264 METAZOAN PHYLA 



the different appendages ; metameres are formed ; and the embryo gradu- 

 ally assumes the characteristics of the adult. Hatching takes place in 

 from five to eight weeks, but the larvae remain clinging to the swimmerets 

 of the mother for about four weeks longer. During this time they grow, 

 shedding the exoskeleton at intervals but undergoing no metamorphosis. 

 The process of shedding, which is an adjustment to permit growth, is 

 known as molting, or ecdysis. This occurs seven or more times during 

 the summer. It is said that the life of the crayfish covers a span of from 

 three to four years. They reproduce annually after the second year. 



299. Regeneration and Autotomy.- — Crayfishes have the power of 

 restoring lost appendages, and under normal conditions the same sort 

 of appendage is restored as that which was lost. Under experimental 

 or abnormal conditions, however, an abnormal appendage may replace 

 the lost one. Crayfishes also have the power of autotomy, breaking off 

 a walking leg at a point near the base known as the breaking point. 

 This enables a crayfish to escape if its leg is grasped by an enemy or 

 closed upon by the valves of a mussel buried in the bottom over which 

 the crayfish is walking. The structure of the leg is modified at the 

 breaking point to make autotomy easier, but the action is under the 

 control of the individual. 



300. Economic Importance.— In many localities crayfishes serve as 

 food, but in most parts of this country they are used only as fish bait. 

 They are an agency in the destruction of decaying animal bodies in the 

 water and from this standpoint are beneficial. Since they make holes 

 in dams and levees they may cause serious damage. The cotton growers 

 of the South also suffer because the crayfish eats young cotton plants. 

 Especially in the clay lands of Alabama and Mississippi do they interfere 

 with the raising of both corn and cotton. 



