Leeches are related to earthworms and lugworms (Oligochaetes), and bristle worms (Polychaetes). But unlike other worms they have a sucker at each end – one for feeding, the other for hanging on while they feed. They all belong to a group of legless invertebrates (animals without a skeleton) called Annelids – “anulus” is the Latin for “ring”. This means that the body is divided into separate segments (which look like rings) connected by a continuous gut, a nerve and a blood vessel. They use external bristles (chaetae) to pull themselves along in a sort of concertina-like motion.
Not all worms are Annelids. Some belong to a group called Nematodes. Many nematodes are internal parasites, often living in the guts of mammals (including humans). Leeches are external parasites.
European leeches live in fresh water and damp places. There are a few marine leeches living in the Atlantic Ocean which prey on fishes. In Britain, the medicinal leech ( Hirudo medicinalis) used to be common in ponds, especially near monasteries where people in medieval times were taken for healing. Now there are only a few ponds where this leech occurs naturally. And ponds are disappearing fast. It is almost an endangered species, but many people would say “good riddance” because of its unattractive appearance and reputation.
On the whole leeches do not have bristles, but their flattened body allows them to swim effectively in an undulating way. They lie in wait for a passing animal of the right kind and then home in on it. They can contract their bodies to become short and stubby, or extend them to become long and thin.
Some leeches are blood-suckers, and attach themselves to the skin of animals at a place where blood vessels are near the surface.The mouth end has sharp jaws to cut through the skin of it host. They inject an anti-clotting substance to stop the blood coagulating (thickening) and feed until they are full. Then they drop off to digest their meal, but the puncture bleeds for a while afterwards.
The horse leech (Haemopis sanguisuga), which can be 30 cm long when extended, does not suck horses’ blood, but feeds on earthworms, and decaying flesh.
The medicinal leech was used in medieval times in Europe for certain illnesses because doctors believed that sucking out some of their patient’s blood helped them to recover. To do this they would apply a leech or two and let them feed. Surprisingly, leeches have come back into fashion for medical use. The anti-coagulant they secrete into a wound helps to stop a scab forming, preventing the skin from sealing over too quickly. This promotes healing from the inside outwards. This is especially important where very delicate repairs have been made to torn tissue.
Fancy having two or three leeches hidden under your bandages! In films about tropical rainforest adventures the tough guys are always removing leeches from their legs.
There is commercial company in south Wales (UK) called Biopharm which breeds leeches and sends them all over the world to doctors. They also carry out research. Amazingly, when I went there a few years ago to report on their activities, they used to invite school parties to visit. Perhaps they still do.
The information above was mostly taken from the Oxford Book of Invertebrates, by David Nichols and John Cooke, illustrated by Derek Whiteley. 1971