post, ad 



Figure 342. — Optical section of fully dpvelopcd larva (pediveligcr) of C. virginica viewed from the left side, m swimming 

 position. Composite drawing from a numlier of photomicrographs of live, slightly narcotized larvae and whole 

 mounts m glycerol, a. — anus; ab.c. — aboral circle of cilia; ant. ad. — anterior adductor muscle; b.g. — byssus gland; 

 d.div. — digestive diverticula; e. — esophagus; ey. — eye; f. — foot; f.r. — foot retractor muscles; g. — gill rudiment; h. — 

 heart; int. — intestine; m. — mouth; m.c. — mantle cavity; post.ad. — posterior adductor muscle; r. — rectum; r.v. 

 velar retractor muscles; st. — stomach; stc. — statocysts; v. — velum. 



they contain lartce oval mitochondria close to the 

 rootlets. 



The high degree of specialization of lar\al organs 

 may he regarded as an adaptive organization of a 

 free-swunming organism to its environment and 

 may have no phylogenetic significance. The 

 pelagic larva of a hivalve has a douhle task: to 

 distribute the species and grow into an adnlt. 



The performance of these tasks requires the main- 

 tenance of an equilibrium between the locomotive 

 efficiency and the weight to be carried; this main- 

 tenance is accomplished by the development of 

 the velum. As the shell grows and becomes 

 tiiicker and heavier, the task of swimming becomes 

 more difhcult, and the fully grown larva sinks to 

 tiie bottom more rapidly and possibly more often 



362 



FISH AND WILDLIFE SERVICE 



