220 AN INTRODUCTION TO ZOOLOGY 



from the part removed is termed heteromorphosis. Figure 117 

 shows an antenna which regenerated in place of an eye in a marine 

 crustacean, Pal&mon. Instances of heteromorphosis have also 

 been recorded in experiments on other animals (176). 



Autotomy. Perhaps the most interesting morphological 

 structure connected with the regenerative process in Cambarus 

 is the definite breaking point near the bases of the walking legs. 

 If the chelae are injured, they are broken off by the crayfish at the 

 breaking point. 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 

 stump remaining. This breaking off of the legs at a definite point 

 is known as autotomy, a phenomenon that also occurs in a number 

 of other animals. The leg is separated along the breaking point 

 by several successive muscular contractions. It has been shown 

 " that autotomy is not due to a weakness at the breaking point, 

 but to a reflex action, and that it may be brought about by a 

 stimulation of the thoracic ganglion as well as by a stimulation 

 of the nerve of the leg itself " (182, p. 310). Immediately after 

 the leg has been thrown off, a membrane of ectoderm cells covers 

 the canal through which the nerve and blood passed; five days 

 later regeneration begins by an outward growth of the ectoderm 

 cells which lined the exoskeleton. An interesting point in this 

 new growth is that the muscles of the regenerated part are prob- 

 ably produced by ectoderm cells, whereas in the embryonic de- 

 velopment of the crayfish the muscles are supposed to arise from 

 the entoderm (178). 



The power of autotomy is of advantage to the crayfish, since 

 the wound closes more quickly if the leg is lost at the breaking 

 point. No one has yet offered an adequate theory to account 

 for autotomy. It is probably " a process that the animal has 

 acquired in connection with the condition under which it lives, 

 or in other words, an adaptive response of the organism to its 

 condition of life " (176, p. 158). 



As in the earthworm, the rate of regeneration depends upon the 



