Leeches

Of the roughly 650 species of leeches, some are predatory, feeding on a variety of invertebrates (e.g., snails) while other species feed on the blood of vertebrates such as amphibians, reptiles, waterfowl, fishes, and mammals (including humans). In the 19th Century millions of leeches were used each year to draw blood (for every conceivable malady). After falling into disuse after the first decade of the 20th Century, leeches are now employed after reattachment surgeries (since they set up a sort of artificial venous circulation).