Two view of SepiaFig. 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.
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 benoted,viz.the oral (fig. 111B,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. 111B, 1).
Fig. 111B 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 atmand the anal immediately in front ofan. The gills, nearly covered by the mantle, are seen atbr. Atpare the cephalic swellings, and the eye is seen atoc.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 infig. 111B. 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.
Side views of SepiaFig. 112. Side views of three late stages in the development of Sepia.(After Kölliker.)m.mouth;yk.yolk-sack;oc.eye;mt.mantle.
Fig. 112. Side views of three late stages in the development of Sepia.(After Kölliker.)
m.mouth;yk.yolk-sack;oc.eye;mt.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 infig. 112.
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. 112C) 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 isearly 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 infig. 113A 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 (inf1andinf2), 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 (videp.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 blastoporeis 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. 113C) 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.
Three Embryos of a CephalopodFig. 113. Three Embryos of a Cephalopod with a very small yolk-sack.(After Grenacher.)a.blastopore;br.branchiæ;inf.1andinf.2posterior 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.
Fig. 113. Three Embryos of a Cephalopod with a very small yolk-sack.(After Grenacher.)
a.blastopore;br.branchiæ;inf.1andinf.2posterior 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 ispresent. 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.
Section through a Loligo ovumFig. 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.œsophagus;ds.yolk-sack;chs.shell-gland;mt.mantle;pdh.mesenteron;x.epiblastic thickening between the folds of the funnel.
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.œsophagus;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. 115m) 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 infig. 114. On the posterior side of the mantle, at the point marked infig. 111B,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. Theremainder 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. 114chsand115shs). 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. 114and127cch).
Section of an embryo LoligoFig. 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.
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]:
“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 ofSpirula, has become enclosed by growths of the mantle, and unlike the shell ofSpirula, 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.
“InSpirulawe have an originally external shell enclosed but not added to by the enclosing mantle sack.
“InSpirulirostra, a tertiary fossil, we have a shell very similar to that ofSpirula, with a small guard of laminated structure developed as in the Belemnite (see the figures in BronnClassen u. Ordnungen des Thierreichs).
“In the Belemnites the original nautiloid shell is small as compared withSpirulirostra. It appears to be largest in Huxley’s genusXiphoteuthis. Hence in the seriesSpirula,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 itssuperadded 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 ofLoligoandSepiamust 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 ofSepiaandLoligocorrespond to the entire Belemnite shell minus the phragmacone-nucleus, it is clear that the sack which develops so early inLoligoand which appears to correspond to the shell-gland of the other Molluscs cannot be held to do so. The sack thus formed inLoligomust 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 adultAplysiaand 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 embryoLoligo, etc., is fused with the surviving remnants of an embryonic shell-gland. When the embryology ofNautilus pompiliusis 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 twofolds at first merely form the side of a groove or imperfect tube (fig. 113C and124ff.), 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.
Chiton embryosFig. 116.I.Chiton Wossnessenskii.(After Middendorf.)II.Chiton dissectedto shewo.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.
Fig. 116.
I.Chiton Wossnessenskii.(After Middendorf.)II.Chiton dissectedto shewo.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 formativehalf 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. 116III.andIV.).
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].
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. 116IV.). 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. 116IV.).
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 œsophagus 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. 116V.). 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 œsophageal 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 hasearly branched off from the main group of the Odontophora along a special line of its own, andnotthat 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 bycilia; 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 headis 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. 117B and C). The velum never becomes bilobed.
Three stages in the development of CardiumFig. 117. Three stages in the development of Cardium.(After Lovén.)hy.hypoblast;b.foot;m.mouth;an.anus;V.velum;cm.anterior adductor muscle.
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 ofthe 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.
An embryo of Pisidium pusillumFig. 118. An embryo of Pisidium pusillum.(From Lankester.)f.foot;m.mouth;ph.pharynx;gs.bilobed stomach;pi.intestine;shs.shell-gland.
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 attaineda 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].
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 œsophagus, 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 takeCardium 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 (videp.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. 117A). 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. 117B). 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. 117C). 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. 117C). It consists of a mouth (m) and ciliated œsophagus 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 byLankester (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.