Three views of an embryo of PisidiumFig. 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.
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 cellsmake their appearance, derived in the main from the epiblast, though probably in part also from the hypoblastic vesicle (videfig. 119C,p). The cavity between the hypoblast and epiblast, which contains these cells, is the body cavity.Fig. 119represents 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.
Advanced larva of PisidiumFig. 120. Diagrammatic view of advanced larva of Pisidium.(Copied from Lankester.)m.mouth;a.anus; B. organ of Bojanus;mn.mantle;f.foot.
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 (videp.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 œsophagus 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 (videp.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].
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.
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 extremityand bears numerous sharp spines on its outer side, which are especially large in the median line (videfig. 121A). 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 (videfig. 121A). 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 aGlochidium(fig. 121A).
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.
Views GlochidiumFig. 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.andp.ad.anterior and posterior adductors;al.mesenteron;mt.mantle.
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.andp.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. 121B,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. 1211 B,a.adandp.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. 121B,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].
The mantle lobes undergo great changes, and indeed by Braun the mantle lobes are stated to be formed almost entirelyde 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: œsophagus, 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].
“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, orbrachia; 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 orinfundibulum.”
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 (videp.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 tofig. 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 filledwith 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 (videp.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-œsophagealganglia 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 œsophagus. 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-œsophageal 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 figures124,126, and127.
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-œsophageal 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-œsophageal 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-œsophageal ganglia finds its parallel amongst the Chætopoda.
Eyes of MolluscaFig. 122. Three diagrammatic sections of the eyes of Mollusca.(After Grenacher.)A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod.Pal.eyelid;Co.cornea;Co.ep.epithelium of ciliary body;Ir.iris;Int.Int1...Int4.different parts of the integument;l.lens;l1.outer segment of lens;R.retina;N.op.optic nerve;G.op.optic ganglion;x.inner layer of retina;N.S.nervous stratum of retina.
Fig. 122. Three diagrammatic sections of the eyes of Mollusca.(After Grenacher.)
A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod.
Pal.eyelid;Co.cornea;Co.ep.epithelium of ciliary body;Ir.iris;Int.Int1...Int4.different parts of the integument;l.lens;l1.outer segment of lens;R.retina;N.op.optic nerve;G.op.optic ganglion;x.inner layer of retina;N.S.nervous stratum of retina.
The supra-œsophageal ganglia appear always to develop within the region of the velar area. This area corresponds with the præ-oral lobe of the Chætopod larva, at the apex of which is developed the supra-œsophageal ganglion. Embryology thus confirms the results of Comparative Anatomy in reference to the homology of these ganglia in the two groups.
Optic organs[113].An eye is present in most Gasteropods andin many larval Pteropods. Although its development has not been fully worked out, yet it has clearly been shewn by Bobretzky and other investigators that it originates as an involution of the epidermis, which first forms a cup and eventually a closed vesicle. The posterior wall of the vesicle gives rise to the retina, the anterior to the inner epithelium of the cornea. The external epidermis becomes continued over the outer surface of the vesicle.
The lens is formed in the interior of the vesicle, probably as a cuticular deposit, which increases by the addition of concentric layers. Pigment becomes deposited between the cells of the retina.Fig. 122B is a diagrammatic representation of the adult eye of a Gasteropod.
The Cephalopod eye is formed, as first shewn by Lankester, as a pit in the epiblast round which a fold arises (fig. 123A) and gradually grows over the mouth of the pit so as to shut it off from communication with the exterior (fig. 123B).
Developing eye of a CephalopodFig. 123. Two sections through the developing eye of a Cephalopod to shew the formation of the optic cup.(After Lankester.)
Fig. 123. Two sections through the developing eye of a Cephalopod to shew the formation of the optic cup.(After Lankester.)
The epiblast lining the posterior region of the vesicle gives rise to the retina, that lining the anterior region to the ciliary body and processes. It is important to notice that the condition of the eye just before the above pit becomes closed is exactly that which is permanent in Nautilus (videfig. 122A). After the pit has become closed a mesoblastic layer grows in between its wall and the external epiblast.
The lens becomes formed in two independent segments. The inner and larger of these arises as a rod-like process (fig. 124) projecting from the front wall of the optic vesicle into the cavity of the vesicle. It is a cuticular structure and therefore without cells. By the deposition of a series of concentric layers it soon assumes a spherical form (fig. 125,hl). The conditionof the eye, with a closed optic vesicle and the lens projecting into it, is that which is permanent in the majority of Gasteropods (videfig. 122B). At about the time when the lens first becomes formed a fold composed of epiblast and mesoblast appears round the edge of the optic cup (fig. 124,cc), and gives rise to a structure known in the adult as the iris. Shortly afterwards this becomes more prominent (fig. 125,if), and at the same time the layers of cells of the ciliary region in front of the inner segment of the lens become reduced to the condition of mere membranes (fig. 125B); and in front of them the anterior or outer segment of the lens becomes formed as a cuticular deposit (fig. 125B,vl). At a still later period a fresh fold of epiblast and mesoblast appears round the eye and gradually constitutes the anterior optic chamber (videfig. 122C,Co). In most forms this chamber communicates with the exterior by a small aperture, but in some it is completely closed. The fold itself gives rise to the cornea in front and to the sclerotic at the sides. At a laterperiod another fold may appear forming the eyelids (fig. 122C,Pal).
Head of embryo of LoligoFig. 124. Transverse section through the head of an advanced embryo of Loligo.(After Bobretzky.)vd.œsophagus;gls.salivary gland;g.vs.visceral ganglion;gc.cerebral ganglion;g.op.optic ganglion;adk.optic cartilage;ak.andy.lateral cartilage or (?) white body;rt.retina;gm.limiting membrane;vk.ciliary region of eye;cc.iris;ac.auditory sack (the epithelium lining the auditory sacks is not represented);vc.vena cava;ff.folds of funnel.
Fig. 124. Transverse section through the head of an advanced embryo of Loligo.(After Bobretzky.)
vd.œsophagus;gls.salivary gland;g.vs.visceral ganglion;gc.cerebral ganglion;g.op.optic ganglion;adk.optic cartilage;ak.andy.lateral cartilage or (?) white body;rt.retina;gm.limiting membrane;vk.ciliary region of eye;cc.iris;ac.auditory sack (the epithelium lining the auditory sacks is not represented);vc.vena cava;ff.folds of funnel.
Auditory organs.A pair of auditory sacks is found in the larvæ of almost all Gasteropods and Pteropods, and usually originates very early. They are placed in the front part of the foot, and on the formation of the pedal ganglia come into close connection with it, though they receive their nervous supply in the adult from the supra-œsophageal ganglia.
In a very considerable number of cases amongst Gasteropods and Pteropods the auditory organs have been observed to develop as invaginations of the epiblast, which give rise to closed vesicles lying in the foot,e.g.Paludina, Nassa, Heteropods, Limax, some Pteropods (Clio).
This is no doubt the primitive mode of origin, but in other cases, which perhaps require confirmation, the sacks are stated to originate from a differentiation of solid thickenings of the epidermis or of the tissues subjacent to it.
The auditory sacks are provided with an otolith, which according to Fol’s observations is first formed in the wall of the sack.
In Cephalopods the auditory organs are formed as epiblastic pits on the posterior surface of the embryo, and are at first widely separated (fig. 113,ac). The openings of the pits become narrowed, and finally the original pits form small sacks lined by an epithelium, and communicating with the exterior by narrow ducts, equivalent to therecessus vestibuliof Vertebrates, and named, after their discoverer, Kölliker’s ducts. The external openings of these ducts become completely closed at about the same time as the shell-gland, and the ducts remain as ciliated diverticula of the auditory pits. The widely separated auditory sacks gradually approach in the middle ventral line, and are immediately invested by the visceral ganglia (fig. 124,ac). They finally come to lie in contact on the inner side of the funnel.
On the side opposite Kölliker’s duct, an epithelial ridge is formed—thecrista acustica—the cells of which give rise to an otolith connected with the crista by a granular material. At a later period of development three regions of the epithelium of the sack become especially differentiated. Each of these regionsis provided with two rows of cells, bearing on their free edges numerous very short auditory hairs. The cells of each row are placed nearly at right angles to those of the adjoining row.