Hirudinea

Hirudinea is a class of annelid worms, commonly known as leeches. They are characterized by their elongated bodies and the presence of a sucker at both the anterior and posterior ends. Leeches are found in a variety of environments, from freshwater to marine habitats, and some species are even terrestrial. They are best known for their hematophagous (blood-sucking) behavior, although not all leeches feed on blood.

Anatomy and Physiology

Leeches, like other annelids, have a segmented body. However, their segmentation is not as apparent externally as it is in other annelids due to the presence of a thick, elastic cuticle that covers the body. The number of body segments can vary greatly among different species, ranging from 30 to over 100.

The anterior (front) end of the leech features a small sucker that surrounds the mouth. The posterior (back) end has a larger sucker used for locomotion and attachment to the host during feeding. The mouth of a leech is equipped with three sharp jaws, used to make a Y-shaped incision in the skin of its host.

Leeches have a well-developed nervous system with a brain and a ventral nerve cord. They also have a closed circulatory system, which is a characteristic of annelids. The digestive system of a leech is specialized for its feeding habits, with a crop that can store ingested blood and a posterior intestine where digestion takes place.

Behavior and Ecology

Leeches are predominantly freshwater organisms, but there are also marine and terrestrial species. They are found worldwide, but are most diverse in the tropics.

Many leeches are hematophagous, feeding on the blood of vertebrates. They are ectoparasites that attach to their hosts with their suckers and cut into the skin with their jaws. The leech then secretes an anticoagulant to prevent the blood from clotting while it feeds. Some leeches can ingest several times their body weight in blood during a single feeding.

Not all leeches are blood-suckers, however. Some species are predatory and feed on small invertebrates.

Medical Use

Leeches have been used in medicine for centuries, most notably in the practice of bloodletting. Today, they are used in microsurgery and reconstructive surgery to reduce swelling and improve circulation in areas with poor blood flow.

See Also

• Annelida
• Ectoparasite
• Bloodletting

References

Hirudinea Resources
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