles on a roof There is a distinct snout. The branchife are numerous, and are 
arranged in two longitudinal rows, one on each side in the groove between the foot 
and mantle. The foot (except in Chitonellus) is strongly developed, with a large flat 



VII MOLLUSGA— SYSTEMATIC RE VIE TV 3 

sole for creeping or for attachment. The sexual ducts and the nephridia are paired. 
The sexes are separate. The heart is provided with two auricles. Radula (3 + 1), 
(2 + 1), (1 + 1 + 1), (1 + 2), (1 + 3). Chiton (Fig. 1), Chitonellus. 

Order 2. Aplacophora sive Solenogastres.i 

The body is almost cylindrical, and generally vermiform. There is no shell. 
The much thickened cuticle contains calcareous spicules. The foot is rudimentary, 
a mere ridge being left, and the mantle cavity is reduced to a groove at the sides of 
this ridge, and a cavity (cloaca) at the posterior part of the body, into which the 
intestinal canal and nephridia open, and in which are found, when present, the 
rudimentary gills. The nephridia serve as ducts for the genital products. 

Family 1. Neomeniidse. 

The foot is a longitudinal ridge, which rises from the base of a medio-ventral 




Fig. 2. — Proneomenia Sluiteri, two-thirds natural size. A, From the right side ; B, from 
beneath ; o, mouth ; d, cloaca. 

longitudinal furrow. This family is hermaphrodite. Proneomenia (Fig. 2), Ifeo- 
menia, Lepidomenia, Dondersia. 

Family 2. Chsetodermidse. 
The foot and the pedal furrow are quite degenerated. The sexes are separate. 
Ghcetoderma. 

CLASS 11. Gastropoda (Cephalophora). Snails. 

The body is asymmetrical. The head, which carries tentacles and eyes, is 
generally distinct from the body. The foot is well developed — usually with a fiat sole 
for creeping. The large protruding visceral dome may be flattened down secondarily 
in all the groups. It is covered by a shell, consisting of a single piece, into which the 
animal can withdraw. In all divisions, however, though rarely among the Proso- 

^ Simroth, in the new edition of Bronn's Klassen und Ordnungen des Thierreiches, 
vol. iii., 1893, divides the Solenogastres as follows :— 

Sub-Order. Fam. 

Chstodermatina Ch^todermatidse. 

iNeomeniidae. 
Proneomeniidse. 
Dondersiidse. 
ParameniidEe. 



COMPARATIVE AXATOMY 



CHAP. 




Fig. 3.— Margarita Groenlaudlca (Trochid, after 
Pelseneer). 1, Head ; 2, anterior epipodial lobes ; 
3, foot ; 4, pigmented prominence at the base of the 
epipodial tentacles (5) ; 6, visceral dome. 



hranchia, this shell may become more or less rudimentary (generally in connection 
with the reduction of the visceral dome). 

The pallial complex becomes shifted forward on to the right (seldom the left) 

side, or along this side so as to lie quite 
anteriorly. The visceral dome and 
shell (with some exceptions) are spirally 
coiled. 

In all except the lowest Proso- 
hmnchiu, the asymmetry is evidenced 
by the disappearance of one gill, of one 
kidney, and of one auricle. 

The radula is rarely wanting. 

Order 1. Prosobranchia. 

The pleuro- visceral connectives are 
crossed. The mantle complex is twisted 
round to the front side of the visceral 
dome. In most forms there is only 
one gill, placed anteriorly to the heart, 
and in the heart the auricle lies anter- 
iorly to the ventricle. The Proso- 
branchia are chiefly marine, and are sexually separate. The foot is generally pro- 
vided with an operculum for closing 
the apertm'e of the shell. A shell is 
wanting only in Titiscaniu, a genus 
of the Xi:ritari:a. 



Sub-Order 1. Diotocardia. 

The heart has two auricles (except- 
ing in Docoglossa). There are two 
kidneys. Instead of thepedalganglion 
of other Gastroiioda, there are two 
longitudinal nerves in the foot, sup- 
plied with ganglia and connected with 
one another by numerouscomraissures. 
The gills are feathered on two sides, 
their points projecting freely. The 
epipodium is well developed, and 
there is a circle of more or less 
numerous tentacles around the base 
of the foot. Proboscis, penis, and 
siphon arc all wanting. 

a. Zeugobranchia (Rhipidoglossa, 
Aspidobrancliia). — Two gills ; both 
auricles well developed. Heart tra- 
versed by the rectum. Shell with 
marginal cleft, or with apical perfora- 
tion or with a row of perforations. 
Generally without operculum. 
Marine. Fam. HaliotiJa;, radula 
c^jI. (5.1.5)1=0, Fissurcflldii: {Fissu- 
rella, rad. ccl.(4.1,4)l.«) , with secondarily symmetrical shell. Eincyimda, .S'aituvi 




Fig. 4.— Patella vulgata (from beneath, after 
Lankester). «, Tentacle ; d, eflerent branchial vessel ; 
c, free edge of the shell ; e, free edge of the mantle ; 
J--!/, median line; </, afferent branchial ves.sels ; 
/, branchial laniellie ; li, one of the afferent vessels '; 
i, spaces between the shell muscles ; h, foot. 



VII 



MOLL USOA—S YSTEMA TIC RE VIE W 



-Parmoplwnis), Pkurotomaridcc (Pkurotomaria, Sdsmrclla, Polytrcmaria), Bellero- 
phontidce (exclusively fossil). 

h. Azygobranchia. — One gill, homologous with the left gill of the Zeugo- 
branchia. Right auricle ending blindly. Heart perforated by the rectum. Fam. 
Turhonidcs, rad. ooO.(5.1.5.)0.<» , TrocUdcc (Fig. 3) Stoviatiidce, Neritopsida; rad. 
a>l.(2.0.2.)l.co , marine, Neritidm, rad. ool.(3.1.3.)l.oo (marine, but along the shore 
able to live out of water), Keritinix (marine and fresh-water). The Sydrocoenidce, rad. 
ool.(l.l.l.)l.co, and ffelicinidce, rad. co.l.(4.1.4.)l.oo , have no gills but a lun" 
resembling that of the Pulmonata. The ffelicinidce are terrestrial. 

c. Doooglossa. — Heart with one auricle, and not perforated by the rectum. Left 
kidney shifted to the right side of the pericardium. Visceral dome and shell 
secondarily symmetrical, the latter usually cup-like. Operculum wantino-. Marine. 




Fir,. 6.— Phoras exutus (after Lankester). o, Proboscidal snout or rostrum ; b, tentaclB ; 
0, eye ; d, foot ; e, metapodiuui with operculum/. 

1. Left true ctenidium present. Acmaeidce, vad. 1.2.(1. 0.1.)2.1.; with numerous 
accessory gills in the mantle furrow: Scurria ; — without such gills: Aanaea 
(Teetura). 

2. True ctenidia altogether wanting, accessory gills very numerous in the mantle 
furrow.— Fam. Patellidce (Fig. 4), rad. 3.1.(2.0.2. )1.3. 

3. Neither ctenidia nor accessory gills found (Lepetidce), rad. 2.0.1.0.2. 



Sub-Order 2. Monotocardia (Peotinibranchia). 

Heart with one auricle. A single true ctenidium feathered on one side, the point 
not projecting freely (except in Valvata). Pedal nerve trunks a jare exception, pedal 
ganglia the rule. Only one kidney. Siphon and penis generally present. Epi- 
podium weakly developed or wanting. The Monotocardia are very numerous and 
are chiefly marine. 

a. Architaenioglossa. — Pedal nerve trunks. In Cypraea (and in other forms ?) a 
rudiment of the right auricle persists. Fam. Oypraeidce, rad. 3.1.1.1.3, Paludinidm 
(fresh-water), OyclopJioridM (terrestrial, pulmonate). 



COMPARATIVE ANATOMY 



CHAP. VII 



h. Taenioglossa. — Typical radula, 2.1.1.1.2. Semiproboscidifera. Fam. 
Naticidce (Fig. 98, p. 107), Lamellaridm. Eostrifera. Fam. Valvatidm (fresh-water), 
Ampullaridce (fresh - water), Littorinidce, Cydosto'inidce (terrestrial), Planaxidce, 
Hydrohiidce (fresh- water), Aeiculidce (terrestrial), TruncatelUdce (partly terrestrial), 
Hipponycidm, Capulidm, Calyptraeidce, Pseudomclanid.ce, Melanidce, CerithiidcB, 




Fig. 6.— Eostellaria reotirostris (after Owen), u, Snout ; 6, tentacle ; t, stalked eye ; d, foot ; 
c, raetapodiuni with operculum/; fe, beak (for the siphon). 

VeriaetUlix', Tvrritelluhe, Xcnophoridcc (Fig. 5), Struthiolaridce, Clienopidcc, 
Stromhidce (Fig. 6). Proboscidifera holostomata. Fam. Scalaridm, rad. xOk ; 
SolaridcE, rad. ccOoo ; PyramideUidce, rad. 0; Eulimidce, rad. 0. Proboscidifera 
sipbonostomata. Fam. ColovihelUnidce, Tritoniida:, Cassidiida; (Fig. 7), Doliidm. 




H^(l^^^m.le\^^ >. /./ .\ . 





Fi,;. -.-Cassis suolosa (after Poll), a, Shell; b, beak; c, siphon; d, head ; (,, probo.scis ; e, 
eye ; /, tentacle ; h, foot ; i, operculum. 



Janthmidae, rad. cxOx . Heteropoda (marine Taenioglossa, with foot transformed 
into a perpendicular rowing fin). Fam. AtlantidcE (Fig. 8), Pfrrofradtacidm 
(Fig. 9). 

c. Stenoglossa. — Normal rad. 1.1.1. Eaohiglossa. Fam. rurbindlida- 

Fiistdcc, Jlitridcc, Bua-inldrr, Mvricidce, Purpiiridcc, Haliadm, Camxllariidm 

Volutidce, Olividcc, ilarginellidce, Harpidjc. Toxiglossa. Fam. Pleurotomidic. 
Terehridw, Conidcc. 




Fig. S.— Atlanta Peronii (after Gegenbaur). a, Pliarynx ; &, lauccal ganglion ; c, tentacle ; d, 
eye ; e, cevebral ganglion ; /, aorta ceplialica ; £/, pleuro-visceral connective ; 7t, columellar muscle ; 
f, fc, osphradium ; I, vagina ; 7E, ctenidiura ; n, anus ; o, uterus ; p, nephridium ; <i, aorta ceplialica ; 
/•, auricle ; s, ventricle ; t, aorta visceralis ; u, digestive gland (liver) ; v, ovary ; w, stomach ; x, 
pedal ganglion ; y, operculum ; z, nietapodium ; 1, sucker of the fin-like foot (rudimentary sole) ; 2, 
foot ; 3, auditory organ ; 4, cesoiOmgus ; 5, snout ; 6, salivary gland. 

23 




Fig. 9.— Pterotrachea (Firola) coronata (after Leuckart). u, Pharynx; &, proboscidal snout ; 
*., (.'ye; rf, cerebral ganglion; e, pedal ganglion; /, pedal artery; g, intestinal canal; h, pleuro- 
visceral connective; i, pari etc- visceral ganglion; 7., osphradium ; 1, ventricle; m, auricle; a, 
anus ; o, ctenidium ; //, metapodium ; g, appendage ; r, aorta cephalica ; s, nerve running to the 
metapodium ; t, artery ; ii, foot ; v, common pedal artery ; w, cephalic artery ; x, auditory organ ; 
1/, buccal ganglion. 



COMPARATIVE ANATOMY 



CHAT. 



Okder 2. Pulmonata. 

The pleuro-visoeral connectives are not crossed. The ctenidium has disappeared 
from the mantle complex and is replaced by a lung, or respiratory vascular network, 
on the inner surface of the mantle. The pallial organs lie primitively to the right, 
anteriorly on the visceral dome. The edge of the mantle, with the exception of a 
branchial aperture on the right, imites with the integument of the neck. In terres- 
trial Pulmonata the visceral dome is often 
flattened down and the shell becomes rudi- 
mentary (Slugs). The operculum is always 
wanting. The heart has one auricle, which 
almost always lies anteriorly to the ventricle. 
The Pulmonata are hermaphrodites with herma- 
phrodite glands or ovotestes, and complicated 
efferent ducts. They are either terrestrial or 
fresh -water. 




Fio. 10. — Amphipeplea leuoonensis 
(after Adams). », Lobe of the mantle 
bent back over the .shell ; &, portion of 
the shell uncovered ; c, foot. 



Sub-Order 1. Basommatophora (fresh-water). 

Eyes at the bases of the non-invaginable 
optic tentacles. Genital apertures separate, to 
the right anteriorly, the male in front of the female. Fam. Livmmidcc, (Limnaia, 
Amphipeplea [Fig. 10], Physa [Fig. 11], Planorhis, Aneyhis), Auricvlida: 




Fig. 11.— Physa fontinalis (after L. Reeve), w, Mantle lobes folded back over the shell ; 6, 
evaginated penis. 

Eyes at the tips of the oj)tic tentacles ; tentacles iuvaginable. 



Sub-Order 2. Stylommatophora. 

a. Monogonopora. — With a single genital aperture to the right. Fam. HelicidEe 
(Helix [Fig. 12, A], Arion [Fig. 12, D], Bulimus). Testacellidse {DavxUhardia 
[Fig. 12, B\ Tcstacella [Fig. 12, G]. LimacidSB [Ariophaiita, Umax, Vitrina, 
Zonites, Heliearion). Bulimulidae (Fig. 13), Pupidse (Bxilimimis, Pupa, C'lausifia), 
Succiueidse. 

b. Digonopora.— Shell-less snails with separate male and female genital apertures, 
the male anterior, the female at the posterior end of the body, both to the rio-bt. 
Pallial complex at the posterior end of the body, lung cavity reduced. Fam. Vagi- 
nulidse (terrestrial), Onoidiidse (marine or amphibious) ; respiration partly by means 
of dorsal branchial appendages. 



VII 



MOLL USGA — ,S YHTEMA TIG RKVIE U ' 



/^ A 




Fig. 12.—^, Hellz pomatla ; B, Daudebardla (Helioophanta) lirevlpes ; C, Testaoella hallo- 
tidea; D, Arion ater ; s% shell, in D sliield (from Lankester). 




Fig. is.— Peltella palllolum(Bu?i»n(;i(/, after Ferussao). 



10 



COMPARATIVE ANATOMY 



CHAP. 



Order 3. OpisthobrancMa. 

The pleuro-visoeral connectives do not cross.' There is one auricle placed behind 
the ventricle A shell is sometimes present, more freciuently wanting. An 
operculum is rarely found. Respiration by means of true ctenidia, or of adaptive 
gills, or through the skin. The visceral dome is very often levelled down. Herma- 
plirodites with ovotestes. JIarine. 

Sub-Order 1. Tectibranchia. 

The pallial complex is to the right of the body, and is more or less covered by 
the mantle fold belonging to that side. One true ctenidium (viz. that which was 
originally the right) is always retained in the mantle cavity, but is often very 
incompletely covered by the mantle. The visceral dome tends to disappear. A 
sliell is always present, but tends to become rudimentary. Generally with para- 
podia, and mantle lobes covering the shell. 

A. Reptantia. 
-(. Cephalaspidffi.— With frontal or cephalic disc. Fam. Actaeonidse (with 
operculum), Scaphandridse, BuUidK (Bulla, Accra), GaBtropteridae (Fig. 14), 
Philinidse, Doridiidse. 

6. Anaspidse.— Head without frontal disc ; four triangular or ear-like tentacles. 
Fam. Aplysiidae (A'phjsia, Dolabella, Xutaixlnis). 






,1- — 



O 



Fio. ifi.— Pleurobranoliiis aurantiaous, witli internal 
shell (after Leuckart's W(niillafdn), seen from the right 
side, a, Rhinophores ; 6, labial sail ; c, genital aperture ; 
d, nephridial apei-ture (?) ; c, ctenidium ; /, anus. 



•3 
</ 

Fig. 14. — Gastropteron Meokelil, 
with internal shell (after Vayssi^re). 
1, Cephalic shield (frontal disc) ; 2, pava- 
podium ; 3, ctenidium, left almost un- 
covered by ttie rudimentary mantle fold ; 
4, flagelluiii = appendage of tlie mantle 
fold. 

t. Notaspidae. — Head short, with or without tentacles. Large dorsal disc 
(notreum) in or on which a shell may lie. Fam. Pleurobranchidse (Pleurobra-iichus 
[Fig. 15], Pkurohrandiira, Oscainius), UmbrellidaB {UmlreUa, Tylodina), Peltidse. 



B. Natantia sive Pteropoda.^ 

These formerly constituted a separate class of the Molluscs, but are noAV recog- 
nised to be Tectibranchia adapted to a free-swimming pelagic life. The parapodia of 
the Tectibranchia develop as tins or wing-like swimming organs. 

"' Except in Actccaa, which is streptoneurous, and thus forms a connecting link 
between tlie Opisthohranchia and Pnliiionata on the one liaud, and tlie remaining 
Gastropods on the other [Bouvier and Pelseneer], v. Xnt. Sci., July 1893. 

^ The classification of the Opisthobranchs, which places tlie Pteropoda thecosomata 
with the Ceplialaspidas, and the Pteropoda gymnosomata with the Anaspidfe, is accepted 
on p. 110 and elsewhere. 



VII 



MOLL use AS YSTEMA TIC RE VIE W 



11 



a. Pteropoda thecosomata. — These are nearly related to the Ccphahispldca^ 
and possess a mantle, mantle cavity, and shell. The head is not distinct, and has 
only one pair of tentacles. The fins, at their anterior edges, are fused over the month ; 
the anus lies to the left. Fam. Limacinidae. An external calcareous shell, with 
left-handed or sinistral twist, and a spiral operculum. Anus to the right [Lima- 
cina [Fig. 16], Feraclis). Fam. Cavoliniidaa. External symmetrical shell {Clio, 
Cavolinia). Fam. Cymbuliidse. Internal cartilaginous shell [Cymhulia, Gymhuli- 
opsis, Gleha). The Thecosomata feed chiefly on small Protozoa and Algte. 

(X 




Fir;. K'p. — Limacina Lesueuri (dorsal aspect, 
after Pelseneer). 1, Penis ; 2, fin (parapo- 
dium) ; 3, seminal furrow ; 4, mantle process 
("balancer"); 5, visceral dome; 6, head with 
two tentacles and the seminal furrow 3. 

Fig. 17.— Pneumoderma (diagram from the 
right, after Pelseneer). 1, right evaginated 
process bearing hooks (hook sac) ; 2, proboscis ; 
3, right buccal tentacle ; 4, position of the right 
nuchal tentacle ; 5, right fin (parapodiiira) ; 6, 
seminal furrow ; V, genital aperture ; S, position 
of the jaw ; 9, ventral proboscidal papilla ; 10, 
right buccal appendage provided with suckers ; 
11, head ; 12, aperture for penis ; 13, right an- 
terior pedal lobe ; 14, anus ; 15, posterior pedal 
lobe ; 16, ctenldiuni ; 17, posterior adaptive 
gill ; (.?, c, a, p denote dorsal; ventral, anterior, 
and posterior. 




4» 



b. Pteropoda gymnosomata. — These are nearly related to the Anaspidae. 
They have no mantle, mantle cavity, nor shell. The head is distinct, and carries 
two pairs of tentacles. The fins are separate ; the anns lies to the right. Fam, 
PneumodennatidsB. One otenidium to the right {Dexiobranchcea, Spongiobrancluea, 
Pneumoderma [Fig. 17]). In the last two genera there is an adaptive posterior gill 
as well. Fam. Clionopsidse and Notobranohseidse. No ctenidium, but a posterior 
adaptive gill. Fam. Clionidae. Neither ctenidium nor adaptive gill. All Gymnoso- 
mata are carnivorous, feeding principally on Thecosomata. 

Sub-Order 2. Ascoglossa. 

This sub-order is characterised by the fact that the worn-out teeth of the long 
narrow radula, which consists of a single row of dental plates, are preserved in a sac 



1: 



COMPARATIVE ANATOMY 



CHAP. 



at its anterior end. No jaws. The anus almost always dorsal. Except in the 
Stegcmobraiichia, the disappearance of the mantle and its cavity is accompanied by 
the disappearance of the single ctenidium of the Tectibranchia. 

Section 1 . Steganobranchia. — With mantle, cavity, and ctenidium to the right ; 
with a shell and parapodia. Fam. Oxynoidea (Oxynoe, Lobigcr). 

Section 2. Cirrobranohia. — Leaf- or club-shaped processes found laterally on the 
back. Fam. Hermseidse, Phyllobranchidse. 

Section 3. Pterobranchia. — The sides of the body produced into lobes, in 
which the branches of the glands of the mid -gut spread out. Fam. Elysiadse, 
Placobranchidaa. 

Section 4. Abranchia. — Neither ctenidium, nor dorsal appendages, nor leaf-like 
lateral expansions of the body. Respiration through the skin. The bodj' is almost 
like that of a Planarian. Fam. Limapontiidse. 



Sub-Order 3. Nudibranchia. 

Without mantle fold, shell, or ctenidium. Jaws almost always found. Eadula 




Fig. is.— Aeolis rufibrancliialis (right aspect, after Alder and Hancock). ", Bye ; J, oral 
cephalic tentacle ; d, anus ; i, genital aperture ; /, dorsal respiratory appendages 



tentacle ; 
(cerata). 



generally well developed, with teeth which fall off and are lost. Adaptive gills 
very variously developed, but occasionally wanting. 




Pin. 19. — Phylllrhoe tuoephalum (lateral aspect, after Souleyet, modified). 1, Tentacle ■ 
2, cerebral ganglion; 3, stomach; 4 and 12, intestinal creca (forming the digestive --land) '■ 
5 ventricle ; 6, auricle ; 1, pericardial aperture of the kidney ; S kidney ; 9, external aperhire of 
the same (on the right side) ; 10, anus (on the right side); 11, hermaphrodite glands, the ducts not 
drawn ; 13, genital apertures ; 14, buccal ganglion ; 16, salivary glands. 

Section 1. Holohepatica.— One large uubranched hepatic gland (liver). Fam. 



VII 



MOLL USCA—S YSTEMA TIC RE VIE W 



13 



Phyllidiidse. Numevous branchial lamellae lie in a groove which encircles the body. 
No jaws and no radula. Pharynx transformed for sucking. Fam. Doridopsidse. 
Without jaws or radula ; pharynx adapted for sucking. Branchial rosette round 
the dorsal anus. Dorididse crypto- 
branohiatas. The branchial rosette 
round the dorsal anus can be with- 
drawn into a cavity. (Bathydoris, 
Ardiiduris, Discodoris, Diaidula, Kent- 
roduris, Platydoris, Chromodo7-is, etc.) 
Dorididse phauerobranchiatse. Bran- 
chial rosette not retractile. (Gonio- 
doris, Polycera, Acanthodoris, Idalia, 
Aaciili'f, Eicplocavius, Triopa, etc.) 

Section 2. Cladohepatica. — Diges- 
tive glands more or less broken up into 
separate branched canals spreading 
widely in the body. Variously formed 
dorsal appendages chiefly connected 
with respiration. Anus usually to the 
right. Fam. AeolidiadaB (Acolidia 
[Fig. 18], Berghia, Tergipes, Galvina, 
Coryjjtdla, Iii::^olia, FacelUna, Flabel- 
lina, Fiona, Glaucus, Janus, Hero). 
Fam. Tethymelibidae, without radula 
{Tcthys, Mdibe). Fams. Lomanotidse, 
Dotonidae, Dendronotidae, Bornellidse, 
Soyllaeidae, PhyllirhoidsB (Fig. 19 ; 
marine free - swimming animals with 
narrow laterally - compressed body, 
without foot or respiratory append- 
ages). Fam. Pleurophyllidiidse. Nu- 
merous branchial lamellfe arranged in leaflets 
a single row on each side along a tentacle shield ; 7, 
furrow between the dorsal shield and the foot (Fig. 
Tritoniadae (Tritonia, Marionia). 




Fio. 20.— Pleurophyllldia lineata (from below, 
after Souleyet). 1, Genital apertui-es ; 2, branchial 
anus ; 4, pedal gland ; 5, mouth ; Ij, 
foot. 



20). Fam. Pleuroleuridse, 



CLASS III. Scaphopoda. 

The body is symmetrical, and elongated dorso-ventrally. The mantle is a tubular 
sac with a narrow dorsal and a wider ventral aperture. Posteriorly, the mantle 
cavity reaches to the apical (dorsal) aperture. The shell forms a high tubular cone, 
and, like the mantle, has a small apical and a larger ventral aperture. Ctenidia are 
wanting ; the kidneys are paired. The vascular part of the circulatory system is 
reduced to a ventricle ; without auricles. The sexes are separate. There are no 
special ducts for the sexual products, which are ejected through the right 
kidney. The mouth lies at the end of a barrel-shaped snout, and is surrounded by 
a circle of leaf-like appendages. At the base of this snout there are numerous 
filamentous appendages, which can be protruded through the lower aperture of the 
shell and mantle. The foot is ventrally elongated. A radula is found. Limicolous. 
JIarine. Fam. Dentalium (Fig. 101, p. 113). The foot is relatively short ; it is shaped 
somewhat like an acorn, with a conical central portion and two lateral lobes. 
Siphonodentalium. The foot is long and worm-like, but broadens out at the end 
into a disc edged with papillEe. 



14 



GOMPARATIFE ANATOMY 



CHAP. 



CLASS IV. Lamellibranohia (Pelecypoda, Bivalva, Acephala, Aglosaa). Mussels. 

The body is symmetrical and more or less transversely flattened ; it has two 
large lateral leaf-like mantle lobes, enclosing a spacious mantle cavity large enough 
to contain the foot, which is usually hatchet- or wedge-shaped. The shell consists 
of two lateral valves connected together only at the dorsal hinge. It is closed by means 
of two adductor muscles passing transversely from one valve to the other (Dimyaria) ; 
occasionally the anterior adductor degenerates and only one remains (Monomyaria). 
On each side in the mantle cavity there is a ctenidium. There are no jaws, no 
pharynx, no radula, no tentacles, and no distinct head. The kidneys and genital 
organs are paired, and the latter either have separate ducts or eject their products 
through the nephridia. The heart has two auricles. At each side of the mouth 
there arc two oral lobes. Either sexually separate or hermaphrodite. They live 
in salt or fresh water, and are either limicolous or attached. 

Oeder 1. Protobranchia. 

The gills with two rows of leaflets, in the posterior part of the mantle cavity ; 
they correspond in all i-espects with the ctenidia of the Zeugobranchia, their ends 




Fig. 21.-Nuoula nucleus, left aspect after removal of the left valve and mantle (after 
Pelseneer). n, Anterior adductor ; b, anterior retractor of the foot ; c, elevator of the foot- d genital 
mass ; e, hypobranchial gland ; /, posterior retractor of the foot ; g, posterior adductor'- h cteni 
dium ; i, mantle cavity ; k, creeping sole of the foot (!) ; m, oral lobes Gabial palps) Avith' posterior 
appendages n and o. 

project freely backward into the cavity. The foot has a sole for creeping. The 
pleural ganglion can be distinguished from the cerebral. Fam. Nuculidse (Nucula 
[Fig. 21], Leda, YoUia, Solenomyidce). 



Order 2. Filibranchia. 

The branchial leaflets of the ctenidium have become lengthened out into long 
filaments hanging far down into the mantle cavity. Each is in two parts, the proxi- 
mal descendmg and the distal ascending {cf. Fig. 88 B). Fam. Anomiid^ : mantle open 



VII 



MOLLUSGA— SYSTEMATIC REVIEW 



15 



without siphons ; Monorayarian. Foot small ; body and shell asymmetrical. Attached. 
Branchial filaments entirely free (^Momia, Placvna). Fam. Arcidae : the branchial 
filaments of each row connected by ciliated discs ; Dimyarian. No siphons. Foot 
large (Area., Pectuncuhis). Fam. Trigoniidse : ctenidia like those of the Ardda: ; 




Pig. 22.— Mytilus edulis (after Meyer and Mobius), left aspect, with extended foot attaching a 
byssus thread ; d, byssua threads ; a, exhalent aperture (anal siphon) ; !>, fringed edge of the in- 
halent mantle aperture ; c, object to which the animal is attached. 

Dimyarian. No siphons ( Trigonia). Fam. MytilidsB (excluding Aviculidcs) : cten- 
idia connected by means of non - vascularised trabeculse. The anterior adductor 
is smaller than the posterior (Heteromyarian). With siphons. Foot long. (Mytilus 
[Fig. 22], Modiola, Lithodomus [boring mussel], Modiolaria). 

Ordek 3. Pseudolamellibranohia. 

The consecutive ctenidial filaments of each row are connected by means either 
of ciliated discs or of vascularised trabecule ; and the ascending and descending 



16 COMPARATIVE ANATOMY chap. 

portions of each filament are similarly united (cf. Fig. 88, p. 92). Fani. Peotinid£e: 







Fig. 28.— Pecten Jacobseus, ventral aspect, shell opened. The mantle cleft is seen bet\\een 
the Iringes of the mantle, which are beset with numerous tentacles and eyes (after Leuckart and 
Nitsche, Zool. Wondtafelii). 




Fig. 24.— Anatomy of tlie Oyster (Ostrea edulls), right aspect (after Mobius, Leuckart, and 
Nitsche, Zool. Wn adtafeln). br, Gills ; Pn, posterior mantle nerve ; ,'■, .xi, apertures of tlie cavities 
between the fused jjlates of the two left gills ; .1/, large adductor muscle ; a, anus ; il/iu, posterior 
portion of the adductor muscle ; iVt, mantle ; P, pericardium ; r, heart ; go, gonad (hermaphrodite 
gland) ; tl, intestinal canal ; ?, digestive gland (liver) ; o, mouth ; os, osi, oral lobes (labial palps) of 
the left side ; Oj, cerebral ganglion ; n, kidney ; Im, branchial nerve ; Vg, visceral ganglion ; Pj, abdo- 
minal process ; Pui, nerve of the pallial edge ; m, stomach, with the apertures of the digestive gland. 

^louomyariaii with mantle entirely open, and eyes at its edge. Without siphons. 
l^oot small and linguiform. Valves of the shell equal or unequal. Capable of 



VII 



MOLL USGA^SYSTEMATIG REVIE W 



17 



swimming. {Pccten [Fig. 23], Chhiiitys). Fam. Aviculidae : Monomyarian or Hetcro- 
myariau without siphons. Valves equal or unequal {Avicula [ilelcKgrian], Mal/cus, 
Vulsella, Pcrna, Inoccmmus, Pinna, Mdeagrina manjaritifcm, pearl mussel). 
Fam. Ostreidse : Monomyarian without foot, with completely open mantle, without 
siphons. "\'alves unequal, the left valve attached to the substratum. (Ostrca : 
oyster [Fig. 24]). 

Oedek i. Eulamellibranchia. 

The gills no longer consist of distinct filaments. On the contrary, the filaments 
in each row and the two parts of each filament are so connected by means of 
vascularised trabeculse or sutures as to form together a lamella or trellis -work. 
There are, on either side, two such branchial lamellie (hence the name of Lamelll- 
branchia), which in fact correspond with the two rows of leaflets of the typical 
ctenidium. This order includes the majority of the Lamellibranchia. 



Sub-Order 1. Submytilaoea. 

Branchial lamelloe smooth. The mantle edges usually grown together only 
between the inhalent and the exhalent apertures. Dimyarian. Fam. Carditidse : 
with open mantle and large foot {Cardita, Vciuricardia). Fam. Lucinidse : 
with simple, and as a rule single, siphonal aperture. Foot often vermiform. Fam. 




Fig. 25. — Anatomy of Unio (Margaritana) margaritiferus, left aspect (after Leuckart and 
Nitsclie). o, Mouth ; eg, cerebral ganglion ; Mj, anterior adductor muscle ; a-, cesophagus ; ?, 
digestive gland (liver) ; no, nephxidial aperture ; lo, aperture of the digestive gland in the stomach 
m ; Aa, anterior aorta ; n, nephridimn, the outlines given in dotted lines ; V, heart ; /-, proctodeum ; 
Ap, posterior aorta ; il/o, posterior adductor ; a, anus ; V(j, visceral ganglion ; Br, gill ; Bk, mantle 
cavity ; go, gonad and ducts goi ; Pg, pedal ganglion ; p, toot. The arrows mark the direction 
of the inhalent and exhalent streams of water. 

Erycinldse : mantle closed except at the two siphonal and the pedal apertures. 
Foot long. {Eryci'im, Kellya, Lasoia, Lepton, Galeomma. ) Fam. Crassatellidae : 
mantle open without siphons. Foot moderately developed. Fam. CyrenidES : mantle 
open, two siphons. Foot large. In fresh or brackish water. (Cyrena, Corhicula, 
Sphcerium, Pisidium, Oalalea.) Fam. Dreissensiidae (fluvial). Fam. Unionidje: fresh- 
water ; foot large, hatchet- or wedge-shaped, two simple siphonal apertures or clefts, 
mantle open ( Unio [Fig. 25], Painter's Mussel ; Anodmita, pond Mussel ; Mutela). 
VOL. II C 



COMPARATIVE ANATOMY 



CHAP. 



Sub-Order 2. Tellinacese. 
Dimyarian with completely separate siphons. Foot large. Gills smooth. Fam. 
TelUnid^ (Tellina). Fam. Donaoid* {Donax), Mactridse (Afactra). 

Sub-Order 3. Veneracea. 
Dimyarian with somewhat folded branchial lamellfe. Siphons separate, and foot 
large. Fam. Veneridffi [reinis, Merrtrix [Cijth:iva\ Tapes). Fam. Petricolidae: 
boring muscles. 

Sub-Order 4. Cardiacea. 
Dimyarian or Monomyarian. Branchial lamellse much folded. Mantle closed 
except at the two siphonal and one pedal apertures. Fam. Cardiidse : Dimyarian. 

710, 




Fig. 26.— Anatomy of Gardium tuberculatum, left aspect (after Grobben, Leuckart, and 
Nitsclie, Zool. JVandtofelii)- v, Foot ; go, gonad ; S, shell ; Pa, mantle ; os, labial palps ; o, mouth; 
Ml, anterior adductor muscle ; (s, oesophagus ; m, stomach ; I, digestive gland ; d, intestinal 
canal ; g02, genital aperture ; noi, pericardial aperture of the kidney ; V, ventricle ; At, auricle ; 2^, 
pericardium ; no, aperture of the kidney in the mantle cavity ; n, kidney ; Jfg, posterior adductor ; 
Bl, point of concrescence of the right and left ctenidia beliind the foot ; a, anus ; Ak, anal chamber 
of the mantle cavityj with anal siphon As ; Bk, branchial chamber of the same cavity "with 
branchial siphon Bs ; Br, ctenidium. 

(Ccw'rfmm [Fig. 26].) Fam. Chamidse : Dimyarian. Valves of shell unequal. (Chama, 
Diceras, Requienia.) To these the fossil forms Mmiopleuridce, Caprinidce, Sip- 
puritidce,''EadioUtidtc. Fam. Tridacnidse : Monomyarian. [Tridacna, Hippopus.) 

Sub-Order 5. Myacea. 
Dimyarian with folded branchial lamellse. Tendency towards concrescence of the 
edges of the mantle folds. Siphons very long and foot large. Fam. Psammobiidse : 
pedal cleft of the mantle still very large {Psamonobia). Fam. Hesodesmatidse, 



VII 



MOLLUSCA— SYSTEMATIC REVIEW 



Lutrariidse, Myidse {Mya, Corhula). Fam. Glycymeridse (Glyq/mrris, Saxicava 
[boring mussels]). Solenidse : shell with anterior and posterior cleft ; foot very 
large {Solenocurtus, CuUeUus, Ensis, Solen). 

Sub-Order 6. Pholadacea. 
Dimyarian with closed mantle and well-developed siphons. Foot varies, and is 

M^ jifo .flit jf 




Fig. 2Y.— Anatomy of Plioladidea, left aspect (after Egger). Lettering as before. In addition, 
N]xi, Npp, anterior and posterior nerves of the mantle edge ; mo, anterior aperture of mantle ; Xis, 
sac of the crystalline stylet ; Kr, branchial vein ; ol, anterior upper mantle lobe ; Rpp, posterior 
retractor of the foot ; Ss, partition between the two siphons ; Ms, accessory adductor ; mh, intestinal 
csecum ; x, pericardial section of the kidney, which opens into the pericardium by means of the 
renal funnel at u. 

Rpp ^^ T M = 

M- 




Fio. 28.— Anatomy of Jouannetia Cumingli, left aspect (after Egger). Lettering as in 

last figure. 

sometimes rudimentary. Shell open, often having accessory pieces added to it. 
Fam. Fholadidse: boring mussels {Pholas, Plwlcuiidea [Fig. 27], Jouannetia [Fig. 28], 



20 



COMPARATIVE ANATOMY 



CHAP. 




Fig. 29.— Teredo Navalis in its boring, 
ventral aspect (after Meyer and MolJius). 
The centre is omitted, the calcareous tube 
is for the most part uninjured. 





J- 



Pio. 30.— Shell of AsperglUirm (Ere- 
chites) vaginlferum, dorsal view, a. An- 
terior; p, posterior; d, right; s, left; 1, 
siphonal aperture of the pseudoconch ; 2, 
pseudoconch (calcareous tube) ; 3, true shell 
embedded in the pseudoconch ; 4, anterior 
aperture of the pseudoconch. 



VII 



MOLL use A —a YSTEMA TIG RE VIE W 



21 



Xijlophaga). Fam. Teredinidse : boring mussels ( Teredo [Fig. 29]). Fam. Clava- 
gellidse (Clavngdla , Brechites [Aspergillmn, Fig. 30]). 

Sub-Order 7. Anatinacea. 

Mantle to a great extent closed. With siphons and foot. Hermaphrodite. 
Fam. PandoridsB, Lyonsiidse, Anatinidse (Anatiiia, Thmcia). 

Order 5. Septibranchia. 
The ctenidium on each side is transformed into a muscular septum pierced by 




Fig. 31. — Soft body of Silenia Sarsii (Cuspidaria), after Pelseneer. A, Left aspect after 
removal of the mantle ; B, ventral aspect after removal of most of the mantle ; a, p, anterior and 
posterior ; c?, v, dorsal and ventral ; d, s, right and left ; 1, anterior adductor ; 2, mouth ; 3, anterior 
group of branchial slits; 4, hepatic mass ; 5, branchial septum ; 6, posterior group of branchial slits ; 
7, posterior adductor ; 8, anal siphon ; 9, siphonal tentacles ; 10, valve of the branchial or inhalent 
aperture ; 11, point where the free mantle edges limiting the pedal aperture fuse ; 12, median group 
of branchial slits ; 13, free edges of mantle ; 14, foot ; 15, posterior labial palps ; 16, anterior labial 
palp. 

slits, which divides the mantle cavity into two chambers, one lying above the other. 
Hermaphrodite. Fam. Poromyidse, Cuspidaridaa (Fig. 31 A and B). 



CLASS \'.— Cephalopoda (Cuttlefish). 

Body symmetrical with high visceral dome. The mouth is surrounded by 
tentacles or prehensile arms, which may be considered as portions of the foot 
developed round the mouth. Another portion of the foot forms the siphon. In 



22 



GOMPAEATIVE ANATOMY 



CHAP. 



the posterior mantle cavity there are two or four ctenidia. The heart has two 
or four auricles, and there are two or four kidneys. Gonad unpaired, with single 
or paired ducts. The sensory organs are highly developed, especially the eyes, 
which lie anteriorly and laterally on the "head" (Kopffuss). The jaws and 
radula are powerful. There is sometimes a shell, either external or internal. An 
ink-bag is generally present. The Cephalopoda are large, highly-developed marine 
carnivora. Dioecious. 

Okder 1. Tetrabranchia. 
An external chambered shell, the animal inhabiting the last (and largest) 
chamber. It is symmetrical, and exogastrieally coiled. The mouth is surrounded 
by numerous tentacles without suckers, Avhich rise from large lobes and can be 




Fig. 32.— Nautilus Pompilius, after Owen. Jledian section of shell, a, Cephalic hood; 
6, tentacles; c, infundibulum (siphon); d, eye ; c, projection caused by nidaniental gland; /, point 
of attachment of the adductor muscle ; g, upper portion of the visceral dome ; Ji, last (inhabited) 
chamber of the shell ; i, anterior lobe of the mantle ; /:, last chamber but one ; I, siphuncle. 

retracted into special sheaths. There are four ctenidia, four auricles, and four 
kidneys. The siphon consists of two lateral lobes distinct from one another, which 
by the apposition of their free edges form a tube. Without ink-bag. The eyes are 
simple pits. The only living form is the Nautilus, radula 2.2.1.2.2 (Fig. 32). 
The two large divisions of this order, Nautiloidea and Ammonitidea,^ occur as 
fossils. 



^ The Ammonitidea, owing to the uncertainty concerning their anatomy, are by 
many authorities arranged in a separate order, "Ammouea," and placed between the 
other two. 



YII 



MOLLUSGA— SYSTEMATIC REVIEW 



83 



Fig. 33.— Spirilla prototypos, right aspect (after Cbun and Owen), 
from Leuckart and Nitsclie, Zool. Wandtafuln. Both portions of the 
shell are visiblej the inner portion seen through the mantle. The eye 
should he placed more anteriorly on the " head" (Kopflfuss). 




^ 



B 




Fio, 34.— LoligO vulgaris (after D'Orbigny). A, Dorsal (physiologically ventral) view; B, 
anterior (physiologically dorsal) view. Of the five pairs of anns, the fourth are seen to be 
developed as long prehensile tentacles ; the eyes, the edge of the mantle, the fins, and the 
chromatophores in the skin are depicted. 



24 



GOMPA RATIVE ANA TOM Y 



CHAP. 



Order 2. Dibranchia. 

The shell is either internal, rudimentary, or altogether wanting. When 
present it is endogastrically coiled. There are two ctenidia, two auricles, and two 
kidneys. The mouth is surrounded by eight or ten sucker-bearing prehensde arms. 
The free edges of the two lobes which form the siphon have grown together. The 
eyes are vesicular. An ink-bag is present. 



Sub-Order 1. Decapoda. 
Shell internal and often rudimentary. There are ten arms, the fourth pair being 
developed into long prehensile tentacles, which can be withdrawn into special cephahc 
cavities. The Decapoda are good swimmers ; their bodies are elongated dorso- 
ventrally, and provided with lateral fins. The oviduct is unpaired. Fam. Spirulid^ : 
internal shell spirally (endogastrically) coiled. Spirilla (Fig. 33). Fam. Belem- 
nitidse : fossil forms with internal chambered shell, usually long and straight 
{Belemnitcs, SpiruUrosIra, Belemnoteuthh). Fam. Oigopsidse (Ommastn-plics, radula 
3.1.3, LoUgoixsis, Oranchia, OMroteiithi.t, Oimiia, Thysanotruthis, Onydiotenthis, 
Oiiimnfusfn-phrs). Fam. Myopsidae {Rossia, Sepiola, Scptadarinm, Idioscpioa, Loligo 
[Fig. 34], Sqnutnithis, Belosepia [fossil], Scpin. radula 3.1.3). 

Sub-Order 'J. Ootopoda. 

"Without shell or "guard" (rostrum) ; eight arms; without specialised prehensile 
tentacles. Body thick, generally without fins, and little adapted for swimming. 




Fio. 35. — Female Argonauta, in the swimming position, riglit aspect (after Lacaze-DutMers). 
1, Uncovered pai-t of the shell ; 2, the right arm of the first (anterior) pair, with its lobe-lilte expan- 
sion (sail) 3, covering a large part of the shell ; 4, fourth arm ; 5, third arm ; 6, siphon ; 7, eye ; 
8, jaw; 9, second arm. Tlio second, third, and fourth arms are stretched backwards inside the 
shell. 

Oviducts paired. Fam. Cirrhoteuthidse : with fins. Fam. Philonexidae : Arijonmifn 
(Figs. 35, 36, and 200, p. 243). Female with external irnchambered shell. Philnncxis, 
Trcmoctopus. Fam. OctopodidsB {Octopus, radula 1.3.1. [Fig. 37], EJrdiNX-). 



Vlt 



MOLL USOA —SYSTEMA TIC RE VIE W 



25 




I'lG. 36.— Female of Argonauta Argo (after V^rany). Seconil, third, and fourth pairs of arms 
stretched dowinvards. re, Siphon; 6, eye; c, first pair of arms, covering with its sail d nearly the 
whole shell e. 




Fig. 37.— Octopus vulgaris, after Merouliano (in "Aquarium Neapolitanum"). Above, in 
swimming position ; below, quiescent, watching for prey. 



26 



COMPARATIVE ANATOMY 



CHAP. 




Fif!. 38. — Hypothetical Primitive 
Mollusc, diagrammatic, left aspect, o, 
Mouth ; fc, head ; sm, shell muscle ; oso, 
upper aperture of the shell ; u., anus ; ;(, 
renal aperture; mli, mantle cavity; ct^ 
ctenidium ; /, foot. 



I. Organisation of the Primitive Mollusc. 

The hypothetical primitive Mollusc, reconstructed from the results 
of morphological research, may be described as follows : — 

The body is bilaterally symmetrical and dorsally arched; its 
anterior end carries the mouth, eyes, and 
tentacles, forming a distinct head. The 
ventral side foi'ms a powerful muscular 
foot, distinct from the rest of the body, 
with a flat sole for creeping. 

The soft integument of the arched 
dorsal side forms a fold, which hangs 
down all round the body, and is called 
the mantle or pallium. The mantle 
encloses a circular cavity, the mantle- 
or paUial cavity, which surrounds the 
bodjr, and communicates freely with the 
surrounding medium between the free 
edge of the mantle and the foot. The dorsal integument of the body 
and of the mantle secretes a closely-applied shell, which consists of a 
chitinous matri.x; (conchyolin) in- 
ter-stratified with deposits of car- 
bonate of lime. This shell repeats 
the form of the dorsal surface, and 
is thus bilaterally symmetrical and 
arched. Such a shell detached and 
turned over would resemble a cup 
or plate. Since the dorsal shell 
covers the whole, or at any rate 
the greater part of the body, it 
forms a protection for it and at 
the same time plays the part of a. 
skeleton, to which the muscles run- 
ning more or less dorso-ventrally 
into the foot and head, can be 
firmly attached. 

The mantle is of special im- 
portance as a protective structure. 
Apart from the fact that its edge 
secretes the greater part of the shell 
substance, and in this way adds to 
the shell as the animal grows, it 
covers the delicate gills, which 
thus also share the protection 
afforded by the shell. Analogous 
arrangements are to be found in other divisions of the animal kingdom, 




Fig. 39.— Hypothetical Primitive MoUusc, 

from above, o. Mouth ; uU, ulpl, ulp, primitive 
left cerebral pleural and pedal ganglia ; idpa, 
urpa, primitive left and right parietal ganglia ; 
hJu, primitive left auricle ; uos, uros, primitive 
left and right osphradia (Spengel's organ) ; ukt, 
urcty primitive left and right ctenidia (gills) ; mb, 
base of the mantle ; mr, edge of the mantle ; m, 
mantle cavity ; i', visceral ganglion ; re, ventricle ; 
t(, anus. 



VII THE HYPOTHETICAL PRIMITIVE MOLLUSC 27 

e.g. the dorsal fold or carapace which, in the higher Crustacea, covers 
the branchial cavity, and the operculum of Fishes. The relations 
existing between the branchiae, the mantle, and the shell in the 
Mollusca are of the highest importance ; these organs should always 
be regarded as essentially interdependent structures. 

The branchiae lying in the mantle cavity are paired and symme- 
trical. It may be left an open question whether the primitive Mollusc 
possessed more than one pair of gills. If only one, we must suppose 
that one gill lay on each side of the mantle cavity posteriorly ; if more 
than one, that there was a row of branchias on each side. 

Each gill is feather-like, with a shaft and two rows of very 
numerous leaflets. The shaft stands out freely from the body in the 
mantle cavity. Close to the base of each gill, a sensory organ, con- 
sidered to be olfactory, and called the osphradium, is found. Such 
a gill with an osphradium at its base has a very definite morphological 
value; in order to distinguish it from analogous though not homologous 
respiratory organs found in certain Mollusca, it has been named a 
etenidium. 

The head is provided with one pair of tentacles and one pair of 
eyes. The mouth lies anteriorly and ventrally. The remaining ojDen- 
ings of the inner organs lie posteriorly above the foot ; the anus in the 
middle line, and on each side, between it and the etenidium (supposing 
that there is only one pair of ctenidia), an aperture for the sexual 
organs, and another for the kidney (nephridium). . These five apertures 
are covered by the mantle, and thus lie in the mantle cavity. We 
have thus, to recapitulate, in the posterior part of the mantle cavity 
two ctenidia, two osphradia, and five apertures, the median anus, and 
the paired symmetrical sexual and renal apertures. These, taken 
together, form what is known as the pallial complex. 

The inner organisation may thus be briefly described. 

The intestinal canal. The mouth leads to a muscular pharynx, with 
horny jaws. At its base lies a chitinous rasp-like ribbon called the 
tongue or radula, which carries numerous consecutive transverse rows 
of sharp chitinous teeth. Paired salivapy glands enter the pharynx, 
which passes into an oesophagus, which latter leads into the mid- 
gut. This, which we will suppose to be more or less coiled, runs 
right through the body, passing posteriorly into a very short hind- 
gut, which opens outward through the median anus. The mid-gut 
has large paired glandular diverticula (mesenteric gland, diges- 
tive gland, hepatopancreas, liver). 

Musculature. — The muscles of the foot are powerful, and are 
adapted for the creeping movement. There are, in addition, muscles 
running from the inner surface of the shell into the foot and head 
(eolumellar or shell muscles), and special muscles for the different 
organs. 

Nervous system. — Two well - developed cerebral ganglia lie 
dorsally in the head, and are connected by means of a short cerebral 



28 COMPARATIVE ANATOMY chap. 

commissure, which runs over the oesophagus. Each cerebral ganghon 
gives rise to two powerful nerve trunks which are provided along 
their whole length with ganglion cells ; there are thus two pairs of 
nerve trunks running right through the body longitudinally. One pair, 
the pedal cords, run right and left in the foot ; the other pair, the 
visceral cords, which lie more dorsally and are more deeply embedded 
in the body, run through the body cavitj-. The two visceral nerves 
are connected posteriorly. 

If we leave the Amphineura and Biotocardiu. out of the question, 
the following modified sketch of the Molluscan nervous system holds 
good. Two cerebral ganglia, two pedal ganglia, two pleural 
ganglia lying at the sides of the pharynx, two visceral ganglia 
lying posteriorly in the body cavity. Giving the name connectives 
to such nerves as unite the ganglia of one side of the body, i.e. dis- 
similar tjandia, and that of commissures to the nerves that unite the 
similar ganglia of the two sides of the body, we have the following 
system: Commissures are found — (1) between the two cerebral 
gangha (over the fore-gut) ; (2) between the two pedal gangha 
(under the fore-gut) ; (3) Isetween the two visceral ganglia (under 
the hind-gut). The connectives on each side are: (1) the cerebro- 
pedal connective ; (2) the cerebropleural connective ; (3) the pleuro- 
pedal connective ; (4) the pleurovisceral connective. 

There is a secondary coelom or body cavity lined with endo- 
thelium, which has at least two divisions. In the anterior division, 
the genital chamber, the sexual products arise from the endothelium ; 
this chamber is connected by means of two canals, the genital ducts, 
with the mantle cavitj'. In the p)osterior chamber, or pericardium, 
lies at least one organ, the heart ; this chamber is connected with the 
mantle cavity by means of two nephridial duets or vesicles. 

The circulatory system is partly vascular and partly lacunar. 
The arterial heart lies in the pericardium above the hind-gut. It 
consists of one ventricle and two lateral auricles. 



II. Review of the Outer Organisation characterising the Chief 
Groups of the Mollusea. 

Having given above a general plan of the morphology of the Mollusea, let us 
now see how far the various gi-oups of Molluscs agree with this description in their 
outer organisation. We shall at first only mention in connection with each group 
those special features which are now considered to be typical or characteristic of 
that group. In other words, we shall again give a general scheme of the outer 
organisation of each class of the Mollusea, in order that these more specialised 
schemes may be compared with that of the hypothetical primitive Mollusc above 
described. 

Later sections will deal with the changes which tlie separate organs undergo, 
not only in the different classes, but within one and the same class, so far, that is, 
as these modifications bear on external morphology. 



VII MOLLUSGA— OUTER ORGANISATION 29 

A. Plaeophora or Polyplaeophora (Chitonidse). 

The body of the Plaeophora is bilaterally symmetrical, and dorso- 
ventrally flattened ; viewed from the dorsal or ventral surface its 
shape is that of a long oval. On the ventral side there is a large 
muscular I'oot vi^ith a flat sole, the outline of which runs very nearly 
parallel with that of the body. In front of the foot, and also on the 
ventral side, there is a distinct snout which carries the mouth in the 
middle of its ventral surface. There are no eyes or tentacles on the 
head. Between the mantle, which forms the outer edge of the body, 
and the body and head it covers, there is a deep groove, in the base of 
which lie numerous lancet-shaped gills, arranged in a single row on each 
side. These two rows of gills sometimes approach each other so nearly 
both anteriorly and posteriorly that there is an almost complete circle 
of gills around the foot, or else they are more or less shortened, and 
are in some forms so reduced as only to occupy the posterior third 
of the branchial furrow. The anus lies posteriorly in the median 
line, ventrally, immediately behind the foot. The two apertures 
of the nephridial ducts lie in the branchial furrow on each side, and 
slightly in front of the anus. The two genital apertures lie imme- 
diately in front of the nephridial apertures, also in the branchial 
furrow. 

The median dorsal region is covered by eight consecutive imbri- 
cating calcareous plates. The peripheral dorsal region, between the 
edge of the body and these shell plates, carries calcareous spicules, 
granules, etc. The corresponding peripheral region on the ventral 
side forms one of the boundaries of the branchial groove, and may be 
considered as the mantle. 

B. Aplaeophora, Solenogastres. 

The body is here bilaterally symmetrical and vermiform ; in section 
it is round, and is sometimes long and thin, at others short and thick. 
The large oral aperture lies in the form of a longitudinal slit on the 
ventral surface of the anterior end of the body. The cloacal aperture 
— or common opening for the intestinal canal and the urogenital 
organs — lies ventrally at the posterior end of the body. A narrow 
median ventral groove runs forward from the cloacal aperture 
and terminates anteriorly near the mouth. In the base of this pedal 
groove rises a ciliated ridge or fold which runs along its whole 
length; this ridge, in cross section, is triangular, and represents 
the reduced foot. In the Chcetoderma both foot and pedal groove 
are wanting. The Solenogastres have no distinct compact shell ; 
its place is taken by calcareous spicules embedded in the integu- 
ment. 



30 



COMPARATIVE ANATOMY 



CHAP. 



C. Gastropoda (Cephalophora). 

Although there can be no doubt as to the relationship to one 
another of the Mollusoa grouped together in this class, it is almost 
impossible to give a general scheme of the outer form of the whole 
class. The greatest variation occurs, the body being sometimes out- 
wardly bilaterally symmetrical, sometimes in a high degree asym- 
metrical. Further, forms such as FissureUu, Olim, Turritella, Cleodora, 
Fterotrachea, Phyllirhoe, Linuix, Pleurohranchus, Tlietys, differ so greatly 
in outward appearance that, at the first glance, it is almost impossible 
to believe that they are related. A shell may be present, and may show 
the most marvellous variation in form ; or it may be rudimentary or 
even (in adult forms) altogether wanting. The foot also may assume 
the most varied forms, or may be entirely wanting. The same may 
be said of the mantle fold, the gills, etc. 

Setting aside those forms which are quite one-sidedly differenti- 
ated, it may be said in general — (1) that, in the Gastropods, the 
protective shell consists of one piece, and follows in a remarkable way 
the forms assumed by the body; (2) that the dorsal portion of the 
body, which contains the viscera, becomes constricted almost hernia- 
like from the head and foot, making a sac-like protuberance (visceral 
dome) ; (3) that, for the diminution of its surface, this dome or humj) 
becomes coiled spirally, the shell repeating its shape ; (4) that the head 
and foot, which project through the aperture of this shell for purposes 

of locomotion, can be withdrawn 
into it. The large, long foot 
generally has a flat sole for creep- 
ing. The head is distinct, and 
provided with tentacles and eyes. 
At some part of the body, the in- 
tegument of the visceral dome 
forms a mantle fold which hangs 
downwards, covering and protect- 
ing the respiratory organs. The 
outer surface of this mantle takes 
part with the rest of the integu- 
ment of the visceral dome in the 
formation of the shell. The follow- 
ing are more special descriptions of 
the outer organisation of the chief 
Gastropodan groups. 

1. Pposobranehia. 




Fig. 40.— Diagram of the Organisation of a 
Zeugobranohiate Diotooardlan. o, Anius ; ve, 
ventricle ; ula, right auricle ; urct^ left ctenidium ; 
^tros, left osphradium. 



The large visceral dome is 
coiled spirally, generally to the 
right (dextrally), the shell naturally assuming the same form. The well- 



VII 



M0LLU8GA— OUTER ORGANISATION 



31 



developed foot has a flat creeping sole. On the dorsal side of the 
posterior portion, of the foot, the metapodium, there is a calcareous 
plate, the operculum, which, when the animal withdraws its head and 
foot, closes the aperture 

of the shell. The mantle 'v^ '^'^t 

fold hangs down from 
the anterior side of the 
visceral dome, and covers 
the spacious branchial or 
mantle cavity, in which 
lie certain organs of 
special morphological 
importance. These, 

which may be called the 
mantle or pallial organs, 
are, in such forms as 
may be considered primi- 
tive, (1) the anus, which 
lies, not posteriorly, but 
on the anterior side 
of the visceral dome, 
shifted forwards to- 
wards the mouth ; (2) 
the two apertures of the 
paired nephridia, one on 
each side of the anus; (3) 
the two gills, one to the 
left and one to the right ; 
(4) the two osphradia 
near thebases of thegills. 
In most Prosobranchia, 
however, the organs just 
mentioned as paired are 

unpaired; only the gill, fio. 4l.-DiagramofaProsobraiicliiateMonotocardlan.iTlie 
nephridial aperture, and outerform, shell, mantle, pallial complex, heart and pericardium, 
0=;r>hradiuni to the left nervous system and operculum, are depicted. Lettering mostly 

of the anus being re- 
tained, while the hind- 
gut with the anus moves 
to the right side of the 
mantle cavity. The single genital aperture lies on the right side, in 
the head, or on the floor of the mantle cavity. (In the Prosobranchia 
the sexes are separate.) The abortion of one of each of these originally 
paired organs, gills, nephridia, and osphradia, produces a very striking 
asymmetry of the whole body. The name Prosobranchia indicates the 
fact that the gills lie in front of the heart. 




as in Fig, 39. In addition : /, foot ; si, siphon ; sup, suh, supra- 
and sub - intestinal connectives; op, operculum; ot, auditory 
organ ; p, penis ; sr, seminal groove ; mil, mantle cavity ; hy, 
hypobranchial gland ; (5 , male genital aperture ; r, rectum ; au, 
eye ; t, tentacle. 



COMPARATIVE ANATOMY 



CHAP. 



2. Pulmonata. 

Type : Helix pomatia.— The visceral dome is well developed, and 
protrudes hernia-like from the rest of the body; it is dextrally coiled, 

and has a corresponding shell. The 
foot is large and long, and has a 
fiat creeping sole. The head has 
two pairs of feelers, one of which 
carries the eyes. The mantle fold 
hangs down from the anterior side 
of the visceral dome, and covers a 
spacious mantle cavity (respiratory 
or pulmonary cavity). The free 
edge of the mantle fold unites with 
the integument of the neck near it, 
only leaving an aperture to the 
right, the respiratory aperture. 
This aperture serves for the inhala- 
tion and exhalation of the air. 
The anus and the unpaired nephri- 
dial aperture lie close to the re- 
spiratory aperture, and are thus on 
the right side. There are no gills 
in the mantle cavity, which con- 
tains air. Respiration takes place 
at the inner surface of the mantle 
fold, in which runs a fine network 
of vessels lying in front of the 
heart. The foot, unlike that of 
the Prosobrcmchia, has no operculum. There is a common genital 
aperture on the neck, to the right, in front of the respiratory cavity 
(the Pulmonata being hermaphrodite). Many Pulmonata, however, 
differ greatly in their outer organisation from the Helix type. 




Fig. 43.— Diagram of a Basommatoplioran 
Pulmonate. a?, Respiratory aperture; rgji, vas- 
cular networlc on the inner surface of the mantle. 
The Icidney is incorrectly (lra\\Ti. Fm-ther letter- 
ing as in Figs. 39 and 41. 



3. Opisthobranehia. 

The respiratory organs lie behind the heart. 

(a) Teetibranehia. — The visceral dome is usually not large. It 
may be either spirally coiled or symmetrical, and is covered by a 
variously shaped shell. The foot is large, and usually has a flat 
sole for creeping. The head is variously shaped, and often carries 
tentacles or rhinophores, and unstalked eyes. The small mantle 
fold hangs down from the right side of the visceral dome, and 
often does not quite cover the single gill lying beneath it. The 
anus lies behind the gill, more or less removed from it. The Teeti- 
branehia are, like all Opisthobranehia, hermaphrodite ; the genital 



VII 



MOLLUSGA— OUTER ORGANISATION 



33 



and nephridial apertures lie on the right side of the body in front of 
the anus. 

(b) Nudlbpanehia. — The body is outwardly symmetrical, the 
visceral dome does not protrude from it, but is closely applied to 
the whole length of the foot, from which it is often not distinctly 




Fig. 43. — Diagram of a TeotibrancMate Opistlio- 
■branoMate. Lettering as before. In addition ; <jg, genital 
ganglion ; s, shell ; J , female genital aperture ; Ipp, rpp, 
left and riglit parapodial lobes, that on the right laid back. 



Fig. 44. — Dentalium, diagram- 
matic, leftaspect. g, iSexual glands ; 
kt, cephalic tentacles ; other letter- 
ing as before. 



differentiated. The foot has a flat creeping sole. There is no distinct 
mantle fold, no gill corresponding with that of the Tectibranchia, 
and no shell. The head carries tentacles or rhinophores, and sessile 
eyes. The anus lies either dorsally in the median line, or laterally to 
the rio-ht. The genital and renal apertures lie to the right in front 
of the anus. The gills, which vary much in form, number, and 
arrangement, are found dorsally or laterally, and are not homologous 
with the typical Molluscan ctenidia. 



VOL. 11 



34 



COMPARATIVE ANATOMY 



CHAP. 



D. Seaphopoda. 

The body is symmetrical and long, i.e. the visceral sac is elongated 
dorso-ventrally, and is completely enveloped in a tubular mantle. The 
mantle cavity lies posteriorly, and is prolonged ventrally far enough to 
allow the snout and retracted foot to be completely concealed in it. 
Besides the large ventral aperture, there is a smaller dorsal aperture 
further placing the mantle cavity in communication with the exterior. 
The shell, like the mantle, is tubular, or like a tapering cone, slightly 
curved anteriorly. It has two apertures corresponding with those in 
the mantle. The head is developed into a barrel-shaped snout, and 
has no eyes. The mouth, which lies at its ventral end, is surrounded 
by a circle of leaf-like tentacles. At the base of the snout there arise 
two tassels of long filamentous contractile tentacles, which hang down 
into the mantle cavity and can be projected far beyond the ventral aper- 
ture. Behind the snout, the cylindrical muscular foot rises from the 
body, and can be protruded downwards. There are no gills. The 

median anus lies posteriorly above 
the foot. The two nephridial 
apertures are at the sides of the 
anus. There are no special genital 
apertures (Figs. 44 and 101, 
p. 113). 



E. Lamellibranehia. 

The body is bilaterally sym- 
metrical ; somewhat elongated 
(from before backward). The 
integument forms leaf-like mantle 
folds to the right and to the left, 
which at their bases are attached 
to the trunk along its whole 
length, and grow down ventrally. 
If the body of a Lamellibranch, 
from which the shell has been 
removed (the foot being re- 
tracted), be viewed from the side, 
the outline will be found to be 
formed, dorsally, by the dorsal 
median line of the body ; an- 
teriorly, posteriorly, and ventrally 
by the free edge of the mantle 
fold. The two mantle folds en- 




FiG. 45. — Transverse section of Anodonta 
cygnsea {ordinary fresh water mussel) (after Howes). 
Ig, Ligament ; (i/, typhlosolis ; ki, pericardial gland 
(Keber's organ) ; re, kidney (glandular portion) ; 
sic, chambers at the bases of the gills ; gd, genital 
ducts ; hrli, brl^, outer and inner branchial lamellic ; 
ibc, mantle cavity ; s, shell ; sj, edge of the shell ; 
/l. foot ; 3>iii, pallial muscle ; i, intestine ; plj, right 
mantle fold ; ggl, gonad ; r, rectum ; cp, cerebro- 
pedal connective ; rei, non-glandular vestibule of 
kidney ; rej, renal aperture ; jjc, pericardium. 



close a space whose transverse 
axis ^ IS always markedly shorter than either its dorso-ventral or its 
longitudinal axis, i.e. the animal with its mantle is laterally compressed. 



VII 



MOLL USCA— OUTER 0BGANI8A TION 



35 



Projecting into the mantle cavity, there is a large muscular process of 
the body, the foot, vrhich is directed downward and somewhat forward, 
and can be protruded between the free edges of the mantle. This foot 
is also laterally compressed. Tn certain cases which, though excep- 
tional, deserve special mention, its free end is flattened, and it thus has 
a flat sole. The outer surface of the trunk and mantle folds secretes 
a bivalve shell which covers the whole body. One valve lies to the 
right, the other to the left of the median plane, and the two are exactly 
alike. Each valve repeats the outline of its own side of the trunk 
with its mantle fold. 

The two valves articulate dorsally, and are open anteriorly, ventrally, 
and posteriorly. Two strong muscles (adductors) run transversely 




Fig. 46.— Anatomy of Dnio (Margaritana) margaritifenis, left side (after Leuckart and Nitsohe). 
0, Mouth ; Cg, cerebral ganglion ; Mi, anterior adductor muscle ; r,\ oesophagus ; Z, digestive gland 
(liver) ; no, nephridial aperture ; lo, apertures of the digestive gland in the stomach m ; Aa, anterior 
aorta ; n, nephridiunij the outline given in dotted lines ; V, heart ; r, hind-gut ; Ap, posterior 
aorta ; M^, posterior adductor ; a, anus ; Vij, visceral ganglion ; Br, gill ; Ek, mantle cavity ; go, 
gonads with genital duct go\ ; Pg, pedal ganglion ; p, foot. The arrows indicate the direction of 
the inhalent and exhalent streams of water. 

from one valve to the other. Their contraction serves to shut the 
shell completely. One of these muscles lies anteriorly, the other 
posteriorly. Their points of attachment produce impressions on the 
inner surface of the shell, which are always distinctly visible when 
the shell is removed. 

The mouth lies below the anterior adductor, between it and the 
anterior base of the foot. The anus lies behind the posterior adductor. 
There is no distinct head. Near each side of the mouth, the body 
carries two leaf-like processes, the oral lobes or labial palps. At the 
line of insertion of the foot in the mantle cavity, a longitudinal 
ridge rises on each side in the middle and posterior regions of the 
body ; this carries two rows of long branchial leaflets. There is thus. 



36 



COMPARATIVE ANATOMY 



CHAP. 



on each side of the mantle cavity, one plumose gill, the shaft of which 
is attached lengthwise to the body (Figs. 45, 46, etc.). 

In various divisions of the LamelHbranchia, the outer organisation 
deviates very greatly from the above. 



— twi 



F. Cephalopoda. 

The body is bilaterally symmetrical. The visceral dome is large 
and often much elongated dorso-ventrally. The head is more or less 

distinct, and is surrounded by the foot, 
cL which is transformed in a peculiar man- 

ner. The foot has, in fact, grown round 
the head, and has developed numerous 
differently -shaped processes (arms and 
tentacles) arranged in a circle round the 
mouth; these serve principally for seizing 
and holding prey. In viewing the body 
of a Cephalopod, it must be remembered 
that the apex of the visceral dome (which 
a casual observer might take to be the 
posterior end of the body) is really the 
highest dorsal point, while the head and 
its arms lie lowest. We may thus dis- 
tinguish, both in the visceral dome and 
in the transformed foot which has been 
combined with the head, and drawn out 
into tentacles, an anterior and a posterior 
part (which to a casual observer would 
seem upper and lower parts), and a right 
and a left side. This at first sight seems 
a paradox to those not acquainted with 
the comparative anatomy of the Mollusca, 
since the normal position in the water of 
certain well-known Cephalopods does not 
agree with it. A Sepia, for example, 
swims or lies at rest in such a way that 
the strongly pigmented anterior side of 
the visceral dome and of the "head" 
(Kopffuss) is uppermost, and the posterior 
side lowermost. The accompanying dia- 
gram illustrates the strict morphological 

position of the body, which alone concerns the comparative anatomist 

(Fig. 47). 

On the right and left of the "head" there is a highly-developed 

eye, and near it an olfactory pit. 

The mantle fold hangs down posteriorly from the visceral dome, 

covering a spacious mantle- or respiratory cavity, which communicates 




Fig. 47.— Diagram of Sepia, median 

section from the left side, v, Ventral 
(physiologically anterior) ; d, dorsal 
(physiologically posterior) ; an, anterior 
(physiologically upper) ; po, posterior 
(physiologically lower) ; 1, 2, 3, 4, 5, the 
live arms of the left side ; au, eye ; 
internal shell; go, gonad; rf, pigment gland 
= ink-bag; m, stomach; n, kidney; ct, 
ctenidium ; a, anus ; mJi, mantle cavity ; 
In, siphon. The arrows indicate the 
direction of the respiratory current. 



VII 



MOLLUSGA— OUTER ORGANISATION 



37 



with the exterior at the free edge of the mantle fold, above the 
" head." Within the mantle cavity there are two or four gills, 
arranged symmetrically, the median anus, and the apertures of the 
sexual and excretory organs. Two symmetrical lobes are found on 
the posterior lower side of the visceral dome ; the edges of these 
are apposed in such a way as to form a tube, the funnel or siphon, 
one aperture of which lies in the mantle cavity, while the other 
protrudes from the mantle cleft. The respiratory water enters the 
mantle cavity through the mantle cleft, and escapes through the 
siphon. The faecal masses, waste and sexual products, and the 
secretion of the ink-bag also leave the body through the siphon. 
Originally, no doubt, all Cephalopoda possessed a shell which covered 
the whole visceral dome as well as the mantle fold. In recent 
Cephalopods the shell is rarely developed in this way ; it is often 
rudimentary, and may, indeed, be altogether wanting. Recent 
Cephalopods fall into two entirely distinct divisions, the Tetra- 
branchia and the Dibranchia. 



into consecutive chambers. 



The Tetrabranehia (Nautilus, Fig. 48) 

These have a shell coiled anteriorly (exogastrically) in the plane 
of symmetry, and divided by septa 
The animal occupies 

the last and largest " 

chamber ; the others 
contain gas.'- The septa 
separating the consec- 
utive chambers are 
pierced in the middle 
to allow of the passage 
of a siphunele, which 
runs through all the 
compartments, and is 
attached to the visceral 
dome. That portion of 
the foot which sur- 
rounds the mouth is 
produced into numerous 
tentacles, which can be 
retracted into special 
sheaths. 

The anterior portion of the foot, which lies in front of and over 
the head, is widened out into a concave lobe, the hood ; this is applied 
to the outer surface of the occupied chamber of the shell anteriorly, 
and, when the tentacles are withdrawn, can close its aperture. The 
hood carries two tentacles, and on each side of the head there is an eye. 

1 Or water : v. Ford'.s Introduction to Brit. Mus. Cat., Fossil Cephalopoda, 1889. 




Fig. 48. — Diagram of Nautilus, left view, ve, Ventral ; do, 
dorsal ; t'o, anterior ; hi, posterior ; /, foot (tentacles and siphon) ; 
sm, sliell muscle ; c(, ctenidia ; mh, mantle cavity ; a, anus ; s, 
shell ; si, siphunele ; au, eye ; o, mouth. 



38 COMPARATIVE ANATOMY chap. 

Above the head, the mantle fold encircles the whole body. It is short 
at the sides, but anteriorly it forms a large lobe which is folded back 
over the shell in the way shown in Fig. 32, p. 22. Posteriorly, the 
mantle covers a very deep cavity which contains the whole posterior side 
of the visceral dome. The siphon consists of two entirely distinct 
lateral lobes (epipodial lobes), whose free edges overlap in such a 
manner as to form a tube, open above and below. As we shall see 
later, this siphon is a part of the foot. Deep down in the mantle 
cavity, two pairs of pinnate gills — a lower and an upper pair — spring 
from the visceral dome. Nine apertures of inner organs are also 
found in this cavity ; a single median anal aperture, and four paired 
apertures, viz. one pair of genital, two pairs of nephridial, and one 
pair of viscero - pericardial apertures. The position of these is 
depicted in Figs. 78 and 79, p. 82. 

The Dibranehia. 

With one exception, viz. the female AiyonniitK, which has an 
external unchambered shell, the Dibranehia either have an internal 
shell lying on the anterior side of the visceral dome, covered by an 
integumental fold, or no shell at all. The visceral dome is sometimes 
compact and pouch-like (in reptant animals. Fig. 37), sometimes, in 
the good swimmers, much elongated dorso-ventrally, produced dorsally 
to a point, and flattened antero-posteriorly (Fig. 34). In the latter 
case, the body is further generally encircled by a fin-like integumental 
fold, which marks the limit between the anterior and posterior sides of 
the visceral dome. 

The " head " is usually distinct from the visceral dome, and carries 
to the right and left the well -developed ej'es. The mouth is sur- 
rounded by eight or ten arms for seizing prey ; these are provided 
with suckers on their lower adoral sides. 

The mantle fold covers nearly the whole posterior surface of the 
visceral dome, and thus encloses a very deep and spacious cavity. 
Laterally and anteriorly to the visceral dome, the mantle fold is 
continued as a narrow border which, immediately above the "head," 
covers a shallow groove or furrow. 

The two lateral lobes which form the siphon of the Tetrabranchia 
have in the Dibranehia grown together at their free edges, and form 
a tube open at each end. There are only two gills in the mantle 
cavity, one right, and one left. Near the upper siphonal aperture in 
the mantle cavity lie the anus, and the genital and nephridial apertures 
as well as that of the ink-bag. Details as to the arrangement and 
number of these apertures will be given further on. 



VII MOLLUSGA—INTEGUMENT, MANTLE, VISCERAL DOME 39 



III. The Integument, the Mantle, and the Visceral Dome. 

The whole body is covered by a single layer of epithelium, which, 
in parts not protected by the 

shell, may be more or less ^ .• •' 

ciliated. This layer is very 
rich in glands which are 
almost exclusively unicellular ; 
some of these lie in the epi- 
thelium itself, while some 
have sunk into the subjacent 
tissue, their ducts, however, 
passing between the epithelial 
cells. 

The layer immediately 
beneath this body epithelium 
is called the corium, and con- 
sists of connective tissue and 
muscle fibres. It is, how- 
ever, not distinctly marked 
off from the tissues beneath 
it. 

The pigment is almost 
always found in the cells of 
the subepithelial connective 
tissue. 



3 ' 




Fig. 49.— Section of the integumeiit of Daudebardia 
nifa (after Plate). 1, Epithelium ; 2, 3, 9, various forms 
of unicellular glands ; 4, globular pigment cells ; 5, 7, 
unpigmented cells of tlie connective tissue ; 6, muscle 
libres ; 8, branched and anastomosing cells of the con- 
nective tissue containing pigment. 



A. Placophora. (Of. the sketch of the Outer Organisation, p. 29.) 

The Chitmi is provided dorsally with eight consecutive shell-plates (Fig. 1, p. 2), 
which overlap in such a manner that the posterior edge of each plate covers the anterior 
edge of the nest. These plates are bilaminar. The outer and upper layer which forms the 
dorsal surface is called the tegmentum, the lower hidden layer the axticulamentum. 
As a rule, the tegmentum of the anterior plate only is as large as the articulamentum 
beneath it ; in the other plates, the latter is the larger and projects beyond the 
former laterally and anteriorly. These projecting parts of the articulamentum, 
called apophyses, slide under the plate next in order anteriorly. Between these 
two layers, tissue is found, which is a continuation of the dorsal integument. 
The tegmentum is penetrated by canals of various sizes, which open at its 
surface through characteristically arranged pores. ■■ The tegmentum consists of a 
horny or chitinous substance, which may be considered as a cuticular formation, 
impregnated with calcareous salts. The articulamentum is compact and free from 
canals ; it contains little organic substance, and much calcareous salt. It alone 
answers to the shell of other Molluscs, while the tegmentum must be considered as 
a calcified cuticle covering the true shells (the articulamenta) as a continuation of 
the cuticle of the zone which encircles the eight shell-plates. This zone carries 



^ On the relation of these canals and pores to peculiar sensory organs and eyes on 
the shell of the Chiton, cf. section on Sensory Organs, p. 166. 



40 



COMPARATIVE ANATOMY 



CHAP. 



cliitiuous or calcareous spines, sete, scales, granules, etc., varying in number and 

arrangement according to the genus and species. 

Each spine, as a rule, arises as a globular vesicle within an epithelial papilla and 

above a, very large formative cell (Fig. 
50). As it grows, it is pushed upwards by 
the newly - forming cuticular layers. The 
formative cell at its base persists, but remains 
connected with the epithelial papilla only by 
a protoplasmic process which continually 
lengthens, and may surround itself with a 
nucleated sheath. In fully-developed spines, 
the remains of this cell are still found as a 
small terminal swelling (Endkolbchen). 

There are, however, spines and specially 
flat scale- or plate-like calcareous formations 
in the integument which do not arise from 
single large formative cells, but are probably 
produced by several cells in the base of an 
epithelial papilla. 

Just as we have recognised the tegmentum 
covering the articularaenta to be merely a 
special portion of the general cuticle, so we 
may further recognise in the articulamenta 
the homologues of the calcareous spines, 
scales, etc., which are developed in the integu- 
ment of the mantle. The articulamenta 
more than very large and expanded calcareous scales. 




Fii;. 50.— A, B, C, Three stages in de- 
velopment of a spine in tlie Chiton (after 
Blumrich), rliagramniatic. ^t. Spine ; hz, its 
formative cell ; e, epithelium ; c, thick 
cuticle secreted by the epithelium ; ek, 
terminal swelling (Endkolbchen) = remains 
of the formative cell. 



would thus be nothing 




Fia. 61.— Transverse section through a Chiton near the nephridial apertures, highly 
diagrammatic (after Sedgwick), somewhat modified, l, Pericardium; 2, ventricle; 3, auricle; 
4, branchial "vein"; 5, branchial groove (mantle cavity); U, gill (ctenidium) ; 7, foot; 8, pleuro- 
visceral connective; 9, branchial "artery"; 10, secondary ccelom ; 11, intestine; 12, posterior 
portion of the gonad lying below the pericardium; 13, 14, the two posterior branches of the 
nephridium, one of which (13) opens into the branchial groove (at 16), the other (14) being connected 
in a way not here depicted with the pericardium ; 15, pedal nerves. 

This view, finally, leads to the conclusion that the shell (if it may here be so 



VII MOLLUSCA— INTEGUMENT, MANTLE, VISCERAL DOME 41 



called) of the Molluscs originally consisted of isolated calcareous spicules or spines, 
which were enclosed in a thick cuticle, and projected from the same as in the 
Prcmeomenia, Ncomcnia, etc. (v. below). 

In Cnjptodiiton the shell is internal, i.e. it is entirely covered by a fold of the 
integument, which grows over it from all sides. It consists exclusively of the 
artieulamentum, since the whole dorsal integument is covered by an even cuticle, 
which therefore forms no tegmentum. 

The only part of a Chituu which can be called the mantle fold is the marginal 
zone of the body, the ventral side of which encircles the head and foot and forms 
the lateral boundary of the branchial groove or furrow. Just as the dorsal side of 
this mantle, which is called 

the zone, carries large " "^^ 

spines, setse, or scales, so 
may the under surface be 
covered with small closely- 
crowded spines. The rest 
of the integument is bare, 
being merely covered with 
a simple epithelium. 

The genus Chitonellus 
is of great importance in 
comparing the outer or- 
ganisation of the Placo- 
phora with that of the 
Solenogccstres. The body 
is not dorso-ventrally flat- 
tened, as in the Chiton, 
but nearly cylindrical ; the 
ventral surface, however, 
is flattened (Fig. 52), and 
has a median longitudinal 
groove. The foot is not 
externally visible, but can 
be discovered, much reduced, in the base of the median groove, itself possessing 
a ventral median groove representing a narrow contracted sole. The flat ventral 
surface is therefore the mantle. In the narrow cleft on each side, between mantle 
and foot, in the posterior half of the body, lie the gills, The lateral margin of 
the body in Chiton is represented in Cliitoarlliis by a mere blunted ridge, which 
is almost exclusively caused, as may be seen in transverse sections, by a great 
thickening of the cuticle. 

B. Solenogastres. 

In the Solenogastres (Aplacophora), whose outer organisation has already been 
sufficiently described (p. 29), the shell is altogether wanting, but the cuticle secreted 
by the epithelium over the whole body is usually exceedingly thick (Fig. 53). It 
contains calcareous spicules, which sometimes project above the surface. These, like 
the spines of the Polyplacojihorn , rise from cellular cups, which are connected with 
the basal epithelium of the cuticle by nucleated stalks. There can be no doubt 
that the spicules are form.ed by these cups and nourished by them during growth. 
The foot, as we have seen, is reduced to a narrow ciliated longitudinal ridge, which 
rises from the base of the medio-ventral groove. The term mantle is here inappli- 
cable, except perhaps to the integument which forms tlie lateral boundary of this 
groove. 




Fio. 52.— Transverse section of Chitonellus, diagrammatic, 
adapted from figures by Pelseneer and Blumrich. tj, Shell (artieu- 
lamentum) ; go, gonad ; i, intestine ; a6, vb, branchial arteries and 
veins ; pc, pleuro-visceral nerves ; x, latero-ventral thickening of 
tlie cuticle ; jj, foot ; ct, ctenidinm ; pn, pedal nerve ; ft, digestive 
gland (liver) ; c, secondary coelom ; ao, aorta. 



42 



COMPARATIVE ANATOMY 



CHAP. 



In Chadodsrma the foot finally atrophies, and the medio-ventral gi'oove also 

disappears. 

The'long series of undoubtedly primitive characteristics in these two groups — the 

PfacOjij/jomandSolenogastres — obliges 
us to place them, as we shall have 
repeatedly to point out, near the root 
of the Molluscan phylum. In some 
points the Solenogastres are perhaps 
more primitive than the Folyinlaco- 
■pliora, and the vermiform body, the 
slight development of the mantle, the 
foot and the gills have been thought 
to be primitive characteristics. More 
recently, however, it has been main- 
tained, as the present writer thinks, 
with justice, that these conditions are 
rather the result of secondary adapta- 
tion to a liniioolous habit of life (most 
Solenogastres inhabiting mud). The 
shell, mantle, gills, and foot are such 
essential cliaracteristics of the Mol- 
lusca that we must assume their 
existence in the racial form. 

The series Chiton, Chitonrllus, 
Nriiiacnia, Chcetodmiia does not, there- 
fore, illustrate for us the rise and 
development of typical Molluscan 

characteristics, but rather their progressive obscuration and disappearance. 




Fig. 53. — Transverse section of Proneomenia 
Sluiteri in tlie region of the mid-gut. 1, Mid-gut ; 
'2, rudimentary'foot ; 3, sepia projecting into the mid- 
gut ; 4, testicular portion of tlie gonad ; 5, ovarial 
portion of the same ; 6, thick cuticle secreteil by the 
epithelium. 



C. Gastropoda. (C'f. Sketch of Outer Organisation, pp. 30-33.) 

Integument. 

The free edge of the mantle, which takes the chief part in the 
formation and growth of the shell, is particularly rich in mueous, pig- 
ment, and calcareous glands. 

The epithelium is ciliated over areas of varying extent, especially 
in aquatic Gastropods. In many of the shell-less Opisthohranchia the 
whole surface of the body is ciliated. 

The remarkable marking and colouring of the integument especi- 
ally seen in the Nudihmnchia are caused by pigment cells, which are 
more often found in the cutis than in the epithelium. 

Where there is no firm shell, calcareous granules or spicules may 
be found scattered throughout the cutis. 

In several NmJihranchia stinging cells have been discovered in 
the integument. 

Mantle, Visceral Dome. 

The mantle fold is, as a rule, well developed in Gastropods, and 
covers a spacious pallial cavity. Whenever the fold is small or alto- 
gether wanting, the condition is secondary rather than primitive. 



VII MOLLUSCA— INTEGUMENT, MANTLE, VISCERAL DOME 43 



1. Prosobranehia. 

In the Prosobranehia, the mantle fold develops on the anterior 
side of the visceral dome, and there covers a spacious cavity. It 
further usually extends like a narrow collar right round the base of 
the visceral dome. 

In the symmetrical Fissurdlidce, the mantle cavity is short, and opens out- 
wardly by means of a dorsal aperture through the mantle fold, which corresponds 
with the perforation at the apex of the shell. A circular fold, provided with a 
highly sensitive fringe, is formed by the mantle around the aperture, and projects for 
a short distance beyond the perforation in the shell. The water needed for respira- 
tion passes into the pallial cavity through the slit-like aperture at the free edge of 
the mantle fold, over the nuchal region, and flows out through the apical aperture 
just described. This aperture also serves for the ejection of excretory matter from 
the rectum, which lies immediately behind it. In Riinula, the apical apertures in 
shell and mantle have moved somewhat forward, and lie anteriorly between the 
apex and edge of the shell. In Emargimda, the mantle has an anterior cleft, the 
edges of which, in the living animal, are folded in such a way as to form a tubular 
siphon, which can be protruded through the marginal cleft of the shell. In Par- 
mophorus there is no second opening into the mantle cavity, but the lateral edges 
of the mantle are very much widened, and bent back dorsally over the outer surface 
of the shell in such a way as to cover the gi'eater part of it. 

In Saliotis, the enormous development of the columellar muscle on the right 
side confines the mantle cavity to the left. The mantle fold has a long slit reach- 
ing from its edge to the base of the pallial cavity. This slit lies under a row of 
perforations in the shell which are characteristic of Halioiis, and through these the 
respiratory water is expelled. In the spaces between the consecutive perforations, the 
edges of the mantle cleft are apposed, merely separating beneath each aperture to 
allow of free communication between the cavity and the exterior. The edges carry 
three tentacular processes, which can be thrust outward through the perforations. 
The anus is always found under the posterior perforation. The edge of the mantle 
surrounding the body splits into two narrow lamellae, which bend round to form 
a groove for the reception of the edge of the shell. 

The Trochidce, Turhinidai, Neritidm, and nearly all Mmiotocanlia have no second 
aperture and no mantle cleft. 

In Docoglossa (Patella, etc.) the mantle forms a circular fold round the visceral 
dome, which is in the form of a blunt cone. It covers the edge of the almost 
circular broad-soled foot. The mantle is broadest anteriorly, where it covers the 
head and neck, i.e. the pallial cavity or gi'oove is here deepest. 

The visceral dome, in the Monotocardia, is almost always distinctly constricted 
at the base, and spirally coiled. The pallial cavity occupies its typical position. 
In many Monotocardia, the free edge of the mantle fold is prolonged on the left side, 
projecting forward, sometimes to a great extent ; the lower edges of this p)rojecting 
fold bend round towards each other to form a tube or semi-cylindrical channel, 
which is called the siphon. Through this siphon, the water needed for respiration 
flows into the mantle cavity. It can generally be told, by the shape of the shell, if 
there is a siphon or not, since most Monoloeardia which possess one have either a 
notch in the edge of the shell at the columella, or a process called the canal or beak, 
at this same point, which encloses the siphon. The length of this latter canal need 
not, however, correspond with that of the siphon. 



44 



COMPARATIVE ANATOMY 



OHAP. 



The Moiiotomrdia have even been grouped, according to the presence or absence 
of a siphon, into the Siplioniata or Siphonostomata, and the Asiphoniata or Holo- 

stomata ; but this classification is artificial, 
since siphons are sometimes present and 
sometimes absent in forms which are un- 
doubtedly nearly related. 

In most Monotocardiu, the shell is not 
outwardly covered by the mantle, but in 
some groups, the edges of the mantle bend 
back over the shell, and finally grow over 
it to such an extent as to unite above it. 
The external shell in such cases becomes 
an internal shell. 

In the Harpidce among the FJiachiglosscr, 
the mantle bends back over the columellar 
lip of the shell. In the Margiiielli'/rr, it 
covers a large part of the outer surface, and 
the same is the case in Fyrula among the 
Tncnioglossii^ in most Cyprmichc and in the 
Lamdlnridce. In Lamdlnria, the shell is 
completely grown over by the mantle. In 
Stilifer among the Eulimidce also, the shell 
is more or less covered by the mantle. 

The edge of the mantle may be fringed 
or notched, or (C'y2nrmdce) provided with 
wart -like, tentacular, or branched ap- 
pendages. 

2. Pulmonata. 

In the Pulmonata, the arrange- 
ments of the mantle fold and visceral 
dome and of the shell, which is in- 
timately connected with them, are of 
great interest. AVe have, on the one 
hand, forms such as HiU:r, with large 
protruding spirally- coiled visceral 
dome and large mantle fold enclosing 




Fro. 54. — TestaceUa haliotidea (aftei- 
Lacaze-Duthiers). A, right view ; h, enonnoiis 
pharynx evaginjiteil through the buccal cavity, 
carrying on its surface the radula (a) ; c, open- 
ing of the pharynx into the o?sophagus ; tf, 
position of the genital aperture ; e, latero-dorsal 
groove along the body ; /, latero-ventral groove ; 
g, mantle, rudiment of tlie visceral dome. B, 
dorsal view : a, h, the two pairs of tent.acles ; 
c, the latero-ventral groove ; d, the latero- 
dorsal groove ; e, shell. 



a spacious cavity ; on the other, 
forms such as Oncidium, without 
distinct visceral dome or mantle fold 
and without shell. Between these 
two extremes there are numerous 
transition forms ; indeed, complete 
series of such forms may be found 

even within some of the natural divisions of the Pulmonata. The 

following are a few characteristic types. 

Helix (Figs. 12 A, p. 9 ; 72, p. 75)._The visceral dome is large and spirally 
coiled, and is covered by a spiral shell sufficiently large to shelter with ease the whole 
body. The mantle fold covers a cavity lying anteriorly to the visceral dome (pul- 
monary cavity). Its free thickened glandular edge unites with the nuchal integument 



VII 



MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 45 



near it in a way characteristic of the Pulmonata, leaving only one aperture, the 
respiratory aperture— on the right. (In Pulmonata whose shells have the sinistral 
twist, the respii-atory aperture lies to the left. ) The apertures of the hind-gut and 
excretory organ are close to the respiratory aperture, through which their excreta 
have to pass out. 

In many species of the genus Vitriiia, the shell cannot contain the whole animal. 
The mantle fold projects in front of the shell, and has a process which is hent back 
over the shell, and is used for cleansing it. 

In Daudelardia (Helicoplmnta) (Fig. 12 B, p. 9) the visceral dome and shell are, 
in comparison with the rest of the body, much smaller than in Vitrina. The animal 
cannot be sheltered by the shell. The visceral dome begins to be levelled down to a 
certain extent, disappearing into the dorsal surface of the foot. It lies far hack 
on the body, the respiratory aperture being on its right side. 

A somewhat similar arrangement is found in the genus Homalonyx, in which the 
low visceral dome lies on 
the centre of the back. 
The respiratory aperture 
lies to the right at the edge 
of the mantle. The edge 
of the flat ear-shaped shell 
is fixed into the mantle 
fold. Davdcbardia and 
Hcrmalonijx begin to look 
like slugs. 

In TestcKclla (Figs. 54 
and 55) a visceral dome 
hardly exists. The only re- 
mains of it is a small mantle 
at the dorso-posterior end 
of the body, which 
covered by an ear -shaped 
shell. Beneath the mantle 
lies a reduced respiratory 
cavity. The respiratory aperture lies to the right posteriorly, beneath the edge of 
the shell. The viscera lie dorsally on the foot. 

The common terrestrial snails Limax and Arimi (Fig. 12 D, p. 9) resemble 
TestiiccUa in the reduction of the visceral dome, but in them the mantle or so-called 
shield which takes its place lies anteriorly behind the head. At its right edge lies 
the respiratoiy aperture. lu Limax there is a small round rudimentary shell which 
is internal, i.e. it is entirely enveloped in or overgi'own by the mantle jfold. In 
Arion fhis sheW is represented by isolated calcareous granules. In Onchidiuin and 
J'aginulus there is no trace of a visceral dome, nor, in the adult, of a shell. The 
visceral dome has to a certain extent spread out over the whole dorsal surface of 
the foot, and has disappeared. There is, further, no outwardly recognisable mantle 
fold distinct from the rest of the dorsal integument. A longitudinal furrow still 
divides the dorsal part of the body from the foot. The respiratory aperture with 
the anus lie posteriorly in the median line. 

In the genus Physa (Fig. 11, p. 8), the edge of the mantle takes the form of 
lobe-like or finger-shaped processes, which bend back over the shell, and can be 
applied to its outer surface. In Amphipeplca (Fig. 10, p. 8) the mantle is much 
widened and, when bent back over the shell, covers all but an oval spot on the 
dorsal side of the last coil. 

The dorsal integument of the Onchidia has wart-like protuberances or (in Peronia) 




Fig. 55.— Testaoella haliotidea, posterior portion of the body 
from the right (after Lacaze-Duthiers). The sliell is removed to 
show the rudimentary visceral dome, a, latero-dorsal groove ; &, 
latero-ventral groove ; c, end of tlie muscle attached to tlie shell ; 
, mantle edge of the visceral dome ; </, respiratory aperture. 



46 COMPARATIVE ANATOMY chap. 

branched appendages. These are richly supplied with blood-vessels, and serve for 
respiration. In Peronia there are besides these also dorsal prominences which carry 
eyes. 

The dorsal integument projects all round the body above the foot, and thus 
forms, as in Chiton, a peripheral zone, which is ventrally separated from the foot 
by a groove. In OnckHclla the edge of this zone, i.e. the lateral edge of the body, 
is dentate or fringed. 

3. Opisthobranehia. 

The tyj)ical outer organisation of the Gastropoda here suffers 
even more varied and thorough modification than in the Pulmonata. 
We have, on the one hand, forms with head, foot, visceral dome, shell, 
mantle and gill ; on the other, forms which possess none of these 
organs and nevertheless are both Gastropods and Opisthobranehia. In 
one principal division of this order, the PuUiata or Tedibranchia, the 
mantle fold is retained on the right side of the body, and partially 
covers a typical Molluscau ctenidium ; in other divisions both mantle 
and ctenidia are wanting. "We do not here apply the term mantle to 
the fold or edge of the dorsal integument which surrounds the body 
at the part where the head and foot take their rise ; such an edge is 
more or less developed in most Opisthobranehia and distinctly marks 
off the foot and head from the rest of the body or back. The mantle 
here means only the broader fold which covers the mantle cavity, in 
which lies a typical mollusoan gill. The edge of the mantle never 
forms a distinct siphon in the Opisthobranehia, though there is an 
approach to such a structure in the limijkmliiim. 

(a) Tectibranchia. 

(a) Reptantia. — In this division we have, on the one hand, forms which still 
have a distinctly projecting visceral dome, whose integument secretes a coiled shell, 
into which the whole body can be withdrawn. On the other hand, forms occur in 
which the flattened visceral dome has spread out over the whole dorsal surface of 
the foot, the shell being rudimentary and internal. Examples of the former are 
found in the C'ephalaspidce, e.g. the Actaeonidw, Tnrnaliiiidce, and some Scaphandrirhr 
(Afijs, Cijlic/nui, Amphispltyra), a few Pullidic (Bulla), and the Mi iigietilidce. 

In Scnpltriiidri- among the Scaphaadrida', and Acrrn among the Pullidce, the body 
cannot be completely withdrawn into the shell. 

In the Cephalaspidce, to which so far reference has been made, the shell is 
external. 

In Giisti-opkron the mantle is rudimentary, and is provided posteriorly with a 
filiform appendage. It covers a delicate membranous internal shell, into which the 
body cannot be withdrawn. The same is the case in PhiUm- and Doi-idiirm, where 
there is also a delicate internal shell covering only a small portion of the viscera • this 
shell, in DoruUum, is produced in the form of two lobes, the one to the left endino- 
in a filiform process. 

The visceral dome in the Annsjndce is small as compared with the size of the 
animal, but rises distinctly above the rest of the body, and is covered by a thin 
inconspicuous shell. The mantle and shell often only partially cover the "ill. In 
A'plysia, the shell is internal, i.e. it is entirely overgrown by the mantle ; in Dola- 
bella, this- enveloping overgrowth is not quite complete, as a circular median dorsal 



V]I 



MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 



47 



The mantle 



aperture is left, through which the dorsal surface of the shell is visible, 
m Dolabdla forms a small anal siphon posteriorly. 

Kotarchus has a microscopically minute shell. In certain species of this'genus 
the mtegument forms protuberances or delicately 
branched appendages. ^ 

In the Oxijnoidcii, the shell is only partially covered 
by the mantle, and is, further, much too small to 
shelter the body. 

Among the Kotaspida; the Uiabrdlidce have a 
small flattened cap-like visceral dome lying upon the 
massive foot. The visceral dome is surrounded by a 
mantle fold which, on the right side, covers the gill. 
The integument of the dome and mantle is covered 
by a flattened disc-shaped shell. 

In Pleurobmiichia, the visceral dome is relatively 
large. The right and left margins project as short 
mantle folds, but there are no such folds to the front 
and back, so that at these latter parts the flattened 
visceral dome is not distinct from the rest of the body. 
In Plcibrohranclms, the integument of the flattened 
visceral dome broadens out into a large fleshy disc 
which projects on all sides beyond the large, broad- 
soled foot ; its margin (mantle fold) is separated from 
the foot by a deep continuous groove running right 
round the body ; in this groove, to the right, lies the 
large gill, while in PhurobroMchus a small flat internal 
shell, thin and membranous, is still found ; in related 
forms this may be wanting. The dorsal integu- 
ment is often strengthened by a layer of calcareous 
granules. 





(/3) Natantia. 



Fig. 56. — Diagrammatic trans- 
verse sections of Gastropods, to 
illustrate the arrangement of tiie 
.shell (black, 1), visceral dome and 
man tle(dotted, 2), and foot (streak- 
ed, 3). A, Prosobranchiate with 
outer shell and epipodium (4). B, 
Tectibranctiiate with lobes (0) of 
the mantle turned back over the 
outer surface of the shell. Dorsally 
the shell is still uncovered ; 5, para- 
podia ; 7, ctenidium. C, Tecti- 
branchiate with internal shell, i. e. 
completely overgrown by the lobes 
of the mantle. 



Fteropoda Thecosomata. — The Zii/tannidce have 
a well - developed visceral dome and corresponding 
shell, with sinistra] twist ; the shell can be closed by 
means of a typical operculum. The mantle fold covers 
a cavity which lies anteriorly to the visceral dome. 
The anus is to the right. The animal can with- 
draw into its shell. In the Cavoliniidce the dome 
and shell are bilaterally symmetrical, not twisted, and 
the body can be entirely hidden within the shell. 
The mantle cavity here lies posteriorly to the 

visceral dome, on what is usually called its lower side. The symmetrical shell of 
the Cymhiliidce does not correspond with the shell of other Thecosomata ; it is a 
cartilaginous " pseudoconch " covered with body epithelium. In the Cyminliida: 
the mantle cavity also lies posteriorly. We shall return later -to the varying 
position of this cavity among the Thecosomata. 

The mantle, in the genus CavoUnia, shows peculiarities which can best be described 
in connection with the shell. In the latter, two surfaces are distinguished, a slightly 
arched anterior surface (usually described as the upper), and an arched posterior 
surface. The anterior surface projects forwards and downwards beyond the 
posterior for a third of its length. The shell has three slit-like apertures, one 



48 COMPARATIVE ANATOMY chap. 

anterior and ventral, through which tlie fin-like processes of the foot can be protruded, 
and two lateral apertures stretching far up, so that the shell appears almost bivalve. 

At these lateral slits, which admit water to the mantle cavity, the mantle bends 
round on to the outer surface of the shell, covering the greater part of it ; and, at 
the upper angles of the slits, has two freely projecting processes. 

Fteropoda Gymnosomata.— In these, the long outwardly symmetrical body is 
naked and without a mantle, and the foot, which is much reduced, is found on 
the ventral side of the most anterior part of the body. 

(6) Ascoglossa and Nudibranchia. 

In mature Ascoglossa and Xudibranchia, with the single exception of the Slegmio- 
branchia, a shell is always wanting, as also a distinctly demarcated visceral dome. 
The latter, indeed, spreads out over the whole dorsal surface. The dorsal integu- 
ment, nevertheless, forms a circular fold (mantle fold) separated from the foot by a 
groove sometimes deep, sometimes shallow ; but, except in the rhyllidiidcc, no gills 
lie in this groove. Where this groove has nearly disappeared, the animals strongly 
resemble Planaria. 

Phyllidiidae. — In these, the mantle fold is distinct, and can'ies on its lower 
surface, to the right and left, a row of branchial leaves, herein recalling Patella and 
Chiton. 

The genus Bermatoiranchus, which, judged by its organisation, belongs here, 
has, however, no gills. 

Doridldas. — The dorsal integument (notseum), which here covers the body like 
a shield, being generally distinctly demarcated from the foot and the head, contains 
numerous calcareous particles, which give it a firmer consistency. Anteriorly, there 
are two feeler-like processes, the rhinophores, which can generally be withdrawn 
into special sheaths or pits; these are not to be confounded with the tentacles. 
The anus lies in the median line, generally behind the middle of the body, and 
is surrounded by an ornamental circlet of pinnate gills. The notseum is often 
covered with prominences, and in some genera the margin carries variously shaped 
processes. 

Cladohepatica. — Here there are no anal gills. The dorsal integument has 
variously formed and variously arranged appendages ; these may be conical, club- 
or finger-shaped, lobate or branched ; they are, for the most part, very striking in 
colour and appearance. Sacs of nematocysts are generally found at their tips, and 
.cteca of the intestinal canal (branches of the digestive gland) penetrate them. These 
dorsal appendages, which, like the rest of the body, are ciliated, have, at least 
partly, a respiratory function. In many forms they easily fall off, and are later 
regenerated (Fig. IS, p. 12). 

Many Cladohepatica have a certain external likeness to Planaria with dorsal 
papillte (Thysanozoon), but this likeness is still more marked in the following 
.family : — 

Ascoglossa.— Anal gills and also, as a rule, dorsal appendages are here wanting. 
The whole body is naked and ciliated. Tlie back is indistinctly demarcated from 
the head. 

Phyllirhoe.^This Nudibranchiate genus, of all Opisthobranchia, shows least of 
the tyijical external organisation of the MoUusca. The body here is naked and 
laterally compressed, with sharp dorsal and ventral edges. It has neither foot 
Mr gills (Fig. 19, p. 12). 



VII MOLLUSGA^IKTEGUMENT, MANTLE, VISGEBAL DOME 49 



D. Seaphopoda. ((/. Review of Outer Organisation, p. 34.) 

E. Lamellibranehia. 

From each side of the body there typically hangs a large leaf-like 
mantle fold of the same shape as the shell-valve formed by it. These 
mantle folds project beyond the body in front, below, and behind, and 
enclose a mantle cavity which everywhere, except dorsally, opens 
outward by means of the slit left between the edges of the folds. 
This large single cleft serves for the admission of nourishment and 
water into the mantle cavity, and for the expulsion of the excreta, 
genital products, and respired water ; through it also the foot is 
protruded. Such a primitive mantle is thus completely open, its 
simple edges (i.e. without folds, papillae, tentacles, or eyes) are quite 
free, coalescing nowhere. 

The above serves for a description of the mantle of A^vcnla — 
one of the FrotoWanchia — and must be considered as the primitive 
arrangement. 

In most Lamellibranchia, however, special differentiations of the 
margin of the mantle occur ; these take the form of folds, thickenings, 
protuberances, papillae, tentacles, glands, eyes, etc., and this is the 
case both in forms which have an open mantle and in those in which 
the mantle is partially closed. 

The partial closing of the mantle is brought about by the con- 
crescence at one or more points of the free edges of the mantle 
folds. 

A. A completely open mantle, i.e. one single large cleft entirely separating the 
edges of the mantle, is found : 

(a) Among the Protobranchia in Niicula. 

{b) Among the Filibranchia in the Anmniichc, Arcidcc, Triyoniidce, and a few 
Mijtilidce (Pinna). 

(c) In all Pseudolainellibranchia except Mrhrtririna. 

{d) Among the Eulanuillibranchia, only in a few species of Crassatella . 

B. The mantle folds of the two sides grow together at one point. — In this 
case the point of concrescence almost always lies high up posteriorly ; and marks 
off a small aperture from the originally simple cleft. This aperture, occurring on 
a level with the anus, forms the exhalent or anal aperture of the mantle. Its edge 
may be more or less prolonged posteriorly to form an anal siphon, which can be 
protruded beyond the valves of the shell. 

At a point a little below this exhalent aperture, the mantle edges usually become 
applied to one another, although no concrescence takes place. Above this point, 
between it and the anal siphon, they separate to form an inhalent or branchial 
aper-ture. The edges of this aperture also may be produced posteriorly into a 
branchial siphon, which, however, in this case, has a cleft extending along the 
whole of its lower side, which is a continuation of the large cleft of the mantle. A 
branchial siphon formed in this way, by mere apposition of the mantle edges, is found 
in the genus Mnlhiin among the Prulobrandiiu . 

VOL. II E 



50 



COMPARATIVE ANATOMY 



CHAP. 



An anal aperture, separated by a point of concrescence from the large mantle cleft, 
is found in the following Lamellibranchia : 
(a) Among the Protohranchia in Malletia. 
(V) Among the Filibranchia in most MytiUdcc. 

(c) Among the Pseudola-melUhranchia in the AvicuHdce (genus Mdeagrina). 

(d) Among the EulaiiwllibraiicMa, in the Carditidce {Venericardia, Cardita 
Milneria), the Astartidce, and most Crassatellidce ; among the Cyrenida!, in the genus 
Pisidimn; among the Unianidce in the Unioiiiiue {Unto, Anodonta) ; and among 
the Luciimcca, in Cryptodon Mosdcyi. 

In Soleiwimja, among the ProtobrancUa, the two mantle edges grow together 
only at one point, but to such an extent as to close the whole posterior half of the 




Pig. 67.— Diagrams to illustrate tlie various ways in which concrescence of the mantle 
and formation of siphons take place in the Lamellibranchia. The foot (7) protruded forward 
through the mantle cleft ; A, mantle completely open ; B, mantle open, but with its edges applied 
to one another at two points, thus giving rise to incompletely separated anal and respiratory 
cavities ; C, edges of the mantle grown together at one point (1), the anal or exhalent aperture of 
the mantle (4) is separated ; D, edges grown together at two points (1, 2), the branchial or inhalent 
aperture (5) is also separated, the mantle has three apertures ; E, mantle closed by the extension 
of the place of concrescence (2), three limited apertures remain, viz. the anal, branchial, and pedal 
apertures — the first two are produced into siphons; F, a third concrescence (3) takes place. 
Mantle with four apertures (4, 5, 6a, 66), the most anterior (6b) for the protrusion of the foot. The 
siphons have united. 

ventral mantle opening. In this way the mantle cleft is divided into two ; the anterior 
aperture serves for the protrusion of the foot, while the posterior serves at the same 
time as inhalent (branchial) and exhalent (anal) aperture. Solenomya is the only 
bivalve in which this arrangement is found. 

C. The mantle folds grow together at two points, thus forming three 
apertures. — This condition arises in consequence of the complete separation (through 
concrescence) of the branchial aperture from the rest of the large anterior mantle 
cleft. The anal and branchial aperturcf may remain as slits, or may be produced 
into longer or shorter anal and branchial siphons. The large anterior and ventral 
mantle cleft serves for the protrusion of the foot, and is called the pedal cleft. 
These two points of concrescence are found : 

[a) Among the ProtdbrancMa, in Yoldia and Lata. 



vii MOLLUSCA— INTEGUMENT, MANTLE, VISCERAL DOME 51 

(6) In most Hulamellibranchia, viz. in most Luciukhc, most Ci/reidcke ; among 
the Vnionidm, in the MuUliiue, in the Donacidce, Psaniriiohiidai, Tellinidm, Scrobi- 
ctilariidce ; among the Veneracea, in the Veueridce, in the Cardiidce, the Mactiida, 
Mesodcsiiuitidic, and the Solenidce (excepting Solen and Lutraria). 

(c) In all Ssptibraiichia (Poromyia, C^ispidaria). 

In the above forms the mantle is still wide open, i.e. the points of concrescence 
are small and local. But these points may become lines of concrescence of con- 
siderable length. In the Chamacea, for example, and especially in the Tridacnidce 
among the EulcmwlUhrmichia, the three apertures of the mantle are found at con- 
siderable distances from one another, being divided by long intervals where the 
edges have gro^vn together. 

In some groups of Lamellibranchia, the concrescence between the anal and 
branchial apertures or siphons remains short, i.e. the one aperture lies directly 
below the other, but in such eases the edges anterior to the branchial aperture 
grow together to a greater extent, so that the pedal aperture becomes reduced to 
a small anterior fissure. In this condition the mantle is closed. Such a mantle is 
found : 

Among the EulanieUibrcmchia in the Modiolarca, Dreisseiisia, Petricola, all 
PholadidcB (Pholas, Pholadidca, Joiiannetia, in which the pedal aperture is said 
to close entirely in old animals, Xylophaga, Martesia) ; in the Tcredinidcc, and 
among the Paiidaridce, Paiulora, the Vcrticordiidce and Lymwiidce (Anatioiacca). 

D. There are some Lamellibranchia with closed mantle, in which a fourth 
aperture is added to the three found in the above groups, the mantle thus having 
three points of concrescence. The fourth aperture is always small, and is found 
between the pedal and branchial apertures ; it probably corresponds with a rudi- 
mentary fissure for the byssus. 

This arrangement is found in the Eulmiullibranchia, ; among the Solenidm, in 
Solcn and Lidraria ; among the Pandoridce, in Myochaina ; in Olycymeris ; among 
the A'lUiiiiiacea, in the genus Thracia ; in the Pholadomyidce and the GlavageUidce 
(Clavagella and Brechites [Aspergillmn]) ; and, finally, in Lyonsia norvegica. 

The anal aperture is often and the branchial aperture nearly always 
fringed, or in various ways edged with protuberances, papillae, or ten- 
tacles, and this is the case whether these apertures are found on the 
edge of the mantle or at the ends of (longer or shorter) siphons. 

The siphons can be contracted and extended, and either wholly 
or partly withdrawn into the shell, by means of special muscles. 
These muscles are attached on the inner surface of the shell-valves to 
the right and left posteriorly, and their line of attachment forms the 
pallial sinus, which will be described later on. 

The length of the siphons varies greatly. Specially long siphons are found in 
the Mactridce, Donacidce, Psavimobiidce, TeUinidce, Scrobiculariidce, many Veiieracea 
and Cardiidm, the Mesodesmatidcc, Lutraria, the Pholadidce, Teredinidce, Anatinidce, 
and Clavagellidm. 

The siphons may be separated throughout their whole length, and often diverge 
one from the other {e.g. Galatea among the Cyrenidce, the Donacidce, Psammobiidce, 
TeUinidce, Scrobicularidm (Fig. 58), Mesodesmatidcc, Pharus, etc.). 

In other forms they coalesce along their entire length ; they may even look like 
a single tube, which is, however, always internally divided into an upper (anal) and 
a lower (branchial) channel. This common siphon is sometimes protected by 
a, special sheath of epidermis, particularly in those forms in which it cannot be 



52 COMPARATIVE ANATOMY chap. 

withdrawn into the shell. Siphons united throughout their whole length are found 
in the Miictridtr, a few Tcnemcea, Lutniriii, Soleiiocurtus, Sohii, the Pholndkhc, 
many Anatinidcc, and the C'lavnijcUidcc. 

In some cases, siphons which are united for some distance at the base, sepai'ate 
near their ends and even diverge, e.g. in Petricoh( among the Vfnenicea, Teredo, etc. 

The two siphons are often of unequal length. In the MoiUnhiria (MiitUidu ) only 
one, the anal, is developed, while the branchial aperture remains unseparated from 
the large mantle cleft. The reverse is the case in iJivisseiisia and Scrohicularia, 
where the branchial siphon is nuioh longer than the anal. 

The siphons are sometimes provided with valves ; these occur more often in the 
anal than in the branchial siphon. 

Significance of the development of the Anal and Branchial Apertures 
and Siphons. 

Most Lamellibranchia inhabit mud or sand, into which they sink the anterior part 
of the body, burrowing by means of the protrusible foot. The water necessary for 
bathing the gills and for respiration can only be received into and expelled from the 
mantle cavity through the cleft at the posterior end of the body which projects above 
the mud. The fiecal masses from the anus near this point must also here be ejected 
from the cavity. The development of localised inhalent and exhalent apertures is 
explained by the fact that a constant regulated stream of water into and out of the 
mantle cavity is necessary both for respiration and for conducting particles of food 




Fir:. 5S.— Scrobioularia plperata buried in mud. The inhalent siphon takes m mud as 
nourishment ; the anal siphon stands erect (after Meyer and Mdblus). 

to the moirth. The most advantageous point for the exhalent aperture is obviously 
directly behind the anus. 

Siphons attain development in consequence of the habit of life of many bivalves, 
which bury themselves deep in mud, sand, wood, and even rock. By means of their 
siphons they can still remain connected with the water which bathes the surface of 
their place of concealment, and, as long as the animals remain undisturbed, a con- 
stant current enters the mantle cavity through the branchial and leaves it through 
the anal siphon. 

"Where the mantle folds have grown together to a large extent (closed mantle) the 
siphons are always well developed. Such closing of the mantle is found principally 
among bivalves which bore into wood, clay, rocl<, etc., and in which the foot of the 
adult is weakly developed, or altogether rudimentary. The degeneration of the foot 
leads to the shortening of the pedal aperture which originally served for its pro- 
trusion. 

The mantle is found completely open with only slightly developed anal and 
branchial apertures or none at all, in bivalves which do not burrow, hut live 
surrounded by Avater, either attached to the bottom or lying freely on it. 



VII MOLLUSGA— INTEGUMENT, MANTLE, VISCERAL DOME 53 

In such animals the surrounding water can circulate through the usually open 
mantle cleft and the mantle cavity. AVe here find protuberances, papillfe, tentacles, 
etc., carrying sensory organs, all along the free edges of the mantle, whereas, in 
bivalves which inhabit mud or bore into wood, rook, etc., such organs are mostly 
found massed together round the edges of the branchial and anal apertures. 

The Edge of the Mantle. 

The edge of the mantle often forms a number of diverging folds, which in trans- 
verse section look like finger-shaped processes. The outermost fold is always applied 
to the shell. The edge of the mantle may also be beset with one or more rows of pro- 
tuberances, papillffi, or tentacles, and often contains unicellular or multicellular 
glands, mucous glands, and others which have been considered to be poison glands for 
protective purposes. Tactile sensory cells are very common. Eyes are rarely developed 
on the edge (</. section on the Sensory Organs). 

In the Pectiiiidce, Spuiiifi/Iidcc, and Limidcc, the inner fold of the mantle has a 
somewhat broad border, which, when the shell is open, projects from the mantle 
towards the median line of the body (Fig. 23, p. 16). The free opposite edges of 
these folds (flaps, or curtains), springing from right and left, may meet in such a 
way as to shut off the central part of the mantle cavity even while the shell is open, 
apertures only remaining anteriorly and posteriorly. 



F. Cephalopoda. 

Integument. 

The integument of the Cephalopoda consists of an outer cylindrical 
epithelium, and a subjacent cutis in the form of thick connective tissue. 
In this cutis, not far removed from the epidermis, and above a layer 
of connective tissue plates (which are refractive and often shimmer like 
silver), there are large pigment cells or ehpomatophores which, by their 
alternate contraction and expansion, bring about the well-known changes 
of colour in these animals. 

These chromatophores are single cells containing yellow, brown, black, violet or 
carmine pigment, either as fluid or in small granules. The layers containing them 
are either single or double ; in the latter case, the colour of the pigment in the one 
layer of chromatophores diff'ers from that of the chromatophores in the other. 
Radial fibres, arising from the suiTounding connective tissue, are attached to each 
ehromatophore, round that equator which lies parallel to the integument. The 
chromatophores are enveloped in a special, possibly elastic, membrane, and when 
contracted are almost globular ; the pigment corpuscles are then crowded together. 
The chromatophores expand equatorially, diminishing the distance between their 
poles, i.e. they become much flattened. In this condition, they may, further, throw 
out fine branches, the pigment gi-annles being thus spread out over a large surface. 
It was formerly believed that the expansion of the chromatophores was caused by the 
contraction of the radial fibres, which were thought to be muscular, but more recent 
investigations have shown the fibres to be of the nature of connective tissue. The 
changes of colour, which are of great physiological and biological interest, and which 
are partially under the control of the animal, are brought about by the alternate 
contraction and expansion of these variously coloured chromatophores 



54 GOMPABATIVE ANATOMY chap. 



Mantle, Visceral Dome. 

Some of the most important points connected with the mantle and visceral dome 
have already been mentioned (pp. 36-38). 

In JVmitihis, the body is attached right and left to the inner surface of the 
shell of the last or inhabited chamber by powerful muscles, which may make 
a slight impression on the shell. Between the points of attachment of these 
lateral muscles, the integument of the visceral dome coalesces with the inner surface 
of the shell of the inhabited chamber in a naiTow circular zone, so that the gas_^ 
enclosed in the upper chambers of the shell cannot escape. While the integu- 
ment and mantle beneath this zone of concrescence {i.r. towards the free apertm-e 
of the last chamber) are rough, fleshy, and muscular, the integument of that 
portion of the visceral dome which lies above the zone and is applied to the last 
septum is delicate and soft. The siphuncle, which arises at the dorsal end of the 
visceral dome and passes through all the septa, is membraneous and hollow and filled 
with blood. It is said to communicate with the pericardium. In the female Nau- 
iihis, the nidamental gland (see Genital Organs, p. 241) lies in the free mantle fold, 
near the point at which it separates from the visceral dome. We thus have parts 
which usually lie in the visceral dome wandering into the mantle fold. 

Among the Dibranchia, which are good .swimmers, fins are found. In the 
Octopoda, which are distinguished by the round, compact form of the visceral dome, 
these are wanting, except in the remarkable genus Cirrliolculhis. Fins are universal 
among the Decapoda, and vary much in form, size, and arrangement. 

In Sejjia (Fig. 80, p. 83) and Sepintcuthis, the fins ai-e inserted on the lateral 
edges of the body, along the whole height (length) of the visceral dome, forming the 
boundary between the anterior and posterior (physiologically the dorsal and ventral) 
surfaces of the latter. In liossia, Scpiola, and Sepiu/oidea they are almost 
semicircular, and are like distinct appendages situated on the anterior surface of 
the dome, about half-way up it. This is also the case in Cirrhotfu/Ids, where the 
more or less circular fin-lobes rise from the body on stalk -like bases. 

The triangular or semicircular fins of Oranchia, Histioteuthis, Onychotcut.his, Loligo 
(Fig. 34, p. 23), LoUgojpsis, Ommastrcphes, etc., are found at the dorsal end of the 
visceral dome, on its anterior side. 

In many Dibranchia, there is a concrescence of the free edge of the mantle fold 
with the integument of the "head" (Kopffuss), which lies below it. This connec- 
tion is effected by means of a muscular band, which passes over the neck (nuchal 
band). In most Dcatpoda, this connection is wanting, and the edge of the mantle 
is free all round the body ; the exceptions are the genera Sepiola, Oranchia, and 
Loligopsis, which have a narrow connection of this sort. All Oiiopuda have this 
concrescence, commencing with the Argonauta ; it lengthens in Philcmexis and 
Octopus, till in Cirrhu/cufhis it spreads to the posterior surface (physiologically the 
ventral surface), so that the edge of the mantle remains free only at the aperture 
through which the funnel or siphon is protruded. 

Arrangements for fastening the mantle fold to the adjacent 
body wall are very common. Such attachment is either temporary 
or permanent. In the former case, there are prominences with cor- 
responding depressions for locking the mantle {ajjjmreil de resistance) ; 
in the latter case, dermal or muscular fusions take place between the 
mantle and body wall. 

' Of. note, p. 37. 



VH MOLLUSGA—THE SHELL 55 

1. Apparatus for locking the mantle. — These are pah-ed or unpaired. The former 
are to be found on the posterior side of the body, iu the mantle cavity, near its 
lower end ; they lie to the right and left at the base of the funnel, and on the 
corresponding points of the inner surface of the mantle fold. The unpaired, on 
the contrary, are found on the anterior surface of the neck. Since all the arrange- 
ments serve the purpose of cutting off the mantle cavity from the external medium, 
it is easy to see that their development is in inverse ratio to the extent of the con- 
crescence of the edge of the mantle round the neck before mentioned. "Where no 
concrescence is found, as in Sepia, the arrangements for locking the mantle are 
highly developed ; while, where the line of concrescence is very long, as in Octopus, 
the locking apparatus may be altogether wanting. The looking apparatus consists, 
in general, of cartilaginous prominences (often accompanied by depressions) on the 
inner surface of the mantle fold, i.e. the surface turned towards the mantle cavity, 
which exactly fit corresponding cartilaginous depressions accompanied, as the case 
may be, by prominences, on the opposite body wall (c/. Fig. 80). The special forms 
of the mantle and nuchal locking cartilages are of importance in classification. 

The cartilaginous arrangements for locking, which are almost always found in the 
Decapoda (they are wanting only in Owenia and Oranchia), are still retained in a 
few Oetopoda in the form of more or less modified fleshy processes (ArgonaiUa, 
Tremoctopus). The nuchal locking apparatus is the first to disappear on the rise of 
the pallio-nuohal concrescence. It has disappeared among the Decapoda in the 
genus Sepiola, where the mantle is firmly attached to the neck. 

2. Permanent oonneotions between the mantle fold and the adjacent body wall 
traversing the mantle cavity are found only in those Cephalopods which have no 
locking apparatus. Thus in Octopus and Medone the mantle is attached to the body 
wall by means of a median muscle above the funnel. This muscle consists of two 
closely - applied lamella, liaving the anus between them. In Granchia the free 
dorsal edge of the funnel (at its so-called base) has become united by an integu- 
mental band on the right and left with the mantle fold, and a similar arrangement 
is found iu Loligopsis. 

Water pores. — Near the moutli, or at the bases of the arms, or laterally on the 
head, in many Cephalopods, there are apertures leading to integumental pouches of 
varying size. The function of these organs is unknown. 



IV. The Shell. 

General. 
The Shape of the Shell, and its Relation to the Soft Body. 

All the various forms of shell found in the Mollusoa are deducible 
from a cup- or plate -like shell covering the dorsal region. Such a 
shell affords sufficient protection for animals such as Fissurella, Patella, 
etc., which can firmly and almost immovably attach themselves to a 
hard surface by the sucker-like foot. The soft body is in this case 
protected on one side by the shell, and on the other by the surface 
of attachment. Free -moving Mollusca, however, show a tendency 
to protect the whole body exclusively by means of their shells, and 
this object is attained in various ways. 

In the Chitonidce, for instance, the shell is made up of consecutive 



56 COMPARATIVE ANATOMY chap. 

joints, overlapping in such a way as to be movable one upon the other. 
This segmented shell can protect the whole body, since it allows the 
Chiton to roll up like an Armadillo or a Wood-louse. 

In the Lamellibranchia, the protection of the whole of the soft body 
is provided for by the development of a bivalve shell, from which the 
foot can be protruded, and which, by the closing of its two valves, 
completely envelops the soft body as well as the retracted foot. 

In the Gastropoda, Saiphopoda, and Cephalopioda, the most complete 
protection on all sides of the body by means of the shell is attained 
on another plan. The shell becomes much elongated and turret- 
like, and is thus so capacious that not only the visceral dome but the 
head and foot also can find place in it. Even the only remaining 
unprotected aperture, the one weak point of this fortification, can very 
often be completely closed by a hard operculum. 

A long, turret-like shell is an inconvenient burden for a freely 
moving animal, being, in consequence of its large surface, a hindrance 
to locomotion. A reduction of the surface is brought about in the 
Gastropoda and Cephalopoda by the coiling of the shell, either in one 
plane or in a conical spiral. 

In the latter case the spiral twist is almost always right-handed or 
dextral. 

Ill order to decide the direction of the twist, the shell should be held in such a 
manner that the point of the spiral is uppermost, while the aperture is directed 
downwards and towards the observer (Fig. 60, p. 60). If, in this position, the 
aperture lies on the riglit of the axis, tlie shell has a dextral twist if to the left its 
twist is left-handed or sinistral. 

We have a striking and in most cases unexplained phenomenon in 
the reduction and even complete disappearance of the shell, which takes 
place not only in nearly all the classes, but even within some of the 
smaller groups of Molluscs, e.g. the Solenogasires among the Amphineura, 
a few Heteropoda and Titiscania among the Prosohranchia, many Pid- 
mowda, very many Opisthohranchia, and most extant Cephalopoda. 

In almost all cases the forms in which the shell is rudimentary or 
wanting can be shown to be derived from forms in which it is well 
developed. All shell-less snails (slugs) have shells in the early stages 
of their development. 

The process of the gradual reduction of the shell to a rudiment, 
which will be more fully described later on, is often as follows : (1) 
the shell becomes internal ; (2) it decreases in size, so that it no 
longer can cover the body ; (3) the visceral dome disappears ; (4) the 
shell is only to be found in the form of isolated calcareous particles 
in the dorsal integument ; (5) even these vanish, and the shell is only 
to be found in the embryo. 

Only in a few cases is it possible distinctly to recognise the reason 
or the advantage of this reduction of a protective covering so useful 
to and exercising so profound an influence on the organisation of the 



VII MULLUSGA — THE SHELL 57 

whole race. The following are a few cases in which the utility of 
the reduction of the shell in adaptation to special conditions is to 
some extent evident: (1) In free-swimming marine forms, where the 
shell is too heavy and increases friction ; (2) in Tcdacella and allied 
forms, which prey upon earthworms, where a large shell would prevent 
them from following their prey into narrow holes and passages ; 
(3) in Gastropods, which browse among thick tangles of Corals, 
Bryozoa, Hydroida, or Algte (e.g. many Nudibranchia). 

The loss of the shell is generally followed by compensatory 
adaptations for protection, such as great capacity for regeneration, 
especially of the easily detachable appendages, voluntary amputation of 
portions of the body, stinging cells, and colouring which may be 
protective in various ways. 

The carnivorous Cejihulopods are protected (1) by their extraordinary 
swimming powers, which are in keeping with their highly developed 
organisation; (2) by their well-developed sight ; (3) by great muscular 
strength ; (4) by strong jaws ; (5) by the discharge of the secretion 
of the ink-bag ; (6) by their partly mimetic changes of colour, etc. 

Certain peculiarities of organisation, which can only be under- 
stood as remains of a shelled condition (e.g. the lateral position of the 
genital and renal apertures and also to some extent of the anus in the 
Nudibranchia), always persist after the shell has disappeared. 

Chemical Composition of the Shell. 

The shell of the MoUusca consists principally of carbonate of lime, with traces 
of phosphate of lime and of an organic substance related to chitin, — conohyolin. 
Besides tliese, various colouring matters may occur. 

Structure of the Shell. 

The shell of the Lamellihranchia consists of three layers, the innermost layer 
being applied to the surface of the mantle. The shell is to be regarded as a 
cuticular structure. 

The outer layer (shell-integument, epidermis, cuticle, periostracum) is, so far as 
its physical constitution is concerned, horny and wanting in lime. It generally 
disappears off the older portions of the shell. 

The middle layer (columnar, prismatic, or porcelanous layer) consists of slender 
prisms of carbonate of lime, usually perpendicular to the surface of the shell and 
closely crowded together. 

The inner (nacreous) layer has a finely lamellated structure. The very delicate 
transparent laminse of which it is composed are thrown into slight waves ; these 
cause the wavy lines on that surface of the shell which lies on the mantle, which, 
by interference, produce the characteristic nacreous lustre. The pearls of the pearl 
oyster are formed of the same substance as this layer. 

The constitution of these three layers varies greatly in details both in the Lamclli- 
braiich.ia and in other MoUusca. The outer and middle layers are formed at the free 
margin of the mantle, the inner layer is yielded by the epithelium of its whole outer 
surface. 

The shell in the Gastropoda and Geplialopoda consists principally of the middle 



58 COMPARATIVE ANATOMY chap. 

or porcelain layer, wliioli, however, has a structure very different from that of the 
same layer in the Lamellibrairichia. This layer is generally, if not always (at least 
in the young), covered by a periostracum. The inner (nacreous) layer is very often 
wanting. 

Growth of the Shell. 

In the Arthropoda, the chitinous exoskeleton, which we may 
compare with the Molluscan shell, develops at the surface of the 
whole body and its appendages. This skeleton, when once formed 
and hardened, encases the body on all sides within fixed boundaries, 
and is incapable of growth. Hence the moults of the Arthropoda, by 
which alone growth becomes possible. 

The Molluscan shell, on the contrary, is open. In the Gastropoda 
and Cephalopoda, it assumes the shape of a conical mantle, coiled round 
a single axis and open at the base of the cone. By continual additions 
at the edge of its aperture, it grows with the growth of the animal, 
without materially altering its form. The lines on the surface of the 
shell of the adult snail register its phases of growth. During growth, 
the oldest, uppermost coils or whorls of the shell either continue to be 
filled by the apex of the visceral dome (in many Gastropods), or are 
deserted by the animal which, as the shell grows, withdraws farther 
and farther from its tip. These whorls may remain empty, or may be 
partially or completely filled with shell substance. In the latter case, 
they may be successively thrown off. The Nautilus and allied forms, 
during growth, periodically form transverse septa, so that the forsaken 
parts of the shell become chambered and filled with gas,^ the animal 
occupying the largest and last-formed chamber, which opens externally. 
In the Lamellihranchia, the growth of the shell keeps pace with the 
growth of the body in exactly the same manner, the free edge of the 
shell valve continually receiving additions of shell substance from the 
edge of the mantle to form the periostracum and the prismatic layers, 
while the whole external surface of the mantle yields an additional 
nacreous layer. The consecutive phases of growth are here also 
registered by the concentric markings on the surface of the shell. 

Special. 
A. Amphineura. (Cy. pp. 39-42.) 

B. Gastropoda. 

A few details concerning the shell of the Gastropods must here be added. As a 
rule, the shell is coiled spirally round an axis. This spiral is, in rare instances, so 
flattened that the coils come to lie almost in one plane, giving rise to a nearly 
symmetrical shell (e.g. Planorbis). 

There are, however, among the Gastropoda, uncoiled shells which are symmetrical, 
and these require special attention. The most important are the cup-shaped or more 
or less bluntly conical shells of the PatelUdce and Fissurdlidce. Since (1) we derive 



Cj. note, p. 37. 



VII 



MOLLUSGA—THE SHELL 



59 



the Gastropoda from bilaterally-symmetrical ancestors with symmetrical shells ; and 
since (2) the Fissurellidm imdoubtedly possess the most primitive organisation of all 
the Gastropoda, and thus stand nearest to the racial form, and are moreover (3) 
strikingly symmetrical in their organisation, it seems, at first sight, natural to 
consider this syinmetry a primitive feature. Certain peculiarities of the nervous 
system, however, especially the crossing of the pleuro-visoeral connectives, taken in 
connection with other conditions explained more fully elsewhere, make it certain 
that the cup-shaped shell of Fissurella is only secondarily symmetrical, i.e. that 
Fissurella is descended from forms which possessed a spirally coiled shell. The same 
is the case with the Patellid.ce. 

The following important facts are in harmony with this conclusion : (1) the 
young shell of Fissurella is asymmetrical and coiled, and it only gradually assumes 




Fig. 59.— Shells of— A, Pleurotomarla ; B, Polytremaria ; and E, Emarginula ; D, Haliotis ; 
F, Fissurella ; G and H, stages In the development of the sheU of Fissurella ; I, shell of the 
twisted racial form of the Gastropoda with marginal cleft ; K, the same with apical aperture ; 
L, shell of Lamellibranch ; M, shell of Dentalium, seen (roin the apical aperture. Tlie lioles and 
clefts of the shells are black ; o, nioutli ; a, anus ; ct, etenidiuni. 

the symmetrical form (Fig. 59, G, H) ; (2) the apparently symmetrical shape of some 
forms nearly related to Fissurella and Patella prove on closer inspection to be 
somewhat asymmetrical, the apex especially being more or less excentric ; (3) 
other forms nearly allied to Fissurella, such as Haliotis, Scissiirella, and Pleuro- 
tomaria, have spirally coiled shells (Fig. 59, A, B, C, D). 

In the Fissurellidm, many PleurotomariidcE, and the Haliotida:, i.e. in the most 
primitive Gastropods, peculiar and noteworthy perforations of the shell occur, such 
as are occasionally found in other divisions. These perforations lie above the slit 
in the mantle which is characteristic of this order (c/. p. 43), and they everywhere 
establish communication between the mantle cavity and the exterior, especially 
needed when the mouth or edge of the shell is closely applied to the object on which 
the creature crawls. 



60 



GO MP A RA TI \ 'E A NA TOM Y 



CHAP. 



In Scisstiirlhi, Pleurotomaria, and Emarginiaa, there is a median indentation in 
the anterior edge of the shell, which corresponds with an incision in the mantle 
edge. This is the case in the young FissiircUa, but, during further development, 
the edge of the shell grows across the incision, so that in the adult animal the 
aperture lies near the apex. Beneath it is the anus, placed high up in the mantle 
cavity. If such a cleft were to arise at both the anterior and posterior edges, and 
to become very deep, a double shell would result comparable with the bivalve shell 
of the LamcHibrandda. It is in fact probable that this notching of the shell edge 
is of great phylogenetic siguificauee. 

In Hahotls we have a row of perforations of the shell, the jirocess of formation 
of the perforation in Fissurdla being often repeated ; the older apertures are always, 
however, closed by shell substance, and the younger only remain open as long as 
they lie immediately over the respiratory cavity. 

In very many Prosobranehia (the Siplinniala of earlier writers), there is, at the 
coliunnar edge or lip of the shell, a notch which gives passage to a channel-like fold 
of tlie mantle margin. This channel keeps up communication between the mantle 
cavity and exterior, even when the shell is closed by the operculum. Instead of a 




Fig. 60.— a, Dextrally twisted ; B, sinistrally twisted shell of Helix pomatia. 

notch, a more or less long process or beak may enclose a corresponding process of the 
mantle, the siphon. The latter may become a tube by the apposition of its edges. 

It has already been mentioned that the shells of most C4astropods are 
dextrally twisted. There are, however, a few families, genera, or species in which 
the shell has a sinistral twist ; and in some species where the twist is dextral, a few 
individuals with sinistral twdst may occur, and vice versa. It is a curious fact that 
some species, in which the shell has a sinistral twist, show the asymmetry of the 
dextral twist in the soft body, wdiereas, in others, the asymmetry of the soft body 
corresponds with the twist of the shell. "VVe shall return to this point. 

For details as to the growth of the shell, and the capacity of the animal to 
dissolve the shell already formed, both of which are points full of interest, we must 
refer to special works on Conchology, as also for detailed descriptions of forms of the 
shell and opercula, and differences due to age. 

Piogressive reduction of the shell occurs in each of the three divisions of 
the (Gastropoda. In the Prosohranchiu, this has only been observed in marine, 
free-swimming Hcteropoch and in Titiwrniia ; in the Pvlmonata, it is much more 
common ; and in the OpislJiobranchia, so frequent that nearly all the members of 
this division have more or less rudimentary shells. Many adult Ojnsthobranchia 
have even lost every trace of a shell {Pteropocla ijiimnvsomnia, XiidibrancHa, and 
most Ascoglossa), although, in their earliest stages at least, they possessed a coiled 



vii MOLLUSCA — THE SHELL 61 

shell, for the closing of which there may even be an operculum, secreted by the foot, 
as in the Prosobnnichiti. 

The following are some of the principal stages and concomitant phenomena of 
the reduction of the shell : (a) The well-developed shell ceases to be large enough to 
shelter the whole body, (b) The shell, which becomes thinner and smaller, is dorsally 
overgi-own, partially or altogether, by extensions of the mantle, (c) As the shell 
(which is then either cup- shield- or ear-shaped) becomes continually smaller, the 
visceral dome begins to be levelled down, till it no longer rises above the rest of the 
body, its contents spreading out to a certain extent over the dorsal surface of the foot. 
(d) The external asymmetry of the body passes by degrees into symmetry, whereas 
the internal asymmetry never entirely disappears, (c) The shell is reduced to a 
number of isolated calcareous particles in the integument of the flattened visceral 
dome. (/) There is at last no trace of a distinct visceral dome ; calcareous particles 
are to be found in the dorsal integument of the long and now naked Gastropod. 
((/) Even these particles finally disappear. 

In connection with the reduction of the shell in Opisthohrcmchia and Pulmonata, 
compare the section on the mantle, pp. 43-48. 

The Heteropoda present the following interesting series : — 

Atlanta. The shell is very light and thin, but large and spirally coiled (with 
an incision at its aperture) ; the animal can entirely withdraw into it, and close 
it by means of an operculum developed on the distinct metapodium. 

Carinaria. The shell is thin, light, and delicate ; it is cup-shaped, and covers the 
large stalked visceral dome, but is incapable of sheltering the long and thick 
cylindrical body and foot. There is no operculum. 

Pterotrachea. The visceral dome is small, and there is no shell and no operculum. 

C. Lamellibranchia. 

The two lateral valves of the Lamellibranch shell are connected, at their 
dorsal edges, by means of a hinge and a ligament. The ligament counteracts the 
muscles of the shell, which will be described later on, and which, by their contraction, 
close the shell. It is usually composed of two layers, the inner layer being elastic, 
while the outer is not. The outer non-elastic layer passes into the epidermis or 
periostracum of the shell. The inner layer of the ligament is elastic and calcareous, 
and is often called cartilage, but this is histologically incorrect. 

The ligament lies either externally, distinctly seen dorsally between the 
prominences of the hinge edges of the valves, or internally, stretched between the 
apposed edges of the hinge, which are furnished with depressions for its reception. 
These depressions can easily be distinguished from those belonging to the hinge 
itself, since the former are alike on the two valves, whereas the furrows and other 
depressions belonging to the one face of the hinge correspond with teeth, ridges, 
etc., on the opposite face. 

When the elastic "cartilage" of the ligament is at rest, as in a dead bivalve, or 
when the adductor muscles of the living animal are relaxed, the valves open. W]ien 
the adductors contract, the "cartilage" is— ajiparently in all cases— compressed. 
On the other hand, when the adductors are relaxed, the elasticity of the ' ' cartilage " 
forces the shell open again (Fig. 61). 

The continuity established between the two valves, by means of this dorsal 
ligament, causes the Lamellibranch shell to appear to consist, strictly speaking, of 
one dorsal piece, developed to the right and left ventrally into two valves. The 
constitution of the ligament and hinge are of importance in classification. 

We must refer the reader to systematic zoological works for the special forms 
taken by the shell, and content ourselves with the following remarks :— 



62 



COMPARATIVE ANATOMY 



CHAP. 



The Lamellibranch shell is originally symmetrical, that is to say, the two valves, 
apart from the almost invariable asymmetry of the hinge, are exactly alike 
(equivalve). This is the case in most of the Lamellibranchia. The two valves 
may, however, become unlike, i.e. the shell (and to a nmoh lesser extent, and only 
in \mimportant details, the soft body also) may become asymmetrical. As far as 
we can at present judge, this asymmetry is caused by adaptation to an attached 
manner of life. 

The left valve of the Oyster is firmly cemented to the surface on which it 
rests. This valve is thicker, more convex and spacious, and forms a sort of 
basin in which the soft body lies, while the right valve acts rather as a lid, and 
is thinner and flatter, ^¥e have thus an 'upper" (the right) and a "lower" (the 
left) valve, but it is hardly necessary to point out that this use of the terms upper 
and lower has as little morphological significance as in the Pleuronectidse among the 
fishes. The attached valve is sometimes the right, sometimes the left, and this 




Fig. lil.— Diagram in illustration of the mechanism for opening and closing the Lamelli- 
branch shell. 1, 2, 3, The three layers of the sliell— 1, prismatic layer ; 2, cuticle or periostracum ; 
3, nacreous layer. A, Shell closed by the contraction of the adductor muscle (6), by means of 
which the elastic inner portion of the ligament (5) is compressed. B, Shell opened by the elastic 
pressure of the inner portion of the ligament during the relaxation of the adductor muscle. 4, 
Non-elastic outer portion of the ligament, which passes into the periostracum. 

variation may occur within one and the same genus (Chama), or even species 
(Aetheria). 

Besides the above-named, the following bivalves are also attached, and have 
dissimilar valves : Spondyhis, Gnjphcca p. p., Kcogi/ra p. p., and especially the 
fossil Hip2)uritr,s {Rudistes), in which the right valve assumes the form of a high cone 
attached by its point, while the left looks like a lid. The conical valve has, however, 
no corresponding internal cavity, but is almost entirely filled up with shell substance, 
so that, in spite of the form of the shell, the space occupied by the animal between 
the two valves is very limited. 

This same condition is found in certain fossil Chamacca. In Requwnia, the left 
valve is produced spirally and is attached by its point, while the spirally-coiled 
flattened right valve covers it like a lid, so that the whole shell closely resembles 
a Gastropod shell closed by its operculum. 

There are also free, unattached bivalves with unequal valves, e.g. many 
Pectinidic. In these animals, however, many peculiarities of organisation, such as 



VII 



MOLLUSGA—THE SHELL 



63 



the rudimentary foot, the constitution of the mantle edge, and the absence of siphons, 
indicate descent from sedentary forms. In the case of other forms with unequal 
valves, however, no such descent can be established. 

In Anomia we have an example of an inequivalve bivalve, in which the valve 
turned to the surface it rests on is iiat and the 
upper arched. The lower valve is here the right 
one, and takes the exact imprint of the surface 
on which it rests, so that, for example, the mark- 
ings of the shell of the Pecten or the Oyster, to 
which Aiioiiiia frequently attaches itself, are 
exactly reproduced. In this right attached valve 
there is a perforation into which a shelly plug, 
the calcified byssus, fits ; by means of this, the 
animal fixes itself to its substratum. The ex- 
planation of this perforation is seen in the course 
of development. It commences as a simple notch 

at the edge of the shell, as found also in other ment of the shell valves of Anomia. 
bivalves, for the passage of the byssus. By the ^' V"'*' y°™g sliell ; B, older shell with 




Fig. 62. — Three stages in the develop- 



notch for the byssus ; C, still older shell, 
the byssus notch surrounded by the shell 



further gi'owth of the shell, this notch to a 

certain extent is grown round, and thus ap- Tnd pe7srsVing"a7aTo"le"(rfter"Morse'r 

parently travels away from the edge of the shell, 

with Avhich, however, it is still really connected (Fig. 62). In related forms {Curolia) 

this aperture becomes quite filled up by a homogeneous calcareous mass. 

Impressions on the inner surfaces of the shell. — Various organs of the Mollusc, 
attached to or adjacent to the inner surface of the shell, leave more or less distinct 
impressions on this surface, which are visible when it is empty. These impressions 
are of great importance, especially to the palfcontologist, for by their means fairly 




Fig. 63. — Dimyaria, inner surface of the left shell valve. A, Cytherea cMone (SiiiupciUiafc); 
B, Lucina Pennsylvanica (Integripalliata) ; 1, impression of the anterior ; 2, impression of the 
posterior, adductor ; 3, sinus of the pallial line (4) ; 5, ligament. 

safe conclusions may be arrived at as to certain points in the organisation of the 
soft body which has disappeared. 

1. The most distinct impressions are those caused by the adductor muscles. 
Where there are two powerful adductor muscles, one anterior and the other posterior 
(Dimyaria), there are two impressions in the corresponding parts of the inner surface 
of the shell (Fig. 63). In cases where the anterior muscle is rudimentary, while 
the posterior is unusually powerful, and has moved anteriorly towards the middle 
of the shell (Monomyaria), there is only one large impression (Fig. 64). The anus 



64 



COMPARATIVE ANATOMY 



CHAP. 




Ephippium. 1. Hinge edj 
pression of adductor. 



e : 2, irii- 



is always to be found close to the posterior (which in the Monomyaria is the only) 
adductor. 

2. Parallel to the edge of the shell, aud more or less removed from it, we tiud 
oil the inner surface of the shell the so-called pallial 
line, caused liy the nmscle fibres which attach the 
edge of the mantle to the valves. 

The course taken by this line undergoes charac- 
teristic modification in such Lamellibranchs as have 
siphons ; at the posterior part of the shell it suddenly 
liends forward and upward, and then again passes 
backward and upward towards the loAver edge of the 
posterior adductor. The pallial line, in this case, 
forms an indentation, leaving a sinus or bay opening 
posteriorly, the pallial sinus, which has been utilised 
for systematic purposes {Siniqxilliatii, latcgripaJ- 
liafii, Fig. 63). The sinus marks the line of attach- 
Fio. 64.— Monomyarian, internal ment of the sipho-retractor muscles ; the stronger 
surface of a shell valve of Pema these retractors and the 1 letter developed the siphons 

the larger and clearer is the sinus. 

3. The foregoing imjiressions are the most dis- 
tinct and constant, but others may occur as well, caused by the protractors and 
retractors of the foot, by the muscles or ligaments which attach the visceral dome 
to the shell, etc. ; but these cannot be further described. 

In most Lamellibrancliia, when the shell is closed, the edges of the two valves 
meet exactly, so that the soft bofh" can be entirely enclosed and cut off from the 
exterior (closed) shell. There are, however, shells in which, in the closed condition, 
the valves gape posteriorly, or, more frequently, both posteriorly and anteriorly 
(r.;/., Mijidcr, (Hyc!imri-iih:i\ Soh'iiiOw). This is accoimted for by the great develop- 
ment of the siphons and of the foot, which can only partially (Myidce, Solenocurtus) 
or with difficulty be withdrawn into the shell. Such gaping shells are found in most 
boring bivalves, whose shell formation is specially interesting owing to the develop- 
ment of accessory valves or calcareous tubes. In this respect Pholas, Fholadidfn, 
and Jouiiiinetiii. represent the most important stages in a remarkable series. 

The shell of Pluilns is elongated longitudinally, and gapes anteriorl}- and ventral! v 
for the passage of the short club-sliaped foot, and posteriorly for that of the strongly 
developed siphons. As many as three accessory valves are developed dorsallv 
(prosoplax, mesoplax, metaplax). 

The shell of Pholadiden somewhat resembles that of Pholas. In the yount; 
animal it gapes anteriorly, as in Phulns, for the passage of the foot. Posteriori}', 
each valve is produced into a horny process, which is succeeded by an accessory 
piece (siphonoplax), hollowed out like a trough. The siphonojilax of the one valve 
often fuses with that of the other to form a single tube for the reception of the 
siphons. There are two pieces of prosoplax, while the meso- and metaplax are 
rudimentary. In the adult the boring activity is suspended, and the anterior 
opening becomes entirely closed by the secretion of au accessory piece, the callum 
(hypoplax). The funotionless foot atrophies, and the animal can move no farther 
in the substance into which it has bored. 

The shell of the adult Joiumnrtvi. is much shortened longitudinally, and is 
globular, and the animal cannot move in the round hole it has bored for itself in a 
block of coral. Any alteration in its position in the hole, which might lie fatal to 
the animal, is avoided by means of a posterior tongue-like process of the shell 
which, however, only belongs to the right valve. The shell is completely closed 
anteriorly, and a foot is wanting (c/. also Figs. 27, 28, p. 19, and 66, p. 67). 



VII MULLUSCA—THE SHELL 65 

The adult condition of JuitauneHa is explained by its developmental history. 
The shell of the young animal is like the segment of a sphere, whose greatest height 
is hardly half of the radius. It covers the dorsal upper portion of the body, its free 
edges thus bounding a very wide aperture, which corresponds with the anterior pedal 
gape of Pliolas. 

In this PAote-stage, in fact, Jouannetia really possesses a foot. Twisting the 
body about and rasping the stone with the anterior edge of the shell, the animal 
excavates a hole, which is spherical in consequence of the shape of its shell. "When 
this hole is made, new accessory shell material is secreted at the free edge of the 
shell ; this forms the "callum," and as the edge of the mantle follows the lines of 
excavation, the form of the accessory shell is here (as in Trrrdo) determined by the 
form of the hole, and the sphere of which the original shell was but a segment is 
completed. 

Setting aside a few related forms {Martesia, TcrcdiiiK., Xylopliaga, Gastrochaena, 
and Fistulaiia), in which the conditions are somewhat similar, we come to the ship- 
worm Teredo (Fig. 29, p. 20). This animal has a long tubular mantle which is 
produced posteriorly in two long siphons. The body lies at the anterior end of the 
mantle. Teredo bores cylindrical passages in wood. The valves of the shell are 
very small in comparison with the body ; they take the form of tri-lobate pieces, 
which encircle the anterior end of the mantle. This rudimentary shell gapes 
anteriorly for the passage of the pestle-shaped foot, and very widely posteriorly. 
The mantle further secretes over its whole surface a calcareous tube which lines its 
buiTow, but which does not fuse with the shell valves. Two small accessory shell- 
pieces, the so-called "palettes," lie at the place where the siphons separate. If the 
anterior portion of the animal reaches {i.e. if it Lores through to) the water, the 
calcareous tube is rounded off and closed. 

Aspergillum (Brechites, Fig. 30, p. 20, and Fig. 65) and Clamtgella show similar con- 
ditions. In the club-shaped shell, which inserts its anterior thicker end into rock, 
shell, coral, or sand, we can distinguish a true and a false shell. The false shell 
forms by far the larger portion of the tube, and corresponds with the secreted tube 
of Teredo, and with a callum like that of Pholas. The true shell is very small 
and lies anteriorly. The two valves of this true but rudimentary shell are, in 
Aspergillum, placed saddle-like over the anterior end of the tube, with which they 
are firmly fused (Fig. 30, p. 20). Were they isolated, their gape would be 
unusually wide, not only anteriorly and posteriorly, but ventrally. The shell-tube is 
open posteriorly, over the apertures of the siphons ; anteriorly, however, it is closed 
(in the adult) by means of a disc perforated like the rose of a watering-can, which 
corresponds in position with the callum of the Pholadidcc. The perforations at the 
edge of the disc, or even over its whole surface, are sometimes produced into cal- 
careous, and at times dichotomously branched tubules. In the middle of the disc 
there is sometimes found a narrow slit-like aperture corresponding with the pedal 
aperture in the mantle beneath, but this is often wanting. Less frequently, we find 
another aperture in the ventral middle line, corresponding with the fourth mantle 
aperture above described (p. 51). 

Aspergillum buries its anterior end in mud or sand, but its whole organisation, 
and especially its shell an-angement, point to a former boring mode of life. 

Clavagella, which is nearly related to Aspergillum, bores into rock or the cal- 
careous shells of various other animals. The arrangement of its shell differs from 
that of Aspergillum chiefly in the somewhat gi-eater size of its true valves, and in 
the fusion of only the left valve with the calcareous tube, the right lying free 
within that tube. 

In the Phokcdidce, the ligament, which is still foimd at the hinge, no longer acts 
for opening the shell. In consequence of a peculiar arrangement of the anterior 
VOL. II r 



ee 



COMPARATIVE ANATOMY 



CHAP. 







VII MOLLUSOA—THE SHELL 67 

adductor, the opening of the shell, such as it is, is brought about by the muscles. 
The anterior and upper edges of the valves are bent outward, and to these edges the 
anterior muscle is attached. We thus have external instead of internal points 




1 
FiCr. 66. — ^Pbolas dactylus, right valve, internal aspect (after Egger). 1-2, Axis round which the 
valves move upon one another ; 3-4, longitudinal axis of the shell ; 5-8, line connecting the shell 
muscles ; 6, anterior muscle ; 7, posterior muscle ; 0, rotating point of the valves ; 10, anterior and 
upper edge of the shell, which is bent outwards, and to which the muscle 6 is attached ; 6-0, shorter 
anterior ; 9-7, longer posterior arm of the lever. 

of attachment, and the whole shell may be compared to a double -armed lever 
acting along the longitudinal axis of the body, its fulcrum being at the point where, 
in other bivalves, the hinge is found. When the anterior muscle contracts, the shell 
opens posteriorly and ventrally ; when the posterior adductor contracts, the shell 
closes (Fig. 66). 

D. Cephalopoda. 

The Cephalopoda are all to be derived from an ancient fossil form which possessed 
a chambered shell, in the last and largest portion of which the animal lived, leaving 
the rest of the shell empty, or rather filled with gas (or water) and traversed by 
the siphon or siphuncle. Such a shell is now found only in the sole living repre- 
sentative of the Tetrairanchia, the Nautilus, an animal of great importance to the 
comparative anatomist. Many fossil forms allied to the Nautilus, and grouped 
in the order Nautiloidea, possessed such a shell, as did also the Ammonoidca, with 
their enormous wealth of forms which, rightly or not, are generally considered to 
be nearly related to the Nautiloidea, i.e. to belong to the Tetrahraiiehia. In nearly 
all these animals the shell, when coiled at all, is, unlike the Gastropod shell, coiled 
anteriorly or exogastrically. 

One group of the Nautiloidea, the Endocei-atidce, which includes only very old 
forms (Cambrian and Lower Silurian), is distinguished by the fact that the chambers 
of its straight shell, which were filled with gas (or water), lay at the side of and not 
behind the inhabited chamber. There was no real siphuncle, but the upper end of 
the visceral dome, much narrowed by the air chambers, stretched as far as to the 
apex of the shell. 

In other Nautiloidea, jthe air chambers always lie, as in Nautilus, 'above the 
occupied chamber, and are traversed by a thin membraneous siphuncle, which, how- 
ever, in old forms, is much thicker, and represented the narrow prolonged portion of 
the visceral dome (Fig. 32, p. 22). 

Some forms of Nautiloidea have shells coiled endogastrically ; this is never the 
ease, however, when the shell forms a complete spiral. The sutures, which corre- 
spond with the lines of insertion of the septa, are simple in the Nautiloidea, as 



68 COMPARATIVE ANATOMY chap. 

compared with those in the Amntunuidca, in which they are folded in a comi)licated 
manner. 

Nautiloidea. — In the following table we have the chief forms of the shell among 
the y<futi/oidca : ^ — 

{a) (hiluiceras group.— Shell straight or slightly bent. Silurian— Trias. 

(b) Cjirti'crras group. — Shell curved like a horn, but not regularly spirally 

coiled. Cambrian — Permian. 

(c) I'liinicems gi'oup. — Shell regularly spirally coiled, the coils, however, not 

touching each other. Silurian — Permian. 

(d) XaiUilus group. — Shell regularly spirally coiled, the coils touching, or the 

outer clasping the inner. Silurian — recent. 
(c) Liluitcs. — Shell at first regularly spirally coiled, straightening later. 
Silurian. 

The siphuncle runs either through the centre of the septa, or through their 
anterior or posterior sides. 

Ammonoidea. — The shells of the (fossil) Ammonoidea are distinguished by very 
comjilicated sutures, their zigzag lines are like the outlines of sharply-indented leaves 
or richly-branched mosses, they are due to the extraordinary folding of the edges of 
the septa, which are attached to the inner surface of the shell. The siphuncle is 
always very thin in the Ammonoidea, and almost always pierces the septa on the 
posterior side. 

The following quotation summarises the chief peculiarities in the form of the 
Ammonite shell : — - 

"The shell, as a rule, forms a closed symmetrical spiral, the coils touching or 
clasping one another. Some of the oldest forms are straight, or in youth incom- 
pletely coiled. In certain groups of the Ammoiioidea^ we find a tendency repeated at 
different times (Trias, Jurassic, Chalk) to depart from the close symmetrical spiral, 
and to adopt what are called accessory forms. The first step in this process of change 
is in most cases the detachment of the occupied chamber from the next inner whoi'l ; 
then, little by little, the inner whorls also separate, though they still remain in the 
same plane — the Criuecrus stage. Sometimes the shell grows straight for a time, 
then becomes hooked — the Anciihio-rKsa.nA Samites sta.ges, and, if only the occupied 
chamber separates from the coiled part — the ScnpJi itrs stage. Finally, entirely straight 
shells arise in the Baeulites stage. Rarely, the coils leave the symmetrical plane and 
assume the shape of a snail's shell ; in this case, the shells may be either closely or 
loosely coiled, — the Turrilites stage." 

Dibranchia. — The shells of all known Dibrcmehia, extinct or recent, are more or 
less rudimentary, since they are never capable of sheltering more than a small portion 
of the animal. Further, they are always internal, on the anterior side of the visceral 
dome, and are overgrown by a fold of the integument. In Spiru/a (Fig. 33, p. 23) 
alone, the shell is not completely overgrown, a portion at the apex of the visceral 
dome remaining uncovered. 

The shell of the (fossil) Belemnites (Fig. 67 C) is straight, conical, and chambered; 
the septa are near one another, and are pierced on the posteiior or ventral side by 
the thrcail-like siphuncle, which is enclosed in short, calcareous sheaths. The apex 
of the shell (phragmocone) is protected by a conical calcareous sheath (rostrum or 
guard), the only part usually preserved. The anterior wall of the last chamber 
is produced downwards into a broad thin process, the pro-ostracum. 

In Sjiii-aliri/stm (Fig. ti? D), the jihragmocone begins to bend posteriorly (endo- 
gastrically). The rostrum is triangular and pointed at the top. 



Steinmann-Ddderlein, Elemente der Palaontologie, 1890. 2 jhid. 



VII 



MOLLUSC A— THE SHELL 



69 



In Spiruta (E), the shell is colled spirally and endogastrically. The siphuncle 
is thick, and is surrounded along its whole length hy septal envelopes. The rostrum 
is rudimentary, and there is no pro-ostracum. 

Starting again from the Belemnites, the modification of the shell may take another 
direction. The phragmocone may become smaller and shorter in comparison with 
the continually lengthening pro-ostracum {e.g. Ostracoteulhis, F). The rostrum 
also may become thinner and smaller. Finally, the shell may be reduced to a very 




Fig. 67.— A-H., Diagrammatic median sections tlirougli the shells of eight extant or fossil 
Dibranchla, from the left side. The point of the visceral dome is turned downward.s, the posterior 
side of the shell is to the left and the anterior to the right (c/. the position of the Ceplialopod body, 
p. S6). A. Sepia ; B, Belosepla (fossil) ; C, Belemnlte (fossil) ; D, Spirulirostra (fossil) ; Ji, 
Spirula; if, Ostraooteuthls (fossil); ff, Ommastrephes ; H, Lollgopsls ; p/i, chambered shell = 
phragmocone; pr, pro-ostracum; r, rostrum (guard); s, siphuncular canal, or space which con- 
tains the siphuncle ; 1, 2, 3, last three septa (the most recent) ; a, anterior wall of the siphuncle ; p, 
posterior ; x, anterior edge of the first septal or siphuncular envelope = anterior or posterior cilge of 
the siphuncular canal. 

small hollow cone at the end of a long narrow horny lamella which corresponds 
with the pro-ostracum, and is called, in the extant Decapoda, the gladius or calamus 
(or pen) {Loligo, Ommastrephes (G), Onydwteuthis). In Dosidicus, this terminal cone 
is almost solid, and in LoUgopsis (H) it is nothing more than a thickening at the 
upper end of the gladius ; in other Decaiwda, there is no trace of it on the gladius. 
In the Odopoda, the shell has completely disappeared. 

Again starting from the Belemnitc, the shell may develop in a third direction 
to form the Sepia shell. The transition form is found in Belosepia (B) (Eocene), 



70 



COMPARATIVE ANATOMY 



CHAP. 



that is, if this interpretation is correct. This shell is somewhat bent, the septa 
are crowded together and slope downwards anteriorly. They are penetrated 
posteriorly by an extremely thick siphon, which is enclosed throughout its whole 

length in an envelope with a very thick anterior 
wall. The completely enclosed siphuncular 
space is thus a wide funnel running through the 
chambers of the shell on its posterior side (Fig. 
67 B). The phragmocone is enclosed in a thick, 
strongly - developed rostrum, and its anterior 
and lateral walls are produced downwards into 
a broad, posteriorly concave shell (pro- 
ostracum ?). 

These arrangements seem to have culminated 
in the extant Sepia (Figs. 67 A and 68). The 
siphuncular space fits over the visceral dome 
like a moiUd. The anterior portions of the 
septa slope downward much more obliquely 
from behind anteriorly, so that, in a back view 
of the shell, the whole area of the last septum is 
visible at the surface (Fig. 68, 1). The septa 
are thin calcareous lamellae, closely superim- 
posed one upon the other, with very narrow air 
chambers between them ; and these latter are 
traversed by perpendicular trabeculfe. The shell 
is thus very light, its specific gravity is less than 
that of water. Behind the siphuncle, on the 
posterior very much shortened side of the shell, 
the short septa are closely contiguous, without 
any intervening air spaces. 

The dorsal end of the shell is enclosed in a 
small pointed rostrum. The whole anterior sur- 
face is covered by a thin lamella of conchyolin, 
which projects laterally beyond the edge of the 
shell, and is itself covered by a calcareous layer 
which is an anterior and ventral extension of 
the rostrum. 

The female Argonaut is the single exception 
to the rule stated above, that in the Octopoda 
the shell has entirely disappeared. This animal 
has a light, thin external shell coiled anteriorly 
or exogastrically, which is not firmly attached 
to the body at any point, and serves more for 
receiving the eggs (Figs. 35, 36, pp. 24, 25) than 
for protecting the body. This shell is sur- 
rounded and secured by lobate processes of the 
anterior pair of arms. It has no nacreous layer, but is porcelanous, and is 
apparently produced from the integument of the visceral dome and the mantle. 
The dorsal pair of arms is said only to deposit the so-called black layer on its 
surface. 

It is usually considered that this Argonaut shell is not the homologue of the 
shell of other Cephalopods, but is a formation peculiar to the Argonant female. An 
opposite view has, however, recently been very ably advanced — that the Argonaut 
shell is an Amtnvnite shell which has lost its septa and siphuncle and also its 




a 

Fio. 68. — Shell of Sepia aculeata. 

Posterior (physiologically ventral) aspect. 
Lettering as in Fig. 67. The last septum 
1 is seen in its whole extent ; s, the mouth 
of the broad, slipper-shaped siphnncular 
cavity ; I, lateral wall of the cavity ; . 
line of the section which in Fig. (57 A is 
diagrammatised. The two figures should 
be compared (principal details after 
D'Orbigny). 



vil MOLLUSCA—THE PALLIAL COMPLEX 71 

nacreous layer. ^ Should this view prove correct, the Cephalopods would have to 
be differently classified. The division into Tetrabranehia and DibrancMa would 
have to disappear, as wo cannot tell whether the fossil Ammonoidea were Tetra- 
branehia, and are also ignorant as to when the Dibranchia developed from the Tetra- 
branehia. The Cephalopods would then have to be divided into (1) Nautiloidm 
with the extant genus Ncmtilus ; (2) Ammonoidea with the still living Odopoda ; and 
(3) Belemnoidcit Avith the extant Decapoda. 

Bivalve shelly plates called aptychi have been found sometimes in the last 
chamber of the Ammonoidea, sometimes isolated. These have been proved to belong 
to the bodies of certain species of Ammonoidea, and have been considered by some to 
be protectives for the nidamental gland, by others as opercula, and by others again 
as the analogues or homologues of the infundibular cartilage of the Decapoda. No 
one of these three views has as yet been generally accepted. 



V. Arrangement of the Organs in the Mantle Cavity and of 
the Outlets of Inner Organs in that Cavity. 

A discussion of this subject at this stage will help to explain the asymmetry of 
the Gastropoda and to simplify the discussion in later chapters. 

There are, in the mantle cavity, many important organs crowded.together in a 
comparatively small space, and into it also open all the apertures of the inner organs 
except the oral aperture of tlie alimentary canal. The term " circuni-anal complex," 
though especially apjilicable to the arrangement in the Gastropoda, is not so suitable 
as "pallial complex," which applies to nearly all Mollusca, and comprises not only 
the pallial organs themselves, but the apertures of inner organs that lie in the 
mantle cavity. 

The most important constituents of the pallial complex are the cteuidium, the 
osphradium (Spengel's organ, olfactory organ, or accessory gill), the hypobranchial 
gland, the anus, and frequently the rectum as well, the nephridial apertures and 
often the renal organ also, the genital apertures, and frequently the pericardium, 
with the enclosed heart. 

Starting with the Chitonidcc, which, as has already been described (p. 42), 
must be considered as the most primitive of all living Molluscs, we have : — 

The median anus, lying at the posterior end of the body in the mantle groove ; 
on each side of it anteriorly the nephridial apertures, and again on each side, in 
front of these, the genital apertures. 

Assuming this to be the primitive arrangement, we have the following important 
variations. 

A. Gastropoda. 

1. Prosobranchia. 

n. Diotocardia. — In Fissurella, the pallial complex is still quite symmetrica], 
but instead of lying posteriorly, as in Chiton, it, together with the mantle and the 
pallial cavity, lies on the front of the visceral dome. We have to imagine that the 
whole complex has shifted forward along the right side of the body, so that the gill 
originally on the left has come to lie on the right anteriorly, and that originally on 
the right now lies anteriorly on the left, and the same applies to the other organs 
belonging to the complex. 



Steinmann, Bericht Freiburg Gesellsch., iv. pp. 113-129. 



GOMPARATIl'E ANATOMY 



CHAP. 



In order to prevent confusion, the liypotlietical original position of each organ 
will be denoted by ur ( = originally right) and iil ( = originally left) in brackets. 

In the upper part of the mantle cavity in Fissurella, beneath the median 
aperture in the mantle and shell, lies the anus, and immediately to its right, the 
right (ul) nephridial aperture, immediately to its left the left («(r) nephridial 
aperture ; the right (ul) and left (iir) ctenidia, again, lie symmetrically to the right 
and left. There are no distinct osphradia, and the genital apertures are wanting as 
the genital gland opens into the right nephridium. 

Haliotis. — The mantle cavity has here shifted to the left, and the rectum, 
attached to the mantle fold, runs forward some way through it, so that the anus is 
at a considerable distance from the posterior apex of the cavity. On the right of the 
rectum lies the right (m?), and to its left the left (itr) ctenidium, both fastened to 
the mantle, and stretching far forward. The right and left nephridial apertures 
lie near the bases of the ctenidia, in the upper and posterior part of the mantle 




Fig. 
side. 



70.— Tlie same specunen from the left 
Lettering as before ; o, mouth. 



Fig. 69. — Anterior portion of Patella, from 
above, after removal of the mantle folil (after 
Ray Lankester). a, Tentacle ; b, foot ; (, 
pedal muscles (shell muscles) ; rf, osphradia ; 
(7, mantle fold ; /, aperture of the right neplii i- 
dium ; g, anal pajjilla and anus ; 7(, papilla and 
aperture of the left nephridium ; i, left nephri- 
dium ; 7u, right nephridium ; I, pericardium ; 
It, digestive gland (liver); ?», cut ed.^T of the 
mantle ; ji, snout. 

cavity. Between the rectum and the left ctenidium, also on the mantle, is found 
the long, well-developed hypobranchial gland (mucous gland), which stretches as far 
forward as the gill. Only a small portion of the gland lies to the right between 
the rectum, as far as it runs, and the right ctenidium. There are two osphradia 
which run as bands along the axes of the ctenidia facing the mantle cavity. 

Turbinidse and Trochidse. — Only the left {%ir) ctenidium of HoUotis is here 
retained ; it lies far to the left on the roof of the mantle cavity, i.e. on the mantle. 
The rectum runs far forward along this roof. Two nephridial apertures lie on 
papilla? in the base of the cavity, at the sides of the rectum. The hypobranchial 
gland is found in various stages of development, the highest being attained in the 
Tvrbinidcc. It is largest between the rectum and ctenidium, i.,-. between the 
right side of the latter and the left side of the former. In the Tvrhiniikc, however, 
a portion of it lies to the right of the rectum. There is a diffuse osphradinm on the 
axis of the gill. 

Neritina.— There is here only one gill (the left («r) h\Haliotis)A\iiteA somewhat far 
to the right. The rectum lies asymmetrically to the right in the respiratory cavity, 



YII 



MOLLUSGA—THE PALLIAL COMPLEX 



73 



reaching so far forward that the anus is found near the right edge of the mantle cleft. 
There is only one nephridial aperture to the left of the base of the ctenidium, far up in 
the mantle cavity. The inner surface of the mantle, between the rectum on the right 




Fio. 71.— Pyrula tuba, male, taken out of the shell (after Souleyet). The mantle is cut open 
along its base and right side, and laid back to the left ; the position of the pallial organs is thus 
reversed. 1, Proboscis; 2, snout ; 8, foot ; 4, penis ; 6, seminal duct, which is continued at 16; 
6, floor of the pallial cavity = nuchal integument; 7, columellar muscle; 8, intestine; 9, heart ^in 
tiic opened pericardium ; 10, digestive gland (liver) ; 11, testes ; 12 and 13, renal organs ; 14, renal, 
aperture ; 16, seminal duct ; 10, rectum ; 17, hypobranchial gland ; IS, anus ; 19, ctenidium (gill) ; 
20, mantle ; 21, osphradium ; 22, respiratory siphon. 

and the gill on the left, is glandular, and represents the slightly differentiated hypo- 
branchial gland. The genital aperture lies close to the anus. 

Docoglossa.— In the Patelliihi: (Figs. 69, 70) a short conical portion of the 



74 COMPARATIVE ANATOMY chap. 

rectum projects into the small mantle cavity. This anal coue is not median, but 
is distinctly shifted to the right. To its right and left lie the nephridial apertures, 
raised on short conical papillae. There is no separate genital aperture. In some 
forms (Tfctvm, Scuviia, Acnuea) one ctenidium is found attached to the mantle, on 
the left side of the pallial cavity. Further details as to the gills in the Patellidce 
will be given later on. AYe further find, on the floor of the cavity, on each side, 
traces of an osphradium in the shape of a small patch of sensory epithelium, which 
may be raised on a prominence. It is doubtful if the prominence found in Patella 
close to each osphradium, containing a blood sinus divided up by septa, can be 
considered as a rudimentary gill. These prominences rise from the floo7- of the 
mantle cavity, whereas in Tectum, for example, iu which a true gill still occurs on 
the left, it lies far removed from the left osphradium, in the usual position on the 
roof of the cavity, i.e. on the inner surface of the mantle. 

b. Monotocardia. — In this division, the numerous forms of which show little 
variety of organisation, the arrangement of the pallial complex is very uniform. 
The single genital aperture is always distinct from the single nephridial aperture. 
The position of the organs in the spacious pallial cavity (Fig. 71), from right 
to left, is as follows : — 

1. To the extreme right, lies the afferent duct of the genital organs (ovary or 
seminal duct), which runs more or less far forward, in tlie mantle cavity. 

2. In contact with this, but quite on the roof of the cavity, is the rectum. 

3. To the left of the rectum, far back in tlie base of the mantle cavity, lies the 
slit-like nephridial aperture, which pierces the wall separating the cavity from the 
renal organ behind and above it. Exceptions occur in Paliulina and Valvata, in 
which this aperture is shifted forward to the end of a urinary duct which runs on 
the mantle. 

4. On the roof of the mantle cavity are found the hypobranchial glands (mucous 
and purple glands), which are developed in varying degrees. 

5. Qriite to the left, and also on the roof of the cavity, the ctenidium, feathered 
on one side (the left (vr) oi Haliotis and Fisswrella), at whose base, deep back in the 
cavity, the pericardium is visible with the ventricle and auricle seen through it. 

6. Finally, to the extreme left, lies the osphradium, which is always well 
developed and sharply circumscribed, and is either filamentous or feathered on two 
sides, and attached to the roof of the pallial cavity. 

The position of the organs in the pallial complex of the Heteropoda, certain forms 
of which, such as Atlanta, are closely related to the other MoiwfoennHa, requires to 
be re-investigated. The osphradium lies at the base of the gill. 

2. Pulmonata. 

In the Pulmonata, the single or double ($ and i ) genital aperture (Fig. 72) no 
longer belongs to the pallial complex, but lies outside the mantle cavity laterally on 
the head or neck. In Oncidium the male aperture lies anteriorly under the right 
tentacle, the female posteriorly, near the anus. 

Bearing in mind that the mantle or pulmonary cavity communicates with the 
exterior only by means of the respiratory aperture lying on the right, we have the 
following arrangement of the pallial complex as typical (excluding such aberrant 
forms as Daudebardia, Tcstacella, and Oncidium). 

1. On the extreme right of the pulmonary cavity lies the rectum, the anus 
opening in the respiratory aperture. 

2. On the roof at the back of the cavity lies the nephridium (kidney). 

3. To the left, near the kidney, also far up in the cavity, and on its roof, lies 



VII 



MOLLUSGA—THE PALLIAL COMPLEX 



75 



the pericardium, oontaining the ventricle and auricle, the latter lying in front of 

s 




Fig. 72. — Helix aspersa, fully extended from the right (after Howes), a, Anus appearing in the 
respiratory aperture, pZa ; s, shell ; />, edge of shell apertiu-e ; ga, genital aperture ; ^i, optic tentacle ; 
t, anterior tentacle ; U, upper lip. 

the former. From the ventricle the aortic trunk runs upward and backward, and 
from the auricle rises the pulmonarj' vein, 
which riins forward along the roof of the 
pulmonary cavity. 

4. The respiratory vascular network 
spreads over the whole remaining surface 
of the roof of the pulmonary cavity, and 
is thus in front of the kidney and peri- 
cardium. 

5. An osphradium has till now only 
been found in the Basomviatopliora {Plan- 
orbis, Physa, Limnaeus), near the respira- 
tory aperture, and among the Stylommato- 
phora in Testacella on the floor of the 
pulmonary cavity at its extreme posterior 
angle. 

The floor of the pulmonary cavity (the 
dorsal nuehal integument) is smooth and 
devoid of organs. 

The arrangement of the efferent ducts 
of the renal organ varies and deserves 
special description (Fig. 73). 

1. The anterior side of the renal sac 
opens on a simple papilla in the mantle 
cavity {Bidimus ohlaiigits, and some species 
of Planorbis) (Fig. 73 A). 

2. The papilla lengthens and runs for- 
ward as a straight ureter (primary ureter). 
This occurs in most BasominaUyphora, and 
some species of BuHjiius, Cimiella, Pupa, 
Helix (B). 

3. The ureter runs backward along the 
kidney, and opens at the base of the respiratory cavity, 
of Helix (C). 

4. A secondary urinary duct is added, becoming constricted from the wall of 




Fig. 73. — Six diagrams Illustrating the 
variations in the renal ducts in the Pul- 
mouata. The organs are supposed to be seen 
through the mantle above them. 1, Free edge 
of mantle ; 2, respiratory aperture ; 3, rectum ; 
4, kidney ; 5, pericardium ; 6, auricle ; 7, ven- 
tricle ; 8, primary urinary duct ; 9, secondary 
urinary dnct, which, in D, is a groove. Further 
explanations found in the text. 



Testacella, and some forms 



76 GOMPARATIVE ANATOMY chap. 

the pulmonary cavity, and at first forming a more or less closed channel along 
which the urinary discharge can be forwarded from the base of the cavity to the 
respiratory aperture. Some species of Bulimus and Helix (D). 

5. The secondary urinary duct becomes closed, and opens either alone or with 
the anus into the pulmonary cavity. Some species of Buliimis, Eelix, Dmidebardia, 
Vitrina, Hyalinia, Zonites, Arion, etc. (E). 

6. The end of the secondary urinary duct and the end of the rectum together 
form a cloaca which is distinct from the pulmonary cavity," and opens close to 
the respiratory aperture. Umax, Amalia, and some species of Davdebardia (F). 

When the primary urinary duct runs back along the kidney it is externally in- 
distinguishable from the substance of the latter, and it thus often appears as if the 
duct rose from the posterior end of the renal organ. 

The variations which occur in the position of the organs of the pallial complex 
in the carnivorous Pulmonata are specially interesting. In a series of car- 
nivorous forms, commencing probably with HyaUmia among the Stylommatophm-a, 
and proceeding through Daudebardiu. to the extraordinary genus Testacella, we find 
progressive diminution of the visceral dome and its displacement to the posterior 
end of the body, simplification and diminution of the shell, and further, a shifting 
back of the liver and genital organs from the visceral dome into the nuchal portion 
of the ccelom, which now is found along the whole length of the dorsal surface of the 
foot. Finally, in Testacrlla and certain Daudehardia, the visceral dome completely 
disappears, and the pulmonary cavity covered by the shell is alone left, the cavity 
reaching up to the apex of the shell. The floor of this cavity, and indeed the whole 
cavity, with the mantle and the shell, sink down into the body. In this way 
Testacella, which follows its prey, the earthworm, into its underground passages, is 
admirably adapted to its manner of life ; its body is slender, and the somewhat 
flat shell at its posterior end, which does not stand out above the surrounding sur- 
face of the body, in no wa}' hinders its movements. These alterations, however, 
especially the displacement of the visceral dome to the posterior end of the body, 
are accompanied by important alterations of position in the pallial organs, which 
finally lead to the condition called opisthopneumonic. 

It is important to note that concrescence of the mantle and the subjacent dorsal 
integument is complete except at the respiratory aperture on the right, and that the 
latter shifts farther and farther back, in its relation to the pulmonary cavity, till, in 
Testacella, its position is almost terminal. 

The first important step in the displacement of the pallial organs is seen in 
Daudelardia rufa. The pericardium, instead of lying far back at the base of the 
pulmonarj' cavity, here lies far forward on its roof, so that by far the greater portion 
of the vascularised pulmonary tissue lies on the roof behind the pericardium (Fig. 
74 A). Daudehardia rufa is thus actually opisthopneumoniG. But in this case the 
relative position of the ventricle and auricle is still unaltered. The auricle is, as 
before, placed in front of the ventricle ; the pulmonary vein from the auricle is thus 
obliged to bend round in order to run backward, while the aorta, which becomes 
almost exclusively the anterior or cephalic artery, supplying that portion of the body 
which lies in front of the visceral dome (by far the greatest part), must bend forward 
from the ventricle. 

In another Daiidebardia, D. saulcyi, the case is somewhat similar, but the 
kidney and pericardium together form a sort of sac which hangs down into the pul- 
monary cavity from its roof. In this sac, the ureter lies dorsally and the peri- 
cardium ventrally to the kidney. The floor of the cavity sinks right and left deep 
into the subjacent region of the body. 

If we imagine that the pulmonary vein which runs back from the anteriorly 



VII 



MOLLUSGA—THE PALLIAL COMPLEX 



77 



placed auricle, and the aorta which runs forward from the chamber lying behind the 
auricle have pulled these chambers round in such a way that the flow of blood can 
have a straight course (c/. diagram, Fig. 74), the ventricle will then come to lie 
in front of the auricle. Indeed, the pericardium (with the ventricle and auricle) 
has actually twisted round 180°. In this twisting it has been followed by the 
kidney, which is connected with it by the reno-pericardial aperture, so that the 
latter organ no longer lies to the right but to the left of the pericardium, the ajier- 
ture of the urinary duct remaining at its former place. The whole reno-pericardial 
complex, as compared with its typical position in the Pulmonata, is quite reversed. 
This reversal is characteristic of Te.stacr/la. 

It is, further, noteworthy that, in TestaceUa, the floor of the pulmonary cavity 
becomes invaginated anteriorly into the body below it to form a large air sac. The 
walls of this sac are not supplied with blood vessels, and it seems to serve merely as 
a reservoir of air. In many Testace/lidcc the reno-pericardial complex hangs down in 
the shape of a sac into this air sac from the roof of the pulmonary cavity. 

In the VagiiiuHdce and the Oncidia the arrangement of the organs, originally 
belonging to the pallial complex, deviates still further from the type. A shell is 




Fig. 74. — Diagrams to illustrate tlie changes of position in tlie pallial organs of Daude- 
bardia and TestaceUa (adapted from figures by Plate). :\Iaiitle organs drawn as in Fig, 73. A, 
Daudebardia nifa ; B, Hypothetical stage, the pallial comple-K of A twisted round 90° ; C, 
TestaceUa. 1, Respiratory aperture ; 2, kidney ; 3, ureter or urinary duct ; 4, reno-pericardial 
aperture (renal funnel) ; b, ventricle ; 6, auricle ; 7, aorta ; S, pulmonary vein ; 9, pulmonary 
vascular network. 



wanting in the adult and a mantle also ; and the mantle- or pulmonary cavity 
seems in consequence to have atrophied. The pericardium lies posteriorly to 
the right, sunk into the integument, the ventricle lying, as in TestaceUa, in front 
of the auricle. Respiration takes place principally through the skin ; in the amphib- 
ious Oncidia it is assisted by dorsal papilla3. In Vugiindiis, the urinary duct joins 
the proctodaeum to form a cloaca which somewhat widens at the point of junction, 
and opens externally at the posterior part of the body. The same is the case in 
most Oncidia, but in OnrMivia cclficum, the urinary duct and the rectum emerge 
separately, but one close to the other, at the posterior end of the body. Close to 
these apertures lies, in all cases, the female genital aperture ; the male aperture, 
however, lies anteriorly to the right below the tentacle. 

The cloaca just mentioned, which is iilled with air, has given rise to interesting 
discussions. From its wall there rise into the lumen closely packed folds, which 
may also be continued along the posterior portion of the urinary duct. The cloaca 
has therefore been considered by some to be a rudimentary pulmonary cavity, into 
which the urinary duct and the rectum open. The present writer holds the opinion, 



78 



COMPARATIVE ANATOMY 



CHAP. 



provisionally, that this cloaca has arisen by the junction of the terminal portions of 
the secondary ureter with the rectum, as in other Pulmonata, but that here the pul- 
monary cavity having atrophied, it opens outward direct, /. e. no longer through a 
respiratory aperture. Others, again, have thought the arrangement in Oncidium 
and Vaginmlus to be primitive, the pulmonary cavity appearing here first as an 
insignificant widening of the terminal portion of the primary ureter. 

If this were the case, then the condition described above (p. 76, 1) for Bulimus 
oblongus, where the kidney opens on a papilla direct into the base of the pulmonary 
cavity, would be thus explained : the pulmonary cavity would have to be considered 
as a much widened primary urinary duct. Then, in this primary ureter (pulmonary 
cavity) would follow the successive stages of the development of the secondaiy 
ureter, at first an open and later a partially closed channel, and finally a closed 
tube, so that at last, as in Helix pomatia, the primary ureter is divided into two 
distinct portions, viz. the much widened pulmonary cavity and the secondary 
ureter. But in the Liinncvidcc, for example, the pulmonary cavity admittedly 
corresponds with the mantle cavity of other Gastropods. The Pulmonata would 
thus fall into two groups, the Nephropneusta {Stylommatophora), in which the 
pulmonary cavity = the widened primary ureter, and the Branchiopneusta {Basoin- 
matophora, p. parte), in which the pulmonary cavity = the mantle cavity of other 
Gastropods. 

We consider this view incorrect because of the uniformity of the whole organisa- 
tion in the Pulmonata, and especially because of the occurrence of an osphradium in 
the pulmonary cavity of a Styloinmatophore (Nephropiwiista), viz. in the genus 
Testaeella. For the osphradium invariably belongs to the mantle cavity, being 
primitively connected with the etenidium, it never lies in the urinary duct. 



3. Gastropoda Opisthobranchiata. 

We can here speak of a pallial complex only in connection with the Tectihi-anchia, 
since in them alone is a distinct mantle fold developed on the right side of the 




Fig. 75.— Aplysia, right aspect, the right parapodiuni (15) turned downwards; the pallial 
complex is seen under the mantle fold 7 (after Lankester). 1, Anterior tentacle ; 3, eyes ; 3, 
posterior tentacle (rhinophore) ; 4, left parapodiuni ; 5, seminal furrow ; 6, genital aperture ; 7, 
mantle fold ; 8, gland ; n, osphradium ; 10, outline of some inner organ seen through the integument ; 
11, nephridial aperture; 12, etenidium; 13, anus; 15, right parapodiuni; 16, anterior portion 
of the f Kit. (There should he no connecting line between 6 and 9.) 

body. The general order of the organs in the pallial cavity (Fig. 75) is as follows :— 

1. Far back, and often hardly or not at all covered by the mantle, sometimes at the 

summit of a conical prominence, lies the anus, and near it occasionally an anal gland. 



VII 



MOLLUSGA—THE PALLIAL COMPLEX 



79 



2. In front of tlie anus, between it and the otenidium, is the nephridial aperture. 
Following these there may be — 

3. A hypobranchial gland. 

4. The otenidium. ■ ^- - 

5. At the base of the otenidium or on its axis, 
the osphradium. 

Were this complex of organs to be shifted along yKt\ . 8 

the edge of the body, we should have the arrange- 
ment found in the MonotocanHa. among the Frosu- 

braiichia. The correspondence is, however, appar- . I Y^'j-V 9 

ently marred by the position of — "1 ( / P in 

6. The genital aperture, which in the Opistho- 
branohia lies farthest forward of all the pallial 
organs. -i^ 

In all other Opisthobranchia (after excluding the 1 --'^ H 

Tectibranchia) the pallial complex is broken up ^ i _, ., 

when the mantle and the true otenidium disappear. 5 ]_f A \ 

The only e.xoeption to this is found in the PJiyl- 6 -'" 

lidiidce, where, apart from the gills, a similar 1 / X.V°W" "» 

arrangement to that in the Tectibirmchia occurs. / Y\ ^5 

The single or paired genital aperture always lies , 

asymmetrically on the right side in front of the / 1 'O 

anus, which is sometimes found asymmetrically on 

the right side, and sometimes has a median dorsal 

position between the middle and the posterior end I ..s' 1/ A 

of the body. The renal aperture lies between the \ ^^^'%l(f>3^y- ffi 

anus and the genital aperture, sometimes close to 
the latter. 

In the Pteruixjdu gymnosomata. (Fig. 76) the 
shell and mantle are wanting. The otenidium, I ■i ? 1 1 »? 

when retained, as in the Z)f.j;ioft;ff)ic/ifa and P/iciHio- ' ^^ "* 

derma, lies somewhat far back on the right side of 
the body, far behind the anus. On the disappear- ^ 

ance of the mantle, it evidently shifted back from ^^^ V6.-Pneumoderma, from the 
its original position between the anus and the genital right side, diagrammatic (after Pel- 
aperture, while the osphradium, which is generally Senear). 1, Right process bearing 
found close to the otenidium, has, as far as has yet "^^°°^^ (Hakensack) evaginated ; 2, 
1 , 1 J. • 1 -J. • • 1 -J.- proboscis ; 3, right buccal tentacle ; 

been observed, retained its original position. ■, -i- « *? ■ , i 1 , * 

' SI' 4^ position of the right nuchal ten- 

The anus lies anteriorly behind the right fin ; tacle ; 6, right flu (parapodium) ; 6, 

the nephridial aperture lies close by, either distinct seminal furrow ; 7, genital aperture ; 

or united with the anus at the base of a common S, position of the jaw ; 9, ventral pro- 

1 ,j . T j'^-i-fj-i-i-i- boscidal papilla : 10, right buccal ap- 

cloacal depression. Immediately m front 01 this , 1- , ,,, ; 

^ -^ pendage bearing suckers ; 11, head ; 

lies the osphradium, then follows, considerably 12, aperture of penis; 13, right anterior 

farther forward on the neck, to the right behind pedal lobe ; 14, anus ; 15, posterior 

the base of the right fin, the genital aperture, from Pe^al lobe; 16, otenidium; 17, pos- 

which, as in many TedihrancUa, a ciliated furrow *"'"'■ f Pf™ «"' ^ '\ ".' «■ V' <""^^^l> 

' -^ p /. 1 1 T ventral, anterior, posterior, 

runs forward along the surface 01 the body to tlie 

aperture of the penis, which lies to the right in front of the foot. 

All Thecosomata have a mantle and a mantle cavity, and often a shell as well ; 
in the CymhuUidce, the latter is replaced by a cartilaginous pseudoconch, a, sub- 
cutaneous formation of the mantle. 

Among the Thecosoinata, the Limacinidce indicate the primitive arrangement ; 
they possess a dorsal or anterior mantle cavity, a coiled shell, and an operculum. 



80 



GOMPABA TI VE A NA TO MY 



CHAP. 



The ctenidium, however, is wanting. In the base of the pallial cavity, to the left, 
lies the pericardium, and immediately in front of it the kidney, with a narrow 
aperture into the cavity ; then follow the osphradium (where this has been found), 
and, at the extreme right of the cavity, the anus with the anal gland. The mantle 
gland (liypobranchial gland, shield) is found on the roof of the pallial cavity. The 
genital aperture lies to the right anteriorly in the cephalic region ; from it a 
ciliated channel or furrow runs dorsally to the aperture of the penis, which lies 
anteriorly between the fins. 

As compared with the Limacinidce, i.e. the Tliceosomata with coiled shell, the 




Pro. V7.— A, B, C, Three diagrams to illustrate tlie relation of the Limacinldse to the 
Cavollnudee (after Boas). A, LimacinidEe ; B, hypothetical intermediate stage between 
the Limacinidas and the CavoUniidse. The visceral douie twisted 90^ C, Cavoliniidse. All the 
diagrams from the ventral or posterior side. In A the visceral dome is drawn straight, whereas it 
is in reality coiled. 1, Right fin (parapodium) ; 2, foot bent forward ; 3, genital aperture ; 4, ten- 
tacular appendage of the mantle edge ; 5, anus ; 6, masticatory stomach ; 7, gonad. 

Oavoliniidcc and CymbidiUhe, or Thecosoinata with straight shell, show a very 
different arrangement of the pallial complex, which can only be explained by the 
supposition that the larger posterior portion of the body (the visceral dome) of 
the Limaeiuiilic, with all the pallial organs belonging to it, has twisted round the 
longitudinal axis of the body 180°, in relation to the cephalic region with the 
genital apertures belonging to it. Such a twist gives the organs the position they 
actually occupy in the Cavoliniidm and CymhuUidce ; the posterior (ventral) pallial 
cavity containing, on the left the anus, on the right the pericardium and kidney and 
the osphradium, the genital aperture occupying its original position to the rio-ht. 
The cause and significance of this twist are at present unknown. 



B. Scaphopoda. 

There is no gill in the posteriorly placed mantle cavity. The anus lies in the 
middle line above the foot, having a nephridial aperture on each side of it. There 
.are no distinct genital apertures. 



VII MOLLUSCA—THE PALLIAL COMPLEX 



C. Lamellibranchia. 

The general arrangement of the organs in the mantle cavity of the Lamelli- 
branchia has already been described. The strict symmetry of the body in this 
class must again be pointed out. All originally paired organs remain paired and 
symmetrical. 

The two nephridial apertures lie on the body above the base of the foot, or 
farther bach near tlie posterior adductor muscle ; they usually lie beneath the point 
of attachment of the gill-axis, between it and the line of concrescence of the (inner) 
ascending lamella of the branchial leaf Avith the foot, where such concrescence takes 
place. In the Scptibranchia, on the contrary, the apertures open into the upper 
pallial chamber. 

The outer genital apertures may be wanting, and in this case the genital 
products are ejected through the nephridial apertures, which is the primitive 
arrangement. "When present, in diceceous bivalves, they are always found in one 
pair, and lie on each side just in front of the nephridial apertures, sometimes in the 
base of a common pit or furrow, less frequently at some distance from these aper- 
tures. There are no special copulatory organs. 

In hermaphrodite Lamellibranchia the arrangements may vary as follows : — 

1. Both kinds of se.xual products may be ejected on each side through a 
common aperture {Ostrcca, Pcetcn, Cyclas, Pisidium, etc.). 

2. There may be, on each side, two distinct apertures, one male and the other 
female [Anatiiiacca). 

3. The seminal ducts and the oviducts may unite before opening to form a 
short, common, terminal piece (SeiMbranchia). 

The osphradium is paired in the Lamellibranchia, and always lies near the 
posterior adductor muscle over the visceral ganglion, at the point of insertion of the 
branchial axis on the body. A pair of sensory organs is found in many Lamelli- 
branchia, one on each side of the anus (abdominal sensory organs), or to the right 
and left on the mantle at the inner aperture of the siphons of the Siphoniata (pallial 
sensory organs). 

Hypobranchial glands have been found in the Protohrancliia [Kuculida and 
Solenoniyid(e). They are large and well developed, and belong to the mantle, lying 
in the posterior part of the body above the base of the gill on each side, to the right 
and left of the pericardium, and in front of the posterior adductor. 

The leaf-like oral lobes (labial palps), one occurring on each side of the mouth, 
between it and the anterior end of the base of the gill, will be described more in 
detail in another jslace. 

D. Cephalopoda. 

In the Cephalopoda the primitive symmetry of the pallial complex is on the 
whole retained. 

If we cut open the mantle of the Nautilus (Figs. 78 and 79), which covers the 
posteriorly placed pallial cavity, and lay it back on all sides, the following organs 
are revealed : — 

1. On each side there are two gills, an upper and a lower. 

2. The anus lies on the visceral dome, between the bases of the four gills. 

3. Below the base of each gill is found a nephridial aperture — making four in all. 
i. Close to the two upper nephridial apertures lie the two so-called visoero- 

pericardial apertures. 

5. Between the bases of the lower gills there are in each sex, two genital 
VOL. II G 



82 



COMPARATIVE ANATOMY 



CHAP. 




Pig. 78.— Palllal complez and siphon of Nautilus pompilius ? (after Bourne and Lankester). 
V, Valve of the siphon ; ro, right genital aperture ; m, the mantle fold, with the nidamental gland, 
folded back ; an, anus ; cp, left aperture of the secondary coeloni ; Vin, left upper nephridial aper- 
ture ; lo, aperture of the left rudimentary oviduct ; Ivn, left lower nephridial aperture. The four 
ctenidia are not lettered. 




3C «•!• V15cfc)t^ 



Fig. 79.— Pallial complex of Nautilus pompiUus i (after Bourne and Lankester). jjc. Penis ; 
a, muscle band of the siphon ; (s;i, aperture of the left rudimentary seminal duct ; mcpfta, iiephp, 
lower and upper nephridial aperture of the left side ; olf, left ospluradium ; viscper, left aperture of 
the secondary coelom ; an, anus ; j, supra-anal papilla of unknown significance ; c, mantle cut off. 



VII 



MOLLUSGA—THE PALLIAL COMPLEX 



83 




Fig. 80.— Sepia Savignyana, from behind (after Savigny). The ^'reater part of the mantle cut 
open and laid back on the right side (left in the figure), a, Prehensile tentacle ; &, oral arm ; c, 
mouth with jaws ; d, lower aperture of siphon ; e, eye ; /, locking apparatus of the mantle g ; Tt, right 
ctenidium ; i, siphon ; Ic, locking apparatus of the mantle on the visceral dome ; I, upper aperture 
of siphon ; m, anus ; n, depressor infundibuli ; o, penis ; p, right nephridial aperture ; g, posterior 
integument of the visceral dome ; r, fin. 



84 COMPARATIVE ANATOMY chap. 

apertures, but only that on the right side is functional. In the male, the aperture 
is jiroduced into a tubular penis. 

6. Above the bases of the lower gills there is an osphradium on each side placed 
on a papilla. 

7. Above the anus there is a large median papilla of unknown significance. 

8. The nidamental gland lies dorsally in the mantle. 

If we compare with the above the pallial complex of a dibranchiate Cephalopod, 
such as Syna (Fig. 80), Ave find the following arrangements : — 

1. There is one gill on each side. 

2. Along the median line of the visceral dome, the rectum and the duct of the 
ink-bag descend together, to open through a common aperture at the tip of a papilla 
at the base of the siphon. 

3. On each side near the rectum, above the anus, a nephridial aperture occurs 
on the point of a papilla. 

4. Of the two paired genital apertures only the left has been retained in Sepia 
and many other Cephalopods ; this lies near the left nephridial aperture at the 
summit of a large papilla (penis). In the female Octopus, the genital apertures are 
paired and sjmimetrical, and lie to the right and left of the rectum. 

5. The two nidamental glands (in Dcmpoda) lie in the visceral dome, sym- 
metrically with regard to the median line ; they open above the nephridial apertm'es 
into the mantle cavity. 



VI. The Respiratory Organs. 
The True Grills or Ctenidia. 

The most important of the palHal organs in the Mollusca is the gill, 
for it is in order to protect it that the mantle, and with it the pallial 
cavity, develo23. The gill found in the mantle cavity is throughout 
all the divisions of the Mollusca a homologous organ, to be derived 
from the gill of a common racial form. But since this gill is wanting 
in certain Mollusca (e.g. many Opisthohranchia), and is functionally 
replaced by new organs which are morphologically altogether uncon- 
nected with it, it has been found useful to distinguish the primitive 
Molluscan gill by the name of etenidium. This word, therefore, has 
a special morphological significance. 

The ctenidia of the Mollusca are originally paired and symmetri- 
eally arranged ciliated processes of the body wall, carrying two 
rows of branchial leaflets, ami projecting into the mantle cavity. 

Venous blood flows into the gills through afferent vessels 
(branchial arteries), and after becoming arterial by means of the 
respiration, flows through efferent vessels (branchial veins) back to 
the body, passing first through the heart. At or near the base of 
each etenidium there always lies a sensory organ, which is considered 
as olfactory, the so-called osphradium or Spengel's organ. 

Such primitive ctenidia are met with first in that group of the 
Mollusca which has undoubtedly retained more primitive characteristics 
than any other, viz. the Chifoiiidiv among the Amphineura. They are, 
further, found in all other Mollusca which have retained the original 



Til 



MOLLUSCA— RESPIRATORY ORGANS 



85 



bilateral symmetry of the body, such as the Lamellihranchia, the 
Cephalopoda, and — a point of great importance — also in the primitive 




Fig. 81.— Ctenidia of various Molluscs (after Ray Lankester). A, Chiton ; B, Sepia ; C, 
Fissurella ; D, Nucula ; E, Paludina. ft. Longitudinal branchial muscle ; a&y, afferent branchial 
vessel ; ehv, efferent branchial vessel (branchial vein) ; fjl, paired lamellas (leaflets) of the feathered 
gill ; in D : d, position of the axis ; a, inner ; 6 and c, outer rows of branchial lamellas ; in E : i, 
rectum ; &r, branchial Jilaments ; o, anus. 

Gastropoda, the Zeugohranchia. In the latter, however, the left ctenidium 
was originally the right and vice versd, but this will be dealt with more 
in detail later. 



86 COMPARATIVE ANATOMY chap. 

With regard to the number of gills originally present on each side 
of the body, opinions are divided. Those who hold that there were 
several seem justified by the arrangement in Chiton, where numerous 
consecutive ctenidia lie in a longitudinal row in the branchial furrow 
(mantle cavity) on each side, and also by that in the Nautilus, which 
is rightly considered the most primitive of extant Gephalopods, where 
four gills are found (Tetrabranchia). We shall, however, see later that 
the other view, viz. that the Mollusca originally possessed only one 
pair of ctenidia, has, to say the least, equal claim to be accepted. 

In all other Mollusca with paired ctenidia, including the Lamelli- 
branchia, there is only one pair at the posterior part of the body. 
Further, in the racial form of the Prosobranchia, a single pair of gills 
must be assumed to have occupied a posterior position in a mantle 
cavity which, with them, shifted forward later to the anterior position. 
The Zeugobranchia still retain this single pair of gills. 

In most Prosobranchia, the asymmetry of the body is also seen in 
the gills, only the left gill of the two in the Fissurellidce and Haliotidce 
being retained, the right completely disappearing. In the forms which 
most resemble the Fissurellidm and Haliotidm, the single-gilled Dioto- 
cardia (Turbinidm, Trochidce, etc.), the gill is still feathered on both 
sides, but in all Monotocardia it has only a single row of leaflets. 

In one division of the Opisthobranchia, the Tectibranchia, one 
ctenidium is still retained, that on the right side. Other Opistho- 
branchia have lost the true ctenidium together with the mantle cavity ; 
it may be/replaced by analogous (but not homologous) respiratory 
organs, such as adaptive gills. 

The Ptdmonata, in consequence of their adaptation to aerial 
respiration, have lost the ctenidia. 

The blood, which has become arterial in the ctenidia, reaches the 
heart through the auricle, and passes into the body through the 
arteries. It is therefore evident that a close relation must exist 
between the gills and auricles. This relation is briefly as follows : 
where the gills are paired, the auricles are paired, and unpaired gills 
are accompanied by a single auricle on that side of the body on which 
the gill is retained. Where gills are paired, there is almost always 
only one pair, and then there is one right and one left auricle. 

The Nautilus has four gills, and, to correspond, two right and two 
left auricles. The Ghitonidce, on the other hand, in spite of their 
numerous pairs of gills, have only one right and one left auricle. 

The Scaphopoda possess neither true ctenidia nor any other localised 
gills. Respiration may take place at the various soft-skinned surfaces 
which come in contact with the water, such as the inner surface of the 
mantle, the tentacles, etc. 

A. Araphineura. 

Chitonidse. — A single ctenidium of a Chiton (Fig. 82) may serve as a type of the 
Molluscan gill with its two rows of leaflets. The plumose ctenidium rises freely from 



VII 



MOLLUSCA— RESPIRATORY ORGANS 



87 



the base of the branchial groove (mantle cavity). The axis here takes the shape of 
a thin septum. At each side, on the broader surface of the septum, extending from 





Fig. 82.— Structure of the otenidlum of a Chiton (after B. Haller). A, Single otenidium with 
its double row of branchial leaflets. B, Transverse section of the gill along the line a-h in Fig. A. 
1, Narrow blood sinus in the branchial leaflet ; 2, septum in its axis ; 3, longitudinal muscle ; 4, 
afferent branchial vessel ; 5, eflferent branchial vessel ; 6, nerves ; 7, long cilia on the branchial 
axis. C, 2 pairs of branchial leaflets cut through at right angles to their surfaces, along the line c-f 
in Fig. B. 1, Same as in Fig. B ; S, space between the consecutive branchial leaflets. D, Longi- 
tudinal section of the ctenidlum somewhat laterally to the axis, and parallel to its septum, along 
the line c-d in Fig. A. This section is part of a transverse section of the body. Lettering as in 
Figs. B and C. In addition : 9, olfactory ridge of the branchial epithelium ; 10, general afferent 
branchial vessel ; 11, general efferent branchial vessel ; 12, pleuro-visceral strand of the nervous 
system. The branchial epithelium is everywhere indicated by a thick black line. 

base to tip, there is one row of smooth, delicate branchial leaflets. In outline they 
are more or less semicircular, and stand crowded together in great numbers almost 
like the leaves of a book. The entire surface of the branchial epithelium is 
ciliated ; on the axial epithe- 

A B C- 



lium, the cilia are remarkably 
long. On that side of the axis 
which is turned towards the 
foot, a blood-vessel runs from 
base to tip, conducting venous 
blood to the gill (afferent 
branchial vessel). On the op- 
posite side, which faces the 
mantle, another vessel, the 
branchial vein, runs from the 
tip to the base of the gill, and 
carries the blood, which has 
become arterial by respiration, 
to the general branchial vein, 
and through it to the auricle. 
These vessels have no special 
endothelial walls, but are surrounded by circidar muscle fibres. The branchial vein 
is accompanied by a powerful longitudinal muscle. At the base of each branchial 
leaflet, the blood flows out of the branchial artery through an aperture into the 
narrow cavity of the leaflet, and passes through a similar aperture on the opposite 
side of the axis to enter the branchial vein. Nerves are supplied to the ctenidium 
from the pleuro-visceral nerve which runs close to its base. 




Fig. S3,— Diagrams illustrating the arrangement of the 
gills in the Chitonid^. in, Mantle ; o, mouth ; k, snout ; /, 
foot ; ct, ctenidia ; ft, anus. 



COMPARATIVE ANATOMY 



CHAP. 



The number of 




Fig. S4.— Posterior 
end of the tody of 
Cheetoderma (dia- 
gram after HubrecM). 
1, Gouad ; 2, pericar- 
dium ; 3, rectum ; 4, 
nepliridimn ; 5, anus ; 
6, ctenidium ; 7, 
cloaca. 



ctenidia in each row varies very mucli in the diflferent species of 
ChitonidcB ; it ranges from 14 to 75. The row extends along the 
whole length of the branchial furrow (Fig. 83 A), or else (in 
Chiton Icevis, 0. Pallasii, and Ghitmiellus) is confined to its 
posterior half (B, C). 

Solenogastres. — [Proneomcnia, Neomenia, C'Juctoderma). 
The mantle cavity, in these forms, is much reduced, consisting 
only of the groove on each side of the rudimentary foot ; it 
opens into the cloacal cavity, or rather widens to form that 
cavity. The cloaca is thus the posterior portion of the mantle 
cavity. In Chcctoderma (Fig. 84) the foot has disappeared, and 
the mantle cavity is reduced to the cloaca, in which one typical 
gill lies on 'each side of the anus. These gills are regarded as 
the last ctenidia of the rows found in the Ohitonidce, which in 
Ohitondlus and some species of Chiton are already confined to 
the posterior half of the body. In Neomenia, there is no longer 
a pair of ctenidia, but a mere tuft of filaments rising from the 
wall of the cloacal cavity, and in Proneomenia, there are only 
irregular folds of the cloacal wall. 

On the relation of the gills in the Chitonidce to certain 
pjatches of epithelium, which may perhaps be considered as 
osphradia, see the section on Olfactory Organs, p. 165. 



B. Gastropoda. 

The Fisstirellidce (Fig. 85, A and B) among the Prosohranchia stand nearest to 
the racial form of the Gastropoda. The mantle cavity is anteriorly placed ; into it 
from behind and above project two long gills feathered on each side ; these lie 
symmetrically to the middle line, and to the right and left of the anus. The 
posterior portion of their axes is connected by a band with the floor of the respiratory 
cavity, while the anterior pointed portion projects freely. 

The fact that in the FissurelUdcc (and related forms) the gills are paired and 
symmetrical is very significant. It points to the primitive character of these forms, 
and enables us to compare their gills with those of the lower Lamellihranchia, i.e. 
the Protobranohia, and of the Cephalopoda. We must, however, again emphasise 
the generally-assumed fact that the left gill of Fissurella answers to the right gill of 
the Lamellibranchia and Cephalopoda, and the right gill of the former to the left of 
the latter, these latter having retained their primitive symmetry in this respect. 
This assumption becomes the more plausible when we consider that the mantle 
cavity with its organs originally lay posteriorly on the body, and shifted forward 
secondarily along its right side. 

The Saliotidce are closely connected with the Fissurellida;. Their spacious mantle 
cavity is, however, forced to the left side by the great development of the columellar 
muscle. There are two gills, feathered on both sides, of which the right is the 
smaller. The axis of each gill has united, for nearly its whole length, with the 
inner wall of the mantle, and only its anterior end is free ; its tip even projects a 
short distance beyond the respiratory cavity. 

Although the FissurelUdce and Haliotidce still possess two gills, other Diotocardia 
have retained only the left (ur) and larger gill of Haliotis. This gill is, however, 
still feathered on both sides, although this characteristic is obscured in a peculiar 
manner. The septum or axis of the gill, to the broader surfaces of which the branchial 
leaflets are attached, and one edge of which had, in Haliotis, already fused with the 



VII 



MOLLUSGA— RESPIRATORY ORGAA^S 



89 




inner wall of the mantle, becomes attached to the mantle by its other edge 
also (viz. that along which the branchial artery rnns), somewhat to the right 
of the first line of concrescence. In this manner,, which is illustrated by the 
accompanying diagrammatic sections (Fig. 86), the mantle cavity is divided 
by the branchial septum into two unequal parts, which open into one another 
anteriorly. 

Into the much smaller upper division the one row of smaller branchial leaflets 
projects, while the opposite row of larger leaflets hangs down into the lower and 
larger chamber. The anterior end of the 
gill, however, is still free, its point pro- 
jecting anteriorly {Trochidcc, Tarbinidw, 
Neritidce). 

In the Docoglossa (PatcUida) the ar- 
rangement of the gills is very varied. 
While the Lepctidci' have no gills whatever, 
we find in Patella a single row of numerous 
small branchial leaflets right round the 
body, on the inner or under side of the 
short encircling mantle fold, between it and 
the foot. This row is broken only in one 
place anteriorly on the left. It is, how- 
ever, evident that these gills, which some- 
what resemble those of the Chitonidce, are 
no true ctenidia, from the fact that there 
are Docoglossa (e.g. some forms of Tcdura 
and Scurria) which possess, in addition 
to this marginal row of leaflets, a typical 
ctenidium corresponding in every way with 
that of the Tiirbiaidce, Trochidcc, etc. 
Other forms, such as Acmccn, have only the 
true ctenidium and no marginal branchial 
leaflets. 

In the large second division of the 
Prosobranchia — the Monotocardia — tlie 
arrangement of the gills is, on the whole, 
remarkably uniform. There is only a 
single gill feathered on one side (Fig. 71, 
p. 73), united to the mantle along almost 
its whole length ; this gill corresponds 

with the left gill in Fissurdla and Haliotis, and the single gill in 
Trochus. It generally lies quite to the left in the mantle cavity. 

The rise of this gill can best be explained by recalling the arrangements already 
described in Turbo and Trochus. We have only to assume that the row of small 
leaflets turned towards the mantle in Turbo disappears, and that the branchial 
septum unites with the mantle across its whole width (Fig. 86, C, D). 

A few anomalous forms alone require special mention. 

1. In a series of terrestrial Monotocardia, aerial respiration has taken the place of 
aquatic respiration, and the ctenidium has disappeared [Acicula, Cydostoma, Cxjclo- 

phonis, etc.). 

2. The Ampullaria are amphibian Prosobranchia. A doubling of the mantle 
gives rise to a very spacious pulmonary sac, on the inner surface of which the respira- 
tory vascular network spreads out. The lower wall of this pulmonary sac, which 
forms at the same time the roof of the mantle cavity, is perforated by an aperture 




Fig. 85. — Subemargimila after removal of the 
shell (after Fischer). A, from above ; B, from 
right side. Tlie mantle cavity is exposed by 
bemling back the mantle fold 4. 1, Snont ; 2, 
tentacle, with the eye on its short stalk behind 
it ; 3, right ctenidium ; 4, mantle fold ; 5, shell 
muscle ; 6, edge of the mantle encircling the 
body ; V, epipodiuni ; S, foot. 



Turbo and 



90 



COMPARATIVE ANATOMY 



CHAP. 



for the inhalation and exhalation of air.i The ctenidium is placed to the extreme 
right of the mantle cavity, a position which is in some way connected with the gi-eat 
development of the pulmonary sac. It nevertheless answers to the left gill in other 
Monotocardia, as can be seen from its innervation. 

3. The genus Valvata is unlike all other Monotocardia, in that its gill is 
feathered on both sides and projects freely. It can, further, be protruded from 
the pallial cavity. 

4. In Atlanta, among the Hcteropoda, the gill is well hidden in the spacious 
mantle cavity. In Carinaria, it is only slightly protected in consequence of the 
small development of the mantle fold. In Pterotrachea there is no mantle fold, and 
the filamentous branchial leaflets project free and uncovered. Firoloides has no 
gills. 

Opisthobranohia. — A true ctenidium is here found only in the Tedibranchia and 





Fio. 86.— General Morphology of the gills of the Prosohranchia. Diagrammatic sections 
in the region of the mantle cavity, from behind. F A, Haliotis ; B, Trochus, anterior portion of the 
pallial cavity. C, Trochus, inicklle or posterior portion of the cavity. D, Monotocardia. 1, 
Mantle cavity ; 2, rectum or anus, r right, I left gill of Haliotis (A), which latter is the only gill 
present in the AzygoiraiicMa (B, C) and i!oiwtocanUa (D). i, Branchial leaflet of the inner row ; 
e, ditto of the outer row, between them the branchial axis or septum with the afferent and efferent 
branchial vessels (8 and 4) ; 5, position of the mantle slit in Haliotis (cf. p. 43). Further explana- 
tions in the text. 

in the Steganobranchia among the Ascoglossa. It lies, often incompletely covered, 
in the mantle cavity which is developed on the right, and is, in some cases at least 
(e.g. Pleurobranclius), distinctly feathered on both sides. 

In the Pteropoda, which must be derived from the tectibranchiate Opistho- 
branchia, the ctenidium, when present, is little developed, and lies on the right side 
of the body. It answers to the tectibranchiate ctenidium. 

In the Gymnosomata, this true gill is retained only in the Pneumodermidm as a 
simple, or less frequently (Pneumoderma) fringed, process on the right side of the 
body (Fig. 76, p. 79). New gills, on the other hand, may develop at the posterior 
end of the body, occurring either together with the true ctenidium {Spongiobranchaa, 
Pneumoderma), or alone (CHonopsis, Notohranclima), until they in their turn dis- 
appear (Olione, Halopsyehe). 

Among the Thecosomata, the Cavolimidce alone (Fig. 87) possess a gill which 



^ In this and in the closely-allied Lanistes there is in addition 
on the left side (v. Fischer and Bouvier, C R. cxi. p. 200). 



protrusible siphon 



VII 



MOLLUSCA— RESPIRATORY OEOANS 



91 



rises in the form of a series of fold-like elevations of the body wall in the pallial 

1 2 ,, 



Fio. S7. — Anatomy of Cavolinia 
tridentata (after Souleyet). Shell 
and mantle removed, and visceml 
dome partly opened, seen from behind 
and below, d, Right ; s, left ; 1, aper- 
ture of the penis ; 2, mouth ; 3, left lin 
(parapodium) ; 4, foot ; 5, asophagus ; 
6, part of the efferent genital appara- 
tus ; 7, ventricle ; 8, auricle ; 9, herma- 
phrodite gland ; 10, lateral processes 
of the mantle ; 11, columellar muscle ; 
12, intestine ; 13, digestive gland 
(liver) ; 14, stomach ; 16, ctenidium ; 
1*3, genital aperture ; 17, anus. 




cavity, and which, running in a wavy line, forms a semicircle, open anteriorly, the 
greater portion of it, however, lying on the right side. 



C. Lamellibranchia. 

The Lamellibranchia also possess typically two symmetrically placed gills, each 
provided with two rows of branchial leaflets. The opinion which until lately was 
common, that the Lamellibranchia possessed two gills on each side of the mantle 
cavity, has been shown to be incorrect — these two gills in reality answering to the 
two rows of branchial leaflets of one typical gill. 

It is worth while to follow, step by step, the interesting series of modifications 
undergone by the original gill in the Lamellibranchia. 

(a) The primitive arrangement is found in the Protobranohia. Taking Nticula 
(Fig. 21, p. 14) as an example, we find a gill like that of Fissurella, consisting of 
an axis along which the branchial artery and the branchial vein run, and which is 
attached by a short membraneous band to the posterior and upper portion^ of the 
body or visceral dome, and to the posterior adductor muscle. On this axis are 
attached two rows of short flat branchial leaflets. These two plumose gills converge 
posteriorly, and project with their free tips into the mantle cavity. The leaflets of 
both rows are directed somewhat downwards, so that they are at right angles to one 
another. In Malletia and Solenmnya, on the contrary, they lie in the same plane, 
the two rows standing out on opposite sides of the axis. In Malletia, this plane is 
horizontal, but in Solenomya it trends downwards and inwards. The number of leaflets 
on the very slender gill of Malletia is much smaller than on that of Nueula ; they 
are consequently neither so crowded nor so flattened. Each leaflet contains a blood 



92 



COMPARATIVE ANATOMY 



CHAP. 



sinus, which is a continuation of the branoliial artery. Two rods of connective 
tissue run along the lower edge of each leaflet from the axis to its tip, and serve for 
its siipport. Similar supports are found in almost all Lamellibranchia and in 
many Gastropods. 

The epithelium of tlie branchial leaflets is beset with long cilia — (1) at the 
ventral edge ; (2) on both (anterior and posterior) surfaces, near the ventral edge. 

The first-named cilia form, with regard to the whole gill, a longitudinal row along 
the free ventral edge of each row of leaflets, and bring about a current in the water 
along this edge from behind forward. The other cilia mentioned above, mingling 
together like the bristles of two brushes Avhich are pressed together, form a loose 
connection 1 letween the successive leaflets of the row. 

(6) In the Filibranchia (Fig. 88 B) the leaflets in each of the two rows are very 
long and filamentous, and hang down far into the mantle cavity. The branchial 
filaments of the two rows are recurved and bent back upon themselves, so that in 
each filament a descending and an ascending portion can be distinguished. The 
prolongation of the filaments corresponds with a necessary increase of the respiratory 




Fig. ss.— Morphology of the gills of tlie Lamellibraacliia, dingrainiuatic transverse sections. 
A, Protobranchla. B, Filibranchia. C, Eulamellibranchia. D, Septibranchia. 1, Mantle ; 
2, body (visceral dome) ; 3, foot ; e, in A, branchial leaflets of the outer row in the feathered gill, 
in B, branchial filaments of the outer row, in C, outer branchial leaf ; i, brancliial leaflets or 
filaments of the inner row or inner branchial leaf; ei, ascending branch of the outer filament, 
or lamella of the outer leaf ; fj, ascending branch of tlie inner filament, or lamella of the inner leaf ; 
in D, s, signifies tlie gill which has become transformed into a nmsculai' septum which divides the 
mantle cavity into an upper (4) and a lower (.5) chamber, the two communicating by means of slits 
(o) in the septum. Further explanations in the text. 

surface. By this liending back of the filaments, the gills make the most of the 
limited space aff'orded by the mantle cavity. Eacli filament of the outer row is bent 
outwards, and of the inner row inwards. 

The filaments of each row may be so crowded together that the wdiole row looks 
like a leaf or fringe. This branchial leaf consists of two closely contiguous lamelhi', 
one the descending and the other the ascending, the two passing into one another 
at the lower edge of the leaf. The descending lamella is formed by the descending 
portions of the filaments, and the ascending by the ascending portions. On the 
outer leaf, the ascending lamella is the outer one, on the inner leaf the inner. 

In the Filibranchia, tlie separate branchial filaments retain their independence — 
they are free, i.e. the separate filaments of a series are unconnected with one 
another, and the descending and ascending portions of one and the same filament 
are in no way united. There are, however, on both the airterior and posterior sides 
of the filaments places covered with long cilia closely crowded together. These 
ciliated tults on adjoining filaments mingle, and so give rise to a sort of connection 
between the filaments of each leaf. 

In the MiitiUihc, so-called interfoliar junctions or trabeculte occur at certain 



VII MOLLUSOA—RESPIBATOliY ORGANS 93 

points bet\\ec'n the ascending and descending portions of the branchial iilanients, 
but no blood-vessels run into them. 

In Anuinia, the dorsal ends of the ascending portions of the outer lamella are 
free, but in the Arcichr united, although their internal cavities are not in commimi- 
cation. In sucli cases, the interior of each filament is divided by a longitudinal 
septum into two canals. In one of these the blood flows from base to tip, and in the 
other back from tip to base, i.e. to the axis. In the Mjjtilidce, the dorsal ends of the 
recurved portions of the filaments of each branch have grown together, and their 
blood-vessels coranmnicate at the points of junction, i.e. along the upper edge of the 
ascending lamella. 

(c) Pseudolamellibranchia. — Each leaf of the gill is here folded, to secure increase 
of surface. The plications run longitudinally with regard to the filaments, and are 
thus almost dorso-ventral. There are, therefore, distinct alternate ridges and furrows 
on each leaf, the ridges on the one surface corresponding with those on the other, and 
the furrows corresponding with furrows. Each ridge or furrow is formed by one 
filament ; the filament forming the furrow is in some way, such as greater breadth, 
distinguished from the others. The two lamellse of each leaf of the gill are united 
here and there by trabeculse, which may or may not contain blood-vessels. They occur 
either between the opposite furrows or between the opposite ridges, i.e. between the 
ascending and descending portions of the filaments which lie either in the furrows 
or ridges. The upper edge of the ascending lamella of the outer leaf may unite with 
the mantle. The consecutive filaments of the same leaf are only connected by means 
of tufts of cilia. 

(d) Eulamellibranchia (Figs. 89-91). — The branchial leaves are either smooth or 
folded, but there is always organic connection, by means of numerous vaseularised 
junctions, not only between the ascending and descending lamella;, but between the 
successive filaments. The junctions are therefore both interfoliar and interfilamentar. 
This leads to the entire disappearance of the original filamentous structure of each 
leaf, which now becomes an actual leaf or lamella with perforations or slits, the 
remains of the spaces between the original filaments, leading into an internal system 
of sinuses or canals, which in their turn are the remains of the spaces between the 
ascending and descending lamellse. This peculiar arrangement was formerly con- 
sidered typical of the Lamellibranehia, and was the origin of their name. It was 
supposed that the animals of this class had two leaf-like gills on each side of the 
mantle cavity, i.e. four altogether, birt we now know how the two branchial leaves on 
each side arose, that they are in fact the two, modified, rows of leaflets of the original 
plumose gill of the Protobraiichia. The Lamellibranehia in reality possess only one 
gill on each side in the mantle cavity. 

The blood now no longer flows through the primitive filaments of the lamellse 
of the gills and back again, but the afferent and efferent channels lie in the trabecular 
network between the two lamellfe of a branchial leaf. 

Instead of the two leaves of a gill hanging down into the mantle cavity parallel 
to one another, the outer leaf may stand up dorsally in the cavity, so that the two 
come to lie in the same plane {TcUinidie and Anatiiiacca). 

The ascending lamella of the outer leaf may be wanting {Anatinacea, Lastca), 
and in fact the entire outer leaf may be absent {L^icina, Corlis, MontaaUa, 
Cryptodon). 

In all Lamellibranehia, with the excejition of the Protobraiichin, and further, of 
the Arcidce, Trigonidce, and Fcetinidcc, the gill and mantle unite, the dorsal edge of 
the ascending (outer) lamella or, where this is wanting, the free edge of the single 
lamella of the outer leaf becoming fused with the mantle. In the same way, the 
dorsal edge of the ascending (inner) lamella of the inner leaf may become fused with 
the upper part of the foot (Fig. 88 C). " If the two gills, which have fused with the foot. 



94 



CUMPABATIVE ANATOMY 



CHAP. 



fuse with each other behind the foot in the middle line of the mantle cavity, they form 
a septum which, uniting ivitli the septum formed by the mantle between the inhalent 
and exhalent siphons, divides the cavity into an upper and a lower chamber. The 
water flows through the lower (inhalent) siphon into the large lower chamber, bathes 
the gills, and, streaming forward, conveys the particles of food it contains to the 
mouth. It then flows back along each side of the foot in the upper chamber of the 
mantle cavity (which is itself divided into two canals by the line of insertion of the 




Fig. 89. — Part of a transverse section of the outer branchial leaf of Dreissensia polymorplia 
(after Peck). /, The separate filaments ; /, sub-epithelial fibres ; ch, supporting substance of the 
filaments ; lac, lacunar or alveolar tissue ; pig, pigment cells ; he, blood corpuscles ; fe, epithelium 
of the free edge of the branchial filaments ; Ifei, Ife^, two rows of lateral epithelial cells of the 
branchial filaments, carrying long cilia (ciliated tufts) ; Irf, tissue of the interfllamentar junctions. 
Two -interfoliar junctions are shown in the figure. 

gill) into the single posterior and upper chamber behind the foot, and escapes through 
the upper (exhalent) siphon (Fig. 26, p. 18). 

(c) Septibranchia (Fig. 31 A and B, p. 21 ; and Fig. 88 D, p. 92).— These Mussels 
were formerly erroneously considered to be gill-less. As a matter of fact, the 
branchial septum just described has in them been much modified in structure, and 
has become a muscular septum, running across the mantle cavity in a horizontal 
direction and joining the siphonal septum posteriorly, while anteriorly it passes 
round the foot. This septum is broken through by various perforations and slits, 
which allow of communication between the upper and lower chambers of the mantle 
cavity, and vary in the different genera. 



VII 



MOLLUSGA— RESPIRATORY ORGANS 



95 




Fig. 90.— Portions of transverse sections of tlie branchial lamellae of Anodonta (after Peck). 
A, Outer ; B, inner lamella. In each leaf the cross sections of both lamellje are seen, and also the 
interfoliar as well as the interfilamentar junctions. C, A part of B much magnified, ol, Outer ; il, 
inner lamella of the same leaf; v, blood-vessels ; /, the separate filaments of which the lamellfe 
consist; lac, lacunar tissue; ch, supporting tissue of the filaments, with firmer supporting 
rods, chr. 




Fio. 91. — Portion of the ascending lamella of the outer branchial leaf of Anodonta, 
diagrammatic (after Peck). /, The separate filaments, connected by interfilamentar junctions ; 
trf, connective tissue of the latter ; v, blood-vessels ; ilj, interlamellar junctions ; the perforations 
in the lamella (of a darker shade) are the spaces remaining between the filaments and their 
junctions, through which the water needed for respiration can flow. 



96 



COM PA BA TI VE ANA TOM Y 



CHAP. 




D. Cephalopoda. 

The gills of the Cephalopoda are always feathered on both sides. Those of the 
SiluriiicJiia have been the most thoroughly investigated. In Sepia, eaeh gill has 
the shape of a slender cone, its "whole length being applied to the visceral dome in 
the mantle cavity, in such a way that the ba,se is directed dorsally towards the ape.K 

of the visceral dome, and the point ventrally 
towards the free edge of the mantle fold or 
the mantle cleft (Fig. 80, p. 83). The 
points of the two gills diverge. 

The two rows of flat triangular branchial 
leaflets (Fig. 92) are carried by the two 
branchial vessels, each leaflet being attached 
by one end of its base to the branchial artery 
and by the other to the branchial vein. In 
the axis of the gill between the two vessels, 
and also between the bases of the two rows 
of leaflets, a channel is formed which com- 
municates by a slit between each successive 
pair of leaflets with the mantle cavity ; 
through this canal the respiratory water 
freely flows. The slits in this axial channel 
are arranged alternately on each side, like 
the leaflets between whose bases they lie. 
The branchial vein forms the jiosterior sup- 
port of the gill turned towards the mantle, 
and the branchial artery the anterior support 
turned towards the visceral dome. The 
artery is united along its entire length with 
the integument of the visceral dome by a 
membrane of connective tissue. The an- 
terior edge of each leaflet (that facing the 
visceral dome) is connected with this mem- 
brane, which may be called the gill-suspen- 
sor, by means of another triangular mem- 
brane. A special vein runs along the 
posterior free edge of each leaflet, and enters 
the general branchial vein at its base ; and 
a special artery runs along the anterior edge, 
i.e. along that edge of the leaflet which is 
fastened to the suspensor. Each leaflet is wrinkled in suclua way that the folds 
on the two surfaces alternate, each fold being creased in its turn. These two systems 
of folds «;ross each other at right angles, and serve to increase the respiratory surface. 
At the point where the suspensor of the gill passes into the integument of the 
visceral dome, it contains a cellular body, which is traversed by a system of inter- 
cellular blood-channels. This may perhaps be a blood-making gland. It receives 
venous blood from branches of the principal branchial artery and of the special 
arteries of the leaflets, and returns the same along two^veins which run back to the 
base of the gill, there, with others, to open into the venous sinus of the renal organ ; 
from this organ the blood passes for the second time along the branchial artery into 
the gill. We thus find that not all the venous blood which is conducted by the 
branchial artery towards the gills enters the leaflets for purposes of respiration ; part 



MO 

Fig. 92.— Diagram to illustrate tlie struc 
ture of tlie gill of Sepia (after Joubin). I, 
Branchial vein (containing arterial blood) ; •!, 
brancliial canal ; 3, brancliialj artery (contain- 
ing venons blood); 4, special branchial vein 
(vas efferens) of each leaflet ; 5, special branchial 
artery (vas alferens) of each leaflet ; r., suspensor 
' if the gill, which attaches tlie brancliial artery 
(3) to the posterior integnment of tlie visceral 
dome (12) ; 7, suspensor of eacli leaflet to tlie 
general suspensor (6) ; S, one of tlie connecting 
vessels between tlie branchial artery and tlie 
"blood-making gland" (9), tlirougli which 
\enous blood flows; 10, 11, vessels carrying 
the venous blood which has passed through 
the " blood-making " gland baclv to the venous 
sinus at tlie base of the gill. The arrows in- 
dicate the direction of tlie blood-stream. 



VII MOLLVSGA— RESPIRATORY ORGANS 97 

of it streams througli the ' ' blood-making " gland, and returns to the venous branchial 
heart still unpurified. There are, further, certain fine branchings of the branchial 
artery which serve for nourishing the gill and its suspending membranes. The blood 
in these returns to the venous sinus through a special vessel which runs parallel to 
the branchial artery on its anterior side. 

A powerful nerve enters the gill at its base and ramifies through it. A muscle 
spreads over the surface of the "blood-making" gland, and a special musculature 
brings about the contractions of the principal branchial vein. 

The gills of the Odopoda differ considerably, though not essentially, in structure 
from those of the Decapoda. The branchial channel is much larger, and the leaflets 
are not only folded, but have on each side alternating lamellie, which in their turn 
may carry similar lamellae of the second order, and so on till in some cases the 
seventh order of subsidiary lamellfe is reached. The leaflet is thus an extremely 
complicated, folded, or feathered structure with its surface increased to an 
extraordinary degree. 

Adaptive Gills. 

The Scaphopoda and many Gastropoda possess no true ctenidia. 
In the Fulmonata and the few air-breathing Fwsobrcmchia, the ctenidia, 
as organs adapted for aquatic respiration, have disappeared. It is, 
however, at present difficult to determine the cause of their dis- 
appearance in Ojnsthobranchia which inhabit water, and in the gill-less 
forms of the Fteropoda, all the more so, as in most Opistholwanchia 
they are replaced by adaptive gills, which are new structures in no 
way comparable morphologically with ctenidia. These adaptive gills 
may even appear [Pneumoderma) before the true ctenidia have dis- 
appeared. The Scapfwpoda and many OpisthobrancMa have no gills 
whatever, and in these respiration evidently takes place at various 
suitable parts of the surface of the body. In many cases, also, where 
epipodial or parapodial processes are developed as well as gills, or 
the mantle possesses extensions, these may help the gills in the 
function of respiration. 

Adaptive gills are found in most Asmglossa and in the Niidi- 
branchia ; also, as mentioned above, in the gymnosomatous Pteropoda. 
In the latter, they consist of small fringed or plain ridges at the 
posterior end of the body ; these may be of various shapes ; a 
description of them would be of no special interest to the comparative 
anatomist. 

The principal forms of adaptive gills of the Nudibranchia are : 
(1) the anal gills of the Dorididm , (2) the longitudinal rows of 
branchial leaflets to the right and left under the mantle fold of the 
so-called PJiyUidiidce , (3) the dorsal appendages or eerata of the 
Nudibrancliia and most Ascoglossa. 

1. The Anal Gills (Fig. 9-3). — These take the form of delicate leaflets, generally 
feathered on both sides, which, in the Doridida:, form a rosette round the anus, 
which has a median dorsal position towards the posterior half of the body. Cerata 
may occur with the anal gills {Polyceridm). The view that these gills are ctenidia 
has as yet no sufiicient foundation. 

VOL. II H 



98 



GOMPABATIVE ANATOMY 



CHAP. 



2. The Longitudinal Rows of Branchial Leaflets (Fig. 20, p. 13).— These organs, 
which lie to the right and left of the body in the Phyllidiidce and Pleuwphyl- 
lidiidce, bear the same relation to the (lost) true ctenidium as do the respiratory struc- 
tures of the Patellidce above described to 
the same organ, which in them is some- 
times present, sometimes wanting. The 
longitudinal rows consist of numerous 
small lamellse which project from the lower 
side of the enveloping mantle fold into the 
shallow pallial cavity. There is either one 
long row of these lamellse running along 
the whole length of the mantle fold and 
only interrupted anteriorly (PhyUidia), or 
a row interrupted posteriorly as well 
(Pleurophyllidia) ; or again, the rows of 
lamellfe are confined to the posterior end 
of the mantle fold (Hypobranchima). The 
genus Derinatolranchus has no gills. 

3. Dorsal Appendages (Cerata) (Fig. 
18, p. 12). — These processes vary very 
much in form, being sometimes simple, 
and sometimes branched ; they differ also 
greatly in number and arrangement. At 
their tips there are often cnidophore sacs ; 
these are invaginations of the ectoderm in 
which stinging cells with stinging capsules 
are developed. Diverticula of the intestine 
(digestive gland) enter the cerata, and may 
open outward at their tips. The cerata are 
generally striking and beautiful both in 
colour and markings. In some cases they 
may serve for protection and concealment, 
in others, where the brilliant colouring is 
combined with stinging properties, they 
may serve as a warning. They often break 
off easily at the base (as a protective ar- 
rangement), and are always quickly regen- 
erated. They no doubt assist, like the 
rest of the body surface, in respiration, 
especially where they are much branched 
and richly supplied with blood-vessels. 
Certain Opisthobranchia are altogether gill -less, e.g. the Mysiidce, Limapontidce, 
and PliyllirrJioidcc. 

Among the Pulmonata, the shell-less genus Oiwhidium has developed adaptive 
gills. The species of this genus are amphibious, living on the sea-coast, within reach of 
the tide.. Their pulmonary cavity is very small ; respiration therefore takes place by 
means of the richly vascularised dorsal integument, and especially of the simple or 
branched dorsal papillte, in which there is a rich vascular network, which receives 
the lilood from an afferent vessel and gives it off to an efferent vessel. 




p. Fig. '.13. — Respiratory and circulatory 
system of Doris, after Leuokart ("Wand- 
tafeln "). a, Rhinopliore ; h, posterior edge of 
the visceral dome ; c, end of the foot ; d, plumose 
gills ; di, two gills cut ofl' ; e, anus ; /, auricle ; 
</, ventricle ; /i, aorta ; t, circular vein around 
the anus, which receives the arterial blood from 
the gill, and sends it through the branchial vein 
into the auricle ; k, circular artery, which receives 
the venous blood coming from the body ; .t, two 
vascular trunks, which conduct venous blood 
direct to the heart. 



VII 



MOLLUSCA— RESPIRATORY ORGANS 



99 




Lungs. 

The total disappearance of the typical molluscan ctenidium is 
characteristic of the Pulmonata, and is connected with their terrestrial 
life and aerial respiration. Instead of water, air enters and escapes 
from the mantle cavity which lies either anteriorly or laterally on 
the visceral dome, and thus the mantle cavity becomes a pulmonary 
cavity. The free edge 
of the mantle fold, which 
forms the roof of this 
cavity, unites with the 
nuchal integument be- 
neath it, except at one 
point on the right, where 
the respiratory aper- 
ture, which can be closed 
at will, allows of the 
entrance and egress of 
air. Along the line of 
its concrescence with the 
integument, the edge of 
the mantle is much 
thickened, forming the 
mantle border, and is 
very rich in lime-secret- 
ing glands. The inner 
delicate surface of the 
mantle, which forms the 
roof of the cavity, is 
overspread by a close 
respiratory vascular net- 
work. A circular vein 
runs alon^ 

collar. From it spring 
numerous fine anastomosing vessels which ramify on the mantle. 
These vessels are again collected into larger trunks, which enter the 
laro'e pulmonary vein. This vein runs upwards and backwards, 
along the right side of the pulmonary cavity, to the left of and almost 
parallel with the rectum, and enters the auricle. The circular vein 
contains venous blood, but the pulmonary vein conducts blood which 
has become arterial through respiration in the vascular network, to 
the heart. 

Since, in most Pulmonata, as in the Prosobmnchia, the respiratory 
oro-an and the pallial cavity in which it is found lie in front of the 
heart, this order is prosopneumonic. An account of the opistho-. 
pneumonic condition of certain Pulmonata, which results from the 



Fig. 94.— SligMly oblique transverse section tirougli the 
body and sbeU of Helix taken just in front of the columella 
(after Howes), pgl, Pedal gland ; fs, lateral pedal blood sinus ; 
ao, cephalic aorta ; gd, genital duct (uterus) ; /y, retractor 
muscle of penis ; plm, pallial muscle, the pallial edge having 
united with the nuclial integument ; si, salivary gland ; cr, crop, 
or widening of the oesophagus ; s, shell ; ms, floor of the pulmon- 
ary cavity = dorsal integument of the posterior nuchal region 
which is covered by the mantle ; sp, spermatheca = stalk of the 
receptaculum seminis ; p?i, pulmonary cavity ; py, afferent pul- 
monary vessels ; rei, renal ducr ; r, rectum ; hgl, hermaplirodite 
gland or ovotestis ; I, digestive gland (liver) ; hd, hermaphrodite 
thp mantle tluct; rm, columellar muscle ; agl, albumen gland ; i, intestine ; 
$t, stomach. 



100 



COMPARATIVE ANATOMY 



CHAP. 



displacement of the visceral dome and mantle to the posterior end of 
the body, will be found in Section V., p. 76. 

Certain Pulmonata (LimiuHdce) have become readapted to aquatic life, but their 
respiration is the same as that of the terrestrial forms, they rise periodically to the 
surface of the water to take in air. The respiratory cavity, is, however, filled with 
water when the animal is young, and it is then a water breather. In Limnma 
a deep-water form found in the lake of Geneva, this form of aquatic 




1 R 
Fio. 95. — Helix. The roof of the puhnonary sac cut along the rectum, and along the edge 

unitmg with the nuchal integument, and turned back to show the arrangement of the blood 
vascular system, after Howes. The pulmonary veins are of a lighter shade than the afferent 
pulmonary vessels and the venous sinuses ; aa, hh, show the cut edges which belong to each other ; 
1, afferent pulmonary ^-essels which draw their \enous blood from the large circular venous 
sinus (9) ; this latter nceives its blood from the large sinuses of the body, two of which, 
that of tlip visceral dome (6) and that on the right side of the -foot (7) are shown. The efferent 
pulmonary vessels collect the blood which has become arterial on the roof of the pulmonary 
chamber, and conduct it through the pulmonary vein (2) to the auricle (3) ; 4, ventricle ; 5, renal 
circulatory system. 

respiration continues throughout life, and the pulmonary chamber, in no way 

modified, is constantly filled with water. 

In certain terrestrial P/usoi?'a?ic/iZ(r (Cyclostoina, Gydophorus^ etc.) the 
respiratory cavity becomes transformed, as in the Fiiliitniuda, into a 
pulmonary chamber, and its roof is covered with a respiratory 
vascular network. Bat there is here no concrescence of the edsre of 

o 

the mantle with the nuchal integument. Cijclostoma still retains a 
rudiment of a prosobranchiate gill, but this is lost in Cf/cloj^hm-vs. 
The amphibian Amjmllaria possess both a gill and a pulmonary 
sac/ and can breathe either water or air. 



See note ante, p. 



Til MOLLUSCA—HYPOBRANCHIAL GLAND, HEAD 101 



VII. The Hypobranehial Gland. 

(Slime gland of the Prosoh'anchia, epithelial shield of the Pteropoda, 
etc., anal gland, etc.) 

This is an organ very commonly found on the molluscan mantle, 
always occurring near the ctenidium, at its base or between it and 
the rectum. Of. on its position and occurrence Section V. 

The hypobranehial gland varies considerably in shape, but is 
never a multicellular, acinose, or tubular gland with efferent ducts. It 
is originally a more or less extended area of the epithelium of the 
mantle cavity (generally of the inner surface of the mantle) in which 
epithelial glandular cells are particularly numerous. In this condition 
it is not very distinct from the parts around it, but it may become 
more definitely localised, and may assume a definite shape ; and in 
this latter case, the glandular epithelium of which it consists may also 
become folded in order to obtain a larger secretory surface, the folds 
being more or less closely crowded together and projecting into the 
mantle cavity. This gland often secretes a large quantity of mucus. 
The purple gland of certain Prosobranchia {Purpura, Murex, Mitra) is a 
hypobranehial gland, the slimy secretion of which is, immediately 
after ejection, colourless or only slightly coloured, but under the 
influence of light becomes violet or red. In Purpura, the gland consists 
of two parts which differ slightly in structure. 



VIII. The Head. 

If by the word head is meant an anterior portion of the body 
more or less distinct from the rest, possessing a mouth and specific 
sensory organs, the Lamellihranchia must be considered headless, and 
as such have been distinguished as Acephala from other Mollusca. 
This absence of a head in the Lamellihranchia cannot be regarded as 
a primitive condition, ^ but is to be accounted for by their general 
habit of living in mud, and by the strong and peculiar development 
of the mantle and shell, which, by cutting off the anterior portion of 
the body (with the mouth) from direct contact with the outer world, 
renders specific sensory organs useless. In those Molluscs which 
have to seek, seize, and crush their food, a projecting head carrying 
sensory organs and furnished with buccal armature is of great use. 
Bivalves, however, feed on particles brought to the mouth by the 
water which by the motion of cilia is driven through the mantle 
cavity ; buccal armatures are thus unnecessary. 

In the Cephalopoda, the head is strengthened by the incorporation 
with it of the foot, here transformed into a circle of arm-like prolonga- 

1 Hence the term " Lipocephala,'' suggested by Lankester. 



102 COMPARATIVE ANATOMY chap. 

tions for seizing the prey. We thus have a combined head and foot 
(Kopffuss), on each side of which, anteriorly, lies a iarge highly- 
developed eye. This head is more or less separated from the rest 
of the body (the visceral dome) by a neck. 

The Gastropoda, with very few exceptions, possess a head which on 
its anterior lower side is provided with an oral aperture, on its upper 
side with eyes and tentacles, and often asymmetrically (generally on 
the right side) with a genital aperture or a copulatory organ. This head 
is distinctly separated ventrally by means of a groove or furrow from 
the foot behind it ; dorsally it passes gradually into the neck. Further 
details of this Gastropod head are given below. 

A. Gastropoda. 

1. Prosobranchia. 

The head in this order ahvays carries tentacles, which are solid, simply contractile 
(not invaginable) processes of the cephalic wall. It may be assumed that there 
were originally two pairs of tentacles, an anterior and a posterior pair. The 
posterior are called ommatopliores and carry eyes at their tips. Most Diotocardia 
possess anterior tactile tentacles, and posterior and slightly lateral optic tentacles. 

The ceplialic tentacles are ahvays innervated from the cerebral ganglion, and are 
thus distinguishable from the tentacular processes which may occur near them on 
the head or neck, but belong to the epipodium, and are innervated from the pedal 
or pleural ganglia. 

In the Dococjlossa and most MoiuMcanliK the optic tentacles do not rise sejiarately 
from the head, but are to a greater or lesser extent fused with the tactile tentacles. 
Starting \rith the tentacular arrangements existing in Dolium, Stromhus, Mostcllaria, 

we find the tactile and optic tentacles 
A B C D E r fused for a certain distance from the 

// ^ n r\ f\ base, but separating later, the tips 

lyy \ \^ \\ 11 projecting independently (Fig. 96, B). 

yl '. li 1\ 1 1 1^ t^^^ *-^^° tentacles were of the 

_/i L. ^ :' L J : I ) H_ J L.»- same length, and were fused for their 

whole extent, there would only be one 
Fm. 96.-R6lations of the tactile and optic tentacle on each side of the head, which 
tentacles m tlie Prosobranoliia. Description in . . ' 

the text would carry the eye at its tip ( Tercbra 

(J). But if the optic tentacle is shorter 
than the tactile, the eye might be met with at any point between the base and tip 
of the latter, on a projection which answers to the tip of the fused optic tentacle (D 
and E). Finally, the eye may be altogether sessile, i.e. it may lie near the base of 
the sensory tentacle in the integument of the head (F). 

The snout, which carries the mouth and is anterior to the tentacles, is very 
variously developed in the Prosobranchia. 

1. It is short and truncated in the Diotocardia, and especially in the herbivorous 
Tcenioglossa. 

2. It is prolonged like a proboscis (rostrum), but is only contractile, not invagin- 
able (Capulidcc, Strombidce, Otenopidce, Calyptrceidce), or else can be invaginated, 
commencing at the tip {Cy2}rceida;, Lmncllaridce, Naticida\ Scn/aridai, Solaridm). 

3. It is transformed into a long proboscis with the mouth at its anterior end. 
This proboscis can be invaginated in such a way that the invaginated base forms a 
proboscidal sheath for the non-invaginated anterior portion or tip. Gastropods 



VII MOLLUSGA—HEAD 103 

with such proboscides are nearly all carnivorous (the Tritonickc, Doliidai, and Cassi- 
didce, among the Stenoglossa the Bachiglossa, and a number of Toxiglossa). 

Most male Monotocardia have a non-invaginable penis, which varies in shape, on 
the right (rarely on the left) side of the head or neck, near the tentacle ; this organ 
in most cases belongs morphologically to the foot, being innervated from the pedal 
ganglion ; less frequently it is a cephalic appendage, and is then innervated from the 
cerebral ganglion (Fig. 71, p. 73). 

The head of the Setcropoda carries two tentacles (occasionally rudimentary : Ptero- 
trachea, Firoloidea). The eyes are sessile or placed on small prominences near the 
bases of the tentacles on their outer posterior sides. That part of the head which 
lies in front of the tentacles is prolonged to form a large proboscis-like non-invagin- 
able snout. 

2. Opisthobranchia. 

The shape of the head in this order varies to an extraordinary degree, and can 
here be only generally described. It usually carries two pairs of tentacles ; the 
posterior pair, which are called rhinophores, are perhaps olfactory. Their surface is 
often increased by the formation of circular folds. They frequently rise from the base 
of pits into which they can be withdrawn. The head is rarely prolonged into a 
proboscidial snout. The eyes are sessile. 

Among the Tectlbranchia, the Ccphalaspidce are distinguished by peculiarities of 
the head. It carries dorsally a flat fleshy disc, the cephalic or tentacular disc 
(Fig. 14, p. 10), which is regarded as the result of the fusion of the tentacles, and 
which, by its shape, recalls the propodium of the Naticidm or Olividie among the 
Prosobranchia. This cephalic disc carries the sessile eyes on its dorsal side, and its 
posterior lobe, which is sometimes produced in the shape of two lateral tentacular 
processes, shifts about over the anterior portion of the shell. The shape of this disc 
varies considerably in details. 

Of the very numerous Nudibranchia we shall only notice two extreme forms : 
Tethys and PhylUrrhoe. 

In Tethys, the head takes the form of a large flat disc, almost semicircular in 
shape and fringed at the edge ; this carries on its upper surface two conical rhino- 
phores, which can be retracted into large sheaths. 

In Phyllirhoe (Fig. 19, p. 12), the head is produced into a short proboscidial 
snout, which caixies only two very long curved tentacles ; the bases of these are 
encircled by integumental folds, and they may be considered as rhinophores. 

Pteropoda gymnosomata. — The head is distinct, and carries two pairs of 
tentacles, one labial and the other nuchal. The former answers to the anterior, and 
the latter to the posterior tentacles or rhinophores of the Tectihrandiia, especially 
those of the Aplysiidce. The nuchal tentacles are generally small or rudimentary, 
the rudiments of the eyes lying at their bases. 

Nearly all the G-ymnosomata, as highly-developed carnivorous animals, are provided 
with a proboscidial snout which, commencing at its tip, can be completely invagiuated, 
and carries at its base, when evaginated, buccal appendages innervated from the 
cerebral ganglion. 

Definite compensatory relations exist between the proboscidial snout and the 
buccal appendages : — 

1. "When the proboscis is specially long, the buccal appendages are wanting 
{Olionopsis). 

2. When the proboscis is of median length, it carries suckers at its base, or a 
pair of long appendages provided with suckers {Pneumodermidce, Fig. 76, p. 79). 

3. When the proboscis is short, there are long anterior tentacles, and at the base 



104 COMPARATIVE ANATOMY chap. 

of the evaginated proboscis three pah-s of conical processes (cephalic cones), with 
special nerve endings and glands whose sticky secretion helps in the capture of prey 
(Clionidce). 

i. The proboscis may be wanting. There is then on each side of the mouth a 
long extensible buccal appendage carrying at its base the labial tentacle. 

Pteropoda thecosomata. — The head is, as a rule, not sharply separated from the 
body, and has no invaginable snout, but one pair of tentacles which answer to 
rhinophores, and sometimes lie in sheaths at their bases. The left tentacle may 
become rudimentary. In the Thecosomata the male cojiulatory organ lies on the 
upper side of the head, near the tejitacle. 

3. Pulmonata. 

The head is here distinct from the foot ventrally, but passes dorsally into the 
neck. It carries two or four tentacles. The Stylom- 
iiialophm-a, which are terrestrial, have four tentacles 
(Fig. 97), an anterior and a posterior pair. The 
posterior, which are usually the longer, carry the eyes 
on their tips. The tentacles are hollow tubes filled 
with blood and connected with the blood spaces of 
the head. They can be invaginated from the very 
tip into the head, special muscles acting as retractors 
which, when the tentacle is evaginated, run from the 
head to the tip of the tentacular cavity. 

The Basoiniiiatophora, which are aquatic, have 
only one pair of tentacles which are usually triangular 
Fig. 117.— HeUx, front view, creep- and flat. They are solid, and not invaginable, but 

ing with extended tentacles (after merely contractile. The eyes lie on the inner side of 

Howes), s, Shell ; ^i, optic tentacle ; ^ligij. leases 

/, anterior tentacle : m, inoutli : ?i , t i. • r, i , i m j • ^ •* /-i 

, ,. ' In certain rahaotutta [Glaimi/ui, Zoiutes, (Jnci- 

diiim) the upper lip may be drawn out into a lobe or 

labial palp on each side. This'labial palp in Glandinu can move very freely, and is 

the seat of a fine sense of touch. 

On the right, behind the right tentacle, lies the common genital aperture, or, in 

cases where the male and female apertures are distinct, the male aperture. 

B. Seaphopoda (Fig. 101, p. 113). 

Ill this order the non-invaginable snout is ovoid or barrel-shaped, 
and projects from the body, over and in front of the foot, downwards 
into the mantle cavity. At its extremity lies the mouth, surrounded by 
a circle of dentate oral lobes shaped like oak-leaves, — four on each side. 

At the boundary between the bases of the foot and of the snout, 
to the right and left of the cerebral ganglion, a shield-shaped lobe 
rises from the body on each side ; this is attached, at the centre of its 
inner side, by a short slender stalk to the body wall, concrescence also 
taking place at its lower edge. This shield carries numerous filamentous 
or \'ermiform glandular tentacles, which move very freely and can be 
protruded far beyond the mantle aperture. 

The ends of the tentacles are swollen into the shape of a spoon, and can become 
attached to foreign objects like suckers. Each swelling has long ciliary hairs on 




VII MOLLUSCA—ORAL LOBES OF THE LAMELLIBRANGHIA 105 

its concave surface, the cilia being continued in a band all along the tentacle to its 
base. Tentacles of this sort are found in all stages of development ; they rise 
chiefly from the inner surface of the shield, and easily become detached or broken 
off, and are then regenerated. They are no doubt chiefly useful as organs of touch, 
and serve for seizing particles of food (Foraminifera, etc.). They may further assist 
respiration in the absence of localised gills, by causing increase of surface. The 
tentacles are innervated from the cerebral ganglion through the stalk of the shield 
on which they stand. 

C. Cephalopoda. 

In Nautilus, there are on each side one tentacle above and one 
below the eye. It is not improbable that these two tentacles cor- 
respond with the two pairs of tentacles in the Gastropoda. 



IX. The Oral Lobes of the Lamellibranehia. 

The oral aperture of the Lamellibranehia is produced right and 
left in the form of a groove, which runs backward along the surface 
of the body to the anterior end of the base of the gill, or to some 
point near it. This groove is bordered by two projecting ridges 
above and below it. The two upper ridges, at the point where they 
meet, form a sort of upper lip over the mouth, the lower ridges, in 
the same way, forming a lower lip. The groove between the ridges 
serves for conducting to the mouth the particles of food which are 
swept past the gills by the cilia. 

The length of the groove is naturally determined by the distance 
between the anterior ends of the gills and the mouth. 

The two ridges just described are continued posteriorly in the 
shape of thin lamellae, which hang down into the mantle cavity. These 
lamellae, between which the groove becomes a deep, narrow cleft, are 
the oral lobes or labial palps of the Lamellibranehia. They are more 
or less triangular, one side of the triangle forming the base by which 
the lobe is attached to the body. 

In cases in which the gills lie far behind the oral aperture, the bases of these 
lobes are long, but in others, where they begin near the mouth, the bases are short, 
and each lobe then usually forms a long, free, pointed process. The surfaces of these 
two oral lobes are ciliated, and, further, the surfaces which face each other, i.e. 
which have the groove between them, are striated at right angles to their bases. 
This striation is caused by parallel ridges, and gives the lobes a superficial resem- 
blance to gills. The lobes contain blood lacunic, and it is probable that, besides their 
chief function of conducting food to the mouth, they may assist in respiration. 

In certain forms, the free edge of the upper lip folds over that of the lower 
{Ostrea, Tridacna) ; in others, the two edges are closely apposed and interlocked by 
means of processes and folds {Pcdeit, Spondyhis), so that a closed cavity rises in 
front of the mouth, into which the gi'oove brings particles of food from each side. 
The edges of the upper and lower labial palps may even grow together (Lima). 

Nucula (Fig. 21, p. 14), in which the ctenidium lies far back, and has a very 
small respiratory surface, may serve as an example of very highly developed oral lobes, 



106 COMPARATIVE ANATOMY chap. 

which were formerly considered to be gills. The base of the lobe here sti-etehes 
along the whole length of the base of the foot, and is further prolonged posteriorly 
in the shape of a free appendage with a groove running along it. This process can 
be protruded beyond the shell, and probably assists in conducting food to the 
mouth. 



X. The Foot and the Pedal Glands. 

The ventral side of the body in the Mollusea is characterised by the 
pronounced development of its musculature, which enables the animal 
to creep, a fleshy foot, provided vi^ith a flat sole suited for creeping, 
distinct from the rest of the body and especially from the head, being 
developed. This strong ventral musculature must be considered as the 
remains of the dermo-muscular tube of the racial form, which attained 
greater development on the ventral side in adaptation to a creeping 
manner of life, while it degenerated on the dorsal side, being rendered 
functionless and useless by the hard shell. 

The flat form of the foot with a sole for creeping must be con- 
sidered the primitive form. Such a foot is found in the Chitonidce 
among the AnvpJiineura, in most Gastropoda, and in certain Lamelli- 
hranchia, especially in the Frotohranchia, which for other reasons also 
must be considered the most primitive form of Lamellibramhia. 

The musculature of the foot and of all parts which become differ- 
entiated from it are innervated from the pedal ganglia or pedal nerve 
cords. 

The foot may become much modified in adaptation to various 
methods of life and of locomotion, — in fact, it may entirely lose all 
resemblance to the primitive organ. It may, by constriction or by 
the formation of lobes or folds, fall into several parts, of which the 
following are the most important : — 

1. Proceeding from before backward we have the ppopodium, an 
anterior portion distinct from the rest, and the metapodium behind 
the former and seldom very distinct, which carries the operculum 
when this is present. 

2. From below upward there are the parapodia, lobe-like exten- 
sions of the edge of the ventral sole, and the epipodium, a projecting 
ridge or fold round the base, i.e. round the upper portion of the foot. 
Tentacular processes are often developed on this ridge. 

Taking the different groups in order, the following variations of 
the foot and the pedal glands (mucous glands and byssus gland) are to 
be noted. 



A. Amphineura. 

{Of. Section II., p. 29). The foot is here not divided into separate consecutive 
portions, and there are no parapodia or epipodia. 



VII 



MOLLUSGA—THE FOOT AND ITS GLANDS 



107 



B. Gastropoda. 

1. Prosobranchia. 

Witli rare exceptions, which will be described later, the foot, which is well 
developed in this order, has a simple (undivided) flat sole for creeping. 

Propodium. — In a few cases, however, the anterior portion of the foot forms a 
propodium well marked off from the rest of the organ. This is especially the case 
in the Monotomrdia {OHvidce, Harpidce, certain species of Pyrulidm, StromUdm, 
Stromhus, Pterocera, Terehdlum, Rostellaria [Fig. 6, p. 6], Xenoplioi-idce [Fig. 5, p. 5], 
NariHdcB, Naticidm [Fig. 98]). 

Among the above, the propodium is pai-ticularly well developed in Oliva, sepa- 
rated from the rest of the foot by a transverse furrow and forming a semicircular 
disc. 

In the large foot of Natica (Fig. 98), the propodium is also very distinct. It 
has an anterior lobe which bends back over the shell, and so covers the head. 




Fio. 98.— Natica Josephina, with protruded proboscis, from tlie right side (after Schiemenz). 
1, Propodium ; 2, suclcer-lilce boring appendage of the proboscis (3) "vvith boring gland ; 4, siphon 
(here formed by the foot) ; 5, tentacle ; 6, lobe of the metapodium, which usually covers a large 
part of the .shell from behind, and carries the operculum on its inner side ; 7, metapodium. 

Sometimes the propodium forms a sort of siphon on the left side, and in other cases 
the lobe which bends back over the shell shows a bulging. Both these arrangements 
serve to conduct water to the respiratory cavity. The metapodium also, which, 
when swollen and expanded, spreads out widely, carries on its dorsal side a lobe 
which bends forward over the shell, and carries the operculum on the side nearest 
the shell. 

In most Prosobranchia the metapodium carries, on its dorsal side, a hornj' or 
calcareous operculum which serves to close the shell. 

Epipodium. — The epipodium is very commonly present in the Diotocardia. It 
is most strongly developed in Haliotis (Fig. 105, p. 121), where it surrounds the base 
of the foot in the form of a large integumental fold. This fold, which may aptly 
be called the ruff, has fringed or digitate appendages as well as long contractile 
tentacular processes. The tentacles here, as in other Prosobranchia, are organs of 
touch, and may be provided at their bases with so-called lateral organs. In the 
Fissitrellidce this epipodial ruff is replaced by a row of numerous tentacles or 
papillse, rising on each side from the base of the groove between the base of the foot 
and the visceral dome. Among the other Diotocardia also, the epipodium is well 



108 COMPARATIVE ANATOMY chap. 

developed as a simple or fringed border, which cariies a few tentacles (usnally four 
on each side) of varying length (Fig. 3, p. 4). At the base of each tentacle there 
is a lateral organ. Eyes are said to occur at the bases of the epipodial tentacles in 
Eimuirgerita and Scissurella. 

The epipodium is, as a rule, wanting in Docoglossa, but one is found beset with 
l)apillse in the genus Hclcion, and in Patimlla and Nacella it is fringed ; these 
I'pipodia correspond in position with those of other Diotocardia, 

A well-developed epipodium rarely occurs among the Monotocardia, but lanthina 
has a typical epipodial border, and the Litiopidce and many Eissoidoi have an 
epipodium with several (1-5) tentacles on each side. Many other Monotocardia 
have retained either the anterior or posterior portions of the epiisodium. 

(a) Anterior vestiges of the epipodium are found in Vermetus in two anterior 
pedal tentacles, and in Paludinn and Ampullarta in two nuchal lobes, which 
must not be confounded with true cephalic tentacles. In Paludinn, the right 
nuchal lobe, and in Ampullaria the left, forming a longitudinal groove, becomes a 
sort of siphon. Calyplrcea possesses on each side under the neck a semicircular 
epipodial fold. 

(h) Posterior vestiges of the epipodium are found in Lacuna in the form of an 
i-'liipodial fold with a process on each side above the foot. Nan'ca has, above the 
metapodium on each side, a iving-like epipodial lobe. 

(c) Median and posterior vestiges of the epipodium are found in Choristes, where 
there is a median papilla on each side, and posteriorly a pair of tentacles below tlie 
operculum. 

The epipodium is always innervated from the pedal nerve cords or the homolo- 
gous pedal ganglia, or from the pleural ganglia which separate off from the latter. 

The foot of HipjMjDjx undergoes a curious transformation. Hippamj.v is a 
Monotocardian genus, with a conical shell ; the animal attaches itself firmly to 
rocks or the shells of other Molluscs, which it excavates, either directly or by 
means of a shell plate, which probably answers to the operculum. The median part 
of the sole of the foot has lost its muscle layer, and its edge has united with the 
edge of the mantle, leaving only an anterior aperture through which the head can 
be protruded. On the lower side of the foot, the eobimellar muscle which descends 
from the shell gives rise to a liorseshoe- shaped muscular area surrounding the 
central non-muscular part. 

Without going into details as to the method of locomotion of the Prosobranchia, 
it may be stated that most of them creep or attach themselves by means of the flat 
sole of the foot. 

Heteropoda. — The Heteropoda are pelagic Prosobranchia {Monotocardia), which 
liave exchanged the creeping for the swimming manner of life. The foot has in 
them become peculiarly adapted to this new method of locomotion. The propodiuni 
has become changed into a narrow vertical rowing fin (carinate foot), which when 
the animal is in its swimming position is turned upward. 

The development of this vertical fin can be traced almost step by step within 
this division, starting with Oxygyrus, and proceeding through Atlanta and Cariiiaria. 
to Ftcriitrndiea. In this series, the typical outer appearance of the Prosobranchiate 
(its shell, visceral dome, mantle, and gills, which are still retained in O-ajgyrus and 
Atlantii), gradually disappears owing to development in another direction. 

Oxygyrus (Fig. 99, A) still has the characteristics of a Prosobranchiate. The 
foot consists of (1) a propodium, the creeping sole of which has been somewhat 
]iollowed out or deepened ; anteriorly it possesses a fin-like outgrowth, which is used 
as a propelling organ in swimming ; and (2) a distinct metapodium directed backwards 
like a tail, and bearing an operculum. The derivation of such a foot from that of 
certain Prosobranchia, which have distinct propodia and metapodia, such as the 



VII 



MOLLUSOA—THE FOOT AND ITS GLANDS 



109 



saltatory Strombidai, is clear. The sole of the foot in Oxygyrus, although it can be 
used for creeping, is looked upon as a sucker. 

In Atlanta (B), the arrangements of the foot are similar to those in Oxygyrus, 
but the iin-like outgrowth of the propodium has become its most important part, 
the comparatively reduced sole or sucker appearing merely as an appendage to it. 

In Oarinaria (C) both the foot and the general external appearance of the whole 




Fio. 99.— Comparative Morpliology of tlie Heteropoda. A, Oxygyrus. B, Atlanta. C, 
Carinaria. D, Pterotrachea 9 , adapted from figures by Souleyet. 1, Visceral dome and shell ; 
2, head with eyes and tentacles and proboscidal snout (3) ; 4, gills ; 5, foot with sole, which latter in 
B and C is rednced to a sucker, and in D is wanting ; 6, fin-like appendage of the foot ; 7, meta- 
podium with, 8, operculum. 

animal are much changed. The metapodium, which here has no operculum, appears 
as a mere tail-like posterior prolongation of the body. The fin is much broader and 
longer, and the sucker seems to have shifted backward along its free edge. 

Finally, in the Pterotrachea (D), the sucker (the original sole of the foot) is still 
further reduced, and only present in the male. 

The Heteropoda are said to attach themselves occasionally by means of the sucker. 



2. Pulmonata. 

The foot is here almost always undivided, and provided with a, large flat sole 
for creeping. In a few Aurieulidce, however [Melampus, Leuconia, Blauneria, 
Fedipes), it is divided into two portions by a. temporary or permanent transverse 
groove. 

3. Opisthobranchia. 

In almost all Opisthobranchia the foot has a well-developed sole for creep- 



no 



COMPARATIVE ANATOMY 



CHAP. 



A 




ing. There is no division into parts, and the adult rarely {Actmon) carries an 
operculum. 

The epi podium is wanting. 

The parapodia, on the contrary, i.e. lateral lobes or fold-like extensions of the edges 
of the sole, are highly developed in many Opisthobranchia {e.g. the Elysiadce among the 
Ascoglossa, and very many Tedihraiichia, such as the Scaphandridce, HuUidce, Aplus- 
trldcc, Gastropteridcc (Fig. 14, p. 10), Philinidce, Doridiida, Aplysiidce (Fig. 75, 
p. 78), Oripioeidce). The parapodia are often bent 
back over the shell, their edges sometimes touching, 
so that the shell may be entirely roofed over by them. 
In many forms which are provided with parapodia 
{Gastroptcriila; PhUinidtr, Doridiidm, Aplysiidce) the 
mantle also bends back over the shell, more or less 
completely covering it. In these cases the shell is 
to some extent doubly internal, being covered first 
by the mantle and then (not in Ph iliiic and Doridium) 
by the parapodia (Fig. 100). 

The parapodia may fuse posteriorly along their 
upturned edges {Aphjslidiv, Oxynoe). In Lobiger each 
parapodium is transversely slit, so that two long 
wing-like processes are formed on each side. Many 
Opisthobranchia {Aplysiidce, Oxynoe, Gastropteridm) 
can propel themselves through the water by means of 
the Avaving motion of their parapodia. PhyllirhoS 
is a Nudibrandi which appears to have become 
adapted to a pelagic swimming manner of life by the 
compression of its body into the shape of a long 
narrow leaf with sharp dorsal and ventral edges ; it 
travels through the water with an undulating motion 
(Fig. 19, p. 12). The foot has disappeared. 

Pteropoda. — The Pteropoda, which are Tecti- 
branchiate Opisthobranchs, have, like the Proso- 
brancMate Scteropoda, become pelagic animals adapted 
for swimming. 

"While in the Rctcrupoda the propodium becomes 
transformed into a medio-ventral vertical rowing fin, 
in the Pteropoda the paired Tectibranchiate parapodia 
which, as we have already seen, can be used for swim- 
ming, develop into the paired fins or wings of these 
animals (Figs. 16 and 17, p. 11 ; 87, p. 91). 

In the Thecusomata (Fig. 87, p. 91), which must 
be derived from Caphalaspidm {BuUoidea), in which 
the parapodia lie on each side as direct prolongations 
of the reptant surface of the foot, this organ, i.e. the 
foot, has become confined to the anterior end of the 
body, and consists of three portions — the median un- 
paired mesopodiiun and the two lateral parapodia or fins. The mesopodium is small, 
and the ventral side of it (which corresponds with the sole of the CephalaspidcB, but 
can no longer be used for creeping) is strongly ciliated. The ciliary movement is from 
behind forward, i.e. towards the oral apertm-e which lies anteriorly on the foot, 
and no doubt serves for conveying to it the minute marine animals on which the 
creature feeds. On the dorsal side of the mesopodium, which projects freely back- 
wards, the Limacinidcc carry a delicate transparent operculum, which often becomes 



'">%^': 




Fig. 100.— Diagrammatictrans- 
verse sections of Gastropods, to 

illustrate the arrangement of the 
shell (black, 1), visceral dome and 
inantle(dotted, 2), and foot (streak- 
ed, 3). A, Prosolaraiicliiate with 
outer shell and epipodium (i). B, 
TectibranoMate with lobes (6) of 
the mantle turned back over the 
outer surface of the shell. Dorsally 
the shell is still uncovered ; .5, para- 
podia ; 7, ctenidium. C, Tecti- 
brancliiate with internal shell, i.e. 
completely overgrown by the lobes 
of the mantii^ 



vii MOLLUSGA—THE FOOT AND ITS GLANDS 111 

detached.^ The parapodia are large, fin- or wing-like, and anteriorly inserted on each 
side of the median portion of the foot ; they unite in front of and above the month. 

The Gymnosomata (Fig. 16, p. 11) are to be derived from the Aplysiidoe, in 
which the parapodia are not exactly lateral extensions of the sole of the foot, but 
arise somewhat above the edge of the sole on each side. This may be explained by 
supposing that they are fused for a certain distance from their bases with the lateral 
wall of the body. In the Gymnosomata, also, the foot is distinctly separated from 
the two lateral fins or parapodia. The mesopodium and the fins lie anteriorly on 
the ventral side of the body, behind the head. 

The foot itself, which is distinct from the head, consists of three parts — a pair of 
anterior lobes, which converge anteriorly till they unite, and a median posterior lobe 
drawn out to a point posteriorly. The fins never unite in front of or above the head. 

Pedal g-lands of the Gastropoda. — Many Gastropods, and especially 
most Frosohnnchia and Pulmonata, possess, besides the various unicellular 
glands scattered over the upper and lower sides of the foot, larger multi- 
cellular localised pedal glands. These belong to two morphologically 
distinct groups. 

1. In the Frosobranchia an anterior pedal gland opens at the 
anterior edge of the foot. In those forms in which this anterior edge 
is divided into an upper and a lower lip, this " labial gland " opens 
between the lips. In the Fulnwnata it opens externally between the 
head and the foot. It consists of an epithelial tube of varying length, 
not infrequently as long as the foot itself ; this tube runs backward 
in the median line mostly through the base of the foot ; less frequently 
it lies upon this base, projecting into the body cavity. 

This tube serves both as reservoir and duct for the numerous 
unicellular mucous glands which lie in the surrounding tissue of the foot 
and open on its walls. It secretes mucus, though it has been incorrectly 
described as an olfactory organ. It undergoes considerable modifica- 
tions with regard to its size, the form of its lumen, and the number and 
arrangement of its glandular cells. 

2. Among the Frosobranchia, opening on the sole of the foot, there 
is commonly found an unpaired gland. Its outer slit-like aperture is 
median, and lies behind the anterior edge of the foot. It leads into a 
cavity in the foot which serves as a reservoir ; the epithelial wall of 
this cavity projects in the form of folds into its lumen. As in the 
former case, unicellular glands pour their secretions into it through 
ducts which pass between the epithelial cells. This sole gland in the 
Frosobranchia has rightly been considered homologous with the byssus 
gland of the Lariiellibranchia. It is developed in varying degrees, and 
not infrequently is altogether wanting. Its slimy secretion forms 
threads hj means of which many Frosobranchia attach themselves to 
objects in the water. Some terrestrial Pitteowato also lower themselves 
from a height (from plants) by means of the tough threads which t\\e.j 
secrete. 

' With regard to the derivation of the The.cosmnata from the Cephalaspidm, which, Uke 
other Opisthobranchia, have as a rule no operculum, it must be noted tl\sX Actceon, which 
is in many respects a primitive Gepludaspid genus, possesses an operculum. 



112 GOMPABATIVE ANATOMY chap. 

Besides these two, other pedal glands are occasionally found. Only one need be 
mentioned, which is found in some Oj)isthobranehia (FleurobrancJms, Pleurobraiichcm, 
Pleurophyllidia). It lies at the posterior end of the sole, and consists of glandular 
ciieca, each of which opens separately. 



G. Seaphopoda. 

The foot of DemtaZiitH! (Fig. 101) is almost cylindrical ; it projects 
downwards into the tubular mantle cavity, and can be protruded 
through its lower aperture. The free end of the foot is conical ; the 
base of the cone carries on each side a fold or ridge which has been 
compared, with questionable propriety, to an epipodium. These two 
lateral folds or ridges encircle the base of the conical end without 
uniting either anteriorly or posteriorly. A groove runs along the 
anterior middle line of the foot. 

In Siphonodentalium both this groove and the lateral lobes are 
wanting, and the anterior end of the foot is broadened into a round 
disc carrying on its edge small conical papillae. 

D. Lamellibranehia. 

The foot in this class is, as a rule, laterally compressed, and has a 
sharp edge directed downwards and forwards, which can be stretched 
out beyond the shell. It may be called hatchet-shaped {PeUcypoda) or 
linguiform, and is especially suited for forcing its way into mud by 
means of alternate contraction and expansion. 

This peculiar shape must be considered as acquired. Originally 
the foot of the Lamellibranehia also possessed a flat sole for creeping. 
The Protobranchia, in fact, have a foot with a ventral disc (Fig. 21, 
p. 14), and so has Pecfiiwulus. The edge of this pedal disc is notched 
or toothed. When the foot is retracted, this disc folds down the 
middle line. 

The foot in the Lamellibranehia varies much in details, according 
to the manner of life or of locomotion of the animal, and according to 
the development of the byssus. One of the special characteristics of 
the Lamellibranchiate foot is the gland which secretes the byssus, the 
latter being a bundle of tough threads varying in thickness, and 
resembling horn in their physical properties. The Lamellibranch, with 
these threads, anchors itself to foreign objects. The byssus can 
generally be thrown off and replaced by a new one, and many forms 
can move about on a smooth perpendicular pane of glass by means of 
alternate attachment and rejection of portions of the byssus applied 
by means of the foot. 

Stationary bivalves, i.e. those attached by one of the shell valves, 
are in the first instance attached by means of the byssus, for a byssus 
is, as a rule, present in the young stages of those bivalves which do 
not possess it as adults. 



VII 



MOLLUSGA—THE FOOT AND ITS GLANDS 



113 



Fig. 101.— Anatomy of Dent- 
alliim entale, after Leuckart 
(Wandtafein) and Lacaze-Du- 
tMers. The riglit half of the 
shell and the lower portion of 
the raantle are removed. a, 
Pallial nerve running up from 
the visceral ganglion ; b, shell ; 
c, space between the raantle and 
shell ; d, anus ; e, visceral gan- 
glion ; /, mantle cavity ; g, 
mantle ; }i, lower, t, upper buccal 
ganglion ; i, auditory organ ; k, 
pedal ganglion, m, lateral folds 
of the foot ; n, terminal pedal 
cone ; o, filamentous tentacles ; 
I, lower edge of the mantle; p, 
leaf- like oral appendages; q, 
snout; r, cerebral ganglion; s, 
shell or columellar muscle, cut 
through ; u, right nephridial (and 
genital) aperture ; v, digestive 
gland (liver) ; w, gonad ; x, upper 
end of the columellar muscle ; y, 
upper open end of the mantle. 




VOL. 11 



114 



GOMPABA TI VE A NA TOM Y 



CHAP. 



The complete byssus apparatus (Fig. 102) consists of : (1) a cavity 
in the foot, into which the byssus gland opens ; (2) a duct connecting 
this cavity with the exterior; (3) a groove which runs from the 
aperture of the duct along the ventral edge of the foot to its anterior 
end ; and (4) a crescent-shaped or cup-like widening of the groove at 
its anterior end. 



(1) The byssus cavitj^ is divided into narrow shelves by numerous folds, which 
project from each side into its lumen. A septum, descending from its roof, further 
divides it into two lateral parts. The byssus secretion is yielded partly liy the cells 
of the epithelial walls, and partly by glandular cells which lie in the surrounding 
tissue, their ducts passing between the epithelial cells. The secretion takes the 
form of the cavity, and is thus held fast as -with roots by the numerous lamella; 
which occupy the shelves. As the amount of the secretion in the cavity increases, 
these lamellfB are pressed into the duct (2), where they unite to form the main stem 
of the byssus. 

The walls of the groove (3) and its terminal expansion (4) are also glandular. 
When a bivalve attaches itself it forms a byssus thread 
in this groove, which fuses with the end of the main 
stem. The tip of the foot presses against some surface, 
such as a rock, and attaches the thread by means of a 
cement secreted by the widened terminal portion (4) of 
the groove. In this way the main stem of the byssus 
may be fastened to a rock by means of numerous threads 
successively secreted in the groove. 

The relation existing between the development of 
the foot and that of the byssal apparatus may be 
sketched as follows : — 

1. The foot in its primitive form, with a flat sole 
and no groove, has a simple invagination without 
liyssus (Solenomya). 

2. With the same foot, a small lamella rises from 
the base of the simple invagination ; the byssus is 
very slightly developed {Kurula, Leda). 

3. The invagination becomes differentiated into a 




-Byssus of a Lamel- 



FlG. 102. 
libranoli with its cavity >na 
duct. 1 Diafi-ammatic trans- cavity and a duct, and the byssus and its glands are 



I'erse section tlirough the foot ; 2, 
niain stem ; 3, terminal threads 
attachinj; the byssus to a foreign 
object. 



strongly developed. In consequence of this the foot 
ceases to be a locomotory organ ; its flat sole 
disappears, and it becomes finger- or tongue-shaped, 
often more or less reduced in size, and serves for 
attaching the byssus. In very many cases the groove is formed from the end of the 
duct, widening at the tip of the foot as above described. This is especially the case in 
forms which anchor themselves by the byssus to stones, plants, or the shells of other 
Molluscs. This attachment may be more or less firm, and may be temporary or 
permanent (Liiiiidce, Spondiilidce, Pectiiiidu:} Mi/lilidcc, Arcidiv,^ Carditidcc} Ery- 
cinidic, lialeommklcc, Tridnciiidu:, Cjiprinidce,^ Veneridtv,^ Glycymcridce, Alyidce,^ etc.) 
When the byssus is very lijghly developed, some of the pedal muscles become 
attached to the byssus gland and form the retractors of the byssus. 

4. Many Laniellibranchs, in the adult state, have neither byssus nor byssus 
glands, but tlie cavity, the duct, and even the retractors {e.ij. Trigonia) may be 



Pro 2>arte. 



VII MOLLUSCA—THE FOOT AND ITS GLANDS 115 

retained. The byssal apparatus may be found, in closely-related forms, sometimes 
mtli and sometimes without the byssus itself. In the latter case the foot is 
generally more strongly developed, and serves for locomotion, i.e. for forcing a way 
forward into sand or mud, which most of these forms inhabit, or for the saltatory 
motion of Trigonia. In these cases it is linguiform, or wedge- or hatchet-shaped 
[Ai'i-ida-,'^ CarditidcB,^ Oi/prinidce,^ Tellinidce, ScrobiculariidcB, ilyidm,^ Cardiidcc,'' 
iHc-»«(Vte (foot vermiform), Donacidce, etc.). 

5. When the linguiform, or hatchet-shaped, and often bent, foot becomes more 
strongly developed as a fleshy and extensible organ, every trace of the byssus and its 
apparatus disappears, at least in the adult {Unimiidx, many Veneridce, Cyrenidcc, 
Psammoliidcc, Mesodermatidce, Solenidce, Mactridce). All these live in mud. The fleshy 
foot of the Solenidce, which is directed forwards, is so strongly developed that it can 
often no longer be wholly withdrawn into the shell, which therefore gapes anteriorly. 
The foot is thick and linguiform in Solenocurttis ; club-shaped and truncated at the 
tip in Pharus, Cultellus, Siliqua, and Ensis ; and cylindrical, with an egg-shaped 
tip, in Solen. 

6. In forms where one of the valves has become firmly attached to some hard 
substance, the foot (the byssus being absent) may become rudimentary (Chamacea), 
or may altogether disappear ( OstrcidcB). In forms which inhabit mud or excavations 
made by themselves in stone, etc., and which surround the body with an accessory 
calcareous tube (Gastrochcenidce, Ctavagellidce), the foot is also reduced to a small, 
usually finger-shaped rudiment. The series of boring Fholadidce is specially 
interesting. Pholas has a pestle- or sucker-shaped foot, which, projecting tlii'ough 
the shell cleft, serves to attach the animal while boring. In Pholadidea and 
Jouajinclia only the young while boring their habitations possess such a foot ; 
as soon as they have finished this work the pedal aperture of the mantle closes, the 
anterior cleft of the shell is also closed by means of an accessory shell-piece called 
the callum, and the foot completely atrophies, so that the animals are no longer 
capable of locomotion. 

In the attached Anoinia, also, the foot is small : it is of great importance, how- 
ever, as bearer of the byssal apparatus. The shelly plug (see p. 63), by means of 
which the animal is fastened to the ground, and which occupies the deep notch cut 
by the byssus into the right or under valve, must be regarded as a calcified byssus. 

Many Lamellibranchs (Crenella, Lima, Modiola) weave a byssus web which they 
inhabit like a nest, and which they strengthen by the addition of foreign bodies 
attached by byssus threads. 

E. Cephalopoda. 

The question, what part of the body in Cephalopoda corre- 
sponds with the foot of other Mollusca, has led to much discussion and 
careful investigation. It may now be considered as pretty well estab- 
lished that the foot in Cephalopoda forms : (1) the arms, (2) the siphon. 

The arms are considered as lateral processes of a Mollusoan foot 
which have pushed past the head to the right and left, and have 
united in front, so that the head is entirely encircled by the foot, and 
the mouth has come to lie in the middle of the ventral pedal surface, 
i.e. at the centre of the circle of arms or brachial umbrella. That this 
circle of arms is a derivative of the foot is supported by important 
anatomical and ontogenetic facts : (1) The arms are innervated from 

1 Pro parte. 



U6 



COMPARATIVE ANATOMY 



CHAP. 



the brachial ganglion, which lies under the oesophagus, and is an 
anterior division of the pedal ganglion. (2) The arms do not 
occupy, in the embryo, their definitive position round the mouth, but 

rise on the ventral side behind the 
mouth, between it and the anus, in a 
row on each side. These two rows shift 
secondarily forward to form the circle of 
arms round the mouth. (According to 
another view, the arms are cephalic ap- 
pendages, comparable with the cephalic 
tentacles of the Pteropoda.) 

The pedal nature of the siphon or 
funnel has rarely been doubted. It is 
innervated from the pedal ganglion. Its 
two lateral lobes, which in the Nautibts 
remain separate throughout life, but in 
the DibmncMcita overlap, may be con- 
sidered as epipodia. The accompany- 
ing figure of a Cephalopod embryo con- 
firms this opinion ; the rudimentary 
siphon is seen in the typical position of 
epipodia in the shape of two lateral folds 
running backward above the foot and 
under the visceral dome. 
In Nmdilus and the Decapoda (excluding the Loligopsidce) a valve 
is present within the siphon. For the form of the siphon, see p. 38. 




Fig. 103.— Embryo of a Oephalopocl, 

seen obliquely from the left posterior side 
(after Grenadier). 1, Mantle ; 2, anus ; 
3, right ctenidium ; 4, rudimentary 
siphon ; 5, auditory organ ; 6, arms ; 7, 
yolk-sac ; 8, left eye. 



1. The Arms of the Tetrabranchia (Nautilus). 

The "head" of the Nautilus (Fig. 104) carries numerous tentacles placed in a 
circle round the moutli ; these do not rise directly from the integument around the 
mouth, but stand upon special lobes which are differently developed in the two 
sexes. These lobes may be compared with the arms of the Dibranichia, and the 
tentacles they carry, perhaps, with the suckers on those arms. Each tentacle can be 
retracted into its own basal portion as into a sheath. 

If the head be viewed from the ventral side, so that the mouth appears lying in 
the centre of the extended lobes and tentacles, we see in the female (lower figure) 
three inner lobes close to the mouth, two lateral and one posterior. The posterior 
inner lobe consists of two fused lateral lobes, the line of fusion being indicated by a 
lamellated (olfactory ?) organ. It carries twenty-eight tentacles, fourteen on each side. 
Each lateral inner lobe carries twelve tentacles. Besides these three inner lobes, 
the foot develops a muscular circular fold ; this is ptti-ticularly thick anteriorly, and 
here forms a lobe, the so-called liood (Fig. 32, a, p. 22), which, when the head is 
retracted, covers the aperture of the shell like an operculum. The outer circular 
fold carries nineteen tentacles on each side. 

Besides these tentacles which belong to the foot, there are two more on each side 
which probably belong to the head, one lying above and the other below the eye. 

In the male Nautilus (upper figure) the posterior inner lobe is rudimentary. 
Each of the lateral inner lobes is divided into 'two portions. In the right lobe, the 
anterior portion carries eight tentacles and the posterior (antispadix) four, three of 



VII 



MOLLUSGA—THE ARMS OF THE CEPHALOPODA 117 



which have a common sheath. The anterior portion of the left lobe also carries 
eight tentacles, and the posterior portion forms the conical spadix, which, instead 




Fig. 104.— Ciroumoral ring of tentacles In Nautilus pompilius (after Lankester and Bourne). 
From the oral or ventral side. Upper figure male, lower female, a, Shell ; h, circular fold or hood 
with its tentacles, g ; c, the two lateral inner lobes, in the male the left inner lobe forms the spadix 
or hectocotylus p, and the right the antispadix q ; d, the posterior inner lobe, reduced in the male ; 
11; lamellated organ (olfactory?) ; e, jaws in the buccal cone ; /, the tentacles of the outer muscular 
circular fold ; I, eye ; m, paired lamellated organ ; o, siphon or funnel. 

of tentacles, carries imbricated lamellae. This spadix is looked upon as the hecto- 
cotylised limb of the Nautihis, and probably takes some part in copulation (see 
the Copulatory Apparatus, p. 242). 

2. The Arms of the Dibranohia. 

The Dibranchia have either eight or ten arms, which stand in a circle round the 
mouth and carry two longitudinal rows of suckers (acetabula) ; rows of cirri may 
accompany the suckers, and the cirri may here and there become transformed into 
hooks or claws (e.g. Onychotcuthis). 



118 COMPARATIVE ANATOMY chap. 

In many Odo-pada, the long arms are connected by means of membranes near 
their bases, and occasionally as far as their tips. In the latter case the circle of 
arms has the appearance of an umbrella, of which the arms are the ribs. The 
mouth lies in the centre. The Octopoda can creep by means of their circle of arms, 
the visceral dome standing erect. In this position they may best be compared 
with snails, the ventral side of the circle of arms functioning like the sole of the 
snail's foot. 

The Dccapoda have ten arms ; eight of these correspond with the eight arms of 
the Oct02)oda, but are shorter and are never connected by membranes. The two 
others, the prehensile tentacles, are inserted between the third and fourth Octo])odan 
arms on each side and differ from the latter in structure, being long and vermiform, 
with swollen ends armed with suckers, hooks, etc. The prehensile tentacles are 
very contractile, and in many Dccapioda (e.g. Sepia) are concealed in special cavities 
of the head when the animal is at rest. These cavities probably correspond morpho- 
logically with the water pores, which often occur elsewhere at the bases of the arms 
or on the head. When pursuing prey the Decapods dart these tentacles out of their 
cavities with great force. 

One (less frequently two) of the' eight or ten arms of the male Dibranchia is 
almost always transformed (hectocotylisecl) to assist in copulation. In some 
Octopoda it even becomes detached from the bodj' and is regenerated. 

The hectocotylised arm is, in the Octopoda, usually the third arm on the right 
side, and in the Decapoda the fourth on the left. (The arms are counted from 
before backward. ) 

In the female Argonaut, each arm of the first pair is widened into a sail-like 
expansion, which stretches back over the outer surface of the shell. 

All Cephalopods, even the more massive Octopoda, are good swimmers. In swim- 
ming, the mantle and funnel play the chief parts. Water is alternately taken into 
the mantle cavity through the mantle cleft, and expelled from it forcibly through 
the funnel, the reaction propelling tlie animal backwards. When the water is being 
ejected, the mantle cleft is closed by the locking apparatus, so that all the water in 
the mantle cavity has to pass out through the fuunel. Many Decapoda can also 
swim with the head directed forward, the lower (distal) end of the funnel being bent 
round, so that the water is expelled in the direction of the visceral dome. In 
swimming the arms are apposed to one another, so as to diminish the friction as 
mucli as possible. Some Octopoda,, especially those which have interbrachial 
membranes, assist themselves in swimming by opening and shutting their circle of 
arms like an umbrella. 



XI. Swelling' of the Foot (Turgescence). 

Imbibition of Water. 

The foot in many LamellibrancJiia and Gastropoda may swell when 
it has to be protruded from the shell and used for locomotion. Until 
recently opinions varied very much as to the way in which this swell- 
ing or expansion took place. Many believed that water was taken 
up from without into the blood vascular system or into a special 
water vascular system, but there was difference of opinion as to the 
manner in which it was taken in. On the one hand it was said to 
enter through apertures or pores in the foot, which, however, do not 
exist, the only pores found being the apertures of the pedal glands 



VII • MOLLUSCA— MUSCULATURE 119 

(byssus and sole glands). On the other hand, the water was sup- 
posed to enter the foot through intercellular ducts between the 
epithelial cells, but this theory has also been disproved. 

Others, again, maintained that the water was conducted by the 
nephridia to the pericardium, and conveyed thence through the blood 
vascular system ; but the pericardium has been shown to be entirely 
separated from the vascular system. Indeed, many theories on this 
subject have been put forward and disproved. 

It is now the received opinion that, except in the case of one 
animal, which will be presently described, the foot is swelled by a 
rush of blood which, flowing into the foot, is prevented from returning 
to the body by sphincter muscles. 

The exceptional case is that of Natica Josepliina. In this animal 
there can be no doubt that water is taken in to swell the foot. The 
swelling takes place very quickly — in less than five minutes. When 
the foot is stimulated it gives out an amount of water which would 
fill the empty Natica shell two or three times. The water is taken in 
through very small slits, invisible to the naked eye (probably indeed 
through a single very narrow slit, lying at the edge of the foot), and 
finds its way to a system of water sinuses, quite distinct from all 
other cavities of the foot, and also distinct from the blood vascular 
system (which in Natica is closed). There can thus be no question of 
a direct taking in of water into the circulatory system. The water 
slits at the edge of the foot can be closed by muscles, which extend 
from their upper to their lower edges. 



XII. Musculature and Endoskeleton. 

This chapter has for its subject simply the general musculature of the body. It 
would be impossible to describe in detail the musculature of special organs, such as 
the intestine, the heart, and the copulatory organs, that of the cutis, or even that of 
the most muscular of all the organs — the foot ; since, owing to the varied development 
and functions of this organ, its musculature is liable to innumerable modifications. 

The character of the general body musculature of the Mollusca is 
determined by the degree of development of the shell, whose function 
is to protect the soft portions of the body. In order to make this 
protection complete, the Molluscan body is, as a rule, though differing 
greatly in details, so arranged that the soft parts can be entirely con- 
cealed in the shell, which can itself in many cases be closed. The shell 
thus functions as skeleton and passive locomotory organ, to which are 
attached such muscles as draw the body into the shell by their contrac- 
tion, and such as partially or wholly close the shell. 

It is obvious that the arrangement of the musculature becomes 
much modified secondarily in cases where the shell aborts or altogether 
disappears. 

The musculature of the Mollusca is not transversely striated. 



120 aUMPARATIVE ANATOMY chap. 



A. Amphineura. 

The musculature of the Chitonidce has neither been sufficiently 
investigated nor systematically described. According to the figures 
of various writers on tlie subject there are — (1) a large longitudinal 
muscle mass on each side above the foot ; (2) numerous muscle fibres 
which run down from the latero-dorsal region and radiate into the 
sole ; and (3) the special fibres of the foot, which run through it in 
various directions. The muscle fibres mentioned under (2) no doubt 
correspond with the shell muscles of the Fissurellidce, etc., and the 
columellar muscle of other Gastropods. Some of the fibres descending 
from one side cross those from the opposite side. These crossings are 
very marked in the median plane between the two pedal nerve 
cords. 

Among the Solenogastres, the muscular system of Proneomenia has 
been the most thoroughly investigated. In connection, no doubt, 
with the degeneration of the foot and the vermiform development 
of the body, a kind of dermo-muscular tube has been formed ; its 
layers, consisting of muscle fibres running in various directions, are 
very thin in comparison with the thick epidermis. This muscular 
tube lies immediately under the epidermis. Its outer layer consists 
of circular muscle fibres, then follows a layer of diagonal fibres, cross- 
ing each other at right angles, but crossing the circular and longi- 
tudinal fibres at an angle of 4.5°. The innermost layer consists of 
longitudinal fibres, and is most strongly developed on the ventral 
surface on each side of the ventral groove. Groups of fibres are 
detached from the circular layer on both sides, and converge towards the 
base of the rudimentary foot, some of them crossing above it. The 
bundles which arise from the lateral and upper walls of the body run 
within the septa which separate the consecutive lateral diverticula of 
the intestinal canal. 

So far as a comparison between these animals and the Chitonidce is jiossible, the 
abortion of tlie foot and vermifovra shape of tlie body being taken into account, and 
Chitouellus taken as the transition form, it may be assumed that the circular muscle 
layer, and in particular the gi-oups of fibres converging towards the foot, correspond 
with the dorso-ventral muscles of Ohitoii, and the longitudinal layer with their lateral 
longitudinal muscle masses. 

B. Gastropoda. 

The only important muscle to be considered in this class is the 
columellar muscle. This muscle is attached inside the shell to the 
columella, along which it runs on the right side of the visceral dome 
and along the right edge of the mantle cavity; it then enters the dorsal 
side of the foot in which it spreads out. The columellar muscle acts 
as a retractor to withdraw the animal into its shell. 



YII 



MOLLUSOA—MUSGULATUBE 



121 



1. Prosobranehia. 

The columellar muscle is here always developed in its typical form. 
It is attached at one end to the columella in the last coil of the shell, 
and at the other to the operculum, which lies on the dorsal side of the 
metapodium. 

A few Prosobranehia, such as most Fissurellidce, Haliotidce, and Docoglossa, use 
theu- foot chiefly as a sucker for 
attaching themselves to some firm 
surface. Tliese forms have no 
operculum. The columellarmuscle 
descends vertically into the foot, 
and by its contraction presses the 
shell against the surface to which 
it is attached. In Ealiotis (Fig. 
105), the ear-sliaped shell of which 
is coiled, this muscle is cylindrical 
and is very highly developed ; it 
runs somewhat to the right of the 
median plane, at right angles to 
the pedal disc, thereby pushing the 
mantle carity and the viscera to 
the left. In many FissureUidco 
and tlie Docoglossa, the shell has 
become cup - shaped and sym- 
metrical ; the columellar muscle, 
Avhich is very much shortened, 
descends direct from the inner 
surface of the shell to the foot, 
and is no longer cylindrical. The 
whole muscle has the form of a 
short truncated hollow cone, open 
anteriorly, which is attached to 
the shell by its upper horseshoe- 
shaped sectional surface, and, by 
its base of the same shape, to tlie 
sucker-like foot. The viscera are 
contained in its hollow axis (Fig. 
106). The same arrangement 
occurs in all cases where the shell 
is flatly conical, cup- or plate-shaped, as in the Hipponycidce and tlie Capiilidm 
among the Monotocardia. 

Heteropoda. — In this order, in which the atrophy of the sliell, the transformation 
of the foot, and the gradual obliteration of all resemblance to a Gastropod can be 
traced, step by step, the musculature deserves special attention. 

In Atlanta, where the head and foot can still be completely withdrawn into the 
shell, the columellar muscle retains its typical form. It descends from the shell, 
dividing into three strands ; the strongest median strand stretches out into the fin and 
the sucker, the posterior into the operculum-bearing metapodium, and the anterior, 
which is the smallest, into the head and snout. 




Fig. 10.!i. — Haliotis, from above, after removal of the 
shell, the mantle, and the entire dorsal intngument (after 
Wegmann). t, Snout ; s and %>, salivary glandy ; pi, lateral 
poclvets of the ojsophagus ; i, mid-gut ; a, CESophagus ; r, 
rectum ; e, stomach with caecum (c) ; 7i, digestive gland 
(liver), its right-hand portion which lies next the large 
columellar muscle (m) is covered by the genital gland. 
A fringed epipodium encircles the body. 



122 



COMPARATIVE ANATOMY 



CHAP. 



The cutis in Atlanta is still comparatively thin. The network of muscles lying 
immediately beneath it is not more strongly de\'eloped than in other Gastropods. 
A special system of crossing muscle fibres independent of the other dermal muscula- 
ture lies on each side under the cutis of the 
' ^"^ fin. This is the case in all Scteropoda. 

The integument greatly increases in 
thickness in the typical Helcropoda 
(Cariiutria, Pterotrachca), and the sub- 
cutaneous muscular tube also grows 
thicker. Over the body the latter con- 
sists of two superimposed layers of fibres 
crossing one another diagonally. In the 
outer layer, the fibres run from above 
anteriorly downwards posteriorly, and in 
the inner from below anteriorly to above 
posteriorly. On the head and snout, the 
visceral dome and the tail-like metapo- 
dium, the diagonal fibres of both layers 
run longitudinallj'. In addition, an ex- 
ternal circular musculature is found in 
Carinaria nearly all over the body, in 
Pterotrachca only in the snout. 

Turning now to Carinaria, which 
still possesses a delicate, easily detachable 
shell covering the visceral dome, but un- 
able to protect any otlier part of the body, 
we find the columellar muscle still per- 
sisting in the form of two bands descending from tlie visceral dome into the fin and 
radiating out to its edge. 

In Pterotrachca, where the shell is wanting and the visceral dome rudimentary, 
the columellar muscle is also reduced. It has now no connection witli the visceral 
dome, and commences half-way up the body wall as three short bands running down 
into the fin and radiating out to its edge. 

The columellar muscle, which originally served for drawing the foot back into 
the shell, now serves chiefly to bring about the lateral movements of the vertical 
rowing fin into which the foot has been transformed. 




erti/ 



Fig. 100. — Patella, from above, after removal 
of the shell (after Lankester). c. The separate 
bundles of the shell muscle, the section of which 
is horseshoe-shaped ; I, pericardium ; Ix, fibrous 
septum behind the same ; n, digestive gland ; nil, 
intestine ; k, larger right nepliridiuiii ; t, smaller 
left ditto ; c, mantle border, widening anteriorly 
into the mantle fold ; ccr, em, edge of mantle. 



2. Opisthobranchia. 

The columellar muscle is well developed in forms possessing a shell 



into 



which the body can be partly or wholly withdrawn. Where, however, the shell 
is rudimentary or wanting, as is the case with most Opisthobranchia, the 
columellar muscle atrojjhies or perhaps forms part of the pedal musculature. The 
subcutaneous dermo - muscular tube, on the other hand, develops in jiroportion 
to the activity of the animal. It consists of longitudinal, circular, and diagonal 
muscle fibres, which occasionally form a regular network. The pedal musculature 
is merely a thickened portion of this dermo-muscular tube in which longitudinal 
fibres predominate. The development of the musculature varies nmcli in detail. 
"Where movable or contractile dorsal appendages, gills, oral lobes, oral discs, para- 
podia, etc., are developed, their musculature is detached from the dermo-muscular 
layer, and the latter, in combination Avith the occasionally tough skin, forms a 
passive organ of support for the former. 

A columellar muscle is further found in the Ptcropoda thecosomata. It is ventral 



VII 



MOLL USGA—MUSGULA TUBE 



123 



in the Liiiuu-iniihc, but dorsal in the Cavoliniidcc, in which family the body, as 
compared with the head, seems to have been twisted through an angle of 180' 
(p. 80). The muscle divides anteriorly into two lateral branches, which radiate 
out into the fins. 

3. Pulmonata. 

In the sliell-bearing Pulmonata, the columellar muscle is strongly 
developed. It is paired, and attached 
at one end by many roots to the foot, 
behind the buccal mass, and at the 
other to the columella of the first coil 
or whorl of the shell. It gives off' 
three subsidiary branches — (1) the 
retractor muscles of the optic and 
other tentacles ; (2) the retractors of 
the buccal mass ; (3) muscles running 
to the viscera. 




Fig. 10".— Shell of Helix, in longitudinal 
section through tlie columellar axis (after 
Howes). c, Columella ; nn, columellar 
muscle ; p, edge of oral aperture (peritreme). 



In the Daudchardia and Testacelluhe, in 
which the dwindling visceral dome with the 
shell which covers it have shifted to the 
posterior end of the body, and in which all 
possibOity of the retraction of the body into 
the shell has ceased, only parts of the colmnellar muscle are retained, and naturally 
those parts which are still functional. In the Daudebardia and Tcstaccllidcc these 
are the retractors of the tentacles, and in Daudchardia the retractors of the pharynx. 
The tentacular and pharyngeal retractors are distinct. 

The retractors of the tentacles, in Daudebardia rufa, run back separately to the 
base of the visceral dome, not entering it, but fusing witli the body wall on each 
side of it. In D. saulcyi the retractors do not run so fqr back, but the two on the 
right fuse with the two on the left, and pass into the pedal musculature in the 
anterior half of the body. The same is the case in the Testaccllidu:. 

The Retractors of the Pharynx. — In Daudebardia rufa there are found, attached 
to the pharynx, two retractors, which, passing through the oesophageal nerve ring, 
fuse to form one muscle,. which runs back along the base of the pharyngeal cavity 
somewhat to the left, then ascends in the visceral dome to be attached to the 
columella of the last coil of the shell. In D. saulcyi, where there is no projecting 
visceral dome, and the shell merely covers a mantle cavity, the oesophageal re- 
tractors, which are not in this case fused together, no longer run up into the shell, 
but end in the middle of the body, where they enter the pedal musculature. 

The numerous oesophageal retractors which, in Testacella, are arranged in two 
asymmetrical rows, cannot for several reasons be considered as the remains of a 
columellar muscle. 

Oncidium when adult has neither shell nor columellar muscle, but its shell- 
bearing larva also possesses a columellar muscle. 



C. Seaphopoda. 

In Dentaliwrn (Fig. 101, p. 113) two closely contiguous muscle 
bands run on each side along the anterior side of the body, and are 
attached anteriorly to the dorsal end of the tubular shell. At the 



124 COMPARATIVE ANATOMY chap. 

base of the foot these two bands unite to form a single muscle on each 
side, which enters the foot and radiates out through it in the form of 
numerous longitudinal bundles. This then is a paired eolumellar 
musele which retracts the foot, and draws the whole of the lower 
portion of the body back into the upper part of the shell. 

D. Lamellibpanehia. 

The two principal groups of muscles to be considered in this 
class are : — 

1. The pallial musculature. 

2. The pedal musculature. 

The former is principally developed near the free edge of the 
mantle, and consists of three systems : (1) Fibres which run in the 
plane of the mantle fold towards and at right angles to its edge. 
These are, in the narrower sense, the muscles of the pallial edge, 
and leave on the shell the scar known as the pallial line. (2) 
Fibres running parallel with the edge of the mantle. (3) Short trans- 
verse fibres running more or less straight between the inner and the 
outer surfaces of the mantle. In the siphons, which are formed from 
the mantle, these three systems become circular, longitudinal, and 
radial layers. The retractors of the siphons are a special differentia- 
tion of the pallial musculature ; their development is in direct relation 
to the size of the siphons ; their crests of attachment to the shell 
valves cause the scar known as the pallial sinus (cf. p. 64). 

The important adductor muscles for closing the shell must 
also be regarded as differentiations of the pallial musciilature. These 
are exceedingly thick and powerful and run transversely from the 
inner surface of one valve to the corresponding surface of the other 
valve. They counteract the ligament at the hinge, their contraction 
causing the two valves to approach one another, till the shell is closed. 
These adductors leave scars on the inner surfaces of the valves. 
Typically, there are two adductors, an anterior and a posterior 
(Dimyaria), situated nearer the dorsal than the ventral edge of the 
valves. In the Mi/tilacea, the posterior adductor is larger than the 
anterior (Heteromyaria as opposed to Isomyaria). In one large 
series of forms the anterior adductor completely atrophies, and the 
posterior adductor, which is all the more strongly developed, shifts 
forwards towards the middle of the shell. These forms are grouped 
together as Monomyaria ; but this is no natural group, since nearly- 
related forms (e.g. within the Muelleriacea) may possess either one or 
two adductors, and widely different forms {e.g. Tridacna, Anomia, 
Muelleria, Aspcrgillum) agree in having only one. The Anomiidm, 
Ostreidce, Sponclylidce, Liniidce, Pedinidce, AvicuUdce, Muelleridce, etc., are 
Monomyarian. 

The adductor often {e.g. Pecten, Ostrca, Nucula) consists of two apparently 
different parts, one containing smooth fibres and the other fibres which appear 



VII 



MOLLUSGA—MUSGVLATUEE 



125 



transversely striated, although their striation does not correspond with that of 
Arthropod and Vertebrate muscles. 

The pedal musculature, taken as a whole, ansvi^ers to the 
columellar muscle of other Molluscs, especially of the Gastropoda. 
It consists of symmetrical pairs of muscles attached at one end to the 
inner surface of the shell on which they leave impressions, the other 
ends entering the foot. The correspondence of this musculature with 
the columellar muscle of the Gastropoda is best seen by comparing a 
Protobranchiate with Patella or Fissurella. In Nucula or Leda, for 
example, there is an almost continuous series of muscle bundles 
running down to the foot on each side between the anterior and 
posterior adductors. The two series taken together, seen from above 
or below, have an oval outline answering to the horseshoe-shaped or 
almost oval form of the section of the columellar muscle in Patella 
(Fig. 106) or Fissurella. 

In most cases in which the foot is developed, the following muscles on each side 
are distinguished in order from before backwards (c/. Fig. 108) : (1) the protractor 



AV 




Fig. lOS.— Pliodon Spelcei, from the left (after Pelseneer). The shell, mantle, gills, and oral 
lobes of the left sides removed. AA, Anterior ; AP, posterior adductor ; OA, anal ; OB, branchial 
aperture of the siphon ; V, visceral mass ; p, foot ; 1, protractor pedis ; 2, retractor pedis anterior; 
3, elevator pedis ; 4, retractor pedis posterior. 



pedis ; (2) the anterior retractor pedis ; (3) the elevator pedis, and (4) the posterior 
retractor pedis. 

Where there is a byssus, the posterior retractor becomes the byssus muscle. It 
is then usually highly developed, runs far forward, and may break up into several 
bundles. 

In those oases in which the foot is rudimentary and the byssus wanting, the pedal 
muscles degenerate. 

In Pecten the pedal retractors are asymmetrically attached, i.e. only to the 
left valve. The same is the case in Anomia, where the shelly plug which lies in 
the byssus notch of the right valve, and corresponds with the byssus, is attached to 
the left (or physiologically upper) valve by two highly-developed retractors. These 
two muscles leave scars near that of the adductors. This fact gave rise to the 
erroneous opinion that the Anomia were Trimyaria. 



126 COMPARATIVE ANATOMY chap. 



E. Cephalopoda. 

In the Cephalopoda, a cartilaginous endoskeleton is developed. 
This not only serves for the attachment of various muscles and 
muscular membranes, but is also a protection for important organs, 
especially for the central portion of the nervous system and the eyes. 
Of the different cartilages forming this endoskeleton the only constant 
one is the eephalie eartilag-e. 

1. Tetpabranehla (Nautilus). 

iSTautilus possesses only the cephalic cartilage. This is shaped 
somewhat like an X, with thick limbs. The cesophagus runs up 
between the one pair of limbs, the other pair serving as supports for 
the funnel and as surfaces of attachment for its muscles. 

The most important of the muscles is the large paired shell 
muscle, which corresponds with the columellar muscle of other 
Molluscs. It arises from the cephalic cartilage, and runs on each 
side into the band (annulus), by which the body of the Nautilus is 
attached to the inner wall of the body-chamber (f/. Fig. 32, p. 22), and, 
like the band itself, is attached "to the shell. The muscle leaves a 
deep scar on the shell (the lobate sutural line). From the lateral 
edges of the cephalic cartilage, especially that portion of it which 
supports the funnel, a broad muscle-band, the museulus eollaris, runs 
forward on each side embracing the nuchal part of the body. The 
two unite on the neck to form the muscular nuehal plate. The 
ventral lower side of the cephalic cartilage serves for the attachment 
of the musculature of the tentacles. 

2. Dibranehia. 

The cartilaginous skeleton is much more developed than in 

Xautilus, owing perhaps, to some ex- 
tent, to the atrophy of the shell. Fins, 
with their supporting cartilages, for 
example, are developed only in those 
forms with internal, degenerated 
shells. 



'f The cephalic cartilage (Fig. 109) is every- 

Fio. lOti.— CepiaUo cartilage of Sepia, ■^^■here well developed. It encloses all those 
1. Central aperture through -n-hich the ceso- central portions of the nervous system which 
phagus passes ; 2, preorbital cartilage ; 3, are crowded round the cesophagus, beine in 
chamber for the eye ; 4, cartilaginous 4.if„rii, . i, ° 

auditory capsule. ^^^ ^°™ "^ '^ ^1°!'°^^' circular capsule traversed 

by the resophagus. Processes of this cartilage 
assist in supporting the eyes, and in conjunction with independent, preorbital 
cartilages form a, kind of cartilaginous eye socket. A baaibrachial cartilage is 
found at the base of the anterior arms in some Decapoda, ^Ye have further to 




VII 



MOLL USGA —MUSCULATURE 



127 



mention the nuclial cartilage and the cartilages for looking the cleft of the mantle 
cavity (p. 55). In the diaphragm, i.e. in the posterior wall of the visceral dome, 
over which the inantle depends, there is in the Decapoda a cartilage near the 
funnel, the diaphragm cartilage. Finally must be mentioned a dorsal cartilage, 
which is specially strongly developed in Sepia. It lies, posteriorly, on the anterior 
border of the mantle, where the latter pro- 
jects over the neclc ; it bears the same 
relation to the nuchal cartilage as does 
the cartilaginous projection on each side of 
the mantle to the cup-shaped socket at each 
side of the base of the funnel or siphon (c/. 
Fig. 80). 

In Sepia the dorsal cartilage is continued 
in the shape of a cartilaginous rod running 
up on each edge of the shell. The inner 

edges of these rods have a groove into which 

the edge of the shell fits, and thus form a 

kind of fold round its lateral edges. 

In the Odopoda there is a cartilaginous 

band on each side in the dorsal integument 

wlrich may correspond with the dorsal carti- 
laginous rods in Sepia. It is possible that 

the "internal shell" of the only Octopod 

in which a shell is found, viz. Cirrho- 

teittkis, is not in reality homologous with 

the shell of the Decapoda, hut corresponds 

with the cartilaginous bauds of Octcypus 

fused in the middle line. 

The (basipterygial) cartilages, univers- 
ally found at the bases of the fins in the 

Decapoda, complete the list. 



With regard to the musculature 
of the Dibranohia, that of the mantle, 
the fins, and the arms cannot be 
described in detail. We note, how- 
ever, that the pallial musculature 




Pig. 110.— Diagram of the more important 
parts of the Dibranchiate musculature. 
Body seen from the left side, v, Ventral ; cI, 
dorsal ; a, anterior ; p, posterior ; 1, depressor 
infundibuli; 2, retractor capitis lateralis; 3, 
retractor capitis medianus; 4, niusculus col- 
is principally attached to the shell laris; 5, adductor infundibuli; e, shell; 7, 
or to the dorsal cartilage, the fin- dorsal cartilage; 8, nuchal cartilage; 9, cephalic 

cartilage ; 10, mantle cavity ; 11, cartilaginous 



musculature to the fin-cartilages, and 



socliet of the locking apparatus on the posterior 
wall of the visceral dome ; 12, corresponding 
cartilaginous Itnob on tlie inner wall of the 
mantle, which fits into 11 ; 13, funnel or siphon 
(infundibulum) ; 14, diaphragm cartilage. 



the brachial musculature to the an- 
terior side of the cephalic cartilage, 
and partly to the basi-brachial carti- 
lage when such is present. 

The remaining musculature can be best explained with the assist- 
ance of the accompanying diagram (Fig. 110), which represents the 
musculature of Enoploteuthis. 

The strong paired depressor infundibuli (1) rises from the shell 
on each side (or from the dorsal cartilage), and runs downwards and 
backwards to the base of the funnel and to the cartilaginous socket. 
From it spring most of the muscles of the anterior wall of the funnel. 



128 COMPARATIVE ANATOMY chap. 

The retractor capitis lateralis (2), which is also paired, rises from 
the same point as the depressor infundibuli, runs into the head, and 
is attached to the cephalic cartilage. The retractor capitis medianus 
(3), originally paired, but usually become single by fusion, arises at 
the posterior (inner) side of the shell, and also runs into the head, 
and is attached to the cephalic cartilage. 

In the Dibranchia, the first muscles which fuse are the two median retractors of 
the head (Onychoteuthis), these tlien fuse more comjjletely with the lateral retractors 
(Ommaslrephes, Sepioteutliis, Loligo, Sepiola), so that finally (lycyia) the whole of the 
musculature running from the shell into the head forms a muscular sheath open 
posteriorly. This sheath encloses the lower portion of the visceral cavity, which is 
principally occupied by the digestive gland or liver, and thus forms a kind of 
muscular hepatic capsule. The posterior opening in this capsule may finally become 
completely' closed by the depressor infundibuli, in that, on the one hand, its 
anterior edges fuse with the posterior and median edges of the capsule, and, on the 
other, it sends out numerous muscles to the diaphragm, forming the diaphragma 
musculare. 

The muscular hepatic capsule, i.e. all the muscles forming it, the 
retractors of the head and the depressors of the siphon, may without 
doubt be accepted as the homologue of the columellar muscle of other 
Molluscs. Like the latter, they run down from the shell or its vicinity 
into the head and foot (represented by the siphon). 

The adductors of the funnel (5) have still to be mentioned. 
They rise from the cephalic cartilage and run upwards and backwards 
to the funnel. Finally, the collaris (4) is a strong muscle which runs 
forwards right and left from the wall of the funnel, and is attached 
to the lateral edges of the nuchal cartilage. lu the Odopoda and Sepiola, 
where a pallio-nuchal concrescence (cf. pp. 54, 55) has rendered a 
nuchal locking cartilage unnecessary, the collaris passes uninterruptedly 
over the neck like a saddle, forming a closed circle round the nuchal 
portion of the body. 



XIII. The Nervous System. 

(As a general introduction to this section the reader may be referred to pp. 27, 28.) 

A. Amphineura. 

The nervous system of the Amphineura is very significant from 
the point of view of the comparative anatomist. Its most important 
peculiarities may be briefly described as follows : — 

1. The ganglionic cells are either not at all or not exclusively 
localised in definite ganglia. 

2. Four nerve cords run through the body from before backward. 
These contain not only nerve fibres, but ganglion cells distributed along 
their whole length. They might suitably be called medullary cords, 
and must be considered as belonging to the central nervous system. 



VII MOLLUSOA—THE NERVOUS SYSTEM 129 

One pair of these cords run along the body laterally, these are the 
lateral or pleurovisceral cords ; the second lie ventrally, and are the 
pedal cords. The visceral and the pedal cords of each side unite 
anteriorly, and when so united become connected vi'ith those on the 
opposite side by a transverse commissure, vphich runs in front of and 
over the oesophagus and contains ganglion cells ; this is the cerebral 
or upper half of the oesophageal ring. The pleurovisceral cords unite 
posteriorly above the rectum, forming a visceral loop. The pedal 
cords are connected both inter se and with the pleurovisceral cords by 
anastomoses, so that the whole nervous system strikingly recalls the 
ladder nervous system of the Turbellaria and Trematoda. 

a. Chitonidse (Figs. Ill and 51, p. 40). — The scheme just given 
is founded upon the nervous system of Chiton. The typical ganglia of 
the central nervous system of the Mollusca are not yet, in Chiton, found 
as distinct ganglia united by means of commissures and connectives, 
but the ganglion cells are equally distributed along the commissures 
and connectives, an arrangement which is probably primitive. The 
upper oesophageal ring thus corresponds with the cerebral ganglia and 
the commissures connecting them, and in the same way the pedal 
cords contain the whole central portion of the pedal nervous system, 
and the pleurovisceral cords the central portion of the visceral, pallial, 
and branchial nervous systems. Only in one single species of Chiton 
(C rubicundus) two distinct (cerebral) ganglia occur near each other 
in the middle line in the upper half of the oesophageal ring. 

Looking more closely at the nervous system of the Chitonidee, ve have to 
observe : (1) the arrangement of the cesophageal ring and the medullary cords ; (2) 
the peripheral ganglia ; (3) the nerves of the ladder-like nervous system ; (4) the nerves 
running from the central nervous system (cesophageal ring and medullary cords). 

1. Form and arrangement of the central nervous system. — The visceral cords 
run back one on each side in the lateral body wall above the branchial groove ; these 
two cords unite above the anus. The pedal cords run in the dorsal part of the 
pedal musculature somewhat near one another, from before backward, to end without 
uniting where the rectum commences. The oesophageal ring consists, in the first 
place, of the semicircular portion mentioned above, which, on account of the peculiar 
shape of the body of the Chiton, lies in the same plane as the visceral cords. Poste- 
riorly, each limb of this semicircle divides up into the pedal and visceral cords. At 
the point where the pedal cord rises from the ring, a cord with a thickened base 
separates from it and runs inwards ; this, uniting below the mouth with a similar 
cord from the other side, foi-ms the lower half of the oesophageal ring. The upper 
and lower halves together form the closed oesophageal ring. 

2. Besides this central nervous system there are peripheral ganglia connected 
with it by nerve cords consisting only of nerve fibres. 

(a) The buccal ganglia together form a horseshoe-shaped ganglionic mass below 
the oesophagus, which mass is connected on each side by the cerebrobuccal connective 
with the thickened portion of the lower oesophageal ring. The buccal ganglionic 
mass in 0. ruMcunchis divides into two paired ganglia and one unpaired ganglion 
joined to one another by connectives. The buccal ganglia innervate the oesophagus 
as far as the stomach and also the oral aperture. 

(6) On each side, from the lower half of the cesophageal ring, somewhat further 
VOL. II K 



130 COMPARATIVE ANATOMV chap. 

in than the hiiccal connective, a nerve (the subradular connective) rises and runs 




Fig. 111.— Diagram of tlie nervous system of Chiton siculus (after B^la Haller). Tlie mantle 
removed on the right side. In the centre and to the left the upper part of the foot removed, to 
expose the pedal nen'-ous system. F, Foot ; A", last gill ; A, anus ; 0, upper, U, lower half of the 
cesophageal ring ; 1, 2, nerves of the oesophageal ring ; c, connective to the anterior visceral ganglia ; 
p, connective to the ganglia of tlie subradular organ n (ahove on the left) ; Es, pi euro visceral and 
pedal cords; mn, gastric nerve; So, point of attachment of the sphincter oris ; ti. (below on the 
right), ni, n-2, nephi-idial nerves ; m, pallial nerves ; p (to the right below), cardial nerves; v, a 
dorsal nerve of one of the pedal cords. The commissures between the pedal cords are seen, and 
the nerves running outwards from the latter. 

forward and inward to the subradular ganglion. This ganglion lies in the sub- 



VII MOLLUSGA—THE NERVOUS SYSTEM 131 

radular organ which is situated on the floor of the buccal cavity. The two sub- 
radular ganglia are united by a short commissure. 

(c) Two small gastric ganglia, connected by a fine commissure, lie at the anterior 
end of the stomach, and are joined on each side to the anterior end of the visceral 
cord by a long connective. 

3. The nerves of the ladder-like nervous system. — The two pedal cords are con- 
nected by anastomosing commissures along their whole length, hut no nerves are 
given off by these commissures to the pedal musculature. In Chiton ruiicunclus the 
visceral and pedal cords are united by numerous connectives, which, in other 
ChitonidiE, appear either to be wanting or to be reduced to one single anterior or 
posterior anastomosis. 

4. The nerves running from the central nervous system : — 

{a) Nerves of the oesophageal ring. — Numerous nerves rise from the upper or 
cerebral portion of the ojsophageal ring to innervate the cephalic part of the mantle, 
the snout, the upper and lower lips, the gustatory buds on the lower wall of the 
oral cavity, and the musculature of the buccal mass. The lower portion of the 
cesophageal ring, besides the connectives to the buccal and subradular ganglia, 
sends off from its median portion another pair of nerves, which run along the base 
of the buccal cavity. 

(i) Nerves of the pleurovisceral cords. — Each of the pleurovisceral cords 
gives ofiF two nerves to each gill. Besides these they send many nerves to the 
mantle, and, posteriorly, nerves which enter the body cavity, probably running to 
the kidneys and the heart. 

(c) Nerves of the pedal corda. — The pedal cords give off on each side seven or 
eight nerves outwards to the lateral musculature of the body, and specially numerous 
nerves run down fi'om it to the pedal musculature (inner and outer pedal nerves). 
These pedal nerves are richly branched, and, anastomosing with one another, form 
a complete neural network in the foot. 

b. Solenogastres. — The central nervous system of the Solenogastres 
differs from that of the Ghitmiidce principally in a tendency to form 
distinct ganglia ; the pedal and pleupoviseeral cords, nevertheless, 
still retain their outer coating of ganglion cells along their whole 
length. Fig. 112 is a diagrammatic representation of the structure of 
the nervous system of Proneomeiiia Sluiteri. The fused cerebral ganglia 
in the middle line are very large. On both the pleurovisceral and 
the pedal cords ganglionic swellings can be distinguished : (1) three 
pairs of posterior visceral ganglia ; (2) two anterior pedal ganglia. 

The posterior visceral ganglia are connected by cords, which run 
transversely over the rectum and correspond, to some extent at least, 
with the loop by which the two visceral strands in Chiton are united. 

The two anterior pedal ganglia are connected by a strong trans- 
verse commissure, which may correspond with the ventral half of the 
cesophageal ring of Chiton. 

Further, the pleurovisceral cords are joined with the pedal cords, 
and the latter are also connected inter se by transverse connections 
along their whole length. The pleurovisceral cords likewise are con- 
nected by arched transverse commissures.'- 

1 These connectives and commissures, however, do not seem to run uninterruptedly 
from one cord to the other. 



132 



COMPARATIVE ANATOMY 



CHAi>. 



On each side of the cerebral ganglion, a nerve rises, which runs to 
a ganglion below the pharynx and behind the radular sheath, this is 
the sublingual ganglion ; this latter is united with the corresponding 
ganglion on the other side by a short transverse commissure. These 
sublingual ganglia probably correspond with the buccal ganglia of 
Chiton. 

Dmiclersia is specially noteworthy because distinct ganglionic swellings occur at 
regular intervals along the pedal cords ; this is particularly marked in the anterior 
part of the body. The equally regularly repeated transverse commissures joining the 
pedal cords, and the connectives between the pedal and visceral cords, start from 
these distinct ganglia. 

In Lepidoincnla hyslrix, one ganglion occurs posteriorly and one anteriorly in 
each longitudinal trunk (whether pleurovisceral or pedal), and each is connected 
with a similar ganglion of the opposite side by a transverse commissure. 

In Xemnenia and Cluvtoderma, no connectives between the visceral and pedal 




Fio. 112.— Nervous system of Proneomenla Sluiteri (original dr.iwing by J. Heuscber). 
1, Cerebral ganglia ; 2, pleiirrivisceral cords ; 3, 4, 5, pust^Tior ganglia of the pleurovisceral eonls ; 
6, sublingual ganglia; 7, anterior pedal ganglia; 8, right pedal cord; 9, left pedal cord; 10, 11, 
strong posterior eommissni-es between the pedal cords; 12, anterior pedal commissure; 13, sub- 
lingual commissnrn. 

cords have been observed, and, so far as is at present Icuown, in Cfiatoderma, the 
commissures lietween the pedal cords are also wanting. Further, in Cha-lodcrma, 
the visceral and pedal cords of each side unite together posteriorly to form one 
single cord, which becomes connected with the similar cord on the other side by a 
transverse cord which runs over the cloaca.-' 



B. Gastropoda. 

The nervous system of the Gastropoda is of great interest to the 
comparative anatomist on account of the crossing of the pleurovisceral 
connectives in the Prosohranehia, which will be further described in this 
section. 

The nervous system of this class consists typically of those parts 
which we have alreadj'' mentioned in our scheme of the organisation of 
the MoUusca, viz. ; — 

1 For further details see Simroth's new edition of Bronn's Klassen inid Ordnungen 
des Thier-reiches, vol. iii. 



VII MOLLUSCA—THE NERVOUS SYSTEM 133 

1. Two cerebral ganglia near or above the resophagus, which 
aie connected by a cerebral commissure. 

2. Two pedal ganglia below the oesophagus, connected with each 
other by a pedal commissure, and with the cerebral ganglia by two 
cerebropedal connectives. 

The cerebral and pedal ganglia with the commissures and con- 
nectives belonging to them form a ring encircling the oesophagus, 
which may be compared with the oesophageal ring of the Annulata 
and Arthropoda. 

3. Two pleural or pallial ganglia (between the cerebral and 
pedal ganglia), which are connected with the cerebral ganglia by two 
eerebropleural, and with the pedal ganglia by two pleuropedal con- 
nectives. 

4. A simple or complex visceral ganglion lying below the in- 
testine, united to the pleural ganglia by two pleurovisceral con- 
nectives. 

5. A ganglion, which may be called parietal, almost always occurs 
in the course of each pleurovisceral connective. The parietal ganglion 
divides the connective into two parts, an anterior pleuroparietal and 
a posterior viseeroparietal connective. 

The cerebral, pedal, and pleural ganglia are (with unimportant 
exceptions) always arranged symmetrically to the median plane in all 
Gastropoda. The pleurovisceral connectives and their ganglia, how- 
ever, are only found in such a position in some Gastropoda. In fact, 
only in the Opisthobranchia (including the Pteropoda but excepting ^cteow) 
and the Fidmonata are they symmetrical, in the sense that the right 
connective and its ganglion lie entirely on the right, and the left 
connective and its ganglion entirely on the left side of the body. 
The Opistliohrancliui and Pulmonata are therefore called euthyneurous 
Gastropoda. 

In the Prosohranchia and Adceon, the pleurovisceral connectives are 
asymmetrical, inasmuch as they cross one another, the connective 
springing from the right pleural ganglion running aver the intestine to 
the left before joining the visceral ganglion, while the connective 
from the left pleural ganglion runs under the intestine to the right side 
of the body. In consequence of this crossing, the parietal ganglion of 
the connective which springs from the right pleural ganglion becomes 
the supraintestinal ganglion, which lies on the left side, and the 
parietal ganglion of the connective springing from the left pleural 
ganglion becomes the infra-intestinal ganglion which lies on the right 
side. The Prosobraiichiaund Adceon are thus streptoneurous Gastropoda. 

The Areas of Innervation of the various Ganglia. 

1. The cerebral ganglia innervate the eyes, the auditory organs, 
the tentacles, the snout or proboscis, the lips, the motor muscles of the 
proboscis and buccal mass, and the body walls lying at the base of the 



134 COMPARATIVE ANATOMY chap. 

snout. Even when the auditory organs are found in close proximity 
to the pedal ganglia, or in close contact with them, they receive their 
nerves from the cerebral and not from the pedal ganglia. 

2. The pedal ganglia supply nerves to the musculature of the foot, 
and occasionally to the columellar muscle also (Patella). 

3. The pleural ganglia send nerves chiefly to the mantle, the 
columellar muscle, and the body walls lying behind the head. 

4. The parietal ganglia innervate the ctenidia and osphradium, 
and also send some nerves to the mantle. 

5. The visceral ganglia supply nerves to the viscera. The con- 
nectives and commissures also may give off nerves which belong to the 
areas innervated by the neighbouring ganglia. 

6. The buccal ganglia, which will be described below, innervate 
the muscles of the pharynx, the salivary glands, the oesophagus, the 
anterior aorta, etc. 

A comparison of the tj'pical nervous sj-stem of the Gastropoda with that of the 
AiTvpliincura reveals the following homologies ; — 

1. The cerebral ganglia of the Gastropoda correspond with the cesophageal ring 
of Chiton, with the exception of the central portion of its lower half ; and further 
with the cerebral ganglia of the Solenogastres. 

2. The pedal ganglia of the Gastropoda answ-er to the pedal cords in the A>n- 
phineura, concentrated each into a single ganglion. The arrangement in the Dioto- 
cardia, which are the more primitive Prosobrandiia , is very interesting in this con- 
nection ; in the Diotocardia the pedal ganglia are continued posteriorly as two 
true pedal cords, which, like those of the Amphineura, are connected by transverse 
commissures. 

It is more difficult to compare the pleural, parietal, and visceral ganglia of the 
Gastropoda with nerves found in the Amjiliiiicvra. The most satisfactory view 
seems to be that this whole complex of ganglia, together with its connectives, corre- 
sponds with the pleurovisceral cords of Chiton. The areas of innervation coincide, 
these being the mantle, ctenidia, osphradia (Chiton ?), and viscera. 

3. If this last assumption is correct, the pleural ganglion must be supposed to 
have arisen by the concentration into one ganglion of that part of the pleurovisceral 
cord of Chiton which contains the pallial ganglionic cells, this concentration having 
taken place at the anterior end of the cord, where it leaves the cesophageal ring. If, 
then, the two component portions of each side of the ring, the cerebropedal and the 
pleural, move further apart, and at the same time the cerebral and pedal ganglia of the 
ring become more individualised as ganglia, a double cerebropedal connective comes 
into existence on each side. One of these connectives shows no ganglion in its course, 
and is the true cerebropedal connective of the Gastropoda. The second, however, has 
the pleural ganglion in its course, and from this latter spring the visceral cords ; this 
second connective is thus divided into a cerebropleural and a jileuropedal connective. 

4. Chiton has numerous gills on each side, each of which receives two nerves 
from the pleurovisceral cord near it. The Gastropoda have at the most two gills, 
one on the right and one on the left. In correspondence with this reduction, the 
ganglionic cells of the pleurovisceral cords belonging to the branchial nerves of 
Chiton have become concentrated on each side into a single ganglion belonging to the 
single gill. The parietal ganglion is thus accounted for. That portion of each 
pleurovisceral cord which lies between the pleural and the parietal ganglia becomes 
the pleuroparietal connective, which consists of fibres only without ganglion cells. 



VII MOLLUSGA—THE NERVOUS SYSTEM 135 

5. There is no nerve in Chiton homologous with the visceral ganglion or ganglia of 
the Gastropoda ; this is the chief difficulty in the comparison of the two nervous 
systems. In the Amphineura, the pleurovisceral cords unite above tlie intestine ; 
in all other Molluscs the point of junction (which is the visceral ganglion) lies below 
the intestine. 

In Proneomeiiia the posterior commissures between the pleurovisceral cords are 
merely a more strongly developed part of a general commissural system. 



Origin of the Crossing of the Pleupoviseeral Connective 

(Chiastoneury) (Figs. 113-116). 

Several attempts have been made to explain the peculiar crossing 
of these connectives in the Prosohranchia. The one here given is in a 
high degree probable if not altogether satisfactory. 

We must start with a supposed racial form which was perfectly 
symmetrical, even in its nervous system, and possessed an organisation 
somewhat like that of our hypothetical primitive Mollusc (p. 26). 
Such an organisation agrees in most important points with that of the 
extant Chitonidce , only one gill, however, was present on each side. 

Further, the parietal ganglia innervated the gills and the osphradia, 
and were thus, closely connected with these organs. 

The racial form of the Gastropoda may have been surrounded by 
a mantle border which widened posteriorly, i.e. covered a somewhat 
deeper mantle cavity which contained the pallial complex, viz. the 
median anus, to the right and left of which were the ctenidia and 
osphradia, and between the ctenidium and anus on each side the 
nephridial aperture. 

If we suppose this pallial complex to have changed its position, 
shifting gradually forward along the right mantle furrow, each cteni- 
dium would drag along with it its parietal ganglion. The heart and 
its auricles which are connected with the ctenidium would also become 
shifted. 

As long as the pallial complex had not moved far forward to the 
right, the pleurovisceral connectives would not cross, but would only 
be shifted to the right (Fig. 114). We find the Tectibranchia among 
the Opisthohranchia apparently at this stage, the only difference being 
that they have already lost the original left ctenidium and also the 
original left auricle (Fig. 43, p. 33). 

If the pallial organs are still further shifted forward along the 
mantle furrow (Figs. 115, 116) till they come to lie quite ante- 
riorly, and once more symmetrically, above and behind the neck, the 
original left ctenidium comes to lie on the right, and the original right 
ctenidium on the left in the anteriorly placed mantle cavity. The 
original right ctenidium has, however, dragged its parietal ganglion 
over the intestine to the left side, and the latter becomes the 
suppaintestinal ganglion. The original left ctenidium, on the 
contrary, has dragged its ganglion below the intestine to the right 



136 



COMPARATIVE ANATOMY 



CHAP. 



side, and this ganglion becomes the infpaintestinal gang-lion. The 
pleurovisceral connectives, in which these ganglia lie, now cross and 
give rise to the condition called ehiastoneupy. The visceral 



wlct 




Pios. 113, 114, 116, 110.— Diagrams to illustrate the sUtting forward of the palUal complex 
along the right side of the body and the development of ohiastoneury. jj. Mouth ; nk, vlpl, 
iilp, original left cerebral-, pleural-, and pedal-ganglion ; -ulpa, iirpu, original left and original right 
parietal ganglion ; tda, original left auricle ; »os, nros, original left and original right osphradiuni ; 
nht, iwcf, original left and original right ctenidium ; ml), base of the mantle ; mr, edge of the .same ; 
>ii, mantle cavity ; v, visceral ganglion ; ve, ventricle ; a, anus. 

ganglion in which these connectives terminate posteriorly lies as 
before under the intestine. 

It is unnecessary to show in detail how this displacement also affects 
the heart and its auricles, the osphradia, and the nephridial apertures. 



VII MOLLUSCA—THE NERVOUS SYSTEM 137 

Although chiastoneury may be satisfactorily explained by this 
theory of displacement, the cause of the displacement itself has still to 
be sought {cf. § xiv. p. 149). 

Special Remarks on the Nervous System of the Gastropoda. 

1. Frosobranchia. (a) Diotocardia. — These are the most primitive Gastropoda. 
The ganglia are not yet very distinct, thus recalling the Amphineura. The cerebral 
ganglia are connected by two long commissures, the cerebral commissure running 
fonvard over the pharynx, and the labial commissure running under the oesophagus. 
The indistinctly separated buccal ganglia together form a horseshoe-shaped figure, 
and are united on each side by a connective with the tliickened root of the labial 
commissure. 

The pleural ganglia lie close to the pedal ganglia, so that no distinct pleuro- 
pedal connectives can be distinguished. The pedal commissure is very short, and 
(Contains ganglion cells. From each pedal ganglion, a long pedal cord runs back into 
the foot ; these two pedal cords contain ganglion cells along their whole length, and 
are connected by transverse commissures. These cords and commissures thus exhibit 
the same arrangement as in the Amphineura. The pedal cords innervate the mus- 
culature of the foot and the epipodinm. There is only one indistinct visceral 
ganglion, which is joined to the pleural ganglia by two pleurovisceral connectives, 
crossed in the typical way. 

In Fissiirella only does a ganglion occur on the supraintestinal pleurovisceral 
connective. In no other Diotocardiaii is there a ganglion at the point of departure 
of the strong branchial nerve from the pleurovisceral connective ; this nerve, how- 
ever, forms the branchial ganglion just below the osphradium at the base of tlie 
gill. Where a ctenidium, or merely an osphradium, is found on each side, there is a 
branchial ganglion close to it ; where only the left (ur) gill is retained (Turhinida, 
Trochidce), only the left branchial ganglion is found. Since, as a rule, the parietal 
ganglia are wanting in the Diotocardia, and the branchial ganglia in the Mouotocardia, 
the branchial ganglia of the Diotocardia have been considered, with much prob- 
ability, as intestinal ganglia, which have shifted away from the pleurovisceral connec- 
tives and towards the bases of the gills. As, however, Fissiirella possesses both a 
supraintestinal and a left branchial ganglion, it would be necessar}' to assume that 
an originally single ganglion had here become divided into two. 

The symmetrical pallial nerve is always connected by a pallial anastomosis 
with the asymmetrical pallial nerves on the same side of the body. The symmetri- 
cal pallial nerve rises out of the pleural ganglion, the asymmetrical nerves out of 
the parietal ganglion, or the pleuroparietal connective. 

The nervous system of the Neritidc and Hclicinidce are peculiar, in that the supra- 
intestinal pleurovisceral connective and its corresponding ganglion are wanting. 

Docoglossa. — The only essential diilerence between the nervous system of Patella 
(Fig. 117) and the typical .system of other Diotocardia lies in the fact that the 
pleural and pedal ganglia are joined by a distinct pleuropedal connective. 

(6) Monotocardia (Fig. 118). — The parietal ganglia are always present. The 
cerebral commissure is short, and lies behind the pharynx. The labial commissure 
is wanting (except in the Paludinidce and Amp^Ularidce). The pedal cords and 
transverse connnissures are wanting (except in the Architccnioglossa : Paludinidce, 
Cyclophorida;, Cyprccida). The number of visceral ganglia varies from one to 
three. 

The progressive development of so-called Zygoneury is noteworthy. In the 
Diotocardia, a pallial anastomosis exists between the symmetrical and asymmetrical 



138 



COMPARATIVE ANATOMY 



CHAP. 



pallial nerves on each side. If this anastomosis were to shift along tlie two pallial 
nerves of one side to their places of origin, i.e. the ganglia from which they spring, 
it would become a pallial connective uniting the pleural and parietal ganglia of 
the same side of the body. There would thus arise a new accessory pleurointestinal 

connective, which would be symmetrical 
and not twisted, and thus unlike the asym- 
metrical twisted connective already existing. 
Zygoneury thus depends on the development 
of such a pallial connective. In the large 
majority of cases in which it occurs it takes 
place on the right side (a few Mostrifera, 
viz. some of tlie Ccrithiiikc, Anqmllariidce, 
Tu riteHidce, Xenoplioridcc, Struthiolariidu; 
Chenopidcc, Slrombidm, Calyptrceidcc, and in 
all Prohosddifera siphonostomata and all 
Stenoglossa). Less frequently, zygoneury 
takes place on the left side (Ampullariidce, 
a few CrepiduUda:, Naticida:, Lamellariidcc 
Oypraddce). In other Prosohranchia there 
is only a pallial anastomosis on each side, as 
in the Diotocardia ; the nervous system is 
then called dialyneurous. 

The progi-essive concentration of the 
central nervous system of the Monotocardia, 
which keeps pace with the development of 
zygoneurj-, must be emphasised. The con- 
nectives rmiting the various ganglia con- 
tinually shorten, so that at last anteriorly 
on the oesophagus there is a collection of 
ganglia ; these are the cerebral, pleural, 
pedal, infraintestinal, and supraintestinal 
ganglia, all lying close together, to which 
must be added the small buccal ganglia. 
Only the visceral ganglia remain far back 
iu the visceral dome. 

In Natica, where the anterior part of the 
foot is strongly developed, and is bent back 
over the head (Fig. 98 ), a propedal ganglion he- 
FiG. 117. —Nervous System of Patella comes differentiated from the pedal ganglion, 
(adapted from figures by Pelseneer and The nervous system of the Heteropoda 

Bouvier). 1, Cerebral ganglion ; 2, cerebral j-equires fresh investigation. So far as we 
commissure ; 3, labial ganglion ; 4, buccal gau- , , , j.i j. . n t 

glion ; 5, cerebropleurJcormective ; 6, cerebro- ^^ P'^'^^f * ^now, they certamly have crossed 
pedal connective ; 7, nervus acusticus ; 8, visceral connectives, and are therefore Proso- 
auditoi-y vesicle ; 9, pleural ganglion ; 10, pedal branchia, and, as the rest of their organisa- 
commissure ; 11, right, 12, left osphradium ; tjon shows, Monotocardia. The cerebral 
13, visceral ganglion ; 14, supraintestinal gan- „„„„!,• „„j ^-l.„ ,.^j i t / i n i 

,1,1 ,,»■,»• « ganglia ana the pedal ganglia (pleuropedal 
15, pedal cords; 16, indication of an ,. „, . ' 6 = \F* "i"i"i>JO'i 

ganglia ?) are far apart, so that the cerebro- 
pedal connectives are very long.^ 
II. Opisthobranchia. — The nervous system of this order, in which the typical 
Gastropodan ganglia are developed, is further characterised : (1) by the absence of 




jlion ; 15, pedal cords ; 16, indication of an 
infraintestinal ganglion. 



1 Cf. Pelseneer's Introduction I'etude des MoUusqwes, 8vo, Bruxelles, 1894, pp. 
104, 105. 



VII 



MOLLUSCA—THE NERVOUS SYSTEM 



139 



uhiastoiieury, i.e. the pleurovisceral connectives do not cross (except in Actceon) ; and 
(2) by a marked tendency to concentration of the ganglia around the posterior end 
of the pharynx. 

(a) Tectibranchia. — As a rule only the right parietal ganglion is found (in Actmon 
the left is also present). A nerve rises from it which innervates the ctenidium, the 
osphradium, and the mantle, and foi'ms a branchial ganglion at the base of the gill. 
A delicate lower cerebral commissure is often found, which runs along the pedal 




Fig. us.— Nervous System of Oyclostoma elegans (after Lacaze-Duthiers). 1, Tentacular 
nerve ; 2, eye ; 3, cerebral ganglion ; 4, pedal ganglion ; 5, infraintestinal ganglion ; 6, visceral 
ganglion ; 7, osphradium ; S, supraintestinal ganglion ; 9, auditory vesicle ; 10, pleiu-al ganglion. 

commis.sure below the pharynx, and may be compared with the labial commissure 
of the Diotocardia. 

As types of the Tectibranchia we may take Sulla as reja-esentative of the 
Cephalaspidce, and Aplysia as representative of the Anaspidw (Aplijsiidce). 

Fig. 119 gives the nervous system oi Bulla hydatis ; only three points concern- 
ing it need be mentioned : (1) The pleural ganglia have shifted till they lie close 
to the cerebral ganglia, the cerebropleural connectives becoming correspondingly 
shortened. (In Actaion these ganglia have even fused, and are no longer to 
be distinguished externally.) (2) There are three visceral ganglia. (3) The 
commissures are comparatively long. (4) The parapodia are innervated from 
the pedal ganglia. 

In many Cephalaspida:, moreover, no distinct right parietal ganglion exists. It 



140 



COMPARATIVE ANATOMY 



CHAP. 



seems to Lave moved up to the riglit pleural ganglion, or to have fused with it, so 
that the nerve running to the branchial ganglion rises direet from the right pleural 
ganglion. 

The nervous system of the Pteropoda thccosomata, which we derive from Cephala- 
spidu; bears a general correspondence to that of the latter, especially in the fact 
that the pleural ganglia shift near to or fuse with the cerebral ganglia. The 
pleurovisceral connectives are so much shortened that the ganglia occurring in 
their course lie close to the cerebral and pedal ganglia. There are usually two such 





I'll,. 1111.— Nervous System of Bulla liydatis (after Vayssifere). 

I , Buccal ganglion ; 2, cerebral ganglion ; 3, pleural ganglion ; 4, 
pedal ganglion ; 5, part of the right pleural ganglion (?); 7, eye ; 
s, cerebral commissure ; 9, pedal conimis.sure ; 10, auditory vesicle ; 

II, riglit parietal ganglion; 12, 13, 14, visceral; 15, branchial 
ganglia. 



Fig. 120.— Nervous System 
of Aplysia, diagi-ani, combined 
from several sources. 1, 
Buccal ; 2, cerebral ; 3, pleural ; 
4, pedal ; 5, right parietal ; 6, 
visceral ganglion ; 7, osphra- 
dinm ; 8, genital ganglion ; 9, 
branchial ganglion. 



ganglia (the right parietal and a visceral ganglion ?), less frequentlj' three (two 
intestinal and one visceral ganglion ?). The pedal ganglia also innervate the fins, 
which correspond with the parapodia of the Cephalaspida:. 

Fig. 120 represents the nervous system of Aplysia, one of the Anaspidai. The 
two cerebral ganglia have moved close to each other in the middle line. The pleural 
ganglia here, unlike those of the Cephalaspida:. lie close to the pedal ganglia, so 
that the jileuropedal connectives are much shortened. The pedal commissure is 
double, the anterior commissure is, relatively speaking, short and thick, the posterior 
long and thin. The long pleurovisceral connectives run back from the pleural 



VII 



MOLLUSGA—THE NERVOUS SYSTEM 



141 



fQ>. 



ganglia, and enter tNvo ganglia lying side by side ; that to the right represents the 

right parietal ganglion, innervating chiefly the gill and osphradium, the nerves 

running to these organs forming a ganglion at the base of 

each ; that to the left is the viseeral ganglion. One of the 

nerves which run from the latter forms a genital ganglion at 

the base of the accessory glands connected with the genital 

organs. In other Anaspiche, such as Notarchus (Fig. 121), 

the pleurovisceral connectives are so much shortened that 

the parietal and visceral ganglia lie close to the periceso- 

phageal group of ganglia, which then consists of two cerebral, 

two pedal, and two pleural ganglia, and further, the right 

parietal and the visceral ganglia. The two cerebral ganglia 

are further connected by a thin lower commissure. The 

parapodia are always innervated from the pedal ganglia. 

The nervous system of the Pteropoda ijipnuosomata, which 

are nearly related to the Aimspuhe. corresponds in all essential 

points with the nervous system of the latter, being of the 

same type as that of Notarchus. 

[b) Nudibranohia and AscoglosBa. — The nervous system 
is here characterised by very great concentration of the 
typical Molluscau ganglia, and by a tendency to the forma- Buccal ; 2, cerebral ; S 
tion of numerous accessory ganglia (at the bases of the pleural ; 4, peilal ganglia 




Fio. 121 
System of 
punctatus (after 

diagrammatic. 1 



— Nervous 
Notarohus 



5, right parietal ganglion ; 

6, visceral ganglion. 



tentacles and rhinophores, and at the roots of their nerves, 
in the course of the genital nerves, etc.). The pleural gan- 
glion has moved close to the cerebral ganglion, and may fuse ^\'ith it. The pedal 

ganglia have also moved towards the cerebral ganglia 
so that now the whole esophageal complex of gan- 
glia lies almost entirely on the dorsal side of the O'so- 
Ijhagus. The pedal commissure which runs under 
the gullet, and is sometimes double, is thus very 
much lengthened. The pleurovisceral connectives 
are short, and occasionally enter an unjiaired visceral 
ganglion, which has also been drawn into the u-so- 
phageal complex. This single ganglion of the visceral 
connectives may be wanting (Fig. 122) ; in that case 
the two visceral connectives appear like a commissure 
between the two pleural ganglia runningunder the a;so- 
phagns and parallel with the pedal commissure, some- 
times even united with it. The fusion of all the 
ganglia belonging to the peri-a'sophageal complex is 
carried very far in such animals as Te.thys, where the 
pleural and pedal ganglia of each side may fuse with 
the cerebral ganglion. The jileuro - cerebropedal 
ganglion thus formed shifts towards the dorsal 
middle line close to the similar ganglion of the other 
side, with which it foi-ms a large supra-oesophageal 
ganglionic mass. Its composition out of the six 
typical ganglia can, however, be made out by the 
grouping of the ganglion cells and the arrangements 
of the nerve tracts. A nerve leaves this mass on 
each side, the two uniting under the gullet. These 
form the pedal commissure, which Avhen closely examined is found to be double. A 
third delicate commissure running under the cesophagus connects the lateral portions 




Fir. 122. —Nervous System of 
Janus (after Pelseneer simplified). 
1, Buccal ; 2, cerebral ; 3, pleural ; 
4, pedal ganglia ; 5, commissure be- 
tween the two pleural ganglia, which 
corresponds with the two pleuro- 
visceral connectives of other Mol- 
lusca ; 6, pedal commissure ; 7, 
auditory vesicle ; 8, eye ; 9, ganglion 
of the rhinophore. 



142 



COMPARATIVE ANATOMY 



CHAP. 



III. Pulmonata (Fig. 123). 



of tlie supra-cesophageal mass, and represents the visceral commissure, in which is 

found a small visceral ganglion. 

Among the Nudibranohia the two buccal ganglia are always found on the 

posterior and lower wall of the pharynx. They are connected with each other by a 

buccal commissure, and with the brain by two cerebrobuccal connectives, in whose 

course accessory ganglia may be found. 

The whole peri-oesophageal comple.x of ganglia is in the Nudibranchia enclosed in 

a capsule of connective tissue. 

—The central nervous system here possesses all the 
typical ganglia of the Gastropoda. These, grouped 
together as in so many OpisthohrmicMa and many 
Frosohranchia, immediately behind the pharyngeal 
bulb, form the peri-cesophageal complex, into which 
even the parietal and visceral ganglia have been 
drawn. The cerebral ganglia lie close to each other 
dorsally, and all the other ganglia, which are also 
close together, lie ventrally. The cerebropedal and 
cerebropleural connectives are consequently always 
easily distinguished. In Testacella they are even of 
some length, in adaptation, no doubt, to the special 
shape and the great development of the pharyngeal 
bulb. All other connectives and commissures, on the 
contrary, are much shortened, so that the ganglia 
connected by them lie close together. A visceral 
ganglion is always found, and usually also in each 
pleurovisceral connective a parietal ganglion. When 
an osphradiuni is present {BasoDimatophora) it is 
innervated from the parietal ganglion of the same 
Fig. 123.-C6ntral portion of the '^i'^^- In Pulmonata with a dextral twist, the osphra- 

Nervous System of Helix pomatia dium lies on the right, and in those with a sinistral 

twist on the left ; in the former the right parietal 
ganglion is the larger, and in the latter the left. 
The smaller parietal ganglion may also fuse with 
the neighbouring pleural ganglion. Lobes are often 
formed in the cerebral ganglia, in which certain 
groups of nerves have their origin. The pedal com- 
missure is often double. Buccal ganglia are always 

found. They lie posteriorly on the pharynx below the cesophagus, and are joined to 

one another by the buccal commissure and to the cerebral ganglia by cerebrobuccal 

connectives. 




(after Bohmig and Leuckart), some 
what diagrammatic, tlie ganglia 
being in reality less distinct. 1, 
Buccal ganglion ; 2, optic nerve with 
thickened root (3) arising from the 
cerebral ganglion (4) ; 5, pedal ; 6, 
pleural ; 7, parietal ; S, visceral 
ganglion. 



C. Seaphopoda. 

The nervous system of the Seaphopoda (Fig. 101, p. 113) is 
symmetrical ; the visceral connectives are not crossed. The two 
cerebral ganglia lie very near one another in front of (or, if the 
intestine is regarded as horizontal, above) the gullet over the snout ; 
the two pedal ganglia, close to one another, lie on the anterior side 
of the foot, more or less at its centre, and are joined to the cerebral 
ganglia by two long cerebropedal connectives. The two pleural 
ganglia lie close to and above the cerebral ganglia, so that the 
cerebropleural connective is very short. The pleuropedal connective 



VII MOLLUSGA—THE NERVOUS SYSTEM 143 

at once fuses with the cerebropedal, the two entering the' pedal 
ganglion as one connective. Posteriorly, to the right and left of the 
rectum, near the anus, there are two visceral g'ang'lia of the pleuro- 
visceral connectives, joined to one another by a commissure running 
behind the intestine. There are no special parietal ganglia distinct 
from the visceral or the pleural ganglia. 

There are fom' buccal ganglia, two behind the gullet or below it (if the intestine 
is supposed to be horizontal), and two lying laterally and anteriorly to (or above) 
the muscular mass of the radula. The anterior are connected with the posterior, and 
these to the cerebral ganglia by connectives, and the two posterior and two anterior 
inter se by commissures running behind (under) the cesophagus. Nerves run from 
the posterior buccal ganglia to the small ganglia of a subradular organ. 

D. Lamellibranehia. 

The nervous system (Fig. 124), like the whole organisation of the 
Lamellibranehia, is perfectly symmetrical, and consists typically of 
three pairs of ganglia: (1) the eerebpopleural ; (2) the pedal; and 
(3) the viseepoparietal ganglia. These three pairs of ganglia lie, as 
a rule, far apart, and the connectives uniting them are therefore long. 
The two pedal ganglia lie close together, while the two cerebropleural 
and the two visoeroparietal ganglia are connected by distinct com- 
missures beset with ganglion cells. 

1. The cerebropleural ganglia are the result of the fusion of the 
cerebral with the pleural ganglia. In the Protoh'anchia, however, the 
pleural ganglia are still distinct, and lie immediately behind the 
cerebral ganglia at the commencement of the visceral connectives. In 
Nucula, the pleuropedal connectives are distinct for some distance, 
and then unite with the cerebropedal connectives. In Solenqmya 
they still have separate roots, but are otherwise fused along their 
whole length with the cerebropedal. 

The cerebropleural ganglia are supracesophageal, and are in 
contact with the anterior adductor muscle, when this is present. 
They send nerves into the oral lobes, the anterior adductor, and the 
mantle. 

2. The pedal ganglia lie at the base of the foot. 

3. The third pair of ganglia, which correspond with the ganglia 
of the visceral connectives in the Gastropoda, lie posteriorly beneath 
the rectum, behind the foot, and are generally in contact with the 
posterior adductor muscle ; in the Protohranchia, however, they lie 
much farther forward. Their area of innervation corresponds with 
that of the combined parietal and visceral ganglia of the Gastropoda, 
for these visceroparietal ganglia supply with nerves the two ctenidia, 
the two osphradia, the posterior portion of the mantle, the posterior 
adductor, and the viscera. 

The buccal or stomodseal nervous system is much reduced ; this reduction is 
connected with the absence of a muscular pharynx and of all buccal armature. The 



144 



COMPARATIVE ANATOMY 



CHAP. 



anterior jiortion of the intestine reeeives nerves from tlie visceral connectives. Since 
the fibres of these nerves have been proved to originate in the cerebral ganglia, we 
may assume that, on the degeneration of the pharyn.v, the buccal connectives united 
with tlie visceral connectives, so that the intestinal nerves now rise from the latter 
and do not come direct from the brain. In the Pholadida' and Teredinidcc the 
visceral connectives are united in front of the i-iscernparietal ganglia by a second 




Fig. 124.— Nervous system of Cardium edule (afler Drost), seen fruni Hie \ ciitral .skle. The 
left mantle (the right in the figure) has been reniriveil and the right bent back ; tlie foot has been 
laid on one side. 1, Oral lobes ; 2, 3, 4, pallial nerves, running nearly parallel to the edge ; 2, the 
nerve of the pallial edge ; d, mantle ; C, gill ; V, point of junction of the principal pallial nerves ; S, 
jnantle edge of the respiratory aperture ; (», ditto of the anal aperture ; 10, posterior adductor ; 11. 
viscero-parietal ganglion ; 12, branchial nerve ; 13, foot ; 14, pedal ganglion ; 15, left cerebroxjleural 
ganglion ; 111, mouth ; 17, right cerebropleural ganglion ; Is, anterior adductor. 

conmiissure, which runs under the intestine, and may perhaps lie considered as a 
buccal commissure shifted far back. 

The mantle is innervated, as is clear from the above, partly from the cerebro- 
pleural, and partly from the visceroparietal ganglia. 

The two anterior pallial nerves, which rise from the cerebropleural ganglia, run 
back along the edges of the mantle, to join the two posterior jiallial nerves which 
originate in the visceroparietal ganglia. A nerve thus runs jiarallel to the edge of 
the mantle on each side (nerve of the pallial edge), and like a connective, unites the 
anterior cerebropleural ganglion with the posterior visceroparietal ganglion. This 



VII 



MOLLUSCA~THE NERVOUS SYSTEM 



145 



pallial nerve gives off branches to the organs at the edge of the mantle and to the 
siphons, and is further connected with a rich nerve plexus in the mantle fold, in 
which certain connecting nerves, further from the edge of the mantle, but running 
parallel to it, are particularly strongly developed. A varying number of small 
peripheral ganglia attain development in the pallial plexus and in the siphonal 
nervous system. 

E. Cephalopoda. 

The symmetrical nervous system of all Cephalopoda is marked by 
the great concentration of the typical Molluscan ganglia, including 
those of the visceral connective. 

In the following description of the nervous system, we shall consider the body in 
its physiological, not in its true morphological position, i.e. we shall imagine the 
pharynx and cesophagus to be running 
horizontally as in other JIoUuscs (c/. p. 
36). The true morphological position 
will be given in brackets after the con- 
ventionally accepted position. 

1. Tetrabranehia (Figs. 125, 126). 

In the complex of ganglia 
which in Nautilus surrounds the 
cesophagus behind the great buccal 
mass, and which is not yet com- 
pletely enclosed in the cephalic 
cartilage, the ganglia are not very 
distinct from the commissures and 
connectives. The cerebral ganglia 
(14, in Figs.) are represented by a 
broad band-like nerve cord running 
over (morphologically in front of) 
the cesophagus, and from them run 
two ganglionic cords, one anterior 
(lower) and one posterior (upper), 

which pass just below (behind) the Jlermg). l, Buccal ganglion; 2, pharyngeal 

CeSOphaaUS. The anterior (3) re- g™glia ; 3, pedal commissure ; 4, infundibular 

, ,-, , , J ii, 4. ■ nerve ; 5, nerve in the female for the tentacles Of 

presents the pedal, and the posterior ^j^^ posterior and inner lobes ; this nerve soon 

(15) the combined pleural and swells to form a ganglion (c/. Fig. 126); 6, nerves 

vi«PPT>nl S-ane-lia ^°'' *'''* °*^'^'' t^'iolfs; 7, pedal cord (=pedal 

visociai s"'"S""" _ ^ ganglia); 8, auditory organ; 9, olfactory nerve; 10, 

The cerebral cord gives rise opti^ ganglion ; ll, nerve of the optic tentacles ; 

laterally to the large optic nerves 12, connective to the pharyngeal ganglion ; 18, 

/ 1 p 1 * T_ i. „ ,,,-^n« 4^+^ labial nerves'; 14, cerebral cord (=cerebral 

(each of which at once swells into g.^glia) ; 15, pleurovisceral cord. 

an optic ganglion), numerous nerves 

to the lips, the nerves for the optic tentacles, the auditory and olfactory 

nerves, and the cerebrobuccal connectives. 

From the pedal cord, nerves run to the tentacles round the mouth 
and to the funnel. In the female, the nerves for the inner circle of 
VOL. II L 




Fig. 126.— Nervous system ot Nautilus (after 



146 



COMPARATIVE ANATOMY 



CHAP. VII 



tentacles come from a brachial ganglion, which, however, does not 

supply all the tentacles (Fig. 126, a); 
this is joined to the pedal ring by a 
brachiopedal connective. 

The pleupovisceral cord gives off 
numerous pallial nerves (there is no 
stellate ganglion), and two strong vis- 
ceral nerves which run near the middle 
line accompanying the vena cava, inner- 
vate the gills, the osphradia, and the 
blood-vessels, and form a genital gan- 
glion high up in the visceral dome. 




Fig. 12(;.— Nervous systemof Nautilus, 
from the right side. I^uiiibering the same 
as in Fig. 12rj. a, Ganglion for the ten- 
tacles of the posterior and inner lobes in 
the female. 



The sympathetic nervous system consists 
of an infra-cesophageal commissure, which rises 
from the cerebral ganglion, and passes close under the oesophagus in the musculature 
of the buccal mass ; two ganglia, a pharyngeal and a buccal ganglion, are found on 
each side in its course. 



2. Dibranehia (Figs. 127, 128). 

The peri-oesophageal mass of ganglia, comprising the whole of the 
central nervous system, is entirely enclosed in the cephalic cartilage. 
The large typical ganglia are so crowded together that it is extremely 
difficult to distinguish them one from another, and the connectives 
and commissures are not visible externally. The whole complex has 
a continuous cortical layer of ganglion cells. 

The more or less distinct separation of each pedal ganglion into 
two, one anterior (lower) and one posterior (upper), is characteristic 
of the Dibranehia. The former of these is the brachial ganglion, and 
innervates the arms, which must be considered as parts of the foot ; and 
the latter is the infundibular ganglion, and innervates the siphon, 
which may be regarded as the epipodium. This differentiation of the 
pedal ganglia can be traced to the great development of that part of 
the foot (viz. the arms) which surrounds the head. In the same way 
in Natica, where the anterior part of the foot is strongly developed, 
and is bent back over the head, a propedal ganglion becomes 
differentiated from the pedal ganglion. The brachial ganglia become 
joined in the Dihraiidiia to the cerebral ganglia by cerebrobrachial 
connectives. In Eledone and Octopus, they are further connected with 
one another by a thin supraoesophageal commissure. 

The pleural ganglia lie laterally in the pericesophageal mass, while 
the ganglia of the visceral connectives, i.e. the parietal and visceral 
ganglia which lie close together, their connectives having shortened as 
much as is possible, form the posterior (upper) portion of the infra- 
oesophageal mass. 

The following are the connectives which are revealed by sections 
through the peri-cesophageal mass : — 




Fig. 127.— Anatomy of Octopus (after Leuckart and Milne Edwards). The body is cut open posteriorly, the mantle laid 
back to the right and left, and the liver removed. 1, Brachial artery ; 2, brachial nerve ; 3, pharynx ; 4, buccal ■ 5 cerebral 
ganglion ; 6, efferent duct of the upper salivary glands ; 7, funnel ; 8, upper salivary glands ; 9, crop ; 10 anus • 11 afferent 
branchial vessel (branchial artery) ; 12, left renal aperture ; 13, efferent branchial vessel (branchial vein) ; 14, gastric t^anglion - 
15, left auricle; 16, spiral caecum of the stomach; 17, renal sac; 18, water canal; 19, ventricle; 20, ovary 21 rectum' •''l' 
efferent ducts of the digestive gland (liver), cut through near its opening into the intestine ; 23, mantle ; 24, stomach ■ 25' 
right ctenidium ; 26, aperture of the right oviduct ; 27, stellate ganglion ; 28, nerve to the gastric ganglion ; 29, upper salivary 
gland ; 30, aorta ; 31, oesophagus ; 32, optic ganglion ; 33, lower salivary glands. 



148 



COMPARATIVE ANATOMY 



CHAP. 



(1) Two cerebro-brachial ; (2) two cerebro-infundibular ; (3) two 
cerebropleural ; (4) two brachio-infundibiilar ; (5) two pleuro-infundi- 
bular; (6) two pleurobrachial connectives. The close proximity of 
the visceral ganglia to the peri-cesophageal mass makes it impossible 
any longer to distinguish the visceral connectives. 



The cerebral ganglia give rise to the two optic nerves (which soon swell into the 

enormous optic ganglia at 

A 

5 







Fig. 128.— Central nervous system of various DibranoMa, 
from the right side. All the figures after Pelseneer. A, Ommato- 
streplies ; B, Sepiola ; C, Loligo ; D, Sepia ; E, Octopus ; F, Argo- 
nauta. 1, Cerebral; 2, pedal; 3, visceral; 4, brachial; 5, upper 
buccal ganglion ; 6, iiifundibular nerve ; 7, visceral nerve ; 8, optic 
nerve cut tlirough; 9, pallial nerve; 10, brachial nerves; and in 
Fig. B the ph<arynx (pTt), and oesophagus (ffi) are drawn in black. 



the bases of the eyes), the 
auditory nerves, the olfac- 
tory nerves (which for a 
certain distance fuse with 
the optic nerves), and the 
connectives of the buccal 
ganglia. 

The brachial ganglia 
send off separate nerves to 
the arms, which nerves are 
connected by a hoop-like 
commissure round the base 
of the circle of arms. Run- 
ning through the arms, the 
nerves swell into succes- 
sive ganglia which corre- 
spond with the transverse 
rows of acetabula. 

The separation of the 
pedal ganglion into a bra- 
chial and an infundibular 
ganglion can be proved on- 
togenetically and anatomi- 
cally. There is no such 
separation in the male 
Nautilus, the brachial and 
infundibular nerves spring- 
ing from one and the same 
ganglion. In Argonauta 
(Fig. 128, F) the separation 
is not externally visible, 
but in Octopus (E) we see 
the first traces of it ; in 
Sc]yia (D), Loligo (C), and 
Sepiola (B), it becomes 
more and more evident, till 
finally in Onunatostrephes 



(A) the distinct brachial ganglion has moved away from the infundibular ganglion, 
with which it is joined by a slender externally visible connective. 

In this same series, the separation of the so-called up})er buccal ganglion from 
the cerebral ganglion also takes place, the buccal remaining united to the brachial 
ganglion by the brachiobuccal connective. 

The parietal ganglia give rise to the two large pallial nerves. Each of these runs 
backward and upward, and enters the stellate ganglion on the inner surface of the 



vii MOLLUSGA^THE ASYMMETRY OF THE GASTROPODA 149 

mantle. Numerous nerves radiate into the mantle from this ganglion, one of them, 
which runs dorsally, looking like the direct continuation of the pallial nerve through 
the ganglion. The pallial nerve often divides into two branches sooner or later after 
it has left the parietal ganglion ; one of the branches running to and through the 
stellate ganglion, to unite beyond it with the other branch which runs past the 
ganglion. The two stellate ganglia are often connected by a transverse commissure. 

The visceral ganglia give off, near the middle line, two visceral nerves, which 
innervate the rectum, the ink-bag, the gills, the heart, the genital apparatus, the 
kidneys, and certain parts of the vascular system. The two genital branches of 
these nerves are connected by a commissure. 

The sympathetic nervous system consists of a buccal ganglion lying beneath 
(behind) the oesophagus in the buccal mass ; this ganglion is joined to the upper 
buccal or pharyngeal ganglion by a buccal connective. Two nerves run up along 
the oesophagus from the lower buccal ganglion to the gastric ganglion, which lies 
on the stomach, and innervates the greater portion of the intestine and the digestive 
gland (liver). 



XIV. An Attempt to explain the Asymmetry of the Gastropoda. 

1. 

Chiastoneury, i.e. the crossing of the two pleuro-visceral connectives in the 
Prosohranchia, may be explained on the three following assumptions. 

1. The ancestors of the Prosohranchia were symmetrical animals ; the mantle 
cavity lay behind the visceral dome and in it the pallial complex, that is, the ctenidia, 
osphradia, nephridial apertures, genital apertures, and, in the centre, the median 
anus. 

2. The visceral commissure or ganglion lay beneath the intestine. 

3. The pallial complex shifted gradually from behind forward, along the right 
side of the body {cf. p. 136). 

The position of the pallial complex in the Tcctibranchia, among the Opisthohranchia 
on the right side, can also be thus explained. The pallial complex in its forward 
movement in these animals has either not yet reached the anterior position or, 
having reached it, has shifted back again. ^ The visceral connectives are therefore 
not crossed. 

The above assumptions do not, however, explain — 

1. The asymmetry which is brought about in some Gastropoda by the dis- 
appearance of one ctenidium, one osphradium, and one renal aperture. 

2. The coiling of the visceral dome and shell, especially the dextral or sinistral 
spiral twist. 

3. The relation existing between the manner in which the visceral dome and 
shell are coiled, on the one hand, and the special asymmetry of the asymmetrical 
organs (ctenidia, osphradia, nephridia, anus, genital organs) on the other. 

4. The cause of the shifting forward of the pallial complex. 

2. 

It is unnecessary to discuss the first of the above assumptions, viz. that the 
ancestors of the Gastropoda were symmetrical animals, since all Molluscs except 
the Gastropoda are symmetrical, i.e. the Amphineura, the Lamellibranchia, the 
Scaphopoda, and the Cephalopoda. 

1 See note to § 13, p. 158. 



150 COMPARATIVE ANATOMY chap. 

The assumption that the pallial complex originally lay posteriorly is also well 
founded. In all symmetrical Molluscs, the anus lies as the centre of the complex 
posteriorly in the middle line, and further, in all symmetrical Molluscs, the nephridial 
and genital apertures lie posteriorly at the sides of the anus. When the ctenidia 
and osphradia have been retained in symmetrical Molluscs, they lie symmetrically 
on the posterior side of the visceral dome. This is the case in the Cephalopoda, and 
in the most primitive LamelUiranchia, the Protobranchia {Nucula, Zeda, SoUnomya), 
and even in some Chitonidce, and those Solenogastres which still have rudiments of 
gills. 

In keeping with the posterior position of the pallial complex, the mantle fold 
which hangs down round the base of the visceral dome is, in symmetrical Molluscs, 
mdest posteriorly where it has to cover the complex ; at this part the mantle 
furrow deepens into a mantle cavity. 

In connection with the second assumption, it still remains unexplained why in 
the Amphineura the commissure between the pleuro-visceral cords runs over the 
intestine ; whereas on the other hand, in all other sjmimetrical Molluscs, the 
visceral ganglion lies, as in the Gastropoda, below the intestine. 



The third assumption, that the pallial complex has shifted forward, req^uires 
separate discussion. 

If the pallial complex did thus shift forward, chiastonenry must necessarily 
have taken place ; the original left half of the complex must necessarily have become 
the present right half, and irice versa. Further, the right pleuro-visceral connective 
would have to become the supra-intestinal connective and tlie left the infra-intestinal 
connective ; the original right parietal ganglion the supra-intestinal ganglion, and 
the original left parietal the infra-intestinal ganglion. But ivhy did such a shifting 
take place ? "We shall here attempt to answer this question. 

Cause of the shifting forward of the pallial complex. — We have assumed the 
symmetrical racial form of the Gastropoda (with posterior mantle cavity and sym- 
metrical pallial complex) to be a dorso-ventrally 
flattened animal with a broad creeping sole, a 
snout-like head with tentacles and eyes, and a 
somewhat flat cup - shaped shell covering the 
dorsal side of the body (Fig. 129). It therefore 
resembled in outward appearance a FissxircUa, a 
Patella, or a Chiton, if we assume the imbricated 
shell of the last to be replaced by a single shell. 

„ , ^. , . .^. The body of such a racial form was only pro- 
FiG. 129. — Hypothetical primitive , , , , ,, , ^, , ,, ^, , , „ 
Gastropod, from the side, o, Mouth ; ^^"^^^ dorsally by the shell. The hard surface 
7,-, head ; sm, shell muscle ; oso, apical along which the animal slowly crept served to 
shell aperture ; a, anus ; «, renal aper- protect its lower side, the dorsal shell being 
ture ; mh, mantle cavity ; c(, ctenidium ; pressed firmly against the substratum, when 
°° ' necessary, by the contraction of a powerful shell 

muscle {cf. Fig. 1 06, p. 122). When the shell was thus pressed down, communica- 
tion between the pallial cavity and the exterior (for the purpose of inhaling and 
exhaling the respiratory water, and ejecting the excreta, excrement, and genital 
products) was rendered possible by means of a cleft in the posterior edges of the 
mantle and shell. 

Unlike their racial form, all known Gastropoda (except those whose body form 
has been secondarily modified, generally in connection with the rudimentation of the 
shell) are distinguished by the fact that the viscera with their dorsal integumental 




VII MOLLVSCA—THE ASYMMETRY OF THE GASTROPODA 151 

covering protrude hernia-like in the form of a high spire-like visceral dome, with 
which the shell corresponds in shape. The uncoiled shell of every snail is as a 
matter of fact spire-shaped. 

The development of such a shell and dome has already been recognised as due to 
the increased protection needed by the body when the capacity for creeping becomes 
developed. Tlie whole of the softer part of the body can be withdrawn into such a 
shell, and, further to increase the protection, an operculum is often developed on the 
foot for closing the aperture of the shell, when the animal has retired into it. The 
shell muscle of the racial form no longer serves for pressing the sliell against the 
surface on which it rests, but for withdrawing the head and foot into the shell. It 
becomes the columellar muscle (Fig. 131, sin). 

Taking in turn the different stages in the development of the Gastropod 




Fig. 131. 

(Lettering in this and in the following three 

figures the same as in Fig. 129.) 



Fig. 180.— Hypothetical primitive Grastropod, 
from above, o, Mouth ; ulc, u2pl, ulp, original 
left cerebral, pleural and pedal ganglia ; ulpa, 
urpa, original left and right parietal ganglia ; 
wto, original left auricles ; uos, itros, original left 
and right osphradia (Spengel's organs) ; ulct^ nrct, 
original left and right ctenidia (gills) ; m&, base 
of the mantle ; mr, edge of the mantle ; m, mantle 
cavity; v, visceral ganglion; ve, ventricle; u, 
anu.s. 

shell, we have as the first and most important its dorsal spire-like prolongation. 
In this way the cup-shaped shell of the racial form becomes a high conical shell like 
that of Deiitalmmi. 

Such a shell earned vertically by the animal (Fig. 131) would, when the latter is 
at rest, be in a state of unstable equilibrium, which would be upset by movement or 
by the slightest pressure from without. It is also evident that when the animal is 
in motion a vertically placed spire-like shell would be extremely awkward. 

If we assume the shell to be carried at some other angle to the body, we have 
the following possible positions : — 

1. The shell might be carried inclined forward (Fig. 132). Such a position is 
the most unfavourable imaginable for locomotion, for the functions of the mouth, 
and for the sensory organs on the head. 



152 



COMPARATIVE ANATOMY 



CHAP. 



On the other hand, such a position is the most favourable imaginable for the 
functions of the organs belonging to the posteriorly placed pallial complex, wlrioh 
now lie dorsally, since in this position tlie mantle cavity is subjected to least pressure 




Flo. 132. 



from the viscera and from tlie columellar muscles. Tlie downward pressure of 
tlie visceral mass wliich now takes place would tend indeed to widen tire cavity. 

2. Tire shell might be carried inclined backwards (Fig. 133). This position is 
the most favourable imaginable for locomotion and for the functions of the organs 




Fig. 133. 

of tlie head, which would thus be free on all sides. It is, however, the most 
unfavourable imaginable for the functions of the organs of the pallial complex, 
which now lie beneath the visceral dome. The mantle cavity has to bear the whole 
pressure of the visceral mass, and especially that of tlie columellar muscle ; it would 

be squeezed together, so that the 
i'' circulation of tlie respiratory water 

would be prevented or at least 
rendered more difficult, as would 
also the ejection of tlie excreta, ex- 
crement, and sexual products. 

3. Finally, the shell may be 
carried inclined to the right or left 
(Fig. 134). This is neither the most 
favourable nor the most unfavour- 
able position for locomotion, for the 
liead, and for the pallial complex. 
It is an imaginable intermediate 
position. 

In this position there is no dead point, as shifting of the parts would always be 
possible, and the shell be enabled to take up the position most suitable for locomo- 
tion and for the functions of the cephalic organs, and the mantle cavity that best 
suited for the exercise of the functions of the pallial complex lying within it. 

Assuming that the shell is inclined to the left (Fig. 135), the pressure brought 
to bear on the mantle cavity would vary in amount in different areas of that cavity. 
It would be greatest on the left side, and would continually decrease towards the 




Fio. 134. 



VII MOLLUSGA—THE ASYMMETRY OF THE GASTROPODA 153 



right. On the left there would be a, pressure from the front which would, so to 
speak, squeeze out the pallial complex backwards over to tlie right. It must further 
be noted that the point subjected to least lateral pressure and to the greatest down- 
ward pull lies on the right, 
which has become the upper 
side of the visceral dome. At 
this point the mantle furrow 
will most easily deepen, and 
become more spacious. Into 
such a deepening the organs of 
the pallial complex which are 
being pressed from the left 
have room to move forward to 
the right. Here we have the 
first step in the shifting for- 
ward of the pallial complex 
along the right mantle furrow. 
Further, as soon as the least 
shifting of this sort has taken 
place, the shell and visceral 
dome can move slightly from 
their present position on the 
left, towards that backward 
position which we have seen ^i°- 136.— Diagram illustrating the variations of pres- 
. 1 , , , /. 11- sure to wMcIi tlie shell and visceral dome are subjected 

to be the most favourable im- „^„„ i„„n„„/i ♦„ +k„ i„«. rrv. n ■ i * ti, ,. ■ 

. when mcluiea to the left. The thickness of the coBcentric 

agmable for locomotion and for ii„es indicates the amount otthe pressure, a, Point of greatest 
the functions of the cephalic pressure ; h, point of least pressure. The arrows give the 
organs. direction in wliich shifting takes place. It is evident that 

If we suppose this process ^^^ ^"^^ '"*" "' ^^^ P'""''' '=°'"P'"'' '" subjected to greater 
. -, , , 1 , T pressure than the right, 

gradually to be completed, the 

shell and visceral dome finally gain the most favourable backward position, and the 

pallial complex is gradually shifted forwards along the right mantle furrow. The 

pallial complex thus lies anteriorly on the upper side of the visceral dome, which 

now points backwards. This anterior position is that of the least upward pressure, 

or rather of the greatest downward pull, i.e. it is the point at which the mantle 

cavity can most easily deepen and widen, and where the pallial organs can best 

fulfil their functions. 

The position of the shell and the pallial complex characteristic of the Gastropoda 

is now attained, and with it chiastoneury and the inverse position of the organs of 

the pallial complex. 




The second stage in the development of the Gastropod shell is the coiling in 
one plane of the visceral dome and shell. 

If the Gastropod visceral dome assumes the most favourable inclined position above 
described, it Avill, under normal conditions, change its conical shape. The side which 
lies uppermost will become arched and the lower side concave. This change of form 
is caused by the stronger growth of the integument of the visceral dome and mantle 
on that side, which, in the inclined position of the visceral dome, is the most 
stretched or pulled. The visceral dome also becomes curved in one plane, and the 
shell naturally adapts itself to the changes of shape of the dome. Again, the shell 
could not remain conical, because a large part of the dorsal integument (base of the 
visceral dome) would then be uncovered, and in consequence of the increase of those 



154 COMPARATIVE ANATOMY chap. 

parts of the body not covered by the shell there would come a time when the body 
could no longer be completely withdrawn into it. 



Before discussing the third stage in the development of the Gastropod shell, we 
must consider its growth. This, from a geometrical point of view, is of three kinds : 
growth in height, peripheral growth, and radial growth or increased thickness of 
the shell wall. This last does not here concern us. 

Supposing, for simplicity's sake, the shell to be conical, growth in height occurs 
at the base (or aperture of the shell), and takes place by means of continual deposits 
of bands of new material at the edge of the aperture, by the growing edge of the 
mantle. 

Peripheral growth is the enlargement of the circumference of the base or aperture 
of the shell. 

If the height and the peripheral growth remain uniform round the whole 
aperture of the cone (which is assumed to be round), -the cone increases without 
altering its shape. 

If, however, the growth in height is not uniform, but steadily and symmetrically 
increases along each side from an imaginary minimum point to a diametrically 
opposite maximum point, the peripheral growth, however, remaining uniform, a 
spirally twisted hollow cone is produced. 

If the minimum and maximum points in this growth continue throughout in 
one and the same plane, a symmetrical shell coiled in this plane of symmetry 
results. 

If, however, as growth increases, the maximum point shifts from the symmetrical 
plane, say to the left (the minimum point shifting in the opposite direction to the 
right), the maximum and minimum points no longer trace on the spirally coiled 
shell straight but spirally twisted lines, and the conical shell is then not coiled 
symmetrically in one plane, but asymmetrically in a screw-like spiral. We then 
have what conchologists call a dextrally twisted shell. 

The growth of the Gastropod shell actually takes place in this last manner. 



This, the dextral (or sinistral) coiling of the Gastropod shell, is the last stage to be 
discussed. If the visceral dome and shell which are twisted in one plane pass, in growth, 
from an incline to the left to a backward incline, this is equivalent to the continual 
shifting of the point of maximum growth to the left and that of minimum growth 
to the right ; the necessary consequence being a dextral screw-like spiral twist. 

It must be borne in mind — 

1. That the peripheral growth remains constant, i.e. that the outline of the 
growing edge of the mantle remaining uniform, the increasing aperture of the shell 
also retains the same form. 

2. That the additions to the shell by the mantle edge are made in the form of 
bands of new material, the already formed firm shell not altering in shape. 

3. That the growing edge of the mantle, which secretes the shell substance, does 
not, in the course of the gradual change from the left to the backward incline, itself 
become twisted, but retains its position in relation to the rest of the body. It is 
thus only the maximum and minimum points of growth in height which become 
shifted along the edge of the mantle. 

i. It must be noted that this description of the manner in which a dextrally 
twisted shell arose only applies to that stage in the ontogenetic or phylogenetic 



Yii MOLLUSGA—THE ASYMMETRY OF THE GASTROPODA 155 

development of the shell during which its displacement in a backward direction and the 
shifting forward of the pallial complex occur. When once the result most favourable 
to the animal, i.e. the anterior position of the mantle cavity and the backward 
direction of the shell, are attained, further displacement, which would be dis- 
advantageous, does not take place. It is, then, not at first sight evident why, 
when the need for displacement ceases, its action still continues, i.e. why, though 
displacement ceases, the visceral dome and shell continue to gi'ow in a dextral 
twist and not symmetrically. This point will be explained below. 



For the sake of clearness we have treated separately the three important factors 
in the development of the Gastropod shell, viz. (1) the formation of a tall conical 
shell, (2) the spiral coiling of the same, and (3) the special manner of coiling in a 
dextral twist. In reality these three factors do not denote special stages, but all 
operate simultaneously. The continually increasing protrusion of the visceral dome 
was accompanied by the dextral twist, as a consequence of the twisting of the 
visceral dome from its incline to the left to the most favourable backward incline, 
by which the pallial complex was shifted forward. 

8. 

The results of ontogenetic research favonr the theory here advanced. We 
have first to note the fact that the anus (the centre of the pallial complex) and the 
mantle fold originally lie posteriorly. They come to lie anteriorly in the embryo 
not by active shifting, but by the cessation of growth on the right side between the 
mouth and anus, and its continuation on the left side. There is, however, no 
difBculty in harmonising this ontogenetic method of gaining the object with the 
phylogenetic method. 

9. 

So far we have placed mechanical and geometrical considerations in the fore- 
ground. But these necessarily coincide with utilitarian considerations. Every 
alteration in the direction we have been considering means an improvement in the 
organisation of the animal, an advantage to enable it the better to maintain the 
struggle for existence. The formation of a spire-like shell, which has been recog- 
nised as the starting-point in the development of the asymmetry of reptant Gastro- 
pods, was the only method by which complete protection of the whole body could 
be attained, and must therefore be considered to have been advantageous under the 
circumstances. We might further conclude this from the fact that the possession 
of such a shell actually distinguishes the Gastropoda from the primitive MoUusca, 
which the Ghitonidce are rightly considered most nearly to represent. 

10. 

One apparently important objection to the theory here set forth must be mentioned. 
If the first factor in the asymmetry of the Gastropod body is the development of a 
high spire-like shell, and if the arrangement of the nervous system is necessarily 
connected with the coiling of the shell in a definite direction, how can we account 
for forms such as Fissurella ? This Diotocardian genus actually belongs to the most 
primitive Gastropods, because the symmetry of the pallial complex is still retained. 
But it possesses an asymmetrical nervous system and the. typical chiastoneury of 
the Prosohranchia, and nevertheless a flat cup-shaped symmetrical shell. We thus 
here have secondary characteristics of the inner organisation combined with an 



156 



COMPARATIVE ANATOMY 



CHAP. 



apparently primitive shell. The latter is, however, only apparently primitive, as can 
be proved systematically and ontogenetically. The forms most nearly related to 
Fissurella, such as the primitive genus PUurotoiTuiria (Fig. 136 A), Polytrcmaria 
(Fig. 136 B), and Scissurella, have spacious spirally coiled dextrally twisted shells. 
In Haliotis (Fig. 136 D) the shell becomes flat and the coiling indistinct, as is also 
the case to some extent in Emarginula (Fig. 136 C), till fiually in Fissitrella (Fig. 




Fio. 136.— Shells of A, Pleurotomarla ; B, Pol3rtremarla ; C, E, Emarginula ; D, Haliotis ; 
F, Fissurella ; Cr, H, stages in the development of the shell of Fissurella ; I, shell of 
the Gastropod racial form, with marginal cleft ; K, the same, with apical perforation ; 
L, Lamellibranch shell : M, shell of Dentalium, seen from the apical cleft. The shell clefts 
and perforations are black, o, Mouth ; a, anus ; ct, ctenidium. 

136 F) it again secondarily becomes flattened or cup-shaped and symmetrical. Fis- 
surella even passes ontogenetically through an Emarginula stage, in which the shell 
is distinctly spirally coiled (Fig. 136 G, H). We may therefore conclude, with as 
much certainty as is possible in morphological questions, that the outwardly sym- 
metrical Fissurella descends from forms with high spirally coiled shells. Its return 
to a flat symmetrical shell may have been determined, as in the Patellidce, Capulidw, 
etc. , by adaptation to certain biological conditions. 



11. 

The explanation given above seems to throw new light on many as yet unsolved 
problems in the morphology of the MoUusca, such as the asymmetry of the pallial 
complex in most Gastropoda. Many Diotoeardia, all Monotocardia, all Opistho- 
branchia, and all Pulimmafa show marked asymmetry in the pallial complex. The 
asymmetry consists principally in the absence of one gill, one osphradiura, and one 
nephridial aperture. The inner organisation also shows reflections of this asym- 
metry in the nervous system, and the absence of one kidney and one auricle. On 
closer inspection, it is found that it is the original left half of the pallial complex 



VII MOLLUSGA—THE ASYMMETRY OF THE GASTROPODA 157 

(which in a Prosobranch would lie to the right in the mantle cavity near tfie anus) 
which is wanting. The anus is no longer the centre of the pallial group of organs, but 
lies outermost on one side. While in the Prosobranchia, for example, the original 
left half of the pallial complex (which would now lie on the right) has disappeared, 
those organs of the complex (the original right) which are retained, shift from 
the left to occupy the empty space. Consequently, we find the anus no longer 
anteriorly in the middle line, but on the right side, close to the extreme right of the 
mantle cavity. 

But what is the reason of the disappearance of the left half of the pallial complex 
in the Monotocardia, Opisthobranchia, and Pulmonata ? 

In answering this question we must refer back to paragraph 3, where it was seen 
that if the spire-like shell assumes the only possible lateral inclination, the mantle 
cavity and the pallial complex within it are subjected to unequal pressure. If the 
shell is inclined to the left, the side of the posterior mantle cavity subjected to the 
greatest pressure is the left, and the pressure continually decreases towards the right. 
These variations of pressure are also retained during the whole time in which the 
backward displacement of the shell and the forward displacement of the pallial 
complex takes place. In other words, i.e. described in terms of our theory, from the 
very commencement of the development of the Gastropod organisation, the original 
left organs of the pallial complex were subjected to unfavourable conditions. In 
this left -sided compression of the mantle cavity the ctenidinm especially would 
necessarily be reduced in size and become rudimentary, and might entirely disappear. 

As a matter of fact, the original left half of the pallial complex (which would 
now lie on the right) has entirely disappeared in many Diotocardia (the so-called Azy- 
gohrancMa), in all Monotocardia, and in the Opisthobranchia. The fact that the 
original right gill, the only one remaining, has also disappeared in the Pulmonata 
is accounted for by the change to aerial respiration. It is an interesting fact that 
in the Basominatophora the original right osphradium is retained. 

If, however, the original left gill did not quite disappear, but only became 
smaller, we should have to expect that in such Diotocardia as still possess two gills, 
the original left (now the right) would be the smaller. This would be the case at 
least in the more primitive forms with shells still twisted. Saliotis and Fisstirella 
are the only Molluscs to which this applies. In Saliotis, whose shell is still 
twisted, the right (originally left) gill is in reality the smaller. But in Fissurella 
and Subemarcjinula, where the asymmetry of the mantle cavity has been secondarily 
lost, the inequality in the size of the gills has also disappeared. 

12. 

Another unsolved problem remains. Why does the shell continue to gi-ow 
asymmetrically coiled with a dextral twist, after the cause of this asymmetry, viz. 
the change from the incline to the left to the backward incline of the shell, simultane- 
ously with the shifting forward of the mantle cavity and pallial complex, has ceased 
to act, i.e. when the shell has definitely assumed the posterior, and the pallial 
complex the anterior, position ? The explanation of this lies in the asymmetry so 
early apparent in the mantle cavity, which from the beginning is more spacious 
to the right (now left) than to the left, the consequence being that the left half of 
the pallial complex atrophied. This asymmetry of the pallial complex and mantle 
cavity remained after the displacements of shell and pallial complex had been 
definitely accomplished in the Prosobranchia, i.e. the asymmetrical growth, and 
therefore the continuous coiling of the visceral dome and shell in a spiral twist, 
continued. 

In altogether exceptional conditions, which rendered a flat cup-shaped shell 



158 



COMPARATIVE ANATOMY 



CHAP. 



useful, the return to symmetry in the pallial complex and mantle eavity or fold 
would be advantageous, since then symmetrical growth of the shell could take 
place. If the differeuce between the maximum and minimum growth in height 
is but slight the shell would be but slightly coiled, and if the peripheral growth 
is pronounced, while the gi-owth in height is insignificant, a flat cup-shaped shell 
would result (Haliotis, Emargimtla, Fissurella, Fatclla, etc.). 



13. 

Chiastoneury only takes place when the original right half of the pallial complex 
crosses over to the left of the median line anteriorly. 

This crossing of the line of symmetry has actually taken place in the Froso- 
Iranckia. The original right gill in them lies quite to the left of the mantle cavity. 
In the Azygobrandiia and Monotocardia the hind-gut with the anus has at the same 
time become displaced into the right (original left) narrower gill-less half of the 
mantle cavity, which, however, is still spacious enough to contain the rectum. The 
Prosobranchia are streptoneurous. 

In the Tectibrandiia and Opisthobranchia the pallial complex is found on the right 
side of the body, and has nowhere crossed the median line anteriorly. There is 
therefore no chiastoneury among the Opisthobranchia, i.e. their visceral connectives 
are never crossed.^ 

In the Pulmonata the pallial complex has shifted far forward, but it has not 
passed the middle line with any organ which, drawing the parietal ganglion and the 
visceral connective with it, could have brought about chiastoneury. For the left 
(original right) gill, the only one elsewhere retained,'disappeared (apparently very early) 
in the Pulmonata. The osphradium, which is retained in aquatic Pulmonata, is the 
original right, and still lies on the right side. In considering the arrangement of 
the nervous system, it is really immaterial whether we assume that the hind-gut 

has shifted back to the right 
secondarily, and the osphra- 
dium moved to near the re- 
spiratory aperture, or that 
the hind-gut never reached 
the median line, and that 
the osphradium never passed 
over it. The Pulinmiata are 
thus euthyneurous. 

14. 

We saw, in paragraph 
3, that with a strongly de- 
veloped visceral dome and 
posteriorly p)laced pallial 
complex, a shell inclined 
forward or coiled forward is 
an impossibility for a rep- 
lant Gastropod. But such 
a shell is not an impossibility for an animal which does not creep. For example, 
in a swi mm ing animal, whose shell, partly filled with air, serves as a hydrostatic 
apparatus, there is no reason why a much developed visceral dome and shell should 

^ Except in Artwon, an exception which makes it probable that in the Ojnstho- 
branchia the pallial complex has secondarily returned from au anterior position. 




Fig. 137.— Nautilus, diagram, do, Dorsal ; re, ventral ; 
anterior ; hi, posterior. 



VII MOLLUSGA—THE ASYMMETRY OF THE GASTROPODA 159 

not become coiled forward, the original posterior position of the pallial complex 
being retained as the most favourable under sucli circumstances. As an example 
of this we have the Nautilus, all Nautiloidea and Ammonitidea, with their exogas- 
trically (anteriorly) coiled shells and posteriorly placed pallial complexes (Fig. 137). 
The coiling of the shell of Spirula forms an exception to that of all other Mol- 
lusca, being endogastric. With regard to this we have to consider first, that the 
shell of Spirula is internal and rudimentary, and that the backward coiling does not 
in any way affect the posteriorly placed mantle cavity ; and second, that only the 
modern genus Spirula has such a shell. The Miocene genus Spirulirostra has its 
phragmacone endogastrically bent but not coiled, and the older Belemnitida never 
have either curved or coiled shells. Moreover, the shell of this whole group, being 
internal and, as far as the original purpose of a shell, protection of the body, is con- 
cerned, rudimentary, does not come under consideration in the present discussion. 



15. 
In an animal living in mnd, like a limicolous bivalve, there appears no reason 

0»0 





Fig. 139.— Hypothetical transition 
form between DentaUum (Fig. iss) 
and tlie racial form of the Gastropoda 
(Fig. 140), from the left side. 




Fig. 138.— DentaUum, diagram from 
the left side, g. Genital gland ; Tct, 
cephalic tentacles. 



Fig. 140.— Hypothetical racial form 
of the Gastropoda, tram the left side. 



why the shell should not simply become elongated, and why the mantle cavity and 
pallial complex should not retain the posterior position. DeiUaUuin (Fig. 138) is 



160 COMPARATIVE ANATOMY chap. 

distinctly in this condition, being the symmetrical primitive Gastropod adapted to 
life in mud, and provided with a turret-like shell and posterior pallial complex. 
The perforation at the upper end of the shell, which freely projects from the 
mud, is of great morphological importance, corresponding physiologically with the 
siphons of the limicolous Lamellibranchia. A comparison between Dentalium and 
a FissurcUa with its pallial complex twisted back, and with a long and turret- 
like shell, is, from our point of view, very appropriate. A FissurcUa, so transformed, 
would almost exactly resemble the hypothetioal symmetrical racial form of the Gas- 
tropoda, in which, however, we should have to assume a mantle- and shell-cleft 
reaching to their edges (c/. Fig. 136, I). 

The anatomy of the Protolrandiia, which has recently been more closely studied, 
and especially the posterior position of the two gills, the fiat sole for creeping, and 
the presence of the pleural ganglia, justify us in deriving the Lamellibranchia also 
from the racial form of the Gastropoda, in which the cleft edge of the mantle would 
correspond with the posterior or siphonal edge of the mantle in the former. This 
edge of the mantle, having a similar physiological function, often possesses tentacles, 
papilla;, etc., in both groups. 

Dentalium further fits in with our theory, for the forward curve and the position 
of the columellar muscle on the anterior side of the visceral dome which would be 
disadvantageous to a freely reptant, is not so to a limicolous, animal. 

16. 
The Dextral and Sinistral Twists. 

Most Gastropods have the visceral dome and shell twisted dextrally. The direction 
of the twist has been determined by the fact that the visceral dome and shell origin- 
ally inclined to the left, and then more and more backward, thus pushing the 
pallial complex along the right mantle furrow. It cannot be determined why the 
incline to the left was originally chosen. The shell might just as well have inclined to 
the right at first, and then more and more backward, pushing the pallial complex along 
the left mantle furrow. The consequent asymmetry would then have been exactly 
reversed. To take a concrete example : in a Monotocnrdian, with visceral dome and 
shell twisted sinistrally, the original left parietal ganglion would become the supra- 
intestinal ganglion on the right. The original right half of the pallial complex 
would disappear, and the left half wdiich persisted would lie to the right of the anus 
or rectum, which would take up its position to the left of the median line. 

Gastropoda with sinistrally twisted shells are actually known, many of them 
havipg the asymmetrical organs in the inverse position which corresponds with this 
twist. ' Such are, among the Prosobranchia, Neptmiea contraria, Triforis, and occa- 
sional specimens of Bucciimm; among the PvJmonaia, Physa, Glausilia, Helicter, 
Ampliidromus, and occasional specimens o( Helix and Limmaca. In Bulimns ^jcr- 
i-ersus, individual specimens with either sort of shell are found, with the special 
asymmetry of the organs belonging to it. 

17. 
There are, however, snails whose shells are dextrally twisted, but which possess 
the organisation of animals with sinistrally twisted shells. This is the case among 
the Prosobranchia in the sinistrally twisted sub-genus Lanistcs of the genus Ampul- 
laria ; among the Pulmunata, in Choaitomjihalus MaacH and Pompholyx solida ; 
among the Opisthobrancliia, in those Pteropoda which, whether as adults {Lima- 
cinidce) or larva; {Oymbuliidcc), have a twisted shell. This fact is entirely against 
our theory in explanation of the asymmetry of the Gastropoda, for this theory 



VII MOLLUSGA—THE ASYMMETRY OF THE GASTROPODA 161 



points to a causal connection between the spiral coiling of the visceral dome and 
shell on the one hand and the special asymmetry of the asymmetrical organs on the 
other. The above-mentioned exceptions to the rule can, however, be explained as 
follows. The spiral of a dextrally twisted shell can by degrees become flattened in 
such a way that the shell may be simply coiled in one plane or may nearly approach 
that condition. In this case the spiral might again assert itself, but on the side 



B 



D 




Fig. 141.— Seven forms of Ampullaria sliells (diminished in various degrees), seen in the upper 
row from the aperture of the shell, in the lower from the dorsal side. The head, foot, and oper- 
culum are arbitrarily drawn merely for the purpose of facilitating a comparison between dextrally 
and sinistrally twisted shells. 

opposite to that on which the umbilicus originally lay, and in this way a false 
spiral might form on the umbilical side and a false umbilicus on the spiral side. 

The transition from a dextrally twisted to a falsely sinistrally twisted shell, which 
latter was, however, genetically dextrally twisted, is illustrated in Fig. 141 by 
means of the shells of seven species of the genus Ampullaria. Ampullaria Swain- 
soni Ph ? (G) and A. Geveana Sam (F) are dextrally twisted with distinctly project- 
ing spiral. In A. crocastmna Ph (E) the spiral is flat, in A. (Oeratodes) rotula 
Mss. (D) and A. (Oeratodes) chiquiteusis d'Orb (C) the spiral is already pushed 
through or sunk, yet we iind a true umbilicus 
on the umbilical side. In A. (Zanistes) Bol- 
teniana Ghemn. (B), and still more in A. 
purpurea Jon. (A), the false spiral appears on 
the umbilical side, and on the spiral side a false 
umbilicus is found. 

However plausible this explanation may 
appear, it can only be proved to be correct if 
it is found that where a spiral operculum 
occurs, the direction of its spiral is opposite to 
that of the spiral of the shell (Fig. 142, A, B, 
C), and the commencement of the spiral is 
always turned to the umbilical side of the shell, 
operculum, but such occur in the Pteropoda. 
VOL. II 




Pig. 142. 

Zanistes has not a spirally twisted 
In those Pteropods which combine a 
M 



162 COMPARATIVE ANATOMY chap. 

sinistrally twisted shell with the organisation belonging to a dextrally twisted 
Gastropod, the operculum exactly corresponds with that of a dextrally twisted shell. 
In Peradis, in the larvse of the Cyvibuliidce and in Linmcina retroversa Flemming, 
the operculum (the fi-ee surface of which must be Tiewed) is sinistrally twisted, and 
the starting-point of the twist faces the (false) spiral, which in these falsely sinistrally 
twisted Gastropods lies in the place of the original umbilicus. 

This apparent exception is thus shown to be quite in keeping with the rule above 
established. 



XV. The Sensory Organs. 
A. Integumental Sensory Organs. 

In the integument of the Mollusca there are epithelial sensory cells 
(Flemming''s eells), which vary in number and arrangement, and 
may be scattered over large areas. Two kinds of these cells may be 
distinguished according to their form. One kind, which is found only 
in Lamellibranchs, consists of large epithelial cells with large terminal 
plates which form part of the body surface and carry tufts of pro- 
jecting sensory hairs (" paint-brush cells," Pinsel-Zellen). The second 
kind of cells are found in all classes of Mollusca. They are long, 
filiform, or spindle-shaped, swelling at one point where the nucleus 
lies. They sometimes carry a tuft of sensory hairs, sometimes none. 
Each kind of cell is continued at its base into a nerve fibre, which 
runs into the nervous system. A distinct specific function can hardly 
be attributed to these epithelial cells. They may respond to very 
various stimuli, chiefly mechanical and chemical, and thus may act in 
an indefinite way as tactile, olfactory, and gustatory cells. 

They may become more specialised in function, when crowded 
together in certain areas of the body, and may then represent special 
sensory organs. Between the individual cells composing such a 
sensory organ, however, other epithelial cells (glandular, ciliated, 
and supporting cells) are always found. 

1. Tactile Organs. 

The tactile function of the integumental sensory cells is likely to 
assert itself at exposed parts of the body surface, such as the ten- 
tacles, epipodial processes, siphons, at the edge of the mantle in the 
Lmnellibranchia, and at the edge of the foot, etc. We cannot, how- 
ever, assume that even in these places the sensory cells are sensitive 
only to mechanical stimuli. 

2. Olfactory Organs. 

{a) The Osphradium. 

"As has been proved to be the case in the Frosobranchia, sensory cells 
occur scattered among the other epithelial cells throughout the whole 



VII MOLLVSGA—THE SENSORY ORGANS 163 

epithelial lining of the mantle cavity. Here, as in other parts of the 
body, three kinds of epithelial cells can be distinguished : (1) undiiTer- 
entiated cells, which may contain pigment, and are usually ciliated ; 
(2) glandular cells ; (3) sensory cells. The proportions in which 
these three kinds of cells appear varies in different regions of the 
mantle. If glandular cells prevail on a certain area, that area assumes 
a glandular character, and may even develop into a sharply localised 
epithelial gland {e.g. the hypobrauchial gland). On the gills, undiffer- 
entiated ciliated cells predominate. Where sensory cells predominate 
a sensory character is given to the region ; such a region, if sharply 
circumscribed, the sensory cells continually increasing in number, 
becomes a pallial sensory organ. The gradual development and con- 
tinuous differentiation of such an organ may be particularly well 
traced in the Prosohranchia, the sensory organ developed being the 
osphradium. In consequence of its position in the mantle cavity, and 
especially on account of its proximity to the gillj it has been assumed 
that its principal function is ! to test the condition of the respiratory 
water, or, in other words, that it is an olfactory organ. 

The osphradium among the Prosohranchia is least differentiated in the Dioto- 
aardia. In the Fissurellidcc it does not exist as a sharply localised organ. In the 
Monotocardia it becomes more and more differentiated, and has a special ganglion, 
and finally in the Toxiglossa, it reaches the maximum of its development. 

A review of the position and number of the osphradia has already been given in 
another place (§ T. p. 71). As an example of the special form and structure of this 
organ we select the highly developed osphradium of a Toxiglossa, Cassidaria 
tyrrhena. 

The osphradium of Cassidaria is a long organ, pointed at both ends, which lies 
to the left of the ctenidium on the mantle in the mantle cavity. As in other highly 
specialised Monotocardia (Fig. 71, p. 73) it lookslike a gUl feathered on both sides, and 
has on that account been regarded and described as an accessory gill. It consists of 
a ridge rising from the mantle, which in transverse section is almost square, and 
carries on each side 125 to 150 flat leaflets, which stand at right angles to the 
surface of the mantle, and are so closely crowded that their surfaces are in contact. 
The ridge consists almost exclusively of the long osphradial ganglion. Each leaflet 
receives from this ganglion a special nerve, which runs along its lower projecting 
edge, and sends off four princiijal branches into it. In its dorsal pallial side each 
leaflet contains blood sinuses, which communicate with a sinus lying above the 
ganglion in the ridge. 

These principal nerves in the leaflets branch, and their last and finest ramifica- 
tions penetrate the supporting membrane between the epithelium and the sub- 
epithelial tissues. These become connected with the branches of the interepithelial 
ganglion cells, each of which again is connected mth a spindle-shaped epithelial 
sensory cell. The branched interepithelial cells are connected together by their 
processes. 

The sensory epithelium above described is developed on the lower surfaces of the 
osphradial leaflets, i.e. those turned to the mantle cavity, the indifferent, non-ciliated 
cells on these surfaces being filled with granules of yellow pigment, while in the upper 
surfaces of the leaflets these cells are devoid of pigment and ciliated. Glandular 
cells are also found definitely arranged in the epithelium of the osphradial leaflets. 



164 COMPARATIVE ANATOMY chap. 

The osphradial nerve usually springs from the pleuro-viseeral connective (from 
the parietal ganglion when this is present) ; in the Lavwllibra'iichia it comes from 
the parieto-visceral ganglion. The osphradial nerve is generally a lateral branch of 
the branchial nerve. 

In the Lamellibranchia, the important fact has been demonstrated that, although 
the osphradial nerve comes from the parieto-visceral ganglion, its fibres do not 
actually rise from this ganglion ; but they pass along the pleuro-viseeral connective 
and have their roots in the cerebral ganglion. 

{b) Olfactory Tentacles. 

Certain experiments, to which, however, some exception might be 
taken, seem to show that the large optic tentacles of terrestrial 
I'nlmonahi are also olfactory. It is also generally accepted, though 
still not certainly established, that the posterior or dorsal tentacles 
(rhinophores) of the Opisthobranchia are olfactory organs. These 
rhinophores (Fig. 93, p. 98) often show increase of surface, usually 
in the shape of more or less numerous circular lamellae surrounding 
the tentacle like a collar. The rhinophores are also often ear-shaped 
or rolled up conically. Not infrequently they can be retracted into 
special pits or sheaths. They are innervated from the cerebral 
ganglion by means of a nerve which forms a ganglion at the base of 
each. 

At the lateral and lower edge of the cephalic disc of the Cephalaspidcc, an organ 
which is considered to have arisen by the fusion of the labial and cephalic tentacles, 
there are structures which are thought to be olfactory, and which, where most 
developed, take the form of several parallel " olfactory lamellse " standing up on 
the disc. 

(c) Olfactory Pits of the Cephalopoda. 

In the JDibranchia there is on each side, above the eye, a pit which 
is considered to be olfactory. Its epithelial base consists of ciliated 
and sensory cells, and underneath it lies, close to the optic ganglion, 
an olfactory ganglion. The nerves running .to this ganglion come 
from the ganglion opticum, but really originate in the cerebral 
ganglion. It looks as if these olfactory organs were the remains of 
the posterior tentacles of the Gastropoda, and were comparable with 
the rhinophores of the Opisthobranchia. In Nautilus the place of the 
olfactory pit is occupied by the upper optic tentacle. We have 
already seen that Ncmtilus still retains true osphradia. 

{d) The Pallial Sensopy Organs of the Lamellibranchia. 

Several Jsiphoniida have, in addition to the osphradia, epithelial 
sensory organs, which lie on small folds or papillae to the right 
and left of the anus, between it and the posterior end of the "-ill. 
These are innervated by a branch of the posterior pallial nerve. 

Epithelial sensory organs of various forms (plates of sensory epithelium, sensory 
lamella", or papillce, tufts of small tentacles) are found on the mantle in the 



VII MOLLUSGA—THE SENSORY ORGANS 165 

Siphoniata ; these lie on the retractor muscles of the siphons and at the base of the 
branchial siphon. These pallial sensory organs also are innervated by the posterior 
pallial nerves, and may correspond with the anal sensory organs of the Asiphonia/a. 
Their function is unknown, but is supposed to be analogous to that of tlio 
osphradia. 

(e) Olfactory Organs of the Chitonidse. 

In the mantle furrow of the Chitonidce there are epithelial sensory 
organs which are considered to be olfactory. These are ridges and 
prominences with extraordinarily high epithelium, consisting of 
glandular cells and thread-like sensory cells. In Chiton Icevis and 
C. cajetanus there are, on each side of the mantle furrow, two sensory 
ridges extending along the whole length of the row of gills ; one of 
these, the parietal ridge, belongs to the outer wall of the furrow, 
while the paraneural ridge runs along the base of the furrow, above 
the bases of the gills and under the pleuro-visceral cord. The para- 
neural ridge is continued a short distance along the inner surface of 
each gill, so that each gill has an epibranchial sensory prominence. In 
front of the first pair of gills and near the last the sensory cells in the 
paraneural ridge become far more numerous in comparison with the 
glandular cells. Chiton siculus, C. Polii, and Acanthochiton (in which 
the numerous gills reach far forward) have no parietal and paraneural 
ridges. The sensory epithelium in these animals is confined to two 
prominences, paraneural in position, behind the last pair of gills, 
and connected with a high epithelium covering the pallial wall of 
the most posterior part of the furrow. 

All these sensory epithelia seem to be innervated from the 
pleuro-visceral cords. 

The question as to the relation of tliese sensory epithelia in the Ohitonidce to the 
osphradia of other Molluscs, which here presents itself, is difficult to answer. In 
position the osphradia best con'espond with the epibranchial prolongations of the 
paraneural ridges in Chiton Icevis and C. cajetanus. 

3. The " Lateral Organs " of the Diotoeardia. 

At the bases of the epipodial tentacles of Fissurella and the 
Trochidce, and at the base of the lower tentacles of the epipodial ruff 
of Haliotis, and also in other parts near the ruff, sensory organs are 
found which have been compared with the lateral organs of Annelids. 
They consist of patches of sensory epithelium, which may form 
either spherical projections or pit-like depressions. The epithelium 
of these sensory organs which lie at the lower side of the bases of the 
epipodial tentacles, consists of sensory cells, each of which is provided 
with a sensory seta, and pigmented supporting cells. Each of these 
sensory organs is innervated by the nerve of the tentacle near it, 
which nerve originates in the pedal cord and forms a ganglion in the 
base of each epipodial tentacle. 



166 COMPARATIVE ANATOMY chap. 



4. Gustatory Organs. 

Folds and prominences found in the mouth in some divisions of 
the Mollusca have been taken for gustatory organs, although there are 
no physiological and hardly any histological grounds for this opinion. 
The existence of so-called gustatory pits on a prominence in the buccal 
cavity has been proved only in a few Chitonidce and Diotocardia (Haliotis, 
Fissurdla, Trochus, Turho, and Patella). This " gustatory prominence " 
(which has been best examined in Chiton) lies on the floor of the 
buccal cavitj', close behind the lip. A few gustatory pits are found 
in its epithelium, sunk somewhat below the surrounding epithelium. 
They consist of sensory cells with freely projecting sensory cones, and 
of supporting cells. 

On each side of the mouth in the Pulmonata lies an oral lobe, 
and under its deep epithelium, which is covered by a thick cuticle, 
lies a ganglion. Smaller ganglia are found in the small lobes at 
the upper edge of the mouth. All these ganglia receive nerves which 
radiate from a branch of the anterior tentacle nerve. These oral 
lobes (Semper's organ) are considered to be gustatory organs. 

5. Subradular Sensory Organ of Chiton. 

In the buccal cavity of Chiton a subradular organ of unknown 
physiological significance has been found. It is described as " a pro- 
minence lying below and in front of the radula," and in shape re- 
sembles two beans with their concave edges turned to one another, 
the ends touching ; the space between them forms a channel into 
which a small gland opens. Below this organ lie two ganglia, the 
subradular or lingual ganglia (c/. section on the nervous system). The 
epithelium of the subradular organ consists of green pigmented 
ciliated cells and two kinds of sensory cells. A similar organ occurs 
in Patella, but has not been thoroughly examined, and at the same 
part in various Diotocardia there is a prominence, which, however, has 
no sensory cells. The Scaphopoda also possess a subradular organ. 

6. The Sensory Organs on the Shell of Chiton. 

There are numerous organs definitely arranged on the shell of 
the Chitonidce which have, no doubt correctlj-, been considered as 
sensory, i.e. tactile organs (Fig. 143). They are called sesthetes, and 
lie in pores on the tegmentum (r/. p. 39) ; they are club-shaped or cylin- 
drical, and each carries a deep cup-like chitinous cap. Each megal- 
sesthete gives oflf all round numerous fine branches or mieraesthetes, 
each of which ends in a swelling which carries a small chitinous 
cap. The body of the aesthetes consists principally of long cells like 
glandular cells ; it is produced into a fibre which runs along the 
base of the tegmentum, and from here passes together with the 



VII 



MOLLUSGA—TEE SENSORY ORGANS 



167 



fibres of the other aesthetes of the shell-plate, between the tegmentum 
and articulamentum to the surrounding pallial tissue, or else pene- 
trates the articulamentum. 

The significance of the separate constituent parts of the aesthetes and their 
fibrous strands is not yet certainly known. It is probable that they are innervated 
from the dorsal lateral branches of the pleuro-visceral cords. It is even not known 
whether the fibroxis strands are their nerves, or whether the clear fibres running 
through them are long sensory cells whose nuclei may lie between the glandular 
cells, and in connection with nerve fibres. 

We are perhaps justified in assuming that the aesthetes are merely modifications 







Fig. 143.— Section of tlie tegmentum of OMton Isevis showing an assthete (after Blumrioh). 
mk, Micrsestliete ; per, periostracum ; sk, principal aesthete ; t, tegmentum ; dz, cpIIs resembling 
glandular cells ; Jif, clear fibres ; fs, fibrous strand ; c, chitinous cap. 

of the spines with their papilla and formative cells, which are so common in the 
integument of the Chitmiidce. The chitinous cap would then represent pai't of the 
chitinogenous base of tlie spine. 

The sensory nature of the aesthetes is rendered highly probable 
by the circumstance that in a few species of Chiton individual megal- 
jesthetes are transformed into eyes. 

Each eye is furnished with a pigmented envelope, which is pene- 
trated by the micrsesthetes, and outwardly covered by an arched 
layer of the tegmentum which forms the cornea. Under this is a 
lens, and under this again a cell layer, which is regarded as a retina, 
and to which is attached a fibrous strand (optic nerve ?) corresponding 
with the fibrous strands of the ordinary aesthetes. 



B. Auditory Organs. 

All Mollusca except the Amphineura possess auditory organs, which 
appear very rarely in the embryo. They take the form of two almost 



168 



COMPARATIVE ANATOMY 



CHAP. 



closed auditory vesicles (otoeysts), whose ejjithelial walls usually con- 
sist of ciliated and sensory cells. The interior of the otocyst is filled 
with fluid and contains a varying number of otoliths (1 to over 100). 
These vary in size, form, and chemical constitution, and in the living 
animal oscillate in the fluid in wliich they are suspended. 

The otoeysts are usually found on or near the pedal ganglia, rarely 
far from it. It is, however, well established that the auditory nerve 
does not originate in this ganglion but in the cerebral ganglion, though 
it often runs along close to and even in contact with the fibres of 
the cerebropedal connective. 

In most cases the otoeysts arise as invaginations of the outer 
epithelium. An interesting discovery has recently been made, that in 

primitive Lamellibranchs 
/^,\ . {Nucula, Leda, Yoldia) each 

of the otoeysts even in the 
adult still opens by means of 
a long canal on the surface 
of the foot. In such cases 
the otoliths are particles of 
sand or other foreign matter 
taken in from outside. In 
Cephalopods, the remains of 
the canal of invagination is 
retained (Kolliker's eanal), 
but it ends blindly. 

The auditory organs are 
most highly developed in 
those Molluscs which are 
good swimmers, especially in 
the Cephalopoda and Hetero- 
poda. Among these, maculae 
and cpistae aeustiese are 
developed. 

Heteropoda. — The struc- 
ture of the auditory organ 
of Pterotrachea (Fig. 144), 
which has been thoroughly 
examined, is as follows : — 
The wall of the otocyst consists in the first place of a structure- 
less membrane surrounded by muscle and coimective tissue. Inside 
the vesicle, which is filled with fluid, a calcareous otolith, built up of 
concentric layers, is suspended. The inner surface of the vesicle is 
lined by an epithelium, containing three different sorts of cells : 
auditory, ciliated, and supporting cells. The auditory cells, which 
carry immobile sensory hairs, are found on the wall of the otocyst at 
a point (macula acustica) diametrically opposite to the place where the 
auditory nerve enters. At this spot there is a patch formed of 




- -5 



& 

Fig. 144. — Auditory organ ol Pterotrachea (after 
Glaus). 1, Auditory ne^^■e ; 2, structureless membrane ; 
3 and 4, ciliated cells ; 5, otolith ; 6, auditory cells ; 7, 
supporting or isolating cells ; S, large central auditory 
celN. 



VII 



MOLLUSGA—THE SENSORY ORGANS 



169 



numerous auditory colls, and in their midst, separated from the rest by 
four supporting or isolating cells, one large central auditory cell. 
On the larger remaining surface of the wall of the otocyst, separated 
by undifferentiated cells, are found flatter ciliated cells, which carry 
very long cilia or setae, exhibiting peculiar movements. They some- 
times lie flat along the inner wall of the vesicle, and at other times (it 
is said in response to strong auditory stimuli) stand upright, projecting 
towards the centre of the vesicle, and supporting the otolith. 

The auditory nerve, which enters the otocyst at a point exactly 
opposite the central cell, at once radiates in the form of fibres over the 
whole wall of the vesicle " as meridians radiate from the pole on a 
globe," finally innervating the bases of the auditory cells. 

The two otocysts of the Cephalopoda are still more complicated ; 
they lie in two spacious cavities of the cephalic cartilage. The sensory 
epithelium is here found on a macula acustica and on a kind of ridge, 
the crista acustica, which projects inwards. Otoliths are only found 
on the macula acustica. The auditory nerve divides into two branches, 
one going to the macula, and the other to the crista acustica. Kolliker's 
canal, above mentioned, which is internally ciliated and ends blindly, 
runs out of the otocyst as the remains of the aperture of the original 
invagination. 

Experiments made on C'ephalopods have shown that one of the 
functions of the otocysts is to regulate the position of the animal 
while swimming. 



C. Visual Organs. 



They are cup-shaped 



1 - 



1. Optic Pits. 

These are the simplest form of visual organ 
depressions of the body 
epithelium, which at the 
base of the cup forms the 
retina. The depression is 
sometimes very shallow, at 
other times deep, and like 
a wide bottle with a short 
narrow neck. The optic 
nerve enters at the base of 
the depression and spreads 
out over it. The epithelial 
wall or retina consists, ap- 
parently in all Gastropoda, 
of two kinds of long thread- 
like cells : (1) clear cells 
without pigment, and (2) 
pigmented cells. Whether 
either or possibly both of these kinds can be considered as retinal cells 




Pig. U5.- 
eavity (pit) ; 
visual cells ; 
optic nerve. 



Eye of Nautilus (after Hensen). 1, Optic 
2, layers of rods ; 3, pigment layer ; 4, layer of 
5, layer of ganglion cells ; 6, branches of the 



no 



COMPARATIVE ANATOMY 



CHAP. 



is still a disputed question. In certain cases it has been proved that 
the pigment in the second kind lies peripherally ; the axis is free 
from pigment, and may perhaps be considered as the sensitive portion 
of the cell. In this case, the clear cells would be undifferentiated 
supporting cells, or secreting cells. The retina is covered, on that 
side of it vrhich faces the cavity, by a thick gelatinous cuticle, or the 
whole cavity is filled by a gelatinous body often called a lens. The 
clear or secreting cells have been thought to yield this gelatinous mass, 
but there is a tendency to regard them now rather as retinal cells. 

Optic pits are, among the Gastropoda, only found in such Diotocardia as show 
primitive characteristics, r. g. Haliotiche, PatelUdce, Trochiihc, Delphimdidm, and Stoina- 
illdrc. 

In connection witli the claim that A'avtibis (Fig. 145) is the most primitive form 
among extant Cephalopoda, it is interesting to find that both its eyes are optic pits. 
Each sensory cell of the retina, i.e. of the epithelial wall of the depression, possesses 
a cuticular rod projecting towards the cavity, and a layer of ganglion cells is 
intercalated between the ramifications of the optic nei've and the retina. 



2. Optie Vesicles or Vesicular Eyes. 

Optic vesicles are developed from optic pits both ontogenetically 
and phylogenetically by the approximation of the edges of the pit, 

which finally fuse. A vesicle is thus 
formed, over which there is a continuous 
layer of epithelium (Fig. 146). The 
outer epithelium is free from pigment 
over the eye, and is called the outer 
cornea, while the immediately subjacent, 
and also unpigmented, epithelial wall of 
the vesicle forms the inner cornea. The 
epithelial base of the original depression 
here again forms the retina ; its cells 
contain distinct rods projecting towards 
the cavity of the vesicle, which is filled 
with a gelatinous inass. The optic 
nerve usually swells into a peripheral 
ganglion opticum before reaching the 
retina. 

The tentacular eyes of most Gastro- 
except those Diotocardia which 




Fio. 140.— Eye of a Pulmonate. 1, 
Outer, 2, inner cornea ; 3, body epithe- 'pOCm, 



Hum ; 

ganglion opticum ; 7, optic nerve. 



vitreous body; 5, retina; (i, have cup-like eyes, are of this simple 
character. 



3. The Eye of the Dibranchiate Cephalopoda. 

This is one of the most highly-developed eyes in the whole animal 
kingdom. It is a further development of the cup-shaped and vesicular 



VII 



MOLLUSGA—THE SENSOSY ORGANS 



171 



eyes. In the Tetrabranchiate Nautilus, as we have seen, the cup-shaped 
eye persists throughout life. 

These lower stages (i.e. the cup-shaped and vesicular stages) of the 
eye are passed through ontogenetically. First a cup-like depression is 
formed (primary optic pit), then this becomes constricted to form a 
vesicle (primary optic vesicle), the inner wall of which becomes the 
retina, while the outer (which corresponds with the inner cornea of 
the vesicular eye) becomes the inner corpus epitheliale. This em- 
bryonic optic vesicle then becomes further complicated ; the integu- 
ment over it (the outer cornea of the vesicular eye) rises in the form 




Fig. 147.— Development of the eye of tlie dlbrancMate Cephalopoda. 1, Body epithelium, 
which becomes the outer corpus epitheliale ; 2, inner wall of the optic depression, which becomes 
the retina ; 3, outer wall of the optic vesicle, which becomes the inner corpus epitheliale ; 4, fold 
which forms the iris ; 5, fold which forms the secondary cornea ; 6, portion of the lens formed by 
the outer corpus epitheliale ; 7, portion of the hame formed by the inner corpus epitheliale ; S, rod 
layer of the retina. 

of a circular rampart, and then grows forward towards the axis of 
the eye like a diaphragm, which forms the iris, the aperture left in 
the same being the pupil. The integument which spreads out over 
the circular base of the iris is in close contact with the inner corpus 
epitheliale, and becomes the outer corpus epitheliale. 

The inner corpus epitheliale forms towards the cavity of the primary 
vesicle an almost hemispherical lens, the outer corpus epitheliale form- 
ing a similar lens outwards towards the pupil. The two hemispheres 
lie in such a way as to form something like a complete sphere ; its 



172 



COMPARATIVE ANATOMY 



CHAP. 



two-fold origin, however, always remains evident, its equatorial plane 
being traversed by the double lamella of the corpus epitheliale. 

A new circular fold grows over the eye, forming a fresh cavity 
over it ; this is the secondary eornea of the dibranchiate eye, which 
must not be confounded with the primary cornea of the optic vesicle 
here represented by the corpus epitheliale. In most forms the circular 
fold (cornea) does not altogether close over the eye ; an aperture 
remains through which the water can enter the anterior chamber of 



H-~l 




--il 



11 



Fig. 148.— Section of tlie eye of Sepia ofificinalis, somewliat diagrammatic (after Hensen). 
1 - S, As in Fig. 147 ; 1+3, corpus epitlieliale ; 9, anterior chamber of tlie eye opening outward at 
10; 11, cartilaginous capsule; 12, ganglion opticum = retinal ganglion; 13, nervus opticus; 2a, 
pigment layer of tlie retina. 

the eye. In some animals, however, the secondary cornea closes com- 
pletely. 

We thus obtain, ontogenetically, some idea of the general structure 
of the dibranchiate eye. A few details of the structure of the adult 
eye are given below (Figs. 148 and 149). 

1. The retina (Fig. 149) consists of two kinds of cells — (1) pig- 
mented visual or rod cells, and (2) limiting cells. Since the nuclei 
of the visual cells form, with relation to the centre of the vesicle, an 
outer, and the nuclei of the limiting cells an inner layer, and since, 
between these two layers, a limiting membrane traverses the inter- 
stices between the retinal cells, the retina appears to be laminated, 
whereas it in reality consists of one layer of cells. The rods of the 



VII 



MOLLUSOA—THE SENSORY ORGANS 



173 



retinal cells lie on the inner side of the limiting membrane, and are 

thus turned to the source of light _ajid at the same 

time to the cavity of the primary vesicle. The '■r-:':r:\^-^k:w.fl. t 
retina is covered on its inner side by a somewhat y:f,^^:fvgX-f:'-^!ry-- 
thick membrana limitans. 

2. The eye is surrounded, except on the side 
turned to the surface of the body, by a eartila- 
ginous capsule, which resembles the sclerotica in 
the vertebrate eye ; this cartilage, where it covers 
the retina, is perforated like a sieve, so that the 
optic nerves can pass through it. 

3. Immediately underneath the cartilaginous 
floor of the retina lies a very large ganglion opticum, 
in the form of a massive cerebral lobe. From this 
rise the nerves which run to the retina through the 
perforations of the cartilaginous capsule. 

4. The two halves of the lens, which are unequal 
in size (the outer being the smaller), consist of 
homogeneous concentric laminae. 

5. The cavity of the primary vesicle (between 
the retina and the lens) is filled with perfectly 
transparent fluid. 

It has been proved that, as in the Arthropoda 
and Vertebrata, the pigment granules of the rod 
cells, which in the dark lie at the base of the cell, 
under the influence of light travel towards its free 
end. 




i. 



in 



and the Eyes 
Pecten (Fig. 



Fig. 149.— Two retinal 
cells of a Ceplialopod, 
much magnified (after 
Grenadier). 1, Meui- 
lirana limitans ; 2, piii- 
ment ; 3, secreted 
tlire.ads ; 4, nerve fibre ; 
5, rod ; 6, pigment ; 7, 
limiting cell ; 8, limiting 
menjbrane ; 9, retinal 
cell ; 10, nerve fibre. 



The Dorsal Eyes of Oneidium 
at the edge of the Mantle 
150) and Spondylus. 

These eyes have been said to resemble vertebrate 
eyes in structure, because in them the visual rods 
are turned away from the light, being directed 
inwards towards the body. 

They are vesicular eyes, but in them it is the 
outer wall of the vesicle, that turned to the light, which becomes the 
retina, while the inner wall (which in other Molluscs forms the retina) 
is a pigmented epithelium. At the same time the outer or retinal wall 
is invaginated towards the inner pigmented wall, as is the endoderm 
towards the ectoderm in the formation of the gastrula. The conse- 
quence of this is, that the cavity which in other Mollusca is filled by the 
gelatinous mass (lens) disappears, and the vesicle becomes a flattened 
thick-walled plate (Peden) or cup {Oneidium), consisting of a pigment 
layer and a retina. The body epithelium which passes over the eye is 
unpigmented and transparent, and here becomes the cornea. Beneath 



174 



COMPARATIVE ANATOMY 



CHAP. 



the cornea, within the optic cup or on the plate, lies a cellular lens, 
which in the dorsal eyes of Oncidium consists of a few (5) large cells, 
but in the pallial eyes of Peden and Spowhjlus of very numerous cells. 




Fig. 150.— Section tlirougli tlie eye of Pecten (after Patten), c, Cornea ; I, lens ; ep, pig- 
mented body epithelium ; £/, layer of ganglion cells ; r, retina ; st, rod layer of the retina ; d, tap- 
etuni ; e, pigmented epithelium ; /, sclerotica ; n, optic nerve ; ni and ii2> its two branches. 

The development of this lens is unknown ; it is perhaps formed by a 
thickening or invagination of the embryonic ectoderm which covers 
the eye. 

In Oncidium, the optic nerve penetrates the Avail of the optic cup, as in the 
vertebrate eye, to spread out on the inner surface (with regard to the centre of the 
vesicle) of the retina, and to innervate the retinal cells. 



VII 



M0LLU8GA—THE SENSORY ORGANS 



175 



In Pectcn, the optic nerve which runs to each eye from the nerve for the pallial 
edge, divides, close to the eye, into two branches. One of these runs to tlie base of 
the optic plate, and there breaks up into fibres, which radiate on all sides to the edge 
of the plate, then bend over towards the retina to innervate some of its cells. The 
other branch runs direct to the edge of the plate, there bends round at a right angle 
and supplies nerves to the rest of the nerve cells. The fibres of this branch are not, 
however, directly connected with the retinal or rod cells, as there is a layer of anas- 
tomosing ganglion cells interposed between the two. Between the pigmented epi- 
thelium and the rod layer of the retina, a tapetum lucidum is found, which gives the 
eye of the Pecten its metallic lustre. 

Dorsal eyes are found in many species of Oncidium. They lie at the tips of the 
contractUe papillse found on the dorsal integiiment of this curious Puhiwiiate ; on each 
papilla three or four such eyes ocoui'. Besides these, Oncidium has the two normal 
cephalic eyes usually found in Gastropods. 

The pallial eyes of the Lamelliiranchiates, Pecten and Spondylus, are' found in 
large numbers on the edge of the mantle, between the longer tentacles, and on the 
tips of shorter tentacles. The rods of the retina in Pecten, when fresh, are of a very 
evanescent red colour (visual purple ?). 



5. The Eyes on the Shell of Chiton. 

These have already been described (p. 167). Their morphological significance 
cannot be determined as long as their development is unknown and their histological 
structure imperfectly investigated. 



6. The Compound Eyes of Area (Fig. 151) and Peetunculus. 

These are found in great numbers at the edge of the mantle, and 
are epithelial organs which do not in any way agree in structure with 
the other visual organs found in 
Mollusca, but rather resemble 
certain simple Arthropodan 
eyes. 

In form they resemble an 
externally convex shell. The 
unilaminar epithelial wall of 
the shell passes, at its edge, 
into the surrounding pallial 
epithelium. In section, its com- 
ponent elements appear to be 
arranged like a fan ("Facher- 

auge "). These elements are of ^^^ isi.-Seotlon of the eye of Area tarbata 
three kinds: (1) conical visual (adapted from Rawltz). l. Retinal ceUs with rod-like 
cells, with their bases turned todies (2); S, pigment cells; 4, slender interstitial 

outwards ; (2) a sheath of six 

cylindrical pigment shells surrounding each visual cell. Each group, 
consisting of one visual cell and its surrounding pigment cells, may be 
considered as a single eye or ommatidium of the simplest structure, in 
which the retinula is represented by one single visual cell. (3) Slender, 
almost thread-like interstitial cells which stand between the ommatidia. 




176 COMPARATIVE ANATOMY chap. 



7. Degeneration of the Cephalic Eyes. 

It is becoming nioie and more probable tliat the cephalic eyes of the various 
Mollusca are homologous structures, and that they primitively occurred in all forms. 
They may, however, under certain biological conditions become rudimentary, and 
even disappear, as in boring animals and those living in mud or in the deep sea 
and in parasitic Molluscs. The LamcUibranchia and Ohitonida' {'!) even have 
cephalic eyes appearing temporarily during development ; they disappear later, 
when, covered by the shell, the}' are useless. They may be replaced by secondarily 
acquired visual organs arising at more suitable parts of the body, and thus we 
have eyes on the mantle edge in some bivalves and on the shell of some Chitonidce. 



XVI. The Alimentary Canal. 

The alimentary canal is well developed in all Molluscs, and is 
composed of (1) the buccal cavity ; (2) the pharynx or oesophageal 
bulb; (3) the oesophagus or fore -gut; (4) the mid-gut with the 
stomach ; (5) the rectum or hind-gut with the anal aperture. The 

mouth originally lies at the anterior, and the anus at the posterior 
end or side of the body, the latter in the mantle furrow or cavity. 
The former always retains its original position, but the latter, as 
central organ in the paliial complex, becomes shifted more or less far 
forward along the right (less frequently the left) side, in the mantle 
furrow. 

When the visceral dome grows out dorsally in such a way that 
the longitudinal axis becomes shorter than the dorso-ventral axis, as 
is the case in many Gastropods and Cephalopods and in Dentalivm, the 
mid-gut at least, with its accessory gland, the so-called liver, runs up 
into this dome, filling the greater part of it. The intestine then forms 
a dorsal loop, consisting of an ascending portion running up from the 
fore-gut and a descending portion running down to the anus. In the 
Gastropoda, where the anus is shifted more or less forward, the 
descending portion bends forward to the right (rarely to the left) to 
reach it. 

Besides this principal visceral loop, which is caused by the 
development of the visceral dome and modified by the displacement of 
the paliial complex, the intestine, in nearly all Molluscs, forms secondary 
loops or coils which add to its length. These loops are found 
principally in the tubular portion of the mid-gut which follows the 
stomach. They are as a rule most pronounced in herbivorous 
animals, which thus have longer alimentary canals than carnivorous 
forms. 

The large digestive gland, usually called the liver, enters the 
stomachal division of the mid-gut. Functionally, this organ only 
very slightly corresponds with the vertebrate liver, if indeed it may 
be said to correspond at all with that organ. It agrees more nearly 



VII MOLLUSCA~THE ALIMENTARY CANAL 177 

with the pancreas, and perhaps combines the functions of the different 
specialised digestive glands of Vertebrates. 

There is a radical difference between Lamellibranchs and other 
Molluscs,! in the fact that in the latter the anterior portion of the fore- 
gut which follows the buccal cavity is developed as a muscular pharynx 
(oesophageal bulb, buccal mass), and carries at its base on a movable 
lingual cushion a file-like organ, the radula, which is beset with 
numerous hard teeth composed of conchyolin or chitin. The radula 
serves chiefly for mastication, but is sometimes used in seizing, holding, 
and swallowing prey. 

None of the Lamdlihranchs have a pharynx provided with a radula, 
they are therefore called Aglossa as opposed to all other Molluscs, which 
are Glossoplwra. 

Hard jaws, composed of conchyolin, are almost always found in 
varying number and arrangement in the buccal cavity of the Glosso- 
phora, but are wanting in all Lamellibranchs. 

One or two pairs of glands open into the pharynx in the Glossoplwra; 
these are usually called salivary or buccal glands, although they very 
slightly if at all correspond physiologically with the glands so named 
in the Vertebrata. Glands may also open into the buccal cavity. The 
Lamellibranchs have no salivary glands. 

The absence of the pharynx, tongue, jaws, and salivary glands in 
the Lamellibranchia is accounted for by their manner of life. They do 
not have to seek their food. Some of them are attached and others 
feed in the same way as attached animals on small particles suspended 
in the respiratory water (animalculse, microscopic algee, and particles of 
detritus) which are brought to the mouth by means of ciliary move- 
ment. These fine particles require no mastication before being 
swallowed. 

This method of feeding also affects the outer organisation of the 
Lamellibranchia, which have lost the cephalic portion of the body with 
the tentacles and eyes : Aglossa = Aeephala = Lipoeephala, and 
Glossophora = Cephalophora. 

In some Gastropoda (Murex, Purpura) and in Dentalium there is in 
connection with the last part of the hind-gut an anal gland, and in 
the Cephalopoda (excepting Nautilus) a gland known as the ink-bag. 

The alimentary canal of the Molhisca runs through the primary 
and often also through the secondary body cavity, attached in various 
ways by fibres or bands of connective tissue. Its walls consist of an 
inner epithelium usually to a great extent ciliated, an outer muscular 
layer in which longitudinal and circular fibres occur, not always in 
regular layers, and, where it passes through the primary body cavity, 
an outer envelope of connective tissue. 

The pharynx and perhaps sometimes part of the oesophagus, and a 
part, in all cases very short, of the hind-gut, arise ontogenetically out of 
the ectodermal stomodaeum and proctodaeum. But the exact limits 

1 For the rare exceptions to this rule, see p. 183. 
VOL. II N 



178 COMPARATIVE ANATOMY chap. 

of the ectodermal and the endodermal portions of the intestine are 
difficult to determine. 



A. Buccal Cavity, Snout, Proboscis. 

The alimentary canal has an oral aperture bordered by variously - shaped lips, 
and in many Glossophora (in nearly all Gastropoda) leads into a vestibule or 
anterior cavity roofed over by the lips and lined by a continuation of the outer wall 
of the head. The dermal glands are not unfrequently (many OpistholrancMa and 
a few ProsoJiranchia) more strongly developed on the lips as labial glands. In 
many Gastropods, when the lips open, the mouth is able to seize and hold prey like 
a sucker. 

Where the snout is short it is simply contractile (the CMtonidce, the Dioto- 
cardia, most herbivorous Tcenioglossa, and many Pulmonata and NudHyrancliia). 
In this case the parts immediately surrounding the mouth are so strongly con- 
tractile that when contraction takes place the mouth is drawn in somewhat so as 
to lie at the base of a depression. An exaggeration of this arrangement, combined 
■with the prolongation of the snout, leads to the formation of the retractile or 
proboscidal snout. The snout can in such cases be invaginated from its tip, i.e. 
from the oral aperture into the cephalic cavity, the mouth then lying at the base of 
the invagination (many Tectibranchia, Capulidce, Stromiidce, CJienopidce, Calyptrceidce, 
Cypraeidce, Lamellariidce, Naticidm, Scalaridm, Solariidce). 

Finally, in many carnivorous ProsohrancJiia (Tritoniidce, DoUidce, Oassididce, 
Rachiglossa, and a few Toxoglossa) a proboscis, often very long and enclosed in a 
special proboscidal sheath, is developed (Figs. 71 and 152) ; this sheath lies in the 
cavity of the head, which is often prolonged like a snout, and may even stretch back 
into the body cavity. The oral aperture lies at the free anterior end of the 
cylindrical proboscis, and we have to regard the proboscis with its sheath as a very 
long snout, the base of which, however, is permanently invaginated into itself. In 
this way the proximal portion of the snout forms the permanent proboscidal sheath, 
while the distal portion with its terminal oral aperture forms the proboscis. Neither 
of these portions can be invaginated or evaginated ; it is merely a zone lying 
between them which takes part in the retraction of the proboscis into the body 
cavity. This zone, when so invaginated, forms a temporary backward prolongation 
of the proboscidal sheath, but when the proboscis is protruded forms the basal 
portion of the latter. The permanent portion of the proboscidal sheath is connected 
with the wall of the head by bands which make its evagination impossible, and the 
inner.wall of the permanent proboscis is connected by muscles or bands with the 
(esophagus lying within it, so that this portion of the organ cannot be invaginated ; 
the oral aperture can thus never lie at the base of the proboscidal sheath. 

When the proboscis is retracted, there is therefore an aperture at the anterior 
end of the snout or the head, which is not the oral aperture, Ijut that of the 
proboscidal sheath. When the proboscis is protruded, it projects beyond the 
aperture of the sheath and cames at its point the oral apertirre. 

The proboscis is retracted by means of muscles attached at the one end to the 
body wall and at the other to its (invaginable) base. In its protrusion, a flow of 
blood towards the snout probably plays the chief part, accompanied by contraction 
of the circular muscles of the head and proboscis. 

The (carnivorous) Ptcropod'a gymnosomata also have a protrusible proboscis (Fig. 
17, p. 11) provided with so-called buccal appendages. The same is present in the 
allied AplyHidre, but is weakly developed. The Tliecosoriiata have no proboscis. 

The buccal cavity of Dentalium is noteworthy. It extends throughout the 



YII 



MOLLUSGA—THE ALIMENTARY GANAL 



179 



whole length of the freely -projecting egg-shaped snout, which carries leaf-like labial 
appendages. On each side of the buccal cavity there is a pouch, tlie so-called cheek 
pouch, which is lined with glandular'epithelium and opens into the cavity anteriorly. 




Fig. 152.— Diagram of the proboscidal apparatus of tlie ProsobranoWa. A, proboscis 
retracted. B, The same protruded, a-c, Cephalic integument ; c, edge of the aperture of the 
proboscidal sheath ; c-d, immovable wall of the proboscidal sheath ; d-e, movable (evaginable and 
invaginable) wall of the same ; e-/, immovable wall of the proboscis ; /, edge of the oral aperture, at 
the anterior end of the proboscis ; g, pharynx ; 7t, cesopliagus ; i, retractor muscle ; ?c, salivary 
glands ; Z, cephalic cavity. 

An exact comparative investigation of the mechanism of the proboscidal 
apparatus, the contractile snout, etc. of the Prosobranchia is still a desideratum. 

There are other forms of proboscis, differing greatly from the one just described 
(e.g. that in the Terebridm). 

In the Heteropoda, the head forms a long snout which is often described as a 
proboscis. The name is inappropriate, as this snout is not retractile and the mouth 
is always found at its anterior end. 



180 COMPARATIVE ANATOMY chap. 



B. The Pharynx and Jaws, the Tongue and Salivary Glands. 

The mouth or buccal cavity is followed in all Molluscs except the 
Lamellibranchia by the pharynx or oesophageal bulb (buccal mass). 
The pharyngeal cavity opens anteriorly into the buccal cavity, and 
posteriorly into the oesophagus. The pharynx is characterised by the 
possession of (1) jaws, which lie anteriorly at the boundary between 
the buccal and pharyngeal cavities ; (2) a lingual apparatus at its 
base, and (3) salivary glands, which usually open laterally near its 
posterior boundary. 

1. Jaws are almost universal, and are sometimes, especially in 
carnivorous animals, very highly developed ; less frequently they are 
rudimentary or wanting. They are hard cuticular formations of the 
epithelium of the anterior pharyngeal region, and no doubt composed 
of conchyolin or some related substance, in a few cases hardened by 
calcareous deposits (e.g. Nautilus). 

The jaws serve for seizing prey or particles of food. The great variations in 
number, form, and arrangement of the jaws can best be understood by assuming 
that they originally extended completely round the entrance to the pharynx ; and 
that of this ring sometimes only upper and lower or sometimes only lateral portions 
have been retained. 

Such a complete circle of jaws is found at the entrance to the pharynx in some 
forms, such as Umbrella and Tylodina (Opisthobranchia). 

The fresh-water Ptdmoiiates have an upper and two lateral jaws. 

Most Prosobranchia and OpistlmbrancMa have two lateral jaws. These may 
approach so near one another as almost to touch {Haliotis, Fissurella). Terrestrial 
Pulmonala have an upper jaw and occasionally a weak lower jaw as well. 

The jaws are particularly strongly developed in the Cephalopoda, which have an 
upper and a lower jaw, the two together resembling in shape the beak of a parrot. 

In the OpisthobrancMatc family Aplysiidce, Notarchus, Accra, Dolabella, and 
Aplysiella have, besides tlie lateral jaws, numerous hooks or small teeth on the roof of 
the pharyngeal cavity. The hook sacs (Fig. 17, p. 11) of the Pteropoda gymnoso- 
mata, which are wanting only in Halopsyche, are perhaps to be derived from these 
pharyngeal teeth. 

The hook sacs are paired dorsal outgrowths of the pharyngeal cavity, which vary 
in length and lie in front of the radula. The walls of the sacs carry hooks project- 
ing inward. AVhen the proboscis of these carnivorous animals is protruded, the sacs 
are completely evaginated, so that the hooks come to lie outside (Fig. 17, p. 11). 

Jaws are wanting or rudimentary in the Atiiphincura and the Scaphupoda ; 
among the Prosobranchia, in the Toxoglossa, Pijramidellidm, Eidimidce, many 
TrocMdcB, the Hetcropoda, and in many Nudibranchia {Tcthys, ilelibe, Doridopsis, 
Phyllidia) ; in the Ascoglossa, and in certain Tectibranchia (Actceon, Doridiuvi, 
Philine, Ulriculus, Scaphander, Lobicjer). Among the Pnlmonata they disappear 
in a series of Tesfiwllidcc, being present in Daudebardia rufa, rudimentary in 
D. Saulcgi, and wanting in Testacella. 

2. The lingual apparatus (Figs. 153, 154) is highly characteristic 
of all Molluscs except the Lamellibranchia, i.e. of all Glossophara. It 
may be said that every animal with a radula is a Mollusc. 



YII 



MOLLUSGA—THE ALIMENTARY CANAL 



181 



The ventral and lateral walls of the pharynx are thickened and 
very muscular. On the floor of the cavity rises a tough longitudinal 
muscular cushion, the tongue. Its surface, which projects into the 




9 



8 h 



Fig. 153.— Longitudinal section (not quite median) through the snout of a Prosohranohiate, 
to illustrate the pharyngeal apparatus. 1, Dorsal wall of the head ; 2, mouth ; 8, jaw ; 4, 
radula ; 5, lingual cartilage ; 6, muscular wall of the pharynx ; 7, muscles attached at one end to the 
pharynx and at the other to the ventral wall of the head (S) ; 9, cavity of the head ; 10, radular 
sheath ; 11, cesophagns ; 12, aperture of the salivary gland ; 13, infolding behind the radular sheath. 

pharyngeal cavity, is covered by a rough cuticle consisting of chitin 
(or conchyolin ?) ; on this basal membrane are found very numerous 
hard chitinous teeth, often many thousands, arranged in close transverse 




im 
Fio. 154.— Median longitudinal section through the anterior part ot the hody of Helix 
(after Howes), ce, CEsophagus ; rot, radular sheath ; mc, cerebral ganglion ; si,, aperture of the 
salivary gland ; oe, muscle mass in the ventral pharyngeal wall ; rd, radula ; hj, upper Jaw ; Ij, l^, 
lips of the oral aperture ; im, pharyngeal muscles ; rm^, retractors of the pharynx ; pgl, pedal 
gland. 

and longitudinal rows. The basal membrane and the teeth together 
form the radula of the tongue. 

The anterior end of the tongue projects freely into the pharyngeal 
cavity, the radula bending down over this end so as to cover for a 



182 COMPARATIVE ANATOMY chap. 

certain distance its lower surface. Immediately in front of the tongue 
there is always a depression in the ventral pharyngeal wall, forming 
a sort of pocket. The radula, at the posterior extremity of the 
tongue, sinks into a narrow more or less long tube, the radular sheath, 
which is an outgrowth of the pharyngeal cavity running downward 
and backward. The radula, always lying upon the anterior or 
ventral wall of this sheath, which is anteriorly thickened to form 
the tongue, extends to the base of the sheath, which is the place 
of its formation. 

The tongue witli the radula on it is movable, and in most cases its movements can 
be compared with those of the cat's tongue when licking, but are usually slower. 
This action helps to rasp the food which lias been seized, and often also broken up, by 
the jaAvs. The tongue can either move inside the pharyngeal or buccal cavities, or 
can be extended to the oral aperture or even protruded more or less far beyond it. 

In or under the fleshy tongue, a lingual cartilage is very commonly found, 
consisting of two or four or even more pieces. This cartilage forms a support for 

the radula, and affords firm points of attachment 
for certain muscles belonging to the lingual 
apparatus. 

The musculature of the pharynx, which can 

be separated into bundles or strands, and is often 

very complicated, consists first of the muscles 

which form the wall of the pharynx, and which, 

being principally developed ventrally and laterally 

round the radula, determine the special licking 

Fio. 156. -Four transverse rows movements of the tongue ; secondly, of muscles 

of the radular teeth of Cyclostoma ^^,^^^^_^ ^^^^ ^^^ ^^^^^ pharynx or the whole of the 

Elegans (after Claparede). , . , . . j t j , 

lingual apparatus, evaginatmg or protruding them. 

The second group consists, speaking generally, of protractors and retractors, attached 
at the one end to the pharynx and at the other to the body wall after running 
through the cephalic or body cavity. Pres.sure of blood may also take some part in 
the protrasion of the pharynx. 

The tongue and radula further often serve for seizing prey (e.r/. in the carnivorous 
Heteropoda, in Testcwella, etc.). 

The radula is of great importance in classification. Further details concerning 
it must be sought in special works and in text-books of conchology. The points to 
be specially noticed are (1) the size and form of the whole radula, (2) the number of 
longitudinal and transverse rows of teeth, and (3) the form of the teeth in each of 
these rows. As a rule the transverse rows resemble one another, but exceptional 
rows differently constituted from those immediately preceding or following them 
recur at intervals. 

Three kinds of teeth have been, as a rule, distinguished. First, there is usually 
a single median longitudinal row of central or rachial teeth. On each side of this 
row are several rows of more or less similar lateral teeth or pleurse. Finally, at the 
lateral edges of the radula, there are single or very numerous longitudinal rows of 
marginal teeth or uucini. 

Dental formulsE are used for the radular teeth, in the same way as for the teeth 
of mammals ; in these the number of central, lateral, and marginal teeth in a 
transverse row are given. 

The reader will find the dental formulae of some of the Molluscs in the Systematic 
Review. 




VII 



M0LLU8GA—THE ALIMENTARY GANAL 



183 



The total number of radular teeth varies very greatly, from 16 in EoHs Drum- 
mmidi to 39,596 in Selix Ghiesbrcghti. 

As a, rule, the teeth are most numerous and finest in lierbivorous animals. In 
carnivorous Molluscs we have two extremes ; (1) great development of the proboscis, 
with weak development of the pharynx and radula, and a comparatively small 
number of teeth (carnivorous Prosobranchia) ; (2) absence of a protrusible proboscis, 
with great development of the pharyngeal apparatus and the radula, and numerous, 
often large, teeth {Hetcropoda, carnivorous Pulnwnata and Cephalopoda). 

The muscular pharynx is most developed in carnivorous Pulnwnates. In these 
it may be half {Davdehardia) or even more than half as long {Testacella) as the 
whole body, and may occupy a very large part of the body cavity. It is protruded 
in such a way that the tongue with the radula occupy the anterior end of the 
evaginated pharynx (Fig. 54, A, p. 44). 

In very rare cases (apart from the Latnsllibranchia) the radula completely 
atrophies ; this is the case in parasitic Gastropoda {Stilifer, EuUma, Thyca, Ento- 
concha), in the CoraUiophilidce (CoraUiophila, Leptoconchus, Magilus, Mhizochilus), 
among the Nudihranchia in Tethys and Melibe, among the Amphineura in Neonunia, 
and certain species of the genera Dondersia and Proneomenia. In Chcetoderma, a 
single tooth of the radula is retained. 

Even in certain carnivorous Prosobranchia which are furnished with a proboscis, 
the above-mentioned reduction of the whole pharyngeal apparatus goes so far that 
the radula disappears (certain species of Terebra). 



Fonuation of the Radula. 

The teeth of the anterior transverse rows of the radula become worn out by use, 
and are continually being replaced by new teeth which are pushed forward. The 
formation of new transverse rows of teeth 
is constantly taking place at the posterior 
blind end of the radular sheath. In Pul- 
'))ionata and Opisthobranchia they appear as 
cuticular formations secreted by several 
transverse rows of large epithelial cells — 
the odontoblasts (Fig. 156) ; the basal 
membrane which carries the teeth is secreted 
by the anterior row or rows, the teeth them- 
selves by the posterior rows. 

Each group of odontoblasts which has 
formed a tooth is not replaced by another, 




FiGi 156.— Longitudinal section through 
the posterior end of the radular sheath of 
but continues to produce new teeth behind a Pnlmonate (after Eossler), diagram. 1, 2, 
those already formed, so that for each loDgi- 3, 4, Formative cells of the ladular teeth ; 6, 
tudinal row of teeth there is at the base of 



formative cells of the basal membrane ; 6, 
teeth of the radula ; S, basal membrane. 



the radular sheath a group of odontoblasts 

which has produced all the teeth belonging to that row. A layer of "enamel " is 

deposited on the teeth so formed by the epithelial roof of the radular sheath. 

In the Ohitonidce, Prosobranchia, and Cephalopoda, the odontoblasts are very 
numerous narrow cells, which form, at the base of the sheath, a cushion divided into 
as many parts as there are teeth in a transverse row of the radula. 

The radular sheath in the Pulmonata, Scaphopoda, Opisthobranchia, and Cephalo- 
poda is short, and is contained in the ventral and posterior muscular wall of the 
pharynx, very seldom projecting posteriorly beyond it ; but in many Prosobranchia 
it is long and narrow, and reaches back into the cephalic cavity or even right into 
the body cavity. This latter is especially the case in the Diotocardia ; in the 



184 COMPARATIVE ANATOMY chap. 

Docoglossa {Patella) the sheath, which lies above the foot on the floor of the body 
cavity, is even longer than the body (Fig. 158). 

3. Salivary glands (buccal glands, pharyngeal glands) are universally 
found in Glossophora, i.e. in Molluscs which have a pharynx and lingual 
appatatus. They are universally absent in Lamellibranchs. They may 
occur in one or two pairs. The posterior or in other cases the only 
pair often lies on the wall of the oesophagus, and sends forward two 
ducts which enter the pharynx laterally, usually somewhat behind the 
point where the radular sheath opens into the pharyngeal cavity. 
Very little is known of the function of these glands ; an exact 
morphological comparison of the various pharyngeal glands of the 
Gastropoda is at present hardly possible. 

Amphlneura. (a) Chiton. — Two small delicate buccal glands lie 
on the roof of the buccal cavity and open into the mouth. They can 
therefore hardly be regarded as pharyngeal or salivary glands. 

(b) Solenogastres. — Salivary glands are here found in all genera 
except Neomenia, and in Ghcetoderma. They are present in some species 
but appear to be absent in others. A pair of long glandular tubes 
with high glandular cells ^ and strong muscular walls lie anteriorly under 
the intestine and are produced in the form of two narrow ducts, which 
enter the pharyngeal cavity on the tongue either separately or through 
a common terminal portion. Besides these there is another pair in 
some species [Paramenia impexa, Param. palifera, Proneomenia vagavs, 
Dondersia flavens) ; the ducts of these open together through an un- 
paired terminal portion on the dorsal wall of the pharyngeal cavity, 
at the point of a papilla which rises from the base of a pit-like 
depression. 

Gastropoda, {a) Prosobranehia. — In most cases there is only 
one pair of salivary glands. These are usually lobed or branched 
glandular masses, which lie, in the Diotocardia, at the sides of the 
pharynx, in the Monoiocardia, at the sides of the cesophagus. In the 
former case, the ducts are short and do not pass through the cesophageal 
ring formed by the nerve centres and their connectives and commis- 
sures, which in these forms surrounds the anterior end of the pharynx. 
In the Monotocardia, the ducts are long, and generally accompany the 
oesophagus through the oesophageal ring (which lies behind the pharynx), 
and open on the posterior lateral wall of the latter. 

Two pairs of salivary glands are found in certain Diotocardia [e.g. 
HaUoti.-<, Fissurelki), and further in Patella, the Scalariidce, lantJiinidce, 
certain Purpuridce, Muricidce, and in the Cancellariidce. 

One of the two pairs of glands in Haliotis is developed in the form 
of large lateral glandular sacs covering the pharynx on the right and 
left (Fig. 105, p. 121). 

In the Ampidlariida', the ducts of the salivary glands do not pass 

^ This differs somswhat from the description found in Simroth (Bronn's Klassen 
uiid Ordnungen, vol. iii, pp. 183-185). 



VII MOLLUSGA—THE ALIMENTARY CANAL 185 

through the oesophageal ring, which here, as in the Dwtocardia, sur- 
rounds the anterior end of the pharynx. 

Whereas the salivary glands are, as a rule, branched tubes or 
acinose, they are sometimes {Scalariidce, lanthinidce, Cancellanidce) 
simply tubular or (Doliidce, Xeiiophoridm, etc.) sac-like. 

The passage of the ducts of the salivary glands through the 
oesophageal ring in the Moiwtocardia may have come about by the 
shifting back of the ring along the pharynx from its former position 
in the Diotocardia, where it encircles the anterior end of the pharynx 
in front of the apertures of these ducts. The salivary ducts would 
thus necessarily become surrounded by the ring. 

The ducts in the Moiwtocardia become the longer the further the 
nerve ring shifts back from the mouth and pharynx. They are very long 
in animals provided with a protrusible proboscis, where the ring lies 
far back on the oesophagus, behind the non-evaginable portion of the 
proboscis. The ducts here run along the whole length of the latter. 
But in those cases in which the oesophageal ring has shifted back more 
quickly than the ducts have lengthened, the glands lie in front of the 
ring. In the event of the subsequent lengthening of their ducts, the 
glands might stretch back outside the ring. The arrangement of 
the glands in the Toxoglossa and Rachiglossa would thus be explained ; 
here the greater part of the glands lies behind the ring, although 
the ducts are said not to pass through it. 

The acid secretion of the sali\'ary glands of certain Prosoh'anchia 
(species of Dolium, Cassis, Cassida.ria, Tritonmm, Murex) and Opistho- 
hrancliia (Pleurobranchus, Fleurobmnchidium) contains 2'18-4'25 percent 
of free sulphuric aeid. These carnivorous animals are able, by means 
of their proboscides, to bore into other Molluscs and Echinoderms which 
are protected by calcareous skeletons. The sulphuric acid in their 
glands probably serves for transmuting the carbonate of lime into 
sulphate of lime, which can then easily be worked through by the 
radula. 

(i) Pulmonata. — Two salivary glands (Fig. 157, 10) are always 
found, their ducts entering the pharynx to the right and left of the 
boundary between it and the oesophagus. The glands lie on the 
oesophagus and the anterior part of the stomach in the shape of long, 
lobate, jagged leaves. In some cases they are acinose or round and 
compact. 

(c) Opisthobranehia. — The salivary glands, of which only one pair 
is almost always found, here vary in size and shape still more than in 
the Pulmonata. These glands, which enter the pharynx, must not be 
confounded with other glands which in many Opisthobranehia enter 
the buccal cavity, and are sometimes more strongly developed than 
the salivary glands. 

Dentalium has no salivary glands opening into the pharynx, for 
the glandular "cheek pouches" enter the buccal cavity, and two 
diverticula which lie further back belong to the oesophagus. 



COMPARATIVE ANATOMY 



CHAP. 



The Cephalopoda have a posterior and an anterior pair of salivary 
glands. Were the fore-gut, which here rises vertically in the visceral 
dome, to occupy the horizontal position it has in the Gastropoda, the 
anterior pair would lie dorsally and the posterior ventrally with regard 
to it. The two posterior glands (Fig. 127, 29, p. 147) are always 
present (except in Cirrhoteuihis and Loligopsis, in which they are said to 
he wanting), and lie on the oesophagus. Each gland has a duct, which 
soon unites with that from the other gland, forming a terminal portion 
which accompanies the oesophagus through the cephalic cartilage, and 
opens above the radula into the pharyngeal cavity. The posterior 




Fig. 157. — Alimentary canal of Helix, Jissected out and seen from the right side (after 
Howes). 1, 3, Tentacles ; 2, constrictor pharyngis ; 4, levator pharyngis ; 5, depressor ; 6, pro- 
tractor pharyngis ; V, pharyngeal bulb ; S, radular sheath ; 9, coluniellar muscle, divided into a 
retractor pedis and retractor pharyngis ; 10, salivary glands ; 11, digestive gland (liver); 12, ducts 
of the same (gall ducts) partly cut open ; 13, hermaphrodite gland ; 14, stomach cut open, in its 
base are seen the apertures of the gall ducts 15 ; 16, mid-gut ; 17, hind-gut ; IS, anus. 



glands occasionally {e.g. in Oegopsidw) fuse behind the gullet, in which 
case the duct is single throughout its whole length. 

The anterior salivary glands are specially well developed in the 
Odopoda (Fig. 127, 33, p. 147), and lie on the pharynx, into which 
they empty their secretions by a duct, which seems always to be 
unpaired. In the Decapoda the anterior glands are much smaller or 
rudimentary ; they are generally represented by a single gland hidden 
within the muscular wall of the pharynx. 

Nautilus has no posterior salivary glands, but there are glandular 
outgrowths of the pharyngeal cavity on each side of the tongue, 
which perhaps correspond with the anterior salivary glands of other 
Cephalopods. 

The Cephalopoda ( 1 without exception) have an additional acinose 



rii MOLLUSGA—THE ALIMENTARY CANAL 187 

lingual gland, opening into that part of the pharyngeal cavity which 
lies between the tongue and jaws. 

The Lamellihanchia, as already mentioned, have neither pharynx, 
jaws, tongue, nor salivary glands. In the Nuculidce, however, which 
are rightly considered to be primitive forms, the mouth leads into a 
widening of the intestine, on each side of which a glandular pouch 
opens. These pouches perhaps correspond with the oesophageal 
pouches of the Chitonidce and JRhvpidoglossa, which will be described 
later. 

Natica, which bores through the shells of living Lamellilranchs and 
feeds on their bodies, has a sucker-like organ on its proboscis (Fig. 98, 
p. 107). The epithelium of the concave side of this organ, which is 
applied to the shell attacked, forms a gland for secreting acid — prob- 
ably sulphuric acid — which serves for dissolving the carbonate of 
lime of the bivalve shell, which is then at once thrown out in the form 
of powdered sulphate of lime. 



C. The CEsophagus. 

That portion of the intestine which lies between the pharynx (or 
the mouth in Lamellibranchs) and the stomach is called the oesophagus, 
the stomach being here used as the name of that widening of the 
intestine into which the gland of the mid-gut opens. It is always 
easy to detect the anterior boundary of the oesophagus. In Lamelli- 
branchs it lies at the mouth, but in the Glossophora at the posterior and 
upper end of the pharynx. The posterior boundary, however, can 
often only arbitrarily be defined, as the oesophagus, which is usually 
narrow and tubular, often widens very gradually into the stomach, 
the structure of its walls at the same time gradually changing. In 
other cases, widenings of the alimentary canal occur before the 
stomach, and it is difl&cult to decide whether these are anterior 
divisions of the stomach or posterior widenings of the oesophagus. 

In LamellibrancJiia, terrestrial Pulmonata, most Opisthobranchia, and 
the Cephalopoda Becapoda the oesophagus is a simple ciliated tube 
running to the stomach, being often provided with longitudinal folds, 
and therefore extensible ; in other divisions, however, complications 
occur, which are caused by glandular outgrowths or muscular en- 
largements. 

In a few SoUnogastres {e.g. Froneomenia), on the boundary between the short 
oesophagus and the mid-gut, a more or less long blind diverticulum occurs ; this is 
single, and runs forward dorsally to the pharynx, and may extend over the cerebral 
ganglion to the end of the head. 

In Chiton there are two lateral glandular sacs (sugar glands) connected with the 
short oesophagus ; their inner glandular walls project into the lumen in the form of 
villi, and their secretion changes boiled starch into sugar. 



li 



COMPARATIVE ANATOMY 



CHAP. VII 



Similar glands, which communicate with the anterior part of the oesophagus, are 
found in the Rhipidoglossa (e.g. Haliotis, Fissurella, Turbo). The glandular epithe- 
lium in these also projects in the form of villi or folds into the lumen. 

The so-called crop of the Docoglossa {Patella) no doubt corresponds with the two 
lateral cesojihageal sacs in the Chitmiidce and Rhipidoglossa. This is a saccular 
widening of the cesophagus (Fig. 158, in), which, on account of the constitution of 
its walls, has been compared mth the psalterium of a Kuminant. A similar widen- 
ing of the esophagus is found in Cyprmidas and Naticidce, which must be counted 
among the most primitive of the Monotocardia. 

In those Monotocardia which are provided with a proboscis, the length of the 
thin oesophagus is in proportion to that of the proboscis. 

The mouth lies at the tip of the proboscis, then follows a short and often rudi- 
mentai-y pharynx, and then the long oesophagus, which rans through the whole 
length of the non-protrusible portion of the proboscis, passes through the oesophageal 




Fig. 15S. — Median longitudinal section througli PateUa (after Ray Lankester). hrv, Efferent 
branchial vessel ; hra, afferent ditto ; asd, duct of salivary gland sd ; go, anus ; no, right nephridial 
aperture ; sd, salivary gland ; cor, heart ; pe, pericardium ; up, kidney ; d, intestine ; Ttp, hepatic 
gland (liver) ; v, blood vessel ; m (to the right), border of mantle covering the gills ; r, radular 
sheath ; g, gonads ; m, crop ; pli, pharynx ; yd, radula ; odm, masses of muscle and cartilage of the 
lingual apparatus ; o, mouth ; /:, head or snout. 

ring, and may be even further prolonged posteriorly. When the proboscis is re- 
tracted, the posterior portion of the oesophagus becomes coiled ; when the proboscis 
is extended, it lies in the protruded or evaginated basal portion. 

Not infrerjuently in carnivorous Monotocardia there is a glandular widening in 
that section of the oesophagus which follows the long proboscidal portion. The 
cfisophagus is most complicated in the Rachiglossa and many Toxoglossa, where this 
widening, in the form of a large compact accessory gland, can become separated from 
the intestine (Leiblein's gland, poison gland), and where other glands and widen- 
ings may occur (Fig. 159). It seems probable that in certain Prosobranchia diges- 
tion and resorption takes place even in the fore-gut. 

In the Pidmonata and Opislhobranchia, there is sometimes a widening (crop, fore- 
stomach) anteriorly to the stomach, and in the same way the short cesophagus of 
the ScapJmjjoda has a glandular widening, or two lateral glandular diverticula. 

Among the Cephalopoda, the Decapoda have a simple thin tubular cesophagus ; 




Fia. 161. 



Fig. 159.— Alimentary canal of Mnrex trun- 
culus (after B^la Haller). 1, Pharynx ; 2, ducts of 
the salivary glands (5) ; 3, oesophagus ; 4, 6, and 7, 
glands of the fore-gut (8) ; 9, digestive gland (liver) ; 
10, stomach ; 11, hind-gut; 12, gland of the hind- 
gut ; 13, anus. 

Fig. 160.— Alimentary canal of Sepia. 1, Jaw ; 
2, pharynx ; 3, posterior buccal ganglion ; 4, duct 
of the salivary gland (5) ; 0, digestive gland (liver) ; 

7, anus; 8, rectum; 9, efferent duct of the pigment gland (ink-hag), 10; 11, stomachal ccecura ; 12, 
stomach ; 13, ganglion gastricum ; 14, " pancreatic appendages " of the gall ducts of the digestive gland. 

Fig. 161.— Sketch of the anatomy of Limacina helicina, from the right side, after removal of the 
mantle, heart, and kidney (after Pelseneer). 1, Fin (parapodiuin) ; 2, foot ; 3, central nervous system 
(oesophageal ring) ; 4, oesophagus ; 5, anus ; 6, columellar muscle ; 7, duct of the hermaphrodite gland, 
7a ; 8, intestine ; 9 and 10, dental plates of the stomach ; 11, accessory glands of the genital apparatus ; 
12, mantle cavity ; 13, seminal groove or furrow. 



190 



COMPARATIVE ANATOMY 



CHAP. 



the cesopliagus of tlie Odopoda, however, is provided with a lateral pouch, the crop 
(Fig. 127, p. 147), whose walls are not glandular. This may 
serve as a reservoir of food when the stomach is already- 
full. In Nautilus, the crop is a very large saccular widening 
of the cEsophagus, larger than the stomach itself. 



5 — 



n 



• --H. 



■13 



- .10 



Fig. 162.— Diagram of the 
anatomy of Clio striata, 
from the right side ; the 
heart, Icidney, and mantle of 
this side removed (after 
Pelseneer). l,Fiu(parapo- 
dium) ; 2, aperture of the 
penis ; 3, right tentacle ; 4, 
genital aperture ; 5, penis ; 
6, oesophagus ; 7, dental 
plates of the stomacli ; S, 
ducts of the gonad ; 9, gonad; 
10, intestine ; 11, digestive 
gland ; 12, ducts of the same 
(cut off) ; 13, accessory 
glands of the genital appar- 
atus ; 14, mantle cavity ; 15, 
terminal portion of the geni- 
tal ducts ; 16, central ner- 
vous system (ganglion ring); 
17, foot ; 18, pharynx. 



D. The Mid-gut with the Stomach and 
Digestive Gland (Liver). 

The oesophagus leads into a wider portion of 
the alimentary canal, the stomach. Into this the 
ducts of a gland open ; this gland is strongly 
developed in nearly all Molluscs, and is usually 
called the liver, but may be more appropriately 
named the digestive gland, since it in no way 
fulfils the functions of the vertebrate liver. As 
far as is at present known, it functions rather as 
a pancreas, or it combines the functions of the 
various digestive glands of the vertebrate intes- 
tine, no such thorough division of labour as is 
found in the Vertebrates having taken place. The 
digestive gland is, in most cases, a richly-branched 
tubular or acinose gland, which to the naked eye 
appears a compact lobate body of a brown, 
brownish-yellow, or reddish colour. Its glandular 
epithelium consists of three sorts of cells — 
hepatic, ferment, and calcareous cells. In 
many Nudihranchia the gland breaks up into 
branching intestinal diverticula, which spread 
through the body almost like the gastro-canals or 
intestinal branches in the Turhellaria, and run up 
into the dorsal appendages of the body (clado- 
hepatio Nudihranchia). 

Chceioderma, among the Solenogastres, has a 
simple midgut diverticulum, which may corre- 
spond morphologically with the digestive gland 
of other Molluscs ; but in Proneomenia, Neomenia, 
etc., the straight mid-gut is provided throughout 
its whole length with narrow lateral glandular 
sacs arranged closely one behind the other at 
right angles to it. 

A part of the mid - gut gland (the part 
nearest to the point where the duct leaves it) 
and the glandular epithelium of the duct may be 
specially differentiated in Cephalopoda, and may, 
finally, form a distinct system of glands called the 
pancreas (Fig. 160). 



VII 



MOLLUSOA—THE ALIMENTARY GANAL 



191 



The stomach is not infrequently a lateral outgrowth of the mesen- 
teric wall, so that the aperture (cardia) leading into it from the oeso- 
phagus and that leading out of it into the small intestine (pylorus) 
are more or less near one another. A sort of connection between 
these apertures may arise, a ciliated furrow or channel bounded by 
longitudinal folds running between them, and in some cases continued 
into the adjoining sections of the alimentary canal. 

In the Cephalopoda, the duct of the digestive gland (the so-called 
hepatic or gall duct) does not open direct into the stomach, but into a 
ccecal outgrowth of the stomach, the spiral eoeeum. 

In very many LameUibranchia there is a diverticulum of the 
stomach which contains within its lumen a rod-shaped gelatinous cuti- 
cular formation, called the crystalline stylet. Similar structures occur 
in the Prosobranchia, and especially in the Rhipidoglossa and Toxoglossa. 

In many Opisthoh'anchia, the inner wall of the stomach carries 
variously-arranged cuticular teeth, dental plates, jaw plates, etc., which 
serve for triturating the food. In such cases the muscular wall of the 
stomach is strongly developed. 

The stomach is succeeded by a narrower tubular section of the 
mid-gut, called the small intestine (intestinum), which usually forms 
coils or loops ; these are more numerous in herbivorous or detri- 
tivorous than in carnivorous Molluscs. 

The stomach, small intestine, and digestive gland, together with 
part of the sexual organs, compose the whole or by far the largest 
portion of the visceral dome, where this is present. 



1. Amphlneura. 

The ChitonidiE show the typical division of the mid-gut into stomach, digestive 
gland, and small intestine. The stomach 
lies far forward, and has a wide outgrowth 
on one side, which is, functionally, a reser- 
voir of secreted matter. The cardia and 
the pylorus lie near one another. The 
digestive gland is paired ; the larger liver 
to the right has four apertures, while the 
smaller one to the left has only one prin- 
cipal aperture into the stomach. The 
small intestine is more than four times as 
long as the body, and it forms many loops 
which are constant in their arrangement. 
Chiton feeds on small or even microscopic 
algfe. 

Unlike the Chitonidce, the Solenogasti-es 
show no separation of the mid - gut into 
stomach and small intestine. The mid-gut 
runs straight through the body, the greater 
part of which it fills. The glandular lateral 
coeca found in Neomenia, Proneomeiiia, 
etc., and called hepatic diverticula, are caused by the projection into the lumen from 




Fig. 163.— Part of a liorlzontal median 
section througli Proneomeuia Sluiteri. 
Septa of the first, second, third, and fourth 
order are seen projecting from the right and 
left into the lumen of the mid-gut. In the 
baelcground is the dorsal wall of the gut, with 
the groove which outs into the hermaphrodite 
gland (of. Fig. 68, p. 42). 



192 



COMPARATIVE ANATOMY 



CHAP. 



S-- 



each side of narrow septa arranged at right angles to the gut, or transversely (Fig. 163) ; 
in tliese septa, muscle fibres run down to the rudimentary foot, and blood lacunaa 
abound. In Proneomenia Sluiteri, septa of tlie first, second, third, or fourth order can 
be distinguished, as seen in the figure. Tlie septa on the right alternate with those 
on tlie lelt side of the body. In the dorsal middle line tlie mid-gut forms a narrow 
ciliated longitudinal groove which cuts deep into the gonad, cilia are also found on its 
medio-ventral surface. 

2. Gastropoda. 

The digestive gland of the Gastropoda falls into two or more lobes, between 
which the stomach and the coils of the small intestine lie embedded. One, two, or 

more ducts of the gland may open into the 
stomach. The walls of the digestive gland 
show the same division into layers as the wall 
of the alimentary canal. For details as to the 
ferment, hepatic, and calcareous cells forming 
the epithelium of the gland, and their physio- 
logical constitution, the reader must be re- 
ferred to special histological and physiological 
treatises. 

In the KudibrancMa, as already mentioned, 
the digestive gland breaks up into a system of 
glandular diverticula (the so - called ' ' diffuse 
liver"). The Aeolidiadce (e.g. Teryipes) afford 
an instructive instance of this. Three diver- 
ticula rise from the stomach, two anterior and 
lateral, and one posterior and unpaired. These 
ramify in the body cavity, and finally send up 
their last ramifications or lobes into the dorsal 
appendages. The contents of the intestine can 
penetrate into these last ramifications of the 
" diffuse liver " (Fig. 164). 

Further, within the NudilrancMa the break- 
ing up of the compact digestive gland to form a 
"diffuse liver," i.e. the loosening from one 
another, and the spreading out of the glandular tubes which are in close contact in 
the compact gland, can be followed almost step by step. In the Tritonidm the gland 
is a gi-eat compact mass. In other families, such as the Tethymelibidce, Lomanotida:, 
Dendronotida; Bornellidcc, Scyllccidcc, it divides into two anterior accessory livers 
and a posterior principal liver, from which diverticula run up into the dorsal append- 
ages. Finally, the accessory and principal livers break up into separate "hepatic 
branches" [Aeolidai), which in some cases anastomose. The posterior principal 
branch of the " diffuse liver " gives off specially numerous lateral branches ; it often 
widens out to a pouch, and may then be compared to an extended gall bag, or a 
posterior diverticulum of the stomach. In PhyUirhoe, a pelagic form, without 
dorsal appendages, the "diffuse liver" is simplified, consisting of four unbranched 
blind tubes, the two anterior opening into the stomach separately, the two posterior 
entering it together (Fig. 19, p. 12). 

Tlie stomach of many Opisthobranchia consists of two divisions separated by a con- 
striction. In some forms, such as the BuUidce among the Tcctibranchia, the Ftero- 
poda thccosomata, and the TetJiymelibidce, Borndlldcc, Scyllaidie, among the Nudi- 
brancMa, it is armed with hard ohitinous plates, spines, teeth, etc., occurring in 
varying number and arrangement on its inner wall (Figs. 161 and 162) 




Fio. 104.— Alimentary canal of Aeolis 
(after Souleyet). 1, Pharynx ; 2, stomach ; 
3, branched digestive gland (liver) ; 4, 
anus ; 5, rectum. 



VII 



MOLLUSGA—THE ALIMENTARY GANAL 



193 



3. Scaphopoda. 

The mid-gut of Deiitalium (Fig. 165) consists of a looped stomachal tube bent 




Fig. 165. — Alimentary canal, kidney, and sezual organs of Dentalium, from behind (after 
Lacaze-Duthiers and Leuckart combined), a, Mouth ; &, leaf-like oral tentacles ; c, snout ; d, 
entrance to pharynx ; e, pharynx with radula, /; g, hind-gut ; h, right kidney ; i, anus ; k, right 
nephridial aperture ; I and q, ducts of the digestive gland, n : m and o, gonad ; n and p, digestive 
gland (liver) ; r, left nepliridial aperture ; s, left kidney ; t, stomach ; w, pharynx ; v, lobes or sails 
on which the filamentous tentacles are placed. 

back on itself, and of a small intestine lying in a tangled coil behind the oesophagus. 
VOL. II 



194 COMPARATIVE ANATOMY chap. 

Two digestive glands, lying in the upper part of the body, open through wide 
apertures into the stomach. Their form can be gathered from Fig. 165. 

4. Lamellibranchia. 

In the Lamellibranchia the cesophagus, which lies under the anterior adductor, 
widens at the anterior base of the foot to foim the stomach. This descends some- 
what into the foot. At the posterior base of the stomach lie two apertures ; one of 
these is the pylorus, and leads into the small intestine which runs more or less 
coiled within the base of the foot ; the other leads into a tubular diverticulum, the 
sheath of the crystalline stylet. The large richly-branched acinose digestive gland 
(liver) opens through several apertures into the stomach, with which it lies in 
the anterior part of the pedal cavity. In Pholas, Jouannetia, and Teredo, the 
stomach has another ccecum besides the sheath of the crystalline stylet. In all 
bivalves there is, on the inner wall of the stomach, a gelatinous cuticular structure 
(dreizackiger Kbrper, fleche tricuspide), which varies in thickness, and is continued 
into the gelatinous crystalline stylet. This latter is secreted in concentric layers as 
a cuticular structure by the epithelium of the sac in which it lies. A plausible 
suggestion has recently been made as to the use of these gelatinous structures, viz. 
that they serve for surrounding with a slimy envelope foreign particles, such as 
sharply-pointed grains of sand, which enter the alimentary canal with the food ; in- 
jury to the delicate walls of the intestine is thus avoided, and the travelling of such 
particles along the digestive tract is facilitated. The point of the crystalline stylet 
projects freely into the lumen of the intestine. In some forms it does not lie in a 
separate sac, but in a groove {JVajada, Cardiimn, Mytilus, Pecten, etc. ). The tricuspid 
body and the crystalline stylet appear temporarily, and are renewed periodically. 
■ Similar structures have been observed in the stomachs of various Gastropods. 
Haliotis has a stomachal cceeum which can be compared with the sheath of the 
crystalline stylet. 

In the lower Lamellibranchs, the Nuculidas and Solenomyidce, the stylet is either 
I'cry slightly developed or wanting. In the Arcidce also, it is only slightly 
developed. 

The Septibranchia [Poromya, Cuspidaria) are distinguished from all other 
Lamelliiranchia by the absence of coils, and the consequent shortening of the small 
intestine (cf. on the intestine of the Lamellibranchia, Figs. 24, 25, 26, 27, 28, 
pp. 16, 17, 18, 19). 

5. Cephalopoda. 

The stomach in the Cephalopoda always lies in the dorsal portion of the 
visceral dome in the shape of a sac with a strong muscular wall. It always has a 
ccEoal appendage (stomachal or spiral ccecum. Figs. 166, 160), which varies in shape 
and size ; into this the digestive gland (liver) opens. This ccecum is a reservoir for 
the secretion of the digestive gland. Food never enters it, there are even valves 
at the point of entrance into the stomach, which allow the secretion collected in 
the eoicum to pass into the stomach, but prevent the entrance of the contents of the 
latter into the ccecum. 

In Nautilus, the ccecum does not open into the stomach, but into the commence- 
ment of the small intestine, and is in the form of a small round vesicle with lamella; 
projecting into its lumen. In Sepia and Sepiola also, the coicmn is more or less 
round ; in Rossia, it is slightly developed ; in Loligo and Sepioteuthis, very long and 
ending in a point ; in all Oegopsidce and Octopoda, more or less spirally coiled at the 
blind end. 

The well-developed digestive gland seems to arise as a paired organ, even when 



VII 



MOLLUSCA—THE ALIMENTARY CANAL 



195 



unpaired in the adult. The whole of the much hranohed gland is surrounded by a 
common integument, and it thus outwardly appears to be compact. 

The digestive gland of Nautilus consists of five lobes (four paired and one 
unpaired), which lie around the crop. They have two ducts, which enter the crecum 
through a short common terminal portion. 

In the Dibranchia also, the digestive gland always lies on the ventral side of the 
stomach, close to the ascending oesophagus. 
It is undivided, and round or oviform in the 
Octopoda, Oegopsidce, and Sepiola. In Lolicjo 
and Scpioteuthis, it is traversed by the oeso- 
phagus and the aorta ; in Enoploteuthis, its 
dorsal half is cut into two points by these 
organs ; and the same is the case in Eossia. 
In Sepia and Spirula, the gland forms two 
lateral lobes which are distinct in Sepia, but 
connected along the middle line in Spirula. 

There are always two ducts (gall ducts) 
which rise near the median plane fi'om the 
upper part of the gland, and open into the 
stomachal coeoum separately or through a 
common terminal portion. 

The following facts have been ascertained 
as to the function of the so-called pancreas 
of the Cephalopoda. It is originally a 
specially differentiated portion of the diges- 
tive gland, and is easily distinguishable in 
the Octopoda by its different colour ; it lies 
near that part of the gland from which the 
ducts spring. In Loligo, the pancreas is 
found in the much thickened wall of the 
ducts themselves. In this case it consists 
of numerous glandular anastomosing out- 
growtlis of the epitheliiun of the ducts into 
their wall. In other Decapoda, these gland- 
ular outgrowths pass from the wall of the 
ducts into the surrounding body cavity, and 
then each duct appears throughout its whole 
length to be covered with acinose or ramified 
"pancreatic appendages." The pancreatic pylorus; 8. 

secretion contains diastase, and appears to ™ t^"^ ^ ^ ^^ ■ j. j.-^ , ■ , 

, » , p . . „ . , bag ; 7, aperture of the same into the hind- 

carry out only one part of the functions of the ^^^ 

digestive gland, viz. that part which corre- 
sponds with the digestive functions of the salivary glands in the higher Vertebrates. 
The small intestine, in which among all Molluscs the resorption of the digested 
food chiefly (if not exclusively) takes place, is short in the carnivorous Cephalopoda, 
and foi-ms several coils only in Tremoetopus violaceiis. 




Fig. 166. ^ Alimentary canal of Loligo 
sagittata (without pharynx and salivary 
glands) partly cut open (after Gegenbauer). 
1, Oesophagus ; 2, probe, inserted into the 
stomach ; 4, stomachal coBCum 
with spiral coeeum 5 ; 6, hind-gut ; S, ink- 



E. Hind-gut (Reetum). 

This is generally short in Molluscs. Where it is sharply marked 
off from the small intestine, it usually differs from the latter in being 
thicker and more muscular. 



196 COMPARATIVE ANATOMY chap. 

In the majority of Lamellibraiichs, and in nearly all Diotocardiu , the 
reetum traverses the ventricle ; this fact, with many others, supports 
the relationship of these two groups. 

In certain Molluscs, viz. the Scaphopoda, a few Prosobranchia 
(Mwicidw, Purpuridce), and the Cephalopoda, the hind -gut has an 
accessory (anal) gland, which is well known in the Cephalopoda as the 
ink-bag. 

The rectal gland iu Dentalhim is a branched acinose gland opening into the hind- 
gut, according to one account through six separate ducts, and according to another 
through one single duct. Eggs and spermatozoa have been met with in the lumen 
of this gland, and it has been supposed that they have been accidentally drawn out 
of the mantle cavity by the swallowing-like action of the hind-gut, which has been 
observed in Dentalium. 

The anal gland found in some Scichiglossa {Monoceros, Purpura, Murex) is always 
dark in colour (brown or violet), and is either tubular with many bulgings of its 
wall, or acinose with an axial duct. It always enters the hind -gut near the 
anus. 

A gland has been found near the rectum in the Pteropoda thecosomata (Clio, 
Cavolinia) and the Bulloida, and has been described as an anal gland, but this 
requires further investigation. 

The ink-bag of the Cephalopoda (Fig. 167), which is wanting only 
in Nautilus, is a much developed anal gland. It enters the hind-gut 
near the anus. The ink or sepia pigment secreted by it consists of 
extremely minute particles which are ejected with vehemence from 
the bag and discharged through the funnel. The pigment quickly 
mixes with the water, and envelops the animal in a pigment cloud, 
thus screening it from its enemies. 

Form and position of the ink-bag (c/. Figs. 160, p. 189 ; 177, p. 213 ; 178, 
p. 214). — The typical position of the ink-bag is in front of the rectum, i.e. in the 
loop formed by the intestine in ascending from the mouth and then descending to 
the anus. In Spirula, Enoplotevthis, and Sepioteuthis, the ink-bag is very small ; 
it progressively increases in size in series both of Decapoda and of Octopodn, its 
division into a saccular portion and a duct opening into the hind-gut in front of the 
anus becoming more and more distinct. In the Octopoda, it lies embedded in the 
upper part of the liver within the muscular hepatic capsule (cf. p. 128). It is still 
found in this position (between the liver and the rectum) in Sepiola. In other 
Decapoda, hoAvever, the ink-bag is found shifting higher and higher in the visceral 
dome, its duct at the same time increasing in length. Finally, in Sepia (and the 
fossil mhrandiia), it is found at the top of the visceral dome, behind the gonad. Its 
duct runs along the right side of the hind-gut, bending round somewhat before 
reaching the anal section of the rectum so as to enter the latter anteriorly. Onto- 
genetically, however, even in Sepia, the ink-bag arises as an anterior outgrowth of 
the rectum. 

Structure of the ink-bag in Sepia (Fig. 167 A). — The ink-bag in this instance 
consists of three parts: (1) the pigment gland which secretes the "ink" ; (2) the 
pigment reservoir and the duct, which forms (3) an ampulla with a glandular «all near 
its aperture. The pigment gland is a sac at the base of the ink-bag on its anterior 
wall (that turned towards the gonad). It projects into the cavity of the ink-bag, 



VII 



MOLLUSGA~THE ALIMENTARY CANAL 



197 



which serves as reservoir and duct for the pigment, 
in the gland, passes through an 
aperture in its wall into this 
reservoir. The cavity of the 
gland is traversed by numerous 
perforated and richly vascular- 
ised lamellas of connective 
tissne, which are inter - con- 
nected in such a way as to form 
a kind of sponge-like structure. 
New lamelljB are continually 
being put forth by the formative 
zone of the gland, which is a 
narrowed portion bent back 
downwards, while the oldest 
lamellfe, which lie nearest the 
aperture of the gland, become 
detached and degenerate. All 
the lamellse are covered by a 
glandular epithelium and the 
formation of the pigment can 
be traced in all its stages from 
its appearance in the epithelial 
cells of the formative zone to 
its condition in those of the 
oldest lamellae. In the forma- 
tive zone, the young glandular 
cells are at first colourless. In 
the succeeding lamellae, how- 
ever, pigment granules increase 
in number and from the older 
lamellae are emptied into the 
cavity of the gland, the epi 
thelial cells then becoming 
detached and breaking up. 

Both the gland and the reser- 
voir are surrounded by a vascul 



The latter, after being formed 




Pio. Iij7.— Morphology of the pigment gland (ink-bag) of 
the Cephalopoda (after P. Girod). A, Median longitudinal 
section through the ink-bag of an adult, c. Anus ; 1, terminal 
portion common to the rectum (2) and the duct of the inlc- 
bag ; 3, ampulla ; 4 and 5, sphincter muscles of the ampulla ; 
6, duct of the ink-bag ; 7, pigment reservoir ; S, opening of 

arised integument of connective ""* pigment gland into the reservoir ; 9, portion of the gland 

tissue ; 



,1 . , , traversed by lamelliie ; 10, formative zone of tlie lamellae. 

tne same integument ^^ various stages in the development of the pigment 

forms the framework of con- giand ; B, anal papilla ; 0, inv.igination in the same ; D, ap- 

nective tissue running through pearance of two new depressions at the base of C ; these 



increase in depth, the one becoming the pigment gland b, the 
other the rectum 2. In F, the formative zone has appeared 
in the gland, in G, the first lamellfe and the duct. H, I, K, 
changes in the relative positions of the rectum and gland in 
the course of development, seen from the posterior (mantle) 
side. In H, tlie rectum lies behind the ink-bag. In I, the 
latter has shifted, and in K lies behind the rectum (on the 
mantle side). 



the lamellfe or trabeculse within 
the gland. 

The ink-bag is further envel- 
oped as a whole in a tough integu- 
ment consisting of three layers : 
(1) an inner glittering silvery 
layer (argentea), similar to the 
corresponding layer in the outer integument ; (2) a central muscle layer (inner 
longitudinal and outer circular muscles ; and (3) an external layer of connective 
tissue. 

The terminal ampulla has, at its two narrow ends, folds projecting inward and 
functioning as valves ; it can be closed at these parts by sphincter muscles. The 



198 COMPARATIVE ANATOMY chap. 

ampulla itself also forms longitudinal folds on its inner surface, between which 
glandular tubes open. 

The anus, in the Cephalopoda, always carries two lateral projecting appendages, 
which are often lancet-shaped. 

The short and narrow hind-gut of the Sulenogastres opens into the 
dorsal portion of a cavity, the eloaea, which lies at the posterior end 
of the body ; this, again, communicates with the exterior by means of a 
ventral and very extensible longitudinal slit. Into this cloaca the 
ducts of the genital organs, which are morphologically to be regarded 
as nephridia, also open. 

In the LameUibrandtia, after the hind-gut has traversed the heart, 
it runs straight backward over the posterior adductor, to open through 
the anus into the posterior and upper portion of the mantle cavity 
(anal chamber). 

On the position of the anus, cf. Section V. on the arrangement of 
the organs in the mantle cavity. 



X"\'II. The Circulatory System. 
A. General. 

All Mollusca have a circulatory system ; in some divisions, 
especially in the Cephalopoda and some Pivsobranchia, this may attain a 
high level of complication by the development of a closed arterial and 
venous vascular system. The heart, as the central organ of propulsion, 
is never wanting. It lies enclosed in the pericardium, a division of 
the secondary body cavity ; its primitive position is median, above 
the hind-gut. In the Lamellibranchia and Diotocardia, it is traversed 
by the hind-gut, in other Gastropoda it lies near it. It is always 
arterial, i.e. it pumps the blood flowing from the respiratory organs 
back into the body. 

In those symmetrical Molluscs in which the dorsal portion of the 
body rises as a high visceral dome, the intestine first ascending into 
the dome and then descending to the anus, the heart comes to lie 
behind the hind-gut (Dentalium, C'ejjhalopioda). 

In asymmetrical Gastropoda, its position depends upon that of the 
pallial complex. Where the hind-gut and anus have shifted with the 
pallial organs to the anterior side of the visceral dome, the heart also 
lies anteriorly {Prosolrancliia, Pulmonata, a few Tedibranchia). 

The heart gives rise, as a rule, to two large arteries (aorta), one 
of which runs to the head, the other to the visceral dome, to suppl}' 
blood to the viscera. Not infrequently they leave the heart as one 
large vessel. Where the circulatory system is not closed, the arteries 
sooner or later convey the blood to the primary body cavity or coelom, 
i.e. into the lacunar system. The venous blood is sometimes conveyed 
along distinct vessels, sometimes in channels without proper walls into 



VII 



MOLLUSGA—THE OIRGULATORY SYSTEM 



199 



the gills, where it becomes arterial and flows back through the auricles 
(atria) into the heart. 

There is, typically, one pair of auricles, one on each side of the 
ventricle. This is the case in all Molluscs provided with two sym- 
metrical gills. The arterial blood flows out of the left gill into the left 
auricle and thence into the ventricle, and out of the right gill into the 
right auricle and thence into the ventricle (Diotocardia, Zeugobranchia, 
Lamellih-ancMa, Cephalopoda JDibranchia). Again, where a longitudinal 



1- 



7— - 




Fia. 168.— A-H, Diagrams illustrating the relation between tlie otenidia, the heart, and 
the aorta. A, OUton ; B, LameUihranchia ; C, Dibranohiate Cephalopoda ; D, Tetra- 
branchiate Cephalopoda ; E, Prosobranohia Diotocardia Zeugobranchia ; F, Prosobranohia 
Diotocardia Azygobranchia ; G, Prosobranohia Monotooardia ; H, Opisthobranchia Teoti- 
branchia. 1, Ventricle ; 2, 3, 2a, 2b, 3a, 3b, auricles ; 4, vena braiicliialis = efferent branchial vessel ; 
5, aorta ; 5a, aorta cephalica ; 5&, aorta visceralis ; 6, aorta posterior vel superior ; 7, ctenida. 

row of numerous gills is found on each side in the mantle furrow 
(Chitonidce), the heart lies posteriorly above the hind-gut, and has one 
auricle on each side of the ventricle. This fact appears quite as much 
to support the view that one pair of gills and one pair of auricles w^ere 
present in primitive Molluscs, as does the arrangement in Nautilus 
{Cephalopoda Tetrahranchia) the other view, that there were two pairs 
of gills and also two pairs of auricles. 

In the majority of Gastropoda, where one of the two original gills 
has disappeared, the auricle belonging to it has usually also disappeared. 



200 COMPARATIVE ANATOMY chap. 

The original right gill and right auricle are usually retained in Gastro- 
pods with shells dextrally twisted. In Gastropods with a true sinistrally 
twisted shell, the left gill and left auricle are retained. 

There is, however, a whole division of the Prosolranchia, the 
Diotocardia, in which both auricles are retained. It is evident that the 
gills are more liable to disappear than the auricles, since in some 
groups both auricles remain when one gill has disappeared (for 
details see opposite page). 

When, in Gastropoda with only one auricle, the pallial complex has 
shifted to the anterior side of the visceral dome, the respiratory organs 
lie in front of the heart, and the single auricle in front of the ventricle 
(FrosobraiKhia, Monotocardia, most Pulmonata, a few Opisthobranchid). 
In those Gastropoda, however, in which the pallial complex lies on one 
(usually the right) side of the body, the gill is placed behind the heart 
and the auricle behind the ventricle. This is the case in nearly all 
the OpisthobrancMa. In a few Pulmonates also, such as Testacella, 
Oncidium, etc., the auricle lies behind the ventricle, as a consequence 
of special organic modifications. 

The blood, or rather the h^molymph, is a fluid rich in dissolved 
albumen (hsemocyanine), which assists in nourishing the body and in 
respiration. Amoeboid cells, the lymph cells or amoebocytes, are 
suspended in the hsemolymph. Haemoglobin is occasionally found 
dissolved in the hsemolymph or combined with special blood corpuscles. 
The Ij'mph cells either become detached from the wall of localised 
blood-making glands, which may vary in position, or, in a more 
diffused manner, from large vascular areas. They seem, from their 
origin, to be cells of connective tissue. 

The walls of the heart and of the walled vessels consist of smooth 
muscle fibres thickly felted, and (on the heart) of an external endo- 
thelium which belongs to the pericardium. An inner endothelium is 
wanting, so that the muscle fibres are directly bathed by the blood. 

The wall of the ventricle is always more muscular than those of 
the auricles. At the point where the auricles open into the ventricle, 
valves projecting into the lumen are always found, which, when the 
latter contracts, prevent the return of blood into the auricle. Besides 
these atrio-ventricular valves, there are occasionally other valves 
between the ventricle and the aorta. Valves may also occur in the 
peripheral blood channels, when these form contractile enlargements 
{e.g. the valve between the branchial heart and the afferent branchial 
vessels of the Cephalopoda). 

In various Gastropods and in Chiton a network of ganglion cells 
and nerve fibres has been found in the wall of the heart, innervated 
by two nerves of different origin. The nerve which runs to the 
ventricular plexus originates, in the Prosobranchia, in the left parietal 
ganglion, that running to the auricle from the left parieto-visceral 
connective. Where there are two auricles, they are innervated from 
the branchial ganglia. 



VII M0LLU8GA—THE CIRGULATORY SYSTEM 201 



B. Special. 

1. Amphineura. 

a. Chitonidse (Polyplacophora). — The heart is symmetrical, with two lateral 
am'icles. 

The ventricle and the two auricles are long tubes. The auricles are iu open 
communication with the ventricle about the middle of their length. Besides this, 
the two auricles pass into one another posteriorly, the posterior end of the ventricle 
also opening into them at this point. 

The ventricle lies against the dorsal wall of the pericardium, to which it is 
attached by a median band of endothelium. The ventricle passes into an aorta 
which allows the blood to flow into the oojlom through apertures in its wall. With 
the exception of the pedal arteries, the rest of the circulatory system is lacunar ; 
there are no vessels with walls of their own. 

The venous blood is collected from the lacunar system of the body (primary 
ccelom) into longitudinal channels which run on each side under the pleurovisceral 
cords. From these channels it flows into the gills, where it becomes arterial, and 
returns through other longitudinal channels which run above the pleurovisceral 
cords. Two transverse channels in the region of the heart (c/. Fig. 51, p. 40) 
convey the arterial blood into the auricles. 

The two pedal arteries lie laterally and ventrally with regard to the pedal cords ; 
they probably draw their blood from the aorta and pass it on to the lacunar system 
of the foot. 

b. Solenogastres. — The heart lies above the hind-gut on the dorsal side of the 
pericardium. It does not lie freely in the latter, nor is it suspended by an 
endothelial band, but simply projects into the pericardium from above, so that only 
its under surface is covered by the pericardial endothelium. The pi-esence of two 
auricles has not been proved. The rest of the circulatory system is purely lacunar. 
Specially large blood channels lie in the depths of the principal septa which project 
into the mid-gut, and bulge these out. Large blood sacs are also occasionally found 
in folds which project into the pharyngeal cavity from its wall, and there are more 
or less large sinuses in the folds, which, in Neomeiiia and Chcetoderma, project 
into the cloaca and may be regarded as gills. In all these parts the intestinal 
epithelium separating the sinus from the intestine is ciliated, and respiration no 
doubt takes place. 

2. Gastropoda. 

Relation of the auricles to the ventricle. — The lowest Gastropods, i.e. the 
Diotocardia among the ProsohrancMa, have a heart with two auricles. This is not 
only the case in the ZeugobraiwMa (FissurcUa, Haliotis, etc.), which have two gills, 
but also in the Azygobranchia (Turbinidm, Trochidcs, Neritid,m), in which only the 
left (originally the right) gill has been retained. No branchial vein then enters the 
smaller (rudimentary) auricle on the right, the veins having atrophied with the gill. 
In the Zeiigobranchia, the long ventricle lies in a line with the hind-gut, which runs 
length-wise through it. In the Azygobranchia, the ventricle lies transversely with 
respect to the hind-gut which runs through it, the left auricle lying in front of the 
ventricle, and the right auricle behind it. The left branchial vein enters the 
anterior (left) auricle. If we suppose the posterior (right) auricle to have disappeared 
altogether, as is the case in all other Gastropoda, the heart consists of a ventricle 
and one auricle lying in front of it, which receives the branchial or pulmonary vein 
from the gill or lung in front of it. 



202 COMPARATIVE ANATOMY chap. 

This serial order of the ventricle, auricle, branchial or pulmonary vein and 
respiratory organs is characteristic of the AsijgobrancMa, Monotocardia, and most 
Pidmonata. 

The Docoglossa (Patella and allied forms) have only one auricle ; the ventricle 
in Patella (not in Acmcea), however, is divided into two parts. 

Among the Mmotocardia, only C'yprma (as far as is at present known) has a 
rudimentary right auricle, closed on all sides except at its aperture into the 
ventricle. 

Among the Pulmonata there are forms in which the auricle lies behind the 
ventricle. This must be regarded as a secondarily acquired position, determined by 
the shifting back of the anus and the mantle cavity to the posterior end of the body 
( TestoJitlla, Oncidium). In Daudebardia, the auricle still lies in front of the ventricle; 
nevertheless this genus, like several other shell-less Pulmonates, is opisthopneumonic, 
i.e. its respiratory network lies chiefly behind the heart. In Testacella, the auricle 
also lies behind the heart {cf. p. 77). 

In the Opistliohranchia, the auricle lies behind the ventricle ; this is connected 
with the position of the gill at the posterior end of the body, or where no true 
ctenidium is found, but whei'e respiration takes place by means of anal gills, or 
dorsal appendages, or through the integument, with the point of entrance of the 
branchial vein into the heart from behind. 

In a few Tectihranchia, e.g. Actmon, Accra, Gastroptcron, the gill lies some- 
what far forward, and the auricle is then placed laterally, to the right of the 
ventricle rather than behind it. 

It is of great importance, with regard to the position of these 
organs in the Lamellibranchia, to note the fact that, in many Dioiocardia 
{e.g. Fissurella, Haliotes, Turbinidm, Trochidce, Neritidce, Neritopsidce, etc.) 
the ventricle is traversed by the hind-gut, while in all other Gastropods 
the intestine merely runs past it. 

Circulation, (a) Prosobranchia. — A large vessel, the aorta, springs from the 
ventricle. This soon divides into two branches : (1) the anterior or cephalic aorta 
(A. cephalica), and (2) the posterior aorta (A. visceralis). 

The anterior aorta conveys blood to the anterior part of the body (head, pharynx, 
proboscis, oesophagus, stomach, copulatory organs) and to the mantle, and gives off 
among others the important arteria pedalis ; this latter soon breaks up into 
separate arteries, which run longitudinally through the foot. In some cases the 
cephalic aorta is richly branched, breaking up into numerous fine vessels which 
spread out in and on the above-mentioned organs ; in others, the arteries, without 
branching, open into arterial sinuses. Among these, the large cephalic sinus into 
which the anterior aorta opens {e.g. in Haliotis) deserves special mention. Where 
the cephalic aorta runs beyond the cesophageal ring formed by the central ganglia 
and their commissures, it passes through this ring. 

The aorta visceralis supplies the organs which lie in the visceral dome, especially 
the digestive gland, the genital glands, and the mid-gut. 

The venous blood collects in the lacunar spaces of all parts of the body, and 
flows into a large venous sinus, i. e. into the space in which the stomach, salivary 
glands, intestine, digestive gland, and genital organs lie. This space or primary 
body cavity is somewhat spacious round the stomach, but very limited in the 
visceral dome, where the lobes of the digestive gland, the walls of the intestine, 
and the genital glands with their accessory parts are so crowded together as to leave 
very narrow spaces between them. 



VII 



MOLLUSGA—THE CIRCULATORY SYSTEM 



203 



The blood passes out of the large venous sinus back into the heart by three 
channels. 

1. A large part of it flows through laounfe or vessels into the paired or unpaired 
branchial artery (afferent branchial vessel). In the course of branchial respiration 
the blood becomes arterial, and collects in an eflferent branchial vessel (cf. section on 
the respiratory organs, p. 84), which, as branchial vein, conducts it to the auricle 
of the heart. "Where there are two gills, there are naturally two branchial arteries 
and two branchial veins, the latter conducting the arterial blood to the two auricles. 

2. Another part of the venous blood flows through the kidney, then again 
collects in lacunse or vessels which lead to the gills, and finally reaches the heart 




Fig. 169.— CSroulatory system of Faludina vivipara (after Leydig). The animal is seen from 
the left side. 1, Eye ; 2, cerebral ganglion ; 3, efferent branchial vessel (branchial vein) ; 4, gill 
(ctenidlum) ; 5, afferent branchial vessel ; G, kidney ; 7, aorta visceralis, winding up close to the 
columella; 8, ventricle; 9, auricle; 10, aorta cephalica ; 11, venous sinuses of the body; 12, 
auditory vesicle ; 13, pedal ganglion. 



through the branchial veins. Less frequently, the veuous blood, after passing 
through the kidney, enters the auricle more or less directly, i.e. without passing 
tlirough the gills, and there mixes with the arterial blood coming from the gills. 

3. A certain part of the venous blood, passing by both the kidney and the gill, 
flows direct into the branchial veins leading to the auricle. 

The arterial blood in the heart is thus mixed with venous blood. 

(6) Pulmonata. — (Examples: Helw pomatia, Limax, Figs. 170, 171, 95, p. 100). 
The blood vascular system is like that of the Monotocardia. The only important 
deviation is caused by the occurrence of pulmonary respiration. Various veins col- 
lect the venous blood out of the large body sinus and the lacmiar system, and unite 
to form one large vein, which accompanies the hind-gut, and, as vena circularis, 



204 



COMPARATIVE ANATOMY 



CHAP. 



runs along the thickened edge of the mantle which concresces with the nuchal integu- 
ment. From this vein spring numerous venous vessels which spread out on the 




.•#»!**..^V- 



FiG. 170. — Pulmonary veins, heart, and arterial system of Helix (after Howes). Tlie mantle 
(roof of pulmonary cavity) is cut open and turned back. 1, Pulmonary vein (efferent pulmonary 
vessel); 2, kidney; 3, auricle; 4, ventricle; 5, rectum, cut through; 0, hermaphrodite gland; 
7, columellar muscle ; S, aorta vi^ceralis ; P, salivary glands ; 10, aorta cephalica. 

nnder surface of the mantle, i.e. on the roof of the mantle cavity, and there form a 
delicate respiratory network. In this network the blood becomes arterial, and is 



^Z Ax 




Fiu. 171. —Vascular system of Limax, after drawings combined by Leuckart fiom Delle CMaje 
and Simroth. The veins carrying tlip venous blood out of the body into the lungs are black. 
A, auricle ; V, ventricle ; Vli, venous circular sinus of the pulmonary cavity ; Ax, aorta cephalica ; 
Ay, aorta visceralis ; M, muscular stomach ; ZD, hermaphrodite gland ; //, digestive gland ; /, in ■ 

testine ; AL, respiratory aperture ; ,Y, arteria genitalis. 

next conducted through many vessels into the large pulmonary vein (vena pulmon- 
aris), which runs back almost parallel to the rectum along the roof of the mantle 



VII MOLLUSGA—THE GIRGULATOBY SYSTEM 205 

cavity, to enter the auricle. The vessels of the respiratory network form projecting 
ribs on the surface of the mantle. The pallial epithelium in the mantle cavity is 
ciliated. 

The efferent pulmonary vessels, which, near the kidney, run along the right side 
of the pulmonary vein, first enter the kidney and break into a fine vascular network 
before passing into that vein. 

The cephalic aorta does not pass through the oesophageal ring, but runs between 
the pedal and visceral ganglia ; this is said to be the case in most Opisthobranchia. 

In Opisthopnenmonic Pulmonata {e.g. Daudehardia, Testacella), in which the 
small or rudimentary visceral dome has shifted to the posterior end of the body, and 
the organs elsewhere found in the dome (liver and genital organs) now lie in the body 
cavity above the foot, and thus in front of the posteriorly placed heart, the posterior 
aorta (A. visceralis) is much reduced, but the anterior aorta (A. cephalica) is strongly 
developed. The posterior aorta supplies only the posterior lobes of the liver and the 
hermaphrodite gland, and the anterior aorta (cephalic aorta, A. ascendens) has thus 
to supply the anterior lobes and even part of the genital organs, which usually receive 
their blood from the posterior aorta. 

In Oncidium, there is an arteria visceralis corresponding with the posterior 
aorta, which branches off soon after the aorta leaves the heart, but it here rims 
anteriorly. 

(c) Opisthobranchia. — Here also the ai-rangement is essentially the same as in 
the Frosobranchia, though modified by the different position of the gills, as has been 
already briefly noted. 

Gastropteron attbrds a good illustration of the circulatory system of tlie Tecti- 
branchia. The heart, which is enclosed in a spacious pericardium, lies to the right, 
in front of and above the base of the gill. It lies transversely, the larger and more 
muscular ventricle to the left, the auricle to the right. Out of the ventricle spring.s 
the aorta, which at once divides into a posterior and an anterior aorta. The anterior 
aorta enters the cephalic cavity, giving off as its principal arteries : (1) the artery of 
the copulatory organ. (2) The two large pedal arteries, each of which again soon 
divides into two branches, viz. (a) an anterior artery, which branches richly in the 
parapodia ; (6) a posterior artery, which runs back on each side parallel to the 
median line of the foot. (3) The arteries of the cephalic disc. (4) The arteries 
of the oesophageal bulb and of the cesopliagus. (5) The anterior end of the aorta 
itself branches in the tissues surrounding the mouth. The following are the chief 
branches of the posterior aorta : (1) The gastric artery. (2) The hepatic arteries. 
(3) The genital arteries. The venous blood flows back from all parts of the body 
through richly - branched channels into two large venous sinuses, one of which 
represents the cephalic and the other the body cavity. "Wide but sliort vessels 
convey the venous blood out of these sinuses into the kidney, which contains a 
rich venous lacunar system. From the kidney it flows direct into the afferent 
branchial vessel, becomes arterial in the gills, and collects in the efferent branchial 
vessel, which, as the branchial vein, soon enters the auricle. 

All the venous blood in Gastropteron, therefore, on its way back to the heart, 
passes first through the kidney and then through the gill, so that only arterial blood 
flows through the heart. 

This is, however, not by any means the case in all Tectibranehia. For examiile, 
in Pleurobranchus, a large part of the venous blood passes from a dorsal circular 
sinus through a very short but wide passage direct into the branchial vein close to 
its point of entrance into the auricle, passing by both the kidney and the gill. 

Dorididse. — "Without going into details as to the circulatory system of this group, 
it may be mentioned that part of the venous blood passes directly through two 
lateral vessels into the auricle. Another part flows into an inner venous circumanal 



206 COMPARATIVE ANATOMY chap. 

sinus, whicli lies at the base of tlie circle of gills. From this the blood rises into the 
gills, becomes arterial, flows back into an outer circumanal vessel, and thence back 
through the branchial vein into the auricle (Fig. 93, p. 98). 

Nudibranchia. — The heart, enclosed in the pericardium, almost always Ues in 
front of the centre of the body, in the median line. The aorta, which springs from 
the ventricle, divides into an anterior and a posterior aorta, each of which breaks up 
into an arterial system, the arteries having walls of their own. The finer branches of 
these arteries open into the lacunar system of the body, which occasionally fonns 
canals resembling vessels, and is connected with the large cephalic and visceral 
sinuses. Veins, apparently with walls of their own, run from the lacunar system of 
the dorsal appendages or the integument, and carry the arterial blood back to the 
auricle. The blood usually finally enters the heart through three "branchial" veins, 
— two lateral and one median posterior, — Avhich open into the posteriorly-placed 
auricle. 

3. Scaphopoda. 

The circulatory system of Dentaliuin, but for the recently-discovered rudimentary 
heart, is entu'cly lacunar, consisting of systems of canals, sinuses, and spaces, the 
special arrangement of which cannot here be described. 

The pericardium with the heart lies on the posterior side of the body, dorsally to 
the anus. If we imagine the intestine of Dentaliuin straight and horizontal, the 
heart would occupy the typical position on the dorsal side of the hind-gut. It has 
no auricles, and is merely a sac-like bulging into the pericardial cavity of its 
anterior wall. It is connected by fine slits with the suiTounding sinuses of the body. 

4. Lamellibranehia. 

The Heart. — In nearly all bivalves, the heart, which is traversed by 
the hind-gut, possesses tvi^o lateral auricles, and lies in a pericardium. 

There are, however, isolated exceptions to this rule. In Nucula, 
Area, and Anomia, the ventricle lies over (dorsally to) the hind-gut. 
This dorsal position must be regarded as the primitive position of the 
Lamellibranchiate heart, since the above genera are among the most 
primitive bivalves, and, further, since the heart of the Amphineura, the 
Scaphopoda, and the Cephalopoda also lies over or behind the hind-gut. 
The perforation of the heart by the hind-gut must have arisen by the 
bending of the ventricle down round the latter. 

The heart in the above-mentioned genera is further distinguished by the fact 
that the ventricle is more or less elongated in the transverse direction, its lateral 
ends being swollen, while the central part, which Ues above the intestine, becomes 
narrower and thinner. This modification goes furthest in Area Nom, where there 
seem to be two lateral ventricles unconnected by a central portion. This separation 
of the ventricle into two lateral parts has here brought about a separation of the two 
aorta. The two anterior as well as the two posterior branches, however, after a 
comparatively short separate course, unite to form an unpaired anterior and an un- 
paired posterior aorta. 

Although these genera have, as a rule, a heart lying above the hind-gut, in some 
specialised forms the heart is placed under the hind-gut, e.g. Meleagrina, Ostrea, 
Teredo. The cause of this modification must lie in the increasing distance between 
the base of the gills and the original region of the heart, the auricles and the ventricle 
having shifted with the latter. The auricles, however, no longer lie laterally to the 



VII 



MOLLUSGA—THE GIRGULATOBY SYSTEM 



207 



ventricle, but arc drawn down to its lower side, where they grow together, communi- 
cating through a more or less large aperture. Pinna, Avimila, and I'erna exhibit 
the consecutive stages in the displacement of the heart to the lower side of the hind- 
gut. The shifting of the gills from the original region of the heart just mentioned 
is caused by the shifting forward of the posterior adductor, which grows more and 
more massive and finally reaches a median position on the shell valve. It has already 
been mentioned that this posterior adductor, by the continuous reduction and final 
disappearance of the anterior adductor, becomes the one adductor of the Mono- 
myaria. 

In Teredo also, the heart lies on the under side of the hind-gut. This is con- 
nected with the approximation of the hind-gut with the anus to the mouth dorsally. 





Fig. 172. 



Fio. 173. 



Fig. 172.— Transverse section through Anodonta, to Illustrate the course of the circulation 
of the gills and the kidneys, and the branchial veins (after Howes). Ir, Gills ; hre, eflerent 
branchial vessel (branchial vein) which opens into the large branchial vein brej, running along the 
base of the gills, and here cut through transversely ; ^v, pallial vein ; vc, large venous sinus of the 
body;; fc&, pericardial gland ; au^, auricle ; rj, rectum ; v, ventricle ; re and rci, renal vessels ; hra-i, 
afferent branchial vessel (branchial artery), running along the base of the gills ; &j'a, lateral branches 
of the same running in the gills. The veins or sinuses conveying venous blood are black. 

Fig. 173.— Another section through Anodonta (after Howes). Lettering as in Fig. 172. tiu, 
auricle ; s&c, spaces at the base of the gills, bathed by the water and communicating with the 
mantle cavity, between the ascending and descending branchial lamella. 

while the gills, remaining in their original position, retain the heart on the lower 
side of the hind-gut. 

Circulation (Fig. 25, p. 17). — The arteries have walls of their own, and branch 
into fine vessels, which discharge the blood into the lacvmar system of the body. 
The venous system seems to have no distinct vessels with walls of their own, 
although it forms more or less wide channels resembling true vessels. 

An anterior and a posterior aorta spring, as a rule, from the ventricle. The 
anterior aorta runs forward above the intestine and breaks up into various arteries. 
The arteria visceralis supplies the intestine, the digestive gland, and the genital 
gland ; the pedal artery .supplies the foot ; the anterior pallial artery spreads out 
over the anterior part of the mantle and the oral lobes (labial palps). 



208 COMPARATIVE ANATOMY chap. 

The posterior aorta leaves the ventricle posteriorly and runs along the lower side 
of the hind-gat. It soon divides into two large lateral arteries, — the posterior pallial 
arteries. The principal branches of tlie anterior and posterior pallial arteries run 
along the free edge of the mantle on each side and then unite, forming together the 
arteries of the pallial edge. From the roots of the jiosterior pallial artery smaller 
arteries spring, which supply with blood the hind-gut, the pericardium, the posterior 
adductor, the retractors of the siphons, etc. The venous blood is collected out of 
the lacunar system of the body through converging channels into one longitudinal 
venous sinus ; this lies under the pericardium (Fig. 172). 

From this sinus, the greater part of the blood flows through the complicated 
system of venous canals in the kidneys, after which it is collected on each side into 
a branchial artery which runs along the base of the gills, and thence enters the 
two branchial lamellse. It becomes arterial through respiration in the gills, flows as 
arterial blood into a branchial vein parallel with the branchial artery, and thence 
into the auricle. 

Part of the venous blood, however, passes by direct channels out of the venous 
sinus into the branchial artery (passing by the kidneys), and part even flows direct 
into the pericardium. In this way some venous blood comes to be mixed with the 
arterial blood flowing through the heart from the gills. 

Not all Lamellibranchia have an anterior and a posterior aorta springing out 
of the heart. In the lower groups of the Protohranchia and Filihranchia there are 
numerous forms [Ahicuht, SoUnomya, Anomia, Mytilidce) in which only one anterior 
aorta leaves the ventricle ; this soon, however, gives off the arteria visceralis, which 
supplies blood to those parts which, in other Lamellibranchia, are fed by the aorta 
posterior. In their possession of a single aorta rising from the ventricle, the above 
lower Lamellibranchiates agree with Chiton and the Gastropoda. The rise of this 
aorta from the posterior end of the ventricle in the Prosobrajichia and in most 
Pulmonata is a secondarily acquired arrangement, caused by the shifting forward 
of the pallial complex. 

It must further be noted that in a very specialised bivalve. Teredo, the posterior 
aorta fuses with the anterior, and thus the two leave the heart as one vessel. 

In those Lamellibranchiates which have siphons, a muscular and contractile 
widening occurs in the posterior aorta near the point where it leaves the ventricle ; 
this is called the bulbus arteriosus. Its special function is perhaps that of bringing 
about pressure of blood, to assist in the extension of the siphons. The backward 
flow of the blood into the ventricle in the contraction of the bulbus arteriosus 
(systole) is prevented by a linguiform valve which projects from its anterior wall. 

5. Cephalopoda. 

Heart (Figs. 127, 168, pp. 147, 199, and 174).— We must here again point out the 
important fact that Nautilus has a heart with four auricles, while the Decapoda and 
Octopoda a heart with only two auricles. This difference is connected with the 
difference in the number of the ctenidia : four in Nautilus {Teirabranchia), two 
in the Decapoda and Octopoda (Dibranchia). 

In Nautilus, the heart is an almost square sac drawn out to two points on eacli 
side ; the four auricles which open into the four points of the ventricle are lonf 
tubes, more like widened branchial veins than auricles. 

The strongly muscular ventricle of the Dibranchia is almost always elongated 
into a tube. In the Octopoda it lies transversely, the two auricles being in the same 
plane with the ventricle. In the Ocgopsidm, the ventricle lies along the longitudinal 
axis of the body, i.e. it is elongated dorso-ventrally, and the auricles are at rio-ht 



VII 



MOLLUSGA—THE CIRCULATORY SYSTEM 



209 



angles to it. The heart of the Myopsidce occupies a position halfway between those 
just mentioned. 

The heart here described is the arterial heart, which corresponds with the heart 
of the other Mollusca. It is called arterial to distinguish it from the venous hearts, 
wliieh will be described below. 

Circulation. — It is important to note that the circulatory system is at least 
partially closed. There is not only a richly-branched arterial, but a richly-branched 
venous system, the vessels of which have walls of their own. These two systems 
pass into one another in certain parts of the body, e.g. the integument and certain 
muscle layers, through a system of capillary vessels. In other parts, however, the 
arterial branches conduct the blood into a lacunar system ; when it has become 




Fig. 174.— Circulatory system, venous appendages of tlie uepbridial system, and gills of 
Sepia officinalis, anterior view (after Hunter). 1, Aorta cephalica ; 2, ctenidiuni ; 3, vein leading 
to the ctenidium : 4, branchial heart ; 5, appendage of the branchial heart (pericardial gland) ; 
ti, venous appendages of the nephridial system ; 7, aorta abdominalis ; 8, Aona abdominalis ; 9, 
lateral veins ; 10, vena cephalica ; 11, auricles ; 12, ventricle (c/. Fig. ISG). 



venous, the blood collects out of this into sinuses (especially into a peripharyngeal 
cej)halic sinus), and flows to the gills through veins with walls of their own. 

Two aorta rise from the ventricle: (1) the aorta cephalica, which runs downward 
(upwards in the figure) to the head, and (2) the aorta abdominalis, which runs up 
towards the apex of the visceral dome. The former is much stronger than the latter. 
The aorta cephalica first gives off branches to the mantle and to the anterior wall of 
the body, and then provides the stomach, the pancreas, the digestive gland, the 
resophagus, the salivary glands, and the funnel with arteries. After accompanying 
the oesophagus, it divides in the head into two branches, which run to the bases of 
the arms, and there break up into as many arterise brachiales as there are arms. 

The aorta abdominalis supplies with arteries the liind-gut, the ink-bag, the 
genital organs, the dorsal part of the body wall, and the fins, when these latter are 
present. 

Only in the Oegopsidce are the aorta limited to the two, above described, springing 
from the heart. In the Odopoda and the Mycrpsida;, there are other arteries rising out 
of the ventricle, and running to the same part of the body as the aorta abdominalis 
VOL. II P 



210 COMPARATIVE ANATOMY chap. 

in the Oegopsidce ; among these are the arteria genitalis, which runs to the genital 
glands, and, in the Myopsidm, a fine vessel called the arteria anterior. 

At certain places, the arteries may swell out to form small muscular and con- 
tractile widenings, called peripheral arterial hearts. 

In the venous system of Sepia, the venous blood in each arm collects (partly 
through capillaries and partly through lacuuie) into a vein running down the inner 
side of the arm. All the brachial veins convey their blood to a circular cephalic 
sinus surrounding the buccal mass, which is the reservoir for collecting the venous 
blood from the whole head region. Out of this sinus springs the large vena 
cephalica, which runs up along the posterior side of the oesophagus and the liver 
into the visceral dome, collecting on the way venous blood from the liver, the funnel, 
etc. A little below the stomach it forks, forming the two venae cavse, which open 
into the two contractile venous hearts at the bases of the gills. From the upper 
part of the visceral dome the blood collects into several abdominal veins, the most 
important of which are an unpaired vena abdominalis, opening into the vena 
cephalica exactly at the point where it divides into the vense cavs, and two lateral 
abdominal veins, which open into the latter near their point of entrance into the 
branchial hearts. 

In the region of the heart, all these veins carry acinose or lobate appendages 
(venous appendages), which are hollow, and communicate at many points with the 
veins, so that they are richly supplied with blood. The cavity into which these 
appendages project is that of the renal sacs, and the epithelium which covers them 
belongs to the epithelial wall of the kidneys (cf. Fig. 186, p. 224). "VVe thus see 
that here the blood flowing back fi;om the body has abundant opportunity of giving 
off its excretory constituents to the kidneys. 

Appendages are found on both the branchial hearts ; these are the pericardial 
glands, which will be further described later. The two branchial hearts, by their 
contraction, drive the venous blood into the afferent branchial vessel. The blood, 
which has become arterial in the gills, flows through the efferent branchial vessel 
(the so-called branchial veins) into the auricles of the heart, and thence into the 
ventricle (on the branchial circulation, cf. p. 96). 

In the Cephalopoda, unlike the other Mollusca, the whole of the blood, in 
returning from the body, flows through the gills, so that the heart contains only 
arterial blood. By far the greater part of the blood, before entering the gills, comes 
into contact with the kidneys in the venous appendages. 

In the Ootopoda, the venous system shows some not unimportant modifications. 
In Octopus, two veins, connected with one another by anastomoses, run along the 
outer side of each arm and collect the venous blood. At the bases of the arms these 
veins become connected in pairs, and unite later in such a way as to form on each 
side a lateral cephalic vein. 

These two veins unite to form the large vena cephalica, which runs up in front 
of the funnel and behind the oesophagus. The brachial veins do not here, as in 
Sepia, convey their blood first to the venous cephalic circular sinus, but are directly 
connected with the cephalic vein. A cephalic sinus nevertheless exists in Octopus ; 
it is not, however, connected with the vena cephalica, but with a large sinus which 
fills the whole visceral dome, and is, in fact, the primary body cavity, in which 
the viscera lie bathed by the venous blood. The latter flows out of this lar^e 
venous sinus through two wide veins, the so-called peritoneal tubes, into the 
upper part of the vena cephalica, near the point where this divides into the two 
venas cava. 

Nautilus is chiefly distinguished by the absence of the branchial hearts. 
Further, each of the two venae cavaa divides into two branches which run as 
afferent vessels, to the gills. 



VII MOLLUSCA~THE BODY CAVITY 211 

XVIII. The Body Cavity. 
Primaxy and Secondary Body Cavity, Pericardium, Pericardial Gland. 

The Mollusca are said to have a primary and a secondary body 
cavity. The former is the system of lacunae and sinuses, into 
which the arteries open, and out of which the veins, where these are 
present, draw their blood. It has no epithelial walls of its own, its 
boundaries are formed by connective, nerve, or muscle tissue, or by 
epithelia, which, however, belong to other organs, such as the intestine, 
the kidneys, or the body wall. 

The so-called secondary body cavity or coelom is, in most Mollusca, 
very much reduced, usually consisting of only two small cavities, the 
perieardium and the cavity of the gonads (testes, ovaries, or her- 
maphrodite glands). The coelom is always lined by an epithelium of 
its own, the coelomic epithelium, and corresponds with the true coelom 
of the Annelida, which also possesses such an epithelium. Like the 
latter, it is connected, by means of the nephridial funnel, with the 
nephridia, which lead to the exterior, and in Molluscs are usually 
found only in one pair. A probe can therefore be introduced through 
the kidney into the coelom, i.e. into that part of it which, containing 
the heart, is called the pericardium. The germinal layers must be 
considered as proliferations of the coelomic endothelium. The epi- 
thelium of the pericardium is, in very many Molluscs, differentiated 
into glands, called the pericardial glands ; these probably may be 
classed together with the kidney as excretory. 

We should be justified in assuming, a priori, that the lumen of the 
genital glands of the Mollusca is part of a true coelom, and that 
the germinal layers themselves, i.e. that complex of cells which yields 
the eggs and spermatozoa, are outgrowths of the endothelial wall of 
this coelom. Direct support is, however, given to this assumption by 
the fact that in the Solenogastres, Sepia, and Nautilus, the sac of the 
genital glands is in open communication with the rest of the coelom, 
forming, in fact, an only partly distinct division of the same. 

In the Solenogastres {e.g. Froneomenia), the hermaphrodite gland lies above the 
mid-gut as a long tube, which In transverse section appears heart- or kidney-shaped, 
as its lower part bulges out on each side. Its shape is determined by the fact that 
the mid-gut forms dorsally a narrow but deep furrow, which cuts into this glandular 
tube from below. The tubular gland is divided into two lateral spaces by a partition, 
whose endothelial wall is the place of formation of the eggs ; these lateral chambers 
may again be traversed by septa, on which the genital products develop. This 
division is especially distinct at the posterior part of the tube, the two chambers 
being there completely isolated, and entering the pericardium separately as genital 
ducts. 

If the secondary body cavity of Froneomenia is compared with that of an Annelid, 
we iind the following differences : 



212 



COMPARATIVE ANATOMY 



CHAP. 



In Proneomenia, the dorsal vessel is wanting in the region of the mid-giit. The 
ca-lom is much less spacious, and instead of surrounding the intestine lies only on 
its dorsal side. It is developed merely as a hermaphrodite glandular sac, its endo- 
thelial wall yielding the genital products. 

In the region of the hind-gnt, the vessel lying in the dorsal mesentery is developed 
as a heart, the coeloni being here represented by tlie pericardium. 




Fig. 175.— Diagrammatic sections througli an Annelid (A) and a Solenogastrid (B and C), to 

illustrate the relation of the ccelom to the genital glands and nephridia. B, Region of the cloaca ; 
C, region of the mid-gut ; 1, dorsal mesentery ; 2, dorsal vessel or heart ; 3, genninal epitlieliuni ; 

4, crelom — in B = pericardium, in C = hermaphrodite gland (in the ccelom are genital products); 

5, nephridia ; 6, intestine ; 7, cloaca. 

The pericardium is connected with the cloaca by two canals ; these may be 
considered as the morphological equivalents of nephridia {cf. Fig. 175). 

As the genital glands have been recognised as part of the ccelom in the Soleno- 
gastres, Nautilus, and Sepia, they must necessarily fall under the same category in 
all other Molluscs, even when no longer in direct connection or in open communica- 
tion with the same. 

In the Chitonidw, tlie ccelom is large, and falls into three distinct divisions. One 
contains the intestine and digestive gland (liver), whicli are accordingly outwardly 




Fig. 170.— Diagrammatic longitudinal section througli Chiton, to illustrate the relation 
between the various parts of the ccelom (after Haller). 1-8, Position of the eight dorsal shell- 
plates ; il7, anterior portion of the dorsal integument; L, snout; m, mouth; I, digestive gland 
(liver); d, intestine;/, foot; n, kidney; p, pericardium; e, portion of the creloin surrounding the 
intestine; ft, heart; /p, band connecting pericardium and genital gland ; grt?/-, genital gland; ?«, band 
connecting the genital gland and the posterior portion of the creloni which surrounds the intestine. 

(i.e. on the side turned to the crelom) covered with an endothelium. The mesen- 
teries, however, which originally attached tlie intestine to the body wall, and 
along which the parietal endothelium passed into the visceral endothelium of the 
intestine and liver, have disajipeared, with the exception of portions retained on 
the hind-gut. The two other divisions of the cicloin are : (1) the pericardium, and 



VII 



MOLLUSGA—THE BODY CAVITY 



213 



(2) tlie genital gland. Certain bands, by means of wliicli the three divisions are 
connected together, have been regarded as tlie constricted remains of communications 
between the three divisions of the originally single ccelom (Fig. 176). 

The Cephalopoda may with advantage be considered in connection with the 
Aiiiphinrura. In Nautilus and the Becapoda {e.g. Sepia, Fig. 177) a spacious 
secondary body cavity is found in the dorsal part of the visceral dome. It is incom- 
pletely divided by a projecting dorsal septum into two cavities, one lying above the 
other ; the lower of these contains, as pericardium, the heart with the arteries and 
veins running out of and into it, the branchial hearts, and the pericardial glands ; 
while the upper holds the stomach and the genital glands. This double cavity, 



Fig. 177.— Diagram sliowiag tlie 
secondary "body cavity of Sepia (after 
GroblDen). Median longitudinal section 
ihrough the body, in which, however, some 
organs are represented which, being paired 
and -syniinetrieal, do not properly come 
into the plane of the section. The outlines 
of the ccelom are indicated by thicker lines. 
1, Female germinal body, with eggs (2) pro- 
jecting into the genital cavity (the ovarial 
division of the ccelom) ; 3, shell ; 4&, an- 
terior portion of the renal sac ; 5, pancreatic 
appendage of the efferent duct (bile duct) 
of the digestive gland (liver) ; 4«, anterior 
venous appendage of the renal system ; 6, 
aperture (funnel) of the kidney into the 
ccelom ; 7, outer or pallial aperture of the 
kidney ; 8, digestive gland (liver) ; 9, 
"head" (Kopffuss); 10, funnel; 11, end of 
the oviduct with female genital aperture ; 
12, mantle cavity; 13, mantle; 14, posterior 
portion of the renal sac ; 15, intestine ; 14i, 
posterior venous appendage of the renal 
system (pericardial gland); IS, fold, in- 
completely dividing the coelom into an 
upper and a lower portion ; 19, stomach ; 
20, upper division of the ccelom (principally 
genital cavity); 21, pigment gland (ink- 
bag) ; 22, aperture of the oviduct into the 
genital cavity; d, dorsal; v, ventral; a, 
anterior ; j), posterior. 




which is called the viscero- pericardial cavity, is covered by endothelium, which also 
covers the organs within it. It is connected by two ciliated funnels with the two 
renal sacs. In Nautilus it also opens direct into the mantle cavity by two canals, 
whose apertures lie- close to the renal apertures. 

While the coelom in Nautilus and the Decapoda is very spacious, in the Odopoda, 
on the contrary, it is very much reduced. It consists merely of a narrow system of 
canals, which, however, have thick walls ; this was formerly called the water vascular 
system. The organs, which in Nautilus and the Decapoda lie in the ccelom, viz. 
the arterial heart with its afferent and efferent vessels, the branchial hearts and the 
stomach, are no longer found within the body cavity, but outside of it, and are 



214 



COMPARATIVE ANATOMY 



CHAP. 



therefore no longer covered with endothelium. Nevertheless this canal system of 
the Octopoda shows the same morphologically important characteristics as the coelom 
of the Decapoda. There are, for instance, on each side three canals which open 
together, one entering the renal sac, the second widening round the pericardial 
gland (appendage of the branchial heart) to form a flask-shaped capsule, and the 
third running to the genital gland to be continued into its wall. In so far as in the 
Oc<qpo(Z« the heart is excluded from theccelom, which has been reduced to the "water 
canal system," the reduction of this cavity has gone further in these Mollusca than 
in any others, which all retain at least the heart in one portion of the ccelom, the 
pericardium. 

In the LamelUhrancMa and Gastropoda, the only part of the coelom retained, 
besides the genital glands, is the pericardium. The pericardium and the gonad are, 
. however, entirely separated 

from one another. InZamelli- 
'^ branchs, there is in the peri- 

cardium, besides the heart, a 
part of the hind - gut which 
traverses it ; in the Gastropoda 
(except in those Diotocardia 
in which the hind-gut pene- 
trates the heart), only this 
latter organ. Rarely {e.g. 
PhyllirJioe) the auricle also 
is excluded from the peri- 
cardium. 

The pericardial gland is 
found in most Mollusca. It 
is a glandular differentiation 
of the endothelial wall of the 
pericardium, and perhaps, as 
already suggested, shares the 
excretory functions of the 
kidney. Its position in the 
pericardium varies, hut it 
seems in all cases shut off from 
the blood vascular system, 
with which it is, however, 
functionally connected. Its 
secretions or excretions must be discharged into the pericardium, and thence out- 
wards through the kidnej'. 

Among the Prosobraiichia, in the Diotocardia, the pericardial gland is found on 
the auricle, its walls forming dendriform branched outgi'owths into the pericardial 
cavity, these being covered with pericardial endothelium. Where pericardial glands 
are found in the Monotocardia, they lie on the wall of the pericardium Itself. 
Similar lobate formations occur among the OpisthobrancJiia, in Aplysia, and 
Notarchus, on the anterior aorta which runs along the pericardial wall ; in Pleuro- 
hranchus and PlcurohrancTicea on the lower, in Doridopsis and Phyllidwa on the dorsal 
pericardial wall. The lateral furrows of the pericardium of Doris form niches, which 
may again have accessory niches. These enlargements of the surface of the peri- 
cardial epithelium have also been considered as pericardial glands. 

Pericardial glands are much more common among the LamelUhrancMa than 
among the Gastropoda, but are wanting in the most primitive forms {Nucula, 
Solenomya, Anoniia). The gland is usually of a rusty red colour, and occurs in two 




Fig. 17s.— Eledono moscliata. This figure corresponds with 
Fig. 177 of Sepia (after Grobben). 8i, Efferent duct of the 
digestive gland; 17o, pericardial gland (appendage of the 
branchial heart) ; 23, "vvater canals. 



VII MOLLUSGA—THE NEPHRIDIA 215 

forms, consisting either of glandular protrusions of the endothelial wall of the 
auricles into the pericardial cavity, or of glandular tubes protruding from the 
anterior corner of tlie pericardium into the mantle (Keber's organ, red-brown 
organ). The first form is found specially strongly developed in Mytilus, Lithodomus, 
and Saxicava, more or less developed in Dreissena, Unio, Anodonta, Venus, Car- 
dium, Scrobicularia, SoUii, Pholas, and Teredo, and more or less rudimentary in 
Pecten, Spondylus, Lima, Ostrea. The second form has been observed in Unio, 
Anodonta, Venus, Cardium, Scrobicularia, Solen, Pholas, Montacuta, and Dreissensia. 
Pericardial glands may also occur singly in other parts of the pericardium, as in 
Meleagrina (as a projecting ruff in the posterior base of the pericardium), and in 
Chama on the ventricle, etc. 

The pericardial gland of the Cephalopoda is the so-called appendage of the 
branchial hearts. This is a structure connected with the branchial heart, and 
covered with peritoneal endothelium, which projects into the viscero-pericardial 
cavity, or, in the Octopoda, into a flask-like widening of the water-canal system 
(which has been recognised as a division of the ccclom). In Sepia this appendage is 
conical. A deep furrow on the surface which projects into the viscero-pericardial 
cavity leads into a richly-branched system of canals, the glandular epithelium of 
which is a continuation of the peritoneal epithelium. Blood sinuses from the 
branchial heart penetrate in between the canals of this system. In other Cephalo- 
poda, the pericardial gland varies in form and structure ; details of these variations 
cannot, however, be here given. Nautilus possesses two pairs of pericardial glands ; 
this fact is connected with its possession of two pairs of gills, with their two pairs 
of afferent vessels, and on these the two pairs of pericardial glands occupy positions 
corresponding with those of the branchial hearts. 



XIX. The Nephridia. 
Kidney, Organ of Bojanus. 

The organs which serve for excretion are homologous in all 
Mollusca. 

They consist typically of two symmetrical sacs, which, on the one 
hand, open into the mantle cavity, through the two outer renal 
apertures, and on the other are connected by two inner apertures 
(renal funnels, ciliated funnels) with the pericardium or coelom. The 
nephridia always lie near the pericardium. Their walls are richly 
vascularised, indeed a large part of the venous blood, in returning 
from the body, flows through the renal walls and gives off excretory 
matter before it enters the respiratory organs. The renal walls 
are traversed exclusively by venous blood. 

The nephridia are paired in all symmetrical Molluscs, and also in 
those Gastropoda which have paired gills and two auricles {Diotocardia). 

In all other Gastropoda, along with the original right ctenidium 
(which, in the Prosohranchia, lies to the left), and the corresponding 
auricle, only one kidney (the corresponding one) is retained. 

Nautilus, which has four gills and four auricles, has also four 
kidneys ; only two of these, however, communicate with the viscero- 
pericardial cavity. 



216 



COMPARATIVE ANATOMY 



CHAP. 



A relation between the nephridial and genital systems similar 
to that found in the Annelida exists in the Solenogastridce, the 
nephridia functioning as ducts for the genital products, the latter 
passing from the hermaphrodite gland (genital chamber of the coelom) 
into the pericardium. 

In a few Lamellibranchia, Diotocardia, and in the Scaphopuda, there 
is a relation between the genital glands and the nephridia, the former 
opening into the latter ; so that a certain part of the nephridium 
functions not only as renal or urinary duct, but also as efferent genital 
duct. In all Diotocardia, it is the right nephridium which functions as 
genital duct. In the Moiiotocardia, in which the right nephridium of 
the Diotocardia has atrophied as such, its duct persists as genital duct. 
In all other Molluscs the genital ducts are entirely distinct from the 
urinary passages. 

A. Amphineura. 

The kidneys of the Solcnogastrichr and the Chitonidcc differ greatly from one 
another in structure. 

1. In the Solenogastridce, two canals spring from the pericardium, embrace the 
hind-gut, and open into the cloaca beneath it through a common terminal portion 




« 15 



Fig. 179. — Paramenia impexa. Posterior end of the body ; the integument must he supposed 
tn be removed on the right side, and also a piece of the wall of the right nephridium ; diagram (after 
Pruvot). 1, Integument ; 2, ovarial portion of the hermaphrodite gland ; 3, testicular portion of 
the same, near the point where the latter opens into the pericardium (4) ; 5, glandular appendage 
of the right nephridium ; 6, dorsal cominissure of the pleurovisceral cords ; 7, organ called the 
sensory bud ; 8, aperture of the hind-gut into the cloaca ; 9, gill ; 10, cloaca ; 11, common aper- 
ture of the nephridia into the cloaca : 12, lower portion of the nephridium ; 13, upper portion of 
the right nephridium, which opens above into the pericardium ; 14, hind-gut. 

(Fig. 179). These canals function as ducts for the genital products. It is also 
certain that they correspond morphologically with the kidneys of other Molluscs, 
even though their excretory activity has not been proved. They are covered with 
an extraordinarily deep epithelium of long filiform glandular cells. 

In some Solenogastridce, an accessory gland opens into each nephridial canal. 

2. In the Chitonida., the strongly-developed paired nephridia function exclusively 
as excretory organs. 

Each nephridium (Fig. 180) consists of a wide canal shaped like a long Y, 



VII 



MOLLUSGA—THE NEPHRIDIA 



217 



the diverging portions being directed backward, and the undivided portion 
forward. These Y-shaped kidneys run longitudinally along each side of the body 
through its whole length. One of the paired limbs of the Y opens outwar<l into 
the posterior part of the mantle cavity, the other into the peiicardium, which also 
lies in the posterior part of the body. In this way the pericardial and outer aper- 




FiG. ISO.— Nephridial and genital systems of CMton, diagramiuatic, from above, after tlie 
figures and accounts of various authors. 1, Mouth ; 2, gills ; 3, unpaired-portion of the nephridiuni 
which runs forward, with its lateral branches ; 4, gonad ; 5, efferent ducts of the gonad ; 6, portion 
of the nephridium running to the outer aperture (10) ; 7, portion running to the reno-pericardial 
aperture (9) ; 8, genital apertures ; 9, reno-pericardial funnel ; 10, nephridial aperture ; 11, peri- 
cardium, indicated only in outline ; 12, anus. 

tures of the kidney lie near one another. The third limb of the Y ends blindly 
anteriorly. Secondary lobules or lobed canals open into all the three parts of the 
kidney, and are specially abundant in its anterior portion. Except in the terminal 
portion of the efferent branch, the epithelium of the limbs as well as that of the lobes 
is cubical and ciliated. 

B. Gastropoda. 

1. Prosobranchia. (a) Diotooardia. — Among all the Gastropoda, Fissurella 
alone possesses a symmetrical excretory apparatus, in the sense of having two 



218 



COMPARATIVE ANATOMY 



CHAP. 



nephridia opening into the mantle cavity to the right and left of the anus. The 
left nephridium is, however, much reduced, while the right, which is strongly 
developed, sends its lobes everywhere into the spaces'between the lobes of the liver, 
the intestine, and the genital organs. There are no reno-pericardial openings. The 
genital gland does not open direct into the mantle cavity, but through the right 
kidney. 

In Haliotis, Turbo, and Trochus, both nephridia are present. The left nephri- 
dium has, however, almost entirely lost its excretory function, but is still connected 
both with the pericardium and the mantle cavity. It is called the papillar sac, its 
walls projecting into its cavity in the form of numerous large papillic. The blood 
lacunfE which penetrate into the papillce communicate direct with the auricles, and 
are thus supplied with arterial blood. In these lacunse of the papillie a crystalloid 
substance (albumen ?) is deposited. It has been thought that these papillar sacs 
serve as reservoirs of nutritive material (in the form of the crystalloids just men- 
tioned), and when needed yield it up to the blood. 

The right nephridium is exclusively excretory in function. It is divided into 
two lobes, one behind the other, which communicate by means of a wide aperture ; 
the anterior lobe lies under the floor of the mantle cavity, bulging it upward. A 
spongy network, covered with excretory epithelium, rises from part of its wall into 
the cavity of the nephridial sac. The meshes of the network are penetrated by a 
system of vessels with walls of their own. Nearly all the venous blood, before 

reaching the gills, passes through the 
vascular system thus developed on the walls 
of the kidneys. The right nephridium is 
in no way connected with the pericardium. 
The Neritidae have only one nephridium 
to the right of the heart, which opens 
through a slit in the base of the mantle 
cavity. The renal sac is traversed by trabe- 
culie, many of which reach from one wall to 
the other, forming a spongy structure. The 
trabeculfe are covered by a glandular epithe- 
lium on the surfaces turned to tlie spaces of 
the sac. 

Patella (Fig. 181) still has two nephridia, 
both functioning as excretory organs. The 
apertures lie at the two sides of the anus. 
Fig. 181.— Diagram of the two nephridia xhe right kidney is, however, much larger 
of PateUa (after Lankester). tea. Anterior ^j^^^ ^^^ ^^^^_ ^^ ^^^^ jj^ ^^ ^^^ ^^^^ ^^ 
and upper lobe of tiie large right kidney ksl ; -^ ° , 

ksi, lower subvisoeral ; l;sp, posterior lobe of the pencardmm, but there are no reuo-peri- 
the same ; /, subanal tract of the large right cardial apertures. Tlie internal structure 
kidney ; g, anal papilla with the portion of the of the right kidney is spongy, but the left 
rectum which runs to it ; ft, papilla with the ^^^^ ^ ^^ ]^^ ^^^jt j^^q ^i^^^^-^^ f^^As 
aperture of the left kidney (which is not • , r ,^ n .1 . 

drawn); /, the same of the right kidney ; ?, P™J^«t f™™ ^"^^ ^^H^" ^ 1*^™"'*'' 8?^*'''" 
pericardium, indicated by a dotted outline; without special walls traverses the trabecular 
the existence of the reno-pericardial aperture network of the right kidney, but is com- 
figured near/, is now denied. pletely cut off from its cavity ; the venous 

blood from the body passes through this 
system before entering the gills. The lacunar system of the left kidney communicates 
directly with the auricle. 

In Haliotis and Patella also the genital products pass, as in Fissurella and the 
Diotocardia generally, out of the genital gland into the right kidney, and are ejected 
through the right renal aperture. 




VII MOLLUSGA—THE NEPHBIDIA 219 

(6) Monotocardia. — The Moiiotooardia have only one nephridium functioning as 
an excretory organ, viz. the left of the Diotocardia. This takes the form of a sac 
lying immediately below the mantle cavity on the right side of the pericardium, 
directly under the integument. It is generally found to the left of the hind- 
gut ; less frec[uently {Cassidaria, Tritoniidce) the kidney is traversed by the rectum, 
or the latter runs forward below it. The slit-like pallial aperture of the kidney, 
however, is always found to the left of the hind-gut, quite at the base of the mantle 
cavity. This position of the kidney, and especially of its outer apertures, had 
already led to the assumption that the Monotocardian nephridium corresponds with 
the left kidney of the Diotocardia, before this fact was established. The assump- 
tion was all the more plausible because of the occurrence of a gland called the 
anal kidney in a few Monotocardia (e.g. Dolium) ; this gland opens to the right 
near the anus, and might represent the right kidney of the Diotocardia. 

The kidney is always connected by means of a canal (tlie reno-pericardial canal) 
with the pericardium. 

Lamellffi or trabeoulfe, covered with the glandular epithelium of the kidney, 
project inward from the lateral walls of the renal sac. These are especially 
strongly developed in fresh -water Prosobranchia (excepting Valvata), traverse 
the whole kidney, and impart to it a spongy structure. The venous blood always 
flows through the whole of the glandular part of the kidney, either in special 
vessels or in lacunae, before jmssing on to the gills ; but an open communication 
with the renal cavity is never found. 

In the Tcenioglossa Proboscidifera the kidney forms two lobes similar in struc- 
ture. In Natica and Oyprcea the lobes begin to differ, and among the Stenoglossa 
this difference becomes more and more marked in a way which need not here be 
described. 

In Paludina and Valvata the kidney no longer opens into the posterior base of 
the mantle cavity, but is continued as a urinary duct (ureter), which runs forward 
in the mantle and opens at its edge. 

The above-mentioned theory that the single kidney of the Monotocardia corre- 
sponds with the left kidney of the Diotocardia has recently been ably opposed, 
another theory being put forward in its place. Attention is specially drawn to the 
fact that in the Diotocardia the left kidney is always the smaller, that in Patella it 
is shifted to the right side of the pericardium, and that in Saliotis, Turbo, and 
Trochus (as papillar sac) it is not excretory in function. In Saliotis, Turbo, 
Trochus, and Patella the lacunar system developed in the wall of the left kidney is 
in direct communication with the auricles. 

In most Monotocardia there is a differentiated part of the kidney, viz. that 
which is called the nephridial gland. This consists of two principal parts : (1) 
canals, covered with ciliated epithelial cells and opening into the kidney. These are 
merely protrusions of the renal wall, which project into the organ ; their epithelium 
is a continuation of the renal epithelium. (2) Between these canals, the organ is fiUed 
with cells of connective tissue and muscles, and contains blood lacunse, one of these 
being specially large and communicating with the auricle. This latter portion of 
the organ perhaps plays the part of a blood-forming gland. 

This nephridial gland may perhaps be the persistent excretory portion of the lost 
nephridium, i.e. the right of the Diotocardia. The duct of this lost nephridium is 
now known to persist as genital duct. As we saw above, all Diotocardia discharge 
the genital products through the right nephridium. 

2. Pulmonata (Fig. 182). — The Pulmonata have only one kidney, which lies 
in the mantle at the base of the pallial cavity, between the rectum and the peri- 
cardium. The renal sac is of the so-called parenchymatous type, the excretory 
epithelium of its wall projecting into the cavity in the form of numerous 



220 



COMPARATIVE ANATOMY 



CHAP. 



folds and lamelLu in such a way as to leave hardly any central free space. The 
kidney always communicates by means of a ciliated canal (renal funnel or renal 
syringe, " Nieren-Spritze ") with the pericardium. The position of the kidney and 
tlie morphology of the urinary duet have already been explained (pp. 74-78). 

3. Opisthobrancliia— Teotibranchia.— Only one kidney is found in the usual 
[losition on the right side of the body, with the pericardium in front of it and the 
hind-gut behind it. It is of the parenchymatous type, and is connected by a 
ciliated canal with the pericardium. It opens at the base of the gill in front of the 
anus. 

In the Pteropoda the delicate-walled kidney is not parenchymatous, but is a 




---« 



F](:. 1S2. — Nephridium and pericardium of Daude- 
bardia rufa, from abo^e, diagram (after Plate). 1, Peri- 
cardium ; 2, reno-pericardial aperture (renal funnel) ; 3, 
nepliridium ; 4, primary ureter ; 5, rectum ; ti, secondary 
ureter ('■/. Fig. 74, p. 77). 



Fio. 183.— Nephridlum of Bornella (after Hancock). 
1, Kidney ; 2, part connecting it with the reno-pericardial 
aperture (pyriform vesicle, renal syringe); 3, part of the 
pericardial wall ; 4, ureter; 5, nephridial aperture. 




simple hollow cavity lined with epithelium, and always communicates with the 
pericardium, against which it lies. 

Nudibranchia (Fig. 183). — The kidneys of the Nudibranchia are strikingly 
different in form from those of the Tectihranchia, The unpaired kidney is here 
somewhat similar to the paired kidney of the Ohitonidcc. It is a somewhat wide 
tube (renal chamber) traversing the cavity of the body, to a greater or less extent ; 
branches entering it from all sides. This tube is connected at one end with the 
pericardium by a duct (renal syringe, jiyriform vessel), which varies in length, and 
at the other opens outward through a ureter at the base of or near the anal 
papilla. 

It is saiil that PleiirviranchoM, a Tret ihranchiate, from which the Nudibranchia 
may perhaps be derived, possesses a Nudibranchiate kidney. 

In PliylUrhoe, the ui'inary chamber has no branchings ; it runs back from the 



VII 



MOLLUSGA~THE NEPHEW JA 



2-21 



pericardium as a simple median tube. Anteriorly it is connected with the peri- 
cardium by a funnel, and near tlie middle communicates with the exterior l>y means 
of a lateral urinary duct (Fig. 19, p. 12). 

C. Scaphopoda (Fig. 165, p. 193). 

Dcntalium has a pair of symmetrical kidneys, one on each side of the hind-gut. 
Each nephridium consists of a sac provided with short diverticula. The two nephri- 
dia are connected by a tube above the anus, and open into the mantle cavity l)y 
two apertures at the sides of the anus. If, as maintained by all authorities, there 
are no reno-pericardial apertures, the Scaphopoda would be the only group of Molluscs 
in which these apertures are entirely absent. Apart from the symmetry of tlie 
kidneys, a fact to be specially noted is that the genital piroducts pass out of the 
genital gland into the right kidney (either by the bursting of the wall between the 
two organs or through an aperture), and only reach the exterior, i.e. the niantlp 
cavity, through the right renal aperture. 

It must, further, be noted that near the anus on each side, between it and the 
renal aperture, a pore, the water-pore, occurs, the function of which is still doubt - 
fid. If these pores really lead into the blood lacunar system of the body, as wah 
formerly maintained, and is still held to be possible, this would be the onlj' known 
ease of the direct imbibition of water into the blood. 



D. Lamellibranohla. 

The nephridium (organ of Bojanus) is always paired and symmetrical, and lies 
below the pericardium and in 
fi'ont of the posterior adductor. 
Each nephridium is tubular or 
sac-like, opening at one end 
through a funnel into the peri- 
cardium, and at the other into 
the mantle cavity. This com- 
nmnication of the kidney with 
the mantle cavity always takes 
place above the cerebrovisceral 
connective. 

The lowest Lamellibranchia 
{ProtoirancMa, Nucula, Lcthi, 
SoleTwmya) are distinguished 
in two ways. (1) Each nephri- 
dium is a simple tube, with a 
free cavity not traversed li}- 
trabeculse or lamellfe. This 
tube consists of two portions 
which unite posteriorly at an 
angle ; the anterior end of one 
of these portions enters the 
pericardium through the renal Pericardiuin 
funnel, the other end opens venous sinus 
into the mantle cavity. (2) 
The paired genital glands do 
not open outward directly, but 
enter the kidneys near their pericardial funnel — a fact which is very important 




Pig. 184.— Transverse section through the hody of Ano- 
donta, showing the pericardiuin, the lieart, and tlie Ividneys, 
combined :^nd diagraniiiiatised from figures by Griesbach. 
Not all the parts represented occur on the same section. 1, 
2, ventricle ; 3, auricles ; 4, hind - gut ; 5, 
6, reno-pericavdial aperture (funnel) ; V, renal 
sac or cavity ; 8, vestibular cavity, whicli at 9 enters tlie 
mantle cavity through tlie nepliridial aperture ; 10, genital 
aperture ; 11, base of the foot. 



222 COMPARATIVE ANATOMY chap. 

connection witli the arrangement in tlie Solciiogastridm, the lower Prosohrmwhia {i.e. 
the Diotocardia), and the Scaphopoda. 

In other Laraellibranchia also there is a relation between the genital glands and 
the kidneys. In the Pedinidce and the AiwmiidcB the genital gland opens into the 
kidney, but near its outer aperture. In Area, Ostnca, Cyclas, and Montaeuta, the 
kidney and the genital gland open on each side into the base of a common depres- 
sion (nrogenital cloaca) ; in all other bivalves the outer nephridial and genital aper- 
tures are separate. 

The simple structure of the Protobranehiate kidney becomes complicated in 
other Lamellibranchia in the following manner : — 

1. That portion of the renal tube ivhich opens outward forms an external cavity 
(vestibular cavity, external sac) ; this cavity has no excretory epithelium ; it 
encircles the outer side of the peiicardial portion of the kidney, the renal sac (Fig. 
184). The latter alone is developed as an excretory organ. Folds or trabeoulfe, 
covered with glandular epithelium, project inward from its walls, forming a paren- 
chymatous or spongy structure. The renal sac is connected with the pericardium 
by means of a nephridial funnel of varying length. 

2. The two renal sacs communicate freely in the median plane. The connecting 
part is widest in the most specialised bivalves {Pholadaeea, Myacea, Anatinacea, 
Septibranchia). 

In Anomia, where all the parts are asymmetrical, the two kidneys, which do 
not communicate with one another, are also asymmetrical. 

Venous blood flows through the kidneys on its way to the gills. The afferent 
renal vessels seem to have walls of their own, but the eflferent vessels appear to be 
lacunar. Open communication between the blood vascular system and the kidneys 
is nowhere found. 

E. Cephalopoda. 

(Cf. Figs. 185, 186, and the sections on the ccelom and the blood 
vascular system, pp. 213 and 208). 

The Cephalopoda have two (Dibranehia) or four (Tctrabranehia) spacious sym- 
metrical renal sacs, in the posterior and upper part of the visceral dome. These 
communicate in the typical way at the one end with the ccelom, and at the other 
with the exterior (mantle cavity). Only one of the two pairs of kidneys in Nautilus, 
however, possesses ca'lomic funnels. 

The large veins returning from the body to the heart run along the anterior wall 
of the urinary sac. These veins bulge out into the cavity of the sac to form the 
venous appendages already mentioned. The epithelium of the urinary sac which 
covers these appendages is no doubt the seat of the excretory function. The excretory 
matter is discharged into the urinary sac (the wall of which is otherwise smooth), 
and passes out thence through a ureter of varying length into the mantle cavity. 
The renal aperture is found on the median side of the base of the gill, and in Nautilus, 
the Oegopsidce, and Scpioteuthis among the Myopsidcc, it is simple and slit-like ; in the 
other Myopsidce and in the Octopoda, however, it lies at the end of a renal papiUa 
which projects freely into the mantle cavity. 

The two renal sacs in the Oetopoda are entirely distinct. Near the point where 
the renal sac passes into the ureter lies the renal funnel, which corresponds with 
the pericardial aperture of other Molluscs, and which here leads to the ccelomic 
cavity, now reduced to the " water vascular system." 

In the Decapoda, the two renal sacs communicate with one another in the median 
plane. In Sepia, there are two points of communication, one above and the other 
below. The lower junction is bulged out to form a large sac,, which rises towards 



VII 



MOLLUSGA—THE NEPHRIDIA 



223 



the apex of the visceral dome on the anterior side of the paired renal sacs {cf. Fig. 
177, p. 213). The veins returning from the body to the heart run in the partition 
between the unpaired anterior and the paired posterior sacs, and may here bulge out 
to form venous appendages, not only posteriorly, i.e. into the cavities of the two 
paired renal sacs, but also anteriorly, into that of the unpaired connecting sac. Near 




Fig. 186.— Renal sac, coelom, genital organs, etc., of Sepia. A, female; B, male. Tlie 
visceral dome is seen from behind ; the mantle, the body wall, the ink-bag, and in A the hind-gut 
and the nidamental gland are removed (after; Grobben). a, Heart; b, genital vein ; c, genital 
artery ; d, stomach ; e, female germinal body ; /, aperture of the oviduct in the ovarial cavity ; 
g, oviduct ; h, unpaired anterior renal sac ; i, abdominal vein ; k, appendage of the branchial 
heart (pericardial gland) ; I, branchial heart ; m, paired posterior renal sac ; n, gill ; o, canals of 
the ccelom leading to the kidneys ; p, gland of the oviduct ; q, female genital aperture ; r, renal 
aperture. In B, 1, testes ; 2 (the indicator points rather beyond the right place), aperture of the 
male germinal body into the genital cavity or capsule ; /, aperture of the seminal duct into the 
male genital capsule ; 3, section of the ccfilom containing the vas deferens (peritoneal sac) ; 5, anus ; 
6, rectum ; g, male genital aperture. 

the point where each renal sac is produced into the ureter, the reno-pericardial canal 
springs from it, opening into the secondary body cavity which contains the heart, 
and corresponds with the pericardium of other Molluscs. 

The form of the renal sac is at least partly determined by the form and position 
of the surrounding viscera, the stage of maturity of the genital organs, and the 
different shape of these organs in the two sexes. All viscera which press against the 
renal wall from without, bulging it inward, are naturally covered at the points of 



224 



COMPARATIVE ANATOMY 



CHAP. 



contact with the epithelium of the renal sacs. The same is the case with all organs 
which, like the stomach, the gastric ccecum, and the efferent ducts of the digestive 
glands in the Decapoda (Sepia), apparently lie inside the spacious renal sacs. These 
organs really lie outside of them, being only suspended into them, like the intestine 
of an Annelid, which apparently lies within the body cavity, but is entirely separated 
from it by the peritoneal endothelium. 

It has been already mentioned that only one of the two pairs of renal sacs of 
XuutiJus, viz. the upper pair, has reno-pericardial apertures. This fact was 




VaS 



Fir;. ISO.— Diagram showing the posterior paired renal sacs of Sepia officinalis, and the vein 
running along its anterior wall with its venous appendages, from behind (after Vigelius). vc, 
Vena cava ; r/?i, right nephridial aperture ; i/j, left reno-pericardial aperture, the outlines of the 
secondary body cavity are indicated by a dotted line ; vij, vena genitalis ; rvc, right branch of the 
vena cava ; rjn/, right pallial vein ; vii, right vena abdominalis ; vha, vein of the ink-bag ; vai>, left 
vena abdominalis ; cc, section of the secondary body cavity (capsule of the branchial heart), which 
surrounds the branchial heart cZj, and the appendage of the same (pericardial gland) x ; vps, left 
pallial vein; li'c, left branch of the vena cava cephalica ; vm, left vena genitalis; rj(f, secondary 
body cavity (viscero-pericardial sac) ; y, left reno-pericardial ai)erture (renal funnel) (r/. Fig. 174). 



brought forward in support of the view that the two pairs of renal sacs arose by 
the division of one single jiair, corresponding with that of the Dibraiichia. Accord- 
ing to this view, the lower pair of gills, and the two auricles are also to be considered 
to be new acquisitions. Indeed, the whole question of the original metamerism of the 
JloUuscan body, which has so often been asserted, rests on very weak foundations. 
It gains no support from the CliUonidcv, where, in spite of large numbers of pairs of 
gills, only two auricles occur, and where no relation exists between the number of the 
shell plates and that of the gilLs. 



Til MOLLUSGA^GENITAL ORGANS 225 

XX. Genital Organs. 

A. General. 

In treating of the genital organs of the Mollusca, we shall have to 
consider — (1) the gonads or germinal glands, those most important 
organs, in which the reproductive cells (eggs and spermatozoa) are 
formed ; (2) the duets through which these cells reach the exterior ; 
and (3) the eopulatory organs. 

1. The gonads or germinal glands have already, in Section XVIII., 
been recognised as completely or incompletely demarcated portions 
of the secondary body cavity, and have been described in their 
relation to the other divisions of that cavity. 

The gonads are paired and symmetrical in the Lamellihranchia and 
Solenogastres, occurring in one pair. In all other Mollusca, only one 
unpaired gonad is found. In very rare cases, such as that of some 
hermaphrodite Lamellibranchs, which will be described later, there are 
two pairs of gonads, one female and one male. 

The sexes are separate, among the Amphineura in the Ohitonidm 
and Ohcetoderma, in many Lamellibranchs, in the Scapliopoda, among the 
Gastropoda in the Prosobranchia (excepting a few Marseniadce and the 
Valvata), and in all Cephalopoda. Hermaphroditism prevails among 
the Amphineura in Proneomenia, Neomenia, and allied forms ; in many 
Lamellibranchs, among the Gastropoda in the Pulmonata, Opisthobranchia , 
and in the Prosobranchiate family of the Marseniadce.. 

In hermaphrodite animals, it is the rule that the same gland, the 
hermaphrodite gland, produces both eggs and spermatozoa, but in 
exceptional cases there are in the same individual distinct male and 
female gonads (testes and ovaries). This is the case, as already 
mentioned, iu certain bivalves, viz. the Anatinacea and the Septi- 
branchia, which possess two testes and two ovaries. 

Position of the gonads. — The long tubular hermaphrodite glands 
of the Solenogastres, which are separated from one another by a median 
septum, lie in the anterior prolongation of the pericardium, over the 
intestine. In the Chitonidce, the gonads are found in a similar 
position, but are not in open communication with the pericardium. 
In the Gastropoda they lie in the visceral dome, usually in its upper- 
most part, between the lobes of the digestive gland. Where the 
visceral dome has disappeared, the gonad with the intestine and the 
digestive gland shift back into the primary body cavity above the 
foot. The gonads in the Smphopoda occupy a position similar to that 
of the Gastropodan gonads, lying dorsally in the high visceral dome, 
above the anus and the kidneys. The same is the case in the 
Cephalopoda. The paired much-lobed genital glands of the Lamelli- 
branchia lie in the typical position in the primary body cavity, above 

VOL. II Q 



226 COMPARATIVE ANATOMY chap. 

the muscular part of the foot, between the coils of the intestine. 
They may lie behind the " liver," or else, passing between its lobes, 
spread out at the sides of and below the kidney. 

The epithelium which lines the gonads is, morphologically, the 
endothelium of the secondary body cavity. The reproductive cells 
may either be produced from any part of the epithelium of the gonad, 
or from definite areas of this epithelium (Cephalopoda), which areas 
may then be called germinal epithelium or germinal layers. It may 
then appear as if the germinal gland lay in or on a special sac, 
whereas this sac is, in reality, the gonad itself, and the germinal 
gland is only the much-developed germinal layer of the gonad. 

The ripe reproductive cells become detached from their place of 
formation, and fall into the cavity of the gonad, i.e. into a part of the 
secondary body cavity, from which they pass out in various ways. 

2. The gonads either have separate ducts {GhitonidM, Monotocardia, 
Pulmonata, Opisthohrandiia, Cephalopoda, many Lamellibranchia) or they 
utilise the nephridia as ducts. In the latter case the genital products 
either pass direct into the kidney, and reach the exterior through 
the nephridial aperture (all Diotocardia, the Scaphopoda, and many 
Lamellibranchia), or they first pass into the pericardium, and then are 
ejected through the nephridia (Solenogasires). Where the gonads 
open into the kidneys, their apertures may lie in various parts of 
these organs ; either in the proximal part, which communicates with 
the pericardium by means of the renal funnel, and is usually widened 
into the renal sac, or in the distal part (ureter) which opens externally, 
or into a shallow urogenital cloaca. 

The gonads therefore open into : — 

a. The pericardium (Solenogasires). 

b. The proximal or pericardial part of the kidney. 

c. The distal part or ureter of the kidney. 

d. The urogenital cloaca. Or : — 

e. They open externally, quite apart from the kidney. 

Paired gonads have paired ducts (Solenogasires, Lamellibranchia). 
Where there is a single unpaired gonad, there is either a single 
efferent renal duct, or a single renal duct is made use of (Gastropoda, 
Scaphopoda, Cephalopoda, etc.) ; the duct is then always asymmetrical 
and usually lies on the right side. A paired duct, belonging to an 
unpaired genital gland, is, however, found in the Chitojiidce and in 
many Cephalopoda. 

When the genital glands have special efferent ducts, various 
sections of the latter may be differentiated into accessory sacs and 
glands, copulatory apparatus, etc., which, especially in the Pulmonata, 
Opisthobranchia, and Cephalopoda, transform the ducts into a very 
complicated apparatus. In males, this complication arises through 
the development of copulatory organs, and of special glands which 
form the capsules of the spermatophores, and of seminal vesicles, etc. ; 
in females, through the development of albuminous glands, shell 



VII 



MOLLUSCA— GENITAL ORGANS 



227 



glands, receptacula seminis, vagina, etc. Since, in hermaphrodite 
Molluscs, both kinds of complication occur simultaneously in the 
same genital apparatus, the most complicated arrangement is found 
in the (hermaphrodite) Pulmonata and Opisthobranchia. 

3. Copulatory organs are wanting in many Molluscs, such as the 
Amphineiira (see below), nearly all Diotocardia, the Scaplwpoda, and 
all Lamellibranchia. They are present in the Monotocardia, the 
Pulmonata, Opisthobranchia, and Cephalopoda. In the Gastropoda, in 
the nuchal region, to the right, there is a male apparatus, consisting 
sometimes of a freely projecting muscular penis, sometimes of an 
organ which can be protruded or evaginated through the genital 
aperture. In the Cephalopoda, this is a definite arm' in the male, 
which is specially modified (hectoeotilised), sometimes in a very 
remarkable manner, and which plays a more or less important part in 
copulation. 

B. Special. 

a. Gonads. (1) Amphineura. — The long hermaphrodite gland of Proneomenia 
and allied forms has been called paired. As a matter of fact it is divided into two 
more or less distinct lateral tubes, by a median much-folded septum. In the lower 
portion of each tube, that 

which lies next the intestine, 2., ?■ , ■ "^ A 

the germinal epitheliiun pro- 
duces spermatozoa, in the 
upper portion eggs. Pos- 
teriorly, these tubes sepa- 
rate for a certain distance, 
and open as a, pair of dis- 
tinct ducts into the anterior 
end of the pericardium. 

The male or female gonad 
of the Ohitoiiidce lies as a 
long unpaired sac on the 
dorsal side of the intestine, in 
front of and partly under the 
pericardium. In the ovary, 
numerous pear-shaped tubes 
(Fig. 187) project from the 
epithelial wall into the 
cavity. Each of these tubes Pia. 187.— Section througli tlie wall of the ovary of Chiton 
is a stalked foUicle, with egg (diagram after Haller). 1, Eggs at different stages of develop- 
cells surrounded by folUcular "«»' : 2, germinal epithelium ; 3, egg sac or tubes ; 4, follicular 
cells. These follicles are ePitMium ; 6, egg tube after the discharge of the egg. 
found in all sizes and at all stages of development. Each egg is at first a simple 
ovarial epithelial cell, which is distinguished by its size from the surrounding 
epithelial cells. As it grows and becomes more and more rich in yolk, it sinks 
down under the ovarial epithelium, bulging out this latter towards the ovarial 
cavity, and thus forming a young follicle. The wall of the pear-shaped testicle 
also rises into its cavity in the form of numerous folds, in which the epithelium 
becomes multilaminar, and produces the mother cells of the spermatozoa. 

The fact that the gonad of Chiton has two ducts makes it probable that it was 




228 COMPARATIVE ANATOMY chap. 

originally paired. The two ducts, i.e. the two seminal ducts in the male and the 
two ovarial ducts in the female, open into the mantle furrow on each side, somewhat 
in front of the renal aperture (Fig. 180, p. 217). 

(2) Gastropoda. — The gonads of the Prosobranchia offer but few points of 
interest to the comparative anatomist. In the Pulmunata and Opisthobranchia, the 
germinal gland is a hermaphrodite gland, in which spermatozoa and eggs are 
produced simultaneously. This gland is much lobed, or else consists of numerous 
converging diverticula ; the spermatozoa and eggs arise intermingled on the walls, 
become detaehed at one of the stages of their development, and then lie free in the 
cavity of the gonad. The same applies to the large hermaphrodite gland of the 
TectibrancMa, which varies much in its outer form. It lies in the posterior part of 
the body, on the digestive gland, penetrating at times between its lobes ; it is itself 
more or less lobed, its lobes consisting of secondary lobes (vesicles or acini). In 
all these acini, spermatozoa and eggs are simultaneously produced. It is only in 
the Fleurobranchcea and allied forms that the parts of the gland which produce 
spermatozoa and those which produce eggs are localised ; this arrangement resembles 
that in the Nudibranchia, which will presently be described. The constituent lobes 
or vesicles are either male or female, the former producing only spermatozoa, the 
latter only eggs. This is the arrangement found also in some Nudibranchia 
(Amplwrina, Capellinia), but in most Nudibranchs the male and the female 
germinal regions become separated in such a way that the terminal acini yield eggs 
only, but open in groups into lobes of the gland which produce only spermatozoa. 
Each lobe has its duct ; these ducts, uniting together, finally form the duct of the 
hermaphrodite gland. This gland thus forms an extensive organ spread out in the 
larger posterior part of the primary body cavity ; where there is a compact diges- 
tive gland it covers this organ. Phyllirhoe has 2 to 6 (usually 3) separate globular 
acini whose long and thin ducts combine to form a hermaphrodite duct (Fig. 
195, p. 238). 

The hermaphrodite ^a.\\di oi the Pteropoda {Tectliranchia natantia) always lies 
in the upper (dorsal) portion of the visceral dome ; it is sometimes acinose and 
sometimes consists of converging tubular follicles or of lamina closely crowded 
together. The eggs are always produced at the peripheral part of the acini, tubes, 
or lamellie, while the spermatozoa arise in the central parts, near the ducts. These 
two parts are generally separated by a membrane, which the eggs have to break 
through to reach the hermaphrodite duct. The Pteropoda are protandrously 
hermaphrodite, i.e. the spermatozoa are produced before the eggs, an arrangement 
found in many hermaphrodite Molluscs. 

(3) Scaphopoda. — The gonad (testis, ovary) in these animals is a long spacious 
sac, provided with lateral diverticula ; it lies above the anus, rising high up into the 
visceral dome along the posterior side of the body. In the Solenopoda (Siphmio- 
dentaliuvi, etc.) a large part of the gonad stretches into the mantle. In young 
animals, the gonad is closed on all sides, but in adults its wall appears to fuse with 
the right kidney, and in the partition wall so formed an aperture arises which 
establishes communication between the gonad and the right nephridium. 

(1) Lamellibranchia. — The gonads are here found in the form of much-branched 
tubular or lobate masses lying on each side in the primary body cavity, surrounding 
and partly penetrating between the other internal organs. In some cases (Anomiidai, 
Mytilid(c), the gonad on each side stretches into the mantle. In others {Axinus, 
Mmitacuta), it bulges out the body wall in such a way that branched outgrowths, 
containing the germinal tubes, project from the body into the mantle cavity. 

In most Lamellibranchia the sexes are separate, but hermaphroditism sometimes 
occurs. There are (1) whole groups of bivalves which are hermaphrodite ; e.g. the 
most specialised forms, such as the Anatinacea and Septibrancliia ; (2) families 



VII MOLLUSGA— GENITAL ORGANS 229 

with a few hermaphrodite genera : C'ychis, Pisidium, Entovalva ; (3) genera {Ostrcea, 
Pcdcn, Cardium) with a few hermaphrodite speeies ; (4) oceasional cases of herma- 
phroditism in species the sexes of whicli are usually separate : Anodonia. The 
hermaphroditism of the Lamellibranchia is, however, always incomplete in the sense 
that the spermatozoa and the eggs do not ripen simultaneously. 

In the Anatinacea and Septibranchia, there are on each side entirely separate male 
and female gonads, whereas all other hermaphrodite Lamellibranchs have a her- 
maphrodite gland on each side. 

(5) Cephalopoda. — The sexes are always separate in this class. It has already 
been mentioned that the germinal sacs form a part of the secondary body cavity, with 
which they are in open communication. 

One single unpaired gonad is always found, lying in the uppermost part of the 
visceral dome. It is a variously-formed sac (peritoneal sac or genital capsule), lined 
on all sides by an epithelium often to a great extent ciliated, which is in reality the 
peritoneal epithelium of the secondary body cavity. The whole of the epithelium 
covering the wall of the gonad is not, however, germinal, but only that on its anterior 
side (that turned to the shell). The germinal layer here forms what may be called, in 
tlie narrower sense, the ovary or the testis, which is then said to be contained in a 
peritoneal sac or an ovarial or testicular capsule, or else to project into or be suspended 
in such sac or capsule. The whole apparatus is really a gonad, in which the places 
of formation of the reproductive cells are localised on the anterior wall. 

From this it is clear why the testes and ovaries do not appear to possess efferent 
ducts of their own, but to empty their products into their respective capsules, these 
products passing out into the mantle cavity through the duets of these capsules 
(oviducts and seminal ducts). Since, however, the entire germinal sac corresponds 
with the genital gland of a Gastropod or a LamelUbranch, the reproductive pro- 
ducts in reality merely fall into the cavity of this gland (the testicular and ovarial 
capsules), and pass out through the ovarial and seminal duets, which exactly corre- 
spond with the same ducts in the Gastropoda, LatnellibraiicMa, and CMtonidm. 

The genital cavity has also another means of communication with the exterior, 
since, in the Cephalopoda, it is in open communication with the remaining part of 
the secondary body cavity, whether the latter forms a viscero-pericardial cavity (Deca- 
poda) or is reduced to the "water canal system " {Octopoda). This latter part of the 
body cavity again is connected, by means of the nephridia, with the mantle cavity. 

In this way, the genital cavity communicates with the mantle cavity directly by 
means of the oviduct or seminal duet, and indirectly through (1) the viscero-pericardial 
cavity or the "water canal system," and (2) the nephridia. This second way of 
communication, however, is never used for discharging the genital products. 

The female germinal layer or ovarial layer (the ovary in the narrow-er sense) is 
always found on theanteriorwall of the gonad, and varies considerably in,structure (Fig. 
188). "We can always distinguish (1) the eggs, and (2) the ovigerous wall. The former 
are stalked, and project from the wall into the cavity of the gonad (the cavity of the 
ovarial capsule). The largest and oldest eggs are covered by a follicular epithelium, 
and this latter by the general epithelium of the wall of the gonad, which also covers 
the stalk. Each egg has a separate stalk. The youngest eggs are mere prominences 
on the wall, which in the process of gi'owth acquire a stalk, by means of which they 
remain connected with the wall from which they project. This arrangement is 
exactly like that in the Chiton. When the eggs are mature, the follicle bursts, they 
fall into the genital cavity, and thence reach the exterior through the oviduct. 

In Nautilus (Fig. 188, A) and EUdone the whole wall of the gonad, with the 
exception of the posterior surface, can produce eggs ; these stand out from it all over 
on simple stalks. In Argonauta (Fig. 188, £) and Tremoctopus also, the whole ova- 
rial capsule except the posterior wall produces eggs, but the egg-bearing region (to 



230 



COMPARATIVE ANATOMY 



CHAP. 



obtain increase of surface) projects into the genital cavity in the form of numerous 
dendriform processes, the eggs being attached by simple stalks to the stems and 
branches. In Parasira {Tremodopiis) catenulata there is a central region containing 
more than twenty large "egg trees " surrounded by a circle of smaller "trees." On 
the anterior wall of the gonad in Octopus there is a single but very richly-branched 
" egg tree " (C). In Sepia, Sepiola, and Rossia the egg-bearing surface bulges out 
in the shape of a ridge on the anterior wall of the gonad. This ridge, in Loligo, 
becomes a nan-ow fold, the free edge of which is produced into filaments, which carry 
on all sides simply -stalked eggs. In the OegopsidcE {Ommastrephes, Fig. 188, D, 
Onychoteuthis, Thysanoteuthis) the region which carries the eggs is only attached by 
its upper and lower ends to the wall of the gonad, and forms an otherwise free spindle- 
shaped body traversing the genital cavity, and beset all over with stalked eggs. 

In Octopus and Eledone all the eggs in a given ovary are found at the same 
stage of maturity. 

A peculiar transformation of the follicular epithelium takes place in the ovarial 
eggs of the Cephalopoda when nearly mature. An extraordinary increase of surface 
occurs in the shape of numerous folds, which run longitudinally along the egg, either 
reticulating or remaining parallel to one another, and projecting far into the yolk 




Fio. 188.— -4-D, Four diagrams of the female gonads of the Cephalopoda. A , Nautilus type. 
B, Argonaut type. C, Octopus type. B, Ommastrephes type. 1 , Aperture of the oviduct into 
the gonad ; 2, cavity of the gonad (a section of tlie secondary body cavity) ; 3, egg-carrier. 



of the egg which they surround. This ari-angement may be connected with the 
nutrition of the egg. 

The male germinal layer (germinal body, or testis in the narrower sense) is a 
variously-shaped (often globular or oviform) compact organ, which usually lies free 
in the genital cavity, suspended to its anterior wall by a thin ligament (mesorchium) 
in which the genital artery runs. The germinal body is everywhere covered with 
epithelium, which is continued over the mesorchium into the epithelium of the wall 
of the gonad (endothelium of the testicular capsule). On the surface of the germinal 
body which is turned away from the mesorchium, there is a funnel-shaped depression 
(Fig. 189, A) ; towards this, from all sides, the tubular testicular canals which form 
the male germinal body converge, in order to open into it. In these testicular canals, 
between which there is a slight framework of connective tissue, the spermatozoa are 
produced, and are passed on to the genital cavity through the depression into which 
all the canals open ; they reach the exterior by means of the seminal duct. The 
testicular canals originally possess a multilaminar germinal epithelium, which yields 
the spermatozoa, and which passes at the common aperture into the outer epithelium 
of the germinal body, and so into the epithelium of the germinal sac. 

This description applies to the male germinal body of most Cephalopoda. In 



VII 



MOLLUSGA—GENITAL ORGANS 



231 



Loligo (B), however, the funnel-shaped depression into which all the testicular 
canals open is replaced by a longitudinal furrow, into wliich these converging canals 
open. In Sepia (C), the germinal body has no ligament, but lies immediately in 
front of the anterior wall of the gonad, and is thus outside the genital cavity. The 
germinal body here has a central channel towards which the radially arranged 
seminal canals converge from all sides, and which they enter. This channel, again, 
opens through an efferent duct into the genital cavity, from which the spermatozoa 
are conducted to the exterior by the seminal duct. 

The spermatozoa of the Mollusca are of the common pin shape. In many species 
of Prosobranchia two different forms of spermatozoa, the liair-shaped and the vermi- 
form, occur in one and the same individual. This phenomenon has by some been 
taken as an indication of developing hermaphroditism, and by others as pointing to 
a fonner hermaphrodite condition ; in the first case tlie vermiform spermatozoa 
would be the eggs beginning to form, in the second the rudiments of eggs. There 
is, however, no solid foundation for eitlier of these views. 

With regard to the question whether the hermaphrodite or the dioecious condition 
is the original condition, the latter alternative may be considered as the more prob- 
able. Of the five classes of the Mollusca, two, the Scaphopoda and the Cephalopoda, 




Fio. 189.—^, B, C, Three diagrams of tlie male gonads of, the Cephalopoda. A, ordinary- 
type. B, Loligo. C, Sepia. 1, Seminal duct ; 2, cavity of the gonad ; 3, space into which all the 
canals of the testis open, and which itself opens into the cavity of the gonad, in Sepia, by means 
of a canal (4) ; 5, suspensor of the male germinal body, attaching it to the^anterior wall of the 
gonad. 

are altogether dioecious. Among tlie Amphineura, the Ohitonidce, which most 
recent observers hold to be less specialised than the Sole7wgastres, are sexually 
separate. Among the Lamellihranchia, the sexes are separate in the Protobranchia, 
which are rightly considered as primitive forms ; and most other bivalves are also 
dioecious. Among the Gastropoda, the sexes are separate in the Prosobranchia, 
especially in the Diotocardia, which are universally considered to be the lowest and 
least specialised Gastropods. 

b. The ducts. — The manner in which the sexual products are conducted to the 
exterior in the Amphineura, Scaphopoda, and Lanullibranchia need not again be 
discussed, as it has already been described in the general part of this section, and in 
the section on the nephridial system. We thus have now only to treat of the very 
complicated ducts of the Gastropoda and the Cephalopoda. 

(1) Gastropoda. — It has been seen that in all Diotocardia (Haliotis, Fissurella, 
Patella, etc.) the genital products are ejected through the right kidney. In 
the Monotocardia, the right kidney has atrophied as such, but, according to the 
most recent investigations, its duct persists as genital duct. In the Pulmonata 
and Opistlwbranchia, the genital aperture is no longer in the mantle cavity. 



232 COMPARATIVE ANATOMY chap. 

but has shifted far forward along the right side of the neck, probably in con- 
nection with the development of the copulatory apparatus. The position of this 
aperture is thus not necessarily affected by any further displacement of the pallial 
complex, or indeed of the whole visceral dome, which explains the fact that, in Daude- 
bardia and Testacella, the common genital aperture, and in Oncidium, the male 
aperture, lies far forward on the right side of the body, although the pallial complex 
has shifted completely to the posterior end of the body. 

In the Opisthobranchia also, the single or (secondarily) double genital aperture 
lies to the right in front of the anus and even in front of the kidney. This position 
seems inexplicable except by the supposition of a shifting back of the pallial com- 
plex in which the genital aperture, emancipated from the complex, took no part, 
thus coming to lie in front of the shifted anal and renal apertures. 

Monotocardia. — Unlike the Diotocardia, which, with the exception of the Neritidai, 
have no copulatory organs, the Monotocardia possess a penis, which, however, does 
not lie in the mantle cavity where the genital aperture originally lay. It would be 
unable to function in this position, and is therefore placed on the right side of the 
liead or neck (Fig 71, p. 78), and forms a freely projecting, extensible, muscular 
appendage, which often attains a considerable size. The male genital aperture, 
liowever, in very many, perhaps in most, Monotocardia, remains in its original posi- 
tion in the mantle cavity, to the right, near the rectum. In such cases, a ciliated 
furrow runs forward on the floor of the respiratory cavity, along the right side of the 
neck, to the base of the penis, to the tip of which it is continued as a deep groove. 
This furrow conducts the semen to the penis from the genital aperture. In some 
cases the furrow closes, and forms a canal ; the penis then becomes tabular, and the 
seminal duct enters into it. The genital aperture is thus shifted far forward from 
its original position. The seminal duct, which arises from the testis, usually forms 
coils as it runs along the columellar side of the shell. The vas deferens has no 
special appendages, although it may widen into a vesicle at some point in its course. 

In the female, the genital aperture remains in the mantle cavity, lying to the 
right near the rectum, behind the anus. The duct remains, as a rule, more or less 
simple ; it is divided into the following consecutive sections : (1) an oviduct, rising 
from the ovary, which may bulge out to form one or more receptacula seminis ; 
(2) the uterus, a wider section with thick glandular walls, in which the eggs arc 
provided with albumen and a shell : (3) a muscular sheath, the vagina, which leads 
to the outer genital aperture. In Palitdina, there is a special albuminous gland 
opening into the oviduct. 

In hermaphrodite Prosobranchia {Valvata, a few Marseniadm, e.g. Marsenina, 
Onchidiopsis) a hermaphrodite gland is found. This gland gives rise either to one 
duct, which divides later into a vas deferens and an oviduct, or to a vas deferens and 
an oviduct which are from the first distinct. The vas deferens runs to the penis as 
in the males of dioecious Prosobranchiates ; the oviduct runs to the female genital 
aperture. Both these ducts are, owing to the occurrence of accessory glands, etc., 
more complicated than in other Prosobranchiates. 

Opisthobranchia and Pulmonata. — The ducts in these orders are extremely 
complicated, both by division into many consecutive sections and by the develop- 
ment of various accessory organs. 

In the following descriptions of several types of genital ducts only the most 
important points can be mentioned. We give first the type of duct commonly 
found in the Cephalaspidw (Tectibrancliia). 

1st Type. — The hennaphrodite gland has a single undivided efferent duct, 
opening out through a single genital aperture. From this aperture the fertilised 
eggs pass out direct, but the spermatozoa pass into a ciliated seminal furrow which 
runs along in the mantle cavity, and by which they are conducted to the penis. 



VII 



MOLLUSGA— GENITAL ORGANS 



233 



This lies more or less far forward in front of the genital aperture, near the right 
tentacle. 

If we imagine the testis of a male Monotocardiau transformed into a herma- 
phrodite gland, and the vas deferens into a hermaphrodite duet, the above condition 
would be realised. 

Gastropteron may be chosen as a good example of this arrangement (Fig. 190), 
which is further found in other 
Ccphalaspidcc (Doridium, Philine, 
Scaphander, Bulla) and all Ftero- 
poda. 

The hermaphrodite gland or ovo- 
testis, which lies between the lobes 
of the liver in the posterior part of 
the body, gives rise to a herma- 
phrodite duct, which, after a long 
coiled course, enters a short but 
much widened terminal section 
known as the uterus or genital 
cloaca. This cloaca opens outward 
in front of the base of the gills 
through the genital aperture. Into 
the cloaca open : (1) the common 
efferent duct of two glands, one 
of which, the albuminous gland, 
supplies the egg with albumen, 
while the other, the nidamental or 
shell gland, yields its outer pro- 
tective envelope ; (2) the duct of 
a globular vesicle (receptaculum 
seminis, Sohwanimerdam's vesicle), 
which receives the spermatozoa dur- 
ing copulation. From the genital 
aperture, which has a more or less 
median position on the right side of 
the body, the seminal furrow runs 
forward to the penis. The latter is 
enclosed in a special sheath, out of 
which it can be protruded, and into Fig. 190.— Genital organs of Gastropteron Meokelli 
which it is withdrawn by means of (after VayssiSra). The penis and the seminal furrow 
a retractor muscle. A gland called ^notlra™. l, Comn.on genital aperture ; 2 genital 
, . ° cloaca ; 3, albuminous gland ; 4, nidamental gland ; 5, 

the prostata opens mto the penis, hermaphrodite duet; 6, herniaplirodite gland; 7, re- 
The penis itself lies on the right ceptaculum seminis. 
anteriorly, on the boundary between 

the head and the foot. When it is at rest its sheath lies in the cephalic cavity, 
near the buccal mass. 

The very complicated ducts of Aplysia and Accra do not essentially differ from 
that above described. The hennaphrodite duct, on reaching the region of the 
albuminous gland, coils back upon itself, the ascending and descending portions of 
this coil surrounding the albumen gland with their spiral coils. The penis has no 
prostata. 

2nd Type. — The hermaphrodite gland gives rise to a hermaphrodite duct, which 
soon divides into two parts, the vas deferens or seminal duct, and the oviduct. The 
former runs to the male copulatory apparatus, the latter to the female genital 




234 COMPARATIVE ANATOMY chap. 

aperture. The male aperture and the penis lie in front of the female, far forward on 
the head or neck ; the two apertures are quite distinct, and both lie on the right. 

This second type may be deduced from the first, if we assume not only that the 
common duct of the hermaphrodite gland divided into a male and a female duct, 
but also that the seminal furrow closed to form a canal in continuation of the 
male duct. 

When the duct of this second type split into a male and a female duct, the 
accessory organs also so divided that the male opened into the vas deferens, the 
female into the oviduct. 

To this type belong, among the Fuhnonata, tlie Basommatopliora, a few species 
of Daudehardia (D. SauUyi, in which the two apertures lie close together), the 
Onoidia, and Vaginulidm. In both these latter groups, the female aperture has 
followed that part of the pallial complex which shifted to the posterior end of the 
body, and lies near the anus. The male aperture has, however, retained its anterior 
position on the head, behind the right cephalic tentacle. The two apertures thus 
lie at the opposite ends of the body. Among the OpistJiobrancMa, this second type 
is exemplified in Oscinius [TectibrancMa). 

Taking Limnsea stagnalis and Oncidium as examples, we find in the former 
(Fig. 191) that the hermaphrodite gland which lies embedded in the "liver," high 
up in the visceral dome, gives rise to a thin hermaphrodite duct ; this soon divides 
into a male and a female duct. The male duct first widens into a flattened sac, 
then into a large pear-shaped glandular vesicle (prostata). From this vesicle it runs 
as a long thin vas deferens through pact of the pedal musculature, and finally enters 
the male oopulatory apparatus, which is, in fact, merely the widened muscular and 
protrusible end of the vas deferens. A small penis tube is first formed by the vas 
deferens, and this projects on a papilla into a subsequent larger tube (the penis 
sheath), which is evaginated during copulation. Protractors are attached to the 
sheath, and retractors to the small tube ; the latter alone with its papilla enters the 
vulva during copulation. 

An albuminous gland opens into the female duct immediately after its separation 
from the male duct. It then forms a uterus consisting of wavy folds, and is continued 
into a large pear-shaped body as oviduct, the narrow end of which is the vagina and 
leads to the female genital aperture. The oviduct receives a lateral accessory gland 
called the nidamental gland, and the vagina the efferent duct of the globular 
receptaculum seminis. 

In Oncidium celticum (Fig. 192) the hermaphrodite gland and female accessory 
glands lie in the posterior part of the body, between the lobes of the liver and the 
coils of the intestine. From the gland rises a hermaphrodite duct, which at one 
point carries a small lateral cfecum, and opens into an irregularly-shaped organ, the 
uterus. Within the uterus two projecting folds border a channel ; if these folds 
become apposed, the channel becomes a tube. This channel runs from the point of 
entrance of the hermaphrodite duct to the point where the seminal duct leaves the 
uterus, and serves for conducting the semen. The remaining wider portion of the 
uterus serves as oviduct and egg-reservoir, and carries a large Cfecal appendage ; 
the ducts of the two much-lobed albuminous glands also enter the uterus. 

A comparison of Livwuea and Oncidium shows that in the latter the male and 
female ducts separate from one another further back than in the former. The vas 
deferens in Oncidium is only incompletely separated as a groove in the uterus. Its 
differentiation into a separate duct takes place here, as in terrestrial PulmmiaUs, at 
the distal end of the uterus. The thin seminal duct (vas deferens) passes into the 
body wall to the right, and runs forward along the longitudinal furrow between the 
foot and back, passing again at the anterior end of the body into the primary body 
cavity, where it forms numerous coils, and finally enters the copulatory apparatus. 



Til 



MOLLUSGA— GENITAL ORGANS 



235 



This apparatus, in Limncea, consists of a large evaginable terminal widening, into 
which the vas deferens projects in the form of a papilla. Blood pressure causes the 
penis sheath or prseputium to be evaginated through the genital aperture, into which 




s ^ ■ 



Fig. 191. 








14 



Fig. 192. 



Fig. 191.— Genital organs of Limnsea stagnalis (after Baudelot). 1, Male genital aperture ; 
2, larger penis tube (penis sheath) ; 3, protractors ; 4, smaller penis tube ; 5, vas deferens ; 6, pro- 
stata ; 7, flattened widening of the vas deferens ; S, hennaplirodite duet ; 9, hermaphrodite gland ; 
10, part of the digestive gland (liver) ; 11, albuminous gland ; 12, nidamental gland ; 13, uterus ; 
14, pear-shaped body ; 15, receptaculum seminis ; IG, vagina ; 17, female genital aperture. 

Fig. 192. — Genital organs of Oncidiiim celticum (combined from the figures of Joyeux- 
Laffuie), somewhat diagrammatic ; only part of the vas deferens is drawn. 1, Male genital aperture ; 
2, penis sheath (prteputium) ; 3, penis papilla ; 4, vas deferens ; 5, uterus, the seminal furrow in 
the uterus is indicated by dotted lines ; 6, cagcum of the uterus ; 7, oviduct and vagina ; 8, csecal 
appendage ; 9, receptaculum seminis ; 10, female genital aperture ; 11, albuminous glands ; 12, csecum 
of the hermaphrodite duct, 13 ; 14, hermaphrodite gland. 



it is again withdrawn by means of a retractor. In other species of Oncidium, the 
copulatory apparatus is complicated by the occurrence of accessory penis glands and 
variously -shaped cartilaginous armature. ; 



236 



COMPARATIVE ANATOMY 



CHAP. 



The oviduct which separates from the vas deferens at the end of the uterus is also 
a vagina. It is a simple tube which opens outward to the right near the anus 
through the genital aperture. Near the middle of its course it is joined by the 
stalk-like duct of a globular vesicle, the receptaculum seminis (bursa copulatrix), and 
by a long glandular e«cal appendage. 

3rd Type.— We find this in the Stylommatophora among the Pulmonata, and also 




Fig. 193.— Anatomy of Heliz pomatia (after Leuckart, Wandtafeln). The shell is removed 
and the mantle laid back to tlie left, the organs of the visceral dome and head are isolated and 
separated. To the left (in the figure) are the gen itEil organs. />, Digestive gland (liver) ; Zd, her- 
mapiirodite gland ; ./, intestine ; N, kidney ; V, ventricle ; j¥, fore-stomach ; i^, foot ; A, anus ; 
.1?, edge of the mantle near the respiratory aperture ; jl/r, retractor muscle ; G, cerebral ganglion ; 
Fl, flagellum ; N7:, oesophageal bulb (pharynx) ; P, penis ; R, retractor of the tentacle ; Ps, dart sac ; 
AD, digitate glands ; I't?, vas deferens ; X, lateral bulging of the stalk of the receptaculum seminis 
(iis); Od, portion of the uterus belonging to theoviduct ; 7i;(f, albuminous gland ; 2(7, hermaphrodite 
duct. 

in all Niidibi-anchia and a few Trxtihranchia {e.g. Phurobranclicea). The herma- 
phrodite gland gives rise to a hermaphrodite duct, which, as in the second type, 
sooner or later divides into a male and a female duct. These, however, do not open 
out through distinct apertures, but again unite to form a common atrium genitale or 
a genital cloaca. This third type may be deduced from the second by suppos- 
ing that the male and female apertures became approximated, and finally opened 
together. 



VII 



MOLLUSCA— GENITAL ORGANS 



237 



Helix pomatia and PUurohrandiKa Meckclii afford good examples of this 
arrangement. 

Helix pomatia (Fig. 193). — From the hermaphrodite gland a hermaphrodite 
duct, in zigzag coils, passes into the long folded uterus. The straight band wliicli 
passes along the folds of the uterus is that portion of it which belongs to the seminal 
duct ; the folds belonging to the female ducts. The seminal channel, however, is 
merely a furrow within the uterus, divided from the cavity of the latter by two 
projecting folds, the edges of which become superimposed. A longitudinal glandular 
band, which is regarded as a prostata, accompanies this duct. At the point where 
the hermaphrodite duct passes into the uterus, the large linguiform albuminous 
gland opens into it. At the end of the uterus, the male and female ducts become 
entirely distinct. The thin vas deferens runs in coils to the oopulatory apparatus, 
which again opens into the genital cloaca. The oopulatory apparatus consists of a 
protrusible penis ; at the point where the vas deferens enters this organ, the latter 
carries a long hollow appendage, the flagellum, the glandular epithelium of which 
perhaps yields the substance of the spermatophoral capsules. At the same point a 
retractor muscle is attached to the penis. The 
short oviduct widens before opening into the 
genital cloaca. The widened portion has the 
following appendages : (1) a long stalked pear- 
shaped receptaculum seminis, lying close to the 
uterus, — the stalk has a lateral bulging, which 
is sometimes rudimentary ; (2) two tassel-shaped 
organs, the digitate glands, the milky secretion 
of which contains calcareous concretions, and 
no doubt assists in the formation of the outer 
envelope of the egg ; (3) the dart sac, which 
lies close to the cloaca, and contains a pointed 
calcareous rod, the spiculum amoris, which is 
thrust by each individual into the tissue of the 
other as an .excitant during copulation. 

The common outer genital aperture lies in 
the nuchal region behind the right optic 
tentacle. 

Pleurobranchaea Meckelii (Fig. 194). — The 
hermaphrodite duct, which rises from the gland, 
foi-ms a long ampulla or widening, and then 
divides into a male and a female duct. The 
vas deferens runs in coils to the penis sheath, 
which it enters, coiling up in it almost like a 

watch-spring, and then forms the evaginable Fig. 194.— Genital organs of Pleuro- 
widened end portion which is called the penis, branolisea Meckelii (after MazzarelU). 

1, Conimou genital aperture ; 2, penis 
sheatli : 3, penis ; 4, retractor muscle of 
the same ; 5, vas deferens ; 6, nidaniental 
gland ; 7, albuminous gland ; 8, genital 
cloaca ; 9, oviduct ; 10, receptaculum 
seminis ; 11, widening and c^ecal appen- 
dage of the oviduct ; 12, hermaphrodite 
duct ; 13, hermaphrodite gland. 




and which can be invaginated by « retractor 

muscle. The oviduct has a shorter course, and 

receives the short efferent duct of a globular 

receptaculum seminis. The widened terminal 

portion of the oviduct (the vagina), which enters 

the genital cloaca with the penis, receives the 

ducts of the albuminous and nidamental glands 

(shell and slime glands) ; the second of these may be regarded as the homologue of 

the digitate gland of Helix. 

There is a general agreement between the ducts of the NudihrancMa and those 
just described ; in details, however, extraordinary variety prevails. The male and 



238 



COMPARATIVE ANATOMY 



CHAP. 



female ducts nearly always unite in the base of a genital cloaca, which often lies 
anteriorly on the right, on a papilla. The male and female apertures are rarely 
separate ; when they are so, they lie close together {cf. Fig. 195 of Phyllirhoe). 
The penis is often armed in various ways. 



The important subject of the mutual relations of the three types of genital ducts 
in hermaphrodite Gastropoda has been much discussed, but no satisfactory con- 
clusion has been reached. Ontogenetic research has been appealed to so far in vain. 

It is thus not at present known whether the 
single hermaphrodite duct has arisen by the 
fusing of separate male and female ducts, or 
whether the separate ducts have come into exist- 
ence by the splitting of an originally single 
hermaphrodite duct. The difficulty is increased 
by the fact that the genetic significance of the 
hermaphrodite gland is uncertain. 



~~i 




Fertilisation is mutual in hermaphrodite 
Gastropods. It is, however, certain that, in the 
Pulmonata at least, when copulation does not 
take place, self - fertilisation can occur. The 
hermaphrodite duct not infrequently carries one 
or two lateral cfeea or vesieulae seminales, in 
which an animal can store up its own sperm to 
be used in fertilising its own eggs if cross-fertil- 
isation does not take place. The eggs and the 
sperm are often not ripe at the same time. 

(2) Cephalopoda. — Although the gonad in all 
extant Cephalopoda is unpaired, the ducts are 
originally paired in both sexes. In Nautilus, 
the Oegopsidce, and the Octopoda, there is one 
pair of ducts in the female ; but in the males a 
paired seminal duct occurs only in Nautilus and 
Philonexis carence [Tremoctoims). In Nautilus, 
in which both sexes possess paired ducts, the 
left duct is in both cases rudimentary and no longer functions. It is the so-called 
pear-shaped vesicle, which is attached on one side to the heart and the lower end 
of the gonad, and on the other opens into the mantle cavity at the base of the 
lower gills. 

Where only one duct is retained, it is, in both sexes, the one on the left, as in 
Loligo, Sepia, Sepiola, Sossia, Sepioteutlds, Chiroteuthis, CirrhoteutMs, etc. 

The genital ducts rise on the wall of that part of the secondary body cavity which 
is known as the genital cavity (peritoneal sac, genital capsule), and open into the 
mantle cavity at the sides of the anus, between the nephridial aperture and the base 
of the gills. 

Male ducts, seminal duct. — In the more complicated form of male duct, 
such as that of Sepia (Fig. 196), four principal divisions may be distinguished. 
From the testicular capsule rises a vas deferens, which runs along in close coils, and 
then widens into a vesicula seminalis, the highly developed and much folded epi- 
thelium of which plays an important part in the formation of the spermatophores. 
The vesicula seminalis is continued as a thin vas deferens to the last division, the 
spermatophoral pouch (Needham's pouch), which serves as a reservoir for the 



Fio. 196.— Genital organs of Phylli- 
rhoe (after Souleyet). 1, Vas deferens ; 
2, penis ; 3, oviduct ; 4, male, 5, female 
genital aperture ; 6, vagina ; 7, lierma- 
phrodite gland ; 8, hermaphrodite duct ; 
9, receptaculum .seminis. 



VII 



MOLLUSCA— GENITAL ORGANS 



239 



spermatophores. This pouch is flask-shaped and projects freely, with the end which 
corresponds to the neck of the flask, at which the male genital aperture lies, into 
the mantle cavity. The vas efferens receives (1) the short duct of an oviform gland, 
the prostata, and (2) a simple, lateral, non-glandular csBCum. The prostata takes 
part, like the vesicula seminalis, in the formation of the spermatophores. The 
prostata, cfccum, and vesicula seminalis, in their natural position, form a coil, 





4M^. 



Fig. 197. 



Fig. 196. 



Fig. 196. — Male genital organs of Sepia officinalis. 1, Genital aperture ; 2, spermatoplioral 
poucli ; 3, vas efferens ; 4. CEecura ; 5, prostata ; 6, canalicule, opening into that part of the body 
cavity which surrounds the male duct ; 7, vesicula seminalis ; 8, 9, vas deferens ; 10, gonad, a 
portion of the posterior wall is removed, the genital cavity is revealed, and on its anterior wall is 
seen the aperture of the male germinal body_(12) ; 11, aperture of the seminal duct into the genital 
cavity. 

Fig. 197. — Male genital organs of Octopus vulgaris (after Cuvier). 1, Penis ; 2, muscle, cut 
through ; 3, spermatophoral pouch ; 4, vesicula seminalis ; 5, prostata ; 0, vas deferens ; 7, opened 
genital cavity, on whose anterior wall the testicular canals of the germinal body (8) are seen ; 
9, aperture of the seminal duct into the genital sac. 

whicli lies in a special division of the secondary body cavity, the peritoneal sac. It 
is remarkable that the vas deferens is in open communication with this peritoneal 
sac by means of a narrow tube. 

The male efferent apparatus of Octopus (Fig. 197), as compared with that of 
Sepia, is distinguished chieily by the absence of a separate vas efferens. The long 



240 



COMPARATIVE ANATOMY 



CHAP. 



vesioula seminalis opens into the large prostata near the point where the latter enters 
the spermatophoral pouch. This point lies, not in the base, but in the neck of the 
pouch, where the latter is produced into the long fleshy penis, the point of 
which projects into the mantle cavity. The penis is provided with a lateral 
Cfecum. 

It has already been mentioned that, as far as we know at present, only two living 



2/r^- 






9- 

dext. 







-^ 






s'mist. 



r—f/ 




dorsal. 
Fia. 198. —Female genital organs ot Sepia offlclnalls (chiefly after Brock). Tlie mantle 
cavity is opened, the posterior integument of the visceral dome removed, the ink-bag laid some- 
what to one side, and the oviduct uncovered. The complex of organs thus exposed is seen from 
behind. 1, Funnel ; 2, edge of the aperture of the funnel ; 3, cartilaginous locking apparatus ; 
4, left ganglion stellare ; 5, glandular terminal portion of the oviduct with the female genital 
aperture ; 6, left lateral lobe of the accessory nidamental gland ; 7, gland of the oviduct ; 8, left 
gill ; 9, oviduct filled with eggs which are seen through its wall ; 10, left nidamental gland ; 11, 
mantle ; 12, ovarial sac, opened from behind, the stalked ovarial eggs are seen on its anterior wall ; 
13, ink-bag (pigment gland) ; 14, stomach ; 15, right nidamental gland ; 16, central portion of the 
accessory nidamental gland ; 17, right lateral lobe of the same ; 18, right gill ; 19, right renal 
aperture ; 20, anus. 

Cephalopods, Nautilus and Philcniexis careiia', have paired male ducts. In Nautilus, 
the left duct is rudimentary. Whether the two ducts of Philonexis carence correspond 
with the two ducts which we may assume that the Cephalopoda originally possessed 
is very doubtful. The two vasa deferentia of Philonexis, which arise out of the 
testicular capsule, and differ considerably in structure, unite together later, and 



VII MOLLUSCA— GENITAL ORGANS 241 

both lie on the left side. It is also remarkable that the spermatoplioral pouch has 
two apertures, and that there are thus two genital apertures. 

Female genital organs — Sepia (Fig. 198). — The complicated female efferent 
apparatus consists of two entirely distinct parts, opening separately into the mantle 
cavity: (1) an unpaired oviduct (to the left), the position and aperture of which 
correspond with those of the seminal duct in the male ; and (2) the nldamental 
glands. The two large nidamental glands are pear-shaped organs, lying just 
beneath the integument in the posterior part of the visceral dome, symmetrically, at 
the sides of and anterior to the descending efferent duct of the ink-bag. They open 
into the mantle cavity at their ventral ends. Each gland appears symmetrically 
divided by a series of glandular lamellse, traversing it from side to side. The spaces 
between the lamellfe open into the central slit-like duct ; this structure is to be seen 
even on the exterior of the gland. Besides these two nidamental glands there is an 
accessory nidamental gland lying below and in front of the former. It is of a 
brick-red colour, and consists of a central part and two lateral lobes. It consists of 
numerous coiled glandular canalicules, which open into a glandular area in the 
mantle cavity. This glandular area forms a depression between the central and 
lateral lobes. As the aperture of the large nidamental gland also lies in this 
depression, the secretions of the two glands here mingle. 

The oviduct which rises from the ovarial sac is, during the reproductive season, 
so fuU of eggs, that it becomes much distended, especially at the part which opens 
into the ovarial sac. Before this duct opens outward into the mantle cavity at the 
same point and in a similar manner as the seminal duct in the male, it becomes con- 
nected by means of a freely projecting portion with a doubly-lobed or heart-shaped 
accessory gland, the gland of the oviduct, which repeats the structure of the nida- 
mental gland. The terminal portion also (from the point of entrance of this gland 
to the aperture of the oviduct) is glandular, two symmetrical rows of perpendicular 
glandular leaflets projecting from its wall into its lumen. 

The secretions of the nidamental glands, accessory nidamental glands, and the 
glands of the oviducts yield the outer envelopes of the ovarial eggs. 

Nidamental glands occur, among the Cephalopoda, (1) in the Tetrabranchia 
[Nautilus) ; (2) in the Dibranchia, among the Decapoda, in the Myopsidce (Sepia, 
Sepiola, Bossia, Loligo, Sepioteuthis, etc.); in a few Oegopsidce [Ommastrephes, 
Onycoteutkis, Thysanoteuthis). They are wanting in the Odopoda and in some 
Oegopsidce {Enoploteuthis, Chiroteuthis, Owenia). 

Nautilus is distinguished from all other living Cephalopoda (1) by the possession 
of only one nidamental gland, and (2) by the fact that this gland does not lie in the 
visceral dome but in the mantle. 

Accessory nidamental glands are found only in the Myopsidce. The two glands 
are either separate [Sossia, Loligo, Sepioteuthis) or fused together {Sepia, Sepiola). 

Glands of the oviduct occur in all Cephalopoda, but vary in position and in 
structure. 

Outgrowths of the oviduct, which function as receptacula seminis, occasionally 
occur {Trejiiodopus, Parasira). 

In all Cephalopoda, certain quantities of spermatozoa are collected in extremely 
complicated envelopes, the spermatophores. The substance of these large fila- 
mentous spennatophores is yielded by the prostata and the vesicula seminalis, but 
the mechanism by which so complicated a case is produced is still unknown. 
When touched, or when they reach water, the spermatophores burst at definite 
points, and scatter their contents. At the reproductive season the spermatophoral 
pouch is entirely filled with spermatophores. In Philonexis carence, however, only 
one very long spermatophore is produced. 

VOL. II R 



242 



COMPARATIVE ANATOMY 



CHAP. 



c. The copulatory apparatus — Hectoootylisation in the Cephalopoda. — The 

copulatory apparatus of the Gastropoda, and the penis which projects into the 
mantle cavity in certain Cephalopoda have already been described. 

One of the most remarkable and enigmatical phenomena in connection with 
the Cephalopoda is their hectoootylisation. This consists in the transformation of 
one of the oral arms of the male into a copulatory organ and 
spermatophore-carrier. This arm is said to be hectocotylised ; 
during copulation it becomes detached, and finds its way into 
the mantle cavity of the female. 

Typical hectoootylisation (Fig. 200) is found only in the 
Octopodan genera Argonauta, Philonexis, and Trcmnctopus. In 
Tremoctopus and Philonexis (Parasira) the third arm on the 
right is the one transformed, in Argonauta the third on the left. 
The arm is at first enclosed in an outwardly pigmented sac (Fig. 
200 A), when this bursts, the arm becomes free, and then its 
special form can be recognised (B). The folds which formed 
the sac bend back so as to form a new sac, which receives the 
.spermatophores and is now inwardly pigmented. An aperture 
leads from this sac into a seminal vesicle inside the hectocotylised 
arm ; this vesicle is continued into a long thin efferent duct, 
which runs the whole length of the arm and opens outwardly 
at its end. The end of the arm is transformed into a long 
filamentous penis, which at first is also enclosed in a special sac. 
When the penis is evaginated the sac remains as an appendage 
at its base. 

The spermatophores then pass from the pigmented sac into 
the seminal vesicle, and are ejected through the efferent duct 
which opens at the tip of the penis. 

It is probable that Cephalopods grasp one another, during 
copulation, with their arms, in such a way that their mouths face 
each other. In this position the hectocotylised arm of the male 
becomes detached, and in some way or other forces its way 
into the mantle cavity of the female. Detached arms are often 
found in the mantle cavity of the female, as many as four have 
been found at one time. 

We still do not know (1) how the hectocotylised arm fertilises 
the eggs of the female, or (2) how the spermatophores reach the 
hectocotylised arm. 

The males and females in the above-mentioned genera differ 
phoreof Serta'cafter f™™ "'^^ another, apart from the sexual dimorphism caused by 
Milne Edwards), a, the development of the hectocotylised arm. The males are 
Outer case ; 6, inner much smaller, and in Argonauta the female only has a shell, 
case ; c, spermatozoal jj. jg yg^y probable that the detached hectocotylised arm 

sac ; d, e, /, a, various i i i , 

parts of the ejacula- ^^^i ''« replaced by a new one. 

tory apparatus. Although a true hectocotylised arm, which can be detached, 

is only developed in the three genera above mentioned, it has 
been proved that in all other Cephalopoda (even Nautilus, cf. p. 117), a certain arm 
or portion of the head in the male is in some way modified, differing in some 
(often unimportant) manner from the other arms. Such an arm is said to be 
hectocotylised, and it is assumed that it plays some part in copulation, although 
its exact function is unknown. In Sepia and Nautilus it is even diSioult to 
imagine what part it can take in copulation. The constant occurrence of a 
hectocotylised arm is the more remarkable as it is by no means always the 



Fig. 199.— Spermato- 



VII 



MOLLUSC A— GENITAL ORGANS 



243 



same arm that is thus transformed. In the Octopoda, as a rule, it is the third 
on the right side, but in the Ootopodan subgenus Scceurgus and in Argonauta 




Fig. 200.— Male of Argonauta argo (after H. MuUer). (Female, Figs. 35, 36, pp. 24, 26.) A, 
With the hectocotylised arm enclosed in the sac (d). B, with the arm free, a. Funnel ; &, edge of 
the mantle fold ; c, left eye ; d, .sac ; di, hectocotylised arm ; e, mouth. 

it is the third on the left. In the Becapoda the hectocotylised arm is generally 
the fourth ou the left, but in the genus Enoploteuthis it may be the fourth on 




/F'^'A 



Fig. 201. — Hectocotylus of PMlonexis 
(Octopus) carense (after Louckart). a, 
Spermatophoral pouch ; &, seminal vesicle ; 
c, eflferent duct of the same ; d, appendage 
= remains of the penis sac ; e, penis ; /, 
sucker. 




the right, or even in one and the same species of Ommastrephes, it is sometimes 
the fourth on the left and sometimes the fourth on the right. In Sepiola and 



244 



COMPARATIVE ANATOMY 



CHAP. 



JRossia, it is the first arm which is hectocotylised. Finally, both the arms of one 
pair may be thus transformed ; in Miosepion and Spirula this is the case with the 
fourth pair, in Bossia with the first. 

The difference in size between the male and the female, which has been mentioned 
as occurring in those forms which have true hectocotylised arms, is also found, 
though not to the same degree, in many other Cephalopoda, in which the male is 
sliglitly smaller than the female. 



XXI. Parasitic Gastropoda. 

1. Thyca ectoconcha (Fig. 202) is a Prosobranchiate Gastropod whicli is parasitic 
on the Star-fish Linckia muUi/oris. The chief points in its organisation are shown 
iu Fig. 202, a longitudinal section in which, however, several organs which lie 
laterally to the section are also represented. The organisation of the Gastropod is as 
yet little influenced by its parasitic manner of life. It possesses a shell, shaped 




Fio. 202.— Longitudinal section through Thyca eotoconoha (after P. and F. Sarasin). Some 
organs not actually belonging to the section are included, cer, Cerebral ganglion ; d, alimentary 
canal ; fl, folds ; fs, foot ; k, gill ; I, liver ; ml, mantle ; oc, eye ; o(, otocyst ; peel, pedal ganglion ; pr, 
proboscis ; sf, false foot ; si, oesophageal bulb ; vl, cephalic fold. 



somewhat like a Phrygian cap. In the mantle cavity lies the gilh The alimentary 
and nervous systems also are in no way remarkable. It has eyes and auditory 
organs, and a short powerful snout, and muscular oesophageal bulb, whicli penetrates 
the Star-fish between the calcareous parts of its integument into the tissues. There 
is no radula. The base of the snout is surrounded by a muscular disc consisting 
of an anterior and a posterior part. This disc, the so-called false foot, is the 
grasping organ by which the animal attaches itself to the integument of its host 
so firmly that it cannot be torn away without injury. The rudiment of « foot 
(fs) occurs without an operculum. 

2. The Gastropodan organisation is somewhat more strongly modified in 
Stillfcr Linckice (Fig. 203), which is parasitic on the male Linckia. The whole 
body of this parasite penetrates into the calcareous layer of the integument of the 
host, on which it raises pathological globular swellings, and further causes the 
peritoneum to bulge inwards towards the body cavity. The parasite communicates 



VII 



MOLL USCA—PARAHITIG GASTROPODA 



245 



with tlie outer world only by means of a small aperture at the tip of the swelling. 
The parasite, thus established in the integument of its host, is surrounded on all 
sides by a fleshy envelope {sm). This envelope is only broken through by an 
aperture at the point where the apex of the dextrally twisted shell lies ; this 
aperture corresponds in position with the aperture above mentioned as occurring 
at the tip of the pathological swelling. Tliis envelope is called the false mantle, 
and corresponds morphologically with the false foot of Thyca, much increased in 
size and bent back on to the shell. There occur besides a true mantle, a gill, a 
rudimentary foot without an operculum, eyes, auditory organs, and a typical 
Prosobranchiate nervous system. The development of the remarkable false mantle 




Fig. 203,— Longitudinal section tlirougli Stlllfer Llnoklse (after P. and F. Sarasln). 6c, 
Buccal ganglion ; H, blood sinus ; mr, cerebral ganglion ; d, alimentary canal ; fs, foot ; fc, gill ; I 
liver ; ml, mantle ; n, proboscidal nerve ; oo^ eye ; ot, otocyst ; ^ed, pedal ganglion ; pr, proboscis ; 
s^n, false mantle ; sut, subintestinal ganglion ; sit^, supraintestinal ganglion. 



no doubt signifies that, although the animal is embedded deep in the integument 
of the host, communication with the exterior is retained. Water for respiration can 
enter and flow out of the mantle cavity, and the faecal masses and genital products, 
and perhaps also the larvse can pass into the cavity of the false mantle and be 
ejected through its aperture. The sexes are separate. The snout has lengthened 
into a very long proboscidal tube which pierces the tissues of the integument of the 
Star-fish, which are rich in blood, and draws from them the necessary nourishment. 
-Both oesophageal bulb and radula are wanting. 

3. The two parasites just described are typical Gastropods, and are easily 
recognised as such when carefully examined ; there are, however, two other 
parasitic Gastropods in which the typical organisation is so much modified that it 



246 



GOMPARATIVE ANATOMY 



CHAP. 



would be difficult to recognise them as Gastropods, or even as Molluscs, were it not 
proved that the larvae of one of these forms at least are distinctly Gastropodan 
larvae. The incomplete state of our knowledge of the development of these two 
parasites, and the absence of any transition forms between them and the typical 
organisation, make them very difficult to understand. 

Entocolax Ludwigii inhabits endoparasitically the body cavity of a Holothurian 
(Myriotrochus Riiikii), one end of its vermiform body being attached to the body 
wall of its host. Its organisation, a scheme of which is given in Fig. 205, can be 
best studied with the help of some hypothetical transition forms, through which a 

A B 




Fio. 204.— A, B, C, D, Hypothetical transition stages between Thyca and Stillfer onltlie 
one side and Entocolax (Fig. 205) on the other (after Schlemenz). o, Anus ; fd, pedal gland ; 
I, liver (digestive gland) ; Id, hepatic intestine ; m, mouth ; mag, stomach ; o, ovary ; of, aperture of 
tlie false mantle ; sf, false foot ; sm, false mantle ; u, uterus ; w, body wall of the host. 

Gastropod of the type of TTiyca or Stilifer might pass in developing into an 
endoparasitic parasite like Entocolax. Fig. 204 A shows the first stage, which still 
much resembles Thyca, and is still ectoparasitic ; Fig. 20i B, C, D are further 
stages in development. In proportion as the animal becomes endoparasitic, and 
gives up its relations to the external world, do the sensory organs, the shell, and the 
mantle cavity with the gill disappear. The stomach, as a separate section of the 
intestine, degenerates, the digestive gland (liver) becomes a simple unbranohed 
diverticulum of the intestine, which loses the rectum and anus. All organs for the 
purpose of mastication at the anterior end of the alimentary canal are lost. The 



VII 



MOLLUSCA— -PARASITIC GASTROPODA 



247 



false mantle becomes larger and larger, and envelops the small visceral dome, which 
gi-adually becomes rudimentary, and finally contains merely the genital organs. At 
the stage D the whole animal already projects freely into the body cavity of the 
host, attached to its wall by a displaced portion of the false foot, and connected with 
the exterior only by the aperture of the false mantle. If this last means of 
communication with the exterior is also abandoned, i.e. if the whole false mantle 
with its aperture becomes enclosed in the body cavity of the host, we have a form 
corresponding with the endoparasite Entocolax Ludwigii (Fig. 205). In this form, 
the cavity enclosed by the false mantle, into which the ovary and its receptacula 




•3MX 




Fig. 206. 



Fig. 205. — Entooolax Ludwigii, sketch 
after Voigt. Lettering the same as that in 
the preceding figure. 



Fia. 206.— Entooonoha mirahills, sketch 
by Sohiemenez (after Baur). Lettering as 
in Fig. 204. liod. Testes? 



seminis open, serves as a receptacle for the fertilised eggs, which were found in it in 
their first stage of segmentation in the one (female) specimen discovered. 

Entoconcha mirabilis, an endoparasite which has been found in a Holothurian, 
Synapta digitata, is even more deformed than Entocolax. The body of this parasite 
is a long vermiform coiled tube, attached by one end to the intestine of the host, 
while the rest of the tube floats freely in the body cavity of the latter. Its 
organisation has as yet been imperfectly investigated. Fig. 206 is » very simple 
diagram, which is introduced for comparison with Fig. 205 of Entocolax. It is 
impossible to say how far such a comparison, which the lettering is intended to 
facilitate, is justifiable. Up to the present time, no aperture leading from the ovary 
into the brood-chamber, which is thought to be the cavity of the false mantle, and is 



248 COMPARATIVE ANATOMY chap, vii 

filled with embryos (not represented in the figure), has been observed. In a widening 
of the tube near its attached end, a number of free " testicular vesicles " have been 
found, but their real significance can only be discovered by further research. 

The embryos found in the brood cavity oi Entoconcha have the same general 
structure as Gastropodan larva. They have a spirally twisted shell, into which 
the body can be withdrawn ; an operculum, a small velum, the rudiments of two 
tentacles, two auditory vesicles, [a foot, and an intestine, which, according to one 
observer (the most recent), consists of only a mouth, pharynx, oesophagus, and the 
rudiment of a liver, but according to an older authority is complete. There is, 
further, a branchial cavity with a transverse row of long cilia. Nothing further is 
known of the development and life history of Entoconcha. 

Some details of parasitic Lamellibranchiate larva; ( Unimiidce) will be given in the 
section on Ontogeny. 



XXII. Attached Gastropoda. 

Of the several forms of attached Gastropods known, only Vermetus, whose inner 
organisation has been carefully investigated, can be shortly described in this place. 
Vermetus has a shell which, instead of being coiled like the well-known shell of the 
snail, is a calcareous tube, which rises freely from the bottom of the sea, to which its 
tip is cemented. This shell is very like the calcareous tubes of tubicolous worms 
such as Serpula. The larva of this form, however, possesses a typically coiled shell, 
and even the young animal, after it has attached itself, has such a shell. In the 
course of growth, however, the coils become loosened, and the shell finally grows out 
as a tube. 

The typical organisation of the Monotocardian Frosobra-nchiates, to which Vermetus 
belongs, is little affected by the attached manner of life. The visceral dome, like 
the shell, is much elongated and almost vermiform. The intestine, the circulatory 
system, the kidneys, the mantle, the gill, and the nervous system are typically 
developed. The sexes are separate, and copulatory organs, which could not be 
used by attached animals, are wanting. The head is well developed, and the 
pharynx well armed. When the animal is slightly irritated, it is said not to 
withdraw at once into its shell, like other Gastropods, but to bite. The foot has 
the form of a truncated cylinder, and is directed anteriorly, ventrally to the head. 
It cannot, of course, function as a locomotory organ, but carries the operculum for 
closing the shell, and, by means of the pedal gland, secretes mucus. Vernutus is 
5aid to produce great r[uantities of this secretion, which it allows to float in the 
water for a time like a veil, and then swallows together with all that has become 
attached to it. In this way it fishes for the small organisms which form its food. 



XXIII. Ontog-eny. 

A. Amphineura. 

1. Ontogeny of Chiton Polii (Fig. 207). The egg possesses little nutritive yolk. 
The segmentation is total and somewhat unequal ; a coelogastrula is formed by 
invagination. 

(a) The blastopore of the gastrula larva marks its posterior end. - A pair of 
endoderm cells near the dorsal edge of the blastopore are specially large. A 
longitudinal section shows two dorsal and two ventral ectodermal cells with larger 




Fig. 207.— Development of Chiton Polii (after Kowalevsky). A-F, Six stages in the develop- 
ment of the gastrula into the young Chiton ; sections nearly median. G, frontal section through 
stage C, oblique, from the upper part of the velum to the blastopore. H, I, K, L, transverse 
sections of four stages of development behind the mouth. 1, Blastopore ; 2, archenteron or 
midgut ; 3, mesoderm ; 4, ectoderm ; 5, velum or preoral ciliated ring ; 6, stomodsum or oesophagus ; 
7, mouth ; 8, radular sac ; 9, body cavity ; 10, pedal gland, in I cesophagus ; 11, foot ; 12, anus 
with proctodaeum ; 13, cerebral ganglion ; 14, pretrochal tuft ; 15, pleuro visceral cords ; 10, pedal 
cords ; 17, mantle furrow ; 18, eye ; c, shell ; C1-C7, the seven shell plates first formed. 



250 COMPARATIVE ANATOMY chap. 

nuclei ; these belong to a double row of cells on which is developed the preoral 
ciliated ring which, in MoUnscs, is called the velum (Fig. 207 A). 

(J) At a later stage, the blastopore appears shifted somewhat towards the ventral 
side, and an inward growth of ectodermal cells begins at its edge ; this is the 
commencement of the formation of the ectodermal stomodseum. At the posterior 
and upper edge of the blastopore, there is, in the figure, a cell lying between the 
endoderm and the ectoderm ; this is, no doubt, a mesodermal cell (B). 

(c) The larva elongates ; a distinct stomodseum (embryonic oesophagus), leading 
through the blastopore into the archenteron, is formed by the continuous gi-owth 
inward of the ectodermal cells ; this organ becomes shifted still further forward 
along the ventral surface (C). 

{d) Fig. 207 G is an oblique section from an anterior upper to a posterior lower 
point through a slightly older larva, which shows the stomodasum, and, at the sides 
of the blastopore, the first mesoderm cells. These are probably derived from the 
endoderm, and are symmetrically placed at the two sides of the blastopore. 

(e) A median section through the next stage (D) shows no mesoderm cells as yet 
in the median plane. The mouth, however, appears shifted forward along the 
ventral side as far as the ciliated ring or velum, the double row of cells in the latter 
being very clear. 

(/) Transverse section of an older stage (H). The mesoderm cells have increased 
in number, and are arranged in two groups at the sides of the stomodteum, between 
the ectoderm and the endoderm. 

(g) At a later stage, a longitudinal section of which is given in Fig. 207 E, the 
principal feature is a stronger development of the mesoderm, in which a space, the 
body cavity, now appears. A bulging backward of the stomodseum forms the first 
rudiment of the radular sac. Behind the mouth, a sac-like depression is formed, 
evidently by the ectoderm ; this has been called the pedal gland, although it has 
not yet been discovered what becomes of it in the adult animal. 

{h) When the body cavity forms, the cells of the mesoderm become divided into 
two layers, the inner visceral layer becoming applied to the intestine, and the outer 
parietal layer to the ectoderm (c/. Fig. 207 I). In the transverse section, we see, 
deep down in the ectoderm, the first rudiments of the pleurovisceral cords. The 
pedal cords arise in the same way, and anteriorly, in the cephalic area, which is 
encircled by the preoral ciliated ring, the rudiments of the supra-oesophageal 
central nervous system form as a neural plate, i.e. as a thickening of the ectoderm, 
which carries a tuft of long cilia. 

(i) At later stages (F, K, L), the central nervous system with the pleurovisceral 
and pedal cords become detached from the ectoderm and take up their mesodermal 
position. The rudiments of seven shell-plates appear on the back as cuticular 
formations ; the eighth only appears later. A posterior invagination of the ectoderm 
represents the rudiment of the proctodeum (the embryonic hind-gut with the anus). 
The first teeth appear in the radular sac. The whole of the cephalic area and the 
region of the foot become covered with cilia. On the dorsal ectoderm, on the parts 
that are not covered by the shell-plates, the first calcareous spines appear. In the 
posterior part of the body, a great accumulation of mesodermal elements evidently 
marks the position of a formative mesodermal zone. 

At this stage, the larva leaves the egg envelope, and swims about freely, and, on 
the degeneration of the ciliated ring, sinks to the bottom transformed into a young 
Chiton. During this last transformation two lateral larval eyes appear on the 
anterior ventral side of the body. The development of the circulatory system, the 
nephridia, the genital organs and the otenidia has not been followed. 

2. Solenogastres. — The ontogeny of this order is as yet only known through 
a very incomplete account of the development of Dondersia banyulemis. The 



VII 



MOLL USGA— ONTOGENY 



251 



segmentation is unequal and total, and takes place through the formation of 
micromeres. The process of gastrulation seems to occur in a manner half way 
between epibole and invagination. The blastopore marks the posterior end of the 
larval body, which is divided by two circular furrows into three consecutive regions. 
The anterior region consists of two circles of cells, and evidently corresponds with 
the pretrochal area. It is partially ciliated, and carries in the middle a group of 
longer cilia, one of which is sometimes to be distinguished from the rest as a fiagellum. 
The second region, which consists of a single row of cells, carries a circle of long 
cilia, and evidently represents the velum. The third region consists of two rows 
of cells carrying short cilia ; the second row edges the blastopore. At an older 
stage, the posterior part of the larva appears to be withdrawn into an invagination 
of the anterior part. The whole or by far the greater part of the body of Dondersia, 
is said to be produced from this posterior part (the " embryonic cone ") alone. On 
this embryonic cone, there appear, first of all, on the two sides of the middle line, 
three pairs of consecutive imbricated spiculse, still retained in their formative cells. 




Fig. 208. — Dondersia banyulensis. A, Larva 36 liours old. B, Larva 100 hours old. 
Dondersia immediately after transformation (7th day), after Pruvot. 



C, Young 



They soon break through these cells, and their number is increased by the appear- 
ance of new ones anteriorly. The embryonic cone lengthens, becomes curved ven- 
trally. The anterior part of the body with the velum and the pretrochal area becomes 
reduced and finally appears as a sort of collar at the anterior end of the body. The 
larva sinks to the bottom, and throws otf the whole anterior part of the body with 
the velum and the pretrochal area. Such throwing off or resorption of parts of the 
body which have been of great physiological importance in the larva is very common 
in the animal kingdom ; see sections on the ontogeny of the Worms [e.g. Nemertina, 
Phoronis, etc., vol. i. p. 272), of the Arthropoda (Metamorphism of Insects, vol. i. 
p. 490), and of the EcMnodermata. 

On the dorsal region of the young Dondersia, seven consecutive, imbricated, but 
only slightly overlapping, calcareous plates can now be distinguished, consisting 
of rectangular rods lying close alongside of one another (Fig. 208, C). This 
fact is very significant with regard to the .shell of the Chiton, which in the adult 
consists of eight, but in the larva of only seven plates. If it could be proved that 
the SoUnogastridcB pass through a Chiton stage, the view that they are more 
specialised animals than the Polyplacophora, and are to be derived from Chiton-like 
forms, would receive almost decisive support. 

Besides the seven dorsal calcareous plates, the young Dondersia has numerous 



252 



COMPARATIVE ANATOMY 



CHAP. 



circular calcareous spicules, covering it laterally ; the ventral side is, however, 
naked. A mouth is still wanting, the endodermal mass is not yet hollow, and on 
each side, between the endoderm and the integument, there is a solid mesodermal 
streak. 



B. Gastropoda. 

As a type of the development of the Gastropoda, we may take Paludina vivipara 
(Figs. 209 and 210), the ontogeny of which has recently been again very carefully 
investigated. Development here takes place within the body of the mother. The 
egg is comparatively poor in yolk. A coelogaatrula is formed by invagination, the 
blastopore of which marks the posterior end of the germ, and becomes the anus. 
No proctodeum is formed. The whole of the intestine from the stomach to the 

A 






Fio. 209.— Development of Paludina vivipara (after v. Erlanger). A and B, Stage after 
gastrulation, witli the rudiments of the mesoderm and the coslom as outgrowths of the archenteron. 
A, Median optical longitudinal section. B, Horizontal optical longitudinal section. C, Horizontal 
optical longitudinal section through the emhryo, after the entire separation of the ccfilomic sac 
from the intestine. D, Sagittal optical longitudinal section through an embrj'o, in which the 
mesoderm has broken up, the cells becoming spindle-shaped. 1, Velum ; 2, segmentation cavity ; 
3, archenteron ; 4, crelom ; 5, blastopore ; 6, mesoderm cells ; 7, shell-gland. 

anus proceeds from the endoderm. The mesoderm arises as a ventral hollow out- 
growth of the archenteron, which soon becomes constricted from the intestine, and 
lies between the intestine and the ectoderm in the segmentation cavity as a vesicle 
with two points directed forward (Fig. 209 A, B, C). This vesicle spreads out to 
the right and left dorsally round the intestine, finally closing round it dorsally. Its 
outer wall of cells, which becomes applied to the ectoderm, forms the parietal 



VII MOLLUSGA— ONTOGENY 253 

layer, wliile its inner wall, whicli is applied to the intestine, forms the visceral 
layer of the mesoderm. The cells of the mesoderm soon become detached from one 
another (Fig. 209, D) ; they assume the spindle shape and finally fill the segmenta- 
tion cavity like a network. 

In the meantime the velum has appeared, and, between it and the anus, the shell- 
gland forms. The oesophagus arises as an invagination of the ectoderm, which soon 
becomes connected with the midgut. By the addition of a paired primitive kidney, 
the typical MoUuscan Trocophora is formed ; this at first is quite symmetrical, 
the anus lying posteriorly in the middle line. 

After the development of the cesophagus, a mass of mesoderm cells collects on 
each side of and below the intestine, this mass soon becoming hollow. In this way 
two mesodermal sacs are formed which approximate towards the middle line till 
they touch, and then fuse to form one sac, the double origin of which is still, for a 
time, evidenced by the presence of a median septum. The sac which thus arises is 
the pericardium. Fig. 210 A shows a somewhat further developed embryo seen 
from the right side. Below and behind the mouth are seen the projecting rudiment 
of the foot, on which to the right and left the auditory vesicles have arisen as 
invaginations of the ectoderm. In the pretroehal area, protuberances to right and 
left represent the rudiments of the tentacles, at the bases of which the eyes have 
appeared as ectodermal pits. The shell gland has secreted a shell. The greater 
growth of that side of the body which is covered by the shell has caused a bending 
by which the anus is shifted towards the ventral side. Immediately behind the 
anus, the ectoderm bulges out to form the rudiment of the mantle fold, so that the 
anus comes to lie in a shallow depression, the rudiment of the pallial or respiratory 
cavity. It is important to note that at this outwardly symmetrical stage, the mantle 
cavity and the anus lie posteriorly. The fore-gut ((Esophagus) has greatly lengthened. 
The digestive gland has grown out from the stomach ventrally in the form of a 
wide sac, but is still connected with the latter by a wide aperture. The pericardium, 
in which the septum is still visible, has already somewhat shifted from below the 
stomach to its right side. The rudiments of the definite nephridia next form in 
the following way (Fig. 210, D). In each division of the pericardium (the left 
division being smaller than the right) the wall bulges out ; the right outgrowth 
becomes the secreting portion of the permanent kidney ; the left degenerates, but 
must be regarded as a temporarily appearing rudiment of the (original) left kidney. 
The mantle cavity, which lies beneath the pericardium, presses into it to the right 
and left in the form of two projections. The right projection, continuing to grow, 
becomes connected with the rudiment of the right kidney and forms its efferent duct. 
The left projection does not grow further, nor does it become connected with the 
rudiment of the left kidney. 

A further stage is depicted from the right side in Fig. 210 B. The following are 
the most important alterations. The optic pit has become constricted into an optic 
vesicle. The mantle fold has grown further forward, and has become deeper to the 
right. The undivided pericardium has shifted altogether to the right of the 
stomach, and lies above _ the rectum, which bends forward and downward. The 
body is already asymmetrical. 

At the following stage (Fig. 210, C) the posterior and dorsal region of the body 
rises distinctly from the rest as a visceral dome ; the shell covering this part of the 
body has increased considerably in size. The mantle fold has become much broader, 
and the mantle cavity much deeper ; the latter now lies chiefly on the right side of 
the body. The looping of the intestine is far more marked. On the posterior and 
dorsal side of the pericardium, the pericardial wall sinks in the form of a channel, 
which soon closes and forms a tube ; this is the rudiment of the heart. The two 
apertures of the tube, where the wall of the heart passes into that of the pericardium. 



254 



COMPARATIVE ANATOMY 



CHAP. 



communicate with the body cavity. The heart tube becomes constricted in the 
middle, the anterior division forming the auricle and the beginning of the branchial 
vein, the posterior, the ventricle and the rudiment of the body aorta. 







Fig. 210. — Development of Paludlna vivipara (after v. Erlangor) A, Right aspect of an 
embryo, in which the pericardium Is divided into two parts by a septum. B, The same of a some- 
what older embryo, with an undivided pericardium. C, The same of an older embryo, in which the 
first rudiment of the heart has appeared. D, Ventral aspect of the posterior end of an embryo, in 
which the asymmetry of the visceral dome begins to appear. The anus is still median, but the 
mantle cavity is already deeper on the right (the left in the figure). 1, Velum ; 2, raid-gut ; 3, 
digestive gland (liver) ; 4, pericardium ; 4a and 4&, divisions of the same formed by a septum ; 5, 
free edge of the shell ; 6, shell groove ; 7, anus ; 8, mantle cavity ; S5, base of the mantle cavity = 
base of the mantle fold ; 9, free edge of the mantle ; 10, foot ; 11, auditory organ ; 12, oesophagus ; 
13, cephalic tentacle ; 14, eye ; 15, efferent duct of the (originally) right nephridiura ; 15?j, rudi- 
mentary efferent duct of the (originally) left nephridium ; 16, primitive kidney ; 17, rudiment of 
the heart ; 18a, right nephridium ; 18&, rudimentary left nephridium. 



Fig. 211 A shows a somewhat older embryo which already resembles in form the 
adult animal. The velum is reduced, and a ventral bulging of the anterior division 
of the oesophagus represents the rudiment of the radular sac. The ventricle and 



VII 



MOLL U80A— ONTOGENY 



255 



the auricle are distinct. An ectodermal depression on the foot forms the operculum. 
The mantle cavity which lies on the right side, and into which the rectum opens, 
now also stretches to the left on the anterior and dorsal side of the sharply 
demarcated visceral dome. The gill appears in the form of a protuberance on the 




Fig. 211. — Development of Faludlna vlvipara (after v. Erlanger). A, An embryo in which 
the first rudiment of the gill has appeared. B, A nearly mature embryo. Both are seen from the 
left side. Lettering as in Fig. 210. In addition, 17a, Auricle ; 17b, ventricle ; IS, nephridium ; 19, 
rectum ; 20, rudiment of the radular sac ; 21, rudiment of the gill ; 22, osphradium (Spengel's 
organ) ; 23, rudiment of the genital duct ; 24, rudiment of the gonad ; 25, operculum. 

inner surface of the mantle cavity, and the osphradium at the left of the gill as an 
ectodermal protuberance. 

Fig. 211 B finally shows us an embryo in which the mantle cavity has assumed 
the anterior position on the visceral dome. The ctenidium and osphradium have 
developed further. The velum is very much reduced and can only be seen in 
sections. This stage is important on account of the appearance of the rudiment of 
the genital organs, which is identical in the two sexes. A depression of the (meso- 



256 



COMPARATIVE ANATOMY 



CHAP. 



dermal) pericardial wall, which becomes separated from the pericardium, forms the 
rudiment of the gonad, while an ingrowth from the base of the mantle cavity runs 
towards this, and is the (ectodermal) rudiment of the genital duct. The latter 
arises on one side of the anus, just as the efferent duct of the permanent kidney 
rises on its other side ; this ontogenetic fact confirms what was stated above (p. 219) 
that the genital duct of the Monotocardia corresponds with a part of the right (which 
originally, and in the young embryo, is the left) kidney of the Diotocardia (apparently 
wanting in the Monotocardia). 

The vascular system arises very early in the form of spaces between the mesoderm 
and ectoderm or entoderm, round which the mesoderm cells grow, and which become 
secondarily connected with the heart. 

All the ganglia of the nervous system, the cerebral, pleural, pedal, parietal, and 
visceral ganglia arise separately as ectodermal thickenings, which become constricted 
off from the ectoderm by delamination. They only secondarily become connected 
with one another through tlie growing out of the nerve fibres. The parietal ganglia 
arise to right and left in the middle region of the body, but soon become shifted by 
the displacement of the organs of the visceral dome, one above the intestine and the 
other below it. The rudiment of the visceral ganglion is said to appear dorsally to 
the hind-gut and to move later to its position beneath the same. 

The observations on the development of Faludina vivipara, here briefly described, 
are in many ways of great importance, and confirm in the most unmistakable 
manner the results arrived at by comparative anatomy. The following are specially 
noteworthy. 

1. The manner in which the pericardium originates favours the opinion that it 
is a secondary body cavity. It is important to note that the pericardium is at 

first paired, being divided into 
3 !* 5 two lateral halves by a septum, 

i 2 J,-— — l~-~-~J which afterwards disappears. 

2. The fact that the gonad 
arises as an outgrowth of the 
pericardium, confirms the view 
arrived at by comparative 
anatomy, that the genital cavity 
also is a secondary body cavity. 

3. The anus and the mantle 
cavity originally lie symmetri- 
cally at the posterior end of 
the body, but, through asym- 
metrical growth, come to lie 
first on the right side of the 
visceral dome, and finally on 
its anterior side. 




The development of other 
Gastropods cannot here be de- 
scribed in detail. We refer the 
reader to the bibliography at 
the end. As a rule, nutritive 
yolk is present in larger 
quantities than in the viviparous Faludina, in which the small provision of 
yolk is evidently connected with the favourable conditions of nutrition of the 
embryo. 

The blastopore generally corresponds in position with the future mouth ; it often, 



FiQ. 212.— Larva of Oncidium oeltioum, from the left 
side (after Joyeux Latfuie). 1, Cerebral ganglion ; 2, edge 
of the mantle ; 3, rudiment of the gonad ; 4, larval shell- 
muscle ; 5, hiud-gut ; 6, rudiment of the digestive gland ; 
Y, auditory organ ; 8, pedal ganglion ; 9, foot ; 10, oesophagus ; 
11, eye ; 12, branched muscle cells of the velum ; 13, velum. 




vn MOLLUSGA— ONTOGENY 257 

perhaps usually, remains open ; notwithstanding this, the oesophagus arises by the 
sinking in of ectoderm cells. 

Paludina is, as far as is known, the only Mollusc in which the mesoderm 
originates as an outgrowth of the archenteron. This fact is no doubt connected 
with its poverty in nutritive yolk. In other Gastropods, the mesoderm arises in the 
manner already described for other Molluscs, as two large symmetrical primitive 
cells, at the posterior edge of the blastopore ; these cells look more like endodermal 
than ectodermal cells, and soon pass into the segmentation cavity. 

A Veliger larva, i. e. a Trochophora -with Molluscan characteristics, always forms 
(1) a dorsal shell gland with the embryonic shell, 
and (2) a ventral i-udiment of the foot. .„„, .jw^. 2_ 

The outward appearance of the Veliger larva, 
however, varies much in different groups, the 
variations being connected with the manner of 
life and of feeding of the embryo. 

In the marine Gastropods, i.e. in the majority 
of the Prosohranchia (including the Hetei-opoda), 
the Pulmonate genus Oncidium, and all Opistho- 
brajichia, the embryo leaves the egg envelope early, 
as a free-swimming Veliger larva. In all these 
forms, the preoral ciliated ring is well developed. 
The ectodermal floor of the ciliated ring usually Fig. 213. — Larva of Cymlmlla 
bulges out anterioi'ly, so that the cilia appear to (Pteropod), from the left side (after 
be carried by a distinct circular ridge. This ridge Gegenbaur). 1, Velum ; 2, shell ; 3, 
even grows out laterally to form a lobe of varying l^^Z°t) ^'*°''' *' ''""' ™"' "''"'" 
size, which carries at its edge long and strong 

cilia, and is occasionally itself produced into an upper and a lower lobe. This is 
the true velum of the free-swimming Gastropod larva, and is its only organ of 
locomotion. It is internally traversed from wall to wall by contractile mesoderm 
cells (muscle cells), which make it highly contractile. In the older larvse, the head 
with the velum can be withdrawn into the shell. 

It is probable that the velum of the larva also serves for respiration, and perhaps 
for bringing about a circulation of the body fluid by means of its contractility. 

The embryos of fresh-water and terrestrial Gastropods, where these animals are 
not viviparous, remain longer in the egg, and leave it only after their transformation 
into young Gastropods, the larval organs (the velum, the primitive kidney, the 
cephalic vesicle, and the pedal vesicle or podocyst) having degenerated within the 
egg envelope. Even in these forms, the mass of nutritive yolk contained in the egg 
is not very great, but there is a large q^uantity of albumen stored up within the egg 
capsule, which serves as food for the developing embryo ; this is either absorbed 
through the body wall or swallowed. The egg capsules are always large, in some 
cases (in tropical terrestrial Gastropods) as large as the egg of a small bird ; but 
their size is not, as in the Cephalopoda, determined by that of the egg contained, 
but by the quantity of albumen in which the small egg is embedded. The mature 
egg capsule contains a young Gastropod of considerable size with a well-developed 
shell. 

In terrestrial and fresh-water forms, the velum is not needed as a locomotory 
organ, and is therefore reduced to a single ring of cilia or to two lateral ciliated 
streaks. It is entirely wanting in the embryos of a few terrestrial Gastropod snails. 
The respiratory and circulatory functions, which were originally merely accessory 
functions of the velum, here become of greater importance. The nuchal region 
becomes much bulged forward, and forms a cephalic vesicle (Fig. 214), which is 
sometimes very large, and undergoes regular pulsations. The posterior division 
VOL. II S 



258 



COMPARATIVE ANATOMY 



CHAP. 




of the foot, in the same way, is often widened into n pulsating pedal vesicle or 

podocyst. Towards the end of larval life 
the cephalic and pedal vesicles and other 
similar "larval hearts'' degenerate. 

The embryonic shell is either retained 
throughout life or is thrown off at an early 
stage, and replaced by the rudiment of the 
definitive shell. Even a second temporary 
shell occasionally attains development. 

It must again be noted that shell-less 
Gastropods, to whatever natural division 
they belong, pass through a typical Veliger 
stage, and at the older Veliger stage have 
a distinctly demarcated coiled visceral 
dome, with a corresponding shell, and 
usually an operculum on the metapodium. 
In the larva of the gymnosomatous Fteropoda three postoral accessory ciliated 

rings are developed on the body. 



Fia. 214.— Embryo of Helix Waltonl (4 mm. 
long), from tlie right side (after P. and P. 
Sarasin). 1, Cephalic vesicle ; 2, npper (optic) 
tentacle ; 3, eye ; 4, lower tentacle ; 5, oral 
lobe ; 6, sensory plate ; 7, podocyst. ' 




»- 



G. Soaphopoda. 

Ontogeny of Dentalium. — Segmentation, in these animals, leads to the formation 
of a cceloblastula, from which a cfelogastrula arises by invagination. The blastopore 
at first lies posteriorly on the ventral side, but 
gradually shifts, as in Chiton, more and more 
forward along the ventral side. The stomodseum 
arises as an ingrowth of the ectoderm, the blasto- 
pore nevertheless remaining open. A typical 
Molluscan Trocophora is developed, although no 
primitive kidney has been found. The velum is 
a thick ridge round the body of the long oviform 
larva. This ridge consists of three rings of very 
large ectoderm cells, each row carrying a circle 
of long cilia. The shell gland spreads out at an 
early stage, its lateral edge soon growing out 
ventrally and posteriorly as the mantle fold. 
The free edges of the two folds fuse at a later 
stage below the body. The anus forms very late. 
The development of the cerebral and pedal 
ganglia and of the auditory organ has been 
specially carefully observed. On the ventral 

side of the pretrochal area, in front of the velum ^^^^ ^j^^ ^^^^^^.^^, ^^^, j^^^^^ ^^^^^^ 
and behind the tuft of cdia, two symmetrical (^jj^ Kowalevsky). 1, Cephalic tuft ; 
invaginations of the ectoderm form the cephalic 2, rudiments of the cerebral ganglion 
sacs or tubes. These become constricted from the (cephalic tubes) ; 8, yelmn, consisting 

, J 4. 1 4. 4. 4.-U ■ 1.,™ ,i„„n„ of three rows of cilia ; 4, raouth, hidden 

ectoderm at a later stage, their lumen gradually , ,, ., .,, ' ', ' ,, 

° 1 -1 ii . n under the ridge of the velum ; 5, mantle 

narrows and finally disappears, while their walls f^y^ 

become thick and multilaniinar by the con- 
tinuous growth of the cells. The two cell masses which thus arise become connected 
in the middle line above and below the oesophagus, and form the cerebral ganglion. 
The otocysts arise at the base of the pedal rudiment on each side as ectodermal 
epithelial pits, which soon become detached from the ectoderm in the form of 
epithelial vesicles. Immediately beneath these auditory vesicles, certain ectoderm 




Fio. 216.— Larva of Dentalium, 



VII 



MOLL USGA— ONTOGENY 



259 



cells sink below the surface, and form on each side an ectodermal cell mass, which 
becomes detached from the rest of the ectoderm, sinks into the mesoderm of the 
foot, and fuses with the similar mass on the other side to form the pedal ganglion. 







D. Lamellibranchia. 

1. Development of Teredo (Figs. 216 and 217). Segmentation is here total and 
unequal. The gastrula, formed by epibole (Fig. 216 A, B) consists of (1) two 
large endoderm cells (macromeres), a thick cap of ectoderm cells (micromeres) 
closely covering these, and 

two symmetrical primitive A E 

mesoderm cells of medium 
size at the posterior edge of 
the blastopore. The blasto- 
pore closes from behind for- 
ward, the ectoderm cells by 
continual division growing 
entirely round the endoderm 
cells ; during this process the 
two mesoderm cells become 
covered by the ectoderm and 
come to lie between the latter 
and the endoderm (Fig. 216 
C). Somewhat anteriorlj' on 
the ventral side, a depression 
of the ectoderm forms a pit, 
the stomodaeum (D). The 
ectoderm separates off from 
the two - celled mesoderm, 
thus giving rise to a seg- 
mentation cavity, or primary 
body cavity. A double 
preoral ciliated band is 
formed (D, E). The two 
large endodei-m cells, by 
fission, produce other smaller 
cells. Cilia appear over the 
whole surface of the germ, 
with the exception of the 
posterior dorsal surface, where 

the ectoderm cells, which fj^. 216.-A-G, Stages in the development of Teredo (after 
have become cylindrical, sink Hatsohek). A, 0, D, E, F, G, from the right side, B in optical 
in to form the shell gland (F). horizontal section. 1, Ectoderm; 2, macromeres = endoderm 
The latter secretes the first "^^'^^ • 8, primitive mesoderm cells ; 4, segmentation cavity ; 
rudiment of the shell in the '' 'to^a^m (cesophagns); 6, ™outh; Y, preoral ciliated 
band ;, 8, shell gland ; 9, shell ; 10, larval muscle cells ; 11, 
form of a simple cuticular cephalic plate with tuft; 12, anal invagination, anus; 13, 
membrane. The endoderm endodermal mid-gut. 
cells begin to collect to form 

the intestinal wall. After the formation of the first rudiment of the shell, the shell 
gland flattens and spreads out ; its edge can still be found as a ridge running under 
the edge of the shell. The endoderm now forms a large globular hollow mid-gut, 
into which the oesophagus breaks through. Each of the primitive mesoderm cells 





260 



COMPARATIVE ANATOMY 



CHAP. 



has given rise to two or three smaller cells. The thin cuticular shell becomes 
bivalviilar hy the appearance of a mediodorsal boundary line. 

A further stage is distinguished first by the appearance of a small posterior 
ectodermal invagination, the proctodseum, which produces the rectum and anus. 
An ectodermal thickening, the neural plate, appears in the pretrochal area, carrying 
three flagella. Some of the mesoderm cells become muscle cells (Fig. 216 G). 

The next stage may be calle(J. that of the Trochophora larva. This larva differs 
from a typical Annelidan Trochophora only by possessing a shell, which now covers 
the greater part of the body, and by a mantle which appears, at first, posteriorly, and 




Pio. 217.— Older Larva of Teredo, from the right side (after Hatscliek). Lettering as in Fig. 
216. In addition, 14, rudiment of the digestive gland (liver) ; 15, preoral ciliated band (velum) ; 
16, postoral ciliated band ; 17, primitive kidney ; 18, auditory vesicle ; 10, rudiment of the pedal 
ganglion ; 20, rudiment of the gill ; 21, mesodermal streak. 



then at the sides, as a fold, and continues to grow from behind forward. The region of 
the body which lies behind the cephalic area has spread out on each side to form a 
broad fold, which becomes outwardly applied to the shell. The neural plate has 
become inultilaminar, and the proctodeum has broken through into the mid-gut. 
The primitive mesoderm cells have given rise to a short mesoderm streak on each 
side. At the anterior end of each mesoderm streak a somewhat long body, the 
primitive kidney, has formed ; this contains a channel which opens externally, and 
whose lumen is ciliated at a later stage. The rudiment of the digestive gland 
appears in the mid-gut as a paired semi-spherical outgrowth. The body is no longer 
ciliated all over ; cilia are retained only on the neural plate and in the anal region. 
The double preoral ciliated band now becomes very distinct, and a postoral band is 



VII MOLLUSOA— ONTOGENY 261 

now added. The region between the two ciliated bands also carries cilia and forms 
an adoral ciliated zone. 

A further stage of development is depicted in Fig. 217. The rudiment of the 
pedal ganglion can be recognised as an ectodermal thickening on the ventral side, 
and that of the gill as a thick epithelial ridge. The stomach has formed a caecum 
posteriorly, and the narrow mid-gut has formed a loop. The two auditory vesicles 
containing otoliths have arisen between the mouth and anus as ingrowths of the 
ectoderm which have become detached. The mesoderm consists of branched muscle 
cells, branched cells of connective tissue, the primitive kidney and the still 
undifferentiated cells of the mesoderm streaks. 

The ectodermal thickening, which represents the rudiment of the pedal ganglion 
at a later stage, becomes rounded off and detached from the ectoderm, at the same 
time becoming surrounded by the cells of the mesoderm streak, which have rapidly 
multiplied, and which unite in front of it to form a median mass of cells. This 
median mass of mesoderm cells increases greatly by rapid division, bulging forward 
the ectoderm in the anterior ventral region, and thus forming the rudiment of the 
foot. In the growing branchial fold slits occur, a single slit appearing first, and 
another soon following in front of the first. The further development of this 
larva is unknown. 

The development of other marine bivalves runs very much the same course as 
that of Teredo, the same larva being formed. The ciliated band is very strongly 
developed in all marine bivalves {Teredo, Ostrea, Modiolaria, Cardium, Montacuta, 
etc. ), and is generally carried by a collar-like expansion of the integument, or velum, 
which is often divided into two lateral lobes. The velum, which on account of the 
band of strong cilia it carries is the locomotory organ of the free-swimming larvae of 
these Lameliibranchs, can be protruded out of and withdrawn into the shell. 

Among fresh-water Lameliibranchs there is one form, Dreissensia polymorpha, 
whose larva is free-swimming and carries a well-developed velum. This form is 
said to have migrated from salt to fresh water in (geologically speaking) recent 
times. 

Special arrangements are found among the other fresh-water forms. The eggs of 
Pisidium and Cyclas, for instance, develop in special brood capsules in the gills of 
the mother animal, and leave these as young bivalves. The Trochophora stage is, 
nevertheless, passed through, but the velum, not being used for locomotion, remains 
rudimentary. 

2. Ontogeny of Cyclas cornea (Figs. 218 and 219). — We shall here only mention 
the points in which the development of Cyclas differs from that of Teredo, and 
describe such observations as complete those made on the latter. The blastula 
consists of a cap of small cells (ectoderm cells) and a floor made of three large cells, 
one very large primitive endoderm cell and two symmetrical primitive mesoderm 
cells. The primitive endoderm cell yields through fission a disc of endodermal cells. 
The two primitive mesoderm cells become overgrown by the ectoderm cells, and 
thus reach the segmentation cavity. The endoderm invaginates in such a way that 
a slit-like blastopore arises, which reaches from the region of the future mouth to 
that of the future anus. This blastopore closes completely. The oesophagus arises 
as an ingi'owth of the ectoderm. A Molluscan Trochophora is formed with a 
shell gland, a rudimentary foot, a mid-gut, a stomach, anus, primitive kidney, and a 
neural plate. The velum is reduced to a ciliated area lying at the sides of the 
mouth (Fig. 218) ; this reduction is connected with the fact that the Trocho- 
phora of Cyclas is not a free-swimming larva, for the eggs of Cyclas pass through 
the whole course of their development within the gills of the mother animal. 
Above the neural plate the ectoderm cells are large and flat, and form a projecting 
cephalic veBicle. The mesoderm consists of (1) scattered cells, which lie under 



262 



COMPARATIVE ANATOMY 



CHAP. 



the ectoderm of the cephalic cavity, in the foot and on the intestine (especially on 
the oesophagus, where they are already changed into muscle cells) ; and (2) two 
mesoderm streaks lying at the sides of the intestine. The pedal ganglia arise 
together with the paired rudiment of the byasus gland, as thickenings of the 
ectoderm at the posterior end of the foot. The auditory vesicles originate as 
ingrowths of the ectoderm. The mantle forms by degrees from behind forward as a 
ridge, which grows more and more ventrally downwards. At the same time the 




Pig. 21S.— A-D, Four stages in the development of Cyclas cornea, from the right side (after 
Ziegler). 1, Membranous shell ; 2, rectum ; 3, anus ; 4, free edge of the mantle ridge or fold ; 
5, rudimentary byssus cavity with gland ; 6, rudiment of the pedal ganglion ; 7, foot ; S, velar 
region ; 9, cesophagus ; 10, stomach ; 11, calcareous shell ; 12, pericardium ; 13, kidney ; 14, rudi- 
ment of the gonad ; 15, edge of the membranous shell ; 16, edge of the calcareous shell ; 17, rudi- 
ment of the gill ; IS, byssus thread ; 19, visceral ganglion ; 20, posterior adductor ; 21, glandular 
part of the kidney ; 22, lateral wall of the pericardial vesicle ; 24, median wall of the same ; 
25, digestive gland (liver) ; 26, cerebral ganglion ; 27, mouth ; 28, auditory vesicle. 



shell gland, which at its edge secretes the delicate shell membrane, spreads out and 
becomes flattened. Beneath the shell-membrane the rudiments of the permanent 
shell valves are produced from two small round areas lying to the right and left of 
the dorsal middle line (B). The digestive gland (liver) develops from two lateral 
globular outgrowths of the wall of the stomach. The gonads arise from cells of the 
mesoderm streaks, which are larger than the rest and also in other ways differen- 
tiated, so that they can very early be distinguished. In the anterior and dorsal 



VII 



MOLLUSGA— ONTOGENY 



263 



part of eacli mesoderm streak a group of cells surrounds a cavity, wliicli at first is 
very small, but becomes continually larger. The two vesicles thus formed, the 
cavities of which represent the secondary body cavity, form the pericardium. 
Behind these the mesoderm cells collect in such a way as to form on each side a 
strand, which becomes hollow ; this is the rudiment of the nephridium, which at 
once becomes connected with the pericardial vesicle, and, growing further towards 
the ectoderm, soon opens outward. The two pericardial vesicles lengthen posteriorly 
and upward, each becoming divided into two parts, one lying behind the other, by a 
constriction, the parts still communicating dorsally with one another (Fig. 219 A). 
The two double vesicles grow towards one another above the rectum, and finally 
fuse in the dorsal middle line (B). In a similar manner they fuse below the 
rectum. The inner wall of the pericardial, vesicle becomes the wall of the ventricle 
(C), and its lateral wall becomes 
that of the auricle. At the points A 

where the lateral vesicles were con- 3 

stricted He the slits through which 
the auricles communicate with the 
ventricle, and the atrioventricular 
valves. 

The visceral ganglion arises at 
the posterior end of the mantle 
furrow from an ectodermal thicken- 
ing. The pleurovisceral connectives Fig. 219.— A-0, Diagrams illustrating the develop- 
form, in all probability, throughout ment of the pericardium and heart of Cyclas cornea 

(after Ziegler). 1 and 2, The two lateral pericardial 
vesicles ; 3, rectum ; 4, pericardial cavity ; 5 and 6, 
invaginations of the lateral walls of the pericardinm= 
rudiments of the two lateral auricles ; 7 and S, median 
walls of the two latei-al pericardial vesicles, in B partly 
fused to form a median septum (above and helow the 
intestine), which in C has disappeared ; 9, rudiment of 
the ventricle. 




their whole length, through con- 
striction from the ectoderm. The 
gill arises on each side as a fold on 
the dorsal edge of the inner surface 
of the mantle. It develops from 
behind forward. In the contrary 
direction furrows form on the 
branchial fold, commencing from below upwards ; these are found on the inner as 
well as the outer surface, and exactly correspond. The inner furrows join the outer 
right through the gill, and thus give rise to the branchial slits. 

3. The development of the Unionidae [Anodonta, Unio) is much influenced by 
the parasitic manner of life of the larva. The fertilised eggs reach the outer leaf 
of the gill of the female, and there run through the first stages of their develop- 
ment. Segmentation leads to the formation of a cceloblastula, in which the rudi- 
ment of the shell gland appears very early as an incurved plate of large and 
high cells of the blastula wall. The archenteron forms by invagination at a very 
late stage ; this is no doubt connected with the later parasitism of the larva. 
Before this invagination occurs the mesoderm has begun to form ; its two primi- 
tive cells lie in the blastoccel at the part where, later, the enteric invagination 
appears. 

The embryo known as Gloohidium paraslticum has, in the last stage of its 
development, which is passed through in the gills of the mother auimal but within 
the egg-shell, the following structure (Fig. 220). It is bilaterally symmetrical, and 
has a bivalve shell. Each valve has, at its ventral edge, a triangular process, 
the exterior of which is beset with short spines and thorns. Between the two valves, 
which are markedly concave, lies the soft body, which lines the shell internally in 
such a way that its ventral epithelial layer might, incorrectly, be called a mantle. 
It may be called the false mantle. If this false mantle is examined from below, 
when the shell is open, it is seen to have on each side four sensory cells furnished 



264 



COMPARATIVE ANATOMY 



CHAP. 




B 



witli long sensory hairs ; three of these cells lie near the shell process, and the fourth 
near the middle line. Between the two more median sensory cells a long adhesive 
filament projects from the opening of the gland which secretes it. Behind this 
gland are found — (1) the oral sinus ; (2) a small prominence, the pedal swelling ; 

(3) the ciliated lateral pits, 
H one on each side ; and (4) 

furthest back of all, the 
ciliated shield or patch. 
Between the mantle and 
shell the embryonic adduc- 
tor runs across from the one 
valve to the other. Besides 
these are only found a few 
isolated muscle fibres, and 
the rudiment 'of the mid- 
gut, the latter as an epi- 
thelial vesicle, which be- 
comes entirely separated 
from the ectoderm, and in 
no way communicates with 
the exterior. 

The embryo at this stage 
leaves the gills, at the same 
time emerging from the egg 
shell. Its adhesive filament 
floats in the water. If a 
passing fish comes in contact 
with such an embryo, the 
latter can, by closing its 
shell, attach itself by means 
of the triangular processes 
mentioned above, to its in- 
tegument, into which the 
spines on these processes 
penetrate. The embryo of 
Anodonta attaches itself 
chiefly to the fins, that of 
Unio to the gills of the fish. 
The epithelium of the part 
of the fish attacked grows 
very rapidly in such a way 
as in a few hours to surround the parasite completely. The embryonic false mantle 
grows out from each valve of the shell as a fungus-like body to penetrate the tissues 
of the host, and probably serves for nourishing the embryo. During this endo- 
parasitic life, which lasts for several weeks, the transformation of the embryo into 
the young Mussel is completed. In the course of this process of transformation 
some larval organs are resorbed, and also serve for nutrition ; first the sensory cells 
disappear in this way, then the gland of the adhesive filament with the remains of 
the filament itself, then the adductor, and finally the false mantle. The rudiment 
of the definitive mantle and .shell then appear. The vesicular mid-gut joins the oral 
sinus ; the pedal swelling grows into the linguiform foot, and, in this, the rudi- 
mentary byssus gland appears as an ingrowth of the epithelium. The rudiments 
of the inner branchial leaves, the digestive gland, the nephridium, the heart, the 




~C<|^J^'V 



Fig. 220.— GlooMdium larva of Anodonta, from the outer leaf 
of the gill of a female. A, from below, the shell being open 
(after ScMerholz). B, in optical transverse section (after 
Flemming). 1, Sensory setfe ; 2, adhesive filament ; 3, shell- 
process ; 4, false mantle ; 5, lateral pits ; 6, oral sinus ; 7, pedal 
swelling ; 8, ciliated patch ; 9, embryonic adductor ; 10, shell. 



VII MOLLUSCA— ONTOGENY 265 

cerebral, pedal, and visceral ganglia, and the auditory vesicle appear during the 
pai'asitio stage, in the same way as in other Lamellibranchs. 

During the last week of parasitic life the capsule formed by a growth of the 
tissue of the host which surrounds the embryo becomes thinner ; the parasite breaks 
through it, and falls to the bottom of the water as a young Mussel. The only 
organs still wanting are the genital organs, the outer leaves of the gills, and the oral 
lobes. 

£. Cephalopoda. 

Tetrabranchia. — Nothing is known of the development of Nautilus. 

Dibranchia. — The egg is usually very large, and contains, like that of the sharks, 
reptiles, and birds, a great quantity of nutritive yolk. It belongs to the telolecithal 
meroblastie type, and is enclosed in a capsule. A number of such capsules may 
become cemented together to form strings. The partial segmentation takes place 
at the animal pole of the egg, and leads to the formation at that point of a germinal 
disc (blastoderm). 

Ontogeny of Sepia. — The blastoderm grows so very slowly round the yolk, that 
long after all the outer organs of the embryo are quite recognisable in the region 
of the original germinal disc, the opposite pole is still occupied by the yolk. The 
germ lies in such a way that the centre of the germinal disc or animal pole is placed 
dorsally, and corresponds with the uppermost point of the visceral dome of the adult 
animal, while the mass of nutritive yolk lies ventrally. 

1st Stage (Fig. 221 A). — In the centre of the germinal disc there appears an 
oval-rhombic bulging ; this is the rudiment of the visceral dome and the mantle. 
On each side of this there arises a bean-shaped prominence, the rudiment of the eye. 
Behind the eye, on each side, a long narrow ridge rnns backward in a curve ; about 
half way down this ridge a small prominence, the rudiment of the funnel cartilage, 
forms close to its outer side. The part of the ridge lying in front of this prominence 
becomes the muscle which runs from the funnel to the nuchal cartilage ; the 
posterior part (which lies behind the rudiment of the visceral dome and mantle) 
forms the paired rudiment of the funnel itself. Between the two rudiments of 
the funnel two other prominences rise symmetrically — the rudiments of the gills. 
A pit in the centre of the rudiment of the visceral dome has been indicated as the 
rudiment of a shell gland (?). 

2nd Stage (Figs. 221 B and 222 A). — The rudiments just described stand out 
more prominently. On the outer and posterior sides of the rudiments of the funnel 
the rudiments of the two posterior pairs of arms first appear as prominences, then 
those of the third and fourth pairs. The first indications of the head are seen in 
the form of a large double swelling on each side, the outer and anterior part of 
which carries on each side the rudiment of the eye. The embryo becomes covered 
with cilia. At the extreme anterior end the mouth appears in the middle line, 
forming the opening of the oesophagus, which begins to sink inwards. 

3rd Stage (Fig. 221 C). — The whole embryo has become more arched dorsally, 
and more marked off' from the yolk. On the latter, the blastoderm, which consists 
of two layers, an external ectoderm and an internal yolk membrane, has spread out 
further towards the ventral (vegetative) pole of the egg. At the posterior edge of 
the rudiment of the visceral dome, the mantle fold has grown out forward in such a 
way as to form a small mantle cavity, which already partly covers the rudiments of 
the gills. In the space between the rudiments of the funnel and the gills the 
proctodseum has formed by invagination, and its aperture, the anus, can be seen. 
The rudiment of the fifth pair of arms appears. 

4th Stage (Figs. 221 D and 222 F, G). — The visceral dome projects further. 



266 



COMPARATIVE ANATOMY 



CHAP. 



and has a free mantle edge all round its base. The gills have shifted further 
into the mantle cavity, which is now larger, and lies posteriorly. The rudiments 
of the funnel also now lie close to the mantle, and are so approximated pos- 
teriorly as nearly to touch. The rudiments of the arms have shifted from 
behind further forward round the rudiments of the head. As the whole embryo 
projects more distinctly from the yolk, the rudiments of the arms shift nearer to 




Fig. 221. — Ontogeny of Sepia (after Koelliker). A-E, Five stages of development. The free 
surface of the germinal disc whicli lies on the yollv is seen, its centre corresponding with the dorsal 
point of the visceral dome of the adult Sepia. The anterior side of the embryo lies lowest in the 
figures. ('., Visceral dome with mantle ; &, rudiment of eye ; c, rudiment of gill ; d, halves of the 
funnel ; e, rudiment of the funnel cartilage belonging to the apparatus for closing tlie mantle ; 
/, periplieral part of the blastoderm, which, growing all round the yolk, forms tlie yolk-sac ; 
(/, mouth ; h, posterior cephalic lobe ; i, anterior cephalic lobe ; h, anus ; 5, anterior or first pair 
of arms ; 4, 3, 2, 1, second, third, fourth, and posterior pairs of arms. 

one another and under the rudiments of the bead. The anus is already covered by 
the mantle fold. 

5th Stage (Figs. 221 E and 222 B, H).— The arms shift still nearer to one 
another (i.e. towards the axis of the embryo), grouping below the rudiments of the 
head (A^hich have become fused), and form a somewhat narrow circle on the ventral 
side ifi such a way that, when the embryo is seen from the dorsal side, some of them 
are hidden by the head. As a consequence of this the embryo, which is already 
recognisable as a young Sepia, now becomes sharply constricted from the yolk 
beneath it. The free edges of the rudiments of the funnel fuse and move to a position 
within the mantle cavity. 

6th Stage (Fig. 222 C). — The rudiments of the head and arms have now 
assumed the typical position to form the "head" (Kopffuss). The embryo is now 
altogether distinct from the yolk, to which it merely hangs instead of, as before, 
lying upon it. The blastoderm finally grows round the yolk and so forms a yolk 
sac. At first this sac is four or five times the size of the embryo, but in proportion 



VII 



MOLL USOA— ONTOGENY 
D 



267 




Fic. 222.— Various stages in the development of Sepia (after Koelliker). A, B, 0, D, 
Anteriorlvlew ; E and F, from the left side ; G and H, from behind. Lettering aa m Pig. 221. In 
addition : d, rudiment of the funnel-nuchal muscle (coUaris) ; di, paired rudiment of the funnel 
proper ; jj, yolk ; aj, edge of the mantle ; t, optic invagination (?) ; «, region of the shell ; 5, edges 
of the two rudiments of the funnel bent round ; r, fins. In G the mantle fold is raised np in;H 
cut off. 



268 COMPARATIVE ANATOMY chap. 

as the latter grows at the expense of the yolk and develops further, the sac becomes 
smaller, so that when the embryo is hatched the size of the yolk-sac is only one third of 
that of the young animal (Fig. 222 D). It must further be mentioned with regard to 
the yolk sac that it is at no time in communication with the intestine. As the 
embryo becomes constricted from the yolk the latter divides into two parts — an inner 
part, lying inside the embryo, and an outer part, filling the sac. These two parts 
are connected by means of the stalk of the yolk sac, which projects down-ward from 
the "head." The yolk within the embryo is divided into three unequal parts, the 
largest of which fills the visceral dome ; another mass of considerable size fills the 
" head," and these two masses are connected with a smaller portion lying in the 
nuchal region. 

Loligo and Argouauta have a, smaller yolk sac, round which the blastopore 
gi-ows at an earlier stage than in Sepia. The yolk sac of Argcniauta is entirely 
taken into the body before the latter has completely closed ventrally. 

The quantity of nutritive yolk is still less in a Cephalopod {Ommatostrephes ?), 
the spawn of which floats in the sea. Segmentation is in this case also partial and 
discoidal, but the blastoderm almost completely encloses the yolk before any organ 
develops on the germ, and no external yolk sae is formed. 

The results of the investigations hitherto made with regard to the germinal 
layers, the development of the inner organs, and the inner differentiations of the 
outwardly visible organs are so contradictory and in many cases so incomplete, 
that no description of them is here attempted. Further investigation is much 
needed. The development of the eye has already been described (p. 171), and that 
of the hind-gut and ink-bag was illustrated (p. 197). 

Two important facts in the ontogeny of the Dibranchia should be noted. (1) 
In considering the arms as parts of the foot, it is important to notice that they 
arise behind the rudiments of the head, and only secondarily come to lie round and 
below the latter. The mouth, at quite a late stage, lies at the anterior end of the 
circle of arms (Fig. 222 C). (2) The funnel consists of two separate lateral rudi- 
ments, the free edges of which fuse secondarily. This point is important in connec- 
tion with the separation of the two lobes of the funnel, which lasts throughout life 
in Nautilus. For the view of the funnel as epipodinm, cf. p. 116. 

The fact that the velum is wanting in the Cephalopod embryo must also be 
noted. The absence of this organ is explained by the direct development of the 
CeplwClopoda, within the egg capsule at the expense of a large quantity of nutritive 
yolk. 

Investigations as to the development of the shell, and as to the nature of the 
organ which has been called the shell gland, are much needed. 



XXIV. Phylogeny. 

No actual points of connection between the Molluscan phylum and any other 
division of the animal kingdom have as yet been found ; the origin of the MoUnsca 
is therefore merely a matter of speculation. The present writer favours the view 
that the Mollusca descended from animals like the Turbellaria, which had become 
differentiated from the modern Platodes by the acquisition of a hind-gut and a heart, 
and the (at least partial) transformation of the genital cavity into a secondary 
and primitively paired body cavity. There is a striking agreement iu the nervous 
system of the lower Molluscs {Chiton, Solenogastres, and in some respects the Dioto- 
cardia) and that of the Platodes; in both there is a ladder-like nervous system 
with the principal trunks beset along their whole length with ganglion cells ; the 



VII M0LLU8GA— LITERATURE 269 

pleurovisoeral cords answer to the lateral trunks of the Plciiodes, and the pedal 
cords to the ventral longitudinal trunks of the latter. If such a hypothetical racial 
form were to secrete a dorsal shell, perhaps at first in the form of a thick cuticle 
containing calcareous particles, a typical Mollusoan organisation would he produced. 
The development of a shell would deprive the greater part of the surface of the body 
of its original respiratory function, and would lead to the formation of localised 
gills. By means of the development of a mantle fold these delicate-skinned organs 
could be brought under the protection of the shell. The musculature on the dorsal 
side, which the shell covered, would disappear, and with it the dorsal longitudinal 
nerve trunks. The musculature on the ventral side, which was already strongly 
developed in the Planaria, would become strengthened in the development of the 
foot with its sole for creeping. A part of the dorsoventral muscnlature would be 
changed into the shell muscle. 

In this derivation of the Mollusca their characteristic larval form might be 
explained, without any need for tracing it to the Annelidan Trocophora, in the 
following way. It would correspond to a Turbellarian larva (Miiller's Polyclade 
larva, etc. ), on to which certain Molluscan characteristics such as the shell gland, 
the shell, the anus, and the foot had been shifted back. The preoral ciliated band 
(the velum) of the Molluscan larva would correspond with the same structure in 
the Turbellarian larva. The primitive kidney of the former would answer to a 
sijnplified water vascular system, while the permanent nephridia as ovarial and 
seminal ducts might be homologised morphologically with the ducts of the 
genital products in the Turbellaria. 



Review of the most important Literature. 

Comprehensive Works. Text-Books. General, Works. Investigations 
treating of all or several Classes. 

Boll. Bcitrdge zur Vcrgleich. Histologic des Molhiskerdypus. Arch, fur mikr. Anat. 

Supplmnentbaiid. 1869. 
H. G. Bronn. Die Klassen und Ordnmigen des Thierreichcs. 3 Bd. Malacozoa. 

I. Malacozoa acephala. 1862. II. Malacozoa ceplialophora, von W. Kcfcrstcin. 

1862-1866. (New edition now appearing, i. Simroth.) 
G. Cuvier. Mimoires pour scrvir d, Vhistoirc et d V anatomic des Mollnsqucs. Paris, 

1817. 
G. B. Deshayes. Traiti elimentaire de Conchyliologie. 3 vols. Paris, 1839-1867. 

Histoire naturelle des MoUusqucs {Exploration de V Ahjirie). 1848. 

Eydoux and Souleyet. Voyage autour du monde sur la corvette la Bonitc. Histoire 

naturelle. Zoologie. Paris, 1852. 
Paul Fischer. Manuel de Conchyliologie et de Paliontologie emuhyliologique. His- 
toire naturelle des Mollusques vivants ctfossiles. 2 vols. Paris, 1887. 
H. von Jhering. Verglcichende Anatomic des JHervcnsy stems, und Phylogenie der 

Mollusken. Leipzig, 1877. 
Keber. Bcitrdge zitr Anatomie und Physiologic der Weichthicre. Konigsberg, 1851. 
E. Ray Lankester. Mollusca. Encyclopaedia Britannica. 9th edit. Vol. XVI. 

1883. 
R. Leuckart. Zoologische Untersuchungen. Heft 3. Giessen, 1854. 
Pelseneer. Introduction d I'itucle des Mollusques, Bruxelles, 1894. 
Poll. Testacea utriusque Sieiliae eorumque historia et anatome. 3 Bd. 1791-1795. 



270 GOMPARATIVE ANATOMY chap. 

H. Simroth. Ueher einigo Tagesfragen der Malacozoologia, iMiiptsdchlieh Convergen- 
zerscheinungen Tjetreffeiid. Zeitschrift Nattmviss. Halle. 62 Bd. 1889. 

The early parts of the new edition of vol. lii. of Bronn's Klassen itnd 

Ordnungen des Thierreichs. 

J. Thiele. Die Stammesverwandscliaft der Mollnsken. Mn heitrag zur Phylogenie der 
Thierc. Jetiaisehe Zeitschr. f. Naturwissnisch. 25 Bd. 1891. 

S. P. Woodwaxd. A Maiiual of the Mollusca. 4th edit. 1880. 



Amphineura. 

L. Graff. Anatomie des Olmetoderma nitidiUwtn. Zeitschr. f. wissen. Zoologie. 
26 Bd. 1876. 

Neo'menia mid OfuMtoderma. Zeitschr. /. wissen. Zoologie. 28 Bd. 1877. 

B. Haller. Die organisation der Chitonen der Adria. Arb. aus dcm Zool. Instil, in 
Wien. I. Theil. i Bd. 1882. II. Theil. 5 Bd. 1883. 

G. A. Hansen. Anatom. Beskrivelse of Chmtoderma nitidiilum. Nyt magaz. for 
naturvidenskai. Vol. XXII. 1877. 

Neomenia, Pj-oneomenia und Cluctoderma. Bergen Mus. Aarster. f. 1888. 

J. Heuscher. Zur Anatomie und Histologic von Prcmeamenia Sluiteri. Jena. Zeitzch. 

Vol. XXVII. 1893. 
A. A. W. Hubrecht. Proneomenia Sluiteri. Niederl. Arch. Zool. Suppl. I. 1881. 

A Contrihutimi to the Morphology of Amphineura. Quart. Journ. Micr. 

Science. Vol. XXII. 1882. 

Dondersia festiva gen. et spec. nov. Vonders Festhundel. Nederl. Tijdschr. 

Geneesk. 1888. 

J. Koren and D. C. Danielssen. Descriptions of iiew species belonging to the genus 

Solenapus, with some observations on their organisation. Ann. Nat. Hist. (5). 

Vol. III. 1879. 
A. Kowalevsky and A. F. Marion. Contributions A I'histoire des Solenogastres ou 

Aplaeophores. Ann. Mus. H. N. Marseilles. Tome III. 1889. 
A. Th. V. Middendorff. Beitrdge zu einer Malacozoologia rossica. I. Beschreibung. 

und Aiuitomie neuer oder filr Pussland neuer OJiitonen. Mim. de I'Acadimie 

St. Petersbourg. Tome VI. 1849. 
G. PrUTOt. Sur I'organisation de qiielques N(ominiens des c6tes de France. Arch. 

Zool. expir. 9?. Vol. IX. 1891. 
A. Sedgwick. On certain points in the anatomy of Chiton. Proceed. Roy. Soc. 

No. 217. Dec. 1881. 
Simroth. See above. 
T. TuUberg. Neomenia, a new genus of invertebrate animals. Svenska Vet. Akad. 

Handl. Bd. III. 1875. 
Axel Wiren. Studien iiber die Solenogastres. I. Monographic des Chaitoderma niti- 

dulum Lovin. Kongl. Sveiiska Vetenskaps - Akademiens Handlingar. Vol. 

XXIV. Stockholm, 1892. 
In addition, works of Van Bemmelen, Dall, Pelseneer, etc. 

Gastropoda. 

Alder and Hancock. A inonograph of the British Nvdibranchiate Mollusca. 

London, 1860-1851. 
R. Bergh. Beitrdge zu einer Monographic der Polyceraden, /., //., ///. Ver- 

handl. der k. k. Zoolog. Botan. Gesellsclwft zu Wien. 29, 30, 33 Bd. 1879- 

1883. 



vii MOLLUSCA— LITERATURE 271 

R. Bergh. Ueber die Verwandischaftsbeziehungen der Onckidien. Morpli. Jahrl. 10 
Bd. 1884. 

Meport on the Ntidibranckiata of the " Clmllenger" Expedition. Chall. Report 

Zool. Vol. X. 1884. 

Die Titiscanien, eine Familie der rhipidoglossen Gastropoden. With three 

Plates. Morphol. Jahrb. 16 Bd. 1890. 

Die Marseniaden. Zool. Jahrb. 1 Bd. 1886 ; compare also Semper's Seisen 

im Arehipel der Fhilippinen. 2 Theile. IVissensch. Resultate. Suppl. Heft 3. 
1886. 

Die cladohepatisehen Nndibranchien. Zool. Jahrb. Abth. fur Systematik. 

5 Bd. 1891. 

Die cryptobranehiaten Dorididen. Zool. Jahrb. Abth. fur Systematik. 6 

Bd. 1891. 

In addition, numerous monographs of various families, genera, and species 

of the Opisthobranchia in various journals. 

J. E. V. Boas. Spolia atlantiea. Bidrag til Pteropodemes Morfologi og Systematik 
saint til Kuiidskahen om deres geografiske Udbredelse. Danske Vid. Selsk. 
Skr. (6). 4 Bd. 1886. 

Zur Systematik tmd Biologie der Pteropoden. Zool. Jahrb. 1 Bd. 1886. 

L. Boutan. Recherehes siir I'anatomie et le developpement de la Fiss^irelle. Arch. 

Zool. expir. (2). Tome III. 1886. 
E. L. Bouvier. Systk'me nerveux, morphologie gindrale et classification des Gastro- 

podes prosobrancJies. Ann. des Sciences Nat. (7). Tome III. 1877. 
E. Claparede. Anatomie und Entwickelungsgeschichte der Neritina fluviatilis. 

Mailer's Archiv. 1857. 
P. Gamault. EecJierches anatomiqiies et histologiques stir le Cyclostoma elegans. 

Arch. Soc. Linn. Bordeaux, 1887. 
C. Gegenbaur. Untersnchungen ilber Pteropoden und Heteropoden. Leipzig, 1853. 

E. J. Harvey Gibson. Anatomy and physiology of Patella vulgala. Part I. Ana- 

tomy. Trans. Roy. Soc. Edinburgh. Vol. XXXII. 1887. 
B. Haller. U-ntersuehungen iiber marine Rhipidoglossen. I. Studie. Morph. Jahrb. 
9 Bd. 1883. //. Studie. 11 Bd. 1886. 

Die Morphologie der Prosobranchier, gesammelt auf einer Erdiimsegelung 

dureh die XSnigl. ital. corvette " Vettor Pisani." I. Morph. Jahrb. 14 Bd. 
1888. //. ibid. 16 Bd. 1890. 

Huxley. On the morphology of the ceplialoas Mollnsca as illustrated by the anatomy of 
certain Heteropoda and Pteropoda, etc. Philos. Transactions. 1853. 

J. Joyeux-LafFuie. Organisation et diveloppement de VOncidie {Onehidium eelticum 
Guv.). Arch. Zool. expir. Tome X. 1882. 

H. de Laoaze-Duthiers. Sistoire et monographic dti Pleurobranche orangi. Annates 
des Sciences Nat. (4). Tome XI. 1859. 

M(moire swr la Pourpre. Annates des Sciences Nat. (4). Tome XII. 

1859. 

Mimoire swr le systime nerveux de V Haliotide. Ann. des Sciences Nat. 

(4). Tome XII. 1859. 

Memoire sur I'ariatomie et V embryoginie des Vermets. Ann. des Sciences 

Nat. (4). Tome XIII. 1860. 

■ Sistoire de la Testacella. Arch. Zool. expir. (2). Tome V. 1888. 

A. Lang. Versuch einer Erkldrung der Asymmetric der Gastropoden. Viertel- 
jahrsschrift d. Naturf. Gcsellsch. Zurich. 36 Bd. 1891. 

F. Leydig. Ueber Palwdina vivipara. Zeitschr. f. wiss. Zool. 2 Bd. 1850. 
Milne Edwards. Note stir la classification nattirelle des Mollv,sqties Gastropodes. 

Ann. des Sciences Nat. 1848. 



272 COMPARATIVE ANATOMY chap. 

G. Moquin-Tandon. lieclterches anatomiqiics mtr I'ombrelle de la meditcrrmiie. Ann. 

des Sciences Nat. (6). Vol. XIV. 187.5. 

H. Miiller and C. Gegenbaur. Ucber PhylHrhoe buceplmhtm. Mull. Arch. 1858. 

A. Nalepa. Bcitrdge zur Anatomic der Stylommatojphorcii. Sitz.-Bcr. Akad. Wien. 

87 Bd. 1883. 
Nordmann. MonograpJiiede Tergipes Edn-nrdsii. Mem. Acad. Imp. St. Pitcrshmirg. 

Tome IV. 1843. 
J. Paneth. Be itrdge zur Histologic cler Ptrropoden mid Heteropiodcn. Archiv. fur 

mikrosl: Aiiut. 24 Bd. 1884. 
J. I. Peck. Oil tlm anatcny and histology of Cymhuliopsis calceola. 4 Plates. 

Studies Biol. Labor. Johns HopTc. Univ. Vol. IV. 
Paul Pelseneer. Beport on the Pteropoda collected by H.M.S. " Challenger " duriny, 

etc. Parts I. II. III. Chall. lieport Zool. Part LVIII. 1887 ; Part LVI. 

1888 ; Part LXV. 1888. 

Tlie cephalic aiipendages of the gymnoso^natous Pteropoda, and espiccially 

of Olioiw. Q\mrt. Jonrn. Mia-osc. Science {1). Vol. XXV. 1885. 

L. Plate. Studien iibcr opistJwpncninviie Lnngenschncckcii. I. Daiidebardia und 
Testacella. Zool. Jali7-biicher. Abth.f&r Anatomic und Ontogeiiie. 4 Bd. 1891. 

Quatrefages. Memmrc sur les Gastropodes phlehentires. Ann. des Sciences Nat. 
Tomes III. and IV. 1844 and 1845. 

Bang and Souleyet. Histoire nnturclle des Mollusqxies Ptiropodes. Paris, 1852. 

B. Sharp. Beitrdge zur Anatomic von Ancylus fluviatil is (0. F. Miill) und Aneyhis 

lacustris (Geoffroy). Inaug. -Dissert. Wiirzburg, 1883. 
H. Simroth. Ucber die Bewegung und das Bewegungsorgan des Cyclostoma clegaiis 
und der einheimischen Sehneckcn ilbcrhaupt. Zeitselir. f. Wiss. Zool. 36 Bd. 
1881. 

Versuch eincr Naturgeschichte der deulscJocn Nacktschnccken und ihrcr 

europdisclien Verwandten. Zeitsehr. f, wiss. Zoologie. 42 Bd. 1885. 

In addition, many other treatises on the Pulmonata in various journals. 

S. Trincliese. Matcriali per la fauna maritima italiaiia. Aeolididae c famiglie 

affini. Atti acead. Lincei (3). Mem. Vol. XI. 1883. 
Troschel. Beitrdge zur Kenntniss der Ptrropioden. Arch. f. Naturg. Tome XX. 

1854. 
M. Vayssiere. BecJierches anatomiques sur les Molhisques de lafamille des Bullidis. 

Ann. Hist. Ned. Zool. (6). Tome IX. 1880. 
• Becherches arMtomiques svr les genres Pelta {Eimcina) et Tylodina. Ann. 

des Sciences Nat. (6). Tome XV. 1883. 

Becherches zoologiques et anatomiques sur les Mollusques opisthobranches du 

(jolfe de Marseilles. Part I. Tectibranches. Ann. Mus. Hist. N. Marseilles. 
Tome II. Mem. 3. 1885. Part II. Ibid. Tome III. Mem. No. 4. 1888. 

Nicolas Wagner. Die iviriellosen Thiere des weissen Meeres. 1 Bd. Leipzig. Fol. 

1885. 
H. Wegmann. Contribution d Vhistoire ncdurelle des Haliotidcs. Arch. Zool. expir. 

(2). Tome II. 1884. 

Note sur I' organisation de la Patella mUgaia L. Becueil. Z. Suisse. Tome 

IV. 1887. 

Emile Yung. Contributions d Thistoire physiologique de Vescarcjot (Helix po^ncdia). 
Mim. Com: Acad. Belg. Tome XLIX. 1887. 

Scaphopoda. 

Henn. Fol. Sur V anatomic microscopiique du Deutale. 4 PI. Arch. Zool. cxpiir. (2). 
Tome VII. 1889. 



VII MOLLUSG A— LITERATURE 273 

H. de Lacaze-Duthiers. Histoire da V organisation ei du dimloppement du Deniale. 

Ann. des Sciences Nat. (4). Tomes VI., VII., and VIII. 1856-57-58. 
L. Plate. JHenierkungen zur Organisation der Dentalien. Z. Anzeiger. 11 Jahrg. 

1888. Ueier das Hers der Dentalien. Ihid. 14 Jahrg. 1891. 
M. Sars. Om Siphonodentalium vitreum, etc. Christiania, 1861. 



Lamellibrauchia. 

Ernst Egger. Jotiannetia Gumingi Son. Eine morphol. Untersuclmng. Arbeit. 

Zool. Instit. Wilrzburg. 8 Bd. 1887. 
Gamer. On the anatomy of the LameUibranchiate Gonchifera. Transact. Zool. Soc. 

London. Vol. II. 1841. 
H. de Laoaze-Duthiers. iUmoire sur l' organisation de I'Anomie. Ann. des Sciences 

Nat. (4). Tome II. 1854. 
■ Morphologie des Aciphales. 1 Mem. Anatomic de VArrosoir {Aspergilhini 

dichotommm). Arch. Zool. expir. (2). Tome 1. 1883. 
Leydig. Aaaiomie nnd Entivickehtng von Gyclas. Mailers Archiv. 1835. 
H. A. Meyer and Moebius. Fauna der Kieler Bucht. Leipzig, 1865. 
Paul Pelseneer. Report on the anatomy of the deep-sea Mollusca collected by H.M.S. 

" Ghallenger." Report Ghall. Zool. Part 74. 1888. 

Gontribniion d VHnde des Lamellibranches. Archives de Biologic. Tome XI. 

1891. 

A. de Quatrefages. Mlnwire siir le genre Taret. Ann. des Sciences Nat. (3). Tome 
XL 1849. 

Cephalopoda. 

A. G. Bourne. The differences betv)een the males and females of the pearly Nautilus. 

Nature. Vol. XXVIII. 1883. 
J. Brock. Studien ilber die Verwandtschaftsverhdltnisse der dibranchiaten Gephalo- 
poden. Erlangen, 1879. 

Versxich einer Phylogenie der dibranchiaten Gephalopoden. Morph. Jahrbuch. 

6 Bd. 1880. 

Zur Anatomie und Systematik der Gephalopoden. Zeitschr. f. unss. Zool. 36 

Bd. 1882. 
Delle Chiaje. Memorie su' Cefalopodi. Memorie sulla storia e notmnia degli ani- 

mali senza vertebre del regno di Napoli. Napoli, 1829. 
F^russac and d'Orbigny. Histoire naturelle g&iUrale et particulUre des G^phalopodes 

acitabulifires vivants et fossiles. Paris, 1835-1845. 
L^on Frederlcq. Recherches sur la physiologic du Poulpe commun {Octopus vulgaris). 

Arch. Zool. expir. Tome VII. 1879. 
Carl Grobben. Zur Kenntniss der Morphologie und Verwandtschaftsverhdltnisse der 

Gephalopoden. Arb. Z. Iiist. Wien. 7 Bd. 1886. 

B. Haller. Beitrage zur Kenntniss der Morpihologie Nautilus Pompilius. Zool. d. 

Semon's Forsehungsreise in Australien. Vol. V. 1895. 
H. von Jhering. Ueber die Verwandtsehaftsbeziehungen der Gephalopoden. Zeitschr. 

f. miss. Zool. 35 Bd. 1880. 
Van der Hoeven. Beitrag zur Kenntniss von Nautilus. Amsterdam, 1856. 
Will. E. Hoyle. Observations on the anatomy of a rare Gephalopod (Gonatus Fabricii). 

Proc. Z. Soc. London. 11. 1889. 
H. Miiller. Ueber das ManneJien von Argonawta argo mid die Hcetocotylen. Zeitschr. 

f. loiss.. Zool. 1855. 
R.Owen. Mem/}ir on the piearly Nautilus, aia. London, 1832. 

VOL. II T 



274 COMPARATIVE ANATOMY chap. 

E. Owen. Description of some iww and rare Cephalopoda. Traits. Zool. Soc. London. 

Vol. II. 1841. 

Cephalopoda. Todd's Cyclopcedia, etc. Vol. I. London, 1836. 

Suppleinentary Observations on the Anaimny of Spirilla australis Lam. Ann. 

of Nat. Hist. (5). Vol.111. No. 13. 1879. 

J. B. Verany. Mollusques midlterraniens dbservis, dierits, fignris et chrmnolitho- 

graphiii d'apris le vivant. Part I. Cilphalopodes de la M&Iiterran4e. Genes, 

1847-1851. 
Verany and Vogt. Mimoires sur les Hectocotyles, etc. Ann. des Sciences Nat. 

Tome 17. 1852. 
W. J. Vigelius. Untersuchmigen an Thysanoteuthis rhombus Frosch. Eiii Beitrag 

zur Anatomic der Cephalopoden. llitth. Zool. Station zu Neapel. 2 Bd. 1880. 

F. Ernest Weiss. O-a some oigopsid cuttle-fishes. Quart. Journ. Micr. Science. 

Vol. XXIX. 1889. 



Treatises on Single Organs or Groups of Organs, — Integument, Mantle, Shell, 

Oral Lobes. 

F^lix Bernard. Recherches sur les organes palUaux des GastrojMdes prosohranches. 

Thise. Paris, 1890. Also in Annates des Sciences Nat. VII. 1889. 
F. Blochmann. Ueber die Drilsen des Mantelrandes bei Aplysia und verwandten 

Formen. Zeitschr. f. wissensch. Zool. 38 Bd. 1883. 
Jos Blumrich. Das Integument der Chitonen. Hit ciner VorbemerTcung von Prof. 

Hatschek. Zeitschr. f. loiss. Zoologie. 52 Bd. 1891. 
Bowerbank. On the structure of the shells of molluscous and conchiferous animals. 

Trans, of Micr. Soc. I. London, 1844. 
W. Carpenter. On the microscopic structure of shells. Report British Assoc. 1843 

and 1847. London, 1844 and 1848. 
E. Ehrenbaum. Untersuchmigen iiber die Structur und Bildung der Schale der in 

der Kieler Bucht hiiufig vorkommende7i Muscheln. Zeitschr. f. wiss. Zool. 

41 Bd. 1884. 
P. Girod. Uecherchcs sur la peau des Cephalopodes. Arch. Zool. experiment 12). 

Tome I. 1883. 
H. Meckel. Mikrographie einiger Driisenapparate der niederen Thiere. Mailer's 

Archie. 1846. 
Felix Miiller. Ueber die Schalenbildung bei Lamellibranchiaten. Zool. Beitrdge 

Schneider. 1 Bd. 1885. 
E. Owen. On the relative 2}ositions to their Constructors of the chambered shells of 

Cephalopods. Proc. Zool. Soc. London. 1878. 
Bemhard Rawitz. Der Mantelrand der Acephalen. 1. Ostracea. Jeimische 

Zeitschr. f. Naturiinss. 22 Bd. 1888. 2. Arcacea, Mytilacea, Unionacea. Ibid. 

24 Bd. 1890. 
G. Steinmann. Vorldufige Mittheilung iiber die Organisation der Ammoniten, 

Ber. Nat. Oes. Freiburg. 4 Bd. 1889. 
G. Steinmann and L. Doderlein. Elemente der Paldontologie. Leipzig, 1890. 
Johannes Thiele. Die Mmullappen der Lamellibranchiaten. Zeitschr. f. wiss. 

Zoologie. 44 Bd. 1886. 
T. TuUberg. Studien iiber den Bau und das Wachsthum des Hummerpanzers und 
der Molluskenschaleii. Eongl. Svensk. Vetensk. Akad. Haiulling. 19 Bd. 
1882. 
Sari A. Zittel. Uandbuch der Paldontologie. I. Abth. Paldozoologie. II. Band. 
Mollusca uiul Arthropoda. Miinolien and Leipzig, 1881-1885. 



vii MOLLUSCA— LITERATURE 275 



Musculature, Foot, Pedal Glands, the Taking in of Water. 

Th. Barrois. Lcs glandes clu pied et Us pores aquifires des Lamellibrmwhes. Lille, 

1885. 
J. Carri^re. Die Driisen im Fttsse der LamellibrancMaten. Arieit. aus d. zool. 

Iiistitut Wurzhurg. 5 Bd. 1879. 

Die Fiissdrilseii der Prosobranchier imd das Wassergefdss-system der Lmnel- 

libranchier xiiid Gastri/poden. Archiv. f. milcroslc. Anatomie. 21 Bd. 1882. 

C. Grobben. Zur Morphologie des Fusses der Heteropoden. Arh. Zool. Inst. Wien. 

7 Bd. 1887. 
A. Fleisclunann. Die Bewegimg des Fusses der Lamellihranchiaten. Zeitschr. f. wiss. 

Zoologie. 42 Bd. 1885. 
Georg Ealide. Beitrag zur Kenntniss der Museulatur der Heteropoden unci Ptero- 

poden, zughich eiii Beitrag zur Morphologie des MoHuskenfusses. Zeitsehr.f. wiss. 

Zool. 46 Bd. 1888. 
J. H. List. Zur Kenntniss der Driisen im Fusse von Teihys finvbriata L. Zeitsclir. f. 

wiss. Zool. 45 Bd. 1887. 
Paul Pelseneer. Sur la valeur mmphologiqiw des tras et la composition du systime 

nerveux central des Oiphalop)odes. Arch. Biol. Tome VIII. 1888. 

Sur I'epipodium des Mollusques. Bull. Seientif. France et Belg. Tome 19. 

1888. Tome 22. 1890. Tome 23. 1891. 

Bemhard Eawitz. Die Fussdrilse der Opisthohranchier. Abhandl. Afcad. Berlin. 

1887. 
Ludwig Beichel. Ueber die Bildung des Byssus der LamellibrancMaten. Zool. 

Beitrdge, Schneider. 2 Bd. 1888. 
P. Schiemenz. Ueher die Wasserauf'iuihme hei Lamellihranchiaten und Gastropoden 

(einschliesslich Pteropoden). Mitth. Zool. Station Neapel. 5 Bd. 1884. 2 

Theil. 7 Bd. 1887. This paper contains the further literature on this 

subject. 
Jap. Steenstrup. Bectocotylus dannelsen hos Octopods, etc. K. DaiisTc. Vidensk. 

Selskabs Skrifter. 1856. 

Nervous System. 

L. Bohmig. Beitrdge zur Kenntniss des Centralnervensystems einiger pulmonatcn 

Gastropoden. Inaug.-Diss. Leipzig, 1883. 
E. L. Bouyier. Systime nerveux, morphologie ginirale et classification des Gastropodes 

prosobranches. Annates des Sciences Nat. (7). Tome III. 1887. 
Louis Boutan. Contribution a I'itude de la masse ncrveuse ventrale {cordons palUo- 

visceraux) et de la collerette de la Fissurelle. Arch. Zool. expirim. (2). Tome 

VI. 1889. 
J. Brock. Zur Neurologic der Prosobranchier. Zeitschr. f. wiss. Zool. 48 Bd. 

1889. 
0. Biitschli. Bemerkungen iiber die wahrscheinliche Herleitung der Asymmetrie der 

Gastropoden, spec, der Asymmetrie im Nervensystem der Prosobranchier. Morph. 

Jahrb. 12 Bd. 1886. 
Charon. Reeherches sur le systime nerveux des Cephalopodes dibranchiaux. Annales 

des Sciences Nat. (5). Tome V. 1866. 
Earl Drost. Ueber das Nervensystem und die SinnesepUhelien der Herzmuschel 

[Cardium edule), etc. Morph. Jahrbueh. 12 Bd. 1886. 
Duvemoy. Mimoires sur le systime nerveux des Mollusq^tes aciphales. Mimoires: 

de I'Acad&mie des Sciences. Tome XXIV. 1854. 



276 COMPARATIVE ANATOMY chap. 

B. Haller. Zur Kenntniss der Muriciden. Eine vergL-anat. Studie. I. Theil. 

Anatoinie des Xerrensystcms. Benkschr. math.-naturw. Klasse Akad. JFissensch. 
JFien. 45 Bd. 1882. 

Untcrsucluingcn iiber marine lihijndoglossen. II. Textnr des Central-nermn- 

systems uiid seiner Hullen. Moiyli. Jahrl. 11 Bd. 1885. 

H. Ton Jhering. Vergleiehende Anatomie dcs Nervensy stems und Phylogenie der 

Mollnskcn. Leipzig, 1877. 
Lacaze-Duthiers. Du systime iicrreux dcs Molhtsques gastropodes pulmonis aqua- 

tiques. Arch, de Zool. exp. Tome 1. 1872. 
Bemhard Eawitz. Decs centrale Ncrvensystem der Acephalen. Jenaische Zeitschr. f. 

Natiiriviss. 20 Bd. 1887. 
Paul Pelseneer. Sur la valeur morphologique des Iras et la composition dn systime 

neri'cux central des Oep}halopodes. Arch. Biol. Tome VIII. 1888. 

Rccherches sur le systeme nerveux des Pteropodes. Arch. Biol. Tome 

VII. 1887. 

C. Semper. Ueber Sehorgane vom Typus der TFirielthieraugen. "Wiesbaden, 1877. 
H. Simroth. Das Fussncrvcjisystem von Paludina vicipara. Zeitschr. f. wiss. Zool. 

35 Bd. 1880. 
Uehcr das Kcrvcnsystem und die Bcwegung der dcntschen Binnenschnecken. 

Progr. d. Bealschule. 2 Ordnung. Leipzig. No. 503. 1882. 
J. W. Spengel. Die Gcruchsorgaiw und das Nerveiisystem der Malluskcn. Zeitschr. 

f. wiss. Zool. 35 Bd. 1881. 

Sensory Organs. 
F^lix Bernard. Recherehes sur les organes palliaux des Gastropodes prosolranches. 

Ann. des Sciences Nat. (7). Tome IX. 1890. Containing researches on the 

osphradia of the Gastropoda. 
J. Brock. Ueher die sogenannten Augen von Tridacna und das Vorkommen von 

Pseiidochlorophyllkorpern im Gefass-system der Muschcln. Zeitschr. f. wiss. Zool. 

46 Bd. 1888. 
0. Btitschli. Niitiz zur Morphologic des Augcs der Muscheln. Festschr. iOQ-jcihrig. 

Bestehens der Buperto-Carola dargci. V. Nat.-Med. Ver. Heidelberg. Nat. Theil. 

1886. 
Justus Carriere. Die Sehorgane der Thiers vergleichend - anatmnisch clargestellt. 

Miinchen and Leipzig, 1885. 

Ueber Molluskenaugen. Arch. f. mikrosk. Anat. 33 Bd. 1889. 

C. Glaus. Das Gehororgan der Heteropoden. Arch. f. mikrosk. Ancd. 12 Bd. 1875. 
P. Fraisse. Ueber Molluskenaugen mit cmbryonaleM Typus. Zeitschr. f. wiss. Zool. 

35 Bd. 1881. 
W. Flemming. Untersuchungen iiber Sinnesepithelicn der MoUusken. Arch. f. mikr. 

Anat. Tome VI. 1870. 
H. Grenacher. Abhandlungcn zur rcrgieicltendeu yj^natomie des Auges. I. Die 

Retina der Cephalopoden. Abhandl. Naturf. Gesellsch. z. Salle. 16 Bd. 1884. 

//. Das Aucje der Heteropoden. Ibid. 17 Bd. 1886. 
V. Hensen. Ueber das Auge einiger Cephalophor'en. Zeitschr. f. wiss. Zool, 

15 Bd. 1865. 
C. Hilger. Beitrdge zur Kenntniss des Gastropodenauges. Morph. Jahrb. 10 Bd. 

1884. 
Lacaze - Duthiers. Otocystes on, capsules auditives des Mollusques {Gastropodes). 

Arch. d. Zool. exp. Tome I. 1872. 
E. Ray Lankester and A. G. Bourne. On the existence of Spengel' s olfactory organ 

and of paired genital ducts in the 2Jearly Nautilus. Quart. Jour. Micr. Science. 

Vol. XXIII. 1883. 



VII MOLLUSOA— LITERATURE 277 

F. Leydig. Ucter das Gchororgan der Gastropoden. Anliiv. f. viikrosk. Anatomie. 

7 Bd. 1871. 
H. N. Moseley. On the presence of eyes in the shells of certain Ohitonidcc ami on 

the structure of these organs. Quart. Journ. Micr. Science (2). Vol. XXV. 

1885. 
Ph. Owsjannikow and Kowalevsky. Ueler das Centralorgan uiul das Geh'&rorgan 

der Cephalopoden. St. Petersburg, 1867. 
W. Patten. Eyes of Molluscs and AHhropods. Mitth. Zool. Stat. Neapel. 6 Bd. 

1886. 
Paul Pelseneer. Sur I'oiil de quelques Mollusqiies gastropodes, et Les organes des 

sens^che:: les Mollusques. Aanales SociiU Bdge ilicrosc. (Memoires). Tome XV I. 

1891. 
Rawitz. See above under heading Integument. 
P. B. Sarasiu. Ueher drei Simiesorgane und die Filssdriise einiger Gastropoden. 

Arbeit. Zool.-zoot. Inst. JVilrtzhurg. 6 Bd. 1883. 

B. Sharp. On the visual organs in Lamellibranchiata. Mitth. Zool. Stat, in Neapel. 

5 Bd. 1884. 
D. Sochaczewer. Das Sieclwrgan der Landpulmonaten. Zcitschr. f. loiss. Zool. 

35 Bd. 1880. 
J. E. Tenison-Woods. On the anatomy and life history of Mollusca peculiar to 

Australia. Proc. Roy. Soc. N.S. Wales. Vol. XXII. 1889. 
Johs. Thiele. Die abdominalen Simiesorgane der Lamellibranchier. Zeitschr. f. 

wiss. Zool. 48 Bd. 1889. 

Intestine, Ink-bag. 

D. Barfurth. Ueher den Bau iiiul die T!idtiglceit der Gastropodenleber. Arehiv. 

f. mikr. Anatomie. 22 Bd. 1883. 
Th. Barrois. Le stylet erystallin des Lamellibranches. Revus biol. du Nord de la 

France. Tome II. 1890. 
Em. Bourquelot. Reelierehes sur les phinomenes de la digestion cliez les Molhisques 

ciphaloimdes. Arch, de Zool. exp. (2). Tome III. 1885. 
J. Frenzel. Mikrographie der Mitteldarmdrilse (Leber) der Molluskcn. . I. Allge- 

ineine Morphologic und Physiologic des Drusenejnthels. Nova acta Acad. Goes. 

Leap. -Carol. 48 Bd. 1836. 
Heinrich Haria Gartenauer. Ueber den Darmkanal einiger cinheimisclien Gastro- 

j>oden. Inaug-Diss. Strassburg, 1875. 
Patrick Geddes. On the mechanism of the odontophore in certain Mollusca. Trans. 

Zool. Soc. London. Vol. X. Part II. 1879. 
Paul Girod. Secherehes sur la poche du iwir des Ciphalopodes des c6tes de France. 

Arch, de Zool. exp^rim. Tome X. 1882. 
Maodonald. General classification of the Gastropoda. Trans, of the Linn. Soc. of 

London. Tome XXIII. 1860. 
Panceri. Gli organi e la secrezioiw dell' acido solforico nei Gastropodi eon wn aprpen- 

dice, etc. Atti delta S. Accad. delle seienze fisiche. Tome IV. 1869. 
Eosaler. Die Bildvag der PMclula bei den cephalophoren Molluskcn. Zeitschr. f 

wiss. Zool. Bd. XLI. 1885. 

C. Semper. Zwnfeineren Bau der Molluskenzunge. Zeitschr. f. iviss. Zool. 9 Bd. 

1868. 

H. Troschel. Das Gebiss der Schneeken. 1 Bd. Berlin, 1856-1863. 

W. J. Vigelius. Vergleichend-anatomische Untersuehnngen Uber das sogenannte Pan- 
creas der Cephalopoden. Verhandl. k. Akad. Wetenseh. Amsterdam. Ded 22. 
1881. 



278 COMPARATIVE ANATOMY chap. 

Respiratory Organs, Circulatory System. 

Felix Bernard. Rcclierclies siir les organes palUaux des Gastropodes jJ^osobrmiehes. 

These. Paris, 1890. 
Bojanus. Ueber die Atliem- imd Kreislaufswerksaeuge der zweischaligen Musdieln. 

Isis, 1817, 1820, 1827. 
L. Cuenot. iHiides sur le sang et les glandes lyviphatiques dans la sirie animale. 

2 Partie. Invertehris. Arch, de Zool. expiriin (2). Vol. IX. 1891. 
Carl Grobben. XJeber den Ijnlbus arteriosus uud die Aortenklap2Kn der Lanwlli- 

Iranchiaten. Arbeiten a. d. Zoologischen Institute der Universitdt Wien. 9 Bd. 

1891. 
W. A. Herdmann. On the structure and fuiietion of tlie cerata or dorsal papillce in 

some Nudibranchiate Mollusca. Quart. JoMrn. Microsc. Science. Vol. XXXI. 

Part I. 1891. 
L. Joubin. Structiire et ddveloppement de la branchie de quelqnes Ciphalopodes des 

c6tes de France. Arch, de Zool expirim. (2). Vol. III. 1885. 
Langer. XJeher das Gefdss-system der Teichmuschel. Denkschriften der Wiener Aka 

demie. 1855 and 1856. 
A. Menagaux. Eeeherchcs sur la circulation des Lavwllibranches inarins. Sesan^on, 

1890. 
K. Mitsukuri. On the structure and significance of some aberrant forms of Lamelli- 

branchititc gills. Quart. Journ. Microsc. Science. N. S. 21. 1881. 
H. L. Osbom. On the gill in some forms of prosobranchiate Mollusea,. Stud. biol. 

labor. J, Hopkins Univ. Vol. III. 1884. 
R. Holman Peck. The structure of the LamcUibranchiate gill. Quart. Journ. Micr. 

Science. Vol. XVII. 1877. 
C. Posner. Ueber der Ban der Najadenkieme. Arch. f. mikrosk. Anat. Bd. 

XI. 1875. 



Secondary Body Cavity, Nephridia, Genital Organs. 

Bandelet. Secherches sur Vappareil ginir. des Ilollusques gastropodes. Ann. Sci. 

Nat. (4). Tome XIX. 1862. 
Th. Behme. Beitrdge ::ur Anatomie und Entioickelungsgeschichte des Harnapparates 

der Ltmgenschnecken. Arch. Naturg. Jahrg. 55. 1889. 
J. Brock. Ueber die Geschlechtscrrgane der Cepluilopoden. Erster Beitrag. Zeitschr. 

f. wiss. Zool. 32 Bd. 1879. 
J. T. Cuningham. The renal organs {nephridia) of Patella. Quart. Jotirn. Micr. 

Science. Vol. XXIII. 1883. 

Note on the structure and relations of the kidney in Aplysia. Mith. Zool. 

Station in Neapel. 4 Bd. 1883. 

R. von Erlanger. On the paired Nephridia of the Prosobranchs, the homologies of 
the only renudning nephridium of most Prosobranchs, and the relations of the 
nephridia to the gonad and genital duct. Quart. Journ. Micr. Science. Vol. 
XXXIII. 1892. 

C. Grobben. Morpholog. Studien Uber den Ham- und Geschlechtsapparat, soivie die 
Leibeshohle der C'ephalopoden. Arb. Zool. Inst. JFien. 5 Bd. 1884. 

Ueber die pericctrdialdriise der Lamellibranchiaten. Ein Beitrag zur Kennt- 

niss der Anatomie dieser Molluskenklassen. Arb. Zool. Inst. Wien. 7 Bd. 
1888. 

Die pericaixlialdrilsen der Gastropoden. Arbeit. Zool. Inst, der Univ. Wien. 

9 Bd. 1890. 



VII MOLLUSGA— LITERATURE 279 

A. C. Haddon, On the generative and urinary ducts in Chiton. Proceed. Soyal 

Dublin Soc. (2). Vol. IV. 1885. 

B. Haller, Beitrage zur Kenntniss der Niere der Prosohranchier. Morph. Jahrb. 

11 Bd. 1885. 
A. Hancock. On the, structure and homologies of the renal organ in the Molluscs. 

Trans, of the Linn. Soc. Vol. XXIV. 
P. P. C. Hoek. Zes organes de la giniration de VhuUre. Tijdschr. Nederl, Dierlc. 

Vereen. Suppl. D. 1. 1883. 
H. von Jhering. Ueher den uropneustischen Apparat der Heliaeen. Zeitschr. f. wiss. 

Zool. 41 Bd. 1884. 
J. Kollmann. Ueier Verhindungen zwischen Oijlom wild Nephridien. Baseler 

Festschrift zum Wurtzburger Juhilduin. 1882. 
A. Kowalevsky. Ein Beitrag zur Kenntniss der Excretionsorgane. Biol. Centralhlatt. 

9 Bd. 1889. 
E. Ray Lankester. On the originally bilateral character of the renal organs of 

Prosobranchia, and on the homologies of the yolk sac of Cephalopoda. Ann. of 

Nat. Eist. (5). Vol. VII. 1881. 
■ Observations on the Pondsnail, etc. Quart. Journ. Micr. Science. Vol. XIV. 

1874. 
E. Ray Lankester and A. G. Boiime. On the existence of Spengel's olfactory organ 

and of paired genital ducts in the pearly Nautilus. Quart. Journ. Micr. 

Science. Vol. XXIII. 1883. 
G. F. Mazarelli. Intorno all' anatomia dell' apparato riproduttore delle Aplysice del 

golfo di Napoli. Z. Anz. 12 Bd. 1889. 
Intorno all' apparato riproduttore di alcuni Tectibranchi (Pleurobranehoea, 

Oscanius, Accra). Zool. Anz. 14 Jahrg. 1891. 
0. NUsslin. Beitrage zur Anatomic und Physiologic der Pulmonaten. Habilita- 

tionsschrift (Carlsruhe). Tiibingen, 1879. 
R. Owen. On the exterruil and structural characters of the male Spirula australis. 

Proceed. Zool. Soc. London. 1880. 
R^my Perrier. Recherches sur TanMomie et Thistologie du rein des Gastropodes 

prosobranches. Annates des Sciences Nat. (7). Tome VIII. 1890. 
Walter Rankin. Ueber das Bojanus'sche Organ der Teichmuschel {Anodonta cygnea 

Lam.) Jenaische Zeitschr. filr Naturwissensch. 24 Bd. 1890. 
A. Schmidt. Der Geschlechtsapparat der Stylotmnatophoren, etc. Abh. des Nat. 

Vereins fur Saehsen und Thuringen. 1 Bd. 1885. 
P. Stepanoff. Ueber Geschlechtsorgane und Entwickelung von Ancylus fluviatilis. 

St. Petersburg, 1886. 
W. J. Vigelius. Bijdrage tot de Kennis van het excretorisch Systeem der Cephalopoden. 

Acad. Proefschrift. Leiden, 1879. 
Ueber das excretionssystem der Cephalopoden. Niederl. Arch. f. Zool. 5 Bd. 

1880. 

Parasitic Gastropoda. 

Albert Bauer. Beitrage zur Naturgeschichte der Synaptct. III. Die Eingeweide- 

schnecke in der Leibeshohle der Synapta digitata. Nova Acta Academ. Cces. 

Leop-Carol. Tome XXXI. 1864. 
Max Braun. Ueber parasitische Schnecken. Zusammcnfassender Bericht im Cen- 

tralbl. f. Baktcriologie u. Parasitenkunde. 5 Bd. 1889. 
Johannes Miiller. Ueber Synapta digitata und die Erzeugung von Schnecken in 

Holothurien. Berlin, 1852. 
Paul and Fritz Sarasin. Ueber zwei parasitische Schnecken. Ergebn. Naturw. Forsch. 

auf Ceylon in 1884-1886. 1 Bd. Wiesbaden, 1887. 



280 COMPARATIVE ANATOMY chap. 

P. Schiemenz. Parasitische Schnecken. Kritisches Meferat. Biol. Centmlhlatt. 

9 Bd. 1889-1890. 

Walter Voigt. Entocolax Ludwigii, ein neuer seltsamer Parasit aus einer Holothwric. 
Zeitschr. f. miss. Zool. 47 Bd. 1888. 

Ontogeny. 

F. Blochmaun. Ueler die Entn-iokelung von Nerilina flumatilis, Mull. Zeitschr. 

f. wiss. Zool. 36 Bd. 1881. 

Beitrdgc ziir Kcnidniss dcr EiitAoickelmig der Gastropoden, Zeitschr. f. wiss. 

Zool. 38 Bd. 1883. 

W. K. Brooks. The development of the Squid (Loligo Pealii, Lesueur). Annivers. 

Mem. Boston Soc. Nat. Hist. Boston, 1880. 
E. von Erlanger. Zur Entwickching von Paludina vivi^iara. I. and II. Mor- 

•phologisches Jahrhuch von Gegcnbauer. 17 Bd. 1891. 
Hermann Fol. Etudes sur le diveloppenwnt cles Mollusqucs. I. Sur le diveloppement 

des Ptiropiodes. Archives de Zool. expirim. Tome IV. 1875. //. Sur le 

diveloppeiiwnt emlryonnaire et larvairc des Hitcropodes. Tome V. 1876. ///. 

Sur le dereloppement des Gastropodes pulinone. Tome VIII. 1879-1880. 
H. Grenacher. Zii,r Entmickelungsgeschichte der Cepihalapoden, zugleich ein Beitrag 

zur Morphologic der hoheren Molluskcn. Zeitschr./. wiss. Zool. 24 Bd. 1874. 

A. C. Haddon. Notes on the development of Mollusca. Quart. Journ. Micr. Science. 

Vol. XXII. 1882. 

B. Hatschek. Ueher Entiviekelungsgeschichte von Teredo. Arb. a. d. Zool. Instit. 

Universitut Wien. Tome III. Heft 1. 1880. 
E. Herat. Emhryogime de llmitre. Tijdschr. Nederl. Dicrk. Vcr. Suppl. Deel. 1. 
1884. 

Development of the European Oyster. Quart. Journ. Micr. Science. Vol. 

XXII. 1882. 

A. Kolliker. Enticickelungsgeschichte der Cepihalop)oden. Ziirioh, 1884. 
A. Kowalevsky. £tude sur I'enibrijogenie du Dentale. Annales du Music d'histoire 
naturellc de Marseilles. Zoologie. Tome I. 1883. 

Embryocjinie du Chiton Polii (PhiUppi) avec quelques remarques sur le 

dii'elop2)cment des autres Chitons. Ann. Mus. N. H. Marseille. Tome I. No. 5. 

A. Krolm. Beitrage zur Entwickelungsgeschichte der Pteropoden und Heteropoden. 
Leipzig, 1860. 

E. Ray Lankester. On the d.cvelopmental history of the Mollusca. Philos. Transact. 
London. 1875. 

Observations on the development of the Cephalopoda. Quart. Jour. Micr. Science. 

Vol. XV. N.S. 1875. 

S. Loven. Beitrage zur Kcnntniss der Mollusca, acepluda la^nellibranchiata. Stock- 
holm, 1879. 

J. Playfair MacMurrich. A contribution to the embryology of the 2'rosobranch 
Gastropods. Sttid. Biol. Lab. J. Hopkins Univ. Vol. III. 1886. 

William Patten. The embryology of Patella. Arbeit. Zool. Inst. Wien. 6 Bd. 
1885. 

G. Pruvot. Sur le cUveloppement d'un SoUnogastre. Comptes rend. Paris. Tome 

CXI. 1890. 
Carl Eabl. Ueber die Entieickclimg der Tellerschnecke. 3Iorph. Jahrb. 5 Bd. 1879. 

Eie Ontogenie der Susswasserpulmonaten. Jcnaische Zeitsehrift. 9 Bd. 

1875. 

Ueber die Entiviekelungsgeschichte der Malermuschel. Jcnaische Zeitsehrift. 

10 Bd. 1876. 



vii RHODOPE VERANII 281 

W. Salensky. I^tudes sur le d4vel6ppement du Vermet. Arch. Biol. Tome VI. 

1887. 
Beitriige zitr Entwkhlungsgeschichtu der Prosobranchirr. Zeitschr. f. wiss. 

Zool. 22 Bd. 1872. 
P. B. Sarasin. Enlwickclungsgeschidite der Bithynia tcntaculata. Arh. Zool-Zool. 

Instit. WiiHzburg. 6 Bd. 1882. 
PaiU and Fritz Sarasin. Aus der Enlwickclungsgeschidite von Relix Ifaltonii. 

Ergebn. Nat. Forsch. Ceylon, 1884-1886. 1 Bd. Wiesbaden, 1888. 
P. Schiemenz. Zusamnwnfassende Darstclhmg der Beohachtungen von Eisig, 

Fiouzaud, Joiirdain, Brock, etc., ilber die Entwickelung der Oenitalorgane der 

Gastropoden Biol. CciitralWatt. 7 Bd. 1888. 
C. Schierholz. Ueler Enticickelung der Uiiioniden. Denkschr. Akcul. IFien. 55 

Bd. 1888. 
F. Schmidt. Beitrag zur Kenntniss der posteinlryonalen Entwickelung der Najaden. 

Archiv. fur Naturgeschiclde. 51 Jahrg. 1885. 
M. Ussow. Untersuchungen iiber die Entwickelung der Cephalopoden. Arch. Biol. 

Tome II. 1881. 
L. Vialleton. Eecherches sur les premiire phases dzi- direloppcment de la Seiche. 

Annal. Sc. Kat. (7). Tome VI. 1888. 
Wladimir Wolfson. Die embrymiale Entwickelung des Lymnccus stagnalis. Bullet. 

Acad. Imp. Sc. St. Petersbourg. 26 Jahrg. 1880.' 
H. E. Ziegler. Die Entwickelung von Cyclas cornea, Lam. Zeitschr. f. wiss. Zool. 

41 Bd. 1885. 

Appendage. 

Rhodope Veranii. 

This small animal (circ. 4 mm. in length) is long and sjiindle-shaped, and out- 
wardly bilaterally symmetrical. Tlie body epithelium is ciliated all over. There 
is a dermo-musoular tube, inside which, embedded in tlie connective tissue (paren- 
chyma), are found numerous irregularly shaped calcareous particles. 

Alimentary Canal. — The mouth lies at the anterior end of this canal, and leads 
into a wide buccal or cesophageal cavity, into the first part of which two acinose 
salivary glands open. A radula and jaws are wanting. A narrow oesophagus con- 
nects the cesophageal cavity with the tube-like mid-gut, which runs through the 
whole length of the body. The midgut possesses a well-developed muscular wall, and 
is continued anteriorly, above the point where the oesophagus enters it, in the form 
of a diverticulum, which runs forward over the brain. There is no separate digestive 
gland. The right side of the mid-gut gives rise to a short, thin, ciliated rectum, 
which runs through the posterior third of the body, and opens through the anus to 
the right. 

The nervous system consists of two pairs of ganglia lying so close together above 
the oesophagus as almost to form one mass, and of one infra-oesophageal ganglion, 
which lies somewhat asymmetrically to the left. The two ganglia of each of the 
upper pairs are connected by transverse commissures, and the posterior dorsal pair 
with the lower ganglion by two connectives which embrace the oesophagus. Two 
lateral nerves which run backward are the most strongly developed. They arise out 
of the posterior upper pair of ganglia, close to which lie a pair of eyes and a pair of 
ciliated auditory vesicles, each of the latter containing an otolith. 

Genital Organs. — Rhodope is hermaphrodite. The gonads consist of about 20 
follicles which lie ventrally in the median and posterior thirds of the body ; the 
anterior follicles produce eggs and the posterior spermatozoa. , The ducts of all the 



282 COMPARATIVE ANATOMY chap. 

follicles are said to unite to form a common duct. If this is really the ease, then 
the gonadial follicles together form a hermaphrodite gland. The hermaphrodite 
duct, which runs forward, is said to divide into an oviduct and a vas deferens. 
The latter leads to the muscular penis, which can be protruded from the male genital 
aperture on the right anteriorly. With the oviduct are connected a receptacu- 
lum seminis and a gland (albuminous or nidamental gland). The female genital 
aperture is said to lie on the right side, behind, and distinct from, the male 
aperture. 

A differentiated blood vascular system has not been found. A well-developed 
body cavity is, however, present, filled with colourless nutritive fluid, in which blood 
corpuscles are suspended. 

Special respiratory organs are wanting. 

The nephridial system has been described as follows. To the right, in front of 
the anus, between the latter and the genital aperture, lies the outer nephridial 
aperture. It leads through a short ciliated canal into a spacious renal chamber, 
which is a widening of a longitudinal canal. The renal chamber bulges out at 
several points to form short cseoa. Into this chamber nine or ten small flask-like 
organs open ; these resemble the excretory ciliated cells of the Platodes, inasmuch 
as "flames"^ arise at the base of each flask, the neck of which opens into the 
chamber. 

Development is direct. At no stage are there any indications of a shell gland, a 
shell, or a foot. 

Systematic Position. — Rhodope is by some classiiied among the Turhellaria 
(near the Rhabdoccelidm), by others among the llolhisca (near the Nvdibranchia), 
while others again are inclined to see in it a transition form between these two 
phyla. 

Tliere is apparently only one single point to support the theory of the relation of 
Rhodope to the Turhellaria, viz. the presence of the ciliated excretory cells in the 
nephridial system. On the other hand, the derivation of the nephridial system of 
Rhodope, with its renal chamber and aperture to the right, from that of the Niidi- 
branchia appears far more probable than its derivation from the water vascular 
system of the Platodes. The presence of a rectum and anus, and of an infra-oeso- 
phageal ganglion (pedal ganglion), is difficult to reconcile with a, relationship to the 
Turhellaria. The occurrence of an infra-resophageal commissure in one isolated case, 
thaX oi Microstoma lineare (cf. vol. i. p. 166), is hardly a convincing argument. The 
genital apparatus of Rhodope is much nearer to the Nudibranchiate than to the 
Turbellarian type. 

There are, no doubt, serious obstacles in the way of those who seek to establish 
the relationship of these animals with the MoUusca. The chief of these is the want 
of a heart and the entire absence of a shell and a foot, even in the embryo. The 
question to be decided is whether it would be possible for a Mollusc which had lost 
foot, gills, and shell {e.g. Phyllirhoe) by the further loss of the heart, so far to depart 
from the typical organisation of the MoUusca, that these organs would not appear, 
even temporarily, in the course of development. If this question is answered in the 
affirmative, then the asymmetry of Bhodope,' and especially the position of the 
genital, nephridial, and anal apertures on the right side, which entirely agrees 
with their position in the Nudibranchia, affords strong support to its claim to be 
related with the MoUusca. 

The view that Rhodope is a transition form between the Turhellaria and the 
MoUusca need hardly be treated seriously. 

^ Cf. vol. i. p. 152, where flame cells are described. 



VII BHODOPE VEBANII— LITERATURE 283 

Literature. 

L. Tou Graff. Ueher Mhodope Veranii. Koell. { = Sidonia elegans, M. Sohulze). 

Morph. Jahrhuch. 8 Bd. 1883. 
A. Eoelliker. Rhodope, nuovo geiwre di Gastropodi. Giornale dell' Istituto B. Lom- 

bardo di scicnze e.c. Tome 16. Milaiio, 1847. 
S. Trinchese. Nuovo osservazione imlla Rhodope Veranii. Koell. Rendic. dell' 

Accad. di Napoli. 1887. 



CHAPTER VIII 
SEVENTH EACE OR PHYLUM OF THE ANIMAL KINGDOM 

ECHINODERMATA. 

The Echinodermata are, as a rule, essentially radiate in structure. 
They, however, always deviate from strict radial symmetry in minor 
l^oints, both in the skeletal system and in the arrangement of 
the inner organs ; sometimes they may become almost bilaterally 
symmetrical. The Echinodermata possess a skeleton of calcare- 
ous matter deposited in the deeper connective tissue layers of 
the integument. This skeleton is in texture a fine rigid sponge- 
work. It consists either of microscopically small isolated calcareous 
bodies (Holothurioidea) or of larger plates which often carry spines, and 
are connected togethei' either movably or immovably (other Echino- 
derms). The coelom is spacious. There is a blood vascular system. 
The intestine, which is provided with a mouth and anus, is completely 
separated from the ccelom. The Echinodermata possess a peculiar sys- 
tem of canals or tubes — the water vascular system. This system, on 
the one hand, takes in water from the exterior through a stone canal 
(sometimes several such canals are present), which primitively opens 
outwards, and, on the other hand, sends out terminal canals to ex- 
ternal extensible appendages arranged in the radii or ambulacra. 
These are the ambulacral feet or tentacles, which in free forms serve 
principally for locomotion, but also for respiration ; in attached forms, 
for respiration, and also perhaps for conducting food. The sexes are 
almost always separate. Development is accompanied by metamor- 
phosis. The larvse are free-swimming and pelagic ; they are bilater- 
ally symmetrical, with ciliated bands, general^ produced on processes. 
The Echinodermata are exclusively marine, and contain a great number 
of fossil forms ; certain extinct types attained a great development 
during the palaeozoic age. 

The race of the Echinodermata is divided into five classes — Holo- 
thurioidea, Eehinoidea, Asteroidea, Ophiuroidea, and Pelmatozoa. 



CHAP. Via EOHINODERMATA— SYSTEMATIC REVIEW 



286 



Systematic Review. 

CLASS I. Holothurioidea. 

The body is elongated along its principal axis ; it is cylindrical or vermiform. It 
shows more or less distinct bilateral symmetry. The integument is soft or leathery, 
and contains irregularly arranged, generally microscopically small, calcareous bodies. 
The mouth lies at the oral (anterior) end of the principal axis of the body, and is 
surrounded by feelers. The anus lies at the apical (posterior) end of the principal 
axis. Ambulacral or tube-feet are either present or wanting. An external madre- 
porite is usually not found. 

Order 1. Actinopoda. 

All the outer appendages of the water vascular system arise from the radial 
canals, and take the form of feelers round the mouth 
and of tube-feet (and ambulacral papillae) in other 

parts of the body ; such feelers are always present, ,/ -' h 

/,' "!)■ ,v«. ';.: 'V _ / 

Z 



the feet and papillse, however, may be wanting. 



i- 



Family 1. Aspidochirotse. 

Tube -feet present. Mouth often more or less 
ventral in position. Body usually shows distinct 
flattening of the ventral surface. 18-30 peltate 
tentacles. Tentacular ampuUse well developed. 
Stone canals often numerous. Retractor muscles 
wanting. Respiratory trees pi-esent. Cuvier's 
organs often present. MiiUeria, Holothuria, 
Stichopus. 

Family 2. Elasipoda. 

Tube-feet pi'csent. Mouth more or less ventral 
in position. Body almost always distinctly flattened 
on the ventral surface. 10, 15, or 20 tentacles, more 
or less peltate in shape. Stone canal always single, 
and not infrequently in direct communication with 
the exterior through the integument. Retractor 
muscles wanting. Respiratory trees wanting or 
quite rudimentary. Cuvier's organs wanting. 
Sub-fam. Psyohropotidse : Psychro2yotes (Fig. 223), 
Benthodytes. Sub-fam. Deimatidse : Deima, Pan- 
nychia, Zaetmogone. Sub-fam. Elpidiidse : Elindia, 
Kolga, Peniagone. 



Family 3. Pelagothmiidse. 

Tube-feet wanting. Mouth and anus terminal. ^^^^ 223.-Psyoliropot6s longi- 
Body cylmdrical ; round the crown of tentacles it oauda (after Tli^el). 1, Oral ten- 
widens out into a thin disc, the edge of which is tacle; 2, mouth; 3, 4, 8, ambulacral 
produced into long rays. 13-16 tentacles. Re- appendages of the (ventral) trivium 
tractor muscles wanting. Neither respiratory trees, 
nor ciliated organs, nor Cuvier's organs present. 
Calcareous bodies altogether wanting. Pelagic, swimming by means of the disc. 
Single genus and species : Pelagothuria natatrix (Figs. 224 and 225). 




5, anus ; 6, dorsal appendage with 
its two posterior processes (7). 



286 



COMPARATIVE ANATOMY 



CHAP. 




Fig. 224.— Pelagotliurla natatrix (after Ludwlg), completed ; from above. I, Body ; 2, amis. 




Fig. 225.— Pelagotliurla natatrix (after Ludwlg) ; front view, i.e. from the oral pole. 1, Mouth ; 
2, oral tentacles ; 3, disc ; 4, canals of the disc. 



VIII 



EGHINODERMA TA —SYSTEM A TIG RE VIE W 



287 



Family 4. Dendrochirotse. 

Tiibe-feet present. Mouth dorsal or terminal. Anus also often dorsal. Body 
cylindrical, or pentagonal, or with a distinctly marked creeping sole. 10-30 arbor- 










^'^ifil 














Fig. 227. — Psolus ephippifer, 
young female, from the dorsal side 
(after Thdel). 1, Oral valves ; 2, 
anus. 



Fig. 226.— Cuoumaria planci (original). 1, The 
two smaller ventral oral tentacles ; 2, mouth ; 
3, anus. 

ascent tentacles, often of unequal size. Tentacular 
ampullse not distinct. Not infrequently more than 
one stone canal. Retractor muscles well developed. 
Respiratory trees present ; Cuvier's organs only 
occasionally found. Cucumaria (Fig. 226), Thyone, 
Phyllophorus, Colockirus, Thielia, Psolus (Figs. 227 
and 228), Mhopalodina. 



Family 5. Molpadiidae. Fio. 228. — Psolus ephlppifer, 

female, dorsal aspect (after Th^el). 
Tube-feet wanting. Mouth terminal. The pos- i, Oral valves, opened ; 2, anus ; 
terior end of the cylindrical body often narrowed to 3, oral tentacles ; 4, dorsal cal- 
a shorter or longer tail-like piece, which is more or <»r'^ous scales, 
less distinct from the trunk. 15 tubular or digitate 

tentacles normally present. Tentacular ampullse present. A single stone canal. 
Retractor muscles distinct only in the genus Molpadia. Respiratory trees present. 
Cuvier's organs almost always absent. Molpadia, CaudiTW,, Trochostonia, An- 
kyroderma. 




288 



COMPARATIVE ANATOMY 



CHAP. 



Order 2. Paractinopoda. 

Only some of the outer appendages of the water vascular system arise from the 
radial canals, the rest from the circular canal, and the only form taken by them is 
that of tentacles round the mouth. 



Family 1. Synaptidse. 
Tube-feet wanting. Mouth terminal. Body cylindrical, more or less elongated 
and vermiform. 10-27 feathered or digitate tentacles. Stone 
canals occasionally numerous. Retractor muscles sometimes 
present. Respiratory trees and Cuvier's organs wanting. Sexual 
glands often hermaphrodite. Synapta (Fig. 229), Ohirodota, 
Myriutrochus. ^ 






CLASS II. Echinoidea (Sea-urchins). 

The body of these Echinoderms is covered by a usually firm 
but sometimes flexible test, which contains the Offilomic cavity 
and the viscera. The test varies in shape, from spherical to a 
form which is flatly compressed in the direction of the principal 
axis. It consists of numerous pentagonal or hexagonal closely 
contiguous plates, which, arranged in meridional rows, form five 
ambulacral and five interambulacral areas. It is covered by the 
outer layer of the integument, and carries spines articulating 
with it. At the apical pole there is a system of plates, consisting 
of five basal plates, five radials, and the anal plate. The mouth 
is usually in the middle of the oral surface, less frequently shifted 
towards the edge in what is called the anterior direction. An 
anus is always present, either at the apical pole or at some part 
of the posterior interambulacral area. The apertures of the 
madreporite lie in the apical system, generally in one of the basal 
plates ; they are connected not only with the stone canal but 
with the so-called dorsal organ. The ambulacral vascular system 
has outer appendages developed as tube-feet and gills. Mouth 
with or without teeth. In the former case a complicated 
masticatory apparatus is developed within the test for the move- 
ment of the teeth ; the muscles moving this apparatus are 
attached to a perignathous apophysial ring developed at the 
edge of the oral aperture of the test {i.e. round the peristome). 
Sexually separate or hermaphrodite. The genital ducts open 
externally through pores in the basal plates or outside these 
latter. Development direct (with care of the brood), or with 
metamorphosis (free-swimming larvte). 



Sub-Class 1. PalEeechinoidea. 

Either only one row or more than two rows of plates in each 

dleltata (original) interambulacral area. Two or more meridional rows of plates in 

each ambulacral area. Plates of the test do or do not imbricate. 

Oral aperture of the test (with peristome) in the middle of the oral surface. Jaws 

^ The arrangement of the classes and families of the Holothurioidea by Ludwig in 
Bronn's Klassen wad Ordinniijen des Thierreichs, 1892, is here followed. 



VIII 



ECHINODERMA TA—8YSTEMA TIG RE VIE W 



present. Anal area either within the apical system, or outside it, in the posterior 
interanibulacral area. Palieozoic forms. 



Order 1. Bothriocidaroida. 

Regular Ecliinoidea, with a more or less spherical, firm test. In each inter- 
radius there is only one meridional row of plates ; in each ambulacral area there are 
two. Anal area, with anus within the apical system. Mouth in the centre of the 
oral surface. Bothriocidaris. 

Order 2. Perisohoeohinoida. 

Kegular Ecliinoidea. More than two meridional rows of plates in each inter- 
radius. Two or many meridional rows in each radius. Test thick and rigid, or 




Fig 230 — PalEeeohinus elegans M Coy 
(after Baily). 



Fig. 231. — Tiarechinus princeps Laube (after Loven). 
1, Genital aperture ; 2, anus; 3, basal; 4, radial; 6, ambu- 
lacrum ; 6, the 3 upper plates of an interambulacrum. 



thin ; in this latter case more or less imbricated. Jaws present. Fam. Archaeo- 
cidaridse : Lepidocentrus, Archceocidaris { = Ec'hinocrinus), Falceechinus (Fig. 230) 
Fam. Melonitidae : Melonites. 

Order 3. Flesiocidaroida. 

Test small and rigid, almost hemispherical. Apical system very large, with 
large united basal plates and central anal area. Ambulacra narrow, with two meri- 
dional or vertical rows of plates. Interambulacra with one single peristome plate, 
followed by three plates separated by vertical sutures. Tiarechinus (Fig. 231). 



Order 4. Cystocidaroida. 

Test irregular (exocyclic), spherical or ovoid, thin and flexible. Madreporite 
central. Ambulacral areas narrow, with two vertical rows of plates. Interambu- 
lacral areas broad, with numerous vertical rows of scale-like movable plates. Anus 
in the posterior interambulacrum above the ambitus. EcMiiocystis ( = Cystocidaris). 
VOL. II U 



290 COMPARATIVE ANATOMY chap. 



Sue-Class 2. Euechinoidea. 

Echinoidea with two vertical rows of plates in each ambulacral and in eacli 
interambulacral area. Mouth on the oral side, rarely shifted towards the edge 
(anteriorly). Teeth and jaws present or wanting. Anus either within the apical 
system, or outside it, i.e. somewhere in the posterior interradius. 



Order 1. Cidarolda. 

Mouth central, anus within the apical system. No external gills. With jaws 
and almost perpendicularly placed teeth. Perignathous apophysial ring interrupted. 
Both the ambulacral and the interambulacral plates are continued over the peri- 
stome on to the oral area as far as the mouth. On the oral area they are imbricated. 
Ambulacra narrow. Large principal and small accessory spines. Sphferidia want- 
ing. Cidaris. 

Order 2. Diadematoida. 

Mouth central, anus within the apical system. So-called internal gills well 
developed, or rudimentary, or wanting. With external gills, and incisions in the 
peristome. With jaws and teeth. Perignathous circular apophysial ring closed. 
Only the ambulacral plates are continued over the peristome on to the oral area, 
where they often appear as separate buccal plates. Sphferidia present. 

Sub-Order 1. Streptosomata. 

Test more or less flexible, with inner dorso-ventral longitudinal muscles. Botli 
external and internal gills present. The ambulacral plates (and only these) are 
continued over the peristome on to the oral area. Fam. EohinothuridEe : Pdan- 
echinus, Echinothuria, Phormosoma, Asthenosovia. 

Sub-Order 2. Stereosomata. 

Test rigid, without internal longitudinal muscles. External gills present, in- 
ternal gills rudimentary or wanting. The ambulacral plates on the oral area are 
replaced by isolated buccal plates. Fam. 1. Saleniidse : Peltastes, Salenia (almost 
exclusively fossil). Fam. 2. Hemicidaridse : Hemicidaris, Acrocidaris, Gonio- 
pygus, etc. (fossil). Fam. 3. Aspidodiadematidse : Aspidodiadema. Fam. 4. 
Diadematidse : Diadema, Diplopodia, Pcdina, Echinothrix, Astropyga, Codechinus, 
Orthopsis, Peronia, Echinopsis, etc. (fossil and extant). Fam. 5. Cyphosomatidse, 
Gypliosoma, etc. (almost exclusively fossil). Fam. 6. Arbaciidse : Arhacia, Eohi- 
nooidaris (Fig. 232), Ocelopleurus, Podocidaris (extant and fossil). Fam. 7. Tem- 
nopleuridse : Ghjphocyphus, Temnopleurus, etc. (extant and fossil). Fam. 8. 
Echinometridse : Echinometra, Parasalenia, etc. , Spongy locentrotus, SphairecMnus 
(mostly extant). Fam. 9. Echinidas : Echinus, Toxopneustcs, Trijmeustes (extant 
and fossil). 

Order 3. Holectypoida. 

Mouth central. Anus outside of the apical system in the posterior interradius 
(exocyclic). With external gills. Only one pair of pores or- a single pore on each 
ambulacral plate. Jaws weak ; teeth perpendicular ; both jaws and teeth may be 
wanting. Sphseridia present, (a) Ambulacral apophyses present : Holcotypus, 



VIU 



ECHINODEBMA TA —SYSTEM A TIC HE VIE W 



291 



Fi/iliistcr, etc. (principally fossil), {b) Ambulaoral apophyses rudimentary or want- 
ing : Discoidea, C'owxhjpeus (fossil). 

Order 4. Clypeastroida. 

Mouth central or sub-central. Anus outside of the apical system in the posterior 
interambulacrum. With external gills. With tentacle pores in the interradii. 








-^1:^A>,^ 






Pio. 232. — Ecliiiiocidaris (Arbacia) pustulosa, from the apical side (original). The spines have 
been removed from part of the shell. 1, Interambulacrum ; 2, ambulacrum. 

More than one pair of pores on each ambulacral plate. Tentacles differ in one and 
the same animal. Teeth usually almost horizontal, rarely vertical. The jaws lie 
above the apophysial ring, which is interrupted. SphEeridia present. 

The test is seldom much arched ; it is usually more or less flattened, and often 
even disc-like. It often has many incisions and perforations, and is usually bilater- 
ally symmetrical. Its dorsal wall is connected internally with its ventral wall by 
means of pillars, needles, septa, etc. Basal plates of the apical system fused. The 
ambulacra form petaloids in the apical region. 

Fam. 1. Fibulariidse : Echinocyamiis, Fibularia, etc. (extant and fossil). Fam. 
2. ClypeastridsB : Olypeaster (Fig. 233), etc. (extant and fossil). Fam. 3. Laganidsa : 
Laganum (extant and fossil). Fam. i. Scutellidse. In all the genera of this family 



292 



COMPARATIVE ANATOMY 



CHAP. 




Fig. 233. — Clypeaster sp., test from the apical side (original). 




Fig. 234. — Scutella sexforis, test from tlie apical side (original). 



vm EGHINODERMATA-^SYSTEMATIG REVIEW 293 

the shell is very Hat : Sculella (Fig. 234), Echimdiscus, Micojk, Mii//ita (Fig. 235), 
Rofi'/ri, Ai-achiuiid-s, etc. (extant and fossil). 




Fig. 2S5.~Mellita testudinata? from the oral side (original). 



Order 5. Spatangoida. 

llouth central, suh-central, or on the anterior edge of the oral surface of the test 
Anus outside the apical system, in the posterior interradius. External gills, jaws, 
teeth, and perignathous apophysial ring wanting. Sphferidia present. The ambu- 
lacra generally form apical petaloids. The test is bilaterally symmetrical, arched, 
often heart-shaped. 

Sub-Order 1. Cassiduloidea. 

Fam. 1. Eohinoneidae : Echinoconus, Echinoneiis, OUyopyyus, Eclmiolrissus, etc. 
(extant and fossil). Fam. 2. Cassidulidse : Cassidulus, Cato2')ijgus, Cbjjxus, Pygurus, 
Echinolampas, etc. (mostly iossA). Fam. 3. CoUyiitidse : CoUyrUes,Dysaster, etc. 
(fossil). Fam. 4. Plesiospatangidse : Eolam-pas, Archiacia, etc. (fossil). 



Sub-Order 2. Spatangoidea. 

Fam. 1. Anan-chytidse : Echinocoi-ys, Holastcr, Scmijmeustes, Oardiaster, Uir- 
cltiims, Cystechiiius, Calymiw, etc. (the last three genera extant, the rest fossil). 
' Fam. 2. Spatangidse — Group 1, Adetes : Isaster, EcMnospatagus, Heterolampas, 
Hriiiipatagus, etc. (almost exclusively fossil) ; Group 2, Prynmadetes : Hcmiastcr, 
Ffi/jriaa, Linthia, ScMzastcr (Fig. 236), Agassizia (extant and fossil) ; Group 3, 
PrymnodeBmia : Micrastnr, Brissus, Spiatangmnorpilia, Brissopsis, Spatmigus, Palceop- 
ii'.ustes (Fig. 237), Echinocard him, Luvrnia, etc. (extant and fossil); Group 4, 
Apetala : fknicopatagus, Pa/ccoirissus, Accste, Aeropic, etc. (extant and fossil). 



294 



COMPARATIVE ANATOMY 



CHAP. 




Fm. 236. — Schizaster lacu- 
nosus ? from the apical side 
(original). The spines, and the 
protuberances on which they 
stand, are not depicted. 1, The 
anterior unpaired ainbulacruna ; 

2, thu right anterior ambulacrum ; 

3, fapciole ; 4, the right posterior 
interanibulacnim ; 5, the right 
posterior ambulacrum ; 6, the 
unpaired posterior interanibula- 
crum ; 7, anal region. 



Fia. 237.— Paleeopneustes 
Murray! (after Agassiz), 
from the oral side. 1, The 
anterior ambulacrum ; 2, 3, 

the anterior right and the 
posterior right ambulacra ; 
4, peristome ; 5, anal region. 



VIII ECHINODERMATASYSTEMATIG REVIEW 295 

Fam. 3. Leskiidse : Fahcostoma (extant). Fam. 4. Pourtalesiidae : Fourtalesia 
(Fig. 238), Spatcwjocystis, Echinocrepis (extant).^ 




Sa*! 



Fig. 238.— Fourtalesia Jeffreys!, from the side (after Lovftn). The smaller tubercles are not 
depicted, ap, Apex ; os, oral pole ; on, anal region. The numbers are explained in the text, in the 
section on tlie perisomatic skeleton of the Echinoidea, p. 342. 



CLASS III. Asteroidea (Stelleridea), Star-fish. 

Eehinodermata, with body flattened in the direction of the principal axis, the 
radii being produced laterally into longer or shorter arms. The arms are usually five 
in number, but their number may be increased to forty or more. They are not dis- 
tinctly marked off from the central part of the body (the disc) ; and besides the radial 
blood vessels, nerves, and ambulacral vessels, diverticula of the intestine and con- 
tinuations of the genital organs run into the ccelomic cavities of the arms. The 
body is usually covered with calcareous plates, but is flexible. The calcareous 
plates carry spines, and often pedicellarise as well. Along each arm runs a ventral 
fuiTow, within which there is a longitudinal row of paired ambulacral plates. The 
consecutive pairs are niovably articulated with one another. Besides these, there 
are, on the arms, adambulaeral, inframarginal, supramarginal, and dorsal plates. 
The ambulacral gi-ooves run from the central mouth on to the arms, and along these 
on their oral (ventral) side, below the ambulacral plates, to their tips. The tube- 
feet (tentacles) rise from the base of this groove, to which they are limited. Anus 
apical (i.e. in the centre of the upper side), rarely wanting. Madreporite also on the 
apical side of the disc. The sexes are separate. Development is in most cases with 
metamorphosis (free-swimming pelagic larvae) ; when the brood is protected develop- 
ment is direct. 

Sub-Class 1. Palseasteroidea. 

Palseozoic Asteroidea, in which the ambulacral plates in the two longitudinal 
rows in each arm, at least in the middle of the arm, are arranged alternately (not 
opposite or in pairs). Aspidosmna, Palmaster, Palccoconui, etc. (all Palsozoie forms). 

1 The classification of the Echinoidea here given is after Martin Duncan, A Revision 
of the Oenera and Great Groups of the Echinoidea. London, 1889. 



296 



COMPARATIVE ANATOMY 



CHAP. 



Sub-Class 2. Euasteroidea. 
Asteroidea with paired, i.e. opposite ambulacral plates or " vertebra;." 



Order 1. Phanerozonia. 

Asteroidea wiVa large, strongly developed, marginal plates. The inframarginal 
and supramarginal plates are closely fitted together. Papula; (branchial vesicles) 
only occur on that surface of the body which is surrounded by the supramarginal 




FiQ. 239. — Ctenodiscus procurator (after Sladen), from the oral side. A Ga.stropod in the 
stomach is visible through the mouth. 

plates, i.e. on the apical or upper side. Ambulacral plates broad. In each ambu- 
lacral furrow there are two longitudinal rows of tube-feet. The adambulaoral plates 
are prominent in the oral skeleton. Where pedicellarise occur they are sessile. 

Fam. 1. Archasteridse : Pararchaster, Dytaster, Plutnaaster, Pseudarchastcr, 
Archctster, etc. Fam. 2. Porcellanasteridse, the centre of the apical S3'stem pro- 
duced into a more or less long oittgrowth : Porcellanaster, Hyphalastcr, Ctenodiscus 
(Fig. 239), etc. Fam. 3. Astropectinidse, without anus and usually without pedicel- 
larisE : Aslropecten, Sathybiaster, llyaster, Luiella, etc. Fam. 4. Pentagonasteridae : 
Prittagmiaskr, Astroguaium, Neclria, CalliasUr, Stellaster, Honindiseus, Mimaster, etc. 
Fam. 5. Antheneidse : Anthenea (Fig. 240), Gnnldster, etc. Fam. 6. Pentacerotidse : 



VII[ 



ECHINOBERMA T A— SYSTEM A TIC RE ] 'IE W 



297 



Pcntaenros, AmpJiiaster, Ciilcila, Asterodiseus, etc. Fam. 7. Gynmasteriidse : Guiii- 
nastcria, Tyl(tstcr, Astrropsis, 
ilaryinastcr, etc. Fam. 8. As- 
terinidse : Garn'rUi, Aster ina^ 
Pii/nu'pcs, etc. 



Order 2. Cryptozonia. 

Asteroidea, in which the mar- 
ginal plates are indistinct and 
more or less rudimentary in the 
adult. The supramarginal plates 
are often separated from the in- 
framarginal by intermediate plates The 
papuliE are not limited to the apical sm 
face, but often occur also between the 
marginal plates and on the oral (lowei 



nr 



\ 




Fi*;. 240. — Anthenea tuberculosa, Gray? 

jiiv. (after Sladen). 1, Supramarginal plates ; 
2, pedicellariii.- ; 3, iiiadreporite ; 4, anus. 




Fig. 241.~Cneimdaster Wyvillii (after Sladen). <k, Dorsocentral ; r, radials ; hn, basals ; 
s)?i, supramarginals ; d, dorsals ; f, terminals. 

surface of the body. Ambulacral plates narrow, closely crowded. Tube- feet 'often 
in four rows. In the oral skeleton the ambulacral or interanibulacral plates are 
prominent. Pedicellarire sessile or pednnculate. 



298 COMPARATIVE ANATOMY chap. 

Fam. 1. Linckiidse : Chaetaster, Ophidiasler, Linckia, Metrodira, etc. Fam. 2. 



^ffim 




Fic. 242. — Hymenaster cselatus (after Sladen), with arms bent back. 

Zoroasteridse : Zoroaster, Cnemidastcr (Fig. 241). Fam. 3. Stiohasteridae : Sti- 
ehastcr, etc. Fam. 4. Solasteridse : Solastei; Crossnstcr, Oorethraster, etc. Fam. 5. 




Fnj. 248.— Hymenaster nobilis (alter Sladen), from the oral side, | natural size. 

PterasteridsB, with brood cavity on the apical side of the disc : Pteraster, Retastcr, 
Hymenaster (Figs. 242 and 243), Myxastcr, Bcntkastcr, Pythonaster, etc. Fam. 6. 



VIII 



EGHINODEBMA TA—S YSTEMA TIG RE VIE W 



299 



EchinasteridSB : AcantJuistcr (numerous arms), Mithrodia, Orihrella, Eddnastcr, 
Vnlvasicr, etc. Fam. 7. Heliasteridse, with numerous short arms : Seliaster. Fam. 
8. Pedicellasteridae : Pedicellastcr. Fam. 9. Asteriidae, tube -feet in four rows : 
Asterias, Uniophora, Coronaster, etc. Fam. 10. Brisingidse, with numerous very 
long arms, marked off from the small disc : Brisiaga, Labidiaster, etc.'- 



CLASS IV. Ophiuroidea. 

Echinoderniata flattened in the direction of the principal axis of the body, the 
radii of which are produced into five long, round, simple or nmch branched slender 
arms. The arms are sharply marked off from the central part of the body, and do 
not contain either cieca of the intestine or extensions of the genital organs. The 




Fio. 244.— Ophiolepis elegans, LUtken (after Lyman), ds, Dorsal shields ; ss, lateral shields ; 
etc, dorsocentral ; i&, infi-aba-sal ; &n, basal ; i\i, radial shields ; r, radial. 

axial part of the arms is occupied by a longitudinal row of vertebral ossicles, articu- 
lated together, and consisting of two fused lateral ambulacral plates or ossicles. 
The body is usually covered with calcareous plates. On the arms we can distinguish 
a longitudinal row of ventral shields on the oral side, two longitudinal rows of 



^ The classification of the two orders of the Euasteroidea is that of W. Percy Sladen, 
Report on the Asteroidea collected hy H.M.S. Challenger. London, 1889. 



300 



COMPARATIVE ANATOMY 



CHAP. 



lateral spine-bearing shields, and a longitudinal row of dorsal shields. On the 
apical surface of the disc larger radial shields are found at the sides of the bases of 
the arms : thus ten in all. On the oral side of the disc there are five interradial 
plates which are distinguished by their great size ; these are the buccal shields. One 
of these plates is at the same time the niadreporitic plate. Mouth at the centre of 
the lower side. Anus wanting. The ambulacral tube-feet appear on each side on 
the arms between the ventral and lateral shields. On the lower side of the disc, close 
to the bases of the arms laterally, there are in all ten or twenty slit-like apertures — 
the bursal apertures. These lead into blind sacs projecting into the ccelom ; these 
are the bursfe, which serve for respiration and for the reception and ejection of tlie 
genital products. Development direct (viviparous and with care of the brood), or 
with metamorphosis (free-swimming pelagic larvie). 



Order 1. Ophiurse. 

Arms uubraneheJ, movable in the horizontal plane, usually distinctly 
Buccal shields, one of them at the same time the madreporitic plate, distinctly 
developed. 

Fam. 1. Ophioglyphidae : Ophinra, Fectinum, OpMolcpis (Fig. 244), Ophio-Mua, 
Ophioglypyia, Opliioden, Ophiomufduvi. Fam. 2. Amphiuridse : OjJhiadis (Fig. 2i5), 



-^A\ ^-1-^ //<^ 







Fig. 24:1— Ophiactis poa, Lym. (after Lyman). Disc and basal ijortioiis of *lie arms ; from tlie 
oral side. 1, Vpiitral .shields ; 2, spines on the lateral shields (4) ; 3, tentacle scales ; 5, lateral 
buccal shields ; o, bursal apertures ; 7, buccal shields ; S, first ventral shield of the arm ; 0, torus 
angularis ; 10, oral papillje. 



.imphmra, (ipJiiocnida, Opii-iocoiaa, Ox>liiacantha, Ophiothrix. 
myxidse, disc and arms covered by a thick naked integument : 
euryalc. 



Fam. 3. Ophio- 
Ophiumyxa, Heiiii- 



Till 



EOHINODERMA TA—t< YSTEMA TIC RE VIE IV 



301 



Order 2. Euryalse. 

Arms simple or branched, can be rolled up vertically towards the nioutli. Only 
rudimentary shields are found below the soft but thick outer integument. Without 
spines. In forms with unbranched arms there are usually 5 buccal shields, one of 
which is the madreporitic plate. Most of the forms with branched arms liave no 




Fig. 246. — Astropliyton Lincki (Miillor and Troscliel), from the oral side (original). 

distinct buccal plates. There is then either a single raadreporite in an oral inter- 
brachial area or else there are 5 interbrachial madreporites. 

Single Fam. Astrophjrtidae : Astrophyton (Fig. 246), Gorgonoceplmlus, EuryaU, 
Trichaster (arms slightly and only at their tips, dichotomously branched), Astroclon 
(the same), Astrocnida (the same), Astroporpa (arms undivided), Astrogmn^jlius (the 
same), Astrochele (the same), Astrotoma (the same), Astroschema (the same), Opliio- 
creas (the same), etc.^ 

' For a more recent classification of Opliiuroidea, see F. J. Bell, Proc Zool. Soc. 
London, 1892, pp. 175-183. 



302 COMPAEATU'E ANATOMY chap. 



CLASS V. Pelmatozoa. 

Echmodermata which are either permanently or temporarily ' attached by the 
centre of the apical surface, so that the oral surface (with the mouth, as a rule, in its 
centre) looks upward. The body is usually raised upon a jointed stem attached to 
it at the apex. An axial canal, in which are blood vessels and nerves, runs through 
the stem. This stem is sometimes found only in the young, the body becoming 
detached later, and further in a few attached forms no stem at all is developed. 
The apical system of plates consists of 5 basals and 6 radials, to which 5 infra- 
basals and a varying number of interradials are often added. The plate in the 
embryo Antedon, which becomes fixed to the ground and is subsequently lost, is 
called "dorsocentral," and is supposed to belong to the apical system. The number 
of the principal rays is rarely i or 6. The plates just mentioned form a cup 
(dorsal cup), which either simply carries or else more or less completely encloses the 
visceral mass. The cup carries jointed appendages, — arms or pinnulfe or both. 

The oral side (in these animals turned uppermost) is often provided with 5 oral 
plates, which surround or cover the central mouth, and it may further be protected 
in very various ways by radially and interradially situated plates (ambulacrals, 
interambulacrals, and orals), which together form the tegmen calycis. Or again this 
cover of the calyx may be either naked or set with very small isolated calcareous 
pieces. The anus lies usually at the end of a longer or shorter tube, excentrically in 
an interradius of the tegmen, occasionally, however, at the boundary between the 
cup and the tegmen. The circumcesophageal canal of the water vascular system does 
not communicate direct with the exterior. The radial canals of this system run 
into the arms. Each of the latter has a food groove on its oral (uppermost) side. 
The tube-feet, which rise from the edge of this furrow, are tentacular, and do not 
serve for locomotion, but for respiration, and possibly for conducting food. 
Development, so far as is known, with metamorphosis. 

Sub-Class 1. Crinoidea. 

Pelmatozoa with long usually branched arms. The arms are jointed, the con- 
secutive ossicles being connected by muscles and bands. The arms can be expanded, 
and closed up together, or again can roll up orally. They may carry jointed, 
unbranched appendages, the pinnulte, which are probably modified branches. The 
nervous .system is generally said to be "double," i.e. there is an abactinal and an 
oral system. The abactinal nervous system consists of a central portion lying in 
the apex of the dorsal cup and of radiating strands which ruu through the skeletons 
of the stem, the arms, and the pinnulte. The oral nervous system consists of a 
circumoral nerve ring, and radiating strands which run into the arms through the 
epithelium at the base of the food grooves, and which branch with the arms. The 
food grooves of the arms pass at their bases on to the tegmen, running in it to 
the central mouth. Anibulacral tentacles may be wanting. The circular canal of 
the water vascular system is connected with the body cavity by means of several 
stone canals, and the body cavity is in open communication with the exterior by 
means of water pores. The mouth is in the centre of the tegmen (exc. Actinovietra). 
The sexual organs extend right into the basal parts of the arms, and even into 
their pinnulse. In pinnulate crinoids, so far as is known, however, the genital 
products only ripen in the pinnula;. 

1 There is, however, no evidence to show that AlarsupUes was attached even in the 
larval stage ; imlike Antedonidis, it has no trace of a stem. 



vm 



EGHINODERMA TAS Y ST EM A TIG RE VIE W 



303 



The old division into PalfKocnnoidca and Neocriiiuidea seems artificial ; that here 
adopted also cannot be considered as definitive.^ 

Order 1. Inadunata. 

Calyx comparatively small ; dorsal cup with monocyclic or dicyclic base ; the 
basals in the former, and infrabasals in the latter case may be fused to 4, 3, 2, or 
1. The only other plates in the apical capsule are 5 radials. In the posterior 
interradius there are very often 1-3 asymmetrically placed anal plates, but no plates 
in the other interradii. 

The tegmen calycis varies. In some Inadunata (Larviformia) there are 5 large 
oral plates, which, rising at the edge of the calyx directly above the radials, form a 
closed pyramid covering the food grooves of the disc, and the mouth. In many 
other forms the orals (which may be partly resorbed) lie at the centre of the tegmen 
calycis. The posterior oral plate is often larger than the others, and is shifted for- 
ward between them. The ambulacra appear at the surface of the tegmen calycis be- 
tween the oral plates and the edge ; they are bordered on each side by rows of small 
lateral pieces, the ambulacral groove being also roofed in by small covering pieces. 
Plates of various shapes, size, and arrangement are found in the interambulacral 
regions. In the posterior ambulacral region the tegmen calycis often bulges out in 
the form of n plated sac, the so-called ventral sac {Fistulata) ; this varies in form 
and size (sometimes reaching beyond the arms), and may sometimes have contained, 
besides the rectum, a large part of the body cavity. The anus lies at its tip or on its 
anterior side. 

Arms free, i.e. not included in the dorsal cup (hence the name Inadunata), 
simple or branched, with or without pinnulse. The food grooves of the arms are 
roofed in by two or more rows of alternating, wedge-shaped, interlocking, ambulacral 
plates ; these plates could probably be erected. 

Almost exclusively paleeozoic forms. 



A. Monocyclica. 
With monocyclic basis (without infrabasals ; several radials often horizontally 

A. 3. 




Fig. 247.— Haploorinua mespillformis (after Waclismuth and Springer). A, from tlie anal 
side ; B, from the oral side. 1, Orals ; 2, oral pole ; 3, anus ; 4, radials ; 5, right posterior infer- 
radial or radianal ; 6, basals ; 7, first brachial ; 8, facet for attachment of the arm. 

bisected). Saplocrinus (type of the so-called Larvifomnia, without anal plate) (Fig. 

1 Classification chiefly after the recent works of Waohsmuth and Springer and Her- 
bert Carpenter. See Bibliography, p. 651. 



304 



COMPARATIVE A ^' ATOMY 



CHAP. 



L'47). Heter(xriiius,Hcr})rtocrinus, C'alceocrimis, Catillucrmus, Pisucriiius, Hyhocrimis, 
Tocriiuis, Sijmbathocrinus, BeUmnocrinus, Gastrocoma (?), C^qiressocrinus. 



B. Dicyclica. 

With dicyclic base (with infrabasals). Fam. Dendroorinidse : Dendrocrinus, 
Huriwcrinus, Potcriocrinus. Fam. DeoadocrinidsB : Boiryocrinics, Barycrinus, 





Fig. :24S. — Encrinus liliiformis (origiiKil). 
ti, '■■^, Costals or prhiiibrachials ; r, radials ; 
(_■", stern ; p, pinnulEe. 



l''iG. 249. — Cyathocrinus 
longjmanus (after Angelin). 
j'l; Ventral sac ; !, place where an 
avju-branch has been removed ; 
r, radials ; &a, basals ; ilj, infra- 
basals ; col, stem ; a-, anal plates ; 
CO, costals or priraibracliials. 



Atelestucrinus, Deeadocrmus, (Jrcqjhiocnuus, Encrinus (Fig. 248), (without anal 
plates, ventral sac reduced to a short cone, Trias), Cromyocrinus, Agasslzo- 
crinus. Fam. Cyathocrinidse : Oyathocrinus (Fig. 249), Gissocrinus, Lecythocrinus, 
Hyjpocrinus. 

The genus Marsupites from the Chalk, and the following extant families are 
perhaps to be classed near the Inadunata ; in these latter five large separate orals 
occur, the ventral sac being reduced to an anal tube, and no anals appearing in the 
dorsal cup. Holcrpkhe (Fig. 250) (Lias, to present time), Eyocrinidce (Fig. 251) (Lias, 
present time), BatAycrinidcc (extant). 



VIII 



ECHINODERMA TA —S YSTEMA TIG RE VIE W 



305 




Pig. 250.— Holopus Bangl d'Orbigny, from the trivial 
side (after P. H. Carpenter). 



VOL. II 



306 COMPARATIVE ANATOMY chap. 



Order 2. Camerata. 

Plates of the calyx firmly connected by means of sutures. The apical capsule 
shows a tendency to develop a very rich system of plates, incorporating the 
proximal brachials to a greater or lesser extent. These brachials are connected 
together in the interradii by interradial plates, which vary in number, and to which, 
in the anal interradius, special anal plates may be added. In those cases in which 
the arms are incorporated in the calyx to such an extent that they branch in the 
latter before they become free from it, their branches may be connected by inter- 
calated plates. Each of the five radials is usually followed by two brachial plates, 
formerly called 2nd and 3rd radials. The tegmen calycis is richly plated with firmly 
connected pieces, and is often much arched, forming a so-called vault. The mouth, 
lyhich lies in the centre of the tegmen, is covered with five firmly united oral 
plates ; the hindermost of these, which is often the largest, projects in between 
the four others. The ambulacra, with their lateral and covering plates, are mostly 
not visible from outside, as the interambulacral plates which border them laterally, 
and which are often very numerous, close together over them by means of processes, 
and thus cover them externally. The ambulacra, in their course on to the bases of 
the free arms, divide as many times as the arms have already divided on the 
dorsal cup. The interradials of the dorsal cup often pass, without any sharp 
boundary, into the interradially arranged interambulacrals of the tegmen calycis. 
The subcentral (less frecjuently central) anus, which is surrounded by firm anal 
plates, is either sessile or else comes to lie at the tip of a chimney-like prolongation 
of the tegmen ; this anal tube, formerly thought to be a proboscis, may project 
beyond the arms. Arms branched ; in adults, almost without exception, the 
brachials become arranged in a double row with primitive articulation, and pinnules 
closely folded together. Dorsal canals (in the brachials) have never been observed. 
Exclusively palaeozoic forms. 

Family 1. Reteocrinidse. 

Apical capsule, with monocyclic or dicyclic base. Four or five basals. Inter- 
radial and interaxillary regions deeply sunk, plated with a large number of irregular 
immovable pieces, which are continued on to the interambulacral areas of the tegmen 
calycis. Posterior interradial region broader, and divided by a perpendicular row 
of somewhat large anal plates. Anus subcentral. Arms composed of a single row of 
calcareous joints. Pinnules strong. licteocrinus. Xciwcrin us. 

Family 2. Ehodocrinidse. 

Apical capsule with dicyclic base. The circle of the five radials interrupted by 
that of the five first interradials, which are in direct contact with the basals. 
Interradial area plated with regular definitely arranged pieces. Posterior interradial 
area differs but slightly. Tegmen calycis thickly plated. The plating of the apical 
interradial region passes without break into that of the tegmen calycis. Ambulacra 
not externally visible. Orals often indistinct. Anus subcentral. Rhodocrinus, 
Gilbertsocrmus, FJiipidocTinus. 

Family 3. Glyptasteridffi. 

Base dicyclic. "With the exception of the first anal plate, which is in contact 
with the posterior basal, the interradials do not touch the basals. Interradial 
region of the apical capsule and tegmen calycis as in the lihodocrinidcc. Oral plates 
distinct. Anus subcentral. Glypiaster. 



VIII 



ECHINO DERMA TA—SYH TEMA TIG BE VIE W 



307 



Family 4. Melocrinidse. 

Base moiiooyclie, 3-6 basals. The basals in contact only with the radials. 
Interradial areas of the apical capsule with numerous large regularly arranged plates. 
Plates of the tegmen calycis often small and regular. Orals distinct. Anus sub- 
central, ilelocri iius {'¥\g. 252), JIariacriiiu.% Glijptocriinis, Stelidiocrinus. 



Family. 5. Actinocrinidss. 

Base monocyclic, 3, rarely 4, basals. The first anal plate rests upon the circle of 
basals ; the first interradials otherwise being in contact only with the circle of 





Fig. 202.— Melocrlnus typus, Br. 

p, PinnuliE ; br, arms ; di, disticlials ; 

ci, cn, first and second costal ; ;-, radial ; ^ 

6a, basal ; co, stem ; ir and id, inter- ^ v V %!\^^^ "^^ '^^ "^-^iidcj-) — ClOr 

radials. 



Fig. 253.— Batoorlnus pyriformis, 
Slmm. (after Meek and Wortlien). 

i'/:, Ventral capsule ; br, arms ; p, pin- 
nulas ; dl, disticlials ; cj, Ca, costal-s ; 
r, radials ; 6a, basals ; co, stem ; ir, 
interradials ; a6r, points of insertion 
nf tlie arms. 



radials. Tegmen calj'cis usually much arched, consisting of numerous firmly 
connected plates, some of which at least are large, arranged in definite order. The 
ambulacra of the tegmen calycis with their skeleton hidden, or only visible in forms 
with flat tegmina. Anus subcentral. Orals usually distinct. Caiyocrinus, Agnrico- 
crinus, Feriechocrimis, Merjistocrinus, Actiiiocriuns, Tdeiocrinus, Steganoerinus, 
Aiiiphoracriiius, Physetocrinus, Strotocrinus, Batocrinus (Fig. 253), Erdmocriniis, 
Dorycrinus. 

Family 6. Platycrinidse. 

Base monocyclic, 3 basals, which are unequal. Anal and interradial plates not 
in contact with the basals. The very large radials together with the basals form 



308 



COMPARATIVE ANATOMY 



CHAP. 



almost the whole of the apical capsule. Each radial is connected with a short and 
small costal plate. The various brachials which follow (distichals, palmars, etc.) 

are free, i.e. belong to the freely out- 
standing arms. In each interradius 
there are at least three interradials, 
which, however, appear more or less 
shifted on to the oral side. In the 
jjroximal (apical) interradial ring there 
are no special anal plates, this ring 
consisting in each interradius of 3-5 
transversely placed plates, the central 
one being the largest. Orals large. 
Tegmen calycis mostly much arched. 
The ambulacra and their covering 
plates often appear at the surface. 
Anus subcentral. Platijcrinus (Fig. 
254), Marsiqnocriiins, Eucladocrinus. 

Family 7. Crotalocrinidse. ^ 

Base dicyclic. The apical capsule 
consists exclusively of the typical 
plates of the apical system (infrabasals, 
basals, and radials), to which is added 
an anal plate. The brachials of the 
separate rays (to the fourth order) 
firmly united by sutures. Arms very 
mobile, uniserial, long and much 
branched ; branches free or connected 
together in such a way as to form a net- 
work around the calyx ; this network 
is either continuous or else divided into 
five leaf-like lobes corresponding with the rays. Arms and their branches traversed 
by large axial canals. Tegmen calycis flat, richly plated with distinct orals, inter- 
radials, and anals ; ambulacra externally visible, with large rigid covering plates, 
which combine with the other plates to form the solid tegmen. Anus subcentral. 

(This family is distinguished from all other Camerata by the presence of axial canals, 
and by the mobility of the free joints of the arms.) Crotalocrinus, Enallocrinus. 




Fio. 264.— Platyorinus triacontadactylus (after 
M'Coy). di, DistichaLs ; c, co-stals ; r, radial ; &o, 
basal ; co, stem ; ii\ interradials ; vl', ventral ca; 



Family 8. Hexacrinidse. 

Base monocyclic. 2 or 3 basals. The first anal plate rests on the circle of 
basals, and resembles the radials in shape. In other respects like the P/atycrinidcc. 
Hcxacrinus, Talarocrinus, Dichocrimis. 

Family 9. Acrocrinidse. 

Base monocyclic. 2 basals, separated from the radials by a broad zone of 
small plates arranged in circles round the basals ; these form the largest ]iart of 
the apical capsule. Each radial is followed by 2 costals. The radials and 



^ This family, originally placed near Cyathocrinus, was referred by Wachsnmtli and 
Springer, first to the Ichthyocrinoidaj and then to the Camerata ; Bather, however, 
would refer it to its original position in the luadunata. 



VIII ECHINODERM AT A— SYSTEMATIC REVIEW 309 

costals of the 5 rays laterally distinct. Interradials in two circles ; in the first 
circle there are two plates to each interradius, and in the second circle only one, 
which, however, is larger than the former two. Posterior interradius much larger, 
with twice as many interradials, between wliich there is, further, an intercalated 
vertical row of anal plates. Acrocrimis. 

Family 10. Barrandeocrinidse. 

Base monocyclic. 3 basals. The first anal plate rests on the circle of basals. 
The interradials rest upon the sloping oral ends of the radials. Arms bent back on 
the calyx, fusing laterally with one another by means of their pinnulse in such a 
way as to form a firm envelope around the calyx. Barrandeocrinus. 

Family 11. Eucalyptocrinidse. 

Base monocyclic. The apical capsule consists of 4 basals, 5 radials, 2x5 
costals, 2x10 disticlials, 3x5 interradials, and 1x5 interbrachials. No anal 
plates. The tegmen calycis consists of 5 large interradials, 5 large and 10 small 
interbrachials, the oral plates, and two other plates lying further up towards the 
apex. Anus shifted quite to the centre. The plates of the tegmen form 10 niches ; 
in the bases of these niches ambulacral grooves (two in each) run to the bases of 
the 10 pairs of arm -branches, which are received into the niches. Eucalyptocrinus. 
Oallicri'mis. 

Order 3. Articulata (Ichthyocrinidse). 

Skeleton flexible. Anal plates often occur in the posterior interradius of the 
calyx. Base dicyclic. Three infrabasals of unequal size, which are usually hidden 
by the uppermost joint of the stem. Radials perforated, with one or more 
primary bracbials. Tlie circle of the combined radials and primary brachials is 
closed, or else interrupted by one or more plates in each interradius. The brachials 
of the first, second, and often also of the third order are incorporated in the calyx. 
The radials and the separate brachials are articulated together. Arms uniserial. 
Pinuulse appear to be wanting. Interradials irregular and varying in shape, size, 
and arrangement, inconstant (may be either present or wanting in one and the 
same species). In the posterior interradius there is often one asymmetrical plate. 
Tegmen calycis only known in a few forms, soft and flexible, the plates lying in it 
not being firmly fused together. Five separate orals of unequal .size grouped round 
the open mouth, the posterior oral being the largest. Ambulacra with their cover- 
ing plates appear at the surface. Between them, there are interambulacral plates 
which are occasionally distinguished by their remarkable size. Interambulacral 
areas often sunk. Food grooves of the arms enclosed by movable covering plates. 
A plated process (anal tube with anus ?) is found excentrically in the posterior 
interradius of the tegmen. 

Fam. Ichthyocrinidse — Palteozoic forms : IcMhyoc.rinus, Forhesiocrinus, Ohio- 
criiius, Taxocrinus (Fig. 255), etc. 

The unstalked genus Vintacrinus, from the upper Chalk, and the extant unstalked 
genus Thaumatocrinus (Fig. 256), ought probably to be classed here. In the latter 
the uppermost ossicle of the stem is retained as eentrodorsal. The dorsal cup 
consists, apart from the eentrodorsal, of 5 basals, 5 radials, and 5 interradials, 
which last rest on the circle of basals, and alternate with the radials. Tegmen 
with central open mouth, which is protected by a. pyramid of 5 large separate 
orals. Between the orals and the edge of the calyx (or the oral edge of the 
interradials of the dorsal cup) the tegmen is covered with small irregular plates 



310 



COMPARATIVE ANATOMY 



CHAP. 



indistinctly arranged in two to three rows. The anal interradial carries a short 





Fio. 255.— Taxoorinus multlbraohiatus, Ly. 
and Cass, ir, iri, and irg, Interrailials ; di, dis- 
tichals ; ha, basals ; i&, infrabasals ; co, stem ; r, 
radials ; Cx, cg, and C3, primary brachials. 



Fig. 256. — Tliaumatocrinus renovatus, 
P. H. C. (after P. H. Carpenter). Calyx 
from the anal side, cj, c^, and cg, Primary 
brachials ; r, radials ; «', points of insertion 
of tire cirri ; cd, centrodorsal ; ii; inter- 
radials ; i", interradialia analia ; jjc, proces- 
sns analis ; ta, tubus analis ; p, pinnule. 



jointed appendage, 
arms with pinnulie. 



Besides this there is a short anal tube. Five iinbranehed 



Order 4. Canalioulata. 



Calyx symmetrically five-rayed. Base dicyclic, the infrabasals usually not 
separate, but atrophied or fused with the proximal columnal 5 basals, occasionally 
not externally visible. Each radial is followed by 2 costals. Anal plates always 
wanting (hence the regularity of the calyx). Interradials with few exceptions 
wanting. Arms simple or divided (one to ten times). Tegmen calycis usually flat, 
with open mouth and ambulacra appearing at the surface. Orals rarely present. 
Tegmen calycis often plated with small loose-lying plates. Stem present either only 
in young forms or also in adults. Basals and radials perforated by dorsal canals. 
To this order belong, besides Mesozoic and Tertiary forms, most of the extant 
Crinoids. 

Family 1. Apiocrinidse. 

Calyx consists of 5 basals of equal size, 5 radials and 2x5 primary brachials. 
Distichals may also take part in its formation. Interbrachials and interdistichals 
may occur. Tegmen flexible, with small plates. Arms more or less branched, con- 
sisting of a single row of joints. Stem without cirri, usually expanding in its 
proximal region to the same width as the calyx, but not containing the viscera. 
.Jurassic, to present. Apiocrinus, Millcrkrinus, and the extant Calattiocrinus. 



Family 2. Bourgueticrinidae. 
Calyx consists of 5 basals and 5 radials. Brachials connected in pairs by 
syzygial sutures. Five orals in the tegmen calycis. Interambulacral region other- 
wise not plated. Ambulacra with covering plates, but without lateral plates. Stem, 



VIII ECHINOVERMATA—SYSTEMATIG REVIEW 311 

\yith root-like processes at its base, or -with iiregularly arranged cirri ; its proximal 

P 



Hlf//(M/>/>^4^ . 




Fig. 257.— Metaorinus Murray! (after P. H. Carpenter). Most of the arms and tlie larger part 
of the .stem broken off. p, Pinnute ; a, cirri : iij, node. 

ossicle usually enlarged. Upper Jurassic, Chalk, Tertiary, Eecent. Rhizocrinus, 
Bourguetwrinjjii?. 



312 



COMPARATIVE ANATOMY 



CHAP. 





— * 



Fig. 259.— a, Cystoblastus Leuchtenbergi. 
1, Interrailial ; 2, 3, radial; 9, basal; 10, infra- 
basal ; 8, anus ; 6, genital aperture. B, From 
the oral side (after Volbortll). 4, Jlouth ; 6, 
ambulacrum. Fig. 295, p. 382, shows the apical 
side. 







Fig. 2.'.S.— Antedon inclsa (after P. H. 
Carpenter). 1, Anns; 2, cirri. 






Fig. 260.— Protocrinus ovlformls, Elcliwald 
(after Volborth). 2, Anus ; 1, third aperture ; 3, 
ambulacrum. 



VIII 



ECHINODEEMA TA—S YS TEMA TIG BE VIE IF 



313 



Family 3. PentacrinidK. 

Calyx small as compareil with the stem ami the arms ; it consists of 5 basals 
and 5 radials. (In the genns E.draa'inus the infrabasals are separate). Rays 
divided one to ten times. Stem surrounded at intervals by whorls of cirri. No 
root-like processes on the stem. One or more free primary brachials. Orals wanting 
in the adult. Trias, to Recent. Pcntacriinis, Mctacrinus (Fig. 257), Exircurinus, 
Balanocrinus. 

Family 4. Comatulidae. 

Adult free, larva stalked. The calyx is closed apically by the uppermost ossicle 
of the larval stem, which is fused with the larval infrabasals ; this ossicle carries cirri 
and becomes detached fi-om the rest of the stem. It is called " centrodorsal. " 
The basals are externally visible, or else form an internal hidden rosette. Five or 
ten simple or branched rays. The radials of the radial circle are usually followed, 
in forms with divided arms, by two fixed primary brachials. Interradials wanting. 
Orals wanting in the adult. AWccrinus (basals externally visible), Evdiocrimis, 
Anteclon (Fig. 258), Promachocrinvs, ActinoniHra (the only Cnnoid genus with 
excentric mouth). Since Jurassic times, many living species. 



Sue-Class 2. Cystidea. 

Body (calyx) oviform or spherical, plated with numerous very variously shaped 
pieces, which are rarely quite regularly, and often irregularly arranged ; stalked, sessile, 
or (rarely) free. Arms in many cases unknown, perhaps wanting in many forms ; when 
present, weakly developed, resembling pinnules, and rising near the mouth. Food 





-Fiu. 2()1.— Orooystls Helmliackeri, „ ... . . ^ 

Baur (after Barrande). 1-8, The Fig. 2(,2.-Agelaormus omcmnatensis. 

three apertures. 

grooves, arranged irregularly on the calyx, radiate from the mouth. At some dis- 
tance from the mouth a second aperture (anal aperture), and between the two a third 
aperture of unknown significance. Double pores or " pectinated rhombs " on some or 
all of the plates. Palfeozoic Pelmator:oa, whose organisation is still little understood. 

Order 1. Cystocrinoidea (c/. the section on the perisomatic skeleton of the 
CystUku) : Porocrinus, C'aryocrinus, Echiiiocncriims, OystoMastns (Fig. 259 A and B). 

Order 2. Eucystidea : Protocrinus (Fig. 260), iVyptosphceritrs, Orocystis (Fig. 
261), EdiiiiiiHpfurira, Aristocystii, Ascocystis, Mcsitc.s, AgdacriiDis (Fig. 262), 



314 



GOMPAHATIVE ANATOMY 



CHAP. 



Sue-Class 3. Blastoidea. 

Armless Pelmatozoa, either pear-shaped, club-shaped, oviform, or spherical. 
Body usually regularly radiate. Base monocyclic. Three basals, one small and 




Fig. -itiS.— Pentremites, from 

tlie side, without pinimles. 1, 



Fio. 265.— Codaster bilobatus, M'Coy, from tlie oral 
side (after Etlieridge and Carpenter). 1, Hydrospire 
Interradial = deltoid ; 2, 3, radials ; slits ; 2, lateral plates ; 3, ambulacral groove ; 4, mouth ; 
4, basal ; 5, ambulacrum ; il, spir- 5, radial ; (3, suture between two radials ; 7, anus ; 8, inter- 



aclp. 



radial ; 0, ridge on an interradial. 





s^'?-- vl ^j^ i\'"^?r\ — *~' 



Fia. 264. — Granatocrinus Norwoodl 
(after Etheridge and Carpenter) ; from 
the apical side, with stem. 




Fig. 266.— Orophocrinus stelliformis (after Ethe- 
ridge and Carpenter) ; from the oral side. 1, Lateral 
plates ; 2, covering plates of the ambulacra ; 3, hydro- 
spire slits ; 4, anus ; 5, ambulacral groove ; 6, points 
of attachment of the pinnules. 



two larger. Five radials, more or less deejily cut out for the reception of the five 
ambulacra. Five interradials lying above the five radials, and siu'rounding the 



EGHINODERMA TA—SYSTEMA TIG RE VIE IF 



315 



peristome. One of these is perforated by the anus. The ambulacra are bordered 
along each side by a single or double longitudinal row of jointed pinn\ile-like 
appendages. Ambulacra with lateral and accessory lateral plates. In each ambu- 
lacrum, under the lateral plates, there is a lancet-like piece, which is penetrated 
lengthwise by a canal, and in which a radial ambulacral vascular trunk probably 
ran. Ten groups of " hydrospires " on the radials and interradials. Peristome 
covered by small plates, which are continued into the covering plates of the ambu- 
lacra. For details cf. the section 
on the Skeletal System, p. 328. 
Palfeozoic forms. 



Order 1. Eegulaxes. 

Stalked Blastoids with sym- 
metrical base. The radials resemble 
one another, as do the ambulacra. 

Fam. 1. Pentremitidse : Pen- 
tremites (Fig. 263), Pentremitidrii, 
Misohlastus. Fam. 2. Troosto- 
blastidse : Troostocrimis, Mctahlas- 
tiis, etc. Fam. 3. Nucleoblastidge : 
Elceocrinus, SchizoUastus, Ci-ijpto- 
hhtstus. Fam. 4. Granatoblastidas : 
'ririiiatocriiijis (Fig. 264), Hetero- 
blastus. Fam. 5. CodasteridsB : 
Codaster (Fig. 265), Plucnoschisma, 
Cryptosehisnm, Orophocrinus (Fig. 
266). 




Fio. 267. — Astroorinus Benniei (after Etlieriage 
and Carpenter). 1, 4, 6, Iiiterradi.ils or deltoid plates ; 
2, radials ; 6, the modified radial ; 3, ambulacrum ; 9, 
the modiHcd ambulacrum ; 7, basal ; S, notch-like sinus. 



Order 2. Irregulares. 

Unstalked Blastoids, in which one ambulacrum with its radial is differently 
developed from the rest. 

Single family, AstrocrinidsB : Eleutlmrocrinus, Astrocrinus (Fig. 267), Pejife- 
phyUum. 



I. General Morphology of the Eehinoderm Body. 

The body of most Echinoderms, superficially observed, appears to 
be of strictly radiate structure, but more careful examination reveals 
that even in apparently perfectly radiate forms, e.g. regular Sea-urchins 
and Star-fish, strict radiate symmetry is not found either in the 
external or in the internal organisation ; in the latter, indeed, the 
asymmetry is evident. Nevertheless, in order to facilitate a simple 
description of the position and arrangement of the organs, terms are 
habitually used which assume a strictly radiate structure. For the 
purposes of description we may imagine the Eehinoderm body to 
be spherical or egg-shaped. Two poles may be distinguished in it. 
At the oral, adaetinal, or ventral pole there lies, in most Echinoderms, 
the oral aperture, while at the other apical, abaetlnal, or dorsal 
pole in many forms is found the anal aperture. The line which 
connects the oral and apical poles is called the principal axis. 



316 



COMPARATIVE ANATOMY 



CHAP. 



Round this principal axis many important parts of the body are 
grouped in a radiate manner. The typical number of the rays is, 
with few exceptions, five. In the Echinoderms, as in the radiate 
Ccelenterates, rays of the first, second, and third order may be distin- 
guished. The radii or radial regions of the first order, in which the 
principal organs lie, are called perradii, ambulaeral radii, or simply 
radii. The five radii of the second order, which regularly alternate 
with these five principal radii, are the interradii or interambulaeral 




Fir.s. 1268 and 2(i:i.— RepresentatlTes of the principal diyisions of the Eohinodermata. In 
Fig. 208, in the morphological position; in Fig. 260, in the natural position witli regard to the 
sea-floor. A, Holotliurian. B, Sea-urcbin. C, Star-fish. D, Crinoid — a, Apical pole ; o, oral 
pole ; an, anns. 

radii. The far less important ten radii of the third order, each of 
which lies between a perradius and an interradius, may be called 
adradii. Between the two poles, at right angles to the principal axis, 
we have the equator. In those Echinoderms which are provided with 
large skeletal plates, the body and skeleton is further divided into two 
zones, separated from one another by the equator ; these are the oral, 
adaetinal, or ventral zone, and the apical, abaetinal, or dorsal zone. 
In the centre of the former lies the mouth. 



vui EGHINODERMATA— MORPHOLOGY OF ^KELF.TON 317 

While these terms facilitate the mopphologieal description of the 
body they do not take into account its position in the water, or 

with regard to the sea-floor, which is assumed to be liorizontal. 
Thus the normal position of the Star-fish and Sea-urchin is such that 
the oral zone is directed downwards and the apical zone upwards ; 
while the very reverse is the case in the Crinoids, where the oral 
zone faces upwards and the body is attached to the substratum by a 
stem which is inserted at the apical pole. In the Holothurians, again, 
the principal axis of the body lies parallel to the substratum, and the 
oral pole forms its anterior, the apical pole its posterior end. 

For particulars as to the form of the body and the external 
organisation of the various classes and orders of the Echinodermata, 
cf. the Systematic Eeview, and also specially the two sections which 
treat of the skeletal and ambulacral systems. 



11. Morphology of the Skeletal System. 
Meaning of the Most Important Lettering of the Figures. 



a 


Apical pole. 


ian 


Anal interradials or anals. 


am 


Ambulacral plates. 


ib 


Infrabasals. 


an 


Anus or anal area. 


id 


Interdistichals or iiitersecundi- 


ap 


Ambulacral pores. 




brachs. 


B 


Buccal plates. 


ir 


Interradials. 


ha 


Basals. 


■III 


Madreporite, pore - openings of 


br 


Brachials, arms. 




the stone canal. 


f-'i 


First costal or primibraoli. 


n 


Nodal columnal. 


c, 


Second costal or primibrach. 





Oral pole, mouth. 


ca 


Points of insertion of the cirri. 


ur 


Orals, or mouth-plates. 


cd 


Centrodorsal. 


P 


Pinnules. 


ce or ( 


: Central plate. 


pa 


Anal. 


ci 


Cirri. 


rs 


Radial shields. 


CO 


Column, stem. 


r 


Eadials. 


cpa 


Covering plates of the ambulacral 


ss 


Lateral shields. 




grooves. 


t 


Terminals. 


D 


Dentes, teeth. 


ta 


Anal tube or ventral sac. 


ch 


Dorsocentral. 


vk 


Tegmen calycis. 


di 


Distichal or secundibrach. 


1-5 


Interradii or interambulacral 


ds 


Dorsal shields. 




areas of the Echinoidea. 


fjo 


Genital aperture. 


I-V 


Radii or ambulacral areas of the 


ia 


Interambulacral plates. 




Echiiioidi'a . 



(In many of the diagrams of the apical system of various Echiuoderms tlie 
infrabasals are dotted, the basals shaded with concentric lines, and the radials 
marked black. The brachials of the Crinoids are shaded with radial lines.) 



Introduction. 

The extensive comparative and ontogenetic researches which have 
been made on the Echinoderms have shown that it is to some degree 



318 



GOMPABATIVE ANATOMY 



CHAP. 



probable that certain pieces or plates of the skeleton are homologous 
in all the divisions. It may be assumed that these plates composed 
the primitive Eehinoderm skeleton. From this primitive arrange- 
ment, the skeletons of all known Echinoderms, whether extant or 
extinct, appear to be derived, on the one hand, through the loss of 
certain pieces of this primitive skeleton, and, on the other hand, by 
the acquisition of new or secondary pieces of varied form, number, and 
arrangement. 

The hypothetical primitive Eehinoderm skeleton consists of two 
pi'incipal groups or systems of plates : (1) the oral, and (2) the 
apical. 

The oral system has five interpadlally placed oral plates, 
arranged radially round the oral pole. This oral system develops 

round the left coelomic vesicle, 
out of which the oral portion 
of the coelom rises. 

In the apieal system the 
following plates occur : (1) a 
central plate at the apical 
pole ; (2) a circle of five 
radially placed plates, the 
infrabasals ; (3) alternating 
with these, five interradially 
placed plates, the basals ; and 
(4) around these, five radially 
placed plates, the radlals. 
The apical system develops 
on the right coelomic vesicle, 
from which the apical portion 
of the coelom is derived. 

The stalked larva of 

Antedon (Fig. 270) has 

retained this system of plates 

less altered than in any 

It has all the typical pieces of the oral and 




Fig. ^70. —Diagram of the apical system of the 
Antedon larva, combined from various stages. Ex- 
lilauatiou of the lettering on p. 317. The number of 
infrabasals is here sliown as 3, but these are produced 
by fusion of 5, which number has also been seen. 



known Eehinoderm. 

apical (or aboral) system of plates. 

All the skeletal plates of the Echinodermata consist of carbonate 
of lime. Their microscopical structure is very characteristic, so that 
small fragments can at all times be recognised and distinguished 
from similar fragments belonging to the skeletons of other animals. 
The structure is a sponge-work ; and thin sections of the skeletal plates 
or of the microscopic calcareous bodies appear to be perforated in a 
lattice-like manner. The finer structure, especially of the spines of 
Sea-urchins, is of great systematic importance. 



VIII 



ECHINODEBMATA— MORPHOLOGY OF SKELETON 



319 



A. The Apical System (Calyx). 

I. Echinoidea. 

The part of the test which in the Sea-urchins is formed by the apical 
system varies greatly in size. In the older and more primitive forms, 
the regular Echinoidea, it is still somewhat extensive as compared with 
the rest of the test (Fig. 271), but in modern, especially irregular forms 
(Clypeadridw and Spantangidm), it continually diminishes in relative 
size till it is nothing more than a minute region at the apical pole. It 
is possible to deduce the apical system of the Echinoidea directly from 
the hypothetical primitive form by the help of certain Saleniidce (Fig. 
:272). It is true that in the apical system of this family, as in that 
z 




Fig. 271.— Tiareohlnusprlnoeps, Laube (after LovSn). 
1, Genital aperture ; 2, anus ; 3, basal ; 4, radial ; 5, 
ambulacrum ; 6, the three upper plates of an inter- 
ambulacrum. 



of all other Echinoidea, the infrabasals are entirely wanting, but all 
the other typical plates are present : i.e. a central plate, and round it 
five basals, and outside these, alternating with them, five radials. In 
the right posterior interradius each of the three plates, the central 
and the two basals, is incomplete at the point where they meet. A 
circular region, the anal region, in which the anus lies, is thus formed. 
The anus, therefore, here lies asymmetrically in the apical system, and 
this' is the case in most Falceechinoidea and in most regular Euechinoidea. 
According to the universally accepted terminology, it lies in the right 
posterior interradius. 

The typical system above described for the adult Saleniidce has 
been found to be repeated in very young specimens of other Sea- 
urchins examined for this purpose (Echinus, Fig. 273 ; Toxopneustes, 
Fig. 274). 

Apart from these cases, where a primitive condition is shown by 



320 



COMPA RA TIVE A NA TOMY 



CHAP. 



the presence of the central plate, most regular Echinoidea show the 
following typical composition of the apical sj'stem : In the centre of 



m 




CMn 



Fig. 273. — EcMnus sp. (1 to 2 lain. long). 
Apical system (after Lovfen). For lettering see 
p. 317. 




Fig. 274. — Toxopneustes drcebaohiensls, 
O.F.M. (10 mm. long). Apical system (after 
Lovfen). For lettering see p. 317. s7i, Tubercles 
carrying spines ; op, anal plates. 



the system lies the anal area, with a few large, or many small, calcareous 
plates. A central plate cannot be distinguished. Within the anal 

area lies the anal aperture, 
usually excentrio, less fre- 
quently central. Round the 
anal area are found the 
circles of plates present in 
all Echinoids, viz. the proxi- 
mal circle of five basal 
plates, and the distal circle 
of five radial plates (Fig. 
275). One, or several, or 
even all of the radials may, 
however, become wedged in 
between the basals apically, 
and finally may take part 
in the limitation of the anal 
area. 

The ontogeny of Toxo- 

Fio. 275. — Toxopneustes droebaoWensls, O.F.M. 'pitusfes shows that there is 

Apical system of the adult (after Lovta). For lettering at first in the anal area of 

see p. 817. . 

very young Echmoidea one 
large central plate (Fig. 274:). Near this central plate, which ceases 
to grow and degenerates, accessory plates appear. Among these 




cm. 



VIII EOHIXODERMATA— MORPHOLOGY OF SKELETON 321 

accessory plates, which continually increase, in number, the anal 
aperture then forms, somewhat excentrically (Fig. 274). After a time 
the central plate can no longer be distinguished from the accessory 
plates. 

As a rule, i.e. in the greater number of eases, the basals and 
radials attain, in Echinoidea, the following special significance : 

1. Each basal is perforated by a large pore or hole, through 
which one of the Ave genital glands opens externally. On this 
account the basals of the Echinoidea have long and almost universally 
been called genital plates. 

2. Each radial is also perforated by a narrow canal, which opens 
at its surface through a single (rarely double) aperture. In this 
canal lies the terminal tentacle of the water vascular system, the 
frequently pigmented end of which projects somewhat beyond the 
aperture. Since these collections of pigment were formerly considered 
to be eyes, the plates (radials) carrying them were called the ocular 
plates. 

3. The fine, and usually very numerous, apertures of the water- 
vascular system perforate one of the five basals (genital plates), which 
becomes the madreporite (m). This is the right anterior plate. 

It must, however, be noted that (1) the genital apertures are not 
necessarily connected with the basal (genital) plates. The latter 
must not be regarded as terminal appendages of the genital ducts, 
but as independent portions of the test. For (a) the basals are solid 
when first developed, and are only perforated by the genital pores 
after the genital ducts have completely developed ; (i) the genital 
apertures in some Echinoidea lie outside the basals. For example, 
among the Clypeadroida, in some species of the genera Laganum, Encope, 
Mellita, etc., the genital pores lie outside the apical system, between 
its- edge and the first two interradial plates which border it ; in 
Cbjpeaster rosaceus, they lie in the five sutures between the interradial 
plates, and are separated from the apical system by two or three pairs 
of interambulacral plates (Fig. 277) ; and further, in another true 
Echinoid, Goniopygus, the genital apertures lie interradially quite outside 
the whole apical system. (2) The madreporite, through which water 
flows into the stone canal, is not necessarily exclusively connected with 
the right anterior basal (genital) plate. On the contrary, the neigh- 
bouring genital plates, indeed, all the five plates, and in isolated cases, 
even the neighbouring interradial plates of the corona may be perforated 
by the afferent ducts of the stone canal. In Palceechinus each basal plate 
is perforated by three pores, which are perhaps apertures of the stone 
canal, perhaps genital apertures, or else partly the one and partly the 
other. In no case, however, do the madreporic apertures extend to 
the radials (ocular plates). 

In the Echinoidea the primitive character and especially the 
radiate structure of the apical system may be more or less strongly 
modified. The original cause of such modification is principally to 
VOL. II Y 



322 



GOMPABATIVE ANATOMY 



CHAP. 



be sought in the shifting of the anus and anal area out of the 
apical system into the posterior interradius ; by this shifting the anus 
may come to he at any point between the (aboral or dorsal) apical 
system and the oral (or ventral) area. In its posterior and downward 
shifting the anus thus does not carry the apical system with it, but 
the latter remains on the dorsal side, although it is often shifted 
somewhat excentrically anteriorly, rarely posteriorly. The whole body 
is then bilaterally symmetrical, and when seen from above it is oval or 
heart-shaped, etc., in outline. The line connecting the mouth with the 
anus, which in the regular endoeyelic Echinoidea altogether or nearly 

IS. 



in 





Fig. 276,— Holectypus depressus, Cot- 

teau. Apical system and neighbouring 

parts of the perisorae (after Lovfen). For 

lettering see p. 317. 

Fig. 277.— Clypeaster rosaceus, L. Apical 

system and neighbouring parts of the perisoine 

(after LovJn). For lettering see p. S17. 

coincides with the vertical (principal) axis, now becomes the more in- 
clined, i.e. approaches the more nearly to the horizontal, the further the 
anal aperture is removed from the apical system into the posterior 
interradius, and is shifted on to the under side (into the oral or actinal 
region). Those Echinoidea in which the anal aperture has been 
shifted outside the apical system are called exoeyelie or irregular. 

Among the PalceecJiinoidea the genus Echinocystis {Cystocidaris) 
alone is exocyclic. It appears that in this form the whole apical 
system consisted merely of one madreporic plate. 

Among the Euechinoidea the three orders Holedypoida, Clyjyeastwida, 
and Spatanrjoida are exocyclic. 

a. Holectjrpoida (Fig. 276). In consequence of the wandering of the anus out 
of the apical system, the posterior basal plate .has lost its genital aperture, probably 
in connection with the disappearance of the related genital gland (the place of which 
has been taken by the rectum) ; in Gmiochipeus and Ga/r-ropyc/zis this plate has even 



VIII 



EGHINODERMATA— MORPHOLOGY OF SKELETON 323 



altogether disappeared. In some more recent species of the genus Hohctypns 
the genital pore of the posterior basal plate reappears secondarily. The space in 
the apical system, vacated by the anal area, is occupied by the madrepore (the 
right anterior basal plate), which greatly increases in size, or else all the five basal 
plates shift together towards the apical pole, the pores of the stone canal being then 
distributed over several or all of them. The plates of the apical system may fuse 
to a greater or less extent. 

b. Clypeastroida (Figs. 277 and 278). The whole apical system is here extra- 
ordinarily reduced in extent ; it is, indeed, very minute. All the five basals are fused 
together, and sometimes also fused with the radials. At least four 'genital pores are 
retained. Where there are only four, it is always the posterior which is wanting. 
The pores of the stone canal open in very various ways in the region of the fused 
basals. Many scattered pores are sometimes found, or one single large pore, or the 
pores open into irregular pits or grooves. In the family of the Clypeastridcc the 



Awn 




Fio. . 278.— Laganum depressum, Less. Apical Fio. 279.— Apical system and neigh- 
system and neighbouring parts of tlie perisome (after bourlng parts of the perisome of Meoma 
Lovto). rp, Ocellar pores in the fused radials ; mp, ventrloosa, Lamk, (after Lov6n). For 
pores of the raadreporite in a branched furrow. For lettering see p. 317. 
lettering see p. 317. 

genital pores have wandered out of the apical system ; they lie either at its edge, or 
further removed in the sutures between the (paired) rows of interradial plates. 

c. Spatangoida. The apical system of these exoeyolic Eohinoids is much reduced 
in extent, although not so much so as that of the Clypeastroida. It varies much in 
detail, and in a few extreme forms [e.g. Pmirtalesia) the primitive condition is to a 
very great extent obliterated and destroyed. 

1. In many geologically ancient forms the influence of the wandering of the anus 
out of the apical system is seen in the disappearance of the posterior basal plate 
(together with the genital pore belonging to it), and in the absence of a central plate. 
The other basals and the radials (each of which has its pore) have all shifted, 
and occupy an area which is sometimes circular or almost regularly pentagonal, 
sometimes lengthened in the direction of the plane of symmetry. In the latter case 
(Fig. 283), the two middle radials touch in the median line, separating the two 
anterior from the two posterior basals. The apertures of the stone canal are found 
in the right anterior basal, which is occasionally somewhat enlarged. 

2. In most recent fossil forms and in the great majority of the extant Spatangoida, 



3-24 



COMPARATIVE AXAT02IY 



CHAP. 



when their development is also taken into account, we find the following condi- 
tions. The posterior basal plate again appears, but never has a genital pore. The 
central plate also reappears. The apertures of the stone canal spread out from the 
right anterior basal towards the centre, i.e. on to the central plate. From this they 





Fig. 2S0.— Apical system 
of Abatus cavernosus, 
PMl. (after LovSn). For 
lettering see p. 317. 



Fig. 2S1. — Apical system of 
Spatangus purpureus, s:; imn. in 
size (after Lov6n). For lettering 
.see p. 317. 



pass on to the posterior basal plate, and the three plates fuse together. The sutures 
between them disappear, and so a large central madreporio plate is formed, which 
in very many forms shows a tendency to increase in size and to spread out in the 
direction of the jjosterior interradius, and thus more or less to press asunder the two 




Fig. 2S2.— Apical system and neighbour- 
ing parts of the perlsome of iVIicraster coran- 
guinum (after Lovfen). For lettering see p. 317. 




Fk :. 2S8.— Apical system and neigh- 
bouring parts ot the perisome of 
Holaster suborbioularis, Defr. (after 
Lov^n). For lettering see p. 317. 



posterior radials (Figs. 279-281). The genital aperture on the right anterior basal 
may disappear, in which case only three genital pores remain. In isolated cases, 
the left anterior basal plate may also lose its genital pore. 

•3. A method of dissolution of the apical system, unique among the Echinoidae, 
is found in many Co//>jritiilcc (Fig. 284). If we imagine that the elongated apical 
system, described under heading 1 (Fig. 283), becomes very much more elongated 
in the direction of the plane of symmetry, and breaks into two groups, one anterior 
and the other posterior, we have the condition in these animals. The anterior 
group contains the four basals, of which the right anterior is the madreporitie 
plate, and three radials, viz. the anterior unpaii'ed, and the right and left anterior. 



VIII EGHINODERMATA— MORPHOLOGY OF SKELETON 325 

The posterior group consists of the two posterior radial plates ; the posterior 
impaired (fifth basal) plate is wanting. The anterior group is separated from the 
posterior by a row of plates which belong to the right and left posterior interradii, as 
can be seen by comparing the figures. This arrangement is found in no other Echi- 
noid. As in all Echinoids, however, the radials remain connected with the apical 
ends of the five double rows of ambulacra! plates, so that these latter divide in a 



■E 




Fig. 2S5. — Apical system and neighbour- 
ing part of the perisome of Pourtalesia 
Jeffreysi, W. Th. (after Lovln). For letter- 
ini;' see p. 317. 



Fig. 2S4.— Apical system 
of Collyrites elliptioa, Lam. 
(after Lovto). For lettering 
see p. 317. 

remarkable manner into three anterior ambulacra (trivium), the anterior unpaired 
and the anterior right and left, and two posterior ambulacra (bivium). 

4. The apical system is most of all reditced and obliterated in the peculiar 
.Spatangoid family of the Pmirtalrsiidir (Fig. 285). Let us take as an example F. 
Jeffreysi, The whole system, which is irregularly pentagonal in outline, is shifted 
forward, and separated from the apical ends of the two posterior ambulacra by 
the uppermost plates of the posterior unpaired and of the right and left posterior 
interradii. It, almost certainly, consists of four basal plates, each perforated by a 
genital pore, but fused together into one single piece in which no suture can be seen. 
In the central and anterior portion of this plate lie the scattered pores of the stone 
canal. No radials can be recognised. 



326 



COMPARATIVE ANAWMr 



CHAP. 



Although there are good palaeontological reasons for the generally accepted belief 
that all known exocyclic (irregular) Echinoidea are descended from endocyclio 
(regular) forms, it has been conjectm-ed that these latter may themselves have had 
exocyclic ancestors (which, indeed, are unknown to us). Thus the modern Spatan- 
tjoida and Clypcastroida, for example, by the position of the anus in the posterior 
unpaired interradius, may secondarily have attained a primitive condition. The 
anus would then have wandered first from the posterior unpaired interradius to the 
centre of the apical area, and then, in the exocyclic forms known to us, have shifted 
back again in the same direction. This suggestion, which is of special signiiicauce 
with reference to the primitive Fchncdozoa, receives some (not very satisfactory) 
support from the fact that in the very old family of the SaJcniida: among the regular 
Echinoidea, the anus lies at the posterior edge of the apical system in the oldest 
forms, but during geological development approaches more and more near the centre 
of the system, near which it is found asymmetrically (posteriorly to the right) in 
the modern forms. 



II. Asteroidea. 

The typical plates of the apical system are not present in most 
adult Star-fish, or at any rate cannot be made out among the numerous 
calcareous pieces embedded in the dorsal area of the disc. There are, 

however, exceptions to this rule. For 
instance, in species of the genera Penta- 
gonaster, Tosia, Astrogonium, Stellaster, 
Ni'dria, Ferdina, Fentaceros, Gymnasteria, 
Scijtader, Ophidiaster, Zoroaster, the central 
plate, the five basals and the five radials 
can still be more or less clearly recognised 
in the adults. Occasionally (in species 
of Pcnfaijuiaider, Gymnasteria, Fentaceros, 
and many Gonictsteridce) there are even to 
be found plates which in position corre- 
spond with the infrabasals. The whole 
apical system is specially well developed 
in young specimens of the deep-sea Star- 
fish Zornaster fulgens (Fig. 286). The 
aperture of the stone canal lies in the 
right anterior interradius, outside the 
basal ; the anus in the right posterior 
interradius, inside the basal. In all 
Asteroids, the madreporic plate and anus lie in these interradii of the 
apical region (cf. the Echinoidea, Figs. 272-275). 

The typical apical system can also be proved ontogenetically in 
Star-fishes, even in forms in which it is absent or unrecognisable in 
the adult. Five basals, a central plate and five radials are actually 
among the first plates formed in the embryo Star-fish, in the very 
order in which they are here named, though always after the terminals, 
presently to be described, which appear first of all. Small plates, 
appearing radially within the circle of basals, have been considered to 




- m 



€l*\ 



Fig. 2S0.— Apical system of plates 
in a young specimen of Zoroaster 
fulgens (after Sladen). For lettering 
see p. 317. 



VIII 



EGHINODERMATA— MORPHOLOGY OF SKELETON 327 



be infrabasals. This view is, however, not certain, because other 
new and also radially arranged plates may be added to these, which 
may thus also themselves possibly be accessory structures. 



III. Ophiuroidea. 

In this class, the plates of the apical system do not appear in the 
embryo in exactly the same order as in the Asteroidea. First the five 
radials and the central plate form, and, somewhat later, between the circle 
of radials and the central plate, the five basals and the five infrabasals 
appear. In many Ophiuroidea, an embryonic condition of the apical 
system is retained in the adult, the central plate being surrounded by 
the circle of five radials, while the basals and infrabasals are wanting 





Fio, 2S7. — Plates of the apical system of the disc of 
Ophiomusium validum (after P. H. Carpenter). For 
lettering see p. 317. 






Fig. 28S.— Apical system of a 
young Amphiura squamata (after 
P. H. Carpenter). For lettering 
.see p. 317. 



(species of the genera Ophioglyj^hO', Ophiomastix, Ophiopyrgus, Ophiura, 
Hemipholis, Ophioceraniis, OpMopholis, OphiotrocJms). In many others, 
however, there are, besides the radials, the five basals, which may 
vary greatly in size (species of the genera Ophioglypha, Ophiomastix, 
Ophiomusium, Ophiura, OpMopholis, Ophiozonn, Ophiadis, Ophiolepis). 
In Ophiomitra exigua there is only the central plate with five basals 
around it. In some Ophiuroidea a complete apical system is developed, 
infrabasals being added to the basals, the radials and the central 
plate (isolated species of Ophioceraniis, Ophioglypha, Ophiozona, Ophio- 
musium (Fig. 287), Ophiolepis). In very many Ophiuroidea the 
calcareous plates developed at the apical surface of the disc are so 
numerous that it is then impossible to recognise among them the 
typical plates of the apical system. The adult Ophiuroidea have no 



328 COMPAIUTIVE ANATOMY chap. 

anus. The apertures of the stone canal are not found on any of the 
apical plates, but ventrally, on one of the oral shields. 



IV. Pelmatozoa. 

In no other class of the Echinodermata do the plates of the apical 
system form so large a part of the skeleton of the body wall (apart 
from the arms) as in the Pelmatozoa. The body of these Echinoderms 
consists of a central calyx, which contains the viscera, and usually 
carries jointed appendages, radially arranged at its edge ; these are 
the arms and pinnulse. Typically the Pelmatozoa are attached to 
the sea-floor by their apical poles, with or without the intervention 
of a stem ; in some the stem becomes separated from its attachment 
(Pnitarrinus), and may dwindle in size (MiUericriniis), or may be present 
only in the embrj'onic stages (Antedon), or there may be no trace of 
either stem or attachment {Marsupites). The oral side of the calyx 
(and also of the arms) is thus turned upwards, while the apical side 
of the calyx (the dorsal eup) is turned downwards and either 
surrounds the viscera like a bowl or carries them like a dish. The 
plated test of this bowl or dish consists exclusively, or for the greater 
part, of the plates of the apical system : the basals and the radials, 
to which infrabasals may be added. The anal aperture always lies 
interradially, usually on the oral side of the body and not con- 
nected with the apical system. 

Sub-Class 1. Cplnoidea. 

There are a good many Crinoids in which the apical system is 
completely developed. The five radials and the basals are constant, 
although the latter may be hidden. The infrabasals are inconstant. 
The Crinoids in which the latter are present are said to have a 
dicyelie base, those in which they are absent have a monoeyelle 
base. 

A central plate has been observed in the larva of Antedon. It 
occurs at the distal or root end of the larval stem, and ultimately 
becomes severed from the animal. 

The part taken by the plates of the apical system in the construc- 
tion of the apical capsule varies greatly. In the stalked larva of 
Antedon they alone form the skeleton of the apical side of the calyx ; 
although an anal interradial has a transitory existence. The same is 
the case also in many other adult Crinoids, which in this respect show 
a primitive or an embryonic character (manjr Inadunata larviformia 
and many Inadunata fistiilata, Encrinus, Mtd'siqnfc!', Holaims, Hyocrinus, 
Bathycriims, and a few Ciiiialiculafa . Rhizooriniis, Fentacrin.ui). 

In most Crinoids, on the other hand, the plates of the typical apical 
system, i.e. the infrabasals (where these occur), basals and radials do 
not form the whole skeleton of the apical capsule, but only a certain 



VIII 



ECHINOVERM ATA— MORPHOLOGY OF SKELETON 35!) 



(often even very small) part of it ; other plates take part in its 
structure, as we shall see more in detail when describing the peri- 
somatic skeleton. The border of radials round the apical capsule 
becomes more or less markedly disturbed by the appearance of 




<is 




^ ^ 

#(^i 





Fig. iS9.— Apical system of Cyatho- Fin. 2fio.— Marsupltes ornatus. Plates of 

crlnus. For lettering .see p. 317. ta«, An.%1 the dorsal cup. For lettering see p. 317. 

interradial. 

special '' anal plates " in the posterior unpaired interradius ; these 
specialised anals occur very frequently in palaeozoic Crinoids (Fig. 291). 

The Crinoids with dieyclie base (with infrabasals, Figs. 289 
and 290) are: {a) most Inadunata , (h) among the Camerata, the 
families of the lieteocrinichi' p. p., 
Ehodocrinidce, Gb/ptasterida', and 
Cwtcdocrinidm , (c) the Articidata 
(IcMhyocrinidce) ; (d) the Canalicu- 
lata, in which, it is true, the infra- 
basals are often either fused with 
the uppermost joint of the stem 
or atrophied, at least in the adult ; 
.such are convenientlj' termed 
Pseudomonoeyelie. 

The Crinoids with monoeyelie 
base (without infrabasals, Fig. 291) 
are, apart from a few Inadunata, 
the Camerate families of the 
Melocrinida, Adinocrinidm, Platy- 
crinidce, Hexacrinidce, Acrocrinida, 
Ba.rrandeocrinidce, Eucidyptocrinidce. 

Instead of the typical five infrabasals and five basals there are 
very often found four, three, or even only two plates in these rings ; 
this is especially the case in extinct Crinoids belonging to the orders 
Inadunata, Camerata, and Artkulata. The plates are then almost 




Pig. 291.— Aotinoorinus proboscidalls. Plate 
of the dorsal cup. For lettering see p. 317. 



330 COMPARATIVE ANATOMY chap. 

always of unequal size, and it appears not unlikely that the reduction 
of their number was caused by the fusing of neighbouring plates. 
These characteristics necessarily destroy the strictly radial symmetry 
of the dorsal cup. 

Still further fusions may occur (among the Canaliculata). 

The relative sizes of the plates of the infrabasal, basal and radial 
circles vary greatly, but this is of no great interest to the comparative 
anatomist. 

Sub-Class 2. Blastoidea. 

The Blastoidea are palaeozoic Pelmatozoa, whose stalked armless 
body very often has the appearance of a bud (Fig. 263, p. 314). 
Seen from the side, the body is an oval, truncated sometimes at 
the apical, sometimes at the oral end. Seen from the oral or 
aboral pole, its outline is in by far the greater number of (regular) 
forms regularly pentagonal with rounded projecting angles, some- 
times not unlike a short - 
"■^ ' armed Star-fish (Figs. 265 

• and 266, p. 314). In the 

Jfy irregular Blastoids, on the 

pi/ contrary (Elentherocrinus, As- 

^ ^ trocrinus, Fig. 267, p. 315), 

_ " the radiate structure is dis- 

_^^^ ^^^f turbed by the modified form 

^^^^^H Ri' \~-> ^^^^^^ °^ "-"^^ °^ ^^® ambulacra. 

^^^^^»^^^^ The outline of the ovoid 

^^.^^ .A^^.^^. ^<x body 01 Lleutlierocnnus, seen 

/"^ 7 ^KKI '•■ ^1^^ ^^^"~^\ ^i"o™ ths apical or oral pole, 

U^ Wf \ ^K^^ '® irregularly pentagonal, 

U ^^ with three shorter and two 

i longer sides, the latter be- 

Cjy longing to the left posterior 

) i ( and the unpaired posterior 

V interradii. In Aitrocmiu% 

! the body is flattened in the 

OB ' ^ 

^^ direction of its jjrincipal axis, 

Fic. 292. -Apical system otPentremites. ,mM>, Axis and, when seen from the oral 

passiit'; thi'outrli jnouth and aims ; .'■, the smaller : w, and ^ ^ ^ ^ l 

z, the two larger basals ; ir, interradials ; r, radials. Or ^boral pole, almOSt sym- 

metrically four-lobed, the 
lobes being of unequal size. The largest of the lobes lies diametrically 
opposite the abnormally shaped ambulacrum, which is_ on the smallest 
truncated lobe. The two other middle-sized lobes are almost alike in 
form (Fig. 267, p. 315.) 

The whole body of the Blastoids is plated. The test consists, 
apart from the ambulacra, of three circles of plates (Fig. 292), two of 
which belong to the typical apical system of the Echinodermata, while 



VIII 



EGHINODERMATA— MORPHOLOGY OF SKELETON 



331 



the third consists of perisomatic plates, which, in all probability, 
correspond with the primary interradii of the Crinoids. 

The first circle at the apex is that of the (interradial) basal plates. 
There are always three of these, one smaller and two larger of equal 
size, as also occurs in the Crinoids. The monocyclic base of the 
Blastoidca is thus symmetrical. But the line of symmetry (the so- 
called dorsal axis), which passes between the two larger plates and 
through the small unpaired plate, does not coincide with the 
symmetrical (ventral) axis of the body, which passes through the 
mouth and the anus, the latter lying in the posterior interradius on 
the oral surface. The smaller unpaired basal plate lies in the left 
anterior interradius. If we imagine the two larger basal j)lates cut 
into two similar parts by radial lines of division, we obtain the five 
equal-sized, strictly radially arranged, and interradially placed basals 
of most other Echinoderms. The uppermost ossicle of the stem 
is inserted at the point where the three basals of the Blastoids meet. 

The circle of the basals is immediately surrounded by that of the 
radials. The typical number of five is always retained in these, 
which, in regular Blastoids, are strictly radiate in 
their arrangement. These are called fork-pieees, : 

because each of them is produced upwards, i.e. 
orally, in the shape of a tuning-fork, the two 
limbs holding between them the distal end of an 
ambulacrum. The radials form a closed circle, 
their lateral edges being contiguous. 

The third circle of plates is in immediate 
contact with the radials, and surrounds the peri- 
stome. It consists of five interradial plates, 
which, in regular Blastoids, are strictly radial ; 
these are the iuterradials or deltoid plates. 
These plates do not foroi a closed circle, as they 
are separated from one another by the five 
ambulacra. The apical edges of each deltoid 
plate rest on the oral edges of the contiguous 
forks of two consecutive radials or fork pieces. 
The relative sizes of the basals, radials, and iuter- 
radials of the Blastoids vary greatly (cf. figures). 
One of the five interradials, which is distinguished 
as the posterior, is perforated by the anus. 

In the irregular Blastoids (Fig. 293), which are without stems, 
all the plates of the regular forms are found, but are, naturally, 
irregularly developed. The radial which supports the modified ambu- 
lacrum is smaller than the other radials and differently shaped. It 
appears shifted quite on to the oral surface. At the same time, the 
pair of basals (y and z) which flank this radial are much prolonged 
orally as naiTow plates. 

It cannot at present be decided whether there are skeletal pieces 




u 



Fig. ii93. — Eleuthero- 
crinus Cassedayi, from 
the apical side (after Ethe- 
ridge and Carpenter). 
aa-bb, Axis passing througli 
the mouth and the anus ; 
X, the smaller, y and z, the 
two larger basals; r, radials; 
an, anal side. 



332 



GOMPARATirS ANATOMY 



CHAP. 



in other Echinodermata homologous with the interradials of the 
Crinoidea and the Blastoidea. In all endeavours to answer this 
question the following plates should be kept in mind . in the 
Ojihiuroiiica the interradially placed plates between the circle of radials 
and the oral side (Fig. '2S7, p. 327), and among the Echini iklca, in 
Tiarechiims (Fig. 271, p. 319), the central of the three interradial plates 
of an interambulacral area. 



Sub-Class 3. Cystidea. 

The spherical, pear-shaped, egg-shaped, or cup-shaped body of the 
Cystidea is also enclosed in calcareous plates. In one of the principal 
groups, that of the Euajstidea, the plating consists of numerous con- 
tiguous plates arranged without any recognisable order. In this case 




l^? 




Fig. 294.— System of plates of the apical capsule 
of Caryoorlnus ornatus, spread out (after Hall). For 
lettering see p. 317. 



Fia. 295.— Cystoblastiis Leuchten- 
bergi, from the apical side. 11, Point 
of insertion of the stein ; S, anus ; 
10, infrabasals ; 12, pectinated rhombs. 



a typical apical system of plates cannot be distinguished. In the 
other principal group, the Cystocrinoideu , certain forms of which show 
near relationship to the Criiioidcn. and Blastoidea, the test consists 
of a relatively small number of plates, and a true apical system can 
be found round the apical pole. 

The forms assumed by this apical system may be grouped around 
two central types : Caryoerinus and Eehinoenerinus. The group 
(Juri/ocriiius {Cm i/locrinus, Hoinamnites, Juglanducriinis) has its plates 
arranged in six rays ; while the group Ediinoi'nciinus [Cullocystis, 
LepaduciiiLiis, Apiocijstis, CystoUastui^, Glyptocystis, Fleurocystis, I'rmio- 
cydis, Fseudurnitm, etc.) shows the typical five-rayed arrangement of 
the. plates. In both groups the base is dicyclic, i.e. there is a circle 
of infrabasals inside the circle of basals. 

Caryoerinus, six-rayed (Fig. 294). — The circle of infrabasals 
consists of four plates, two larger (which are contiguous) and two 
smaller. Each of the two larger plates is double. Outside the circle 
of the infrabasals lies a closed circle of six interradial basals, and 



viii ECHiyODERMATA—MOKPHOLOtiY OF SKELETON 



333 



this is surrounded by a closed circle of six radials. These plates, 
together with two accessory plates (interradials ?), form the Avhole 
test of the cup of the attached Oari/ncriims, from the point of 
insertion of the stem to the base of 
the arms. The anus lies excen- 
trically on the oral surface, in the 
(interradial) prolongation of the 
suture between the two larger infra- 
basals ((/. Figs. 294, 295). 

Eehinoenerinus, five-rayed (Fig. 
296). — The circle of infrabasals 
consists of four plates, one large 
posterior plate and three smaller 
ones. The larger plate is double 
{i.e. consists of two fused plates). 
Outside the circle of infrabasals 

comes the closed circle of the five Pio. 2!iii.— system of plates of the dorsal 
basalS, and outside this that of the ™P "f Eoliinoenorinus armatus, spread out 
' (after Forbes). For lettering see p. 317. 

five padials, between which acces- 
sory pieces are intercalated, the homologies of which cannot be made 
out. The anus lies posteriorly to the right. In Cystohlashis the 
radials, like the radials or fork-pieces of the Blastoidea, have deep 
incisions on the oral side for the reception of the ambulacra (c/. Fig. 
2.59, A and B, p. 312, and Fig. 295). 




B. The Oral System of Plates. 

In certain Echinodermata (I'ehnatozoa and Opliiuroklea) there is a 
system of plates surrounding the oral (ventral, actinal) pole, and thus 
diametrically opposite to the apical system. This system develops 
round the left ccelomic vesicle of the larva in a waj^ similar to that in 
which the apical system develops round the right vesicle. The oral 
system is, however, much simpler than the apical, and consists of one 
single circle of five plates (less frequently six, in the six-raj-ed arrange- 
ment of the whole system) ; these plates, placed interradially, corre- 
spond in the oral system with the basal plates of the apical system, 
and are called oral plates. 

In our considerations of this oral system we again find the best 
starting-point to be the stalked larva of Anteclon {Pentacrinus stage). 
In a young stage of this larva the oral surface of the calyx apjiears 
vaulted over by a roof closed on all sides. The surface of the calyx 
thus forms the floor, and the vault the roof, of a closed cavity, which 
is called the oral or tentacular vestibule. At the centre of the floor 
the oral aperture breaks through, connecting the intestine with the 
vestibule. The mouth is thus at this stage not connected with the 
exterior. The fifteen primary tentacles, which rise on the disc of 
the calyx, also cannot project externally, but are covered over by the 



334 



GUMPAL'A TIVE ANA TOMY 



CHAP. 



roof of the vestibule. This roof is formed of five iuterradial lobes, 
supported by five interradial skeletal plates, the oral plates. An aper- 
ture only arises secondarily at the apex of the roof, and the five oral 
lobes separate in such a manner that the tentacles can project through 
the clefts between them. The mouth is now in open communication 
with the exterior. 

At first the five oral plates rest directly on the oral edges of the 
basal plates of the apical system. But in proportion as the calyx 
increases in size, and the arms grow out, the distance between the 
basals and the newly-formed radials, which support the arms, on the 
one hand, and the oral plates on the other, becomes greater and greater, 
since the latter remain at the centre of the tegmen calycis, surround- 
ing the mouth. There thus arises, between the bases of the arms and 
the circle of the oral plates, which in comparison with the continually 

growing calyx be- 
comes more and more 
insignificant, a cir- 
cular zone, the peri- 
pheral zone of the 
tegmen calycis. The 
food grooves running 
out from the mouth, 
passing between the 
five oral lobes, tra- 
verse this peripheral 
zone of the tegmen 
to the bases of the 
arms. This peri- 
pheral zone continu- 
ally increases in size, 
while the central part, 
surrounded by the five oral lobes, does not grow further, and forms 
an ever-diminishing central region of the tegmen calycis. Finally, the 
oral plates, with the lobes, are entirely resorbed, and the minute 
central zone can no more be distinguished ; the whole oral surface of 
the Antedon calyx is a free disc, by far the greatei' part of which has 
been formed outside the base of the oral pyramid. In the centre of 
this oral disc the mouth lies uncovered, and on the surface of the 
disc the food grooves are visible running out radially to the bases 
of the arms. 

Among the immense array of forms comprised under the crinoids 
we find a few groups vrith five oral plates forming, as in the larva of 
Antedon, the whole skeleton of the tegmen calycis. In the Inadunata 
larviformia, type Haplocrinus (Fig. 297), there is actually a closed 
pyramid of five oral plates, which, at the edge of the calyx, rest on 
the radials of the dorsal cup. Only at the bases of the arms do 
the five oral plates separate to form five radial apertures, through 




.—Haplocrinus mespiliformis (alter Wachsmutli and 
A, From the anal side ; B, from the oral side. 1, Orals ; 
2, oral pole ; 3, anus ; 4, radials ; r>, interradial ; 6, basals ; 7, first 
brachial ; Sj point of attachment of the arm. 



Fig. iV 
Springer), 



VIII EGHINODEBMATA— MORPHOLOGY OF SKELETON 



335 




which the food-grooves pass out on to the arms. The posterior oral 
plate is somewhat larger than the others, and has a perforation 
which may be the anus (1). 

The same condition is found in the extant genera Holopus and 
Hi/ocrinus (Fig. 298), the extant unstalked genus Thaumatociiims, 
and the extant canaliculate 
genus Rhi-ucrinus. All these 
genera possess five oral plates, 
which, however, are separate, 
and do not form a closed 
pyramid ; the niouth, there- 
fore, is in open communica- 
tion with the exterior between 
them. Compared with the 
larva of Antedon and with 
Htqilocrinus, Holopus shows 
the most primitive (or em- 
bryonic) condition, since in 
it the oral pyramid is large, 
covering nearly the whole of 
the tegmen, so that between 
its base and the edge of the 
calyx only a very small peri- 
pheral zone remains. In 
Hyocrinus (Fig. 298) also, and 
Thaumatocrinus the orals are 
still of considerable size, but the peripheral zone, which is beset with 
small closely -crowded plates, is somewhat broader than in Holopus 

(about one-fifth the diameter 
of the whole tegmen). In 
Bhizocrinus lofotensis the orals 
are smaller, and in Bhizocrinus 
cjoct/ Bawsoni they are almost rudi- 
mentary, so that the zone 
which surrounds them forms 
the greater part of the 
tegmen. 

In the Gyathocrinidce {In- 
adunata fistulata), five large 
plates can sometimes be dis- 
tinctly made out in the centre 
of the plated tegmen ; some- 
-System of plates of the tegmen of times, however, 



Fig. 308. —Hyocrinus Bethellianus (after P. H. 
Carpenter). Tegmen calycis. 1, Axial canal of the 
brachials ; 2, extension of body cavity in the arm ; 
3, food groove of the arm ; 4, smaller plates of the 
tegmen ; 5, orals ; 6, anal cone ; 7, oral edges of the 
radials. 




Fig. 



Platyorlnus tuberosus (after Waclismutli and pieces 
Springer). For lettering see p. 317. 



irregular 



are found in their 
places. When they are dis- 
tinct, the posterior plate is the largest, and is. sometimes shifted 
anteriorly between the others. In all cases they cover the mouth in 



336 COMPARATIVE ANATOMY chap. 

such a way as to hide it. These plates are by some regarded as 
orals. 

In the Cnmcmtu (Fig. 299) Ato supposed oral plates {or) can almost 
always be distinguished in the centre of the richly and rigidly plated, 
often highly arched, tegmen. They close together firmly over the 
mouth. The posterior oral is larger than the rest, and jjresses in 
between them. 

As far as is known, in the ArtimJata (Ickthyocrinoidea) also, five 
orals can be distinguished at the centre of the richh' but loosely 
plated tegmen. But, in this case, they are separate, and surround an 
open mouth. The posterior plate is larger than the rest. 

In the Canalicniatu, (with the exception of the above-named genus 
Rhizociinus) the orals are altogether wanting in the adult. 

In the Blastoidea the oral region is covered by a roof consisting 
of numerous small plates usually without definite arrangement, which 
are continued as covering plates over the ambulacra. In a few forms, 
however, and especially in Stephanociiiais, five orals can be made out. 
In Stqiliaiiocrinus these five interradial orals, resting on the inter- 
radials {i.e. the deltoid pieces), form a closed pyramid over the oral 
region. 

In many Cystidea, also, the mouth is arched over by an oral 
pyramid. In Cyathocystis, the five oral plates forming this pyramid 
are more or less equal in size, but in species of the genera Sphceronis, 
Glyptospluvra, and Pirocystis the posterior oral is, as in so many 
L'umerata, larger than the rest. In the six-rayed Cystid Caryocrinus 
this latter is the case, one of the six orals having shifted from behind 
forward between the other five, which surround it symmetrically. 

In the Ophiupoidea, on the oral (lower) side of the disc, there is 
in each interradius a plate, usually distinguished by greater size. One 
of these plates, which are called buccal shields (Fig. 245, p. 300), 
is, as madreporite, perforated by the pores of the water-vascular 
system. In the pentagonal larva of Amphiura these buccal shields 
appear at the edge of the oral side. They have been homologised, 
probably correctly, with the orals of the Pehnatozoa. 

In the Asteroidea, on the lower surface of the disc at the edge of 
the mouth, in each interbrachial region, there occurs a skeletal p)late 
of very various shape, which is called the odontophore (Fig. 310, p. 
352). These plates, which might be described as the proximal or 
basal plates of the interbrachial system, may correspond with the 
orals of the Pelmatosoa and the oral shields of the Ophiuwidea, although 
they may be pushed below the surface by the oral plates (the first 
pairs of adambulaoral plates), and are usually completely covered 
externally. They arise early in the larva of Asterias (after the five 
terminal plates, the five basals, the apical central plate, the ten oral 
ambulacral plates, and twenty other ambulacral plates are formed), 
interbrachially between the oral ambulacrals. 

Orals have not been discovered in the Echinoidea. "Whether 



VIII ECHINODEBMATA— MORPHOLOGY OF SKELETON 337 

certain pieces of the calcareous ring of the Holothupioidea corre- 
spond with the orals of other Echinodenns cannot at present be 
determined. 



C. The Pepisomatie Skeleton, i 



All those skeletal pieces which protect the body, between the apical 
and the oral systems, taken together, form the pepisomatie skeleton 
of the Echinodermata. It is obvious that the extent of the periso- 
inatic skeleton must varj' inversely with that of the polar (apical and 
oral) systems. Where the polar systems form only a small part of 
the body wall the perisomatic skeleton is the more strongly developed, 
and vice versCt. In the Blastoidea, for 
example, nearly the whole of the test 
is formed by the polar systems 
(especially the apical), while in most 
Echinoidea, Asteroidea, and Ophiuroidea, 
the perisomatic system covers nearly 
the whole body. Where the equatorial 
zone of the body is produced into 
variously shaped branched or un- 
branched arms, as in most Pelmaiozoa, 
Asteroidea, and Ophiuroidea, the skeleton 
of these arms is exclusively formed by 
perisomatic pieces. It is at present 
impossible to prove any definite 
homologies between the parts of the 
perisomatic systems throughout the 
Echinodermata. 




I. Holothupioidea. t^ »„« ,t. • , 

Fig. 300.— Microscopic calcareous bodies 
T .1 . - ^ ii TT 1 11 * • 7 of Holothiiiioidea. 1, Aiichor and anchor 

In the cutis of the EoMlWlOtdea, pi^j, „f gynapta Inhaerens, O. P. M. ; 2, 

as well in the body wall as in the "stool"of Cucumaria longipeda, Semp; 3, 

wall of the tentacles, ambulacra, tube- ""^if"™ tody of cucumaria crucifera, 

J. J ^ . , ..- - Semp; 4, rod from one of the tube-feet of 

leet, and ambulacral papulae, there are sticopus japonicus ; s, supporting piate 
found enormous numbers of micro- from one of the tube-feet of stychopus 
scopically minute calcareous bodies of JJ''''"^"^ ^ ", "stool" of Hoiothu™ 

J J^ , , . . Murrayi ; i.rod from the ventral ambula- 

dennite shapes (Fig. 300). These give cral appendages of Onelrophanta mutabilis, 
the integument a firm and rough ^heel ; 8, latticed hemisphere of Colochirus 

consistency. Their principal signifi- 
cance may well be that of protection. 
These small calcareous bodies may be called, according to their shapes, 
"anchors," "wheels," "rods," "anchor plates," "crosses," "lattices," 
"stools," "buckles," "biscuits," "cups," "rosettes," etc. 

' On the author's use of the term " perisomatic," see footnote, p. 362. 
VOL. II Z 



cucumis, Semp; 9, "wheel" of Acantho- 
trochus mirabilis, Dan. and Kor. 



338 COMPARATIVE ANATOMY chap. 

The shape and method of association of these bodies is of importance for 
classification, especially for distinguishing one species from another. Nearly all 
their various forms can be traced back, in a way which cannot here be further 
described, to a common form, viz. to a very short rod, which tends to branch 
diohotomously at each end. In some DciulrocMrotcc {Psolus, Theelia, etc.) the 
calcareous bodies upon the (physiologically) dorsal side of the body attain a 
specially large size (1 to 5 mm.), so that the back appears to the naked eye to be 
covered with scales or plates (Fig. 228, p. 287). 

In the Dendrochirotce an anterior part of the body, the proboscis, is invaginable. 
At the posterior boundary of this proboscis (when evaginated) five calcareous oral 
valves are found in a few genera. When the proboscis is invaginated these eome to 
lie close together in the form of a rosette, which protects the aperture. In Psolus 
these five oral valves are placed interradially, and each is a large triangular calcareous 
plate (Fig. 228, p. 287) ; in Colochirus, Adiiwcucumis, etc., they are arranged radially 
and consist of compact masses of calcareous granules and ambulacral papilla. In 
many Asjiidodiirota and Dcndrochirota radially or interradially arranged anal 
valves (anal plates or anal teeth) also occur round the anus. 

II. Eehinoidea. 

The skeleton of the Eehinoidea forms a plated covering called the 
test, which encloses the viscera. The greater part of this test is 
composed of the plates of the perisomatic system, since, as a rule, the 
plates of the apical system (the central plate, the periproctal plates, 
the basals and radials) occupy but a small, and even sometimes a 
minute, area at the apical pole. There are, however, exceptions to 
this rule, e.g. the Triassic genus Tiarechinus, in which a considerable 
portion of the test is formed by the plates of the apical system (c/. 
Fig. 231, p. 289). 

The form of the shell is thus, as a rule, in the Eehinoidea, deter- 
mined by the perisomatic skeleton. The horizontal outline of the 
shell, i.e. the outline seen when an Echinoid shell is viewed from the 
oral or the aboral pole, is called the ambitus. This ambitus in 
regular Echinoids is, as a rule, strictly circular, or else pentagonal with 
rounded comers ; less frequently it is oval, in which case the greatest 
diameter of the ambitus need not coincide with the symmetrical axis. 
In irregular Eehinoidea the ambitus is symmetrical, being generally 
elliptical (lengthened from before backward), or else egg- or heart- 
shaped. 

In all Eehinoidea, except the Spatangoida, the mouth lies at the 
centre of the oral surface of the test ; in the Spatangokla it has shifted 
anteriorly on this surface. The mouth, however, always remains the 
centre round which the plates of the perisomatic skeleton are 
grouped. 

AVe have already seen that in regular endocyclic forms, the anus 
lies in the centre of the apical system, but in exocyclic forms it leaves 
the apical system and enters the posterior interradius, where it may 
approach the ambitus, or even cross it on to the oral surface, always, 
however, remaining in the posterior interradius. 



VIII ECHINODERMATA— MORPHOLOGY OF SKELETON 339 

The whole perisopie, from the mouth to the apical system, falls 
into two sections : (1) a small portion surrounding the mouth, the 
pepistome or oral area ; and (2) the larger remaining portion be- 
tween the peristome and the apical system, the corona. In the peri- 
stome the skeletal pieces are usually loosely embedded near one 
another, or imbricate one with the other, remaining movable one 
against the other. Sometimes the peristome is membranous, without 
skeletal pieces. In the corona the skeletal pieces are usually firmly 
connected with one another by means of sutures, like the plates of 
the apical system, together with which they form a rigid test. In 
dead EcJiinoidea, and in nearly all fossil forms, this test remains 
intact, while the skeleton of the peristome falls to pieces, and is 
therefore rarely preserved. 

The perisomatic skeleton in all Echinoidea consists of two systems 
of plates, which run from the apical system over the ambitus to the 
mouth as ten meridional zones ; five of these zones or systems of 
plates are placed radially, and these are called the ambulacra. These 
five zones, on which the tube-feet rise, are always in contact with the 
five radial (ocular) plates of the apical system, so that each ambulacrum 
touches an ocular plate with its apical end. The ambulacral plates 
are perforated for the passage of the ambulacral vessels, which serve 
for swelling the tube-feet. The five other zones or systems of plates 
are interradially placed, and are called interambulaera or interambu- 
lacral plate systems. They alternate regularly with the ambulacra. 

Considering the perisomatic skeleton of the Echinoidea more closely, the follow- 
ing special points are worth attention. 

(a) The Number of the Vertical or Meridional Rows of Plates in the Ambulacra 
(radii) and Interambulaera (interradii). 

In all JEuechinoidea (from Devonian times up to the present), the corona consists 
of twenty meridional rows of plates, ten of which united in pairs belong to the 
ambulacral system, and ten also in pairs to the interambulaeral system. Five 
double rows of ambulacral plates thus regularly alternate with five double rows of 
interambulaeral plates. 

In the exclusively Palteozoic PalceecMnoicUa, the number of meridional rows of 
plates in both ambulacra and interambulaera varies. The number of rows in all 
the five ambulacra and in all the five interambulaera of individuals of one and the 
same species is, however, always the same. 

In the ambulacra, however, the number of rows of plates in the Palaeechinoidea 
is usually two. The Melonitidm (Fig. 301) form the only exception, having four to 
ten rows in each ambulacrum. 

In the interradii, on the other hand, the number of rows of plates varies. 
Bothriocidaris has only one single row of plates in each interradius. In all other 
Palceechinoidea there are more than two (3-11) rows of plates in each interradius 
(Fig. 230, p. 289). The interesting genus Tiarechinus (Fig. 231, p. 289) is dis- 
tinguished by the great simplicity of its interradial system of plates ; in each inter- 
radius there are only four plates, a single one at the edge of the peristome — the 
large marginal plate of the peristome — and three intercalated between this and the 



340 



COMPARATIVE ANATOMY 



CHAP. 



adjoining apical system, tliese plates being seijarated by meridional (perpendicular)' 
sutures. 

The plates of the Ecliiaovh'a are most frequently pentagonal. In the two ]ier- 

pendicular rows of an ambulacrum or 
c\,\yy an interamliulacruni the consecutive 

^ plates usually alternate in such a way 

that the suture between the two row.s 
forms a zigzag line. The sutures 
'•< x>-^Ti^ / i ^^^ between the plates, which lie one 

,Y~^S^i}Z^\'^C- \ below the other in a row, usually run 

\' ''*• '--^ \\r~? ' horizontally (Fig. 232, p. 291). 

D 

P<^5^v^ ^ . r "VnWT^' ) (*) "^^ Pores perforating the Plates 

^TIXk^CI^^^ .^^^^^yAvVY °^ *^^ Ambulacra! System. 

■ - ^^li/Ttv'- / As a rule, in the Echinoidea, the 

$f^Sj^~r^ jl<\kl/> pores occur in pairs. These double 

' (^ jy \5iir§5S2l""^/-C\ ' ' pores occur only on the ambulacral 

TVjII/iy^jri, -> i^lates. One double pore belongs to 

/■~\^^-;^:j^^^ each ambulacral foot.^ From the 

. . , ^ ^ J. . . . amiiuUa, under the test (at its inner 

Pig. 301.— Apical system and adjommg pen- ■j\ + + i- i 

some of Melonitas multipora, Norw. (after Meek side), two canals run out, which, 
and Worthen). For lettering see p. 317. running separately through the plate, 

unite at the base of the tube-foot to 
form a single canal, which runs through the foot and ends blindly at its tip. 
Originally, there was only one pair of pores on each ambulacral plate. Where two 
or more pairs occur on one plate, the plate can bo proved to be composed of just as 
many fused plates as there are pairs of pores. Primary plates are such as reach 
from the lateral edge of a two-rowed ambulacrum as far as the median suture 
between the two rows of ambulacral plates. Half plates are such as do not reach 
the suture, and included plates such as do not reach the edge of the ambulacrum. 
Isolated plates reach neither the edge nor the median suture of the ambulacrum. 

Besides the double pores there are, in the GlypcnstniUhi, and Spatangoida, single 
pores as well, to which small tentacles belong. The arrangement of these pores 
varies, and they are often not confined to the ambulacra, but are also found on the 
iuterradii, especially on the oral surface. Occasionally they are scattered, often in 
grooves, the so-called ambulacral grooves, which radiate out from the peristome, 
and may stretch more or less far towards the ambitus or even beyond it, and may 
be more or less branched. 




(c) The Symmetry of the EcMnoid Shell. 

The test of the regular Echinoids {Cidaroida, Diadematoida, and most 
PalccecMiioidea), viewed superficially, appears to be strictly radiate. The anal area 
lies at the apical, and the oral area at the diametrically opposite oral pole. All the 
ambulacra and interambulacra appear similar one to the other, and the ambitus, 
with few exceptions, is circular or regularly pentagonal with rounded corners. In 
the Holectypoida also the test, as a rule, appears radial, with regard both to the 
circular (or regularly pentagonal) form of the ambitus and to the similar develop- 
ment of the ambulacra and interambulacra. The peristome occupies its place at the 
centre of the oral surface. Notwithstanding this, the longitudinal axis and the 



^ For the different forms and arrangements of these feet or tentacles, cf. section on the 
ambulacral system, p. 416 et seq. 



Yiii EOHINODERMATA^MORPHOLOGY OF SKELETON 341 

plane of symmetry uan be recognised in the Hoh'eiijpukla at the first glance, because 
the anal area has shifted out of the apical system, and into that interradius which is 
called the posterior interradius. The same is the case in the Chjpcastroida, and, in 
a still higher degree, in the Spatangcrida. In the Clypeastroida the peristome with 
the mouth still remahis in the centre of the oral surface, or only very slightly shifts 
away from this piosition. But the ambitus is no longer circular or regularly 
pentagonal ; its outline appears symmetrically lengthened or shortened in the 
direction of the longitudinal axis, in such a way that, even in a superficial view, the 
plane of symmetry is discoverable. Apart from the fact that the posterior inter- 
radius is at once recognisable by the anus lying in it, it is often further distin- 
guished in the ScKlellidic by a pierforation through the test (lunula), which never 
occurs in the other interradii. Further, in the Sctitf/lida; the bilateral symmetry 
is often distinctly indicated by the number and arrangement of the radial lunulse, 
or of the marginal incisions (Figs. 233-235, pp. 292, 293). 

The bilateral .symmetry, which is most pronounced in the Spatmigoida, culmi- 
nates in the remarkable family of the PourtaUsiidw. The ambitus, which varies 
greatly in details, is frequently egg-shaped, or heart-shaped, and in Pourtalesia 
flask -shaped. Xot only does the anus always lie somewhere in the posterior inter- 
radius, but the oral area also shifts from the centre of the oral .surface, moving more 
or less far along this surface anteriorly. In the CassidvJidcc all the transition stages 
between a central and a frontal position of the oral area occur. Since the mouth, 
with the oral area, always forms morphologically the centre of all the systems of radii, 
in shifting anteriorly it necessarily draws along with it the systems radiating out 
from it. AVe shall return later on to the dissimilarity in the ambulacra, and especially 
to the abnormal development of the anterior ambirlacra, and consequent formation of 
the bivium and trivium, to the special form of the peristome of the Spatangokla, etc. 

The apical system also does not always remain at the dorsal centre of the test, 
but shifts more or less far forward (less frequently backward), and the highest point 
of the test may then come to lie in front of (less frequently behind) its central 
point (Figs. 236-238, pp. 294, 296). 

We have seen that in exocyclic Echinoidea (in which the anal area lies some- 
where in the posterior interradius) the longitudinal axis and the plane of symmetry 
can easily be made out even in a superficial examination, they can also be dis- 
covered by careful observation, even in regular endocyclic Echinoidea, which are 
apparently strictly radiate. When describing the apical system, the constant 
relation of the outer apertures of the pores of the stone canal to the right anterior 
basal plate, was pointed out. These relations never quite disappear, and where 
the apical system is retained, they define with certainty the longitudinal axis 
and the plane of symmetry. 

Further, even where the apical system has not been retained, it is always 
possible, as has been proved by a very careful investigation of the Echinoid test, 
to determine the longitudinal axis and the plane of symmetry by the definite and 
constant arrangement of the plates of the test, both in regular and irregular endo- 
cyclic and exocyclic Echinoids. This constant relation of the plates to one another 
is expressed in Loven's law. 

Let the test of any Spatangoid be laid with the dorsal (apical) side on a perpen- 
dicular surface, in such a way that the mouth is directed upward, and the posterior 
unpaired interradius (between the bivium) downward. Let the five ambulacra be 
then marked with the figures I, II, III, IV, V (Fig. 302), starting from the left 
lower ambulacrum (the right posterior of the animal) and proceeding in the direc- 
tion in which the hands of a watch travel. Two plates of each ambulacrum, the 
so-called marginal peristome plates, take part in forming the boundary of the 
peristome. The first marginal plate which is met with in each ambulacrum, Avhen 



342 



COMPARATIVE ANATOMY 



CHAP. 



moving from left to right may be marked a, the second h, and these letters may- 
further indicate the rows to which these plates belong. In this way we can 
name each of the ten ambulacral plates bordering the peristome. Examining 
these ten plates carefully, we see that those indicated by la, lla, IIU, R'a, \h 




-=="=31 



Fig. 302. — Kleinia luzonica (Gray). Apical system, spread ont (after Lov^n). /a, Fascicles. 
Further e.xplanations in the adjoining text. 

are larger and possess two pores each, while the smaller plates 16, 116, Ilia, IV6 
and \n have only one pore each. Only the ambulacra I and V, i.e. the two 
posterior ambulacra, are thus bilaterally symmetrical, while the two (paired) 
anterior ambulacra II and IV, and the two rows of plates of the anterior .unpaired 
ambulacrum III, are asymmetrical. This law holds for all Echinoidea (not only 
for adults hut for their young stages also) ; the plates la, Ila, III6, I\^a, \h are 



vin ECHINODEBMATA— MORPHOLOGY OF SKELETON 343 




Fig. 303. — Toxopneustes droebachiensis juv., 4 mm. in diam. The whole system of plates 
spread out in one plane (after Lov6n). B, Peristome plates. D, teeth. 

marked by common characters, and are distinguished from the plates K, IIS, Ilia, 
IV6, Va, which also resemble one another. These different characters are, it is true, 
often not very evident. 



344 



GOMPARA TIVE ANATOMY 



CHAP. 



As a further example, let us take the test of a young Toxopncustrs drcebachiciisis, 
4 mm. in diameter (Fig. 303). If we examine it we shall find that, of the ten 
ambulacral plates bordering the peristome, five, belonging to different ambulacra, 
are of greater size (consisting each of three primary plates), and show three double 
pores, while the five others are smaller (consisting of but two primary plates) and are 
perforated by only two double pores, ff e can place the test in only one position, viz. 
that given in the figure, in which the formula la, Ila, I1I6, IVa, VJ, and lb, lib, Ilia, 
IVJ, Va holds good. In this we see that a median plane, corresponding with that of 
the iircijular Echinoidea, can be established also for the regular Echinoidea. The 
accuracy of this law can be proved by investigating the position of the madreporite. 
In the above'case this actually lies in the right anterior basal plate between the radii 
II and III. 

Loven's law also ajiplies to other plates besides the ambulacral marginal plates of 
the peristome. 

It may be remarked in passing here that the system of marking above described 
can be used for naming all the plates of the Eohinoid test ; in this way we have 
the ambulacra I-Y, the ambulacral rows of plates la, Vi, 11a, lib, Ilia, III6, IVa, 
IV6, Va, and V6, and in the apical system the radials I-V. If we mark the inter- 
radii (interanibulacra) 1 -5, starting from the one lying to the left of ambulacrum I, 
.ind proceeding in the direction of the hands of a watch (viewing the test orally), 
we get the interambulacral rows of plates la, 16, 2a, 2b, 3a, 3J, 4a, 46, 5a, 56, and 
the basals 1-5. The madreporite lies in basal 2. The consecutive plates, counting 

along each row of ambu- 

3 



lacral and interambulacral 
plates, start from the edge 
of the oral disc. 

The arrangement of 
plates revealed by Loven's 
law, taken together with 
the special position of the 
madreporite, and with the 
excentric position of the 
anus in the anal area of 
the regular Eohinoids, show 
us that, strictly speaking, 
no Echinoid is either radi- 
ally, or bilaterally, sym- 
metrical. 

{d) The Relation of the 
Ambulacral and In- 
terambulacral Plates 
to the Peristome. 

Three cases must be 
distinguished. 

1. The plates, both of 
the ambulacraand of the interambulacra, arecontinued in a modified form over the edge 
of the peristome, and on the peristome itself, towards the mouth (Cidaroida, Fig. 304). 
2. Only the ambulacral plates are continued on to the oral integument (Biade- 
matoida), forming either several concentric rings of plates (Slreptosomatd, Ecldno- 
thuridce), or as five pairs of plates lying isolated in the integument, the so-called 
buccal plates {Stereosomata). 




Fig. 304. —Peristome and neighbouring parts of tlie test 
ot Cidaris hystrix, Lamk. (after Lov^n). 



VIII 



EGHINODERMATA— MORPHOLOGY OF SKELETON 



345 



3. Neither the ambulacral nor tlie interambulacral plates are continued on to the 
peristome (Holectypoida, Objprastriiii/a, Spatangoida). 

Among the Palacchlaoidea also there are forms in which the perisomatic plates 
reach as far as the mouth ; in Lcpidocciitrus, indeed, they do this in such a way as 
to make it impossible to distinguish the coronal from the peristomal plates. 

Apart from the peristome plates just mentioned, the oral area is beset all over 
with small irregularly arranged calcareous bodies. 

With regard to the number of coronal plates which border the peristome (mar- 
ginal plates of the peristome), it is to be noted that in regular Echinoidea {Cidaroida, 
Dituhmatoida), and even in most Holectypoida, ten pairs occur, live ambulacral and 
five interambulacral. There are, however, certain Holectypoida in which, in one or 
several interradii, only a single marginal plate occurs. In the Clypeastroida (Fig. 
306) and Spatangoida (Fig. 802) the peristome is, as a rule, bordered by five pairs of 
ambulacral and five single interambulacral marginal plates. Exceptions to this I'ule 
are found in the Spatangoid division, the Cassiduloidca, where, for example, among 
the Echiiioiwid.a;, Echinoiieus and Amblypygus have two marginal plates in their 
second and fourth interradii and only one in the others. 

(c) Manner in which the Skeletal Plates are Connected. 

In most Euechiiwidca the plates of the skeleton, at least those of the corona, are 
firmh' and immovably connected together by means of sutures, and thus form a 
rigid test. This is not the case in very many Palcecchinoidca, and among the 



,ouoo 




Fig. 305.— Oral area of Cidarls papiUata, Leske, from within (after Lov6n). 
apo, Perignathous apophyses. 



Euecliinoidea in the Diculematoid EchinotAnriclce ; also, as far as the skeleton of the 
peristome is concerned, in the Oidaroida (Fig. 305). The edges of the plates here 
overlap, i.e. they are imbricated. In the -Bc/tMioiAttWcto; the plates are divided from 
one another by strips of uncalcified connective tissue, which, to some extent, 
allow the test to change its shape. The imbrication of the ambulacral plates is in 
a direction opposite to that of the interambulacral. Viewing the test from without. 



346 



COMPARATIVE ANATOMY 



CHAP. 



the imbrication of the ambulacra is adoral, i.e. the oral edge of each plate overlaps 
the apical edge of the next in order below it, whereas, in the interambnlacra, the 
imbrication is apical. Lateral imbrication also occasionally occurs. 
Slight imbrication is also found in certain S'j>ata)igoirln. 

(/) Special Modifications of the Ambulacra. 

In all Echinoidea, in which the mouth remains at the centre of the oral surface, 
the five ambulacra are alike in length, breadth, and in the arrangement of their 






|S*i!"Z)> 






J /I' 






J pis 

l" 



w^r/i&A 



Fio. 306— System of plates of a Clypeastrold (Enoope Valenoiennesi, 

(after LoT&n). 



), spread out 



pores, prominences, etc. They only vary in length when the apical system, towards 
wliich, radiating from the peristome across the ambitus, they converge, is shifted 
from the centre of the apical hemisphere to a somewhat anterior (less frequently 
posterior) position. If the test of such an Echinoid, in which the ambulacra 



\-iii EGHINODEEMATA— MORPHOLOGY OF SKELETON 347 

are of unequal length owing to the shifting of the apical system, be viewed from 
the oral side, the ambulacra still form a regular, or almost regular, five -rayed 
star round the central oral aperture or peristome. Where, however, as in the 
Spataiiijinda, the peristome with the mouth has moved from the centre of the oral 
surface (on wliioh the Echinoids creep), and is shifted more or less anteriorly, and 
finally, where in the Pmirtalesia it comes to lie quite on the anterior ambitus, the parts 
taken by the five ambulacra in the formation of the oral surface are necessarily very 
different. The unpaired anterior ambulacrum (III) and the two anterior and 
lateral ambulacra (II and IV) shorten and form an ever smaller portion of the 
whole ambulacral area of the oral (ventral) surface, in proportion as the peristome 
with the mouth shifts forward. They form together the trivium. Conversely, the 
two posterior radii at the same time lengthen and form an increasingly large 
portion of the ambulacral area of the ventral surface. They form tlie bivium. The 
length of the ambulacra of the trivium and the bivium in the apical direction is of 
course determined by the position of the apical system. If this system shifts for- 
ward, the ti'ivium is shortened apically ; if backward, the ambulacra of the trivium 
(especially the anterior unpaired ambulacrum) are lengthened, while those of the 
bivium are shortened. This grouping of the ambulacra into an anterior trivium 
and a posterior bivium is especially clear on the apical surface of those Spataiujokhi 
which have a diffused apical system, i\cj. the Collyritidce and Pourtalcsiidm {cf. pp. 
324, 325). Since the apical ends of the ambulacra are always in contact with the 
radial plates of the apical system, and since, further, in the diffused apical system 
the two posterior radials I and V, which are separated from the anterior, are 
shifted posteriorly, the apical ends of the two posterior ambirlaera (the bivium) are 
also necessarily separated from the three anterior ambulacra (the trivium) by a con- 
siderable space (Fig. 284, p. 325). 

In the Palcccc/iinoith-a, and among the SuecMuuir/ra in the Cidarukla, the Dkule- 
matoida, nearly all Holectypoida, and many Spatamjoida, the ambulacra throughout 
their whole courses have a similar structure, and are similarly provided with pores. 
In the Clijpeastridu- and many Spatangoida, however, the ambulacra are modified on 
tlie apical side in a characteristic manner ; they are petaloid, each ambulacrum forming 
a petalodium (Figs. 233, 234, p. 292 ; 236, p. 294, and 306). Such a petalodium 
arises by the divergence of the two rows of large double pores of each ambulacrum 
from one another immediately on leaving the apex, and their reapproximation and 
junction before they reach the ambitus. The two rows of pores of each petalodium 
make a figure like a lancet-shaped leaf, and the five petaloids together form round 
the apex a graceful rosette of leaves, which recalls the petals of a flower. On the 
remaining plates of the ambulacra, i.e. those not forming the petalodium, the pores 
are single and small ; they are, further, few in number and scattered. Between 
the regular ambulacra and those which have apical petaloids there are many transi- 
tion forms, occurring often within one and the same family. One of these transi- 
tions i.s specially frequent ; the two rows of pores of a petalodium do not unite 
at their oral ends but remain open. The ambulacra are then called sub-petaloid. 
Such petaloids are often very long. 

The petaloids often sink in (Fig. 236, p. 294), and then, not infrequently, serve 
as brood cavities, or marsupia, for containing the young. 

Just as the ambulacra occasionally form petaloid rosettes round the apical system, 
so, in the family of the OassidiUidcc (sub-order Cassiduloida of the order Spatan- 
goida), can they form rosettes of so-called phyllodes round the peristome (Fig. 307). 
The five phyllodes, in which the well-developed double pores lie thickly crowded 
together, sink in, while the five interradial marginal plates of the peristome between 
them are contrariwise bulged out. The five interradial cushions form, together witli 
the five radial phyllodes, what is called a floscelle. 



348 COMPARATIVE ANATOMY chap. 

The anterior unpaired ambulacrum in many exocyclic Echinoidea differs greatly, 
both ill shape and in the number, arrangement, and form of its pores, from the other 



ffl 




a 
V 

Fig. 807.— Oral porlsome of Cassidulus paoifious, Ag., with the live phyllocles (after Lovto). 

four. This variation in the anterior ambulacrum is found almost exclusively in the 
order Spatani/Dida, especially in the Gassiduloid family Pli'.sios2mtangidcc and in the 
sub-order Spatangoidea (here especially, and, to a very marked degree, in the family 
of the Spatciiiijida-). 

(g) Special Modifications of the Interradii. 

We can here only point out certain conditions occurring in the order Spatan- 
ffulda. 

In the sub-order Spataiiijoidi:a an extraordinary asymmetry of the two 
posterior interradii 1 and 4 prevails {of. Fig. 302, p. 342). The right posterior in- 
terradius 1 is always so modified near the peristome that two plates fuse, thus con- 
trasting with the left posterior interradius, which remains only slightly if at all 
modified. Tliis fusion takes place either between the second and third plates of the 
row In, or the two second jilates of rows la and 16, or the second and third plates 
of row l) and the second plate of row a. In the last case, the second plates of the 
two rows of interradius 4 are also fused. 

Since, in the Spatangoida, the peristome, with the mouth, is shifted forward on 
the oral surface, the posterior unpaired interradius occupies a considerable portion of 
the ventral surface (and this is also the ease in the Cassiihilokl.ea with mouth shifted 
forward). It is often somewhat bulged out, and the region occupied by it on the 
oral side is known as the plastron. It takes part in the limitation of the peristome 
by means of a single crescent-shaped plate, which is known as the labrum in those 
forms which have a projecting under-lip to the transverse peristome (c/. Fig. 302, 
p. 342). Ill many Sptdarigoida the labrum is followed posteriorly by two large sym- 
metrically arranged plates (sternum), which again are followed by two smaller but 



via EGHINOBERMATA—MORPHOLOOY OF SKELETON 349 

still not insignifieant plates (episternum). The test is then amphisternal. In 
other forms, however, the arrangement of the plates on the plastron (apart from the 
labrum) approaches the usual arrangement, i.e. the plates of the two rows alternate 
more or less distinctly, so that the median suture which divides them forms a zig- 
zag line. This arrangement, as compared with that first described, is older and more 
primitive. The test is then called meridosternal. 

In most Clypeastrithi' the iiiterambulacra are interrupted, i.e. they do not run 
continuously from the apical system to the peristome, but, near the latter, are 
crowded out by the broad plates of the ambulacra which touch one another inter- 
radially, so that the five interradial marginal plates of the peristome are completely 
isolated from the remaining portions of the interambulacra (Fig. 306). Not infre- 
quently, the paired interambulacra are interrupted and the unpaired posterior inter- 
ambulacrum is uninterrupted. 

(A) Form of the Peristome. 

In most Eohinoidea, i.e. in those in which the peristome retains its central posi- 
tion, its shape is pentagonal, or decagonal, or round, less frequently oval or oblique, 
or quite irregular, often with branchial incisions. But where the peristome is 
shifted anteriorly, as in the sub-order Spatangoidea, the peristome is transverse and 
crescent-like, with depressed anterior upper-lip and raised posterior under-lip. The 
peristome, however, is always central in the embryo, and is originally pentagonal. 

{i) Ornamentation. 

The outer surface of the plates of the Echinoid test are beset — in many different 
ways, which are of importance in classification — with numerous larger or smaller pro- 
minences, granules, etc., on which spines and pedieellariie are planted. 

In the sub-order Spatangoidea, narrow, finely gi-anulated streaks or bands run, in 
definite aiTangement, along the surface of the test, and carry small rudimentary 
spines or pedicellarise. These are called fascioles or somites (Fig. 302, p. 342). The 
following systematically important forms of fascioles are to be distinguished : — 

1. The peripetaloid fasciole encircles the apical rosette of petaloids. 

2. The lateral or marginal fasciole runs round the shell near the ambitus. 

3. The lateral subanal fasciole branches off from the peripetaloid fasciole and 
runs below the anus. 

4. The subanal fasciole forms a ring below the anus (between the latter and 
the peristome). They may give off anal branches which run up on each side of the 
anus, and occasionally unite above it to form an anal fasciole. 

5. The internal fascioles run around the apex and the anterior ambulacrum. 
The tentacles and plates in those regions which are encircled by the internal 

and subanal fascioles are modified. 

One very varied form of ornamentation of the Echinoid test, which arises early 
during postlarval development, is due to the deposit of calcareous substance on the 
plates, and is known as epistroma. 

ije) Marginal Incisions or Perforations. 

These are often to be found in the flat disc-shaped test of the Seidellidm, in 
■ some or all of the ambulacra, and not infrequently also in the posterior inter- 
ambulacrum. The edge of the shell is at first entire, but during growth marginal 
"indentations and incisions make their appearance, and these may close to form per- 
forations (lunula). (Figs. 234, 235, pp. 292, 293, and 306, p. 346.) 



350 



COMPARATIVE ANATOMY 



CHAP. 



(Q The Perignathic Apophysial Girdle (Figs. 308, and 348, p. 402). 




ffkM-\, cwn. 



In all Ecltiiwidea in which the mouth is armed with five teeth, moved by a com- 
plicated masticatory apparatus, i.e. in all Ecldnoidea except the Sj)atan(joida and a 
few Eokctypoida, processes, directed apically inwards, are found at the peristomal 
edge of the test ; these serve for the attachment of the muscles and bands of the 
masticatory apparatus. They either consist solely of the ambnlacral or inter- 
ambulacral marginal plates of the peristome bent round inwards, or else a few of the 
plates next in order also take part in their formation. 

These processes may be divided into those which rise on the ambulacral marginal 
plates, and those which rise on the interambulaoral marginal plates. The former 

may be called the ambulacral apophyses, 
the latter the interambulacral apophyses. 

The apophysial circle is closed or inter- 
rupted. In the former case, which is best 
illustrated by the Diadeviatoida (Fig. 308, A), 
an apophysis rises on the peristomal margin 
of each ambulacral area on each side of the 
ambulacral suture. The two apophyses of 
one and the same ambulacrum usually unite 
at their free ends, which project into the 
body, in such a way as together to form a 
kind of arch ; this is called an auricle, and 
affords passage for some of the important 
organs (for the trunks of the radial ambulacral 
vessels, of the nerves, etc. ). There are thus, 

T??®,'il'^'^*''"','^?°'"'^-^°^ ™ '^^^' ^^^ ambulacral apophyses, which may 
unite in pairs to form five auricles. The 
interambulacral apophyses project less far into 
the interior of the body. The two apophyses 
of one and the same interambulacrum together 
form a ridge w'hich runs along the edge of 
the peristome, and connects two neighbour- 
ing auricles ; these ridges are generally fused 
with one another and with the auricles. 
Such a closed apophysial ring, which rises on the edge of the peristome and pro- 
jects into the body, may be compared to a circular wall with high arched gateways 
at five radially arranged points. The five arched gateways would represent the 
auricles, i.e. the five pairs of ambulacral apophyses, and the circular wall would 
be formed of the five pairs of interambulacral apophyses. 

In the Cidaroida (Fig. 308, B and C) the apophysial ring is interrupted. The 
ambulacral apophyses are wanting, but the interambulacral apophyses are all the 
more strongly developed, and form ear-shaped processes. The two apophyses of an 
interambulacrum are connected by a suture at their bases, but diverge at their tips. 
When the two interambulaoral apophyses standing at the sides of an ambulacrum 
approximate above it (the ambulacrum), but without fusing, a false auricle may 
be formed. 

The ambulacral apophyses are also wanting in a few Holectypoida ; where they 
are present, they do not unite in pairs to form auricles. 

In all Clypeastroida, the apophysial ring is interrupted, and consists either of 
ambulacral or of interambulacral apophyses. 



Fio. 

of a radius and of the two neighbouring 
interradii of various Eohinoidea. A, 
Diadematoid. The apophyses of the 
ambulacral plates {am) form true auriculie 
(aur). B, Cidaroid. Apophyses are formed, 
not by the ambulacral but by the iuter- 
ambulacral plates, forming what are called 
false auricles. In C (also a Cidaroid) these 
interambulacral plates have fused. 



viii ECHINODERMATA— MORPHOLOGY OF SKELETON 351 



III. Asteroidea. 

Here also the perisomatic portion forms by far the greater part 
of the whole skeleton. Only in a few forms does the apical system 
constitute a distinctly appreciable element in the skeleton. Further, 
the oral system also, even if we include, besides the orals (odonto- 
phores, proximal plates of the interbrachial system), the terminals, as 
radials belonging to the oral system, forms but a very small fraction 
of the whole skeleton. 

The skeleton of the Asteroidea is distinguished from that of most 
Ecliinoidea by its mobility. It is not a rigid capsule, but its principal 
plates are articulated one with another, and are movable one upon 
another by means of muscles. The arms can bend upwards and 
downwards, and also occasionally, to a certain degree, laterally (in the 
horizontal plane). The ambulacral furrows may be deep, or shallow. 
The disc is sometimes shortened in the direction of the principal axis, 
i.e. flattened. 

In the perisomatic skeleton of the Asteroidea three principal parts 
may be distinguished : (1) the ambulacral, (2) the interambulacral, 
and (3) the accessory. 

(a) The Ambulacral Skeleton. 

From the free end, or tip, of each arm or ray a large median groove 
runs on the oral side to the centre of the disc, and here runs into the 




Fio. 309.— Transverse section through the hrachial skeleton of Astropecten aurantia- 
cus (Gray) ; original. For lettering see p. 317. sa, Supports of tlie ambulacral plates or supra- 
ambulacral plates ; ad, adambulacral plates ; j), paxilla; ; 1, position of the radial canal, etc. ; 
2, ampullje ; 3, ambulacral feet. 

mouth. In the base of this ambulacral furrow rise the ambulacral, 
or tube-feet in two or four longitudinal rows (Figs. 239, 243, pp. 296, 
298, and 343, p. 396). The plates of the ambulacral skeleton, which 



352 COMPARATIVE ANATOMY chap. 

may be compared with vertebrae, and are the principal pieces of the 
skeleton, form a long roof over the ambulacral furrow, which opens 
downwards. In a transverse section through the arm of an Asteroid 
(Fig. 309) we see that the roof of the furrow invariably consists of 
four skeletal pieces. Two of these pieces — the ambulaeral ossicles 
(am) — form the greater part of the roof. They lie symmetrically to 
the median plane of the arm, and articulate with one another along 
the ridge of the roof. The two other skeletal pieces — the adambu- 
laeral ossicles (ad) — meet the diverging edges of the ambulacral 
ossicles, and so lie at the edge of the furrow, or, in other words, at 
the lower lateral edges of its skeletal roof. 

The general form of the ambulacral ossicles is that of transversely elongated 
clasps. They are arranged in two longitudinal rows in close proximity to one 
another, and in this way form the roof, which arches over the groove along the whole 
of its course, from the tip of the arm to the mouth. 

In the Euasteroidea (to which sub-class all recent forms belong) the ambulacral 
ossicles of the two rows are arranged in pair.s, each ossicle on one side of the roof 




Fio. 310.— Solieme of the oral skeleton of tlie Asteroidea, from tlie inner side (after Ludwig). 
or, Oral plate (odontophore) ; Mj, first lower transverse muscle of tlie iambulacral furrow ; Mi, the 
ititerradial muscle ; I-Vl, first to sixth ambulacral ossicles ; 1-6, first to sixth adambulacral 
ossicles ; a, &, c, d, e, /, apertures for the ampulla of the tube-feet. 

corresponding with one on the other side. In the Palmasteroidea, on the contrary, 
the ossicles alternate, at least in the middle part of the arm. 

The (smaller) adambulacral plates usually alternate regularly with the ambulacral 
plates. 

We must here emphasise the important fact that the ambulacral ossicles of the 
Asteroidea lie much deeper than the skeletal pieces of the same name in the 
Echinoidea. In the latter class they are quite superficial, the radial trunks of the 
water vascular system, as well as the radial nerves and the spaces of the schizocoel, 
are to be found on their inner side ; whereas, in the Asteroidea, these organs lie on 
the outer side under the ambulacral roof. Of the whole ambulacral vascular system 
only the ampullfe lie on the inner sides of the ambulacral ossicles, i.e. that turned 
towards the general body cavity. 

Between every two consecutive ambulacral ossicles there is one (and never more 
than one) aperture for the passage of a tube -foot. The number of ambulacral 
ossicles in a row thus always corresponds quite accurately with the number of the 
tube-feet on the same side of the ambulacral furrow. 

Each aperture for the passage of a tube-foot normally lies in the corner between 



viii ECHINODERMATA— MORPHOLOGY OF SKELETON 353 

two ambalacral ossicles and an adambulacral ossicle {cf. Fig. 310). In those 
Asteroids which have four longitudinal rows of tube-feet, however, these apertures, 
at some distance from the month, alternate regularly in such a way that the 
laterally placed aperture of one interstitium is followed by a more median aperture 
in the next interstitium, the next again being lateral, and so on. The connecting 
line between the apertures of one and the same side of an ambulacrum in this case 
forms a zigzag, the angle of which is the more pointed the narrower the ambulacral 
ossicle. The consequence of this is, that the tube-feet which stand in the corners of 
the zigzag line appear arranged in two rows, that is, in the whole ambulacrum, in 
four rows. 

The oral aperture, which always lies in the centre of the ventral 
surface of the disc, and into which the ambulacral furrows of the arms 
converge, is surrounded by a circle of firmly connected calcareous 
pieces, the external edges of which are in immediate contact with the 
ambulacral and adambulacral ossicles. This circle forms the oral 
skeleton of the Asteroidea. It is extremely probable that its separate 
pieces (which in the five-rayed forms number thirty, and in forms with 
a greater number of rays are six times as numerous as the rays) are 
merely the transformed and more firmly connected proximal ossicles 
of the ambulacral and adambulacral rows. In this case, in each ray 
or arm, the first two pairs of ambulacral and the first pair of adambu- 
lacral plates of these rows (in Ctenodiscus, the first three ambulacral 
and the first two adambulacral pairs of plates) would take part in the 
formation of the oral skeleton. The oral skeleton is ambulacral (in 
many C'ryptozonia) or adambulacral (in the Phanerozonia and some 
Ori/fitozonia), according as the ambulacral or the adambulacral portions 
of the circle project the further into the oral cavity. 



(b) The Interambulacral Skeleton. 

This comprises the ambitus, i.e. the whole surface of the body 
between the oral (or ventral) and the apical (or dorsal) regions, on both 
of which, however, interambulacral plates may be found. The inter- 
ambulacral skeleton thus forms the lateral walls of the arms. The 
pieces constituting it are called marginal plates, and are arranged in 
each lateral wall in two rows, one above the other. The upper row 
consists of the supramarginal (Fig. 309 sm) and the lower of the infra- 
marginal (im) plates. It only rarely happens (e.g. in Luidia) that 
the marginal plates agree in number and length with the ambulacral 
ossicles. The marginal plates, which in the order Fhanerozonia are 
large and well developed, become reduced in that of the Cryptozonia, 
being difficult to distinguish externally. They may be altogether 
wanting, or else represented merely by microscopically small rudi- 
ments. The row of inframarginal plates may be separated from that 
of the adambulacral ossicles by a row of small intermediate plates. In 
the same way a row of small intermediate plates may be intercalated 
between the two rows of marginal plates. 

VOL. II 2 a 



354 COMPARATIVE ANATOMY chap. 

(c) The Accessory Skeletal System. 

In this system may be included all those plates or ossicles which occur in those parts 
of the body not covered by the anibulacral and marginal systems. This accessory system 
is very variously developed, and a comparative study of it cannot here be under- 
taken. The plates differ greatly in size, form and ornamentation, and arrangement, 
sometimes being scattered or lying loosely near one another, or else closely approxi- 
mated, sometimes imbricating or reticulating by means of anastomoses of skeletal 
pieces. 

Not infrequently either the whole, or parts, of the accessory skeleton are reduced. 
It is often covered by a considerable layer of integument, and is difficult to dis- 
tinguish externally. Its plates may diminish gi'eatly in size, even becoming micro- 
scopically small, but they are rarely altogether wanting. 

Three sub-divisions of the accessory skeleton may be distinguished : — 

1. The dorsal, abactinal, or apical acoeBSory system, when present, consists 
of skeletal plates developed in the dorsal integument of the disc and in the arms. 
We have seen above that in the Asteroidea the apical system only rarely takes any 
recognisable part in the formation of the dorsal skeleton. There are, nevertheless, 
forms {e.rj. Cnemidaster) in which the large and distinct plates of the apical system 
form almost the whole of the dorsal protection of the disc. 

2. The ambital accessory system consists of the intermarginal plates already 
mentioned as occasionally being intercalated between the supra- and the infra- 
marginal rows of plates. 

3. The ventral, actinal, or oral system in the same way consists of the already 
mentioned intermediate plates which may occur between the inframarginal and the 
adambulacral plates. It is most developed in those forms in which the disc 
increases in size at the expense of the arms, i.e. in forms whose outline is more or less 
pentagonal. The ventral accessory plates then fill up the larger or smaller triangular 
regions between the ambulacral farrows on the lower side of the disc. 

Finally, two other skeletal systems which occasionally occur in the Asteroid 
body must be mentioned. 

In a certain number of Star-fish each ambulacral ossicle is connected by a skeletal 
plate, or more rarely by a row of two to three firmly united plates, through the 
body cavity, with a marginal plate of its own side, or else with a laterally placed 
accessory plate. These simple or compound skeletal pieces, which are limited to 
the arms, and which here correspond in number with the anibulacral ossicles, are 
called supports to the ambulacral ossicles or supraambulacral plates (Fig. 309 sa). 

The other skeletal system, which occurs especially in Asteroidea with large discs, 
but is altogether wanting in many forms, is called the interbrachial system. It 
continues the divisions between the arms, either completely or incompletely, into 
the interior of the disc, and consists either of interbrachial walls, running from the 
oral to the actinal skeleton, or of interbrachial chains of skeletal plates descending 
vertically to the oral skeleton. In each interradius a proximal plate of this inter- 
brachial skeleton, however, always enters into closer relations with the oral skeleton. 
These plates are the orals, already mentioned in the section on the oral system. 

At the free end of each arm in every Asteroid there is to be 
found a single median skeletal plate, which is sometimes of consider- 
able size and distinctly visible, sometimes small and inconspicuous ; 
it carries on its lower side a visual organ. These plates are called 
oeular plates or terminals. According to recent investigations they 
develop very early (apparently first of all the plates) over the left 



VIII ECHINODERMATA— MORPHOLOGY OF SKELETON 355 

coelomic vesicle. They must thus belong to the oral system, and 
perhaps, in this system, correspond with the radials in the apical 
system. 

In the development of the Asteroidea the formative centre of each 
newly appearing plate in a radius of the perisomatic system is always 
immediately proximal to the ocular plate of the arm. At these 
points new plates continually appear between those last formed and the 
ocular plates, which thus always remain at the free tips of the arms. 

(d) Comparison of the Ferisomatiq Skeleton of the Asteroidea with that of 
the Echinoidea. 

The ocular plates (terminals) of the Asteroidea bear to the newly appearing 
plates of the perisomatic skeleton relations altogether similar to those which the 
radials (also "oculars ") of the apical system in the Echinoidea bear specially to the 
ambulacral plates. Since it has not been proved that the radial plates of the 
Echinoidea arise over the right ccelomio vesicle, it is possible that they, although 
lying high up at the apex, belong genetically to the oral system, and correspond 
with the terminals of the Asteroidea. The radials should then not be represented in 
the apical system of the Echinoidea. 

In a comparison of the skeletons of the Echinoidea and the Asteroidea we should 
then have to suppose that in the former the ambulacra have been lengthened round 
over the ambitus to the apex ; and that, further, the body took on the form 
of a pentagonal pyramid, by the abbreviation of the arms and the elongation of 
the principal axis of the body ; and that, therefore, the whole region occupied by 
the accessory skeleton of the Asteroid has disappeared. The marginal plates of the 
Asteroid would then correspond with the interambulacral plates of the Echinoid, 
and the adambulacral ossicles of the former with the ambulacral plates of the latter. 
A comparison of the ambulacral plates of the Echinoid with the plates of the same 
name in the Asteroid is rendered difficult by the difference of position of the two, 
the former being superficial, epiambulacral, and epineural, and the latter deeper, 
suhambulacral, and subneural. The ambulacral ossicles of the Asteroid would thus 
not be represented in the skeleton of the Echinoid. 



IV. OphiuFoidea. 
[a) Skeleton of the Arms. 

The brachial skeleton of the Ophiuroidea consists typically of six 
longitudinal rows of plates, a dorsal row (dorsal shields), a ventral 
row (ventral shields), two lateral rows (lateral shields), and a double 
row of internal ossicles lying in the axis of the arm. This system is 
jointed, or segmented, in quite a regular manner — one- dorsal, one 
ventral, one axial piece and two lateral pieces together composing 
a skeletal segment (Fig. 311). 

The external pieces together form, in each arm, a jointed tube, 
which determines the shape of the arm. Most of the lateral shields 
carry spines ; on each shield there are usually four of these, one 
above the other, so that each longitudinal row of shields is armed 
with four longitudinal rows of spines. The tube-feet emerge at 



356 



COMPARATIVE ANATOMY 



CHAP. 



regular segmental intervals through apertures which lie on each side 
between the ventral shields and the lateral shields belonging to them 



dj- 



cl 



ac 



ss -- 




arw 



— oe 



utc 



6,1 ntro 

Fig. 311.— Transverse section througli the arm of an Ophiurid (after Ludwlg). Diagram. 
ss, Lateral shields ; (is, dorsal shields ; i(, cavity of the arm (ccelom) ; ac, spines ; tun, the ambu- 
lacral plates (vertebra) ; r, loop of tentacle canal in the groove on the distal face of the ossicle (c/. 
next tig., .\ 4); de, tentacular canal of the radial vessel (m) of the water vascular system; U, 
feeler (tentacle) ; rv, radial pseudohsemal vessel ; rii, radial nerve strand ; !)S, ventral shield. 




Fig. 312.— Vertebral ossicles (ambulacral plates) of Ophiarachna inorassata (after Ludwig), 
to show the articulating prominences and depressions, etc. A, Three vertebral ossicles from the 
side. B, Vertebral ossicles from the proximal (adoral), and 0, from the distal (aboral) side. D, Three 
vertebral ossicles from the ventral side, pr, Proximal ; di, distal ; ra, radial trnnlts of the water 
vascular system; rTi, radial nerve trunk; ru, radial pseudolitemal canal. 1, Point at which the 
branch of the radial water vascular trunk running to the tube-foot passes out of the substance 
of the vertebral ossicle at its distal side; 2, point where this branch re-enters the ossicle; 
4, channel between these two points, which receives the loop of the branch belonging to the 
tube-foot ; 3, depression for the lower intervertebral muscle ; 5, channel for the radial water 
vascular trunk ; 6, depression for the tube-foot ; 7, channel for the branch of the nerve running to 
the tube-foot ; 8, pseudoheemal vessel to the same ; 9, nerve branch to tlie same ; 10, brancli of the 
water vascular system to the same, wliich at 12 passes into the substance of the ossicle, and at 13 
out of the latter and into the tube-foot ; 11, point at which the nerve branch (14) running to the 
upper intervertebral muscle, enters the vertebral ossicle. 

(c/. Fig. 245, p. 300). At the edge of these apertures there are smaller 
spines or scales. 

The axial double plates are called veptebral ossieles, a very suit- 




am. 



viii EGHINOnEBMATA— MORPHOLOGY OF SKELETON 357 

able name, since they play a part altogether similar to that of the 
vertebrae of the axial skeleton in Vertebrate animals. In a large 
majority of cases the two lateral portions of a vertebral ossicle are 
fused in the median plane in such a way that no sutures are now to 
be seen. These ossicles, however, arise ontogenetically as two, at first 
entirely distinct, lateral pieces, which only fuse later. There are, 
further, certain deep-sea Ophiuroidea (Ophiohelus, Fig. 313) in which 
each vertebral ossicle consists, even in the adult, of two distinct slender 
pieces, articulated one with the other. 

The vertebral ossicles fill up the greater part of the skeletal tube 
formed by the dorsal, ventral, and lateral shields. Between them and 
the tube, in dried skeletons. 

Ml if 



only small spaces are to be 

found, which dorsally contain 

continuations of the body K^fy?' 

cavity of the disc, while ven- 

trally they contain the radial 

water vascular trunk, the 

radial nerve cord, the epineural 

canal, and the pseudohtemal 

vessel. The lateral branches 

of the radial vessels of the 

water vascular system, before fig. 313.— OpWohelus umbeUa, Lym. a macerated 

enterinjy each tube-foot pass joint from near the tip of an arm, from the dorsal side 

, ^ -I ■^' ^.r^ (after Lyman), ds. Dorsal; ss, lateral .shield; cnii, 

tnrough, on eacn Siae, tne ambulacral ossicles ; apa, hook spines. 

substance of the vertebral 

ossicle of the corresponding segment, nearer the distal than the 
proximal end of the ossicle. The consecutive vertebral ossicles of the 
arms articulate one with another, and are connected by means of four 
intervertebral muscles. The contraction of the two upper inter- 
vertebral muscles brings about the upward curving, and the contrac- 
tion of the two lower, the downward curving, of the arms. The 
horizontal (lateral) movement is brought about by the contraction of 
the upper and lower muscles of the same side. The vertical movement 
of the arms is very slight in true Ophiuridae, whereas in the Euryalidce 
the arms can be completely rolled up orally {cf. Fig. 246, p. 301). 

Small accessory plates may occur in addition to the dorsal shields. The super- 
ficial brachial skeleton is much reduced in the AstropMjtidcc (Euryalidce) and the 
Ophiomyxidce, and the arms are, in these animals, covered hy a soft integument, in 
which only small skeletal pieces occur. In other forms the brachial skeleton is so 
covered by an integument, often containing small embedded skeletal pieces, that it 
is either partly or altogether invisible externally. 

At the distal end of each arm in the Ophiuroidea there is, as in 
the Asteroidea, an unpaired median terminal, which surrounds the 
tip of the radial water vascular trunk (the terminal tentacle) in the 
form of a short skeletal ring. Since, in the Asteroidea, the terminal 



358 COMPARATIVE ANATOMY chap. 

plate receives the terminal tentacle in a channel on its ventral side, it 
is important to note that " the terminal plate in the Ophiuroidea also 
originally forms a channel opening downwards, and only later closes 
to form a ring," 

The relation between the terminals and the developing brachial 
skeleton is the same in the Ophiuroidea as in the Asteroidea. The 
oldest skeletal segment is the one lying most proximally (orally), and 
of the following segments the more distal are always the younger. 
The plates which compose each newly appearing skeletal segment 
always arise at the end of the arm, on the proximal side of the ter- 
minal, which thus remains at the extreme tip of the arm. 

^Yhen we consider the paired elements of the vertebral ossicles and the relative 
positions of the skeletal plates and the water vascular system, we are able to estab- 
lish the following homologies between the components of the brachial skeletons of 
the Ophiuroidea and Asteroidea. 

Ophiuroidea. Asteroidea. 

The two lateral halves of the vertebral Ambulacral ossicles. 

ossicles. 
Lateral shields. Adambulacral ossicles. 

Ventral shields. Not represented. 

(b) The Oral Skeleton. 

The most important and constant plates of the oral skeleton, in 
the Ophiuroidea, as in the Asteroidea, are the specially modified 
proximal plates of the brachial skeleton. The most satisfactory view 
which has been propounded as to the morphological worthy of the 
oral skeleton is that it consists essentially of the ambulacral ossicles 
(the halves of the vertebral ossicles), adambulacral ossicles (lateral 
shields), and ventral shields of the first and second proximal skeletal 
segments of the arms. 

If we look at the oral region of any Oi^hiuroid from without, i.e. 
from the free oral surface of the disc, or from within, i.e. after removal 
of the apical cover of the disc and the viscera, we see the mouth in the 
centre of the disc as a rosette-like, or star-shaped, aperture. The slits 
arranged radially round the centre are called the buccal Assures. Be- 
tween them lie the triangular oral-angles (Figs. 245, p. 300, and 314). 
Five pairs of large plates form the frame surrounding the mouth ; 
these are the oral -angle plates (Fig. 314). At the interradial 
angle of each of these, i.e. the angle which projects towards the 
centre of the oral aperture, two neighbouring angle plates meet. 
Each angle plate has, on the side facing a buccal fissure of the oral 
aperture, two depressions for receiving the first tube-feet which have 
shifted into the oral aperture, and are known as oral tube-feet, or oral 
tentacles. There are often in addition, in the dorsal side (that facing 
the body cavity) of the circle of oral-angle plates, two circular furrows 



VIII 



EOHINOBERMATA— MORPHOLOGY OF SKELETON 



359 



or channels, one of which receives the nerve ring and the other the 
water vascular ring. 

In AsifojiJii/ton part of the water vascular ring is entirely enclosed 
within the oral-angle plates. 

Closer examination reveals the fact that each oral-angle plate 
consists of two fused plates, a proximal and a distal. The former, 




pfei> 



am^-^ad^ 



Fig. S14.— Oral skeleton of the OpUopya longispinus, Lym., from within; above, an inter- 
radial. region of tlie cover ot the di.sc. rs, Eadial shields ; nm, vertebral ossicle ; ami, peristomal 
plates ; pteS, depressions for the oral tentacles ; ceMij+ocZi, oral-angle plates ; fh, bursal apertures ; 
ta, torus angularis ; H, teeth ; ihr; interbrachial region ; sj/c, bursal scale ; fflj, genital plate 
(after Lyman). 

directed towards the centre of the mouth, fuses with the corre- 
sponding piece of its associated oral-angle plate, the two forming the 
oral angle. The distal plate at its distal end is in contact with a 
corresponding plate on the opposite side of the buccal fissure. The 
former of these constituents of each oral-angle plate is regarded as an 
adambulaeral plate of the first brachial segment, taking part in the 
formation of the oral skeleton, while the distal plate is regarded as an 



360 COMPARATIVE ANATOMY chap. 

ambulaeral ossicle of the second skeletal segment. It is the latter 
which is provided with furrows for the nerve and the water vascular 
rings, and with depressions for the oral feet (two on each piece). 
The distal portions of each pair of oral-angle plates, which together 
border a buccal fissure, would thus correspond with the lateral halves 
of a brachial vertebral ossicle, not fused together. 

In viewing the under (oral) side of the disc of an Ophiuroid (Fig. 
245, p. 300) we can easily recognise the interradially placed buccal 
shields (scuta buccalia), which are usually large, and have already 
been mentioned as belonging to the oral system. At the sides of 
each buccal shield, between it and the neighbouring oral-angle plates, 
lie two skeletal plates, which are known as lateral buccal shields 
(scutella adoratia). That these last-mentioned plates belong to the 
same row as the adambulacral plates (lateral shields) of the arms can 
generally easily be seen. They are the adambulacpal plates of the 
second segment taking part in the formation of the oral skeleton. 
The third pair of adambulacral plates is thus the first pair of lateral 
shields in the arm. 

Again viewing the oral skeleton from the dorsal or apical side (Fig. 
314), we see that above the ten oral-angle plates lie ten other plates, 
which usually to a greater or lesser extent roof over the water vascular, 
and the nerve furrows. These, the peristomal plates, thus lie upon 
the inner sides of the oral-angle plates, i.e. the sides facing the body 
cavity. The peristomal plates belonging to two neighbouring radii 
meet interradially, and may fuse together to form single plates. The 
two peristomal plates belonging to one and the same radius may, in 
the same way, touch one another (in which case the ten plates 
together form a closed circle), or their radial ends may remain more 
or less apart. Accessory peristomal plates sometimes occur ; in other 
cases these are altogether wanting. The peristomal plates are con- 
sidered to represent the ambulaeral ossicles (halves of the verte- 
bral ossicles) of the first segment of the oral skeleton, a view 
which does not appear to be certainly established, chiefly because they 
are in no way connected with the tube-feet. The two pairs of tube- 
feet of each radius of the oral skeleton, as has been pointed out, belong 
to its two oral-angle plates. 

At the distal end of each of the oral slits radially, viewed from 
without, there is, in many, indeed, in most Ophiuroidea, a plate which 
also takes part in the limitation of the oral cavity (Fig. 245, p. 300). 
This plate can at once be recognised as the most proximal plate in 
the row of ventral shields. It is the ventral shield of the second 
segment of the oral skeleton. The lateral shields belonging to them 
are the lateral buccal shields. 

In a row with, but dorsally to, this ventral shield, within the buccal 
fissure, there is a second plate (which, however, may occasionally be 
wanting) ; this varies greatly in size and form, and is to be regarded 
as the ventral shield of the first segment of the oral skeleton. 



VIII ECHINODEEMATA— MORPHOLOGY OF SKELETON 361 



The following table embodies this view of the oral skeleton, viz. 
that it consists of modified pieces of the first two skeletal segments of 
the radii (arms). 



Skeletal Segment of the arm. 



2nil (Distal) Segment of the oral 
skeleton. 



1st (Proximal) Segment of the 
oral skeleton. 



The two peristomal 
plates of a radius (Fig. 



The two halves of the ! The distal portions of 
vertebral ossicle (ambu- ■ the two oral-angle plates 
lacral plates) (Figs. 311 ' belonging to a radius 314 avii). 
and 314 am). j (Fig. 314 amo + ac?i). 

The two lateral shields The two lateral buccal The proximal portions 
(adambulaoral plates) j shields of a radius (Fig. [of the oral -angle plates 



(Figs. 311 ss, and 245, 4, 
p. 300). 

The ventral shields 
(Figs. 245, 1, p. 300, and 
311 bs). 



245, 5, p. 300). 



Externally visible ven- 
tral shield of each radius 
of the oral skeleton (Fig. 
245, 8, p. 300). 



belonging to a radius (Fig. 
j 314 aiih:: + adj). 
I 

Inner ventral shield of 



the oral skeleton. 



Accessory Parts of the Oral Skeleton. 

At each oral angle (at the point where two neighbouring oral-angle plates meet 
interradially), on the side facing the buccal cavity, there lies a vertical row of small 
skeletal pieces, which may fuse together to form the torus angularis (Fig. 386, ta, 
p. 486). This can'ies the teeth (D) which project into the buccal cavity. The oral- 
angle plates themselves carry, at the edges which are visible externally, i.e. from 
the ventral side, small spine-like skeletal pieces. Of these those which project into 
the buccal fissures are called oral papillse ; while those which rise at the tips of the 
oral angles, and are turned to the axis of the buccal cavity, are called dental 
papillse. Consequently, in each oral angle, the teeth above mentioned lie dorsally 
til the dental papillfe. 

Accessory Skeletal Plates of the Disc. 

Lower side. — The pieces already described as appearing at the surface on the lower 
side of the disc, and which belong to the oral system (oral shields) or to the oral 
skeleton (oral-angle plates, lateral buccal shields, ventral shields), hardly ever form 
the whole ventral carapace of the disc. On the contrary, between the roots of the 
arms (interbrachially or interradially) these plates leave free spaces (Figs. 245, p. 300, 
and 314 ibr) ; these are often triangular, and are sometimes covered with plates which 
vary in size and number, and frequently imbricate, or else they consist of soft integu- 
ment with small skeletal granules scattered through it. These interbrachial regions 
of the disc may be armed with spines of varying length. 

On either side of the root of each arm, on the ventral surface of the disc, there 
are one or two fissures or slits ; if two, one iiroximal and the other distal. These 
bursal apertures (Figs. 245, 246, pp. 300, 301, and Fig. 314) lead into the bursa", 
which will be described later. The adradial edge of each of these slits is usually 
supported by a single skeletal piece, the genital plate, while the interbrachial edge 
is plated with a row of scales, which is directly continued into the plating of the 
neighbouring interbrachial region. 

Upper (apical) side of the disc. — It follows from what was said above (j). 327) 



362 OOMPARATIVE ANATOMY chap. 

that the apical system, whether complete or incomplete, forms, in many Ophiuroidea, 
even in adults, the greatest, or at any rate a considerable, part of the dorsal carapace 
of the disc. Those regions which are not covered by the apical system are plated 
by the perisomatie skeleton. The plates of this skeleton vary much in size, foi-m, 
number, and arrangement, and not infrequently, especially in oases where the apical 
system does not consist of large distinct plates, the dorsal integument of the disc is 
soft, and only provided with scattered skeletal pieces, which are sometimes micro- 
scopically small. 

Ten large perisomatie plates appear most constantly (even more constantly than 
any of the circle of plates of the apical system) ; one pair of these lies near the base 
of each arm. These are called the radial shields (Figs. 244, p. 299, and 314 rs), 
and are often present even when there are no large plates in the rest of the dorsal 
carapace of the disc. Sometimes the radial shields, covered with a soft integument, 
reach from the base of the ami to near the centre of the disc, their presence being 
then outwardly marked by a graceful rosette formed of five pairs of radial ridges. 

V. Crinoidea. 

{Of. the apical and oral systems of this class, pp. 328-333). 

The perisomatie skeleton of the Crinoidea consists of: (1) The 
perisomatie skeleton of the calyx ; (2) the skeleton of the arms and 
pinnulae ; (3) the skeleton of the stem. 

((() The Perisomatie ^ Slieleton of the Calyx. 

In this are included all the skeletal pieces of the calyx, which do 
not belong either to the apical system (central, infrabasals, basals, and 
radials) or to the oral system (orals). 

In the young stalked larva of Antedon the skeleton of the calyx 
has no perisomatie pieces ; it consists exclusively of the typical plates 
of the oral and apical systems (Fig. 270, p. 318). 

The only forms in which this stage persists throughout life are 
those of the type Inadunata lapvifopmia, e.g. genus Haplocrinnti 
(Fig. 297, p. 334). 

In all other living and extinct Crinoidea a perisomatie skeleton is 
developed, although it varies in extent to an extraordinary degree. 

This skeleton may consist of very various systems, and may be 
developed both in the dorsal cup and the tegmen. 

If. One, or several, or even many, pieces may appear only in the 
posterior or anal interradius, especially in the dorsal cup supporting 
or bordering the anus. These anals, which characterise the posterior 
interradius, more or less markedly disturb the regularly radial 
structure of the calyx. 

h. In all the five interradii one or many pieces may occur, both 

' Mr. F. A. Bather (Natural Science, vol. vi. pp. 418, 419 : 1895), in reviewing the 
origuial German eilition of this Work, adduces strong reasons against the use of the term 
" perisomatie " as here employed by the author. Tlie term is, nevertheless, retained in 
this English version because its exclusion seemed to necessitate the entire rearrangement 
of this Section V.— Te. 



VIII ECHIXODERM AT A-' MORPHOLOGY OF SKELETON 363 

in the dorsal cup and in the tegmen. They are called interpadlals. 
In the tegmen they develop in the zone between the orals and the edge 
of the calyx, and belong to the interambulacral system of plates. 
Usually, only the interradials of the dorsal cup are so called, although 
they are not infrequently continued, between the bases of the arms, 
into the interradial system of plates of the tegmen without any sharp 
boundary. 

c. The proximal portions of the arms may, to a greater or lesser 
extent (to their first, second, etc. divisions), be taken into the calyx, 
in which case the skeletal segments of the arms (brachials) become 
perisomatic plates of the dorsal cup, and are known as flxed brachials 
(primary, secondary, etc., formerly called radials of 1st, 2nd, etc. 
orders). (For the meaning of these names, see below, the section on 
the brachial skeleton, p. 370.) 

(/. Just as interradials may appear in the dorsal cup between the 
five radials and the fixed branchials of the five radii, so the branches 
of each arm incorporated into the calyx may themselves be connected 
by intercalated plates. Those which lie bet'vi^een brachials of the 
second order are then called interdlstichals or intersecundibraehs, 
those between brachials of the third order (after the second forking) 
Interpalmars or intertertibraehs, etc. 

When more than five free arms rise from the edge of the calyx, i.e. 
when some length of the arms and their branches is incorporated into 
the calyx, the food-grooves running over the tegmen from the mouth 
to the periphery divide dichotomously in such a way that the number 
of grooves ultimately corresponds with that of the free arms. The 
regions between the branches of the five primary radial food-grooves 
are as a rule also plated with small interambulacral pieces. 

e. The food-grooves running over the tegmen from the mouth 
to the bases of the arms very often have a skeleton of their own, 
which may be continued into the ambulacral furrows of the arms and 
their branches. This ambulacral skeleton may consist of lateral 
plates (which border the furrow laterally) or of covering plates (which 
cover the furrows, changing them into passages or tunnels), or of both 
these sorts of plates. Subambulacral plates may also occur. 

Special Remarks on the Perisomatic Skeleton of the Crinoid Calyx. 

In the Inadunata larviformia (Type : Haplocrimis) there is no perisomatic 
skeleton of the calyx. This latter consists exclusively of the plates of the apical 
and oral systems (five basals, five radials, three of which are transversely divided, 
and five orals). 

The first perisomatic plate of the calyx occm-s in related forms, interradially, in 
the radial circle, and rests upon the posterior basal ; it is the anal. 

As a type of the Inadunata fisiulata, we shall first select Cyathocrinus. In the 
dorsal cuji only one perisomatic plate appears, which is found resting on the 
posterior basal, between the two posterior radials (Fig. 289, p. 329). The apical 
capsule thus altogether resembles that of the Larviformia. The tegmen calycis, 
on the contrary, shows an entirely different condition, which, however, may vary 



364 



GUMPABATIVE ANATOMY 



CHAP. 







considerably in the diflferent species, and indeed in different individuals of one 
and tlie same species. Tlie orals now no longer occupy the whole of the tegmeu, 
but are supposed by some writers to be represented by certain plates which occur 
at its centre, and vary in number and regularity, often being replaced by small 
irregularly arranged perisomatic plates. The mouth is always hidden beneath 
them. From these supposed orals the five ambulacral furrows run over the tegmeu 
to the bases tif the five much branched arms. Each furrow is covered or bordered 
by two or four rows of alternating covering plates. Interradially there are 5 plates 
(deltoids), the edges of which meet beneath the ambulacrals and form the floors of 
the furrows. They sometimes appear at the surface for a certain distance between 
the ambulacrals ; in other cases, they are even here covered by more or less numerous 
interambulacrals. 

In some species of Cyciihocrinvs, and in many related Inadwnata the posterior or 
anal interradial area bulges out to form a ventral or anal sac, which is sometimes 
cylindrical, sometimes club-shaped or bladder-like (Fig. 316). This anal sac, besides 

the hind-gut, probably contained a large part of 
the body cavity. It is covered with numerous 
plates arranged in vertical rows. The plates 
of the neighbouring rows alternate in some 
species. The anus lies sometimes near the tip 
of the sac, sometimes on its anterior side, and is 
often encircled by special plates. The anal sac 
may attain such dimensions that it is as long 
as, or even longer than the arms. The first 
tendency to the formation of such an anal sac is 
met with in Hyhocrinus, in which the posterior 
interradial region of the tegmeu is somewhat 
though still only slightly bulged. 

The Iiw.duna.ta so far mentioned are palseo- 
zoic forms. From them certain more recent 
types may be derived. In Encrinus (Trias) the 
anal sac has again become a short cone. In 
forms closely related to this genus, and in 
MarsupUcs (Chalk), the anal pieces as well 
have disappeared, so that, while the base is 
dicyclic, the regularly radial dorsal cup consists 
only of the plates of the apical system, periso- 
matic pieces being, in this system, altogether 
wanting. 

The same is the case in the dorsal cup of the 
family Holopiikc (Lias to present time), HyocrinidcR (Lias to present time), Batliy- 
crbiidce (present time). In the tegmeu calycis of these forms we first notice that the 
large anal sac of the Cyathocrinidce is reduced to a small anal tube. In Holopus, 
between the base of the open oral pyramid and the edge of the calyx, there is only a 
very narrow zone beset with irregular perisomatic plates. This zone is still wider in 
Hyocrinus {cf. Fig. 298, p. 335), and is thickly covered with numerous small plates. 
Between the ambulacral furrows lie the interambulaoral plates ; the furrows, im- 
mediately on emerging from between the oral jjlates, are bordered and covered by 
lateral and covering plates. In the posterior interambulacral area, near the edge of 
the tegmen, sometimes excentrically, there rises the short conical plated anal tube, 
with the anus. In Bathycrinus, where the orals are either wanting or reduced, the 
interradial region is either naked or plated with small pieces. The ambulacral 
furrows have lateral plates only. The anus lies on a very short papilla- like anal cone. 




Fig. 315.— Cyathocrinus longimanus, 
after Angelln, from the anal side, after 
removal of the greater part of the arms. 
l>r, Ventral sac ; .r, anal plate ; r, radials ; 
&a, basals. 



via 



ECHINOBERMATA— MORPHOLOGY OF SKELETON 



365 



The Camd iculaia, like the more recent Inadunala (Lias to present time), are 
distinguished by the regular radial structure of the dorsal cup, in which interradials 
only exceptionally occur, and special plates in the posterior interradius (anals) never 
occur. Very often {Apiocrinus, Mhizocrinus, Aiilahnidcc) two or njore brachial 
plates following the radials of the calyx are incorporated as "fixed brachials' into 
the dorsal cup. 

In connection with the tegmen calycis, it must be noted that among the Canali- 
cnlata, orals appear in the adult only in Rhizocrinus. As a rule, the tegmen calycis 
is plated in the interambulacral regions with numerous loosely connected skeletal 
pieces, which vary in size according to the species and genus. These small plates 
are perforated by pores. This skeletal covering is not infrequently continued on to 




fa la, 



Fig. 316.— Tegmen calycis of Metaorlnus angu- 
latus, P. H. Carp, (after P. H. Carpenter), o, Mouth; 
hr, amis ; p, pinnulK (both broken off ) ; ta, anal tube, 
near which there is a second abhonnal tube to] ; cpci, 
covering plates of the ambulacral furrows. 



Fig. 317.— Actinometra strota, P. H. 
Carp, (after P. H, Carpenter). Tegineu 
calycis. o. Mouth ; oit, anus ; o/n, food 
grooves of the anns ; afd, the same of the 
tegmen ; pi, two pinnulffi, which tal^e the 
place of one of the two posterior arms. 



the bases of the arms, and occasionally runs out between these apically in such a 
manner as to be visible in the interradii of the dorsal cup. 

The ambulacral furrows of the tegmen calycis are rarely open, but usually covered 
with covering plates and often bordered by lateral plates (Fig. 316). Occasionally 
the mouth also may be covered with perisomatic plates, but it is usually open. 

The anal tube in the posterior interradius varies in size and in its position within 
this interradial area. Its plating agi'ces with that of the interambulacral area on 
which it is found. 

The interambulacral areas may also be naked, i.e. covered with integument 
containing only very small calcareous corpuscles. 



Adinomdra is the only recent Crinoid in which the mouth is found 
placed quite excentricall}' (anteriorly) on the tegmen, and the anus, 
which lies in the enlarged posterior interradial area, comes to lie 



366 



COMPARATIVE ANATOMY 



CHAP. 



almost centrally (Fig. 317). In consequence of this shifting the 
ambulacra are, of course, very unequal in length. 

The (palteozoio) Oavierata are distinguished by the tendency to strong development 
of the perisonmtic skeleton in the calyx, and by the plates being so firmly inter- 
connected as to form a rigid test. In the formation of the dorsal cup, the 
bases of the arms are incorporated to a certain extent in such a manner that their 
lower brachials become fixed plates of the cup. In the five interradii of the dorsal 
cup, interradials appear, to which, in the posterior interradius, special anal 
plates are often added. In those cases in which the arms take part in the formation 
of the capsule beyond their first branchings, interdistichals, etc. , may connect the 
branches firmly together. 

The tegmen calycis also consists of plates which are usually very numerous and 
firmly connected together. Just as the mouth is always covered by characteristically 
arranged, closely fitting, orals, so also the ambulaoral furrows are never open, but are 
always arched over by large covering plates, some of which may be distinguished by 

A B 




l>, — ti 

Fig. 318. — Actinometra (after P. H. Carpenter). Diagrams to illustrate tlie coiuses of the food 
grooves over the tegmen calycis. Aj-Eo, the directions of the hve pairs of arms. In the centre the 
anal tube. 

their greater size. In the older forms, the tegmen is, as a rule, rather flat, and the 
covering plates of the ambulaoral skeleton appear at the surface. In the course of 
the geological development of the palieozoic epoch, however, the tegmen bulges out 
more and more, and finally forms a high, firmly plated "vault" or dome (Figs. 
253, 254, pp. 307, 308), which, immediately behind its centre, may be prolonged 
to form a tube, often of greater length than the arms, with the anus at its tip. 
Where such a highly arched dome is developed, the interambulacral plates, which 
border the ambulaoral furrows, send out processes over the latter. The processes 
(which are closely joined to one another) from one side of the ambulacra meet and 
become firmly connected with those from the other, so that the ambulaoral furrows 
with their skeletons are completely arched over, and are not externally visible. 

(This condition was until quite recently wrongly explained as follows. The 
Camerata possessed an inner, naked, or merely loosely plated tegmen, in which the 
ambulacra ran from the mouth in the centre to the periphery, and this tegmen 
was arched over by a firmly plated vaidt in such a way, that between the tegmen 
and the vault there was a free space. ) 

The interradial plates of the tegmen are often continued directly, i.e. without a 
boundary line, into the interradial plates of the dorsal cup. 

The anus, surrounded by special plates, lies in the posterior interradius. 



VIII EGHINODERMATA— MORPHOLOGY OF SKELETON 367 



a. The apical capsule or dorsal cup. — In Platycrinus, the dorsal cup (cf. Fig. 
254, p. 308) still consists exclusively of the plates of the apical system (three basals 
and iive large radials). The arms are free from their bases. A plate which is found 
in each interradius, between the bases of the free arras and between the radials, may 
be considered to belong almost as much to the tegmen as to the dorsal cup. In 
Hexacnnus the radial structure of the apical capsule is essentially disturbed by the 
appearance of an anal plate, which presses in between the two posterior interradials 
in the posterior interradius, and to which, in the direction of the tegmen, two or 
three other anals may be added. Further, in each radius, the one small primary 
brachial plate present has become a fixed plate of the apical system. As a further 




Fig. 319.— Gilbertsoorinus tuberoulosus, Hall (after Waohsmutli and Springer). The sy.steni 
of plates of the dorsal cup and of the interradial appendages IB. Ba, point of attachment of the 
arms ; B/, commencement of the free portions of the arms. For other lettering see p. 317. 

example we may take Bimerocrinus (Glyptasteridcv), in which the dorsal cup is still 
more complicated. In each radius the radial is followed by two primary brachials, 
which are incorporated into the dorsal cup. In each case the second of these 
brachials is followed by two or three secondary brachials, which are also fixed in 
the dorsal cup, the last of them carrying a free arm. In each interradius there are 
several interradials ; iirst a large plate which lies between the primary brachials, and 
then two more lying at the level of the secondary brachials. The posterior interradius 
is broader than the others. The first plate here lies between the radials, and 
agrees with them in size, then follows a second row of three plates, and, orally 
from these, various small plates which lead over on to the tegmen calycis. Inter- 
distichals may also occur. Melocriuus (Fig. 252, p. 307) and Dorycrinus, etc. agree 
with Dimerocrinus in these points. 

In Gilbertsocrinus {Bhodocrinidce) also, the two primary brachials and the two or 
three secondary brachials are incorporated into the dorsal cup (Fig. 319). In each of 



368 



COMPARATIVE ANATOMY 



CHAP. 



the five iuterradii there are several (twelve) interradials, the arrangement of which 
is shown in the figure. The anal interradius is hardly distinguishable from the 
other interradii. The distichals or secondary brachials are connected by smaller 
interdistichals. 

The perisomatic skeleton of the dorsal cup of Aclinocrinus (Fig. 291, p. 329) 
is very like that of Gilbertsocriims ; but the anal interradius is much larger than 
the others, and its plates are divided into two lateral groups by the intercalation of 
a vertical row of anal plates. This is also the case in Batocrinus (Adinocrinidce). 
Here, however, not only the 5x2 primary brachials and the 10x2 secondary 
lirachials, but also the 20 x 2 tertiary branchials are incorporated into the dorsal 
cup. In Strotocrinus (regalis) an extreme form is found (Fig. 320). Tlie calyx 




FiG. 320.— Strotocrinus regalis (after Waohsmuth and Springer). The apical border. The 
conical portion of the dorsal cnp is broken away (as far as the distichals di) and shows the 
tegnjen with the anus, the mouth and the food grooves. The dotted lines denote the manner of 
branching of the fixed arms, mi, anus ; hf, fixed joints of the arms, which form the border ; B/, the 
free arms which run out from the edge of the border ; ia, interambulacral region of the tegmen 
calycis ; am, ambulacra ; ^(/, tixed pinnnlw. 

is very large. The dorsal cup consists of a small conical portion above the stalk, 
followed by a border spread out horizontally. In each radius each radial is followed 
l>y two primary brachials. The second costal is in each case followed by the two 
secondary brachials (making ten in all). Up to this point the above mentioned 
plates, together with the apical system, form the conical part of the dorsal cup. 
The plates which follow form the horizontally expanded border. Each distichal is 
followed by a principal row of (six) plates, which runs radially to the edge of the 
border, where the last plate carries a free arm. Accessory rows branch alternately 
from these principal rows, three on each side. These also run to the edge of the 
Ijorder, and the last plate of each row carries an ai-m-branch. Seventy free arm- 
branches in all thus rise from the edge of the border. In the interradii, in the 
interdistichal regions, and between all the further branches of the fixed arms, inter- 



VIII ECHINODERMATA— MORPHOLOGY OF SKELETON 



369 



radials, interdistichals, etc. are found binding the brachials into the rigid liorizontal 
border. Tlieir number and arrangement are best elucidated by the figure. The anal 
interradius is not distinguished from the others in any marked manner. 

b. The Tegmen calycis. — The tegmen of ilarsupioorinus (ccelatus) is only slightly 
vaulted. It is plated with numerous small, firmly connected pieces (Fig. 321). 
Among these, we can easily distinguish the covering plates of the ambulacra, which 
thus here come to the surface, and 
can easily be distinguished from 
the somewhat larger interradial 
and interambulacral plates. In 
the centre of the tegmen lie the 
five orals, arranged in the manner 
which is characteristic of the 
Cameratif, and behind these, 
subceutrally, the anal aperture, 
surrounded by special plates. 

If the ambulacral covering 
plates are larger and more massive, 
as in many species of the genus 
Platycrinus, it is then more 
difficult to distinguish the inter- 
radial plates of the tegmen from ^'°- 32l.-Tegmen oalyois of Marsupioorinus ooelatus 
, (after Wachsmutll and Springer), or, Orals ; om, am- 

bulacra ; cj), covering platps of the ambulacral furrows ; 

The genus Agaricocrimis {„, interambulacral region, 
affords examples of the specially 

strong development of single covering plates of the ambulacral skeleton, which are 
called radial dome plates. The tegmen is highly vaulted. 

An e.xtraordiuarily highly vaulted tegmen is found in the Adinocrinidce 
{Actinocrinus, Batocrimis, Figs. 253, 254, pp. 307, 308). It is regularly and firmly 




an. 



cs; 



,.or&i 









^^ 




=x 



Fig. 322.— Part of tlie dorsal cup of Forbesiocrlnus, spread out. For lettering see p. 317. 
In addition, 10, one of the four similar interradial regions; iA, the deviating anal interradial 
region ; pal, palmars. 

plated with large stroug plates more or less equal in size. Nothing can be seen 
of the ambulacral skeleton externally, it having been pressed down, or rather, over- 
grown, as already described, by the interambulacral plates. In the posterior inter- 
VOL. II 2 B 



370 COMPARATIVE ANATOMY chap. 

radius, immediately behind the centre of the tegmen, this dome is produced still 
further into a long tube similarly plated ; this is the anal tube, on the tip of which 
lies the anus. 

The Articvlata, so far as the perisomatic plates of the calyx are concerned, 
agree with the Camerata in that the ossicles of the arms are sometimes incorporated 
into the dorsal cup as far as to their second or third divisions (Fig. 322), the 
primary, secondary, and often also the tertiary brachials becoming fixed plates 
of the dorsal cup. The number of the brachials in each arm and its branchings 
varies. Three primary brachials are often found in each radius. But these fixed 
brachials are not, as in the Camerata, rigidly connected inter so and with the radials, 
but are articulated. The spaces on the dorsal cup between the radii and between 
their branchings are filled either with quite small, loose, and irregular calcareous 
corpuscles or scales, or with small, definitely arranged plates (interradials, inter- 
distichals, etc.). In the posterior interradius there are often, in addition, special anal 
plates frequently asymmetrically arranged. 

The tegmen calycis of one species of the genus Taxocrimis is well known. The 
radii and their branchings are bulged out while the interradii are depressed. From 
the central mouth, which is open and surrounded by five orals, the five ambulaoral 
furrows run out, dividing diehotomously in correspondence with the branching of 
the arms. Each ambulacral furrow has a floor of two longitudinal rows of sub- 
ambulacral plates, is bordered by lateral plates, and closed in by two longitudinal 
rows of covering plates. The covering plates in the two rows are alternately 
arranged, their interlocking forming a zigzag line, and it is very probable that they 
were movable, i.e. that they could be raised and depressed. The interambulacral 
regions contain a large number of small, loose, irregular plates. In the posterior 
interradius, at the edge of the tegmen, there is a plated process (anal tube ?). 

For Tliaumatoerimis, see "The Systematic Review" (p. 309). 

(6) The Brachial Skeleton. 

The calyx of the Crinoidea carries at its edge (on the boundary 
between the tegmen calycis and the dorsal cup) five arms, which 
are rarely simple, but usually branched, and in the living animal are 
beautifully extended. The arms can be made by stimulation to 
fold together over the tegmen. They are found in this position 
also in dead animals, and therefore almost always in fossilised 
individuals. 

The arms, which contain important inner organs, are supported 
by a special brachial skeleton. This consists of consecutive calcareous 
pieces, the braehials, which are either firmly connected or articulated 
with one another. The brachials are deepened on their oral side, 
that which is directed upwards when the arms are spread out, to form 
a more or less deep longitudinal groove along the arms and all their 
branches ; this is the ambulaeral groove. In the base of this groove 
lie the most important inner organs of the arms (radial canals, water 
vessels, outgrowths of the body cavity, etc.). The soft integument 
which covers these organs, and stretches over the ambulacral grooves 
of the brachial skeleton is in its turn depressed to form a channel. 
These integumental channels, which accurately correspond with the 
ambulacral grooves of the skeleton, are called food grooves. At the 



VIII ECHINODERMATA— MORPHOLOGY OF SKELETON 371 

bases of the free arms they pass into the ambulaoral grooves or food 
grooves of the tegmen calycis, which run to the mouth. 

The arms, when divided, as they normally are, usually branch 
dichotomously ; occasionally, however, they give off alternating 
branches, which may again branch alternately. In most Crinoids the 
arms and their branches carry, at the sides of the ambulacral grooves, 
closely crowded and alternating processes ; these are rod-shaped, and end 
in a point, and are known as pinnules. The skeleton of these pinnules 
resembles that of the arm, and, like the latter, is jointed. The pinnules 
may best be regarded as the ultimate branchings of the arms, and it is 
very probable that in the numerous palaeozoic Inadunata that have no 
pinnules the last branches of the arms fulfilled their functions. 

The brachial skeletons in the Crinoidea are always direct continuations of the 
radials of the apical capsule. The first plate which follows the apical radial radially 
must be considered, morphologically, as a brachial or ossicle of the arm, although 
it is only rarely (e.g. in the Inadmutta) a free ossicle. The terms introduced to 
denote the various orders of brachials are almost as numerous as the writers them- 
selves. It is the clearest plan to speak of them as brachials of the first, second, 
third, etc. orders, or as primary, secondary, tertiary, etc. brachials. Such a plan 
was, however, soon found too cumbrous for practical purposes, and was supplanted 
by the terms costals, distichals, palmars, and postpalmars. To these terms, how- 
ever, considerable exception may be taken, and it seems simplest to adopt the intel- 
ligible and congruent terms primibrachs, secundibrachs, tertibraclis, etc., which are 
capable of indefinite extension, and are readily symbolised as IBr, IlBr, etc. 

It has already been pointed out, in the section which treated of the perisomatio 
plates of the calyx, that brachials are incorporated into the dorsal cup in many, 
indeed, in tlie great majority of Crinoids. We can accordingly distinguish free 
brachials from fixed brachials, the latter being those which have become peri- 
somatic plates of the dorsal cup. The first brachials to be so incorporated are 
naturally the primibrachs, the next the secundibrachs, the tertibrachs may also then 
become fixed. In describing the skeleton in detail, therefore, the terms fixed primi- 
brachs, fixed secundibrachs, etc. are used, and the number of these plates in eacli 
arm is given. The arrangements found in the various divisions of the Crinoidea 
in this respect have been already briefly described in the preceding section. That 
of the Iiiadunata is the simplest, since in them the arms are free from their very 
bases (hence the name), the first primibrach being a free ossicle of the arm ; the most 
complicated condition is that of certain C'amerata [Actinocrinoidea, etc.), in which 
the brachials of several orders are incorporated into the calyx, and being connected 
by interradials, interdistichals, etc. lend to the dorsal cup its rich plating. 

In branched arms those joints above which the divisions or branchings take place 
are called axillary, e.g. we have an axillary costal, axillary distiohal, or, as they 
may more simply be called, primaxil, secundaxil, etc. (lax, Ilax, etc.). 

With regard to the distribution of the pinnulse, it is the rule, at least in modern 
Crinoids, that the axillary joints never carry pinnulEe, and that where two joints 
are connected by syzygial sutures or by ligaments, pinnulfe are also wanting on the 
lower or proximal joint. 

There are three different ways in which the free brachials may be arranged. The 
arms may consist of a single row of joints, the brachials being superimposed in a 
single series with parallel surfaces of contact (uniserial). Again, the joints may 
"alternate," if they are wedge-shaped, and if, in the row, the thick and the thin sides 



372 



GOMPABATIVE ANATOMY 



CHAP. 



of the wedges regularly altei-nate. Or again, the joints may be arranged in two series 
or rows, the contact-surfaces of the one row alternating with those of the other, and 
the two rows themselves interlocking along a zigzag line (biserial). 

Tlie Articulata, many Canaliculata, and the recent Inaduiiatn have the joints 
of their arms arranged in single rows. This condition has been proved to be 
ontogenetically and phylogenetically primitive, i.e.. for the palEeozoic Inadunata 
and the C'amei'atn. The majority of palfeozoic Inadunata have uniserial arms, 
but towards the end of the palfeozoic period forms appeared with alternating rows 
(e.g. Foferiocrimcs), and finally some genera in which the brachials may be biserial 
at the tips of the arms (EupacAycrinus, Erisocrinus, Hydreionocrinus). 

Jlost of the Camerata (an order limited to the paleeozoic age) have biserial 
arms. But by far the greater number of the Lower Silurian species have uniserial 



6r br^ bt' 



Fii:. 323.— Part of tlie arm 
of a Crinoid. Diagram showing 
tlie transition from tlie uniserial, 
through the alternating, to the 
biserial arrangement of the 
brachials. 




2 

Via. 324.— Part of the disc formed ty the arms 
of Crotalocrinus rugosus (after Wachsmuth and 
Springer). 2, The trabecnlii; connecting the arms ; 
br, the arms with the covering plates icpa) over their 
food grooves ; in 3 these covering plates are removed. 



arms. In the Upper Silurian, however, but few forms persisted with such arms, 
and they are found side by side with species and genera with alternating, or with 
two rows of, brachials. 

In Crinoids whose arms have two rows of joints, the uniserial and the alternate 
stages are passed through ontogenetically. It must, further, he specially emphasised 
that not a. single case is known of arms being formed of two rows of brachials 
throughout their whole length, i.e. from the radials of the calyx to their tips. At 
their bases the arms always, for a certain distance, have a single series of brachials, 
then they have alternating brachials, and finally two rows. The transformation of 
the uniserial arm into an alternate, and finally into a biserial oire commences, 
ontogenetically and phylogenetically, at the tip of the arm, and proceeds from that 
point towards the base. 

The food grooves of the arms resemble those of the calyx. They are sometimes 
naked and open, and at others provided with a variously developed ambulacral 
skeleton, consisting either only of lateral plates, or of lateral and covering plates. 
Subambulacral plates may also occur in the floor of the food grooves, dividing them 
from the subjacent organs of the ambulacral furrows of the skeleton (body cavity of 
the arms, genital strands, pseudohfemal canals, etc.). Where covering plates are 
present there are two rows which alternate and interlock in such a way as to form a 



via EGHINODERMATA— MORPHOLOGY OF SKELETON 373 

median zigzag line. Tliese plates can be raised and depressed in the living animal ; 
when they are raised the food groove is open, when depressed, it is shut. 

An altogether peculiar arrangement is found in the arras of the genus Crotalo- 
crlnus (Upper Silurian, England. Sweden), which is thought by some to belong to 
the Cumcrata. The free arms branch extraordinarily frequently, the separate branches 
being closely crowded together, and forming together a wide expanded coherent 
disc round the calyx, resembling the fully open corolla of a flower. As many as 500 
to 600 branches may in some forms reach the edge of this disc (C. rugosus, Fig. 324). 
Each ossicle of the arms has two lateral processes, which become connected with 
similar processes of the corresponding ossicles of the neighbouring arras or branches, 
so that the disc formed in this way by the .skeleton of all the free arms is lattice-like. 
At definite distances from the calyx the brachials are of e(iual length, so that they, 
as well as the sutures which lie between the consecutive brachials, seem to be arranged 
in regular concentric rings round the calyx. The whole brachial disc was very flexiljle, 
and coidd be rolled up over the calyx from its periphery. In C. pulchrr, the brachial 
disc falls into five broad radial lobes, which, when the disc closes over the calyx, over- 
lap like the petals of a bud. The food grooves are covered by double longitudinal 
rows of alternating covering plates. 

(c) The Stem (Columna). 

The great majority of Crinoids are attached to the bottom of the 
sea by means of a jointed stem. Among recent Crinoids only the Ante- 
donicke and Thaumatocrinns are, in the adult condition, non-pedunculate 
and unattached. The stalked condition is undoubtedly the more 
primitive, for (1) the Crinoids show very markedly the habitus 
characteristic of many attached animals, and (2) all free and unstalked 
Antedunidce pass through an early stalked and attached stage. The 
stem, which varies greatly in length and thickness, consists of a series 
of calcareous ossicles one above the other, the uppermost of which is 
connected with the centre of the apical system, and carries the calyx 
with its arms". 

The ossicles of the stem (columnals) vary greatly in shape. They may be flat and 
disc-like, or long and cylindrical ; sometimes they are gradually thickened towards 
each end in such a way as to resemble dice-boxes. Further, the columnals in 
different parts of one and the same stem may be very different. The external outline 
of the ossicles in transverse section is sometimes pentagonal, sometimes round, 
rarely elliptical. They are connected with one another more or less firmly by sutures, 
or else are movably articulated. The stem throughout its whole length is pene- 
trated by a central canal (axial canal), which thus runs through all the consecutive 
columnals. Within this canal run the ccelomic canals (continuations of the chambered 
organ) and nerves. The size of the canal in transverse section differs as much as 
its shape. The outline of its section seems most frequently to be pentagonal or quinque- 
lobate, but it is not infrequently round. Occasionally also the central canal is sur- 
rounded by five narrower peripheral canals. 

New ossicles are added, as the animal grows, at the upper end ; at first they are 
small and flat, and often concealed within the stem. The most constant place of 
their appearance is between the uppermost coluranal and the base of the calyx. 
New ossicles may, however, also be intercalated between two already formed ossicles, 
but this almost always takes place at the upper end of the stem. In a gi-owing stem 
the ossicles in the upper part vary gi'eatly in length, the shortest being the youngest. 



374 



COMPARATIVE ANATOMY 



CHAP. 



At definite intervals the stem ma}' carry whorls of so-called 
eiPPi. These are jointed processes of the stem, pointed at their tips, 
and perforated by a longitudinal canal which communicates with the 
central canal of the stem (Figs. 257, 258, pp. 311, 312). 

The cirri are, as observations on living animals have shown, very mobile. Five 
cirri, as a rule, belong to one whorl, being inserted on the five sides of the nodal 
ossicle. Between two consecutive nodes there are a varying number of columnals 
which do not carry cirri. These together form an intemode. Whereas in the 
Lmdunata, Articulata, and Gamerata cirri are, as a rule, wanting, or only present 
at the lower part of the stem, in the Ganaliculata {Pentacrinidcc) nodes are found 
along the whole length of the stem between the consecutive internodes. In the 




Fig. 826.- 



-Diagram to elucidate Waohsmuth and Springer's rule. A, Crinoid with dicyclic 
base. B, Crinoid witli monocyclic base. For lettering see p. 317. 



recent species of Pentacrinus and Mctacrinus, each nodal ossicle is connected with 
the next ossicle of the intemode below it by a syzygial suture. 

Peculiar relations exist between the stem and the base of the apical capsule, 
according to the "rule of AVachsmuth and Springer," given in the diagram Fig. 325. 
In Crinoids with dicyclic base (i.i'. where the base consists of basals and infrabasals, 
with pentagonal stem and five-rayed central canal, the five edges or angles are interradi- 
ally arranged, while the five rays of the central canal and the five cirri of each whorl 
are radially arranged. In Crinoids with monocyclic base {i.e. where the base consists 
exclusively of the basals. Fig. 325 B) the reverse is the case. In those Crinoids which 
possess cirri, and in which the stem and central canal are not round, the character 
(monocyclic or dicyclic) of the base of the calyx can be determined — apparently with 
great certainty — from an examination of the stem. This is of importance in forms 
in which the infrabasals are very small, or, being covered by the uppermost joint 
of the stem, are hidden, or when they occur only in a young stage. Such forms are 
said to be constructed on a dicyclic plan, and have been called "pseudo-monocyclic." 
It is possible that certain genera in which "Wachsmuth and Springer's rule appears 
to be violated may eventually be proved pseudo-monocyclic. Meanwhile, however, 
the rule is not absolutely universal. 

The lower part of the Crinoid stem is called the root. It serves, 
in various ways, to attach the body to the sea floor. If the latter 



VIII EOHINODEEMATA— MORPHOLOGY OF SKELETON 375 



be muddy or sandy, the base of the stem puts out lateral branches, 
the so-called root-eirri, the numerous ramifications of which penetrate 
the sea floor in all directions. The end of the stem itself may at 
the same time branch like the root-cirri. "When the sea iloor is 
rocky, the root-cirri spread out more horizontally, accommodating 
themselves to the surface to which they are attached, and becoming 
cemented to it at their ends by means of a calcareous secretion. 

Further, it is almost certain that individuals of some species, e.g. Pentacrinida: 
and some Palfeozoic crinoids, are capable of free locomotion when the stem is either 
voluntarily or accidentally 

broken. Such locomotion is -£' C 

no doubt chiefly promoted by 
the movements of the arms, 
the cirri serving rather for 
attachment. 

In Holopus (Fig. 250, p. 
305) the stem is wanting. 
The calyx, which resembles 
a reversed cone in shape, is 
cemented to the substratum 
by means of an irregularly 
expanded calcareous mass. 

The Antedonidw are 
stalked and attached 
only in the young stage. 
The larval stem re- 
sembles that of the 
Bourgueticrinidse (Fig. 
326) ; the cirri are 
developed only on its 
uppermost ossicle, on 
which five radially ar- 
ranged cirri first appear, 
then five interradially 
arranged 

stage, differing in the 
different species, the 
calyx together with the uppermost columnal (i.e. the one carrying cirri), 
which is fused with the infrabasals to form the eentrodorsal, separates 
from the stem, the latter remaining attached to the place where it was 
fixed. Above the cirri already formed, i.e. between them and the base 
of the calyx, new whorls of cirri continuously appear on the centro- 
dorsal, which constantly increases in size, so that we are tempted to 
consider this piece as part of the stem, consisting entirely of nodal 
ossicles fused together without intervening internodes. 

The Comatulidae can both swim by the rowing movements of their arms, and 
creep by means of the cirri and of the ai-ms. They can, further, anchor themselves 
by their cirri, the arms being then directed upwards. 




Fig. 826. — Several stalked young stages of Antedon 
At a certain phalanglum (A) ; Antedon spec. (B) ; Antedon tuberosa (C) ; 
and Antedon multispina (D), after P. H. Carpenter. For 
lettering see p. 317. cia, Points of attachment of the cirri. 



376 COMPARATIVE ANATOMY chap. 



(d) The Manner of Connection between the Skeletal Pieces.^ 

Under this head we have to consider the method of connection between the 
ossicles of the arms and of the pinnulse, between the plates of the apical capsule, and 
between the cohmmals. 

Perhaps the clearest view of the great diversity which prevails in this matter is 
obtained by assrmiing that the plates composing an echinoderm skeleton develop 
in a stroma of connective tissue fibrils ; all the plates niiglit thus be supposed to have 
been originally but loosely united by such fibrils. 

This condition persists in what is known as the loose suture. The ossicles of the 
jiinuules in many living Crinoids are united in this way. 

From this loose suture we can obtain all the many variations which are found 
either in the direction of greater rigidity or of gi-eater flcxilnlity. 

In tlie former ease we have : — 

1. The close suture, also known as synostosis, in which the connecting fibres 
are short, and "the joints closely and immovably fitted together, though they can be 
separated by the action of alkalies," e.g. the radials of Antedon. 

2. The syzygy, which is a special case of close suture, viz. that in w"hich, if 
pinnules or cirri are carried hj the ossicles, the lower one loses its pinnule or cirrus. 

The two components of a syzygy are termed epizygal and hypozygal. 

3. Anchylosis, in which the tAVO jilates or ossicles are immovably cemented 
together by an unbroken deposit of calcareous substance, which, hoAvever, is less 
solid than that of the plates themselves (e.g. the basals of Bathycrinus and the 
radials of Rhizocrinus). 

On the other hand we have the modifications in the direction of greater flexibility 
which lead up gradually to the development of true muscles, the original undiffer- 
entiated fibrils becoming muscularly contractile. Such muscular articulations may 
indeed be very highly sjiecialised, with interlocking ridges and teeth on the opposed 
facets of tlie ossicles. 

As all these different modifications pass into one unother by imperceptible stages, 
it is not always easy to say whether in any special case we have to do with nniscular 
articulation or with a less specialised form of connection. It now seems probable 
that many of the fibrous connections which were at one time thought to be only 
elastic fibres are really muscles. 

For instance in the arms of living Crinoids, a pair of muscles on the oral side are 
counteracted by a pair of bundles of "elastic " fibres on the dorsal side. The action of 
the muscles in contracting rolls the arms up orally, and on the muscles rela.xing, the 
" elastic " fibres expand the arms again. Now it is clear that if these fibres were 
thus simply clastic, Crinoids would die with their crown of tentacles expanded, 
whereas the reverse is the case. 

Again, the cirri are actively movable, often {e.g. in Pentacrinus) more so than 
the arms, although no muscular articulations occur in them. 

From these facts it is rightly argued that the fibrils in these cases, though ditt'er- 
ing histologically from the true muscles, are yet to some extent muscular. 

That all these various connections are in reality derivations from some common 
primitive form of connection, we gather from the fact that in the stems of Crinoids 
we may have anchylosis, close suture, syzygy, loose suture, and true muscular 
articulation. 



^ This passage was rewritten in accordance with the tenour of Mr. Bather's criticism 
in Natural Science, vol. vi. pp. 420, 421. — Tr. 



VIII EGHINODERMATA—MORPHOLOGY OF SKELETON 377 

(f) The Nerve Canals of the Arms and of the Apical Capsule. 

(Figs. 327-330). 

The skeletal joints of each arm (the brachials) are perforated by 
an axial canal, which is continued to the tip of the arm, and into the 
pinnulfe. Where the arms fork or branch in various ways, the axial 
canal does the same. It contains nerve strands, and may thus be 
suitably considered as a nerve canal. This canal is continued right into 
the base of the dorsal cup, perforating the radials, basals, and also the 
infrabasals, when present. All the nerve canals, and the nerves within 
them, converge towards the apex of the calyx, where, either in the base 
of the dorsal cup itself (surrounded by the basals in stalked Crinoids), or 
partly enclosed within the centrodorsal {Ankdonidce), lies the central 
organ of this nervous system, which surrounds the so-called five- 
chambered sinus in the shape of a cup or capsule. From this point 
the already mentioned central, or axial, canal runs through all the 
ossicles of the stem, giving off lateral branches to the cirri. 

The nerre strands arise from this apical central nervous system at five interradial 
points. The five interradial strands fork either in the basals or in tlie radials. 
"Within the radials each branch of a strand becomes connected with a branch of the 
neighbouring strand, and from these radially arranged points of junction the radial 
nerve strands originate which pass from the radials into the costals, and are 
continued into the ossicles of the arms and of their branchings. AVitliin the circle 
of radials there is, besides, a. circular commissure between the radiating nerve 
strands ; the following diagrams illustrate the courses taken by these. 

In the Fentacriiiidw, EncrinidcE, and AnJrdoiiida;, where the nerve strands 
divide in the first axillaries, there is a peculiar chiasma nervorum brachialium, 
which is shown in the diagrams. 

In Eiicrinus, and it is said also in Pentacrinus, the nerve strands which run 
through the ossicles of the arms are double. But whereas, in Encriims, tliey run 
separately, and are enclosed in separate canals, in Pentacrinus they lie in a common 
canal. 

Many palfeozoie Crinoids, and above all the Camerata (witli the exception of the 
Crotalocrinoids), appear to have no differentiated nerve canals in the arms. 



(/) The Water Pores. 

In the Canaliculata {e.g. Pentacrinus, Aiitedon, Adinometra) the 
tegmen calycis, whether naked or plated, is perforated by so-called 
water pores, whose significance will be discussed more in detail 
later on. 

If the tegmen is plated, many or all of the plates of the interambulacral areas 
are perforated by one or more such pores. In Pentacrinus dcforns, as many as twenty 
pores are found on one plate. The total number of pores varies greatly in different 
genera and species. In Ant^don rosacea it has been estimated at 1500, and in other 
forms may be even greater. The pores are usually limited to the tegmen, and are 
least plentiful in the posterior interradius. They may, however, also occur on the 



L 




Fia. 32' 



Figs. 827-330. —Diagrams to illustrate 
tlie course of the axial canals and the 
nerre strands within them in the dorsal 
cup and the first hraohial joints of En- 
crinus (Fig. 327, after Beyrioh), Rhizoorinus 
lofotensis (Fig. 32S, after P.' H. Carpenter), 
Antedon rosaoeus (Fig. 329) and Bathy- 
crinus aldrichianus (Fig. 330, after P. H. 
Carpenter). In Fig. 327 only the distal ends 
of the Interradial canals are represented. 
The parts which are transversely streaked 
rnn superficially on the inner side of the 
basal plates. 





Fig. 330. 



CH. VIII ECHINOBERMATA— MORPHOLOGY OF SKELETON 379 

edge of the calyx between the bases of the arms, and in the genus Actinomclra, 
where they are chiefly developed near the ambulacral furrows, they have occasionally 
been observed on the lowest pinnulte as well, and even on pinnuloB in the middle 
or towards the ends, of the arms. 

In lihizocriii »s, in each interradius of the tegmen calycis there is only one water 
pore perforating the oral plate. In Hyocrinvs the anal oral plate is perforated by 
two pores ; the other oral plates either have one jiore each or else none at all. 
Further, in this genus, 2 to 7 pores occur in the plates of the interainbulacral areas 
lying between the oral pyramid and the edge of the calyx, except in the posterior 
interambulacral area, where they are wanting. 

It is impossible to decide with certainty whether the pores which, in certain 
Camei'ata (Actinocrinidm, Melocrinidci:, lUwdocrinida), occur at tlie edge of the calyx at 
the bases of the arms (and correspond with the arms in number) are the homologues 
of these water pores. The same applies to the slit-like pores which are supposed by 
some writers to perforate the edges of the plates of the ventral sac in the Inadunata 
fistvJata (along the sutures), and to the pores which are found along the brachial 
furrows in the Inadunata larviformia. These pores may, in some cases, have 
been connected with hydrospires (see the next two sections). In many Inadunata 
there is what appears to be a true madrejioric plate in the posterior interradius of 
the tegmen. 

VI. Blastoidea. 

One part of the perisomatic skeleton of the Blastoidea has already 
been described in connection with their apical system. There are 
five intepradial or deltoid plates, which surround the oral region 
(peristome), and radiate out from it (Fig. 331). These deltoid plates 
do not form a closed circle, i.e. do not touch one another laterally, 
but are separated from one another by the proximal portions of the 
five ambulacra. 

In describing the rest of the perisomatic skeleton, which, apart 
from the stem, consists exclusively of the skeleton of the ambulacra, 
it is useful to select a few typical forms. 



(a) The Ambulaeral Slieleton. 

1. Pentremites. — Fig. 263, p. 314, shows a representative of 
this genus in profile. Fig. 331 from the oral side. The five ambulaeral 
regions, or ambulacra, together form a five-leaved rosette round the 
peristome (Fig. 331, A, B, C, D, E). They are separated from one 
another by the five (interradial) deltoid plates (3). The larger, distal 
part of each ambulacrum fits in between the two limbs (10, 10'') of 
the forked radials. 

On the egg- or pear-shaped body the ambulacra stretcli as far as 
to the equator, or even further, towards the apical pole. 

The skeleton of each ambulacrum, when most complete, consists 
of the following parts : — 

{a) One lancet plate (6). 

(J)) One lower lancet plate (12). 



380 



COMPARATIVE ANATOMY 



CHAP. 



((■) Two rows of side plates (5). 

(d) Two rows of accessory side plates (8). 

(e) Two rows of pinnule (2). 

(/) Two groups of folds of' hydrospire pouches 
(g) A double row of covering plates (1). 



13) 



Ji. 




Fig. 331.— Diagram of tlie organisation of a Pentremites (origintLl). A, B, C, D, E, the five 
ambulacra. A, Aiiibiilacruni with covering plates (1) and extended pinnulw (2). B, Ambulacrnin 
with depressed pinnulic. C, Ambulacrum after removal of the pinnulie and covering plates. D, 
After tlie further removal of the side plates and accessory siile plates (except 3). E, After removal 
of the lancet plate as well. In the centre is seen the mouth with the spiracles surrounding it, in 
the po^tpriur interradius tht* anus. 1, Covering plates; 2, pinnulte; 3, deltoid plates; 4, their 
sloping ambulacral edges ; 5, side plates ; G, lancet plate ; 7, pores ; 8, outer or accessory side 
pieces ; 9, furrow of unknown significance on each side plate ; 10, radials = fork plates ; 11, aperture 
of the ambulacral canal ; 12, lower lancet plate ; 13, hydrospire folds. 

Passing over the covering plates, which are very rarely retained, we have, at the 
centre of each ambulacrum, a skeletal plate about half as wide as the ambulacrum 
itself, and more or less similar in shape. This is the so-called lancet plate (Fig. 



vm EGHINODEEMATA-^MOBPHOLOGY OF SKELETON 381 

331, 6). On its outer, i.e. oral surface, this plate shows a more or less deep longi- 
tudinal furrow, which gives off alternating lateral furrows to right and left. This 
longitudinal furrow of the laniet plates is universally considered to have the same 
significance as the food grooves on the tegnien calycis, and on the arms, of Crinoids. 
Each lancet plate is longitudinally pierced by a canal, the so-called ambulacral 
canal. 

The space on each side between the lancet plate in tlie middle, and tlie lateral 
margin of the ambulacrum (which latter is formed by the sloping edges of a deltoid 
jilate and of one limb of a fork plate (radial)), is occupied (a) by a longitudinal row 
of large side plates (5), and {b) by a longitudinal row of smaller accessory aide 
plates (8). The number of side plates and accessory side plates cori'esponds witli 
tliat of the lateral branches of the median ambulacral furrow on the lancet 
plate. Each side plate consists of a narrow portion which is directed towards the 
edge of the ambulacrum, and of a broad portion which is in contact with the lancet 
plate. The broad parts of the consecutive side plates in a row are in contact with 
one another, but between the narrow parts of these same plates are spaces, in each 
of which lie an accessory side plate, and a hydrospire pore (7). This latter leads 
below the surface to the hydrospire pouches under the ambulacrum. The hydro- 
spire pores, the accessory side plates, and the narrow ends of the side plates follow 
one another regularly in tliis order. 

The margins of the ambulacra carry thin long jointed appendages, 
the pinnules (2), which may be compared with the structures of the 
same name in the Crinoids. 

The pinnules ai'C retained only in I'arc cases, and are then depressed from the 
two sides orally over the ambulacral area (ambulacrum B, Fig. 331). There can, 
liowever, be no doubt that they could be raised and opened out (ambulacrum A). 
The number of the pinnules corresponds with that of the side plates in a longitudinal 
row, as also with that of the accessory side plates and of the hydrospire pores. The 
points of attachment of the pinnuhu lie between the consecutive pores. Each 
pinnule consists of a large number of skeletal pieces, which, near the base, alternate 
in two rows, bnt above this are arranged in one row. 

If the lancet plate of an ambulacrum is removed (E, Fig. 331), 
the (smaller and thinner) lower lancet plate (12), which lies close below 
it, appears at the surface. 

This plate resembles the lancet plate in shape. If the pinnule, the side plates, 
and the accessory side plates be removed, the edges of the plates which border the 
ambulacrum (deltoid plates, limbs of the fork pieces) are seen sloping towards the 
floor of the ambulacram. These sloping edges carry a longitudinal row of transverse 
ridges, alternating with depressions, into which latter the narrower (outer) portions 
of the side plates fit. On each side, between the lower lancet plate and the sloping 
lateral walls of the ambulacrum, some of the parallel clefts and folds of the hydro- 
spire pouches (13) are seen lying parallel to the longitudinal axis of the ambulacrum. 
At the central or proximal end of the ambulacrum {i.e. near the peristome), tlie 
interradial deltoid plates meet, and are joined by a radial suture. An aperture in 
this suture, the ambulacral aperture (11), leads to the interior of the calyx. Through 
this ambulacral aperture, the ambulacral canal, which traverses the lancet plate 
longitudinally, is connected with a circular canal which surrounds the cesophagus. 

In closest proximity to the peristome there are five large inter- 



382 



COMPARATIVE ANATOMY 



CHAP. 



radial apertures, the so-called spiraeles. Each of these apertures 
leads into the hydrospire pouches in such a way that the halves of 
two adjoining ambulacral areas have one common spiracle for their 
hydrospire pouches. 

Each spiracle forms a depression in the central part of the corresponding deltoid 
plate, and is further bordered by the proximal side plates, and by the proximal 
ends of the lancet plates. Occasionally, each spiracle is more or less distinctly 
divided into two by a vertical median ridge (septum) projecting into it from the 
deltoid plate. In the posterior interradius, the spiiacle is confluent with the 



The hydrosplpes (Fig. 332) are calcareous pouches or tubes 
united in groups. 

A group of such calcareous pouches is arranged symmetrically on each side of 
the middle line of, and in close connection with, each ambulacrum. The pouches. 



1 \ 




Fig. 33-2. — Section througli an ambulacrum of Pentremites, diagrammatic. 1, Deltoid, while 
lower down is tlie radial ; 2, hydrcspire pore ; 3. accessory side plate ; 4, side plate ; ^, lancet plate, 
witli its ambulacral canal 7 ; 6, covering plates ; S, common channel, into which the hydrospire 
pouches (11) enter at 9 ; 10, base of a pinnula ; 11, hydrospire pouches ; 12, lower lancet plate. 

which hang down into the cavity of the calyx, are parallel to one another, and 
stretch from the distal end of the ambulacrum to its proximal end, as far as to the 
spiracle, through which they open externally. In addition to this opening, each 
hydrospire pouch possesses a slit-like aperture extending along its whole length, in 
the ambulacral area. These hydrospire folds are hidden, lying partly under the side 
plates and partly under the lancet plate. After the removal of these latter pieces, 
as we have seen, they appear at the surface. They vary in number from three to 
nine. The concealed hydrospire canal (8), into which, on eacli side, the hydrospire 
pouches open through their clefts, communicates with the exterior by means of the 
hydrospire pores already mentioned. 

The hydrospire pouches or tubes have thus a double manner of communication 
with the exterior, viz. through the five or ten spiracles round the mouth, and 
through the numerous hydrospire pores at tlie lateral edges of the ambulacra. 

In certain species, the peristome was overarched by a roof of covering plates, 
for the most part irregularly arranged (Fig. 331, 1) ; at their centre, five oral plates 



VIII EGHINODERMATA—MORPHOLOGY OF SKELETON 383 

can sometimes be distinguished. The covering plates, which are rarely retained 
complete, are occasionally continued on to the food grooves of the ambulacra, where 
they are arranged in two longitudinal rows. Perhaps they could be raised and 
depressed ; if not, it is difficult to see how the food groves with their lateral furrows 
could function : the lateral furrows would then have to pass under the covering 
plates so as to be in communication with the principal groove. In rare cases the 
covering plates even spread sideways over the spiracles. 

■1. Codaster (Fig. 265, p. 314).— The arrangements here differ considerably from 
those of Fentrcmilcs, just described. The food gi-ooves are deeply sunk into the 
lancet plates, which are hollowed out on each side for the reception of the side plates. 
Spiracles are wanting. A certain number of the hydrospire clefts which run parallel 
to the ambulacrum always appear at the surface of the calyx, laterally to the 
ambulacrum (Fig. 333). These clefts run at right angles across the suture between 




Fig. 383. — Transverse section tirougli an ambu- 
lacrum of Codaster (after Etherldge and Carpenter), 
diagram. 1, Deltoid plate or possibly a radial ; 2, aiiibu- 
lacral canal ; 3, food groove ; 4, lancet plate ; 5, side plate ; 
n, apertures of the hydrospire pouches ; 7, lower lancet 
plate ; S, hydrospire pouches. 



Fio. 334.— Eleutherocrinus Cas- 
sedayi, from tlie oral side (after Ethe- 
ridge and Carpenter), aa-ljb, Axis, 
passing through the mouth and anus ; 
;', radials ; ir, interradials ; ri, the 
radial of the differently shaped am- 
bulacrum ; z, y, the two larger basals. 



the radial and the deltoid plates. One or more hydrosjjire clefts may be covered by 
the side plates of the ambulacra. At those sides of the two posterior ambulacra 
which are turned towards the anus, the clefts are altogether wanting. 

3. In Orophocrinus (type 0. stdliformis, Fig. 266, p. 314) there are no hydro- 
spire pores on the ambulacra, but depressions occur between the consecutive side 
plates for the reception of the bases of the piunules. The hydrospire clefts lie quite 
hidden below the surface within the ambulacral sinus, covered by the lower lancet 
plate. The spiracles, on the contrary, which are ten in number, appear at the sides 
of the ambulacra as long wavy slits. The two spiracles of the posterior interradius 
are distinct from the anus. The ambulacra, round the mouth at least, are covered 
by covering plates. 

4. The Irregulares {Astrocriims and Eleutherocrinns) are chiefly distinguished 
by the fact that one of the four ambulacra is developed quite differently from the 
others (Figs. 334, and 267, p. 315). 



384 GOMFARATIVE ANATOMY chap. 

The bundles of hydrospire tubes or pouches of the Blastoidea 
have been compared with the " bursae " of the Ophiuroidea. They are 
said to have served, like these latter, for respiration and for the 
ejection of the genital products. The similarity of these apertures is 
specially marked if we compare Orophocrimis with an Ophiurid. 

(b) The Stem. 

With the exception of the genera Pentephtjllum, Eleutherocrinus, 
and Astrocrinus, which, at least in the case of all known adults, were 
non-pedunculate, the Blastoidea were attached to the substratum by 
a jointed stem without cirri (Fig. 264, p. .SI4). 



VII. Cystidea. 

The study of the skeleton of this ancient class, which is limited to 
the palaeozoic age, has no very great comparative interest. The organi- 
sation of the very heterogeneous groups which are classed together 
under this heading can only to a small extent be understood from 
their skeletal remains. According to the structure of the skeleton, we 
can perhaps distinguish two principal groups : the Cystocrinoidea, whose 
skeleton consists of comparatively few definitely arranged plates, and 
which in some forms approximate the Grinoidea ^ and the Eucystidea, 
whose skeleton consists of a very large number of plates, showing no 
definite recognisable order. 

It is characteristic of most Cystidea, that all or some of the 
plates of the skeleton are perforated in various ways by pores, which, 
however, never seem to establish communication between the interior of 
the calyx and the exterior. It is difficult to ascertain the significance 
of these pores. They could not serve for the passage of the ambu- 
lacral feet, since the pore canal, as already said, does not stand in 
direct communication with the interior of the calyx. It is now pretty 
generally accepted that, as the water passed through them, they served 
for respiration. The following principal forms of pores can be 
distinguished : — 

1. Scattered single pores. 

2. Scattered double pores (the pores being united in pairs) (Fig. 
260, p. 312). 

3. Double pores arranged in rhombs. In this case, the two pores 
of a double pore are found on two neighbouring plates, and are con- 
nected by a furrow or a canal, which sometimes runs at the outer 
and sometimes at the inner side of the plate. This canal or furrow 
lies at right angles to the suture between the two plates, and 
the suture itself lies diagonally to the rhomb formed by the pores. 
Such pore - rhombs may occur on all the plates of the Cystid 
test, or again may occur singly. In the latter case, the two halves 



vm ECHINODERMATA— MORPHOLOGY OF SKELETON 385 

of a rhomb are not infrequently divided by a smooth intermediate 
region. 

Since it is difficult or even impossible to give any comprehensive description of 
the perisomatic skeleton of the Cystidea, it is advisable to treat a few of the best 
known forms separately. 

Cystoorinoidea. 

Porocrinus is a form which only differs essentially from a simple Crinoid of the 
order Inadunata in the presence of the pectinated rhombs. 

Caryocrinus. — The calyx is almost pear-shaped, and is carried by a long stalk 
perforated by a wide axial canal. At the edge of the calyx there are 6 to 13 
thin, jointed, biserial arms, each provided with a furrow on the oral side. The 
hexagonal dorsal cup, apart from two plates lying interradially in the circle 
of the radials, consists exclusively of the already described plates of the apical 
system (four infrabasals, six basals, six radials). The tegmen calycis is formed of a 
great number of plates, and at their centre six orals, which show the characteristic 
arrangement before described (Fig. 294, p. 332), completely cover the mouth. The 
ambulaoral furrows are not externally visible. An excentrically placed aperture, roofed 
over by a pyramid formed of six triangular plates, is regarded as the anus. The 
pectinated rhombs are found on all the plates of the dorsal cup, and on them alone. 
The two pores of a double pore are connected by a canal on the inner side of the plate. 

Echinoencrinus. — The almost egg-shaped calyx consists of the already described 
plates of the apical system (four infrabasals, five basals, five radials) arranged in five 
rays, and five other perisomatic plates in contact with the circle of the radials, and 
partly pressing in between them. At the oral pole there is a depression, round which 
short uniserial tentacles rise, and in the centre of which the star -shaped oral 
aperture lies. The anal aperture has shifted across the equator of the calyx 
apically, and lies to the right posteriorly above the basals. Three pectinated 
rhombs are developed, whose position is shown in Fig. 296, p. 333. The calyx is 
carried by a short thick stem, the ossicles of which are perforated by a wide canal. 

Cystoblastus (Figs. 259, p. 312, and 295, p. 332) shows some resemblance to 
the Blastoidea. The egg-shaped or spherical calyx consists of sixteen plates and the 
ambulacra. Of these sixteen plates, which are arranged in four circles, fourteen 
belong to the apical system (four infrabasals, five basals, and five radials). The 
radials exactly resemble the radials (fork plates) of the Blastoidea. Each radial 
clasps an ambulacrum between its two limbs. Between two neighbouring radials, 
in four of the interradii, a lancet-shaped plate, which is keeled in the middle, 
presses in, recalling the deltoid plates of the Blastoidea. In one interradius this 
plate is wanting, so that here the neighbouring radials touch one another. At the 
centre of the rosette formed by the five ambulacra lies the mouth, and from it a 
furrow runs to each ambulacrum, passing down its whole length and dividing it into 
two lateral halves. The principal groove of each ambulacrum gives off alternating 
lateral furrows, which end in distinct p