100 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



and the thoracic part of the nervous S} r stem ; while, in the 

 abdomen, 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.m 



XVI XV 



pep. 



FlG. 22. Astacua fur tat His. A longitudinal section of the body to 

 show the principal muscles and their relations to the exoskeleton 

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

 e.m, extensor, and/.w. flexor muscle of abdomen ; #>*, oesophagus ; 

 jjf/i, procephalic process ; t.t', the two segments of the telson ; 

 xv xx, the abdominal somites ; 120, 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 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 



