annelid

Introduction
General features
- Distribution and abundance
- Size range and diversity of structure
  - Polychaetes
  - Oligochaetes
  - Leeches
- Importance
Natural history
- Life cycle
  - Reproduction
  - Development
  - Life span
  - Regeneration
- Ecology
- Locomotion
- Food and feeding
- Behaviour and associations.
Form and function
- External features
- Internal features
  - Tissues and fluids
  - Nervous system
  - Digestive system
  - Excretory system
  - Respiratory system
  - Circulatory system
  - Hormones
Evolution and paleontology
Classification
- Distinguishing taxonomic features
- Annotated classification
- Critical appraisal

References & Edit History Quick Facts & Related Topics

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

Quizzes

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

Animal Group Names

Deadliest Animals Quiz

Match the Baby Animal to Its Mama Quiz

The bearded vulture (Gypaetus barbatus) is seen in Switzerland on December 2024

Animal Factoids

Related Questions

How are annelids different from flatworms and roundworms?
How are leeches related to oligochaetes?
How do annelids breathe through their body walls?
How do annelids regulate water and salt balance?
How do aquatic annelids remove nitrogen wastes?
How do circular and longitudinal muscles work together in earthworm movement?
How do earthworm setae prevent the body from sliding backward?
How do earthworms, leeches, and polychaetes differ?
How do earthworms reproduce using the clitellum?
How do free-moving polychaetes use parapodia to crawl?
How do leeches move with their anterior and posterior suckers?
How do leeches swim?
How do polychaetes reproduce in ocean water?
How do scale worms form commensal relationships with other marine animals?
How do transverse rings give annelids their name?
How do tube-dwelling polychaete worms build their tubes?
How does a coelom help an earthworm move?
How does a feather duster worm use its tentacular crown?
How does an earthworm cocoon protect developing young?
How does an earthworm move through soil?
How does bioluminescence help some polychaete worms reproduce?
How does the annelid body wall help protect the animal?
How many annelid species are known?
What are the functions of a leech's two suckers?
What are the functions of gills in polychaete worms?
What are the three major classes of annelids?
What is the difference between polychaetes and oligochaetes?
What is the role of the gizzard in an earthworm?
What is the typhlosole in an earthworm intestine used for?
What senses do earthworms use to respond to light and touch?
Where do polychaete worms live?
Where is the earthworm brain located?
Which animals belong to the phylum Annelida?
Which annelid fossils are found most often?
Which annelids live in freshwater habitats?
Which polychaete worms can produce light?
Why are annelid bodies divided into segments?
Why are earthworms important for soil?
Why can some marine polychaetes not survive sudden drops in salinity?
Why do earthworms and leeches have no free-swimming larval stage?
Why do earthworms avoid strong light?
Why do earthworms excrete more urea than marine annelids?
Why do leeches lack setae?
Why is the annelid fossil record limited?
What is a tardigrade’s survival mechanism?
What kills tardigrades?
What is an arthropod?
What are the main characteristics of arthropods?
What are the major groups of arthropods?
What is a dipteran?
Why are flies important?
What is a key physical characteristic of true flies?

Science Insects & Other Invertebrates Worms & Annelids

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External Websites

The Royal Society Publishing - Proceedings of the Royal Society B - The impact of fossil data on annelid phylogeny inferred from discrete morphological characters
UEN Digital Press with Pressbooks - Biology and the Citizen - Mollusks and Annelids
University of Hawaiʻi at Mānoa - Exploring Our Fluid Earth - Worms: Phyla Platyhelmintes, Nematoda, and Annelida
The Canadian Encyclopedia - Annelida
Encyclopedia of Arkansas - Annelids
National Center for Biotechnology Information - PubMed Central - Phylogeny of Annelida (Lophotrochozoa): total-evidence analysis of morphology and six genes
The University of Hawaiʻi Pressbooks - Concepts of Zoology Â– Hawaiʻi Edition - Annelids
University of Minnesota Libraries - Introductory Biology: Evolutionary and Ecological Perspectives - Mollusks and Annelids
Digital Commons at Michigan Tech - Invertebrates: Annelids (PDF)
Animal Diversity Web - Annelida

annelid

invertebrate

Also known as: Annelida, segmented worm

Written by

Donald J. Reish

Emeritus Professor of Biology, California State University, Long Beach. Author of Marine Life of Southern California.

Donald J. Reish

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

Britannica Editors

Last updated

Nov. 7, 2026 •History

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...

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