Many leeches are blood suckers they attach themselves with a sucker to fish, snails, or mammals, and either insert straw-like mouthparts into their host, or slice its skin and then suck up the flow. The common name leech comes from the Old German lAhhi, which means physician. This is a reference to the once common practice of treating illness by attaching leeches to the patient and allowing them to suck out the "bad blood." For much of modern history, calling a doctor a leech was common practice!
The majority of worms in this class are freshwater, but some are terrestrial
and a few live in marine waters. Piscicola geometra feeds exclusively
on the blood of fishes. It occurs widely throughout the Northern Hemisphere.
Glossiphonia complanata, which feeds on the fluids in snails, is common
in North America and occurs on the Canadian Arctic mainland where its hosts
live. This species carries its eggs on its body to care for them until they
hatch. Not all leeches suck blood about one quarter of species are predators
of other worms, snails, and insects.
Leeches possess a few characteristic features that set them apart from other
worms. Firstly, they have two suckers: one at the front end and one at the hind
end. They also possess four muscle layers in comparison to the two layers of
other annelids. Another difference is that leeches lack the hairs that are present
on the bodies of other worms to anchor themselves. As a consequence of these
structural differences, leeches crawl and swim, but are unble to burrow. In
order to crawl, a leech uses its two suckers to alternately anchor itself to
All leeches are hermaphroditic, with every individual having both male and female reproductive parts. They cannot self-fertilize, however, but must mate because their eggs and their sperm develop at different times. Unlike other annelids, leeches cannot reproduce asexually or regenerate damaged body segments. After copulation, eggs are laid in a cocoon, which contains a nutritious protein called albumin the same protein that is found in egg whites. Most leech species release the cocoon into the environment, or attach it to the substrate, vegetation, or the host.