062 MOLLUSCA [PULMONATA. of the visceral liump, in the centre of which the shell-sac existed for a brief period. In Clausilia, according to the observations of Gegenbaur, the primitive shell-sac does not flatten out and disappear, but takes the form of a flattened closed sac. Within this closed sac a plate of calcareous matter is developed, and after a time the upper wall of the sac disappears, and the calcareous plate continues to grow as the nucleus of the permanent shell. In the slug Testacella (fig. 69, C) the shell-plate never attains a large size, though naked. In other slugs, namely, Limax and Arion, the shell-sac remains permanently closed over the shell-plate, which in the latter genus consists of a granular mass of carbonate of lime. The permanence of the primitive shell-sac in these slugs is a point of considerable interest. It is clear enough that the sac is of a different origin from that of Aplysia (described in the section treating of Opisthobranchia), being primitive instead of secondary. It seems probable that it is identical with one of the open sacs in which each shell-plate of a Chiton is formed, and the series of plate-like imbrications which are placed behind the single shell-sac on the dorsum of the curious slug, Plectrophorus, suggest the possibility of the formation of a series of shell-sacs on the back of that animal similar to those which we find in Chiton. Whether the closed primitive shell-sac of the slugs (and with it the transient embryonic shell-gland of all other Mollusca) is precisely the same thing as the closed sac in which the calcareous pen or shell of the Cephalopod Sepia ccn Fiu. 72**. Comparative diagrams of an embryo Slug, Limax (left), and an embryo Cuttle-fish, Loligo (right), sh, internal shell ; pk, embryonic renal organ (Stiebel s canal) in Limax ; mt, edge of the mantle-flap in Loligo ; op, cephalic eye ; t, cephalic tentacle ; m, position of the mouth ; Ft, the foot ; Fu, the hinder part of the foot drawn out to form the funnel of Loligo ; con, the contractile yelk-sac or hernia-like protrusion of the mid-region of the foot, corresponding to the line of closure of the blastopore in LimnDeus. N.B. The blastopore in the embryo of Loligo, which, like that of a bird, is much distorted by excess of food-yolk, does close at the extremity of the yelk-sac con. (Original.) and its allies is formed, is a further question, which we shall consider when dealing with the Cephalopoda. It is important here to note that Clausilia furnishes us with an exceptional instance of the continuity of the shell or secreted product of the primitive shell-sac with the adult shell. In most other Mollusca (Anisopleurous Gastropods, Pteropods, and Conchifera) there is a want of such continuity; the primitive shell-sac contributes no factor to the permanent shell, or only a very minute knob- like particle (Neritina and Paludina). It flattens out and disappears before the work of forming the permanent shell commences. And just as there is a break at this stage, so (as observed by Krohn in Marsenia = Echinospira) there may be a break at a later stage, the nautiloid shell formed on the larva being cast, and a new shell of a different form being formed afresh on the surface of the visceral hump. It is, then, in this sense that we may speak of primary, secondary, and tertiary shells in Mollusca, recognizing the fact that they may be merely phases fused by continuity of growth so as to form but one shell, or that in other cases they may be presented to us as separate individual things, in virtue of the non-development of the later phases, or in virtue of sudden changes in the activity of the mantle- surface causing the shedding or disappearance of one phase of shell-formation before a later one is entered upon. The development of the aquatic Pulmonata from the egg offers considerable facilities for study, and that of Limnaeus has been elucidated by Lankester, whilst Rabl has with remarkable skill applied the method of sections to the study of the minute embryos of Planorbis. The chief features in the development of Limnasus are exhibited in the woodcuts (figs. 3, 4, and 72***). There is not a very large amount of food-material present in the egg of this snail, and accordingly the cells resulting from division are not so unequal as in many other cases. The four cells first formed are of equal size, and then four smaller cells are formed by division of these four so as to lie at one end of the first four (the pole corresponding to that at which the " directive corpuscles " dc are extruded and remain). The smaller cells now divide and spread over the four larger cells (fig. 3) ; at the same time a space Via. 72**"*. Embryo of Limn/rus stagnalis, at a stage when the Trochosphere is developing foot and shell-gland and becoming a Veliger, seen as a transparent object under slight pressure, ph, pharynx (stomodaal inragination) ; r, r, the ciliated band marking out the velum ; ng, cerebral nerve-ganglion ; re, Stiebel s canal (left side), probably an evanescent embryonic nephridium ; sh, the primitive shell-sac or shell-gland ; pi, the rectal peduncle or pedicle of imagination, its attachment to the ectoderm is coincident with the hindmost extremity of the elongated blastopore of fig. 3, C ; tge, mesoblastic (skeleto- trophic and muscular) cells investing <js, the bilobed arch-enteron or lateral vesicles of invaginated eudoderm, which will develop into liver ; /, the foot. (Original.) the cleavage cavity or blastocoel forms in the centre of the mulberry-like mass. Then the large cells recom mence the process of division and sink into the hollow of the sphere, leaving an elongated groove, the blastopore, on the surface (fig. 3, C, and fig, 4, G). The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron ; the outer cells are the ecto derm. The blastopore now closes along the middle part of its course, which coincides in position with the future "foot." One end of the blastopore becomes nearly closed, and an ingrowth of ectoderm takes place around it to form the stomodaeum or fore-gut and mouth. The other extreme end closes, but the invaginated endoderm cells remain in continuity with this extremity of the blastopore, and form the "rectal peduncle" or "pedicle of invagination " of Lankester (see also the account and figures (fig. 151, A) of the development of the bivalve Pisidium), although the endoderm cells retain no contact with the middle region of the now closed-up blastopore. The anal opening forms at a late period by a very short ingrowth or proctodseum coinciding with the blind termination of the rectal peduncle (fig. 72*** pi).. The body-cavity and the muscular, fibrous, and vascular
tissues are traced partly to two symmetrically-disposed