ONCOPOD

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. The evolution of oncopods, as of other very old groups with few or no fossils, is a speculative subject, on which no consensus has yet been reached. Oncopods, as well as arthropods, originated in Precambrian times from marine, bottom-dwelling forms that may also have been ancestral to modern annelids. The adaptation that resulted in the evolution of oncopods and arthropods was the acquisition of lobopodia, locomotory appendages that could work independently of the waves of contraction of the body. The acquisition of lobopodia led to the dissolution of separate coelomic compartments and to the formation of a hemocoel; it also permitted the development of a firmer cuticle, which made molting necessary. The development of the cuticle also led to the loss of external cilia. Tardigrades evolved toward small size, with a consequent simplification of body organization. Some of their primitive characteristicse.g., the cuticle of protein, the brain, and the absence of specialized legs on the headsuggest that their ancestors diverged very early from the oncopod-arthropod stem. Tardigrades show interesting similarities to some aschelminths (roundworms). The evolution of pentastomids has been greatly influenced by their endoparasitic (internal-parasitic) mode of life; it also entailed simplifications. Their chitinous cuticle, striated muscles, and brain form indicate that they had ancestors more active and advanced than those of tardigrades; it is possible that they shared a common ancestor with the arthropods. Pentastomids could have evolved from free-living forms, first as parasites of fishes, then becoming parasites of terrestrial vertebrates as they appeared in the Devonian Period (408 to 360 million years ago). The formation of a strong skeleton under a soft epidermis is a characteristic of onychophoran evolution and permits the body to be deformed. Among oncopods, onychophorans have the most extensive combinations of primitive featurese.g., structure of nerve cords, persistence of cilia, smooth muscleswith oncopodan characters or basic arthropodan characters (e.g., chitin and molting, hemocoel, gonads, embryonic development) or both. They also developed specializations of their own (e.g., dermis, tracheae, viviparity). Both onychophorans and arthropods could have arisen in Precambrian seas from a Xenusion-like ancestor, arthropods acquiring sclerites (i.e., hard body plates) and joints and onychophorans developing a strong dermis while keeping a thin cuticle. According to some authorities, the myriapod-insect line originated from onychophoran ancestors, independently from the other arthropods. See also onychophoran; pentastomid; tardigrade.