leech

Any of a group of annelid worms. Leeches live in fresh water, and in tropical countries infest damp forests. As bloodsucking animals they are injurious to people and animals, to whom they attach themselves by means of a strong mouth adapted to sucking. (Class Hirudinea.)

Formerly, the medicinal leech (Hirudo medicinalis) was used extensively for ‘bleeding’ for a variety of ills. It is still cultivated as the source of the anticoagulant hirudin.