Imp. Pétersbourg_, Vol. V. Ser. 7, 1863.
(224) F. Küchenmeister. "Ueber d. Umwandlung d. Finnen Cysticerci in
Bandwürmer (Tænien)." _Prag Vierteljahrsschr._ 1852.
(225) ---- "Experimente üb. d. Entstehung d. Cestoden. 2o Stufe
zunächst d. Coenurus cerebralis." Günsburg, _Zeitsch. klin. Med._ IV.
1853.
(226) R. Leuckart. _Die Menschlichen Parasiten_, Vol. I. Leipzig,
1863. Vide also additions at the end of the 1st and 2nd volume.
(227) R. Leuckart. "Archigetes Sieboldii, eine geschlechtsreife
Cestodenamme." _Zeit. f. wiss. Zool._, Vol. XXX. Supplement, 1878.
(228) El. Metschnikoff. "Observations sur le développement de quelques
animaux (Bothriocephalus proboscideus)." _Bull. Acad. Imp. St
Pétersbourg_, Vol. XIII. 1869.
(229) W. Salensky. "Ueb. d. Bau u. d. Entwicklungsgeschichte d.
Amphilina." _Zeit. f. wiss. Zool._, Vol. XXIV. 1874.
(230) Von Siebold. Burdach's _Physiologie_.
(231) R. von Willemoes-Suhm. "Helminthologische Notizen." _Zeit. f.
wiss. Zool._, Vol. XIX. XX. XXII. 1869, 70 and 73.
CHAPTER VIII.
ROTIFERA.
For many reasons a complete knowledge of the ontogeny of the Rotifera
is desirable. They constitute a group which retain in the trochal disc
an organ common to the embryos of many other groups, but which in most
other instances is lost in the adult state. In the character of the
excretory organs they exhibit affinities with the Platyelminthes,
while in other respects they possibly approach the Arthropoda (_e.g._
Pedalion ?). The interesting _Trochosphæra æquatorialis_ of Semper
closely resembles a monotrochal polychætous larva.
Up to the present time our embryological knowledge is mainly confined
to a series of observations by Salensky on _Brachionus urceolaris_,
and to scattered statements on other larval forms by Huxley, etc.
In many cases Rotifers lay summer and winter eggs of a different
character. The former are always provided with a thin membrane, and
frequently undergo development within the oviduct. They are hatched in
the autumn. The winter eggs are always provided with a thick shell.
The summer eggs are of two kinds, viz. smaller eggs which become
males, and larger, females. On the authority of Cohn (No. 232) they
are believed to develop parthenogenetically. Males are not found in
summer, and only seem to be produced from the summer eggs. Cohn's
observations, especially on _Conochilus volvox_, are however not
quite satisfactory. Huxley (No. 234) came to the conclusion that the
winter eggs of Lacinularia developed without previous fertilization.
The following are the more important results of Salensky's
observations (No. 236) on _Brachionus urceolaris_.
The ovum is attached by a short stalk to the hind end of the body of
the female, in which position it undergoes its development. It will be
convenient to treat separately the development of the female and male,
and to commence with the former. The female ovum divides into two
unequal spheres, of which the smaller in the subsequent stages
segments more rapidly than the larger. The segmentation ends with the
formation of an epibolic gastrula. The solid inner mass of cells
derived from the larger sphere constitutes the hypoblast, and is more
granular than the epiblast. The evolution of the embryo commences with
the formation of a depression on the ventral surface, at the bottom of
which the stomodæum is formed by an invagination. At the hinder part
of the depression there rises up a rounded protuberance which
eventually becomes the caudal appendage or foot. Immediately behind
the mouth is formed an underlip.
On the sides of the ventral depression are two ridges which form the
lateral boundaries of the trochal disc. They appear to unite with the
under lip.
In a later stage the anterior part of the body becomes marked off from
the posterior as a præoral lobe, and the hypoblast is at the same time
confined to the posterior part. The supra-oesophageal ganglion is
early formed as an epiblastic thickening on the dorsal side of the
præoral lobe.
The first cilia to appear arise at the apex of the præoral lobe. At a
later period the lateral ridges of the trochal disc meet dorsally and
so enclose the præoral lobe. They then become coated by a ring of
cilia, to which a second ring, completing the double ring of the
adult, is added later.
[FIG. 100. EMBRYO OF BRACHIONUS URCEOLARIS SHORTLY BEFORE IT IS
HATCHED. (After Salensky.)
_m._ mouth; _ms._ masticatory apparatus; _me._ mesenteron; _an._
anus; _ld._ lateral gland; _ov._ ovary; _t._ tail, _i. e._ foot;
_tr._ trochal disc; _sg._ supra-oesophageal ganglion.]
In the trunk an indication of a division into two segments makes its
appearance shortly after the development of the præoral lobe. Before
this period the proctodæum is established as a shallow pit immediately
behind the insertion of the foot. The latter structure soon becomes
pointed and forked (fig. 100, _t_).
The complete establishment of the alimentary canal occurs late. The
stomodæum (fig. 100) gives rise to the mouth (_m_), oesophagus and
masticatory apparatus (_ms_). The mesenteron is formed from the median
part of the hypoblast; the lateral parts of which appear to give rise
to the great lateral glandular structures (_ld_) which open into the
stomach, and to the ovaries (?) (_ov_) etc. The proctodæum becomes the
cloaca and anus (_an_). The origin of the mesoblast is not certainly
known. The shell is formed before the larva is hatched--an occurrence
which does not take place till the larva closely resembles the adult.
The early developmental stages of the male are closely similar to
those of the female; and the chief difference between the two appears
to consist in the development of the male being arrested at a certain
point.
The larvæ of Lacinularia (Huxley, No. 234) are provided with a præoral
circlet of cilia containing two eye-spots[98], and a peri-anal patch
of cilia. They closely resemble some telotrochal polychætous larvæ.
[98] In Leydig's figure of the larva, _Zeit. f. wiss. Zool._
Vol. III. 1851, the eye-spots lie just outside the ciliated ring.
Salensky has compared the larva of Brachionus to that of a
cephalophorous Mollusc, more especially to the larva of Calyptræa on
which he has made important observations. The præoral lobe, with the
ciliated band, no doubt admits of a comparison with the velum of the
larva of Molluscs; but it does so equally, as was first pointed out by
Huxley, with the ciliated præoral lobe of the larvæ of many Vermes. It
further deserves to be noted that the trochal disc of a Rotifer
differs from the velum of a Mollusc in that the eyes and ganglia are
placed dorsally to it, and not, as in the velum of a Mollusc, within
it. The larva of Lacinularia appears to be an exception to this, since
two eye-spots are stated to lie within the circlet of cilia. More
important in the comparison is the so-called foot (tail), which arises
in the embryo as a prominence between the mouth and anus, and in this
respect exactly corresponds with the Molluscan foot.
If Salensky's comparison is correct, and there is something to be said
for it, the foot or tail of Rotifers is not a post-anal portion of the
trunk, but a ventral appendage, and the segmentation which it
frequently exhibits is not to be compared with a true segmentation of
the trunk. If the Rotifers, as seems not impossible, exhibit
crustacean affinities, the 'foot' may perhaps be best compared with
the peculiar ventral spine of the Nauplius larva of _Lepas
fascicularis_ (vide Chapter on Crustacea) which in the arrangement of
its spines and other points also exhibits a kind of segmentation.
BIBLIOGRAPHY.
(232) F. Cohn. "Ueb. d. Fortpflanzung von Räderthiere." _Zeit. f.
wiss. Zool._ Vol. VII. 1856.
(233) F. Cohn. "Bemerkungen ü. Räderthiere." _Zeit. f. wiss. Zool._
Vol. IX. 1858, and Vol. XII. 1862.
(234) T. H. Huxley. "Lacinularia socialis." _Trans. of the
Microscopical Society_, 1853.
(235) Fr. Leydig. "Ueb. d. Bau u. d. systematische Stellung d.
Räderthiere." _Zeit. f. wiss. Zool._ Vol. VI. 1854.
(236) W. Salensky. "Beit. z. Entwick. von Brachionus urceolaris."
_Zeit. f. wiss. Zool._ Vol. XXII. 1872.
(237) C. Semper. "Zoologische Aphorismen. Trochosphæra æquatorialis."
_Zeit. f. wiss. Zool._ Vol. XXII. 1872.
CHAPTER IX.
MOLLUSCA[99].
[99] The classification of the Mollusca adopted in the present
chapter is shewn in the subjoined table:
I. ODONTOPHORA. II. LAMELLIBRANCHIATA.
1. Gasteropoda. _a._ Dimya.
_a._ Prosobranchiata. _b._ Monomya.
_b._ Opisthobranchiata.
_c._ Pulmonata.
_d._ Heteropoda.
2. Pteropoda.
_a._ Gymnosomata.
_b._ Thecosomata.
3. Cephalopoda.
_a._ Tetrabranchiata.
_b._ Dibranchiata.
4. Polyplacophora.
5. Scaphopoda.
Although the majority of important developmental features are common
to the whole of the Mollusca, yet at the same time many of the
subdivisions have well-marked larval types of their own. It will for
this reason be convenient in considering the larval characters to deal
successively with the different subdivisions, but to take the whole
group at once in considering the development of the organs.
_Formation of the layers and larval characters._
ODONTOPHORA.
Gasteropoda and Pteropoda. There is a very close agreement amongst the
Gasteropoda and Pteropoda in the general characters of the larva; but
owing to the fact that the eggs of the various species differ
immensely as to the amount of food-yolk, considerable differences
obtain in the mode of formation of the layers and of the alimentary
tract.
The spheres at a very early stage of segmentation[100] become divided
into two categories, one of them destined to give rise mainly to the
hypoblast, the other mainly to the epiblast. According as there is
much or little food-yolk the hypoblast spheres are either very bulky
or the reverse. In all cases the epiblast cells lie at one pole, which
may be called the formative pole, and the hypoblast cells at the
opposite pole. When the bulk of the food-yolk is very great, the
number of hypoblast spheres is small. Thus in Aplysia there are only
two such spheres. In other cases, where there is but little food-yolk,
they may be nearly as numerous as the epiblast cells. In all these
cases, however, as was first shewn by Lankester and Selenka, a
gastrula becomes formed either by normal invagination as in the case
of Paludina (fig. 107), or by epibole as in _Nassa mutabilis_ (fig.
105). In both cases the hypoblast becomes completely enclosed by the
epiblast. _The blastopore is always situated opposite the original
formative pole._ In the large majority of cases (_i.e._ Marine
Gasteropoda, Heteropoda, and Pteropoda) the blastopore becomes
gradually narrowed to a circular opening which eventually occupies the
position of the mouth. It either closes or remains permanently open at
this point. In some cases the blastopore remains permanently open and
becomes the anus. The best authenticated instance of this is _Paludina
vivipara_, as was first shewn by Lankester (No. 263).
[100] The reader is referred for the segmentation to pp. 98-102,
and to the special description of separate types.
In some instances the blastopore assumes before closing a very narrow
slit-like form, and would seem to extend along the future ventral
region of the body from the mouth to the anus. This appears, according
to Lankester (No. 262), to be the condition in Lymnæus, but while
Lankester believes that the closure proceeds from the oral towards the
anal extremity, other investigators hold that it does so in the
reverse direction. Fol (No. 249) has also described a similar type of
blastopore. In an undetermined marine Gasteropod, with an embolic
gastrula, observed by myself at Valparaiso, the blastopore had the
same elongated form as in Lymnæus, but the whole of it soon became
closed except the oral extremity; but whether this finally closed
could not be determined. It is probable that the typical form of the
blastopore is the elongated form observed by Lankester and myself, in
which an unclosed portion can indifferently remain at either
extremity; and that from this primitive condition the various
modifications above described have been derived[101].
[101] Rabl (No. 268) describes a blastopore of this form in
Planorbis which closes at the mouth.
Before the blastopore closes or becomes converted into the oral or
anal aperture, a number of very important embryonic organs make their
appearance; but before describing these it will be convenient to state
what is known with reference to the third embryonic layer or mesoblast.
This layer generally originates in a number of cells at the lips of
the blastopore, which then gradually make their way dorsalwards and
forwards, and form a complete layer between the epiblast and
hypoblast. The above general mode of formation of the mesoblast may be
seen in fig. 107, representing three stages in the development of
Paludina.
In some cases the mesoblast arises from certain of the segmentation
spheres intermediate in size between the epiblast and hypoblast
spheres. This is the case in _Nassa mutabilis_, where the mesoblast
appears when the epiblast only forms a very small cap at the formative
pole of the ovum; and in this case the mesoblast cells accompany the
epiblast cells in their growth over the hypoblast (fig. 105).
In other cases the exact derivation of the mesoblast cells is quite
uncertain. The evidence is perhaps in favour of their originating from
the hypoblast. It is also uncertain whether the mesoblast is
bilaterally symmetrical at the time of its origin. It is stated by
Rabl to be so in Lymnæus[102].
[102] Rabl (No. 268) has quite recently given a more detailed
account than previous observers of the origin of the mesoblast in
Planorbis. He finds that it originates from the posterior one of
the four large cells which remain distinct throughout the
segmentation. By the division of this cell two 'mesoblasts' are
formed, one on each side of the middle line at the hinder end of
the embryo. Each of these again divides into two, an anterior and
a posterior. By the division of the mesoblasts there arise two
linear rows of mesoblastic cells--the mesoblastic bands--which
are directed forwards and divided transversely into two parts, an
anterior continued from the front mesoblast, and a posterior from
the hinder mesoblast.
If Rabl's account is correct, there is a striking similarity
between the origin of the mesoblast in Mollusca and in Chætopoda.
It appears to me very probable that the mesoblastic bands are
formed (as in Lumbricus) not only from the products of the
division of the mesoblasts, but also from cells budded off from
one or both of the primary germinal layers.
In the case of Paludina the mesoblast becomes two layers thick, and
then splits into a splanchnic and somatic layer, of which the former
attaches itself to the hypoblast, and gives rise to the muscular and
connective-tissue wall of the alimentary tract, and the latter
attaches itself to the epiblast, and forms the muscular and
connective-tissue wall of the body and other structures. The two
layers remain connected by protoplasmic strands, and the space between
them forms the body cavity (fig. 107). In most instances there would
appear to be at first no such definite splitting of the mesoblast, but
the layer has the form of a scattered network of cells between the
epiblast and the hypoblast. Finally certain of the cells form a
definite layer over the walls of the alimentary canal, and constitute
the splanchnic mesoblast, and the remaining cells constitute the
somatic mesoblast.
[FIG. 101. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM. (From
Lankester.)
_f._ foot; _ot._ otocyst; _m._ mouth; _v._ velum; _ng._ nerve
ganglion; _ry._ residual yolk spheres; _shs._ shell-gland; _i._
intestine.]
We must now return to the embryo at the time when the blastopore is
becoming narrowed. First of all it will be necessary to define the
terms to be applied to the various regions of the body--and these will
best be understood by taking a fully formed larva such as that
represented in fig. 101. The ventral surface I consider to be that
comprised between the mouth (_m_) and the anus, which is very nearly
in the position (_i_) in the figure. As a great protuberance on the
ventral surface is placed the foot _f_. The long axis of the body, at
this period though not necessarily in the adult, is that passing
through the mouth and the shell-gland (_shs._): while the dorsal
surface is that opposite the ventral as already defined.
Before the blastopore has attained its final condition three organs
make their appearance, which are eminently characteristic of the
typical molluscan larva. These organs are (1) the velum, (2) the
shell-gland, (3) the foot.
The velum is a provisional larval organ, which has the form of a
præoral ring of cilia, supported by a ridge of cells, often in the
form of a double row, the ventral end of which lies immediately dorsal
to the mouth. Its typical position is shewn in fig. 101, _v_. There
are considerable variations in its mode and extent of development
etc., but there is no reason to think that it is entirely absent in
any group of Gasteropoda or Pteropoda. In a few individual instances,
especially amongst viviparous forms and land Pulmonata, it has been
stated to be absent. Semper (No. 274) failed to find it in Vitrina,
Bulimus citrinus, Vaginulus luzonicus, and Paludina costata. It is
very probably absent in Helix, etc.
In some cases, _e.g._ Limax (Gegenbaur), Neritina (Claparède),
Pterotrachæa (Gegenbaur), the larva is stated to be coated by an
uniform covering of cilia before the formation of the velum, but the
researches of Fol have thrown very considerable doubt on these
statements. In some cases amongst the Nudibranchiata (Haddon) and
Pteropoda there are one or two long cilia in the middle of the velar
area. In many Nudibranchiata (Haddon) there is present a more or less
complete _post-oral_ ring of small cilia, which belongs to the velum.
The cilia on the velum cause a rotation of the larva within the
egg-capsule. Cilia are in most cases (Paludina, etc.) developed on the
foot and on a small anal area.
The shell-gland arises as an epiblastic thickening on the posterior
and dorsal side. In this thickening a deep invagination (fig. 101,
_shs._) is soon formed, in which a chitinous plug may become developed
(Paludina, Cymbulia? etc.), and in abnormal larvæ such a chitinous
plug is generally formed.
The foot is a simple prominence of epiblast on the ventral surface, in
the cavity of which there are usually a number of mesoblast cells
(fig. 101, _f_). The larval form just described has been named by
Lankester the trochosphere larva.
Before considering the further external changes which the larva
undergoes, it will be well to complete the history of the invaginated
hypoblast.
[FIG. 102. EMBRYO OF A HETEROPOD. (From Gegenbaur; after Fol.)
_o._ mouth; _v._ velum; _g._ archenteron; _p._ foot; _c._ body
cavity; _s._ shell-gland.]
The hypoblast has after its invagination either the form of a sack
(fig. 102) or of a solid mass (fig. 101). Whether the mouth be the
blastopore or no, the permanent oesophagus is formed of epiblast
cells, so that the oesophagus and buccal cavity are always lined by
epiblast. When the blastopore remains permanently open the outer part
of the oesophagus grows as a prominent ridge round the opening.
The mesenteric sack itself becomes differentiated into a stomach
adjoining the oesophagus, a liver opening immediately behind this,
and an intestine. The cells forming the hepatic diverticula and
sometimes also those of the stomach may during larval life secrete in
their interior peculiar albuminous products, similar to ordinary
food-yolk.
The proctodæum, except when it is the blastopore, arises later than
the mouth. It is frequently developed from a pair of projecting
epiblast cells symmetrically placed in the median ventral line behind
the foot. It eventually forms a very shallow invagination meeting the
intestine. Its opening is the anus. The anus, though at first always
symmetrical and ventral, subsequently, on the formation of the pallial
cavity, opens into this usually on the right and dorsal side.
In the cases where the hypoblast is not invaginated in the form of a
sack the formation of the mesenteron is somewhat complicated, and is
described in the sequel.
From the trochosphere stage the larva passes into what has been called
by Lankester the veliger stage (fig. 103), which is especially
characteristic of Gasteropod and Pteropod Mollusca.
The shell-gland (with a few exceptions to be spoken of subsequently)
of the previous stage flattens out, forming a disc-like area, on the
surface of which a delicate shell becomes developed, while the
epiblast of the edges of the disc becomes thickened. The disc-like
area is the mantle. The edge of the area and with it the shell
now rapidly extend, especially in a dorsal direction. Up to this time
the embryo has been symmetrical, but in most Gasteropods the shell and
mantle extend very much more towards the left than towards the right
side, and a commencement of the permanent spiral shell is thus
produced.
[FIG. 103. LARVÆ OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE.
(From Gegenbaur.)
A. and B. Earlier and later stage of Gasteropod. C. Pteropod
(Cymbulia). _v._ velum; _c._ shell; _p._ foot; _op._ operculum; _t._
tentacle.]
The edge of the mantle forms a projecting lip separating the dorsal
visceral sack from the head and foot. An invagination appears, usually
on the right in Gasteropods, and eventually extends to the dorsal side
(fig. 103 B). It gives rise to the pallial or branchial cavity, and
receives also the openings of the digestive, generative and urinary
organs. In most Pteropods it is also formed to the right, and usually
eventually extends afterwards towards the ventral surface (fig. 103
C). In the pallial cavity the gills are formed, in those groups in
which they are present, as solid processes frequently ciliated. They
are coated by epiblast and contain a core of mesoblast. They soon
become hollow and contractile.
The velum in the more typical forms loses its simple circular form,
and becomes a projecting bilobed organ, which serves the larva after
it is hatched as the organ of locomotion (fig. 103 B and C). The
extent of the development of the velum varies greatly. In the
Heteropods especially it becomes very large, and in Atlanta it becomes
six-lobed, each lateral half presenting three subdivisions. It is
usually armed on its projecting edge with several rows of long cilia,
and below this with short cilia which bring food to the mouth. It
persists in many forms for a very long period. Within the area of the
velum there appear the tentacles and eyes (fig. 103 B). The latter are
usually formed at the base of the tentacles.
The foot grows in most forms to a very considerable size. On its
hinder and dorsal surface is formed the operculum as a chitinos plate
which originates in a depression lined by thickened epiblast, much in
the same way as the shell (fig. 103 B and C, _op_). In the typical
larval forms it is only possible to distinguish the anterior flattened
surface of the foot for locomotion and the posterior opercular region,
but special modifications of the foot are found in the Pteropods and
Heteropods, which are described with those groups. The foot very often
becomes richly ciliated, and otic vesicles are early developed in it
(fig. 101, _ot_).
All the Gasteropods and Pteropods have a shell-bearing larval form
like that first described, with the exception of a few forms, such as
Limax and perhaps some other Pulmonata, in which the shell-gland
closes up and gives rise to an internal shell.
The subsequent metamorphosis in the different groups is very various,
but in all cases it is accompanied by the disappearance of the velum,
though in some cases remnants of the velum may persist as the
subtentacular lobes (Lymnæus, _Lankester_) or the lip tentacles
(Tergipes, _Nordmann_). In prosobranchiate Gasteropods the larval
shell is gradually added to, and frequently replaced by, a permanent
shell, though the free-swimming veligerous larva may have a long
existence. In many of the Opisthobranchiata the larval shell is lost
in the adult and in others reduced. Lankester, who has especially
worked at the early stages of this group, has shewn that the larvæ are
in almost every respect identical with those of prosobranchiate
Gasteropods. They are all provided with a subnautiloid shell, an
operculated foot, etc. The metamorphosis has unfortunately been
satisfactorily observed in but few instances. In Heteropods and
Pteropods the embryonic shell is in many cases lost in the adult.
The following sections contain a special account of the development in
the various groups of Gasteropoda and Pteropoda which will complete
the necessarily sketchy account of the preceding pages.
Gasteropoda. To illustrate the development of the Gasteropoda I have
given a detailed description of two types, viz. _Nassa mutabilis_ and
_Paludina vivipara_.
[FIG. 104. SEGMENTATION OF NASSA MUTABILIS. (From Bobretzky.)
A. Upper half divided into two segments. B. One of these has fused
with the large lower segment. C. Four small and one large segment,
one of the former fusing with the large segment. D. Each of the four
segments has given rise to a fresh small segment. E. Small segments
have increased to thirty-six.]
Nassa mutabilis. This form, the development of which has been very
thoroughly worked out by Bobretzky (No. 242), will serve as an example
of a marine Gasteropod with a large food-yolk. The segmentation has
already been described, p. 102. It will be convenient to take up the
development at a late stage of the segmentation. The embryo is then
formed of a cap of small cells which may be spoken of as the
blastoderm resting upon four large yolk cells of which one is
considerably larger than the others (fig. 104 A). The small and the
large cells are separated by a segmentation cavity. The general
features at this stage are shewn in fig. 105 A, representing a
longitudinal section through the largest yolk cell and a smaller yolk
cell opposite to it. The blastoderm is for the most part one cell
thick, but it will be noticed that, at the edge of the blastoderm
adjoining the largest yolk cell, there are placed two cells underneath
the edge of the blastoderm (_me_). _These cells are the commencement
of the mesoblast._ In the later stages of development the blastoderm
continues to grow over the yolk cells, and as it grows the three
smaller yolk cells travel round the side of the largest yolk cell with
it. As they do so they give rise to a layer of protoplasmic cells
(fig. 105, _hy_) which form a thickened layer at the edge of the
blastoderm and therefore round the lips of the blastopore. These cells
form the hypoblast. The whole of the protoplasmic matter of the yolk
cells is employed in the formation of the hypoblast. The rest of them
remains as a mass of yolk. A longitudinal section of the embryo at a
slightly later stage, when the blastopore has become quite narrowed,
is represented in fig. 105 C. The greater part of the dorsal surface
is not represented.
[FIG. 105. LONGITUDINAL SECTION THROUGH THE EMBRYO OF NASSA
MUTABILIS. (After Bobretzky.)
A. Stage when the mesoblast is commencing to be formed.
B. Stage when the yolk is half enclosed. The hypoblast is seen at
the lips of the blastopore.
C. Stage when the blastopore (_bp_) is nearly obliterated.
D. The blastopore is closed.
_ep._ epiblast; _me._ mesoblast; _hy._ hypoblast; _bp._ blastopore;
_in._ intestine; _st._ stomach; f. foot; _sg._ shell-gland; _m._
mouth.]
Two definite organs have already become established. One of these is a
pit lined by thickened epiblast on the posterior and dorsal side
(_sg_). This is the shell-gland. The other is the foot (_f_) which
arises as a ventral prominence of thickened epiblast immediately
behind the blastopore. The hypoblast forms a ring of columnar cells
round the blastopore. On the posterior side its cells have bent over
so as to form a narrow tube (_in_), the rudiment of the intestine.
In the next stage (fig. 105 D) the blastopore completely closes, but
its position is marked by a shallow pit (m) where the stomodæum is
eventually formed. The foot (_f_) is more prominent, and on its hinder
border is formed the operculum. The shell-gland (not shewn in the
figure) has flattened out, and its thickened borders commence to
extend especially over the dorsal side of the embryo. A delicate shell
has become formed. In front of and dorsal to the mouth, a ciliated
ring-shaped ridge of cells, which is however incomplete dorsally,
gives rise to the velum. On each side of the foot there appears a
protuberance of epiblast cells, which forms a provisional renal organ.
The hypoblast now forms a complete layer ventrally, bounding a cavity
which may be conveniently spoken of as the stomach (_st_), which is
open to the yolk above. Posteriorly however a completely closed
intestine is present, which ends blindly behind (_in_).
The shell and with it the mantle grow rapidly, and the primitive
symmetry is early interfered with by the shell extending much more
towards the left than the right. The anus soon becomes formed and
places the intestine in communication with the exterior.
With the growth of the shell and mantle the foot and the head become
sharply separated from the visceral sack (fig. 106). The oesophagus
(_m_) becomes elongated. The eyes and auditory sacks become formed.
[FIG. 106. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF NASSA
MUTABILIS. (After Bobretzky.)
_f._ foot; _m._ mouth; _ce.v._ cephalic vesicle; _st._ stomach.]
With further growth the asymmetry of the embryo becomes more marked.
The intestine takes a transverse direction to the right side of the
body, and the anus opens on the right side and close to the foot in
the mantle cavity which is formed by an epiblastic invagination in
this region. The cavity of the stomach (fig. 106, st) increases
enormously and passes to the left side of the body, pushing the
food-yolk at the same time to the right side, and the point where it
communicates with the intestine becomes carried towards the posterior
dorsal end of the visceral sack. The walls of the stomach gradually
extend so as to narrow the opening to the yolk. The part of it
adjoining the oesophagus becomes the true stomach, the remainder the
liver; its interior is filled with coagulable fluid.
Paludina. Paludina--Lankester (No. 263) and Bütschli (No. 244)--is a
viviparous form characterised by the small amount of food-yolk. The
hypoblast and epiblast cells are distinguished very early, but soon
become of nearly the same size.
In the later stages of segmentation the epiblast cells differ from the
hypoblast cells in the absence of pigment. The segmentation cavity, if
developed, is small. A perfectly regular gastrula is formed (fig. 107
A and B), which is preceded by the embryo assuming a flattened form.
The blastopore is at first wide, but gradually narrows, and finally
assumes a slightly excentric position. _It becomes not the mouth, but
the anus._
When the blastopore has become fairly narrow, mesoblast cells (B,
_me._) appear around it, between the epiblast and hypoblast. Whether
they are bilaterally arranged or no is not clear; and though coloured
like the hypoblast, their actual development from this layer has not
been followed.
[FIG. 107. FOUR STAGES IN THE DEVELOPMENT OF PALUDINA VIVIPARA.
(Copied from Bütschli.)
_ep._ epiblast; _hy._ hypoblast; _me._ mesoblast; _bl._ blastopore;
_an._ anus; _st._ stomodæum; _sh._ shell-gland; _V._ velum; _x._
primitive excretory organ.]
The velum appears about the same time as the mesoblast, in the form of
a double ring of ciliated cells at about the middle of the body (B and
C, _V_). The mesoblast rapidly extends so as to occupy the whole space
between the epiblast and hypoblast, and at the same time becomes
divided into two layers (C). Shortly afterwards a space--the body
cavity--appears between the two layers (D) which then attach
themselves respectively to the epiblast and hypoblast, and constitute
the somatic and splanchnic layers of mesoblast. The two layers remain
connected by transverse strands.
By a change in the relations of the various parts and especially by
the growth of the posterior region of the body, the velum now occupies
a position at the end of the body opposite the blastopore. Immediately
behind it there appear two organs, one on the dorsal and one on the
ventral side. That on the dorsal side (_sh_) is a deep pit--the
shell-gland--which is continuous with a layer of columnar epiblast
which ends near the anus. The other organ (_st_), situated on the
ventral side, is a simple depression, and is the rudiment of the
stomodæum. Between it and the dorsally placed anus is a slight
prominence--the rudiment of the foot. On the two sides of the body,
between the epiblast and hypoblast on a level with the shell-gland are
placed two masses of excretory cells, the provisional kidneys (D,
_x_). These are probably _not_ homologous with the provisional renal
organ of Nassa and other marine Prosobranchiata. At a later period a
ciliated cavity appears in them, which probably communicates with the
exterior at the side of the throat.
In the later stages the foot grows rapidly, and forms a very prominent
mass between the mouth and the anus. An operculum is developed
somewhat late in a shallow groove lined by thickened epiblast.
A provisional chitinous plug is formed in the shell-gland which soon
becomes everted. The shell is formed in the usual way on the everted
surface of the shell-gland. The thickened edge of this part becomes
the edge of the mantle, and soon projects in the neighbourhood of the
anus as a marked fold.
With the rapid growth of the larva the invaginated mesenteron becomes
relatively reduced in size. In its central part yolk-spherules become
deposited, while the part adjoining the blastopore (anus) becomes
elongated to give rise to the intestine. The stomodæum grows greatly
in length and joins the dorsal part of the archenteron which then
becomes the stomach. The part of the mesenteron with yolk-spherules
forms the liver. With the development of the visceral sack the anus
shifts its position. It first passes somewhat to the left, and is then
carried completely to the right.
The development of _Entoconcha mirabilis_ (Joh. Müller, No. 265), a
remarkable Prosobranchiate parasitic in the body cavity of Synapta,
which in the adult state is reduced to little more than an
hermaphrodite generative sack, deserves a short description. It is
viviparous, and the ovum gives rise to a larva which from the hardly
sufficient characters of the foot and shell is supposed to be related
to Natica.
There is nothing very striking in the development. The food-yolk is
scanty. The velum, as might be anticipated from the viviparous
development, is small. The tentacles are placed not within, but behind
the velar area. There is a nautica-like shell, a large mantle cavity,
and a large two-lobed foot.
In Buccinum, and Neritina only one out of the many ova included in
each egg-capsule develops. The rest atrophy and are used as food by
the one which develops.
Opisthobranchiata. It will be convenient to take a species of
Pleurobranchidium (Aplysia), observed by Lankester (No. 239), as a
type of Nudibranchiate development. The ovum first divides into two
segments, and from these small segments are budded off, which
gradually grow round and enclose the two large segments. The small
segments now form the epiblast.
At the aboral pole the epiblast becomes thickened and invaginated to
form the shell-gland, and shortly afterwards the velum and foot are
formed in the normal way, and a stomodæum appears close to the ventral
edge of the velum (fig. 101). The two yolk cells (_ry_) still remain
distinct, but a true hypoblastic layer (probably derived from them,
though this has not been made out) soon becomes established. Prominent
cells early make their appearance at the base of the foot, which
become at a later period invaginated to form the anus. Otolithic sacks
(_ot_) become formed in the foot, and the supraoesophageal ganglia
from a differentiation of the epiblast (_ng_).
At a later period the shell-gland becomes everted, and a nautiloid
shell developed. The alimentary tract becomes completed, though the
two yolk cells long retain their original distinctness. The
shell-muscle is developed, and peculiar pigmented bodies are formed
below the velum. The foot becomes prominent and acquires an operculum.
The metamorphosis of Tergipes has been more or less completely worked
out by Nordmann and by Schultze (No. 271).
In _Tergipes Edwardsii_ worked out by the former author, the larva
when hatched is provided with a large velum, eyes, tentacles, an
elongated operculated foot, and mantle. In the next stage both shell
and operculum are thrown off, and the body becomes elongated and
pointed behind. Still later a pair of gill-processes with hepatic
diverticula becomes formed.
The velum next becomes reduced, and two small processes, which give
rise to the lip tentacles and a second pair of gills, sprout out. An
ecdysis now takes place, and leads to further changes which soon
result in the attainment of the adult form.
In _Tergipes lacinulatus_, observed by Schultze, the velum atrophies
before the shell and operculum are thrown off.
Pulmonata. The development of the fresh-water Pulmonata appears from
Lankester's observations on the pond-snail (Lymnæus) to be very similar
in all important particulars to that of marine Branchiogasteropoda.
The velum is however less developed than in most marine forms. The
shell-gland, etc. have the normal development. In Lymnæus the
blastopore has an elongated form and it is still a matter of dispute
whether it closes at the mouth or anus.
In the Helicidæ there is a gastrula by epibole. The shell-gland, as
may be gathered from Von Jhering's figures, has the usual form, and an
external shell of the usual larval type is developed. There is a
ciliated process above the mouth, which extends into the lumen of the
mouth. This process is often regarded as a rudimentary velum, but
probably has not this value. There is no other organ which can be
homologous with the velum.
The development of Limax presents some peculiarities. The yolk-spheres
(hypoblast) form a large mass enclosed by the epiblast cells. A
shell-gland is formed in the usual situation, which however, instead
of being everted, as in ordinary forms, becomes closed, and in its
interior are deposited calcareous plates which give rise to the
permanently internal shell. The foot grows out posteriorly, and
contains a large provisional contractile vesicle, traversed by
muscular strands which contract rhythmically.
Although an external shell is present in Clausilia in the adult, the
shell-gland becomes closed in the embryo as in Limax, and an internal
plate-like shell is developed. The shell is at first covered by a
complete epithelium, which eventually gives way in the centre, leaving
covered only the edges of the shell. It thus comes about that the
original internal shell becomes an external one. It is very difficult
to bring this mode of development of the external shell into relation
with that of other forms. Clausilia like Limax develops a large pedal
sinus.
In both Limax and Clausilia cilia are early developed and cause a
rotation of the embryo, but how far they give rise to a distinct velum
is not clear.
Heteropoda. The Heteropod embryos present in their early development
the closest resemblance to those of other Gasteropods. The
segmentation takes place according to the most usual Gasteropod type;
(vide p. 99) and after the yolk cells have ceased to give origin to
epiblast cells they divide towards the nutritive pole, become
invaginated, and line a spacious archenteron. The epiblast cells at
the formative pole gradually envelop the yolk (hypoblast) cells, and
the blastopore very early narrows and becomes the permanent mouth.
Simultaneously with the narrowing of the blastopore, the shell-gland
is formed at the aboral pole, and the foot on the ventral side. The
velum appears as a patch of cilia on the dorsal side, which then
gradually extends ventrally so as to form a complete circle just
dorsal to the mouth.
The larva, after these changes have been completed, is represented in
fig. 102.
In later stages the shell-gland becomes everted, and a shell is
developed in all the forms both with and without shells in the adult.
The foot grows very rapidly, and an operculum is in all cases formed
behind. A bilobed invagination in front gives rise to the mucous
gland. The velum enlarges and becomes bilobed.
Though the blastopore remains permanently open as the mouth, the
oesophagus is formed as an epiblastic ingrowth. The rudiment of the
proctodæum appears as two epiblastic cells symmetrically placed behind
the foot, which subsequently pass to the right side, and give rise to
a shallow invagination which meets the mesenteric sack. In the latter
structure the cells of part of the wall develop a peculiar nutritive
material, and form a nutritive sack which eventually becomes the
liver. The part of the sack connected with the epiblastic oesophagus
becomes constricted off as the stomach. The remainder, which unites
with the proctodæum, forms the intestine.
The structural peculiarities of the adult are formed by a post-larval
metamorphosis. The caudal appendage of Pterotrachea and Firoloidea is
formed as an outgrowth of the upper border of the hind end of the
foot. The so-called fin arises as a cylindrical process in front of
the base of the foot, which is eventually flattened laterally. In the
Atlantidæ it is in some cases at first vermiform, and in other cases
attains directly its adult structure. The embryonic foot itself gives
rise in Pterotrachea, Firoloidea and Carinaria to the tail, on the
dorsal and posterior side of which the operculum may still be seen in
young specimens. In Atlanta it forms the posterior part of the foot on
which the operculum persists through life.
The embryonic shell is completely lost in Pterotrachea and Firoloidea,
and the shell is rudimentary in Carinaria. With its atrophy the mantle
region also becomes much reduced.
The velum is enormously developed in many Heteropods. In Atlanta it is
six-lobed, each of the two primitive lateral lobes being prolonged
into three processes, two in front, and one behind. As in all other
cases, it atrophies in the course of the post-larval metamorphosis.
[FIG. 108. EMBRYO OF CAVOLINIA (HYALEA) TRIDENTATA. (After Fol.)
_m._ mouth; _a._ anus; _s._ stomach; _i._ intestine; sigma.
nutritive sack; _mb._ mantle; _mc._ mantle cavity; _Kn._ contractile
sinus; _h._ heart; _r._ renal sack: _f._ foot; _pn._ epipodia; _q._
shell; _ot._ otolithic sack.]
Pteropoda. The early larval form of the Pteropods is closely similar
to that of marine Gasteropods. There are usually only three
hypoblastic spheres at the close of the segmentation in the
Thecosomata, and a somewhat larger number in the Gymnosomata. The
blastopore closes at the oral region, on the nutritive side of the
ovum, and the shell-gland is placed at the original formative pole.
The velum, shell-gland and foot have the usual relations. Although
many of the adult forms are symmetrical, there is very early an
asymmetry visible in the larva, shewing that the Pteropods are
descended from asymmetrical ancestors. In the Gymnosomata there is a
second larval stage after the loss of the shell when the larva is
provided with three rings of cilia (fig. 109). In most forms of
Pteropods the dorsal part of the body, covered by the mantle, is
produced into a visceral sack like that of the Cephalopoda (fig. 108).
The velum varies considerably in its development in different forms.
In the Hyaleidæ it is comparatively small and atrophies early; while
in Cymbulia (fig. 103) and the Gymnosomata it is large and bilobed,
and persists till after the foot has attained its full development.
The free edge of the velum is provided with long motor cilia, and its
lower border with small cilia which bring the food to the mouth. In
Cleodora there is a median bunch of cilia in the centre of the velum
like that in the Lamellibranchiata, Nudibranchiata, etc.
The shell-gland forms a pit at the aboral end of the body, and in
Cymbulia a chitinous plug appears to be normally formed in this pit.
The pit afterwards everts itself. The edge of the everted area becomes
thickened and gradually travels towards the anterior end of the body.
On this everted area a small plate is developed, which forms the
commencement of the embryonic shell with which the larvæ of all
Pteropods are provided.
The remainder of the embryonic shell is secreted in successive rings
by the thickened edge of the mantle, and grows with this till it
reaches the neck (fig. 108). The permanent shell is added
subsequently, usually on a very different model to the larval shell.
The fate of the embryonic shell is very various in different forms. In
the Hyaleidæ the animal withdraws itself from the larval shell, which
becomes shut off from the permanent shell by a diaphragm. The larval
shell then becomes detached.
In the Styliolidæ the permanent shell becomes twice the size of the
embryonic shell while the animal is still in an embryonic condition,
but the larval shell persists for life. In the Cymbulidæ there is an
embryonic and secondary shell, which persist together during larval
life. They are eventually cast off at the same time and replaced by a
permanent shell.
[FIG. 109. FREE-SWIMMING PNEUMODERMON LARVÆ. (After Gegenbaur,
copied from Bronn.)
The velum has atrophied in both larvæ.
In A three ciliated bands are present, and the auditory vesicles are
visible.
In B the tentacles with suckers and the epipodia have become
developed.
_an._ anus.]
In the Gymnosomata an embryonic shell is developed, and a secondary
shell added to it during embryonic life. Both are cast off before the
adult condition is attained. After the shell has been cast off three
ciliated rings are developed (fig. 109). The anterior of these is
placed between the velum and the foot, and the two hinder ones on the
elongated posterior part of the body.
The ciliated rings give to these larvæ a resemblance to Chætopod
larvæ; but there can be no doubt that this resemblance is a purely
superficial one. The anterior ring atrophies early (fig. 109 B), and
the second one soon follows suit. It is probable that the hindermost
one does not persist through life, although it has been observed in
forms with fully developed sexual organs. Most of these larvæ have not
been traced to their adult forms. They have been referred to
Pneumodermon, Clio, etc.
The most characteristic organ of the Pteropods is the foot, which is
prolonged into two enormous lateral wings, the epipodia. These develop
at different periods in different larvæ, but are always distinct
lateral outgrowths of the foot.
In the Hyaleidæ the foot is early conspicuous, and soon sends out two
lateral prolongations (fig. 108 _pn._) which develop with enormous
rapidity as compared with the medium portion, and give rise to the
epipodia. The whole of the foot becomes ciliated.
In the Cymbulidæ, though not in other forms, an operculum is developed
on the hinder surface of the foot (fig. 103 C). The epipodia are late
in appearing.
In the Gymnosomata the foot is developed very early, but remains
small. The epipodia do not appear till very late in larval life (fig.
109 B).
In Pneumodermon and some other Gymnosomata there appear on the hinder
part of the head peculiar tentacles with suckers like those of the
Cephalopoda (fig. 109 B). It is not certain that these tentacles are
genetically related to the arms of the Cephalopoda.
Cephalopoda. The eggs of the Cephalopoda are usually laid in special
capsules formed in the oviduct, which differ considerably in the
different members of the group.
In the case of Argonauta each egg is enveloped in an elongated capsule
provided with a stalk. By means of the stalk the eggs are attached
together in bunches, and these again are connected together and form
transparent masses, which are placed in the back of the shell. In
octopus the eggs are small and transparent: each of them is enclosed
in a stalked capsule. In Loligo the eggs are enveloped in elongated
sack-like gelatinous cords, each containing about thirty or forty
eggs. The cords are attached in bunches to submarine objects. In Sepia
each egg is independently enveloped in a spindle-shaped black capsule,
which is attached to a stone or other object.
In a decapod form with pelagic larvæ, described by Grenacher (No.
280), the eggs were enclosed in a somewhat cylindrical gelatinous
mass. In each mass there were an immense number of eggs arranged in
spirals. Each ovum was enclosed in a structureless membrane, within
which it floated in a colourless albumen.
The ovum itself within the capsule is a nearly homogeneous granular
mass, without a distinct envelope. Development commences by the
segregation, at the narrow pole of the ovum opposite the egg-stalk, of
the greater part of the protoplasmic formative material[103]. This
material forms a disc equivalent to the germinal disc of meroblastic
vertebrate ova. The germinal disc in Sepia and Loligo does not,
however, undergo a quite symmetrical segmentation (Bobretzky, No.
279). When eight segments are present, two of them close together are
much smaller and narrower than the remainder; and when, in the
succeeding stages small segments are formed from the inner ends of the
large ones, those derived from the two smaller segments continue to be
smaller than the remainder: so that throughout the segmentation one
pole of the blastoderm is formed of smaller segments, and the
blastoderm exhibits a bilateral symmetry[104]. The partial
segmentation results in the formation of a blastoderm covering one
pole of the egg, but, unlike the vertebrate blastoderm, formed of a
single row of cells. This blastoderm very soon becomes two or three
cells deep at its edge, and the cells below the surface constitute the
layer from which the mesoblast and hypoblast originate (fig. 110
_ms_). The origin of the mesoblast at the edge of the blastoderm is a
phenomenon equivalent to its origin at the lips of the blastopore in
so many other types. The external layer forms the epiblast.
[103] In Octopus and Argonauta (Lankester) as soon as the
blastoderm is completed the egg reverses its position in the
egg-shell; the cleavage pole taking up a position nearest the
stalk.
[104] I do not know the relation of this axis of symmetry to the
future embryo.
[FIG. 110. SECTION THROUGH THE BLASTODERM OF A LOLIGO OVUM AT THE
BEGINNING OF THE FOURTH DAY. (After Bobretzky.)
_ms._ mesoblast; _d._ cell at the edge of the blastoderm; _c._ one
of the segmentation cells.]
The whole blastoderm does not take its origin from the segmentation
spheres, but, as was discovered by Lankester (282), a number of nuclei
arise spontaneously in the yolk outside the blastoderm, around which
cell bodies become subsequently formed. They make their appearance
near to, but not at the surface, extending first in a ring-like series
in advance of the margin of the blastoderm, but subsequently appearing
indiscriminately over all parts of the egg. They take no share in
forming the epiblast, but would seem, according to Lankester, to
assist in giving rise to the lower layer cells, and also to a layer of
flattened cells which eventually completely encloses the yolk, and may
be called the yolk membrane. The cells of the yolk membrane first of
all appear at the thickened edge of the blastoderm. From this point
they spread inwards under the centre of the blastoderm (fig. 115
_m´_), and, together with the epiblast cells, outwards over the yolk
generally; so that before long (on the tenth day in Loligo) the yolk
becomes completely invested by a membrane of cells.
In the non-germinal region the blastoderm is formed of two layers, (1)
a flattened epiblast, and (2) the yolk membrane. In the region of the
original germinal disc the epiblast cells become columnar, and below
them is placed a ring of lower layer cells, which gradually extends
towards the centre so as finally to form a complete layer. Below this
again comes the yolk membrane just spoken of.
Before describing the further fate of the separate layers it is
necessary to say a few words as to the external features of the
embryo. In the adult Cephalopod it is convenient, for the sake of
comparison with other Mollusca, to speak of the narrow space enclosed
in the arms, which contains the mouth, as the ventral surface; the
aboral apex as the dorsal surface; and what is usually called the
upper surface as the anterior and the lower one as the posterior.
Employing this terminology the centre of the original blastoderm is
the dorsal apex of the embryo. In the typical forms with a large
yolk-sack the whole embryo is formed out of the original germinal
disc; the part of the blastoderm which is continued as a thin layer
over the remainder of the egg forms a large ventral yolk-sack appended
to the head of the embryo. The following description applies
especially to two types, which form the extremes of the series in
reference to the development of the yolk-sack. The first of these with
a large yolk-sack is Sepia, of which Kölliker in his classical memoir
(No. 281) has published a series of beautiful figures. The second,
with a small yolk-sack, is the pelagic larva of an unknown adult
described by Grenadier (No. 280).
In a young blastoderm of Sepia viewed from the dorsal surface, a
series of structures appear which are represented in fig. 111 A. In
the middle is a somewhat rhomboid prominence which forms the rudiment
of the mantle (_mt_). In its centre is a pit which forms the
shell-gland. On each side of the mantle is a somewhat curved fold
(_f_). These folds eventually coalesce to form the funnel. They are
divided into two parts by a small body which forms the cartilage of
the funnel. The smaller part of the fold behind this body gives rise
to the true funnel, the part in front becomes (Kölliker) the strong
muscle connecting the funnel with the neck-cartilage. In front and to
the sides are two kidney-shaped bodies (_oc_), the optic pits. Behind
the mantle are two buds (_br_), the rudiments of the gills.
[FIG. 111. TWO SURFACE VIEWS OF THE GERMINAL DISC OF SEPIA. (After
Kölliker.)
_mt._ mantle; _oc._ eye; _f._ folds of funnel; _br._ branchiæ; _an._
posterior portion of alimentary tract; _m._ mouth. 1, 2, 3, 4, 5,
arms; _p._ cephalic lobe.]
In the somewhat later stage rudiments of the two posterior pairs of
arms make their appearance outside and behind the rudiments of the
funnel. The head is indicated by a pair of lateral swellings on each
side, the outer of which carries the eyes. The whole embryo now
becomes ciliated, though the ciliation does not cause the usual
rotation. At a slightly later stage the second, third, and fourth
pairs of arms make their appearance slightly in front of those already
present. The posterior parts of the funnel rudiments approach each
other, and the anterior meet the rudiments of the neck-cartilage. The
gills begin to be covered by the mantle-edge, which now projects as a
marked fold. At a slightly later period two fresh rudiments may be
noted, viz. the oral (fig. 111 B, _m_) and anal invaginations, the
latter of which is extremely shallow and appears at the apex of a
small papilla which may be spoken of as the anal papilla. These
invaginations appear at the two opposite poles (anterior and
posterior) of the blastoderm. Shortly after this the rudiment of the
first pair of arms arises considerably in front of the other
rudiments, at the sides of the outer pair of cephalic swellings (fig.
111 B, 1).
Fig. 111 B represents a view from the dorsal surface of an embryo at
this stage. In the centre is the mantle with the shell-gland which is
now very considerably raised beyond the general surface. Concentric
with the edge of the mantle are the two halves of the funnel, the
anterior half meeting the dorsal or neck-cartilage and the posterior
halves approaching each other. The oral invagination is shewn at _m_
and the anal immediately in front of _an_. The gills, nearly covered
by the mantle, are seen at _br_. At _p_ are the cephalic swellings,
and the eye is seen at _oc_. The arms 1-5 form a ring outside these
parts. The whole of the embryo, with the exception of the gills, the
funnel, and the outer border of the blastoderm, is richly ciliated.
The embryo up to this time has had the form of a disc or saucer on the
surface of the yolk. After this stage it rapidly assumes its permanent
dome-like form, and becomes at the same time folded off from the yolk.
The blastoderm is very slow in enveloping the yolk, and the whole yolk
is not completely invested till a considerably later stage than that
represented in fig. 111 B. As soon as the blastoderm covers the
yolk-sack cilia appear upon it. The mantle grows very rapidly, and its
free border soon projects over the funnel and gills. After the two
halves of the funnel have coalesced into a tube, it comes to project
again beyond the edge of the mantle.
On the completion of the above changes the resemblance of the embryo
to a Cuttle-fish becomes quite obvious. Three of the stages in the
accomplishment of these changes are represented in fig. 112.
[FIG. 112. SIDE VIEWS OF THREE LATE STAGES IN THE DEVELOPMENT OF
SEPIA. (After Kölliker.)
_m._ mouth; _yk._ yolk-sack; _oc._ eye; _mt._ mantle.]
To the ventral side of the embryo is attached the enormous external
yolk-sack (_yk_), which is continuous with an internal section
situated within the body of the embryo. The general relations of the
embryo to the yolk will best be understood by reference to the
longitudinal section of Loligo, fig. 127.
The arms gradually increase in length, and the second pair passes in
front of the first so as eventually to lie completely in front of the
mouth. The arms thus come to form a complete ring surrounding the
mouth, of which the original second pair, and not, as might be
anticipated, the first, completes the circle in front. The second pair
develops into the long arms of the adult.
After the embryo has attained more or less completely its definite
form (fig. 112 C) it grows rapidly in size as compared with the
yolk-sack. The latter structure is at first four or five times as big
as the embryo, but, by the time of hatching, the embryo is two to
three times as big as the yolk-sack.
Loligo mainly differs from Sepia in the early enclosure of the yolk by
the blastoderm, and in the embryo exhibiting the phenomena of rotation
within the egg-capsule so characteristic of other Mollusca.
In Argonauta the yolk-sack is still smaller than in Loligo, and the
yolk is early completely enclosed by the blastoderm. A well-developed
outer yolk-sack is present during early embryonic life, but is
completely absorbed within the body before its close. Cilia appear on
the blastoderm very early, but vanish again when the yolk is about
two-thirds enclosed. There is, during embryonic life, no trace of a
shell, but the mantle and other parts of the body become covered by
peculiar bunches of fine setæ. The shell-gland develops normally in
both Octopus and Argonauta, but disappears again without closing up to
form a sack (Lankester).
The pelagic Decapod larva described by Grenacher, which forms my
second type, must be placed with reference to the development of the
yolk-sack at the opposite pole to Sepia. Segmentation, as in other
Cephalopods, is partial, but the blastoderm almost completely envelops
the yolk before any organs are developed; and no external yolk-sack is
present. At a stage slightly before the closure of the yolk-blastopore
the mantle is formed as a slight prominence at the blastodermic pole
of the egg, and even at this early stage is marked by the presence of
chromatophores. The edge of the blastoderm is ciliated. At a slightly
later stage the embryo becomes more cylindrical, the edge of the
mantle becomes marked by a fold, which divides the embryo transversely
into two unequal parts, a smaller region covered by the mantle, and a
larger region beyond this. The yolk is still exposed, but rudiments of
the optic pit and of two pairs of arms have appeared. The first-formed
arms are apparently the anterior, and not, as in Sepia, the posterior.
At a still later stage, represented in lateral and posterior views in
fig. 113 A and B, considerable changes are effected. The
yolk-blastopore is nearly though not quite closed. The mantle fold
(_mt_) is much more prominent, and on the posterior side on a level
with its edge may be seen the rudiments of the gills (_br_). The
funnel is formed as two independent folds on each side (_inf1_ and
_inf2_), which apparently correspond with the two divisions of the
funnel rudiments in Sepia. The eye has undergone considerable changes.
Close to each rudiment of the funnel may be seen a fresh sense
organ--the auditory sack (_ac_). The ventral (upper in the figure) end
of the body now forms a marked protuberance, probably equivalent to
the foot of other Mollusca (_vide_ p. 225), at the sides of which are
seen the rudiments of the arms (1, 2, 3). To the two previously
present a third one, on the posterior side, has been added. The
blastopore is placed on the anterior side of the ventral protuberance,
and immediately dorsal to this is an invagination (_os_) which gives
rise to the stomodæum. The ciliation at the edge of the blastopore
still persists, but does not lead to the rotation of the embryo.
In later stages (fig. 113 C) the blastopore becomes closed, and the
mantle region increases in length as compared with the remainder of
the body. The ventral halves of the funnel, each in the form of a half
tube, coalesce together to form a single tube (_inf_) in the same
manner as in Sepia. A shallow proctodæum (_an_) is formed between the
two branchiæ. The eyes (_oc_) stand out as lateral projections, and
the arms become much longer.
[FIG. 113. THREE EMBRYOS OF A CEPHALOPOD WITH A VERY SMALL
YOLK-SACK. (After Grenacher.)
_a._ blastopore; _br._ branchiæ; _inf.1_ and _inf.2_ posterior and
anterior folds of the funnel; _g.op._ optic ganglion (?); _oc._
eye; _wk._ white body; _ac._ auditory pit; _os._ stomodæum; _an._
anus; _mt._ mantle; 1, 2, 3. 1st, 2nd, and 3rd pairs of arms.
Still later a fourth pair of arms is added as a bud from each of the
posterior pair, and with the growth in length of the arms the suckers
make their appearance. The mouth is gradually carried up so as to be
surrounded by the arms. The ciliation of the surface becomes more
extensive.
During the whole of the above development the interior of the embryo
is filled with yolk, although no external yolk-sack is present. The
internal yolk-sack falls into three sections; a cephalic section, a
section in the neck, and an abdominal section. Of these, that in the
neck is the first to be absorbed. The cephalic portion fills out the
ventral protuberance already spoken of. The hinder section becomes
occupied by the liver which exactly fits itself into this space as it
absorbs the material previously there.
It will be convenient at this point to complete the account of the
Cephalopoda by a short history of their germinal layers, and by a
fuller description of the mantle, shell, and funnel than that given in
the preceding pages.
[FIG. 114. LONGITUDINAL VERTICAL SECTION THROUGH A LOLIGO OVUM WHEN
THE MESENTERIC CAVITY IS JUST COMMENCING TO BE FORMED. (After
Bobretzky.)
_gls._ salivary gland; _brd._ sheath of radula; _oe._ oesophagus;
_ds._ yolk-sack; _chs._ shell-gland; _mt._ mantle; _pdh._
mesenteron; _x._ epiblastic thickening between the folds of the
funnel.]
It has already been shewn that in the region of the germinal disc a
thick layer of cells becomes interposed between the epiblast and the
yolk membrane. This layer (fig. 115 _m_) is mainly mesoblastic, but
also contains the elements which form the lining of the alimentary
tract. Its cells first become differentiated into mesoblast and
hypoblast after the shell-gland has become a fairly deep pit. The mode
of differentiation is shewn in fig. 114. On the posterior side of the
mantle, at the point marked in fig. 111 B, _an_, a cavity is formed
between the yolk membrane and the mesoblast cells (fig. 114, _pdh_).
This cavity is the commencement of the anal extremity of the
mesenteron, and the columnar cells lining it constitute the hypoblast.
The remainder of the lower layer cells are the mesoblast. The
mesenteron gradually extends itself till it meets the stomodæum (fig.
127). The proctodæum is formed as a shallow pit close to the first
formed part of the mesenteron.
The mesoblast gives rise not only to the organs usually formed in this
layer, but also to the nervous centres, etc.
The mantle and shell. The mantle first arises as a thickening of the
epiblast on the dorsal surface of the embryo. The thickened
integument, with the subjacent mesoblast, soon forms a definite
projection, in the centre of which appears a circular pit (figs. 114
_chs_ and 115 _shs_). This pit, which has already been spoken of as
the shell-gland, resembles very closely the shell-gland of other
Mollusca. The fold around the edge of the shell-gland grows inwards so
as gradually to circumscribe its opening, which before long becomes
completely obliterated; and the gland forms a closed sack lined by
epiblast which grows in an anterior direction (figs. 114 and 127
_cch_).
[FIG. 115. DIAGRAM OF A VERTICAL SECTION THROUGH THE MANTLE REGION
OF AN EMBRYO LOLIGO. (From Lankester.)
[This figure is turned the reverse way up to fig. 114.]
_ep._ epiblast; _y._ food-yolk; _m._ mesoblast; _m´._ cellular yolk
membrane; _shs._ shell-gland.]
The edges of the mantle now begin to project, especially on the
posterior side (fig. 127), and within the cavity formed by this
projecting lip there are placed the anus (_an_), gills, etc. The
projecting lip of the mantle is formed both of epiblast and mesoblast.
The whole of the anterior side of the mantle is filled by the
elongated shell-sack (_cch_), within which the shell or pen soon
becomes secreted.
There are certain difficulties in comparing the shell-gland of the
Cephalopoda with that of other Mollusca which will best be rendered
clear by the following quotation from Lankester[105]:
[105] "Development of Pond-Snail." _Quart. J. of Micro.
Science_, 1874, pp. 371-374.
"The position and mode of development of the shell-gland of the
Cephalopoda exactly agree with that of the shell-gland as seen in the
other Molluscan embryos figured in this paper. We are therefore fairly
entitled to conclude from the embryological evidence that the pen-sack
of Cephalopoda is identical with the shell-gland of other Mollusca.
"But here--forming an interesting example of the interaction of the
various sources of evidence in genealogical biology--palæontology
crosses the path of embryology. I think it is certain that if we
possessed no fossil remains of Cephalopoda the conclusion that the
pen-sack is a special development of the shell-gland would have to be
accepted.
"But the consideration of the nature of the shell of the Belemnites
and its relation to the pen of living Cuttle-fish brings a new light
to bear on the matter. Reserving anything like a decided opinion as to
the question in hand, I may briefly state the hypothesis suggested by
the facts ascertained as to the Belemnitidæ. The complete shell of a
Belemnite is essentially a straightened nautilus-shell (therefore an
external shell inherited from a nautilus-like ancestor), which, like
the nautiloid shell of _Spirula_, has become enclosed by growths of
the mantle, and unlike the shell of _Spirula_, has received large
additions of calcareous matter from those enclosing overgrowths. On
the lower surface of the enclosed nautilus-shell of the Belemnite--the
phragmacone--a series of layers of calcareous matter have been thrown
down forming the guard; above, the shell has been continued into the
extensive chamber formed by the folds of the mantle, so as to form the
flattened pen-like pro-ostracum of Huxley.
"Whether in the Belemnites the folds of the mantle which thus covered
in and added to the original chambered shell, were completely closed
so as to form a sack or remained partially open with contiguous flaps
must be doubtful.
"In _Spirula_ we have an originally external shell enclosed but not
added to by the enclosing mantle sack.
"In _Spirulirostra_, a tertiary fossil, we have a shell very similar
to that of _Spirula_, with a small guard of laminated structure
developed as in the Belemnite (see the figures in Bronn _Classen u.
Ordnungen des Thierreichs_).
"In the Belemnites the original nautiloid shell is small as compared
with _Spirulirostra_. It appears to be largest in Huxley's genus
_Xiphoteuthis_. Hence in the series _Spirula_, _Spirulirostra_,
_Xiphoteuthis_, _Belemnites_, we have evidence of the enclosure of an
external shell by growths from the mantle (as in Aplysia), of the
addition to that shell of calcareous matter from the walls of its
enclosing sack, and of the gradual change of the relative proportions
of the original nucleus (the nautiloid phragmacone) and its superadded
pro-ostracal and rostral elements tending to the disappearance of the
nucleus (the original external shell). If this view be correct as to
the nature of these shells, it is clear that the shell-gland and its
plug has nothing to do with them. The shell-gland must have preceded
the original nautiloid shell, and must be looked for in such a
relation whenever the embryology of the pearly Nautilus can be
studied. Now, everything points to the close agreement of the
Belemnitidæ with the living Dibranchiata. The hooklets on the arms,
the ink bag, the horny jaws, and general form of the body, leave no
room for doubt on that point; it is more than probable that the living
Dibranchiata are modified descendants of the mesozoic Belemnitidæ. If
this be so, the pens of _Loligo_ and _Sepia_ must be traced to the
more complex shell of the Belemnite. This is not difficult if we
suppose the originally external shell the phragmacone, around which as
a nucleus the guard and pro-ostracum were developed, to have finally
disappeared. The enclosing folds of the mantle remain as a sack and
perform their part, producing the chitino-calcareous pen of the living
Dibranch, in which parts can be recognised as corresponding to the
pro-ostracum, and probably also to the guard of the Belemnite. If this
be the case, if the pen of _Sepia_ and _Loligo_ correspond to the
entire Belemnite shell minus the phragmacone-nucleus, it is clear that
the sack which develops so early in _Loligo_ and which appears to
correspond to the shell-gland of the other Molluscs cannot be held to
do so. The sack thus formed in _Loligo_ must be held to represent the
sack formed by the primæval upgrowth of mantle folds over the young
nautiloid shell of its Belemnitoid ancestors, and has accordingly no
general significance for the whole Molluscan group, but is a special
organ belonging only to the Dibranchiate stem, similar to--but not
necessarily genetically connected with--the mantle fold in which the
shell of the adult _Aplysia_ and its congeners is concealed. The pen,
then, of Cephalopods would not represent the plug of the shell-gland.
In regard to this view of the case, it may be remarked that I have
found no trace in the embryonic history of the living Dibranchiata of
a structure representing the phragmacone; and further, it is possible,
though little importance can be attached to this suggestion, that the
Dibranchiate pen-sack, as seen in its earliest stage in the embryo
_Loligo_, etc., is fused with the surviving remnants of an embryonic
shell-gland. When the embryology of _Nautilus pompilius_ is worked
out, we shall probably know with some certainty the fate of the
Molluscan shell-gland in the group of the Cephalopoda."
The funnel. The general development of the funnel has already been
sufficiently indicated. The folds of which it is formed are composed
both of epiblast and mesoblast. The mesoblast of the anterior part of
each half of the funnel would appear to give rise to a muscle passing
from the cartilage of the neck to the funnel proper. The posterior
parts gradually approximate, but meet in the first instance ventrally.
The two folds at first merely form the side of a groove or imperfect
tube (fig. 113 C and 124 ff.), but soon the free edges unite and so
give rise to a perfect tube, the primitive origin of which by the
coalescence of two halves would not be suspected. In Nautilus the two
halves remain permanently separate but overlap each other, so as to
form a functional tube.
[FIG. 116.
I. CHITON WOSSNESSENSKII. (After Middendorf.)
II. CHITON DISSECTED to shew _o._ the mouth; _g._ the nervous ring;
_ao._ the aorta; _c._ the ventricle; _c´._ an auricle; _br._ the
left branchiæ; _od._ oviducts. (After Cuvier.)
III., IV., V. STAGES OF DEVELOPMENT OF CHITON CINEREUS. (After
Lovén.)
The figure is taken from Huxley.]
Polyplacophora. The external characters of the embryo of Chiton have
long been known through the classical observations of Lovén (No. 285),
while the formation of the layers and the internal phenomena of
development have recently been elucidated by Kowalevsky (No. 284). The
eggs are laid in April, May, and June, and are enclosed in a kind of
chorion with calcareous protuberances. The segmentation remains
regular till sixty-four segments are formed. The cells composing the
formative half of the ovum then divide more rapidly than the
remainder; there is in this way formed an elongated sphere, half of
which is composed of small cells and half of larger cells. In the
interior is a small segmentation cavity. From its eventual fate the
hemisphere of the smaller cells may be called the anterior pole, and
that of the larger cells the posterior. An involution of the cells at
the apex of the posterior pole (though not of the whole hemisphere of
larger cells) now takes place, and gives rise to the archenteron. At
the same time an equatorial double ring of large cells appears on the
surface between the two poles, which becomes ciliated and forms the
velum. At the apex of the anterior pole a tuft of cilia, or at first a
single flagellum, is established (fig. 116 III. and IV.).
In the succeeding developmental period the blastopore, which has so
far had the form of a circular pore at the posterior extremity of the
body, undergoes a series of very remarkable changes. In conjunction
with a gradual elongation of the larva it travels to the ventral side,
and is prolonged forwards to the velum as a groove. The middle part of
the groove is next converted into a tube, which opens externally in
front, and posteriorly communicates with the archenteron. The walls of
this tube subsequently fuse together, obliterating the lumen, and
necessarily causing at the same time the closure of the blastopore.
The tube itself becomes thereby converted into a plate of cells on the
ventral surface between the epiblast and the hypoblast[106].
[106] There is a striking similarity between the changes of the
blastopore in Chiton and the formation of the neurenteric canal
of Chordata; especially if Kowalevsky is correct in stating that
the pedal nerves are developed from the ventral plate.
While the above changes have been taking place the mesoblast has
become established. It is derived from the lateral and ventral cells
of the hypoblast.
After the establishment of the germinal layers the further evolution
of the larva makes rapid progress. A transverse groove is formed
immediately behind the velum, which is especially deep on the ventral
surface; and the stomodæum is formed as an invagination of the
anterior wall of the deeper section of the groove. Behind the
stomodæum the remainder of the ventral surface grows out as a
flattened foot.
The dorsal surface behind the velum constitutes the mantle, and
becomes divided by six or seven transverse grooves into segment-like
areas, which may be called mantle plates (fig. 116 IV.). These areas
would seem (?) to correspond to so many flattened out shell-glands.
Immediately behind the velum the eyes appear as two black spots (fig.
116 IV.).
While the above external changes take place the archenteron undergoes
considerable modifications. Its anterior section gives rise, according
to Kowalevsky, to a dorsal (?) sack in which the radula is formed;
while the liver arises from it as two lateral diverticula.
From the above statements it would appear that Kowalevsky holds that
the oesophagus and radula sack are both derived from the walls of the
archenteron and not from the stomodæum. Such an origin for these
organs is without parallel amongst Mollusca.
The larva becomes about this time hatched, and after swimming about
for some time attaches itself by the foot, throws off its larval
organs, cilia, etc., and develops the shell.
The shell appears first of all during larval life in the form of
spicula on the middle and sides of the head, and later on the middle
and sides of the post-oral mantle plates (fig. 116 V.). The permanent
shell arises somewhat later as a series of median and lateral
calcareous plates, first of all on the posterior part of the velar
area, and subsequently on the mantle plates behind. The three
calcareous patches of each plate fuse together and give rise to the
permanent shell plates. The original spicula are displaced to the
sides, where they partly remain, and are partly replaced by new
spicula.
The nervous system is formed during larval life as four longitudinal
cords:--two lateral--the branchial cords, and two ventral--the pedal.
Paired anterior thickenings of the pedal cords meet in front of the
mouth to form the oesophageal ring. The pedal cords and their
derivatives are believed by Kowalevsky to be developed from the
lateral parts of the plate formed by the metamorphosis of the
blastopore. The median part of the plate is still visible after the
formation of these parts.
The chief peculiarity of the larva of Chiton (apart from the peculiar
ventral plate) consists in the elongation and dorsal segmentation of
the posterior part of the body. The velum has the normal situation and
relation to its mouth. The position of the eyes behind it is however
abnormal.
The elongation and segmentation of the posterior part of the trunk is
probably to be regarded as indicating that Chiton has early branched
off from the main group of the Odontophora along a special line of its
own, and _not_ that the remaining Odontophora are descended from
Chiton-like ancestral forms. The shell of Mollusca on this view is not
to be derived from one of the plates of Chiton, but the plates of
Chiton are to be derived from the segmentation of a primitive simple
shell. The segmentation exhibited is of a kind which all the
trochosphere larval forms seem to have been capable of acquiring. The
bilateral symmetry of Chiton, which is quite as well marked as that of
the Lamellibranchiata, indicates that it is a primitive phylum of the
Odontophora.
Scaphopoda. The external characters of the peculiar larva of this
interesting group have been fully worked out by Lacaze Duthiers (No.
286).
The segmentation is unequal and conforms to the usual molluscan type.
At its close the embryo becomes somewhat elongated, and there appears
on its surface a series of transverse ciliated rings. As soon as these
become formed the larva is hatched, and swims about by means of its
cilia. Six ciliated bands are formed in all, and in addition a tuft of
cilia is formed in a depression at the anterior extremity.
The larva thus constituted is very different in appearance to the
larvæ already described, and its parts very difficult to identify; the
next stages in the development shew however that the whole region of
the body taken up by the ciliated rings is part of the velar area,
while the small papilliform region behind is the post-velar part of
the embryo. This latter part grows rapidly, and at the same time the
ciliated rings become reduced to four; which gradually approach each
other, while the region on which they are placed grows in diameter.
The rings finally unite, and form a single ring on a projecting velar
ridge. In the centre of this ring is placed the terminal tuft of cilia
on a much reduced prominence.
By the time that these changes have been effected in the velum, the
post-velar part of the embryo has become by far the largest section of
the embryo, so that the velum forms a projecting disc at the front end
of an elongated body. The mantle is formed as two lateral outgrowths
near the hinder extremity of the body which leave between them a
ventral groove lined by cilia; on their dorsal side is formed a
delicate shell. The mantle lobes continue to grow, and by the time the
above changes in the velum are effected they meet and unite in the
ventral line and convert the groove between them into a complete tube
open in front and behind. A stream of water is driven through this
tube by the action of the cilia. The shell, which is at first
disc-shaped like the shell of other molluscan larvæ, moulds itself
upon the mantle and is so converted into a tube. At the front end of
the mantle tube, which does not at first cover the velum, there is
formed the foot. It arises as a protuberance of the ventral wall of
the body, which rapidly grows forwards, becomes trilobed as in the
adult, and ciliated.
On the completion of these changes the larva mainly differs in
appearance from the adult by the projection of the velum beyond the
edge of the shell. The velum soon however begins to atrophy; and the
larva sinks to the bottom. The mantle tube and shell grow forward and
completely envelop the velum, which shortly afterwards disappears. The
mouth is formed on the ventral side of the velum at the base of the
foot; at its sides arise the peculiar tentacles so characteristic of
the adult Dentalium.
LAMELLIBRANCHIATA.
The larvæ of Lamellibranchiata have in a general way the same
characters as those of Gasteropods and Pteropods. A trochosphere stage
with a velum but without a shell is succeeded by a veliger stage with
a still more developed velum, a dorsal shell, and a ventral foot.
The segmentation is unequal, and in a general way like that of
Gasteropoda, but the specially characteristic Gasteropodan type with
four large yolk spheres is only known to occur in Pisidium, and a type
of segmentation similar to that of Anodon (p. 100) appears to be the
most frequent.
There is an epibolic or embolic gastrula, but the further history of
the formation of the germinal layers has been worked out so
imperfectly, and for so few types, that it is not possible to make
general statements about it. What is known on this head is mentioned
in connection with the description of the development of special
types.
The blastopore in some cases closes at the point where the anus
(Pisidium), and probably in other cases where the mouth, is eventually
formed. In Anodon it is stated to close at a point corresponding
neither with the mouth nor the anus, but on the dorsal surface!
The embryo assumes a somewhat oval form, and in the free marine forms
there appears very early in front of the mouth a well-developed velum.
This is formed according to Lovén from two papillæ, and takes the form
of a circular ridge armed with long cilia. In the centre of the velar
area there is usually present a single long flagellum (fig. 117 B and
C). The velum never becomes bilobed.
[FIG. 117. THREE STAGES IN THE DEVELOPMENT OF CARDIUM. (After
Lovén.)
_hy._ hypoblast; _b._ foot; _m._ mouth; _an._ anus; _V._ velum;
_cm._ anterior adductor muscle.]
In the later stages, after the development of the shell, the velum
becomes highly retractile and can be nearly completely withdrawn
within the mantle by special muscles. It forms the chief organ of
locomotion of the free larva.
In some fresh-water forms, which have no free larval existence, the
velum is very much reduced (Anodon, Unio, Cyclas) or even aborted
(Pisidium). In these forms as well as in Teredo and probably other
marine forms (_e.g._ Ostrea) the central flagellum is absent. It has
been suggested by Lovén, though without any direct evidence, that the
labial tentacles of adult Lamellibranchiata are the remains of the
velum. The velar area is in any case the only representative of the
head. In some marine forms a general covering of cilia arises before
the formation of the velum; and in Montacuta and other types there is
developed, as in many Gasteropoda, a circumanal patch of cilia.
A shell-gland appears at a very early period on the dorsal surface in
Pisidium, Cyclas and Ostrea, and probably in most marine forms (fig.
118, _shs_). It is somewhat saddle-shaped, and formed of elongated
non-ciliated cells bounding a groove. It flattens out and on its
surface is formed the shell, which appears usually to have the form of
an unpaired saddle-shaped cuticle, on the two sides of which the
valves are subsequently formed by a deposit of calcareous salts. In
Pisidium the two valves are stated by Lankester to be at first quite
independent and widely separated, and it has been suggested by
Lankester, though not proved, that the ligament of the shell is
developed in the median part of the groove of the shell-gland.
The mantle lobes are developed as lateral outgrowths of the body: they
usually have a considerable extension before they are covered by the
shell. In Anodon and Unio the larval mantle lobes are, however, formed
in a somewhat exceptional way, and are from the first completely
covered by the valves of the larval shell. The larval mantle lobes and
shell in Anodon and Unio are subsequently replaced by the permanent
structures.
[FIG. 118. AN EMBRYO OF PISIDIUM PUSILLUM. (From Lankester.)
_f._ foot; _m._ mouth; _ph._ pharynx; _gs._ bilobed stomach; _pi._
intestine; _shs._ shell-gland.]
The adductor muscles are formed soon after the appearance of the
shell. The posterior sometimes appears first, _e.g._ Mytilus, and at
other times the anterior, _e.g._ Cardium.
The foot arises in the usual way as a prominence between the mouth and
anus. In comparison with Gasteropoda it is late in appearing, and in
many cases does not become prominent till the shell has attained a
considerable size. In its hinder part a provisional paired
byssus-gland is developed from the epidermis in Cyclas and other
forms. In other cases, _e.g._ Mytilus, the byssus-gland is permanent.
The byssus-gland occupies very much the position of the Gasteropod
operculum, and would appear very probably to correspond with this
organ. The anterior part of the foot is usually ciliated.
The gills appear rather late in larval development along the base of
the foot on either side, between the mantle and the foot (fig. 120,
_br_). They arise as a linear row of separate ciliated somewhat
knobbed papillæ. A second row appears later. The two rows give rise
respectively to the two gill lamellæ of each side.
The further history of the development of the gills has been studied
by Lacaze Duthiers (No. 297) in Mytilus. The first row of gill papillæ
formed becomes the innermost of the two lamellæ of the adult. The
number of papillæ goes on increasing from before backwards. When about
eleven have been formed, their somewhat swollen free extremities unite
together, the basal portions being separated by slits.
The free limb is formed by the free end of the gill lamella bending
upon itself towards the inner side and growing towards the line of
attachment of the lamella. The free limb is at first not composed of
separate bars, but of a continuous membrane. Before this membrane has
grown very wide, perforations are formed in it corresponding to the
spaces between the bars of the attached limb.
The outer gill lamella develops in precisely the same way as, but
somewhat later than, the inner. The rudiments of it appear when about
twenty papillæ of the inner lamella are formed. Its first papillæ are
formed near the hind border of the inner lamella, and new papillæ are
added both in front and behind. Its free limb is on the outer side.
In Mytilus the two limbs (free and attached) of each bar of the gill
are joined at wide intervals by extensile processes, the
'inter-lamellar junctions,' and the successive bars are attached
together by ciliated junctions. In many other types the concrescences
between the various parts of the gills are carried much further; the
maximum of concrescence being perhaps attained in Anodon and Unio[107].
[107] R. H. Peck, "Gills of Lamellibranch Mollusca." _Quart. J.
of M. Science_, Vol. XVII. 1877.
Large paired auditory sacks seem always to be developed in the foot;
and clearly correspond with the auditory sacks in Gasteropoda.
Eyes are frequently present in the larva, though they disappear in the
adult. In Montacuta and other types a pair of these organs is formed
at the base of the velum on each side of the oesophagus, not far from
the auditory sacks. They are provided with a lens.
A row of similar organs is present in the larva of Teredo in front of
the foot.
Cardium. As an example of a marine Lamellibranchiate I may take
_Cardium pygmaeum_, the development of which has been studied by Lovén
(No. 291). The ova, surrounded by a thickish capsule, are impregnated
in the cloaca. The segmentation takes place much as in Nassa (_vide_
p. 101), and the small segments gradually envelop the large hypoblast
spheres; so that there would seem to be a gastrula by epibole. After
the hypoblast has become enveloped by the epiblast, one side of the
embryo is somewhat flattened and marked by a deepish depression (fig.
117 A). From Lovén's description it appears to me probable that the
depression on the flattened side occupies the position of the
blastopore, and that the depression itself is the stomodæum. At this
stage the embryo becomes covered with short cilia which cause it to
rotate within the egg-capsule.
Close above the mouth there appear two small papillæ. These gradually
separate and give rise to a circular ridge covered with long cilia,
which encircles the embryo anteriorly to the ventrally-placed mouth.
This structure is the velum. In its centre is a single long flagellum
(fig. 117 B). Shortly after this the shell appears as a saddle-shaped
structure on the hinder part of the dorsal surface of the embryo. It
is formed at first of two halves which meet behind without the trace
of a hinge (fig. 117 C). The two halves rapidly grow and partially
cover over the velum, and below them the mantle folds soon sprout out
as lateral flaps.
The alimentary tract has by this time become differentiated (fig. 117
C). It consists of a mouth (_m_) and ciliated oesophagus probably
derived from the stomodæum, a stomach and intestine derived from the
true hypoblast, and an hepatic organ consisting of two separate lobes
opening into the stomach. The anus (_an_) appears not far behind the
mouth, and between the two is a very slightly developed rudiment of
the foot (_b_). The anterior adductor muscle (_cm_) appears at this
stage, though the posterior is not yet differentiated.
The larva is now ready to be hatched, but the further stages of its
development were not followed.
Ostrea. The larvæ of Ostrea, figured by Salensky (No. 293), shew a
close resemblance to those of Cardium. The velum is however a simple
ring of cilia without a central flagellum. The proctodæum would appear
to be formed later than the stomodæum, and the earliest stage figured
is too far advanced to throw light on the position of the blastopore.
Pisidium. The development of Pisidium has been investigated by
Lankester (No. 239). The ovum is invested by a vitelline membrane and
undergoes development in a brood-pouch at the base of the inner gill
lamella.
The segmentation commences by a division into four equal spheres, each
of which, as in so many other Mollusca, then gives rise by budding to
a small sphere. The later stages of segmentation have not been
followed in detail, but the result of segmentation is a blastosphere.
An invagination, presumably at the lower pole, now takes place, and
gives rise to an archenteric sack.
[FIG. 119. THREE VIEWS OF AN EMBRYO OF PISIDIUM IMMEDIATELY AFTER
THE CLOSURE OF THE BLASTOPORE. (After Lankester.)
A. View from the surface.
B. Optical section through the median plane.
C. Optical section through a plane a little below the surface.
_ep._ epiblast; _me._ mesoblast; _hy._ hypoblast; _p._ cells
apparently budding from the hypoblast to form mesoblastic elements.]
The embryo now rapidly grows in size. The blastopore becomes closed
and the archenteric sack forms a small mass attached at one point to
the walls of the embryonic vesicle (fig. 119, _hy_). In the space
between the walls of the archenteron and those of the embryonic
vesicle stellate mesoblast cells make their appearance, derived in the
main from the epiblast, though probably in part also from the
hypoblastic vesicle (_vide_ fig. 119 C, _p_). The cavity between the
hypoblast and epiblast, which contains these cells, is the body
cavity. Fig. 119 represents three views of the embryo at this stage. A
is a surface view shewing the epiblast; B is an optical section
through the median plane shewing the hypoblast and some of the
mesoblast cells; and C is an optical section shewing the mesoblast
cells. A prominence on one side of the embryo now develops which forms
the commencement of the foot, and the archenteric sack grows out at
its free extremity into two lobes, but remains attached to the
epiblast by an imperforate pedicle. The next organ to appear is the
stomodæum. It arises as a ciliated epiblastic ingrowth which meets the
free end of the archenteric sack, fuses with it, and shortly
afterwards opens into it (fig. 118, _ph_). Between the mouth and the
attachment of the enteric pedicle is placed the foot (_f_), which
becomes ciliated. On the dorsal side of the enteric pedicle there
appears a saddle-shaped patch of epiblast cells bounding the sides of
a groove (_shs_). This is the rudiment of the shell-gland.
The enteric pedicle, or intestine as it may now be called, soon
acquires a lumen, though still imperforate at its termination where
the anus is eventually formed. Ventral to the intestine is placed a
mass of cells--the rudiment of the organ of Bojanus. It is stated to
be developed as an ingrowth of the epiblast.
In a slightly later stage the shell-gland rapidly increases in size
and flattens out, and on the two sides of it there appear the
rudiments of the two valves, which are at first quite distinct, and
separated by a considerable interval (fig. 120). Before the appearance
of the valves of the shell, the mantle folds have already grown out
from the sides of the body.
[FIG. 120. DIAGRAMMATIC VIEW OF ADVANCED LARVA OF PISIDIUM. (Copied
from Lankester.)
_m._ mouth; _a._ anus; B. organ of Bojanus; _mn._ mantle; _f._
foot.]
At a somewhat later stage the gills appear as a linear series of small
independent buds within the folds of the mantle behind the foot (fig.
120, _br_). The anterior adductor also becomes differentiated.
The alimentary tract in the meantime has undergone considerable
changes. The primitive lateral lobes dilate enormously and become
ciliated. At a still later stage their walls undergo peculiar changes,
the nature of which is somewhat obscure, but they appear to me to be
of the same character as those in many Pteropods and Gasteropods,
where the cells of the hepatic diverticula, to which the lobes of
Pisidium apparently correspond, become filled with an albuminous
material.
The later stages in Pisidium have not been followed.
It is remarkable that in Pisidium a veliger stage does not occur. This
is probably due to the development taking place within the
brood-pouch. The late development of the otocysts is also remarkable.
A byssus-gland was not formed up to the stage observed. In Cyclas
calyculata (Schmidt), a byssus-gland also appears to be absent.
Cyclas. The development of Cyclas as described by Von Jhering is very
unlike that of Pisidium, and the differences would seem to be too
great to be accounted for except by errors of observation.
The segmentation of Cyclas is similar to that of Anodon (_vide_ p.
82), and a mass of large cells enclosed by the smaller cells gives
rise to the hypoblast. In the interior of this mass there appears a
lumen, and a process from it grows towards and meets the epiblast, and
gives rise to the oesophagus and mouth,--a mode of development of
these parts without parallel amongst Mollusca. A very rudimentary
velum would appear, according to Leydig (No. 290), to be developed at
the cephalic extremity. A shell-gland is formed of the same character
as in Gasteropods. According to Leydig the shell appears as a single
saddle-like structure on the dorsal surface; the lateral parts of this
become calcified, and give rise to the two valves, but are united in
the middle by the membranous median portion. At the two sides of the
body the mantle lobes are formed, as in Pisidium.
Very shortly after the formation of the shell the byssus-gland appears
as a pair of small follicles in the hinder part of the foot. It
rapidly grows larger and becomes a paired pyriform gland, in which are
secreted the byssus threads which serve to attach all the embryos at a
common point to the walls of the brood-pouch.
The foot is large, and ciliated anteriorly. Otolithic sacks and peda
ganglia are developed in it very early.
Unio. The ovum of Anodonta and Unio is enveloped in a vitelline
membrane, the surface of which is raised into a projecting
trumpet-like tube perforated at its extremity (fig. 12). This
structure is the micropyle. The micropyle disappears in Anodonta
piscinalis when the egg is ripe, but in Unio persists during the whole
development. The ova are transported, in a manner not certainly made
out, into the space between the two limbs of the outer gills of the
mother, and there undergo their early development. The animal or upper
pole of the egg is placed at the pole opposite to the micropyle.
The segmentation is unequal (__vide__ p. 100) and results in the
formation of a blastosphere with a large segmentation cavity. The
greater part of the circumference of the egg is formed of small
uniform spheres, but the lower (with reference to the segmentation)
pole is taken up by a single large cell. The small spheres become the
epiblast, and the large cell gives rise to hypoblast and
mesoblast[108].
[108] The account of the remainder of the development till the
larva becomes hatched is taken from Rabl, No. 292.
The single large cell next divides into two, and then four, and
finally into about ten to fifteen cells. These cells form an especial
area of more granular cells than the other cells of the blastosphere.
Most of them are nearly of the same size, but two of them (according
to Rabl), in contact with each other, but placed on the future right
and left sides of the embryo, are considerably larger than the
remainder. These two cells soon pass into the cavity of the
blastosphere, while at the same time the area of granular cells
becomes flattened out, and then becomes involuted as a small sack with
a transversely elongated opening, which does not nearly fill up the
cavity of the blastosphere. This involuted sack is the archenteron.
The two large cells, which lie in immediate contact with what,
following Rabl, I shall call the anterior lip of the blastopore, next
bud off small cells, which first form a layer covering the walls of
the archenteron, but subsequently develop into a network filling up
the whole cavity of the primitive blastosphere. The space between
these cells is the primitive body cavity. For a long time the two
primitive mesoblast cells retain their preponderating size[109]. At
the hinder end of the body, and at the end opposite therefore to the
two mesoblast cells, are placed three especially large epiblast cells.
[109] In this description I follow Rabl's nomenclature. According
to his statements the ventral part of the body is the original
animal pole--the dorsal the lower pole; the anterior end the
mesoblastic side of the opening of invagination.
In Anodonta and Unio tumidus there appears at this period a patch of
long cilia at the anterior end of the body. These cilia cause a
rotation of the embryo and would appear to be the velum. In Unio
pictorum they do not appear till much later.
Immediately following this stage the changes in the embryo take place
with great rapidity. In the first place a special mass of mesoblast
cells appears at the hinder end of the archenteric sack; and becoming
elongated transversely gives rise to the single adductor muscle. On
the subsequent formation of the shell the muscle becomes inserted in
its two valves. The blastopore next becomes closed, and the small
archenteron grows forwards till it meets the epiblast anteriorly, and
at the same time detaches itself from the epiblast in the region where
the blastopore was placed. Where it comes in contact with the wall of
the body in front a small epiblastic invagination arises, which meets
and opens into the archenteric sack and forms the permanent mouth.
While these changes have been taking place the shell is formed as a
continuous saddle-shaped plate on the dorsal surface. From this plate
the two valves are subsequently differentiated. On the dorsal surface
they meet with a straight hinge-line. Each valve is at first rounded,
but subsequently becomes triangular with the hinge-line as base. The
valves are not quite equi-sided, but the anterior side is less convex
than the posterior. At a later period a beak-shaped organ is formed at
the apex of each valve in the same manner as the remainder of the
shell. This organ is placed at about a right angle with the main
portion of the valve. It is pointed at its extremity and bears
numerous sharp spines on its outer side, which are especially large in
the median line (_vide_ fig. 121 A). It is employed in fixing the
larva, after it is hatched, on to the fish on which it is for some
time parasitic. The shell is perforated by numerous pores.
After the shell has become formed a new structure makes its appearance
which is known as the byssus-gland. It is developed as an invagination
of the epiblast at the hinder end of the body: Rabl was unable to
determine whether it was formed from the three large epiblastic cells
present there or no. It subsequently forms an elongated gland with
three coils or so round the adductor muscle on the left side of the
body, but opening in the median ventral line. It secretes an elongated
cord by which the larva becomes suspended after hatching.
For some time the ventral portion of the body projects behind the ends
of the valves of the shell, but before these are completely formed a
median invagination of the body wall takes place, which obliterates to
a large extent the body cavity, and gives rise to two great lateral
lobes, one for each valve. These lobes are the mantle lobes.
Before the mantle lobes are fully formed peculiar sense organs,
usually four in number, make their appearance on each lobe. Each of
them consists of a columnar cell, bearing at its free end a cuticle
from which numerous fine bristles proceed. Covering the cell and the
parts adjoining it is a delicate membrane perforated for the passage
of the bristles. The largest and first formed of these organs is
placed near the anterior and dorsal part of the mantle. The three
others are placed near the free end of the mantle (_vide_ fig. 121 A).
These organs probably have the function of enabling the larva to
detect the passage of a fish in its vicinity, and to assist it
therefore in attaching itself. When the embryo is nearly ripe there
appears immediately ventral to and behind the velum a shallow pit on
each side of the middle line, and the two pits appear to be connected
by a median transverse bridge. These structures have been the cause of
great perplexity to different investigators, and their meaning is not
yet clear. According to Rabl the median structure is the somewhat
bilobed archenteron, and according to his view it is not really
connected with the laterally placed pits. The cilia of the velum
overlie these latter structures and make them appear as if their edges
were ciliated. They are regarded by Rabl as the rudiments of the
nervous system.
With the development of the shell, the mantle, and the sense organs,
the young mussel reaches its full larval development, and is now known
as a Glochidium (fig. 121 A).
If the parent, with Glochidia in its gills, is placed in a tank with
fish, it very soon (as I have found from numerous experiments) ejects
the larvæ from its gills, and as soon as this occurs the larvæ become
free from the egg membrane, attach themselves by the byssus-cord, and
when suspended in this position continually close and open their
shells by the contraction of the adductor muscle. If the mussels are
not placed in a tank with fish the larvæ may remain for a long time in
the gills.
[FIG. 121.
A. GLOCHIDIUM IMMEDIATELY AFTER IT IS HATCHED.
_ad._ adductor; _sh._ shell; _by._ byssus cord; _s._ sense organs.
B. GLOCHIDIUM AFTER IT HAS BEEN ON THE FISH FOR SOME WEEKS.
_br._ branchiæ; _au.v._ auditory sack; _f._ foot; _a.ad._ and
_p.ad._ anterior and posterior adductors; _al._ mesenteron; _mt._
mantle.]
Before passing on to state what is known with reference to the larval
metamorphosis, it may be well to call attention to certain, and to my
mind not inconsiderable, difficulties in the way of accepting in all
particulars Rabl's account of the development.
In all Gasteropod Molluscs the lower or vegetative pole of the ovum is
ventral, not dorsal as Rabl would make it in Unio. The blastopore in
other Molluscs always coincides either with the mouth or anus, or
extends between the two. The surface on which the foot is formed is
the ventral surface. On the dorsal surface are placed, (1) the velum
near the mouth, (2) the shell-gland near the anus. In Anodon the velum
is placed just dorsal to the mouth, then according to Rabl follows the
blastopore, and in the region of the blastopore is formed the shell.
The blastopore is therefore dorsal in position. It occupies in fact
the ordinary place of the shell-gland, and looks very much like this
organ (which is not otherwise present in Anodon and Unio). Without
necessarily considering Rabl's interpretations false, I think that the
above difficulties should have been at any rate discussed in his
paper. More especially is this the case when there is no doubt that
Rabl has made in his paper on Lymnæus a confusion between the mouth
and the shell-gland.
Investigations on the post-embryonic metamorphosis of Glochidium have
been made by Braun (No. 287), and several years ago I made a series of
observations on this subject, the results of which agree in most
points with those of Braun. I was however unsuccessful in carrying on
my observations till the young mussel left its host.
The free Glochidia very soon attach themselves to the gills, fins, or
other parts of fish which are placed in the tank containing them;
after attachment they become covered by a growth of the epidermic
cells of their host, and undergo their metamorphosis.
The first change that takes place is the disappearance of the byssus
and the byssus organ. This occurs very soon; shortly afterwards all
traces of the velum and sense organs also become lost.
At the time of the disappearance of these bodies, at the point of the
projection from which the byssus cord arose, and very possibly from
this very projection, the foot arises as a rounded process which
rapidly grows and soon becomes ciliated (fig. 121 B, _f_).
The single adductor muscle begins to atrophy very early, but before
its entire disappearance rudiments are formed at the two ends of the
body, which at a later period can be distinctly recognised as the
anterior and posterior adductor muscles (fig. 121 B, _a.ad_ and
_p.ad_).
After the formation of these parts the gills arise as solid and at
first somewhat knobbed papillæ covered with a ciliated epidermis, on
each side of, but somewhat in front of (!) the foot (fig. 121 B,
_br_). In the foot there soon appear the auditory sacks (_au.v_), and
the foot itself becomes a long tongue-like ciliated organ projecting
backwards[110].
[110] The position of the foot and gills in the larva represented
in Fig. 119 B would be more normal if the convex and not the
flatter side of the shell were the anterior. I have followed Rabl
and Flemming in the determinations of the anterior and posterior
end of the embryo, but failed to rear my larvæ up to a stage at
which the presence of the heart or some other organ would
definitely confirm their interpretation. I originally adopted
myself the other view, and in case they are mistaken, the
so-called velum would be a circumanal patch of cilia, while the
position of the primitive mesoblast cells as well as of the
byssus would better suit my view than that adopted in the text on
the authority of the above observers.
The mantle lobes undergo great changes, and indeed by Braun the mantle
lobes are stated to be formed almost entirely _de novo_. The permanent
shell is (Braun) formed on the dorsal surface of the still parasitic
larva in the form of two small independent plates. I have not followed
the changes of the alimentary canal, etc., but at an early stage there
is visible, dorsal to the foot, a simple enteric sack.
By the time the larva leaves its host all the organs of the adult,
except the generative organs, have become established.
The post-embryonic development of the organs of Glochidium is similar
in the main to that of other Lamellibranchiata. This fact is of some
importance on account of the peculiarities of the earlier
developmental stages.
The byssus organ, the toothed processes of the shell, and the sense
organs of the Glochidium can hardly be ancestral rudiments, but must
be organs which have been specially developed for the peculiar mode of
life of the Glochidium. Whether the single muscle is to be counted
amongst such provisional organs is perhaps a more doubtful point, but
I am inclined to think that it ought to be so.
If however the single muscle is an ancestral organ, it is important to
observe that it entirely disappears as development goes on and the two
adductor muscles in the adult are developed independently of it.
_General review of the characters of the Molluscan larvæ._
The typical larva of a Mollusc, as has been more especially pointed
out by Lankester, is essentially similar to the larva of a number of
invertebrate types, and especially the Chætopoda, with the addition of
certain special organs characteristic of the Mollusca.
It has a bent alimentary tract, with a mouth on the ventral surface
and a terminal or ventral anus. The alimentary tract is divided into
three regions: oesophagus, stomach, and intestine. There is a
variously developed præoral lobe with a ring of cilia--the velum, and
a peri-anal lobe, often with a patch of cilia (Paludina, etc.). In all
these characters it is essentially similar to a Chætopod larva. The
two characteristic molluscan organs are (1) a foot between the mouth
and anus, and (2) an invagination of the epiblast on the dorsal side
at the hinder end of the body, which is connected with the formation
of the shell.
The larvæ of most Gasteropoda, Pteropoda, and Lamellibranchiata
present no features which call for special remark; but the larvæ of
the Gymnosomata amongst the Pteropoda, and of the Scaphopoda,
Polyplacophora and Cephalopoda present interesting peculiarities.
The larvæ of the Gymnosomata are peculiar in the presence of three
transverse ciliated rings, _situated behind the velum_ (Fig. 109).
These rings might be regarded as indications of a rudimentary
segmentation; but, as already indicated, this view is not
satisfactory. There is every reason for thinking that these rings have
been specially acquired by these larvæ.
At first sight the larvæ of the Gymnosomata might be supposed to
resemble those of the Scaphopoda, which are also provided with
transverse ciliated rings; but, as shewn above, the rings of the
Scaphopoda are merely parts of the extended velar ring.
Thus, the ciliated rings of the two larvæ--so similar in
appearance--are in reality structures of entirely different values,
being in the one case parts of the velum, and in the other special
developments of cilia behind the velum.
The great peculiarity of the early larva of the Scaphopoda is the
enormous development of the præoral lobe, which gives room for the
development of the ciliated rings. In the presence of a central tuft
of cilia, at the anterior extremity, the larva of the Scaphopoda
resembles that of the Lamellibranchiata, etc.
The larva of the Polyplacophora resembles that of Lamellibranchiata in
its anterior flagellum, and that of the Scaphopoda in the large
development of the præoral lobe; but is quite peculiar amongst
Mollusca in the transverse segmentation of the mantle area.
The embryo of the Cephalopoda agrees very closely with that of normal
Odontophora in the formation of the mantle and (?) of the shell-gland,
but is quite exceptional (1) in the almost invariable presence of a
more or less developed external yolk-sack, (2) in the absence of a
velum, (3) in the absence of a median foot, and in the presence of the
arms.
The presence of a yolk-sack may most conveniently be spoken of in
connection with the foot, and we may therefore pass on to the question
of the velum.
The velum is one of the most characteristic embryonic appendages of
the Mollusca, and its absence in the Cephalopoda is certainly very
striking. By some investigators the arms have been regarded as
representing the velum, but considering that they are primitively
placed on the posterior and ventral side of the mouth, and that the
velum is essentially an organ on the dorsal side of the mouth, this
view cannot, in my opinion, be maintained with any plausibility.
Various views have been put forward with reference to the Cephalopod
foot. Huxley's view, which is the one most generally adopted, is given
in the following quotation[111].
[111] _The Anatomy of Invertebrated Animals_, p. 519.
"But that which particularly distinguishes the Cephalopoda is the form
and disposition of the foot. The margins of this organ are, in fact,
produced into eight or more processes termed arms, or _brachia_; and
its anterolateral portions have grown over and united in front of the
mouth, which thus comes, apparently, to be placed in the centre of the
pedal disk. Moreover, two muscular lobes which correspond with the
epipodia of the Pteropods and Branchiogasteropods, developed from the
sides of the foot, unite posteriorly, and, folding over, give rise to
a more or less completely tubular organ--the funnel or _infundibulum_."
Grenacher, from his observations on the development of Cephalopoda,
argues strongly against this view, and maintains that no median
structure comparable with the foot is present in this group: and that
the arms cannot be regarded as taking the place of the foot, but are
more probably representatives of the velum.
The difficulty of arriving at a decision on this subject is mainly due
to the presence of the yolk-sack, which, amongst the Cephalopoda as
amongst the Vertebrata, is the cause of considerable modifications in
the course of the development. The foot is essentially a protuberance
on the ventral surface, between the mouth and the anus. In Gasteropods
it is usually not filled with yolk, but contains a cavity, traversed
by contractile mesoblastic cells. In this group the blastopore is a
slit-like opening (_vide_ p. 187) extending over the region of the
foot, from the mouth to the anus, the final point of the closure of
which is usually at the oral but sometimes at the anal extremity. In
Cephalopods the position of the Gasteropod foot is occupied by the
external yolk-sack. In normal forms the blastopore closes at the apex
of the yolk-sack, and at the two sides of the yolk-sack the arms grow
out. These considerations seem to point to the conclusion that the
normal Gasteropod foot is represented in the Cephalopod embryo by the
yolk-sack, which has, owing to the immense bulk of food-yolk present
in the ovum, become filled with food-yolk and enormously dilated. The
closure of the blastopore at the apex of the yolk-sack, and not at its
oral or anal side, is what might naturally be anticipated from the
great extension of this part.
Grenacher's type of larva, where the external yolk-sack is practically
absent, appears to me to lend confirmation to this view. If the reader
will turn to fig. 113, he will observe a prominence between the mouth
and anus, which exactly resembles the ordinary Gasteropod foot. At the
sides of this prominence are placed the rudiments of the arms. This
prominence is filled with yolk, and represents the rudiment of the
external yolk-sack of the typical Cephalopod embryo. The blastopore,
owing to the smaller bulk of the food-yolk, reverts more nearly to its
normal position on the oral side of this prominence.
If the above considerations have the weight which I attribute to them,
the unpaired part of the Cephalopod foot has been overlooked in the
embryo on account of the enormous dilatation it has undergone from
being filled with food-yolk; and also owing to the fact that in the
adult the median part of the foot is unrepresented. The arms are
clearly, as Huxley states, processes of the margin of the foot.
Both Grenacher and Huxley agree in regarding the funnel as
representing the coalesced epipodia; but Grenacher points out that the
anterior folds which assist in forming the funnel (_vide_ p. 253)
represent the great lateral epipodia of the Pteropod foot, and the
posterior folds the so-called horse-shoe shaped portion of the
Pteropod foot.
_Development of Organs._
The epiblast. With reference to the general structure of the epiblast
there is nothing very specially deserving of notice. It gives rise to
the whole of the general epidermis and to the epithelium of the organs
of sense. The most remarkable feature about it is a negative one, viz.
that it does not, in all cases at any rate, give rise to the nervous
system.
The epiblast of the mantle has the special capacity of secreting a
shell, and the integument of the foot has also a more or less similar
property in that it forms the operculum, and a byssus in some
Lamellibranchiata, other parts of the integument form the radula, setæ
in Chiton, and other similar structures.
Nervous system. The origin of the nervous system in Mollusca is still
involved in some obscurity. It is the general opinion amongst the
majority of investigators that the nervous ganglia in Gasteropods and
Pteropods are formed from detached thickenings of the epiblast. Both
Lankester (No. 239) and Fol (No. 249-251) have arrived at this
conclusion, and Rabl has shewn by sections that in Planorbis there are
two lateral thickenings of the epiblast in the velar area; from which
the supra-oesophageal ganglia become subsequently separated off. The
observations on the pedal ganglia are less precise: they very probably
arise as thickenings of the epiblast of the side of the foot.
According to Fol, the nervous system in the Hyaleacea amongst the
Pteropoda originates in a somewhat different way. A disc-like area
appears in the centre of the velum, which soon becomes nearly divided
into two halves. From each of these there is formed by invagination a
small sack. The axes of invagination of the two sacks meet at an angle
on the surface. The cavities of the sacks become obliterated; the
sacks themselves become detached from the surface, fuse in the middle
line, and come to lie astride of the oesophagus. Fol has detected a
similar process in Limax. The exact origin of the pedal ganglia was
not observed, but Fol is inclined to believe that they develop from
the mesoblast of the foot.
A very different view is held by Bobretzky (No. 242), whose
observations were made by means of sections.
The supra-oesophageal and pedal ganglia are formed according to this
author as independent and ill-defined local thickenings of cells which
are apparently mesoblastic. The two sets of ganglia appear nearly
simultaneously, and later than the rudiments of the auditory and optic
organs.
In the Cephalopoda there seems to be but little doubt, as first
pointed out by Lankester, that the various ganglia originate in what
is apparently mesoblastic tissue.
There is still very much requiring to be made out with reference to
their origin, unless details on this subject are given in Bobretzky's
Russian memoir. It would seem however that each ganglion develops as
an independent differentiation of the mesoblast (unless the optic and
cerebral ganglia are from the first continuous)[112]. The
corresponding ganglia of the two sides become subsequently united and
the various ganglia become connected by their proper commissural
cords. The ganglia are shewn in figures 124, 126, and 127.
[112] Ussow states that they are independent.
In Lamellibranchiata the development of the nervous system has not
been worked out.
The two points which are most striking in the development of the
nervous system of Mollusca are (1) the fact that in the Cephalopoda at
any rate it is developed from tissue apparently mesoblastic; and (2)
the fact that the several ganglia frequently originate quite
independently, and subsequently become connected.
With reference to the first of these points it should be noticed that
the supra-oesophageal and pedal ganglia are at first respectively
connected with the optic and auditory organs, and that these sense
organs are in some cases at any rate developed anteriorly in point of
time to the ganglia. It seems perhaps not impossible that primitively
the ganglia may have been simply differentiations of the walls of the
sense organ, and perhaps their apparent derivation from the mesoblast
is really a derivation from cells which primitively belonged to the
walls of these sense organs. Bobretzky's observations on Fusus fit in
well with this view.
In the Hyaleacea and in other Pteropods, where the eyes are absent in
the adult, Fol finds the supra-oesophageal ganglia resulting from a
pair of epiblastic invaginations. May not these invaginations be
really rudiments of the eyes as well as of the ganglia? Fol also, it
is true, describes a similar mode of origin for these ganglia in
Limax. It would be interesting to have further observations on this
subject. The independent origin of the pedal and supra-oesophageal
ganglia finds its parallel amongst the Chætopoda.
[FIG. 122. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA.
(After Grenacher.)