Members of the phylum Annelida are wormlike animals that usually possess an extensive internal cavity, extending the length of the body, subdivided into consecutive discrete segments. The segments may be separated from one another internally by septa and indicated externally by outgrowths from the body wall (parapodia) and/or by bundles of bristles (setae).
Three major groups of the phylum Annelida are recognized: The class Polychaeta (polychaetes), almost exclusively marine animals, is characterized by the presence of bundles of many setae on each segment, by heads provided with sensory structures, and by separate sexes. The class Oligochaeta (earthworms) consists mainly of terrestrial and freshwater forms, possessing few setae per segment, lacking sensory structures on their heads, and reproducing as cross-fertilizing hermaphrodites. The class Hirudinea (leeches), primarily found in freshwater and terrestrial habitats (although a few are parasitic on marine fish), lacks setae,