A leech is any segmented worm of the subclass Hirudinea (phylum Annelida). These worms are characterized by a small sucker, which contains the mouth, at the anterior end of the body and a large sucker at the posterior end. All leeches have 34 body segments, and their length ranges from minute to about 20 cm, or even longer when the animal stretches. Leeches primarily occur in fresh water and on land, but members of the order Rhynchobdellida can be found in the sea as well as in fresh water. Some species of leeches are predators of other animals, some eat organic debris, and others are parasitic. Many leeches that attack humans belong to the family Gnathobdellidae.

What type of animal is a leech?

A leech is any segmented worm belonging to the subclass Hirudinea within the phylum Annelida . Annelids are invertebrate animals characterized by their segmented bodies, the presence of a body cavity, and movable bristles. Leeches have 34 body segments and range in size from minute to about 20 cm. Leeches primarily live in fresh water and on land, with some species also found in the sea. They can be bloodsuckers, predators, or feed on organic debris. Some aquatic leeches feed on the blood of fishes, amphibians, birds, and mammals, while others consume snails, insect larvae, and worms. Terrestrial leeches feed exclusively on mammalian blood.

Leeches, a segmented worm in the phylum Annelida, feed through a small sucker containing a mouth at their anterior end. Most are bloodsuckers that use their mouthparts to cut into the host's skin. Their saliva contains substances that act as an anesthetic, dilate blood vessels to increase blood flow, and prevent clotting. Hirudo medicinalis , a medicinal leech, has three jaws with around 100 sharp teeth each. It attaches its sucker to the skin, exposes its teeth, and cuts into the skin. The saliva contains hirudin, an anticoagulant, which causes extensive bleeding. After feeding, leeches detach and may not feed again for up to 18 months. Some leeches are carnivorous and feed on snails and oligochaetes.

What role do leeches play in the ecosystem?

Leeches, belonging to the phylum Annelida, play varied roles in their ecosystems. While some are predators, others feed on organic debris, and some are parasitic bloodsuckers. As non-migratory macroinvertebrates, leeches are useful indicators of environmental health in streams and waterways because they are highly tolerant of pollution. Some leeches are scavengers, feeding on decaying organic material, micro-algae, or protozoans. Some species, like the medicinal leech Hirudo , feed on the blood of mammals, fish, and amphibians, using hirudin to facilitate blood flow. Carnivorous leeches consume oligochaetes and snails.

How have humans used leeches in medicine?

For centuries, humans have used leeches in medicine. Ancient physicians in India and later in Greco-Roman cultures used them to bleed patients, believing it could treat various conditions from mental illness to skin diseases. In the 19th century, leeching was integrated into bloodletting practices, with millions of leeches used annually in hospitals across Europe and America. Today, the medicinal leech is used to restore blood flow to damaged veins after an appendage has been reattached or tissue has been grafted. Leech saliva contains hirudin, a polypeptide that inhibits blood clotting, which helps in these procedures.

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European medicinal leech (Hirudo medicinalis).

Head of the giant Amazon leech (Haementeria ghilianii). The retractable proboscis is used to pierce the skin and suck blood from the host.

A medicinal leech, Hirudo medicinalis, with a dark, striped body, is attached to human skin.

At a Glance

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Quick Facts

Related Topics:: Arhynchobdellida; marine leech; land leech; Branchiobdellida; Rhynchobdellida

On the Web:: National Center for Biotechnology Information - PubMed Central - Leeches in the extreme: Morphological, physiological, and behavioral adaptations to inhospitable habitats (Oct. 15, 2026)

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leech

annelid

Also known as: Hirudinea

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Oct. 15, 2026 •History

Top Questions

What is a leech?
A leech is any segmented worm of the subclass Hirudinea (phylum Annelida). These worms are characterized by a small sucker, which contains the mouth, at the anterior end of the body and a large sucker at the posterior end. All leeches have 34 body segments, and their length ranges from minute to about 20 cm, or even longer when the animal stretches.

Leeches primarily occur in fresh water and on land, but members of the order Rhynchobdellida can be found in the sea as well as in fresh water. Some species of leeches are predators of other animals, some eat organic debris, and others are parasitic. Many leeches that attack humans belong to the family Gnathobdellidae.
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What type of animal is a leech?
A leech is any segmented worm belonging to the subclass Hirudinea within the phylum Annelida. Annelids are invertebrate animals characterized by their segmented bodies, the presence of a body cavity, and movable bristles. Leeches have 34 body segments and range in size from minute to about 20 cm.

Leeches primarily live in fresh water and on land, with some species also found in the sea. They can be bloodsuckers, predators, or feed on organic debris. Some aquatic leeches feed on the blood of fishes, amphibians, birds, and mammals, while others consume snails, insect larvae, and worms. Terrestrial leeches feed exclusively on mammalian blood.
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Where do leeches commonly live?
Leeches, segmented worms in the phylum Annelida, primarily inhabit fresh water and land, but some can be found in the sea. Aquatic leeches may feed on the blood of fishes, amphibians, birds, and mammals, or they may eat snails, insect larvae, and worms. True land leeches feed only on the blood of mammals and await their victim in damp vegetation. Aquatic leeches are widely distributed, similar to oligochaetes.
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How do leeches feed?
Leeches, a segmented worm in the phylum Annelida, feed through a small sucker containing a mouth at their anterior end. Most are bloodsuckers that use their mouthparts to cut into the host's skin. Their saliva contains substances that act as an anesthetic, dilate blood vessels to increase blood flow, and prevent clotting.

Hirudo medicinalis, a medicinal leech, has three jaws with around 100 sharp teeth each. It attaches its sucker to the skin, exposes its teeth, and cuts into the skin. The saliva contains hirudin, an anticoagulant, which causes extensive bleeding. After feeding, leeches detach and may not feed again for up to 18 months. Some leeches are carnivorous and feed on snails and oligochaetes.
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What role do leeches play in the ecosystem?
Leeches, belonging to the phylum Annelida, play varied roles in their ecosystems. While some are predators, others feed on organic debris, and some are parasitic bloodsuckers. As non-migratory macroinvertebrates, leeches are useful indicators of environmental health in streams and waterways because they are highly tolerant of pollution. Some leeches are scavengers, feeding on decaying organic material, micro-algae, or protozoans.

Some species, like the medicinal leech Hirudo, feed on the blood of mammals, fish, and amphibians, using hirudin to facilitate blood flow. Carnivorous leeches consume oligochaetes and snails.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How have humans used leeches in medicine?
For centuries, humans have used leeches in medicine. Ancient physicians in India and later in Greco-Roman cultures used them to bleed patients, believing it could treat various conditions from mental illness to skin diseases. In the 19th century, leeching was integrated into bloodletting practices, with millions of leeches used annually in hospitals across Europe and America.

Today, the medicinal leech is used to restore blood flow to damaged veins after an appendage has been reattached or tissue has been grafted. Leech saliva contains hirudin, a polypeptide that inhibits blood clotting, which helps in these procedures.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

leech, (subclass Hirudinea), any of about 650 species of segmented worms (phylum Annelida) characterized by a small sucker, which contains the mouth, at the anterior end of the body and a large sucker located at the posterior end. All leeches have 34 body segments. The length of the body ranges from minute to about 20 cm (8 inches) or even longer when the animal stretches. Leeches occur primarily in fresh water and on land. Members of the order Rhynchobdellida occur in the sea as well as in fresh water. One member of this order, the giant Amazon leech (Haementaria ghilianii), can grow to a half metre (about 18 inches) in length. This leech uses its 15-cm (6-inch) proboscis as a hypodermic needle to suck blood from its host. Some species of leeches are predators of other animals, some eat organic debris, and others are parasitic.

Leeches breathe through the skin. The digestive system contains a crop, or pouch, in which food can be stored for several months. One to four pairs of eyes are located at the anterior end. Individuals are hermaphroditic; that is, functional reproductive organs of both sexes occur in the same individual. Leeches are not self-fertilizing, however, for the sperm of one individual fertilizes only the eggs of other individuals. The eggs are laid in a cocoon, which may be deposited on land or in water. Development and growth are direct, without a free-living larval stage.

Aquatic leeches may feed on the blood of fishes, amphibians, birds, and mammals, or they may eat snails, insect larvae, and worms. True land leeches feed only on the blood of mammals. Three jaws set with sharp teeth make a Y-shaped incision in the flesh. The leech’s saliva contains substances that anesthetize the wound area, dilate the blood vessels to increase blood flow, and prevent the blood from clotting. The anticoagulant hirudin, which is extracted from the body tissues of the European medicinal leech (Hirudo medicinalis), is used to prevent blood clots following surgery; another chemical isolated from Amazonian leeches is used to dissolve existing blood clots.

Sea otter (Enhydra lutris), also called great sea otter, rare, completely marine otter of the northern Pacific, usually found in kelp beds. Floats on back. Looks like sea otter laughing. saltwater otters

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Land leeches await their victim in damp vegetation, poising one end in the air. The victim is often unaware of being bitten until blood is discovered running from the wound; blood flow may continue because of the anticoagulant still present.

Many leeches that attack humans belong to the family Gnathobdellidae. Some species have been used medically for centuries; in Europe the use of leeches to drain off blood reached its height of popularity in the 19th century. Diseases commonly treated with leeches included mental illness, tumours, skin disease, gout, and whooping cough. A common treatment for headache was to apply several leeches to each temple and allow them to draw blood. Leeches have also been used to prevent excessive blood buildup following certain surgical procedures (e.g., reattachment of severed fingers).

In addition to H. medicinalis of Europe, the Algerian dragon (H. troctina) was used. Gnathobdella ferox was commonly used in Asia. After H. medicinalis was introduced into North America, it established itself there as a wild species. It grows to 10 cm (4 inches) in length and is green, with four to six brown stripes.

Other land leeches that attack humans are primarily of the genus Haemadipsa in Asia, the Philippines, the East Indies, and Madagascar. Leeches of the genus Philaemon are parasitic on humans in Australia.

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Aquatic leeches, particularly Limnatis nilotica, may enter the body in drinking water. Some may enter the excretory openings of persons who bathe in infested waters. L. nilotica, which inhabits lakes and streams of southern Europe, North Africa, and the Middle East, attains lengths of up to 12 cm (4.75 inches), but younger, smaller specimens are most likely to enter the body. When ingested with drinking water, they may first attach themselves to the linings of the nose or throat and then be inhaled into the lungs.

Tyrannobdella rex, a member of the family Praobdellidae and native to remote parts of the upper Amazon River in Peru, appears to prefer the mucous membranes found in the nasal cavity of mammals. This leech seeks out its victims as they bathe, making an attachment by means of a single jaw made up of eight large teeth.

A person infected with many such leeches may suffer from anemia resulting from blood loss. In external wounds secondary infection is more likely to occur than anemia. Leeches can cause suffocation and death of the host by blocking the breathing passages; in Asia, in particular, domestic animals commonly die in this way.

This article was most recently revised and updated by John P. Rafferty.

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