496 



The Animal Kingdom 



variation on the different segments of the body, it is termed serial 

 homology. This is one of the reasons that the crayfish is such a vakiable 

 form for ilkistrating this basic concept. 



Fig. 163. — The appendages of the crayfish showing the homology of the 

 parts. They are numbered from I to XIX. 1, Protopodite; 2, endopodite; 3, exo- 

 podite; 4, epipodite; 5, epipodite with gill filaments; 6, gill with gill filaments; 

 7, chitinous threads. (From Beaver: The Science of Biology, The C. V. Mosby 

 Company.) 



