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iii
24503377736
TEXT-BOOK
OF THE
EMBRYOLOGY OF INVERTEBRATES
By Prors. KORSCHELT ann HEIDER.
TEXT-BOOK OF THE
EMBRYOLOGY OF INVERTEBRATES.
Vou. I.—Porifera, Cnidaria, Ctenophora, Vermes, Enteropneusta,
Echinodermata. 15s.
Vou. I1.—Phoronidea, Bryozoa, Ectoprocta, Brachiopoda,
Entoprocta, Crustacea, Palaeostrica. 12s.
Vou. ITI.—Arachnida, Pentastomidsae, Pantopods, Tardigrada,
Onychophora, Myriopoda, Insecta. 14s,
Vou. IV. 188,
vi PREFACE,
The Tunicates, more than any other group, seem of recent
years to have occupied the attention of embryologists, and the
large amount of work which has been done on this group,
especially in France, with regard to both the sexual and the
asexual methods of reproduction, will be gathered from the
additional literature appended to Chapter xxxv., only a small
proportion of which could be referred to in the footnotes.
In the Mollusca also a great deal of work has been done,
eapecially in connection with cell-lineage, and the formation of
the mesoderm and of the larval kidney, in spite of which the
last two points still remain obscure. Since I am more familiar
with the Mollusca than with any other group of Invertebrata,
LT have revised the chapters dealing with this phylum some-
what more thoroughly than the rest of the volume: I have
appended numerous notes, inserted some fresh paragraphs, and
wade certain alterations in the text which appeared justified
by recent: investigations
T must again express regret that so long a time has inter-
vened between the publication of the German edition of this
work and the appearance of the last volume of the English
translation, Volumes ii. ili. and iv. came into the hands of
the translator only in 1897, and the task of bringing them out
being necessarily somewhat lengthy. it bas been impossible
suoner to offer the completed work to the English-speaking
student. to whom it should be of great service.
MARTIN F. WOODWARD.
Rov Consus vr Sciescs. Loxpos.
Fre, W9OU.
CONTENTS OF VOL. IV.
Caarren XXIX. AMPHINEURA. By E. Korscuetr.
I. The development of Chiton . See eS
1. Tho cleavage and the formetion of the germ-layes
2, The development of the larval form . .
8. The further development of the larva and metamorphosis.
I, The development of Dondersia =. 0. ww wwe
Literature .
Cuarrer X LAMELLIBRANCHIA. By E. Korscurut
1. Oviposition and care ofthe brood . . 1 7 wt
2. Cleavage and formation of the germ-layers Ree 1S
8. Development and structure of the Trochophore larva.
A. The Trochophore stage ass free-swimming larva.
B. The Trochophore stage of fresh-water Lamellibranchia .
4. The transformation into the adult . . . . .
Divergencies in the metamorphosis accompanying the
Monomyarian condition. . 3 . . . 2.
5. The development of the Unionidae. . . .
A. Development of the early stage
B. The development of the embryo into the parasitic larva
C. The transition tothe adult. . . . . .
6. The formation of the organs ‘
A.Thesholl. . . 1 we
B.Thenervoussystem. . . . 1 wee
C. The sensory organs. ww wee
The eyes of Pecten Bite! Loni He 8
The otocyuts. 5
Spengol's olfactory organs, abdominal vensory organs.
D. The alimentary canal gs
E.The gills.
F. The body-cavity, the blood-vascular sytem and the
kidney 2.
‘The formation of the heart ee
G. Musculature and connective tissue . . a a
H. The genital organs. ws
Literature . . . . ‘s
Ss
viii CONTENTS.
Cuarrer XXXI. SOLENOCONCHA. By E. Korscuett .
1, Cleavage and formation of the germ-layers .
2. The development of the form of the larva =. .
3. The transformation of the larva into the adult .
Literature =. ae = . . . > + < .
Cuapren XXXII. GASTROPODA. By E. Korscnetr .
1. Oviposition and character of the egg-capsules and the eee
2, Cleavage and formation of the germ-layers ‘
8. The formation of the germ-layers .
The mesoderm. .
4. The rise of the larva and its relation to the adult form .
The ontogeny of Patella . . .
The Trochophore stage . . .
The Veliger stage. a
The development of Paludina |.
The primitive kidneys . ue fs
The shape and transformation of the blastopore. .
Considerations relating to the asymmetry of the Gastropoda
. The development of the external form of the body in the
different divisions of the Gastropoda
A. Prosobranchia . . . . . . .
Pa
B.Heteropoda . 1 www
!, Opisthobranchia 2 . . 1 ee
D. Pteropoda 6 wwe a oS
Thecosomata oP REM St on a ces ce NS
Gymnosomata . 5 we
&. Pulmonata . . .
The ontogeny of the Stylomatophorous ‘terrestrial
Pulmonates . Fs . . .
6. The formation of the organs. we wee
A. The shell . . . . . . . . .
B. The nervous system .
(. The sensory organs .
Theeyss . . .
Theotocysts. 2. 0. ewe
Theosphradium . 7 ww ee
D. The pedal glands
E. The alimentary canal
F.Thegils. 9.2. :
G, The differentiation of the mesoderm. rudiment, the
development of the body-cavity, the nephridial and
circulatory systems 5 ww ewe
H, The genital organs.
Literature . . . . .
III, CEPHALOPODA. By E. Korscnxtt .
CHapTeR XX:
1. Oviposition and the constitution of the egg :
2. Cleavage and formation of the germ-layers . . .
x CONTENTS.
Cuarren XXXV. TUNICATA—continued.
Themantle. . © . . e. $8
The nervous system. 7 :
The ciliated pit . 3 5 3
The chorda . - . . . c
Mesoderm, body-cavity, musculature * : ie
The alimentary canal . a z . a
Peribranchial, atrial or cloacal cavity . . . . .
The heart, the pericardium and the epicardium .
E. Review of the organisation of the free-swimming larva.
F. Fixation and retrogressive metamorphosis
‘The development of new gill-slits . .
‘The development of the genital organs =. |
G, The abbreviated development of the Molgulidse
. Doliolum. . a er Ha sis! aah” VG oe
7 Pyrosoma. . bg ia, headiee a
4. Cleavage and formation of the germ-layers |.
B. Development of the Cyathozooid 5
C. Development of the primary Tetrazooid colony.
D. Further development of the Cyathozooid
E. Development of the four primary Ascidiozooids
5. Tho Hemimyaria (Salpidae) . ae
A. Forms without covering folds . . . . . .
B. Forms with covering folds...
General considerations on the embryonic development
oftheSalpidsee. . . . . .
Il. Asexual reproduction... wt ‘
1. Social and composite Ascidians
‘4. Reproduction through transverse fission .
B. Stolonic gemmation .
C. Pallial gemmation of the Botryllidae
D, Budding of the Didemnidse and the Diplovomidae
E. Development of the organs in pi secrneny produced in-
dividuals . . . . . . .
2. Doliolidae . . Boe be As
Alternation of generations in Doliolum . . .
Alternation of generations in Anchinia . . . z
Alternation of generations in Dolchinia .
3. Pyrosoma. i
A. Development of the proliferating stolon #
B. Further development of the buds.
4. Salpidae . S ‘ :
4. Structure and development of the proliferating stolon
B. Development of the buds on the stolon . .
C. Development of the organs inthe bud . .
5. The interpretation of the alternation of generations in the
Tunicates . . . . . . .
III. General considerations on the “punicates
Literature
CONTENTS.
Cmapren XXXVI. CEPHALOCHORDA (Aeros): By K.
HEIDER . . . .
Amphioxus . eee eral
A. Oviposition, cleavage and ‘gastrulation a és
B, Development of the medullary tube, the primitive seg-
ments and the notochord
C. Farther development up to the time when the mouth
and the first gill-cleft form . - . . . .
D. Later larvalstages . . . ww
General considerations on Amphioxus. - www.
Literature . . . . . . . . . . .
Susszcrs Inpex Be
Aurnors INDEX . . . . : ots
vi PREFACE,
“The Tunicates, more than any other group, seem of recent
years to have occupied the attention of embryologists, and the
large amount of work which has been done on this group,
especially in France, with regard to both the sexual and the
asexual methods of reproduction, will be gathered from the
additional literature appended to Chapter xxxv., only a small
proportion of which could be referred to in the footnotes.
In the Mollusca also a great deal of work has been done,
especially in connection with cell-lineage, and the formation of
the mesoderm and of the larval kidney, in spite of which the
last two points still remain obscure. Since I am more familiar
with the Mollusca than with any other group of Invertebrata,
I have revised the chapters dealing with this phylum some-
what more thoroughly than the rest of the volume; I have
appended numerous notes, inserted some fresh paragraphs, and
made certain alterations in the text which appeared justified
by recent investigations,
I must again express regret that so long a time has inter-
vened between the publication of the German edition of this
work and the appearance of the last volume of the English
translation. Volumes ii., iii. and iv. came into the hands of
the translator only in 1897, and the task of bringing them out
being necessarily somewhat lengthy, it has been impossible
sooner to offer the completed work to the English-speaking
student, to whom it should be of great service.
MARTIN F. WOODWARD.
Royar CoLircr or Sciexcr, Loxpon,
June, 1900.
CONTENTS OF VOL. IV.
Carrer XXIX. AMPHINEURA. By E. Korscuerr.
I. The development of Chiton re
1. The cleavage and the formation of the germ-layera . .
2. The development of the larval form
3. The further development of the larva and metamorphosis
IL. The development of Dondersia A
Literature . . . . . & “
CHapren XXX. LAMELLIBRANCHIA. By E. Korscueit
1. Oviposition and care of the brood . 5 is 7 .
2. Cleavage and formation of the germ-layers pea
3. Development and structure of the Trochophore larva
A. The Trochophore stage as 8 free-swimming larva .
B. The Trochophore stage of fresh-water Lamellibranchia .
4, The transformation intotheadult. . . . .
Divergencies in the metamorphosis accompanying the
Monomysrian condition. . 3 . . 3 .
5. The development of the Unionidae . . * . .
A. Development of the early stage
B. The development of the embryo into the parasitic ‘larva
C. The transition to the adult o
6. The formation of the organs. ‘ 2 ts
A. The sholl . o
B. The nervous system . Ca as ky Ae
©. Thesensory organs. ww ww wtt
TheeyesofPecten . . . . . .
The otocysts .
Spengel's olfactory organs, abdominal sensory organs .
D. The alimentary canal ie ts
E. The gills .
F. The body-cavity, the blood-vascular sytem and the
kidney .
The formation of the heart . . . .
G, Musculature and connective tissue . . . .
Hi. The genital organs .
Literature. if 5 v “i
»
=
&
™
SBSSSSSE HSSSREE BRawnwwe
o
8
BSERBSEB
2 AMPHINEURA,
I. The Development of Chiton.
The external characters of the larva of Chiton and the develop-
ment of the same were described many years ago by Lovin
(No. 10), while the formation of the germ-layers and the various
internal changes have since been investigated with the assistance
of modern methods by Kowauevsky (Nos. 6-8).*
Oviposition and Nature of the Eggs, The eggs pass from the genital
duct into the mantle-groove of the female in an immature condition,
and are there fertilised by the sperm which the male has discharged
into the water ; after this they are deposited either singly or in groups.
They are found floating freely in the water; but, in some cases, as
in C. Polii, the eggs are retained, until the embryo is mature,
within the mantle-cavity of the female. ach egg is enclosed
within a spiny shell, the surface of which is marked out into poly-
gonal areas, The form of these spines varies in the different species
and genera. The egg itself does not appear to be yery rich in yolk,
1, The Cleavage and the Formation of the Germ-Layers.,
The cleavage is total and practically equal, the egg being divided
by two meridional grooves first into two and then into four blastomeres.
of almost equal size. At the eight-blastomere stage, however, a
slight differentiation of the cleavage-spheres lying at the animal pole
takes place, four larger (vegetative) and four smaller (animal) blasto-
meres being distinguishable. The animal pole is marked by the
polar bodies which lie almost exactly over the point of intersection of
the cleavage-planes. During the further course of cleavage, the
blastomeres of the vegetative half are at first to be distinguished by
their larger size, the smaller size of those of the animal pole being due
to their more rapid division. In the later stages, as in the earliest,
there is also a certain resemblance to the conditions found in the
Gastropoda which are characterised by a stage consisting of four
macromeres and four micromeres (Fig. 40 C’, p. 109). We find here the
tres are Mollusca at all, According to Sepewick, the most important
Gitterence between the Solenogastres and all other Mollusca is that in the
former the gonad opens directly into the pericardium. This distinction does
not appear to us to be so vital when we remember that in some Molluscs the
gonad communicates directly with the kidney, the latter in turn opening into
the pericardium, and further that the cavities of all three organs are parts of
the primitive coelom.—Ep,]
*[Mercaxx (No. L.) has since reinvestigated the embryology of Chiton, pay-
beepers Seaton! Ne the cleavage and cell-lineage, his observations entirely
confirming those made by Kowanevsky,—Ep.]
THE CLEAVAGE AND THE FORMATION OF THE GERM-LAYERS. 3
expression of a kind of radial symmetry, which is still more marked
in the Gastropoda at a somewhat later stage (Fig. 40 D and Z), and,
aceording to Kowavevsky's figures, is sometimes also met with in
Chiton. At first the vells increase in unmber in a more regular
manner than in the later stages,
Cleavage results im the production of a somewhat flattened, and
therefore hemispherical blastula, the vegetative pole of which consists
of comparatively few but very large cells (Fig. 1 A.) As the cells
:
= sections embryos of Chiton Polit at the blastula
2 Sain bi, blastopore ; m, rudiment of the mesoderm
ent 0 y ring (velum]. ‘
divide, an invagination of the vegetative half (B) takes
this way the cleavage-cavity, which was never large, is
The invagination-gastrula (B) which at first is
at depressed, now elongates in the direction of the invagina-
‘The archenteron also grows larger. In its wall, near the
re, there appear two cells which, as compared with the rest,
». 1.) finds a ba 2 blastocoele which is not wholly obliterated
ent.—Ep.)
4 AMPHINEURA.
are specially large (C, m). These cells which, as well as others
situated near them, at first lie in continuity with the entoderm,
represent the rudiment of the mesoderm. They soon shift out of the
series of entoderm-cells into the cleavage-cavity (D, m). The meso-
derm-rudiment which thus arises seems at first to have a regular
bilateral arrangement in keeping with its origin, t.e., two groups of
large cells lying near the blastopore can be seen, but this regularity
is soon lost, the cells, which subdivide further, becoming scattered.
In this respect, and perhaps also in the manner of its origin, the
mesoderm of Chiton may be compared with that of other Molluscs
(Lamellibranchia, p. 29, Gastropoda, p. 117).
2. The Development of the Larval Form.
Even before the development of the germ-layers has progressed
thus far, altorations take place in the external shape of the embryo.
Two adjacent rows of cells in the ectoderm of the gastrula become
distinguished from the rest as bearing cilia (Fig. 1 C, w), and these
divide the larva into an anterior and a posterior section. Similarly,
@ group of cells lying at the pole furthest away from the blastopore
becomes covered with cilia. These two groups of ciliated cells are
the rudiments respectively of the ciliated ring [velum] and of the
frontal or apical ciliated tuft (Figs 2 aud 3, 2 and ws). Very similar
embryonic stages are met with in the ontogeny of other Mollusca, ¢9.,
Patella (Fig. 50, p. 124). The pre-oral ciliated ring in the Lamelli-
branch larvae is aleo formed of two rows of cells. Indeed, the
ciliated ring seems usually to be biserial ; though, in Patella, there
are three rows of cella (Figs. 52 and 53, pp. 126, 127).
As the body extends in the direction of its principal axis, the blas-
topore, which has hitherto lain at the posterior pole, assumes another
position and form. It shifts to that side of the larva which later
becomes the ventral surface, and, owing to the growth of the dorsal
surface, gradually approaches the ciliated ring (Fig. 1 BD). The
blastopore, as it shifts its position, loses its circular form, and, as far
as we can make out from the figures, assumes the form of a trans-
verse slit (Fig. 3 4). Meantime, the continuous growth of the dorsal
surface causes the aperture to shift more and more towards the
ciliated ring, and it is finally found immediately behind it (Fig. 2 4}
This slit-like aperture, however, no longer corresponds fully to the
blastopore, since the ectoderm surrounding the latter has sunk below
the surface, and the uctual primitive mouth thus comes to lie at the
inner end of a laterally compressed ectodermal tube which for some
‘THE DEVELOPMENT OF THE LARVAL FORM. 5
time longer continues to deepen (Fig. 2 A, oe). This ectodermal
invagination, the stomodaenm, represents the rudiment of the fore-gut
(buceal mass and oesophagus). In connection with it there appears
later, as a ventral outgrowth, the radular sac (Fig. 2 B, 7).
‘The “shifting " of the blastopore just described agrees closely with the
processes to be met with in the Gastropoda (p, 141), and we are inclined in both
cases to assume that we are really dealing with the closing from behind forward
of an originally slit-like blastopore.
Pro. and 2, median longitudinal sections through embryos of Chitow Polit at
SS Se os Tel
‘The more active growth of the part lying behind the ciliated ring
is accompanied by reduction of the anterior section which formerly
preponderated (Figs. 1 and 2). The embryo, which is now almost
pear-shaped, may become free at this stage (¢.9., Chiton marginatus,
Loves). The larvae of this latter form carry a large ciliated tuft at
the frontal pole (Fig. 3.4). The embryos of other Chitones remain
Jonger in the egg, and before attaining free life approach more nearly
‘the form of the adult (Fig. 3 ©).
‘The larwne of the Chitones resemble those of the Annelida, and since a
phore exceedingly like that of the Annelida is found in other Molluscs
(Pigs: 1S, 51, 59), we are justified in instituting such a comparison here also,
a gh the resemblance is not so close. We have here a pre-oral
ting, and the origin and position of the different sections of the
tinal canal is the same asin the Trochophore, The larva, at first, has no
terminal segment of the alimentary canal only appears later ab
“the posterior end of the body in the form of an ectodermal invagination, the
oko
6 AMPHINEURA.
proctodaeum (Fig. 9). An organ which is of great importance in interpreting
the larva, the apical piate, is not present in the eariy stages of Chiton, but
the cerebral ganglia arise later in the position which this organ occupies in
the Annelida; in the free-swimming larva of Chizon Polit these ganglia are
found beneath the ciliated tuft on the frontal pole (Fig. 5.), and may therefore
be regarded as representing the apical plate. Thus, to make the comparison
complete, only the primitive kidneys are wanting. So far these have not
been found, although they occur in other Molluscs (pp. 39 and 136).
Fis, 7A, larva of Chilo marginatus (after Lovés); B, embryo; and C, larva of
Polit (after KowAanewsky) ; ; &, rudiments of sbeli-plates; in, month ;
a spines; 1, ciliated ring wel}; ys apical ciliated tuft.
3 The Further Development of the Larva and Metamorphosis.
The changes that now take place in the larva are introduced by
considerable growth of the posterior region of the body (Figs. 2 and 5),
8 process which recalls the development of this same section in the
Annelidan Trochophore into the trunk region of the worm (Vol. i.,
Fig. 120, p. 269; cf. also Chapter xxxiii.). In the case of Chston,
it is especially the dorsal surface which increases in size (Fig. 5).
In some cases, these changes take place even in the embryo, and as
THE FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 7
these forms have received more attention than the others, they will
serve for description, as these processes have a very similar course in
all Chitones.
To the organs which we have already found in the larva, a new
one is udded at a somewhat early stage: this is a sac-like gland,
opening behind the oral aperture (Fig. 2 3, fd.), According to
Kowateysky this organ, whieh he regards as the pedal gland, is
unpaired and originates as a thickening and depression of the
ectoderm, but this origin has not been fully established. Structures
corresponding to the “pedal gland” will again be met with in the
Gastropoda. The nature of this organ in Chiton is not yet quite
clear, and, as it appears to degenerate later, its efferent duct closing
first, Kowanevsry regards it as a larval organ.
As the post-oral portion of the body elongates, the intestine also
increases in length (Fig. 5); but: its posterior end does not as yet
communicate with the exterior. It has already been shown that, in
the Annelidan 7'rochophore, the anus is wanting, or else the intestine
is in @ very backward stage of development.
The mesoderm which, as has already been described, first arose as
two groups of cells continuous with the entoderm, and shifted later
into the cleavage-cavity near the blastopore, has become richer in
cells, and with the progressive growth of the larva has extended
between the ectoderm and the entoderm. Now, as when it first
appeared, it shows « bilateral arrangement, i., in young embryos
(aged twelve hours or slightly older, Fig. 3 B), the mesoderm takes
the form of two compact cell-bands situated on the ventral side of
the embryo and lying internal to the ectoderm. We may claim
these as mesoderm-bands, since they later undergo division into a
splanchnic layer applied to the embryonic intestine and a somatic
| layer lining the ectoderm (Fig. + 4 and #). The cavity lying
between therm (coe) which is paired, is consequently to be regarded
| ae a trne coelom. It develops earlier in the anterior than in the
posterior part of the body. Thus, when two distinct cavities almost
‘lined with epithelium are to be found in front, there is still a solid
‘mass of cells in the posterior region. The reader will here doubtless
‘reeall conditions prevailing among the Annelida. It should also be
‘mentioned here that the regular arrangement of the mesoderm just
described ‘is soon lost, as the mesoderm-cells grow into the primary
in 4 manner similar to that observed in other Molluscs
(Figs. 5, 9). A large complex of mesoderm-cells, which at first
‘remains lying at the posterior end of the body (Fig. 5) yields later
}
8 AMPHINEUBA.
the chief material for the formation of various organs (circulatory,
excretory and genital organs)
‘THE FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 9
According to the figures before us, the mesoderm in Chiton shows a specially
primitive condition, the coelomic sacs being very Iurge, and by far the grater
part of the mesoderm formed up to this time being related to them (Fig. 4, 4
and §). In other Molluses they are much smaller and do not assume the
characteristic shape found in the Annelida. As rule, the mesoderm-bands
disintegrate very early. In the remains of thése bands, if not also in the
Seattered portions of mesoderm, a division into layers takes place which gives
rise to the pericardial cavity (which is thus coelomic in origin). It would be
‘of great value to ascertain the relation of the pericardium to the first (bilateral
and bilaminar) mesoderm-rudiment in the Amphineura, especially as in some
of them we find very primitive conditions prevailing in that the genital glands
are immediately connected with the pericardium and the genital products thus
pass direct into the latter, as, in the Annelida, they pass into the body-
cavity, and are conducted thence to the exterior by the nephridia (Soleno-
gastres).
The central nervous system in Chiton consists of the oesophageal
below). The Fra, 5,—Longitustinal section thi bryoni
© Sel earalle tre ger tied tet
f commis- ‘matt hg, viseeral ganglion ; m, mouth ;
geet mes, en: oe, eto Yr,
? lular sac; vell-rudiment ; 0, cil
from the ectoderm by — [velum); mv, & ated tuft. tee
of cell-strands, stages of which may be seen in Fig. 4.
4-0, pn, In.
~
10 AMPHINEURA,
We have already mentioned that we thought ourselves justified in comparing
the radiments of the cerebral ganglia with the apical plate of the Annelidan
Trochophore (p. 6). ‘The connection of this rudiment with the lateral
tranks of the nervous system in the larva cannot as yet be clearly established.
Kowanrysky speaks of the two organs as discontinuous. The cell-mass at
the posterior end of the body described as the visceral ganglion (Fig. 5, jig) is
(as regards its ganglionic nature) of the same significance and origin.
Actual ganglia, indeed, are not developed in the nervous system of Chifon,
where we find the ganglionic cells distributed along the entire length of
the longitudinal commissures, nevertheless it appears from KowaLEvsky’s:
ontogenetic researches that these trunks are greatly swollen both anteriorly
and posteriorly. The transverse commissures no doubt form in the same way
as the longitudinal trunks.
When the embryo (Chiton Polit and C. olivaceus) has attained a
stage somewhat like the one described above, it leaves the egg and
becomes a free-swimming larva. At this stage, a fresh differentia-
tion makes its appearance in the form of a segmentation affecting the
dorsal surface. Here, seven consecutive segments may be dis-
tinguished, separated from one another by shallow grooves (Figs. 5
and 9), These structures are indications of the future shell ; this.
dd
)
,
BA 2 cg
Fics. 6 and 7.—Two sections through the mantle-epithelium of Chiton Poli (after
Buomaren). A, oe papilla without spine; B-D, papillac in various stages of
spine-formation ; #, later of the same; bs, formative cells of the spines; ¢,
onticular covering of the body ; «p, mantle-epithelium ; m, mantle-tissue; s¢, spines,
eventually consists of eight plates, but the eighth segment only
appears at a later stage.
The ectoderinal skeleton in C/iton falls under two categories: (1)
the eight calcareous dorsal plates and (2) isolated spines and plates.
situated anteriorly, posteriorly and laterally to the former (Fig. 8,
at). These spines are of special interest, being a characteristic feature
not only of Chifon but also of all other Amphineura.
FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 11
According to KowauEvsky, the spines of the larval Chiton originate
within the cells—the cells, at those points where the spines arise
Inter, being richly vacuolated ; within these cells, the spicula are said
to appear as rudiments and only later break through the surface of
the body. The description of the embryonic development of the
spines given by Kowatnvsky is not easily reconcilable with the
observations made by other zoologists on their formation in the adult
Chiton (e.g., those of Reinoxs, No. 14, and recently those of
-Buosrrtcx, No. 1), and other Amphineura (Proneomenia, Tarene,
No, 1). According to these observers, the spines arise as cuticular
secretious in depressions of the mantle-epithelium, the whole being
covered by « thick cuticle (Figs. 6-8, ¢).
In the mantle-epithelium there are papilla-like swellings (Fig. 6 4) in which
‘at @ later period the formation of spines takes place. These papillae become
‘differentiated in the following way: broad cells appear at their bases, and to
these smaller cells become applied laterally (Fig. 6 B), the whole structure thus
Assuming the character of an cotodermal depression. A special basal cell (bz)
is said to be pre-eminently the formative cell of the spine, which is first to be
‘recognised as a small rounded structure within the papilla, It then increases
‘in size (Fig. 6 B), presses apart the cells of the papilla and passes out of the
latter (C and D). As the epithelial cells round the papilla continually secrete
cuticular substance, the spine is pressed upwards. The basal (formative) cell
from which it formerly rose is during this process drawn out like a thread
(Pig. 7 2), the spine for a long time remaining in connection with it. The
base of the spine, in contradistinction to the main calcareous shaft, consists of a
chitinous substance and surrounds the latter like a cup, the whole being now
_far removed from the smaller papilla-cells. The latter, probably, give the
characteristic markings, The formation of the spine is completed
y the secretion of @ peg-like terminal knob by the formative cell, from which
“spine then detaches itself. The neighbouring cells may also secrete
“another chitinous ring composed of several pieces round the base of the
actnal spine.
‘Tt is & remarkable fact that these spines somewhat resemble in their origin
‘the setae found in the Annelida in ectodermal depressions; this has already
Tepentedly been pointed out (Reixcke, Suwrur, v. Jnenrnc) and Harscune
‘and Taree have again recently laid great stress on this point. We shall refer
to it again later (Chapter xxiv.)
While the spines show a formation sui generis, the dorsal plates
are comparable with the shell-structures of other Molluscs. Their
“position on the back of the larva corresponds to that of the shell-
gland in the larvae of the Lamellibranchia and Gastropoda (c/. Figs.
4and 5; Figs. 56, p. 135, and 57, p. 137). As in these latter forms,
nN first appears above the epithelium (Fig. 9, c) and, beneath
euticle, the calcareous substance is then secreted (Fig. 9, 4).
12 AMPHINEURA,
Each plate begins to form independently ; the calcareous secretion
takes place first at the anterior border of each segment, and proceeds
backwards from this point. 5
‘The segmentation of the shell is a peculiarity of the Chitones and does not
occur in other Molluscs. Although the impression of segmentation is given
(Figs. 3 C, and 9), this term is not, strictly speaking, applicable here, since there
is no corresponding segmentation of the body. The shape of the shell is perhaps
rather due to its mode of origin. We have already seen that the spines are
cuticular structures, and this is of special interest since, in Molluscs as
primitive as the Solenogastres, they are the only hard structures of the integu-
Fic, 8.—Portion of a transverse section through a (hiton (diagrammatic, chiefly after
BrUMgicH): a, articulamentum ; ae, aesthetes; c, mantle-cuticle; ep, ectoderm
cavesing body and mantle, forming papillae in the mantle-region : Ay foot ; ys, cornice
asd Banh a mantle; &, gill, with the branchial artery cut through on its left
and the branchial vai on its right, between the two lies the lateral (visceral) uerve-
strand (m); vity ; wm, mantle; st, spines; ¢, tegmentum, with another
thinner and Mooctalle ‘differentiated cuticular layer above it.
ment besides the cuticle. It has already been mentioned that the spines may
assume a plate-like shape, and if we bear in mind the development of the
spines in such forms as very young Dondersia (of. Fig. 10 C, p. 17), it appears
probable that the dorsal plates of the Chitones may have developed out of such
modified spines,* either through the broadening of individual spines or through
the fusion of several to form a single plate. In this respect Dondersia is of
Special interest, for this form, in which actual dorsal plates do not occur,
* The derivation of the dorsal plates of Chiton from its spines was attempted
by Georynave (in his Grandriss der vergl, Anatomie) as early as 1878; his
views have recently heen adopted by BLuamicu, but THinLe assumes difforent
origins for the dorsal plates and the spines, and attempts to explain a part of
the former (the articulamentum) as an inner integumental skeleton,
FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 13
shows, in its youth, a remarkable resemblance to Chifon in the arrangement
of its broad, leaf-like dorsal spicula. The first appearance of the shell in the
Molluses will be discussed further in Chapter xxxiy.
‘The dorsal plates of Chiton consist of two layers, an upper layer which is
continued into the thick cuticle of the mantle, the so-called tegmentum, and
‘a subjacent calcareous layer, the articulamentum (Fig. 8, / and a), In tracing
the origin of the shell from spines, we should have to imagine the articula-
mentum as arising from the latter, which, as can easily be explained by their
origin, remained lying beneath the cuticle where they became expanded, The
cuticle above the calcareous plates which have thus arisen becomes the teg-
mentum of the shell. The retention of single modified spines or complexes of
spines, which is determined by the segmentation of the shell into separate
plates, would then be explicable by the manner of life of the animal, the body
of which, at first perhaps of considerable length, was able to roll up.
The shell of Chiton is characterised not only by its segmenta-
tion, but by the presence within it of cellular strands. These are
the aesthetes (Fig. 8, ae) which may be simple or branched, and are
accordingly composed of a smaller or a larger number of long cells.
‘These sometimes occur in the tegmentum and extend from its outer
surface, where each is covered by a cuticular cap, to a cornice-like
fold of the mantle at the lateral margin of the shell (Fig. 8, gs).
These cell-strands have developed from the epithelial cells of the
mantle, which underwent great elongation during the secretion of
the enticle through pressure of the surrounding cells. While that
part of the mantle to which they belong, and which secretes the
tegmentum, undergoes marked lateral displacement during the growth
of the shell, they, in consequence of their length, are able to retain
their primitive connection with the surface of the shell, and yet to
remain attached by their bases to the epithelium of the lateral
cornice of the mantle (Fig. 8). This latter epithelium is also partly
concerned in the secretion of calcareous shell-substance and, in this
way, the aesthetes appear to perforate the articulamentum which has
in reality been secreted round their bases. These structures have
been held to be sensory organs. They can hardly be tactile organs,
but they may perhaps serve other sensory purposes which are un-
known to us. The eyes discovered by Moseuey (No. 11) on the
shell of a few exotic Chitones must doubtless be regarded as further
modification of the aesthetes.
‘Figs. 5-9 show cloarly that the shell of Chiton is, just like that of other
Molluscs, a cuticular structure. While the articulamentum, in consequence
of lying immediately above the epithelium, can readily increase in thickness,
the tegmentum, from its position, must grow chiefly at its margin.
14 AMPHINEURA.
The development of the shell commences even during larval life.
But, since some Chitons leave the egg-membrane only at a very late
stage, the free-swimming stage is in their case of comparatively short
duration. The commencement of metamorphosis is indicated by the
compression and final degeneration of the cells of the ciliated ring
which from the first were distinguished from the surrounding ecto-
derm by their size and structure (Figs. 5 and 9 t). It must be
further mentioned that in the older larvae, two eves are said to be
Fic, 9—Median longitedinal section through a young CR, just emerging. from
the larval stage (after KOWALEVSET). a, rn ; ¢, cuticle of the shell,
darkened parts indicating, according to RowaLxvskt, those. points, at w
formation of the shell-plates is
the
h the
place ; ag, cerebral ganglion ; ecpola aod
Praleided portion of the shell mouth nc, cnteron aes messleney edule
sac: or, sub-radular organ ; sf. spines; w, ciliated ring.
present, which curiously enough lie, not in the actual pre-oral section,
i+. im front of the ciliated ring, but behind it (Fig. 3 C). These
eyes are still to be found in the young Chiten, but, whereas they
were at first superticial in position, they are now found beneath the
epidermis (Fig. 4 (’) and are consequently still nearer than before
to the lateral nerve-trunk.®
© [Peisexeen (App. Lit. on Lamellibranchia, No. IV.) regards these
exes of the larval Chiton as homologous with the cephalic eres which have
recently been discovered in the adult Myfilidae under the branchial filaments.
In the larva, they are situated outside the velar area as is the case also in
Chiton, whereas the eves of the typical Gastropods arise within the velar area.—
THE DEVELOPMENT OF DONDERSIA. 16
When the larva changes into the adult, the external form is modi-
fied not only by the continuous lengthening of the posterior region
of the body but also by the rotation of the pre-oral region towards
the ventral surface., A comparison of Figs. 5 and 9 shows clearly
the beginning of this process,
A further approximation towards the shape of the adult is bronght
about by the development of a fold growing out on either side of
‘the body at its dorso-lateral angle, thus giving origin to the mantle.
At the same time, the ventral surface becomes specialised by the
growth of its cells, which multiply and give rise to the flattened foot.
The area between the mantle and the margin of the foot becomes
the slightly invaginated mantle-cavity (Fig. 4 C).
Nothing, to our knowledge, is known of the origin of the gills, but,
considering the simplicity of their form, it is very probable that they
arise a8 papilla-like prominences on the body-surface. In the adult
they form on each side a row of consecutive bipectinate ctenidia.
Among the internal changes we note the formation of an ecto-
dermal invagination (KowaLEvsky) at the posterior end of the body ;
this is the proctodueum which gives rise to the anus and the base
of the intestine (Fig. 9, a). In the stomodaeum, the radular sac has
increased considerably in size, and the radula itself has already
appeared within it. In front of the latter another ventral outgrowth
‘of the wall of the stomodaeum has formed ; this widens later, and at
its base a thickened cell-mass can be recognised. We here appear to
have the so-called sub-radular organ of the adult, which has been
aeourately described by Hatter (No. 2). A further differentiation
of the alimentary canal is caused by a ventral swelling of the enteron,
which is perhaps the first indication of the liver (Fig. 9).
The further development of the larvae of Chiton has not'as yet
become known, but it is evident that they already resemble the
adult in various ways, apart from the incompleteness of their
internal organisation.
IL The Development of Dondersia.
The development of Dondersia banyulensis * has been investigated
‘by Pruvor. This form is allied to Proneomenia (Fig. 147, Chap.
xxxur, Huprecur, No. 5), possessing like the latter, a vermiform
body (40 mm. long and 1 mm. broad) capable of coiling up spirally.
[Smmora (Zeitschr. f, wiss, Zool., Bd. ivi., 1893) has created the genus
Beemets Pi tis species. Ee)
a
16 AMPHINEURA.
The doraal and lateral parts of the body bear spicules ; on the back,
these are inclined towards one another and form a projecting ridge
in the doraal-median line, elsewhere (especially at the sides) the
apines become flattened and imbricated.* A groove is found on
the ventral surface as in many other Solenogastres (Fig. 147 B,
Chap. xxx.)
Dondersia lays its egy singly. They are opaque and surrounded
by a delicate envelope which is only developed after the egg has
passed from the pericandium into the nephridia which serve as genital
ducts Acconting to Pruvort. the envelope is formed by the latter,
which ne longer possess the function of excretory organs.
The cleavage is from the first, slightly unequal ; the egg breaks up
iwtw tee unequal blastomeres which, through division, yield three
stwall cleavage spheres and one lanre one (77. Lamellibranchia, Fig.
WLC. The next stage. ane af eight cells. is reached by the biparti-
taon af the micromeres aani the unequal division of the macromere.
No forther monomers are cut off from the macromere. which now
Yreaks cp into tes ani then inte four macromeres, while the seven
primsry moomeees te in fourteen and then inte twenty-eight.
Tn thos squge. tie errno begins and the entadenm-celis dis
spear wks ite aa foemed by the more rapéily dividing eetoderm-
col, Thr enter:
The TevacTaI AC
Time. Vie sik agg. ctuated ares arsine ac the frontal pole. o.,
Tat thssteoure Pacvor. as well as round
Tome aia: demas WEN eocizeies the whole body
vw Sas a conical caplike shape. the aperture of
Thy uct +
shoot nal war
mar he oomneres
suhsted ting trees 63 ibe gastric la ssc.
lam anceramr
mit & dew spermlly
al The ace Hertuc, cettvimg the
THOT leg Gece anor aku Scumat of
yey ce we ad ak atthe ant ce wiant nes ie szectim 28 amesgimi-
yee DAEWOT cage te note ah Te imcgem. sd que vow
aa Qomencing af iw: srs oe
semng and long a.
aban tng and |
2 Ohne eremans wlnol sor onacmeme oR embiue meu umbenamd Te
SO aoa, RO Basar Sere ster oracee we Teron Na C2) om
Ph Oativevanaa Tre qustur mnemis wu guiiisa
Soap the x ment deencmmerc om she Summ,
hes NUISRA § ScARTM nt Shenae af squther
Veen ce nastics
THE DEVELOPMENT OF DONDERSIA. lq
that at first it is somewhat elongate and reaches (on the dorsal side ?)
almost to the ciliated ring, but later becomes circular (Fig. 10 A).
The further development of the larva, as described by Provor (4 and
B) makes his interpretation of this depression (at least in the later
stages) appear somewhat doubtful.
About this stage the embryo appears to become free, for the stage
depicted im Fig. 10 A is called by Prouvor a larval stage, The
wperture of invagination has already narrowed, and is terminal in
The further transformation of the larva is said to take place in the
following way, From within the invagination a button-like structure
gradually grows out, which corresponds to the posterior end of the
animal (Fig. 10, 8 and C), and carries at its end the remains of the
Fro. 10.—A-0,, stages in the metamorphosis of Dondersia banyulensis ; A and B, larva
Shertanorpons te Pver ee ae cnerr seven ie eats
. + dp. jo pero Pe ar
‘calcareous spicules
blastopore. This latter must be regarded as having been previously
displaced inwards. With the button-like terminal region a conical
part is also gradually evaginated (2), and this is destined to yield
the greater part of the definitive body. On this the leaf-like spicules
‘are already appearing (2, sp). These are said to arise, as asserted for
Chiton (jp. 11), as intracellular secretions, only breaking out of these
cells ax they increase in size. The conical part now grows consider-
ably, and new spicules continue to be formed ; the anterior part of
the larva, on the contrary, degenerates rapidly, and finally appears
only as a small collar at its anterior end. The larva is at last no
longer able to swim about in the water, for the cilia degenerate
together with the cells that carry them. The two posterior rows of
cells also (those of the third section, Fig, 10, 4 and 2) are thrown off.
S c
be
18 AMPHINEURA.
This section of the body is called by Pruvor the mantle-section in the
older larvae (B), and this name seems to be justified by its relation
to the adult body which is in process of formation. Between the
process of invagination just described and the formation of the adult
body, various other formative processes take place, but these apparently
occur within the larval body, and have so far not yet been investi-
guted. Essential differences are to be found between the development
of Doudersia and that of Chiton ; although, in the latter also, it is
chiefly the part of the larval body lying behind the ciliated ring that,
by its elongation, becomes changed into the body of the adult. The
pre-oral part of the larva (i.e., the part lying in front of the ciliated
ring) is here retained, as in Chiton, and yields the corresponding part
of the adult body. It appears that, in Donderaia banyulenais, there
are two lobe-like projections at the anterior end of the body, and that
these are already distinct in the young animal (Fig. 10 C). The
mouth is said to be wanting until metamorphosis commences; the
entoderm is present in the form of a solid mass, at the sides of which
two mesoderm-bands lie. This latter condition would agree with
that described for Chiton (p. 7), and is interesting inasmuch as
the mesoderm-bands in other Molluscs are not usually so distinct.
Unfortunately, no further details are known either of the internal or
of the external development of Dondersia. It should, however, be
stated that seven calcareous plates are found on the back of the young
animal which has only just undergone metamorphosis (Fig. 10 C),
these being formed of rectangular spicules arranged one behind another.
The resemblance of this stage to a young recently metamorphosed
Chiton is striking, the latter also possessiug seven such plates. In
what way this condition can be reconciled with that of the adult
Dondersia we do not know: this cau only be made clear by the
investigation of transition-stages. The voung animal has lateral
spicules which are plate-like and imbricated, and which, in the adult,
seem to be less developed. as far at least as can be judged from the
statements as yet made on the subject.
The form of the larva of Dondersia, like that of the Chiten, may be
traced back to that of the Trochuphore. We have already pointed
out various specially striking poiuts of resemblance between the
development of Dewiersia and that of Chiten. The elongation of
the posterior section of the body recalls here almost more than in
Chiten the growing of the post-oral seetion of the Annelidan Trocho-
pho into the trunk of the worn :Vol. i, Fig. 120, p. 269. This is
specially striking if we examine the Mirna: .. Fig. 124,
aa darva (Vol.
=
THE DEVELOPMENT OF DONDERSIA. 19
p. 276), in which the posterior end of the body is at first also sur-
rounded by the cap-like anterior part of the larva and hidden within
it. Since the Amphineura are very primitive representatives of the
Molluscan type, such comparisons are not unjustifiable, although the
‘unusual length of the body in this form alone determines a greater
ebange in the more compact larva, We shall return to this point.
again in diseussing the relationships of the Mollusea (Chap. xxxiv.)
[Pavvor Pre. TL.) bas neg sider @ preliminary account of the
stages ination-
identical in the two forms, Similarly, Proneomenia
Ac a divided into shses its and provided with a
tong. flagellum. Jay tl invagination (? archen-
Ree sceasiot corsapond 2 definitive entoderm, but gives rise to all the
‘issues of the trunk. By the tangential division of its cells, it gives rise to a
entodermic upon a single layer of cells; the latter in-
oreases by the radial division of its cells and infol forming three
; of these the middle one, which remains open, becomes the
while the two lateral ones close and are transformed
incontact with the proctodaeum behind. The next important change
eee a of three ventral invaginations of the larval ectoderm, just
circle of cilia on the middle segment; the median of these
eeamnsirs, the stomodaeum, is merely transitory, while the two
ones sre concerned in the formation of the ectoderm and mesoderm
vRaleee ‘These two unite, forming a transverse band capping the anterior
e the entodermic mass and prolonged posteriorly at two points to meet
trunk ; this portion appears to form the muscles,
more dorsal elements of the invagination form the cerebral ganglia.
of the plate seem to take no in the formation of the
Cepeda of the head ae to form entirely from
invaginations, while that of the trunk develops from the
¥ pos invagination. The latter is now completely evaginated,
the provisional imbric
Proneomenia is developed under cover of of kg ar ectoderm which
aw
+ the moment of meta-
is perfectly distinct and arises quite independently of
__A very striking resemblance, possibly of great significance, is to be noted
etween ee and certain Lamellibranchia (Drew,
ia, No. IL). In Foldia, we find the young, Molluse
a pair of invaginations at the antero-ventral
20 AMPHINEURA,
end of the larva. As in the Solenogastres, the larval locomotory ectoderm is
thrown off during Pheciad Fone ‘The resemblance in the early cleavage
has been already pointed ont, These yery striking resemblances, suggesting
as they do a connection between the Solenogastres and the Lamellibranchia,
require further and more complete investigation.—Ep.]
LITERATURE.
rar
. Buvaricn, J, Das Integument der Chiton. (With a pre-
face by Hatschek, treating of the position of the Chitones
or Amphineura). Zeitschr. f. wiss. Zool., Bd. Iii, 1891, A
critical account of this treatise, with remarks on the integu-
ment as well as on the systematic position of the Chitones,
by Thiele. Biol, Centralbl. Bad. ii. 1891.
2. Haier, Bena. Die Organisation der Chitonen der Adria.
Theil. i. u. ii. Arb. Zool. Inst. Univ. Wien. Bad. iv. a. v.
1882 u, 1884.
3, Hansen, G. A. Neomenia, Proneomenia und Chiitoderma,
Bergens Museums Aars beretning for 1888, Bergen 1889.
Husrecat, A. A. W. (1) Proneomenia Sluiteri. Nederland.
Archiv. Zool. Suppl. 1., 1881. (2) A contribution to the
morphology of Amphineura. Quart. Journ. Micro, Sci. Vol,
xxii. 1882.
5. Huprecat, A.A. W. Dondersia festiva. Donders-Feestundel
Nedert, Tijdschr. 1888,
6, Kowanevsry, A, Ueber die Entwicklung der Chitonen, Zool.
Anz, Jabrg. ii, 1879.
7. Kowanevsky, A. Weitere Studien iiber die Entwicklung
der Chitonen. Zool. Anz. Jabrg.v. 1882.
8, Kowanevsky, A. Embryogénie du Chiton polli ayee quel-
ques remarques sur le développement des autres Chitons.
Ann. Mus. Hist. Nat. Marseille Zool. Tom. i. 1883.
9. Kowanevsty, A., et Manton, A. F, Contributions a Ihistoire
des Solénogastres. Ann. Mus, Hist. Nat. Marsville. Zool.
Tom. iii, 1887.
10. Lovéx, 5. Ueber die Entwicklung von Chiton. Archiv. f.
Naturgeschichte, Jahrg. xxii. 1856. Oefversigt af Konyl.
vetenskaps. Acad. Forhand. 1885. Translated in Ann. Mag,
Nat, Hist. Vol. xvii. 1856.
11. Moseney, H. N. On the presence of eyes in the shells of
certain Chitonidae and of the structure of these organs.
Quart. Journ. Micro. Sci. Vol. xxv. 1885.
>
13.
14.
It.
LITERATURE. 21
. Pruvot, G. Sur quelques Néoméniées nouvelles de la Médi-
terranée. Archiv, Zoul expér. (2). Tom. viii., p. 22 (Notes and
review). 1890. 3
Pruvor, G. Sur le développement d’un Solénogastre. Comp.
rend. Acad, Sci. Puris. Tom. cxi., p. 689. 1890.
Reincxe, J. Beitriige zur Bildungsgeschichte der Stacheln, ete.,
im Mantelrande der Chitonen. Zeiferhr. f. wiss. Zool. Bd.
xviii. 1868.
APPENDIX TU LITERATURE ON AMPHINEURA.
- Mercatr, M. M. Contributions to the Embryology of Chiton.
Johns Hopkins Univ, Studies. Vol. v. 1893.
. Pauvor, G. Sur l'embryogenie d’une Proneomenia. Comp.
rend. Acad. Sei. Parix, Tom. cxiv. 1892.
Smunots, H. In Bronn'x Klass. u. Ord. d. Thierreichs. 1892-
94. Anatomy, Ontogeny, Phylogeny and Literature.
CHAPTER XXX.
LAMELLIBRANCHIA.*
Systematic :—
I. Prorosrancalia, gills bipectinate, branchial processes plate-like
and not reflected, foot with creeping sole.
Nucula, Yoldia, Solenomya.
Il. Fivrrancai, gill-filaments distinct and reflected; solid
interlamellar junctions, cilia forming interfilamentar junctions.
_ Arca, Mytilus, Modiolaria, etc.
III. PsEUDOLAMELLIBRANCHIA, the gill-filaments reflected and
loosely connected by ciliated discs or vascular junctions, interlamellar
junctions vascular; gills folded and filaments at base of grooves
modified.
Pecten, Ostrea, ete.
IV. EvLAMELLIBRANCHLA, gill-filaments of plate-like gill con-
nected by vascular interfilamentar and interlamellar junctions, re-
flected.
Cardium, Teredo, Cyclax, Unio, Venus and most Lamellibranchs.
1. Oviposition and Oare of the Brood.
The eggs of the Lamellibranchia may be discharged into the
water and there fertilised (Modiolaria and Mytilus edulis, LOVEN,
Barrois, Winson ; Ostrea riryiniana, Brooks, No. 16; and pro-
* We have, chiefly for practical reasons, placed the Lamellibranchis be-
fore the Gastropods, because their larval forms appear to be more primitive,
and their further development is usually more simple. The relation of the
Lamellibranchia to the other divisions of the Mollusce will be discussed
later (Chap. xxx.). In the classification of the Lamellibranchs, we have
followed PztseNeen’s recent works, but it should be mentioned that GROBBEN
has quite recently adopted a new stand-point in classifying the Lamelli-
branchia, and hes expressed himself as opposed to the use of the gills as the
determining feature in their classification (Beitrige zur Kenntniss des Baues
von Cuspidaria (Neaera) cuspidata, ete. Arb, Zool. Inst. Wien. Bd.x. 1892).
24 LAMELLIBBANCHIA.
2 Cleavage and Formation of the Germ-layers.
Tn those forms in which the cleavage of the egg has been carefully
investigated (Caio, Anodonta, Cartiam, Cyclas, Teredo), its course
is so uniform that we may conclude that it is the same in those eggs
of which only a few but similar stages have been observed (Osreu
edulix, Perten, Mytilus edulis, Méprus, Horst. Fcuartos. Barros
and Witsox). Cleavage is always unequal : the first cleavage-plane
divides the egg
into two cells, a
very large macro-
mere and a much
smaller micromere
(Fig. 11 A) In
Teredu, a corres-
ponding differen-
tiation is indicated
eveu before cleav-
age by the different
constitution of the
protoplasm at the
Pro. 11.—.-F, diacrams illustrating the cleavage of the Veeetative and at
ewe in the Lamellibranchia The lines connecting the the animal poles
nackei of two cells indicate that the pair has arisen from
the division of one vel. of the ey. The
plane dividing the
two blastomeres passes through the point where the polar bodies
lie. The micromere next divides into two (Fig. 11] B). and almost
at the same time. or else a little later, the macromere gives origin
toa new micromere (CL The new cell then divides, and the process
of the abstriction of a micromere from the lange cleavage-sphere is
repeated (D) again and again. the lange cell viekling micromeres
which then divide (£1. Finally, the micromeres. seen from the
surface. resemble a cap placed upeo the remaius of the macromere,
which at last also divides into two similar cells (macromeres)
(Fig. 11 Pe
eh is common) ¥ hed thas che entaden arses soleir from the macromeres:
ema and thai. ai the foar-
—
‘OLEAVAGE AND FORMATION OF THE GERM-LAYERS. 25
The cells do not necessarily always divide in exactly the way described.
For example, a new micromere may be constricted off before the one last
formed has divided; but this does not indicate any essential deviation from
the course above desoribed. This is also the case in the apparently divergent
method of cleavage seen in Modiolaria and Ostrea virginiana, as was recognised
jong ago by Lovéx, and was again pointed out by Zrecuer. In the two
Lamellibranchs just named, during the first stages of cleavage, a very
temarkmble process tukes place, a part of the large sphere rising up from it
like a distinct blastomere, but not, like the true blastomeres, entirely separating
from it; ata later stage, this protuberance is withdrawn into the macromere,
‘On account of this process, which is probably determined by the relative
‘Aistribution of the protoplasm and the yolk in the egg, the first stages of
Mediolavia and Ostrea virginiana difier in appearance from the diagrams given
wboye; they may, however, be referred to these, as is evident after the de-
generation of the false blastomere.
‘Ray Lasnesrer long ago described the cleavage of the egg in Pisidinm
pusitlam, « form nearly related to Cyclas, into four spheres of equal size, from
each of which a smaller cell became constricted (No. 29). Tf this is really the
ease, this method of cleavage would not correspond to that known to cour in
other Lamellibranchs, but would rather closely resemble the cleavage of the
Gastropod egg (p. 108). This condition of the egg of Pisidiwm is however so
peculiar when compared with that of other Lamellibranch eggs that it requires
to be further 4
In describing the stages of cleavage, we have so far dealt only with
their outward form. Although the manner in which this arises in
the various egys is very similar, nevertheless certain differences of
‘Internal constitution are very soon evident. In one case, a cavity,
ee nn. soon appears between the micromeres and the
macromeres. [In Cyelos, at the 13-celled stage, Sravrracumn.]
“This increases considerably in size, as the division of the cells
continues, and leads to the formation of a blastula, such as is found
in Cyelas, Pixidium and the Unionitae, the wall of which is not
uniform in thickness. In other cases, the cleavage-cavity is not so
| Tange, especially at first (Mytitus),* while in Teredo, as well as in
to be precisely similar to that described above, but neither
the second eee ep the salen from a vegetative
asserted, this separati Litre, only oceurri
i ‘and thus the entoderm arises both from the epee me
‘Lintne suggests that the peers cleavage in Unio is due
‘the rudiment of the immense shell-gland is to be found in
‘and he further accounts for the minute size of the entomeres
remains undeveloped until late stage.—Eb.]
eae mists on Mytilus (No. 1) are only known to
in the Jahrsberichte, but, taken together with the
os (No. 59) are probably to be understood in the way
26 LAMELLIBRANCHIA.
Ostrea virginiana, it is altogether wanting (Figs. 12 and 14 A). The
micromeres then lie immediately upon the macromeres, the con-
sequence being that, as they multiply, they surround the latter.
An epibolic gastrula is
thus produced (Figs.
12 and 14 A), such as,
according to Lov#n,
is found in Modiolaria
and Cardium. In the
last stages of cleav-
Frc. 12—1-C, embryos of Tered» during the for- i
mation of the germlarers after Harscutx:, The 98% the two primary
entoderm-celisare lightly dotted. whilethemesoderm- germ-layers are
cells are more darkly market; the unsbaded partis Sieeady differentiated,
the micromeres corre-
sponding to the ectoderm and the macromeres to the entoderm.
This also applies to those cases in which a cleavage-cavity forms and
the gastrula arises through inragination (fresh-water Lamellibranchs,
Ray Laswesrer, Zreccer, Livuie). In Cyclas, for instance,
a shallow depres-
sion forms in the:
blastula (Fig. 13
A) the vegeta-
tive pole of which
is no longer to be
distinguished by
the larger size of
its cells, and. by a
farther invagina-
tien of the cells at
this point. a small
arehenteron is
formed (Fig. 13
Bu This is also
the case in Pis-
Erg peters cre pp pA
iastopore ater ZIBGLER. NW. tlassapore: topore takes the
Pow miewnferm : es, rudiment of the S00 Go of a slit lying
in the median line,
and in this way the embryo early assumes a bilateral symmetry.
The blastopore won closes, « that the arvhenteron kees its
connection with the exterior and lies as a closed svc in cuntact
OLEAVAGE AND FORMATION OF THE GERM-LAYERS. a7
with the ectoderm (Fig. 13 C), It is not known whether the
loses from behind forward, so that its relations to
the mouth and anus are still uncertain. [In Unio (Lannie) the
‘blastopore is said to close by the forward growth of its posterior
lip, the ventral plate.)
Tp the Unionidae also there is an invagination-gastrula, but the
arehenteron is here still smaller than in Cyclas (Gorrrs, Fig. 23
ALC, #, p. 51). ¥
‘The presence of an invagination-gastrula in the Unionidae was first ob-
served by Rape in 1876 (No, 43), and Scurernouz in 1878 (Nos. 47-49), and
the gastrala was said to have a specially large archenteron (Fig. 22, p. 50), but it
is impossible to reconcile either the position or the large size of this arch-
enteric invagination with the later development of the embryo, especially as
the alimentary canal is at first very inconspicuous. According to Gorrre's
recent description (No. 15), this deep depression represents the shell-gland
which is here specially strongly developed, the archenteron, on the contrary,
‘being reduced (Fig. 28 4-C, sd and ¢). The subject will be discussed more
in detail in connection with the further development of the Unionidae (p. 49).
[Lanxzm (No. II.) has shown definitely that the large invagination observed
‘by Rast and Scumnnoiz was the temporarily inturned shell-gland, the true
‘erchonteron being very small.)
Between the extreme cases of epibolic and embolic gastrulation,
such as are offered by Cyclas on the one band and Teredo on the
other, Ostrex forms to a certain extent a transition. In the Euro-
pean as well as in the American Oyster, the micromeres have been
observed to grow round the macromeres, of which there are only one
‘or two present at this stage (Fig. 14 A). Hors and Brooks agree
in denying the presence up to this stage of a cleavage-cavity, but
‘such a cavity arises as soon as the macromeres increase in number.
‘As the micromeres even during epibole projected slightly beyond
the macromeres ut the vegetative pole, a depression arises in this
region. When the macromeres now divide, a stage arises, with an
almost triangular blastopore, which cannot be distinguished from an
ecctns cin a. 14.8). During these processes, important
‘alterations have taken place in the form of the embryo ; an invagina-
tion closely resembling the archenteron in form, the so-called shell-
gland, has appeared (Fig. 14 B and ©, «!). ‘To this and the further
transformation of the embryo (C-£) we shall return later.
Conditions similar to those in the Oyster are found also in the Lamelli-
Jbranchs examined by Loviix (Modiolaria and Cardium), in which the abund-
determines the varly ciroumerescence of the entoderm-elements,
i a
28 LAMELLIBRANCHIA.
which latter only then commence to divide. A cavity then appears to arise
between the ectoderm and the entoderm, and stages occur exactly resembling
Fig. 14 B. In Teredo, the separation of the two primary germ-layers and the
increase of the entoderm-cells takes place at later stages (Figs. 12 and 15,
pp. 26 and 81).
During the act of gastrulation (Teredo, Unionidae) or even before
it commences (Cyclas), the rudiment of the mesoderm appears in the
embryo. In the epibolic gastrula of Tvredo, there are two large cells
which, according to HaTscHEs, are to be traced back to the macro-
@ & &
Fro, 11. —A-E, various stages of development of the Oyster (a of Ostrea virginiana
after BRooxs, B-£, of edulis after Honst). a, anus; bl, blastopore ; a,
mouth ; ma, stomach ; mes, mesoderm-cells ; rk, polar bodies ; s, anal (in D, ux
paired embryonic shell-rudiment) ; ad, shell-gland ; «v, the anterior adductor muscle;
4o, pre-oral ciliated ring.
meres, lying symmetrically to the median plane at the posterior
edge of the blastopore (Fig. 12 A and #). They are soon grown
round by ectoderm and are thus drawn into the interior of the
embryo (Fig. 12 €). In Ostrea edulis, corresponding cells are
found in a similar position (Fig 14 C), and conditions similar on the
whole are also found in Cyclas.* These two cells have been assumed
to be primitive mesuderm-cells [mesodermal teloblasts] homologous to
*[In Cyclas, after the macromere has given origin to the last miecromere
(about the 30-celled stage), it divides into two cells of equal size, from each of
which a large cell segments off into the cleavage-cavity. These are the two
primary mesoderm-cells. The two smal] remnants of the macromeres form
the entoderm (Sravrracuzr, No, VI),—Ep.]
CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 29
Dieses the! Annelida, from which by repeated ‘division the meeoderm-
bands are formed. In this way, the bilateral symmetry of the body
would find an early expression in the rudiment of the mesoderm,
Two mesoderm-bands, which, however, are not nearly so regular in their
arrangement as in the Annelida, have also been discovered by Rann and
Harsenex. Honsr has described similar conditions in Ostrea, and ZimoLen’s
account of Cyclas also, on the whole, agrees with the above. The latter
author, however, does not exclude the idea of o further participation of the
ectoderm in yielding the mesoderm-elements, and Ray LaNgesren also was
formerly in favour of the partial derivation of the mesoderm from the ecto-
derm {Pisidiwm). There was therefore a general inclination not to derive the
whole of the mesoderm in the Lamellibranchia from the primitive mesoderm-
cells.
Unio, a form in which the mesoderm and the germ-layers in general were
first demonstrated by Rast, although indeed not very accurately (c/, pp.
7 and 50), shows most markedly the method of formation of the primitive
tmesoderm-cells [mesodermal teloblasts] and the mesoderm-bands, But since
the entodermal nature of the large invagination in the Unionidae must be
considered as refuted, these conditions also cannot be regarded as sufficiently
established. Rar found, in Unio, two cells which even in the blastula-stage
are distinguished by their size from the rest. At the commencement of
gastrulation, these pass into the cleavage-cavity, and then lie symmetrically
tothe median line. The active multiplication of these two cells is said to
give origin to the mesoderm-bands,
‘It must here be mentioned that the presence of the large cells that were
found by Rant within the young embryo is confirmed by the later descriptions
of Scurensonz (No. 49) and Gorrre (No. 15) (Fig. 28 4, p. 51). According
to Gorrre's figures, these might also lie near the blastopore, since the latter
is apparently not far removed from the shell-invagination which Rast mis-
took forthe archenteron (Fig. 23 4). The mesoderm-bands in the Lamelli-
branchis cannot, as a rule, be said to be very distinct.
[The primitive mesoderm-cells in Unio lie in the cleavage-cavity imme-
‘diately posterior to the blastopore, and give rise to two mesoderm-bands by
teloblastic . There are in addition certain mesoblastic cells (the larval
mesoblast of Liztze) situated anteriorly to the archenteron, which have the
—— and possibly form the larval adductor muscle and
Summary. The differentiation of the germ-layers in the Lamelli-
‘branchia takes place very early. Even during cleavage the two
primary Jayers can be clearly distinguished, and the rudiment of
the middle germ-layer can also be recognised very early (Figs.
1214), After the gastrula-stage is reached, the mesoderm is found
"tn the form of more or less massive accumulations of cells (mesoderm-
, bands), apparently proceeding from the posterior pole, between the
“tetoderm and the entoderm. The bilateral symmetry of the germ
‘
30 LAMELLIBRANCHIA.
ourly finds expression in the rudiment of the mesoderm and in the
position of the blastopore.
8. Development and Structure of the Trochophore Larva.
There ix, in the dovelopment of the Lamellibranchia, a stage
which more or less closely resembles the 7'rochophore larva of the
Annolida, and which hus therefore received the same name (Ray
Lanxestser, HaTscuek). This stage is most marked, as we should
naturally expect, when it is represented by a free-swimming larva,
much ax ig found among the marine Lamellibranchs (Teredo, Car-
dium, Mytilus, Ostrea, ete.) but can be clearly recognised also in
other forma (Cyelas, Pisidinm). In the Unionidae, the Trochophore
tayo hax undergone much greater modification. Thus among the
marine Lanellibranchs we find, as a rule, that the primitive larval
form has been retained in a less specialised condition than among the
fresh-water forma, and this affords a further confirmation of a pheno-
menon which is very wide-spread in the animal kingdom. One fresh-
water Lamellibranch, however, Dreissensin polymorpha (evidently in
consequence of its late transference to fresh water) exhibits a larva
agreoing exactly with those of the marine Lamellibranchia (Kor-
woment, No 27, BLocumans, No. 3, Wentsgp, No. 58).
‘Vhe structure and development of the Trochuphore larva have been
boat investigated by Hatscnen in Teredo ; in addition, Brooxs and
Hows have published observations upon the larva of the Oyster,
aud Loves upon those of various other Lamellibranchs (Modiolaria,
Cwntinn, Montacata). The TrochopAure stage of the fresh-water
Lameltibranehs has been carefully investigated in Cycluz by ZUBGLER.
We shall here for the mest part follow HatscHEx’s account of the
larva of Tero, since this form, of all these as yet known, most
clearly exhibits the Trochephoran condition. The larva of Oxrea
aiscis which, with regant tw the furmation of the alimentary canal,
whows (aceunting to Horst) a still simpler conditinn. agrees very
eloeely with Tee te
A. The Trochophore stage as a free-swimming larva.
We have stremiy on. 25) dewnbed a few stages iz the development
wo fest Son whieh am ett gastrula ws formed (Fiz. 12 4-c)
Wurther chars barn by the overgrowsh of the meadern-veills bring
at the aise of the Nastomare by the ectaiers: . tie Samer thas
eerre encet in the ember, the Nastonwe cies im con
‘THE TROCHOPHORE STAGE AS A FREE-SWIMMING LARVA. 31
‘sequence of the further growth of the ectoderm (Fig. 12 C). The
relation of the blastopore to the mouth which now forms could not
‘be established in Teredo, but the closure of the blastopore on the
ventral side seems to take place in the neighbourhood of the future
mouth. This latter arises at a somewhat later stage in the form of
ain ectodermal invagination (Fig. 15 A). A comparison of this stage
with those that lead to the Trockuphore shows us that the longitu-
rior
by an invagination of pyc, 15.—4-C, embryos and Inrva of Teredo (after
the ectoderm, the HATSCHEK). began entoderm-cells are lightly the
a cells dark ;
mesoderm
ce etaatoe af a x nell la Meee
mi m, mou!
‘dlastopore thus pers ell Bishan
stomodaecum and the enteron. The transformation
‘of the archenteron into the enteron of the larva can take place
‘more simply in Ostrea, inasmuch as the embryonic entoderm here
‘consists, even at an early stage, of a large number of cells (Fig.
14 4-D). In Teredo, on the contrary, the two large entoderm-cells
are retained for a very long time, only few smaller entoderm-cells
abstricted from them (Fig 15 2). The development
of the enteric cavity and its close connection with the stomodaeal
Wwyagination thus occur later (Fig, 15 €).* Consequently, the in-
‘testine of Teredo can only become capable of functioning at a much
* The statement of Buoons that, in the American Oyster, the blastopore
‘and the mouth and anus appear as new structures in no way connected
ft, cannot be reconciled with the account given by Horst, We should
‘@condition such as is found in the fresh-water Lamellibranchia
| a condition would have to be regarded as a specialised one, and
allel expect to find it in the free-swimming larvae of
32 LAMELLIBEANCHIA.
later stage than that of Oxfrea. The proctodaeum also seems to
develop earlier in the latter. In 7-redo, according to HarscHex,
the terminal portion of the intestine arises as an ectodermal invagina-
tion ut the posterior ent of the body (proctodaeum), which afterwards
becomes connected with the enteron (Fig. 15 C, «).
Even before the processes just described are completed, other im-
portant changes, especially affecting the external shape of the embryo,
have taken place. At the time of the formation of the stomodaeum,
the ectoderm began to separate from the entoderm, thus giving rise
to the primary body-cavity and at the same time causing a striking
alteration in the shape of the embryo | Figs. 14 and 15). The latter,
which hitherto was almost egg-shaped. now broadens anteriorly, the
pre-oral part assuming the shape of a somewhar flattened cupola,
while post-orally the body tapers slightly ; in fact. the larva assumes
the shape with which we became familiar in the Annelidan Trochophore
(Vol. i. pp 265, ete.
During this alteratien in the shape of the embryo. the ciliation
characteristic of the Trwhopdore also appears, two rows of cells lying
in froat of the mouth and encircling the cephalic area becoming
covered with cilia (Fig. 15 4. The preoral ciliated ring consisting
o€ a double row of cells thus aries. but, in the following ontogenetic
stages of To~ia, this & the less distinet. as the whole body becomes
covered with cilia, overs of which are ket again later. Then. only
the biverial preoral ciliated ring persists, while behind the moath
are seen the first inicatinns of a peet-oral ciated img. These are
gradually continued towands the dorsal shie until the chowed post-oral
universal ciliated rims predaed Fix. IS ©. Between the two
Yims. a wee of more delete cilia is retaimad: chis i= called by
Hiarsommx the adural ciliated sme. Rebind the anus. ale a small
ciisted area umd. A tuft of rene Ga of a single thick cillium,
dounsi =m cuany Lamelitranch “arvae xz the centre of the cephalic area,
Muaces the Skeness to the Anoeldan Fewiced 7. alreniy produced
ay the fem cf Dady ani dembote of the cia cil more striking
Fir is.
Ware the nets clan mor sad the aioe wee Do doubt
2 thas fameman it alwars found
Preserves errae. iz which the
Sate cay Saremerate. This important
TAY RV ‘
Jew) mmr ami tae
ermetorr cmc sct&ss =
Tle agrerur cart -€ he Sy aera
34 LAMELLIBRANCHIA.
larvae of the last-named form are met with swimming freely on the surface of
the water before a:taining the Troctupduwr stage as well as at that stage.
Before indicating further points of resemblance between the
Lamellibranch larvae and the Annelidan Trovhuphore connected speci-
ally with the internal organisation. we most first draw attention to
a character, not hitherto considered. which distinguishes these larvae
at once from all other (non-Mollasean! larvae. This is the so-called
shell-Nawi. At a somewhat early peried in (#¢ra, as early as the
wastrulaetage (Fig. 14 8), in Tesi. rather later, a part of the
ectoderm, which is smewhat thickened by the lengthening of its
cells, forms a trough-like depression on the dorsal surface near the
posterior pole (Fig. 15 8). This depression. which represents the
ridiment of the shell-gland. s»a deepens considerably. so that it
appears like a blind tube (Fixx 14 4. and 22. p. 50) It has a
vlandular chanecter, inasmuch as its cells show the longitudinal
striatinn characteristic of many glandular cells: it son also begins
to secrete a substance which can be seen as a thin integument over
the external aperture ami the margin of the shell-gland (Figs 14 C,
amd 15 B) This is the first indication of the shell, and it is thus
seen that the latter in its earliest rudiment is unpaired.
In the further coure of development the invagination of the
shell-gland atten: out again. first becoming redaced tw a shallow
ciepression eovered by the rudiment of the shell (Figs. 14 D, and 23),
waned later ppearing alteether. The shell at the same time in-
e. am pew. Uke a siddle, covers a part of the donal
réaces Firs 130. 14 2 ami 23. C1. By the extension
ze-tmangin of the adult shell : it is indi-
by ht line on the back of the
the uiethead of of the detinitive shell in
tained by the shell in
The ianse size
Ling larva is to be seen
srorect: teyoud the body. a condition only rep-
muatinn of the right ami left mantle-folds
‘lace. These folds are fwmed as Lateral out-
‘mal cells being in
he face of the outgrowth
<i-shaped veutral nrx~a of the larva by a
sleet nure—the mantl-vavity (HaTScHEK’. The reader should
=
THE TROCHOPHORE STAGE AS A FREE-SWIMMING LanvA. 85
compare this with the formation of the mantle in Cyclay at a later
stage (p. 43).
‘The shell, in the condition just described, is already a real pro-
tection to the larva, for, on account of the contractility of the velum,
the whole body can be withdrawn between the two valves. The
shell increases in size as the larva grows ; in Dyeissensia concentric
bands of growth can soon be recognised, their number increasing
more and more with age. The growth of the larva of Dretssensia,
and also that of the larvae
A 2
P16, 17, —1-C, ne Dlactyd Dreissensia polymorpha i
various : af, surface sree thy abit 5
B, wntero-ventral + © (older Jarva), seen
from the side (original) ; m, oral region ; 4, shell.
‘The velum, in Aa strongly pig.
mented. In C, retractors are faintly seen running
twek from the velum.
tween the mouth and the anus, and the gill-rudi-
ch are first indicated by papilla- or ridge-like outgrowths
mm : but these changes will be dealt with later when
transformation of the larva into the adult. Before
subject, we have to describe an important char-
lore larva itself, which marks still more strongly
the Annelidan Trochophore.
ne 118 B, p.265). Fine hairs are attached
the centre of each is a strongly refractive round
es embedded in them, which Lovén observed in a
larvae (¢.g., Mytilus), apparently arise at a Tutor
cently discovered that these eyes are retained
Arvicila, where they are situated at the base
filament of the internal branchial lamella;
in. They are not homologous with the larval
. occur on the velum, and are therefore true
sibly homologous with the larval eyes of Chiton
have remained only slightly differentiated,
retained in this primitive condition for a very.
it is evident that they contain, stored up
nutritive material, which is gradually used up in t
larval body ; the presence of this food renders an
of the intestine, such as takes place in Oxtrea, unmec
14). At first the intestine makes but a simple b
resemble in shape that of the Anuelidan
rudiment and its derivatives, which are, nevertheless,
ance. According to Rann and Harscres, the symmeti
mesoderm-bands run forward from the two primitive 1
{mesodermal teloblasts} which at first lie near the
afterwards (ventrally) at the sides of the anus. The
these mesoderm-bands are, as in the Annelida,
of the primitive mesoderm-cells, which long retain
character of the blastomeres (Fig. 15). ‘The
Lamellibranchia do not appear so distinct or so highly developed as
those of the Annelida, since, from an early period, cells bud off
from the main mass of the mesoderm which become distributed in the
primary body-cavity.® ‘These give rise to the muscles of the larva;
the originally round cells leagthen, send out processes, and, finally,
by assuming a fibrous structure, produce the fibres of the retractor
muscles (Figs. 15 C, and 18). ‘The retractors of the velum which
run from the posterior part of the shell to the cephalic area form first.
‘Thon several shorter muscles are added, also running from the inner
surface of the shell in the region of the hinge, and finding points of
insertion in the post-oral region of the body (Fig. 18). These muscles
seom to serve chiefly for closing the shell (Harsomes), but this fune-
tion is carried out principally by the muscles which, soon forming
from long mesoderm-cells, traverse the body-cavity dorsally to the
intestine, running from one shell-valve to the other. This shell-
adductor appears very carly in the larvae of many Lamellibranchs
(Fig. 16, sm),
“(According to Lrutre (No. LIL), this larval mesoderm has in Unie an
quite distinct from the mesodermal teloblasts which form the mesoderm-
bands. ‘The larval has more the character of a
is situated in front of the whereas the mesodermal are
situated behind the blasto) c| former ves ccicha 60 the Se eae
9 and adult m is well seen in Figs.
i
‘The fact that this degeneration has taken
cated form of the alimentary canal. In Cyclas and
the Unionidae, the blastopore closes; the ectoderm then |
Fig. eles Cyclas cornen nt the Trock- the Trochophore,
(combined from E, ZrecuEn's Seuss)
lion; d, enteron
a
very much reduced in those forms which do not swim about freely at the
Trochophore stage. In Cyclas, all that remains of the ciliated apparatus of
the Trochophore is. small ciliated area extending above and below the mouth
and at its sides. Zreater has homologised this ciliated area with the ad-
oral ciliated zone of the Trochophore, and believes that the part of the yelam
connected with feeding is partly retained while the part chiefly connected with
locomotion and which was no longer used has completely disappeared. The re-
duction of the velum has led to « corresponding reduction of the larval muscles.
It has alreudy been mentioned that the larval kidney is found in
Cyelas. Of the other Trochophoral organs, ZIEGLER was only able
* The statements that the blastopore becomes directly transformed into
anus (as in Pisidiwm, Ray Laxxester) require confirmation, since the more
Aire Lamellibranchia show an entirely different relationship =
seated account of the processes in Cyclas, emerges
anus arises at the Lsancies end of the ditlike blastopore which has a
ee A proctodseal invagination seems in any case to be absent in
i
42 LAMELIDIBRANCHTA.
48 a hollow outgrowth of the ectoderm into which a great mass of
mesoderm-cells find their way. The gills arise in its neighbourhood
either as two ectodermal ridges one on either side of the body at the
point where the inner lamella of the mantle-fold is continuous with
the body-epithelium (Teredo and Cyclus), or else at the same spot is
# single row of papillae (Mytilus, Dreiasensia
and the Unionidae, and, acvording to Jacksox,
in Ostrer).
In those stages of Cyclas and Pistdiuin
which must be regarded as the equivalents
of the Troehophore, the foot has already
uttained a considerable size. It oceupies
the whole of the ventral surface between
the mouth and anus in the form of a massive
projection of the ectoderm (Fig. 19 f). At
a later stage it extends in length and, in form
and position, shows the same relation to the
rest of the body as it does in the adult
pa — Oiler tere. of ig. 21),
Drveissensict inonyphé
(original). In the free-swimming larva, the foot is
a f already of considerable size. Althongh at
first merely a truncated structure projecting
only slightly below the shell, it soon grows in length, and can be
protruded far beyond the shell when it appears to make vermiform
movements and to function as a tactile organ (Fig. 20).
At this stage, therefore, the larva, besides its provisional lecomotory organ,
the velum, also possesses the locomotory organ of the adult Lamellibranch,
Farther, the foot usually, as far as we can judge from Dreissensia, is retracted
while the larva (which is still very active) swims about; in this respect Fig.
20, which depicts the foot as extended in a larva with an expanded yelum,
is not quite true to nature. Such a condition does, however, occasionally
occur when a larva has just extended the velum and is beginning to retract
the foot which was protruded beyond the shell as a sensory organ,
The surface of the foot, at the Trockophore stage as well as Inter,
is covered with fine cilia. At the posterior upper boundary of this
ciliated area, in Cyclas, pit-like depression of the ectoderm is found
on each side of the middle line, lying exactly above the mass of cells
from which the pedal ganglion develops (Figs. 19 and 21). This is
the paired rudiment of the byssal gland.
|
44 LAMELLIBRANCHIA,
an outgrowth of the ectoderm arises (Fig, 19 mr), forming a fold
which extends anteriorly and posteriorly over the body (Fig. 21 4,
mr;
vids Teredo (p. 34), the mantle extends with the shell from the
dorsal to the ventral side, The outer lamella of the mantle is closely
applied to the shell, while its inner lamella bounds the
which has arisen with the growth of the mantle. per pc
the mantle also increases more and more in size, and th two now
enclose a large part of the body (Figs. 21 B, and 31, p,
The siphons, where found, appear to develop at a very late stage.
They are formed from the edges of the mantle, which become closely
applied and fuse, the parts destined to form siphons then growing
out into long, tabular structures.
and B, pes op. co weet (after bp ae to = a, anus; by, byssus ;
ment ei, mined ak m+é, stomach and liver ; wr,
Faust of Tne vighh calcarnoue whet-ratee sad cate pe 1; wel, stomodaennt moma wi,
velar area.
Within the mantle-cavity another ectodermal fold appears on either
side of the body, developing from behind forward like the mantle
itself. This paired fold is the first rudiment of the gills (Pig. 21
B, k), which arise in the same way in Teredo. The branchial fold
is covered by fine cilia. Thus, in Cyelas and Teredo, the earliest gill-
rudiment exhibits the form of a plate, and at first shows no indication
of the slits and bars so characteristic of the adult. The further
differentiation of the gills, resulting in the formation of the gill-slits
and filaments, takes place from the anterior end of the fold. Starting
from the lower free edge, a series of vertically placed parallel grooves
Zetete
‘the latter, a gill-plate first appears, but
extend to the ventral edge of the fold,
distinct gill-filaments are not formed, but
with slits, the gill-plate retaining its
ow the slits
the external form of the Lamellibranchia we
the labial palps (oral lobes). "These, in the
upper (anterior) and « lower (posterior)
to ZieGLER, they arise in the following
surrounding the mouth becomes divided
ver portion by a groove which runs out on either
of the mouth. ‘The first of these must be reck-
d the second as the lower lip. These two areas
re rise to the labial palps. At the time when
over the upper lip, a median depression ap-
4 similar depression is to be found in the
s being thus divided into two lateral portions.
out as folds, and develop into the labial
s the two halves of the upper and lower
aboye and below the mouth, the divi-
46 LAMELLIBRANCHIA.
sion into right and left palps being more apparent than real. This
condition appears to be brought about by the stronger growth of the
lateral parts of these structures. ]
This origin of the labial palps partly confirms the assumption made by
Lovim that the velum of the larva may pass over directly into the adult
palps. In the double nature of this organ Lovin finds agreement with the
double velum of the Gastropoda, a resemblance which is strengthened by
Yxecuen's observation of the median division of the ciliated area of Cyclas.
‘The small section of the ciliated area lying above the mouth might then
‘be regarded as the last vestige of the former ciliated ring of the Trocho-
plore, The ciliated area of Cyclas, however, as we haye just shown, seems
rather to correspond to the ad-oral ciliation of the Iarva (p. 40). And since
this serves for feeding more than for locomotion we see that a part of the larval
body passes over into a similarly functioning organ of the adult animal.
‘The significance of the labial palps lies principally in their relation to the
capture of food, in which they assist through their position and their ciliation
({Tarere, No. 55). An exact knowledge of the fate of the entire velum, fe.
of the pre-oral ciliated portion of the body ina marine Lamellibranch, would
be of great value,
The metamorphosis of the Lamellibranch larva into the adult is
characterised chiefly by the complete degeneration of the pre-oral
part of the body which was so large in the former. In the larva, the
highly developed velum spreads out between the mouth and the
shell (Figs. 15-18, pp. 31-36 and Fig. 19, p. 40), but, as develop-
ment proceeds, this‘area becomes contracted (Fig. 21 4), and finally
almost entirely disappears, in keeping with the condition of the
adult, in which the cephalic region is almost completely lost,
While the external changes of form just described have been taking
place in the embryo, a marked advance has also taken place in the
inner organisation, but this will be entered into later on. The young
of Cyclas and Pisidium leave the mother only when they possess, on
the whole, the same organisation as the adult.
Divergencies in the Metamorphosis accompanying the Monomyarian
Condition.
Among those forms classed by the older malacologists as the
Monomyaria, the transition from the larva to the adult has only been
well investigated in Ostrec, and their development, judging from
this form, seems up to a certain point to agree closely with that of
other marine Lamellibranchia. We have already seen how close the
agreement is in the early stages of development (p. 28 Fig. 14
and Fig. 16, p. 33). The Tvochuphore larva already possesses an
Didairigislab aed atdacKon. to the\tks valgen
‘This adductor in Ostrea lies dorsally to the alimentary
(ANS mcr empscaliec sence ea
es ops gee eae
be homologised with this latter, The adductor of
er therefore cannot be the same as that of the adult.
h has been. emphasised by several investigators
ete.) by the study of the later
panel
which ouly one adductor musele (the anterior) is
Perea eee etn s better developed than the
via Pion, In the Unionidae also the anterior
first, as, indeed, is the case in nearly all
Retr tigd fe this reap
we ie ntrary, according to Zimanun, the posterior adductor
A sok e anterior, but it has already been pointed out that in the.
F di the anterior appears first. Lacazn-Durarens (No. 28)
rior adductor develops first in Mytilus, but this
larval stages examined by this author as well as by
0 old, According to Wiisow (No. 59), in the young
adductor develops earlier than the posterior, and
the nearly related form Preissensia (Korsennur,
adductor has appeared in Osxtrea, a posterior
ly to the intestine arises in the same manner
chs mentioned above (JACKSON). Ostrea, und
Monomyaria as well, possess for » time two
and a posterior) of almost equal strength, and
the Dimyaria. era ee eetesy of these vee
och larvac do for time possess only one ad-
as eS 2 roe aly ome ot
‘permanent condition of the Monomyaria as having
of development in this direction, i., through the
a second muscle. The Dimyaria hence do not pass
an stage in the proper sense of the term, but the
48 LAMELLIBRANCHTA.
Monomyaria probably invariably possess in youth the two typical adductors
of the Dimyarian,
(The fact that the anterior adductor almost invariably
‘before the posterior, not only in the Dimyaria, but also in the
Monomyaria, in which latter group it is only a larval structure,
might seem to suggest that this muscle was a phylogenetically older
structure than the posterior adductor, and that the Lamellibranchia
were originally Monomyarians, not, like the existing Monomyaria,
with a single muscle represented by the posterior adductor, but with
the anterior adductor alone developed (Jackson). Palaeontology
does not, however, bear out this view; the oldest known Lamelli-
Dranchs found in the Cambrian belong to the Nuculidae and Areidae,
which are typically Dimyarian,]
The young of Osfrea agrees, not only in the possession of two ad-
ductors, but also in other points of its organisation, with the larvae
of other Lamellibranchs, but at a later stage it changes from a free
to an attached manner of life.
The larva, which has hitherto swum about freely, possessing two
quite symmetrical shell-valves and two adductor museles, attaches
itself by means of a secretion produced by the left lobe of its mantle ;
the latter stretches beyond its valve, and, applying itself to the stone
or shell to which the valve is to adhere, secretes shelly matter which
serves to cement the valve to its support (Huxney, Ryper). In the
further development, we now from this time find an inclination to
that radial symmetry which can be recognised in the adult Oyster,
and which is often found in animals that assume an attached manner
of life. The anterior adductor now degenerates and the only remain-
ing adductor muscle (the posterior adductor) enlarges and shifts
almost to the centre of the animal, The anterior part of the body
gradually rotates (round its vertical axis)* through an angle of
about 90°, so that the mouth, which at first lay very near the free
edge of the shell, comes to lie near the umbo. This rotation also
explains the great and almost circular extension of the gills and the
mantle found in the adult. The condition of the foot in the Lamelli-
branchia depends largely upon its use or disuse. In Osfyea, the shell
becomes permanently attached at the close of the free-swimming
Trochophove stage; the foot is therefore unnecessary before fixation
Dates in relation to Wee el the true transverse axis of the
animal. ‘is rotation is possibly due in large measure to the degeneration
‘of the anterior adductor muscle and of the velum.—Ep,)
of the Unionidae.
Unionidae differs 40 essentially from that
attached to the gills or to the integument:
in the Unionidae, superadded to the normal
Ib din the marine Lamellibranchs, an
jal form which cannot be compared with
and which possesses characters not present
by Fontartoy, from
‘o. 14), we ir that
jbranchs up to the later
adult was not observed.
'
a
50 LAMELLIBRANCHIA—UNIONIDAE. :
The ontogeny of the Unionidae bas been studied by a number of
zoologists. Frewantnc, Rast, Gorrre and ScureraoLz have in-
vestigated their embryonic development, while the later stages of
their development, which were examined by Foren (No. 13), Layne
(No, 32), Braun (Nos. 4 and 5), Barrour, F. Soumupr (No. 50)
and others, have recently been reinvestigated by Scniernonz and
Gorrrr.
[Still more recently, Laure (No. 111.) has reinvestigated the entire
course of development in Unio complanata, paying special attention,
however, to the cell-lineage.]
A. Development of the Early Stage.
It has already been mentioned that the Unionidae show an in-
yagination-gastrula (p, 27, etc.) and that, before the latter develops,
large mesoderm-cells bud off from the wall of the blastula and enter
the cleavage-cavity. Before the formation of the very insignifi-
cant archenteron
which, like that of
other Lamellibranchs,
is derived from the
macromeres resulting”
from the unequal
cleavage, a depression
appears ou the blastula
and deepens more and
more (Fig, 22, ad).
This depression is
formed by large cells
which are granular
o and therefore appear
Fio, 22—Embryo of Anodonta in the vitelline mem- ark, and ite whole
brane (after Scurenno1z) ; ase eatsinayeaene: form is such that we
ie msglrmadl some of rich have tured nto can easily understand
interal cells: re, posterior cilinted nrea [rentral plate). WAY it was long mis-
taken. for the archeu-
teron (p. 27). This depression, however, does not occur en the ventral
side of the embryo, but upon its dorsal surface ; it gradually flattens
out again and above it the shell-integument appears (Figs. 23 4-C, 24
A). This structure is therefore, as Gonrrs proved, the shell- od
* [According to Linum (No, II1.) the entomeres which eventually become
invaginated to form the archenteron are derived from all the four cleavage.
—
‘DEVELOPMENT OF THE EARLY STAGE. 51
eon S exactly the same way as in the marine Lamellibranchs
coro and 15, p. 81) and in Cyelas (Pig. 19, p. 40), except that
specially large and appears very early. This early develop-
i ee a he. ie ae
estrada all in its free life which we shall discuss later,
and, in the same way, we may explain the degeneration of the intestine by
‘the parasitic life of the larva, in consequence of which the intestine is not
‘required to fulfil its ordinary functions until » late stage.
In spite of the highly modified character of the larvae of the
Unionidae, we are able to make a comparison between their organs
and those of the typical Trochophore larva. Apart from the ento-
dermal and mesodermal parts which have already been mentioned
Tater ciate area Lago Plate].
the most aie feature is the ciliated area (Figs,
is evidently the last vestige of the ciliation of the
. This ciliated area, termed the ventral plate,
sat first supposed, correspond to a remains of the
the whole ventral surface plus the posterior
y, and is therefore rather to be compared with the
the first, two divisions, that is to say, neither the first
e-p divides the egg into an animal cell and a vege-
Lirxrm further finds that the small archenteron
of the malgad which latter, however,
ner. Of the two groups of mesoderm-cells represented
those above the at shellegiand would correspond with
while those below this structure and behind the
23 A, represent the adult mesoderm
ir of mesoderm-cells. For more
52 LAMELLIBRANOHIA—UNIONIDAE,
larva (Figs. 15 and 18). The broad part of the body in the embryo
of Anodonta which lies in front of the shell and of the entodermal
vesicle would correspond to the velum, i.e, to the pre-oral part of
the Zrochuphore, In the younger embryo depicted in Fig. 22, this
part appears to be formed solely by a somewhat thin layer of cells,
while, in the embryo represented in Fig. 24 A, it has a thick wall,
consisting of cells containing vacuoles such as ZrEGLER has described
in the reduced velum of Cyclas, The shape of this part of the
embryo recalls the swollen pre-oral portion or cephalic vesicle of the
Gastropodan embryos, a condition still more marked in them than in
Cyclas, as ZIEGLER pointed out. [Lroure also regards this area as
the head-vesicle}.
In this region of the embryo, the polar bodies are occasionally met
with (Figs. 22 and 23 C) and these afford an indication for the
correct orientation of the embryo which is not otherwise very easy to
determine, and which was usually misinterpreted by the earlier
investigators,
B, The Development of the Embryo into the Parasitic Larva.
It is evident from the above that the peculiarities in the develop-
ment of the ('nionidae appear very early and affect both the inner
and the outer organisation of the embryo. In the stage up to which
we have followed its development, it resembles a rounded vesicle of
somewhat irregular form consisting of a single layer of ectoderm on
the inner side of which there appear here and there single musele-like
cells ; these belong to the mesoderm, the cells of which have increased
in number, some becoming lengthened (Figs. 23 and 24). In this
young embryo, the shell at first lies like a saddle upon the dorsal
side (Fig. 23 C, and 24, «).
The rudiments of the shell-valves appear later beneath the
unpaired cuticular shell. The shell probably arises here in the same
way as in Cyclas; each shell-valve in the Unionidae appears to be
three-sided and has its ventral point bent like a hook, a modification
connected with the manner of life of these forms. The shell carries
on its outer surface a number of small hooklets which, together with
the two terminal hooks just mentioned, serve for attaching the
Lamellibranch to the body of its host during its parasitic life (Figs.
25 and 26, sk). Before the shell has developed thus far, a radical
transformation of the whole body takes place. The ventral part
of the body, which was not previously covered by the shell but
—————
THE DEVELOPMENT OF THE EMBRYO INTO THE PARASITIC LARVA, 53
projected beyond it (Fig. 24 #), now becomes withdrawn or rather
invaginated towards the hinge of the shell, iv., towards the dorsal
side (Fig. 25 A), and the whole body is thus divided into two halves,
each belonging to one of the valves of the shell (Fig. 25 4 and B).
‘The mantle arises in this way and ut this stage is remarkably large,
greatly over the rest of the body, which only later
redevelops by the outgrowth of the central portion of the body (Fig.
25).
Before the withdrawal of the central portion of the body, four groups
of bristles were developed from the ectoderm on either side of the
embyro (Fig. 24 B,s0); during the invagination-processes just described
these organs lengthen and, in consequence of these changes, are then
ae, * ent,
Sn
oS al de eater {4 ee OT FLEMMING, somewhat
a 0 veil sek with he outline of the sel
ou 3 i entoderm (arc! dimen i
eek eral chatty oo wath pg, beneory. briatles? Wy vitae res
found on the inner surface of the mantle. Kach of these organs
consists of a long columnar cell which gives origin to a number of
Jong and fine sensory bristles (at first four to ten in number, later as
*) that perforate the thin ectodermal cuticle (FLEMMING).
organs are apparently of importance to the larva in the
‘attaching itself to the fish-host and are acquired at a late
gly regard these sensory organs as differentiations
and can hardly consider them to be related to the
(OLY was led to believe on account of the position
. This particular organ occupies an isolated position
in front of the oral aperture (Fig. 26 A).
‘54 LAMELLIBRANCHIA—UNIONIDAE,
These peculiar organs are believed to communicate to the larva the stimu-
lus produced by coming into contact with a fish, and thus to give rise to the
muscular movements which cause the shell-valves to close and the larva to
hecome hooked on to the host.
There are a few more important organs to be mentioned in
connection with the further development of the larva of the
Unionidae, the first of these being the powerful adductor muscle
of the shell. This arises very early through the increase in number
of the mesoderm-cells, which are still only slightly differentiated
{larval mesenchyme, Lroure] (Fig. 23 C, sm), these cells lengthening
and becoming attached to the shell-valves, The short but broad
muscle thus passes through the body-cavity from one valve to the other
(Fig. 25 B, sm). Besides this large muscle, there are a number of
BO
Fro, 25.—Older embryo (within the egg-envelope) and free larva (@lochidium) of Ano-
donta (after ScurenHoLz and Roane) , larval filaments ; 9, lateral pits; s, shell ;
sh, shell-hooks; «m, adductor muscle; 's, tufts of setae representing the sensory
organs ; 1, ciliated wrea,
other weaker muscles in the form of long mesoderm-cells attached in
various directions to the ectoderm, like the muscles which, in the
Lrochophore, bring about the contractions of the larval body.
Scmrernouz [and Lior] ascribe to the continuous contraction of
these muscles the withdrawal of the central part of the embryo above
mentioned. There are also, according to Scumipt, special muscles
in the form of modified mantle-cells connected with the shell-hooks-
A peculiar and characteristic larval organ arises in the median line
between the two halves of the mantle as an invagination of the
eotoderm (Rast), It grows inwardly as a long glandular tube which
coils several times round the adductor of the shell and secretes 4
filament of tough substance which projects from the aperture of the
gland (Figs. 25 B and 26 A,/'). This organ has been regarded as
——
THE DEVELOPMENT OF THE EMBRYO INTO THE PARASITIC LARVA, 55
a byssal gland corresponding to the homonymous organ of other
Lamellibranchs, but this view, in spite of the similar function of the
two organs, is not justified, since the two organs do not agree in
position, and since two ectodermal invaginations appear later on
the foot of the larva which must be considered as the homologues
of the byssal gland (Carriere, F. Scuarpr, Scurernoz), The
glntinous filament must therefore be regarded as a distinct larval
organ.
‘The position of this filament is very remarkable in so far as it is
said to be pre-oral (Fig. 26 4). The mouth has been pressed un-
usually far back and, like the intestinal canal (cd), now belongs to the
small posterior part of the larva. This displacement has been traced
to the great development of the adductor muscle (wm), bat the
conditions of this larval stage as compared with
the former Trochophove-like stage seem to us to require further
elucidation.
| Lannr (No. 111.) deseribes the thvead-gland as arising from one of
the ells of the head-vesicle ; this cell elongates and grows backward
beneath the hinge-line until it reaches the posterior end of the body.
The cell now becomes tubular, the thread occupying the lumen of the
gland, Lite believes that the thread is formed as an actual meta-
morphosis of the substance of the cell; he regards this gland as
primarily excretory, and thinks that the utilisation of its secretion
as an attaching filament was secondarily acquired. During the
transformation of the larva into the Glochidium, the aperture of the
thread-gland undergoes a remarkable change in position, shifting
from its former antero-dorsal situation to the middle of the ventral
surface. |
Between the brush-like sensory organs and the ciliated arca and near the
posterior angle of the valves, two ectodermal depressions are to be seen on
| these are the so-called lateral pits, as to the significance of
which authors are not yery clear. If we rightly understand the somewhat
‘obsonre description given by Scurennorz, he implies that large cells at the
of these pits (no doubt corresponding to the lateral cells of the young
rise to the pedal ganglia. But since the pedal ganglia, as in
the depressions which yield the byssal gland, these early-
)may be related to the latter (?). In Cyclas also the paired rudi-
gland sppears very early (Figs. 19 and 21). Taking these
together with the position of the pits with relation
ble that this interpretation of the lateral pits as the first
glands is correct. The formation of the actual byssal
however, seems to take place at a later stage,
56 LAMELLIBRANCHIA— UNIONIDAB.
When the embryo has attained the stage of organisation just
deseribed, it is ready for ejection from the mother ; on coming into
free contact with the water, the egg-envelope bursts and the embryo
emerges from it. The larva thus set free is known as the (lochidinm.
‘The embryos found by the older investigators (RATHKE, JACOBSON)
Beste illest Lael esc ac sce ae
called Glochidium parastticum,
The remarkable fact that the Glochidia remain for a time parasitic
on fishes was discovered by Leypic (No. 32) and then further in-
vestigated by Braun. FP. Scummr and Scutersousz have recently
given a detailed description of what takes place, and we shall here
follow chiefly their account. The larvae, when freed from the mother,
become connected together in large masses by means of their glutinous
filaments, and in this form rest at the bottom of the water, occa~
sionally rotating upwards.* Chance brings them no doubt into
contact with fish, and « few of them succeed in attaching themselves
to these by the help of their shell-hooks. Unio only attaches itself
to the gills of fish, but the Glochidia of Anodonta, which are more
richly provided with hooks, may also become attached to the fins and
the skin.
The hook-apparatus, according to Scurexmocz, is less developed in Unio,
and it is an interesting fact that it may be altogether wanting in certain
North American Unionidae (Lea). The same is the case, according to ¥.
JueRine's recent observations (No, 25), in the larvae of South American
Unionidae, which are devoid of the tufts of setae, and perhaps also of the
larval filament. In these American Unionidae, therefore, the biological con-
ditions seem to differ somewhat from those of the European forms, and it
would be interesting to ascertain the conditions of parasitism in the larvae of
these Lamellibranchs.t The larval filament and the shell-hooks are wanting
in the Glochidium of Anodonta complanata, although in other respects the
organisation of these larvae is the same as that of other Glochidia, and they
lead « parasitic life (ScureRHOLz).
* (It is often stated that the Glochidia are only discharged when fish are in
the neighbourhood, but Larren (Proc. Zool, Soc,, 1891) found that he could
produce a discharge of Glochidia by gently stirring the water in which the
‘Anodons were lying. He also observed cord-like ejection of Glochidia from an
undisturbed Anodon in its mative water. The Glochidia cannot swim, but
when discharged sink to the bottom, where they lie on their dorsal beets!
the thread streaming up into the water. In this position the Glochidinm lies
powerless to move in any direction, and here, too, it dies unless a suitable
“host” is brought into contact with its thread, Ep. ]
+ We do not know of any other statements upon this subject, although ft is
not impossible that such may exist among the mass of malacological literature
which is difficult to review; v. Juxnic mentions that he found Unionid
lurve on fish in South America,
THE TRANSITION TO THE ADULT. 57
A cyst soon forms from the tissues of the fish and encloses the
parasitic Glochidium. A peculiar mushroom-like growth formed by
large cylindrical cells of the embryonic mantle serves for absorbing
the tissues of the host, and especially the fin-rays in which the shell-
hooks are embedded, The larva is no doubt nourished in this way
until its intestinal eanal becomes functional.
The time during which the Glochédium remains parasitic on the
fish appears to be determined by the favourable or unfavourable
conditions of temperature, and varies from a few weeks to several
months. ScurerHonz and Braun found that the larvae remained
seventy-two to seventy-three days on the fish, during which time
they develop the definitive form.
‘The South American relations of our Anodonta have larvae differing greatly
Tn shape, so that v. JHeninG, who found these larvae within the mantle-cavity
of the patent, would have taken them for parasites had not all doubt as to
their being Lamellibranchs been removed by the agreement of the egg-
envelope and its micropyle with the envelope of the ovarian eggs (No. 25). In
these forms the embryos are found in the inner gills, not, as in our native
Unionidae, in the outer gills, The body in the South American forms is com-
posed of three sections: (1) 1 conical anterior portion covered with cilia;
(2) large middle portion containing internally the entoderm-elements and
two kidney-like structures (byssal glands?) ; the dorsal side of this region is
only partly covered by a delicate shell-integument; (3) the short caudal end,
which forks, and consequently terminates in two rounded prominences, beset
with bent hook-like setae.
A very peculiar organ possessed by these larvae is a very thin but broad and
flat band considered by v. Jn=ntxo to be the byssus. This band is almost at
the middle of the body and is attached to the ventral surtace, from which it
runs forward. It is somewhat broader than the body, and six to ten times as
long. ‘anid also to be connested with the anterior part of the body.
to the somewhat yague account given by v. Jamntne of the larva
‘by him “Lasidium,” and.in the absence of any statements as to the
‘of this larval form, it is at present impossible to compare it with
the entirely different Unionid larvae ((lochidia) or with the larvae of other
Lamollibranebs.
©, The Transition to the Adult.
Very soon after attachment, as early as the second day, the larval
‘organs which enabled the Glochidiuim to establish itself on its host, =,
the glutinous filament and the brush-like sensory organs, degenerate.
depression of the ventral surface arises behind these
organs as they degenerate ; this depression involyes the two lateral
pits already present in the embryo (Fig 26 A and B, g), At this
58 LAMELLIBRANCHIA—UNIONIDAE.
place, the foot now appears as a blunt cone and soon grows rapidly.
‘The wall-like outer margins of the two lateral pits also increase in
height. These prominences become the rudiments of the gills which
first appear in the form of two knobbed papillae (F. Scumpr).
Fig. 26 C shows the rudiments of the gills at a somewhat later
stage. The foot is here
found well developed,
and both it and the
gills are ciliated. The
posterior ciliated area
of the embryo (w),
which was still visible
when the foot had
attained a considerable
size, now disappears,
Of the larval organs,
the shell-hooks aud the
large adductor muscle
are still Lo be seen,
The first are for the
present retained, the
shell in other respects
also retaining its em-
bryonie form until the
young Lamellibranch.
leaves the fish ; indeed
the embryonic shell can
still be made out in the
shell of theadult. The
longer of the two free
= sides of the three-sided
i: embryonic shell must
P16, 26.—A-C, larvae of A nodonte (after Scurernowz). be considered to corre-
Spork Ce Pes ah Pe Pian zt a spond to the anterior
ao a aun tes sry end of the animal, and
in this position it can
actually be found as a small prominence on the umbo of the adult
shell (Braun).
The powerful adductor muscle of the larva agrees in position with
the anterior adductor of the marine Trechophore larva. Tt is, aceord-
ing to Braun and F, Scuimpr, merely « larval organ, and degene-
‘THE TRANSITION TO THE ADULT. 59
nutes completely later, so that the two adductors of the adult must
be regarded as new formations. In opposition to this view we haye
the statement of SchierHoLz that the larval muscle only partly
degenerates, some of it passing over into the anterior adductor of
the adult. ‘This latter condition would agree with the fact that the
anterior adductor appears first in most Lamellibranchs, and for along
time is the only adductor present (p. 48); Braun, however, has
maintained his original view against that of ScureRHoLz.
‘The formation of the intestine is also apparently greatly influenced
by the specialised conditions of the larva. The archenteron had
already lost: its connection with the ectoderm before the commence-
ment of parisitism, and lay in contact with the ectoderm as an
entodermal vesicle closed on all sides. In this condition it remains
for « very long time ; the larva either does not reyuire nourishment
or obtains it as described above through the mushroom-shaped growth
of the mantle. The small entoderm-vesicle is now found in the pos-
terior part of the larva lying rather closely applied to the ectoderm.
‘The swelling carrying an invagination known by authors as the oral
shield (Fig. 26 A, m) has also shifted posteriorly, The sac of the
oral or middle shield of authors is the rudiment of the stomodaeum,
and appears as a transverse slit (Fig. 26, m). By the development
of the foot this organ is pressed forward, The entoderm-vesicle also
lengthens from behind forward and fuses with the ectodermal rudi-
ment of the stomodacum. At the posterior end where the entoderm
yesicle is in contact with the ectoderm, the anus now breaks through,
without the formation of an ectodermal invagination (F. Scratrpr,
Scnrernonz). The formation of the other organs, in so fav as they
present peculiar features, will be described later.
When the young Lamellibranch leaves the fish, it moves about
with great activity by means of its foot, which has in the meantime
hecome perfected, having lengthened very much and become geni-
enlate. On its lower surface it carries a groove which represents the
rudiment of the byssal gland. The latter arises, as in Cyelas, in the
form of two pits situated posteriorly on the pedal swelling. In conse-
quence of an invagination which forms later, these pits come to lie at
the base of a funnel-shaped pit which is afterwards continued into
groove just mentioned. The persistent byssal gland
of other Lamellibranchs exhibits similar morphological conditions
to those wlready described in connection with the (Unionidae and
Dyelas,
ai
i 2)!
60 LAMELLIBRANCHIA,
6. The Formation of the Organs.”
A. The Shell,
The shell, as in the Gastropoda, is unpaired in its origin, and is
formed by « secretion of the epithelium of the shell-gland (Figs. 14,
p- 28, and 15, p. 31). This embryonic cuticular shell is retained
and passes over into the periostracum (epidermis) of the adult shell.
The latter arises through a secretion of granular calcareous substance ;
this at first accumulates in two complexes lying symmetrically at the
two sides of the body beneath the cuticular shell (Fig. 21 A, p. 44),
which, by further increase, yield the shell-valves. These grow out
dorsally until they meet. The dorsal part of the cuticular shell that
lies between the two calcified shell-valves which are growing towards
one another yields the ligament of the shell (Zrecnmr, p. 43).
The larval shell which thus arises and which has a very simple
structure, is retained in Ostrea, the Unionidae and, as has recently
been proved, in a number of other Lamellibranchs (RYDER, JACKSON,
Bravy, Scureruonz). It is found asa minute prominence on the
umbo of the large shell when the latter has not, as is often the case
with the Unionidae, been de-
a. stroyed through mechanical
causes.
The larval shell usually differs in
shape from the shell of the adult,
and is even sometimes very unlike
the latter, As a rule, the shell
changes very much during develop-
ment. The youngest stage of the
bivalve shell is characterised by a
straight hinge-line and the slight
development of the umbones. This
can be seen in Fig. 27 4, and still
better in Figs. 14-16, pp. 28-33), This
Fig.
larval shell of Ostrea edudix, seen from
27.—A and B, early stages of the
the side and from behind ; (, somewhat
later stage (‘‘ prodissoconch ") of the
larval shell of Ostrea virginiana, oblique
side view. ‘This particular larwa had
just attached itself, the left shell being
stage (Fig. 15 C, p. 81), follows
directly upon that of the unpaired
cuticular shell, and must be supposed
to have arisen out of the latter by the
already lightly fastened to the enb-
stratmm (after JACKSON). gradual swelling and upward growth
of the shell on either side of the dorsal
middle line, causing a slight infolding along this line which gives origin to
the straight hinge. This straight hinge changes, as the shell soon begins to
* We here treat of the formation of the organs only in so far as this as not
already been considered.
THE FORMATION OF THE ORGANS. 61
curve and appears more arched (Figs. 27 B and 18, p. 36), and finally the
umbones appear. The form of the shell seems to be developed in this way in
many Lamellibranchs, as may be seen from the figures given by Lovin and
Jackson and other authors, Jackson (No. 22) gives a special description of
its development in Ostrea (Fig. 27 A-C), and we were ourselves able to observe
it rise in a very similar manner in Dreissensia (No. 27).
It is hardly necessary for us to point out the great difference that exists in
the two forms just named between the larval and the adult shell, and this is
‘still more the case in such Lamellibranchs as Pecten and the Aviculidae, the
larval shell of which resembles that depicted in Fig. 27 C. This latter form
of shell (the prodissoconch of Jackson) * represents a stage passed through by
many Lamellibranchs. Jackson is therefore inclined to consider this form
Sel ax specially peitve. In keeping with this view, we find that Nucula,
whieh, in consequence of other features may be regarded as a primitive form,
and which is found in the lower Silurian [Ctenodonta, one of the Nuetlidae,
sours in the Cambrian], has a shell of somewhat the same shape as the
above.
The young shell grows by the secretion from the mantle-cells of
new calcareous material ; this is deposited both on its inner surface
and at its margin. These deposits give rise to the lamellate and
zoned character of the adult shell, In older stages, these growth-
processes take place chiefly at the peripheral parts of the mantle.
There appears to be no essential difference between the manner of
formation of the inner (nacreous) and the outer (prismatic) layers of
the shell ; the one may pass into the other. ‘The prismatic structure
of the onter layer is apparently due to the fact that the originally
rounded calcareous granules became polygonal through mutual
pressure, For details as to the formation of the Lamellibranch shell
we are indebted especially to the researches of Tunnnene (No. 56),
Exnensavm and F, Mincer (Nos. 11 and 38).
growth of the periostracum also takes place at the edge where
pisses over into a fine cuticle which covers the (ectodermal)
epithelium of the pallial margin. At the periphery, where the cal-
careous shell ends, we thus have the same condition as is shown in
r embryonic times by the whole shell; the shell-integument, as
we covers the mantle-epithelium. Indeed, the whole of the
Tamellibranch shell is to be regarded as a cuticular structure covering
‘the mantle-epithelium.
"In contradistinction to the adult shell (the “ dissoconch") Jackson has
-arched form with well-developed umbones (Fig. 27 C) the
because it usually precedes the adult form of shell, as has
62 LAMELLIBRANCHIA,
Tu the case of Lamellibranchs inhabiting a tube, ¢g., the Gustro-
chaenidae, this latter is secreted in the same way as the shell; the
free edges of the mantle bend over the wide open shell-valves and are
thus able to form the tube (Snurrer, No. 53). The structure of the
calcareous tube of (astrochaena seems closely to resemble that of the
shell.
We still have to mention the relation of the muscles to the shell. The
adductor muscles, as is well known, are inserted into the shell and the rela-
tion of the epithelium to these muscles is interesting. At the points of inser-
tion, the epithelium must cither be modified or must altogether degenerate.
EBRENBAUM assumes that it degenerates, and ascribes to the muscles them-
selves the capacity for secreting the shell-substance (!). As the animal and
its shell grow, the adductor muscles, especially the posterior adductor, con-
tinually change the position of their attachments on the shell. At their
points of insertion, a shell-substance is produced, the so-called transparent
substance, and the presence of this substance on the inner side of the shell
indicates the course taken by the wandering muscle, But that this substance
is seoreted by the muscle itself is very improbable, and we therefore profer to
follow the older necount of TuLLBERG, according to which there is, between
the muscle and the shell, an epithelium which produces the shell-material.
B, The Nervous System.
All the ganglia originate as thickenings of the ectoderm, which
subsequently become separated from the latter. The ganglia arise
separately and become connected later by commissures.
Tt has already been mentioned that the cerebral [cerebro-pleural]
ganglion arises in the Trochophore larva as a neural plate (Figs. 15,
p- 31, 18, p., 36), This consists at first of large closely crowded cells
which, by active division, give rise to a multilaminar cell-plate. The
upper layer of this plate which remains continuous with the body
epithelium becomes raised up, the lower cell-mass becoming detached
from it in the form of two groups of cells. These are the two halves
of the cerebral ganglion, the connecting commissures of which no
doubt arise in the same way, becoming severed from the ectoderm
(this, according to Ziman, is probably the case in Qyelas). FP.
Scumipt, indeed, has claimed for the cerebral ganglia of the
Unionidae distinct origins and secondary connection by means of a
commissure, a condition which will be described in connection with
the Gastropoda (Chap. XXXII.}. In Anodonta, the two halves of the
ganglion arise near the mouth and are separated by the stomodaeum,
above which the commissure extends as a loop.
THE SENSORY ORGANS. 63
‘The pedal ganglia, in Cyelas und the Unionidae, according to the
somewhat similar accounts of ZreauER and F. Scumrpr, with which
also that of ScutenHonz can be harmonised, in their formation are
associnted with the byssal gland, Shortly before the paired byssal
gland becomes invaginated (Fig. 19, p. 40), at the point where it is
to form, 4 number of cells become detached from the ectoderm.
These at first lie beneath the floor of the invagination, but then
separate from the latter and shift further forward, at the same time
coming closer together, forming the rudiments of the pedal ganglia
(Fig. 21 B, p. 44, and Fig. 31, p. 75).
In Teredo, the pedal ganglion, according to Harscuek, arises as an
ectodermal thickening even before the foot begins to form (Fig. 18,
wy p> 36). It occupies at first a large part of the ventral surface,
but appears to decrease in size after its detachment from the ecto-
derm. During its severance, the mesoderm grows round it. The
‘livision into two parts is not so distinct here, but is indicated by a
median line. The two halves of the ganglion are thus in this case
connected from the first. When the foot rises up and grows out on
the ventral side of the larva, the ganglion remains lying at its base.
Tn their manner of formation the visceral ganglia agree closely with
the cerebral and the pedal ganglia. They arise in the groove
between the gills and the body, almost at the posterior end of
the foot.
‘The cerebro-visceral [pleuro-visceral) commissure has its origin,
according to Zrecner, in a cell-strand which becomes detached from
the ectoderm in the groove between the gill and the body, and rans
forward from the visceral ganglion, and later becomes x commissure.
[In Nueula and the Protobranchia generally, distinct pleural
wanglia are present. These are situated immediately behind the
cerebral ganglia at the commencement of the visceral commissures ;
here, also, the pleuro-pedal commissures are for some distance in-
dependent of the cerebro-pedals. In other Lamellibranchs, the
pleural ganglia are fused with the cerebral. Drew was, however,
unable to trace a distinct origin for the pleural ganglia in Yoldia.]
©. The Sensory Organs.
‘The Byes. Tt may be stated with some certainty that the
simply constituted eyes of the border of the mantle, é.2., the so-called
Jneaginations, or optic pits, and the compound ryes arise through «
comparatively slight differentiation of the mantle-epithelium,
lia
64 LAMELLIBRANCHIA,
The ineaginations, the optic nature of which is, indeed, doubtful, are pit-
like depressions of the epithelium, in the cells of which pigment is deposited,
while the pit itself becomes filled with m mass of what appears like a secretion
(conjectured to be a lens).
The compound eyes arise as convex thickenings of the mantle-epithelium
at certain points. In these, conical sensory cells are distinguished from the
pigment-bearing and supporting cells lying between them by the development
of crystal cones and acornea, The visual cells are connected with the fibres
of anerve which is a branch of the mantle-nerve (Canine, Parren, Rawrrz).
‘The eye which thus arises shows some similarity to the compound eye of
the Annelida as recently described by Anpnews.* These eyes of the Lamelli-
branchia cannot well be compared with the compound eyes of the Arthropoda,
since the latter are far more complicated in structure, It is evident that
in neither case can there be any real homology,
The Eyes of Pecten. The mantle-eyes of Pecten, the morphology
and physiology of which are still somewhat obscure, were investigated
from the ontogenetic point of view by Parren (No. 39), but his study
of them was not altogether satisfactory, so that we must content
ourselves with a short reference to them.
The eyes of Peeten, unlike the two modifications of the edge of the mantle
just described in other Lamellibranchs, are highly developed orgaus (Fig.
28). The principal constituents of the eye of Pecten areas follows: there is a
cornea behind which lies a large lens; behind the lens comes @ retina com-
posed of a ganglionic layer, followed by a layer of rod-bearing cells, the most
remarkable feature of the retina being that the rods are directed away from
the light and towards the posterior wall of the eye, This latter is covered
behind by an integument of pigment-cells, in front of which lies the tapetum,
which has a metallic lustre, The innervation of the eye is double, and takes
place by means of a nerve (Fig. 25, 7), which sends out one branch to the
base of the eye, and thence direet to the optic cells, while the second branch
enters the eye laterally, becoming connected first with the ganglionic layer,
and through it coming into contact with the optic cells, For the further com-
plications found in this eye we must refer our readers to the special works of
Canrréne, Bétsceut, Parren and Rawrrz,
ParreEn was able to establish ontogenetically that the eyes at the
edge of the mantle in Pecten arise as knob-like thickenings of the
ectoderm, As these thickenings rise up and become more and more
distinct from the surrounding ectoderm, an ectodermal cone grows
down from the surface towards the interior. While active increase
in number of the cells brings about the growth of the whole struc-
ture, the ectodermal mass directed inwards becomes marked off from
the outer epithelium, a process which is assisted by the growth of
* Compound eyes of Annelids, Jownal of Morphology, Vol, ¥-, 1891,
THE SENSORY ORGANS. 65
connective tissue-cells between the inner ectoderm-mass, this tissue
forming a continuous layer between the two. From this, #.e., from
mesodermal elements, the lens, according to Parren, is formed, while
the inner ectodermal mass yields the principal constituent of the eye.
Eee (alter Sears tae Harsonen's Teel-
1, cornea; 2, 3, pigment stoderm ; blood-
Pieter! wits mpertslas chnriisele tages tnd betwee
fone Si 6, pigment-layer, with the tapetum lying in front of it; 7, optic
‘The way in which the various layers, the ganglionic cell-layer, the
retina, ‘argentea, and the tapetum, ete., arise out of this mass
is described, but these difficnlt points are not made sufficiently
clear.
Purther details concerning the ontogeny of these very peculiar eyes aud
especially ns to the origin of the rods are much to be desired. ‘The solution
¥
a
66 LAMELLIBRANOHIA.
of these problems seems all the more desirable as the eye of Pecten,* in tts
structure stands almost alone among Molluscan eyes, With regard to their
morphological interpretation, we are inclined to agree with Birscuit (No,
7) who showed how the pigment cell-layer of the posterior wall of the eye
passes over into the retina, a closed vesicle being thus formed in the eye, its
anterior wall consisting of the retina and its posterior wall of the
integument. This vesicle must be supposed to have arisen by invagination
and abstriction from the ectoderm, a view with which Patren's observation
of a solid ingrowth can be reconciled. The description given by Parrmy also
of the rise of the lens outside of the optic vesicle supports such a condition if we
do not assume a mesodermal origin for the lens but rather imagine a second
process of invagination such as occurs in the Cephalopodan eye. The rise of
the lens outside of the optic vesicle makes it possible for the more superficial
wall of the latter to be changed into the retina, a change which is impossible
where the lens has itself arisen from this outer wall, as is the case in the
Gastropoda and in some of the Cephalopoda also. The position of the rods
is hereby explained (Bison), Since these always arise at the free ends
of the cells, they are directed forward when the deeper wall of the optic
vesicle is transformed into the retina (Gastropoda, Cephalopoda); but are,
on the contrary, directed backward when the retina is derived from the outer
or superficial wall of the vesicle, The latter must originally have been the
case in Pecten.
The otocysts arise, in Terelo and Anodonta, near the pedal gang
lion as invaginations of the ectoderm which then become abstricted
from the latter and provided with otoliths and sensory hairs (Fig. 18,
vt, p. 36). In Cyecias, the otocysts lie at the two sides of the embryo,
behind the lateral end of the ciliated area. [In the Protobranchia the
otocysts retain their connection with the exterior throughout life.]
Spengel’s olfactory organs and the abdominal sensory organs
(TH1zLx) show, by their structure, that they are mere modifications
of the body-epithelium, a
D. The Alimentary Canal.
The structure of the alimentary canal, being greatly influenced by
adaptation to different conditions of life, varies in certain points in
the different forms. In Ostrea, for example, the archenteron is said
to pass over direct into the definitive intestine the blastopore remain:
ing open, while in T¢redo, as well as in Vyclas and the Unionidae, the
blastopore closes and a true stomodaeum forms. his condition, and
#5) jylus has eyes similar in structure to those of Pecten, The
found on the dorsal papillae of Onchidium also resemble those of Pecten
in so far os the rods in them are turned away from the light. We thus
find similar complicated structures, which must have arisen in altogether
different ways.
THE ALIMENTARY CANAL. 67
tts relation to the other ontogenetic processes, have already been
described in a former section (p, 30). The ectodermal invagination
yields the oesophagus; the stomach, liver and intestine are ento-
dermal. The anus, in the majority of cases observed, seems to have
been formed by direct fusion of the entoderm with the ectoderm, so
that the posterior part of the intestine would be entodermal ; in
Teredo, however, there is, according to Harscwex, a proctodaeal in-
vagination, und a similar invagination is described by Vorurzkow
as oceurring in Entovalva (No. 57).
The further development of the intestine consists in its increase
in length, as a result of which it becomes coiled. A circular con-
striction marks off the stomach from the intestine. As early as the
Trochophore stage, a pair of sac-like outgrowths appear in connection
with the stomach; this is the rudiment of the liver (Fig. 16, p. 33)
with which the yolk-laden remains of the macromeres become incor-
porated (Fig. 18, p. 36). A peculiar phenomenon in connection with
these two liver-sacs, which at first are spherical, is the occurrence of
rhythmical movements ; these are no doubt to be traced back to the
action of mesoderm-cells which have become apposed to the entoderm
wall (Lovéx, Zrecier). The passages from the liver into the
stomach which at first are wide, become narrow later and form the
efferent ducts; the bulgings found on the liver-sacs mark its separate
lobes and lobules (Fig. 31, /, p. 75).
In the stomodaeum of Cardium, Lovin observed a small bulging of the
ventral wall which involuntarily recalls the radula-sac of other Molluses, an
organ which is known to be wanting in the Lamellibranchia, It cannot be
connected with the crystalline style-sac, as this is invariably an entodermal
derivative. The sac which contains the crystalline style is formed as an out-
growth of the wall of the stomach. This structure which, as has long been
known, occurs also in the Gastropoda, appears, according to the most recent
view, to yield a secretion (the crystalline style) which serves for enveloping
solid particles of food, and thus protects the wall of the intestine (Bannors).
No statements ns to the ontogenetic formation of the crystalline style-sac
are known to us.
‘The layer of muscle and connective tissue which forms the outer
wall of the intestine is yielded by the mesoderm-cells distributed in
the primary body-cavity, which become applied either to the ento-
derm or to the ectoderm.
a
68 LAMELLIBRANOHIA,
E, The Gills.
Tu those Lamellibranchs in which the formation of the gills has
been studied, they are found to arise in one of two [three, ¢/. p. 45]
different ways which are somewhat difficult to harmonise in their
early stages. According to one method, which has already been
described for Cyelas and Teredo (pp. 42 and 44), a fold resembling the
mantle-fold rises between the latter and the foot, and develops from
behind forward, The outer and inner surfaces of these folds show
groove-like depressions lying at right angles to the longitudinal axis
of the folds ; these grooves deepen and, meeting those of the opposite
surface, fuse together. As the gill-fold becomes perforated along
these lines, fissures result which extend in from the free margin
of the folds towards their bases (Fig. 31, p. 75). The gill now con-
sists of a series of consecutive lobes which decrease in size from
before backward.
According to the other method of gill-formation, which has been
observed in Mytilus, Dreissensia, Ostrea (a somewhat similar method
being found also in the Unionidae),* a papilla arises on each side of
the body between the mantle and the median visceral mass, and
behind these new papillae arise (Fig. 26 C), A longitudinally
placed row of papillae thus arises by the continued development of
fresh papillae behind those already formed. These, by the develop-
ment of interfilamentar junctions, form the inner branchial leaf,
while the outer leaf is produced by a similar row of papillae which
arise somewhat later.
The further development of the papillae was studied by LacazE-
Durusiers in a form belonging to the last category, viz, in Mytilus
edulis (No. 28), Jackson also has recently investigated the forma-
tion of the gills in Ostrea, and has arrived on the whole at the same
results as Lacazu-Duruimrs (No. 22).
During the development of the inner branchial leaf, the papillae
increase in number, new ones continually budding out posteriorly.
* This seems also to be indicated by the observations made by Lovity on
Montaouta. The filiform permanent gill, of Peefen at any rate, arises as
papillae, and Ray Lankesrnr states that the gills of Pisidiwm a first in
the form of papillae, although these, from the figures, at first like the
mere swellings of a fold. These statements recall the condition in the nearly
related genus Cyclas, in which also papilla-like structures produced by the
splitting of a leaf are found as the rudiments of the gills. It is, however,
possible that Pisidiwm, in the formation of its gills, may be somewhat nearer
‘the primitive condition.
THE GILLS. 69
The papillae are thickened at their free ends (Fig. 29 A). The
continued extension, anteriorly and posteriorly, of these free ends
leads to fusion of the papillae, so that the series may now be regarded
44 a membrane perforated by parallel vertical slits, this membrane
representing the rudiment of the inner branchial leaf. In most
Lamellibranchs, however, each leaf consists of two lamellae. The
second or ascending lamella of the inner leaf arises by the bending
inward of the free edge of the primary fold formed by the fusion of
the papillae (Fig. 29 B); this new lamella then grows npward parallel
to the (now outer or descending) lamella towards the base of the
latter, The inner lamella thus formed is at first an unbroken menm-
brane, the slits only appearing in it when it has increased in size,
Fin 24.—Diaeram of the development of the gills fu a Lamellibranch possessing tw
Deeeen eves ie cork ella Th tunes cr ontor leach leaf, fost ve, mantic,
The outer branchial leaf now appears and becomes applied to the
posterior half of the base of the inner leaf when the latter consists
of about twenty papillae and when its inner or ascending lamella is
partly formed (Fig, 29 C). The outer leaf forms on the whole in
the same way as the inner, but, in it, papillae are said to form
‘as well as posteriorly, and the leaf, in order to yield a
second lamells, bends outwards and not inwards (Fig. 29 D). The
fusions of the free edge of the inner lamella of the inner leaf and the
buter lamella of the outer leaf with the integument of the body take
place Tater, and vary in extent greatly in different Lamellibranchs,
being altogether wanting in some.
an
70 LAMELLIBRANCHIA,
In Mytilus, as in some other Lamellibranchs (e.g., Pecten, Arca) the gills,
even in the adult, consist of individual filaments which, however, are arranged
in just the same way as the branchial leaves of other forms, The inner row
becomes bent inward to form the ascending lamella of the inner leaf, while,
in the case of the outer leaf, the filaments are bent outward (Fig. 29 ). A
section of these gills has the form of a W, and thus resembles 4 section of the
gill-leaves in the Eulamellibranchs (Fig. 30 £). The free ends of the fila
ments seem to be connected by continuous strand of tissue running parallel
to the length of the gill-leaf. This latter must be regarded as the modified
representative of that transverse connection found uniting the free ventral
ends of the papillae when the gill first arose, shifted dorsally, The papillae
themselves correspond to the filaments of the adult gill. Since, in Mytilus
also, the reflected or ascending portion of the gill is at first represented by a
solid plate (see the above description of the development of the gill) in which
the slits arise secondarily, the Mytilus gill, in its later stages, passes through a
condition resembling that seen in the earliest gill-rudiment in such Lamelli-
branchs as Cyclas and Teredo [or better still, in Pholas, StsGenroos], the
wills of which originate as leaves. There is therefore some difficulty in
regarding, with many authors, the later filiform condition of the gill as am
original condition, This difficulty is increased by the fact that the gills of
Mytilus, Pecten, ete., which consist of single filaments, have, when regarded
as a whole, the general characters of a branchial leaf with descending and
reflected or ascending lamellae, the descending and ascending limbs of the
same filament being united together by fusions of tissue at certain points, the
so-called interlamellar junctions; further, the adjacent filaments of the same
row, both im the ascending and descending limbs, are held together by the
interlocking of some specially long cilia, Wherever, therefore, we have gills
consisting of independent but reflected filaments, the assumption that these
filaments might have arisen by a secondary separation of the gill-bars in a
primary branchial plate is suggested (p. 72).
It appears that the papillae correspond to the gill-burs or, as they
are generally termed, filaments of the adult and the slits to the
interstices between these bars, The differentiation of the bars would
then have to take place from the posterior end of the gills, The gill
of the adult Lamellibranch is usually a much more complicated strue-
ture than the larval gill up to the stage we have described. Between
the filaments of each lamella, as well as between the ascending and
descending lamellae of each leaf, there are connections which may
consist of solid cell-strands, of hollow vascular junctions, or simply
of interlocking cilia, so that the leaves are connected by longitudinal
interfilamentar and by transverse lamellar junctions. The mesoderm
of the papillae yield the connective tissue, the blood-vessels and the
skeletal rods which support the gill-bars, and from thence extend
in certain forms into the complicated junctions found in most
Lamellibranchs (Eulamellibranchs and Pseudolamellibranchs).
THE GILLS. val
To cases in which, as in Cyclas, the rudiment of the gill is leaf-like
and only breaks up later into consecutive lobes through the slits
which arise in it, we may assume that these lobes unite later, like
the papillae, to form the branchial leaf.
‘If we compare the origin of the gills in Teredo and Cyclas on the one hand
and Mytilus, ete.,on the other, we might at first feel inclined to regard the
method seen in the former as the more primitive, since the formation of the
leaf precedes that of the papillae. The gill originates ns a Jeaf, and is only
later broken up by incisions into separate lobes which are arranged in the
same way as the papillae in other cases. This view, which is founded on the
ontogeny of 4 few forms such as T'vredo and Cyclas, which in other respects
‘are undoubtedly specialised, cannot, however, in any way be reconciled with
the morphological conditions of the definitive gill in the different Lamelli-
branchs. A comparative study of these latter suggests rather that the origin
of the gills in the form of papillae, as in Mytilus, was the primitive condition,
‘Unfortunately very little is as yet known as to the mode of formation of the
gills, but if we examine the apparently carefully investigated development of
these organs in Mytilus and Ostrea, we find that certain ontogenetic stages can
be most exactly matched in the shape of the gills of certain adult Lamelli-
‘branchs. Thus, in Dimya, according to Dat, the gill on each side consistsof one
row of branchial filaments (Fig, 30 2) and in Avusiwm Dalli (and as it appears
also in Arca vctocomata) there are two such rows on each side (Fig. 30 C)."
‘The branchial filaments are not connected, and thus represent the ontogenetic
stage at which there are one or two rows of papillae. The further develop-
‘ment of the gills may be imagined to have taken place by the free ends of the
branchial filaments becoming connected, in the manner illustrated in the
of Mytilus (p. 68), In this way the row of branchial filaments gave
rise leaf. This leaf doubled back on itself, when an increase
‘of surface was needed and growth in a straight direction was not possible on
account of the want of room in the shell (Fig. 29 B-K). The ascending (re-
fected) lamella of the branchial leaf thus arose; the free edge of which may
finally fuse with the mantle, as ix. the case, for example, with the ascending
‘Jamelia of the outer branchial leaf in the Unionidaa (Pig. 30 F).
of gill which consists of single filaments, bent back upon them-
indicating the two lamellae of the later branchial leaf (Fig. 30 D)
‘uns repeatedly been held to be very primitive and has been thought to repro-
‘sent the stage succeeding that in which the gills consisted of two straight rows
ae rs: 00). Such gills are found in rigonia (Pexseneen) and
whieh may be considered as very old forms. The gill-leaf consisting
‘was thought to have arisen from the union of these reflected
To us, the reflection of the single filaments and their regular,
‘arrangement, such as is seen in the gills of Peete and Mytitus
the accounts given by Persesxen, Dave and MimsuKuni
sepa of the Lamellibranch gills, It is impos-
far these may represent primitive conditions or may to
tion-phenom aay, for it is evident that these latter do
72 LAMELLIBRANCHIA,
and even in Arca, is very difficult to explain, When isolated filaments for the
sake of increase of surface grow in length, they are not likely to retain such:
a regular arrangement, even if we bear in mind their position im one row,
the limited space within the Lamellibranch shell, and the circulation of the
water between them, We therefore think ourselves justified in assuming, in
the case of those Lamellibranch gills which, while filiform in structure, show
such a regular leaf-like shape, a secondary breaking up of a gill which
originally consisted of two plates to which allusion has already been made
(p. 70). A satisfactory explanation of these obscure points can, however,
only be obtained by comprehensive investigation not only of the gills them-
selves but also of the whole structure of those Lamellibranchs which may be
regarded as transitionary forms.
B. cs
ate
ae
“ b
Fro, 30,—Diagrams illustrating the position of the gills in the Lamellibranchia, aay
Foldia; B, Dimya; O, Anusinm Dali; 1, Arca none: B, Anodonta; 7, foots
m, mantle; i, inner, ¢, outer branchial leat.
We may regard as the most primitive form of the Lamellibranch gill a
ridge [the ctenidial axis] with two rows of branchial filaments. In place of
the filaments, triangular leaflets must originally have been present, with
vertically expanded surfaces, placed transversely to the long axis of the ridge,
@ condition permanently retained in the gills of Nucula and Yoldia (Fig. 30
A, Mirscxor). Taking into account the similar form of the gills in the
Aspidobranchiate Gastropoda, this latter condition might be regarded as the
original condition, It-is, indeed, not essentially different from that with the
double row of papillae, since the leaflets correspond in every respect to the
still unreflected papillae.
THE KODY-CAVITY, ETC. 73
‘The leaflets by lengthening and narrowing gave rise to the filaments, The
gill of Nucwla is further primitive in its free pointed posterior termination, and
may without further question be directly homologised with the bipectinate
gill of the lowest Gastropods. ‘This last view of the Lamellibraneh gill, which
was advanced years ago by Lecernant (No. 90), bas recently, owing to the
esearches of Persennen (Nos. 40 and 41), Menecavx (No, 35), and others,
received great support and has become almost universally adopted. The
ontogenetical fact that one of the rows (the inner row) appears first and the
other (outer) row only much later does not, indeed, appear to be in harmony
with it. In tracing the gill back to that primitive form, we should expect
that the two rows of papillae would arise almost simultancously.
The rise of the gills in the form of leaves, as in Teredo and Cyclas, may,
according to the present state of our knowledge, best be compared to the pro-
duction of the branchial filaments or papillae from the ridge. We should,
indeed, require to understand more exactly the way in which the second
branchial leaf found in these animals arises. We must be careful not to
ascribe too great significance to the method of formation of the gills in Teredo
and Cyclas, because these are, as has already been shown, highly specialised
Lamellibranchs, and because, in the nearly related Pisidium, the leaf-like
rudiment of the gills is far less distinct (according, at least, to Rav Lan-
sesrem), These varied conditions are somewhat difficult to reconcile, and
their explanation is very desirable. So far, there are many indications that,
in the development of the Lamellibranch gills, great modifications have been
introduced which render it very difficult to form conclusions os to their
original constitution.
P. The Body-cavity, the Blood-vascular System and the Kidney.
The development of the closely related structures, the body-cavity,
the blood-vascular system and the kidney, have been investigated in
the Unionidae and in Cyclas, but are best known in the latter, Our
information on these points is due to the investigations of Leypic,
Srepanorr, Gantw and y. Juerixc, which have recently been ex-
tended and supplemented by Zmauen. The bistory of the meso-
dermal structures, in Cyclas and the remaining Lamellibranchs has,
indeed, not yet been exhausted, as will be evident from the following
account.
‘The first rudiment of these mesodermal structures appears at a
time when the embryo, through the development of the foot and the
formation of the mantle-folds passes out of the Tvoehophure stage, i,
ut & stage occurring between the two depicted in Figs. 19, p. 40, and
al Zane a4.
Trochophure there is on each side of the intestine a compact
ia ‘mesoderm-cells (Fig. 19, mes) which ZrmGceR claims as the
‘mesoderm-bands. In the anterior end of each of these masses, u
74 LAMELLIBRANCHIA.
cavity arises which soon, by the regular arrangements of its cells into
an epithelium, assumes the form of a vesicle. This is the paired
rudiment of the pericardium.
The rise of the paired pericardial vesicles out of the bilateral
ainesoderm-rudiment so nearly resembles the formation of the primitive
segments in the Annelida and the Arthropoda that we must regard
the pericardial vesicles as coelomic sacs and their cavities as the
secondary body-cavity. The coelom in the Lamellibranchs, however,
only attains a very small size, and the definitive body-eavity which
contains the organs arises independently of the former as a pseudocoele.
‘The view that the pericardial sacs must be regarded as the coelom
rests chiefly on the fact that the kidney shows the same relation
to this cavity (Fig. 32) as do the nephridia in the Annelida to the
cavities of the primitive segments (secondary body-cavity). This
relationship is very early developed in the embryo of Cyclas.
The kidney (organ of Bojanus), Behind the pericardial vesicle,
the mesoderm-cells soon beeome grouped in the form of a tube, the
lumen of which communieates with the cavity of this vesicle. This
tube, which at first rans upwards, and then again bends downwards,
is the rudiment of the organ af Bojanus (Fig. 21 A, n, p. 44). Its
upper end, which opens into the pericardial vesicle (Figs. 21, 32) is
lined with cilia, The resemblance thus brought about between the
organ of Bojanus and a nephridium is heightened later when the
lower end of the canal fuses with the ectoderm and communication
with the exterior is thus established (Fig. 31, »,).
From Zircrer's description, it is not clear whether the formation of the
efferent duct takes place through the direct fusion of the lower end of the rudi-
ment of the kidney with the ectoderm, or whether on invagination of the ecto~
derm takes part in it. ZmGbEeR’s statements on the whole support the first
hypothesis, which also agrees with the manner of formation of the nephridia
in the Annelida as described by Benou.* But since, as we shall see, in the
Gastropoda and also in the Annelida (Vol. i., p. 297), an ectodermal invagination
takes part in the formation of the nephridia, this question cannot here be
decided.
The statements which have been made as to the rise of the kidneys as mere
depressions of the ectoderm (Ray LanxkestER, Gaxtx) must be considered as
refuted, especially as the morphological agreement of the organs with the
nephridia of the Annelida points to a similar method of formation. We are
indeed led to look for a still closer relation of the nephridia, when forming,
with the coelomic sacs, and such a relation will be found in the Gastropoda.
*R. S. Benon. Neue Beitrige zur Embryologie der Anneliden, I. Zur
Entwicklung und Differenzirung des Keimstreifens von Lumbricus. Zeifsch.
J. wiss. Zool. Ba, 1. 1890,
THE PERICARDIUM AND HEART. 17
32.A, x”). The right and left pericardial vesicles now grow towards
each other and unite above the intestine at the two sides of which
they formerly lay; in exactly the same way they unite below the
intestine, i.e., ventrally to it (Fig. 32 A-D), the intestine having been
previously invested by certain of the mesoderm-cells which were
distributed in the primary body-cavity.
The circumerescence of the intestine by the pericardial vesicles
and the fasion of these latter, as described by Zreener, strikingly
recalls the fusion of two primitive segments in the Annelida to form
& segmental cavity (Vol. i., p. 290). We have already drawn atten-
tion to the relation of the kidneys (nephridia) to the pericardial
cavity. The walls of the pericardial vesicles which come into contact
and which, in following the comparison, would be the equivalent of
the intestinal mesenteries, seem completely to degenerate, so that the
cavities of the two pericardial vesicles unite together to form a
common cavity. The formation of the heart, which will be described
immediately, takes place outside of this space, ¢¢., outside of the
secondary body-cavity and within the primary body-cavity. This
also would agree with the condition in the Annelida, where the dorsal
vessel arises between the splanchnic layer of the mesoderm and the
entoderm, and therefore in the primary body-cavity (Vol. i., p. 291).
‘The formation of the heart is introduced by the cirewmerescence of
the intestine by the pericardial vesicles, The wall of the vesicles
which is turned to the intestine yields the wall of the ventricle.
This statement made by ZreGueR must be taken to mean that,
from that wall of the vesicle, elements are produced by delamination
which yield the heart, while the wall of the pericardial vesicle itself
represents the investing peritoneal epithelium (Fig, 32 B and 0).
‘The same process would be repeated in the formation of the auricles.
‘These latter had already arisen as the invaginations of the pericardial
vesicles described above (Fig. 32 A). These invaginations unite
with the opposite wall of the pericardial vesicle and the auricles,
which form by the widening of the originally narrow invaginations,
fuse with the rudiment of the ventricle (Fig. 32 2-D). At the
points of junction, the apertures and valves between the ventricles
and the auricles arise.
‘The efferent and afferent vessels of the heart (aortae and branchial
veins) arise separately from the rudiment of the heart and are no
doubt formed by the grouping together of those mesoderm-cells
‘which are derived from the wall of the pericardium, or were already
present in the body-cavity, i.e, they originate as cavities between
rs
ves LAMEDLIBRANCHIA,
eavity arises which soon, by the regular arrangements of its cells into
an epithelium, assumes the form of a vesicle, This is the paired
vudiment of the pericardium.
The rise of the paired pericardial vesicles out of the bilateral
mesoderm-rudiment so nearly resembles the formation of the primitive
segments in the Annelida and the Arthropoda that we must regard
the pericardial vesicles as coelomic sacs and their cavities as the
secondary body-eavity. The coelom in the Lamellibranchs, however,
only attains a very small size, and the definitive body-cavity which
contains the organs arises independently of the former as a psencdocoele.
The view that the pericardial sacs must be regarded as the coelom
rests chiefly on the fact that the kidney shows the same relation
to this cavity (Fig. 32) as do the nephridia in the Annelida to the
cavities of the primitive segments (secondary body-cavity). This
relationship is very early developed in the embryo of Cyclas.
The kidney (organ of Bojanus), Behind the pericardial vesicle,
the mesoderm-cells soon become grouped in the form of a tube, the
lumen of which communicates with the cavity of this vesicle. This
tube, which at first rans upwards, and then again bends downwards,
is the rudiment of the organ of Bojanus (Fig, 21 A, n, p. 44). [ts
upper end, which opens into the pericardial vesicle (Figs. 21, 32) is
lined with cilia, The resemblance thus brought about between the
organ of Bojanus and a nephridium is heightened later when the
lower end of the canal fuses with the ectoderm and communication
with the exterior is thus established (Fig, 31, »,).
From ZixG.er’s description, it is not clear whether the formation of the
efferent duct takes place through the direct fusion of the lower end of the rudi-
ment of the kidney with the ectoderm, or whether an invagination of the ecto-
derm takes part in it. Zrmqnen’s statements on the whole support the first
hypothesis, which also agrees with the manner of formation of the nephridia
in the Annelida as described by Bercu,* But since, as we shall see, in the
Gastropoda and also in the Annelida (Vol. i., p. 297), an ectodermal
takes part in the formation of the nephridia, this question cannot here be
decided.
The statements which have been made as to the rise of the kidneys as mere
depressions of the ectoderm (Ray LanxesTer, GANtN) must be considered as
refuted, especially as the morphological agreement of the organs with the
nephridia of the Annelida points to a similar method of formation. We are
indeed led to look for # still closer relation of the nephridia, when forming,
with the coelomic sacs, and such a relation will be found in the Gastropoda.
“BR. 8S. Bencu, Neue Beitrige zur Embryologie der Anneliden. I. Zur
Entwicklung und Differenzirung des Keimstreifens von Lumbrieus. Zeitach.
J. wise, Zool, Bd. 1, 1890.
ee
THE KIDNEY. 75
As the kidney develops further, its tube becomes coiled (Figs. 21
A p. 44, and 31), Three sections can then be made out in it; a short
ciliated section, a long glandular section and an efferent section.
‘The latter, which is not ciliated in the embryo, shows ciliation at a
later stage when the efferent duct of the genital orgaus opens into
it near its end, and it thus serves to transmit the genital cells.
‘The three sections of the embryonic kidney are the same as those
that can be distinguished in the adult organ, but the latter is further
modified in so far as the middle section is more coiled. This gives
rise to the renal sac and to the more complicated portion, the renal
d. v. pack
un ; the middle segment is that portion of the kidney
on takes place. In the Uionidae, and in other Lamelli-
idle section is not so much coiled as in- Cyclas, but.
secretory epithelium is increased by the formation
- In the primitive forms (Nuewa, Solenomya), the
‘the form of a slightly coiled tube, the inner wall
‘no great increase of surface,
longer, as in the Unionidae, the gills also lengthen
us takes up @ somewhat different position. Its original
pericardium and the posterior adductor which is illus-
78 LAMELLIBRANCHIA.
‘the mesodermal tissues of the latter, The passage of these out of
the pericardium that surrounds them is, owing to the nature of their
origin, easily understood (Fig. 32 Cand D).
‘This method of formation of the heart from the mesial walls of the pericardial
yesicles explains how, in the adult, the intestine traverses the heart. Phylo-
genetically, this condition is supposed to have arisen through a blood-sinus
surrounding the intestine developing thicker walls and thus becoming the
heart (GropsEN). Since the vessels arise distinot from the heart, such an
origin of the latter is not in any way improbable. On the other hand she
fact that, in the Lamellibranchia, a paired heart lving dorsally to the
intestine, and with each half enclosed in a separate pericardium may occur
(Area) has led to the conclusion that the unpaired heart which, in the higher
forms, surrounds the intestine, might have arisen by the fusion of these two
hearts (Turmux, Chap. xxx.). ‘This view seemed to be supported by the fact
that the double heart is found in just those forms that are very primitive, and,
further, that a double heart is also present in various Annelids,
‘The paired origin of the heart (Figs. 32 and 33 C), might perhaps be regarded
‘as primitive feature and as indicating that the heart was originally a paired
vessel, but this view is not justified, since it is supported merely by the paired
development of the ecoelom and the part taken by the latter in the formation
of the heart. A comparison with the manner in which the heart arises in the
Annelida and its formation in the Lamellibranchia should help to elucidate
these points (cf. Vol. i., p. 291).
In the Annelida, the paired origin of the heart is still more
marked than in the Lamellibranchia. Even during the growth of the
primitive segments towards the dorsal middle line the rudiment of
the dorsal vessel appears on that side of the splanchnic layer which
is turned towards the entoderm (Fig. 33 A, I. and IL). The dorsal
vessel is therefore paired and, as the primitive segments grow further,
shifts towards the dorsal line (A IT. and IIL.) On this line, the two
rudiments of the heart finally meet (A 1V,) and fuse to form the
unpaired dorsal vessel, except in those forms in which they remain
distinct in the adult. With this latter condition in which the dorsal
vessels remain distinct, the heart of Arca shows the greatest agree-
ment. We must suppose that, in Arca, each of the two pericardial
sacs, by the invagination of its inner wall, developed a ventricle (Fig,
33 B, L-IV., 1). The fusion of the pericardial sacs above and below
the intestine did not take place, and in this way the union of the
two rudiments of the heart was prevented. In most Lamellibranchs,
on the contrary, the cireumerescence of the intestine takes place <
the whole median wall of the pericardial vesicles takes part in the
formation of the ventricle, and the latter thus surrounds the intestine.
(Fig. 33 C, 1,-1V.). The rise of this single ventricle from distinat
THE BODY-CAVITY, ETC, 79
rudiments is suggested here also, and the double character is still
more recognisable in the rise of the auricles, which originate as
invaginations of the outer walls of the pericardial vesicles (Fig. 32).
Bat this double character may, as already mentioned, be derived
from the connection of the formation of the heart with the paired
evelomic sacs. Further, the paired character of the heart, as repre-
sented in the adult condition, seems to us easily explained by these
ontogenetic processes,” ‘The fact that, in the paired heart of Arca,
On Goo CS. iad
020). Oc)
the formation of the heart. A, in the Annelida,
#, in Avon, ©, in other Lamellibranchs, (The auricles are omitted for the sake of
intestine 4, anes rudiment of the heart (united, in A IV. and
wv, R the two pericardial vesicles (united in CTV, to
19, ee 4p, splanchni« layer of the coelomic sac (primi-
‘there i is a common anterior and posterior aorta, seems to point rather
to the sine up of an originally single heart than to the union of
‘ distinct, hearts. The paired dorsal vessel of the Armelida often
between the two parts,+ and this also might be a
Lo lg ea such a view of the Lamellibranch heart, speaks
“heey stage through an arrest of develo}
Soo that the method of acuabon described would
as double heart in cases in which such a heart
to he animal,
and Acanthodrilus show the recurrence of conneo-
hearts. In another Acanthodrilus almost the whole
paired and is without transverse connections, but in its
still mcomnection. ag omg Note on the Paired Dorsal
Proc, Roy, Phys, Soe, Edinburgh. Vol. viii
—
80 TLAMELLIBRANCHIA,
consequence of its having developed from an originally single
rudiment. -
An attempt has been made to explain the rise of a paired heart (as
the original condition) through the relation of the two parts to the
gills lying at the two sides of the body (Tarene, Chap, xxx.). If
the paired heart really represents the primitive condition, this ex-
planation would be very plausible, but the Annelida, Arthropoda
and Amphineura all agree in showing us the heart as an unpaired
organ lying dorsally to the intestine.
We have still to mention that the heart in a few more modified
forms (Teredo, Ostrea, Mulleria) lies ventrally to the intestine. In
these cases, the union of the pericardial vesicles to form the unpaired
heart has no doubt taken place beneath the intestine. [In Nweniu,
Arca and Anomia, the heart is dorsal to the intestine.)
The condition of the secondary body-cavity and the kidneys in the
Lamellibranchia recalls very strikingly those found in the Crustacea
and Peripatus (af, Vol. ii, p. 180, and Vol. iii., p. 204). In these
latter, a part of the coelom is directly incorporated in the kidney, with
which it is also functionally united. We are perhaps justified in
regarding the pericardium of the Lamellibranchia, into which the
renal fannel opens as it does in the Arthropoda into the cavities of
the primitive segments (or coelom), as the homologue of the end-sae
of the excretory organs in these forms. The fact that the coelom-
sacs of the two sides are here united, can make no difference, for this
does not cause the heart to lie, as may at first appear, in the secondary
hody-cavity, but it is still found outside that cavity, as it is also in
the forms mentioned above.
If the pericardium possesses the morphological significance ascribed
to it, we might perhaps expect that its physiological function should
be modified in the same way as in those forms in which the secondary
body-cavity has entered into such close relation to the kidney. ‘This
assumption seems actually to be confirmed, when we consider the
so-called pericardial gland, ‘This gland, the so-called red-brown
organ or Keber’s organ, arises as outgrowths of the epithelium of the
pericardial wall and lies either on the auricles or on the anterior part
of the pericardium (Grossen). This organ is most probably ex-
cretory and, since it owes its origin to the pericardial epithelium, it
seems not unsuitable to ascribe to the latter a similar significance,
‘The close relation in point of position existing: between both the
pericardium and the pericardial gland and the blood-yascular system,
makes such a view appear possible,
MUSCULATURE AND CONNECTIVE TISSUES. 81
According to our present anatomical and ontogenetical knowledge,
the communication between the pericardial cavity and the blood-
vascular system which was formerly assumed, does not exist. The
idea of an admixture of water with the blood which was also held
must be regarded as exploded, quite apart from the fact that the
‘transmission of water from outside through the organ of Bojanus
into the pericardium seems from recent researches to be highly im-
probable (Rawkry). ‘The structure of the organ itself as well as the
direction of the cilia within it are unfavourable to such « process.
Tndeed the whole idea of the reception of water into the body of the
Lamellibranch from without, which has often been adopted as an
explanation of the swelling of the foot, must be regarded as refuted.
‘The pores which were supposed to conduct water from without into
the foot could not be demonstrated ontogenetically (ZimcLER). The
swelling of the foot, as is evident from the statements of a number of
authors (CanmeRe, Freiscamann, Scuremenz, Ranxry, ete.), is
rather due to the fact that the greater part of the blood is driven
into this organ. This is brought about through the blood being
retained in the foot, the valve at the entrance to the sinus venosus
being closed and the blood which was emerging from the foot being
thus retained within it. Besides this, the quantity of blood already in
the foot is increased through the flow of fresh blood from the anterior
aorta. When the foot is extended, the sphincter at the point where
the posterior aorta emerges from the heart contracts, so that the
greater part of the blood is obliged to flow through the anterior aorta
into the foot, During this process, a certain amount of blood still
circulates in the heart, so as to prevent an arrest of the whole circula-
tion. When the valve in the sinus venosus opens, the blood flows
ont of the foot, and as the latter ceases to extend, the sphincter of
the posterior aorta opens again, until, when the animal moves on
again the same process is repeated.
G. Musculature and Connective Tissue.
“The only organs as yet referred to as differentiations of the mesodern
have been the coelom, the kidney and the blood-vascular system, but.
there are other structures mesodermal in origin, which, indeed, up to
the present have received little attention from zoologists; these are the
imusenlature and the connective tissue, and, further, the genital organs,
which will be dealt with immediately. The muscle-cells are formed
by the detachment of single cells from the mesoderm-inass, the distri-
a
—
82 LAMELLIBRANCHIA,
bution of these in the pseudocoele, and the further growth of the
isolated cells into contractile fibres. When considering the larval
forms, we pointed out that these fibres become applied to one another
to form larger complexes which are the muscles of the larva and the
adult (Fig. 15, p. 31; Fig. 18, p. 36), The musculature of the foot
arises from the gret increase in number of the cells detached from
the mesodermal mass, and the massive connective tissue both of the
foot and of the rest of the body has the same origin.
H, The Genital Organs,
The ontogeny of the genital organs has not as yet been sufficiently
studied. In Cyclas, the genital glands originate from the two meso-
derm-bands and lie, as a rather large mass of cells, beneath the peri-
cardial vesicle and close under its wall (Zrecuer). A somewhat later
stage in the development of these glands is depicted in Fig, 21 A, g,
p. 44. At w still later stage, they form two club-shaped russes,
the broad surfaces of which meet in the middle plane, lying above the
cerebro-visceral commissure (Fig. 31, 7).
From what is as yet known of these glands in the Lamellibranchs, they do
not bear any direct relation to the pericardial sacs, i., to the epithelium of
the secondary body-cavity, as was found to be the case in the Annelids and as
we shall presently see that they do in other Mollusea. Our knowledge of the
subject is far too slight to justify further conclusions, but we may suggest that
the close relation of the coelom to the kidney has led to an alteration in the
conditions and thus to a gradual shifting of the genital rudiment out of the
coelom. The efferent ducts must at the same time have undergone alteration,
but with respect to these points, ontogeny fails us and we can only draw our
deductions from the anatomical conditions,
The relation of the efferent genital ducts varies in the Lamellibranchia,
Most usually, they open on the surface of the body independently of the
nephridia. In by far the greater number of the Eulamellibranchia they open
into the supra-branchial cavity near the external aperture of the kidneys.
In other Lamellibranchs, they and the efferent ducts of the kidneys open into
acommon cloaca (Area, Pinna, Ostrea, Cyclas); in others again, they emerge
further back in the organ of Bojanus (Anomia, Spondylus, Pecten, Lima), and
only in a few primitive forms (Nuenla, Solenomya) do the genital products
pass into the kidney, not far from the reno-pericardial aperture (PELsENnER,
No. 41).
Distinct efferent ducts for the kidneys and the genital organs are found in
such forms as, from their structure, may be considered as phylogenetically
younger than the others, while the two organs are connected in those
Lamellibranchs which, by their organisation and their early geological
necurrence, are proved to he of greater age (v. Juenina). These facts indicate
84 LAMELLIBRANCHIA.
12. Fremminc, W. Studien in der Entwicklungsgeschichte der
Najaden. Sifzungaber. k. Akad, Wiss, Wien. Math. Nat.
Cl. Bd. li. Abth. iii. 1875.
13, Foret, F. A. Beitriige zur Entwicklungsgeschichte der Najaden.
Med. Inaug. Diss, Univ, Wirzbur7. 1867.
14, Funzarton, J. H. On the Development of the Common Scallop
(Pecten opercularis). Eighth Annual Report of the Fishery
Board for Scotlaul. Part iii. Edinburgh, 1890.
15, Gorrre, A. Bemerkungen uber die Embryonalentwicklung der
Anodonta piscinalis. Zettsehr. 7. wie, Zool. Bd. li, 1891.
16. Grosnen, C. Die Pericardialdriise der Lamellibranchiaten.
Arh, Zool. Inst. Griv, en. Bd. vii. 1888.
17. Groppen, C. (1) Die Pericardialdriise der Lamellibranchiaten
und Gastropoden. Zool. Anz. 1886. (2) Die Pericardial-
driise der Opisthobranchier und Anneliden, ete. Zovl. An-
seiyer. 1887, (3) Die Pericardialdriise der chiitopoden Anne-
liden, ete. Sitzungxber. k. Akud. Wien. Math. Nat. Cl. Bd.
xevii. 1888.
18, HatscHex, B. Ueber . Entwicklungsgeschichte von Teredo.
Arb, Zool, Inst. Unir, Wien, Ba. iti, 1881.
19. Horst, R. On the Development of the European Oyster (Ostrea
edulis). Quart, Journ. Micro, Sei, Vol. xxii. 1882.
20. Horst, R. Embryog¢énie de I’huitre (Ostrea edulis). Tijdschrift
dor Nederlandsche Dierkuwliqe Vereeuiquaig. Supplement Deel
1. 1883-84.
21, Huxuey, T. H. Oysters and the Oyster Question. Enjlish
Hlustr, Muy. 1883.
22. Jackson, R. T. The Development of the Oyster, with Remarks
on Allied Genera, Pro. Bastin Sor, Nat. Hist. Vol. xxiii.
188s,
Jackson, R. T. Phylogeny of the Peleeypoda. The Avulidae
and their Allies. Men, Boston Sor. Vat. Hist, Vol. iv. No.
1890. See also + Studies of Pelecypoda and the
1 Origin of Structure in Peleeypods ”. Amerivan
Vol. xxv. (p. 1.132), and xxv. (p. 11). 1890-
Zur Morphologie der Niere der sog. Mollusken.
Leiterhe 1, wis Ba. xxix 1887.
Jrerisc. H. vo Anodonta und Glibaris, Zool. Anz. Jahn.
i 1s91.
26. Juprinc., Ho v. Ueber die Ontogenie von Cyclas und die Homo-
33.
34.
35.
36.
37.
41.
LITERATURE. 85
logie der Keimblatter bei den Mollusken. Zetxchr. J. wixa,
Zool, Bd. xxvi. 1876.
. Korscnet, E. Ueber die Entwicklung von Dreissena poly-
morpha Pallas. Sitzwngsber. Gesellech. Naturforsch. Freunde.
Berlin, July, 1891.
. LacazE-Dutaiers, H. Mémoire sur le développement des
branchies des Mollusques acéphales lamellibranches. Ann.
Sei, Nat. Zool. (iv.) Tom. v. 1856.
. Langester, E. Ray. Contributions to the Developmental History
of the Mollusca. Phil. Trans. Roy. Soc. London. Vol. elxv.
Parti. 1875.
. Leuckart, R. Ueber die Morphologie und Verwandtechaftsver-
haltnisse der wirbellosen Thiere. Brunswick, 1848.
. Leypic, F. Ueber Cyclas cornea Lam. Archiv. Anat. und
Phys, 1855.
. Leypic, F. Mittheilung fiber den Parasitismus junger Unioni-
den an Fischen in Noll: Tithing. Inaug.-Dissert. Frank-
furt u, M. 1866.
Lovén, S. Bidraytil kanned om. Utreckl. af. Moll. Acephala
Lamellibr. Vetenxk. akud. Handl, 1848, and Archiv. f. Nuturg.,
1849.
Martens, E. von. Eine eingewanderte Muschel. Der Zvolo-
gieche Garten, Jahrg. vi. 1865.
Méx&caux, M. (1) Sur le coeur et la branchie de la Nucula
nucleus. (2) Sur la branchie chez les Lamellibranches et sur
la comparaison avec celle des Scutibranches. Bull. Suc.
Philom, Parix, (viii) Tom. i. 1888-89,
Mitsuxurt, K. On the Structure and Significance of some
Aberrant Forms of Lamellibranchiate Gills. © Quart. Journ.
Micro, Sei. Vol. xxi. 1881.
Ménius, K. Die Auster und die Austerwirthschaft. Berlin,
1877.
. Miuuer, F. Ueber die Schalenbildung bei Lamellibranchiaten.
Schneider's Zool, Beitrage. Bd. i. Breslau, 1885.
. Partes, W. Eyes of Molluscs and Arthropods. Mittheil.
Zool. Stat, Neapel. Ba. vi. 1886.
. PELSENEER, P. Sur la classification phylogénétique des Pélécy-
podes. Bull. xi. France of Belyique (A. Giard). Tom. xx.
1889,
PELSENEER, P. Contributions u l'etude des Lamellibranches.
Archiv, Biol, Tom. xi, 1891,
46.
47.
LAMELLIBRANCHIA.
2. QuaTREFAaGEs, M. A. DE. Mémoire sur l’embryogénie des
Tarets (Teredo). Ann. Sei. Nat. Zool. (ii) Tom. xi. 1849.
3. Rast, C. Ueber die Entwicklungsgeschichte der Malermuschel.
Jen. Zeitachr. 7. Natur, Bd. x. 1876.
. Rangin, W. M. Ueber das Bojanus’sche Organ der Teich-
muschel etc. Jen. Zeitachr. 7. Naturw. Bd. xxiv. 1890.
. Rawitz, B. Der Mantelrand der Acephalen. Jen. Zeitechr. 7.
Nature. Bd. xxii. and xxiv. 1888 and 1890.
Ryper, J. A. The Metamorphosis and Post-larval Stages of
Development of the Oyster. Annu Report of the Commis-
xioners of Fish and Fixheries for 1882. Washington, 1884.
ScurerHouz, C. Zur Entwicklungsgeschichte der Teich- und
Flussmuschel. Zeitechr. f. icixx, Zool, Bd. xxxi. 1878.
. ScHERHOLZ, C. Zur Entwicklungsgeschichte der Teich- und
Flussmuschel. Berlin, 1878.
. SCHIERHOLZ, C. Ueber die Entwicklung der Unioniden.
Denkachr. k. Akad. wins. Wien. Math. Vat. Cl. Bd. xiv. 1889.
. Scumipt, F. Beitrag zur Kenntniss der postembryonalen
Entwicklung der Najaden. 9 Archiv. 7. Natury. Jabrg. li.
1885,
. Scnapt, Osc. Ueber die Entwicklung von Cyclas caliculata.
Archiv, f. Anat. n. Phys. 1854.
2. SHarp, B. Remarks on the phylogeny of Lamellibranchiata.
Ann, May. Nat. Hist. (vic) Vole ii, 1RRR,
. SLuITER, C. Px. Ueber die Bildung der Kalkriéhren von Gas-
trochaena. Naturkund, Tijdschrift Nederlandech, Indié, Ball.
1890.
. STEPANOFF, P. Ueber die Geschlechtsorgane u. die Entwick-
lung von Cyclas comea. Archie. f Natury. Jahrg. xxxi.
1865.
. THIELE, TH. Die Mundlappen der Lamellibranchiaten. Zettachr.
J. wise, Zool. Ba. xliv. 1886.
. TULLBERG, T. Studien iiber den Bau und das Wachsthum des
Hummerpanzers u. der Molluskenschalen. A‘g/. Svenska
Vetenskaps-Akal. Handlingar, Ba. xix. No. 3. 1882.
. VorLttzkow, A. Entovalva mirabilis. eine schmarotzende
Muschel aus dem Darm einer Holothurie. Zool. Jahrb.
Abth. 7. Systematik, ete. Ba. v. 1890.
.» Wextner, W. Zur Entwicklung von Dreissensia. Zool. Anz.
Jahrg. xiv. 1891.
. Witson, Joan. On the Development of the Common Mussel
LITERATURE, "87
(Mytilus edulis L.). Fisth Annual Report of the Fishery
Bourd sor Scotland (jor the year 1886). Edinburgh, 1887.
60. Zrecuer, E. Die Entwicklung von Cyclas cornea Lam. Zeitschr.
J. weiss, Zool. Bd. xli, 1885.
APPENDIX TO LITERATURE ON
LAMELLIBRACHIA.
1, Bernagp, F. Scioberetia australis, type nouveau de Lamelli-
branche (Anatomy, Embryology, etc.). Bull. Sei. France et
belyique. Tom. xxvii. 1896.
I. Drew, A. G. Notes on the Embryology, Anatomy and Habits
of Yoldia limatula, Say. Johns Hopkins Univ. Circ. No. 132.
1897,
MI. Linure, F. K. The Embryology of Unio complanata. Journ.
Morphol. Vol. x. 1895.
IY, PeELsENEER, P. Les yeux cephaliques chez les Lamellibranches.
Compt. Rend. Acad. Sei. Paria, Tom. exxvii., p. 735. 1898.
V. Sincerroos, C. P. The Pholadidae. I. Note on the Early
Stages of Development. Jvhnx Hopkins Univ. Cire. 1895.
II. Note on the Organisation of the Larva and the Post-larval
Development of the Shipworm. Op. cif. 1896.
VI. SraurracHEr, H. Kibildung und Furchung bei Cyclas cornea.
L. Jen. Zeitechr. f. Naturwins. Ba. xxviii. 1894.
Vi. Sraurracuer, H. Die Urniere hei Cyclas cornea. Zeiterhr.
J. ties, Zool. Ba. \xiii. 1898.
VIII. MzissennEmer, J. Entwicklungsgeschichte von Dreigsensia
polymorpha. Marburg. 1899. Zool. Centralbl. Jahrg. vi.
CHAPTER XXXI.
SOLENOCONCHA (Scapuopopa).
(DENTALIUM.)
Tue ontogeny of Dentalinm was investigated many years ago
(1857) by Lacaze-Dururers, and more recently (1883) by Kowa-
LEVSKY with the aid of sections; the researches of KowALEVSKY,
however, do not extend so far into the life of the animal as do those
of LacazE-DuTuiers, the former having been able to observe the
larva only up to the sixth or seventh day, while the latter was able
to keep the larvae alive until they were thirty-five days old. We
therefore still have to refer, for many points, to the older accounts of
LacazE-DuTHIERs.
The genital products are discharged into the water through the
right renal aperture, fertilisation taking place outside of the body.
The eggs, which are not very rich in yolk, are surrounded by a thin
envelope.
1. Cleavage and Formation of the Germ-Layers.
The cleavage is total, the egy dividing into two cleavage-spheres,
one of which is somewhat larger than the other. The larger sphere
then, by division, gives rise to a new sphere, and the smaller sphere
also divides into two, so that we have now one macromere and three
micromeres. It is possible that additional micromeres are segmented
off from the larger sphere. The former divide repeatedly, so that
there is soon a great number of the micromeres lying upon a single
macromere which still remains rather large. This latter also finally
divides into two and then into four macromeres. This method of
cleavage shows considerable resemblance to that most common among
the Lamellibranchia. Further division and the formation of a
central cavity give rise finally to a blastula, one half of which con-
sists of small and the other of lurge cells (Fig. 34 4). The animal
a
90 SOLENOCONCHA.
2, The Development of the Form of the Larva.
As early as the gastrula-stage, the embryo becomes free and is
capable of active locomotion, some of the ectoderm-cells being already
covered with cilia (Fig. 34 C). Besides those ciliated cells which lie
at the cephalic pole and later form the ciliated tuft, the young larva
has three rows of such cells lying one behind the other at the middle
of the body of which they form a large part (Figs. 34 C and 35 A).
Since these ciliated cells represent the pre-oral ciliated ring, the post-
oral part of the larva is very little developed. At this early stage,
the larva consists of comparatively few cells, which are still very large,
ax is evident from a glance at Fig, 35 A. In later stages, as the larva
grows in size and as its cells increase in number, the rows of ciliated
Fig, 35,—A-C, three larvae of Dentadinn aged respectively 12, 24 aud 87 hours (after
Kowarevsky). 41, blastopore ; m, mantle-fold ; moe, permanent posterior aperture
of the mantle; p, posterior part of the hody; W, ciliated ving; a, apical ciliated
tuft.
cells are less conspicuous as compared with the rest of the body (Fig.
36 A), and finally appear as 4 single though somewhat broad ciliated
ring (Fig. 35 A-C). Meantime, the ciliated tnft at the cephalie pole
has become more conspicuous, and a large part of the anterior section
of the body has also become covered with delicate cilia (Fig. 35 8),
In the youngest larvae, eiz., at the gastrula-stage, the blastopore
was terminal, i.¢., opposite to the cephalic pole (Fig, 34 C), but it
soon changes its position, shifting forward towards the ciliated rmg
along the future ventral surface (Fig. 35 A). The larva thus
assumes a somewhat irregular shape, the flattened ventral surface
being somewhat backwardly inclined. At the same time, the pre-
oral part of the larva, that lying in front of the ciliated ring, hus
92 SOLENOCONCHA.
of a certain character typical of the Molluscan larva. Thus, a dorsal
invagination of the ectoderm (Fig. 36, «/) becomes differentiated at a
very early stage (A), then deepens and flattens out again later ; this
organ, from its development and subsequent modification, as well as
in its position, is seen to be the shell-gland, a structure peculiar to
the Mollusca. A comparison of the figures of the Dentalium larva
with those of the Lamellibranch and Gastropodan larvae (Figs. 14,
p. 28, 15, p. 31, and Fig. 50, p. 124) will enable the reader without
further assistance to recognise the great resemblance in the position
of the organs in these different larval forms. As the shell of
Dentalium is secreted on the dorsal surface of the posterior section of
the body, just where the shell-gland appears, it shows the same
manner of origin and shape as the young shells of other Molluscs.
It shows special resemblance with that of the Lamellibranchs, since
it extends like a saddle frum the back on to the two sides of the body,
but, whereas the young Lamellibranch shell soon becomes bivalve,
the shell of Denta/ium remuins single, /.¢., it remains to a. certain
extent at a stage which, in the Lamellibranchs, was found to precede
the bivalve shell (p. 60).
Before the shell develops, further important changes take place in
the free-swimming larva of Dentulinm, the post-oral region being the
first affected by them. During the early stages, this section is very
inconspicuous (Fig. 34 C), but it soon increases in size. This region
by its growth gives rise to the greater part of the adult body, the pre-
oral section degenerating almost completely. We find in this respect
a similarity between Jrufalinm and the Amphineura (pp. 5 and 6),
and when treating of these processes in the latter, they were compared
with the corresponding processes of metamorphosis in the Annelida.
At an early stage, the pre-oral portion of the body becomes
somewhat swollen and distinctly marked off from the post-oral part
(Figs. and 36 B). The definitive mouth is derived from an
invagination lying immediately behind the ciliated ring (Fig. 36, m).
The depression on the dorsal side which is to be regarded as the shell-
gland (./) has already been mentioned. When the post-oral section
has increased still further in size, two folds laterally placed arise on
it: these grow out towards the ventral middle line and at a some-
what later stage meet, at first near the posterior end (Fig. 35 C, m).
These folds, the free edges of which fuse later, represent the rudi-
ment of the mantle which thus rises here very much in the same way
as in the Lamellibranchia. The folds enclose a ventral swelling,
the foot (Fig. 38 B, 7), at the base of which the otocysts are to be
— |
‘THE DEVELOPMENT OF THE FORM OF THE LARVA. 93
| recognised early. These lie rather near the ciliated ring and arise as
| depressions of the ectoderm which become detached from the latter
as closed vesicles. The pedal ganglia also develop as paired thicken-
ings of the ectoderm neur the otocysts; at a later stage, they also
become detached through delamination. In the middle line of the
foot, there seems to be an invagination which perhaps corresponds to
the pedal gland deseribed for Chiton.
While these changes are taking place in the post-oral part of the
larva, the pre-oral section which, in consequence of the preponderance
| of the former region, appears comparatively reduced, also under-
goes modification. ‘Thus, two ectodermal depressions appear close to
the ciliated tuft; these at first are shallow, but deepen more and
more (Fig. 37 4 and &, cy) and eventually give rise to two closed
vesicles which are the paired rudiment of the cerebral ganglion,
‘The cells lining these depressions are, at first, direetly continuous
with the ectoderm of the cephalic pole, the two depressions being
connected together by the cells surrounding the apical ciliated tuft,
and thus they represent a common brain-rudiment. The invagina-
tions, which have become tubular, grow in further and further until
they reach the walls of the stomodaeum (Fig. 37 B). At the same
time, by the active proliferation of their cells, they become con-
siderably thickened. They also finally become detached from the
ectoderm (C) and undergo differentiation into fibrous masses and
ganglionic cells, so that there is no room for donbt as to their
ic character. Only at a later stage does a commissure form
between the two halves of the ganglion which now lose their vesicular
character.
‘The pedal ganglia, ax above shown, arise by delamination from the ectoderm,
while the cerebral ganglia originate as invaginations. This is somewhat re-
markable, since the cerebral ganglia arise, as a rule, through delamination,
other Mollusca. Considering the greater contractility of the Iurva, the
Presence of such invaginations suggests a more or less temporary infolding of
‘the surface. Kowatevsxy assumes that these ganglia first arose as a surface
uf and explains the invagination of the ganglionic rudiment as due
% je absence of room for surface-expansion owing to the limitation of the
pre by the forward concentration of the ciliated ring. The develop-
‘ment of the cerebral ganglion in Pentalinm recalls the condition which we
‘shall find in various Gastropods, where it undoubtedly arises by invagination
- Since, in these latter cases, we have to do with more specialised
would be desirable, in instituting a comparison with Dentalium, to
1 in what way the cerebral ganglion arises in the more primitive
‘especially in the Diotocardia.
94 , SOLENOCONCHA.
While the nervous system is forming through the processes just
described, both the ciliated tuft and the ciliated ring undergo reduc-
$ tion (Fig. 37). This is especially
the case with the latter which, in
accordance with the nomencla-
ture used for other Molluscs,*
is here also called the velum.
The velum is the chief swim-
ming organ and, when it degene-
rates, the larva has to adopt
another method of locomotion.
At the stage depicted in Fig.
38, the velum appears still
greatly developed, but, as the
conical apical pole has degene-
rated, the anterior section of
the larva now! seems flattened
and plate-like. When the
velum is more reduced and
the other parts of the body
(the shell, the foot, ete.) better
developed, the larva sinks to
the bottom, where it still swims
to some extent by means of the
velum, but also creeps with
the assistance of its foot, just
as do other Molluscan larvae
when passing over to the adult
form (ef. p. 42 and Figs.
54, 67, etc.). The free-swim-
ming life of the larva lasts
quite four days, during which
time it does not, like the
larvae of the Lamellibranchia
and the Gastropoda, move at
the surface of the water, but appears to maintain itself at various
depths (acazE-DUTHIERS).
es
the formation of the brain (after Kow
Levsky). ey, rudiment of the c
ganglion; 1, mantle ;
x, cephalic pole; 1, pre
‘tol
al ciliated ring.
* CF. on this point pp. 33 and 125.
THE TRANSFORMATION OF THE LARVA INTO THE ADULT, 95
3, The Transformation of the Larva into the Adult.
Even at the time when the larva sinks to the ground, though still
‘at first moving with the help of the velum, the principal organs of
the adult are already present as rudiments. This last period of its
is therefore marked by the growth and the further
development of rudiments already present in it.
If we examine the larva externally (Fig. 38 B), we find that the
shell has grown much larger. At first it was a dise-like structure
lying on the back, but then it became saddle-shaped, growing down the
sides of the larva till its free edges united in the ventral middle line
(Fig. 38 A), In the
ventral parts of the
shell, « parallel
strintion can be recog-
nised (Fig, 38 2),
representing lines of
growth, su that the
growth takes place
here in the same way
as in the shells of the
Lamellibranchia (Fig.
2%, p. 60), as the gig, u8—Lormne of Denlalium, A at. the end of the
shell increases in size, second day and A on the third or fourth day. 1,
4 = seen from “the ventral side, B, seen somewhat obliqu ely
‘the fusion of the the oame side (after Lacans-Durutans).. /, toot
weil margins e-file
comes closer, At first
the anterior aperture of the shell is still considerably wider than the
& condition connected with the shape of the larva (Fig.
38 2B), but when the velum degenerates and the shell lengthens, the
anterior uperture becomes relatively smaller. The shell now appears
almost cylindrical, its anterior aperture being somewhat wider than
its posterior aperture. Its increase in size is caused by the secretion
of new shell-material from the anterior tubular margin of the fused
mantle-folds, the newly formed parts being marked off from the older
parts by circular boundary lines ; these latter give the shell the
appearance, especially in older animals, of being segmented (Fig. 39).
“At « later stage the shell assumes « dorsal curvature and gradually
‘sequires the tubular conical shape found in the adult. The anterior
gud posterior apertures, which originated through the lateral growth
96 SOLENOCONCHA.
and ventral fusion of the shell-plate (Fig. 39 A-and 2), are retained
throughout life.
The shape of the shell, which is at first cylindrical and then tusk-
like, is due to the mantle first assuming this form. The latter has
already been mentioned as growing out, like the shell, from the back
in the form of two folds, which fuse ventrally. Like the shell also
it remains open anteriorly and posteriorly. Anteriorly it grows to-
gether with the shell in the form of a tube for some distance over
the body which lies entirely hidden within it. The foot which, as
=A, a larva of Dentalinn undergoing metamorphosis ; B, anterior portion of
young Dentalivm (after Lacaze-DUTHIERS), d, intestinal canal ; 7, foot ; mor,
posterior aperture of the mantle ; s, shell; ¢, tentacle-rudiment ; r, velum,
we saw, originated as a large ventral swelling behind the oral aper-
ture, can be extended for some distance beyond the anterior aperture
of the mantle. It soon assumes the triangular form characteristic of
Dentulinm (Fig. 39 A and B, f). In spite of the early development
of this exceedingly characteristic shape, it is not to be considered
a primitive feature, but must be regarded rather as a later acquisi-
tion, as it is wanting in a few genera PLATE (No. 3). In Stphono-
dentalium and Caduus the two lateral lobes are wanting, these genera
apparently exhibiting a more primitive form of foot.
THE TRANSFORMATION OF THE LARVA INTO THE ADULT. 97
At a somewhat later stage, at which the velum is still retained, the
foot is found protruded from the shell (Fig. 39 A). This stage, as
well as the younger one depicted in Fig, 38 B, recalls that stage
in the Lamellibranch larva in which the larval and the adult organs
ef locomotion are present and functional at the same time (Fig. 20,
p. 42). At the posterior end of the larva, an early specialisation
of the mantle-folds produced a well-inarked channel, lined with
‘powerfully ciliated cells (Fig. 38 and 39, 1). This ciliation is
eonnected with the circulation of the water, which is further pro-
moted by the ciliation of the mantle-cavity.
The foot, as already mentioned, lies in front of the oral aperture.
Tt is here that the prominences arise which give origin to the
tentacles (Fig. 39 6, f). According to Lacaze-Duraters, there are
at first three of these, two lateral and one smaller median prominence
(Fig. 39 B). These structures, by lengthening, give rise to the
tentacular filaments which are so numerous in the adult. The
deseription given does not explain the relution of the filaments to the
prominences and to the oral aperture, but the condition of the
tentacles in the adult enables us to form some conclusions on this
subject. In the adult, the mouth lies surrounded by leaf-like labial
appendages at the apex of an egg-shaped projection which, together
with the tentacular filaments that are innervated from the cerebral
ganglion, must be regarded as the cephalic region. The tentacular
filaments arise from two lobes lying at the base of the cephalic
projection, so that here also, there are three prominences which
might be traced back to those found in the larva. We should then,
as in the Gastropoda, consider the middle prominence as the rudiment
of the oral cone, and the lateral prominences as the two original
tentacles, from which later the tentacular filaments arise.
A similar view of the tentacle-filaments of the adult is taken by THrene
a to Chapter xxxiii.}, who compares the two lobes or tentacular
shields with the large tactile lobes of Haliotis which are beset with tufts.
‘These Intter, if lengthened, would result in structures resembling the ten-
teoular filaments. Quite recently PLarr (No, 3) also has accepted this view,
ortega prominences on the head of the young animal the
‘The radular sac arises during the later stages of larval life as an
gutgrowth of the stomodaeum, The anus also appears in the larva
‘6 aslight depression of the ectoderm behind the base of the foot.
‘The enteron, according to Kowarevsxy, becomes connected with it
‘direct, without the formation of an ectodermal rectum.
H
98 SOLENOCONCHA.
Further ontogenetic processes especially connected with the development of
the inner organs, are described by Lacaze-Doratens, but these processes, which
are evidently very difficult to make out, could, at the time when he wrote,
only be studied in the complete animal, and could not thus be clearly under-
stood. The above account, in which the most essential ontogenetic phenomena
are described, must here suffice, and for further information we must refer the
reader to the original treatise on the subject (No. 2),
LITERATURE.
1. Kowauevsky, A. Etude sur Vembryogénie du Dentale. Aun.
Musée Hist. Nat. Murseille Zool. Tom. i. 1883,
2. Lacaze-Duraters, H. pg. Histoire de l’organisation et du
développement du Dentale. Ann. Sci. Nat. (4.) Tom. vi.
and vii. 1856 and 1857.
3. Puate, L. Ueber den Bau und die Verwandtschaftsbezichungen
der Solenoconchen. Zool. Juhrh. Atth. f. Anut. Bd. v.
1892.
APPENDIX TO LITERATURE ON SOLENUCONCHA
(SCA PHOPODA).
I. Sumrota, H. In Bronn’s Klass. u. Ord. d. Thierreichs. 1894-95.
Anatomy, Ontogeny, Phylogeny and Literature.
CHAPTER XXXII.
GASTROPODA.
Systematic Order :—
L Prosoprancata (Streptoneura).
The gill or gills lie in front of the heart. The pleurovisceral
connectives are crossed. The sexes are distinct (save in
Vilvata, Marsenina and a few parasitic forms).
Sub-order 1.—Diotocardia. The heart has usually two
auricles. The ctenidia are bipectinate and free distally.
The pedal centres form long ganglionic cords connected
by transverse commissures and closely associated with
the pleural centres. Gonad opens into right nephridium
(save in Neritidae). The nephridium is generally paired.
(«) Zygobranchia. Ctenidium paired, ventricle tra-
versed by rectum, two nephridia ; shell with apical or
inarginal slit or row of” perforations.
Haliotix, Fissurella, Pleurotomaria.
(+) Azygobranchia. One ctenidium (left of Zygo-
branchs); two auricles (right ending blindly); heart
traversed by rectum (except in Helfeinidue); nephridium
generally paired, operculate.
Turbo, Trochws, Neritina (one kidney, distinct genital
aperture). Aelicina (pulmonate, no ctenidium, one
auricle).
(c) Docoglossa. Gill single or absent; heart with
single auricle, ventricle not traversed by rectum; two
osphradia ; two kidneys.
Patella (ctenidia absent), Aemaee,
Sub-order 2.—Monotocardia. Heart with one auricle;
kidney and gill unpaired, the latter monopectinate and
100 GASTROPODA,
attached for its whole length (save in Valvata). The
nerve-ganglia distinct and concentrated round oesophagus ;
pedal commissures rare. (senital aperture distinct,
dioecious with rare exceptions.
To this order belong by far the greater number of
Prosobranchia, all, indeed, of those the development of
which is dealt with here except the forms named above.
Junthina, Murex, Buccinun, Purpura, Newa, Fulque,
Fusux, Farciolaria, Strombus, Rostellaria, Crepidula,
Calyptraca, Vermetus, Bythinia, Paludina, Thyea, Stilt-
ter, Entoconcha, ete.
Sub-order 3.—Heteropoda (Nucleobranchia). The char-
acter of the nervous system, the position of the gill,
ventricle and auricle the same as in the Monotocardia.
Foot developed into a fin.
Oxyyyrus, Atlanta, Pterutrachea, Carinarin, Firoloidu,
II. Op1stHOBRANCHIA.
The gill and auricle generally behind the ventricle (except in
Actaeun). — Pleurovisceral commissures rarely crossed
(Actaeonidae). Hermaphrodite, marine.
Sub-order 1.—Tectibranchia. Shell generally present, often
much reduced and internal, wanting in Runcina and
Plenrobranchea ; with mantle-cavity containing a cteni-
dium.
Artaron, Bullu, Avera, (asteropteron, Philine, Aplusia,
Plearobrauchus, Pleurobranchea, Umbrella.
Sub-order 2.—Nudibranchia. Without shell in adult stage ;
mantle, ctenidium and osphradium wanting.
Tritonia, Doris, Chromuduris, Palyeeva, Teryipes, Rlysia,
Arolis, Doto, Fiona,
111. Preropopa.
Pelagic Gastropods in which the head is much reduced, and
the foot is developed like a fin; now generally classed
with the Opisthobranchia,
Sub-order 1.—Thecosomata. With calcareous or carti-
laginous shell, with mantle and mantle-cavity.
Spirinlix, Limacina, Tiedemaunia, Cymbnlia, Crvolinia,
Hyalorylie, Styliola, Cleudurn, Cresvia,
OVIPOSITION AND CHARACTER OF THE EGG-CAPSULES AND EGG, 10]
Sub-order 2.—Gymnosomata. Without shell and mantle.
Clicne, Preumodermon.
IV. Ponmonara.
Principally fresh-water or terrestrial. Ctenidium wanting ;
mantle-cavity modified as a lung. The pleuroyisceral
commissures are not crossed. Hermaphrodite,
Sub-order 1.—Onchidiacea. Marine or littoral, without
shell; anal and pulmonary orifice posterior,
Ouchidivn, Vaginulus.
Sub-order 2.—Basommatophora. Fresh-water and terres-
trial (usually maritime) Pulmonates. Eyes at the bases
of the tentacles.
Limnaea, Planorhis, Ancylus, Auricula.
Sub-order 3.—Stylommatophora. Terrestrial Pulmomates.
Eyes at the tips of the tentacles.
Suceinea, Vitrina, Clausitia, Bulinus, Helix, Testacella,
Dawlebardia, Limac, Arian.
1. Oviposition and Character of the Bgg-capsules and Egg.
‘The Gastropoda * are mostly oviparous, but oviposition takes place
in such « variety of ways that we can only give a few examples.
An exceedingly simple method of oviposition is found in Patella,
the eggs of which are laid singly and are apparently fertilised in the
water, as copulatory organs ure wanting in this genus. Tt was there-
fore possible to fertilise these eggs artificially (ParrEn, No, 52).
Each egg is surrounded by a somewhat thick radially striated en-
yelope which has a funnel-like projection with a wide aperture (the
micropyle).
Th most Gastropoda, however, fertilisation takes place within the
body of the mother, and the eggs are not laid singly but unite to
form larger or smaller masses of spawn, The spawn may have the
form of dise-shaped or long hyaline gelatinous masses (fresh-water
Pulmonates). Each egg within the gelatinous mass is further
* A more detailed description of the spawn of land and fresh-water Gastro-
is by Pyenren (No. 88). A detailed account of oviposition in
! and notices of the literature on the subject are given by Kurer-
ee and reneat obtained ere the treatises setecies a3 a
account of the egg-capsules will found in
ameraele Comic 1887.—Eb. mn =
102 GASTROPODA.
surrounded by a transparent membrane. In certain marine Gastro-
poda, ¢.y., in various Opisthobranchia, this gelatinous spawn attains
a great size, forming long, ribbon-like coils (Acol/s) or round cords
repeatedly bent back on themselves (Aplysia). In these cords, the
eggs either lie irregularly or else are arranged in one or more rows.
‘The mass of spawn often takes the form of a ribbon which is spirally
coiled (Doris, Doto, Pleurobrauchux, ete.). These gelatinous masses
frequently contain a very large number of eggs, the spawn of a single
Dorix having been estimated to contain 600,000 eggs. The spawn
sometimes has the form of a gelatinous sac attached to the substratum
by a stalk and containing thirty to forty eggs (Teryipes, according to
Sauenxa, No. 114).
The eggs of the Heteropoda are also laid in gelatinous masses
which take the form of long ropes (Carinaria, Pterotrachea, Firu-
loida) according to Fou (No. 31); only the Atlantidae (Atlanta,
Oxrygyrus) seem to lay their eggs singly, each surrounded by a
gelatinous envelope. The eggs of the Pteropoda also are found in
gelatinous masses which are usually tubular in shape. These tubes
contain a great number of eggs placed either one behind the other
or else close together. The spawn less frequently appears in the
form of a thin membranous plate (Creseis aeirulata), or as round
balls containing a large number of eggs (C/ione).*
In Fixeurelle also the spawn forms a gelatinous mass containing a
large number of eggs and deposited on stones. The Prosobranchia for
the most part differ greatly from the above in their method of
oviposition, A variable number of eggs are usually enclosed in an
egy-capsule, the shape of which varies in different forms. Besides
the eggs, this capsule contains a fluid or viscid substance which
serves as nourishment to the embryo. We are hereby reminded of
the Oligochaeta and Hirudinea (Gathobdellidar) in the cocoons of
which several embryos are found floating in a nutritive fuid (Vol. i.,
pp. 281 and 391). The comparison becomes all the more striking
when we find that ina few Prosobranchs, as in the Oligochaeta (p.
281), not all the eggs ina capsule develop, but a few, or it may be
a luge number disintegrate, and serve as food for those that survive.
In many Prosobrinchia, however, all the es in a capsule develop,
in Fudyur, from 12 to 14, in Nasea, from 5 to 15, ete. In Purpnra
A sules contain many egys, all of which undergo
Hana, the ea
* Detailed statements as to the oviposition in the Pteropoda and also in the
Heteropoda are found in the works of Fon (Nos. 31 and 32).
OVIPOSITION AND CHARACTER OF THE EGG-CAPSULES AND EGG, 103
cleavage, some of the embryos, however, develop no further, but
perish, their remains being devoured by the other embryos, This
ix also the case, according to McMurrics, in a few species of
Cepidala, and in Urosalpine (BRooxs). Fasciataria lays about 200
eggs in each capstile, but only 4 to 6 of these develop, and this is also
the case with Buceinnm undatum. Each capsule of Purjara lapillus
contains 400 to 600 eggs, only 10 to 16 of which develop into mature
embryos (SenunKa). The egg-capsule of Neritina fluviatilis also
shelters « large uumber of eggs (according to Buocnmann 70 to, 90)
although only « single embryo in it attains complete development.
(Ccavantpe). In this case, the unfertilised eggs divide soon after
the polar bodies have formed, and break up into irregular heaps of
protoplasmic spheres, being in this way distinguished from the eggs
undergoing cleavage.
Tn shape and structure, these egy-capsnles vary greatly. As a
rule they are formed of tough leathery or parchment-like integument
and are in some cases approximately spherical, but appear flattened
on the side by which they are attached to foreign objects. This is
the case in Ner/ins. the older cocoons of whieh easily divide into
two hemispherical halves. ‘T'o allow the brood to eseape, the capsule
oceasionally hus an aperture closed hy a delicate membrane, situated
opposite to the point of attachment. Several capsules are usually
found together, as in Bucrinum wulatum, Fusus antiquus and others,
the capsules of which are piled one upon another, thus forming an
enormous mass of spawn, ‘They oceasignally appear laterally com-
pressed and, in one species of Fusus observed by Bosrerzky, are
round plano-convex dises, attached by the flattened side. The
capsules of Busyron (Fulgur) also are leaf-like or rather dise-shaped ;
these are arranged in « row like a roll of coins, aud are attached to a
| common filament, ‘These capsules have an aperture opposite to the
points of attachment for the escape of the brood.
tn Wessa neutabilis, the capsules are cup-shaped and attached by
the obliquely troneated end, the opposite pointed end carrying an
aperture ut first closed by a membrane. The surface of these capsules
shows polygonal markings which form rib-like or membranous ridges,
“They are found united into large clumps on sea-weeds and worm-tubes.
The cup-like capsules of many Prosobranchia are arranged in
groups attached by their narrow ends drawn out into stalks (Maree),
Here also, the aperture of the cup is closed by a membranous cover,
Which opens when the brood is ready to hatch, In Purpura dapillus
10 to 15 such capsules, which, however, are more flask-shaped and of
—
104 GASTROPODA.
leathery consistency, are fastened to a similarly constituted, strueture-
less membrane which, in its turn, is attached to a stone, The same
is the case with the capsules of Fusciolaria tulipa, in which the edges
of the cup are continued into a wavy membrane. The form of this
latter capsule seems to come nearest that of the well-known Janthina,
the cup-like capsules of which are attached to a kind of raft by
means of which the animal floats in the sea. This float is a large
spindle-shaped body formed of the same substance-as the capsules
and containing air-spaces. It is connected by its pointed end with
the foot of the animal and from its lower side the capsules hang.
The float is also found in the male, and without its aid the animal
cannot move about freely in the water, so that it must not be
regarded merely as an organ connected with oviposition, although it
may perhaps be considered to have been primarily developed for this
purpose,
The conditions of oviposition in the terrestrial Gastropoda differ
somewhat from those of the aquatic forms. The eggs of the for-
mer may also be surrounded by & gelatinous albuminous substance
and may be connected together into spawn-masses which resemble
rows of beads (imac) or else may be massed in larger numbers to
form gelatinous balls (Onchidium), In the latter case, the structure
of the spawn is rather more complicated, each egg being surrounded
by « mass of albumen which is enclosed in a transparent but resist-
ant envelope. The latter lengthens in a line with the two opposite
poles of the oval albuminous mass, forming at each end a thread
which is continued into the envelope of another egg, so that the
eggs constituting the spawn are connected into wreathlike chains
which are again surrounded by the common gelatinous mass. The
albuminous mass surrounding the egg usually, in the terrestrial
Gastropoda, becomes still further protected by a firm membrane
impregnated by lime-sults. A more or less thick calexreous shell
is thus formed around the egg; this, even in Heli pomatia, is of
somewhat firm consistency. The eggs are usually deposited in great
numbers (60 to 80 in elie pomatia) in small holes in the ground pre-
pared by the parent animal and are then covered over with earth.
Species of Bulimasx which live on trees roll up leaves into the form
of cornucopiw and Jay in these their soft-shelled eggs.
The egys of the terrestrial Pulmonata attain a considerable size.
Even the eggs of Helic pomtia measure 6 mm. in diameter. Those of
the Ceylon form, Helie (Acirne) Waltout ave as large as a sparrow's
egg (P. and F, Sarasin, No, 102), and those of an American species
i ="
OVIPOSITION AND CHARACTER OF THE EGG-CAPSULES AND EGG. 105
of Brilimus which are oval, measure 5 cm. in length and are there-
fore larger than the eggs of pigeons. ‘These eggs, in consequence of
their firm, smooth shell, closely resemble the eggs of birds, but are
distinguished from the latter by the fact that the actual egg (the
yolk) is always very small and floats in a great mass of viscid trans-
parent matter enclosed within the egg-shell. But although the yolk
or the egg-cell, as compared with the size of the egg is almost nil,
the mature embryo almost completely fills the shell, having increased
im size to this extent at the expense of the surrounding mass of
nutrient material.
Some Gastropods take care of their eggs. Those species of Crepn-
dula which are immovably fixed to one spot (C. surnicata, plana,
and conrera, McMunnion, No. 70, Consnin, No. IV) retain the egg-
capsules, which ure attached to the substratum, under cover of
the shell. The wall of the capsules thus protected are naturally of
delicate nature. Verimetus attaches a few capsules to the inner sur-
face of its shell, near the aperture of the latter (Lacaze-Duraiers),
Tn comparatively few Gastropods, the whole development is passed
through within the body of the mother. These forms are therefore
viviparous. The best known example is Paludina (Viviparns) vivi-
para, the eggs of which develop in the oviduct, which functions
‘a5 a uterus, until the form of the adult is reached. Its course of
development, however, exactly resembles that of other Prosobranchin.
The egg is surrounded by « conspicuous layer of albumen, which
again is enclosed in a membrane that runs out into a twisted stalk,
s¢ that « kind of cocoon is formed, As a rule, only one egg lies
within this envelope, but two are sometimes found in it (Leypia,
No. 68), the resemblance to the eggwapsules of other Prosobranchia
‘being thus heightened. Similarly, in a few species of Melanéa [in
Typhobie sud Nassopsis), the embryos develop in the uterus, and
are only boru when they have attained the adult form.
{in some species of Melania and in Spelia, the embryos develop in
‘# special brood-pouch formed by an ectodermal invagination near
‘the right cephalic tentacle, The viviparous habit appears to be
hugely confined to fresh-water forms. |
A few Pulmonata are, like the Prosobranchia above-mentioned, vivi-
parous, the development of the embryo here also taking place in
‘the oviduct which is transformed into a uterus, This is the case in
“afew species of Clausitia, Pupa, Helix and Vitrina, Nearly related
+ osnligaahtaad found to differ greatly in their methods of repro-
ie
ion, some being oviparous, and others viviparous (No. 102).
1
il
106 GASTROPODA.
The actual egg of the Gastropods, if not specially large, is ‘fairly
rich in yolk which is often yellow, but occasionally of some other
colour (blue-green in Pafell) ; this frequently renders the egy quite
opaque. A clear protoplasmic region can frequently be distinguished
from a more opaque region laden with yolk, the difference between
the animal and the vegetative pole being thus indicated (Fig. 40 A
and B).
In some Gastropod egys there is less yolk than in others. /aln-
dina may be cited as an extreme case on the one hand, and Vase
and Fusux on the other. The egg itself is usually surrounded by a
clear viscid mass which, in its turn, is again enclosed in a transparent
envelope. It has already been mentioned that other envelopes may
be udded, and that several egzs may be enclosed in a common
capsule.
2, Cleavage and Formation of the Germ-Layers.
In spite of the great number of forms among the Gastropoda and
the different development of the several divisions, we can, in every
case which has been investigated, recognise a common plan in the
cleavage of the egy, although at times this is more or less obscured
by modifications introduced by the variations in the amount of the
yolk present.
In this respect we have resemblance to the Lamellibranchs, but
the course of cleavage itself is different in the Gastropoda. The
phenomena of cleavage have been studied in a large number of
Gastropods and may therefore be considered as pretty accurately
understood. As early as 1850, the cleavage of the Gastropod egg
was described by WaknEck (No. 130), very completely, considering
the time at which he wrote. And since then it has been investigated
by a number of zoologists, among whom we may mention For,
Bonretzky, Rani, Mark, Buocu Sand others (see the literature
appended to this chapter).
In all Gastropods, as far
may be equal
is known, cleavage is total; at first it
but it very soon becomes unequal. The egg, in many
cases, is divided up into two large blastomeres of almost equal size
by a median groove which cuts it below the polar bodies (Fig. 40 4).
A second furrow, which is also meridional, divides the egg into four
almost equal blastomeres (/, /-/1). These four cells, owing to the
nature of the second cleavage, often lie in such a way that two are
in contact with one another in the centre of the egg and thus
CLEAVAGE AND FORMATION OF THE GERM-LAYERS, 107
separate the other two (Fig. 48 A, p. 120) [see Conkran, No. 1V, pp.
44-53 on this point]. At the line of junction between the first two
blastomeres, the transverse axis of the future embryo can already be
seen, while the plane at right angles to this represents the sagittal
plane of the embryo. _The position uf the axes is thus determined
very early as was seen to be the ease in other animals.*
An equatorial furrow cuts otf from these four cleavage-cells four
smaller cells so that the embryo now consists of four macromeres and
four micromeres (Fig. 40 C, L/V and J'-7V"), the latter lying at the
animal pole. The relative size of these blastomeres varies eon-
siderably im different Gastropods. In Patella (Parren, No. 83),
and in Paludina, for instance, the micromeres are not much smaller
than the macromeres, whereas in Fudyur, the micromeres in com-
ferison with the macromeres, which are very rich in yolk, are hardly
visible (MeMurricn, No. 70). These differences are no doubt de-
termined by the amount of yolk present in the eggs.
As cleavage proceeds, four moré micromeres arise (D, 1*-1V"), as
before, from the macromeres. As a rule, this process is repeated
once more, and in this way three generations of micromeres arise
from the macromeres ; this, however, is not the case in all forms.
The nuclear spindles in the macromeres depicted in Fig. 40 /, show
Unt these cells ure preparing to divide to form the third generation
of micromeres. In the meantime, the already formed micromeres
have, by division, increased still further in number; sometimes,
however, they do not multiply antil later, In Voritins, according
to BuocuMann, the first and second generation of micromeres (/-/"
aud 1-4) divide first, but in Planordix the twelve cleavage-spheres
which now compose the embryo undergo almost simultaneous division
(Rast). In the example depicted in Fig. 40 4, the twenty-four-
celled stage is reached by a division of the first generation of
micromeres quickly followed by the division of the second generation
and the abstriction of the third row of micromeres from the macro-
meres as indicated by the nuclear spindles in /’-7V" and L-JV, &.
The macromeres, through this last division, are either considerably
(This seems to hold good for ull those forms in which the position of the
‘first two cleavages has been investigated in relation to the axis of the adult
. CONKLIN, for instance (No. 25), concludes that, in Crepiduda, the first
to the transverse axis and divides the egg into an anterior
aod o half, while the second furrow lies in the longitudinal axis
denotes the division of the egg into a right and a left half. Heywons
(No. X11) finds a similar condition in Umbrella. Cf. also the position of the
“Asis at & somewhat later stage, as given on p. 143.—Kp.]
108 GASTROPODA,
reduced in size (Fig. 40 #), or, as is most frequently the case, are
retained for some time longer as specially large cells (F aud @), the
micromeres which have arisen from them shifting towards the animal
pole. The multiplication of the latter continues and leads to the
development of a cap of smaller cells which lies upon the macromeres
(F und (, ete.).*
The great agreement of the various stages of cleavage in the Gastropoda
with those of the Turbellaria is very striking, as may be seen from a com-
parison of Fig. 40 with Fig. 75, Vol, i., p. 162. This is most marked in
stages C-F of Fig. 40, but the later stages F’ and @ also show great re-
semblance to the corresponding stages in the Turbellaria (Vol. i., Fig. 75 F
and Z), The radial structure does not, indeed, in the Gastropoda, extend as
far as in the Turbellaria, and cannot be directly compared with that of the
Polyclad embryo, because the radial cells of the latter are to be regarded as
the rudiment of the mesoderm (Vol. i., Fig. 75 C and £), while, in the
Gastropods, they are ectodermal. The radial structure in the two cases is,
however, only apparent, since the axes of the body can early be demonstrated
both in the Turbellaria and the Gastropoda. In « similar way we might
» speak of a radial symmetry in connection with the stages of cleavage of many
other animals. Further, this radial symmetry is very soon lost in the
Gastropod embryo by the appearance of the paired rudiment of the mesoderm
which gives the embryo a marked bilateral character. It has, however, been
asserted (Manvnept, No, 72), that in Aplysia, when the embryo consists of
eight blastomeres only, the mesoderm arises as four cells through division of
the four micromeres ; this, if true, considerably strengthens the resemblance
"(Since this work was published, a large number of observations relating
to the sy pee in the cleavage of the erie yea egg have been recorded,
allof which lend additional support to the belief expressed above (p. 106) that
4 common type or plan could be recognised in the segmentation of the egys of
all the various divisions of the Gastropoda.
Thus, it is possible to trace the origin cf the ectoderm in every case to three
quartettes of micromeres which are cut off successively from the macromeres,
complication being introduced, in some cases, by the secondary division of the
first and second quartettes before the separation of the third and last quartette
from the macromeres, and in other cases, by the lesser development of the
‘olk and consequent slighter differentiation of the blastomeres, thus making
it difficult eA identify the macromeres. woe E
A very striking feature, common to the development of most Gastropodan
eggs and well shown in Fig. 40 €, D and E, Pas beea tecaned abe taee
cleavage. Thus, as carly as the third cleavage, é., the formation of the first
quartette of micromeres, « curious obliquity becomes evident. This obliquity
is visible in the nuclear spindle even before the completion of the division,
but becomes more apparent at its close, when the cells of the upper quartette
micromeres) lie in the furrows between the cells of the lower quartette
Lercreind This “spiral" character is generally more than
represented in Fig, 40 (, but is well shown in D in the case of the second
uartette of micromeres, Spiral cleavage is of particular interest in view of
the fact that, in sinistral Gastropoda, the obliquity takes the reverse inelii
to that which is found in dextral forms (Cramprox, No, V and Houmes, No.
XIIa). Fora oes discussion of the significances of the forms of cleavage
in the Gastropodan egg see Conxiin (No LV, pp, 185-192),—Ep,]}
a
CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 109
to the Turbellaria, but the mode of formation of the mesoderm described by
this author so little agrees with what is found in other Gastropods, that it
must be regarded as quite improbable, especially when we remember that
Brockasy, who investigated the ontogeny of Aplysia at the same time as
Maxrnept, saw nothing of this process, and MazzaneLui who, quite recently,
has made similar investigations, describes the formation of the mesoderm in
an entirely different way,
of the germ-layers develop, as in the Amphineura
very early. In Planorhis, according to Raby,
110 GASTROPODA.
the posterior of those two macromeres which are in contact with one
another mesially, divides into two cells, the smaller of which shifts
towards the centre of the egg. The other three macromeres also give
off such a small cell towards the centre, so that there are now four
small entoderm-cells (Fig. 40 #). The posterior macromere then
divides into two large cells of about equal size (//, mes) and the other
imacromeres also divide (H, ent). In Neritina, a similar process takes
place, but the size and position of the cells is somewhat different (Fig.
40 G, wes and ent). In Crepidula and Umbrella also (Fig, 48 B, p.
120), one of the posterior macromeres gives rise to an entomere and
to a cell which divides into « right and a left half. These two last
cells are the primitive mesomeres and, according to their origin,
either already lie in the primary body-cayity (@) or else are pressed
into that cavity later. This latter is the case when, as in Playurbis
(Fig. 40 H), these cells (wes) at first form a continuous circle with
the large cells (ent). A cleavage-cavity is sometimes first developed
at this stage, by the partial separation of the layer of micromeres
from the macromeres, or else it forms still earlier, so that even before
the stage represented in Fig. 40 H, the embryo may exhibit the
form of a blastula with a wall much thickened at the vegetative pole,
in which case an invagination-gastrula results (Planorbis).* Tn the
first case, however, in spite of the fact that the formation of an
epibolic gastrula has already commenced (G@) or has been actually
attained through the failure of the micromeres to rise up from the
macromeres, an invagination may also take place later owing to the
appearance of a rather large cleayage-cavity. In this latter case,
however, the germi-layers may also already have appeared as rudi-
ments. The macromeres next give off at the vegetative pole a few
small cells (G and HH, ent) which, together with the former, repre-
sent the rudiment of the entoderm. The rudiments of the three
germ-layers are now visible; the ectoderm has arisen from the micro-
meres, the entoderm is represented by the macromeres and their last
derivatives, and, finally, the mesoderm is found in the form of two
cells (derived from one of the [posterior] macromeres).+
, [the cleavage-cavity seems to be very variable in the Gastropoda, and even
in those forms in which it is most conspicuous, it is found to vary at different
stages of cleavage. This variation is most noticeable in Limaa, and Koro
(No. XTV) thinks that this cavity is connected with the excretory of
the blastomeres. The cavity is most developed in those Gast in which:
the gastrula is embolic and, during invagination, it becomes temporarily
obliterated, but re-appears later (Planorbis, Rant, No. 90).—Ep.
+ (It will be seen that if the interpretations given on p. 107 of the relation
between the first and second cleavage-planes and the axis of the adult body
CLEAVAGE AND FORMATION OF THE GERM-LAYERS. di
‘The formation of the germ-layers does not take place in all Gastro-
pods in the manner just described, indeed, the layers form in very
different ways in diverse Gastropods, as might be expected from the
variations found in the manner of cleavage. It has already been
mentioned that such variations occur in spite of strong general
reseinblance. The method of cleavage described above applies, with
slight modifications, to many Gastropods. We append a list of a few
genera chosen as representatives from the different divisions in which
this is the case: among the Prosobranchia, Fissurella (No, 12),
Neritina (No. 7), Crepidula (Nos. 24 and 25), Bythinia (Nos. 91,
101 and 28), Vermetus (No. 99), Fusus (No. 11), Entoconeha (No. 76) ;
among the Heteropoda, Firoloida and Pterotrachea (No. $1); among
the Pulmonata, Plaworbis (No. 91), Limnwea (Nos. 130 and 131),
Fr, 4. Jn the clenvage of Covolinia tridentata (A) and Aplysia ti B
ations Rupes) Fra Tecan pane Ariveia femacina (B
: and the polar bodies
Limex (Nos. 130 and 73), Onchidinm (No. 51); among the Opistho-
Dranchia, Voto, (No. 91), Ercolenia (No. 124), Tethys (No. XXVI),
Uabrella (No, X11); among the Pteropoda, Cavolinix, Cyynebutia
(No. 32), Clione (No. 55).
Certain modifications in the cleavage are no doubt principally
determined by the amount of yolk in the egg. These are connected
specially with the size of the macromeres, In Cavolinia and Cymtuliv,
Are correct, then there must be two anterior and two posterior macromeres,
and it is from one of the latter that the primitive mesomere is now said to
arise. [1 seems further ble that the first mesomere arises from the left
| macromere in and from the right in sinistral Gastropoda. In
| ;, of the large amount of evidence which is accumulating in favour
| ‘view we must, when we consider the great difficulty in tracing the rela-
the early cleavage-planes, wait for further observations, especially on
rs before we finally conclude that this origin of the mesoderm is
‘ypical of all a, See footnote, p. 119.—Kp.]
112 GASTROPODA. -
for instance, one of the four macromeres is markedly smaller than the
others, although the cleavage, in other respects, follows the usnal
course (Fig. 41 A). At the four-celled staye in Aplysia, two blasto-
meres are distinguished by their smaller size, a difference which can
be recognised in the later stages also (Fig. 41 2), Although the two
smaller macromeres are still visible at this stage (8, 77. and /V.), yet
in later ontogenetic stages, only the two larger ones are still distinet,
and these are apparent until grown over by the micromeres (epibolic
gastrulation, Ray LANKESTER, Chap. xxvi., Lit, No. 29; MaNrREpi,
No. 72, Buocumann, No. 8). Another Opisthobranch, Avera, re-
sembles Aplysia in this respect (RaBL, No. 91).
Fig, 42,—A-£ stages of cleavage in Vasa mutabilis (after Bonnerzky from BaLvoun's
Text-book), A-C, formation of the macromeres, on which, in, four, and in A «
large number of micromeres lie,
‘The first stages of cleavage, in Nassa mutabilis, are very striking
and peculiar (Bosretzky, No, 11), The egg contains a large
amount of food-yolk, and the formative protoplasm is aggregated at
the animal pole, over which the polar bodies are situated. An
equatorial and a vertical furrow, the former near the animal pole,
appear simultaneously, and divide the ovam into three segments,
two smaller blastomeres which are produced by the vertical furrow
and one large brown sphere, minus a nucleus and consisting entirely
of yolk-material (Fig. 42 A). The two blastomeres thus rest upon
this sphere somewhat like a germ-disc, except that the yolk has in
this case not attained to any great size. This condition soon dis-
THE FORMATION OF THE GERM-LAYERS, 113
appears, the yalk-sphere fusing with one of the blastomeres (Fig. 42
B); at the four-celled stage, produced by a triple segmentation of the
large sphere and division into two of the. small blastomeres, it re-
appears (C). The yolk-sphere, which at this stage is distinct from
the blastomeres, again fuses with one of the cleavage-spheres, and it
thus happens that the eight-celled stage (D) does not essentially
differ from the usual condition (Fig, 40 C) except for the fact that
one of the macromeres is specially large, the greatest mass of the
yolk having accumulated in it. The further cleavage seems to take
place in a regular manner and in a way similar to that above
described. Finally, here also, a large number of very small micro-
meres lie like a dise or cap upon the four macromeres (Fig. 42 2).
At a later stage, in Nassa, there is one large cell which is specially
distinguished from the rest. While the other cells divide further
it remains, on account of its large amount of yolk, almost unchanged.
Tt represents a kind of food-yolk which, in a much more specialised
form, will be found again in the Cephalopoda.
‘The preponderance of one macromere over the three others is found
to a striking degree in Purpura (Sepenna, No, 115) and in Uyo-
ealpine (Broors, No, 17; Corgis, No, 24), forms which in their
ontogeny seem to resemble Naas.
3. The Formation of the Germ-layers.
‘The first appearance of the germ-layers in a few forms has already
been alluded to in connection ‘
with the phenomena of cleav-
age, but in other forms these
layers arise in a somewhat
different way, their origin in
some cases being so differently
described by authors that this
point calls for special atten-
tion.
‘Gastrulation is attained in
different ways in accordance
swith the variations in cleay-
age. In the simplest cases, Fig, 43-—Blatalastage of Patlla (ater
tively large cleavage-cavity
“arises (Fig. 43). The vegetative pole of the blastula is formed by the
1
114 GASTROPODA.
macromeres and consequently appears much thickened. After the
mesoderm has become differentiated, the entomeres begin to increase
in number (Fig. 40 H,+nt), and the whole entoderm becomes in-
vaginated into the cleavage-cavity, and thus a typical invagination-
gastrula forms (Planorbis, Rabu). In Patella, on the contrary, an
extremely large solid ingrowth of macromeres takes place from the
vegetative pole of the blastula (Figs. 49 and 50, p. 124). From this
ingrowth, the mesoderm and entoderm become differentiated and,
at a later period, an archenteric cavity forms within the till now solid
entoderm (Patten, No. 83).
In a few Gastropods, such as Bythinia and Limnuea, a cleavage-
cavity is present at an early stage, but this soon disappears; the
WH.
Fic. 44 —.1-C, embryos of Firnluida Desmuresti in the stage of gastrula-formation
(after Fou). 4. blastopore ; raf, ectoderm ; rk, polar bodies.
blastula now becomes flattened, the macromeres prepare to invaginate,
and the micromeres, advancing towards the vegetative pole, grow over
the mesoderm which, has already formed, and a part of the entoderm
(Ray Lankester, No. 63: Wotrson, No. 131; ERtancer, No.
28). Gastrulation follows the same course in Paludina, with the
distinction that, in this form, the cleavage-cavity is from the first
very small, and the mesoderm only later becomes recognisable
(cf. p. 134, Biirscuit, No. 18). In the Heteropoda also (Firoluida
and Carinuria) a more or less flattened blastula with a slit-like
blastocoele, the animal end of which is composed of small and the
vegetative of large cells (Fig. 44 4), gives rise by a similar process
THE FORMATION OF THE GERM-LAYERS. 115
to the gustrula (Fig. 44 B). When gastrulation commences, and
during its course, the cleavage-cavity is but slightly developed, or
even entirely degenerates, but enlarges considerably at a later
period through the greater development of the ectoderm. The
archenteric cavity also is large (Fig. 44 C), and the archenteron thus
represents a wide sac (Fou, No, 31). These stages resemble those of
Paludina.
‘The partial circumcrescence of the macromeres by the ectoderm,
as it occurs in the last-named form, is a first indication of the
transition to the epibolic gastrula which is formed at an early stage
in the Pteropoda (Cymbulia, Clione). The cleavage-cavity here is
either entirely reduced or but slightly indicated. The thin layer of
ectoderm-cells then lies in close contiguity to the entoderm (Fig. 45
A), But even here an invagination takes place. The middle ento-
a
45.—A and B, embryos of Clione limacina showing the formation of the germ-
Kexrowrrsen). 61, Seep ect, hater eta ose ntoderm ; se, Pe
derm-cells shift upwards, the ectoderm at the same time growing out
still further towards the vegetative pole and thus narrowing the
blastopore, and the epibolic gastrula thus has the appearance of an
(Fig. 45 B). A similar process was described
in connection with the Lamellibranchia (Ostrea, p. 27).
“The gastrula arises by epibole in Fueus (Boprerzxy, No. 11),
Aplysia (Buocumans, No. 8), Crepidula (Conxti, No. 24) and
Vermetus (Savensky, No. 99). In these forms, the ectoderm, as a
thin Tayer, surrounds the four yolk-laden macromeres, from which,
‘at w later stage, small cells become detached, chiefly at the vegetative
pole, that is, in the neighbourhood of the blastopore ; by the deyelop-
‘ment of these small cells an archenteron is formed, bounded dorsally
‘by the four macromeres and ventrally by these small cells. In Neri=
tina, these cells form early, before the circumerescence of the macro-
116 GASTROPODA.
meres has proceeded so far (Fig. 40 G, ent). According to BLocuMANN
(No. 7), the smaller entoderm-cells shift beneath the layer of ectoderm
towards the animal pole and here form, above the macromeres, a kind
of cap (Fig. 46). In this way an archenteron arises, which is bounded
partly by smaller
entoderm-cells and
partly by the mac-
romeres. eritina
in this point more
nearly resembles the
forms considered
above, in which there
was a transition from
; an epibolic to an in-
Fro. 46.—Embryo of Neritina sluciatilis in optical Vagination - gastrula.
section (after LOCHMANS). Al, blastopore ; ert, ecto- A capofmicromeresat
derm ; ent, entoderm ; mes, mesoderm.
first lies on the large
macromeres, somewhat as in Fig. 40 F and G, but a cleavage-cavity
soon appears between the micromeres and the macromeres. As the
circumcrescence of the macromeres advances, the archenteron de-
velops, although in a way which deviates from that commonly met
with.
In Uroralpins, Fulgur, Purpura and Nasea also, gastrulation takes
place through epibole (Brooks, No» 17; McMurricn, No. 70;
Bosretzky, No. 11), and in these forms, on account of the great
abundance of yolk, other variations in the formation of the germ-
layers are caused. It has already been shown that in Nassa mutabilis
the one of these forms which has received most attention, as well as
in Urosalpine and Purjnra, one of the macromeres which is specially
rich in yolk is far larger than the others (Fig. 42 D). The micro-
mere-layer lies on the macromeres in the form of a disc or cap (Fig.
42 FE). When the micromeres grow out towards the vegetative pole,
the three smaller macromeres also take part in the process of shifting
and in so doing increase in number (Fig. 47 B, hy). Finally, these
cell-complexes, which represent the rudiment of the entoderm, become
more and more shifted towards the vegetative pole (Fig. 47 C and
D). They line a cavity which corresponds to the future lumen of
the enteron. It is the protoplasmic parts of the macromeres that
are at first used for the formation of the epithelium of the enteron ;
the rest forms a kind of food-yolk upon which the cells of the germ-
layers lie like a germ-dise (Fig. 47 B). As far as can be seen from
EE
THE FORMATION OF THE MESODERM. 117
Brooks’ description, the entoderm forms in an exactly similar way
in Urosalpine. A mass of food-yolk is also formed in Fusus, Ver-
Aplysia, etc, by those macromeres which attain to so con-
siderable a size,
The Mesoderm. In connection with the account given of the
processes of cleavage, it was stated that the middle germ-layer arises
very early, In Planorbis, one of the posterior of the four macro-
A
eel sections tl sh embryos of different ages of Nass muta-
‘Bonnerzky, from BaLroun's Text-book). 6/, blastopore ; ap, ectoderm ;
Sees hy, entoderm ; in, intestine ; m, mouth ; me, mesoderm ;
shell-gland ; enteron,
* divides, giving rise to un entomere and to the primitive meso-
which latter eventually yields the two primitive mesoderm-
as already shown (p. 110), These are soon pressed into the
ity, and, by their increase in number, give rise to the
ape eemotierns-bands. This seems also to be the case in the Ptero-
© 8 [This coll is believed to be homologous in all Gastropods and is now desig-
“nated D by studente of cell-lineage.— wt. J a
118 GASTROPODA,
poda (Clione, Kxreowrrscn, No. 55). In this case also, the division
of one of the four macromeres is said to give rise to two cells which
are soon driven inwards, these two symmetrically placed cells denot-
ing the posterior end. Kxrrowrrscr conjectures that in those
Pteropoda in which, according to Fon, one of the macromeres is
distinctly smaller than the others, this smaller macromere yields the
primitive mesoderm-cells (Fig. 41 4, ///). In Clione, each of the
two cells which arise by the division of the macromere again divides
into two large cells (mesoblasts, Fig. 45 B, mes), which now take up
a symmetrical and bilateral position at the posterior end and, by
continuous multiplication, give rise to smaller cells.
‘The radial character of the cleavage, which is so marked during the
early stages (Fig. 40 C-#), is much modified by the differentiation
of the mesoderm, and, when the two mesoderm-cells appear, the germ
attains a true bilateral symmetry (Fig. 40 A). This is the case
in Planorbis and a similar condition is shown in Bythinia also. As
in Planorbis, the primitive mesoderm-cells in Bythinia arise from one
of the posterior blastomeres, which is to be regarded as a mesentomere,
i... it divides into two cells, one of which remains as an entomere
in the position oecupied by the posterior macromere, while the other
shifts slightly forward. This latter cell divides into two cells in
such a way that the two lie side by side; these are the mesodermal
teloblasts which give rise to the mesoderm-bands (y. ErnancEr, No.
28). The mesoderm rises in a similar manner in Crepidula (CONKLIN,
No. 24) * and Neritina (Buocumann, No. 7), although a few slight
modifications are here brought about by gastrulation taking place
through epibole in consequence of the large size of the macromeres,
or by a near approach to this form of gastrulation. In Weritina,
a cell becomes detached from one of the posterior macromeres which,
by division, gives rise to the two mesoderm-cells (Fig. 40 G@). This
process can be made out very distinctly in the eggs of an Opistho-
branch (Umbrella) examined by Heymons (Fig. 48). Here also a
smaller entoderm-cell and a larger mesoderm-cell (B, ent and m)
arise through the division of one of the posterior macromeres (A and
B). This primitive mesomere divides into two laterally placed meso-
derm-cells (C, wm) which soon give rise to the two mesoderm-bands,
formed of a few large cells containing yolk and other smaller cells
(D and E£). The rise of the mesoderm from one of the posterior
*(Conxuis (No, IV.) now finds that, in Crepidula, the mesoderm ae ae
arise until after two further divisions, but regards this as an
dition,—Ep,]
120 GASTROPODA.
the mode of formation of this layer in Nossa as given by BoprerzKy
(No. 11) to the method described above. In sections made through
such a stage in the egg of Nussa (Fig. 42 £), under a cover of
smaller cells, a few larger cells can be seen projecting into the
cleavage-cavity. The projecting cells detach themselves and yield
a few somewhat large cells which from this time lie in the cleavage-
cavity. These are the first mesoderm-vells and, since the cells
from which they were abstricted evidently correspond to one of
the smaller macromeres (Fig. 42 #), the mesoderm has an origin
similar to that in the cases previously considered. The smaller
Fie. 48.—A-#, a few stages of the cleavage and formation of the i-layers of
Umbrella {afer Heymons). A shows the four macromeres ; F, {he ‘Uvicios of the
fo
ect, ectoderm ; ent, entoderin 5 Rly the: primitive mesomere ; mesoderm ; ram, the
pained mesodermn-cells (mesodermal teloblasts) resulting from the division of #,
mesoderm-cells at first present soon again divide (Fig. 47 A), and
here also seem to yield structures akin to mesoderm-bands (Fig.
47 B).
The mesoderm is also found to arise from the macromeres in various other
forms, e.g., in Limnaea (Woursox, No. 131) and Fulgur (McMcrrice, No.
70), and Janthina, in which form, according to Happow (No. 40) it becomes
Separated from the macromeres at the top of the blastopore, At a stage ip
which the ectoderm-cap has not completely grown round the macromeres, the
peripheral macromeres yield the mesoderm-cells. Happox's account is too
slight and his figures too vague to allow any conclusions to be arrived at
THE FORMATION OF THE MESODERM. 121
regarding the origin of the mesoderm in Janthino. There are also various
other descriptions of the origin of the mesoderm in the Gastropoda which, as
they are still less well founded, cannot here be considered.
‘The mesoderm arises in Patella, as above, in connection with the
entoderm, but at a stage when further differentiation has taken place
in the embryo (Parren, No. 83). In Patella, there is a blastula,
from which the entoderm arises, as already shown, by the ingrowth
of large cells at the vegetative pole (Figs. 49 and 50). There are at
first four large cells, occupying the same relative positions as do the
four macromeres in other forms. Now, however, according to
Parren, a cell arises on each side of these four blastomeres which,
by division, gives off into the cleavage-cavity another rather large
cell. These two cells are regarded by Parren as mesentoderm-tells
(Fig. 50 A, em), and from them the two primitive mesoderm-cells
(mesodermal teloblasts) are derived. These lie near the blastopore,
at the posterior end of the larva and increase in number later from
behind forward (teloblastically). In this way the mesoderm-bands
arise, these being, according to Parren, developed with special
regularity in Patella (Figs. 51 and 52, p. 126).
‘The mesoderm, in the Gastropods, has generally been considered to arise in
eonnection with the primitive entomeres before the formation of the arch-
enteron, but it has recently been asserted that it arises in the form of coelomic
‘sacs, an assertion which was specially startling because it was supposed that
the Molluses showed no sign of the formation of enterocosles, such condi-
tions having so far never been observed. In the differentiation of the meso-
derm, especially in the development of the pericardium, the Mollusca, it is
‘true, show great agreement with certain ‘ Enterocoelia," and there is no doubt
‘that, like these, they possess a secondary body-cavity, but, in this respect, they
‘most nearly towards the Annelida, the formation of the meso-
-derm from mesodermal teloblasts being like that in the latter group. Con-
‘sidering all that is as yet known of the formation of the mesoderm, we cannob
agree with the results obtained by Ertancen for Paludina,and must continue
‘to be sceptical about them until they are better supported or are actually
‘confirmed by new investigations (if possible made on other forms as-well),
y. Exnaxoen's account is as follows: From the rather wide archenteron of
“Paiudina a bilobed outgrowth appears which gives the impression of a double
eoelomie sac such as occurs for instance in various Echinoderms (Vol. 1., pp.
407-409). This sac, which rises from the archenteron near the blastopore,
‘becomes detached later from the entoderm and now represents a vesicle
@losed on all sides and symmetrical in form, The outer and inner walls
‘approach the ectoderm and the entoderm respectively so that at this stage
‘speak of a somatic and a splanchnic layer, It is evident that, up
point, the condition of the mesoderm closely resembles that of the
sacs in otheranimals. This, however, soon changes, for the coclo-
sacs, by giving off single cells, break up altogether, leaving only two
.
122 GASTROPODA.
insignificant vesicular vestiges surrounded by irregularly distributed meso-
derm-cells on the ventral side of the archenteron. These will be referred to
again later.*
Other descriptions in which the middle germ-layer is derived direct from
the ectoderm are difficult to reconcile with the accounts we have given of the
formation of the mesoderm. Such an ectodermal origin is attributed to the
mesoderm in Fusus (BOBRETZKY, No. 11) in Termetus (SALENSKY, No. 99) and
in various other Gastropods (For). The eggs of Vermetus are very rich in yolk.
The ectoderm lies as a thin layer upon the macromeres, almost entirely en-
closing them. Near the blastopore, the increase in number of the cells of the
ectoderm is said to give rise to a thickening which is the rudiment of the
mesoderm. In Fusus, BoBRETZKY regards the latter as arising by a prolifera-
tion of cells from the lips of the blastopore. According to SaLensxy, this
mesoderm-rudiment is bilaterally symmetrical like the mesoderm-bands, but
another independent formation of mesoderm is said to take place in the
neighbourhood of the shell-gland. Sazensky is inclined to regard this part
of the mesoderm as having arisen through delamination from the ectoderm
near which it lies, i.e., from the dorsal part of the body. There is some
similarity between this last view and the account given previously by P.
Sarasix (No. 101) of the origin of the mesoderm. According to SaRasix,
growths of the ectoderm occur at certain points of the body from which
mesodermal elements become detached. This takes place partly at an early
stage of embryonic development and partly later. Since this material becomes
abstricted at various times and at different parts for the formation of those
organs which are usually regarded as mesodermal, SanasiN is unable to assume
the existence of one uniform mesoderm-layer and therefore takes somewhat
the same stand-point as that adopted later by KLEINENBERG in so decided
manner for the Annelida (Vol. i., pp. 292 and 293). Primitive mesoderm-
cells and mesoderm-bands in Bythinia have been more recently described by
ERLANGER (No. 28) and, according to the very definite account of SARASLN, we
should have to show whether, besides this distinct mesoderm-rudiment, 6
further formation of mesodermal elements takes place from the ectoderm, as
* [In spite of the more recent investigations on this point, the true origin of
the mesoderm in P’aludina must still be regarded as undecided. In his
most recent publication, ERLANGER (No. X.) gives figures which are difficult to
interpret in any other way than he has done. Consequently, he still regards
Paludina as enterocoelic, but he finds, besides the coelomic sac, paired
primitive mesoderm-cells near the blastopore which may be the forerunners
of the cells which form the enterocoeles. He suggests that the sparsity of
yolk has made Paludina more primitive in this respect than other Gastropoda.
‘SNNIGES (No. XXV.), who has specially investigated this point in Paludina,
concludes, but without reference to ERLANGER'S latest work, that the meso-
derm arises shortly after the formation of the gastrula by a wandering in of
ectoderm-cells from that portion of the ventral surface which is formed by the
closing of the blastopore; the mesoderm then spreads out to form a ventral
sheet which extends by growth on either side of the archenteron. Soon, how-
ever, its cells become scattered in the cleavage-cavity without forming a second-
arycoelom. Scuwipt (Nos. XX. and NXI.\, who has confined his attention to
Pulmonates, finds no support for ERLAS| iews in the origin of the mero-
derm of these forms. _An investigation on this point in some of the primitive
Prosobranchia is very desirable. —Ep.}
THE RISE OF THE LARVA, ETO, 123
bas been assumed or conjectured in connection with other forms (Annelida,
Echinodermata) and specially for the Mollusca (cf. Cyclas, p. 29). It must be
regarded as a striking fact that even those zoologists who, like Ertanorn,
are very decided as to the derivation of the whole mesoderm from the meso-
derm-bands, allow that some of the elements of the connective tissue arise
from the ectoderm. For example, the so-called “nuchal” cells on the pos-
terior edge of the velum on the “neck,” ic, am accumulation of specially
large ectodermal cells, pass inward so as to become distributed in the con-
Weetive tissue. Although of different appearance from the other elements of
the connective tissue, they appear to belong to the latter.”
4 The Rise of the Larva and its Relation to the Adult Form.
‘The variations which we have found in the development of the
germ-layers among the Gastropoda.naturally lead us to expect varia-
tions in the external form of the embryo. In the development of the
latter, an important part is played by the smaller or larger amount
of yolk contained in the egg. Besides this, however, adaptation to
the manner of life of the various forms has to be considered, for the
greater number of Gastropod larvae swim about freely for a long
time before assuming the adult form. Now although the larvae, in
essential points, can be traced back to a fundamental form, the
differentiations found in the various divisions are somewhat far-
reaching, so that we are obliged to consider the different larval forms
upart. We shall first, however, describe the development of a few
specially forms so as to give the reader a general
‘idea of the subject and to make possible a comparison with other
| tlivisions of the Molinsca.
The development of the larval form of Patella has been deseribed
‘in detail by Parren (No. 83), and since this Prosobranch, which
telongs to one of the most lowly groups, apparently most nearly
attains the typical larval form, its ontogeny will occupy us first.
Unfortunately the development of this form has only been followed
by Parren up to a stage at which the larva is still far removed from
‘the shape of the adult.
of Patella shows primitive conditions in so far as
is thrown off very early, even while cleavage is still
fhe aboye somewhat conflicting accounts of the rise of the mesoderm,
im cont ion with the more recent observations of ConKiin (No, IV.),
(0 oe ), and Wrerzessk1 (No. XXVIL.), seem to render it highly
a ae Sane aaeet layer has, in all Gastropoda, os has
hia, a double origin: (1) from primitive
sing oven to the lateral mesoderm-bands ; and (2) from
stage as paired differentiations nearer the anterior
the body.—Ep.]
124 GASTROPODA.
going on. Since cilia appear as early as the blastula-stage (Fig. 49),
the embryo is very soon able to move about freely and thus becomes
alarva. In this way, Patella resembles a Lamellibranch, but such
early locomotion is not common among the Gastropods, most of the
larvae hatching at a much later stage. The ingrowth of entoderm
Flos. 49 and 50,—Embryos of Patella at the blastula-stage and at the commencement
and comple gastrulation (after PaTTRN). 8/, blastopore ; em, mesentomere ;
ent, entoderm ; mex, mesoderm ; sd, sbell-gland ; 17, ciliated ring.
and the differentiation of the mesoderm take place, as already de-
scribed (pp. 114 and 121), from the thickened vegetative pole of the
blastula (Figs. 49 and 50). The blastopore lies at the vegetative
pole which at the same time corresponds to the posterior end of the
larva. The principal axis of the larva, at this stage, passes through
THE RISE OF THE LARVA, ETC. 125
the middle of the blastopore and the opposite pole at which later the
apical plate develops. Through the appearance of the mesoderm,
the larva becomes bilaterally symmetrical. The blastopore soon
changes its position, shifting forward on the ventral surface, as a
consequence of the active growth of the dorsal surface. The rudi-
ment of the velum which was indicated at the blastula-stage has now
become more distinct (Figs. 49 and 50). In later stages, the dis-
placement of the blastopore becomes much more striking, and recalls
the condition already described in connection with Dentalium (Figs.
34 and 36, p. 91). The blastopore, during this process, changes
from its round form and becomes slit-like (Fig. 51 8). At its
Bei eettateat petal vets can be oes ear the coeslsr Uincapore Ts Bie
Rope agitate. Near ié cen to recognised the xndimenta of the two
‘hind it the anal ciliated eet
posterior end, two cells are distinguished by their special size. They
soon become covered with cilia (Figs. 51 and 52), and may well be
compared to the anal cells of other Gastropods which will be described
later (p. 142). The slit narrows and closes in from behind forward.
‘The anterior part of the blastopore remains in the form of a round
pit in the position of the future mouth; later, the blastopore is
earried inwards by a depression of the ectoderm, the stomodaeum,
which occurs at this point. This depression represents the rudiment
of the oesophagus (Fig. 50 /), the blastopore persisting as the opening
etween the stomach and oesophagus. Out of this solid mass of
cells, which still represents the entoderm, the enteron forms later
be
126 GASTROPODA.
through a rearrangement of the cells which also increase greatly in
number (Fig. 52), From the posterior end, where the mesoderm-
cells lie, two very regular mesoderm-bands grow out (Fig. 52), The
shell-gland appears dorsally before this stage as a depression formed
of columnar ectoderm-cells ; over this gland, the shell-integument is
secreted later.
Parren asserts that the foot arises at a very early stage in a
remarkable manner. It is said to be produced from two prominenees
which lie ventrally at the posterior end of the body (Fig. 51 A). These
flank the blastopore on
either side at a time when
the latter still is a round
aperture, As soon as it is
displaced anteriorly, they
shift together and unite to
form the foot, the donble
origin of which can be
recognised even in later
stagesthrough thepresence
of a median groove,
Up to this stage, the
pre-oral region was speci-
ally large and bell-shaped
(Fig. 51). It is separated
aera from the posterior section
Mio, Pua ater Protas). "a, ciated anal PY the pre-oral ciliated
osetia aa mesoderm ; 4, spit ring, which is composed
of three rows of cells, the
middle row being provided with the strongest cilia (Fig. 52). A tuft
of long cilia appears on the apical plate, and near it lie two promi-
nences bearing stiff cilia (Fig. 51 B). These would recall the cephalic
tentacles of the Annelida did not each of them consist of a single cell.
As development advances, the pre-oral part flattens out considerably,
and the apical plate, which has already appeared as a median
thickening (Fig. 52, *), now takes up # considerable part of the pre-
oral section (Fig. 53, sp). At the posterior end of the larva also, a
tuft of long cilia can be seen; these belong to the anal cells aboye-
mentioned. The shell-gland which was previously invaginated has
now flattened out, and the dorsal surface even appears convex. The ~
epithelium, which was formerly very thick in this region, now consists
merely of flattened cells (Fig. 53). The shell itself has become eup-
THE RISE OF THE LARVA, ETC. 127
shaped. The somewhat swollen edge which is seeu bordering the shell
represents the margin of the muntle, the mantle itself being covered
‘by the thin horny shell. The enteron has considerably widened and
is now suc-like and, connected with it posteriorly, a pointed appendage
can be seen; this unites later with the ectoderm to form the anus.
In the stomodaeum, which has now enlarged, an outgrowth (r) is
visible ; this is the rudiment of the radular sac which was found to
i
i
uu
[
Hl
f
ty Fyo, 53.—Median longitudinal section through the
Prominence and in Eat a hal ore stage (after
4, the ciliated homey at the post
feo Ba month mul, enteron ; mes, meso-
derm has lost its regular arrangement, single cells becoming detached
from the mesoderm-bands, and being distributed in the primary
body-cavity ; these no doubt represent the rudiment of the covering
of the ectodermal and entodermal organs already formed. Certain
of these cells elongate and give rise to musele-fibres, a number of
which become attached to a point on the dorsal surface, where they
finally become firmly connected with the shell, and yield the retractor
muscle by means of which the larval body can be withdrawn into
the shell, as soon as the latter has attained the proper size.
Tn the stages depicted in Figs. 51 and 52, and even in the later
‘condition, a median section of which is given in Fig. 53, the Patella
larva closely resembles the Tvochophore stage met with in the
la
128 GASTROPODA.
Lamellibranchs (cf. Figs. 15 and 18, pp. 31 and 36). This re-
semblance is not only an external one, but extends to the inner
structure also. There is thus a Trochophore stage in the Gastropods
aleo (Ray LANKESTER, No. 63); it is not, indeed, usually developed
in so typical a way as in Patella, but shows certain modifications.
These modifications are either definite characteristics of the Gastropod
larva or are transformations undergone by the primitive larval form
as a consequence of altered conditions of life, especially by the
presence of a greater abundance of yolk causing an abbreviation or
the suppression of the larval stage and many modifications of the
processes of development.
Besides the outward resemblance to other Molluscan larvae
(Lamellibranchia, Scaphopoda, Amphineura) which is at once evident,
we have the inner organisation correspondingly developed. We have
already mentioned the apical plate and the pre-oral ciliated ring (Figs.
52 and 53), but we have to add to these the post-oral ciliated ring
which has been demonstrated in the Gastropod larvae, e.g., in Crepidula,
Fulgur, Faseivlaria and other Prosobranchia, as well as in Heteropoda,
Opisthobranchia and Pteropoda (GEGENBAUR, Kroun, Fou, Brooks,
McMorricu, etc.). It consists of a row of cilia which lie immedi-
ately behind the mouth and run parallel with the pre-oral ciliated
ring (Fig. 54, p,,, p- 130). Between this and the pre-oral ring there
are also delicate cilia which correspond to the so-called ad-oral ciliated
zone of the Lamellibranch larva. The whole apparatus, in any case,
serves, as in the Lamellibranchs, for forwarding particles of food to
the mouth, while the pre-oral ring, as the velum proper, is chiefly
of locomotory significance. The ciliated tuft at the cephalic pole
completes the resemblance to the Trochophore of other Molluscs (Fig.
3, p. 6, and Fig. 36, p. 91) and the Annelida (Vol. i., Fig. 118, p.
265). In the pre-oral section, in the region of the apical plate, eye-
spots may occur. The post-oral otocysts lying at the sides of the
mouth have already been mentioned.
The alimentary canal, like the other organs, shows the same
structure as in other Trochophore larvae. It is composed of the
entodermal mid-gut, the enteron, and of an ectodermal fore-gut, the
stomodaeum, and perhaps also of an ectodermal hind-gut, the procto-
daeum (1): at a later stage, the radular sac, that special character of
the Gastropods which distinguishes them from the Lamellibranchs,
appears in the stomodaeum (Fig. 53, r).
Among the organs found in the Gastropod larva, one is of special
significance when comparison is made with the Annelidan Trocho-
THE RISE OF THE LARVA, ETC. 129
phore, viz., the paired primitive (larval or head-) kidney. This organ,
as already noted, is conspicuous in the larval Lamellibranchs and in
the Annelida (¢/. p. 39 and Vol. i., p. 267). It has not, indeed, been
discovered in Patella, but we may reasonably expect that it will be
found im this form which in most other points is so primitive,
especially as it is found in other Gastropods of a less simple type of
development, such as the fresh-water Prosobranchia (Bythinia, Palu-
dina, p. 136) and the Pulmonata (p. 178). A tubular primitive
kidney has recently been described as occurring in the larva of an
unidentified marine Gastropod (v. Eruancer, No. 28). The
primitive kidneys in their original form appear as tubular structures,
the relation of which to the primary body-cavity is probably the
same as in the Lamellibranchs, and these organs open outwards on
the ventral side of the body behind the velum, These primitive
excretory tubes are either quite short (Palwiina, Fig. 59 B, un, p. 139)
or else longer, as in Planorbis, in which case each kidney consists
of a V-shaped tube (Fig. 78, un, p. 177).
Besides the primitive tubular kidney, various groups of ectoderm-cells have
been claimed as primitive excretory organs. Bopretzky thus interpreted two
rounded cell-growths which appear near the rudiment of the foot, Similar
organs have been found by McMurstca in Fulgur (No. 70). Sanasin de-
seribes, in Bythinia, ectoderm-cells of exeretory nature which are connected
with the velum. (In Crepidula, Conxxn (No. IV.) finds paired groups of ecto-
dermal cells, situated just behind the velum, which are eventually cast off ; he
regards them as excretory.) These ectodermal cells frequently contain concre-
tions which are said to be extruded, a fact which has led authors to attribute
an excretory function to them, They are very soon to be recognised owing to
their granular contents; in Nerifina, such granular cells, which later give rise
to velar cells, may be clearly distinguished even during cleavage among the mic-
romeres (BLocuxAsn, No. 7). Two rows of granular cells which liealong the edge
of the velum have been described in Onchidium by Joysux-Larrure (No. 51).*
*[(Hermons describes, in Umbrella, the presence of red groups of ecto-
dermal excretory cells, situated near the aia of ‘hen the right woop alone
attains functional development and sinks under the surface of the ectoderm.
‘He regards these as homologous with the similar cells situated near the velum
im the Prosobranchia. Conxur (No. IV.), however, thinks that they aré
only, since arise from totally distinct blastomeres ; the anterior
sina eas are found in three of the great Gastropodan orders.
“MAZ#ARELLI pe rae} who has studied the ectodermal or anal kidney of
‘that it does not disappear, but represents the rudiment of the definitive
Ghee internal primitive kidney is so far known to occur only in
ees eae in two Pestonsker Esoertapelde oe) possibly in es
concerning which Entanour is unable form us whether
it was an uh or & Prosobranch, Thus it will be seen that this
itive organ is found to be most highly developed in those most
forms, the Pulmonata, and that it is only elsewhere known to
occur in two Prosobranchs. A further search for this in some of the
more primitive marine Prosobranchia is much needed.—Ed.)
K
130 GASTROPODA.
It is only in comparatively few Gastropods that the T'rochophore is
developed in such a pronounced manner as in Patelin, this being no
doubt due to the fact that, in most Gastropods, a great part of the
development of the embryo takes place under the protection of the
egg-shell or in the egg-capsule. The Z'vochophore stage is neverthe-
less to be found in all Gastropods, although it is more distinet in some
than in others. In these latter, as a rule, the larva attains free life
at a stage in which its shape has already undergone several modifica-
tions. The later ontogenetic stages of Patella are not known, but
the larva, at the stage in which it is provided with a foot and a fairly
developed shell, still resembles the T'rochophore, so that we may
assume that it does mot undergo any further changes except those
which are determined
by its transformation
into the adult. This
also seems to be the
case in Fisswrella as
far as its develop-
ment is known
(Boutax, No. 12).
In this Gastropod,
the velum broadens
somewhat and as-
sumes « bilateral
} form. This was also
. A already described in
"Sjchlemuon{f fou; ol aperture; pp para, the Lamellibranch
ea tte iene: ad a larva, and Fiseurella
does actually show a
certain resemblance to the later stages of these forms (Fig. 17, p. 35),
if we leave out of consideration the shell which in the one case is
single and in the other bivalve.
In the two last-named primitive Gastropods, the velum does not
differ essentially from the Lamellibranch velum, but in most other
forms its shape becomes modified in a manner specially characteristic
of the Gastropoda. The bilateral development of the velum which
is already indicated in Fixsure/la is, in those Prosobranchs the eggs
of which are rich in yolk (Neritina, Vernetus, Fulgur), evident when
the velum first appears as a rudiment. This organ appears in the
embryo at first in the form of two specially marked réws of cells
(Neritina) or two curved ridges which unite only later to form the
EE
THE VELIGER LARVA. 131
yelum, the dorsal union of the two ridges often taking place very
late. In its later development, in the Prosobranchia and especially in
the Opisthobranchia, Heteropoda, und Pteropoda, the velum by its
great lateral growth, assumes a bilobed form (Fig. 55 4-C), It
hecomes at the same time very large and is a most efficient locomotory
organ. It is beset with large, strong cilia, which may be replaced by
much smaller cilia at the junction of the two wing-like lobes, the bila-
a B
Pro, 56.—A, embryo, B and C, Velirer larvae Ch peat tn rioieg
A, dorsal aspect; 2, ventral aspects O; lateral aspect
fr é rudiments of the cerebral ganglia; f. foot; m, wonth; of, otocyst; op.
{4 shell, ¢, tentacle ; v, velum.
-teral character thus becoming still more apparent (Fig. 72, p. 162). The
larval stage which is provided with this very characteristic locomotory
apparatus has been called the Veliger stage (RAY LANKESTER). ‘The
‘great size which may be attained by the velum can be seen from
_oapreoes represents the Veliger larva of a Prosobranch (species
—
132 GASTROPODA.
unknown). Each of the velar lobes is drawn out longitudinally so
that the whole velum appears to consist of four lobes. In Atlanta,
the velum is very large and here each of the lateral parts splits up
into three, the velum thus consisting of six lobes (Fig. 67, p. 155).
In other respects the development of the body has advanced consider-
ably at the Veliger stage. The shell, which at first is cup- or cap-shaped,
increases in size through the addition of new layers, this fact being
indicated, as in Dentalium and the Lamellibranchs, by the appearance
of lines of growth. But as the addition of new material takes place
in an irregular manner, i.e., as the new layers of shell are not all of
equal width and, further, follow the curvature of the visceral sac, the
shell soon loses its symmetrical shape and begins to coil (Fig. 55).
The visceral sac is separated by the projecting lip-like edge of the
mantle from the rest of the body, especially from the head and trunk.
A slit-like depression usually appears on the right side in front of the
edge of the mantle; this depression extends posteriorly eo that the
mantle now covers a cavity, the mantle- (or pallial) cavity, in which
the gills arise later as outgrowths of the body-wall. The intestine
opens into this cavity, the anus having arisen as an ectodermal
depression primarily situated somewhat ventrally at the posterior end
of the body. At first this lies in the median plane, but is usually
displaced to the right side later, shifting at the same time forward,
and somewhat dorsally. This displacement is a result of the fixed
and rigid nature of the shell covering a large part of the body (c¢/.
p. 146).
The rudiment of the foot appears early and may attain large pro-
portions in the Veliyer larva. In Vermetus, it is paired at least
anteriorly (Fig. 55 B and C). In this larva the foot, however, is
more complicated than Fig. 55 would lead us to believe. The double
character of its rudiment is noteworthy as a peculiarity which recurs
in other Gastropods (Patella, Fig. 51 A; Limnaea, Ray LANKESTER,
No. 63; Succinea, F. Scumipt, No. 109) in very early stages. In
Succinea, the foot arises in the form of two distinct prominences
separated by a broad furrow; these outgrowths afterwards approach
each other and fuse to form the median foot, a process similar to that
described for Patella (p. 126).
On the postero-dorsal surface of the foot, a plate composed of the
same substance as the shell is secreted (Fig. 55 C, op). This is the
operculum. The otocysts lie in close contact with the foot (B, vf).
In the young stages of the Veliyer larva two prominences appear
on the velur area ; these soon extend and lengthen and can be recognised
THE VELIGBR LARVA. 133
‘os the tentacles (Fig. 55, #). At their bases, the eyes (a) arise.
Both the tentacles and the eyes are, by their origin, indisputably
proved to belong to the primary cephalic section, and it is of special
imterest that the tentacles occupy the same position as the cephalic
tentacles of the Annelida and the Annelidan larvae (Vol. i., Figs. 120
B, p. 269, and 121, p. 270).
The velum may still persist after the foot has deaced a consider-
able size and when the development of the other organs also is far
advanced, but it gradually diminishes in size and finally degenerates,
the larva thereby passing over to the adult condition which, indeed,
had already been nearly approached. ‘Two small rounded ciliated lobes
may persist near the mouth as the remains of the velum, as was
observed by Ray LANKEsTER in Limnaea (No. 63), and Joynux-
Lapeure in Onchidlinm (No. 51). These are said to give rise to the
sub-tentacular lobes or lip-tentacles ; these two structures would thus
have an origin similar to that which we felt inclined to assume for
the oral lobes of the Lamellibranchs (p. 45).
_ The perfectly developed Veliyer larva is found almost exclusively
among the marine Gastropoda, the young of which swim about freely
for a long time. Among fresh-water Gastropods, Neritina passes
through a stage with a well-developed bilobed velum resembling that
depicted in Fig. 55 (WVermetus), but the Veliger larva does not lead a
free life, but passes through this stage within the egg-capsule, When
the embryo leaves the capsule it shows the adult form (CLararéps,
No. 23). Neritina is one of those fresh-water forms which can also
live in salt water. This fact, and the presence of the well-developed
Veliger stage, suggest that it has only recently adopted a fresh-water
existence. In other fresh-water Prosobranchs, as well as in aquatic
and terrestrial Pulmonates, the Veliger stage is much reduced,
Onchidium, however, among the Pulmonates, in this respect resembles
Neritina.
- Onchirlinm, a Pulmonate living between tide-marks, not only passes
through a Trochophore stage but, while still within the egg-shell,
becomes a Veliger larva with coiled shell and a large bilobed velum.
In the course of further development, the velum degenerates, only
two rounded lobes which lie laterally to and somewhat in front of the
mouth being retained as the lip-tentacles. The embryo, on leaving
the egg-shell has, on the whole, the same shape as the parent
Goveux-Larrvir, No. 15). The condition we have just described
would be very remarkable ina Pulmonate, did not the organisation and
the manner of life of this form give some cause for the assumption
ae
134 GASTROPODA.
that it may have been derived from a marine ancestor (possibly an
Opisthobranch). Onchidium lives in the littoral zone, within the reach
of the tides, hidden in rocky fissures, where it lays its gelatinous
egg-masses, These are washed by the sea water, and JoyEux-
Larrure was able to develop them by keeping them damp and
immersing them from time to time in sea water. The eggs therefore
develop under conditions not very different from those of marine
Gastropods.
Although, as a rule, the Veliger stage is much reduced in fresh-
water and terrestrial Gastropods, the 7rochophore form is still more
or less distinctly developed in them. In the Pulmonates, the
Trochophove stage is present but is not very conspicuous (p. 177);
in Paludina, however, it is unmistakable, although this Gastropod
is viviparous (Fig. 56). Palwdina, in many. other respects besides
the retention of the Trochophore stage, is an archaic form and, as its
ontogeny has been so carefully studied, we shall give a special
account of its development, The principal ‘sources of our knowledge
concerning the ontogeny of Paludina are the works of Leypie (No.
68), Ray Lankester (No. 64), Biirsontr (No. 18) and vy, Exuancer
(No. 27), and we have also observations made by Rasu (No. 92) and
Brocumany (No. 8), Ertanaer’s account is the most recent and
the most complete in every respect.
The Development of Paludina. The fertilised egg of Paludina
viciyara develops into an almost spherical blastula which becomes
somewhat flattened later and contains a distinct cleavage-cavity.
The flattening takes place in connection with gustrulation, the
cleavage-cavity during this latter process being almost completely
obliterated by the development of the archenteron, so that a stage
is here brought about similar to that which occurs in other Gastro-
pods, especially in Firoloida (Fig. 44 B, p. 114). The gastrula,
which at first is almost kidney-shaped, with a wide
expands by growth and becomes bell-shaped in Firoloida (Fig. 44 C).
The blastopore narrows to a slit.
The formation of the mesoderm has already been described. On
this point, we follow the older statements of Bitrscutt, according to
which the mesoderm is present in the form of two mesoderm-bands
which, during the gastrula-stage, consist of few cells but increase
later (Fig. 56 A), ie., show the same condition as in other Gastropods
(ef. p. 121). The greater part of these bands soon undergoes dis-
integration, breaking up into separate cells which become irregularly
distributed in the cleavage-cavity.
THE DEVELOPMENT OF PALUDINA. 135
Meanwhile, by the development of two rows of large ciliated
ectoderm-cells, placed transversely to the gastrula-axis, the Trocho-
phore stage is reached. The pre-oral ciliated ring thus borders
the cephalic area, which has become larger by the increase in
number of the cells (Fig. 56 A). The blastopore marks the posterior
end of the embryo, but in consequence of somewhat stronger growth
and the consequent bulging of the ventral surface it is shifted slightly
dorsally. The blastopore is said to be retained in Paludina and to
pass over into the anus (Birscxt, v. Enuancer). It has, however,
beon asserted that the blastopore closes (RABL) and that the mouth
and the anus are only indirectly related to the primitive mouth, as
we shall describe later (cf. p. 141).* A large, somewhat sunken
Fie, 56.—A frontal and B sagittal section of two embryos of Paludina of different
ages m, Tegion where the mouth develops at a later stage ;
; mes,
bands (im My and scattered mesoderm-cells (in B); sd, shell-gland ; vd,
vy, velum,
area, which lies dorsally in front of the blastopore and consists of
columnar ectoderm-cells (Fig. 56 B, ed), represents the shell-gland,
above which the chitinous shell soon appears. An ectodermal de-
pression (1) which appears on the ventral side behind the ciliated
ting, and which becomes connected later with the archenteron, yields
the stomodaeum. At this stage, the anterior part of the embryo has
‘lost its former bell-shape and has become more flattened (Fig. 56 8).
The mesoderm has lost its regular arrangement and has become for
* (Téwmiers (No. XXV.), the most recent investigator of the development of
Paludina, finds that the oval biastopore closes trom before backward, and
‘What it does not give rise to the anus, which, as a secondary formation, appears
‘st the point where the blastopore closes —Ep.]
136 GASTROPODA,
the greater part distributed in the form of isolated spindle-shaped
cells in the primary body-cavity. Its further development will be
described later, but we must here refer to one of the organs formed
from the mesoderm, the primitive kidney, since this is essentially «
larval organ.
Each of the primitive kidneys arises from a compact mass of
mesoderm-cells, two such ‘masses lying at the sides of the embryo
behind the velum. A lumen now appears in the mass, which, by
lengthening sornewhat, becomes a short tube and, coming into con-
tact with the ectoderm, fuses with the latter and thus opens externally
not far behind the velum. The ectoderm sinks in somewhat at this
point later; in Byt/inia, this ectodermal invagination is even very
deep and forms the longer, distal part of the primitive kidney (v,
Exvancer). The inner surface of the tube becomes covered with
cilia especially at the blind end. The primitive kidney remains short
in Paludina, but, in the Pulmonata, appears as a long bent tube.
This is said to possess an internal aperture, that is to say, it eom-
municates with the (primary) body-cavity (p. 179). v. ERLANGER
was unable to convince himself of the presence of such an aperture in
Paludina and Bythinia* and, taking into consideration the condition
of the primitive kidneys in the Annelida, we may conclude that it is
wanting in these forms and that the two renal tubes end blindly.
This is certainly the case in the earlier stages. At the inner end of
each kidney, there is a bundle of spindle cells which in all cases
extend to the ectoderm, and serve for suspending the renal tube,
‘This latter attains its highest degree of development at the somewhat
advanced stage shown in Fig. 99, and degenerates later (Bitrscxut,
vy. ERLANGER).
The Trochophore form of the embryo is now specially modified by
the development of the foot on the ventral surface as a massive pro-
minence (Figs. 56 B and 57, /). The appearance and rapid increase
in size of this organ leads to a considerable displacement of the other
parts of the body (Figs. 56 Band 58). The pre-oral part of the body
becomes still more flattened out. The mouth shifts to the anterior
end and the velum finally appears displaced to a dorsal position (Fig.
58), The oral aperture and the anus lie at the two opposite ends of
*(v. Ertancer (No. 71), however, describes an internal aperture in
Pulmonstes. Mutssensemer (Nos, XVII. and XVII.) has made a most
careful investigation of this point in Lima and is firm im his belief that
there {s no internal opening in that Pulmonate. He derives the entire
organ from the ectoderm.—Ep.]
THE DEVELOPMENT OF PALUDINA. 137
the body. The shell-gland now becomes modified by the invagination
-of its greatly thickened epithelium and by the appearance within the
invagination of the brown ‘‘chiton-plug '’ described by Biirsonnr
(Fig. 57). During the further growth of the embryo, the gland
‘becomes flattened out and its cells lose their long columnar character
Pros. 57 and 55.—Sagittal section of two embryos of Palwdina vivipara (after Ton.
SIGES). a, anus; enf, entoderm; /, rudiment of foot; /, rudiment of liver; m,
{ tad, enteron ; mes, mesoderm-cells ; w/, tirst indications of the mantle-fold ;
A sheli-gland; f, shell-groove ; v, velum.
‘and the epithelium finally becomes very thin (Fig. 58). At this
stage, lying above the shell-gland which is now slightly depressed,
there can be seen not only the remains of the chitinous plug but the
shell-integument itself («). The shell now extends rapidly over the
138 GASTROPODA.
dorsal surface by the growth of its free edge which is still in close
contact with a thickened layer of ectoderm, the cells of this
thickening being concerned in the secretion of the shell. Beyond
this thickening, the mautle-fold (Fig. 58, m/), as a slight upgrowth
of the ectoderm, is situated dorsally to the anus. When the latter is
displaced forward by the more rapid growth of the posterior dorsal
part of the body (Fig. 59 A), the mantle either grows out further or
else the surface of the body behind the mantle and in front of the
anus sinks in somewhat ; the depression which is thus formed is the
rudiment of the mantle-(pallial) or branchial cavity (Fig. 59 A, mh).
The anal aperture now comes to lie in this depression.
Turning to the internal organs, we find that the fusion of the
stomodaeum with the enteron has now taken place (Fig. 58). The
rudiment of the liver appears ventrally as a sac-like outgrowth
of the enteron, and the radular sac arises from the stomodaeum.
As the mesodermal structures (the pericardium, the heart and the
kidneys, Fig. 59) become differentiated, those on the right side
attain a greater size than those on the left, so that a marked asym-
metry is already evident in these internal organs. The rectum, which
formerly ran directly backward, now comes to lie at right angles to-
the longitudinal axis in consequence of the displacement of the anus
described above and, later, runs obliquely to the right side.
The inner asymmetry precedes the outer, and has therefore been
used as an explanation of the asymmetrical structure of the body
(Bythinia, P. Sarasin, No. 101). When we spoke above of a dis-
placement of the anus the expression used was not strictly accurate,
since the distance between the mouth and the anus remains almost
the same. Marked growth, on the contrary, takes place first in the
dorsal surface and later especially in the left posterior part of the body.
Although the area lying between the mouth and the anus does not-
grow appreciably, considerable increase in size occurs in the posterior
region (Fig. 59 A-C), and it results that the parts that have not grown
now seem to belong more to the anterior portion of the body which
as a whole is now much larger. Birscxri bas paid special attention
to these processes in Paludina (No. 19). ‘The left posterior part of
the body, in consequence of the processes of growth just described, is
much swollen and this leads to the formation of the visceral sac
directed backward to the left and to the (apparent) shifting forward
to the right of the anus and the parts surrounding it. The swelling
of the posterior dorsal parts of the body to form the visceral sac is
determined by the advancing growth of the inner organs, The
THE DEVELOPMENT OF PALUDINA. 139
subject of the asymmetrical shape of the body will be alluded to
further on (p. 143).
vanes ina, vivipare of different (after v. Sem
eyes foot; ear: ¥, Ser; de eft peri sac; m, mouth ; mé,
of the mantle; na, eflerent
eee
It bas already been mentioned that the anal aperture lies im the
mantlecavity. This latter has deepened during the processes just
Aa
140 GASTROPODA.
described through the rising up and growth of the margin of the
mantle, but it also becomes affected by the asymmetrical develop-
ment of the embryo. It is soon evident that the part of the cavity
lying on the right side is much deeper than that lying on the
left, and the cavity shortly becomes confined almost entirely to the
right side in consequence of the twisting of the embryo. On
this side there opens into it not only the rectum, but the efferent
ducts of the now developed definitive kidney and of the genital
organs. At a later stage, the mantle-cavity extends dorsally and
thence over to the left side. Above the mantle lies the shell which
passes from its earlier flat shape to a more arched form, till it becomes
a somewhat deep cup (Fig. 59 A and 8), and, finally, in consequence
of its one-sided growth, becomes coiled (c/. p. 147).
While the above processes have been going on, the anterior part of
the body also has undergone essential alteration. The velum has
degenerated more and more, while the foot has greatly increased in
size (Fig. 59 A-C). At its base, the otocysts (o/) have appeared as
ectodermal depressions which soon became cut off as closed vesicles.
In the posterior dorsal part of the foot, the operculum is secreted in
a manner similar to the secretion of the shell (Fig. 99, op, and Fig.
92, op).
The tentacles arise on the yelar area as two very large swellings
which soon increase in height and thus become conical (Fig. 59 4-C,
t), At their bases the eyes appear. At the stage depicted in Fig.
59, both these organs can be recognised as belonging to the
velar area, since the ciliated ring is still present as a narrow band.
In these later stages, when, in keeping with the shape of the body,
the velum has become almost bilobed, it may be compared with the
sail of the Veliger larva of the marine Gastropods which, however, is
much more distinctly bilobed.
The further development of the embryo is chiefly determimed by
the continued growth of the visceral sac as a result of the perfecting
of the inner organs, and by the increase in size of the foot and of the
tentacles. This may best be seen by comparing Fig 59 with Figs, 99
and 100.
Before closing this section we must deal with one or two other
morphological points which could not earlier receive the consideration
they deserve. The first of these concerns the shape and transforma-
tion of the blastopore, In its simplest form, the blastopore has been
described as a rounded aperture appearing at the yegetative pole;
this aperture, without undergoing essential change of form, may pass,
‘TRANSFORMATION OF THE BLASTOPORE. 141
by gradually narrowing, into the mouth. An ectodermal depression
does, indeed, regularly accompany this process, pushing the actual
blastopore some distance inward. Such a direct passage of the
blastopore into the mouth has been claimed by Bosrerzky for Fusus
and by For for the Pteropoda and Heteropoda. The narrowing of
the blastopore may, further, lead to its direct closure, but even in
such cases, the stomodaeum forms at the same spot, as, for instance,
in Vaxsa and Neritina (BonsetzKy and Buocumann). The pot
at which the blastopore closes and where the adult mouth eventually
forms, no longer corresponds to the vegetative pole, ie., to the end
of the embryo which is turned away from the animal pole, but, in
consequence of the growth of the postero-dorsal region, has shifted
somewhat towards the animal pole and is found behind the velum.
This last condition of the blastopore is that which is by far the most
frequent among the Gastropoda, Here also the blastopore is at first
round and may have a considerable diameter, but it soon becomes
narrow and slit-like (Planorbis, Patella, Palwfina and many other
Gastropods). The slit-like blastopore closes from behind forward, ©
and its anterior end either passes direct into the mouth, as in Plan-
orlis, Limnaea, and Patella (according to Rasy, Ray LAnKEsTer,
Wotrson, Parren) or closes completely, in which case, at the last
point to close, an ectodermal depression forms which yields the
stomodaeum. This latter is the case in Aplysia, Bythinia, and
Orpidula (BLocumany, Sanasry, y, Exvaxcer, Conxnry). The
formation of the definitive mouth is always connected with an
invagination of the ectoderm.
The slit-like blastopore, if regarded as open from its posterior to
its anterior end, seems to occupy the whole length of the later ventral
surface. Its posterior end no doubt still corresponds approximately
to the former vegetative pole, its anterior end lying immediately be-
‘hind the velum. Now while, in the majority of cases as yet known,
the blastopore closes from behind forward, in Paludina, as already
described, the posterior part of it is said to persist and to yield the
anus in the same way as the anterior part in the above cited cases
yielded the mouth. If this is actually the case, it can only be
x by means of the view adopted by Btrscunt, according
te which both the mouth and anus arise by the differentiation of the
Blastopore. Biirscxt: found an indication of this in Ray Lan-
KESTER'S observations on Limnaea, in which form, the slit-like
blastopore, the anterior end of which becomes the mouth, extends as
far as to the anal region. Since that observation was made, other
142 GASTROPODA.
cases have become known of certain relations existing between the
anus and the blastopore. v. Ertan, for instance, described the
blastopore of Bythinia as a slit, the posterior end of which lies at the
spot where the anus forms later (Fig. 96 B, p. 210), and even in
Palwdina itself it appears indisputable that the slit-like blastopore
extends almost to the velum, #.v., to the spot which corresponds to
the mouth that forms at a later stage. Further support for this view
is found in the condition of some Opisthobranchs (Doris, Aplyxia,
according to LancrrHans and BLocumann), in which the anal cells
are found exactly at the posterior end of the blastopore which here
also is slit-like. It should be mentioned further of those two anal
cells that, in various Gastropods, they mark at an early stage and in
a striking manner the position of the anus (pp. 154 and 160). They
coincide in position with the two cells which, in Patella, appear be-
hind the blastopore (p. 125). The anterior end of the blastopore in
the forms just named, also, becomes the mouth, so that the relations
of the blastopore to the month and to the anus in these forms are
"specially distinct and the apparently divergent condition of Paludina
is thus explained.
Greater attention has been paid to the form and the transformations of the
Dlastopore in the Gastropoda than in other animals, and the subject there-
fore has received special consideration from us. It was not our intention to
give an exhaustive account of the observations made in connection with it
chiefly because these are to some extent unreliable. We have therefore
selected only such statements as seem to some degree well-founded, though
even these need more careful examination. It seems, however, to be proved
by these observations that, in the Gastropoda, there are relations between the
blastopore on the one hand and the mouth and anus on theother. We conse-
quently find, in the Mollusca, conditions similar to those previously met with
by us in the Arthropoda, in which class also, the mouth and anus are either
directly or indirectly related to the blastopore (cf, Vol, iii., p. 412). The eondi-
tion of Paludina may recal] the Echinoderms, in which the blastopore passes
direct into the anus (Vol. i., p. 359). 'The condition of the Gastropods is, how-
ever, in any case, to be| traced back to corresponding processes met with in
the formation of the Annelidan Trochophore (Vol. i., p. 265). Tn the latter, the
Dlastopore at first lies at the vegetative pole of the embryo, It then extends
and occupies the whole length of the ventral surface which, however, is not
very great, When it closes from behind forward, its anterior end passes over
into the mouth, this latter lying behind the pre-oral ciliated ring, as in the
Gastropoda. The anus, however, arises at the posterior end of the larva which
previously corresponded to the vegetative pole and thus to the position of
the blastopore. The conditions here are thus evidently very like those in the
Molluscs,
RELATING TO THE ASYMMETRY OF THE GASTROPODA. 143
‘These last considerations lead us to the changes of shape undergone
by the embryos in early stages. Even before the Gastropod egg is
affected by cleavage, and while it is undergoing this process, the animal
and vegetative poles may be distinguished. The blastopure at first
corresponds to the vegetative pole, but, as it lengthens, it encroaches
upon the future ventral surface, while the animal pole appears to lie
on the dorsal surface. The part of the ectoderm that forms at the
wuimal pole seems to shift later to the anterior end of the embryo
around which the velum is developed, The axis which passes through
the animal and vegetative poles of the early stages does not, there-
_fore, in the Gastropoda, correspond, as might be supposed, to that
passing through the apical plate and the anus of the larva, but lies
more or less at an angle to the latter. It has already been shown
that the definitive axes are laid down at an early stage in the embryo
107). The identification of these axes is by no means easy, especi-
Wiis shape of the larva undergoes « certain amount of modifi-
eation according to the quantity of yolk deposited in the egg, On
this account, Fon’s statement that the shell-gland appears at the
animal pole requires further investigation. It is a striking fact,
however, that in the Cephalopoda it actually has such a position,
a fact which will be discussed later.* The shell-gland, as is well
known, lies dorsally on the embryo, whereas the pedal prominence
arises on the ventral side between the mouth and the anus. In their
most primitive condition the embryos, or larvae of the Gastropoda,
are quite symmetrical ; only later does the body become asymmetrical
through displacement of the internal and external organs.
Considerations relating to the asymmetry of the Gastropoda.
The development of the body during its ontogeny follows the
course which we are inclined to believe was taken by the Gastropoda
phylogenetically in attaining their present asymmetrical condition.
‘There can be no doubt that the Gastropoda arederived from symmetrical
forms, for we find the other members of the Molluscan phylum, which
had the same ancestors as the Gastropoda, symmetrically developed.
‘This is confirmed by ontogeny, for the symmetrical form is long retained
in the embryo although it is eventually lost in consequence of the
‘unequal growth of the various regions of the body. It is especially
“(Por a review of the facts relating to the shifting of the larval axes see
@oxxtax (No. TV.) and Lrutae (App. to Literature on Lamellibranchia,
‘No. II).—Ep.]
144 GASTROPODA.
the left side that grows more actively, and this is the reason why the
posterior parts (especially the anus and the organs surrounding it)
B. €.
Fro. 60,—.1-4, Diagrams illustrating the displacement of the pallial complex and the
manner in which the asymmetry of the Gastropod body was developed {constracted
after BOrscHit and Lana}. ‘The pallial complex shifts first to the right and then for-
ward. In £, it has passed the median line, and here the mantle-cavity has sunk in
more deeply. The gills grow back and thus sink deeper into the cavity. The heart,
auricles and anterior aorta are outlined in red, the intestinal canal in blue, the nerve-
ganglia and visceral loop in black. a, anus; ao, anterior aorta; og, cerebral
ganglion ; f, foot ; k, gills; 1m, mouth ; 1, renal aperture ; peg, pedal ganglion ; plg,
pleural ganglion; r. edge of the mantle and shell ; tc, visceral commissure.
RELATING TO THE ASYMMETRY OF THE GASTROPODA. 145
are displaced anteriorly to the right, but at the same time they
retain their original position with relation to the anterior end, because
the region lying between them and the anterior end on the right side
does not grow. These phenomena have been described by various
zoologists who have treated of the ontogeny of the Gastropoda (P.
Sanastx, For, Boprerzxy, ete.).* Spencer (No, 122), also, has
made them the subject of detailed consideration in adult animals,
and more recently Bi'rscuut especially has given a careful descrip-
tion of them (No. 19). Lana has recently made a further attempt to
explain them from a phylogenetic point of view (No. 61).
‘Ontogenetically as well as phylogenetically, the asymmetry rests
im any case upon the greater growth of one side, usually the left,
and the consequent shifting of the left posterior part of the body
to the right and of the whole posterior region anteriorly. In this
process we start with a very simple, Ci ifon-like Molluse, whose dorsal
surface with its investing shell is only slightly arched. The foot.
Projects only a little way beyond the visceral sac. The anus lies at
the posterior end, the paired apertures of the nephridia and the gills
lying near and symmetrically to it (Fig. 60 A). The mantle-cavity
also, to which these organs belong, is found at the posterior end.
The way in which the shifting forward to the right of this posterior
complex of organs (pallial complex) may be imagined to have taken
place may best be seen from the diagrams given in Fig, 60 (Biirscan1
and Lane). The asymmetry which is brought about by the shifting
of the pallial complex to a position near the anterior end of the body
(D) is found in the Opisthobranchia and Pulmonata; when the
pallial complex, in shifting forward, crosses the median line (2), as in
the Prosobranchia (including the Heteropoda), the pleuro-visceral
commissures become crossed (chiastoneury, streptoneury, Fig. 60 £),
@ condition not found in the two divisions mentioned above, and
indicating a specially high degree of asymmetry.+
*(It is manifestly impossible in a work of this nature to review all the
aka theories relating to the etry of the Gastropoda. The views
adopted by our authors are those of BirscHii and Lana, but the reader should
consult Smmorn's account of the Mollusca in Brown's Klass. u. Ordnung, d.
Thierreichs, Ba. iii. Lief. 22 u. 23, 1896, where an excellent summary of
both the earlier and the more recent views, including those of PersENRER and
Puate, will be found.—Ep.)
+[In Actazon, a form which, in spite of its uliarities, must be regarded
Aa ee allied to the Opisthobrancha we find that pleuro-visceral
connectives & streptoneurous condition, and in certain other forms
also (Philine, Aplysia, foneurous condition is also found in the
etc., a strep
Poritoae eae Oesiina) ‘an indication of this condition is to be seen. The
met in these forms is thought to be a highly specialised one,
7
—
146 GASTROPODA.
The cause of this asymmetry is to be sought in the manner of life of
the Gastropoda, i.e., in the development of the foot ax a massive creeping
organ and in the simultaneous development of the shelly covering of the
dody. At first the visceral mass was fairly equally distributed over
the body, which was covered only by « flattish shell. ‘These original
forms no doubt most nearly resembled the Chitones, apart from the
segmentation of the shell found in the latter, So as to give greater
freedom to the head which carried the sensory organs und the mouth,
to allow the foot to grow larger and also to make it independent of
the rest of the body, this organ became restricted to a smaller part
of the body. This led to the formation of the high visceral sac, to
which, as the part specially needing protection, the shell also became
restricted, although the head and foot might still be drawn in under
the latter, which consequently had to be of larger size than would be
necessary in a merely protective covering. The animal was thus
obliged to carry not only the high visceral dome, but a calcareous
shell capable of accommodating the whole body. If this heavy
mass became too high, it would be in a state of unstable equilibrium
and would naturally become inclined, the best inclination being
backward, as hindering the animal least in creeping, But since the
mantle-cavity with its important organs (the gills, the apertures of
the intestinal canal, the kidneys and the genital organs) lay at the
posterior end ofthe body, such a backward inclination of the visceral
mass would be so unfavourable as to be at first impossible and the
only inclination which seems possible would be to the side. This
lateral projection of the sac, however, too greatly impeded locomotion,
and in spite of the disadvantages mentioned above, the visceral dome
tended to incline backward, Tf we assume that the visceral dome
inclined to the left side, the great pressure from the left would tend
‘to squeeze the pallial complex towards the right. Herein, therefore,
Jay the cause of that displacement to the right and then forward
which has been described above (Fig. 60). Ontogenetically, this pro-
cess takes the form of more active growth of the posterior part of the
body on the left side, which leads to the bulging of the visceral sae,
and the forward displacement of the anus then follows-(¢/. p. 138).
It would not be surprising if the pressure of the inclined visceral
for, from the study of other points in their anatomy it has long been concluded
that the Opisthobranchs and Pulmonates (ie., the Huth; are to be
derived from the Prosobranchia after the latter attained the streptoneurous
condition. If this is the ease, we must regard the condition met with in the
Euthyneura as a retrogressive one and not as an arrested stage in the rotation
of the pallial complex.—Ep.)
RELATING TO THE ASYMMETRY OF THE GASTROPODA. 147
mass led, not only to the shiftings we have mentioned, but also to
the degeneration of single organs. Lane, in this way, traces back
the absence of the organs originally forming the left purt of the
pallial complex (the left gill and the left renal aperture, etc.) which
is to be noticed in various Gastropods (#.y., the Monotocardia among
the Prosobranchia and the Opisthobranchia) to the fact that the left
side was exposed to specially strong pressure, through which these
organs were prevented from functioning and degenerated. In other
eases (Haliotis) the right (originally the left) gill is said to be smaller
than the left (originally the right), and there is also an inequality in
the kidneys of those Gastropods (Haliotis, Patella) in which the
excretory organ is paired.*
The inclination of the visceral sac naturally led to its becoming
coiled. Lan rightly traces this to the fact that, in order to avoid
distortion, the upper side has to grow more than the lower. This
unequal growth gives rise finally to the spiral coiling of the sac, which
is followed in its shape by the shell. In those shells that are inclined
to the left, further room for extension is given on this side, especially
when the shell and visceral sac are directed backward. This unequal
growth determines the formation of the so-called dextrally twisted
shell. An original inclination to the right must be assumed for the
shell that shows the sinistral twist. In other respects the process is
the sume in the two cases. The causes that lead to the inclination
to one side or the other are difficult to determine, indeed, at the
present time, they are hardly known,+
Some of the sinistrally twisted Gastropods have their inner orgars
arranged in the same way as the ordinary dextrally twisted forms.
Tn such cases we have a false coiling which, it has been assumed,
arose through the flattening of a dextrally twisted shell to such an
extent that it became coiled in one plane. In this case the spiral
might again assert itself on the side 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 (Sim-
Bora, ¥. JuERtNG, Lana, No. 61). An indication of such a process
development of the gills is very marked in Pleurotomaria,
Uttar bere left) gill being much the shorter of the two; this is the
sence Diseerie eg in the Monotocardia, Curiously enough the kidneys
Hatiotis, Patella) show exactly the reverse condition
to th iat a seen in the fees tee wile
the right (primary left) kidney is much larger
AE nevertheless, it is apparently the latter
eddie which persists in the Monotocardia.—Ep.}
+ [See footnote, p. 108, on the cleavage of the egg of sinistral Gastropods.—
148 GASTROPODA,
is found in the Pteropoda that have a sinistrally twisted shell, but
in other respects show the structure of dextrally twisted forms ;
these have the operculum also sinistrally twisted, whereas spiral
opercula elsewhere always have a twist opposite to that of the shell
(Prtseneer, No. 86).
[Forms with a dextral organisation in a sinistral shell, and which
are supposed to have arisen as above, have been termed ultra-dextral.
The commencement of an ultra-sinistral coiling is seen in Planurbis
corneus, which possesses a true sinistral organisation with a shell
which otherwise would be regarded as a flattened dextral coil. The
embryo of this Gastropod, however, possesses a well-marked sinistral
shell.
The asymmetry characteristic of the Gastropoda may, however, be-
come more or less marked by the acquisition of a secondary bilateral
symmetry. This is the case in forms which, like the Pteropoda, have
become adapted to a free-swimming manner of life. In such cases
the principal cause of the asymmetry, which we found to be the
ereeping manner of life in connection with the development of a
high visceral mass, falls into the background. The fact, however,
that there are Gastropods which again become almost symmetrical
while still leading a creeping life, but in which the shell has al-
together or partly degenerated, as is the case in Onehidium and the
Limacidae, shows what an important part is played in these processes
by the covering of the body.
5. The Development of the External Form of the Body in
the Different Divisions of the Gastropoda.
A. Prosobranchia.
We have already repeatedly alluded to the development of the
larval form of the Prosobranchia and to its transformation into the
adult,* the principal features in these processes, the development of
the Trochophore and Veliger larvae and their transformation into the
adult are thus known to the reader. Certain divergences, however,
occur among the Prosobranchia, especially in the earlier ontogenetic
stages, causing a modification of the external form of the body, and
thus requiring special consideration.
Tt has already been stated (pp. 112, 116) that the eggs of many
Gastropods are very rich in yolk, and this influences not only tho
* Cf. pp. 123, 131 and 134 on the development of Patella, Vermetus and
Paludina, also Figs. 49-59.
DEVELOPMENT OF THE EXTERNAL FORM—PROSOBRANCHIA. 149
formation of the germ-layers but also the development of the external
shape of the body. This is the case, for instance, in Vassx, Pasis,
Fulgur, Natica and others. Even in Vermetus, the Veliger stage of
which we became acquainted with (Fig. 55), the T'rochophore form is
no longer distinctly developed. The velum appears at first in the form
of two wavy cell-bunds at the anterior end of the ventral surface and
near it appear the rudiments of the tentacles ; immediately behind
them are the mouth and the pedal swelling. The latter appears as
& rudiment when the velum is only slightly developed and is far
from complete dorsally. The rudiments of the organs, with the
exception of the dorsally placed shell-gland, are thus here crowded
together into a very limited area of the very large embryo. This is
the case toa far greater extent when the egg is still richer in yolk,
as, for instance, in Pulgur (McMurricn, No. 70). The first rudi-
ments of the organs are here so crowded together that we might
almost speak of a germ-dise in contrast to the large yolk-mass of
the egg. We should then see the commencement of processes which,
in « fur higher degree, will be met with in the Cephalopoda. Thus
im eggs very rich in yolk we may speak of «a “blastoderm” which
grows round the yolk, i.e., the macromeres, and, indeed, the layer of
micromeres is here greatly reduced as compared with the yolk-mass
of the macromeres, as may be seen by a glance at Fig. 42 D and &,
p- 112, and Fig. 47 A and A, p. 117. If we compare these figures with
those of the blastula and invagination-gastrula of Putel/a (Figs. 49
and 50), Plaworbis or Paluedina, it is evident that these altered condi-
tions must bring with them modifications in the external shape of
the body.
In Nossa motabilis, which we select for description as the best
investigated if not the most extreme form in this respect, there is
a point at the vegetative pole which remains for some time uncovered
by cells (Figs. 61 4, b/, and 47 C, bp). This is the blastopore which
closes later, the stomodaeum arising in this region (Fig. 47 D, m).
Iu Fuss, the eggs of which exhibit « similar condition, the blastopore
is said to persist and to pass over into the mouth (Bosrerzky).
‘The foot appears very early as a broad swelling behind the blastopore,
even before the rudiment of the velum has arisen (Fig. 61 4, /),
Neur it lie the groups of ectoderm-cells (ez) which have been claimed
4s an exeretory apparatus (external kidney). The velum (x) appears
in front of the blastopore, advancing from the ventral to the dorsal
side. Dorsally, the shell-gland appears, and over it the shell-integu-
ment. At a later stage, the anterior part together with the foot
150 GASTROPODA.
becomes marked off from the principal part of the embryo which
contains the yolk (Fig. 61 D), the anterior part becoming swollen up
like a vesicle (Fig. 63, ce, v). This phenomenon can be observed, still
better than in Vassa, in a species of Fueus examined by Boprerzky.
Tn this form, the foot and especially the anterior part of the body
appear to be swollen into a large vesicle (Fig. 62 A and B, kb), and
this part is therefore here also sharply marked off from the posterior
A. B. €.
Fra, 61.—A-#, embryos of Nasea. mutabiliv of different ages (after BOBRETZKY).
Diastopore ; «, posterior tubnlar portion of the enteron ; dr, yolk; er, group
ectodermal excretory cells ; /, foot ; fd, peilal gland ; A, rudiment of the heart 5
posterior hepatic lobe, near whick can be seen, to the left, the anterior hepatic Tol
and above the latter the intestine () and the anus ; 2, rodiment of gill; &h,
vl
A,
cavity ; d, larval heart ; op, operculum ; r, margin of the shell (s) ; », velum.
part of the embryo. This swollen part, which corresponds to the
pre-oral section of the Tyoehophore larva, and which is found in other
Prosobranchs, as well as in various other Gastropods (Pulmonates),
has been called the cephalic vesicle. The embryo in consequence
presents a very characteristic appearance (Figs, 62, A/, and 81, 00).
The condition of the entoderm or yolk is of special significance for
the embryos now under consideration. The sac-like rudiment of the
DEVELOPMENT OF THE EXTERNAL FORM—PROSOBRANCHIA. 151
enteron is seen to be open towards the yolk (Figs. 47 Cand D, 62 B,
and 63), which occupies the posterior and dorsal portion of the em-
bryo. The enteron consists of an anterior wider section and a posterior
tubular section (Figs. 47 D, and 62, md). The latter is ut first
pérallel to the longitudinal axis, but soon lies obliquely to it, becomes
connected with the ectoderm, and opens out through the anus which
still lies in the ventral middle line (Fig. 61 @). Ata later stage,
the posterior part of the
intestine assumes a still
more oblique position
and the anus comes to
lie on the right side
(Fig. 61 D and £).
Here also the pallial
cavity arises as a sickle-
shaped depression of the
ectoderm, this cavity in
Navsu being altogether
restricted to the right
side of the embryo. The
seems still
more marked here than
in Paludina (p. 138).
‘The shell also shares
in this asymmetry; by
(Fig, 61 D) The py, 62,—A, surface view, ant 8, median I tudinal
cperoulum appears as a setion rose an entre ot er alt ci
delicate plate in the i, liver; m, mouth; wd, enteron; mg, stomach 5
posterior oral part of cuit; tal ees tau
To the foot can be seen a ventral tubular ectodermal depression,
which is no doubt the rndiment of the pedal gland (Figs. 61 F
and D, and 63), The velum, which is not yet closed dorsally,
has lost its former almost circular shape through the shifting
forward of the mouth and the appearance at this point of a noteh
(Fig. 61 @). It at the same time increases in size and thus assumes
the bilobed form which we have already described in connection with
152 GASTROPODA.
other Gastropod larvae. Massa now shows a strong general resem-
blance to such larvae, as is evident from Fig. 61 £. This is also the
case with #usws, the embryos of which also at first deviate in
several points from the usual shape and resemble those of Vassa.!
Bosrerzky has described in connection with Nassa and Fusus an organ of
which no account has as yet been given ; this is the so-called “ larval heart,”
which has also been found in other Gastropoda (¢g., by Sacensky in
Calypiraea, No. 98). This larval heart (Fig. 61 F, Ji) is said to be part of
the eotoderm lying dorsally behind the velum, which is connected with
mesodermal! elements and carries on contractile movements. Other parts of
the embryo, such as parts of the cephalic vesicle and the foot, are said to be
capable, like this region, of contractile movements.
Remarkable transformations
in the shape of the body take
place in some Prosobranchs
which become adapted to a
parasitic life on or in various
Echinoderms (Asteroids, Echi-
noids and Holothuroids). An
excellent example of this is
afforded by Entoconcha miralili«
described by Jon. Mi~uer
Pe ity carhy of Span
eeret Troe Birigs Totton, attached to the wall of the
imagers soamictt wt ciemy intestine, ‘The only of this
from the yolk which forms the posterior animal has the form of a long
Pattar the embryo. cov: cepbalicvesicle; vermiform oiled tube which
in no way recalls that of a
Gastropod, but its brood-cavity contains embryos very like those
of other Prosobranchs. These have a velum (not, it is true, very
highly developed), « spirally coiled shell, a foot with an opereulum,
otocysts, etc. Their further development is not known, but it is
probable that they live freely for a time, like the young Hnfovaloa
(p. 43), and only later wander into a Holothurian.
In explaining the remarkable transformation undergone by Enfo-
couche in consequence of its parasitic life, two Prosobranchs described
by P. and F. Sarasix (Zhyca entoconcha and Stilifer Linchiae) are of
great value (No. 103). These forms live parasitically on Asteroids,
either piercing the integument by means of a proboscis-like structure
(Thyca) or else sinking bodily into it (Stilifer). Even these ecto-
DEVELOPMENT OF THE EXTERNAL FORM—HETEROPODA. 153
parasitic Gastropods show decided changes in their structure, and
this would be still more the case if they were to penetrate through
the integument-of the host and reach the body-cavity. The possibility
of such a breaking in of the parasite from without is shown by the
Stilifer, which has already buried itself deep in the skin. The
external shape as well as the inner organisation finally undergo, as
im many other parasites, such a far-reaching alteration, that there ix
hardly any resemblance left to the former Gastropod, the parasite
having degenerated into a mere tube, like Hnfocolax or Eutoconcha, ou
which are devolved the functions of feeding and reproduction alone
4{W. Votar, No. 129; Braun, No. 15; Scarmmenz, No. 108).
B, Heteropoda.*
The ontogeny of the Heteropoda closely resembles that of the
Prosobranchia to which in other respects also they are nearly related,
but the special form of the adult Heteropod determines certain
variations especially affecting the later stages of development. The
ontogeny of the Heteropoda has been made the subject of special
study by Leuckart (No. 67), GzGennaur (No. 37), Kronn (No.
58a) and Fon (No. 31).
We have already become acquainted with a few of the younger
stages of the embryo of Firo/oida (Fig. 44
A-C, p. 114), The oldest of these stages
was an invagination-gastrula. The inner
end of the archenteron soon assumes a
remarkable bilobed form, which recalls the
enterocoelie formation of the mesoderm as
deseribed by ExuanGenr in connection with
Palwdina (p. 121), but which is no doubt
explained by the fact that the shell-gland Fra, 644—Embryo of Firvloide
which arises dorsally grows as a conical — Desmmuresti (after Fou), ¢
the primary body-eavity ; 9,
invagination towards ne oe archeteris a iat
causing a depression in the latter. hen 2 {ook 5, shell gland;
the shell-zland begins to flatten out again
(Fig. 64), the archenteron also assumes a more regular form, becoming
wider and sac-like. The blastopore passes over into the permanent
mouth (Fig. 64, 0). The shell-gland at first appears filled by « plug
of brownish substance (#'); in Padudina, where « similar feature was
or Nucleobranchia, are very generally regarded as a
* (The
minor branch of the Prosobranchia, being classed under the Monotocardia as
« subdivision of the Taenioglossa.—Ev.)
154 GASTROPODA.
observed by Birscurt, the plug was said to be expelled before the
actual shell formed, whereas For believes here that this mass which
fills the shell-gland passes directly into the shell when that depression
flattens out again.
In the stage depicted in Fig. 64, the pre-oral ciliated ring has
made its appearance and in this way the velar area (vv) becomes
bounded. Behind the mouth, the rudiment of the foot (p) appears
as 4 prominence which widens and thus assumes the form of a plate
(Fig. 65 B). On either side at its base, the otocysts (of) appear,
while, anteriorly, the bilobed pedal gland forms as an ectodermal
invagination. The posterior part of the foot at this early stage
secretes a thin plate (op) which, in position and function, corresponds.
Fic, 65.—Embryos of Firolvida Desinavesti. «
the ventral d,
to the operculum of the Prosobranchia. Fine caleareous concretions
become deposited beneath the shell-integument, and lead to the
development of the caleareous shell. Unequal growth here also
causes the shell soon to assume a coiled form, at least in the later
stages. In Ftroloida and Pterotrachea, the shell has only two.
whorls; in Carinaria and Atlanda it coils several times.
Up to this point, the alimentary canal is without an anus.
According to Fox, two large cells which appear behind the foot
indicate, even in the stage depicted in Fig. 64, the position of this
organ, and at this point the enteron, which is bent anteriorly, be-
comes connected with the somewhat depressed ectoderm (Fig, 65, ae)..
DEVELOPMENT OF THE EXTERNAL FORM—HETEROPODA, 155
These specially marked cells lay originally in the ventral middle line ;
they however shift towards the right side in consequence of the un-
equal growth which takes place also among the Heteropoda, and the
anus is thus found on the right
side, as we have already seen
to be the case in various other
Gastropods (p. 142). At this
stage, the embryo is almost in
the condition of the 7'rocho-
plore. Tt then soon passes
over to the Veliger stage, the
yelum being bilobed (Iig. 66).
This bilobed character is at first
made evident by the mouth
shifting into a notch of the : A, mdi
pre-oral ciliated ring. the base of whie a i is visible ;
So far, the course of de- the lotetantacle teat vention tartoeege
velopment in the various peasy an aera
Heteropoda seems to be very similar (Fou). The round embryo,
which is now provided with a bilobed velum, a foot and a cup-shaped
shell, moves about by means
of its cilia within the gela-
tinous egg-rope, which has
become hollow; it, however,
soon leaves this to swim ubout
asa free larva (Fig. 66),
circling slowly in the water
(Groensaur), The move-
ments become more rapid and
the larva more active when
the lobes of the velum increase
in size and are able to act
"independently of one another.
to Krosn, in
Firoloida and Pterotrachea, ARE
the yelum becomes drawn out ue
oa each side into wo Ing Mt des el
and very narrow streamers, rudiment of the fin; op, operculum ; 0¢,
Raita presenting an otocyst ; s, shell ; v, velum.
similar to that of the Veliger larva depicted in Fig. 54,
p. 130. In At/anta, the velum is drawn out into three streamers
156 GASTROPODA.
(Fig. 67), which, however, are considerably shorter than those just
mentioned. In Carinaria, again, the streamers are longer, and the
lobes, cut up into three parts, cause this larva greatly to resemble
that of Firoloida (GuGeNpAun and Kronn).
The great development of the locomotory organs in the Heteropoda
causes their metamorphosis to be very marked. At its commence-
ment, a cylindrical process with a rounded free end appears on the
anterior side of the foot immediately in front of its base (Figs. 66 and
67, A); this soon lengthens and carries on continuous swinging
movements. This is the rudiment of the fin which, in its origin, must
be regarded as belonging to the foot. In the course of metamor-
phosis, the cylindrical process becomes flattened laterally, and thus
Fic. 68.—Lateral aspect of a Cerinaria (after SOULKYET feces GsouxaauR}. a, aus >
ag, sbdominal ganglion auricle; au, eye; ég, buccal ganglion; bin,
mass; cg, cerebral ganglion; i, intestine ; /, tentacles; fl, fin; &, gill; d, liver; m,
month vas, stomaaeh we, Kidhay t 3p, petal ganglion 'y, sucker ; sc, shell; sp,
salivary gland ; #1, tail; tel, oesophagus ; ve, ventricle,
approaches the form of the keel-like fin of the adult (Fig, 68, jf).
‘This flattening extends from behind forward ; for a time, in the already
keel-shaped fin, a portion of the former cylindrical process is found ;
this, in Firvloida, is attached somewhat nearer the anterior margin,
but in Pterofruchea somewhat further back. By degrees this also is
drawn into the flattened fin (Krous). In some species of Atlanta,
the fin appears from the first asa laterally flattened projection on the
anterior side of the foot and thus here more nearly resembles its
definitive shape. In this form, also, the sucker can already be seen,
lying close to the posterior margin of the keel-like foot, this position
DEVELOPMENT OF THE EXTERNAL FORM—HETEROPODA. 157
showing it to be the principal part of the Gastropod foot. This is
evident from the fact that the fin originates at the anterior end of
the foot, the posterior side of the latter being covered by the
operculum (Figs. 66, 67); the intermediate part, é.., the actual
rudiment of the foot, must therefore in any case be concerned in the
formation of the sucker (Fig. 69 A), unless we are to regard the
latter as a secondary formation, The sucker usually appears much
later; in Firoloida, it is only found in the male, and has therefore
here become merely a
sexual character. Here
and in Carinaria (Fig.
68), the sucker lies some-
what far down at the
margin of the fin, and
thus becomes absorbed
in the latter, its pedal
character being in this
way still more marked.
That the sucker is not
merely a supplementary
differentiation of the fin
is proved by forms such
as Oxygyrus in which it
is independent of the fin
and lies behind the
latter (Fig. 69 A). We
have here great agree-
ment with the condition
of some Prosobranchs
Fie. 69.—A
B, Strombus, each viewed
ia, Strombus, from ihe te (er ‘SouzgvEr and Kunean), a,
« . it iA, posterio foot 5
Fig. 69.8), in which the $2°sifeatum's \ protoacloy a; sckess se aboll
posterior part of the sw, tail (most ior section of the foot); #,
Sea ibi tailor of th anterior part of the foot.
carrier 1c
operculum, is sharply marked off from the anterior part. This view
corresponds on the whole with that adopted by Gr@mnsaur and
recently especially by Groppen (No. 38), as to the significance of
the foot in the Heteropoda,
‘The tail found in the Heteropoda (Fig. 68, se), also arises from the
foot, in Atlanta as a projection lying close behind the sucker (Kxouy).
As it increases in size, this process, which also is cylindrical, presses
that part of the foot which bears the operculum more to the dorsal
158 GASTROPODA.
side, a position which is constant in forms like Af/anfa, in which the
operculum is retained throughout life. The operculum, however, as
well as the shell, is frequently thrown off during the metamorphosis
While these changes have been taking place in the foot, the velum
has gradually attained its highest development and then commences
to degenerate, The rudiments of the tentacles have already appeared
on the velar area;. these arise, curiously enough, quite asym-
metrically. One tentacle only is present at first (Fig. 66, f). At
the bases of the tentacles, the eyes appear. In some forms, the
tentacles may be reduced again (Pterofrachea), Before the shell is
thrown off, the velum has for the most part degenerated, only a few
traces of it being still found near the eyes.
The body increases greatly in length, not only on account of the
development of the caudal section just described, but also through
the extension of the anterior part (the development of the so-called
proboscis, Fig. 68). In consequence of the great lengthening of the
foot and the anterior part of the body, the visceral sac lies, as in
other Gastropods, on the upper side of the body (Fig, 68). At the
junction of the visceral sac and the dorsal wall of the anterior part
of the body, the mantle-cavity has arisen and the gill has formed.
Where the shell is retained, it covers the visceral sac (Carinaria,
Fig. 68), and in At/anta, which also as an adult possesses a shell, the
whole animal can still be withdrawn into it.
©. Opisthobranchia.
The ontogeny of various forms of the Opisthobranchia has been
studied by many zoologists. The embryonic development and the
younger stages of the free-swimming larvae were those usually in-
vestigated, the animals being difficult or even impossible to keep in
confinement during the later stages. Sans (Nos. 104 and 105) and
Lovin (No, 69) established the chief features of their ontogeny,
while at a later period ADLER and Hancock (No, 1), NorpMANN
(No. 80), Voor (No. 127), Scuunrze (No. 113), and KerersTein
(Nos. 52 and 53), occupied themselves principally with the develop-
ment of the larval forms and of the shape of the body. Ray Lan-
KESTER (Lamell. Lit., No. 29), Trinonese (No. 125), BLooHMANN
(No. 8), Rao (No, 93), turned their attention also to the internal
processes, especially to the earliest ontogenetic stages. References
DEVELOPMENT OF THE EXTERNAL FORM—OPISTHOBRANCHIA. 159
to the other authorities on this subject will be found in the literature
asppended to this section and in the course of our account.*
In consequence of the rich supply of yolk in the egg, gastrulation
seems usually to take place by epibole-+ The blastopore, at one
period, is # slit of variable length (e., in Fiona and Llysta, Happon,
No. 40; Zrcolania, TaincuEse ; Aplysid, BuocuMann). This slit
loses from behind forward and, in some cases, altogether disappears,
the mouth then arising as an ectodermal invagination at the point at
which it closes. This is the case, according to BLocamann, in
Aplysia and & similar condition may, according to Voer's account,
be found in Hlysia. In Fiona, according to Happon, the slit-like
blastopore closes from behind forward and either passes directly into
” the mouth or the latter is invaginated at the spot where the former
finally closes. Such a condition can be gathered from the descriptions
given by Trrncuese (No. 125) and Lanceruans (No. 62) of the
Aeolidae and Doris, but these accounts are not very clear. From
all these statements, however, it appears tolerably certain that the
mouth corresponds in position to the anterior end of the slit-like
blastopore.
The changes that take place in the large entoderm-cells are
significant in connection with the further shaping of the embryo.
These have been specially observed in Aplysia by Buocumann.
Cleavage is unequal from the first and, at the four-celled stage, two
of the cells are very much larger than the remaining two, and this is
still apparent after the abstriction of the micromeres, when we find
two very large and two smaller macromeres (Fig. 41 8). In conse-
quence of the smaller amount of the yolk contained in the latter,
they soon divide and give rise to a mass of small entoderm-cells,
while the two large macromeres (Fig. 41 J and J/) are retained in
their full size. The ectoderm-cells grow over the entomeres and the
smaller entoderm-cells separate from the two large macromeres which
[Recent — a this oh md have devoted themselves mainly to the
uaa ot mymons (No. XIT.) and Vicurer (No. XXVI.).
ens Bo. fa i y is however, made some additional observations on
Pattee a fo. , who has investigated the early stage in the
ontogeny of Tila, hes that the gastrula is here intermediate between
the epibolic and the embolic type, as is the case in so many other Gastropods.
‘His work, which is an important one, deals largely with the cell-lineage and
the early je stages. Umbrella, in its fens me , appears to conform to
‘the normal type, the process of ent formation is quite
unlike that described by Buocumann in Aplysia, the yolk being equally
distributed between the four macromeres and the entodermic epithelium
arising in a more normal manner.—Ep.)
160 GASTROPODA.
are still undivided. An archenteron forms between them, lined partly
by the small entomeres and partly by the two persistent macromeres
(Fig. 70). Here also there
isa suggestion of a condition
intermediate between an
epibolicandan invagination-
gastrula, as is said to be the
case in other Gastropods
(cf, p- 115), The closure
of the blastopore and the
sinking in of the stomo-
—l of A, limacina in oy 1 alread: il
Pest ir Beta ian ip owe ira saci
immediately after thisstage.
Where the macromeres are not in direct contact with the ectoderm,
the smaller entoderm-cells spread out (Fig. 71). The gut, still partly
bordered by the macromeres which have shifted apart, now resembles a
closed sac.* Up to this point there has been no sign of the mesoderm-
rudiment which, according to BLocHMANN, appears late in the form of
two small masses of cells lying to the right and left of the stomodaeum,
the origin of which could not be established. TrincHess, on the other
hand, described, in the deolidac, two large and distinct primitive
mesoderm-cells which may be traced back, like those of the mesoderm-
rudiment found by Ruo in Chromodoris, to the macromeres. [See
the more exact work of Huymons (No. X11.) on Umbrella in this
connection, and footnote, p. 119].
At the side of the embryo opposite to the mouth, the shell-gland
arises as a depression which at first is shallow, but deepens later
(Fig. 71 A, sd); above this, the shell-integument is soon seereted.
‘Two cells (az), which lie on the ventral side in front of the shell-
gland and which, in consequence of their large size, rise above the
surface (Fig. 71), mark the position of the anus, which appears late.
These anal cells could be seen in Aplysia in earlier stages, lying at
the posterior edge of the blastopore. They had been already described
by Lancernans in several Opisthobranchs (Acera, Aeolis, Doris)
and had been connected with the formation of the anus; the same
*(Mazzarecwi (No. XVI.) does not appear to have traced the ultimate fate
of the two smaller macromeres, but one would imagine, from his ee
that they form part of the ectoderm. He regards the small entomeres seen in
Fig. 71 as derivatives of the two lanes macromeres and, judging Sor his Fig.
12, Pl. x., small cells are constricted off from the macromeres. His observa-
tions are not clear, but they seem to differ from those of BLocuMaNN.—Ep.]
DEVELOPMENT OF THE EXTERNAL FORM—OPISTHOBRANCHIA, 161
significance is aseribed to them by TxixcaEse in the Aeolidae and
by For in the Heteropoda and Pteropoda.
‘The ciliated cells of the pre-oral ring have already become differ-
entiated, the velar area being thus marked off (Fig. 71, »). Ventrally
bebind the mouth, the foot appears as a swelling (/); behind it can
be recognised the wnal cells (az). The shell-integument has already
developed further. The T'rochophore stage is here less marked than
im many other Gastropods, as the embryo undergoes certain modifiea-
tions in consequence of the richer supply of yolk. Such a stage,
however, has been distinctly recognised by Ray Lankester and
Trrvcnrse and other observers in Opisthobranchs which have been
Fre. 71, —Two stages in the development of Apiysia dimacine (after BLOCHMANN), a3,
Saal cells; eet, ectoderm ; ent, entoderns; #7 loot; m, mouth; mes, imesoderm ; mr,
mmmrgin of the mantle ; 4, shell’; sf, shell-ziand ; #h, shell-integument ; e, velum,
investigated by them. An embryo of Aplysia figured by Ray Lan-
kesTER * shows the greatest resemblance to the embryos of Firo-
Joida depicted in Fig. 65 A.
‘The Trochophore stage, by the transverse extension of the velum,
passes into the Veliger stage, in which, owing to processes of growth
similar to those already described, the symmetrical shape undergoes
certain modifications. In most respects, indeed, the ontogenetic
processes Which now follow closely resemble those described for the
Prosobranchia, so that we need here only touch upon the principal
* (Bee Lit. to Lamellibranchia, No. 20, Pl. 8, Fig. 17.)
M
i
[enna
162 GASTROPODA.
The two lobes of the velum are very large and give the larva »
characteristic appearance (Fig. 72, v). They remain undivided, are
very broad and are beset with long thick cilia ; the fissure between
them, however, usually carries short and delicate cilia, the bilobed
character of the velum being in this way emphasised, The shell has
lost its flat and (later) cup-like shape and, in the free-swimming larva,
has already become coiled. The foot (f) develops an operculum on
its dorsal surface. We thus find, in the Opisthobranchia, the same
general condition already met with in the Prosobrauchia and the
Heteropoda, Although most Opisthobranchs, as adults, are entirely
Fig. 72.—Veliger larva of an Opisthobranch. @, anus; ad, anal gland (? probably an
excretory organ, like »);«, alimentary canal! di, divertionlam of the stomach ; /,
foot ; m, mouth; mm, mascle (retractor of the velum) ; op, operculum ; al, otoeyst |
#, shell; », velum.
devoid of a shell, or else, as in the majority of the shell-bearing forms,
have a highly specialised, often greatly reduced, or even internal
shell, the larva possesses a coiled, often nautiloid ‘shell into which
the body can be withdrawn and which can be closed by an operculum
(M. Sars). The shell is usually thrown off and the operculum al-
most invariably has the same fate. When, however, the adult
possesses a shell, we may fairly safely assume that this has been
derived from the larval shell, In only a few Opisthobranchs is the
shell of the adult so large that the whole body of the animal can be
DEVELOPMENT OF THE EXTERNAL BODY—OPISTHOBRANCHIA. 163
withdrawn into it; the retention of the operculum as in Actaeon
(Tornatella) is quite exceptional. According to Trrxceese, the
larval shell in some forms (Saceoglossa) shows « delicate reticulate
structure on its surface ; in most other larvae it is smooth.
Passing now to the internal organisation of the Veliger larva, we
notice first that, from the oral aperture which lies at the ventral
incision of the velum, the stomodaeum (whieh only at a later stage
is provided with a radula) runs backward and becomes connected
with the large enteron. From this latter, there are two lateral
outgrowths which differ in size (Fig. 72, di); these are formed of
specially yolk-laden cells and thus no doubt owe their origin to
the macromeres. The intestine also arises as a diverticulum of the
entoderm-sac ; it then lengthens considerably, bends forward and,
after uniting with the ectoderm at the right side of the body, opens
outward rather far forward, near the edge of the shell (Fig. 72).
Little is as yet known as to the differentiation of the mesoderm in
the Opisthobranchia. A strong muscle, sometimes composed of two
branches, runs back from the velum, becoming attached to the shell
posteriorly (Fig. 72, mu). Another shorter retractor of the velum
extends between the base of this organ and that of the foot. This
arose from single spindle- or star-shaped mesoderm-cells which came
to lie on the right side of the larva in this region.
‘This latter muscle carries on regular rhythmical movements and, on this
account, has, according to Trinciese, falsely been regarded by several
observers as a heart, The so-called larval heart which has been described
iu connection with the Prosobranchia (Nassa, Fig. 61 E, lh, p. 150) differs
somewhat in position from this retractor, but is, like it, composed of long
mesoderm-cells.
According to the accounts of the Opisthobranchs now under con-
sideration, no primitive kidneys resembling in shape those occurring
in the Prosobranchia (Paludina) and Pulmonata (p. 136) have been
found in them, but vesicular structures which lie in the dorsal
region behind the velum to the right and left of the oesophagus
have been described as primitive kidneys. These have been regarded
&s excretory organs chiefly because they are filled with strongly
refractive concretions. They seem never to possess efferent ducts.*
‘The views taken of the excretory organs of the Opisthobranchia seem
‘to us to be somewhat confused. Tarxcuesz, for instance, has described a
or unpaired sac-like gland with a longer or shorter efferent duct which
*(r ‘appear to be ectodermal in origin (Heymons) and analogous to the
‘ anal kidney of the Prosobranchia (p. 129 and No. XV.),—Ed.}
am
|
164 GASTROPODA.
opens out near the anus as an anal gland. In Hrcolania, this gland is un-
paired and strongly pigmented. A glandular structure described by Ruo in
Chromodoris is said also to open near theanus. This involuntarily recalls the
rudiment of the kidney of the adult, a view which has recently been adopted.
by Mazzangnui (No. 74 and No, XV.). This author derived similar structures
from the mesoderm. Oue organ especially which, curiously enough, was
assumed to be an “anal eye,” excited attention. This lies in various Opistho-
branch larvae (in Aplysia, Philine, Pleurobranchus, Doris, Aeolis, Lacaze-
Dorarers and Provor, No. 60) on the ventral side, near the anus; it is
strongly pigmented and is no doubt identical with the glandular structures
above mentioned. According to Mazzareuoi, as already mentioned, it is
derived from the mesoderm, but Lacaze-Duruiens and Pruvor, who in-
vestigated the origin of this hypothetical larval eye more closely, traced it
back to the ectoderm, This was also the result of the ontogenetic researches
of Heymons as to the origin of this structure, and it cannot therefore be
regarded as a nephridium, but must rather be compared with those excretory
organs which, like the sub-velar cells described in the Prosobranchia, are
yielded by the ectoderm (p. 129), The excretory character of the organ seems
indisputable, but no decision as to its homology can be arrived at until its
development and future fate in the different forms of Opisthobranchs are
better known.
Among the sensory organs of the larva, the large otocysts wt the
base of the foot deserve special mention. As in other pelagic larvae,
strong cilin appear at the ceutre of the velar area in various forms
(Fiona, Polycera, Elysia, Phitine, Happow, No. 40). In the Aeotidae,
the end of the foot carries a few long stiff cilia. Eyes are found on the
velar area in those cases at any rate in which tentacles also appear
there ux rudiments, but are altogether wanting in many larval forms.
The greatly modified forms found among the Opisthobranchia,
such as the genera Limapontia and Phyllirhue, like the more primitive
forms, have larvae with bilohed yelum and shell provided with au
operculum (ADLER and Hancock, No. 2; Scunerper, No. 112).
Our knowledge of the transformation of the larva into qhe adult
rests principally upon the statements of Max Scnunrze and Norp-
MANN made with regard to Vergipes Edwardeii and T. lacinulatus
(Nos. 80 and 113).
The larva of Tergipes Edwardeit, when still provided with « shell,
already seems to have lengthened somewhat. The two velar lobes
are unusually large and oval. On the velar area are sitnated a pair
of tentacles and, at the base of these, the eyes. The larvae probably
swim about for some time at this stage. The mantle then with-
draws from the shell and comes into closer contact with the body.
The way is thus prepared for the casting of the shell which takes
place while the velum is still fully developed. We thus have a
DEVELOPMENT OF THE EXTERNAL FORM—OPISTHOBRANCHIA. 165
Veliger larva without shell or operculum, which presents a very
peculiar appearance (Fig. 73 A). This larva, in the length of its
body already shows a distinct approach towards the adult condition.
Tn the Tergipes observed by M. Scuunras, the passage frem the larva
to the adult is somewhat different, the velum degenerating in this
form before the shell is thrown off. In this last case, the larva must
have adopted earlier the creeping manner of life. ‘The shell-less larvae
of Tergipes Edwardsit, with their large vela at first swim about with
even greater rapidity than the shelled forms, but then gradually
begin to creep, as the body increases in size (Fig. 73 B). The lobes
of the velum commence to degenerate until they are reduced to a
pair of rounded processes lying in front of the mouth (Fig. 73 @),
which, it has been assumed, change into the labial palps.
a. 3B. ¢.
be Th —A-D, SEAMEN hae young 0 of the Tergipes Kéwartsit (after NompMany),
lorsal papillae.
‘This view of the transformation of the remains of the velum into the sensory
organs near the mouth, has been adopted especially by Lovin, who already
held a similar view as to the origin of the oral lobes in the Lamellibranchs
ip. 46), Ray Lanxesrer holds that, in Limnaea, the remains of the velum
PASS over into these subtentacular lobes ; but this point has been disputed
iw connection with this form. It has already been stated (p. 198) that the
observations made by Ray Lankesrer for Onchidiam were confirmed by
Joveex-Larrvie.
‘While the larva is still provided with the large velar lobes, one pair
‘of the dorsal appendages (cerata) arise which are so characteristic of
the Nudibranchs and into which the diverticula of the enteron soon
extend (Fig. 71 C). Another pair of these intestinal diverticula has
already formed and these belong to the next pair of cerata. As other
Processes develop, the young animal approuches more and more
il
166 GASTROPODA.
nearly to the adult form, but has first to pass through a moult
(Norpmany), during which it remains entirely quiescent, surrounded
by the cast skin as by a transparent sheath. This membrane is no
loubt to be regarded as the cast off cuticle.
D. Pteropoda.
The early development of the Pteropoda closely resembles that of
other Gastropods. We have already seen that the embryo at first
has the form of an epibolic gastrula and passes from this to an
invagination-gastrula (Fig. 45 A and Bp. 115). ‘The entoderm, at a
later stage, by the great increase in number of its cells, is transformed
direct into the epithelium of the archenteron; but, in some forms,
the macromeres seem to be retained for a long time, the transition to
the definitive entoderm being then less simple. The blastopore is
slit-like and situated at the vegetative pole. After its closure, the
mouth arises at the same spot throngh an ectodermal depression.
From the published accounts, we may assume that the mouth then
shifts its position or, in consequence of the further growth of the
embryo, changes its shape. At one end of the embryo a circle of
strongly ciliated cells marks off the velav area, immediately behind
which the mouth now lies. At a point almost opposite the cephalic
area, on the dorsal surface of the embryo, an ectodermal depression
appears which varies in size in the different genera; this is the
shell-gland. The whole of the interior of the embryo is filled with
yolk-laden macromeres. The yelum becomes more distinet, and,
behind the mouth, the foot appears as a large outgrowth. When
the otocysts arise near the foot and the two anal cells (whieh also
occur in the Pteropoda) behind it, the embryo passes into the
Trochophore stage which greatly resembles that met with in the
Opisthobranchs, or the corresponding stage in Firoloida (Fig. 65),
At the stage just described, or even earlier, the embryo may
become free and may swim about actively, since it is already provided
with a velum, Up to this point the different Pteropods develop in
much the same way, but differentiations soon appear in the develop-
ment of the larval form, especially with regard to the shape of the
velum and the shell. The Gymnosomata also diverge from the other
forms in so far as the Veliger stage gives rise to a peculiar larval
form encircled with several ciliated rings.
A certain differentiation in the development of the early larval stages is
also caused by the fact (stated by For) that the order in which the organs
167
(velum, mouth, shell-gland, foot, ete.) appear, varies greatly in different forms.
‘The comparison of corresponding stages is in this way rendered somewhat
‘more difficult, but the final result is, as already stated, very similar,
The embryonic development of a large number of Pteropoda (Cavolinia
(Hyalea), Hyalocylix, Creseis, Styliota, Cleodora, Cymbutia, Clione) has been
closely studied by For, who has also described the further development and
the metamorphosis of these animals (No. $2). The phenomena connected
with metamorphosis had previously been investigated especially by Jon.
Mé ten, Georxeaur, and Knounin the above genera as well as in Tiedemannia
and Preumodermon (Nos. 77-79, 37 and 58a),
Thecosomata. The Tvochophore stage soon passes into the Veliger
stage, a dorsal and a ventral incision appearing in the velum, which
thus becomes bilobed. This organ is bordered anteriorly by a circle
of strong cilia serviug for locomotion, and posteriorly by weaker cilia
which conduct food to the mouth
(Greennaur, Fou). In Cleodora
4 band of cilia appears on the velar
area at a time when the larva is
still at the Troekuphory stage.
Other Pteropods, e.y., Cavolinia,
carry on the velar area a central
ciliated tuft, such as has been met
with in other Molluscan larvae.
The size of the velum varies
greatly. In Cavolinia, where it
is retained for only a short time,
it is less extensive. In Fig. 74,
we see the velum in a slightly
older larva of such « form. In
Cleodora, Cymbulia, Tiedlemannia
(Fig. 75 A and #) the velum is
much larger, and each of the two
DEVELOPMENT OF THE EXTERNAL FORM—PTEROPODA,
Fre. 74.—Larva of Cavolinia tridentata,
lobes in aguin subdivided, so that
the whole appears to consist of
four lobes. This condition is
specially distinct in a larva be-
longing to the genus Crese/x und
described by Grosnpaun (Fig.
7h ©), in which the velum is still
seen from the right and veutral side
Coles Fou, from Baurour's Text-
). a, anal region, with the two
anal cells; /, mesopodium; A, heart ;
é, intestine; 4n, contractile dorsal
sinus; m, oral region; mb, mantle;
me, mantle-cavity ; of, otocyst; pn,
rudiment of fin ; 9, shell ; 1, renal sac;
», stomach ; «, food-yolk.
of considerable size when the shell has grown to « great length. A
‘strong retractor starts from the anterior part of the body and is
inserted nt the posterior end of the shell (Fig. 76 A, 1).
—
168 GASTROPODA.
The shell originates from the shell-gland which has shifted towards
the end of the body. Aceording to Fox, a plug of strongly refractive
substance is very often to be found in the shell-yland ; in some cases,
this plug is perhaps formed abnormally, but in Cymbutia it no doubt
represents the normal condition. The substance is then said to
spread out under the shell which is secreted as a cuticular integu-
ment, after the shell-gland has gradually flattened out. It is at first
shaped like a watch-glass, then deepens and becomes cup-shaped
(Cavolinia, Cleodora, ete.), or else it becomes rounded and almost
oviform like the embryonic chamber of the Cephalopoda. This is
Fra. 75.—Larvae of Tiedemennia (A), Cyututia Peronit (B); and Oveseis acicula (C)
{after Kron and GeornBaun). d, operculum ; f, foot Sei fins; s, shell ; », velum,
the case in Cress, Cymbulia, and the Gymnosomata. The shell,
which now becomes calcified, grows by the addition of new layers to
the margin of the embryonic shell, their boundaries being recognisable
as zones of growth. In this way, the large larval shell which, in the
Cavoliniidae and Gy mnosomata is long and in the Cymbuliidas coiled
is formed (Figs. 74, 9, 75 A-C, 76 A, #).
In the Cavolinitdae, the shell of the adult forms very simply from
the larval shell, by the addition of further layers to its anterior
DEVELOPMENT OF THE EXTERNAL FORM—PTEROPODA. 169
margin, but the latter is marked off by a constriction from the part
which represents the adult shell; here also, in Cavofinin, a transverse
wall is seereted, after the body of the animal has withdrawn from
the posterior part of the shell, This larval shell is afterwards lost.
In other Cuvoliniidae, the larval shell is retained even in the adult
(Styfiola), the posterior part of the body not being withdrawn from
it (Cresei*). The coiled larval shell of the Limucinidae passes directly
over into the adult shell, new coils merely being added to those
already present (Limacina, Spivialis) In the Cymbulitdac, the
larval shell can hardly be distinguished from that of the young
animal undergoing metamorphosis. This calcareous shell is thrown
off, the cartilaginous shell of the adult surrounded by the mantle
then appearing ; this shell arises by the thickening of the connective
tissue and can therefore not be in any way compared to a true
Mollusean shell (PELSENEER).
The transformation of the shell just described is one of the most
features among the external alterations undergone by
the larva. In the Cavoliniidae, the shell lengthens, and, in the Cym-
tuliidae and Limacinidae, becomes rolled up (Fig. 75 A and B).
Even in the straight shells of the Cucoliniidae we find a slight flexure
of the posterior end which gives the shell the shape of a hunting
horn. It is a curious fact that the concavity of this slightly bent shell
does not correspond to the ventral side, but lies dorsally, This must
‘be connected with « twisting undergone by the posterior part of the
body in these forms (Boas, Nos. 9 and 10). The coiled shell in any
cause represents the more primitive condition and persists throughout
life in the Limacinidue, which ave also provided with an operculum.
The development of the foot exercises a great influence on the
changes that take place in the external form of the body. The foot
originally was a large projection lying behind the mouth. While
the middle part of the foot does not increase greatly in size, and at
first is conical or linguiform, two projections arise at its sides and
grow out rapidly (Fig. 74, pv, and 75 A, fl) in the form of two large
lobes, the so-called fins (Fig. 75 4, fl). The great size which may
be attained by the fins in the further course of metamorphosis is
already sufficiently known. The median lobe of the foot also in-
creases in size. In the Cymbuliidar, a filiform appendage develops
on it posteriorly, Ontogeny proves indisputably that the fins owe
‘their origin to the foot, as was observed long ago by Jon. M0LLER
: ie
The Vetiver tarvn of the Pteropoda shows great agreement with
—_—
170 GASTROPODA,
that of the Opisthobranchia, a fact which is specially evident in
the forms that have « coiled shell (Figs 75 4, and 72, p. 162). The
posterior part of the foot here also usually carries an operculum
which, in the Limerinidee, is retained throughout life, aud in the
Cymbuliilae, is thrown off after the shell has been lost ; but in those
Pteropods that have straight shells an operculum is not found A
well-developed primitive kidney is not known to occur in the Ptero-
poda; they may, in this respect, resemble the Opisthobranchia, a
comparison which would be supported by their internal organisation.
We have here, for instance, as in the Opisthobranchs, the two sacs
filled with food-yolk as appendages of the enteron, In the formation
of the alimentary canal, the entoderm becomes differentiated in such
a way that the median (ventral and dorsal) parts become the epi-
thelium of the archenteron, while the lateral parts which appear
composed of large cells rich in yolk, by growing out into cacea, be-
come the nutritive diverticula. These caeca have been supposed to
yield the liver, but this organ, according to Fou’s statements, forms
independently of them as an outgrowth of the archenteron. A pos-
terior tubwlar diverticulum of the archenteron runs out towards the
ventral surface and fuses with the ectoderm at the spot where the
anal cells lie to form the anus. This lies either in the middle line
behind the foot, shifting secondarily to the left side (Cavoliniidae) or
else it lies from the first on the right side of the body (Cymbulitdae,
Gymnosomata). There are also other indications of asymmetry, such,
for instance, as the lateral position of the mantle-cavity. This indi-
cates that the Pteropoda which, as adults, are somewhat symmetrical
in structure, are derived from asymmetrical forms.
As the fins increase in size, the velum gradually degenerates. The
mouth takes up its final position between the fins, The disappeur-
ance of the velar area leads to the yreat reduction af the large section
of the tarcat body which lies in front uf the fact. At w later stage,
the two tentacles bud out in this region, carrying the eyes, This
reduction of the anterior part of the body as compared with the
massive foot, which has shifted far forward, is specially characteristic
of the Thecosomata, In 7'edemunnia, however, the oral region be-
comes raised up to form the proboscis which bends backward, After
the growing shell has reached the base of the foot, a slit-like invagina-
tion appears in the Cuvolinitiar (according to Fou) on the right side
between the base of the foot and that of the velum, extending then
dorsally and ventrally. The mantle-cavity thus formed finally
encircles the body (visceral dome) on three sides, so that the latter
DEVELOPMENT OF THE EXTERNAL FORM—PTEROPODA. 171
is connected with the mantle or shell (in the Cavoliniidae) only on
the left dorsal side.
‘The ventral position of the mantle-cavity in the Cavoliniidae is very striking,
as this cavity, in obher Gastropods, is dorsal in position. According to Boas,
the visceral dome and the shell connected with it have undergone torsion.
‘This view is supported by the fact that, in younger larvae, the bent apex of
the shell is directed not, as in the adult, dorsally, but to the left, It is at
‘once evident that this process may be classed with those already described in
connection with the acquisition of asymmetry by the Gastropoda (p. 143), but
in this case other changes have been added in adaptation to a different manner
of life.
We shall not here give any special aceount of those ontogenctic
processes such as the formation of the otocysts, the radular sao, ete.,
which take place in the sate way as in other Gastropods,
Gymnosomata. The 7rochophor is followed by a larva provided
with « large bilobed velum
and a pointed foot (Fig.
76 A, /f). The shell, which
at first is cup-shaped but
later oviform, as it crows in
length, becomes a tube
widening out anteriorly
(Fig. 76 A), on which, as a
rule, the zones of growth
are recognisable as intervals
varying in width. The
larva does not long retain
at this stage, in which it
closely resembles — the
straight-shelled — Thecoso-
mata. The shell is thrown
re 76 Band 77.A). In
those larvae that develop /
ciliated rings even before Fria. 76,— Larvae of Clione at two Afferent stages
the disappearance of the ee itn ane ee ase:
yelum and the casting of retractor muscle; s, shell; , velums 1,
chiated rings.
buted in such a Way that the most anterior ring lies between the
a
172 GASTROPODA.
velum and the foot, and the posterior ring immediately in front of the
aperture of the shell. In this case, the posterior part of the body is
still of some length; in other larvae, the posterior ciliated ring is
found almost at the end of the body (Fig. 76 B). The velum seems to
Dear no relation to the ciliated rings. After it degenerates, the larva
presents an appearance which, for a Molluse, is very peeuliar, recalling
rather the Annelid larvae which are encircled with several ciliated
rings. These also are at u stage following the Trochuphure larva,
as already mentioned (Vol. i., p. 277), and as we were able to see
in various polytrochan larvae. This comparison to an Annelid
larva has already been instituted by Grcensavr, and the fact has
been emphasised that the resemblance is accidental and of no great
significance.
Our knowledge of the ontogeny of the Gymnosomata relates entirely to Clione
and Prewmodermon, two forms which seem to agree pretty closely in the
general features of their development, as shown by Jou. Mirten, Gecexeaun,
Kronsand Fou, Asmost of these larval Gymnosomate have not been traced to
the adult stage, it is by uo means certain that the larvae examined belonged
to these genera.
The mouth lies on the anterior, proboscis-like projection, and the
anus, which is displaced to the right, ventrally between the first and
second ciliated rings. Two pointed outgrowths lying near the mouth
represent the rudiment of the so-called cephalic cone (Fig. 77 2).
Somewhat further back, but
in any case in front of the
anterior ciliated ring, the rudi-
ments of the acetabuliferous
appendages appear (JoH.
Miinner), (These, according
to PEUSENEER, are derivatives
of the proboscis,] When the
proboscis is evaginated at a
later stage, these seem shifted
Fio, 77.—Two larvae of Pnenmodermm at further back, being now situ-
dierent aged, Gucesaaces OM ated on its posterior pat
(Fig. 77 B). The foot was
previously referred to as a pointed ventral appendage, lying behind
the first ciliated ring. Before this stage, an anterior, horseshoe-
shaped lobe forms in the posterior concavity of the pointed part
of the foot. Immediately behind the anterior lobe of the foot, on
either side of the posterior lobe, the first rudiment of the fins can
VIS —
DEVELOPMENT OF THE EXTERNAL FORM—PTEROPODA. 173
be seen as very small, rounded lobes projecting from depressions in
the body (Krowy).
The further metamorphosis consists in the growth of these parts
and the degeneration of the ciliated rings, The most anterior of
these is the first to disappear and then the middle ring; the posterior
ring is still to be found when the young animal attains its full size,
but no doubt degenerates later.
We must here add a few words of explanation as to the position assigned by
us to the Pteropoda, Until recent times, the Pteropoda were often regarded.
a 8 special elass equivalent to the Gastropoda, Cephalopoda, ete., although
some zoologists objected to such a classification. For anatomical and onto-
genetic reasons the Pteropoda are now classed with the Gastropoda,” being
placed specially near the Opisthobranchia, as is indicated by the form of
the central nervous system and their circulatory apparatus, as well as by the
structure of their genital ducts and by their hermaphroditism. Another im-
portant factor in classing the Pteropoda is found in the organ which gives the
body its characteristic shape, viz,, the swimming apparatus, The manner in
which the fins arise proves that they are derived from the transformed lateral
parts of the foot. It is an interesting fact that in some Pteropoda (the
Gymnosomata) the propodium has still retained its function as a creeping
‘sole, serving, like the sucker of the Heteropoda, for attachment (SounkYET,
No. 121; Gnospes, No. 39), The fins have been regarded by some as epipodia,
bat Petsesesn, on the contrary, considers them to be widenings of the whole
margin of the foot. Such fin-like widenings (swimming lobes) are found in
‘certain Opisthobranchs, and the derivation of the Pteropoda from such forms
Seems to be suggested. Gnonnen, as well as Boas and Petseneer (No. 84),
‘the two more recent investigators of this subject, have recently given active
adherence to this view. Lateral widenings of the sole of the foot are found in
Avera, Gasteropteron. These Opisthobranchs which, like the Ptoropoda, can
swim freely by flapping these fin-like foot-lobes have therefore been regarded
us the starting-point for the latter group. From such Opisthrobranchs the
‘Thecosomata would first have to be derived, as has been done by Petsexner,
‘who traced back the Thecosomata to forms like Acera among the Bulloidea,
whereas he derives the Gymuosomata from forms like Aplysia, in whioh latter
‘the swimming lobes are, as in the Gymnosomata, somewhat more dorsal in
position. Persexxun, in his classification of the Opisthobrunchia, places the
‘Thecosomata directly after the Bulloidea, and the Gymnosomata near the
bag Boas also regards the Pteropoda as very nearly related to the
1 Se ten cadcl of this view we may mention For, Spexeen,
iN In R. Henrwie's text-book, the Lar ey
8 Sota of the Gastropoda, and Craus also recentl
vinallar |, Placing them after the Opisthobranchia, [ Pract.
now class the Pteropoda with the Gastropoda and most
according to which they find their nearest allies
‘Opisthobranchs. PrLsexmer further separates the
om the osomata, placing the latter with the Bulloidea
r with the Aplyscidea (see Challenger Reports, Vol. xxiii,)—
174 GASTROPODA.
Opisthobranchia and points out the great similarity existing between the in-
ternal organisation of the Bulloidea and that of the Thecosomata, Between
the Gymnosomata and the Thecosomata he finds a great distinction, since he
cannot regard the fins in the two divisions as homologous. Since, however,
according to him, the Gymnosomata, like the Thecosomata, are to be traced
back to Tectibranchia, they have in any case a common root. It appears to
us that their developmont is in favour of a connection between them. Their
larval forms agree closely, the resemblance between the long, straight shell of
the Gymnosomatous larva and that of the Thecosomata being specially strik-
ing. This is a feature which points to a long period of pelagic life of the
adult, for the larvae of the Opisthobranchs also live in the sea. We might
therefore assume that the Gymmosomata are to be traced back to forms
resembling the ancestors of the Theeosomata, which only later underwent
the changes now found in their structure and development. We can hardly
regard as of much importance the apparent retention of a primitive feature
in presence of a small creeping foot in the Gymmosomata, since single
primitive characters may be retained in forms which in other respects are
highly specialised. It is also by no means certain that this character has
not been secondarily acquired.
We have felt justified in treating the Pteropoda separately from the
Opisthobranchia on account of the great deviations found in the structure of
the body. In so doing, we do not wish in any way to deny their relation to
forms like the Bulloidea and especially Gasteropteron. It is possible that
there may be even closer ontogenetic relationship to these forms than is at
present known. This would be the case if the ontogeny of a Cephalophoran
described by C. Voor were really found to refer to Gasteropteron, as was con-
jectured by Gucensavn (No. 123). This Veliger larva develops two fin-like
structures, and yet, in consequence of various other characteristics, is nob
comparable to a Pteropod-larva. The conical shell with its transverse lines
of growth, further, resembles the shell of the Gymnosomata and would be
little suitable to an Opisthobranch. It is thrown off even within the egg-
shell, The view that the larva now under consideration belongs to Gasterop-
teron hus been directly denied by Knons (No. 58b) who regards another larva
‘as being that of Gasteropteron, Weare not acquainted with any more recent
accounts of this very interesting larva which may be of great importance in
determining the view which should be taken of the Pteropoda.
E. Pulmonata,
The transition from the ontogeny of the Opisthobranchia to that
of the Pulmonata is afforded by Onchidium, a form which has already
been alluded to p. 133. This amphibious form, which lives on the
seashore, develops embryos with a large bilobed velum. The two
lobes are beset with long cilia, while small and delicate cilia are found at
the incisions between the lobes. This embryo thus greatly resembles
the Veliger larva of the Opisthobranchia. Although the adult is shell-
Jess, the embryo has a coiled shell like that of a marine Gastropod,
==E==E
DEVELOPMENT OF THE EXTERNAL FORM—PULMONATA. 175
The operculum, on the contrary, is wanting according to Jormux-
Larevte (No. 51) and the foot which, even in the Veliger stage, is
very large is also covered at its anterior and dorsal side with delicate
cilia, The shell is thrown off during embryonic life, and the velum
also degenerates within the egg-shell.
With regard to the absence of the operculum which, according to
Jovevx-Larrore can hardly be doubted, it should be pointed out
that this organ is as a rule not found in the Pulmonates,
The marine Amphibola, however, has an operculum showing the usual
structure and position (i.c,, lying posteriorly on the back of the foot, No. 66).
‘Unfortunately, this Australian form is little known; a more accurate know-
ledge of its anatomy and ontogeny is very desirable. According to Semper
(No, 118, ii, p. 100), the embryos of Auricula and Scarabus have opercula.
In Onchidium, after the shell has been thrown off, the mantle,
with the reduced palmonary cavity, shifts dorsally and, with the
kidney, opens by a median aperture at the posterior end of the body.
‘The hitherto asymmetrical anus (lying on the right side) also assumes
median position at the posterior end of the body. Iu some species,
the pulmonary, renal and anal orifices open through a common
aperture on to the surface of the body. The loss of the shell thus
leads to the acquisition of a secondary symmetrical position of the
organs, 4 phenomenon that may also occur in other slug-like forms
(a8 also in various Opisthobranchs).
With regard to the further development of Ovehé lium, it need here
only be noted that the form of the adult is attained within the egy.
The Vaginulidas, forms usually placed near to Onchidium, no
Jonger possess, according to Semper und y, JHERING, either the fully
developed bilobed velum or the larval shell (No, 116), although the
spawn appears to have the same constitution as that characteristic of
Onchidium (p. 104). These forms would therefore appear more
adapted to a terrestrial existence, if the short statements as to their
development should be corroborated.
— Onehidinm and Vayinulus are both opisthopneumonic, and this
fact, taken together with the other features of their organisation as
well as their ontogeny, suggests that they represent forms which,
from « condition like that of the marine Opisthobranchs, have become
bac ged to a terrestrial existence. The classification of Onchidinm
nd Vayinulus among the Pulmonata which might, on account
of the peculiarities above mentioned, appear doubtful (Joreux-
Larrcte), has been strengthened by the more recent observations on
a
i
176 GASTROPODA.
this subject (v. Jazrine, No. 46; Stora, No. 120).* Since the
Veliqer stage may still be found even among the undoubted Pulmon-
ates, although usually in a somewhat reduced condition, no objection
can be made to this classification from the ontogenetic stand-point.
‘Their development, however, shows in an unmistakable manner that
we have to do with transitionary forms, a fact which is further con-
firmed by their manner of life, especially by that of Onchicdiun
(p. 133).
The velum, it should be mentioned, is, according to Semper, well
developed in some tropical forms (Auricala, Searabus No. 118); in
the same way as in Ovchidium (R. Bara, No. 5, p. 175). Sampur
assumes that the larvae of these forms swim about freely in the
sea. Since they, as already stated, also possess an operculum, they
bear a great resemblance to the Opisthobranch larvae. As a rule, the
yelttm is much reduced in the Pulmonates. These puss throngh the
invagination-gastrula stage, the manner in which this gastrnla arises
being modified in many ways according to the varying amount of the
yolk. ‘Thus the archenteron, in consequence of being composed of ~
the large, yolk-laden cells, appears at first as a massive structure
with « narrow lumen, bit at a later stage widens out and becomes
4 Spacious sac. The originally narrow cleavage-cavity also gradually
widens out. The embryo is now spherical. Its animal pole is often
marked by the presence of the polar bodies ; at the opposite vegetative
pole is found the blastopore, which at first is wide, but narrows later
and usually becomes slit-like. It closes from behind forward, but,
apparently, a small anterior aperture may remain. At this pomt,
in the midst of un ectodermal depression, the mouth forms, and,
when the blastopore is retained, it becomes displaced somewhat far
inward by the stomodaeum to the point at which the stomach eom-
mences (Fon, No. 33; Rann, No. 91; Wonrson, No. 131),
The spherical or often somewhat ventrally flattened form of the
embryo undergoes some alteration in consequence of the appearance
of the shell-gland, the foot and the velum. The shell-gland arises
as an ectodermal invagination on the dorsal surface opposite the
mouth (Fig. 78). It may sink in so deep that it has repeatedly been
mistaken for the rudiment of the proctodaeum, It flattens out again
* [Pate (Zool. Jahrb, Anat., Ba. vii., 1894) who has roan tly nee
study of the anatomy of Onchidium, concludes that while these forms are
true Pulmonates, they nevertheless show affinities with the Tectibranchiate
Opisthobranchs. He places the Onchidiadae and Vaginulidae as direct
derivatives of the primitive pulmonate on a branch quite independent of the
Stylommatophora or Basommatophora,—Ep.]
===
DEVELOPMENT OF THE EXTERNAL FORM—PuLMONATA, 177
later, secreting the shell in the usual way; in Linaz, however, the
shell of which is at first internal, the shell-gland is pouch-like and
becomes ubstricted from the ectoderm (Fon). A swelling of the body
behind the mouth indicates the position of the foot (Fig. 78), The
velum appears in the form of two transverse swellings (formed of
large, richly vacuolated cells) in front of the mouth, which run as
bands round a large part of the anterior body, but for a time do
not meet, or else, as in Planorlis, in consequence of the very much
reduced condition of the velum, never completely unite (Fig. 78, v).
At this stage, we may, with Ray Lanxesrer, consider the embryo
as equivalent to the Trochophore ; occasionally, as in Limnaea, even
a ore tecgaterd ane, een the Liimline Rast). aw, “yes m, argh
‘enteron and ive gland (large cells); mes, mesoderm ; of, otocyst ;
radular seo; *, shell; sd, shell-gland ; «p, apical plate ; wm, primitive kidney ; as
;
the external form of the Trochophore is preserved, « large pre-oral
portion of the body being marked off from the posterior portion by
the yelum (Ray Lasxesrer, For), A thickening at the pre-oral
pele denotes the apical plate. That the bilobed character of the
velum so characteristic of the Velijer larva is found here also is due
to its mode of origin, Asa rule, not only the Veliger stage but the
Trochophore stage as well is much reduced, the principal features of
the latter, however, are still to be found.
At the stage which more or less corresponds to the Trochophore,
the alimentary canal consists of a stomodaeum from which a radular
sac s00m grows out ventrally (Fig 78, r) and the still undivided and
e N
a
|
178 GASTROPODA.
exceedingly large archenteron (sd). Some of the cells of the latter, in
consequence of the albuminous matter which has been brought from
without through the mouth into the lumen of the intestine, have a
swollen appearance (Figs. 70-80); others, however, which lie pos-
teriorly and ventrally are smaller, indeed, through more active
division, they may even be specially small. They form u diverti-
culum of the entoderm which is directed backward (Fig. 78) and
represent the rudiment of by far the greater part of the enteron.
The cells containing albumen, which continue to increase in size, pass
over into the formation of the liver later. At first the intestinal
cavity appears bounded partly by a large-celled and partly by a
small-celled epithelium. The posterior diverticulum of the enteron
comes into contact with the ectoderm in the ventral middle line,
behind the foot. This part at first bulges somewhat outward, form-
ing the anal prominence, Later on, the entoderm-diverticulum here
fuses with the ectoderm to form the anus.
The resemblance of the stage just described to the Trochophore
stage is heightened by the presence of a paired primitive kidney,
which, in the fresh-water Pulmonates, has a very characteristic
origin and shape (Figs. 78-80, wn) Even at an early stage, a
remarkably Tange cell cau be seen on each side below the dorsal part
of the velum; these two cells yield the principal constituents of the
primitive kidney, and have been claimed as velar cells which have
entered the body-cavity (Wourson), this view being no doubt sug-
gested by the vacuolated character of the velar cells as well as by
the condition of those Prosobranchs in which complexes of ecto-
dermal cells which are certainly excretory are apparently closely
related to the velum. This view can, however, hardly be correct,
and, taking into consideration the usual method of formation of the
primitive kidneys, we prefer the view of Rast that these large cells
are to be derived from the mesoderm.* ‘They lie at the posterior
part of the mesoderm-bands which are already disintegrating. In
each of these cells, a cavity which at first resembles a vacuole ap-
pears, lengthening as soon as the cell itself lengthens. The cell then
becomes bent and forms the principal part of the primitive kidney,
the canal of which is thus intra-cellular in its origin (Gani, No. 35 ;
Rast, No. 91; Wourson, No. 131). The large cell yielding the
primitive kidney is joined by a few of the adjacent mesoderm-cells
and the canal, by becoming connected with the ectoderm, opens
[See footnote, p. 179.—Ep.]
DEVELOPMENT OF THE EXTERNAL FORM—PULMONATA, 179
externally. The apertures of the two kidneys lie at the two sides of
and behind the velum. The inner end of the primitive kidney is
usually regarded by authors as communicating with the primary
body-cavity by a ciliated aperture.* In the terrestrial Pulmonates
this has been maintained with certainty for Helix (Acavus) by
P_and F. Sarastn (No, 102) and Jourpary, as well as Mevrox
(Nos. 50 and 75), arrived at the same result.
The primitive kidneys of the terrestrial Pulmonates, which were
early recognised by O. Sonmipr and GuGrnnaun, are somewhat
differently constituted from those of the aquatie forms. They also
have the form of bent tubes opening externally through wide aper-
tures in front of the border of the mantle, but they are composed
of a large number of cells arranged like an epithelium, none of which
are distinguished by their special size (Jourparn, Meuron, SARasty).
De Mevnon considers that, in Helix, the primitive kidney arises chiefly
from the ectoderm, but holds also that the innermost part may be derived
from the large mesoderm-cells. But since these latter, in the aquatic Pul-
monates, yield the principal part of the primitive kidneys, the derivation of
these organs from the mesoderm appears more probable. We need not,
however, exclude the supposition that, as in the primitive kidneys of the
Prosobranchs, an ectodermal invagination takes part in the formation of
the peripheral part and that this latter, in terrestrial Palmonates, is specially
extensive,
At the time when the primitive kidneys attain their full develop-
ment, the external form of the embryo also undergoes further altera-
tion. The shell-gland begins to lose its pouch-like form and gradually
flattens out. The ectodermal epithelium belonging to the shell-area
still appears formed of columnar cells. Over this area lies the shell
AY Easvascen (No, VII.) has since described the detailed structure of the
kidney in P! isand Limnaea ; he finds a specialised ciliated cell
{the funnel-cell) which puts the tube into communication with the body-cavity,
and then a long tubular segment containing a flagellum and a terminal por-
tion which on to the exterior, this latter portion Entancur thinks may
in the Euthyneura, while the remainder is mesodermal. In
he finds a swollen ampulla at the junction of the two seg-
ments, The develo] t of this organ has been more recently investigated
Morsenvneimen (No. XVIL.), and this observer maintains that, in Limar,
primitive kidney is wholly ectodermal, and here he is at variance with most
observers. As he also maintains that the heart and definitive kidney
a arise from a common ectodermal rudiment, we think that his views
confirmation before we can accept them. Mutssexnetmen (No.
has also given a most elaborate account of the structure of this organ
ins he differs from EnLaxcer in one important respect, vir., he is un
able to find any opening into the body-cavity and thinks that ExtancEr
mistook a =H vacuole which is invariably present in the end-cell for an
RAGES
180 GASTROPODA.
which has now become cap-like, The margin of the shell seems
buried in a groove, a swelling of the ectoderm, the margin of the
mantle, having formed here. The whole embryo has somewhat
lengthened, and the foot stands out more distinctly (Fig. 79).
The foot in Limnaea, which ut first appears as an unpaired swelling, is
said to assume a bilobed form (Ray-Lanxesrer). Such a bilobed foot seems
often to occur among the Gastropoda, We have already met with it in
Succinea, Patella and Vermetus (p, 132). Fou also observed this later develop-
ment of the bilobed form in the foot of Limnaea, as well as in Planorbis and
F 4
Fie. 79.—Older embryo of Planorbis, seen trom the side (after am, cye
Se Sear | Fe) pedal makin oe, Sataee a 3 um, primitive
kidney ; v, velum,
Ancylus, though in these last two animals it was less striking. Ray Law-
KESTER compares this to the transformation of the foot into the paired fin in
the Pteropoda.
The outgrowth of the body-epithelium to form the foot causes a
considerable enlargement of the ventral portion of the inner cavity
of the larva, and a similar cavity is produced pre-orally by the dilata-
tion of the part which is encircled by the velum. A similar process
has already been met with in the Prosobranchia (p. 150), The anterior
swollen part of the embryo is known as the cephalic vesicle and the
DEVELOPMENT OF THE EXTERNAL FORM—PULMONATA, 181
wide space as the cephalic cavity. Special attention has been directed
to this part in consequence chiefly of the pulsating movements which
may occur here, a peculiarity also found in the nuchal and the pedal
regions of the embryo.
Tt has repeatedly been stated that certain regions of the body-covering,
those to which a large number of ‘mesoderm-cells became attached, carry on
contractions which sometimes follow one another with considerable regularity,
this last fact having led to their being called ‘Jarval hearts.” The cireulation
of the body-fluid is, in any case, promoted by these contractions, but it seems
doubtful whether they should bedlescribed as actual pulsations, Sometimes
the movements that thus occur are somewhat irregular, and Rast found that,
occasionally, contraction of one part of the body is followed by extension of
another part, but we cannot consider this to be regular rhythmical move-
ment. The embryo moves in consequence of these contractions, It is well
known, however, that Gastropod embryos are able in addition, in ing go
of their rich ciliation, to rotate within the egg-envelope.
Since the embryo, by taking in the albuminous fluid contained within the
egg-shell, feeds independently and also has « circulation of its own and special
excretory organs, the velum may serve as respiratory apparatus, this func-
tion being also exercised by it in addition to its locomotory function in the
free-swimming larvae. In the embryos of terrestrial Pulmonates, a special
respitatory organ develops, the caudal vesicle (podocyst), which will be further
described below.
The very large apical plate of the embryo has considerubly thickened
and has become bilobed. According to Rasn, the cerebral ganglion
is derived from it, though in other Pulmonates the formation of this
ganglion has been thought to arise differently (p. 191). At the
posterior end of the ‘apical plate” the eyes arise as ectodermal pits,
‘Two large superficial prominences, which soon become conical, arise
Isterully to the optic vesicles and represent the rudiments of the
tentacles. Both eyes and tentacles belong to the pre-oral section,
whereas the otocysts arise behind the velum (Figs. 79, 80 aw, #, of).
Up to this point, the embryo is fairly symmetrical in shape, but
this symmetry is disturbed chiefly by the further development of
the shell which grows towards the right more strongly than towards
the left (Fig. 80). The edge of the mantle, which now bulges out
more than before, is of course also affected by this unequal growth.
‘The anus is pressed out of its median position to the right. It is
evident from this that processes occur in the later development of the
Pulmonates similar to those already met with in the metamorphosis
of other
As the mantle eitendd further, its growth takes place more rapidly
‘on the right than on the left side. In front of the anus an indenta-
am
182 GASTROPODA,
tion forms which at first is shallow but soon becomes deeper; this
is the rudiment of the respiratory cavity which continues to widen
and thus comes to include
the anus and the aper-
tures of the adult kidney.
This cavity itself opens
externally only through
a narrow aperturey the
respiratory aperture,
which lies rather far
forward on the right
side of the body,
The formation of the res-
Fic, 80.—Older Planorbis embryo, seen from the Pitatory cavity has nlso been
a OTe tire ee x al
; foot ; ma, id, as ion taki ince
eetentacles un, priitive ‘Kidneys relum; Between the margin of the
wd, stomodacum, mantle and the body, only
a small aperture being left,
which, as respiratory aperture, leads into the greatly deepening cavity. In
this way, the respiratory cavity is shown to be the transformed pallial or
branchial cavity, In the Basommatophora this is indisputable, as a gill
is in some forms found in the cavity (dmphibola). 'The respiratory cavity
in the Stylommatophora has, ou the contrary, been regarded as not homo-
logous with the branchial cavity, but rather as the ureter transformed
into a respiratory organ. On this account v. JnBRtNo termed the terrestrial
Pulmonates the Nephropneusta, thus distinguishing them from the aquatic
Pulmonates, which he named the Branchiopneusta (Nos, 45 and 46.) We
ourselyes do not find anything in the manner of formation of the respira-
tory cavity in land Pulmonates to justify so different an interpretation of it.
‘The mantle-cavity in the Prosobranchia may also at first, as here, arise appar-
ently in the form of an ectodermal depression, The homology between the
respiratory cavity of the land Pulmonates and that of the water Pulmonates,
which in itself is so probable, is further supported by the fact that in some of
the former (Testacellidae, Puate, No, 89) a sensory organ is present in it which
corresponds to SrenGeu's olfactory organ found lying near the gills in the
mantle-cavity of other Gastropods.
Towards the end of that period of embryonic life during whieh the
embryo may be compared with the larva of other Gastropods, the
sinuses in the head and the foot which gaye rise to the embryonic
circulation above described undergo gradual degeneration. In the
same way the primitive kidney disappears and the permanent kidney
functions in its stead.
The final form of the animal is reached by the growth of the parts
DEVELOPMENT OF THE EXTERNAL FORM—PULMONATA. 183
now present. ‘The respiratory cavity and the edge of the mantle
extend more to the left, the shell taking the same course. The head
becomes more distinet, rising up from the foot, which, in its turn has
increased considerably in size and has approached nearer its definitive
form. The velum has disappeared, a portion of it, according to
Ray LaNkEstER, giving origin to the labial palps (p. 133). This
latter view seems quite in keeping with the position of the velum,
but is set aside as improbable by For and is directly refuted by
‘Worrson.
Fig. 81,—Embryo of Helive lafter Fox). a,
peepee ie mouth ? md, enteron and ive gland
iar suo; i, shel ‘kidney
‘The shell is still cup-shaped, but is already asymmetrical. Further
unequal growth on one side leads to coiling both of the shell and the
has, so far, referred chiefly to the development of the
set Put especially to that of a few forms which have
‘been particularly carefully investigated, such as Linnaea and Plan-
‘orbis. These latter have been described in detail by Rav LankesT2r
Wo 68), Base (No. 91), For, (No. 33), and Worrson (No. 131) to
- whose descriptions we must refer the reader for further details. Fou
184 GASTROPODA.
has also included various other fresh-water Pulmonates as well as
terrestrial Pulmonates in his comprehensive researches. ‘These latter
forms, which had already been studied by Gucunsaur, differ from
the aquatic Pulmonates in some points of their development and
therefore require separate treatment.*
The ontogeny of the stylomatophorous terrestrial Pulmonates
is characterised by the development of exceedingly large provisional
organs, viz, the cephalic and pedal vesicles, ‘These larval organs
appear early. At a stage which corresponds somewhat to the
Fro. treo of Heli powatia, ten a day 9 old, awe tro u the side (after Fou). a
anus ; coat ie vesicle ; 3 md, enteron and
aiqertive sani genre ots lan wn, ‘print kidney,
Trochophore stage, the embryos (of Limaz, Arion, Helix, Clausilia)
are distinguished by the great swelling of the pre-oral section of the
body. At the stage of which we speak, this cephalic vesicle is so
large as almost to eclipse the rest of the embryo, At a rather later
stage also (Fig, 81), the cephalic vesicle (id) is still very large, but
* [See also the more recent works of Honates (No. XIII. eer ee
Metsenaanren (No. XVIL), Scammpr (No. XX.) and Wrerzessnt ‘ eva,
‘These deal for the pgp with the cleavage and cell-lineage.
‘wermER's researches on Limax, however, are carried anreee Soa outa te
= in connection with the development of the Stylommatophora. —
D.
THE ONTOGENY OF THE STYLOMMATOPHORA. 185
the foot now bulges ont and also commences to swell up into a vesicle.
Little now remains of the Trochophore shape. At a stage somewhat
younger than that depicted in Fig, 81, a slight vestige of the velum
is atill to be found in two transverse ciliated ridges which lie on
either side of the mouth and run towards the shell-gland, These,
however, do not extend up to the mouth, and soon disappear. In
Arion and Limax, no traces of the velum are to be found (For).
‘These embryos, like those of the aquatic Pulmonates, are able to
rotate within the egg, being covered with cilia,
¥iq. 8, —Older ombryo of Limax marinus, seen from the side (after Fou), au, eye;
’d, yolk-material :'7, (oot; , labial palp: ima, tustle-fold :
pele eee ve gland; of, upper lip; pd, podocyst; py, pedal gunglion ;
rs, vadular sac; 9, shell; 1, tentacle ; wn, primitive kidney, ey Mt
‘The position of the different organs of the embryo can be nnuder-
‘stood most easily by reference to Fig. 81. The oesophagus is followed
by the enteron from which the digestive gland composed of large
albumeniferous cells is already becoming differentiated, posteriorly
the enteron is lined by smaller entoderm-cells, The anus lies behind
the pedal swelling, and behind it again, marking the dorsal side, is
found the shell-gland. A pit lying near the mouth represents the
rudiment of the radular sac which, according to Fou, arises in the
stomodaeum, which has not yet fully sunk in, and is thus near the
‘oral aperture, but is soon drawn into the buccal cavity. Near the
—
186. GASTROPODA.
enteron can be seen the tube of the primitive kidney which is as yet
unbent and which, according to Fon, opens outward at the posterior
base of the foot. Almost in this region, but somewhat behind the
foot, lies an organ described by For as the larval heart.
The so-called larval heart (Fig. 82, /h) consists of a bulging of the ectoderm
with which numerous mesoderm-cells become connected. This specially
differentiated part of the coyering of the body which, when the mantle-
cavity forms later, is drawn into it and thus comes to lie more to the right,
carries on regular pulsations and is regarded by Fo. ns an organ for promoting
the embryonic circulation. It thus belongs to the category of larval hearts
which have already been alluded to (p. 152).
While the cephalic vesicle in the later stages decreases in size, the
foot lengthens considerably, At first it is cylindrical, but it soon
spreads out more and more and now becomes a massive olub-shaped
organ (Fig. 83), which is known as the caudal vesicle, and more
recently has been named the podocyst (Jourpain, Sarasty), As it is
richly supplied with mesoderm-cells which become applied to its wall,
it is capable of contraction and carries on rhythmical movements which
alternate with those of the cephalic vesicle. It is evident that this
large vesicular swelling is a circulatory or respiratory apparatus and
it may be that it also serves for nutrition, since diosmotic processes
take place in it,
The podoeyst is specially large in the embryos of various species of
MHelic (Gucennavr, v. JuexinG, Fou, Saxasin). It here spreads
out laterally, and thus assumes the form of a broad plate which,
towards the end of the “larval period,” lines the whole of the inner
cavity of the egg-shell. P.
and F. Sarasry, in describing
a Helix (Acavus Waltoni, Fig.
84) found im Ceylon, show that
the podocyst covers like a cap
the shell of the very large
embryo in which several coils
Ee have already developed. In this
Fic. $1.—Eimbryo of Helix (Acaeus) form also, in which the pedal
Pat, thie ane waite in apeially igh
ss agg ele) veloped, pleating movenents
4, upper, ¢’, lower tentacle. were perceived in that organ.
When it has reached its highest
development, two wide canals within the foot start from the vesicle,
one passing to the brain along the ventral side and the other running
THE FORMATION OF THE ORGANS—THE SHELL, 187
dorsally towards the viscera which are surrounded by a blood-sinus.
A provisional circulation thus exists side by side with the definitive
circulation.
‘Towards the end of embryonic life, the pedal vesicle decreases in
sive, It remains at first as an appendage to the foot, but this vestige
also disappears, being absorbed. The foot thus assumes its final
shape. A median invagination, which only appears at a late stage
on the foot near the mouth and lengthens out posteriorly into a tube,
represents the rudiment of the pedal gland (For).
Apart from the development of these embryonic organs which are
here specially large, the further development of the embryo resembles
that of other Gastropods and especially of the aquatic Pulmonata.
This also applies to the shell where this is not vestigial and internal
as in many terrestrial Gastropods. Where there is an internal shell,
in Limar and Arion, the shell-gland becomes disconnected from
the ectoderm, as already explained. The shell remains internal, being
hidden beneath the mantle, and is a vestigial structure. In Arion it
eonsists merely of a number of disconnected calcareous granules.
Tt is « striking fact that, in Clausilia, according to Gecunsaur, the shell
algo ab first lies internally enclosed in the epithelium of the shell-gland,
Only when this latter, as well as the mantle-tissue above it, disappears, does
this internal shell become external, develop and become coiled. As far us
we know, this somewhat inexplicable observation of GuGessaun has not been
corroharated.* We feel inclined to explain the phenomenon described on
the belief that there is retained a small aperture over the shell as it lies
within the shell-gland, this gland flattening out at an unusually late stage.
6. The Formation of the Organs.
A. The Shell.
We have already, in treating of the external shape of the body,
repeatedly alluded to that of the shell, so that only a few further
remarks need be added. The shell arises from the shell-gland, and,
when the latter has flattened out, appears cap-like, At first, there-
fore, there is great resemblance in this point to the Lamellibranchs.
Here also a shell-integument forms first, beneath which the calcareous
substance is deposited later. The further Processes are altogether like
‘those in the Lamellibranchs as given more in detail on p. 60, The
add ) has since confirmed Grounpavr's observations that
she thal a Seely and later opens out again both in Clausilta
188 GASTROPODA.
unequal growth which leads to the coiling of the shell, has already
been described (p. 147) and so have the special shapes assumed by
the shell (¢.7., Pteropoda) and the partial or total loss of the shell in
the Heteropoda, Opisthobranchia, Pteropoda and Pulmonata,
Tt is a striking fact that a few specially low forms of Gastropoda
such as Haljotis and still more Patella and Fiseurella, ave dis-
tinguished by a reduction of the coils and the udoption of a flat eup-
shaped shell. In youth, the shell was, as in other
distinctly coiled. This can be seen particularly well in Pissurella
(Fig. 85 A-C), The margin of the shell is at first unbroken, but a
slit appears in it later lying above the slit which occurs in the
mantle of these forms (Fig. 85 A). The shell-slit is of special interest
because it is present in two of the oldest fossil Gastropods, ag.,
Pleurotomuria and Belleraphon, both of which are found in the
Cambrian.* The ontogeny of Fissurella would suggest that these
forms with slit shells have been derived from forms in which the
margin is not slit. In many forms the slit is retained as such
(Scisrurella, Emarginula, and fossil as well as recent Pleurotomariae),
in others, as the shell grows further ; the most posterior portion of
the slit becomes cut off by shelly matter from the rest of the slit and,
4s this continues to take place throughout life, we find in such forms
as Haliotis a series of consecutive apertures in the shell ; in other
cases, the slit becomes to a great extent closed by a shell-substance
of peculiar structure which is seen extending alung the length of the
whorls as the slit-band. In Fisswrefla, the margin, as it grows
further, is unbroken (Fig. 85 B). The reduction of the coiled part
of the shell and the fairly equal growth of the whole margin leads
finally to the slit taking up a central position near the apex of the
adult shell (Fig. 85 C). The shell of Fixsure//a has now passed from
a coiled form to that of a flattened cone; this change is due, as in
other Gastropods with similarly simple shells, to the manner of life
and, as ontogeny shows, must be regarded as a phenomenon of
degeneration. The symmetry of the shell is thus of a secondary
character.
B. The Nervous System.
The nervous system usually arises by delamination (Fig. 88, eg,
pl, p, p- 194), but it cannot be doubted that, according to recent
*A description of the development of the Gastro ot the different
geological epochs has been given by Koren (No, 56). See also Zrrren’'s
Palacontologie,
THE FORMATION OF THE ORGANS—THE NERVOUS SYSTEM. 189°
researches, the cerebral ganglion or part of it, is, in certain cases,
formed by an invagination of the ectoderm. So far as is as yet
known, the cerebral ganglion alone has such an origin; all the other
ganglia arise as ectodermal thickenings which later split off from
this germ-layer.
An acourate knowledge of the structure of the nervous system of the adult
is very desirable as a help to understanding the processes of development,
especially as some confusion prevails as to the naming of the different parts
of that system, one and the same ganglion sometimes bearing several different
85.—A-C, three stages in the development of Misswrella showing the changes in
isfiet BoorAN), ‘The anitoal’ as depicted in'O, has very nearly attained
shell
adult form. f, part of the foot ;'ma, mantle; ms, mantle-slit; «, shell; sn,
sont; ap, apex of the shell; ss, shell-oleft ; ¢, tentacles.
names, while, on the other hand, different ganglia receive similar names,
‘We shail therefore describe side by side some of the principal types of nervous
system found in the Gastropoda (Fig. 86 4-C).
‘The nervous system of the Gastropoda consists of the two cerebral ganglia,
whieh are connected by the supra-oesophageal cerebral commissure (4-C, cg).
Below the oesophagus, and connected with the cerebral ganglia by connectives,
lie the pedal ganglia (peg), which innervate the foot and are joined together
by a commissure. In this way a ring corresponding to the oesophageal ring
of the Annelida and Arthropoda is formed. The resemblance ceases when we
a
190 GASTROPODA,
come to the other constituent parts of the nervous system. A large nerve
runs back from the cerebral ganglia on each side, swelling to form two lateral
ganglia, the pleural ganglia (A-C, ply). ‘These are connected with the pedal
ganglia by the pleuro-pedal connectives. From the pleural ganglia, again,
two lateral strands ran back and end in the one or two connected abdominal
ganglia (Fig. 85 B, aby). Another lateral ganglion is formed in each of these
lateral strands which are known as the pleuro-visceral commissures, These
two last ganglia may be called the visceral ganglia (B and C, vg). In the
Prosobranchia, the pleuro-visceral commissures undergo displacement in
consequence of the twisting of the body already described (cf, p. 145 and
go
spp)
Fig, $6.—A-C, Diagrams of the nervous system of a Prosobranch (4), an Opisthobranch
(B), anda Pulmonate (€}. aby, abdominal ganglia; 4g, buccal ganglia; og, cerebral
gunglin; d, alimentary cival diagrammatically represented a8 a etraight tbe 5 pap
pedal ganglia; pig, plewral ganglia; sog, sub-, and spg, supra-intestinal ganglion ;
vy, visceral ganglia.
Fig. 60), the right commissure coming to lie above and the left commissure
below the intestine (Fig. 86 A). The original right visceral ganglion is thus
displaced to the left side and becomes the supra-intestinal ganglion (spg),
while the original left visceral ganglion now lies on the right side and is
known as the sub-intestinal ganglion (sy). The abdominal ganglia (abg),
in consequence of the twisting, come to lie dorsally to the intestine. In
this way arises the crossing of the pleuro-visceral commissures (chiastoneury)
characteristic of the Prosobranchia.
Tn the Pulmonata, the commissures are, as a rule, decidedly shorter than
in the other divisions, and the whole of the nervous system appears con-
centrated round the oesophagus (Fig. 86 C).
THE FORMATION OF THE ORGANS—THE NERVOUS SYSTEM. 191
The cerebral ganglia might at once be referred back to the apical
plate of the Trachophore, were it not for the fact, about which authors
seem to be fairly unanimous, that the ganglia here appear in the
form of two distinet thickenings of the ectoderm (Fig. 68, cy) which
only unite later by the formation of the cerebral commissure. P.
Sanasry, indeed (No, 101), has stated for Byfhinia, that the two
eetodermal thickenings at first are connected by a median ectodermal
growth, and thus (in their origin at any rate) suggest a common
rudiment, but this method of formation, which in itself is very
probable, has been directly denied, not only for Bythinia but for the
related form Paludina (v. ERLANGER, Nos. 27 and 28). The two
thickenings belong to the velar area, lying laterally in it in front of
the mouth. Even if the cerebral ganglion forms with the help of
au invagination, its rudiment is paired, In the Pulmonata, in which
this method of formation of the braia is best known, there are at
first the two ectodermal thickenings which here also yield the
principal mass of the cerebral ganglia in the usual way. Then,
when these are already partly detached from the ectoderm, a
depression of the ectoderm occurs at the lower edge of the posterior
tentacles; this becomes continually deeper, and thus forms a tube
(Sanasin’s cerebral tubes). According to P. and F. Sarasry, in
Helix (Acavus) Walton/, there wre two such cerebral tubes on each
side (Fig. 87 A, ct) while, in Limax, only one is found on each
side (H=ncxman, No. 42; F. Scumrpt, No. 110). The blind ends of
the cerebral tubes become applied to the rudiments of the cerebral
ganglia which have become further differentiated (Fig. 87 A, ct, cg),
and fuse with these to form that part of the brain which is known as
the accessory lobe (Fig. 87 B, ct). They then become abstricted
from the superficial epithelium. Their lumina can still be recognised
as fissures (Fig. 87 B), but these soon entirely disappear, the forma-
tion of the brain being thus practically completed. A differentiation
of the principal part of the brain into ganglionic cells and fibrous
tissue had already taken place.
_ SaRAstn’s observations with regard to the cerebral tubes, which were on the
whole confirmed by the researches of F. Scumrpt and Hexcamax, afford an
‘explanation of the apparent contradiction involved in the two views of the
origin of the cerebral ganglia, which were derived by one author by invagina-
‘tion, and by another in the same or related forms by delamination, Both
these views are founded on fact, each being observed at a different stage of
i Tn this respect, those forms in which the brain arises as two
depressions of the velar aroa, us is the case, according to For, in the Pteropoda,
‘require more careful investigation. The two invaginations are no doubt
i
192 GASTROPODA.
present, as we gather from Fou's description, but}the question arises whether
they yield only a part or the whole of the cerebral ganglion, From what we
as yet know, the latter view is the more probable, and is further rendered
possible by the fact that in a Prosobranch (Vermetus) also, the whole of the
cerebral ganglion originates from two invaginations (Sacensky), These first
appear on the velar area as two thickened plates which then sink inwards,
Fic, 87.—A and B, transverse sections through two embryos of Helix
Waltoni, at different stages (diagrammatic after P. and F. SARAsty).
doraal, ia, the ventral ope the ‘section is same, con
jet, corel (in B, as the ac lobes) j
am Seda gland ; Us, body-cavity ;'mes, pay sat? Ba
‘the salivary ducts); s/, buccal mass (in A,
tentacles.
The two tubes that arise in this way unite to form the brain and become
detached from the superficial ectoderm. The cerebral ganglia were seen to
form in exactly the same way in Dentalium (p. 93). Tt would in any ease be
interesting to learn in what way this condition may be reconciled with that
described for the Pulmonata. The rise of the brain through delamination,
ee
THE FORMATION OF THE ORGANS—THE NERVOUS SYSTEM. 193
which was observed in various Prosobranchs (Sanasix, Woursox, Happox,
MeMorrtcs, vy. Extaxcen, etc.), in Heteropoda (For) and perhaps also in
Opisthobranchs (Rar Lanxesrer) appears in any case to be the more usual.
‘The pedal ganglia arise laterally or rather on the under surface of
the foot, near the otocysts, the positionof which has already been
described more than once (Fig. 88 B, p). These ganglia at first are
not connected with each other nor with any other ganglia. The
commissures and connectives* are secondary structures, év., they
arise only after the detachment of the ganglia from the ectoderm as
outgrowths of the ganglia, « point on which the statements of all
observers agree. Where, as in the Pulmonates, the ganglia lie close
together, the distinct ganglia, in the eourse of growth become
connected at an enrly period.
Besides the original commissure connecting the pedal ganglia in the
Palmonates, second smaller commissure appears lying more posteriorly.
Since this second commissure is also present in adults, it was thought that
it might belong to a second pair of ganglia, but this view is not supported
by ontogeny, as each of the two ganglia first appear as distinct structures,
the apparent division in them arising only Secondarily (F. Senmipt), [This
second commissure appears to be specially developed in the Opisthobranchs,
where it is known as the parapedal commissure.)
The commissures and connectives, so far as their origin has been traced,
arise by the growing out of peripheral parts of the gunglia, and the same
origin has been assumed for the peripheral nerves (SaLENskY, Hexcaumax,
y. Earanoen, F. Souampr, etc.). P. Sanasis, indeed, as above stated, main-
tained that the two halves of the cerebral ganglion separated as one connected
organ from the ectoderm, and Rast assumed, as we saw (p. 181), that they
arose from 4 common rudiment, the apical plate. It is therefore in any case
probable that the cerebral commissure may have arisen from the middle part
of the common ectodermal thickening. Such an origin for the commissures
snd the cormectives is on the whole very probable, but is not supported by
the observations so far made, indeed, in Bythinia, investigated by Sanasry,
the common origin of the two cerebral thickenings has been denied (v.
Extaxcen, No. 28),
‘The buceal ganglia, ax was first shown by Sarasin and as has been
confirmed by subsequent investigators, arise as cell-growths of the
stomodacum, The wall of the stomodaeum becomes thickened, and
while the cells lying on the inner side retain the eylindrical shape, a
number of smaller cells appear on the outer side (Fig. 88 B, dg).
These become differentiated into two swellings which lie near the
~ Lacaze-Dvrnizns and Srenoet (No, 122), we distinguish the
strands connect the ganglia of one and the same side as connectives
0 ‘transverse strands which connect the right and left halves of a pair
these latter being commissures.
o
—
194 GASTROPODA.
stomodacum and the radular sac and form the rudiments of the
buccal ganglia.
The formation of these ganglia recalls to some extent that of the frontal
ganglion in the Insecta, which also arises from the stomodaeum (vol. iii.,
Pp. 328).
The pleural ganglia form, in Paludina and Bythinia, as two lateral
ectodermal thickenings lying somewhat ventrally behind the velum
Fra, 88,—Transverse sections through the anterior region of two embryos of Paludina
at the stage represented in Fig. 59, A and H, p. 139 (after v. ERLanagn). 6g, buccal
ganglia; a7, cerebral ganglia : wes, mesodermal tissue ; os, oesophagus ; uf, otocyst ;
p. pedal ganglia ; j/, pleural ganglion; r, radular sac; ¢, rurliment of ‘tentacle;
us, primitive sinus: r. velum.
(Fig. 88 A, pl), and the two visceral ganglia also arise ventrally
and laterally, though farther back than the pleural ganglia (v.
Ervancer). They lie near the edge of the mantle, and near the
‘THE FORMATION OF THE ORGANS—THE NERVOUS SYSTEM. 195
constriction which divides the visceral mass from the cephalic and
pedal parts of the body, laterally and somewhat ventrally to the
alimentary canal, It is important to note that these two pairs of
ganglia, on their first appearance, are quite symmetrical, and that
the asymmetry which is so characteristic of the Prosobranchia
appears in them only later as a consequence of unequal growth of
the different regions of the body, This leads to the right visceral
ganglion shifting first dorsally and then over the oesophagus, while
the left lies below this tube. In this way, these ganglia become
the supra- and sub-intestinal ganglia,
This process has already been theoretically examined, and illustrated by
Fig. 60 A-E, p. 144. Ontogenetically, the process is similar, but is less
distinct on account of the connecting strands of the visceral loop, which are
either wanting or difficult to make out, Indeed, the study of the develop-
ment of the nervous system is often rendered difficult by the fact that the
ectodermal rudiments are so indistinctly marked off from the mesoderm, as
may be gathered both from the text and the figures of older and more recent
authors. This difficulty no doubt gave rise to the view thnt the nervous
system in the Gastropoda was of mesodermal! origin, which was supported by
Bosuerzky's observations (No. 11).
The abdominal ganglion arises, in Paludina and Bythinéu, us an
unpaired ectodermal thickening on the floor of the mantle-cavity at
its posterior end, being found dorsally to the heart.
As already mentioned, all the ganglia are said to arise independently and
to become secondarily connected by commissures, Where, as in Bythinia,
the ganglia lie very near each other, they are, according to Emuaxcer, more
distinct in the embryo and only later shift nearer to one another. P.
Sans, in these very forms, derived the pedal and intestinal ganglia as well
as the abdominal ganglion from a common ventral ectodermal thickening,
‘and therefore was able to compare them to the ventral chain of ganglia of
the Annelida, whereas the prevailing opinion now is that only the pedal
génglia or rather the pedal strands (which in some Prosobranchia are pro-
vided with transverse connecting strands) can be considered as the true
of the ventral cords.
Even in the Pulmonata, in spite of the great concentration of the nervous
‘system peculiar to this division, the ganglia appear as distinct rudiments, and
‘only become connected later. We have repeatedly alluded to the conditions
in the Pulmonates, which have recently been very thoroughly examined by
A. Hexcumas and F. Scumrr, although we have dealt principally with the
Prosobranchia, in which the processes can be more easily understood on
account of the intervals between the ganglia being greater. The formative
processes fn the Pulmonata agree on the whole with those in the Proso-
branchia.
196 GASTROPODA.
C. The Sensory Organs.
The appearance of the tentacles as prominences on the velar area
has already been several times alluded to in connection with the
external form of the body (Figs. 54, 55, 59, 78, 79, ete.). They lie
immediately above the rudiment of the brain and, when this becomes
detached from the ectoderm, remain as large thickenings of the latter
(Fig. 88 B, t). In position, they correspond to the cephalic tentacles
of the Annelida, a correspondence which would be all the more strik-
ing if we could definitely homologise the rudiment of the cerebral
ganglion with the apical plate. The anterior and lower tentacles in
the terrestrial Pulmonates arise somewhat later in the closest prox-
imity to the bases of the posterior tentacles (ophthalmophores). In
the terrestrial Pulmonates, the cephalic region in which these organs
originate, and where also the cephalic invagination occurs, has been
called the sensory plate. P. and F. Sarasin found here in Helis
(Acavus) a number of sinall bulb-like specialisations of the ectoderm
(Fig. 39 4, s, and B) which, by the similarity of their structure to
the lateral line organs of the Vertebrata, were shown to be sensory
organs. On each side of the section of the embryo given in Fig. 89
A, two of these organs can be seen in a depression, and it is possible
that the cerebral tubes which arise at these points originate from
them. These lateral organs are found in other parts of the body as
well, and have also been met with in a somewhat similar form in
adult Gastropods. It is, however, probable that the organs now
under consideration do not persist and we must therefore regard
them as temporary larval organs.
A similar significance is ascribed by P. and FP. Sanasty to the cerebral
tubes described above (p. 191) as taking part in the formation of the brain,
these being also considered ax vanishing sensory organs. They may have
actually functioned in the ancestors of the Gastropoda, as is assumed to be
the case with the conjectural olfactory organ of the Annelida. They now
give rise to part of the brain, just as in the Annelida, where the origin of the
brain is traced to the pre-oral sensory organs (KLEINENBERG, Vol. i., p. 288).
The eyes develop in a very simple way. They first appear almost
simultaneously with the rudiments of the tentacles, at the ventral
edge of which a depression takes place. This deepens to form a
vesicle which finally becomes detached from the ectoderm and is then
found below the integument. Each optic vesicle frequently, as in
Paludina, lies on a prominence at the base of the tentacle. Where
the eyes are found on the tentacles themselves, as in the posterior
THE FORMATION OF THE ORGANS——THE SENSORY ORGANS. 197
tentacles of terrestrial Pulmonates, they are raised up as the tentacle
grows. The eyes are already visible when the tentacles are first
indieated (For).
The next step in the development of the eyes is the deposition of
pigment in the proximal part of the optic vesicle. The cells of this
region increase and yield the retina, while the distal part forms the
cornea. Two structures arise within the optic vesicle as secretions
of the cells; these are the lens and the vitreous body which are
at first homogeneous and strongly refractive. In the latter, delicate
fibres appear which run from the retina to the lens (v. ERLANGER,
No. 27).
im ‘49, —A, Transverse section through a young embryo of Melix (tcavusy Wallond,
j. one of the Interal organs (.1, s) highly magnified (after P. and FP. Sanastn). cet,
‘ectoderm ; mes, mesoderm ; #, lateral organs of the sensory plate ; al, posterior part
of the bucea! mass, including transverse section of the oesophagus the mdalar
mac; mm, sensory cells ; «¢, supporting cells.
According to Sarensxy, the formation of the eyes, in Vermetus, is connected
with the invagination of the cerebral ganglion. They appoar as rounded
thickenings at the edge of the plate which later sinks in to form the brain ;
these xeon become hollow and form vesicles similar to those described above
‘as rudiments of the eyes, and then shift inwards in connection with the
invagination. Only somewhat late, when the lens has already been secreted,
do they lose their connection with the cerebral invagination. The further
development of the eyes apparently takes the course described above. This
198 GASTROPODA.
method of formation of the eyes recalls the condition in those Gastropods in
which they lie upon the brain (many Opisthobranchs), The eyes of Viermwtus,
however, so far as we know, lie externally at the bases of the tentacles,
B
Fro, 0,—Byex—A, Patella rede; B, Trvchnn onus Twrtes creniferwe :
Murex betadacias (after HILcERL by, connective tixsae ectodern: ; gf, vitreonm
body; d, lems: a, optic nerve; p, pigment ; ¢, retina; sf, rods
The ontogeny of the Gastropod eye is of great interest in so far as
it may, at some of its stages, be compared with the adult condition
of the eye in various forms. Pafella, for instance, has eyes which are
placed in the usual position, but which are mere pit-like depressions
of the surface (Fig. 90 4). In Haliutis, Trochme, ote., the pit is deeper
THE FORMATION OF THE ORGANS—THE SENSORY ORGANS. 199
and becomes a vesicle which, however, remains open (Fig. 90 2). Its
tumen is filled with a strongly refractive gelatinous mass (g/) known
as the vitreous body. In other forms the vesicle has closed (()
and, finally, the higher form of Gustropod eye (10) provided with
& so-called lens and vitreous body is developed (FRaisse, No. 34;
Hiner, No. 43; Pauseneen, No. 85). [In most Diotocardia the
optic vesicle is open, but in the specialised Helicénidae and
Neritidue (the Gymnopoda of Fisenen) and in the Turbinidae it is
‘losed as in all the Monotocardia.)
~ The first-named Prosobranchs are held on other yrounds to be
primitive forms, and the simple stracture of the eye seems therefore
probably a primitive condition. If this supposition is correct, we
‘should here see with special clearness the gradual development of
the optic organ up to its present level.
According to Cannréne (No. 22), in cases where the eyes are regenerated,
their formation takes place in the same way as when they arise ontogene-
Hieally, The ectodermal epithelium is thus at a later time also capable of
giving rise to the sensory organs.
The otocysts, which are specially distinct in the larva, and the
origin of which has already been alluded to several times (Figs. 55,
: 79), appear as depressions of the ectoderm on either side
rudiment, near the pedal yanglion, with which, however,
t come into any closer relation as they are innervated from
sin (LAcazE-Durniers). When cut off from the ectoderm,
is are still formed of long cylindrical cells which flatten
for a time, the anterior and ventral part of the vesicles
thick. From this part of the wall, the otolith or otoliths
) are secreted ; these structures become detached from the
nd rest upon the sensory hairs which have arisen on the
‘Spengel’s (olfactory) organ (osphradium) only develops at a later
stage (Palutina). It arises as an ectodermal thickening composed
of several layers of cells. Where, as in Palulina, pits are found in
the organ, these are caused by depressions in the thickened ectoderm
& ‘Extaxoun).
condition of this organ, which is found in many Gastropods,
‘a similar way. The organ was originally paired and lay near
“45 may still be the case in Zygobranchiate Diotocardia. Where
jn the Monotocardia and the Euthyneura, this is in all cases
connected with the asymmetry caused by the torsion of the visceral mass.
i
rises
200 GASTROPODA.
D. The Pedal Glands.
In the larvae of various Gastropods, e.g., Nassa (Figs. 61 D and F, 68),
Vermetus, Murex, Firoloida (Fig. 65), etc., a deep tubular or aac-like ectodermal
depression has been described in the foot; this shows great agreement in
position with the pedal gland found by KowaLevsxy in the embryos of Chiton.
Such a rudiment is perhaps also present in Denfalium. In Nassa, this gland
forms a rather long tube, and in Murer it has a similar form (BoBRETZKY,
No. 11); in Firoloida, it is said to be much shorter and bilobed (Fo, No. 31,
Fig. 65, fd.). Savensky describes, in Vermetus, the formation of two ectodermal
invaginations in the foot, the one lying at the anterior and the other at the
posterior end. The canals lengthen inwards and fork, eo as to yield the
glandular portion. Various glands are known in adult Gastropods also lying
one behind the other in the sole of the foot (CanRrzreE, No. 21). The connec-
tion of the rudiments we have just described with these glands does not as
yet appear to be clearly demonstrated. It is well known that various
glands also occur at the anterior end and in the sole of the foot in Lamelli-
branchs which have been homologised with the anterior and posterior pedal
glands of the Prosobranchia (Barnots, No. 8), but whether such a homology is
correct still seems doubtful.*
E. The Alimentary Canal.
The Stomodaeum first appears as an ectodermal depression in
which can soon be recognised a ventral outgrowth, the radular sac
(Figs. 53, p. 127, 78, p. 177 and other figures). This sac sometimes
appears even before the stomodaeum is completely invaginated and
consequently lies near the aperture of invagination, as in Helix
igs, ST and 82. p. 184). When the radular sac lengthens, it
ves dorso-ventral flattening. — Its lateral margins then bend
upw rd so that it assumes the form of a channel. the dorsally directed
vity of which is filled with a mass of connective tissue. The wall
of the chanel is formed of the upper and the lower epithelium, the
latter taking the principal part in the formation of the radula.
The first indication of this organ is found early in the form of a thin
cuticle in the radular sac. The formation of the radula, an organ
which has been studied in the adult by R6ssLeR (No. 95), and RUcKER
(No. 86), and others. takes place in’ the following way: The teeth
theniselves are seereted by the cells which lie ventrally at the blind
end. while the basal membnare upen which the teeth are bore is
2 enormous development in the Pulmocata:
ere the giand takes the form of a
siong the greater part of the foot.
ad and foot. as in Heiss, or 21
_—Ep.”
THE FORMATION OF THE ORGANS—THE ALIMENTARY CANAL, 201
yielded by the lower epitheliam (Fig. 91 4). The large groups of
tooth-forming vells (odontoblasts) form a kind of cushion or bed upon
which the teeth are modelled (Fig. 91 4 and B). In the shape of
this cushion, the future form of the tooth is already shown. In the
i ‘ia and the Pulmonata, a special differentiation occurs,
ouly « few (four to five as seen in longitudinal section) * very large cells
undertaking the formation of one tooth (Fig. 91 B, od); the most
waterior of these large cells is said to yield the part of the basal
membrane that underlies the tooth now in course of formation.
OT
i ead we a
Fie, 91, —A and £8, Longitudinal sections through the radular Oct
frudgaria (a) and Helge memorutis (H) (after Réssuxu), bn, basal membrane ;-od,
" 3% @» “pper epithelium ; arm, subsratular membrane; m, ep lower
Hina; 2, teoth,
The tooth thus produced fuses with the basal membrane and with
the prolongation of the basal part of the last tooth (Fig. 91 2),
When « tooth is thus completed, this cell-group undertakes the
rof gach tooth, the cells being arranged in two parallel sories, so that, in a
section, like that shown in Fig. 91, only one raw is seen at a time,
Te that the three most posterior pairs of these odontoblasts secrete
the main body and hook of the tooth, the next transverse pair secreting the
base, while the most anterior pair secretes the sub-radular membrane.—Ep.}
202 GASTROPODA.
formation of the next tooth of the same longitudinal row. The
number of teeth in a transverse row corresponds to the number of
groups of odontoblasts. The formation of the radula is, however,
not altogether completed by the processes just described, for the
upper epithelium yields a viscid fluid secretion which forms an
enamel-covering to the teeth. The gradual shifting forward of the
newly-formed teeth to replace those which are continually being
worn away in front, is brought about to a great extent by the
growth of the surrounding tissues, and is no doubt also caused by
the action of the muscles at the anterior part of the odontophore
(RossLeR).
The radula appears to form in other Molluscs that are provided
with it (Cephalopoda, Fig. 91 4, and Amphineura) in just the
same way as in Gastropods ; it will not, therefore, be necessary to
describe it in detail again.
The salivary glands arise somewhat late us diverticula of that
part of the stomodacum which lies in front of the radular sac.
The enteron, in various Gastropods, arises to a certain extent in
a different way, as the accumulation of yolk or of a secondary
nutritive mass at various points of the gut frequently retards its
development and may even, where the mass is very voluminous,
strongly influence the manner of formation of the intestinal canal.
In many cases, however, the formation of the enteron takes a very
simple course, the invaginated entoderm-vesicle increasing in size by
the continuous division of its cells, fusing anteriorly with the stomo-
daeum and growing ont posteriorly into a conical terminal section
which becomes connected with the ectoderm to form the anus. It
has already been explained that the posterior section of the enteron
may at first run straight back, but may later bend forward to the
right, and that this is connected with the acquisition of asymmetry.
The coils made by this section of the gut as it lengthens are uot of
essential importance and need not therefore be specially described.
There are, however, other important alterations brought about by the
deposition of nutritive masses in the enteron. ‘This process of deposi-
tion takes place in a very simple manner in Palwiina (BUTSCHLI).
The ventral part of the entoderm here becomes even at early stages
especially large through the increase in size of the cells and the
deposit in them of drops of secondary yolk (Figs. 57, 58 and 59, p.
137, ete.). This thickening of the wall of the enteron is evidently
due to the absorption of the surrounding albumen ; this albumen being
received especially into the ventral entoderm and deposited there. At
THE FORMATION OF THE ORGANS—THE ALIMENTARY CANAL, 203
a later stage, the whole of the sac-like anterior part of the enteron is
affected by these deposits, which, however, are always greatest on
the ventral side, The dorsal and anterior part, with which the
becomes connected, is marked off into a sac-like stomach,
while the part that lies ventrally and more posteriorly, and which
contains by far the largest amount of deutolecithal constitnents,
yields the liver. The latter, originally spherical, soon becomes lobate,
Ley describes the gradual development which commences with a
few large lobes; then, by subdivision of these, an increasing number
of small ones arise, until, when the embryo is ready for birth,
continued division has led to the formation of numerous long
follicles.
It has been observed in most cases that those parts of the entoderm
that are laden with nutritive substance pass over into the liver or
else wre connected with its formation; it appears donbtful to us
whether this is invariably the rule, since these parts vary greatly in
the position they occupy in the enteron, as will be shown later.
‘The accumulation of nutritive material in the ventral entoderm is
still more striking in the Heteropoda than it is in Paludina. Fou,
in connection with the Hetcropoda, speaks of a ventral nutritive sac
formed of immense, greatly swollen cells which is abstricted from the
stomach so as to become the rudiment of the liver, its glandular
eharacter being soon proved by the development of several lobes. A
ventral nutritive sac is also found in the later stages of Limnuea ;
but it is expressly stated that this does not take part in the forma-
tion of the liver, but that the latter arises quite independently of
it as two small caeca which grow out at the end of the stomach
(Wotrsox, No. 131).
Ik is impossible to ascertain the correctness of the various statements made
‘8s to the manner of formation of the liver. These statements differ so greatly,
and in the present state of our knowledge are so difficult to compare with one
another, that we are justified in assuming that more careful research will
greatly modify them. This is all the more probable as it is evident that the
Processes under consideration are difficult to interpret.
In the Pteropoda also, the liver is said to arise as x finger-shaped
outgrowth of the ventral wall at the posterior end of the stomach,
‘near which a second outgrowth soon appears (Fo), In the Pteropod
Jarva, the nutritive material is stored up in the cells forming the
walls of two suc-like outgrowths of the stomach, which are at times
from the latter by stalks. These nutritive sacs, one of
Which is usually larger than the other, differ slightly in position in
a
204 GASTROPODA.
the different forms, but generally appgar to open into the stomach at
its postero-ventral end, so that # relation between the nutritive sacs
and the liver seems probable. As the liver continues to develop, the
sacs decrease in size.
The two entodermal stomach-diverticula of different sizes are, as
already mentioned, also found in the larvae of the Opisthobranchia
(Fig. 72. di, p. 162). They are here said to belong rather to the
dorsal and anterior part of the stomach (RHo, No. 93; FiscHEk,
No. 30). According to FiscHer, they become transformed direct
into the liver, forming the outgrowths which enter the dorsal
papillae (cerata). The
left diverticulum yields
the principal lobe of
the liver, while the
right, in the Nudi-
branchia, is of small
size.
To the Pulmonata, the
position of the nutritive
masses is somewhat
different. It has been
asserted that, in them,
the principal mass of
large cells filled with
albumen lies at the
dorsal side. The large
dimensions attained by
this part of the ento-
derm is evident from
Figs. 18-83, pp. 197-185,
depicting the embryos
of Planurhis, Helis and
Limax. The entoderm-
Fic. 02,0 amd B. embryos of Byt
at liflerent stages catter ¥.
. verebral ganglion ; F. foot;
cells in other parts,
abe ot however, remain small,
especially ventrally and
posteriorly, and these
parts give rise to the
posterior portion of the
intestine which takes the course already deseribed. The small-
celled portion of the entoderm spreads out further at a later
THE FORMATION OF THE ORGANS—THE ALIMENTARY CANAL. 205
stage, and the albuminiferous cells seem to be pressed more to
the left (Fig. 78).
The complexes of nutritive cells are said to be dorsal in position in
the land Pulmonates also, and the direct rise of the liver from them
has been described (Jourparx, No. 49). It appears, however, from
the figures of Pulmonates, especially of the land-form before us, that
the large-celled mass extends well to the ventral side of the stomach,
so that there is here perhaps after all a near approach to the con-
93.—A-C, sagittal sections of the embryos of Fuse at various stages (after
RMR Gy ae eclks 7, tock di, cephalic vesicles 7, livers mm, month} ma,
‘eniterou ; wg, stominch ;'4, shell; a, shell-gland ; ed, stoniodacum,
ditions described above. The fact that the intestine, the stomach
and the liver are not clearly marked, makes it diftioult to ascertain
the exact relation of these parts which is further complicated by
a frequent displacement of these organs. In Bythinia, the intestine
arises from the posterior part of the conical enteron, while the larger
part gives rise to the liver and stomach (P. Sarasin, v. ExuancEx).
"Phe liver appears in the form of a very wide anterior and a smaller
=
206. GASTROPODA,
posterior outgrowth (Fig. 92, vl, Ai), while the stomach (my) arises
from a small dorsal part of the enteron lying between these two,
Into the stomach open the oesophagus, the intestine and the two
hepatic sacs.
In the eases so far considered, the enteron bas at first 1 sac-like
form ; this, however, soon becomes differentiated by the concentration
of the nutritive yolk or by the absorption of albumen by the cells in
one part of the enteron. In other cases, however, the accumulation
of food-yolk in the entoderm is so great that the sac-like rudiment of
the enteron is not able to develop at once. In Fusus, for instance,
according to Bopretzky, at a time when the oesophagus, the shell-
gland and the mesoderm are already well developed, the entoderm
consists of only a few large cells which are to a great extent filled
= sub-velar cells, 3
with yolk, having a small protoplasmic portion directed towards the
mouth (Fig. 93 4). At this point, the division of the macromeres
gives rise to new entoderm-cells which are much smaller and soon
rise up from the macromeres, thus forming the rudiment of the
midgut, especially that of the stomach, which then, through the
formation of a posterior conical process, gives rise to the intestine
(Fig. 93 A and B, m7), The increase in number of the entoderm-
cells is continued at the expense of the food-yolk, which is now
pressed further back. While, ventrally, the stomach becomes more
distinctly marked off (Fig. 62, p. 151 and 93, mg), the recently
developed dorsal parts of the entoderm become filled with deuto-
lecithal spherules and thus have a glassy appearance like the
THE FORMATION OF THE ORGANS—THE ALIMENTARY CANAL. 207
albuminous cells of other Gustropods described above. The yolk-
tmass, which is still very large, limits directly the lumen of the
entoderm-vesicle (Figs, 92 and 93), This latter is already found to
be partly filled with disintegrated yolk-substance (Fig. 62 B), this
being taken up by the large entoderm-cells, which, according to
Bowagerzky, represent the rudiment of the liver (Figs. 62, 93 and
94,4). The large-celled “hepatic vesicle '’ may be said to form the
Fic, 95.—A-), lougitudinal sections thre
ey from BaLroun’
ra it of foot; Ay, entoderm ; in,
‘@, shell-gland ; #f, lumen of the enteron.
gh embryos of Noses ssilabilts ob diferent
xt-book). bp, blastopore ; ep, ectoderm ;
epithelium of the enteron ;'m, mesoderm ;
dorsal and posterior part of the entoderi-sac, if the rudiment of the
intestine is left out of consideration (Figs. 93 and 94, md). It
ocoupies the left side of the body while the food-yolk is pressed more
to the right. From the sections given in Figs. 93 and 94 « good
‘idea of the relative positions of these parts and of the stomach may
be gained. The food-yolk still directly limits the lumen of the
jntestine, but is gradually absorbed as development advances,
208 GASTROPODA.
A still further specialisation of the enteron along the lines seen in
Fusns is found in the egy of Naxa which is still more richly supplied
with yolk. The formation of the germ-layers in this egg has already
been described (p. 116). The entoderm is found here as a slightly
developed single layer of cells on the ventral side of the embryo.
The stomach and rudiment of the intestine appear when the massive
food-yolk which, at first, presses closely upon the entoderm, separates
from it (Fig. 95 Cand D). Owing to this origin of the enteron, its
lumen is here also directly bounded on one side by the yolk, which,
even at a later time, is very extensive (Fig. 61 D and £, p. 150),
and fills the whole of the posterior part of the body. The intestine
still appears open towards the yolk-mass (Fig. 63, p. 152) and, in its
further development, no doubt follows the same course as that of
Faana,
The nutritive substance is, as we have seen, stored up in various
parts of the entoderm, and seems frequently to influence the develop-
ment of the liver. It is inherently probable that the liver originates
from definite parts of the entoderm, always appearing in the same
region of the enteron, but this process nay be modified through the
various ways in which the nutritive mass is deposited. From the
different conditions found, we scem to be able to conclude with some
certainty that the whole of the anterior part of the enteron was
originally specially utilised for the storing of the nutritive material.
The anux forms in most cases through the direct fusion of the
entodermal intestine with the surface of the body, though some
authors (WoLFson, No. 131; P. Sarasin, No. 101; Jourpain, No.
49, ete.) speak of the development of a proctodaeum. As the latter
is said to occur in other Molluses, ¢.7., Chiton, Teredo, Entovalva, and
as it is found in the Annelid larvae, the structure of which is
remarkably similar to that of the forms we are now considering, its
presence cannot be regarded as @ priori improbable. In by far the
greater number of Molluscan embryos, however, a proctodaeum is not
developed.
F. The Gills.
The gills have been found to develop in some Prosobranchia as
consecutive prominences on the ectoderm, These prominences corre
spond to single branchial leaflets. Mesoderm-cells enter into them
and form a septum in each leaflet. The gill commonly seems to
appear only after the mantle-cavity has formed, arising within the
latter (Figs. 61, A, p. 150. and 99 and 100, p. 214), but occasionally it
=
THR DIFFERENTIATION OF THE MESODERM-RUDIMENT, ETC. 209
may be found at an earlier period on the surface of the body, as in
Faseiolavia (Osporn, No. 81).
Bipectinate plumose gills, a pair of which is found in Fisswredia and Hadiotis,
are considered as the most primitive, and we may assume that the single
monopectinate gill of the Monotocardia is to be derived from these, one of
the gills (originally the left) disappearing through the shifting of the pallial
complex while the other (originally the right), by fusion with the inner wall”
of the mantle-cavity, loses one of the rows of its leaflets.” So little attention
has aa yet beon bestowed on the development of the gills in the Gastropoda
that itis impossible to confirm by their ontogeny this view which in any
case ix very probable. ‘The derivation of the single gill from the double gill is
also plausible because the former is found not only in the most primitive
Gastropods, but also in the Amphineura, the lowest Lamellibranchs and the
Cephalopoda, i.e., in all the principal divisions of the Mollusca,
G. The Differentiation of the Mesoderm-rudiment, the
Development of the Body-cavity, the Nephridial
and Circulatory Systems.
Apart from the primitive kidneys (pp. 136, 178) little has yet
been recorded of the formation of the mesodermal organs. We have
already shown that the mesoderm appears as « bilateral rudiment
whieh is soon found in the form of two cell-masses, comparable to
the mesoderm-bands of the Annelida, at the posterior end of the
body near the blastopore (Figs. 96, 48, 51, 52, 56). The distinctness
of these two cell-masses varies in the different forms; they may also
be considerably reduced in size at an early stage, single cells being
detached from them and becoming distributed in the primary body-
cavity, By the development of u cavity in each of these cell-masses,
right and left coelomic sacs are formed (Fig. 56 4 and C), in which
a somatic and a splanchnic layer can be distinguished. As a rule,
however, this process is not so simple as that described for Bythinia
by v. Exnancer. The detachment of the cells from the two masses
usually occurs very quickly, the two coelomic sacs being then much
more difficult to recognise. They represent, in the main, the rudi-
*{In Trochus, the septum, which separates the two sets of leaves of the
gill, is attached (except at the free end) to the mantle-wall along both
its margins; in this way one set of gill-leaves becomes enclosed in a small
cavity which only communicates with the general pallial cavity in front.
are much reduced in size as compared with the set which
t main mantle-cavity, and it is easy to see that a further stage
this process might result in a complete fusion of the septum with the
mantle-wall and thus cause 4 suppression of the one set of gill-leaves. There
‘is every reason to believe that the monopectinate gill nrose in this way—Ep.}
p
:
—
210 GASTROPODA.
ment of the pericardium ; the process is therefore very similar to
that described (p. 74) in connection with the Lamellibranchs. The
lumen of the saes is to be regarded, here ulso, as further development
shows, as the secondary body-cavity, while the definitive body-cavity
proceeds from the cleavage-cavity which contains numerous scattered
mesoderm-cells,
The whole mesoderm-rudimeut is not, as already mentioned, used
up in the formation of the coelomic sacs; ovcasionally even vom-
pact masses of mesoderm remain which have been distinguished as
Pie. 96.—Dingranimatic represeutations of Rev! embryos of Bythinia tentaculata »
A ital section; B and (*, from the right side (alter v. OER), a, anal
Fey 3 bd, Buco ¢, coelom; ent, entoderm ; m, mouth; mes, ‘
rudiment ; s¢, shell-gland ; ¢. ectodermal thickening, from which the tentacles and
the cerebral ganglion are produced ; 7, velum.
cephalic or trank-mesoderm and from which, by delamination, somatic
and splanchnic layers have been derived. According to this view,
the definitive body-cavity would arise at least to some extent in
the form of a coelom. This subject will be referred to again later
(p. 217). Tt is generally assumed that the definitive body-cayity
arises out of the primary body-cavity in which the cells detached
from the mesoderm-bands become distributed, yielding comneetive
tissue and musculature. With regard to the latter, the origin of
the columellar muscle has been somewhat more carefully examined :
THE DIFFERENTIATION OF THE MBSODERM-RUDIMENT, BTC. 211
it is found to arise by the concentration of mesoderm-cells at the
base of the foot.
The development of the mesoderm and of the parts connected with
it has recently been specially studied by v. Enuancer in Paludine
and Bythinia, As vy, EXtaNeeR found that these organs developed
here in the same way as in the Lamellibranchs, and, since the
investigwtions of other zoologists which were less comprehensive led
to less satisfactory results, we shall here follow principally the state-
ments of this author.
The two mesoderm-sacs, above mentioned, approach each other
and come to lie ventrally between the archenteron and the ectoderm,
Fro. 97.—Transverse section hyd the pericardial region of an embryo of Paludina
" at the stage depicted in Fig. 59 B, p (after v. ERLANGER). /, liver ; Lh,
i stomach ; wes, mesodermal tissue; mf, mantle-fold; mh, muntle-
envity 7 ty nt of detinitive, n’, of abortive kidney ; na, na’, rndiments of
efferent duets of the same; p, pericardium ; », shell.
where they fuse. Oceasionally, in later stages, a septum is retained
as an indication of the former partition-wall (Fig. 98 A, sp). In the
further course of development, the right half of the sue grows much
more vigorously than the left, and the whole sac extends dorsally to
the right side (Figs. 59 A, and 97). Differentiation now sets in, the
walls of the two later ventral angles of the sac becoming thickened
‘and subsequently forming distinct outgrowths (Fig. 97, » and n’).
These outgrowths, according to Ertanaer, are the rudiments of the
definitive kidneys which are consequently, like the pericardial sacs,
paired on their firstappearauce. The left rudiment soon disappears,
212 GASTROPODA.
while the right forms a sac (Fig. 101, ») and unites with the ecto-
derm to form the efferent duct. In Bythinia, the kiduey can at this
stage be recognised as a derivative of the posterior part of the peri-
cardial sac (Fig. 92 B,n). At. later stage, a process grows out
from its postero-ventral part and becomes connected with the ecto-
derm of the mantle-cavity, so that the lumen of the kidney now
communicates with the latter. In Palwdina, the formation of the
efferent renal duct (ureter) takes place from the mantle-cavity, which
at an earlier stage sank in on the right ventral side. The pallial
Fira, 98.—A, transverse section through the goo of an em
Paludina vivipara. at the stage Sg rahe in . 90. B, eee ets
mature Paludine embryo (after V. INGER). d, intestine ; =
, liver; Uy body-cavity ; m, stomach ; mes, mesodermal tissue
abortive kidney ; na, efferent duct of the former: ve, aperture Edy to
the dhe porter Pp, pericardium ; sy, pericardial Spee tee are he of
depression is prolonged in the direction of each of the kidney-rudi-
ments (Fig. 97, na and na’). The branch running towards the right
kidney is specially distinct, being longer than that ranning towards the
left rudiment ; the latter, indeed, has no permanent significance on
account of the degeneration of this left rudiment. The right branch
of the mantle-cavity, however, then fuses with the right kidney, sa
thus becomes its ureter (Figs, 59 4, and 98, na). an
=
THE DIFFERENTIATION OF THE MESODERM-RUDIMENT, ETC. 213
‘The ectodermal origin of the ureter can be recognised even at a lator stage
in its histological structure. The duct formed.as above has been distinguished,
‘a primary ureter, from the secondary ureter met with in the terrestrial
Pulmonates. In some of these latter, the primary ureter opens into the
pulmonary cavity in the way above described. In others, it is continued as
Pee eee aes ie AT naan Mes age this channel partly
closes and, becoming finally altogether detached from the wall
iad eaptiney aunt, yields the secondary uyeter which, in the most
oxtreme cases, such as Heli pomatia, runs alongside of the rectum and,
with it, ends near the respiratory aperture (vy. JamRrxc, No. 46; Bravx,
No. 14).
The origin of the ureter as w part of the pulmonary cavity which at first
is channel-like but closes to form a tube later, gathered with some certainty
from the study of comparative anatomy, is entirely confirmed by ontogeny
(Buavs, Beuwe, No, 4). The kidney, in the embryos of Heli pomatia, opens
es embryo of Palwlina wiviy (after v. Entanogn). /, liver; wn,
3% Velumn ; the rest of the lettering as in Fig. 100,
‘neur the primitive kiduey in a depression of the body which represents the
‘diment of the respiratory cavity. As this cavity deepens, the glandular
part of the kidney and the primary ureter become differentiated. At the
_ posterior part of the pulmonary cavity, the latter passes into o channel which
sums through the whole cavity and ends only at the respiratory aperture.
a broad and is distinguished by its high cylindrical
the rest of the respiratory cavity which is lined with
jum, The channel closes later, its edges bending together
and fusing from behind forward, and the secondary ureter thus formed now
ees Saeary tube which opens in the neighbourhood of the anus.
‘The secondary tireter is a new acquisition within the division of the Stylom-
‘ as Vv. Jaeninc has shown. It ocours only in the so-called
Ma. Ainong these, however, in one and the same genera, forms
possessing the secondary ureter and others exhibiting the much
a
214 GASTROPODA.
more primitive conditions found in the aquatic Pulmonates, as Brion has
shown. From this we may gather that the division of the Pulmonates into
Branchiopneusta and Nephropneusta is not justifiable. It has already been
shown (p. 182) that we cannot regard the pulmonary cavity us a transformed
ureter, but must consider it as the pallial cavity, corresponding to that in
other Gastropods,
The kidney now enlarges considerably and its walls become folded
(Fig. 98 8). At first only a few such folds are formed, and the renal
cavity is still spacious, but at a later stage the lamellae almost com-
pletely fill it. vy. ERpanGer points out that the complicated kidney
Fro, 100.—An almost mature
ryo of Paludina s Vivipere (after v. ERLANGER). a,
anus; «, auricle; am, eye; f, foot; g, sential yiaw efferent genital duct ; ky,
gill; 'm, mouth ; wel, intestine ; mg, stomach ; raptor ere edge of the
Thantle! 2, kidney ; na, ureter: o; aperture of the unter fut the mantleenti
e, reno-pericardial pore ; op, operculum ; ut, otocyst ; ium ; r, raduler
sac; ap aeetippietn #/, spines on the shell ; t tae ventricle,
of Paludina thus passes through a stage which is retained through-
out life by the very simple kidney of Haliotis. The opening of the
kidney into the efferent duct gradually approximates to the reno-
pericardial pore (Fig. 100). he latter is (with some exceptions)
known to be retained in the Gastropoda, so that the details given
under the Lamellibranchia (p. 74, ete.) as to the connection between
the coclom and the nephridia are applicable here.
Most of the Gastropoda possess only one kidney, but those
Prosobranchs that are provided with two auricles (Diotocardia, sueh
THE DIFFERENTIATION OF TH! MESODERM-RUDIMENT, ETC. 25
as Haliotia, Palella, Fisturella, Trockus) have a second kidney. Iv
is an interesting fact that this original paired character still finds
expression in the development of the kidney in Palwfine. In the
adult, this kidney lies, as in inost Gastropods, to the left of the
rectum and must therefore, as was shown above, have been the right
kidney before the twisting of the posterior part of the bedy took
place (Fig. 100 A-2, p. 214). This view is admirably supported by
vy. Excanoen’s researches, since, according to his account, it is
the rudiment of the right kidney which develops further, while the
left degenerates, P. Sarasin’s researches also show that, in
Bythinia, the rudiment of the kidney lies on the right side and is
displaced to the left later.
The one kidney which persists in most Prosobranchs (Monotocardia) thus
corresponds to the (definitive) left kidney which, before the twisting took
place, was the right kidney of those forms which still possess two renal organs.
In these latter, however (Haliotis, Fissurella, Turbo, Truchus) the right kidney
is usually well developed, the left, on the contrary, being reduced. It thus
appeared possible that the permanent kidney of the Monotocardia might
correspond to the right kidney of the Diotocardia, a view which has been
put forward several times (Perrine, No, 87). Ontogeny, however, as well
as the fact that, in the Diotocardia, the right nephridium serves for con-
ducting to the exterior the genital products (see below, p. 220) indicate that
it is the left (which before torsion is the right) kidney that persists and is
alone retained in the Monotocardia (Ray Lankesrer, No. 65; v, ERtancer,
No. 29).
‘The pericardial sac has several times been mentioned. ‘The term
pericardial is here hardly correct, since the kidney also originates
from this sac, to which, further, the /eart owes its origin. This
organ has now become very large and has thin walls (Fig. 99).
Dorsally, and to the left of the renal outgrowth of the pericardium,
a channel-like invagination representing the rudiment of the heart
(Fig. 98, 4) appears and occupies the whole length of the sac. The
channel becomes more and more marked off from the pericardium /.,
it becomes a tube which at first still remains open toward the primury
body-cavity. ‘This tube, by finally closing and remaining connected
with the wall of the pericardium only at its two ends, gives rise to
the heart which now, as a tube, lies within the pericardinm, its two
ends opening into the primary body-cavity. At a somewhat earlier
stage, a constriction appears near the middle of this tube, by means
of which the auricle and ventricle are divided from one another
(Figs. 99 and 100).
‘The Vessels arise as inter-cellular spaces in the mesodermal tissue of
216 GASTROPODA.
the primary body-cavity, and ave thus at first quite independent of
the heart. We have already repeatedly spoken of embryonic or
larval blood-sinuses, some of which, being capable of carrying on
rhythmical moyement, have been assumed to be larval hearts. The
rudiments of the vessels first appear as such blood-sinuses of different
sizes; in Palmdlina, for instance, » large sinus is found beneath the
intestine (Fig. 88 B, ux, p. 194). The yradual narrowing of these
spaces, which are surrounded by i luyer of flat cells, and their con-
nection with the open ends of the heart gives rise at the end of the
ventricle to the aorta and at that of the auricle to the efferent
branchial veiu. The other vessels arise in « corresponding manner.
The heart, in the Gastropoda, forms in a less primitive way than in the
Lamellibranchia (p. 76). This is not surprising, since the circulatory and
respiratory organs of the Gastropoda have wudergone far-reaching alterations
in consequence of the asymmetrical shape of the body. The presence of two
auricles, however, and the perforation of the heart by the alimentary canal
in o few Prosobranchs (Diotocardia) point to conditions resembling those
found in the Lamellibranchs. We might even believe that the heart in both
divisions arose ontogenetically in a similar way, and might then consider the
region at which the heart formed in the pericardium as the boundary between
the two coelomic sacs.
Tt is an interesting fact that we have, persisting throughout life, in
Dentalium, & condition similar to that seen in the developing heart in the
Gastropoda, which, as we have seen, arises as an infolding of the pericardium.
According to Prare (Solenoconch. Lit., No. 3), the heart of Dentalium
represents a sac-like invagination of the pericardium, and the blood-vessels
also are found in a condition similar to that in Gastropod embryos, being
mere spaces in the mesoderm between the other organs. The structures re-
garded by PLare as pericardium und heart, however, are but slightly developed,
and the nephridia are not connected with the pericardium, It is well known
that Dentalinm is a form already highly differentiated, but it may be possible
that in this respect a primitive character is retained. It appears also that,
among the Amphineura, the Solenogastres show a similar primitive condition,
while, in the Chitonidae, the circulatory system is much more highly organised,
the heart being entirely surrounded by the pericardium and provided with
efferent and afferent vessels,
The different positions assumed by the heart in the various
divisions of the Gastropoda, which are considered of great systematic
significance, are connected with the shifting of the different regions
of the body to which allusion has already repeatedly been made
(p. 144). One of the auricles, as was seen, is almost always Jost in
the process. If the pallial complex is only displaced to the side, the
gill lies behind the heart, the auricle behind the ventricle (this is
notably the case in the Opisthobranchia); but, if the pallial eom-
THE FORMATION OF THE ORGANS—THE GENITAL ORGANS. 317
plex shifts quite to the front, the gill will be found in front of the
heart and the auricle in front of the ventricle (Prosobranchin).
Other descriptions of the rise of the pericardium, the kidney and the
heart." In the formation of the pericardium as deseribed above, this organ
Was treated as if it corresponded to the whole of the coelom, but Vv, Hntaxomn's
observations on Paludina and Hythinia may also be interpreted as showing
that only « part of the original coelom persists as the pericardium while the
rest disintegrates, as we saw to be the case in the formation of the definitive
body-cavity in the Arthropoda. Sanensicy also, at a somewhat later stage of
the embryos of Vermetus, speaks of « somatic aud a splanchnic layer which
&re apposed to the ectoderm and the entoderm respectively and which enclose
@ large space as a (temporary) secondary body-cavity, The two layers of the
mesoderm are, however, so indistinct in the Mollusca that we are unable to
speak of them with wny certainty and, until more detailed statements are
wads, must regard ther as only definitely differentiated in the pericardium.
Sausinsky, who regarded this large space as the coelom, considers that the
heart urose from it im a way similar to that above described. With this may
be reconciled the earlier accounts of Gaxrx (No. 85), Bétsenus (No. 18) and
especially of P. Sanasuy (No. 101) and Scaanrenw (No. 106) which refer
partly to the Prosobranchia and partly to the Pulmonata.
It is easier to reconcile the older and more recent researches with regard
to the rise of the heart than with respect to the origin of the kidney, This
organ was indeed early derived from the mesoderm by constriction from the
pericardium (ScuaLrxew) or at least in the neighbourhood of the latter
(SALessky), the efferent ducts being derived from au (ectodermal) invagina-
Hon of the mantle-cavity, but the majority of authors trace back the whole
Kidney to an ectodermal invagination. After what hos been said above
{p- 74) 4s Lo the formation of the nephridia in the Lamellibranchia and the
Annelida, it cannot be doubted that the first method is the more probable. t
H, The Genital Organs.
‘The development of the genital organs bas been best observed in
Paludina, a form belonging to the Prosobranchia in which the sexes
are distinct (v. Extanexr). In these animals, the condition of the
* The literature connected with the formation of the iaplaetaal organs is,
like that connected with the ontogeny of the Gastropoda in general, rivh in
\erteeaid ‘statements. Where recent researches may be considered to
older statements, we have ignored the latter. Lack of space
oa peateom, taking into cousideration all the published data of a
nature. A summary of these is to be found in v. Exuancnn's
works (Nos. 27 und 28).
recent investigatious made by Memsennuimer (No. XVIL) on the
tence. in Lima do not at present help to clear up the
to their origin, as they are too startling to be accepted
Misses aerten maintains that the heart and kidney arise from
‘ectodermal rudiment, a condition which, so fur as we are aware,
opps. to be quite unique.—Ep.)
—
218 GASTROPODA.
genital organs is far simpler than in the hermaphrodite Pulmonates
which have repeatedly been studied but are far from being fully
understood, and we shall therefore consider Palwilina first. The
first rodiment of the genital organs here appears at a time when the
velum is still present, and the primitive kidney at its highest de
velopment, #.*., somewhat at the stage of Fig. 99. The male and
female genital rudiments are similar.
The germ-gland arises as a rounded outgrowth of the pericardial
sac near the rudiment of the (original) left kidney (Fig. 101, 9), which,
ad
Fra, 101,—Transvetse section through the posterior end of an embryo of Palwabinn
in the stage depicted im Fig. 9 (after v. ERLANGEM). ay,
iduct; d, intestine; y, ru
tissue; mA, mantle-cavity ;
we, ureter.
ut of the genital gland; /, liver: wes,
, kidney: 0, reno-pericartial pore; pr, pericnndimur ;
as has been stated, degenerates. This outgrowth becomes separated
from the pericardial sac as a spherical vesicle which approaches the
efferent genital duct (a) that has now also appeared as a rudiment.
The latter arises as an ectodermal invagination from the mantle-
cavity, and, according to v. ERLANGER, it is very probable that the
duct of the (original) left kidney changes direct into the genital duct.
It grows out further (Fig. 102, ug) and becomes anited with the
vesicular rudiment of the germ-gland (Fig. 100, g and ga), The
genital gland and its efferent duct now increase considerably in length
THE FORMATION OF THE ORGANS—THE GENITAL ORGANS, 219
and begin to coil, but there is as yet no sexual differentiation save
that this increase in growth takes place at «an earlier stage in the
male than in the female.
‘The male genital apparatus of Palwdina is composed of the germ-
gland just described,
which becomes the
testis, of the efferent
duct, which has also
been to some extent
described, and of uw
much Jonger section,
the vas deferens; the
latter, which leads to
the penis, develops in a
somewhat different way.
This part of the vas
deferens arises in the
form of « groove in the
base of the mantle-
cavity into which the
previously formed : ere peo Pee youn iawn) a
(primary) efferent duct — giferent duct: , intestine; y, rudiment. of
opens. ‘The groove —_eetiltal gland; liver; wine, mesodermal tissue ;
choees and, ip the form wl, mautle-cavity,
of a tube, extends as far as the right tentacle where the penis
develops.
In some Prosobranchs, this seminal groove is retained throughout life and is
continued from the male genital aperture in the mantle-cavity to the tip of
the penis. We thus find here, as in the formation of the secondary ureter of
the Pulmonates, conditions which are permanent in other forms appearing a8
consecutive ontogenetic stages,
‘The female genital apparatus of Pifui/inc does not develop as early
as that of the male, Whereas the chief features of the latter caw
be recognised at the end of embryonic development, those of the
female cannot be distinctly made out \ntil several weeks after birth.
At this stage, the rudiments of the albumen-gland appear in the
form of eight to twelve tubular outgrowths near the point where the
ectodermal efferent dict unites with the germ-gland. The ovary is
still represented by a tube lined by undifferentiated opithelium. The
short tube which extends from the ovary to the albumen-gland is
said to be derived from the mesoderma) rudiment, like the short piece
Fi, 102-—Pertion of mgitial section of an, embryo
—
220 GASTROPODA.
which, in the male apparatus, runs from the testis to the commence-
ment of the so-called primary (ectodermal) efferent duct. The whole
of the remaining efferent apparatus in the female corresponds to the
primary efferent duct in the male, and the former, like the latter,
opens into the mantle-cavity. There is, in the female, no part corre-
sponding to the secondary efferent apparatus of the male. Apart from
this last portion the genital apparatus in the male and in the female
thus agree closely in their development, the principal constituent
parts being apparently quite homologous (v. ERLANGER).
The relation of the germ-glands to the pericardium, which was only con-
jectured to exist in the Lamellibranchia (p. 82), is definitely proved by v.
ERLANGER to exist in the Gastropoda. The genital glands arise as growths
of the pericardial wall, and thus bear to this latter the same relation as do the
genital products in the Annelida to the peritoneal epithelium (Vol. i., p. 297).
In this way we obtain a further support for the coelomic nature of the peri-
cardial sac. Since, in the lower forms, the nephridia function in conducting
tho genital products to the exterior, it appcars as if, in the Gastropoda, the
nephridium, which no longer functions as a kidney, might become directly
modified as the efferent genital duct. As we have seen in the Solenogastres
{p. 9), the nephridia transmit the genital products, and even in some Proso-
branchs (i.e. the majority of the Diotocardia) the right nephridium serves
in addition as a genital duct; such a modification of the efferent renal ducts
ix therefore not surprising.
‘The hermaphrodite genital organs of the Pulmonateshave repeatedly
been made the subject of careful ontogenetic research (Ersta, No. 26 ;
Rouzaup, No. 94; Brock, No. 16; Simkotu, No. 119; Kiorz, No.
54), but so far no satisfactory conclusion as to their origin has been
arrived at. The conditions are here very complicated and obscure.
The point of greatest importance is to ascertain the relations between
the various ducts of the hermaphrodite forms and the simple efferent
apparatus of the dioecious forms and finally to trace the former to the
latter. We cannot state definitely that the separation of the sexes ix
the primitive condition, although this seems highly probable, since the
older Gastropods (the Diotocardia) are dioecious and the most special-
ised forms (Opisthobranchia, Pulmonata) are hermaphrodite. As the
accounts so far published do not enable us to obtain a clear concep-
tion of the development of the hermaphrodite genital organs, it is
only possible to consider them by the light of the better understood
development of the dioecious Prosobranchia.
At the very outset of this investigation, however, a difficulty is
asioned by the question as to whether the genital apparatus is
derived from one common rudiment or from two or three distinct
THE FORMATION OF THE ORGANS—THE GENITAL ORGANS. 221
rudiments. The whole apparatus, the yerm-gland included, has
been derived from a single ectodermal thickening which extends and
becomes differentiated later (RouzauD). ‘There can, however, be
but little hesitation in at once excluding this view, inasmuch as the
hermaphrodite gland is, in any case, yielded by the mesoderm. [The
yonai is probably derived from the apparently undifferentiated blasto-
meres.} With regard to the ectodermal part, i.e. the efferent ducts
and accessory structures, there may certainly be two distinct types
of development according as the copulatory organ lies separately
from the female genital aperture or is united with it in a common
atrium. In the first case, the genital apparatus would, as in the
Prosobranchs (Paludina), consist of three parts, riz., of the germ-
gland, of the primary (nephridial), and of the secondary (ectodermal)
efferent ducts together with the penis. This is evidently the case in
Linnaea, as we may conclude from the observations of Ersia and
Kuorz.
The genital apparatus ‘of Limnwes appears as a rudiment even
before the hatching of the yonng animal.
an ectodermal invagination at the
base of the right tentacle. The
hermaphrodite duct arises inde-
pendently of it as a strand-like
structure. The mesodermal
character which has repeatedly
been assumed for it appears doubt-
ful. We are inclined rather to
consider it as ectodermal, a view
which is supported in the literature
on the subject. This strand is of
‘The penis appears first as
special importance, as it splits
later into two parts, one of which
represents the rudiment of the
uterus and the other that part of
the vas deferens which is known
us the prostate (Fig. 103, »f, w/).
The hermaphrodite gland arises
independently of this strand from
the mesoderm [? primitive blastomeres} .
ed/--
Fra, 103.— Diagrans representing a later
stage in the development of the genital
organs of a Pulmonate, alb, albumen-
gland ; p, penis ; ra, spermetheca ; u/,
d, vas deferens; 2, hermaph-
te 4 Hy hermaphrodite duet ; 3
rs ‘ant? , xenital apertures,
A xhort process of the
mesodermal rudiment yields the proximal portion of the efferent
duct, while the distal part arises from the cellular strand mentioned
above (Fig. 103, zy, Brock, Kiotz).
The spermetheca becomes
222 GASTROPODA.
abstricted from the uterine portion of the common duct in a way
similar to that in which the prostatic part of the vas deferens
arose from it earlier. These abstrictions take place by means
of longitudinal folds which grow into the common canal that arose
when the strand became hollow. The albuminiparous yland arises
in the form of a number of tubular outgrowths near the proximal
end of the uterus (Fig. 103, ab).
‘The origin of the male and female ducts through the division of a common
tudiment may be demonstrated with some certainty in the various herma-
pbrodite forms that have been investigated, When we take into consideration,
in this connection, that, in the Opisthobranchia, the transmission of the two
kinds of genital products takes place through « common duet (Fig. 104 B)
and that, in the Palmonates also, their transmission takes place for a longer
or shorter distance through the same duct, the division inte male and female
duets occurring later (Fig. 104 C), we may with safety assume that these two
ducts have arisen phylogenetically also through the splitting of one duct and
that thus the Opisthobranchs exhibit the more primitive condition. If we
then take one step further back, we may trace back the common efferent duct
of the hermaphrodite Gastropoda to the efferent apparatus of the dioecious
forms. We here naturally presuppose that we regard the separation of the
sexes as the primitive condition and hermaphroditism as the derived condi-
tion. Since also in dioecious animals, ova are often met with in the testix
and rice versd, and, further, in other divisions of the animal kingdom in which
separation of the sexes is the rule, hermaphroditism occurs in a few highly
differentiated forms, such an assumption is not inadmissible.
The question now arises how the connection is established between
the penis and the genital aperture in those forms in which these two
arise separately. In the Opisthobranchia, a groove runs from the
aperture of the common duct to the introvertible penis which here
also is found near the right tentacle (Fig. 104 8). Where the com-
mon duct becomes divided up into a female and a male portion the
channel starts from the aperture of the latter, and, by the closure of
the groove and its detachment from the ectoderm, the part of the
vas deferens arises which lies nearer the penis, the process being
similar to the formation of the secondary vas deferens in the Proso-
branchia (Fig. 104 C). It is in any case probable that, ontogenetic-
ally, the formation takes place in this way, although this has not yet
been. proved.
In Fig, 104 4-# we have attempted to give some idea of the way in which
these processes may have taken place. The modifications brought about by
the earlier closing of the channel, by the later separation of the male duct,
and by the invagination, in the course of ontogeny, of the rudiment of the
penis (2) are self-evident,
THE FORMATION OF THE ORGANS—THE GENITAL ORGANS. 223
Tn cases in which, finally, the penis becomes shifted towards the
female genital aperture, and the two join to form a common atrium, as
in the Stylommatophora (Fig. 104 £), the rudiment of the ectodermal
parts form from a common rudiment. In these eases, we have only
to distinguish between the mesodermal rudiment of the hermaphro-
dite gland (or the hermaphrodite organ) and the ectodermal rudiment
of the primary and secondary ducts and copulatory apparatus.
"The significance of these processes is still little understood, and it is doubt-
ful if ontogeny will throw much light upon the subject. Summaries and
critioal desoriptions of these ontogenetic processes are given by Rouzaun,
Brock, Semper (No, 117), Scurmmenz (No. 107), and Kuorz (No, 54).
Pra. 104,—.A-K, diagrams illustrating the manner in which the genital apparatus opens
Bak ab Were’ ontogenetic stages. A, in n dioccious Gastropod; Bed, in hetma-
Phrodite Gastropods. os, ovosemninal duct ; p, peuis; wd, vas deferens: 8 and 9
genital apertures or the terminal portions of the corresponding efferent dnots,
[The i of the complicated conditions met with in the herma-
sipeclialia of the Sige saree and Pulmonata is one of those
upon which ontogeny throws little light, We think there
can be doubt that it will be found more profitable to leave the onto-
side alone and accept the obvious and comparatively simple interpre-
offered Wy the study of the comparative anatomy of these organs.
ee existing Gast we seem to have every stage in the development of
genil
SSS founda many
‘is in many of the hermaphroditic Tectibranchia, which, how-
ever, show
—
224 GASTROPODA.
groove-like vas deferens leads down to an introvertible cephalic pen'
next change which occurs is the closure of the groove-like vas deferen:
separation from the ectoderm asa tube. This brings us to the condition seen
in Actaeon and then to the Basommatophorous Pulmonata, where the male
aperture is distinct from the female. The only break in the series is that
between the Basommatophorous and Stylommatophorous Pulmonata, due to
the development in the latter of wascondary oviduct which extends from the
primitive genital aperturo (now closed) down to the cephalic penis, and opens
with that structure through an atrium. With regard to the origin of this
secondary oviduct, two possible interpretations present themselves, one being
that the secondary oviduct has arisen as a groove, like the vas deferens, and,
like that structure, has secondarily closed together with the primary genital
aperture, so that both products are discharged by a single anterior aperture.
or it may be that the primitive genital orifice has shinies by growth down the
xide of the body towards the penis and. finally, an atrial involution has caught
up both these apertures, so that they now communicate with the exterior by
common aperture. The fact that these stages are not recapitulated in the
ontogeny is not, we think, of much importance, for we know that ontogeny
by no means always recapitulates phylogeny, and that this is especially the
case in forms which, like these Mollusca, have a considerable amount of yolk.—
Ep.]
LITERATURE,
1. ALDER, and Hancock, A. Observations on the structure
and development of Nudibranchiate Mollusca. Ann. May.
Vat. Hist. Vol. xii) 1843.
2. Auber, J., and Hancock, A. On a proposed new order of
Gastropodous Mollusca. Ann. Mag. Nat. Hist. (2). Vol. i.
1&48,
3. Barrois, TH. Les glandes du pied et les pores aqniféres chez
les Lamellibranches. Lille, 1885. (Compt. Rend. Acad, S
Paris. ‘Tom, c.)
4. Benmg, TH. Beitrize zur Anatomie und Entwicklungsgesch-
ichte des Harnapparates der Lungensehnecken. Archie. 1.
Naturg. Jabrg. lv. 1889.
5. Berau, R. Ueber die Verwandtschaftsbeziehungen der
Onehidien. Morphol. Jahrb. Bd. x. 1885,
6. Bernarp, F. Recherches sur les organes palléaux des Gastro-
podes Prosobranches. Aun. Sei. Nat. Zou. (7). Tom. ix.
1x90.
7. Biocumany, F. Ueber die Entwicklung der Neritina fluviatilis.
Zeitsehr, ¢. wiss, Zool, Bao xsxvi. 1882,
Biocumasy, F. Beitraége zur Kenntniss der Entwicklung der
Gastropoden, Zeitsehr, f wiss, Zool. Ba xxx 1883.
% Boas, J. E. vy. Spolia Atlantica, Bidrag till Pteropodernes
Morfologi och Systematik, ete. Videuskap. Selek. Skr.
Kjtlenharn, 6 Riekke. 1886, Abstracted by Kobelt ii
LITERATURE. 225
Nachrichisblatt Dentech. Malakozool. Gesellsch. Jahrg. xix.
1887. -
10. Boas, J. E. v. Zur Systematik und Biologie der Pteropoden.
Spengel's Zool. Jahrb, Bd. i. 1886, (Contains extracts from
No, 9.)
11, Bongerzxy, N, Studien iiber die embryonale Entwicklung
der Gastropoden. Archiv. f. miler, Anat, Bad. xiii. 1877.
12. Bouran, L. Recherches sur l'anatomie et le développement de
Ja Fissurelle. Archiv. Zool. exp. gen (2). Tom. iii, Suppl.
1885,
13, Bouvier, E. L. Systime nerveux, morphologie générale
et classification des Gastéropodes Prosobranches. Ann, Sei.
Nat. Zool. (7). Tom. iii, 1887.
14. Braus, M. (1) Ueber den Harnleiter bei Helix. (2) Ueber
die Entwicklung des Harnleiters bei Helix pomatia. Nach-
richishlatt Deutsch. Malakozool. Gesellsch. Jahrg. xx. 1888.
15. Braun, M. Bericht iiber parasitische Schnecken. Centralbl.
J. Bacteriol, und Purasitenkunde, Bad, v. 1889,
16. Brocs, J. Die Entwicklung des Geschlechtsapparats der
stylommatophoren Pulmonaten, ete. Zeitschr. f. wise. Zool.
Bd. xliv. 1886.
17. Brooks, W. K, Preliminary observations upon the develop-
ment of the marine Prosobranchiate Gastropods, Chesapeake
Zool. Laboratory. Johns Hopk. Univ. Scient. Results. 1878.
18. Birscun1, O. Entwicklungsgeschichtliche Beitrige. Ueber
Paludina vivipara. Zerfschr. /. wiss, Zool. Bd. xxix. 1877.
19. Birscati, O. Bemerkungen tiber die wahrscheinliche Her-
leitung der Asymmetrie der Gastropoden, spec. der Asymmetrie
im Nervensystem der Prosobranchiaten. Morphol. Jahrb.
Bd. xii. 1887.
20. Carrenter, W. On the development of Purpura. Ann,
Mag. Nat. Hist. (2). Vol. xx. 1857.
21. Carrikre, J. Die Fussdriisen der Prosobranchier, eto, Archiv.
S. mikr. Anat. Ba. xxi. 1882.
22. Canri®ee, J. Die Sehorgane der Thiere. Miinchen und
Leipzig, 1885.
23. Craparkpe, E. Anatomie und Entwicklungsgeschichte der
Neritina fluviatilis, Muller's Archiv. f. Anat. Phys. 1857.
24. Conus, E.G. Note on the embryology of Crepidula fornicata
and of Urosalpinx cinerea. Johns Hopk. Univ. Cire. Vol. x.
No, 88. 1891.
a
226 GASTROPODA.
25. Conxuin, E.G. The cleavage of the ovum in Crepidula forni-
cata. Zool. Anz. Jahrg. xv. 1892.
26. Exeta, H. Beitriige zur Anatomie und Entwicklungageschichte
der (Jeschlechtsorgane von Lymniius. Zettechr. f. wiss. Zool.
Bd. xix. 1869.
Eruanagr, R. v. Zur Entwicklung der Paludina vivipara.
Th. I. und II. Morphol. Juhrb, Bd. xvii. 1891.
28. Eruanogr, R. v. Beitrage zur Entwicklungsgeschichte der
Gastropoden. Erster Theil. Zur Entwicklung von Bythinia
tentaculata. dfitth. Zool. Stat. Neapel. Bd. x. 1892.
29. Kruanagr, R. v. On the paired Nephridia of Prosobranchs,
the homologies of the only remaining Nephridium, etc.
Quart. Journ, Micro, Sci. Vol. xxxiii. 1892.
30. Fisongr, H. Sur le développement du foie chez les Nudi-
branches. Compt. rend. Acad. Sci. Paris, Tom. cxii. 1891.
Archiv, f. Naturgesch. Bd. lvii. 1891.
31. Fou, H. Etudes sur le développement des Mollusques. Hétéro-
podes, Archiv. Zool. erp. gen. Tom. v. 1876.
33. Fou, H. Sur le développement des Ptéropodes. Archiv. Zool.
exp. gen, Tom. iv. 1875.
33. Fon, H. ftudes sur le développement des Gastéropodes
pulmonés, Archiv. Zool. exp. gen. Tom. viii. 1880.
34. Fratssz, P. Ueber Molluskenaugen mit embryonalem Typus.
Geitschr, f. wiss, Zool, Bd. xxxv. 1881.
35. Ganin, M. Zur Lehre von den Keimblittern bei den Weich-
thieren. (Russian.) Warschauer Cnicersititeberichte. 1873.
No.l. Zeitechr. f. wise, Zool. Bd. xxii, 1872.
36. GrarnnauR, C. — Reitrige aur Entwicklungsgeschichte der
Landgastropoden. Zeitechr. 7. wies, Zool, Bd. iii, 1851.
37. Quawsnaur, C. Untersuchungen iiber Pteropoden und Hetero-
peden. Leiprig. 1855.
38. Growney, C. Zur Morphologie des Fusees der Hetel len.
Ard, Zool, Inst. Unie. Wier Bd. vii, 1888.
39. Groregy, C. Zur Morphologie des Preropodenkirpers. 471).
Joe, Dist. Caie, Wein, Bd. viii, 1889.
Notes on the development of Mollusea. Qu rt.
27.
Porpura, ava. Map. Nas. Hie. 3). Volo sii, 1883.
42. Hescumas is PL The origin and development of the
central nervous system of Liman maximus, Bal, Mux Comp.
ww. Harvari Co Volos 1290,
LITERATURE, 227
43. Hineer, C. Beitriige zur Kenntniss des Gastropodenauges.
Morphol. Jahrb. Bd. x. 1885.
44. Jaerive, H. von. Entwicklungsgeschichte von Helix. Jen.
Zeitachr. f. Naturw, Bd. ix. 1875,
45. Jueninc, H. vox. Vergleichende Anatomie des Nervensystems
und Phylogenie der Mollusken. Leipzig, 1877.
46. Jwenine, H. von. Ueber den uropneustischen Apparat der
Heliceen. Zeitechr, f. wiss. Zool, Bd, xli. 1885,
47. Jugrine, H, von, Die Stellung der Pteropoden. Machrichts-
blatt der Deutsch. Malakozool, Gesellsch. Jahrg. xx, Frank-
furt, 1888.
48. Juenina, H. von. Sur les relations naturelles des Cochlides
et des Ichnopodes, Bull. Sci. France et Belgique. Tom.
xxiii, 1891.
49. Jourpats, 8. Sur le développement du tube digestif des
Limaciens. Compt. rend. Acad. Sei. Paris. Tom. xeviii,
1834,
50. Jourpars, 8, Sur les organes segmentaires et le podocyste des
embryons des Limaciens. Cumpt. rend. Acad. Set. Paris.
Tom. xeviii. 1884.
51, Joveux-Larroure, J. Organisation et développement de
TOneidie, (Oncidium celticum.) Archiv. Zool. exp. -gén.
Tom. x. 1882.
52. Kererstem, W. Malacozoa cephalophora, Sronn’s Klassen
und Ordnungen des Thierreichs. Ba. iii. Leipzig und Heidel-
berg, 1862-66.
53. Kurerstern, W., und Exavers, HE, Beobachtungen iiber die
Entwicklung von Aeolis peregrina. Zoologische Beitrage.
Leipzig, 1861.
54. Krorz, J. Beitrag zur Entwicklungsgeschichte und Anatomie
des Geschlechtsapparats von Lymimniius. Jen. Zeitsehr. f.
Naturw. Ba. xxiii, 1889.
55. Kxrrowrrscos, N. Zur Entwicklungsgeschichte von Clione
Timacina, Biol, Centralbl. Bd. xi, 1891,
56. Koxen, EB. Ueber die Entwicklung der Gastropoden von Cam-
brium bis zur Trias. Neues Jahrbuch f. Min. Geol. und
Paldont. Beilageband vi. 1889.
57. Koren und Danterssex. On the development of the Pectini-
branchiata. Ann. Mag. Nat. Hist. (2). Vol. xix. 1857.
58a. Kroun, A. Beitriige cur Entwicklungsgeschichte der Hetero-
poden und Pteropoden, Leipziy, 1860.
ai
228
GAB8TROPODA.
58+. Kroun, A, Ueber die Schale u. Larven des Gasteropteron.
59.
60.
61.
64,
66,
7.
72.
73.
Archiv. f. Naturgesch. Jabrg. xxvi. 1860.
Lacazg-Dutuiess, H. pg. Mémoire sur l’anatomie et l’embryo-
génie des Vermets. Ann. Sci. Nat. Zool. (4). Tom. xiii. 1860.
LacazE-Dutuiers, H., et Pruvot, G. Sur un ail anale
larvaire des Gastéropodes opisthobranches. Compt. rend.
Acad, Sct. Paris. Tom. ev. 1887.
Lana, A. Versuch einer Erkliarung der Asymmetrie der
Gasteropoden. —Vierteljahreschrift Naturfursch. Gesellech.
Zirich. Bd. xxxvi. 1891.
. Lanceruans, P. Zur Entwicklung der Gastropoda Opistho-
branchiata. Zettechr. f. wiss. Zool. Bd. xxiii. 1873.
. LANKESTER, E, Ray. Observations on the development of the
Pond-Snail (Limnaeus stagnalis) and on the early stages of
other Mollusca. Quurt. Journ. Micro. Sci. Vol. xiv. 1874.
LanxestER, E. Ray. On the coincidence of the blastopore
and anus in Paludina vivipara Quart. Journ. Micro. Sci.
Vol. xvi. 1876.
Lankester, E, Ray. On the originally bilateral Character of
the renal organs of Prosobranchs, ete. Ann. Mug. Nat. Hist.
(5). Vol. vii. 1881. :
Lexmann, R. Anatomie von Amphibola nux Avellana. Malako-
zoologixche Blatter. Bd. xiii. Caseel, 1866.
Levucxart, R. Der Bau der Heteropoden. Zuvol. Unter-
suchungen. Heft. iii. Gtessen, 1854,
. Leypiec, F. Ueber Paludina vivipara. Zeitechr. 7. wies. Zool.
Bad. ii. 1850.
Loven, S, Bidrag till kiinnedomen of Molluskernas utvickling.
Kongl. Vetenskaps Academiene Handlingar for 1839. Stock-
holm, 1841.
. McMurricn, J. P. A contribution to the embryology of the
Prosobranch Gastropods. Stwi. Biol. Lab. Johns Hopk. Univ.
Vol. iii. Baltimore, 1887.
McMorricn, J. P. On the existence of a post-oral band of cilia
in Gastropod Veligers. Ann. Mug. Nat. Hist. (5). Vol. xvi.
1885.
Manrrepi, L. Le prime fasi dello sviluppo dell’ Aplysia.
Atti R. Accad, Sci. Vol. ix. Napoli, 1882.
Mark, E, L. Maturation, Fecundation and Segmentation of
Limax campestris. Bul. Mus. Comp. Zool. Harvard College.
Vol. vi. 1881.
LITERATURE, 229
74. Mazzanenut, G. Intorno al preteso occhio anale delle larve
degli Opistobranchi. Rend. R. Accad. Linesi. (5), Vol. ic
Fasc. iii. 1892.
75. Meuron, P. ve. Sur les organes rénaux des embryons ’ Helix.
Compt. vend. Acad. Sei. Paris. Tom. xeviii, 1884.
76. Mturer, Jon. Ueber Synapta digitata und die Erzeugung von
Schnecken in Holothurien. Berlin, 1852.
77. M@urer, Jon. Ueber die Entwicklungsformen einiger niederer
Seethiere. Ber, Akad. Wiss. Berlin, 1852.
78. Miuver, Jon. Bemerkungen aus der Entwicklungsgeschichte
der Pteropoden. Ber, Akad. Wiss. Berlin, 1857.
79. Miter, Jon. Bemerkungen aus der Entwicklungsgeschichte
der Pteropoden, Monatsher. k. Akad. Wiss. Berlin. 1857.
(1858).
80, Norpmany, A. y. Essai d’une Monographie du Tergipes
Edwardsii, Arm. Sei, Nut, Zool. (3). Tom. v. 1846,
Sl. Ostorx, H. L. Development of the Gill in Fasciolaria. Stud.
Biol. Lab, Johns Hopk. Univ. Vol. iii. 1884-7.
82. Parren, W. Artificial fecundation in the Mollusca. Zool.
Anz, Jabrg. viii. 1885,
83. Parrex, W, The embryology of Patella. Arb. Zool, Institut
Univ. Wien. Bd. vi. 1886.
84, Pecsenreer, P. Sur le pied et la position systématique des
Ptéropodes, Ann. Soe, Roy. Mulacologique Belgique. Tom.
xxiii, 1888.
85. Petseneer, P. Sur I'wil de quelques Mollusques Gastropodes,
Amn. Soe. belg. microwop. (Mém.) Tom. xvi. Brucedles,
1891,
86. Pexsexerr, P. Sur la dextrosité de certains Gastropodes dits
“sénestres."” Compt. rend, Acal, Sei. Paris, Tom. exii.
1891,
87. Perrier, R. Recherches sur l'anatomie et l'histologie du rein
des Gastéropodes Prosobranchs. Ann, Sei, Nat. Zool. (7).
Tom. viii. 1889.
88. Preirrer, C. Systematische Anordnung und Beschreibung
deutscher Land- und Siisswasserschnecken. Cussel, 1821.
89. Puate, L. Studien uber opisthopneume Lungenschnecken,
I. Die Anatomie der Gattungen Daudebardia und Testacella.
Zool. Juhrb. Anat. Ba, iv. 1891.
90. Rapp, ©. Die Ontogenie der Siisswasser-Pulmonaten. Jen.
Zeitachr. f, Naturw. Ba, ix, 1875,
—
280
GASTROPODA.
Q9la. Rasy, C. Ueber die Entwicklung der Tellerschnecke.
Morphol. Jahrb. Bd. v. 1879.
916. Rant, C. Ueber den ‘pedicle of invagination” u. das Ende
92,
93.
94.
95.
101,
103.
los.
is
Wh.
10s.
der Furchung von Planorbis. Morphol. Jahrb. Bd. vi.
1880.
Rast, C. Beitriige zur Entwicklungsgeschichte der Proso-
branchier. Sttzungeber. der k. Akad. Wiss. Wien. Bad. Ixxxvii.
Abth. iii, 1883.
Rao, F. Studii sallo sviluppo della Chromodoris elegans.
Atti R. Accnd. Sci. (2). Vol. i. Napoli, 1888.
Rovzaup, H. Recherches sur le développement des organes
génitaux de quelques Gastéropodes hermaphrodites. Travaux
Lab. Zool. Faculté Sci. Montpellier, 1885.
Résuter, R. Die Bildung der Radula bei den cephalophoren
Mollusken. Zeitechr. f. wiex. Zool. Bd. xli. 1885.
. Riioxgr, A. Ueber die Bildung der Radula bei Helix pomatia
23 Ber. Oberheextsch. Gesellech. fiir Natur und Heilkunde.
Giexsen, 1883.
. Rypagr, J. Notes on the development of Ampullaria depressa.
Amer, Nat. Vol. xxiii. 1889.
Sauensxy, W. Beitrige zur Entwicklungageschichte der
Prosobranchier. Zeitechr. f. wise. Zool. Bd. xxii. 1873.
Sauensxy, M. Etudes sur le développement du Vermet
Archiv. Biol, Tom, vi. 1885.
. Sauensky, M. Zur Entwicklungsgeschichte von Vermetas.
Biel, Central, Bdov. 1885-6.
Sarasix, P. Entwicklungsgeschichte der Bithynia tentaculata.
Artveit. Zool. Inst, Wurzburg. Bd. vi. 1882.
Sarasis, P. vu. F. Aus der Entwicklungageschichte der Helix
Waltoni. Ergedu. Nuturi. Forsch. raf Ceylon. Bd.i. Heft.
ii, Wrestarden, 1 Abstract in Zod. Asc, 1887.
Sarasix, P. cv. F. Ueber awei parasitische Schnecken.
Enyhe. nat, Forech. auf Coytion, Bd i. 1887.
Sars, M. Zur Entwicklungsgeschichte der Mollasken und
Zoophyten. A-cAic f. Netureewd. Bd iii, 1837.
Sars, M. Beitras zur Entwicklumggreschichte der Moitasken.
§ Naterwat. Rio 1840. . Supplementary paper
Biosi 1S.
Swuarrepw, Mo Schimkewiteh.: Sur ie déveloprement do
ear des Motuayres rolmeress ete. Zed de Jakeg.
Iss.
LITERATURE. 231
107. Scuiemenz, P. Die Entwicklung der Genitalorgane bei den
Gastropoden. Biol, Centralbl. Bd. vii, 1887-8.
108, Scmremenz, P. Kritische Betrachtungen uber die parasitischen
Schnecken, Biol. Centralbl. Ba. ix. 1889-90.
109, Scusupr, F. Die Entwicklung des Fusses der Succineen.
Sitzungsber. Nat. Ges. Univ. Dorpat. Bad. viii, 1889.
110. Scuapt, F. Studien zur Entwickhingsgeschichte der Pul-
monaten, I, Die Entwicklung des Nervensystenis. Sitzungsber.
Nat, Gea, Univ. Dorpat. Ba, viii. 1891, Ann, and Mag.
Nut. Hist, (6). Vol. viii. 1891,
111. Semsnpr, O. Ueber die Entwicklung von Limax agrestis.
Arch. f. Anat. und Phys, 1851,
112. Scuwemer, A. Ueber die Entwicklung der Phyllirhoe
bucephalum. Avehiv. f. Anat. und Phye, 1858,
113. Scuunrze, Max. Weber die Entwicklung des Tergipes
Jncinulatus, Archiv. f, Naturg. Jahrg. xv. 1849.
114. Setenca,E, Entwicklung von Tergipes claviger. Néederldnd
Archiv. f. Zoologie. Ba. i, 1871-3.
115. Setenka, E. Die Anlage dér Keimblitter bei Purpura lapillus,
Niederlind Archiv. 7. Zoologie. Bad. i, 1871-3.
116, Semper, ©. Entwicklungsgeschichte der Ampullaria polita,
ete. Utrecht, 1862.
117, Semrer, C. Ueber Brocks Ansichten iiber Entwicklung des
Molluskengenitalsystems. Arb, Zool. Inst. Wiirzburg. Ba.
viii, 1887,
118. Semper, K. Die natiirlichen Existenzbedingungen der Thiere.
Leipziy, 1880.
119, Sotnorx, H. Weber die Genitalentwicklung der Pulmonaten,
etc. Zeitechr. f. wise. Zool. Bd. xlv, 1887.
120. Stunorn, H. Ueber das Vaginulidengenus Atopos, Zeitschr,
f- wis. Zool. Ba. li. 1891.
121. Sounzyer, M. Hétéropodes. Voyage autour du monde 1836-7
sur la Corvette La Bomite, etc, Tom, ii. (with atlas). Paris,
1852.
122. Srxnoex, J. W. Die Geruchsorgane und das Nervensystem
der Mollusken. Zuitsehr. f. wiss. Zool. Bd. xxxv. 1881,
1234. Sreraxor, P. Ueber Geschlechtsorgane und Entwicklung
von Ancylus fluviatilis, Mem. Acad, Imp. St. Petersbourg (7).
Tom. x. No, viii, 1866.
123b, Sruanr, A. Ueber die Entwicklung einiger Opisthobranchier,
Zeitachr, f. wits. Zool. Ba. xv. 1865,
(A
232
124,
125.
126.
127.
128.
129.
130.
131.
GASTROPODA.
TrcHesg 8. I primi momenti dell’ evoluzione nei Molluschi.
Atti R. Accad. Lincei (3). Mem. Vol. vii. Roma, 1880.
TrincHeEse, S. Per la funna marittima italiana. Aeolididae
e familie affini. Atti R. Accad. Lincei (3). Mem. Vol. xi.
Roma, 1881.
TrincuEsE, S. Ricerche anatomiche ed embriologiche sulla
Flabellina affinis. Mem. R. Acca. Sci. dell’ Inatituts di
Bologn (4). Tom. viii. 1887.
Voat, C. Recherches sur l’embryogénie des Mollusques Gaste-
ropodes. Ann. Sci. Nat. Zool. (3). Tom vi. 1846.
Voat, C., und Gecenpaur, C. Beitrag zur Entwicklungs-
geschichte eines Cephalophoren. Zeitschr. f. wins. Zool.
Bd. vii. 1856.
Vorat, W. Entocolax Ludwigii, ein neuer seltsamer Parasit
aus einer Holothurie. Zeitechr. f. wise. Zool. Bd. xlvii.
1888,
Warneck, A. Ueber die Bildung und Entwicklung des
Embryos bei Gasteropoden. Bull. Soc. Imp. Natural.
Moscou. Tom. xxiii. 1850.
Woxrson, W. Die embryonale Entwicklung des Limnaeus
stagnalis. Bull. Acud. Sci. St. Petersboury. Tom. xxvi. 1880.
APPENDIX TO LITERATURE ON GASTROPODA. $
I. Buocu, L. Die embryonale Entwicklung der Radula von
Paludina vivipara. Jew. Zeitachr. f. Nature. Bd. xxx.
1896.
Il. Bouran, L. Sur le développement de l’Haliotide. Compt.
rend, Assoc. Franc. Tom, ii. 1892.
III. Boutan, L. Sur le développement de I’Acmaea virginea.
Compt. rend. Acad. Sri. Paris. Tom, exxvi. 1898.
IV. Conkuin, E. G. The Embryology of Crepidula. Journ.
Morphol. Vol. xiii. 1897.
Vv. Crampton, H. E. Reversal of Cleavage in a Sinistral
Gastropod. Ann. NV. York. Acad. Vol. viii. 1894.
Vl. Drew, G. A. Some observations on the Habits, Anatomy
and Embryology of members of the Prosobranchia. Anat.
Anz, Bd. xv. 1899.
VII. Ertancer, R. von. Bermerkungen zur Embryologie der
Gasteropoden. Urnieren. Biol. Centralbl. Bd. xiii.
1893. Bd. xiv. 1894. Bd. xviii. 1898. Also Archiv.
Biol, Tom, xiv. 1895. :
LITERATURE, 233
VIL Ercaxcer R. voy. Mittheilimgen tber Bau und Entwick-
lung einiger marinen Prosobranchier. I. Ueber Capulus
hhungaricus. Zool. Anz. Jahrg. xv. 1892.
1X. Brnancer, R. von. Ueber einiger abnorme Erscheinungen
in der Entwicklung des Cassidaria echinophora. Zool.
Anz, Jabrg. xvi. 1893.
X. Extanomr, R. von. Zur Bildung des Mesoderms bei des
Paludina vivipara. Morphol. Jahrb, Bd. xxii, 1894.
XI. Fuarra, T. Preliminary note on the mesoderm formation
of Pulmonata, Zool. May. Tokyo. Tom. vii. 1895.
XI. Heymons, R. Zur Entwicklungsgeschichte von Umbrella
Mediterranea. Zeitsch, f. wiss. Zool. Bd. lvi. 1893.
XIIL, Honams, 8, J. The cell-lineage of Planorbis. Zool. Bull,
Vol. i. 1897. XIIL a. Ancynus, Amer. Nat. 1899.
XIV. Koror, C. A, On the early development of Limax. Bud.
Mus. Coup, Zool, Havard. Vol. xxvii. 1895.
XV. Mazzarenut, G. Bermerkungen iiber die Analniere der
freilebenden Larven den Opisthobranchies. Biol. Centralbl,
Ba. xviii, 1898.
XVI. Mazzarenot, G. Monografia delle Aplysiidae del Golfo di
Napoli (sistematica, biologia, anatomia, fisiologia, ed
embriologia). Mem. Soc, Ztal, ‘Tom. ix, 1893,
» XVIL Mzrssexnenmenr, J, Entwicklungsgeschichte von Limax
maximus L. Theile i., und ii. Zeitschr. f wiss. Zool.
Ba. Ixii. und Ixiii, | 1898.
XVILL. Murssennerwen, J, Zur Morphologie der Urnieren der
Pulmonaten, Zeitachr. f. wise, Zool. Bd. Ixy. 1899.
XIX. Poate, L. Bermerkungen iiber die Phylogenie und die
Entstehung der asymmetric des Mollusken. Zool. Jahrb.
Anat, Bad. ix. 1895.
XX. Scamipr, F. Beitrige der Entwicklungsgeschichte der
Stylommatophoren, Zool. Jahrb, Anat. Ba. viii. 1895.
XXL. Scmmopr, F. Die Furchung und Keimblitterbildung der
Stylommatophoren. Zool. Jahrb, Anat, Ba. vii, 1894,
XXIT. Smrrore, H. Mollusea, Bronn’s Klass. u, Ordn. d. Phier-
reiehs. Bad. iii. Lief. 22 und 23. 1896, (Summary of
all recent views on the asymmetry of the Mollusca with
complete literature, )
XXIIL Siseote, H. Weber die miogliche oder wahrscheinliche
Herleitung der asyinmetrie der Gastropoden. Biol.
Centralbl, Bad. xviii. 1898.
Di
284 GASTROPODA.
XXIV. Taree, J. Zur Phylogenie der Gastropoden. Biol.
Centrabl. Bd. xv. 1895.
XXV. Ténnicgs, C. Die Bildung des mesoderms bei Paludina
vivipara. Zettechr. f. wiss. Zool. Bd. lxi. 1896.
XXVI. Vieurer, C. Contribution a l'étude du développement
de la Tethys fimbriata. Archiv. Zool. exper. (3). Tom.
xvi. 1898.
XXVIL Wierzessxi, A. Ueber die Entwicklung des Mesoderm bei
Physa fontinalis. Biol. Centralbl. Bad. xvii.
CHAPTER XXXIII.
CEPHALOPODA.
Systematic :—
I, TerraBrancaia, with two pairs of gills, two pairs of auricles,
two pairs of kidneys, external chambered shell and a large number
of tentacles round the mouth. Funnel consisting of two lobes ; with-
out ink-sac.
1. Nautiloidea.
2. Ammonoidea. .
Il. Drprancuia, with one pair of gills, one pair of auricles, one
pair of nephridia, with internal shell, the chambers of which are
seldom distinct (Spirula, Belemnites) often reduced or absent. Round
the month eight to ten arms. The two halves of the siphon united
to form a tube ; an ink-sac generally present. 3
1, Decapoda, with ten arms.
(a) Phragmophora.
Spirulidae.
Belemnitidae.
Belemnoteuthidae,
Acanthoteuthidae.
(B) Oigopsida.
Ommastrephi:lae.
Onychoteuthidae.
Cranchitdae.
Chiroteuthidae.
(c) Myopsida.
Loliginidae.
Sepioluiae.
Sepiidae.
2. Octopoda, with eight arms.
Cirrhoteuthidae, with fins.
Philonexidae, Tremoctopus, Philonexis.
Argonautidae.
Octopodidae, Octopus, Eledone.
236 CEPHALOPODA,
1, Oviposition and the Constitution of the Egg.
The egg of a Cephalopod, before it is mature and at the time of
oviposition, is surrounded by « protective envelope which, in the
different forms, may assume very various shapes. A large number
of eggs are usually laid at one spot, forming a large mass of spawn.
In Sepia, the eggs constituting the mass are distinct from one
another, Each of them is surrounded by a compact, spindle-shaped,
black capsule of leathery consistency which, at one end, runs out
into a process, by means of which the eggs are attached close to one
another to some firm object. The egg-capsules attain about the
size of a hazel nut. In Rossia and Sepiola * also, the eggs are laid
separately and are attached to some object or else to each other,
but the envelope is less thick and is even transparent (Sepiola).
The eggs of Lotigo, on the contrary, are laid in gelatinous tubes,
each tube containing a large number (in Loligo vulgaris, as many
4s eighty or more), The tubes are attached by one end to some firm
substratum. As they stand out from their point of attachment
radially, they form a kind of tassel. Such large tassels are found
attached to plants, pieces of wood, stones, etc.
The eggs composing a mass of spawn, resembling Cephalopodan
spawn—dredged by Grenacuer off the Cape de Verde Islands and
attributed by STEENSTRUP to one of the T'euthidae (i.¢., to a form
something like Onvmastrephes, No. 14), are also surrounded by a
gelatinous mass, but are not contained in distinct tubes. This spawn
forms @ gelatinous mass 75 cm. long and 15 em. thick which
resembles sausage. Within the gelatinous eover, the violet-coloured
spherical eggs are arranged in fairly regular spiral coils, their number
amounting to thousands. Each egg, as in Zoligo, is surrounded by
a firm envelope. A similar enyelope which must be regarded as the
chorion (p. 246) ulso surrounds the eggs of the Octopoda.
In Octopus and Argonauta, the chorion of the oval egg is drawn
out into a stalk. The stalks of a number of eggs become connected
various authts (2 van evpeay Mevachnorh Uesow) flee rato to &
species of Loligo: further obscurity being due to the fact that the masses of
spawn found and investigated have been attributed to Cephalopods to which
the did not belong. Hgg-masses produced by Loligo vulgaris, w are
said to have been ascribed to Ommastrephes sagitiatus, a form to
the group of the Oigopsida. This led to the inaccurate conclusion forms
remote from one another in systematic position showed ut in
their development. According to Sremysrrvr, this resemblance in
ment is due rather to the fact that they all belong to the genus Lolago, and
theoretical conclusions founded on this similarity would thus be of no value.
OVIPOSITION AND THE CONSTITUTION OF THE EGG. 237
together, large egg-bundles being thus produced, the bundles again
uniting to form aciniform masses (Argonauta). In Hledone also, the
threads from the chorion of the long eggs unite to form a stronger
strand, which then becomes attached to the substratum (Jousry, No.
21). We have ourselves found that the eggs of Eledone (apparently
£. moschata) ave attached by their stalks to the substratum in pairs
or in groups of two or four, A number of such groups are found in
close juxtaposition, giving rise to a spawn-mass consisting of sixty
to seventy eggs. The long ovate eggs of this Eledone are very large,
measuring (including the envelope) 15 mm. in length, while those
described by Joupin are only about half this length,
_In Sepia, the stalks of the individual eggs become twisted together,
the result being a rather large strand of eggs closely arranged round
a central axis. These strands are attached to rocks, the female
covering them with her body and, by the promotion of a continuous
flow of water, assisting in their development (ScHMIDTLEIN).* The
egus of Aryonuuta are still further protected by the mother, as the
spawn is attached to the inner side of her shell and carried about by
her. [Nautilus (Wituey, No. IV.) lays solitary eggs of great size,
each egg-capsule measuring 45 mm. by 16 mm, and containing one
egg 17 mm. long and very rich in yolk.—En.]
The capsules or the gelatinous masses which surround the eggs and the
cementing substance by means of which they are attached are secreted by
special glandular portions of the oviducal wall and by the nidimental glands.
Where there is no such special development of glands in the genital apparatus,
a5 in the Octopoda, the eggs wre surrounded by the chorion alone. This latter,
however, is also found round eggs surrounded by firm capsules or gelatinous
masses. At the animal pole of the egg, the chorion is perforated by the
amicropyle (Fig. 105, m).
‘The conditions under which copulation and the fertilisation of the egg take
place in the Cephalopoda ure so peculiar that we must devote some attention.
ftothem, In the Octopoda, fertilisation probably takes place in the oviduct.
‘The spermatophores are introduced by the help of the hectocotylised arm into
the mantle-cavity or the oviduct. In Argonauta, Tremoctopus and Philonexis,
it is well-known that the detached hectocotylised arm of the male is found in
‘the mantle-cavity of the female. The female in the two last-mentioned forms
possesses a receptaculum seminis in the form of an outgrowth of the oviduct
which serves for the reception of the sperm (Brock).
‘Tn the Oigopsida, as in the Octopoda, fertilisation takes place within the
mantle-cavity, the spermatophores being introduced into that cavity and
Sttached to various parts of its inner wall, Among the Myopsida, in the
OSD ag ge Iona die rn mr und Ejablage verschiedener Seethiere.
Zool. . Neapel, Bd, i. 1879.
238 CEPHALOPODA.
female of Hossia (according to the verbal statements of F. C. v. MarResrTHaL)
there is a well-marked area near the mouth of the oviduct within which the
spermatophores sre attached. In the nearly related Sepiola (also according
to researches by v. MazReNTHAL not yet published), there is a pouch-like
depression of the integument lying laterally to the mouth of the oviduct
for the reception of the spermatophores; this has been hitherto erroneously
regarded as s terminal portion of the oviduct itself.
In the other Decapoda, copulation takes place in an exceedingly peculiar
way, the spermatophores not being brought into the mantle-cavity, but
attached near the mouth on the outer integument of the lip (buccal membrane)
of the female. Glandular invaginations of the integument, in which the
spermatozoa that escape from the spermatophores are stored, are found in
this position in Sepia and Loligo (ViaLLeTox), in Sepioteuthis and no doubt
also in the other genera (v. MazRENTHAL).
In this last case, it is evident that fertilisation takes place only when the eggs
are expelled through the funnel and are retained for s time near the mouth by
the arms. The future leathery egg-capsule (of Sepia) is either still soft at
this time and penetrable by the spermatozoa (?) or only forms after these
have penetrated the egg (through the micropyle of the chorion), the still fluid
glandular secretion being subsequently poured over the eggs by the funnel.
This would then also no doubt apply to the gelatinous mass (in Loligo). It
is an interesting fact that artificial fertilisation of mature eggs taken from
@ female Loligo Pealii was brought sbout by means of seminal fluid found in
the spermatophores of the buccal
membrane (WataskE, No. 50). The
same conditions sre found in Rossia.
The eggs of the Cephalopoda
are unusually rich in yolk, and
consequently attain a considerable
size, a point in which they are
essentially distinguished from
those of other Molluscs. The
eggs of Sepia, for instance, are
fully as large as a pea (Sepia
officinalis). The eggs of Eledone
may be still larger and are ex-
ceedingly rich in yolk (p. 237).
Fio, 105,-—The upper pole of the ogg of
*¢Aryonauta argo in optical section.
71, before fertilisation; 8, shortly
after fertilixation (after Ussow). ch,
chorion, yolk : bs, germlise; mm,
micropyle ; pl, peripheral protoplasm :
Ph,fpolar bellies ace
longitudinal diameter.
The eggs of other Cephalopods,
such as Loligo and Octopus, are
less rich in yolk and therefore
distinctly smaller ; those of Aryo-
nauta are even rather small,
measuring, however, 1:3 mm. in
The food-yolk, which consists of rather fine
OVIPOSITION AND THE CONSTITUTION OF THE EGG, 239
granules, always constitutes by far the greatest mass of the egg. The
egg, in shape, is usually oval (Loligo, Eledone, Octopus, Argonauta)
or spherical, as in Sepia and the Cephalopods investigated by
GrenacuEn (p. 267). The massive food-yolk is completely invested
by « comparatively thin layer of the formative protoplasm, which
thickens to form a dise-shaped accumulation at the upper or future
animal pole of the egg beneath the micropyle. This is the future
germ-dise (Fig. 105, ks) and is sharply marked off from the food-yolk.
In consequence of this peculiarity, and others to be described later,
the eggs of the Cephalopoda afford the most perfect examples of
meroblastic cleanage.
wv
Pro, 106,—1 tle longitudinal sections thro the egg of Loligo Peadii (after
Warainy 5B ia mei tlt, aa oi mt ti ok
eo re
; While. tin food-york ia aadeL. a, decal thier ¢, ventral alder
; 0, anterior ; /, lef ee
A bilateral symmetry can be recognised not only in the early stages
of cleavage but even before cleavage sets in; this symmetry bears a
definite relation to the later development of the embryo. In the oval
‘egys of Lolige Pealii, it is expressed not only in the form of the egg,
‘but also in the way in which the germ-disc spreads over it (WaTASE,
‘No. 50). The eggs of this Cephalopod appear somewhat flattened on
one side while, on the opposite side, they are arched (Fig. 106 B).
‘The positions of the anus and the mouth in the embryo correspond
‘to these two sides. At the part which becomes the anterior end (vo),
the protoplasm of the germ-dise extends further down towards the
‘equator than on the opposite side (4). The germ-disc, however,
ma CEPHALOPODA.
syrenin ty right and left for an equal distance (Fig. 106 A, r and /).
A conuparinm of Fig. 106 4 with the median sagittal sections of
later embrymmic stages (Figs. 132, 133, p. 282) shows that the
animal pole of the egg (¢) corresponds to the area of the dorsal
. Murface, the vegetative pole (c), on the contrary, to the ventral
surface,
In the yerm-dise we find the egg-nucleus and later the first cleay-
age-nucleus, Neither of these, however, quite coincides with the
animal pole of the germ-dise (Fig. 105 A) but occupies a somewhat
oxcentric position, i.«., it is slightly nearer the posterior edge of the
dine, The nucleus appears to be surrounded by a more hyaline zone
of protoplasm which passes externally into a more granular proto-
plasm, The protoplasm of the germ-disc is in any case distinguished
hy its granular character from the thin protoplasmic layer that
surrounds the whole egg. The thickness of the disc appears to vary
in different casos, so far as can be gathered from the statements of
authors, This is perhaps to be accounted for by the various stages
of development at which the disc is represented.
A vitelline membrane is apparently never developed in the eggs of
the Cephalopoda. The mombrane, which is perforated by the micropyle
anc in often very tough, is secreted by the follicular epithelium (Ussow,
ViaLLETON) and may therefore be termed the chorion. Between this
ogg-nhell and the surface of the egg there is a somewhat wide space
filled with clear albuminous fluid. This enables the embryo to extend
considerably within the chorion, which is itself said to be extensible,
and thus to admit. of further growth on the part of the embryo.
At the animal pole, in the mature egg, there are two polar bodies,
one of which has been observed to divide (Loligo and Argonauta) and
ive rive to a third body (Fig. 105, rk), Such division is said by
VIALLETON not to oceur in Sepia, but the fact that one of the two
polar bodies possesses two nuclei shows that these bodies, even in
Sepia, are quite normal, VIALLETON denies that there is any con-
atant relation between the polar bodies and the first cleavage-plane,
but the deviations from such a relation are very rare, and from his
own figures and those of other authors this line of cleavage is seen to
oeour in the closest proximity to the polar bodies.
& Cleavage and Formation of the Germ-layers.
The cleaene of the on: ts incomplete, a fact connected with the
great abundance of the food-yolk in it and the distribution of this
CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 241
and of the formative yolk ; cleavage is af first limited to the germ-
dise. The conditions of this process therefore differ essentially from
those in the other Mollusca, The principal features of these pro-
cesses have been known for many years, being described in Kénni-
keR’s famous work on the development of the Cephalopoda (No. 24).
‘These investigations which, from the nature of the case are far from
being exhaustive, appeared in 1843, and were followed by observa-
tious made by Boprerzxy (No. 4), Ussow (Nos. 44-46), VIALLETON
(No. 48) and more recently by WATAsE (No. 50). We shall here
follow the last-named author, dwelling especially on the detailed
descriptions of the processes of cleavage in Loligo Pealii as given by
him and of Sepia officinalis as described by Vianueton, So far as
4 B
L I
e ee
Z
Pie. 107.— Se reesiardl the polar Voalec (th after ee first (J) and second (/7)
js at present known, cleavage seems to take place in the different
Cephalopods in very much the same way.
The spindle of the first cleavage-nucleus which is preparing for
division lies with its longitudinal axis running from right to left
(Warase), and therefore in the plane depicted in Fig. 106 4.*
It therefore, curiously enough, does not appear to lie, according to
the usually accepted rule, in the direction of the widest extension of
the formative protoplasm.
The first line of cleavage runs, in correspondence with the position
of the spindle, from before backward, cutting the axis of the spindle
at right angles. The first furrow thus lies in the median sagittal
* Of. p. 239 on the bilateral symmetry of the egg before cleavage.
R
242 2 CEPHALOPODA,
plane of the egg or later embryo, 7e., in the plane given in Fig. 106
B. This first furrow (1) above which the polar bodies are, as a rule,
found (Figs. 107 and 105 2, rh), starts from the middle part of the
germ-disc where the cleayage-nucleus lies and is continued to the
periphery of the disc. It cuts deepest in the centre of the dise,
dividing the whole of the protoplasm at this point into two segments
(Fig. 106 B), while, further back and especially beyond the actual
Fro. 108. —1 of Lotiyo Peatii representing various stages of the cleavage of the
-disc, the bilateral of which can be after W. a.
Se eae ogee kath fee
disc where it is also prolonged (Figs. 107 and 106 A), it forms
merely « shallow groove in the formative protoplasm which vanishes
towards the equator of the egg. The same is the ease with the next
furrows which are like the first meridional lines of cleavage (Migs
107 to 109),
The second furrow runs at right angles to the first (Fig. 107 B,
ZT). Tn consequence of the furrows passing beyond the germ-dise
@LEAVAGE AND FORMATION OF THE GERM-LAYERS. 243
into the thin layer of peripheral protoplasm, these first blastomeres
as well as those that follow (Figs. 107 to 109) are not distinetly
bounded externally but fade away into the peripheral protoplasm.
‘This is still the with the peripheral cleavage-cells of later stages
(Pigs. 108 @ to 111). Vianneron defines these as /asfocones ws
opposed to the /axtomeres.
A further step in the cleavage is marked by the breaking up of
each of the four segments now present into two new segments (Figs.
£™=
Lor
ay eS
Pasi et ct at
ihe sl {a directed upwards, and the posterior side downwards, ¢, leit side
fright side; £-V, directions of the first five meridional planes of cleavage,
109 A and 108 B). ‘Of these new furrows (J//' and JJ") only the
anterior ones (7/7') make an angle of 45° with the median plane and
therefore cut the two anterior segments into almost equal halves.
‘The two posterior furrows (J//") ran somewhat parallel to the median
plane (Figs. 105 2 and 109 4), and it thus happens that here, at the
posterior part of the germ-disc, two narrow segments bounded by
parallel sides are cut off. Through this course of cleavage, concerning
he
244 OEPHALOPODA.
which observers are agreed, the germ-disc becomes markedly bilateral,
a character which is retained in the later stages also (Figs. 108 and
109 B and C, 110), the two segments mentioned above and the
blastomeres that proceed from them retaining their characteristic
shape, i.e., retaining their regular position with regard to the median
plane, the other cleavage-cells also continuing to be symmetrically
arranged with regard to that plane.
The broad segments of the eight-celled stage (Figs. 109 A and
108 B) are directed forward, the narrow segments, on the contrary,
backward (Watase, Ussow). In this way the bilateral symmetry
of the germ-dise after cleavage and the relation to the form of the
adult animal are shown still more distinctly than in the egg before
cleavage (cf. p. 239 and Fig. 106 4 and &).
It should be added that ViaLLeTon also describes the striking bilateral
symmetry of the germ-disc after cleavage which is evident from Fig. 109, but
does not assume so definite a symmetry in the shape of the egg before cleavage.
It may be more difficult to establish these points with certainty in the spheri-
cal egg of Sepia. The identity of the median sagittal] line of the blastoderm
with that-of the embryo which is emphasised by Ussow is also apparently
assumed by VIALLETON as probable, so that, according to him also, the
bilateral symmetry of the germ-dise after cleavage corresponds to that of
the embryo.
Since, in consequence of the continuous division of the cells, the shape of
the germ-dise is less regular in the later stages (Fig. 111). it is very difficult
to prove that the bilateral symmetry of the germ disc passes directly over
into that of the embryo: this hax, indeed, not yet been exactly proved, so
far as we can see. But the bilateral character of the germ-dise found in
Cephalopods otherwise very different from one another (Loligo, Sepia, Argon-
auta*) makes relation to the form of the embryo appear more than
probable, and we must therefore for the present hold to the view of Watase,
although, indeed, this seems to be somewhat conjectural.
As cleavage advances further, an equatorial furrow cuts off from the
two narrow posterior segments, towards the-middle of the germ-disc,
two small blastomeres (Fig. 109 8); a continuation of this equatorial
furrow cuts off, in Lolizo, similar blastomeres from the large segments
in front (Fiz. 109 @). In Sepia, however, additional meridional
furrows appear first and divide these segments into narrower sections
(Fig. 109 B, 7V and V), after which they becéme divided by an
* With regard to Argonauta, we have to rely on the statement repeatedly
made by Ussow that he saw cleavage taking place, in the forms observed by
him, in a similar manner to the above. A confirmation of these statements
with respect to the Octopoda is, indeed, very desirable. With reference to the
conjectural Sepiola, also investigated by Ussow, see the remarks made p. 296.
CLEAVAGE AND FORMATION OF THE GERM-LAYERS, 245
equatorial furrow. The two narrow posterior segments, so character-
istic in shape and position, still remain unaltered even after this
division, but two further blastomeres gradually become detached
from them by equatorial division and pressed towards the centre
(Fig. 109 €).
The number of segments increases more and more through the
‘ppearence of other furrows, some meridional and others equatorial
(Figs. 108 @, 110). From the accounts of Kénntkex and ViannE-
‘TON, it appears that the segments at the middle of the germ-dise are
not at first in close contact with each other (Figs. 107 and 109). Ax
cleavage advances, this space in the centre disappears.
) ee 4
rim
Ah
Fi6, 110, —Cerm-dise of Baer teal, at a later ee stars of cleavage, the bilateral sym-
¢ blastomeres and peor ae a
aici ae Yel ater Wotan). ry, ateror sh pnterr Pkt nates ei
a symmetry evident up to this time and still visible in some-
what Inter stages is still further heightened by the division of the cells taking
place wt somewhat different times, a fact which finds expression in the
conditions of their nuclei. Such a case is represented, for instance,
110, in which the posterior colls lying near the middle line contain
‘nuclei while the nuclei of all the other cells are found to be dividing.
t is also frequent in the larger cell-complexes. The almost
-dingrn “aspect thus produced corresponds, as Wavase expressly states,
‘946 CEPHALOPODA,
to the actual condition of the disc. This is also confirmed by Viatteron's
earlier description (e.., Figs. 25 and 26, No. 48), in which the various condi-
tions of the nuclei in symmetrical division are given. Further, whole
complexes of cells such as the posterior cells or those of a luteral part may
advance more rapidly in their division, whilst the division of others may. be
retarded, this being again visible in the state of their nuclei.
In the germ-dise depicted in Fig. 110 two complexes of segments lying
symmetrically and marked off by the planes JJ and JJ7', are distinguished by
the fact that the furrows lying between them are less distinct than those jp
other parts of the germ-disc, These segments are thus shown to be connected
together, and have most probably proceeded from the segments bounded by
the furrows J7 and JZ7' of the stage represented in Fig. 109 4, which is passed
through in Loligo and in Sepia in the same way (Warasr). Such a condition
also renders the bilateral symmetry specially clear.
bee
bulev
Fre. 101,—Germ f Sepia officinadie at a late stage of cleavage (after ViauuETos)
A, blastomeres ; Ale, hlastocones ; «, yolk.
Up to this point, the origin of the cells is clearly recognisable in
their arrangement ; their position with regard to the middle line also
is very regular, As cleavage proceeds further, and the blastomeres
continually decrease in size, this regularity of form is no longer
perceptible in the germ-disc, A certain regularity of position may at
first be retained in the peripheral cells, especially in the blastocones,
but even this is finally lost as the cells continue to inerease in
number. The animal pole of the egy now appears covered by a
CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 247
unilaminar “disc'’’* of polygonal plate-like cells (Fig. 111), The
cells at the edge of the disc (blastocones), however, are not polygonal,
having no outer edge (Fig. 111, b/c), but passing over into the
remaining mass of formative yolk. In Fig. 111, a certain difference
of size can be perceived in the blastomeres, but this apparently
cannot yet be related with certainty to the later form of the embryo.
The germ-dise spreads owt and increases in size as is evident in
Figs. 107-111, at first no doubt chiefly at the expense of the formative
yolk in contact with it; only later is the abundant food-yolk utilised
us nourishment for the growing embryo.
Fie, 112. —Germ-dise of Sepia officinalis at the commencement of the formation of the
germ: fafter VIALLETON), f, yolk: ¢, unilaminar portion of the germ-dise :
ed, (moultilaminar) portion of the yerm-dise (area opaca); #, cells in the
act of separating from the gern-lisv.
After the cleavage-cells have considerably increased in number, the
peripheral cells somewhat change in shape; their free ends narrow
and they show a tendency towards becoming detached from the
germ-dise (Fig. 112 7). They finally become rounded off and move
away from the disc. They are then found scattered round the latter
at the surface of the food-yolk. It should here be mentioned that
+ Phis so-called disc is actually a more or less arched cap.
248 CEPHALOPODA.
these cells, according to ViALLETON, wander beneath the cell-material
of the germ-dise, there becoming arranged into a connected cell-layer
which is said gradually to spread over the whole of the food-yolk.
But this brings us to the formation of the germ-layers, and in order
to comprehend this, we must first refer to another process and also
mention the views which have hitherto prevailed on the subject.
The problem of the formation of the germ-layers in the Cephalopoda
must be regarded as exceedingly complicated. The fact that the
significance of the different cell-layers forming in the germ has not
been recognised and that it has not yet been possible satisfactorily to
trace back the method of their formation to corresponding processes
in the other Molluscs or in the members of other animal groups, is
evidently due to the highly modified conditions under which the
Cephalopodan egg develops on account of the large amount of food-
yolk deposited in it and the marked distinction between the food-
yolk and the formative yolk.
Fra. 115.—Longita dinal section hous a an egg of mt Lali at the which the
Sigs aete the gorm-dise becomes thickened (after frasl Teer Terk
), ¢ peripheral cells; @ and ms, the [reser ot the | edge.
‘The following is a brief statement of the riew hitherto held as to the formation
of the germ-layers. The germ-dise consisting of a single layer of cells which
at first covered only a small part of the animal pole of the egg, at a certain
time, undergoes at the periphery a thickening of the cell-layer (Fig. 118),
‘The layer which thus arises atid which soon gains considerably in size by the
active increase in number of its cells, and spreads out beneath the whole of
the upper layer of the dise, has been derived either through delamination
from the cell-layer already present (Murscaxrxorr, Ussow) or else through
the bending in of this layer (BosRETZKx).
In contrast to the upper (ectodermal) cell-layer, the massive cell-accumula-
tion has been claimed as chiefly mesodermal although the mid-gut epithelium,
which elsewhere is always entodermal in origin, is said to be derived from this
accumulation. Beneath the latter which, for the present, we must regard os
mesoderm, a new layer is now found which, in its origin and relation to the
other layers is of special interest. This is the “ vitelline membrane” of authors
which, starting from the periphery of the germ-dise, no doubt spreads below
the dise (and the ‘mesoderm ") as well as over the whole food-yolk, satround-
ing it as a unilaminar cell-integument,
CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 249
Ray LasKesrer assumes, with considerable inherent probability, that the
vitelline membrane owes its origin to cells (or nuclei) freely distributed in the
yolk, these being at once comparable to the yolk-cells and merocytes in the
‘eggs of the Arthropoda and Vertebrata which are also very rich in yolk.
These nuclei are said to shift to the surface, to become surrounded with
protoplasm and to unite ta form the vitelline membrane.
Other investigators (e.g., Ussow) have been inclined to derive the cells of
the vitelline membrane rather from the deeper layers of the germ-dise, and
thus from the “mesoderm.” Tn any case, this enveloping membrane of the
yolk has the same significance as the vitellophags in the Arthropoda and the
Vertebrata. As the terms vitelline membrane and yolk-integument are not
an happy, being commonly used in another sense, we shall give this
cellular integument another name (also applied to it by Ray LaNKesrEn),
ealling it the yolk-epithelium.
During the formation of the yolk-epithelium, the superficial cell-layer, the
ectoderm, has spread over the whole of the food-yolk, which thus, beyond.the
germ-disc, is covered by two cell-layers. Besides this, there is, as far as the
extends, the “mesodermal” cell-mass lying between the ectoderm
and the yolk-epithelium, The three germ-layers are thus apparently repre-
sented, if we may for the time assume the yolk-epithelium to be the entoderm.
The difficulty arises, however, that the yolk-epithelium is not found to be
connected with the formation of the enteron, which owes its origin rather to
the breaking up of the middle layer.
Having now arrived at some comprehension of the germ-layers
from which the Cephalopodan body is built up, we must trace the
way in which the latter originates, as described in the most recent
accounts.
We traced the origin of the germ up to the point at which the
animal pole of the egg is covered by a unilaminar plate of polygonal
cells, and at which the irregular peripheral cells of this plate begin
to detach themselves from it (Fig 112), Almost simultaneously with
this process, the thickening of the edge of the plate already mentioned
occurs, ie., the plate here becomes multilaminar through more active
increase in number of its cells (Figs. 112, vd, and 118). This is the
process which was described by earlier authors us the formation
(delamination) of the mesoderm.
Before the layer which has formed in this way loses its close connection
with the superficial layer of the germ-disc, the cells previously detached from
that dise undergo, uccording to Viautntos, an essential alteration, Their
cellular character disappears, they are no longer distinetly although irregularly
bounded, but now appear as a syncytium, ic., as nuclei lying in the thin
Tayer of protoplasm that surrounds the food-yolk, There can be no doubt
that we have before us, in them, the same nuclei which, according to
Lanxesrer, give rise to the yolk-epithelium, This last significance actually
belongs to these cellular structures which, according to VrauLeTon, arise
a0 Cw CEPHALOPODA. ve Wet
from the: germ-dise; they increase greatly in namber and at first unite to
form o cell-layer which not only penetrates beneath the disc (Fig. 114 4-C,
de), but also extends beneath the ectoderm over the whole of the yolk. The
formation of this yolk-epithelium and the thickening of the edge of the
dise which affects the whole of its periphery are clearly illustrated in Fig.
114 4-C. These figures show, at the same time, how the yolk-epithelium
Presses in towards the middle of the dise, as assumed by both VrALLeToN and
WATASE.
et hspreneehe ree ee », yolk-epil Sanita’ r, the the aed aaonat of =
germ-dise,
The separation from the superficial or ectodermal layer of the cell-
mass, formed by the thickening of the margin of the disc, gives rise
to the layer claimed by earlier authors as the mesoderm. So as to
understand the real significance of this layer, we must follow its
further fate. Since, to do so, we should have to pass on to some-
what later stages, it will be advisable first to turn our attention to
the external changes taking place in the germ-dise or in the whole
blastoderm.
3, The Development of the External Form of the Embryo.
While the formation of the yolk-epitheliam and the simultaneous
thickening of the edge of the germ-dise are taking place, another rapid
increase in number of cells occurs in the superficial layer of the dise,
and results in the gradual extension of this upper cell-layer over the
whole egg. ‘This layer is known as the blastoderm, although this
term is not quite correct for, apart from the fact that a differentia-
tion of this layer has already taken place round the animal pole, it
DEVELOPMENT OF THE EXTERNAL FORM OF 'THE EMBRYO, 251
is soon followed in its cireumerescence of the egg by a second layer, the
yolk-opitheliam, ‘I'wo areas can now be distinguished in the egg.
‘The germ-dise, which forms the embryonic radiment and which now
increases by the thickening of its margins (Figs. 112 and 113) only
ater extends over the yolk, and a second, the yolk-sac, which at
first is bounded by the two cell-layers, the ectoderm and the yolk-
epithelium. A middle layer is added to the two cellular integuments
which at first cover the yolk-sac, this third layer appearing either
simultaneously with them or very soon after them (p, 277).
The extension of the part known as the germ-dise over the yolk
varies greatly in different Cephalopods. In the egg of Sepia, which
is unusually rich in yolk, the embryonic rudiment is represented only
by a small cup-shaped region of the spherical egg. Here, therefore,
we can best speak of a germ-disc, and the yolk-sac is from the first
very large. In other Cephalopods, indeed, so far as is yet known, in
the majority, the embryonic rudiment and thus also the sv-called
germ-dise spread over a much larger portion of the egg (Fig. 115),
but as, at a luter stage, the embryonic rudiment again draws back
more towards the animal pole, a yolk-sac is formed in these cases also
(Figs. 116-118), In the Cephalopod investigated by Grexacuer,
there is hardly any development of an external yolk-sac (Fig. 126,
p- 270) and the yolk-mass which is comparatively small is here found
enclosed by the embryonic rudiment at a very early stage. This
form would thus have to stand at the end of a series, the starting
point of which would be formed by Sepia with its unusually large
development of yolk. ‘Transitionary forms between Sepia and the
Cephalopod described but not identitied by GrenactEen would be
found in such forms as Loligo, Octopus, Arqonauta, in which the yolk-
sae is more and more reduced and the embryonic rudiment at the
first contains the larger part of the yolk.
‘If we may judge from the large amount of yolk in the egg (p. 297) and the
Jarge size of the yolk-sac in older embryos (No, 26) the condition of Kledone
‘Th this respect may resemble that of Sepia.
Ttshould here be mentioned that the cleavage and the formation of the
germ-loyers in forms which differ somewhat in their later development is, 80
far as is known, yery much alike, and takes place in the way described above.
Ta forms in which the rudiment of the embryo can early be dis-
o.,.. from the large yolk-sac, the ectoderm, in the region of
the germ-dise consists of cubical cells while in the yolk-region it is
formed of flat cells. During its gradual extension over the yolk,
the blastoderm becomes ciliated. The ciliation may extend over the
262 CEPHALOPODA,
whole of the blastoderm or may at first be found only at definite
parts, appearing, for instance, especiully at the growing edge of
‘the blastoderm (Figs, 115 and 116, p. 255). If this ciliation develops
strongly and spreads over the whole of the blastoderm which covers
the greater part of the yolk, as is the case in Loligo, the embryo
rotates within the egg-integument, a phenomenon which recalls the
free-swimming larvae in other divisions of the Mollusea. In Argo-
nauta and Octopus, the only movement that takes place in the embryo
is the shifting of the pole at which cleavage began away from the
micropyle-region to that of the egg-stalk (Ray LankKEsTER). Some
of the embryos of Octopus are, in fact, found lying at the micropyle
end of the egg and others at the opposite pole ; in the older embryos
examined by us the last position was the more frequent.
The ciliation of the embryo either soon disappears, or else is
retained for a long time, as, for instance, in Sepia, where it is found
both in the embryonic region, which is already far developed, and on
the yolk-sac. The embryos of Sepia, however, notwithstanding this
ciliation, do not rotate like those of Loligo, a fact evidently due to
the large amount of the yolk (KénnmER),
Even before the circumerescence of the yolk by the blastoderm is
completed, indications of the future shape of the Squid appear in the
blastoderm or germ-dise (Figs. 115 and 116 4). We shall describe
the rudiment and further development of the body-form at first: in
one of those species in which the blastoderm grows round the yolk
very early and the embryonic rudiment at first encloses the greater
part of the yolk, The ontogeny of such « form, Loligo Peafii, was
studied in a very thorough manner by Brooks (No. 7). Bay Lan-
KESTER has also published investigations as to the ontogeny of
Loligo (No. 30) and representatives of this genus, as has already been
mentioned, were studied by earlier zoologists (P. van BungepEn, No.
3; Merscuniorr, No. 32). Our own investigations, made with
very rich material of Lolige oulgaris, as well as Octopus vulgaris and
Argonouta have enabled us to supplement the discoveries of earlier
observers.
A. Development of the Embryonic Rudiment through its
extension over the greater part of the egg with
subsequent development of a Yolk-sac.
(a) Loligo.
In the region in which cleavage began, i.e., at the animal pole of
the egg, 4 swelling forms in consequence of the thickening of the
DEVELOPMENT OF THE EMBRYONIC RUDIMENT—LOLIGO. 253
cell-layers ; this corresponds to the highest point of the dorsal side,
as may be seen by a comparison with the later embryonic stages.
‘This swelling, which is the rudiment of the mantle, soon becomes
Jarge, and, in the Loligo examined by Brooks, rests like a cap upon
the embryo (Fig. 115 A), but, in Loligo vulgaris, is less distinctly
marked at this stage. At the stage depicted in Fig. 115 A, the
greater part of the circular margin of this cap-like swelling has risen
up in the form of a fold from the embryonic body (Figs. 116 and 117,
ma), the mantle thus forming in the same way as in the other
Mollusca, As already mentioned, the swelling lies on the dorsal side
and a structure appears here which is comparable with the shell-gland
‘of other Molluscs. This is an ectodermal depression (Fig. 115 B,
and 116 A, s?) which at first is shallow but deepens later, and does
A
d
Fie. 115.—Two early stages iv the development of Loligo Pealii (after Brooks). ar,
Trotman of arma ; au, rudiment of eye; @, yolk; 4 rodiment enenary: the
‘ciliated margin of the blastoderm ; sd, Shell land.
not flatten ont again as in other Molluses (pp. 34, 92, 126), but
becomes a.large pouch (Fig. 132, sd, p. 282). This peculiarity is
connected with the fact that Loligo possesses an internal shell.* In
those land-Pulmonates that are provided with an internal shell, we
also found (p. 187) that such a shell-pouch formed from the shell-gland.
‘There is a certain similarity between the swelling which becomes
differentiated into the rudiment of the mantle and the shell-gland
and the first indications of the eyes which also arise in the form of
‘swellings each carrying a pit-like depression (Fig. 115 B). They lie
t the two sides of the body below the mantle (Fig. 116 B, au).
Of. p. 987, on the significance of the internal shell of the Cephalopoda,
254 OEPHALOPODA, ’
Other much larger paired prominences are found lower down at the
sides of the body in Loligo Pealii us the first indications of the arms
(Fig, 115 A, av, Brooxs). It seems to be specially characteristic of
this form that the rudiment of the embryo spreads over a very con-
siderable portion of the egg; in other forms this is not the ease, as
may be seen by comparing Figs. 115 A and 116 2B,
Orientation of the Cephalopod body. A few words must be said as to the
position in which we have represented the embryo, since it is not that formerly
ascribed to Cephalopodan embryos, which are generally placed with the head
and arms pointing upward. Our orientation of the embryo is in conformity
with the now universally accepted view as to morphology of the Cephalopodan
body, the ventral side being directed downward as is usual in other animal
forms. If we regard the part of the body which lies between the mouth and
the anus as the ventral surface, we have to consider by far the greater part of
the body as the dorsal surface. That end of the body which, as opposed to
the head appears as the posterior end must, according to this yiew, which was
first adopted by Levckaanr (No, 81), be regarded as the apex of the dorsal
surface. In such an assumption, according to which the ventral surface lies
in the horizontal line, the embryo ought really to be placed obliquely, but
this position was departed from for practical reasons and the head with the
arms was simply directed downward. The (ascending) part of the dorsal
surface which is directed forward will be called the antero-dorsal, and the
(descending) part which is directed backward the postero-dorsal surface.
These points are best illustrated by the median sagittal section of an older
embryo (Fig. 133, p. 283) which cuts through the mouth and the anus.
In the two ontogenetic stages of Leligo (Fig. 115 A and B) as yet
considered, the circumerescence of the yolk by the blastoderm is not
yet completed, although the rudiments of the mantle (m), the eyes
(au) and indications of the arms (ur) are present, At a somewhat
later stage, the yolk appears completely enclosed by the blastoderm
and a number of new structures appear, especially on the ventral
surface. Among these, the oral aperture deserves special mention ;
this appears as a transverse oval pit somewhat in front of or between
the optic rudiments, this pit arising like those rudiments very eurly.
In Fig. 116 #2 the mouth is seen at a somewhat later stage.
In front of the oral aperture a swelling appears (Fig. 116 B, ar),
which runs right round the embryo, being divided into separate pro-
minences. These latter are the rudiments of the arms which thus
appear in Loligo vulgaris in x manner somewhat different from that
described by Brooxs in the case of Loliyo Pealii. In the latter, the
arms first appear in a very early stage as a pair of prominences, one
on each side of the body (Fig. 165 4, ar), the individual arms
appearing later by the breaking up of these. There is no mention of
DEVELOPMENT OF THE EMBRYONIC RUDIMENT—LOLIGO. 255
a circular swelling such as foreshadows the development of the arms
in L, vulgaris (Fig. 116 2, dv). Further, in L, vu/yarts, the indi-
vidual arms develop more distinctly in consecutive order, the first,
which appear as button-like prominences, being those which lie next to
A
Fito. 116.—Various stages in the development of Loligo vulgaris (original). A, earl:
sage st whe mich th Stee yen atid the dhell-gtand appear’ 2, on ftom tha oral alte;
‘the anal side. J is seen obliquely from above, and for the sake of
‘learness, rather more of the yolk-sac is shown in this figtre than is actual visible.
te mae: € first three eats Se ey au, renee of &
that carry the optic pits; d, yolk ; ds, yolk-sac; Mf, posterior
; between the two branchial promivences lies the rudiment of the mond
faeries mantle ; of, otoeysts ; r, edge of the blastoderm ; ad, sh
at anterior funuel-fold.
the funnel. At this period, the rudiments of various other organs are
already visible, the most striking of these being the paired branchial
fold (Fig. 116 @, k) which lies immediately in front of the mantle
_—
256 CEPHALOPODA.
(m). In front of and somewhat laterally to these folds, a rather long
paired ridge appears (C, Aff) running at first almost parallel with the
branchial folds; it then passes back round the “mantle” at the
margin of which it can be seen even on the opposite side of the embryo
(i.e, when the embryo is viewed from the oral side, B, héf). This
paired ridge, which appears very early, assists in the formation
of the funnel, the chief part of which, however, is derived from a pair
of folds which arises somewhat later more anteriorly (C, vf). These
two pairs of ridges will be distinguished as the posterior (ht/) and
anterior (vf/) funnel-folds. At first they are not prominent, and this
is still the case with the anterior pair at the stage depicted in Fig.
116 C, but they rise up more and more and then become very con-
spicuous (Fig. 141, tr, p. 297). Between the anterior ends of the two
posterior folds a slight curved prominence appears, apparently uniting
the folds of the two sides (J). A very narrow superficial prominence
now also connects the anterior folds; this latter fold is the first
indication of the complete connection of the two anterior folds which
takes place in later stages (Figs. 117 and 118).
When the anterior funnel-folds appear, two vesicular depressions
of the surface are seen behind them (Fig. 116 C, of; Fig. 141 4,
p. 297); these are the otucysts which, when the posterior funnel-folds
shift forward, are found lying near them (Figs. 116 D and 117 A and
B). They lie also in the closest proximity to the large swellings
which carry the optic pits. These swellings, which are very large
even in an early embryonic period (Figs. 116 A and 115 B, au),
continue to increase in size, and give the Cephalopodan embryo a
highly characteristic appearance at the stage just described and
especially in the following stage (Figs. 117 and 118). Only a part
of these large projections is yielded by the eyes themselves which
first appear on them in the form of depressions (Fig. 116 8). These
depressions close later, a second depression then forming above the
first (primary) optic vesicle, and the lens being secreted inwards at
this point (cf. Figs. 142 and 143, pp. 298 and 299).
The formative processes hitherto described affect merely a limited
part of the egg, for the embryonic rudiment which extended earlier
over the larger part of the egg (Fig. 115) has drawn back more
towards the animal pole. There is therefore a part entirely free from
the rudiments of organs, formed only of the yolk-mass and enclosed
by several cellular integuments (Fig. 133, p. 283). This is the
yolk-sac, which in later stages is much more distinct, the embryo
becoming marked off from it by a constriction (Figs. 117119). The
==
DEVELOPMENT OF THE EMBRYONIC RUDIMENT-—LOLIGO. 257
yolk-sac may contract (KéniiKER, Merscuntgorr) and, according
to Ray LANKesTer, carries out rhythmical movements which are
made possible by the fact that its envelopes do not consist, as is
generally supposed, merely of a layer of ectodermal cells and another
of entodermal cells, but also of « mesodermal layer intercalated
between these (Fig. 133) which evidently contains contractile elements.
This mesodermal layer, the presence of which in a number of other
Cephalopods (Sepia, Sepiola, Octopus Argonauta) has been established
through the examination of sections, seems only slightly developed
in the forms which have smaller yolk-saes, but in those with large
yolk-saes as, for instance, Sepiv, this layer is highly developed, long
straight, fibre-like cells here lying one above the other.
It appears that, by the contraction of the yolk-sac, its contents
are pressed into the interior of the embryo, extending far into the
embryonic rudiment (Figs, 132 and 133, p. 282). A distinction is
therefore made between the outer and the inner yolk-sac, the latter
extending as far as to the mantle and into the optic swellings. Here
also it is enclosed in the ‘‘ yolk-epithelium” and does not, as has
been assumed, stand in direct communication with the intestinal
canal of the embryo, so that the yolk-substance can be utilised by
the embryo only with the help of the yolk-epithelium, As the yolk-
sac extends so far into the embryo, the surfaces are in contact over
«very large area, a fact which explains the absorption of the yolk
without direct communication existing with the intestine and with-
out the intervention of special vessels. The whole embryo (including
the yolk-sac) increases in size during development, the later stages
being much larger than the eurlier,
Although the great development of the external yolk-sae at first
recalls the conditions which prevail in the Vertebrates, some difference
is brought about in the Cephalopoda by the fact that the yolk-sac
is Were devoid of any direct communication with the intestine.
Special vessels also xeem to be wanting in the yolk-sae, as already
mentioned, and it therefore enters far into the embryonic body.
Further, the yolk-sac in the Cephalopoda is ventral, its position at
the anterior end of the body surrounded by the arms being merely
apparent. In reality, it lies ventrally (between the mouth and the
anos) and a comparison of Figs. 116 4 and D, and 113, p. 248,
shows that the mouth lies at one side of the yolk-sac (the anterior
side) and the anus at the other (posterior) side,
‘Tt hus already been mentioned (p. 254) that the mouth appears ws
arndiment at an early embryonic period (Fig. 116 8). The anus
8
258 CEPHALOPODA.
does not appear until later, sinking in at the middle of a slight
prominence, the anal papilla (Fig. 116 D); the posterior part of the
intestinal canal, starting from this point, runs towards the mantle,
its course being externally marked by a slight rising on the surface
(Figs. 117 and 118).
In the embryo of Loligo Pealii, at a stage intermediate between
those depicted in Figs. 116 B and C we find, in the oral region,
starting from the two angles of the mouth and running first to the
optic pits and then passing anteriorly, two rows of cilia ; these have
been compared by Brooks with the velum, ée¢., with the pre-oral
ciliated ring of other Molluscan larvae. We should, in this case,
have to regard the part including the eyes and the very limited
region between them as the pre-oral part of the body.
Among the changes which take place on the dorsal side of the
embryo, those that occur in the mantle exercise a special influence
on the external form of the animal. The pit representing the shell-
gland which, at first, is very wide (Fig. 116, «/) narrows with time,
and in later stages is only a small aperture (Fig. 117 B) and finally
closes entirely. The external aperture of the continually deepening
pit becomes surrounded by a kind of circumvallation, the surface
again sinking in, though not to any great extent (Fig. 116 D). The
almost rhomboidal swelling (m) which surrounds this shallow de-
pression represents the marginal part of the mantle which now
begins to rise above the rest of the body. At a slightly later stage
(Fig. 117 A), we find the mantle becoming marked off by the
swelling of its margin. ‘The large depression round the circular wall
of the shell-gland flattens out again and becomes rounded off like the
edge of the mantle itself (Fig. 117 2). Two pointed prominences,
the rudiments of the fins, are visible upon it (Fig. 117 4 and 8).
Another change has taken place in the mantle-region on the
dorsal side, the posterior funnel-folds having shifted more tovards
the middle line, there ending in a kind of plate which is the rudiment
of the nuchal cartilage (Fig. 117 B). The posterior funnel-folds are
thus recognisable as the broad muscle-bands (the so-called nuchal
muscles) which even in the adult appear as lateral continuations of
the funnel. They run to the nuchal cartilage and become attached
to it.
In tracing the further development of the funnel, we find that the
anterior folds form the essential factor in determining the alterations
that take place in its principal parts. These anterior folds, which
soon rise much higher than the posterior folds, become united in the
DEVELOPMENT OF THE EMBRYONIC RUDIMENT—LOLIGO. 259
ventral middle line (Figs. 116-117). At first they form together
merely a slightly undulating line (Fig. 117 A), but their lateral ends
soon bend back further (Fig, 117 #) and as the folds at the same
time swell up more, the later form of the funnel becomes indicated
(Fig. 118 4 and #). At the same time, the posterior funnel-folds
become modified in shape, now appearing forked at the point where
they come into contact with the anterior folds (Fig, 118 4). This
is caused by the rise of a new fold at this point, which runs towards
the mantle, This is, like the other folds, an expression of the
greater growth of the mesodermal tissue and represents a part of the
retractors of the funnel (muse, depressores infundibuli).
Fre, 117.—Two in the development of Loliga oulgaris (original). A, vege from
cpeera jsiche iby obliquely from above. ( Sights vi 116
ree irs of arms, a, optic swelling; ds, “BAC 5
Mp ee atoe hustled k, Ae Solas ch giles ok, masiies Sh, wate cad,
lage 5 oe otocysts; off, anterior funnel-fold. ‘The vireular fold ont of which the
arise can still be recognised, especially in A. In B, the long i-
nences indicating a pair of arms Prin Bebind @,) still form part of it, The perl
and /f) meet in the middle line. “Between the two gills (k) lies the
anal papilla; on the mantle, are the two prominences representing the
of the fins (¢/. Fig. 118, f).
Tu the adult, these muscles are attached to the funnel laterally, some of
them running further forward to end within the fannel and in its dorsal parts.
‘This ix perhaps indicated even in the embryo (Figs. 117 and 118) by the
course of the anterior folds, but we have hitherto been unable to make any
careful examination of the fate of these raised parts which anteriorly become
connected with the posterior funnel-folds, and must therefore refrain from
‘conjectures as to their significance,
260 CEPHALOPODA,
It is evident from the above that not only do the anterior funnel-
folds from the two sides unite to form a common fold but that the
posterior folds also fuse with the anterior, At the point of junction
of these two sets of ridges, the posterior folds appear as a mere pro-
longation backward of the anterior (Figs. 118 and 119), By this
process and by the simultaneous extension of the anterior folds, the
funnel hus already approximated closely to its final shape (Fig. 119).
The free edges of the two anterior folds become apposed, but do not
as yet fuse (Figs. 141, fr, p. 297 and 143, #, p. 299) and the funnel
Fie, 118.—'Two embryos of Latin ewdguri, seu from the. prsteror ge fuels
. ital Qy-dg, ATMS; ce, Hae de, wget Jt, fins;
ills; me, mw retractor of be funnel ; of. eee
faunal elds.” Between the tive pills (A) ithe anal papilla
has thus attained the stage which is retained throughout life in
Nautilus. \n this primitive Cephalopod the funnel, though acting
as a tube, is actually formed by the bending round posteriorly and
the overlapping of two distinct folds.
As development advances, the two half-tubes composing the funnel
fuse in the middle line (Fig. 120 4); the exhalent aperture, however,
which could already be recognised at an early stage, being retained,
The formation of the funnel is thus completed in all essential points,
Laterally, the posterior funnel-folds run to the nuchal cartilage (Fig.
DEVELOPMENT OF THE EMBRYONIC RUDIMENT—LOLIGO. 261
120 A and FB, Am). They continue to increase in breadth and
represent the nuchal muscles (musculi collires) which, together with
the retractors (Fig. 120 A, 7) that run backward direct to the
mantle, form a kind of lateral chambers, not communicating with
the actual funnel, /e., with the middle fannel-cavity. The funnel-
valve wppears in the anterior part of the funnel, 4, in the wall
whieh is in contact with the body, as an unpaired fold (BRooKs).
While the changes just described
have taken place in the funnel,
the mantle also has undergone
repeated modifications of form. It
now rests like a capon the end of
the body (Fig. 118, ma) as its
edge has extended forward further
and has become raised from the
hedy. The further growth of this
overhanging margin leads to the
formation of the mantle-cavity,
into which the gills are drawn, these
not having essentially changed
their shape (Fig. 118, 4). The
i in size of the mantle is now
chief feature of its develop-
(Figs. 118-121). The fins
its upper end also increase in
z The optic swellings have ds
become very Iarge. It has already P16. 119.—Oller embeyo ot Lolyyo
wen pointed out that in this way tees oral. I era
the enabryo wequires specially The Mo the Itong le
charseteristic appearance which it by the overhanging margin of the
Gelindihs leter tages uso (Figs. cet between them is the anal
120 A and &),
We have 86 far mentioned only the first stage in the rise ur the
urine (p. 254). From the circular fold which runs round the whole
embryo at the boundary line between the embryonic rudiment and
the yolk-suc (Fig. 116 4, ar), the separate prominences which repre-
sent the arms become differentiated, each first appearing as u long
swelling which soon assumes a bntton-like form (Fig. 116 2-2). The
first pairs of arms to become distinet are the two that lie nearest
the funnel, the second pair of arms, however, in Loliyo attains a far
higher degree of development than the first and than the one that
262 CEPHALOPODA,
follows it, a distinction which is retained in later stages. ‘This pair
of arms, the prehensile arms, is soon followed by a third pair (Pig.
116 C and JY). When the first three pairs have become distinetly
differentiated us button-like prominences, the other two which lie
nearest the mouth are still mere transverse swellings ; the first of
¥ ia. 120, -Two older embryos af Lotigo vulgaris, A, seen trom the Iwunel site, Br
from the oral side (original). aj-@,, arms; aw, eyes: ds, yolk-se ; Jf, tins ; fom,
nuchal muscle ; ma, mantle ; rt, retractor of the funnel (tr). “In 1, the gills project
helow the mantle ; hetween them is the anal papilla.
these to become distinct is the fourth, then follows the fifth* As
‘the embryo continues to develop, the arms grow in length, and the
sucking dises appear on them. The change of shape undergone by
* In speaking here of the first to fifth pairs of arms, they are numbered
according to the order in which they originate in the embryo, uot
to the order somotimes adopted (for which we can see no good reason)
which, on the contrary, the pairof arms whieh lies furthest dorsally counts
‘as the first and the most ventral pair as the fourth, ths pees arms being
reckoned separntely, evidently on account of their different development,
which seemed to give them a claim to a special position,
DEVELOPMENT OF THE EMBRYONIC RUDIMENT—LOLIGO. 263
the embryo in rising up from the yolk prodaces a change in the
position of the arms which shift from the funnel-side more towards
the oral side (Fig. 117-120).
In Loligo Pealii, the three pairs of arms depicted in Fig. 115, ar, develop
first and only when these have become differentiated, does the pair whieh
hhes nearest the funnel appear (Brooks). ‘The fifth pair seems to form very
late. The same is said to be the case in
(Gmenxacnen’s Cephalopodan embryo (p. 271).
The second pair of arms, the prehensile
arms, grow with special rapidity, out-
stripping in development not only the first
but the other arms also (Figs. 119-121) ;
external investigation, however, reveals no
special peculiarity in the rudiments of these
arms. This corresponds with the adnit
condition in other more primitive Decapods
(Ommastrephes). Only later, when the
embryo hatches, are the prehensile arms
distinguished by the fact that the basil
part is free from suckers, this part lying in
a depression. In younger embryonic stages
and even in older stages (Fig. 120 4 and
A), tive pairs of arms can easily be made
out, but the fifth develops much less than
the rest, so that in later stages (e.g., in
that depicted in Fig. 121) it appears
merely as two small cones which can hardly
he made out. ‘This fact no doubt led to 'y, {al puter en
rT i the posterior or fimnel-side
the assumption that only four pairs of arms ee ete ape
appeared as radiments in the embryo, the the necond ‘pair and. be-
i tween them the first pair
fifth forming later. of arms; dm, eyes; de,
Sinde the yolk-sae occupies a ventral ‘olk-sae 5 ft, fins; &, gills;
between the two Is is
position (between the mouth and the anus), the anal papillu; ¢r, funnel,
the same position must be ascribed to the Thesis Is, already
arms that surround it. This is especially ——_phores.
noticeable in the younger stages (Fig.
116 BD), although it can also be recognised in older embryos
(Fig. 120 4 and &). The mouth here still lies outside of the circle
of arms, but seon shifts within it (Fig. 120 B) or rather is surrounded
‘by the arms as they shift dorsally. This occurs simultaneously with
the degeneration of the yolk-sae that now begins, The mouth finally
264 CEPHALOPODA.
occupies the place of the yolk-sac, /.-., is surrounded by the arms, a
position which, as is well-known, is occupied by it in the adult.
When the yolk-sac begins to degenerate, i.¢., in the last stages of
development, the embryo approaches ever nearer the form of the
adult (Fig. 121). The arms are still rather small, the eyes still
remind us of their former condition, but the funnel, the mantle, the
gills and the anal papilla have almost attained their final form. The
chromatophores have already formed and they also give the embryo
a characteristic appearance more like that of the adult. The
chromatophores appear first on the mantle on the posterior (postero-
dorsal) side and are only somewhat later found on the arms and on
the head. When the embryo leaves the egg, the external yolk-sac
has, to a great extent, disappeared.
(b) Octopus.
Although, systematically, Octoprux is far removed from Loligo, the
course of development of the two forms is very similar. The em-
bryonic rudiment extends at first over a larger part of the egg and
later withdraws again more towards the animal pole, just as in Loliyo.
‘The shell-yland appears very early, at a time when the blastoderm
has not yet grown round the yolk, as a depression at the animal pole.
This fact is of special interest, because Octopus, like most other
Octopoda (Cirrhoteuthix perhaps forming an exception?) has no
internal shell. The shell-gland in this case, therefore, has the
significance of a vestigial organ. At somewhat later stages, when
thé rudiments of the external organs appear, it can be recognised as
a distinct depression at the apex of the rudiment of the mantle, and
even later is perceptible (Fig. 122 4). According to Ray LANKESTER.
it disappears without having closed.
At a stage somewhat earlier than that illustrated in Fig. 122 4, «
pair of small prominences appear on the mantle, resembling, in shape
and position, the fins described in Loligo. Indeed, the aspect of the
mantle-rudiment in Ocfopux in the younger stages closely resembles
that of the mantle-rudiment of Lofigo. The prominences are retained
for some time; they can be recognised in Fig. 122.4 and 8, and in
later stages are still present, but, finally, they decrease in size and
altogether disappear. They cannot be regarded as anything else than
the vestiges of a pair of fins, and must be considered as an indication
that the Octopoda originally carried fins, like the Decapoda. This
fact affords further support to the view which in itself is probable
DEVELOPMENT OF THE EMBRYONIC RUDIMENT—OCTOPUS. 265
that the Octopoda must be regarded as derived and the Decapoda ax
primitive forms. The conjecture as to the nature of these promin-
ences is further rendered probable by the fact that fins occur in some
adult Octopoda (Octopus membranaceux, Pinuoctopus, Cirrhoteuthia).
The oral aperture appears in Octopus at a-very early stage and
soon takes the shape of a semicircular groove bordering a swelling.
The appearance of the rudiments that lie on the ventral side closely
resembles that described for Loligo and will best be understood by
a study of Fig. 122 A and &, and by comparing this figure with the
illustrations of the Loliyo embryo given in Figs. 117 and 118.
It should be noted that the funnel forms, in the Octopoda, in the
same complicated way as in Loligo and the other Decapods. A paired
posterior fold appoars and, in the course of development, unites with
an anterior fold which is also paired to form the funnel and its lateral
parts (Figs. 122 and 123, Wé/, vty and rf).
do.
Fic, 122 — Two embryos of Ortyyns rulguria at different ages, seen from the posterior
de (original). ar, arm : au, optic swellings ; ds, yolle-xae : h(7, posterior fu
; Ay gills; om, mantle ; of, otocyste; rtf, anterior funuel-fold,
In comparing Loligo and Octopus, we are struck by the fact that, in the
latter, the separate organs appear very early, but do not develop further with
corresponding rapidity, so that, as contrasted with organs that appear later,
their development is retarded. This peculiarity wax pointed out by Brooks
(No. 7), in comparing the form examined by him (Loligo Pealiiy with
Gresacuen’s Cephalopodan embryo. In (efopus, for instance, the rudi-
ments of the arms appear early, their number being complete even earlier
than the stage depicted in 122 A, but they then develop very slowly.
With regard to the order in which the arms appear, this seemed to ux to be
the same as in Loligo, ie. from the rudiment of the funnel towards the
mouth, Two pairs of arms appear as slight swellings in front of the optic
266 CEPHALOPODA.
prominences; @ third and a fourth pair are soon added; this latter pair, iv.,
the one near the mouth, the most dorsal of the
arms, is always the least developed. The pairs
of arms appear very quickly one after the other,
so that the above statements are made with a
certain reserve,
In the development of Octopus, it is of
xpecial interest that the yolk-sac is some-
what less developed than in Loltyo, as is
evident from a comparison of Figs. 122 and
123 with Figs. 118 and 119, p. 261. In
earlier stages, this difference is less marked ;
later, on the contrary, it is still more notice-
able. In itself, this comparatively slight
difference in the development of Octopux
da would be hardly worthy of note, but it
Moca, wee ‘Row te fords a transition to those forms in which
funnel - side (original). ~— the yolk-sac develops still less (Argouutay
ur, arms; au, optic swell- A 5 ‘
ings; Am, nuchal muscle; OF; indeed, is almost altogether wanting
et, retractor muscle of the . “3
founel (0), (GRENACHER’S Cephalopod).
(c) Argonauta.
Aryonauta, in its development, also closely agrees with the forms
hitherto considered. Special interest attaches here to the appearance
of the shell-gland at an carly embryonic stage and to its retention
for a long period, although the shell of the adult does not develop in
it, having another origin and significance, as will be shown later (p.
294). The xhell-gland, as in Octopus, is said gradually to flatten out
again (Ray La TER, No. 29; Ussow, No. 44, p. 352), and the
Argonaut shell forms after embryonic life is over, as was shown by
KGLLIKER in opposition to former statements (Nos. 24, 1 and 9).
The embryos of Argonauta show, at various stages, in the rise of
the different organs, great similarity with the Cephalopods already
considered (cf., the late stage depicted in Fig. 124) especially with
the embryos of Octopus and Loliqn of about the same age (Figs. 123
and 119), but the small size of the yvolk-sac scems to determine a
more compact form of bod.
As in Loligo, the embryonic rudiment at first extends over a lange
part of the egg, but becomes at a later stage more concentrated,
withdrawing more to the animal pole, and rising from the yolk, thus
DEVELOPMENT WITHOUT ACTUAL YOLK-SAC. 267
giving origin to an external yolk-sac which, however, is not nearly
so large as in cases already mentioned
(Fig. 124). As development advances, this
volk-sac decreases in size, and, in mature
embryos, at hatching, there is not any
trace of it.
The differences in size and shape of body
existing between the two sexes of Argonauta,
also find their first expression during post-
embryonic life. No sexual dimorphism
could be observed in any of the many
embryos examined by us.* This applies
also to the striking heetocotylised arm of Fic. 124. — Embryo
i , aAryonauta argo, Wi
the male, which in other Cephalopods funnel still incomplete!
Lala leveloped (original),
also, becomes differentiated only as sexual eloped (original)
maturity is gradually attained. Mature ee ee tunnel
at, tm, .
embryos of Octopus, Loligo and Sepia show :
no sign of this modification; this is the less strange as the arms
are still far from being fully developed in these embryos.
B. Development without actual yolk-sac.
The description of the above forms may best be followed by that
of the Cephalopod observed by GRENACHER (No. 14), a form the
systematic position of which has not yet been determined. It
probably, however, belongs to the large division of the Oigopsida,
perhaps to the genus 7wthis, STREENSTRUP (No. 42) believes
that it resembles Ommastrephes (see also p. 236). The eggs, unlike
those of the forms hitherto mentioned, are spherical and distinguished
by the violet colour of the yolk which elsewhere ix yellow. They
are about 1 mm. in diameter, and thus much smaller than those
of Argumuuta. ‘This is a remarkable fact since, judging from the
quantity of spawn and the number of eggs contained in it (p. 236),
this Cephalopod is inost probably a large animal. The small size
of the eggs, and the small quantity of yolk contained in them afford
an indication of the manner of development of this form, which is
marked by an almost entire absence of the external yolk-sac.
©The number of eggs laid by the female Argonaut is very large, so that a
considerable number of embryos are to be found in the bunches within
the shell. The eggs to which we had access belonged to various females and
were at different stages of development, and, although no fully mature embryos
were found by us, we were justified in forming the above conclusion from those
in a late stage of development.
|
268 CBPHALOVODA.
Since, in Guesacten'’s Cephalopod, we apparently have a comparatively
ptimitive form, it has been thought that paucity of yolk might be regarded
as the primitive condition, So long as the first ontogenetic processes are not
known, no decided opinion can be given on this point, but the
of this form in general shows such close similarity to that of other Cephalopods
that a reduction of the yolk seems more probable. We may also regard the
early appearance of the chromatophores which elsewhere appear late as
secondary (see below), Although the Oigopsida are very primitive forms among
the Cephalopods now living (Nautilus and Spirula excepted) they themselves
appear highly specialised when fossil Cephalopods are taken into account.
‘There is a great gap between them and the forms with chambered shell, and
yet Nautilus even has large eggs rich in yolk, as is shown by examination of
the ovary (Owen, No. 38), [The egg-capsule measures 45 x 16 mm. and the
actual egg is 17 mm. long (WuLex, No. IV.).—Ep.]
The earliest development seems to agree with that of the other
Cephalopods. A blastoderm forms which, however, in this case very
soon grows round the yolk, Kven
beforedistinct rudiments of organs
can be seen on the blastoderm, it
has grown almost to the vegetative
pole, leaving only a small portion
of the yolk free (Fig. 125), The
growing edge of the blastoderm is
beset with cilia, but these do not
lead to any rotation of the embryo,
The first indication of organs is
the appearance at the animal pole
of star-like cells containing red
pigment, which rapidly increase
Fic. 125,— Young eubryo of Guawa- in number (Fig, 125, rf). ‘These
oe Mio pba eeerone are the chromatophores whieh thus
the yolk (after Gnexacksn). ch, chro- form, not asin other Cephalopods,
matophores: yolk; v, mang of th towards the end of the embryonic
period (Fig. 121), but, strange to
say, arise quite at the commencement of that period. They soon
spread over the upper third of the blastoderm, at the edge of which
a circular fold now forms. The upper pigmented part of the embryo
thus rises from the rest of the body as the rudiment of the mantle,
this process beginning at the postero-dorsal surface, i.¢., in the
neighbourhood of the future anal pit, and proceeding towards the
antero-dorsal surface. During these differentiations, the originally
spherical embryo becomes cylindrical. In Fiz, 126 4 and B the
DEVELOPMENT WITHOUT AOTUAL YOLK-SAC. 269
rudiment of the mantle is seen in a later stage thickly covered with:
chromatophores.
It is curious that Gresacien makes no mention of a shell-gland, as an in-
ternal shell usually oceurs in the Oigopsida, and a shell-gland even appears
in those Cephalopods which are devoid of the internal shell (Octopus,
Argonauta). The region of the animal pole in general, in GrewacuEr’s
Cephalopod, differs from the same region in the other Cephalopods con-
sidered by us, and it appears probable that the intermediate stages were
missed in consequence of the far from favourable conditions under which the
observations were carried out. The mantle is rounded at its end, and the fins
which, in Loligo, form so early, appear only at a later stage.
We have already, in a Loligo, seen the arms arise not far from the
vegetative pole (Fig, 115 A, p. 253). In GrRENACHER’s embryo,
the rudiments of the first two pairs of arms appear as fold-like
prominences directly at the edge of the blastoderm which has not
yet completely closed, ¢.¢., quite near the vegetative pole, so that
wt this stage and still more at a somewhat later stage, when the
cireumerescence of the yolk by the blastoderm is completed, the
whole yolk with the exception of a sinall portion is enclosed in the
embryonic rudiment. But here also, later, a process takes place
similar to that described for some other Cephalopods, viz, the
ic rudiment withdraws to some extent from the vegetative
pole (Fig. 126 A) a process which, in the cases before mentioned,
led to the formation of a yolk-sac (Figs. 115 and 116, p. 253).
Here, indeed, this takes place only to an inconsiderable eegree, and
thus gives rise to the mere indication of a yolk-sac (Fig. 126 A and
#B, de) which, however, becomes somewhat more distinct later (Fig.
127, ds). At the time when this swelling, which corresponds to the
yolk-saec of other Cephalopods, forms, and in consequence of it, the
aperture of the blastoderm, which is still present, undergoes displace-
ment, being pressed further away from the vegetative pole towards
the so-called nuchal region. This part is for a long time marked by
a ciliated area lying somewhat in front of the disappearing aperture
(Fig. 126, w). Anteriorly, the oral aperture (m) appears as an
vetodermal invagination.
When considering the ontogeny of Loligo, a ciliated area lying near the
tmouth was mentioned (p. 258). The ciliation occurring here in GneNacuen’s
Cephalopod in the oral region recalls that area which was compared by
Brooks to the velum of other Molluscs. A comparison of the two is, how-
ever, inadmissible, since the ciliated area in the present case lies in front of
or below the arms, as is evident from Fig. 126 4, and still more from Grey-
4schmn’s figures of later stages. The ciliation thus belongs to the region of the
|
270 CEPHALOPODA.
yolk-sac and consequently lies ventrally, In Loligo, on the coutrary, it is
found between the rudiments of the arms and the eyes,
‘The position of the oral invagination in the neighbourhood of the former
‘aperture of the blastoderm might appear remarkable (Fig. 126 4), but may
possibly be accidental, being caused by the great reduction{of the yolk-sno.
We shall have to point out later that we are inclined to regard the slight
development of a yolk-sac in Grenacner's Cepbalopod as a secondary
phenomenon,
Fra, 126,—Two stages of Grexacten's Cephalopod embryo, A, seen from the side
B, from the ventral ee. pone surface (after cae My-O, AIMS; aw
rudiment of ©) Qi Hever ch, chromatophores ; ds, valk saa i, ‘posterior
ftunnel-fold fears gill; «, month: mu, mantle ve, apertre of the
blastoderm ; of, otooyat (7, anterior funnelfold ; w, elliated ares.
About the time when the rudiments of the arms appear, a large
swelling arises behind them on cither side of the embryo, in connec-
tion with a depression, the rudiment of the eyes (Fig. 126 A, au).
Soon after, the rudiment of the funnel appears in the form of two
pairs of folds. The anterior, slightly undulating folds are inclined
‘one to the other (Fig. 126 2). They unite later, in the way already
described for Loligo, to form the funnel (Fig, 127), The posterior
folds, which were originally distinct from the anterior though
running in the same longitudinal direction, fuse later with these latter,
and in any ease yield the lateral portions of the funnel, that is, the
so-called nuchal muscles (Figs. 126 2 and 127). At the points
where the anterior and posterior folds fuse, a process rans inward
a
DEVELOPMENT WITHOUT ACTUAL YOLK-SAC, 271
which gives rise to the retractor of the funnel. We thus sev here
again that the funnel forms in much the same way in other forms.
The course of development
of the other organs, stich as
the gills and the intestine, the
otocysts and the eyes, can be
made out by reference to Figs.
126 and 127, as they resemble,
in their origin and development,
the similar structures in Loligo.
The arms, on the contrary,
must be referred to more in
detuil, since, according to
Gmexacuer's statement, they
“Appear in an order somewhat
different from that seen in
Loligo vulgaris. We have
ready mentioned two pairs of
arms at an earlier stage, and
there was some uncertainty as
to the presence of a third pair,
whieh in any case soon follows
the others, Of these pairs of
arms, those which appeared first
are said to be the more dorsal
in position, At the base of the
which perhaps lead to
‘the formation of the foarth pair ; ab, pe
vesicle; au, rudiment of eye; oh, chro-
_matophores: ds, Ik-aue; gu, optiv
j Am, nuchal (collar) muscle ;
‘teral portion of the funnel ; &, rudiment
of gill; ma, mantle ; of, otocyst ; rt, re-
tractor of the funvel ; fv, funnel; sk,
“white body",
subsequent third pair a fourth
grows out, diverging to a certain extent from it. A fifth pair has not
‘been observed in this Cephalopod, but probably develops later,
‘If Gaesacuen is correct in his statements, the arms, in this Cephalopod,
‘appear in an order the reverse of that which prevails in Loligo and Sepia, A
certain departure from the accepted order was also found in the Loligo
‘examined by Brooks (p. 263). We must await further observations, made
for preference on related forms, to solve this difficulty. We shall not attempt
to desoribe the arms of this Cephalopod embryo, although the difference in
‘size between them and those of the embryos of Loligo in younger stages
tp, 261) seems to demand examination, and the different result as to the order
of appearance of the arms in Grenacuen’s embryo might thus be attained.
Ty consequence of the small amount of yolk they contain, the
embryos soon resemble the adult in the shape of the body more
closely than do those of other Cephalopods. ‘The embryo depicted in
272 - CEPHALOPODA.
Fig. 127, for instance, resembles the adult more than do embryos of
Loligo and Sepia at somewhat corresponding stages (Fig. 119, p. 261,
and Fig. 129 C, p. 275). By the further growth of the mantle,
the appearance on it of fins, 2s well as the closing of the funnel which
is grown over by the mantle and, like the gills, partly enclosed in
the mantle-cavity, the embryo approaches the adult form more and
more. The eyes which, in this embryo, are very large, also decrease
in size, as their internal structure gradually develops. The arms
grow and become covered with suckers. The swelling between them
(the external yolk-sac) gradually disappears, and the internal yolk-
sac, which here also fills a large part of the embryo, is gradually
but very slowly absorbed. The oral aperture, in consequence of this
process of growth, shifts downward to between the arms, to take up
its final position at the anterior end of the body. At about this
stage, the embryo breaks through the egg-envelopes and, during the
period of pelagic life which now ensues, assumes the final form of
the adult.
C. Development through the formation of the Embryonic Rudi-
ment on a limited portion of the egg with simultaneous
development of a large Yolk-sac.
The type of development now to be considered has only been
observed in the egy of Sepia (KOLLIKER, No. 24; VIALLETON, No.
48), but may occur in other specially large Cephalopodan eggs as
well. This forin of development is brought about by the-abundance
of yolk in the egy, and is characterised by the restriction of the
embryonic rudiment toa small, cap-like portion of the egg (the germ-
dise), the rest of the yolk being enclosed merely by the thin cell-
integuments already mentioned (ectoderm, mesoderm and entoderm,
p. 257). There is a further difference between this form of develop-
ment and that in which the embryonic rudiment extends over a
considerable part of the egg, inasmuch as the embryo appears on a
surface only slightly arched (Fig. 128 A and 8). This makes the
processes of development somewhat more difficult to understand, but
we are assisted in following them by comparison with the same
processes in eggs less rich in yolk.
The first indications of the embryo on the germ-disc take the form
of various prominences and swellings which soon show bilateral
symmetry in their shape and arrangement. This symmetry most
probably corresponds to that already evident in the germ-dise during.
DEVELOPMENT OF SEPIA. 273
cleavage (see Fig. 109, and Fig. 110, of Loligo), The swellings
of the surface which at first are very vague and indistinct, become
gradually clearer and can soon be recognised as the rudiments
of the different organs, one of the first to grow distinct being a
cirenlar depression at the centre of the germ-dise. This becomes
surrounded with a flat wall which is more or less pentagonal with
rounded corners (Fig. 128 4, sd, and ma). This represents the
rudiments of the shell-gland and the mantl-, in the early appearance
of which Sepia resembles
those highly developed
structures which, in other
Cephalopods, we regarded
as the optic swellings.
In Fig. 128 A, they are
represented at a some-
what later stage (Jt).
‘The divergent character
of the development of
Sepia is specially shown 5 .
‘in these structures, which a. | \
here appear on an almost 2
Mae) curface; claowhere "1% 228. — Germdiscr with young embryonic
they form two large and and KGutaxxn). "a, auus; ay-ay, the five pairs of
: arms ; au, rudirents of posterior -
very prominent swellings, fold ; X, wills; Kl, ce} m a totes bs, ich 3
one-on ether side of the (yy MMs"! (tna fia
‘The germ-disc, on the side opposite to the optic swellings, ix
bordered by w narrow band-like prominence which at first is almost
semicircular but soon extends round the greater part of the germ-
dise and then resembles an incompletely closed circular swelling.
‘This corresponds to the circular swelling which, in Loligo, runs round
T
274 CEPHALOPODA.
the whole embryo (Fig. 116 B, ar, p. 255) and contains, here as there,
the rudiments of the arms. Those of the two ventral pairs are to
be seen first, becoming differentiated from the circular swelling as
rounded prominences. These are followed very soon by a third pair.
When these three pairs have become distinct, the swelling containing
them is prolonged laterally as a narrow process. As this becomes
more massive later, the fourth pair of arms develops from it, this
being finally followed by the fifth which lies the most dorsally (Fig-
128 4, aas). The order in which the arms arise is therefore the
same in Sepia as in Loligo vulgaris (p. 261).* Here also the pre-
hensile arm, as a rudiment, does not seem to be characterised in
any special way.
Another characteristic of the germ-dise at this early stage (Fig.
128 A) is a pair of rather long, arched folds near the mouth,
the later development of which leads us to recognise them as the
posterior funnel-folds. Between these and the mantle lie two
comma-shaped prominences, the gills. The anterior funnel-folds also
(tf) are already present as rudiments. In their neighbourhood, lie
the cirenlar otocysts (Fig. 128 A, of). These are the organs which
K6LtikER, as VIALLETON pointed out, regarded as the nuchal
cartilages.
We have here, as in former descriptions of the first rudiments of external
organs, repeatedly had to speak of folds (eg,, the funnel-folds), but this
designation is not altogether justifiable, since, as WiacceTon has shown in
sections of young Sepia embryos, these and other prominences, are, for the
present, mere thickenings of the superficial cell-layer accompanied by slight
bulging of the same, but these thickenings soon rise inte actual folds. This
is especially the case in the further development of the mantle and the funnel
in the next stages,
While the organs mentioned have appeared as rudiments, the
germ-dise only at first extends slightly over the yolk, occupying
merely.a small part of it. The blastoderm also does not nearly cover
the yolk, but appears in the form of a broad ring beneath the germ-
disc. Over the rest of the egg, the yolk lies freely at the surface.
The germ-dise and the blastoderm are covered with cilia which are
also present in later stages, when the embryo has risen up from the
yolk (Fig. 129 ©),
* Our statements as to the rise of the arms rest upon our eal observations.
Since the observations of Kénumer and Meet oe bes ae
would be no need to mention this, especially as only he
this stage were available, had not the order of devalopeaiied sraecar arms bean
less clearly or otherwise described in other cases.
DEVELOPMENT OF SEPIA. 275
The development of Sepia (Fig. 128 #) consists for a time in the
further growth of the rudiments of the organs already present. The
edge of the mantle begins to rise from the germ-dise, and already
covers the gills, only parts of which project from beneath it (Fig.
128 B,k). The rudiment of the anus (a) appears between the gills.
A depression which at first is crescent-shaped appears on the opposite
side of the mantle, quite at the edge of the germ-disc ; this is the
oval aperture (m).
The funnel-folds also deserve mention. They have now united,
sud the principal parts of the
funnel ean already be made
out in them, viz, the anterior
folds (vt?) which yield the chief
part of the funnel, the lateral
parts (nuchal musele, Am) which
run buck to the nuchal curtilage
(nk) and, finally, the retractor-
folds (vt) which run towards the
gills, These parts are, indeed,
for less distinct than in Loligo
(Fig. 117 A and B, p. 259),
‘but are nevertheless homologous
with the similarly named struc-
‘tures found in that genus. The
origin of the funnel from two |
jualves is, on the other hand,
more distinct, that organ being
formed here, as in Loligo,
through the rising of the two pal side fale: Race) rae
anterior folds which, after fm, nuchal muscle ; A(y, fonnel-
obtaining in this way an increase real isc hens ees tn
ann
‘The principal part of the funnel derived from two half tubes,
thus comes to lie in front of the mantle; its posterior aperture is
tumed toward the latter and opens into the mantle-cavity after the
mantle bas grown over the funnel. The narrow anterior aperture
ix turned away from the mantle.
In adult Cephalopods, the efferent aperture of the funnel is directed to-
wards the mouth, for the funnel lies on the ventral surface between the anal
and oral apertures (Figs. 120 and 121, p. 262). In the early stages of Sepia
ae
276 CEPHALOPODA.
embryos, this is not self-evident, as apparently different conditions are brought
about by the intercalation of the yolk-mass and the superficial extension of
the germ. The efferent aporture of the funnel does not appear to be directed
towards the mouth but rather turned away from it (Fig. 128 8). A com-
parison with the embryos of oligo shows, however, that quite the same
conditions prevail in the two cases. If we imagine e Loligo embryo, such as
the one seen obliquely from above in Fig. 117 B, p. 259, spread out flat, we
should find that the anterior aperture of the funnel is here also turned away
from the oral aperture which lies on the further side of the mantle and, apart
from the modifications peculiar to the Loligo embryo, we should have a view
something like that given in Fig. 128 B.
The further development of the *Sepia embryo is characterised by
the fact that its superficial extension is arrested and it rises up from
the yolk-sac, the yolk then pressing in beneath the arched embryonic
rudiment ; in this way, an internal yolk-sac is formed as already
i described for Loligo. The rise
abl of the embryo from the yolk
is very gradual, and has a fairly
similar effect on the different
organs. Fig. 129 A-C’ repre-
sents a few stages in this
Saas process. In the first two, the
. ¢ embryo is seen from the oral
side. The posterior funnel-
folds (kéf) at first still lie rather
far from the mantle, but the
latter, by extending laterally,
soon projects beyond them. At
the same time, the mantle
bends over and thus begins to
assume its final shape. The
embryo in its further rise from
the yolk is also followed by
oh the cephalic lobes and the
Ftc. 130.—-Oler embryosof Sep eyes, organs which, as in Lolign,
_ Tren -_ pealidde. (after K eae at this time and even later form
be narrower. Lettering as in Fig. 129. the largest part of the embryo
(Fig. 129 B). Fig. 129 C.
representing the embryo at a somewhat later stage from the anal
side, shows its great resemblance in external shape to the
embryos of Loli at about the stage depicted in Fig. 119, p. 261.
The gills are not yet completely grown over by the mantle; the
FURTHER DIFFERENTIATION OF THE GERM-LAYERS, ETC. 277
junction of the two posterior funnel-folds has taken place but these
folds have not yet fused in the median line, On the mantle, the
fins are visible, these apparently forming late in Sepia, The mouth
lies at the opposite side; its position can be ascertained from
Fig. 130 of a later stage. The yolk-suc is still very large, and is
directly connected with the internal mass of yolk, As the yolk-sac
is gradually absorbed, the large cephalic section decreases in size,
approaching the final shape of the adult Sepia. By the time that
the embryo is mature and ready to hatch, the greater part of the
external yolk-sac has been absorbed. Only after its complete reduc-
tion can the arms occupy their fillal position round the mouth. At
hatching, they are still rather short, und only attain their full length
during free life. The prehensile arms which, as rudiments in Sepia
also, are not specially distinguished from the others (Figs, 128 and
129), become characterised at later stages not only by their length
(Wig. 130, a) but also by their bases sinking into the body. The
depression thus caused is, in older embryos, visible even externally
in the form of a horse-shoe-shaped fold lying above. the first pair
of arms.
4. The further differentiation of the Germ-layers and the
formation of the Organs.
A. The separation of the Germ-layers and the formation of the
Yolk-epithelium and the Alimentary Canal.
Only after the external form of the body is known is it possible to
enter in detail into the further development of the germ-layers.
‘These, in the Cephalopods, develop very late, or rather, they do not
appear so distinctly here as in the other divisions of the Mollusea,
As we have already remarked, the great accumulation of yolk in the
embryo has, in this direction also, essentially modified the ontogenetic
processes.
Our description of the body-layers was interrupted at the stage at
which there had formed, at the margin of the germ-dise which covered
only a small part of the egg, a peripheral thickened ring (Fig 112,
p- 247 and 131 A) and cells became detached from the edge which,
according to the view adopted by ViALLETON, wandered beneath the
superficial cell-layer (Fig. 114, p. 250) there to give rise to a con-
neeted layer. ‘This lower cell-layer then extends over the whole yolk,
and is followed by a middle layer * so that the yolk is now surrounded
* See also p. 126, ete.
278 CEPHALOVODA.
by three cell-integuments, Yolk-cells are not found in the Cephalo-
poda, and the yolk thus attains, in them, « greater independence
than in the eggs of most Arthropoda and Vertebrata, whieh are also
very rich in yolk.
Very different conclusions haye been arrived at as to the signifi-
cance of these cell-layers. We shall consider first the inner layer,
the so-called yolk-epithelium.
The yolk-epithelium is formed of large fattened cells which are
swollen in the region of their nuclei, The only function of this layer
is to yield an envelope for the yolk, and to bring about its utilisation
by the embryo. In later stages it fs found surrounding the yolk still
in the same state as before. The yolk, especially in the typical forms
of Cephalopodan development in which it is plentiful, is accumulated
principally in the external yolk-sac. This latter is directly continuous
with the mass which lies now within the developing embryo (Fig.
132); at a later stage, in consequence of the processes of growth
that take place in the embryo, a constriction forms in the region of
the arms (Fig. 120, p. 262), and a rather narrow duct here “in
connecting the external with the internal yolk-sac (Fig. 133, a, ds,
and #, ds). In this latter, again, various parts may be distinguished
as lying respectively in the cephalic, the pallial, or the nuchal region,
‘The part lying in the head gives off two outgrowths into the optic
stalks, the nuchal portion narrows later and leads to the voluminous
pallial portion, The embryo no doubt absorbs the yolk in the follow-
ing way: the external yolk-sac passes on its contents to the internal
sac, partly in consequence of the rhythmic contractions of its wall and
partly in consequence of the processes of growth in the embryo itself,
then the uutritive masses are conducted to the embryo from the inner
yolk-sac through the intervention of the yolk-epithelium, Since, so
far as we know, there are no vessels in the external sac, its dis-
appearance cannot be accounted for in any other way.
Tt has already been pointed out that the yolk-epithelium ix a
closed unilaminar layer of cells extending round the whole yolk. In
consequence of its close connection with the yolk, the yolk-epithelium
must certainly be regarded as the inner layer of the Cephalopod
germ. The question now arises, What is its relation to the future
entaderm ? This latter first becomes perceptible in the following
ways
About the time when the first rudiments of organs appear ox-
ternally on the embryo, there is seen, on the ventral side, contignons
to the yolk, an epithelial plate which at first consists of only a few
THE SEPARATION OF THE GERM-LAYERS, ETC. 279
cells, This is the first indication of the enteron, which soon increases
somewhat in size (Fig. 131 D, md) and finally separates from the
yolk and appears sac-like (Fig. 132 A, md). Beneath it, the yolk-
epithelium is now seen, which, in earlier stages, was wanting at this
spot (KorscuEnt, No. 25).
The sae-like rudiment of the enteron was known long ago and it
was assumed, with great probability, that it might be connected
with the yolk-epitheliam, and thus to a certain extent might be
regarded as an outgrowth of the latter (Ray LANKEsTER, VIALLETON,
Brucr). The two together would represent the entoderm, the
vesicle being regarded as the permanent and the yolk-epithelium as
the provisional part of the entoderm. Bosrerzky, on the other
hand, considered that the vesicle arose as a mere differentiation of
the lowest cell-layer of the middle layer, i.e, of the so-called
‘mesoderm. There, indeed, appeur to be no yolk-epithelium at the
stage when the epithelial plate above described as the first indica-
tion of the enteron forms, either beneath that plate or in its near
neighbourhood, and this has Jed to the assumption that the cells of
the yolk-epithelium as well us the enteric plate arose as differentia-
tions of a cell-layer which grew from the edge of the germ-disc
towards its centre.
This last view seems not without justification because it is very
difficult to decide whether the yolk-epithelium presses beneath the
germ-dise (Fig. 114, p. 250) or whether these cells are differentiated
from the whole cell-mass. The cells which are directly in contact
with the yolk actually bear a very close resemblance to those of the
middle layer.
These questions are of importance as determining the manner in
which the germ-layers form. We have to imagine that the cell-mass
pressing from the edge towards the centre of the germ-dise (Fig. 131
A and B) represents the meso-entoderm. The whole process is then
to be regarded as a much modified invagination. The edge of the
germ-lisc corresponds to the blastopore which is filled by the large
yolk-plug. The yolk-mass also fills the whole of the archenteric
cavity (Fig. 131 2).
In the Molluses considered earlier in this work, especially in the Gastro-
poda, we have already found the mouth related to the blastopore, In Gnen-
achen’s Cephalopodan embryo, we saw that the oral wperture arises in the
neighbourhood of the aperture of the blastoderm which closes only at a very
Tate stage (p. 270). Since we regard the latter as the blastopore, relations
between it and the mouth may exist here also.
280 CEPHALOPODA.
Fie. 131. — oy illustrating the formation of the germ-layers, .1, thickening of
the edge of the germ-lise ; B and C, differentiation of the yolk-epithelium, Further
extension of the germ-dise over the yolk. ), differentiation of the rudiment of the
enteron and the mesoderm; de, yolk-epithelium ; ect, ectoderm ; md, rudiment of
enteron; wes, mesoderin ; erf (in 1), the rudiment of the cerebral ganglion,
THE SEPARATION OF THE GERM-LAYERS, ETC. 281
At a later stage, as already described, the lowest cell-layer, that
which is in contact with the yolk, becomes differentiated and yields
the provisional and the final entoderm (Fig. 171 Cand D, de, md).
‘The cell-material which remains between these two and the ectoderm
corresponds to the mesoderm, which, in its development is, like the
entoderm, greatly influenced by the excessive abundance of the yolk.
During these processes, the germ-dise has extended far over the ege
(Fig. 131 @ and D).
We need hardly point out that the formation of the germ-layers
in the Cephalopoda, as compared with that in other Molluses is much
modified ; transitions between the two types of formation may, how-
ever, be found in the eggs of many Gastropods that are rich in yolk
(e.g Navsa, Fig. 95, p. 207) although, in these latter cases, the
modifications undergone by the process are not nearly so great.
The entoderm, indeed, was there seen forming in « disconnected
manner and the rudiment of the enteron was open towards the yolk
(Figs. 123-135), as in the Cephalopoda, except that, in the
latter, the yolk is still covered by the thin epithelial layer,
The alimentary canal now develops, the enteron (Fig. 132 A, mu)
inereasing in size and soon dividing into two parts, as is seen in Fig.
132 C which represents « later stage. The lower part, which appears
sac-like (#)), represents the rudiment of the ink-sac, and the upper
part, which is open towards the yolk-epithelium, the actual stomach
and intestine. Where this comes into contact with the ectoderm (a),
fusion subsequently takes place and results in the anus, There is
at this point only an inconsiderable depression of the ectoderm, and
there is therefore no proctodaeum of any size, as is evident from the
fact that the ink-sac, which originates from the entoderm, opens into
the intestine in the adult quite near the anus.
The enteron grows ont towards the apex of the yolk-sac, this
being still more marked in later stages, It is growing toward the
stomodaeum which is approaching it from the other side of the
yolk-sac. This latter rudiment arose as an ectodermal depression
‘on the opposite side of the blastoderm and farther down, é¢., more
anteriorly.
‘The positions of the mowth and the anus have already been meu-
tioned and are made clearer by the sections now before us (Figs. 132
and 133, also Figs. 116-120, pp. 255-262), The oral invagination arises
below the ectodermal thickening depicted on the left in Fig. 131 A,
wi. Another depression (sp), at first lying outside of the stomodaeum,
represents the rudiment of the posterior or large salivary glands, At
282 CEPHALOPODA.
a later stage, the rudiment of the radudar sac is added (Figs. 132 and
133, r), and a tubular depression, the rudiment of the anterior salivary
glands, appears farther forward,
Fre, 132,—Sagittal sections through embryos of Loliyo vn/gavis at various stuges, some-
what diagrammatic (original). 3, section throught the region of the mouth. a,
ent ; ¢, cerebral commissure ; ey, cerebral ganglion ;
: eet, ectoderm ; ve, mouth; ma, wdge of mantle; Corea
mes, mesoderm (indicated dia autnatically); vy, ractilar sac; aif, ; =
salivary gland ; 4, ink-sae; ed, stomodacum,
‘We may here summarise the development of the stomodaeum according to
the numerous and careful investigations of Gnenacaen, Bosneraky, Ussow,
‘Watase, Jovsrs, as follows. The rudiments of the salivary glands very soon
divide into two branches assuming in this respect the final form of the adult
i" _ eal
THE SEPARATION OF THE GEHRM-LAYERS, BTC. 283
organ. The posterior rudiment is specially large. The two branches, which
diverge at a wide angle, carry diverticula and thus form the lobed masses of
the lower pair of salivary glands (Grenacuer, No. 14; Jousrn, No. 22), The
anpaired part of the rudiment, which corresponds to the common section of
Fi, 138. ittal section through an older embryo of Loligo vulgaris, somewhat
diogrammatic (original). «, anus; acs, external yolk-sac; ar, arm-rudiment ; bs,
cerebral ganglion ; , Jyolk; de, yolk-epithelium ;' ect,
art ‘ig ot yolkaast m mouth ; md, Intestine} mes
7 mg, Fea re me of mantle; of, otocyst ; py, gun in
radular aac ; salivary: giant idey nme) edoanal we bee:
daecum : 1, oat ei
the efferent duct, greatly lengthens, for, in the adult, the lower salivary glands
lie far back, The rudiment of the radular sac also undergoes modification,
‘the part of its wall which is turned backward and inward thickening and the
radula being secreted in the way described above (p, 201 and Fig. 91 A).
—
284 CEPHALOPODA.
Various prominences and folds appear in the epithelium of the stomodaeum
near the mouth, and the jaws also arise here as cuticular secretions (JOUBIN.
GrenacHER, BoBRETZEY).
Long before the differentiation of the stomodaeum advanced to this
point, it became united to the rudiment of the enteron, the two having
grown up towards each other from opposite sides of the yolk, and
having met near the apex of the inner yolk-sac, there fusing. Fig.
133 represents a later stage, in which the enteron has dilated
anteriorly and has thus formed the rudiments of the stomach (mg)
and of the stomach-caecum (bx).
At a somewhat earlier stage, the rudiment of the enteron shows a
remarkable peculiarity,
being wide open to-
wards the yolk-epi-
thelium, as is evident
in sagittal sections
(Fig. 132 C’) and still
more in transverse
sections (Fig. 134 A
and 8). It here
appears as a plate, a
large part of which is
in close contiguity to
the yolk. The in-
folding of the edges of
this plate (Fig. 134 B)
gives rise to the two
k. a : 7
Pio, 184.—The venteal portions of two transverse see. hepatic tubes which
tions‘of embryos of align rnigaris at different. aj open into the enteron
(original). @, anal region; de, yolk-epitheliu aa
ret, ectoderm ; &, branchial rudiments; /-liver; md i the region of the
enteron; mes, mesoderm ; r, vascular spaces in the Gaecum.
mesoderm. .
The wide opening
of the enteron towards the yolk-epithelium gradually narrows,
but can still be distinctly made out even in later stages (Fig.
13. Below it, large cells of the yolk-epithelium can be seen
lying, and sending off lateral processes into the yolk (Fig. 133).
The yolk is thus completely covered by the yolk-epithelium even
beneath these gaps in the intestinal epithelium, where a con-
nection of the lumen of the intestine with the yolk-sac might be
assumed, as well as below the former wide aperture of the euteron.
There tx, therefore, no direct communication between yolk-sac and enteron.
THE SEPARATION OF THE GERM-LAYERS, BTO. 285
The yolk-material thus does not pass directly into the intestinal
cavity, but hus first to puss through the yolk-epithelium. The latter
consequently promotes the absorption of the yolk by the embryo, this
being its principal function. A specially active inception of nutritive
substance probably takes place at the open part of the enteron by
means of the yolk-epithelium. This view is confirmed by the rise of
the liver in this region, this organ showing in other Molluscs also
a close relation to the nutritive substance of the egg, and also by
the appearance of the large rhizopod-like cells in the yolk-epithelium
which separate the cavity of the enteron and the yolk-sac.
When the gap in the intestinal epithelium closes as development
advances, the internal yolk-sac appears to lie in the body-cavity
quite unconnected with the enteron. The external yolk-sac gradually
decreases in size as its contents are transported to the internal sac
whence they are absorbed and, when completely taken up, the internal
suc itself shares the same fate, the yolk-epithelium being the last to
disappear, its Function being now fulfilled.
‘The function of the yolk-epithelium resembles that of the yolk-cells of the
Arthropeda and the Vertebrates, described above. This comparison has
already been made by Ray Lankesten, ViacceTox, Warase and others, and
the resemblance is heightened by the fact that the yolk-cells may also appear
as a peripheral layer, the outer merocyte-layer of the Selachians or the so-
called periblasts of the Teleosteans. Some difference between these and the
yolk-epithelium of the Cephalopoda still, however, remains, since, in the
Vertebrates, these cells are at first distributed throughout the yolk and then
collect into a continuous layer, whereas, in the Cephalopoda, the cells are
never found in the yolk, the yolk-epithelium being produced direct from the
cells of the germ-disc. The type of the meroblastic egg is thue specially
marked in the Cephalopoda,
Tn order to complete our description of the development of the
alimentary canal, some reference must be made to the ink-sae. We
have already seen that it arises from the rudiment of the enteron as
a vesicular structure (Fig. 132 C, md). It soon deepens, and grows
out asa tube which is surrounded by mesoderm-cells. In this tube,
two sections can soon be distinguished, the inner blind end, the walls
of which are much folded (Grrop), and the superficial part which
opens externally and becomes greatly dilated, but remains lined with
au simple epithelial layer. The inner part of the ink-bag represents
the glandular secreting part, while the dilated portion which finally,
through a long efferent duct, opens into the intestine near the anus,
forms the reservoir for the secretion. As has already been pointed
out (p. 281), the fact that this entodermal structure opens close to
—
286 CEPHALOPODA.
the anus proves conclusively that there is here no long proctodaeum,
Nevertheless, a very considerable part of the alimentary canal has
actually, by some authors, been attributed to a proctodacal invagina-
tion. Although we considered this view as disproved, we mention it
here as having formerly been supported by the majority of authors
(Merseunrgorr, No, 32; Grexacuer, No. 14; Ussow, No. 44;
Gop, No. 12; Warvasn, No. 49).
According to these authors, the structure spoken of as the enteron was
already connected with the ectoderm from its earliest development, and, as
this gives rise to the intestine, the latter must therefore have arisen in the
form of an ectodermal depression, iv., as a proctodaeum, Ussow, as well as
Girop and Watase, who later investigated this subject very thoroughly, must
be regarded as having adopted this view, the details of which are here un-
necessary. The hypothetical proctodaeum grows up over the yolk, becoming
differentiated in the way described above for the enteron. As to the point to
which the proctodaeum extends forward (or the stomodseum backward)
opinions are divided, but according to this view, the liver, the eaecum and the
stomach are all derived from the ectoderm, since the whole alimentary canal
is produced by the union of the stomodaeum growing backward and the
proctodaeum advancing forward. The whole of the entoderm is represented
by the yolk-epithelium and is quite transitory (Warass). Similar statements
ws to the ectodermal origin of the alimentary canal have been made in
connection with other animals (e.g., Insects, Gantn, Wrrnaczi, Grane,
Heywons), but are, in such cases also, altogether improbable, as is proved by
the condition of nearly related forms.
As opposed to the view to which we have just briefly referred, we have given
that maintained by Ray Lanxusrer (No. 29); VIALLETON (No. 48); Bosrerexy,
(No. 4) and confirmed by ourselves (No. 25) in much earlier stages of de-
velopment, as, apart from its greater probability, this derivation of the
alimentary canal seems much better founded.
B, The Covering of the Body and the Shell.
The ectoderm which covers the body of the embryo seems to pass,
with only slight modifications, direct into the body-epithelium
(epidermis) of the adult. The shell appears as the secretion of a
specially modified part of the ectoderm. In studying the embryonic
formation of this organ which is of such importance for the compre-
hension of the Cephalopodan body, we are unfortunately restricted to
those forms in which it no longer attains full development. Only in
a few recent Cephalopods such as Nautilus and Spirula * is the shell
* [It is very questionable if the shell of Spirula can be regarded as perfect ;
it is almost internal and evidently much reduced —Ep.} _
INTERPRETATION OF THE SHELL IN RECENT OEPHALOPODS. 287
found in its perfect form, and the development of these rare animals
is not known, ‘The shell of Argonauta exemplifies other conditions
which will be discussed later (p. 294). In those recent Cephalopods
in which the development of the shell has been investigated, it lies,
enclosed in the mantle (and is thus internal) on the antero-dorsal
surface of the body, and either develops into the so-called pen, of a
horny character (Onunastrephes, Loligo, and others) or consists of
numerous calcareous layers built upon a horny foundation (Sapia).
It forms in an invagination of the ectoderm, the shell-gland.
In a very early embryonic period, a depression appears at the
centre of the mantle-rudiment, this being the first indication of the
shell-gland (Figs. 115, 116, p, 255, Fig. 131 D, p. 280), At first, in
Loligo, this is a wide, shallow pit, but the margin of the pit soon grows
inwards and constricts its aperture, the pit finally assuming the form
of a sac connected with the exterior by a small opening. This sac is
. lined with an epithelium composed of cells which, at its base,
are specially long, and is surrounded by mesodermal tissue. The
aperture of invagination completely closes at a later stage, and the
shell-gland then lies internally as a closed sac, surrounded by the
mesodermal tissue (Fig. 132 C). It extends later especially anteriorly
and then occupies a large part of the antero-dorsal side of the mantle
(Fig. 133). The secretion of the shell then takes place within it
(Ussow, Borrerzky).
The Interpretation of the Shell in Recent Cephalopods.
There can be no doubt that we have in this case an internal shell,
but the question remains, What is its relation to the large external
shell met with in the (living) Vawtifus, in the Ammonites and other
extinct forms? This is a point of importance in studying the manner
of formation of the Cephalopodan shell and its relation to that of
other Molluses. In solving this question it is necessary to institute
comparisons with the shells of various extinct forms.
The shell of the recent Dibranchia is very differently developed in the
various forms. It occasionally consists merely of a long, narrow, horny,
plate, shaped like » symmetrical bird’s feather, but otherwise not specially
differentiated (e.g., in Loligo). In other cases, the plate is less simply formed,
its posterior end forming u hollow cone, the whole shell thus being slipper-
shaped (Ommastrephes, Fig, 140),
The calcareous shell of Sepia (Figs. 137, 188 4) is' much more complicated
in structure, and is composed of many calcareous layers. Its structure is still
Hot fully understood, but it has been compared with the more highly
a
288 . CEPHALOPODA.
developed Cephalopodan shells. Among the living Dibranchia, the shell
which appears to approach most nearly to those of the extinct forms is that
of Spirula; here we find a spirally coiled and chambered shell which, how-
ever, is grown over and enclosed by the mantle. This approximates most
nearly to the shell of certain Belemnites which was also an internal shell
though, so far as its chambered portion (phragmocone) was concerned, it was
less highly developed than that of Spirula. “
The shell of the Belemnite is not, like that of Nautilus or most Ammonites,
spirally coiled, but is straight like that of the Orthoceratidae (Nautiloides).
It is characteristic of the Belemnite that in addition to the part of it, the
phragmocone, that may be regarded as corresponding to the actual cham.
bered Cephalopodan shell (that of the Ammonites
and Nautilus) there is a calcareous pointed external
investing piece, the so-called guard or rostrum. A
good example of this latter is afforded in the fossil
Spirulirostra (Fig. 185) the phragmocone of which
is curved and thus bears a certain resemblance to
that of ‘Spirula, but has, addition, dorsally and
late rally, a large conical rostrum. It has also been
maintained that traces of such a rostrum are to be
found in Spirula, although these were not found by
us in an examination of a large number of shells.*
The shell of the Belemnite is, as already men-
tioned, straight. Two parts can be distinguished in
it. The chambered part (phragmocone) is provided
with siphon and is surrounded by the actual shell-
wall (the ostracum); this latter becomes specially
dilated towards the head of the animal as the su-
called proostracum, and is not here followed by the
phragmocone. This first portion of the shell might
be compared with the Ammonite or Nautilus shell,
but on the side opposite to the proostracum, sur-
oe rounding the posterior part of the phragmocone and
- Longitudinal i satis 3
section through the prolonged in the same direction, there is a second
shell of a Spiru/i- part, a very large rostrum, which is usually the
rustra ir only part of the whole Belemnite to be preserved
cae ithau,’ i @ fossil condition. ‘That this shell was internal.
i,e., surrounded by the mantle, seems to be highly
probable from observations made on forms resembling Belemnites but better
* The position of Spirula as a branch of the Belemnite stock connected to
it by forms like Spirudirustra seems to us doubtful. While the position of
the siphon and the orientation of the shell with regard to the body would
seem to favour this view, the constitution of the shell, on the other hand.
with its well-retained chambers and the siphon renders it improbable that
it has undergone a process of degeneration which led to the total loss of the
rostrum. The shells of other recent Cephalopods, in which such degeneration
took place, underwent, iu consequence, great change of structure. We must.
in any case, take into account the view that Spirula may have separated from
the Decapodan stock before acquiring a rostrum.
INTERPRETATION OF THE SHELL IN RECENT CEPHALOPODS.
289
preserved, and as is also shown by the so-called vascular impressions on the
external surface of the rostrum of many
Relemnites,*
Attempts have been made to deduce the
constituent parts of the shell of Sepia from
the above parts of the Belemnite shell
(Voutz, RigrstaHL, No. 39). The shell of
Sepia, or, as it is generally termed, the cuttle-
bone, is very complicated. The whole forms
an oval shield-like structure, which, for the
most part, is biconvex, but in the more dorsal
region it becomes concave on its posterior
or inner side, its shape is well shown in
Figs. 187, 138 A. Its antero-dorsal surface
is covered externally by a roughly calcified
shagreen-like layer, the outer layer, under
which is a deposit of horn-like matter (con-
chyolin), the middle layer, the latter being
freely exposed at the margin of the shell
(Fig. 137, mp). Dorsally, the shell is
produced into a small pointed structure
(Figs. 137 and 138, k, d) which consists
essentially of a prolongation of the outer
calcified layer but has become covered by 8
secondary development of horny (conchyolin)
matter which is quite distinct from the
horny middle layer. This calcareous spine
may in some species project freely on to
the exterior (8. andreanoides). On the
posterior or internal surface of the shell is
a prominent swelling produced by a great
deposit of calcareous material arranged in
thin oblique layers (Fig. 138 -1, 1), separated
from one another by air-spaces closed by
calcareous supporting trabeculae; this
structure which is most developed near the
antero-ventral portion of the shell serves as
@ float. Dorsally this portion of the shell
is less developed and consequently the mar-
gins of the shield are here much more pro-
nounced. Surrounding the posterior remnaut
of the calcareous swelling is a modified forked
or V-shaped area (Fig. 137, 9), the two ends
of which are directed forward ; this ledge,
Fio. 136,—Median longitudinal
vection’ through the shell of
Belerunite, somewhat diagram-
matic, ek, embryonic chamber;
ph, phragmocon -
tracum ; 7, rostrum ; , siphon.
The dotted lines indicate the
anterior edge of the shell the
limits of which are at present
not accurately known,
* We do not here follow out this point, since, for our purposes, it ix of
no special importance that in certain Belemnites the posterior part of the
Tostrum projects as a more or less long spine beyoud the mantle, giving the
posterior end of the animal the shape of an arrow, an adaptation favourable
to locomotion.
u
290 CEPHALOPODA.
the posterior part of which is somewhat raised and arched forward, forms
& cavity which in some cases (8. officinalis) is shallow, but in other species
(S. aculeata, Fig. 137) tolerably deep.
These parts of the Sepia shell have been homologised with those of the
Belemnite shell in the following way. The spine, together with the outer
calcareous plate, have been thought to correspond to the rostrum of the
Belemnite (cf. Figs. 186 and 137) which is continued far on to the actual
shell, almost as if the proostracum of the Belemnite were covered by an
externa! calcareous investment connected with the rostrum, as perhaps may
actually be the case. A similar condition is found in Spirulirostra, in which
the rostrum embraces a large part of the shell (Fig. 185, r). -
The actual shell of the Belemnite, i.e., the
phragmocone and proostracum, corresponds
to the two inner layers of the shield in Sepia,
the prominence and the fork (Figs. 196-188,
w and g). While the two outer layers are to
be regarded as the wall of the shell (ostracum).
the lamellae of the prominence may perhaps
be considered as partition-walls between the
chambors. These lamellae end posteriorly
in free edges (Figs. 187 and 188); laterally,
however, they are continued into corres-
ponding lamellae of the forked ledge. As
this latter rises anteriorly over the posterior
end of the prominence and forms a rather
deep cavity posteriorly, a structure arises
which actually resembles the phragmocone
of the Belemnite. If the comparison is
carried further, the wide aperture of the
posteriorly directed partition-walls of the
chambers might be compared to the siphonal
spaces in the Ammonites, which, however,
iw aculedtt. have become very wide and in which a con-
from the surface
i d.spine; y, forked Siderable part of the body lies.
dee ie donee ay th: This view seems to be supported by the
vy promiuence,, showing ‘the CDdition of a fossil form (Belosepia) the shell
frve edges of the lamellae. of which bears a general resemblance to that
of Sepia but still shows the phragmocone
fairly distinctly (Fig. 188 3 and C). In place of the siphon there is, in this
form, a wide cavity (Fig. 188 4), and this may be regarded as marking a
transition to the condition of Sepia. The rostrum in these forms resembles
that of Sepia, but is still more strongly developed (Fig. 138 4 and B).*
If the lamellae are regarded as partition-walls of the chambers, it appear~
remarkable that these latter extend anteriorly; that is, that the proostracum
thus completely disappears, for the whole prominence would then correspond
to the phragmocone.
* We refrain from comparing the antorior part of the shell (of which usually
only the hind or dorsal end is retained) with the shell of Sepia, but it seems
possible that here also resemblance can be found.
INTERPRETATION OF THE SHELL IN RECENT CEPHALOFODS. 291
‘The lamellae are connected by numerous delicate calcareous trabeculae, so
that the spaces between them are not empty, as one might expect if they
represented chambers. But this can hardly be reckoned as an argument
against the above interpretation of the Sepia shell, as such a modification of
the shell in adaptation to new functions (as a float) is quite explicable,
The development of the different parts of the Sepia shell has beer investi-
gated in detail by Arretérr, but has, so far as we know, been deseribed only
in short Swedish treatise (No. 2) from which it is impossible to judge
whether factors of import-
ance for the morphological
interpretation of the shell
have been discovered,
Finally, it appears neces-
sary to point out that this
whole comparison is still
far from being well founded,
although it may be con-
sidered as extremely
plausible,
Other recent Gephalo-
pods, especially those which
are universally regarded
as more primitive than
Sepia, have a shell of very
simple structure, carrying
‘ab its end at the most a
hollow oone (Ommastrephes,
Fig. 140). If this structure
is compared with the phrag-
mocone, the rest of the
shell would have to be
regarded as the proostra-
cum, and this suggests the
idea that, in Sepia also,
the fork which bifurcates Rae A ee i
solerty and she con- 1854, agmatine of
tained cavity may be ‘ville’, seen from the side; C, posterior part of the
regarded as such a much- same, seen from the ventral side ( and C after
Stiatepireamcoave: [he senee spe pl pliragmocone; r. rostrum ;
whole of the part lying
anteriorly to it (the prominence) would then have to be considered as the
Proostracum, which has perhaps attained such a large size in adaptation
to its present function, the diminution of the specific gravity of the body,
‘The Iamellate stracture of the prominence would then be traceable to a
secondary modification causing the secretion of the shell in layers, to form
the air-cavities of the float; under these cireumstances the higher morpholo-
ical significance could not be ascribed to it. Further light upon the subject
of the significance of the shell of Sepia and its relation to the shells of the
292 ‘CEPHALOPODA,
fossil forms may be expected from detailed palacontological investigations and
perhaps also from more comprehensive ontogenetic researches:
A comparison of the shell of Sepia with that of Belemnites, such as was
attempted above, suggests a relation between the Dibranchia in general and
the Belemnitidae, This indeed seems a bold proceeding, since nothing certain.
is known of the aoft parts of these Cephalopods, but forms like Belemnotouthis,
a Cephalopod living in Triassic times with
well-preserved phragmocone and arms
carrying hooks, as well as Acanthoteuthis, a
Decapod much nearer the recent Decapods
and also armed with hooks (Jaren, No.
17), indicate with coisiderable certainty
that the Dibranchia or at least the Deca-
~ poda are to be derived from forms resemb-
ling the Belemnitidae.
There can be no doubt that the shell of
the Belemnitidae was internal and was
almost completely enclosed in the mantle.
Of the transitionary forms, moreover,
Belemnoteuthis shows on the phragmocone
which is still provided with chambers and
siphon a large and distinctly bounded
proostracum (Fig, 139, po) recalling, in its
shape, the Sepia shell. The rostrum, in
these forms, is either wanting or, when
present, is only a slight appendix to the
phragmocone. In this respect, these forms
would approximate to the recent forms
in which the shell has a posterior cup
(Ommastrephes, Onychoteuthis, Taowius,
pi
Fic, 139,—Shell of Belomnoteuthis
from the lower Lias, Lyme
Regi somewhat Giagremarratin
iginal).* ‘The shell is seen
the funnel-side with the
ink-sac lying (#) upon it. In
the phragmocone (p/h), the most
rior ‘is wanting aud is
indicated by dotted lines. The
partition-walle orn then chambers
are seen on the surf aos owing to
the posterior portion being
broken away ; po, proostracum.
Leachia).
It is of interest that, in Ommastrephes,
regular transverse striation is found on the
hollow cone (Fig. 140); this is quite distinet
from the lines of growth in other parts of
the shell and may perhaps be regarded as
the last vestige of the chambering of the
phragmocone, [Genatus Fabricii, according
to Srnexsrrur, has a series of chambers
at the end of its horny pen.) Such « view
does not appear unjustifiable, as Omuwmax-
trephes is among the most primitive of the extant Cephalopoda, JamKeL
has already pointed out (No.17) that Ommastrephes also, in the possession of
small hooks, shows\a primitive character and recalls the hook-bearing transi-
tionary forms mentioned above (Lelemnoteuthis, Acanthoteuthis),
Fig. 189 represents a very instructive and as yet undescribed
isan e collection of Dr, O, JawKer, kindly placed at our Our
thanks are due to him also for revising the figure,
INTERPRETATION OF THE SHELL IN RECENT CEVHALOPODS. 293
The reduction of the shell goes still farther in other recent Cephalopods ;
the terminal cone, in older specimens of Dovidieus, is found to be solid,
whereas, in the younger animals, it was hollow (Stwexsrrup). In some
Cranchiidae the hollow cone is still present at the end of the shell, in others
it has disappeared and, in its place, there is mere solid swelling.
Finally, @ simple horny plate develops, as in Lolig. The
. hollow cone does not even appear ontogenetically, so far as we
know at present,
The above comparison seems to show with certainty that the
shell in the Cephalopoda is internal, the manner in which it
arose from an external shell being still exemplified in a living
form, Spirula. Nautilus, only a small part of the mantle
covers the exte shell, but the process of circumcrescence
of the shell went farther, until the shell became covered, though
incompletely, by the mantle, asin Spirula. During this process,
the size of the shell as compared with that of the animal became
reduced in most cases; at the same time, it changed its position
and gradually degenerated, since it no longer functioned in the
same way, Externally, new calcareous layers became added
to the primitive shell, for we find that only the inner part of
the shell of Belemnites or Sepia corresponds to the shell of
Nautilus, the rostrum and its continuation as a covering of the
proostracum are secondary structures, no doubt secreted by the
mantle-sac which surrounded the shell. We are here again
brought to the question of chief interest in connection with this
subject, viz., the manner of formation of the shell.
We saw above that the shell is formed in an ectodermal
depression, the shell-gland. It would be well to discover
whether this shell-gland is homologous with the organ of the
game name in the Lamellibranchia and Gastropoda or not.
This question has been raised before now by Ray Lanxestun
(No. 28) who maintained the negative because he believed that
the shell of Sepia corresponds to the Belemnite shell and conse-
quently must be formed in a mantle-sac and not in the primitive
shell-gland, Ray Lankesten was obliged to take up this
position decisively since he considered the Sepia shell as
homologous with the external part only of the shell of
Belemnites and lett the phragmocone out of account.
In deciding the question as to the significance of the shell-
gland in the Cephalopoda, we are inclined from the first to
ascribe to this organ which appears so early, in consequence
of its position and development, the same significance as is
possessed by the shell-gland in the other Mollusca, and thus
Pia. 140.—Posterior veal of the shell of poate secre the a Sacre seen
from the surface (original coni at D
Peale sialk ehh voters, Aocaliy: vente Leonieting ent an the. cotical
appendage: aE the strong horny ledges betwers which the shell consists merely of &
membrane strengthened by ribs.
—
294 CEPHALOPODA.
to consider it as fully homologous with this latter structure. The most
natural course is to seek for a confirmation of this view in the ontogeny of
those Cephalopods which are provided with an external shell. Since the
development of the shell is unknown both in Nautilus and Spirula, em-
bryos being unobtainable at present, we might turn to the only Cephalopod
with external shell which is more accessible, viz., Argonan/a, if the conditions
in this case were not essentially modified.
Tt has already been shown (p, 266) that a shell-gland does indeed appear in
the embryo of Argonauta, but that it disappears later and does not give rise
to the shell of the adult. The latter is not formed within the egg-shell, as
Was assumed by a few of the older authors (Port, Dette Cxraze, No. 9) but
rises later, as was observed by Mrs. Power, Apams and Kéuumer (Nos, b
and 24). The statements made as to the origin of the shell are somewhat
peculiar and obscure, According to the almost universal view, the shell is
secreted by the expanded surfaces of the dorsal arms which cover the shell
when fully formed. ‘his view, which at first sight is rather improbable, is
rendered still more so by the fact that the regeneration of parts of the shell
which are lost is said to take place from within, The mantle might also be
regarded as a source of the shell, but it is not closely connected with she
latter and so this view also has no support.
‘The disappearance of the shell-gland in the embryo of Argonauts shows
that the adult shell, in this case, is not a structure which can be homologised
with the shells of other Cephalopods, If, as is stated, the shell-gland actually
flattens out, the shell may have arisen from a part of the mantle which
originally corresponded to the shell-gland. The position of the animal with
respect to the shell is the same as in the Ammonites and Nantilus, while, in
Spirula, on the contrary, the orientation is different, the concave and not the
convex side of the shell here corresponding to the ventral side,
An attempt has receutly been made to derive the Argonaut shell directly
from that of Scaplites, the external form in the two cases having a certain
similarity (STenvmasx, No, 43). We are unable to accept such a view be-
cause we do not regard the Argonaut shell as directly homologous with the
Ammonite shell, apart from the fact that a long period elapsed between the
disappearance of Scaphites and the appearance of Argonauta, the latter, more-
over, belongs to the order Octopoda wnd is thus closely related to the other
living Dibranchia.
If the shell of Argonauta is to be derived from that of Scaphites, a com-
paratively quick disappearance of the chambering of the shell without
essential modification of the external form must be assumed, The chamber-
ing of the shell, however, and the manner in which the animal is connected
with it reappears in so marked a manner in all Cephalopoda (in which the
shell is well preserved) that we are not warranted in assuming that 4rgonauta
relinquished the chambering shell and received sea-water into the living
chamber, a change which would involve a complete alteration of the manner
of life of the animal.
From what is known of the modifications undergone by the Cephalopod
shell, it always appears to take place in the same way as in the forms with
internal shells. Although, in them also, the significance of the shell is
essentially modified, the chambering is retained (Belemnites) and disappears
4 _
THE SENSORY ORGANS. 295
only when the degeneration of the shell has reached its highest limit (horny
shell of the Dibranchia). We might presuppose a similar process in the
ancestors of Argonauta, and thus claim the shell which occura only in the
female and is altogether wanting in the male, ax a new formation.®
The only Cephalopod provided with an externa] shell, the ontogeny of
which is at present known, is thus not adapted to assist in the solution of
the problem as to the significance of the shell-gland; we are therefore
restricted to the embryological facts concerning the forms with internal
shell.
Since the invagination known as the shell-gland secretes the whole shell,
and therefore also the part of it which corresponds to the rostrum of the
Belemnites, and as this is a part added to the primitive (Ammonite or
Nautilus) shell, there can be no doubt that the invagination ix the equivalent
of at least that part of the mantle-sac which covers the internal shell. The
extraordinarily early appearance of the invagination indicates, however, that,
in the Cephalopoda as in other Molluscs, the original shell owes its origin to
@ shell-gland. If this is the case, we might assume that in the course of
development the shell-gland became connected with this secondary mantle-
sac. The position of the primitive invagination is easily reconcilable with
this assumption. This question can be satisfactorily answered only when the
formation of the shell in recent Cephalopods with external shell such as
Spirula and above all Nautilus, is understood. Since the first of these forms
is closely allied with the Decapoda and since, from the examination of the
ovaries in the latter (OWEN, No. 33) it is known that, like other Cephalopoda,
it has large eggs rich in yolk, it is uot too much to assume that similar
processes of development took place in these forms also, the conditions
of development of the shell especially being similar.
C. The Sensory Organs.
The sensory organs of the Cephalopoda (olfactory, auditory and
visual organs) show much similarity in their development, all appear-
ing first as invaginations of the ectoderm. The olfactory organs are,
throughout life, mere ectodermal invaginations, but the auditory
organs and especially the eyes become partly or altogether separated
from the ectoderm and reach a high degree of development. In
these cases also, however, traces of the connection with the ectoderm
may be retained either as vestigial or specialised structures (as in
the auditory organs) or by the retention of the original aperture of
invagination, this latter condition being exhibited in the eye of
Nautilus,
* Cirrhoteuthis is said, unlike other Uctopoda, to possess a shell, the nature
of which, however, is not well understood. If it isa true shell, it no doubt
arises from shell-gland as in the Decapoda, and we should be justified in
assuming that forms like Octopus and .trgonauta, in which a shell-gland
occurs, once possessed vestigial shells. The case inhabited by Argonuuta
could then no longer be homologised with a true Cephalopodan shell.
296 CEPHALOPODA.
The Olfactory Organs.
These organs, unlike the eyes and the auditory vesicles, appear in the
embryo at a very late stage. In Sepia, in which they were observed
by ZeRnor¥ (und previously by K6LLrKER) at a time when all the
arms, the funnel and the chromatophores have already formed, there
appears, behind each of the eyes, a round prominence. The edges of
this rise and curve over towards the centre and thus an invagination
of the ectoderm is brought about which at first is rather shallow and
sac-like, its floor being much thickened, while the covering consists
of thinner cell-layers. Some of the ectodermal elements in the floor
of the olfactory sac take the form of spindle-shaped sensory cells pro-
duced into stiff setae. This is the condition in the adult Eledone,
while in Sepia and Loligo the intermediate and supporting epithelial
cells become much lengthened and invested with movable cilia. The
organ at a later stage becomes deeper and sac-like this being the
usual adult condition.
The papillae which, in Aryonauta, usually take the place of the
olfactory pits, are considered by KGLLIKER. as the equivalent of
the prominences which, in other Cephalopods, ontogenetically precede
the invagination, this author consequently regarding them as a lower
stage of development of the olfactory organ.
These organs cannot be homologised with the osphradial olfactory organs
which are so strikingly developed in the Prosobranchia, and in a lesser degree
in many other Molluscs, since the latter are found in the mantle-cavity near
the gills, whereas the former occur on the head near the eye. In Nautilus,
true osphradia occur near the gills. [According to WILLEY, two pairs of
these organs are present. This author also described two pairs of olfactory
tentacles, the pre- and post-occular tentacles.]
Otocysts.
The rise of the otocysts has been observed in most of the forms.
of which the ontogeny is known, and these organs have already been
depicted in many of the figures here ziven (Figs. 116-118, p. 260, 1
p. 265, 126 p. 270 and 128 p. 273). KOLLIKER examined in detai
their (later) structure, and they were subsequently carefully studied
by Ray LankesTER and GRENACHER.
The position of the otocysts in the embryo may be ascertained
from the above-mentioned figures. They form as depressions of the
ectoderm (Fig. 141 A) which gradually deepen and become vesicular
(Fig. 141 B and (). The aperture of invagination does not close
THE SENSORY ORGANS—OTOCYSTS. 297
for some time, and its connection with the sue becomes elongated
and tubular (2 and @). This appendage, which was described by
K6tnrmer and by GrenacHer, named Kéiorker’s duct, seems at
first to communicate with
the exterior, but is said
later to become separated
from the surface and to
end blindly. [ts interior
is lined with cilia directed
towards the aperture of
the otocyst which are in
constant undulating move-
ment, This appendage
is also found in the adult.
Baxrovr compares it with
the recessus vestiould of the
Vertebrates, the — blind
appendage of the primitive
auditory vesicle which re-
presents its former connec-
“tion with the point of
invagination.
Tn that part of the wall
of the auditory vesicle
which lies almost opposite
to the point at which
KGturken’s duct enters,
the epithelial cells thicken
‘to form the crista acustica,
and it is here that the
secretion of the otolith
takes place (Fig. 141 D).
The furtherdevelopment
Fig. 141. —Sections through the funnel-region of
of the otocysts is brought several advanced embryos of Loligo wulgurin
r me (original). A.C, transverse sections, /), sagittal
about by the differentiation pection, sonemtint Mingramuatic, "The yolk
i i as been omitted. de, yolk-epithelium ; ect,
OF the erixta: aewatica which ectoderm ; mex, mesoderm ; Hh otoeyst ; fF,
extends fur over the wall fonnel-foltts.
of the vesicle. The cells
of the evista acusticn lengthen, the inner free ends developing «
nimber of tine hairs. In this way arise the sensory epithelia which
compose the auditory ridges described by Kowannvsky and
i
298 CEPHALOPODA.
Owssannikow (and also probably the auditory plate which only
develops later). In examining the formation of this terminal sensory
apparatus, GRENACHER thought he could also recognise the nerves
which give off branches to the cells in the form of delicate fibrous
strands.
While the internal structure of the otocysts is thus developing,
the organs change their position, gradually shifting from a lateral
position to below the funnel (Fig. 141 4-C) where, as large closed
sacs, they are found in close contact with the pleuro-visceral ganglia
(Figs. 133, of, p. 283 and 143, ac), Finally they come into close
contact with one another and flatten hy mutual pressure, as was
observed in GRreNAcHER's embryo and in Sepia. Their detinitive
position being attained, the cephalic cartilage develcps round them-
The Eyes.
The origin of the eyes in the Cephalopoda has been carefully
studied by Grenacner (No, 14), Ray Lankester (No. 29), and
Boprerany (No. 4).
Figs. 115-119, pp. 253-261 will help the reader to understand the
orientation of the eyerudiments in the embryo. These organs
originate in connection
A with the large swellings
(Fig. 115, au) as two
lurge, rather — shallow
ectodermal depressions
(Fig. 142). The floor of
eA each depression —_ s00n
Yas thickens considerably and.
Na Sian” its margins grow up and
9, Trees esi Droeh ine acs ae
of the eve in Rolige (after Ray L mu from (Fig. 142 2). A
Seren oe ie eae ee a produced with
a thinouterand thickened
inner wall and this is connected with the exterior by a small aperture.
The inner wall of this vesicle yields the retina, while the outer wall
yields a part of the lens aud the ciliary body.
It is an interesting fact that this stage of development is retained
throughout life in the eye of one Cephalopod, Nawfilus (Fig. 145).
The adult eye, in Nautilus, corresponds to the primitive optic vesicle,
the cavity of which is lined by the retina, 7.¢., modified ectoderm, and
THE SENSORY ORGANS—THE EYES. 299
communicates with the exterior through an aperture. The sensory
epithelium is consequently directly bathed by the sea water, and its
ectodermal character thus becomes very evident. Eyes thus simply
constituted have already been met with in a few primitive Gastropoda
(Fig. 90, p. 198).
In the Dibranchiate Cephalopoda, the eye reaches « higher grade
of development. The first advance is the closing of the primitive
optic vesicle and its abstriction from the ectoderm. Mesoderm-cells
then press in between the latter and the outer wall of the vesicle, a
process the commencement of which is indicated even in Fig. 142 3.
After the abstriction of the optic vesicle, this stage may be compared
to the permanent condition of the eye in the majority of the
Gustropoda (Fig. 145 2).
ve
Fie. 143.—Tyansverse section through the head of an advanced embryo of Lotige (after
Bonnerzxy, from Bauroun's Text-book). ae, otocyst ; adk, optic cartilage ; ak and
y, lateral cartilage and white body ; cc, iris; /f, funnel-fold ; go, cerebral ganglion ;
gm, membrana limitans; gfe, duct of the salivary gland ; (7.op), optic ganglion; (g.1) .
ganglion ; rt, retina; ec, vena cava; vd, stomodaenm ; of, ciliary region
of the eye; a, thickened ectoderm in the floor of the funnel.
A secdnd circular ectodermal fold now rises above the optic vesicle,
enclosing a depression which strongly resembles the primitive optic
pit (Fig, 145.2). Almost simultaneously, the (cuticular) secretion of
a conical structure (Fig. 143) commences on the inner surface of the
external wall of the vesicle; this is the first indication of the /ens.
‘This rudiment increases in size through the deposit of concentric
layers (Fig, 144 4),
300 CEPHALOPODA,
The lens of the Cephalopodan eye is not yielded by the outer wall
of the primitive optic vesicle alone, since the floor of the second
invagination above the vesicle also takes part in its formation (Fig.
143). From this latter is formed an anterior and ‘smaller section of
The cell-layers formerly lying above
the first-formed lens are in
this way gradually used up
(Figs. 143 and 144).
The primitive —_ optic
vesicle and the invagination
lying above it also yield the
ciliary body, the wall of the
former, which is directed
outward, and the floor of
the latter uniting for its
formation (Fig. 144 4 and
B). The mesoderm present
between these two ecto-
dermal cell-layers no doubt
gives rise tothe musculature
‘of the ciliary body. The
anterior or outer part of
the invagination (or rather
fold) becomes the iris (Figs.
144 and 145), Mesoderm-
elements find their way in
large numbers into this
the lens (Fig. 144 4 and 2).
*\ ec Ke
Fig. 144,—Sections through the eye of oligo
in two stages of development (after BOBRETZRY,
from Batvour’s Text-book). a and a’, the
epithelium lining the anterior optic chamber ;
af, and i, iris fold ; aq, equatorial pactllage
cc, stall ectoderm-cells of the ciliary body ;
gz, large cells of the ciliary body (nm); ht,
inner part of the lens; ms, mesoderm-tissne of
the ciliary body ; r¢, inner, r?!’, outer layer of
the retina; sf, rods; ef, anterior part of the
leus; , epithelium of funnel,
anterior fold also.
Another circular fold now
grows over the eye at this
stage of its development
and gives rise to the cornea
(Fig. 145 @). This fold, in
many Cephalopoda (¢.g.,
Oigopsida) does not close,
the cornea retaining its
aperture which is often somewhat wide and through whieh the
sea water can enter the external optic chamber, while in others
(eg., the Myopsida among the Decapoda and the Octopoda), the
cornea either retains only a minute aperture or else completely closes,
thus precluding any communication between the optic chamber and
THE NERVOUS SYSTEM. 301
the sea water. Another fold round the eye gives rise to the eye-
lid found in some Cephalopoda (especially in the Octopoda, Fig. 145
C, Int *).
Nop
Fre, 145,—Diagrams rey mting the eyes of Vewtilus (A), a Gastropod (4), and one
7afte pook). Cb
of the Oigopsida (0) (after Grenacner from Batrour's Text-h ) commen 5
Ch.ep, epithelium of the ciliary body ; G.op, optic ganglion; [nt-Jul4, integument
(ectoderm) ; Fr, iris ; J, lens; 7', outer section of the lens; Nop, optic nerve; N.S,
nerve-layer of the retina ; Pal, eyelid ; RK, retina ; ©, outer layer of the retina.
The course of development of the Cephalopod eye described above
shows that it attains a high degree of perfection, as may indeed be
seen from an examination of the adult eye.
D, The Nervous System.
All investigators of the origin of the nervous system in the Cephalo-
poda were formerly unanimous in tracing it back to the mesoderm
(Ray Lanxesrer, Ussow, Boprerzky). More recently, the forma-
tion of the nervous system has been studied by ViALLETON, who found
that the ganglia arise through the thickening of the external layer ;
these thickenings, however, yield at the same time mesodermal tissue.
Other similar thickenings of the ectoderm are principally formative
centres for the mesoderm.* This view also, therefore, does not
establish any sharp distinction between the central nervous system
in the process of formation and the mesoderm. At the same
time, we have to emphasise the fact that in the Cephalopoda as in
other Molluscs, the nervous system is of purely ectodermal ovigin
(Korscurit, No, 25).
# See further, p. 907.
302 CEPHALOPODA.
‘The cerebral ganglion arises in the form of an ectodermal thicken-
ing above the rudiment of the stomodaeum. Before the latter sinks
in, the ectoderm above this region becomes multilaminar (Fig. 131
Fic, 146,—Sagittal sections through embryos of Lolign of various ages, somewhat
diagrammatic (0 is thle in the region of the mouth. a, anal
region; a7, arn-ruliment; ¢ cerebral commnisire; of, cerebral ganglion ;
de, yolk-epithelinm ; ert, ectoderm ; m, mouth ; ud, mantle-fold ; md,
mesoderm (diagrammatic) ; 7, radular sac; a7, shell-gland ; sp. salivary gland;
ink-sac ; ev, stomodaeum,
D, ect., p. 280). ‘This ectodermal layer, which at first is thin,
thickens greatly after the stomodacum has become.invaginated (Fig.
146 A, cg), and a large ganglionic mass forms at this point, consisting
THE NERVOUS SYSTEM. 303
of two parts connected together by a commissure which at first is
broad but .narrows later; these are the two halves of the cerebral
yanglion, which become detached from the superficial ectodermal
layer and, in sagittal section (Fig. 146 8, cg), appear as spindle-
shaped bodies. The ectoderm, at the formative centre of the
cerebral ganglia (above the mouth) is now again a thin layer (Fig.
146 Band C). In the median section (C’), the cerebral commissure
(c) can here be recognised.
The paired rudiment of the cerebral ganglion was early observed
(by the above-named zoologists as well as by GRENACHER). The
two ganglia greatly increase in size and finally fuse together. In
consequence of processes of growth, the relative position of the
cerebral ganglion and the mouth is modified in a striking manner
(Fig. 133, p. 283).
The optic ganglia also form as massive thickenings of the ectoderm
in the cephalic region. They are directly connected with the ecto-
dermal thickenings which become the cerebral ganglia and soon press
in below the optic vesicles, a process which evidently gave rise to the
statement that the optic ganglion arises on the posterior side of the
optic vesicle, from the mesodermal cell-elements there present. The
optic ganglion soon shows a close connection with the optic vesicle,
a connection which leads to the formation of the optic nerve.
The pedal and the pleuro-visceral ganglia, like the other ganglia,
are said to originate through differentiation of the cell-masses lying
between the ectoderm and the alimentary canal. In reality they are
due to ectodermal thickenings in the ventral part of the embryo.
The pleuro-visceral and pedal ganglia, like the cerebral ganglion, are
each composed of two parts of distinct origin. The two cell-mnasses
which yield the pleuro-visceral ganglion lie behind the otocyst, aud
the masses that produce the pedal ganglion in front of that vesicle
(Bosretzky, Ussow, Fig. 133, p. 283).
During the course of development, the three ganglia just mentioned
gradually shift nearer one another and, as the yolk-sac disappears,
move towards the stomodacum, where they finally fuse, the pleuro-
visceral and pedal ganglia uniting to form the sub-oesophageal mass
which is connected with the cerebral ganglion by two short com-
missures, a broad posterior and a narrow anterior commissure.
The anterior section of the sub-oesophageal mass, which is, to a
certain extent, marked off from the rest, and into which the narrow
commissure enters, is distinguished as the brachial ganglion. This
part becomes differentiated from the pedal ganglion at a time when
304 CEPHALOPODA.
the latter has not yet fused with the visceral ganglion (PELSENEER).
According to the above author, who is confirmed by Bopretzky,
the buccal ganglia originate in the same way, becoming abstricted
from the cerebral ganglia and shifting forward, but Ussow states that
they have a separate origin like the principal ganglia, and unite with
these only at a later period. A similar origin is claimed by Ussow
for the splanchnic and the stellate ganglia.
The ganglia can easily be recognised by the fact that, soon after
their appearance, a differentiation into an outer cellular layer and
a central fibrous mass takes place.
The connection of the ganglia with the sensory organs and the
peripheral parts of the body takes place only at a somewhat later
stage. Ussow assumes that the cells of the layer which produced the
ganglia lengthen and thus yield the nerve-fibres, as described above
(p. 193) for the Gastropoda.
The above interpretation of the nervous system is not accepted by all
anatomists. The anterior part of the sub-oesophageal mass, the brachial
ganglion, has been regarded as e part of the cerebral ganglion, which in con-
sequence of the great lateral extension of its anterior section, finally stretched
below the oesophagus, fused here in the middle line, the anterior part being
constricted off and remaining connected only by a commissure with the
cerebral ganglion (v. JHERING, Gropeen, No. 16).
The brachial ganglion supplies the arms with nerves; some of these nerves,
however, are said not to arise from the brachial ganglion but to pass into
the cerebral ganglion through the anterior commissure (DieTL, No. 10). This
connection and the conditions found in Nautilus have led to the view that
the brachial ganglion belongs to the brain. In Nautilus, the majority of
the tentacle-nerves spring from the part of the middle oesophageal ring which
might be indicated as the pedal ganglion, but some originate above the roots
of the optic nerves, and are thus thought to be cerebral in origin. According
to this interpretation, the part of the oesophageal mass known as the pedal
ganglion would then also have to be reckoned as belonging to the cerebral
ganglion, since we cannot claim some of the tentacle-nerves as cerebral
nerves and the others as pedal nerves.
This question is of importance in connection with the interpretation of the
arms which, if innervated from the brain, would have to be regarded as
cephalic appendages, while, if they derived their nerves from the pedal
ganglion, they would have to be considered as parts of the foot. Some of the
facts of ontogeny and comparative anatomy have recently been shown to be
opposed to the first of these views and to favour the second. According to
Perseneer, as already mentioned, the brachial ganglion at first forms by
abstriction from the pedal ganglion, and is thus not in any way closely
connected (as assumed) with the cerebral ganglion.
Before the brachial ganglion separated from the pedal ganglion, a nerve-
strand ran from the anterior part of the (primitive) pedal ganglion to the
THE GILLS. 306
arms, a similar strand running from the posterior part of the ganglion to the
funnel, The latter, the pedal nature of which can hardly be doubted, thus
{according to PxiseNuEn), at this early stage, receives its nerve-elements front
the same ganglion as the arms.
Another argument in favour of the pedal nature of the arms is found in the
statement of Jarta (No. 18) that the brachial nerves have their roots in the
pedal gunglion and that they do not pass into the cerebral ganglion as has
been stated, Prrsexern further rightly points out that the arms in their
origin (and also in the manner of their innervation) are clearly ventral
organs, and those that assume a dorsal position only reach it in a late stage
of development (p. 263). Only as the arms are displaced anteriorly, and to
some extent dorsally, does the part of the central nervous system innervating
them and originally belonging to the pedal ganglion shift in » dorsal direc-
tion (PeLsenzen, No. 38). J. Srerwer's physiological researches,” according
to which the destruction of the pedal ganglion led to paralysis of the arms
and thus proved them to belong to the foot-region, are also significant in this
connection.
We thus have strong reason for regarding the arms, not* as cephalic ap-
pendages, but rather as greatly modified parts of the foot (of. Chap. XXXIV.).t
B. The Cartilaginons Skeleton.
The various parts of the cartilaginous skeleton are regarded by some
authors (Merscuniorr, Ussow) as ectodermal structures which have gradu-
ally shifted inward. Bosrerzky's figures of the optic cartilage seem to
confirm this method of origin which at first sight is surprising. Around each
eye (Fig. 143 ad, adk), the ectoderm is seen to become greatly thickened; these
thickenings, in consequence of the development of the folds described above,
come to lie deeper, Even if these figures appear convincing, we must not
exclude the conjecture that some part of the superficial cell-layers may
have already become differentiated, und in this case might be reckoned as
mesoderm.
P. The Gills.
The gills were found to arise as papilla-like prominences in front
of the mantle near the anal region (Fig, 85 C and D, &, p. 189), to
be gradually grown over by the latter (Figs. 87 and 88 &, p. 194),
* Sitz.-Ber. Akad. Berlin, 1890,
iad recently Kenn (No. 11.) has opposed the pedal nature of the arms of
2 and reverted to the older view of Grousen that they are cephalic.
While ad ing several weighty arguments against some of the evidence
which has been put forward regarding their pedal nature, he practically gives
‘no striking reasons in favour of their cephalic nature and we think that at
epee at any rate, we cannot unhesitatingly accept his conclusions,
question seems to rest on the justification of the attempts to homologise
the anterior sub-oesophageal ganglionic mass of the Cephalopoda with one of
the highly differentiated ganglia of the Prosobranchia,.—Ep.]
x
306 CEPHALOPODA.
and thus eventually to lie within the mantle-cavity. They had
previously become flattened, and their surfaces folded in such a
way as to produce the bipectinate gills characteristic of so many
‘Molluscs (pp. 72 und 208). |
The development of the gills in Seva was very carefully studied
by Jounin (No. 20). Further foldings of the leaflets take place,
and as these processes ure repeated in the secondary leaflets, a some-
what complicated gill is produced consisting of three systems of folds
one above the other ; this is peculiar to the Cephalopoda.
According to Jounrn, the mesoderm is the chief factor in the
development of the gills which, by its own increase, determines the
growth of these organs and the modifications in their shape. The
gill-rudiment, when still papilla-like, was composed of the superficial
ectoderm-layer and a massive mesoderm (Fig. 134, &, p. 284). The
young gill, also, when only slightly differentiated, consists largely
of mesodermal tissue, in which, at a later stage, the cavities of the
blood-vessels form in the usual way, through the moving apart of
the cells. Only the larger vessels develop walls of their own. The
afferent (venous) and efferent (arterial) vessels, lying in the axis of
the gill (the middle lamella) become connected with the principal
blood-vessels of the body, and the afferent vessel of each gill is
said at its base to give rise to au auricle, In other Molluscs we saw
the auricles form from the coelomic sacs, like the ventricles ; we must
therefore accept these statements with caution.
G, The Mesodermal Structures.
The rise of the mesoderm, and of the organs derived from it, is
still very imperfectly known in the Cephalopoda, and can therefore
only be briefly noticed, We shall have to follow chiefly the older
researches of Bosrerzky and Ussow, and shall have repeatedly to
refer to the conditions found in the adult animals.
We have already described the first rudiment of the mesoderm
as being contained in the peripheral thickening of the germ-dise
(Figs. 112-114, p. 247, ete., and Fig. 131, p. 280), and as lying, after
the differentiation of the entoderm, between it and the ectoderm,
Thus at first it forms a circular thickened layer, which then extends
both towards the centre of the germ-dise and towards its edge, In
this way it attains a size which, as compared with that of the
other layers (ectoderm and entoderm), is very considerable (Figs. 131
and 132).
THE MESODERMAL STRUCTURES. 307
According to the view of ViacceTox, which has already been mentioned,
a delamination of the mesodermal part from the outer layer takes place even
at the time when, according te the account here given, the germ-layers have
long been differentiated. At a much later stage also, when the rudiments
of the organs have appeared and have to some extent developed, the ecto-
derm continues to yield mesodermal cell-material. ViauceTon compares this
process with the development of mesoderm in the Annelida as described by
Keersessens (Vol, i., p. 286), and refuses to acknowledge this mesoderm as
the equivalent of a distinct germ-layer. Such a yiew can best be understood
in connection with the Cephalopoda in which the formation of the germ-
layers is evidently greatly influenced by the large amount of yolk in the
egg, the distinction between the layers appearing almost obliterated, but in
this onse, as in that of the alimentary canal and the nervous system, we
must postpone a decision until further light is thrown upon the matter.
Tn the Cephalopoda, especially in Vautilua and the Decapoda, the
secondary body-cavity (the coelom) is very well developed and sur-
rounds the heart with its afferent and efferent vessels, the branchial
hearts, the pericardial glands, the genital organs and the stomach,
4nd is connected with the cavities of the nephridia (Groppen, No.
15). The last two organs lie in the posterior section of the body-
cavity which is incompletely separated from an anterior section by
a septum,
In the Octopoda the body-cavity is much reduced, being represented
merely by a system of narrow canals formerly claimed as a water-vascular
‘system. In consequence of its slight development, the coslom no longer
surrounds the heart, the branchial hearts and the stomach (Gropary),
As the condition of the coelom in the Cephalopoda is so primitive, we
are led to expect the coelomic sacs to appear distinctly in their ontogeny.
But the conditions of the formation of the mesoderm above described seem
im this respect to be unfavourable. A splitting of the mesoderm into a
somatic and a splanchnic layer has indeed been asserted by Ussow, but
is not noticed by other observers and, in any case, does not lead to the
formation of coelomic and pericardial sacs such as were met with in the
Gastropoda or such as we might expect in connection with the well-developed
eoclom of the adult Cephalopod. We may observe in passing that there
‘are reasons for believing that structures of this sort may yet be found in
the
The kidneys, in the Cephalopoda, show the same primitive condition as in
other Molluscs. In the Octopoda, they are represented by two sacs lying
symmetrically; in the Decapoda these have united to form a single sac.
The renal sacs open out through ureters on either side of the anus.
A comparison of the Cephalopodan kidney with the nephridia of the
segmented animals seems specially suitable on account of two pairs of rena!
sacs being found in Nawtilus, each pair opening out through a distinct
aperture. One of these pairs, however, is without an aperture into the
coelom, and its significance as a true nephridium is therefore doubtful; we
308 CEPHALOPODA.
must rather imagine it to be a new structure formed from the posterior pair
(which alone is originally present), This process may have been connected
with the development of four gills, which some believe fo be a secondary con-
dition and which has led to the assumption that the Tetrabranchia (Nartilts)
were derived from Dibranchia (v. Jaenixc, No. 19), Although Nautilus is,
without doubt, a very ancient and primitive form, there are certain signs (¢.9.,
the degeneration of the efferent genital duct) that it was already specinlised
ima definite direction, and thus might have acquired a second pair of gills.
The same argument would apply in the case of the kidney. This view would
find some support from the fact that, in other Mollusca, only one pair of
nephridia and, with the exception of Chiton, only one pair of gills are present,
‘The certain information we possess as to the development of the kidneys
is as yet too scanty to enable us to settle this question. It was shown by
Bosretzky that, in Loligo, the kidneys arise directly under the covering of
the postero-dorsal surface as two distinct sacs in the mesoderm, and only
later unite, and assume the close relation to the veins which they show in
the adult. The inner wall of the kidneys is much folded, and thus yields the
aciniform structures known as venous appendages (GRonaEN).
Our knowledge of the development of the genital organs also is still very
incomplete. The genital glands arise as thickenings of the pericardial
epithelium near the heart (Bonretzky, Scuukewrrsca). This primitive
relation to the pericardium or coelom is preserved by them throughout life,
but at a later period a (genital) capsule is formed round the glands by a
peritoneal fold ; the cavity of this capsule, however, remains permanently in
connection with the body-cavity, and thus forms a part of the latter (Brock,
Grospen).
The efferent ducts, of which there is one pair, are connected with the
eapsule. When, asin most Dibranchia, there is only one efferent duct, this
must be considered as due to degeneration, as is proved by the presence of
paired ducts in the Octopoda, in Ommastrephes (x Decapod) and in Nautilus,
this latter form having one functional and one reduced oviduct.
This relation of the efferent genital ducts suggests that they are modified
uephridia (see Vol, iii, p. 42 footnote), and the question thus again arises
whether the Cephalopoda possessed two pairs of nephridia and in this respect
& segmentation (which, however, would be incomplete). Such a view is by
no means justified by what is known of those Molluses which on the whole
show conditions more primitive than those of the Cephalopoda. These
Molluscs afford no convincing evidence of segmentation. We must therefore
regard efferent ducts as having formed independently of the nephridia, or else
as derived by fission from nephridia, but cannot consider them ns independent
nephridia.
The blood-vascular system. Even in the case of this system of organs, so
well developed in the Cephalopoda, very little is known ontogenotically,* and
*Bosrerzxy appears to have studied the development of the
apparatus in detail, but as his work is in Russian, we are limited to the
descriptions of the figures and a short abstract in Anat. Hofmann
and Schwalbe, Bd. vii., 1878, and this applies also to our former references to
his observations, .
kL =—
THE MESODERMAL STRUCTURES. 309
we must content ourselves with a brief summary of the accounts given of its
origin.
According to Bosrerzky, the arterial heart is derived from two sac-like
ergans which first appear as cavities in the massive mesoderm near the
radiment of the intestine and the yolk-snc. Round these, the cells become
regularly arranged, and the two sacs or vesicles thus produced then unite to
form the heart, The formation of the auricles has already been mentioned
(p. 806).
‘The arteries arise as canals in the mesoderm, their limits being marked by
the regularity of arrangement in the cells; at a very early period blood is
driven through them, in consequence of the commencing pulsations of the
heart. After the two sacs have united to form the heart, the two pericardial
sacs are said to extend towards the latter, so as to enclose the heart in the
same way as in other Molluscs (Scammewrrscn). The branchial hearts abut
on that part of the body-cavity which encloses the heart, and are also covered
by the peritoneum. The pericardial glands (the so-called branchial appen-
dage) develop from the latter as growths of the epithelium; these glands are
connected with the branchial hearts and are held to be excretory organs.
The branchial hearts are said to be differentiated from the mesoderm at the
broad bases of the gills, and the whole venous system, the venae cavae (chiefly
the anterior vena cava) being specially noticeable in the embryo, arises as
Incunar spaces in the mesoderm, some of these spaces changing into actual
veins and others into irregular blood-sinuses (BosreTaxy).
Scarm«kewrrsce attributes the origin of the blood-corpusecle to the increase
in number and migration of cells of the yolk-epithelium in the posterior part
of the body, and thus assumes for them an origin similar to that of the blood-
cells in the Arachnida, the latter being formed from migrating yolk-cells
(Vol. iii., p.88). We refrain for the present from expressing an opinion on
these somewhat improbable statements.
Chromatophores, subcutaneous tissue, musculature. ‘The layers of
the mesoderm lying beneath the ectoderm become transformed into
the so-called fibrous layer, while the deeper layers yield the con-
nective tissue and musele-fibres of the cutis and also, iu any case, the
muscles of the external organs. The chromatophores, also, are said
by nearly all authors to originate here, but a somewhat different view
of their origin has recently been propounded.
The time of the appearance of the chromatophores in the different
forms varies greatly (¢/. Figs. 120 and 121, p. 263). In Loligo, for
instance, they uppear very late, but in the Cephalopod described by
GW®ENACHER at an early stage, before the circumcrescence of the
yolk by the blastoderm is completed and before the organs have
appewred (Fig. 125, p. 268). In the last case, a very early differen-
tiation of these mesoderm-layers seems to have taken place.
The chromatophores are said to be derived from mesoderm-vells
. Which are distinguished from the surrounding cells by their large
ee
310 CEPHALOPODA,
size and by the early deposit of pigment in their protoplasm (Gmop,
No. 13). Ata later stage, they are covered by a thick envelope;
the cells in the neighbourhood stretch out into spindles and become
connected with the chromatophoral cells. In this way arises the
well-known appearance of the (contractile) fibre-bundles connected
radially with the chromatophoral cells, The change of shape in the
pigment-cells which is accompanied by change of colour was usually
attributed to the contractility of these bundles, é¢., they have been
regarded as muscle-fibres, while some authors have aseribed a capacity
for contraction to the pigment-cells themselves, the radial fibres being
considered as merely connective tissue which, it was assumed, held
the actual chromatophores in position (Grrop).
Another account of the origin of the chromatophores has recently
been given (Joupin, No. 23). According to this view, the ectoderm-
cells, which are especially distinguished by their size, sk inwards
through a funnel-like depression. In the large cell at the base of
the depression, the protoplasm becomes differentiated and, later, pig-
ment appears, ‘The cell then loses its connection with the ectoderm,
A number of mesoderm-cells which could be scen even earlier regularly
arranged below it, and which soon multiply still further, yield the
radial fibres. The chromatophores would thus be due to the com-
bined action of the outer and the middle germ-layers (Joust),
LITERATURE.
1. Apams, A. Mollusca. The Zoology of the Voyage of H.M.S.
Samarung. London, 1848.
2. Arrenuérr, A. Om skalets bildning hos Sepia officinalis. Gfver-
sigt kangl, Vet.-Akad. Férlandl, Stockholm. No.7. 1887.
3, Benepes, P. J, van, Recherches sur l’embryogénie des
Sépioles. oun. mém. Acad. roy. sci. Bruxellee, Tom, xiv.
1841.
B KY, N. On the Development of the Cephalopoda.
Esvyest. imp. Obsheh. lynbit. estestr. Antrop. i
. Ma Tom. xxiv. 1877.
Ueber die Gesehlechtsorgane der Cephalopoden.
iss. Zool. Bd. xxxii. 1879.
einer Phylogenie der dibranchiaten Cepha-
phot. Jahrb, Ba. vi. 1880. *
_
LITERATURE. 311
7. Brooxs, W. K, The development of the Squid (Loligo pealii).
Anniv. Mem. Nat. Hist. Soe. Boston, 1880.
8. Brvon, A. T.. Segmentation of the egg and formation of the
germ-layers of the Squid (Loligo pealii). Johns Hopkins Uni-
versity Circulars. Vol. vi. No. 54. 1886.
9. Curase, St. Dette, Memoire sulla storia e¢ notomia degli
animali senza vertebre del regno di Napoli. Napoli, 1823-
1829. Vol. iv. (2). (2nd Ed. Vol. i., Wapodi, 1841, bis 44.)
(Contains quotations from Pott and Mrs. Power as well as
notes on the Formation of the Shell by Apams (No. 1),
Frrrvusac and d’Orpreny. See also P. FiscHer. Manuel
de Conchytiologie. Paris, 1887.)
10. Disrn, M. J. Unters, iiber die Organisation des Gehirns
wirbelloser Thiere, ete. Séteungsber, k. Akad. Wien. Bad.
Ixxvii. 1878.
11. Fon, H. Note sur le développement des Mollusques Ptéropods
et Céphalopodes, Arch. Zool, exp. gén. Tom. iii, 1874
12, Grrop, P. Recherches sur la poche du noir des Céphalopodes
des cites de France. Areh. Zool. exp. yen. Tom. x. 1882.
13. Gmrop, P. Recherches sur la peau des Céphalopodes. Arch.
Zool. exp. gén. (2). Vol. i. 1883.
14, Grenacuer, H. Zur Entwicklungsgeschichte der Cephalopoden.
Zeitechr, f. wits, Gool. Ba, xxiv, 1874.
15. Groppex, C, Morphologische Studien iiber den Harn- und
Geschlechtsapparat sowie die Leibeshéhle der Cephalopoden.
Arb. Zool. Inst. Wien. Bd. ve 1884,
16. Groppen, C. Zur Kenntniss der Morphologie und der Ver-
wandtschaftsverhaltnisse der Cephalopoden. Arb. Zool. Inst.
Wien. Bad. vii. 1886.
17. darken, O. Ein Acanthoteuthis aus dem unteren Lias von
Lyme Regis in England. Sitzwngsher. Gesellech, Naturforsch.
Freunde, Berlin, Mai 1890,
18. Jarra, G. La innervazione delle braccie dei Cefalopodi. Boll.
della Soc, di Naturatisti in Napoli, Vol. iii. 1889.
19. Jagnia H, von, Ueber die Verwandtschaftsbeziehungen der
Cephalopoden. Zeituchr. /. wiss, Zool. Bd, xxxv, 1881.
20. Jour, L. Sur la structure et le développement de la branchie
de quelques Céphalopodes des cétes de France. Arch, Zool.
exp. gé. (2). Vol. iii, 1885.
21. Jousim, L. La ponte de l’Elédone et de la Stche. Arch, Zool.
» exp. gén (2). Tom. vi. 1888,
ie
312
22.
23.
24,
25.
26.
28.
29.
33.
34.
35,
36.
CEPHALOPODA.
Jounin, L. Kecherches sur la morphologie comparée des
glandes salivaires des Céphalopodes. Arch. Zool. erp. gén. (2).
Tom. v. Suppl. 1887-90.
Jounin, L. Sur le développement des chromatophores chez
les Céphalopodes. Compt. rend. Acad. Sei. Paris, Tom.
exii. 1891.
Kourixer, A. Entwicklungsgeschichte der Cephalopoden.
Ziirich, 1844.
Korscuett, E. Beitriige zur Entwicklungsgeschichte der
Cephalopoden I. Die Entstehung des Darmkanals und Nerven-
systems in Beziehung zur Keimblitterfrage. Festschrift 70
Geburtstage Leuckarts. Leipzig, 1892. Abstract in Verhandl.
2. Vers. D. Zool. Gexellech. Berlin, 1892.
Korscnett, E. Ueber den Laich und die Embryonen von
Eledone. Sttzwngxher. Ges. Naturforsch. Freunde. Berlin,
Febr. 1893.
. Lankester, E. Ray. Summary of Zoological observations made
at Naples in the winter of 1871-2. Anu. Mag. Nat. Hist.
(4). Vol. xi, 1873.
‘Lankester, BE. Ray. Observations on the Development of the
Pond-Snail (Lymnaea stagnalis) and on the early stages of
other Mollusca. Quart. Journ. Micro. Sci. Vol. xiv. 1874.
Lankestrr, E. Ray, Observations on the Development of
the Cephalopoda. Quart. Journ. Micro, Sei. Vol. xv. 1875.
. LANKESTER, E. Ray. Article ‘“ Mollusca”. © Eneyclopuedia
Britannica, 9th Ed. 1883.
. Leuckart, R. Ueber die Morphologie und Verwandtschafts-
verhiltnisse der wirbellosen Thiere. Braunechireig, 1848.
MetscuntkorF, E, Le développement des Sépioles. Extract
from the Russian work by E, Cuaparkpe in Bibl. Univ. et
Rem Suisse. Arch. dex xe. phys, et nat. Nouv. Période, Tom.
Xxx. Genéve, 1867.
Owen, R. Memoir on the Pearly Nautilus, London, 1832.
Owen, R. Spirula Peronii. The Zoology of the Voyage of
H.M.S. Samarang, edited by A. ADAMs. Mollusca. London,
1848,
Owen, R. On the External and Structural Characters of the
Male of Spirula australis. Proc. Zool. Soc. London, 1880.
OwsJANNIKOW und Kowauevsky, A. Ueber das Central-
nervensystem und des Gehororgan der Cephalopoden. Mém.
de Vacad de St. Pétersbourg (7)., Tom, xi. 1867.
42.
43.
44,
45.
46.
47.
48.
49.
LITERATURE. 313
. PELSENEER, P. Sur la valeur morphologique des bras et la
composition du systtme nerveux central des Céphalopodes.
Arch, de Biologie. Tom. viii. 1888.
. Pevsenger, P. Sur la nature pédieuse des bras des Céphalo-
podes. Ann. Suc. Roy. Malacolog. de Belgique. ‘Yom. xxiv.
1289.
. Rrerstany, E. Die Sepienschale und ihre Beziehungen zu den
Belemniten. Palaeontographica. Bd. xxxii. 1886.
. ScuimkEwrtTscH, W. Note sur le développement des Céphalo-
podes. Zool. Anz. Bd. ix. 1886.
. Stzenstrup, J. De Ommatostrephagtige Blackspruthers ind-
byrdes Forhold. Om Ommatostrephernes Aeglaegning og
Udvikling. Oversigt over ud. k. Danske Vidensk. Selsk.
Forhawil. 1880.
Sreenstrup, J. Zur Orientirung iiber die embryonale Ent-
wicklung verschiedener Cephalopoden-Typen. Biol. Centrathl.
Bd. ii, 1884-5.
Sremmann, G., u. Dopervein, L. Elemente der Palaonto-
logie. Leipzig, 1890.
Ussow, M. Zoologisch-embryologische Untersuchungen. Die
Kopffiissler. Arch. 7. Natury. Jahrb. xl. 1874.
Ussow, M. On the Development of the Cephalopoda. (Russian,
with abstract in German by Stieda.) Jzryext. imp. Obxhch.
lyubit. extesty, Antrop. i Ethnog. Moxcow, 1879.
Ussow, M. Untersuchungen iiber die Entwicklung der Cephalo-
poden. Arch. de Biol. Tom. ii. 1881.
ViatteTon, L. Sur la fécondation chez les (éphalopodes.
Comptes rent. Acai. Sei. Paris. Tom, ci. 1885,
ViaLLeton, Iu, Recherches sur les premiéres phases du dé-
veloppement de Ia Seiche (Sepia officinalis). daw. ai, nat.
(7). Zool. Tom. vi. 188%.
Watase, S. Observations on the development of Cephalopods :
Homology of the germ-layers. Stu. Biol, Lah. Johns Hopkinx
Univ. Baltimore. Vol. vi. 1888.
50. Wartase, S. Studies on Cephalopods. 1. Cleavage of the Ovum.
Journ. Morphol, Vol. iv. 1891.
. ZErnorr, D. Ueber dasx (eruchsorgan der Cephalopoden.
Bull. de la Soc. imp. d. Naturatlister de Moxcou. Tom. xii.
1869.
. Zrrrer, K. Handbuch der Paliiontologie. Bl. i. Cephalopoda.
Munchen unt Leipzig, 1884.
314 CEPHALOPODA.
APPENDIX TO LITERATURE ON CEPHALOPODA.
I, Fausser, V. Zur Cephalopodentwicklung. Zool. Anz. Jahrg.
xix. 1896. i! :
II. Kerr, J. Granam. On some points in the Anatomy of Nautilus
pompilius. Proc. Zool. Soc. London. 1895.
III. Korscnevt, E. Ueber den Laich und die Embryonen von
Eledone. Sifzungsber. Gra. Naturf. Berlin. 1893 (1894).
IV. Wrutey, A. The Oviposition of Nautilus macromphalus. Proc.
Roy. Soc. London. Vol. ix. 1897.
CHAPTER XXXIV.
General Considerations on the Mollusca.”
Tn attempting to combine into one yeneral scheme the de-
velopmental phenomena described in the preceding chapters, in
connection with the different divisions of the Mollusca, we cannot, of
course, take into account all the very different ontogenetic processes,
but can only select from among them those that are of the more
general significance.
Even in the cleavage of the egg, great variety prevails, Thus we
find, on the one hand, that the meroblastic type of cleavage attains
its highest development among the Mollusca (Cepbalopoda) but, on
the other, that total and, at first equal, but soon after unequal
cleavage is still more common in eggs of this phylum (Chifon,
Gastropoda), and so also is a type of cleavage which from the first is
wnequal (Solenogastres, Lamellibranchia, Solenoconehae), The cause
of this difference is to be songht in the varying amount of yolk
contained within the eggs but, even among eggs showing total
cleavage, there are some that, through a secondary separation of the
yolk-mass from the blastomeres, lead us to the meroblastic method
of cleavage (Vassa). The latter is to be explained as due to the
extraordinarily large amount of yolk in the egg, and this same
peculiarity further determines so great a modification of the early
ontogenetic phenomena in the Cephalopoda, that this division must
be left almost entirely out of account in our comparative review.
Within each separate division of the Mollusca, the phenomena of
cleavage are very similar and very regular.
‘The formation of the primary germ-layers takes place through the
invagination of a blastulu with a more or less wide cleavage-cavity,
or, where the latter is wanting, and the egg is richer in yolk, through
epibole. These two processes are found in nearly related forms, or
* See note, p. 1.
316 GENERAL CONSIDERATIONS ON THE MOLLUSCA.
else an invagination-gastrula appears as a stage following the epibolic
gastrula, as, for instance, in Osfrea and various Gastropods, in which
a cleavage-cavity arises only at a later stage, and the macromeres
which are continuing to multiply press in towards it. There is also
no essential difference between these two types of formation of the
germ-layers. In the Cephalopoda also the germ-layers may be traced
back to the same processes, although these are influenced in a marked
manner by the large amount of yolk in the egg.
The middle germ-layer arises in a very similar way in all those
forms in which it has been investigated. It originates in two
primitive mesoderm-cells derived from one of the macromeres (primary
entoderm). In the Gastropoda, in which this point has been best
examined, the formation of the primitive mesoderm-cells from the
macromeres was found to be very regular.* The large primary cells
of the mesoderm have been discovered to exist in all the following
divisions of the Mollusca: the Amphineura, the Lamellibranchia, the
Solenoconchac and the Gastropoda. In the Cephalopoda, on the
contrary, the development of the mesoderm has been considerably
modified by the conditions mentioned above.
The two mesoderm-bands arise through the multiplication of the
primitive mesoderm-cells. [t has repeatedly been stated that
mesodermal tissue is not due to the multiplication of these cells, but
is yielded later partly by the ectoderm, as described for the Annelida.
The first of these views, i.., the derivation of the mesoderm from the
primary cells is, so far, the more probable, but the other view should
not be summarily dismissed, and in any case deserves more careful
investigation.+
It is very characteristic of the Mollusca that the mesoderm-bands
are retained for only a short time. They soon disintegrate, single
cells separating from them and becoming distributed in the cleavage-
cavity as the so-called mesenchyme. Before this happens, however,
or else during this process, a cavity appears in each of the mesoderm-
bands : this is bounded by a more or less regular epithelial wall and
is thus recognisable as the coelom. ‘This process is the same already
met with in the Annclida (Vol. i., p. 290), and Arthropoda (Vol. iii..
p. 413). In these latter it leads to the formation of two mesodermal
layers, one applied to the ectoderm and the other applied to the
entoderm ; these are the somatic and splanchnic layers. This seems
* [See footnote, p. 119, Ep.] t [See p. 29, Ep.]
GENERAL CONSIDERATIONS ON THE MOLLUSCA. 317
likewise to be the case in the Chitons which in other respects also-
appear to be very primitive animals (Fig. 4 4 and B, p. 8); they
do not, however, show any suggestion of the segmentation of the
mesoderm (formation of the primitive segments) so characteristic of
the segmented animals,
The Amphineura, even as adults, show the secondary body-cavity
well preserved. It is still very large, and contains the principal
organs, such as the alimentary canal, the liver and the heart. In the
higher Mollusca (Lamellibranchia and Gastropoda) it is much more:
veduced im comparison with the primary bedy-cavity and is even
quite small. The primary body-cavity with the mesenchymatous
tissue distributed in it is very large and becomes the definitive body-
cavity, just as in the Arthropoda (Vol. iii., p. 423). In spite of this,
the Molluses have been regarded as typical Schizocoelata, Ae, as forms
devoid of a true coelom. Such a true coelom, however, is retained
by them, though only slightly developed.
While in the Arthropoda, the coelomic sacs (primitive segments)
nsnally completely disintegrate or at the most persist to a small
extent in the genital glands (Peripatus, Myriopoda) the coelom of the
Mollusca is always retained in the form of the pericar/ium from which
the wephridia and the genital glands are formed (Gastropoda) in a
manner recalling the primitive conditions in the Annelida, Where
the connection between the kidneys, the genital organs and the-
pericardium has not yet been made evident in the embryo in conse-
qnence of insufficient investigation, or else where, in consequence of
secondary modifications in the forms examined, it can no longer be
demonstrated, the anatomical condition of the systems of organs
clearly proves such a connection. In various Molluses, the cavity of
the genital glands is directly connected with the pericardial cavity
(Amphineura, Cephalopoda), the nephridia entering the latter through
an open funnel, a condition which recalls the open connection existing
between the nephridia of the Annelida and the secondary body-
cavity (Amphineura, Lamellibranchia, Gastropoda, Cephalopoda).
There can consequently no longer be any doubt that the pericardium of
the Mollusea should tw regarded as the secondary body-eavity ; and,
apart from the absence of segmentation, the resemblance in this point
to the Annelida is very great.
The condition of the mesoderm and the structures belonging to
it is thus evidently of great importance in interpreting the Mollusea 5
it has therefore been considered in connection with the early onto-
genetic processes. Hardly less important, however, is the larval
ie
.
318 GENERAL CONSIDERATIONS ON THE MODLUSCA.
_form which also in several ways throws light upon the relationships
‘of the Molluses.
Although the larvae of the different divisions, e.y., those of the
Amphineura, the Solenoconchae, the Lamellibranchia, and perhaps
also those of the Heteropoda and the Opisthobranchia appear with
very different forms, all of them may without any difficulty be traced
back to the Trochophore, the typical form of which was met with
in the Annelida. In some cases, such, for instance, as the larvae of
Dondersia and Dentalium (Figs. 10, pp. 17 and 138, p. 291) as well as
in those of a few Gastropoda (perhaps of the Gymnosomatons Ptero-
poda, Fig. 77, p. 172) this is less evident, while in the larvae of other
Gastropods such as Patella, Paludina and most Lamellibranchia, the
resemblance is exceedingly striking (Figs. 51-53, p. 127; 56, p. 135;
14, p. 28; 18, p. 36), But even in larvae in which the resemblance
is not so great (Figs. 66 and 67, p. 155; 72, p. 162; 75, p. 168), eom-
parison with other forms or the examination of the younger stages
enable us easily to trace back the larval form to the T'rochophore
(Figs, 64 and 65, pp. 158 and 154; see also p. 166). In the greatly
modified ontogeny of the Cephalopoda, traces of the larval form have
hitherto not been established with certainty,
The development of the Molluscan Trochophore closely resembles
that of the Annelidan larva, It arises from the gastrula-stage,
several rows of cells at the anterior region becoming covered with
strong cilia and thus yielding the pre-oral ciliated ring, the velum
of the Molluscan larva. The blastopore, which at first lies at the
posterior end, usually becomes slit-like ; it probably closes from
behind forward and generally, at its anterior end, passes into the
mouth through the formation of an ectodermal depression, the
stomodaeum, A post-ovw/ ciliated ring appearing behind the mouth
heightens the resemblance to the typical 7'rochophore, the external
form of the Molluscan larva now also agreeing with that of the
latter, the anterior end being much widened and the posterior
narrowed.
At the posterior end of the body, and thus at the point where the
Dlastopore at first lay and from which its closure commenced, the
anus now forms. It thus appears as if it also bore some relation
to the blastopore ; indeed, an attempt has been made to prove this
to be the case in some Molluscs (¢.g., the Opisthobranchia), and in
another Gastropod, Palwline, the direct transformation of the blasto~
pore into the anus has been assumed. We thus find in the Mollusca
conditions altogether similar to those met with in the Annelida and
Lb al
GENERAL CONSIDERATIONS ON THE MOLLUSOA. 319
the Arthropoda, in which also relations were proved to exist between
the mouth and the anus on the one hand and the blastopore on the
other,
The intestine, which like the oesophagus, is usually formed by an
ectodermal invagination in the Invertebrata, appears to be derived
entirely from the entoderm in the Mollusca; thus the enteron
usually fuses direct with the ectoderm, without any depression of
the latter. This has been regarded as an important peculiarity in
the organisation of the Mollusca, and much stress has been laid
upon it, but if, as has been stated, an ectodermal intestine actually
occurs in a few Molluses (Chiton, Tervdo, ete., pp- 15, 66, 208), the
usual absence of the proctodaeum cannot be considered as a dis-
tinctive feature.
At the opposite end of the larval body, i.¢., at the cephalic pole,
and in the midst of the velar area, an ectodermal thickening known
aa the apical pinte is found, occupying the same position as in many
other pelagic larvae. In the Annelida, the supra-oesophageal ganglion
is thought to originate from the apical plate, and in the Mollusca
also the cerebral ganglion is said to arise from it (Chifon, Lamelli-
branchia) or else to bear some relation to it (Dentalium). In the
Gastropoda, indeed, the cerebral ganglia originate as two ectodermal
thickenings of the pre-oral part of the bedy, but these also do not
differ much in position from the apical plate, so that even here there
may be some relation to the latter.
An organ of special importance in comparing the Molluscan 7'rocho-
phore with other larval forms is the primitive kidney. This arises
from cells derived from the mesoderm-bands,* in the same way as
in the Annelida, as a tubular paired structure. Its relations to the
primary body-cavity also seem to be the same as those met with in
the Annelida and in the case of the adult excretory organ of the
Plathelminthes.
It is evident from the above that the resemblance of the Molluscan
larva with the Trochophore of other animal phyla, and especially
with that of the Annelida is exceedingly striking not only in external
form but in internal structure. Attempts have been made to account
for this resemblance by the supposition that the larvae of these
phyla gradually assumed the same organisation as a consequence
of similarity in their manner of life. We do not share this view,
and can only explain the remarkable external and internal similarity
* [See footnote, p. 179. Ep.)
320 GENERAL CONSIDERATIONS ON THE MOLLUSCA-
through actual relationship to one another of all those groups which
have the 7yevhophore as their larval form.
If we accept this latter view, there can be no doubt as to the
significance to be attributed to the larval form. Its reappearance in
the development of phyla so different as the Annelida, the Mollusca,
and the Molluscoida, points to a racial form haying this structure.
This brings us to the difficult and much diseussed question of the
origin of the Mollusca.*
Of the theories as to the origin of'the Mollusca, two seem to us
to demand our special attention, these being the theory that the
Mollusca are derived from Turbellaria-like forms and that which |
derives them from a 'rochophore-like ancestor.
The derivation of the Mollusea from Turbellaria-like forms has much
in its favour, especially as it affords a partial explanation of the
perplexing conditions of the nervous system. The pedal strands,
according to this view, would correspond to the ventral longitudinal
nerves of the Turbellaria, while the plearo-visceral strands correspond
to the lateral nerves. The resemblance is specially striking in the ner-
vous system of the Amphineura (forms which somewhat resemble the
‘Turbellaria in shape) which consists of four longitudinal strands with
connecting commissures. A similar ladder-like nervous system with
a ventral and a lateral strand on each side occurs in the Turbellaria
(Triclada and especially Gunda). The anus, which is wanting in the
Turbellaria, was acquired later, and so was the blood vaseular system.
The coelom is to be explained by the dilation of the genital glands,
for the genital products originate, as was shown, from its epithelial
wall, The shell, an important component part of the Molluscan
organisation, arose for the protection of the body in the form of
a cuticular secretion of the dorsal surface in which were deposited
calcareous concretions. The foot, an equally essential part of the
organism, resulted from the transformation of the ventral surface of
the body, which was used for creeping, into a muscular sole, or else
is assumed to be a modification of the ventral sucker.
* A great deal has been written on the relationships of the Mollusca, Pu
vefrain from discussing the different and often opposite views which have been
propounded on this subject as they would merely add length to our
and make it the lessclear. We shall only allude in passing to the
theory adopted by Ray LankesTer and still more ardently by HatscHek, and
to Lano’s theory of the derivation from Turbellaria-like forms. ‘list of the
most important works on the subject will be found at the end of deeien
Lasa's view has recently been published in his Text-book of Comparative
Anatomy (Engl. Trans., Vol. ii.).
+THrexe, like Lac, derives the Mollusca. from Turbellaria-like forms and
regards the ventral sucker of the Polyclada as the organ from which, without
Lk __ al
GENERAL CONSIDERATIONS ON THE MOLLUSOA. 321
The above may, on the whole, be described as possible, but this
origin has the disadvantage of starting from very highly differentiated
animals ; and, a point which appears to us as very important, affords
no clear explanation of the striking resemblance existing between the
larvae of the Mollusca and those of the Annelida, This resemblance
is made possible if we go back farther, to a form from which may be
derived both the Trochophore and the ancestors of the Turbellaria,
Since, in such speculations, it is only right to try to start from existing
forms, the common ancestral form of the Trochophore and the Tur-
bellaria has been sought among the Ctenophora, but it is impossible
to construct from them the desired ancestor without unduly forcing
the comparisons, It is true that the Ctenophora, on account of their
locomotion by means of cilia, their possession of the apical plate and
the condition of the entoderm and mesoderm may be brought into
relation with the Turbellaria and perhaps even with the T'rochophore,
but it appears to us still more certain that in them we have forms
already strongly specialised in a definite direction and thus no longer
suited to serve as the ancestral forms of the Mollusca. Instead then
of attempting to derive these forms from known and specialised groups,
it seems to us simpler and at least equally justifiable to fall back
upon some form with a more primitive organisation.
In considering the ontogeny of the Annelida, we started from a
gastrula-like form, ciliated all over, which developed at the cephalic
pole « ciliated tuft, and in which also the locomotory cilia were
specially arranged in the form of a ring running round the body (the
later pre-oral ciliated ring or velum). The primitive mouth (blasto-
pore), originally lying at the posterior end of the body, becomes dis-
placed forward owing to the development of a ciliated apparatus in
this very limited region ; thus the mouth shifts towards the locomotory
wpparatus which at the same time serves for conducting food to the
mouth, a8 is still seen to be the case with the ad-oral zone and the
poat-oral ring of the Trochophore, 1n the primary body-cavity of this
form, mesodermal elements are already found; among these lie the
gonads which no doubt are derived from the entoderm as well as from
the mesoderm [probably from distinct blastomeres which are neither
mesodermal nor entodermal but are the germ-teloblasts}, and which
—
322 GENERAL CONSIDERATIONS ON THE MOLLUSGA.
ure either still connected with the gastral cavity or already open
externally through special efferent ducts (nephridia t). A specially
important organ of this hypothetical form which ulso lies in the
primary body-cavity is the excretory organ, the origin of which is
one of the most difficult points to explain. Since we see the ex-
eretory organs rising from the mesoderm, and are inclined to derive
this latter from the entoderm, we hold it as most probable that the
primitive excretory organ was a diverticulum of the entoderm which
became secondarily connected with the ectoderm. At a later stage,
it loses its connection with the entoderm and thus becomes the
structure known as the primitive kidney (protonephridium), |
From a form possessing such an organisation, the Plathelminthes
also nay be derived. Their excretory system remiains on the level
of the protonephridium, merely branching and extending further
through the body. Their larval form corresponds more or less to
that described, but does not possess a primitive kidney. The Pilidiwm
of the Nemertini already shows a certain similarity to the T'roche-
phore, ond it has been pointed out that transition forms between it
and the MGLvER's larva of the Turbellaria are to be found (Volei.,
p- 168). The Pilidiwm is distinguished, like the Troehophore, by the
possession of an apical plate.
Through the coucentration of the ciliated apparatus and the
acquisition of an anus, the ancestral form ascended to the level of
the Trochophore, and then became the starting-point for the
Rotatoria, the Mollusca, and the Molluscoida, Its relation to the
Rotatoria and the Annelida, é.c., its ascent to the level of the latter,
has already been discussed (Vol. i., p. 342). The most important
features in this process are the appearance of segmentation and the
rise of the coelom, the latter being perhaps explicable through the
enlargement of the gonads ‘of the primitive form, As
mentioned, the genital products originate from the epithelial wall
of the eoelom, a fact which fayours such an origin, The Mollusea
exhibit, on the whole, similar conditions, but show an important
difference in the absence of segmentation, remaining in this respect
more like the primitive form.
We therefore assume that the Molluscan larva (Z'rochophore) still
closely resembles the ancestral form; it already shows, however,
some new characters which clearly indicate differentiation in a certain
direction, two of these, the shell and the foot, being specially dis-
tinctive of the whole organisation of the Mollusca. The first of these,
especially, can very early be recognised through the appearance of
i a
GENERAL CONSIDERATIONS ON THE MOLLUSCA. 323
the shell-gland on the dorsal surface of the larva, and thus gives the
larva the special character of the Molluse without at first affecting its
general appearance (Figs. 14, 15, 18, pp. 28, 31 and 36; Figs. 51 and
56, pp. 125 and 135), Somewhat later, but also ata very early stage,
the foot appears on the ventral side of the larva, The very early rise
of this organ which may in « few cases be found even before the
Trochophore form fully develops, must be regarded as a shifting back
to an early period of embryonic development of this feature which was
only a recent acquisition. It is all the more easy to admit this, as the
shell-gland is found to arise exceptionally early in those forms, the
development of which show marked specialisation, as, for instance, in
the Unionidae (Fig. 22, p. 50) and in the Cephalopoda (Figs. 116
A, p, 255 and 131 D, p. 280). The shifting back of the shell to the
earliest poasible embryonic period can easily be explained by its im-
portance as a protection to the larva, a fact which may be observed
in every Lamellibranch or Gastropod larva whether young or old ;
the slightest disturbance causes the animal rapidly to retreat into its
shell and thus to sink to the bottom of the water.
‘The larvae of the Amphineura have no true shell-gland, a peculiarity
that would increase the resemblance between them and the Annelida
if their organisation were better understood. It must at present be
confessed that the larvae of the higher Molluses are far more like those
of the Annelida than are the larvae of these more primitive forms, in
which we should expect a closer resemblance. The shell-plates of
Chiton, further, arise in the same region as the typical shell of the
higher forms (Fig. 5, p. 9).
‘The manner in which the shell appears in the embryo favours the
view that it is derived phylogenetically from « cuticular dorsal cover-
ing, within and beneath which caleareous concretions were deposited.
‘The shell-plates of Chiton have, indeed, with some probability, been
traced buck to the transformed spines of this animal (p. 12), but on
the whole it seems more likely that the Chiton shell, which consists
of a number of plates, arose in consequence of a secondary distribution
(determined by the manner of life of the animal) of an originally
continuons dorsal carapace.* From this flattened, bowl-shaped shell
* The shell has also been regarded as a partly internal dermal skeleton in
consequence of its condition in Chifon, where it is traversed by strands of
connective tissue (Fig. 8, p. 12), and the retractors of the body inserted into
it have been thought to an important part in its development (Tarete,
No. 20). This last factor is in any case of importance in connection with the
‘various modifications of the shell (where the latter is already present),
324 GENERAL CONSIDERATIONS ON THE MOLLUSOA.
were derived later all the varied shapes of shell met with ih the
different divisions of the Mollusca,
‘The protective dorsal covering which became the shell was, in any
case, of great significance for the further development of the Mollusca.
Since, starting from the back, the shell had to cover a large part of
the body so as to be able to shelter it as completely as possible, the
locomotory organ could only develop on the ventral surface, the
ereeping manner of life leading to the development of the foot, an
equally important organ and one highly characteristic of all the
Mollusea.
It has already been mentioned that attempts have been made to
derive the foot of the Mollusca from the ventral sucker of the
Polyclada, but it appears to us that the exclusive use of the ventral
surface ag creeping sole, simultaneously with the development of the
dorsal shell which necessitated a firmer adhesion to the sub-stratum,
alone suffices to explain the greater development of the ventral part
of the body into a muscular foot. Ina few Annelida and Annelidan
larvae, there is a ventral ciliated area extending between the mouth
and the anus which evidently assists the animal in creeping. Such
a differentiation may in any case be ascribed to the primitive form,
and this, together with a strengthening of the ventral musculature,
led to the formation of the foot when the primitiye form became
adapted to a creeping manner of life, an adaptation which, again,
was connected with the development of a shell.
In the Solenogastres, which are elongated and evidently very
lowly Molluscs, the foot is only slightly developed and appears
as a ciliated ridge lying in the ventral longitudinal groove. This
gtoove, which is represented in Fig. 147 A and B, might be compared
with the ciliated area mentioned above as occurring in the Annelida,
in some forms becoming depressed in such a way as to produce.»
ventral ciliated groove. In C/uetoderma, there are no signs of either
a ventral groove or a foot, and we are much tempted to regard these
elongated worm-like creatures (Fig. 147 4) which are totally devoid of
shell, as worms rather than as Molluscs, a view which has repeatedly
been adopted. In any case, it appears possible that they are tramsi-
tion-forms between the Vermes and the Mollusca, this view being
supported by the fact that the formation of the spines in the
Amphineura shows great similarity to that of the setae in the
Annelida.
Although there cannot be any doubt that we have, in the Am-
phineura, forms which stand very low among the Mollusca, we may
\
ete — |
GENERAL CONSIDERATIONS ON THE MOLLUSOA. 325
hesitate to attribute to them the significance of transitionary forms.
We have already pointed out that the spines which cover the body
of the Amphineura (Figs. 147 B, and 6-8, p. 10) show striking agree-
ment in their origin with the setae of the Chaetopoda and it was”
mentioned that some authors had regarded this as proving relation-
ship between the Amphineura and the Annelida. We do not attach
wny great importance to this resemblance, since these spines are
found irregularly distributed over the body, while the setae of the
Amnelida are, as is well known, very regularly and segmentally
arranged. Spines also occur in some forms while the related forms
A
dies the anterior ends the tos." skerorsadothe sae gta
from the vewtral side, and showing the oral aperture and, behind it, the apertare of
Uhe pedal gland and the ventral groove (after KowaLevsxy and Manqon),
show no such structures. As an example of this we would recall the
Turbellarian described by v. Grave, Enantia spinifera. Here we
have true cuticular spines which can only be considered as analogous
to those of other animals, but in connection with the origin of such
structures this illustration is of interest.
Tn comparing the spines of the Amphinevira with the setae of the
Annelida it must be remembered that we ean certainly not derive
the Mollusca from such highly developed forms as the Chaetopoda,
‘The most primitive Annelida, however (the Archi-Annelida), have no
‘setae, but even these are certainly too much differentiated to serve as
Ma
326 GENBRAL CONSIDERATIONS ON THE MOLLUSCA,
starting points for the Mollusca in which segmentation is altogether
wanting,
The temptation is certainly very great to derive the elongate,
‘worm-like Solenogastres which are provided with a coelom and
nephridia (Fig, 47 A) from the Annelida, but they also show no
segmentation. Either segmentation or the distinct remains of it
must, however, be found if they are really to be more nearly related
tothe Annelida. We are thus inclined to regard the elongate forns
of ‘the Solenogastres rather as a secondary phenomenon and to oon-
sider as such also the growing out of the posterior part of the larva
into the adult body which recalls similar processes in the Annelida
(Vol. i., p. 268). It is possible that the development of the Soleno-
gastres, when better known, will throw further light upon their origin.
In the young Dundersia, seven calcareous plates are said to cover the
back, as in Ch/fou. This stage thus resembles that of Chifon and
this perhaps supports the conjecture that we have in the Amphineura
a less primitive form than was assumed a priori. The absence of
the shell also would no longer have to be regarded as a primitive
feature, nor would the slight development or absence of the foot.
We still indeed have to take into account the important fact that
the shell is wanting in these forms. The covering of the body with
spines and the very primitive internal organisation in any case
indicate that the Solenogastres are very primitive forms. If we
actually have, in them, merely an aberrant branch of the Molluscan
stock, ‘this branch in any case diverged very near the root. ©
If we are unable to find any direct relations between the Solenc-
gastres and the Annelida, such might perhaps be found between
them and other divisions of the Vermes, such as the Turbellaria or
the Nemertini. The coclom, that very important part of the internal
organisation might, as above shown, with some probability be derived
from the dilation of the gonads in these forms, The conditions of
the coelom in the Mollusca agree iu such a striking manner with
those in the Annelida that it is difficult to believe that two stractures
so remarkably alike arose in different ways; in this case we should
have to derive both the Mollusca and the Annelida from Turbellaria
or some similar form. This brings us, however, buck to our former
view as to the racial form of the Mollusca made in ion with
the Trochophore larva (p. $21) which, however, was wt tv to
the derivation of the latter from the Turbellaria,
Starting from a creature still more simple in organisation: ‘hss
Trochophore, we arrived at the form of the latter and traced the
L _— |
GENERAL CONSIDERATIONS ON THE MOLLUSCA. 327
acquisition of those characters which determine the typical Molluse.
We then touched chiefly upon features of the outer organisation, but
some points of internal organisation were also pointed out, such as
the supposed rise of the coelom from the gonads of the primitive form
and the primary exeretory organ, the primitive kidney. A further
important characteristic of the Mollusea is the occurrence of the
(adult) nephridia and their connection with the coelom (pericardium).
We bolieve the origin of the adult nephridia to be the same as
in the Annelida, ie., we derive them from the protonephridium.
Although nothing is known ontogenetically on this subject, the adult
nephridia, like the primitive kidney, are derived from the middle
germ-layer, a faet which, indeed, indicates a common origin, if only
because the mesoderm originally distributed in the body-eavity
(mesenchyme) and the coelomic mesoderm (the former gonads) had
im any case the same origin, 4+, were derived from the entoderm.
‘The connection now existing between the nephridia and the coelom
is secondary, for it is wanting in the primitive kidney. ‘The nepbridia
took over the transmission to the exterior of the genital products,
when special efferent ducts for them were not developed.*
‘The circulatory system of the Mollusca, like the coelom and the
nephridia, shows great resemblance to that of the Annelida, a fact
which inclines us to ascribe it to the primitive form from which the
two stocks are derived. The simplest form of circulatory system was,
im any case, that of a contractile sac open at one end and lying
dorsally. The heart belonged to the primary body-cavity. Where
the coelomic sacs were specially large it was found squeezed in
between them and the intestine, dorsally to the latter. The rise of
the heart between the entoderm and the splanchnic layer of the
mesoderm, still characteristic of many forms, and which, in the
Lamellibranchia, even leads to its development round the intestine
(Figs. 31-33, p. 75), led some authors to trace back the heart to one
of the blood-sinuses which encircles the intestine.+ According to
this view, the blood-sinus would have to be located dorsally to the
intestine. The blood passed into the muscular sac which represented
the primitive heart and which carried on rhythmical contractions, by
means of which the blood was again driven out. There were no
vessels, but the blood ran through spaces und slits in the mesodermal
“(Seo footnote p. 179 and Goonricn, (Quart. Jour, Micro, Sci. Vol,
XXXVIL—Ep.}
+ This view which was adopted by Gronben has been discussed in con-
nection with the formation of the heart in the Lamellibranchia (p. 79).
328 GENERAL CONSIDERATIONS ON THE MOLLUSCA.
tissue of the primary body-cavity, a condition which is still passed
through by the embryos of the Mollusca, in which the heart arises
independently of the vessels (pp. 77 and 216).
The blood-vessels must have first arisen when the gills developed.
These later, in any case, arose as very simple leaf-like or tubular out-
growths of the body-wall, such as are found as the most primitive
gills in the Aunelida, the Arthropoda and the Eehinoderma, Au
increase of surface soon took place and led to the bipeetinate gill,
the so-called cfenidium so characteristic of the Mollusca, This gill
was paired, i.¢., a ctenidium was found on either side of the body
concealed in a cavity formed by a fold of the integument. The fold
is the mantle which grew out on each side from the back, and de-
veloped simultaneously with the shell and the gills. As the primitive
Molluse we must thus imagine an animal somewhat flattened dorso-
ventrally, whose back was covered by a bowl-shaped shell while its
ventral side formed a muscular and slightly projecting creeping sole.
Beneath the lateral parts of the shell lay the mantle enclosing the
pallial cavity and within this the gills. On the pre-oral part of the
body (the head) there were perhaps also the eyes and two tentacles,
corresponding to the cephali¢ feelers of the Archi-Annelida., In the
oesophagus, the radular sae with the radula became differentiated as
outgrowths, these occurring even in the most primitive of known
Molluses (Amphineura, Solenogastres), The anus lay at the posterior
end, the nepbridia opening out at either side of it. These latter
opened inwardly into the coelomic saes with which the genital glands
were connected. ‘The two coelomic sues, dorsally to the intestine, held
the heart between them. The primary body-cavity was traversed
by mesodermal tissue, which became differentiated into connective
tissue and muscles.
From such u simple form of Molluse can be derived the types
represented in the different divisions known to us. Nearest to it
stand the Chitones, the characteristics of which have been drawn upon
for the above description. The somewhat aberrant conditions of the
Amphineura have already been described above (p, 326). After the
Chitones come the most primitive Gastropods (Diotocardia), the
Chitones themselves being for a long time regarded as Gastropods,
‘The foot of the Gastropoda, apart from exceptions to be mentioned
presently, has the primitive form of the creeping sole. ‘The marked
development of the head which carries the tentacles and eyes is
characteristic of most Gastropoda. The shell also has attained higher
development and is a constant feature of the Gastropoda; where it
‘ _— |
GENERAL CONSIDERATIONS ON THE MOLLUSCA. 329
is wanting, it has evidently degenerated. To the actual shell, the
operculum has been added. ‘The position of this latter is like that of
the shell itself, 4., it lies on the dorsal side of the foot at the point
where the latter passes over into the back. The shifting of the anus
makes it difficult to establish the origin of the operculum. It has
been snggested that it arose through abstriction from the shell, but
its independent development and position in the embryo point rather
to an independent origin.
The simple bowl-shaped shell, assumed by us for the primitive
form is no longer retained in this form in the Gastropoda, for the
shell has become twisted. This condition is connected with the
asymmetry of the body which, again, is the result of the one-sided
development of the visceral sac, a feature which is specially character-
istic of the Gastropoda. This one-sided development brings about
displacements of both external and internal organs, and leads to-
processes of degeneration (¢.g., in the gills, the kidneys, parts of the
circulatory and nervous systems) ; these now occur on one side of the
body only and thus still further increase its asymmetry.* In forms
which lead a pelagic life, such as the Pteropoda, or in creeping forms
that have lost the shell (Onchidiun, Opisthobranchia, Limacidae, ete.)
there is a more or less complete return to the symmetrical shape.
In the form of the gills, the paired character of the kidneys and
the auricles, the relations of the coelom and of the nephridia, the
Diotocardia are among the forms most nearly resembling the primitive
Molluse, but they appear essentially differentiated from it, as the
asymmetry of the body is already found in them.
The relation of the Gastropoda to the primitive form is easier to
trace than is that of the other great branches of the Molluscan stock
(the Solenoconchae, the Lamellibranchia and the Cephalopoda).
‘The Solenoconchae may be derived from the primitive form by the
extension of the body in a dorsal direction ; the head is much reduced
but develops a large number of tentacular filaments. The foot
becomes the long burrowing foot, the mantle is found to be influenced
by the above-mentioned growth of the body, but on the whole
exhibits the usual features, except that it retains an aperture at its
dorsal apex. In the same way also we can explain the shape of the
shell which resembles a tube open at both ends. According to the
most recent investigations as to the structure of Dentalium it seems
most probable that this form is related to the Gastropoda, the
* Cf. on the asymmetry of the body, Chapter XXXIL,p. 148.
supposed relation to the Lamellibranchia and the Cephalopoda being
untenable.* The Solenoconchae are an aberrant although insignificant
branch of the Molluscan stock, certain resemblances between them
and the Lamellibranchia are to be explained by the fact that
both branched off from the same primitive form. , ,
‘The Lamellibranchia also are much specialised, but may still,
through their lowest representatives, be related to the primitive form,
The Protobranchia, for instance, still possess a foot with a creeping
sole, as well as bipectinate gills. The foot and the gills in the
higher forms, however, though modified, may still be traced back to
the fundamental type. The creeping sole is, in any ease, lost in
consequence of their burrowing habit, bot, on the other hand, one of
the pedal glands which are found in the different divisions of the
Mollusca develops into the byssal apparatus.
The reduction of the head and absence of the radula, structures
which are such constant features in nearly all other Molluscs, are
characteristic of the Lamellibranchia. It has been said, no doubt
rightly, that the radula has been lost ; it is oceasionally also wanting
in other forms whose relations possess it as, for instance, im various
Opisthobranchia (Phyllidia, Doridinm, Doricopsis, Tethys, ete)A
‘The shell of the Lamellibranchia has a specially. typical -develop-
ment. At first it is shaped like « shallow bowl, lying apon the back,
like the shell assumed to have been possessed by the primitive form ;
later, however, it bends over on the two sides, calcifies as two pieces,
and thus assumes the typical bivalve form, The formation of the
mantle corresponds to that of the shell,
Aniong the internal organs, the nervous system, the circulatory
apparatus, the pericardium (coelom) and the nephridia form in the
way usual among the Mollusea, and this probably also may be said of
these organs in the Cephalopoda,
‘The Cephalopoda. Of the external organs of this elass, the mantle
and gills also resemble those of other Molluscs. ‘The shell too may
be derived from a simpler form, as is evident from the rounded
chambers found in the embryo. The highly complicated form of the
* Prats, in his recent work on the anatomy of Dentalinm, gives a:
account of the relationships of these forms (see Literature to
XXXL, No. 3, p. 98). re UG
+ According to Sumorn (No. 17), Tethys (and the related form Melibe), for
instance, does not require the radula because its food is soft. A Proso
also (Magilus) has no radula. It lives ins tube covered by corals and feeds
‘on the offal of these animals. (The Prosobranch families and
Eulimidoe are also devoid of radulae.—Ep).
GENERAL CONSIDERATIONS ON THE MOLLUSGA. 331
chambered shell of Vautifus, the highest development of the Mollus-
can shell, was attained later.
The head, in the Cephalopoda as in the Gastropoda, is well de-
veloped. The arms, which surround the mouth, at first sight appear
to belong to it, and yet, according to the results of more reeent re-
search, another significance must be ascribed to them, ie., they must
be regarded us parts of the foot. A fact which causes surprise and,
at first sight, is unfavourable to this view, is that some of the arms
shift to the dorsal side of the head, being here found. behind the
mouth. This makes it difficult to understand how the foot could
have become transformed in this way, but when we see how extremely
plastic it is, judging from different transformations which it has
undergone in the Lamellibranchia, the Prosobranchia, the Hetero-
poda and the Pteropoda, further modification is not so inconceivable,
especially when we find thut the lateral parts of the foot in certain
Prosobranchia (Diotocardia) even develop tentacular’ structures,
Another part of the foot of the Cephalopoda has at any rate be-
come changed into the fannel, which at first is paired, The pedal
character of this structure has never been doubtful, its origin being
at once betrayed by its position between the mouth and the anus.
The change undergone by the foot in yielding the funnel is also very
great, We cannot here enter into the question as to whether, as has
been assumed, we have in this case to do with epipodia. The eom-
parison of the different parts of the foot known as the propodium,
mesopodium, metapodium, parapodium and epipodium becomes very
difficult in modified forms, the inter-relationships of which alone pre-
sent great difficulties. The conditions of transformation and adapta-
tion may, in the various forms, have developed and modified very
different parts of the foot.
In the Cephalopoda, the organisation of the Mollusca has attained
its highest development. In the structure of the adult the most far-
reaching differentiation has taken place and thus also in its ontogeny
we have the greatest complications found in the Molluscan stock, the
greatest departure being made from those features (total cleavage
of the egg, larval forms, ete.) which we have learnt to regard as
primitive in the lower as well as the higher representatives of the
Mollusca,
16.
17.
LITERATURE. 333
Rouue, L. Considération sur l’embranchement des Trochozo-
aires. Ann. Sci. Nat. (7). Zool. Tom. xi. 1891.
StumnotH, H. Ueber einige Tagesfragen der Malacozoologie, etc.
Zeitechr. f. Naturw. Bd. |xii. Halle, 1889.
. Spenaex, J. W. Die Geruchsorgane und das Nervensystem
der Mollusken. Zeittschr. f. wiss. Zool. Bd. xxxv. 1881.
. TareLE, J. Ueber Sinnesorgane der Seitenlinie und das Nerven-
system von Mollusken. Zeitschr. f. wiss. Zool. Bd. xlix.
1890.
. THrELE, J. Die Stammesverwandtschaft der Mollusken, etc.
Jen. Zeitachr. f. Naturw. Bd. xxv. 1891.
CHAPTER XXXV.
TUNICATA.
Systematic (after Herpman) :—
Order I. Larvacea (Appendicularia).
Order II. Ascidiacea.
1. Ascidiae Simplices.
2. Ascidiae Compositae.
3. Ascidiae Luciae (Pyrosoma).
Order IIT. Thaliacea.
1. Cyclomyaria (Duliolum).
2. Hemimyaria (Sulpa, Octacnemus).
IL. Sexual Reproduction.
1. Larvacea (Appendiculuria).
Very little is as yet known as to the development of Appendi-
cularia, since the small eggs which are discharged into the surround-
ing water are not easy to obtain and exceedingly difficult to examine.
Fou (No. 1) and KowaLEvsky (treatise on Amphioxus) have, how-
ever, stated that the development of this form closely resembles that
of the Ascidiacea. The paired respiratory tubes of Appendicularia
are formed in the same way as the first gill-slits of the Ascidian
larvae (p. 366), an ectodermal invagination appearing at each side and
a diverticulum of the pharynx growing out to meet it until the blind
ends come into contact, perforation taking place at the point of
junction.
2. Ascidiae Simplices and Compositae.
A. Oviposition, Fertilisation and Egg-envelopes.
The eggs of most of the solitary Ascidians, soon after passing from
the oviduct into the atrial (peribranchial) cavity, are ejected into the
ASCIDIAE SIMPLICES AND COMPOSITAE. 335
surrounding water where they pass through their embryonic develop-
ment, being supported at the surface by the large, foam-like follicle
cells (Fig. 149, ¢). Fertilisation usually takes place either in the
peribranchial (atrial) cavity or after the egg is laid, but exceptions
to this rule are found in the genera Cynthia and Lithonephrya
(Gtarp), these forms passing through their embryonic development
148.— Three stages in the development of the apy of Mhalducia wean witlata
WALEVSKY, adapted from Kuryrer, For and others). basal membrane of the
le; b su layer of peversent-epitheliua ; , follicle-cella; , chorfon ;
4 tontcelld ; /, eug-cel
within the peribranchial cavity of the mother. In Clavelina also,
and in all composite Ascidians, development up to the time when
the free-swimming, tailed larva is hatched takes place in the peri-
branchial cavity of the mother, or else in peculiar divertioula of
this cavity known as brood-spaces. The composite Ascidians differ
from the solitary forms in the large amount of yolk contained in
the ogg. Savensky (No. 49) recently observed in a few Polyclinidae
336 TUNIOATA.
(Amaroucium, Circinalium, Fragarium) a fusion of the embryo with
the atrial wall of the mother. At this spot a thickening, the placenta,
is formed, derived in part from the atrial wall (placenta. materna),
from the follicle-epithelium enveloping the embryo and from an
accumulation of test-cells (/alymmocytes).
These Ascidiacea are hermaphrodite. Self-fertilisation seems, in-
deed, to be prevented, in most, cases, by the maturation at different
times of the male and the female genital products, but is not im-
possible in other cases, in which both products ripen simultaneously.
{In Ciona (Casrtx, No. I1,), although the products ripen at the same
time, self-fertilisation does not occur.)
The mature eggs are surrounded, at the commencement of em-
bryonic development, by a complicated system of envelopes, which
‘we are inclined to regard as derivatives of the original egg-follicle-
In this respect we agree with KowaLEysky, whose views were con-
firmed later by Van Benepen and Junt (No. (0) as well as by
Moraan (No. 46), but we must point out that the origin of these
enyelopes is still an epen question. This poimt will be further dis-
cussed in the section on the formation of the egg under the heading
of general considerations.
Very young eggs, while still in the ovary, appear to be surrounded
by a pavement-epithelium consisting of flattened cells (Fig. 148 A, ¢).
The elements of this primary follicular epithelium (0) are derived
from undifferentiated cells of the ovary (VAN Benepen and Juni).
A structureless basal membrane (a) seems to be secreted early at the
surface of the follicular epithelium, The follicle-cells multiply by
division, and soon lose their flattened form and become cubical; a
few of these cells are displaced inwards (Fig. 148 B, ¢) in such a
way as to become deposited on the surface of the egg into which
they may even find their way, passing into the most superficial layer
of the vitellus, These cells, which can usually be distinguished by
their yellow colour, have been called fest-elis (e) because it was
erroneously supposed that they gave rise to the cells of the cellulose
mantle (test) of the adult Ascidian, a view which was refuted by
0. Herrwic (No. 25).* These test-cells, which soon increase very
iphones 3{ more resent Eathors. ‘Toote seme be ba Saal esata
moc
‘about the fate of these cells. Sanensky (No. XXIX.), from his
on the compound Ascidians, regards them as giving rise bets the ne p
356) whereas the similarly ‘named cells of Salpa et tephra os
nutritive structures (Hetpen, No. XIII; Kor XVII;
No. XXIV. ; Pizox, No. XXVIL), the most recent “orga oh oa
the follicle- and test-eella seams to agree with the given above,—Ep.]
ASCIDIAE SIMPLICES AND COMPOSITAE. 337
greatly in number, at first form an inner epithelial layer over the
surface of the egg known as the fes/-cell-luyer (Fig. 148 C,e). In
later stages they undergo a process of degeneration.
the regularity of their arrange-
ment and are found embedded
separately in a gelatinous mass
secreted over the surface of the
egg. Their original cellular
character is then less distinct
and has been altogether denied
by some authors (SEMPER, Fot).
After the development of the
test-cell-layer, a structureless
membrane (Figs. 148, 149, «/)
is secreted between it and the
They then lose
actual follicular epithelium, and
this, being derived from the
follicle-cells, may be described
as the chorion,
Fig, 149.—Mature egg from the oviduct
of Ascidia canina* (after KUPFFER). c¢,
follicle-cells (foain-like cells) ; «, chorion ;
e, test-cells; /, egg-cell; +, gelatinous
substance,
In the meantime, a delicate erternal puvement-cpithelium (Fig.
148 C, 4) has developed on the outer surface of the follicle which
is in close contact with the basal membrane : this epithelium is pro-
bably to be regarded as an outer layer of follicle-cells. Like the
basal membrane, it seems to disappear, not being found in eggs after
they have been laid.
In the free eggs of the solitary Ascidians, the cells of the actual
follicle-layer, at a later stage, assume a peculiar character. Their
protoplasm becomes filled with numerous vacuoles (Fig. 148 C, 149, c)
and thus resembles foam. The cells, which increase in size and form
papilla-like structures (Fig. 149, c), are concerned in supporting the
egg as it floats in the water.
The mature Ascidian egg thus possesses the following envelopes,
reckoned from without inwards :—
(a) T= basal membrane of the follicle.
(6) The external pavement-epithelium.
(c) The actual follicle-epithelium (layer of foam-like cells).
(@) The chorion.
(e) The layer of test-cells (kalymmocytes, abortive eggs].
* ( =Ciona intestinalis, rar.—Ep.]
Z
338 TUNICATA.
According to Cuasry (No. 13), another fine. structureless membrane cover-
ing the external surface of the test-cells should be added to the above. In
the solitary Ascidians, after the egg is laid, the interspace between the chorion
and the surface of the egg is increased (Fig. 149) through imbibition of water
and consequent swelling of the gelatinous matter secreted at the surface of
the egg.
We must here briefly allude to views as to the origin of the egg-envelopes
which differ from those given above. According to SaBaTiER, Fo (No. 21),
and Route (No. 47) the follicle-cells are produced by the egg itself. For and
Rocze think that this is brought about by a process of budding from the
germinal vesicle, but SaBatrer considers that the follicle-cells arise through
free cell-formation in the yolk.# It seems to be fairly established that an
ejection of chromatin-elements actually takes place from the nucleus of the
ovum but we incline to the view that this process has nothing to do either
with the rise of the follicle-cells or with that of the test-cells. Such an
origin was assumed for the latter by Route, DaviporF (No. 14), and P1zox
(No. XXVIL.), while a number of other authors (Semper, Fou, Sabatier and
others, following Kcerrer, No. 34) thought that the test-cells formed freely
in the protoplasm of the egg.
It is of interest to note that the eggs of Appendicularia also are enveloped
while in the ovary by a follicle (A. B. Lee, DaviporF). When laid, however,
they are without covering, and only after fertilisation has taken place in the
water do they become surrounded by a delicate vitelline membrane (Fol.
No. 21).
B. Cleavage.
The free-swimming, tailed Ascidian larvae were known to the older
authors and were carefully described by Mitye-Epwarps, P. J.
vAN BENEDEN, and others. Our first knowledge of the embryonic
development of the Ascidians, however, was due to the researches of
Kowatevsky (Nos. 29 and 30), which were soon supplemented by
the accounts of Kuprrer (Nos. 34 and 35) and MeTscuntxorF (No.
41). Among later workers we must mention SEELIGER (No. 50) and
van BENEDEN and Jubtn (Nos. 7-10); the eggs of the Ascidiae
compositae which are rich in yolk have been investigated by Mauricr
and Scrutatn (No. 39) and DaviporF (No. 14).+
The cleavage of the Ascidian egg is total, and, seeing that the
blastomeres at first differ only to an inconsiderable extent iMsize and
structure, may be described as almost equal. The term ‘ adequal
* [As we have already stated, free cell-formation is now generally discredited
by cytologists.—En.]
+(Still more recently, SaLeNsKY (No. XXIX.) has worked at the develop-
ment of the Ascidiae compositae. CastLE (No. II.) has worked at Ciona
intestinalis. The work of the last-mentioned observer is very important and
exhaustive.—Ep.]
ASOIDIACEA—CLEAVAGE. 339
cleavage,’ which was applied by Harsonex to the process in the
egg of Amphiowus, is equally suited to the very similar processes met
with in the Ascidians. Certain characteristic irregularities are, indeed,
to be observed, antl these are referable to the early foreshadowing of
certain important differentiations.
The furrow which appears first and divides the egg into two equal
halyes corresponds to the plane of bilateral symmetry, and the two
blastomeres that result from this division represent the right and
left: halves of the body (SeELIcER, van Benepe, Junin and Castim).
From this stage onward throngh all the other stages of development,
the bilateral symmetry of the embryo is clearly recognisable, The
Fira. 150,—"Two a the cleavage of Clavelina (after gre LIGER). «A, four-celled
stage, seen eeotienrsbore . The two smaller cells v rej to Sie:
the anterior half of the body, and the larger cells Rergerees i. B, Intera!
view of the cight-celled stage ; a, blastomeres of the Sain hal halt 6, blastomeres of
the vegetative half,
next furrow is also meridional and cuts the first at right angles. The
four blastomeres thus produced (Fig. 150 A) are not of exactly the
same xize, two being larger (/) and two smaller (v).* According to
the most usually adopted orientation of the cleavage-stages in these
forms, the plane of cleavage now under consideration has a transverse
direction so that (according to vAN BenepEN and Juris) the two
Jarger blastomeres represent the future anterior half of the body and
the two smaller the later posterior half, but SeEniGER takes an
exactly opposite view (Fig. 149 4). In Distaplia, according to
Daviporr, the four blastomeres are exactly equal in size.
“(In Ciona (Casrur, No. IT.) no difference in size is to be seen between the
Ay the four-celled stage. The polar bodies are found to corre-
spond to the future vegetative pole—Ev.]
a
340
a
TUNICATA,
The third plane of cleavage is equatorial and, in the eight-celled
stage that follows (Fig. 150 #), separates four smaller cleayage-spheres
from four larger.
ee
A
Fie. 151. Oy cant pgver chorea tnd
SBELIGER). 4. ula-stage 5 e
= sah G, ‘after
According to all observers, the later ventral half
of the body ig thus divided from
‘the dorsal half. The four smaller
spheres (2) which lie near the
animal pole, and are said to
represent the ventral surface of
the body, are purely ectodermal
in character, while the four larger
blastomeres (2) which belong to
the vegetative half (the future
dorsal half) are said by vax
Brnnpen and Juni to show a
mixed character, They give rise,
through division, to the large
entoderm-cells, smaller eetoderm-
elements being
simultaneously
-abstricted from them, these latter
then joining with those of the
ectodermal half of the body.
According to SEELIGER and
Davroorr, these cells are, on the
contrary, purely entodermal.*
Even at this stage, certain
displacements of the blastomeres:
can be observed, and these inter-
fere with the regularity of the
later course of the sleavage.
This regularity is also disturbed
by the fact that the ectoderm-
cells, from this time onwards,
divide more rapidly than the
entodermal elements, It is,
however, possible to distinguish
a sixteen-cell stage ‘brought
about by meridionial furrows, «
thirty-two-cell stage caused by
equatorial division, and a later
* [According to both Castrx (Nos. IL. and ITT.) and Samassa
nie roi and Jun, together with SeELicen, were Pen
ee
lll
ASOCIDIACEA—FORMATION OF THE GERM-LAYERS, 341
sixty-four-cell stage. For further details as to the cleavage we must
refer the reader to the works of SeeticeR (No. 50), van BeneDEn
and Sutin (No. 8), Cuanry (Nos. 12 and 13) [and especially Casi
(No. IL.)).
Even at the four-celled stage a cleavage-cayity is found open at
both the animal and vegetative poles (Fig. 150). This cavity, at the
sixteen-celled stage, appears to be closed on all sides, In later stages
it disappears (Fig. 151 A, ) in consequence of a flattening of the
embryo, which begins at the poles and which is specially marked
in the entodermal half of the body; this flattening precedes the
invagination of the entodermal cell-layer which results in the
gastrula-stage.
©. Formation of the Germ-layers. Appearance of the Medullary
Tube and the Notochord.
Through the changes just described, the embryo passes from the
blastula-stage into a stage which we may, with Birsouts, call the
plaeula (Fig. 151 A). The lens-shaped embryo is now composed of
two layers, an entodermal layer (en) consisting of large high cells
and a small-celled ectodermal layer (ec) which already covers the
former like a cap.* In a fissure (f) between these two layers, we
recognise the remains of the much compressed cleavage-cavity. The
gastrulation which now takes place (Figs. 151 B, and 152) is due
essentially to the curvature of the bilaminar embryo, the flattened
area of entoderm-cells becoming invaginated in this process, while
the ectodermal layer continues to spread out over the surface of the
orientation of the Ascidian egg. CastTL concludes that SeeuicEn determined
the dorsal and yentral sides of the egg correctly, but reversed the anterior and
jor ends in all his figures of the carly stages. vas Buwepes and JUcin
‘were correct in their determination of the anterior and posterior ends, but
reversed the dorsal and ventral surfaces in all stages prior to the forty-four-
celled Lat In consequence, the latter authors state that the four small cells
of the eight-celled stage give rise to ectoderm only, while the four larger cells
produce both entoderm and ectoderm ; whereas, as « matter of fact, neither
group produces ectoderm exclusively. It is the four larger, not the four smaller
cells, which give riso to the greater portion, perhaps the whole, of the ectoderm,
‘The vegetative pole, which is marked by the polar bodies and the four smaller
blastomeres, is dorsal, while the animal pole with the four larger cells is
ventral. The future mesoderm arises from both primary layers. CasTun's
work is so complete, and he traces the cell-lineage in such detail, that his
interpretation must, we think, be accepted in preference to the above,—Ep.}
* (The large cells at this stage would not, according to CasTLE, correspond
with the e cells of the eight- and sixteen-celled stage; the latter by their
more rapid division have become gradually smaller, whereas the originally
«mailer cells, by their slower division, have not decreased in size to the same
_oxtent and are now the larger of the two.—Ep.]
se
342 TUSICATA.
eubrry. Gastrulation in the Ascidians has omsequently frequently
been dexribed as transitional tetween the typical epibolic and
embolic conditions.
The gatrela-stam that thus arises (Fig. 151, ("is saucer-shaped.
The arched surface of the embryo is covered with the small ectoderm-
cells, while the flattened side of the body is occupied by the large,
round blastopore. This side is said to change into the later dorsal
surface of the embryo. while the arched side bhecomex the ventral
surface.
If we take as the principal axis of this gastrula-stage (Fig. 152 4) that
which connects the animal] with the vegetative pole of the egg in the first
mages of cleavage (a-b.. we find that this axis passes through the apex of the
arched portion on the one hand and through the centre of the blastopore on
the other. The future longitudinal axis of the body. on the contrary, would
lie at right angles t this principal axis, since, according to all observers, the
blastopore corresponds to the later dorsal surface. We should then havea
condition differing from that of other Bilateralia, in which the primary axis
corresponds approximately to the longitudinal axis of the adult. It seems
probable that the blastopore shifts secondarily from the vegetative pole of the
embryo, at which the posterior end of the body now develops to # position on
the dorsal surface. According to this orientation, the dorsal and ventral
surfaces would, as in the first ontogenetic stages of most of the Bilateralia,
occupy a meridional position. After what has been stated above, we may
conjecture that, in Ascidians, the displacement of the blastopore to the dorsal
side of the body is first brought about by shifting caused by growth in the
later gastrula-stages such as is indicated by the orientation adopted in Fig.
152 A-C. A comparison with the ontogeny of Amphiorus to a certain extent
supports this supposition (Chap. XXXVI). According to this view, the
orientation of the cleavage-stages given by some authors, in which the animal
pole of the body is said to be related to the future ventral half and the
vegetative to the later dorsal half, does not appear altogether suitable, and
can only be admitted with certain reservations,*
Bilateral symmetry can be recognised in the gastrula-stage, even
at the first, by the distribution of the cells. In later stages this
*(CasTLe, in his work on Ciona, supports the latter view, that is, he finds
that the vegetative pole with the entoderm and blastopore is always dorsal in
position. The apparent shifting of the blastopore is due to the fact that it
closes more rapidly from the anterior margin and from the sides than from
behind. Consequently it comes to lie in the posterior portion of the dorsal
surface of the embryo. He is therefore in agreement with Samassa (No.
XXXIL) when ho states that there is no rotation of the axes during gastrula-
tion as conjectured on theoretical grounds by KorscHELT and HEIpeR, but
that the primitive axis corresponds with the vertical axis of the larva and
the longitudinal axis is at right angles to this. According to this inter-
pretation, the orientation of Fig. 152 is incorrect, since the blastopore, in
each of the threo stages, is represented on a different surface, whereas it ix
always dorsal in its position.—Ep.]
ASCIDIACEA—FORMATION OF THE GERM-LAYERS,
‘343
symmetry is still more marked, the future anterior end of the body
becoming swollen in consequence
of the increased curvature of its
two layers (Fig. 152 8). This
arching is connected with the
gradual narrowing of the blastopore
which takes place on the dorsal
side of the embryo in such a way
that its last vestige lies near the
posterior end of the body (Fig.
153 C). Originally, the blastopore
is a wide oval aperture, but in
later stages it is pear-shaped, and
it finally becomes a small posterior
wperture (Fig. 153, 6-4"). This
narrowing of the blastopore is
caused principally by the inward
growth of its anterior and lateral
margins, the posterior edge re-
maining unchanged. We have
here conditions exactly similar to
those in Amphioous, and we may
assume a continuous closure from.
befgre backward of the blastopore
which originally extended along
the whole length of the dorsal
surface.
During these stages even, the
embryo becomes somewhat elon-
gated in the direction of the
longitudinal axis (Fig. 152 C).
The dorsal side is recognisable by
its flatter condition, and shows, at
its posterior end, the remains of
the blastopore (p); the ventral
side, on the contrary, is arched,
van Bryepen and Junin have
pointed out that the posterior end
of the body, at the gastrula-stage,
is always marked by the presence
F1G, 152.—Three consecutive gastrula-
stages of Phaldusia wmumoidata (after
Kowaxnysky). A, the invagination
commencing ; B, appearance of the
bilateral symmetry; C, narrowing of
the blastopore ; «2, principal axis of
the gastrula-stage ; ee", later lougi-
tudinal axis of the body; d, dorsal
side; cc, ectoderm ; en, entoderm ; /,
cloavage-cavity; p, blastopore; |v,
ventral side,
of two small wedge-shaped ectoderm-cells lying at the edge of the
pire ‘TUNICATA. '
blastopore and representing the boundary between the ectoderm and
the entoderm (Fig. 154, w). 4
In these later gastrule-stages the commencement of histological
: differentiation is already evident.
This does not consist merely in
the distinction between the eeto-
dermal and the entodermal elements,
although the latter are larger, :
strongly granular and darker in
colour; but differentiations are
already. to be found within these
germ-layers. ‘The ectoderm-cel
which bound the blastopore,
instance (Fig. 154 A,
tinguished by the lange
nuelei, their greater 7
carmine stain, and their |
shape from the other ectoderm-cells, which soon become
‘This ring of cells is the first rudiment of the central nervon
and, as the blastopore closes more and more, changes ix
iq, 153.—Dorsal aspect of an embry
of Claveline (after SERLIOER),
bY,
4”, outlines of the blastopore at
consecutive stages of development.
«
Fro, 154.—Gastrula-stage of Clawlina Risswaie (after VAN BRNRDEN Jv
A, dorsal area wadion sagittal section. 4, blastopora: ee, ectodern a
ontoderm ; m, cells of the nerve-ring ; ic, small, wedge-shaped cells, + ae
lll
ASCIDIACEA—FORMATION OF THE GERM-LAYERS. 345
medullary plate* This radiment, when it first appears, consists, at
the sides of the still open blastopore, of a single row of cells, while,
in front of the blastopore, it is composed of several rows of cells.
Tu later stages (Fig. 154), as the blastopore narrows, the part of
the medullary plate in front of it extends more and more, and, at the
time when the blastopore is represented by merely a small aperture
(Fig. 155), is a large and slightly depressed area, ‘The medullary
groove thus formed, which is open anteriorly, is bounded by two
teen thar
dorsal aspect; 5, median sagittal Dlastoy ch, Peace ot
5 B, n , ;
chorda; ec, ectoderm ; em, entoderm ; m, Malaherpaonnn ar cells of the nerve-
Jateral swellings (medullary swellings, m) which pass into one another
behind the blastopore, thus forming a semicircle,
A differentiation similar to the above is evident in the entoederm
(Fig. 158). The cells of the latter, as a rule, are large and turgid,
but in the region of the dorsal wall smaller cells appear which are
originally arranged so as to form a ring encircling the blastopore, one
af to Casrie (No. IL.), the cells lying behind the blastopore aud
edn ta Pigs 154 and 155 are not part of the rudiment of the central
ees, ‘as stated by Juni and vas Bennpny, but are in reality
The rudiment of the nervous system is situated entirely in
front of the blastopore. In Ciona the posterior margin of the blastopore
Pied ed forward over the blastopore, coveriug in the medullary canal, as
by VAN Beweven and Jutes in the case of Clavelina.—Ep.}
i
ASCIDIACEA—FORMATION OF THE GERM-LAYERS. 347
stages as a narrow slit, The entoderm-cells also, in this latter form, do not jong
retain their unilaminar arrangement, but become distributed in a radial direc-
the formation of the
3 the cell-boundaries drawn are
nenropore, still very large.
tion, the entoderm thus becoming multilaminar. The gastrula-stage is here
im reality reached through epibole (Fig.
156), In the anterior region of the body
this overgrowth is especially marked, while in
the posterior half, a small pit-like depression
(Fig. 156 B) indicates the Inst remains of an
invagination-cavity. This cavity, however,
completely disappears after the blastopore has
closed, The entoderm then represents a solid
mass, the cells of which are soon found to
vary in size. The position of the blastopore
is occupied by the neural plate (7).
During these stages, the medullary
plate, which isalready somewhat invagin- yy, yg9
ated, changes into a closed medullary hohe “i
tube, its lateral walls, the medullary Tous: a pS mcrae the
folds, growing towards one another and chord jec, ectoderm ; ca, ento-
derm ; ma, lerm-~diverti
. 160) 1; m0, folds
ee ae aes
The medullary tube develops from behind forward, a special part being
taken in the process by the fold which connects the two medullary swellings
—
348 TUNICATA.
posteriorly in the form of a semicircle (Figs. 155 and 158). It then appears,
especially in longitudinal sections (Fig. 158 B), as if the medullary tube was
formed solely by the posterior lip of the blastopore growing over the anterior
lip. We must, however, bear in mind that, as this posterior medullary fold
Fic, 160,—Transverse sections through an embryo of Clavelina Riswuna, at the same
stage as in Fig. 158 (adapted from vaX BENRDEX and JULES) A, through the
anterior, B, through the middle, and C, through the posterior part of the body.
ch, chorda ; d, lumen of intestine : en, entoderm; #, medullary plate ; wr, medullary
tube.
grows forwards, the lateral medullary swellings are drawn into it, so that the
posterior fold does not actually represent the lip of the blastopore, but « purely
ectodermal! fold lying at this point; in this way, after the medullary tube has
developed, its roof (n’) which is derived from the inner layer of the folds is
also to be regarded as ectodermal. This is a point which deserves to be
emphasised in opposition to the view of METscaNiKorF (No. 42). [Cf. foot-
note, p. 345.)
etxe sections through an embryo of Clacelina Risen, at the same
162 (adapted from VAN BENEDEN and JULIS). ‘through the
anterior, 8, through the middle, and C, through the posterior section of the body.
ch, chorda; rn, eutodermn : ms, mesoderin : ar, medullary tube; ., cells which com
plete the dorsal wall of the intestinal ¢
We shall see (Chap. NNXVL.) that, in mphiosus, the medullary tube forms
by the sinking in of the somewhat depressed medullary plate and its separa-
tion as such from the ectoderm; hence it is only covered by a single
of cells, Only at a later stage does the plate curl round under the ectoderm
toformatube. This manner of formation is probably @ moditication of the
ASCIDIACEA—FORMATION OF THE GERM-LAYERS, 349
origin from folds and is stated by SzeLiGeR (No. 50) as occurring in Clavelina
also, but his observations on this subject were not confirmed by van BuwepEn
and Juri (No. 10). These latter authors also do not agree in SEELicEn’s
view that the medullary groove, at the time when it appears, lengthens
posteriorly beyond the blastopore, -
At the time when the medullary tube develops, the blastopore has not com-
pletely closed, The remains of it, which originally lie in the floor of the de-
veloping medullary groove, are retained for some time longer as the meurenteric
canal and form # communication betweeen the lumen of the intestine and the
central canal of the medullary tube (Fig. 158 B).
Fre, 162.—Stage at which, in Olavelina Rissoena, the trunk- begins to separate
from the caudal region (after VAN BENEDEN and JULIN). tae erteal sation;
B, lateral aspect. ch, chorda; @, archenteric cavity; ec, ectoderm ; en, entoder
on, snb-chordal entoderm-strand ; ms, anterior portion of the tmesoderm-bands
composed of small cells; ms’, Roars jor portion of the same composed of large calls ;
np, neuropore; nr, medullary tube
As the medullary tube develops from behind forward, the aperture
at its anterior end, known as the newropore (Fig. 162, mp), is retained
for a long time. The separation of the mesoderm from the chorda
dorsalis takes place simultaneously with the development of the
medullary tube. These two rudiments arise, as may be ascertained
from the detailed accounts of van BenepEN and Juni, essentially
through the same processes of development as in Amphiowus, although
the conditions are in this case not so evident, and seem specially
modified in the posterior region of the body. The embryo soon
assumes « long, pear-shaped form (Fig. 162), the posterior, narrowed
a :
350 ‘TUNICATA,
region corresponding to the future tail of the larva. Tn the anterior,
dilated region, the mesoderm arises through the development of
paired diverticula of the archenteron (Figs. 159, 4, ms), the lumens
_ of these sacs very soon disappear and the cells of their walls,
which originally were arranged in two layers (the somatic and the
splanchnic layers), then appear irregularly distributed between the
ectoderm and the entoderm. Between the two coelomic diverticula,
the roof the archenterie wall is completed by entoderm-cells (Figs.
159, 160 A, 161 A, ch) which represent the rudiment of the chorda,
At a later stage, these cells become shifted into closer proximity, and
thus form a solid strand which, in cross-section, is round. ‘The fact
that this strand is to be traced back toa median fold of the arch-
enteron is proved by transverse sections through the most anterior
part of the rudiment of the chorda (Figs. 160 4, 161 A), where the
infolding of the cell-plate which represents the rudiment of the |
chorda can actually be seen (van BenepEen and Juni, No. 10).
In Amphiocus, according to HarscHex, the median fold of the intestinal
wall is not completely absorbed in the formation of the chorda. Its lateral |
cells, which are in contact with the coelomic diverticula, yield the cells which
complete the dorsal wall of the intestine after the chorda has separated from
the mesoderm. van BenepEeN and Juin conjecture that similar conditions
exist in the anterior part of the chorda-rudiment in the Ascidians (ef, Fig-
161 A, 2).
A certain guarantee of the accuracy of the observations made by vay
Benepe and Jur seems to be afforded by the striking resemblance to the
formation of the mesoderm in Amphiowus. Their observations, however,
have not been confirmed either by Daviporr (No, 14), who investigated the
formation of the mesoderm in Clavelina and Distaplia or by Witney (No.
54a), According to the former author, the mesoderm-cells become abstrigted:
from definite entoderm-cells at the margin of the blastopore (mesoderm-
gonads), and become arranged in an originally single layer below the
entoderm, This would be a kind of mesoderm-formation by delamination.
Davrporr was unable to find coelomic diverticula. The “mesoderm-gonads*
‘ave said to remain in connection with the entoderm, and, after the mesoderm
has been produced, some of them are said to take part in the formation of the
chorda and others in that of the alimentary canal. *
In the posterior region of the body (the future caudal region), the
condition appears to be modified through the early disappearance of
the lumen of the intestine (Fig, 160 @), The strand-like chorda is
*([In Ciona, the mesoderm-cells form temporarily of the wall of the
archenteron between the chorda and the entoderm, Eventually they become
displaced outwardly and the entoderm and chorda come into contact,
soning to Ciexcs (No. J) thera dose ‘not appeal Seiad setae SS
formation in this genus.—E.]
t ill
ASCIDIACEA—FORMATION OF THE GERM-LAYERS. 351
here pressed inward by the developing medullary tube, so that it
then fills the whole lumen of the intestine. The lateral cells of the
wall of the enteron, three of which are usually seen on either side
Fro, 163 — — Hater tae of development of Clavelina Rixsoene (after Yay BENEDEN and
JULEN) , median angittal section ; B, lateral aspect. , chord:
on, 3 en’, subchordal entoderm-strand in the caudal region; me, anterior
celled of the mesoderm-bands; ms’, posterior large-celled caudal
weotion of the aamo; vp, neuropore; mr, medullary tube,
in cross-section, pass directly into the large mesoderm-elements
which cover the chorda laterally and subsequently yield the caudal
musculature, There then still remain the entoderm-cells that lie
_
352 TUNICATA.
in the ventral middle line (Figs. 160 Cand 161 C, en) ; these, which
are arranged in two parallel rows, retain the character of ordinary
entoderm-cells and form a permanent cell-strand connected with the
intestine, in which we recognise the vestiges of a caudal section of
the alimentary canal (Fig. 162 A, 163 A, en’).
Fic. 184.—Median sagittal sections of two stages of development of Distaplia magui-
Jarta (after Davivorr), ¢, rudiment of the caudal section of the intestine; cA,
rudiment of the chorda;: ‘d, enteric cavity; ec, ectoderm; en, entoderm; *.
medullary plate; ap, neuropore; 7, medullary tube.
In the caudal region, the separation of the mesoderm and the chorda takes
place in a vory simple way, the archenteron merely breaking up into the two
rudiments. These structures, however, are probably to be derived in & way
similar to that described above for the anterior region of the body. We
ASCIDIACEA—FORMATION OF THE GERM-LAYERS. 363
shall have here to assume (with vAN BENEDEN and JuLin) in these regions,
the presence of a lumen of the archenteron compressed through the growth of
the chorda-rudiment to the shapo of @ crescent. The question here arises
whether the mesoderm-layer of the caudal section is‘to be referred to the
splanchnic or the somatic layer of the former mesoderm-rudiment. van
Benepex and Juuin incline to the first assumption, and SEELIGER (No. 50)
has also pointed out the resemblance of the mesoderm-cells of the caudal
section to to the cells of the inner layer of the mesoderm in the anterior
region of the body. :
Fio. 165, —A later stage in the development of istaplia maynilarce (after DavIDOFF).
¢, caudal prolongation of the alimentary ; ch. rudiment of the chorda ;
vity ; ee, ectoderm; en, entodermn ; /, adhesive papillae; ma, mesen-
; ar, medullary tube. :
The mesoderm and the chorda are therefore derivatives of the
primary entoderm.* Their origin can be traced back to the form of
folding which is also found in Amphiocus. The principal distinction
between the process here and in Amphiorns seems to be that, in the
mesoderm-rudiment of the Ascidians, no trace is to be found of the
segmentation which appears so early in Amphiorus, In the Ascidians
the mesoderm-bands appear composed of two different parts (Figs.
162 B, 163 B); an anterior part (ms), consisting of several layers of
small cells, which has arisen through folding in the anterior part of
the archenteron, and a posterior part composed of a single layer of
large cells (ms’) belonging to the caudal region. The anterior part of
the mesoderm, at a later stage, forms 2 mesenchyme filling the
® [See footnote, p. 340.—Ep.}
AA
354 : TUNICATA.
primary body-cavity (Fig. 167, ms) which yields the blood-corpuseles,
the connective tissue, the body-musculature, as well as the genital and
excretory organs, while the posterior part gives rise to the larval
caudal musculature (Fig. 168 B, m).
In Distaplia, in which, after the closure of the blastopore, the
entoderm forms a solid cell-mass (Figs. 157, 164 A), the enteric
cavity arises only later through the shifting apart of these cells (Fig.
164 8). In this way posterior part of the body is marked off; in
this the cells of the entoderm separate into the rudiment of the
chorda (Fig. 164, ch) and into that of the solid sub-chordal enteric
process (c), while the large entoderm-cells in the anterior region of
the body (Fig. 165, en) mix later with the mesenchyme and probably
disintegrate. In other respects there is no essential ditference be-
tween the development of Distaplia as described by Daviporr (No.
14) and that of other Ascidians. It should be pointed out that the
elements of the food-yolk here appear equally distributed in all the
tissues (the ectoderm, the mesoderm and the entoderm).
D. Development of the free-swimming larva.
External form of the body. We have already pointed out that,
at the time when the medullary tube develops, the embryo becomes
elongate and pear-shaped. In this way a broader anterior section is
inarked off from a narrower posterior section (Fig. 162) which gives
rise to the tail of the larva. This latter part of the body next grows
yreatly in length, less, as SEELIGER points out, by the multiplication
than by the elongation of the cells composing it. At the same time,
it becomes more distinctly constricted from the anterior section and
curves round ventrally (Fig. 163). As the tail, which is now bent
downwards and forwards, increases in length, its posterior end not
only reaches the anterior end of the body but even grows upwards
again at the right side of the latter. In this proccss, the tail be-
comes twisted on its longitudinal axis, so that the nerve-tube appears
shifted to the left side of the embryo (Fig. 170, p. 368).
The anterior region of the body, which at first appears more or less
spherical, lengthens later, and in the larva is ovate (Figs. 167 and
168). ‘Three prominences, arising as thickenings of the ectoderm,
can soon be seen at its anterior end; these are the rudiments of the
papillae for attachment (Figs. 167, 168, 170, 4), through which, by
means of a secretion yielded by the glandular epithelium, the fixation
of the larva takes place.
ASCIDIACEA—DEVELOPMENT OF THE FREE-SWIMMING LARVA. 3565
Baxrour has pointed out that, since a similar attaching apparatus is found
in Amphibian larvae and (in front of the mouth) in the larvae of many
Ganoids (Acipenser, Lepidosteus) we may perhaps have here an inherited
feature common to the Chordata. It seems doubtful, however, to what ex-
tent these structures are really homologous and not merely analogous.
After the egg-envelope has burst, the larva straightens out. The
tail then forms a direct posterior continuation of the longitudinal
axis of the body (Figs. 167 and 173 A, p. 375).
Fro. 166,—Transverse section through an attached larva of Phallusia mammillate
(after Kowaevsky). a, mesenchyme-cells in the act of passing through the
ectoderm ; 4, mexeuchyme-cells in the cellulose mantle; d, alimentary canal ;
ec, ectoderm; ms, inexenchyme-celln; of, otolith ; s, transverse section” through
the sensory vesicle; f, cellulove mantle,
The mantle, The ectoderm-cells, which originally were somewhat
cubical but assumed a more flattened form, at the time when the
caudal region develops, secret, at their outer surface, a homogeneous
cuticular layer which, from its first appearance, gives a cellulose
reaction. This is the first rudiment of the Ascidian test or tunic.
In the caudal region, this layer grows out to form a median dorsal
and a ventral fin (Fig. 169, ff, p. 363). While, in Doliolum and
Appendicularia, such a simple, homogeneous cuticular layer is
retained throughout life, in the Ascidiacea and Hemimyaria it is
considerably thickened, single cells immigrating into the cellulose
356 TUNICATA.
layer. It has hitherto been believed that, as O. Hertwia (No. 25)
maintained, the cells that wandered into the cellulose substance came
from the ectoderm, but Kowauevsky (No. 32) has recently proved *
that the mantle-cells were derived from the mesoderm, being meso-
dermal cells which traverse the ectoderm and thus migrate outwards
(Fig. 166). Subsequently, in the cellulose substance, they assume
Fic, 167.—Embryos of Phallusie wammillate at a later stage (after Kowal
al, lateral aspect ; 8, dorsal aspect. a, eye; eh, chorda: cl, cloacal vesicle; ,
rudiment of the alimentary canal; ex, entoderm- ciliated pit ; h,
papillae; é, mouth ; ms, mesenchyme-cells ; trunk-section,
section of the medullary tube ; sb, sensory oI sub-chordal
strand; r, vacuoles between the cells of the chorda,
the character of star-shaped connective-tissue cells. We have to
regard the mantle of the Tunicates as a cuticular gelatinous secretion
permeated by phagocytes (mesoderm-cells).+ When, in the composite
Ascidians, some of the individuals disintegrate, the phagocytes of the
* SALENSKY's recent statements as to Pyrosoma (No. 74) fully agree with this
observation (see below, p. 401).
[tSreticer (No. XXXIII) agrees with Kowavgvsxy that the true test-
cells are mesodormal. In Qikopleura, howover, the cells of the “ Haus” are
ectodermal. --Ep.]
ASCIDIACEA—DEVELOPMENT OF THE FREE-SWIMMING LARVA, 357
mantle play an important part in the process (Maurtox, No, 40).
The histological character of the mantle-tissue may undergo further
modification, such as the vesicular transformation of the mantle-cells
in. Phallusia, and the appearance of fibrillae in the ground-substance
in Cynthia.
Since the surface of the embryo is, from the earliest stage, surrounded by
@ gelatinous covering, in which lie embedded the yellow test-cells, it was
formerly thought that this layer was to be regarded as the rudiment of the
future mantle (KowaLevsky, Kurrrer), an error to which the test-cells owe
their name, Zoologists were inclined to consider the mantle of the Ascidians
as a persisting embryonic envelope. ©, Huntwic first proved that the test-
cell-layer is lost and that the mantle arises from the ectoderm. The im-
migration of the mantle-cells was only recently ol by Kowanavsxy.
Sanensxy, however, in a recent treatise (No. 49, also No. XXIX.) has returned
to the older view, ascribing to the test-cells (kalymmoocytes) in Distaplia the
principal part in the formution of the cellulose mantle [see footnote, p. 836.]
The nervous system. ‘he rudiment of the central nervous system
which has hitherto been called the medullary tube, from the early
stages onward, shows a dilatation of its anterior section (Fig. 163,
nr, p. 351). In the later stages which lead to the development of
the free-swimming larva, this dilated anterior part gives rise to a
vesicle, the so-called cerebral or sensory vesicle (Fig. 167, sb, vésieule
antéricure ou vérébrale of vas Bexepen and Junin) while the
posterior, narrowed part yields the caudal section (région caudate) of
the nerve-cord (4). These two parts appear connected by a middle
part (r) with » narrow central canal and thickened wall which
Kowarervsky (No. 30) has called the trunk-ganglion (portion viscérale
du myelencéphale of VAX Brxupex and Junin). The former con-
nection between the neural tube and the exterior (the neuropore)
completely closes even before the appearance of the oral aperture,
whieh lies near the same point.
The cerebral or sensory vesicle represents the most anterior part of
the medullary tube swollen out into a vesicle by the dilatation of its
central canal. [ts walls consist for the most part of pavement-
epithelium, but the dorsul wall is thickened and divided into a right
and a left awelling by a median furrow (VAN Beyepen and Junty,
No. 7). The two organs known as the eye and the ofocyat (au and ot,
Fig. 168) soon appear in the form of accumulations of pigment. ‘The
eye, which is derived from the right dorsal swelling (Fig. 168 8), is
4 cup-like deposit of pigment at the inner ends of several radially
placed columnar cells, the cavity being occupied by « lens with a
superimposed meniscus (Fig. 168).
,
ASCIDIACEA—-DEVELOPMENT OF THE FREE-SWIMMING LARVA. 359
the ventral wall of the vesicle while the free ond carries a pigment:
cap. This view of the formation of the auditory organ is supported
by Koprrer’s statement (No. 35) that the cells of the ventral wall
of the sensory vesicle which surround the otolith-cell and which
differ somewhat in histological character from the rest, are provided
with fine, stiff setae projecting towards the otolith-cell, According
to Kuprrer, « vesicular cavity is found in the crista acustica
directly below the otolith-cell, oa
According to KowAuevs«y, the otolith-cell, on its first appearance, is
situated on the dorsal wall of the sensory vesicle and only shifts later ovey
its right side to the ventral surface. Our knowledge of the structure and
development of both the sensory orguns is, however, very inadequate.*
The frank-vection of the nervous system (Figs. 167 and 168, r)
(the trunk-ganglion) is, according to vAN BenepEn and Junin,
the direct continuation backward of the left dorsal swelling of the
sensory vesicle. The cells of the wall of this swelling show the same
histological character as those which, in a single layer, line, like an
epithelinm, the narrow central canal of the trunk-ganglion, On
these cells, however, on the ventral side, there is superimposed a
great mass of ganglion-cells (Fig. 171). According to the distri-
bution of these cells we can recognise the division of the trank-
yanglien into an anterior and a posterior section, the anterior section
being still included by Kurrrer in the cerebral sensory vesicle as
4 ganglionic portion. In the posterior section, the ganglion-cells
enclose a nucleus of nerve-fibrillae, The trunk-region of the nerve-
cord lies above the most anterior end of the chorda (Fig. 168 4)
* [According to Waunex (No, XXXVL) who has investigated the develop-
ment of these organs in dscidia mentula and Clavelina lepadiformis, the first
indication of these sensory organs consists in the deposition of black pigment-
grapules in the dorsal wall of the cerebral vesicle. The most anterior of the
cells containing nt-granules becomes distinguished by the larger size
of its granules and the swollen nature of the cell itself. This pigment-cell
soon separates itself from the others and becomes gradually transferred by
a differential growth of the wall of the vesicle down the right wall to its
‘final ‘ion on the ventral side of the vesicle. This cell is the otocyst, and
the pi granules become consolidated together to form the otolith, The
other pigment-cells of the dorsal wall of the vesicle collect themselves
er and form a slight protuberance in the right dorso-lateral corner
of the vesicle, The pigment-granules become concentrated toward the cavity
‘of the vesicle. Subsequently two or three cells from the adjoining wall of
the yesiole take up a position, one above the other, in front of the mass of
pigment and by an alteration of their contents give rise to the lens of the
eye. The original pigment-producing cells constitute the retina, which retains
its primitive position as of the epithelial wall of the brain. See also
Sauessky (No, XXX,).—Ep.)
360 TUNICATA.
According to vaAN BENEDEN and Junin, however, the chorda does
uot reach so far forward in Clucelina as in Phallusia.
The cdudal section of the nerve-cord (Figs. 167 and 168, s) is a
tube the walls of which consist of a simple pavement-epithelium.
Tn cross-section (Fig. 169, nr, p. 363), four cells are usually found,
two lying laterally, one dorsally and one ventrally.
This section extends to the postgrior end of the body. The reduction of
the lumen of the alimentary canal in the caudal region of the embryo is
accompanied by the obliteration of the neurenteric canal which represented
the posterior continuation of the central canal of the nerve-cord.
KupFFer observed the important fact that, in the larva of decidia
mentula, lateral bundles of fibrillae are given off by the caudal
section of the spinal cord; these we may claim as spinal nerves.
The first pair of these was found on the boundary between the
trunk and caudal regions, and the following two pairs at intervals
more or less corresponding to the length of a caudal muscle-cell.
We nay regard this as an indication of the segmentation of the
caudal region, The same significance may be attributed perhaps
to those cell-groups found by LaniLie (about ten in number) in
the caudal nerve-cord of the Distaplia larva, and also occurring in
alppendientaria (Noctse, LANGERHANS, and others).
The ciliated pit. In connection with the central nervous system,
we must deseribe a ciliated diverticulum which opens into the dorsal
wall of the anterior section of the alimentary canal (branchial sae
or pharynx) and which has been claimed as a homologue of the
Aupophysis corchri of the Vertebrates. In adult Ascidians its form
is more complicated. A 0 glandular mass, the sub-neural yland
talamb -hypophysnire or sub-anglionic bedy) can then be recognised
in close proximity to the brain. and an efferent duet runs forwant
to enter the pharynx through a complicated apparatus, the doral
tuberele. iu the doral middle line between the two ciliated bands
(Mons yo weds which run upwards from the endostyle. The
opening of this duct has erroneously been assumed to be an olfactory
orgn (olfactory tuberele. see TuLts, Nos, 26 and 27),
According tov. and Juris (No. 7) aud SEBLIGER
(No. 50), the first rudiment of this ciliated diverticulum arises quite
independently of the nervous system pit-like invagination of
the entodermal wall of the pharynx. At a later stage. the blind
end of this diverticulum is said to become closely applied laterally
to the sensory vesiel on the side which is tumed away
ASCIDIACEA—DEVELOPMENT OF THE FREE-SWIMMING LARVA. 361
from the eye, te., as a rule, on the left side, although LaHILLE
(No. 37) considers that this condition varies in the different forms.
Lanitte (No. 37), SHELDON (No. 52), Wiuvey, (No. 54), and
Hyorr (No. 59), on the contrary, have been led by their researches.
to confirm m almost all points the older observations of KowaLev-
SKY as to the rise of the ciliated pit, ie., to regard it, in its origin,
as much more closely connected with the central nervous system.
After the neuropore has completely closed, the most anterior section
of the cephalic vesicle lengthens and fuses with an ectodermal
depression, the stomodaeum (rudiment of the larval mouth, branchiak
aperture). At this point perforation takes place (Figs.-167 A, 168
A, 7), so that now the cephalic cavity, by means of this short tube
which represents the rudiment of the ciliated pit, communicates with
the most anterior ectodermal section of the alimentary canal. Only
in later stages when, after the fixation of the larva, the larval
nervous system degenerates, is the connection between the ciliated
pit and the nervous system lost. The pit then forms a blind in-
testinal diverticulum contiguous with the definitive ganglion (Fig.
173 4).*
According to these statements, the ciliated pit opens into the ectodermat
or stomodaeal portion of the branchial sac, and we thus have a condition
agreeing with that of the hypophysis in the Vertebrata,
The chorda. The chorda arises through the transformation of a
plate-like rudiment (Fig. 159, ch), which originally formed the roof
of the archenteron, into a cell-strand which, in cross-section, is
round (cf. Figs. 160 and 161, ch). We have seen above (p. 348)
that we must suppose this to have been brought about by the forma-
tion of a groove (as in .Amphiorus). In cross-section, the chorda-strand
is originally composed of several cells. Both in lateral (Fig. 162 4)
and in dorsal aspect. it appears composed of two rows of cells, the
ends of which dove-tail with one another. This dove-tailing of
the cells denotes the commencement of a change of position which in
most cases leads to the chorda-cells appearing arranged one behind
the other in a single row like a roll of coins (Fig. 163 1). In those
*[Witury (No. XXXVI.) has recently reinvestigated this point both in
Ciona and Clavelina. He is convinced that van BENEDEN and JULIN were
quite mistaken in their interpretation of the origin of the hypophysial tube
in Clavelina. The whole is derived essentially from the neural tube, and
thus the lumen of the hypophysis is at first in direct communication with
the lumen of the central nervous system, the opening of this structure into.
the pharynx being, according to WILLY. a reopening of the neuropore.—
‘D.)
362 TOUNICATA.
later stages which ure connected with a lengthening of the caudal
section, the cells of the chorda-strand also lengthen (Figs. 167 4,
170, ch). The chorda then begins to undergo a transformation which,
at its commencement, is comparable to the changes in the chorda
of Amphiorus, but, in the Ascidians, leads to peculiar modifications in
this organ. Between cach two consecutive cells there appears a
vacuole filled with a gelatinous substance (Fig. 167, v; ¢f. also Fig.
170). These vacuoles, which, at first, lie in the axis of the chorda-
strand, as they enlarge, compress the chorda-cells in such a way that
the latter soon assume the biconcave form of fish-vertebrae and, as
the gelatinous mass extends further, can be recognised merely as
thin vepta between its different sections. These sections soon come
into contact and fuse, and in this way a strand of homogeneous
gelatinous substance arises which at first resembles a string of beads
but is later uniform and cylindrical (Fig. 168 A), while the chorda-
cells which are pressed out to its surface surround it as a kind of
sheath (KowaLEvsky, KupFFER, SEELIGER. and others).
‘The transformation of the chorda is not, in all Ascidians, so complete.
Avconding to SEELIGER, in Clurelina, it does not advance beyond the stage in
which the chonda-cells assume the form of transverse septa.
Mesoderm, body-cavity, musculature. The two mesoderm-bands
accompauy the chorda along its whole length and project a little
beyond it anteriorly. Two parts can be distinguished in them (Figs.
182 4.163 4). Lathe posterior part (aie) where they consist of a single
layer of large cells arminged in three longitudinal rows, they yield
the musculature of the larval tail, The cells of this part lengthen
in later stag.
while. on th
fibrils of contractile substamee 1Fix. 188.
af development (Fig. [6S 2, ma) becoming hexagonal,
inner ant outer surfaces they produce longitudinal
1 which appear to lie
somewhat obliquely to the lonzitudinal axis of the bedy in such a
way that che fibres of the inner Liver emes these of the outer layer
at an acute angle (SEE
ada] musculature of the larva
tmuusverse striation.
mes sierm-bands consist of
vely crowded together (Figs.
Myer which ties neat to
the myoblast-larer of
asfomuations as the
ASCIDIACEA—DEVELOPMENT OF THE FREE-SWIMMING LARVA. 363
somewhat loosened ; they assume a spherical form and constitute a
mesenchyne which fills the primary body-cavity of the trunk-section
(Fig. 168, ms).
We have seen (p. 350) that, according to VAN BENEDEN and JuLIN,
the mesoderm in this anterior part becomes detached from the arch-
enteron in the form of paired coelomic
diverticula (Figs. 159 and 160 A). The
true coelom that arises in this way
belongs, however, merely to the earliest
embryonic stages, and disappears even
as early as the time when the mesoderm
completely separates from the entoderm.
The mesoderm then fills the space
between the ectoderm and the entoderm,
which, in later stages, becomes consider-
ably widened and, as it appears, filled
with a gelatinous mass ; this forms the
yround - substance of the mesenchyme
that arises through the transformation
of this anterior portion of the mesoderm.
The lacunae which arise later in this
mesenchyme must be regarded, like
the blood-vessels (which, according to
vaN BENEDEN and Jurin are entirely
devoid of an endothelial wall) as the
pseudococle.
The mesenchyme of the trunk-region,
which is derived in the above way, yields
Fis.
through the caudal portion of
169, — Transverse section
the mesodermal organs of the adult
Ascidian. Its histological differentiation
gives rise to the connective tissue, ax
well as to the pigmented clements,
and to the body-musculature of the
adult which appears arranged in radial
and circular muscles surrounding the
the free-swimming larva of
Clarelina (after SRELIGER), ch.
chorda ; er, ectoderm; fl,
median’ fin; m, _ cellulose
mantle ; mf, muscle-tibrillae in
transverse section ; mz, muscle-
celle; ur, neural tube; s, sub-
chordal entoderm - strand ; +
mantle cells,
inhalent and exhalent orifices as well as into longitudinal muscles
of the trunk, etc. Single cells of the mesenchyme, which become
free and reach the pseudococle, become changed into blood-
corpuscles. We shall see later (p. 380) that the genital organs
also, with their efferent ducts, and the urinary organs originate in
the mesoderm.
364 TUNICATA.
It must be mentioned that at a later stage (especially during the transforma-
tion connected with fixation) the mesenchyme becomes apparently enriched
by elements which, when the entodermal strand (Fig. 169, s) and the larval
nervous system disintegrate, become free. KowaLEvsky thought that these
cells became changed into blood-corpuscles, and later, SEELIGER (No. 50)
utilised this fact in constructing his theory of the budding in compound
forms. It, however, appears doubtful to us whether these elements take any
part whatever in the formation of the organs of the adult or of their buds or
whether they do not rather, after reaching the blood, undergo degeneration.
The alimentary canal. The rudiment of the alimentary canal is
derived from the archenteron by the separation of the mesodenn
bands and the chorda-strand. In early embryonic stages (Figs. 162,
163, p. 351) it consists of an anterior pre-chordal dilatation (en) and,
following this, of a narrowed purt lying already below the chorda but
still belonging to the trunk-region. This narrowest part is directly
continued into the sub-chordal entoderm-strand of the caudal region
(en') which must be regarded as the intestinal rudiment of this
part of the body.
Since the mesoderm and the chorda (as seen above, p. 350) arise
by a process of folding from the dorsal wall of the archenteron, the
dorsal wall of the intestine is defective at this point; this gap be
comes closed in later stages by certain cells as described above
(p. 350). This defect is only found in the trunk-region in the
posterior narrowed part of the alimentary canal, as the rudiment of
the caudal region undergoes no further advance in form and, on the
other hand, the pre-chordal section of the intestine takes no part in
the foldings which give rise to the mesoderm and the chorda. This
narrowed sub-chordal trunk-portion of the intestine, after the gap
just mentioned has closed, forms a blind diverticulum projecting
backward (Fig. 167 .1, ¢, p. 356), the end of which, according to
Kowavevsky, at those stages in which the caudal section becomes
more sharply marked off from the trank, bends slightly towards the
dorsal side, thus severing its connection with the cellular entoderm-
band of che caudal region (xe). In this way is introduced the de-
generation of this last part of the intestine mentioned above, which
leads to the cells of this band becoming disconnected and assuming
a resemblance to blood-corpuscles. We then tind, beneath the chorda
in the caudal region, a cavity apparently filled with blood-corpuscles
and in direct communication with the spaces of the pseudocoele.
According to KowaLevsky, with whom the majority of later
authors (KUPFFER, SEELIGER) agree, the branchial sac or pharynx
is derived from the pre-chordal dilatation of the intestinal rudiment,
ASCIDIACEA—DEVELOPMENT OF THE FREE-SWIMMING LARVA. 365
while the sub-chordal diverticulum, which is directed backward (d),
yields, through simple growth, the other parts of the alimentary
canal (the oesophagus, the stomach and the intestine proper). This
diverticulum, as the larva lengthens, is said to form a coil in which
we can distinguish a right descending portion, a ventral connecting
piece, and a left ascending part which ends blindly. The right
portion is said to give rise to the oesophagus, the connecting piece
to the stomach and the left ascending portion to the intestine (ef.
Fig. 168, ¢ and e/ with Fig. 170, of, m, and ed). The anus only
arises later, during larval life, by the blind end of the intestine be-
coming connected with one of the two so-called cloacal vesicles (that
on the left), these latter being ectodermal invaginations which will
be described further later.
The account given by VAN BENEDEN and Junin (No. 10) differs
from that given above in so far as they derive only the descending
portion of the intestinal coil (consisting of the oesophagus and the
stomach) through direct growth from the posterior diverticulum
mentioned above, while they believe that the intestine proper arises
from the ventral surface of the dilated stomach as a secondary out-
growth. The point of origin of this secondary caecum, which is
directed to the left and upward, is said to lie somewhat far forward,
4e., to be almost pre-chordal. We shall see below (p. 521) that these
authors ascribe some significance to this observation.
The oral aperture of the larva (which gives rise to the inhalent or
branchial aperture of the adult) is formed only shortly before the
larva hatches. The anterior pointed end of the alimentary canal,
just before reaching the sensory vesicle, bends dorsally. It there
comes into contact with an invagination which has arisen from a
thickened dise of ectoderm-cclls. By the apposition of these two
structures and a breaking down or separation of the cells, the oral
aperture is formed (Figs. 167 and 168, p. 358).
The endoxtyle develops as a ciliated furrow in the antero-ventral
region of the branchial sac (pharynx), owing to the formation of two
lateral longitudinal swellings. We cannot here enter further into
the histological details of this structure, but must refer the reader
for these to the treatises of R. Hertwie, Fou and SEELIGER (No.
50). We may, however, mention that this furrow is not purely
ventral in position, its anterior portion extending up towards the
dorsally placed oral aperture (Fig. 170, es). According to a recent
treatise by Wiubry (No. 54a), the rudiment of the endostyle
originally lies in the most anterior part of the branchial sac in which
366 TUNICATA.
it has a dorso-ventral position. Only later does it shift farther back
and come to lie ventrally. This observation is of importance in
connection with the condition of this organ in Amphtoxrus, where
it undergoes a similar displacement.
The digestive or pyloric gland arises as a caecal outgrowth at the
boundary between the stomach and the intestine; it, however, soon
branches repeatedly, and these ramifications extend over the surface
of the intestine where, by anastomosing, they form a network (Fig.
175 A, dr, p. 379). It has been homologised by WiuLeEy (No. 54a)
with the hepatic caecum of Amphiorus.
Peribranchial, atrial, or cloacal cavity. The first rudiments which
lead to the development of the peribranchial cavity are found, shortly
before the larva hatches, in the forin of a pair of ectodermal invagina-
tions lying dorsally at the boundary between the sensory vesicle and
the trunk-ganglion, called by MEerscHNIkorF, who was the first to
observe them, the cloacal vesicles (Fig. 167, cl, p. 356). Two
diverticula grow out from the pharynx towards these invaginations,
one on each side, and fuse with them, thus giving rise to the first
gill-slits (Fig. 168, %, p. 358). According to KowALEvsky, a second
pair of these slits (k”) soon forms in Phallusia behind the other in
the same way. If the interpretations of KowALEvsky and SEELIGER
are correct, the cloacal vesicles, by enlarging, give rise to the paired
halves of the peribranchial cavity. In this case, the latter would be
lined throughout with ectoderm, and the wall of the pharynx, which
is perforated by the gill-slits, would on its inner side be covered with
entoderm and on its outer with ectodermal epithelium. We should
then perhaps be justified in homologising the peribranchial cavity of
the Ascidians with the atrium of Amphiorus: we can hardly, in any
case, doubt the homology of the gill-slits in these two groups.
Another view has, however, been adopted by vaN BENEDEN and
Jur (Nos. 9 and 10). According to these observers, the first gill-
slit arises through the fusion of a rather long entodermal diverticulum
with the cloacal vesicle of the same side which, according to these
authors, is never very large. The Ascidian larva at this stage is
exactly in the condition in which Appendicularia remains through-
out life, the pharynx, in the latter, communicating through a
branchial passage on either side with the exterior. These passages
represent a pair of gill-slits, and this pair, in the Ascidian ax in
Appendicularia, remains the only pair. In the Ascidian, the branchial
passages are considerably enlarged secondarily, and in this way the
peribranchial or atrial cavity arises. Since these passages, according
ASCIDIACEA—DEVELOPMENT OF THE FREE-SWIMMING LARVA. 367
to vAN Benepen and Junin, are for the most part of entodermak
origin, a large although not sharply marked portion of the peri-
branchial cavity must be lined with entoderm. These observers
therefore maintain that these spaces cannot be homologised with the
atrial cavity of Amph/owus, and the future perforations in their inner
walls can in no way be homologised with the gill-slits of Amphiorus
and the Vertebrates, and are therefore distinguished from true gill-
slits by these authors as branchial stigmata. We have some hesita-
tion in accepting the view of vAN Buyepxn and Jens. The origin
of the peribranchial cavities in the Ascidians does not appear to us
sufficiently understood to justify its utilisation in the formation of
such important conclusions, It should be pointed out that, in the
embryo of Pyrosoma, the purely ectodermal origin of the peribranchial
eavities can hardly be doubted (see below, p. 394), and this seems
also to apply to Doliolum (see the statements as to the Anchinia buds,
p. 481). Again, we are probably not justified in concluding without
further evidence that the condition of Appenlicularia is primitive.
Since an Appendieularian was found by Moss (No. 5) possessing
many gill-clefts like those of Doliofwm, we may have to regard the
apparently simple respiratory organs of other Larvacea as degenerate.
We shall have to speak of the further transformations in the
branchial region when describing the metamorphosis of the attached
larva, although, in some cases, the multiplication of the gill-slits
commences in the free-swimming larva (Botryllus, Distaplia, Fig.
230, p. 457),
Laninee (No, 38) and Wittey (No. 54a) have both recently expressed their
belief in the purely ectodermal origin of the peribranchial sacs. According to
Lanu.e, they arise, in the Didemnidae, through the enlargement of the
cloacal vesicles. Their outer aperture then disappears and an unpaired
ectodermal invagination appears on the dorsal side; this is the cloacal in-
yagination proper, which only secondarily becomes connected with the
peribranchial sacs. Wrotry (No, 54a) observed, in Clavelina, as the pre-
cursor of the cloacal vesicle, a longitudinal furrow, from the posterior end of
which the cloacal vesicle develops. Wruuny is therefore inclined to assume
Shat the peribranchia!l cavity of the Ascidians is homologons with that of
Amphionus.
The outer apertures of the two peribranchial spaces (Fig. 231,
p. 460) shift continually towards each other and towards the dorsal
middle line till they meet and fuse. In this way arises the cloacal,
atrial or inhalent orifice (Fig. 230, ¢). van Benepen and Jou
(No. 9) have pointed ont that, during this fusion, the part of the
ectoderm which lies between the two apertures becomes depressed
i
368 TUNICATA.
and this depression gives rise to the true cloacal cavity, i.e. to the
unpaired part which connects the two peribranchial cavities and
which is thus lined with ectoderm.
The terminal portion of the intestine which had, at an earlier
stage, become connected with the left cloacal vesicle (see above,
p- 365) now opens into the common peribranchial cavity.
The heart, the pericardium and the epicardium. We owe to
SEELIGER the proof that the heart and the pericardium of the
Ascidians are entodermal structures, the first rudiment of which can
be recognised in the form of an outgrowth from the pharynx arising
between the posterior end of the endostyle and the entrance to the
oesophagus. A vesicle which becomes abstricted from this caecum
es ep we
Taevline obey (attet SEELIGER). 01, e)
hor xhalent aritive ; «/, . epivardial outgrowth : es, endost
7, iufolding of the body-snttace in anticipation of the rotation that takes pl
fixation h. adhering papillae; é iuhaleut oritive; de, gill
esophagus: 4, anditory ong
; sh, sensory vesicle,
Lett lateral asp
(Fig. 170, pe) is the common rudiment of the pericardiam and the
heart. The remainder of the caecum (4) has been named by vay
Bexepes and Junin the egveardium.*® These authors were able
essentially to contirm the statements of SEELIGER, although they
Aly ditfer from him in points of detail. They also recognised
titicance of the epicardium in connection with budding, and
the originally double rudiment of these structures,
The tirst rudiment of these org: observed in the form of two
solid cell-strands which ran side by side in close contiguity to the
repute
the
aS Ww
*(Dastas (No. IX.) finds that in Ciona the epicardium has a paired origi:
and opens by two distinct orifices into the pharynx.—ED.]
ASCIDIACEA—DEVELOPMENT OF THE FREE-SWIMMING LARVA. 369
ventral wall of the alimentary canal near the point at which the
oesophagus enters the pharynx. The origin of these cell-strands
(procardium) is less clearly stated. vax Brnepen and Juin have
no doubt that they become detached from the entodermal intestinal
epithelium. Wuttey has recently (No. 54a) made the same observa-
tion, finding, however, that the procardial rudiment is unpaired and
also not entirely agreeing with vay BeyepEN and Juni with regard
to its further development. The left procardial strand always appears
stronger than the right. The strands soon develop lumina and thus
become tubes, In later stages, the posterior ends of these tubes fuse,
while, anteriorly, they open into the branchial sac, ‘The whole rudi-
Pip, 171.—Pares comeoutvy franaveres sections throng the trunk: region of a Clare
(diagrammatic, after VAN BewEDEN aud JuLIN). A, shows the pe |
ind nacre branchial auc or pharynx with the apertures of
epicardial tubes (~p.0). 8, shows the connection hetween the epicardial Sire
isha he the nde rete (os ©, shows the blind ends’ of the epicardial
¥
ip tctoterm |,
teas e °, Pina en ends of the plonrdial tabex
wear their ‘openings os the TX; thy heart; m, pte wntle ; m, neural
inent now consists of wn unpaired posterior caecum (Fig. 171 B, ep
and pe), which forks anteriorly into two tubes that open separately
into the branchial sac (Fig. 171 4, ep). From the posterior
caecum, a vesicle becomes abstricted (Fig. 171 C, pe) and this
represents the common rudiment of the heart and the pericardium.
The lumen of this vesicle (pe) is the future pericardial cavity.
form of the yesicle is complicated in consequence of the invagina-
tion of its dorsal wall as a furrow running along its whole length ;
this makes the vesicle crescent-shaped in cross-section, The lumen
of this invagination is the future cavity of the heart (4). The in-
BB
‘TUNICATA, =
vaginated part of the wall of the vesicle
‘ ‘the noninvaginated part |
Th tn tt hn in
and 4, p. hoo age it ceates tans Sea
ventral wall is drawn in to close the dorsal aperture of
duction of the buds. By extending farther and f
reaches the stolon (Fig. 173 C, st) in which it fo
partition. Th thiv' process it becomes \so! minal
of the stolon (Fig. 229, 2, p. 456,) the two bloo
each other at this point. Werrhall hers tose
oats tetova Wace aah ri
vertebrate pericardium. In later stages, the cell
the heart secrete, on the surface turned to the lu
ASCIDIACEA—ORGANISATION OF THE FREE-SWIMMING LARVA. 371
muscle-fibrils in which transverse striation can be distinctly seen.
An endocardium is wanting in the Ascidian heart, and its yessela
have no endothelial lining.
E. Review of the Organisation of the Free-swimming
Larva (Pigs. 168-173 A).
It should be mentioned that there is considerable variation in the
degree of development attained by the organisation at the moment
of hatching in the different species and even in the individuals of the
same species.
The form of the larva, the development of which has just been
traced in detail, recalls somewhat that of a tadpole. Anteriorly,
there is an oral region (cephalic and trunk-region) the end of which
carries the three adhesive papillae (Fig. 173, hp); this region is
followed by a long, flatly compressed swimming tail. This latter,
which shows markings like those of fin-rays (Fig. 173 A) attains its
characteristic form through the great development of the mantle-
substance (Fig. 169, #) which further covers the whole surface of
the body, even passing over the oral and cloacal apertures.
The axis of the tail is oceupied by the chorda which extends
anteriorly into the trunk-region. Above it lies the neural tube,
while a cell-strand that runs below it and soon disintegrates repre-
sents the remains of that part of the intestine which belongs to the
caudal region. Running along this part of the body and extending
along its whole length, are the massive muscle-bands.
In the anterior region, the neural tube swells out to form a sensory
vesicle (Fig. 173, s+) and a swollen trunk-section (1) immediately
following the latter. The primitive enteron gives rise to the
pharynx or branchial sac, the oesophagus, the stomach and the
intestine. The oral aperture (or inhalent orifice, ‘) is established
and is distinguished by its dorsal position. Near it, the ciliated pit
(A), which extends as far as the base of the sensory vesicle, opens
into the pharynx. The intestine opens into the atrial cavity which
has formed by the union of the two originally separate peribranchial
sacs. There is now a single atrial aperture (or exhalent orifice, e).
The number of gill-slits which have at this stage developed varies
in different species.
The heart (}, the pericardium and the epicardium (ep) have
developed. The heart already pulsates ; the endostyle (hypobranchial
groove) has developed. The mesenchyme becomes differentiated into
ASCIDIACEA—FIXATION AND METAMORPHOSIS. 373
(Fig. 173 B), to which they are then appended as # short truncated
process. The internal organs of the caudal region are retracted more
and more into the trunk (Fig. 172). They there coil up spirally, but
the separate elements (the chorda, the muscle-bands, the neural tube)
which compose this coiled strand retain for a long time their relative
Fic, 172.--Degeneration of the candal region during metamorphosis in the larva of
Phatlusie mummillate (atter KOWALBVSKY). itudinal section through an
early stage’; B, posterior portion of a longi ction through an older stage.
-cells ; ¢c, ectoderm ; eva, invaginated epidermis of the tail; me, mesen-
chyme-cells in the act of passing through the ectoderm ; mt, cellulose mantle ; ms,
inuscle-cells of the tail : #1, nerve-cells of the caudal section ;’ns, neural tube,
positions (cf. Fig. 172 B, 2, where the strand is cut through trans-
versely). The degeneration of the chorda commences, according to
KowaLevsky's most recent researches (No. 32), by the disappearance
of its gelatinous substance, while its cells (Fig. 172, ch) again become
374 TUNIOATA,
arranged into a simple strand, The degeneration of the chorda-cells
which are distinguished by their granular contents, is finally brought
about by phagocytosis. This is also the case with the musele-cells.
The ectoderm-cells, as the caudal section shortens, become larger
(Fig. 172 A), and spherical, strongly refractive bodies appear in
them, so that they come to resemble the so-called granulated spheres
of the pupa of the Muscidae (Vol. iii., p. 379), When the internal
organs of the caudal section have teat completely taken up into
the body-cavity of the trunk, the ectoderm is invaginated (Fig. 172
B, ens), This invagination soon becomes completely abstrieted from
the epidermis of the larva and then forms a closed vesicle lying
within the body-cavity ; the cells of this vesicle soon lose their
cohesion, the lumen disappears, und finally nothing remains but
a mass of detached and gradually disintegrating granular cells. The
gelatinous envelope (Fig, 173 ©, ss) of the caudal region is, finally,
lost either by being simply absorbed aceording to Kuprrer’s obser-
vations, or thrown off, as SkEnIGER and Munr-Epwarps agree in
maintaining. f
Since the attachment of the larva is accomplished by means of the
anterior end of the body, the oral aperture (inhalent orifice) appears
to lie near the point of attachment (Fig. 173 B). In the adelt
Ascidian, on the contrary, the oral aperture lies at the end of the
principal axis of the body opposite to the point of attachment (Fig,
173 C). ‘This shifting of the position of the oral aperture is the
result of a rotation made by the body round its transverse axis
after attachment, during which the part of the body between the
mouth and the point of attachment lengthens, This lengthening, in
Clavelina, according to SEELIGER, is made possible by the develop-
ment of a deep infolding of the surface of the body (Figs. 170, 173
A and B, f) whieh slightly separates a pre-oral portion carrying the
adhering papillae from the rest of the body. This pre-oral region
represents the basal section of the young Clavelina from which the
branching stolon soon grows out. The folding just mentioned
renders it possible for the Clavelina, which originally was placed
with its longitudinal axis at right angles to the basal plane (surface
of attachment), first to bend sharply towards this plane and then
to lie with its longitudinal axis parallel to it, finally however, to
rise up from it in such a way that the oral aperture comes to
lie opposite to the point of attachment. During this rotation round
the transverse axis, which was pointed out first by Kurrrer and
later by SeexicER, the angle passed through is one of almost 180."
a
ASCIDIACEA—FIXATION AND METAMORPHOSIS. 875
picardial
th at , heart; i, adhering papillae; i, bra H
By cavity; r, trunk-portion of the medullary tube; s, partition-wall of the
376 TUNICATA.
It is interesting to compare the transformations undergone by the
Ascidian larva after attachment with that of the Cirripedia and the
Pedicellinn larvae, in which to some extent analogous conditions
prevail.
A degeneration similar to that undergone by the organs of the
caudal region algo takes place in the larval nervous system. The
anterior sensory vesicle breaks down, its elements become spherical
and lose their connections. For some time after the sensory organs
have disappeared a mass of pigment remaining from them may be
“observed in the body-cavity of the young Ascidian. The degenerated
elements. later reach the latory system, where they probably
completely disintegrate (p. 364). A similar disintegration is suffered
by the tissue on the ventral side of the trunk-ganglion which consists
of large ganglionic cells. The central nervous system of the young
Ascidian, according to VAN BENEDEN and JULIN (No. 9), consists of
those elements which surround the central canal in the region of the
trunk-ganglion, and are continued anteriorly on to the left swelling
of the sensory vesicle. The elements derived from the cephalic vesicle
thus yield the definitive ganglion, while the elements of the trunk-
ganglion produce aganglionic cell-strand (rordonganglionnaire viscéral)
discovered by vAN BENEDEN and Juin, which, first running back-
ward in the dorsal median line, becomes applied to the dorsal wall of
the pharynx but then diverges to the left, runs along the left side of
the oesophagus and ends between the two hepatic diverticula, After
the gelatinous cover has been perforated by the apertures of ingestion
and egestion, the admission of water and of food into the alimentary
canal becomes possible.
In the further development of the Granchial network, the chief im-
portance attaches to the appearance of new slits, each of which, as a
rule, arises through the fusion of a shallow diverticulum growing out
from the entodermal wall of the pharynx with the lining of the atrial
cavity. At the point of fusion, the slit is first visible as a very small
aperture. In this way, according to KowauEvsky, in Phallusio
mammillata, after the first gill-slit has formed, a second appears
behind it (Fig. 168, 4”) this being apparently of the same size as
the first. Later, nccording to Kroun (No. 33), two new slits appear
between these two and behind the last slit (the second in order of
formation) one more. In this way five primary gill-slits form in a
longitudinal row. Each of the trabeculae between every two gill-slits
contains a blood-sinus (branchial vessel, Fig. 168, 4). Above and be-
low this primary row of slits, other rows are said to form later, the
ASCIDIACEA—FIXATION AND METAMORPHOSIS. 317
number being then further increased by the intercalation both of new
slits and new rows between those already formed. ‘The gill-slits are
at first elongated in the transverse direction, but their shape changes
later, as they lengthen longitudinally.
According to van Brenepen and Junin (No. 9) the formation of
the additional gill-slits in Phadlusia (Ascidiella) scabroides follows an
entirely different rule. A longitudinal.row of six primary slits here
first forms (Fig. 174 A, 1-6). Of these, the fourth (4) in the row is
suid to appear first, the first in position (1) being the second to form.
The fifth (5) forms next and then the second (2), while those that
oceupy the third and sixth positions (3 and 6) form last. These six
primary gill-slits ure markedly elongate transversely (Fig. 174 B)
and each becomes divided by projecting outgrowths of the trabeculae
lying between them, In this way the six primary slits give rise to
six rows of secondary slits. In later stages, new slits are said to
break through between these.
Tn Clavelina, according to SkrcicEeR, two transverse rows of gill-slits are
found even in the free-swimming larva (Pig. 173 A). A new row forms in
front of these and behind them another is added after fixation. In this case
the number of slits is increased by the appearance of new independent perfora-
tions (and thus not by the division of those already present), This is also the
case, acoording to GansTana (No. 21a) with the buds of Bofryllus, while, in
the larva, the number of slits is increased through the division of thoxe
formed first. According to this author also, Pyrosoma, with its transversely
elongated slits in a single row, represents a specially primitive condition.
The rise of the six primary gill-slits (Fig. 174 A, 1-6) has recently
been carefully examined in Ciona by Witney (No. 54a). Slits 1 and
4 arise first, simultaneously and apparently independently of one
another. Wuinney is inclined to regard these two as parts of a single
slit separated as in Amphiorus by a tongue-bar, The next slits to
appear (2 and 3) form by abstriction from | and 4, while 5 and 6
arise independently. Witory thus regards slits 1-4 as parts of a
single primary slit separated by abstriction. The stage depicted in
Fig. 174 4 would then possess only three actual primary slits. As
the six slits present divide in the manner stated by VAN BENEDEN
and Jur, six transverse rows of slits are formed, these all at first
being elongated transversely but later lengthening longitudinally
(parallel to the endostyle). Further division of the apertures leads
to the intercalation between these six rows of other transverse rows
(Witxey).
‘Even at an early stage, an entodermal fold can be seen projecting
inward on the imner side of the branchial region between every two
Ui
= |
* 378 TUNICATA,
primary slits (Fig, 174, tr). These folds are the rudiments of the
transverse bars; prominences (p) form on them both anteriorly
p
Fie, 174.—Two young stages of Phallusia (Ascidietla) idles (after Wax
and JULIN, somewhat altered), 1-6 in A, the six primary gill-elefts, in B, the
of gill-clefis derived from them, cl, atri 5
and posteriorly and give rise to papillae. The papillae on the eon-
secutive branchial arches lie so near each other as to come into
i — |
ASCIDIACEA—FIXATION AND METAMORPHOSIS. 879
contact and fuse, giving rise to the internal longitudinal bars (Ir)
of the branchial region.
At an early stage, the first rudiment of the pericoronal circle
(peripharyngeal bands, ciliated arch, Fig. 174 A, 6), as well as that of
the coronal circle, which lies in front of it and is beset with tentacles,
can be seen. The arrangement of the tentacles when they first appear
is bilateral.
The development of the genital organs has still to be described.
In the compound Ascidians, the individual that develops from the egg
has no genital organs, but multiplies exclusively through budding.
This is also the case with the social Ascidians (GaNrn). In studying
the development of the genital organs we are consequently restricted
to the simple Ascidia if we wish to study their origin in a sexually
Fio, 175.—.1, dorsal aspect of the intestinal coil in the bud of Perophora Listeri with
the rudiment of the genital organs; B, somewhat older genital vesicle (after VAN
BeNngnes and Juuty). g, genital vesicle ; gs, yenital strand ; dr, digestive gland ;
ve, oesophagus; a, stomach ; é, intestine,
produced embryo, otherwise we must trace their rise in the asexually
produced buds of other Ascidians. The course of development in
these two cases is, however, so uniform that we may take as our
example the buds of Perophora which were examined with special
reference to this point by vAN BEenEDEN (No. 10).
The Ascidians are hermaphrodite. The male and female genital
rudiments, however, are derived from a common rudiment which is
always unpaired and lies on the intestinal loop medianly and dorsally
(Figs. 174 B, gs’, 238 EZ, g). This is found at the point at which the
efferent duct of the digestive gland firat branches (Fig. 175 A). It
consists of an accumulation of cells indistinguishable from ordinary
THE ABBREVIATED DEVELOPMENT OF THE MOLGULIDAR. 38%
comes into contact with it. At the same time, the vas deferens has
been gradually split off from the oviduct so that finally the two
canals enter the cloaca separately, The male and female genital
rudiments have thus become altogether distinct. The further de-
velopment of the genital glands manifests itself chiefly in the con-
tinuous formation of lobes. In the epithelium lining the ovary, the
eggs can soon be distinguished from the surrounding undifferentiated
cells, which form the follicular epithelium. As the eggs increase in
size, they shift, enveloped in their respective follicles, into the
surrounding stroma, so that, finally, they are only connected, like
the grapes on a bunch, by means of the thin efferent ducts of the
follicles with the ovarian epithelium.
The urinary vesicles (Fig. 174 B, rs) form in the same way as the
first rudiment of the genital organs as accumulations of mesenchyme-
cells in which a cavity appears, containing at first only serous fluid,
but, later, urinary concretions (VAN BENEDEN and Jury, No. 9).
G. The Abbreviated Development of the Molgulidae.
The development of those forms the eggs of which up to the time
when the tailed larva hatches remain within the body of the mother
(Ascidiae compositac, Cynthia, Lithonephria) is, in many ways, some-
what modified and abbreviated. One of the Molgulidae which lays
its eggs affords us, however, curiously enough, the most marked
example of abbreviation of development. The caudate larval stage,
in this case, is ultogether omitted (Lacaze-Durnrers, No. 36; and
Kurrrer, No. 35); even the chorda does not appear to develop, In
other respects, the ontogeny of this form, so far as it is known, does.
hot seem essentially to differ from that of other Ascidians, After the
cleavage of the very opaque egg has taken place, the thick-walled
primary enteric vesicle can be recognised inside the embryo, and near
it an accumulation of large cells filled with reserve nutritive material.
This accumulation ean be seen for a long time, as development pro-
ceeds, lying near the posterior end of the body (Fig. 177, r). It may
perhaps be regarded as the equivalent of the suppressed larval tail and
may be compared to the claeoblast of Pyrosoma and Salpa. As the
internal organs develop, five finger-shaped outgrowths appear on the
surface (Fig. 177, f); these vary greatly in size and position and
degenerate later, They do not serve, as might be supposed, for the
fixation of the embryo. The nervous system (mn) seems to develop
from an ectodermal depression. The primary enteric vesicle repre-
S|
382 TUNICATA.
sents the rudiment of the branchial sac (pharynx). The rest of the
alimentary canal (d) develops as an outgrowth at the posterior dorsal
Fig, 177.—Embryo of Molgula spb a tere (after ae
alimentary pal es, endastyle ; ae Inet Of the of the imhalen
orifice; m, nervous system ; 7, spherules Chtcamereaettiaale
angle of the primary enteric vesicle. The inbalent and exhalent
orifices, the endostyle, the gill-slits, ete., develop as in other Ascidians,
3. Doliolum.
The development of the egg in Doliolum seems to form a direct
sequence to that in other Ascidians. As in the latter, a larval form
occurs which propels itself by means of a swimming tail. ‘This larva
was first described by Kron (No. 85), and later by GEGENBAUR
(No. 78), Kerensrety and Exuers (No. 81), Gronpen (No. 79), and
Unsanin (No. 86), To the latter author we owe, further, almost
the only statements we have as to the embryonic development of
Doliolum, which is very insufficiently known.
‘The mature egg of Dotiolum, surrounded by follicle-cells, reaches
the atrial cavity of the mother from which it is soon ejected inte the
surrounding water. Occasionally, however, part of the embryonic
development seems to take place within the atrial cavity. As a rule,
the egg is fertilised only after its expulsion, and then surrounds
itself with a homogeneous membrane. This membrane (Fig. 178, =)
which soon rises up from the surface of the egg in such @ way
that a space filled with fluid can be seen between it and the egg,
: al
DOLIOLUM—CLEAVAGE, 383
has been called the vitelline membrane by ULsanty, and for a long
time shows, on the external surface, remains of the original follicular
epithelium (f). Since, however, both Grospen and Fon (No. 21)
observed within this membrane cells which without doubt correspond
to the test-cells of the Ascidiacea we may perhaps rather regard it
as the equivalent of the chorion of the Ascidian egg.
Fro. eet er eae, ee of meer Miilleri Lali d rd rls
Ree icecths os ons toat (cncciosys ovr, menor sw totinon! or tha eevee
Cleavage is total and almost equal, and closely resembles that
described above in connection with the solitary Ascidians. The
blastala and invagination-gastrula (Fig. 178 A) observed by Unsantn
were comparable to the similar stages in the Ascidiacea.
We have very incomplete accounts of the subsequent stages, At
the next stage observed by Utsaniw and depicted in Fig. 178 B,
the embryo is pear-shaped, and within it we can see three distinct
rudiments. A large dorsal cell-mass (m) is regarded by Unsantn as
a...
384 TUNICATA,
the rudiment of the central nervous system, which here does not take
the form of a tube but of a solid ectodermal growth, while 4 ventral
cell-mass (eh) is assumed to be the rudiment of the chorda and a
posterior mass the mesoderm (ms). According to Unsanry, the arch-
enteron is used up in the formation of the chorda and the mesoderm ;
the rudiment of the adult intestine on the contrary owes its origin
to an independent ectodermal invagination which occurs later.
In the next stage (Fig. 178 C), the embryo appears to be folded
several times within the egg. A dilated anterior region principally
Fie, 179. = ire sorcalled larval’ stagos of Zoliofwm Allert iter (aoe Uist ay
pase B, older stage, ch, chorda ; cl, atrial :
heart ; 2m, Ser eet ahle ma’, anterior mesodernt-1 vadtaheee ma", :,
ridiment; 1, nervous system ; p, pharynx ; x, mesodermal rut imeut of the stalon ;
yy mesoderm-masses,
occupied by the large ganglionic rudiment (w) can be distinguished
from a caudal region bent back upon itself, in which the chorda (ch)
is seen to be already developed. ‘wo lateral mesoderm-bands (ma)
run along the whole length of these two regions of the body. Similar
stages were also observed by Fon (No. 21).
In the stages which follow (Fig, 179 A), the embryo straightens
itself out within the egg-shell and is now able to raise itself from
the bottom of the sea on which the egg rests and to swim about
«a
DOLIOLUM—LARVAL DEVELOPMENT. 385
by means of its long caudal region, und is therefore usually called
a farve although it is still enveloped in the much-distended egg-shell
(m) iu which traces of the follicle-cells ean be found, We do not
know for certain when this egg-shell is cast off, During these pelagic
ontogenetic stages in which the Doéfolum resembles the Ascidian
larva, the body is elongate (Fig. 179 4) and the middle of it is
occupied by a vesicular ectodermal swelling (¢4) caused by the
accumulation of a clear fluid. ‘This vesicle divides the body into a
posterior and an anterior region. ‘I'he anterior develops into the
young Doliolum (the first asexual or “nurse” form, the /aatozaoid)
while the ectodermal vesicle and the caudal region must be regarded
nm
Fic. 130.—Young “nurse” form of Lotiolum Khrenbergii, with remains of the larval
tail (after Unsaxrn). ch, chorda ; dé, so-called dorsal stolon ; ¢, endostyle ; A, heart
and pericardium ; m, egg-shell ; #, nervous system ; +, rosette-shaped organ (rudi-
ment of the weutral stolon).
as provisional larval organs and degenerate later (Figs, 179 #, and
180). The structure of the caudal region corresponds to that of
the same region in the Ascidian larva. It consists of a chorda and,
laterally, of muscle-plates derived from the mesoderm-bands. At the
anterior end of the caudal region, a part of the mass of mesoderm-
cells (Fig. 179, ms’) is not transformed into spindle-shaped muscle-
fibres. ‘I'wo cell-masses (y) are subsequently given off from this into
the ectodermal vesicle, where they break up and change into blood-
corpuscles.
The anterior region of the body contains the very large rudiment
of the central nervous system (Fig. 179 A, ”) and the anterior
co
F
386 TUNICATA.
portion of the lateral mesoderm-bands (ms) which also give off into
the ectodermal vesicle from their posterior ends elements that change
into blood-corpuscles. An ectodermal invagination can also be seen
forming ventrally (p) and from this is derived the whole intestine
of the ‘‘ nurse” form. This invagination first gives rise by its dilata-
tion to the pharyngeal cavity (Fig. 179 B, p), while the intestine
proper is derived from a solid cone of cells which develops at the
base of the invagination. This cone very soon develops a lumen at
first closed at both ends, and this becomes differentiated into the
esophagus, the stomach and the intestine, the rudiment of the
digestive gland also becoming visible. ‘The rudiment of the intestine
opens only later into the atrial cavity (cl). The latter develops later
than the pharyngeal cavity from an independent dorsal ectodermal
invagination (Fig. 181, c/) which, as it enlarges, comes into close
contact with the posterior wall of the pharynx. In this way the
transverse and somewhat diagonally-placed branchial lamella arises,
in which the four pairs of gill-clefts found in this generation (Fig..
245, p. 475) soon appear in the form of small round perforations.
According to Unsanrn, the two pairs that lie dorsally develop before
those that lie ventrally.
um Maileri (after ULAsINi. 7.
lion; 1d, branchial uerve.
181, -Dersal region of an older lar
atrium : 77, ciliated pit; 0, uuscle-ho«
Only the middle part of the rudiment of the central nervous system
(Fig. 181, 7) retains its original massive character, while the anterior
and posterior ends soon become narrower In the anterior narrowed
portion, an irregular cavity develops and breaks through into the
pharyna. At this point, the ciliated pit (7) appears and a delicate
connects it with the sub-ganglionic body (the homologue of
‘ylanle hypophysaire” of the Ascidians). From the middle
swelling of the neural rudiment, the actual ganglion and the sub-
ganglionic body develop, while the posterior narrowed portion gives
rise to an unpaired nerve which runs back wards (nernus branchialis, 9.
DOLIOLUM—LARVAL DEVELOPMENT. 387
Unsanty) in which we perhaps have the homologue of the ganglionic
cell-strand discovered by van BeyepeN and Joan in the Ascidians
(p. 376), ‘The peripheral nerves and the sensory organs develop
later, and among these the vesicular auditory organ which belongs
to the left side of the body deserves special mention (Fig. 245 A, ot);
the vesicle itself urises as an ectodermal invagination into which a
cell wanders and develops into the otolith. According to Unsantn,
the auditory organ of Doliolum AMfilleri remains throughout life a
mere cup-shaped ectodermal invagination,
ae
‘Pie. 182.— Transverse section through two outogenetic stages of Doliolum (after
ULsantx). A, section through anterior ion of the body at a stage somewhat
holder tan that depicted in Fig. 179 4; 2, section through an older stage. d,
Bester ee corer ee iach rotitents av tre hears ond pertains
ya, mesoderm; max’, mesoderm of the ventral stolon; a, rudiment of the nervous
system ; p, pharynx,
The mesoderm of the anterior region of the body gives rise
principally to the musele-hoops (Fig. 181, m), the pericardial rudi-
ment (Fig. 182 B, hk) and the mesoderm-mass (ms’) of the ventral
proliferating stolon of the “nurse” stage (the rosetie-skaped organ
of Kerersrein and Exvers), Two cell-groups become separated
posteriorly und ventrally from the mesoderm-layer which envelops
the pharyngeal cavity like a mantle ; one of these, in close contact to
the ectoderm, becomes the mesoderm of the ventral stolon (ms’),
while the other, near the rudiment of the alimentary canal, changes
into the pericardial vesicle (A), a cavity appearing within it round
which the cells become arranged into an epithelium, As in other
‘Tunicates, the heart is derived from a dorsal invagination of this
388 TUNICATA.
vesicle.* It should here be mentioned that the dorsal closure of the
cardiac tube is brought about by a histologically differentiated lamella
(the “ mittelfeld” of GRoBBEN) as to the development of which, how-
ever, we have no detailed information, but we are reminded of the
participation of the epicardial lamella in the formation of the heart
in the Ascidians (vAN BENEDEN and Juuin, p. 370).
The muscle-hoops develop in the way described by Leuckarr for
Salpa democratica (see p. $31), through the fenestration of the meso-
dermal lamella, these perforations separating one muscle-hoop from
another.
In this way, the general course of the most important systems of
organs occurring in the first ‘‘nurse” generation is indicated (Figs.
180, 243, 245, p. 475). To these, two stolons connected with the
formation of buds have to be added. One of these (Figs. 180, r, and
245 A, rs) lies behind the fifth muscle-hoop, close to the posterior
end of the pericardial vesicle; this we shall call the ventral stolun
(the rosette-shaped organ). The second or dorsal stolon (Figs. 180, d,
245, ds) + arises from the dorsal surface in the seventh intermuscular
space and forms a geniculate process pointed posteriorly, into the
base of which an open coil of the seventh muscle-hoop extends (Fig.
243, st’). We shall have to enter into the details of the structure
and development of these stolons and of their relation to the forma-
tion of the subsequent generations later (p. +70).
After the young barrel-shaped ‘“ nurse” has developed fully, the
provisional larval orgaus gradually atrophy. While the internal
parts undergo futty degeneration and the cells become mixed with
the blood, the ectodermal envelope yradually shortens so that the
ectodermal vesicle and the larval tail soon form merely a rounded
prominence on the body of the nurse”. This outgrowth strikingly
resembles an embryonic organ consisting of reserve nutrition found
in the Thaliacea, the so-called elaeoblast, a fact which makes the
derivation of the latter from the transformed tail of the Ascidian
larva, attempted by SALENSKy, appear somewhat probable (see below
p. 432) :
The first “nurse” generation of Doliolum, at a later period, as
Fou first pointed ont, undergoes a remarkable metamorphosis, the
*GRoBBEN’s statements as to the formation of the heart in the larval
Doliolwm have been misunderstood and misrepresented by ULsanin.
+ (This is better termed the dorsal outgrowth, as it does not itself give rise
to buds, but receives those structures in rudiment from the ventral stolon and
only gives attachment to them (pp. 472-476).—Eb.}
PYROSOMA—EMBRYONIO DEVELOPMENT. 389
gills, the endostyle and the whole of the alimentary canal degenerat-
ing completely, while the muscle-hoops considerably increase in size,
and the neryous system develops correspondingly. ‘The “nurse”
then, like a swimming bell of a Siphonophoran stock, carries out the
locomotory function, while the nutritive and respiratory functions
of the whole stock are fulfilled by certain laterally- placed buds (tropho-
avoids) on the dorsal outgrowth.
4, Pyrosoma, |
The development of Pyrosoma from the egy resembles in many
respects that of the Thaliacea, Embryonic development takes place,
as in them, within the body of the mother and is consequently direct
or abbreviated. It even takes place, as at first in the Thaliacea,
within the egg-follicle. Pyrosoma is, however, specially distinguished ;
(1) by the large amount of food-yolk in the egg, which leads to a
discoidal cleavage and the development of a germ-dise and (2) by
the early asexual multiplication of the embryo, The primary indi-
vidual which develops from the embryo and which has been called
the Cyathorooil by Huxuery, at an early embryonic stage, gives rise
by a kind of transverse fission to four more individuals, the first
Ascidiozooids of the colony (Fig, 193, ete),
We owe our knowledge of the embryonic development of Pyrosoma
ebiefly to Huxney (No. 72), Kowanevsxy (No. 71), and SALeNsky
N. 74).
A. Cleavage and Formation of the Germ-Layers.
Only a single egg matures in the genital rudiment of the Axscidio-
zooid which has arisen through budding, as also is the case in the
Thaliacea. Part of the remaining cell-material of the so-called genital
strand becomes arranged round the egg as the follicle, while another
part is used up in forming the rudiment of the testes and of the
oviduct which appears as an outgrowth of the follicle. The egw
wrows greatly by the addition of food-yolk, so that finally the forma-
tive yolk and the germ-vesicle within it form «# mere prominence
upon the large yolk-sphere (Fig. 183 A). After the oviduct has
hecome connected with the atrial cavity, spermatozoa pass into it
and remain in it until the egy is ready for fertilisation ile the
oviduct partly degenerates, At the same time, an active immigra-
tion of follicle-cells takes place into the space extending between
the surface of the egg und the folliculur epithelium (Fig. 183 A, 2).
|
390 TUNICATA.
These cells, which have been called by Kowangvsky inner follicular
cells and by Sauensky kalymmocytes, aud as to the det of
which from the follicle-cells there can be no doubt, are homol
with the test-cells of the Ascidiaus and the inner follicle-cells of the
Thaliacea (SALENSKY's gonoblasts). According to SauEnsiky, they
take a certain part in the formation of the embryo here as in the
Thaliacea, The statements on this subject, however, appear ty us
somewhat inconclusive.
An epithelial lamella further becomes separated from that part
of the inner surface of the follicle which lies next the oviduct (Pig.
184, ds); this covers the germ-dise like a cap und represents a
secondary germ-envelope that takes no further part in the develop
ment of the embryo, ‘This has been called by SaneNsky the cover
ing layer.
Fic. 183.—A, lateral aspect of the ogg of Pyrosonut, showing the first ane,
the germ-line of Pyronoma at the sit-oellad stage, viewed. from eae eee eae
LEVSKY), ft, inner follicle-cells.
The cleavage of the egg of Pyrosoma, first made known through
the investigations of Kowatevsky, is discoidal and recalls that of
the Teleostei. The first stages seem to have a fairly regular course,
the germ becoming divided into two, four and eight blastomeres by
the successive appearance of meridional furrows (Fig. 36). ‘The stage
of three blastomeres observed by SALENSKY and that of six found by
Kowaueysky (Fig. 183 2) must be regarded as accidental
larities. We have no further details as to the course of cleavage,
but its result is a so-called morula-stage (Fig. 184 B) in whieh the
yerm-prominence is composed of blastomeres apparently irregu
arranged and already forming several layers.
‘The numerous inner follicle-cells (kalymmocytes) wander by imeans of
amoeboid movements into the spaces between the blastomeres
and are even able to penetrate the cell-substance of the latter,
- il
PYROSOMA—CLEAVAGE OF THE EGG. 391
bethe case, however, only in the first stages of cleavage and to have no further
significance, since the follicle-cells, as it appears, do not remain inside the
blastomeres. Many inner follicle-cells, as cleavage advances are, however,
found scattered between the blastomeres (Fig. 184 2B, fe) and, as the distinc-
jon of size between the two kinds of cells disappears when the blastomeres
divide further, and the original histological character of the inner follicle-cells
canalso no longer be recognised, it is finally impossible to distinguish the follicle-
cells from the actual germ-cells or blastomeres. For this reason, and because
Pie, 184. —Sections of two germ-dises of Pyrosoma (diagrammatic after SALENSKY),
he aight celled stage ; B, older sage. 6, blastomeres ; do, food-yolk ; dx, covering
layer ; dz, yolk-cells ; f, egg-follicle ; fs, immigrated follicle-cells ; ar, oviduct. ri
Sanensky was unable to find follicle-cells showing the commencement of dis-
integration, this author concluded that the inner follicle-cells take part in the
formation of the embryo, a view resembling that held by him in connection
with the Thaliacea (p. 420). We consider it to be more probable that the
embryo here; as in the Thaliacea, is produced solely by the blastomeres, and
that the follicle-cells which wander in between the blastomeres undergo final
disintegration, [See footnotes, pp. 420 and 421}.
392 TUNICATA.
Mention must now be made of cells which, in the later stages of
cleavage, are found in large numbers in the yolk, near the point at
which the germ-dise lies on it, and which may be called yolk-cells
(Fig. 184, dz). Sauensky, who traced back these cells to follicle-
cells that had immigrated into the yolk, has named them the yolk-
kalymmocyter. Since, however, as we shall see below (p. 395), these
yolk-cells take part in the formation of the intestinal wall, we are
inclined to regard them as blastomeres belonging to the entoderm-part
of the germ-mass. We here have a repetition of the conditions found
in the meroblastic egg of the Vertebrata, in which also volk-cells (to
be considered as entoderm) are said to take a similar part in the
formation of the intestinal gland-cells.
For information as to the formation of the germ-layers in Pyrosoma
we are dependent entirely on SaLeNsky's statements. The embryo,
after a number of cell-
divisions, lies on the yolk
as & prominence composed
of uniform polygonal cells
which are irregularly distri-
buted. This prominence
soon becomes bilaterally
symmetrical, the largest
mass of cells collecting in
the posterior half of the
germ-dise, so that the pos-
ities terior slope is more abrupt
do foods te velkells than the anterior (Fig. 183).
terior : r, anterior. According to SALENSKY, the
separation of the germ-layers
takes place throngh delunination, the most superficial cell-layer (er)
first becoming arranged inte an cpithelium (ectoderm) ; the mass that
remains (the meso-entoderm) then undergoes a similar transformation,
the lowest layer, that in contact with the yolk, becoming separated
as the intestinal epithelium (entoderm). Between the ectoderm and
the entoderm the mesoderin extends, being greatly developed in the
posterior half of the germ-dise while, in the anterior half, it is want-
ing or elxe is represented merely by a few cells (Fig. 186.1 and &).
Taking into account the process of formation of the germ-layers in the
meroblastic eggs of the Vertebrata, we may perhaps be allowed to conjecture
that in Pyrosoma also the separation of the germ-layers is not an actusl
delamination, but an invagination or infolding of the posterior edge of the
germ-dise, such as, for instanc ‘urs in the Selachians.
=
PYROSOMA—DEVELOPMENT OF THE CYATHOZOOID, 393,
At the time when the separation of the germ-layers takes place
three systems of cavities appear in the mesoderm (SaLENsKy) these
being connected with invaginations on the lower (entodermal) surface
of the germ-dise. One of these invaginations is rather large and lies
near the posterior edge of the germ-dise (Fig. 185, ch), It is con-
nected with a system of cavities ranning forward in the median line
of the disc. The two other (paired) invaginations (Fig. 186 4, «)
Fie. 186.—Two transverse sections through a young rin-dise of Pyrosome (after
SabeNeky), A, through the posterior, and 4, through the anterior region. 6, coclom ;
oh, cavity of the chordn ; ec, ectoderm ; en, entoderm ; ms, mesoderm ; u, nervous
system.
lie laterally and somewhat in front of the first and probably com-
municate with the lateral system of cavities.* These are regarded
hy Saensky as the rudiments of the coelomic sncs, and the axial
system of cavities as the equivalent of the chorda, SabENsky was
unable to decide whether there are here a number of separate spaces
serjally arranged or a continuous, but somewhat bent longitudinal
canal.
B. Development of the Uyathozooid.
The next changes to be noticed in the germ-lisc are the appearance
of the rudiment of the nervous system of the Cyathozooid and the
development of the peribranchial sacs. The nervous system arises as
an ectodermal thickening in the anterior part of the germ-dise (Fig.
187, n), which later becomes depressed us a furrow, and in this way
* [Savensky figures the paired coelomic invaginations at an earlier stage
than that shown in Fig. 186 A, and further he regards the depression seen on
the right of the swelling containing ch as the coelomic invagination, and in
his figure, of which the above is a copy, letters it as such.—Ep.]
—
394 TUNICATA.
orms the vesicular rudiment of the ganglion. In enoss-seetions this
anterior part of the germ-<disc is seen to be bilaminar (Fig. 186 2),
as the mesoderm of the germ-dise does not extend so far forward,
Fic, 187.— Two germ-dises of Pyrosoma (after Kowanevsky). ”, radi of the
hervous system ; 0, aperture of one rats peribranchial tubes ; p, peribranchial
cavity (tube).
The two peribranchial sues or tubes appear as ectodermal invagina-
tions (Fig. 187 4, p) directed from before backward, which seo
lengthen (Fig. 187 /) and show, at the anterior end, the original
aperture of invagination (9).
The two anterior ends with
their apertures unite in.
front of the rudiment of
the nervous system (x)
(Kowaneysky), and thus
give rise to the atrial
aperture (Fig. 189, ef) of
the Cyathozooid. Aceord=
ing to Savensky, on the
contrary, the latter is pro~
duced by an unpaired ecto
dermal invagination with
which the anterior ends of
Fro, 188,—‘Trausverse sections through the germ- ; z
\lise of Pyriwmn, ut the stage depicted in the peribranchial tubes
Fig. 187 1 (after Sanensky), A, through the ii
auterlor part of the disc, with théxudiment of °Ome into, contaay the |
the nervons system ; 2, through the zladiaae original apertures of
ity;
withthe peribranchial snes, dh, enteric em 3
ds, yolk-cells ; ec, ectoderm ; ev, entoderm; n, ttbes having closed
rndimant of the nervous system ; », invagina- the formation of the
tions of the peribranchial sacs.
(SALENSKY),
PYROSOMA—DEVELOPMENT OF THE CYATHOZOOID. 395.
In the meantime the germ-dise has separated somewhat from the
surface of the food-yolk (Fig. 188). The cavity thus formed is the enteric
cavity, which originally appears covered by the entoderm (e) only on
its upper surface. At a later stage the entoderm covers the whole of
el
pe
Fra, 189.—Germ-dise of Pyrimona with the atrial otilice sleveloped (after Kowa-
LBVSKY). cloaca ; en, endostyle ; x, nervous system ; p, peribrauchial tubes ;
pe, pericardial snc ; pe’, the posterior tubular continustion of the same,
the cavity, its lateral edges bending downward and growing towards
one another (KowaLevsky). According to SaLensky, the yolk-cells
also take part in this ventral closure of the enterie rudiment (Fig. ”
188, dz) by coming to the surface of the food-yolk and changing into
Pig, 190. —Transverse section through the eeaeinaat region a germ-ise at the stage
apes in Fig. 189 (after Savenskr). ch, enteric envity ; ec, ectoderm ; en, entox
A ef inet if the ‘erslontyle i Muay, meeodormn ; po, paribranchlal Gabo | pc;
parla tube,
‘epithelial cells of the entoderm (p. 392). While the enteric rudiment
in this way becomes a tube closed on all sides (Fig. 190), a median
Gnfolding, the rudiment of the endosty/e (es) becomes visible in the
Posterior half of its upper wall.
Mi
396 TUNICATA.
‘The transformations undergone, in the further course of develop-
ment, by the paired coelomic snes, the lumina of which had become
Fic. 191.—Three
cavity ; 3, 1
Py peribranchial t va
connected posteriorly, are of importance, Only the rig]
sac is retained (Fig. 191, re), while the left * undergoes di
(ig, 191 4-C), its lumen becoming smaller and its cells Tos
8
-tlises of 1 diagrammatic (afer Sabey
Samos fa ‘a it fei wa einen of the
ial sae = ve, Tight enelomic sae,
wo embryos of Myrosene (after KowaLeveky),
il. em, endostyle-fol tern 5
"ut tube; pr, per eee
ith y dial aac ; 2, posterior part of thes germ:
the surface of the egg (rudiment of the stalon); =, cell-some,
"The terms " right” and “left refer to the arrangement of |
‘the adult Cyathozovid, in which the atrial aperture denotes the |
‘of the body. As, in our orientation of the germ-dise, the atrial |
at the anterior edge of the disc, the right and left organs to
Onur orientation of the germ-dise is, however, intentional (p. 404),
apenas: in accordance with the views of authors, since ox
id also lead to certain difficulties in describing the procosses (és
jn connection with the development of the ‘Ascidionsnid),
PYROSOMA—DEVELOPMENT OF THE CYATHOZOOID. 397
epithelial continuity, so that finally, only a mass of irregularly
arrange cells remains to take part in the formation of the mesenchyme
which is developing in the primary body-cavity. A similar disin-
tegration is undergone by the median strand which was regarded as
the equivalent of the chorda, and which, after the disappearance of
its lumen, retains its independence only for a short time, and is called
by SALENSKY the axial mesoderm-strand, ‘The right coelomic sac
gives rise to the pericardial rudiment (Figs. 192 C, 189, 190, po),
thozooids with their tirst-formed buds (utter KoWALEVSKY, some-
I straight stolon; 2, with curved stolon ; the Cyathozooid
from the surface of the food-yolk (id), cf, atrial aperture ; ,
. rudiment of the endostyle ; 2, body-cavi ¢ Cyathozooid |p;
vathozooi
3 comme
food-yolk ;
peribranchial tubes; yc, perivardial sav of the
which soon becomes club-sxhaped, a dilated, anterior, sac-like part
changing into the pericardial vesicle of the Cyathozooid, while the
tube that runs backward from this part does not develop further but
soon loses its lumen : the connection of its cells then becomes loosened.
It appears that these elements then mingle with the mesenchyme
and assist in the formation of the mesoderm of the Ascidiozooid. In
the pericardial sac of the Cyathozooid the surface adjacent to the
intestinal wall is seen to thicken, this part then becoming invaginated
and forming the rudiment of the heart proper.
body-cavity of the Cyathozooid (Fig. 193 B, 2).
up into separate islands (Fig. 194, 2) which are
visible near the surface of the food-yolk. Kowa
the elements of this cell-zone took no further i
of the embryo, or at the most changed into bloo
‘SADENSKY axeribes to them a very important part
of the mesoderm of the Ascidiozooid (see below, p
©, The Development of the Primary T
The edge of the germ-dise, by continu
the yolk-sphere (Figs. 193 B, 194), which
merely by the follicular epithelium. The fo
comes to lie inside the Cyathozooid, i.e, in
part is taken in this circumerescence, howe
of the elongated dise (Fig. 192 B, x), Tt
vows out into a long sac-like appendage (Fig.
by transverse furrows into four sections (1
the tape-worm); these sections are ede
Aseidiorovids. This chain of Ascidiozooids,
stolon, and is evidently homologous with the m
and Salpa, is originally straight, lying parallel to t
of the Cyathozooid (Fig. 193 4). Later, howe
it curves and finally lies equatorially (Figs, 194,
the Ascidiozooids form a ring surrounding the
Cyathozooid. The individual Ascidiozooids at t
their positions ; at first they lay with their k
same direction as that of the whole stolon (Fig.
is a tendency for these axes to lie purallel to the pr
Cyathozooid (Fig. 196). The stolon then, as a
of zig-zags, as the thin, drawn ont trabeculae
necting the individual Ascidiozooids lie obliquely, a:
posterior end of one zooid to the anterior end of
YROBOMA—THE PRIMARY TETRAZOOID COLONY. 399
While the four Aastdioacias continue to increase in size and develop
the structure of the adult individual (Figs. 194-196), the Cyathozooid
whieh lies in the midst of them gradually utrophies (Fig. 196, ¢).
Only now (Fig. 196 B) does the colony, which is enveloped in « large,
common cellulose mantle, attain an independent existence. It passes
out of the parental brood-sac into the cloaca of the colony, and thence
to the exterior, The youngest free colonies of Pyrosoma are only
found at considerable depths (Cuun), but older and larger stocks are
met with at the surface of the water.
Ftc, 194,—T'wo staze> in the development of the colony of /yrosum (after Kawa~
Levsky). In A, the yolk-mass (do) is partly surrounded by the Cyathozooid (¢),
while, in B, it Hes entirely orth the body-cavity of the latter. ¢; Cynthozooid :
el, atrial pore ot the Cyathozooid ; d, alimentary canal of the Cyathozooid ; do,
io ; el, elacoblast ; en, endostyle of the Ascidiozooid ; 1, olllated
snglion of ibe Cyaan: i, imhalent orifice of the Asoidiouoaid ; a, gill-
oer ae of id ‘Cyathozooid ; m, cellulose mantle ; m, nervous system of
MNORSSIA Pe ribranchial cavity ; sn, Interal nerve ; 0, eutodermal canal
ee the Ascidiozooids with one another ; 2, remains of the cell-zone.
The organ-rudiments of the young Ascidiozooid chain are, originally,
direct continuations of the imperfectly developed organ in the Cyatho-
woid (Figs. 192 8, 193). The ectoderm of the chain is in continuity
with that of the germ-disc, The intestine of the Cyathozooid is
continued into the enteric rudiment of the Ascidiozooid, and the
endostyle-fold also proceeds direct from the rudiment of this fold in
the germ-dise (Figs. 192,193,en). In a similar way the peribranchial
400 TUNICATA,
tubes (p) on either side of the intestine, and, on the right side of the
body, the pericardial tube (Fig. 192, pc), are continued direct from
the Cyathozooid into the chain of Ascidiozovids. The central nervous
system, on the contrary, arises independently in the Ascidiozovids
(SaLENsky). From this point the development of the Cyathozooid
and that of the Ascidiozooids will be treated separately.
D. Further Development of the Cyathozooid.
The structure of the Cyathozooid is fairly simple. One of the poles
of the body is marked by the presence of an ectodermal invagination,
the atrial invagination (Fig. 193 B, 194, cl) which occupies the most
anterior end of the germ-dise (Fig. 189, cl), and the origin of which
has already been discussed (p. 394). This invagination originally
communicates with the peribranchial tubes (Figs. 189, 192 B, 393
A, p). Very soon, however, that part. of these tubes which lies in the
Cyathozooid degenerates and completely disappears (Fig. 193 8).
The atrial invagination, on the contrary, in which a narrow thick-
walled portion can be distinguished later from a thin-walled portion,
the actual cloaca, becomes connected with the alimentary canal of
the Cyathozooid, the lamellae that separate the two cavities being
perforated (Fig. 194). he alimentary canal (Fig. 194, d) of the
Cyathozooid is a simple thin-walled sac, narrowed in the shape of
a funnel posteriorly, which adopts a somewhat curved position in
accordance with the curving of the stolon in later stages. — Its
posterior, narrowed end passes over into the enteric rudiment of
the Ascidiozooids. There is no sign of endostyle-folds in the intestine
of the Cyathozooid, that part of the organ in which, in the germ-dise,
the rudiments of these folds appeared (Fig. 189 ¢2) being nsed up
in the formation of the Ascidiozooids (Fig. 193, e7).
The rudiment of the nervous system of the Cyathozooid, whieh is
derived from an ectodermal invagination lying close behind the atrial
rudiment near the anterior margin of the germ-dise (Fig. 189, 1) is
originally a somewhat long and completely closed vesicle which, when
the alimentary canal changes its position, in consequence of the
curvature of the stolon, also shifts from its original position. The
posterior end of the neural vesicle now enters into open communica
tion with the enteric cavity (Figs. 194 B, y, and 197). This is the
rudiment of the ciliated pit (ff). The anterior part of the vesicle
now becomes divided by a farrow from that part which is used for
the formation of the ciliated pit: it swells and changes into the rudi-
PYROSOMA—FURTHER DEVELOPMENT OF THE CYATHOZOOID, 401
ment of the ganglion (y). This part gives off two lateral processes,
the rudiments of the lateral nerves (sv) which clasp the enteric canal
(d) laterally and end in the lower wall of the atrial invagination.
Within the ganglionic rudiment of the Cyathozooid there is never any
development of punctated nervous tissue (Punktsubstanz),
The development of the
heart has already been de-
scribed (Fig. 193, pe).
The cetoderm of the
Cyathozooid yields the cel-
lulose test of the young
colony, The secretion of
this layer begins even before
the ciroumeresence of the
yolk-sphere by the Cyatho-
zooid is fully completed
(Fig. 194 4, m). The area
occupied by the Cyathozooid
is then indicated by the
extension of the cellulose
mantle which, at a later
period, encloses the four
primary Ascidiozooids (Figs.
194, 195). ‘The ectoderm
of the Asoidiozooids does
not, according to SALENsKy,
take part in the formation
. Fi, 195,—Later ontogenetic stage of a tetrazooid
of the cellulose investment colony of Pyrosowa (after SaLensxy). The
of the young colony, which mn calelies mantle Gf the thetheoaa kas
is yielded exclusively by the grown round the Ascidiozooids. ci, atrial
Cyathowoid. ‘The process ual en, euostle 1, inion? orice ES
by which this mantle is gill-clefts ; m, mantle ; x, cell-lamellae of the
cellulose mantle ; =, remains of the cell-rone,
secreted agrees pretty
closely with that described by Kowaxevsy for the Ascidiacea (p, 355),
According to Sanansky, single mesoderm-cells (mesenchyme-cells),
wandering through the ectoderm, come to lie on its external surface
(Fig. 198, ms) and are the cells found later in the test. The secretion
of the latter which now takes place proceeds from the ectoderm-cells
(ec), each cell giving off externally a perpendicular plate-like process, so
that the cellulose layer which is secreted seems broken up by these
processes into separate prisms. At the same time, the mantle-cells
DD
ae
‘TUNICATA.
(wandering mesoderm-eells) become arranged in a
way in Pyresoma. ‘They form rows, which are note
‘way as to produce hexagona] areas (Figs. 195, 1 pers
later stages the processes of the ectoderm-cells
test are again withdrawn. ‘The hexagonal p
by the arrangement of the mesoderm-cells is, | 10
some time.
Fig, 196.—Two later ont
KoWAaLevsky). ¢,
Boao oni
, lnnguets ; +, youn! ive strand
ery sand 4”, Gunectins versus Veerouaaae ene
of the cell-zone.
According to Sanensky, the ectoderm of =
capable of secreting cellulose substance on its
accounts for the fact that the ground sultans
which fills a part of the primary body-cavity is
the development of this -intermediate layer,
becomes removed « considerable distance from
Cyathozooid.
The simplicity of structure of the Cyatho :
we consider that its sole function is to give asi y
four primary Ascidiozooids, As soon as these
PYROSOMA—FURTHER DEVELOPMENT OF THE CYATHOZOOID, 403
degree of development, the Cyathozooid begins gradually to degenerate,
[t can then be seen as an oval body (Fig. 196, ¢) gradually decreasing
in size, at the centre of the young tetrazooid colony. — Its atrial
wperture (ef) closes, and it is gradually
absorbed till not a trace of it is left.
Kowavevsky thought that the atrial cavity
of the Cyathozooid persisted as the common
cloacal cavity of the whole colony; this,
however, is in opposition to the above
account by Sauensky. In accepting the
latter account we must assume that the
common cloacal cavity of the colony repre-
sents a depression of the surface of the
cellulose mantle which appears later.
We have still briefly to describe the way
in which the structure of the Cyathozooid
is to be interpreted and to compare it with
that of a typical Tunicate (Fig. 199), and
in connection with this we must explain
why, together with KowaLevsky and
Savensky, we regard the ectodermal depres-
sion in the Cyathozooid just mentioned as
the atrial cavity (c). The fact that the
peribranchial tubes open into this cavity is
in favour of interpreting it in this way, and
this view is still further confirmed by the
system with relation to this invagination.
Big). d, Ge ne
sea sole
a
position of the nervous
If it is to be regarded as
the inhalent aperture, the ciliated pit (/l) of the central nervous
system would have to
be directed towards it,
but this is not the
case. The ciliated pit
forms the part of the
central nervous system
(g) which is turned Fic, 198.—Transverse section through the developing
test of the Cyathozooid in the stage depicted in ns
away from the invagi- 195 (after SateNsKY),
¢, cellulose substance ; eo, ecto-
nation. Acomparison — derm; ms, mesenchyme-cells ; ma’, mesenehyme-cells
of a diagram illustrat-
forming hexagonal pattern in test (of. Fig. 196).
ing the relative positions of the most important organs in the body
of a solitary Salp (Fig. 199 #) with a diagram of a Cyathozooid
ie
one representing the intestinal tube, the —
consecutive individuals, and the two lateral
tubes (p). These tubes are originally
ions of the corresponding org:
192 B). When, at a later period,
PYROSOMA—THE FOUR PRIMARY ASCIDIOZOOIDS. 405
become more markedly constricted from one another, these rud
ments also are cut up into sections corresponding to the different
individuals. The peribranchial tubes become completely dissevered
at the boundaries of the individuals (Fig. 193 2), each Ascidiozooid
then containing a pair of lateral closed sacs, the peribranchial cavities
(p). The remains of the peribranchial tubes in the Cyathozooid, as
already mentioned (p. 400) then atrophy. The enteric rudiment,
however, does not at first undergo such complete constriction
between the individual Ascidiozooids, but a canal is retained in the
connecting trabeculae which establishes communication between the
consecutive individuals (Fig. 194 B, »). This canal does not dis-
integrate until. the Ascidiozooids separate completely on attaining
their full development. A vestige of it plays an important part in
‘the later development of buds, changing into the so-called endostyle-
process or entoderm-process of the stolou (SEEDIGER, p. 486).
The rudiment of the endostyle-furrow was recognisable in the
primary enteric rudiment of the Cyathozooid even before the chain
of Ascidiozooids began to form (Figs. 189, 190, en). That part of
the alimentary canal of the Cyathozooid which was distinguished by
the presence of the endostyle-rudiment was the ‘part which, by
lengthening, changed into the common enteric rudiment of the
four primary Ascidiozooids, Consequently, the endostyle-rudiment
originally runs through all the four Ascidiozooids (Fig. 193, en). At
‘a later stage, however, only the posterior part of it is retained in each
individual (Figs. 194, en, 200, es), becoming the definitive endostyle
of the zooid, In cross-section, the endostyle-rudiment originally has
the form of a broad fold projecting into the lumen of the intestine
(Fig. 203 A), the lateral part of which shows epithelial thickenings
which, in a surface view, appear as dark bands. The depression
between them flattens out later, but, on either side, the paired
endostyle-folds project inwards (Fig, 203 B, es). It is not yet
clearly understood in what way these lateral endostyle-folds together
with the middle part yield the endostyle of the adult.
Tn front of the anterior end of the endostyle-rudiment there is, in
the developing Ascidiozooid, a pit-like ectodermal depression (Figs.
194, 200, #), the rudiment of the inhalent or branchial aperture.
‘The space in front of this is occupied by the rudiment of the central
nervous system (Figs. 194, 200, n).
The lateral walls of the branchial sac of the Ascidiozooid are in
contact with the peribranchial tubes (Fig. 193, p). Here the gill-slite
(ks) break through (Fig. 200), a small entodermal outgrowth fusing
a
406 TUNICATA,
on either side with the wall of the peribranchial sacs, and the
perforation then taking place in the base of the outgrowth. The
entodermal lamella thus takes a more active part in the development
of the gill-slits than does the ectodermal wall of the peribranchial
sac, The vertical bars between the adjacent gill-slits whieh, in
cross-section, are almost quadrangular, are not merely covered on
their inner surfaces with entoderm, but their lateral surfaces also
which are turned towards the slits belong to the entoderm. Only
the covering of the external surface is derived from the ectodermal
Fic, 200,—Diagramumatic views of an Ascidiozooid at the depicted in Fig. 1% |
(iollowing SaukNsky). 4A, viewed from above; B, from I thy i |
x (branchial sae) ; éd, rectum ; el elaeoblast ; ia rudiment of is
pha at inhalent ot branchial aperture; ‘ka,’ gill-slits ; stomach ; #,
oe, oesophagus ; p, peribranchial sacs; pe, vesicle; me,
7
rudiment of the lateral nerves.
wall of the peribranchial cavities, The gill-slits in Pyrosoma, acoorl-
ing to SALENSKY, appear in order from before backward, the most
anterior slit forming first. After the gill-slits have broken through, |
they very soon lengthen ; those of Pyrosoma, indeed, are distinguished
for their length. The internal longitudinal bars, which cross the
slits at right angles and give rise to the characteristic latticelike
appearance of the branchial wall, develop later as independent in-
growths from the vertical bars which become secondarily connected.
The gill-slits, as Servicer has pointed out, seem always to lie
at right angles to the endostyle (Fig, 201, 4s and ¢s), Since the
endostyle of the Ascidiozooids originally runs horizontally, as may
be seen in the diagram Fig. 201 4, and then later adopts a vertical
position (Fig. 201 C) the gill-slits pass gradually from a vertical t9 |
PYROSOMA—THE FOUR PRIMARY ASCIDIOZOOIDS, 407
an oblique position and finally lie horizontally. This leads us to the
general changes of form which characterise the development of the
Ascidiozooid, The longitudinal axis of the adult Ascidiozooid (Fig,
201 C) seems to be marked by the branchial and atrial apertures (i) ;
in the bud, however, these two apertures do not lie at the ends
! 4
Fig, 201,—Three consecutive ontogenetic stages of two Ascidiozoolds, side view,
diagrammatic (following Saumnsky). cl, atrium; @, alimentary canal; ¢, atrial
apertnre; ¢s, endostyle ; i, branchial aperture ; ks, gill-clefts; m, nervous system ;
p, peribranchial cavity.
of the longitudinal axis (Fig, 201 4). This shows that through the
changes brought about by growth during the course of development,
the longitudinal axis of the bud is replaced by a new one running
at right angles to it. The longitudinal axis of the young Ascidio-
zovid, the two poles of which are represented by trabeculae join-
ing one Ascidiozooid to another becomes, later, the transverse axis of
i
PYROSOMA—THE FOUR PRIMARY ASIDIOZOOIDS, 409
cavity. The rudiment of the stomach and intestine is found on the
lower surface of the entoderm-tube in the form of a blind horseshoe-
shaped diverticulum (Fig. 200 B) closely bent upon itself, the free
ends being directed anteriorly, The two limbs of the horseshoe are
seen cut through in the cross-section given in Fig. 204 (o¢ and ed),
One limb (ed) of this rudiment separates early from the entoderm-sac
and, as a blind diverticulum, represents the rudiment of the intestine,
while the other gives rise to the stomach and oesophagus. The com-
munication between this latter limb and the pharyngeal cavity is re-
tained as the entrance to the oesophagus (oe). Only in later stages
does the blind end of the intestine become connected with the atrial
cavity and the alimentary canal attains complete development with
the appearance of the so-called digestive gland,
The central nervous system arises in the most anterior region of
the Ascidiogooid as an ectodermal invagination (Fig. 202, n) on the
upper surface of the body, which soon becomes separated from the
ectoderm as an elongate closed vesicle. This, at a later stage, becomes
triangular (Fig. 200, n). The anterior part * of this vesicle forms the
rudiment of the ganglion proper principally by a growth of its upper
wall. From this region, two lateral hollow processes arise (Fig 200,
mm) which, at a later period, grow and embrace the sides of the ali-
mentary canal. These are the rudiments of the lateral nerves which
thus arise here in a way similar to that described by SeenicER for
the later buds of Pyrovoma, The narrowed portion of the neural
rudiment whieh is directed towards the branchial aperture becomes
connected with the ectoderm of the alimentary canal, and, by a per-
ce in relation to the long axis of the bud, posterior in the adult.—
ia clotoeaa cae nella ane ;
a bn OEM EE eee
| which is to some extent derived from inner
but for the greater part from elements of the disit
sac, is the principal source of the whole of th
diozooids, It must indeed be pointed out tl
as x prolongation of the pericardial rudiment
breaking up into separate cells, may also co
Ascidiozooids in the cell-zone which, in its tu
from the elements of the disintegrated left co
gration of the mesoderm into the germ-stock (
zooids) takes place first in the form of an
cell-masses, Later, when the cell-zone has b
islands, detached cell-groups or single elen
hody-cavity of the Ascidiozooids.
The mesodermal elements become distribute
cavity of the Ascidiozooids. Two groups of the
up a definite position in the posterior region of
side of the body, and their elements are found
AN
PYROSOMA—THE FOUR PRIMARY ASCIDIOZOOIDS.
411
layers, the external layer (Fig. 203 A, e/) being the rudiment of the
elacoblast, and the inner that of the so-ealled pericardial strands,
which must not be confounded with the pericardial tube mentioned
above, an organ that disappears at an early stage (Fig. 203 A, pe and
pe’). The cells of the elacoblast-rudiment soon increase in size and
form a rather high eylindrical
epithelium. At a later stage,
they are less regular in their
arrangement, vacuolesdevelop
within them and they change
into large elements, resem-
bling vegetable parenchyma-
cells, and thus assume the
features characteristic of the
elacoblast-tissue (Fig. 203 C/).
The elacoblast is here at first
paired and consists of rounded
groups of cells lying in the
posterior part of the body
which cause the surface of
the body to bulge out some-
what (Figs. 194 and 200 ed).
The inner layer of the
paired mesodermal rudiment
just described gives rise to
two cell-strands which develop
differently (Fig. 203, pe, pc’).
The strand to the right
extends somewhat further
forward than the other, and
its anterior end becomes
transformed into a closed
vesicle, the pericardial vesicle
(Fig. 203 # and C, pe). The
heart develops through the
thickening of that wall of the
vesicle which is in contact
Fic, 203.—Transverse sections through the
distal region of an Ascidiozooid of
in three consecutive stages of development
(after Sacessky),
ment of elaeoblast; en, entoderm; es, paired
endostyle-folds; g, genital strand ; pc, right
pericardial strand, or pericardial vesicle ;
pe, left pericardial strand.
ec, ectoderm ; el, rudi-
with the alimentary canal and its invagination into the vesicle,
The heart consequently forms here in the same way as in all other
Tunicates.
The right pericardial strand (Fig. 203, ye) is not completely used
i
a
412 TUNICATA.
up in the formation of the pericardial vesicle, but is continued pos-
teriorly. This mesodermal strand and the corresponding strand of
the left side (the so-called left pericardial strand, Fig. 203, pe’) do
not for the present develop further, but at a later stage, when the
Ascidiozooid has become independent and prepares to give rise to
fresh buds, these strands pass over into the proliferating stolon which is
developing and, on either side of the endostyle-process (entoderm-tube)
that is also continued into the stolon, form the mesoderm-rudiment
of the latter structure (SALENSKY), According to SALENSEY, there-
fore, the paired pericardial strands of the Ascidiozooid give rise, on
the right, to the pericardial vesicle of the Ascidiozooid, ancl, in their
further course, to the to mesoderm-strands of the proliferating stolen
(p. 486).
By the development of the peribranchial sacs and the above-men-
tioned mesoderm-formation in the lateral parts of the embryo, the
primary body-cavity is divided into two longitudinal sinuses, ont
above and the other below the alimentary canal (Fig. 203), called by
SALENsKy the supra-intestinal and the sub-intestinal blood-vinuses
In the region of the sub-intestinal sinus the mesenchyme-cells collect
(Fig. 203 4, g) and unite to form the genital strand belonging to the
posterior region of the Ascidiozooid. In later stages, according to
SALENSKY, a lumen appears in this strand (Fig. 203 €, g), =
which the cells become arranged
like an epithelium, but the lumen
disappears again in the course of
further development. The genital
strand not only represents the
genital rudiment of the Ascidio-
zooid in which it appears, but gives
rise to the genital strand of the
proliferating stolon of this zooid,
as will be described later (p. 484),
The rise of the genital organs in
the first four Ascidiozooids has 7 -
recently been deseribed in detail beter earn est Boose
by SeRLIgER (No. 76a). As it — SpeGNeeth ai
resembles that of the corresponding hee
organs in the zooids that develop branebial ca >
later we must refer the reader to
the description given on p. 493, merely adding here hanya
first four Ascidiozooids, the ovary degenerates, It was well-known te
PYROSOMA—THE FOUR PRIMARY ASCIDIOZOOIDS. 413
KowAatevsky that the small Pyrosoma colonies contained only male
zooids, females being found in the large colonies,
Auteriorly the supra-intestinal blood-sinus surrounds the rudiment
of the central nervous system, Further back it is continued into the
furrow which seems to be formed by the infolding of the endostyle-
rudiment. Since, between the nervous system and the endostyle-
rudiment the branchial aperture occurs, the stream of blood would
be interrupted but for a peculiar adaptation for conducting it further,
the upper wall of the intestine becoming folded in at this point
somewhat in the manner of a typhlosole (Figs. 204, 205, dh), This
fold within which the blood now rans becomes completely separated
from the dorsal wall of
the intestine, and then
forms a tube ranning
freely through the intes-
tine from the neural
rudiment (Fig. 205, n)
to the beginning of the
endostyle - furrow (en),
This peculiar structure,
which has been called
by Sanensky the pha-
bps greene P26. — Longa tion Shrvagh ay Asc
yy Huxney the diapha- of Pyrosomne MALENSKY). db, diapha-
ryngeal band, may be Sansui apeciers snp stomach? “ei spl De
compared with che gil mutt orn se eno oop
of the Thaliacea with
which SALENSKY even homologises it, although topographically it is
clearly only an analogous structure. The diapharyngeal band is
merely a provisional adaptation which atrophies in the further
course of development.
‘Two structures, the significance of which is obscure, the clongate and the
lenticular cell-masses (Kerersteix and Extmrs) are to be traced back to the
mésoderm. The lenticular masses are paired and symmetrical accumulations
‘of cells lying at the entrance to the branchial sac between the wall of the
peribranchial cavities and the entoderm (Figs. 196, 1, and 106, lm, p).
Sauenskr considers that they are to be derived from the kalymmocytes
{inner follicle-cells), They are said to be phosphorescent organs, The
elongate masses, which lie on the neural side “near the gills in the blood-
sinus,” form later from an unpaired accumulation of mesoderm-cells which
originally lies below the so-called languets of the entoderm (Figs. 196, d,
106, cm).
Mi
THE HEMIMYARIA (SALPIDAE). 416
points, such as the cleavage, the formation of the germ-layers and the
development of the placenta, the recorded investigations are ineom-
plete and contradictory, and we have statements which we must
hesitate to accept because they are at variance with all that is known
of the development of other Tunicates (and of animals in general).
Tm our account we shall have these difficulties to contend with
and must restrict ourselves to giving a brief survey of what at the
present time seems to be fairly well established. We cannot enter
upon the many contradictory uud obsenre points in connection with
this subject.
Among the Sadpidae, the sexual individuals (the forms belonging
to the chain) are hermaphrodite, but the time of maturation of the
male and female products differs. The individuals of the young
chain set free from the stolon of the “nurse” generation (solitary
FiG, 206,—Side-view of democraticu-mucronata (combined after con a
‘SALENSKY), a a) es trial aperture ; end, eudostyle ;
band ; i, brand ‘gill; n, nerv ion} niu, Hud!
en -folllele) ; gare cavity
ov, (conslating of a
the oviduct.
form) are at first female; they are fertilised by the individuals of
another chain, and each develops one embryo, Only as this develops,
do the testes become functional.
The Salpidae, as a rule, develop only one egg, The whole ovary
(Fig. 206, ov) consists, in such cases, of a single follicle which con-
tains the egg and is connected with the epithelium of the atrial
cavity * by a strand-like oviduct in which two portions can be dis-
% nperture of
"(Owing to the peculiar relations existing between the pharynx and the
ssi scanty fn Salva, i it becomes extremely difficult to do their limits ;
ratory Eoertanite (Athemhdhle) in commonly loosely applied to the
Aree chamber. We have, however, thought it za isable to d:
and to use in its stead the more specific terms atrial cavity anc
8 cavity in those cases in which we were able to determine the
‘of the general cavity which was being referred to.—Ep.]
i
|
416 TUNICATA.
tinguished (Fig. 207). One of these (st) is directly connected with
the follicle and consists of a single row of cells (the so-called stalk
of
narrowed
of the follicle) and the other is a dilated efferent portion (od), with
a distinct lumen. The egg-follicle lies at the base and to the right
ll
THE HEMIMYARIA (SALPIDAE). 417
of the nucleus near the oesophagus (Fig. 206) and is surrounded by
ramifications of the circumvisceral network of blood-vessels. The
oviduct also appears to be accompanied until near its aperture by
a vascular network (Fig. 207 B, 4). The point at which the oviduct
opens (Fig. 206, x) is found on the right side of the body behind the
penultimate musele-hoop above the nucleus. Round the aperture,
the epithelinm of the atrial cavity is thickened into a shield (Fig.
207 A, ep) and projects slightly inward (Fig. 207 B, ep). This
swelling is the rudiment of the epithelial prominence of SALENSKY
which Toparo calls the uterus.
The question now arises whether the members of a Salpa-chain, after the
birth of the mature embryo, remain sterile or are able to produce a new ovary
which may yield a new embryo, Some of Savunsky’s observations seem to
favour the latter yiew. The individuals composing a chain grow consider-
ably in size while the embryo is developing within them, so that the largest
individual contains the most advanced embryo, Savensky, in such a large
individual, found the remains of a placenta,
which indicated the previous expulsion of
an embryo and, side by side with this, «
mature egg or a quite young embryo.
Exceptions to the rule that each individual
of a chain produces only one embryo are
found in Salpa zonaria (Ouasisso and Escu-
micut), S. Thilesii (Known) and S. heragona
(Travstept), in which several embryos
develop simultaneously, although they are
not all at the same stage of development,
In consequence of this and of the presence
of a special point of attachment for each
embryo, Lxuckarr (No. 98) concluded that
several egg-follicles with distinct ducts must
bé present, These forms have recently been
united to form the genus Jasis (LAnILLE,
No. 38), the above feature being one of its
yeneric characters.
In many of the Saipidae (S. maxima, 8.
pinnata, 8. punctata) the, follicle appears to <
he incompletely divided by a longitudinal ~omal
Seem Oy) inisstwo chambers, one waaoiais euien eee
of which (the ovarian sac, ov) contains the, which this Cnt
egg during the stages of its maturation, while
the other (the embryonic sac, em) receives it
during the first embryonic stages. In many
forms (¢.7., S. maxima) the embryonic sac is
continued into a pointed process (s) which
soon degenerates. The remains of this process, in later stages, whew |
RR
——
416 TUNICATA.
tinguished (Fig. 207). One of these (st) is direetly connected with
the follicle and consists of a single row of cells (the so-called stalk
Fro, 207.—Female genital apparatus, .t, of Salpa pinata (after
‘Saipa wirgula (after Toano}. a, epithelium lintng
ipa vingula ( ( an
nt animal; 0, blood-sinus; ¢, egg-cell ; em, embryonic chamber;
Tromivenoe 7 fllicle; m, aparfure of the oviduct oj distal diladale i
oviduct ; ov, ovarian chamber; a, process of the pote ‘ehaunber 5
part of the oviduct (so-called stalk of the follicle).
of the follicle) and the other is a dilated efferent portion (od), with
The egg-follicle lies at the base and to the right
a distinct lumen.
<li
THE HEMIMYARIA (SALPIDAE). 417
of the nucleus near the oesophagus (Fig. 206) and is surrounded by
ramifications of the circumvisceral network of blood-vessels. The
oviduct also appears to be accompanied until near its aperture by
a vascular network (Fig. 207 B, 4). The point at which the oviduct
opens (Fig. 206, ») is found on the right side of the body behind the
penultimate muscle-hoop above the nucleus, Round the aperture,
the epithelium of the atrial cavity is thickened into a shield (Fig.
207 A, ¢p) wnd projects slightly inward (Fig. 207 B, ep). This
swelling is the rudiment of the eptthelial prominence of SALENSKY
which Toparo calls the uterus.
The question now arises whether the members of a Salpa-chain, after the
birth of the mature embryo, remain sterile or are able to produce a new ovary
which may yield a new embryo, Some of Satensky’s observations seem to
favour the latter view. The individuals composing a chain grow consider-
ably in size while the embryo ix developing within them, so that the largest
individual contains the most advanced embryo. Savensky, in such a large
individual, found the remains of a placenta
which indicated the previous expulsion of
an embryo and, side by side with this, «
mature egg or a quite young embryo,
Exceptions to the rule that each individual
of a chain produces only one embryo are
found in Salpa zonaria (Camisso and Escu-
ricut), S, Thilesii (Kroax) and 8. heragona
(Travstep?), in which several embryos
develop simultaneously, although they are
not all at the same stage of development.
In consequence of this and of the presence
of a special point of attachment for each
embryo, Levckarr (No. 98) concluded that
several egg-follicles with distinct ducts must
be present. These forms have recently been
united to form the genus Jasis (Lanmux,
No, 38), the above feature being one of its
generic characters.
Tn many of the Salpidae (S. maxima, 5.
pinnata, 8. punctata) the follicle appears to
be incompletely divided by a longitudinal je 999 Dorsal aspect of Sal
furrow (Fig. 207 8), into two chambers, one bicaudata (' ). @, point
of which (the ovarian sac, ov) contains the aia Sea oie
egg during the stages of its maturation, while {Cane idin ateial sere
the other (the embryonic sac, em) receives it end, endostyle ; /, periphar-
during the first embryonic stages. In many _YPSeal band; g, genital tubs;
M +, branchial aperture ; &, gill;
forms (¢.g., 3. maxima) the embryonic sac is nm, nerve-ganglion ; wi, nuolews.
eontinwed into a pointed process (s) which
soon degenerates. The remains of this process, in later stages, when the
EE
418 TUNICATA.
embryonic sac, in consequence of the shortening of the oviduct, shifts towards
the epithelial prominence, serves for attaching the sac to the epithelium.
The development of the female genital apparatus of Salpa (Pegea) bicaw-
data ix quite exceptional. Here (Fig. 208) at about the middle of the body
of the individual of the chain, at the right side, there is an outgrowth of the
body-wall with a somewhat curved end (genital tubv, g. SaLExsKy. No. 10H.
sin the development of Sipe bicandata (after SALBSSKY). 1
ined figure 3 SKY), the embryo ¢ still lies at the hase of the
T ovi In B; the embryo, at a more advance!
ae «c, wall of the respite:
forming 1 . umbilical cont
venbryn and the placenta): ¢ embryo in the dilated ovituct
xenital fold ; 7, ciliated pit: y, genital tube:
erture of the > mit, Mantle; 4, net
2 pe perivardinn ; sf, stolon.
The lumen of this tube communicates with the atrial cavity (Fig. 209 4).
The short oviduct opens fur back in the base of this tube between two epithelial
folds projecting into the lumen of the genital tube (incubatory folds, f. Iv
spite of this peculiar arrangement, which must be regarded as a modification
of the part of the atrial wall surrounding the aperture of the oviduct, it seem=
THE HEMIMYARIA (SALPIDAE)—CLEAVAGE. 419
(SaLENsKY) that the embryonic development of Salpa bicaudata does not, in
essentials, diverge greatly from that of other forms. The greater the increane
in size of the embryo and of the placenta which has attached itself at the base
of the tube, the shorter does the tube become. The embryo finally passes into
the atrial cavity of the parent through the opening of the tube (Fig. 209 B).
The first change in the genital apparatus which precedes the
fertilisation of the egg, occurs in quite young Salps, in the act of
detaching themselves from the stolon of the “nurse” or just after
detachment. It consists of a continuous shortening of the oviduct,
especially affecting the part that has been described as the stalk of
the follicle, the cells of which shift towards each other in such a way
that they soon form several layers, while a lumen appears, so that
Fig. 210. —Two aegonetie stages of Supa pinnate (ufter SALENSKY). In uf, the
embryo consists of four blastomeres, two of which are cut through in the
is of a larger number of blastomeres and of numerous smaller cells, kz,
tt, blood-sinus ; ¢, epithelial prominence ; ¢, moditied part of the
K's ectoderm-gerin) ;,/A, enveloping fold ; fin, follicle-
wlls, kalymmocytes ; £2, smaller celle of the embryo
(SALENSK Y's, qenlantay ¢ », shortened oviduct.
the oviduct is now hollow throughout its whole length. During this
abbreviation of the oviduct (Fig. 210) the egg, together with the
follicle, shifts continually nearer the aperture of the duct, the process
suggesting that seen in the descent of the mammalian testes.
Through the now open oviduct the spermatozoa obtain access to
the follicle, and fertilisation takes place. According to Toparo (No.
112), the spermatozoon enters the egg and the male pronucleus
develops after the expulsion of the first polar body and before the
development of the second.
Cleavage is total (Figs. 210 4,211 A and B). Egys have been
observed divided into two and into four and also in the later stages
FF
420 'TUNICATA,
of cleavage; we are, however, fur from having obtained a clear
insight into the details of the process. In individual cases, cleay-
age seems to be unequal, Sanensky figured the embryo of Salpa
mucronata in the four-celled stage consisting of four equal blasto-
meres, but both he and Toparo observed an inequality of the
blastomeres in S. pinnata and 8. punctata at this stage. [Judging
from recent investigations (Nos. XIII. and XXIa,), unequal segmenta-
tion is the rule rather than the exception in Sa/pa-]
‘The processes of cleavage are here specially difficult to follow in
detail because of the cells which become detached from the wall
of the follicle and become applied to the embrydénie mass (Fig. 210 4,
fs), and even wander in between the blastomeres; these cells are
known as the follicle-cells or kalymmocytes. This immigration of
cells, which we may compare to the test-cells of the Ascidiacen (jp. 336)
and the inner follicle-cells of Pyrosome (p. 390), is so profuse that
the blastomeres at the later stages of cleavage seem actually enveloped
in them (Fig, 210 B), appearing to be embedded in u matrix of
gonoblasts (as SALENSKY terms them, No. 104), Topano, who was the
first to notice this multiplication and immigration of the follicle-cells
(Nos. 108 and 109), described them as yolk-cells (cel/ulew lMcitiches)
and holds that they serve for the nourishment of the embryo which
forms from the blastomeres. ‘Uhey are said to undergo granular dis-
integration, to be taken in and assimilated by the blastomeres, and,
finally, to disappear altogether. Sanensky (No. 104), on the contrary.
sees in these cells the actual constituent elements of the future
embryo and therefore calls them gonoblasts.* According to him,
the large blastomeres, the protoplasm of which soon bevomes divided
up in a peculiar way, do not subdivide further and are, in general,
incapable of any special further development, They are said finally
* (The recent and exhaustive investigations made by Brooxs
Heer (No, XIII), Konornerr (Nos. XVIUL-XXIc.), and
XXIV.), prove undoubtedly that Sanmysky was in error when he
a formative role to the kalymmocytes, since, however much these
differ from one another in detail, they were all that the
embryo are eventually wholly formed from the the v
majority being that. the Bais aD ao play an entirely passive rile
development of the embryo, being merely nutritive structures.
The account given in the following is largely based upon §
work on the development of Sapa. Unfortunately, the conclus
at by this observer have, in many cases besides the one n i
not confirmed by subsequent investigators, This renders out
incomplete and, in some particulars, inaccurate, so that the
well to consult the original monographs of Hmrpen, Konorxerr m
‘We have, however, endeavoured, in footnotes, to draw attention to |
serious error.—Ep.)
THE HEMIMYARIA (SALPIDAE)—CLEAVAGE, 421
to disintegrate, while the embryo is built up by the gonoblasts which
form the greater part of all the later rudiments of organs. SALENSKY,
therefore, considers the embryonic development of the Sa/pidae as
4 process intermediate between the development of an egg and
budding, beginning with a regular cleavage, but the resultant blaste-
meres play no further part in the development, the embryo being
for the greater part built up out of derivatives of the egg-follicle.
SALENsKyY consequently describes the embryonic development of the
Salps as follicular tudding. A priori, SauEnsky’s view as to the
part taken by the follicle cells in the embryonic development of the
Salpidae must be regarded us extremely improbable ; it is also by no
means proved by what SALENSKY says of the decisive stages. We
must therefore for the present accept Topano’s views as the more
probable. *
We have already seen (p. 390) that Savensky also ascribes a considerable
part in the building up of the embryo of Pyrosoma to the immigrated follicle-
cells or kalymmocytos, and he has recently attributed to these cells a share
in the development of the cellulose mantle of Distaplia (No. 49, see also
p- 357).
The shortening of the oviduct mentioned above (Fig. 210) is not
due merely to the dilation of its lumen and the consequent shifting
of the cells of its wall, but is also directly connected with the immigra-
tion of cells already described. In this way, a large amount of cell-
material is given off by the wall of the oviduct and the follicle to the
embryo. In its abbreviated condition, the oviduct forms a short wide
chamber (Fig. 211 A) communicating with the follicle through a
harrow aperture which, however, soon widens. The two cavities
finally unite to form a single capsule, the lumen of which is almost
{According to Brooks and Mrtcany (Nos. I. and XXIV.) these kalym-
mocytes first block out the embryonic tissues and organs, but are eventually
blastomeres, after which the former degenerate and probably serve
a5 food-material for the latter cells. Even at an early stage Taivacidovied
degenerate and their nuclei migrate into the large blastomeres, forming the
so-called Si bgt Hetper (No. XIII.), while agreeing that the kalym-
mooytes are taken up by the blastomeres, thinks that the entire cell, not merely
its nucleus, enters the protoplasm of the latter, Korornerr (Nos. XXa. and
XXIa.), however, regards the masses seen in the blastomeres as true yolk-
masses and not degenerating kalymmocytes. The last two observers regard
the latter cells as playing « passive rdle in the development and do not agree
with Baoox’s view that the embryonic organs are blocked out in kalymmocytes.
According to these two authors, the embryo is made up of Jarge and small
blastomeres und kalymmocytes, the small blastomeres being indistinguish-
able from the latter and hence, they suppose, Brooxs’ and SALENSsKY's error
arose, the cells which the latter took to be kalymmocytes being in reality
small blastomeres.—Ep,}
| ae
—7
422 TUNICATA.
completely filled by the embryo (Fig. 211 8). The wall of this
capsule, which is produced by the union of the oviduet and the
follicle is from this time called (although not very accurately) the
Solticular epithelium.
During the aboye changes, the shield-like thickening of the epi-
thelium round the aperture of the oviduct has risen up more and
more, and now forms a mound-like swelling (epithelial prominence,
Fig. 210, e, 211, a) projecting into the posterior portion of the atrial
cavity. As the oviduct continues to shorten, the follicle, with the
embryo, is brought into ever closer proximity to this prominence and
finally passes into it. At later stages, the prominence becomes con-
stricted at its base (Fig. 213), remaining connected with the wall of
the atrial cavity only by a thin stalk. ‘The embryo now, enclosed in
a kind of brood-sac, projects into the interior of the atrial cavity.
a B
e
aller Saueneaeh
In A, the embryo undergoing cleavage still lies in the atte tn
embryo has need into the cavity of the oviduct (O ‘The follicular ‘tai >
i ¢ placenta, a later
uinence = outer lamella of the broods
cof the bmood-sac 5 S Ciicila SS =
7 JF blastomeres ; 5, immigrated follicle-cells (not represented tn A
The wall of this brood-suc, which, as we shall see, is merely proe-
visional, is double. The external wall (Fig. 211 C, a) is a modified
portion of the epithelium of the atrial cavity, and, in the inner wall
(Fig. 211 C, 4) we recognise the follicular epithelium. It appears
that, in this stage, the opening of the oviduct into the atrial cavity
has completely closed.
After cleavage has ended, the embryo forms a solid, rounded body
composed of numerous cells which, according to SALENSKY, are
mostly derived from the follicular epithelium, but, to
Topano, have been produced by fission from the blastomeres, We
shall for the present adopt the latter view as the more
lll
* [See editorial note, p. 421).
SALPIDAE—FORMS WITHOUT COVERING FOLDS. 423
According to Toparo, the cells derived from the follicle-epithelinm
which are fated to disintegrate, can still be distinguished between
these embryonic cells, It should, indeed, be mentioned that
SaLENsKY was able to find within the embryo, long after cleavage
had ended, at a time when the first rudiments of organs grow dis-
tinet, a few large blastomeres definitely arranged. The significance of
these is obscure, and we are altogether in the dark as to those stages
in the development of Salpa which lie between cleavage and the be-
ginning of the formation of the organs, i.¢, the stages in which we
should expect the germ-layers toform.* ‘The cleavage-cavity appears
to be wanting in all Salpidae [Savensky, Heer and Korotnerr].
Brooks, however, suggests that the follicular cavity may be thus
interpreted.
Certain divergencies ave found in the different species of Sadpa in
the further processes of development, but these apparently are not
due to any fundamental difference. The development of most: species
being as yet only very partially known, we shall restrict ourselves to
# closer account of the two forms which haye been best investigated,
viz., 8. democratica-mucronata and 8. pinnata. These represent two
types of development which are to be distinguished by the absence
or presence of a covering fold and by the stracture of the placenta.+
A. Forms without Covering Folds.
The embryo of Salpa (Thalia) democratica-mucronata, in the stages
that mark the completion of the cleavage-processes (Fig. 211 C) pro-
jects in the form of a cone into the atrial cavity of the parent. It
lengthens later and becomes more cylindrical, its end being rounded
(Fig. 212 B). It is still surrounded by the two walls of the brood-
*(Recent investigations tend to show that germ-layers, as we understand
them in other animals, cannot be said to exist in Salpa, since the various
ls, which give rise to the different organs, appear to be mixed in an
way throughout the germ-mass. Gradually, however, certain of the
blastomeres take up definite positions, and, from what is known of their sub-
sequent history, can now be definitely stated to give rise to certain tissues.
‘The first of these rudiments to be yh ee is the ectoderm ; this takes the
form of certain large blastomeres which migrate to the surface tee the
laconta and there give rise to the ectoderm and its derivatives. Other large
Eiimomeres which remain nearer the placenta have been found to give rise to
* the entoderm and possibly some erm; the latter layer, however, appears
to arise largely from the smaller blastomeres which, with the kalymmocytes,
make up the main mass of the germ.—Ep.)
+(Konorserr (No. XVIIL) expresses grave doubts as to the advisability of
Oe can be no doubt from recent invest tions saat
development ferences between these two divisions have n tly
‘exnggerated.—Ep.} bo
— x
SALPIDAE—FORMS WITHOUT COVERING FOLDS. 4265
It appears that the inner lamella of the brood-sac is very soon
reduced (Fig. 212 2) and completely degenerates. This degeneration
at first affects a zone running obliquely round the embryo, leaving
only a cap of the lamella covering the anterior end of the embryo and
a posterior cup-like portion connected with the rudiment of the
placenta, which completely unites with the latter at a subsequent
period ; the anterior cap appears soon to disintegrate. The embryo
is then covered by only one envelope, the outer lamella of the brood-
mac (a).
The above is in accordance with SALENSKY'S earlier description of the fate
of the inner lamella (No. 100). The more recent statements of this author
suggest that the inner lamella does not disintegrate, but enters into close con-
nection with the embryo, finally changing into the ectoderm of the latter.
The ectoderm in S. democratica-mucronata would then have to be traced
back to the transformed epithelium of the oviduct, a view which is a priori
improbable, and less in accordance with our own investigations than the older
statements. [The ectoderm, like the other embryonic organs, is now generally
regarded as arising from the blastomeres. See footnote, p. 424, aud KoRoTNEFY
(No, XVIII.) on S. democratica.)
In the next stage (Fig. 212 2) important differentiations are evident
in the embryo. The mesoderm (m) has appeared between the ecto-
derm and the entoderm in the form of a cell-accumulation, which
spreads out like a germ-layer to right and left over the sides of the
embryo, The central nervous system (n) is algo found as a cell-
growth proceeding from the ectoderm. Its position marks the plane
of symmetry and the anterior end of the body in the embryo.*
Diametrically opposite to it is another cell-accumulation (/), which
SALENSKY also traces back to the ectoderm, and which seems to be
the first rudiment of the elaeoblast. That part of the ectoderm which
is in contact with the rudiment of the placenta is already distinguished
by the large size and the height of its cells (x). This is the rudiment
of the lunella, which takes part in the formation of the epithelium
covering the placenta.
The fundamental features of the organisation of the embryo of
Salpa, which are thus already sketched out, appear still more dis-
tinctly in the following stage (Fig. 213), in consequence of the
development of a system of cavities. The inuer cell-mass severs
* [Kororsrrr believes that the nervous system ix formed as a closed vesicle,
which lies at first quite independently in the mesoderm without any relation
w the ectoderm or to the pharynx. The elacoblast also arises from the
cinbryonic blastomeres and not from follicular cells, ax SALENSKY stated —
Ep.)
THE HBMIMYARIA (SALPIDAE)—OCLEAVAGE, 421
to disintegrate, while the embryo is built up by the gonoblasts which
form the greater part of all the later rudiments of organs. SaLENSKY,
therefore, considers the embryonic development of the Salpidae as
#& process intermediate between the development of an egg and
budding, beginning with a regular cleavage, but the resultant blasto-
meres play no further part in the development, the embryo being
for the greater part built up out of derivatives of the egg-follicle.
SaLEnsky consequently describes the embryonic development of the
Sulps as follicular budding, <A priori, Saumnsky's view us to the
part taken by the follicle cells in the embryonic development of the
Salpidae must be regarded us extremely improbable ; it is also by no
means proved by what SALENSKY says of the decisive stages. We
must therefore for the preseut accept Topano’s views us the more
probable *
We have already seen (p. 390) that Sacensky also ascribes a considerable
part in the building up of the embryo of Pyrosoma to the immigrated follicle-
cells or kalymmocytes, and he has recently attributed to these cells a share
in the development of the cellulose mantle of Distaplia (No. 49, see also
p. 357).
The shortening of the oviduct mentioned above (Fig, 210) is not
due merely to the dilation of its lumen and the consequent shifting
of the cells of its wall, but is also directly connected with the immigra-
tion of cells already described. In this way, « large amount of cell-
material is given off by the wall of the oviduct and the follicle to the
embryo. Iu its abbreviated condition, the oviduct forms a short wide
chamber (Fig. 211 A) communicating with the follicle through «
harrow aperture which, however, soon widens. The two cavities
finally unite to form a single capsule, the lumen of which is almost
* (According to Brooxs and Mrcacy (Nos. I. and XXIV.) these kalym-
mocytes first block out the embryonic tissues and organs, but are eventually
replaced by blastomeres, after which the former degenerate and probably serve
Lneey oat the latter cells. ere a ye stage Mets coy
and their nuclei migrate into the large blastomeres, forming the
so-called yolk-particles. Hen (No. XITI.), while agreeing that the vl:
mocytes are taken up by the blastomeres, thinks that the entire cell, not merely
its nucleus, enters the protoplasm of the latter. Kororxnry (Nos. XXa. and
XXIa.), however, regards the masses seen in the blastomeres as true yolk-
masses and not de erating kalymmocytes. The last two observers regard
the latter cells as playing « passive rdle in the development and do not agree
with Buoor’s view that the embryonic organs are blocked out in kalymmocytes,
to these two authors, the embryo is made up of large and small
blastomeres und kalymmocytes, the small blastomeres being indistinguish-
able from the latter and hence, they suppose, Brooks’ and SaLensxky's error
arose, the cells which the latter took to be kalymmocytes being in reality
small blastomeres.— Ep, |
SALPIDAR—FORMS WITHOUT COVERING FOLDS. 423
According to Toparo, the cells derived from the follicle-epithelium
which are fated to disintegrate, can still be distinguished between
these embryonic cells. It should, indeed, be mentioned that
Sauensky was able to find within the embryo, long after cleavage
had ended, at a time when the first rudiments of organs grow dis-
tinct, a few large blastomeres definitely arranged. ‘The significance of
these is obscure, and we are altogether in the dark as to those stages
in the development of Sadpa which lie between cleavage and the be-
ginning of the formation of the organs, i. the stages in which we
should expect the germ-layers to form.* The cleavage-cavity appears
to be wanting in all Salpidae [Savensky, Herper and KororNerr].
Brooks, however, suggests that the follicular cavity may be thus
interpreted.
Certain divergencies are found in the different species of Sa/pa in
the further processes of development, but these apparently are not
due to any fundamental difference. The development of most species
being as yet only very partially known, we shall restrict. ourselves to
a closer account of the two forms which have been best investigated,
viz., 8. democratica-mueronata and S. pinnata. These represent two
types of development which are to be distinguished by the absence
or presence of a covering fold and by the structure of the placenta.+
A. Forms without Covering Folds.
The embryo of Salpa (Thalia) democratica-neucronata, in the stages
that mark the completion of the cleavage-provesses (Fig. 211 C) pro-
jects in the form of a cone into the atrial cavity of the parent. It
lengthens later and becomes more cylindrical, its end being rounded
(Fig. 212 4). It is still surrounded by the two walls of the brood-
*[Recent investigations tend to show that germ-layers, as we understand
them in other animals, cannot be said to exist in Salpa, since the various
-cells, which give rise to the different organs, appear to be mixed in an
irregular way throughout the germ-mass. Gradually, however, certain of the
blastomeres take up definite positions, and, from what is known of their sub-
sequent history, can now be definitely stated to give rise to certain tissues,
‘The first of these rudiments to be ised is the ectoderm; this takes the
form of certain large blastomeres which migrate to the surface oppocite the
lacenta and there give rise to the ectoderm and its derivatives. Other large
jlastomeres which remain nearer the placenta have been found to give rise vo
* the entoderm and possibly some waestenn ; the latter layer, however, appears
to arise largely from the smaller blastomeres which, with the kalymmocytes,
make up the main mass of the germ.—Ep.)}
+[Konorserr (No. XVILI.) expresses grave doubts as to thé advisability of
this subdivision, and there can no doubt from recent investigations that
the developmental differences between these two divisions have Sse greatly
exaggerated.—Ep.]
SALPIDAB—FORMS WITHOUT COVERING FOLDS. 495
It appears that the inner lamella of the brood-sac is very soom
reduced (Fig. 212 2) and completely degenerates. This degeneration
at first affects a zone running obliquely round the embryo, leaving
only a cap of the lamella covering the anterior end of the embryo and
a posterior cup-like portion connected with the rudiment of the
placenta, which completely unites with the latter at a subsequent
period ; the anterior cap appears soon to disintegrate. The embryo
is then covered by only one envelope, the outer lamella of the brood-
sno (a).
The above is in accordance with SaLexsky's earlier description of the fate
of the inner lamella (No. 100), The more recent statements of this author
suggest that the inner lamella does not disintegrate, but enters into close con-
nection with the embryo, finally changing into the ectoderm of the latter.
‘The ectoderm in S. democratica-mucronata would then have to be traced
back to the transformed epithelium of the oviduct, a view which is a priori
improbable, and less in accordance with our own investigations than the older
statements. [The ectoderm, like the other embryonic organs, is now generally
regarded as arising from the blastomeres. See footnote, p, 424, aud Konorserr
(No. XVITL) on S. democratica.)
In the next stage (Fig. 212 8) important differentiations are evident
in the embryo. The mesoderm (m) has appeared between the ecto-
derm and the entoderm in the form of a cell-accumulation, which
spreads out like a germ-layer to right and left over the sides of the
embryo. The central nervous system (n) is also found as a cell-
growth proceeding from the ectoderm. [ts position marks the plane
of symmetry and the anterior end of the body in the embryo.*
Diumetrically opposite to it is another cell-accumulation (/), which
SavENsky also traces back to the ectoderm, and which seems to be
the first rudiment of the elaeoblast. ‘That part of the ectoderm which
is in contact with the rudiment of the placenta is already distinguished
by the large size and the height of its cells (x), This is the rudiment
of the lamella, which takes part in the formation of the epithelium
covering the placenta,
The fundamental features of the organisation of the embryo of
Salpa, which are thus already sketched out, appear still more dis-
tinetly in the following stage (Fig, 213), in consequence of the
development of a system of cavities, The inner cell-mass severs
*(Konorserr belioves that the nervous system is formed as a closed vesicle,
whieh lies at first quite independently in the mesoderm without any relation
to the ectoderm or to the pharynx. The elacoblast also arises ‘from the
embryonic blastomeres ard not from follicular cells, as SALeNsKy stated.—
SALPIDAE—FORMS WITHOUT COVERING FOLDS, 427
a tube, the inner cells being transformed into blood-corpuscles which
pass into the blood.
Before passing on to describe the other changes that take place in
the embryo we must dwell for a moment on the degeneration of the
brood-sac and the development of the placenta, After the inner
lamella of the brood-sae has degenerated as described above, the
embryo remains surrounded solely by the very thin epithelium of the
outer lamella (Fig. 212 8, a), which consists of a differentiated part
of the atrial epithelium of the parent. This outer lamella is unable
to keep pace with the further increase in size of the embryo ; it
becomes ruptured at the point at which the aperture of the oviduct
was originally situated and shrinks downwards over the embryo (Fig.
213). In consequence of this contraction of the outer lamella the
embryo, which originally lay in the follicle (Fig. 211 4), and then
shifted forward into the dilated oviduct (Fig. 211 4), protrudes
into the atrial cavity of the parent, in which from this time it lies
freely. -
We have already seen (p, 424) that a compact cell-mass is attached
to the lower surface of the embryo (Fig. 212, p); this, which
represents the first rudiment of the placenta, is derived from the
transformed cell-material of the egg-follicle. The outer lamella of the
brood-sac now shrinks completely back over this cell-mass, and finally,
as a constricted funnel-like annulus, surrounds and strengthens the
connection between the rudiment of the placenta and the parent (Fig.
214, a), The placenta-rudiment would lie exposed, after the with-
drawal of the outer lamella, were it not covered by « thin ectodermal
layer of the embryo (Fig. 214, ee), which develops as the brood-sac
‘is withdrawn. Through this cireumerescence of the placenta by an
ectodermal lamella, which was not observed by Sauensky, but of
which we were able clearly to convince ourselves, the placenta is
incorporated in the embryo and then appears enclosed in a capsule
derived from the ectoderm of the embryo, The lateral walls of this
capsule are formed by the thin lamella just mentioned ; its upper
wall or the so-called roof (Fig. 213, ¢), on the contrary, is yielded by
the thick ectoderm-layer, the origin of which was traced above (p. 425),
[This is of follicular origin according to Konornerr (No. XXe.).] On
its under side the ectodermal capsule of the placenta possesses an
aperture through which the placental cavity communicates with the
blood-vascular system of the parent. The placental cavity arises in
the form of gaps or clefts in the placental tissue, which thus assumes
a loose structure. Some of the cells of this originally compact tissue
latin
SALPIDAE—FORMS WITHOUT COVERING FOLDS. 429
the brvod-sac which can still be recognised for some time as x kind
of corpus luteum. The embryo, at birth, passes out through the
atrial aperture of the parent.
The development of the final shape of the body goes hand in hand
with the development of the placenta. The embryo at first formed
4 cone projecting into the atrial cavity of the mother (Fig. 212), its
principal axis corresponding to the future dorso-ventral axis, It now
lengthens at right angles to this axis, é.¢., in accordance with its
future longitudinal axis (Fig. 214). It soon becomes cylindrical and,
after the cellulose mantle has developed, resembles a tetragonal prism.
The mantle-substance develops in just the same way as in the
Ascidians (p. ). It arises on the outer surface of the ectoderm
aga secretion into which single cells soon wander. Finally, the two
conical processes characteristic of Salpa democratica (solitary form)
develop (Fig. 262, p. 495).
The nervous system has been seen to arise as a solid ingrowth of
the ectoderm.* ‘This soon severs its connection with the latter, a
cavity develops within it, and it becomes a vesicle characterised by
its size and the thickness of its walls (Fig. 214, »). KowALEvsky
(No. 96) was aware of the fact that this vesicle lengthens later and
is indistinctly divided by constrictions into three consecutive parts
which show a certain resemblance to the primary cercbral vesicles
of the vertebrate embryo (Fig. 214 8B, n). The anterior vesicle
becomes closely connected anteriorly with the adjacent wall of the
pharynx, and this connection which is at first solid soon develops
a lumen which puts the neural cavity into communication with the
pharyngeal cavity. The canal thus formed is the first rudiment of
the future ciliated pit (Fig. 216, 4). While this organ develops
further, the walls of the ganglionic vesicle thicken, it shortens, its
lumen disappears and the vesicular rudiment thus gradually assumes
the character of the definitive ganglion of the adult. A conical
process rising on the dorsal side of the ganglion on which three
accumulations of pigment appear represents the rudiment of the
eye,
Further details as to the development of the eye have recently been pub-
lished by Mercanr (No. 99) and Biirscuti (No. 94).+ ‘The eyes seem to develop
differently, not only in the different species but also in the solitary and the
colonial forms of the same species. According to Bé-Tacutt, the simplest form
* [See footnote, p. 425.—En.]
t[See also Mxrcan® in Brooks’ Monograph (No. I.), and
No. 94a).—Ep.]
a
430 TUNICATA.
of eye is a mound-like swelling of the brain (Fig, 215 4); at the sides of this
swelling are arranged the pigment-cells while, from the centre, closely packed
rod-bearing cells are found radiating
towards the surface, the nerve-fibres A r
being connected with the inner ends of
the rods. In this eye, the rods are
therefore turned directly towards the P
source of light, In other forms, the
rudiment of the eye becomes differenti-
ated into three parts which either remain
united in the shape of a horse-shoe, or
form three entirely distinct eyes, the
unpaired median eye retaining the B
original simple condition (Fig. 216 B, «)
while the two latetal eyes are formed
on the plan of an inverse eye (b), i.c., the
rods are directed away from the surface
and the nerve-fibres are connected with
their outer ends, Bérscrur, assuming
an optic vesicle which cannot be observed,
homologised the median non-inverted
eye with the cephalic eye of the Verte- Ft oe iteeens eee
brates, and the lateral inverted eyes with single eye; J, typical
the paired eyes of the Vertebrates, but, ts niet yi 6 a
independent of this theoretical vesicle, iagthent-eclie sy title y
the structure of the lateral eye of the
Ascidian larva seems directly to suggest the paired vertebrate eyes by thr
fact that the rods are directed towards the cerebral cavity (see, however, the
objections raised by Metcary, No, a, and Gérrxnt, No, 94a).
‘The first rudiment of the pharynx and the development of the gill
have already been described (p. 426). The wall of the pharyngeal
cavity is formed by a simple epithelium, the cells are either eubieal
or somewhat flattened. The rudiment of the endostyle (hypobranehial
furrow) appears in the form of paired folds of this epithelium origin
ating at some distance from each other (Fig. 216, es); these shift
towards one another later and then form the boundaries of # hypo-
branchial furrow which runs from the peripharyngeal bands to near
the entrance of the oesophagus. The rudiments of the
bands which ran towards the anterior end of the gill from the anterior
end of the endostyle, encircling the aperture of the respiratory cavity,
first appear as similar prominences. ‘The rudiments of the branchial
and atrial apertures (Fig. 216, i and ¢) appear only in later stages
in the form of transverse depressions of the ectoderm which break
through into the pharynx and atrial cavities, ‘The rudiment of the
atrial aperture originally lies almost at the centre of the dorsal side
~
_—
SALPIDAE—FOKMS WITHOUT COVERING FOLDS. 431
of the body (Fig. 216), but later, as the part of the body known as the
nucleus decreases in size, it shifts further back.
The radiment of the alimentary canal, in the strict sense of the
term (Fig. 214 4, d), originally forms a posteriorly directed diverti-
culum of the pharynx, This caecum becomes divided later by two
folds rising into it from below into three spaces (Fig. 216, oe, m, ed),
the anterior space being the rudiment of the oesophagus, while the
posterior space represents the intestine. The middle spuce is the
rudiment. of the stomach-caecum. The intestine curves upward
fio, 216,—Later embryonic stage of Safpa democrution-mucronata (aftr SALENSKY).
¢, atrial aperture ; ¢, elavol 1, intestine ; ex, ondosty! , ciliated pit;
Trauchial (oral) apertuice ; gill; m, stomnach-eaecunt ; #, ganglion | v7, oesophh fag:
2 per rial sac; pol, placenta ; xf, stolon; f, so-called ‘root of the placenta (basal
towards the left, till its blind end comes into contact with the
atrial wall, perforation leading later to the formation of the anal
aperture.
Tt has been shown that the mesoderm (Fig. 213 B, ms) spreads out
over the right and left sides of the embryo in the form of two lamellae
ju close contact with the entoderm. These lamellae, according to
Levoxart, yield the ruscle-hoops, a kind of fenestration taking place
in the lamellae and separating the mesoderm-bands which correspond
to the different hoops. The transversely striated contractile substance
develops later in the muscles. The heart also, according to SALENSKY,
432 TUNICATA.
owes its origin to the mesoderm, i.¢., to the lamella lying on the
right side of the body, which is continued back over the posterior end
of the wall of the branchial cavity, and there forms a vesicle (Fig.
216, p) in which we recognise the first rudiment of the pericardial
vesicle.* A swelling of the thickened dorsal wall of the vesicle.
which projects into its lumen, and which, though at first solid
becomes hollow later, is the rudiment of the heart proper; this con-
sequently develops in just the same way as in the Ascidiacea (p. 370).
The blood-vessels apparently arise as spaces within the gelatinous
connective-tissue which, in later stages, fills the primary body-cavity.
It should be mentioned that, in the Salpidae, as Toparo has pointed
out and figured, the blood-vessels seem to be lined throughout with
w cellular intima (Fig. 207 3B, 4). In this respect this group
would seem to differ from the Ascidiacea, in which, according to
van BENEDEN and Junin, such an intima is wanting (qf. pp. 363
and 371).
The elaeoblast (Fig. 216, eb), the rudiment of which has already
been described (p. 425), attains its full development only in the later
stages of embryonic life, and, after the birth of the embryo, under-
yoes gradual degeneration [by phagocytosis, according to KoroTNEFF
(No, XIX.)]. It is a mass of large polygonal cells, which are filled
with reserve nutrition. The remarkable resemblance between the
claeoblast and the degenerating larval tail of Doliolum (p. 388) caused
SALENSKY to assume that this problematical organ is the homologue
of the tail and the chorda of the Ascidian larva. But the presence
of the rudiment of the claeoblast, as we shall see in the buds of the
Nalpidae and of Pyrosoma, is not very favourable to this view.
Physiologically, the elacoblast is probably, as Leuckart suggests.
aqeservoir of food-material, which is gradually used up as the
embryo develops.
At a later stage of development the rudiment of the stolon can
be seen (Fig. 216, st). This consists first of a diverticulum of the
pharyngeal wall lying at the posterior end of the endostyle, and is
turned toward the left side of the body. The ectoderm soon bulges
over this entodermal diverticulum. The space between the two
layers is, according to SEELIGER (No. 105), filled with mesenchyme:
cells, the short conical stolon thus consisting of three germ-layer
(p. 495).
* According to Konorserr (No. XVIIL), the pericardium arises as in other
Tunicates as a diverticulum of the pharynx.—F:
8ALPIDAE—FORMS WITH COVERING FOLDS, 433
B. Forms with Covering Folds.
The development of the forms belonging to this type (5. (Cyclosapa)
pinnata, 8, africana-macima, S. runcinata-fusiformis, S. prunetata)
differs in many essential points from that of S. democratica-mucronata_
The principal distinction consists in the presence in the former of an
external covering which after the degeneration of the primary brood-
sac (present also in WS. democratica-mucronata), forms a secondary sac
investing the embryo, and in the peculiar development of the placenta,
The development of the organs also appears to follow another type.
The forms just mentioned seem to agree fairly well in their develop-
ment, which has been studied by many soologists, especially by
Fra, 217.—Two ontogenctlo stages of Salpe pinnate (after Saueveny) forming
sequence to Fig. 210 8. }, blastomeres ; bk, ‘* blood-forming bud"; Af, blogd-
cavities in the ta; BY, median hlood-sinus’; of, roof of the placenta (tusal late) =
¢, lower part of the epithelial prominence, known {ater as the placental membrane s
#, upper part of the epithelial prominence (SALENSKY's ectoderm-gerin) ; /, follicular
cavity ; fA, covering fold ; fiw, follicle-wall.
Topako, Barrors and Savensxy, and more recently by Brooks
(No, I.), Herpzr (No, XIII.), and Korotyerr (Nos. XX., XXa.,
and XXIa.). The following account relates chiefly to 8, pinata, a
comparatively well-known form,
Starting with the embryonic development of S. pinnata at the
stage depicted in Fig, 217 A, we find conditions in fairly close agree-
ment with those described in connection with 8, democratiea. The
embryo consists of large and of small cells. ‘The protoplasm of the
large cells (6) breaks up in a peculiar way into polygonal portions
* (See footnote, p.423 and p, 445,—Eb.]
FF
aA
‘The embryo lies in a sac (Fig. 217 A, fw)
union of the follicle and the dilated oviduct.
been deseribed in 8. demoeratica-mueronata
brood-sac. At the posterior end of this sac a
in the forms now under consideration ; this
distinet. (Fig. 217 2, 4h), and is to be traced |
wall of the follicle. While, in S. demoe
ing forms the rudiment for the whole of the p
the embryo?). This part has been called by
(bottone ematogene, Figs. 217 B, and 218, bk).
The embryo almost entirely fills the cavity
217, /). On one side, it appears to be
lamella of the sac. According to SALENSKY,
later haemal side of the body, so that we are.
to orient the embryo. [According to Bi
contrary, marks the middle dorsal line of the |
The outer lamella of the brond-sas (Fig: 3
from the thickened part of the atrial epitheliu
is pushed out into the atrium by the growth o
it, and which is called by SAnENsKY the epitl
‘Two parts can soon be distinguished in it. '
covers the greater part of the embryo, co
cells, while the lower part (e) is composed
This latter yields later the lateral walls of the p
Ep ay ee ee that the embry
sists of two kinds of Blastomores 4. Tang nd
appearance indistinguishable from the ke
SALPIDAE—FORMS8 WITH COVERING FOLDS. 435
by Topaxo the placental membrane or germoblastica [the supporting
ring of the placenta (Brooxs)].
A differentiation somewhat resembling that just deseribed in the
outer lamella is also found in the inner lamella (splanchnic layer of
the follicle) which represents the transformed epithelium of the ovi-
duct and follicle. The lower half of the lamella (Fig, 217 2, @), in
this case, becomes connected with the thickened epithelium of the
atrial cavity known as the placental membrane or supporting ring
of the placenta (e), and forms the roof of the placenta (Fig. 218 A, dp),
in the centre of which the ‘ blood-bud ” is attached. The placenta
is thus hollow, its lateral walls (supporting ring, Fig. 218 A, mp)
being yielded by the epithelial prominence of the atrium, and its
roof (dp) by the inner lamella of the brood-sac, “e., by the follicle
(placental portion of the follicle). The “ blood-bud” (#4) hangs
from the roof into the cayity which is part of one of the blood-
channels of the parent. SALENsKyY distinguishes in this cavity two
communicating sinuses, au afferent and an efferent sinus, and, be-
tween these two, a third vascular space round the “‘ blood-bud ” (Fig.
217 A, 41), the relations and significance of which are unknown.
The placenta, which, from the first, is a greatly swollen structure,
now becomes constricted at its base (Fig. 218), and thus forms a
stalked structure on the upper surface of which the embryo rests.
Tt becomes saddle-shaped later, the parts lying at the sides of the
embryo growing upward. This is why, in a horizontal section (Fig.
219) only the lateral parts of the placenta (p) are seen cut through,
The actual relations of this organ are still very obseure.
While, in this way, the primary brood-sac undergves essential
alteration, a circular fold of the atrial epithelium grows up from
the base of the epithelial prominence (Figs. 210 B, and 217, fh)
and completely overgrows the placenta and the embryo (Fig. 218, /),
and thus forms a new secondary brood-sac (embryo-sac) such as is
not found in 8. democratica-mucronata, ‘This is known as the cover-
ing fold or amnion. It continues to grow upwards as a circular fold
round the embryo, over which, however, it never fuses, but remains
Separated by a variable aperture through which the embryo eventu-
ally passes out into the atrium.
There are, in the different species, characteristic variations in the form of
this fold, In S, africana-marima, the aperture is elongate and the margins
of the fold project and take the form of a semicircular crest; this is seen in
eross-section in Fig. 221, c; in S. fusiformis, this crest is abruptly truneated.
Tn S, pinnata and 8. punctata, on the contrary, such a crest is wanting.
a |
We haye seen that the placenta is formed from the lower parts of
the primary brood-sac, the upper halves of the outer and inner
lamellae taking no part in it (Figs. 217 and 218). We are still quite
in the dark as to the future fate of these latter parts, which cover
the embryo like a cap. According ta Haxnars (NG 187) Sienna
(No. 110), they are cast off and disintegrate.
According to Sanmnsmy, on the contrary, they are pepnaigy gs!
becoming closely connected with it and participating in its formation. That
part of the outer lamella (Fig. 217, e’) of the primary brood-sac, derived from
the maternal atrial wall, which is not concerned in the formation of the
placental membrane, is said to yield the ectoderm of the embryo (Fig, 218, ¢),*
and the upper half of the outer lamella, which consists of flat cells, has there-
fore been called by Satunsny the ectoderm-germ. The inner lamella (Fig. 217,
fr), on the other hand, which can be traced back to the transformed epi-
thelium of the oviduct, is said to yield, together with those parts that are not
used up for forming the roof of the placenta, chiefly the mesoderm-tissue of
the embryo (Fig. 218); the enteric rudiments, however, are also said
originate in this layer. We must admit that we feel sceptical as to these state-
ments. According to them, the embryo results from separate rudiments derived
from various parts of the body of the parent, The epithelium of the atrial cavity
of the latter would yield the ectoderm, the oviduct, a part of the mesoderm
and the enteric rudiments, while the rest of the embryo would be derived
from immigrated follicle-cells (for, according to Sanensxy, the blastomeres
take no part in the building up of the embryo, p, 421). We are inclined to
think that errors of observation or of interpretation must here basen in,
[See editorial notes, pp. 420-425].
At the stage when we should expect the development of the germ
layers,+ and the first rudiments of the organs, there is a considerable
gap in our knowledge, We, at least, have been anable, from the very
fragmentary statements of the stages that follow those described
ubove, to obtain any clear idea of these processes of development
which we desire to compare with the facts known im connection
with the ontogeny of the other Tunicates or that of other animals.
* (This layer, (¢') the epithelial capsule of Buooxs, is o
temporary protective membrane which disintegrates at a later stage.
no part in the formation of the ectoderm, which is, on the contrary,
from the blastomeres (Brooks, Herprr and Kororserr).—Eb.]
+In connection with the formation of the Ftc in the |
special stress is to be laid on a “ gastrula-stage" by
in which an invagination is fan on the lower side of the embryo, n
towards the placenta. Fi
‘According to Brooks, the ye in Salpa co to ie pat
ns cording is to be sought nae stage ike | that given in 217, the cavi
of the follicle, which becomes the posrearart being the
and the blastopore coinciding with the attachment of the ce
blastomeres and follicle-cells to the inner layer of the
segmentation of the egg is much retarded, but the
in follicle-cells. This view is not accepted by other
h
SALPIDAE—FORMS WITH COVERING FOLDS, 437
In the next stages we find ourselves on firmer ground (Fig. 218 4),
the most important organs having already developed. This stuge is
characterised by the appearance of a cavity which, from its relations
to the rudiments of organs (corresponding with those described for
S. democratica-mucronata), we may regard as the body-cavity. Into
this cavity, a club-shaped mass of cells, the early rudiments of the
organs, hangs down from the upper surface of the embryo. The peri-
cardial rudiment (pe) is, however, distinguished by being further
attached at its lower end.
Pia, 218. Two oiitogeictic stage of Salo pinata (after Sauk). A; lingram
matic median soction of a younger stage combined from various figures hy SaueNsky +
Holes 4, blastomeres ; bi, ™ blood-forming hud."” ; Bl, blood-spaces in the
, atrial cavity ; d, enteric rudiment ; te roof of the placenta ; #2, ecto-
covering fold; h, rudiment of heart gill; m, mnscle-hoopa mp,
fred membrane; a, rndimont of the nervous system ; p, placenta Tee pat
cardia} rudiment ; pls, pharyngeal cavity.
This cavity has been called by SALENSKY the secondary follicle-
cavity. Since, according to this author, only some of the rudiments
of the internal organs (the nervous system and the pericardial
rudiments) are formed from the inner cell-mass of the embryo, the
body-wall (together with the rudiment of the intestine) being
derived from the wall of the primary brood-sac; this cavity has, for
SALENsKY, the same significance as the original cavity of the primary
brood-sae (Fig. 217, f). The latter, which is called by Sauensky
the primary follicle-cavity, is said completely to disappear in those
obscure stages which lead up to the stage now being considered, and
438 TUNIOATA.
to be replaced by a secondary follicle-cavity that appears im the sate
place.*
The body-cavity (Sanensiy’s secondary follicle-cavity)
the rudiments of the organs laterally and ventrally from the
wall. In the latter we can now distinguish an external layer, the
ectoderm (Fig. 218 A, ec), from an inner layer, the cells of whieh
wander into the body-cavity, filling it with a mesenchyme. We are
consequently enabled to recognise in this inner layer a part of the
mesoderm-rudiment,
Tf, in order to obtain a correct idea of the relative positions of the
¥'1G, 219,—Horizontal sections through two embryos of a ie
Salpa pinnata, wade jo the direction of the Tt first parts of the
‘ig. 218 A'(after SALENSKY). 6, the blastomeres ;
i, enteric rudiment; @’, layer Ceres the intestine ; enteric rudiment
ec, sotoderm ; 1, slip lateral parts of the (pharynx) to develop
ts of the placenta § 7, is its fat ‘Antoni!
nerye-rudiment ;
P,
pericardial rudiment; pl, blood wavities of the placenta, a
*[Brooxs considers that this second cavity is probably the ori al avity
of the follicle opened a second time by the growl of of the r .
He would thus derive the body-cavity in Salpa from the ,
cavity, the latter he believes to represent the cleavage-cavity
—Ep.j
SALPIDAE—FORMS WITH COVERING FOLDS, 439
is at first a paired rudiment of the pharynx only slightly connected
in the median line. According to SaLEnsxy, the first recognisable
rudiments of this system of organs are found as two entirely distinct
accumulations of cells derived from the wall of the primary brood-sac
(wall of the follicle). Ina similar way, in 8. democratica-mucronata
also, the rudiment of the pharynx develops in the form of paired
cavities (p. 426). Two layers may be distinguished in the enteric
rudiment ; an inner layer consisting of deep cylindrical cells (ento-
derm) and an external layer, the so-called covering layer of the
intestine (d’), in which a few larger blastomeres (enteric blastomeres,
Fig. 219 A, 6) can still be recognised and which may perhaps be
considered as belonging to the mesoderm.
In horizontal sections the rudiment of the nervous system has at
first a curious trilobate form (Fig. 219 4), but this is less marked in
later stages. Toparo (No. 107) has given to the two paired lobes
(n’) the name of the dorsal cise,
and regards them as belonging wo
the mesoderm. He considers them
to be provisional and homologous
to the chorda. The pericardial
rudiment (pc) is distinguished by
the regular arrangement of the
blastomeres found in it, two columns
of the latter in which the cells are
arranged in pairs running through
it. Both the pericardial rudiment
and the neural rudiment project
slightly at their upper ends above
the surface of the embryo (peri- Fi, 250 — Hortoontal section through
cardial and neural projections, Fig. pared ob ave reenate (ate
220, n'); in later stages a dorsal cula of the enteric rudiment (gill-slits
longitudinal furrow runs between ef Backes Beer tS ti
them, but as to the significance of iy Projection j 2, ‘ol wiht
these structures we are still in the somatic layer of the follicle, BRooxs).
dark,
In these stages the embryo lies like a flat disc on the placenta (Fig.
218 A). Only its upper surface appears covered by the cap-like
ectoderm-layer (Fig. 221, ¢c). Lt is not clear in what way the basal
.
met be ule Inaconsabe-als ail probably tas arly exisoed ay
e atrial cavity, but even that structure is not derived from
the wall of the follicle but from the ectodermal blastomeres,—Ep. }
al
SALPIDAE—FORMS WITH COVERING FOLDS. 44d
bridge, first elongates. The trabecula, which is retained between
the two diverticula of the developing respiratory cavity (Figs. 220, d,
and 222, cl) that run upward, represents the rudiment of the gill (4)
which, when the diverticula unite over it, becomes detached from the
dorsal wall of the respiratory cavity.
Fia, 222.—Median section through « later ontogenetic stage of Salpa pinnate (after
Saukysky). ef, wtrinl diverticulum of the enteric rudiment; #, enteric rudiment
frodimoent of the respiratory eavliyy no, ectoderm ; /, ciliated pit; 4, rndiment of
wart; me, mesenchyme; 7, ganglion : pe, pericardium ; pl, tissue of the placenta.
The part of the respiratory cavity which is to be regarded as the
cloacal cavity is not, consequently, according to Sauensky, derived
from an independent rudiment, but arises through the formation of
diverticula from the rudiment of the pharyngeal cavity. Toparo
* (The recent observations of Brooks, Herper aud Konornere are so abso-
lutely antagonistic to those of SALENSKY, that We must now regard the above
account of the origin of the pharyngeal and atrial cavities and of the gill as
inaccurate. Heapsn and Kororxerr are fairly in agreement, and together
they differ in some important points from Brooxs. All three however,
that the atrium develops first, and is probably ectodermal in origin.
The pharynx, on the other hand, appears as a cleft in the embryonic mass
which becomes lined with entodermal blastomeres. The gill arises from the
septum between these two cavities by the breaking down of the lateral parts
of this septum, the clefts thus fanned. being the gill-slits. Saumnaky’s view
that the gill and atrium of Sapa are not homologous with the similarly named.
structures in the other Tunicates may be regarded as disproven,—Ep.}
SALPIDAE—FORMS WITH COVERING FOLDS. 443
begins to assume a simpler form (Fig. 219 B, m) ; at still later stages
it is found as a cell-mass running obliquely downwards and forwards ;
within this mass a cavity appears which communicates anteriorly with
the respiratory cavity (Fig. 222). The part lying nearest to the aper-
ture of the central canal represents the rudiment of the ciliated pit
(/), while the blind end that is directed backward and upward forms
the ganglion proper (n). In the course of development these two
sections of the neural rudiment become more sharply marked off from.
one another by a constriction. At the same time, in the ganglionic
part of the rudiment, the central canal becomes segmented, breaking:
up into three consecutive cerebral vesicles, a condition first noticed
by Kowavevsky, and similar to that
seen in S. demoeratica (p. 429). In
later stages the cerebral rudiment:
becomes completely separated from
the ciliated pit, and the two radi-
ments shift apart, although they seem
still to remain connected by a nerve-
strand that runs forward from the
brain to the ciliated pit. The central
cavity of the cerebral rudiment disap-
pears, and the interior of the rudiment
then seems filled with punctate nervous
tissue (Leypia’s “ Punktsubstanz ”).
A process running towards the surface
leads to the development of the eyes,
in which single cells of the ganglion
become changed into elements sensi-
tive to light, while other cells of the
most superficial layer become filled
with pigment (p. 430). No details
are known of the development of the
paired auditory vesicles which lie in
228. — Horlzontal | setion
through an embryo of Solpa
inmeta (after SALBNSKY), 5,
d, enteric rudi-
ment ; m, mesenchyme-cells ; 1,
rudiment’ of the nervous system |
Fra,
wh, neural cavity; pe, pericardial
rudiment; a, stib* pericardial
coll-strand ; £, cells in the lumen.
ofthe pharynx (atrium, BRooKs).
vontact with the brain, and were first
observed by H. Mtinrer and further described by Toparo (No.
107). The ciliated pit, by the folding of its walls, assumes a com-
plicated form approaching that of the same organ in the adult (Fig.
24, B, fl).
The pericardial rudiment (Figs. 218, 219 pe), whieh is originally
4 cell-strand running from above downward, divides into two parallel
strands (Figs. 223,224 4); the anterior strand, near the enteric
THE EMBRYONIC DEVELOPMENT OF THE SALPIDAE. 445
eavity appears within each of the developing muscle-hoops and these
cavities have been compared by SALeNSKyY with those in the primitive
muscle-plates of the Vertebrates (cavities of the primitive segments).
The elacoblast (Fig, 224, eb) seems to be derived trom cells of the
mesenchyme.
The development of the other species of Salpa that are provided
with the enveloping fold (S. africana, S. fusiformis and S. punctata)
seems to follow essentially the same course as that of §, pinnata,
Judging from the very fragmentary statements as to the ontogeny
of these forms (Barrors, No. 87, Savensky, No. 104), however, there
appears to be considerable variation in points of detail [see Hemmer
(No, XIIL) and Konornerr (Nos. XX., XXa., and XXIa.)}.
General Considerations on the Embryonic Development of
the Salpidae.
{The embryonic development of the Salpidae is still, in spite of the recent
investigations of Brooks, Hemer and Korornerr, anything but satisfactorily
understood. The confusion arising from the immigration of the kalymmo-
cytes and the difficulty of discriminating between these cells and the smaller
formative blastomeres of Hemer and Korornerr, still requires to be swept.
away before we can make any satisfactory comparisons of the development of
Salpa with that of other Tunicates. When, further, we compare the mono-
graphs of the two last authors with that of Brooxs, we must {sel that additional
confirmation of one or other of the views put forward by these writers is
necessary before one of them is finally accepted, It is obvious, however, that
we are dealing with a highly speci form of development which has
possibly arisen in connection with the viviparous habit of Salpa and has been
further complicated in connection with the peculinr life-history of this form.]
The embryo undergoes direct development in accordance with its
retention during the whole embryonic and larval periods within the
body of the parent. We have already seen a similar omission of
metamorphosis among the Ascidiacea in the Molgubidae. The Salpidae
in this respect show a more specialised condition in so far as the
embryo becomes closely connected with the tissues of the parent,
and a placenta develops for its nourishment, Since we may, with
some probability, derive the Salpidae from the attached Aseidialike
forms, we might be tempted to trace back the fixation of the embryo
on the wall of the respiratory cavity of the parent to this original
attached manner of life,
[Considerable stress was formerly laid upon the ontogenetic differences
described by SALENSRY as occurring in the various species of Salpa, especially
on the presence or absence of covering folds and on the structure of the
placenta. According to Sauunsky, the covering folds were completely wanting
THE EMBRYONIC DEVELOPMENT OF THE SALPIDAE. 447
ment, is nevertheless very remarkable, this organ having been con-
sidered as the typical feature of the Tunicate plan of organisation.
To explain this fact, we might be tempted to assume that there are,
among the Tunicates, forms only slightly removed from some primi-
tive ancestor which did not possess a chorda, and in which, conse-
quently, the development of this organ is not sufficiently established
aus a constant feature.*
Sanensky holds that a rudiment of the larval tail is found in the
Salp embryo in the elaeoblast, that problematicul provisional organ
(p. 432) to which Tovaxo (No. 107) no doubt erroneously attributes
such a great significance in comection with the development of the
proliferating stolon. The significance of this structure as a vestige
of the larval tail seems to be rendered somewhat probable by a
comparison with the tailed embryos of Doliolum. [During the
development of this organ, however, there is no suggestion of an
entodermal origin, if anything an ectodermal one is suggested.
Functionally, the elaeoblast appears to be nutritive. ]
Turning to the rudiments of organs, we must first trace the deyelop-
ment of the respiratory cavity. In this we can always distinguish
‘two separate cavities divided from one another by the gill which runs
between them obliquely (Fig. 224, &). The anterior and ventral
eavity known as the pharyngeal cavity (ph) is considered as the
equivalent of the respiratory cavity of the Ascidian, while the pos-
terior cavity which lies dorsally to the gill is regarded as the atrial
cavity (c/). The two large apertures through which these two
cavities communicate on either side of the gill are regarded as un-
usually dilated gill-clefts. In this view, which is favoured by the
condition of the gill in Doliolum, this one pair of gill-clefts occurring
in the Salpidae has been thought to arise by the fusion together
of several smaller clefts, This view is opposed by vAN BeNepEN and
Juni (No. 10), according to whom only one pair of true gill-clefts
develops in the Ascidiwns also, the many perforations of the wall of
the gill which develop later being secondary structures (branchial
stigmata, p. 367). ‘The Salpidae in this case would, like the Larvacea,
exhibit a very primitive character in the presence of a single pair
of gill-clefts. Toparo (No. 113), who adopts this view on the whole,
has extended it by explaining certain ciliated invaginations which
are found arranged im rows at the sides of the gill in some Sulps
® We must, however, bear in mind that the body of the Thaliacean represents
principally the pre-chordal region of the Ascidian larva,
448 TUNICATA.
(S. pinnata, S. bicaudata, etc.), us the homologue of the secondary
gill-clefts in the Ascidians (branchial stigmata). We do not think
there are sufficient grounds for such an explanation, since these
invaginations, which were already known to Fou, may also be
secondary acquisitions resulting from the need for increasing the
respiratory surface. We have pointed out above that the condition
of Appendicularia is probably not to be regarded as primitive.
Some statements as to the occurrence of a true coelom in the
embryos of the .yalpidae have still to be noticed. TopaRo (No. 107)
considers that the coelomic sacs originated through dehiscence taking
place in a mesodermal layer surrounding the intestine. According to
SALENSKY, a cavity first arises in cach of the already distinct muscle-
hoops, and this cavity also is considered as the equivalent of the coelom
in the Vertebrates (#.¢., of the cavities in the primitive segment plates,
p. 445).* =
The origin of the pericardial sac may perhaps, according to
SALENSKY, be traced back to the mesoderm, whereas, in the
Ascidiacea, it is of entodermal origin (p. 368). [Entodermal accord-
ing to Korornerr, see footnote, p. 432].
II. Asexual Reproduction.
Asexual reproduction, both by fission and budding, is of wide
occurrence among the Tunicata, and frequently leads to the forma-
tion of stocks. Before describing these reproductive processes, we
must point out that the capacity for regeneration also occurs to &
large extent in this class. The experiments made by LoEn and con-
tinued by Mrncazzixi have shown that solitary Aseidians (Ciont
intestinalis) are capable of regenerating distinct portions of the body.
If, for example, the central nervous system is removed artificially, it
can be regenerated. In some cases, sinilar processes of regeneration
Hy. This was observed by DELLA VALLE (No.
zona violacea, in which, under unfavourable con-
seem to occur nom
70) in colonies of Di
ditions, the anterior part of the body (the branchial sac and other
organs) degenerate in the individuals of the colony. There is then
found in this region an accumulation of yellow mesoderm-cells filled
with nutritive material. The organs of the posterior half of the body
*(Brooxs (No. L) considers the transitory body-cavity of Salpa as a r-
. opening of the follicular cavity, and this latter he attempts to homologise with
the cleavage-cavity of the normal gastrula, He thus regards the body-cavity
of Salpa as the equivalent of the primary body-cavity (cleavage-cavity), and
not as the secondary body-cavity (coelom’ proper). The cavity becomes filled
later with mesenchyme-cells from which the muscle-hoops arise.—Ep.]
ASOIDIACEA— TRANSVERSE FISSION. 449
(the intestinal loop and the heart) remain unaffected by degeneration,
and, should the conditions of existence again become more favourable,
this posterior half is able to regenerate the anterior part. An increase
in the number of individuals forming the colony, by means of division,
may even be connected with this regeneration. The yellow body then
becomes lobate, and divides into several parts, each of which develops
into a new Ascidian. The details of these interesting processes are,
however, still unknown.
The occurrence of such far-reaching regenerative processes and the
capacity for asexual reproduction in the Tunicata at first sight seems
surprising, when we take into account their comparatively complicated
organisation and their near relationship to the Vertebrata. We must,
however, remember that the saine capacity ix found in the Annelida
and the Echinoderma, groups which, in the condition of their
organisation, may at least be compared with the Tunicata.
1. Social and Composite Ascidians.
The asexual reproduction which takes place in these groups is
usually called budding. In the Polyelinidae, however, asexual
multiplication takes place through the segmentation of the post-
abdomen, This kind of reproduction, therefore, must, strictly
speaking, be defined as transverse fission, and must be considered
as distinct from budding. :
A. Reproduction through Transverse Fission.
This is the kind of multiplication which was defined by Giarp
(No. 57) as “ bourgeonnement ovarien ” and which has become better
known through the researches of KowaLevsky (No, 61) in conncetion
with Amearoucium proliferun,
[Since this description was published, further investigation of the budding
processes in the composite and the social Ascidians has shown us that, while
the account given in the following pages is correct in so far as it derives all
the important internal organs from the inner sac, yet it obscures the actual
state of affairs by always speaking of this structure ag entodermal. While it
is probably true that this inner sac is derived from the entoderm in mort cases,
yet, in one group, the Botryllidae, if the observations of Hsont (No. XIV.) and
Pizon (No. XXVI.) are correct, this does not appear to be the case. Thexe
observers find that the stolon is purely ectodermal, the epicardia arising from
the peribranchial sacs of the parent which, in the first instance, ic., in the
larva, are of ectodermal origin. From this ectodermal epicardium, the bud
arises much in the way described above. Thus we find that, in one family, all
the organs of the bud are of ectodermal origin, while, in the majority, they
arise from the entoderm.—Ep.]
Ga
primary b
ee od uineiie aia
4 transverse partition-wall (s) into a dorsal
(#). The partition-wall itself is hollow,
flat diverticulum of the branchial sac.
ately behind the posterior end of the eu
entrance of the oesophagus, which runs bh
en and ends blindly near its
heart (Fig. 227 A, A)ourved into cl
the entodermal process just
is identical with the tube in Clavslinais
Junin (No. 10) the epicardial tube (see F
ASCIDIACEA—TRANSVERSE PISSION. 451
is also evidently the homologue of the entodermal or endostyle-process
in the proliferating stolon of the Salpidae and Pyrosoma, but the rela-
tive position of the heart distinguishes this so-called epicardial tube
in the Polyelinidae from
the similar tube in the Bie PE vcigg
above-mentioned — forms:
The finer anatomical
features of this tube have
been described by Mavu- a A
RICE (No. 40) in Fraga-
roides, The tube forks at a
its anterior end; the two
prongs of this fork huve D
been distinguished by vax A
Benaven and Juum as
epicardial tubes from the 7
posterior undivided epi- y
cardial sac (see above, p. 1G. 226.—Dingrams illustrating the condition of
370). The two Mploardial bs! ai te PA aide view ps
tubes arise on cither side — “Termparetion atthe level ab 0 atthe leva cas
of the median line behind Pp Lite sac; x, forked end of the epl-
J cardial sac.
the posterior end of the
endostyle from the pharynx. The posterior end of the sac is, according
to Maurice, also forked (Fig. 226 d and D, ep). [t embraces the
ereacent-shaped pericardial vesicle (p). In Clavelina, the epicardial
tube enters into close relation to the heart (p, 370), completing
the dorsal wall of that organ, but this is not the case, according to
Mavnicg, in the Polyclinidae. The heart-tube (/) in these latter is
an invagination of the outer wall of the U-shaped pericardial vesicle
(Fig. 226 © and D).
It should be mentioned that the paired apertures of the two epicardial tubes
ean be recognised only in larvae and in jquite young asexually-produced
individuals, They could not be found by Kowacevsry in the adult xooid,
and Maurice also has recently stated that the two epicardial tubes, although
they approach close to the wall of the branchial suc, seem no longer to com-
municate with its cavity.
‘The epicardial sac in the Polyclinidae divides the body-cavity into
a dorsal and a ventral half (Fig. 225, 4, 4’). Through each of these
halves a blood-stream flows, but, in the dorsal half, the direction of
the stream is opposed to that in the ventral half, the blood flowing
in the one half towards the heart and in the other from the heart to
452 TUNICATA.
the body. Similar conditions are found in the proliferating stolon of
other Tunicates (¢.g., Clavelina, p. 370). In this form also, in the
stolon, a dorsal blood-current is separated from a ventral current
flowing in the opposite direction by the epicardial sac which forms a
partition extending almost to the end of the stolon (Fig. 229, x). A
transverse section through the proliferating stolon of Clavelina shows,
on the whole, remarkable agreement with one through the post-
abdomen of the Polyclinidue, though important variation is found in
the position of the heart. In the Polyclinidae, the heart lies at the
posterior distal end of the post-abdomen (Figs. 226 and 227), which,
consequently, contains important organs belonging to the organiss-
tion of the parent (the heart and the genital organs). In Clavelina,
the pericardial vesicle and the heart have shifted to the point of
origin of the stolon (Fig. 173 C, p. 375). The heart lies at the
proximal end of the epicardial sac, on the ventral side of the latter,
so that the epicardial lamella, as shown above (p. 370), can be utilised
in the formation of the dorsal wall of the heart. The proliferating
stolon of Clavelina does not contain any important organ of the
parent body, but is a process of the body dedicated exclusively to a
sexual reproduction. The same is the case with the stolon of other
Tunicates (Thaliacea and Pyrosuma, etc.). We might imagine the pro-
liferating stolon as derived from the post-abdomen of the Polyclinidae,
if we chose to assume that the genital organs and the heart withdrew
to the proximal end of the post-abdomen, which then became dedicated
exclusively to the function of reproduction. Such an assumption
would satisfactorily explain the remarkable divergence in the position
of the heart above alluded to. While, in Clavelina, the heart lies on
the ventral side of the e
dorsal side of the so-called endostyle-process (Fig. 2
If, then, we assuine that in these two groups the heart origin:
as in the Polycl/nidae, at the distal end of the stolon, it is not difficult
to imagine that secondary shifting took place, in the Clarelina to the
ventral and in Pyrosom to the dorsal side.
icardial sac, in Pyrosoma it is found on the
The ontogeny of the Pyrosoma would, indeed, rather lead us to consider
the change of position of the heart as the consequence of lateral shifting of
the organs. The heart there lies originally on the right side of the entoderm-
process, and only later shifts to the dorsal side (SEELIGER) ; cf. p. 493.
It is evident from the above that the asexial reproduction of the
Polyclinidae (through the segmentation of the post-abdomeu) and
the stolonic budding found, for instance, in Clavelina, are related one
to the other. Giarp (No. 57) has already pointed out that the
ASOIDIACEA—TRANSVERSE FISSION. 458
elongated, branched post-abdomen of many Polyclinidae (e.g., Circin-
alium), part of which creeps horizontally along the substratum, is
remarkably like the stolon of Clavelina. These two methods of
reproduction are thus connected with each other by transitional
Fra. 227.—.1, young Amaroucium before the commencement of asexual reproduction ;
B, Amaroucium with segmented bdomen (after Kowauevaky). a, thorax;
8,'abdomen ; ¢, post-abdomen ; A, heart; », partition-wall ; s', anterior part of the
i y. separated portions of the post-abdomen ; &, anterior swollen
-wall in the posterior separated portion.
The commencement of asexual reproduction in the post-abdomen
of Amaroucium is marked by its elongation and the abstriction of
its soft part from the point of attachment to the rest of the body.
The heart continues to beat after the separation of the post-abdomen
from the abdomen is accomplished. Soon after, the post-abdomen
(Fig. 227 B) breaks up, through transverse fission, into a varying
a
454 TUNICATA,
number of parts, each of which develops into a young Aseidian.
The first sign of development is shown in a widening of the proximal
end (A) of the ectoderm-tube (segment of the epicardial sac) which
lies in every segment. his proximal dilation is the rudiment of
the whole alimentary canal of the young animal. The non-dilated
part of the entoderm-tube becomes the epicardial tube of the yeung
individual. The heart of the parent-animal that has remained in
the distal segment now degenerates. Somewhat older colonies
(Fig. 228) are of a different shape. The young animals, which
originally lay in a row one behind the other, show a tendency to
Fie, 228.—Two young colonies of Amermuetum (after RowALkvakY), A, younger.
B, older stage. a, parent-animal ; 4, an advanced bud; ¢ younger
shift upward within the test towards the parent-individual, and the
whole colony thus beeomes broader and shorter, The parent-indi-
vidual now begins to develop, through regeneration, a new heart
and post-abdomen (Fig. 228 4, a). The danghter-individuals in the
figure seem to be at different stages of development. One of thew
(6) exhibits the almost perfect organisation of the adult Aseidian,
while the three others show the rudiments of the different parts uf
the body, but these are very slightly developed, Tn all these young
individuals the post-abdomen is still comparatively short. Only
later (Fig. 228 8) does it grow out to a greater length, and come
bh ay
ASCIDIACEA—STOLONIO GEMMATION. 455
to resemble more and more the post-abdomen of the parent before
the commencement of asexual multiplication. The anterior part of
the body of the young individual has shifted upward. The branchial
and atrial apertures have broken through, and reproduction by means
of tranverse fission now takes place in the daughter-individuals that
have arisen as described above.
We thus find, in this case, the first rudiments of the young animal
im the portion of the post-abdomen, whieh becomes separated by
transverse constriction, and which, through processes of regeneration,
is able to develop into a perfect new individual. For details of these
processes, see p. 470.
Whether, in the segmentation of the post-abdomen in the Polyclinidac, we
actually have a more primitive form of asexual multiplication, from which
the stolonic gemmation of other Tunicates is to be derived, must still be
regarded as doubtful, It is also possible that the transverse division of the
Polyclinidae is to be derived from stolonic budding, A comparison with the
conditions of development of Pyrosoma shows that caution must be exercised
in such speculations, At first sight we should feel inclined to describe the
rise of the first four Ascidiozooids in the Pyrosoma embryo as transverse
fission (Fig. 193, p. 397), but more careful examination reveals the fact that
the later longitudinal axis of the Ascidiozooid is at right angles to that of the
proliferating stolon. We have, consequently, to regard the rise of these first
Ascidiozooids also as stolonic budding, which is not essentially distinguished
from the budding of the zooids that are produced later (see pp. 404 and 484).
B. Stolonic Gemmation.
The typical form of stolonic gemmuation is found in the so-called
social Ascidians, in Clavelina and Perophora, The single individuals
here send out a creeping proliferating stolon which branches re-
peatedly (Fig. 229), and at the end of which the buds appear as
club-shaped swellings. In structure the stolon closely resembles the
post-abdomen of the Polyclinidae. Here also we find, as has already
been shown, the blood-space of the stolon (primary body-cavity)
divided by the extension of the epicardial sac (stolonic septum, s)
into a dorsal and a ventral half, the blood circulating through these
in opposite directions. Since the partition-wall does not reach quite
to the distal end of the stolon (s) the two sinuses communicate at
this point, where also the stream of blood changes from the one
direction into the other.
The buds here originally appear in the form of bilaminate vesicles
tén). The outer layer of the vesicle, the ectoderm of the bud, is
continuous with the ectoderm of the stolon and of the parent-
456 TUNICATA.
individual. The inner layer, the entoderm, arises as was first proved
by Kowauevsky (No. 60) for Perophora, as a diverticulum of the
stolonic septum (epicardial sac) with which it long remains connected.
Between the ectoderm and the entoderm, the primary body-cavity
of the bud appears filled with
mesenchyme - cells, We shall
presently, following the accounts
of Kowatevsky (Nos. 60 and
61), SeEticER (No. 66), and
vaN BENEDEN and Juuin (No.
10), describe more in detail the
further development of the bud.
We can here merely mention
that, in Clavelina, the connection
of the bud with the stolon, and
through the latter with the other
individuals of the colony, is
usually retained even in the adult.
Not all the root-like processes of
Clavelina seem to be capable of pro-
ducing buds. Many of the ramifica-
tions seem intended merely forattach-
ment or to serve as reservoirs of blood
(SKELIGER). The latter then have
no epicardial process extending into
them. A sharp distinction must be
made between these sterile body-
processes, which may be compared
with the mantle-vessels, and the
ramifications of the actual prolifer-
ating stolon.
Among the Composite .\scidians
Flu, 229,—Portion of n_ proliferating the family of the Dixtomidae
atolon of Pecophuca (after Kowatevsk¥). belongs to the type just described,
ee, ectoderm of the bud ; en, entoderm ha
. the entoderm-vesicle of the bud
of the bud; kn, buds
septum (epicardial lamella} here also becoming abstricted
tion of the stolon,
from a process at the posterior end
of the endostyle. In the Botryllidae, the Didemnidac, and the
Diplosomidae, on the contrary, budding of another type occurs.
The family of the Déstomidae seems to be distinguished by the fact
that its buds separate very carly from the proliferating stolon. They
are then found within the common cellulose mantle scattered bet ween
ASCIDIACEA—STOLONIC GEMMATION, 457
the separate individuals as small, rounded bodies, each provided with
acavity. In individual cases a longer proliferating stolon seems to
occur, as in the stalked colony of Colella pedyneniata, in which
Hexpman found that each individual gave off into the common stalk
a body-process (evidently the proliferating stolon). On this, then,
the young buds arise and detach themselves, shifting upward in the
common mantle-substance in proportion as they develop further. In
another form belonging to this group, described by KowAnevsky
as Didemnum styliferum, but which, according to Denna Vane
(No, 68), belongs to the genus Distaplia, the small buds which early
become independent, arise, as KowALEVSKY conjectured, on a process
eK
280, —Freeswimming larva of Déstaplia (utter Data VALLE). cl, atrial cavity;
¢, atrial aperture ; en, endostyle ; hit, intestine ; 4, branchial aperture ; , detached
Hud; &, bud in the avt of dividing ;'m, stomach! n, ganglion ; oe, 4
adhering organ ; sf, stolon ; <r, larval tail,
directed posteriorly, which must evidently be regarded as a pro-
liferating stolon, This is also the case, according to LAHMmLE (No.
38), with the individuals of Distaplia magnilaroa, but Denna VALLE
(No. 68) assumes thut these processes are merely mantle-vessels.
‘The special characteristic of the Distomidae is the early detachment
of the small buds within the still free-swimming larva (Fig. 230). The
large larvae of Distaplia magnilarva. in which, according to Denia
Vane, the organisation of the adult Ascidian is almost perfectly
attained while the animal swims about freely, also show, between the
endostyle and oesophagus, a short proliferating stolon (st), from which
small buds that multiply by division (4, 4’) are abstricted.
lai
BUDDING OF THE DIDEMNIDAE AND THE DIPLOSOMIDAE. 459
individuals are arranged in such a way that their atrial apertures turn towards
each other, In this way commences the union of the individuals round a
common cloaca (Kroux, No. 63). This young tetrazooid colony shows con-
siderable resemblance to the youngest Pyrosoma colonies. The individuals
which are produced later by budding are always given off by the parent
laterally, and thus occupy the spaces between the parent-individuals. They
themselves are very soon capable of multiplying in their turn. The daughter-
individuals at first lie somewhat away from the common cloaca, towards which
they only shift later. Systems consisting of two concentric cycles may thus
develop, the inner cycle containing the parent-individuals and the outer the
doughter-individuals, While the latter shift to positions round the common
cloaca, the individuals of the inner cycle disintegrate (Jourpars). The
common cloaca is nothing more than a pit-like depression of the outer surface
of the common cellulose mantle. This is also the case with the common
cloaca in Pyrosoma (cf. p. 408).
New circular systems are produced in the Bofryllidae, when one of the buds
belonging toa cycle does not shift towards the common cloaca, but moves
away from it. This individual, by reproducing itself through budding,
becomes the founder of a new cycle.
D. Budding of the Didemnidae and the Diplosomidae.
The conditions of budding in the families of the Didemnidae and
the Diplosomidae are very peculiar. Since, in these fathilies, the
buds remain vonnected with the parent until fully developed, re-
markable double individuals are produced whieh long since attracted
the attention of zoologists. Since, further, in the Diplosomitie, the
first budding process takes place during larval life, free-swimming
and still caudate larvae are found in which two branchial sacs are
well developed. On closer inspection, it is not difficult to distinguish
the branchial sac of the larva from that of the bud. In the brain of
the primary individual, moreover, the larval sensory organ can be
recognised, while in the bud it is wanting.
Budding, in these two families, follows the type defined by Garp
(No. 57) as “ bourgeonnement pylorique."’ According to GaGENBAUR,
Gants (No. 55), Denia Vanue (No. 68) and other authors who have
investigated this method of budding, the newly formed individual
here arises through the concrescence of two originally distinct buds
(Fig. 85), one of which (the thoracic bud, k’) yields the branchial
region with its organs, the peribranchial sacs, and the pharynx, while
the other (the abdominal bud, k) gives rise to the intestinal loop, the
genital organs and the heart. The first rudiment of the abdominal
bud (Fig. 232 A, 4) is found as an outgrowth of the oesophagus of
the parent; the thoracic bud, on the contrary (4’) lies further down
on a level with the stomach, on the right side of the body and,
——
460 TUNICATA.
according to DeLLa VALLE, is derived from a simple outgrowth of
the body-wall (consisting of ectoderm and the parietal layer of the
wall of the peribranchial sac), and thus arises in the way described
above for the buds of the Botryllidar. JourDAIN (No. 64) has
recently stated that these two buds (the thoracic and the abdominal
buds) arise through the division of an original single bud, and that
the connection between the two halves is still maintained for some
time. No further details, however, are known on this point. G1agp
(No. 58) holds that the first rudiment of the bud can here also be
traced back to the epicardial tube.
The «atdominal Ind (Fig. 232 A, &) is thus at first apparently an
outgrowth of the oesophagus of the parent which, however, soon
becomes more sharply marked off (Fig. 231 4 and 2) in such a way
that it then forms a caecum connected with the oesophagus only at
its anterior end (Fig. 231 C'), this becomes U-shaped and can now
be recognised as the rudiment of a new intestinal loop, in which the
ophageal bud of Trufidenswn ® (after
y AL bud: ae, stomach.
tomach, intestine) become differentiated.
y-formed intestinal loop is connected with
of the parent (4+). The intestine of the bud becomes
at of the parent (Fig.
Various parts (oesophagus,
The oesophagus of the new
the oesophagu
B, 4) and enters into com-
munication with it. The parent-individual now possesses two fully
applied tot
developed intestinal loops, with continuous lumina. It is not yet
clearly known how the heart and the genital rudiments of the bud
develop, but Detta VacLce believes that the latter are perhaps
derived direct from the genital rudimeuts of the parent. .
At the
fully (Fis:
sue time, the thorneie hud (Fig. 232 1. &) alse develops
2. 8). We cannot here enter in detail into the some-
Tire genus Trididemnum is included by HinpMax in the genus Didemnum
Savigny.—Ep.]
BUDDING OF THE DIDEMNIDAE AND THE DIPLOSOMIDAE. 461
what unsatisfactory statements of authors ns to the way in which the
organs develop in this bud, but may mention that peribranchial sacs
develop at the sides of the central enteric cavity, that gill-clefts break
through, and that the ganglion, and the branchial and atrial apertures,
appear as rudiments. An oesophageal tube and a short rectum seem
also to appear. The former (Fig. 232 2, a) now becomes connected
with the oesophagus of the parent (o-) near the point at which the
oesophageal tube of the abdominal bud («) enters it. At this one
point, therefore, three oesophageal tubes seem to be connected, viz.,
that of the parent and those of the two buds. This is also the case
with the rectum, the short rudiment of that organ belonging to the
Fig, 232,—Budding in Trididemnin (after DELLA VALLE).
intestinal loop of the
dividual with the rudiment of the abdominal (4), and thoracic (A’) buds ;
pare
Bh individual with the tyo buds at Tater stage of developinent, oesophageal
rudiment of the bud &; «’, oesophageal rudiment of the bud 4’; 4, rudiment of the
intestine in the bud &; 6’, the sane rudiment in the bud A” fntetine af the pore
individual; &, abdominal bud ; 4. thoracic bud ; w, stomach ; o, oesophagu
constricting ectoderm-ring,
thoracic bud (/') entering the rectum of the parent (¢) at the point
at which the rectum of the abdominal bud (4) joins it. If, now, the
oesophagus of the thoracic bud became more closely connected with
that of the abdominal bud, and such connection were also to be
established between the intestines of the two buds, the two halves of
the daughter organism would then at last be united. But althongh
the alimentary canal of the bud is now completed, it still, for a long
period, remains connected with that of the parent, both in the
oesophageal region and through the rectum (Fig. 233).*
* [For further details concerning the development of the complex buds of
the Diplosomidae and Didemmidae, see the recent works of CacLERY (Nos, V.-
VII.) and SaLeNsky (No. XXIX.). Cauzery finds epicardial tubes in the
adults, and from these he derives the thoracic and abdominal buds.—Ep.]
462
TUNICATA.
The thoracic and the abdominal buds do not always develop equally.
In some cases only the abdominal bud develops. This leads to an
Fic. 233, —Late stage in the budding
vrid idemnni (ater DELLA VALLB).
The alimentary canal of the perfectly.
developed bud still remains connected
with that of the parent. ¢, atrial
aperture ; en, endostyle ; é, branchial
aperture: m, stomach ; ‘x. ganglion ;
or, oesopliagus ; r, rectum,
abnormality, consisting of one
branchial region with two fully
developed intestinal loope (Fig.
234 B). In such cases the intestine
of the parent may degenerate later.
This condition is regarded by Datta
VALLE as a rejuvenescence, and
consists in the development of an
individual, the anterior halfof which
belongs to the parent, while its
posterior half develops anew (see
also Oxa, No. 646). A similar
process may occur in the anterior
part of the body, when the thoracic
bud alone develops (Fig. 234 A).
The process is often still further
complicated by the appearance of
the bud-rudiments ofa third genera-
tion arising .from the not yet fully
developed bud. In this way
remarkable combinations of the various halves of the body are
produced.
Vauue, simplified).
B, two intestin:
ASCIDIACEA—FORMATION OF ORGANS IN THE BUD. 463
In the Diplosomidae the process of budding and the formation of double
individuals begins even in the free-swimming larva, but in the Didemnidae
this is not the case.
But forthe statements made by Jourpain we might be tempted to derive
the remarkable budding processes in these two families from a primitive
Jongitudinal fission of the parent,
E. Development of the Organs in Asexually Produced
Individuals.
The development of the inner parts of the bud has been described
by Merscanrkorr (No, 41), Gants (No. 55), KowALevsny (Nos. 60
and 61), Grarp (No. 57), Dena Vane (No. 68), SEELicER (Nos.
66 and XXXIV.), vay Bewxepen and Juvin (Nos, 10 and XVIL),
Hyorr (Nos. 59 and XVI), Oxa (No. 642), and in the still more recent
works of Caunery (No. IV.), Lerivee (No. XXIL.), and Rirrer
(No. XXVIIL). In our account of these processes we have followed
Kowatevsky, whose careful researches have been confirmed with re-
gard to the development of the nervous system and the pericardial
vesicle hy vaN Bsnepen and Junin.
The bud is at first a hollow body consisting of two or three layers
(Fig. 229, kn). The outer layer is the ectoderm (ee) which is in
continuous connection with the ectoderm of the stolon, The inner
layer, the entoderm (en), encloses the primary enteric cavity of the
bad, which, in Claveltna and the Distonidae, originated as a diyerti-
culun of the epicardial sac (entodermal stolonic septum, ‘ cloison ").*
The connection between the entoderm-vesicle of the bud and the
epicardial snc is retained in the social Ascidians (Clavelina and Pero-
phora) for « very long time, often throughout life. According to
vAN Bexepen and Juuis, the stalk-like portion which connects the
bud with the stolonic septum represents the rudiment of the epicardial
sue und of the pericardial vesicle of the budding individual. The
primary body-cavity extends between the ectoderm and the entederm
of the bud ; into it mesoderm-elements soon immigrate, and these are
the first rudiment of the mesoderm of the bud. In many cases
(expecially in the buds of the Distomidae and the Botryllidae) the
*{In Perophora, according to Rirrun (No. XXVIII.) and Lerkver (No,
XXIIL), the developing blastozooid (bud) is connected with the stolonic
septum, not by its branchial sae but by the left peribranchial sac. Rrrren
expresses some doubt concerning the origin of the pericardium; he thinks
that it arises from the inner vesicle, but even if it is produced by the aggrega-
tion of mesenchyme-cells, as Larivan states, it is still probable that its
ultimate source is the entoderm, since the mesenchyme-cells are probably
produced from that layer.—Ep.]}
464 TUNICATA.
genita] rudiments can also be distinguished at an unusually early
stage.* The above is the case in the buds of Distaplia (Kowa-
LEvsky’s Didemnium styliterum), which, as free bodies detached from
the stolon, are found scattered in the cellulose substance of the colony.
In the genital strand (Fig. 235, 7) of the youngest of these buds,
several young egg-cells can always be recognised. These buds,
however, are capable of multiplying by fission (Fig. 235 4), and in
this case the eggs become distributed so that one occurs in each
portion of the original bud. KowALevsky is, therefore, inclined to
regard the buds which form first as stolons which have separated
Fig, 235.—.1, younger, #, older stage of development of Distaplia stylivera (afer
Kowacevsky). In B, the bud is dividing into two. e¢, ectoderm ; ex, entoderm;
7. genital strand ; ms, mesoderm,
from the Ascidiozsoids, the parts resulting from fission alone repre-
senting the true buds, and Laninue has recently adopted this view.
The entoderm-sac is the seat of the most important transformations
through which the bud develops into the young Ascidian. This sac
increases in size and its anterior margin becomes trilobed (Fig. 236).
The middle lobe must be regarded as the rudiment of the branchial
sac (pharynx), while the two lateral parts represent the rudiments of
the peribranchial sacs. These sacs are, therefore, in the bud of
Distaplia, distinctly entodermal in their origin.
The peribranchial sacs grow round the sides of the middle ves-
icle and, at the same time, a process grows from each sac towards
No. XXVla.) the sexual cord is continued
*[In the Didemnidae (Piz
D.)
from the parent into the bud.
ASCIDIACEA—FORMATION OF ORGANS IN THE BUD, 465
the dorsal middle line (Fig. 236 4, »). These processes grow
towards each other and fuse, and thus the single atrial cavity arises
(Fig. 237, cl). In the meantime the two peribranchial sacs have
become completely disconnected from the central cavity.
The peribranchial sacs arise in a slightly different manner in Perophora
(Kowatrvsky, Ritter), in which form, instead of separating as two distinct
sacs and fusing together at a later period, o single bilobed sac separates from
the inner vesicle ; thus the definitive atrial cavity is formed at an earlier
period, the development being apparently abbreviated. SFELIGER also states
that, in Clarelina, an unpaired vesicle becomes abstricted from the dorsal side
Fie, 236.—Two stages in the development of the buds of Dixtaplia atylifera (after
Kowa: -l. younger, B, older stage. «, alimentary canal ; dr, digestive
gland ; 7, rudiment of the stolon ; 7, genital rudiments ; «, nerve-tube; p, right
peribrainchial sac.
of the enteric sac, which persists as the atrium and, growing round the sides
of the pharynx, forms the peribranchial sacs. Hort (No. 59) again, recently
stated that, during the budding of Botryllus, a saddle-shaped vesicle becomes
separated from the inner vesicle of the bud and gives rixe to the atrial cavity
and the paired peribranchial sacs.
At the time when the peribranchial sacs form, an unpaired caecum
grows out from the posterior end of the entoderm-vesicle (Fig. 236
A, d); this soon bends to the left and thus becomes the rudiment.
of the intestinal loop (Fig. 236 4, /) in which latter the different
sections (vesophagus, stomach and intestine) can be more distinctly
made out. A diverticulum (dr), rising from the pyloric region,
develops into the rudiment of the so-called digestive gland (Fig. 237).
L
is
The rudiment of the genital organs appears dorsally to the intee
tinal loop immediately below the nerve-strand (Figs. 238 £, g, and
237, g). The strictly median position of this rudiment is specially
distinct in the buds of Clavelina, where it appears only in later
stages. It apparently takes its rise as an agglomeration of closely
crowded mesenchyme-cells, in which a cavity nevertheless soon
appears ; this is the primary genital cavity round which the cells —
become grouped like an epithelium. For the development of the
male and female genital organs from this uniform rudiment we must
refer the reader to p. 379. |
468 TUNICATA,
Fig. 239.—Three consecutive ontogenetic stages in the so-calli
(after Kowangvsky), 4, dorsal view of the anterior part of
of an older stage ; C, dorsal view ofa still more advanced. vanced stage.
ment; ec, ectoderm ; ea, entoderm ; ” epicardial process; m,
nervous system ; p, peribranchial adc
bud of 4
Important observations on the condition of the pericardial yesiele |
and the epicardial tube have been made by vaw Benepen and Junin. |
Following these authors, we shall first deseribe the condition of «
more fully developed bud traced in a series of cross-sections,
last section of this series (Fig. 238 7) passes through the
U-shaped intestinal loop (i) and, ventral to this, the stolonie
(st) is seen, In anterior sections, we find that the latter
connection with the pericardial vesicle (Fig. 238 D, pe). 4
(%) here also has arisen through the invagination of the
pericardial sac (pe). A section cut further forward (J
shows the double or forked end of the epicardial sae
im close contact with the pericardial vesicle. Further
pericardial vesicle decreases in size (Fig, 238 C@, pe)
ASCIDIACEA—FORMATION OF ORGANS IN THE BUD. 469
disappears (Fig. 238 B), while the epicardial sac (¢p) can be followed
forward to the point at which its wide paired aperture (Fig, 238 4)
enters the pharynx (branchial
sac). In other words, the epi-
cardial sac arises from paired
apertures situated ventrally to
the entrance of the oesophagus
and extends backward, its forked,
blind end becoming applied to
the pericardial vesicle. The
latter is continued direct into
the stolonic septum, At an
earlier stage, the forked end of
the epicardium is found to he
continuous with the wall of the
pericardial vesicle, and the cavity
of the epicardium of the bud
communicates at this point with
the pericardial cavity. Conse. — Kow.
quently, the cavity of the cenit: Piseeial Hewes ks,
stolonic septum, the pericardial pa Pic Arata SIMO. PPE
sac and the epicardium are
merely differentiated portions of one and the same system of cavities,
Tn the larva, however, according to vaN BexepEN and Jucr, the
relation of the parts is different (p. 368).*
With regard to the further development of the bud, we can merely
briefly mention that the gill-clefts appear as perforations of the
contiguous walls of the pharyngeal sae and the peribranchial sacs ;
that the muscles are derived from strands of mesenchyme-cells, and
that the branchial and atrial apertures owe their origin to ectodermal
invaginations which become connected with the pharynx and the
atrium. ‘The endostyle appears in the form of « fold of the ventral
wall of the pharynx.
Fio, 240,—]
so-onlledd
* [Jou (No. XVIL) has recently investigated the origin of the pericardium
in Distaplia, and finds a condition which differs somewhat from that described
above as occurring in Clavelina. As in Clavelina, a pair of epicardial (pro.
cardia!) tubes prow out from the pharynx on either side of the oesophageal
Aperture; of these the left becomes divided transversely into an anterior
portion, which then fuses with the left epicardial tube to form the epicardial
sac, and a posterior portion which gives rise to the pericardium and heart.
From: the recurved end of the right sac the primary bud is given off before
the tube fuses with that ou the left to form the median epicardial sac, —Ep.]
470 TUNICATA.
Ontogenetic processes altogether similar to those just described are
found in cases where the detached parts of the post-abdomen in the
Polyclinidae regenerate (Fig. 239). Here also the central entoderm-
vesicle is the first seat of transformation. As has already been men-
tioned (p. 454), the first change to occur is a widening of the proximal
end of the epicardial septum (Fig. 239 8). An entoderm-vesicle is
in this way formed in the proximal part of the young individual, and
this is continued backward into the part of the stolonic septum which
did not widen. The proximal dilatation is the rudiment of the whole
alimentary canal of the new individual, while from the non-widened
part are derived the epicardial sac (Fig. 239 B, ep) and probably
also the pericardium. The entoderm-vesicle here also divides up in
the same way as in the bud into three lobes (Fig. 239 A and 8), the
middle lobe being the rudiment of the branchial sac and the two
lateral lobes the rudiments of the peribranchial sacs. The complete
abstriction of the latter, their interconnection (Fig. 239 C) to form
a median unpaired dorsal part (atrium), and the development of the
gill-clefts all occur in the same way as in the formation of buds
The rudiment of the alimentary canal is here also a small, unpaired
caecum (Fig. 239 B, d), which grows out at the dorsal side in the
posterior part of the entoderm-vesicle and curves into the shape of
the letter U. In the development of the more important systems
of organs, we thus have here complete agreement with the pmcees
of gemmation.
2 Doliolidae.
We have already mentioned (p. 385) that two stolons are apparently
found in the * nurse” (blastozeoid) generation of Ddidaae. one ventral
(es) and the other dorsal (Ze. the dorsal outgrowth de i
much greater size than the true ventral stolen, and finally
to hetermorphous individuals kuown as lateral
(the gastrozovids and phorozooids:. The two st
si usiidle bunks
nS Vary xTeatiy in
structure. As will be seen later. the scalled dorsal stok
to the buds derived from the true ventral p
it is better to restrict the tern stelon te
speak of the dorsal structure as the dora
The ve ntea std Figs. ISO, rp) B83: Bo: oe. foreriy
kuown as the neette-like onnin, and first reeounised by GBOS REN
Tein its true charmeter as proliferating <<
DOLIOLIDAE—AS8EXUAL REPRODUCTION,
471
shaped projection rising from an ectodermal depression ; in cross-
section, this is found to be composed of seven parallel cell-strands
(Fig. 242) four of which (4 and ») are
arranged symmetrically in two pairs,
while the other three (a, z, and m) are
unpaired. Groppen and Unsaniy differ
considerably as to the origin and signifi-
eance of these seven strands which, with
the ectoderm that envelops them, form
the primary rudiment of the | buds.
These two authors agree that two cones
grow downwards from both the pharyn-
geal and the atrial cavities of the
“nurse” (Fig. 241, ph and cl), enclosing
between them a mesoderm-mass (we), the
origin of whieh has already been traced
(p. 387; see also Fig. 182 B, 1 and
ms’, yx 387). According to Unsanin,
the stolon is thus originally composed
of five strands, two pharyngeal and two
atrial strands and a middle mesoderm-
strand (Fig. 241). The number of these
NIN).
ut-anitul ; ct’, out
‘all
weynx of the paren
Satgrowth of the wall of the
pharynx.
strands becomes augmented to seven when the atrial strand (c/’)
becomes bent on itself, its reflexed end
giving rise to a new pair of strands (c/").
The fusion of this pair, according to
Unsanty, yields the future neural rudi-
ment (x), while the pharyngeal strand ( p)
yields, by dehiscence, an unpaired middle
strand (#) in which this author sees the
rudiment of the pharyngeal cavity of the
bud. The unpaired strand (7) is said to
represent the pericardial rudiment, while
the pharyngeal strands (») change into
the genital rudiment, and the atrial
strands (4) into those of the muscle-plates.
According to Grospgy, on the con-
‘trary, the pharyngeal strands (») represent
the rudiment of the pharyngeal cavity
and intestine of the bud, the atrial strands
Fic. 242,—Transverse section
through the ventral stolon or
a primitive bud of Juliolun
( m after Gnoppes and
UIJANIN). ¢¢, ectoderm ; h,
rwusole-rudiments ; i, peri-
cardial radiment ; a, newral
rudiment; p, genital rudi-
mont, pharyngeal rmdi-
ment (UisANIN).
(&) the later rudiment of the atrium, and the unpaired mass x
bi
472 TUNICATA.
(resulting from the fusion of paired strands) is assumed to be the
genital rudiment.
wn ¢
Fic, 243,- Dorsal view of the posterior part of the
body in a large * nurse" (blastozooid) Dulinfua
(after BaRRots). 7, lateral Inds (gastrozooids) ;
im, median buds (phorozooids) ; mss", four
posterior muscle-hoops ;p, perieardinm ; r.
Tosette-like organ ; sf, ventral stolon ; at’, dorsal
outgrowth ; 4, primitive buds wandering to the
ids.
veutral side of the “* nurse"; 1’, primitive
wandering to the dorsal side: 1, prin
bads on the dorsal outgrowth,
The unpaired strand m is regarded as the rudi-
ment of the pericardial sac,
and the mass n, which from
the first is unpaired, as that
of the nervous system.
The buds are produced
through the transverse con-
striction of the ventral
stolon (Fig. 243, r and st),
from which they eventually
become completely de-
tached. In structure, they
resemble the stolon itself,
being composed of an ecto-
dermal envelope and the
seven strands above de
scribed. They are not
capable, however, of de-
veloping further on the
ventral stolon. GROBBEN
has therefore regarded the
ventral stolon, which is
evidently homologous with
the proliferating stolon of
the other Tunicates, as a
vestigial stolon, and con-
siders the buds produced
by it as abortive.
A cross-section of the
dorsal outgrowth (Fig. 243,
at’) of the “nurse” genera
tion (blastozooid) reveals
an essentially — different
structure (Unsantn). Like
the ventral stolon, it is
covered superticially by a
layer of ectoderta, thickened
on the dorsal side, but the
interior of this outgrowth
fis occupied merely by two
DOLIOLIDAE—ASEXUAL REPRODUCTION. 473
blood-vessels separated by a purtition-wall of connective tissue. The
transverse section of this outgrowth, indeed, strikingly recalls
sections through the stolon of certain Ascidians, although it must
be noted that, in these latter, the partition-wall is formed by an
entodermal lamella (epicardial lamella) of which nothing can be seen
in Doliolum. ;
The buds which develop from the dorsal outgrowth as lateral and
median buds (Fig. 243, /, m) do not arise independently from this
structure. GROBRBEN conjectured that they were all abstricted from
a “primitive bud” found at the base of the stolon. Uxsanin, on the
other hand, observed that the parts which become abstricted from the
ventral stolon are capable of wandering along the surface of the body
of the parent, and in this way reach the dorsal outgrowth (wu, x’).
These wandering cell-masses are the primitive buds, from which the
lateral and median buds of the dorsal outgrowth arise by constriction.
The primitive buds that first reach the dorsal outgrowth remain at
its base and, through fission, produce buds which become arranged
along each side of the stolon, developing into the lateral buds or
gastrozooids (/) ; consequently the buds towards the distal end of the
row are more highly developed than those near its base. Those
primitive buds which reach the dorsal outgrowth later are distributed
along the whole of the middle dorsal line (w’’), and by gemmation give
rise to the median buds or phorozooids (m). These buds are arranged
in groups alternating on either side of the row of primitive buds (w”).
The buds of each group develop unequally, but here also there is
an advance in development towards the distal end of the stolon.
According to Unsantn, the ventral stolon is thus the only prolifer-
ating stolon of the ‘ nurse” generation ; the dorsal outgrowth cannot
be regarded as a true proliferating stolon, but is merely a body-process
serving for the nourishment of the buds attached to it, which can be
traced back to the mantle-vessels of the Ascidians.
The Doliolidae thus show an early detachment of the buds from the
proliferating stolon such as takes place in the Distomidae (pp. 456 and 463), in
which family also the detached buds are able to multiply further through
fission. The wandering of the primitive buds and the development of their
descendants in their secondary position in the Doliolidue are very remarkable.
The statements made on this subject have been confirmed for Doliolum by
Barkols (No. 77), and similar processes have been observed in Anchinia and
Dolchinia, so that little room is left for doubt on this point. According to
Uxsanty, the buds are able to move by means of pseudopodia-like processes
of their ectoderm-cells. According to Barkors, on the contrary, there are,
ou each side of the ventral stolon of Doliolum, large amoeboid cells, arranged
DOLIOLIDAE—ASEXUAL REPRODUCTION. 475
ventral process (s/) near the posterior end of the body ; this process
is derived from the peduncle counecting the median bud with the
Fie, 245.—The three generations of Doliotum Millleri (utter GKONBEN). A, young
“nurse” or blastozooid; 14, the phorozooid ; C, sexually mature individual or
gonorooid. d, alimentary canal; as, dorsal outgrowth of the “nurse” form; 4
endostyle; #, peripharyngeal band; y, buds of the sexual generation; h, testi
Aa, integumental sensory organ; hx, gill-clefts; , ganglion ; of, auditory organ ;
wm, ovary; p, pericardial vesicle and heart; «/, peduncle for the attachment of the
onosoolds (7) to the phorozooid ; ex, ventral stolon of the nurse’ form (so-called
rosette-like organ).
476 TUNICATA.
dorsal outgrowth of the “nurse,” and carries the buds of the sexual
generation or gonozooids (g). The latter was formerly thought to be
produced from the phorozooid itself, and these buds, when free, were
consequently known as the second “nurse” generation. The sexual
generation, however, is produced from a primitive bud which has
become attached to the base of the peduncle of the phorozooid. This
bud, according to Uxsanry, is not yielded by the phorozooid itself,
but is a direct descendant of the primitive buds (Fig. 243, «”) which
wandered over to the dorsal outgrowth of the first “nurse "’ genera-
tion. Utsantn therefore regards the median bud merely as the foster-
mother of the buds which develop into sexual animals.
There is thus, according to Unsantn, in the whole cycle of genera-
tions of Doliolum, only a single true proliferating stolon, viz., the
ventral stolon, which alone is capable of producing primitive buds.
All the individuals derived from these primitive buds which attain
development represent, according to this author, only the heterv-
morphous forms of one and the same generation, the sexual gener-
tion either, as nutritive animals or as foster forms, losing their genital
organs, or else changing into actual sexual animals (Fig. 245 €).
According to GRoBBEN, on the contrary, who practically follows
GEGENKAUR, the cycle of generations of Doliolum consists of two
successive asexual generations and one sexual generation. GROBBEN
consequently regards the form yielded by the egg, which differs
essentially from the sexual animal, as the first “nurse” form 4.
This gives rise to the two heteromorphous forms (lateral buds Z, ant
median buds ./): of these. the median buds, in their capacity of
second “nurse ” generation, produce the sexual generation 4G. The
abortive buds yielded by the ventral stolon represent a lateral branch
of the eyele of generations (A). These two opposite views may be
tabulated as follows :-
Alternation of Generations in Doliolum.
GROBRED's view : Tianis’s view :
A (first “nurse ” form) A ("nurse ” form)
K-
L. = M (second * nurse ~ form) Lo + M + G (sexual generation!
1
G (sexual individual)
We see from the above that if ULranty’s view is contirmed. the
alternation of generations in Defiolun closely resembles that in the
ALTERNATION OF GENERATIONS IN DOLIOLUM. 477
Salpidae, consisting in both cases of an asexual generation followed
by a sexual generation. The only difference between the two would
be that the primitive buds produced by the asexual generation,
in Doliolum, multiply by fission, and that the sexual generation
develops in three different forms (L, M and G). In this way also
the fact that the median buds entirely agree in structure with the
sexual generation would be explained (Fig. 245 2 and C).
We have still to describe the development of the young buds after
their detachment from the primitive buds. According to Unsanin,
all these buds, whether lateral, median or sexual, develop more or
less in the same way, so that an account of the development of the
lateral buds will suttice. The young buds, immediately after abstrie-
tion, resemble in structure the primitive bud and the ventral stolon,
consisting of an external layer of ectoderm and of the seven strands
mentioned above. We have already mentioned (p. 471) the different
views held by GRoBBEN and ULJANIN as to the significance of these
seven strands in connection with the further development of the bud.
We have as yet comparatively few statements as to the way in which
the young bud develops out of these seven primary rudiments, and
further investigation of this point is very desirable.
The young bud (Fig. 246 4), from the time when it becomes
detached from the primitive bud, is a completely independent organ-
ism enclosed in ectoderm, but attached externally, like a parasite,
to the body of the “nurse “ or to the foster form. This attachment
is brought about by means of a thickening of the ectoderm (ec). In
the youngest buds observed by Unsanix, the body was already
elongated, and the organs, as compared with the seven cell-strands,
had already changed their positions. The dorsal side can now be
distinguished by the presence of the large rudiment of the nervous
system (n), while, on the ventral side, the pericardial rudiment (p)
is to be observed. Between these two, the pharyngeal rudiment (ph)
can be seen, while the paired genital rudiment, forming a common
cell-mass, has shifted into the neighbourhood of the point of attach-
ment of the bud. The ectodermal invagination (el), behind the
nervous system, represents the rudiment of the atrium. This is one
of the principal points in which Unsantn’s description differs from
that of GRoBBEN. According to the latter author, the atrium arises
from paired rudiments (the strands & in Fig. 242) already present in
the primitive bud. At the two sides of the body, the muscle-plates
(m), lying in close contact with the ectoderm, have extended con-
siderably.
478 TUNICATA.
In the course of further development a cavity forms in the pharyn-
geal rudiment and gradually enlarges (Fig. 246 B, ph) ; this soon
opens externally through an aperture (branchial aperture, ¢) resulting
from an ectodermal invagination which forms opposite to the atrial
aperture. The muscle-hoops which lie in the neighbourhood of the
atrial and branchial apertures now become separated from the muscle.
plates. The pharyngeal cavity gives off a pair of flat lamella-like
diverticula (2) towards the dorsal side, and these, as GROBBEN had
already observed, embrace the neural rudiment (n) laterally. | Acoord-
ing to Uxsantn, these diverticula are concerned principally in the
Fro, 246 --Diagrams illustrating three st
Dotiolum, seen from the side(.1 and B,
d, enteric rudiment ;
atyle-rudiment; 9
growths of the
ment >
in the development of the lateral buds of
ter ULJANIN ; C, after GROBBEN), ¢/, atrium;
ectodermal thickening ; es, endo-
enital ruc é rat || aperture ; &, gill; 7, Interal out-
ine of the muscle-phites ; 2, neural rudi-
rudiment ; ph, pharyngeal rudiment.
formation of the branchial lamellae. A process (d) running back
from the pharyngeal cavity develops into the intestine of the bud, its
blind end becoming applied to the wall of the dilating atrial cavity
(el). The latter extends specially towards the sides of the body, 80
that, as GROBBEN observed, at a certain stage it resembles a pair of
spectacles. Its lateral extensious become applied to the wing-like
processes of the pharyngeal cavity (/). This juxtaposition of the
atrial and pharyngeal walls gives rise to the branchial lamellae (Fig.
246 C,é), in which the gill-clefts then break through. The rudiment
of the central nervous system undergoes a transformation closely
ALTERNATION OF GENERATIONS IN DOLIOLUM. 479
resembling that described above for the larva on leaving the egg
(p. 386). An anterior narrowed portion becomes the ciliated pit,
4% posterior process changes into an unpaired nerve running from the
ganglion, while a third part of the rudiment develops into the gang-
lion and the sub-ganglionic body. The pericardial sac (p) and the
heart seem, as in the larva, to develop according to the type preva-
Jent among all Tunicates. The genital radiment (y) degenerates in
the lateral buds. It can still be recognised for some time to the left
side of the intestinal loop as a mass of cells. In the buds which
become Sexual individuals this rudiment breaks up into two unequal
portions, the anterior smaller part developing into the ovary and the
posterior lurger portion into the testis.
The later differences found in the lateral and median buds and the
sexual individuals are explained by the varied forms which these
finally attain. The median buds and the sexual individuals tend to
assume the characteristic barrel-shape, while the lateral buds increase
in height and, as mentioned ubove, adopt the somewhat asymmetrical
spoon-shape, owing to the dilation of the atrial aperture and
cavity.
‘Two more remarkable and insufficiently understood genera, Anchinia and
Dolehinia, which, in the structure of their gills, form a transition between
Pyrowoma and Doliolum, have still to be added, In these genera only parts
of detached stolons are known, the asexual “nurse” form, developed from
the egg which produced the stolon, being still undiscovered. We shall have
to compare these stolons with
the dorsal outgrowth of the first
“nurse” generation of Doliolum.
Asa rule the stolon, in Anchinia
and Dolchinia, consists of a tube
which, in cross-section, is round
{the colonial tube, Pig. 247, ¢),
und seems to be formed of a
‘single layer of flat ectoderm-cells.
The Interior of the tube is filled Fre, 247.— matic oroar-section through
with a gelatinous mass, in which the colonial tube (dorsal outgrowth) of
ase embedded. mesoderm -cells Dolchinia (atter Konorxery), ¢, colonial
varying insbape, Ths external ‘via’ tan’ "© the 8
surface of the ectodermal tube
which is covered by a cellulose mantle carries the various buds (2) which, in
later stages of their development, seem to grow out on stalks from thickened
parts of the ectoderm. The buds are thus here, as in Doliolum, ouly attached
to the exterior of the so-called stolon. They appear irregularly arranged along
the stolon, quite young buds being found among others half-developed, and
others again fully developed, In a transverse section of a stolon, however, the
480 TUNICATA.
youngest buds are found to occupy the dorsal middle line, while the older
buds shift to the sides of the atolon (Fig. 247). "
Three different forms have been found in the colonies known of Anchinia,
but these are regarded by Barrois (No. 77) as fragments of the same stolon in
different stages of development.
I. There are fragments with zooids which, even when developed, remain
comparatively small, which are without genital organs and are incapable of
reproducing themselves by budding. These zooids are distinguished from
those of the sexual generation (III.)
by the absence of the taree pigment-
spots and of the papilla-like pro-
cesses above the apertures of the
body. An accumulation of pigment
is found, on the other hand, at the
base of the peduncle (Fig. 248 4,
pda). On the dorsal side of the tube
which bears these zooids a slightly
coiled thread is found ranning longi-
tudinally (Fig. 249, st); this consists
apparently of ectoderm and ento-
derm (Fig. 249 B), and is assumed
by Barros to be the actual pro-
liferating stolon, from which the
buds of this generation grow out
laterally.
I. Fragments with zooids re-
sembling in shape the sexual forms
(III.) in which also the rudiment»
of genital organs appear. The-e
rudiments. however, degenerate
later. These zooids, which may
be compared to the phorozooids of
Doliolum, do not seem to reproduce
themselves either sexually or asexu-
ally. They do not, however, directly
nourish the buds of the sexual
eration Of generation, for these grow out inde-
; By sexual generation (after ri
ii alimentary. vanals +s, pendently on the colonial tube. In
o jk. gill; p, papilla above the — the tube on which thexe zvoids are
branchial ‘aperture; ‘p’, papilla above found, the structure above described
the atrial aperture ; 7,’ peduncle. : Apa ig
as the proliferating stolon is ne
longer to be seen ; but there are clusters of very smal] buds which Barrot:
holds to be derived from the disintegrated proliferating stolon.
III. Fragments with sexually mature zooids (Fig. 249 3). Each of thee
zovids ix distinguished by the possession of a papilla-like process ;p, p') above
the branchial and atrial apertures, that over the latter being specially large.
On these processes, there are accumulations of pigment ; a third pigment-spet
ovcupies the middle of the body. These zooids are further distinguished by
the great depth of the body and the abbreviation of the endostyle (es). The
ALTERNATION OF GENERATIONS IN ANOHINIA, 481
young buds from which they are derived are found scattered between the
developed zooids on the colonial tube.
Barpots compares the zooids of type I, to the lateral buds of Doliolum, and
those of the second type to the phorozooids or foster forms of Doliolum, The
three different forms of the asexually produced generation are believed to
develop in succession in the colony of Anchinia, the one replacing the other.
On the youngest stolons, zooids of the first type develop; later, when the
proliferating stolon proper breaks up into portions, only zooids of type TI.
develop, and these finally are replaced by the sexually mature forms.
The budding of Anchtwia has been described most in detail by Bannots
(No. 77) and is in many respects of great interest, It appears that the develop-
mont of the three types of buds takes place as a rule in a very aniform manner,
although considerable
variety prevails in the m
time and manner of de-
velopment of the organs
into which we cannot
here enter further. In
Doliolum, the prolifer-
ating stolon is composed
of & number of longi-
tndinal strands and, ¢
consequently, even the
youngest buds show the
separate rudiments of
the most important
organs, bub the stolon
of Anchinia (Fig. 249 1) =
is composed merely of B
ectoderm and an inner
cell-mass called by atin
Bangows entoderm. The rs beer os stolon of ante iB, oe of
mame structure is ex- Im PROTON AIRE SOLE; OES “CURA Ly .
hibited by the very Toke tuitions, ethene a
smal] or youngest buds liferating stolon.
in which an ectodermal
Jayer and a central cell-mass can be distinguished. The latter becomes
differentiated, in a way as yet insufficiently known, into the nervous system,
the enteric canal (pharynx + intestine), the pericardial vesicle and (in
types II. and IIL.) into the rudiment of the genital organs which appears
very early, Certain features of the later development of type I. are of special
interest, as showing close resemblance to the manner of development of the
Ascidian embryo, ‘The nervous system is found in the form of a tube running
along the whole dorsal side of the bud (Fig. 250 A,n). The anterior part
of this tube gives rise to the ganglion and the ciliated pit, while the posterior
part changes into an unpaired nerve-strand (Fig. 250 B) which runs back-
ward and ends in a visceral ganglion. This nerve no doubt corresponds to
the strand observed by Unsaxry in Doliolwm and called the branchial nerve,
Tt may be regarded as the homologue of the “ cordon ganglionnaire viscéral "
1
wt A
recall the atrial vesicles of the Ascidian larva and |
in the embryo of Pyrosoma. These paired ectoder:
Fie. 251.—Portion of « colonial tube of
Dolehinia with its 2ooids (after Konor-
NEFF). a, points of attachment of older
nooids; c, colonial tube; y, buds giving
rise to the sexual individuals ; g’, priiuitive
bnd of the sexaal gemmae; v, wandering
primitive buds; 2, z00id.
ALTERNATION OF GENERATIONS IN DOLCHINIA. 483.
pf the endostyle (es). From this diverticulum, the pericardial vesicle becomes
separated in late stages. It is probable that a part of the diverticulum is
retained here as an epicardium (p 368,).
Of the form known as Dolchinia (Kornornerr, No. 82), the only colonies
which have been found have zooids corresponding to types Il, and IIL. of
Anchinia (i.c., foster-forms and sexual gemmae, Fig. 251). The individuals
belonging to type Il. (phorozooids) are here closely crowded on the common
colonial tube, while the sexual gemmae (g) develop on the peduncles of the
foster forms (in the'same way, therefore, as in Doliolum). These foster forms
(phorozooids) of Dolchinia (2) become very easily detached from the colony
and then lead an independent pelagic existence. In the same way, the sexual
individuals sever themselves from the phorozooids. Migrating primitive buds
(u) may be seen on the colonial
tube. These are able to move 2 A a
from place to place (as was
stated by Barrors for Doliolum
and Anchinia) by means of
large, amoeboid cells (c) adher-
ing externally to the bud on
each side (Fig. 952 A,a), Small
secondary portions (b) become
detached from the primary
primitive buds by fission, and
these either attach themselves
to the colonial tube itself and
grow into foster forms (phoro-
sooids), or else settle on the
peduncle of one of the develop-
ing foster forms and there
change into primitive buds of
the sexual gemmae carried by
that individual (Fig. 251, 9). bac Foapeey eign sag is of Dolchinia
( two. deta ds (b); J, Transverse
The primitive bud seems to ection throngh an attached tnd of Dolehowia
produce only sexual gemmao, after Konorxnyy). a, primitive bud; },
and does not itself develop letached buds; ¢, amoeboid transporting
further, For the further de. “M81, ¢¢ eotoderm'; m, muscle-rudiments ;
. ph, pharyngeal rudiment; st, epithelium of
velopment of the buds, which the colonial tube ; , mass of large cells.
has not been made out quite
clearly, we must refer the reader to the statements of Koxorxnvy (No. 82),
Here also only an outer and inner layer of cells can at first be dis-
tinguished. In the latter, a mass of large cells (Fig. 251 B, x) soon
from a mass of small cells. The large cells are said to be the
rudiment of the nervous system and the genital organs, while the mass com-
posed of small cells breaks up into three strands, the median strand (ph)
representing the rudiment of the pharynx, while the lateral strands are
thought to represent the muscle-rudiments. The atrium arises as an ecto-
dermal invagination, and the pericardial vesicle as a diverticulum of the
pharynx.
PYROSOMA—DEVELOPMENT OF THE PROLIFERATING STOLON. 485
large cells are seen in it which can be recognised as young egg-cells.
The rudiment of the genitul strand has, in its turn, become abstrieted
from the genital rudiment (0, 1) of the parent (of. ma, in individuals
1 and I),
Fra, 258-—A clin of three individuals of Pyrowma (after Sunuione), 1; youngest,
proximal bud ; 7?, middle and Z/7, oldest, distal bud (nearly fully developed). 2,
point at which the endostyle-process of the parent enters ; d, endostyle- or entoderm-
process ; de, rudiment of the alimentary canal ; dm, elongate cell-mnass ; ¢, rudiment
of the atrial aperture ; eb, elaeoblast ; Hc, ectoderm of the parent ; ec, ‘ectoderm-
plate of the stolon-rudiment ; #s, endostyle of the parent ; es, endostyle ;” fy, ciliated
pit g, ganglion ; fh, teatis; hd 'intostine ; As, heart; iy rudiment of thé branchial
aperture ; &i, atrium ; ks, gill-clefts ; {f, internal longitudinal gill-bars ; Zm, lenti-
cular phosphorescent organ ; m, stomach ; ma, genital strand ; n, rudiment of the
nervous system ; 0, ovary (égg-follicle with egg); oe, oesophagus ; p, dotted line
indicating the boundary of the peribranchial sacs ; pe, pericardium ; rz, languets ;
¢, tentacle-rudiment ; 0, duot connecting the enteric cavity of the second’ individual
with that of the third. +
These two rudiments (those of the entoderm-process and the
genital strand) can also be recognised in a transverse section through
this region of the body of the parent (Fig. 255). We then also see
that the entoderm-process is still accompanied by isolated mesen-
chyme-cells (Fig. 254, ms). At the two sides of the process especially,
mesenchyme-cells can be seen arranged in two strands (ms). The
486 TUNICATA.
strand on the right is continued as far as the pericardial vesicle of
the parent. These strands are the so-called mesoderm-strands of the
proliferating stolon mentioned above, the development of which in
the four primary Ascidiozooids was followed by SaLENsky (No. 74).
The part played by these mesoderm-strands in the further develop-
@o
rs
Fia, 254.—Transverse sec-
tion through the ento-
derm - process (en) of a
very young stolon-rudi-
ment of Pyrasoma (al
SBRIIGRR). m4, xurrou
ing mesenchyme-cells,
ment of the proliferating stolon and of the
buds is still obscure, and therefore no
further note will be taken of it.
That part of the ectoderm towards which
the entoderm-process is directed (Fig. 253.
ec) seems somewhat thickened even in the
early stages. ‘I'he actual stolon now begins
to form in this region as a rapidly increasing
swelling. The conical process that results
(Fig. 256 A) is the stolon at the distal end
of which transverse constrictions soon lead
to the abstriction of individuals (Fig. 256 C).
In transverse sections through a developing stolon (Fig. 255), the
rudiments of the peribranchial tubes (p) can already be seen at either
side of the entoderm-process. How these rudiments arise is not yet
accurately known, but as they are found connected by their distal
ends in a certain way
with the — genital
strind, SEELIGER is
inclined to derive
them from the latter.
He therefore reganls
the peribranchial
tubes in the buds of
AS meso
structures,
although, in the
Cyathozooid and in
the first four Aseidio-
zooids, they are un
doubtedly derived
SIudging by what is known of the development
of these organs in other Tunicates, and of the relations of the senital
rudiment in all other animals, it appears to us in the highest degree
improbable that there is any connection between the two. structures.
‘ey peribranchial tube and genital strand, or between the latter amd
the nervous system].
Pyrowma
dermal
ction through two very young
Of Pyrosona (after SEBLIGER),
genital strand; 0. voung wgeeell: p.
branchial tubes,
from the ectoderm.
PYROSOMA—DEVELOPMENT OF THE PROLIFERATING STOLON. 487
The rudiment of the nerve-twhe of the stolon also, according to
Seevicer, is derived from the genital strand. In very young stolons
(Fig. 256 A) the distal end of the latter appears to bend round to
the upper side, (The lower side of the stolon is marked by the
position of the genital strand y), This upper part of the genital
strand becomes, in the further course of development, separated
from the lower part, and, according to Seenicer, becomes the
rudiment of the neural tube (Fig, 256 C, n), a lumen very soon
appearing inside it.
SeeiGex thus holds that not only the genital organs of the bud
but also its peribranchial tubes, and a large part of all ita mesodermal
structures, are derived from the genital strand. The group of cells,
Fe, 266,—Th ih in the dod ment of the proliferating stolon of Pyrvmma
(uttar Su nC) the separation of the two individuals (7 and 21) is already
indicated. ppt , ectoderm ; es, endostyle of the parent; gy
strand ; hs, first cr m, rudiment of the alimentary canal proper 5 ™,
rudiment of the neural tube,
which we described as the simple rudiment of the genital strand, has
thus, in his eyes, a far wider significance in the development of the
proliferating stolon. He has therefore designated it as the germ-
strand or the mesodermal germ-mass.
‘The fact that SreriGER traces back the peribranchial tubes and the neural
tube in the buds of Pyrosoma to the mesoderm cannot fail to awaken surprise
when it is remembered that the rudiments of these organs are undoubtedly
derived from the ectoderm, not only in the Cyathozooid but also in the four
primary Ascidiozooids. In our attempts to interpret the structure and sig-
nificance of the proliferating stolon, we naturally seek for agreement between
the development of its buds and that of the first four Ascidiozooids. When
we consider that the proliferating stolon must evidently be traced back to the
PYROSOMA—FURTHER DEVELOPMENT OF THE BUDS, 489
the enteric rudiment still connected with that of the next bud
(Fig. 258, »). The distal individual of the stolon is always the most
developed (¢f. also Fig. 267). When five individuals have become
marked off on the stolon through the appearance of constrictions,
the distal individuals always seems to become detached. More
than five individuals are consequently never found on one stolon.
B. Purther Development of the Buds.
A fact already pointed out in connection with the development of
the four primary Ascidiozooids (p. 407) must again be emphasised
here, viz., that the longitudinal axis of the stolon does not coincide
with the later longitudinal axis of the developing buds, but rather
appears to lie at right angles to it (Figs. 258, 267). The longitudinal
axis of the segments of the stolon changes into the dorso-ventral axis
of the developed bud. The axis which becomes the later longitudinal
axis of the body is one which we may imagine as cutting the
longitudinal axis of the stolon at right angles, é.e., from the neural
side to that of the genital strand, The ends of such an axis would
be indicated by the rudiments of the branchial and atrial apertures.
From the ectoderm of the stolon is developed the integument of the
bud which, on its outer surface, secretes the cellulose mantle. Accord-
ing to SEELIGER, this secretion takes place here in the way described
for other Tunicates by Semper, Herrwic and others. When, there-
fore, SALENSKY emphasises the fact that the first rudiment of the
cellulose mantle is yielded by the Cyathozooid, the four primary
Ascidiozooids ut first taking no part in the secretion of the cellulose
substance, his remark applies only to the first stages of development.
‘The further enlargement of the cellulose envelope proceeds from the
Ascidiozooids.
The branchial and atrial apertures (Figs. 253, 259, ¢) first appear as
4imple ectodermal invaginations. These invaginations fuse with the
wall of the branchial sac or atrial cavity, the apertures breaking
through later at the points of fusion, Near the point at which the
branchial aperture forms, the branchial sac (7.¢., the entoderm) gives
off the first bud-like rudiments of the crown of tentacles that en-
circles the entrance to this part of the alimentary canal (Fig. 253, ¢).
The entoderm-sac of the bud first forms the Aranchial sac or
pharynx. It has already been meutioned (p. 488) that this is cruci-
form in transverse section (Fig. 257), two of the outgrowths being
directed upward and two downward. The upward outgrowths are
separated from each other by a median fold in which ean be recognised
PYROSOMA—FURTHER DEVELOPMENT OF THE BUDS. 491
the contiguous walls of the entoderm-sac and the peribranchial sacs
(Fig. 258, p), but they very soon become elongate (Fig. 259)
The inner longitudinal bars of the branchial sac (Fig. 253, (/) now
develop at right angles to the clefts. As the slits break through
chiefly in consequence of an outgrowth of the entoderm-sac, they
wppear to be lined with entoderm.
‘The two peribranchial sacs, which early lost their connection with
those of the adjacent bud, by growing towards each other on the lower
side of the stolon and fusing, give rise to the unpaired atrial cavity
(Figs. 253, kl, 259, el),
Finally, the outgrowths of the dorsal wall of the branchial sac
known as the /anguets (Fig. 253, r2) develop.
The radiment of the central nervous system appears at first as a tube
running along the whole length of the wpper side of « stolon-segment
Fig, 258,—Stolon of Pyrosoma with the rudiments of two individuals, 2 and 27 (after
Swesicer). .1, side view; 2B, seen from the side on which the ital strand ix
«i eb, rudiment of elueoblast; ec, ectoderm ; ex, endostyle-rudiment; g,
genital strand ; As, gill-clefts ; m, rudiment of the stomach ane intestine; 1, nervous
of bud J; 2’, nervous system of bud 27; p, peribranchial tubes ; s, rudiment:
the lateral nerves (in B, seen in transverse section).
(Fig. 256, 2); later, however, the proximal part of this tube develops
into « large vesicle, while the thinner, distal part disappears, Two
lateral outgrowths of the proximal part of the neural tube (Figs. 258, ,
259) can be seen very early (Fig. 257 A, sn); these soon change
into hollow processes which encircle the alimentary canal and unite
on its lower side. ‘These are the rudiments of the so-called /ateral
nerves. A growth of the cells on the dorsal wall of the cerebral
vesicle yields the rudiment of the ganglion proper, which later gives
off the lateral nerves that run backward along the dorsal surface of
the adult in the form of solid strands with terminal ganglionic
swellings. The remains of the cerebral vesicle develop a connection
with the branchial sac and become transformed into the ciliated pit
PYROSOMA—FURTHER DEVELOPMENT OF THE BUDS. 493
fibrillae increase in size, they become bund-like. In transverse sec-
tion, they then appear radially arranged, while the centre of the
musele-bundle is oecupied by
the undifferential protoplasm
and nuclei of the cells.
The first rudiment of the
pericardial vesicle is also traced
back, by SEELIGER, toa small
group of mesenchyme-cells
which can be seen beneath the
distal end of the right peri- yy, 260—Two early stages in the du
branchial tube. In thie cell. ment of the rcepen teeny! in the bu
{after SEKLIORR). “ev, entoderm
group (Fig. 260, pe) a lumen (wall of the enteric rudiment); ec, ectoderm ;
soon appears which is the
pericardial cavity; at a later
Line reper hae Pe, rudiment of the
vesicle,
stage, the rudiment of the heart (/z) arises through the invagination
of that part of the wall of the pericardial vesicle which is in contact
with the wall of the intestine (Fig. 261).
The pericardial vesicle originally lies on
the right side of the bud, but wanders
later towards the median plane and thus
reaches the dorsal side of the endostyle,
process (Fig. 253, iz). It has therefore
in Pyrosoma, a position unlike that
oceupied by it in the other Ascidiacea,
where, as deseribed above (p. 370), the
heart lies on the ventral side of the
endostyle-process (epicardium) in such
way that the epicardial lamella brings
about the closure of the cardial tube,
Tn each bud, the genital strand (Fig.
209, g) gives rise to the genital organs
of the individual and to the genital
strand of the stolon produced by that
individual, At an carly stage, the
genital strand is found to break up into
a distal and a proximal part. The
distal part, which is embedded in the
elacoblast-tissue (c+), becomes abstricted
and gives rise to the genital strand of
de
fe
Fic, 261,—wo later stages in
the development of the peri-
cardial vesicle in the bud of
Pyvosoma (after Seettoer).
ec, ectoderm ; ef, elaeoblast ;
en, entoderm (wall of intes-
tine) 5 ed pericardial vesicle ;
he, ridiment of heart,
the stolon. In the proximal part, which represents the rudiment of
SALPA—DEVELOPMENT OF THE STOLON. 495,
In Salpa pinnata, 8. affinis and S. dolichosoma, it is more or less
straight and runs forward along the ventral side of the body, In 8.
vuncinata-fusiformis and costata-
Tilesit, it at first rans forward on
the ventral side, but then bends
round and runs backward on the
left side of the nucleus, and only
passes out behind this organ. In
S. democratica - mucronata, 8.
acutigena - confederata and WS,
cordliformis - zonaria, the stolon
winds spirally round the nucleus
(Fig. 262).
The individual buds here, as in
Pyrosoma, become marked off by
transverse constrictions — which
appear on the stolon. While,
however, in Pyrosoma, the separate — yry4, 469 — Posterior end of tha body in
parts of the stolon remain in a « solitary form of ipa ie
* + vy mucronate, from the dorsal side (after
single row, in the Salpidae, Lavckan). ¢, alimentary canal; h,
secondary shifting leads to a bi- Pees ae el
serial arrangement. A large :
number of buds consequently can be crowded on to a short piece
of the stolon.
A. Structure and Development of the Proliferating Stolon.
Our knowledge of the earliest stages of the development of the
stolon is due chiefly to Spmnrcer (No, 105), The stolon, as already
mentioned, is at first a small, conical
outgrowth on the left side of the
body near the posterior end of the
endostyle. Three layers can be dis-
tinguished in it ; ectoderm, mesoderm
and entoderm. ‘The ectoderm of the
stolon passes direct into that of the 51, 963, —Youngest. stage of de-
parent. The entoderm-sac of the velopment of the stolon in. Sapa
stolon (Fig, 263, en) is adivertioutum — {Mer Swartcnu) f, Dlood-ainus
of the wall of the parent's branchial Process; mm; mesoderm ; ms,
Ae 7 mesenoliyme-vells.
sac arising immediately behind the
endostyle. This diverticulum exactly corresponds to the organ known
SALPA—DEVELOPMENT OF THE STOLON. 497
round which the cells are arranged like an epithelium (Fig. 265, ¢),
although SEELIGER was not able to come to a definite conclusion on
this point. He called these strands the /ateral strands, whereas the
other authors speak of them as the peribranchial, perithoracic or cloacal
tubes, ‘They correspond to the peribranchial tubes in the stolon of
Pyrosoma. In the fourth, lower strand (g), young egg-cells early
appear, and it must therefore be called the genetal strand. Besides
these strands, scattered mesenchyme-cells are found in the primary
hody-cavity (Fig. 264, mz).
In later stages of develop-
tment of the stolon (Figs. 264
B and C, 265), we find,
farther, two large blood-
sinuses lined with an endo-
thelium, one lying above (0)
and the other below (u) the
entoderm-tube, Since this
tube does not reach as far as
to the end of the stolon, com-
imunication between the two
sinuses is possible, At this
point the blood flows from
one sinus to the other.
Another constituent ele-
ment of the stolon in Sadpa
has been pointed out by — Fi, 265,—Transvorse section through « youn;
Buooxs, who found, om either HoH of et (ates, MS a clen
side of the stolon, between tube; g, genital strand, im, muselo-tubes ;
the peribranchial tubes and epee: Ns Ea a OSes
the ectoderm, another tube
which, he considers, gives rise to the muscular system. ‘These tubes
Brooks culls the musowlar tubes (Fig. 265, m).
In these six longitudinal strands running through the stolon we
have the rudiments of the most important organs of the buds, ‘The
ventral ontoderm-tube (em) yields the rudiment of the branchial sac
(pharynx) and of the alimentary canal ; the neural tube yields the
ganglion and the ciliated pit, while from the peribranchial tubes (c)
the single atrial cavity arises, from the genital strand (7) the genital
organs, and from the muscle-tubes (m) the body-musculature, When,
later, through transverse constriction, the separate individuals (buds)
begin to detach themselves from the stolon, the longitudinal strands
KK
SALPA—DEVELOPMENT OF THE BUDS ON THE STOLON. 499
ectoderm of the stolon must, however, be considered probable for the
neural tube. We should add that the examination of these onto-
genetic processes in the stolon is exceedingly difficult.*
B. Development of the Buds on the Stolon.
Tt has already been shown (p. 495) that the buds arise through
transverse constriction of the stolon (Fig. 266). The stolon is in this
way cut up by a kind of transverse fission into a series of consecutive
individuals which continuously increase in size while remaining con-
nected together by the narrowed parts of the stolon, The buds thus
arise in Salpa in exactly the same way as in Pyrvsoma (Figs. 253
and 256 C), Only in later stages do the buds of the Salpidae undergo
important changes of position which lead to their biserial arrangement
on the stolon, In order
clearly to understand
these changes, we must
first recall the condition
of the Pyrosoma stolon,
According to the more
detailed account given
on p. 484, the individuals
are arranged on the
Pyrosoma stolon in a
single row, one behind — pio, a6. — Disgsamimatio longitudinal section
Jentae _ through the stolou of a Pyrvioma (constructed b
the other. ‘The orienta- Boggs after Huxuay ad Kowauevart), 2
tion of each individual —— parent-individual; J, JJ, 117, tirst second and
Slab Shat ok third buds; 6, ‘branchial-sd; c ateigim; of,
resembles that of the alimentary Cana; , eetoderm of the connecting
strand ; em, entoderm of the same; es, endostyle ;
parent’: The haemal or m, nervous system; o, segment of the genital
ventral side of the buds strand ; s, young stolon of the third individual.
(marked bythe position
of the endostyle, ««) is directed towards the distal end of the stolon,
‘The right half of the body of every individual agrees in position with
the right side of the parent; the left half of the bud corresponds to
the left half of the parent and has been produced by the left half of
the stolon, just as the right half of the bud has been produced by
the right half of the stolon.
*[According to Brooxs (No. I.), the nerve-tube-and the two peribranchial
tubes are probably derived from the ectoderm of the stolon; the upper blood-
sinus is continuous with the cavity of the heart, and the lower sinus with the
body-cavity of the nurse-form.—Ep.)
SALPA—DEVELOPMENT OF THE BUDS ON THE sroroN. 401
younger stages, that the distal individuals of a group are somewhat
further developed than the
proximal individuals of the
same group.
The individuals are in reality
arranged on the stolon in two
alternate rows (Fig. 269) in
such a way that the correspond-
ing individuals of the two rows
turn their ventral (haemal)
sides (A) to each other, while
their dorsal (neural) sides (n)
are directed outward, This
arrangement can be clearly
made out in the transverse
section (Fig, 275) through an
older part of a stolon. We
thus have an arrangement of
the individuals like that de-
picted in the diagram (Fig.
269). The median plane of the
individuals does not coincide
with that of the stolon, but
lies at right angles toit. This
biserial arrangement of the
bads results from a lateral
shifting of the segments of the
stolon which move alternately
to the right and left side.
Each bud at the same’ time
rotates round its longitudinal
axis, passing through an angle
of 90° ; consequently, the dorsal H €
side of the bud which origin-
ally (Fig, 268) was directed F
Fw. 270,—Horizontal longitudinal section through an advanced stolon of Sade
{after Brooxs), ‘The individuals are out through obliquely, the longitnetinal section
Pasting through them. in such a way that, in those placed most distally (1, 2, ete-)
the most anterior region of the body is seen and, in the most proximal indi als
(20, 21, etc.), the posterior region of the body. J», distal; P, proximal; #, right;
, left. a, anal aperture; c, atrium; ef, elaeoblast; es, endostyle; y, gill; h,
heart; As, hasmal side of the bud ; é, intestine ; 2, loft half of the branchial ao >
im, opening of the oesophagus; 1, nervous system; as, neural side of the bud; o,
ovary ; rh, right half of the branchial sae ; #, storaaeh,
SALPA—DEVELOPMENT OF THE BUDS ON THE STOLON.
the individuals on the stolon of Sal/pa can be traced back to the
original monoserial arrangement which is retained in the stolon of
Pyrosoma.
Special interest attaches to the condition of individuals 3-7 of
We can here
Fig. 270, diagrammatically illustrated in Fig. 271.
clearly see the relation of the buds to
the stolon. The stolon itself is cut
through obliquely, the section being
intermediate between a longitudinal and
4 transverse section. We can see the
entoderm-tube of the stolon (en), an
teriorly, the upper blood-sinus (0) and
posteriorly the lower blood-sinus (wu, ¢/.
Figs. 264 and 265), In individual 5,
we ean trace the connection between the
right (72) and the left (/0) halves of the
branchial sac with the entoderm-tube
(en) of the stolon, All the other indi-
viduals are cut through at a different
level. In individuals 4 and 6, for
instance, the two blood-sinuses are inter-
calated between the right and left halves
of the respiratory cavity,
We have reproduced other diagrams
by Brooks (Figs. 272 and 273) which
still further illustrate this condition.
Fig. 272 gives the upper or oral aspect of
« stolon as it would appear if the primary
position of the buds had been retained
unaltered (¢/. Fig. 268). The right half
of each bud is seen to have arisen from
the right side of the stolon and the left
half of the bud from the left side of the
stolon, Vig. 273 shows the buds shifted
alternately to the two sides of the stolon,
x condition attained through the repeti-
tion of the process taking place in indi-
vidual 5 in Figs. 270 and 271. The
buds 1, 3 and 5 have moved toward the
Fo, 272 —
of Salpa as it would ay
if no secondary shifting of
the individuals were to take
place (after Brooks), J’,
pro: iD, di it
right; ZL, left of the stolon ;
rand /, right and left aides
of the individuals; es, endo-
style-folds; , ganglion,
right side of the stolon while 2, 4 and 6 have moved toward the
left side. The rotation of the individuals, however, is not yet visible.
504 TUNICATA.
Originally, therefore, the rudiments of the individuals (buds) are
nothing more than the consecutive segments of the stolon marked off
one from the other by transverse furrows. These furrows, as seen
from above or below, are soon no longer transverse, but run somewhat
obliquely (Fig. 274), in alternating directions, yo that furrows 1, 3
and 5 are parallel on the one hand,
and furrows 2, 4 and 6 on the other.
In this oblique course of the furrows,
we see the first preparation for the
later biserial arrangement; it is
the expression of the lateral displace-
ment of the tissues of the buds
forming the different segments of
the stolon. When, therefore, in
later stages, the segments shift to
the two sides, it appears as if the
buds arose as lateral ontgrowths
from the stolon. This erroneous
view of the budding of the Scdpt ix
actually adopted by many of the
older authors. Brooks was the first
to trace back the origin of the buds
to the transverse division of the
stolon.
The individuals become more and
more sharply marked off from one
another in proportion as the june
their lateral positions and project out
Diagram of a stolon or ftom the stolon, ‘They then appear
Nepa after the lateral shifting of to be hanging on to the remains of
the buds hast place; the einer
Nepme in this diagram are all repre. the stolon like grapes on a bunch
jou of individual Seay iehca .
yulivictwil (Fig. 275 8). ‘The remains of the
nies len stolon (xt), however, are nothing
halves of the 5
ating one indi- more than the consecutive strands
or, ectoderm 5
RB. right ;
1
which connect the — individuals.
en, entoderm,
These, as may be seen from the
diagram (Fig. 272), originally ran from the haemal side of each
individual to the neural side of the next older bud. They are
originally inserted at the middle of the body. In later stages,
when the individuals have taken up the lateral position and bave
undergone rotation, the connective strand forms a continuons longi-
SALPA—DEVELOPMENT OF THE BUDS ON THE STOLON. 505
tudinal tube to which the buds adhere laterally. The blood-vessels
running in this strand, and the entoderm-tube which persists within
it and connects the branchial sacs of the individuals,
are of importance for the nourishment of the buds.
This longitudinal strand may be called the remains of
the stolon. Its position with relation to the buds
changes, as its points of attachment wander more and
more upward, ie, toward the branchial aperture
of the individual (Fig. 275), while the individuals sink
downward on either side of it. This change of position
can be most distinetly traced in the ganglia. The
-neural tube of the stolon originally lies above the
entoderm-tube as ix seen in Fig. 264. The ganglia
derived from the neural tube must consequently seem
to lie in the median line above the entodermal con-
neetive canals (see diagram, Fig. 268). When, Inter, Fra, 2
the buds become marked off laterally, the ganglia sink [nstrating
lower down, and come to lie at the sides of the con- hee oh oures
necting canals; indeed, in the diagram (Fig. 268) _ furrowing in
they appear alternately to cover the canals and tobe Mazel
covered by them. Later, the ganglia and, with them, from above.
the individuals, sink still farther down.
The sinking down of the buds on either side of the remains of the
stolon solves a ditticulty which apparently presents itself in connee-
tion with their rotation. In examining individual 5 in Fig. 271,
and still more in considering the diagram Fig. 273, it may oceur to
the observer to wonder in what way the endostyle-fold which, in
individual 5, lies on the left side, passed over to the right, since it
appears separated from the right side of the body by the entoderm-
tube of the stolon. We have tried to make this process clear by the
diagrams given in Fig. 276. The connecting strand a4, which
represents the remains of the stolon, originally rans from the neural
side of each individual to the hacmal side of the next (proximal)
individual. After rotation has taken place, these strands would
assume a zig-zag course, as indicated in Fig. 276 2. Later, as the
buds sink down, the connecting strands, which are alrendy attached
near the anterior region of the body, shift further forward to a
pesition quite near the branchial aperture, whereas the endostyle-
folds, which remain unaffected by this change, do not extend so far
forward. ‘here ix therefore no obstacle in the way to prevent the
union of the connecting strands ; by this union these strands appear
SALPA—DEVELOPMENT OF THE BUDS ON THE sTOLON. 507
sare hollow, and each of them contains a blood-vessel. These might
be compared to the mantle-vessels of the Ascidians, The final
connection between the individuals is formed by the processes of
neighbouring individuals growing out towards one another and
fusing. Each of the individuals belonging to a chain possesses, as
a rule, eight of these processes placed in four longitudinal rows ;
two ventral and two lateral. The lateral couplings serve for attach-
ment to the individuals lying in the same longitudinal row, while the
‘ventral processes establish a connection between the two parallel rows
i
Bg
Fro, 276. —Diagram illustrating the relatious of the connecting strand (st) to the bads
‘on the stolon of a Salpa, A, represents the stolon from Fig. 268 viewed from above ;
B, shows the condition after rotation has taken place ; C, the condition after the
buds have sunk downward. a, distal end of the counecting strand, 4, proximal end
of the same; a, distal part of stolon ; es, endostyle-folds ; , hactal or ventral side ;
nm, wervons system and neural or dorsal side ; », proximal part of stalon ; sf, connect-
ing strands.
of individuals seen in the diagram (Fig. 277 A). The long axes of
the individuals are inclined somewhat obliquely to that of the chain,
a position which results from the fact that the couplings are not
inserted at the same level on the right and the left sides of the body
(Fig. 277 B). This oblique position may have given rise to the
horizontal one seen in Salpa fusiformis. In S. (Cyelosalpa) pinnata
the individuals are found arranged in the form of a rosette, each of
the buds giving off a single process only from the ventral side which
runs towards the centre of the rosette, where all the processes meet
508 TUNICATA.
together like the spokes of a wheel. Such an arrangement is charac
teristic of the genus Cyclosalpa.
This union between the individuals of the Salpa-chain must be
regarded as colony-formation. Whereas, in the composite Ascidians
and in Pyrosome, the individuals which arise through budding remain
connected by means of a common cellulose mantle, connection here
takes place through special connecting organs.* This connection is
lustrating the interconnection existing between the individuals
in seen trom above; B, lateral view. re, etudostyle ;
lateral connecting’ processes ; A, Ventral processes
not very close. When the fully developed chain passes ont from the
cavity in the cellulose inantle of the parent and separates from the
proliferating stolon, it very easily breaks up into smaller portions:
individuals even become de
hed from the chain and continue their
existence independently.
C. Development of the Organs in the Bud.
In the above account of the relation of the buds to the stolon we
have in all points followed the short but important deseription given
hy Brooks (No. 92). The most important investigations made in
* See [1
Kant’s description, No. 28.
SALPA—DEVELOPMENT OF THE ORGANS IN THE BUD. 509
coonnection with the development of the organs in the bud are those
of SaLEensky (Nos. 10] and 102) and SEELIGER (No. 105). Although
the researches of these investigators yield much valuable material for
working out the organogenesis of the buds of Sadpa, yet, owing to the
fact that these observers did not recognise the rotation of the bud
round its longitudinal axis, as made out by Brooks, their observations
are somewhat vitiated through the adoption of an erroneous orienta-
tion of the bud, taken from its later stages and applied in describing
the earlier stages. SALENSKY and SEELIGER hold that the buds
arise simply by the lateral shifting of the segments of the stolon,
According to them the dorsal part of the bud develops from one of
the lateral surfaces of the stolon, while its ventral side corresponds
to the opposite side of the stolon, According to Brooks, on the
contrary, the right side of the stolon gives rise to the right half of
the body of the bud and the left side of the stolon to the left half.
In the following account we shall merely give a brief outline of the
genesis of the organs in Nadpa which probably closely resembles that
in Pyrosoma.*
[t must once more be pointed out that we have the first rudiment
of the bud in a transverse segment of the stolon. The proximal
parts of this segment become the dorsal side of the bud, and the
distal parts its ventral side. From the upper region of the stolon is
developed the anterior part of the body of the bud, while its posterior
end develops from the lower region (see diagram, Fig. 268). The
organs of the bud are formed from sections of the tubes and longi-
tudinal strands which are to be seen in the transverse section of the
atolon (Fig. 265).
That part of the central entoderm-tube found in cach segment of
the stolon gives rive to the pharynx of the bud. The entoderm-tube,
in a cross-section through the stolon, bears some resemblance to the
expanded wings of a butterfly (Fig. 264 (), an upper and a lower
indeutation and two lateral indentations being found in it. Its form,
in cross-section, later resembles that of the letter H, also found in a
similar section of a Pyrosoma bud (Fig. 257). [In the latter, the two
portions of the tube that point upward are connected with the
development of the endostyle-folds, while those that extend down-
wand, yield the stomach and intestine (p. 490). It is at present
impossible to say whether similar conditions prevail in the Salpa bud.
*(Brooxs’ memoir on the genus Salpa (No. I.) should be consulted in this
connection ; he gives a full and detailed account of the development of the
ohain-form, including its organogenesis.—E.}
SALPA—DEVELOPMENT OF THE ORGANS IN THE BUD. 511
origin of these organs, According to Brooks, the muscle-tubes give
rise to the body-musculature. We may no doubt assume that the
paired segments of these tubes spread out on either side of the bud
and thus yield musele-plates, which become fenestrated and then
break up into the musele-hoops.
The rudiment of the genital organs is yielded by the genital strand
(Figs. 264, 265, g). We have already seen (p. 497) that young egg-
cells can early be recognised within this strand, These at first are
very plentiful, but many of them disintegrate later and seem to serve
as food for the developing eggs. When the genital strand is ready
to break up into segments, the eggs become arranged in such & way
that only one oceurs in each division. The smaller peripheral cells
of the genital strand yield the egg-follicle, the oviduct and (according
to SEELIGER) probably also the rudiment of the testis. Even in
Fie. 278.—Chain-form of Salpa democraticn-mucronata from the distal part of an
advanced stolon (after Skeutasr), ¢, atrinm ; ¢, atrial aperture; eh, elacoblast ;
4s, endostyle; fy, cilinted pit: y, ganglion ; h, radiment of tostis': hy; connecting
Prvoens | Ks, heart; ¢ branchial aperture : sm, tatesting ; &, gill me, stomachs
oc, eye; od, oviduct ; v¢, oesophagus ; on, ogg-follicle ; ph, pharynx,
early stages, it is evident, in cross-sections through the stolon, that
group of cells becomes detached in each segment from the lower
part of the genital strand, and this probably is to be regarded as the
rudiment of the testis (Fig. 264, c, A).
The rudiment of the genital organs originally lies at the posterior
end of each bud. Later the egg-follicle shifts farther forward on the
dorsal side and then lies above the intestinal loop in the dorsal median
line. From this point the oviduct turns to the right with an S-shaped
curve and becomes connected on the right side of the body with the
epithelium of the atrial cavity (Fig. 278, ov and od). The wateh-
glass-shaped rudiment of the testis (4) remains longer at the posterior
end of the body curved round the posterior end of the intestinal loop.
At a later stage it breaks up into separate tubes which unite to form
612 TUNICATA.
a common efferent duct that opens out into the atrial cavity between
the intestine and the stomach on a puapilla-like prominence (see
Savensky, Nos. 101 and 102, aud SEELIGER, No. 105). The testis
develops comparatively late in the chain-form.
5. The Interpretation of the Alternation of Generations
in the Tunicates.
Alternation of generations is found among the Tunicates in
marked form in Pyrosomea, Doliolum and Salpa. This fact has
long attracted the attention of zoologists, who have attempted to
explain it in many different ways. We shall adopt the view first put
forward by Leuckart (No. 98) and accepted later by Craus* and
GropsEN (No. 79) that alternation of generations in the Tunicates
must be regarded as having arisen in consequence of the formation of
stocks through division of labour, and we shall follow GropBEs’s
clear exposition of this view. Among more recent descriptions we
may specially mention those of Unsantn (No. 86) and SgELIGER
(No. 106).
The Larvacea, which are conjectured to be the most primitive of
all existing Tunicates, develop through sexual reproduction. This
seems to suggest that the capacity for asexual reproduction (by means
of buds) was acquired as a consequence of the attached manner of
life. Asexual reproduction is indeed very common among. attached
animals. We may, with GroBBEN, suggest as the cause that the
abandonment of locomotion left a larger proportion of the substance
of the body available for reproduction, so that it was possible for
attached animals to introduce a new method of multiplication inte
the cycle of their development. It may also, however, be added that
when, in consequence of attachment, cross-fertilisation became mor
difficult, the capacity for asexual reproduetion would become of special
importance for the maintenance of the species.
It is evident that, originally, all the individuals of forms which had
developed this character were equally able to reproduce themselves
cither sexually or asexually.
Asexual multiplication led to the formation of stocks. the bud»
being either altogether incapable of separating from the parent or
else able to do so only incompletely, All individuals were thus at
first capable, by asexual multiplication, of increasing the size of the
colony to which they belonged, or else hy sexual multiplication ot
* Grundz. d. Zool. 4 Aufl.
ALTERNATION OF GENERATIONS IN THE TUNICATES. 513
founding new colonies. Such a condition is found, for example, in
the Ascidiozooids of Pyrosoma, which produce stolons and also maturate
genital products.
The distribution of these two kinds of reproduction among various
individuals of the colony must be regarded as a later, derived condi-
tion through which the first individuals arising in a colony then
beeame adapted exclusively for increasing the colony by budding,
while the lateral individuals formed new colonies by sexual reproduc-
tion. Such an arrangement, in which we see the first commencement
of ulternation of generations, is found in the composite Ascidians.
Ganiy, following the investigations of Krom, established that, in
this case, the individuals developing from the egg are capable of
asexual reproduction only, and in this way lay the foundation of
the colonies, while the descendants of these individuals, produced
by budding, again develop genital products.
In Salpa we find this condition more marked, and established
as a regular alternation of two generations, one reproducing only
asexually and the other only sexually, At the same time the two
generations vary to a certain degree in the structure of the body.
Gropsen bas rightly pointed ont that these variations may be
explained by the different conditions of life and the different work
to be carried out by the two generations. The points that project
wt the posterior end of the “ nurse’ of Salpa democrativa-mucronata
(Pig. 262) serve as a protection for the proliferating stolon which
cceurs in that region of the body. This nurse” of S. demveratioa-
mucroneta. further possesses one muscle-hoop more than the sexual
animal, and this is explained on the ground that, owing to the
presence of the massive proliferating stolon, greater muscular power
is required in this case to enable the animal to swim with equal
rapidity. On the other hand, the form of the sexual individua
(that belonging to a chain) may be explained by the crowding of the
buds on the chain,
It follows from the above that the solitary form and all the indi-
viduals of the chain produced by it must be regarded as members of
one and the same colony. The solitary (“nurse”) form is the
founder of the colony, while the individuals of the chain produced
asexually give rise to the foundation of new colonies in producing
sexually new solitary forms.
‘The heteromorphous development of the individuals of the Salpa
colony recalls the polymorphism of the Siphonophora. Polymorphism
is still more strongly marked in the Dolioltdae, where we have, itv
LL
ALTERNATION OF GENERATIONS IN THE TUNICATES. 515
why we should not assume that the capacity for division was
possessed by the adults from the first, indeed, this capacity may
ibly have been inherited directly from older pelagic ancestors
the Tunicates. For, even if the circumstance that the Larvacea
not multiply asexually seems to indicate that this form of re-
luction was acquired only after attachment, we are not sure that
this was the case,
SketiGeR, who attributes to the mesoderm the principal part in
development of the proliferating stolon, and who derives the
of the stolon, at any rate in Pyrosoma, from the genital
ues of the parent, finds in the limitation of sexual reproduction
cause for the development of buds, the material left in the
after the production of a single egg is utilised in its plastic
ity as the mesoderin of the stolon. Bnt even if we make this
ption, the manner in which budding was acquired remains
obscure,
Farther confusion has been introduced into the views as to the
alternation of generations in the Tunicates by the fact that the exgg-
cells. frequently maturate very early in the buds. They can even be
4 distinguished in the genital strand of the stolon in Pyrosoma and
Salpa. This has, in many cases, led to the view that the ovary
actually belongs to the solitary form and is merely deposited in the
forms composing the chain, This view, which is adopted by Brooxs
Nos. 88-91) does not seem justifiable to us. We prefer SEELIGER’s
view that the egg with its follicle is just as much an organ of
the individual of the chain as are all the other organs. The early
differentiation of the egg-cells is to be traced back to the effort on
the part of the organism to arrive at sexual maturity as early as
| possible. The same hastening of sexual maturity is found in the
Hydroids and, indeed, in the parthenogenetic Cladocera and Aphidae
and also in the Diptera (polar cells).
Bnooxs regards all the ovaries of the individuals of a Salpa-chain taken
together as the germ-gland of the solitary form shifted into the stolon. He
considers the solitary forms not as sexless but as females, while he regards the
individuals of the chain as males which have arisen from the females as buds,
‘The solitary form deposits an egg in each male, and this, when fertilised,
develops inton female. Bnooxs therefore reduces the alternation of genera-
tions of Salpa to a kind of sexual dimorphism.
We have already stated (p. 496) that Tonaro, by deriving all the buds from
certain embryonic germ-cells (germoblasts), and by tracing these latter directly
to the blastomeres of the embryo, regards the individuals of a Salpa-chain not
as the descendants of the solitary form but merely as younger members of the
GENEBAL CONSIDERATIONS ON THE TUNICATES. 517
composite Ascidians. We must await the result of further researches
before coming to any definite conclusion, .
Tn any case, the development of the bude must always be considered
quite apart from embryonic development, since these two methods
of development are to be traced back to different principles. In
the embryo, the primary organs arise anew from an originally
undifferentiated mass of blastomeres, while in budding, which is
evidently deducible from division, parts of the most important
primary organs are taken over from the organisation of the parent
into the bud. Although the literature on the budding of the Tuni-
cates at present is far from supporting the statement that all the
more important organs in the bud or the stolon are to be derived by
abstriction from the corresponding organs of the parent, indications
are not wanting that the solution of the problem as to the origin of
the organs of the bud is to be sought in this direction (p. 487). It
appears, for instance, that the strands which compose the rosette-like
organ of Doliolum ave direct continuations of all the more important
organs of the parent. In the four primary Ascidiozvoids of Pyrosonut
also, the peribranchial tubes and the pericardial rudiment of the
Cyathozovid are directly continued. It may be mentioned further
that, according to Kowavevsky, the peribranchial tubes in the stolou
of Salpa are derived from the atrium of the parent. These state-
ments which, however, are in direct contradiction to many observa-
tions on other forms suggest that none of the more important organs
arise anew in the bud, but that all the more important rudiments of
organs pruxs over from the parent into the xtolon and thence into the
buds, while the actual new formation of organ-rudiments takes place
andy in the embryo. The nervous system would probably have to be
considered as an exception to this rule. Since we know that the
brain of the Ascidian can be regenerated after excision, it appears
possible that it arises anew also in the buds, though probably only
from the ectoderm.
Turning to the embryonir development, we tind that the different
divisions of the Tunicata here also vary greatly. ‘The embryonic
development of the Sa/pidae is, indeed, so little understood that we
are hardly in a position to say anything definite about it. When we
see that, according to SALENSKY, all the species examined show
important variation in their methods of development, it is evident
that there ix here an ample field for further research.* We may,
[* See footnote, p. 423 and pp. 445-446.—Ep.]
GENERAL CONSIDERATIONS ON THE TUNICATES, 519
tion of the dorsul region found in the Ascidians and the approxima-
tion of the branchial and atrial apertures thus brought about is
traced buck to attached forms, the position of the anus in the
Larvacea, which must evidently be regarded as primitive, would
indicate that these animals are descendants of those hypothetical
Tunicate ancestors which still retained the original pelagic life.
On the other hand, we find in Appendicu/aria a series of unmistakable
degenerative phenomena tending to support the assumption that we
must regard the racial form of the Larvacea as an attached Tunicate.
It is evident that they must be considered as sexually mature larval
forms, sexual maturity being shifted continually further back to
earlier stages so that, finally, the mature adult form no longer
developed. What form can we imagine this latter to have assumed ?
Was ita free-swimming Ascidian form intermediate between Amphiorus
and the Ascidian larva, or an already attached Ascidian form? The
last assumption seems the more probable. The appearance of the
eollulose mantle and hermaphroditism and the indistinctness of the
segmentation of the body must be regarded as features acquired as
& consequence of attached manner of life. As these characters are
found in the Larvacea, we are to a certain extent justified in con-
sidering them as the sexually mature larvae of an already attached
form of Tunicate.* In any case, however, all phylogenetic specula-
tions concerning the Tunicates must rest upon careful consideration
of the structure of Appendicularia and the Axscidian larvae.
Among the Ascidiacea the solitary forms are probably the more
primitive. ‘The composite forms lead on to Pyrosoma, which may be
regarded as a eomposite colony that has not become attached and
that is distinguished by a large common cloaca. In the interesting
family Coelocormidae we see the development of the whole colony in
w similar direction. Here also the colony is not attached but, on the
other hand, the internal cavity cannot be compared to the common
clouwea of Pyrosoma (see Herxoman, No. 24). Pyrosoma forms
4 transition to the free-swimming Doliolidae. This was pointed out
by Huxuey with reference to the structure of the gill and the
opposite position of the two apertures of the body (uf. also Gnopuen,
No. 79). The Dotiolidue (Cyclomyaria), among which Doliopsis
(Anchinia) exhibits the most primitive characters, must be regarded
as phylogenetically the oldest Thaliacea. The Salpidae (Hemimyaria)
must be considered as derived from them, We shall, therefore, have
*Witiey (No. 54a) also has lately maintained that the Larvacea are
degenerate forms.
GENERAL CONSIDERATIONS ON THE TUNICATES, 521
disappearance of the segmentation of the body. Only in the caudal
region of the Ascidian larvae and of Appendicudaria are there any
traces of the segmentation which must have been present in their
ancestors. [n the caudal portion of the nervous system in the
Ascidian larva, spinal nerves are given off segmentally, as KuprFER
first noticed, In Appendicularia these are connected with paired
ganglionic swellings in the dorsal cord, LanGERHANs (No, 2), by
the use of reagents, was further able to prove that the caudal musen-
lature breaks up into ten consecutive muscle-segments (myomeres)
which are provided with segmentally arranged pairs of motor nerves.
To individual cases Lancuraans (No. 2) found that, in the posterior
caudal region of Appendicularia, the spinal nerves of the left side
have shifted somewhat forward as compared with the corresponding
nerves of the right side, A similar condition is found in Amphioaus.
In the anterior region of the body, on the contrary, no traces of
segmentation are retained.
Although we must recognise a great general agreement in struc-
ture and development between Amphiorus and the Tunicates, it is
very difficult to establish exactly in detail the homologies between
the organs of the two groups. A special attempt of this kind was
made by vAN Brewepen and Juni, but we are not able to regard all
their deductions as convincing. Starting from the view that, in the
‘Tanieates, a large part of the alimentary canal (in the caudal seetion)
hus degenerated, VAN BENEDEN and JucIn regard the rectum and the
aval wperture of the Tunicates as new acquisitions which are not
homologous with the homonomous structures of Amphiorus. They
find the homologue of the rectum of the Tunieates in the ‘ club.
shaped gland” of Amphiorus, which belongs to the first trunk-metamere
(p- 549) and opens ont near the mouth.* Consequently the whole of
the precaudal part of the body in the Ascidian larva corresponds
to only the small anterior region of Amphionus, viz., to the anterior
cephalic part—the first trunk-segment. This view leads these
authors further logically to deny the strict homology of the endostyle
with the hypobranchial groove of Amphionus. Since, however, in the
Cephalochorda and the Vertebrata also, the anal aperture has evidently
undergone a secondary shifting forward and the eandal section of the
alimentary canal degenerates, there is nothing which compels us to
doubt either the homology of the rectum throughout the Chordata,
*(Wittey (No. XXXVIL) regards the club-shaped gland as the right
primary gill-slit,—Ep,)
GENERAL CONSIDERATIONS ON THE TUNICATES. 519
tion of the dorsal region found in the Ascidians and the approxima-
tion of the branchial and atrial apertures thus brought about is
traced back to attached forms, the position of the anus in the
Larvacea, which must evidently be regarded as primitive, would
indicate that these animals are descendants of those hypothetical
Tunieate ancestors which still retained the original pelagic life.
On the other hand, we find in Appendicu/aria a series of unmistakable
degenerative phenomena tending to support the assumption that we
must regard the racial form of the Larvacea as an attached Tunicate.
It is evident that they must be considered as sexually mature larval
forms, sexual maturity being shifted continually further back to:
earlier stages so that, finally, the mature adult form ne longer
developed. What form can we imagine this latter to have assumed ?
Was ita free-swimming Ascidian form intermediate between Amphiorus
and the Ascidian larva, or an already attached Ascidian form? The
lust assumption seems the more probable. The appearance of the
cellulose mantle and hermaphroditism and the indistinctness of the
segmentation of the body must be regarded as features acquired as
4 consequence of attached manner of life. As these characters are
found in the Larvacea, we are to a certain extent justified in con-
sidering them as the sexually mature larvae of an already attached
form of ‘Tanicate.* In any case, however, all phylogenetic specula-
tions concerning the Tunicates must rest upon careful consideration
of the structure of Appendicularia and the Ascidian larvae.
Among the Aseidiacea the solitary forms are probably the more
primitive. The composite forms lead on to Pyrosoma, which may be
regarded as a composite colony that has not become attached and
that is distinguished by a large common cloaca. In the interesting
family Coclocormidae we see the development of the whole colony in
«similar direction. Here also the colony is not attached but, on the
other hand, the internal cavity cannot be compared to the cormmon
cloaca of Pyrosoma (see Hirpman, No. 24). Pyrosoma forms
a transition to the free-swimming Doliolidac. This was pointed out
by Hoxney with reference to the structure of the gill and the
opposite position of the two apertures of the body (77. also Gronuen,
No. 79). The Dotiolidae (Cyclomyaria), among which Doliopsis
(Anchinia) exhibits the most primitive characters, must be regarded
ws phylogenetically the oldest Thaliacea. The Salpidae (Hemimyaria)
must be considered as derived froin them. We shall, therefore, have
*Witiey (No. Sda) ulso has lately maintained that the Larvacen are
degenerate forms.
GENERAL CONSIDERATIONS ON THE TUNICATES. 521
disappearance of the segmentation of the body. Only in the caudal
region of the Ascidian larvae and of Appendiewaria are there any
traces of the segmentation which must have been present in their
ancestors. In the caudal portion of the nervous system in the
Ascidian larva, spinal nerves are given off segmentally, as Kurrrer
first noticed. In Aypendiewlaria these ave connected with paired
vanglionic swellings in the dorsal cord. LanGERHANs (No. 2), by
the use of reagents, was further able to prove that the caudal museu-
lature breaks up into ten consecutive muscle-segments (myomeres)
which are provided with segmentally arranged pairs of motor nerves.
In individual cases Lancrrnans (No. 2) found that, in the posterior
caudal region of Appendicularia, the spinal nerves of the left side
have shifted somewhat forward as compared with the corresponding
herves of the right side, A similar condition is found in Amphioxus.
In the anterior region of the body, on the contrary, uo traces of
segmentation are retained,
Although we must recognise a great general agreement in struc-
ture and development between Amphiowus and the Tunicates, it is
very difficult to establish exactly in detail the homologies between
the organs of the two groups. A special attempt of this kind was
made by van Benepen and Juni, but we are not able to regard all
their deductions us convincing. Starting from the view that, in the
‘Tunicates, a large part of the alimentary canal (in the caudal section)
has degenerated, vax BENEDeN and Junty regard the rectum and the
anal aperture of the Tunicates as new acquisitions which are not
homologous with the homonormous structures of Amphiorus. They
find the homologue of the rectum of the Tunicates in the “ club-
shaped gland” of Amphionus, which belongs to the first trunk-metamere
(p- 549) and opens out uear the mouth.* Consequently the whole of
the precaudal part of the body in the Ascidian lurva corresponds
to only the small anterior region of Amphiorus, viz, to the anterior
vephulic part—the first trunk-segment. This view leads these
authors further logically to deny the strict homology of the endostyle
with the hypobranchial uroove of Amphiorus. Since, however, in the
Cephalochorda and the Vertebrata ulso, the anal aperture has evidently
undergone a secondary shifting forward and the caudal section of the
alimentary canal degenerates, there is nothing which compels us to
doubt either the homology of the rectum throughout the Chordata,
*(Wineey (No. XXXVIL) mgards the club-shaped gland as the right
primary gill-slit.—Ep.}
GENERAL CONSIDERATIONS ON THE TUNICATES. 521
disappearance of the segmentation of the body. Only in the caudal
region of the Ascidian larvae and of Appendicularia are there any
traces of the segmentation which must have been present in their
ancestors. [In the caudal portion of the nervous system in the
Ascidian larva, spinal nerves are given off segmentally, as KupFFER
first noticed. In Appendicn/uria these are connected with paired
ganglionic swellings in the dorsal cord. LanGErnans (No. 2), by
the use of reagents, was further able to prove that the caudal muscu-
lature breaks up into ten consecutive muscle-segments (myomeres)
which are provided with segmentally arranged pairs of motor nerves.
In individual cases LaNGeRHans (No. 2) found that, in the posterior
caudal region of Appendicularia, the spinal nerves of the left side
have shifted somewhat forward as compared with the corresponding
nerves of the right side. A similar condition is found in Amphious.
In the anterior region of the body, on the contrary, no traces of
seginentation are retained.
Although we must recognise a great general agreement in stru
ture and development between mphiorux and the Tunicates, it is
very difficult to establish exactly in detail the homologies between
the organs of the two groups. A special attempt of this kind was
made by vAN BENEDEN and Jury, but we are not able to regard all
their deductions as convincing. Starting from the view that, in the
Tunicates, a large part of the alimentary canal (in the caudal section)
has degenerated, vaN BENEDEN and JuLIN regard the rectum and the
anal aperture of the Tunicates as new acquisitions which are not
homologous with the homonomous structures of Amphiorus. They
find the homologue of the rectum of the Tunicates in the ‘“ club-
shaped gland” of Amphiorus, which belongs to the first trunk-metamerce
(p. 549) and opens out near the mouth.” Consequently the whole of
the precaudal part of the body in the Ascidian larva corresponds
to only the small anterior region of Amphiorus, viz., to the anterior
cephalic part—the first trunk-segment. This view leads these
authors further logically to deny the strict homology of the endostyle
with the hypobranchial groove of Amphiorus, Since, however, in the
Cephalochorda and the Vertebrata also, the anal aperture has evidently
undergone a secondary shifting forward and the caudal section of the
alimentary canal degenerates, there is nothing which compels us to
doubt either the homology of the rectum throughout the Chordata,
VIL) regards the club-shaped gland as the right
522 TUNICATA.
or the derivation of the anterior region of the body in the Tunicate
larva through fusion from a large number of trunk-metameres.
We have already mentioned (p. 367) that vaN BENEDEN and JuLix
deny the homology of the branchial slits and the peribranchial or
atrial cavity in the Tunicates with those of other Chordata. Only
the two clefts which form first in the Tunicates are really to be
regarded as true gill-slits. This view results from the ascription by
these authore to the entoderm of a considerable part in the develop-
ment of the peribranchial sacs. In the same way, VAN BENEDEN
and Junin doubt the homology of the heart in the Tunicates with
the heart of the Vertebrata. We shall only be able to judge of this
last view, which indeed receives decided support from the absence of
the heart in the Amphiocus, when the way in which this organ arises
in the Tunicates is fully established. While SEELIGER, like va‘
BeNepEN and Junin, derives the pericardial sac in the Ascidian
larva from the entoderm, most of the statements of other writers
seem to render its mesodermal origin probable. We must, however,
constantly bear in mind that an actual endocardium is altogether
wanting in the heart of the Tunicata.
This view, shared by many of the more recent writers (BALFOUR,
van BENEDEN and Juin, HatscHEk) that the Tunicates and the
Cephalochorda, to which the Vertebrates are allied, represent distinct
branches of the Chordate type connceted together only at their roots,
is opposed to that of Donrx (Nos. 15-19), who regards the Tuni-
cates as degenerate fish. The ¢
author thought to repress
clostoma and Amphiarus are by this
distinct in the series of degenera-
tive processes through which the organisation of the Tunicates is to
be derived from that of the fishes. This view rests principally upon
the proof which Dour attempted to establish that the hypobranchial
groove (endostyle) as well as the peripharynyeal ciliated bands of the
Tunicates, the homologue of which was discovered by SCHNEIDER in
Ammocortes, ave to be regarded as transformed gill-clefts, and the
thyroid-gland, the homology of which with the hypobranchial groove
had been maintained by W. MiLLER was said to represent a branchial
sac lying between the spiracle and the first branchial cleft, while the
ciliated arch is the homologue of the spiracular cleft (the pseudo-
Iranch of the Teleosteans).* The endostyle and the ciliated arch
teas
* (Done stands alone in his belief that the pseudobranch of the Teleosteans
ix formed from the anterior wall of the original spiracular cleft and that, by
the later suppression of the cleft, the pseudobranch comes to lie in the first
branchial cleft. Most vertebrate imorphologists regard this pseudobranch as
GENERAL CONSIDERATIONS ON THE TUNICATES, 523
would thus be met with in the Tunieates in a secondarily modified
form, This view rests especially on observations of the development
of the homologous structures in A mmocoetes.
Even though it appears to us that Donn carries these homologies
too far, since we are not inclined to assume direct genetic relations
between the Fishes and the Tunicata, and still less to regard the two
groups as independent branches derived from a common, primitive
racial form (Protochordata), we consider that it entirely justifies the
conviction that the Tunicates cannot be utilised to bridge over the
gap existing between the Chordata and the other branches of the
animal kingdom. Haroxen and GEGENBAUR have been specially
prominent in their advocacy of this view. The hypothetical primitive
form of the Tunicates is represented as a typical Chordate with all
the features generally ascribed to that type. But no characteristics
are to be found either in the anatomy or the ontogeny of the Tunicata.
which ally them directly to any one branch of the Invertebrata. The
Tunicates appear to us no more nearly related to the Invertebrata
than are Amphiorus or the Vertebrata. The specially striking
features in the Tunicates, viz., the absence of segmentation, of the
coelom and the nephridia, the occurrence of asexual reproduction are
all characters which we cannot regard as primitive. They have been
newly acquired in connection with the attached manner of life. ‘The
way in which we have to reconstruct for ourselves the primitive
Chordata (the common hypothetical racial form of the Tunicata, the
Cephalochorda and the Vertebrata) can only be discovered through
careful comparison of the ontogeny and anatomy of these three
groups, and in this we are convinced that the chief stress must be
laid on Amphiorns. Such a reconstruction is at present specially
diffioult, indeed is rendered almost impossible by the fact that our
knowledge is as yet too fragmentary to enable us to establish exactly
the homologies of the different organs in the three groups of the
Chordata, In illustration of this it may be mentioned that our
knowledge of the origin of the peribranchial cavities in the Tunicates
is still incomplete, and the question as to the homology of the ciliated
pit in the Tunicates with the hypophysis cerebri and other problems
are as yet also insoluble,
a development of the anterior wall of the first branchial cleft in no way con-
nected with the spiracular cleft or with the pseudobranch of Elasmobranchs.
Consequently, if there is any truth in the compurison given above it would be
More in aé-ordance with the generally accepted homologies to read
branchiae of Elasmobranchs for pseudobranchiae of Teleosteans.—Ep.
GENERAL CONSIDERATIONS ON THE TUNICATES. 519
tion of the dorsal region found in the Ascidians and the approxima-
tion of the branchial and atrial apertures thus brought about is
traced back to attached forms, the position of the anus in the
Larvacea, which must evidently be regarded as primitive, would
indivate that these animals are descendants of those hypothetical
Tunieate ancestors which still retained the original pelagic life.
On the other hand, we find in Appendicu/aria a series of unmistakable
degenerative phenomena tending to support the assumption thar we
must regard the racial form of the Larvacea as an attached Tunicate.
It is evident that they must be considered as sexually mature larval
forms, sexual maturity being shifted continually further back to
earlier stages so that, finally, the mature adult form no longer
developed. What form can we imagine this latter to have assumed ?
Was it a free-swimming Ascidian form intermediate between Amphiowus
und the Asvidian larva, or an already attached Ascidian form? The
lust assumption seems the more probable. The appearance of the
cellulose mantle and hermaphroditism and the indistinctness of the
segmentation of the body must be regarded as features acquired as
@ consequence of attached manner of life. As these characters are
found in the Larvacea, we are to a certain extent justified in con-
sidering them as the sexually mature larvae of an already attached
form of Tunicate.* In any case, however, all phylogenetic specula-
tions concerning the Tunicates must rest upon careful consideration
of the structure of Appendicularia and the Ascidian larvae.
Among the Ascidiacea the solitary forms are probably the more
primitive. The composite forms lead on to Pyrosoma, which may be
regarded as a composite colony that has not become attached and
that is distinguished by a large common cloaca. In the interesting
family Coelocormidae we see the development of the whole colony in
uw similar direction, Here also the colony is not attached but, on the
other hand, the internal cavity cannot be compared to the common
clouea of Pyrosoma (see Herpmax, No. 24). Pyrosoma forms
4 transition to the free-swimming Doliolidae. This was pointed out
by Huxney with reference to the structure of the gill and the
opposite position of the two apertures of the body (¢f. also GrosBEN,
No. 79). The Doliolidae (Cyclomyaria), among which Doliopsis
(Anchinia) exhibits the most primitive characters, must be regarded
us phylogenetically the oldest Thaliacea. The Sadpidae (Hemimyaria)
must be considered as derived from them, We shall, therefore, have
"Winter (No. 54a) also has lutely maintained that the Larvacea are
degenerate forms.
GENERAL CONSIDERATIONS ON THE TUNICATES. 521
disappearance of the segmentation of the body. Only in the caudal
region of the Ascidian larvae and of Appendicularia are there any
traces of the segmentation which must have been present in their
ancestors. In the caudal portion of the nervous system in the
Asecidian larva, spinal nerves are given off segmentally, as Kuprren
first noticed. In Appensicwlaria these are connected with paired
ganglionic swellings in the dorsal cord. Lancerans (No. 2), by
the use of reagents, was further able to prove that the caudal musen-
lature breaks up into ten consecutive muscle-segments (myomeres)
which are provided with segmentally arranged pairs of motor nerves.
tn individual eases Lancerans (No. 2) found that, in the posterior
caudal region of Apperdécularin, the spinal nerves of the left side
have shifted somewhat forward as compared with the corresponding
nerves of the right side. A similar condition is found in Amphionus.
In the anterior region of the body, on the contrary, no traces of
segmentation are retained.
Although we must recognise a great general agreement in struc-
ture and development between Amphiorns and the Tunicates, it is
very difficult to establish exactly in detail the homologies between
the organs of the two groups. A special attempt of this kind was
made by van Bexepen and Juni, but we are not able to regard all
their deductions as convincing. Starting from the view that, in the
‘Tunicates, a large part of the alimentary caual (in the caudal section)
has degenerated, vaN BENEDEN and Jucrn regard the rectum and the
anal aperture of the Tunicates as new acquisitions which are not
homologous with the homonomous structures of Amphiorus, They
find the homologue of the rectum of the Tunicates in the ‘ club.
shaped gland” of Amphiorns, which belongs to the first trank-metamere
(p. 549) and opens out near the mouth.* Consequently the whole of
the precandal part of the body in the Ascidian larva corresponds
to only the small anterior region of Amphiowus, yiz., to the anterior
cephalic part—the first trunk-segment. This view leads these
authors further logically to deny the strict homology of the endostyle
with the hypobranchial groove of Amphionus, Since, however, in the
Cephalochorda and the Vertebrata also, the anal aperture has evidently
undergone a secondary shifting forward and the caudal section of the
alimentary canal degenerates, there is nothing which compels us to
doubt either the homology of the rectum throughout the Chordata,
“(Witter (No.
primary gill-slit.—!
XXVIL) regards the club-shaped gland as the right
a)
528 TUNICATA.
57..Giarp, A. Recherches sur les Synascidies. Archiv. Zod.
expér, Vol. i. 1872.
58, GiarD, ALFR. Sur le bourgeonnement des larves d'Astellium
spongiforme Gd. et sur la Poecilogénie chez les Ascidies.
Compt. Rend. Acad. Sri. Parix. Tom. exii. 1891.
59. Hsort, J. Zum Entwicklungscyclus der zusummengesetzten
Ascidien. Zool, Anz. Jahrg xvi. 1892.
60. Kowabevsky, A. Sur le bourgeonnement du Perophora Listeri.
Rewue. Sci. Nat. Montpellier, 1874. .
61. Kowavevsky, A. Ueber die Knospung der Ascidien. Arrhir.
f. mikro. Anat. Bd. x. 1874.
62. Kroan, A. Veber die Fortpflanzungsverhaltnisse der Botryl-
liden. drehie. f. Natury. Ba. xxxv. 1869.
63. Krony, A. Ueber die friheste Bildung der Botryllenstécke.
Archiv. f. Naturyrach, Ba. xxv. 1869.
64. Jourpary, S. Sur les Ascidies composées de la tribu dex
Diplosomidae. Compt. Rem, Acad. Sei. Paris, Tom.e. 1885.
64a. Oxa, A. Ueber die Knoxpung der Botrylliden. Zeitachr. 5.
wise, Zoul, Bd. liv. 1892.
644. Oxa, A. Die periodische Regencration der oberen K6rperhalfte
bei den Diplosomiden. Biol. Contralhl, Bd. xii. 1892.
65. Pizon, A. Sur la blastogénése chez les Botryllides. Bull.
Philom. Paris, (8). ‘Tom. iii. 1891-92.
6. SernicerR, O. Zur Entwicklung der Ascidic
Knospung von Clavellina lepadiformis.
Wien. Bd. Ixxxv. 1882.
67. Unrasix, B. Bemerkungen uber die Synascidiengattuy
Distaplia, Zool, Anz. 1885,
68. Denia Vanue, A. Nuove contribuzioni alla: storia naturale
delle Aseidie composte del golfo di Napoli, Mem, Re Aceadl.
Linrré (3). Vole x. 1881,
69 DELLA VALLE, AJ Sur le bourgeonnement des Didemnides et
Botryllides et sur le type entérocoelien des Ascidies. A rehir.
Ital. Biol, Tom. 1&3.
70. Deuba Vane, A. Sul ringievanimento delle colonie de
Dinzona violacea. Sav. Cum. prelim. Avehir. Ital, Biol.
Volkov. D884.
Kibildung unt
unysher, Aewl,
Pyrosoma.
71. Kowauevsky, A. Ueber die Entwicklungsgeschichte der
Pyrosoma, Arehir. fi miko, Anat. Bal xi. 1
LITERATURE. 529
72. Huxuey, To. H. Anatomy and Development of Pyrosoma.
Trans. Linn, Soe. London, Vol. xxiii. 1860.
73. Joumt, L. Btudes anatomiques et embryogéniques sur le
Pyrosoma giganteum, ete. Paris, 1888.
74. Sanensky, W. Beitriige zur Entwicklungsgeschichte der
Pyrosomen. Zool. Juhrb. Abth. f. Anat. Bd. iv. 1891.
Bd. v. 1892.
75. Seenicer, O. Bemerkungen zu Herrn Prof. Salensky’s
“Beitriige zur Embryonalentwicklung der Pyrosomen.” Zool.
Anz. Jahrg. xv. 1892.
76. Sennicer, O. Zur Entwicklungsgeschichte der Pyrosomen.
Jena. Zeitschr. f. Nature. Bd. xxiii. 1889.
76a. Seericer, O, Ueber die erste Bildung des Zwitterapparates
in den jungen Pyrosomenstocken. Festschrift fiir Leuekart.
Leipaig, 1892.
Doliolum.
77. Barros, J. Recherches sur le cycle génétique et le bourgeon-
nement de l’Anchinie. Journ. Anat. Phys. Paris. Ann. xxi.
1885.
78. Geornpaur, C. Ueber den Entwicklungscyclus von Doliolum
nebst Bemerkungen tiber die Larven dieser Thiere. Zeitschr.
J. wiss. Zool. Ba. vii, 1856.
79. Gropgen, ©. Doliolum und sein Generationswechsel. Ayd.
Zool. Inst. Wien. Bd. iv. 1882.
80. Huxiey, Ta. HH. Remarks upon Appendicularia and Doliolum.
Phil. Trans. London, 1851.
81. Kererstem unp Enters. Zoologische Beitrage. Leipziy,
1861.
82. Korotnerr, Au, pe. La Dolchinia mirabilis. Mitth, Zool.
Stat. Neapel. Bd. x. 1891,
83. Korotnerr, A. Die Knospung der Anchinia. Zeitsehr. f.
wiss, Zool. Bd. Ix. 1884.
84. Kowanevsxy, A., er Barrors, J. Matériaux pour servir a
Vhistoire de l'Anchinie. Journ. Anat. Phys. Paris, Tom.
xix. 1883,
85. Kromn, A. Ueber die Gattung Doliolum und ihre Arten.
Archiv. ¢. Natury. Ba. xviii. 1852.
86. Unsanin, B, Die Arten der Gattung Doliolum im Golfe von
Neapel. Fauna und Flora wn Neapel. Monogr x. 1884.
MM
530 TUNICATA.
Salpa.
87. Barnois, J. Mémoire sur les membranes embryonnaires des
Salpes. Journ. Anat. Phys. Paris. Ann. xvii. 1881.
88. Brooxs, W. K. On the Development of Salpa. Bull. Mus.
Comp. Anat. Harv. Coll. Camb. Vol. iii. 1871-1876. Cf.
Archiv. fiir Naturg. Bd. xlii. 1876.
89. Brooxs, W. K. The Origin of the Eggs of Salpa. Stud. Biol.
Lab. Johns Hopkins Univ. Baltimore. Vol. ii. 1882.
90. Brooxs, W. K. Chamisso and the Discovery of Alternation of
Generations. Zool. Anz. Jahrg.v. 1882.
91. Brooxs, W. K. Is Salpa an Example of Alternation of Genera-
tions? Natwre. Vol. xxx. 1884.
92. Brooxs, W. K. The Anatomy and Development of the Salpa-
chain. Stud. Biol. Lab. Johns Hopkins Univ. Baltimore.
Vol. iii. 1886.
93. Brooks, W. K. On the Relationship between Salpa and
Pyrosoma. Johns Hopkins Univ. Cire. Vol. 9. 1890.
94. Biscuur, O. Kinige Bemerkungen uber die Augen der Salpen.
Zool. Anz. Jahrg. xv. 1892.
94a. GoprErRt, E. Untersuchungen iiber das Sehorgan der Salpen.
Morph. Jahrb, Bd. xix. 1892.
95. Huxuey, Tu. H. bservations on the Anatomy and Physiology
of Salpa and Pyrosoma, Phil. Trans. 1851.
96. Kowavevsky, A.
Machy, kyle
97. Knoxy, A. Observations sur la géncration et le developpement
des Biphores. dun. Se/, Not. (3). Tom. vi. 1846.
gx. Leuckart, R. Salpa und Verwandte. Zool, Untersuchungen,
Heft. ii. Giessen, 1854.
99. Metcatr, M. The Anatomy and Development of the Eyes
and Subneural Gland in Salpa. Johuws Hopkins Ouie. Cire.
No. 97. 1892.
99a. Mercaur, M. On the Eyes, Subneural Gland and Central
Nervous System in Salpa. Zool. Anz. Jahrg. xvi. 1893.
100. Sanensky, W. Ueber die embryonale Entwicklungsgeschichte
der Salpen, Zeitsehr, f, miss, Zool. Bd. xxvii. 1876.
101. SaLtensky, W. Ueberdie Knospung der Salpen, Morph Jala}.
Bad. 1877.
102. Satensky, W. Ueber die Entwicklung der Hoden und uber
den Generations we
Beitrag zur Entwicklung der ‘Tunicaten.
Useh. Wissenseh. Gottingen. 186%.
sel der
Bd. xxx. Suppl. 1878.
alpen. Zeitschr. f. wise, Zool.
LITERATURE, 531
103. Sanensky, W. Folliculare Knospung der Salpen und die
Polyembryonie der Pflanzen. Bini. Centralbl. Bd. v. 1885,
104. Sanensky, W. Neue Untersuchungen iiber die embryonale
Entwicklung der Salpen, Mitth, Zool, Stat. Neapel, Ba. iv.
1883, In two parts,
105, Szeuicer, Osw. Die Knospung der Salpen. Jen. Zeitschr.
fiir Naturw, Bad. xix. 1886.
106, Szenicer Osw. Die Entstehung des Generationswechsels der
Salpen. Jena. Zeitschr. f. Naturw. Bd. xxii. 1888.
107. Toparo, Fr. Sopra lo sviluppo ¢ l’anatomia delle Salpe. Atti.
R. Accad. Lincet. Trans. (2). Vol. ii. 1875.
108, Toparo, FR Sui primi fenomeni dello sviluppo delle Salpe,
Atti. R. Accad. Lincei. Trans. (3). Vol. iy. 1880,
109. Toparo, Fr. Sui primi fenomeni dello sviluppo delle Salpe.
24 communic. preliminare. Atti, R. Accad. Lincei. Trans.
Vol. vi. 1882. (Also in Archiv. Ital. Biol. Tom. ii, 1882.)
110, Toparo, Fr. Sui primi fenomeni nello syiluppo delle Salpe.
3* comm. prelim. Atti. R. Accad. Lineei. Trans. Vol. vii.
1883. (Also in Archiv. Ital. Biol. Tom. iii, 1883.)
111. Topaxo, Fx. Sopra i canali e le fessure branchiali delle Salpe.
Atti, R. Accad. Lincei. Trans, Vol. viii. 1884.
112, Toparo, Fr. Studi ulteriori sullo sviluppo delle Salpe. Atti,
R. Avead. Lineei. Mem. (4). Vol. i. 1886.
113. Toparo, Fr. Sull’ omologia della branchia delle Salpe con
quella degli altri Tunicati, Afi. R. Accad. Lincei. Rend. (4).
Vol. vi, 1889. (Also in Archiv, Ital, Biol, Tom. xi, 1889),
APPENDIX 70 LITERATURE ON TUNICATA,
I. Brooks, W. K. The genus Salpa. Mem. Johns Hopkins
Univ, 1893.
Il. Casrozn, W. B. Early Embryology of Ciona intestinalis.
Bull, Mus, Harvard. Vol. xxvii. 1896.
Ifl. Castne, W. BE. On the Cell-lineage of the Ascidian egg.
Proc. Amer. Acad. Vol. xxx. 1894.
TV. Caununy, M. Contributions a l'Btude des Ascidies com-
posées. Bull. Sei, France et Belgique. Tom. xxviie
1895,
V. Caunery, M. Sur la Morphologie de la Larve composée
dune Synascidie (Diplosomoides Lacazii, Giard). Compt.
Rend. Acad. Sci. Paris, Tom. exxv. 1897.
532 TUNICATA.
VI. Cauuery, M. Sur le Bourgeonnement des Diplosomidae
et des Didemnidae. Compt. Rend. Acad. Set. Paris
Tom. cxxi. 1895.
VII. Cautery, M. Sur I'Interprétation morphologique de la
larve, etc., du genre Diplosoma. Compt. Rend. Acad.
Sci. Paris. Tom. cxxi. 1895.
VII. Caunery, M. Sur les Synascidies du genre Colella, et le
polymorphisme de leurs bourgeons. Compt. Rend. Acad.
Sei, Paris, Tom. exxii. 1896.
IX. Damas, D. Les Formations épicardiques chez Cions
intestinalis. Archiv. Biol. Tom. xvi. 1899.
X. Fuoperus, M. ‘ber die Bildung der Follikethiillen bei
den Ascidien. Zeitechr. f. wise. Zool. Bd. lxi. 1896.
XI. Garstanc, W. Budding in Tunicata. Setence Progress.
Vol. iii. 1896.
XII. Grarp, A., ET CaunERy, M. Sur I'Hivernage de la
Clavelina lepadiformis. Compt. Rend, Arad. Sci. Parix.
Tom. cxxiii, 1896.
XIN. Herper, K. Beitrage zur Embryologie von Salpa fusi-
formis. Frankfurt, 1895. (bh. Senkenb. Gea. Bd.
XV.
XIV. Pile J. Beitrage zur Keimblitterlehre und Entwick-
lungsmekanik der Ascidienknospung. Anat. Anz. Bd. x.
1895.
XV. Hyort, J. Ueber den Entwicklungcyelus der zusammen-
gexetzten Ascidien. Mittheil. Zool. Stat. Neapel. Bd. x.
1893.
XVI. Hyort, J., uxp Bonneviz, Fr. Ueber die Knospung von
Distaplia magnilarva. Anat. Anz. Bd. x. 1895.
XVI. Juni, GC. Recherches sur la blastogéntse chez Distaplia
magnilarva et D. rosea. Congr. Zool. Leyden. 1896.
XVII. Korotserr, A. Embryonale Entwicklung der Salpa demo-
eratica. Biol, Contralbl, Ba. xiv. 1894: and Zettarhr
foaiss, Zool, Ba. lix. 1895.
XIX. Korotnerr, A. Tunicatenstudien. Mitth. Zool. Stat.
Neupel. Ba. xi. 1895,
XX. Korotyerr. A. Zur Embryologie von Salpa cordiformis-
zonaria und musculosa-punetata, — Mitth. Zool. Stat.
Noupel. Ba. xii. 1896.
XXa. Korotyerr, A. Zur Embryologie von Salpa runcinata-
fusiformis. Zeitschy #. miss, Zool. Bd. Ixii. 1896.
XXL
XXIa.
XXII.
XXIIL,
XXIV.
XXY.
LITERATURE, 533
Kororyerr, A. Zur Entwicklung der Salpen. Biol.
Centralbl. Bd. xv. 1895.
Kororyerr, A, Zur Embryologie von Salpa maxima-
Africana, Zeittschr. f. wiss. Zool, Bad, Ixvi, 1899.
Lerevrr, G. Budding in Ecteinaseidia. Anat. Anz. Bd.
xiii. ; and Johns Hopkins Uwiv, Cire. 1897,
Lereven, G. On budding in Perophora. Jolns Hopleins
Univ. Cire, Vol. xiv, 1895,
Mercatr, M. The follicle-cells of Salpa. Zool. Anz.
Jabrg. xx. 1897; and Jvhns Hopkins Unio. Cire.
1897.
Pizon, A. Contributions a I'Embryogénie des Ascidies
simples, Compt. Rend. Acad. Sei. Paris, Tom. exx.
1895.
XXVI. Przon, A. Histoire de la Blastogénése chez les Botryllides.
XXVIa.
XXVIL.
XXVIII.
XXIX,
XXX.
XXXL.
XXXII,
XXXII.
XXXIV,
Ann. Sei. Nat. Tom. xiv. 1893.
Pizox, A. Evolution des elements sexuels chez les
Ascidiens composts. Compt. Rend. Acad, Sei, Paris,
Tom. exix. 1894.
Pizon, A. Les Membranes embryonnaires et les Cellules
de rebut chez les Molgules, Compt, Rend. Acad, Sci.
Pavis. Tom. exxii, 1896,
Rirrer, W. E. Budding in Compound Ascidians based
on studies on Goodsira and Perophora. Journ. Morphol.
Vol. xii, 1896.
Sauensky, W. Beitrige zur Entwicklungsgeschichte a.
Synascidien (Diplosoma and Didemnum). Mitth. Zool.
Stat. Neapel. Bd. xi. 1895.
Sauenscy, W. Morphologische Studien an Tunicaten,
(Nervous system and Metamorphosis of Distaplia). Morph.
Jahrb. Ba. xx. 1893.
Sauensky, W. Ueber die Entstehung der Metagenesis bei
Tunicaten. Biol. Centralbl. Ba. xiii, 1893.
Samassa, P, Zur. Kenntniss der Furchung bei dem Asei-
dien. Arch mikro. Anat. Ba. xliy. 1894,
Szerioer, O. Kinige Beobachtungen iiber die Bildung
der auesseren Mantels der Tunicaten. Zeitschr. f. wise.
Zool. Bad. li. 1893.
Srzticer, O. Ueber die Entstehung des Peribranchial-
raums in den Embryonen der Ascidien. Tom. eit.
534 TUNIOATA.
XXXV. Toparo, F. Sopra lo Sviluppo della parte anteriore del
corpo delle Salpe. Atti. R. Acad. Lincei. Rend. (3).
Vol. vi. 1897.
XXXVI. Wingy, A. Studies on the Protochordata. Parts IL and
III. (nervous system, sense-organs and mouth). Quart.
Journ. Micro, Sci. Vol. xxxv. 1893.
XXXVII. Wimuey, A. Amphioxus and the Ancestors of the verte-
brates. London and New York. 1894,
CHAPTER XXXVI.
CEPHALOCHORDA.
Amphioxus.
Tux earlier statements concerning the development of Amphiorus
made by Max Scnruttze (No. 18), Leuckart and PAGENSTECHER
(No. 15) referred merely to a few of the later larval stages; our
knowledge of the ontogeny of this form is, therefore, founded princi-
pally on the investigations of KowaLevsky (Nos. 10 and 11), and was
extended by HarscHek (Nos. 4 and 8). The metamorphosis of
Amphiorus has recently been described by Ray LankesTER and
Wiuey (No. 13) and by Wiuvey alone (No. 23). The develop-
ment of the genital organs has been investigated by Bovert (No. 3).
This last author (No. 2) as well as SPENGEL (No. 19), Ray LANKESTER
(No. 12) and van W1sHE have also published treatises on the anatomy
of the adult Amphiorus to which we shall have occasion to refer.*
A. Oviposition, Cleavage and Gastrulation.
The mature genital products of Amphtorus pass from the genital
chambers, through rupture of their walls, into the atrial cavity and
thence they were said to pass through the gill-clefts into the pharynx
and to be ejected through the mouth (KowaLEvsky, HatscHEk).
According to Ray LankEsTER and WILLEY, however, they are, in
most cases, ejected through the atriopore. Fertilisation takes place
in the surrounding water. The newly laid egg is surrounded by a
vitelline membrune at first only slightly separated from it, but, under
the influence of the sea water, the interval between the egg and the
*[More recently, Soporra (No. XI.) and Sticut (No. XII) have rein-
vestigated the maturation and fertilisation of the egg, and the former,
Kuaatscn (No. LV.) and Macsripr (No. VIIL) have re-examined the forma-
tion of the germ-layer.—Ep.]
636 CEPHALOCHORDA.
membrane becomes greater. ‘I'here is no micropyle. The spermatowa
pass through this elastic membrane to reach the egg.*
‘The first stages of development closely resemble those of the
Ascidians. Cleavage is total and almost equal (adequal type of
Harscuxk). ‘The first furrow is meridional and appears first at
the animal pole, where for some time it is deepest; it eventually
divides the egy into two exactly equal parts (Fig. 279 8). The
second furrow, which is also meridional, is at right angles to the first
and leads to the rise of four blastomeres of equal size which leave free
between them a cavity open above and below ; this is the cleavage-
cavity (Fig. 279 C and D). The eight-celled stage (Fig. 279 £) is
brought about by an equatorial furrow which lies somewhat nearer
the animal than the vegetative pole and leads to the first differentia-
tion between the blastomeres of the animal and vegetative halves.
‘The embryo consists of a circle of four smaller blastomeres near the
animal pole and another circle of four larger blastomeres belonging
to the vegetative pole. Further meridional furrows divide theve eight
cleavaye-spheres into sixteen, the sixteen-celled stage then conxisting
of acirele of eight smaller and another of eight, larger blastomeres
(Fig. 279 F).
Even at this sixteen-celled stage, according to WILSON, certain individual
differentiations are found which influence the further course of cleavage. The
regular stage described by Hatscuex, in which the eight cells of the apper
cirvle are found resting regularly on the eight cells of the lower
comparatively rarely observed by Witsos. The blastom
cance often appear shifted spirally in relation to those of the
to be the ease in the Annelida and Mollu
the sixteen-celled stage a bilateral (or strictiy speaking a sirad: a.
is evident in the armingement 0
tate haif be
larger cells
sma
ang divided into four
Tove
CLEAVAGE AND GASTRULATION. 537
pol
more rapid «
he act of ng; II, the same stage
blastula, in section
5. divin
of protoplaam ;'(, four-celled stage
colled stage sixteonsrollod stage
animal pole: one of the circles of cells
in section ; /, blaxtula, surface view ;
=f
axis, and the wall at ‘he vegetative pole, /.e., the p
the egg, is composed of somewhat larger cells richer i
‘This represents the entodermal region of the embryo.
ing occurs, and this soon passes into an invagination:
which leads to the development of « cap-shaped
invagination causes the cleavage-cavity to decrease in s
completely to disappear, the two primary g !
close contact (Fig. 280 2).
The gastrula-stage now passes through certain phas
which the bilateral symmetry which, according to Wi
evident in the stages of cleavage, becomes more distit
same time, the embryo elongates in the direction
longitudinal axis. The apex of the ectoderm of the
sponds to the animal pole, while the vegetative pole
coincide with the centre of the at first circular ap
show a point at which the curve is more abrupt ;
coincide with the animal pole but lies somewhat ey
sponding to the anterior end of the later pri
DEVELOPMENT OF THE MEDULLARY TUBE, ETC. 539
end coinciding with the posterior edge of the blastopore. The
definitive principal axis thus forms an acute angle with the primary
axis. The blastopore has undergone shifting to the dorsal side of
the embryo ; it now gradually decreases in size, chiefly through a
backward growth of its dorsal or anterior border. The posterior
{ventral) edge of the blastopore, on the contrary, remains stationary
during this narrowing process. It is always marked by two larger
entoderm-cells lying symmetrically to the median plane (Fig. 280 @) ;
these are claimed by Harscuex as pole-cells of the mesoderm.*
Finally, the embryo becomes more elongated. The ventral surface
is distinguished by being arched, while the dorsal side, which was
originally oecupied by the blastopore, is distinctly fattened. The
posterior end of the dorsal side is occupied by the now very small
vestige of the blastopore (Fig. 280 @). Even at this stage, the
external surface of the embryo is covered with short flagella which
enable it to rotate within the egg-envelope.
Tn the position of the blastopore and the conditions under which it closes,
there is close resemblance between Ampiiorus and the Ascidia (cf, p. 342).
_ Inour description of the transformations undergone by the gastrula-stage,
we have for the most part followed Hatscnex. A rather divergent account
has recently been given by Lworr (No. 17) who, in dealing with the closure
of the blastopore, lays the chief stress on the independent and rapid growth
of the ectoderm at its dorsal margin. (Stress should also be laid on the
protrusion of the lateral edges of the blastopore, a point which was overlooked
by Lworr), From this rapidly growing dorsal edge, ectodermal elements are
said to be invaginated dorsally into the gastrula-cavity and there finally to
force the proper entodermal elements from the dorsal to the ventral side of
the cavity. According to this view, the cells forming the dorsal wall of the
archenteron are ectodermal and it is from this cell-layer that Lworr derives
the chorda. Lworr was not able to convince himself of the presence of the
primitive cells of the mesoderm which Hatscu@x found marking the posterior
end of the longitudinal axis of the embryo.
B. Development of the Medullary Tube, the Primitive Segments
and the Notochord.
The next ontogenetic stages of Anmphiorue are characterised by the
increase in length of the whole body, The median part of the dorsal
surface, at the same time, sinks in somewhat (Fig. 281), and this
* (Sonorra (No. X.) agrees with Lworr (No. 17) in denying that the biasto-
pore closes from before backward. They believe that it gradually diminishes
on allsides. Sonorra was unable to find the pole-cells and does not believe
that they exist, at any rate at the early stages figured by Harscuex.—Ep,]
540 CEPHALOOQHORDA,
depression leads to the development of the medullary tube, The
latter is here formed, not, as in the Ascidia and many Vertebrates,
through the fusion in the middle line of
two lateral medullary folds, the process,
although it may be deduced from the
above type, being somewhat modified.
It might be described us lateral over
growth. The medullary plate (Fig, 282
A, mp) sinks down somewhat and its
lateral edges become detached from the
rest of the ectoderm. The ectoderm (/})
now grows inward from either side above
the medullary plate and unites in the
middle line before the plute hus become
curved into a tube (Fig. 282 8). “The
c . dorsal groove, although completely
wiead with the reduaeny of covered externally, ia still /opadiieTiaa
eee aera iti, under the integument” (Fig. 283). Only
Henn je later does the medullary plate curve round
tube; ws’, first primitiveseg- dorsally, and, through the fusion of itt
ee Primitive Jateral edges, form a closed tube, the
medullary tube (Fig. 284).
The union of the lateral ectodermal growth above the medulliry
plate takes place from behind forward, commencing near the remains
Fig, 282. —A, transverse section through au embryo of a
of the first primitive segment (after ATCHIRK, {hom 0,
transverse section through an embryo of Amy
primitive segments (after Hatsenes, from 0,
ch, chorda-rudiment ; dh, archenteric cavity ; Ab, lay
the medullary plate; i, entoderm ; fh, body.
medullary plate,
DEVELOPMENT OF THE MEDULLARY TUBE, ETO. 541
of the blastopore, which thus becomes covered by a layer of ectoderm
(Fig. 281). The blastopore thus does not open externally but into the
neural canal; this connection between the intestine and the neural
tube is long retained, being known as the neurenterie canal. [See
Korscn, No. V.]
The medullary plate does not reach the most anterior end of the
embryo, but extends for about three-quarters of its length. At the
point where it stops, which lies somewhat in front of the anterior
edge of the first primitive segment, the medullary tubes retain an
external aperture which is at first wide but gradually narrows
later (neuropore, Figs. 285, 286, up), As we shall presently see,
the neuropore in Amphioxue
only closes in a very late stage
(Kurrrer). The cells of the
medullary tube, like the other
ectodermal cells, carry flagella.
These, which are long and ex-
ceedingly fine, project into the
lumen of the tube and, are
directed backward,
The development of the
medullary tube leads to a Fic. 283.—Tvansverse section through an
pressing inward of the middle Ss — Bae papa
part of the dorsal wall of the oor from 0. Henrwio's Text-
entoderm - sac (Fig. 282 A).
This median swelling is accompanied by two latero-dorsal out-
growths of the entoderm-sac (Fig. 282 3B, mic). hese paired
longitudinal folds, the so-called mesoderm-folds, yield the material
which becomes the mesoderm and can be traced posteriorly as far as
to the neighbourhood of the two primitive mesoderm-cells,* although
the most posterior part of the folds is indistinctly marked off from
the rest of the entoderm-sac. Segmentation very soon appears in
the anterior region, the mesoderm-folds becoming cut up by trans.
verse indentations, into consecutive portions, the primitive segments
(Fig. 281, ws’, ue"). The segments which, owing to their origin, must
be regarded as archenteric diverticula, develop regularly from before
backward, We thus see, in Fig. 281 two, in Fig. 286 five, and in
Fig. 286 nine segments distinctly marked off. Later, the primitive
segments become completely cut off from the entoderm-sae, and they
[* See footnote, p. 539.—Ep.]
1
DEVELOPMENT OF THE MEDULLARY TUBE, ETO. 543
contact in the median line dove-tail into one another. The rudiment,
of the chorda is then a solid strand-like thickening of the dorsal
entoderm-wall, from which it soon becomes severed as an independent
strand (Fig. 284, ch). In the meantime the cells of this rudiment
press between each other in such a way that each cell finally extends
transversely across the entire rudiment.
The chorda develops, on the whole, from before backward. It
commences to develop in the region of the primitive segments. The
anterior part of the chorda, which extends above the first primitive
segment towards the anterior end of the body, only develops later,
Fig, 286.—Eimbryo of Amphiccus with tive pairs of primitive segmonts (after HaTscren,
from 0, Hxntwro's Tevt-book), A, lateral aspect; , viewed from the dorsal side.
; om, neurenteric canal; dh, enteric cavity; ik, ectoderm; mh,
mesoderm-fold ; #, neural tube; wd, archenteron; ws’, first primitive segment; ws,
cavity of primitive segment; V, anterior, 1, posterior,
and then not through the independent growing out of the already
developed rudiment, but through « separation of cells at the most
wnterior part of the archenteron from which no primitive segment is
abstricted,
The whole of the entoderm-plate which lies between the mesoderm-
folds is not, however, used up for the formation of the chorda, but only
the median part of it, the lateral parts, according to HaTsoneK,
being utilised for the completion of the dorsal wall of the alimentary
EARLY LARVAL DEVELOPMENT. 545
288, 290, ©) does not arise as an epithelial fold, but as a simple
thickening due to the ectoderm-cells increasing greatly in height.
The medullary tube (Fig, 288, mr), in the central canal of which
backwardly directed cilia can still be seen, has an anterior swelling,
in which not only are the walls thicker but the central canal wider.
Even in very young stages a pigment-spot is found in the ventral
wall of the medullary tube in the fifth metamere (Fig. 288), Later,
a similar spot, functioning as an eye-spot, appears at the anterior end
of the cerebral swelling (Figs. 289, 290). ‘The posterior end of
the medullary tube uppears dilated (Fig. 288), and this swelling
contains the undifferentiated material which, as growth advances,
continually produces new parts of the medullary tube, This dilated,
posterior end is bent round the posterior end of the chorda, At this
point the communication with the alimentary canal is found (nenren-
terie canal, Fig. 288, en).
In cross-sections, the medullary tube is found to consist of a
single layer of cells (Fig. 234, »), the first perceptible nerve-fibres
being found in the ventro-lateral corners, in contact with the chorda.
The rudiment of the mesoderm consists of the series of consecutive
primitive segments (Fig. 286, us’, ux") and of a posterior unsegmented
region, the mesoderm-folds (mf) which (according to Harscnex)
terminate in the pole-cells of the mesoderm (mp). In this posterior
region, the coelom is still for a time in communication with the
enteric cavity. Later, however, the mesoderm-folds become com-
pletely separated from the entoderm-sac, and, as the posterior end of
the chorda-rudiment becomes similarly abstricted from the alimentary
canal, the neurenteric canal, in later stages, forms a simple communica-
tion between the ventrally-curved posterior end of the medullary tube
and the most posterior end of the intestine.
Even in the stage with eight primitive segments, we find indica-
tions of that asymmetry which affects the later development of the
body. The primitive segments of the right side of the body lie
somewhat further back than those of the left (¢/. the boundaries of
segments fully outlined with those in dotted outline in Fig. 287).
‘This asymmetrical shifting of the primitive segments takes place to
such an extent that the junction between two segments on one side
coincides with the centre of a segment on the opposite side,
In the stage with nine primitive segments, the most anterior
segment sends off dorsally, at the side of the chorda-rudiment, a
hollow process (Fig. 286, m) which gradually grows forward to the
anterior extremity of the body. This cephalic process of the mesoderm
NN
546 CEPHALOCHORDA.
yields the mesodermal structures in the anterior region of the body.
It has recently been regarded by HaTscHEK (No. &) as the rudiment
of an independent pair of primitive segments, in which case, the
segment hitherto called the first would actually be the second.
The walls which separate the consecutive segments, the so-called
dissepiments, at first run straight from the dorsal to the ventral side
(Fig. 285, 4). Later, they curve backward (Fig. 286) and finally
they develop the characteristic angulation at the level of the upper
part of the chorda dorsalis (Figs. 287, 288).
ec om
Fic. 286.—Stage iu the development of Amphiv.cus in which there are nine primitive
segments (after HaTscHek). In the fifth, sixth and seventh primitive segments,
the muscle formative cells (2) are clearly marked. du, anterior entoderm-divert-
culum ; rc, ectoderm ; en, entoderm; m, cephalic process of the first primitive
segment ; mf, mesoderm-folds ; mp, pole-cells of the mesoderm ; m3, muscle
formative cells ; 1p. neuropore ; us’, first primitive segment; we”, second primitive
segment.
The primitive segments origi
ally lie dorsally above the alimentary
canal (Fig. 283, wk). Later, they extend ventrally and thus grow
round the alimentary canal (Fig. 284). Those parts of the coelom
also which surrounded the canal are originally distinet from one
another, but at the stage in which the mouth forms, the ventral part
of the dissepiments degenerate
and consequently the originally
distinct coelomic cavities run together and form the splanchnocorle,
which, enclosed by the lateral plates, rans through the whole length of
the body as a continuous cavity. The right half of the splanchno-
coele is in Open communication with the left half below the alimentary
canal. According to HaTscHEK, previous to this union of the ventral
part of the primitive segments that grow down on either side, 4
simple ventral lamella is formed which extends between the ectodenu
and the alimentary canal. [n this ventral lamella the rudiment of
the first recognisable blood-vessel, the swh-(ntestinal vessel appears,
a vessel w
ich lies later on the ventral side of the alimentary canal
covered by the splanchnopleure. The first rudiment of this vessel
EARLY LARVAL DEVELOPMENT, 547
can be followed forward from the posterior end of the body as a cloar
canal (Figs. 288, 289, av). Near the second segment it is deflected
to the right side of the body by the first gillaudiment forming in the
ventral median line, and ends, apparently blindly, near the club-
shaped gland. The origin of this vessel is still obsenre. KoWALEVSKY
derives it from cells lying free in the body-eavity which form a solid
strand that becomes hollowed out later.
As the cavities of the primitive segments gradually increase in size
the cells of their walls become more and more flattened. This does
not apply, however, to the cells in contact with the sides of the noto-
chord which, on the contrary, become greatly elongated as columnal
or club-shaped cells and form the rudiment of the lateral trunk-museles
(Fig. 284). These cells contain nuclei in their free club-shuped ends,
while the basal portion, that is turned to the chorda, becomes trans-
formed into muscular tissue. Each of these musele-cells traverses the
te be
Fig, 287,75! of Amphiorus with thirteen primitive segments (aftr HaTscHek).
The boundaries of the segmecuts of the left sida are iodicated by uninterrupted lines,
and those on the tight by dotted lines. om, ueurenteric canal ; dr, rudiment of the
club-shaped gland, de’, right, de”, left, anterior entoderm-divertioulum ; np,
neuropore,
whole length of the primitive segment to which it belongs (Fig. 286,
mz), and its axis lies in a line with the corresponding cells of the
neighbouring primitive segments. The muscle-fibres formed by these
cells fuse together and ran continuously through the consecutive
segments. Each muscle-fibre is thus formed from a series of con.
secutive muscle-cells. Transverse striation can early be made out
in the fibres. The fact that the walls of the primary gut-pouches
(enterocoeles) give vise not only to the lining of the body-cavity but
also to the trunk-muscles, has caused some observers to speak of the
primary cavities as the myocoelomic pouches, At an early period
after the ventral extension of the mesodermal somite described above,
its cavity becomes divided into two by the formation of a horizontal
partition which separates a ventral splanchnocoele from a dorsal
myocoele (see pp. 564 and 565),
EARLY LARVAL DEVELOPMENT, 549
essentially agrees with that of the Vertebrates, consisting of vesicular
flattened cells. The structures that were described as chorda-plates
appear to be a kind of artifact.
Great changes also take place in the eatoderm, Two lateral
diverticula first become abstricted from that anterior part of the
entoderim-sac which lies in front of the first primitive segment ; these
diverticula lie beneath the cephalic process of the first primitive
segment (Fig. 286, do), and are called by Harsoren the anterior
entoderm-sacs. This wuthor hus recently claimed them as an anterior
pair of branchial saes (No. 8). The future fate of these two diverti-
cula, which at first resemble each other, differs greatly. The one on
the right (Fig. 287, dv) increases considerably in size and, pressing
back the anterior end of the alimentary canal, completely fills the
anterior cavity beneath the chorda in the snout-like prolongation of
the body (Fig. 288, A). The left diverticulum (Fig. 287, dv), on the
contrary, remains small and does not shift forward. In later stages
it lies transversely beneath the chorda and opens outward in front of
and above the oral aperture (Fig. 288, 1), A strongly ciliated efferent
portion can be distinguished from a smaller blind portion lying to the
right. This vesicle to which, later, « nerve runs, was regarded by
Kowa.evsry as « peculiar sensory organ of the larva. The pre-oral
pit becomes enclosed by the developing oral hood and thus comes to
lie within the adult mouth ; the ciliated epithelium lining its efferent
section increases in extent and gives rise to the so-called wheel-organ
(Raderorgan), while its sensory organ persists in the adult as the
groove of HaTscHEK.
The two anterior entoderm-diverticula have been interpreted in many
different ways. We have already mentioned that Harscuek (No. 8) regards
them as the most anterior pair of branchial sacs, vas Wise (No, 22)
recently claimed the aperture of the left entoderm-vesicle as the primary
mouth (autostoma) of Amphionus, homologous with the mouth of the Tunicata.
The ciliated organ which develops from the entoderm-vesicle, together with
Hatscuen's pit, has been compared by van WisHe to the ciliated pit of the
Tunicata. The right entoderm-vesicle, on the contrary, which is not morpho-
logically the antimere of tho left, corresponds to the anterior cephalic somite of
the Selachians, from which are developed the optic museles innervated by the
culo motorius. Baresox (No. 26) las compared the two anterior vesicles to
the proboscis-coelom of Balanogiossus and the uperture of the left vesicle to
the proboscis-pore, a conclusion with which Wiuny (No. XIIT.) agrees.
A further derivative of the entoderm is found in the so-called eld
shaped gland first seen by M. Scauntze and later by Lauckarr and
PagenstecHeR (Figs. 288,289, /). This lies near the first primitive
550 CEPHALOCHOEDA.
seginent and arises as a trausverse yrouve in the cher oi the enteno
(F 87, dr) specially distinct on the right side of the teodiy and
running thence ventrally to the left side. This gmevve leepen: and
becornes constricted off fran the enteron to form an independent
tube, and then represents the club-shaped gland. the right, bind
portion of which is dilated, while the narrowed left section opens
externally in front of the oral aperture (Fig. 288, 2°). In later stages,
the right, blind end of this yland enters into communication with the
Iumen of the intestine (Ray Laskester and WILEY.
dota larva somewhat older than that depicted im Fig. 238,
end of the same seen trom th
; d, alimentary canal ; de, di:
dissepiments of the other side
, club-shaped gland ; ka, gill-cleft : mm. mouth ; sp,
‘mr, medullary tube ; my, posterior end of the medullary
fube sap. neuropore ; xr, sub-intestinal vein ; 1, ciliated organ ipre-oral pith.
lhumediately in front of the club-shaped gland there is a transverse
ciliated band (Fig. 289 A, fl) which, according to WILLEy, is the
first rudiment of the endoxtyle.
The oral aperture (Fig. 289, 1) forms on the left side of the bedy
in the region of the first segment. A dise-like thickening of the
ectoderm first appears, to the inner side of which the intestine
becomes closely applied, since, in this region, the mesoderm does not
LATER LARVAL STAGES. “651
extend so far ventrally. In the middle of this ectodermal thickening,
the larval oral aperture forms at first as a narrow perforation which,
however, soon widens. Consequently, the oral aperture is surrounded
by a thickened ectoderm-wall.
The first gill-cleft forms soon after (Fig. 289, kx) in the ventral
region of the second body-segment. A small outgrowth of the ento-
derm here forms, round which the entoderm-cells become elongated
and less granular. An annular entodermal thickening is thus formed
in the middle of which is the above-mentioned depression, the cells
of which soon fuse with the ectoderm, and a perforation, the first
gill-slit, takes place. Round the first gill-cleft, the ectoderm is not
thickened, but the entoderm forms a circular wall representing the
inner edge of the gill-cleft. The first primary yill-cleft which arises
dn this way soon shifts to the right side of the body (Fig. 289 4).
After the mouth and the first gill-clefts have broken through, the ciliated
organ, the pre-oral pit (derived from the left enteric vesicle), and the elnb-
shaped gland also open externally.
The anal aperture breaks through at the most posterior end of the
alimentary canal on the left side of the body. At the same time,
the communication between the intestine and the ventrally curved
end of the medullary tube (neurenteric canal) (Fig. 289 2) is lost.
After all these apertures have formed. the |
taking in food.
va is capable of -
D. Later Larval Stages.
The further development of the larva falls into three periods :—-
I. Behind the first yill-slit, a series of other so-called primary yill-
xlitx (ax many as fourteen, WILLEY) develop ; most of these shift: to
the right side of the pharynx. The metapleural folds arise and the
atrinm begins to form and to close from behind. The primitive seg-
nents increase in number till the condition of the adult is in’ this
rexpect reached (sixty-one segments forming in Amphiocus lanceolatue),
The unpaired tin of the adult with its cavity develops (Figs. 290, 291).
Il. On the right side, above the row of primary gill-clefts, a second
row forms (xeronlary clefts of WILLEY). After the atrium has closed,
the primary gill-clefts xhift to the left side, while the secondary
remain on the right side. The larval mouth becomes changed into
the velum, and the definitive mouth-aperture arises as a consequence
of the formation of pnired folds, the oral hood, round the larval
mouth. The mouth shifts into the ventral median line. The oral
32 CEPEALOCRORDA
eirr: feveing ami the cbit-+hapert siami deyenerates. In consequence
o€ che jeveinpment xf >art—bers im oamectie sith the gill-diw,
the aumber of the latter » ienbled. The hepatic caecum form»
(Figs. 292, 293, 255.
TIL The iarva, which in cwential points now chwely resembles the
adtoit. has Sven ap jeiagie ite ami baries itself in the aand. The
gillclef=- sirewiy éemei. whieh at tirst were arranged metamerically,
shift nearer «gether. ami their number is further increased by the
additing of painal *-;tin-, ters (Writer). These tertiary clefts
evatinie t) inerease in camber throagbout life.
Tt has alrewiy been mentinoed (pp 351) that the first primary yill-
deft which aree in the ventral median line of the second truank-
segment ~»n shifts t» the right side of the body. In an exactly
similar was. oew yill-elefts form successively in the body-segments,
that follow : Fizs. 290. 291), these clefts also lying in the ventral
median line ami shifting later to the right side of the body. The
row of préwary clefts now lying on the right side is destined later
to take up its tinal position en the left side. The number of primary
clefts which thus arise one after the other varies from twelve to
fifteen. and ix usually fourteen. They hare a strictly metameric
orranpwent aml, aceoniing to Hatscaer (No. 8), are intersegmental.
The gill-clefts thus correspond to the boundaries of the segments.
HatscHEeK No. >) regards the above-mentioned entoderm-vesicles as the most
ft... Thexe vesicles correspond to the posterior boundary
reirepresented by the cephalic process of the mesoderm
as au undeveloped pair of primitive segments).
ithe anterion ciliated arch, which is homologous
Tunicates) was regarded by him as the second
> which were described above as the first true
rdin. to this interpretation, represent the third pair.
somewhat smaller than the others: the clefts which
int. are ai at first smalier than the rest and do not deviate from
their median position to tie right.
With the i
pair of
gill-clefts would a
This pair ison
follow the
The median ventral blooLvessel which, in the pharyngeal region,
constitutes the branchial artery, turns sumewhat to the right in the
branchial region and then runs forward above the row of primary
will-clefts (Fi wr). We have ly seen (p. ) that the
course of this vessel is diverted to the right by the rudiment of the
Inost anterior pair of gill-clefts. The course of this vessel marks the
future ventral median line of the pharyns.
A longitudinal ridge now soon
ss above the
rauchial artery on
the right side of the body (Fig. 292, 4); this is composed of con-
LATER LARVAL, STAGES.
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pu waLSHASY] AVY “amqyn) sqrianizes-Spoq JO (ono-Saxys) coquane yp) 949 AP Ap "162 “OL
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Sarnypraqdon § AaHOSLY A sys gid yuso-aad ‘a ‘ gaodoxnan “dS yy
qmamypna) pumg poanytio “af = qwaws ce come ‘p ! mpsoyo ‘yo ay [wpnVa Tn
Aawontad 99a) 840 “ST “(ATLA PUY BALSTANT AVY ay) ‘squmuities-Apoq, Xt
LATER LARVAL STAGES, 355
aperture has meanwhile shifted from the left side and taken up a
more anterior and ventral position. The branchial artery, which
lies beneath the rudiment of the endostyle, now ocenpies the ventral
‘middle line, while the secondary gill-clefts remain on the right side of
the body (Fig. 293). A reduction in the number of the primary
clefts takes place at the same time, the most anterior and some of
the most posterior closing and completely disappearing (twelfth in
Fig. 293), so that. finally the number of clefts on the right equals
that on the left. These changes have been defined as the process of
symmetrisation of the branchial region or the equalisation of the -
gill-slits. They lead to a final stage in which an equal number of
clefts (seven to nine, usually eight) is found on each side. This
stage, which marks « long pause in development during which only
the endostyle-rudiment yrows further posteriorly and the clefts
Fic. 293,—Ventral side of an Amphionws lurve wt o later stage (after WILLEY), 2,
second, 12, vestige of twelfth prnery gill-cleft; F-VITI, first eight secondary
lefts; be, bnecal cirri: of, chordn ; ex, endostyle: m, mouth; v, velum.
increase in height, has been named by Winey the critical stage of
larval development. Wurnrey points out that the number of clefts
at this stage approximately agrees with the typical number of gill-
clefts in Vertebrates.
The gill-clefts hitherto present were segmentally arranged, but
this relation to the body-segments is lost in the tertiary clefts which
are added later in pairs behind the clefts already formed. The most.
anterior, originally seginental clefts (primary and secondary) are also
then displaced forward.
The primary clefts become early lengthened in the transverse
direction of the body, #., vertically (Figs. 292, 293). . The secondary
cleits, on the contrary, are, when they first appear, lengthened at
right angles to this direction and consequently parallel to the
longitudinal axis of the body (Fig. 293). Only in later stages do
they also extend vertically.
LATER LARVAL STAGES, 557
The ateral folds at first lie very near together on the ventral side
of the larva (Fig. 294). They are found in larvae in which nine or
ten primary gill-clefts have developed. Behind this region, the
lateral folds are bilaterally symmetrically placed at the sides of the
median line. Near the gill-clefts, on the contrary, they diverge to
the right (Fig. 294). The right lateral fold (xf) runs forward along
the right side of the body above the clefts, in front of which it again
bends toward the median line. It covers externally the upper part
of the clefts. The left lateral fold (Jf) is at first only slightly
developed anteriorly and rans almost along the median line in the
branchial region.
The lateral folds border a groove running along the ventral side
which is the first rudiment of the atrial cavity. The posterior part
op
Fic: 204.—'Three larval stages of Amphiowus, from the ventral aspect (after Ray
LANKgSTER and Writey). A, the atrium is still entirely open; B, the ae
in ly closed CG, the atrium is almost compl
fk gill ates 4 Teh 'raclaplenral fds me, mouths ay; rete moterlened
of this groove closes first (Fig. 294 B), closure being effected by the
appearance of two solid ridge-like projections of the inner opposed
faces of the metapleurs (sub-atrial ridges of Ray Lankester and
Wittey); these ridges grow out towards one another (Fig. 295
A, sl) and fuse (Fig. 295 B), Through the fusion of the sub-atrial
ridges, the floor of the atrial cavity is formed, in which later that
part of the body-cavity called by Harsonmx (No. 8) the cavity of the
lower folds appears (Fig, 311, uf, p. 572). The atrial cavity, which
is at first tubular and open at both ends, gradually extends farther
forward (Fig. 294 B and C), till, finally, the anterior part of the atrial
cavity is completely closed towards the exterior, Only in the posterior
part of it do the metapleurs still remain separate. The aperture here
558 CEPHALOCHORDA.
retuined is the atriopore (Fig. 294, ay). The lateral folds are also
found in the adult as projecting ridges of the body-wall, running from
the mouth to the atriopore.
The atrial cavity thus formed is at first a tube with a compara
tively narrow lumen (Fig. 295 8) which only secondarily widens ;
this widening, which is accompanied by a shifting apart of the meta-
pleural folds, is brought about by the lateral growth of the atrial
cavity between the wall of the intestine and the body-wall pressing
into the coelom so as almost completely to grow round the alimentary
canal (Fig. 311, p. 572), the coelom being proportionately reduced as
us to illustrate the
¥). ao, aorta; ¢,
Ph, dorsal fin-cavity ; m, myomere ; , neural tube; p, atrial cavity
‘fh, metaploural cavity; “si, sub-iutestinal vein! sk, selera:
ridge ; sp, voelom.
7, metapleur;
pal, suleatrial
the atrium enlarges. Part of the outer wall of the atrial cavity (the
epipleura of Ray LankestTeR) therefore does not arise through the
formation of folds, but is a modified portion of the body-wall which
has undergone no displacement. [See also MacBripg, No, VIL. «.]
The formation of the atrial cavity in Amphiorus recalls to some extent
that of the similarly placed cavity in the Ascidians which, as we have seen.
originated in the form of paired invaginations (p. 366) which also o
secondarily grow round tho pharynx.
The outer wall of the atrial cavity cannot be homologised with the oper-
culum of the fishos, the latter being a fold which bolongs exclusively to the
hyomandibular arch.
LATER LARVAL STAGES. 559
The metapleural folds have frequently been homologised with the
primary paired lateral fins of the Vertebrata (Ray Lankester and
Wiuury, Harscuek). According to HatscHek (No. &), they are
merely special parts of « system of ventral folds which in the most
anterior part of the body develops as the unpaired ventral fin of the
rostrum, in the oral region forms the lateral buecal wall, in the
branchial region the metapleural fold. and, finally, behind the atrio-
pore, the unpaired ventral fin which extends in front of and behind
the anus. The “cavities of the lower folds” which develop in the
sub-atrial ridges are said to be the cavities of the unpaired ventral
fin.
We have already alluded (p. 550) to a ciliated band lying on the
right side of the pharyngeal wall in front of the club-shaped glud
(Fig. 289, fl). This ciliated band, in which a somewhat clearer inner
zone can be distinguished, resembles in shape the club-shaped gland,
so that, in the illustration, it looks like a shadow or a reduplication
of the latter. It is the rudiment of the endostyle and, even in early
stages, appears bent on itself (Fig. 292 A, ex), being divided into a
shorter upper and a longer lower half. The endostyle-rudiment then
proceeds backward (Fig. 292 B) passing over the club-shaped gland
into the space between the primary and secondary clefts. It thus
comes to overlie the branchial artery. During the shifting mentioned
above as taking place in the branchial region, the endostyle-rudiment
passes from the right side of the body to the ventral median line
ig. 293, -x). The upper half of the bent rudiment becomes the
right and the lower half the left part of the endostyle of the adult.
At an early stage, two ciliated arches, the peripharyngeal bands, are
to be seen ascending from the anterior end of the eudostyle-rudiment
to the dorsal side of the pharyngeal wall (Fig. 296, #1), and then
continue backward on either side of the dorsal middle line as the
ciliated hyperpharyngeal (epipharyngeal) bands. The proximal part
of them corresponds to the ciliated arches (pericoronal arches or
peripharyngeal bands) of the Ascidian (W1ILLEY).
The formation of the definite oral aperture has been described in
detail by Wiiuey. The oral aperture shifts from the left side of
the body forward and ventralwards, so that it finally occupies a
median symmetrical position un the ventral side. At the same time,
it becomes grown over by a secondary fold of the body-wall (Fig. 296,
mr), the stomodaeum or oral hood being thus formed. ‘The primary
oral aperture of the larva shifts to the back of the buccal cavity and
its lips are retained as the velumn (r) : here the first rudiments of the
LATBR LARVAL STAGES, 561
Wine, like the larval mouth, an organ belonging to the left side of the body,
in spite of its apparently symmetrical position.
It is difficult, from Wiuury’s description, to gain a clear idea of the shiftings
which occur in the oral region. According to this author, the longitudinal axis
of the slit-like larval mouth rotates about a vertical axis through an angle of
90". Originally it lies parallel to the axis of the body, but is at right angles
to it later. Consequently, the anterior part of the larval mouth sinks in and
passes to the right side to give rise to the right half of the velum, while the
left half of that organ is derived from the posterior corner of the mouth.
Similar shifting of the labial folds takes place.
VAN WisHE maintains that the mouth of Amphiorus is not homologous
with that of the Craniata. He also doubts the homology of the velum of
Amphiorus with that of the Cyclostomi. According to him the mouth of
Amphiorus is a gill-cleft lying on the left, and the club-shaped gland is the
corresponding organ on the right side of the body. van WisHe homologises
it with the left spiracle of the Selachians and with the left gill-cleft of
Appendicularia (2.
In the later larval stages, when eight secondary gill-clefts have
already developed and the tongue-bars have begun to form, the
elub-shaped gland degenerates, and finally, as it appears, reaches
the interior of the alimentary canal where it is perhaps absorbed.
The aperture of the gland into the intestine which was mentioned
above (p. 550) seems to withstand disintegration longer than any
other part of the organ (Wi1zy).
In these later stages an organ is found which was discovered by
Harscuek (No. 5) and was figured and described both by him and
by Ray LankEsTER and Winury (Nos. 12 and 23) as the kidney
(nephridium, Fig. 290, x). This is developed only on the left side in
the form of a mesodermal ciliated funnel and canal lying in front of
the mouth in the region of the first metamere (between the pre-oral
pit and the mouth). It lengthens posteriorly later and is found, in
the adult, as a strand extending on the left side beneath the chorda
from the anterior edge of the mouth to immediately behind the
velum. HAaTSCHEK conjectured that this canal opened into the
pharynx. [See MacBrrpe, No. VIIL. «.]
This last observation has recently been confirmed by vAN WisHE (No. 22)
who considers this organ, which he calls the oesophageal process, as the
remains of the original communication between the intestine and the left
anterior entodorm-diverticulum.
We have already mentioned that, in the larval stages which follow
the stage with one primary gill-cleft, the number of primitive segments
is continually increasing through the formation of new segments from
the mexoderm-folds at the posterior end of the body. At the time
00
LATER LARVAL STAGES, 563
relation of these compartments to the segmentation of the body cannot
be made out. —
The anal aperture which originally lay at the posterior end of the
body shifts later further forward (Fig. 291), and this leads to the
development of the post-anal region of the body characteristic of
the Vertebrates,
We have seen above (p. 545) that the anterior end of the medullary
tude, which lies in the region of the first true primitive segment and
the so-called cephalic process of the mesoderm (anterior primitive
segment of HatscnEx) is widened (Figs. 288, 289). This part,
according to HarscHex’s recent statements (No. 8), becomes dif-
ferentiated, in the young Amphioxs, into three consecutive sections
which correspond to the three primary cephalic vesicles of the Craniata,
Fig. 297.—A, transverse sections en glen the brain of a young Amphiowue (after
Hatscuek). /, through the (primary) first ventricle ; ah through the (primary)
second ventricle (aquaeductus Syivii); HH, through the (primary) third ventricle
(fossa rhombnidalis). B, transverse section through the spinal ord,
Fie. 298.
Fic. 298.—Brain with the most anterior nerve-roots of a ike J Amphioxus (after
Harscnex), Ch, chorda dorsalis; N, ciliated pit, to the wall of which
the olfactory nerve runs ; £, 7, 12, the three primary ventricles (diagrammatic),
The anterior section, the primary fore-brain, shows, in transverse sec-
tion, the well-known dilation of the medullary tube which constitutes
the first primary ventricle (Figs. 297 4, 7; 298 J). The anterior end
of this ventricle is continued towards the neuropore (é.c., towards the
ciliated olfactory pit) into the infundibulum (the lobus olfactorius of
LAnGeruans) which, in Amphiorus, is curved upward. The second
part of the brain (the mid-brain) contains within it the second
primary ventricle, which is represented by a narrowed portion of the
564 CEPHALOCHOERDA.
medullary canal (aquaeductus Sylvit, II). In the third part (the
hind-brain) the central canal shows a narrow ventral tube, while its
dorsal portion is independently dilated so as to form a vesicle covered
by a thin membrane, the fossa rhombowlalis of HatscnEk (111).
The medullary tube originally opens externally through a neuropore
bordered by closely crowded flagella. This is pressed out of the
middle line to the left through the development of the dorsal fin.
An epithelial depression forms later at this point ; this is KOnLIKER's
olfactory or ciliated pit (Fig. 298, x), at the base of which the neuro-
pore opens. The ciliated pit is the homologue of the olfactory organ
and of the hypophysis of the Craniata. The short, unpaired olfactory
nerve discovered by LANGERHANS runs from the brain to the posterior
wall of the ciliated pit. The continuation of the first ventricle towards
the neuropore corresponds, as above mentioned, to the infundibulum
of the Craniata. In Ammuocoetes, the hypophysis and the olfactory
organ are still connected together, and the external aperture of the
hypophysis still retains its original dorsal position. The inner end of
the hypophysis, on the contrary, and the infundibulum have moved
to the ventral surface of the brain.
In identifying the anterior end of the medullary tube of Amphiorus with
the infundibulum of the Craniata we have followed HatscHex. It should,
however, be mentioned that Kuprrer has recently been lod by his researches
on Acipenser (No. 38) to homologise the anterior end of the cranial axis of
Amphiorus with his lobus olfactorins impar which lies near the lamina
terminalis above the anterior commissure, and indicates the point where the
medullary tube remained longest in direct connection with the ectodenn
precisely as in Amphiocus, and to regard the infundibulum as a secondary out-
growth of the ventral side of the brain.
It should here be pointed out that Kout (No. 9) occasionally noticed, on
the right side of the head of .Lmphioxus, a pit resembling the olfactory pit of
the left side. Konu is inclined to consider this as the half of an originally
paired olfactory organ in a vestigial condition. The relations of the neuropore
to the olfactory pit were denied by Kout, The remains of the neuropore are
said to be found in an ectodermal depression lying somewhat behind the
olfactory pit.
Our knowledge of the important transformations that take place in
the primitive segments and the body-cavity is due chiefly to HATSCHEK
(No.7). Each primitive segment becomes divided by a transverse
partition into two portions, as has already been stated (p. 547), viz..
a dorsal portion (proto-vertebra) and a ventral part (lateral plate,
Fig. 300, / and J/). Only in the dorsal portion is the primitive
segmentation retained, the adjacent walls of the segments persisting
as the transverge septa (myosepta). In the region of the lateral
LATER LARVAL STAGES. 565
plates, the septa disappear and the segmental cavities enclosed by
them flow together to form a common cavity surrounding the intestine
(splanchnocoele). The disappearance of the ventral mesentery leads
to intercommunication between the right and left halves of the
splanchnocoele.
The proto-vertebrae enclose segmentally-arranged cavities which,
however, ure not quite symmetrical in relation to the median plane
(p. 545). These are the cavities of the proto-vertebrae or the
myococles (Fig. 153, Z). The walls of each proto-vertebra consists of
a parietal (7) and an inner (2) layer. The parietal layer (/), which
consists of flat cells, applies itself closely to the ectoderm, and since
Fic. 299.—Transverxe section Fig, 300,— Diagrammat
from the middle of the body ing of the same sec
of an .tinphiocus larva with
five branchial clefts (after
HaTScHEK).
A, epidermis; 8, medullary tube; (, chorda; (,
epithelium ; #, sub-intestiual’ vessel. is-layer: 2, muscle-layer (lateral
‘trunk-muac! 3 sclera-layer ; }. boundary cells of the proto-vertebra ; 6, somato-
plenre; 4, splanchnopleure. -/, myocoele ; 77, splanchnocoele,
inner chorda-sheath ; /), intestinal
it yields the cutis of the adult, is sometimes termed the cutis-layer.
The cells of the inner layer, at the sides of the chorda and of the
medullary tube, elongate transversely and longitudinally, as described
above (p. 548), and, as they give rise to the muscle-tibres of the
myotome, this part is spoken of ax the muscle-plate.
At first, each muscle-cell contains only one fibrilla, but as the
number of fibrillae increases later they are arranged in regular order
one above the other. Several such groups of fibrillae occur in each
muscle-cell. The nuclei of the muscle-cells lie on the outer side
of the muscle-layer that turns towards the myocoele. Towards the
ventral side, the muscle-layer is continued into a pavement-cpithelium
LATER LARVAL STAGES, 567
(selerotome). The musele-plate is then connected merely at its dorsal
edge by means of a mesentery-like band to the wall of the proto-
vertebra and otherwise hangs quite freely into the cavity of the
same. Its chief points of attachment are to the dissepiments. The
inner wall of the selerotome becomes applied to the chorda and the
medullary tube, and here forms the skelefogenous layer which yields
the outer sheath of the chorda (4) and the neural continuation of the
Jatter, The outer layer of the selerotome becomes applied to the
inner side of the muscle-layer and forms the internal sheath or
Juscia-layer (3). The lateral trank-muscle of Amphiocus is not
entirely surrounded by fascia, since this layer only develops on its
inner side.
All these layers, derived through differentiation from the wall of
the proto-vertebra, shift ventrally, pressing in between the ectoderm
and the somatopleura. he cutis-layer in this way comes to lie in
the ventral middle line, where it yields the lining of the cavity of the
ventral fin (7,,), and the dorsal fin-cavities (/.) seem in the same way
to be lined by the cutislayer. From the floors of these cavities, the
fin-rays grow up later. The skeletogenous and the muscular layers
also shift ventrally.
The method of formation of the layers agrees in all essential points
with that of the Craniata. In this type, beneath the epidermis, lies
the cutis-layer (1) which, in Amphioxus, retains its simple epithelial
character throughout life. This is followed by the myocoele, which
also here persists throughout life. Then comes the muscle-layer (#)
and on the inner side of the latter lies the fascia-layer (3) ; m the
Craniata the fascia is also developed on the outer surface of the
muscles ; then comes the cavity of the sclerotome, and, finally, the
skeletogenous layer (4). The latter (as chorda-sheath) encloses the
chorda and also the medullary tube in the dorsal half of the body ;
in the lower half, it becomes applied to the somatopleure. ‘The two
layers (skeletogenous layer and somatopleure) here form a delicate
partition-wall, the intercoelic membrane (5 and 6) which separates
the cavity of the primitive vertebrae from the splanchnocoele, On
the inner side of the splanchnocoele lie the splanchnopleure and the
entodermal intestinal epithelium.
In the branchial region (Fig. 311) the condition of the body-cavity
is complicated through the development of the atrial cavity (p), which
presses in between the splanchnocoele and the ventral part of the
myocoele. The latter then lies in the peribranchial fold and breaks
up into sections called by HarscneK (No. 8) the upper and lower
LATER LARVAL STAGES. 569
part along the ventral edge of the protovertebrac where the cutis-layer
passes into the skeletogenous layer, small cells filled with deeply
staining nuclei can be made out. These are accumulations of primi-
tive genital cells which represent the rudiment of the genital gland,
and which may perhaps be traced back to HaTscHEx’s large boundary
cells mentioned above (Fig. 299, 4, p. 565). These agglomerations of
cells are repeated at definite intervals in series of cross-sections.
Nince they are related to the dissepiments between the mesodermal
somites, they are affected by the asymmetry of those structures and,
like them, alternate on the right and left sides of the body. In
superficial views of later stages (Fig. 303), the rudiments of the
Fre. 305, Fic, 306,
—Later stages of developmeut of the genital rudiment
(after (BovEnt).
307, Ge
an Lmphiorus
{after Bovert).
ital rudiment in
» Samm. long
genital gland can be seen as rounded kuobs (gd) lying in small
outgrowths in the posterior lower angles of the myocoeles (rudi-
ments of the genital pouches).
Examination of the genital rudiment in the youngest stages reveals
that it arises as a modified part of the epithelium on the anterior
wall of each myococle (Fig 304). is epithelial growth then forms
a hernia-like invagination into the cavity of the preceding segment
(Figs. 305, 306), and in this way constitutes a knob which at first is
hollow but is provided later with a himen (Fig. 307), and which is
covered superficially with an epithelium derived from the posterior
wall of the segment into which the knob projects, the latter being
Ftc, 309,—Two consecutive yenital vesicles of an
BOVERI).
So as to complete our deseription of the
we must, in conclusion, briefly deseribe the re
Bovent (Nos. 1 and 2) in the adult animal.
found in the region of the pharynx which
coelom (Fig. 311, sc) with the atrial eavi
GENERAL CONSIDERATIONS. 571
mesial side of the sub-chordal coelomic sac, and open into the atrial
cavity at the uppermost angle where the wall that covers the outer
side of the gill-bars bends outward to
form, together with the wall of the
sub-chordal coelomic sac, the thin
partition-wall known as the ligamen-
tum denticulatum (/d). This latter, in
the primary gill-bars, extends further
down than in the secondary (Fig. 312,
7 and ZJ). The nephridial canals
recur regularly in the pharyngeal
region and open on the tongue-bars
(Fig. 312). In each canal (nt) an
anterior ascending branch can be
distinguished from a short posterior
branch. Each of these branches
opens at its end into the sub-chordal
coelomic cavity (nc, funnel), and in Pours aeraoerrioeciae
the course of the canal three or four the represented in Fig. 309
other apertures occur. Round these on Pag esl fy
apertures (nc) the cells of the walls of etnct land; ee bee
the sub-chordal coelom are peculiarly ‘ Mid ts
modified. Highly refractive spherical cells are found in the body-
cavity connected with the apertures by means of delicate filaments
(thread-cells), Round the nephridial canals, the upper ends of the
branchial vessels, by anastomosing, form a vascular network which
we may call the glomerulus (Fig. 311, g@).
Bovent regarded these canals as homologues of the pronephros of the
Craniata, and the atrial cavity of Amphiorus as the homologue of the
pronephric duct of the Vertebrata. For further details on these points we
must refer the reader to the works of this author. Nothing is as yet known
of the development of these renal canals, We are also in the dark as to their
relation to the larval organ mentioned above and described by HaTscHek as
the nephridium, but it should be mentioned that the figure of this organ given
by Harsousx (No. 14) shows a certain similarity to the canals discovered by
Bovent, Objections have recently been raised to Bovent’s generalisations by
Semon and van WisHe (No. 22).
General Considerations.
With regard to the systematic position of Amphiorus, we adopt
the view now accepted by most zoologists that this animal is to be
regarded as the representative of a very primitive group which served
GENERAL CONSIDERATIONS.
ditions of the blood-vascular system, and especially to the development
of the body-cavity which, as made known by Harscunx, yields the
key for understanding the formation of the layers in the Vertebrates,
Tt must further be pointed out that the development of Amphiorus
without doubt shows very primitive characters, a view to which
however, objections have been raised by Donrn. The occurrence of
free-swimming ciliated larvae, nevertheless, can hardly be interpreted
in any other sense.
T Ir I ia T
eg rete ete ge
Se lpr at entfael ontg' Gs toe eal Oe Sense
cavity De synaptionlum ; Z, primary Mite Te tongue-bar.
Considering the agreement prevailing between Amphioxrus and the
Vertebrates, it is important to emphasise the distinctions between the
two forms. Amphioxus is specially distinguished by the rostrum-
like prolongation of the notochord, by the secondary increase in
number of the gill-clefts, and by the duplication of sueh primary clefts
through division (development of the tongue-bars), this last character-
istie recalling the multiplication of the gill-clefts in the Ascidia.
With regurd to the condition of the urogenital system, we must refer
to the accounts of Bovert (No. 2), according to which the renal
canals of Amphioxus are to be considered as the homologue of the
pronephric canals of the Vertebrata, and the atrial cavity of
kidney in the latter. sates dates.
must still recognise certain distinctions be
whieh, however, would not then be of a fi
‘Through the great development of the atrial
assumptions of Doxrn (p. 522), that A
derived from the Craniata. We do not deny th
consequence of its half-sedentary manner of life
may have undergone a certain degree of
tion. Above all, we might in this way explai
of the brain and the sensory organs and also omo
It is naturally difficult to determine how far the simple st
Amphinens vests upon primary peculiarities o
peculiarities secondarily acquired through deg
of the facts known to us as to the ont 1
Amphiorus seem to indicate that we have in-
with a very primitive form.
specially marked in the larval forms, but is also
extent in the adult (position of the olfactory pi
the hepatic caecum, conditions of the Vv
the definitive mouth according to van Wisner). Wr
that the Amphiorus larva lies when at rest at |
on the right side of the body seems to ind
is acquired in the same way as in the
dpe ‘Witney, ith
this point, fot toad the yood, whan Ranh 7
GENERAL CONSIDERATIONS, 575
We therefore regard Amphioxus as a very primitive chordate form
very closely related to the hypothetical ancestor of the Craniata, but
somewhat more distantly related to the Tunicates. Every specula-
tion as to the origin of the Vertebrates and the Chordata must
necessarily take account of Amphiocus as the most primitive repre-
sentative and the starting-point of the whole series. Among all the
hypotheses* which have hitherto been advanced as to the origin
of the Chordate stock, that which derives it from the Annelida has
at present most adherents. As the founders and most eminent
upholders of this hypothesis we must name Semprrr (No, 46) and
Dourn (Nos. 30 and 31), while, more recently, a whole series of
renowned zoologists have taken part in its further development. The
view that the Vertebrates are descended from Annelids rests chietly
on the similarity in the segmentation of the body and in the produc-
tion of new segments at the posterior end of the body; further, on
the agreement in the position of the more important organs, if we
assume that the hypothetical Annelid ancestor of the Vertebrata
underwent such a rotation round its longitudinal axis that the former
ventral side became the dorsal side, an assumption which lay at the
root of Gxorrroy Sr. Hruare’s statement that ‘* Insects are
Vertebrates running on their back.” When such a rotation is
assumed, the ventral ganglionic chain of the Annelida corresponds
to the medullary tube of the Chordata, the ventral longitudinal
vessel of the Annelida becomes the aorta, while the dorsal vessel
corresponds to the sub-intestinal vessel. The Annelidan hypothesis
obtained its strongest support when Sxarsr discovered a remarkable
bottom and fall ou one side, is due to the physical impossibility to rest in any
other position, and not to a pressing desire or instinct to assume this position,
The asymmetry of Amphioxus is of a totally different character to that of the
Plouronectidae. For a full account of these views, see Wiumy, Amphioxua
and the Ancestry of the Vertebrates. Columbia Univ. Biol. Series, 1894.—Ep. J
* We have no intention of entering upon the much-disputed point of the
origin of the Chordata except in a way. A detailed investigation of
this difficult problem would require a study of vertebrate embryology which
does not fall within the scope of the present work. We have already, in the
foregoing chapter (p. 523), stated that the Tumicates contribute little to the
solution of this question. cay ose to be considered as degenerate members
of the Chordate stock, of which Amphiorus is to be regarded as the most
primitive form. Of the many theories on the subject of the relationship of
the Chordata we have here alluded only to those two which appear best
founded on actual morphological facts, viz., the derivation of the Chordata
from CASA and the assumption of relationship between the Chordata and
The hypothesis of the relationship to the Nemertines has
been briefly Parkidea Soveowe (vol. i., p. 291). We do not consider it nec
to refer to the relationship of the ertebrata to the Arthropoda which has
recently been again assumed.
576 CEPHALOCHORDA,
similarity of structure between the primitive kiduey-tubules of the
Selachian embryo and the segmental organs of the Annelida. Our
acceptance of this homology, however, has been recently made ian-
possible by the researches of vAN WusHE (No. 48), Rickert
(No. 44), and Boverr (No. 2) which cause us to doubt the serial
homology of the pronephros with the primitive kidney and to regard
the latter as a secondary acquisition which in any case has nothing
to do with the segmental organs. From this point of view it would
be the pronephros which requires consideration.
The derivation of the Vertebrates from the Annelida is, in fact,
accompanied by certain difficulties which are not insurmountable.
The most important of these is the position of the mouth. While,
in the Annelida, the oesophagus perforates the central nervous system
at the oesphageal ring in such a way that the supra-ocsophageal
ganglion comes to lie above and the ventral cord below the gut,
no such relation between the stomodaeum and the central nervons
system is to be found in the Vertebrates. The most varied hypotheses
have been suggested to obviate this difficulty. ScHNEIDER thought
that he could discover in the connection of the hypoglossal and tri-
geminal nerves an equivalent for the oesophageal ring. vAN BENEDEN
and Junin (No. 29) have assumed that, in the primitive ancestors
of the Vertebrates, the oesophagus shifted forward in the median
plane between the still unconnected cephalic lobes of the brain, while
other zoologists like KLEINENBERG, BEarp, No. 27, v. KENNEI.
(No. 35a) believe that the supr-vesophageal ganglion atrophied and
that the brain and the dorsal chord of the Vertebrates only are the
equivalents of the ventral cord of the Annelida. In connection
with this idea, we have the question as to the primitive mouth of
the Vertebrates. Under the assumption that the pro-cephalon of
the Vertebrates corresponds to the supra-oesophageal ganglion of these
Annelida, we should have to conjecture that the original vertebrate
mouth, which perforated the oesophageal ring, has disappeared. The
definitive mouth of the Vertebrates would have to be regarded as a
secondary formation, and is by many zoologists thought to have arisen
through the fusion of gill-slits. ‘There is actually mueh to support
the view that the mandibular arch is a branchial arch which has
been drawn into the mouth. This, however, does not decide the
question of the derivation of the vertebrate mouth from gill-clefts.
In the case of Amphiorns, there is no ontogenetic indication of the
derivation of the mouth from gill-clefts. The mouth here arises in a
different manner from the gill-slits (p. 550) and in a different position.
GENERAL CONSIDERATIONS. 577
Harscuex (No. 8) has therefore maintained that the velar opening
of Amphioxus is not to be regarded as a gill-slit.
The attempt has often been made, in following out these ideas
further, to discover vestiges of the primary vertebrate mouth, which
has been sought in the rhomboidal fossa, the pineal gland and the
hypophysis, Only recently, Brarp (No. 27) and Kurrrer (No. 38)
have pointed to the hypophysis as the primary mouth (palaeostoma)
of the Vertebrata.
An altogether satisfactory solution of the difficulties connected with
all these questions is, however, at present wanting.
Attempts have been made in other directions also to find the con-
nection between the Vertebrates and the Annelida. We may recall
the discovery by Eisia of organs in the Capitellidae comparable to
the lateral line of the Vertebrata, the homologising of the spinal
ganglia of the Vertebrates with the parapodial ganglia of the Annelida
by KiLEiNenberc (No. 36), the derivation of the unpaired fin of the
Selachians from fused Annelidan parapodia by P. Mayer (No. 39),
the attempt"to derive the vertebrate eye from the Annelidan eye
by v. Kennet (No. 35a), etc. For its support, the Annelidan
hypothesis has required a number of sub-hypotheses. Nevertheless,
we must admit that the gap which divides the Annelida from the
Chordata is even now very considerable, and, as BaLFrour pointed
out, the two most typical organs of the Chordata, the notochord and
the gill-clefts, are not foreshadowed in the Annelids. Attempts have
not been wanting, it is true, to find the equivalents of these organs
in the Annelids. The origin of the mouth from paired rudiments in
the buds of Nais and Chastogaster (SkmPER) was compared to the
formation of gill-clefts, while the most varied structures in the
Annelida were regarded as homologues of the chorda. With regard
to this latter point, the view that most deserves attention is that of
Exvers (No. 32) and E:ste (No, 33), who see in the so-called
accessory intestine of the Cwpitellidae and the Kunietdae (and in
similar structures in the Gephyrea) the homologue of the notochord.
On the other hand, it should be mentioned that the researches of
Kreierserc (No, 36) on this point did not lead to satisfactory
results. “In the development of most Annelids,” says KLEINENBERG,
“there is no trace of the accessory intestine; I found it only in the
larvae of those forms which possess it when adult, viz., the Caprfel-
Ndae and the Hunicrdae. Ina larva belonging to the last of these
families it hangs as a somewhat short loop beneath the principal
intestine and opens both anteriorly and posteriorly into the latter.
PP
578 CEPHALOCHORDA.
In the Capitellidae, a diverticulum forms early at the most posterior
part of the archenteron and grows out anteriorly. I believe that this
is the rudiment of the accessory intestine, but am not quite certain
on this point.” With regard to the origin of the gill-clefts. we may
assume that the originally blind intestinal diverticula secondarily
acquired external openings, so as to allow of the outtlow of the
respiratory water. The assumption that such perforations occur
is supported by the actual presence of pores of communication at the
ends of the tentacles in the Actinia, and in the hepatic tubes of some
Acolidue.
We are not, however, able to regard the Annelidan hypothesis as
resting on altogether certain foundations. It seems to us that we
have no convincing proof that the many points of agreement which
actually exist between the Annelida and the Chordata rest upon true
homology. The crucial point of the whole question lies in the
decision as to whether it is necessary for us to refer the similar
method of segmentation of the body in the two groups to their
derivation from a common ancestor. It is evident that, as BaTEson
(No. 26) has shown, it is not a decisive objection to this assumption
that the characteristic segmentation in each of the two groups (the
Annelida and the Chordata) has arisen separately, or, in other words,
that the common ancestor of the two groups was still unsegmented.
Batrovr (No. 25) in this connection wrote “that we must look for
the ancestors of the Chordata, not in allies of the present Chactopoda,
bat in a stock of segmented forms descended from the same un-
segmented types as the Chaetopoda, but in which two lateral
nerve-cords, like those of Nemertines, coalesced dorsally, instead of
ventrally to form a median nerve cord.”
If, after w tid, we do not regard the derivation
of the Chordata from the Annelida as certainly proved. and institute
has just been
comparison with the other invertebrate groups, we are confronted first
of all by Balanuylossus which, in the possession of gill-clefts in the
phi
striking agreement with the type of the Chordata. This form was
formerly thought to be nearly related to the Chordata by GEGENBAUR
and Huxvey and more recently by Bateson, HaEcKEL, ScHi-
kKewrrscH, MoxGas, Route and others. We are far from regarding
as established the various homologies assumed by BaTEsoN between
ad on this point refer the reader to
isin of the best qualified judge in this matter—
EL (No. 47, p. 721, ete.) who denies all relationship between
vnygeal region and the nerve-strand ronning dorsally shows
Balsnoglossus and Amphiorus,
the detailed crit
Ss
GENERAL CONSIDERATIONS. 579
the Chordata and Balanoyloxsux. Bateson (No. 26) homologises the
dorsal nerve-strand in the collar region of Balanoglossus (the so-called
collar-cord) with the medullary tube of the Vertebrates. The anterior
intestinal diverticulum (the so-called proboscis-intestine) of Bulano-
gloxsus (Vol. i., Fig. 165, di, p. 375), according to him and to KozHLER,
is the homologue of the notochord. The rudiment of the so-called
proboscidal coelom is homologised with the anterior unpaired entoderm-
diverticulum of Amphiocus (Fig. 285 B). The externil aperture of
the left anterior entoderm-diverticulum of Amphiocus is assumed to
correspond to the proboscis-pore of Bi/nugloxsux. A posterior fold
in the collar-region, called by BaTEson the “operculum,” is said to
correspond to the epipleura of Amphiorus. Finally, even GEGENBAUR
compared the ventral nutritive section of the pharynx in Balanoyloseus
(Vol. i., Fig. 166, d, p. 377) to the endostyle of the Tunicates.
SpenGEx (No. 47) has pointed out the difficulties that stand in the
way of such a homology, and lays special stress on the great difference
in the position of the gills which, in Bu/anoglossus, are dorsal, and, in
Amphiorns, veutral—the development of the blood-vascular system
and the genital organs in the two groups. The bare fact of the
presence of gills in Ba/anoglossus, indeed, and their remarkable and
detailed agreement in structure and arrangement with those of
Amphiocus (U-shaped form in consequence of the growth of tongue-
bars, chitinous skeletal structures in the form of prongs, presence of
synapticula) seem to us of such significance that we cannot avoid
the thought that we have, in Ba/anoglossus, the only living form of
Invertebrate which is closely related to the Chordata. But, if we
adopt this assumption, the Chordata naturally become somewhat
more remote from the Annelida, since Bulanoglossus is only distantly
related to this last group. How far it is possible to remove the dithi-
culties which now lie in the way of establishing a stricter homology
between Balunoglosxux and Amphiorns must be decided by further
research.
The problem of the derivation of the Chordata is not solved by
assuming a relationship between them and the Enteropneusta, since
the latter themselves occupy an unusually isolated position. Only
through the structure of the Ba/anoygloxsus larva is an indication
given of remote connection with the Echinoderma. We must
resign ourselves to the thought that we are not ut present in a
position to state from what primitive form the Chordata and, with
them, Balanglossus are to be derived. The origin of the Vertebrates
is lost in the obscurity of forms unknown to us.
580
1,
=,
4.
10.
11.
12.
13.
14.
CEPHALOCHORDA.
LITERATURE.
Bovert, Tx. Ueber die Niere des Amphioxus. Miinchener
Med. Wochenschr. No. 26. 1890.
Bovert, Tu. Die Nierenkanalchen des Amphioxus. Ein Beit-
rag zur Phylogenie des Urogenitalsystems der Wirbelthiere.
Zovl. Jahrb. Abth. f. Anat. Bd. v. 1892.
Boveri, To. Ueber die Bildungsstiitte der Geschlechtsdriisen
und die Entstehung der Genitalkammern beim Amphiorus.
Anat, Anz, Jahrg. vii. 1892.
Hartscaex, B. Studien iiber Entwicklung des Amphioxus.
Arb, Zool. Inst. Wien. Bd. iv. 1881.
Hartscuex, B. Mittheilungen tiber Amphioxus. Zool. Anz.
Jahrg. vii. 1884.
. Harscnex, B. Zur Entwicklung des Amphioxus. Vers. D.
Naturf. Aerzte in Berlin, Tagebl. lix., p. 271. 1886.
. Harscuex, B. Ueber den Schichtenbau von.Amphioxus. Anat.
Anz, Jabrg. iii, 1888.
. Harscuer, B. Die Metamerie des Amphioxus und des Ammo-
coetes. Verhandl. der Anat. Gesellech. 6. Vers. in Wien.
Anat. Anz. Jahrg. vii. 1892.
. Kort, C. Einige Bemerkungen iiber Sinnesorgane des Amphi-
oxus lanceolatus. Zool. Anz. Jahrg. xiii. 1890.
Kowaevsky, A. Entwicklungsgeschichte des Amphioxus
lanceolatus. Mém. Acad. Impér. St. Pétershourg (7). Tom. ii.
1867.
Kowauevsky, A. Weitere Studien iiber die Entwicklungs-
geschichte des Amphioxus lanceolatus. Archir. 7. mikro.
Anat. lid. xiii, 1877.
Lanxester, E, Ray. Contributions to the Knowledge of Am-
phioxus lanceolatus Yarell. Quart. Journ. Micro. Sei. (2).
Vol. xxix. 1889.
Lanxester, E. Ray anp WinueEy, A. The Development of the
Atrial Chamber of Amphioxus. Quart. Journ. Micro. Sci. (2).
Vol. xxxi. 1890.
Leuckart, R. unp NitscHe, H. Zoologische Wandtafeln.
Cassel, Plate 72 contains a drawing of a hitherto unfigured
stage of Amphioxus. Cf. also the accompanying text by
Harscnek.
. Leuckart, R. unp PaGenstecHER, A. Untersuchungen iiber
niedere Seethiere. 1. Amphioxuslanceolatus. Archiv. f. Anat.
Phys. 1858,
LITERATURE. 581
16. Lworr, B. Ueber Bau und Entwicklung der Chorda von Amphi-
oxus, Mittheil. Zool. Stat. Neapel. Bad, ix. 1889,
17. Lworr, B. Ueber einige wichtige Punkte in der Entwicklung
des Amphioxus. Biol. Centraldl. Bad. xii, 1892.
18. ScnunTz", M. Beobachtung junger Exemplare von Amphioxus.
Zeitschr. f. wise, Zoot. Ba. iii, 1851.
19. Spencen, J. W. Beitrage zur Kenntniss der Kiemen des
Amphioxus. Zool. Juhrb. Abth. f. Anat. Bd. iv. 1890.
20. Wass, F. E. Kxeretory Tubules in Amphioxus lanceolatus,
Quart. Journ, Micro. Sei. (2). Vol. xxxi. 1890. ~
21, van Wuue,J,W. Die Kopfregion der Cranioten bei Amphioxus,
ete. Anat, Anz. Jahrg. iv. 1889.
22. van Wune, J. W. Ueber Amphioxus. Anat. Anz. Jahrg. viii.
1893.
23. Wituny, A. The Later Larval Development of Amphioxus.
Quart. Journ, Micro. Sei. Vol. xxxii. 1891.
23a, Witsoy, E. B. On Multiple and Partial Development in
Amphioxus. Anat. Anz. Jahrg. vii, 1892.
On the Phylogeny of the Chordata,
24. Banrour, F.M. A Monograph on the Development of Elasmo-
branch Fishes. London, 1878.
25. Baurour, F/M. A Treatise on Comparative Embryology-
Vol. ii., chap. xii, p. 258. 1881.
26, Barzson, W. The Ancestry of the Chordata, Quart, Journ.
Micro. Sci.(2). Vol. xvi. 1886.
27. Buarp, J. The Old Mouth and the New. A Study in Verte-
brate Morphology. Anat. Anz, Jabrg. iii, 1888,
28. Buarp, J. Some Annelidan Affinities in the Ontogeny of the
Vertebrate Nervous System. Nature. Vol. xxxix. 1889.
29. Van Benepen, BE. er Junin, Cx. Recherches sur la Morpho-
logie des Tuniciers. Archiv. Biol. Tom. vi. 1887.
30. Donry, A. Der Ursprung der Wirbelthiere und das Prineip
des Functionswechsels. Leipzig, 1885.
31. Donen, A. Studien zur Urgeschichte des Wirbelthierkérpers.
Nos, 1 and 2 in Mittheil. Zool. Stat. Neapel, Ba. iii, 1882.
No. 3, ibid, Bd. iv, 1883, Nos. 4, 5, 6, ibid. Bd, v.
1884. Nos. 7, 8, 9, 10, tid. Bd. vi, 1886. Nos. 11, 12,
ibid. Bd. vii. 1886-87. Nos, 13, 14, hid. Bd. viii. 1888.
No. 15, ibid, Bd. ix. 1889-91. Nos, 16, 17, ddd. Bd. x.
18v1.
582 CEPHALOCHORDA.
82. Envers, E. Nebendarm und Chorda dorsalis. Narhr. Gea.
Wiss. Gottingen. 1885.
33. Erste, H. Monographie der Capitelliden. Fauna und Flora
dex Golfea von Neapel. Monogr. xvi. 1887.
34. Hagcken, E. Anthropogenie. Aufl. iv. Letpziy, 1891.
35. Husrecut, A. A. W. The Relation of the Nemertea to the
Vertebrata. Quert. Journ, Micro. Sri. (2). Vol. xxvii.
1887.
35a. v. Kennet, J. Die Ableitung der Vertebratenaugen von den
Augen der Anneliden. Dorpat, 1891.
36. KiEINENBERG, N. Die Entstehung des Annelids aus der Larve
von Lopadorhynchus, ete. Zeitachr. f. wise, Zool. Bd. xliv.
1886.
37. Kornver, R. Sur la parenté du Balanoglossus. Zool. Anz.
Jahrg. ix. 1886.
38, Kuprrer, C. von. Studien zur vergl. Entwicklungsgeschichte
des Kopfes der Cranioten. Heft. i. Miinchen Leipziy, 1883.
39. Mayer, P. Die unpaaren Flossen der Selachier. Mitthet!. Zool.
Stat. Neapel. Bd. vi. 186.
40. Mayer, P. Ueber die Entwicklung des Herzens und der grossen
Gefiisstiimme bei den Selachiern. AMitthe. Zool. Stat.
Neapel. Ba. vii. 1887.
41, Morgan, T. H. The Growth and Metamorphosis of Tornaria.
Journ, Morph. Vol. vy. 1891.
42. Rovie, L. Les attinités zoologiques des Vertébrés. Rene
Srieutiriyue. Tom. xlix, 1892.
43. Rickert, J. Ueber die Entstehung der endothelialen Anlagen
des Herzens und der ersten Gefi
Biol, Centralhl, Ba. viii. 1888,
44. Rickert, J. Ueber die Entstehung der Excretionsorgane bei
Selachiern. Archiv, Anat. Phys. Anat, Abth. 1888,
45, ScuimKewirscH, W. Ueber die morpholog. Bedeutung der
Organsysteme der Enteropneusten. Anat, Anz. Jahrg. v.
1890.
46. Semper, ©. Die Stanmverwandschaft der Wirbelthiere und
Wirbellosen. Arh. Zool, Inst, Wiirzhurg. Bd. ii. 1875.
47. Spexcen, J. W. Monographie der Enteropneusten. Fauna nnd
Flora dos Golfes ron Neapel. Monogr. x 1893.
48. van Wong, J. V Veber die Mesodermseginente des Rumpfes
und die Entwicklung des Exeretionssystems bei Selachiern.
Archiv. f. mikro, Anat, Ba. xxxiii, 1888.
se bei Selachier-Embryonen.
LITERATURE. 583
49. van Wine, J. W. Ueber die Entwicklung des Excretions-
systems und anderer Organe bei Selachiern. Anat. Anz.
Jabrg. iii. 1888.
APPENDIX TO LITERATURE ON CEPHALOCHORDA.
I. Bennam, -W. B. The Structure of the Pharyngeal Bars of
Amphioxus. Quart. Journ. Micro. Sei. Vol. xxxv. 1898.
Il. Ersmenp, J. Zur Ontogenie der Amphioxus lanceolatus.
Biol, Centralbl. Ba. xiv. 1894,
IIL. Hawmay, J. A. Zur Kenntniss der Lebenentwickehing bei
Amphioxus. Anat. Anz. Bd. xiv. 1897-98.
IV. Kvaatscn, H. Bemerkungen uber die Gastrula des Amphioxus.
Morph. Jahrb. Ba, xxv. 1897,
V. Kopscu, G. Ueber Bildung und Bedeutung der canalis
neurentericus. Sifzangxber. Gesell, Nating. Berlin, 1897.
VI. Kravse, W. Die Farbenempfindung des Amphioxus. Zool,
Anz Jahrg. xx. 1897.
VII. Macsripg, E. W. The Relationship of Amphioxus and
Balanoglossus. Pree, Cambridge Phil. Soe. Vol. ix. 1897.
VII. Macnripr, E. W. The Early Development of Amphioxus.
Quart. Journ, Miro, Sei. Vol. xl. 1898,
Villa, MacBrivg, E. W. Further remarks on the development of
Amphioxus. Quart. Journ. Micro, Sei, Vol. xliii. 1900.
IX. Minor, C. 8. Cephalic Homologies. A Contribution to the
Determination of the Ancestry of the Vertebrates. Amer.
Nat. Vol. xxxi. 1897.
IXa. Samassa, P. Studien iiber den Einfluss des Dotters auf die
Gastrulation und die Bildung der primiren Keimbliitter der
Wirbelthiere N. Amphioxus. Archiv, f. Entiv, Mech. Bd.
vii, 1898,
X. Sonorra, J. Beobachtung uber den (Gastrulationsvorgang
beim Amphioxus. Verh. Gex. Wurzburg (2). Bd. xxxi,
1897.
XI. Sonorta, J. Die Befruchtung des Kies von Amphioxus
lanceolatus. Anat, Anz. Bd. xi, 1895.
XII. Sticut, O. van DER. Ln Maturation et la fécondation de Pouf
W@Amphioxus lanceolatus, Bull. Acad. Belg. (3). Tom. xxx,
1895; and Archir, Biol. Tom, xiv. 1896.
XIII, Winey, A. Amphioxus and the Ancestry of the Vertebrates.
London and New York, 1894.
SUBJECTS INDEX.
AL
Acanthodrilus, 79,
Acanthoteuthidae, 235,
Acanthoteuthis, 292.
Acavus, 104, 179, 186,
191, 192, 196, 197.
Acera, 100, 160, 173.
Acipenser, 355, 564.
Acmaca, 99,
Actaeon, 100, 145, 162,
163, 294,
Avolidae, 159, 161, 164.
‘Aeolis, 100, 102, 160, 164,
Ammonoidea, 235, 287,
288,
Amphibola, 175, 182.
Stophiness, 1-21.
Sy ener S34, 389,
348-850, 958,
or 366, 377, 520-524,
536-579.
Amusium Dalli, 71, 72.
Saree 367, 479-483,
Aaya, 101, 180,
Anodonta, 23, 24, 50-54,
58, 62, 66, 72.
Anomia, 82,
Aplysia, 100, 102, 103,
112, 115, 117, 129, 141,
145, 159-161, 164, 173.
—_ 29," 70-72, ‘78, 79,
Arniiae, 48.
uta, 235-240, 251-
, 257, 266, 267, 287,
‘294-296.
Arion, 101, 184-187.
Articulamentum, 13.
Ascidia canina, 337,
— mentula, 359,
Ascidiacea, 334,
a Compositae,
Atlanta, 100, 102, 154-
158.
Auricula, 101, 175, 176.
Aviculidae, 37, 61,
‘Aaygobranchia, 99,
B
bpd pet 542, 549,
5T5-5i
Raeeecaamtoee 101.
Belemnites, 235, 288-295.
Belemnoteuthis, 235,
292,
Bellerophon, 188,
Belosepia, 290, 291,
Botryllidae, 372, | 449,
456, 457, 460, 463.
Botryitus, 467, 877, 465,
Beeauhionancene 182,
Buccinum, 100, 103,
Bulimus, 101, 104, 105.
Bulla, 100.
Busycon, 103.
Bythinis, 100, 111, 114,
118, 121, 129, 136- =
191-195, 204, 205, 209
212,
c.
Cadulus, 96.
Oulyptruca, 100, 152.
Capitellidae,82, 577, 578.
30,
Carinaria, 100, 102, 114,
1
Cavotinia, 100, 111, 167-
Cophilochorda, 536-579.
Chiton, 1-18, 87, 98, 200,
top eae 215, 308, 315, 319,
Angee 5, 6
— olivaceus, 10,
— Poli, 2, 3, 6, 8, 10.
Chromodoris, 100, 160,
164.
Cirrhoteuthidae, 235,
Cirrhoteuthis, 265, 295.
Clausilia, 101, 105, 184,
187.
Clavelina, 386-340, 344-
351, 360-364, 367-377,
452, 463-469,
— le;
liformis, 359.
ma, 344-351,
Cleodora, 100, 167.
Clione, 101, 102, 115,
118, 167, 171, 172.
‘Coelocormidae, 519,
Coletia pedunciilata, 457.
Cranchiidae, 235, 293.
Crepidula, 100, 108, 107,
110, 111, 115-119, 128,
129, 141,
— conyexa, 105.
— fornicata, 105.
— plana,
105.
Cress, ‘100, 102, 167-
Oyclomyaria, 394, 519.
586
Cyclosalpa affinis, 495.
— dolichosoma - virgula,
416.
*— pinnata, 433, 507, 608.
Cymbulia, 100, 111, 115,
167, 168-170.
Cynthia, 335, 357, 381.
D.
Daudebardia, 101.
Dentalium, 88-98, 182,
192, 200, 216, 318, 319,
329, 330.
Diazona violaces, 448.
Dibranchia, 235.
Didemnidae, 367, 372,
pins 457, 459-464,
518,
357, 364-367, 872, 457,
469.
— magnilarva, 346, 353,
457.
= veslifera, 464-466.
Distomidae, 372,
457, 463, 473.
Docoglossa, 99.
Dolchinia, 473, 479, 482,
483,
456,
Doliolidac. 470-483, 514,
51s.
Doliolum.
355, 367,
447, 470-
4
— banyule enisis, 15, 17-19.
— festive.
Doridia: .
Doridopsis, 330.
Doris. 100, 102, 159, 164.
Dosidicus,
Doto, 100, 1
sis 30, BB.
9, 45, 47. 49, 61,
K
Eledone, 235-
Elysia, 100, 1
Emarginula, 18k.
SUBJECTS INDEX.
Enantia spinifera, 325. |
| Entocolax, 153. |
Entoconcha, 100, 111.
— mirabilis, 152. |
Entovalva, 43, 67, 208.
: — mirabilis, 23. |
Ercolania, 111, 159, 164.
Eulamellibranchia, 22. ,
Eunicidae, 577. ;
Euthyneura, 146.
|
F.
Fasciolaria, 100,
104, 128, 209.
. Filibranchia, 22.
Fiona, 100, 159, 164.
' Firoloida, 100, 102, 111,
134, 153-158, 166, 200.
— Desmaresti, 114, 158, j
154.
108, |
| Fissurella, 99, 111, 180,
i 188, 189, 209, 215,
Fragarium, 336.
Fragaroides, 450, 451.
Fulgur, 100, 102, 103,
107, 116, 120, 128, 180, ;
149. i
Fusus, 100, 106, 111, 115,
117, 121, 141, 149, 150:
152, 205, 206.
— antiquus, 103. 1
G.
Gasteropteron, 100, 173,
174.
Gastrochaena, 43, 6:
Gastropoda, 1, ¢
Glochidium,
Gnathobdellida
Gonatus Fabricii, 2
Gunda. 320.
Gymmosomata, 101, 171.
H.
97, 99,
3 2i4.
Haliotis,
18s, 194,
Helicarion, 200.
Helicinidae, 99, 199.
Helix, 101, 105. 179, 186,
200, 204.
—nemoralis, 201,
— pomatia, 104 185,184,
13.
147, |
eAvaliani 104, 179,
186, 191, 192, 196, 197.
' Hemimyaria, 414-448,
519.
i Heteropoda, 100, 102, ,
153.
Hyalea, 167.
Hyalocylix, 100, 167.
J.
Janthina, 100, 104, 120.
K.
Kalymmocytes, 336. 390,
420-424.
L.
Lamellibranchia, 1, 22-
87, 380.
Larvacea, 834.
Lasidium, 57.
Leachia, 292.
Lepidosteus, 355.
Lima, 82.
Limacidue, 329.
Limacina, 100, 169.
Limapontia, 164.
Limax, 101, 104, 110,
111, 119, 186, 177, 179,
184-17, 191. 204, 217.
Limneea, 101, 111, 114,
120, 132, 141, 177-183,
203, 221.
Lithonephrya, 885, 381.
Loliginidae, 235.
Loligo, 236-246, 251, 252-
277, 287, 296, 298-
. Lucernaria, 520.
Lumbricus’ trapezoides,
574.
M.
Marvenina, 99.
Megascolex, 79.
Melania, 105.
Melibe, 330.
Michrochaeta, 79.
Mitraria larva, 18.
Modiolaria, 23, 25, 27,
30, 33, 47.
Molgula— macrosipho-
nica, 382.
Molgulidae, 381, 445.
Mollusca, a
Monotocardia, 99, 147.
Montacuta, 30, 47, 64.
Miilleria, 80.
Murex, 100, 103, 200.
— brandaris, 198.
Muscidae, 374.
ytilui
87, 89, 45, 47, 68-71.
— edulis, 22, 24, 68.
Myzomenia, 1, 15, 19.
N.
Nais, 577.
Nassa, 100. 102, 106,
118, 116-121, 141, 149,
152, 163, 200, 208, 315.
— mutabilis, 108, 112,
116, 117, 150, 152, 207. |
Nassopsis, 105.
Natalins, 200.
Natica, 149.
Nautiloidea, 285,
Nautilus, 287, 268, 286-
Nemertini, $22
Neomenia, 1.
Nephropneusta, 182.
Neritidae, 99, 199,
Neritina, 99, 107, 110,
111, 116, 118, 129, 180,
138, 141.
— fluviatilis, 103, 116,
Nucleobranchia, 100,
158.
Nucula, 22, 37, 61, 63,
72, 75, 82.
Nuculidae, 48, 61.
Nudibranchia, 100.
oO.
Octacnemus, 334.
Octopoda, 235.
Octopus, ' 236-289, 251,
253, 257, 264-267, 295.
— membranaceus, 265.
—yulgaris, “201, 252,
Oigoncid, 235.
copleura, 356.
Ontneieni, 236, 267,
287, 291-298, 308.
Onchidium, 66, 101, 104,
111, 129, 138, 174-176,
329.
* Onychoteuthidae, 235.
Onychoteuthis, 292.
Opisthobranchia,
158.
100,
Orthoceratidae, 268,
Ostracum, 288.
SUBJECTS INDEX.
Ostrea, 22-38, 45-49, 66,
TA, 80, 82, 115, 316.
— edulis, 28, 28, 38, 60.
— virginiana, 22, 26, 60.
Oxygyrus, 100, 102, 157.
P.
Paludina, 100, 106, 114,
121, 122, 129, 134-142,
148, 149, 151, 153, 163,
191-199, 202, 211-221,
318.
— vivipara,
105, 187,
139, 212-214, 219.
99, 101,
Patella, 4, 91,
— oyes of, 64-66.
Pedicellina, 376.
Pegea bicaudata, 418.
—seutigera - confoode- |
rata, 446, 495.
Peripatus, 80, 317.
Perophora, 379, 445, 456, ;
463, 465.
879, 380.
346, 357, 360,
366, 372.
— mammillata, 335, 348,
355, 356, 358, 373, 376.
— scabroides. 377, 378.
Philine, 100, 145, 164.
Philonexis, 235, 287.
Pholas, 45, 70.
Phragmocone, 288.
Phragmophora, 235.
Phyllidia, 330.
Phyllithoe, 164.
Physa, 119.
Piliditan larva, 322.
Pinna, 82.
Pinnoctopus, 265.
Pisidium, 23-26, 29, 30,
39, 46. 47, 68, 73.
Planorbix, 101. 107, 109-
114, 117, 118, 129, 141,
148, 149, 177-183, 204.
Pleurobranchea, 100.
Pleurobranchus, 100,
102, 164.
Pleuronectidae, 574.
Pleurotomaria, 99, 147,
188.
Pneumoderma, 101, 167-
172.
Polycers, 100, 164.
587
, Polyclinidae, 995, 449,
452-455, 470.
Prodissoconch, 61.
Proneomenia, 1, 15, 19.
aglaopheniae, 325.
| Pr Prcusbrenehias 99, 102,
48.
Protebranchia, 22.
Pseudolamellibranchia,
Praropoda, 100, 102, 166,
Preroeianses 100, 102,
111, 154-158.
Pulmonata, 101, 174,
| 187.
| Pups, 105,
Purpura, 100, 118, 115.
— floridana, 102.
— lapillus, 108.
Pyramidollidae, 330.
Pyrosoma, 334, 356, 367,
| 377, 381, 389-414, 420,
482, 451, 455, 459, 479,
| 484-500. 508, 508, 512-
520.
R.
| Rossia, 28
Foanctiora: 100, 157.
| Runcina, 100.
Ss.
i Saccoglossa, 168.
' Sagitta, 54:
, Salpa, 334, 336, 881, 414-
| 448, 494-518.
— affinis, 495.
, — africana - maxima,
| 417, 483, 485, 440, 442,
445,
i — bicaudata,
448.
— costata-Tilesii,
495.
, — demovratica - mucro-
nata, 415, 420-446, 495,
496, 511, 513.
— dolichosoma - virgula,
416, 495.
— hexagona, 417, 446.
— pinnata, 416-420, 423,
433, 435-448, 495, 507.
— punctate, 417, 420,
433, 435, 445.
— runcinata-fusiformis,
438, 435, 442, 445, 507.
| —scutigera - confoede-
rata, 446, 495.
417-419,
417,
588
Salpa zonaria - cordi- |
formis, 417, 495.
Salpidee, 414-448, 451,
477, 488, 494-512, 516-
520,
Scaphites, 294.
Scaphopoda, 87-98.
Bearabue, 175, 176.
Scioberetia, 45.
Scissurella, 188.
Sepia, 236-252, 272-277,
287-298, 296, 298.
— aculeata, 290.
— andreanoides, 289.
— officinalis, 238, 241-
247, 250, 273-276,
Sepiidas, 235.
Sepiola, 236, 244, 257,
Sepiolidae, 235.
Sepioteuthis, 238.
Siphonaria, 119.
Siphonodentalium, 96.
Solenoconcha, 1, 88-98,
829.
Solenogastres, 1.
Solenomya, 22. 75, 82.
Spekia, 105.
Spirialis, 100, 169.
Spirula, 235, 268, 286,
288, 298, 204.
Spirulidae, 235.
Spirulirostra, 288, 290.
Spondylus, 66, 82.
Stilifer, 100.
— Linckiae, 152.
Streptoneura, 99.
Strombus, 100, 157.
Styelidae, 372.
Styliola, 100, 167, 169.
Stylomatophora, 101,
184.
SUBJECTS INDEX.
Succinea, 101, 192, 180,
187.
Synapta, 43.
— digitata, 152.
T.
Taenioglossa, 158.
Taonius, 292.
Tectibranchia, 100.
Tegmentum, 13.
Teredo, 23-39, 42, 44, 66-
78, 80, 208, 819.
Tergipes, 100, 102.
— Edwardsii, 164, 165.
— lacinulatus, 164.
Testacella, 101.
Testacellidae, 182.
Tethys, 111, 119, 830.
Tetrabranchia, 235.
Teuthidse, 236.
Teuthis, 267.
Thalia democratica-mu-
cronata, 415, 420-446,
495, 511, 518.
Thaliacea, 334.
Thecosomata, 100.
Thyca, 100.
— entoconcha, 152.
Tiedemannia, 100, 167-
170.
Tornatella, 163.
Tremoctopus, 235, 287.
Triclada, 320.
Trididemnum, 460-462.
Trigonia, 71.
Tritonia, 100.
Trochophore, 5-7, 10, 18,
30-43, 91, 125-128, 142,
161, 167, 177, 326.
; Urosalpinx,
Trochus, 99, 198, 200,
magus, 198.
Trophezocid, 289.
Tunicata, 384-534.
Turbellaria, 108, 320.
Turbinidae, 199.
Turbo, 99, 215.
— creniferus, 198.
Typhobia, 105.
Uw.
Umbrella, 100, 107, 110,
111, 118-120, 129, 159.
Unio, 22-29, 38, 119.
Unionidae, 25-29, 30, 39,
49-59, 66-71, 73-76.
108, 113,
116, 117.
v.
Vaginulidae, 175, 176.
Vaginulus, 101, 175.
Vaivata, 99, 100.
Veliger ‘larva, 130-134,
162, 169, 177.
Venus, 22.
Vermetus, 100, 105, 111,
115-122, 130-132, 148,
180, 192, 197, 200, 217.
Vitrina, 101, 105.
¥;
Yoldia, 19, 22, 63, 72.
Z.
Zygobranchia, 99.
AUTHORS INDEX.
A.
Apams, A.
Cephalopoda, 294.
Apter, J. and Han-
ook, A.
Gastropoda, 158, 164,
Awsprews, E. A.
Lamellibranchia, 64.
APPELOFF,
Oapaaiipetes 291.
B.
Bakr, K, E. v.
Tunicata, 524.
Baurour, F. M.
fay eS 577,
Latnelibrenshis, 50,
Tunicata, 955, 514,
522.
Barnors, J.
Tunicata, 433,
445, 472-474, 480.
Basnots, TH,
Lamellibranchia, 22,
24, 66.
Barrson, W.
Cephalochorda, 649,
578, 579.
Brann, J,
Cephalochorda, 576,
577.
Brppanp, F.
Oligochaeta, 79.
Brune, Ta.
Gastropoda, 213.
Bexepes, Ep. vas, and
Junin, On.
Cephalochorda, 576.
Tunicata, 338-388, 432,
447, 450, 451, 456,
465-469, 482, 520-
$22,
Beno, R.
Gast 176.
Buror, R. 8.
Lamellibranchia, 74.
Bernarp, F,
Tasnallibranchis, 45.
Lamellibranchia, 30.
Gastropoda, 105-109,
111, 112, 115-118,
134, 141, 149, 158-
161,
Bromnron, J.
Amphineura, 10-12.
J.B. y.
ropoda, 114, 134,
138, 141, 144, 145,
153, 202, 217.
oarpr he a 64-
Tunicata, 429, 430,
Cephalopoda, 241, 248,
Tro, sea, ba6, "208!
309. roe
Gastropoda, 103, 5
112, 115-117, 120,
122, 129, 141, 149-
152, 195, 200, 205-
207,
Booran, L.
Gastropoda, 130, 189.
Bouvirn, B. Le
Gastro) 225.
Bovest,
Oepbaloaerdiy 535,
556, 566, 568-576.
Braon, M.
Gastropoda, 153, 213,
Lamellibranchia, 60,
56, 58-60, 83,
Gophat poda, 237,
Cephalo;
Gastropi 220-223,
Cephalopoda, 252-254.
Gastropoda, 103, 118,
116, 128,
Lamellibranchia, 7,
98, 1.
Tunicata, 420, 423,
Cephalopoda, 294,
CLapanior, B.
» 178.
Jone 415, 612,
Coreen
cornea 108, 119.
D.
Dart, W. H.
Lamellibranchia, TL,
590
Damas, D.
Tunicata, 368.
Davivorr, M. v.
54.
Tunicata, 386, 388, | Faatssz, P.
| Fuaita, T.
350-354, 520.
Detua VauLE, A.
Tunicate, 448, 457-463,
Diett, M. J.
Cephalopoda, 304. |
Dopverueiy, L. |
Cephelopoda, 318.
Dourn, A. |
Cephalochorda, 573, |
575.
Tunicata, 522, 528. |
Drew, G. A.
Gastropoda, 232.
Lamellibranchia,
63.
19, +
|
E.
EHLERS,
Cephelochorda, 577.
Exnrenpauy, E.
Lamellibranchia, 61.
Eists, H.
Cephalochorda, 577.
Gastropoda, 220, 221.
E1smunp, J.
Cephalochorda, 583.
Ervaneer, R. v.
Gastropoda, 114, 118-
122, 129, 184-136,
139, 141, 153, 179,
191-199, 204, 209-
220.
Escuuicut, D. F.
Tunicata, 417, 494.
|
:
tropuda, 101,
cen, H.
Gastropoda, 201,
FLmIscHMANN, A.
Lamellibranchia, 81.
Fresaixc, W.
Tamellibranchia, 50,
199,
Frop d
Tunicata, 532.
Fou. H.
Gastropoda, 102, 111,
114, 128, 153, 154,
166-187, 191, 192,
197, 200, 202
Tunicata, 335-338, 365,
474.
| Garstana, W.
a ee ee poo
AUTHORS INDEX.
Foret, F. A.
Lamellibranchis, 50,
Gastropoda, 199.
Gastropoda, 283.
Forranton, J. H.
Lamellibranchia, 22,
24, 49.
G.
Ganis, M.
Gastropoda, 178, 217.
Insecta, 286.
Lamellibranchia, 73,
74.
Tunicata, 379, 458, 463,
513.
Tunicata, 377.
GeGenaaur, C.
Amphineura, 12.
Cephalochorda, 578,
579.
Gastropoda, 128, 158,
155-157, | 167-174,
179, 184-187.
sarees 459, 476,
Sian a
Mollusca, 382.
Tunicata, 449, 452, 458,
463.
Garp, A.,und CaULERy,
M
Tunicata, 532.
Girop, P.
Cephalopoda, 285, 286,
310,
Goren
Tunic
Gorter, A.
27,
Goopnicn,
Molluse
Graper, V.
Thsecta,
Gare, L1. von.
Mollusca, 3:
GRENACHER, H,
Cephalopoda,
Grosnen, C.
Cephalopoda, 304-307.
Gastropoda, 157, 173.
Lamellibranchia, 22, ,
78, 80. i
Tunicate, 362, 388,
470-478, 512, 519.
H.
Happon, A. C.
Gastropoda, 120, 159,
164, 193.
Haxcket, E.
Cephalochorda, 578.
Tunicata, 523.
Hauer, Beta.
Amphineura, 15.
Hamman, J.
Cephalochorda, 588.
Hancock, A. See ADLER
and Haxcock.
Hansen, G. A.
Amphineura, 20.
Harscuex, B.
Amphineura, 11.
Cephalochorda, 535-
577.
Lamellibranchia, 23,
26-39, 63, 67.
Tunicata, 350, 522.
Heer, K.
Tunicata, 336, 416,
420, 423, 426, 433,
436, 441, 445.
HEncuMay, ANNIE P.
Gastropoda, 191, 195.
Herpman, W.
Tunicata, , 457,
460, 519, 520.
HERTWIG, O.
540-
Cephalochorda,
543.
33
Herrwic, R.
Cephalochorda, 541.
Gastropoda, 173.
Tunicata, 3
Hermoss, R.
Gastropoda,
118-120,
Gastropoda, 198.
Hyont, J.
Tunicata, 361, 449,
458, 463-466, 516,
Horr, and Boxne-
vig, Fr.
Tunicata, 532.
Hotes, 3. J.
Gastropoda, 184.
Horst, R.
Lamellibranchia,
30, 47.
23-
Huprecar, A. A. W.
Amphineura, 15, 16.
Hoxzey, T. H. |
Cephalochorda bit
Tunicata, $89, 413,
484, 494, 499, 519.
J.
Jackson, R. T.
Lamellibranchia, 42,
47-49, 60, 61, 68. ‘
Jaconson, L.
Lamellibranchia, 56.
JARKEL, O.
Cephalopoda, 292.
data, G.
Cephalopoda, 305.
Juxnina. H.
‘Amphineure, 1, 56. |
Cephalopoda, 304, 368.
Gastropoda, 147, 175, '
182, 166, 218. I
Lamellibranchia, 78,
82, 83.
Mollusca, 332.
Jouixt, L.
Tunicata, 484, 490.
Jounin, L.
Cephalopoda, 237, 283,
306, 310.
Jourparn, S.
Gast yoda, 179, 186, |
Tonkate 460, 463. '
Joreux-Larrurm, J.
Gastropoda, 133, 165, \
175. i
Juux, Ci. See Bane: |
DEN and JUIN.
Juuin, CH.
Tunicata, 860, 469.
K.
Kerenstetn, W.
Gastropoda, 101, 158.
Tunicata, 382.
Kererstery, W., and
Eu.ens, EB. 1
_ Tunicata, 387, 413.
J. Ve
lochorda,
576,
Kerr, J. Grawan.
Cephalopoda, 305.
Krener, L.
_ Gastropoda, 157.
Tunicata, 526.
AUTHORS INDEX.
Kuaatscu, H.
Cephalochorda, 535.
KLEINENBERG, N.
Cephalochorda,
577.
Cephalopoda, 307.
Gastropoda, 122, 196,
Kuorz, J.
Gastropoda, 220-228,
Kuirowrtscn, N.
Gastropoda, 115, 118.
Korurer, R.
Cophalochorda, 579.
KOLLIKER, A.
Cephalochorda, 564.
Cephalopoda, 241, 245,
252, 273-276, 204-
297.
Tunicata, 458.
Kororp, C. A.
Gastropoda, 184.
Kou, C.
Cephalochorda, 564.
Koken, E.
Gastropoda, 188.
Konen, J., and Dante.s-
sen, D.C.
Gastropoda, 227.
Konorserr, A.
Tunicate, 336, 420,
423-427, 432, 433,
436, 441, 442, 445-
448, 479, 482, 483.
Koxscuenr, K.
Cephalopoda, 279, 801.
576,
Lamellibranchia, 28,
30, 47. .
1.
535.
542, 547, 549, . 556,
Cephalopoda, 297.
Gastropoda, 200.
Mollnsea, 825.
Solenoconcha, 88-98,
376, 389, “391, 395-
403, 408, 429, 443,
448-457, 463 - 469,
484, 494, 498, 499,
516-520.
Kowanevsxy,
Bannows, J.
Tunicate, 529.
Kowanevsky, A.,
Marton, A. F.
Amphineura, 20.
A, and’
and |
591
Kroun, A.
Gastropoda, 128, 188,
155-158, 167-173. °
Tunicata, 376, 382,
417, 458, 513.
Koprres, C. v.
Cephalochorda, 541,
577.
Tunicata, 335 - 388,
357, 364, 372, 874,
381, 382, 521.
P L.
Lacaze-Dutuiers,
DE.
Gastropoda, 105, 131,
199.
H.
Lamellibranchia, 47,
Solenoconcha, 88, 94-
§
Tunicata, 381.
Lacaye-Durarmas, H,
pn, and Pavvor, G.
Gastropoda, 164.
Lanier, F.
Tunicata, 360, 361,
367, 372, 417, 457,
464.
Lana, A.
Gastropoda, 144. at:
‘Mol sca, 3 3:
Gastropoda, 142, 159,
160.
Tunicata, 360, 521.
Lankester, E. Ray.
Cephalochorda, 585,
556-562.
Cephalopoda, 249, 252,
}, 285, 286, 298,
296, 301.
Gastropoda, 112, 114,
128, 191-1834, 141,
161, 177, 180, 183,
193, 215.
Lamellibranchia, 25-
30, 40, 68 74
Mollusca, 820, 332.
Lankester, K. Ray, and
WILury, A.
Cephalochorda, 535,
550-553, 556-561.
Latter, O. H.
Lamellibranchia, 56.
Lea, I.
Lamellibranchia, 56.
Ler, A
Tunicata, 388.
Rarakn, H.
‘Lamellibranchia, 56,
Rawitz, B,
Lamellibranchia, 64.
Mollusca, 388.
Cephalochorda, 568,
Rypver, J.
Gastro 230.
Lamellibranchia, 48,
60.
5.
Sapatinr, Ap.
Tunicata, 338.
Sr, Hiarre, GEorrroy.
Cephalochorda, 575.
Savensky, W.
Cephalochorda, 587.
Gastropoda, 115, 122,
192, 197, 200, 217.
Tunicata, 835, 338,
Saaassa, P,’
141,
215,
Sanasin, P. and F,
Gastropoda, 104,
152, 179, 186,
197.
Sars, M,
Gastropoda, 158.
Tunicata, 458,
Scuanreerw, M,
Gastropoda, 217,
122,
191- |
Amphineura, 1.
Seecicer, Osw.
. 339,
405,
=
5T5-
caatropds, 175, 176,
anteate, 337, 338,
489.
Snarp, B.
Lamellibranchia, 86.
Suxxpon, L.
quia 527.
Smoove,
‘Amphineura, 1b,
Gestecpoti, 147, 220,
StxcErroos, C. P.
Lamellibranchia, 45,
Suurres, 0, Pa,
ellibranchia, 62,
QQ
Toparo,
Tunicata, 416-421, 426,
432-448, 494-498,
515, 516.
Ténsices, 0.
aera 185, 187.
Tunicata, a.
‘TRINCHESE,
ecnenee® 158-168.
Tuiusena, T.
Lamellibranchia, 61,
Uv.
Ursasiy, B.
‘Tunicata, 382-388, 385
471-478, 512, 514.
Ussow, M.
Ge) 240,
Br Beads 302
286, 801, 303-306.
Vv.
Ve RS
“ti Fs
594
ViautER, C.
Gastropoda, 159.
VoErLtTzKow, A.
Lamellibranchia, 28,
Voar, C.
Gastropoda, 158, 174.
Tunicata, 494.
Cephalopoda, 289.
WwW.
‘Wakneck, A.
Gastropoda, 106.
Watasg, 8.
Cephalopoda, 241-246,
Wuues, F. E.
Cephalochorda, 581.
AUTHORS INDEX.
| Wettner, W. Wirson, Jour.
| “Lamellibranchia, 30. ! Lamellibranchia, 2,
Wirrzrsski, A. 24, 39, 47.
| Gastropoda, 119, 128, ° Wrrtacziz, Em.
184. Insecta, 286.
Wisue, I. W. van. Wotrsox, W.
Cephalochorda, 535, Gastropoda, 114, 120.
| 860, 571, 574. 141, 176, 178, 183,
| Winury, A. 198, 202, 908.
Cephalochorda, 585, -
549, 558-561, 574. a
575. : on
Cephalopoda, 287. “y ),
Tunicata, 350, $59, PCephalipe 26.
361, 365-969, 377. | pod,
519-521. See also | ZIEGLER, KE.
q igreo. Lamellibranchia, 23-
Lankesten = and) “59°80 Gg gs GOSR
tag anna 66, 73-77, 81-83.
; bi ter nar | Zrrren, K.
‘ephalocho 536, '
jerry Cephalopoda, 291.
Gastropoda, 188.
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