MOLLUSCA. 



231 



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 to- 

 wards the left than towards the right side, and a commencement 

 of the permanent spiral shell is thus produced. 



The edge of the mantle forms a projecting lip separating the 

 dorsal visceral sack from the head and foot. An invagination 

 appears, usually on the right in Gasteropods, and eventually 

 extends to the dorsal side (fig. 103 B). It gives rise to the 



FIG. 103. LARVAE OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE. 

 (From Gegenbaur.) 



A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia). 

 v, velum ; c. shell ; p. foot ; op. operculum ; /. tentacle. 



pallial or branchial cavity, and receives also the openings of the 

 digestive, generative and urinary organs. In most Pteropods it 

 is also formed to the right, and usually eventually extends after- 

 wards towards the ventral surface (fig. 103 C). In the pallial 

 cavity the gills are formed, in those groups in which they are 

 present, as solid processes frequently ciliated. They are coated 

 by epiblast and contain a core of mesoblast. They soon become 

 hollow and contractile. 



The velum in the more typical forms loses its simple circular 

 form, and becomes a projecting bilobed organ, which serves the 

 larva after it is hatched as the organ of locomotion (fig. 103 B 

 and C). The extent of the development of the velum varies 

 greatly. In the Heteropods especially it becomes very large, and 

 in Atlanta it becomes six-lobed, each lateral half presenting 

 three subdivisions. It is usually armed on its projecting edge 

 with several rows of long cilia, and below this with short cilia 



