STRUCTURAL EFFECTS 139 



but 50 per cent females strictly determined at 

 hatching, then they transform into females and 

 are able to effect development of an indifferent 

 larva into the male needed for fertilizing their 

 eggs, if one comes near. 



The outline of the development of sex in the 

 American marine snail, Crepidula, is essentially 

 similar. These almost sessile mollusks live in the 

 shells inhabited by hermit crabs. If a young 

 neuter animal settles near a larger one, whether 

 the larger snail is male or female, the small neuter 

 individual develops into a functional male. If 

 later the larger individual is removed, the smaller 

 animal reverts to an indifferent stage and then 

 develops into a functional female. If the neuter 

 animal settles in an unoccupied shell, it develops 

 directly into a female. The causal factors are 

 not so clear in this case as with Bonellia but the 

 survival values of the case are practically identical. 



In nematode parasites of grasshoppers and other 

 animals, if there are many parasites present in 

 one host, all or the great majority tend to be 

 males; when few are present, the majority are 

 females. The causal relations here are not 

 understood. 



With water fleas, relatives of Daphnia, the 

 whole group of which is commonly called Clado- 

 cera, the whole sexual history is different. These 



