The phylum comprising the multisegmented, invertebrate wormlike animals, of which the most numerous are the marine bristle worms and the most familiar the terrestrial earthworms. The Annelida (meaning little annuli or rings) include the Polychaeta (meaning many setae); the earthworms and fresh-water worms, or Oligochaeta (meaning few setae); the marine and fresh-water Hirudinea or leeches; and two other marine classes having affinities with the Polychaeta: the Archiannelida (meaning primitive annelids) are small heteromorphic marine worms, and the Myzostomaria (meaning sucker mouths) are parasites of crinoid echinoderms.