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The tongue worms, a group of parasitic wormlike animals considered to form a distinct phylum thought to be descended from primitive arthropods, although modified by parasitism to form elongate, pseudosegmented, wormlike organisms with two to three pairs of budlike degenerate limbs in the larva and anterior, hollow, fanglike hooks in the adult. Adults are usually parasitic in the lungs or respiratory tract of vertebrates, usually in snakes and other reptiles, although one group parasitizes the air sacs of birds and one family (Linguatulidae) has become adapted to the lungs of mammal carnivores (families Felidae and Canidae). Larvae are found in the viscera of many hosts that serve as prey of the final hosts (insects, fish, amphibians, chiefly frogs, and mammals, chiefly rodents). Dogs may develop adult Linguatula serrata in their nasal passages from infective larvae (nymphs) in the viscera of sheep, cattle, or rabbits, which became infected from water or vegetation contaminated with eggs passed by infected dogs; humans also can develop a larval infection from this source. Human infection of liver, spleen, and lungs has been reported in Africa from Armillifer armillatus and in China by A. moniliformis from contaminated water or vegetation or from handling infected snakes.