Annelid is the common name for about 9000 species of wormlike invertebrate animals with well-developed segmentation. The three major classes of the annelid phylum are the bristle worms (about 5300 species), which are mainly marine and often luminescent; the oligochaetes (about 3100 species), which are mostly freshwater or inhabit the soil, such as the earthworm; and the leeches (about 300 species), which are mainly freshwater but may also be marine or terrestrial.

Characteristics
Although annelids are diverse in form, the bristle worms, which generally are considered the ancestral stock, can exemplify their structure. The elongate body is rounded in cross section, with a mouth (sometimes jawed) at one end and an anus at the other, and it exhibits bilateral symmetry. The body is made up of several similar units, or segments, separated externally by furrows and internally by septa (partitions). The segments generally bear parapodia (lobes) with setae (bristles) that are used in locomotion. The coelom (fluid-filled body cavity) provides a kind of structural support, so that the body is flexible. The gut is a straight tube and the nervous system fairly simple, with sense organs poorly developed. The head may bear eyes, feelers, or tentacles; a few bristle worms have highly developed eyes. Bristle worms grow by adding segments at the posterior end.

Oligochaetes have fewer setae and no parapodia. The body is streamlined and well adapted to burrowing, and the head does not have eyes or feelers. Leeches are flattened and can swim or crawl by looping movements; they usually lack setae.

Bristle worms generally have separate sexes and reproduce by releasing sperm and eggs into the water. By contrast, the basic pattern in earthworms and leeches is hermaphroditism, with cross-fertilization occurring internally. These animals produce cocoons, and some leeches care for the young.

Ecology and evolution
Some bristle worms are active predators. Others are sedentary and extract particles from the water or take up deposits from the bottom. Earthworms feed mainly by passing soil through their bodies to extract nutrients, enriching and aerating the soil and thus playing an essential role in soil ecology. Most leeches suck the blood of other animals; they are sometimes used medically for bloodletting, and an extract prepared from their blood is used as an anticoagulant. A few species of leeches are predators.

Fossil annelids are rare because their soft bodies do not preserve well. Existing fossils date from more than 500 million years ago. They may be descended from the flatworms and are thought to have given rise to the arthropods. Some aberrant marine annelids are often placed in separate classes; others may even be in separate phyla.

Scientific classification
Annelids make up the phylum Annelida. Bristle worms make up the class Polychaeta, oligochaetes the class Oligochaeta, and leeches the class Hirudinea.