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Classification, Habitat, Movement, Feeding,
Medical uses.
Classification &Relationships
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.
Habitat
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.
Movement
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.
Feeding habits
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.
Medical uses
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.
References
The information above was mostly taken from the Oxford
Book of Invertebrates, by David Nichols and John Cooke, illustrated by Derek Whiteley.
1971
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