294 ANIMAL AGGREGATIONS 



initiation and the final results were less complete than when attach- 

 ment to living proboscides was allowed. 



The situation, so far as Bonellia viridis is concerned, has been 

 summarized by Baltzer as essentially follows: 



Sex determination is partly predetermined, partly epigenetic. 

 Both sex tendencies are probably predetermined in the fertilized 

 egg in varying degree, with the male tendency predominant. The 

 fertilized egg, as well as the indifferent larvae which is not yet sex- 

 ually differentiated in its organization, is most probably hermaphro- 

 ditic. Only in their further development do they become wholly 

 male or female. In this development the evolution of the organiza- 

 tion in males or in females proceeds in an unlike manner, cor- 

 responding to the strong sexual dimorphism of the species. 



With few exceptions the development of males is possible only 

 when the indifferent free-swimming larva finds an opportunity to 

 live parasitically on the proboscis of an adult female. During this 

 period there is an absorption of sex-determining substance from the 

 host by the larva. If this opportunity for parasitism exists, all the 

 larvae usually become males. If it is lacking and the larvae are 

 forced to live freely, females arise almost exclusively, though there 

 are occasional hermaphrodites. Females appear only after a longer 

 indifferent period during which development is approximately at a 

 standstill; during this time the female tendency gradually gains in 

 intensity and finally assumes ascendancy over the male tendency. 

 The females that arise late are protandric hermaphrodites and 

 contain sperm in the coelom, but in further development become 

 typical females. 



If opportunity for parasitic development is given to swarming 

 indifferent larvae, and if the parasitism is prematurely interrupted 

 by separating the larvae from the proboscis of the host, then, after 

 further culture in the free state, hermaphrodites arise, together with 

 a few males and females. The duration of the parasitic period de- 

 termines whether hermaphrodites actually bisexual are obtained, 

 or gynandromorphs in which only the secondary sexual characters 

 are combined. In Bonellia these secondary sexual characters include 

 most of the organizational characters. 



