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collective name for three groups of animals: Onychophora and Tardigrada, which are free-living forms, and Pentastomida, which are parasitic. Onychophora and Tardigrada are considered to be separate phyla, although some authorities have included them within the Arthropoda. Pentastomida is usually considered to be a subphylum or class of Arthropoda. The various groups considered together as oncopods have several common features: similar paired locomotory appendages, called oncopodia, at some stage in the life cycle; a body cavity (hemocoel); a cuticle (skin) secreted by surface cells and shed periodically (molting); a gut that is usually a straight tube; and separate sexes and gonads. The groups also differ: only the onychophorans have well-developed organ systems; those of pentastomids and tardigrades are reduced or lacking. The groups are of interest chiefly because they bear strong resemblances to primitive annelidan forms as well as to the phylum Arthropoda (e.g., insects, crustaceans)-the most highly evolved invertebrates.