xir 



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. 



sd '.. 



A 



( 



i 



bl. 



D. 



E. 



meSr 



FIG. 593. Five stages in the development of Ostrea. a. anus ; bl. blastopore ; m. mouth ; 

 ma, stomach ; mes. mesoderm : rk, polar bodies ; s. shell ; sd, shell-gland ; siii. 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 stomodseum (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 Chastopods (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 Polychajta. The pelecypod 



s 



S3.' 



Fie. "ii4. Veligcr larva of Ostrea. . anus; dm. 

 dorsal longitudinal muscle ; 1. " liver " ; m. mouth ; 

 ma, stomach ; .s. shell ; *//?. adductor muscle ; ss. 

 hinge of shell ; Vc) . velum ; rut. ventral longitu- 

 dinal muscle. (From Korschelt and Heider.) 



