{145}_Trichocephalus dispar_ Rud. (_hominis_ Gmel.) is common in man, and also occurs in some species of monkey. It does not live freely within the intestine, but buries its long whip-like anterior end in the mucous lining of the caecum or colon. The eggs pass out of the body of the host. The development of the embryo is slow, lasting many months; whilst still in the egg-shell the embryos are swallowed, and give rise to the sexually-mature parasite without the intervention of an intermediate host. They are by no means uncommon. Davaine calculated that about 50 per cent of the inhabitants of Paris were infested with them, but they give rise to little disturbance, and only very occasionally cause serious harm. _T. affinis_ Rud. infests sheep; _T. crenatus_ Rud. the pig; _T. depressiusculus_ Rud. the dog; and _T. unguiculatus_ Rud. the hare and rabbit. [Illustration: FIG. 71.—_Trichocephalus dispar_ Rud., attached to part of the human colon. × 2.] The genus _Trichosoma_, with many species, is as a rule found in birds, but it occurs also in mammals, as _T. plica_ Rud. in the bladder of the fox and wolf, _T. felis cati_ in the bladder of the cat, _T. aerophilum_ Duj. in the trachea of the fox and marten. The chief interest of this genus is that, at any rate in _T. crassicauda_ Bel., which infests the rat, the dwarf males live two, three, or four at a time within the uterus of the female, a condition of things which recalls the similar arrangement found in the Gephyrean _Bonellia_. _Trichina spiralis_ is the cause of the well-known disease trichinosis, which appears in two forms, intestinal and muscular, according to the habitat of the parasite. The mature forms of {146}both sexes are found in the intestine of man and many other mammals. They have been experimentally developed in birds, though in the latter the larval forms have never been observed. By keeping such cold-blooded animals as the salamander at a constant temperature, Goujon and Legros succeeded in infecting them, but the larvae perished as soon as the artificial heat was withdrawn. Muscular trichinosis is unknown in fishes, but the sexual form develops in their intestine.