258 METAZOAN PHYLA 



cells are transferred by the first two pairs of swimmerets of the male 

 from the opening of the vasa deferentia to the seminal receptacles of 

 the female. The seminal receptacles are cavities inclosed in folds of the 

 cuticula between the fourth and fifth pairs of walking legs. There the 

 sperm cells remain until the eggs are matured. At this time the latter 

 are passed out of the oviducts, which open at the bases of the third pair 

 of legs, and backward in a groove between the bases of the legs of the 

 two sides of the body, receiving sperm cells and being fertilized on the 

 way. The eggs finally become attached to the swimmerets by a gluelike 

 secretion, masses of them appearing like so many bunches of grapes 

 (Fig. 158). They remain attached during development, their aeration 

 being assisted by movements of the swimmerets. 



Fig. 158. — A female crayfish with eggs attached to her swimmerets. (From Andrews, in 



Am. Natur., vol. 38.) 



Cleavage is superficial and the embryo develops from a thickening 

 of the blastoderm on one side. Limb buds appear, which correspond to 

 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. 



297. 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. 



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

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



