The Filaria Perstans is a form that is found in the blood at all times, day or night. The embryo alone is known. The Filaria diurna occurs in the daytime only.

Filariasis is common in Egypt, India and Africa, and the intermediate host is a mosquito, one of many species belonging to several genera.

Fig. 121. - Embryo of Filaria Bancrofti. Microfilaria Bancrofti. Living specimen (Bulletin No. 1, Medical Department, U. S. Army).