Illustration of medicinal leeches (subclass Hirudinea). Leeches are parasitic worms that feed on blood. They attach themselves to the skin using suckers and create a wound with three sharp jaw plates, which contain over 600 teeth. Their saliva contains hirudin, a chemical which prevents blood from clotting. From New Dictionary of Practical Medicine and Surgery by Dr Jaccoud Vol. 39 (1882).