The Incidence from Sexual Characters. 173 



sexes arc sepanite in all the Rotifera. In some forms 

 the males are even more sim])lilicd than in JS'otonimata, 

 while in others they are less so, and in a few they are 

 like the fern iles in size and siructure, and have the di- 

 gestive organs perfeetly developed. 



Annelids. — Among the marine polych^etons annelids 

 there is often considerable difference between the sexes, 

 and the points in which the male differs from the female 

 are also points in which the males of various species dif- 

 fer from each other. 



Arthropoda. — Among the Arthropods, the Insects, 

 Crustacea, etc., the female is often very gl-eatly modi- 

 fied, and in some cases the females of allied species dif- 

 fer from each other much more than the males, and in 

 other cases it is hardly possible to say whether the males 

 or the females of allied species differ most, but, taking 

 the group as a whole, the Anthropods seem to follow 

 the law which prevails in other groups of animals, and 

 male modifications are more numerous than female 

 modifications. 



In the Branchiopod Crustacea the males are smaller 

 than the females, and are much less abundant. The 

 male differs from the female in the possession of a num- 

 ber of sccondarv sexual characters. The second anten- 

 nae of the male are more richly supplied with sensory 

 hairs than those of the female, and various appendages 

 of the male may be so modified as to form clasping 

 organs for holding the female. In Branchippus the 

 second antennse of the male are greatly modified for 

 this purpose. Figure 3 shows the head of a female 

 specimen of Branclii'ppii8 Griibci, figure 4 the head of 

 the male of the same species, and figures 5 and 6 the 

 heads of the males in two closely allied species. These 

 figures show how much the males of the various species 



