These are the leeches, and are considered the most advanced annelid class. They differ from the other two Classes in 3 important features.
Leeches are predominantly freshwater animals, but some are marine and a few terrestrial.
They range in size from 1 - 30 cm in length, but most are between 2 - 5cm long. The longest is the Giant Amazonian leech, Haementeria ghiliana.
Like the oligochaetes they are hermaphroditic, but their clitellum only appears during the breeding season.
Gas exchange occurs through the skin, except for some of the fish leeches with gills. They can have a number of tiny black eyes. They have two brains - one at the head end and the other at the tail end.
Leeches have a fixed number of segments, this is unlike the other Annelid Classes; the number of segments varies with species but is most commonly 17, 31 or 34. They appear to have many more segments because externally each has superficial grooves.
Leeches have a looping movement, but are also very good and graceful, swimmers, they flatten their body into a ribbon-like shape and propel themselves forward in a series of vertical undulations.
Most leeches are fluid feeders. Blood-sucking leeches drink as much blood as they can from their victims. This blood is stored in special sacs at the side of their intestine and digested later.
Most can consume 3 times their own body weight in one meal. This allows the leech to survive long periods without food.
Freshwater leeches attach to their hosts only long enough to have one blood meal.
Eggs are laid in batches inside a cocoon or capsule attached to a solid object, or carried around on the underside of the adult, depending on species.
Above the horse leech, Haemopsis sanguisuga
The Medicinal leech, above, is the biggest freshwater leech in the UK - lengths of 20 cm have been recorded. It used to be fairly widespread but once bridges replaced fords and drinking troughs replaced ponds it became very difficult to find in the wild as it no longer had easy access to its victims.
It has strong teeth which make a Y-shaped cut in the skin, and an anticoagulant in its digestive juices stops the clotting of the victim's blood.
It can consume 2 - 5 times its own body weight in blood, and go without feeding for 18 months. A full meal can take around 200 days to digest.
Medicinal leeches are raised commercially to obtain hirudin - the anti-coagulant used to prevent blood clots. For more details see the table below.
Feeds mainly on turtles, but will also feed on alligators if there are no turtles.