[Illustration: 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 {252}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.