THE SPERM-RECEPTACLE IN CAMBARUS. I7I 



and long, between the wings of the fourth legs and a rather flat 

 plate that may be regarded as belonging to that same somite. 



Deciding that the wedge-plate really belongs to the somite of 

 the fourth legs we find no difficulty in homologizing it with the 

 annulus of the crayfish. We will call the wedge-plate an an- 

 nular plate. The only reason that this is not self-evident is that 

 the annular plate of the lobster is secondarily pushed far back 

 and fused to the sternal skeleton of the fifth leg somite. Even 

 in the crayfishes there is a marked recession of the annular 

 plate and in the lobster which has not the freedom of motion 

 between the fifth and fourth somites that is found in the crayfish, 

 the backward migration and fusion could take place to a greater 

 extent. 



However, to establish the homology of the wedge with the 

 annular plate it is not necessary to show that they are on the 

 fourth leg somite, but only that they are on the same somite : 

 so that if the endosternites be taken as the exact bounds of the 

 somites both the annulus and the wedge would belong to the 

 somite of the fifth legs, but still be homologous. 



While the receptacle of the lobster is a space between the 

 side plates and an annular plate the receptacle of crayfishes is 

 entirely within the annular plate itself : hence the two recepta- 

 cles are not homologous, though one element of each is the 

 same, the annular plate. 



The general facts regarding the receptacle of the crayfish 

 are as follows : In the crayfishes of Europe, which are all of the 

 genus Astactis, it is said that the sperm is scattered over the pos- 

 terior part of the sternum of the thorax of the female, enclosed 

 in tubes of paste-like material, the spermatophores. Yet there 

 is, in both sexes, an annular plate, slightly marked off from the 

 great fused sternal mass that ends between the fourth legs. The 

 same is presumed to be true of all the crayfishes of Asia and of 

 North America, west of the Rocky Mountains as they are all of 

 the same genus, Astacus. On the other hand in the three score 

 and more crayfishes of the genus Cambarris, found east of these 

 mountains the annular plate is variously sculptured and provided 

 with a suture. Hagen, who discovered this annular plate 

 (Monograph N. A. Astacidae, 1870), supposed it had some gland- 



