CEUSTACEA 269 



first pair of abdominal legs, which form copulatory organs as 

 in the majority of the stalk-eyed Crustacea. In the first 

 form the tip of this appendage is less membranous than in 

 the second, the hooks on it are horny, the teeth or bifid ends 

 are longer and more separated, the hairiness in those species 

 where hairs occur more profuse. But the first form differs 

 from the second also in other characters. The whole body is 

 more robust and more sculptured, while the claws are longer 

 and broader. Also the hooks on the third joint of the third, 

 or in some species of the third and fourth pairs of walking 

 legs are larger and more developed. The two forms occur 

 in about equal numbers in collections. 



Walter Faxon, in his revision of the Astacidas, 1 states 

 that no intermediate condition between the two forms 

 exists, and that there is no constant relation between the 

 special characters and the size of the individual. In 

 the second form the testes are smaller, and the vasa 

 defer entia shorter than in the first, and Faxon at first 

 came to the conclusion that the second form was sterile, 

 although he could offer no explanation of the existence of 

 sterile males. He afterwards found on keeping the first 

 form alive in captivity that after moulting it changed into 

 the second form, and proved thus that the two forms are 

 alternating conditions in the life of the individual, the first 

 form beingassumed during the pairing season, the second during 

 the interval between the pairing seasons. It would appear 

 from this that Cambarus moults twice in the year, and that 

 this is the reason of the existence of the two forms. Hence 

 it is impossible to avoid the conclusion that the peculiarities 

 of the sexually active form have been determined by the 

 special actions of the breeding male. The peculiarities are 

 precisely those which are related to the relations of the sexes. 

 The copulatory organs are developed and specialised in rela- 



1 Mem. Mus. Harv. x. 4, 1885. 



