INTERNAL ANATOMY.—In correspondence with the external {249}metamerism there is an internal repetition of parts. For, except in the anterior segments, where the powerful protrusible pharynx is situated, the body-cavity or "coelom" is divided into a series of chambers, by means of muscular septa inserted, on the one hand, into the body-wall at the level of the grooves between the external segments, and, on the other, into the wall of the alimentary canal. Each of these coelomic chambers contains a pair of nephridia, a portion of the intestine, of the vascular system, and of the nervous system, as will be seen in Fig. 124. The epidermis, which forms the outer part of the _body-wall_, consists of a single layer of cells, covered externally by a thin, tough cuticle. The latter is usually stated to consist of the chemical substance known as chitin, but since the cuticle differs from true chitin by dissolving in caustic potash after a time, Eisig[300] has suggested that its substance is merely a stage in the formation of chitin. The epidermis contains gland-cells, which are especially abundant on the lobes of the parapodia. Below the epidermis lies the circular coat of muscles by whose contraction the worm diminishes its diameter: it is interrupted on each side at the junction of the parapodium with the body. Deeper still lie the longitudinal muscles, which form four great bundles, two dorsal, separated by the insertion of a small mesentery and dorsal blood-vessel, and two ventral bundles separated in the middle line by the nerve-cords. These longitudinal muscles, by their contraction, bend the worm from side to side, and are continuous from segment to segment. A very characteristic muscle, present in all the Polychaeta, is an obliquely transverse sheet of fibres passing from the body-wall at the side of the nerve-cord to the parapodium, where it spreads out and serves to move the parapodium (Fig. 124). All these muscles consist of smooth fibres, as in the earthworm. The ALIMENTARY CANAL may be divided into the following four regions:—(1) buccal or eversible region, (2) pharynx, carrying the great jaws, (3) oesophagus, (4) intestine. The first two regions constitute an "introvert" (Lankester[301]). When fully everted the whole of the buccal region is turned inside out, and the terminal aperture leads directly into the pharynx, which is not everted but merely protruded. {250}Throughout the following chapters the word "buccal" region is used for that part—if any—which is thus everted (Figs. 125, 126). Both the buccal and pharyngeal regions are wrapped round by several coats of muscle, to form apparently a single muscular organ (Fig. 127, _sh_), which occupies about eight segments in a condition of complete introversion. The septa are absent from the anterior part of the body.