100 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



and tlie tliomcic part of the nervous system ; wliile, 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 



Fig 22. — Antaeiitt ffuviatilii^. — A longitudinal section of the body to 

 show the principal muscles and their relations to the exoskeleton 

 (nat. size). //, the vent : add.m, adductor muscle of mandible ; 

 e.ni, extensor, and/..w, flexor muscle of abdomen ; ces, oesophagus ; 

 ]icp, procephalic process ; t,i', the two segments of the telson ; 

 AT — A'.v, the abdominal somites; 1 — 20, the appendages;- x, x, 

 hinges between the successive abdominal somites. 



abdomen has a slight curve, dependent upon the form and 

 the degree of elasticity possessed by its different parts ; 

 and, in a living ci'ayfish at rest, it will be observed that 

 the curvature of the abdomen is still more marked. 

 Hence it is read}' either for extension or for flexion. 



A sudden contraction oi the flexor muscles instantly 

 increases the ventral curvature of the abdomen, and 



i 



