Pentastomida (pĕnˈtəstōˌmə) [key], 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.