18 ASTACIDJE. 



hooks and teeth. We find also in Cambarus the tip 01 the first abdom- 

 inal legs bifid, and the two branches more or less elongated, equal or 

 not in length and breadth, straight or curved, and very well adapted to 

 form specific characters. Having examined a very great number of 

 specimens, I am able to state that these different forms are very con- 

 stant in the same species. Naturally here, as elsewhere throughout the 

 group, it is not possible to find an exact mathematical identity, but a 

 constancy within certain limits, and I was aide to observe and figure 

 some variations. 



The rolled part of the second pair of abdominal legs — I have re- 

 marked before that it is rolled in the opposite direction — is apparently 

 analogous to this formation in the first pair, and is formed in the Euro- 

 pean species exactly in the same manner as in the first abdominal legs. 

 In the Cambarus the apical end of the dilated plate is considerably more 

 rolled than the basal end, assuming in this manner a triangular shape. 

 It is interesting to find the same arrangement in the true American 

 a istaeus. 



The Purpose of this Structure. — The purpose of this structure of the 

 first two pairs of abdominal legs is easily explained. The seminal fluid 

 coming out of the basal part of the fifth legs by an open circular aper- 

 ture, must be conveyed to the sexual aperture of the female, situated 

 farther forward in the inner basal part of the third legs. The first pair 

 of the abdominal legs of the male, being situated closer to the venter, is 

 very well adapted by its channelled shape (represented in the Cambarus 

 by the shallow suture) to direct the seminal fluid to the designed part in 

 the female. It is likewise well to notice that the transformed shape of 

 the second abdominal legs gives a considerable help for this purpose in 

 two ways. First, the dilated part, especially in the Cambarus, is well 

 adapted to be inserted into the sutured part of the first legs, and 

 mechanically to support the first legs in a horizontal position. The 

 rolled part also serves to complete the channel made by the first legs 

 for the direction of the seminal fluid, and it is well to remark that the 

 inverse manner of rolling, as before stated, serves more completely to 

 prevent any of the ejected seminal fluid from taking a wrong direction. 

 In the Madagascar and most of the Australian Astacidaa these modified 

 abdominal legs, fitted for use in coition, are entirely wanting, and the 

 conveyance of the seminal fluid is perhaps more simple and imperfect. 



The Females, — Regularly in the articulated animals, especially in 

 the insects, we find in the genera or families in which the organiza- 

 tion of the sexual parts diners specifically in the males, that the 

 females also exhibit differences more or less adapted to the male or- 

 gans. The striking differences in the male organs of the Cambari 

 made the presence of analogous differences in the females probable. 

 But these are not to be found. The female sexual aperture is always 



