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Pentastomida(pĕn`təstō'mə), tongue worms, small phylum of fewer than 100 species of parasites living in the upper respiratory passages of reptiles, and occasionally of birds and mammals. They are closely related to arthropods, but zoologists disagree as to their exact placement in that phylum. Recent evidence indicates a possible origin from parasitic crustaceans. The tapering wormlike body, varying in length from 1-2 in. to 5 in. (1.3–13 cm), is unsegmented. The front of the body bears five short appendages; one bears the mouth, the other four attach to host tissues. Internal organs are much reduced, except the reproductive organs, which occupy most of the internal space. Eggs are released by the mature pentastomids and hatch when eaten by a suitable intermediate host. When the intermediate host, in turn, is eaten, the larvae migrate to the respiratory passages of the final host, where they take up permanent residence and mature.
(more accurately, Linguatulida), a unique class of parasitic invertebrates, whose place in the system of animal classification is unclear. Members of the class most closely resemble arthropods, and they are usually added to the phylum Arthropoda as an extra class. There are about 60 species, distributed mainly in the tropics.
The body, which reaches 14 cm long, is wormlike, not infrequently ligulate. It consists of a short, unsegmented anterior section and a longer, segmented posterior section. The mouth is on the underside of the anterior section, and along its sides there are two pairs of claws. The animal is covered with a cuticle. Under the skin there is a layer of annular and then longitudinal striated muscles. In most members of the class the ventral nerve cord is concentrated in a subesophageal ganglion. The digestive tract is tubular, and there is an anus at the posterior end of the body. There are no respiratory or circulatory organs. The sexes are separate.
Adult individuals parasitize the lungs and nasal passages of reptiles and mammals. The eggs, which are swallowed by an intermediary host (also a vertebrate), develop into larvae with two pairs of short lateral legs. The larvae then become nymphs, which develop into adults after they are swallowed by the terminal host.
A. V. IVANOV