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Gnathustomulida (nā´thyəstəmyōō´lĬdə), phylum of microscopic worms described in 1956 that have elongated bodies, cilia, and are hermaphroditic. The 80 known species in this group are geographically widespread; they live mainly in sandy marine sediments, where there is organic material to feed on. Although they resemble flatworms, they differ primarily in their feeding mechanism and the fact that each epithelial cell has only one cilium.