f^ 



Figure 343. — Photomicrograph of a tangential, slightly 

 slanted section of the larval eye of C. virginica preserved 

 in osmie acid. 





than it does at the straight-hinge stage. When 

 the larva attaches to the substratum, the velum 

 and the foot are no longer needed. Their disap- 

 pearance marks the transition from free-swimming 

 to a sedentary mode of life. Garstang (1929) 

 expresses the correct opinion that larval organs 

 should be regarded as an adaptation to the con- 

 dition of life during development and need not 

 affect the organization of the adult. His charming 

 book on larval forms (Garstang, 1951) summarizes 

 in a somewhat unorthodox way the ideas and 

 theories concerning the significance of various 

 larval forms in the evolution of aquatic animals. 



MORPHOLOGY OF LARVAL SHELL 



The morphology of the larval shell differs from 

 that of an adult oyster primarily in the greater 



t 



# 



|l 



t "i^. 



m 



\ 



V. 



X 



\« 



\ 



Micron 



Figure 344. — Electron micrograph of a tangential section of a portion of a ciliated cell of the velum of the larva of C. 



virginica. 



LARVAL DEVELOPMENT AND METAMORPHOSIS 



363 



