SOCIAL TRANSITIONS 253 



In many of these mollusks he has found that the sex 

 ratios vary greatly in different environments, and 

 has reached the conclusion that frequently among 

 these animals the expression of an innate sexual 

 tendency may be in part suppressed or stimulated, 

 as the case may be, by the environment in which 

 any given animal is living. 



A pertinent case is that of a set of marine snails 

 of the genus Crepidula. Three of these "boat-shell" 

 snails are common animals in the coastal waters of 

 southern New England. Their sexual history follows 

 similar outlines. After a juvenile period which is 

 essentially asexual, the growing Crepidula becomes 

 first a male and then later, sometimes only at long 

 last, it transforms into a female. A typical species to 

 follow through this transformation is Crepidula for- 

 nicata. 



When young, these animals move about, but as 

 they become older and larger they settle down in 

 one place on a wharf piling or a rock or another 

 shell. If the larger, older animals are broken loose 

 the soft parts are usually destroyed by some predator 

 before they can reattach themselves, leaving behind 

 the relatively heavy shell. Frequently they form 

 large chains of individuals, of which a simple exam- 

 ple is shown in Figure 45. The large, bottom snail 



