No. 545] THE BIOLOGY OF THE CRAYFISH 291 



including the continuous study of several specimens for 

 twenty-four hours, I have concluded (21) that the cray- 

 fish shows his greatest activity at nightfall and at day- 

 break. In nature the crayfish is less active during the 

 day than he is in captivity, since, as a rule, he has more 

 hiding places in his natural habitat. 



Pearse has stated (40) that the number of matings 

 occurring in two boxes, one being painted black and 

 closed entirely, and the other being exposed to light, did 

 not vary to any extent. It is obvious from the work of 

 Andrews and myself that the fact that crayfish in a state 

 of sexual tension, stimulated by transference to different 

 receptacles, copulate as well in the light as in the dark, 

 does not bear on the question of normal activity. The 

 difference between night and day must not be assumed 

 to be entirely that of light, in experiments on higher 

 invertebrates. 



It is possible that the tendency of the crayfish to re- 

 main in hiding during the day is to some extent lessened 

 when sexual feeling is strong, but this seems rather im- 

 probable under natural conditions. 



In my specimens hibernation was well marked. I was 

 careful to change the water daily in my still-water 

 aquarium, thus keeping it fairly cool. Several of my 

 crayfish hibernated as long as six weeks at a time, in 

 closed burrows in the bank of this miniature pond. 



