Every movement of the body, or of any organ of the body, is an effect of one and the same cause, namely, muscular contraction. Whether the crayfish swims or walks, or moves its antennæ, or seizes its prey, the immediate cause of the movements of the parts which bring about, or constitute, these bodily motions is to be sought in a change which takes place in the flesh, or muscle, which is attached to them. The change of place which constitutes any movement is an effect of a previous change in the disposition of the molecules of one or more muscles; while the direction of that movement depends on the connexions of the parts of the skeleton with one another, and of the muscles with them. The muscle of the crayfish is a dense, white substance; and if a small portion of it is subjected to examination it will be found to be very easily broken up into more or less parallel bundles of fine fibres. Each of these fibres is generally found to be ensheathed in a fine transparent membrane, which is called the _sarcolemma_, within which is contained the proper substance of the muscle. When quite fresh and living, this substance is soft and {91} semi-fluid, but it hardens and becomes solid immediately after death. [Illustration: FIG. 19.—_Astacus fluviatilis._—A, a single muscular fibre; transverse diameter 1‐110th of an inch; B, a portion of the same more highly magnified; C, a smaller portion still more highly magnified; D and E, the splitting up of a part of fibre into fibrillæ; F, the connexion of a nervous with a muscular fibre which has been treated with acetic acid. _a_, darker, and _b_, clearer portions of the fibrillæ; _n_, nucleus of sarcolemma; _nv_, nerve fibre; _s_, sarcolemma; _t_, tendon; 1–5, successive dark bands answering to the darker portions, _a_, of each fibrilla.] Examined, with high magnifying powers, in this {92} condition, the muscle-substance appears marked by very regular transverse bands, which are alternately opaque and transparent; and it is characteristic of the group of animals to which the crayfish belongs that their muscle-substance has this striped character in all parts of the body. A greater or less number of these fibres, united into one or more bundles, constitutes a muscle; and, except when these muscles surround a cavity, they are fixed at each end to the hard parts of the skeleton. The attachment is frequently effected by the intermediation of a dense, fibrous, often chitinous, substance, which constitutes the _tendon_ (fig. 19, A; _t_) of the muscle.