100 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
and the thoracic part of the nervous system ; 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 
Fia 22.—Astacus fluviatilis.—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 ; 
em, extensor, and f.m, flexor muscle of abdomen ; @s, cesophagus ; 
pep, procephalic process ; ¢,t’, the two segments of the telson ; 
xv—xx, the abdominal somites ; /—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 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 
