198 HETEROPODA. 



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 development of the external shell into relation with that of other 

 forms. Clausilia like Liniax 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. 81) 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 envelope 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 

 cesophngus is formed as an epiblastic ingrowth. The rudiment of the 

 proctodreum appears as two epiblastic cells symmetrically placed behind the 

 foot, which subsequently pass to the right side, and give rise to a shallow 

 invagiuation which meets the mesenteric sack. In the latter structure the 

 cells of part of the wall develope 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 

 stomach. The remainder, which unites with the proctodseum, forms the 

 intestine. 



The structural peculiarities of the adult are formed by a post-larval 

 metamorphosis. The caudal appendage of Pterotrachea and Firoloidea is 

 formed as an outgrowth of the upper border of the hind end of the foot. 

 The so-called fin arises as a cylindrical process in front of the base of the 

 foot, which is eventually flattened laterally. In the Atlantidpe it is in some 

 cases at first vermiform, and in other cases attains directly its adult struc- 

 ture. The embryonic foot itself gives rise in Pterotrachea, Firoloidea and 

 Cariuaria to the tail, on the dorsal and posterior side of which the 

 operciilum may still be seen in young specimens. In Atlanta it forms 

 the posterior part of the foot on which the operculum persists through 

 life. 



The embryonic shell is completely lost in Pterotrachea and Firoloidea, 

 and the shell is rudimentary in Cariuaria. With its atrophy the mantle 

 region also becomes much reduced. 



The velum is enormously developed in many Heteropods. In Atlanta it 



