The buccal region is lined with chitin, which is specially thickened at certain definite spots, forming small "denticles" or "paragnaths" (Fig. 125), which have a different arrangement in the various species.
The cavity of the pharynx is narrow and the walls thick and muscular; each side wall carries a large, dark, chitinous "jaw" (Fig. 127, J), which is hollow at the base, into which the muscles serving to move it are inserted, whilst the apex is solid, curved, and more or less notched. These two great jaws are used not only for tearing prey, but for seizing it; for when the pharynx is entirely protruded the two jaws are wide apart, and when retraction takes place they come together and grasp the prey.
Eversion of the apparatus is partly effected by protractor muscles (Fig. 126, A, p) and partly by the pressure of the coelomic fluid, compressed by the muscles of the body-wall; the eversion is stopped at a certain stage by a sheet of muscular tissue or "diaphragm" (Fig. 127, diaph) inserted round the buccal region and attached to the body-wall in the second segment. The introversion is effected partly by the contraction of this diaphragm and partly by the action of powerful retractor muscles (Fig. 126, r) inserted into the hinder end of the pharynx and passing to the body-wall (these are removed in Fig. 127). The movement of the jaws themselves and of the wall of the apparatus is due to other muscles.
The oesophagus is quite short; into it opens a pair of sacculated diverticula or glands. Then follows the intestine, which extends through the rest of the body as a thin-walled tube, slightly dilated at the insertion of the septa.
Fig. 127.—Nereis, laid open by removal of the dorsal body-wall. br, Cerebral ganglion, from which three pairs of nerves are represented as arising; a pair to the tentacles, a pair to the palps, and a pair (con) passing one on each side of the buccal region, to join the ventral nerve-cord; mo, mouth, exposed by removal of the dorsal wall of the buccal region (Buc); pgn, the paragnaths in its wall; Ph, pharynx; J, the large "jaws" embedded in its wall; J.mus, the muscles which work the jaws; sh, the muscular sheath; diaph, the "diaphragm"; oes, oesophagus; gl, its glands; st, stomach; int, intestine; Sept, septum; d.v, dorsal blood trunk; p.v, perivisceral branches, one pair in each segment; P, palp; t, tentacle; per, peristomium; per.ci, two of the four peristomial cirri; ppIII, the first parapodium which belongs to the third true, but second apparent, segment; cir.m, circular muscular coat; lg.m, longitudinal muscular coat of the body-wall.
The vascular system consists of a contractile dorsal vessel and of a non-contractile ventral vessel extending along the whole length of the body, from each of which paired and segmentally-arranged vessels pass to the intestinal wall and to the body-wall, and here form extensive capillary networks (Fig. 124, p. [247]). This type of vascular system is pretty generally adhered to throughout the Order, but in the Terebelliformia, Scoleciformia, and Cryptocephala the dorsal vessel and capillary plexus on the intestine are replaced by a continuous blood sinus, situated in the substance of the gut-wall. This "perienteric sinus" has the same relation to the segmental vessels as the dorsal vessel has in the Nereidiformia, and from it a tubular dorsal vessel arises anteriorly. In Arenicola the sinus is preceded in the young stage by a network the branches of which gradually enlarge, meet, and fuse to form the sinus.[[302]] Whether it is in all cases secondary is a moot point.
This system of vessels in the majority of Chaetopoda contains a respiratory fluid coloured red[[303]] by haemoglobin in solution; in it float a very few small oval nucleated non-amoeboid corpuscles. But the place of this red pigment is taken by a green one, named "chlorocruorin," in the Chlorhaemidae and many Sabelliformia;[[304]] whilst in Magelona[[305]] the blood is tinted madder-pink by a number of globules of "haemerythrin." The blood (or "haemal fluid") is driven forwards in the dorsal vessel, and passes backwards in the ventral vessel. Respiration in Nereis is carried on by the whole surface of the body, but naturally with greater activity in the surface of the parapodia, the lobes of which, with their extensive vascular plexus, may be termed "gills"; but it must be borne in mind that these organs have other functions as well.
The coelomic fluid, which fills the general body-cavity, is colourless, and contains amoeboid corpuscles or "leucocytes." It corresponds to the lymph of Vertebrates, being nutritive in function, in that it conveys absorbed material from the wall of the intestine to the organs of the body, and at the same time removes any waste substances from these organs; these waste substances contain nitrogen, and are ultimately removed by the nephridia. In Ophelia many of the corpuscles contain a curious dumb-bell-shaped rod of chitin, and it has been shown[[306]] that this substance is a highly complex form of excretory material,—more complex than guanin, for instance, which exists in the corpuscles of the Capitelliformia.
In Glyceridae, Capitelliformia, and Polycirrus haematodes (a Terebellid), the vascular system is absent, and the coelomic corpuscles become coloured by haemoglobin, and in order that the coelomic fluid may be distributed to the organs of the body, the peritoneum is ciliated along certain definite tracts. The fluid in these "anangian" worms thus combines originally separate functions, and behaves like the "blood" of Vertebrates.