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Archiannelida (phylum Annelida) A class of small, simple, annelid worms which are metamerically segmented (see METAMERIC SEGMENTATION) and most of which have narrow, tube-like bodies. In some, the nervous system is primitive. Parapodia and tentacles may be present. Separate male and female forms generally occur. The group was formerly regarded as a distinct phylum; most authorities now suggest it represents an assortment of largely unrelated annelids, most of which are marine and members of the interstitial fauna of sand grains. There are three families, all found in aquatic habitats.