298 ANIMAL AGGREGATIONS 



does not take place when a small male is near a larger one. Though 

 males are normally more motile than females, movement is not nec- 

 essary, as is shown by those males the conformation of whose shells 

 proves that they have not recently moved about. Neuters will de- 

 velop into males without showing movement. Male development 

 here is not a matter of food, for neuters will develop into males re- 

 gardless of the richness or scantiness of the food supply, provided 

 only they are near large animals. Starving or feeding mature males 

 does not affect their state of sexual development if they are near 

 large females. 



Preliminary experiments designed to test for the presence of a 

 secretion from the larger animal which would affect the development 

 of the sexual organs of the smaller have been run without positive 

 results. There is no specific ovarian secretion concerned, since the 

 presence of a large male has somewhat the same effect as that of a 

 female. Males in one finger bowl were covered with water from 

 another finger bowl which contained a number of females. The 

 water was changed daily and was replaced with water in which the 

 females had stood. Degeneration of the male organs occurred. Simi- 

 lar results were obtained when water from a dish containing 20-25 

 large females was led into a finger bowl containing 20-25 males. 

 The experiment ran for a month, and during this time the males 

 were kept separated as carefully as possible. Again the degenera- 

 tion of the sexual organs was shown by the condition of the penes. 

 A similar experiment in which water from the mature females flowed 

 over a number of small neuters did not reveal any tendency for penis 

 development. Neither was there any initiation of male development 

 from adding extract of crushed adult females twice daily to finger 

 bowls containing neuters. 



The stimulus to sex development or recession does not depend 

 on the presence of the hermit crab with which these Crepidida are 

 normally associated in nature, since the whole gamut of sex can be 

 run under the artificial conditions of the laboratory, with glassware 

 in place of shells, and with hermit crabs entirely absent. 



Gould recognizes that his experiments concerning the causal rela- 

 tions involved in sex determination in Crepidula are inconclusive. 



