230 GASTEROPODA AND PTEROPODA. 



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 invagination 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 

 oesophagus is formed of epiblast cells, so 

 that the oesophagus and buccal cavity 

 are always lined by epiblast. When the 

 blastopore remains permanently open the 

 outer part of the oesophagus grows as a 

 prominent ridge round the opening. 



The mesenteric sack itself becomes 



FIG. 102. EMBRYO OF 



differentiated into a stomach adjoining A HETEROPOD. (Fom Ge- 

 the oesophagus, a liver opening immedi- s enbaur ; a * er FoL) , 



,,,,,. , o. mouth; v. velum; g. 



ately behind this, and an intestine. The archenteron; /.foot; c. body 



cells forming the hepatic diverticula and cavit y ; s ' shell -s land - 

 sometimes also those of the stomach may during larval life 

 secrete in their interior peculiar albuminous products, similar to 

 ordinary food-yolk. 



The proctodasum, 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 

 invagination meeting the intestine. Its opening is the anus. 

 The anus, though at first always symmetrical and ventral, subse- 

 quently, 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 



