THREAD-WORMS. The thread or round worms are members of the class Nematelminthes, order Nematoidea, and are so called from their slender round thread-like body. The dense skin is not segmented, and the body-cavity (ecelome) is not lined with epithelium, but is directly bounded by the muscles of the body. There is a definite digestive canal. Two excretory canals open in front on the ventral side of the body, while the nervous system consists of a ring around the pharynx, from which two main nerve cords pass backward. The true thread-worms undergo no metamorphosis. They are mostly parasitic and usually bisexual. Some of them are free, living coiled up under stones between tide-marks; certain minute species occur in fresh water or damp earth or mud. A few live in plants, and Tyleneha triad damages wheat.
The more common parasitic forms are species of Asearis, Trichina, Oxyuris. etc. Of Ascaris, the human roundworm (Asearis lumbricaides) is remarkable for its great size, being 5-6 inches long, and about a tenth of an inch in diameter; it has three papillie around the mouth and is milk-white. The common pin-worm vermicularis) lives in the rectum of children; the palisade-worm (Eustrongylus gigas), one female of which was 39 inches in length and the thickness of a quill, the male being one-third as long, has been found living in man; allied species occur in the brain or brain-cavity of birds.
Among the most formidable human, parasites of this group are the Trichina (q.v.), the guinea worm (q.v.). and the species of sanguinis-hoiniwis, a microscopic th,read-wo•m found living in the blood of the mosquito in In dia and China, is thought to occasion the disease known as elephantiasis. The formidable disease called beriberi is supposed to be due to a nema tode worm, whose eggs and embryos swarm by millions in the soil and dirty puddles around the villages. In certain species of the family Anguil lulidar there is an alternation of generations (see PARTHENOGENESIS), from an hermaphroditic internal parasitic to a free dicecious generation. Thus Rhabditis (Rhabdonenta) nigro•enosa lives in mud, and gives rise to a second form living in the lungs of frogs.