The annelids, formally called Annelida, are a large phylum of segmented worms, with over 22,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments.They are bilaterally symmetrical, triploblastic, coelomate organisms. They have parapodia for locomotion. Although most textbooks still use the traditional division into polychaetes, oligochaetes and leech-like species, research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes.
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.
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