THE CRAYFISH AND ARTHROPODS IN GENERAL 217 



in adult crayfishes as well as in young and adults of many other 

 animals. In the larval crayfish the cuticle of the first stage be- 

 comes loosened and drops off. In the meantime the hypodermal 

 cells have secreted a new covering. Ecdysis is necessary before 

 growth can proceed, since the chitin of which the exoskeleton is 

 composed does not allow expansion. In adults it is also a means 

 of getting rid of an old worn-out coat and acquiring a new one. 



In the second larval stage the young crayfish is again supported 

 immediately after casting off its covering by a thread extending 

 from the new to the old telson. When the larva becomes strong 



FIG. 116. Larva of crayfish 24 hours after hatching ; claws fastened to stalk 

 of eggshell and abdomen fastened to inside of shell. (From Andrews in 

 Am. Nat.) 



enough, it grasps the old larval skin or swimmerets, and the telson 

 thread drops off. The duration of the second larval stage is 

 about six days. 



No telson thread is present after the molt which ushers in the 

 third larval stage, but the young is able at once to cling to the old 

 cuticle. In about a week the third larvae become entirely inde- 



