36 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



The third maxillipede may be regarded as the 

 mean type of the average thoracic appendages. 

 Compared with it the other appendages exhibit a 

 true alternative development of their parts. When 

 the exopodite is large the endopodite is small, and 

 vice versd. Besides these general modifications in 

 which the degenerative evolution of the entire 

 appendages is exhibited, each one of the segments 

 undergoes certain modifications of its own. 



FIG. &. Astacus fluviatilis. Third left maxillipede 

 (1,5/1): co, coxopodite,and 6a, basipodite, forming 

 pr, protopodite ; br, gill ; ex, exopodite ; en, en- 

 dopodite (Huxley). 



FIG. 9. Astacus fluviatilis. Second 

 left maxillipede (1,5/1) : co, coxo- 

 podite, and ba, basipodite, forming 

 pr, protopodite ; en, endopodite, 

 ex, exopodite ; br, gill (Huxley). 



In the second maxillipede (fig. 9), which much 

 resembles the third, the exopodite (ex.) is large, and 

 the endopodite (en.) is small, the prodopodite (pr.) is 

 better developed, and the podobranch (br.) has 

 begun to atrophy. 



The same evolution is exhibited by the first 



