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Of the 650 species of leech listed, Hirudo medicinalis is one of the most extensively studied. This annelid from the subclass Hirudinea has 2 suckers, one housing the mouth and its 3 jaws. It lives mainly in the stagnant waters of marshes and ponds. When it comes to reproduction, the animal is a hermaphrodite. Inseminated leeches lay their eggs during the spring and summer months, outside the water, with cocoons containing as many as thirty young.
Classes and subclasses of annelida. The three major groups of annelids are the polychaetes (largely marine annelids, with over 5,500 species); the oligochaetes (earthworms and freshwater worms, with over 3,000 species); and the hirundinea (leeches, with about 500 species). However, biological classification of annelids can vary widely among taxonomists.