Leeches are highly specialized Annelids constituting the order Hirudinea or Discophora. They are distinguished from most other annelids by the nearly complete obliteration of the coelom or body-cavity, owing to the development of parenchymatous connective tissue, muscles, etc., the presence of an anterior or oral sucker and a posterior or subanal sucker, and by the absence of setae, except in Acanthobdella. In all leeches which have been carefully studied there are exactly 34 segments or somites, each represented by a ganglion in the central neryous system, and being of smaller size and simpler structure toward the ends than in the middle of the body, where each is divided into from 2 to 12 rings, one of which, sometimes regarded as the first, sometimes as the middle ring, bears metameric, eye-like sense organs.
   Most leeches are temporary parasites, a few nearly permanent parasites; the rest are predatory hunters or scavengers, or they may change from one mode of life to another. They are marine, fresh-water or terrestrial. The first class is most abundant, both in indi­viduals and species, in cold seas, the second is both temperate and tropical and the third is confined to warm regions. Four families are distinguished: the Ichthyobdellidae or fish-leeches, the Glossiphonidae or tortoise and snail leeches, the Herpobdellida or worm-leeches and the Hirudinidae or jawed leeches. The first two families possess a long protrusible proboscis and are much more closely allied than the Herpobdellidae and Hirudinidae, which have no proboscis. The Ichthyobdellidae are chiefly parasitic on fishes and, except a few-fresh-water forms, are marine. Some of them, as Branchellion, are branchiate. The Glossiphonidee are richly represented in the fresh-water lakes and streams of North America by a great variety of species, most of which attach themselves to tortoises, whose blpod they suck, or else they devour water-snails and small worms. A few are parasitic on fishes. In all of them the oral sucker is small and the eyes in one to four pairs placed near the median line.