STRUCTURAL EFFECTS 137 



Other cases are known in which sex, a second 

 fundamental property of organisms, is determined 

 by crowding. It will not be feasible here to 

 review all the known cases and the following 

 brief account must suffice: Bonellia is a marine 

 worm which lives in restricted regions in the Bay 

 of Naples. The two sexes differ decidedly in 

 appearance. The female is fairly large, three 

 inches or so in length with a long, extensible 

 proboscis at the anterior end. The males are 

 minute flatworm-like animals about a millimeter 

 in length and are totally different in appearance 

 from the females. The female has a large body 

 cavity with a well developed alimentary canal. 

 The male lacks the proboscis and its reduced 

 alimentary canal is without an external opening. 

 It lives as a parasite within the uterus of the 

 female. 



The eggs develop into freely swimming larvae 

 which at first are sexually indifferent. If these 

 swimming larvae come into contact with a female, 

 they settle upon the proboscis and start develop- 

 ing as males. After a few days in this position, 

 they enter the alimentary canal of the female and 

 continue their development for about two weeks, 

 after which they make their way to their final 

 position as parasites within the uterus of the 

 female. The developing male takes no food during 



