CONJUGATION IN THE CRAYFISH, CAMBARUS AFFINIS 239 
seized another male, which at once escaped. As he did not see male 
seize male on other occasions, he came to the conclusion that 
crayfish use their eyes as one mode of recognizing the sex of their 
fellows, though he also surmised the sense of smell might play 
some part in the sex recognition. 
While carefully devised experiments would be necessary to 
disprove the use of the sense of sight and of smell in sex recog- 
nition in these crayfishes, there is as yet no evidence that the 
crayfish recognizes the sex of another with which he is not in 
actual contact. On the other hand the behavior of the sexes once 
they are seized is so different that this alone would account for 
the carrying out of the processes of conjugation between any 
members of the opposite sexes. 
\Mien the males are approached they brandish their claws and if 
there are two males one withdraws, after more or less deliberate 
holding and shoving of one another's claws. WTien a female is 
approached she generally retreats, but may show fight and more or 
less successfully keep off the male. In confined spaces, however, 
the female is almost always overcome by the male even though 
he be much smaller or lacking in one claw. A few females amidst 
many hundred males will alwaj^s be found out and clasped by 
males; but when very many males were kept together only one 
case was seen in which a male had seized and held, as if in conju- 
gation, a small male, but this one was dead. That this selection 
of females by the males may be merely a matter of adj ustment of 
males to the different amount and character of resistance offered 
by the females and males when seized is, perhaps, supported by 
the fact that a difference in muscular tonus is rather distinctive 
of the sexes. In fact it was found possible to sort a lot of cray- 
fishes into two sets, one male, one female, with one's eyes shut, 
merely by their differences in muscular contraction when taken 
hold of, the males have a much more pronounced habit of violently 
contracting their limbs and trunk muscles, so as to pass into a 
rigid state that may last a long time when the animal is out of 
the water. The females are, as a rule, notably more relaxed. 
WTien the male acutally seizes a female or a male he seems to 
judge of the amount or the character of the resistance by pushing 
