The muscular system of mollusks is also well developed. The muscle fibres, both of bivalves and gastropods, are all of the unstriated sort, because of the slow movements of the animals. These fibres occur usually in distinct bands or sheets, and frequently are combined for greater power to form very large muscles, such as the two abductors of Anodonta, which close the shell. Other important muscles are those of the foot, including two protractors and one retractor; and in the snail those that work the radula of the mouth, the retractors of the horns and penis, and those that pull the head into the shell. The muscles are fastened to the shell, upon which they make certain distinct markings at the points of attachment, called 'scars.' The musculature of segmented worms, like that of the roundworms, is composed of a dermo musculature tube which is composed of an outer circular and an inner longitudinal layer of fibres. The fibres are in the form of bands. The pharynx, mouth, gut, parapodia, and septa have special muscle fibres. In leeches, in addition to the circular and longitudinal bands of muscle fibres, there are bands reaching diagonally from the dorsal to the ventral surface of the body.
Among arthropods the of the Crus taeea is well developed and very complex, but there is little evidence remaining of the dermo-musenlar tuhe of worms. It is assumed that the dorsal and ventral pair of longitudinal muscles corre spond to the four similarly situated bands of muscle in the Polyelerta. Perhaps traces of the circular musculature of annelids are found in the muscles that are attached to the basal parts of the appendages on the one hand, and to the integument of the body on the other. Certainly the firmer exoskeleton makes greater localization of the musculature possible. In the limbdwaring portion of the abdomen and the thorax there are paired dorsal and ventral muscle bands. In the limbless part of the abdomen there is a thick layer or tube of longitudinal muscle fibres, inter rupted at each septum by the connective tissue that separates the myomeres. Thus the muscle itself is separated into myomeres. The abdomen is bent upward, downward, or sidewise by the cmitrartion of the muscle fibres in the corre spc portion of the body. The appendages are moved by muscles that pass out into them from the trunk. The muscles are either attached to the cuticular outer covering or to inward pro jecting part: of it. and often !dere, that are white and are composed of bundles of eross-striated fibres—the kind of fibre that characterizes all rapidly moving muscle. I Peripatus. however, the muscles are unstripped, those Which work the jaws. In insects, the thorax and metathorax, as in the ease of the grasshopper. are crowded with leg and Wing mus cles. In the cockroach, where the wings are little used, the wing muscles art' poorly developed.
The musculature of Amphioxus.like that of the abdomen of Crustacea. is divided into metameres. Of these muscular segments or myomeres there are about sixty, V-shaped on surface view, with the apex of the \' extending forward. The myo
meres are composed of striated muscle fibres which extend longitudinally and are attached to the septa. immediately in front and behind them. The myomeres of the two sides alternate. The musculature of the dorsal wall is the thicker.
In eyclostomes, as well as in Amphioxus, the muscles of any myomere are attached to the anterior and posterior myocomata of that seg ment. In selaehians a significant change has occurred—the first step toward the complicated conditions of higher vertebrates. Certain of the ventral lateral muscles on either side of the median line become differentiated from the others, in that their fibres move tint from the general level in correspondence with their greater functional activity. Thus while the dorsal trunk muscles retain their primitive character, the ventral muscles have become separated into two masses: a pair of strong mid-ventral ones which are known together as the reetu: abdominis, and Ina, We next find that in this latter mass the direetion of the muscle fibres, which in Amphioxu: and eyelostomes is horizontal. has become in selaehians oblique: so that. from the mid-ventral line. they pass (torso posteriorly. The remaining less differentiated portion of the ventral inusculatnre is known as obliques abdominis. or oblique abdominal muscle. Two kind, of changes have now occurred in the vertebrate musculature: I 1) There is the func tional differentiation of parts of muscles by wltiiel they become cut off from the remainder, and (2) ehanges in the direction of muscle fibres by which originally axial muscles have become oblique. In the lowest vertebrates the anterior and pos terior ends of muscles ore attached to the ad jacent myoNunata. \\*Rh the development of ribs from the fibrous tissue of the niyoconinta. muscular attachment has. in part, fallen upm them. The derma, indeed, is differentiated from the same tissue as the myocomata hence it is in t surprising that. in the migrations on the of muscles to adapt themselves to the new COTO TI it ion., we c find that some muscles are attached to the derma. Finally. some museles. whose ends come thus to lie outside of the myomeres. may extend alongside of several somites without signs of metamerism. Thus, by these simple modifica tions, we gain the complex musculature of the higher vertebrates, where there are trunk mus cles of ditTerent sizes and lengths, often without metamerism and attached to bony parts such as ribs, or to the skin. In Amphioxus the myomeres arc arranged with reference•to the sagittal plane from which they extend obliquely outward and backward. In cyclostomes the obliquity is so great that the myomeres overlap each other like the shingles on a roof. so that a cross-section of the body may cut three or four myocomata on each side. In many fishes the free edge of the myoeomata, as it appears at the surface, is not merely bowed, but is zigzag.