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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.
Most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species. Cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes.ach segment has the same sets of organs and, in most polychaetes, has a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly defined or absent in others, and Echiura and Sipuncula show no obvious signs of segmentation.