660 



ZOOLOGY 



SECT. 



phase of development. The shell soon becomes "bivalved and 

 extends ventrally on each side, paired processes of the dorsal 



E. 



'r/C. 



c. 



D. 



E. 



Or. 



FIG. 55(3. Five stages in the development of Ostrea. a. anus ; /*/. blastopore ; m. mouth ; 

 ma, stomach ; mes. mesoderm ; rk, polar bodies ; s. shell ; sil, shell-gland ; sni, anterior adductor ; 

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



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

 A projection STOWS out from the ventral surface, between mouth 



1 71 



and anus, and becomes 

 the foot (Fig. 558 /), and 

 on the sides of the body 

 the gill-filaments (k) arise 

 as a row of delicate pro- 

 m cesses, at first simple, but 



afterwards becoming bent 

 \ upon themselves so as to 



A cm assume a V-shape. Eyes 

 are often present in the 

 larva at the base of the 

 ry " tt velum. 



/ General Remarks. 



Although none of the 

 Pelecypoda are micro- 

 scopic, they present a con- 

 siderable range in size, 

 from the little fresh-water 

 Cyclas, about 1 cm. long 

 to the Giant Clam ( Tri- 

 dacna f/if/as) of the Indian and Pacific islands, which is sometimes 

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



s.m. 



nur. 



Fie. :,',!. Voliger larva of Ostrea. a. anus ; <? llt . 

 dorsal longitudinal muscle; /. liver; ,,>. mouth; 

 ma, stomach; *. shell; ,x,, t , adductor muscle; as, 

 hinge of shell ; ?d. velum ; cm. ventral longitu- 

 dinal muscle. (From Korschelt and Heider.) 



