]()() THK PHYSrOLOGY OF THE COMMON CRAYFISH. 



and the thoracic part of the iiervous system ; while, in the 

 ahdomen, its strands are attached to the sternal exoske- 

 leton of all the somites and extend, on each side of the 

 rectum, to the telson. 



When the exoskeleton is cleaned by maceration, the 



add .n 



Fig 22. — Axfricii.t Jtuvidtilix. — A longitudinal section of the body to 

 show the principal muscles and their relations to the exoskeleton 

 (nat. size). «, the vent ; ttdil.m, adductor muscle of mandible ; 

 e.w, extensor, and/./,v, flexor muscle of abdomen ; a'x, oes >phagus ; 

 ■pep, procephalic process ; //', the two segments of the telson ; 

 AT — XX, the abdominal somites ; 1 — 20, the appendages ; x , x , 

 hinges between the successive abdominal somites. 



ahdomen has a slight curve, dependent upon the form and 

 tlie degree of elasticity possessed by its different jiarts ; 

 and, in a living crayfish at rest, it will be observed that 

 the curvature of the " abdomen is still more marked. 

 Hence it is ready either for extension or for flexion. 



A sudden contraction of the flexor muscles instantly 

 increases the ventral curvature of the abdomen, and 



-/o 



