IQ4 AN INTRODUCTION TO ZOOLOGY 



and the steadily increasing demand for crayfishes, will undoubt- 

 edly soon make it worth while to raise the latter for market (162). 



External Features. EXOSKELETON. The outside of the body 

 of the crayfish is covered by an extremely hard chitinous cuticle 

 impregnated with lime salts. This exoskeleton is thinner and 

 flexible at the joints, allowing movement. A delicate cuticle 

 of the same substance (chitin) was noted in the earthworm 

 (p. 166). 



REGIONS OF THE BODY. Unlike the earthworm, the body of 

 the crayfish shows two distinct regions, an anterior rigid portion, 

 the cephdothorax, and a posterior series of segments, the abdomen. 



FIG. 100. Transverse section through the abdomen of the crayfish. DA, 

 dorsal abdominal artery ; EM , extensor muscles of the abdomen ; 

 EP, epimeron ; FM , flexor muscles of abdomen ; M, muscles of append- 

 age ; N, endopodite ; NG, nerve ganglion ; P, protopodite ; PL. 

 pleuron ; PR, intestine ; S, sternum ; T, tergum ; V, ventral abdominal 

 artery ; X, exopodite. (From Marshall and Hurst.) 



The entire body is segmented, but the joints have been obliterated 

 on the dorsal surface of the cephalothorax. 



