_Filaria sanguinis hominis nocturna._—The female of this parasite has been described as living in the lymphatic glands of man. The embryos escape from it into the lymph, and thus reach the blood. According to Manson the intermediate host is the mosquito, in whose stomach the embryos undergo their larval changes. When the mosquito dies the larvae escape into the water, and then make their way into the alimentary canal of man, where they are believed to pair, and whence the female makes its way to the lymphatics. The presence of this _Filaria_ causes great functional disturbance. One of the most remarkable features of it is that the larvae, which are very numerous in the blood during the night, disappear during the day, and are not to be found. Recently Manson[186] has described two new varieties: _F. san. hom. diurna_, in which the conditions of things are reversed, the larvae being found by day and not by night; and _F. san. hom. perstans_, in which the larvae occur both by day and by night. The larvae are long-lived, and were found by Manson in the blood of a negro who had not been in Africa, where it is endemic, for six years. The same observer is inclined to associate the presence of _F. san. hom. perstans_ with the fatal disease known as "sleeping sickness." He also suggests that the mature form of the variety _diurna_ is the _F. loa_, which is not uncommon in the eyes of negroes, and that its intermediate host may be one of the blood-sucking flies so common on the west coast of Africa. The genus _Ichthyonema_ is confined to fishes. The male is very minute and the female partly degenerate. It has no anus and no external opening to its generative organs. The uterus fills up almost the whole of the body-cavity. _I. sanguineum_ Rud. is found encapsuled in the peritoneum of many fish. _Hystrichis_ and _Dispharagus_ are confined to birds, where they occur in the oesophagus and stomach. _Spiroptera reticulata_ {150}Crep. occurs in horses, twisted in a spiral round tendons and muscles, forming tumours which require to be opened. V. FAMILY MERMITHIDAE. Nematodes without anus and with six mouth papillae. Two spicules in the males and three rows of numerous papillae. Genera: _Mermis_, _Bradynema_, _Atractonema_, _Allantonema_, _Sphaerularia_, and others. As a rule the Nematoda show but little trace of their parasitic mode of life, but in this family there is considerable degeneration, and in extreme cases the body of the female is reduced to a simple sac crowded with eggs. They are exclusively parasitic in insects. In some respects their structure shows a transition towards _Nectonema_ and the Gordiidae; especially is this the case in the structure of their ventral nerve-cord.