[Illustration: FIG. 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; _e.m_, extensor, and, _f.m_, flexor muscle of abdomen; _œs_, œsophagus; _pcp_, procephalic process; _t,t′_, the two segments of the telson; _XV–XX_, the abdominal somites; _1–20_, the appendages; ×, ×, hinges between the successive abdominal somites.] When the exoskeleton is cleaned by maceration, the 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 {101} throws the tail fin, the two side lobes of which are spread out, forwards; while the body is propelled backwards by the reaction of the water against the stroke. Then the flexor muscles being relaxed, the extensor muscles come into play; the abdomen is straightened, but less violently and with a far weaker stroke on the water, in consequence of the less strength of the extensors and of the folding up of the lateral plates of the fin, until it comes into the position requisite to give full force to a new downward and forward stroke. The tendency of the extension of the abdomen is to drive the body forward; but from the comparative weakness and the obliquity of its stroke, its practical effect is little more than to check the backward motion conferred upon the body by the flexion of the abdomen. * * * * *