Tongue worms are bloodsucking endoparasites with a flattened, tongue-like body, and are in the phylum Linguatulida. The final host of these parasites is a predaceous vertebrate, usually a reptile, but sometimes a mammal or a bird. The intermediate host (in which the parasite lives as a larva) can be any of a number of animals, including insects, fish, amphibians, reptiles, and mammals. The 70 species of tongue worms are primarily tropical and subtropical in distribution.
Adult tongue worms live in the lungs or nasal passages of the host. Mature females reach a length of up to 15 cm (6 in) and a width of 0.4 in (1 cm), while males are 0.8-1.6 in (2-4 cm) long and 3-4 mm wide. The mouth is sited at the anterior end of the worm, and has a set of hooks for attachment to the tissues of the host. In most species of the tongue worm, the body has superficial ringlike markings, giving it the appearance of segmentation. The only internal body structures are the digestive and reproductive organs, and certain glands. Respiratory, circulatory, and excretory organs are absent.
An typical life cycle of a tongue worm is that of Porocephalus crotali, whose adult stage lives and reproduces in the lungs of the rattlesnake, and whose larval stage is found in the muskrat and other hosts which constitute the rattlesnake's food. Following copulation, the female tongue worm releases her eggs into the host's lungs. The eggs leave the host's body in its respiratory secretions, and might be accidentally ingested by the intermediate host. In the digestive tract of the intermediate host, the eggs hatch to produce four-legged larvae which superficially resemble the immature stage of a mite. The larva penetrates the wall of the host's digestive tract, migrates to another tissue such as the liver, and encysts. If the intermediate host is now eaten by a rattlesnake, the parasite is transferred to the snake's digestive tract. Here it emerges from the cyst and migrates up the host's esophagus and into its lungs, where it eventually matures. Armillifer and Linguatula are two other well known genera of tongue worms, each with different final and intermediate hosts, but with an essentially similar life cycle.
Zoologists consider linguatulids to be closely related to the arthropoda, and in some classifications the tongue worms are included in that phylum.