288 



COLLEGE ZOOLOGY 



entia (38) and out of the genital apertures (8), one in the base of 

 each fifth walking leg. In the female the eggs arise in the bilobed 

 ovary (Fig. 205, 2, 3), pass through the paired oviducts (1), and 

 out of the genital apertures (4), one in the base of each third 

 walking leg. 



The spermatozoa are transferred from the male to the seminal 

 receptacle of the female during copulation, which usually takes 

 place in the autumn. The seminal receptacle is a cavity in a 

 fold of cuticle between the fourth and fifth pairs of walking legs. 

 The eggs are laid in April and are probably fertilized by the 

 spermatozoa at this time. They are fastened with a sort of glue 



Fig. 206. — Female crayfish aerating eggs by raising and straightening 

 abdomen and waving swimmerets back and forth. (From Andrews in Am. 

 Nat.) 



to the swimmerets, and are aerated by being moved back and 

 forth through the water (Fig. 206). 



The cleavage of the egg is superficial (Fig. 207, A), and the em- 

 bryo appears first as a thickening on one side (Fig. 207, B). 

 The eggs hatch in from five to eight weeks, and the larvas cling 

 to the egg-shell. In about two days they shed their cuticular 

 covering, a process known as molting or ecdysis. This casting 

 off of the covering of the body is not peculiar to the young, but 

 occurs in adult crayfishes as well as in young, and adults of many 

 other animals. In the larval crayfish the cuticle of the first stage 

 becomes loosened and drops off. In the meantime, the hypo- 

 dermal cells have secreted a new covering. Ecdysis is necessary 

 before growth can proceed, since the chitin of which the exo- 

 skeleton is composed does not allow expansion. In adults it is 



