292 ANIMAL AGGREGATIONS 



larvae are at first sexually indifferent. About 10-12 days after fer- 

 tilization the first signs of transformation into females may appear, 

 providing the larvae remain out of contact with the proboscides of 

 older females, although the larvae can remain indifferent from 2 to 

 4, or even 5 weeks, and then develop into females. 



When they develop into females, the larvae first lose their ciliary 

 bands, the posterior part of the body enlarges, due to the develop- 

 ment of the coelom, and the peristaltic contractions, characteristic of 

 mature females, also begin. The alimentary tract differentiates, setae 

 and anal vesicles are formed, and the animal sinks to the bottom, 

 where it lives from now on. It normally depends on yolk for its 

 food for about another week, before it begins active feeding on mud 

 and detritus. 



Male development is almost always initiated by the effect of a 

 special metagametic factor or factor complex present in the proboscis 

 of the female. If the indifferent larvae settle upon the proboscis 

 lappets of a female, male development starts. It is not necessary 

 that the female should be adult, for immature females have the same 

 effect. This sessile period lasts some 3 days. During this period 

 the typical male pigmentation develops, together with the shorten- 

 ing of the forward end, which is also a characteristic male feature. 

 In the later part of the sessile period spermatogenesis is initiated. 



After 3 or 4 days on the proboscis lappets the developing male 

 creeps into the foregut of the female, where development as a male 

 is completed; and after 16 or 18 days from the beginning of male 

 differentiation the adult male makes its way into the uterus of the 

 female by way of its external opening. During the time spent on 

 the proboscis the developing male takes no food, living on its stored 

 yolk. The food relations of the male during its life in the gut and 

 later in the uterus, though presumably of a parasitic nature, are not 

 exactly known. 



There is good evidence, both from vital staining and from experi- 

 mental removal of the sessile males at different periods after attach- 

 ment to the proboscis lappets of the female, that the attached larvae 

 receive material from the female which induces male development. 

 If the larvae are removed after a very short time on the proboscis 



