190 



GASTEROPODA AND PTEROPODA. 



The foot is a simple prominence of epiblast on the ventral surface, 

 in the cavity of which there are usually a number of mesoblast cells 

 (fig. 101, /). The larval form just described has been named by 

 Lankester the trochosphere larva. 



Before considering the further external changes which the larva 

 undergoes, it will be well to complete the history of the invaginated 

 hypoblast. 



The hypoblast has after its imagination either the form of a 

 sack (fig. 102) or of a solid mass (fig. 101). Whether the mouth be 



the blastopore or no, the permanent oeso- 

 phagus is formed of epiblast cells, so that 

 the oesophagus and buccal cavity are always 

 lined by epiblast. When the blastopore re- 

 mains permanently open the outer part of 

 the oesophagus grows as a prominent ridge 

 round the opening. 



The mesenteric sack itself becomes dif- 

 ferentiated into a stomach adjoining the 

 oesophagus, a liver opening immediately 

 behind this, and an intestine. The cells 

 forming the hepatic diverticula and some- 

 times also those of the stomach may du ring- 

 larval life secrete in their interior peculiar 

 albuminous products, similar to ordinary 

 food-yolk. 



The proctodgeum, except when it is the blastopore, arises later 

 than the mouth. It is frequently developed from a pair of projecting 

 epiblast cells symmetrically placed in the median ventral line behind 

 the foot. It eventually forms a very shallow imagination meeting 

 the intestine. Its opening is the anus. The anus, though at first 

 always symmetrical and ventral, subsequently, on the formation of 

 the pallial cavity, opens into this usually on the right and dorsal side. 

 In the cases where the hypoblast is not invaginated in the form 

 of a sack the formation of the mesenteron is somewhat complicated, 

 and is described in the sequel. 



From the trochosphere stage the larva passes into what has been 

 called by Lankester the veliger stage (fig. 103), which is especially 

 characteristic of Gasteropod and Pteropod Mollusca. 



The shell-gland (with a few exceptions to be spoken of subse- 

 quently) of the previous stage flattens out, forming a disc-like area, 

 on the surface of which a delicate shell becomes developed, while 

 the epiblast of the edges of the disc becomes thickened. The 

 disc-like area is the mantle. The edge of the area and with it the 

 shell now rapidly extend, especially in a dorsal direction. Up to this 

 time the embryo has been symmetrical, but in most Gasteropods the 

 shell and mantle extend very much more towards the left than to- 

 wards the right side, and a commencement of the permanent spiral 

 shell is thus produced. 



c 



FIG. 102. EMBRYO OF A 

 HETEROPOD. (From. Gegen- 

 baur; after Fol.) 



o. mouth; v. velum; g. 

 archenteron ; p. foot; c. 

 body cavity ; s. shell-gland. 



