ifl
sassst
)GY LIBRARY
TEXT-BOOK
OF
COMPARATIVE ANATOMY
BY
DR. ARNOLD LANG
PROFESSOR OF ZOOLOGY IX THE UNIVERSITY OF ZURICH
FORMERLY RITTER PROFESSOR OF PHYLOGEXY IX THE UNIVERSITY OF JEXA
TRANSLATED INTO ENGLISH BY
HENRY M. BERNARD, M.A. CANTAB.
AND
MATILDA BERNARD
PAET IL
ILontron
MACMILLAN AND CO, LTD.
XEW YORK : MACMILLAX & CO.
1896
All rights reserved
QL9
V.
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 Echinodermata. 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. AVoodward 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 Jeffery 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 much 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
Nachwort which appeared at the end of the fourth German part.
In it the author answers the only serious charge against the work
as a text-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. BEENAED.
AUTHOB'S PEEFACE TO THE SECOND VOLUME
\YiTH 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 University 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.
AKNOLD LANG.
ZUKICH, July 1894.
CONTENTS
CHAPTER VII
MOLLUSCA
PAGE
Systematic Review ........ 2
Class I. AMPHINEURA . . . . . . . 2
II. GASTROPODA (CEPHALOPHORA) ..... 3
III. SCAPHOPODA ....... 13
IV. LAMELLIBRANCHIA (PELECYPODA, BIVALYA, ACEPHALA,
AGLOSSA) . . . . . 14
V. CEPHALOPODA . . . . . . .21
I. ORGANISATION OF THE PRIMITIVE MOLLUSC ... 26
II. REVIEW OF THE OUTER ORGANISATION CHARACTERISING THE
CHIEF GROUPS OF THE MOLLUSCA .... 28
A. PLACOPHORA OR POLYPLACOPHORA (CHITONID^E) . . . .29
B. APLACOPHORA, SOLENOGASTRES ..... 29
C. GASTROPODA (CEPHALOPHORA) ..... 30
D. SCAPHOPODA . . . . . . .34
E. LAMELLIBRANCHIA ....... 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. LAMELLIBRANCHIA ....... 49
F. CEPHALOPODA ....... 53
IV. THE SHELL ... ... 55
A. AMPHINEURA .....'.. 58
B. GASTROPODA 58
: COMPARATIVE ANATOMY
PAGE
C. LAMELLIBRANCHIA ....... 61
D. CEPHALOPODA ....... 67
V. ARRANGEMENT OF THE ORGANS IN THE MANTLE CAVITY, AND
OF 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. AMPHINEURA . . . . . . .86
B. GASTROPODA ....... 88
C. LAMELLIBRANCHIA . . . . . . .91
D. CEPHALOPODA . . . . . . .96
ADAPTIVE GILLS . . . . 97
LUNGS ........ 99
VII. THE HYPOBRANCHIAL 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 . . . . . . .115
XL SWELLING OF THE FOOT (Turgescence) . . . .118
XII. MUSCULATURE AND ENDOSKELETON .... 119
A. AMPHINEURA . . . . . . .120
B. GASTROPODA . . . . . . .120
C. SCAPHOPODA . . . . . . .123
D. LAMELLIBRANCHIA . . . . . .124
E. CEPHALOPODA . . . . . . .126
XIII. THE NERVOUS SYSTEM . . .128
A. AMPHINEURA ....... 128
B. GASTROPODA 132
CONTENTS xi
PAGE
1. THE AREAS OF INNERVATION OF THE VARIOUS GANGLIA . 133
2. ORIGIN OF THE CROSSING OF THE PLEUROVISCERAL CON-
NECTIVE (CHIASTOXEURY) . . . . V 135
3. SPECIAL REMARKS ON THE NERVOUS SYSTEM OF THE GAS-
TROPODA ....... 137
C. SCAPHOPODA . . . . . . .142
D. LAMELLIBRANCHIA ....... 143
E. CEPHALOPODA ....... 145
XIV. AN ATTEMPT TO EXPLAIN THE ASYMMETRY OF THE GASTROPODA 149
XV. THE SENSORY ORGANS . . . . . .162
A. INTEGUMENTAL SENSORY ORGANS .... 162
1. TACTILE ORGANS ...... 162
2. OLFACTORY ORGANS ...... 162
3. THE "LATERAL ORGANS" OF THE DIOTOCARDIA . . 165
4. GUSTATORY ORGANS . . . . . .166
5. SUBRADULAR 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 OF ARCA AND PECTUNCULUS . 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 DIGESTIVE GLAND
(LlVER) ........ 190
1. AMPHINEURA ....... 191
2. GASTROPODA ....... 192
3. SCAPHOPODA ....... 193
4. LAMELLIBRANCHIA ... . 194
5. CEPHALOPODA ....... 194
E. HIND-GUT (RECTUM ...... 195
XA~II. THE CIRCULATORY SYSTEM ...... 198
A. GENERAL . 198
xii COMPARATIVE ANATOMY
PAGE
B. SPECIAL ........ 201
1. AMPHINETJRA . . ... . . .201
2. GASTROPODA ....... 201
3. SCAPHOPODA ....... 206
4. LAMELLIBRANCHIA ...... 206
5. CEPHALOPODA ....... 208
XVIII. THE BODY CAVITY . . . . . . .211
XIX. THE NEPHRIDIA . . .... 215
A. AMPHINEURA ....... 216
B. GASTROPODA . . . . . . .217
C. SCAPHOPODA ....... 221
D. LAMELLIBRANCHIA . . . . . . .221
E. CEPHALOPODA . . . . . . . 222
XX. GENITAL ORGANS ... ... 225
A. GENERAL . . .'.... . . . . 225
B. SPECIAL ........ 227
XXI. PARASITIC GASTROPODA ...... 244
XXII. ATTACHED GASTROPODA ...... 248
XXIII. ONTOGENY ........ 248
A. AMPHINEURA ....... 248
B. GASTROPODA ....... 252
XXIV. PHYLOGENY .... ... 268
Review of the most Important Literature . . . .269
APPENDAGE, RHODOPE VERANII . . . 281
CHAPTER VIII
ECHINODERMATA
Systematic Review . . . . . . . .285
CLASS I. HOLOTHURIOIDEA ..... 285
II. ECHINOIDEA ....... 288
III. ASTEROIDEA ....... 295
IV. OPHIUROIDEA ....... 299
V. PELMATOZOA ....... 302
1. CRINOIDEA ....... 302
2. CYSTTDEA ....... 313
3. BLASTOIDEA . . . . . . .314
I. GENERAL MORPHOLOGY or THE ECHINODERM BODY . . 315
II. MORPHOLOGY OF THE SKELETAL SYSTEM . 317
CONTENTS xiii
PAGE
INTRODUCTION .... 317
A. THE APICAL SYSTEM (Calyx) . . 319
1. ECHINOIDEA .... . 319
2. ASTEROIDEA ....... 326
3. OPHIUROIDEA ....... 327
4. PELMATOZOA ....... 328
(a) CRINOIDEA ....... 328
(6) BLASTOIDEA ....... 330
(c) CYSTIDEA ....... 332
B. THE ORAL SYSTEM OF PLATES ..... 333
C. THE PERISOMATIC SKELETON ..... 337
1. HOLOTHURIOIDEA ...... 337
2. ECHINOIDEA ....... 338
(a) THE NUMBER OF THE VERTICAL Rows OF PLATES . 339
(6) THE PORES OF THE AMBTJLACRAL SYSTEM . . 340
(c) THE SYMMETRY OF THE ECHINOID SHELL . . 340
(d) THE RELATION OF THE AMBULACRAL AND INTERAMBU-
LACRAL PLATES TO THE PERISTOME . . . 344
(e) MANNER IN WHICH THE SKELETAL PLATES ARE CON-
NECTED ....... 345
(/) SPECIAL MODIFICATIONS OF THE AMBULACRA . . 346
(g) SPECIAL MODIFICATIONS OF THE INTERRADII . . 348
(h) FORM OF THE PERISTOME ..... 349
(i) ORNAMENTATION . . . . . . 349
(&) MARGINAL INCISIONS OR PERFORATIONS . . . 349
(1) THE PERIGNATHIC APOPHYSIAL GIRDLE . . . 350
3. ASTEROIDEA ....... 351
(a) THE AMBULACRAL SKELETON . . . .351
(b) THE INTERAMBULACRAL SKELETON . . . 353
(c) THE ACCESSORY SKELETAL SYSTEM . . . 354
(d) COMPARISON OF THE PERISOMATIC SKELETON OF THE
ASTEROIDEA WITH THAT OF THE ECHINOIDEA . . 355
4. OPHIUROIDEA ......: 355
(a) SKELETON OF THE ARMS ..... 355
(b) THE ORAL SKELETON ..... 358
5. CRINOIDEA ....... 362
(a) THE PERISOMATIC SKELETON OF THE CALYX . . 362
a. THE APICAL CAPSULE OR DORSAL CUP . . 367
b. THE TEGMEN CALYCIS ..... 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
xiv COMPARATIVE ANATOMY
PAGE
(/) THE WATER PORES ..... 377
6. BLASTOIDEA ....... 379
(a) THE AMBULACRAL SKELETON .... 379
(6) THE STEM ....... 384
7. CYSTIDEA ....... 384
D. THE SPINES AND THEIR DERIVATIVES THE SPHJERIDIA AND
THE PEDICELLARI^G . ... 387
E. THE MASTICATORY APPARATUS OF THE ECHINOIDEA. (Aris-
totle's Lantern) ....... 400
F. THE CALCAREOUS RING OF THE HOLOTHURIOIDEA . . 403
G. FURTHER DEPOSITS OF 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 COSLOM ........ 436
A. THE BODY CAVITY . .... 437
B. THE BRACHIAL CAVITIES ...... 440
C. THE PERICESOPHAGEAL 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^EMAL SYSTEM . . . . . 447
IX. THE EPINEURAL SYSTEM . ... 448
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 461
CONTENTS xv
PAGE
XII. THE SENSORY ORGANS ...... 462
A. THE AMBULACRAL APPENDAGES AS SENSORY ORGANS . . 462
B. NERVE ENDINGS IN THE INTEGUMENT .... 466
C. AUDITORY ORGANS, ORGANS OF ORIENTATION . . . 468
D. EYES ........ 468
XIII. THE BODY MUSCULATURE . . . . . .470
A. HOLOTHURIOIDEA ....... 471
B. ECHINOIDEA .... . . 471
C. ASTEROIDEA ....... 472
D. OPHIUROIDEA ....... 474
E. CRINOIDEA ........ 474
XIV. THE ALIMENTARY CANAL ...... 474
A. GENERAL REVIEW ....... 474
B. HOLOTHURIOIDEA ....... 476
C. ECHINOIDEA ....... 479
D. CRINOIDEA ........ 481
E. ASTEROIDEA ....... 483
F. OPHIUROIDEA ....... 485
XV. RESPIRATORY ORGANS ...... 485
A. THE (INNER) RESPIRATORY TREES OF THE HOLOTHURIOIDEA . 487
B. REVIEW OF THE RESPIRATORY ORGANS OF THE ECHINODER-
MATA ........ 487
XVI. THE CUVIERIAN 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. OPHIUROIDEA ....... 494
1. THE BURBLE ....... 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 FOR REGENERATION AND ASEXUAL REPRODUCTION . 504
XXI. ONTOGENY ........ 506
A. THE VARIOUS LARVAL FORMS OF THE ECHINODERMATA 506
xvi COMPARATIVE ANATOMY
PAGE
B. ONTOGENY OF THE HOLOTHURIOIDEA . .510
C. ONTOGENY OF THE ECHINOIDEA ..... 519
D. ONTOGENY OF THE ASTEROIDEA . . . 524
E. ONTOGENY OF THE OPHIUROIDEA . . . 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 MEMBRANES, THE PROBOSCIDAL SKELETON, AND THE
BRANCHIAL SKELETON . . . . . .579
IX. THE BLOOD VASCULAR SYSTEM ..... 581
X. THE GONADS ...... .585
XI. ONTOGENY ........ 586
XII. PHYLOGENY ........ 591
Literature . ... . . . . . 595
APPENDAGE TO THE ENTEROPNEUSTA
I. CEPHALODISCUS ....... 596
II. RHABDOPLEURA . .... 600
Literature . . . . . . . .602
INDEX . . 603
CHAPTEE VII
SIXTH RACE OK PHYLUM OF THE ANIMAL KINGDOM
MOLLUSCA.
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, Avhich 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 coelom (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
cerebral, 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 Yeliger
larva, typical of the Mollusca.
These general characteristics of the Molluscan body have to be modified for each
class. In each class there are series of forms which deviate from the typical
organisation in 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 Molluscan 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 Pretre, in the Voyage de V'Astroldbe).
provided with ganglion cells throughout their whole length ; these pass anteriorly
into the cerebral ganglion. Special sensory organs are reduced. Marine.
ORDER 1. Placophora (Polyplacophora) sive Chitonidse.
On the dorsal side there are eight consecutive shelly plates overlapping like the
tiles on a roof. There is a distinct snout. The branchiae 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
MOLLUSCA SYSTEMATIC REVIEW
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. 1
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 iu which are found, when present, the
rudimentary gills. The nephridia serve as ducts for the genital products.
Family 1. Neomeniidae.
The foot is a longitudinal ridge, which rises from the base of a niedio-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), Neo-
menia, Lepidomenia, Dondersia.
Family 2. Chsetodermidae.
The foot and the pedal furrow are quite degenerated. The sexes are separate.
Chcetoderma.
CLASS II. 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 flat 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-
1 Simroth, in the new edition of Bronu's Ktassen und Ordnunyen des ThierreiclieS)
vol. iii., 1893, divides the Solenogastres as follows :
Sub-Order. Fain.
Chaatodermatina .... Chsetodermatida?.
Neomeniidae.
Proneomeniidae.
Dondersiidae.
Xeomeniiua
Parameniida?.
OF THF
UNIVJL,
COMPARATIVE ANATOMY
CHAP.
6
FIG. 3.-Margarita Groenlandica (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.
branchia, 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-
brancMa, 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
domei Jn mogt formg there ig on]
one gA 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 aperture of the shell. A shell is
wanting only in Titiscania, a genus
of the Neritacea.
Sub-Order 1. Diotocardia.
The heart has two auricles (except-
ing in Docoglossa). There are two
kidneys. Instead of the pedal ganglion
of other Gastropoda, there are two
longitudinal nerves in the foot, sup-
plied with ganglia and connected with
one another by numerous commissures.
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 are all wanting.
a. Zeugobranchia (Rhipidoglossa,
Aspidobranchia). Two gills ; both
auricles well developed. Heart tra-
versed by the rectum. Shell with FlG . 4 ._p a tella vulgata (from beneath, after
marginal cleft, or with apical perfora- Lankester). , Tentacle ; d, efferent branchial vessel
tion or with a row of perforations. c > free ed s e of the she11 ; e > free ed s e of the mantle
Generally without operculum. *-*> "^ "*' ?, afferent branchial reweta
* * i i /i branchial lamellae; h, one of the afferent vessels
Marine. Fam. ffahotidce, radula i? spaces be tween the shell muscles ; b, foot.
ool.(5.1.5)loo, Fissurellidce (Fissu-
rella, rad. ool.(4.1.4)l.oo, with secondarily symmetrical shell. Emarginula, Scutum
vii MOLLUSC A SYSTEMATIC REVIEW 5
= Par>nophonis), Plev rotomaridce (Pleurotomaria, Stissurella, Polytrcmaria), Bellero-
phontidce (exclusively fossil).
b. Azygobranchia. One gill, homologous with the left gill of the Zeugo-
branchia. Right auricle ending blindly. Heart perforated by the rectum. Fam.
Turbonidce, rad. a>0. (5.1.5. )0.oo , Trochidce (Fig. 3) St&matiidce, Neritopxidce, rad.
ool. (2.0.2.)!. oo , marine, Neritidce, rad. ool.(3.1.3.)l. (marine, but along the shore
able to live out of water), Neritince (marine and fresh-water). The Hydrocoenidce, rad.
ool.(l.l.l.)l.o), and Jfelicinidce, rad. oo 1. (4.1.4.)!. oo , have no gills but a lung
resembling that of the Pulmonata. The Helidnidce are terrestrial.
c. Docoglossa. 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 wanting. Marine.
Fi<~;. 5. Phorus exutus (after Lankester). a, Proboscidal snout or rostrum ; b, tentacle ;
. eye ; d, foot ; e, metapodium with operculum /.
1. Left true ctenidium present. Acmaeid.ee, rad. 1.2.(1.0.1.)2.1.; with numerous
accessory gills in the mantle furrow: Scurria ; without such gills: Acmaea
(Tectura).
2. True ctenidia altogether wanting, accessory gills very numerous in the mantle
furrow. Fam. Patellidce (Fig. 4), rad. 3.1. (2.0.2.)!. 3.
3. Xeither ctenidia nor accessory gills found (Lcpetidce), rad. 2.0.1.0.2.
Sub-Order 2. Monotocardia (Pectinibranchia).
Heart with one auricle. A single true ctenidium feathered on one side, the point
not projecting freely (except in Valvata). Pedal nerve trunks a rare 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. Cypraeidce, rad. 3.1.1.1.3, Paludinidce
(fresh-water), Cydvphoridce, (terrestrial, pulmonate).
COMPARATIVE ANATOMY
CHAP. VII
b. Taenioglossa. Typical radula, 2.1.1.1.2. Semiproboscidifera. Fam.
Naticidce (Fig. 98, p. 107), Lamellaridce. Rostrifera. Fam. Falvatidce (fresh-water),
Ampullaridce (fresh - water), Littorinidce, Cyclostomidce (terrestrial), Planaxidce,
Hydrobiidce (fresh- water), Aciculidce (terrestrial), Truncatellidce (partly terrestrial),
Hipponycidce, Capulidce, Calyptraeidce, Pseudomelanidce, Melanidce, Cerithiidce,
FIG. 6. Rostellaria rectirostris (after Owen). , Snout ; b, tentacle ; c, stalked eye ; d, foot ;
e, inetapodium with operculuin /; h, beak (for the siphon).
Vermetidce, Turritellidce, Xenophoridce (Fig. 5), Sir uthiolar idee, Chenopidce,
Strombidce (Fig. 6). Proboscidifera holostomata. Fam. Scalaridce, rad. ooQco ;
Solaridce, rad. ooQoo ; Pyramidellidce, rad. 0; Eulimidce, rad. 0. Proboscidifera
siphonostomata. Fam. Colombellinidce, Tritoniidce, Cassidiidce (Fig. 7), Doliidce.
Fie. 7. Cassis suclosa (after Poli). a, Shell; &, beak; c, siphon; d, head; g, proboscis; e,
eye ; /, tentacle ; h, foot ; i, operculuin.
Janthinidse, rad. ocO oo . Heteropoda (marine Taenioglossa, with foot transformed
into a perpendicular rowing fin). Fam. Atlantidce (Fig. 8), Pterotrachaeidce
(Fig. 9).
c. Stenoglossa. Normal rad. 1.1.1. Rachiglossa. Fam. Turbinellidce,
Fusidce, Mitridce, Buccinidce, Muricidce, Purpuridcc, Ifaliadca, Cancellariidce,
Volutidce, Olimdce, Marginellidce, Harpidce. Toxiglossa. Fam. Plcurotomidce.
Terebridce, Conidce.
FIG. S. Atlanta Peronii (after Gegenbaur). a, Pharynx ; b, buccal ganglion ; c, tentacle ; d,
eye ; e, cerebral ganglion ; /, aorta cephalica ; [/, pleuro-visceral connective ; h, columellar muscle ;
i, fc, osphradiuni ; /, vagina ; m, ctenidium ; n, anus ; o, uterus ; p, nephridium ; q, aorta cephalica ;
r, auricle ; s, ventricle ; t, aorta visceralis ; u, digestive gland (liver) ; v, ovary ; w, stomach ; x,
pedal ganglion ; y, operculum ; 2, metapodium ; 1, sucker of the fin-like foot (rudimentary sole) ; 2,
foot ; 3, auditory organ ; 4, oesophagus ; 5, snout ; 6, salivary gland.
21
FIG. 9. Pterotrachea (Firola) coronata (after Leuckart). a, Pharynx; b, proboscidal snout ;
c, eye ; d, cerebral ganglion ; e, pedal ganglion ; /, pedal artery ; g, intestinal canal ; h, pleuro-
visceral connective ; i, parieto-visceral ganglion ; t, osphradium ; 7, ventricle ; m, auricle ; ?z,
anus ; o, ctenidium ; p, metapodium ; q, appendage ; r, aorta cephalica ; 5, nerve running to the
metapodium ; f, artery ; n, foot ; r, common pedal artery ; u% cephalic artery ; a-, auditory organ ;
y, buccal ganglion.
COMPARATIVE ANATOMY
CHAP.
OHDER 2. Pulmonata.
The pleuro-viseeral connectives are not crossed. The ctenidmm 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, unites 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.
\^~~ r
FIG. 10. Amphipeplea leuconensis
(after Adams), a, Lobe of the mantle
bent back over the shell ; b, 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. Limnceidce, (Limncea,
Amphipeplea [Fig. 10], Physa [Fig. 11], Planorbis, Ancylus), Auriculidce.
FIG. 11. Physa fontinalis (after L. Reeve), a, Mantle lobes folded back over the shell ; b,
evaginated penis.
Eyes at the tips of the optic tentacles ; tentacles invaginable.
Sub-Order 2. Stylommatophora.
a. Monogonopora. With a single genital aperture to the right. Fam. Helicidse
(Helix [Fig. 12, A], Avion [Fig. 12, D\ Bulimus}. Testacellidse (Daudebardia
[Fig. 12, E\, Testacella [Fig. 12, C]. Limacidse (Ariophanta, Limax, Vitrina,
Zonitcs, Helicarioii). Bulimulidae (Fig. 13), Pupidse (Buliminus, Pupa, Clausilia},
Succineidse.
&. 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 right.
Pallial complex at the posterior end of the body, lung cavity reduced. Fam. Vagi-
nulidae (terrestrial), Oncidiidse (marine or amphibious) ; respiration partly by means
of dorsal branchial appendages.
MOILUSCASYSTEMA TIC REVIEW
Fit;. I*. A, Helix pomatia ; B, Daudebardia (Helicophanta) brevipes : C, Testacella halio-
tidea ; A Arion ater : s, shell, in D shield (from Lankester).
~^~\//
];. Peltella va.\liolum(BuUmnUd, after Ferussac).
10
COMPARATIVE ANATOMY
CHAP.
ORDER 3. Opisthobranchia.
The pleuro- visceral connectives do not cross. 1 There is one auricle placed behind
the ventricle. A shell is sometimes present, more frequently 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-
phrodites with ovotestes. Marine.
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
shell is always present, but tends to become rudimentary. Generally with para-
podia, and mantle lobes covering the shell.
A. Reptantia.
a. Cephalaspidse. With frontal or cephalic disc. Fam. Actseonidse (with
operculum), Scaphandridse, Bullidse (Sulla, Acera), Gastropteridae (Fig. 14),
Philinidae. Doridiidse.
6. Anaspidse. Head without frontal disc ; four triangular or ear-like tentacles.
Fam. Aplysiidse (Ajjlysia, Dolabella, Notarchus).
FIG. 14. Gastropteron Meckelii, FIG. 15. Pleurobranchus aurantiacus, with internal
with internal shell (after Vayssiere). shell (after Leuckart'S Wandta.feln), seen from the right
1, Cephalic shield (frontal disc) ; 2, para- side, ft, Rhinophores ; b, labial sail ; c, genital aperture ;
podium; 3, ctenidium, left almost un- d, nephridial aperture (?); e, ctenidium ;/, anus,
covered by the rudimentary mantle fold ;
4, flagellum = appendage of the mantle
fold.
c. Notaspidae. Head short, with or without tentacles. Large dorsal disc
(notseum) in or on which a shell may lie. Fam. Pleurobranchidse (Pleurobranchus
[Fig. 15], Pleurobranchcca, Oscainius), Umbrellidae (Umbrella, Tylodina), Peltidse.
B. Natantia sive Pteropoda. 2
These formerly constituted a separate class of the Molluscs, but are now recog-
nised to be Tectibranchia adapted to a free-swimming pelagic life. The parapodia of
the Tectibranchia develop as fins or wing-like swimming organs.
1 Except in Actceon, which is streptoueurous, and thus forms a connecting link
between the Opisthobranchia and Pulmonata on the one hand, and the remaining
Gastropods on the other [Bouvier and Pelseneer], v. Nat. Sci., July 1893.
2 The classification of the Opisthobranchs, which places the Pteropoda thecosomata
with the Cephalaspidse, and the Pteropoda gymnosomata with the Anaspidse, is accepted
on p. 110 and elsewhere.
VII
MOLLUSCA SYSTEMATIC REVIEW
11
a. Pteropoda thecosomata. These are nearly related to the Cephalaspidea,
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 mouth;
the anus lies to the left. Fam. Limacinidse. An external calcareous shell, with
left-handed or sinistral twist, and a spiral operculum. Anus to the right (Linia-
[Fig. 16], Peraclis). Fam. Cavoliniidse. External symmetrical shell (Clio,
Cavolinia). Fam. Cymbuliidse. Internal cartilaginous shell (Cymbulia, Cymbuli-
opsis, Gleba). The Thecosomata feed chiefly on small Protozoa and Algae.
CL
1
FIG. Iti. 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 tin (parapodium) ; 6,
seminal furrow ; 7, genital aperture ; 8, 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, ctenidium ; 17, posterior adaptive
gill ; d, v, o, p denote dorsal, ventral, anterior,
and posterior.
17
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 anus lies to the right. Fam.
Pneumodermatidse. One ctenidium to the right (Dcxiobranch/, ulp, primitive
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
afforded bv the shell.
left cerebral pleural and pedal ganglia ; iilpa,
urpa, primitive left and right parietal ganglia ;
nla, primitive left auricle ; iios, uros, primitive
left and right osphradia (Spengel's organ) ; itlct,
urct, primitive left and right ctenidia (gills) ; nib,
base of the mantle ; mr, edge of the mantle ; m,
protection mantle cavity ; r, visceral ganglion ; re, ventricle ;
Analogous
arrangements are to be found in other divisions of the animal kingdom,
vii THE HYPOTHETICAL PRIMITIVE MOLLUSC 27
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 branchiae 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
ctenidium.
The head is provided with one pair of tentacles and one pair of
eyes. The mouth lies anteriorly and ventrally. The remaining open-
ings of the inner organs lie posteriorly above the foot ; the anus in the
middle line, and on each side, between it and the ctenidium (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 salivary 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 divertieula (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 ganglion
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 cavity. The two visceral nerves
are connected posteriorly.
If we leave the Amphineura and Diotocardia 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 ganglia, 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
ganglia (over the fore-gut) ; (2) between the two pedal ganglia
(under the fore-gut) ; (3) between 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 eoelom 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 cavity. In the posterior 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 groups 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 the 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 MOLLUSC A OUTER ORGANISATION 29
A. Plaeophora OP Polyplaeophora (Chitonidse).
The body of the Placophora 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 foot with 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 Mollusca 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 Fissurdla, Oliva, Turritella, Cleodora.
Pterotrachea, Phyllirhoe, Limax, Pleurobranchus, Thetys, 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 hump
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.
FIG. 40. Diagram of the Organisation of a
Zeugobranchiate Diotocardian. a, Anus ; ve,
ventricle ; ula, right auricle ; nrct, left ctenidium ;
uros, left osphradiuin.
1.
Prosobranehia.
The large visceral dome is
coiled spirally, generally to the
right (dextrally), the shell naturally assuming the same form. The well-
VII
MOLLUSCA OUTER ORGANISATION
31
ol
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 opereulum, which, when the animal withdraws its head and
foot, closes the aperture
of the shell. The mantle ^
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 the bases of the gills.
In most Prosobranchia,
however, the organs just
mentioned as paired are
unpaired; only the gill, Fio.4i.-DiagramofaProsobrancMateMonotocardian. The
nephl'idial aperture, and outer form, shell, mantle, pallial complex, heart and pericardium ,
OSphradilim tO the left nervous system and opereulum, are depicted. Lettering mostly
as in Fig. 39. In addition : /, foot ; si, siphon ; sup, sub, supra -
01 tne anUS being re- ailc ^ sub -intestinal connectives; op, opereulum; ot, auditory
tained, while the hind- organ ; p, penis ; sr, seminal groove ; mh, mantle cavity ; hy,
gut with the anus moves **??Zf* ' 6 ' male genital apertnre ; r ' rectum ; au '
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.
32
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
flat 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 Prosobranchia, 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. 42. Diagram of a Basommatophoran
Pulmonate. ul, Respiratory aperture ; rgn, vas-
cular network on the inner surface of the mantle.
The kidney is incorrectly drawn. Further 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
711
MOLLUSCA OUTER ORGANISATION
33
and nephridial apertures lie on the right side of the body in front of
the anus.
(b) Nudibranehia. 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 Tectibranchiate Opistho-
branchiate. Lettering as before. In addition : gg, genital
ganglion ; s, shell ; , cerebro-
pedal connective ; re lf non-glandular vestibule of fold. The two mantle f olds en-
kidney ; re,, renal aperture ; pc, pericardium. cloge a gpace whoge transver se
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
MOLLUSC A OUTER ORGANISATION
35
Projecting into the mantle cavity, there is a large muscular process of
the body, the foot, which is directed downward and somewhat forward,
and can be protruded between the free edges of the mantle. This foot
is also laterally compressed. In 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. 4o. Anatomy of Unio (Margaritana) margaritiferus, left side (after Leuckart and Nitsche).
o, Mouth ; Cg, cerebral ganglion ; MI, anterior adductor muscle ; a>, oesophagus ; I, digestive gland
(liver) ; HO, nephridial aperture : lo, apertures of the digestive gland in the stomach m ; Aa, anterior
aorta ; ?i, nephridium, the outline given in dotted lines ; I", heart ; r, hind-gut ; Ap, posterior
aorta ; J/ 2 . posterior adductor ; a, anus ; Vg, visceral ganglion ; Br, gill ; Bk, mantle cavity ; go,
gonads with genital duct goi ; 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. Xear 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 Lamellibranchia, the outer organisation
deviates very greatly from the above.
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,
<* 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 tody, 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
five arms of the left side ; au, eye ; co,
internal shell; go, gonad; d, pigment gland
= ink-bag ; m, stomach ; n, kidney ; ct,
ctenidium ; a, anus ; mh, mantle cavity ;
in, siphon. The arrows indicate the
direction of the respiratory current.
vii MOLLUSOA 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 faBcal 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.
The Tetrabranehia (Nautilus, Fig. 48).
These have a shell coiled anteriorly (exogastrically) in the plane
of symmetry, and divided by septa into consecutive chambers.
The animal occupies
the last and largest
chamber ; the others
contain gas. 1 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 Fic 4S ._ Diagram of Nautilus> Mt ^ rt , Ventral . do>
tentacles, Which Can be dorsal ; m, anterior ; hi, posterior ; /, foot (tentacles and siphon) ;
retracted into Special snl > she11 muscle ; ct, ctenidia ; mh, mantle cavity ; a, anus ; s,
i ,r shell ; si, siphunele ; a, eye ; o, mouth.
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 ; c. Ford's Introduction to Brit. Mus. Cat., Fossil Cephalopoda, 1889.
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 Argonawta, 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 eyes. 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 MOLLUSCA 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. 3
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
j {- FIG. 49. Section of the integument of Daudebardia
, . rufa (after Plate). 1, Epithelium ; 2, 3, 9, various forms
I he pigment IS almost of unicellular glands ; 4, globular pigment cells ; 5, 7,
always found in the Cells of unpigmented cells of the connective tissue ; 6, muscle
the subepithelial connective JJ^^^^^ISSr^ ^^^*^
tissue.
A. Placophora. (Cf. the sketch of the Outer Organisation, p. 29.)
The Chiton 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 next. These plates are bilaminar. The outer and upper layer which forms the
dorsal surface is called the tegmentum, the lower hidden layer the articulamentum.
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. 1 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 calcined 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
1 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.
chitinous or calcareous spines, seise, 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 articulamenta to be merely a
FIG. 50. A, B, C, Three stages in de
velopment of a spine in the Chiton (after
Blumrich), diagrammatic, st, Spine ; bz, its special portion of the general cuticle, so we
formative cell ; e, epithelium ; c, thick may further recognise in the articulamenta
cuticle secreted by the epithelium; el; the homologues of the calcareous spines,
terminal swelling (Endkolbchen) = remains
of the formative cell.
ment of the mantle. The articulamenta
scales, etc., which are developed in the integu-
ment of the mantle. The articu
would thus be nothing more than very large and expanded calcareous scales.
y* ^
FIG. 51. Transverse section through a Chiton near the nephridial 'apertures, highly
diagrammatic (after Sedgwick), somewhat modified. 1, Pericardium ; 2, ventricle ; 3, auricle ;
4, branchial "vein"; 5, branchial groove (mantle cavity); 0, gill (ctenidium) ; 7, foot; 8, pleuro-
visceral connective; 9, branchial "artery"; 10, secondary coelom ; 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 MOLLUSC A 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
Proneomenia, Neomenia, etc. (v. below).
In Cryptochiton 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
articulamentum, since the whole dorsal integument is covered by an even cuticle,
which therefore forms no tegmentum.
The only part of a Chiton 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, setae, 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
Soleitor/'.'.xf /*. 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
FIG. 52. Transverse section of Chitonellus, diagrammatic,
adapted from figures by Pelseneer and Blumrich. g, Shell (articu-
lamentum) ; go, gonad ; i, intestine ; ab, vb, branchial arteries and
veins ; pv, pleuro-visceral nerves ; x, latero-ventral thickening of
the cuticle ; p, foot ; ct, ctenidium ; pn, pedal nerve ; h, digestive
gland (liver) ; c, secondary ccelom ; ao, aorta.
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 ChitoncUus 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 Polypli'i.cophorn, 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 formed 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 the lateral boundary of this
groove.
42
COMPARATIVE ANATOMY
CHAP.
In Chaetoderma the foot finally atrophies, and the medio-ventral groove also
disappears.
The'long series of undoubtedly primitive characteristics in these two groups the
Placophora and Solenogastres obliges
us to place them, as \ve shall have
repeatedly to point out, near the root
of the Molluscan phylum. In some
points the Solenogastres are perhaps
more primitive than the Polyplaco-
phora, 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 limicolous habit of life (most
Solenogastres inhabiting mud). The
shell, mantle, gills, and foot are such
essential characteristics of the Mol-
lusca that we must assume their
existence in the racial form.
The series Chiton, Chitoncllus,
Ncomenia, Chcetoderma does not, there-
FIG. 53. Transverse section of Proneomenia
Slulteri in the region of the mid-gut. 1, Mid-gut ;
2, rudimentary "foot ; 3, sepia projecting into the mid-
gut ; 4, testicular portion of the gonad ; 5, ovarial
portion of the same ; 6, thick cuticle secreted by the
epithelium.
fore, illustrate for us the rise and
development of typical Molluscan
characteristics, but rather their progressive obscuration and disappearance.
C. Gastropoda. (Of. 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 mucous, pig-
ment, and calcareous glands.
The epithelium is ciliated over areas of varying extent, especially
in aquatic Gastropods. In many of the shell-less Opisthobranchia the
whole surface of the body is ciliated.
The remarkable marking and colouring of the integument especi-
ally seen in the Nudibranchia 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 Nudibranchia 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 Prosobranchia, 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 Fissurellidce, 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 Rimula, the apical apertures in
shell and mantle have moved somewhat forward, and lie anteriorly between the
apex and edge of the shell. In Emarginula, 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 greater part of it.
In Haliotis, 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 Haliotis, 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 lamellse, which bend round to form
a groove for the reception of the edge of the shell.
The Trochida', Turbinidce, Neritidce, and nearly all Monotocardia 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 groove 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 projecting
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 Monotocardia 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
CHAP.
The Monotocardia have even been grouped, according to the presence or absence
of a siphon, into the Siphoniata 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 Monotocardia, 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 Rhacliiglossa,
the mantle bends back over the columellar
lip of the shell. In the Margimllidm, it
covers a large part of the outer surface, and
the same is the case in Pynda among the
Taenioglossa, in most Gyprceidcc and in the
Lamellaridcc. In Lamellaria, the shell is
cqmpletely 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 (Cypraeidce) provided with
wart -like, tentacular, or branched ap-
pendages.
e..
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. We have, on the one
hand, forms such as Helix, with large
FIG. 54. Testaceiia haiiotidea (after protruding spi rally -coiled visceral
Lacaze-Duthiers). A, right view ; b, enormous dome and } mant l e f ^ enclosing
pharynx evaginated through the buccal cavity, __;*._ ^ ^
carrying on its surface the radula (a) ; c, open-
ing of the pharynx into the oesophagus ; d,
position of the genital aperture ; e, latero-dorsal
groove along the body ; /, latero- ventral groove ;
g, mantle, rudiment of the visceral dome. B,
dorsal view : a, b, the two pairs of tentacles ;
c, the latero-ventral groove ; _ r=
66
COMPARATIVE ANATOMY
CHAP.
1 1 11 i
xa 3 ;> O " 3
|& =5 *" I | |
13 *~.g
5 2 2 |
llffl
^ Z 03 cS
si 8 "
tC-g -
s? "
ilii:-i
i-int
liJijl
-Hill
ill*3
7- ^-> W ^ g O
> ^ g
* 3 *S oo c
1 * 2 ' ? 1
^= S- ^3 OJ P*
Q) 0,0^ OS
1:1^91
-^ os = S P 3 3
^ " rt O) "r- O
.sp-s .-a-s <
c * . s *
vii MOLLUSCATHE 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
FIG. 66. Pholas 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 ; 9, 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 ; 6-9, 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 Tetrabranchia, 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 Ammonoidea, 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 Tetrabranchia. 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 Endoceratidce, 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, fc the 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
case, 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 Ammonoidea, in which they are folded in a complicated
manner.
Nautiloidea. In the following table we have the chief forms of the shell among
the Nautiloidea : ]
(a) Orthoceras group. Shell straight or slightly bent. Silurian Trias.
(b) Cyrtoceras group. Shell curved like a horn, but not regularly spirally
coiled. Cambrian Permian.
(c) Gyroceras group. Shell regularly spirally coiled, the coils, however, not
touching each other. Silurian Permian.
(d) Nautilus group. Shell regularly spirally coiled, the coils touching, or the
outer clasping the inner. Silurian recent.
(e) Lituites. 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
complicated 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 : 2
"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 Ammonoidea 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 whorl ;
then, little by little, the inner whorls also separate, though they still remain in the
same plane the Crioceras stage. Sometimes the shell grows straight for a time,
then becomes hooked the Ancyloceras and Hamites stages, and, if only the occupied
chamber separates from the coiled part the Scaphites stage. Finally, entirely straight
shells arise in the Baculites 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 Dibranchia, 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 Spirula (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 posterior or ventral side by
the thread-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 Spirulirostra (Fig. 67 D), the phragmocone begins to bend posteriorly (endo-
gastrically). The rostrum is triangular and pointed at the top.
1 Steinmann-Doderlein, JSlemente der Paldontologie, 1890. 2 Ibid.
VII
MOLLUSCATHE SHELL
69
In Spirula (E), the shell is coiled spirally and endogastrically. The siphuncle
is thick, and is surrounded along its whole length by 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. Ostracoteuthis, 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 through the shells of eight extant or fossli
Dibranchia, from the left side. The point of the visceral dome is turned down wards, the posterior
side of the shell is to the left and the anterior to the right (cf. the position of the Cephalopod body,
p. 36). A. Sepia; B, Belosepia (fossil); C, Belemnite (fossil); D, Spirulirostra (fossil); E,
Spirula; F, Ostracoteuthis (fossil); G, Ommastrephes ; H, Loligopsis; ph, 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 en velope = anterior or posterior edge 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), Onychoteuthis). In Dosidicus, this terminal cone
is almost solid, and in Loligopsis (H) it is nothing more than a thickening at the
upper end of the gladius : in other Decapoda, there is no trace of it on the gladius.
In the Octopoda, the shell has completely disappeared.
Again starting from the Belemnite, 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 mould. 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 trabeculae. 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 conchy olin,
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 Argonaut female. An
opposite view has, however, recently been very ably advanced that the Argonaut
shell is an Ammonite shell which has lost its septa and siphuncle and also its
-i4 s
r
FIG. 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 siphuncular
cavity ; I, lateral wall of the cavity ; a-/3,
line of the section which in Fig. 67 A is
diagrammatised. The two figures should
be compared (principal details after
D'Orbigny).
fi UNI V
vn MOLLUSCA*-THE PALLIAL COMPLEX 71
nacreous layer. 1 Should this view prove correct, the Cephalopods would have to
be differently classified. The division into Tetrabranchia and Dibra/ichia would
have to disappear, as we cannot tell whether the fossil Ammon&idea Avere Tetra-
branchia. and are also ignorant as to when the Dibrauchia developed from the Tetra-
branchia. The Cephalopods would then have to be divided into (1) Nautiloidea
with the extant genus X'. Diotocardia. In Fissure? la, the pallial complex is still quite symmetrical,
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.
1 Steinmann, Bericht Freiburg Gesellsch., iv. pp. 113-129.
72
COMPARATIVE ANATOMY
CHAP.
In order to prevent confusion, the hypothetical original position of each organ
will be denoted by ur ( = originally right) and ul ( = originally left) in brackets.
In the upper part of the mantle cavity in Fissu-rella, 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 (ur) nephridial
aperture ; the right (ul) and left (itr). 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 (ul), and to its left the left (ur) 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. 70. The same specimen from the left
side. Lettering as before ; o, mouth.
FIG. 69. Anterior portion of Patella, from
above, after removal of the mantle fold (after
Ray Lankester). a, Tentacle ; b, foot ; c,
pedal muscles (shell muscles) ; d, osphradia ;
e, mantle fold ; /, aperture of the right nephri-
dium ; g, anal papilla and anus ; h, papilla and
aperture of the left nephridium ; i, left nephri-
dium ; A-, right nephridium ; I, pericardium ;
n, digestive gland (liver) ; m, cut edge of the
mantle ; p, 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.
Turbinidae and Trochidse. Only the left (ur) ctenidium of Haliotis 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
papillae 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
Turbinidce. It is largest between the rectum and ctenidium, i.e. between the
right side of the latter and the left side of the former. In the Turbinidce, however,
a portion of it lies to the right of the rectum. There is a diffuse osphradium on the
axis of the gill.
Neritina. There is here only one gill (the left (ur) in Haliotis) shifted somewhat far
to the right. The rectum lies asymmetrically to the right in the respiratory cavity,
VII
MOLLUSCATHE 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
_^
20
Jui
FIG. 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 ; 3, foot ; 4, penis ; 5, seminal duct, which is continued at 15;
6, floor of the pallial cavity = nuchal integument; 7, colmnellar muscle; 8, intestine; 9, heart in
the opened pericardium ; 10, digestive gland (liver) ; 11, testes ; 12 and 13, renal organs ; 14, renal
aperture ; 15, seminal duct ; 16, rectum ; 17, hypobranchial gland ; 18, 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 Patellidce (Figs. 69, 70) a short conical portion of the
74 COMPARATIVE ANATOMY CHAP.
rectum projects into the small mantle cavity. This anal cone is not median, but
is distinctly shifted to the right. To its right and left lie the nephridial apertures,
raised on short conical papilla?. There is no separate genital aperture. In some
forms (Tectura, Scurria, Acmcca) 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. We 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 floor of the
mantle cavity, whereas in Tectura, for example, in 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 the 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 the 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 Paludina 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. Quite to the left, and also on the roof of the cavity, the ctenidium, feathered
on one side (the left (ur) of Haliotis and Fissurella), 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 Monotocardia, requires to
be re-investigated. The osphradium lies at the base of the gill.
2. Pulmonata.
In the Pulmonata, the single or double ($ and 6 ) genital aperture (Fig. 72) no
longer belongs to the pallial complex, but lies outside the mantle cavity laterally on
the head or neck. In Onddium 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, Testacella, and Onddium).
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 MOLLUSCATHE PALLIAL COMPLEX 75
the pericardium, containing the ventricle and auricle, the latter lying in front of
S
7
r,
fffr,
FIG. 72. Helix aspersa, fully extended from the right (after Howes), a, Anus appearing in the
respiratory aperture, pl- 2 ; s, shell ; p, edge of shell aperture ; ga, genital aperture ; ti, optic tentacle ;
t, anterior tentacle ; ?->, upper lip.
the former. From the ventricle the aortic trunk runs upward and backward, and
from the auricle rises the pulmonary vein,
which runs 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 Basommatophora (Plan-
orbis, Physa, Limnaeus}, near the respira-
tory aperture, and among the Stylommato-
ph&ra in Testacella on the floor of the
pulmonary cavity at its extreme posterior
angle.
The floor of the pulmonary cavity (the
dorsal nuchal 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 (Bulimus oblongus, 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 Basommatophora, and
some species of Eulimus, Cionella, 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-
monata. 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 duct, which, in D, is a groove. Further
explanations found in the text.
Testacella, and some forms
76 COMPARATIVE 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 Bulimus, Helix, Daudebardia,
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. Limax, Amalia, and some species of Daudebardia (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 Hyalinm among the Stylommatophora,
and proceeding through Daudebardia 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 Testacella and certain Daudebardia, 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 way 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
Daudebardia rufa. The pericardium, instead of lying far back at the base of the
pulmonary 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). Daudebardia rufa is thus actually opisthopneumonic. 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 Daudebardia, 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
MOLLUSCATHE 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 (cf. 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 aper-
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 Testacella.
It is, further, noteworthy that, in Testacella, 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 Testacellidce 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 Vaginulidce 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 the changes of position in the pallial organs of Daude-
bardia and Testacella (adapted from figures by Plate). Mantle organs drawn as in Fig. 73. A,
Daudebardia rufa ; B, Hypothetical stage, the pallial complex of A twisted round 90 ; C,
Testacella. 1, Respiratory aperture ; 2, kidney ; 3, ureter or urinary duct ; 4, reno-pericardial
aperture (renal funnel) ; 5, ventricle ; 6, auricle ; 7, aorta ; 8, 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 Testacella, in front
of the auricle. Respiration takes place principally through the skin ; in the amphib-
ious Oncidia it is assisted by dorsal papillae. In Vaginulus, 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 Oiicidium cclticum, 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 filled 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, i.e. no longer through a
respiratory aperture. Others, again, have thought the arrangement in Oncidium
and Vaginulus 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. 75, 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 Limnccidce, 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 (Basom-
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 Stylommatophore (Nephropneusta], viz. in the genus
Testacella. For the osphradium invariably belongs to the mantle cavity, being
primitively connected with the ctenidium, it never lies in the urinary duct.
3. Gastropoda Opisthobranchiata.
We can here speak of a pallial complex only in connection with the .Tectibranchia,
since in them alone is a distinct mantle fold developed on the right side of the
FIG. 75. Aplysia, right aspect, the right parapodium (15) turned downwards; the pallial
complex is seen under the mantle fold 7 (after Lankester). 1, Anterior tentacle ; 2, eyes ; 3,
posterior tentacle (rhinophore) ; 4, left parapodium ; 5, seminal furrow ; 6, genital aperture ; 7,
mantle fold ; 8, gland ; 0, osphradium ; 10, outline of some inner organ seen through the integument ;
11, nephridial aperture ; 12, ctenidium ; 13, anus ; 15, right parapodium ; 16, anterior portion
of the foot. (There should be 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
MOLLUSGATHE PALLIAL COMPLEX
79
2. In front of the anus, between it and the ctenidium, is the nephridial aperture.
Following these there may be ^
3. A hypobranchial gland.
4. The ctenidium.
5. At the base of the ctenidium or on its axis,
the osphradium.
Were this complex of organs to be shifted along
the edge of the body, we should have the arrange-
ment found in the Monotocardia among the Proso-
brn.n<:hia. The correspondence is, however, appar-
ently marred by the position of
6. The genital aperture, which in the Opistho-
branchia lies farthest forward of all the pallial
organs.
In all other Opisthobranchia (after excluding the
Tectibranctiia} the pallial complex is broken up
when the mantle and the true ctenidium disappear.
The only exception to this is found in the Phyl-
lidiidcc, where, apart from the gills, a similar
arrangement to that in the Tcctibranchia occurs.
The single or paired genital aperture always lies
asymmetrically on the right side in front of the
anus, which is sometimes found asymmetrically on
the right side, and sometimes has a median dorsal
position between the middle and the posterior end
of the body. The renal aperture lies between the
anus and the genital aperture, sometimes close to
the latter.
In the Ptcropoda gymnosomata (Fig. 76) the
shell and mantle are wanting. The ctenidium,
when retained, as in the Dcxiobranchia and Pni'iiii<.>-
il.-t'iiM, 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 Flo/ 76 ._pn eiimod erma, from the
its original position between the anus and the genital r j g ht side, diagrammatic (after Pel-
aperture, while the osphradium, which is generally seneer). 1, Right process bearing
found close to the ctenidium, has, as far as has yet
been observed, retained its original position.
The anus lies anteriorly behind the right fin ;
the nephridial aperture lies close by, either distinct
or united with the anus at the base of a common
cloacal depression. Immediately in front of 1 this
lies the osphradium, then follows, considerably
farther forward on the neck, to the right behind
the base of the right fin, the genital aperture, from
which, as in many Tectibranchia, a ciliated furrow
runs forward along the surface of the body to the
aperture of the penis, which lies to the right in front of the foot.
All Thccosoniata have a mantle and a mantle cavity, and often a shell as well ;,
in the Cyinbuliidce, the latter is replaced by a cartilaginous pseudoconch, a sub-
cutaneous formation of the mantle.
Among the Thccosomata, the Limacinidce indicate the primitive arrangement ;
they possess a dorsal or anterior mantle cavity, a coiled shell, and an operculum.
hooks (Hakensack) evaginated ; 2>
proboscis ; 3, right buccal tentacle ;
4, position of the right nuchal ten-
tacle ; 5, right fin (parapodium) ; 6,
seminal furrow ; 7, genital aperture ;
5, position of the jaw ; 9, ventral pro-
boscidal papilla ; 10, right buccal ap-
pendage bearing suckers ; 11, head ;
12, aperture of penis ; 13, right anterior
pedal lobe ; 14, anus ; 15, posterior
pedal lobe ; 16, ctenidium ; 17, pos-
terior adaptive gill ; d, v, a, p, dorsal,
ventral, anterior, posterior.
80
COMPARATIVE ANATOMY
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 (hypobranchial 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 Thecosomata with coiled shell, the
-7
FIG. 77. A, B, C, Three diagrams to illustrate the relation of the Limacinidae to the
Cavoliniidae (after Boas). A, Limacinidae ; B, hypothetical intermediate stage between
the Limacinidae and the CavoliniidaB. The visceral dome twisted 90. C, Cavoliniidae. 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 tin (parapodium) ; 2, foot bent forward ; 3, genital aperture ; 4, ten-
tacular appendage of the mantle edge ; 5, anus ; 6, masticatory stomach ; 7, gonad.
Cavoliniidce and Cymbuliidce, or Thccos&mata 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 Limaciwidas, 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. Sifch a twist gives the organs the position they
actually occupy in the Cavoliniidce and Cymbuliidce ; 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 right.
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 MOLLUSCATHE PALLIAL COMPLEX 81
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 back near the 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 with the foot, where such concrescence takes
place. In the Septibranchia, 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 sexual products may be ejected on each side through a
common aperture (Ostrcea, Pecten, Cyclas, Pisidium, etc.).
2. There may be, on each side, two distinct apertures, one male and the other
female (Anatiiiacea).
3. The seminal ducts and the oviducts may unite before opening to form a
short, common, terminal piece (Septibranchia).
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 Protobranchia (Nuculidcc and
Solcnomyidce}. 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 occiirring 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 place.
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.
4. Close to the two upper neplmdial apertures lie the two so-called viscero-
pericardial apertures.
5. Between the bases of the lower gills there are in each sex, two genital
VOL. II G
82
COMPARATIVE ANATOMY
CHAP.
CMX
FIG. 78. Pallial complex and siphon of Nautilus pompilius ? (after Bourne and Lankester).
v, Valve of the siphon ; TO, right genital aperture ; m, the mantle fold, with the nidameutal gland,
folded back ; an, anus ; cp, left aperture of the secondary coelom ; Ihn, left upper nephriclial aper-
ture ; lo, aperture of the left rudimentary oviduct ; Ivn, left lower nephritlial aperture. The four
ctenidia are not lettered.
FIG. 79. Pallial complex of Nautilus pompilius (after Bourne and Lankester). pe, Penis ;
a, muscle band of the siphon ; hp, aperture of the left rudimentary seminal duct ; nepJia, nephp,
lower and upper nephridial aperture of the left side ; olf, left ospliradium ; viscper, left aperture of
the secondary coelom ; an, anus ; x, supra-anal papilla of unknown significance ; c, mantle cut off.
VII
MOLLUSCATHE PALLIAL COMPLEX
83
FIG. 80. Sepia Savignyana, from behind (after Savigny). The greater part of the mantle cut
open and laid back on the right side (left in the figure), a, Prehensile tentacle ; b, oral arm ; c,
mouth with jaws ; (?, lower aperture of siphon ; e, eye ; /, locking apparatus of theinantle g ; h, right
ctenidium ; i, siphon ; k, locking apparatus of the mantle on the visceral dome ; I, upper aperture
of siphon ; 7/1, anus ; n, depressor infundibuli ; o, penis ; p, right nephridial aperture ; q, 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 produced 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 Sepia (Fig. 80), we 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 nephridiai 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 nephridiai aperture at the
summit of a large papilla (penis). In the female Octopus, the genital apertures are
paired and symmetrical, and lie to the right and left of the rectum.
5. The two nidamental glands (in Decapoda) lie in the visceral dome, sym-
metrically with regard to the median line ; they open above the nephridiai apertures
into the mantle cavity.
VI. The Respiratory Organs.
The True Gills or Ctenidia.
The most important of the pallial organs in the Mollusca is the gill,
for it is in order to protect it that the mantle, and with it the pallial
cavity, develop. 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 Opisthobranchia), 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 etenidia of the Mollusca are originally paired and symmetri-
cally arranged ciliated processes of the body wall, carrying two
rows of branchial leaflets, and 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 etenidia are met with first in that group of the
Mollusca which has undoubtedly retained more primitive characteristics
than any other, viz. the Chitonidce among the Amphineura. They are,
further, found in all other Mollusca which have retained the original
VII
MOLL USC A RESPIRATORY ORGANS
85
bilateral symmetry of the body, such as the LameUibmnchia, the
Cephalopoda, and a point of great importance also in the primitive
FIG. 81. Ctenidia of various Molluscs (after Ray Lankester). A, CMton ; B, Sepia ; C,
Fissurella ; D, Nucula ; E, Paludina. ft, Longitudinal branchial muscle ; abv, afferent branchial
vessel ; ebi; efferent branchial vessel (branchial vein) ; gl, paired lamellae (leaflets) of the feathered
gill ; in D : d, position of the axis ; a, inner ; 6 and c, outer rows of branchial lamellae ; in E : i,
rectum ; br, branchial filaments ; a, anus.
Gastropoda, the Zeugobranchia. In the latter, Jiowever, 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 Cephalopods, 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 Haliolida
being retained, the right completely disappearing. In the forms which
most resemble the Fissurellidce and Haliotidce, the single-gilled Dioto-
cardia (Turbinidce, 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 TectibrancMa, 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 Pulmonata, 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. Kespiration 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. Amphineura.
Chitonidae. -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
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 ctenidium of a Chiton (after B. Haller). A, Single ctenidium with
its double row of branchial leaflets. B, Transverse section of the gill along the line a-b in Fig. A.
1, Narrow blood sinus in the branchial leaflet; 2, septum in its axis; 3, longitudinal muscle; 4,
afferent branchial vessel ; 5, efferent 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 e-f
in Fig. B. 1, Same as in Fig. B ; 8, space between the consecutive branchial leaflets. D, Longi-
tudinal section of the ctenidium 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. Tlie 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-
Hum, the cilia are remarkably A
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 circular 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. Xerves 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 Chitonidae. m, Mantle ; o, mouth ; A:, snout ; /,
foot ; ct, ctenidia ; a, anus.
88
COMPARATIVE ANATOMY
CHAP.
The number of ctenidia in each row varies very much in the different species of
Chitonidce ; 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 Chitonellus] is confined to its
posterior half (B, C).
Solenogastres. (Proneomenia, Neomenia, Chcetoderma}.
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 Chcetoderma (Fig. 84) the foot has disappeared, and
the mantle cavity is reduced to the cloaca, in which one typical
gill lies on r each side of the anus. These gills are regarded as
the last ctenidia of the rows found in the Chitonidce, which in
T f Chitonellus and some species of Chiton are already confined to
end of tne body of r * . .
Chaetoderma (dia- the posterior half of the body. In Neomenia, there is no longer
gram after Hubrecht). a pair of ctenidia, but a mere tuft of filaments rising from the
1, Gonad ; 2, pericar- wa n o f the cloacal cavity, and in Proneomenia, there are only
dium ; 3, rectum ; 4, irregular folds of the c l oa cal wall.
nepnridium ; 5, anus ; , . ., , . , , . , .
6 ctenidium- 7 ^ n 't ne relation of the gills in the Chitonidce to certain
cloaca. ' patches of epithelium, which may perhaps be considered as
osphradia, see the section on Olfactory Organs, p. 165.
B. Gastropoda.
The Fissurellidce (Fig. 85, A and B) among the Prosobranchia 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 Fissurellidce (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 Lamellibranchia, i.e.
the Protobranchia, 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 Haliotidce are closely connected with the Fissurellidce. 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 Fissurellidce 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
MOLLUSC A RESPIRATORY ORGANS
89
inner wall of the mantle, becomes attached to the mantle by its other edge
also (viz. that along which the branchial artery runs), 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 (Trochidce, Turbinidce,
Nentidas).
In the Docoglossa (Patellidce} the ar-
rangement of the gills is very varied.
While the Lepctidce 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 Tectura
and Scurria) which possess, in addition
to this marginal row of leaflets, a typical
ctenidium corresponding in every way with
that of the Turbinidce, Trochidce, etc.
Other forms, such as Acmcea, have only the
true ctenidium and no marginal branchial
leaflets.
In the large second division of the
Prosobranchia the; Monotocardia the
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 Fissurella and Haliotis, and the single gill in Turbo and
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, Cyclostoma, Cyclo-
phorus, 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. Subemarginula after removal of the
shell (after Fischer). A, from above ; B, from
right side. The mantle cavity is exposed by
bending back the mantle fold 4. 1, Snout ; 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 ; 7, epipodium ; 8, foot.
90
COMPARATIVE ANATOMY
CHAP.
for the inhalation and exhalation of air. 1 The ctenidium is placed to the extreme
right of the mantle cavity, a position which is in some way connected with the great
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 Heteropoda, 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.
Opisthobranchia. A true ctenidium is here found only in the Tectibranchia and
FIG. 86. General Morphology of the gills of the Prosobranchia. Diagrammatic sections
in the region of the mantle cavity, from behind. ' A, Haliotis ; B, Trochus, anterior portion of the
jJallial cavity. C, Trochus, middle 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 Azygobranchia (B, C)and Monotocardia (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 (3 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. Pleurobranchus), 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 Pneumodermidaz 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 (Clionopsis, Notobranchcea), until they in their turn dis-
appear (Clione, Halopsyche).
Among the Thecosomata, the CavoliniidcR alone (Fig. 87) possess a gill which
1 In this and in the closely-allied Lanistes there is in addition a protrusible siphon
on the left side (t;. Fischer and Bouvier, C. R. cxi. p. 200).
VII
MOLL USC A RESPIRATORY ORGANS
91
rises in the form of a series of fold-like elevations of the body wall in the pallial
FIG. ST. Anatomy of Cavolinia
tridentata (after Souleyet). Shell
and mantle removed, and visceral
dome partly opened, seen from behind
and below, d, Right ; s, left ; 1, aper-
ture of the penis ; 2, mouth ; 3, left tin
(para podium) ; 4, foot ; 5, oesophagus ;
ti, part of the efferent genital appara-
tus ; 7, ventricle ; 8, auricle ; 9, herma-
phrodite gland ; 10, lateral processes
of the mantle ; 11, columcllar muscle ;
12, intestine ; 13, digestive gland
(liver); 14, stomach; 15, ctenidium ;
16, 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 Protobranchia. Taking Nucula
(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 Solenomya, 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 Xucula ; 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 branchial artery. Two rods of connective
tissue run along the lower edge of each leaflet from the axis to its tip, and serve for
its support. Similar supports are found in almost all Lamellibranchia and in
many Gastropods.
The epithelium of the 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 which are pressed together, form a loose
connection between 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. 88. Morphology of the gills of the Lamellibranchia, diagrammatic transverse sections.
A, Protobranchia. 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, branchial leaflets or
filaments of the inner row or inner branchial leaf; ej, ascending branch of the outer filament,
or lamella of the outer leaf; ij, ascending branch of the inner filament, or lamella of the inner leaf;
in D, s, signifies the gill which has become transformed into a muscular septum which divides the
mantle cavity into an upper (4) and a lower (5) chamber, the -t\vo communicating by means of slits
(o) in the septum. Further explanations in the text.
surface. By this bending back of the filaments, the gills make the most of the
limited space afforded by the mantle cavity. Each 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 whole row looks
like a leaf or fringe. This branchial leaf consists of two closely contiguous lamellse,
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, the 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 anterior and posterior sides
of the filaments places covered with long cilia closely crowded together. These
ciliated tufts on adjoining filaments mingle, and so give rise to a sort of connection
between the filaments of each leaf.
In the Mytilidce, so-called interfoliar junctions or trabeculse occur at certain
vii MOLLUSGA RESPIRATORY ORGANS 93
points between the ascending and descending portions of the branchial filaments,
but no blood-vessels run into them.
In Anomia, the dorsal ends of the ascending portions of the outer lamella are
free, but in the Arcidce united, although their internal cavities are not in communi-
cation. In such 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 Mytilidce, the dorsal ends of the
recurved portions of the filaments of each branch have grown together, and their
blood-vessels communicate 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 lamellae of each leaf of the gill are united
here and there by trabeculae, 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 vascularised
junctions, not only between the ascending and descending lamellae, 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 lamellae. This peculiar arrangement was formerly con-
sidered typical of the Lamellibranchia, 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, but 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 Protobranchia. The Lamellibranchia 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 lamellae
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 (Tellinidce and Anatinacea).
The ascending lamella of the outer leaf may be wanting (Anatinacea, Lascea],
and in fact the entire outer leaf may be absent (Lucina, Corbis, Montacuta,
Cryptodon}.
In all Lamellibranchia, with the exception of the Protobranchia, and further, of
the Arcidce, Trigonidce, and Pectinidce, 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
COMPARATIVE ANATOMY
CHAP.
fuse with each other behind the foot in the middle line of the mantle cavity, they form
a septum which, uniting with 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 polymorpha
(after Peck). /, The separate filaments ; /, sub-epithelial fibres ; ch, supporting substance of the
filaments ; lac, lacunar or alveolar tissue ; pig, pigment cells ; be, blood corpuscles ; fe, epithelium
of the free edge of the branchial filaments ; lfe\, lfe->, two rows of lateral epithelial cells of the
branchial filaments, carrying long cilia (ciliated tufts) ; Irf, tissue of the interlilamentar 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).
(e) 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
MOLLUSC A RESPIRATORY ORGANS
ot A
95
FIG. 90. Portions of transverse sections of the branchial lamellae of Anodonta (after Peck).
A, Outer ; B, inner lamella. In each leaf the cross sections of both lamellae are seen, and also the
interfoliar as well as the interfilamentar junctions. C, A part of B much magnified, ol, Outer ; if,
inner lamella of the same leaf; r, blood-vessels;/, the separate filaments of which the lamellae
consist ; lac, lacunar tissue ; ch, supporting tissue of the filaments, with firmer supporting
rods, cltr.
.f
trf
FIG. 91. Portion of the ascending lamella of the outer branchial leaf of Anodonta.
diagrammatic (after Peck). /, The separate filaments, connected by internlamentar junctions ;
trf, connective tissue of the latter ; r, bloodvessels ; 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
COMPARATIVE ANATOMY
CHAP.
D. Cephalopoda.
The gills of the Cephalopoda are always feathered on both sides. Those of the
Dibranchia have been the most thoroughly investigated. In Sepia, each 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 base is directed dorsally towards the apex
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
arid 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 posterior 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-
" blood-making gland" (9), through which
venous blood flows ; 10, 11, vessels carrying
the venous blood which has passed through
FIG. 92. Diagram to illustrate the struc
ture of the gill of Sepia (after Joubin). 1,
Branchial vein (containing arterial blood) ; 2,
branchial canal ; 3, branchial] artery (contain-
ing venous blood); 4, special branchial vein
(vas efferens)of each leaflet ; 5, special branchial
artery (vas afferens) of each leaflet ; 0, suspensor
of the gill, which attaches the branchial artery 1
(3) to the posterior integument of the visceral terlor ed S e of each leaflet ( that facm g tlie
dome (12) ; 7, suspensor of each leaflet to the visceral dome) is connected with this mem-
general suspensor (6) ; 8, one of the connecting brane, which may be called the gill-suspen-
vessels between the branchial artery and the 8Qr> by meang of another triangular mem-
brane. A special vein runs along the
posterior free edge of each leaflet, and enters
the "blood-making" gland back to the venous the general branchial vein at its base ; and
sinus at the base of the gill. The arrows in- a spec i a i arte ry runs along the anterior edge,
dicate the direction of the blood-stream. . , , c , f -, a , -. . -,
i.e. along that edge of the leaflet which is
fastened to the suspensor. Each leaflet is wrinkled in such a way that the folds
on the two surfaces alternate, each fold being creased in its turn. These two systems
of folds cross 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
vii MOLL USGA RESPIRATORY ORGANS 97
of it streams through 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 Octopoda 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 lamellse, which in their turn
may cany similar lamellae of the second order, and so on till in some cases the
seventh order of subsidiary lamellse 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 Pulmonata and the few air-breathing Prosobranchia, 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 Opisthobmnchia which inhabit water, and in the gill-less
forms of the Pteropoda, all the more so, as in most Opisthobranchia
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 Scaphopoda and many Opisthobranchia 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 Ascoglossa and in the Nudi-
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 Dorididce ; (2) the longitudinal rows of
branchial leaflets to the right and left under the mantle fold of the
so-called PhyllidHdw ; (3) the dorsal appendages or eerata of the
Nudibranchia and most Ascoglossa.
1. The Anal Gills (Fig. 93). These take the form of delicate leaflets, generally
feathered on both sides, which, in the Dorididce, 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 (Poly cer idee). The view that these gills are ctenidia
has as yet no sufficient foundation.
VOL. II H
98
COMPARATIVE 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 Pleurophyl-
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 lamellae 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 (Phyllidia), or
a row interrupted posteriorly as well
(Pleurophyllidia) ; or again, the rows of
lamellre are confined to the posterior end
of the mantle fold (HypobrancMcea). The
genus Dermatobranchus 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
93. - Respiratory and circulatory colour and markings. In some cases they
system of Doris, after Leuckart (" Wand- may serve for protection and concealment,
tafeln "). a, Rhinophore ; ft, posterior edge of in others, where the brilliant colouring is
the visceral dome ; c, end of the foot ; d, plumose com bined with stinging properties, they
gills ; dj, two gills cut off; e, anus ; /, auricle ; & *
g, ventricle ; h, aorta ; i, circular vein around ma y serve as a warning. They often break
the anus, which receives the arterial blood from off easily at the base (as a protective ar-
the gill, and sends it through the branchial vein rangement), and are always quickly regen-
into the auricle ;fc, circular artery, which receives erated Th no doubt ^^ nke th(?
the venous blood coming from the body : x. two / .. i i i c
vascular trunks, which conduct venous blood rest f the bod y surface > m respiration,
direct to the heart. especially where they are much branched
and richly supplied with blood-vessels.
Certain Opisthobranckia are altogether gill-less, e.g. the Elysiidw, Limapontidce,
and Phyllirrhoidcc.
Among the Pulmonata, the shell- less genus Onchidium 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 papillae, in which there is a rich vascular network, which receives
the blood from an afferent vessel and gives it off to an efferent vessel.
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 man tie fold, which i.^t f
forms the roof of this f,
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 FIG. 94. Slightly oblique transverse section through the
very rich in lime-secret- body and sheu of Helix ***** J 1 "* in front of the oommdia
* - , . (after Howes), pgl, Pedal gland ; fs, lateral pedal blood sinus ;
ing glands. Ine inner a0j cephalic aorta; gd, genital duct (uterus); rp, retractor
delicate Surface Of the muscle of penis ; plm, pallial muscle, the pallial edge having
mantlp wVnVh if rm: thp Ulli ted with the nuchal integument ; si, salivary gland ; cr, crop,
jie, WHIG ine or widenmg of the oesophagus ; s, shell ; ms, floor of the pulmon-
1'Oof of the Cavity, is ary cavity = dorsal integument of the posterior nuchal region
Overspread by a close WQich is covered by the mantle; sp, spermatheca = stalk of the
receptaculum seminis ; pli, pulmonary cavity; pv, afferent pul-
'" inonary vessels ; re^ renal duct ; r, rectum ; hgl, hermaphrodite
WOrk. A Circular Vein gland or ovotestis ; I, digestive gland (liver) ; hd, hermaphrodite
runs alon " the mantle duct; w, columellar muscle ; aj/Z, albumen gland ; i, intestine;
st, stomach.
collar. Jbrom it spring
numerous fine anastomosing vessels which ramify on the mantle.
These vessels are again collected into larger trunks, which enter the
large 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
organ 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
Jps- u**%&
T* OF THK n r
UNIVERSITY"
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 (Limnceidce) 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, tilled with
water when the animal is young, and it is then a water breather. In Limncea
abyssicola, a deep-water form found in the lake of Geneva, this form of aquatic
cr
FIG. 95. Helix. The roof of the pulmonary sac cut along the rectum, and along the edge
uniting 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 ; , bl>, show the cut edges which belong to each other ;
1, afferent pulmonary vessels which draw their venous blood from the large circular venous
sinus (9); this latter receives its blood from the large sinuses of the body, two of which,
that of the 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 Prosobrandiia (Cydostoma, Cydophorus, etc.) the
respiratory cavity becomes transformed, as in the Pulmonata, into a
pulmonary chamber, and its roof is covered with a respiratory
vascular network. But there is here no concrescence of the edge of
the mantle with the nuchal integument. Cydostoma still retains a
rudiment of a prosobranchiate gill, but this is lost in Cydophoi'us.
The amphibian Ampullaria possess both a gill and a pulmonary
sac, 1 and can breathe either water or air.
1 See note ante, p. 90.
vii MOLLUSCA HYPOBRANCHIAL GLAND, HEAD 101
VII. The Hypobranehial Gland.
(Slime gland of the Protobranchia, epithelial shield of the Ptcropoda,
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. Cf. on its position and occurrence Section V.
The hypobranchial gland varies considerably in shape, but is
never a multicellular, acinose, or tubular gland wifch 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 (Pwpwra, Murex, Mitra) is a
hypobranchial 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 Lamellibranchia must be considered headless, and
as such have been distinguished as Acephala from other Mollusca.
This absence of a head in the Lamellibranchia cannot be regarded as
a primitive condition, 1 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 large 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 always carries tentacles, Avhich 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 ommatophores and carry eyes at their tips. Most Diotocardia
possess anterior tactile tentacles, and posterior and slightly lateral optic tentacles.
The cephalic tentacles are always 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 Docoglossa and most Monotocardia the optic tentacles do not rise separately
from the head, but are to a greater or lesser extent fused with the tactile tentacles.
Starting with the tentacular arrangements existing in Dolium, Strombus, Rostellaria,
we find the tactile and optic tentacles
fused for a certain distance from the
base, but separating later, the tips
projecting independently (Fig. 96, B).
If the two tentacles were of the
same length, and were fused for their
whole extent, there would only be one
FIG 96.-Relations of the tactile and optic ten tacle on each side of the head, which
tentacles in the Prosobranchia. Description in
the text. would carry the eye at its tip ( Terebra
(7). 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, Ctenopidce, Calyptrceidce), or else can be invaginated,
commencing at the tip (Cyprccida', Lamellaridce, Naticidce, Scalaridce, Solaridce).
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 n on -invaginated anterior portion or tip. Gastropods
vii MOLLUSCAHEAD 103
with such proboscides are nearly all carnivorous (the Tritonidce, Doliidce, and Cassi-
didce, among the Stenoglossa the Rachiglossa, 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 Heteropoda carries two tentacles (occasionally rudimentary : Ptero-
trachea, Firoloidect). 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 Tectibranchia, the Cephalaspidce 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 Natitidce or Olividce 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 Phyllirrhoe.
In Tcthys, 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 carries 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 rhiuophores of the Tectibranchia, especially
those of the Aplysiidce. The nuchal tentacles are generally small or rudimentary,
the rudiments of the eyes lying at their bases.
Xearly all the Gymnosomata, as highly-developed carnivorous animals, are provided
with a proboscidial snout which, commencing at its tip, can be completely invaginated,
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
(Clionopsis).
2. When the proboscis is of median length, it carries suckers at its base, or a
pair of long appendages provided with suckers (Pneumodcrmidce, 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 pairs of conical processes (cephalic cones), with
special nerve endings and glands whose sticky secretion helps in the capture of prey
(Clionidce).
4. 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 copulatory organ lies on the
upper side of the head, near the tentacle.
3. Fulmonata.
The head is here distinct from the foot ventrally, but passes dorsally into the
neck. It carries two or four tentacles. The Stylom-
liiatophora, 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 JSasommatophora, which are aquatic, have
only one pair of tentacles which are usually triangular
FIG. 97.-Helix, 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 ; ti, optic tentacle ; ^gjj. bases
; Zl> In certain Pulmonata (Glandina, Zonites, Ond-
dium) the upper lip may be drawn out into a lobe or
labial palp on each side. This labial palp in Glandina 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).
In 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 vermiform 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 MOLLUSCAORAL LOBES OF THE LAMELLIBRANCHIA 105
its concave surface, the cilia being continued in a baud all along -tlite 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, an<| easily become detached or broken
off, and are then regenerated. They^ aje no doubt chiefly useful as organs of touch,
and serve for seizing particles oftfood (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 lacunae, 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
(Ostrra, Tridacna) ; in others, the two edges are closely apposed and interlocked by
means of processes and folds (Pccfe/i. Spondylus), so that a closed cavity rises in
front of the mouth, into which the groove 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 stretches
along the whole length of the base of the faot, 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 Mollusca is characterised by the
pronounced development of its musculature, which enables the animal
to creep, a fleshy foot, provided with 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 Chitonida>
among the Amphineura, in most Gastropoda, and in certain Lamelli-
branchia, especially in the Protobfanchia, which for other reasons also
must be considered the most primitive form of Lamdlibranchia.
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 propodium, 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 MOLLUSCATHE FOOT AND ITS GLANDS 107
B. Gastropoda.
1. Prosobranchia.
With 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 Monotocardia (Olividce, Harpidcc, certain species of Pyrulidce, Strombidce,
Strambus, Pterocera, Tercbellum, Rostellaria [Fig. 6, p. 6], Xenophoridce [Fig. 5, p. 5],
Xaricidcc, Naticidce [Fig. 98]).
Among the above, the propodium is particularly 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.
FIG. 98. Natica Josephina, with protruded proboscis, from the right side (after Schiemenz).
1, Propodium; 2, sucker-like boring appendage of the proboscis (3) with 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 horny or
calcareous operculum which serves to close the shell.
Epipodium. The epipodiuin 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
Fissurellidce this epipodial ruff is replaced by a row of numerous tentacles or
papillae, 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 carries a few tentacles (usually 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
Eumargerita and Scissurella.
The epipodium is, as a rule, wanting in Docoglossa, but one is found beset with
papillae in the genus Helcion, and in Patinella and Nacella it is fringed ; these
epipodia 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 Rissoidce have an
epipodium with several (1-5) tentacles on each side. Many other Monotocardia
have retained either the anterior or posterior portions of the epipodium.
(a] Anterior vestiges of the epipodium are found in Vcrmetus in two anterior
pedal tentacles, and in Paludina and Ampullaria in two nuchal lobes, which
must not be confounded with .true cephalic tentacles. In Paludina, the right
nuchal lobe, and in Ampullaria the left, forming a longitudinal groove, becomes a
sort of siphon. Oalyptrcea possesses on each side under the neck a semicircular
epipodial fold.
(6) Posterior vestiges of the epipodium are found in Lacuna in the form of an
epipodial fold with a process on each side above the foot. Narica has, above the
metapodium on each side, a wing-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 the
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 Hipponyx undergoes a curious transformation. Hipponyx 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 columellar muscle which descends
from the shell gives rise to a horseshoe - 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
have exchanged the creeping for the swimming manner of life. The foot has in
them become peculiarly adapted to this new method of locomotion. The propodium
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 Carinaria
to Pterotrachea. In this series, the typical outer appearance of the Prosobranchiate
(its shell, visceral dome, mantle, and gills, which are still retained in Oxygyrus and
Atlanta], 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
hollowed 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
MOLLUSC A THE FOOT AND ITS GLANDS
109
saltatory Strombidce, 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 fin-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 Carinaria (C) both the foot and the general external appearance of the whole
FIG. 99. Comparative Morphology of the 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 reduced 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 PterotracJiea (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 Auriculldce, however (Melampus, Leuconia, Blauneria,
Pedipes), it is divided into two portions by a temporary or permanent transverse
groove.
3. Opisthobranchia.
In almost all Opisthobrauchia the foot has a well-developed sole for creep--
110
COMPARATIVE ANATOMY
CHAP.
ing. There is no division into parts, and the adult rarely (Adceon] carries an
operculum.
The epipodium 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 Tectibranchia, such as the Scaphaiidridce, Bullidm, Aplus-
Gastropteridce (Fig. 14, p. 10), Philinidce, Doridiidcc, Aplysiidce (Fig. 75,
p. 78), Oxynoeidce). The parapodia are often bent
A 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
(Gastropteridce, Philinidce, Doridiidce, Aplysiidcc) 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 Philine and Doridium}
by the parapodia (Fig. 100).
The parapodia may fuse posteriorly along their
upturned edges (Aplysiidce, Oxynoe}. In Lobiger each
parapodium is transversely slit, so that two long
wing-like processes are formed on each side. Many
Opisthobranchia (Aplysiidce, Oxynoe, Gastropteridce)
can propel themselves through the water by means of
the waving motion of their parapodia. Phyllirhoe
is a Nudibranch 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-
branchiate Heteropoda, become pelagic animals adapted
for swimming.
While in the Heteropoda the propodium becomes
transformed into a medio-ventral vertical rowing fin,
in the Pteropoda the paired Tectibranchiate parapodia
FIG. 100. Diagrammatic trans-
verse sections of Gastropods, to
illustrate the arrangement of the
shell (black, 1), visceral dome and
mantle (dotted, 2), and foot (streak-
ed, 3). A, Prosobranchiate with which, as we have already seen, can be used for swim-
outer shell and epipodium (4). B, min& develop into the paired fins or wings of these
animals (Figs. 16 and 17, p. 11 ; 87, p. 91).
In the Thecosomata (Fig. 87, p. 91), which must
be derived from Cephalaspidce (Bulloidea), 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 mesopodium and the two lateral parapodia or fins. The mesopodium is small,
and the ventral side of it (which corresponds with the sole of the Cephalaspidce, but
can no longer be used for creeping) is strongly ciliated. The ciliary movement is from
behind forward, i.e. towards the oral aperture 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
Tectibranchiate 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-
branchiate with internal shell, i.e.
completely overgrown by the lobes
of the mantle.
vii MOLLUSCATHE FOOT AND ITS GLANDS 111
detached. 1 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 mouth.
The Gymnosomata (Fig. 16, p. 11) are to be derived from the Aplysiidce, 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 Gym nosomata, 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 glands of the Gastropoda. Many Gastropods, and especially
most Prosobranchia 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 Prosobranchia 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 Pulmonata 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 ProsobrancMa, 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
Prosobranchia has rightly been considered homologous with the byssus
gland of the Lamdlibranchia. It is developed in varying degrees, and
not infrequently is altogether wanting. Its slimy secretion forms
threads by means of which many Prosobranchia attach themselves to
objects in the water. Some terrestrial Pulmonata also lower themselves
from a height (from plants) by means of the tough threads which they
secrete.
1 With regard to the derivation of the Thecosanuita from the Cepkalaspidce, which, like
other Opisthobrauchia, have as a rule no operculum. it must be noted ih&tActceon, which
is iu many respects a primitive CepJudaspid genus, possesses an operculum.
112 COMPARATIVE ANATOMY CHAP.
Besides these two, other pedal glands are occasionally found. Only one need be
mentioned, which is found in some Opisthobranchia (Pleurobranchus, Pleurobranchcea,
Pleurophyllidia). It lies at the posterior end of the sole, and consists of glandular
caeca, each of which opens separately.
C. Seaphopoda.
The foot of Dentalium (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 (Pelecypoda) 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. 1 4), and so has Pectunculus. 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
MOLLUSCATHE FOOT AND ITS GLANDS
.
OF
FIG. 101. Anatomy of Dent-
alium entale, after Leuckart
(Wandtafeln) and Lacaze-Du-
thiers. The riglit half of the
shell and the lower portion of
the mantle are removed. a,
Pallial nerve running up from
the visceral ganglion ; b, shell ;
c, space between the mantle and
shell ; d, anus ; e, visceral gan-
glion ; /, mantle cavity ; g,
mantle ; h, lower, t, upper buccal
ganglion ; i, auditory organ ; A-,
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 ; i(, 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. II
114
COMPARATIVE ANATOMY
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 cavity 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 by 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 lamellse
which occupy the shelves. As the amount of the secretion in the cavity increases,
these lamellae 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
byssus (Solenomya).
2. With the same foot, a small lamella rises from
the base of the simple invagination ; the byssus is
very slightly developed (Nucula, Leda).
3. The invagination becomes differentiated into a
cavity and a duct, and the byssus and its glands are
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
Molmscs. This attachment may be more or less firm, and may be temporary or
permanent (Limidoe, Spondylidce, Pectinidcc, 1 Mytilidce, Arcidce, 1 Carditidcc, 1 Ery-
cinidce, Galeommidce, Tridacnidce, Cyprinidce, 1 Fenendce, 1 Glycymcridce, My idee, 1 etc.)
When the byssus is very highly developed, some of the pedal muscles become
attached to the byssus gland and form the retractors of the byssus.
4. Many Lamellibranchs, in the adult state, have neither byssus nor byssus
glands, but the cavity, the duct, and even the retractors (e.g. Trigonia] may be
FIG. 102. Byssus of a Lamel-
libranch with its cavity and
duct. 1, Diagrammatic trans-
verse section through the foot ; 2,
main stem ; 3, terminal threads
ie byssus to a foreign
1 Proparte.
vii MOLLUSCATHE FOOT AND ITS GLANDS 115
retained.^ The byssal apparatus may be found, in closely-related forms, sometimes
with 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
(A re idee, 1 Carditidte, 1 Cyprinidce, 1 Tellinidce, Scrobiculariidce, Myidce? Cardiidce, 1
Lucinidm (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 (Unionidce, many Veneridce, Cyrenidce,
Psammobiidce, 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 Solenocurtus ; club-shaped and truncated at the
tip in Pharus, Cultellus, Siligua, 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 (Ostrcidce). In forms which inhabit mud or excavations
made by themselves in stone, etc., and which surround the body with an accessory
calcareous tube (Gastrochcenidce, Clavagellidce), the foot is also reduced to a small,
usually finger-shaped rudiment. The series of boring Pholadidce is specially
interesting. Pholas has a pestle- or sucker-shaped foot, which, projecting through
the shell cleft, serves to attach the animal while boring. In Pholadidea and
Jouannelia 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 callurn, and the foot completely atrophies, so that the animals are no longer
capable of locomotion.
In the attached Anomia, 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 Molluscan 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.
116 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 Nautilus
remain separate throughout life, but in
the Dibranchiata overlap, may be con-
sidered as epipodia. The accompany-
ing figure of a Cephalopod embryo con-
FIO. los.-Embryo of a Cephaiopod, fi thi opinion . t h e rudimentary
seen obliquely from the left posterior side * . > . . . J
(after Grenacher). 1, Mantle; 2, anus ; Siphon IS seen in the typical position Ol
3, right ctenidium ; 4, rudimentary epipodia in the shape of tWO lateral f olds
y ^c;\^eye y . 0rSan; "' ^ ' '' running backward above the foot and
under the visceral dome.
In Nautilus and the Decapoda (excluding the Loligopsidce) a valve
is present within the siphon. For the form of the siphon, see p. 38.
1. The Arms of the Tetrabranchia (Nautilus).
The "head" of the Nautilus (Fig. 104) carries numerous tentacles placed in a
circle round the mouth ; 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 Dibranchia, 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 particularly thick anteriorly, and
here forms a lobe, the so-called hood (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
MOLLUSCATHE 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
t s
FIG. 104. Circumoral ring of tentacles in Nautilus pompilius (after Lankester and Bourne).
From the oral or ventral side. Upper figure male, lower female, a, Shell ; b, 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 ;
n, 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 Nautilus, and probably takes some part in copulation (see
the Copulatory Apparatus, p. 242).
2. The Arms of the Dibranchia.
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 Octopoda, 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 Decapoda have ten arms ; eight of these correspond with the eight arms of
the Octopoda, but are shorter and are never connected by membranes. The two
others, the prehensile tentacles, are inserted between the third and fourth Octopodan
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 Decapoda (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 (hectocotylised) to assist in copulation. In some
Octopoda it even becomes detached from the body 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 the 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 funnel. 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
much 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 1 of the Foot (Turgescence).
Imbibition of Water.
The foot in many Lamellibranchia 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 MOLL USCA MUSCULATURE 1 1 9
(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 Josephina. 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 COMPARATIVE 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 the 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 45. 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 possible, the
abortion of the foot and vermiform shape of the body being taken into account, and
Chitonellus taken as the transition form, it may be assumed that the circular muscle
layer, and in particular the groups of fibres converging towards the foot, correspond
with the dorso-ventral muscles of Chiton, 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.
VII
MOL L USOA MUSCULATURE
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
their foot chiefly as a sucker for
attaching themselves to some firm
surface. These 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 Haliotis (Fig.
105), the ear-shaped 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 cavity and the viscera to
the left. In many Fissurellidce
and the Docoglossa, the shell has
become cup - shaped and sym-
metrical ; the columellar muscle,
which 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- FlG - 105. -Haliotis, from above, after removal of the
, . i ,. j i. shell, the mantle, and the entire dorsal integument (after
shaped sectional surface, and, by Wegmann) . f)Snou t; sand p, salivary glands ; ft , lateral
its base of the same shape, to the pockets of the oesophagus ; i, mid-gut ; a, oesophagus ; r,
sucker-like foot. The viscera are rectum ; e, stomach with caecum (c) ; h, digestive gland
contained in its hollow axis (Fig. ( liver >' its right-hand portion which lies next the large
106). The same arrangement
occurs in all cases where the shell
is flatly conical, cup- or plate-shaped, as in the Hipponycidce and the Capulidce
among the Monotocardia.
Heteropoda. In this order, in which the atrophy of the shell, 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.
columellar muscle (m) is covered by the genital gland.
A fringed epipodium encircles the body.
122
COMPARATIVE ANATOMY
CHAP.
ecr
The cutis in Atlanta is still comparatively thin. The network of muscles lying
immediately beneath it is not more strongly developed 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 Heteropoda.
The integument greatly increases in
thickness in the typical Hetcropoda
(Carinaria, Pterotrachea), 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 longitudinally. In addition, an ex-
ternal circular musculature is found in
Carinaria nearly all over the body, in
Pterotrachea only in the snout.
Turning now to Carinaria, which
env
FIG. 106. 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 ; int, still possesses a delicate, easily detachable
intestine ; fc, larger right nephridium ; i, smaller s h e ii covering the visceral dome, but un-
left ditto ; e, mantle border, widening anteriorly We to ^ other t Qf the bod
into the mantle fold ; ecr, em, edge of mantle. *
we find the columellar muscle still per-
sisting in the form of two bands descending from the visceral dome into the fin and
radiating out to its edge.
In Pterotrachea, where the shell is wanting and the visceral dome rudimentary,
the columellar muscle. is also reduced. It has now no connection with 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.
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 atrophies or perhaps forms part of the pedal musculature. The
subcutaneous dermo - muscular tube, on the other hand, develops in proportion
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 much 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 with 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 USCAMUSCULA TUBE
123
in the Limacinidce, but dorsal in the Cavoliniidce, 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 shell-bearing Pulmonata, the columellar muscle is
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.
strongly
In the Daudebardia and Testacellidce, in
which the dwindling visceral dome with the
shell which covers it have shifted to the
FIG. 107. Shell of Helix, in longitudinal
section through the columellar axis (after
Howes). c, Columella ; mi, columellar
posterior end of the body, and m which all muscle; p> edge of oral aperture (peritreme).
possibility of the retraction of the body into
the shell has ceased, only parts of the columellar muscle are retained, and naturally
those parts which are still functional. In the Daudebardia and Testacellidce these
are the retractors of the tentacles, and in Daudebardia 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 with the body wall on each
side of it. In D. saulcyi the retractors do not run so far back, but the two on the
right fuse with the two 011 the left, and pass into the pedal musculature in the
anterior half of the body. The same is the case in the Testacellidce.
The Retractors of the Pharynx. In Daudebardia rufa there are found, attached
to the pharynx, two retractors, which, passing through the cesophageal 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 cesophageal 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 cesophageal 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.
In Dentolium (Fi<
C. Seaphopoda.
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
muscle which retracts the foot, and draws the whole of the lower
portion of the body back into the upper part of the shell.
D. Lamellibranehia.
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 musculature. 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 Mytilacea, 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, Aspergillum) agree in having only one. The Anomiidce,
Ostreidce, Spondylidce, Limidce, Pectinidce, Aviculidce, Muelleridce, etc., are
Monomyarian.
The adductor often (e.g. Pecten, Ostrea, Nueula) consists of two apparently
different parts, one containing smooth fibres and the other fibres which appear
vii MOLLUSCA MUSCULATURE 125
transversely striated, although their striation does not correspond with that of
Arthropod and Vertebrate muscles.
The pedal musculature, taken as a whole, answers 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
Protobmnchiate 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 (cf. Fig. 108) : (1) the protractor
.-OA
-OB
FIG. 10S. Pliodon Spekei, from the left (after Felseneer). 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.
AY here 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 cases 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 cephalic cartilage.
1. Tetrabranehia (Nautilus).
Nautilus possesses only the cephalic cartilage. This is shaped
somewhat like an X, with thick limbs. The oesophagus 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 (cf. 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 collaris, runs
forward on each side embracing the nuchal part of the body. The
two unite on the neck to form the muscular nuchal 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
Nautilus, 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.
The cephalic cartilage (Fig. 109) is every -
Fio. 109. Cephalic cartilage of Sepia, where well developed. It encloses all those
1. Central aperture through which the ceso- central portions of the nervous system which
phagus passes ; 2, preorbital cartilage ; 3, are crow ded round the oesophagus, being in
chamber for the eye ; 4, cartilaginous ,-, ? / i -n i
auditory capsule. the form of a hollow circular capsule traversed
by the oesophagus. Processes of this cartilage
assist in supporting the eyes, and in conjunction with independent, preorbital
cartilages form a kind of cartilaginous eye socket. A basibrachial cartilage is
found at the base of the anterior arms in some Decapoda. We have further to
VII
MOLL USC A MUSCULA TURE
127
mention the nuchal cartilage and the cartilages for locking the cleft of the mantle
cavity (p. 55). In the diaphragm, i.e. in the posterior wall of the visceral dome,
over which the mantle 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 neck ; 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 (cf.
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 Octopoda there is a cartilaginous
band on each side in the dorsal integument
which 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-
teuthis, is not in reality homologous with
the shell of the Decapoda, but corresponds
with the cartilaginous bands of Octopus
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 Dibranchia, that of the mantle, ^ G - HO-Diagram of the more important
, , , , ' parts of the Dibranchiate musculature.
the tins, and the arms Cannot be Body seen from the left side. , Ventral ; d,
described in detail. AVe note, how- -dorsal; a, anterior ;p, posterior; 1, depressor
ever, that the pallial musculature SS^'SSLTfLSSTii
IS principally attached to the shell laris; 5, adductor infundibuli; 6, shell; 7,
Or tO the dorsal Cartilage, the fin- dorsal cartilage ;S,nuchal cartilage; 9, cephalic
musculature to the fin-cartilages, and ZS^X-^-ESSS
the brachial musculature tO the an- wall of the visceral dome ; 12, corresponding
terior side Of the Cephalic Cartilage, cartilaginous knob on the inner wall of the
j , ,, i-i i -i ,- mantle, which fits into 11 ; 13, funnel or siphon
and partly to the basi-brachial carti- (ill fundibuiuin) ; 14, diaphragm cartilage,
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 eapitis 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 then fuse more completely with the lateral retractors
(Ommastrephes, Sepioteuthis, Loligo, Sepiola), so that finally (Sepia) 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. In the Octopoda 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 ganglionie 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 MOLLUSCATHE 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 with those on the
opposite side by a transverse commissure, which runs in front of and
over the oesophagus and contains ganglion cells ; this is the cerebral
or upper half of the cesophageal 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. Chitonidae (Figs. Ill and 51, p. 40). The scheme just given
is founded upon the nervous system of Chiton. The typical g Qn g|ja ^f
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 cesophageal 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 cesophageal ring.
Looking more closely at the nervous system of the Chitonidae, we 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 cesophageal 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, forms the lower half of the cesophageal ring. The upper
and lower halves together form the closed cesophageal 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 cesophageal ring. The buccal ganglionic
mass in C. rubicundus 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.
(b) On each side, from the lower half of the cesophageal ring, somewhat further
VOL. II K
130 COMPARATIVE ANATOMY CHAP.
in than the buccal connective, a nerve (the subradular connective) rises and runs
n ^ ,
,0
FIG. 111. Diagram of the nervous system of Chiton siculus (after Bela Haller). The mantle
removed on the right side. In the centre and to the left the upper part of the foot removed, to
expose the pedal nervous system. F, Foot ; K, last gill; A, anus ; 0, upper, U, lower half of the
resophageal ring ; 1, '2, nerves of the cesophageal ring ; c, connective to the anterior visceral ganglia ;
p, connective to the ganglia of the subradular organ n (above on the left) ; Es, pleurovisceral and
pedal cords ; mn, gastric nerve ; So, point of attachment of the sphincter oris ; n (below on the
right), ni, tt2 nephridial 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 MOLLUSCATHE 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, but no nerves are
given off by these commissures to the pedal musculature. In Chiton rubicundus the
visceral and pedal cords are united by numerous connectives, which, in other
Chitonidae, 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 oesophageal 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.
(&) Nerves of the pleurovisceral cords. Each of the pleurovisceral cords
gives off 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 cords. The pedal cords give off 011 each side seven or
eight nerves outwards to the lateral musculature of the body, and specially numerous
nerves run down from 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 Chitonidce principally in a tendency to form
distinct ganglia; the pedal and pleuroviseeral 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 Proneomenia 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
03sophageal 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
1 These connectives and commissures, however, do not seem to run uninterruptedly
from one cord to the other.
132 COMPARATIVE ANATOMY CHAP.
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.
Dondersia 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 Lepidomenia hystrix, 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 Neomenia and Chcetoderma, no connectives between the visceral and pedal
FIG. 112. Nervous system of Proneomenia Sluiteri (original drawing by J. Heuscher).
1, Cerebral ganglia ; 2, pleurovisceral cords ; 3, 4, 5, posterior ganglia of the pleurovisceral cords ;
6, sublingual ganglia; 7, anterior pedal ganglia; 8, right pedal cord; 9, left pedal cord; 10, 11,
strong posterior commissures between the pedal cords ; 12, anterior pedal commissure ; 13, sub-
lingual commissure.
cords have been observed, and, so far as is at present known, in Chcetoderma, the
commissures between the pedal cords are also wanting. Further, in Cha-toderma,
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. 1
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 Prosobranchia, which will be further described in this
section.
The nervous system of this class consists typically of those parts
which we have already mentioned in our scheme of the organisation of
the Mollusca, viz. :
1 For further details see Simrotli's new edition of Bronn's Klassen und Ordnungen
des Thier-reiches, vol. iii.
vii MOLLUSCATHE NERVOUS SYSTEM 133
1. Two cerebral ganglia near or above the oesophagus, which
are 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 cesophageal 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
cerebropleural, 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 pleuroviseeral con-
nectives.
5. A ganglion, which may be called parietal, almost always occurs
in the course of each pleuroviseeral 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 pleuroviseeral 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 A ctceon)
and the Pulmonata 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 Opisthobranchia and Pulmonata are therefore called euthyneurous
Gastropoda.
In the Prosolyranchia and Actceon, the pleuroviseeral connectives are
asymmetrical, inasmuch as they cross one another, the connective
springing from the right pleural ganglion running over 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 Prosobmnchia suidAdceon 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 typical nervous system of the Gastropoda with that of the
Amphineura reveals the following homologies :
1. The cerebral ganglia of the Gastropoda correspond with the oesophageal 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 answer to the pedal cords in the Am-
phineura, concentrated each into a single ganglion. The arrangement in the Dioto-
cardia, which are the more primitive Prosobranchia, 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 Amphineura. 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 conies
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 pleuropedal 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 MOLLUSGATHE 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 pleuro visceral cords unite above the intestine ;
in all other Molluscs the point of junction (which is the visceral ganglion) lies below
the intestine.
In Proneomenia 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 Pleuroviseeral Connective
(Chiastoneury) (Figs. 113-116).
Several attempts have been made to explain the peculiar crossing
of these connectives in the Prosobranchia. 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 Opislhobranchia 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
supraintestinal 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 infraintestinal ganglion. The
pleurovisceral connectives, in which these ganglia lie, now cross and
give rise to the condition called chiastoneury. The visceral
uvct
FIGS. 113, 114, 115, 116. Diagrams to illustrate the shifting forward of the pallial complex
along the right side of the body and the development of chiastoneury. p, Mouth ; ulc, ulpl,
ulp, original left cerebral-, pletiral-, and pedal-ganglion ; ulpa, urpa, original left and original right
parietal ganglion ; ula, original left auricle ; M.OS, uros, original left and original right osphradium ;
ulct, urct, original left and original right ctenidiuni ; mb, base of the mantle ; mr, edge of the same ;
m, 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 MOLLUSCATHE 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.
I. Prosobranchia. () Diotocardia. These are the most primitive Gastropoda.
The ganglia are not yet very distinct, thus recalling the Amphineara. The cerebral
ganglia are connected by two long commissures, the cerebral commissure running
forward over the pharynx, and the labial commissure running under the oesophagus.
The indistinctly separated buccal"gaigiia together form a horseshoe-shaped figure,
and are united on each side by a connective with the thickened 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 epipodium. There is only one indistinct visceral
ganglion, which is joined to the pleural ganglia by two pleuro visceral connectives,
crossed in the typical way.
In Fissurella only does a ganglion occur on the supraintestinal pleuro visceral
connective. In no other Diotocardian 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 the
gill. Where a ctenidiuin, 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 (Turbinidce,
Trochidfc), 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 Monotocardia,
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, Fissurella possesses both a
supraintestinal and a left branchial ganglion, it would be necessary to assume that
an originally single ganglion had here become divided into two.
The symmetrical pallial nerve is always connected by a pallia! 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 Neritidcf and Helicinidce are peculiar, in that the supra-
intestinal pleurovisceral connective and its corresponding ganglion are wanting.
Docoglossa. The only essential difference 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 Ampullaridce). The pedal cords and
transverse commissures are wanting (except in the Architcenioglossa: Paludinidce,
Cydophoridce, Cyprccidcv}. 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 eacli side. If this anastomosis were to shift along the 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 fiostrifcra,
viz. some of the Cerithiidce, Ampullariidce,
Turitellidce, Xenophoridce, Struthiolar ii pe dal cord (=pedai
m, , . . ganglia); 8, auditory organ; 9, olfactory nerve; 10,
The Cerebral COrd gives rise optic ganglion ; ll, nerve of the optic tentacles ;
laterally to the large Optic nerves 12, connective to the pharyngeal ganglion ; 13,
(each of which at once swells into labia ;. ner i yes ' ; 1 14 ' Cereb 1 ral ord < =cerebral
ganglia) ; 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
^
146
COMPARATIVE ANATOMY
CHAP. VII
tentacles come from a braehial ganglion, which, however, does not
supply all the tentacles (Fig. 126, a);
this is joined to the pedal ring by a
brachiopedal connective.
The pleuroviseeral 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
FIG. 126. Nervous system of Nautilus, blood-vessels, and form a genital gan-
from the right side. Numbering the same ^ QU facfa up j n fa Q visceral dome,
as in Fig. 125. a, Ganglion for the ten- c
tacles of the posterior and inner lobes in The sympatll eti c nervous system consists
the female. . * . . .
of an mfra-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 braehial 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 braehial ganglia become
joined in the Dibranehia to the cerebral ganglia by cerebrobrachial
connectives. In Eledone and Octopus, they are further connected with
one another by a thin supraoesophageal commissure.
The pleura! ganglia lie laterally in the perioesophageal 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-
oasophageal 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
>ack to the right and left, and the liver removed. 1, Brachial artery ; 2, brachial nerve ; 3, pharynx ; 4, buccal ; 5, cerebral
anglion ; 6, efferent duct of the upper salivary glands ; 7, funnel ; 8, upper salivary glands ; 0, crop ; 10, anus ; 11, afferent
n-anchial vessel (branchial artery) ; 12, left renal aperture ; 13, efferent branchial vessel (branchial vein) ; 14, gastric ganglion ;
c>, left auricle; 16, spiral c*cum of the stomach; 17, renal sac; 18, water canal; 19, ventricle; 20, ovary; 21, rectum; 22.
fferenfr ducts of the digestive gland (liver), cut through near its opening into the intestine ; 23, mantle ; 24, stomach ; 25,
ight ctenidiurn ; 26, aperture of the right oviduct ; 27, stellate ganglion ; 28, nerve to the gastric ganglion ; 29, upper salivary
land ; 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-infundibular ; (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 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
Sepia (D), Loligo (C), and
Sepiola (B), it becomes
more and more evident, till
finally in Ommatostreplies
(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 upper 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
FIG. 128. Central nervous system of various Dibranchia,
from thenright side. All the figures after Pelseneer. A, Ommato-
strephes ; B, Sepiola ; C, Loligo ; D, Sepia ; E, Octopus ; F, Argo-
nauta. 1, Cerebral ; 2, pedal ; 3, visceral ; 4, brachial ; 5, upper
buccal ganglion ; 6, infundibular nerve; 7, visceral nerve; 8, optic
nerve cut through; 9, pallial nerve ; 10, brachial nerves ; and in
Fig. B the pharynx (ph), and resophagus (ce) are drawn in black.
vii MOLLU8CATHS 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.
i.
Chiastoneury, i.e. the crossing of the two pleuro- visceral connectives in the
Prosobranchia, may be explained on the three following assumptions.
1. The ancestors of the Prosobranchia were symmetrical animals ; the mantle
cavity lay behind the visceral dome and in it the pallial complex, that is, the ctenidia,
osphradia, uephridial 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 Tectibranchia, among the Opisthobranchia,
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. 1 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 nephiidial
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 Lamellibranchia, the Protobranchia (Nucula, Leda, Solenomya),
and even in some Chitonidce, and those Solenogastrcs 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,
widest 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 symmetrical Molluscs, the
visceral ganglion lies, as in the Gastropoda, below the intestine.
The third assumption, that the pallial complex has shifted forward, requires
separate discussion.
If the pallial complex did thus shift forward, chiastoneury must necessarily
have taken place ; the original left half of the complex must necessarily have become
the present right half, and vice versa. Further, the right pleuro-visceral connective
would have to become the supra-intestinal connective and the left the infra-intestinal
connective ; the original right parietal ganglion the supra-intestinal ganglion, and
the original left parietal the infra-intestinal ganglion. But why 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 Fissurella, a
Patella, or a Chiton, if we assume the imbricated
"p 1 **" 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 , . J , ,
Gastropod, from the side, o, Mouth ; tected doi ' sall y b Y the shell. The hard surface
fc, head ; sm, shell muscle ; oso, apical along which the animal slowly crept served to
shell aperture ; a, anus ; n, renal aper- protect its lower side, the dorsal shell being
* u o ; t ^' mantlecavity; c *> ctenidium ; presse d firmly against the substratum, when
necessary, by the contraction of a powerful shell
muscle (cf. Fig. 106, 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 MOLLUSCATHE 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. The 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 shell against the
surface on which it rests, but for withdrawing the head and foot into the shell. It
becomes the columellar muscle (Fig. 131, sm}.
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. 130. Hypothetical primitive Gastropod,
from above, o, Mouth ; ulc, idpl, ulp, original
left cerebral, pleural and pedal ganglia ; ulpa,
iirpa, original left and right parietal ganglia ;
tila, original left auricles ; uos, uros, original left
and right osphradia (Spengel's organs) ; ulct, urct,
original left and right ctenidia (gills) ; mb, base
of the mantle ; mr, edge of the mantle ; m, mantle
cavity; r, visceral ganglion; ve, ventricle; o,
anus.
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 Dcntalium.
Such a shell carried 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, which
now lie dorsally, since in this position the mantle cavity is subjected to least pressure
m el
FIG. 13$
from the viscera and from the columellar muscles. The downward pressure of
the visceral mass which now takes place would tend indeed to widen the cavity.
2. The 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 the 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 the columellar muscle ; it would
be squeezed together, so that the
* circulation of the respiratory water
would be prevented or at least
rendered more difficult, as would
also the ejection of the 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
head, 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
FIG. 134.
vii MOLLUSCATHE 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 the 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
to be the most favourable im-
aginable for locomotion and for ii nes indicates the amount of the pressure, a, Point of greatest
the functions of the cephalic pressure ; b, point of least pressure. Th arrows give the
direction in which shifting takes place. It is evident that
the left side of the pallial complex is subjected to greater
pressure than the right.
FIG. 135. Diagram illustrating the variations of pres-
sure to which the shell and visceral dome are subjected
when inclined to the left. The thickness of the concentric
organs.
If we suppose this process
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.
4.
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 will, 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 conch ologists call a dextrally twisted shell.
The growth of the Gastropod shell actually takes place in this last manner.
6.
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.
4. 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
vii MOLLUSCATHE ASYMMETRY OF THE GASTROPODA 155
development of the shell during which its displacement in a backward direction and the
shifting forward of the pallia! 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 grow 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 favour 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
difficulty 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 Mollusca,
which the Chitonidce 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 Prosobranchia, 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 Pleurotomaria (Fig. 136 A), Polytremaria
(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 finally in Fissurella (Fig.
FIG. 136. Shells of A, Pleurotomaria ; B, Polytremaria ; C, E, Emarginula ; D, Haliotis ;
F, Fissurella; G, 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, Capulidce,
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 Mollusca, such as the asymmetry of the pallial
complex in most Gastropoda. Many ' Diotocardia, all Monotocardia, all Opistho-
branchia, and all Pulmonata show marked asymmetry in the pallial complex. The
asymmetry consists principally in the absence of one gill, one osphradium, 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 MOLLUSCATHE ASYMMETRY OF THE GASTROPODA 157
(which in a Prosobmnch would lie to the right in the mantle cavity near the 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 ctenidium 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-
gobranchia), 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 Basommatophora 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. Haliotis and Fissurella
are the only Molluscs to which this applies. In Haliotis, whose shell is still
twisted, the right (originally left) gill is in reality the smaller. But in Fissurella
and Subemarginula, 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 grow
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 cavity or fold
would be advantageous, since then symmetrical growth of the shell could take
place. If the difference 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 growth in height is insignificant, a flat cup-shaped shell
would result (Haliotis, Emarginula, Fissurella, Patella, etc.).
13.
Chiastoneury only takes place Avhen 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 Proso-
branchia. The original right gill in them lies quite to the left of the mantle cavity.
In the Azygobranchia 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 Tedibranchia 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. 1
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 Pulmonata are
thus euthyneurous.
14.
We saw, in paragraph
3, that with a strongly de-
veloped visceral dome and
posteriorly placed pallial
complex, a shell inclined
forward or coiled forward is
an impossibility for a rep-
tant Gastropod. But such
a shell is not an impossibility for an animal which does not creep. For example,
in a swimming 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
1 Except in Actceon, an exception which makes it probable that in the Opistho-
branchia the pallial complex has secondarily returned from an anterior position.
FIG. 137. Nautilus, diagram, do, Dorsal ; re, ventral ; vo,
anterior ; hi, posterior.
UN.
.
vii MOLLUSCATHE ASYMMETRY OF THE GASTROPODA 159
not become coiled forward, the original posterior position of the pallial complex
being retained as the most favourable under such 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 Belemnitidce 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 mud, like a limicolous bivalve, there appears no reason
940
FIG. 139. Hypothetical transition
form between Dentalium (Fig. 138)
and the racial form of the Gastropoda
(Fig. 140), from the left side.
FIG. 138. Dentalium, diagram from
the left side, g, Genital gland ; W,
cephalic tentacles.
FIG. 140. Hypothetical racial form
of the Gastropoda, from 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. Dentcdium (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 Fissurella with its pallial complex twisted back, and with a long and turret-
like shell, is, from our point of view, very appropriate. A Fissurella, so transformed,
would almost exactly resemble the hypothetical symmetrical racial form of the Gas-
tropoda, in which, however, we should have to assume a mantle- and shell-cleft
reaching to their edges (cf. Fig. 136, I).
The anatomy of the Protobranchia, which has recently been more closely studied,
and especially the posterior position of the two gills, the flat 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,
papillae, 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 Monotocardian, 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 which 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
having the asymmetrical organs in the inverse position which corresponds with this
twist. Such are, among the Prosobranchia, Neptunea contraria, Triforis, and occa-
sional specimens of Buccinum; among the Pulmonata, Physa, Clausilia, Helicter,
Amphidromus, and occasional specimens of Helix and Limmaea. In Bulimus per-
versus, 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 Lanistes of the genus Ampul-
laria; among the Pulmonata, in Choanomphalus Maacki and Pompholyx solida;
among the Opisthobranchia, in those Pteropoda which, whether as adults (Lima-
cinidce) or larvae (Cymbuliidce), have a twisted shell. This fact is entirely against
our theory in explanation of the asymmetry of the Gastropoda, for this theory
vii MOLLUSCATHE 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 shells (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. crocastoma Ph (E) the spiral is flat, in A. (Ceratodes) rotula
Mss. (D) and A. (Ceratodes) chiquitensis d'Orb (C) the spiral is already pushed
through or sunk, yet we find a true umbilicus
on the umbilical side. In A. (Lanistes) Bol-
teniana Chemn. (B), and still more in A.
purpurea Jon. (A), the false spiral appeal's 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
FIG. 142.
Lanistes has not a spirally twisted
In those Pteropods which combine a
162 COMPARATIVE ANATOMY CHAP.
siriistrally 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 larvae of the Cymbuliidce and in Limacina retroversa Flemniing,
the operculum (the free surface of which must be viewed) 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 cells), 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
Lamellibranchia, 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 Prosobranchia, sensory cells
occur scattered among the other epithelial cells throughout the whole
vii MOLLUSCATHE 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) undiffer-
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 hypobranchial 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 Prosobranchia, 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 gill, 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 Prosobranchia is least differentiated in the Dioto-
fj.i I'dia. In the Fissurdlidcc it does not exist as a sharply localised organ. In the
Mbnotocardia 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 ofj;he osphradia has already been given in
another place ( V. p. 71). As an example of the special form and structure of this
organ we select the highly developed osphradium of a Toxiglossa, Cassidaria
tyrrliciia.
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 looks like a gill 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 principal 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 with 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-visceral connective (from
the parietal ganglion when this is present) ; in the Lamellibranchia 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-visceral 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
Pulmonata 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 Cephalaspidce, 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 lamellae. " standing up on
the disc.
(c) Olfactory Pits of the Cephalopoda.
In the Dibranchia 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 Opisthobmnchia. In Nautilus the place of the
olfactory pit is occupied by the upper optic tentacle. We have
already seen that Nautilus still retains true osphradia.
(d) The Pallial Sensory Organs of the Lamellibranehia.
Several Asiphoniata 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 gill.
These are innervated by a branch of the posterior pallial nerve.
Epithelial sensory organs of various forms (plates of sensory epithelium, sensory
lamellse, or papillae, tufts of small tentacles) are found on the mantle in the
vii MOLLUSCATHE SENSORY ORGANS 165
Siphoniata ; these lie ou the retractor muscles of the siphons aiid 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 Asiphoniata.
Their function is unknown, but is supposed to be analogous to that of the
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 sicuhis, 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 these sensory epithelia in the Chitonidce to the
osphradia of other Molluscs, which here presents itself, is difficult to answer. In
position the osphradia best correspond with the epibranchial prolongations of the
paraueural 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,
Fismrella, Trochus, Turbo, and Patella). This " gustatory prominence "
(which has been best examined in Chiton) lies on the floor of the
buccal cavit} 7 ", 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 (cf. 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 Scapliopoda 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 correctly, been considered as
sensory, i.e. tactile organs (Fig. 143). They are called aesthetes, and
lie in pores on the tegmentum (rf. p. 39) ; they are club-shaped or cylin-
drical, and each carries a deep cup-like chitinous cap. Each megal-
sesthete gives oif all round numerous fine branches or miersesthetes,
each of which ends in a swelling which carries a small chitinous
cap. The body of the {esthetes 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
MOLLUSCATHE 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 {esthetes and their
fibrous strands is not yet certainly known. It is probable that they are innervated
from the dorsal lateral branches of the plenro-visceral cords. It is even not known
whether the fibrous 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 {esthetes are merely modifications
ttik
FIG. 143. Section of the tegmentum of Chiton laevis showing an aesthete (after Blumrich).
ink, Micnesthete ; j>cr, periostracum ; sk, principal aesthete ; t, tegmentum ; dz, cells resembling
glandular cells ; hf, clear fibres ; fs, fibrous strand ; c, chitinous cap.
of the spines with their papillae and formative cells, which are so common in the
integument of the Chitonidcc. The chitinous cap would then represent part of the
chitinogenous base of the spine.
The sensory nature of the aesthetes is rendered highly probable
by the circumstance that in a few species of Chiton individual megal-
aesthetes are transformed into eyes.
Each eye is furnished with a pigmented envelope, which is pene-
trated by the micraesthetes, 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 epithelial 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 which 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
Cephalopoda, the remains of
the canal of invagination is
retained (Kolliker's canal),
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, maeulse
and eristse 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 connective 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
FIG. 144. Auditory organ of Pterotrachea (after
Glaus). 1, Auditory nerve ; 2, structureless membrane ;
3 and 4, ciliated cells ; 5, otolith ; 6, auditory cells ; 7,
supporting or isolating cells ; 8, large central auditory
VII
MOLLUSCATHE SENSORY ORGANS
169
numerous auditory cells, 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 Cephalopods have shown that one of the
functions of the otocysts is to regulate the position of the animal
while swimming.
They are cup-shaped
C. Visual Organs.
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- FlG 145 ._ Eye of Nautilus (after Hensen). 1, Optic
like Cells : ( 1 ) Clear Cells ravity (pit) ; 2, layers of rods ; 3, pigment layer ; 4, layer of
Without pigment and (2} visual cells ; 5 > la >' er of ganglion cells ; 6, branches of the
pigmented cells. Whether
either or possibly both of these kinds can be considered as retinal cells
170
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 which 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, e.g.Haliotidce, Patellidce, 7rochidce, Delphinulidce, and Stoma-
tiidcc.
In connection with the claim that Nautilus (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 nerve and the retina.
2. Optic 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 mass. The optic
nerve usually swells into a peripheral
ganglion opticum before reaching the
retina.
FIG. i46.-Eye of a Puimonate. i, The tentacular eyes of most Gastro-
Outer, 2, inner cornea ; 3, body epithe- podd, except those DiotoCCirdia which
Hum; 4, vitreous body; 5, retina; 6, have CUp-Hke eyCS, are of this simple
ganglion opticum ; 7, optic nerve. ,
character.
3. The Eye of the Dibranehiate 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
MOLLUSCATHE SENSORY ORGANS
171
eyes. In the Tetrabranchiate ^'auf-ilii*, 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. 14'. Development of the eye of the dibranchiate 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 same 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 cornea 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
FIG. 148. Section of the eye of Sepia officinalis, somewhat diagrammatic (after Hensen).
1-8, As in Fig. 147 ; 1+3, corpus epitheliale ; 9, anterior chamber of the eye opening outward at
10; 11, cartilaginous capsule; 12, ganglion opticum= retinal ganglion; 13, nervus opticus ; -2c.,
pigment layer of the 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
MOLLUSCATHE SENSORY ORGANS
173
5
retinal cells lie on the inner side of the limiting membrane, and are
thus turned to the source of light and at the same
time to the cavity of the primary vesicle. The ^:;^ ;;-.^-^ : ^ t
retina is covered on its inner side by a somewhat !]&&>
thick membrana limitans.
2. The eye is surrounded, except on the side
turned to the surface of the body, by a cartila-
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.
4. The Dorsal Eyes of Oneidium and the Eyes FIG. uy.-Two retinal
at the edge of the Mantle in Peeten (Fig. ceUs of a cephaiopod.
much magnified (after
150) and SpOndylUS. Grenacher). 1, Mem-
brana limitans ; 2, pig-
These eyes have been said to resemble vertebrate ment ; 3, secreted
eyes in structure, because in them the visual rods lhreads '> 4 > erve fibre;
*^ 5, rod \ 6, pi^nncnt * * ,
are turned away from the light, being directed limiting ceii ; s, limiting
inwards tOWardS the body. membrane ; 9, retinal
They are vesicular eyes, but in them it is the c
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 (Pedeii) 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 Pecten and Spondylus of very numerous cells.
FIG. 150. Section through the eye of Pecten (after Patten), c, Cornea ; I, lens ; ep, pig-
mented body epithelium ; g, layer of ganglion cells ; r, retina ; st, rod layer of the retina ; d, tap-
etuin ; e, pigmented epithelium ; /, sclerotica ; n, optic nerve ; n\ and n-2, 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 wall 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
MOLLUSCATHE 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 the 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 Pectcn its metallic lustre.
Dorsal eyes are found in many species of Oncidium. They lie at the tips of the
contractile papilla found on the dorsal integument of this curious Pulmonate ; on each
papilla three or four such eyes occur. Besides these, Oncidium has the two normal
cephalic eyes usually found in Gastropods.
The pallial eyes of the Lamellibranchiatcs, Pectcn and Spondyhis, 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 coloiir (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
three kinds : (1) conical visual
cells, with their bases turned
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.
FIG. 101. Section of the eye of Area toarbata
(adapted from Rawitz). 1, Retinal cells with rod-like
bodies (2) ; 3, pigment cells ; 4, slender interstitial
cells.
176 COMPARATIVE ANATOMY CHAP.
7. Degeneration of the Cephalic Eyes.
It is becoming more and more probable that 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 Lamellibranchia and CMtonidce (?) even have
cephalic eyes appearing temporarily during development ; they disappear later,
when, covered by the shell, they 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 bueeal cavity ; (2) the pharynx or cesophageal
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 pallial 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 Dentalium, 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 pallial 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
.viz MOLLUSCATHE 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, 1 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
(cesophageal 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 Lamellibranchs 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 Glossophora;
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 algae, and particles of
detritus) ^yhicll 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 Mollusca 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 proctodseum. 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 Opisthobranchia and
a few Prosobranchia) 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 Chitonidce, the Dioto-
cardia, most herbivorous Tcenioglossa, and many Pulmonata and Nudibranchia).
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, Strombidce, Chenopidce, Calyptrceidce,
Cypraeidoc, Lamellariidce, Naticidce, Scalaridce, Solariidce).
Finally, in many carnivorous Prosobranchia (Tritoniidce, Doliidce, Cassididce,
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, but that of the
proboscidal sheath. When the proboscis is protruded, it projects beyond the
aperture of the sheath and carries at its point the oral aperture.
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) Pteropoda gymnosomata also have a protrusible proboscis (Fig.
17, p. 11) provided with so-called buccal appendages. The same is present in the
allied Aplysiidcr,, but is weakly developed. The Thecosomata have no proboscis.
The buccal cavity of Dentalium is noteworthy. It extends throughout the
VII
MOLLUSCATHE ALIMENTARY CAXAL
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, the so-called cheek
pouch, which is lined with glandular epithelium and opens into the cavity anteriorly.
FIG. 152. Diagram of the proboscidal apparatus of the Prosobranchia. 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
invagiuable) wall of the same ; e-f, immovable wall of the proboscis ; /, edge of the oral aperture, at
the anterior end of the proboscis ; g, pharynx ; h, oesophagus ; i, retractor muscle ; k, 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 Terebridce).
In the Hctcropoda, 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 Pulmonates have an upper and two lateral jaws.
Most Prosobranchia and Opisthobranchia have two lateral jaws. These may
approach so near one another as almost to touch (Haliotis, Fissurella). Terrestrial
Pulmonata 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 Opisthobranchiate family Aplysiid-ce, Notarchus, Accra, Dolabella, and
Aplysiella have, besides the 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 Halopsychc, 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. When 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 Amphincura and the Scaphopoda ;
among the Prosobranchia, in the Toxoglossa, Pyramidcllidcc, Eulimidoc, many
Trochidce, the Heteropoda, and in many Nudibranchia (Tethys, Melibe, Doridopsis,
Phyllidia] ; in the Ascoglossa, and in certain Tcctibranchia (Actceon, Doridium,
Philine, Utriculus, Scaphander, Lobiger}. Among the Pulmonata they disappear
in a series of Testacellidce, being present in Daudebardia rufa, rudimentary in
D. Saulcyi, and wanting in Tcstacclla.
2. The lingual apparatus (Figs. 153, 154) is highly characteristic
of all Molluscs except the Lamellibranchia, i.e. of all Glossopliwa. It
may be said that every animal with a radula is a Mollusc.
VII
MOLLUSCATHE 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
FIG. 153. Longitudinal section (not quite median) through the snout of a Prosobranchiate,
to illustrate the pharyngeal apparatus. 1, Dorsal wall of the head; 2, mouth; 3, jaw; 4,
raclula ; 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, oesophagus ; 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
sin
FIG. 154. Median longitudinal section through the anterior part of the body of Helix
(after Howes), a-, (Esophagus ; rd, radular sheath ; nc, cerebral ganglion ; sl- 2 , aperture of the
salivary gland ; oc, muscle mass in the ventral pharyngeal wall ; rd, radula ; hj, upper jaw ; Z ls 1 2 ,
lips of the oral aperture; im, pharyngeal muscles; rwio, 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 with 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 has been seized, and often also broken up, by
the jaws. 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
movements of the tongue ; secondly, of muscles
wufch rao the whole ?"7" or the 1' ole , f the
lingual apparatus, evagmating 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 w r all after running
through the cephalic or body cavity. Pressure of blood may also take some part in
the protrusion of the pharynx.
The tongue and radula further often serve for seizing prey (e.g. in the carnivorous
Heteropoda, in Testacclla, 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 pleurae. Finally, at the
lateral edges of the radula, there are single or very numerous longitudinal rows of
marginal teeth or uncini.
Dental formulae 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 formula of some of the Molluscs in the Systematic
Review.
vii MOLLUSCATHE ALIMENTARY CANAL 183
The total number of radular teeth varies very greatly, from 16 in Eolis Drum-
mondi to 39,596 in Helix Ghietsbrcghti.
As a rule, the teeth are most numerous and finest in herbivorous 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 (ffeteropoda, carnivorous Pulmonata and Cephalopoda).
The muscular pharynx is most developed in carnivorous Piilmonates. In these
it may be half (Daudebardid) 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
originated pharynx (Fig. 54, A, p. 44).
In very rare cases (apart from the Lamellibranchia] the radula completely
atrophies ; this is the case in parasitic Gastropoda (Stilifer, Eulima, Thyca, Ento-
concha], in the ComUiophilidce (Coralliophila, Leptoconchus, Magilus, Rhizochilus) ,
among the Nudibranchia in Tethys and Melibe, among the Amphineura in Neomenia,
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).
Formation 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-
monata 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 FlG 156 ._ Longltudil , al sec tion through
formed a tooth is not replaced by another, the posterior end of the radular sheath of
but continues to produce new teeth behind a Pulmonate (after Rossler), diagram. 1, 2,
those already formed, so that for each longi- 3 > 4 ' Formative cells of the radular teeth ; 5,
,1.1 c . , , , , . , , , , f formative cells of the basal membrane ; 6. 7,
tudmal row of teeth there is at the base of ^ of ^ ^^ . g> ^ 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 Chiton idee, 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
apparatus. 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.
Amphineura. (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 Chcetoderma. They are present in some species
but appear to be absent in others. A pair of long glandular tubes
with high glandular cells 1 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 vagans,
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 Monotocardia, at the sides of the oesophagus. In the
former case, the ducts are short and do not pass through the oesophageal
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.
Haliotis, Fissurella), and further in Patella, the Scalariidce, lanthinidte,
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 Ampullariidce, the ducts of the salivary glands do not pass
1 This differs somawhat from the description found in Simroth (Bronn's Klasien
und Ordnungen, vol. iii. pp. 183-185).
vii MOLLU8CATHE ALIMEXTARY CAXAL 185
through the oesophageal ring, which here, as in the Diotocardia, 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, Xemphoridce, etc.) sac-like.
The passage of the ducts of the salivary glands through the
cesophageal ring in the Monotocardia 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 Monotocardia 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 cesophageal 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 salivary glands of certain Prosobranehia
(species of Dolium, Cassis, Cassidaria, Tritonium, Murex) and Opistho-
Iranchw. (Pleuro^ranclius, Pleurobrdnchidium) contains 2'18-4'25 percent
of free sulphuric acid. These carnivorous animals are able, by means
of their proboscides, to bore into other Molluscs and Eckinoderms 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.
(6) 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.
186
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 Cirrhotcuthis and Loligopsis, in w r hich they are said to
be 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, dissected out and seen from the right side (after
Howes). 1, 3, Tentacles ; 2, constrictor pharyngis ; 4, levator pharyngis ; 5, depressor ; 6, pro-
tractor pharyngis ; 7, pharyngeal bulb ; 8, radular sheath ; 9, columellar 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 ; 18, anus.
glands occasionally (e.g. in Oegopsidce) 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 ( ? without exception) have an additional acinose
viz MOLLUSGATHE ALIMENTARY CANAL 187
lingual gland, opening into that part of the pharyngeal cavity which
lies between the tongue and jaws.
The Lajnellibranchia, 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 Bhipidoglossa, which will be described
later.
Natica, which bores through the shells of living Lamellibranchs 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 (Esophagus.
That portion of the intestine which lies between the pharynx (or
the mouth in LameUilranchs) 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 LameUi-
lranchs 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 difficult to decide whether these are anterior
divisions of the stomach or posterior widenings of the oesophagus.
In LameUibranchia, terrestrial Pulmonata, most Opisthobranchia, and
the Cephalopoda, Decapoda 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 Solcnogastres (e.g. Proneonunm), on the boundary between the short
oesophagus and the mid-gut, a more or less long blind diverticuliun 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.
188
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 cesophageal sacs in the Chitonidce and Rhipidoglossa. This is a saccular
widening of the oesophagus (Fig. 158, m), which, on account of the constitution of
its walls, has been compared with the psalterium of a Ruminant. A similar widen-
ing of the oesophagus is found in Cyprceidoe 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-
mentary pharynx, and then the long oesophagus, which runs through the whole
length of the non-protrusible portion of the proboscis, passes through the cesophageal
d 6,
dm
til
FIG. 158. Median longitudinal section through Patella (after Ray Lankester). brv, Efferent
branchial vessel ; bra, afferent ditto ; asd, duct of salivary gland sd ; go, anus ; no, right nephridial
aperture ; sd, salivary gland ; cor, heart ; pe, pericardium ; np, kidney ; d, intestine ; Tip, hepatic
gland (liver) ; v, blood vessel ; m (to the right), border of mantle covering the gills ; r, radular
sheath ; g, gonads ; m, crop ; ph, pharynx ; rd, radula ; odm, masses of muscle and cartilage of the
lingual apparatus ; o, mouth ; k, 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 infrequently in carnivorous Monotocardia there is a glandular widening in
that section of the oesophagus which follows the long proboscidal portion. The
oesophagus 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 Pulmonata and Opisthobranchia, there is sometimes a widening (crop, fore-
stomach) anteriorly to the stomach, and in the same way the short oesophagus of
the Scaphopoda has a glandular widening, or two lateral glandular diverticula.
Among the Cephalopoda, the Decapoda have a simple thin tubular oesophagus ;
Fio. 161.
'J
FIG. 159. Alimentary canal of Murex trun-
culus (after Bela HaUer). 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) ; 6, digestive gland (liver) ;
7, anus; 8, rectum; 9, efferent duct of the pigment gland (ink-bag), 10; 11, stomachal ccecum ; 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 (parapodium) ; 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 oesophagus of the Octopoda, 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 oesophagus, larger than the stomach itself.
5
9 \-
13
FIG. 162. Diagram of the
anatomy of Clio striata,
from the right side ; the
heart, kidney, and mantle of
this side removed (after
Pelseneer). 1, Fin (parapo-
dium); 2, aperture of the
penis ; 3, right tentacle ; 4,
genital aperture ; 5, penis ;.
6, oesophagus ; 7, dental
plates of the stomach ; 8,
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 Nudibranchia the gland breaks up into
branching intestinal diverticula, which spread
through the body almost like the gastro-canals or
intestinal branches in the Turbellaria, and run up
into the dorsal appendages of the body (clado-
hepatic Nudibrancliia).
Choetoderma, 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
MOLLUSCATHE ALIMENTARY CANAL
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 CepJmlopoda, the duct of the digestive gland (the so-called
hepatic or gall duct) does not open direct into the stomach, but into a
coecal outgrowth of the stomach, the spiral eceeum.
In very many Lamellibranchia 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 PJiipidoglossa and Toxoglossa.
In many Opisthofa'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. Amphineura.
The Chitonidffi 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
algse.
Unlike the Chitonidce, the Solenogastres
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
cceca found in Neomenia, Proneomc/na,
etc., and called hepatic diverticula, are caused by the projection into the lumen from
FIG. 163. Part of a horizontal median
section through Proneomenia 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
background is the dorsal wall of the gut, with
the groove which cuts into the hermaphrodite
gland (cf. Fig. 53, p. 42).
192
COMPARATIVE ANATOMY
CHAP.
2
each side of narrow septa arranged at right angles to the gut, or transversely (Fig. 163);
in these septa, muscle fibres run down to the rudimentary foot, and blood lacunre
abound. In Proneomenia Sluiteri, septa of the first, second, third, or fourth order can
be distinguished, as seen in the figure. The septa on the right alternate with those
on the left side of the body. In the dorsal middle line the mid-gut forms a narrow
ciliated longitudinal groove which cuts deep into the gonad, cilia are also found 011 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 Nudibranchia, as already mentioned,
the digestive gland breaks up into, a system of
glandular diverticula (the so - called ' ' diffuse
liver"). The Aeolidiadce (e.g. T*ergipcs] 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 Nudibranchia 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 Tritonidce the gland
is a great compact mass. In other families, such as the Tcthymelibidce, Lomanotidce,
Dendronotidcv, Bornellidce, Scyllceidce, it divides into two anterior accessory livers
and a posteiior principal liver, from which diverticula run up into the dorsal append-
ages. Finally, the accessory and principal livers break up into separate ' ' hepatic
branches" (AeolidcK), 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 Phyllirhoe, 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).
The stomach of many Opisthobranchia consists of two divisions separated by a con-
striction. In some forms, such as the Bullidce among the Tcctibranchia, the Ptero-
poda thccosomata, and the Tcthymelibidce, Borndlidce, Scyllceidce, among the Nudi-
branchia, it is armed with hard chitinous plates, spines, teeth, etc., occurring in
varying number and arrangement on its inner wall (Figs. 161 and 162)
FIG. 164. Alimentary canal of Aeolis
(after Souleyet). 1, Pharynx ; 2, stomach ;
3, branched digestive gland (liver); 4,
anus ; 5, rectum.
VII
MOLLUSCATHE ALIMENTARY CANAL
193
3. Scaphopoda.
The mid-gut of Dentalium (Fig. 165) consists of a looped stomachal tube bent
FIG. 165. Alimentary canal, kidney, and sexual organs of Dentalium, from behind (after
Lacaze-Duthiers and Leuckart combined), a, Mouth ; 6, leaf-like oral tentacles ; c, snout ; d,
entrance to pharynx ; e, pharynx with radula, /,- g, hind-gut; ft, 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 nephridial aperture ; s, left kidney ; t, stomach ; 11, 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 esophagus.
VOL. II O
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 oesophagus, which lies under the anterior adductor,
widens at the anterior base of the foot to form 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 Korper, 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 (Najada, Cardium, Mytilus, Peden, 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 ccecum which can be compared with the sheath of the
crystalline stylet.
In the lower Lamellibranchs, the Nuculidce and Solenomyidce, the stylet is either
very slightly developed or wanting. In the Arcidce also, it is only slightly
developed.
The Septibranchia (Poromya, Cuspidarid) are distinguished from all other
Lamellibranchia 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
ccecal 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 ccecum 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 ccecum 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
MOLLUSCATHE ALIMENTARY CANAL
195
unpaired in the adult. The whole of the much branched 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 ccecum
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 Loliyo
and Sepioteuthis, 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 Rossia.
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 from the
upper part of the gland, and open into the
stomachal ccecum 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-
growths of the epithelium 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
4-3
FIG. itiO. Alimentary canal of Loligo
saglttata (without pharynx and salivary
then each duct appears throughout its whole glands) ^^ cut opeu (after Gegenbauer).
length to be covered with acinose or ramified 1, (Esophagus ; -2, probe, inserted into the
"pancreatic appendages." The pancreatic pylorus; 3, stomach; 4, stomachal coacum
secretion contains diastase, and appears to , with s * )iral ccficum 5 ' *ind-gut; 8, i"k-
L < i / j- f ..I ba ; "> aperture of the same into the hind-
carry out only one part of the functions of the gut>
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 forms several coils only in Trcrnoctopus violaceus.
E. Hind-gut (Rectum).
This is generally short in Molluscs. Where it is sharply marked
oft' from the small intestine, it usually differs from the latter in being
thicker and more muscular.
196 COMPARATIVE ANATOMY CHAP.
In the majority of Lamellibranchs, and in nearly all Diotocardia, the
rectum traverses the ventricle ; this fact, with many others, supports
the relationship of these two groups.
In certain Molluscs, viz. the Scaphopoda, a few Prosobranchia
(Muriddw, 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 in Dentalium 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 Rachiglossa (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 (cf. 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, JEnoploteuthis, and Sepioteuthis, the ink-bag is very small ;
it progressively . increases in size in series both of Decapoda and of Octopoda, 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, however, 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 Dibranchia), it is found at the top of the visceral dome, behind the goiiad. 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 wall 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
MOLLUSCATHE ALIMENTARY CANAL
197
which serves as reservoir and duct for the pigment. The latter, after being formed
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- //3
ised lamella? of connective 5-
tissue, which are inter - con-
nected in such a way as to form
a kind of sponge-like structure.
Xew lamella? are continually
being put forth by the formative
zone of the gland, which is a
narrowed portion bent back
downwards, while the oldest
lamella?, which lie nearest the
aperture of the gland, become
detached and degenerate. All
the lamella? 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 lamella?. In the forma-
tive zone, the young glandular
cells are at first colourless. In
the succeeding lamella?, how-
ever, pigment granules increase
in number and from the older
lamella? are emptied into the
cavity of the gland, the epi- FIG. 167. Morphology of the pigment gland (ink-bag) of
thelial cells then becoming the Cephalopoda (after P. Girod). A, Median longitudinal
j i 1 section through the ink -bag of an adult, n, Anus ; 1, terminal
detached and breaking up. ^^ common to the rec tum (-2) and the duct of the ink-
Both the gland and the reser- bag ; 3, ampulla ; 4 and 5, sphincter muscles of the ampulla ;
voir are surrounded by a vascul- 0, duct of the ink-bag ; 7, pigment reservoir ; 8, opening of
arised integument of connective the Pigment gland into the reservoir ; 9, portion of the gland
. . , , . , , traversed bv lamellae ; 10, formative zone of the lamellae,
tissue ; the same integument B Q Varioug stageg in the development of tne pigm ent
forms the framework of con- gland ; B, anal papilla ; 0, invagination in the same ; D, ap-
nective tissue running through pearance of two new depressions at the base of C; these
the lamella? or trabecula? within increase in depth, the one becoming the pigment gland b, the
,1 -I other the rectum 2. In F, the formative zone has appeared
in the gland, in G, the first lamellae and the duct. H, I, K,
The ink-bag is further envel- ehanges i u the relative positions of the rectum and gland in
oped as a whole in a tough integu- the course of development, seen from the posterior (mantle)
ment consisting of three layers si(le - In H > the rectum lies behind the ink-bag. In I, the
(1) an inner glittering silvery latter has * hifted ' and in K lies behind the rectuin (on the
, J mantle side),
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 Solenogastres opens into the
dorsal portion of a cavity, the cloaca, 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 Lamellibranchia, 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.
XVII. The Circulatory System.
A. General.
All Mollusca have a circulatory system ; in some divisions,
especially in the Cephalopoda and some Prosobranchia, 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 Lamellibramhia 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, Cephalopoda).
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 (Prosobranchia, Pulmonata, a few Tectibranchia).
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 supply
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
MOLLUSCATHE CIRCULATORY 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,
Lamcllibranchia, Cephalopoda Dibranchia). Again, where a longitudinal
7
FIG. 168. A-H, Diagrams illustrating the relation between the ctenidia, the heart, and
the aorta. A, Chiton ; B, Lamellibranchia ; C, Dibranchiate Cephalopoda ; D, Tetra-
branchiate Cephalopoda ; E, Prosobranchia Diotocardia Zeugobranchia ; F, Prosobranchia
Diotocardia Azygobranchia ; G, Prosobranchia Monotocardia ; H, Opisthobranchia Tecti-
branchia. 1, Ventricle ; -2, 3, -la. 2b, 3d, Sb, auricles ; 4, vena branchialis = efferent branchial vessel ;
5, aorta ; oa, 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 were
present in primitive Molluscs, as does the arrangement in Nautilus
(Cephalopoda Tetrabmnchia) 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 Prosobranchia, 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
(Prosobranchia, Monotocardia, most Pulmonata, a few Opisthobranchia).
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 Opisthobranchia. 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 hsemolymph, is a fluid rich in dissolved
albumen (hsemocyanine), which assists in nourishing the body and in
respiration. Amoeboid cells, the lymph cells or amrebocytes, are
suspended in the hsemolymph. Haemoglobin is occasionally found
dissolved in the hsemolymph or combined with special blood corpuscles.
The lymph 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 MOLLUSCATHE CIRCULATORY SYSTEM 201
B. Special.
1. Amphineura.
a. Chitonidae (Polyplacophora). The heart is symmetrical, with two lateral
auricles.
The ventricle and the two auricles are long tubes. The auricles are in 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 coslom 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 (cf. 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 pericardia! endothelium. The presence 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 Neoinenia 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 Prosobranchia, have a heart with two auricles. This is not
only the case in the Zeugdbraiichia (Fissurella, Haliotis, etc.), which have two gills,
but also in the Azygobranchia (Turbi/iidce, Trochidce, Neritidce], 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 Zeugobranckia, 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 Azygobranchia, Monotocardia, and most
Pulmonata.
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 Monotocardia, only Cyproea (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 cavitj^ to the posterior end of the body
( Testacella, 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 Opisthobranchia, 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 where 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 Tectibranchia, e.g. Actceon, Accra, Gastropteron, 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 Lamellibranchw, to note the fact that, in many Diotocardia
(e.g. Fissurella, Haliotes, Turbinidce, 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 oesophageal 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 MOLLUSCATHE 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 lacunte 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 efferent 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 lacunae or vessels which lead to the gills, and finally reaches the heart
FIG. 169. Circulatory system of Paludina vivipara (after Leydig). The animal is seen from
the left side. 1, Eye ; 2, cerebral ganglion ; 3, efferent branchial vessel (branchial vein) ; 4, gill
(ctenidium) ; 5, afferent branchial vessel ; >, kidney ; 7, aorta visceralis, winding up close to the
columella ; 8, ventricle ; 9, auricle ; 10, aorta cephalica ; 11, venous sinuses of the body ; !_',
auditory vesicle ; 13, pedal ganglion.
through the branchial veins. Less frequently, the venous blood, after passing
through the kidney, enters the auricle more or less directly, i.e. without passing
through 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.
(b) Pulmonata. (Examples : Helix 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 lacunar 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
FIG. 170. Pulmonary veins, heart, and arterial system of Helix (after Howes). The 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 ; 6, hermaphrodite gland ;
7, columellar muscle ; 8, aorta visceralis ; 9, salivary glands ; 10, aorta cephalica.
under 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
FIG. 171. -Vascular system of Limax, after drawings combined by Leuckart from Delle Chiaje
and Simroth. The veins carrying the venous blood out of the body into the lungs are black.
A, auricle ; V, ventricle ; VR, venous circular sinus of the pulmonary cavity ; Ax, aorta cephalica ;
Ay, aorta visceralis ; M, muscular stomach ; ZD, hermaphrodite gland ; II, digestive gland ; 7, 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 MOLLUSCATHE CIRCULATORY SYSTEM 205
cavity, to enter the auricle. The vessels of the respiratory network form projecting
ribs on the surface of the mantle. The pallia! 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 cesophageal ring, but runs between
the pedal and visceral ganglia ; this is said to be the case in most Opisthobranchia.
In Opistho2meumonic Pulmonata (e.g. Daudebardia, 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 Onddium, there is an arteria visceralis corresponding with the posterior
aorta, which branches oft' soon after the aorta leaves the heart, but it here runs
anteriorly.
(c) Opisthobranchia. Here also the arrangement is essentially the same as in
the Prosobranchia, though modified by the different position of the gills, as has been
already briefly noted.
Gastropteron affords a good illustration of the circulatory system of the 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 springs
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 arteiy 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 ; (b] 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 cesophageal bulb and of the oesophagus. (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. AVide but short 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 Tectibrancliia. For example.
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, which lies at the base of the 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 lies 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 forms
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, which open into the posteriorly-placed
auricle.
3. Scaphopoda.
The circulatory system of Dentalium, but for the recently-discovered rudimentary
heart, is entirely 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 Dentalium 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 surrounding sinuses of the body.
4. Lamellibranehia.
The Heart. In nearly all bivalves, the heart, which is traversed by
the hind-gut, possesses two 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 lies above the intestine, becomes
narrower and thinner. This modification goes furthest in Area Noce, 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
MOLLUSCATHE CIRCULATORY SYSTEM
207
ventricle, but are drawn down to its lower side, where they grow together, communi-
cating through a more or less large aperture. Pinna, Aricula, and Perna 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,
TO
FIG. 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), br, Gills ; bre, efferent
branchial vessel (branchial vein) which opens into the large branchial vein brei, running along the
base of the gills, and here cut through transversely ; pi; pallial vein ; vc, large venous sinus of the
body ; kb, pericardial gland ; a HI, auricle ; j-j, rectum ; v, ventricle ; rv and /TJ, renal vessels ; bra\,
afferent branchial vessel (branchial artery), running along the base of the gills ; bra, 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. 17 % 2. .
auricle ; sbc, spaces at the base of the gills, bathed by the water and communicating with the
mantle cavity, between the ascending and descending branchial lamellae.
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 lacunar 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-gut. It soon divides into two large lateral arteries, the posterior pallial
arteries. The principal branches of the 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 posterior 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 lamellae. 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 Protobranchia and Filibranchia there are
numerous forms (Nucula, Solenomya, Anomia, Mytilidaz) 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 Prosobranchia 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 (Tetrabranchia), two
in the Decapoda and Octopoda (Dibranchia).
In Nautilus, the heart is an almost square sac drawn out to two points on each
side ; the four auricles which open into the four points of the ventricle are long
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 Ocgopsidce, the ventricle lies along the longitudinal
axis of the body, i.e. it is elongated dorso-ventrally, and the auricles are at right
VII
MOLLUSC A 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,
which 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 the nephridial system, and gills of
Sepia officinalis, anterior view (after Hunter). 1, Aorta cephalica ; 2, ctenidium ; 3, vein leading
to the ctenidium ; 4, branchial heart ; 5, appendage of the branchial heart (pericardial gland) ;
6, venous appendages of the nephridial system ; 7, aorta abdominalis ; 8, vena abdominalis ; 9,
lateral veins ; 10, vena cephalica ; 11, auricles ; 12, ventricle (cf. Fig. 186).
venous, the blood collects out of this into sinuses (especially into a peripharyngeal
cephalic 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
oesophagus, 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 hind-gut, the ink-bag, the
genital organs, the dorsal part of the body wall, and the fins, \vhen 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 Myopsidce, 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 Myopsidce, 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 lacunae) 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 cavse, 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). We thus see
that here the blood flowing back from 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, conies
into contact with the kidneys in the venous appendages.
In the Octopoda, 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 large
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
vense cavse.
Nautilus is chiefly distinguished by the absence of the branchial hearts.
Further, each of the two vense cavae divides into two branches, which run, as
afferent vessels, to the gills.
vii MOLLUSCATHE BODY CAVITY 211
XVIII. The Body Cavity.
Primary and Secondary Body Cavity, Pericardium, Pericaxdial Gland.
The Mollusca are said to have a primary and a secondary body
cavity. The former is the system of laeunse 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 eoelom is, in most Mollusca,
very much reduced, usually consisting of only two small cavities, the
pericardium and the cavity of the gonads (testes, ovaries, or her-
maphrodite glands). The ccelom is always lined by an epithelium of
its own, the ccelomic epithelium, and corresponds with the true eoelom
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 eoelom, 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 eoelom, 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 eoelom. 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 eoelom,
forming, in fact, an only partly distinct division of the same.
In the Solenogastres (e.g. Proneomenia), 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 Proneomenia is compared with that of an Annelid,
we find the following differences :
212
COMPARATIVE ANATOMY
CHAP.
In Proneomenia, the dorsal vessel is wanting in the region of the mid-gut. The
ccelom 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 -gut, the vessel lying in the dorsal mesentery is developed
as a heart, the coelom being here represented by the pericardium.
4
&'
FIG. 175. Diagrammatic sections through an Annelid (A) and a Solenogastrid (B and C), to
illustrate the relation of the coelom to the genital glands and nephridia. B, Eegion of the cloaca ;
C, region of the mid-gut ; 1, dorsal mesentery ; 2, dorsal vessel or heart ; 3, germinal epithelium ;
4, coelom in B= pericardium, in C= hermaphrodite gland (in the coelom 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, the coelom is large, and falls into three distinct divisions. One
contains the intestine and digestive gland (liver), which are accordingly outwardly
n V
FIG. 176. Diagrammatic longitudinal section through Chiton, to illustrate the relation
between the various parts of the coelom (after Haller). 1-8, Position of the eight dorsal shell-
plates; M, anterior portion of the dorsal integument; L, snout; TO, mouth; /, digestive gland
(liver); d, intestine;/, foot; ti, kidney; p, pericardium; c, portion of the coelom surrounding the
intestine ; h, heart ; Ip, band connecting pericardium and genital gland ; gdr, genital gland ; la, band
connecting the genital gland and the posterior portion of the coelom which surrounds the intestine.
(i.e. on the side turned to the ccelom) covered with an endothelium. The mesen-
teries, however, which originally attached the intestine to the body wall, and
along which the parietal endothelium passed into the visceral endothelium of the
intestine and liver, have disappeared, with the exception of portions retained on
the hind-gut. The two other divisions of the ccelom are : (1) the pericardium, and
VII
MOLLUSCATHE BODY CAVITY
213
(2) the genital gland. Certain bands, by means of which the three divisions are
connected together, have been regarded as the constricted remains of communications
between the three divisions of the originally single coelom (Fig. 176).
The Cephalopoda may with advantage be considered in connection with the
AmpJiineura. In NautihLS and the Decapoda (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 showing the
secondary body cavity of Sepia (after
Grobben). Median longitudinal section
through the body, in which, however, some
organs are represented which, being paired
and symmetrical, do not properly come
into the plane of the section. The outlines
of the coelom are indicated by thicker lines.
1, Female germinal body, with eggs (2) pro-
jecting into the genital cavity (the ovarial
division of the coelom) ; 3, shell ; 46, an-
terior portion of the renal sac ; 5, pancreatic
appendage of the efferent duct (bile duct)
of the digestive gland (liver) ; 4a, anterior
venous appendage of the renal system ; 6,
aperture (funnel) of the kidney into the
coelom ; 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 ; 14j,
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 coelom (principally
genital cavity); 21, pigment gland (ink-
bag) ; 22, aperture of the oviduct into the
genital cavity ; rf, dorsal ; v, ventral ; a,
anterior; p,* posterior.
22
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 ccelom in Nautilus and the Decapoda is very spacious, in the Oetopoda,
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 coelom, 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.
\ 19
therefore no longer covered with endothelium. Nevertheless this canal system of
the Octopoda shows the same morphologically important characteristics as the ccelom
of the Dccapoda. 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
Octopoda the heart is excluded from the coelom, 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 Lamellibranchia and Gastropoda, the only part of the ccelom retained,
besides the 'genital glands, is the pericardium. The pericardium and the gonad are,
however, entirely separated
from one another. In Lamelli-
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 (<\g.
Phyllirhoe) 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, but 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 kidney.
Among the Prosobranchia, in the Diotocardia, the pericardial gland is found on
the auricle, its walls forming dendriform branched outgrowths 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 Opisthobranchia, in Aplysia, and
Notarchus, on the anterior aorta which runs along the pericardial wall ; in Pleuro-
branchus and Plcurobranchcea on the lower, in Doridopsis and Phyllidcea 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 Lamellibranchia than
among the Gastropoda, but are wanting in the most primitive forms (Nucula,
Anomia). The gland is usually of a rusty red colour, and occurs in two
FIG. ITS. Eledone moschata. This figure corresponds with
Fig. 177 of Sepia (after G-robben). Si, Efferent duct of the
digestive gland; 17a, pericardial gland (appendage of the
branchial heart) ; 23, water canals.
viz MOLLUSCATHE 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 the 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, Solen, 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
Ohama 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 cffilom). 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 ccelom. 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 Prosobranchia, 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 Scaphopoda, 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 Monotocardia, 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 Solenogastridce 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
FIG. 179. Paramenia impexa. Posterior end of the body ; the integument must be supposed
to be reinoved 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 commissure 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 Chitonidcc, 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
MOLLUSCATHE 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 outward into
the posterior part of the mantle cavity, the other into the pericardium, which also
lies in the posterior part of the body. In this way the pericardial and outer aper-
3-
10
11
FIG. 180. Nephridial and genital systems of Chiton, diagrammatic, from above, after the
figures and accounts of various authors. 1, Mouth ; 2, gills ; 3, unpaired-portion of the nephridium
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) Diotocardia. 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 papilla*. The blood
lacunre which penetrate into the papilla* communicate direct with the auricles, and
are thus supplied with arterial blood. In these lacuna; of the papilla* 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 Avay connected with the pericardium.
The Neritidse 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-
cul, many of which reach from one wall to
the other, forming a spongy structure. The
trabeculae are covered by a glandular epithe-
lium on the surfaces turned to the 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. I8l.-Diagram of the two nephridia The right kidney is however, much larger
"D*k+Al1 /n-ft-^** T AVkl-sk+A-M\ 7. AJ-:, *
of Patella (after Lankester). ksa, Anterior
and upper lobe of the large right kidney Jcsl ;
,, in, m, , .-, v . , , , -
tlian tlie left " The ? b th lie t0 the n S ht f
Tcsi, lower subvisceral ; ksp, posterior lobe of tne pericardium, but there are no reno-peri-
the same ; /, subanal tract of the large right cardial apertures. The internal structure
kidney ^analjjapilla with the portion of the o f the right kidney is spongy, but the left
forms a simple cavity, into which folds
project from the walls. A lacunar system
rectum which runs to it ; h, papilla with the
aperture of the left kidney (which is not
drawn) ; /, the same of the right kidney ; I,
pericardium, indicated by a dotted outline ; without special walls traverses the trabeeular
the existence of the reno-pericardial aperture
figured near /, is now denied.
network of the right kidney, but is com-
pletely cut off from its cavity ; the venous
blood from the body passes through this
The lacunar system of the left kidney communicates
system before entering the gills,
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 MOLLUSC A THE NEPHEIDIA 219
(b) Monotocardia. The Monotocardia 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 frequently (Cassidari", 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 (the reno-pericardial canal)
with the pericardium.
Lamellse or trabeculse, 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 lacuna, before passing 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 Cyprcca 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 Haliotis, Turbo, and
Trochus (as papillar sac) it is not excretory in function. In Haliotis, Turbo,
Trochus, and Po.tdla 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 filled
with cells of connective tissue and muscles, and contains blood lacunae, 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 lamellae 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
the morphology of the urinary duct have already been explained (pp. 74-78).
3. Opisthobranchia Tectibranchia. Only one kidney is found in the usual
position 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
---6
FIG. 182. Nephridium and pericardium of Daude-
bardia rufa, from above, diagram (after Plate). 1, Peri-
cardium ; 2, reno-pericardial aperture (renal funnel) ; 3,
nephridium ; 4, primary ureter ; 5, rectum ; 6, secondary
ureter (cf. Fig. 74, p. 77).
FIG. 183. Nephridium 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 Tectibranchia. The unpaired kidney is here
somewhat similar to the paired kidney of the Chitonidic. 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, pyriform 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 said that Pleurobranchcm, a Tcctibranchiate, from which the Nudibranchia
may perhaps be derived, possesses a Nudibranchiate kidney.
In Plnjllirhoe, the urinary chamber has no branchings ; it runs back from the
VII
MOLLU8CATHE NEPHRWIA
221
pericardium as a simple median tube. Anteriorly it is connected with the peri-
cardium by a funnel, and near the middle communicates with the exterior by means
of a lateral urinary duct (Fig. 19, p. 12).
C. Scaphopoda (Fig. 165, p. 193).
Dentalium 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 by
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 the
kidneys, a fact to be specially noted is that the genital products 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 mantle
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-
ful. If these pores really lead into the blood lacunar system of the body, as was
formerly maintained, and is still held to be possible, this would be the only known
case of the direct imbibition of water into the blood.
D. Lamellibranchia.
The nephridium (organ of Bojanus is always paired and symmetrical, and lies
below the pericardium and in
front 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-
munication of the kidney with
the mantle cavity always takes
place above the cerebrovisceral
connective.
The lowest Lamellibranchia
(Protobranchia, Nucula, Leda.
Solenomya) are distinguished
in two ways. (1) Each nephri-
dium is a simple tube, with a
free cavity not traversed by
trabeculse or lamellte. This
tube consists of two portions
which unite posteriorly at an
angle ; the anterior end of one
of these portions enters the
FIG. 1S4. Transverse section through the body of ao
donta. showing the pericardium, the heart, and the kidneys,
combined and diagrammatised from figures by Griesbach.
Not all the parts represented occur on the same section. 1,
pericardium through the renal Pericardium; 2, ventricle; 3, auricles; 4, hind -gut; 5,
funnel, the other end opens venous sinus ; 6, reno-pericardial aperture (funnel) ; 7, renal
into the mantle cavity. (2) sac or cavity ; 8, vestibular cavity, which at 9 enters the
The paired genital glands do ^ntle cavity through the nephridial aperture ; 10, genital
aperture ; 11, base of the foot,
not open outward directly, but
enter the kidneys near their pericardial funnel a fact which is very important in
222 COMPARATIVE ANATOMY CHAP.
connection with the arrangement in the Solenogastridcc, the lower Prosobranchia (i.e.
the Diotocardia), and the Scaphopoda.
In other Lamellibranchia also there is a relation between the genital glands and
the kidneys. In the Pectinidce and the Anomiidce the genital gland opens into the
kidney, but near its outer aperture. In Area, Ostrcea, Cyclas, and Montacuta, the
kidney and the genital gland open on each side into the base of a common depres-
sion (urogenital cloaca) ; in all other bivalves the outer nephridial and genital aper-
tures are separate.
The simple structure of the Protobranchiate kidney becomes complicated in
other Lamellibranchia in the following manner :
1. That portion of the renal tube which opens outward forms an external cavity
(vestibular cavity, external sac) ; this cavity has no excretory epithelium ; it
encircles the outer side of the pericardial portion of the kidney, the renal sac (Fig.
184). The latter alone is developed as an excretory organ. Folds or trabeculte,
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 (Pholadacea, 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 efferent vessels appear to be
lacunar. Open communication between the blood vascular system and the kidneys
is nowhere found.
E. Cephalopoda.
(Of. Figs. 185, 186, and the sections on the ccelom and the blood
vascular system, pp. 213 and 208).
The Cephalopoda have two (Dibranchia) or four (Tctrabrancliid) 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 coelom, and at the other
with the exterior (mantle cavity). Only one of the two pairs of kidneys in Nautilus,
however, possesses coelomic 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 Ocgopsidce, and Sepioteuthis among the Myopsidce, it is simple and slit-like ; in the
other Myopsidce and in the Octopoda, however, it lies at the end of a renal papilla
which projects freely into the mantle cavity.
The two renal sacs in the Octopoda 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 coelomic
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
MOLLUSCATHE 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. 1S5. Renal sac, ccelom, genital organs, etc., of Sepia. A, female; B, male. The
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). o, Heart ; 5, genital vein ; c, genital
artery ; <1, stomach ; e, female germinal body ; /, aperture of the oviduct in the ovarial cavity ;
y. oviduct ; h, unpaired anterior renal sac ; i, abdominal vein ; fc, appendage of the branchial
heart (pericardial gland); I, branchial heart; m, paired posterior renal sac; , gill; o, canals of
the coelom leading to the kidneys ; />, 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 coelom containing the vas deferens (peritoneal sac) ; 5, anus ;
6. rectum ; q, 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
Nautilus, viz. the upper pair, has reno-pericardial apertures. This fact was
FIG. 186. 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 ; rno, right nephridial aperture ; y\, left reno-pericardial aperture, the outlines of the
secondary body cavity are indicated by a dotted line ; vg, vena genitalis ; rvc, right branch of the
vena cava ; vpd, right pallial vein ; va, right vena abdominalis ; vba, vein of the ink-bag ; vas, left
vena abdominalis ; cv, section of the secondary body cavity (capsule of the branchial heart), which
surrounds the branchial heart cb, and the appendage of the same (pericardial gland) x ; vps, left
pallial vein ; Ivc, left branch of the vena cava cephalica ; vm, left vena genitalis ; vpc, secondary
body cavity (viscero-pericardial sac) ; y, left reno-pericardial aperture (renal funnel) (cf. Fig. 174).
brought forward in support of the view that the two pairs of renal sacs arose by
the division of one single pair, corresponding with that of the Dibranchia. 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
Molluscan body, which has so often been asserted, rests on very weak foundations.
It gains no support from the Chitonidce, 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.
vii MOLL USCA 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 Lamellibranchia 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 Chitonidce
and Chcetoderma, in many Lamellibranchs, in the Scaphopoda, among the
Gastropoda in the Prosobranchia (excepting a few Marseniadce and the
Falvata), and in all Cephalopoda. Hermaphroditism prevails among
the Amphineura in Proneomenia, Neomenia, and allied forms ; in many
Lamellibranchs, among the Gastropoda in the Pulmonata, Opisthobi'anchia,
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, in 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 Scaphopoda 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
UNIVERSITY
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 (Chitonidce, Mo7iotocardia,
Pulmonata, Opisthobranchia, 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 (Solenogaslres). 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 (Solenogastres).
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 (Solenogastres, 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 Chitonidce 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
MOLLUSC A 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
Amphineura (see below), nearly all Diotocardia, the Scaphopoda, and
all Lamellibranchia. They are present in the Monotocardia, the
Pulffioiutta, 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, 3-, .1
the germinal epithelium 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 Chitonidce 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 FIG. 1ST. Section through the wall of the ovary of Chiton
is a stalked follicle, with egg (diagram after Haller). 1, Eggs at different stages of develop-
cells surrounded by follicular ment ' 2 > ei inal epithelium ; 3, egg sac or tubes ; 4, follicular
cells. These follicles are epithelium ; 5, 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 inultilaminar, 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 Pulmonata 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 detached 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 Pleurobranchcea 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
(Amphorina, 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 gland of the Pteropoda ( Tcctibranchia 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 laminae closely crowded
together. The eggs are always produced at the peripheral part of the acini, tubes,
or lamellae, 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 (Siphono-
dentalium, 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.
(4) 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 (Anomiidcv,
Mytilidce), the gonad on each side stretches into the mantle. In others (Axinus,
Montacuta), it bulges out the body wall in such a way that branched outgrowths,
containing th 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 Septibranchia ; (2) families
vii MOLLUSC A GENITAL ORGANS 229
with a few hermaphrodite genera : Cydas, Pisidium, Entovalva ; (3) genera (Ostrcea,
Pccten, Cardium] with a few hermaphrodite species ; (4) occasional cases of henna-
phroditism in species the sexes of which are usually separate : Anodonta. 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 Septibratichia, 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
the 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 ducts of these capsules
(oviducts and seminal ducts). Since, however, the entire germinal sac corresponds
with the genital gland of a Gastropod or a Lamellibranch, 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 ducts, which exactly corre-
spond with the same ducts in the Gastropoda, Lamellibranchia, and Chitonidce.
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 duct, 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 narrower sense) is
always found on the anterior wall 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 growth 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 Eledone 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, B] and Trernodopus 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 (Tremoctopus) 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 narrow 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, Thysanotcuthis) 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
FIG. 188. A -D, Four diagrams of the female gonads of the Cephalopoda. A, Nautilus type.
B, Argonaut type, f , Octopus type. D, Ommastrephes type. 1 , Aperture of the oviduct into
the gonad ; 2, cavity of the gonad (a section of the secondary body cavity) ; 3, egg-carrier.
of the egg which they surround. This arrangement 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
MOLLUSCA GENITAL ORGANS
231
Loligo (B), however, the funnel-shaped depression into which all the testicular
canals open is replaced by a longitudinal furrow, into which 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 hair-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 former hermaphrodite condition ; in the first case the vermiform spermatozoa
would be the eggs beginning to form, in the second the rudiments of eggs. There
is, however, no solid foundation for either 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,
FIG. 189. A, B, C, Three diagrams of the 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 the Amphineura, the Chitonidce, which most
recent observers hold to be less specialised than the Solenogastres, are sexually
separate. Among the Lamellibranchia, 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 Lamellibranchia 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. "VVe 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 Opisthobranchia, 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 Ddude-
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 Neritidce,
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 w r ould be
unable to function in this position, and is therefore placed on the right side of the
head or neck (Fig 71, p. 73), and forms a freely projecting, extensible, muscular
appendage, which often attains a considerable size. The male genital aperture,
however, 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 tubular, 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 are
provided with albumen and a shell : (3) a muscular sheath, the vagina, which leads
to the outer genital aperture. In Paludina, there is a special albuminous gland
opening into the oviduct.
In hermaphrodite Prosobranchia (Valvata, a few Marseniadce, 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 Prosobraiichiates ; 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 Prosobraiichiates.
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 Cephalaspidce (Tectibranchia}.
1st Type. The hermaphrodite 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
MOLLUSC A GENITAL ORGANS
233
This lies more or less far forward in front of the genital aperture, near the right
tentacle.
If w'e imagine the testis of a male Monotocardian transformed into a herma-
phrodite gland, and the vas deferens into a hermaphrodite duct, 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
Cephalaspidcc (Doridium, Philine,
Scaphander, Bulla) and all Ptero-
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 iiidameutal or
shell gland, yields its outer pro-
tective envelope ; (2) the duct of
a globular vesicle (receptaculum
seminis, Schwammerdam'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
7: 6
-K)
FIG. 190. Genital organs of Gastropteron Meckelii
which it is withdrawn by means of ( after Vayssiere). The penis and the seminal furrow
a retractor muscle. A gland called are not draw ?' * Common genital aperture ; 2 genital
cloaca ; 3, albuminous gland ; 4, nidamental gland ; 5,
ie prostata opens into the penis. hermaphro dite duct; 6, hermaphrodite 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 hermaphrodite 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 Pulmonata, the Basommatophora, a few species
of Daudcbardia (D. Saulcyi, in which the two apertures lie close together), the
Oncidia, and Vaginulidce. 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 Opisthobranchia, this second type
is exemplified in Oscinius (Tectibranchia).
Taking Limnaea 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 part of the pedal musculature, and finally enters '
the male copulatory 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 csecum, and opens into an irregularly-shaped organ, the
uteltis. 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 caeca! appendage ;
the ducts of the two much-lobed albuminous glands also enter the uterus.
A comparison of Limncea 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 Pulmonates, 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.
VII
MOLLUSC A 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 prreputium to be evaginated through the genital aperture, into which
FIG. 191.
FIG. 192.
FIG. 191. Genital organs of Limnaea 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 ; 8, hermaphrodite duct ; 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 ; 16, vagina ; 17, female genital aperture.
FIG. 192. Genital organs of Oncidium 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 (pneputium) ; 3, penis papilla ; 4, vas deferens ; 5, uterus, the seminal furrow iu
the uterus is indicated by dotted lines ; 6. caecum of the uterus ; 7, oviduct and vagina ; 8, csecal
appendage ; 9, receptaculum seminis ; 10, female genital aperture ; 11, albuminous glands ; 12, caecum
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.
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 csecal appendage.
3rd Type. We find this in the Stylomtnatophora among the Pulmonata, and also
FIG. 193. Anatomy of Helix pomatia (after Leuckart, Wandtafeln). The shell is removed
and the mantle laid back to the left, the organs of the visceral dome and head are isolated and
separated. To the left (in the figure) are the genital organs. L, Digestive gland (liver) ; Zd, her-
maphrodite gland; J, intestine; N, kidney; V, ventricle; M, fore-stomach; F, foot; A, anus;
Al, edge of the mantle near the respiratory aperture ; Mr, retractor muscle ; G, cerebral ganglion ;
Fl, flagellum ; Sk, oesophageal bulb (pharynx) ; P, penis ; R, retractor of the tentacle ; Ps, dart sac ;
AD, digitate glands ; Vd, vas deferens ; X, lateral bulging of the stalk of the receptaculum seminis
(Rs) ; Od, portion of the uterus belonging to the oviduct ; Ed, albuminous gland ; Zg, hermaphrodite
duct.
in all Nudibranchia and a few Tectibranchia (e.g. Pleurobranchcea). 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 Pleurobranchcea Meckelii 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 which
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 copulatory apparatus,
which again opens into the genital cloaca. The copulatory 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.
Pleurobranchsea Meckelii (Fig. 194). The
hermaphrodite duct, which rises from the gland,
forms 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
widened end portion which is called the penis,
and which can be invaginated by a retractor
muscle. The oviduct has a shorter course, and
receives the short efferent duct of a globular the ^n,e; 5 vas deferens ; 6 nidamental
gland ; , , albuminous gland ; 8, genital
receptaculum seminis. The widened terminal cloaca; 9> oviduct; 10, recepteculum
portion of the oviduct (the vagina), which enters seminis ; 11, widening and caecal appen-
the genital cloaca with the penis, receives the da S e of the oviduct ; 12, hermaphrodite
ducts of the albuminous and nidamental glands duct ; 13 ' hermaphrodite gland.
(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 Nudibranchia and those
just described ; in details, however, extraordinary variety prevails. The male and
FIG. 194. Genital organs 'of Pleuro-
branchaea Meckelii (after Mazzarelli).
1, Common genital aperture ; 2, penis
sheath ; 3, penis ; 4, retractor muscle of
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.
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 cseca or vesicuhe 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 OegopsidcK, 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 (Trcrnoctopus), 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, Rossia, Sepioteuthis, Chiroteuthis, Cirrhoteuthis, 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
FIG. 195. Genital organs of Phylli-
rhoe (after Souleyet). 1, Vas deferens ;
1, penis ; 3, oviduct ; 4, male, 5, female
genital aperture ; 6, vagina ; 7, herma-
phrodite gland ; 8, hermaphrodite duct ;
9, receptaculum seminis.
VII
MOLLUSC A GENITAL ORGANS
ITY
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 etferens receives (1) the short duct of an oviform gland,
the prostata, and (2) a simple, lateral, non-glandular caecum. The prostata takes
part, like the vesicula seminalis, in the formation of the spermatophores. The
prostata, csecuni, and vesicula seminalis, in their natural position, form a coil,
Fin. 196.
FIG. 190. Hale genital organs of Sepia officinalis. 1, Genital aperture ; 2, spermatophoral
pouch ; 3, vas efferens ; 4. caecum ; 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, veslcula seminalis ; 5, prostata ; 6, 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.
which 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 chiefly by the absence of a separate vas efferens. The long
240 COMPARATIVE ANATOMY CHAP.
vesicula 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
caecum.
It has already been mentioned that, as far as we know at present, only two living
ventraL
-4
O --,
dorsal.
FIG.' 198. Female genital organs ol Sepia officinalis (chiefly after Brock). The 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 Philonexis carence, 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 spermatophoral 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 nidamental
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 lamellae, traversing it from side to side. The spaces
between the lamellae 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 full 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 Dibrandiia, among the Decapoda, in the Myopsidce (Sepia,
Sepiola, fiossia, Loligo, Sepioteuthis, etc.) ; in a few Oegopsidce (OmmastrepJies,
Onycoteuthis, Thysanoteuthis). They are wanting in the Octopoda 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 (fiossia, 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 (Tremoctopiis, Parasira').
In all Cephalopoda, certain quantities of spermatozoa are collected in extremely
complicated envelopes, the spermatophores. The substance of these large fila-
mentous spermatophores 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 Hectocotylisation 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 hectocotylisation. 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 hectocotylisation (Fig. 200) is found only in the
Octopodan genera Argonauta, Philonexis, and Tremoctopus. 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
FIG. 199. Spermato- .
phore of Sepia (after fr m one another, apart from the sexual dimorphism caused by
Milne Edwards), a, the development of the hectocotylised arm. The males are
Outer case ; 5, inner much smaller, and in Argonauta the female only has a shell,
case ; c, sperraatozoal It ig v probable that the detached hectocotylised arm
sac ; d, e, f. g, various J
parts of the ejacula- can be replaoed 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 difficult 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
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 Octopodan subgenus Scceurgus and in Argonauta
FIG. 200. Male of Argonauta argo (after H. Miiller). (Female, Figs. 35, 36, pp. 24, 25.) A,
With the hectocotylised arm enclosed in the sac (d). B, with the arm free, a, Funnel ; 5, edge of
the mantle fold ; c, left eye ; d, sac ; di, hectocotylised arm ; e, mouth.
it is the third on the left. In the Decapoda the hectocotylised arm is generally
the fourth on the left, but in the genus Enoploteuthis it may be the fourth on
FIG. 201. Hectocotylus of Philonexis
(Octopus) carenae (after Leuckart). a,
Spennatophoral pouch ; b, seminal vesicle ;
c, efferent 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.
Rossia, it is the first arm which is hectocotylised. Finally, both the arms of one
pair may be thus transformed ; in Idiosepion and Spirula, this is the case with the
fourth pair, in fiossia with the first.
The difference in size between the male and the female, Avhich 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
slightly smaller than the female.
XXL Parasitic Gastropoda.
1. Thyca ectoconcha (Fig. 202) is a Prosobranchiate Gastropod which is parasitic
on the Star-fish Linckia multiforis. The chief points in its organisation are shown
in 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
ml
oc
FIG. 202. Longitudinal section through Thyca ectoconcha (after P. and F. Sarasin). Some
organs not actually belonging to the section are included, cer, Cerebral ganglion ; d, alimentary
canal ; fl, folds ; /*, foot ; k, gill ; I, liver ; ml, mantle ; oc, eye ; ot, otocyst ; ped, pedal ganglion ; pr,
proboscis ; sf, false foot ; si, oesophageal bulb ; vl, cephalic fold.
somewhat like a Phrygian cap. In the mantle cavity lies the gill. The alimentary
and nervous systems also are in no way remarkable. It has eyes and auditory
organs, and a short powerful snout, and muscular cesophageal bulb, which 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 a foot
(/) occurs without an operculum.
2. The Gastropodan organisation is somewhat more strongly modified in
Stilifer Linckia (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
MOLLUSCA PARASITIC GASTROPODA
245
with the 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. This 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
jel
V
FIG. 203. Longitudinal section through Stilifer Linckiae (after P. and F. Sarasin). be,
Buccal ganglion ; U, blood sinus ; ccr, cerebral ganglion ; d, alimentary canal ; fs, foot ; k, gill ; 7,
liver ; ml, mantle ; n, proboscidal nerve ; oc, eye ; ot, otocyst ; ped, pedal ganglion ; pr, proboscis ;
sm, false mantle ; sub, subintestinal ganglion ; sup, 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 O3sophageal 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
COMPARATIVE ANATOMY
CHAP.
would be difficult to recognise them as Gastropods, or even as Molluscs, were it not
proved that the larvse 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 Rinkii], 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
FIG. 204. A, B, 0, D, Hypothetical transition stages between Thyca and Stilifer on!,the
one side and Entocolax (Fig. 205) on the other (after Schiemenz). a, Anus ; fd, pedal gland ;
I, liver (digestive gland); Id, hepatic intestine ; m, mouth ; mag, stomach; o, ovary ; of, aperture of
the false mantle ; sf, false foot ; sm, false mantle ; u, uterus ; w, body wall of the host.
Gastropod of the type of Thyca 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. 204 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 unbranched
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
MOLLUSC A PARASITIC GASTROPODA
247
false mantle becomes larger and larger, and envelops the small visceral dome, which
gradually 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
...fc
FIG. 205.
FIG. 206.
FIG. 205. Entocolax Ludwigii, sketch
after Voigt. Lettering the same as that in
the preceding figure.
FIG. 206. Entoconcha mirabilis, sketch
by Schiemenez (after Baur). Lettering as
in Fig. 204. hod, 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 a 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, vn
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 of Entoconcha have the same general
structure as Gastropodan larvae. 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 larvse (Unionidce) 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 Prosobranchiates, 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. Vermetus is
said to produce great quantities 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. Ontogeny.
A. Amphineura.
1. Ontogeny of Chiton Polii (Fig. 207). The egg possesses little nutritive yolk.
The segmentation is total and somewhat unequal ; a ccelogastrula is formed by
invagination.
(a) The blastopore of the gastrala 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, stomodaeum or oesophagus ;
7, mouth ; 8, radular sac ; 9, body cavity ; 10, pedal gland, in I oesophagus ; 11, foot ; 12, anus
with proctodteum ; 13, cerebral ganglion ; 14, pretrochal tuft ; 15, pleurovisceral cords ; 16, pedal
cords ; 17, mantle furrow ; 18, eye ; c, shell ; c r c 7 , 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 Molluscs, is called the velum (Fig. 207 A).
(6) 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 growth
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 stomodseum, 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 stomodseum, 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 stomodeeum 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 (cf. 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-cesophageal
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 ctenidia 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 banyulensis. The
VII
MOLL USG A 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 flagellum.
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 hours old. B, Larva 100 hours old. C, Young
Dondersia immediately after transformation (7th day), after Pruvot.
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 off 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 Echinodermata.
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 Solcnogastridce 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 mvipara
(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 ccelogastrula is formed by invagination, the
blastopore of which marks the posterior end of the germ, and becomes the anus.
No proctodseum is formed. The whole of the intestine from the stomach to the
FIG. 209. Development of Paludina vivipara (after v. Erlanger). A and B, Stage after
gastrulation, with the rudiments of the mesoderm and the ccelom as outgrowths of the archenteron.
A, Median optical longitudinal section. B, Horizontal optical longitudinal section. C, Horizontal
optical longitudinal section through the embryo, after the entire separation of the ccelomic sac
from the intestine. D, Sagittal optical longitudinal section through an embryo, in which the
mesoderm has brolien up, the cells becoming spindle-shaped. 1, Velum ; 2, segmentation cavity ;
3, archenteron ; 4, coelom ; 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, while its inner wall, which 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 Molluscan Trocophora is formed ; this at first is quite symmetrical,
the anus lying posteriorly in the middle line.
After the development of the oesophagus, a mass of mesoderm cells collects on
each side of and below the intestine, this mass soon becoming hollow. In this way
two inesodermal 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 pretrochal 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 (oesophagus) 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 Paludina vivipara (after v. Erlanger) 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, mid-gut ; 3,
digestive gland (liver) ; 4, pericardium ; 4a and 45, divisions of the same formed by a septum ; 5,
free edge of the shell ; 6, shell groove ; 7, anus ; 8, mantle cavity ; Sb, 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 nepliridium ; 155, 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 USCA 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 Paludina vivipara (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, Via, Auricle ; 176, ventricle ; 18, 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-
UNIVERSITY
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 the 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 Paludina 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
two lateral halves by a septum,
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.
FIG. 212. Larva of Oncidium celticum, from the left The development of other
side (after Joyeux Laffuie). 1, Cerebral ganglion ; 2, edge Gastropods cannot here be de-
of the mantle; 3, rudiment of the gonad; 4, larval shell- scribed in dptail Wp rpfpr t>lp
muscle; 5, hiud-gut ; 6, rudiment of the digestive gland; C ,
7, auditory organ ; 8, pedal ganglion ; 9, foot ; 10, oesophagus ; rea(ler to the bibliography at
11, eye ; 12, branched muscle cells of the velum ; 13, velum, the end. As a rule, nutritive
yolk is present in larger
quantities than in the viviparous Paludina, 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,
vii 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 rudiment of the foot.
The outward appearance of the Yeliger 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 Prosobranchia (including the Hetcropoda).
the Pulmonate genus Oncidium, and all Opistho-
branchia, the embryo leaves the egg envelope early,
as a free-swimming Yeliger larva. In all these
forms, the preoral ciliated ring is well developed.
The ectodermal floor of the ciliated ring usually FIG. 213. Larva of Cymbulia
bulges out anteriorly, so that the cilia appear to (Pteropod), from the left side (after
be carried by a distinct circular ridge. This ridge Gegenbaur) i, velum ; 2, shell ; 3,
J . parapodia (fins); 4, foot with oper-
even grows out laterally to form a lobe of varying ^ lu {^ 5)
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 larvae, 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 quantity 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
FIG. 214. Embryo of Helix Waltoni (4 mm.
long), from the right side (after P. and F.
Sarasin). 1, Cephalic vesicle ; 2, upper (optic)
tentacle ; 3, eye ; 4, lower tentacle ; 5, oral
lobe ; 6, sensory plate ; 7, podocyst.
In the larva of the gymnosomaton
rings are developed on the body.
widened into a 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.
s Ptcropoda three postoral accessory ciliated
C. Scaphopoda.
Ontogeny of Dentalium. Segmentation, in these animals, leads to the formation
of a coeloblastula, from which a coelogastrula 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 stomodieum
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
FIG. 215. Larva of Dentalium, 37
hours old, posterior and lower aspect
and behind the tuft of cilia, two symmetrical (after K owalevsky). 1, Cephalic tuft ;
J_" ^ *! J_1 it f* ,1 1T
of three rows of cilia ; 4, mouth, hidden
under the ridge of the velum ; 5, mantle
fold.
invaginations of the ectoderm form the cephalic 2, rudiments of the cerebral ganglion
sacs or tubes. These become constricted from the (cephalic tubes); 3, velum, consisting
ectoderm at a later stage, their lumen gradually
narrows and finally disappears, while their walls
become thick and multilaminar 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
MOLL USC A 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
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 anteriorly on
the ventral side, a depression
of the ectoderm forms a pit,
the stomodseum (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 endoderm 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
have become cylindrical, sink
in to form the shell gland (F).
The latter secretes the first
rudiment of the shell in the
form of a simple cuticular
membrane. The endoderm
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
FIG. 216. A-G, Stages in the development of Teredo (after
Hatschek). A, C, D, E, F, G, from the right side, B in optical
horizontal section. 1, Ectoderm; 2, macromeres = endoderm
cells ; 3, primitive mesoderm cells ; 4, segmentation cavity ;
5, stomodseum (oesophagus); 6, mouth; 7, preoral ciliated
band ; 8, shell gland ; 9, shell ; 10, larval muscle cells ; 11,
cephalic plate with tuft ; 12, anal invagination, anus ; 13,
endodermal mid-gut.
260
COMPARATIVE ANATOMY
CHAP.
has given rise to two or three smaller cells. The thin cuticular shell becomes
bivalvular by 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 called 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
15
FIG. 217. Older Larva of Teredo, from the right side (after Hatschek). 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 ; 19, 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 multilaminar, and the proctodseum 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 *s 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 MOLLUSC A 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, Montaciria,
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 Lamellibranchs, can be protruded out of and withdrawn into the shell.
Among fresh-water Lamellibranchs 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 oasophagus arises
as an ingrowth 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 vesicle. 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 byssus 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
FIG. 218. 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 ; 0, rudiment of the pedal ganglion ; 7, foot ; 8, velar
region; 9, oesophagus ; 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 ; 18, 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
MOLL USC A ONTOGENY
263
part of each mesoderm streak a group of cells surrounds a cavity, which 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 lie 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-C, Diagrams illustrating the develop-
form, in all probability, throughout ment of the pericardium and heart of Cyclas cornea
their whole length, through con- < after Ziegler). 1 and 2, The two lateral pericardial
striction from the ectoderm. The
gill arises on each side as a fold on
vesicles ; 3, rectum ; 4, pericardial cavity ; 5 and 6,
imaginations of the lateral walls of the pericardium =
rudiments of the two lateral auricles ; 7 and 8, median
the dorsal edge of the inner surface walls of the two lateral pericardial vesicles, in B partly
of the mantle. It develops from fused to form a median septum (above and below the
behind forward. In the contrary "Destine), which in C has disappeared ; 9, rudiment of
J the ventricle,
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 Unionidse (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 coeloblastula, 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 Glochidium parasiticum has, in the last stage of its
development, which is passed through in the gills of the mother animal 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.
, 2
B
with 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,
K. 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
/^B passing fish comes in contact
. 'L ' / ' "<\ % 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
5 penetrate. The embryo of
Fio. 220.-Glochidium larva of Anodonta, from the outer leaf ^iwdanta attaches itself
of the gill of a female. A, from below, the shell being open chiefly to the fins, that of
(after Schierholz). B, in optical transverse section (after Unio to the gills of the fish.
Flamming). 1, Sensory setae ; 2, adhesive filament ; 3, shell- The epithelium of the part
process ; 4, false mantle ; 5, lateral pits ; 6, oral sinus ; 7, pedal f th fi i attac t e d ffrows
swelling ; 8, ciliated patch ; 9, embryonic adductor ; 10, shell.
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
// - \ ^r
; /
vii MOLLUSC A ONTOGENY 265
cerebral, pedal, and visceral ganglia, and the auditory vesicle appear during the
parasitic 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.
E. Cephalopoda.
Tetrabranchia. Xothiug is known of the development of NanMlus.
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
meroblastic 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 runs backward in a <;urve ; 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 iuvagination, 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 which lies on the yolk 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, a, Visceral dome with mantle ; b, rudiment of eye ; c, rudiment of gill ; d, halves of the
funnel ; e, rudiment of the funnel cartilage belonging to the apparatus for closing the mantle ;
/, peripheral part of the blastoderm, which, growing all round the yolk, forms the yolk-sac ;
g, mouth ; It,, posterior cephalic lobe ; i, anterior cephalic lobe ; A', 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 head. 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 (which have become fused), and form a somewhat narrow circle on the ventral
side in 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 USO A ONTOGENY
267
FIG. 222. Various stages in the development of Sepia (after Koelliker). A, B, C, D,
Anterior view ; E and F, from the left side ; G and H, from behind. Lettering as in Fig. 221. In
addition : d, rudiment of the funnel-nuchal muscle (collaris) ; d], paired rudiment of the funnel
proper ; p, yolk ; ai, edge of the mantle ; t , optic invagination (?) ; , region of the shell ; q, edges
of the two rudiments of the funnel bent round ; r, fins. In G the mantle fold is raised up 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 downward 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 Argonauta have a smaller yolk sac, round which the blastopore
grows at an earlier stage than in Sepia. The yolk sac of Argonauta 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 sac 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 epipodium, 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
Cephalopoda 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 Mollusca
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 in 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 MOLLUSCA LITERATURE 269
pleurovisceral cords answer to the lateral trunks of the Platodes, 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 Molluscan organisation would be 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 musculature 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
simplified 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 jWorks. Investigations
treating of all or several Classes.
Boll. Bcitrdge zur Vergleich. Histologie des Molluskentypus. Arch, filr mikr. Anat.
Supplementband. 1869.
H. G. Bronn. Die Klassen und Ordnungen dcs Thierreiches. 3 Bd. Malacozoa.
I. Malacozoa acepliala. 1862. II. Malacozoa cephalophora, von W. Keferstein.
1862-1866. (New edition now appearing, v. Simroth.)
G. Cuvier. Memoires pour servir a I'histoire et a I'anatomie des Mollusques. Paris,
1817.
G. B. Deshayes. Traite elementaire de Conchyliologie. 3 vols. Paris, 1839-1857.
Histoirc ii.<:itiirclle des Mollusques (Exploration de I'Algerie). 1848.
Eydoux and Souleyet. Voyage autour du monde sur la corvette la Bonite. Histoirc
ni't.tu relic. Zoologie. Paris, 1852.
Paul Fischer. Manuel de Conchyliologie et de Paleontologie conchylwloffique. His-
toire naturelle des Mollusques vivants etfossiles. 2 vols. Paris, 1887.
H. von Jhering. Vergleichende Anatomic des Nervensy stems, und Phylogenie der
Mollusken. Leipzig, 1877.
Keber. Beitrdge zur Anatomie und Physiologic der Weichthiere. Kb'nigsberg, 1851.
E. Ray Lankester. Mollusca. Encyclopaedia Britannica. 9th edit. Vol. XVI.
1883.
R. Leuckart. Zoologische Untersuchungen. Heft 3. Giessen, 1854.
Pelseneer. Introduction d I' etude des Mollusques. Bruxelles, 1894.
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270 COMPARATIVE ANATOMY CHAP.
H. Simroth. Ueber einige Tagesfrayen der Malacozoologie, hauptsdchlich Convcfgen-
zerscheinungen bctreffend. Zeitschrift Natunviss. Halle. 62 Bd. 1889.
The early parts of the new edition of vol. iii. of Bronn's Klassen und
Ordnungen des Thierreichs.
J. Thiele. Die Stammesverwandschaft der Mollusken. Em bcitrag zur Phylogenie der
Thiere. Jenaische Zeitschr. f. Naturwissensch. 25 Bd. 1891.
S. P. Woodward. A Manual of the Mollusca. 4th edit. 1880.
Amphineura.
L. Graff. Anatomie des Chaetoderma nitidulum. Zeitschr. f. ivissen. Zoologic.
26 Bd. 1876.
Neomenia und Chaetoderma. Zeitschr. f. wissen. Zoologie. 28 Bd. 1877.
B. Haller. Die organisation der Chitonen der Adria. Arb. aus dem Zool. Instit. in
Wien. I. Theil. 4 Bd. 1882. II. Theil. 5 Bd. 1883.
G. A. Hansen. Anatom. Beskrivelse of Clicetoderma nitidulum. Nyt magaz. for
naturvidenskab. Vol. XXII. 1877.
Neomenia, Proneomenia und Clicetoderma. Bergen Mus. Aarster. f. 1888.
J. Heuscher. Zur Anatomie und Histologie von Proneomenia Sluiteri. Jena. Zeitzch.
Vol. XXVII. 1893.
A. A. W. Hubrecht. Proneomenia Sluiteri. Niederl. Arch. Zool. Suppl. I. 1881.
A Contribution to the Morphology of Amphineura. Quart. Journ. Micr.
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Dondersia festiva gen. et spec. nov. Donders Festbundcl. Nedcrl. Tijdschr.
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J. Koren and D. C. Danielssen. Descriptions of new species belonging to the genus
Solenopus, with some observations on their organisation. Ann. Nat. Hist. (5).
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A. Kowalevsky and A. F. Marion. Contributions a Vhistoire des Solenogastres ou
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A. Th. v. Middendorff. Beitrdge zu einer Malacozoologia rossica. I. Beschreibung.
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G. Pruvot. Sur V organisation de quelques Neomeniens des c6tes de France. Arch.
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A. Sedgwick. On certain points in the anatomy of Chiton. Proceed. Roy. Soc.
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R. Bergh. Beitrage zu einer Monographic der Polyceraden, /., //., ///. Ver-
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vii MOLLUSCA LITERATURE 271
R. Bergh. Ud>^r die VerwandtscJictftsbeziehungen der Onchidien. Morpli. Jahrb. 10
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Report on the Nvdibranchiata of the " Cliallenger" Expedition. Chall. Report
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Die Titiscanien, cine Familie der rhipidoglosscn Gastropoden. With three
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Die Marseniaden. Zool. Jahrb. 1 Bd. 1886 ; compare also Semper's Reisen
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1886.
Die cladoliepatisclien Nudibranchien. Zool. Jahrb. Abth. fur Systematise.
5 Bd. 1891.
Die cryptobmnchiaten Dorididen. Zool. Jahrb. Abth. fur Systematik. 6
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In addition, numerous monographs of various families, genera, and species
of the Opisthobranchia in various journals.
J. E. V. Boas. Spolia atlantica. Bidrag til Pteropodernes Morfologi og Systematik
samt til Kuiidskaben om deres geografiske Udbredelse. Danske Vid. Selsk.
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Zur Systematik und Biologie der Pteropoden. Zool. Jahrb. 1 Bd. 1886.
L. Boutan. RechercJies sur Vanatomie et le dtveloppement de la Fissurelle. Arch.
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E. L. Bouvier. Systeme nerveux, morpJiologie gen&mle et classification des Gastro-
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E. Claparede. Anatomic und Entwickelungsgeschichte der Neritina fluviatilis.
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B. Haller. Untcrsuchungen uber marine Rhipidoglossen. I. Studic. J/or^/t. Jahrb.
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Die Morphologic der Prosobranchier, gesammelt auf einer Erdumsegcliinij
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des Sciences Nat. (4). Tome XL 1859.
Memoire sur la Pourpre. Annales des Sciences Nat. (4). Tome XII.
1859.
Memoire sur le systeme nerveux de VHaliotide. Ann. des Sciences Nat.
(4). Tome XII. 1859.
Memoire sur Vanatomie et V cnibryoge'nie des Vermets. Ann. des Sciences
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Histoire de la Testacella. Arch. Zool. exper. (2). Tome V. 1888.
A. Lang. Versuch einer Erkldrung der Asymmetric der Gastropoden. Viertel-
jahrsschrift d. Naturf. Gescllsch. Zurich. 36 Bd. 1891.
F. Leydig. Ueber Paludina vivipara. Zeitschr. f. wiss. Zool. 2 Bd. 1850.
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272 COMPARATIVE ANATOMY CHAP.
G. Moquin-Tandon. Eecherches anatomiques sur I'ombrelle de la mcditerrane'e. Ann.
des Sciences Nat. (5). Vol. XIV. 1875.
H. Miiller and C. Gegenbaur. Ueber Phyllirhoe bucephalum. Mull. Arch. 1858.
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87 Bd. 1883.
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Paul Pelseneer. Report on the Pteropoda collected by H.M.S. " Challenger" during,
etc. Parts I. II. III. Chall. Report Zool. Part LVIIL 1887 ; Part LVL
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The cephalic appendages of the gymnosomatous Pteropoda, and especially
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L. Plate. Studien ilber opisthopneumone Lungenschnecken. I. Daudebardia und
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Rang and Souleyet. Histoire naturelle des Mollusques Pttropodes. Paris, 1852.
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1881.
Versuch einer Naturgeschichte der deutschen Nacktschnecken und ihrer
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In addition, many other treatises on the Pulmonata in various journals.
S. Trinchese. Materiali per la fauna maritima italiana. Aeolididae e famiglie
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M. Vayssiere. Recherches anatomiques sur les Mollusques de lafamille des Bullidts.
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Recherches anatomiques sur les genres Pelta (Runcina) et Tylodina. Ann.
des Sciences Nat. (6). Tome XV. 1883.
Recherches zoologiques et anatomiques sur les Mollusques opisthobranches du
golfe de Marseilles. Part I. Tectibranches. Ann. Mus. Hist. N. Marseilles.
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H. Wegmann. Contribution a I'histoire naturelle des Haliotides. Arch. Zool. cxpfr.
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Note sur I 'organisation de la Patella vulgata L. Recueil. Z. Suisse. Tome
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vn MOLLUSC A LITERATURE 273
H. de Lacaze-Duthiers. Histoire de V organisation et du dtveloppement du Dentale.
Ann. des Sciences Nat. (4). Tomes VI., VII., and VIII. 1856-57-58.
L. Plate. Bemerkungen zur Organisation der Dentalien. Z. Anzeiger. 11 Jahrg.
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M. Sars. Om Siphonodentalium vitreum, etc. Christiania, 1861.
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Ernst Egger. Jouannetia Cumingi Son. Eine morphol. Untersuchung. Arbeit.
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Morphologic des Acephales. 1 Mem. Anatomie de VArrosoir (Aspergillum
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Leydig. Anatomie und Entwickelung von Cyclas. Mutter's Archiv. 1835.
H. A. Meyer and Moebius. Fauna der Kieler Bucht. Leipzig, 1865.
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H. von Jhering. Ueber die Verwandtschaftsbeziehungen der Cephalopoden. Zeitschr.
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Van der Hoeven. Beitrag zur Kenntniss von Nautilus. Amsterdam, 1856.
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H. Muller. Ueber das Mdnnchen von Argonauta argo und die Hectocotylen. Zeitschr.
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VOL. II T
274 COMPARATIVE ANATOMY CHAP.
R. Owen. Description of some new and rare Cephalopoda. Trans. Zool. Soc. London.
Vol. II. 1841.
Cephalopoda. Todd's Cyclopcedia, etc. Vol. I. London, 1836.
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J. B. Verany. Mollusques mediterrantens observes, dtcrits, figures et chromolitho-
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Fe"lix Bernard. Recherches sur les organes palUaux des Gastropodes prosobranches.
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P. Girod. Recherches sur la peau des Cephalopodes. Arch. Zool. experiment (2).
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H. Meckel. Mikrographie einiger Drusenapparate der niederen Thiere. Mutter's
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Felix Miiller. Ueber die Schalenbildung bei Lamellibranchiaten. Zool. Beitrdge
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Bernhard Rawitz. Der Mantelrand der Acephalen. 1. Ostracea. Jenaische
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24 Bd. 1890.
G. Steinmann. Vorlaufige Mittheilung uber die Organisation der Ammonitcn.
Ber. Nat. Ges. Freiburg. 4 Bd. 1889.
G. Steinmann and L. Doderlein. Elemente der Paldontologie. Leipzig, 1890.
Johannes Thiele. Die Mundlappen der Lamellibranchiaten. Zeitschr. f. wiss.
Zoologie. 44 Bd. 1886.
T. Tullberg. Studien uber den Bau und das Wachsthum des Hummerpanzers und
der Molluskenschalen. Kongl. SvensTc. Vetensk. Akad. Handling. 19 Bd.
1882.
Karl A. Zittel. Handbuch der Paldontologie. I. Abth. Palaozoologie. II. Band.
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vii MOLLUSGA LITERATURE 275
Musculature, Foot, Pedal Glands, the Taking in of Water.
Th. Barrois. Les glandes du pied et les pores aquiferes des Lamellibranches. Lille,
1885.
J. Carriere. Die Drilsen im Fusse der Lamellibranchiaten. Arbeit, aits d. zool.
Institut Wiirzburg. 5 Bd. 1879.
Die Fussdriisen der Prosobranchier und das Wassergefdss-system der Lamel-
libranchier und Gfastropoden. Archiv. f. mikrosk. Anatomie. 21 Bd. 1882.
C. Grobben. Zv.r Morphologic des Fusses der Heteropoden. Arb. Zool. Inst. Wien.
7 Bd. 1887.
A. Fleischmann. Die Bewegung des Fusses der Lamellibranchiaten. Zeitschr. f. wiss.
Zoologie, 42 Bd. 1885.
Georg Kalide. Beitrag zur Kenntniss der Musculatur der Heteropoden und Ptero-
poden, zugleich ein Beitrag zur Morphologie des Molluskenfusses. Zeitschr.f. wiss.
Zool 46 Bd. 1888.
J. H. List. Zur Kenntniss der Drusen im Fusse von Tethys fimbriata L. Zeitschr. f.
wiss. Zool. 45 Bd. 1887.
Paul Pelseneer. Sur la valeur morphologique des bras et la composition du systeme
nerveux central des Cephalopodes. Arch. Biol. Tome VIII. 1888.
Sur Vepipodium des Mollusques. Bull. Scientif. France et Belg. Tome 19.
1888. Tome 22. 1890. Tome 23. 1891.
Bernhard Rawitz. Die Fussdriise der Opisthobranchier. Abhandl. Akad. Berlin.
1887.
Ludwig Reichel. Ueber die Bildung des Byssus der Lamellibranchiaten. Zool.
Bcitrdgc, Schneider. 2 Bd. 1888.
P. Schiemenz. Ueber die Wasseraufnahme bei Lamellibranchiaten und Gastropoden
(eimchliesslich Pteropoderi). Mitth. Zool. Station Neapel. 5 Bd. 1884. 2
Theil. 7 Bd. 1887. This paper contains the further literature on this
subject.
Jap. Steenstrup. Hectocotylus dannelsen hos Octopods, etc. K. Dansk. Vidensk.
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Nervous System.
L. Bohmig. Beitrdge zur Kenntniss des Centralnervensystems einiger pulmonaten
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Louis Boutan. Contribution a Vetude de la masse nerveuse ventrale (cordons palleo-
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276 COMPARATIVE ANATOMY CHAP.
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Untersuchungen iiber marine Rhipidoglossen. II. Textur des Central-nerven-
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H. von Jhering. Vergleichende Anatomie des Nervensy stems und Phylogenie der
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Lacaze-Duthiers. Du systeme nerveux des Mollusques gastropodes pulmones aqua-
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Bernhard Rawitz. Das centrale Nervensy stem der Acephalen. Jenaische Zeitschr. f.
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Paul Pelseneer. Sur la valeur morphologique des bras et la composition dn systeme
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C. Semper. Ueber Sehorgane vom Typus der Wirbelthieraugen. Wiesbaden, 1877.
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W. Flemming. Untersuchungen iiber Sinnesepithelien der Mollusken. Arch. f. mikr.
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viz MOLLUSC A LITERATURE 277
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Heinrich Maria Gartenauer. Ueber den DarmJcanal einiger einheimischen Gastro-
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Patrick Geddes. On the mechanism of the odontophore in certain Mollusca. Trans.
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Paul Girod. Recherches sur la poche du noir des Cephalopodes des cdtes de France.
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278 COMPARATIVE ANATOMY CHAP.
Respiratory Organs, Circulatory System.
Felix Bernard. Recherches sur les organes palUaux des Gastropodes prosdbranches.
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Bojanus. Ueber die Athem- und Kreislaufswerkszeuge der zweischaligen Muscheln.
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L. Cuenot. Etudes sur le sang et les glandes lymphatiques dans la s6rie animate.
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Carl Grobben. Ueber den bulbus arteriosus und die Aortenklappen der Lamelli-
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1891.
W. A. Herdmann. On the structure and function of the cerata or dorsal papillae in
some Nudibranchiate Mollusca. Quart. Journ. Microsc. Science. Vol. XXXI.
Part I. 1891.
L. Joubin. Structure et dtveloppement de la branchie de quelques Ctphalopodes des
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A. Menagaux. Recherches sur la circulation des Lamellibranches marins. Besanqon,
1890.
K. Mitsukuri. On the structure and significance of some aberrant forms of Lamelli-
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H. L. Osborn. On the gill in some forms of prosobranchiate Mollusca,. Stud. biol.
labor. J. Hopkins Univ. Vol. III. 1884.
R. Holman Peck. The structure of the Lamcllibranchiate gill. Quart. Journ. Micr.
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C. Posner. Ueber der Bau der Najadenkieme. Arch. f. mikrosk. Anat. Bd.
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Secondary Body Cavity, Nephridia, Genital Organs.
Baudelot. Recherches sur I'appareil gener. des Mollusques gastropodes. Ann. Sci.
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Th. Behme. Beitrdge zur Anatomie und Entivickelungsgeschichte des Harnapparates
der Lungenschnecken. Arch. Naturg. Jahrg. 55. 1889.
J. Brock. Ueber die Geschlechtsorgane der Cepho.lopoden. Erster Beitrag. Zeitschr.
f. wiss. Zool. 32 Bd. 1879.
J. T. Cuningham. The renal organs (nephridia} of Patella. Quart. Journ. 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 remaining 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 Harn- und Geschlechtsapparat, sowie die
Leibeshohle der Cephalopoden. Arb. Zool. Inst. Wien. 5 Bd. 1884.
Ueber die pericardialdruse der Lamellibranchiaten. Ein Beitrag zur Kennt-
niss der Anatomie dieser Molluskenklassen. Arb. Zool. Inst. Wien. 7 Bd.
1888.
Die pericardialdriisen der Gastropoden. Arbeit. Zool. Inst. der Univ. Wien.
9 Bd. 1890.
MOLL USCALITERA TURE
279
A. C. Haddon. On the generative and urinary ducts in Chiton. Proceed. Royal
Dublin Soc. (2). Vol. IV. 1885.
B. Haller. Beitrage zur Kenntniss der Niere der Prosobranchier. 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. Les organes de la generation de I'huttre. Tijdschr. Nederl. Dierk.
Vereen. Suppl. D. 1. 1883.
H. von Jhering. Ueber den uropneustischen Apparat der Heliceen. Zeitschr. f. uriss.
Zool. 41 Bd. 1884.
J. Kollmann. Ueber Verbindungen zwischen Colom und Nephridien. Baseler
Festschrift zum Wurtzburger Jubildum. 1882.
A. Kowalevsky. Ein Beitrag zur Kenntniss der Excretionsorgane. Biol. Centralblatt.
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. Hist. (5). Vol. VII. 1881.
- Observations on the Pondsnail, etc. Quart. Journ. Micr. Science. Vol. XIV.
1874.
E. Ray Lankester and A. G. Bourne. On the existence of SpengeFs olfactory organ
and of 'paired genital ducts in the pearly Nautilus. Quart. Journ. Micr.
Science. Vol. XXIII. 1883.
G. F. Mazarelli. Intorno all' anatomia delT apparato riproduttore delle Aplysice del
golfo di Napoli. Z. Anz. 12 Bd. 1889.
Intorno air apparato riproduttore di alcuni Tectibranchi (Pleurobranchcea,
Oscanius, Accra). Zool. Anz. 14 Jahrg. 1891.
0. Niisslin. Beitrage zur Anatomie und Physiologic der Pulmonaten. Habilita-
tivnsschrift (Carlsruhe}. Tubingen, 1879.
R. Owen. On the external and structural characters of the male Spirula australis.
Proceed. Zool. Soc. London. 1880.
Remy Perrier. Rccherchts sur I'anatomie et Vhistologie du rein des Gastropodes
prosobranches. Annales des Sciences Nat. (7). Tome VIII. 1890.
Walter Rankin. Ueber das Bojanus'sche Organ der Teichmuschel (Anodonta cygnea
Lam.} Jenaische Zeitschr. fur Naturwissensch. 24 Bd. 1890.
A. Schmidt. Der Geschlcctitsapparat der Stylommatophoren, etc. Abh. des Nat.
Vcreinsfur Sachsen und Thuringen. 1 Bd. 1885.
P. Stepanoff. Ueber Geschlechtsorgane und Entwickelung von Ancylus jluviatilis.
St. Petersburg, 1886.
W. J. Vigelius. Bijdrage tot de Kennis van het excretorisch Systeem der Cephalopoden.
Acarl. Proefschrift. Leiden, 1879.
-- Ueber das excretionssystem der Cephalopoden. Niederl. Arch. f. Zool. 5 Bd.
1880.
Parasitic Gastropoda.
Albert Bauer. Beitrage zur Naturgeschichte der Synapta.
schnccke in der Leibeshohle der Synapta digitata.
Lcop-Carol. Tome XXXI. 1864.
Max Braun. Ueber parasitische Schnecken. Zusammenfassender Bericht
tmlbl. f. Bakteriologie u. Parasitenkunde. 5 Bd. 1889.
Johannes Miiller. Ueber Synapta digitata und die Erzeugung von Schnecken in
Holothuricn. Berlin, 1852.
Paul and Fritz Sarasin. Ueber zwei parasitische Schnecken. Ergebn. Naturw. Forsch.
auf Ceylon in 1884-1886. 1 Bd. Wiesbaden, 1887.
III. Die Eingeweide-
Nova Ada Academ, Cces.
Cen-
280 COMPARATIVE ANATOMY CHAP-
P. Schiemenz. Parasitische Schnecken. Kritisches Re/erat. Biol. Centralblatt.
9 Bd. 1889-1890.
Walter Voigt. Entocolax Ludwigii, ein neuer seltsamer Parasit aus einer Holothurie.
Zeitschr.f. wiss. Zool. 47 Bd. 1888.
Ontogeny.
F. Blochmann. Ueber die Entwickelung von Neritina Jluviatilis, Mull. Zeitschr.
f. wiss. Zool. 36 Bd. 1881.
Beitrage zur Kenntniss der Entwickelung 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.
R. von Erlanger. Zur Entwickelung von Paludina vivipara. I. and II. Mor-
phologisches Jahrbuch von Gegenbauer. 17 Bd. 1891.
Hermann Fol. Etudes sur le developpement des Mollusques. I. Sur le developpement
des Pteropodes. Archives de Zool. exptrim. Tome IV. 1875. //. Sur le
developpement embryonnaire et larvaire des Heteropodes. Tome V. 1876. ///.
Sur le developpement des Gastropodes pulmone. Tome VIII. 1879-1880.
H. Grenacher. Zur Entwickelungsgeschichte der Cephalopoden, zugleich ein Beitrag
zur Morphologic der hdheren Mollusken. Zeitschr.f. wiss. Zool. 24 Bd. 1874.
A. C. Haddon. Notes on the development of Mollusca. Quart. Journ. Micr. Science.
Vol. XXII. 1882.
B. Hatschek. Ueber Entwickelungsgeschichte von Teredo. Arb. a. d. Zool. Instil.
Universitdt Wien. Tome III. Heft 1. 1880.
E. Horst. Embryogenie de Vhuitre. Tijdschr. Nederl. Dierk. Ver. Suppl. Decl. 1.
1884.
Development of the European Oyster. Quart. Journ. Micr. Science. Vol.
XXII. 1882.
A. Kolliker. Entwickelungsgeschichte der Cephalopoden. Zurich, 1884.
A. Kowalevsky. Etude sur I' embryogenie du Dentale. Annales du Musee d'histoire
naturelle de Marseilles. Zoologie. Tome I. 1883.
Embryogenie du Chiton Polii (Philippi) avec quelques remarques sur le
developpement des autres Chitons. Ann. Mus. N. H. Marseille. Tome I. No. 5.
A. Krohn. Beitrage zur Entwickelungsgeschichte der Pteropoden und Heteropoden.
Leipzig, 1860.
E. Ray Lankester. On the developmental 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 Kenntniss der Mollusca acephala lamellibranchiata. Stock-
holm, 1879.
J. Playfair MacMurrich. A contribution to the embryology of the prosobranch,
Gastropods. Stud. 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 developpement d'un Solenogastre. Comptes rend. Paris. Tome
CXI. 1890.
CarlRabl. Ueber die Entwickelung der Teller schnecke. Morph.Jahrb. 5 Bd. 1879.
Die Ontngenie der Susswasserpulmonaten. Jenaische Zeitschrift. 9 Bd.
1875.
Ueber die Entwickelungsgeschichte der Malermuschel. Jenaische Zeitschrift.
10 Bd. 1876.
vii RHODOPE VERANII 281
W. Salensky. fitudes sur le developpement du Vemnet. Arch. Biol. Tome VI.
1887.
Beitrage zur Entwiekelungsgeschichte der Prosobranchier. Zeitschr. f. wiss.
ZooL 22 Bd. 1872.
P. B. Sarasin. Entivickelungsgeschichte der BUhynia tentaculata. Arb. Zool-Zoot.
Instit. Wurtzburg. 6 Bd. 1882.
Paul and Fritz Sarasin. Aus der Entwiekelungsgeschichte von Helix Waltonii.
Ergtbn. Nat. Forsch. Ceylon, 1884-1886. 1 Bd. Wiesbaden, 1888.
P. Schiemenz. Zusammenfassende Darstellung der Beobachtungen von Eisig,
Rouzaud, Jourdain, Brock, etc., uber die Entwickelung der Genitalorgane der
Gastropoden. Biol. Centralblatt. 7 Bd. 1888.
C. Schierholz. Ueber Entwickelung der Unioniden. Denkschr. Akad. Wien. 55
Bd. 1888.
F. Schmidt. Beit rag zur Kenntniss der postenibryonalen Entwickelung der Najaden.
A rch i v. fit r Nat urgesch ichte. 51 Jahrg. 1885.
M. Ussow. Untersuchungen uber die Entwickelung der Cephalopoden. Arch. Biol.
Tome II. 1881.
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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 spindle-shaped, and out-
wardly bilaterally symmetrical. The body epithelium is ciliated all over. There
is a dermo-muscular tube, inside which, embedded in the 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 oesophageal 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-cesophageal 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 case, 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 caeca. Into this chamber nine or ten small flask-like
organs open ; these resemble the excretory ciliated cells of the Platodes, inasmuch
as "flames" 1 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 classified among the Turbellaria
(near the Rhabdoccslidce), by others among the Mollusca (near the Nudibranchia],
while others again are inclined to see in it a transition form between these two
phyla.
There is apparently only one single point to support the theory of the relation of
Rhodope to the Turbellaria, 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 Nudi-
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-ceso-
phageal ganglion (pedal ganglion), is difficult to reconcile with a relationship to the
Turbellaria. The occurrence of an infra-oesophageal commissure in one isolated case,
that of 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 Mollusca. 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 Mollusca, 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 Rhodope, 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 tbe Mollusca.
The view that Rhodope is a transition form between the Turbellaria and the
Mollusca need hardly be treated seriously.
Cf. vol. i. p. 152, where flame cells are described.
VII
RHODOPE VERANII LITERATURE
Utfi
Literature.
L. von Graff. Ueber Rhodope Veranii. Koell. ( = Sidonia elegans, M. Sehulze).
Morph. Jahrbuck. 8 Bd. 1883.
A. Koelliker. Rhodope, nuovo genere di Gastropodi. Giomdle dell 1 Istltuto R. Lom-
bard o di scienze e.c. Tome 16. Milano, 1847.
S. Trinchese. Nuovo osservazione sulla Rhodope Veranii. Koell. Rendic. delV
A>:-:ad. di Napoli. 1887.
CHAPTEE VIII
SEVENTH EACE OE 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
points, 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 together either movably or immovably (other Echino-
derms). The ccelom is spacious. There is a blood vascular system.
The intestine, which is provided with a mouth and anus, is completely
separated from the coelom. 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. [JDevelopment is accompanied by metamor-
phosis. The larvae are free-swimming and pelagic ; they are bilater-
ally symmetrical, with ciliated bands, generally 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
CHAP, vin ECHINODEEMATA SYSTEMATIC REVIEW
285
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, Ar l \ \v
the feet and papillae, however, may be wanting.
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 ampullae well developed,
Stone canals often numerous. Retractor muscles
wanting. Respiratory trees present. Cuvier's
organs often present. Mulleria, Holothuria,
Stichopus.
Family 2. Elasipoda.
Tube-feet present. 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. Psychropotidse : Psychropotes (Fig. 223),
Benthodytes. Sub-fam. Deimatidae : Deima, Pan-
nychia, Laetmogone. Sub-fam. Elpidiidae : Elpidia,
Kolga, Peniagone.
Family 3. Pelagothuriidse.
Tube-feet wanting. Mouth and anus terminal.
Body cylindrical; round the crown of tentacles it
widens out into a thin disc, the edge of which is tacle ; 2, mouth
produced into long rays. 13-16 tentacles. Re-
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).
OwltS-
3, 4, 8, ambulacral
appendages of the (ventral) triviuin ;
5 ' anus ; 6 ' dorsal a PP enda S e *&
its tw P^erior processes (7).
286
COMPARATIVE ANATOMY
CHAP.
FIG. 224. Pelagothuria natatrix (after Ludwig), completed ; from above. 1, Body ; 2, anus.
FIG. 225. Pelagothuria natatrix (after Ludwig) ; front view, i.e. from the oral pole. 1, Mouth
2, oral tentacles : 3, disc ; 4, canals of the disc.
VIII
ECHINODERMATA SYSTEMATIC REVIEW
287
Family 4. Dendrochirotse.
Tube-feet present. Mouth dorsal or terminal. Anus also often dorsal. Body
cylindrical, or pentagonal, or with a distinctly marked creeping sole. 10-30 arbor-
FIG. 2-27. Psolus epliippifer,
young female, from the dorsal side
(after The'el). 1, Oral valves ; 2,
anus.
FIG. 226. Cucumaria planci (original). 1, The
two smaller ventral oral tentacles ; 2, mouth ;
3, anus.
escent tentacles, often of unequal size. Tentacular
ampullae 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, Colochirus, Theelia, Psolus (Figs. 227
and 228), Rhopalodina.
Family 5. Molpadiidse. FIG. 228. Psolus epliippifer,
female, dorsal aspect (after Theel).
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 careous scales,
less distinct from the trunk. 15 tubular or digitate
tentacles normally present. Tentacular ampullae present. A single stone canal.
Retractor muscles distinct only in the genus Molpadia. Respiratory trees present.
Cuvier's organs almost always absent. Molpadia, Caudina, Trochostoma, 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. Synaptidae.
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), Ckirodota,
Myriotrochus. 1
CLASS II. Echinoidea (Sea-urchins).
The body of these Echinoderms is covered by a usually firm
but sometimes flexible test, which contains the ccelomic 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 larvse).
SUB-CLASS 1. Palseechinoidea.
Either only one row or more than two rows of plates in each
dlgltata (orig^f) P a 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
1 The arrangement of the classes and families of the Holothurioidea by Ludwig in
Bronn's Klassen und Ordnungen des Thierreidis, 1892, is here followed.
VIII
ECHINODERMATA SYSTEMATIC REVIEW
289
present. Anal area either within the apical system, or outside it, in the posterior
interambulacral area. Palaeozoic forms.
Order 1. Bothriocidaroida.
Regular Echinoidea, 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. Boihriocidaris.
Order 2. Perischoechinoida.
Regular Echinoidea. More than two meridional rows of plates in each inter-
radius. Two or many meridional rows in each radius. Test thick and rigid, or
/ Z
FIG. 230. Palaeechinus elegans M'Coy
(after Baily).
FIG. 231. Tiar echinus princeps Laube (after Lovfen).
1, Genital aperture ; 2, anus ; 3, basal ; 4, radial ; 5, ambu-
lacrum ; 6, the 3 upper plates of an interambulacrum.
thin ; in this latter case more or less imbricated. Jaws present. Fam, Archseo-
cidaridae : Lepidocentrus, Archceocidaris ( = Echinocrinus), Palceechinus (Fig. 230)
Fam. Melonitidae : Melonites.
Order 3. Plesiocidaroida.
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. Echiuocystis ( = Cystocidaris).
VOL. II U
290 COMPARATIVE ANATOMY CHAP.
SUB-CLASS 2. Euechinoidea.
Echinoidea with two vertical rows of plates in each anibulacral and in each
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. Cidaroida.
Mouth central, anus within the apical system. No external gills. With jaws
and almost perpendicularly placed teeth. Perignathous apophysial ring interrupted.
Both the anibulacral 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. Sphseridia 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. Sphreridia present.
Sub- Order 1. Streptosomata.
Test more or less flexible, with inner dorso-ventral longitudinal muscles. Both
external and internal gills present. The anibulacral plates (and only these) are
continued over the peristome on to the oral area. Fam. Echinothuridse : Pdan-
echinus, Echinothuria, Phormosoma, Asthenosoma.
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, Acroddaris, Gonio-
pygus, etc. (fossil). Fam. 3. Aspidodiadematidse : Aspidodiadema. Fam. 4.
Diadematidse : Diadema, Diplopodia, Pedina, Echinothrix, Astropyga, Codechinus,
Orthopsis, Peronia, Echinopsis, etc. (fossil and extant). Fam. 5. Cyphosomatidse,
Cyphosoma, etc. (almost exclusively fossil). Fam. 6. Arbaciidse : Arbacia, Echi-
nocidaris (Fig. 232), Coslopleurus, Podocidaris (extant and fossil). Fam. 7. Tem-
nopleuridse : Glyphocyphus, Temnopleurus, etc. (extant and fossil). Fam. 8.
Echinometridae : Echinometra, Parasalenia, etc. , Spongy locentrotus, SpJicercchinus
(mostly extant). Fam. 9. Echinidse : Echinus, Tox&pncustcs, Tripneustes (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
ambulaoral plate. Jaws weak ; teeth perpendicular ; both jaws and teeth may be
wanting. Sphseridia present, (a) Ambulacral apophyses present : Holedypus,
ECHINODERMA TA S YSTEMA TIC RE VIE W
291
/, etc. (principally fossil), (b) Ambulacral apophyses rudimentary or want-
ing : Discoidea, C&noclypeus (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.
FIG. 232. EcMnocidaris (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. Sphseridia 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. Fibulariidae : Echinoeiiamus, FibvJaria, etc. (extant and fossil). Fam.
2. Clypeastridse : Clypeaster (Fig. 233), etc. (extant and fossil). Fam. 3. Laganidse :
Lagan urn, (extant and fossil). Fam. 4. Scutellidse. In all the genera of this family
292
COMPARATIVE ANATOMY
CHAP.
FIG. 233. Clypeaster sp., test from the apical side (original).
FIG. 2^4. Scutella sexforis, test from the apical side (original).
VIII
ECHINODERMATA -SYSTEMATIC REVIEW
293
the shell is very flat : Scutella (Fig. 234), EchinodAscus, Encope, MellUa (Fig. 235),
t, Arachnoid's, etc. (extant and fossil).
FIG. 235. Mellita testudinata? from the oral side (original).
Order 5. Spatangoida.
Mouth central, sub-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. Sphseridia present. The ambu-
lacra generally form apical petaloids. The test is bilaterally symmetrical, arched,
often heart-shaped.
Sub-Order 1. Cassiduloidea.
Fam. 1. Echinoneidse : Echinoconus, Echinoncus, Oligopygus, Echinobrissus, etc.
(extant and fossil). Fam. 2. Cassidulidse : Cassi'lv.lus. Catopygus, Clypetis, Pygurus,
Echinolampas, etc. (mostly fossil). Fam. 3. Collyritidae : Collyrites, Disaster, etc.
'fossil). Fam. 4. Plesiospatangidae : EnJanipcis, Archiacia, etc. (fossil).
Sub-Order 2. Spatangoidea.
Fam. 1. Anan-chytidae : Eclnnocorys, Holaster, Hemipmustes, Cardiaster, Ure-
si. Cijstechiiius, Calijmne, etc. (the last three genera extant, the rest fossil).
Fam. 2. Spatangidae Group 1, Adetes : Isastcr, Eehinospatagus, Heterolampas,
H'--,nipof.agus, etc. (almost exclusively fossil); Group 2, Prymnadetes : Hemiaster.
F'">ri:ia. Linthia, 8chi~astcr (Fig. 236), Agassizia (extant and fossil) ; Group 3.
Prymnodesmia : Micmste.r, Bi-issus. Spaianffomorpha, Brissopsis, Spatangus, Palceop-
,, votes (Fig. 237), Echiiiocardiu,,i. Lwiiia, etc. (extant and fossil) ; Group 4.
Apetala : Cfenieopcttoffut^ Pdlrrob'riw*. Ac?stc, Aerope, etc. (extant and fossil).
294
COMPARATIVE ANATOMY
CHAP.
FIG. L'36. Schizaster lacu-
nosus ? from the apical side
(original). The spines, and the
protuberances on which they
stand, are not depicted. 1, The
anterior unpaired ambulacrum ;
2, the right anterior ambulacrum ;
3, fascicle ; 4, the right posterior
interambulacrum ; 5, the right
posterior ambulacrum ; 6, the
unpaired posterior interambula-
crum ; 7, anal region.
Fio. 237. Palaeopneustes
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 ECHINODERMATA SYSTEMATIC REVIEW 295
Fain. 3. Leskiidse : Palccostoma (extant). Fam. 4. Pourtalesiidae : Pourtalesia
(Fig. 238), Spatagocystis, Echinocrcpis (extant). 1
IV
0*
FIG. 238. Pourtalesia Jeffreys!, from the side (after Loven). The smaller tubercles are not
depicted. r: die ell aster. Fam. 9. Asteriidae, tube -feet in four rows:
Aatffias, Uniophora, Coronaster, etc. Fam. 10. Brisingidae, with numerous very
long arms, marked off from the small disc : Brisinga, Labidiasftr, etc. 1
CLASS IV. Ophiuroidea.
Echinodermata flattened in the direction of the principal axis of the body, the
radii of which are produced into five long, round, simple or much branched slender
arms. The arms are sharply marked off from the central part of the body, and do
not contain either ca?ca of the intestine or extensions of the genital organs. The
Fio. -244. Ophiolepis elegans, Lutken (after Lyman). ds, Dorsal shields ; as, lateral shields ;
dc, dorsocentral ; ib, infrabasal ; fac, basal ; rs, 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
1 The classification of the two orders of the Euasteroidea is that of W. Percy Sladen,
Report on the Asteroidea collected by 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 madreporitic 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 bursae, which serve for respiration and for the reception and ejection of the
genital products. Development direct (viviparous and with care of the brood), or
with metamorphosis (free-swimming pelagic larvae).
Order 1. Ophiurae.
Arms unbranched, movable in the horizontal plane, usually distinctly plated.
Buccal shields, one of them at the same time the madreporitic plate, distinctly
developed.
Fain. 1. Ophioglyphidse : Opliiura, Pectinura, Ophiolepis (Fig. 244), Ophiozona,
Ophioglypha, Opliioctcn, Ophiomusium. Fain. 2. Amphiuridse : Ophiadis (Fig. 245),
FIG. 245. Ophiactis poa, Lym. (after Lyman). Disc and basal portions of t.he arms ; from the
oral side. 1, Ventral shields ; 2, spines on the lateral shields (4) ; 3, tentacle scales ; 5, lateral
buccal shields ; 6, bursal apertures ; 7, buccal shields ; 8, first ventral shield of the arm ; 9, torus
angularis ; 10, oral papillae.
Amphiura, Ophiocnida, Ophiocoma, Ophiacantha, Oirfiwthrix. Fam. 3. Ophio-
myxidae, disc and arms covered by a thick naked integument : Ophiomyxa, Hemi-
curyalc.
VIII
ECHINODERMATA SYSTEMATIC REVIEW
301
Order 2. Euryalse.
Arms simple or branched, can be rolled up vertically towards the mouth. 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 have no
FIG. 246. Astrophyton LincM (Miiller a
}Ch.el), from the oral side (original).
distinct bnccal plates. There is then either a single madreporite in an oral inter-
brachial area or else there are 5 interbrachial madreporites.
Single Fam. Astrophytidse : Astrophyton (Fig. 246), Gorgonocephalus, Euryale,
Trichaster (arms slightly and only at their tips, dichotomously branched), Astroclon
(the same), Astrocnida (the same), Astroporpa (arms undivided), Astrogomphus (the
same), Astrochcle (the same), Astrotoma (the same), Astroschema (the same), Ophio-
creas (the same), etc. 1
1 For a more recent classification of Opliiuroidea, see F. J. Bell, Proc. Zool. Soc.
London, 1892, pp. 175-183.
302 COMPARATIVE ANATOMY CHAP.
CLASS V. Pelmatozoa.
Echinodermata which are either permanently or temporarily l 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 5 radials, to which 5 infra-
basals and a varying number of interradials are often added. The plate in the
embryo Antcdon, 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 4 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 pinnulse 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 circumo3sophageal 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 pinnulse, 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 run through the skeletons
of the stem, the arms, and the pinnulse. 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. Ambulacral 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. Adinometra}.
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 pinuulse.
1 There is, however, no evidence to show that Marsupites was attached even in the
larval stage ; unlike Antedonidse, it has no trace of a stem.
VIII
EGHINODERMATA SYSTEMATIC REVIEW
303
The old division into Pakcocrinoidea and Neowinoidca seems artificial ; that here
adopted also cannot be considered as definitive. 1
Order 1. Inadunata.
Calyx comparatively small ; dorsal cup with nionocyclic 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 (Larviformm) 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 a 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 pinnulpe. 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 palaeozoic forms.
A. Monocyclica.
With monocyclic basis (without infrabasals ; several radials often horizontally
\
FIG. 247. Haplocrinus mespiliformis (after Wachsmuth and Springer). A, from the 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). Haplocrinus (type of the so-called Larrifonnia, without anal plate) (Fig.
1 Classification chiefly after the recent works of Wachsmuth and Springer and Her-
bert Carpenter. See Bibliography, p. 551,
304
COMPARATIVE ANATOMY
CHAP.
247). Hcterocrinus,Hcrpctocrinus, Calceocrinus, Catillocrinus, Pisocrinus, Hybocrinus,
locrinus, Symbathocrinus, Belemnocrinus, Gastrocoma (?), Cupressocrinus.
B. Dicyclica.
With dicyclic base (with infrabasals). Fam. Dendrocrinidse : Dendrocrinus,
Homocrinus, Poteriocrinus. Fam. Decadocrinidse : Botryocrinus, Barycrinus,
m
FIG. 248. Encrinus liliifonnis (original).
c l ca* Costals or primibrachials ; r, radials ;
co, stem ; p, pinnulae.
FIG. 249. Cyathocrinus
longimanus (after Angelin).
pr, Ventral sac ; !, place where an
arm-branch has been removed ;
r, radials ; ba, basals ; ib, infra-
basals ; col, stem ; x, anal plates ;
co, costals or primibrachials.
Atelestocrinus, Decadocrinus, Graphiocrinus, Encrinus (Fig. 248), (without anal
plates, ventral sac reduced to a short cone, Trias), Cromyocrinus, Agassizo-
crinus. Fam. Cyathocrinidse : Cyathocrinus (Fig. 249), Gissocrinus, Lecythocrinus.
Hypocrinus.
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. Holopidaz (Fig. 250) (Lias, to present time), Hyocrinidce (Fig. 251) (Lias,
present time), Bathycrinidce (extant).
VIII
EGHINODERMATA SYSTEMATIC REVIEW
305
1
.sll ft.*
FIG. 250. Holopus Rangi 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,
which 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 frequently 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. Reteocrinus. Xenocrinus.
Family 2. Rhodocrinidse.
Apical capsule with dicyclic base. The circle of the five radials interrupted by
that of the five first interradials, which are in direct contact Avith 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,
Gilbertsocrinus, Rhipidocrinus.
Family 3. Glyptasteridae.
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 Rhodocrinidce. Oral plates
distinct. Anus subcentral. Glyptaster.
VIII
EGHINODERMATA SYSTEMATIC REVIEW
307
Family 4. Melocrinidse.
Base monocyclic, 3-5 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. Mdocriiias (Fig. 252), Mai-iacriiius, , Pinnuhe ; br, arms; (//', distichals;
t'l, c-2, first and second costal ; /, radial ;
tin, basal ; co, stein ; it- and id, inter-
radinls.
Fi<;. 253. Batocrinus pyriformis.
Shum. (after Meek and Worthen).
>:k, Ventral capsule ; br, arms : p. pin-
nulit: ; (U, distichals ; GI, Co, costals ;
/, radials; /x?, basals; co. stem; ir,
interradials ; abr, points of insertion
ufthe arms.
~Q^abr
radials. Tegmen calycis 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. Carpocrinus, Agarico-
ff/iiiis, Pcricchocrinus, Jfogistoeriwus, Adinocrinus, Teleiocrinus, Steganocrinus,
Amphoracriniu, Physetocrinus, Strotocriiius, Batocrinus (Fig. 253), Erctiuocriiius.
Dorycrinax.
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
proximal (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. Platycrinus (Fig.
254), Marsupiocrinus, Eudadocrinus.
Family 7. Crotalocrinidse. 1
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, iuter-
radials, and anals ; ambulacra externally visible, with large rigid covering plates,
which combine with the other plates to form the solid teginen. 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, Eimllocrinus.
CO
FIG. 254. Platycrinus triacontadactylus (after
M'Coy). di, Distichals ; c, costals ; r, radial ; ba,
basal ; co, stem ; ir, interradials ; vk, ventral capsule.
Family 8. Hexacrinidae.
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 Platycrinida.'.
Hexacrinus, Talarocrinus, Dichocrinus.
Family 9. Acrocrinidae.
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 part of
the apical capsule. Each radial is followed by 2 costals. The radials and
1 This family, originally placed near Cyathocrinus, was referred by Wachsmuth and
Springer, first to the Ichthyocrinoidse aud then to the Camerata ; Bather, however,
would refer it to its original position in the Inadunata.
viii ECHINODERMATA 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 which there is, further, an intercalated
vertical row of anal plates. Acrocrinus.
Family 10. Barrandeocrinidae.
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, 2 x 10 distichals, 3x5 interradials, and 1x5 interbrachials. Xo 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.
CktUierinus.
Order 3. Articulata (Ichthyocrinidae).
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
primai'y brachials. The 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.
Pinnule 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. Ichthyocrinidae Palseozoic forms : Ichthyocrimis, Forbesiocrinus, Gleio-
cri iiu.s, Taxocrinus (Fig. 255), etc.
The unstalked genus Uintacriiius, from the upper Chalk, and the extant unstalked
genus Thaumatoci-inus (Fig. 256), ought probably to be classed here. In the latter
the uppermost ossicle of the stem is retained as centrodorsal. The dorsal cup
consists, apart from the centrodorsal, 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
ir
CO
FIG. 255. Taxocrinus multibrachiatus, Ly.
and Cass. ir, ir^, and ir. 2 , Interraclials ; di, dis-
tichals ; ba, basals ; ib, infrabasals ; co, stem ; r,
radials ; c\, Co, and 03, primary brachials.
FIG. 256. Thaumatocrinus renovatus.
P. H. C. (after P. H. Carpenter). Calyx
from the anal side, cj, c-2, and 03, Primary
brachials ; r, radials ; c, points of insertion
of the cirri ; cd, centrodorsal ; ir, inter -
radials ; ia, interradialia analia ; pa, proces-
ses analis ; ta, tubus analis ; p, pimmlee.
jointed appendage,
arms with pinnulre.
Besides this there is a short anal tube. Five unbranched
Order 4. Canaliculata.
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. Apiocrinidae.
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, Millericrinus, and the extant Calamocrinus.
Family 2. Bourgueticrinidse.
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 ECHINODERMATA SYSTEMATIC REVIEW 311
with root-like processes at its base, or with irregularly arranged cirri : its proximal
FIG. 257. Metacrinus Murray! (after P. H. Carpenter). Most of the arms and the larger ];ait
of the stem broken off. p, Pinnulse ; ci, cirri : ng, node.
ossicle usually enlarged. Upper Jurassic, Chalk. Tertiary, Recent. Ehizocrinus,
Bourguetiarimu.
312
COMPARATIVE ANATOMY
CHAP.
FIG. 259. A, Cystoblastus Leuchtenbergi.
1, Interradial ; 2, 3, radial ; 9, basal ; 10, infra-
basal ; 8, anus ; 6, genital aperture. B, From
the oral side (after Volborth). 4, Mouth; 5,
ambulacrum. Fig. 295, p. 332, shows the apical
side.
;. -jr.*. Ante don incisa (after P. H.
Carpenter). 1, Anns; 2, cirri.
FIG. 260. Protocrinus oviformis, Eicliwald
(after Volborth). 2, Anns ; 1, tliird aperture ; 3.
ambulacrum.
VI II
ECHINODERMATA SYSTEMATIC REVIEW
313
Family 3. Pentacrinidse.
Calyx small as compared with the stem and the arms ; it consists of 5 basals
and 5 radials. (In the genus Extracrirms 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. Pcntacnnus, Metacrinus (Fig. 257), Extracrinus,
Balanocrinus.
Family 4. Comatulidse.
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 from 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. Atchcrinus (basals externally visible), Eudiocrinus.
Antcdon (Fig. 258), Promachocrinus, Adinomctra (the only Crinoid genus with
excentric mouth). Since Jurassic times, many living species.
SUB-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
*
FK;. 2(51. Orocystis Helmhackeri.
Baur (after Barrande). 1-3, The Fm. -j^.-Agelacrinus cincinnatensis.
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. Palaeozoic Pelmatozoa, whose organisation is still little understood.
Order 1. Cystocrinoidea (cf. the section on the perisomatic skeleton of the
C instilled) : Pcrocrinus, Canjocrinus, Echinocncnnus, CystoWastus (Fig. 259 A and B).
Order 2. Eucystidea : Protocrinus (Fig. 260), WypfnspJuerites, Orocystis (Fig.
261), EchinoqpJuera, Ari*t'Iriinis (Fig. 262).
314
COMPARATIVE ANATOMY
CHAP.
SUB-CLASS 3. Blastoidea.
Armless Pelmatozoa, either pear-shaped, club-shaped, oviform, or spherical.
Body usually regularly radiate. Base mouocyclic. Three basals, one small and
FIG. 263. Pentremites, from
the side, without pinnules. 1,
Interradial = deltoid ; 2, 3, radials ;
4, basal ; 5, ambulacrum ; 6, spir-
acle.
FIG. 265. Codaster bilobatus, M'Coy, from the oral
side (after Etheridge and Carpenter). 1, Hydrospiiv
slits ; 2, lateral plates ; 3, ambulacral groove ; 4, mouth ;
5, radial ; 6, suture between two radials; 7, anus; 8, inter-
radial ; 9, ridge on an interradial.
Fio. 264. Granatocrinus Norwood!
(after Etheridge and Carpenter); from FIG. 266.-Orophocrinus stelliformis (after Ethe-
the apical side, with stem. 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 deeply cut out for the reception of the five
ambulacra. Five interradials lying above the five radials, and surrounding the
VIII
ECHIXODEKMA TA S YS TEMA TIC RE 1 'IE 11 '
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 pinnule-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. 3:2 S.
Pal.eozoic forms.
Order 1. Regulares.
Stalked Blastoids with sym-
metrical base. The radials resemble
one another, as do the ambulacra.
Fam. 1. Pentremitidse : Pen-
< (Fig. 263), Pcnti-fiiiiti'l:"..
Fam. 2. Troosto-
blastidae : Tr*to<:rinn3. M> /-
//'*. etc. Fam. 3. Nucleoblastidae :
ttttS, Xdir.ijllastux. '
. Fam. 4. Granatoblastidse :
"S (Fig. 264 . .
iili.istiis. Fam. 5. Codasteridse :
/ (Fig. 265), PJw:nox<:lii*innt>:-
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, ^faetinal, or ventral pole there lies, in most Echinoderms.
the oral aperture, while at the other apical, abaetinal. 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
Coelenterates, 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, ambulacral radii, or simply
radii. The five radii of the second order, which regularly alternate
with these five principal radii, are the interradii or interambulaeral
FIGS. 2(38 and 209. Representatives of the principal divisions of the Echinodermata. In
Fig. 268, in the morphological position ; in Fig. 269, in the natural position with regard to the
sea-floor. A, Holothurian. B, Sea-urchin. C, Star-fish. D, Crinoid a, Apical pole ; o, oral
pole ; an, anus.
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,
adactinal, or ventral zone, and the apical, abaetinal, or dorsal zone.
In the centre of the former lies the mouth.
vin ECHIXODEHMATA MORPHOLOGY OF SKELETON 317
While these terms facilitate the morphological 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 horizontal.
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 Review, and also specially the two sections which
treat of the skeletal and ambulacral systems.
II. Morphology of the Skeletal System.
Meaning of the Most Important Lettering of the Figures.
" Apical pole. ian Anal interradials or anals.
a in Ambulacral plates. ib Infra basals.
a. it Anus or anal area. i>./ Interdistichals or intersecuudi-
ap Ambulacral pores. brachs.
B Buccal plates. ir Interradials.
IKI. Basals. m Madreporite, pore - openings of
In- Brachials, arms. the stone canal,
q First costal or primibrach. n Xodal columnal.
'_, Second costal or primibrach. o Oral pole, mouth.
ca Points of insertion of the cirri. or Orals, or mouth-plates.
cd Centrodorsal. p Pinnules.
ce or c Central plate. pa Anal.
ci Cirri. rs Radial shields.
co Column, stem. r Radial s.
i-p'.'' Covering plates of the ambulacral ss Lateral shields.
grooves. t Terminals.
D Dentes, teeth. ta Anal tube or ventral sac.
ii<: Dorsocentral. vfc Tegmen calycis.
fU Distichal or secundibrach. 1-5 Interradii or interambulacral
ds Dorsal shields. areas of the Echinoidea.
' (Fig. 285). Let us take as an example P.
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 ANATOMY CHAP.
Although there are good palseontological reasons for the generally accepted belief
that all known exocyclic (irregular) Echinoidea are descended from endocyclic
(regular) forms, it has been conjectured that these latter may themselves have had
exocyclic ancestors (which, indeed, are unknown to us). Thus the modern Spatan-
yoida 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 significance
with reference to the primitive Pelmatozoa, receives some (not very satisfactory)
support from the fact that in the very old family of the Saleniidce 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,
Nectria, Ferdina, Pentaceros, Gymnasteria,
Set/taster, 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 Pentagonaster, Gymnasteria, Pentaceros,
and many Goniasteridce) 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 Zoi'oaster fulgens (Fig. 286). The
FIG. 286.-A P icar s ystem of plates a P erture of fche stone canal lies in the
in a young specimen of Zoroaster right anterior interradius, outside the
sS gen 3 S i7 after Slad6n) ' Forlettering 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
viii ECHINODERMATA 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. Ophiupoidea.
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
Fi<;. 287. Plates of the apical system of the disc of
Ophiomusium validum (after P. H. Carpenter). For
lettering see p. 317.
FIG. 288. Apical system of a
young Amphiura squamata (after
P. H. Carpenter). For lettering
see p. 317.
(species of the genera Ophioglypha, Ophiomastix, Ophiopyrgus, Ophiura,
H'-iiiiphol is, Ophioceramis, Ophiopholis, Ophiotrochus). 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, Ophiopholis, Ophiozona, Ophiactis, Ophiolepis).
In Ophiomifra wigua 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 Ophioceramis, OphioglypJw, Ophiozona, Ophio-
musiu.m (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
OF THF.
ITN^ :TY
328 COMPARATIVE 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 pinnula?. 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
(Pentacrinus), and may dwindle in size (Millericrinus), or may be present
only in the embryonic 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 cup) 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. Crinoidea.
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 monocyclic
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 (many Inadunata larviformia
and many Inadunata fistulata, Encrinus, Marsupites, Holopus, Hyocnuu.^
JBathycrinus, and a few Canaliculata : PJiizocrinus, Pentacrinns).
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
ECHINODSBMATA MORPHOLOGY OF SKELETON 329
(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
- \OA1S
FIG. -2SO. Apical system of Cyatho-
crinus. For lettering see p. 317. <""/<. Anal
interred iaL
FIG. 29u. Marsupites ornatus. P!
the dorsal cup. For lettering see p. 317.
i .-A
special ' anal plates " in the posterior unpaired interradius ; these
specialised anals occur very frequently in palaeozoic Crinoids (Fig. 291).
The Crinoids with dieyelie base (with infrabasals, Figs. 289
and 290) are : (a) most Iimdumtta ; (b) among the Canierata, the
families of the ReteocrinidiR p. p.,
Glyptasterid^, and
(c) the Articulatn.
(Ichthyocrinida) ; (d) the Camilieu-
lafa, 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 conveniently termed
Pseudomonoeyelie.
The Crinoids with monoeyelie
base (without infrabasals, Fig. 291)
are, apart from a few Inadnnata,
the Camerate families of the
I'.iinocrinidce, Phity-
B:irra ndeocrin <-r' ( n idee.}
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
I/<"du/>fii. . and Arfwhtf-.i. The plates are then almost
FIG. 291. Actinocriuus proboscidalis. Plates
f the dorsal J/\^ 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 strictty 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 paleozoic 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
irregular Blastoids, on the
contrary (Eleutherocrinus, As-
trocrinus, Fig. 267, p. 315),
the radiate structure is dis-
turbed by the modified form
of one of the ambulacra.
The outline of the ovoid
body of Eleutkerocrinus, seen
from the apical or oral pole,
is irregularly pentagonal,
with three shorter and two
longer sides, the latter be-
longing to the left posterior
and the unpaired posterior
interradii. In Astrocrinus,
the body is flattened in the
direction of its principal axis,
z, the two larger basals ; ir, interradials ; r, radials. Or aboral pole, almost Sym-
metrically four-lobed, the
lobes being of unequal size. The largest of the lobes lies diametrically
site the abnormally shaped ambulacrum, which is on the smallest
form C (FiJ o?7 p 315 T ther middle - sized lobes are almost dike in
The whole' body of the Blastoids is plated. The test consists,
apart from the ambulacra, of three circles of plates (Fie. 292) two of
which belong to the typical apical system of the Ech noderma a while
viii ECHINODERMATA 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
Blastoidea 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 plates 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-pieces,
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 *i
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 interradials or deltoid plates. CMX
These plates do not form a closed circle, as they & &
are separated from one another by the five FlG 2 o 3 . _ Eieuthero-
ambulacra. The apical edges of each deltoid crinus Cassedayi, from
plate rest on the oral edges of the contiguous ^d e^^m^^^entert
forks of two consecutive radials or fork pieces. aa . bbj A * pa . SS ing through
The relative sizes of the basals, radials, and inter- the mouth and the anus ;
radials of the Blastoids vary greatly (cf. figures). ^^0^
One of the five interradials, which is distinguished an> anal side,
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 narrow plates.
It cannot at present be decided whether there are skeletal pieces
332
< ////.//.<, 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.
Caryocrinus, 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
VIH ECHINODERMATA MORPHOLOGY OF SKELETON 333
this is surrounded by a closed circle of six radials. These plates,
together with two accessory plates (interradials ?), form the whole
test of the cup of the attached Caryocrinus, 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 (cf. 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
O
FIG. 296. System of plates of the dorsal
basalS, and outside this that of the cup of EcWnoencrinus armatus, spread out
(after Forbes). For lettering see p. 317.
five radials, between which acces-
sory pieces are intercalated, the homologies of which cannot be made
out. The anus lies posteriorly to the right. In CystoUastus the
radials, like the radials or fork-pieces of the Blastoidea, have deep
incisions on the oral side for the reception of the ambulacra (cf. Fig.
259, A and B, p. 312, and Fig. 295).
B. The Oral System of Plates.
In certain Echinodermata (Pelmatozoa and Ophiuroidea) 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 way 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-rayed 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 Antedon (Pentacrinus stage).
In a young stage of this larva the oral surface of the calyx appears
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
COMPARATIVE ANATOMY
CHAP.
334
roof of the vestibule. This roof is formed of five interradial 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
FIG. -297. Haplocrinus mespiliformis (after Wachsmuth and arms. This peri-
Springer). A, From the anal side ; B, from the oral side. 1, Orals ; ph era j zone COntittU-
2, oral pole ; 3, anus ; 4, radials ; 5, inferradial ; 6, basals ; 7, first ,. . .
brachial ; 8, point of attachment of the arm. ally increases in S1ZC,
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 greater 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 with 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
via ECHINODERMATA 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 (?).
The same condition is found in the extant genera Holopus and
Hyocrinus (Fig. 298), the extant unstalked genus Thaumatocriims,
and the extant canaliculate
genus Bhizocrinus. All these
genera possess five oral plates,
which, however, are separate,
and do not form a closed
pyramid ; the mouth, there-
fore, is in open communica-
tion with the exterior between
them. Compared with the
larva of Antedon and with
Haplocrinus, 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 FIG. 298. Hyocrinus Bethellianus (after P. H.
Calyx only a Very Small peri- Carpenter) Tegmen calycis 1, Axial canal of the
. J J J . * braclnals ; 2, extension of body cavity in the arm ;
plieral zone remains. In 3, food groove of the arm ; 4, smaller plates of the
HyOCrimiS (Fig. 298) also, and tegmen; 5, orals; 6, anal cone; 7, oral edges of the
Thaumatocnnus 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
Rhizocrinus lofotensis the orals
are smaller, and in Rhizocrinus
Rawsoni they are almost rudi-
mentary, so that the zone
which surrounds them forms
the greater part of the
tegmen.
In the Cyathocrinidce (In-
adunata fistulata), five large
plates can sometimes be dis-
tinctly made out in the centre
of the plated tegmen ; some-
FIG. -299. -system of plates of the tegmen of times, however, irregular
Platycrinus tuberosus (after Wachsmuth and
Springer). For lettering see p. 317.
peces are found in their
placeg When they an) djg _
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
In the Camerata (Fig. 299) five 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 presses in
between them.
As far as is known, in the Articulate (Ichthyocrinoidea) also, five
orals can be distinguished at the centre of the richly 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 Camliculata (with the exception of the above-named genus
Rhizocrinus) 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 Stephanocrinus, five orals can be made out.
In Stephanocrinus 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 Cyatliocystis, the five oral plates forming this pyramid
are more or less equal in size, but in species of the genera Sph&ronis,
Glyptosphcera, and Pirocystis the posterior oral is, as in so many
Camerata, larger than the rest. In the six-rayed Cystid Caryocrinu*
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 Ophiuroidea, 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 bueeal 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 Pelmatozoa.
In the Asteroidea, on the lower surface of the disc at the edge of
the mouth, in each interbrachial region, there occurs a skeletal plate
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 interbraehial system, may correspond with the
orals of the Pelmatozoa and the oral shields of the Ophiuroidea, although
they may be pushed below the surface by the oral plates (the first
pairs of adambulacral plates), and are usually completely covered
externally. They arise early in the larva of Asterias (afteV 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
vin ECHINODEEMATA MORPHOLOGY OF SKELETON 337
certain pieces of the calcareous ring of the Holothurioidea corre-
spond with the orals of other Echinoderms cannot at present be
determined.
C. The Perisomatie Skeleton. 1
All those skeletal pieces which protect the body, between the apical
and the oral systems, taken together, form the perisomatie skeleton
of the Echinodermata. It is obvious that the extent of the periso-
matie skeleton must vary inversely with that of the polar (apical and
oral) systems. Where the polar systems form only a small part of
the body wall the perisomatie skeleton is the more strongly developed,
and vice versd. 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 perisomatie 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 Pelmatozoa,
Asteroidea, and Ophiuroidea, the skeleton
of these arms is exclusively formed by
perisomatie pieces. It is at present
impossible to prove any definite
homologies between the parts of the
perisomatie systems throughout the
Echinodermata.
I. Holothurioidea. FlG 3 oo._Microscopic calcareous bodies
rji 7 of Holothurioidea. 1, Auchor and anchor
In the CUtlS Of the HolottlUnOUlea, plate of Syna pt a inhserens, O. F. M. ; 2,
as well in the body Wall as in the "stool "of Cucuinaria longipeda, Serap; 3,
wall of the tentacles, ambulacra, tube- %%%*&&
feet, and ambulacral papillae, there are sticopus japonicus ; 5, supporting plate
found enormous numbers of micro- from one of the tube-feet of stychopus
. ,, . , , i' f japonicus; 6, "stool" of Holothuria
scopically minute calcareous bodies ot Murray . . 7j rod from the ventral ambula .
definite Shapes (Fig. 300). These give cral appendages of Oneirophanta mutabilis,
the integument a firm and rOUgh Theel ; S, latticed hemisphere ,of Colochirus
_. . ! cucumis, Semp; 9, "wheel of Acantho-
COnSlStency. Their principal Slgmtl- trochus mirabilis, Dan. and Kor.
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.
1 On the author's use of the term " perisomatie," see footnote, p. 362.
VOL. II
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, Ito a common form, viz. to a very short rod, which tends to branch
dichooSsly at each end. In some Dendrochirotce (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 Dendrochirotcc 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 come 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, Actinocucumis, etc., they are arranged radially
and consist of compact masses of calcareous granules and ambulacral papillae. In
many Aspidochirota, and Dendrochirota 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 (cf.
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 corners ; 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 EcMnoidea, except the Spatangoida, the mouth lies at the
centre of the oral surface of the test ; in the Spatangoida it has shifted
anteriorly on this surface. The mouth, however, always remains the
centre round which the plates of the perisomatic skeleton are
grouped.
We 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.
vin EGHINODERMATA MORPHOLOGY OF SKELETON 339
The whole perisome, from the mouth to the apical system, falls
into two sections : (1) a small portion surrounding the mouth, the
peristome 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 Echinoidea, 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 Interambulacra (interradii).
In all Euechinoidca (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 interambulacral system. Five
double rows of ambulacral plates thus regularly alternate with five double rows of
interambulacral plates.
In the exclusively Palaeozoic Palccechinbidea, 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 Mclonitidw (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
Palccechinoidea there are more than two (3-11) rows of plates in each interradius
(Fig. 230, p. 289). The interesting genus Tiarechimis (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, these plates being separated by meridional (perpendicular)
sutures.
The plates of the Eckinoidea are most frequently pentagonal. In the two per-
pendicular rows of an ambulacrum or
an interambulacrum the consecutive
plates usually alternate in such a way
that the suture between the two rows
forms a zigzag line. The sutures
between the plates, which lie one
below the other in a row, usually run
horizontally (Fig. 232, p. 291).
(6) The Pores perforating the Plates
of the Ambulacral System.
As a rule, in the Echinoidea, the
pores occur in pairs. These double
pores occur only on the ambulacral
plates. One double pore belongs to
each ambulacral foot. 1 From the
ampulla, under the test (at its inner
side), two canals run out, which,
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 be 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 Clypeastroida 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
interradii, 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.
FIG. 301. Apical system and adjoining peri-
some of Melonites multipora, Norw. (after Meek
and Worthen). For lettering see p. 31V.
(c) The Symmetry of the Echinoid Shell.
The test of the regular Echinoids (Cidaroida, Diadematoida, and most
Palceechinoidea), 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 Holedypoida 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
1 For the different forms and arrangements of these feet or tentacles, cf. section on the
ambulacral system, p. 416 et seq.
vin ECHINODERMATA MORPHOLOGY OF SKELETON 341
plane of symmetry can be recognised in the Holedypoida 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 Clypeastroida, and, in
a still higher degree, in the Spatangoida. In the Clypeastroida the peristome with
the mouth still remains in the centre of the oral surface, or only very slightly shifts
away from this position. 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 Scutellidcc by a perforation through the test (lunula), which never
occurs in the other interradii. Further, in the Scutellidcc, 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 Spatangoida, culmi-
nates in the remarkable family of the Pourtalesiidce. The ambitus, which varies
greatly in details, is frequently egg-shaped, or heart-shaped, and in Pourtalesia
flask-shaped. Not 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 Cassidulidce 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. We shall return later on to the dissimilarity in the ambulacra, and especially
to the abnormal development of the anterior ambulacra, and consequent formation of
the bivium and trivium, to the special form of the peristome of the Spatangoida, 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, 295).
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, when
342
COMPARATIVE ANATOMY
CHAP.
these ten plates carefully, we see that those indicated by la, lla, I
FIG. 302. Kleinia luzonica (Gray). Apical system, spread out (after Loven). fa, Fascicles.
Further explanations in the adjoining text.
are larger and possess two pores each, while the smaller plates Ib, lib, Ilia, I\ b
and Va 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 17 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 but for their young stages also) ; the plates la, Ha, lllb, IVa, V6 are
vm ECHINODERMATA MORPHOLOGY OF SKELETON 343
IV
FIG. 303. Toxopneustes drcebachiensis juv., 4 mm. in diam. The whole system of plates
spread out in one plane (after Loven). B, Peristome plates. D, teeth.
marked by common characters, and are distinguished from the plates Ib, lib, Ilia,
IVb, Va, which also resemble one another. These different characters are, it is true,
often not very evident.
344
COMPARATIVE ANATOMY
CHAP.
As a further example, let us take the test of a young Toxopneustes drcebachiensis,
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. "We can place the test in only one position, viz.
that given in the figure, in which the formula la, Ila, lllb, IVa, ~Vb, and Ib, lib, Ilia,
I VJ, Va holds good. In this we see that a median plane, corresponding with that of
the irregular 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 applies 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 Echinoid test ; in this way we have
the ambulacra I-V, the ambulacral rows of plates la, Ib, Ila, lib, Ilia, lllb, IVa,
IV6, Va, and V6, and in the apical system the radials I-V. If we mark the inter-
radii (interambulacra) 1 -5, starting from the one lying to the left of ambulacrum I,
and proceeding in the direction of the hands of a watch (viewing the test orally),
we get the interambulacral rows of plates la, Ib, 2a, 26, 3a, 3b, 4a, 46, 5a, 5b, and
the basals 1-5. The madreporite lies in basal 2. The consecutive plates, countino-
along each row of ambu-
$
6 i ^(_o; x^r^ 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 th.e
madreporite, and with the
excentric position of the
anus in the anal area of
the regular Echinoids, show
us that, strictly speaking,
no Echinoid is either radi-
ally, or bilaterally, sym-
metrical.
FIG. 304. -Peristome and neighbouring parts of the test
of Cidaris hystrix, Lamk. (after Loven).
(d) The Relation of the
Ambulacral and In-
terambulacral Plates
to the Peristome.
be
Three cases must
distinguished.
1. The plates, both of
J ambulacraand of the interambulacra, arecontinued in a modified form over the edge
tome, and on the peristome itself, towards the mouth (Cidaroida, Fig. 304).
2. Only the ambulacral plates are continued on to the oral integument (Diade-
tiaida), forming either several concentric rings of plates (Streptosomata, Echino-
vte) or as five pairs of plates lying isolated in the integument, the so-called
buccal plates (Stercosomata).
vin ECHINODERMATA MORPHOLOGY OF SKELETON 345
3. Xeither the anibulacral nor the interambulacral plates are continued on to the
peristome (Ifolectypoida, Clypeastroida, Spatangoida).
Among the Palceechinoidea also there are forms in which the perisomatic plates
reach as far as the mouth ; in Lepidocentrus, 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,
Jti'fderiiatoida), and even in most Ifolectypoida, ten pairs occur, five 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. 302) the peristome is, as a rule, bordered by five pairs of
anibulacral and five single interambulacral marginal plates. Exceptions to this rule
are found in the Spatangoid division, the Cassiduloidea, where, for example, among
the Echijioticidcc, Echinoncus and Amblypygiis have two marginal plates in their
second and fourth interradii and only one in the others.
(e) Manner in which the Skeletal Plates are Connected.
In most Euechinoidca the plates of the skeleton, at least those of the corona, are
firmly and immovably connected together by means of sutures, and thus form a
rigid test. This is not the case in very many Palceechinoidea, and among the
FIG. 305. Oral area of Cidaris papiUata, Leske, from within (after Loven).
apo, Perignathous apophyses.
Encchinoidea in the Diadcmatoid Echinothuridce ; also, as far as the skeleton of the
peristome is concerned, in the Cidaroida (Fig. 305). The edges of the plates here
overlap, i.e. they are imbricated. In the Echinothuridce 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 anibulacral 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 interambulacra, the
imbrication is apical. Lateral imbrication also occasionally occurs.
Slight imbrication is also found in certain Spatangoida.
(/) 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
in
F,o. 306.-Sy*tem of plates of a Clypeastroid (Encope Valenciennesi, Agass.), spread out
(after Loven).
P mi " el ' Ce f S ' etc :, The y 'y y length when the apieal system, towards
from thf J" g f7 . PeriSt me aCTOSS the ambitus ' the y c n ' e ' * shif ted
-he centre of the ap.cal hemisphere to a somewhat anterior (less frequently
1-ostenor) pos.bon. If the test of such an Echinoid, in which the ambulacra
vni ECHINODERMATA 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
Spatangoida, the peristome with the mouth has moved from the centre of the oral
surface (on which the Echinoids creep), and is shifted more or less anteriorly, and
finally, where in the Pourtalesia 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 ambulacra! 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 the 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 trivium 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 Spatangoida
which have a diffused apical system, e.g. the Collyritidce and Pourtalesiidce (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 Y, which are separated from the anterior, are
shifted posteriorly, the apical ends of the two posterior ambulacra (the bivium) are
also necessarily separated from the three anterior ambulacra (the trivium) by a con-
siderable space (Fig. 284, p. 325).
In the Palccechinoidea, and among the Euechinoidea in the Cidaroida, the Diade-
matoida, nearly all Holeetypoida, and many Spatangoida, the ambulacra throughout
their whole courses have a similar structure, and are similar!} 7 provided with pores.
In the Clypeastridce and many Spatangoida, however, the ambulacra are modified on
the 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 is 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 Cassidulidw (sub-order Cassiduloida of the order Spate n-
yoida), 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 with
the five radial phyllodes, what is called a floscelle.
348 COMPARATIVE ANATOMY CHAP.
The anterior unpaired ambulacrum in many exocyclic Echmoidea differs greatly,
th in s^Tlnn the number, arrangement, and form of its pores, from the other
both in s
a
V
FIG. 307.-0ral perisome of Cassidulus pacificus, Ag., with the five phyllodes (after Loven).
four. This variation in the anterior ambulacrum is found almost exclusively in the
order Spatangoida, especially in the Cassiduloid family Plesiospatangidce and in the
sub-order Spatangoidea (here especially, and, to a very marked degree, in the family
of the Spatangidce).
((/) Special Modifications of the Interradii.
We can here only point out certain conditions occurring in the order Spatan-
goida.
In the sub-order Spatangoidea an extraordinary asymmetry of the two
posterior interradii 1 and 4 prevails (cf. 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. This fusion takes place either between the second and third plates of the
row la, or the two second plates of rows la and 1&, or the second and third plates
of row b 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 case in the Cassiduloidca 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 (cf. Fig. 302,
p. 342). In many Spatangoida the labrum is followed posteriorly by two large sym-
metrically arranged plates (sternum), which again are followed by two smaller but
vin ECHINODERMATA MORPHOLOGY OF SKELETON 349
still not insignificant plates (episternum). The test is then amphisternal. In
other forms, however, the arrangement of the plates on the plastron (apart from the
labruni) 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 Clypmstrid.ai the interambulacra 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.
(h) Form of the Peristome.
In most Echinoidea, 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 pedicellariae are planted.
In the sub-order Spatangoidea, narrow, finely granulated streaks or bands run, in
definite arrangement, along the surface of the test, and carry small rudimentary
spines or pedicellarise. These are called fascioles or Semites (Fig. 302, p. 342). The
following systematically important forms of fascioles are to be distinguished :
1. The peripetaloid fascicle encircles the apical rosette of petaloids.
2. The lateral or marginal fascicle runs round the shell near the ambitus.
3. The lateral subanal fasciole branches off from the peripetaloid fascicle 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.
() Marginal Incisions or Perforations.
These are often to be found in the flat disc-shaped test of the Sadellidcc, 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.
(0 The Perignathic Apophysial Girdle (Figs. 308, and 348, p. 402).
In all Echinoidea in which the mouth is armed with five teeth, moved by a com-
plicated masticatory apparatus, i.e. in all Eclmwidea except the Spatangoida and a
few Holectypoida, processes, directed apically inwards, are found at the peristomal
edge of the test ; these serve for the attachment of the muscles and bands of
masticatory apparatus. They either consist solely of the ambulacral or inter-
ambulacral marginal plates of the peristome bent round inwards, or else a few oi
plates next in order also take part in their formation.
These processes may be divided into those Avhich rise on the ambulacral marginal
plates and those which rise on the interambulacral 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, Avhich is best
illustrated by the Diadematoida (Fig. 308, A),
an apophysis rises on the peristomal margin
of each ambulacral area on each side of the
ambulacral suture. The tAvo apophyses of
one and the same ambulacrum usually unite
at their free ends, Avhich 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,
in all, ten ambulacral apophyses, which may
unite in pairs to form five auricles. The
interambulacral apophyses project less far into
The tAvo apophyses
of one and the same interambulacrum together
form a ridge AA'hich runs along the edge of
the peristome, and connects two neighbour-
ing auricles ; these ridges are generally fused
Avith 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 Avail with high arched gateAvays
at five radially arranged points. The five arched gateAvays Avould represent the
auricles, i.e. the five pairs of ambulacral apophyses, and the circular Avail Avould
be formed of the five pairs of interambulacral apophyses.
In the Cidaroida (Fig. 308, B and C) the apophysial ring is interrupted. The
ambulacra! apophyses are A\-anting, but the interambulacral apophyses are all the
more strongly developed, and form ear-shaped processes. The tAvo apophyses of an
interambulacrum are connected by a suture at their bases, but diverge at their tips.
When the two interambulacral 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 Avanting in a feAv Holectypoida ; Avhere 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.
FIG. 308. The perignathic apophyses
of a radius and of the two neighbouring
interradii of various Echinoidea. A,
Diadematoid. The apophyses of the
ambulacral plates (am) form true auriculas the interior of the body,
(cutr). B, Cidaroid. Apophyses are formed,
not by the ambulacral but by the inter-
interambulacral plates have fused.
vin 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
Fir;. 30;<. -Transverse section through the brachial skeleton of Astropecten aurantia-
cus (Gray) ; original. For lettering see p. 317. sa, Supports of the ambulacral plates or supra-
ambulacral plates ; ad, adambulacral plates ; p.^paxillse ; 1, position of the radial canal, etc. ;
2, ampullse ; 3, ambulacral feet.
mouth. In the base of this ambulaeral 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 ossieles (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 pairs, each ossicle on one side of the roof
FIG. 310. Scheme of the oral skeleton of the Asteroidea, from the inner side (after Ludwig).
or, Oral plate (odontophore) ; MI, first lower transverse muscle of the jambulacral furrow ; Mi, the
iuterradial muscle; I-VI, first to sixth ambulacral ossicles; 1-6, first to sixth adambulacral
ossicles ; a, b, c, d, e, f, apertures for the ampullae of the tube-feet.
corresponding with one on the other side. In the Palwasteroidea, 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 ampullae 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 thrs 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 ambtilacral 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 mouth, 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 ambulaeral (in
many Cryptozonia) or adambulaeral (in the Phanerozoma and some
Cryptozonia\ according as the ambulacral or the adambulacral portions
of the circle project the further into the oral cavity.
(b) The Interambulaeral 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 mfra-
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 PJmnerozonia 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 ambulacral 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 greatly 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 accessory 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.g. 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 ambulacral ossicles, are
called supports to the ambulacral ossicles or supraambulacral plates (Fig. 309 .).
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
ocular plates o;- terminals. According to recent investigations they
develop very early (apparently first of all the plates) over the left
vin 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 Perisomatic 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 ccelomic 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,
subambulacral, and subneural. The ambulacral ossicles of the Asteroid would thus
not be represented in the skeleton of the Echinoid.
IV. Ophiuroidea.
(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
regular segmental intervals through apertures which lie on each side
between the ventral shields and the lateral shields belonging to them
."-*
^ s 771 /*#
FIG. 311. Transverse section through the arm of an Ophiurid (after Ludwig). Diagram.
ss Lateral shields ; ds, dorsal shields ; cl, cavity of the arm (ccelora) ; oc, spines ; am, the ambu-
lacral plates (vertebrae) ; x, loop of tentacle canal in the groove on the distal face of the ossicle (c/.
next tig., A 4) ; rte, tentacular canal of the radial vessel (ra) of the water vascular system ; te,
feeler (tentacle) ; re, radial pseudoluemal vessel ; rn, radial nerve strand ; bs, ventral shield.
PIG. 312. Vertebral ossicles (ambulacral plates) of Ophiarachna incrassata (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 C, from the distal (aboral) side. D, Three
vertebral ossicles from the ventral side, pr, Proximal ; di, distal ; ra, radial trunks of the water
vascular system; rn, radial nerve trunk; rv, radial pseudohsemal 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, pseudohsemal vessel to the same ; 9, nerve branch to the same ; 10, branch of the
water vascular system to the same, which 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 vertebral ossicles, a very suit-
vin ECHINODERMATA 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 (OphioMus, 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,
only small spaces are to be
found, which dorsally contain
continuations of the body
cavity of the disc, while ven-
trally they contain the radial
water vascular trunk, the
radial nerve cord, the epineural
canal, and the pseudohaemal a m
vessel. The lateral branches
of the radial vessels of the
water Vascular system, before FiG.313.-OpMohelusimbella,Lym. A macerated
entering each tube -foot, pass J int from near the tj p of an arm > frora the dorsal side
l 'A 4.U (after Lyman). ds, Dorsal ; ss, lateral shield ; am,
through, On each Side, the Lnbulacral ossicles ; p, ho^k 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 OphiuridaB, 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 Astrophytidce (Euryalidce) and the
Ophiomyxidce, and the arms are, in these animals, covered by 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 A*f<''roi'1"ii, 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 A*t>-rmERU ATA MORPHOLOGY OF SKELETON 359
or channels, one of which receives the nerve ring and the other the
water vascular ring.
In Astrophyton 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,
ta
D
FH;. 314. Oral skeleton of the Ophiopya longispinus. Lym., from within; above, an inter-
radial region of the cover of the ilisc. rs, Radial shields ; am, vertebral ossicle ; omj, peristomal
plates ; ptcb. depressions for the oral tentacles ; o.m-t+cdi, oral-angle plates ; fb, bursal apertures ;
tit. torus angularis ; D, teeth ; ibr, interbrachial region ; 9 sge, bursal scale ; gp, 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.
ambulacral 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 bueeal
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 bueeal 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 adambulaeral 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 bueeal shields.
In a row with, but dorsally to, this ventral shield, within the buccal
issure, 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 ECHIXODERIIA TA MORPHOLOG Y OF SKELETON 36 1
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.
- n -> . - . , . ,
removal of the greater part of the arms. ^ f the P ktea f the a P 1Cal 8 y stem > P 6 " 80 '
pr, Ventral sac ; x, anal plate ; r, radials ; niatic pieces being, in this system, altogether
'", hasals. ' wanting.
The same is the case in the dorsal cup of the
family Holopulce (Lias to present time), Hyocrinidce (Lias to present time), Bathy-
crinidce (present time). In the tegmen calycis of these forms we first notice that the
large anal sac of the Cyalhocrinidce 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 interambulacral plates ; the furrows, im-
mediately on emerging from between the oral plates, 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,
ith the anus. In Pathycriniis, where the orals are either wanting or reduced, the
interradial region is either naked or plated with small pieces. The ambulacral
furrows havp lateral plates only. The anus lies on a very short papilla-like anal cone.
la:
vin ECHINODERMATA MORPHOLOGY OF SKELETOX 365
The Canal iculata, like the more recent Inadunuta (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 mterradius (anals) never
occur. Very often (Apiocrinus, PJiizocrinus, Antedonidce) two or more 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-
culata, orals appear in the adult only in PJiizocrinus. 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
afa
FIG. 31'5. Tegmen calycis of Metacrinus angu-
latus, P. H. Carp, (after P. H. Carpenter), o, Mouth;
hr, arms ; p, pinnulae (both broken off) ; ta, anal tube,
near which there is a second abnormal tube ta] ; rpa,
covering plates of the ambiilacral furrows.
FIG. 317. Actinometra strota, P. H.
Carp, (after P. H. Carpenter). Tegmen
calycis. o, Mouth; an, anus; afa, food
grooves of the arms afd, the same of the
tegmen ; p 1} two pinnula?, which take 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 agrees 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.
A'1in.i.tin':ii'i.t. is the only recent Crinoid in which the mouth is found
placed quite excentrically (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 (paleozoic) Camerata are distinguished by the tendency to strong development
of the perisomatic 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 ambulacral furrows are never open, but are
always arched over by large covering plates, some of which may be distinguished by
FIG. 318. Actinometra (after P. H. Carpenter). Diagrams to illustrate the courses of the food
grooves over the tegmen calycis. A r E 2 , the directions of the five 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 ambulacral skeleton appear at the surface. In the course of
the geological development of the palaeozoic 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 ambulacral furrows, send out processes over the latter. The processes
(which are closely joined to one another) from one side of the ambulacra meet and
ecome firmly connected with those from the other, so that the ambulacral furrows
with their skeletons are completely arched over, and are not externally visible.
This condition was until quite recently wrongly explained as follows The
vnerata 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 vault 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 lm<-, into the interradial plates of the dorsal cup.
The anus, surrounded by special plates, lies in the posterior interradius.
viii ECHINODERMATA 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 five large radials). The arms are free from their bases. A plate which is found
in each interradius, between the bases of the free arms and between the radials, may
be considered to belong almost as much to the tegmen as to the dorsal cup. In
Hexacriiius 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. 31t'. Gilbertsocrinus tuberculosus, Hall (after Wachsmuth and Springer). The system
of plates of the dorsal cup and of the interradial appendages IB. Ba, point of attachment of the
anus ; B/, commencement of the free portions of the arms. For other lettering see p. 317.
example we may take Dimerocrinus (Glyptasterida-), 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 ; first 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. Mdvuriiiux (Fig. 252, p. 307) and Dorycrimis, etc. agree
with Dimcrwrinus in these points.
' In Gilbertsocrinus (Rliodocrinidce] also, the two primary brachials and the two or
three secondary brachials are incorporated into the dorsal cup (Fig. 319). In each of
COMPARATIVE ANATOMY
CHAP.
368
the five interradii 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
The perisomatic skeleton of the dorsal cup of Actinocrinus (Fig. 291, p. 329)
is very like that of Gilbertsocrinus ; 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 (Actinocrinida) .
Here, however, not only the 5x2 primary brachials and the 10x2 secondary
brachials, but also the 20 x 2 tertiary branchials are incorporated into the dorsal
cup. In Strotocrinus (regalis) an extreme form is found (Fig. 320). The calyx
&
Fro. 320. Strotocrinus regalis (after Wachsmutli and Springer). The apical border. The
conical portion of the dorsal cup is broken away (as far as the distichals di) and shows the
t.'-iiH-n with the anus, the mouth and the food' grooves. The dotted lines denote the manner of
branching of the fixed arms, an, anus ; If, fixed joints of the arms, which form the border ; B/, the
free arms which run out from the edge of the border ; ire, interambulacral region of the tegmen
calycis ; am, ambulacra ; pf, fixed pinnule.
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
by 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
border, and the last plate of each row carries an arm-branch. Seventy free arm-
branches in all thus rise from the edge of the border. In the interradii, in the
intcidistichal regions, and between all the further branches of the fixed arms, inter-
a/n,
vin ECHINODERMATA MORPHOLOGY OF SKELETON 369
radials, interdistichals. etc. are found binding the brachials into the rigid horizontal
border. Their 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 Marsupiocrinus (ccelatiis) 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 iuterradial
and interambulacral plates. In
the centre of the tegmeii lie the
five orals, arranged in the manner
which is characteristic of the
Camcrata, and behind these,
subcentrally, 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
them.
The genus Agaricocrinus
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 extraordinarily highly vaulted tegmen is found in the Adinocrinidce
(Actinocrinus, Batocrinus, Figs. 253, 254, pp. 307, 308). It is regularly and firmly
&z
an.
FIG. 321. Tegmen calycis of Marsupiocrinus coelatus
(after Wachsmuth and Springer), or, Orals; am, am-
bulacra ; cp, covering plates of the ambulacral furrows ;
ia, interambulacral region.
FIG. 322. Part of the dorsal cup of Forbesiocrinus, spread out. For lettering see p. 317.
In addition, IO, one of the four similar interradial regions ; IA, the deviating anal interradial
region ; pal, palmars.
plated with large strong 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
OF THh
UNIVI
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 Articulata, 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 se 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 Taxocrinus 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 ambulacral
furrows run out, dividing dichotomously 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 Thaumatocrimis, see "The Systematic Review" (p. 309).
(b) 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
he 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
the brachial skeleton is in its turn depressed to form a channel
mtegumental channels, which accurately correspond with the
ambulacral grooves of the skeleton, are called food grooves. At the
vin ECHINODERMATA MORPHOLOGY OF SKELETON 371
bases of the free arms they pass into the ambulacral 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 Inwlumta 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 Inadniiata) 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, tertibrachs, etc., which are
capable of indefinite extension, and are readily symbolised as IBr, IIBr, etc.
It has already been pointed out, in the section which treated of the perisomatic
plates of the calyx, that brachials are incorporated into the dorsal cup in many,
indeed, iu the 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 iu detail, therefore, the terms fixed primi-
brachs, fixed secundibrachs, etc. are used, and the number of these plates in each
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 Iiiadv.nata 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 Camerata (Adinocrinoidca, 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, c.cj. we have an axillary costal, axillary distichal, 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 pinuulse, and that where two joints
are connected by syzygial sutures or by ligaments, pinnulre arc 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
COMPARATIVE ANATOMY
CHAP.
of the wedges regularly alternate. 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).
The Articulata, many Canaliculata, and the recent Inadunata 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 palaeozoic Inadunata
and the Camerata. The majority of palaeozoic Inadunata have uniserial arms,
but towards the end of the palaeozoic period forms appeared with alternating rows
(e.g. Potcriocrinus), and finally some genera in which the brachials may be biserial
at the tips of the arms (Eupachycrinus, JErisocrinus, Hydreionocrinus).
Most of the Camerata (an order limited to the palaeozoic age) have biserial
arms. But by far the greater number of the Lower Silurian species have uniserial
br
Fn;. 323. Part of the arm
of a Crinoid. Diagram showing
the transition from the uniserial,
through the alternating, to the
biserial arrangement of the
brachials.
FIG. 324. Part of the disc formed by the arms
of Crotalocrinus rugosus (after Wachsmuth and
Springer). 2, The trabeeulte connecting the arms ;
/-, the arms with the covering plates (cpa) 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, be 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 one commences,
ontogenetically and phylogenetically, at the tip of the arm, and proceeds from that
point towards the base.
The food grooves of the anus 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, pseudohaemal canals, etc.). Where covering plates are
present there are two rows which alternate and interlock in such a way as to form a
YIII EGHINODERMATA MORPHOLOGY OF SKELETON 373
median zigzag line. These 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 arms of the genus Crotalo-
c i- in us (Upper Silurian, England, Sweden), which is thought by some to belong to
the Caiiicrata. 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 arms 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 equal 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 flexible,
and could be rolled up over the calyx from its periphery. In C. pulchcr, 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.
(<) 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-
doiiidn:' and Thaumatocrinus 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
Antedonidce 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. AVithin 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.
Xew 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 columnal 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 growing stem
the ossicles in the upper part vary greatly in length, the shortest being the youngest.
COMPARATIVE ANATOMY
CHAP.
374
At definite intervals the stem may carry whorls of so-called
cirri 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 internode. Whereas in the
Inadunata, Articulata, and Camerata cirri are, as a rule, wanting, or only present
at the lower part of the stem, in the Cwutliculata (Pentacrinidce) nodes are found
along the whole length of the stem between the consecutive internodes. In the
FIG. 325. Diagram to elucidate Wachsmuth and Springer's rule. A, Crinoid with dicyclic
base. B, Crinoid with inonocyclic base. For lettering see p. 317.
recent species of Pentacrinus and Metacrinus, each nodal ossicle is connected with
the next ossicle of the internode below it by a syzygial suture.
Peculiar relations exist between the stem and the base of the apical capsule,
according to the " rule of Wachsmuth and Springer," given in the diagram Fig. 325.
In Crinoids with dicyclic base (i.e. 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 stern 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
vni ECHINODERMATA MORPHOLOGY OF SKELETON 375
be muddy or sandy, the base of the stem puts out lateral branches,
the so-called root-cirri, 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 floor 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. Pentacrinidce
and some Palaeozoic crinoids, are capable of free locomotion when the stem is either
voluntarily or accidentally
broken. Such locomotion is
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 Antedonidce are
stalked and attached
only in the young stage.
The larval stem re-
sembles that of the
Bourgueticrinidae (Fig.
326) ; the cirri are
developed only on its
uppermost ossicle, on
which five radially ar-
ranged cirri first appear,
then five interradially FIG. 326. Several stalked young stages of Antedon
arranged. At a certain Phalangium (A) ; Antedon spec. (B) ; Antedon tuberosa (C) ;
rTffiori - fV, anfi Antedon multispina (D), after P. H. Carpenter. For
o e ' " lettering see p. 317. cia, Points of attachment of the cirri.
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 arms. They can, further, anchor themselves
by their cirri, the arms being then directed upwards.
376 COMPARATIVE ANATOMY CHAP.
(d) The Manner of Connection between the Skeletal Pieces. 1
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 columnals.
Perhaps the clearest view of the great diversity which prevails in this matter is
obtained by assuming that the plates composing an echinoderm skeleton develop
in a stroma of connective tissue fibrils ; all the plates might 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
pinnules 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 greater flexibility.
In the former case 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 which, if
pinnules or cirri are carried by 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 two plates or ossicles are immovably cemented
together by an unbroken deposit of calcareous substance, which, however, 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 specialised, with interlocking ridges and teeth on the opposed
facets of the ossicles.
As all these different modifications pass into one another by imperceptible stages,
it is not always easy to say whether in any special case we have to do with muscular
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 relaxing, the
"elastic" fibres expand the arms again. Now it is clear that if these fibres were
thus simply elastic, 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 differ-
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.
1 This passage was rewritten in accordance with the tenour of Mr. Bather's criticism
in Natural Science, vol. vi. pp. 420, 421. TR.
viii ECHINODERMATA MORPHOLOGY OF SKELETON 377
(>-) 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
pinnulae, 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 (Antedonulce), 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 nerve strands arise from this apical central nervous system at five interradial
points. The five interradial strands fork either in the basals or in the 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, a'nd are
continued into the ossicles of the arms and of their branchings. Within 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 Pentacrinida.', Encrinidce, and Antedonidce, where the nerve strands
divide in the first axillaries, there is a peculiar chiasma nervorum brachialium,
which is shown in the diagrams.
In Encrinus, and it is said also in Pcntacrinus, the nerve strands which run
through the ossicles of the arms are double. But whereas, in Encrinus, they run
separately, and are enclosed in separate canals, in Pcntacrinus they lie in a common
canal.
Many palaeozoic Crinoids, and above all the Camcrata (with the exception of the
Crotalocrinoids), appear to have no differentiated nerve canals in the arms.
(/) The Water Pores.
In the Canaliculata (e.g. Pentacrinus, AuMm, Adi no metro) 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 Pcntacrinus dccorus, as many as twenty
pores are found on one plate. The total number of pores varies greatly in different
genera and species. In Antcdon 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
FIGS. 327-330. Diagrams to illustrate
the course of the axial canals and the
nerve strands within them in the dorsal
cup and the first brachial joints of En-
crinus (Fig. 327, after Beyrich), Rhizocrinus
lofotensis (Fig. 328, after P. H. Carpenter),
Antedon rosaceus (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
run superficially on the inner side of the
basal plates.
FIG. 330.
CH. viii ECHIXODERMATA MORPHOLOGY OF SKELETON 379
edge of the calyx between the bases of the arms, and in the genus Actinometra,
where they are chiefly developed near the ambulacral furrows, they have occasionally
been observed on the lowest pinnuke as well, and even on pinnule in the middle
or towards the ends, of the arms.
In Rhizocrinus, in each interradius of the tegnien calycis there is only one water
pore perforating the oral plate. In Hyocrinus the anal oral plate is perforated by
two pores ; the other oral plates either have one pore each or else none at all.
Further, in this genus, 2 to 7 pores occur in the plates of the interambulacral 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
L'ERM ATA 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.
2. Codaster (Fig. 265, p. 314). The arrangements here differ considerably from
those of Pent re mites, just described. The food grooves 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. 333. Transverse section through an ambu-
lacrum of Codaster (after Etheridge and Carpenter),
diagram. 1, Deltoid plate or possibly a radial ; 2, ambu-
lacral canal ; 3, food groove ; 4, lancet plate ; 5, side plate ;
at lts mie ^P* however, there are only
Hpine ; 6, longitudinal rows of a ^ ew small pores or eyes. The swollen head which
pores in the shaft (7) of the spine, surrounds the tip of the spine contains a somewhat
large poison sac, with an aperture at its tip, through
the spine may protrude. The epithelium lining the sac passes, at the
aperture, into the outer epithelium of the head. The poison sac and the part of
e spine which runs through it are filled with a clear fluid with floating vesicles
id remains of cells), yielded by the epithelium of the sac. The sac and its
elope of connective tissue are surrounded by a powerful muscular capsule, most
fibres are attached on the one hand to the sac, and on the other to the part
e spine lying below it. The contraction of these muscles causes the sharp tip
spine to protrude through the aperture of the sac. Perhaps, at the same
ECHINODERMATA SPINES, ETC.
391
time, the poison is squeezed into the spine through the lower pores in that part of
it which lies within the sac, and is squirted out through the few pores or eyes at
its tip.
On the fascicles of the Spatangoida, whose course has already been described
(p. 349), there are exceedingly numerous very small, granular tubercles carrying small
seta-like spines, thickened at the tip ; these are sometimes articulated, sometimes
immovable. Such clavulae are covered by a ciliated integument, which very
probably contains sensory cells.
I. The Spines of the Asteroidea. The Asteroid body is also
usually covered with spines and papillae. The form and arrangement
of these vary so greatly that they cannot here be more fully described.
We must refer the reader to the principal systematic works for details.
Their finer structure is
almost entirely unknown,
and we have hardly any
knowledge of the positions
of the sensory organs and
glands which almost cer-
tainly occur.
The spines are often firmly
connected with the skeletal
plates of the body wall, from
which they rise. Spines occur
most constantly at the edges of
the ambulacral furrows, border-
ing them like a palisade. They
are not infrequently movable ;
the}' can be erected, and inclined
over the furrow for its protection
(Fig. 243, p. 298).
Many Phancrozonia, and
specially the Astrapectinidcc,
are characterised by short cal-
careous pillars rising from the integument, which, on their terminal flat surfaces,
carry a usually circular group of small, thickly crowded spines, prominences, or
papillae. These structures are called paxillae (Fig. 309, p. 351).
FIG. 339. Three brachial joints of Ophiopteron
elegans, from the middle part of the arm, lower side (after
Ludwig). 5s, Ventral shields ; te, tentacle ; ss, lateral
shields ; 1, hook ; 2, thorny spine ; 3, supporting rods of
the fins.
c. Spines of the Ophiuridse. In the Ophiuridce, it is principally or
exclusively the lateral shields which carry spines, in a manner already
described (p. 355).
These spines are mostly large, slender, and pointed, and are occasionally provided
with thorns. In the genera Ophiomastix, Astroschema, and Ophiocreas club-shaped
spines occur, together with the ordinary kinds. Over the ends of these spines the
epithelium is thickened, and contains glandular and sensory cells. In Ophiopteron
elegans, numerous small spines of peculiar structure are found on the dorsal side of the
disc. A short stem divides into six long pointed branches, which are connected
by a thin, soft membrane in such a manner as to form a kind of funnel. The whole
.structure somewhat recalls an umbrella turned inside out. In the same species,
each lateral shield carries, besides a hook and a thorny spine, ten long, slender
C,
392 COMPARATIVE ANATOMY CHAP.
spines arranged in a row, which runs up from the ventral to the dorsal side of the
ami ; these spines are connected by a thin, transparent membrane in such a way as
to form a sort of fin (Fig. 339). On the first three free joints of the arm, the fin of
one side of the arm passes into that of the other side dorsally. We are justified
in assuming that the animal is able to swim by means of these large fins on
the arms.
The genera Ophiotholict and Ophiohdus are distinguished by peculiar umbrella-
shaped spines. A stem with a swollen, button -like base, articulating with a
tubercle, carries at its tip a circle of recurved spines, which, during life, are covered
by a common integument. These are found either in groups near the base of the
ordinary brachial spines, as in Ophiotholia, Avhere they first appear at some distance
from the disc, or else replacing the ordinary spines near the end of the arm, as in
Opkiohelus.
Function of the spines. The fact that the spines serve princi-
pally for the protection of the body is at once evident, especially when
they are provided with poison glands.
In response to stimuli, the spines become erect. In Diadema setosum, which is
very sensitive to light, the long spines turn threateningly towards a hand which
approaches them from any side. The spines of most Echinoids further serve for
locomotion, moving in a co-ordinated manner. This has been directly proved in
the Cidaridcc, Arbacia, Echinus, and Spatangus, etc. In the first of these forms,
the long (principal) spines are indeed the chief, or the only, locomotory organs, and
are used as. stilts. Many Echinoids, e.g., Dorocidaris, Arbacia, Spatangus, if laid
on their backs, can turn themselves over again by the help of their spines.
It has also been proved that the spines may serve for seizing prey and for
forwarding it to the mouth. Several spines incline towards the prey, seize it with
their tips, and pass it on to the next group in the oral direction, and so on towards
the mouth. Compare with this the rise of pedicellarise in Asteroids, p. 394.
2. Modified Spines.
a. The Sphseridia of the Eehinoidea. These are small spherical
or ellipsoidal bodies, which, by means of a short stalk, articulate with
a prominence of the test, and are inclined sometimes in one direction,
sometimes in another. They either project freely, or else rise from the
base of a pit-like depression of the test (Fig. 340). This depression
may more or less completely close over the sphamdium. We are
here reminded of the various forms of acoustic tentacles in the
Medusae, which sometimes rise freely from the body, sometimes from
the base of a pit, sometimes on the walls of closed vesicles, which latter
come into existence through the concrescence of the edge of the pit
above the tentacle. Here, however, we have to do not with tentacles,
but evidently with modified spines.
Sphoridia occur in all Echuwidea except the Cidaroida. They are found only
e ambulacra, and always on the peristomal plates, although in many forms
re not limited to these, the area in which they occur stretching out in the
the ambitus, or even beyond it. The number and arrangement of the
sphaendia vary greatly m different groups.
VIII
ECHINODEEMA TA SPH^RIDIA
393
Structure of the Sphseridia (Fig. 341). The sphseridia consist
(1) of a very firm and hard transparent calcareous sphere, which is
concentrically laminated, and does not show the lattice-like perforated
structure of the rest of the skeleton, and (2) of the calcareous stem,
which is perforated like a sponge, and is generally continued into the
interior of the sphere. The calcareous sphere perhaps answers to the
cortical layer of a large spine of a Cidaroid (cf. Fig. 337). Not infre-
quently the head is traversed by a canal which opens at its free end.
The sphseridium is covered by a ciliated epithelium which is often
pigmented ; the waving cilia are very long at the base of the stem,
but gradually diminish in length towards the head. The sphseridia,
like the spines, are surrounded at the base where they articulate with
the tubercle, by a muscular envelope and by a circular ganglion, the
FIG. 340. Portion of an ambulacrum
bordering the peristome in Echinocidaris
nigra, Mol. (after Loven), magnified. 1, Sphseri-
dium in its niche ; 2, ambulacral double pore ;
3, edge of the peristome.
FIG. 341. Longitudinal
section through a sphaeri-
dium, diagrammatic. 1, Cal-
careous mass of thesphflpriclium;
2, epithelium ; 3, calcareous
stem of lattice-like structure ;
4, muscle envelope ; 5, circular
ganglion ; 6, tubercle ; 7, test.
latter lying within the epithelium, which is here specially thickened.
The hair-like cells of this circular thickening of the epithelium are
probably for the greater part sensory.
The sphseridia have always been claimed as sensory organs, and,
on account of their usual position near the mouth, as gustatory or
olfactory organs. They have also been thought to be auditory, or
organs for the appreciation of the movements of the water. They
also remind us of organs adapted for appreciating the position of the
body in the water.
b. The Pedieellarise. These are small seizing organs which rise
from the integument. They occur in all JEchinoidea, most Asteroidea,
and a few Ophiuroidea in very varying number and arrangement, and
in many different forms, between which, again, there are transition
forms. They must be considered as modified spines, or groups of
spines. In one and the same species various forms of pedicellarise,
394 COMPARATIVE ANATOMY CHAP.
definitely arranged, may occur. It is very probable that many of the
different forms of pedicellariae, within certain divisions, have developed
independently out of spines.
1. The pedicellarise in the Ophiuroidea. In Trichaster elegans, from about the
thirty-sixth tentacle pore of an arm onwards, the two tentacle papillae are replaced
on the adoral side of each pore by two hooks movably articulated on a stem. This
stem also is articulated with a ventral lateral process of the corresponding brachial
vertebral ossicle. The skeleton of this apparatus consists of three pieces, belonging
to the stem and to the two diverging hooks. The hooks do not move against one
another, the planes of their movement being nearly parallel. On one side a flexor,
and on the other an extensor muscle connects each hook with the stem. In
Astropliyton also, similar pedicellarise are found, and in Ophiothrix fragilis the end
of the arm is beset with movable hooks provided with flexor and extensor muscles.
Similar hooks occur, further, on the lateral shields of the arms in certain species of
Gorgonoccphalus.
2. The pedicellarise of the Asteroidea (Fig. 342). In some groups, e.g. the
Astcrinidce, Solastcridce, and Pterasteridce, the pedicellarise are altogether wanting :
in the Astropcctinidce they are only very rarely found. v
In the simplest cases, groups of small spines may function as pedicellarise. The
spines of such a group are movably connected with the body, and may be arranged
either in two opposite rows of four to five spines each, these rows approximating or
diverging ; or else at definite points of the body three or four spines stand close
together, forming, when they incline towards one another, a three- or four- sided
pyramid. Two spines even may form a group. For instance, on the dorsal surface
of Asterina gibbosa, spines are found sometimes isolated, sometimes united in larger
or smaller groups. Among these groups there are couples connected at the base by
a transverse muscle, and such spines can move towards one another more energetic-
ally than those of the other groups (Fig. 342, A to F).
In the above cases, we have to a great extent to do with commencing or rudi-
mentary pedicellarise, and we recognise, in the larger and smaller groups of spines,
the material out of which pedicellarise with two, three, or four forceps may be
r developed. (Of. also what has been remarked on p. 392 on the spines of the EcMnoidea
as organs for the seizing and conveying of prey to the mouth, and p. 390 on the
smaller spines of the Cidaroida.}
The true pedicellariae of the Asteroidea usually have two blades or valves, less
frequently three. Stalked and sessile pedicellarise may be distinguished.
a Sessile pedieellariae (Fig. 342, G). The two blades rise
rectly from the integument. Each consists of a calcareous piece
determining its shape, which may be long or short, broad or
narrow, pointed or blunt, flat or spoon-like. The two skeletal pieces
are directly articulated with a skeletal plate of the integument. In
rymnastena carinifem, for example, numerous double-bladed pedi-
llarias rise at the edge of the ambulacral furrow. The two blades
e connected at their bases in a manner illustrated in the figure, by a
transverse muscle, the adductor muscle. Further, each blade at its
ter side (i.e. at the side turned away from the axis of the pedicel-
> also connected with the subjacent calcareous plate of the
by an opening muscle (abductor). The bases of the
A'HI
ECHINODERMA TA PEDICELLARIA
395
pedicellariae are further attached by a strong elastic fibrous band to
this same plate.
b. Stalked pedicellarise (Fig. 342, H, K). Each pedicellaria
rises from a short, soft stalk ; the blades, of which there may be two
FIG. 342. Pedicellariae of Asteroids. A, B, C, D, E, F, Pseudo- or commencing pedicellariae of
various species. G, Sessile pedicellaria from the edge of the ambulacral furrow of Gymnasteria
<:ni'(i(ifera (after Cuenot). H, Stalked straight pedicellaria diagrammatised (a^ter Cuenot). J,
Basal portion of a stalked crossed pedicellaria of Asteracanthion rubens (after Perrier). K, A
similar pedicellaria of Asteracanthion glacialis (after Cuenot). 1, Calcareous blade of the forceps ;
2, basal piece ; 3, occlusor muscle ; 3i, axial muscles of the blades ; 4, opening muscle ; 5, axial
band ; 6, epithelium ; 7, body wall ; S, stem.
or three, articulate with a basal skeletal piece. The double-bladed
(didaetyle) pedicellariae are either straight (forfieiform) or crossed
(foreipiform). Both kinds may be found in one and the same
animal.
We select for description Asterias (glacialis), one of the Asteroids
most richly provided with pedicellariae, whose arrangement is specially
interesting.
A. glacialis has three kinds of pedicellariae, straight, crossed, and
three-bladed.
The crossed pedicellarise are found in very great numbers, thickly crowded
together on a soft cushion, which surrounds the base of the spines, and into which the
latter can be withdrawn (Fig; 344).
396
COMPARATIVE ANATOMY
CHAP.
The straight pedicellarite are far less numerous, and are found scattered over the
integument either singly or in groups.
The three -bladed pedicel lari?e are always found entirely isolated, and may be
altogether wanting in some individuals.
Structure of the straight pedicellarise (Fig. 342, H). Each of the two blades
consists of a hollow toothed skeletal piece, which articulates with a common basal
FIG. 343. A portion of an arm of Asterias stichantha, Sladen, from the lower side (after
Sladen). 1, 2, 3, 4, The four longitudinal rows of ambulacral feet ; 5, forficiform pedicellarht 1 ; 0,
adambulacral spines ; 7, papulae ; 8, inframarginal spines ; 9, forcipiform pedicellarite at the
outer bases of these latter.
piece. Two muscles serve for opening the pedicellaria, the outer side of each blade
being attached by a muscle to the basal piece. The blades are closed by moans
of two muscles which run from the inner sides of their bases to the basal piece,
i. 344. Asterias (Stolasterias) volsellata. Adambulacral plates and neighbouring portion
f the oral integument of an arm. /, Straight ; fc, crossed pedicellarite on a cushion at the base of
a large spine (a<-) ; m- lt smaller spine (after Sladen).
and also perhaps by means of two muscles which, lying for the greater part
hin the calcareous blades, run from their tips to the basal piece. Each pedi-
211am is unrounded by a layer of connective tissue, and covered by body epithelium,
in which glandular cells are scattered.
Structure of the crossed pedicellariae (Fig. 342, K). A crossed pedicellaria
t Unlike a forceps with short handles. It also consists of three pieces, the
VIII
ECHINODERMA TA PEDICELLA RI^
397
two limbs of the forceps, and an intermediate or basal piece, upoii which the
blades move. Each limb of the forceps consists of the blade and the stalk or
handle. The two limbs cross at the two sides of the intermediate piece like the two
parts of a pair of pincers or scissors. "When the two handles are approximated, the
pincers close, when they are drawn apart, they open. The opening and closing of
the pcdicellaria: is effected by means of six muscles. Two small muscles, running
from the outer side of the bases of the blades to the basal piece, by their contraction,
open the forceps ; while two pairs of muscles close it. One of these pairs runs
within the blades to the basal piece, and the two muscles of the other pair run
transversely from the handles of the two limbs to the basal or intermediate piece.
An axial strand of elastic fibres run from the stalk of the pedicellaria to the base
FIG. i>4 5. Pedicellariae of Echinoids. A, Four-bladed pedicellaria of Schizaster canali-
fenis (after KceMer). B, Glandular pedicellaria with glandular sacs on the stalk,
Sphaerechinus granularis. C, Longitudinal section through a decalcified tridactyle
pedicellaria of Centrostephanus longispinus (after Hamann). 1, Adductor muscle ; 2, nerve ;
3. elastic column ; 4, calcareous rod ; 5, longitudinal muscle fibres.
of the forceps. This strand divides into two branches, which embrace its handles.
The fibrous strands of the individual pedicellarise penetrate the cushion, which sur-
rounds the base of the spine (Fig. 344), and finally break up into fibres which become
closely matted together. The whole cushion consists of thickly interwoven fibres of
connective tissue and muscle. Muscle fibres run down from the calcareous piece of
the spine into the cushion, in which they are lost. By means of these muscles, the
cushion can be drawn up the spine, like a sort of sheath. The pedicellarife, like
the cushion from which they rise, are covered by a markedly glandular epithelium.
The three-bladed pedicellariae, apart from the number of their blades, agree in
structure with the straight two-bladed pedicellariae.
3. The pedicellariae of the Echinoidea (Figs. 345 and 346). Pedicellarise occur
in all Echinoids on the integument, between the spines, and in one and the same
species two or more forms of them may be found. The special arrangement of the
various forms of pedicellarife on the body (whether occurring on the ambulacral or
on the interambulacral areas, and whether orally or apically), their distribution,
number, and very varied form cannot here be described in detail, but must be
sought for in systematic works.
398
COMPARATIVE ANATOMY
CHAP.
The pedicellari of the Echinoidea are always stalked, and three-, less frequently
two- or four-, bladed. Two principal forms may be distinguished : seizing pedi-
cellarise (Fig. 345 A and C) and glandular pedi
cellarise (Fig. 345 B and Fig. 346).
a. The seizing pedicellarise. The form of the
blades varies greatly in details. They are some-
times long and slender (p. tridactyke, tetradactylfe),
sometimes spoon-shaped and toothed (p. ophioce-
phake, seu buccales, seu triphyllae), or in other
cases broadened out like leaves (p. trifoliatse).
Each blade is always '. supported by a skeletal
piece, which determines its general shape and the
special form of its teeth, hooks, etc. The stalk
also is always supported by an axial calcareous
rod, which sometimes penetrates the whole of its
basal half (p. tridactylae), sometimes only reaches
a short way into the base of the stalk.
The tridactyle pedicellarise of Centroste-
phanus longispinus (Fig. 345 C) will serve to
illustrate the structure of the seizing pedicellarie.
The three slender blades are connected at their
bases, and on the sides turned to the axis of the
whole forceps, by three transverse adductor
muscles, each of which is attached on the inner
(axial) sides of two neighbouring blades. The
three muscles together form a triangle. These
adductor muscles are counteracted by opening
muscles, which run down on the outer sides of the
bases of the blades longitudinally. A nerve enters
each blade, running towards its tip, and innervat-
ing the musculature and epithelial sensory cells.
The inner surface of each blade is ciliated. Within
the stalk, the supporting calcareous rod reaches
only half way up, ending in a knob. The con-
tinuation of the calcareous rod is formed by an
elastic pillar, which consists of modified connective
tissue, and is enveloped in a sheath of longitudinal
muscle fibres. This arrangement makes it possible
for the distal portion of the stalk with the head
to bend in various directions, and even to bend
right back upon the basal portion. When the
forceps gland ; 10 and 11, opening muscles which bring about such movement are
muscles ; 12, nerve ; 13, calcareous rod relaxed, the distal part resumes the upright
position by means of the elastic pillar it
contains.
The adductor muscles of these pedicellarise
consist of transversely striated muscle fibres ;
consequently Uiese tridactyle pedicellarise are very active seizing organs.
b. The glandular pedicellariae have been carefully investigated, up to the present
time, only in a small number of Echinoids (Sphcerechinus granularis, Echinus acutus,
E. melo, Dorocidaris papillata, Strongylocentrotus lividus, Echinocardium flavesccns],
but it is probable that in time they will be found more widely disputed. In
general structure they resemble the ordinary seizing pedicellarise possessing three
340. Organisation of a glan-
dular pedicellaria of Sphaerechinus
granularis, section. 1, Distal tactile
prominence ; 2, aperture of the gland
'C the forceps ; 3, proximal tactile
lironiinence ; 4, adductor muscle ;
>, skeletal piece of the forceps ; 6, epi-
of the same ; 9, muscle layer of the
in the stalk ; 14, aperture of the stalk
gland (16) ; 15, epithelium of the gland.
(The distal tactile prominence here
represented is wanting in this species.)
vni ECHINODEBMA TA PEDICELLARIA 399
seizing blades, which are opened and closed by special muscles, but the fibres of the
adductor muscles are not transversely striated. In the stalk, the axial calcareous rod
is continued as far as the three-bladed head, an arrangement which greatly decreases
the mobility of this kind of pedicellaria.
The most distinctive characteristic of these pedicellariae, however, is the
presence of a large glandular sac in each blade. This glandular sac, which, as is
shown by recent discoveries, consists of two fused sacs, causes each blade to be pear-
shaped. It is covered by a thick glandular epithelium, and has a muscular wall of
its own, in which the (smooth) fibres run in circular layers. This muscular wall, no
doubt, serves for pressing out the slimy, and probably poisonous, secretion, through
the aperture which lies near the tip of the blade. This aperture appears in most
cases to lie on the outer side of the blade.
At the base of each blade, on its inner side, the epithelium is thickened to form
a tactile prominence or cushion, which (besides cilia) carries immovable sensory
hairs. In Echinus acutus, besides the basal, or lower, tactile prominence, there is,
on each blade, a distal or upper prominence, which also lies on the inner side of the
blade (Fig. 346).
Numerous nerves pass from the stalk of the pedicellaria into its head and its
blades, so as to innervate the muscular and the sensory cells.
In a few Echinoids, glands also occur on the stalk of the pedicellaria ; such
glands are specially strongly developed in Sphcerechinus granularis. These
glands, three in number, encircle the stalk of the glandular pedicellariae (p. gemmi-
forrnes), about half way up. Each gland is a large vesicle with an aperture, through
which, on stimulation, a slimy secretion is discharged. The wall of the vesicle con-
sists of glandular epithelium within a muscular layer. The three glands cause large
vesicular swellings on the stalk of the pedicellariae on which they occur ; they are
covered by unditterentiated outer body epithelium.
If we imagine that, in such pedicellariae provided with stalk glands, the distal
portion of the stalk above these glands degenerated, or was no more developed, we
should have the form of pedicellaria which is called p. globifer. Such globifers,
occasionally still provided with rudimentary seizing forceps, have been discovered in
C- li'rostephan't'.s lonrjispitius and ,V///'" -'"' ''-'hinus granularis, side by side with ordinary
pedicellarise. They are capable of pendulous movements.
The function of the pedicellarise has not yet been satisfactorily decided. The X j
view that, in Echinoids, they play some part in locomotion, has recently been '
decidedly opposed, and it has been maintained that Echinoids move exclusively by
means of their ambulacral feet and spines. It has further been asserted that the
pedicellarue lay hold of foreign objects, algse, etc., and hold them fast on the upper
side of the body in order to hide it, but this view also has been opposed, the function
therein ascribed to pedicellariae being claimed for the tube-feet. Such a function could,
in any case, only be accessory. Another view is that the pedicellarise serve for the"\
holding of prey, and for carrying it to 'the mouth. In the Asteroidea, however,/
which take the food in large pieces (Fish, Crabs, Mussels, Snails, Echinoids, etc.),
they could not well play this part.
The most probable view is that the pedicellariae are protective organs, and fulfil \
the function of cleaning the spine-covered body. They clear away foreign bodies.
Small animals which come into contact with the body are seized, and enveloped in
the slimy secretion of the epithelium, or in the possibly poisonous secretion of the
specialised glands of the pedicellariae, and held until they are dead, and then
" thrown overboard." In this way Echinoids and Asteroids may protect themselves
from animal and vegetable growths, parasitic or otherwise. This would explain the
astonishing cleanness of most members of this group in spite of their spinous
covering.
400 COMPARATIVE ANATOMY
E. The Masticatory Apparatus of the Eehinoidea.
(Aristotle's Lantern.)
In all Eehinoidea, with the exception of the Spatangoida, and per-
haps of a few Holectypoida, the mouth, which lies at the centre of the
peristomal area, is armed with five hard and pointed, interradially
arranged teeth. These teeth are approximated or moved apart by
means of a complicated masticatory or jaw apparatus lying within
the test, and resting on the peristome. This apparatus is known as
the lantern of Aristotle, and is of considerable size ; it is covered on
all sides by a closely applied integument, the lantern membrane, a
continuation of the peritoneum. The spaces within the masticatory
apparatus are completely separated by this membrane from the spacious
body cavity within the test.
The masticatory apparatus resembles a pentagonal pyramid, the
base of which is directed up wards, i.e. projects into the cavity of the
test, while the tip, formed by the five teeth, lies in the mouth. Its
axis is traversed by the oesophagus. It consists essentially of skeletal
pieces, muscles, and ligaments.
a. The skeleton of the masticatory apparatus (Fig. 347) is
composed of twenty-five pieces (including the teeth), radially grouped
around the oesophagus ; some of these pieces have received very un-
suitable names. There are five teeth, five pairs of jaws (alveoli),
five "sickles" (falces), and five radii or rotulse. The "sickles"
may be named intermediate plates, and each pair of jaws forms a
" pyramid."
The principal part of the framework of the masticatory apparatus is formed by
the five interradially placed pairs of jaws. These determine the conical or pyramidal
form of the whole framework. , The two pieces of each pair are firmly connected with
one another on the outer side of the framework by a vertical interradial suture, and
together form a hollow triangular pyramid, the fifth part of the whole pyramidal
framework. Each single pyramid thus has one outer and two lateral surfaces. The
five single pyramids are in contact with one another along these lateral surfaces,
which lie radially to the axis of the whole framework. The edges along which the
lateral surfaces come in contact are the axial edges, i.e. those turned towards the oeso-
phagus. The suture, which divides each single pyramid into two halves or jaws, runs
down the outer surface, exactly halving it. The walls of each single hollow pyramid
are, however, not complete : (1) the two lateral surfaces do not quite meet along
their inner edges, but there is a slit left between them (Fig. 347, E) ; (2) the basal
wall (that turned upward) is wanting ; when the soft parts are removed an aperture
is found 4iere, the foramen basale, which leads down into the cavity of the pyramid ;
(3) a large incision (foramen externum) is found, at the base of the outer wall, and
is either confluent with the foramen basale or is separated from the latter by an
arch, the arcus.
-in-1,- pyramids (or pairs of jaws) are the supports and carriers of the teeth.
ach tooth is a long, slender, and hard skeletal piece, curved, so that its convex side
outwards-; it traverses the cavity of the pyramid, and projects beyond it at both
VIII
ECHINODERMATA ARISTOTLE'S LANTERN
401
ends. The lower end, which projects beyond the tip of the pyramid, is short and
pointed, and forms the externally visible part of the tooth lying in the mouth. The
upper end, which is directed aborally, is called the root of the tooth, and projects
considerably beyond the foramen basale, it is usually coiled inwards (towards the
axis of the masticatory framework). The growth of the tooth no doubt takes place
principally at this root end. On its inner side, the tooth usually has a longitudinal
ridge, the carina, and on its outer side is firmly attached to the outer wall of the
FIG. 347. Masticatory apparatus of an Echinus, original. A, In profile. B From the apically
directed basal side. C, External view of a single pyramid. D, Side view of the same. E, Internal
riew of the same. F, Tooth. 1, Arcus ; 2, intermediate plate ; 3, freely projecting portion
of the teeth ; 4, median portion of a tooth ; 5, upper portion of the same ; 6, the limbs of a forked
radius (7) ; 8, single pyramid in situ.
pyramid which it traverses, in such a way that it cannot move by itself, but only
with its pyramid.
The fine structure of the teeth differs essentially from that of the other skeletal
pieces of the body (cf. on this subject the special treatises mentioned in the Biblio-
graphy).
The intermediate plates are five more or less flat, oblong, skeletal masses lying
on the base of the masticatory apparatus, like the spokes of a wheel round its central
axis. Each of these intermediate plates rests on the bases of the two contiguous
lateral walls of two pyramids [or pairs of jaws], and therefore between two foramina
basalia.
Finally, lying apically upon these falces, are the five forked radii, which are also
arranged like the spokes of a wheel. Each radius consists of a slender central
stalk, and of two peripheral diverging prongs, and each is bent downwards in such a
VOL. II 2 D
402
COMPARATIVE ANATOMY
way that its prongs point down towards the peristome over the edge of the base of
of the masticatory apparatus (Fig. 848,.-We
here'refer back to what has been said of the perignathous apophysial ring, for the
^^ and the apophysial ring are, physiologieally, closely connected.
Th most Important muscles and bands of the masticatory apparatus connect its
component pieces with the apophysial ring, and the latter must be regarded merely
as a folding inwards of the edge of the peristome, which has come into existence
Fin. 348. Masticatory apparatus of an Echinoid (Toxopneustes) in its natural position at the
centre of the oral side of the shell, which has been broken off, original. 1, Root of the tooth ; 2,
intestine ; 3, accessory intestine (?) ; 4, axial sinus with stone canal ; 5, forked radii ; 6, arcus of a
single pyramid ; 7, muscles of the forked radii ; 8, perignathous apophysis (auricula) ; 9, ligaments of
the forked radii ; 10, adductor muscles of the teeth ; 11, opening muscles of the teeth ; 12, radial
canal of the water vascular system ; 13, ampullae ; im, interambulacrum ; am, ambulacrum. The
delicate transparent lantern membrane which covers the whole of the masticatory apparatus is
not represented.
for the insertion of the masticatory muscles. The two apparatus are either absent or
present simultaneously.
Round the masticatory apparatus, ten thin ligaments (9) connect the forked radii
with the interradial apophyses of the perignathous girdle. The two bands which
belong to each fork are attached to the prongs, and continue their lines down-
wards to the apophysial girdle ; they are inserted into the two neighbouring inter-
radial apophyses near the interradial sutures.
The two bands proceeding from each radial fork thus diverge downwards, and
the two proceeding from each interradial apophysis of the perignathous ring diverge
upwards.
These bands appear merely to serve for the attachment of the masticatory appa-
viir ECHINODERMATA CALCAREOUS RING 403
ratus, and for its maintenance in the upright position over the oral area. Further
investigations, however, must decide whether the bands consist only of elastic fibres,
or whether muscle fibres also occur in them.
The adductor muscles of the teeth (musculi adductores dentium, 10). These
are present in five interradial pairs ; they are strongly developed as broad bands.
The two muscles of a pair are attached, above, along the outer edge of the arcus of the
pair of jaws (pyramid) to which they belong ; and below, along nearly the whole
length of the corresponding interambulacral apophysis of the perignathous ring. If
these muscles contract, the upper ends of the pair of jaws (or pyramids) are drawn
outwards and downwards, forcing the lower ends, with the teeth, inwards, i.e.
towards the centre of the mouth. In other words, the externally visible pointed
lower ends of the teeth are pressed together.
The opening muscles of the teeth (musculi abductores dentium sive dilatatores
oris, 11). These are five radially arranged pairs of muscles, which run horizontally.
The two muscles of each pair are attached on the one side to the inner surface of the
ambulacral apophyses (auriculae), and, on the other, to the halves of the jaws nearest
them, close to the ends which point downwards. These muscles counteract the
adductor muscles ; when they contract, the lower ends of the five pairs of jaws, and
with them the tips of the teeth, are moved, centrifugally, towards the auriculae. The
teeth move .apart, and the mouth opens.
The intermediate jaw muscles (musculi intermaxillares) connect the apposed
lateral surfaces of the five pyramids with one another. The five pyramids close
firmly together, when these muscles, which together act like a kind of sphincter,
contract.
The muscles of the forked radii (7) lie on the upturned base of the masticatory
apparatus, forming together a pentagonal ring by connecting the five handles of the
forks for about half their length. As to the function of these muscles, we can
only imagine that they depress the whole masticatory apparatus by their contraction,
and thus cause the oral integument to project conically, especially if the adductor
muscles of the teeth contract at the same time. It is well known that Echiuoids are
assisted in locomotion by the bulging forward of the tooth -carry ing portion of the
oral area, which is supported by the masticatory apparatus.
In the Clypeastroida, the frequently asymmetrical masticatory apparatus is more
or less flattened, usually indeed quite flat. The teeth are not vertical, but slope
towards one another quite obliquely, or are even arranged horizontally. The radii
are wanting, and the intermediate plates are rudimentary.
r
F. The Calcareous Ring of the Holothurioidea.
In the Holothurioidea, the oesophagus is surrounded by a circle of
ten calcareous skeletal pieces (Fig. 349, 3 and 13), five of which are
radial and the other five interradial. This calcareous ring protects
the nerve ring at its inner side. For a certain distance it supports
the radial water vascular trunks and the tentacular vessels, and may
indeed be regarded as the inner skeleton of the oral region of the
body. The five longitudinal muscles or pairs of muscles of the body,
and, where such are present, the five retractor muscles of the oral
region, are attached to this ring, i.e. to its radial portions. The
calcareous ring is altogether wanting in the remarkable free-swimming
form Pelagothnria (Fig. 224, p. 286).
COMPARATIVE ANATOMY
CHAP.
404
The form and size of the calcareous ring and its separate parts vary greatly. The
radials are often lengthened backwards (apieally) into two prongs of varying length,
between which the radial water vascular trunks run.
It not infrequently happens that the separate parts become partially or altogether
broken up into single pieces, which are connected together like a mosaic.
The number of pieces in the ring may increase or decrease. Where there are
more or fewer than ten pieces, it is always the interradials which either increase or
diminish in number. This is comprehen-
sible when we remember that the longi-
tudinal muscles of the body are attached
to the radials.
The iriterradial portions are wanting in
species of the genera Phylloporus, Cucum-
aria, and Trochostoma, and in many Elasi-
poda, especially in the whole family of the
Elpidiidae.
More than ten pieces are found in many
Synaptidce, viz. in nearly all those forms
which possess more than ten tentacles.
The number of extra interradials then
usually corresponds with that of the super-
numerary tentacles.
Six - rayed specimens] of Cucumaria
Planci have been described, whose cal-
careous ring consists of six radials and six
interradials.
The ring which is originally radiate may
become bilaterally symmetrical. Its plane
of symmetry then agrees with the general
plane of symmetry of the body, and passes
Fio. 34','. The oesophagus and half the through the fifth interradius (the so-called
oral tentacles of a dendrochirote Holo-
thurian (after Lud-wig). 1, Genital aperture ;
J, genital duct ; 3, radial pieces of the cal-
careous ring; 4, retractor muscles ; Sjjnadre-
jporite ; 6, stone canal ; 7, dorsal mesentery ;
dorsal interradius in which the genital
aperture lies) and the central (first) radius
of the ventral side. The symmetry is
determined either by the fact that the
9, Polian vesicles; 10, circular portions of the ring on the ventral side
canal ;,11, continuation of the radial calcareous differ in f ^ and manner of con nec-
pieces ; 12, proximal portions of the radial ,. f , -, , , -, , . -, -,
canals of the water vascular system; 13, inter- tlon frOm those on the dorsal side, or else
radial pieces of the calcareous ring ; 14, one of by the presence of a larger number of such
the two small ventral tentacles. portions, in consequence of an increased
number of interradials in definite sym-
metrical interradii. For instance, Synapta digitata has seven interradials, one each
in the mediodorsal and in the two ventral interradii, and two each in the dorso-
lateral interradii.
The portions of the calcareous ring are more or less closely united together
by means of connective tissue (never by means of muscles) ; in some cases they are
firmly fused together.
Structures corresponding to the calcareous ring of the Holothurioidea have long
been sought for in the other classes of the Echinodermata. It was thought that
in the Echinoidea it might perhaps be represented either by the teeth or by the
perignathous apophysial ring, or in certain parts of the masticatory apparatus.
The horaology of the teeth of the Echinoidea with the calcareous ring of the
viii ECHINODERMATAENDOSKELETON, ETC. 405
Holothurioidea is no longer maintained. The two structures are altogether differently
related to the nervous and water vascular systems.
The hornology of the calcareous ring with the perignathous apophysial ring of
the Echinoidea is equally doubtful. The radials of the calcareous ring were in this
case compared with the auriculae (ambulacral apophyses). But each auricule is
paired and consists of two processes/or folds, of the edge of the peristome, which
may or may not be connected together by an arch ; the radials, however, are
from the first unpaired. Only the arches of the auricula? could be compared with
the radials ; the arch, however, is not a single plate, but is formed by apposition
of the two neighbouring ambulacral apophyses of one and the same ambulacrum.
The comparison of the calcareous ring with the masticatory apparatus or
Aristotle's lantern of the Echinoidea still remains. The five radials have been
compared with the five fork pieces, and the five iuterradials with the five arches
of the pairs of jaws (pyramids) of the lantern. This comparison is in many ways
plausible, but here, as before, many difficulties appear when the subject is carefully
investigated. The arches of the jaws are paired structures, and cannot therefore be
compared with the iuterradials, which are from the first unpaired. Moreover, it
is very doubtful whether they represent independent skeletal pieces ; they appear
rather to be merely muscular processes of the halves of the jaws. Further, the
sinews which proceed from the forks of the radial fork pieces are attached to the
perignathous apophysial ring interradially (i.e. to the interambulacral apophyses),
while the muscles which are attached to the radials of the calcareous ring of the
Holothurioidea run strictly radially.
G. Further Deposits of Calcareous Matter.
Deposits of calcareous corpuscles and masses may occur in the
connective tissue of the walls of various internal and external organs,
especially in the ambulacral and alimentary systems. These will be
considered in connection with the systems to which they belong.
We shall here only mention certain calcareous deposits in the
Cl-ypeastroida. An endoskeleton is here formed. On the oral, as well
as on the apical, inner surface of the test, needles, pillars, lamellae,
etc. rise, sometimes only at the edge, sometimes over large areas.
These may traverse the whole depth of the test, connecting its oppo-
site walls. They more or less completely separate the ambulacral
structures from the other internal organs, such as the intestine, the
genital organs, etc., and may in some cases attain such great develop-
ment that, as in Encope, they form a sponge-like or cellular calcareous
framework throughout the whole interior of the test, in which larger
spaces are left for the masticatory apparatus, the intestine, the
ambulacra, etc. Xot infrequently, the ambulacral vessels are completely
vaulted over b} r deposits of calcareous matter.
H. Concluding Remarks on the Section on the Skeletal System.
In the above section, I have adopted the views of those investigators whose wide
and for the most part difficult researches have convinced them that at least the
plates 'of the apical and oral systems (the central, infrabasals, basals, radials, and
orals) are homologous throughout the whole group of the Echinodermata. These
plates therefore must be ascribed to the common racial form. But these plates are
406 COMPARATIVE ANATOMY CHAP.
in reality only characterised according to their position in the adult animal, whether
radial or interradial, apical or oral, and according to the place of their first appear-
ance (above one or the other ccelomic vesicle). They have no other distinctive
characteristic by which, for example, a radial could be recognised throughout the
class of the Echinodermata. It is therefore still possible that such correspondence
may be merely superficial, merely the expression of the radiate structure so common
among Echinoderms. There is nothing astonishing in the fact that the skeleton
of a radiate animal commences at the poles either with radially or with interradially
arranged plates. Such correspondence, then, as far as it goes, is described as homo-
logy. But nothing is really gained by insisting that such and such Ophiurids
" possess infrabasals," because the system of plates commences at the apex with five
radial plates, which are followed by another outer row of radial plates. Is it, after
all, certain that the infrabasals are wanting when the skeletal system at the apex
begins with interradial plates (which are on that account called basals) ?
III. The Outer Morphology of the Holothurioidea.
The Holothurioidea form an exception to the rule which applies to all other
Echinoderms that the outer form of the body is accurately reproduced in the test of
skeletal plates. This test gives us, as a rule, exact information as to the position
of the outer apertures of the internal organs, and as to the relation of the radii
or ambulacra, to the interradii or interambulacra. But in the Holothurioidea, in
whose integument only microscopically small and isolated calcareous bodies occur,
this is not the case. Having treated of the external morphology of the Echinoidea, the
Asteroidea, the Ophiuroidea, and the Pelmatozoa in the section on the skeletal system,
we must now give some account of the outer morphology of the Holothurioidea.
We shall begin with those forms in which the body, elongated in the
direction of the principal axis, is, in section, circular or pentagonal with
rounded edges (cf. for example, Cucumaria Planci, Fig. 226, p. 287). At
the oral pole of the principal axis (i.e. in the Holothurioidea, anteriorly)
lies the mouth, surrounded by feelers ; at the opposite, apical (posterior)
pole, the anus. Along the body, from before backward, run five ridges,
corresponding with the radii, and causing the pentagonal form of the
transverse section. On each edge there are two longitudinal rows of
tube-feet.
Careful examination shows that the radiate structure of Cucumaria
is even externally disturbed by certain characters which make it bi-
laterally symmetrical. There is only one genital aperture, at the oral
margin of an interradius, which we will arbitrarily call the dorsal
interradius. Further, of the ten oral feelers, two adjacent feelers
are much smaller than the rest. They lie exactly opposite the genital
aperture, and distinguish the middle ventral radius. The plane which
passes through the dorsal interradius and the middle ventral radius, in
the direction of the principal axis (i.e. longitudinally through the body)
is the plane of symmetry.
If the animal is opened, it is seen that this external symmetry
corresponds with an internal symmetry ; the anterior limb of the
A_intestme is attached by a mesentery to the body wall in the dorsal
X ! The stone canal and the genital glands lie in the dorsal
interradius, and the Polian vesicle in the middle ventral radius.
vni ECHINODERMATA MORPHOLOGY OF HOLOTHURIOIDEA 407
The application of the terms "ventral" and "dorsal," in very
many Holothurioidea, is fully justifiable, since there arises, parallel
to the principal (longitudinal) axis, a flattened creeping sole, along
the middle of which the above-named ventral radius runs, while the
middle of the vaulted dorsal surface opposed to this creeping sole is
occupied by the middle dorsal interradius.
The radii and interradii now become arranged in such a way that
three radii (one middle and two lateral) belong with their ambulacral
rdd
Trimum
FIG. 350. Diagrammatic section illustrating the symmetry of the Holothurioidea. De-
velopment of the bivium and the trivium (mainly after Ludwig). imd, Medio-dorsal interradius ;
ids, left dorsal ditto ; isr, left ventral ; idv, right ventral ; idd, right dorsal interradius ; rds, left
dorsal radius ; rst; left ventral ditto ; rmv, medio-ventral ; rdv, right ventral ; rdd, right dorsal
radius ; mi, anterior or dorsal mesentery ; m^, middle or left ; 7/13, posterior or right mesentery ;
i i, io, 13. tirst. second, and third, or anterior, middle, and posterior limbs of the intestine ; vd and
vv, dorsal and ventral intestinal vessels ; bd and bs, right and left branchial tree (aquatic lung) ;
go, gonad ; dg, genital duct ; a, body cavity.
feet to the creeping sole, and form the trivium ; while on the dorsal
surface two radii (one right and the other left) form the bivium. Two
interradii, on the other hand, belong to the creeping sole, and three
to the dorsal surface (cf. the diagram, Fig. 350).
The creeping sole usually runs along over the whole length of the body, less fre-
quently (Psolus, Psolidium) it is limited to a circumscribed region between the
anterior and posterior ends.
The difference between the ventral and the dorsal side (the trivium and the
408
COMPARATIVE ANATOMY
CHAP.
bivium) becomes still more accentuated by the different development of the ambu-
lacral feet in the two regions. On the ventral side, these feet are altogether or princi-
pally locomotory tube-feet (ending in suckers), on the dorsal side (the bivium) they are
exclusively or chiefly non-locomotory papillae (with more or less pointed ends). This
difference between the dorsal and ventral tube-feet is found both in those forms in
which the ainbulacral feet, limited to the radii, are arranged in one or more longi-
tudinal rows, and in those in which they are also found in the interradii and arranged
irregularly.
In the genus Psolus, the distinction between dorsal and ventral, and consequently
the bilateral symmetry of the body, becomes still more marked by the entire absence
TVS '
rmv
Fio 351 -Derivation of Bhopalodina (A) from an ordinary Holothurian (B) (after Ludwig).
n.'elio? In 1 ter ? iedi ? te form ' ** " , left dorsal, left ventral, medioventral radius; imd,
.1 interradms ; o, mouth ; an, anus ; go, genital aperture ; , water vascular ring.
of ambiilacral appendages on the bivium. The tube-feet of the middle ventral radius
o be wanting in some species of this genus.
Where ventral and dorsal are sharply distinguished, the mouth and the anus
tend to shift on to the ventral side.
P
genUS Rh P alodi ( F ig- 351) is quite peculiar.
th ' and produced into a lon stalk - At ^ end
betwee tti ,l! e '"i 0t ^ He the m Uth and the anus > and
odv UP f ^ al apertUre ' n the swolle11 P rtion ^ the
I' ^ f te ^ dou ^. lon S itudinal ^ws of ambulacral feet, so that
onlvfiv P T M T - P SSeSSeS ten radli ' Whereas {i ^ realit y
only five To obtain this condition we have to imagine (1)
n ? ^ de j ldrochirot Holothurian bent upward an-
b t Tl y ' and (2) the W^imation of the anus and
by the great shortening of the dorsal interradius. The accom-
VIII
EGHINODERMATA RADIAL ORGANS
409
panying diagram will help to make this clear. The genus Ypsilo-
thurin seems to have become fixed while in the act of being similarly
modified.
In the genus Psychropotes (Fig. 223, p. 285) the dorsal surface is
prolonged beyond the anus into a long caudal appendage directed
posteriorly. Peniagone is distinguished by an anteriorly inclined comb,
rising transversely from the neck. On the swimming disc of Pelago-
thurio, cf. Figs. 224 and 225, p. 286.
IV. Position and Arrangement of the Most Important Organs
in the Radii.
The position and arrangement of the organs in the radii can best
be explained by describing cross -sections. In the Asteroidea, the
.
FIG. 352. Transverse Section of a radial region of the body wall of a Holothurian,
partly diagrammatic. 1. Eudothelinm of the body cavity ; 2, circular musculature ; 3, longitudinal
musculature ; 4, motor nerve ; 5, radial water vascular* canal ; 6, radial blood lacuna ; 7, radial
ridge of the deeper oral nervous system ; S, ampulla ; 9, cutis ; 10, epidermis ; 11, tube-foot canal
of the vascular system ; 12, tube-foot ; 13, nerve of the same ; 14, vessel of the same ; 15, radial
nerve strand of the superficial oral nervous system ; 16, epineural canal ; 17, peripheral nerve ; 18,
pseudohsemal canal.
Ojtl'iuroidea, and the Crinoidea, in which the body is produced radially
into arms, the sections to be described will be those of the arms ; in
the Holothurioidea and Echinoidea the sections are of a radial region of
the body wall.
Holothurioidea (Fig. 352). In a transverse section through a
410 COMPARATIVE ANATOMY CHAP.
radial region of the body wall of an actinopodan Holothmian we find,
proceeding from without inwards :
(a) The outer body epithelium (10).
(b) The cutis, or the connective tissue layer of the body wall, wit
the calcareous corpuscles (9).
(c) The epineural canal (16).
(d) The radial nerve trunk of the superficial oral system (
(e) The radial nerve trunk of the deeper oral system (7).
(/) The subneural pseudohsemal canal (18).
(a) The radial blood lacuna (radial blood vessel) (6).
FIG. 353. Transverse Section through a radial region of the body wall of an Echinoid, partly
diagrammatic. 1, Ampulla, traversed by muscle filaments ; 2 and 3, the two canals traversing the
test and connecting the ampulla and the tube-foot canal (5) ; 4, circular nerve in the terminal disc
of the tube-foot ; 5, tube-foot canal ; 6, nerve of the tube-foot ; 7, integumental nerve ; 8, calcareous
substance of the ambulacral plate ; 9, nerve plexus in the depths of the body epithelium ; 10, suture
between two plates of the two contiguous rows of ambulacral plates ; 11, body epithelium ; 12,
epineural canal ; 13, endothelium of the body cavity ; 14, pseudohsemal canal ; 15, radial blood
vessel ; 16, radial canal of the water vascular system ; 17, radial nerve strand ; 18, lateral canal of
the radial canal of the water vascular system to the ampulla.
(h) The radial canal of the water vascular system (5), and the tube-
foot canal branching from it transversely (11), and finally
also the ampulla of the tube-foot (8).
(i) The circular musculature of the body (2).
(k) The longitudinal musculature of the body (3).
(1) The endothelium of the body cavity (1).
The figure also illustrates the relation of a tube-foot to its canal
and ampulla.
This description does not apply to the Paradinopoda (Synaptitto) in
so far as, in these latter, the radial canals of the water vascular system
are altogether wanting.
VIII
ECHINODERMATA RADIAL ORGANS
411
Echinoidea (Fig. 353). In a transverse section of an anibulacral
area we find :
(a) The outer body epithelium (11).
(b) The cutis, almost entirely calcified, as ambulacral plates (8).
(c) The epineural sinus (12).
(d) The radial nerve trunk (17).
(e) The subneural sinus pseudohaBmal canal (14).
iz
ti
10
21
Fi< -. :;;>4. Transverse Section through the arm of an Asteroid, , radial trunk of the deeper oral nervous system; 10, radial pseudohsemal canal; 11, peripheral
branch of the radial nerve trunk ; 12, spine ; 13, lower (oral) intervertebral muscle cut across ; 14,
lateral shield ; 15, vertebral ossicle ; 16, upper (apical) intervertebral muscle ; 17, dorsal canal of
the brachial cavity (coeloin) ; 18, ciliated strip of endothelium ; 19, dorsal shield ; 20, radial canal of
the water vascular system ; 21, lateral portions of the brachial cavity, which are segmentally re-
]>at."l ; 22, branch of the .water vascular system running to the tentacle ; 23, ganglion at the base
of the spine ; 24, motor branch of the nerve (of the deeper oral system).
(e) The radial canal of the water vascular system (2), with the
canals of the tube-feet branching from it. (All these are
separated from one another by thin layers of connective
tissue.)
(/) The ambulacral plates (30), with the transverse muscles which
connect them (28).
((]) Still further in, and projecting into the body cavity, are the
ampullae (26) of the tube-feet.
VIII
ECHINODERMATA RADIAL ORGANS
413
(/>) The endothelium of the body cavity (21).
The figure also shows the relations of the ampullae to the tube-feet
and their canals, and the organs of the apical side of the arm
of an Asteriod.
Ophiuroidea (Fig. 355). In a section through an arm, proceeding
from the lower (oral) to the upper (apical) side, we find :
(a) The body epithelium.
(b) The ventral shield (5).
Q
FIG. 35<5. Transverse Section through the arm of a Crinoid, diagrammatic. 1, Radial nerve
trunk of the superficial oral nervous system ; 2, radial pseudohsemal canal ; 3, radial canal of the
water vascular system ; 4, the paired deeper longitudinal nerves of the arms ; 5, 7, and 11, the three
radial sinuses of the brachial coelom ; 6, genital sinus with genital rachis ; 7 (see 5) ; 8, nerve
trunk of the apical nervous system ; 9, end of the nerves at the surface ; 10, branch connecting 4
and 8 ; 11 (see 5) ; 12, tentacle nerve ; 13, tentacle canal of the water vascular system ; 14, sensory
cone on the tentacle ; 15, food groove of the arm.
(c) The radial epineural canal (6).
(d) The radial nerve trunk of the superficial oral nervous system (7).
(e) The radial nerve trunk of the deeper oral system (9).
(/) The (subneural) radial pseudohsemal canal (10).
(g) The radial canal of the water vascular system (20).
(h) The calcareous mass of the vertebral ossicle (15), which is
traversed by the tentacle canals (22), and the intervertebral
musculature (16 and 13).
414 COMPARATIVE ANATOMY CHAP.
(i) The endothelium of the body cavity.
(k) The much diminished body cavity itself (enterocoel, 17 and 21).
(I) The dorsal (apical) body wall, which in this connection is of no
further interest.
Crinoidea (Fig. 356). On the section of the arm of a Crinoid,
proceeding from the oral to the apical side, we find :
(a) The body epithelium covering the food groove.
(b) Deep in this epithelium, the radial nerve trunk of the super-
ficial oral system (1).
(c) Below the epithelium (not invariably present) a small schizo-
ccel canal (pseudohremal canal (2).
(d) The radial canal of the water vascular system (3).
(e) At its two sides, the paired subepithelial longitudinal nerves of
the arms (4).
(/) The three radial sinuses; viz. two paired sinuses (5 and 11),
separated by a vertical septum (the so-called ventral or sub-
tentacular canals), and a third unpaired sinus (7), the
dorsal canal, separated from the first two by a horizontal
(transverse) septum.
All these parts lie embedded in somewhat sparse connective tissue.
In the middle between them run :
(g) The narrow genital sinus (6), with the genital tube (rachis)
within it.
(h) The skeletal ossicle of the arm, or (according to the plane of
the section) the apical and oral muscles and bands, uniting
the ossicles.
(i) In the centre of the joint we find the section of the nerve canal
(axial canal) with the radial trunk of the apical nervous
system (8) which it encloses.
The figure also shows the tentacles, and the nerves which connect
the paired radial nerves of the oral with the radial trunk of the apical
nervous systems.
V. The Integument.
The integument of the Echinodermata consists of (1) the uni-
laminar body epithelium which covers the whole body with its processes
and appendages, and (2) a strong subjacent connective tissue layer
(the cutis or corium) of mesenchymatous origin, in which the various
skeletal structures develop. The cutis forms by far the largest part
the body wall. Internally, it is either directly lined by the endo-
thelium of the body cavity or else is separated from the endothelium
by musculature (Holothuria, Asteroidea).
(1) The body epithelium. (a) This is distinct from the subjacent cutis in the
ulca, Asteroidea, many Holothurioidea, and on the oral surface of the disc and
arms of the Crinoidca ; also in the Euryalce.
In the Ophiuroidea (excluding Eurydlas), and on the apical side of the disc and
ECHINODERMATA THE INTEGUMENT 415
anus of the Crinoidea, there is no sharp line of distinction between the body
epithelium and the cutis. Such a distinction is, however, demonstrable in very young
stages. In later stages of development the elements of the two forms of tissue seem
to mingle, and skeletal substance forms right up to the surface of the integument.
In many Holothurioidea also, the body epithelium, as such, is very indistinct.
In Cucuinaria, for example, the cutis appears at the surface of the integument, and
the body epithelium is found in the form of nests of cells scattered within the peri-
pheral layer of the cutis. Each cell sends a thin process to the surface of the
integument.
(&) The body epithelium is usually covered by a cuticle of varying thickness.
(c) The body epithelium is ciliated over the whole surface of the body in the
Axtd-oidea and Echinoidea, but in the Crinoidea only in the food grooves.
The integument of the Ophiuroidea, Crinoidea (with the exception of the food
grooves), and Holothurioidea, is non-ciliated.
(d) The body epithelium of the Asteroidea is rich in glands. The glands are
usually unicellular (goblet glands, granular glands, etc. ), and remain on the level of
the epithelium. In Echinastcr sepositus, large multicellular glands are also found,
whose pear-shaped or spherical bodies dip down into the cutis. In the integument
of the Holothurioidea also glands have been described, and it will probably be dis-
covered that certain epithelial cells of the Echinoidea are of a glandular character.
(e) The integumental pigment may belong to the epithelium as well as to the
cutis ; it not infrequently occurs in both layers.
(/) Epithelial sensory cells, ganglion cells, and nerve fibres will be described in
another place.
(2) The cutis of the Echinodermata is always very thick, although it shows
extraordinary variations in this respect according to the genus and species. It
everywhere consists (a) of a ground- or intercellular substance of gelatinous or carti-
laginous consistency, and (ft) of the nucleated connective tissue cells which secrete
this ground- substance and are embedded in it ; these cells are spindle-shaped, star-
shaped, etc. There are. further, (c) in all Echinodermata, granulated plasm cells
or wandering cells (amoebocytes) similar to those which are to be found in different
body fluids. These can move, like amcebfe, in and through the different tissues.
In Ilolothurioidca, these wandering cells may collect in such quantities in the
deep looser layer of the cutis, as to form a distinct layer (Wanderzellenschicht).
The calcareous skeleton of the body wall of the Echinodermata always lies in
the cutis, whether it consists, as in the Holothurioidea, of isolated calcareous cor-
puscles, or, as in other Echinoderms, of larger plates of lattice-like or spongy struc-
ture. In sections through the decalcified body wall, the spaces in which the skeleton
lay are visible. In other Avords, the connective-tissue fills up all the spaces in the
spongy calcareous skeleton. Since the wandering cells can travel to the surface
through these spaces, they may play an important part in the nutrition of the soft
parts which lie at the surface of the skeleton, especially in Asteroids and Echinoids.
It appeal's that even the intercellular substance may occasionally become differ-
entiated into fibres, which, however, are difficult to distinguish from the fibrous
processes of the connective-tissue cells.
Where two skeletal plates are united by a suture, this suture is formed of thickly
crowded parallel fibres, which connect the ground -substance of one plate with that
of the other.
416 COMPARATIVE ANATOMY CHAP.
VI. The Water Vascular System.
(System of the Ambulacral Vessels : Hydroccel.)
This is a system of canals filled with fluid, the arrangements of
which may be generally described as follows.
An outer aperture, the madreporite, leads first into a vesicular
section of the coelom, the madreporitie ampulla. This again is con-
nected by means of a stone eanal (so called because that portion of its
wall which consists of connective tissue is often calcified) with a ring
canal which surrounds the oesophagus. Into the madreporitie ampulla
there opens, further, the axial sinus of the body cavity, which follows
the stone canal in its course, and surrounds a lymphatic gland, the
axial organ.
The water vascular ring may carry various accessory structures,
whose principal function seems to be that of lymphatic glands, and
which are known as Polian vesicles, Tiedemann's bodies, etc.
From the ring canal there run out into the radii of the body,
either in the body wall or in close contact with it, as many radial
canals as there are radii (usually therefore five). The radial canals
send off, on each side, tube-feet canals, which run into outer append-
ages of the body wall, ending blindly at their tips. These extensible
appendages are usually present in great numbers, and serve either as
tube-feet for locomotion (Holothurioidea, some Echinoidea, Asteroidea),
and are then provided with a terminal sucker, or as tentacles, ten-
tacular gills, etc., for tactile purposes, for respiration, and for conduct-
ing food (some Echinoidea, Ophiuroidea, Crinoidea). In connection with
the tube-feet canals, tube-feet ampullse are very often found (Holo-
thurwidea, Echinoidea, Asteroidea) ; these are accessory contractile
vesicles, which serve for the swelling of the tube- feet. Special
valves are so arranged as to prevent the flowing back of the water
vascular fluid into the radial canals (Fig. 352, p. 409).
The chief departures from this general description met with in
the five classes of Echinoderms, affect the madreporite, the madre-
poritie ampulla, and the stone canal. These will be described in
detail later on.
Structure of the wall of the water vessels. Lining the lumen of the vessels,
there is generally found, first of all, a ciliated epithelium. This is followed, in
most parts (always in the ambulacral appendages), by a longitudinal muscle layer.
Outside this latter lies a layer of connective tissue, and, outermost of all, there is
almost always an external ciliated epithelium. On the ambulacral appendages (the
tube-feet and tentacles) this last is nothing more than the external body epithelium.
But in those parts of the water vascular system which project into or lie in the
body cavity, it is the endothelium of the ccelom. This outer epithelium of the
water vascular system is rarely altogether wanting ; it is, however, absent in such
parts of the systeir as run embedded in the body wall. A circular musculature is
seldom found ; it only occurs locally.
viii ECHINODERMATA WATER VASCULAR SYSTEM 417
Calcareous corpuscles may be formed in the connective tissue layer of the wall in
other parts of the water vascular system besides the stone canal. Such calcification
always takes place in locomotory tube-feet.
The fluid contained in the water vascular system is sea-water
with traces of albumen (in a '5 - 2 per cent solution). Floating in
this fluid are found amoeboid cells (lymph bodies) and coloured cor-
puscles often united into small lumps. The fluid occasionally appears
of a pale yellow, or reddish, colour.
The origin of this fluid is a question of frequent recurrence.
The view which still appears best supported is that sea-water flows in
through the madreporite and the stone canal, but an exactly opposite
view has also been maintained. The observations made on this
subject appear to contradict one another, it being very difficult to
carry on investigations in a decisive and satisfactory manner.
A. Madreporite and Stone Canal.
1. Holothurioidea (Fig. 357). The condition which must be con-
sidered as the original is that in which only one stone canal occurs ;
this is attached to the dorsal mesentery (cf. p. 406), and its
madreporite lies mediodorsally in the integument, and its pore
canal or canals open outward direct.
Such a condition is found in the adult only in certain Elasipoda
and in Pelagothuria.
In the large majority of Holothurioidea, the stone canal loses all
direct communication with the exterior, while at its distal end,
which now lies in the body cavity, a new inner madreporite forms,
through whose canals communication is established between the
stone canals and the body cavity.
In a comparatively small number of Holothurioidea (never in
Molpadiidce and JElasipoda) the number of stone canals increases (the
single canals usually shortening at the same time), and may finally
become very great (over 160).
The inner madreporite is found in the form of a variously shaped swelling on
the stone canal, which is often S-shaped or spirally coiled. Only the primary stone
canal is connected with the dorsal mesentery ; this is never the case with accessory
canals. These latter float freely in the body cavity, and this is also the condition
of the primary stone canal of the Aspidochirotce, which has lost its connection with
both the body wall and the mesentery.
More than one canal is found in only a very small number of forms even among
the Synaptidce, the Dendrochirotce, and Aspidochirotce. The number of accessory
canals varies greatly in different forms ; it does not seem to be of systematic import-
ance, since it varies in individuals of one and the same species. It is probable that
the accessory canals, ontogenetically, bud off* secondarily from the water vascular
system, whereas the dorsomedian stone canal arises primarily from the canal which,
in the larva, connects the hydroccel with the exterior.
Branched stone canals, with a madreporite at the distal end of each branch,
occur in Synapta Beselii, Jag ; and Thyone chilensis, Semp.
VOL. II 2 E
418
COMPARATIVE ANATOMY
CHAP.
The madreporite of the primary (mediodorsal) stone canal The simplest, and
the most primitive condition is found in Pelagothuria and m certain
'
^ sp > ,
ITd zLhodytes. In these the stone canal opens simply through a single
m Idorsal pore, which lies in front of the genital aperture (Fig. 37 A). In other
species of these genera and in species of Psychropotes, Lcetmogone llyodcemon, more
than one madreporite pore is found, their number varying, according to the ^species
from twoorthree to fifty or more (Fig. 357, B). In other cases (species of the Elasipod
zenera Irpa Elpidia, Oneirophanta, Orphnurgus, Benthodytcs, and the Molpadndan
genera Trochostww and Ankyroderma] the distal end of the stone canal still remains
Fio. 357. Diagrams illustrating the various relations existing between the stone canal
and the madreporites in the Holothurioidea. 1, Body wall ; 2, commencement of the radial
canal ; 3, oesophagus ; 4, dorsal mesentery ; 5, stone canal ; 6, outer madreporite ; 61, inner madre-
porite ; 7, genital aperture ; 8, genital duct ; 9, water vascular ring ; 10, Polian vesicle.
embedded in the body wall, but it has lost the pore or pores which formed the com-
munication between it and the exterior. New pores therefore arise laterally at the
distal portion, which still lies in the body wall, and these now open communication
between the lumen of the stone canal and the body cavity, and make this widened
part of the stone canal into an inner madreporite (Fig. 357, C). Other Molpadiidce
and the Synaptidce and Dendrochirota differ from these last only in the fact that in
them the stone canal has become entirely detached from the body wall (Fig. 357, D).
In the Aspidochirota, which also possess an inner madreporite, the latter appears
complicated, in that its pore canals do not open direct into the lumen of the stone
canal, but first into a collecting cavity, which in its turn communicates by means of
an aperture (occasionally through several) with the lumen of the stone canal.
2. Eehinoidea (Fig. 358, 33). In the Echinoidea, so far as is
viii ECHINODERMATA WATER VASCULAR SYSTEM 419
FIG. 358. Diagram of the organisation of a regular Echinoid. Section in the direction of
the principal axis. The surface of the section lies interradially on the left and radially on the
right. The left half is incomplete. 1, External gill (this would not exactly come into this section,
since there are five pairs of interradially placed gills) ; 2, seizing pedicellaria ; 3, oral integument ;
4, tooth ; 5, mouth ; 6, cushion of connective tissue ; 7, nerve ring of the superficial system ; 8,
deeper oral nervous system ; 9, radial epineural canal ; 10, radial blood vessel ; 11, arch of the
ambulacral apophysis (auricula) ; 12, sphseridium in its niche ; 13, radial canal of the water vascular
system ; 14, radial nerve trunk (of the' superficial oral system); 15, 'radial pseud ohsemal canal ; 16,
circular ganglion at the base of the spine ; 17, spine ; 18, glandular pedicellaria ; 19, ambulacral
tube-feet with terminal disc ; 20, 21, ambulacral tentacles (without terminal disc) ; 22, terminal
feeler or tentacle emerging through the pore in the radial (ocular) plate ; 23, apical (genital) ring
sinus ; 24, perianal sinus of the coelom ; 25, anus ; 26, sinus, into which a process (27) of the axial
organ projects ; 27, aboral process of the axial organ ; 28, 'madreporite ; 29, genital aperture on the
genital papilla ; 30, genital duct ; 31, madreporitic ampulla, into which the stone canal and axial
sinus enter from below ; 32, axial organ ; 33, stone canal ; 34, part taken by the blood lacuna in
the formation of the Polian, vesicle ; 35, root of the tooth ; 36, muscle of the forked radii (Fig. 348, 7)
cut through ; 37, arch (arcus) of a jaw pyramid of the masticatory apparatus ; 38, lantern mem-
brane ; 39, ligament of a forked radius; 40, adductor muscle of the teeth; 41, interambulacral
apophysis ; 42, general body cavity (coelom) ; 43, pyramid ; 44, peripharyngeal sinus, lantern sinus
of the coelom ; 45, part taken by the water vascular system in the formation of the Polian vesicle ;
46, circular vessel of -the blood lacunar system ; 47, water vascular ring ; 48, oesophagus ; 49, axial
sinus of the coelom ; 50, hind-gut ; 51, perirectal sinus of the coelom ; at 52 and 54 the section is not
quite radial, so that it does not, as at 22 and 56, take in the radial canal of the water vascular
system, but passes transversely through its lateral canals which lead to the ampullae ; in 53 the
plane of the section lies still more to the side, so that the ampulla is taken in (cf. Fig. 353) ; 57,
abductor muscle of the teeth ; 58, Stewart's organ ; 59, muscles between the pyramids ; 60, inter-
mediate plate ; 61, forked radius ; 62 and 65, intestinal vessels ; 63, accessory intestine ; 64, prin-
cipal intestine. The accessory intestine in reality runs on the axial side of the principal intestine.
420 COMPARATIVE ANATOMY CHAP.
yet known, there is always only one stone canal, and it always com-
municates with the exterior by means of the pores of the madreporite.
This communication is, however, by no means direct. The pores of
the madreporite first lead into a small cavity lying below it, the
madreporie ampulla, into which opens, on the one hand, the ascend-
ing stone canal, and, on the other hand, the axial sinus of the entero-
coel, to be described later. The stone canal, on leaving the ampulla,
traverses the body cavity, following the axial sinus with its lymph
gland, and runs down to the water vascular ring, which in the
Cidaroida and Clypeastroida encircles the oesophagus immediately above
the masticatory framework (Fig. 358), but, in the Spatangoida, imme-
diately above the mouth. In both the former groups the stone canal
is short and more or less straight, but in the Spatangoida it is very
long and runs in coils.
On the possibly great morphological importance of the ampullae, cf. the section
on Ontogeny.
Echinocyamus pusillus, a Clypeastrid, shows an embryonic condition in the
adult in that the madreporite has only one pore. All other Echinoids, examined
with reference to this point, possess as adults several or numerous pores. The
number of pores increases with age and growth.
The pore canals which traverse the madreporites may anastomose with one
another. They may enter the ampulla through several inner pores or else through
one common inner aperture. In the Spatangidce, they traverse the substance of a
large skeletal process (apophysis) of the madreporite, which projects into the cavity
of the test.
The condition of the stone canal in the Spatangoida deserves further investiga-
tion, since the observations hitherto recorded contradict one another. According to
one account, the stone canal (in Echinocardium) breaks up into branches on its way to
the water vascular ring, these branches communicating with the axial blood lacunar
system. According to another, it ends blindly (in Spatangus purpureus), and the
water vascular ring is said in no way to communicate openly with the apical stone
canal. A canal, however, runs from the water vascular ring towards the stone canal,
without reaching it. The existence of any kind of communication with the lacunar
system is emphatically denied by those who hold this latter view.
3. Asteroidea. In all Alferoids the madreporite is external,
and takes the form of a skeletal plate, which is perforated by many
pores, and always lies on the apical side of the disc interradially.
The stone canal, within the axial sinus and attached by a band to its
wall, descends direct to the water vascular ring which surrounds the
oasophagus, and enters this ring interradially. The wall of the stone
canal is generally highly calcified, and its lumen is divided in a more
or less complicated manner into shelves, niches, etc., by projecting
folds which frequently branch. It not infrequently happens in
Asteroids that there is more than one stone canal and madreporie
plate. For example, all Asteroids which reproduce asexually (i.e. by
division) possess more than one canal.
The relations of the madreporite to the axial sinus are interesting.
Not all the pores of the madreporie plate open into the stone canal ;
Till
ECHINODERMATA WATER VASCULAR SYSTEM
421
some of them open direct into the axial sinus. No direct communica-
tion between the stone canal and the axial sinus is found in adult
animals.
The madreporite appears externally marked by furrows radiating from the centre
to the periphery (Fig. 359). In the bases of these furrows lie the apertures of the
pores. The pore canals, which run through the substance of the
madreporite to the stone canal, anastomose in definite ways, which
cannot here be described.
The increase of surface of the inner wall of the stone canal
(Fig. 360) is of some interest. As the middle layer of connective
tissue takes part in the formation of the folds projecting into the
lumen, these folds may calcify. The simplest condition is found in
the Echin aster idee and Astcrias tenuispina, where a projecting longi-
tudinal ridge is formed on the inner wall of the stone canal (Fig. quarter of the
360, A). In Asterina the free edge of this fold splits up into two niadreporic
diverging lamellae, in such a way that the transverse section is Y- or P late 5 h . As '
anchor -shaped (B). The lamellae may become coiled (species of m^ng ( a fte r
Asterias, Pentaceros, Grymnasteria, C). Occasionally the ridge traverses Ludwig).
the whole lumen of the canal as a septum (D), and then carries on
each surface a coiled lamella (species of Astropccten) . The whole lumen, further,
may be traversed by septa which, in transverse section, form a network (Luidia,
Culcita, species of Astropecten and Ophidiaster, F).
Number of the stone canals and madreporitic plates. Several madreporites
and stone canals (two to five and more) are not infrequently found in individuals
FIG. 359. A
FIG. 360. A-F, Transverse sections through the stone canal of various Asteroids. 1, Sus-
pensor of the stone canal to the wall of the axial sinus ; 2, endothelium of the axial sinus ; 3, inner
epithelium of the stone canal ; 4, connective tissue portion of the wall.
with six, seven, or more arms, belonging to species which normally have five arms.
There are, however, some species (having normally five or more arms) which habitu-
ally possess more than one madreporite (Asterias capensis, A. polyplax, Ophidiaster
Gen/iani, Acanthaster echinites and A. Ellisii}. On the other hand, the species of the
genera Solaster, Heliaster, and Luidia, which normally have numerous arms, possess
only one madreporite. When more than one madreporite is present they lie, as a
rule, in different interradii. Cases have, however, been observed in which two stone
canals occurred in one and the same interradius, and even in one and the same axial
sinus.
422
COMPARATIVE ANATOMY
CHAP.
4. Ophiuroidea. In this class, as a rule, one single madre-
porite with one pore aperture and a single stone canal are present.
The pore aperture is not found, as in Asteroids and JEchinoids, on the
apical side of the body, but, in adult Ophiuroids, on the oral side of
the disc, asymmetrically in an interbrachial area, and on that edge of
the oral shield which is turned to the bursal aperture. This oral
shield thus becomes the madreporic plate. The pore aperture leads
first into an ampulla (Fig. 361, 3), which probably corresponds with
the axial sinus of the Askeroidea and Echinoidea. Into this ampulla
the stone canal which descends from the water vascular ring opens.
FIG. 361. Stone canal and neighbouring parts of Amphiura squamata, diagrammatic
vertical section through the madreporic interradius of the disc. 1, Water vascular ring ; 2,
stone canal ; 3, ampulla ; 4, madreporic canal ; 5 and 7, axial sinus (?) ; 6, circular genital sinus ;
8, axial organ ; 9, genital rhachis ; 10, bursal pouch ; 11, oral wall of the intestine ; 12, peristomal
sinus ; 13, irtterradial muscle ; 14, circular nerve ; 15, teeth ; 16, mouth ; 17, oral surface of the disc.
A large part of the ampulla lies on that side of the stone canal which
is turned towards the mouth. In consequence of the position of the
pore aperture, the stone canal, which rises out of the water vascular
ring interradially, runs in a downward (oral) direction.
The diagram (Fig. 361) illustrates in detail (1) the relation of the stone canal to
the axial sinus ; (2) the manner in which the former enters the madreporic ampulla,
the cylindrical epithelium of the former being directly continued into the tessellated
epithelium of the latter ; (3) the opening outward of the ampulla through a madre-
poric canal.
It appears that in many species of the genera Amphiura, Ophiolepis, OpJdo-
plocus, Ophionereis, and Ophiocnida, several or many pore apertures occur at the
edge of the oral shield. This is certainly the case in many Astrophytidce. In
Trichaster, however, only one pore aperture is present ; but this and the stone canal
belonging to it are repeated in each interradius.
- In Ophiactis viretis also, which reproduces itself asexually by division, several
(as many as five) stone canals occur in the adult in different interradii. In young
individuals only one is found.
vin EGHINODERMATA WATER VASCULAR SYSTEM 423
5. Crinoidea. Adult Crinoids have at least five, and usually
many more or even very numerous stone canals, all of which open
into the body cavity. Communication between the exterior and the
body cavity is brought about by
at least five ciliated pores (Kelch-
poren) in the tegmen calycis ;
their number is generally far
greater, and may mount up to a
thousand. Each single pore
corresponds with a madreporite
with one pore canal. We must
not therefore compare the calyx
pores of a Crinoid collectively
with the numerous pores of a
madreporic plate. Originally, ^ ^ _ A st Tegmen calycis ; 2, calyx pore ;
tone CanalS. in Case., in 3> aperture of t i ie stone canal into the body cavity ;
which both Structures are Very 4, intestinal epithelium; 5, intestinal cavity;
numerous, however, no SUch 6, ccelom ; 7, stone canal ; 8, ring canal ; 9, circular
-,,. t i i T r j nerve": 10, oesotfhageal epithelium; 11, oesophagus;
relation can be established. 12> coimec tive tissue.
In many inadunate Crinoids
(cf. p. 303) a madreporite occurs in the posterior interradius of the
tegmen.
Pihizocrinus lofotensis and Adinocrinus verneuilianus have only five
interradial stone canals and five interradial pores in the calyx. The
openings of the stone canals into the body cavity lie directly below
the pores belonging to them.
For the number and arrangement of the calyx pores, cf. the section on the Tegmen
Calycis of the Crinoids, p. 377.
B. The Water Vascular Ring and its Appendages.
1. Holothurioidea. The water vascular ring always encircles the
oesophagus behind (i.e. apically to) the calcareous ring. In all Holo-
thurioidea without exception it carries Polian vesicles. As a rule,
only one Polian vesicle is present.
These pear-shaped or tubular cteca of the water vascular ring, which project
freely backward into the body cavity, vary greatly in size. In extreme cases they
may be half as long as the body.
In the Molpadiidce, and among the Elasipoda in the Psychropotidce and Deimatidce,
more than one vesicle has never yet been observed, and, in the Elpadiidce, there
is, normally, only one. In other divisions, a varying number of species, greatest in
the Synaptidcc, have more than one Polian vesicle. In all such species, however,
there was originally only one vesicle. Where accessory vesicles occur they vary
greatly in number, and appear to have very slight, if any, systematic significance.
OF THR
UNIVERSITY
424 COMPARATIVE ANATOMY CHAP.
Where only one Polian vesicle occurs, it lies in the left ventral
interradius, very seldom in the left dorsal interradius.
Where two or more vesicles occur, they are also mostly found in the ventral
region of the circular canal.
The walls of the Polian vesicles correspond in structure, essentially, with those of
the ring canal. Cells belonging to the inner epithelium become amoeboid and break
away from the wall. These are said to become the lymph cells of the water vascular
system.
2. Eehinoidea. In the Spatangoida (which have no masticatory
apparatus) the ring canal encircles the oesophagus immediately above
the mouth. In other Eehinoidea, however, it is pushed up by the
masticatory apparatus which intervenes between it and the mouth.
The canal therefore surrounds the oesophagus at the point where this
latter emerges from the lantern. The ring canal, as well as its
accessory structures, nevertheless, lie within the lantern membrane,
which envelops the whole masticatory apparatus. The circular vessel
(the lacunar ring) is in close contact with the canal (Fig. 358),
In the Spatangoida and some Clypeastridce (Echinocyamus pusillus),
the ring canal has no accessory structures. In the Stereosomata, on the
contrary, it has, in each interradius, a small outgrowth, which ramifies
and intertwines with similar ramifications of the circular blood vessel
to form together a spongy body, which is known as the Polian vesicle,
and is regarded as a lymph gland. This, which, in the Stereosomata, is
confined to certain localised interradial points, occurs in the Cidaroida,
certain Clypeastroida (e.g. Peronella orbicularis), and the Streptosomata,
along the whole course of the canal, so that the intertwining of the
appendages of the ring canal and of the circular blood vessel gives
rise to a spongy ring.
An intermediate stage is found in Echinodiscus biforis (Clypeastroid), in which
the interradial spongy bodies of the circular canal are longer than in the Stereosomata,
but long radially arranged tracts are still left free, the canal at these parts retaining
its simple lumen.
3. Asteroidea. The circular canal which surrounds the mouth,
following the inner outline of the oral skeleton, here has two kinds
of appendages : Tiedemann's bodies and the Polian vesicles, both
of which lie interradially. Tiedemann's bodies appear to occur in all
Asteroids, whereas the Polian vesicles are wanting in some families,
e.g. the Asteriidce, Echinasteridce, and Linckiidce.
Tiedemann's bodies (Fig. 363, 7) are small tufts of tubules, closely crowded
together, their walls of connective tissue being fused with one another. These
tubules, which open into the circular canal, are lined internally with a cubical
epithelium, and contain within their lumen bundles of cells which have broken
away from the wall. These cells, the protoplasm of which contains pigmented
concretions, become the amoeboid lymph cells which float in the fluid of the water
vascular system. They give the Tiedemann's bodies their more or less distinct
coloration.
VIII
EGHINODEEMATA WATER VASCULAR SYSTEM 425
Two Tiedemann's bodies usually occur in each interradius ; the interradius
containing the stone canal not infrequently, however, forms an exception to this
rule, only one such vesicle being present in it (Asteriidce, Echinasteridce, Linckiidoe,
Aster inidce, Culcitidof}. If the circular canal is viewed internally in the position
shown in Fig. 363, this body lies to the right of the stone canal.
The Polian vesicles (Fig. 363, 6) are large structures with long stalks, and to
them, as to Tiedemann's bodies, the function of lymph glands has been attributed.
In the Asterinidce, Culcitidce, Luidia, and several species of Astropecten, one
10.
FIG. 363. Circular canal, Polian vesicles, Tiedemann's bodies, and ampullae of the water
vascular system of Asterina gibbosa (after Cue'not). Seen from within, i.e. from the body cavity.
1, Mouth at the centre of the oral membrane ; 2, stone canal ; 3, axial sinus ; 4, transverse muscles
of theambulacral plates ; 5, ambulacral plates ; 0, Polian vesicles ; 7, Tiedemann's bodies ; 8, circular
canal ; 9, blood vascular ring(?) ; 10, % ampullae.
vesicle is found in each interradius. Only in the interradius containing the stone
canal is it wanting, or else (in species of Astropecten) two are here found instead of
one. Astropecten aurantiacus has two to four (usually three) Polian vesicles in each
interradius (even in the stone canal interradius). The wall of this vesicle, proceeding
from without inwards, consists of: (1) the ciliated endothelial covering ; (2) a layer
of connective tissue in which run the longitudinal muscle fibres ; (3) a circular
muscle layer, and (4) the inner epithelium, whose cells lie in the interstices of a net-
work of connective tissue.
4. Ophiuroidea. The water vascular ring here possesses one
Polian vesicle, which functions as lymph gland in each interradius
except that of the stone canal. The structure of the wall of this
vesicle resembles that in the Asteroidea, the longitudinal musculature,
however, seems always, and the circular musculature frequently, to be
wanting. The canals to the first two tube-feet arise directly from the
circular canal, commencing usually as a common canal which forks
later, but occasionally the canals are distinct from the first.
Ophiactis virens (Fig. 364) occupies an exceptional position among the Ophiuroidea,
being capable of asexual reproduction by means of fission. This form not only has,
as already mentioned, several stone canals, but in each interradius two to three
426
COMPARATIVE ANATOMY
CHAP.
Polian vesicles and, besides (an altogether unique condition), six to fifteen long thin
accessory vessels in each interradius, which are hollow and end blindly ; these
encircle the intestine and in sexually mature animals penetrate between the genital
organs. The walls of these vessels, which are filled with blood and lymph corpuscles,
and communicate with the circular canal, consist, from without inwards of: (1) the
FIG. 364. A portion of the disc of OpMactis virens in horizontal section, somewhat diagram-
matic (after Cue*not). 1, Oral tentacles ; 2, tooth ; 3, circular canal ; 5, section of the stomachal
sac ; 6, Polian vesicles ; 7, accessory vessels of the circular canal ; 8, stone canal.
endothelium of the body cavity ; (2) a thin layer of connective tissue ; (3) the inner
epithelium. This altogether peculiar development of the water vascular system in
OpMactis virens is considered to be connected with the absence of bursse which serve
for respiration, OpMactis standing alone among the Ophiuroidea in having no such
structures. This peculiar development of the water vascular system is said to be a
supplementary means of respiration.
5. Crinoidea. The circular canal which surrounds the mouth has
here no accessory structures except the stone canals. It is provided
with longitudinal muscle fibres, which are connected with the epithelial
cells (epithelial muscle cells). As in the radial canals, muscle cells
also occur transversely traversing the lumen of the canal. The
circular canal gives off canals direct to the five groups of tentacles
which surround the mouth.
C. The Radial Canals, the Canals of the Tentacles and Tube-feet ;
the Tentacle and Tube-feet Ampullae.
1. Holothurioidea. The Holothurioidea fall into two very
distinct groups, the Synaptidce being- distinguished from all
other members of the class by the fact that, in adults, neither
tube-feet, tube -feet canals, ampullae, nor any traces of radial
vessels are found. The Synaptidce (Paractinopoda) have only oral
viii ECHINODERMATA WATER VASCULAR SYSTEM 427
tentacles and tentacle canals, the latter springing directly out of
the circular canal.
The arrangement in all other Holothurioidea (Adinopoda) may be
described as follows. There are five radial canals, and never more.
The tentacle canals never spring directly from the circular canal, but
arise out of the radial canals. The tentacles are to be regarded as the
first (modified) tube-feet, and the tentacle canals as the first tube-feet
canals.
The canals of the tube-feet and tentacles are usually connected
with ampullae.
Actinopoda (Fig. 365). From the circular canal the radial canals run forward
(anteriorly) along the oesophagus towards the mouth, passing the axial surface of
the calcareous ring (i.e. between it and the oesophagus). They then pass, together
with the radial nerves on whose inner side they lie, through the incisions or apertures
belonging to them in the ring, and run backwards (aborally) in the body wall,
outside the circular musculature, and end blindly near the anus.
In some rare cases, where the ventral surface is sharply distinguished from the
dorsal, and the dorsal ambulacral appendages, i.e. those of the bivium, have entirely
disappeared, the corresponding dorsal radial canals are said also to be wanting. In
a few isolated forms the central radial canal of the ventral side (i.e. of the trivium)
is also said to be wanting.
The tentacle canals branch off from their radial canals just above the calcareous
ring. Their number corresponds with that of the tentacles to which they run.
These canals are often connected, at the anterior edge of the ring, with tentacle
ampullae (Fig. 365, 18). These latter are tubular outgrowths, which vary greatly in
size, stretching back over the outer surface of the calcareous ring, and for the most
part projecting freely into the body cavity. Where such ampullse occur, all the
canals without exception are provided with them. They are entirely wanting in the
families of the Elasipoda and Dendrochirota, but occur normally in the Synaptidce,
MolpadtidtBi and Aspidoclnrota. In Pelagothuria, branches run through the peculiar
swimming disc (cf. p. 286), radially, and reach even to the tips of its processes.
They are evidently to be regarded as modified tentacle ampullae.
The canals of the tube-feet branch off alternately from the radial canals. As a
rule, a separate canal runs from the radial canal to each foot ; but in some cases
(Holothuria tubulosa} one canal, by branching, runs to several (4-6) tube-feet. In
the Molpadiidee, and the above-named Holothurian, it is said that there are tube-feet
canals which end blindly, and thus have no tube-feet answering to them. Except
in the footless Molpadiidee and the Psychropotidce, the tube-feet canals are connected
with egg-shaped, often somewhat long and occasionally branched ampullae. These
either lie as covered ampullae outside of the circular musculature of the body wall or,
as free ampullae, press in between the transverse musculature into the body cavity.
At the point where the ampulla opens into the tube-foot canal, but in that part
of the latter which comes from the radial canal, there is a valve, similar to that
found in Asteroids, which will be described later. This valve is arranged in such a
way as to prevent the return of the fluid into the radial vessel, either from the foot
or from the ampullae. Valves are also found in the tentacle canals.
The walls of the ampullae resemble those of the Polian vesicles in structure.
The radial canals and their branches are chiefly distinguished by the fact that the
longitudinal musculature is only developed in the outer part of the walls.
Paractinopoda. The tentacle canals here spring directly out of the circular
canal, and nearly always agree in number with the tentacles. At the level of the
428
COMPARATIVE ANATOMY
CHAP.
calcareous ring, in each tentacle canal a muscular membrane forms a semilunar
valve which projects from the wall with its concave side directed forwards (orally).
FIG. 365. Section through the oral region of an Actinopod, in the direction of the principal
(longitudinal) axis. On the right, the plane of the section is radial ; on the left, almost interradial.
1, Cutis ; 2, body epithelium ; 3, oral tentacle, cut oft'; 4, water canal of the oral tentacle ; 5, blood
vessel of the oral tentacle ; 6, tentacle nerve ; 7, circular nerve ; 8, oral portion of the coelomatic
perioesophageal sinus ; 9, mouth ; 10, cesophagiis ; 11, perioesophageal sinus ; 12, interradial
portion of the calcareous ring ; 13, water vascular ring ; 14, blood vascular ring ; 15, ventral
intestinal vessel ; 16, intestinal epithelium ; 17, Polian vesicle ; 18, ampulla of the oral tentacle ;
19, endothelium of the body cavity; 20, circular musculature of the body wall ; 21, body cav.ity ;
22 and 26, radial blood vessels ; 23, radial nerve trunk of the superficial system ; 24, radial epineural
canal ; 25, radial perihsemal canal ; 27, radial canal of the water vascular system ; 28, longitudinal
muscles ; 29, commencement of the radial canal of the water vascular system ; 30, radial portion of
the calcareous ring ; 31, retractor muscle ; 32, dorsal intestinal vessel.
This valve prevents the water vascular fluid flowing back out of the tentacles into
the circular canal.
The wall of the tentacle canals consists, from without inwards, of : (1) the endo-
thelium of the body cavity'; (2) a longitudinal muscle layer ; (3) a layer of connective
tissue ; (4) a circular muscle layer ; (5) an inner epithelium.
vni ECHINODERMATA WATER VASCULAR SYSTEM 429
2. Eehinoidea (Fig. 358, p. 419). In the Spatangoida, where a
masticatory apparatus is wanting, the radial canals, on leaving the
circular canal which surrounds the mouth, are already in their
respective radii, and commence at once to send out branches right
and left to the tube-feet. In other Eehinoidea, however, the radial
canals have to descend from the circular canal which encircles the
oasophagus above the lantern to the peristome, and thus, after rising
out of the circular canal, have first to run under the intermediate
plates and over the intermaxillary musculature. They emerge at the
periphery of the lantern and then descend on its outer side, i.e.
outside the intermaxillary musculature to the peristome. Having
reached this latter, they first give off a branch which runs in the oral
region towards the mouth. They then pass through the auriculae,
in order to run up radially towards the apex, on the inner side of
the test, and in the middle lines of the ambulacra. They end
blindly in the pores of the radial plates of the apical system.
While running up on the inner side of the test, the radial canals
give off alternating lateral branches, each of which enters an ampulla
(cf. Fig. 353, p. 410). The ampulla, which projects into the body
cavity, is itself connected, by means of one or two canals, with the
cavity of a tube-foot or tentacle, which latter projects freely on the
outer side of the test. The ambulacral plate at such a point is per-
forated by either a single or a double pore, according as the canal to the
tube-foot is single or double (cf. the section on the Skeletal System).
In all Eehinoidea, the tube-feet in young animals are all alike, and each is
connected with its ampulla by a single pore through the test. This may be
considered to be the primitive arrangement. Tube-feet with single pores are found
in adults in a few Spatangoida : in the Pourtalesiidce, in the Ananchytidan genera
Urcchinus, Cystcchinus, Calymne, in the Spatangoid genus Palaeotropus and the
Cassidulid genus Neolampas.
In all other Echinoids, the tube-feet or tentacles have double pores. In the
regular Echinoids (Cidaroida, Diadematoida), only double pores are found : but in
the Clypeastroida and the Spatangoida, only the pores of the petaloids are double :
those on the remaining ambukcral regions being single.
The ampullae are delicate structures which vary in shape. In cases in which
they, like the tentacles to which they belong, stand at some distance from one
another, they are pear-shaped or spherical ; but where they, like the tube-feet, stand
in compact rows in the ambulacral meridians, as in the regular Echinoids and in the
petaloids of the irregular forms, they are lengthened out horizontally and flattened
vertically (dorso - ventrally ). The walls of the ampullae, from without inwards,
consist of: (1) a ciliated endothelium ; (2) a layer of connective tissue, containing
occasional embedded calcareous corpuscles ; (3) a circular muscle layer ; (4) an inner
ciliated epithelium. The lumen is traversed from wall to wall by fibres, which are
probably muscular. In several Echinoids, at the points where the lateral canals of
the radial canals open into the ampullae, valves have been observed.
The branches of the radial canal which run in the oral integument supply the
tube-feet or tentacles occurring in this region.
3. Asteroidea. The radial canals, in this class, run along the
430 COMPARATIVE ANATOMY CHAP.
bases of the ambulacral furrows of the arms, outside the ambulacra!
plates. At the tips of the arms they end blindly in the terminal
ocular tentacles. In their courses, consecutive widenings and
narrowings are not infrequently found, these correspond with the
segmentation of the arm, but are never very marked. Each radial
canal gives off at regular intervals, which correspond with the
skeletal segments, and at opposite points to right and left canals to
the tube-feet. At the point where such a canal opens into the tube-
foot, a second canal, the ampulla canal, branches off from it. This
canal rises up between two consecutive ambulacral plates to widen out
above these latter into an ampulla which projects freely into the body
cavity (Fig. 354, p. 411).
This ampulla is single in all young Asteroids and many adults (LincJciidce,
Echinasteridce, Asteriidce, Luidia). In other Asteroids (Astropectinidce excluding
Luidia, Asterinidce, Pentacerotidce, e.g. Culcila) two separate ampullae occur to
each tube-foot in the adult.
Valves are found at the points where the canals of the tube-feet
open into the radial canal. A muscular membrane, resembling a
truncated cone with the base attached horizontally round the wall of
the canal, projects into the lumen directed towards the foot. This
valve prevents the fluid pressed out of the ampulla from returning
into the radial canal, either because the membrane is able by muscular
action to close the aperture, or because the pocket surrounding this
projecting membrane is swelled up by pressure of water from the
foot or ampulla, and so closes the valve.
4. Ophiuroidea. The first point to be noted with regard to
the Ophiuroidea is that they have no tube-feet ampullae.
The radial trunks of the water vascular system run in the arms
between the ventral shields and the vertebral ossicles. At the tip
of the arm each trunk ends in a small terminal tentacle. Regularly
consecutive and distinct widenings are found in their courses
corresponding with the regular segmentation of the arms. Between
every two of these consecutive widenings, the radial canal is provided
with a single layer of band-like circular muscle fibres. A narrow
tube-foot canal runs off to right and left from each widening, running
either straight into its tentacle or first forming a V-shaped loop, which
ascends apically into the calcareous mass of the vertebral ossicle. At
the point where the tentacle canal enters the tentacle, the lumen of
the former becomes much widened, and a valve occurs (similar to that
described in the Asteroids), which prevents a flowing back of the
water vascular fluid out of the tube-foot into the radial canal.
The first two pairs of canals to the tube-feet or tentacles (the so-
called oral tentacles) come direct from the circular canal.
5. Crinoidea. Tentacle ampullae are wanting 1 . The radial
canals lie close under the food grooves of the disc, of the arms, and of
the pinnulse, whose courses they exactly follow, so that they branch just
vni ECHINODERMATA WATER VASCULAR SYSTEM 431
as often as do the arms and their food grooves. Their course is more
or less markedly zigzag, and they give off at the angles thus formed
(i.e. alternately) lateral tentacle canals. Each of these latter runs to
a group of three small tentacles at the edge of the food groove, and
here divides into three canals, which enter the three tentacles and
form their cavities.
Tentacle canals are wanting in all cases where food grooves are
wanting, which is the case in Actinometra over a great part of the
arms, and in some species of Antedon in certain proximal pinnulse of
the arms.
All authors agree in maintaining that the inner epithelium of the water vascular
system in the Crinoids differs from that in all other Echinoderms in not being
ciliated. A band of longitudinal muscle fibres runs in the wall of the canals along
the side turned to the food groove. The lumen of the canals is at certain points (i.e.
where the tentacle canals branch, or at the commencement of these canals) traversed
by muscular fibres. This arrangement perhaps fulfils the function of the valves
found in other Echinoderms.
D. The Ambulaeral Appendages.
(Tube-feet, Tentacles, Feelers, Ambulacra! Papillae, etc.)
1. Holothurioidea. The following facts require first of all to be
emphasised.
a. In all Holothurioidea, a smaller or greater number of ambulacral
appendages (10-30) are developed as tentacles near the mouth.
b. The Synaptidce and Molpadiidce have no ambulacral appendages
except these tentacles.
c. In all other Holothurioidea besides the tentacles there are
tube-feet (and papillae) varying greatly in number (often very
numerous), in structure, and in arrangement.
d. These tube-feet (and papillae) are found either only on the
radii, one or two or more longitudinal rows being arranged in each
radius, or else they are distributed, usually in an irregular manner,
over some or all of the interradii. The arrangement of the tube-feet
is not of great systematic importance, since even within one and the
same genus (e.g. Cmumaria), all the intermediate stages between a
strictly radial and an altogether scattered arrangement can be
observed.
e. Where the ventral and the dorsal surfaces are distinctly differ-
entiated, the ambulacral appendages are developed on the ventral side
(in the trivium), normally as loeomotory tube-feet with sucking discs
supported by perforated plates : on the dorsal side, on the other
hand, they take the form of conical non-locomotory papillae, which
have either a rudimentary perforated plate at the narrow tips or none
at all.
432 COMPARATIVE ANATOMY CHAP.
No very sharp distinction between tube-feet and papillae is, however, possible,
either with regard to their distribution, their form, or their structure.
With regard to the number of the tentacles, the following numbers seem to
prevail in the different families : 20 in the Aspidochirotce, 20 in the sub-family
Deimatidce of the Elasipoda, 15 in the Molpadiidcc, 13-16 in the Pelagothuridce, 12
in the Synaptidce, and 10 in the Dendrochirotce and in the sub-family Elpidiidce-
of the Elasipoda.
With regard to form : the tentacles are feathered (Molpadiidce Synaptidce, Fig.
229, p. 288), dendriform (Dendrochirotce, Fig. 226, p. 287), and shield - shaped
(Aspidochirotce, Elasipoda}. In the latter, the disc or shield, the edge of which may
be more or less deeply indented, is carried by a stalk.
The size of the tentacles has already been sufficiently indicated in the systematic
review.
The relation between the arrangement and size of the tentacles on the one hand
and the symmetry of the rest of the body on the other is interesting. In the
Dendrochirotce, (cf. Fig. 226, p. 287), of the ten tentacles, the two ventral are almost
always distinguished by being much smaller than the rest.
In many species of Myriotrochus, Synapta, and CMrodota with twelve tentacles,
these are distributed symmetrically as follows : three occur in each of the two
dorsal interradii, and two in each of the three ventral interradii.
The tentacles may be swelled and extended ; and^on the^ other hand they can be
withdrawn into the body cavity together with the surrounding anterior part of
the body, although not invaginated like the tentacles of a Gastropod.
2. Eehinoidea. Ambulacral feet are developed in all Echinoids
without exception. In early youth, they are always found to resemble
one another, and in both the Echinidce and the Pourtalesiidce this
is still the case in adults, the former having tube-feet with terminal
sucking discs and the latter tube-feet with rounded ends. In most
Eehinoidea, on the contrary, more or less marked polymorphism
occurs, division of labour taking place between the ambulacral
appendages of one and the same individual.
This polymorphism is not very striking in the regular Eehinoidea,
e.g. the Cidaroida, Echinothuridce, Diadematidce, Arbaciidce, Echinometridce,
etc. In these, the ambulacral appendages appear, as a rule, in three
different forms : (1) as loeomotory tube-feet with terminal or sucking
discs ; (2) as tactile or branchial tentacles without terminal sucker ;
and (3) as oral or sensory feet with bi-lobate terminal disc.
All these tube-feet are connected by means of double pores with their ampullae,
which lie w r ithin the test. Without detriment to their principal function, they
may all act as respiratory organs, since the presence of the double pore allows of
a circulation of the ambulacral fluid between the inner ampulla and the outer
ambulacral foot ; the fluid in the foot takes in oxygen, carries it back into the
ampulla, and gives it off through the wall of the ampulla to the fluid in the body
cavity.
The loeomotory tube-feet are found on the oral hemisphere of the body, but
may also occasionally occur on the apical hemisphere as well.
The tactile or branchial tentacles are limited to the apical hemisphere. They
are specially suited for respiratory purposes when the ampullae are large, and have
thin and delicate walls containing no calcareous corpuscles.
vin ECHINODERMATA WATER VASCULAR SYSTEM
433
The oral tube-feet (always ten in number ?) surround the mouth, and especially
when food is being taken in, are subject to active swinging or pulsating movements;
without, however, touching the food. They have been regarded as olfactory or
gustatory organs. They seem to be wanting in the Cidaroida and the Uchinothuridce ;
on the other hand they occur in those Eehinidce which otherwise possess only one
sort of tube-feet, viz. those with sucking discs.
The polymorphism of the ambulacral appendages is much more
marked in the Cbjpeastroida and the Spatangoida. It must first be
noted that the ambulacral appendages, in those apical regions of the
ambulacra which are known as petaloids (cf. p. 347), serve for respira-
tion (ambulaeral gills). They seem to be peculiarly fitted for this
activity by the delicacy of their walls, the want of calcareous corpuscles,
the increase of surface obtained by branching, the possession of
double pores (whereas the ambulacral appendages in other parts of the
body have single pores) and by the size of their ampullae.
In the Clypeastroida t besides the ambulacral gills of the petaloids, three kinds of
appendages have been observed : (1) the ordinary slender tube-feet, with rounded
terminal knobs, scattered on the test ; (2) sessile knobs, with deep sensory
FIG. 366. Longitudinal section through an ambulacral brush of a Spatangoid (after Loven
and Hamann). 1, Body epithelium ; 2, supporting rod ; 3, supporting plate of the terminal disc ;
ta ; 5, canal of the water vascular system ; 6, longitudinal muscles ; 7, nerve ; 8, circular
muscle fibre*.
epithelium (sensory tentacles) ; (3) short, thick tube-feet with truncated ends : these
occur between the ordinary feet on the oral side, and are perhaps locomotory.
Among the Spatangoida, the polymorphism of the ambulacral appendages is very
marked in all the divisions except in the Echinoncidce ; it reaches its highest point
in the families of the Spatanyidcc and Apetala.
The ambulacral gills of the four paired petaloids have been described above.
We note first the characteristic ambulacral brushes which occur in the Spatangoida
more or less near the mouth and the anus, and in the Cassiduloida on the phyllodes
(cf. p. 347). The terminal plate or disc of an ordinary tube-foot (Fig. 366) is here
extraordinarily widened, and carries a number (usually large) of club-shaped or
conical, solid appendages, each of which is supported by a calcareous rod. These
ambulacral brushes are said to play an important part in the taking in of food by
VOL. II 2 F
434 COMPARATIVE ANATOMY CHAP.
stirring up the sand. On other parts of the ambulacra, slender tentacles without
prehensile discs occur, to which a tactile function has been ascribed. Still more
interesting are the ambulacral appendages of the anterior unpaired ambulacrum,
which are certainly to a still higher degree tactile organs. These vary in shape ; in
all young Spatangoida and many adults they are distinguished by their remarkable
size, and help to emphasise the bilateral symmetry of the whole body. In Spatangi'n
and other genera they end in a flat disc, the edge of which is drawn out into short,
solid, knobbed processes, which are supported by calcareous rods. The whole
terminal disc thus looks like a beautiful rosette.
As compared with the ordinary tube-feet, truly gigantic proportions are attained
in the genera Aceste and JErope by the ambulacral appendages, which lie in the
depressed anterior ambulacrum within the peripetaloid fasciole. They are found
only in small numbers, yet in their contracted condition they almost completely fill
the depression from whose base they rise. Their ends are provided with large discs.
Turning to the finer structure of the ambulacral appendages of the Echinoidea,
their wall is found to consist of the typical layers. In the locomotory tube-feet of
the regular Echinoids, the inner layer of the connective tissue is specially modified
as an elastic membrane, with circular fibres. Calcareous corpuscles are wanting
only in the respiratory tentacles of the apical surface of the body. In all other parts
of the body they are found in great numbers in the stalk, while in the terminal discs
of the tube-feet they take the form of delicate, circular, terminal plates usually com-
posed of several pieces. The whole surface of the tentacles, except that of the terminal
discs, is ciliated. A nerve enters each tube- foot, running in the epithelium until
near the tip, Avhere it forms a lateral ganglion, which can be externally recognised as
a swelling. From this ganglion, the terminal apparatus of the tube-foot is innervated.
In tube-feet with terminal discs, the epithelium at the edge of the disc is differen-
tiated as a deep sensory epithelium, and within it runs a basal nerve ring, connected
with the lateral ganglion by two nerves. Where the tentacles end in knobs (tactile
tentacles and sessile knobs of the Clypeastroida) or carry knobbed processes on their
terminal discs (ambulacral brushes, rosette-like tube-feet of the anterior unpaired
ambulacrum of the Spatangoida] these knobs are caused by the great thickening of
the sensory epithelium. In the ambulacral gills of the Clypeastridce (Echinocyamus,
Echinodiscus) the epithelium occasionally thickens to form sensory papillae. The
sensory epithelia appear everywhere to carry stiff sensory setfe or hairs.
The lumen of the tube-feet at its central and basal parts is not infrequently
traversed by transverse muscle fibres ; occasionally it appears to be double a septum
consisting of transverse bands continues for some distance into the tube-foot, the
partition in the test between the two apertures of the double pore. The cavity of the
large terminal discs of the paint-brush tentacles in the Spatangoida is traversed by
concentric, much-perforated septa (Fig. 366). The ambulacral gills and their ampullae
are traversed by bands arranged radially around an axis, in such a way that the
fluid contained is forced to circulate at the periphery.
3. Asteroidea. The ambulacral appendages always take the form
of tube-feet, and stand in two or four longitudinal rows in the ambu-
lacral furrows which run from the mouth to the tips of the arms. It
has already been pointed out (p. 353) that the tube-feet, even when
apparently present in four rows, in reality belong only to two. That
there are only two rows is very clear in young animals. In young
Asteroids all the tube-feet are alike ; all have conical ends, with
rounded tips. This is still the case in many adult Asteroids (Astro-
VIII
ECHIXODERMATA WATER VASCULAR SYSTEM
435
pecten, Luulin, etc.), while in very many other genera (e.g. Asterias,
Solaster) a slight difference in shape occurs. Only the tube-feet near
the ends of the arms retain the primitive shape, while a well-developed
disc is found on all the rest. The former then function chiefly as
tactile tentacles.
The wall of the tentacles shows the typical layers. In the tactile tube-feet, the
epithelium at the conical end is much thickened, and contains very numerous
sensory cells. Within the epithelium, a layer of nerve fibres is developed ; these
run from the base to the tip of the foot. The layer is specially strongly developed
i
10
FIG. 367. Portion of the disc of Hemipholis cordifera, from the oral side (after Lyman). 1,
oral tube-feet ; 2, buccal shields ; 3, jaw = oral-angle plates ; 4, lateral buccal shields ; 5, first veutral
shield ; 6, oral integument, lip ; 7, spines on the marginal plates ; 8, retracted tentacle ; 9, tentacle
scale ; 10, ventral shields ; 11, extended tentacle ; 12, tentacle pore ; 13, tonis angularis ; 14, teeth.
within the terminal sensory epithelium. A similar deep sensory epithelium, consist-
ing of sensory, supporting, and glandular cells, also covers the sucking discs of the
other tube-foot ; these discs have a depression at their centres, while, round their
edges the nerve tissue lying within the epithelium becomes thickened into a nerve
ring.
From the centre of each sucker, radial muscle fibres run out towards the periphery
and are attached round the ambulacral canal which ends below the sucker. These
muscles, by their contraction, cause the sucker to adhere. They are entirely dis-
tinct from the longitudinal muscles of the appendage, which explains the fact that
a tube-foot sticking to an object may be cut off without becoming detached.
This applies almost equally well to the sucking discs of the tube-feet of the
Echinoids.
436 COMPARATIVE ANATOMY CHAP.
4. Ophiuroidea (Fig. 367). In the Ophiuroidea, the ambulacral
appendages have no locoraotory significance ; they resemble tentacles,
and never have suckers. Locomotion is caused by the jointed arms
themselves. The tentacles are always strictly segmentally arranged, i.e.
one pair of tentacles occurs on each brachial segment. Each of these
emerges through an aperture between the ventral shield and the lateral
shield of a segment. The tentacles are not infrequently covered with
a large number of sensory papilla. A nerve, coming from the basal
circular ganglion and running in the deeper portion of the layer of
connective tissue, traverses the tentacle from base to tip.
It has already been mentioned above that the first ten pairs of
tentacles (i.e. the first two pairs of each arm) have shifted, as oral
tentacles, to a position round the mouth, and receive their canals direct
from the circular canal.
5. Crinoidea. The small tentacles which form the ambulacral
appendages of this class have already been sufficiently noticed (p. 414).
They never possess suckers, and have no locomotory function, but
simply serve for respiration, and for conducting food to the mouth.
VII. The Ccelom.
(The Enterocoel. the true or secondary Body Cavity.)
All those cavities of the body which are derived from the entero-
eoelomie vesicles of the larva are considered to belong to the coelom,
which is lined throughout with endothelium, usually developed as
ciliated epithelium. The ecelomie fluid exactly resembles in constitu-
tion the water vascular fluid already described. The ccelom is, how-
ever, except at one single point to be mentioned later, altogether
separate from the ambulacral vascular system.
The coelom is never found as a single cavity, but is always divided
into several cavities, which may be entirely distinct one from the
other. The largest of these cavities is that which contains the viscera,
and which may be termed simply the body cavity.
The body cavity is most spacious in the Echinoidea and the Holothu-
rioidea ; in these forms it occupies almost the whole cavity of the test,
or the sac- or tube-shaped body. In the disc of the Asteroidea it is
somewhat less spacious, and is very limited in that of the Ophiuroidea.
In the Crinoidea, it is traversed by a more or less strongly calcified
network of connective tissue.
Where the body is drawn out into arms in the radii, the body
cavity runs into these, and forms the braehial cavities. The brachial
cavities in the Asteroidea are very spacious, but are much narrowed in
the Ophiuroidea and the Crinoidea, owing to the great development of
skeletal plates (vertebral ossicles, joints) in the arms.
A special section of the ccelom, the pericesophageal sinus (peri-
pharyngeal sinus) encircles the oesophagus or pharynx. In the Echi-
ECHINODERMATATHE CCELOMIC CAVITIES 437
f, this is quite cut off from the body cavity. The membrane
which separates the body cavity from the perioesophageal sinus is
called, in those Echinoids which are provided with a masticatory frame-
work (Cidaroida, Diadematoidu, and Clypeastroida), the lantern mem-
brane. This membrane entirely covers the lantern on the side turned
to the body cavity.
In many Echinoids, this part of the ccelom protrudes externally
at the edge of the peristome, forming the outer gills : in others,
the lantern membrane bulges out into the body cavity, and forms
Stewart's organs.
In the Holothurioidea and Echinoidea, the hind-gut is surrounded by
a small ccelomic sinus, the perianal sinus.
In the E'-hinoidea,Asteroidea, and Ophiuroidea, a part of the coelom,
cut off from the rest, runs from the region of the madreporite inter-
radially to the circular canal of the water vascular system. This is
the axial sinus in which the stone canal runs. It contains also a
lymph gland, the so-called ovoid gland or axial organ.
The axial sinus, in the Echinoids, is in open communication with
the ampulla which lies beneath the madreporite. Recent ontogenetic
researches have shown that this ampulla also is of enteroccelomic origin,
and is thus a section of the coelom. Since the stone canal opens into
the ampulla, an open communication exists at this point, and at this
alone, between a closed division of the coelom (the axial sinus) and a
section of the water vascular system (the stone canal).
A. The Body Cavity.
1. Holothurioidea. The spacious body cavity of the Holothurioidea
is divided up by the mesentery which attaches the intestine to the
body wall. This mesentery may be described as having three parts
corresponding with the three sections of the intestine, viz. the dorsal
mesentery, belonging to the first section of the intestine which runs
backward ; the left dorsal mesentery, belonging to the second
section, which bends forward ; and the right ventral mesentery,
belonging to the third section, which runs backward to the cloaca.
All three parts of the mesentery lie interradially.
The so-called ciliated urns or funnels (Fig. 368) are found only
in the Synaptida. These are funnel-, cup-, or slipper-shaped organs,
each of which is attached to the body wall or the mesentery by a stalk,
and hangs down freely into the bod} T cavity. They are specially
numerous to the right and left of the dorsal mesentery. In Chtrodota,
many funnels have one common stalk, and so form trees of ciliated
funnels.
The funnel consists of three layers, an outer endothelial tessellated epithelium, a
middle and extremely thin layer of connective tissue, and an inner layer of columnar
epithelium, which lines the lumen and carries long cilia. Towards the stalk
the lumen is closed, but it is open towards the body cavity.
438
COMPARATIVE ANATOMY
CHAP.
The vigorous movements of the ciliated urns no doubt serve to promote the
streaming and circulation of the fluid in the body cavity.
2. Eehinoidea. In the Echinoids, the body cavity is partitioned
in a manner similar to that described for the Holothurioidea, by mesen-
teries which follow the intestine
in its windings (see p. 480), and
attach it to the inner surface of
the test. The genital organs
are also attached to the test by
mesenteries. In regular Echin-
oids, the mesenteries are much
perforated, but are only slightly,
if at all, broken through in the
Spatangoida.
In this latter order, where
the mesenteries have to carry
the heavy intestine, filled with
sand, they are specially strong
and tough. The coils of the
intestine are here also united
inter se by mesenteries. Special
bands attach the intestine to the
apical and oral poles of the test,
Fir 368 -Ciliated urns of a Synaptid (after interna l processes or apophyses
Cuenot). 1, mesentery ; 2, circular muscle layer , . . , i j *
of the body wall ; 3, ciliated inner epithelium of being Sometimes developed lor
the urn; 4, endothelium of the body cavity; the attachment of the bands.
Two ^ h apophyses are found
at the apical pole, at the end of
the stone canal, and a third not infrequently occurs at the peristome,
in an interradius.
The axial sinus, with the axial organ and the stone canal, is at-
tached by bands on the one hand to the apical pole, and on the
other to the oesophagus.
For a description of the calcareous pillars, septa, etc., which, in the Clypeastridce,
traverse the cavity of the test, see p. 405.
In the fluid of the body cavity in Echinoids there are found, besides blood cor-
puscles, great numbers of spermatozoa-like cells, with long flagella in vigorous move-
ment. These may set up currents in the fluid of the body cavity.
3. Asteroidea. The body cavity of the disc is not spacious, the
greater part of it being filled by the large digestive sac. Mesenteries
are wanting in the greater part of the intestine, or are only developed
as isolated filaments or strands of connective tissue. In the peripheral
portion of the disc, radially placed bands or septa traverse the body
cavity vertically in the interradii, connecting the dorsal (apical) body
wall with the ventral (oral) wall.
UNI
ECHINODRMATATHE CCELOMIC CAVITU
The lymph gills, branchial vesicles or papulae, which are only
found in the Asteroids, deserve attention. These are small vesicular
bulgings of the body wall, which occur in great numbers between the
skeletal plates. On these bulgings, the body wall is very thin, and, in
order to facilitate osmosis, devoid of calcareous deposits. It consists
of layers similar to those found in other parts of the body : an outer,
strongly ciliated and glandular epithelium ; a middle layer of con-
nective tissue, containing longitudinal and circular muscle fibres ; and
an inner ciliated epithelium, which is nothing else than the endothelium
of the body cavity. The cavities of the papulae are merely diverticula
of the body cavity which bulge out the much-thinned body wall.
The papulae are sensitive, and contract at the slightest touch.
In Asteroids with very thick body wall, diverticula of the body cavity force their
way into it, branching on their way to the surface. On reaching this latter, each
branch enters a branchial vesicle.
In certain forms, a single diverticulum traversing the body wall supplies a whole
group of branchial vesicles.
A constant streaming to and fro of the body fluid can easily be observed in the
branchial vesicles.
Each branchial diverticulum of the body, cavity is surrounded, in the connective
tissue layer of the body wall, by a circular lacuna.
The branchial vesicles occur both 011 the arms and on the disc. In the Phanero-
:S5^6 nm through the (calcined) cutis to
the surface of the body. The
manner in which these nerve fibres
terminate is unknown. Investiga-
tion on this point is the more
difficult as the epithelium appears
to be hardly distinguishable from
the cutis.
At the edges of the food grooves
(on the arms and the oral disc) of
the Crinoidca, alternating with the
trilobed tentacles, groups consisting
of five to six sensory cells with
delicate immobile hairs, occur.
Among the Holothurioidea, a
system of nerve fibres ramifying in
I) . \ the cutis has been described in
'X,^ ,.-. Cucumaria. From these branches
run to the nests of epithelial cells
FIG. 375. Half of a transverse section through , *
an ambulacral tentacle of Ophiothrix fragilis (com- sunk below the surface, which were
bined from figures by Hamann). 1, Body epithelium ; mentioned in connection with the
2, sensory papillse ; 3, cuticular connective tissue
4, nerves to the sensory papillae; 5, longitudinal mus-
culature ; 6, epithelium of the tentacle canal (7) ;
8, tentacle nerve.
integument, p. 415. A similar
arrangement has been found in
other Actinopoda.
In the Paractinopoda (Synapta,
Anapta] numerous scattered sensory or tactile papillae are found on the integument,
which, at such points, bulges out to form prominences. At the centre of such a
prominence a group of sensory cells forms the tactile papilla. A distinct nerve runs
from each papilla to a large tactile ganglion lying in the cutis beneath it. The
epithelial cells surrounding the papilla are differentiated into glandular cells
inhaerens).
ECHINODERMATA SENSORY ORGANS
467
C. Auditory Organs, Organs for Orientation.
Two types of organs for hearing or for orientation have been
observed in Echinoderms : (1) the auditory vesicles (Baur's vesicles,
otoeysts) of certain Holothurioidea ; and (2) the sphseridia of the
Eclrinoidea, which have already been described.
1. Auditory vesicles are found only in the Holothurioidea, and
among these in the Paradinopoda (Synaptidce), and among the Actino-
poda in the Elasipoda.
They have been best observed in the Synaptidce (Fig. 376). In
this family, five pairs of these vesicles occur in the cutis of the body
wall, near the tentacles, at the points where the five radial nerve
trunks emerge from the calcareous ring. On the outer side of each
7 ff
FIG. 376. Section tlirough the two auditory vesicles of a radius of Synapta ''after Cuenot).
1, Epineural sinus ; 2, epithelial wall of the auditor}* vesicle; 3, otoliths; 4, nervus acusticus ;
5, longitudinal muscles ; 6, pseudohaemal canal ; 7, radial nerve trunk.
nerve trunk lies a pair of otoeysts. Each otocyst consists of a vesicle
filled with fluid, with a wall of (ciliated) plate epithelium. Numerous
otoliths are found vibrating in the fluid. These otoliths are vesicular
cells, the cavity of each being filled by a hard mass (phosphate of
lime). The nerves to the otoeysts (nervi acustici) come from the
radial nerve trunk.
The auditory vesicles of the Elasipoda occur in great numbers ;
there may be fourteen to one hundred or even more. Xot infrequently
their manner of distribution is bilaterally symmetrical. For instance,
in Elpidia glacialis, six of the fourteen auditory vesicles occur on the
two lateral radii of the trivium, and one on each of the two radii of
the bivium. In this case no vesicles occur in the ventral median
radius,
2. The sphaeridia of the EchinoiJea, which are regarded as
468
COMPARATIVE ANATOMY
CHAP.
transformed spines, have already been described in the section on the
skeletal system (pp. 392-3). According to recent researches, there is
only a loose connection between the sphaeridium and the shell tubercle,
fibres of connective tissue, and not muscle fibres, uniting them. When
an Echinoid is in the natural position, the sphseridia, which are
developed only on the oral side, hang down perpendicularly in their
niches or chambers, owing to the weight of the dense calcareous mass
which forms their rounded ends. They are thus able, by pressing on
the nerve cushion at their base, to orientate an animal as to its position
in space. Echinoids which are laid on the back, quickly turn them-
selves over again.
D. Eyes.
1. The eye-spots of Asteroids have already been mentioned
(p. 463) in connection with the terminal tentacles. A vivid red eye-
FIG. 377. Section through the optic cushion at the base of the terminal tentacle of an
Asteroid. 1, Cuticle of the optic cup ; 2, pigment cells ; 3, cuticle of the tentacle epithelium (4) ;
5, nerve layer below the surface of the same ; 6, cutis of connective tissue ; 7, epithelium of the
tentacle canal.
spot is found at the base of each of these tentacles, on the side turned
to the mouth. On closer examination each eye-spot is found to break
up into a large number of single eyes, shaped like cups or hollow
cones. The tips of these conical cups are directed inward towards
the highly developed layer of nerve fibres below the surface of the
tentacle epithelium, while their cavities open outward (Fig. 377). The
wall of each optic cup is formed of pigment cells (with interspersed
urpigmented retinal cells). The cuticle of the tentacle epithelium
VIII
ECHINODERMATA SENSORY ORGANS
469
also is continued into each cup. The portions of the cuticle which
belong to the different cells forming the wall of the cup are distinct
from one another, and have been described as rods.
Living Asteroids carry the arm with its tip directed upwards, so
as to enable the eye-spot to function.
2. In Diadema setosum (Euechinoidea diadematoida), an animal
highly sensitive to light, the skin, which is as black as velvet, is
FIG. 37S. Part of a compound eye of Diadema setosum (after P. and F. Sarasin).
p, Pigment cups.
ornamented with numberless shiny blue spots ; these are gradually
lost on the oral side.
Each of these blue spots, when its surface is examined with the
microscope, breaks up into a number of pentagonal or hexagonal
portions. The number of these varies, according to the size of the
spot, sometimes being many hundreds. Each is a refractive body,
which stands in a cup of black pigment (Fig. 378). The blue colour
FIG. 379. Section through the eye of Diadema setosum, diagrammatic (after P. and F. Sara-
sin). 1, Layer of ganglion cells; 2, "cornea" ; 3, refractive body ; 4, pigment cups; 5, nerve
layer, or plexus ; 6, fibres of connective tissue ; 7, collection of pigment below the nerve layer.
of the spots, which are regarded as compound eyes, is due to inter-
ference.
A section made through such an eye (Fig. 379) reveals : (1) that
the body epithelium, which is covered by a ciliated cuticle, spreads,
much thinned, over the whole eye (cornea) (2) that each " refractive
body " consists of a number of vesicular cells (modified epithelial
cells) ; (3) that a pigment cup (consisting of cells which are often
branched and star-like) surrounds the basal portion of each refractive
body ; (4) that the whole eye, with its numerous pigment cups, rests
470 COMPARATIVE ANATOMY CHAP.
directly upon a nerve layer provided with ganglion cells, which, at
the edge of the eye, passes into the usual layer of nerve fibres, found
in all Echinoids below the surface of the body epithelium.
Similar spots are found in other Diadematidce, and species of the related genus
Astropyga.
In the Synaptidce (S. vittata), at the base of each tentacle, two pigment spots
occur. It has now been proved that these " eyes " are sensory organs, but a detailed
account of them has not yet been given.
XIII. The Body Musculature.
The special development of the body musculature of the Echino-
derms is directly connected with the peculiar development of the
skeleton.
Regarding the musculature and the skeleton alone, the Echino-
derms may be divided into three groups.
Holothurioidea. The skeleton here consists merely of isolated,
and usually microscopically small, calcareous bodies. The presence of
these in the leathery integument does not prevent changes of form in
the tubular body. The body musculature is developed as a dermo-
museular tube. By means of co-ordinated contractions of the
circular and longitudinal musculature of which this tube is com-
posed, the animal is able to make slow, worm-like movements.
Asteroidea, Ophiuroidea, and Crinoidea. The body in these
classes is drawn out in the direction of the radii in the form of
arms, which are supported by a jointed skeleton. The dermo-
museular tube breaks up into separate muscles, which connect the
joints of the skeleton together. In the Asteroidea alone, a dermo-
muscular tube persists side by side with these isolated muscles.
Eehinoidea. The skeleton of the Echinoidea (with but few
exceptions) is a rigid test or capsule. A body musculature would
here be useless, and is therefore wanting.
An exception to this rule is found among the Euechinoidea in the
Streptosomata, in which the plates of the more or less flexible test
imbricate. It has now been proved that in the fichinothuridce, five
pairs of muscle lamellae run in meridians from the oral to the apical
side on the inner surface of the test. The contraction of these
muscle lamellae causes a depression of the test.
The musculature of the Echinodermata consists, as a rule, of smooth muscle fibres.
A longitudinally striated fibril of contractile substance lies on one side of the undiffer-
entiated protoplasm of the formative cell which contains the nucleus. Transversely
striated muscle fibres are of less frequent occurrence, but are found in the adductor
muscles of the seizing pedicellarise in the Echinoidea (p. 398), and in the muscles of
the rotating anal spines of Centrostephamis longispinus.
The musculature of the various organs or systems of organs will be described in
the sections dealing with those organs.
ECHIXODESMA TABOD Y MUSCULA TURE 4 7 1
A. Holothurioidea.
The dermomuscular tube consists everywhere of an outer circular
muscle layer, and of five radial longitudinal muscles (Figs. 371 and
3 S3, pp. 451 and 477).
The circular musculature lies immediately within the cuticle. It
is usually interrupted in the five radii, and thus consists of five longi-
tudinal interraclial strips or bands of muscle, the fibres of which run
transversely. In the Paradinopoda (Synaptidce) alone, where there are
no radial canals of the water vascular system, the fibres run uninter-
ruptedly round the body.
The longitudinal musculature consists of five strong muscles or
pairs of muscles traversing the body longitudinally along the radii.
These muscles thus cover, on the side of the body cavity, the radial
organs enumerated on p. 409. Anteriorly (at the oral pole) they are
inserted into the radial pieces of the calcareous ring, posteriorly (at
the apical pole) they end near the anus.
In the DendrochirotcRj the longitudinal musculature is differentiated
in a peculiar manner. At the middle of the body, or somewhat in
front of it, the fibres of each of the five longitudinal muscles divide
into two bundles. One of these bundles is continued simply as a
longitudinal muscle along the body wall, while the other freely tra-
verses the bod}' cavity, and is attached anteriorly to a radial piece of
the calcareous ring (Fig. 349, p. 404). The retractor muscles of the
oral region have in this way been derived by the splitting up of the
originally simple longitudinal muscles, and this specialisation became
more marked as the oral tentacles of the Dendrochirotcf, became more
and more highly developed, and required increasing protection.
Species are to be found in which the separation and branching off of
retractors from the longitudinal muscles has not yet been perfected.
Apart from the Demt rock i rot ce, retractors occur only in the genus
Molptttlia, and in species of the genera Clrirodota and Synapta.
B. Echinoidea.
The longitudinal muscles of the Eclrinothuridce (Asthenosoma) have
the shape of semilunar leaves, the convex sides of which are directed
outwards, and are attached to the inner surface of the test; the
concave edges, on the other hand, face the axis of the test (Fig. 380).
They are inserted into the test at the boundaries between the
ambulacra and the interambulacra, at the lateral edges of the ambu-
lacral plates.
In each muscle lamella, the fibrous bands radiate fan-like (Fig. 380) from a
"centrum tendineum " on the inner edge of the lamella. The uppermost fibres are
attached to the radials, the lowermost to the outer side of the auriculae. Anasto-
moses between the fibres are not infrequent.
472
COMPARATIVE ANATOMY
CHAP.
The five pairs of longitudinal muscles or pairs of muscle lamellae in the Echino-
thuridce invite comparison with the five longitudinal muscles or pairs of muscles in
the Holothurioidea. No true homology can, however, be proved with certainty,
Fig. 380. Test of Asthenosoma, broken open so as to show the longitudinal muscles. 1, In-
terambulacral plates ; 2, ambulacral plates ; 3, radial canals ; 4, centrum tendineum ; 5, muscle
bands ; 6, ambulacral apophysis (auricula) ; 7, opening muscle of the teeth ; 8, retractors of the
masticatory apparatus.
since no direct relation between the calcareous ring of the Holothurian, and any
definite portions of the Echinoid skeleton (such as the auricules, or the pieces of
the masticatory apparatus) can be established.
C. Asteroidea.
On the apical side of the arms and of the disc, a dermomuscular
layer has been observed, which consists of external transverse, and
internal radial fibres, and runs lengthwise in the arm. This does not
appear to spread (as a muscle layer) to the oral side of the body, where
the ambulacral skeleton is developed. It may perhaps, however, have
become differentiated here into the special musculature of the ambu-
lacral skeleton.
This latter is developed as follows :
Ten muscles occur in each skeletal segment.
1. On each side two vertical muscles (or bands), one distal and the other
proximal, connect the adambulacral with the ambulacral plate (cf. Fig. 309, p. 351).
2. On each side an upper longitudinal muscle connects every two consecutive
ambulacral plates on their apical side (that turned to the body cavity). The func-
tion of these muscles is to bend the arm upward (Fig. 381, 2 and 7).
3. On each side a lower longitudinal muscle connects every two consecutive
adambulacral pieces ; this muscle counteracts the upper longitudinal muscle.
ECHINODERMATABODY MUSCULATURE
473
4. An upper transverse muscle connects the two opposite ambulacral plates of
one and the same skeletal segment, on their apical side (that turned to the body
cavity). These muscles, by their contraction, widen the ambulacral furrow (Fig.
382, 3 and 6).
5. A lower transverse muscle connects the two ambulacral plates of a segment
on the lower side, which is turned to the furrow. These muscles, by their contrac-
tion, narrow the furrow.
The musculature of the oral skeleton (Fig. 381) is arranged as follows :
FIG. 381. Oral skeleton and basal part of the brachial skeleton of Pentaceros turritus,
with the musculature of these parts (after Viguier). From within. 1, First adambulacral plates ;
I-IV, first to fourth ambulacral plates ; or, orals ; 2, dorsal longitudinal muscles ; 3, dorsal trans-
verse muscles (for opening the ainbulacral furrow) ; 4, interbrachial pillars ; 5, muscle apophyses
of the first adambulacral plates ; 6, facets of the ambulacral plates for the attachment of the dorsal
transverse muscles ; 7, ditto for the attachment of the dorsal longitudinal muscles ; 8, transverse
muscles between the first adambulacral plates (teeth) ; 9, dorsal transverse muscles between the first
pair of ambulacral plates ; 10, dorsoventral muscle ; 11, stone canal ; 12, crossed ligament ; 13, ab-
ductor dentium ; 14, adductor dentium ; o, aperture for the first ambulacral feet.
1. A single or double radial muscle connects the distal ends of the first adam-
bulacral plates (teeth, 1 in Fig. 310, p. 352) of one and the same radius (Fig. 381, 13),
and opens these plates.
2. A muscle, which connects the distal ends of the first two adambulacral plates
of two neighbouring radii, and is therefore interradial. These interradial muscles,
by contracting, close the pairs of teeth (Fig. 381, 14). This closing action
is assisted by a transverse muscle, which joins the opposing edges of each pair of
teeth (8).
3. The first pair of ambulacral plates of a radius, like all succeeding pairs, are
connected together by means of a dorsal transverse muscle (Fig. 381, 9).
474 COMPARATIVE ANATOMY CHAP, vm
4. Five pairs of dorsoventral muscles connect the first five pairs of ambulacral
plates with the dorsal wall of the disc. By the contraction of these, the apical wall
of the disc is approximated to its oral wall (Fig. 381, 10).
D. Ophiuroidea.
A dermomuscular tube is altogether wanting. The muscles which
move the brachial skeleton (the intervertebral muscles) have already
been described, p. 357.
The musculature of the oral skeleton (cf. Figs. 314 and 386, pp. 359 and 486).
1. Amusculus interradialis externus connects transversely the opposite distal
surfaces of the oral-angle plates of neighbouring arms. This is the most powerful
of the muscles.
2 and 3. The two oral-angle plates of one and the same arm are connected
by an upper and a lower transverse muscle (musculus radialis superior et inferior),
and approximated by means of their contraction.
The three muscles just described form an outer circle, which is followed, orally,
by a second inner circle, consisting of the following muscles :
4. A musculus interradialis internus inferior connects transversely the oral
ends of the oral-angle plates of the different arms.
5. The innermost muscles of the oral skeleton consist of fibres which radiate out-
wards. They run as five interradially placed pairs of muscles, from the oral-angle
plates to the teeth (in Ophiactis only to the upper teeth), for whose movement they
serve. These are the musculi interradiales interni superiores.
E. Crinoidea.
A dermomuscular tube is wanting. The musculature which moves
the jointed skeleton has already been described, p. 376.
XIV. The Alimentary Canal.
A. General Review.
The alimentary canal, which runs through the body cavity, being
attached or suspended to the body wall in various ways by means of
mesenteries or mesenterial filaments, commences with the mouth and
ends with the anus.
The absence of the anus in the Ophiuroidea and in the Asteroid
family Astropedinidce cannot be regarded as an original condition.
In no ease does the alimentary canal run as a straight tube from
mouth to anus, although, in many Synaptidce, its condition -is almost
as simple. As a rule, the secreting and resorbing surface of the canal
is increased in one of two ways :
1. The alimentary canal, between the mouth and anus, becomes
increasingly lengthened, and thus necessarily forms loops, and has a
winding course (Holothurioidea, Echinoidea, Crinoidea).
R
Hblofhuricr
JTntedon, Pentacrmus.
anas
x anus
FIG. 382. Diagram of the course of the alimentary canal in various Echinoderms. The
body is viewed from the oral side, rnd, ms, Mesenteries ; as, food-grooves or ambulacral furrows
of the tegmen calycis ; m, madreporite ; x, commencement of the backward coil of the intestine ;
ii first, L 2 second (backward) coil of the intestine ; s, siphon, accessory intestine ; 1 and 2 (in F)
the two points at which the accessory intestine enters the principal intestine ; i, free portion of
the accessory intestine ; so, portion of the same in contact with the principal intestine ; coec, in-
testinal ccecum. [Combined from several sources.]
476 COMPARATIVE ANATOMY CHAP, vm
2. The alimentary canal runs direct, without coiling, from mouth
to anus, but has sae-like widening^ (Ophiuroidea), and further, in the
Asteroids, sends off branched outgrowths into the arms.
The wall of the intestine, as a rule, in the Echinoderm, consists of
the following layers : (1) a deep inner epithelium, with numerous
glandular cells ; (2) a layer of connective tissue ; (3) a muscle layer ;
(4) an outer epithelium, the endothelium of the body cavity. A
system of blood lacunae (absorbing canals) is developed in the layer
of connective tissue in the Holothurioidea, Ecliinoidea, and Crinoidea.
B. Holothurioidea (Figs. 371, p. 451, and 383).
Course of the alimentary canal. The mouth lies at the oral
pole (i.e. at the anterior end of the body), the anus at the apical pole.
For the exceptions to this rule, especially ffhopalodina, in which the
mouth lies close to the anus, cf. section III., p. 408.
The alimentary canal is, as a rule, considerably longer than the
body (on an average three times as long), and therefore has a looped
or winding course. From the mouth, it first runs backward towards
the anus (first or anterior section), it then bends for the first time,
and runs forward again (second or middle section) ; lastly, it bends
again near the anterior end of the body, and runs backward once
more, this time to the anus (third or posterior section).
In making these bends, the alimentary canal forms a spiral round
the principal (longitudinal) axis of the body ; this is very clearly
seen by following the line of attachment of its mesenteries to the
body wall.
The anterior section is attached to the median dorsal line interradially. From
this, at the first bend, the mesentery passes across the left dorsal radius into the left
dorsal interradius. The whole of the central section is attached in this interradius.
At the second bend, the mesentery passes over the left ventral radius and interradius,
and over the middle ventral radius, into the right ventral interradius. The third
or posterior section is attached in this latter interradius (Figs. 350, p. 407, and
383).
If a Holothurian is placed upright, with the oral pole upward and
the apical downward, and if we project the loops of the alimentary
canal on to a horizontal plane, or, if we simply view the intestinal
loops of a Holothurian from the oral pole, we see that the digestive
tract runs from left to right, i.e. in the direction of the hands of a
clock. In other Echinoderms with coiled intestine, the coils also run
in this direction.
It was mentioned above that the alimentary canal of many Parac-
tinopoda (Synaptidce) is almost straight. This is, however, not an
original condition, as is seen from the following facts : (1) the older
larva and the quite young Synapta have a bent alimentary canal ; (2)
the intestinal mesentery is inserted in the body wall exactly in the
Fig. 383. Organisation of an Aspidochirotan Holothurian. In the dorsal interradius, the
body wall is cut through on the left, near the dorsal mesentery, and is spread out (after Ludwig, in
Leuckart's Tafelwerk). 1, Genital aperture ; 2, radial plates ; 3, interradial plates of the calcareous
ring ; 4, genital duct ; 5, dorsal or anterior mesentery of the intestine ; 6, stone canals with their
inner madreporites ; 7, dorsal intestinal vessel ; 8, gonads ; 9, anterior section of the alimentary
canal ; 10, ventral intestinal vessel ; 11, posterior section of intestine ; 12, longitudinal muscles ; 13,
posterior edge of the dorsal mesentery ; 14, right branchial tree (water lung) ; 15, circular muscula-
ture of the body wall ; 16, longitudinal muscles ; 17, cut edge of the body wall ; 18, longitudinal
muscles ; 19, partly muscular filaments running from the wall of the cloaca to the body wall ; 20, cloa-
cal aperture ; 21, cloaca ; 22, Cuvierian organs ; 23, left branchial tree ; 24 and 25, longitudinal muscles ;
26, middle section of the alimentary canal ; 27, vascular anastomosis ; 28, fore-gut ; 29, Polian
vesicle ; 30, blood vascular ring ; 31, water vascular ring ; 32, commencement of the radial vessel.
UBS*;
OF VHK
UNIVERSITY
478 COMPARATIVE ANATOMY CHAP.
manner above described, and thus runs in a spiral ; (3) in most cases,
close examination reveals that even in the adult the canal is coiled in
a spiral, although drawn out to a great length, and that the first and
second bendings can still be distinguished as slight curves.
If the typical alimentary canal of an Adinopod were to be
shortened until it was of almost the same length as the body, the con-
dition found in the Synaptidce would arise.
The divisions OP sections of the alimentary canal. In the
intestine of the Holothurioidea, consecutive sections have been
distinguished, but these are never very marked microscopically.
Throughout its whole course, the canal retains its tubular shape.
The different sections are distinguished by their sizes and by the
thickness of their walls, by their colour, their vascularisation, and
especially by their histological structure. The boundaries of the con-
secutive sections are usually externally indicated by circular constric-
tions of varying distinctness; these constrictions not infrequently
correspond with circular folds projecting into the canal.
The mouth. Around the mouth, the circular musculature becomes thickened into
a small sphincter muscle.
The more strongly the oral tentacles are developed, the more marked is the
capacity for invaginating the mouth with its tentacles, and with a larger or smaller
portion of the anterior end of the body, into the body cavity. In the Dendrochirotcc,
in which the tentacles are strongly developed, the invaginable portion of the anterior
end of the body is called the proboscis. It is not infrequently distinguished by the
different colouring and constitution of its integument. In all cases, in invagination,
the chief part is played by the retractor muscles (cf. p. 471). At the posterior
boundary of the proboscidal region five (interradial or radial) calcareous valves are
occasionally developed ; these, when the proboscis is invaginated, close the aper-
ture (e.g. Psolus, Figs. 227 and 228, p. 287).
The oesophagus reaches from the mouth to the circular canal of the water
vascular system, or even further. It is attached to the water vascular ring, the cal-
careous ring, the radial canals of the water vascular system, etc. , by means of bands
which run out radially, traversing the pericesophageal sinus (see Fig. 365, p. 428).
These bands are chiefly of the nature of connective tissue, but also contain muscle
fibres. The oesophagus, with the complex of surrounding organs, is sometimes
called the pharyngeal bulb.
The oesophagus is followed by a shorter portion known as the stomach intestine,
and this again by the longest part of the digestive tract, the small intestine. This
last forms the larger posterior portion of the first section of the intestine, the whole
of the second section, and by far the greater portion of the third and last section.
The last part of the alimentary canal, the cloaca or rectum, is distinguished by
special thickness, and is attached by radially arranged strands and filaments to
the neighbouring body wall. These strands consist of connective tissue and muscle
fibres.
Into the cloaca or rectum open the water lungs and the Cuvierian organs,
where these are present. These will be described, pp. 487, 488. In some Elpidiidce,
the anterior part of the cloaca bulges out to form a large caecum, which projects more or
less far into the body cavity, sometimes reaching almost to the middle of the body.
Since the Elpidiidce possesses no water lungs, there is some justification for the
viii ECHINODERMATA ALIMENTARY CANAL 479
suggestion recently made that their cloacal caecum may function as a rudimentary
organ of this kind.
The inner surface of the alimentary canal often shows a longitudinal fold.
Transverse intestinal folds, arranged in longitudinal rows, have been observed in
the small intestine of the Aspicl.odiirotaz.
Finer structure of the alimentary canal. The wall of the digestive tract
consists of the following layers, which may vary greatly in thickness and special
structure in the different sections : (1) An inner intestinal epithelium ; (2) an inner
layer of connective tissue with the blood lacunae ; (3) a muscle layer (generally con-
sisting of an inner layer of longitudinal and an outer layer of circular fibres, but in
some Synaptidce and Aspidochirotce this order is reversed) ; (4) an outer layer of con-
nective tissue (often very thin) ; (5) the ciliated endothelium of the body cavity.
The inner intestinal epithelium is ciliated, especially in the^ small intestine.
Over most of the digestive tract it is found as a very deep epithelium covered by a
fine cuticle, its cells being pallisade or thread cells. Glandular cells of various
sorts are specially numerous in the epithelium of the stomach. The epithelium of
the cloaca resembles the outer body epithelium. Into it open the processes of long
subepithelial glands, which are unicellular and tubular.
The anus can be closed by means of a sphincter muscle. Calcareous plates,
papilla?, etc., may occur round it.
C. Eehinoidea.
For the position of the mouth and the anus, cf. p. 338.
In all adult Eehinoidea, the length of the tubular intestine is
greater than that of a straight line from the mouth to the anus, so
that the course of the alimentary canal is necessarily coiled.
The simplest arrangement is found in the Clypeastroida (Fig. 382,
E, p. 475). The direction of the intestinal coils will here be described
in the same way as in the Holothurioidea, the viscera being viewed
from the oral side. After traversing the masticatory apparatus, the
alimentary canal turns to the right (following the direction of the
hands of a clock), and makes rather more than a complete coil round
the principal axis of the body. So far, the course exactly resembles
that of the intestine in the Holothurioidea. In the Clypeastroida,
however, the canal now bends back upon itself, and runs direct back
to the anus, which in this division of the Eehinoidea lies orally in the
posterior unpaired interradius.
In the endoeyelie or regular Eehinoidea, the arrangement is not
simpler, but still more complicated. After leaving the masticatory
apparatus, the alimentary canal ascends towards the apical system,
then bends round and follows the direction of the hands of a clock
(attached to the inner surface of the test by the mesentery) till it has
run about once round the principal axis. It then bends back upon
itself, coiling in the reverse direction backwards to the apical anus
(Fig. 382, D, p. 475).
The intestine of the exocyclic Spatangoldea resembles in its course
that of the endoeyelie Eehinoidea with one difference, caused by the
facts that the mouth has shifted anteriorly along the oral surface, and
480 COMPARATIVE ANATOMY CHAP.
the anus, out of the apical system into the posterior interradius. The
mouth therefore draws the commencement of the first intestinal coil
(which runs in the direction of the hands of a clock) forward, while
the anus draws back (i.e. posteriorly) the end of the spiral which runs,
as above described, backwards (Fig. 382, F, p. 475).
It is worthy of note that, in quite young Spatangoidea (Hemiaster
cavernosus, 2 mm. long), the intestine, which appears to end blindly,
ascends direct from the oral to the apical pole. At a rather later
stage the mouth is still central, while the apical end of the alimentary
canal has already somewhat shifted, and opens through the anus out-
side of the apical system. At this stage (when the length of the
animal is from two to three mm.) the intestine runs up from mouth
to anus in one single coil, as a spiral in the direction of the hands
of a clock. The complicated arrangement in the adult is thus
secondary, and is no doubt due to the fact that the canal increases in
length more than does the interval between mouth and anus.
Finer structure of the intestinal wall. This agrees essentially with the struc-
ture described in connection with the Holothurioidea. No distinct sections can be
made out in the alimentary canal. That part of it which runs through the mastica-
tory apparatus is often called the pharynx. Its lumen in section is five-rayed, the
layer of connective tissue forming five longitudinal ridges which bulge in the
epithelium. It is connected, in a manner which cannot here be further described,
bv means of five pairs of longitudinal bands of connective tissue, with the surround-
ing masticatory apparatus.
The name ossophagus is generally given to the portion of the digestive tract
which follows the pharynx (and, in the Spatangoidea, to the whole of the first por-
tion of the intestine) as far as the point where, in regular Echinoids, there is a sac-
like widening, and in the Spatangoidea a large csecum. In regular Echinoidea, it
includes that part of the intestine which ascends from the lantern towards the
apical system, together with the first portion of the first spiral. In the Spatangoidea
it runs back from the mouth and then bends forward, forming the commencement
of the first intestinal spiral.
The oesophagus is followed by the first intestinal spiral, which runs in the
direction of the hands of a clock. It commences with a slight sac-like swelling
(regular Echinoids) or a large caecum (Spatangoidea). In this part of the alimen-
tary canal a rich system of blood lacunse is developed in the connective tissue layer
on the inner side of the otherwise weak musculature.
In the second or reverse spiral this lacunar network is wanting. This spiral is
distinguished, more especially in the regular Echinoidea, by its peculiar colouring,
being yellow, whereas the first spiral appears brown.
In regular Echinoidea, the two intestinal spirals have an elegantly undulating
course, regularly ascending and descending.
The second spiral passes without any sharp boundary into the rectum, which, in
the Spatangoidea, runs back from the middle of the body. At its commencement, in
Echinocardium (flavesccns) and Sehizaster, it has a small diverticulum.
The alimentary canal of the Spatangoidea, which is distended with sand and
mud, is thicker and its walls are firmer than in the regular Echinoidea, whose in-
testine usually contains, besides mud, a large number of unicellular algse. There is a
corresponding difference in the mesenteries. In the regular Echinoidea the intes-
tine is attached by means of mesenteries practically only to the test, and these
vin EGHINODERMATA ALIMENTARY CANAL 481
mesenteries are broken through in such a manner as to form elegant arcades. In the
Xpatanyoidea, hoAvever, the different coils of the canal are further connected together
inter se by mesenteries which are not perforated.
Unicellular glands of various kinds are found, chiefly in the epithelium of the
first section of the intestine. In the Spatangoidea, in the commencement of the first
spiral, there are multicellular flask-shaped glands ; these lie in the connective tissue
layer, the neck-like duct alone opening into the lumen of the alimentary canal.
The accessory intestine (siphon), which occurs in nearly all
Echinoidea, deserves special attention. Near the commencement of
the first spiral, the siphon branches off from the main intestine as a
narrow tube, which again enters the intestine at the end of that spiral,
to which it thus belongs. The siphon always runs along the inner
side of the main intestine (that turned to the principal axis of the
body). In regular Echinoids, it follows the main intestine in its
course ; in the Clypeastroida, on the contrary, its course is somewhat
shortened. In the Spatangoida, the first part of its course is shorter
than that of the main intestine, while the rest follows the coils of the
latter.
The Cidaroida (Doroddaris papillata) have no distinct siphon, but
it is very probable that this organ is here represented by a longitu-
dinal furrow bordered by two folds, which furrow is either not yet, or
no longer, shut off from the lumen of the intestine. This furrow
occurs in the same region of the intestine as the siphon, and also on
the axial side of the canal.
In the Spatangoid genera Brissus, Briss&psis, and Schizaster a
second siphon has been discovered, running parallel to the intestine.
The structure of the siphon resembles, in essential points, that of
the main intestine. It has been conjectured that it, like the accessory
intestine of certain worms, subserves intestinal respiration.
D. Cpinoidea.
In this class the alimentary canal is tubular. It descends from
the mouth into the calyx, coiling in the direction of the hands of a
clock (when the body is viewed from the oral side). From the base
of the calyx it again ascends, continuing the same curve, towards the
tegmen calycis, and here enters the anal cone in the anal interradius ;
it then runs through the anal cone, opening outward at its tip through
the anus.
During its course through the calyx, the intestine makes one
complete coil round the principal axis (Fig. 382, B, p. 475). The
alimentary canal of Adinometra affords a striking exception to this
rule, forming, in the same direction as in other Crinoids, as many as
four coils (Fig. 382, C). It may be remembered that Adinwnetra is
also distinguished from all other Crinoids by the eccentric position of
the mouth in the tegmen calycis.
The section of the intestine which lies at the bottom of the calyx
VOL. II 2 I
482
COMPARATIVE ANATOMY
CHAP.
FIG. 384. Radial-interradial section in the direction of the principal axis through the
calyx of Pentacrinus decorus (after P. H. Carpenter), left half radial, right interradial, dia-
grammatic. In many respects the figure is obsolete and incorrect. The most noteworthy points
are emphasised by special type in the following lettering, sa, Subambulacral plates ; rn, radial
nerve trunk (hatched) ; rv, radial pseudoluemal canal (black) ; ra, radial canal of the water vascular
system (dotted); d\, brachial coelom ; pi, spongy organ of the blood vascular system ; art, circular
canal of the water vascular system ; r>, mouth ; mi (to the right), nerve ring ; ar, blood vascular
ring? pseudohsemal ring?; an (to the left), anus; re, rectum; in, plates of the interambulacral
areas ; pa, calyx pores ; d, the coelom, traversed by a spongy network ; it, intestinal coils
(cut through); gp, axial organ; mu, muscles; , first and second costal; di, distichal; si/, syzygial
suture ; gr, genital canal ; uni, apical nerve trunk of the arm.
viu ECHINODERMATA ALIMENTARY CANAL 483
is occasionally somewhat widened, and is then called the stomach.
In Rhizocrinus and Bathycrinus there are, on the external side of the
digestive tract, interradially placed outgrowths. Similar outgrowths
occur in great numbers on the inner side of the tract in Antedon (the
side facing the axis of the calyx). Such a diverticulum, when espe-
cially large and branched, has been called a hepatic caecum, but this
name must not be accepted in any strict sense.
The finer structure of the intestine agrees in essentials with that
in other Echinoderms. The intestinal epithelium is everywhere ciliated
except in part of the rectum. The musculature is weakly developed
or altogether wanting, except near the mouth and in the rectum,
where sphincters are formed. The anal tube or cone consists of the
body wall externally and of the wall of the rectum internally.
Between these two the reduced body cavity is traversed by radially
placed strands of connective tissue.
E. Asteroidea (Fig. 385).
That part of the oral area which is left free by the skeleton is
covered by a soft oral integument, in the middle of which lies the
mouth. This organ can be opened by muscles which run out from it
radially in the oral integument ; it can be closed by circular muscle
fibres, which run round it in the oral integument and in the
oesophagus.
The mouth leads into an oesophagus, which ascends perpendicu-
larly, widens rapidly, and passes over without any sharp boundary
into the stomachal sac.
In Echinaster scpositus, the oesophagus has around it ten outgrowths, whose
walls are very much folded, and whose (inner) epithelium is richly supplied with
glands.
The membranous stomachal sac of the Asteroids is very spacious,
filling the whole disc. Its wall is irregularly folded, and provided
with outgrowths ; it is connected with the wall of the disc by means
of mesenterial strands, partly of connective tissue, partly of muscle.
In the upper (apical) portion of the stomachal sac, five pairs of
braehial divertieula open ; these are the radial caeca, or hepatic
appendages, which stretch more or less far into the arms, according
to family, genus, or species. There is one pair in each arm. These
divertieula of the stomachal sac (which, ontogenetically, develop com-
paratively late) have the following general structure. Each diver-
ticulum consists of a median common tube, which runs in the longi-
tudinal direction of the arm, giving off lateral tubes alternately to
right and left. Each lateral tube receives from each side the openings
of closely crowded glandular lobes, so that the secreting surface is
very large.
484
COMPARATIVE ANATOMY
CHAP.
In the Echimisteridce and Asterinidce, the common tube swells into
a large sac.
At the point where the stomachal sac narrows to form the short
FIG. 385. Alimentary canal and genital organs of an Asteroid, diagrammatic. 1, Bradrial
diverticula of the stomach ; 2, gonads ; 3, base of the gonad, which corresponds with its aperture ;
4, stomachal sac ; 5, anus ; 6, rectal diverticula ; 7, apical circular sinus and trunk ; 8, one of the
ten radial sinuses and trunks running from this latter to the gonads ; 9, stone canal in the axial
sinus ; 10, madreporite.
rectum, i.e. high up in the apex of the disc, it is once more provided
with diverticula. These rectal diverticula, whose number, arrange-
EGHINODERMATA RESPIRATORY ORGANS 485
ment, and size are subject not only to specific and generic, but often
also to individual variations, are, as a rule, much smaller than the
brachial diverticula of the stomach, with which they otherwise agree
in structure.
The anus is wanting only in the Astropectinidce. Elsewhere it
lies somewhat eccentrical^ (never exactly at the apical pole) in the
interradius which follows the madreporitic interradius (in the direction
of the hands of the clock), when the disc is seen from the apical
side.
Finer structure of the intestine. The intestinal epithelium is ciliated. Glandular
cells, as goblet cells, mucus cells, and granular cells, are everywhere found together
with epithelial cells. The last named appear to secrete especially the digestive
ferment ; they preponderate at the commencement and terminal part of the canal,
and are particularly numerous in the brachial and rectal diverticula. The muscle
layer is well developed in the oesophagus, the rectum and the rectal diverticula, less
strong in the stomach, and wanting in the brachial diverticula.
The manner in which the brachial diverticula are suspended to the apical brachial
wall has already been described (p. 440).
The Asteroidea are carnivorous, feeding on other marine animals, especially
Bivalves and Gastropods. When feeding, they evaginate the greater part of the
stomach out of the oral aperture, enveloping their prey with it. The secretion of
the mucus cells yielded during the process appears to be poisonous and to have a
decomposing action. The animals attacked quickly die, and are passed on to the
part of the stomach still remaining within the disc, where they undergo the digesting
action of the secretion yielded by the granular glands (Kornerdrusen).
The evagination of the stomach is brought about by the musculature of the disc, \
and its withdrawal by the (partially) muscular mesenterial bands which attach it to \
the body wall.
The anal aperture certainly does not serve for the ejection of all the fecal masses.
It is impossible that large masses, such as the shells of Bivalves and Gastropods,
which are found in the stomachs of Asteroids, can be ejected through such a narrow
aperture ; they are no doubt passed out again through the mouth.
F. Ophiuroidea (Figs. 386 and 388, p. 494).
The condition of the alimentary canal in this class is simpler than
in any of the others. The somewhat spacious buccal cavity which is
surrounded by the oral skeleton leads into the digestive sac which
fills the body cavity of the disc, in so far as it is not occupied by the
bursae. An anus is wanting. Special intestinal appendages in any
way corresponding with the brachial or rectal diverticula of the
Asteroids are wanting.
XV. Respiratory Organs.
There are no respiratory organs which are homologous throughout
the whole phylum of the Echinodermata. Portions of the body
belonging to very different organs and systems of organs are function-
486
COMPARATIVE ANATOMY
CHAP.
ally modified for the purpose of respiration. All these respiratory
organs (except the respiratory trees of the Holothurioidea) are described
FIG. 380. Radial - interradial section
through the disc and the base of the arm
of an Ophiurid, in the direction of the prin-
cipal axis. Left half interradial, right half
radial (after Ludwig). mu, mu\, mil*, Muscles
of the oral skeleton ; an (black), nerve ring ;
am-2+adi, oral-angle plates; or, oral = ventral
wall of the disc; aav, apical circular sinus
with ring-like strand (c/. Fig. 390). vP, Polian
vesicle ; In, mesenterial filaments between the
stomachal sac and the wall of the disc ;
cl, coelom ; alw, apical wall of the disc ; ra,
radial canal of the water vascular system ;
rph, radial pseudohsemal canal ; n; continua-
tion of the axial organ in the arm(?), radial
blood vessel (?); rn, radial nerve trunk of the
oral system: bsi-bs^, first to sixth ventral
shield ; teo\ and teo->, first and second oral
tentacles ; tn, torus angularis ; D, teeth ;
o, buccal cavity ; la, entrance to the stomachal
sac; iph, peripharyngeal sinus; o.nij, peri-"
stomal plates ; aa-, water vascular ring ;
it, stomachal sac ; am.? - am^, second to sixtli
brachial vertebral ossicles ; spt, septum be-
tween the body cavity and the peripharyngeal
in connection with the systems of organs to which they belong,
review of these organs will be found below.
viii ECHINODERMATA RESPIRATORY ORGANS 487
A. The (inner) Respiratory Trees of the Holothurioidea (Figs. 371
and 383, pp. 451 and 477).
These organs, which are known as water lungs or respiratory
trees, occur as two tree-like delicate- walled branched canals or tubes,
which lie to the right and left in the body cavity, their principal
trunks opening posteriorly into the anterior part of the cloaca. They
open either separately or through a common terminal portion. The
last branches of the respiratory trees end in vesicular widenings,
similar " ampullae " being found also along the branches themselves.
When well developed, the respiratory trees reach far forward into the
body cavity, being attached at various points by muscle fibres or
filaments of connective tissue to adjacent organs, i.e. to the body wall,
the alimentary canal, the pharynx, and the mesenteries. In many
Aspidochiroto the left respiratory tree is associated with the rete mira-
bile of the blood vascular system in the way described on p. 452.
The delicate wall of the organ consists of an inner ciliated epithelium,
a thin layer of connective tissue, a muscle layer (in which an inner
layer of longitudinal fibres and an outer layer of circular fibres can
be more or less distinctly made out), and, finally, of the ciliated
endothelium of the body cavity.
There can be no doubt that the respiratory trees actually function
as respiratory organs. At regular intervals water flows into them
from the cloaca, and is from time to time expelled through the anus,
discoloured by the admixture of faecal masses.
Respiratory trees are wanting in all Paradinopoda (Symptidce),
the Pelar/olhuriidce, and the Elasipoda, unless, in the last-named family,
the diverticulum of the rectum described in the section on the
alimentary canal, p. 478, represents a rudimentary lung.
B. Review of the Respiratory Organs of the Eehinodermata.
(a) Holothurioidea Aetinopoda (excluding Elasipoda and Pelago-
fhuiiidce).
1. The respiratory trees, which open into the cloaca.
2. The oral tentacles, and to some extent the delicate-walled
ambulacral tentacles as well.
(!>) Holothurioidea, Paraetinopoda and Pelagothuriidse.
The whole of the body wall and the oral tentacles. Respiration
is promoted by the circulation of the body fluid, kept up by means of
the ciliated urns.
('.) Eehinoidea.
1. The external gills, as outgrowths of the peripharyngeal sinus,
p. 442.
2. The ambulaeral feet, especially those on the apical surface of
488 COMPARATIVE ANATOMY CHAP.
the body, and more particularly the branchial tentacles on
the petaloids, cf. p. 433.
3. The accessory intestine, in which, at least in regular Echinoids,
a streaming of water takes place which does not interfere
with the digestive processes going on in the principal
intestine, cf. p. 481.
(d) Asteroidea.
1. The papulae, cf. p. 439.
2. The ambulaeral feet.
(e) Ophiuroidea.
1. The bursse (respiratory and genital chambers).
2. The ambulaeral tentacles.
(/) Crinoidea.
1. The ambulaeral tentacles.
2. The anal tube (anal cone), which alternately takes in and
gives out water.
XVI. The Cuvierian Organs of the Holothurioidea (Fig. 383, 22,
p. 477).
In certain Holothurioidea, peculiar accessory structures are found
.connected with the terminal portion of the respiratory trees ; these
are known as the Cuvierian organs. They occur chiefly in the Aspi-
dochirotce (especially in the genera Holothuria and Mnlleria) ; in other
Holothurioidea they only occur in isolated cases (Molpadia chilensis,
Cucumaria frondosa). The Cuvierian organs are usually very numerous,
even as many as a hundred occurring in some of the species provided
with them. Although, as already mentioned, they are usually found
in the terminal portion of the respiratory trees, they may shift higher
up the principal trunk, and may even pass over on to the branches.
It is not improbable that they represent morphologically trans-
formed branches of these trees, the structure of their walls agreeing
in general plan with that of these latter.
Two kinds of Cuvierian organs are distinguished (1) glandular,
and (2) non-glandular.
The Cuvierian organs of the glandular kind are long tubes, the
very narrow axial canals of which open into the terminal section of
the respiratory tree. Each of these axial canals has a spiral course,
and is lined by a unilaminar epithelium. This epithelium is followed
externally by a very thick layer of connective tissue, which projects
into the axial canal in the form of a spiral fold. The next layer
consists (1) of isolated circular muscle fibres, and (2) of external
longitudinal muscle fibres gathered into small bundles. Outside of
the muscle layer there is another layer of connective tissue, which,
on the side of the body cavity, is covered by a peculiarly developed
glandular layer ; this no doubt represents the modified endothelium
of the body cavity.
ECHINODERMATASACCULI OF CRINOIDS 489
In this glandular layer the cells can no longer be recognised except by their
nuclei, no boundaries being distinguishable. The layer is closely packed with
secreted granules. "Wandering cells and calcareous corpuscles are found in the con-
nective tissue wall.
The animal, when irritated, vehemently ejects its Cuvierian organs through the
cloaca. (The susceptibility to irritation which leads to such ejection varies greatly
in different forms. ) In this process the tubes are not turned inside out, but are
thrown out complete, just as they are in the body cavity, probably through a rent
in the cloacal wall. When these tubes are thus thrown out, water is almost certainly
pressed out of the respiratory trees into their axial canals. The discharged Cuvierian
organs are remarkable (1) for their extreme viscidity; (2) for their extraordinary
extensibility. They can be drawn out to more than thirty times their ordinary
length. Their viscidity is no doubt produced by the secreted granules of the
glandular layer. In consequence of these peculiarities, the discharged Cuvierian
organs are weapons of defence ; they remain attached to the body of an enemy, and
impede its movements. They may also be weapons of attack, the prey being
caught and held fast till it dies, when its decomposing remains serve for food.
The non-glandular Cuvierian organs are either tubular, like the glandular, or
branched. They are mostly beset with stalked vesicles. The smooth endothelium
of the body cavity which covers them shows no glandular development of any sort.
The part played by these non-glandular and consequently non-viscid organs is
entirely problematical.
XVII. Excretion.
Special excretory organs are altogether wanting throughout the
Echinodermata. It is probable that fluid excrement is osmotically
given off, together with the carbonic acid, at the respiratory surfaces
of the body. Further, coloured and occasionally crystalline corpuscles,
which are met with in very different parts of the body, chiefly in
the connective tissue layers in most Echinoderms, have been regarded
as products of excretion. They appear to remain in the places of
their formation, this conclusion being arrived at from the fact that
they are present in far greater quantities in old than in young animals.
They are also found within the wandering cells, and it might be
worth investigation whether these wandering cells, which force their
way into the body- and the intestinal-epithelium, do not play some
part in excretion.
XVIII. The Saeeuli of the Crinoidea.
These are peculiar organs which, in certain Crinoids, occur in great
numbers below the integument, principally at the edge of the food
grooves of the pinnulae, the arms, and the disc, less frequently else-
where (intestinal wall, mesenteries). They are globular sacs lying
close below the surface, but having no outer aperture, and are closely
packed with strongly refractive spherules, which, during life, are
colourless, but turn red after death. Close examination shows that
490 COMPARATIVE ANATOMY CHAP.
these spherules are enclosed, at least at first, in cells, each of which
has a nucleus lying in the base, which is turned away from the
surface. These are regarded as
connective tissue cells.
According to recent ontogenetic research,
each individual spherule is the metamor-
phosed product of a mesen chyme cell, and
the sacculi on their first appearance are said
to be nests of such cells.
Sacculi are specially numerous in all
f^^^/^t^^Wv^~^^-^_ species of Antedon, but are also found in
\~^&^^^^ > " 4 Eudiocrinm, Promachocrinus, Pentacrinus,
\/ -.,.,-, ..._,._> Rhizocrinus, Bathycrinus. They are want-
j, ing in Adinometra, Thaumatocrinus, and
FIG. 387. -Diagram of a sacculus. 1, Super- Holopus. Their significance has not been
licial layer of integument passing over the discovered. They have, been regarded by
sacculus ; 2, granular masses within special various authors as calcareous glands, ex-
cells ; 5, the nuclei of these cells; 4, nuclei cret of unicellll i ar a i g8e
of the surrounding cutis (3). , .
and slime glands, but, according to the
most recent opinion, they are proteid corpuscles, deposited in the connective tissue
cells as reserve stuff', to be used as occasion requires, for the regeneration of broken -
off arms or of the viscera.
In other Echinoderms the contents of wandering cells (especially of those cells
which are massed together below the surface of the Holothurian integument) have
also been claimed as reserves of nutrition.
XIX. Genital Organs.
A. General Morphology.
With rare exceptions, which will be dealt with separately, the
sexes are separate in Echinoderms.
The genital organs are throughout distinguished by great sim-
plicity, as evidenced by:
1. The entire absence of every kind of eopulatory organ.
The sexual products are ejected from the body, and fertilisation takes
place in the water (except in cases of care of the brood to be men-
tioned later).
2. The entire absence of accessory glands, of widenings or
outgrowths of the ducts, and of complicated adaptations for the
nourishment of the ripening sexual products.
The genital organs consist of variously shaped tubes, within which
the spermatozoa or eggs ripen, and from which they are discharged
through simple efferent ducts.
These gonadial tubes lie in any part of the body cavity ; in the
most complicated cases their wall consists, from without inward, of
(1) the endothelium of the body cavity; (2) a thin muscle layer;
vin E( 'HIXODERMA TA GENITAL ORGANS
(3) a layer of connective tissue ; and (4) the inner germinal epithelium.
The muscle layer is often wanting.
According to the morphology of the genital organs the Echino-
derms fall into two groups.
In the larger principal group, containing the Echinoidea, Aste-
1-n'nl-n, Ophnrroidea, and Crinoidw, there are several gonads, each with
a duct and an aperture ; they follow, in their arrangement, the radial
structure of the body, showing at the same time close relations to the
axial organ (or to the wall of the axial sinus). The axial organ has
been compared to a trunk, of which the gonads, as direct prolonga-
tions, are the fruitful branches, on which the sexual products ripen.
as fruit.
The direct organic connection between the axial organ and the
gonads persists throughout life in the Asteroidea, Ophiuroidea, and
perhaps also in the Crinoidea ; in the Echinoidea it is only ontogenetic,
and ceases in the adult.
The second group is formed by the Holothurioidea, in which there
is neither axial organ nor axial sinus. The genital organs consist of
a single tuft of gonadial tubes. This tuft lies in the body cavity in
the median (dorsal) interradius, and sends off a duct which runs
forward in the dorsal mesentery, and opens outward in the anterior
region of the body, often very near the mouth.
There is, as a rule, no difference in the microscopic structure
and external appearance of the male and female genital organs in
Echinoderms. In some cases, however, at the time of maturity, they
may be distinguished by their different colouring.
Secondary sexual characters .have been noticed only in very rare
cases.
B. Holothurioidea (Figs. 371 and 383, pp. 451 and 477).
In all Holothurioidea, the gonads are developed as a single tuft of
genital tubes, lying in the dorsal interradius. All the tubes of the
tuft converge towards one point, and open into the base of the gonad.
which lies in the dorsal mesentery, and is often somewhat widened
for the reception of the sexual products.
The gonadial tubes are either simple or branched ; in number and size they vary
within pretty wide limits according to the species and the stage of maturity attained.
They may exceed the body in length. They usually hang from the base of the
gonad, on the two sides of the mesentery, into the body cavity, but there are cases
in which they are developed only on one side the left of the mesentery (in
Hnlnthurifi. Miillri'ia, Labldodcinas among the Aspidochirotce, and in many
///// /<'///. '). The base of the gonad lies in the anterior half of the body, often very
near its anterior end (especially in Synaptidas and Molpadiidce, but also in many
Aspidochirotce and Dcnclrochirotce).
From the base of the gonad the genital duet runs more or less
492 COMPARATIVE ANATOMY CHAP.
far forward in the dorsal mesentery, to pass through the body wall
at some point of the anterior half of the body in the dorsal median
line, and to open outward through the usually single genital aperture.
The distance of this aperture from the extreme anterior end of the body, how-
ever, varies very greatly. It is greatest in the Elasipoda, and becomes smaller in
the Aspidochirotce. In the Molpadiidce and Synaptidce the genital aperture lies
immediately behind the circle of tentacles, and in the Dciidrochirotce it shifts into
that circle, even reaching its inner side. It is found behind the middle of the body
only in Psychropotes longicauda.
The genital aperture is usually inconspicuous. Occasionally it is found on the
tip of a genital papilla ; in species of the genera Thyone and Cucumaria, this is the
case only in males, a slight sexual dimorphism thus arising.
The occurrence of several genital apertures (2, 4, 8, 16 in certain Elasipoda) is
quite exceptional. They always belong to one and the same gonad, and to one
genital duct. This latter in such cases, before emerging, divides dichotomously into
as many branches as there are apertures.
C. Asteroidea (Fig. 385, p. 484).
The genital organs are here developed as five pairs of bundles of
gonadial tubes, or as five pairs of rows of such bundles. These pro-
ject freely into the body cavity ; their bases are attached to the apical
(dorsal) body wall, generally at the apical edge of the supramarginal
plates, or on a level with this edge. Exactly over the point of
attachment, i.e. over the base of each gonadial tuft, the efferent duct
traverses the body wall (between two neighbouring skeletal plates) to
open outward at the surface through one, less frequently through
several, genital apertures. These apertures are quite small, and are
often only clearly visible at the season of sexual maturity, when the
genital products are ejected.
The bases of all the gonadial bundles are still connected with the
axial organ even in the adult (cf. p. 445 on the axial organ and the
axial sinus). The axial organ is continued along the inner apical
(dorsal) body wall (that turned to the ccelom) as a pentagonal strand
running round the apical pole and the anus, which agrees in
structure with the organ itself. At each of the five interradially
placed corners of the ring it sends off a pair of strands which run
peripherally. There are thus in all five pairs of strands radiating
from the ring ; these run to the bases of the five pairs of gonadial
tufts, and where these are in rows, from tuft to tuft of each row,
connecting their bases.
Just as the axial organ is surrounded by the axial sinus, so are
all its derivatives surrounded by a coelomic sinus, a direct prolonga-
tion of the former.
The aboral ring-like strand lies in a ring-sinus, attached to its
wall by a suspensory band. This sinus is also continued along the
five pairs of strands which radiate from the ring-like strand ; when it
viii ECHINODERMATA GENITAL ORGANS 493
reaches the bases of the gonads it is further continued along all the
individual tubes to their tips. The gonadial tubes thus have a double
wall first, their own wall, which is a continuation of that of the out-
growths from the axial organ ; and secondly, an outer wall, which is
a continuation of the wall of the axial sinus. Between these two
walls lies the narrow ccelomic sinus, which is in open communication
by means of the sinuses of the genital strands, with the ring sinus,
and through this latter with tlie axial sinus.
The relations existing between the gonads, the axial organ, and
the system of sinuses, is clearly elucidated by the ontogeny of the
Asteroidea, which shows that in quite young animals the axial organ
grows out apically, and first forms the ring strand. Out of this the
genital strands bud, and from these latter again the bundles of
gonadial tubes arise, which are at first solid outgrowths, and only
become hollow secondarily. During this whole process the growing
axial strand, which finally produces the rudiments of the gonads,
continually carries the axial sinus along with it, so that the ring-like
strand, the genital strand, and the genital tubes are entirely sur-
rounded by a sinus, which constantly remains in open communication
with the axial sinus.
At those points of the genital strands from which the gonadial
bundles bud, i.e. at the future bases of the gonads, the duct which
perforates the body wall is formed from within at a later stage.
The form of the individual gonadial bundles requires little notice. The genital
tubes of which each bundle is composed are usually not long, sometimes they
resemble short sacs and are vesicular, they are rarely branched.
Of much greater interest are the number and arrangement of these bundles.
In the simplest cases, five pairs of gonadial bundles are present ; this is the case,
as far as examination of the various species on this point has taken place, in the
following families : the Aster inidcc, Solastcridce, Echinasteridce, Linckiidw, Asteri-
ii.lr. In these, the five pairs either lie in the disc, one bundle at each side of each
interradius, or have shifted into the bases of the arms (Echinasteridce, Linckiidaz).
More than five pairs of gonads are found in the families of the Astropedinidai,
Pcntaccrotida', and Gymnasteriidcc. They either lie in the disc in rows at the sides
of the interradii, or the five pairs of rows stretch into the arms. This last arrange-
ment is found in the most extreme form in Luidia, where, on each side of each arm,
a row of nine runs to near its tip, one or two pairs occurring on each skeletal
segment.
In all cases, each bundle has its separate genital aperture.
As a rule, each bundle has only one aperture, but it sometimes happens (Asterias,
Solaster] that the duct which traverses the apical body wall branches, and opens
through several genital pores lying near one another.
Asterina gibbosa is an exception to the rule that the genital apertures lie on the
apical side of the disc or arms. The apertures here lie on the oral side, a peculiarity
connected with the fact that these Asteroids do not simply eject their eggs into the
water, but attach them in combs or plates to stones, etc.
It must, finally, be noted that the aboral ring sinus is not always simple, but
may break up into a circular network of sinuses (e.g. Echinaster sepositus).
494
COMPARATIVE ANATOMY
CHAP.
D. Ophiuroidea.
In structure and development the genital organs in this class
strongly resemble those of the Asteroidea. The gonad is connected
with the axial organ by means of an aboral ring-like strand, and both
the gonads and this strand are surrounded by coelomic sinuses, which
communicate with the axial sinus.
The only important difference in the genital organs of the two
classes is caused by the fact that, in the Ophiuroidea, the gonads do not
open outward directly, but by means of five pairs of large sac-like
FIG. 388. Stomach and bursse of a young Ophioglypha albida, in its natural position in
the disc, the dorsal wall of which is removed. 1, Bursse ; 2, cavity of the disc ; 3, interradial ; 4,
radial bulgings of the digestive sac (after Ludwig).
invaginations of the body wall into the coelom of the disc, these
sacs themselves communicating with the exterior through five pairs
of slit-like apertures lying at the sides of the bases of the arms on the
lower (oral) side of the disc. These sac-like invaginations of the
body wall are the bursae or bursal pockets, their outer slit-like
apertures being known as the bursal (genital) apertures, which have
already been mentioned (Figs. 245, 246, and 314, pp. 300, 301,
and 359).
1. The Bursse (Figs. 388 and 389).
These are large sacs within walls, which fill up the body cavity of
thf 1 disc round the digestive sac. Their walls are attached to that of
ECHIXODERMATA GENITAL ORGANS
495
the digestive sac and the apical body wall by means of strands of
connective tissue, and consist of the same layers as the body Aval),
though in the bursae these layers are much thinner. Calcareous
corpuscles may be either present or wanting in the connective tissue.
The inner epithelium of the bursae is in some parts strongly ciliated.
The outer apertures of the bursae lie at the sides of the proximal
portions of the arms, which are included in the disc. Each bursa has,
as a rule, one aperture, but in
the genus Ophinn.i. (formerly
Ophioderma) there are two aper-
tures on each side of the base of
an arm, one distal and the other
proximal. Both these apertures,
however, lead into one and the
same bursa, and the double aper-
ture (in Ophiura) can be deduced
from the ordinary single aperture
by assuming that the margins of
the latter fuse at about 'the
middle of their length.
The gonads are attached to
the wall of the bursa, on the
side turned to the body cavity
(Figs. 391 and 392). The
spinal nrorhirts nass into the of the stomach and the gonads (after Ludwig).
5 6XUal prc a Of the two burs*, that on the left has been removed.
blirsa, and are ejected thence j, Dorsal shields of the ami; 2, dorsal wall of -the
FIG. 389. Part of a preparation of Ophio-
glypha similar to that in Fig. 388, after removal
disc ; 3 > bursa with its tJ P ( 4 >;
through the aperture. This is,
i F ,1 F 6, vertebral ossicle in the base of the arm ; 7, genital
however, Only Olie Of the func- ptate : 8> row of tawal plates or scales.
tions of the bursa, and, in most
Ophiuroidea, as it appears, not the principal function.
The bursse have a more important function as respiratory organs.
Sea-water can enter them, and through their thin walls exchange of
gases can take place between it and the body fluid. It would be in-
teresting if it could be proved that, as in the mouth and oesophagus
of the Corals, the sea-water enters through one (more or less proximal)
part of the bursal aperture, and flows out again through another (more
distal) part. The proximal aperture of each bursa in Ophiura is per-
haps an inhalent, and the distal an exhalent aperture.
In certain Ophiuroidea (e.g. Amphiura squainata, magellanica, Ophia-
ntntha ricijuii'ii. marsupialis, Ophioglypha he.radis, Ophiomym rivipara, etc.),
the bursae serve as brood chambers. The eggs pass through their
Avhole development in them, until all the organs of the young Ophiurid
are formed.
2. The Genital Apparatus (Figs. 390-393).
The most interesting point in connection with the genital apparatus
is the peculiar course of the apical ring sinus with the ring-like strand
496
COMPARATIVE ANATOMY
CHAP.
it contains. Fig. 390, which represents the ring sinus in horizontal pro-
jection, illustrates its course in five outwardly directed radial and five
FIG. 390. Course of the aboral circular sinus, with the ring-like strand contained in it in
the Ophiuroidea (diagram after Ludwig). 1, Gonads ; 2, axial sinus with axial organ ; 3, mouth ;
4, circular sinus with ring-like strand, on the side of the bursal wall turned to the interradius ; 5,
interradial portion of the ring sinus and strand, bent downwards orally (Fig. 386, left aav) ; 6, bursal
aperture ; 7, radial (apical) region of the ring sinus (Fig. 386, right anv) ; 8, lateral branches of
the same on the bursal wall turned to the radius.
inwardly, ie. orally directed interradial curves. In this undulating
course the ring sinus descends on the inner wall of the disc alternately
from the apical to the oral side,
and then again ascends to the
apical side, the radial curves lying
apically and the interradial (those
near the bursse) orally.
This peculiar course is no doubt con-
nected (1) with the orally directed course
of the axial sinus, the axial organ, and
the stone canal which opens outward
orally through its madreporite (Fig.
361, 6, p. 422). For the ring sinus
is the continuation of the axial sinus,
and the ring-like strand is the continua-
tion of the axial strand. It is now im-
possible to determine whether the axial
sinus and the axial organ, in bending
oralty, drew the ring sinus interradially
in the oral direction (in the first place this could of course only apply to the madre-
poritic interradius), or whether, on the contrary, the ring sinus, shifting orally, drew
FIG. 391. Bursa of Ophioglypha, seen from
the side turned towards the interradius (diagram
after Ludwig). 1, The tip of the bursa, lying on
the dorsal side of the digestive sac ; 2, the gonads
sessile on thr bursal wall ; 3, distal portion of a
bursa (that turned to the periphery of the disc) ;
5, proximal portion of the same (that turned to
the centre of the disc) ; 4, the rows of bursal
scales along the aperture.
VIII
EGHINODERMATA GENITAL ORGANS
497
the axial sinus, etc. with it downwards ; i.e., it is impossible to decide which organ
took the lead in shifting. (2) As the gonads which bud from the ring-like strand
open into the bursae, which latter, however, open outward orally, it is to some
extent explicable why the ring-
like strand descends interradi-
ally to the bursre.
The whole problem is still
further complicated by the ques-
tions : (1) What was the original
function of the bursse ? (2) Is
the ventral position of the bursse
the primitive position ? (3) Is
the opening of the gonads into
the bursse a recent specialisation
in the Ophiuroidea ?
The curved-in portion
of the ring -like strand
(with the sinus enclosing
FIG. 39-2. Transverse section through the disc of an
Ophiurid (Ophioglypha) at the base of an arm (after
Ludwig). 1, Dorsal wall of the disc ; 2, bulging of the
it) rUllS along that side Of digestive sac ; 3, bursa ; 4, gonad on the bursal wall ; 5, base
each bursa which is turned of the arm ; 6 - ventral waU of the disc ; 7) bursal ap 61 ^ 6 :
, .. : . , . T
to the interradms. It,
however, gives off a branch to the wall
the radius (of the arm), this branch running along this wall from
its periphery to its proximal part. Both w r alls of the bursa, therefore,
8, genital plate ; 9, bursal scale.
which is turned towards
FIG. 393. Section through an ovary of an Ophiurid (Ophioglypha lacertosa) (after Cudnot).
1, Muscle trunk, cut through transversely ; 2, nerve ring ; 3, bursal wall ; 4, aperture of the ovary
into the bursa ; 5, wall of the genital sinus ; 6, genital sinus ; 7, the endothelium of the genital
sinus, which covers the gonadial wall ; 8, cavity of the gonad ; 9, eggs in a more mature condition
than the rest ; 10, ring-like strand in the aboral ring sinus (11).
the abradial wall, i.e. that turned to the interradius, and the adradial
wall, i.e. that turned to the arm, have a genital strand. The abradial
genital strand of each bursa is merely a part of the apical ring
strand, while the adradial is a lateral branch of that strand. These
VOL. II 2 K
498 COMPARATIVE ANATOMY CHAP.
five pairs of adradial genital strands recall the five pairs of genital
strands of the Asteroidea.
The gonadial tubes are sessile upon the genital strands of the
bursae, and project freely into the body cavity of the disc. These
gonadial tubes are either single pear-shaped tubes, great numbers of
which are arranged in rows along the genital strands, or they are
collected into tufts, and then there is one tuft on the adradial and one
on the abradial wall of the bursa.
In the former case (e.g. Ophioglyplia, Ophiomyxa, Ophiocoma) the two rows of
genital tubes (the adradial and the abradial) stand rather low down on the wall of
the bursa, near its aperture, almost parallel with the edges of the latter. Each
genital tube has its special aperture into the bursa.
In the second case, the points of insertion of the two tufts of gonads within the
ventral region of the bursal wall, i.e. the position of the bases of the gonads,
seems to vary greatly, and each tuft appears to have only one aperture into the
bursa (Ophiopholis, Ophiothrix).
It is still an open question whether the genital apertures are constant in adult
Ophiurids, or whether they only break through into the bursal cavity at the time of
maturity.
The gonadial tufts arise as originally solid buds from the genital strands, and,
while forming, bulge out the wall of the sinus which contains the strand ; the tubes
are thus here also surrounded by a genital sinus, which communicates with the ring
sinus, and through it with the axial sinus (Fig. 393).
The ring-like strand is attached by a thick band to the wall of the ring sinus.
It is solid, and consists of two kinds of cells : (1) cells which entirely resemble
those of the axial organ, of which the ring-like strand is a prolongation ; (2)
enclosed in these cells, there is a strand of cells proved to be genital germ cells
(rachis genitalis). The cells of the former kind progressively decrease in number,
and those of the second kind increase in number the nearer the ring-like strand
approaches the gonads. The former are not even continued into the gonadial
tubes, while the latter kind yield the germinal cell material of the gonads. It is
very probable that, after the sexual products have been ejected, a new formation
of germinal cell material takes place, by some kind of forward movement, from the
rachis genitalis.
The development of the genital system from the axial organ and the axial sinus
proceeds in the same manner as in the Asteroidea.
Ophiactis virens, a form distinguished by reproduction by means of
fission, and by the peculiar arrangement of the appendages of the
water vascular system, is the only Ophiurid in which the bursse are
altogether wanting. The gonads open direct outward on the oral side.
E. Eehinoidea (Figs. 358 and 370, pp. 419 and 443).
Although the genital system of the Eehinoidea appears to resemble
in its development that of the Asteroidea and the Ophiuroidea (a point
on which, however, further research is desirable), marked deviation
takes place in the adults.
The gonads, at least in regular Echinoids, are five in number, and
VIII
ECHINODERMATA GENITAL ORGANS
499
lie in the apical region of the body cavity, in the interambulacra. The
five genital ducts ascend towards the apex, there perforate a ccelomic
circular sinus which surrounds the rectum, pass through the genital
pores of the basals, and then open outward, sometimes at the tips of
projecting papillae.
The gonads. These, in a mature condition, are large acinose
organs, which are suspended to the inner wall of the test by an
exactly interradial principal
suspensor, and by various
other bands of connective
tissue. They are not sur-
rounded by a genital sinus.
The number of gonads
was originally five. Five
are found in all the regular
Echinoids (Cidaroida and
Diademaioidd), and also in
many Clypeastroida. In the
Spatangoida, the Holectypoida,
and many Clypeastroida, the
number is reduced, the pos-
terior unpaired gonad with
FIG. 394. Cystechinus vesica A. Ag. Apical portion
of the test from within, with the three gonads. 1, An-
belonging terior ambulacrum ; 2, left anterior ; 3, left posterior ;
3 4, right posterior gonad ; 5, circular sinus (after A.
Agassiz).
appear. In the Spatangoida,
the reduction may go still further, the right anterior, and in a few
cases the left anterior as well, disappearing (Fig. 394).
the genital
to it beinsr the first to dis-
Further details on this point are to be found in the section on the skeletal system
(cf. pp. 321-324 on the genital pores). It is there shown that these pores are by no
means necessarily limited to the basals.
The genital apertures. The genital papillae, on the tips of which the genital
apertures lie. are specially well developed in the Spatangoida.
The ring sinus encircles the anus with the periproctal sinuses, the stone canal,
and the axial sinus. Its wall is formed on the one side by the test, and on the
other by a circular lamella of connective tissue which is covered on both surfaces by
endotheliuni, on the apical surface by that of the ring sinus, and on the oral by that
of the general body cavity.
The lower wall of the apical ring sinus is broken through in Dorocidaris, so
that the circular sinus is here in open communication with the general body
cavity.
In all other cases, the ring sinus is entirely closed on all sides in adult
Echinoids.
In adults, there is no trace of a ring-like strand enclosed in the
ring sinus. The connection between the axial organ and the gonads
is thus lost.
500
COMPARATIVE ANATOMY
CHAP.
F. Crinoidea (Fig. 395).
In the Crinoids, a genital strand runs through the arms, branching
with them, and entering into their last ramifications the pinnulse.
While this genital strand, which is to be found even below the food
grooves of the tegmen calycis, remains as a rule infertile in the calyx
and in the arms, in the
pinnules the germinal
cells which it contains
give rise to the genital
cells. The genital strand
in the pinnulse becomes
a gonadial tube.
On the position of the
genital strand cf. p. 414
and Fig. 356. It runs
between the three brachial
sinuses of the body cavity
(between the dorsal canal
and the two ventral
canals), below the food
grooves of the arms.
It is contained in a
special coelomie sinus
ovariai (like the ring-like strand
and the genital strands
of the Asteroidea and the
Opliiuroidea), to the wall
5, deeper longitudinal nerves of the pinnula ; 6, tentacle canal ; o f which it is attached by
7, radial canal of the water vascular system ; 8, nerve ridge of fi ] ampnt< , nf rnrmPP ti vp
the superficial oral system ; 10, sacculus, see p. 490.
tissue.
The coelomie sinus is continued on to the gonadial tubes of the
pinnulae and there becomes the genital sinus.
The genital strand is at first solid, but at a later stage becomes a
hollow genital tube. This genital tube widens in the pinnules into
the gonadial tube, which, in mature pinnules, is filled either with eggs
or spermatozoa, these having their origin in the cells of the wall of the
gonadial tubes (the germinal epithelium).
In a transverse section of the genital tubes of the arms, the wall
appears thickened at one part. This thickening is the section of a
ridge whose cells seem to correspond with those of the germinal epi-
thelium of the gonadial tubes.
It is very probable also, that after the ejection of the sexual pro-
ducts from the pinnules in Crinoids, the new formations of these pro-
ducts proceed from the germinal cells, which are pushed forward out
of the ridge of the genital tube into the pinnules.
FIG. 395. Transverse section through an
pinnule of a Crinoid, diagrammatic. 1, nerve trunk of the
apical nervous system in the joint of the pinnula ; 2, genital
sinus ; 3, germinal epithelium of the gonadial rachis (genital
strand or tube) ; 4 and 9, sinuses of the brachial coelom ;
ECHINODERMATA GENITAL ORGANS 501
That the cells of the genital ridge (and, indeed, originally all the cells of the
genital strand) are germinal cells is further proved by the fact that, in exceptional
cases, gonads may develop in the arms also, and even under the ambulacral furrows
of the calyx (e.g. in individuals of the species Antedon and Actinometra, and in one
species not specified).
The gonadial tubes are sometimes long, sometimes egg-shaped. They run
through a larger or smaller number of joints of the pinnule. At the time of
maturity they swell and often bulge out the wall of the pinnule in such a way as
to show at a glance which pinnules contain ripe sexual products.
The manner in which the ripe products are ejected from the pinnules is not yet
satisfactorily explained. There seem to be no constant genital apertures in the
adult. It appears that the ejection takes places through two merely temporary
apertures (one on each lateral wall of the pinnule).
Round the mouth, finally, there are five genital strands with the
sinuses in which they lie, running from the periphery, i.e. from the
bases of the arms below the food grooves of the tegmen calycis. It is
not certainly known what becomes of these genital strands ; according
to some accounts, they are continued round the mouth into the strands
of the axial organ. They are said : also to develop ontogenetically as
outgrowths of that organ (cf. p. 446).
If the axial organ of the Crinoids is homologous with that of the Ophiuroids,
Asteroids, and Echinoids (which homology cannot be considered as certainly estab-
lished), then we should have the same relations subsisting between the axial organ
and the genital organ in the Crinoids as in the other groups above mentioned. But
in the Crinoids the genital strands, which only become fruitful as gonadial tubes in the
pinnulae, are oral outgrowths of the axial organ, whereas in other Echinoderms (apart
from the Holotliurioidca, which are quite isolated) they are apical outgrowths.
G. Origin of the Sexual Products.
The first origin of the sexual products has been accurately described
for the Ophiurid Amphium squamata. They, and the cells of the axial
organ, arise out of one and the same rudiment, which consists of the
endothelial cells of the eoelom. Tie Echinoderms would thus, as
far as the origin of the sexual products is concerned, agree with the
Annulata, the Mollusca, and the Vertebrata.
The specific cells of the axial organ seem incapable of becoming
germinal cells.
H. Hermaphroditism in Eehinoderms.
Hermaphroditism is an altogether exceptional phenomenon in
Echinoderms, and is only of frequent occurrence in one order of the
Holothurians, the Paractinopoda (Synaptidce). Apart from this order,
it is only certainly established in one Asteroid (Asterina gibbosa) and
one Ophiurid (Amphium squamata).
(a) Paractinopoda. All species of the genera Synapta and Anapta, examined
with regard to this point, and a few species of the genus Chirodota, are herma-
phrodite.
502 COMPARATIVE ANATOMY CHAP.
as well as spermatozoa are produced in the gonadial tubes, but the two
products do not ripen simultaneously (Synapta inhcerens}. The spermatozoa only
ripen after the ejection of the eggs.
(>) Asterina gibbosa. Here also the eggs and the spermatozoa are formed in the
same genital organs, again not being simultaneously produced. The young animals
are males, the adults females.
(c] Amphiura squamata. The simple pear-shaped gonads are very few in number.
On the average, the adradial and the abradial walls of a bursa have only one gonad
each sessile on it. The adradial gonads are testes, the abradial ovaries. Only a few
eggs in the ovary and a small number of spermatozoa in the testes ripen at one time.
These two kinds of sexual products here also, as it appears, do not ripen simul-
taneously in one and the same animal. The eggs are developed in the bursse.
I. Care of the Brood and Sexual Dimorphism.
Little by little, somewhat numerous cases of care of the brood have become known
among the Holothurioidea, Echinoidea, Asteroidea, and Ophiuroidea. These are not
infrequently connected with a more or less pronounced sexual dimorphism, the
adaptations for care of the brood being found only in the female.
The eggs of an Echinoderm in which the brood is cared for are, as far as investiga-
tion on this subject has gone, distinguished by remarkable size, and by a rich pro-
vision of nutritive yolk, from those which are ejected into the water, and are destined
to develop into free-swimming larvae.
The following review makes no claim to be exhaustive.
(a) Holothurioidea. In Psolus ej)hippifer (cf. Fig. 228, p. 287) large plates are
found on the back of the female, raised from the dorsal integument by means of
stalks. Between the stalks a brood chamber, roofed over by the contiguous plates,
arises ; in this the fertilised eggs which emerge through the dorsal genital aperture
pass through their development.
In Cucumaria crocea, the developing eggs are retained in a dorsal trough, which
arises by the swelling up and bulging outward of the body wall in the two dorsal
radii.
Another kind of care of the brood is found in Cucumaria, Icevigata and C. minuta.
Two sacs here project from the body wall into the body cavity ; these are brood
pouches, which shelter the developing brood. The sacs are probably mere invagina-
tions of the body wall ; their outer apertures, however, have been discovered only in
C. minuta. The two sacs belong to the two ventral interradial areas ; in C. Icevigata
they lie near the middle of the body, in C. minuta anteriorly.
In Phyllophorus urna and Chirodota rotifera, the body cavity serves as a brood
chamber. It is, however, unknown how the fertilised eggs pass in arid the young
Holothurioidea out of it.
In other Echinoderms, as might be anticipated, we find the spines occasionally
acting as protections for the brood.
(6) Echinoidea. In a few Cidaroida (e.g. C. canaliculaia, C. nutrix, C. mem-
branipora) the eggs are retained on the apical area of the test, and here develop,
protected by the spines, which bend together over them. The same is the case in
many Spatangoida, but the members of this order have become still more specialised
for this function. In certain forms either some or all of the petaloids (cf. p. 347)
sink in deeply, and thus give rise to brood chambers (marsupia) into which the eggs
pass from the genital aperture. The brood developing in such a marsupium is pro-
tected by the bending together of the larger spines which border it. In the Schizaster
figured on p. 294, the anterior impaired petaloid ; in Hemiaster cavernosus, in which
this arrangement is best known, the paired petaloids are the most deeply sunk. As
VIII
ECHINODERMATACARE OF BROOD
503
this is only the case in the female, we here have a striking sexual dimorphism.
Similar adaptations for the care of the brood seem to occur in Moira, Anochanus,
etc.
(c) Asteroidea. Among the Asteroidea, the Pterasterince (Pteraster, Hymenaster)
are very remarkable for the care of the brood. The whole of the apical body wall
carries large peculiarly shaped paxillae or calcareous pillars, from the free ends of
which radiate, like the spokes of a wheel, a varying number of calcareous rods (cf.
p. 391). All these calcareous stars of the paxillee are connected by an integument
in such a way that, between this in-
tegument (supradorsal membrane) and
the dorsal wall of the body beneath it,
a large brood chamber is formed. This
chamber communicates with the ex-
terior at many points : (1) through a
large aperture at the apical pole
(osculum) usually surrounded by five
valves of considerable size (Fig. 396) ;
(2) through numerous contractile pores
or spiracles in the membrane which
covers the brood cavity ; (3) through
regular segmeutally recurring apertures
at the sides of the arms. These aper-
tures can be closed by means of small
spines or scales. These "segmental"
apertures are regarded by the present
writer as ventilating apertures, as they
appear to serve the purpose of keeping
up an active circulation of water in the
brood cavitv.
FIG. 39t5. Hymenaster pellucidus, Wyv. Thorn-
The sexual arrangements in the son> from the apical side> The osculum is seeDj
Pterasterince are unfortunately still un- surrounded by five valves (after Sladen).
known. All specimens as yet described
show the brood membrane. Possibly they are all females, and the males are still
unknown, or the Pterasterince may be hermaphrodite. Or, again, there may be a
far-reaching dimorphism, which has led to the males being described as a separate
species.
Leptoptychaster kerguelenensis, an Astropcctinid, shows us the care of the brood,
seen in the Ptcrasteriiuc, to a certain extent in statu nascendi. The eggs which
emerge from the genital aperture pass into the interstices between the stalks of the
still separate paxillre, and there pass through the first stages of their development.
At a later stage, also, as young Asteroids, they remain for some time on the body of
the mother.
In Asterias spirabilis, similar arrangements are found, but the embryo is con-
nected by means of a stalk to the body wall of the mother.
Other Asteroids (e.g. species of Echinaster and Asterias) protect the brood which
collects on the oral side ; it develops under the shelter of the arms, which simply
bend round over it, so forming a temporary brood chamber.
Ophiuroidea. In the description of the bursse, p. 495, it was mentioned that, in
many Ophiurids, these function as brood chambers, and the best-known cases were
given.
504 COMPARATIVE ANATOMY CHAP.
XX. Capacity for Regeneration and Asexual Reproduction by means of
Fission and Gemmation.
The capacity for regeneration is, as a rule, highly developed in Echinoderms.
Defects in the body wall are in this way easily and quickly repaired in all Echino-
derms. The Echinoidea, even, in which this capacity is in other ways slightly
developed, easily repair smaller or greater defects in the body epithelium which covers
the test. For example, in Dorocidaris papillata, portions of the test over which
the epithelium has been damaged or destroyed are cast off, and as soon as a fresh
integument has formed a new test can undoubtedly be produced below it.
The capacity for regeneration may increase to an extraordinary degree within the
different divisions, and the sensitiveness to external stimulus increases in proportion,
till a stage is reached when voluntary amputation by means of muscular contraction
takes place in response to external stimuli.
The Crinoids completely regenerate lost viscera, and it even appears as if such
loss is not altogether involuntary, in certain species and under certain conditions
voluntary amputation taking place. This, however, is not certain.
Crinoids easily regenerate broken-off portions of arms or whole arms ; several or
indeed all the arms may, under favourable circumstances, be regenerated. The arms
break off easily at their bases ; it even appears as if Antedon, in response to injurious
stimuli, voluntarily throws off its arms.
The regeneration of the portions of arms (bitten off, possibly by enemies) or of
whole arms takes place very easily in many Asteroidea and Ophiuroidea. The
frequency with which Asteroids and Ophiurids with regenerated arms or arm tips are
met with demonstrates both the frequency of mutilation and the great utility of
regeneration.
Species of Asteroids in which the disc with the other arms are regenerated at the
base of broken-off arms are less common. Such regenerations give rise to the well-
known "comet" form of Asteroids (Fig. 397, B). Regeneration of the whole body
from one arm never occurs in Ophiurids. It has been suggested that the difference
between Asteroids and Ophiurids in this respect is connected with the fact that, in
Asteroids, intestinal diverticula project into the arms, and that the genital products
are often developed in them, which is never the case in Ophiurids.
Animals in which half the disc is retained can regenerate the rest of the body
both among the Ophiuroidea and the Asteroidea.
Defects both great and small in the disc are repaired.
In Linckia multifora, an Asteroid distinguished by an extraordinary capacity for
regeneration, cases have been known in which, after the animal has lost the greater
part of an arm, two new tips have been formed by the wounded surface, and in one
case regeneration led to the formation of a complete new Asteroid at such a
point. This latter case is illustrated in outline in Fig. 397 C. The new animal
consists of two discs with their arms, connected by the regenerating stump of the
arm.
Holothurioidea. Here also the capacity for generation seems to be very great.
Not only are tentacles and integumental defects repaired, but the ejected viscera
(intestine, respiratory trees, and even the calcareous ring, the water vascular ring,
and the gonads) can be regenerated. In Synapta, after the body has been completely
cut to pieces, its anterior portion can regenerate the whole. In a Cucumaria, the
two separate halves can grow into complete animals.
Increase in the capacity for regeneration is accompanied by increased irritability.
Many Holothurioidea, especially Aspidochirotce, when strongly stimulated, contract
EGHINODERMATA REGENERATION, ETC.
505
so violently that the intestine is torn out (usually behind the calcareous ring), and
together with the right respiratory tree is ejected through a rent in the cloacal
wall.
In certain Holothurioidea, the integument, when strongly irritated, easily dis-
solves into slirne. A Stichopus was observed to come entirely out of its skin, i.e.
the whole integument dissolved into slime, so that only the dermomuscular tube
FIG. 397. A, OpMdiaster diplax. A specimen with the anus (3, 4, 5) in the act of being re-
generated ; and two (1 and 2) being constricted off (after Haeckel). B, Linckia (Ophidiaster) multi-
fora, a "comet" form. One arm is in the act of regenerating the disc and the other four anus
(after Haeckel). C, The case given in the text of a specimen of Linckia multifora (after P. and F.
Sarasin).
enclosing the viscera remained. That regeneration follows such a phenomenon has
not yet been established by observation.
The SynaptidcB react on stimuli (often quite slight) by falling to pieces, the
circular musculature being at certain points so strongly contracted that the parts
thus constricted break off.
It will no doubt be proved in time that all these manifestations of increased
irritability which coincide with increased capacity for regeneration are of use to the
animal.
Asexual reproduction (schizogony). In certain Echinoderms, the strongly
developed capacity for regeneration has had as a consequence asexual reproduction.
It is, indeed, not certain that, in the cases to be quoted below, the division into
506 COMPARATIVE ANATOMY CHAP.
parts is purely voluntary (i.e. results from causes entirely within the animal itself)
and not to some extent due to external stimuli, however slight. In any case, the
final result of the regeneration which follows is the same the multiplication of
individuals.
Fission of the body into two halves of approximately equal size with subsequent
regeneration has been observed in Ophiuroidea, Asteroidea, and Holothurioidea.
In the two former classes the plane of fission passes through the middle of the disc
(through the mouth and digestive sac), in the Holothurioidea (Cucumaria) it passes
transversely through the tubular body, dividing it into an anterior (oral) and a
posterior (apical) half.
In the Ophiuroidea, reproduction by means of fission has been observed in the
following genera : Ophiactis (Miilleri, Savigny. virens), Ophiocnida (sexradia}, Ophio-
coma (pumila, Valencies}, Ophiothcla (isidicola, dividua).
Among the Asteroidea, schizogony is specially characteristic^ many species of the
genus Asterias (acutispitia, atlantica, calamaria, microdiscus, tenuispina], and is also
found in Asterina Wega, Cribrella sexradiata, Stichaster albulus.
Another kind of asexual reproduction seems to be very common in the family of
the Linckiidce. In these Asteroids, the arms become constricted off at their bases,
after which not only does the disc regenerate the arms which have been cast off, but
each individual arm regenerates the disc and the other arms ("comet" forms of
Asteroids, Fig. 397 A, B).
Asexual reproduction does not, as a rule, appear to take place simultaneously with
sexual reproduction, but there are exceptions to this rule.
XXI. Ontogeny.
In all Echinoderms, except those few forms in which care of the brood occurs, the
fertilised eggs develop into free-swimming, bilaterally symmetrical larvae, which are
transformed into the radially built Echinoderm after passing through an often very
complicated metamorphosis.
The larvse of the different classes of Echinoderms will first be compared ex-
clusively according to their external characteristics.
A. The various Larval Forms of the Echinodermata.
We shall first construct a hypothetical larval form, and then deduce the various
larval forms from it (Fig. 398, A).
The body of the larva is egg-shaped, and concave on the ventral side. In the
base of the concavity lies the larval mouth. Near one of the poles (i.e. near the
posterior end), but still on the ventral side, there is a second aperture (proceeding
from the blastopore of the gastrula larva) ; this is the larval anus. A ciliated band
which runs back upon itself surrounds the mouth along the edge of the ventral con-
cavity ; posteriorly it runs over the ventral side in front of the anus, and is the
circumoral ciliated ring. The aperture of the mouth and its immediate surround-
ing are also ciliated (adoral ciliated band).
1. Holothurioidea. The Holothurid larva known as Auricularia (Fig. 398, A)
differs but little from the hypothetical form. The ventral concavity becomes more
complicated, lengthening on each side posteriorly and anteriorly, while a posterior
median portion -\\ith the anus forms a ventral prominence, the anal area, and a
median portion in front of the mouth forms another prominence, the preoral area.
The ciliated band which runs longitudinally along the ventral depression assumes
ECHINODERMA TA ONTOGENY 507
in consequence a more complicated form, and takes a winding course. This descrip-
tion will be elucidated by the figures.
Here, as in all other Echinoderms, the ciliated rings are mere remains of the
cilia which covered the whole body in an earlier stage, i. c. in the gastrula.
2. Asteroidea (Fig. 398, B). The Asteroid larvae are known as Bipinnarise and
Brachiolariae. The chief distinction between them and the Auricularia is the
preoral ciliated ring. This is a circle on the preoral area, and within the circumoral
ciliated ring, from which it is altogether distinct.
The comparison of a Bipinnaria with an Auricularia led to the conjecture that
the preoral ciliated ring of the former corresponds with a preoral portion of the
common circumoral ciliated ring of the latter, which has become distinct and has
closed to form a ring. Direct observation of the ontogenetic development of the
ciliated ring of the Asteroid larva has entirely confirmed this conjecture.
FIG. 398. A, B, C, Auricularia, Bipinnaria, and Tornaria (Enteropneustan larva), from the
right side, diagrammatic. 1, Pretrochal area ; 2, oral area ; 3, postoral area ; 4, anal area ; I, pre-
oral; II, circumoral : III, anal or principal ciliated ring; 5, neural plate; os, mouth, OH, anus.
The Bipinnaria passes through an Auricularia stage. The general ciliation of
the body, belonging to an early stage, disappears first from the ventral side, which
becomes depressed, then from the dorsal side, in such a way as to leave a band
running back on itself at the edge of the ventral depression ; this corresponds entirely
with the course of the circumoral ciliated band in the Auricularia. In the frontal
region, however, where the two lateral strips of the circumoral band approach each
other in the median line, a ciliated island is for a time retained connecting them
(Asterias rube/is}. The covering of cilia thus forms an X-like cross on the frontal
region. By the disappearance of the ciliation from the centre of the X, the preoral
section of the ciliated ring is separated from the rest, and forms the distinct preoral
ring enclosed within the circumoral ring.
The process in Asterias vulgaris seems to take a somewhat different course, but
has the same final result. On the frontal region, where, in A. rubens, an isolated
ciliated area remained to form a connection between two portions of the circumoral
ciliated band, this connection arises only secondarily by the approximation of the
two portions in the middle line. The further process of separation of the preoral
ring from the rest, which latter then represents the secondary circumoral ciliated
ring, agrees with that in A. rubens.
The ventral depression (in which the mouth lies) which, in the Auricularia, runs
forward to the right and left of the preoral portion of the circumoral ciliated ring,
is now able, after the latter has become constricted off as a ring, entirely to surround
508
COMPARATIVE ANATOMY
CHAP.
that portion ; it forms a moat round the preoral area, which becomes raised up like a
shield.
An adoral ciliated ring, closely encircling the mouth and extending some way
into the buccal cavity, is also present.
The body is produced into longer or shorter processes or arms, in the regions of the
preoral and circumoral ciliated rings. An anterior unpaired frontal process, belong-
ing to the ciliated ring, is distinguished by its constant occurrence and its greater
length.
In some species, the ciliated band disappears on this frontal process, which, on
20-
FIG. 399. Older Auricularia, seen diagonally from the lower and left side (after Semon). 1,
Circumoral ciliated ring ; 2, hydropore ; 3, hydrocoel ; 4, adoral ciliated ring ; 5, median or stomach
intestine ; 6, nerve band ; 7, hind-gut ; 8, left enterocrel ; 9, calcareous wheel ; 10, fore-gut, oeso-
phagus ; 11, right enterocoel.
the other hand, divides into three branches, apparently covered with protuberances
at their tips. Such larvae are known as Brachiolariae.
There are, further, Asteroids whose larvae do not at all resemble the Bipinnarian
and Brachiolarian larvae, or else show only a superficial resemblance to them ; cf.
below the account of the larva of Asterina gibbosa (p. 525).
3. Ophiuroidea. The Ophiurid larva is known as the Pluteus, and can be
just as easily deduced from the hypothetical larval form of the Echinoderms, sketched
above, as the Auricularia and the Bipinnaria. The gastrula stage is followed by
the bilateral stage with depressed ventral surface, in the centre of which lies the
larval mouth. A circumoral ciliated band is retained, running along the edge of this
ventral depression. This band always remains single. While the preoral area (the
larva being viewed from the ventral side) remains very small or is even indistinguish-
able, the anal area appears very large. The body is produced into processes or arms,
which may become very long, and are supported by calcareous rods. These pro-
VIII
ECHINODERMATA ONTOGENY
509
cesses are of two kinds. One kind, which belong to the region of the circumoral
ciliated ring, are paired, and diverge in a forward direction. Two arms are
distinguished by their constant occurrence and special length ; these belong to the
posterior and lateral region of the circumoral ciliated band. Opposed to these paired
processes of the circumoral ciliated band pointing anteriorly is an unpaired, posterior,
postanal process projecting backwards from the posterior end of the anal area ; its
tip may carry a cap of cilia.
In Ophiuroidea in which care of the brood occurs, the typical larval forms are not
developed.
4. Echinoidea (Figs. 400 and 401). The larva of Echinoidea agrees to a great
extent with that of the Ophiuroidea, and is, like it, known as the Pluteus. The only
FIG. 400. Larva of an Echinid (Pluteus) from
the ventral side. 1, Ciliated "epaulettes " ; ant, an-
terior ; post, posterior; dex, right ; sin, left.
FIG. 401. Spatangid larva (Pluteus)
from the ventral side. 1, The three processes
of the anal area.
important difference is that the two lateral arms which, in the Ophiurids, are the
most constant and the longest, seem to be altogether wanting in the Echinoidea.
The Pluteus of Echinus has eight arms or processes, and at the bases of each ot
the four posterior arms a ciliated li epaulette " (Fig. 400).
The larvre of Arbacia and Spatangus (Fig. 401) have no ciliated "epaulettes,"
but Arbacia has two and Spatangus three long posterior processes of the anal area,
which, like all the other processes, are supported by calcareous rods.
Echinoids in which care of the brood occurs develop direct without meta-
morphosis.
5. Crinoidea (Fig. 402). The free-swimming larva of Antedon is long and egg-
shaped. At the frontal pole, the thickened but somewhat depressed ectoderm (the
neural pit or plate) carries a tuft of long flagella. The larva, in swimming, has the
frontal pole, which corresponds with the anterior end of other Echinoderms, directed
forwards.
flp"
OF THE
UNIVERSITY
510 COMPARATIVE ANATOMY CHAP.
The body is surrounded by five ciliated rings, distinct from one another ; these
cannot be ontogenetically derived from one continuous ciliated ring.
The most anterior ring is interrupted on the ventral side.
The second ring runs somewhat diagonally from above downwards and forwards,
the third is directed downwards and backwards, so that there is a large interval
between the second and third rings ventrally.
In this region, the ventral side sinks in to
, u|,,, form a large ciliated vestibular depression.
A smaller depression on the ventral side
between the first and second ciliated ring is
known as the adhesive pit. The larva attaches
itself at this point, by means of a special
secretion yielded by the glandular cells of the
depression.
To the left, between the third and fourth
rings, there is a small aperture, the primary
madreporite (water pore).
The intestine lies as an entirely closed sac in
J :-..- / the posterior part of the larva. The free-swim -
Hpfe. ming larva has neither larval mouth nor larval
\ anus. The definitive mouth breaks through the
floor of the vestibular depression later.
The whole anterior part of the larva, as far
FIG. 402. Free -swimming larva of as the third ciliated ring, becomes the stalk, and
Antedon, from the right lower side (after the posterior part the calyx of the attached
Bury). I- V, The five ciliated rings ; 1, the larya
neural tuft; 2, the adhesive pit; 3, the
vestibular depression; d, dorsal; v, The free-swimming Crmoid larva cannot with
ventral. certainty be derived from the same hypothetical
form as other Echinoderm larvae. The difficulty
consists in the varying number and arrangement of the ciliated rings, which most
recall the condition in the Holothurid larva (pupa), to be described later. The
vestibular depression of the Antedon larva may, however, be compared without
forcing to the ventral depression of the other Echinoderm larvse. A thickening of
the ectoderm, comparable with the neural plate of the Antedon larva, also occurs, as
we shall see in other Echinoderm larvse.
B. Ontogeny of the Holothurioidea.
The segmentation of the ovum is total and equal, and leads to the formation of a
coeloblastula, whose unilaminar cell wall usually consists on one side of somewhat
larger cells. By invagination of this part of the blastula wall, a ccelogastrula is
formed. The invaginated part, i.e. the archenteron, is a blindly ending tube, with
narrow lumen (archenteric cavity), which is far from filling the segmentation cavity.
This latter is filled with an albuminous, fluid or semifluid, mass, the gelatinous
nucleus.
The ectoderm and the endoderm are ciliated.
During the process of invagination (occasionally even during the blastula stage)
cells arise by division from the ectoderm, but more especially from the endoderm,
which, as mesenchyme cells, wander into the enclosed jelly-like substance, multiply
by division and, in ever-increasing numbers, occupy the blastocoel. From them is pro-
duced all the connective tissue of the Holothurian body. The calcareous corpuscles
arise exclusively in the mesenchyme.
VIII
ECH1NODERMATA ONTOGENY
511
The blind end of the lengthening archenteron bends to that side, which becomes
the dorsal side of the larva as it rapidly grows bilaterally symmetrical (Fig. 403, A),
and at the same time it moves somewhat to the left side. As the hydro-enteroccel
vesicle, it soon becomes entirely constricted off from the rest of the archenteron,
which opens outward through the blastopore (Fig. 403, B, C, D, 4, 5).
This constriction from the archenteron, the hydro -enterocoelomic vesicle, is of
the greatest importance, because out of its wall arises the whole musculature of the
body and all the internal epithelia, i. c. the coelomic and water vascular epithelia.
The hydro-enterocrelomic vesicle increases in length, alongside of the intestine,
in the direction of the blastopore, and again divides into two vesicles by means of a
FIG. 403. Formation of the larval mouth
and the hydro - enteroccelomic vesicle in the
gastrula larva of Synapta digitata, diagram-
matic (after Selenka). .4. Gastrula, the archen-
teron bent towards the dorsal side ; B, archen-
teron, opening outward through the hydropore ;
C, hydro-enteroccel, constricted from the intestine ;
D, intestine, opening outward through the larval
mouth ventrally. 1, Segmentation cavity, blasto-
coel ; 2, archenteron ; 3, blastopore ; 4, hydropore ;
5, hydro-enteroccel ; 6, intestine ; 7, esophagus ;
8, mouth; ant, anterior; post, posterior; r, ven-
tral ; d, dorsal.
FIG. 404. Auricularia with the left half of
the ectoderm removed, from the left (after
Ziegler's model). The organs lying in the
segmentation cavity (11) are seen. 1, Cut edge
of the ectoderm ; 2, mouth ; 3, oesophagus ;
4, mesenchyme cells ; 5, mid-gut or stomach
intestine ; 6, anus ; 7, hind-gut ; 8, left entero-
coel, still connected with the hydroccel 10, the
latter showing slight indications of the first
radial outgrowths ; 9, dorsal pore or hydro-
pore ; 11, blastoccel, segmentation cavity.
transverse constriction. The anterior vesicle (that further from the blastopore) is
the hydrocffilomic vesicle, which at once sends off a canal to the dorsal side, which
opens outwards through a pore on the left of the middle line. The canal is the
primary stone canal, and the pore the primary madreporite. The hydroccelomic
vesicle is the rudiment of all the rest of the water vascular system, and in the first
place, of course, of the circular canal (Fig. 404).
The first appearance of the various structures just described does not occur in the
same order in all species of Holothurioidea examined on this point. The hydro-
enteroccelomic vesicle may become connected with the exterior through a stone canal
before it has divided, or even (a unique condition found in Synapta digitata} before
512
COMPARATIVE ANATOMY
CHAP. VIII
it has itself separated from the archenteron (Fig. 403). In the last case, it can be
established that the archenteron which begins with the blastopore opens outward for
a time through a second aperture, the primary madreporite.
After the hydro-enterocoelomic vesicle has become constricted from the archen-
teron, the intestine grows further, its blind end bending to the ventral side (that lying
opposite to the water pore), which commences to become depressed and to sink in.
The blind end of the intestine soon becomes applied to the ectoderm of the
depressed ventral side of the larva, about half way down the body, or a little in
\
OS-
2
20-
U^-
CCJl
FIG. 405. Young Auricularia of
Synapta, from the ventral side (after
Semon). 1, Circumoral ciliated band ;
2, entero - hydrocoel ; 3, calcareous
wheel ; 4, adoral ciliated ring ; os,
mouth ; an, anus ; 5, mid - gut or
stomach intestine ; 6, larval nerve
band.
FIG. 406. Older Auricularia, seen diagonally from the
left lower side (after Semon). 1, Circumoral ciliated band ;
2, hydropore ; 3, hydrocoel ; 4, adoral ciliated band ; 5, mid-
gut or stomach - intestine ; 6, nerve band; 7, hind -gut;
8, left enterocoel ; 9, calcareous wheel ; 10, fore-gut, oaso-
phagus ; 11, right enterocoal.
front of the middle point. Where the two touch one another, an aperture, the mouth,
breaks through.
The median portion of the intestine (the mid-gut) swells up and becomes distinct
both from the fore-gut and from the hind-gut.
In the meantime, the larva has undergone a change of shape through which it
reaches the Auricularia stage, the depression of the ventral side being the most
important part of this change. The general ciliation has disappeared ; of the
complete covering of cilia, only the circumoral ring and the adoral band are
retained, and the region immediately around the mouth has become depressed to
form the oral vestibule (Figs. 404-407).
The transformation of the Auricularia into the barrel-shaped larva (Figs. 408-
413). The Auricularia does not change direct into a young Holothurid, but passes
through an intermediate stage, which was formerly known as the pupal stage,
becfinse during it no nourishment is taken.
Fi<;. 407. Older Auricu-
laria (after Semon). an,
Anus ; os, mouth ; 1, out-
growths (primary and
secondary) of the primitive
lior.se - shoe - shaped hydro-
ccel ; 2, stone canal ; 3, left,
4, right enterocoelomic sac,
which have become closely
applied to the mid-gut.
FIG. 40S. Auricularia,
in which the circumoral
ciliated ring is beginning
to break up into lengths
(after Semon). The horse-
shoe - shaped hydroco?! is
growing round the intestine.
The first pieces of the cal-
careous ring (1) have ap-
peared.
VOL. II
2 L
514
COMPARATIVE ANATOMY
CHAP.
The Auricularia assumes the shape of a barrel. The circnmoral ciliated ring
atrophies in sixteen places, which are indicated in the diagram (Fig. 413). The
sixteen lengths of the ring which remain continue to grow and join, as indicated
FIG. 409. Old Auricularia.
Transition to the barrel - shaped
pupa, the whole body decreasing
considerably in size. 1, The nerve
bands, in the act of forming the
nerve ring ; 2, primary tentacle.
FIG. 410. Intermediate stage be-
tween Auricularia and the barrel-
shaped pupa of Synapta (after
Semon). I-V, the rudiments of the
five ciliated rings. 1, The oral funnel ;
2, the primary ; 3, the secondary out-
growths of the water vascular ring ;
4, pieces of the calcareous ring ; 5,
coelomic vesicle ; 6, water vascular ring.
by dotted lines (Fig. 413), to form five ciliated rings entirely encircling the barrel -
shaped body. The centre of the former oral area becomes surrounded by four lengths
of the ciliated band which join together to make a square. The part of the oral
FIG. 411. Young barrel - shaped
larva (pupa) (after Semon). 1, Oral
funnel ; 2, tentacles ; 3, pieces of the
calcareous ring ; 4, Polian vesicle ;
5, left coelom ; 6, hind-gut ; 7, audi-
tory vesicles ; 8, secondary out-
growths of the hydrocoel ring.
FIG. 41-2. Barrel- shaped larva,
with the tentacles (1) beginning to
project from the opening oral funnel
(after Semon). 2, Water vessels of
the body secondary outgrowths of
the circular canal ; 3, the rapidly
swelling enterocoel.
area enclosed by this ring sinks below the surface, and thus increases the size of the
oral vestibule. The ciliated square itself becomes depressed and forms the oral
shield. The spacious oral vestibule becomes cut off from the exterior, with the
exception of a very narrow aperture, and shifts quite to the front, so that now the
ECHINODERMA TA ONTOGENY
515
three apertures, that of the oral vestibule, the mouth itself lying in its floor, and the
anus, lie almost in the axis of the barrel-shaped body.
In the older Auricularia stages and during the transformation into the barrel -
shaped larva important internal processes take place.
Calcareous corpuscles appear early (even in the younger Auricularia) in the
mesenchyme. In the best known Auricularia, that of Synapta digitata, these
bodies appear iu the form of wheels in the two
posterior tips of the larva (cf. Fig. 404, p. 511,
and following).
The hydroccelomic vesicle assumes the form
of a horse-shoe with the curve towards the dorsal
side. On the convex side of this horse-shoe-
shaped vesicle five outgrowths appear. The two
arms of the horse-shoe then close round the fore-
gut, growing towards each other round it until,
finally, they meet and fuse (probably in the
right half of the body). The horse-shoe-shaped
hydroccel is now the closed water vascular ring
.surrounding the fore-gut It continues, as before,
to communicate with the exterior through the
primary stone canal and the dorsal water pore.
The five outgrowths of this hydroccel ring
now become more distinct. They are originally
directed forwards, but very soon, with further
growth, bend backward, and, as the rudiments
of the radial canals of the water vascular
system, grow further back under the body wall,
in the five radii. The rudiments of the tentacle
canals appear very early on the rudiments of culuria (after Ludwig). The pieces of
the radial canals as orally directed lateral out- the ciliated band are uaarked b * v broad
, black lines, the interruptions being left
:hs - clear. 8, The preoral ; 9, the postoral
The above account of the first processes of intermediate piece of the ciliated ring ;
differentiation in the hydroccel vesicle are those os, mouth. The dotted lines give the
found in Cv.cuiiinna Pland, the ontogeny of direction in which the pieces of the ring
which has recently been carefully investigated. '^Sf&^V^^'
Iii other Holothurioidea, at least in Synapta
diqitata, according to former authors, the ontogenetic processes differed essentially
from these. The first five outgrowths of the hydroccel in Synapta develop exclusively
into the tentacle canals, and only after the appearance of these and alternately
with them, five other outgrowths form the rudiments of the radial canals.
This and certain other discoveries led to the conclusion that the radial canals
in the Holothurians arise interradially and only shift into the radii secondarily,
hence it was inferred that the tentacle canals of the Holothurioidea were homologous
with the radial canals of other Echinoderms, and that the radial canals of the Holo-
thurioidea are not represented in other classes. The above discoveries in the larva
of Cucn.iiw.ria Planci dispose of this suggestion, which must always have appeared
improbable to comparative anatomists.
It is a very noteworthy fact that, in Synapta, the radial canals appear onto-
geuetically, whereas they are wanting in the adult.
The Polian vesicle also arises as an outgrowth of the circular canal ; in Cucu-
nuiri'A Plfiiid. it forms at the point where it lies in the adult, i.e. in the left dorsal
interradius.
The tube-feet arise as outgrowths of the radial canals, which push the ectoderm
FIG. 413. Diagram illustrating the
rise of the five ciliated rings of the
Holothurian pupa from the pieces of the
ciliated bands 1-7 and l'-7' of the Auri-
516
COMPARATIVE ANATOMY
CHAP.
out in front of them (Fig. 415). The first two tube-feet in Cucumaria Planci arise
simultaneously at the posterior end of the body. Both these feet belong to the
medioventral radial canal.
The differentiation of the enteroccel vesicle. After the hydro-enteroca-1
vesicle has divided into the hydrocoel and the enteroccel vesicles, the latter grows
backwards longitudinally, the growing posterior end pushing its way gradually over
the intestine, on to its right side (i.e. into the right-hand portion of the segmenta-
tion cavity). The anterior part of the enterocoel vesicle now lies to the left, the
posterior to the right, near the intestine (Fig.
404). These parts become completely separated
by a constriction which stretches transversely over
the intestine, into a left and a right enteroccel
vesicle.
Each of these vesicles becomes applied to the
intestine, and, increasing in size, takes the shape
of a hollow disc resembling a watch glass.
The nervous system of the larva. On each
side of the Auricularia larva, on the ventral side,
in the oral area, there is a ciliated ectodermal
ridge, beneath the surface of which ganglion cells
lie and longitudinal nerve fibres run. The ridge
consists of two limbs, meeting in an obtuse angle
which is open towards the mouth. From the ends
and angles of these ridges nerve fibres go off to
the circumoral ciliated band.
Formation of the tentacles. The tentacle
vessels of the water vascular system ;
7, hind -gut; 8, calcareous wheels;
9, mid-gut ; 10, madreporite ; 11, stone
canal.
FIG. 414. Young Synapta (Pentac-
tula) (after Semon). 1, Oral tentacles ;
2, auditory vesicles ; 3, pieces of the
calcareous ring ; 4, water vascular
ring ; 5, Poliau vesicle ; 6, radial canals, whether as lateral outgrowths of the radial
canals or direct outgrowths of the water vascular
system, grow towards the oral vestibule, and press
out the ectodermal wall of the latter before them.
The ectodermal covering thus afforded them is
derived from the oral shield, i.e. indirectly from parts of the original circumoral
ciliated ring of the Auricularia larva. The tentacles (five of which form first)
remain hidden in the oral vestibule during the pupal stage.
Transformation of the barrel- shaped larva into the young Holothurian (Figs.
414 and 415). The external changes are as follows. The ciliated rings atrophy.
The tentacles project freely from the expanding and widely opening vestibule, and
increase in number. In the Actinopoda tube-feet are formed in all the five radii.
It is an important fact that, in the comparatively simple transformations of the
Holothurian, not only does the whole of the larval epithelium pass into the body
epithelium of the adult, but none of the larval organs are eliminated.
The following description applies to Cucumaria Planci.
The tentacles. It is somewhat remarkable that the five tentacles first formed do
not each arise from a single radial canal. Two of the five tentacles, on the contrary,
receive their canals from the medioventral, and two others from the left dorsal
radial canal. The fifth tentacle belongs to the right dorsal radial canal. Other
tentacles appear only at a very late stage, two more being added first, a sixth and a
seventh. These belong to the two lateral ventral radii, which up to this time have
been without tentacles.
The stone canal. An anterior outgrowth forms on the primary stone canal ; the
epithelium of this outgrowth flattens, giving rise to the madreporitic vesicle. On
the wall of this vesicle the mesenchyme forms an incomplete shell, perforated like a
lattice
viii ECHINODERMATA ONTOGENY 517
The water pore lying on the right side of the mesentery (now formed) disappears
19
FIG. 415. Longitudinal section of a larva of Cucumaria doliolum (aftp r Selenka). 1, Frontal
prominence with enclosed jelly-like substance; -2, tentacle vessels; 3, 6, 15, 14, 13, radial
vessels ; 5, stone canal ; 8, madreporite ; 4, Polian vesicle cut off ; 7 and 12, coelom, enterocoel
vesicle ; 9, anus ; 10 and 11, the first two tube-feet ; 16, circular canal of the water vascular
system ; 17, oesophagus ; 18, mouth ; 19, mesenchyme cells ; II, III, IV, V, ciliated rings.
later, and still later the madreporitic vesicle opens into the body cavity, and thus
becomes the secondary inner madreporite.
518 COMPARATIVE ANATOMY CHAP.
The radial canals. The five radial canals do not develop with equal rapidity,
nor indeed do the radial nerves and the radial longitudinal muscles. The medio-
ventral organs (radial canal, radial nerve, longitudinal muscle) in all cases arise first,
then follow the organs of the two dorsal radii, and only at a later stage, those of
the two lateral ventral radii.
The tube-feet. In agreement with the order of appearance just described, the
two first tube-feet, already mentioned above, belong to the ventral radius (Fig.
415). The two next in order also belong to the medioventral radial canal, and,
according to the rule which applies to all the newly developing tube-feet, arise in
front of those already present. The fifth tube-foot belongs to the left dorsal radial
canal. (The correspondence of this order with that of the rudiments of the tentacles
should be noted. )
According to observations which have been made, it appears that
when, in a Holothurian, the tube-feet are scattered, this arrangement
is, ontogenetically, secondary. In the same way animals which have
several rows of tube-feet in each radius have only two rows in a young
stage, or a zigzag row of alternating feet.
The nervous system. The first part of the nervous system to
appear is the oral circular nerve, and this arises as an ectodermal
circular ridge on the floor of the oral vestibule in the larva. It
sends out five band-like processes in the direction of the rudiments of
the radial canals ; these are the rudiments of the radial nerves.
In that the rudiments of the circular nerve and the radial nerves
become subepithelial, there arises between them and the body
epithelium which closes over them a narrow space ; this is the
epineural canal.
The rudiments of the five radial nerves grow backwards together
with those of the radial canals.
In Cucumaria Planci there seems to be no larval nervous system.
In Synapta, on the contrary, the larval nervous system yields the
rudiments of the definitive system. The two lateral nerve ridges of the
Auricularia larva, when the oral vestibule of the barrel-shaped larva is
formed, shift into it. Their free ends then become connected from
the two sides to form a ring encircling the mouth, which is the rudi-
ment of the nerve ring.
The intestine shows, at an early stage, the coils characteristic of the adult.
The first portions of the calcareous ring to appear are the five radial pieces : these
arise on the radial canal and, like all the calcareous structures, are produced by the
mesenchyme. The medioventral calcareous piece is from the first the largest.
The enteroccel. The right and the left enteroc The ttrst larval arms; 2 ' rudiraent of tLe
, , larval mouth ; 3, ectoderm ; 4, hvdro-enterocoel
posterior ventral arms, an ectodermal in- rudiment . 5> ^enteron ; 6, blastopore ;
vagination appears (Fig. 419, 3) ; this 7, larval skeleton.
sinks into the blastoccel in the shape of a
flask. This invagination plays an important part in the transformation of the larva
into the young Echinoderm.
Third stage (fully-grown Pluteus larva). The two anterior dorsal and the two
anterior ventral arms continue to grow (cf. Figs. 400 and 401, p. 509). On the dorsal
side, a fifth unpaired calcareous spicule arises, and, in the immediate neighbourhood
of the water pore, sends off processes, two of which enter the anterior dorsal arms
and support them. The body has shortened, and its posterior region has become
rounded off.
Further differentiation of the hydro-enteroccel. We resume the description of
this organ from the stage in which it consisted of two lateral vesicles applied to the
intestine. Each vesicle now becomes divided by a constriction into an anterior
and a posterior vesicle. The two anterior vesicles lie at the sides of the posterior
portion of the cesophagus, the two posterior at the sides of the stomach intestine.
The left anterior vesicle opens outward through a water pore ; the other three are in
no way connected with the future water vascular system ; they are distinguished as
the right anterior, right posterior, and left posterior enteroccel vesicles. Some-
what later, three vesicles are seen on the left side (Fig. 419, 2, 4, 5). The anterior
and middle vesicles are in communication, whereas the posterior is distinct, and
becomes applied to the middle vesicle, assuming a crescent shape. The left
522
COMPARATIVE ANATOMY
CHAP.
anterior (enteroccel) vesicle is the one which opens outward through the water
pore ; it, however, does not become the hydrocoel, but the middle vesicle (which is
in communication with it) now represents the rudiment of the hydrocoel. It is
probable that this is produced by constriction from the left anterior hydro-enterocu-1
vesicle. .
On the left side, the development of the hydro-enterocoel is now as follows.
The water pore leads into the posterior end of a left anterior enteroccel vesicle,
which, again, communicates, by means of a constricted portion, with the hydroccel
ler
post
FIG. 419. Dorsal aspect of an EcMnoid
Pluteus, to illustrate the relations of the hydro-
enterocoel (after Bury), ant, Anterior ; post, pos-
terior ; sin, left ; dex, right ; 1, larval oesophagus ;
2, left anterior enterocoel ; 3, ectodermal in-
vagination ; 4, rudiment of the hydrocoel ; 5, left
posterior enterocoel vesicle ; 6, stomach intes-
tine ; 7, right posterior enterocoel ; 8, hydro-
pore ; 9, unpaired dorsal skeletal piece ; 10, right
anterior enteroccel. The arms are not fully
represented.
FIG. 420. Rudiment of the Echinoid
in the Pluteus larva of Echinocyamus
pusillus (after Theel). The Pluteus is seen
from the dorsal side, and only the left side
is completely drawn. 1, Arms of the
Pluteus ; 2, aperture of imagination of the
sac (3), whose floor will form the oral body
wall of the Echinoid ; 4, outgrowths of the
hydrocoel, which push this wall before them
and form the first ambulacral tentacles;
skeletal rods of the Pluteus ; 6, hydrocoel ;
7, hydropore ; 9, portion of a skeletal piece
lying in the neighbourhood of the hydro-
pore, which will probably become the
madreporite ; 8, stomach-intestine.
vesicle. This latter is embraced posteriorly by the horse-shoe-shaped left posterior
enteroca-1. The stone canal does not arise out of the water pore, but out of the
connecting piece between the left anterior enterocoel vesicle and the hydroccel
vesicle, which becomes drawn out into a canal. The left anterior enteroccel
seems to become the madreporitic ampulla.
(The above description of the differentiation of the hydro-enteroccel must not be
considered as fully established. The observations are not quite continuous, and
do r >t all agree.)
VIII
ECHINODERMA TA ONTOGENY
Transformation of the Pluteus larva into the young Echinoid. This meta-
morphosis is far from having been satisfactorily described, its investigation being
exceedingly difficult.
An important part in the shaping of the Echinoid body is played by the above-
mentioned flask-like invagination of the ectoderm on the left side. The thicken-
ing floor of this invagination grows towards the hydroccel, and becomes externally
applied to this latter as the "Echinoid disc" (Fig. 420). The thin lateral walls of
the capacious flask, which is still connected by its neck with the larval ectoderm,
are known as the amnion.
The hydroccel vesicle assumes the horse-shoe-shape, and at the same time puts
forth five outgrowths which push before them the Echinoid disc, i.e. the floor of
dea
FIG. 421. Dorsal aspect of a larva of Echi-
nocyamus pusillus, about forty-five days old
(after Theel). 1, The larval arms, with their
calcareous rods ; 2, unpaired calcareous rod,
taking part in surrounding the dorsal pore (3) ;
4, spines ; 5 and 6, primary tentacles of the
young Echinoid ; ant, anterior ; post, posterior ;
si/i, left ; dex, right.
FIG. 422. Lateral view of a very
young Echinoid (Echinocyamus
pusillus\ forty - five days old (after
Theel). The first tube-feet and spines
of the Echinoid are seen, and, attached
to its back, the remains of the calcar-
eous rod of a larval arm.
the flask-like invagination. Five hollow tubes thus now project into the cavity of
the flask, which continually becomes more and more spacious ; these are the five
primary tentacles, which receive their covering from the spreading Echinoid disc.
This Echinoid disc forms the oral wall (that is, no doubt, only the epithelium and
the nerves ?) of the young Echinoid, while the apical wall is formed direct from the
larval dorsal ectoderm of the Pluteus.
The fate of the amnion is differently described for different forms. Sometimes
it is said to pass over into the young Echiuoid, the amnion sac opening and spread-
ing out, and yielding the circular integumental region between the apical and oral
surfaces of the body. At other times, again, the amnion sac is said to remain closed
524
COMPARATIVE ANATOMY
CHAP.
and the anmion, together with part of the larval integument, are lost when the
larva changes into the young Echinoid.
The larval arms disappear, and their spicules are for the most part absorbed.
As a rule, one or other of the arms of the Pluteus still adheres to the quite young
Echinoid (Fig. 422).
The intestine, at least the whole stomach, the spreading enterocoel, and the
growing hydrocoel are taken over into the young Echinoid ; the latter, however,
has, at first, neither mouth nor
Becomes the madrepontlC
7, radial skeletal plates ; 8, interradial skeletal plates. basal. Four other plates, which
arise over the right enterocoel
of the larva, become the other basals. In their centre the dorso-central is soon
distinguishable. On the oral side, in the peripheral part of the original Echinoid
disc, where the primary tube-feet developed, the first ambulacral and interambu-
lacral plates appear, with the rudiments of the spines and the sphseridia, both
of which form independently over the plates (Fig. 423). In the future oral area,
which is surrounded by a circle of ambulacral and interambulacral plates, thirty
small calcareous centres form, three in each radius and three in each interradius ;
these are the rudiments of the plates of the masticatory apparatus. The middle
calcareous plates of the interradii become the teeth.
Little or nothing is known of the ultimate fates of the other twenty-five pieces, or
of the enterocoel, of the hydroccel (e.g. the order of appearance of the tube-feet), or as
to the appearance of the nervous system, the origin of the radial plates, etc.
D. Ontogeny of the Asteroidea.
Segmentation is total, and leads to the formation of a coeloblastula, through the
iir. igination of which a ccelogastrula arises. The formation of the mesenchyme
takes place in the manner already described for the Holothurioidea and the Echin-
VIII
ECHINODERMATA ONTOGENY
525
oidca, and commences either in the blastula stage or not until the gastrula stage.
In the former case, that part of the blastoderm which becomes iuvaginated to pro-
duce the archenteron yields the mesenchyme, and, after invagination has taken
place, continues to produce it. In the second case, also, the endoderm is the place
of formation of the mesenchyme cells, which wander into the blastoccel. Such
observations, however, seem to point to the fact that, although most mesenchyme
cells arise from the endoderm, the ectoderm also takes part in their formation.
In the older gastrula stage of Asterias vulgaris, the ectoderm seems to be
thickened at the (aboral) pole opposite to the blastopore. This may be the rudi-
ment of the neural plate.
As a further illustration of the development of the Asteroidea, we shall utilise
the observations made on Asterina gibbosa, in which form, however, a typical
FIG. 424. Asterina gibbosa,
gastrula four days old ; ap-
proximately horizontal longi-
tudinal section, from the ventral
side (after Ludwig). ant, An-
terior ; post, posterior ; dex, right ;
sin, left ; 1, segmentation cavity ;
2 and 3, right and left coelomic
outgrowths of the archenteron ;
4, blastopore.
FIG. 425. Asterina gib-
bosa, larva at the end of the
fourth day, horizontal longi-
tudinal section seen from the
ventral side (after Ludwig).
The enterocoel outgrowths
have grown in length. 2, right
enterocrel outgrowth ; 3, left
or hydro - enterocoel out-
growth ; 5, intestine ; 6, an-
terior unpaired coelom. The
coelom is still in open com-
munication with the intestine.
FIG. 420. Asterina gib-
bosa, larva at the com-
mencement of the fifth
day, horizontal longitudi-
nal section (after Ludwig).
The enteroccel has become
constricted off from the
intestine. Lettering as
before.
Bipinnaria larva does not attain development. In the course of the description
observations made on other Asteroids will be referred to.
In the ovoid gastrula of Asterina, the blastopore does not lie altogether at the
posterior pole, but is shifted somewhat on one side, which in the further develop-
ment of the animal becomes distinguishable as the ventral side. Two sections can be
made out in the archenteron, a short cylindrical commencement (posterior section),
and a vesicular blind terminal part (anterior section). This description applies to
the gastrula on the second day.
Third day. The rudiment of the hydro- enterocoel vesicle. The anterior
vesicular section of the archenteron, which represents the rudiment of the hydro-
enterocoel vesicle, bulges out posteriorly on each side, while its wall becomes
thinner (Fig. 424). The two bulgings grow out longitudinally backwards, at the
sides of the posterior part of the archenteron, and become the two hydro-enteroccel
526
COMPARATIVE ANATOMY
CHAP.
vesicles, which continue to grow backward in proportion as the posterior part of the
archenteron, the larval intestine, grows anteriorly (Fig. 425).
Fourth day. The whole hydro -enteroccel becomes constricted off from the
larval intestine, and is now found as a large vesicle occupying the anterior part of
the larval body, and continued posteriorly in the two long hydro-enteroccel vesicles,
the one on the left being longer than the one on the right (Fig. 426).
An imagination of the ectoderm, somewhat anteriorly to the middle of the
ventral side, represents the rudiment of the larval mouth and oesophagus, and.
towards the end of the fourth day, breaks through into the larval intestine.
Anteriorly a cushion-like thickening of the body appears encircling a depression.
post
FIG. 428. Larva of As-
terina gibbosa four days
Old, just hatched, from
the ventral side (after
Ludwig). 1, Larval organ ;
2, blastopore. Here, and
in the following figures,
ant = anterior ; post = pos-
terior.
FIG. 429. Asterina
gibbosa, larva six days
Old, from the left side
(after Ludwig). v, Ven-
tral side ; d, dorsal
side ; 1, larval organ.
FIG. 427. Asterina gibbosa,
larva five days old, horizontal
longitudinal section seen from the
ventral side. First rudiment of the
hydroccel outgrowth (7) on the left
of the hydro-enterocoel vesicle (3).
The two enterocoel vesicles have
opened into one another posteriorly
at 8.
This circular cushion, the rudiment of the larval organ, slants from above anteriorly
to below posteriorly (Figs. 428-430).
At the end of the fourth day the embryo leaves the egg-envelope, and swims
about freely by means of the cilia covering its entire surface.
Fifth day. The two hydro-enterocoel vesicles grow round the larval intestine
above and below. Where they meet below, somewhat to the left of the median line,
they form a ventral mesentery, which, however, rapidly disappears, the two vesicles
opening into one another at this point. Above the intestine a dorsal mesentery,
lying to tlu right of the median line, arises in a similar manner, and persists.
The left hydro-enterocoel vesicle bulges out laterally somewhat behind its middle
point. This bulging is the rudiment of the hydroccel (Fig. 427).
At this stage, therefore, the hydro-enterocoel consists of the following sections in
widely open communication with one another.
1. Anterior unpaired enterocoel (6 in Figs. 426, 427), lying in the larval organ.
2. Right enterocoel vesicle (2 in the figs.), anteriorly in wide open communica-
tion with No. 1, and ventrally in open communication with
ECHINODERMA TAONTOGEN Y
527
3. The left enterocoel vesicle (3 in the figs.). This vesicle, again, has an out-
growth (7) on the left, which is
4. The hydroccel vesicle.
Simultaneously with the formation of the hydroecel rudiment, that of the water
pore appears dorsally, somewhat to the left of the median line, as an invagination
of the ectoderm, which grows towards the left enterocoel, and breaks through
into it.
Sixth and seventh day. The outer form of the larva has been considerably
modified on the fifth day. The larval organ has increased in size, and its sloping
circular ridge projects considerably beyond the surface of the larval body.
The rudiment of the hydrocoel has grown out further backward, but is still
anteriorly in open communication with the left enteroccel. Five outgrowths (Nos.
I-V in Fig. 435) now appear at its posterior edge ; these are the rudiments of the
five radial vessels. The water pore (dorsal pore, madreporite) still leads into the
post
FIG. 430. The same
specimen of Asterina gib-
bosa viewed from the
left and from the ventral
Side. 1, Larval organ with
ir> dorsal and ventral
lobes ; 2, larval mouth.
FIG. 431. Asterina gibbosa, larva at the
beginning of the eighth day, from the left
side; the larval organ is very largely de-
veloped (after Ludwig).
left enteroccel. "A channel develops on that wall of the hydrocoel which faces the
interior of the body, which soon closes to form a canal. " This canal runs towards
the point where the dorsal pore opens into the left enteroccel. One end of this
canal remains in open communication with the hydroccel, while the other enters
the enterocoel quite near the aperture of the dorsal pore. This canal is the stone
canal of the future Asteroid. The dorsal pore of the larva does not thus lead
direct into the stone canal, but enters it through the left enteroccel (Fig. 436).
Only at a later stage does the dorsal pore come into direct connection with the stone
canal.
Formation of the hydro-enterocoel in other Asteroids. In the larva of Asterias
fulf/aris also, the entero-hydroccel arises in the form of two lateral diverticula of
the blind and somewhat swollen archenteron, whose wall has become thinner. The
two diverticula soon become constricted from the archenteron, and become distinct
vesicles. Each sends off an outgrowth towards the dorsal surface, a growth of the
ectoderm running in towards it. The two meet and fuse, become hollow, and form
the stone canal with the water poro. Thus in the young Bipinnaria larva of
Asterias vulgaris, the bilateral symmetry is so marked that a right as well as a
left stone canal attains development (Fig. 432). The right pore, however, soon
disappears, and the right canal somewhat later.
528
COMPARATIVE ANATOMY
CHAP.
The two lateral mesoderm vesicles lengthen and fuse in front of and above the
mouth, and, further, surround the intestine. On trie left vesicle (hydro-enteroccel
vesicle) a transverse constriction appears, which finally divides it into two vesicles,
an anterior, which at its posterior end opens outward through the stone canal and
water pore, and a posterior (Fig. 433).
Further development of the hydroccel in Asterina gibbosa. After the seventh
day the five outgrowths of the hydrocrel (Figs. 437-440) become trilobate, and
later have five lobes. The unpaired terminal lobe of each outgrowth is the rudiment
of the terminal tentacle, the paired lobes are the rudiments of the first two pairs
ant
2 1-
1-
pott
FIG. 432. Larva 01
Asterias vulgaris, about
four days old, from the
dorsal side (after Field).
1, Circumoral ciliated
band ; 2, n.outh ; 3, right
and left hydro-enterocoel
vesicles, with their hydro-
pores (4) ; 5, oesophagus; 6,
mesenchymatous muscle
fibres; 7, stomach intes-
tine ; 8, anus. The mouth
and the anus lie on the
side turned away from
the reader.
FIG. 433. Dorsal aspect of a
Bipinnaria larva to illustrate the
developmentoftheliydro-enteroccel
(after Bury). 1, Larval oesophagus ;
2, left anterior enterocoel ; 3, hydro-
pore ; 4, rudiment of the hydrocoel ;
5, stomach intestine ; 6, terminals ;
7, left posterior enterocoel vesicle ;
8, dorsal mesentery ; 9, right posterior
enterocoel vesicle ; 10, madreporite ;
11, blood vesicle, pulsating vesicle ;
12, right anterior enterocoel.
FIG. 434. Asterina gib-
bosa, larva six days old,
horizontal longitudinal sec-
tion from the ventral side
(after Ludwig). The hydro-
coel (7) has become con-
stricted posteriorly from the
left enterocoel. An out-
growth of the intestine (8) is
the first indication of the
future oesophagus of the
Asteroid.
of tube-feet. Each new pair of feet arises between the terminal tentacle and the
foot last formed.
The five outgrowths of the hydroccel become outwardly visible, bulging out the
body. On the left side of the seven-days-old larva there are thus visible five flat
protuberances arranged in a convex arch directed upward and backward ; these
protuberances become more marked on the eighth day, and are then divided either
into three or five lobes each (Figs. 438-440). These are the first indications of the
young Asteroid, the rudiments of its ambulacral arms.
The rudiment of the definitive oesophagus appears in the form of a bulging of
the left side of the archenteron, that facing the hydroccel. This arises in the region
which corresponds with the anterior part of the gastrula intestine (Fig. 434, 8), and
has nothing to do with the larval oesophagus. This latter degenerates on the
ei&i th or ninth day, and the larval anus also disappears.
VIII
ECHINODERMATA ONTOGENY
529
The larval organ attains its greatest development on the eighth or ninth day, it
diminishes in size later, and finally is altogether resorbed, without giving origin to
any organ of the young Asteroid. Its wall consists of three layers : (1) an outer
ciliated larval epithelium, (2) the inner epithelium of the unpaired section of the
enteroccel which fills up the whole cavity of the larval organ, and (3) between these
two layers, one of mesenchyme cells differentiated into muscle fibres. The larva uses
the organ for locomotion and for temporary attachment.
Soon after the ambulacral (oral) rudiments of the arms have appeared on the left
side in the form of the five protuberances (1-5, Fig. 437) mentioned above, five
mesenchyme thickenings form, which also bulge out the ectoderm, and represent
the antiambulacral (apical, dorsal) arm rudiments (I-V). Three of these are
22
FIG. 435. Asterina gibbosa. larva six
days old, seen from the left (after Ludwig).
I-V, The five primary bulgings of the
hydroccel (7) ; 3, the left enterocoel, opening
outward through the hydropore or dorsal
pore (11) on the dorsal side ; 5, intestine;
6, anterior enterocoel, enterocoel of the
larval organ ; 9, larval mouth ; 10, mesen-
tery ; 11, dorsal pore ; 12, ectoderm of the
larval organ.
FIG. 436. Asterina gibbosa, larva eight
days old, seen from the dorsal and somewhat
from the left side, optical longitudinal section
(after Ludwig). II, V, second and fifth primary
bulgings of the hydrocoel ; 3, left enterocoel ;
5, intestine ; 7, hydroccel ; 11, dorsal pore ;
13, oesophagus of the Asteroid ; 14, rudiment of
the stone canal.
found on the left ventral side, and two somewhat to the left of the median line, on
the dorsal side of the larva. The five stand in a curved row, the curve opening
anteriorly, the plane in which they lie making an angle with that of the ambulacral
arm rudiments.
These two sets of arm rudiments then shift towards one another, until their
planes are nearly parallel.
Appearance of the skeletal plates. As early as the time when the hydrocoel
bulgings begin to become trilobate, a small calcareous body appears in the mesen-
chyme on each side of each primary bulging, on the proximal side of the lateral
secondary bulgings (i.e. of the rudiments of the first tube-feet). These are the
rudiments of the first five pairs of ambulacral plates. "When a second pair of lateral
lobes appear distally to the first pair on each hydroccel bulging, a second pair of
VOL. II 2 M
530
COMPARATIVE ANATOMY
CHAP.
calcareous bodies form between this and the first pair ; these are the rudiments of a
second pair of ambulacral plates, and so on.
FIG. 437. Asterina gibbosa, larva at the end
of the eighth day, from the left side (after
Ludwig). 1-5, The ambulacral rudiments of the
arms over the primary hydroccel bulgings ; I-V, the
antiambulacral rudiments of the arms.
FIG. 438. Asterina gibbosa, larva ten
days old, seen from the left and some-
what from the ventral side (after Ludwig).
The ambulacral arm rudiments (1-5) now
have five lobes, ft, Terminal lobe, terminal
tentacle.
As early as the seventh day, the rudiments of the apical skeletal plates, eleven
in number, appear. These eleven rudiments lie superficially below the ectoderm of
FIG. 439. Asterina gibbosa, young
Asteroid, with much reduced lateral
organ (fc,) at the end of the tenth day,
from the left side (after Ludwig).
The first rudiments of the ambulacral
skeleton have appeared (five pairs of
ambulacral plates). The mouth of
the Asteroid has not yet formed.
FIG. 440. Asterina gibbosa, young Asteroid
eleven days old, horizontal section immediately
below the oral surface (after Ludwig). 1-5, The five
five-lobed outgrowths of the hydroccel ring (aa) which
has not yet, closed ; ax, the two outgrowths at the two
ends of the horse-shoe-shaped hydroccel, which by
growing out towards one another and opening out
into one another close the hydroccel ; lo, interradius of
the larval organ ; m, interradius of the madreporite.
the apical region. Five of them appear in the mesenchyme of the five apical brachial
ru.liments, and become the terminals of the Asteroid arms, always remaining at the
VIII
ECHINODERMA TAOKTOGENY
531
tips of the growing arms (Fig. 441, ^ - 1 5 ). Five others appear within the anteriorly
open curve made by the five terminals, and alternate with these latter ; these are
the primary interradials (basals) of the dorsal surface of the Asteroid disc (b^ - ba 5 ).
One of these (ba 5 ) always lies on the right near the dorsal pore, and, growing round
this pore later, becomes the madreporitic plate. The eleventh plate lies in the
centre of the two curves just mentioned, and is the rudiment of the central plate (ce).
The basals and the central appear on the right side of the larva over the right
enterocoel. The relation of the terminals to the enterocoel has not yet been
certainly ascertained. It has
been proved that in the Bipin-
iin /'in they appear even before
the rudiment of the five hydro-
coel outgrowths, above the left
enterocoel.
Metamorphosis of the larva
into the young Asteroid.
This is throughout a continuous
process. Only two parts of the
larva are not taken over by
the young Asteroid, viz. the
larval organ and the larval
oesophagus, which are gradually
resorbed. The anus of the
Asteroid does not indeed de-
40,
FIG. 441. Asterina gibbosa, young Asteroid ten days
old, dorsal view (after Ludwig). I-V, The antiainbulacral
arm rudiments ; I, interradius of the larval organ (lo) ; m, in-
terradius of the madreporite (mp) ; bc^ - ba 5 , the five basals ;
h~^5, the five terminals ; ce, central.
velop out of that of the larva,
but at the same point.
The last remains of the
larval organ are found on the
ventral side of the young
Asteroid lying excentrically
in that interradius in which
the hydrocoel closes to form
the water vascular ring ; view-
ing the body apically this interradius follows the madreporitic interradius on the
right (cf. Figs. 440, 441 ; the arrows indicate these interradii).
The mouth and oesophagus of the young Asteroid arise by the outgrowth from
the left side of the archenteron, mentioned above, reaching the body wall and finally
breaking outward through it (thirteenth or fourteenth day). The oesophagus is then
grown round by the hydrocoel, which closes to form the water vascular ring. Only
shortly before this takes place does the hydrocoel become entirely constricted off
from the enterocoel, and the dorsal pore comes into direct connection with the
stone canal.
The intestine widens into a sac, five radially placed outgrowths appearing in it,
directed towards the rudiments of the arms. At the point where the larval anus
formerly lay, in the interradius between the first and second apical brachial rudi-
ments, the definitive anus breaks through.
The two curves formed by the five apical (antiambulacral) and the five oral
(anibulacral) arm rudiments approach one another more and more, the zone of the
body wall which separates them (and which is, with regard to the animal, equatorial)
becoming continually narrower. Finally, the edges of the apical and those of the
oral rudiments touch to form the young Asteroid. During this process the arm
rudiments unite in the following peculiar manner : Number 1 unites with II.
'2 with III, 3 with IV, 4 with V, and 5 with I.
532 COMPARATIVE ANATOMY CHAP.
In the meantime new pairs of outwardly projecting lateral lobes (rudiments of
tube-feet) have appeared on the hydroccel bulgings, which have developed into the
radial water vascular trunks ; these new growths always appear distally to those
already formed and proximally to the median terminal lobe (terminal tentacle).
The nervous system develops as an epithelial circular cushion in the oral area,
even before the mouth has broken through its centre.
The skeleton receives the addition of fifteen new plates on the apical side outside
of the basals, five being radial and five pairs interradial.
In each interradius orally (on the thirteenth day) a plate forms between the
separate proximal pairs of ambulacral plates. These five plates are the rudiments
of the orals (odontophores).
At the sides of the ambulacral plates the adambulacral plates appear. The
remaining pairs of ambulacral and adambulacral plates arise in the same order as
the pairs of tube-feet, always proximally to the terminals of the arms, and distally
to those already formed.
The five first and the five second pairs of ambulacral plates unite with the five
first pairs of adambulacral plates to form the oral skeleton.
The five radial outgrowths of the intestine quickly grow into the arms, forking,
and thus producing the ten brachial diverticula of the digestive sac. Five pairs of
small interradial outgrowths on the water vascular ring represent the rudiments of
Tiedemann's bodies. None of the tube-feet at first have suckers. The formation
of the nerve ring is followed by that of the radial nerve ridges, which, like the
former, are epithelial in position, persisting as such even in the adult Asteroid. The
continuous ciliated covering of the larva is at no time interrupted, but passes
direct into the ciliated covering of the Asteroid.
We shall not enter upon the accounts given of the rise and development of the
blood vascular system, since there is nothing more problematical in the anatomy
of the adult Asteroids than this system.
Where, among Asteroids, a typical Bipinnaria larva is developed, the meta-
morphosis which produces the young Asteroid seems to resemble in essentials that
of Asterina. The rudiment of the young Asteroid is found in the posterior part of
the larva which contains the swollen mid-gut. At first, as in Asterina, this
rudiment is double, i.e. it consists of an oral rudiment, arising close to the hydroccel,
and an apical rudiment, the two uniting round the intestine. The larger anterior
portion of the larval body, together with the ciliated rings of the Bipinnaria, are,
like the larval organ of Asterina, gradually resorbed.
E. Ontogeny of the Ophiuroidea.
According to the present state of our knowledge the development of the Ophiur-
oidea does not appear to differ so greatly from that of the Asteroidea, in spite of
the difference in shape of the larvte, as to need detailed description. We shall
therefore limit ourselves to a few points.
Development of the hydro - enteroccel. The first rudiment of the hydro-
enterocoel has not been observed with as much certainty as could be desired. In
the quite young Pluteus larva an enteroccel vesicle lies at each side of the oesophagus.
Somewhat later the larva possesses, besides this pair of vesicles, another pair of
enteroccel vesicles at the sides of the stomach-intestine, these latter having, as it
appears, been constricted off from the former. The left anterior vesicle at this
stage enters into communication with the exterior through the dorsal pore (water
pore). On the left side there now arises, between the anterior and the posterior
mteroccel vesicles, apparently by constriction from the latter, a third new vesicle,
VIII
ECHINODERMA TAONTOGEX Y
533
the hydroccel vesicle (Fig. 442). This at once becomes entirely distinct, and
lengthens out anteriorly below the left anterior enteroccel vesicle. At its outer left
edge it then produces five outgrowths, the rudiments of the radial portions of the
water vascular system. Between the fourth and fifth outgrowths (reckoning from
before backward) a dorsally directed diverticulum further grows out from the
hydroccel vesicle, which, after
a very short course, comes in t^""^ " nt
contact with the left anterior
enteroccel vesicle, and opens
into it immediately below the
aperture of the water pore.
This diverticulum is the rudi-
ment of the stone canal. Its
connection with the dorsal pore
(madreporite)is thus secondary,
and is bi'ought about by means
of the left anterior enteroccel,
which no doubt becomes the
ampulla.
The long hydroccel vesicle,
with its five outgrowths, then
clasps the larval cesophagus
like a halter, and grows round
it ; this larval oesophagus
apparently becomes the defini-
tive cesophagus, while no
definitive anus replaces that
of the larva.
First appearance of the
plates of the skeleton. Soon
after the formation of the stone
canal, ten skeletal plates appear
on the Plutev.s larva, five on
the left and five on the right
side, i.e. above the left and
right posterior coelomic vesicles. The five on the right side are the radials of the
apical system ; the five on the left are the terminals. In the middle of the right
side the rudiment of the central plate then appears, and on the left side, immediately
in front of the water pore, another plate appears, which is the fifth oral, the one
which becomes the madreporitic plate. The inadreporite thus belongs ontogenetically
to the oral system of plates. The other parts of the skeleton form only after the
metamorphosis.
pott
FIG. 442. Dorsal aspect of a young Ophiuroid Pluteus.
to illustrate the hydro - enteroccel (after Bury). 1, Larval
cesophagus ; 2, left anterior enteroccel ; 3, hydropore ;
4, hydroccel ; 5, left posterior enteroccel vesicle ; 6, stomach-
intestine ; 7, right posterior enteroccel vesicle ; 8, right
anterior enteroccel vesicle.
F. Ontogeny of the Crinoidea.
The Ontogeny of Antedon alone has been investigated.
1. Embryonic Development.
Here also a coelogastrula is formed by the invagination of a cceloblastula. The
transverse slit-like blastopore indicates the posterior end of the larva, which at a
later stage becomes bilaterally symmetrical. The segmentation cavity is filled by
an albuminiferous gelatinous mass (gelatinous nucleus).
534
COMPARATIVE ANATOMY
CHAP.
After the process of invagination has begun, the formation of mesenchyme also
commences, proceeding from the blind end of the archenteron, which here is bi-
laminar. The cells of the layer which is turned to the segmentation cavity Avander
into that cavity, i.e. into the gelatinous nucleus Avhich fills it, and become mesen-
chyme cells (Fig. 443). The formation of mesenchyme proceeds actively during
the whole process of invagination along the Avhole archenteron. but chiefly at its
base. Here, indeed, the formation of mesenchyme is observed long after important
processes of separation and differentiation have been accomplished in other parts
of the archenteron.
The formation of mesenchyme takes place here more actively than in any other
Echinoderm in which it has been observed, so that the large segmentation cavity
soon appears to be croAvded with mesenchyme cells.
The ectoderm becomes covered with cilia.
The blastopore closes completely in the course of the second stage of develop-
FIG. 443. A, Horizontal longitudinal section through an embryo (gastrula) of Antedon
twenty-six hours old ; B, the same of one forty-eight hours old, in which the archenteron, which
has become constricted off, is divided into two sections (after Seeliger). 1, Ectoderm ; 2, mesen-
chyme cells ; 3, place of formation of the mesenchyme cells at the base of the archenteron ; 4,
blastopore ; 5, endoderm ; 6, archenteric cavity ; T, mesentero-hydrocoel A^esicle ; 8, enteroccel
vesicle.
ment. The archenteron then lies as a closed vesicle in the posterior region of
the segmentation cavity.
An important process soon takes place. The archenteric vesicle or archenteron
becomes constricted by a circular furrow (Fig. 443 B). This constriction leads to
a complete division of the archenteron into an anterior and a posterior vesicle.
The anterior is someAvhat larger than the posterior ; the formation of the mesen-
chyme continues actively on its Avail (Fig. 444).
From the anterior vesicle are derived the intestine and the hydroccel ; from the
posterior the ccelom with the chambered sinus, etc. We here have a remarkable
difference between Antedon and other Echinoderms, in Avhich latter, as above
described, the anterior blind end of the archenteron ahvays yields the ccelom.
The anterior vesicle is in close proximity to the ectoderm, on the ventral side.
The posterior vesicle becomes a transversely placed tube, whereas the anterior
is produced into a horn, both dorsally and ventrally. These two horns clasp the
posterior vesicle from its anterior side. The larva is now distinctly bilaterally
symmetrical (Fig. 444).
The next changes to occur are the following :
VIII
ECHINODERMA TA ONTOGENY
535
The two horns of the anterior vesicle grow out towards one another round the
posterior vesicle until they touch, and so form a hollow ring surrounding the
posterior vesicle, but not closed posteriorly.
The posterior vesicle (enteroccel vesicle) assumes the shape of a dumh-bell, its
two lateral parts swelling up, while the transverse connecting piece becomes
narrower. It is this connecting piece which
becomes encircled by the anterior vesicle
(Fig. 444 .
The ectoderm thickens on the ventral side.
The germ, which till now is approximately
spherical, begins to lengthen from before back-
ward (in the direction of the principal axis).
The anterior vesicle forms a large ventrally
directed outgrowth, the first rudiment of the
hydrocoel (Fig. 445, 3). A small outgrowth
of its anterior wall is the rudiment of a sinus,
which has been called by some the parietal
cavity, and by others the anterior enterocoel (2).
The circular anterior vesicle itself becomes the
intestine (5, 7).
In the posterior (enteroccel) vesicle the two
lateral swellings increase in size, while ^the
connecting piece becomes thinner and thinner,
and finally, at a later stage, entirely disappears.
The enteroccel vesicle is thus divided into a
right and a left enterocoel sac.
FIG. 444. Horizontal longitudinal
section through an embryo of Antedon,
fifty -seven hours old (after Seeliger).
1, Point at which the neural plate becomes
differentiated ; 2, ectoderm ; 3, mesen-
Duriug the next period, which more or less cnyme ; 4 , place of formation of the
corresponds with the fourth day of develop-
ment, the embryo increases somewhat more in
length. Anteriorly, in the frontal region, i.e.
meseuchyme ; 5, rudiment of the intes-
tine ; 6, rudiment of the left coelom ;
7, ventral outgrowth of the mesentero-
hydroccel vesicle ; 8, rudiment of the right
at the end of the embryo diametrically opposite coelom . 9> transverse duct, connecting the
to the point where the now vanished blastopore two rudiments of the coelom.
lay, a ciliated tuft forms. Ciliated rings
appear in the arrangement characteristic of the free-swimming larva (cf. Fig. 402,
p. 510).
The ectoderm which carries the neural tuft thickens (neural plate), becomes
multilaminar, and at the same time appears to be slightly depressed (Figs. 446 and
447 . The deep cells become ganglion cells, and nerve fibrillae also appear below
the surface, closely applied to the neural plate and formed by the ectoderm ; these
are the rudiments of the larval nervous system.
Ventrally from the neural plate, close behind it in the median line, a pit-like
depression forms ; this is the adhesive pit, so called because, at a later stage, the
free-swimming larva attaches itself by means of it.
Another depression, which rapidly deepens and increases in circumference, lies
in the thickened ventral ectoderm, and is the rudiment of the vestibule, whose signi-
ficance will be explained later.
The two coelom sacs have become completely detached, the connecting piece
having disappeared. That on the right spreads chiefly dorsally forward into the
segmentation cavity and then over the intestine, even crossing the median line on to
the left side. The left coelom sac, however, spreads chiefly backward and grows
round the intestine posteriorly like a cap, until it touches the posterior wall of the
right sac. Dorsally it touches the latter somewhat to the left of the median line,
and a mesentery is thus formed which runs dorsally somewhat to the left of the
536 COMPARATIVE ANATOMY CHAP.
median line, but on the posterior side shifts the more to the right the nearer it
approaches the ventral side. This is the principal mesentery. Ventrally the two
coelom sacs still remain far apart.
In the anterior vesicle, the hydroccel rudiment, together with the rudiment of
the parietal sinus, becomes separated from the rudiment of the intestine. After
this separation has taken place, the hydrocoel vesicle still remains for a short time
in open communication with the parietal sinus.
The hydroccel vesicle lies close below the thickened ventral ectoderm, shifted
somewhat far to the left.
The rudiment of the parietal sinus becomes a transverse tube.
The rudiment of the intestine changes shape. It is no longer, as before, an
incomplete hollow ring, placed vertically, through which the connecting piece of
the two coelom vesicles passed (Fig. 445).
The connecting piece having degenerated,
the lumen of the tubular ring has room
to expand from before backward, until
the hollow ring becomes a vesicle.
Towards the close of embryonic
development, in the fifth stage, the first
rudiment of the calcareous skeleton
appears. In an embryo one hundred
hours old, the rudiments of the following
plates were found : 5 orals, 5 basals, 3-5
infrabasals, and about 11 segments of
FIG. 445. Posterior end of an embryo of ,, , ,,
Antedon sizty hours old, seen from the right r
side (after Seeliger). 1, The outline of the right Tlie five orals liave a superficial
coelom sac ; 2, rudiment of the parietal sinus ; position at the posterior part of the
3, rudiment of the hydroccel ; 4, mesenchyme ; embryo, making a horse - shoe - shaped
6, ventral, and 7, dorsal process of the , mesentero- arch which ig ant eriorly and down-
hydroccel vesicle ; 6, connecting duct between the , '
right and the left coelom sacs. , wards - The left end of the arch reaches
further forward than the right.
As a rule (with the exception of the first oral, which indicates the end of the
left side of the arch) the five orals lie round the left coelom sac.
The five basals have exactly the same arrangement as the five orals, merely lying
somewhat further forward than the latter. All of them, except the first basal, lie
above the right ccelomic vesicle.
The 3-5 infrabasals again, which are still extremely small, lie in front of the
basals, but further below the surface of the embryo.
In the anterior half of the embryo the infrabasals are followed by the row of
stalk plates. This row forms an arch which is concave towards the ventral surface,
so that the most anterior, the pedal, plate, lies near the floor of the vestibular
invagination.
The newly-arising skeletal plates of the stalk appear at the posterior end of the
row, generally (but not exclusively) immediately in front of the future centrodorsal,
between it and the last formed most posterior stalk plate.
Up to this time the embryo has lain enclosed in the egg-membrane, on the pin-
nule of the mother, but it is now ready to be hatched. Even at this stage its
organisation leads to the conjecture that the calyx will be produced from the larger
posterior half, which alone contains the internal rudiments, while the stalk of the
future attached larva will be produced from the anterior half.
VIII
ECHINODERMA TA ONTOGENY
537
nrd".
2. The Free-swimming Larva (Figs. 402 (p. 510), 446, 447, 448).
The external form of the free-swimming larva has already been described on
p. 510.
The duration of this manner of life differs greatly in individuals of the same
brood, varying from a few hours to several days.
Ectoderm. For the ciliated bands, see above, p. 510.
In the intermediate zones, which are free from cilia, a fine cuticle becomes
differentiated from the ectodermal epithelium, the cells of which later begin to
secrete a homogeneous substance which separates cell from cell, so that the epi-
thelium comes to resemble a connective tissue.
The neural plate, the neural tuft, and with them the larval nervous system attain
their highest development at this stage, but undergo complete degeneration in the
next. The ganglion cells below the neural plate
increase in number, and the layer of nerve fibres
spreads out over the whole anterior end of the larva.
Fine nerve trunks run to the ciliated rings, and two
specially strong ventral nerve trunks run back at
the sides of the vestibular invagination, their an-
terior parts being beset with isolated ganglion cells.
The adhesive pit becomes larger and deeper,
and towards the end of this period loses its ciliation
and assumes a glandular character.
The vestibular invagination spreads over the
greater part of the ventral side. It closes and
becomes a tube, the lateral edges of the invagination
growing towards one another and fusing in the
median line. This process takes place from behind
forwards, and is not fully completed during this
period, a small aperture being retained anteriorly.
The ciliation of the vestibule disappears.
The intestine alters its shape. It spreads out FlG - 440. -Free-swimming larva
somewhat, assuming tirst the form of a hollow plate, ?% 2KS5J8
with the concavity directed ventrally and the con- j_ v The five ^uated r j n g S . j > the
vexity dorsally. In the ventral concavity lies the parietal sinus ; 2, the vestibule,
hydroccel vesicle. At a later stage the intestine already closed posteriorly; 3, the
again becomes rounded and vesicular. hydrocoel ; 4, the hydropore ; 5, left
The two enterocal sacs continue to change their ^ n
positions and to spread out. The right vesicle pro-
duces anteriorly five tubular outgrowths, which become grouped round the principal
axis. These five tubes arise, widened like funnels, from the right coelom, then
narrow anteriorly, and, losing their lumina, run out as strands. They are the
rudiments of the chambered sinus.
The skeletal pieces of the stalk are at this time horse-shoe-shaped, and tend to
enclose the five tubes of the chambered sinus. When they become complete rings
the chambered sinus passes through them.
The hydroccel vesicle becomes completely constricted from the parietal sinus,
flattens in the dorsoventral direction, and at once assumes the horse-shoe shape.
The gape of the horse-shoe points. at first backwards and to the left, and finally to
the left and forwards. Five ventrally directed outgrowths appear on it, out of each
of which, at a later stage, three tentacle vessels arise.
538
COMPARATIVE ANATOMY
CHAP.
The rudiment of the primary stone canal appears at the blind end of the left
limb of the hydroccel, as a dorsal process, running to the left.
The tubular parietal sinus, which is now completely isolated from the hydroccel,
FIG. 447. Median longitudinal section
through a free-swimming larva of An-
tedon, twenty -eight hours old, in the
act of becoming attached (after Seeliger).
[-V, The ciliated rings ; 1, neural plate with
nerve fibres (2), and ganglion cells (3) ;
4, gelatinous nucleus, the mesenchyme
cells which crowd it are not represented ;
5, the tubes of the chambered organ ; 6, in-
testine ; 7, right coelom ; 8, vestibule ;
9, parietal sinus ; 10, right enterocoel ;
11, hydroccel ; 12, adhesive pit ; 13, left
enterocoel.
FIG. 448. Free-swimming larva of
Antedon, forty -eight hours after
being hatched, from the left side,
specially to illustrate the rise of the
skeletal plates. I-V, The ciliated rings ;
?>l-bag, the five basals ; or r or 5 , the five
orals, those lying on the right side re-
presented as discs ; 1, vestibule ; 2, in-
testinal vesicle ; 3, right enterocoel ;
4, calcareous joints of the stalk ;
5, pedal plate.
has shifted to a position in front of and above the latter. Its posterior end grows
out till it touches the ectoderm immediately in front of the fourth ciliated ring
ventrally and to the left, and finally breaks out through the hydropore at this point.
3. Attachment of the Larva and its Transformation into the Stalked Form
(Figs. 449-453).
Attachment takes place by means of the adhesive pit, which yields a sticky
secretion ; and since this pit lies ventrally at the anterior part of the body, the
attached larva has at first a position parallel to the surface to which it is fastened,
and the vestibule lies immediately above that surface. The body, however, soon
becomes erect, and the adhesive pit takes up a terminal position.
. ry soon after attachment the ciliated rings disappear, and so does the
VIII
EGHINODERMA TA ONTOGENY
539
neural tuft ; the neural plate flattens out, and the larval nervous system
completely degenerates.
The ectoderm cells continue to yield an intermediate substance. Many of them
sink below the surface, the consequence being that the distinction between the
body epithelium and the mesenchymatous cntis is entirely obliterated.
The vestibule becomes completely constricted off, the last remains of the aperture
of imagination closing. At the same time it shifts entirely to the posterior end
of the larva, the end which now freely projects, twisting through an angle of 90, so
that its thickened epithelial floor, which before lay parallel to the principal axis, now
eat
FIG. 440. Young attached larva of An-
tedon. forty-eight hours old, from the left
side (after Seeliger). The vestibule has be-
come entirely constricted off, but the distinc-
tion between the calyx and the stalk is not yet
pronounced. 601-603, Basals ; orj-org, orals ;
il>, infrabasals ; 1, pedal plate ; 2, parietal
canal ; 3, hydrocoel outgrowths ; 4, vestibule ;
5, intestinal vesicle ; 6, left crelom sac ; 7, right
ccelom sac ; 8, calcareous joints of the stalk.
FIG. 450. Young attached larva of
Antedon, forty -eight hours after being
hatched, from the left side (after Seeliger).
co, Stalk ; co, calyx ; 601-603, basals ; ori-org,
orals ; ib, infrabasals of the left side ; 1, pedal
plate ; 2, hydropore ; 3, left coelora ; 4, right
crelom ; 5, joints of the stalk.
lies at right angles to it. The larval body becomes club-shaped, the anterior body
forming the handle of the club. The vestibule, which continues to increase in size,
comprises the entire posterior part of the club (the calyx) ; it becomes pentagonal,
and imprints the same shape upon the whole posterior part of the body, and thus
first determines the radiate structure (Figs. 450, 451, and 452).
The anterior end of the larva becomes the apical end of the stalk, the pos-
terior end becoming the oral side of the calyx of the attached Pentacrinus-like
larva.
The hydrocoel undergoes the same twisting and shifting as the vestibule, beneath
540
COMPARATIVE ANATOMY
CHAP.
whose floor it lies after as before the process. It has passed from the horse-shoe
shape to the circular, but the hydroccel ring still remains unclosed for a long time
at the point where the gape of the horse-shoe formerly was. Its five outgrowths
-Li
d.
FIG. 451. stalked larva of Antedon,
eighty-four hours old, with twenty-five
tentacles, from the right side (after
Seeliger). Calcareous plates not repre-
sented. 1, Right ccelom sac ; 2, stomach
intestine ; 3, left coelom sac ; 4, sacculi ;
5, vestibule, still closed ; 6, the fifteen
primary tentacles ; 7, the five pairs of
secondary interradial tentacles ; 8, oeso-
phagus ; 9, hind-gut ; 10, axial organ ;
11, fibrous strands in the stalk, continua-
tions of the axial sinus.
FIG. 45-_'. Transverse section
through the region of the left
or oral ccelom of an attached
larva of Antedon, 108 hours
old (after Seeliger). I-V, The
five radii ; 1, left oral coelom ;
2, oesophagus ; 3, stone canal ;
4, parietal canal.
Fir,. 453. Diagrammatic
transverse section through the
region of the aboral ccelom in
an Antedon larva, 108 hours old
(after Seeliger). I-V, The five
radii; bi-& g , the five basal.s ;
1, right or aboral coelom ; 2, hind-
gut ; 3, axial organ ; 4, parietal
sinus ; 5, oesophagus.
push up the ectoderm of the floor of the vestibule into the vestibular cavity ; they
soon appear to be trilobate, so that in all 5 x 3 tentacles are present, ten more being
added to them, which arise in pairs at the bases of the primary outgrowths.
viii ECHIXODERMATA ONTOGENY 541
The stone canal breaks through into the parietal sinus. The point at which
tliis occurs does not, however, correspond with the point at which the hydroccel and
the parietal sinus originally were in open communication.
The parietal sinus also takes part in the shifting just mentioned. In the free-
swimming larva it lay in front of the hydroccel. This position it retains, while
shifting backward (towards the oral end) together with the hydroccel. It thus
approaches its external pore. Compared with the other growing organs, it remains
small and stationary.
The hydroccel is thus connected by the stone canal with the parietal sinus, and
this latter is in open communication with the exterior through the hydropore.
The intestine. An extraordinary process goes on in the intestinal vesicle.
Numerous cells become detached from its wall, and wander into its cavity, which
they finally completely fill. They fuse for the most part into a large yolk-like mass,
which is entirely resorbed at a later stage as nutritive material.
The floor of the vestibule deepens at the centre, and is produced anteriorly like a
funnel towards the intestinal vesicle. This funnel, which passes through the hydro-
ccel ring, becomes the oesophagus, and joins a posterior process of the intestinal
vesicle which grows out to meet it.
The intestinal vesicle divides into a stomachal section to the left of the larva
and a narrower portion running dorsoventrally on the right side. This latter part,
the hind-gut, rises with a broad base out of the former and ends blindly. The blind
end of the hind -gut then grows over to the left ventrally.
The ccelom. The two ccelom sacs shift and spread out in a peculiar manner.
The left sac shifts quite posteriorly, and becomes the oral ccelom, which grows round
the oesophagus on all sides from above downward, thus assuming the shape of a
hollow horse-shoe which clasps the oesophagus, the stone canal, and the parietal
canal (counting these in order from within outward). The gape of this horse-shoe
is directed ventrally to the left, and since the tips of its two arms grow towards one
another, a short longitudinal accessory mesentery arises. The right ccelom vesicle
passes through changes of form, expansion, and shifting which are difficult to describe
briefly, and becomes the aboral or apical ccelom. In consequence of the shiftings of
these vesicles the longitudinal principal mesentery which separated the originally
right from the originally left coelom vesicle becomes a transverse mesentery,
separating the oral from the apical ccelom, and surrounding the cesophagus like a
diaphragm. Xear the right (now aboral) ccelom also, a longitudinal, somewhat
diagonal accessory mesentery develops, which runs somewhat to the right of the
ventral median line. The walls of the apical (originally right) ccelom are continued
anteriorly into the Avails of the five tubes which form the chambered sinus, but at
the point where they pass into one another they are so pressed together that no
open communication exists between the two sinuses.
The axial organ (genital stolon) of the calyx arises as a thickening in the left
epithelial wall of the longitudinal accessory mesentery of the aboral ccelom, at its
most anterior (apical) end, where the chambered organ commences. As the genital
strands of the arms and pinuulse most probably arise as outgrowths of the axial
organ, it might thus be proved that in the Crinoids also the genital cells are derived
in the last instance from the endothelium of the body cavity. The cushion-like
thickening increases in length, becoming partly constricted from the mesentery ;
posteriorly, it reaches to the oral ccelom ; anteriorly, the axial organ passes into the
stalk up the centre, between the five tubes of the chambered sinus.
The formation of trabeculse begins in the aboral ccelom. Single endothelial
cells lengthen and project like pillars into the ccelomic cavit}'. A similar process
takes place in the hydroccel.
The skeleton. When the vestibule shifts to the posterior end of the larva the
542 COMPARATIVE ANATOMY CHAP.
skeletal plates are also shifted. The horse-shoes formed by the five orals and the
five basals close, and form two flat rings or circles. The circle of the orals shifts
backward and on to the roof of the vestibule in such a way that the five plates together
form a pyramid, the truncated tip of which forms the centre of the vestibular roof, at
the extreme posterior end of the larva. The orals have thus shifted away from the
region of the left (oral) ccelom.
The circle of basals which lies in front of (apically to) that of the orals forms a
pyramid in the body wall of the calyx, around the aboral ccelom, the truncated end
of this pyramid lying at the commencement of the stalk, or at the anterior (apical)
end of the calyx. The orals and the basals together form a pentagonal double
pyramid, truncated at both ends. At the truncated end of the basal pyramid, round
the uppermost (most posterior) joints of the stalk, lie the four or five small infra -
basals. The number of joints in the stalk increases, and the anterior body of the
larva becomes, as the stalk, more and more distinctly demarcated from the posterior
body or calyx, which has now become five-rayed.
The orals and basals alternate with the primary outgrowths of the hydroccel.
i.e. they are interradially placed. If we indicate that primary outgrowth of the
hydroccel which, when the larva is viewed from the oral side, comes next in the
direction followed by the hands of the clock, to the hydropore (which lies ventrally
to the right) as No. I, and those which follow in this direction as II, III, IV, and
V, and if, again, we indicate that oral or basal which lies in the interradius between
radii I and V as the first, and those which follow in this direction as orals (or basals)
2 to 5, it can be proved that the hydropore, in the older stages of the attached larva,
is enclosed by the basal part of the first oral plate. In these stages it is also seen
that the infrabasals fuse to form a single plate, the centrodorsal, at the centre of
which there is an aperture for the passage of the chambered organ.
During the first developmental periods which follow the attachment of the larva,
the sacculi appear. Five of these first arise exactly radially at the bases of the
middle tentacle of each group of tentacles, on the outer side of the circular canal.
These sacculi can be ontogenetically derived from groups of mesenchyme cells.
4. The Stalked Larva after the Vestibule has been Perforated.
(From five days to the sixth week after hatching, Fig. 454.)
The calyx becomes more and more distinct from the stalk.
The roof of the vestibule becomes ever thinner at its centre, an aperture finally
forming. Radial incisions run from this central aperture towards the peripheral base
of the roof, so that this latter becomes divided into five interradial lobes or valves,
each of which contains an oral plate. This pyramid of valves can open 'and shut.
The vestibule has opened outward.
The five tentacles meanwhile lengthen and receive their papillse. They usually
project outwards from between the five oral valves.
The definitive nervous system (which is oral and superficial) rises quite inde-
pendently of the larval nervous system, which entirely disappears. The first ap-
pearance of the nerve ring was observed very late, long after the perforation of the
vestibule. The ectoderm of the oral disc, i.e. the peripheral portion of the original
vestibular floor (the central part having sunk in to form the oesophagus), becomes
thickened in a ring which is bordered by tentacles, and here becomes multilaminar.
The cells of the deeper layer yield the nerve tissue.
Neither the rudiments of the deeper oral nervous system nor those of the apical
sy.stc m have as yet been certainly observed.
VIII
ECHINODERMA TA ONTOGENY
543
16
Alimentary canal. The mouth from the very first does not lie exactly at the
centre of the oral disc, but somewhat eccentrically in the interradial area bordered
by the first and fifth radii.
The stomach becomes a capacious sac, and the yolk-like mass contained in it is
gradually absorbed. The hind-gut arises out of it (in interradius III-Y), being
broad at the base, and then thinning into a
tube which has the following course. Viewing
the larva from the oral pole, the hind-gut runs
(in the direction of the hands of the clock) in
the horizontal mesentery, near the body Avail,
through the interradial space IV- V, then runs
across radius V, and immediately after opens
outwards in the interradial space V-I, through
the anus which has in the meantime broken
through the calyx laterally. This is the same
interradius in which the hydropore lies, on
the original ventral side of the bilaterally
symmetrical larva. The ectoderm takes no
part in the formation of the anus.
In the ccelom sacs profound changes are
going on, which may be briefly summarised
as follows.
(a) The chambered sinus gives up all
connection with the original right, now the
aboral coelom sac.
(b) The mesenteries (both the principal
and the longitudinal accessory mesentery) are
completely resorbed, and as a consequence
the right and left cceloms unite to form one
large body cavity.
(c) The trabeculae (of endothelial origin)
become very strongly developed, and traverse
the body cavity in all directions as a network.
(d) The axial organ becomes differentiated
as an independent solid cell strand, lengthens
till it reaches the tegmen calycis (oral disc),
and at a later stage becomes hollow.
(e) In the parietal sinus, which comes to
post
FIG. 454. Calyx of a decalcified larva
of Antedon, five weeks old, with extended
tentacles, from the left and lower side (after
Seeliger). I-V, The five radially placed
primary sacculi ; 1, axial organ ; 2, right
lie quite in the body cavity, two sections (aboral) coelom ; 3, principal mesentery,
become more and more distinct: the one between the right (aboral) and the left (oral)
i j ,, ,, , ,., coelom; 4. hind -gut which follows the
vesicular, and the other a narrow canal-like stomach; J >oralw 5 om . 6> hydrocoel ring .
section opening outwards through the hydro- 7, two of the ten secondary tentacles;
pore. The former, into which the primary 8, tentacle papillae; 9, primary tentacles
stone canal enters, loses its independent ( nl y seven of the total number, fifteen, are
endothelium, and the thin wall which represented); 10, oral lobes ; 11, stone canal;
12, hydropore ; 13, oesophagus ; 14, stomach ;
separates it from the ccelom also probably 15> continuation of the chambered organ in
disappears, so that it ceases to exist as a the stalk (16).
separate cavity. The stone canal now opens
into the general body cavity, which is thus in communication with the exterior
through the narrower section of the original parietal sinus, and through the
hydropore in the anal interradius.
In the hydrocoel, the water vascular ring completely closes. The whole of the
musculature of the hydroccel is formed by the hydroccelomic epithelium itself. The
544 COMPARATIVE ANATOMY CHAP.
trabeculse within the system of canals increase in number. In the tentacles, the
following changes have taken place. Formerly the twenty-five tentacles were
arranged in five radial groups of five each. The five tentacle canals of a group were
connected by a common tentacle canal rising with the circular canal. Now all the
five tentacle canals of a group rise separately from the water vascular ring.
Further, during the period of the attachment of the stalked larva, four new stone
canals appear, and four new calyx pores form in the other interradii. These and
all that arise later cannot, of course, form in the same way as the primary calyx
pore.
The stage, the development of which has just been described, has been called
the Cystid stage, owing to the absence of arms, to there being no clear division
of the calyx into dorsal cup and tegmen calycis, and to the occurrence of the
rudiment of the genital organs as an axial organ, whereas later the genital glands He
in the arms and especially in the pinnules.
In opposition to the above view it may be remarked :
1. That neither the want of arms nor the absence of division of the calyx into a
dorsal cup and an ambulacral disc is characteristic of the Cystidea.
2. That the skeletal system of the attached larva of Comatula is altogether
radiate, consisting of the three circles, the radials, the basals, and the infrabasals.
On the other hand, the irregular arrangement of the skeletal plates is, as a rule,
characteristic of the Cystidea. Those Cystids which most resemble the larva of
Comatula in the number and radial arrangement of the skeletal plates are also those
which are, of all Cystids, the most nearly related to the Crinoidea.
3. The hydroccel of the stalked Comatula larva consists simply of the water
vascular ring and a circle of tentacles, which receive their canal direct from the water
vascular ring. In the Cystidea, radial canals must have run out from the water
vascular ring, below the food grooves of the ambulacra, giving off tentacle canals to
right and left, and also probably penetrating into the arms.
4. The appearance of the first rudiments of the genital glands in the body merely
proves that the definitive position in the arms is secondary, and this applies to all
Echinoderms.
The position of the anus, indeed, agrees in both.
5. Last Stage of the Attached Stalked Larva Pentacrinus Stage.
(Of. Fig. 326, p. 375.)
This stage is distinguished by the rise of the arms, which begin to grow out in
the radii between the circle of the orals and that of the basals. Each rudiment of
an arm is, from the very first, supported on its apical side by a newly arising skeletal
plate. These plates are the five radials of the dorsal cup. Distally from each
radial on the growing arm, two new skeletal plates follow one another, the first and
second costals. The growing rudiment of the arm then forks, the distichals form,
and so on.
During the formation of the arms the five middle and strictly radially arranged
tentacle canals of the five tentacle groups become the radial vessels, which fork with
the arms. Fresh investigation of this point is, however, much needed.
The interval between the oral pyramid and the bases of the arms increases, and
the tegmen calycis thus comes into existence. The pyramid of five oral valves in
the middle of the latter does not grow further, and the valves with their skeletal
plates finally disappear. Round the anus, which comes to lie in the tegmen calycis,
an anal plate develops temporarily.
At this stage the resemblance of the attached and stalked larva of Antedon
vin ECHINODERMATA PHYLOGEXY 545
to the Inadunata. especially to the so-called Larviformia (cf. pp. 303, 328,etc.),
is so striking as to be at once recognisable.
The calyx, with the arms, sooner or later breaks away from the stalk, and
can either move by using the arms as paddles or catch on to objects by means of its
cirri. When it breaks loose from the stalk, some of the uppermost whorl joints on
which cirri have formed remain connected with it ; these fuse with one another, and
with the centrodorsal. The basals, again, fuse to form a rosette, which is soon over-
grown on all sides by the large apical centrodorsal plate.
XXII. Phylogeny.
Xo other phylum of the animal kingdom is so sharply marked off from all others
as the Echinoderms. Their organisation is in all points strange ; even the radiate
structure is strange, in so far as it is, unlike that of many Ccelenterata, only a mask
which hides a complicated and hitherto inexplicable asymmetry. We are not in a
position to compare an adult Echinoderm with the adult representative of any other
phylum from a phylogenetic standpoint.
The difficulties which meet us in attempting to reconstruct the phylogenesis of
the Echinodermata are still further increased by the fact that the typical charac-
teristic Echinoderm larva cannot at any stage of its development be compared with
the adult or larval form of any other animal. An exception to this statement may,
however, perhaps, be made in favour of the Enteropneusta, which will be described in
the next chapter.
If, taking the gastrsea theory as a foundation, we assume for the Metazoa a
common bilaminar racial form, it seems, in view of the above-mentioned difficulties,
that the racial form of the Echinodermata must have branched off extraordinarily
early, perhaps at a stage corresponding phylogenetically with the gastrula. By such
an assumption, the Echinoderms and their larvae would be removed from the sphere
of comparative anatomy and comparative embryology, except in so far as such com-
parative enquiry were limited to the Echinoderm phylum itself.
It appears to us, however, that attempts to approximate the Echinodermata to
Metazoa standing higher than the Coelenterata should not be abandoned. Recent
anatomical and ontogenetic researches have brought to light facts which open up new
prospects. We may mention the demonstration of a neural plate and of a larval
nervous system, the attempts to demonstrate that there are two pairs of enterocoel
vesicles, the proof that the first rudiments of the gonads proceed from the endothe-
lium of the cu-lom, the suggestion that the stone canal or the hydropore should be
regarded as a nephridial canal, etc.
All this, of course, does not justify us in closely comparing the Echinoderm larva
with other definite forms, adult or larval, belonging to Metazoan classes higher than
the Ct.flenterata, except perhaps the Enteropneusta. But these discoveries and new
views tend to make the Echinoderm body appear somewhat less strange, since we
find in its organisation important points in which it is fundamentally in agreement
with the so-called Triploblastica.
It cannot be doubted, and has never been doubted, that the Echinodermata form
a distinct, naturally marked out phylum of the animal kingdom, or, in the language
of Phylogeny, that all Echinoderms have had a common racial form.
Within the phylum of the Echinodermata, further, the classes are again quite dis-
tinct and naturally marked off from one another. Among known Echinoderms,
there are no intermediate forms between the Pelmatozoa, the Holothurioidea, the
Ec/tinoidca, the Asteroidca, and the Ophiuroidca. Every known Echinoderm can at
once be recognised as either an Echinoid, an Asteroid, a Holothurid, etc. The Cystidea
VOL. II 2 N
546 COMPARATIVE ANATOMY CHAP.
alone, perhaps, form an exception to this rule, "showing decided resemblance to
the Crinoids on the one hand, with an oecasional possible approach to the opposite
extreme, i.e. the Holothurioidea, on the other. It is, however, very difficult to judge
of the Cystidea, since conclusions as to the inner organisation drawn exclusively
from the structure of the skeleton cannot be regarded as altogether trustworthy.
It appears to us that there is not the least justification for deducing the different
Echinoderm classes in any definite way from one another, nor can we at all accept
the recently urged view that the Holothurioidea stand nearest to the racial form.
On the contrary, the morphology of the genital organs leads us to believe that the
Holothurioidea are distinct from all other Echinoderms, with the possible exception
of the Cystidea.
If we review the whole morphology of the Echinodermata, our phylogenetic specu-
lations are, first of all, influenced by the fundamental fact that the radiate, but at
the same time asymmetrical Echinoderm proceeds ontogenetically from a bi-
laterally symmetrical larva, the so-called Dipleurula.
The Dipleurula Larva.
This larva is regarded from two opposite points of view. (1) The bilateral
structure is thought to have been secondarily acquired, within the different groups
of the Echinoderms, in adaptation to the free-swimming manner of life. (2) The
bilateral structure of the larva has been inherited from the common racial form
of the Echinodermata, or from the larva of such a form. The first view is now
generally abandoned. The manner of life might indeed have called forth external
bilateral symmetry of form, but certainly not the marked bilateral symmetry of
structure of the internal organs.
If we now try to sketch a hypothetical phylogenetic stage based upon a com-
parison of the various Dipleurula larva* of the Echinodermata, the following is the
result : The body was freely movable, ovoid, and bilaterally symmetrical ; the mouth
lay anteriorly on the ventral side, the anus at the posterior end, or posteriorly on
the ventral side. In the frontal region there was a nerve centre below the surface
of the ectodermal epithelium which was differentiated into a sensory organ (neural
plate). Running back from the nerve centre along the ventral side, below the surface
of the body epithelium, were two nerve trunks beset with ganglion cells. The intes-
tine was divided into the ectodermal(?) oesophagus, the wider endodermal mid-gut,
and the hind-gut, which was also endodermal. At the sides of the intestine were
two pairs of coelomic vesicles, the anterior pairs at the sides of the (esophagus, the
posterior at the sides of the mid- and hind-guts. The two anterior coelomic vesicles
(or their posterior portions) were connected with the exterior by a canal laterally or
dorsally (cf. the interesting temporary occurrence of a hydropore on the right side in
Asteroids, especially in Asterias vwlgaris, p. 527). The genital products developed
out of the endothelium of the coelom.
Such an organisation has nothing strange about it. It has almost as much claim
to be classed with the Vermes as Sagiita has, for which latter classification, however,
not much can be said. It is further possible that the racial form possessed special
organs for locomotion, respiration, etc., about which nothing can now be positively
affirmed, since they have in all cases disappeared from the ontogeny of the Echinoderms.
It is not at all likely that the ciliated rings have any phylogenetic significance.
vin ECHIXODERMATAPHYLOGEXY 547
Metamorphosis of the Dipleurula Larva.
The larva develops, through metamorphosis, into the young Echino-
derm, which, under its radiate mask, is asymmetrical. The radiate
structure is amalgamated with an asymmetrical structure.
Here again, we are not altogether without light as to the phylo-
genetic significance of this process. We agree with the majority of
modern authors in believing that a radiate structure of body arises
as a consequence of an attached manner of life. We are, therefore,
justified in assuming that the radiate Echinoderm arose from a free-
moving racial form, in adaptation to a newly -acquired, attached
manner of life.
All Eehmoderms must, therefore, once have been attached
animals.
If now, we wish to ascertain in what special manner attachment
took place, we unavoidably turn for an answer to this special question
to the Crinoids. These are the only Echinoderms which, in all prob-
ability, never again gave up the attached manner of life. That the
only Crinoid about whose ontogeny we know anything, Antedon, is a
form which has actually once more become free, i.e. has, as a. secondary
specialisation, given up the attached manner of life, detracts in no way
from the arguments based upon its ontogeny.
All other Echinoderms whose ontogeny we can investigate have
long since given up the attached manner of life, and, with the excep-
tion of certain (analogous) cases among the Asteroids (e.g. Asterina\
do not any longer pass through an attached larval stage. Hence the
methods of development of other Echinoderms, even when simpler
than that of the Crinoids, must in comparison with the latter be regarded,
phylogenetic;illy, with some suspicion.
From the developmental history of Antedon then, we learn that the attachment
of the Dipleurula larva of this animal took place by means of the ventral side of
the anterior end of the body. In a similar way the Dipleurula larva of Aster ina
attaches itself by means of the larval organ which develops anteriorly.
Authors who have recently attacked this problem assume that attachment took
place on the right side ; making this assumption in order to explain the asymmetry
which follows. To us also this assumption appears necessary, but it should be
specially stated that the attachment took place on the right anteriorly.
When this assumption is made we must further ask : What were the changes
which the attached manner of life induced ?
It is difficult, with the embryological material we have at present, to obtain an
adequate idea of the resulting processes, and only a very tentative explanation can
be given.
Judging by analogy from the modifications which, in other parts of the animal
kingdom, result from an attached manner of life, it may be assumed that the arrange-
ments for conducting food were the first to become adapted to the new condition of
existence. The mouth left its unfavourable position and wandered along the ventral
side, first to the left, i. e. to the side which was now uppermost (being opposite to the
point of attachment). In this shifting the oesophagus pushed the median and
548 COMPARATIVE ANATOMY CHAP.
ventral wall of the left anterior coslom in front of it, embedded itself to a certain
extent from without in the ccelomic vesicle, so that this vesicle surrounded it in the
shape of a horse-shoe. Round the mouth, the body wall (and with it the left anterior,
ccelomic vesicle which lay here) grew out into five tentacles which, as in so many
attached animals, served for bringing in food, for the sense of touch, and for respira-
tion. (Compare the tentacles and the horse-shoe-shaped tentacle -carrier of the
Bryozoa Cephalodiscus, etc.) Thus, the left anterior ccelom, which from the very
first was, like the right, connected with the exterior by means of a canal, produced
the primary horse- shoe-shaped hydrocoel with the primary tentacles and the hydro-
pore (stone canal). In this way the first impulse towards the development of the
radiate structure was given. The horse-shoe finally closed to form the circular canal.
The right anterior side of the body, which was used for attachment, could be
produced into a stalk, as is the case in most Pelmatozoa. (The larval organ of
Asterina may be a modified reminiscence of such a stalk.) The right anterior
ccelomic vesicle, which lay in this region, now serving for attachment, lost its efferent
aperture, atrophied, or became a cavity of the stalk (chambered sinus and its con-
tinuation in the Crinoids(1), crelom of the larval organ in Asterina (?).
The body now developed principally in the oral and tentacular region (on the left
anterior side). The posterior portion of the body with the anus near its end was
originally like a lateral outgrowth or shield on the body, which gradually subsided
and disturbed less and less the radiate appearance.
According to this view, the greater development of the anal interradius which is
found in many Crinoids, especially in palaeozoic forms, may possibly be an original
condition, in connection with which we have the occurrence of special anal plates in
the anal interradius. The anus also may originally have lain outside of the circle of
tentacles, a supposition which harmonises with its position in the Cystids and in
the ontogeny of Antedon.
Concurrently with these changes, the left posterior ccelom, which lay nearer than
the right to the mouth, which had shifted to the left, now upper, side, grew round
the oesophagus, and forming a vertical mesentery, became the oral coelom. The
right coelomic vesicle, however, spread out chiefly in the lower (originally the right)
region of the body, and became (also forming a vertical mesentery) the apical coelom.
The mesentery dividing the two (oral and apical) sections of the ccelom would natur-
ally be horizontal (transverse).
In the vertical mesentery, the rudiments of the gonad (the axial organ) arose
as a ridge-like thickening and growth of the endothelium on one side ; in mature
animals, this opened outward through a genital duct and aperture in the region be-
tween the mouth and the anus.
This phyletic stage, deduced as a result of the attached manner of life, may be
called the Pentactsea.
For the protection of the body, calcareous plates developed in the mesenchyme
below the integument, at first, perhaps quite irregularly.
From the Hypothetical (unknown) Pentactsea Stage to the known
Echinoderm.
Most Echinoderms gave up the attached manner of life at a later stage. The
known case of Antedon, in which an animal in the highest degree adapted for the
attached life resumed the free life, is specially welcome and useful in this connec-
tion.
The ancestors of the Holothurioidea were probably the first to renounce the
aci ached manner of life, although not as early as the Pentactrea stage.
vin ECHIXODEEMATAPHYLOGENY 549
The organisation of the Pentactrea would only become completely adapted to the
attached manner of life, when the number of tentacles and the surface for taking in
nourishment increased. Such increase might take place in various ways ; we have
many examples among attached animals belonging to other divisions of the animal
kingdom.
The Pentactaea may have become perfected in one direction as follows :
The interval between the bases of the primary tentacles and the mouth increased,
while the (apical) interval between the primary tentacles and the attached pole
remained the same 'or decreased. By the shifting of the circle of tentacles away
from the mouth, the basal piece of each tentacle canal would be drawn out radially
below the oral body wall, and would become a radial canal, from which new lateral
tentacle canals would bud out alternately to right and left, always proximally to
the shifting primary (now terminal) tentacle, the body wall projecting in the form
of tentacles. There thus arose in each radius a double row of tentacles, which, to
speak exactly, stood in the corners of a zigzag line. Nutritive particles, descending,
would be sent on to the mouth between these rows of tentacles. This adaptation
became perfected when the floors between these rows of tentacles sank in the form of
furrows the food grooves or ambulacral furrows ; these furrows then became pro-
vided with means of transport, in this case with cilia. The epithelial cells lining
the furrows gradually became sensory cells, and epithelial nerve ridges, the radial
nerves, would arise, which would meet round the mouth as the nerve ring.
This rise of the radial nervous system of the Echinodermata as a natural
development of the ambulacral furrows, and of the palisades of sensory tentacles border-
ing these furrows on each side, may be without difficulty followed out in detail,
although there is no space for such an attempt in this volume.
At this stage are found the armless Cystids with their carapace of plates.
The genital organ is enclosed in the body, and opens outward through a single
aperture (the " third " aperture of the Cijstidea).
It was, perhaps, at a similar phyletic stage that the ancestors of the Holothuri-
oidca gave up the attached manner of life. For locomotion, they used the tentacles,
arranged in five meridional double rows, the body elongated, and the anus shifted
to the apical end, which became free. Food was taken in directly at the mouth ; to
assist in alimentation, the oesophagus became modified into a pharynx, and the ten-
tacles lying close to the mouth became specialised. The food grooves lost their
function, leaving, however, behind them the epithelial nerve ridges, which continued
to innervate the tube-feet. The food grooves could now close over to form a tube
for the protection of the nerve ridges ; these latter became the subepithelial radial
nerves, and the lumina of the closed tubes the epineural canals. According to this
view, therefore (and this applies also to the Ophiuroidea and Echinoidea), the sub-
epithelial radial nerves, with their epineural canals, are the original food grooves
closed over to form tubes. They gave up their original function as " food grooves "
in proportion as, with the adoption of a free manner of life, food was taken in direct
at the mouth.
It is a question of subsidiary importance, in the derivation of the Holothurid
body, which still possesses the single genital organ and the single aperture, whether
the condition of its skeleton is to be regarded as original, or whether it has not rather
been derived from the carapace of plates of a Cystid-like animal by means of the
multiplication of the skeletal pieces, their loose arrangement, and their decrease
in size.
The longitudinal and circular musculature may be new, but may just as well have
been inherited from an attached ancestral stage, in which they could be functional,
just as are the longitudinal and circular muscles of Actinia.
After the foregoing description it is obvious that the Paractinopoda (Synaptidce]
550 COMPARATIVE ANATOMY CHAP.
cannot be regarded as primitive forms of Holothurioidea. They are, on the contrary,
highly specialised forms, which, in adaptation to the limicolous life, have lost the
tube-feet and the radial canals, which, however, still occur ontogenetically.
We have thus seen how the Pentactrea might become modified in the direction
of certain Cystids and of the Holothurioidea.
By remaining attached, the Pentactsea might develop in another direction.
The body carried by the stalk might remain small, but become drawn out into
processes or arms in the directions in which the primary tentacles travel from the
mouth, the ends of these arms being always marked by the possession of the primary
tentacles. Secondary tentacles then rose out of the radial vessels which ran along
the arms (the tentacle canals of the primary tentacles) in the way above described ;
the food grooves and their nerve ridges also fovmed in the same way. A still more
complete adaptation to the attached manner of life was attained by the branching of
the arms and the formation of pinnulre. In this way the surface for capturing food
was continually increased.
The direction of adaptation here indicated might be called the Crinoid direction,
the Crinoids having, in fact, gone furthest in this direction.
The development of the crown of arms on a body which remained small had
necessary consequences. The body (calyx, disc) and the stalk (should this latter
develop) would have to gain the necessary stability for carrying the growing arms.
This was provided for by the formation of the more or less firm carapace of plates.
The stalk attained firmness by the development of joints ; the calyx, by that of the
dorsal cup, and here all the facts seem to indicate that in the racial form of
the Crinoids, the dorsal cup had a definite composition, viz. five infrabasals, five
basals, five radials arranged in the typical manner, and the anals. For the
protection of the mouth, five orals were added, forming together a pyramid which
could be opened and closed. For the support of the arms, and in connection with
the developing capacity for unfolding and closing the crown of arms, the jointed
brachial skeleton formed.
As the arms grew out from the small body, the ccelom was produced into them,
and processes of the single rudiment of the gonad (the axial organ) spread in one
way or another into them. They became fertile more or less far from the calyx (or
disc) and yielded the gonadial bundles, each of which opened outwards through one
or more special apertures.
In this point also, the Crinoids are the most extreme forms.
The Echinoidea, Ophiuroidea, and Asteroidea appear also to belong as lateral
branches to this Crinoid development.
First, and probably very early, the Echinoids seem to have branched off. They
became free, used their tentacles for locomotion, and took in food direct through the
mouth, the food grooves, with the nerve ridges, becoming the subepithelial radial
nerves. The arms were again incorporated into the enlarging calyx, or test ; in that
the apical skeleton of the arms degenerated, and thus brought the (ambulacral) ends
of the arms close up to the continually decreasing apical capsule. As this latter
was free, the anus could shift into its centre.
We have come to hold this view of the derivation of the Echinoidea from attached
ancestral forms with arms, a view which, as far as we know, has never before been
published, chiefly for the following reasons :
The Echinoidea possess five pairs of gonads, which are at first connected with the
axial organ by means of an aboral circular strand.
This important distinction from the Holothurioidea, with which the Echinoidea
are usually compared in other points, can only be explained by the assumption that
thchinoderm morphology, it must be acknowledged that in the
majority of cases it does not suffice for a full explanation, and that it cannot indeed
at present be reconciled with many ontogenetical and anatomical facts. The recent
researches in Echinoderm morphology and the attempts at phylogenetic explanations,
which are continually suggesting new points of view, justify us, however, in hoping
that, little by little, many of these interesting and important problems will receive
a satisfactory solution.
Review of the most important Literature.
Comprehensive Works. Text Books. Treatises of Wider Scope. Re-
searches extending over some or all of the Classes.
A. Agassiz. Paleamtoloffieal <>i>d .,,il>njoloii the d^vi-lnpni' nt of the Calcareous Plates of Asterias. Bull. Mus. Harvard
Coll Vol. XVII. 1888.
On the development of the Calcareous Plates of Amphiura. Bull. Mus. Comp.
Zool. Vol. XIII. No. IV. 1887.
Alex. Foettinger. Sur la structure des pedicellaires gemmiformes d'tfchinides.
Arch, de BioL Vol. II. 1881.
E. Forbes. A history of British Starfishes and other animals of the class Echino-
fhrmata. London, 1841.
E. Fraas. Die Asterien des u-cissen Jura von Schwaben und Franken, rait Unt<:r-
554 COMPARATIVE ANATOMY CHAP.
suchungen iiber die Structur der Echinodermen und das Kalkgeriist der Aslerien.
Palaeontogr. 32 Bd. 1886.
Wilhelm Giesbrecht. Derfeinere Ban der Seeigelzdhne. Morph. Jahrbuch. 6 Bd.
1880.
J. E. Gray. Synopsis of the species of Starfishes in the British Museum. London,
1866.
G. Hambach. Contributions to the anatomy of the genus Pentremites, with descrip-
tion of new species. Trans, of the Acad. of Science of St. Louis. Vol. IV.
No. I. 1880.
Clem. Hartlaub. Beitrag zur Kenntniss der Comatulidenfauna dcs Indischen
Archipels. Nova Ada Acad. Cces. Leop.-Carol. Germ. Nat. Cur. 58 Bd.
No. 1. 1891.
C. Heller. Die Zoophyten und Echinodermen des Adriatischen Mceres. Wien, 1868.
0. Jaekel. Beitrage zur Kenntniss der Palceozoischen Crinoiden Deutschlands. Pal.
Abh. New Series. III. 1895.
K. Lampert. Die Seewalzen. Reisen im Archipel der Philippinen von Dr. C.
Semper. 2 Th. Wiss. Resultate. 4 Bd. 3 Abth. Wiesbaden, 1885.
A. Ljungman. Ophiuridea viventia hue usque cognita. Stockholm, 1867.
P. de Loriol. Echinologie helvetique. L, II., III. Geneva, 1868-1875.
Monographic des Crinoides fossiles de la Suisse. Geneva, 1877-1879.
Paleontologie franqaise. Terrain jurassique. Tome XL Crinoides. Part I.
1882-1884. Part II. 1884-1889.
S. Loven. fitudes sur les fichinoidees. K. Svensk. Vet. -Akad. Handl. 11 Bd.
(1873-75). Stockholm, 1874.
On Pourtalesia, a genus of Echinoidca. K. Svensk. Vet. -AJcad. Handl. 19 Bd.
1884.
Echinologica. Bihang till K. Svensk. Vet.-Akad. Handl. XVIII. Afd. 4.
1892.
Chr. Fr. Liitken. Additamenta ad historiam Ophiuridarum. Copenhagen, 1858-
1869.
Hubert Ludwig. Trichaster clegans. Zeitschr. f. wissensch. Zool. 31 Bd. 1878.
Zur Kenntniss der Gatlung Brisinga. Ibid.
Das Mundskelct der Asterien und Ophiuren. Ibid. 32 Bd. 1879.
Ueber den primaren Steinkanal der Crinoideen nebst vergleichend-anatomischen
Bemerkungen. Ibid. 34 Bd. 1880.
Zur Entwickelungsgeschichte des Ophiurenskeletes. Ibid. 36 Bd. 1882.
Ophiopteron elegans, eine neue, wahrscJieinlich schivimmende Ophiuridenform.
Ibid. 47 Bd. 1888.
Ankyroderma musculus (Hiss.), eine Molpadiide des Mittelmeeres, nebst Bemer-
kungen zur Phylogenie und Systematik der Holothurien. Ibid. 51 Bd. 1891.
Holothuroidea of the (< Albatross" Expedition. Mem. Mus. Harvard. XA r II.
3. 1894.
Th. Lyman. Ophiuridce and Astrophytidce. Illustr. Catalogue of the Museum of
Comp. Zool. Harvard College. I. Cambridge, Mass., 1865.
Report on the Ophiuridea. Report on the scientific results of the voyage of
H.M.S. "CJiallenger." Vol. V. Part XIV. London, 1882.
Meyer. Ueber die Laterne des Aristoteles. Arch. f. Anat. u. Physiol. 1849.
J. S. Miller. A natural history of the Crinoidea or lily -shaped animals. Bristol,
1821.
Job. Miiller. Ueber den Bau des Pentacrinus caput Medusce. Abhandl. d. Akad. d.
Wissensch. Berlin, 1841.
Ueber die Gattung Comatula Lam. und Hire Arten. Abhandl. d. Akad. d.
Wissensch. Berlin, 1847.
vin ECHINODERMATA LITERATURE 555
Job. Miiller and Fr. H. Troschel. System der Asteriden. Braunschweig, 1842.
Edm. Perrier. Observations sur les relations qui existent entre les dispositions des
pores ambulacraires a TexUrieur eta Vintericur du test des JZchinides reguliers.
JVtwr. Arch. Museum. Tome V. 1869.
Edm. Perrier. Recherches sur les pedicellaires et les ambulacres des Asteries et des
Ov.rsins. A/males des Sciences natur. (5). Vols. XII. and XIII. 1869-1870.
Revision de la collection de StelUrides du Museum d'histoire naturelle de Paris.
Paris, 1875-1876.
Memoire sur les etoilesde mej\ recueillies dans la mer des Antilles et le golfe du
M'.rique. Paris, 1884.
H. Prouho. Recherches sur le Dorocidaris papillata et quelques autres Bchinides de
la mediterranee. Arch, de Zool. exptr. (2). Tome V. 1887-1888.
F. A. Quenstedt. Petrefactenkunde Deutschlands. 3 Bd. Echiniden. Leipzig,
1872-1875.
Ferd. Rb'mer. Monographic der fossilen Crinoideenfamilie der Blastoideen. Arch.
f. Xaturfjcschichte. 1851.
G. 0. Sars. Memoire pour servir a la connaissance des Crinoides vivants. Chris-
tiania, 1868.
Researches on the structure, etc., of the genus Brisinga. Christiania, 1875.
M. Sars. Bidrag til Kundskaben om Middelhavets Littoral- Fauna. Christiania,
1857.
Oversiyt of Xorges Echinodermer. Christiania, 1861.
E. Selenka. Bcitrdcje zur Anatomic und Systematik der Holothurien. Zeitschr. f.
wissensch. Zool. 17 and 18 Bd. 1867, 1868.
C. Semper. Reisen im Archipel der PMlippinen. 1 Bd. Holothuricn. Leipzig,
1868.
W. Percy Sladen. On a remarkable form of Pedicellaria, and the functions per-
formed thereby, etc. Annals and Mag. of Xat. History (5). Vol. VI. 1880.
On the homologies of the primary larval plates in the test of Brachiatc Echino-
derms. Quart. Journ. Microsc. Science (2). Vol. XXIV. 18"84.
Report upon the Asteroidea collected by H.M.S. "Challenger." Vol. XXX.
1889.
Hj. Theel. Report on the Holothurioiclea collected during the voyage of the
" Challenger." Parti. Vol. IV. 1882. Part II. Vol. XIV. 1886.
On the formation and resorption of the skeleton in the Echinoderms. Ofv. Ak.
Fork. 1894.
Volborth. Ueber Achradocystitcs und C ystoblastus, zwei neu-e Crinoiden-Gattungen.
Mem. Acad. Imp. Sc. St. Petersbourg. 1870. Tome XVI. No. II.
C. Viguier. Anatomic comparee du squdettc des Stclleridcs. Arch. Zool. experim.
Tome VII. 1879.
C. Wachsmuth and F. Springer. Revision of the Palceocrinoidca. Proceed. Acad.
Xat. Sc. of Philadelphia. 1879, 1881, 1885.
(1) Discovery of the ventral structure of Taxocrinu-s and Haplocrinus, and
consequent modifications in the classification of the Crinoidea. Proceed. Acad.
Xnt. Science Philadelphia. 1889.
(2) Crotalocrinus : its structure and zoological position. Ibid.
- The perisomic plates of the Crinoids. Pro. Acad. Nat. Sc. Philadelphia. 1890.
T. Wright. Monograph of the British fossil Echinodcrmata of the bolithic forma-
tion. London, 1857-1880.
Monograph of the British fossil Echinodermata of the cretaceous formation.
London, 1864-1882.
K. A. Zittel. Handbv.ch der Paldontologie. 1 Bd. 1876-1880.
556 COMPARATIVE ANATOMY CHAP.
Anatomical Monographs.
A. Agassiz. Revision of the Echini. Mus. Compar. Anatomy Harvard Coll. Vol.
VII. 1872.
North American Starfishes (Mem. of the Mus. of Comp. Zool. Vol. V. No.
I.). Cambridge, Mass., 1877.
Report on the Echinoidea collected by H.M.S. "Challenger" Vol. III.
London, 1881. (Also contains anatomical details.)
Nic. Christo Apostolides. Anatomie et developpement des Ophiures. Arch. Zool.
exper. gtner. Vol. X. 1882.
Alb. Baur. Beitrdge zur Naturgcschichte der Synapta digitata. Ada. Acad. Gees.
Leop.-Carol. Nat. Curios. 1864.
P. H. Carpenter. Report upon the Crinoidea collected during the voyage of H.M.S.
" Challenger" during the years 1873-1876. Part 1. General Morphology, with
descriptions of the stalked Crinoids. Vol. XI. 1884. Part 2. The Comatulce.
Ibid. Vol. XXVI. 1888. (The second part is almost exclusively systematic
and descriptive.)
William B. Carpenter. Rcsearclies on the structure, physiology, and development of
Antedon rosaceus. Philos. Transactions. Vol. CLVI. 1866. Addendum.
Pro. R. Soc. Vol. XXIV. 1876.
L. Cuenot. Contribution a I'etude anatomique des Asterides. Arch. Zool. experim.
(2). Tome V. Supplementary (1887-1890).
Etudes anatomiques et morphologiques sur les Ophiures. Arch. Zool. exper.
(2). Tome VI. 1888.
D. C. Danielssen and J. Korea. Holothurioidea. The Norwegian North Atlantic
Expedition. 1876-1878. Christiania, 1882.
Fredericq. Contributions a V etude des Echinides. Arch, de Zool. experim. Tome
V. 1876.
0. Hamann. Beitrdge zur Histologie der Echinodermen. 1. Heft. Die Holothurien.
Jena, 1884. 2. Heft. Die Asteriden. Jena, 1885. 3. Heft. Anatomie und
Histologie der Echiniden und Spatangiden. Jena, 1887. 4. Heft. Anatomie
und Histologie der Ophiuren und Crinoiden. Jena, 1889. (Reprints from the
Jenaischen Zeitschr. f. Naturwiss. )
Beitrdge zur Histologie der Echinodermen. 1. Die Holothurien (Pedata) und
das Nervensystem der Asteriden. Zeitschr. f. wiss. Zool. 39 Bd. 1883.
6. Herouard. Recherches sur les Holothuries des c6tes de France. Arch. Zool.
exper. (2). Tome VII. 1889.
C. F. Heusinger. Anatomische Untersuchung der Comatula mediterranea. Zeitschr.
f. organische Physik. Tome III. 1828.
C. K. Hoffmann. Zur Anatomie der Echinen und Spatangcn. Niederl. Arch.
Zool. 1 Bd. 1871.
Zur Anatomie der Asteriden. Niederl. Arch. f. Zool. 2 Bd. 1874.
G. F. Jaeger. De Holothuriis. Diss. inaug. Zurich, 1833.
Et. Jourdan. Reclierches sur Vhistologie des Holothuries. Ann. Mus. H. N.
Marseilles. Tome I. 1883.
R. Kbhler. Reclicrches sur les Ecliinidcs des cdtes de Provence. Ann. Mus. H. N.
Marseilles. Tome I. 1883.
W. Lange. Beitrag zur Anatomie und Histologie der Asterien und Ophiuren.
Morph. Jahrbuch. 2 Bd. 1876.
Leydig. Anatomische Notizen ilber Synapta digitata. Mailer's Arch. 1852.
Loven. EtudcssurlesEchinoid.es. Svensk. Vetensk.-Akad. 11 Bd. 1874.
H. Lvdwig. Br.it rage zur Kenntniss der Holothurien. Arbeit. Zool. Inst. Wilrzburg.
2 Bd. 1874.
vm ECHIXODERUATA LITERATURE 557
H. Ludwig. Beitrage zur Anatomic der Crinoidccn. Leipzig, 1877. Zeitschr. f.
. Zool. 26 Bd. 1877. 28 Bd. 1877.
- Zvr An atomic des Rhizocrinus lofotensis. Ibid. 29 Bd. 1877.
Rltopalodina lagcniformis. Ibid. 1877.
- Beitra : zur Aimtuinie der Asteriden. Ibid. 30 Bd. 1878.
- Ueber Asthenosoma rariv.m Gnibe und iiber ein neues Organ bei den Cidariden.
Ibid. 34 Bd. 1880. Bericlitigung im Zool. Am. 3 Jahrg.
- Ueber den primarcn Stcinkanal der Crinoiden nebst vergl.-anatomischen
Bemerkungen iibcr die Echinodermen iiberJiaupt. Ibid. 34 Bd. 1880.
-- X'-nf Beit rage zur Anatomie der Ophiuren. Ibid. 34 Bd. 1880.
Nochmals die Rhophalodina Jageniformis. Ibid. 48 Bd. 1889.
- Die Seewalzen. Bronns Klassen und Ordnungen des Thicr-Reichs. 2 Bd. 3
Abth. Echinodermata. Leipzig, 1889-1892.
Hub. Ludwig and Ph. Barthels. Beitrage zur Anatomie der Holothurien. Zeitschr.
f. iff iss. Zool. 54 Bd. 1892.
T. Lyman. Report on the Ophiuridea. Voyage of H. M.S. "Challenger" Zool.
Vol. XV. Part 14. London, 1882.
J. Muller. Uebcr den Ban von Pentacrinus caput Medusa'. Abhandl. d. Akad. d.
Jrisscnsch. Berlin, 1841.
- Ueber Synapta diyitata und iibcr die Erzeugung von Schnecken in Holothurien.
Berlin, 1852.
E. Perrier. Memoire mr I' organisation ct le devcloppement de la Comatule de la
Mtditerranee (Antedon rosacea Linck}. Xouv. Archives Mus. Paris, 1886-
1892.
- Recherches sur Tanatomie et la regeneration des bras de la Comatula rosacea.
Arch. Zool. exper. Tome II. 1872.
H. Prouho. Recherches sur le Dorocidaris papillata et quelques autres chinides de
Jo. Medfarrante. Arch, de Zool. exper. (2). Tome V. 1887.
A. de Quatrefages. Mtmoire sur le Synapte de Duvernoy. Ann. Sc. natur. (2).
Tome XIV. 1842.
C. Semper. Knr^ anatomische Bemcrkungen iiber Comatula. Arbeit. Zool. Inst.
irurzburg. 1 Bd. 1874.
C. F. and P. B. Sarasin. Uebcr die Anatomie der Echinothuriden und die Phylo-
genic der Echinodermen. Ergcbnisse Xat. Forschungcn Ceylon 1 Bd. 1888.
G. 0. Bars. R< searches on the structure etc. of the genus Brisinga. Christiania,
1875.
- Memoire pour servir a la connaissance des Crinoides vivants. Christiania, 1868.
Selenka. Beitrage zur Anatomie und Systematik der Holothurien. Zeitschr. f.
wiss. Zool. Two papers in Vols. 17 and 18. 1867-1868.
R. Semon. Beitrage zur Xaturgeschichte der Synaptiden des MUtelmeers. Mitth.
Zool Stat. in Xcapd. 1 Bd. 1887.
Von Siebold. Zur Anatomie der Seesterne. Muller' s Archiv. 1866.
H. Simroth. Anatomie und Schizogonie der Ophiactis virens. Zeitschr. f. wiss.
Zool. Two papers in Vols. 27 and 28. 1877.
Reinhold Teuscher. Beitrage zur Ancdomie der Echinodermen. I. Comatula medi-
f> -i-ranei't. Jenaische Zeitschr. 10 Bd. 1876.
Friedr. Tiedemann. Anatomie der Rdhrcnholothurie, des 'pomeranzenfarbigen
Seestcrns und des Stcin-Seeigcls. Landshut, 1816.
Hj. Theel. Import on the Holothurioidea. Voyage of H.M.S. "Challenger."
Zoology. Part I. Vol. VI. Part XIII. 1882. Part II. Vol. XIV. Part
XXXIX. 1885.
J. V. Thompson. Surle Pcntacrinuseuropaeus, Titat de jcunessc du genre Comatula.
L'Institut. 1835.
558 COMPARATIVE ANATOMY CHAP.
G. Valentin. Anatomie du genre Echinus. Monographies d' Echinodermes, par
L. Agassiz. Neuchatel, 1841.
Alex. Weinberg. Die Morphologic der lebenden Crinoiden mit Beziehung auf die
form Antedon rosacea LincTc. Naturhistoriker. 5 Jahrg. 1883.
Works dealing with Single Organs or Systems of Organs.
H. Avers. On the structure and function of tlie Sphceridia of the Echinoidea. Quart.
Journ. Microsc. Science (2). Vol. XXVI. 1886.
E. W. M'Bride. Tlie development of the genital organs, ovoid gland, axial and aboral
minuses in Amphiura squamata, together with some remarks on Ludwig's Haemal
System in this Ophiurid. Quart. Journ. Microsc. Science. Vol. XXXIV. 1893.
The development oftlie dorsal organs, genital rachis and genital organs in Asterina
gibbosa. Zool. Anz. 16 Jahrg. 1893.
E. Baudelot. Contributions a I'histoire du systeme nerveux des Echinodermes. Arch.
de Zool. cxpfr. 1872.
W. B. Carpenter. On the nervous system of Crinoidea. Proceed. Roy. Soc. London.
Vol. XXXVII. 1884.
L. Cue'not. Etudes sur le sang, son role et sa formation dans la serie animale.
Part II. Invertebrts. Arch. Zool. exptr. (2). Tome IX. 1891.
E. Haeckel. Ueber die Augen und Nerven der Seesterne. Zeitschr. f. wiss. Zool.
10 Bd. 1860.
Otto Hamann. Beitrdge zur Histologie der Echinodermen. 1. Die Holothurien
(Pedata) und das Nervensystem der Asteriden. Zeitschr. f. wiss. Zool. 39 Bd.
1883. 2. Mitth. 1. Nervensystem der pedaten Holothurien. 2. Cuvier'sche
Organe. 3. Nervensystem und Sinnesorgane der Apoden. Ibid.
R. Kohler. Recherches sur Vappareil circulatoire des Ophiures. Ann. Sc. nat. (7).
Tome II. 1887.
F. Leipoldt. Das angebliche Excretionsorgan der Seeigel, untersucht an Sphcerechinus
granularis und Doroddaris papillata. Zeitschr. f. wiss. Zool. 55 Bd. 1893.
Theodore Lyman. The stomach and genital organs of Astrophytidce. Bull. Mus.
Comp. Zool. Harvard College. Vol. VIII. No. 6. Cambridge, Mass., 1891.
A. M. Marshall. On the nervous system of Antedon rosaceus. Quart. Journ.
Microsc. Science. Vol. XXIV. 1884.
C. Mettenheimer. Ueber die Gesichtsorgane des violetten Seesterns. Mailer's Arch.,
1872.
E. A. Minchin. Notes on the Cuvierian organs of Holothuria nigra. Ann. and
Mag. Nat. History (6). Vol. X. 1892.
J. Niemiec. Recherches sur les ventouses dans le regne animal. Recueil Zool. Suisse.
Tome II. Encore un mot, etc. Ibid. 1885.
Owsjanikoff. Ueber das Nervensystem der Seesterne. Bull. Acad. St. Pttersbourg.
1870. Tome XV.
Ed. Perrier. Recherclies sur les pedicellaires et les ambulacres des Asteries et des
Oursins. Ann. des Sciences natur. (5). 12 and 13 Bd. 1869-1870.
Recherches sur I'appareil circulatoire des Oursins. Arch, de Zool. exper.
Tome IV. 1875.
G. J. Romanes and J. C. Ewart. Observations on the locomotor system of Echino-
dermata. Philos. Transact. London. Part III. 1881.
A. Russo. Ricerche citologiclie sugli elementi seminali delle Ophiurece (spermato-
genesi-oogenesi] Morfologia delV apparecchio riprodutore. Internal. Monatschr.
f. Anat. und. Phys. 8 Bd. 8 Heft.
C. F. and P. B. Sarasin. Die Augen und das Integument der Diadematiden.
via ECHINODERMATA LITERATURE 559
Ergebnissc natunciss. Forschungen auf Ceylon in d. Jahren 1884-1886. 1 Bd.
1887.
Rich. Semon. Das Nervensystem der Holothurien. Jenaische Zeitschr. f. Natur-
u-iss. 16 Bd. 1883.
H. S. Wilson. The nervous system of the Asteridce. Transact. Linnean Society.
Vol. XXIII. 1860.
Ontogeny.
A. Agassiz. Xorth American Starfishes, 1864. Mem. of the Museum of Comp. Zool.
Harvard College. Vol. V. 1877.
Revision of the Echini. Illustr. Catalogue of the Museum of Comp. Zool.
Harvard College. 1872-1874.
Nic. Christo Apostolides. Anatomic, et developpement des Ophiures. Arch. Zool.
/. Tome X. 1882.
J. Barrois. Itecherches sur le developpement de la Comatule (C. mediterranea).
Eccucil Zool. Suisse. Tome IV. 1888.
E. W. M 'Bride. The development of the genital organs, ovoid gland, axial and aboral
sinuses in Amphiura squamala, together with some remarks on Ludwig's Hcemal
System in this Ophiurid. Quart. Journ. Microsc. Science. Vol. XXXIV.
Part II. 1893.
The development of the dorsal organ, genital rachis and genital organs in
Astcrina gibbosa. Zool. Anz. 16 Jahrg. 1893.
H. Bury. The early stages in the development of Antedon rosacca. Philos. Transac-
tions. Vol. CLXXIX. 1888.
Studies in the embryology of the Echinoderms. Quart. Journ. Microsc. Sc.
Vol. XXIX. 1889.
- Tlie Metamorphosis of Echinoderms. Ibid. Vol. XXXVIII. 1895.
P. H. Carpenter. On some points in the anatomy of larval Comatula. Quart. Journ.
Mi.: rose. Sc. (2). Vol. XXIV. 1884.
Xotcs on Echinoderm morphology. No. 11. On the development of the apical
plates iii Amphiura, squamata. Quart. Journ. Microsc. Sc. Vol. XXVIII.
1888.
William B. Carpenter. Researches on the structure, physiology, and development of
Antedon rosaccus. Philos. Transact. Vol. CLVI. 1866. Addendum. Pro.
Roy. Soc. Vol. XXIV. 1876.
J. W. Fewkes. On the development of calcareous plates of Amphiura. Bull, of the
Museum of Comp. Zool. of Harvard College. Vol. XIII. 1887.
A. Fleiscnmann. JJic EnticickcJung des Eiesvon Echinocardium cordatum. Zeitschr.
f. iciss. Zool. 46 Bd. 1888.
Geo. W. Field. Tlie larva of Asterias vulgaris. Quart. Journ. of Microsc. Sc. Vol.
XXXIV. Part 2. 1892.
Alexander Goette. Vergleichendc EniicicMungsgeschicMe der Comatula mediter-
ranea. Arch. f. mikr. Anatomic. Tome XII. 1876.
E. Korshelt. Zur Bildung des mittleren Keimblatts bei den Echinodermen. Zool.
J.ihi-b. AUh. Morph. 3 Bd. 1889.
Kowalevsky. Beitrdge zur Eiiticick':hin ventral vessel ; 10, dorsal mesentery ; 11, ventral
branchial pore, leads to the ^m. 12f gi
exterior.
The inner aperture, the gill -slit, is as long as the gill -pouch
itself, and would have the shape of a very long were it not com-
plicated by the formation of the tongue. The intestinal wall projects
from the upper end of the gill-slit in the form of a hollow process
down into the slit, changing the into a very long vertical TJ
(Fig. 457, 12). This hollow process is the tongue. Its cavity is in
open communication with the coelom of the trunk. It either hangs
freely down into the gill-slit (Balanoglossus, Glandiceps), or is attached
to the wall of the gill-pouch by means of rods or buds, the so-called
synaptieulse which run across the limbs of the U-shaped gill-slit
transversely, making the latter fenestrated.
9 8 11 2
FIG. 457. Portion of the branchial region of
an Enteropneustan, cut down through the
middle line, and seen from the cut surface,
diagrammatic. 1, Dorsal vessel ; 2, body wall ;
3, cavity of the trunk ; 4, branchial pore ; 5, con-
568
COMPARATIVE ANATOMY
CHAP.
The partitions between the consecutive gill -pouches are called
septa, and the edges of these which are turned to the intestine are
the septal edges. If a lateral wall of the branchial intestine be
viewed from the intestinal cavity, the septal edges and branchial
JO 11
FIG. 458. Ptychodera minuta, transverse section through the branchial region, somewhat
diagrammatic (after Spengel). 1, Dorsal nerve cord ; 2, dorsal blood vessel ; 3, branchial furrow ;
4, body epithelium ; 5, gonad ; 6, longitudinal muscle layer of the integument ; 7, ventral blood
vessel ; 8, ventral nerve cord ; 9, ccelom of the trunk ; 10, genital pore ; 11, branchial pore ; 12,
branchial tongue ; 13, dividing ridges ; 14, branchial septum ; 15, cavity of the branchial intestine ;
16, oesophagus.
tongues are seen regularly alternating. The septa, like the tongues,
are hollow, their cavities communicating with the coelom of the trunk.
But whereas the septal edges are continued both dorsally and ventrally
into the wall of the intestine, the edges of the tongues turned towards
the intestine, the so-called backs of the tongues, are, of course, only
in connection with the intestinal wall dorsally.
ENTEROPNEUSTA ALIMENTARY CANAL
The epithelial walls of the gill -pouches and of the tongues are
ciliated.
The depth (measured dorsoventrally) of the area occupied by the
gill-slits on the lateral wall of the branchial intestine varies greatly.
In all cases the gill-slits leave only a narrow strip of the intestinal
wall in the dorsal median line ; this strip is the epibranehial streak.
Ventrally they never extend so far towards the median line. They
either leave a narrow strip of the intestinal wall, the hypo-branchial
streak, which is at any rate wider than the epibranehial streak (Schizo-
cardium), or they only extend a very short way on to the ventral wall
(Glandiceps), or again they only reach about half way down the lateral
wall (Balonoglossiis). In the last case the hypobranchial streak
FIG. 459. Vertical longitudinal section
through the anterior part of a row of
gills, and through a collar pore of Schizo-
cardium brasiliense (after Spengel). cpj,
Anterior aperture of the collar canal (into
the coeloin of the collar) ; cpo, posterior aper-
ture (collar-pore) of the same (into the first
gill-pouch) ; bpi-bpQ, first to sixth branchial
pores (outer apertures of the gills) ; bsi-bs^,
first to fifth gill-pouches ; 6/4, bf 5 , fourth and
fifth gill-slits (apertures of the gill-pouches
into the branchial intestine) ; dvm, dorsoven-
tral musculaturp ; cc, cceloin of the collar ;
re, ccelom of the trunk ; v, blood vessels ;
Ic, continuation of the ccelom of the trunk
into the branchial tongues; I, branchial
tongues ; s, branchial septa ; szi, first anterior
septal bar or prong ; lz, tongue bars or
prongs ; crs, septum dividing the trunk
from the collar.
occupies the ventral or nutritive half of the branchial intestine, which
is thus more or less distinct from the dorsal or respiratory half, into
which the gill-slits open. The distinction between these two halves is
still more marked in Ptychodem (Fig. 458, 15, 16), inasmuch as they are
here separated by longitudinal ridge-like projections of the intestinal
wall, which run on each side along the boundary between the two
(13). The two ridges growing towards one another may even touch,
in which case open communication between the branchial intestine
above and the oesophagus below ceases.
The form of the outer apertures of the gill-pouches, the branchial pores, has
been described above. The furrows in which they lie correspond, in Balanoglossus,
Glandiceps and Schizocardium, with the submedian line, which is indicated by the
570 COMPARATIVE ANATOMY CHAP.
interruption of the longitudinal musculature. In Ptychodera, the branchial pores
lie mediad of this line.
The gills are, as a rule, paired, but in species of Ptychodera, those belonging to
one side may be shifted in front of those of the other side by as much as half the
breadth of a gill.
At the posterior end of the branchial region, even in adult animals, new gills are
continually being formed.
In Ptychodera clavigera, each gill -pouch has a long ventrally directed diver-
ticulum.
The cavity of the branchial tongue is lined with endothelium, and traversed in
various directions b} r fibres, some of which, no doubt, are muscular.
The efferent section of the gill-pouches is provided with a musculature, which
cannot here be described. The pores also may be provided with an encircling sphincter
musculature of their own.
In Balanoglossus Kowalevskii, the posterior edge of the collar is continued back-
ward as two outgrowths, which cover the most anterior branchial pores. These
outgrowths have been called the opercula, and the small space they enclose, the
atrium.
For the blood vessels and the skeleton of the branchial intestine, see below,
pp. 584 and 580.
D. The afferent intestine, which follows the branchial intestine,
runs through the posterior gill-less part of the branchio-genital region,
and at its posterior end passes over into the hepatic or stomach intestine.
In some forms the afferent intestine is distinguished by the fact that
it sends off dorsally to right and left short canals, which open outward
on the dorsal surface. These efferent canals are known as the unpaired
intestinal pores, because they are for the most part unpaired.
Special. These unpaired intestinal pores are found in Schizocardium brasiliense,
Glandiceps Hacksii, and GL talaboti. In Schi. brasiliense the openings are irregular,
either paired or unpaired. Twenty-nine in all have been observed, thirteen on the
left and sixteen on the right side, and among them seven pairs. The afferent section
of the alimentary canal is, in this species, distinguished by a strong circular
musculature. In GL Hacksii, nine unpaired pores were observed in the young
animals examined, the most anterior being on the right, and the rest on the left.
In GL talaboti, all the pores in this region are unpaired ; in the animals examined
they are arranged in nine groups at irregular distances from one another. The
efferent canals of each group probably open into a common ampulla, which, on its
part, opens outward through a single aperture.
E. The hepatic or stomachal region of the intestine is, in all
Enteropneusta, distinguished by the fact that its epithelium is ciliated
and contains numerous globules of a secretion, usually green in colour.
This section of the intestine seems to have a musculature of its own
only in Schizocardium brasiliense, in which it is developed as a fine
layer of longitudinal fibres. The hepatic intestine is no doubt the
part of the alimentary canal of the greatest importance for digestion ;
the network of vascular capillaries, which will be described later, is
specially strongly developed in its walls.
The hepatic intestine appears as a specialised section of the
ENTEROPXEUSTA CCELOMIC SACS 571
digestive tract only in those species of Pfychodera and Schizocardium
in which it gives off on each side dorsally a row of finger- or wedge-
shaped outgrowths, which push out the body wall in such a way
as to form the above-mentioned liver-caeca. The aperture of each
caecum into the alimentary canal is a narrow transverse slit. Food
never passes into the liver-caeca. The capillary network is exceedingly
close in their walls, and the intestinal epithelium of the caeca is, as a
rule, much folded.
In Glandiceps Hacksii, an accessory intestine occurs in the hepatic
region : this is a straight canal, ca. 6 mm. long, which branches off
from the median dorsal surface of the intestine proper about the
middle of the region, and again enters it at the posterior end of the
same region.
In Schizocardium brasiliense, Glandiceps Hacksii, Balanoglossus Kowa-
levskii, and B. Merschkovskii (but not in Ptycliodera and not in B. Kupfferi,
and B. canadensis) paired intestinal pores, leading outward dorsally, are
found in the most anterior hepatic region, or in the region immediately
in front of it, intercalated between it and the afferent intestine. Schi.
brasiticnse has one pair, Gl. Hackm three pairs, and Balanoglossus Kowa-
l''c*kii four to six pairs of such pores. They emerge mediad of the
submedian line, and may be provided with cilia and with sphincter
muscles.
F. The hepatic intestine is followed by the efferent section,
which gradually passes into the narrower rectum, this in its turn
opening outward through the anus. Where, in this section, a proper
musculature is found, it is very weakly developed.
VI. The Coelomie Sacs and the Body Musculature.
We here use the expression coelomic sacs rather than eoelomic
cavities, the former implying that they have walls of their own.
Five eoelomie sacs occur in the body of an Enteropneustan, these
being divided among the principal regions of the body as follows :
The proboscis contains one unpaired eoelomie sac.
The collar contains two paired ecelomie sacs.
The trunk contains two paired eoelomie sacs.
The coelomic sacs fill up almost the whole of the space between
the intestinal epithelium and the body epithelium, i.e. the seg-
mentation cavity or blastoeoel of the larva, with the exception of
a system of spaces, serving as the blood vascular system, which will
be described later.
In each coelomic sac there can be distinguished, at the least, a
visceral wall in contact externally with the intestinal epithelium, and
a parietal wall, in contact internally with the body epithelium.
Where the ccelomic sacs are paired, i.e. in the collar and the
572 COMPARATIVE ANATOMY CHAP.
trunk, the two lateral sacs come in contact with one another above
the intestine to form a bilaminar dorsal mesentery, and below the
intestine to form a bilaminar ventral mesentery.
In the adult animal these mesenteries are nowhere retained in
their full extent.
Each coelomic sac has an anterior and a posterior wall. The
posterior wall of the collar sac becomes applied to the anterior wall
of the trunk sac and thus forms a bilaminar transverse and vertical
septum, separating the ecelom of the collar from that of the
trunk.
The walls of the ccelomic sacs, in the larva, are epithelial.
Throughout the greater part of these sacs, however, the epithelial
cells become transformed into muscle fibres to form the musculature of
the body and of the intestine. And this takes place to such an
extent that over large areas no endothelial lining to the body cavity
is any longer demonstrable.
Connective tissue is also produced by the walls of the coelomic
sacs.
The musculature of the Enteropneusta consists exclusively of
smooth fibres.
Lymph cells (probably amoeboid) float in the fluid of the body
cavities : these are presumably produced by the peritoneal endo-
thelium.
A. The Coelom of the Proboscis.
The proboscis coelom is, as above mentioned, unpaired. The
parietal wall lies under the proboscidal epithelium, the visceral wall
envelops not only the proboscidal diverticulum of the buccal cavity,
but a complex of other organs as well, which lie posteriorly in the
base of the proboscis ; these basal organs, to a certain extent, bulge
out the coelomic wall, like the finger of a glove, from behind forward,
into the proboscis cavity.
This cavity has three outgrowths directed backward towards the
neck, one ventral and two, a right and a left, dorsal. The left out-
growth is produced backward into a canal lined with ciliated epithe-
lium, which opens outwards through the proboscis pore. This pore
lies dorsally to, and on the left side of, the neck, at a greater or
less distance from the median line.
In a few forms (constantly in Balanoglossus Kupfferi and B. cana-
densis, and occasionally in Ptyclwdem minuta and B. KowalevsJcii) a
second proboscis pore occurs, through which the right dorsal outgrowth
of the proboscis coelom opens outward. This secondary proboscis
pore rises much later ontogenetically than the primary.
It has been conjectured that water is taken in through these pores
for the purpose of swelling the proboscis. There is no justification
for ascribing to them any excretory function.
The visceral wall of the proboscidal coelomic sac and, in general.
ix EXTEROPXEUSTACCELOMIC SACS 573
the walls of the posterior outgrowths, retain an epithelial character,
while the parietal wall develops muscle and connective tissue. This
parietal wall consists of the following parts :
1. Close under the basal or limiting membrane of the body
epithelium, there is an outermost layer of circular muscle fibres.
2. This latter is followed by a massive layer of longitudinal
muscles, filling up the greater part of the proboscis. The very
complicated course of the longitudinal muscle fibres cannot here be
described in detail. They are stretched like the strings *of an
instrument between two points of the proboscidal wall, one behind the
other, so that they cross one another in every direction.
3. Dorsoventral muscle fibres form a dorsoventral muscle
septum exactly in the median plane of the proboscis. This is,
however, not developed through the whole length of the cavity,
but reaches only as far forward as the proboscidal diverticulum of
the intestine or its vermiform process. This muscle septum thus has a
free anterior edge. The fibres of the septum, which descend from the
median line, when they reach the basal organs, diverge to right and
left, clasping these organs between them, then again uniting beneath
them, form the ventral portion of the septum. This ventral portion
is distinctly a double muscle lamella. The two constituent lamellae
are separated by a structureless limiting lamella, which is a continua-
tion of the limiting membrane of the ventral proboscidal epithelium.
The circular muscle layer of the proboscis passes through the limiting
membrane of the ventral septum in bundles.
The ventral septum is interrupted at its most posterior part, so
that it has a free posterior edge as well as a free anterior edge.
That portion of the proboscidal cavity which is free from muscle
fibres, is to a great extent filled with connective tissue, in which
irregular spaces are found as remains of the cavity. A space free
from connective tissue and varying in size is retained round the basal
organs.
B. The Coelomie Sacs and the Musculature of the Collar.
The collar region of the body contains not only its own two
coelomic sacs, but outgrowths or processes of the trunk ccelom as
well ; these latter have been called peripharyngeal or perihaamal
cavities, and will be described with the trunk ccelom. The two
lateral ccelomic sacs of the collar are, in adults, nowhere completely
separated from one another by mesenteries. The median ventral
mesentery is retained for a short distance in the posterior region of the
collar. The dorsal mesentery extends further forward, but never as
far as to the anterior end of the collar. In Balanoglossus Kupfferi, both
the mesenteries are altogether wanting.
The divisions of the collar ccelom are complicated by the appear-
ance of folds in the inner or visceral wall. The two lamellae of
574 COMPARATIVE ANATOMY CHAP.
these folds lie close to one another, being only separated by a limiting
membrane containing vessels. According to the courses of these
vascular folds which project into the collar ccelom, the Enteropneusta
can be divided into two groups.
G roU p i Balanoglossus, Glandiceps, Schizocardium. A fold commences at the
posterior end of the ccelom on each side, near the ventral median line, and ascends
diagonally in a curve anteriorly to the neck of the proboscis.
Group 2 Ptychodera. A medio-ventral vascular fold runs anteriorly from the
posterior end of the collar region, dividing at a short distance from the anterior
end of that region into two folds, which ascend perpendicularly and encircle the
buccal cavity.
The walls of the collar coelom are for the most part developed as
muscles.
1. The parietal wall consists first of an outer layer of longitudinal muscle
fibres. These commence, it is true, posteriorly in the visceral wall, then run slant-
ingly forwards and outwards towards the integument, traversing the coelom. Only
in the anterior collar region do they run close under the integument to the anterior
end of the collar. Only in this anterior region of the collar also is a circular
muscle layer developed on the inner side of the longitudinal musculature.
2. The visceral wall contains first an inner longitudinal musculature, according
to the arrangement of which the Enteropneusta may be divided into two groups.
Group 1 Schizocardium, Glandiceps, Balanoglossus. A bundle of longitudinal
fibres rises on each side anteriorly, from the proboscidal skeleton, which will be
described later. These muscles spread out fan-like towards the septum between the
collar and the trunk. The fibres composing this fan slope more and more the
nearer the ventral middle line they become attached to that septum. Anteriorly, the
two bundles of fibres surround the efferent vessel of the collar.
Group 2 Ptychodera. The numerous bundles of longitudinal fibres run
parallel to one another. Only a few of them, viz. those lying nearest the median
line, run as far as the neck, enclosing the efferent collar vessel and becoming
attached to the skeleton of the proboscis. None of the rest reach the anterior
end of the collar region, but become attached to the posterior wall of the vascular
fold mentioned above as encircling the buccal cavity.
The visceral wall of the collar coelom further consists of a transverse musculature.
This also is differently arranged in the groups mentioned above, its distribution
being determined by the courses of the vascular folds.
In group 1 (Schizocardium, Gflandiccps, and Balanoglossus) the transverse fibres
run out laterally from the dorsal point of attachment of the ventral mesentery on
each side and then upwards, to become attached to the vascular fold above described
which runs from behind and below, anteriorly and upward.
Where the ventral mesentery is wanting, the transverse fibres run without
interruption from the right to the left vascular fold, surrounding the buccal cavity
ventrally. The transverse fibres are quite short posteriorly and limited to the
ventral side, for here the vascular folds are but commencing to diverge. Anteriorly,
as the folds gradually meet over the buccal cavity, the fibres become longer and
longer ; they finally form, near the insertion of the neck, circular loops almost
completely surrounding the buccal cavity, these loops being interrupted for only a
short distance dorsally.
In group 2 (Ptychodera} this transverse musculature is altogether wanting
ix EXTEROPXEUSTA CCELOMIC SACS 575
throughout the larger part of the collar, viz. in all that part which lies behind the
circular vascular fold. In front of this fold, however, it is as highly developed as
in the same area in group 1. The muscles arise dorsally at each side of the pro-
boscidal skeleton, and form loops encircling the buccal cavity.
3. The anterior wall of the collar ccelom. A strong bundle of muscle fibres
arises to right and left of the neck of the proboscis ; these radiate from the anterior
wall of the collar ccelom to the edge of the collar. Both ventrally and dorsally
the marginal fibres of these two radiating bundles usually pass beyond the median
line, cross one another, and intermingle at these points.
Besides the musculature hitherto described of the parietal visceral and anterior*
walls of the collar ccelom, there are further isolated radial muscle fibres, which]
connect the outer with the inner wall, and also with the anterior wall. These\
fibres together form a system of crossing fibres, some running slantingly from the]
outer wall inward and forward, and others inward and backward.
The cavity of the collar is filled with connective tissue, which penetrates every-
where between the muscles, leaving only certain spaces free. Such a free space is
as a rule found to the right and left in the posterior part of the collar. The collar
ccelom here is continued on each side into a canal lined with a ciliated cylin-
drical epithelium ; each canal opens through a pore (collar-pore), not on the
external surface of the body, but on the anterior wall of the first gill-pouch, near
the branchial pore. The canal projects from this pore freely forward into the
collar ccelom, and, on its outer surface, that turned to this ccelom, is covered with
plate epithelium.
As to the function of these two collar pores, we can say no more than what
was said above about that of the proboscis pore.
In Balanoglossus Kupfferi a cushion-like thickening of the epithelium is found
on each side of the body on the septum dividing the collar from the trunk. This
thickening occurs both on the anterior surface, that turned to the collar ccelom,
and on the posterior surface, that turned to the trunk ccelom. These thickenings
probably function as lymph glands.
C. The Coelomic Sacs and the Musculature of the Trunk.
The Perihsemal and Peripharyngeal Cavities of the Collar Region.
The trunk ccelom is uninterrupted throughout its whole length.
Its composition out of two lateral ccelomic sacs can still be recognised
in the adult animal, the ventral mesentery being entirely, the dorsal
partially, retained.
The ccelomic sacs of the trunk send outgrowths anteriorly into
the cavity of the collar, which push before them the wall of that
cavity ; these are the perihsemal and peripharyng-eal cavities
(Fig/460).
The perihsemal cavities are two dorsal prolongations of the trunk
ccelom, which traverse the collar region and the neck of the
proboscis as far as the proboscidal skeleton. They run below the
collar cord and above the buccal cavity. In the median line the two
cavities are separated by a structureless partition, a limiting mem-
brane, in which the dorsal vessel runs. The perihaemal spaces are
almost entirely filled by longitudinal muscle fibres formed by their
576
COMPARATIVE ANATOMY
CHAP.
dorsal walls. These are the immediate anterior continuations of the
dorsal longitudinal musculature of the trunk. In Ptychodera, a single
weak layer of longitudinal muscle also develops on the ventral walls
of the perihsemal cavities. In Schizocardium and Glandiceps, on the
contrary, a transverse musculature is here developed. In the genus
Balanoglossus, both longitudinal and transverse muscles are wanting
in the ventral wall.
Besides the muscles which have been mentioned, there are fibres
which traverse the perihaemal cavity transversely, chiefly in a dorso-
ventral direction.
The peripharyngeal cavities are also anterior continuations of
FIG. 460. Ptychodera minuta, diagram of the collar
coelom and of the anterior region of the trunk, in an
almost median longitudinal section (after Spengel), some-
what modified. 1, Anterior wall of the collar ; 2, collar
coelom ; 3, peripharyngeal cavity ; 4, perihsemal canal ;
5, buccal cavity ; 6, septum dividing the collar from the
trunk ; V, trunk coelom ; 8, oesophagus.
FIG. 461. Ptychodera minuta,
transverse section of the body through
the genital region, diagram to illustrate
the arrangement of the coelom. d, Dor-
sal ; v, ventral ; 1, dorsal mesentery ;
2, dorsal accessory chambers of the
trunk coelom ; 3, lateral mesenteries ;
4, body epithelium ; 5, parietal wall ;
6, visceral wall of the trunk coelom ;
7, intestinal epithelium ; 8, intestinal
cavity ; 9, principal chamber of the
trunk coelom ; 10, ventral mesentery.
the trunk coelom, which push in between the buccal cavity (pharynx)
on the one side and the collar ccelom on the other, and, in Ptychodera,
surround the buccal cavity. Anteriorly, they end dorsally at the
point where the proboscidal diverticulum of the intestine arises, and
laterally, at the points of attachment of the vascular folds. The
inner wall of the peripharyngeal cavities consists of a layer of circular
muscle fibres which surrounds the buccal cavity, closely applied to
its epithelium, and only separated from it by a limiting membrane.
In Schizocardium, the two peripharyngeal cavities are less exten-
sive, they lie at the sides of the buccal cavity, without surrounding
it either ventrally or dorsally. Each peripharyngeal cavity forms a
ix EXTEROPNEUSTACCELOMIC SACS 577
triangle, whose sides are constituted as follows. The first (posterior)
side corresponds with its origin out of the trunk ccelom ; the second
(dorsal) with the lateral edge of the perihaemal cavity; the third
(anterior and lower) with the line of insertion of the vascular fold.
There is a corresponding limitation and circumscription of the trans-
verse muscle fibres on the lateral walls of the buccal cavity. Never-
theless a closed muscular envelope is formed: (1) dorsally, by means
of the transverse musculature on the lower walls of the perihaemal
cavities ; (2) ventrally, by the transverse musculature of the collar
coelom.
Peripharyngeal cavities are found, not only in Ptychodera and Schizocardium,
but in Balanoglossui Kowalevskii, in the same form as in Schizocardium, but pro-
vided with longitudinal instead of transverse muscles, not belonging, however, to
the inner or visceral wall, but to the outer parietal wall which is in contact with
the collar ccelom.
In Ptychodera, a further complication occurs in the divisions of the trunk
ccelom. In the anterior hepatic and the branchiogenital regions, an accessory or
lateral mesentery (Fig. 461) occurs on each side dorsally in the submedian line, in
addition to the two principal (median) mesenteries. This accessory mesentery runs
from the intestine to the integument, dividing the ccelom at this point into four
chambers, two large, ventral, principal chambers, and two small dorsal accessory
chambers. The accessory chambers open posteriorly into the principal chambers,
the accessory mesentery disappearing ; anteriorly, they narrow and end in the
branchial region. They are here no longer in contact with the intestine, but only
with the integument, the accessory mesenteries here shifting their visceral edges
of attachment on to the integument, in a manner which cannot here be described
more in detail.
By far the greater part of the walls of the trunk coelom are
taken up in the formation of musculature. The parietal wall most
especially becomes differentiated into a powerful dermo-museular tube
which gradually diminishes in strength posteriorly.
The most important and constant part of this dermo-muscular
tube is the longitudinal musculature.
The longitudinal musculature, which is specially strongly developed on the
ventral side of the body, in the genital folds (where these are developed), and on
the dorsal side in the branchial region, is interrupted in the dorsal and ventral
median lines by the median mesenteries. A similar interruption takes place in
the branchiogenital region in the submedian lines, in which the gonads, and, in the
genera Balanoglossus, Gflandiceps, and Schizocardium the gills also, open.
By these four lines of interruption, the longitudinal musculature is divided
into two dorsal and two ventrolateral areas. (B. canadensis has two streaks on
each side free from muscle, and gonads open in both.)
Each longitudinal fibre runs in a curve between two points, one behind the
other, on the limiting membrane of the body epithelium. Each fibre thus crosses
numberless others.
In Ptychodera, in addition to these, an outer circular muscle
layer is also differentiated from the parietal wall of the trunk coelom ;
the fibres of this layer pass through the mesenteries.
VOL. II 2 P
578 COMPARATIVE ANATOMY CHAP.
A true circular muscle layer is nowhere else developed. Such a
layer is, however, functionally replaced by pseudo-circular muscle
fibres which run on the inner side of the longitudinal musculature,
but which in reality do not form a closed ring.
In Schizocardium, the bundles of these pseudo-circular muscle fibres run on
each side from the dorsal edge of the mediodorsal mesentery to the dorsal edge of
the ventral mesentery. Similar bundles arise near the ventral edge of the ventral
mesentery, and break up into fibres on the lateral walls of the body, ascending
along the inner side of the longitudinal musculature, traversing it, and becoming
attached to the limiting membrane of the body. In Glandiceps, this latter system
is repeated (but of course reversed) on the dorsal side.
Balanoglossus has neither the outer true, nor the inner pseudo-, circular mus-
culature.
Radial muscle fibres connect the limiting or basal membrane of
the body epithelium with the limiting membrane of the intestine
throughout the whole coelom. In the genital folds, these fibres are
stretched between opposite points of the integument. In the region
of the lateral mesenteries, similar fibres stretch between these and the
integument.
VII. The "Heart Vesicle" (Figs. 456, 21, p. 566 ; 464, 11, p. 583).
This is one of the names l suggested for a small closed sac which
lies upon the proboseidal divertieulum of the intestine in the basal
part of the proboscis. Its ventral wall bends down somewhat over
the divertieulum to right and left, and it is separated from the latter
by a small blood sinus. Posteriorly, tOAvards the neck of the pro-
boscis, the " heart vesicle " is drawn out to a small tip, which is
traversed by fibres, most probably muscular, chiefly in a transverse
direction, while the rest of the vesicle contains a fluid as clear as
water. The median part of the posterior and dorsal wall is in con-
tact with the body epithelium of the neck of the proboscis.
The ventral wall is formed of a single layer of transverse muscle
fibres and pear-shaped cells, while the rest of the wall is represented
by a plate epithelium. The existence of a closed circular muscula-
ture has not yet been demonstrated.
The " heart vesicle " in Schizocardium (and to a lesser degree in Glandiceps also)
is produced anteriorly into two large symmetrically arranged tips, the auricles.
From the posterior tip of the "heart vesicle " two bundles of muscle fibres arise which
pass anteriorly into these auricles, each one giving off fibres, one after the other, to
the wall of the auricle it enters.
It must be emphasised that the "heart vesicle" does not belong to the blood
vascular system, and does not communicate with it, but is merely in contact with
part of that system. If, therefore, the vesicle propels the blood, this can only
1 Morgan suggests "Proboscis vesicle. " TR.
EXTEROPXEUSTA LIMITING MEMBRANES 579
occur in a way similar to that seen in the lower Crustacea, where the contractions
of the intestines are able to set the body fluid in motion.
The " heart vesicle " appears, according to recent researches, to be of ectodermal
origin, and thus cannot be considered as a ccelomic vesicle.
VIII. The Limiting Membranes, the Proboseidal Skeleton, and
the Branchial Skeleton.
Throughout the whole body of the Enteropneusta, the walls of
organs which are in contact are separated from one another by
structureless limiting membranes, which are to be regarded as
secretions of these walls. These limiting membranes must be thought
of as composed for the most part of two adhering laminae. The blood
vessels lie within the limiting membranes ; they represent a system
of spaces between the two laminae.
In secreting the limiting membranes, the histological character of
the secreting walls is of no consequence. A muscle wall can secrete
a limiting membrane just as well as an epithelial wall.
After the foregoing description the reader will be able to under-
stand without further assistance the occurrence and arrangement of
limiting membranes in the body. He will, for example, know that a
limiting membrane exists everywhere below the body epithelium,
secreted by that epithelium on the one hand, and by the parietal
wall of the coelom on the other.
A similar limiting membrane must also occur between the visceral
wall of the coelomic sacs and the intestinal epithelium, as also between
the anterior and posterior walls composing the septa, which separate
collar from trunk, peripharyngeal cavities from collar ccelom, etc. etc.
At certain points, especially in the proboscis and on the branchial
intestine, the limiting membrane becomes thickened, and forms the
proboseidal and branchial skeletons.
A. The proboseidal skeleton consists of a median body and two
limbs diverging backward. The body of the proboseidal skeleton lies
in the neck of the proboscis, between the neck of the proboseidal
diverticulum of the buccal cavity above, and the ventral body
epithelium of the neck of the proboscis below. The limbs diverge to
right and left into the collar region, clasping from above the entrance
to the buccal cavity, and in close contact with its epithelium.
The proboseidal skeleton is further strengthened laterally by the
ehondroid tissue which becomes attached to it. The ground sub-
stance of this tissue is identical with the substance of the proboseidal
skeleton, and of the limiting membranes generally. It is secreted by
the anterior wall of the collar ccelom, and by the posterior wall of
the proboseidal ccelom, or by the latter alone ; but cell processes of
these walls remain in the secreted ground substance, and these processes
may break up into cell groups or nests, which give sections of the
580
COMPARATIVE ANATOMY
CHAP.
chondroid tissue a certain similarity to cartilage. This chondroid
tissue is most developed, forming a mass thicker than the pro-
S-Ji
FIG. 462. Gill slits and branchial skeleton of an Enteropneustan. The six hindermost
gills seen from the intestine, the three posterior in the act of forming, diagrammatic. The black
parts represent the U-shaped gill slits ; the dotted parts, the skeletal forks. 1, Branchial tongue ;
2, branchial septum ; 3, anterior prong ; 4, median or septal prong ; 5, posterior lingual prong of
a three-pronged skeletal fork.
boscidal skeleton, which always remains at its centre, in the genera
Schizocardium and Glandiceps.
B. The Branchial skeleton (Figs. 462 and 463). (Cf. here pp.
567 and 568 on the gill slits, the branchial septa, and the branchial
tongues.)
The branchial skeleton here, again, consists of local thickenings of
the limiting membrane, which separates
the epithelium of the branchial intes-
tine from the visceral wall of the trunk
coalom of the branchio-genital region.
These thickenings are in the form of
upright three -pronged skeletal forks,
which are arranged on each side, in a
single longitudinal row, throughout the
whole length of the branchial region.
The number of forks corresponds with
that of the gills. The free ends of the
prong are turned downwards, and the
connecting piece upwards. The three
prongs of a fork are arranged as
follows. The middle prong lies in a
branchial septum, under the surface
FIG. 463. The three anterior forks of O f the septal edge, which is turned
l ^ ^ i t y <* the branchial in-
tCStme. This Septal prong f Orks at
Jtg f ree lower Clld, giving off a short
. ,
anterior and a posterior branch.
The anterior prong of a fork lies
on the posterior wall of the branchial tongue, immediately in front of
the septum ; the posterior prong in the anterior wall of the branchial
4
A
i
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n
I
"Y
E
P
A^
I
^%
II
-2
z
f
3
J '
V
J
V
/
I
anterior (I) has only two prongs. 1, A pos-
terior lingual prong ; 2, a septal prong (in
its origin double); 3, an anterior lingual
p ron g
ix ENTEBOPNEUSTA BLOOD VASCULAR SYSTEM 581
tongue, immediately behind the septum. Each fork thus has a
median septal prong, and an anterior and a posterior lingual prong.
Each tongue has two prongs, one anterior and the other posterior,
but these belong to two different forks. Each septum has only one
prong. A very minute examination shows, however, that each septal
prong consists of two fused prongs. Two-pronged forks must, there-
fore, be the ultimate elements of the branchial skeleton. Each fork
would lie with one prong in a tongue and the other in a septum.
The two septal prongs belonging to two consecutive two-pronged
forks, are, however, in every case fused together.
The most anterior skeletal fork, and it alone, has two prongs.
In the formation of the lingual prongs, the branchial epithelium
(belonging to the intestine) and the mesodermal, inner wall of the
lingual cavity (belonging to the visceral wall of the trunk ccelom)
take part, but the septal prongs are secreted exclusively by the
branchial epithelium of the septal edge.
IX. The Blood Vascular System.
The blood vascular system consists of spaces in the limiting mem-
branes of the body. The two lamellae of the limiting membranes
simply remain apart at certain points, thus forming the walls of
the vessels. An endothelium-like covering of the inner side of the
separating lamellae has only been found in Ptychodera, and in isolated
parts in SchizocanUum and Glandiceps. Nothing of the sort has been
observed in Balanoglossus. In Ptychodera, isolated blood cells float in
the colourless blood fluid.
The lacunar blood vessels of the Enteropneusta do not arise by
the separation of the formerly contiguous lamellae of a limiting
membrane. They are, rather, persistent portions of the larval
segmentation cavity or blastoeoel. The organs of the larva lie in
a spacious blastoeoel, which narrows and disappears in proportion as
the organs (especially the ccelomic sacs) increase in size, these
swelling up in such a way that their walls come in contact with one
another and with the body and intestinal epithelium. Certain
cavities, however, persist, which afterwards form the blood vascular
system. The blood cells and endothelial cells of the vascular system
are, in all cases, of mesenchymatous origin.
The arrangement of the vascular system may be roughly described
as follows.
There is a capillary network in all the limiting membranes of
the body, especially in that of the integument and of the intestine.
This network is in connection with larger vessels, i.e. (1) with a dorsal
vessel which, in the dorsal mesentery, runs through the trunk and
collar and communicates with the blood vessels of the proboscis, and
(2) with a ventral vessel which, running in the ventral mesentery of
582 COMPARATIVE ANATOMY CHAP.
the trunk, receives blood from the proboscis through two lateral vessels
or vascular plexuses within the two vascular folds of the collar, these
vessels or plexuses usually uniting in the ventral median line at the
posterior end of the collar region. The dorsal and the ventral
vessels of the trunk have muscular walls, which, however, do not
properly belong to them, but are borrowed from the apposed walls of
the mesenterial portions of the coelomic sacs. In the proboscis, the
blood vascular system, by increase of its surface towards the probosci-
dal ccelom, to right and left of the basal complex of organs, gives
rise to the so-called proboseidal gill or glomerulus.
Special. The finer details cannot be entered upon.
1. Vessels of the trunk. While the dorsal mesentery is retained in the
abdominal part of the body (as already noted, the ventral mesentery persists
throughout the whole trunk) it may disappear in the anterior trunk region with the
exception of the part which contains the dorsal longitudinal vessel. The muscles
of the vascular trunks are transverse or circular muscles, and, in part at least,
continuations on to the mesenteries of the circular musculature of the body. The
ventral vascular trunk of B. Kowalevskii is provided, not with a transverse, but
with a longitudinal musculature. The musculature (which is yielded by the
mesenterial endothelium) always lies on the side of the limiting membrane
away from the lumen of the vessel, and facing the body cavity.
2. The dorsal vessel of the collar is the direct continuation of the dorsal vessel
of the trunk. It runs between the two perihfemal cavities, from whose walls it
borrows its musculature. Passing out again from between these cavities, it loses
its musculature and opens, in the proboscis, into a blood sinus, the basal sinus of
the proboscis. This is a space left between various heterogeneous organs, the
proboscis pore, the diverticulum of the intestine, the posterior tip of the ' ' heart
vesicle," the epithelium of the neck of the proboscis.
This basal sinus communicates, on the one side, with the capillary network in
the wall of the proboscis, and on the other, through a narrow slit, with the central
blood sinus of the proboscis which lies in front of it.
3. The central blood sinus of the proboscis (Fig. 464, 9) is a space in that limit-
ing membrane which separates the "heart vesicle" (dorsally) from the proboseidal
diverticulum of the buccal cavity (ventrally). It has no musculature of its own.
This, however, is supplied by the ventral transverse musculature of the ' ' heart
vesicle " which lies above it. In Schizocardium, the central blood sinus is con-
tinued in a peculiar manner, which cannot here be further described, on to the
two "auricles" of the "heart vesicle."
4. The proboseidal glomerulus (Fig. 464, 10) consists of two lateral principal
portions and a dorsal connecting piece. Each of the principal portions has the
form of a unilaniinar honeycomb, with deep cells. The base of the comb is formed
by the right or left lateral walls of the basal complex of proboseidal organs, i.e. of
the "heart vesicle " and the proboseidal diverticulum of the intestine. The apertures
of the single "cells," however, are turned towards the proboseidal ccelom. The
walls of the cells are formed by folds of the visceral endothelium of the proboseidal
ccelom. They are hollow, and the cavities are blood sinuses, which open into a
common cleft-like sinus in the base of the comb. This latter, again, communicates,
by means of a slit-like transverse aperture, between the "heart vesicle" and the
proboseidal diverticulum, with the central sinus of the proboscis (Fig. 464, 9).
Posteriorly, each lateral principal part of the glomerulus becomes simpler and
.
ENTEROPNEUSTA BLOOD VASCULAR SYSTEM
583
simpler, and its sinus, which anteriorly is so complicated, becomes on each side a
simple vessel in the limiting membrane between the proboscidal intestine and the
visceral coelomic endothelium.
These two vessels are the efferent proboscidal vessels.
Blood reaches the central sinus of the proboscis in the following ways : (1)
from the dorsal vessel of the trunk and collar through the basal blood sinus,
(2) out of the integumental vascular network of the proboscis through vessels or a
vascular plexus, which ascends in the limiting membrane of the ventral septum,
and lastly (3) out of this vascular network through a vessel, which descends along
the free edge of the "heart vesicle."
The " heart vesicle " propels the blood by means of its ventral wall, which lies
upon the central sinus. Its function is considered to be that of driving the blood
Fn;. 4. The efferent proboscidal, and collar, vessels. From their origin out of the
two posterior tips of the lateral portions of the glomerulus, these vessels turn
v< nt rally, and, running very close to one another, traverse the chondroid tissue of
the neck. In the anterior and upper area of the collar region, they enter the two
vascular folds, in whose limiting membranes they run, breaking up into more or
less rich plexuses. Their courses then, naturally, correspond with those of the
vascular folds, which take their name from the vessels within them. In Schizo-
cardiinii, Balanoylossus, and Glandiceps, where the two vascular folds descend
slantingly to the ventral median line of the posterior end of the collar, they neces-
584 COMPARATIVE ANATOMY CHAP.
sarily also have slanting systems of blood vessels, which open into the anterior
end of the ventral vessel of the trunk.
In Ptychodera, on the contrary, where the vascular folds descend perpendicularly
in the anterior part of the collar region, and form a ring around the buccal cavity,
which is then continued as a medioventral fold to the posterior end of the collar,
the vessels which run in these folds naturally also form a similar ring, which
passes into a medioventral collar vessel continuous with the ventral vessel of the
trunk.
The vessels of the collar are distinguished from the principal vessels of the
trunk by the fact that they possess no musculature.
A circular space, running in the limiting membrane of the septum separating
the 'collar from the trunk, is in open communication with the ventral vascular
trunk.
6. The vascular capillary networks of the integument and of the intestine are
everywhere in communication with the two principal vessels. In the collar, a
connection is formed between the integumental and the intestinal plexuses by
plexuses running in the mesenteries. Where peripharyngeal cavities are found
(Ptychodera, Schizocardium} the integumental plexus lies in the peripheral walls of
the cavities, viz. those turned to the ccelom of the collar. Of all the sections of
the intestine, the hepatic is most distinguished by the closeness of its capillary
plexus and its rich supply of blood.
In some species of Ptychodera, dendriform, blindly-ending, vascular cseca project
from the dorsal side of the collar cord and sometimes also from the dorsal septum.
7. Lateral vessels Ptychodera. Two lateral vessels, provided with muscular
walls, run through the branchiogenital and hepatic regions. They originate
anteriorly out of the vascular network of the integument, run backward in the
submedian line, and enter the lateral mesenteries, in whose limiting membranes
they run. At the posterior ends of these mesenteries, at the boundary between
the branchiogenital and the hepatic regions, they pass over on to the intestine,
being continued in two vessels running along close below the liver-caeca of the
intestine, and finally open into the intestinal capillary network. The anterior
portions of the lateral vessels, which might be called the genital vessels, are
connected with the capillary network of the gonadial walls. Similar lateral
vessels also occur in Schizocardium.
In Balanoylossus and Glandiceps there are, usually in the hepatic region, two
lateral vascular trunks of the intestine, which open anteriorly and posteriorly into
its capillary network. Their musculature consists, in Glandiceps, of circular
fibres ; in Balanoglossus, of longitudinal fibres. These perhaps correspond with
the posterior or intestinal portion of the lateral vessels of Ptychodera.
8. The branchial vessels. These vessels have been best investigated in Ptycho-
dera. A branchial capillary network is found in the limiting membranes both in
the branchial tongues and the branchial septa, i.e. in the limiting membrane
which separates the epithelium of the branchial intestine from the visceral layer of
the mesoderm which lies outside it. Into this plexus, vessels having a definite con-
stant course and of large size enter : (1) a vessel along the back of each tongue, (2)
a vessel along the inner side of each lingual prong, i.e. on the side turned to the
lingual cavity, (3) a vessel along that edge of each septal prong which is turned to
the body wall. These last-named vessels run ventrally into the capillary network
of the lower, nutritive part of the branchial intestine (i.e. of the oesophagus),
and must be considered as efferent vessels of the branchial septa.
The branchial capillary network receives its blood from afferent branchial
vessels, which originate out of the dorsal vessel and (in Ptychodera clavigera] have
the following arrangement : Each afferent branchial vessel, soon after its origin
ENTEROPXEUSTA GONADS 585
out of the dorsal vessel, divides into two, one running into a tongue and the other
into the branchial septum next in front. The lingual vessel divides again into
two branches, which are continued into the two above-mentioned vessels of the
lingual prongs ("tongue-bars").
It is not known in what way the blood is again carried out of the branchial
tongues.
X. The Gonads.
The sexes are separate in the Enteropneusta. The gonads are
simple or branched sacs of various shapes which project into the
body cavity of the trunk, towards which, however, they are completely
closed. They form on each side a conspicuous longitudinal row in
the genital region of the trunk, which, however, is not sharply de-
marcated from the branchial region in front of it nor from the
hepatic region behind it. At the posterior end of each row of gonads,
a constant formation of new gonads takes place.
The gonadial sacs open outward through simple efferent ducts
and genital pores, which always lie dorsally in the submedian line
close to, but on the outer side of, the branchial pores (Fig. 458).
These gonads, which open laterally to the branchial pores, form
the row of principal gonads, and their pores are the primary
principal pores.
A certain agreement in the number of the gonadial pores with
that of the branchial pores is sometimes found.
The arrangement of the gonads may become complicated.
A. One and the same gonadial sac may open outward through
accessory pores, which lie either medianly, or laterally, to the
principal pore.
Such accessory pores are found in Schizocardium toasiliense and
Glandiceps talaboti, in the latter in great numbers.
B. Accessory gonads may occur in addition to the principal
gonads, opening outward through secondary genital pores.
In Balanoglossus Kupfferi, such accessory gonads form a complete
row running parallel with the principal row, along its median side.
The same is the case in Glandiceps talaboti, although here the accessory
row is not quite complete. In Balanoglossus canadensis, both principal
and accessory gonads occur, there being several rows of each. The
pores of all the gonads lie in the submedian lines, which are free
from muscle, and are in this case widened into broad streaks.
When accessory gonads occur in species of PtycTiodera (e.g. Pt.
aurantiaca, bahamensis, erytlima) their pores always lie laterally to the
principal pores.
Structure of the gonads. The gonads consist (1) of a Avail turned to the
coelomic cavity and belonging to it, constructed of a tesselated epithelium and
fine muscle fibres, and (2) of a massive inner germinal layer, consisting of germinal
cells and covering or follicle cells ; this layer is continued into the epithelium of
the efferent ducts.
586
COMPARATIVE ANATOMY
CHAP.
Between these two layers lies (3) a limiting membrane, in which a rich capillary
network may be developed, or else the membrane is divided into its two lamellte
by a continuous slit-like blood sinus.
The origin of the gonads is not yet certainly known. They were formerly held to
be derived from the ectoderm, but the most recent researches seem to show that
they arise as local accumulations of the mesenchyme cells which occupy the
blastocoel. In any case the connection of the gonads with the body epithelium
by means of the ducts is secondary. They originally lie isolated between this
epithelium and the parietal layer of the ccelom.
XI. Ontogeny.
The development of the Enteropneusta is sometimes connected with metamor-
phosis, a pelagic larva, the Tornaria larva, being developed. This larva in many
respects recalls the Bipinnaria larva of the Asteroids, and
was at first considered to be an Echinoderm larva. In other
cases development is abbreviated, and is indeed almost direct,
for though a free larva develops from the fertilised egg, it
lives at the bottom of the sea, and shows no signs of many of
the most important characters of Tornaria.
A. Structure and Metamorphosis of the Tornaria larva.
FIG. 465. Very young
specimen of Tornaria
Krohni, from the side
(after Spengel). 1, Ap-
ical plate with eyes ;
2, preoral area; 3, pre-
The egg segmentation and gastrula are unknown.
1. Outer organisation. The youngest larval stages
observed are almost egg-shaped (Fig. 465). At the anterior
pole there is a pair of brown eye-spots, at the posterior the
anal aperture, and in the middle of the ventral side the
long transverse mouth. The thin transparent integument is
oral ciliated ring ; thickened only in the region of two ciliated rings, which
4, oesophagus; 5, mouth; border in a man ner soon to be described, a somewhat
(3, stomach intestine ; , , , , , , ... ,
7 anus- s hind-gut; a ee P ened r al area, at whose centre the mouth lies. The
9] pastoral ciliated ring '; ciliated rings are strictly bilaterally symmetrical. A preoral
10, postoral area ; ii, ciliated ring runs from the anterior ventral edge of the oral
proboscis pore ; 12, pro- area f orwar d s and upwards on each side to the frontal region,
nZSlapSl Plate?' ^ here the ^ lie > and marks ff a P re ral ^ A SeCOnd
ciliated ring runs back on each side, almost longitudinally,
from the frontal region, then bends round on to the ventral side, and here,
behind the mouth, passes into the corresponding ciliated band of the other side
of the body.
This postoral ciliated ring forms the dorsal and posterior boundary to the oral
area, and marks off a postoral area, which comprises the dorsal and posterior (anal)
regions of the larval body. The preoral and postoral ciliated rings unite for a very
short distance at the apical pole. The oral area enclosed within these two rings has
the form of a transverse ventral saddle, drawn out on each side towards the apex.
The next remarkable change which is externally visible is the appearance of a
ciliated ring at right angles to the principal axis. This surrounds the posterior
part of the postoral area, and is the principal ciliated ring (Fig. 466, 9). The
postoral area is by it divided into an anterior and a posterior region. The posterior
region is the anal area, with the anus at its centre. In the anterior region, a
dorsal area can be distinguished from a ventral zone. Behind the principal ciliated
ring, a second weaker ciliated anal ring may appear (Fig. 466, 8).
IX
ENTEROPNEUSTA ONTOGENY
587
During the further transformations which take place in the larva, the anal area
which is bordered by the greatly developed principal ciliated ring remains almost
unaltered, while the oral area, pushing out before it the preoral and the postoral
rings, sends symmetrical extensions (Fig. 466) into the pre- and postoral regions,
as follows :
From the anterior and lateral tips of the oral region which stretch to near the
apical pole, two extensions, one on each side, run posteriorly and ventrally into the
preoral area (13), two more posteriorly and dorsally into that region of the postoral
area which was above distinguished as the dorsal area (2).
In this way the larva, seen from the apical pole, has temporarily a four-rayed
appearance.
From the lateral and dorsal regions of the oral area, two extensions invade a
postoral area dorsally (4). From the posterior lateral edges, two inconspicuous
~^~!6
2 i UT
Fm. 456. A, B, C, Tornaria Miilleri (?). A, From the ventral side ; B, from the dorsal side ; C,
in profile (after Spengel). 1, Apical plate, with the eyes and apical tuft ; 2, anterior dorsal lobes of
the oral area ; 3, " heart vesicle " ; 4, posterior torsal lobe of the oral area ; 5, collar coelora ; 6, trunk
crelom ; 7, anus ; 8, secondary anal ciliated ring ; 9, principal anal ciliated ring ; 10, postoral area ;
11, proboscis pore ; 12, proboscidal ccelom (water sac) ; 13, anterior ventral lobe of the oral area.
14, oral area; 15, ventral "saddle"; 16, ventral zone of the postoral area; 17, anal area; IS,
cesophagus ; 19, stomach-intestine.
extensions may spread posteriorly. The ventral zone, however, bulges forwards
ventrally towards the oral area (15).
These changes bring about the peculiar indented course of the preoral and
postoral ciliated rings, shown in the figures.
The ciliated rings may even become still more folded. Such folding reaches
the highest degree in Tornaria Grenachcri, hardly 1 cm. long, in which the
anterior ventral and the anterior dorsal extensions of the oral area bulge out at
the cilia-carrying edge to form numerous long, narrow, freely projecting accessory
lobes, resembling tentacles.
In the frontal region, on the apical eye-bearing plate, which here becomes
differentiated, a tuft of delicate immobile cilia develops early.
The larva swims in such a way that the anterior or apical pole is directed
upward and the anal pole downward.
The metamorphosis of the Tornaria larva into the young Enteropneustan is
accompanied by the following external processes :
588 COMPARATIVE ANATOMY CHAP.
The body lengthens and its preoral section becomes produced into the proboscis,
at whose tip the eyes are still long visible, until they degenerate together with the
apical plate and tuft.
The preoral and postoral ciliated rings degenerate, but the whole body becomes
covered with cilia.
The principal ciliated ring (Fig. 467) persists for some time, eventually, i.e.
when the anal area has increased in length, surrounding the body about half
way between the mouth and anus. A circular furrow between it and the base
of the proboscis is the first indication of the posterior boundary of the collar
region.
The whole ectoderm of the oral area degenerates during metamorphosis, the
body epithelium proceeding exclusively from the ectoderm of the preoral, postoral,
and anal areas, which increases in thickness. This phenomenon, together with the
lengthening of the larva, causes a very marked diminution in the transverse section
of the body, an essential accompaniment of which is the approximation of the
larval integument to the parietal walls of the ccelomic vesicles.
2. Anatomical. The apical plate of the Tornaria, larva, which completely
degenerates later, consists of a dorsal and a ventral half. Below the surface of the
specially thick dorsal half there is a layer of nerve fibres.
The centre of the apical plate is formed by a small group of long narrow
sensory cells carrying delicate immobile cilia (sensory hairs).
The two eyes which are embedded in the apical plate are optic pits, whose
floors are formed by cells which are pigmented at their bases (retinal cells ?). The
optic pit is filled with a clear substance, which is a continuation of the cuticle
covering the apical plate, and is called the lens. The apertures of the two pits
diverge anteriorly and laterally. Round them in the apical plate the:e is a
deeper layer of elements, which are considered to be ganglion cells.
The alimentary canal. Even the youngest larvae observed showed the division
of the alimentary canal into oesophagus, mid-gut or stomach, and hind-gut, which
is characteristic of all the larval stages.
The oesophagus ascends vertically. It is a flattened tube provided with a
circular musculature, its thickened dorsal and ventral walls being ciliated.
The stomach is a large egg-shaped sac, whose axis lies horizontally, and into
whose anterior pole the oesophagus enters. The epithelial cells of the sac are at
first low, but they lengthen at a later stage. The originally thin-skinned stomach
develops in this way a thick wall, which is probably non-ciliated except at two
points. A ciliated cushion is found ventrally, at the entrance to the stomach, and
the efferent aperture is surrounded by long hairs, which perhaps act as a fish-trap
apparatus.
The hind-gut, in the youngest larvae, is an almost cylindrical tube with thin
walls. At a later stage its anterior part becomes swelled up, so that the hind-gut
as a whole becomes funnel-shaped or conical. As, however, the aperture com-
municating with the stomach remains small, the wall of the hind-gut is applied
over a considerable area to the posterior wall of the stomach. The aperture lies at
the centre of this area. Immediately in front of the anal aperture there is a circle
of cells provided with cilia.
Formation of the gills. The formation of the first pair of gills takes place
shortly before or after "metamorphosis. Two lateral cseca arise at the posterior end
of the oesophagus, grow out towards the integument, and break out laterally and
dorsally through the external branchial pores. The cesophageal apertures of these
branchial diverticula are at first round, later they become U-shaped, by the lateral
dorsal intestinal wall growing out into the diverticulum. This outgrowth is the
rudiment of the branchial tongue.
ix EXTERUPXEUSTA ONTOGENY 589
In Tornaria Ayassizi, it was observed that the pores of the second pair of gills
arise earlier than those of the first.
In other cases, the new pairs of gills always form behind the last produced.
During the process, the growing cesophagus, the posterior part of which is now the
branchial intestine, passes from a vertical to a horizontal position.
Even in the adult animal, new gill-pouches are constantly being formed at the
posterior end of the branchial region. The course of development is always the
same as that of the first gills in the larva.
The collar canals, with their pores, develop shortly before the rudiments of the
second pair of gills arise, most probably as outgrowths of the anterior wall of the
first gill-pouch, near its external pore. These outgrowths run forwards towards the
collar ccelom.
The first rudiment of the proboscidal diverticulum of the buccal cavity has
been observed in Tornaria Agassizi, as a small bulging of the anterior wall of the
larval oesophagus directed anteriorly, and lying immediately above the mouth.
The proboscidal ccelom. The rudiment of the proboscidal ccelom has not as
yet been observed with as much certainty as could be desired. According to one
observer, it arises as an outgrowth of the intestine at the boundary between the
cesophagus and the stomach.
In the youngest stages which have been closely observed, the proboscidal ccelom
(the so-called water sac of the larva) is an almost cylindrical tube lined with
tesselated epithelium, the inner half of which is slightly widened. This tube
becomes attached by its inner end to the anterior wall of the oesophagus, near
the point at which the latter opens into the stomach. Here it sends two processes
(the reins) to right and left on to the lateral walls of the cesophagus, on which
it appears to ride. From the cesophagus, the tube, traversing the blastocoel,
ascends almost vertically. Shortly before it reaches the dorsal ectodermal wall, it
is continued into a short internally ciliated tube, the epithelium of which suddenly
changes its character. This short tube, lined with cylindrical epithelium, is the
rudiment of the proboscidal canal, and the pore to which it leads is the proboscis
pore.
The proboscidal ccelom is further attached to the apical plate by means of a
strand in the manner illustrated in Fig. 466 ; the strand consists of contractile
fibres surrounded by a nucleated envelope, a continuation of the wall of the water
sac. The contractile fibres of this strand are continued on to the tips or reins of
the water sac, which grasp the ossophagus between them.
The further fate of the water sac, briefly stated, is as follows. It swells up,
and soon changes from a tube into a vesicle, the greater part of the wall of which,
during metamorphosis, becomes applied, as parietal wall of the proboscidal ccelom.
to the body epithelium of the anterior or proboscidal section of the body, the
muscular apical strand becoming shorter and shorter, and finally altogether
disappearing.
The manner in which the epithelial walls of the water sac become differentiated
into the proboscidal musculature cannot here be described.
" Heart vesicle." Authorities ditfer as to the first rudiments of this organ. In
the youngest stage investigated by the most recent observers, it is a small cellular
structure with an internal cavity lying so close to the ectoderm that it may be
either ectodermal or mesenchymatous. It appears to the right, in front of and
near the proboscis pore. The body becomes a hollow vesicle, leaves the ecto-
derm, sinks below the surface, and becomes applied to the right side of the water
sac. Transverse muscle fibres develop on its ventral wall. The water sac then forms
posterior outgrowths, which grow round the "heart vesicle' 1 on its right dorsal
and ventral sides. The two dorsal posterior and the ventral posterior sections
590
COMPARATIVE ANATOMY
CHAP.
of the proboscidal ccelom thus come into existence. The "heart vesicle" on its
lower side, however, always remains separated from the dorsal wall of the ventral
posterior outgrowth of the water sac by a space, into which the proboscidal
diverticulum of the buccal cavity grows from behind, but in such a way that
between it (the diverticulum) and the superimposed "heart vesicle" a space
remains, which appears to become filled with blood at an early stage. This is the
central blood sinus of the proboscis.
The coelomic sacs of the collar and trunk. These two pairs of coelomic sacs
appear, in Tornaria, to have a common rudiment in the following way. The
edge formed by the anterior wall of the hind-gut, bending round backwards into its
lateral walls, is produced anteriorly to right and left as hollow sacs, or in other cases
as a pair of solid bilaminar plates. These become
applied to the stomach, but are on the other
hand separated by a large space from the ecto-
derm. These two sacs or plates become con-
stricted from their matrix, the hind -gut, and
grow round the stomach dorsally and ventrally.
In each, apparently, an anterior portion becomes
constricted off. This anterior pair of sacs or
plates is the rudiment of the collar coelomic
sacs, the posterior, which only secondarily
extend backwards along the sides of the hind
gut, is the rudiment of the trunk coelomic sacs
(Fig. 467). These two coeloms are therefore
enteroccels. Where the first rudiments of the
cceloms are solid bilaminar plates, a space soon
arises in them by the separation of the two
lamiiise. These small spaces, whether present
from the first, or formed later, begin to increase
in size at the end of the larval period. The
two pairs of coelomic rudiments become vesicular.
The outer wall becomes applied to the body
epithelium as the transverse section of the
coelom; 5, septum between the collar
and the trunk ; 6, trunk coelom ; 7, ven-
FIG. 467. Collar and trunk of an
Enteropneustan (Tornaria Krohni)
immediately after metamorphosis, from
the ventral side (after Spengel). 1, Pro- growing larva decreases during metamorphosis
boscis; 2, collar; 3, trunk; 4, collar in the way already described, and forms the
dermomuscular tube. The inner wall lies upon
tral mesentery; 8, principal ciliated the intestine, and represents the visceral layer
ring ; 9, wall of the mid-gut ; 10, wall of the ccelomic sac. The dorsal and ventral
of the hind-gut ; 11, anus. mesenteries are formed where the right and
left trunk ccelomic sacs and the right and left
collar ccelomic sacs, in surrounding the intestine, come in contact dorsally and
ventrally in the median plane.
These processes, of course, go hand in hand with a progressive reduction of the
blastoccel, which contains a number (small at first, but increasing later) of
mesenchyme cells of unknown origin. The remains of the segmentation cavity
represent the blood vascular system.
Nervous system. Shortly before the conclusion of metamorphosis, the two
longitudinal nerve trunks arise as local differentations of the body epithelium,
below the surface of which a layer of nerve fibres forms. The collar cord, also, at
first lies superficially in the integument, and is nothing more than the collar
portion of the dorsal epithelial longitudinal cord. This part only sinks below the
surface at a later stage. According to recent observers, the process recalls the
sinking in and constriction of the neural tube in Vertebrates.
ix EXTEROPNEUSTAPHYLOGEXY 591
Gonads. The development of the gonads has already been sufficiently described
above, p. 586.
B. The almost Direct Development of the Balanoglossus Kowalevskii.
"\Ve can only select a few of the principal points in this development for
description. Segmentation is total and equal, and leads to the formation of a
cceloblastula, out of which, by invagination, a ccelogastmla is produced. This
latter becomes covered with cilia, and a ciliated ring forms round the blastopore,
which diminishes in size and finally closes ; this ring corresponds with the principal
ciliated ring of the Tomaria larva. At this stage the larva leaves the egg to live
at the bottom of the sea, without showing any trace of the form, or of the charac-
teristic ciliated rings, of Tornaria.
The differentiations which take place in the archenteron are important. Its
anterior part becomes constricted off as a semilunar vesicle lying transversely. This
takes up the whole of the most anterior part of the blastoccel and becomes the
proboscidal ccelom, which thus, according to these observations, is an enteroccel.
Two pairs of lateral outgrowths become constricted off from the rest of the archen-
teron, the anterior being the rudiment of the collar ccelom, and the posterior that
of the trunk ccelom.
The blastoccel is small from the first.
The mouth is said to arise by the simple breaking through of the intestine out-
wards, and the anus in a similar way, in the place of the original blastopore. Thus
the whole of the intestinal wall is of an endodermal origin.
The developmental processes in B. Kowalevskii cannot here be further described :
we refer the reader to the account of the formation of the organs in the Tornaria
larva, given above.
XII. Phylogeny.
The systematic position of the Enteropneustan must still, or rather again, be
considered as altogether uncertain. In any case, the Enteropneusta are not closely
related to any single large division of the animal kingdom. Special affinities with
the Chordata, the Echinodermata, and the Xemertines have been long suggested.
and in quite recent times also with Cephalodiscus and Rhabdopleura.
A. The relation of the Enteropneusta to the Chordata has been maintained
on the following grounds :
1. The Chordata and the Enteropneusta show a very far-reaching and extra-
ordinary agreement in their gills. This agreement holds good even in details
(branchial tongues, branchial skeleton, synapticulae) if the gills of An^hioxus are
taken for comparison.
2. The proboscidal di verticulum of the Enteropneusta is, in structure and origin,
comparable with the chorda of the Chordata.
3. The proboscidal skeleton of the Enteropneusta corresponds with the sheath
of the chorda.
4. The body cavities in the two groups are of enteroccelomic origin ; the pro-
boscidal ccelom corresponds with the anterior unpaired mesoderm vesicle of
5. The collar cord of Balanoglossus corresponds with the dorsal cord of the
Chordata, and arises in the same way as the neural tube of Vertebrates, by sinking
in and covering over.
The most recent researches have, however, yielded results unfavourable to this
assumed homology.
592 COMPARATIVE ANATOMY CHAP.
1. The great similarity of the Enteropneustan gills to those of Amphioxus in
finer structure remains, but the detailed comparison of point with point makes a
real homology doubtful, and seems to oblige us to consider the resemblance as at
the most a very remarkable case of convergence.
Further, the following considerations must also be taken into account. The
gills of Amphioxits arise ontogenetically as segmental structures, while those of
the Enteropneusta, although standing in a longitudinal row, belong to the unseg-
mented trunk.
The gills of the Chordata receive their blood from the ventral vascular trunk,
those of the Enteropneusta from the dorsal trunk.
Should it be proved that the larval oesophagus of the Enteropneusta proceeds
from an ectodermal invagination, and is a stomodaeum, then the gills of the
Enteropneusta would lie in an ectodermal intestinal region, in contradistinction
to those of the Vertebrata, which belong to an endodermal intestinal region.
2. The proboscidal diverticulum is a preoral outgrowth of the wall of the buccal
cavity, and is lined with epithelium. It does not thus agree with the tissue of
the noto-chord. It is further very questionable whether the buccal cavity,
and, with it, the diverticulum are endodermal formations. The proboscidal
diverticulum lies below the continuation of the dorsal blood vascular trunk (below
the central blood sinus of the proboscis) ; the chorda of Vertebrates, on the con-
trary, lies above the dorsal blood vascular trunk (aorta). No homology between
the two is possible.
3. The proboscidal skeleton, as a thickened limiting membrane, could at the
most be compared only with the inner cuticular sheath of the chorda.
4. The body cavity is an enterocoel, in many different divisions of the animal
kingdom (either constantly or exceptionally) other than the Enteropneusta and the
Chordata. The ccelomic vesicles (mesoderm vesicles, primitive vertebrae) of the
Vertebrata show a segmental arrangement, corresponding with the metamerism of
the other organs, while in the Enteropneusta no such arrangement exists.
5. The collar cord of the Enteropneusta is only the anterior continuation of the
dorsal nerve cord of the trunk. It does not sink below the surface until all its
parts are formed. The corresponding ventral nerve cord of the Enteropneusta does
not exist anywhere in the Chordata.
The following further points must be emphasised.
The gonads of Amphioxus arise segmentally from the endothelium of the body
cavity, while the rows of gonads in the Enteropneusta lie in an unsegmented
region. The first origin of the gonads of the Enteropneusta is, indeed, unknown,
but their rudiments are found in the blastoccel very early. The manner in which
the genital products are ejected in the two groups is altogether different.
In the Chordata, the blood in the dorsal vessel flows from before backward, in
the ventral from behind forward ; the reverse is the case in the Enteropneusta.
A comparison of the two collar pores in the Enteropneusta with the most
anterior pair of nephridia in Amphioxus could only be of value were the develop-
ment of these organs known. In all probability the former are of ectodermal and
the latter of mesodermal origin.
There is nothing we know of in the Chordata comparable with the Tornaria
larva.
These considerations render any relationship between the Enteropneusta and
the Chordata, at least at present, highly improbable.
B. The relationship between the Enteropneusta and the Nemertina is so very
problematical that it cannot here be discussed.
C. Relation of the Enteropneusta with the Annelida. The attempt to bring
the Euteropneusta and the Annelida into even a distant genetic relationship is
ix ENTEROPNEUSTA PHYLOGENY 593
supported chiefly upon a comparison of larval forms. The following characteristics
of the Trochophoran and the Tornarian larvae have been pointed out.
The neural plate, the apical sensory organs, and the muscles which become
attached to the neural plate correspond in the two. The divisions of the intestine
also agree, that is, if the fore-gut of the Tornaria is an ectodermal stomodaeum,
and the hind-gut a proctodseurn. The two pairs of coelomic sacs are comparable
with the two anterior pairs of mesodenn vesicles.
The comparison of the ciliated bands presents difficulties. Three ciliated rings,
a preoral (surrounding the apical area with the neural plate), a postoral, and a
preanal, are typically ascribed to Trochophora, the last of which is supposed to
correspond with the principal ciliated ring of Toraaria, while the preoral and
the postoral rings of Trochophora are wanting in Tornaria. And it is argued
that the absence of these rings in Tornaria has led to the specialisation of the
preanal as the principal ciliated ring.
On the other hand the preoral ciliated ring of Tornaria cannot be compared
with the preoral ring of Trochophora, because it does not surround the apical
plate. This latter lies, on the contrary, outside the frontal area, at the opposite
end of the oral region to the mouth ; the preoral ring passes in front of it.
Compared with the principal ciliated ring, the pre- and postoral rings are,
perhaps, of small morphological significance, since they are wanting in the non-
pelagic larva of Balanoylossus Kowalevskii, whereas the principal ring occurs
in it.
In a comparison of the Tornaria Avith the Trochophora larva, the great import-
ance of the following differences must not be overlooked.
1. Trochophora possesses typically one pair of primitive kidneys, which are
wanting in Tornaria.
2. Tornaria has a preoral coelom, which is absent in the Trochophora larva.
If now we turn to the organisation of the adult animals for light as to this
question of the relationship between the Annelida and the Enteropneusta, we find
immediately that insurmountable difficulties stand in the way of any close com-
parison. Only in the blood vascular system is any fundamental agreement found.
The blood in the dorsal and ventral longitudinal vessels has the same course in
both groups. But, on the other hand, a comparison of the nervous system of the
Enteropneusta with that of the Annelida encounters difficulties similar to those
found in comparing it with that of the Vertebrata. The gills in the two groups
are altogether heterogeneous. The typical Annelidan kidneys are wanting in the
Enteropneusta, for the collar pores, which are probably of ectoblastic origin, can
hardly be regarded as a pair of nephridia.
Thus the relationship of the Enteropneusta to the Annelida appears at the
best to be extremely distant.
D. The relation of the Enteropneusta to the Echinodermata. This relation-
ship is claimed on the ground of the agreement existing between the larval forms.
The similarity of the Tornaria, especially with the Bipinnaria larva of the
Astcroidea, is, indeed, so striking that the first observer of Tornaria expected
for certain that it would develop into an Echinoderm.
A closer comparison yields the following results :
1. If we place the Tornaria and the Bipinnaria similarly with regard to the
position of the mouth and the anus, a striking agreement in the conformation
of the regions of the body and in the ciliated rings bordering them is observed
(Fig. 468). In both we find a separate preoral ciliated ring in the same position,
bordering a preoral area. In both we can distinguish, lying behind this, a
deepened oral area with the mouth in its ventral centre. The postoral ciliated
ring of the Tornaria corresponds with the large circumoral ring of the Bipinnaria.
VOL. II 2 Q
594 COMPARATIVE ANATOMY CHAP.
The region of the body lying behind this occupies in both a large part of the dorsal
posterior side of the body ; in it lies the anus.
2. Whereas, in Tornaria, in the postoral area of the body, a large ciliated
ring bounds an anal area, such a ring is wanting in Eipinnaria.
3. The apical plate with the two eyes and the tuft, which is so sharply marked
in Tornaria, is wanting in the developed Hipinnaria. Too great a significance
must not now, however, be attributed to this fact, since something like a neural
plate (an ectodermal thickening with long cilia) has been observed in the quite
young larvae of Asteroids and Echinoids, and a neural plate with a layer of nerve
fibres, ganglion cells, and ciliated tuft, although without eyes, has been demon-
strated in the apical region of the Antedon larva.
4. The intestine of Tornaria shows the same divisions as are found in that
of the Echinoderm larvse, viz. : oesophagus, stomach -intestine, and hind -gut.
Whether, however, these three sections correspond with one another in the two
FIG. 468. A, B, C, Aurlcularia, Bipinnaria, and Tornaria (Enteropneustan larva), from the
right side, diagrammatic. 1, Preoral area ; 2, oral area ; 3, postoral area ; 4, anal area ; I, preoral ;
II, circumoral ; III, anal or principal ciliated ring ; 5, neural plate ; os, mouth ; an, anus.
groups must remain uncertain so long as the origin of the oesophagus and hind-
gut is not definitely ascertained. In this matter the Echinoderms are in the same
position as the Enteropneusta.
The oesophagus, in the Echinoderms, is sometimes described as an ectodermal
stomodseum, sometimes as a section of the archenteron. The latter, according to
the most recent investigations, is the case, e.g., in the Holothurioidea, and the
former in Antedon and others. In this case the oesophagus, even within the
Echinodermata, is not an homologous structure ! In the case of the Enteropneusta,
in the interest of other views, doubt has been thrown upon the statement that the
larval oesophagus is a part of the archenterou.
The hind-gut in the Echinodermata is, by all authorities, considered to be
endodermal. The same was affirmed of the hind-gut of the Enteropneusta, but
this has recently been strongly doubted.
5. The condition of the coelom in the two larval forms would show great agree-
ment if two pairs of ccelomic sacs can really be attributed typically to the Echino-
derm larva, a point which recent research makes more and more probable, and if
also the endodermal origin of the ccelomic vesicles of the Enteropneusta could be
proved.
It would then be evident that the two anterior ccelomic sacs of the Echinoderm
j (the left of which is the hydroccel) correspond with the two ccelomic sacs of
ix ENTEROPNEUSTA LITERATURE 595
the collar of the Enteropneusta, and the two posterior sacs of the former with the
two trunk ccelomic sacs of the latter. The two anterior sacs are in communication
with the exterior through the collar pores ; this communication (hydropore, water
pore) in the Echinodermata is usually limited to the left anterior sac, i.e. to the
left hydroccel vesicle, but occasionally in Asteroids a matter of great importance
appears on the right side as well.
From these considerations, it seems that the prospect of establishing a funda-
mental agreement in structure between the Enteropneustan larva and that of the
Echinodermata is very hopeful. This relationship between the Enteropneusta and
the Echinodermata seems to rest upon more solid ground than do any of the others
which have been attributed to either of these two groups.
At the same time any attempt to compare adult Echinoderms with adult
Enteropneusta is at present completely futile. The Echinoderms and Entero-
pneusta could, as far as we can see, only be genetically connected through some
common racial form far back in their phylogeny a form which corresponded with
the Tornarian and the Dipleurulan larva?.
Further, before we can feel any certainty on these questions of affinity, new and
more exact ontogenetic researches must be made. The origin of the proboscidal
crelomic vesicle of the Enteropneusta has to be established, as has also that of the
"heart or proboscis vesicle." Attention must be directed to the question as to
whether a preoral section of the body corresponding with the proboscis of the
Enteropneusta is present (if only as a rudiment) in the Echinoderm larvae, as for
instance in the preoral section of the body in the Antedon larva (?), .or in the larval
organs of Astcrina and other Asteroids (?). With reference to the " heart vesicle "
we are reminded of the statement that a "pulsating vesicle" occurs, apparently
not of enteroccelomic origin, in Echinoderni larvae. This has to be confirmed.
The Relationship of the Enteropneusta to Cephalodiscus and Rhabdopleura
will be considered in the Appendage to this chapter.
Literature.
Alex. Agassiz. The history of Balanoglossus and Tornaria. Mem. Amer. Acad.
of Arts and Sc. Vol. IX. 1873.
W. Bateson. The early stages in the development of Balanoglossus. Quart. Journ.
Mlt-rosc. Sc. (X.S.). Vol. XXIV. 1884.
The later stages in the development of Balanoglossus Kou:alevsJcii, with a
suggestion on the affinities of the Enteropneusta. Quart. Joum. Microsc. Sc.
(JV.tf.). Vol. XXV., Suppl. 1885.
Continued account of the later stages in the development of Balanoglossus
Koicalevskii and on the morphology of the Enteropneusta. Quart. Journ.
Microsc. Sc. (N.S.). Vol. XXVI. 1886.
The ancestry of the Chordata. Quart. Journ. Microsc. Sc. (X.S.}. Vol.
XXVI. 1886.
Gilbert C. Bourne. On a Tornaria found in British Seas. Journ. Mar. Biol.
Assoc. (2). Vol. I. 1889.
E. Kb'hler. Rcehc relies anatomiques sur unc nouvelle espece de Balanoglossus.
Bull. Soc. Sc. Nancy (2). Tome VIII. 1886. An almost identical work in
Intcrnat. Monatsschr. Anat. Hist. 3 Bd. 1886.
A. Kowalevsky. Anatomie des Balanoglossus delle Chiajc. Mem. Acad. Imp.
Sc. St. Petersbourg (7). Tome X. 1867.
A. Krohn. Bcobachtungen iibcr Echinodermenlarven. Arch. f. Anat., PhysioL
u. wissciisch. Mtd. 1854.
596
COMPARATIVE ANATOMY
CHAP.
A. F. Marion. ^Etudes zoologiques sur deux especes d 1 Elite" ropncustcs. Arch. Zool.
gtner. et exper. (2). Tome IV. 1886.
E. Metschnikoff. Untersuchungen ilber die Metamorphose einiger Seethiere. 1.
Ueber Tornaria. Zeitschr. f. wiss. Zool. 20 Bd. 1870.
T.H.Morgan. Growth and metamorphosis of Tornaria. Journ. Morph. V. 1892.
The Development of Balanoglossus. Journ. Morph. Vol. IX. 1894.
Job. Miiller. Ueber die Larven und die Metamorphose der Echinodermen. Part 2.
Akad. d. Wissensch. 1848. Berlin, 1850.
Wladimir Schimkewitsch. The Fauna of the White Sea : Balanoglossus Meresch-
kovskii Wagner. St. Petersburg, 1889. (In Russian. )
J. W. Spengel. Die Enteropneusten. Fauna and Flora des Golfes von Neapel.
18 Monographic. Berlin, 1893. The most important recent work.
W. F. R. Weldon. Preliminary note on a Balanoglossus larva from the Bahamas.
Proceed. Roy. Soc. London. Vol. XLII. 1887.
R. v. Willemoes-Suhm. Biologische Beobachtungen uber niedere Meeresthiere. 4.
Ueber Balanoglossus Kupfferi aus dem Oeresund. Zeitschr. f. wissensch. Zool.
21 Bd. 1871.
A. Willey. Amphioxus and the ancestry of the Vertebrates. 1894.
Appendage to the Enteropneusta.
Cephalodiseus and Rhabdopleura.
I. Cephalodiseus (Figs. 469-471).
The body is about 1 mm. long, almost bean-shaped, bilaterally
symmetrical; it is rounded
posteriorly and anteriorly
flat, with a slight backward
slope. The most important
organs which can be distin-
guished externally are found
in this anterior sloping sur-
face, while in the whole of
2 the rest of the body only
one organ appears, viz. a
cylindrical stalk or pedicle,
which rises from the ventral
side of the rounded posterior
end of the body.
In that part of the body
which projects most anteri-
orly, i.e. the anterior end
of the dorsal side, lies the
anus, while somewhat be-
FIG. 469. Cephalodiseus dodecaiophus, from the hind the anterior end of
ventral side (after M'IntOSh). 1, Tentacles; 2, buccal 4-},^ vpr ,t ra l ~\A~ 4->, p nirmtfc
shield = proboscis ; 3, mouth ; 4, buds ; 5, pedicle ; 6, trunk tn V6ntral Slde > tne mOUth,
and between the two the
slope mentioned above, the inter -stomatal region. The median
IX
CEPHALODISCUS
597
line between the mouth and anus is the inter -stomatal middle
line.
In the inter-stomatal region or in its immediate vicinity lie the
following parts of the body : (1) the preoral buccal shield with its
two pores ; (2), the central nervous system ; (3), the twelve feathered
tentacles ; (4), the apertures of the two oviducts ; (5), the postoral
\
15
FIG. 470. Median section through Cephalodiscus dodecalophus somewhat near the median
plane (after Harmer). 2, Nervous system ; 3, anterior paired coelom (collar coelom); 5, fold of the
anterior trunk region ; 8, paired trunk coelom; 9, pharynx ; 10, o?sophagus ; 11, stomach-intestine ; 12,
hind-gut ; 13, buccal ca vity ; 14, pedicle ; 15, ovary ; 16, anus ; 17, oviduct ; 18, buccal shield ; 19,
coelom of the same = proboscidal ccelom ; 20, one of the proboscis pores ; 21, anterior diverticulum
(proboscidal diverticulum) of the pharynx.
lamella ; (6), the two postoral pores of the body cavity ; (7), the
two gill-slits (Figs. 470, 471).
The buccal shield, which is comparable with the proboscis of the
Enteropneusta, is a plate, which projects downwards from the inter-
stomatal region immediately in front of the mouth by means of a
rather short stalk, in such a way that its free surface, the epithelium
of which is enormously thickened, faces ventrally (Fig. 470, 18).
The central nervous system lies in the hypodermis, almost at
598
COMPARATIVE ANATOMY
CHAP.
the centre of the inter-stomatal region. If we describe the mouth
as lying posteriorly and ventrally to the stalk of the buccal shield,
the central nervous system lies exactly opposite, that is, posteriorly
and dorsally to the same structure.
Twelve tentacles rise dorsally at the base of the stalk of the
buccal shield, six to the right and six to the left of the central nervous
system. The nervous system
extends into the dorsal hypo-
dermis of these tentacles (Fig.
471). The tentacles are large,
are feathered on both sides like
the feathers of birds, and are
knobbed at their free ends.
Two pores, placed symmet-
rically to the inter-stomatal
median line, break through
the most anterior part of the
central nervous system. There
is thus open communication
between the body cavity of the
buccal shield (the proboscis) and
the exterior (proboscis pores).
Between the central nervous
system and the anus there is on
each side an aperture. These
two apertures belong to the
oviducts.
Outside of the inter-
stomatal region, but in its
immediate proximity, the
following parts are found.
Immediately behind the
mouth, covered by the oral
disc, a thin lamella hangs ven-
trally and laterally down from
FIG. 47i.-Horizontai section through cephaio- the body like an apron ; this
discus (after Harmer). 1, Tentacles; 2, nervous ,1 i i n /!?
system; 3, anterior paired body cavity (collar coelom); 1S the POStOral lamella (1 Ig.
4, collar pore ; 5, folds of the anterior tnmk region, the 471, 5). In the posterior
r^fd 1 t am t a; t b T h ^ lp0res;7 ' mesentery; an S le formed V this Amelia
8, paired trunk ccelom ; 9, pharynx ; 10, oesophagus ; -~L .1 T i c i
11, stomach ; 12, hind gut. Wltn the body, a pore is found
on each side (collar pore).
These pores lead into the paired middle body cavity, of which we shall
speak later. Immediately behind these pores, and like them over-
arched by the lateral folds of the postoral lamella, two branchial pores
lead from without into the pharyngeal section of the intestinal canal.
Musculature. From near the mouth, longitudinal muscle fibres
run back along the ventral side, and enter the pedicle. Muscles are
ix CEPHALODISCUS 599
also found in the stalk of the buccal shield, which radiate out from
the stalk into the shield.
Body cavity. In a young bud, the body appears to be divided
into three sections (anterior, middle, and posterior) by two circular
furrows. Each of these three sections possesses a separate body
cavity. The most anterior section, out of which the buccal shield
proceeds, has an unpaired body cavity, into which a short intestinal
diverticulum enters from the middle section. The body cavity in
both the middle and posterior sections is paired, the two lateral halves
being separated by mesenteries. The boundary between the middle
and posterior sections, the latter of which becomes the greater part of
the body cavity in the adult, becomes more or less indistinct. The
body cavity of the middle section is retained in the adult in the
postoral lamella and in the region of the central nervous system and
of the feathered tentacles, into which it is continued. The body
cavity of the posterior section in the adult contains the whole of the
alimentary canal and the ovaries, these organs almost entirely filling it.
It is continued into the pedicle.
The alimentary canal forms a loop in the body, with a ventral
section which runs backwards and into which the mouth leads, and a
dorsal section running forwards and opening anteriorly through the
anus. The mouth leads first into the "pharynx," which com-
municates with the exterior by means of the two gill-slits mentioned
above. A thin divertieulum runs out anteriorly from the pharynx
below the nervous system into the stalk of the oral disc. The
pharynx is followed first by an oesophagus and then by a very
spacious stomach or stomach intestine, which occupies by far the
greater part of the body cavity. At the point where the pedicle joins
the body, the stomach passes over into a narrower section of the
intestine, which, immediately behind the stomach, ascends, and then,
bending forward, runs along the dorsal side as the hind-gut to the
anus.
Genital organs. Male genital organs have not been observed.
The female organs consist of two ovaries, lying in the anterior
part of the body ; they are continued into two strongly pigmented
oviducts, which open outward through the apertures already men-
tioned (Fig. 470, 17).
Reproduction. Besides multiplying sexually by means of eggs,
Cephalodiscus also reproduces itself asexually by gemmation. The
buds always form on the pedicle, near to its free end. Almost all
adult individuals have from 1 to 3 buds.
Many individuals of Cephalodiscus live together in a ramifying
and anastomosing system of tubes secreted by themselves, these tubes
having occasional apertures. The animals, throughout life, when
not disturbed, remain in the immediate vicinity of these apertures,
through which they protrude their unfolded crowns of tentacles.
600 COMPARATIVE ANATOMY CHAP.
C. dodecalophus, the only known representative of the genus, was
found in the Magellan Straits at a depth of 245 fathoms.
Systematic position. Cephalodiscus shows in the following
points a remarkable agreement with the Enteropneusta.
1. The body falls into three sections (distinct even in the young
bud), one preoral and two postoral. The preoral section, the so-
called buccal shield, corresponds with the proboscis, the middle section
with the collar, and the larger posterior section and the pedicle with
the trunk of the Enteropneusta.
2. These sections correspond with special sections of the ccelom,
an unpaired ccelom in the buccal shield, and two pairs of cceloms in
the body proper. We recognise here the unpaired proboscidal coelom
and the paired collar and trunk cceloms of the Enteropneusta.
3. The pores of the ccelom of the buccal shield correspond with
the proboscis pores of the Enteropneusta proper, which are also often
two in number.
4. The pores of the pair of cceloms in the anterior body correspond
with the collar-pores.
5. Cephalodiscus and the Enteropneusta have gill-slits, the former
having one and the latter many pairs.
6. The anterior diverticulum of the buccal cavity corresponds
with the proboscidal diverticulum of the Enteropneusta.
7. The central nervous system corresponds with the collar cord
(which, however, in this case is not sunk below the skin) of the
Enteropneusta and with its immediate continuation on to the base of
the proboscis.
The differences existing between Cephalodiscus and the Entero-
pneusta may well be attributed, at least in part, to the tubicolous, half-
sedentary manner of life of the former. These are : (1) the anterior
position of the anus, and the consequent looped course of the
alimentary canal ; (2) the general crowding together of the most
important external organs (apart from the pedicle or stalk) at the
most anterior part of the body ; (3) the presence of a tentacular
crown, consisting of twelve feathered tentacles ; (4) the presence of
the pedicle or stalk; 1 (5) the occurrence of asexual reproduction by
means of gemmation ; (6) the small number of gill-slits and genital
organs ; (7) the form of the body in general and especially that of the
proboscis \ (8) the absence of a blood vascular system.
II. Rhabdopleura.
This form, which was formerly classed with the Bryozoa? is no
doubt somewhat nearly related to Cephalodiscus, but is further
removed than the latter from the Enteropneusta.
1 An apparently homologous structure is, however, figured by Bateson on a young
Balanoglossiis Koivalevskii.
* Iii the first volume of this book, indeed, Rhabdopleura appears among the Bryozoa.
.
RHABDOPLEURA
601
This animal forms colonies by gemmation. Each individual
consists of a body and a contractile stalk, both of which are
enclosed in a horny tube. This tube is supine at first but rises
erect later. It is secreted in successive rings by the buccal shield.
The tentacles can be protruded through the aperture of the tube,
FIG. 472. Rhabdopleura Nonnani Allm.,
individual, from the right side (after Lankester).
1, Buccal shield ; 2, feathered tentacles ; 3, region
of the "collar pore " ; 4, anus ; 5, trunk ; 6, stalk
or pedicle.
the body being withdrawn again into the tube by means of the stalk.
All these tubes are lateral branches of a creeping basal tube which
spreads out and branches on the substratum, and appears to be divided
into chambers by septa.
The stalks of the individuals enter this radical tube, and are
continued in it as thin strands covered with cuticle, which in travers-
ing it perforate the septa.
The axis of each individual stalk is formed by a strand of tissue
602
COMPARATIVE ANATOMY
CHAP. IX
somewhat of the consistency of cartilage. Similar skeletal pieces
support the tentacles and their branches.
Some insight into the chief anatomical features of the individuals will
be gained from the figures (Figs. 472, 473). The principal differences
between Rhabdopleura and Cephalo-
discus are : (1) the gill -slits are
wanting in the former; (2) there
are only two feathered tentacles ;
(3) the proboscis pores, i.e. the
pores of the coelom of the buccal
shield, are wanting.
The genital organs are still
imperfectly known. In some speci-
mens a testicle tube, which runs
longitudinally and asymmetrically
along one side of the body, and
bulges out the body wall, has been
demonstrated : this tube opens out
near the anus. Rhabdopleura, like
Cephalodiscus, is a deep-sea form.
Further research is needed be-
fore we can establish the exact
relationship of Cephalodiscus and
Rhabdopleura to the Bryozoa.
Literature.
FIG. 473. Rhabdopleura Normani,
median longitudinal section, diagrammatic
(after Fowler). 1, Tentacle of one side, in-
dicated by dotted lines ; 2, anterior paired
coelom (collar coelom) ; 3, anus ; 4, posterior
paired or trunk coelom ; 5, hind-gut ; 6, mid-
gut ; 7, buccal cavity ; 8, mouth ; 9, anterior
diverticulum of the buccal cavity (probosciclal
diverticulum) ; 10, ccelom of the buccal shield
(proboscidal coelom) ; 11, buccal shield.
G. J. Allmann. On Rhabdoplcura, a new
form of Polyzoa, from deep-sea dredg-
ing in Shetland. Quart. Journ.
Microsc. Sc. Vol. IX. 1869.
G. Herbert Fowler. The morphology of
Rhabdopleura Normani Allm. Fest-
schrift zum Siebenzigsten Geburtstag R.
Leuckart's. 1892.
Sidney F. Harmer. Appendix to M'Intosh : Report on Cephalodiscus dodecalophus
M'Intosh. Rep. Voy. of the "Challenger" Zool. Vol. XX. 1887.
E. R. Lankester. A contribution to the anatomy of Rhabdopleura. Quart. Journ.
Microsc. Sc. Vol. XXIV. 1884.
W. C. M'Intosh. Report on Cephalodiscus dodecalophus M'Intosh, a new type of the
Polyzoa, procured on the voyage of H.M.S. " Challenger." Rep. Voyage
" Challenger " Zool. Vol. XX. 1887.
G. 0. Sars. On Rhabdopleura mirabilis. Quart. Journ. Microsc. Sc. Vol. XIV.
1874.
INDEX
Numbers in Italics give System atio Position.
refer to Figures
^Numbers in Black Type
Amph. = Amphineura
Ast. = Asteroidea
Blast. =Blastoidea
Crin. Crinoidea
Cyst. = Cystidea
Echin. = Echinodermata
Echinoid. = Echinoidea
Ent. = Enteropneusta
Gastr. = Gastropoda
Hoi. = Holothurioidea
Lam. = Lamellibranchia
Moll. = Mollusca
Oph. = Ophiuroidea
ABATUS cavcrnosus, apical system. 324
Abranchia, 12
Acanthaster, 299
,, echinaster, 421
Ellisii, 421
Acanthochiton, 165
Aca.nUwdoris, 13
Acanthology, 387
Aatiithotrochus rniro.bilis, "wheel." 337
Acephala, 14, 177
Accra, 10
Acetabula (Moll.), 117 ; (Echinoid.), 390
Acicididce, 6
Acnwca ( = TccturoJ), 5
Acmaeidce, 5
Acri'cido.ris, 290
Ac/'"t:rinidce, 308
A?r<:r!.iin*. 309
Actceon, 110
Actceonidce, 10
Actiitc/'inidce, 307
Actiii'icrinus, 307
,, proboscidali.s, apical system,
329
,, cr/-,i(uiliani(s, 423
Actiiuxvcumis, 338
Actinometm, 313 ; food grooves, diagr.,
366
t rota, 365
Actiii'>j)oda, 285 \ oral region, section, 428
Adambulacral ossicles, 352 ; radii, 316
Adetes, 293
Adradii, 316
13
, alimentary canal, 192
nifibranchialis, 12
, 293
^Esthetes, 166
jEtheria, 62
Agaricocrinus, 307
Agassizia, 293
Agassizoerinus, 304
Agelacrinus, 313
cincinnatensis, 313
Aglossa, 14, 177
A media, 76
Ambitus, 338
Amblypygus, 345
Ambulacra, 339
Ambulacral bmsh (Echinoid.), 433; gills
(Echinoid.), 433; ossicles, 352; radii,
316
A mmon it idea , ;? J
Amnion, 523
Amoebocytes (Moll.) 200 ; (Echin.), 415
Amphiaster, 297
A inph idrom us, 1 60
Amphineura, 2
Amphipeplea, 8
,, leuccmensis, 8
A//(phisphyra, 46
Amphisternal test, 349
Amphiura, 300
magellanica, 495
,, squamata, apical system, 327 ;
stone canal, 422
604
COMPARATIVE ANATOMY
Amphiuridce, 300
Amphoracrinus, 307
Ampullae, madreporic, 420
,, tentacle, 430
Ampullaria, 100
,, (Lanistes), Bolteinana, shell,
161
(Oeratodes), chiquitensis, 161
crocistoma, 161
Geveana, 161
purpurea, 161
(Geratodes), rotula, 161
Swainsoni, 161
Ampullaridce, 6
Ananchytidce, 293
Anapta, 466
Anaspidce, 10
Anatina, 21
Anatinidce, 21
Anchylosis (Grin. ), 378
Ancula, 13
Ancyloceras stage, 68
Ancylus, 8
Ankyroderma, 287
Annulus, 126
Anochanus, 503
Anodonta, 17 ; circulation, 207 ; gills,
95 ; larva, 264 ; section, 221
,, cygncea, 34
Aiwmia, 15 ; shell, 63
Anomiidce, 14
Antedon, 313 ; embryo and larva, 510,
534-543
Antedon incisa, 312 ; larva, apical system,
318
,, phalangium, young stage, 375
,, rosaceus, 378
,, tuberculosa, 297
Antedonidce, 373
Anthenea, 296
Antheneidce, 296
Antispadix, 116
Aorta, 198, 202
Apetala, 293
Apiocrinidce, 310
Apiocrinus, 310
Aplacophora, 3
Aplustridce, 110
Aplysia, 10, 78 ; nervous system, 140
Aplysiella, 181
Aplysiidce, 10
Apophyses (Chiton), 39 ; (Echin.), 350
Aptychi (Ceph. ), 71
Arachnoides, 293
Arbacia, 290, 291
Arbaciidce, 290
Area, 15
,, barbata, eye, 175
Noce, 206
Archceocidaridce, 289
Archceocidaris ( = Echinocrinus), 289
Ar chaster, 296
296
Arcliiacia, 295
Archidoris, 13
Architcenioglossa, 5
A rci dee, 15
Arcus (Echinoid. ), 400
Argentea (Ceph.), 197
Argonauta, 24, 24 ; gonads, female, 230 ;
nervous system, 148
argo, male, 243
Anon, 8
ater, 9
Ariophanta, 8
Aristocystis, 313
Aristotle, lantern of, 400-403
Articulamentum, 39
Articulata, 309
Ascocystis, 313
Ascoglossa, 11
Asiphoniata, 44
Aspergillum (Brechites), 21, 20
dichotomum, 66
Aspidobranchia, 4
Aspidochirotce, 285 ; organisation, 477
Aspidodiadema, 290
Aspidodiadematidce, 290
Aspidosoma, 295
Astartidce, 50
Asteracanthion glacicdis, pedicellariaa, 395
rubens, madreporic plate,
421
pedicellariae, 395
Asterias, 299
acutispina, 506
,, atlantica, 506
calamaria, 506
capensis, 421
microdiscus, 506
polyplax, 421
rubens, 507
spirabilis, 503
,, sticantha, arm, 396
,, (Stollasterias), volsellata, arm,
396
tenuispina, 421
mdgaris, 507
Asteriidce, 299
Asterina, 297
,, gibbosa, circular canal, etc., 425 ;
ontogeny, 525-531
Wega, 506
Asterinidce, 297
Asterodiscus, 297
Asteroidea, 295 - 299 ; alimentary canal,
etc., 484; arm, section, 411; optic cushion,
section, 468 ; oral skeleton, 352 ; pedi-
cellariae, 395 ; stone canal, section, 421 ;
water vascular system. 463
Aster opsis, 297
Asthenosoma, 290
urens, spine, 390 ; test, 472 ;
viscera, 443
Astrochele, 301
Astroclon, 301
INDEX
605
Astrocnida, 301
Astrocrln idcr\ 315
Astrocrinu-s, 315
Benniei, 315
Hiphus, 301
Astrogonium, 296
den ai'.rantiacus, branchial skele-
ton, 351
Axtropectinida, 296
Attrophytiffce. 301
Attrophyton, 301
Lincki, 301
Astroporpa, 301
Astropyfja.
Astroschema, 301
Astrotoma, 301
Atelecrinus, 313
Atelestocrinus, 304
Atlanta, 90, 109
Peronii, 7
Atlantidce, 6
Atys, 46
Auricula, 350, 402
Auricularia, 506, 507, 508, 511, 512,
513, 514
of Synapta, 512, 514
A v. ricul idee, 8
Ai-icv.la. 17
Aciadidcf, 17
Axial organ ( = ovoid gland), 437, 445, 446
Azygobranchla, 5
BACULITES stage, 68
Bc^anocrinus, 313
Bcdanoglossus, 562
canadensis, 565
Kwoalevskii, 562 ; branchial
skeleton, 580
i Kiipfferi, 571
Mcrschkorskii, 571
Be. rra n dewin idee, 309
Barrandeocrinus, 309
Barrel-shaped larvae (Echin.), 514
BarycriiiHs, 304
Basals, 318
Basibrachial cartilage (Ceph.), 126
Basipterygial cartilage (Ceph.), 127
Basoiniiifitiiphora, 8
Bathybiaster, 296
Bathycrinida, 304
Bathycrinus Aldrichianus, axial canals,
378
Bathydoris, 13
Batocrinus, 307
,, pyriforiiris, 307
Baur's vesicles, 467
Bdemnites, 24 ', shell section, 69
Belemnitidce, 24
Belcm nocrin us, 304
Bdemiwteuthis, 24
Bellerophontidce, 5
Belosepia, 24
,, shell section, 69
Benthaster, 298
Benthodytes, 285
Berghia, 13
Bipinnaria larva, 507 ; dorsal aspect, 528
Bivalra, 14
Bivium, 325, 347, 407
Blastoidea, 314
Blauneria, 109
Bojauus, organ of, 215, 221
Bornellidce, 13
Bothriocidaris, 289
Bothriocidaroida, 289
Botyocrimis, 304
Bourgueticrinidce, 310
Bourguetinicrinus, 311
Brachiolaria larva, 508
Branchiopneiista, 78
Brechites (Aspergillum), 21, 20
Brisinga, 299
Brisingidce, 299
Brissopsis, 293
Brissus, 293
Buccal shields (Oph.), 336 ; plates (Echin-
oid.), 344
Buccinidce, 6
Buccinnm, 160
Bull-minus, 8
Bidimulidce, 8, 9
Bulimus, 8
,., oblongus, 75
,, percersus, 160
Bulla, 10
,, hydatis, nervous system, 140
Buttidas, 10
Bulloidea (Cephalaspidce), 110
Bursse (Oph.), 494
Byssus, 112-115, 114
CALAMOCRINUS, 310
Calamus, 69
Calcareous cells, 190
Calceocrinus, 304
Calliaster, 296
Callicrinus, 309
Callocystis, 332
Callum, 64
Calymne, 293
Calyptroea, 108
Ccdyptrceidce, 6
Calyx, 319
Camerata, 306
Canaliculata, 310
Cancdlariidce, 6
Caprinidce, 18
C'iptdidce, 6
Cardiacea, 18
Cardiaster, 293
Cardiidce, 18
Cardita, 17
Carditldce, 17
Cardium, 18
ed ule, nervous system, 144 ; tuber-
culatum, 18
606
COMPARATIVE ANATOMY
Carina (Echinoid.), 401
Carinaria, 90, 109
Carolia, 63
Carpocrimis, 307
Caryocrinus, 313
,, ornatus, apical capsule, 332
Cassidaria tyrrhena, 163
Cassidiidce, 6
Cassiduloidea, 293
Cassidulus, 293
pacificus, perisome, 348
Catillocrinus, 304
Catopygus, 293
Caudina, 287
Cavolinia, 11
tridentata, 91
Cavoliniidce, 11 ; diagram, 80
Centre-dorsal, 375
Centrostephanus longispinus, pedicellarisp,
397
Cephalaspidce (Bulloidea), 10
Cephalic cartilage (Gastr.), 126 ; cone, 104 ;
disc, 103
Cephalodiscus dodecalophus, 596 ; sections,
597, 598
Cephalophora, 3, 177
Cephalopoda, 21 - 25 ; eye, development,
171 ; embryo, 116 ; gonad, female, 230 ;
male, 231 ; ink-bag, 197 ; heart, 199 ;
retinal cells, 173
Cerata, 98
Ceratodes (see Ampullaria), 161
Cerithiidce, 6
Cidaris, 290 ; spine of, 389
canaliculata, 502
hystrix, peristome, 344
membranipora, 502
nutrix, 502
papillata, oral area, 345
tribuloides, surface of test, 389
Cidaroida, 290 ; apophyses, 350
Cionella, 75 .
Cirrhoteuthidce, 24
Cirrhoteuthis, 54 ; shell of, 127
Cirrobranchia, 12
Chcetaster, 298
Chcetoderma, 3, 88
Chcetodermatidce, 3
Chcetodermatina, 3
Chcetodermidce, 3
Chama, 18
Chamacea, 51
Chambered sinus, 446
Chamidce, 18
C/ienopidce, 6
Chiasma nervorum brachialium (Grin.),
377, 460
Chiastoneury, 135, 136, 137
Chirodota, 288
rotifera, 402
Chiroteuthis, 24
Chiti. ",3,2; spine, 40 ; sections, 40, 212 ;
ctenidium, 85, 87 ; heart, 199 ;
eye, section, 167 ; ovary, 227 ;
nephridial and genital systems,
217
Chiton cajetanus, 165
Icevis, 88, 165
Pallasii, 82
Polii, 165 ; development, 249
rubicundus, 129
siculus, 165 ; nervous system, 130
Chitonellus, 3 ; section, 41
Chitonidce, 2 \ gills, 87
Chlamys, 17
Choanomphalus Maacki, 160
Choristes, 108
Chromatophores, 53
Chromodoris, 13
Cionella, 75
Cladohepatica, 13
Glaus-ilia, 8
Clavagella, 21
Clavagellidce, 21
Clavulse (Echinoid.), 391
Cleiocrinus, 309
Cleodora, 30
Clio, 11
striata, anatomy, 190
Clione, 90
Clionidce, 11
Clionopsidcc, 11
Clionopsis, 90
Clypeaster, 291, 292
rosaceus, apical system, 322
Clypeastridce, 291 ^
Clypeastroida, 291 ; system of plates, 346
Clypeus, 293
Cnemidaster, 298
,, Wyvilli, 297
Cadaster, 315 ; ambulacrum section, 383
,. bilobatus, 314
Codasteridce, 315
Codechinus. 290
Codopleurus, 290
Cottyrites, 293
elliptica, apical system, 325
Collyritidce, 293
Colochirus, 287
,, cucumis, calcareous body, 337
Colombellinidce, 6
Columella, 123
Columellar muscle, 120-123
Columna (Crin. ), 373
Columuals, 373
Columnar layer of shell (Moll.), 57
Comatulidce, 313
Conchyolin, 26, 57
Conidce, 6
Conoclypeus, 291
Coralliophila, 183
Coralliophillidcc, 183
Corbicula, 17
Corbis, 93
Corbula, 19
INDEX
607
Corethraster, 298
Corium (Moll.), 39 ; (Echin.), 414
Corona, 339
Coronaster, 299
Corpus epitheliale, 171
Corylocrinus, 332
Coryptella, 13
Costals (Grin.), 371
Oranchia, 24
Crassatella, 49
Crassatellidce, 17
Crenella, 115
Crepidididce, 138
Cribrella, 299
,, sexradiata, 506
Crinoidea, 302-316 ; arm, 372 ; arm, sec-
tion, 413 ; ovarial pinnule, section, 500
Crioceras stage, 68
Crista acustica, 169
Oromyocrinus, 304
Orossaster, 298
Crotalocrinidoe, 308
Crotalocrinus, 308
,, pulcher, 373
,, rugosus, arm disc, 372
Cryptoblastus, 315
Oryptochiton, 41
Cryptodon Moseleyi, 50
Cryptoschisina, 315
Gryptozonia, 297
Crystalline stylet, 191
Ctenidium, 84, 85
Ctenodiscus, 296
* ,, procurator, 296
Ctenopidce, 102
Cucumaria, 287
crocea, 502
crudfera, cruciform body, 337
doliolum, section, 517
frondosa, 488
Lacazii, 464
,, losvigata, 502
longipeda, "stool," 337
mimita, 502
planci, 287
Culcita, 297
Cidtellus, 19
Gupressocrinus, 304
Cuspidaria, 21
Cuspidaridce, 21
Cuttlefish ( = Cephalopoda], 21
Cuvierian organs (Hoi.), 488
Cyatliocrinidce, 304
Cyathocrinus, 304 ; apical system, 329
,, lonyimanus, 304, 364
Cyclas, 81
cornea, development, 262, 263
Cyclophoridce, 5
Cydophorus, 100
Cydostoma, 100
,, elegans, nervous system, 139 ;
radula, 182
Cydostomidce, 6
Cylichna, 46
Cynibulia, 11 ; larva, 257
Cymbulidce, 11
Cymbuliopsis, 11
290
Cypraea, 5
Cypraeidce, 5
Cyrena, 17
Cyrenidce, 17
Cyrtoceras group, 68
,, vesica, gonads, 499
idea, 313
CystoUastus, 313
,, Leuchtenbergi, 312 ; apical
side, 332
Cystocidaris ( = Echinocystites), 289
Cystocidaroida, 289
Cystocrinoidea, 313
Cystoid stage, 544
Cytherea (Meretrix), 18
chione, shell, 63
DAUDEBARDIA (Helicophantd), 9
brevipes, 9
/""/#, intestine, section,
39 ; uephridia, 220 ;
pallial organs, 76, 77
saulcyi, 76
Decadocrinidce, 304
Decadocrinus, 304
Decapoda, 24
Deima, 285
DeimoMdce, 285
Delphinulidce, 170
Deltoid plates (Blast.), 331 ; (Grin.), 364
Dendrochirotce, 287
Dendrocrinidce, 304
Dendrocrinus, 304
Dendronotidce, 13
Dentalium, 13, 33 ; shell, 59, 156
,, entcde, 113, 159 ; alimentary
canal, 193 ; larva, 258 ; on-
togeny of, 258
Dermatobranchus, 48
Deziobranchced, 11
Dextral twist (Gastr.), 56, 60, 160
Diadema, 290
setosum, 392 ; compound eye,
469
Diadematidoe, 290
Diadematoida, 290
,, apophyses, 350
Dialyneurous nervous system (Gastr.), 138
Diaphragm (Ceph.), 127 ; cartilage, 127
Diaulula, 13
j Dibranchia, 24 ', eye development, 171 ;
musculature, 127 ; shells, sections, 69
Diceras, 18
Dichocrinus, SOS
Dicyclic base (Criu.), 328
Dicydica, 304
608
COMPARATIVE ANATOMY
Diffuse liver (Gastr.), 192
Digonopora, 8
Dimerocrinus, 367
Dimyaria, 14, 124 ; shell, 63
Diotocardia, 4> 30
Dipleurula larva, 546
Diplopodia, 290
Discodoris, 13
Discoidea, 291
Distichals (Grin.), 371
Docoglossa, 5
Dolabella, 10
Doliidce, 6
Dolium, 102
Donacidce, 18
Donax, 18
Dondersia, 3
is, 251
. 184
Dondersiidce, 3
Dorididce cryptobranchiata, 13
,, phanerobranchiata, 13
Doridiidce, 10
Doridium, 46
Doridopsidce, 13
Doriopsis, 214
Doris, respiratory and circulatory organs,
98
Dorocidaris papillata, 388
Dorsal axis (Blast.), 331 ; cartilage (Ceph.),
127
Dorycrinus, 307
Dosidicus, 69
Dotonidce, IS v
Dreissensia pvlwnorpha, gills, 94
Dreissensiidce, 17
Disaster, 293
Dytaster, 296
ECHINASTER, 299
,, sepositus, 483
EchinoMeridce, 299
Echinidce, 290
JEchinobrissus, 293
Echinocardium, 293
Echinocidaris, 290
nigra, ambulacrum, 393
pustulosa, 291
Echinocotms, 293
Echinocorys, 293
Echinocrepis, 295
Echinocrinus (Archceocidaris), 289
Echinocyamus, 291
,, pusillus, gastrula, 520 ;
Pluteus larva and young,
520-524 ; tentacle, sec-
tion, 464
Echinocystis (Cystocidaris), 289
Echinodermata, alimentary canal, 475 ;
representatives of principal divisions,
316
293
Echinodiscus biforis, 424
Echinoencrinus, 313
armatus, 333
Echinoidea, 288-295; endocyclic, 322;
exocyclic, 322 ; larva, 509 ; organisa-
tion of regular, 419 ; pedicellarise,
397 ; Pluteus, 520-524 ; radial region,
section, 410
Echinolampas, 293
Echinometra, 290
Echinometridce, 290
Echinoneidce, 293
Echinoneus, 293
Echinopsis, 290
Echinospatagus, 293
Echinosphcera, 386
ISchinothrix, 290
Echinothuria, 290
Echinothuridce, 290
Echinus, 290 ; apical system, 318 ; masti-
catory apparatus, 401
,, acutus, 398
Edrioaster, 387
Elceocrinus, 315
Elasipoda, 285
Eledone, 24
,, moschata, secondary body cavity,
diagram, 214
Eleutherocrinus, 315
,, Cassedayi, apical side,
331 ; oral side, 383
Elpidia, 285
,, glacialis, 467
Elpidiidce, 285
Elysiadce, 12
EiYbarginula, 4 > shell, 59, 156
Embryonic cone, 257
Enallocrinus, 308
Encope, 293
,, Valenciennesi, system of plates,
346
Encrinus, 304 ; axial canals, 378
,, liliiformis, 304
Endoceratidce, 67
Eudogastric coil (Gastr.), 67, 68, 159
Enoploteuthis, 127 ; musculature, 127
Ensis, 19
Enter opneusta, 561-596 ; branchial region,
567, 580 ; larvae, 586-590, 594 ; pro-
boscis, section, 583
Entocolax Ludwigii, 246, 247
EntoconcJia, 183
, t mirabilis, 247, 248
Entovalva, 229
Eolampas, 293
Eolis D-rummondi, 183
Epineural canal, 449
Epipodial lobes (Ceph.), 38
Epipodium, 106
Episternum, 349
Epistroma, 349
Eretmocrinus, 307
INDEX
609
JSrisoc-rinus, 372
ErycinO) 17
Erycinidce, 17
Emixteroidea, 296
Eucalyptocrinidce, 309
Euoalyptoervn us, 309
ix, 308
313
Eudiocrinus, 313
Eiocliinoidea, 290
,, diadematoida, 469
Eulamellibranchia, 17 ', gills, 92
Ei'lima, 183
Enliuiidce, 6
tin HI irrgerita, 108
Eupachycrin //.y, 372
E'tptocamus, 13
Enryalai, 301
Ei'ru'i.le, 301
Euthyneurous condition (Gastr.), 133, 158
Exogastric coil (Gastr.), 159
/>/////, 62
313
FACELLINA, 13
Falces (Echinoid. ), 400
Faorina,
Fascioles (semites), 349
F'-riti'nfi, 326
Ferment cells, 190
Fibularia, 291
Fibulariidce, 291
Filibmnchia, 14 ', gills, 92
Fins (Ceph.), 54 ; (Oph.), 392
Fiona, 13
Firulu coronata, 7
Firoloides, 90
F!x*mvlla, 4 ', ctenidium, 85 ; shell, -59,
156
Fistitrellidce, 4
Fistulana, 64
Fistu.lata, 303
Flabcllina, 13
Fleming's cells, 162
Floscelle, 347
Foramen basale (Echinoid.), 400 ; exter-
num (Echinoid.), 400
Forbexincriii //.y, 309 ; dorsal cup, 369
Fork pieces (Blast.), 331 ; (Echinoid.), 400,
401
Funnel (Ceph. = siphon), 265 ; ciliated
(Echin.), 437
Ft'xidce, 6
GALATEA, 17
Galeomma, 17
Galempygus, 322
('"/nata larviformia, 303, 363
lufrabasals, 318
Ink-bag (Ceph.), 177, 196 ; morphology,
197
Inoceramus, 17
I uti'iji'ljiiillinta, shell, 63
Interambulacra, 339
Interambulacral radii, 316
Interdistichals, 363
Internodes, 374
Interpalmars, 363
Inter radii, 316
Intersecundibrachs, 363
Intertertibrachs, 363
locrinn.s, 304
Irpa, 418
Irregidares, 315
Isaster, 293
Isoiiiyaria, 124
JANUS, 13 ; nervous system, 141
Jouanmtia, 19
,, Gumingii, 19
Juglandocrinus, 332
KEBER'S organ (red-brown organ), 215
Kellya, 17
Kentrodoris, 13
Kleinia luzonica, apical system, 342
Kolga, 285
Kollicker's canals (Ceph.), 168
LABIAL palps (Gastr.), 104; (Lam.), 105
La.bidiaster, 299
Labidodemas, 491
Labrum (Echinoid.), 348
Lacuna, 108
Lcetmogcnie, 285
Laganid.ce, 291
Laganum, 291
depressum, apical system, 323
Latnellaridaf, 6
Lamellibraiiclnata, lJ^-21 \ byssus, 114;
diagrams, 50 ; heart, 199 ; shell, 59, 62,
156
Lancet plates (Blast.), 380
Lanistes (see Ampidlaria), 161
Luri-ifnnnia, 303
Lascea, 17
Lateral organs (Gastr.), 165
Lecythocrinus, 304
Leda, 14
Lepctidce, 5
Lepidocentriis, 289
Lepidomenia, 13
hystrix, 132
Leptoconckus, 183
Lepton, 17
Leptopty 'chaster kerguelenensis, 503
Leskiidce, 295
Leuconia, 109
Ligament (Lam.), 61
Lima, 105
Limacidce, 8
Limacina helicina, anatomy, 189
,, Lesuerui, 11
,, retroversa, 160
Liinacinidce, 11 ; diagram, 80
Limapontiidce, 12
Limax, 8 ; vascular system, 204
Limidce, 53
Limncea, 8
abyssicola, 100
stagnalis, genital organs, 235
Limnceidce, 8
Linckia, 298
,, midtifora, 244
Linckiidce, 298
Linthia, 293
Lipocephala, 101, 177
Lithodomus, 15
Litiopidce, 108
Littorinidce, 6
Lituites, 68
Lobiger, 12
Loligo, 24 ; gonad, male, 231 ; nervous
system, 148
,, vulgaris, 23
Loligopsis, 24 ', shell, section, 69
Lomanotidce, 13
Loven, law of, 341-344
Lovenia, 293
Lucina Pennsylvanica, shell, 63
Lucinacea, 50
Lucinidce, 17
Luidia, 296
Lunula, 341
Lutraria, 51
Lutrariidce, 19
Lyonsia Norwegica, 51
Lyonsiidcie, 21
MAGTRA, 18
Mactridcc, 18
Macula, 168
,, acustica, 168
Madreporite, 321, 416-423
Magttus, 183
Malletia, 49
Malleus, 17
Mantle, 26 ; cavity, 26
Margarita Groenlandica, 4
Margaritana (Unio) Margartiifents, 17
Marginaster, 297
Marginellidce, 6
Mariacrinus, 307
Marionia, 13
Marseniadce, 225
Marsupia, 347
Mamipiocrinus, 308
ccelatus, tegmen calycis,
369
Marsupites, 304
ornatus, apical system, 329
Martesia, 65
Megallaesthetes, 166
612
COMPARATIVE ANATOMY
Megistocrinus, 307
Melampas, 109
Melanidce, 6
Meleagrina, 17
,, margaritifera, 17
Melibe, 13
Mdlita, 293
,, testudinata, 293
Melocrinidce, 307
Melocrinus, 307
typus, 307
Melonites, 289
,, multipora, apical system, 340
Melonitidce, 289
Membrana limitans, 173
Meona ventricosa, apical system, 323
Meretrix (Gytherea), 18
Meridosternal test, 349
Mesites, 313; ambulacrum, section, 386
Mesollastus, 315
Mesodesinatidce, 18
Mesoplax, 64
Mesorchium, 230
Metablastus, 315
' Metacrinus, 313
angulatus, tegmen calycis, 365
Murrayi, 311
Metaplax, 64
Metapodium, 106
Metrodira, 298
Micrsesthetes, 166
Micraster, 293
,, coranguinum, apical system, 324
Miller icrinus, 310
Milneria, 49
Mimaster, 296
Mithrodia, 299
Mitra, 101
Mitridce, 6
Modiola, 15
Modiolaria, 15
Moira, 503
Mollusc, hypothetical primitive, 26
chilensis, 488
Molpadiidce, 287
Monocyclic base (Grin.), 328
Monocydica, 303
Monogonopora, 8
Monomyaria, 14, 124 ; shell, 64
Monopleuridce, 18
Monotocardia, 5 ; diagram, 31 ; gills, 90
Montacuta, 93
Muelleria (Moll.), 124
Muelleriacea, 124
Miietteridce, 124
Muileria (Echin.), 285
Murex, 101
trunculus, alimentary canal, 189
Muricidte, 6
Mutela, 17
jHfutelinc', 51
My a, I.
Myacea, 18
Myadce, 64
Myiidce, 19
Myochama, 51
Myopsidce, 24
Myriotrochus, 288
Rinkii, 246
Mytilidce, 15
Mytilus edulis, 15
NACELLA, 108
Nacreous layer of shell, 57
Narica, 108
Naricidce, 107
Natantia, 10
Natica Josephina, 107 ; swelling of foot,
119
Naticidce, 6
Nautiloidecti 22
Nautilus, 22 ; eye, 169 ; diagram, 37 ;
gonad.s, 230 ; nervous system,
145, 146 ; pallial complex,
82 ; tentacles, 117
Pompilius, 22
Nautilus group, 68
Nectria, 296
Needham's (spermatophoral) pouch, 238
Neocrinoidea, 303
Neomenia, 3
Neomeniidce, 3
Neomeniina, 3
Nephropneusta, 78
Neptunea contraria, 160
Neritacea, 4
Neritidce, 5
Neritince, 5
Neritopsidce, 5
Notaeum, 10
Notarchus, 10
punctatus, nervous system, 141
Notaspidce, 10
Notobranchcea, 90
Notobranclweidai, 11
Nuchal plate, 126
NudeoUastidce, 315
Nucula, 14 ; ctenidia, 85
nucleus, 14
Nuculidce, 14
Nudibranchia, 12
OCTOPODA, 24
Octopodidce, 24
Octopus, 24 ; anatomy, 147 ; nervous
system, 148
vulgaris, 25; gonad, female, 231 ;
male, 239
Ocular plates (Echinoid.), 321 ; (Ast.) 354
Odontoblasts, 183
Odontophore, 336
Oigopsidce, 24
Oligopygus, 293
Oliva, 30
INDEX
613
Olividce, 6
Ollacrimts (see Gilbertsocrinus)
Ouimastrephes, 24 ', gonad, female, 230 ;
shell, sectiou, 69
Ommatophore, 102
Ommatostrephes, 24 ; nervous system, 148
Oncididla, 46
Onci'Uidce, 8
0/ici'Uum, 44, 104
celticum, genital organs, 235 ;
larva, 256
Oneirophanta mutabilis, rod, 237
Onychoteuthis, 24
Operculum, 31
Ophiamntha, 300
,, marsupialis, 495 ; vivipara,
495
Ophiactis, 300
MiUleri, 506
poa, 300
Savigny, 506
virens, 498 ; disc, section, 426
Ophiarachna incrassata, vertebral ossicles,
356
Ophidiaster Germani, 421
Ophi'.'ceramis, 327
Ophiocnida, 300
sexradia,, 506
Ophiocoma, 300
pumila, 506
Valencia, 506 ;
Ophiocreas, 301
Ophioderma ( = 0phiura), 495
Ophiodiaster, 298
diplax* regeneration of arms,
505
Ophioglypfui, 300 ; bursa, 495, 496 ; disc,
section, 497
albida, stomach, 494
hexactis, 495
lacertosa, ovary, section, 497
Ophioglyphidce, 300
Ophiohehis umbella, arm joint, 357
Ophiomastix, 327
Ophiomitra exigua, 327
Ophiomusium, 300
ralidum, apical system, 327
Ophiomyxa, 300
I'ii-ipara, 495
Op/iiumyxidce, 300
Ophiontreis, 422
Ophwplocus, 422
Ophiopteron elegans, brachial joints, 391
Ophiopya longispinus, oral skeleton, 359
Op h iopyrgus, 327
Ophiothela dividiia, 506
isidicola, 506
Ophiotholia, 392
Ophiothrix, 300
fragilis, ambulacral tentacle,
section, 466 ; nervous sys-
tem, 456
Ophwzona, 300
VOL. II
Ophiura, 300
Ophiurce, 300
Ophiuroidea, 299-301 ; arm, section, 412 ;
arm, joint, 357 ; disc, section, 486 ;
Pluteus, 533 ; nervous system, 456 ;
ring sinus, 496
Opisthobranchia, 10, 33 ; heart, 199
Opisthopneumonia, 76
Oral angle plates, 358
lobes (Lam.), 105
Orocystis Helmhackeri, 313
Orophocrinus, 315
stelliformw, 314
Orphnurgits, 418
Orthoceras group, 68
Orthopsis, 299
Oscainius, 10
Osphradium, 84, 162-164
Ostracoteuthis, shell, section, 69
Ostrea, 17
edulis, anatomy, 16
Ostreidce, 17
Owenia, 24
Oxygyrus, 108
Oxynoe, 12
Oxynoidea, 12
PAL&ASTER, 295
Palceasteroidea, 295
Palceechinoidea, 288
Pcdceechinus, 289
elegans, 289
Palceobrissus, 293
Palceocoma, 295
Palceocrinoidea, 303
Murrayi, 294
Palceostoma, 295
Palceotropus, 429
Pallial cavity, 26 ; line, 64, 124 ; sinus,
64, 124
Palliata ( = Tectibranchid), 46
Pallium, 26
Palmars (Grin.), 371
Palmipes, 297
Paludina, ctenidium, 85
vivipara, circulatory system,
203 ; development, 252, 254,
255
Paludinidce, 5
Pancreas (Ceph.), 196
Pandvridce, 21
Pannychia, 285
Papula, 439
Paractinopoda, 288
Paramenia impexa, 216
palifera, 184
Parameniidce, 3
Parapodia, 106
Pa rar chaster, 296
Parasalenui, 290
Parasira (Tremoctopus) catenulata, 230
Parelpidia, 418
2 R 2
614
COMPARATIVE ANATOMY
Parmophorus (Scutum), 5
Patella, 72 ; nephridia, 218 ; nervous
system, 138 ; section, 188
,, vulgata, 4
Patellidce, 5
Patinella, 108
Paxillse, 391, 503
Pecten, 17 ; eye, 174
Jacobeus, 16
Pectinibranchia, 5
Pectinidce, 16
Pectinura, 300
Pectunculus, 15 ; eye, 175
Pedicellarise, 393-399
Pedicellaster, 299
Pedwellasteridce, 299
Pedina, 290
Pedipes, 109
Pelagothuria natatrix, 285, 286
Pelagothuriidce, 285
Pelanechinus, 290
Pelecypoda, 14
Pelmatozoa, 302-315
Peltastes, 290
Peltella palliolum, 9
Peltidce, 10
Pen (Ceph.), 69
Peneagone, 285
Pentaceros, 297
turritus, oral skeleton, 473
Pentacerotidce, 296
Pentacrinidce, 313
Pentacrinus, 313
decorus, 377 ; calyx, section,
382
Pentactcea, larva, 548
Pentactula, 516
Pentagonaster, 296
PentagonasteridcK, 296
Pentephyllum, 315
Pentremites, 315, 314 ; ambulacrum,
section, 382 ; apical system, 330 ;
organisation, 380
Pentremitidce, 315
Pentremitidea, 315
Peradis, 11
Periechocrinus, 307
Periostracum, 58
Periscoechinoidea, 289
Peristome, 339
Perna, 17
Ephippiwn, shell, 64
Peronella orbicularis, 424
Peronia (Moll), 45 ; (Ech.), 290
Perradii, 316
Petalodium, 347
Petricola, 52
Petricolidce, 18
Phcenoschisma, 315
Phanerozonia, 296
Pharus, 51, 115
Philine, 46
Philinulce, 10
Philonexidce, 24
Philonexis, 24
(Octopus) carence, hectocotylisa-
tion, 243
Pholadacea, 19
Pholadidce, 19
Pholadidea, 19
Pholadomyidce, 51
Pholas, 19
dactylus, valve, 67
Phormosoma, 290
Phorus exutus, 5
Phragmocone, 68
Phyllidiidce, 13
Phyllirhoe, genital organs, 238
,, bucephalum, 12
Phyllirhoidce, 13
Phyllobranchidce, 12
Phyllodes, 347
Phyllophorus, 287
urna, 502
,, fontinalis, 8
Physetocrinus, 307
Pinna, 17
Pinnules (Crin.), 371 ; (Blast), 381
Piracy 'stis, 336
Pisidium, 17
Pisocrinus, 304
Placobranchidce, 12
Placophora, 2
Placuna, 15
Planaxidce, 8
PlanorUs, 8, 58
Plastidogenic organ, 445
Plastron (Ecbinoid.), 348
Plates, included, 340 ; isolated, 340 ; half,
340 ; primary, 340
Platycrinidce, 307
Platycrinus triacontadactylis, 308
tuberosus, tegmen, 335
Platydoris, 13
Plesiocidaroida, 289
Plesiospatangidce, 293
Pleurae, 182
Pleurobranchcea, 10
Meckelii, genital organs,
237
Pleurdbranchia, 47
Pleurobranchidce, 10
Pleurobranchus, 18
aurantiacus, 10
Pleuroleuridce, 13
Pleurophyllidia lineata, 13
Pleurophyllidiidce, 13
Pleurotomaria, 5 ; shell, 59, 156
Pleurotomaridce, 5
Pleurotomidce, 6
Pliodon Spekei, 125
Pluteus, larva, 508, 509, 522
Plutonaster, 296
Pneumoderma, 11, 79
Pneumodermatidce, 11
INDEX
615
Podocidaris, 290
Podocyst, 258
Polian vesicles, 416, 423
Polycera, 13
Polyplacophora, 2
Polytremaria, 5 ; shell, 59, 156
P&mpholyx solida, 160
Porcelainous layer of shell, 57
Porcellanaster, 296
Porcellanasterid.ee, 296
Pore rhombs (Cyst.), 384
Porocrinus, 313
Poromya, 51
Poromyidce, 21
Postpalmars (Grin.), 371
Poteriocrimis, 304
Pourtalesia, 295
Je/reysi, 295 ; apical system,
325
Paurtalesiidce, 295
Prseputium, 235
Prismatic layer of shell, 57 .
Proboscidifera holostamata, 6
siphonostomata, 6
Promachocrinus, 313
Proneomenia, 3
Sluiteri, 3 ; nervous system,
132 ; sections, 42
vagans, 184
Proneonieniidce, 3
Proostracum, 68
Propodium, 106
Prosobranchia, 4 '> gills* 90 5 heart, 199 ;
proboscis, 179 ; sections, 110 ; snout,
section, 181 ; tentacles, 102
Prosoplax, 64
Prostata, 233
Protobranchia, 14 ', gills, 92
Protocrinus, 313
oviformis, 312
Prunocystis, 332
Pr;j,nnadetes, 293
PrynDiodesmia, 293
Paammobia, 18
Pxiiiiriiiobiidce, 18
Pseudarchaster, 296
Pseudoconch, 47
Pseudolamellibranchia, 15
Pseiidomelanidce, 6
Pseudomonocyclic Crinoids, 329
Psolus, 287
ephippifer, 287
Psychropotes, 285
lonyicavda, 285
Psychropotidce, 285
Pteraster, 298
Pterasteridce, 298
Pterasterince, 503
Pterobranchia, 12
Pterocera, 107
Pteropoda, 10 ; larva, 257
gymnosomata, 11
thecosomata, 11
Ptero&rachceidce, 6
Pterotrachea, 109
auditory organ, 168
(Firola) coronata, 7
Ptychodera, 562
aurantiaca, 585
bahamensis, 585
clavigera, 570
,, erythrcea, 585
minuta, 562 ; branchial region,
section, 562 ; head region,
section, 566, 568 ; ccelom,
diagrams, 576
Pulinonata, 8, 32 ; eye, 170 ; radula, 183 ;
renal ducts, diagram, 75
Pupa, 8
Pupidce, 8
Purpura, 101
Purpuridce, 6
Pygaster, 291
Pygurus, 293
Pyramidellidce, 6
Pyriform vessel (Gastr.), 220
Pyrula, 44
tuba, 73
Pythonaster, 298
EACHIAL teeth, 182
Rachiglossa, 6
Radial shields, 362
Radials, 318
Radiolitidce, 18
Radula, 177, 180-183
Red-brown organ (Keber's organ), 215
Regulares, 315
Reptantia, 10
Requienia, 18
Metaster, 298
Rete mirabile (Hoi.), 452
Retiocrinidce, 306
Retiocrinus, 306
Rhabdopleura, 600-602
Normani, 601 ; section,
602
Rhachiglossa, 6
Rhinophore, 48, 103
Rhipidocrinus, 306
Rhipidoglossa, 4
Rhizochilus, 183
Rhizocrinus, 311
lofotensis, 335 ; axial canals,
378 ; stone canal, 423
Raivsoni, 335
Rhodocrinidce, 306
Rhodocrinus, 306
Rhodope Veranii, 281
Rhopalodina, 287
derivation of, 408
Rimula, 43
Ringiculidce, 46
Rissoidce, 108
Rizzolia, 13
Rossia, 24
616
COMPARATIVE ANATOMY
Rostellaria, 102
rectirostris, 6
Rostrifera, 6
Kostruin (Ceph.), 68 ; (Gastr.), 102
Rotula, 293
Rotulae (Echinoid.), 400
Rudistes (ffippurites), 62
Huff (Gastr.), 107
SACCULI (Grin.), diagram, 490
Salenia, 290 ; apical system, 319
Saleniidce, 290
Saxicava, 19
Scceurgus, 243
Scalariidce, 6
Scaphander, 46
Scaphandridce, 10
Scaphites stage, 68
Scaphopoda, 13
Schizaster, 293
canaliferus, pedicellariae, 397
lacunosus, 294
SchizoUastus, 315
Schizocardium, 562
brasiliense, gills, section,
569
Schizogony, 505
Schwammerdam's vesicle, 233
Scissurella, 5
Scotoplanes, 418
Scrobicularia piperata, 52
Scrobiculariidce, 51
Scurria, 5
Scuta buccalia, 360
Scutella adoratia, 360
Scutella, 293
,, sexforis, 292
Scutellidce, 291
Scutum (Parmorphus), 4
Scyllceidce, 13
Scytaster, 326
Sea urchins ( = Echinoid. ), 316
Semiproboscidifera, 6
Semites ( = fascicles), 349
Semper's organ, 166
Sepia, 224 ; alimentary canal, 189 ; body
cavity, 213 ; cephalic cartilage,
126 ; ctenidiura, 85 ; diagram, 36 ;
gill, 96 ; gonad, male, 231 ; onto-
geny, 266, 267 ; nervous system,
148 ; renal sacs, 213 ; shell, sec-
tion, 69 ; spermatophore, 242
aculeata, shell, 70
officinalis, circulatory system, 209 ;
eye, 172 ; genital organs, female,
240 ; genital organs, male, 239 ;
renal sacs, 224
Savigniana, 83
Sepiadarium, 24
Sepiola, 24 ', nervous system, 148
Sepioloidea, 54
Sepiotei'this, 24
Septibr* nchia, 21 ; gills, 92
Sickles = radii (Echinoid.), 400
Silenia Sarsii, 21
Siliquia, 115
Sinistral twist (Gastr.), 56, 60, 160
Sinupalliata, shell, 63
Siphon (Ceph.), 37 ; = accessory intestine
(Echiuoid.), 481 ; (Gastr.), 43; (Lam.),
exhalent, 49 ; (Lam.), inhalent, 49
SipJwnodentalium, 13
Siphonoplax, 64
Sipfwnostomata, 44
Siphuncle (Ceph.), 37
Solaridce, 6
Solasteridce, 298
Solen, 19
Solenidce, 19
Solenoc^lrtus, 19
Solenogastres, 3
Solenogastridce, section, 212
Solenomya, 91
Solenomyidce, 14
Solenopoda, 228
Spadix, 116
Spatagocystis, 295
Spatangidce, 293
Spatangoida, 293 ; ambulacral brush,
section, 433
Spatangoidea, larva, 509
Spatangomorpha, 293
Spatangus, 293
pvirpureus, apical system, 324 ;
oral region, section, 441
Speugel's organ, 84
Spermatophore, 241, 242
Sphcerechinus, 290
granularis, pedicellarue, 397,
398
Sphseridia (Echinoid.) 392 ; section, 392
Sphcerium, 17
Sphceronis, 336
Spiculum amoris, 237
Spiracles (Blast.), 382
Spirula, 24 ', shell, section, 69
prototypos, 23
Spiwilirostra, 24 ; shell, section, (
Spondylidce, 53
Spondylus, 62
Spongiobranchcca, 11
Spongylocentrotus, 290
Star-fish ( = Asteroidea), 316
Steganobranchia, 12
Steganocrinus, 307
Stelidiocrinus, 307
Stellaster, 296
Stelleridea (= Asteroidea), 295
Stenoglossa, 6
Stereosomata, 290
Sternum, 348
Stewart's organs (Echinoid.), 442
Stichaster, 298
albulus, 506
INDEX
617
298
Stiehopus, 285
j< iconic us, rod and supporting
plate, 337
Mil > ft'/- Liu'-kii'; sections, 245, 246
Stolasterias ( = Asterias volseUata), 396
8t/ //i>li'nru.s, 290
Terebra, 102
Terebridce, 6
T>>rHna, 65
Teredinidce, 21
Teredo. Jl
n