270 



SEXUAL DIMORPHISM 



tion to their employment in copulation, and the greater size 

 and strength of parts of the body, and especially of the chelae 

 or pincers, are related to the exertions of the male either in 

 lighting with rivals or obtaining possession of the female. 

 There is clearly no reason why these peculiarities should exist 



Fig. 28. Gonoplax angulatus, the Angular Crab. A, male ; B, female. 



only during the breeding season of the year, unless they owed 

 their existence to stimulations whose action was confined to 

 that season. 



Amphipoda. These are small sessile-eyed Crustacea which 

 move generally by swimming or jumping. The body is com- 

 pressed from side to side. The number of thoracic appendages 



