MUSCLE AS THE SOURCE OF MOTION. 93 
fibre surrounds a cavity, the cavity is lessened when the 
muscle contracts. This is the whole source of motor 
power in the crayfish machine. The results produced 
by the exertion of that power depend upon the manner 
Fig. 20.—Astacus fluviatilis.—The chela of the forceps, with cne side 
cut away to show, in A, the muscles, in B, the tendons (x 2). 
cp, carpopodite ; pp, propodite ; dp, dactylopodite ; », adductor 
muscle; m’, abductor muscle; ¢, tendon of adductor muscle ; 7’, 
tendon of abductor muscle ; x, hinge. 
in which the parts to which the muscles are attached 
are connected with one another. 
One example of this has already been given in the 
curious mechanism of the gastric mill. Another may be 
found in the chela which terminates the forceps. If the 
