1 68 THE AMERICAN NATURALIST. [Vol. XXXV I II. 



back he seizes all her clawed legs with his two large claws, hold- 

 ing the three left ones of the female with his one right claw and 

 the three right ones of the female with his left. He then moves 

 forward over the supine female to the position indicated in Fig. 

 I . From this figure it is evident that the two crayfish are accu- 



rately and closely adjusted to one another ; not only does the 

 male hold all the claws of the female, but his abdomen is tightly 

 bent around that of the female which is closely coiled up under 

 the male. While all the five right legs of the female may be 

 seen, there appear to be but four legs of the male's set of five 

 left legs. The base only of the male's fifth leg is shown ; poste- 

 rior to this are the peculiar long male pleopods, or appendages 

 of the first abdominal segment and of the second abdominal seg- 

 ment. These four appendages are elevated at an angle of about 

 45 and point toward the ventral surface of the thorax of the 

 female, forward and downward. A view of the right side of the 

 ixiir wculd, however, show the left fifth leg of the male project- 

 m- outward and backward between the fourth and fifth legs of 

 that right side. 



The explanation of this peculiar arrangement is found in a 

 habit of the male which seems necessary for the accomplish- 

 ment of sperm transfer and is a very instructive example of 

 mechanical adjustment amongst several rigid calcified organs. 

 After the male has come forward over the supine female there 

 is a period of ten to twenty minutes of apparent inaction before 



