Annelid

Annelid, common name for about 9,000 species of worm-like invertebrate animals with well-developed segmentation. The three major classes of the annelid phylum are the bristle worms (about 5,300 species), which are mainly marine and often luminescent; the oligochaetes (about 3,100 species), which are mostly freshwater or inhabit the soil, such as the earthworm; and the leeches (about 300 species), which are mainly freshwater but can also be marine or terrestrial.

Although annelids are diverse in form, the bristle worms, which are generally considered the ancestral stock, can exemplify their structure. The elongated 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 partitions (septa). The segments generally bear lobes (parapodia) with bristles (setae) that are used in locomotion. The fluid-filled body cavity (coelom) 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 (rear) end.

Oligochaetes (“few setae”) have fewer bristles and no lobes. 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 move by looping movements; they lack lobes and bristles.

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.

Some bristle worms are active predators. Others have taken to a sedentary way of life and extract food 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 as they do so and thus playing an essential role in soil ecology. Most leeches suck the blood of other animals and are still sometimes used medically to facilitate blood flow after microsurgery; an extract prepared from their saliva is used as an anticoagulant. Some species are predators.

Fossil annelids are rare because their soft bodies are not easily preserved. Known fossils date from more than 500 million years ago. They may be descended from the flatworms (Platyhelminthes) 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.