660 



ZOOLOGY 



SECT. 



phase of development. The shell soon becomes bivalved and 

 extends ventrally on each side, paired processes of the dorsal 



E. 



C. 



mes. 



TO. 



Fir;. 55(1. Five stages in the development of Ostrea. a. anus ; W. blastopore ; m. mouth ; 

 ma, stomach ; me*, mesoderm ; rk, polar bodies ; g. shell ; *<l, shell-gland ; sm, anterior adductor ; 

 if, pre-c.'ral circlet of cilia. (From Korschelt and Heider.) 



region of the body accompanying it and forming the mantle-lobes. 



A projection grows out from the ventral surface, between mouth 



and anus, and becomes 

 the foot (Fig. 558 /), and 

 on the sides of the body 

 the gill-filaments (&) arise 

 as a row of delicate pro- 

 cesses, at first simple, but 

 afterwards becoming bent 

 upon themselves so as to 

 assume a V-shape. Eyes 

 are often present in the 

 larva at the base of the 

 velum. 



General Remarks. 

 Although none of the 

 Pelecypoda are micro- 

 scopic, they present a con- 

 siderable range in size, 

 from the little fresh-water 

 Ci/clas, about 1 cm. long 

 to the Giant Clam ( Tri- 



dacna //<>) of the Indian and Pacific islands, which is sometimes 



60 cm. (two feet) in length and 500 pounds in weight. 



ma..- 



i. a V- p.m.. 



- - a. 



'. s 



(tut. 



KM;. '.:.;. Veliger larva of Ostrea. a. anus; 

 dorsal longitudinal muscle; /. liver; m. mouth; 

 ma, stomach; *. shell; *m, addurtm- muscle; ss, 

 hinge of shell; re/, velum; vm. ventral longitu- 

 dinal muscle. (From Ktirschelt and Heider.) 



