The vascular system of the Oligochaeta forms a system of perfectly closed vessels, which ramify into fine capillary networks in the body-wall, in the coats of the alimentary canal, and upon {356}the other organs of the body. The main trunks are a dorsal and a ventral longitudinal, which communicate directly in the anterior end of the body by large transverse contractile trunks, the so-called hearts (see Fig. 190, 6). The dorsal vessel is also contractile, but not the ventral, or, when it occurs, the subnervian. The vascular system has many degrees of complexity in different families; it is simpler in the smaller aquatic forms. The blood is usually red, and the pigment which is suspended in the plasma is haemoglobin. The blood is corpusculated. EXCRETORY ORGANS.—There appears to be a great deal more variation in the structure of the excretory system than there is in many other groups. For a long time only _Lumbricus_ and a few of the aquatic genera were known as regards their excretory systems. In these there is a pair of excretory organs or nephridia in nearly all the segments. These are much coiled tubes, in which it is always possible to recognise three divisions. The nephridium commences with an orifice of a funnel-like character, fringed with long cilia, and opening into the body-cavity; from this springs a tube, which immediately perforates the septum lying between the segment which contains the funnel and the following one; this tube has the peculiarity first pointed out by Claparède of being excavated in the substance of cells; the glandular part of the nephridium is a row of cells which are bored through by a continuous canal, the walls of which are here and there furnished with cilia. It often happens that the main canal gives off minute lateral ramifications, which may even form a kind of network round the principal canal. The terminal section of the nephridium is a muscular sac which opens on to the exterior by a pore, and from which the products of excretion are from time to time evacuated by contractions of its walls. This is a brief statement of the main facts in the structure of those Oligochaeta in which there is a single pair of nephridia to each segment of the body; small differences of more or less importance occur. In _Chaetogaster_, for example, there is no trace of a funnel; in some genera the terminal sac is much reduced or unusually extended, being even sometimes provided with a caecum of moderate dimensions. In _Acanthodrilus novae-zelandiae_ and a few other species the point of opening of the nephridia varies from segment to segment, though it always bears some relation to the chaetae. In these {357}species the nephridia which open more dorsally are a little different in structure from those which open more ventrally. One set have a caecum, and the other have not. The nephridia of the terrestrial forms are enveloped by a richly developed network of blood capillaries, which is absent in the smaller aquatic genera.