A subclass of bloodsucking crustacean arthropods, parasitic in the respiratory organs of vertebrates, that frequently are referred to as the Linguatulida or tongue worms. The adults are vermiform, with a short cephalothorax and an elongate, annulate abdomen that may be cylindrical or flattened. On the ventral surface the cephalothorax bears two pairs of hooklike, retractile claws. Respiratory, circulatory, and excretory organs are absent. The digestive tract is a straight tube with cuticle-lined anterior and posterior ends. The nervous system consists of subesophageal ganglia and a circumesophageal ring. Paired nerves extend from the ganglia to cephalic structures, and a midventral pair extends posteriad the length of the body. The sexes are separate; the females are two to five times the size of the males. The male genital pore is medial and ventral, at the anterior border of the abdomen. The female genital pore is anterior in the Cephalobaenida but located at or near the posterior end of the body in the Porocephalida.