266 



SEXUAL DIMORPHISM 



the female, and his chelae or pincers especially are larger 

 than those of the female. Darwin believed that the chief use 

 of the chelae was to seize and hold the female, though he sub- 

 sequently remarks that the chelae are well adapted for fighting. 

 In the Brachyura fertilisation is effected by a process of true 

 copulation. The male grasps the female at first by the back. 



Fig. 26. Corystes cassivelaumis, the Masked Crab, male. 



The copulatory organs in the male are the two first pairs of 

 appendages on the " tail " or abdomen. The apertures of the 

 seminal ducts are at the base of the fifth pair of walking 

 legs. The female apertures are on the bases of the third 

 pair of legs. The intromittent appendages of the male are 

 inserted into the female apertures, and the milt of the male 

 is thus introduced. I have not had an opportunity of making 

 detailed observations with the object of discovering whether 



