The genus _Filaria_ is a very large one. Like _Ascaris_, it is confined to Vertebrates, but usually lives in the tissues of the body and not in the intestines. _F._ (_Dracunculus_) _medinensis_ Gmel., the guinea-worm, is well known as a human parasite in hot countries; it also occurs in the horse and dog. The female has an average length of 50 to 80 cm., but gigantic forms with a length of 4 metres have been described. The alimentary canal is degenerate. In adult females the body is completely occupied by a uterus crowded with eggs and embryos, which can only escape by the rupture of the mother's body, as the genital ducts have disappeared. Its original home is tropical Asia and Africa, but it has been introduced into South America with the negroes. The female lives coiled up in the subcutaneous tissues, usually in those of the legs. Its presence gives rise to painful tumours. When these break the female protrudes, and may be withdrawn from the body by very carefully rolling it round a stick or pencil. This must be done very slowly, a few inches a day, as the rupture of the body sets free the contained embryos, and may result in {148}the death of the host. The embryos normally bore their way into the body of the fresh-water _Cyclops_, and are re-introduced into their Vertebrate hosts with the drinking-water. It is usually stated that the female alone is known, and that it is uncertain whether it is hermaphrodite or whether both sexes are present in the _Cyclops_. Recently Dr. Charles[184] has described a specimen found in the mesentery of a human subject, from an orifice in the middle of whose body he was able to draw a much smaller specimen, and he thinks this may be the long-sought-for male. [Illustration: FIG. 73.—A, View of the heart of a dog infested with _Filaria immitis_[185] Leidy; the right ventricle and base of the pulmonary artery have been opened. _a_, Aorta; _b_, pulmonary artery; _c_, vena cava; _d_, right ventricle; _e_, appendix of left auricle; _f_, appendix of right auricle. B, A female _F. immitis_ removed from the heart to show its length. Natural size.] _Filaria immitis_ Leidy, the cruel worm, is common in dogs in China and the East generally. It is not unknown in America and Europe. It occurs in such large clusters in the right ventricle that it is difficult to see how the circulation can proceed. The intermediate host is unknown, but from the prevalence of the {149}disease in marshy country it is probably some aquatic animal. The larvae are said by Manson to disappear from the peripheral circulation of the dog during the day, but not to such a marked extent as do the _F. sanguinis hominis_ Lew., var. _nocturna_ Man. They were found by Galeb and Pourquier in the foetus of an infested bitch, a fact which establishes the transmission of such parasites through the placenta.