B. 



^,■ 100 



Microns 







Microns 



90 



Figure 338. — Photomicrographs of early straight-hinge live larvae of C. virginica. A — larvae resting on bottom, shells 

 closed; B — the slightly narcotized larva (upper part) has its velum protruding from the shell; the lower larva has 

 closed its shell and withdrawn its velum. 



by English-speaking oyster biologists in spite of 

 the obvious contradiction in applying the adjec- 

 tives adult and matiu'e to larval stages. The 

 term "velichoncha" proposed by Werner (1940) 

 and adopted by Rees (1950) refers to the advanced 

 stages of development of bivalve larvae, but the 

 expression is not generally used in malacological 

 literature. The name "pediveliger" was proposed 

 bj" Carriker ('1961) to designate the "swunming- 

 creeping" stage of clam larva, Merce/naria (Venun) 

 mercenaria. The term deserves to be accepted in 

 malacological literature because it indicates the 

 major character, i.e., the presence of a foot, and 

 is applicable to many bivalve species, including 



oysters, in which a larval foot appears during the 

 planktonic period. 



As the larva grows its valves become deeper and 

 almost circular. The liinge develops two bulgings 

 or umb(uies, the one on the left side larger than 

 its opposite number. At these stages the umbones 

 bend toward the posterior end of the shell, which 

 at this tune has pronounced concentric rings, is 

 heavy, and obscures the organs under it. In 

 swuiiming the umbo larva protrudes its large cili- 

 ated foot forward. The larvae of C. virginica 

 now have diameters of more than 300 /i in both 

 length and height. The photomicrograph in 

 figm-e 339 shows the side view of the advanced 



358 



FISH AND WILDLIFE SERVICE 



