The medicinal leech of Europe, perhaps twelve centimeters long on average. It is now rare in nature because of over-collecting and destruction of habitat. It has been introduced into northeastern United States, but is uncommon there. They are hermaphroditic.
This humble and somewhat repulsive animal played an important part in the history of Europe and of European culture as exported elsewhere, and for a long time was important for its production of hirudin,a powerful anti-coagulant.
These are freshwater animals. (Some leeches of other species can be found in marine or terrestrial habitats.) A leech attaches itself to its prey by a posterior sucker, then applies the anterior sucker to the skin and makes a wound, often with the aid of little saw-toothed jaws inside the mouth. It fills its digestive tract with blood, sometimes taking an amount equivalent to 10 times its own weight, then drops off. Hirudin is manufactured by the animal to prevent the coagulation of the blood during the process of drawing it as well as while it is being digested.
European physicians developed the idea that illness was caused by an imbalance of substances (called by them "humors") in the body, which imbalance could be corrected by allowing leeches to draw large amounts of blood from the sick. (This notion goes back as least as far as to Galen (130 AD).) It is probable that George Washington died from excessive "bleeding;" nor was he the only victim. From this practice physicians came popularly to be called "leeches."
Though bleeding is no longer used therapeutically in Europe, hirudin is still important as an anti-coagulant in some kinds of heart surgery. Further, because of the peculiar construction of hirudo's nervous system, it is an important animal for research purposes.
Hirudo medicinalis is difficult to cultivate in the laboratory. For a time researchers turned to Haementeria ghilianii, a giant South American leech. Hirudin can now be made artificially.
(PHYLUM, Annelida; CLASS, Hirudinea; ORDER, Gnathobdellae)