The chief motor organs in the metazoa are the muscles which constitute the "flesh" of the body. Muscular tissue consists of contractile cells--that is to say, of cells with the sole property of contraction. When the muscular cell contracts, it becomes shorter and its diameter increases. This brings nearer together the two parts of the body to which its ends are attached. In the lower metazoa the muscle-cells have, as a rule, no particular structure; but in the higher animals the contractile plasm undergoes a peculiar differentiation, which has the appearance under the microscope of a transverse streaking of the long cells. On this ground a distinction is drawn between striated muscles and simple non-striated or smooth muscles. The more vigorous, rapid, and definite is the contraction of the muscle, the more marked is the streaky character, and the more pronounced the difference between the doubly refractive muscular particles from the simple refractive. The striated muscle is "the most perfect dynamo we know of" (Verworn). The normal heart of a man accomplishes every day, according to Zuntz, a work of about twenty thousand kilogrammetres--in other words, an energy that would suffice to lift to a height of one metre a weight of twenty thousand kilogrammes. In many flying insects (gnats, for instance) the flying muscles make three hundred to four hundred contractions a second. In the lower and higher classes of the metazoa the muscle amounts to no more than a thin layer of flesh underneath the skin. This layer consists of muscular cells, which come originally from the ectoderm in the form of internal contractile processes of the skin-cells themselves, as in the polyps. In other cases the muscle-cells are developed from the connective-tissue cells of the mesoderm, the middle skin-layer, as in the ctenophora. This mesenchymic muscle is less common than epithelial muscle. In most of the askeletal vermalia the subdermal muscle divides into two layers--an outer deposit of concentric muscles and an inner layer of longitudinal muscles; in the cylindrical worms (nematodes, sagittæ, etc.) the latter fall into four longitudinal bands, one pair of upper (dorsal) and a pair of lower (ventral) muscular bands. At those parts of the body which are especially used for locomotion the muscle is more strongly developed, as in the belly-side of the crawling worms and mollusks. This muscular surface develops into a kind of fleshy "foot" (_podium_); it assumes a great variety of forms in the various classes of mollusks. In most of the snails which creep on the solid ground it grows into a muscular "flat-foot" (_gasteropoda_); in the mussels which cut like a plough through the soft slime it forms a sharp "hatchet-foot" (_pelecypoda_). The keel-snails (_heteropoda_) swim by means of a "keel-foot," which works like the screw of a ship; the floating-snails (_pteropoda_) swim unsteadily (like butterflies flying) by means of a pair of head-folds,