PHYLUM ARTHROPODA 



289 



also a means of getting rid of an old worn-out coat and acquiring 

 a new one. The young stay with the mother for about one month, 



Fig. 207. — Stages in the development of the egg of the crayfish. A, super- 

 ficial cleavage of the egg. B, embryo in the Nauplius stage. A, anus; 

 a 1 , antennule; a 2 , antenna; e, rudiment of eye; /, upper lip; m, mandible; 

 la, thoraco-abdominal plate. (From Korschelt and Heider, after Reichenbach.) 



and then shift for themselves. They molt at least seven times 

 during the first summer. The life of a crayfish usually extends 

 over a period of three 

 or four years. 



Regeneration. — The 

 crayfish and many 

 other crustaceans have 

 the power of regenerat- 

 ing lost parts, but to a 

 much more limited ex- 

 jtent than such animals 

 I as Hydra and the earth- 

 worm. Experiments 

 jhave been performed 

 lupon almost every one 

 pf the appendages as 

 Well as the eye. The 

 growth of regenerated tissue is more frequent and rapid in 

 Uoung specimens than in adults. The new structure is not 

 u 



Fig. 208. — Diagram showing antenna-like 

 organ regenerated in place of an eye of Palxmon. 

 (From Morgan, after Herbst.) 



