This order includes the Leeches, and is characterised by the possession of a locomotive and adhesive sucker, posteriorly or at both extremities, and by the absence of bristles and foot-tubercles. The sexes are mostly united in the same individual, and the young do not pass through any metamorphosis.

The Leeches are vermiform, mostly aquatic animals, chiefly inhabiting fresh water, though a few species are marine. Locomotion is effected either by swimming by means of a serpentine bending of the body, or by means of one or two suctorial discs. In those forms in which there is only a single sucker (posterior), the head or anterior extremity of the body can be converted into a suctorial disc. The body is ringed, as many as one hundred annulations being present in the common Leech; but it is not divided into distinct somites, and, with rare exceptions (Branchellion), there are no lateral appendages of any kind. The mouth is placed at the front end of the body, and may or may not be furnished with teeth. The pharynx is muscular; the gullet leads into a stomach with, usually, capacious lateral caeca (fig. 128, B); and the anus is placed in front of or at the bottom of the posterior sucker. The alimentary canal is united with the integument by a spongy tissue, formed of vascular sinuses, which more or less completely obliterate the body-cavity, and in which the blood circulates. The pseudo-haemal system generally consists, in addition to the sinuses just alluded to, of four principal longitudinal trunks, devoid of special dilatations. Respiration is carried on mainly by the soft integument, possibly assisted by the "segmental organs," and, in the case of Branchellion, by the vascular leaf-like appendages on the sides of the body. The "segmental organs " are in the form of a larger or smaller number of sacs (fig. 128, B), which open upon the abdominal surface by so many pores or "stigmata." The function of these sacculi appears to be excretory, and in the majority of the Hirudinea they are closed internally, and only open externally by the " stigmata." In some of the Hirudinea, however, the "segmental organs" agree with those of the great majority of the Annelides in not only opening externally, but in also communicating internally with the perivisceral cavity.

The segmental organs of the Leeches differ further from those of the other Annelida in not being in any way connected with the process of reproduction.

The nervous system consists of a prae-oesophageal ganglion, which gives branches to a number of simple eyes, or ocelli, which are placed on the head, and which is united by lateral oesophageal cords to the ventral gangliated chain. The ventral chain of the common Leech carries twenty-three successive pairs of ganglia, marking the composition of the body out of the same number of segments or "zonites."

Fig. 128. - A, Diagram of the Leech, showing the nervous system, and the ten eye3 placed on the top of the head. B, The leech dissected to show the alimentary canal (2), and the " segmental organs," or so-called "respiratory sacs" (r r) ; as Anterior sucker ; ps Hinder sucker; n n Nervous system ; h Head, carrying the eye-spots.

The sexes are almost always united in the same individual, but the Leeches are nevertheless incapable of self-fertilisation. Reproduction, also, is always effected by means of the sexes, and never by fission or gemmation. The generative products, instead of being discharged into the perivisceral cavity. (as in the Annelida generally), reach the surface by special apertures. The ova are deposited in delicate chi-tinous capsules or "cocoons;" and the young, on being hatched, undergo no metamorphosis, but are essentially similar to the adult in all except their size.

The common Horse-leech, Haemopsis, is only provided with blunt teeth; but the Medicinal Leech (Sanguisuga medicinalis, fig. 129) has its mouth furnished with three crescentic jaws, the convex surfaces of which are serrated with minute teeth. This species is chiefly imported from Germany, Bohemia, and Russia. In Sanguisuga officinalis, the Hungarian Leech, also used in medicine, the abdomen has numerous black spots. In both species the oral and caudal extremities are narrowed before dilating into the suckers, and the body has from ninety to one hundred rings. The anterior sucker is small, with a lancet-shaped upper lip, carrying ten ocelli on its superior surface. The posterior sucker is round, obliquely placed, and separated from the body by a distinct constriction. The alimentary canal, usually termed the "stomach" in its anterior portion, occupies the greater part of the body-cavity, and is furnished with eleven membranous pouches or diverticula on each side. There are seventeen segmental organs on each side, opening by minute stigmata on the lower surface of the body. The opening of the male reproductive organs is in the anterior third of the body, between the twenty-seventh and twenty-eighth rings; that of the female organs, in the form of a small fissure, five rings behind. Impregnation of the hermaphrodite individuals is mutual, and the ova are deposited in moist earth, within a cocoon, where they remain until hatched. The marine Pontobdellae have the body tuberculated, and attach themselves to the bodies of fishes, especially skates. The anterior sucker is separated in these from the body by a distinct constriction or neck. In the little freshwater Clepsines the anterior sucker is wanting, and there is a proboscidi-form mouth. They are found attached to the stems of water-plants, or to aquatic animals of different kinds. Branchiobdella lives upon the gills of Crustaceans; and Branchellion infests the gills of various Fishes, such as the Turbot. A few Leeches inhabit damp situations on land.