lyo THE AMERICAN NATURALIST. [Vol. XXXVIII. 



determine this point reported to me that one male was seen to 

 use the same leg on one side, in several successive unions with 

 different females. 



In one case the fifth leg was seen to project between the third 

 and fourth legs of the opposite side instead of between the 

 fourth and fifth as is the rule. Possibly this may have been in 

 connection with some difference in size between the female and 

 male. . Though the two pairing are about the same size there is 

 often considerable difference in length and to secure accurate 

 interadjustment of such rigid bodies with so many protuberances 

 antl pairs of ap[:>endages is no small problem. To solve it the male 

 at times rela.xes his abdomen and moves forward o\'er the female 

 and when finally the tips of the pleopods have been introduced 

 into the annulus the male again envelojoes the end of the 

 abdomen of the female and firmly holds it as in the hollow of a 

 hand. The persistent union of the two is made more complete 

 by the use of the hooks found on the legs of the males of this 

 and some other species of Cambarus ; apparently these fasten 

 the male so that the abdominal pressure exerted down the pleo- 

 pods against the annulus does not react and push the male 

 upward away from the female. In C. affinis there is one of 

 these hooks or spines on the third segment of each third 

 walking leg and the male fastens these two hooks into the base 

 of the fourth legs of the female. In the above figures the male 

 has raised the third left leg so that the hook is free and far from 

 its projjer socket. When lowered into place the hook depresses 

 the soft membrane that forms the external aspect of the basal 

 segment and thus a temjjorary socket is formed. The outer 

 ventral edge ot this socket is bounded by the stiff calcified ridges 

 that help form the hinge between the first and second segments 

 and It a-ainst this rigid rim that the spine of the male's leg 

 uiuhe^. The use of these male hooks as secondary sexual 

 <»i,uans is thus established and we may expect to find that in 

 those spei-ies of cambarus in which more than one pair of legs 

 are provided with such hooks, that there will be a corresponding 

 increase in comjjlexity of the adjustments of sexual union. 



The union of the male and female is now so hrm that they 

 cannot be readily separated, and if thrown into actively boiling 



