186 Heredity. 



Darwin says that it seems to be a general rule among 

 the Crustacea, that the remarkable differences of struc- 

 ture which distinguish the male from the female, do not 

 make their appearance until the male is nearly mature. 

 In proof of this he refers to the fact that the male sand- 

 hopper, does not acquire his large claspers, which are 

 very differently constructed from those of the female, 

 until nearly full grown, while the claspers of the young 

 male resemble those of the female. 



The history of the abdomen in crabs seems to show 



FIG. 25. Tip of abdomen of male FIG. 86. Tip of abdomen of 



Lucifer. female. 



clearly that this difference is due to the fact that the 

 male has deviated further than the female from the an- 

 cestral type. The long-tailed Crustacea, like the cray- 

 fish, have a long free movable abdomen, ending in an 

 enlarged tail-fin, and composed of a number of seg- 

 ments, each of which carries a pair of appendages. In 

 the female cray-fish the first of these appendages are 

 like those behind, but in the male, the first ones are 

 peculiarly modified and form, copulatory organs. We 

 have ample evidence that the true crabs are the modi- 



