Phylum Annelida

Annelida is a phylum consisting of more than 16,500 described, living species of aquatic or terrestrial animals that are ecologically and systematically important. Two questionable species are the Precambrian Dickinsonia, originally believed to be an annelid but now assigned to the Placozoa, and Spriggina, which has been interpreted as an annelid, a cnidarian, and an arthropod. Annelids are characterised by their segments with coelomic compartments and bundles of chitinous chaetae, both of which show variation. Their evolution is not well understood. The name Polychaeta is synonymous with Annelida. A number of groups that have previously been assumed as separate phyla are now placed within the Annelida, including the Echiura, Pogonophora, Vestimentifera, and Sipuncula. The embryology displays most of the characteristics predicted by the trochaea theory, which firmly places the phylum in the Spiralia. The radiation of the spiralians remains unresolved.