PHYLUM ARTHROPODA 



213 



They molt at least 6 times during the first 

 summer. Before the new exoskeleton hardens 

 an increase in body bulk occurs, probably 

 due to absorption of unusual quantities of 

 water before molting. Several days after a 

 molt, the animal remains in hiding, thus 

 avoiding enemies while it is in a relatively 

 defenseless condition. It takes several weeks 

 for the new shell to become as completely 

 hardened as the one cast off. 



Regeneration 



The crayfish and many other crustaceans 

 have the power of regenerating lost parts, 

 but to a much more limited extent than 

 such animals as the hydra and earthworm. 

 Experiments have been performed upon al- 

 most every one of the appendages as well 

 as the eye. The second and third maxilli- 

 peds, the walking legs, the swimmerets, and 

 the eye have all been injured or cut off at 

 various times, and the lost parts were sub- 

 sequently renewed. Many species of crayfish 

 of various ages have been used for these ex- 

 periments. The growth of regenerated tissue 

 is more frequent and rapid in young speci- 

 mens than in adults. 



The new structure is not always like that 

 of the one removed. For example, when the 

 annulus containing the seminal receptacle 

 of an adult is extirpated, another is regen- 

 erated; although this is as large as that of 

 the adult, it is comparable in complexity 

 only to that of an early larval stage. A more 

 remarkable phenomenon is the regeneration 

 of an apparently functional antennalike or- 

 gan in place of a degenerate eye which was 

 removed from the blind crayfish Cambarus 

 pellucidus testii. In this case a nonfunc- 

 tional organ was replaced by a functional 

 one of a different character. The regenera- 

 tion of a new part which differs from the 

 part removed is termed heteromorphosis. 



Self-amputation ( autotomy ) 



Perhaps the most interesting anatomic 

 structure connected with the regenerative 



process in Cambarus is the definite break- 

 ing point near the bases of the walking legs 

 (Fig. 119). If a cheliped is grasped or in- 

 jured, it is broken off by the crayfish at the 

 breaking plane. The other walking legs, if 

 injured, may be thrown off at the free joint 

 between the second and third segments. A 

 new leg as large as the one lost develops 

 from the end of the remaining stump. This 

 breaking off of the legs at a definite point 

 is known as autotomy, a phenomenon that 

 occurs also in a number of other animals. 

 The breaking plane in decapod crustaceans 

 is near the base of the legs. The leg is flexed 

 by a special (autotomizer) muscle; con- 

 tinued pull of this muscle separates the leg 

 at the breaking point. The muscles are not 

 damaged, and a membrane develops across 

 the inside of the leg on the proximal side 

 of the breaking place. There is a small hole 

 in the membrane through which nerves and 

 blood vessels pass, but this hole is quickly 

 stopped by a blood clot. About S days later 

 regeneration begins by an outward growth 

 of the cells which lined the exoskeleton. 



Autotomy is an adaptation which pre- 

 vents undue loss of blood when a leg is 

 sacrificed to escape an enemy. 



As in the earthworm, the rate of regenera- 

 tion depends upon the amount of tissue 

 removed. If one cheliped is amputated, a 

 new one regenerates less rapidly than if both 

 chelipeds and some of the other walking 

 legs are removed. 



Behavior 



When at rest, the crayfish usually faces 

 outward from its place of concealment and 

 extends its antennae. In this position it 

 may learn the nature of any approaching 

 object without being detected. Activity at 

 this time is reduced to the movements of a 

 few of the appendages and the gills; the gill 

 bailers of the second maxillae move back 

 and forth bailing water out of the forward 

 end of the gill chambers; the swimmerets are 

 in constant motion creating a current of 



