MOLLUSCA 



125 



of the visceral hump, in the centre of which the shell-sac 

 existed for a brief period. 



In ClausiUa, 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 

 MoUusca) is precisely the same thing as the closed sac in 

 which the calcareous pen or shell of the Cephalopod Sepia 



Fig. 72**. — Comparative diagrams of an embryo Slug, Limax (left), and an 

 embryo Cuttle-fish, Loligo (right), sh, internal shell ; pk, emDryonic renal 

 organ (Stiebel's canal) in Limax ; m(, edge of the mantle-llap in Loligo ; op, 

 cephalic eye ; t, cephalic tentacle ; m, position of the mouth ; Ft, the foot ; 

 F'Uf 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 LimnEeus. N.B. — 

 The b^stopore in the embryd of Loligo, which, like that of a bird, is much 

 distorted by excess of food- yelk, 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 sheU-sac with the 

 adult shell. In most other MoUusca (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 MoUusca, 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 lis 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 

 Limnseus has been elucidated by Ltokester, 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 Limnseus are exhibited 

 in the woodcuts (figs. 3, 4, and 72***). There is not a 

 very large amount of food-material jiresent in the egg of 

 this snail, and accordingly the cells resulting from division 

 Eire 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 



Fig. 72**\ — Embi-yo of Limneeus 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 (stomodaeal invagination) ; v, v, 

 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 

 invagination, its attachment to the ectoderm is coincident with the hindmost 

 extremity of the elongated blastopore of fig. 3, C ; tge, mesoblastic (skeleto- 

 tropliic and muscular) cells investing ^5, the bilobed arch-enteron or lateral 

 vesicles of invaginated endoderm, which will develop into liver ; f, the foot. 

 (Original.) 



— the cleavage cavity or blastocoel — forms in the centre 

 of the mulberry-Uke 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 

 ceUs (derived from the division of the fom' 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 

 stomodseum 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 proctodeeum 

 coinciding with the bUnd 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 



