XII 



PHYLUM MOLLUSCA 



709 



frequently takes place i»' v thc water after the eggs are laid. Seg- 

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



mes- 



E. 



dm ../.J* 



TTltS- ^-^ / 



'.—Five stages in the development of Ostrea. a. anus ; M. blastopore ; m. mouth ; 

 ma, stomach ; vies, mesoderm : rk; polar bodies ; s. shell ; sd, shell-gland ; sm. anterior adductor ; 

 tt>, prc-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 stomodaeum (m) 

 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 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 



Fii;. 0!'4. — Veliger larva of Ostrea. «. anus; 4ml 

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

 «<", stomach ; ». shell ; sm. adductor muscle ; st. 

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

 dinal muscle. (From Korschelt aud Heider.) 



