CEUSTACEA 267 



the male exerts his muscular powers in general, and his 

 chelae in particular, most in holding the female, or in 

 fighting with other males. There can be little doubt, how- 

 ever, that the males struggle with one another, and it is 

 certain that their greater size and strength, and especially 

 the greater development of their chelae, corresponds to the 

 more active part they play in the relations of the sexes. 



Fig. 27. Corystes cassivelaunus, female. 



To illustrate the sexual dimorphism of the Brachyura I 

 have given figures of the male and female in two British 

 species, the Masked Crab, Corystes cassivelaunus, and the 

 Angular Crab, Gonoplax angulatus. These figures were drawn 

 from average adult specimens, and correctly represent the 

 proportional dimensions of the males and females. 



Darwin mentions a Brazilian species of Gelasimus in which 

 the male is coloured green and white, while the female is 

 uniformly greyish brown, and states that the male does not 



