MUSCULAR SYSTEM, EVOLUTION OF. The muscular system comprises those tissues or masses of tissue which have the property of con tractility and whose function it is to accomplish the movements of the parts or of the whole of the animal body. Contractility is one of the prop erties of simple protoplasm. Thus, the ameba progresses by the extension of some portion and the contraction of other parts of its body. The cortical layer of Paranneeium is a specially con tractile region in the protoplasm. In the Stentor and the stalk of Vorticella there are protoplasmic differentiations that resemble muscle fibres.
Many of the outer-lying ectoderm cells of Hydra show considerable differentiation; they are conical in shape with the broad surface outward. On this external surface a thin, eutiele-like layer has been demonstrated. Internally the cell ends in a contractile basal portion or process which lies parallel to the long axis of the body of the hydra and between the ectoderm and entoderm. The whole of these cells are contractile, but the long processes are specially so. These processes were formerly known as 'neuro-museular,' but with the discovery of special nerve cells, the mus cular function alone must be conceded to them. Like the muscle cells of vertebrates they con tract when properly stimulated. Here then are eetodermic muscle cells, the entire protoplasm of which, like that of the ameba, is irritable and contractile, but with a portion of the cell more clearly set apart to perform the contractile func tion than in any protozoan. In the jelly-fishes, such as Aurelia, a muscle cell is likewise com posed of two parts: a contractile portion (which shows cross striations), and, attached to the striated portion, a protoplasmic, non-differen tiated, non-contractile portion which may hear cilia on its external surface. This muscle-bear ing epithelium is arranged in a bundle or ring around the edge of the sub-umbrella. It is the contraction of this nngeular zone that propels the jellyfish through the water. In the tentacles and about the lips unstriated muscle-fibres occur. The contraction and extension of the body of the sea-anemone, as \Veil as of its gullet and mesen teries, and its ability to move slowly from the point of attachment, are accomplished by means of a well-developed muscular system. This con sists of bands of longitudinal muscle fibres which run on the mesenteries from the base to the disk; of parietal muscles which pass obliquely across the lower and outer angle of the mesentery; and of a thin sheet of transverse muscles. The con
traction of the longitudinal muscles draws the animal toward the base, and that of the trans verse muscles causes the contracted animal to extend again. A hand of circular muscles at the junction of column and disk causes the disk and contracted tentacles to be inclosed within the body of the anemone. In addition to the bands of museles, scattered fibres occur both in the body wall and in the gullet, which consist partly of spindle-shaped, nucleated fibres, and partly of such striated muscle processes as occur in Hydra. The latter are mostly found in the transverse muscles of the body and the tentacles, and are of entodermal origin, as also are the muscle bands of the mesenteries. Other longi tudinal ninsele,k are of Pet odermic origin. Some of the muscles. however, sink so far down into the middle layer or mesoglea as to seem to belong to it alone. This is significant because all the musculature of animals above the ceelenterates lies between the ectoderm and entoderm.
The musculature of flatworms falls under two groups: ( 1 ) the dermal musculature, and (2) the dorso-ventral musculature. The dermal mus culature lies either under the basal membrane of the epidermis or under the cuticle. It is com posed of distinct layers. In each layer all the fibres run in one and the same direction. There are longitudinal muscles, transverse and diago nal muscles. The musculature is stronger on the ventral creeping surface. In cestodes the diago nal layer is replaced by a second circular layer. The dos o-ventral musculature runs from the dorsal to the ventral surface, and is much dis placed at sexual maturity by the male and fe male germ-glands. Below the cuticular layer of roundworms there is a well-developed muscu lar layer in the form of a tithe, and composed of outer circular fibres and inner longitudinal ones. This muscular tube effects the writhings and Un dulations of the body. As in the ecelenterates, the muscle element is composed of a single cell, made up of a protoplasmic (often glandular) portion and a fibrillar part. The fibrilhe rarely show any evidence of cross striation, the lateral lines are free from muscle fibres, and the longi tudinal sheet is thus broken up into bands.