Xll 



PHYLUM MOLLUSCA 



709 



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

 7nentation is total but unequal, and the gastrula is formed either 



Fig. 593. — Five stages in the development of Ostrea. a anus ; U, blastopore ; m. mouth ; 

 Hia, stomach ; mes. mesoderm : rk, polar bodies ; s. shell ; sd, shell-gland ; s-iii. anterior adductor ; 

 Wf pre-oral circlet of cilia. (From Korsclielt and Heider.) 



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

 formed as an invagination of tlae dorsal surface, a stomodosum (»i) 

 as an invagination of the 



ventral surface, and the 

 larva of most forms, un- 

 like that of Anodonta or 

 Unio, passes into a stage 

 in which it closely re- 

 sembles the trochophore 

 of Cha3topods (Fig. 693), 

 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 



Fig. 594.— Veliger larva of Ostrea. a. anus; dm. 

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

 ma, stoxnaeh ; s. shell ; sm. adductor muscle ; ss. 

 hinge of shell ; Vd. velum ; vm. ventral longitu- 

 dinal muscle. (From Korschelt and Heider.) 



