Leeches are annelids comprising the subclass Hirudinea.
There are freshwater, terrestrial and marine leeches. Like earthworms,
leeches are hermaphrodites. The medicinal leech, Hirudo medicinalis,
which is native to Europe, and its congeners have been used for
clinical bloodletting for thousands of years.
All leech species are carnivorous and have evolved from detritus-eating
earthworms/oligochaete ancestors. Some are predatory, feeding on
a variety of invertebrates such as worms, snails, insect larvae,
crustaceans, while a very few are blood-sucking leeches, feeding
on the blood of vertebrates such as amphibians, reptiles, waterfowl,
fish, and mammals. Given the opportunity, they will also feed on
human blood. The most important predators on leeches are fish, aquatic
insects, crayfish and other leeches.
Haemophagic leeches attach to their hosts and remain there until
they become full, at which point they fall off to digest. Leeches'
bodies are composed of 34 segments. They all have a anterior (oral)
sucker formed from the last six segments of their body, which is
used to connect to a host for feeding, and are known to release
an anaesthetic to remain unnoticed by the host. They use a combination
of mucus and suction (caused by concentric muscles in those six
segments) to stay attached and secrete an anti-clotting enzyme into
the host's blood stream.
Some species of leech will nurture their young, providing food,
transport, and protection, which is unusual behavior in an invertebrate.
The anatomy of medicinal leeches may look simple, but more details
are found beyond the macro level. Externally, medicinal leeches
tend to have a brown and red striped design on an olive colored
background. These organisms have two suckers, one at each end, called
the anterior and posterior sucker. The posterior is mainly used
for leverage while the anterior sucker, consisting of the jaw and
teeth, is where the feeding takes place. Medicinal leeches have
three jaws that look like little saws, and on them are about 100
sharp teeth used to incise the host.
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