This parasite has a worm-like body, but is closely allied to the mites and belongs to the Arachnida. It differs from the mite in having but four short limbs retractile and protractile and furnished with sharp claws. The body is thickest toward the anterior end and prolonged and narrow posteriorly; marked by about 90 rings; head rounded off abruptly, mouth broadly open, with a horny lip; integument with numerous openings or stigmata (respiratory); _male_ 7 lines long by a line broad in its anterior part genital orifice on the front part of the abdomen in the median line; _female_ 3 to 4 inches long, by 3 or 4 lines broad anteriorly; genital opening at the end of the tail. _Reproduction_ oviparous. _Habitat._ Nasal chambers and sinuses of the dog, wolf, goat, and horse. _Pentastoma Denticulata._ The young partially developed _P. Taenioides_. Has all the rings except the two first, garnished with fine sharp recurved spines; legs more slender with accessory booklets; length 2 lines; breadth ½ line. _Habitat._ Cysts in the lungs, liver, mesenteric glands, etc., of the hare, porpoise, goat, sheep and other mammals, not excepting man. _Development._ Leuckart found that the adult Pentastomata copulate in the nasal chambers, as many as half a million of eggs being fertilized in a single female; that these eggs are discharged with the nasal mucus and falling on vegetables are taken in by herbivora; their shells are digested and destroyed in the stomach, and the liberated embryos perforate the intestinal walls and encyst themselves in various organs. The encysted embryo varies from ¹⁄₅₀₀ to ¹⁄₄₀ inch in length, is rounded and blunt anteriorly but very thin posteriorly with the tail slightly curved toward the ventral aspect. It is several months before the feet, cutaneous spine and generative organs are developed, and during this period it undergoes several moultings. Finally it leaves its cyst and may live free in the cavities in the body of its host, and if it does not escape from the body it finally constructs a new cyst and then dies. If the host is eaten by a carnivorous animal the liberated pentastomata reach the nose either from the lips or pharynx and in a few months more acquire their complete development. They must reach the nose of the horse by their presence in the food or water. _Symptoms Caused by the Pentastoma._ No morbid symptoms have been traced to the young encysted condition of the parasite. Yet it would not be surprising if their presence in large numbers in the mesenteric glands and liver should give rise to troubles of assimilation, sanguification, biliary secretion and the like. Frerichs says they are more common in the human liver in Germany than echinococcus, but adds that they have no clinical importance.