32 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



description of which is not of immediate impor- 

 tance. The portion of the appendage which has 

 degenerated is thepodobranch, which has completely 

 disappeared. The reason is obvious ; these ap- 

 pendages, having become a support for the eggs, 

 have lost their respiratory function. The part 

 specially adapted for that function has been allowed 

 to atrophy, the more readily because the presence 

 of gills on the ventral surface of the abdomen 

 would be incompatible with free movement of that 



part of the body when the 

 cray-fish is swimming. 



The appendages of the 

 sixth abdominal segment 

 are greatly modified into 

 a caudal fin (fig. 4). 

 P r This also exhibits the three 



fundamental parts of a 



FIG. 4. Astacus Jtuviatilis.'Left ap- 



pendage of the fith abdominal seg- complete appendage. The 



ment. Fins (1,5/1) : pr, protopodite; , . / N .i-i 



*r,exopodite; e,endopodite(Hu:dey).r0t00<me r. IS thlCK 



and short and has no gill, degeneration being ex- 

 hibited in the loss of the gill, and by the reduced 

 length of the part. The exopodite (ex.), and the 

 endopodite (en.) are modified into two large oval 

 plates which serve as propellers. The actual de- 

 velopment of these two parts is accompanied by 

 degeneration. In a typical appendage, the exopodite 

 and the endopodite terminate in slender parts divided 

 into several rings by false joints ; in the propellers, 

 which should offer the maximum resistance to the 



