ix ANATOMY OF NEREIS 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 ali- 

 mentary 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 1 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 con- 

 traction 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 longi- 

 tudinal 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 earth-worm. 



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 2 ). 

 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. Through- 



1 "Die Capitelliden," Fauna u. Flora d. Golfes v. Neapel, Monogr. xvi. 1887, p. 350. 

 2 Encyclopaedia Britannica, 9th ed., Art. "Mollusca," p. 652. 



