Leeches — Class Hirudinea

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 the substrate.

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.