There are about 100 extant species of pentastomids; all are obligate parasites with correspondingly degenerate anatomy. Adult tongue worms vary from about 1 cm to 14 cm in length, and parasitize the respiratory tracts of vertebrates . They have five anterior appendages. One is the mouth; the others are two pairs of hooks which they use to attach to the host. This arrangement led to their scientific name, meaning "five openings", but although the appendages are similar in some species, only one is a mouth.
Their affinity with other invertebrates is uncertain. They grow by moulting, which suggests they belong to the Ecdysozoa. Historically, they were considered to deserve a phylum of their own, but in 1972, Wingstrand showed similarities in the structure of spermatozoa between pentastomids and branchiurans (fish lice), a group of parasitic crustaceans , and this relationship is supported by molecular evidence .
However, fossils from the Upper Cambrian identified as pentastomids (Heymonsicambria, Haffnericambria, Bockelericambria) suggest that the pentastomids branched very early and may be an outgroup to the other arthropods .
Alternative names for the Pentastomida include Pentastoma (strictly a genus name), Linguatulida, and Acanthotheca.
Subclass Pentastomida Diesing, 1836
Pentastomids live in the upper respiratory tract of reptiles, birds, and mammals, where they lay eggs. The eggs are either coughed out by the host or leave the host body through the digestive system. The eggs are then ingested by an intermediate host. The first larva hatches in the intermediate host and breaks through the wall of the intestine. It then forms a cyst in the intermediate host's body. The pentastomid reaches the main host when the intermediate host is eaten by the main host.