FILARIASIS AND MOSQUITOES--A number of species of Nematode worms belonging to the genus _Filaria_, infest man and other vertebrates and in the larval condition are to be found in the blood. Such infestation is known as _filariasis_. The sexually mature worms are to be found in the blood, the lymphatics, the mesentery and subcutaneous connective tissue. In the cases best studied it has been found that the larval forms are taken up by mosquitoes and undergo a transformation before they can attain maturity in man. The larvæ circulating in the blood are conveniently designated as microfilariæ. In this stage they are harmless and only one species, _Filaria bancrofti_, appears to be of any great pathological significance at any stage. _Filaria bancrofti_ in its adult state, lives in the lymphatics of man. Though often causing no injury it has been clearly established that they and their eggs may cause various disorders due to stoppage of the lymphatic trunks (fig. 119). Manson lists among other effects, abscess, varicose groin glands, lymph scrotum, chyluria, and elephantiasis. The geographical distribution of this parasite is usually given as coextensive with that of elephantiasis, but it is by no means certain that it is the only cause of this disease and so actual findings of the parasites are necessary. Manson reports that it is "an indigenous parasite in almost every country throughout the tropical and subtropical world, as far north as Spain in Europe and Charlestown in the United States, and as far south as Brisbane in Australia." In some sections, fully 50 per cent of the natives are infested. Labredo (1910) found 17.82 per cent infestation in Havana. [Illustration: 119. Elephantiasis in Man. From "New Sydenham Society's Atlas."] The larval forms of _Filaria bancrofti_ were first discovered in 1863, by Demarquay, in a case of chylous dropsy. They were subsequently noted under similar conditions, by several workers, and by Wücherer in the urine of twenty-eight cases of tropical chyluria, but in 1872 Lewis found that the blood of man was the normal habitat, and gave them the name _Filaria sanguinis hominis_. The adult worm was found in 1876 by Bancroft, and in 1877, Cobbold gave it the name _Filaria bancrofti_. It has since been found repeatedly in various parts of the lymphatic system, and its life-history has been the subject of detailed studies by Manson (1884), Bancroft (1899), Low (1900), Grassi and Noé (1900), Noé (1901) and Fülleborn (1910). The larvæ as they exist in the circulating blood, exhibit a very active wriggling movement, without material progression. They may exist in enormous numbers, as many as five or six hundred swarming in a single drop of blood. This is the more surprising when we consider that they measure about 300µ × 8µ, that is, their width is equal to the diameter of the red blood corpuscle of their host and their length over thirty-seven times as great.