The annelids, collectively called Annelida (from Latin annellus "little ring"), are a large phylum of animals, comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches. They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over 3 meters.

Anatomy

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Annelids are triploblastic protostomes with a coelom, closed circulatory system and true segmentation. Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coleom is divided into a sequence of compartments by walls called septa. In the most general forms each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.

Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (to increase surface area) along much of its length. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders which simply ingest the sediments, and blood-suckers.

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