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XL], Manson, 1891. (Plate X., Fig. 2.)

Synonym.—Acanthocheilonema perstans, Railliet, Henry, and Langeron, 1912.

Geographical range.—This parasite is very common in the blood of the natives of large districts in tropical Africa. I have found it in natives from Old Calabar and from the basin of the Congo—both in the coast negroes and in those from the interior. Annett, Button, and Elliot report its presence in many places in Southern Nigeria, in Northern Nigeria, in Lagos, in the Gold and Ivory Coasts, in the Kroo Coast, and in Sierra Leone. In Northern Rhodesia it is very common; I found it in 30 per cent. of a number of the natives whom I examined. On the Congo in parts it occurs in half the population. Prof. Firket, of Liége, has confirmed this observation as regards the Congo district. Cook, Hodges, and Low have found the parasite to be extremely common in Uganda, where, in some districts, Low has found it in 90 per cent. of the population. Sometimes it occurs along with mf. loa, mf. bancrofti, and, in British Guiana, as stated, with mf. ozzardi, in the same individual. I have never found it in West Indian negroes, nor, in fact, in natives of any country except those of tropical Africa, and in the aborigines of Demerara. I have frequently found it in Europeans who had resided on the Congo.

Its characters.—Microfilaria perstans observes no periodicity, being present in the blood both by day and by night. In this respect it resembles mf. demarquaii and mf. ozzardi; and, like these, in size, shape, and anatomical detail it differs very materially from mf. bancrofti and mf. loa. The number of microfilariæ in the peripheral circulation may vary considerably, but there is no marked difference between day and night. Their special seat of selection is not the peripheral blood, but that of the heart, lungs, aorta and other large vessels. They have not been found in the spleen, and only rarely in the liver and pancreas.

The larva in the blood measures, on an average, 0·2 mm.