MOLLUSCA. 239 



The development of Umax 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 rhyth- 

 mically. 



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 develop- 

 ment 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 develop- 

 ment the closest resemblance to those of other Gasteropods. The seg- 

 mentation 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 

 proctodanim 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 mescnteric 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 



