xrt 



PHYLUM MOLLUSCA 



709 



frequently takes place in the water after the eggs are laid. Seg- 

 mentation is total but unequal, and the gastrula is formed either 



E. 



B. 



c. 



si 







bl. 



FIG. 503. Five stages in the development of Ostrea. a. anus ; bl. blastopore ; HI. mouth ; 

 ma, stomach ; me*, mesoderm : rl; polar bodies ; . shell ; sd, shell-gland ; SHI. anterior adductor ; 

 w, pre-oral circlet of cilia. (From Korschelt and Heider.) 



by invagination or by epiboly. A shell-gland (Fig. 593, sd.) is 



formed as an invagination of the dorsal surface, a stomodseuni (m) 



as an invagination of the 



ventral surface, and the 



lavva of most forms, un- ^ ^p 



\V) 



Vel J 



dm, ,-J.Jj? 



like that of Anodonta or 

 Unio, passes into a stage 

 in which it closely re- 

 sembles the trochophore 

 of Chsetopods (Fig. 593), 

 having a pre-oral and a 

 post-oral circlet of cilia, a 

 tuft of cilia round the 

 anus, and an apical tuft 

 in the middle of the pros- 

 tomium. There is also an 

 ectodermal thickening on 

 the prostomium which 

 becomes the cerebral gang- 

 lion, and a similar ventral 

 thickening which gives 

 rise to the pedal ganglion and corresponds with the rudi- 

 ment of the ventral nerve-cord in Polychaeta. The pelecypod 



ma..-' 



' s 



FIG. 594. Veliger larva of Ostrea. a. anus; dm. 

 dorsal longitudinal muscle ; I. "liver" ; m. mouth ; 

 ma, stomach ; s. shell ; stn. adductor muscle ; **. 

 hinge of shell ; Vel. velum ; rm. ventral longitu- 

 dinal muscle. (From Korschelt and Heider.) 



