100 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



and the thoracic part of the nervous 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 hy maceration, the 



Fig. 22. — Astacus Jtuviatilis. — 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/.m, flexor muscle of abdomen ; ces, oesophagus 

 pep, procephalic process ; t,t', the two segments of the telson 

 xv — xx, the abdominal somites; 1 — 20, the appendages; x, x, 

 binges 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 



