MONICA SCHROEDER / SCIENCE PHOTO LIBRARY MONICA SCHROEDER / SCIENCE PHOTO LIBRARY
Illustration of a leech (Hirudo medicinalis officinalis). Leeches are bloodsucking worms with a flattened body which inhabit fresh water and in tropical countries infest damp forests. They are harmful to people and animals to whom they attach by means of a strong mouth adapted to sucking. The saliva of leeches contains an anti-clotting substance, called hirudin, which is injected into the wound before sucking blood.
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