Annelids are invertebrates characterized by their segmented bodies, a body cavity, and movable bristles. The phylum Annelida consists of over 9,000 species divided into three classes: Polychaeta (marine worms), Oligochaeta (earthworms), and Hirudinea (leeches). Annelids evolved in the sea, possibly from a flatworm ancestor, and are now found worldwide in various habitats such as oceans, fresh water, and damp soil. They possess a thin cuticle that is pierced with pores that secrete mucus and chitinous bristles called chaetae.

Why are annelids considered invertebrates?

Annelids, also known as segmented worms, are classified as invertebrates because they lack a vertebral column, or backbone. Invertebrates include a vast array of animal species, distinguished by the absence of this internal skeletal feature. Annelids are characterized by their segmented bodies, a feature from which they get their name because of the transverse rings, or annulations, that divide their bodies. They possess a body cavity and movable bristles. This group comprises over 9,000 species, including earthworms , leeches , and various marine worms .

What are common examples of annelids?

Annelids, invertebrates characterized by segmented bodies, include over 9,000 species divided into three classes: Polychaeta (marine worms), Oligochaeta (earthworms), and Hirudinea (leeches).

How is the body of an annelid organized or segmented?

Annelids, including earthworms and leeches, exhibit segmentation, where their bodies are constructed of repeating parts called metameres or somites. The annelid body plan can be described as a tube within a tube, where the digestive tract is separated from the body wall by the coelom. The head region (prostomium) is followed by a series of similar segments. The body of many annelid species is separated into two or three regions. Segmentation in annelids is conspicuous in adulthood.

Where do annelids live and how do they adapt to their environments?

Annelids, invertebrates also known as segmented worms, thrive worldwide in diverse habitats, including oceanic waters, fresh waters, and damp soils . Polychaetes are mostly marine, existing as free-moving or tube-dwelling species. Oligochaetes, including earthworms, burrow in soil, while some inhabit fresh water, and a few are marine. Leeches prefer freshwater or humid environments and are either carnivorous or parasitic. Annelids adapt through various mechanisms. For instance, earthworms extend their bodies, anchor with setae , and contract muscles to move. Some polychaetes have enlarged nephridia to tolerate salinity changes. Their thin, horny cuticle and secreted mucus protect them, while specialized glands in some marine species create protective tubes.

Why are annelids important in nature and for humans?

Annelids, also known as segmented worms, play significant roles in both natural ecosystems and for humans. In nature, annelids like earthworms contribute to soil health by aerating the soil, improving drainage, and enriching it with organic material, which enhances plant growth. Polychaetes play a crucial role in marine environments by turning over bottom sediments. For humans, earthworms assist plant growth, and some polychaetes, such as the palolo worm, are a food source in the South Pacific. Leeches have been used in medicine to restore blood flow to damaged tissues and produce anticoagulants.

annelid

Images

Diagram of an earthworm showing its nervous system, including labeled parts such as the cerebral ganglia, mouth, circumoesophageal connective, subesophageal ganglion, and ventral nerve cord, with an inset of the whole earthworm body.

At a Glance

annelid summary

annelid

invertebrate

Also known as: Annelida, segmented worm

Written by

Donald J. Reish

Fact-checked by

Britannica Editors

Last updated

Nov. 7, 2026 •History

Top Questions

What is an annelid?
Annelids are invertebrates characterized by their segmented bodies, a body cavity, and movable bristles. The phylum Annelida consists of over 9,000 species divided into three classes: Polychaeta (marine worms), Oligochaeta (earthworms), and Hirudinea (leeches). Annelids evolved in the sea, possibly from a flatworm ancestor, and are now found worldwide in various habitats such as oceans, fresh water, and damp soil. They possess a thin cuticle that is pierced with pores that secrete mucus and chitinous bristles called chaetae.
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1. integument: Annelids
2. annelid
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Why are annelids considered invertebrates?
Annelids, also known as segmented worms, are classified as invertebrates because they lack a vertebral column, or backbone. Invertebrates include a vast array of animal species, distinguished by the absence of this internal skeletal feature. Annelids are characterized by their segmented bodies, a feature from which they get their name because of the transverse rings, or annulations, that divide their bodies. They possess a body cavity and movable bristles. This group comprises over 9,000 species, including earthworms, leeches, and various marine worms.
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What are common examples of annelids?
Annelids, invertebrates characterized by segmented bodies, include over 9,000 species divided into three classes: Polychaeta (marine worms), Oligochaeta (earthworms), and Hirudinea (leeches).
- Polychaetes These marine worms are abundant in oceans and can be free-moving or tube-dwelling. An example is the feather duster worm (Manayunkia speciosa).
- Oligochaetes This class includes more than 1,800 species of terrestrial worms, with earthworms being a prime example. They are commonly found in soil with sufficient moisture and organic content. Lumbricus terrestris is a common species.
- Hirudinea Leeches primarily inhabit fresh water or humid environments. They can be carnivorous, feeding on small invertebrates, or parasitic, feeding on the blood of other organisms.
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How is the body of an annelid organized or segmented?
Annelids, including earthworms and leeches, exhibit segmentation, where their bodies are constructed of repeating parts called metameres or somites. The annelid body plan can be described as a tube within a tube, where the digestive tract is separated from the body wall by the coelom.

The head region (prostomium) is followed by a series of similar segments. The body of many annelid species is separated into two or three regions. Segmentation in annelids is conspicuous in adulthood.
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2. annelid: External features
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Where do annelids live and how do they adapt to their environments?
Annelids, invertebrates also known as segmented worms, thrive worldwide in diverse habitats, including oceanic waters, fresh waters, and damp soils. Polychaetes are mostly marine, existing as free-moving or tube-dwelling species. Oligochaetes, including earthworms, burrow in soil, while some inhabit fresh water, and a few are marine. Leeches prefer freshwater or humid environments and are either carnivorous or parasitic.

Annelids adapt through various mechanisms. For instance, earthworms extend their bodies, anchor with setae, and contract muscles to move. Some polychaetes have enlarged nephridia to tolerate salinity changes. Their thin, horny cuticle and secreted mucus protect them, while specialized glands in some marine species create protective tubes.
5 Britannica Sources
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Why are annelids important in nature and for humans?
Annelids, also known as segmented worms, play significant roles in both natural ecosystems and for humans.

In nature, annelids like earthworms contribute to soil health by aerating the soil, improving drainage, and enriching it with organic material, which enhances plant growth. Polychaetes play a crucial role in marine environments by turning over bottom sediments.

For humans, earthworms assist plant growth, and some polychaetes, such as the palolo worm, are a food source in the South Pacific. Leeches have been used in medicine to restore blood flow to damaged tissues and produce anticoagulants.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

annelid, any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name. The coelom is reduced in leeches, and setae are lacking a few specialized forms, including leeches. A major invertebrate phylum of the animal kingdom, the annelids number more than 9,000 species distributed among three classes: the marine worms (Polychaeta), which are divided into free-moving and sedentary, or tube-dwelling, forms; the earthworms (Oligochaeta); and the leeches (Hirudinea).

General features

Distribution and abundance

Annelids are found worldwide in all types of habitats, especially oceanic waters, fresh waters, and damp soils. Most polychaetes live in the ocean, where they either float, burrow, wander on the bottom, or live in tubes they construct; their colours range from brilliant to dull, and some species can produce light. The feather duster (Manayunkia speciosa) inhabits the Great Lakes and some rivers of the United States. The polychaetes include more than 6,000 known species, which are about evenly divided between free-moving and tube-dwelling forms. The oligochaetes number about 3,250 known species. Oligochaetes, including earthworms, burrow into soil; certain small oligochaetes are found in fresh water, and a few are marine, usually inhabiting estuarial or other shallow waters. Leeches, which number about 300 species, inhabit freshwater or humid environments and are carnivorous or parasitic on other organisms—e.g., all marine leeches are parasitic on fish.

Size range and diversity of structure

The length of annelids varies from a fraction of an inch to more than six metres (about 20 feet). The width may exceed 2.5 centimetres (about one inch) in the contracted state. Free-moving polychaetes and earthworms include the largest species. Leeches attain lengths of about 0.4 metre in the contracted state.

Polychaetes

The body of free-moving polychaetes (see figure) consists of a head, or prostomium, which may bear two or more eyes; a preoral segment, with such appendages as antennae, tentacles, and palpi (fleshy sensory projections); a trunk divisible into distinct segments; and a tail, or pygidium, which may bear anal cirri (fleshy projections) or plaques and a terminal anus. Each body segment following the second segment (peristome) usually has paired parapodia; i.e., fleshy, lateral outgrowths used in feeding, locomotion, or breathing. The parapodia, generally prominent in free-moving polychaetes, bear bundles of setae, which can be extended, and aciculae (needlelike structures), which are used for support.

The heads of sedentary polychaetes (see figure) may be distinct or indistinct. Forms with a distinct head generally lack head appendages. Branchiae, or gills, which serve for respiration and as food-gathering organs, are well-developed in many of the tube-dwelling forms. Some have tentacles at the anterior (front) end, and gills arise from the dorsal (upper) surface of a few anterior segments. In these species food is gathered by the tentacles and respiration is confined to the gills. The rest of the body is divided into thoracic and abdominal regions. Parapodia, if present, are generally simple lobes; frequently the setae project directly from the body wall. Many sedentary polychaetes construct tubes made from a substance secreted from cells that constitute the epidermis, or skin. Tubes may consist of calcium carbonate, parchment, or mucus, to which sediment adheres. The anus is at the posterior tip. Tube dwellers generally have an external fecal groove along which fecal material passes forward. Eyes are occasionally present on gills, along the sides of the body, or on the pygidium in sedentary forms that do not live in tubes.

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Oligochaetes

The body of oligochaetes is uniformly segmented and has conspicuous segmental lines. The prostomium is usually a simple lobe overhanging the mouth and lacking appendages. The microscopically small eyes are scattered over the body. The clitellum, a saddle-shaped thickening of the body wall, is present at sexual maturity. The anus is at the posterior tip. Setae generally arise from the ventral (lower) surface of the body.

annelid

General features

Distribution and abundance

Size range and diversity of structure

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Polychaetes

Oligochaetes