2r.6 



THE SOCIAL LIFE OF ANIMALS 



dition as well as for its prolongation. With these 

 snails the tendency to become first a male and later 

 a female is probably determined by heredity, al- 

 though the hereditary mechanism which promotes 

 such a shift is at present unknown. The point of 

 interest for this discussion is that the association 



CFORNICATA 



Fig. 46. As Crepidula fornicata gets older and larger 

 it passes successively from the sexually immature 

 through the male on into a final female stage. Mated 

 males retain that stage longer than if actively motile. 

 (From Coe.) 



with others, especially among mated males, tends to 

 postpone transformation to the opposite sex. 



Some cases are known in which the presence of 

 other animals of the same species determines the 

 sex. One of the most thoroughly studied is that of 

 the worm Bonellia, (21) in which the sexually un- 

 differentiated larva does not, in nature, become the 

 small parasitic male unless it is associated with the 

 large female. 



Among certain nematode worms which are para- 

 sitic in insects, if few eggs are introduced into, for 

 example, grasshoppers, (3) most of the resulting 



