Ferris, Howard Department of Nematology, University of California, Davis, California.
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An order of nematodes (roundworms) formerly constituting at least two important groups of animal parasites: the mermithids, which are parasites of invertebrates, and the trichinellids, which are parasites of vertebrates. These nematodes are characterized by a pharynx that is narrow and thin-walled anteriorly and is surrounded posteriorly by unicellular, glandular stichocytes, each with a duct into the pharyngeal lumen (Figs. 1 and 2). The pharynx extends one-fourth to nine-tenths of the body length in various taxa and is almost devoid of musculature. The region of the pharynx surrounded by stichocytes is known as the stichosome. Recent phylogenetic analysis based on a synthesis of molecular and morphological data suggests that the stichosome may be an example of parallel evolution and that the trichinellids and mermithids are more appropriately separated as two orders: Trichinellida, with at least six families, and Mermithida, with two families. However, debate on the higher classification of nematodes continues, and opinion will no doubt refocus as new data emerge and are applied. See also: Medical parasitology; Nemata (nematoda)
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