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 {253}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. The EXCRETORY SYSTEM is represented by a pair of nephridia in each segment, with the exception of a few anteriorly and a few posteriorly. The nephridium of _Nereis_ differs from that of most other Polychaetes hitherto examined carefully, and rather resembles that of the Oligochaetous Enchytraeids. It consists of a compact gland-like organ, containing a much coiled tube, ciliated for the greater part of its length, but deprived of cilia in its last coils; this latter part—or duct—leaves the "gland" and pierces the body-wall, opening to the exterior at the base of the parapodium. The ciliated canal passes forwards into the next segment, where it opens by a funnel into the coelom. The lip of the funnel is extremely curious, for the cells constituting it are drawn out into very long, delicate processes covered with cilia.[307] [Illustration: FIG. 128.—Nephridium of _Nereis_. (From Goodrich.) _f_, Funnel; _n_, neck, which passes through a septum; _t_, coiled tubule; _c_, connective tissue; _d_, duct.] In most Polychaetes the nephridium is a wide, sac-like tube as in _Arenicola_[308] (Fig. 129). Its walls are covered by a dense network of blood-vessels, and it not only acts as an excretory organ, but also as a genital duct (see p. 273). Excretion, in the strict sense of the word, is carried out by {254}the cells forming the wall of the tube; they remove waste materials from the blood distributed over the surface of the organ. But, in addition, there is a removal from the coelom, by means of the funnel, of any dead or dying coelomic corpuscles which in their turn have eaten up or otherwise destroyed foreign bodies (such as Bacteria, etc.) that may have entered the animal.