Thread-worms, a popular name for Nematoda, a class of more or less thread-like worms, many of which are parasitic, while others are free-living. The body is unsegmented; there is a well-developed cuticle; there is a complete alimentary canal surrounded anteriorly by a nerve-ring from which six nerves run forward and backward. The sexes are separate, and the life-history is often intricate. Several species are often parasitic in man—Asearis lumbricoides, in the small intestine; Oxyuris vermicularis, in the cæcum and large intestine; Dockmius duodenalis, in the small intestine; Filaria sanguinea hominis, in the blood; Draecunculus medinensis, the Guinea-worm; Trichina spiralis; Trichocephalus dispar, in the cæcum and large intestine. Not a few are parasitic on plants—e.g. several species of Tylenchus, which infest wheat and other crops. Some occur in domesticated animals—e.g. Strongylus armatris, the palisade worm, which causes aneurism in the horse, or Filaria immitis, in the heart of the dog, or Ollulanus, which passes from mouse to cat. Others live freely in water and putrefying substances—e.g. Euoplius and many other genera.
See ASCARIS, GUINEA-WORM, PARASITISM, TRICHINA; Leuckart, Parasites of Man (1876; Eng. trans. 1886); Schneider, Monographie der Nematoden (1876).