40 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



the epipodite are missing ; the endopodiie is reduced 

 to a mere unjointed stem, and only the protopodite 

 retains its two normal component parts. 



The mandibles (fig. 14), the appendages of the 

 fourth segment, are modified entirely for mastica- 

 tion. They consist of a strong transverse piece 

 (pr.) provided at the extremity with an inner 

 surface (st.) for grinding and sawing, and of a 

 three-jointed piece (en.) with bristles which point 

 outwards. The first piece is the result of the 



en 



en 



Fio. 14. Astacus fluvia- FIG. 15. Astacus fluvia- FIG. 16. Astacus fluvia- 



tilit. Left mandible tilts. Left antenna tilis. Left antennule 



(1,5/1) : pr, protopodite; (1,5/1) : pr, protopodite ; (1,5/1) : pr, protopodite; 



en, endopodite; st, rasp- ex, exopodite; en, endo- ex, exopodite; en, endo- 



ing surface of the proto- podite (Huxley). podite (Huxley), 

 podite (Huxley). 



modification of the two parts of the protopodite, 

 which have united to form the organ of mastica- 

 tion ; the second, which represents the endopodite, 

 is the feeler, an organ of sensation. All the other 

 parts of the appendage have disappeared. 



The antenna (fig. 15), which is a tectile ap- 

 paratus, is formed of two parts representing the 

 segments of the protopodite (pr.). The long-ringed 

 process is the endopodite, while the lateral scale of 

 the antenna represents a much reduced exopodite (ex.). 



