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Gnathostomulida A phylum of tiny, acoelomate animals, discovered by Peter Ax in 1956; according to Ax it is the sister group of Platyhelminthes, like which it has no anus. Gnathostomulids are usually less than 1 mm long, elongate (often thread-like) and cylindrical, and externally ciliated (see CILIUM); they are hermaphrodite, there is a pair of lateral, toothed jaws. The 80 or so species live in fine sand in marine environments.