CROWDING AND SEX DETERMINATION 291 



sex may develop, the chances that the first parasite to enter may 

 become male or female are 50 to 50. It is also highly improbable 

 that a second or third nauplius enters only those worms which have 

 been previously parasitized by a copepod predetermined for male- 

 ness. The possibility of differential mortality, however, is not so 

 easily dismissed. 



Malaquin interprets the observed results as being due to crowd- 

 ing, and suggests that the effective agent is the decrease either in 

 amount of food or in available space. With the higher numbers of 

 parasites per host he finds a marked decrease in size of the parasites 

 and, accompanying this, partial or nearly complete suppression of the 

 testes, which he regards as an example of nutritive castration. The 

 statistical facts concerning the tendency toward maleness of crowded 

 monstrilhd parasites appear plain, but the physiological explanation 

 is not so clear; and until necessity compels us to change, we must 

 keep in mind the possibility that the results are due to differential 

 mortality. In this regard, at least, the effects of crowding on sex 

 ratios will be more clear in cases to be reported immediately. 



SEX DETERMINATION IN BONELLIA 



Bonellia is an aberrant annelid worm with strong sexual dimor- 

 phism. In the female, the proboscis is long and extensible and is 

 bifurcated at the anterior end. The enlarged body contains a well- 

 developed alimentary canal, a pair of nephridia, and a single anterior 

 nephridium which is enlarged and serves as a uterus. The male is a 

 small turbellarian-like worm, about i mm. in length, ciliated, lack- 

 ing a proboscis, and with a reduced alimentary canal which lacks 

 both mouth and anus. These and other details are well illustrated 

 in Parker and Haswell's textbook of zoology. Although the adult 

 males differ so much from the females, their structures can be shown 

 to be homologous. A series of studies, chiefly by Baltzer, has pre- 

 sented us with an analysis of the factors leading to this dimorphism, 

 which he conveniently summarizes (1928). 



According to this account, the male living within the uterus of 

 the female fertilizes the eggs there. These are laid, and in about 4 

 days develop into free-swimming larvae about i mm. long. These 



