Pentastomes are also known as tongue worms because their shape is reminiscent of a vertebrate tongue. They are all respiratory parasites of vertebrates with larval forms infecting intermediate hosts such as arthropods and vertebrates. There are about a hundred species world wide and they are most common in tropical and subtropical habitats. Because they infect animals that people keep as pets, there is a potential for infection in humans (zoonotic disease). Most infections in humans are accidental and the most common cause is eating undercooked snake meat.
The relationships of the pentastomes is not clear. Their larvae resemble mite and tardigrate larvae so they probably have arthropod connections. They are coelomate protostomes at any rate. Because they are vertebrate parasites, their simplicity is probably secondary and they may be relatively recently evolved rather than primitive forms. Meglitsch (p. 445) thinks they are probably related to the myriapods (centipedes and millipedes). They have a cuticle and must moult in order to grow.
Pentastomes are most common in reptiles, using hooks to attach themselves to nasal passages or lungs. The mouth is found on the fifth segment, hence the name pentastome. The worm-like body is usually 2-15 cm in length, The hooks wound the respiratory surfaces and the worms suck blood into their digestive tracts. Anti-coagulants are injected from glands into the wounds to keep the blood flowing. The worms have no circulatory, respiratory or excretory organs and the nervous system is a simplified version of the general annelid-arthropod type with a large ventral ganglion and nerves that pass to legs hooks and abdomen.
As is typical in many parasites, the reprodcutive organs and large and complex and the reproductive potential is huge. Males have a single testis, are smaller than femals and more restless. Females have one ovary, eggs are fertilised internally and a single female may carry up to half a million fertilised ova in the uterus. When eggs are shed, they emerge with mucous secretions from the host’s respiratory tract. When the eggs are eaten by an intermediate host, they hatch and relase a migratory larva that pierces the stomach wall and then encysts in the tissues. The larvae has two pairs of legs and looks like a four legged tardigrade. Sometimes they infest vital organs and can cause serious illness or the death of the host. The larvae finally mature when the intermediate host is eaten by the final host. They migrate from the esophagus and trachea to the lungs or nasal passages. After a few months they mature, mate and produce another generation of eggs.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://animals.jrank.org/pages/1876/Tongue-Worms-Pentastomida.html http://parasitology.informatik.uni-wuerzburg.de/login/n/h/1066.html