Mouth in leeches is in the anterior end, either in the bottom of a sucking disc, or under an upper lip, which is formed like a spoon. Mouth is followed by a muscular pharynx. The anterior part of the pharynx, the so-called oral cavity, has different structures in different groups. In some, there is a keratinized chin with smooth edges or with teeth (Figure 1). Some of them have a proboscis and can be extended to the exterior. One-cell salivary glands are present around the pharynx. Canals of these glands open between teeth or to the end of the proboscis.

The salivary glands of the blood-sucking leeches contain “Hirudin”, so that the sucked blood doesn’t coagulate. Stomach (mid-gut), which is the biggest part of the digestive system, follows pharynx in leeches with chins.

In leeches with proboscis, a thin and long esophagus is present between the pharynx and stomach, and it makes a loop when proboscis is pulled back. Stomach is in the form of a large tube with thin walls. Multiple sacs originate from the sides of the stomach, mostly in couples. Number of these sacs depends on species. For example, there are 11 couples in Hirudo medicinalis, only one couple in Heamopis sanguisuga and none in herpobdella. This part of the digestive tube functions as a gizzard and deposits the food taken in from the external environment. Digestion takes place in stomach and chyle intestine. The last part of the intestine is short and anus is located at the dorsal of the leech, at the anterior of the sucking disc. (Barnes, 1974; Brown, 1967; Davies, 1991; Kaestner, 1967; Sağlam and Sarıeyyüpoğlu, 1998; Sawyer 1986.) Not all the leeches are blood-suckers.

Some species feed with invertebrates, members of other classes of annelida, snails and insect larvae. Blood-suckers live as ectoparasites on fish, frogs, turtles, snails crustaceans or vertebrates (Barnes, 1974; Davies, 1991; Kaestner, 1967; Sawyer 1986). Although almost every group of vertebrate animals is host, fish are the most attacked group. (Sağlam, 1998a; Sawyer 1986). Individuals of the Haemodipsidae family that live on land in wet areas of tropical regions, adhere themselves on mouths and noses of mammals and birds.

They climb on plants and then let themselves fall on the bodies or animals passing under. While some of them, for example individuals of the Piscicolidae family that lice as parasites on fish, stay permanently on the host, some others suck blood in certain periods (Barnes, 1974; Davies, 1991; Demirsoy, 1982; Kaestner, 1967; Sawyer 1986). For example, Hirudo medicinalis can suck blood in amounts 5.83 (3-10) folds of their body weight, and then can survive for up to one year without feeding (Kaestner, 1967; Sağlam, 1998b).