The genus was founded for a flagellate parasite, _Prowazekia cruzi_, discovered in a culture of human fæces in Brazil. Various other species have been referred thereto. The genus is separated from _Bodo_ by the possession of a second nucleus, the so-called kinetonucleus or blepharoplast. It differs from _Trypanoplasma_ in the absence of an undulating membrane. It is heteromastigote, that is, it possesses two dissimilar flagella, one anteriorly directed and the other lateral and trailing. The principal species are: *Prowazekia urinaria*, Hassall, 1859. Syn.: _Bodo urinarius_, Hassall, 1859; _Trichomonas irregularis_, Salisbury, 1868; _Cystomonas urinaria_, Blanchard, 1885; _Plagiomonas urinaria_, Braun, 1895. Hassall[47] in 1859 first found Bodo-like flagellates in human urine. He examined fifty samples of urine from patients suffering from albuminuria and from cholera. The reaction of the urine was alkaline or sometimes only feebly acid. The flagellates were only seen after the urine had been standing for several days. Hassall named the organism _Bodo urinarius_, and gave a very good description of it with illustrations. The flagellate, which was round or oval, measured 14 µ by 8 µ. The organism had “one, usually two, and sometimes three lashes or cilia.” In 1868 Salisbury described a similar flagellate in the urine under the name _Trichomonas irregularis_. Künstler in 1883 described the latter parasite under the name _B. urinarius_. In 1885 Blanchard, considering Künstler’s organism a different parasite from Hassall’s, called it _Cystomonas urinaria_. Braun, in 1895, gave the name _Plagiomonas urinaria_. Barrois (1894) considered Künstler’s and Hassall’s organisms to be identical and not to be true parasites of man. Sinton,[48] in 1912, found the flagellate in the deposit, after centrifuging, of a 24-hour old specimen of alkaline urine from a Mexican sailor in the Royal Southern Hospital, Liverpool. Sinton found a kinetic nucleus or blepharoplast in the organism, and therefore placed it in the genus _Prowazekia_. [47] _Lancet_, 1859, ii, p. 503. [48] _Annals Trop. Med. and Parasitology_, vi, p. 245. [Illustration: FIG. 24.--Types of _Prowazekia urinaria_. (_a_) sausage-shaped; (_b_) round; (_c_) carrot-shaped form. (After Sinton.)]