The flagellate stage (fig. 24) of the organism is polymorphic, and may be either (_a_) sausage-shaped, 10 µ to 25 µ in length by 2·5 µ to 6 µ in breadth; (_b_) round or oval, varying from 4 µ in diameter to oval forms 15 µ by 10 µ; (_c_) a carrot-shaped form, of varying size up to 25 µ by 4 µ. The kinetic nucleus is large and pear-shaped. Near it are basal granules, closely applied to one another, from which the flagella arise. There is a small cytostome near the roots of the flagella. There is a well-marked karyosome in the nucleus. The movement is jerky. The shorter, anterior flagellum may be used in food-capture. In life, bacteria have been seen to be ingested. Food-vacuoles tend to accumulate at the posterior (aflagellar) end. A contractile vacuole may be present, near the base of the cytostome, and may really be the dilated fundus of the latter. Division occurs by binary fission. The organism can encyst (fig. 25, _a_), when the flagella are lost, and round or oval cysts are found, 5 µ to 7 µ in diameter. After a time flagella are formed inside the cyst, and the organism emerges therefrom in its typical flagellate form (fig. 25, _b_-_f_). Sinton’s case is interesting. He obtained the flagellate only twice from the same patient, a Mexican then in hospital in Liverpool. The flagellate was not found in the patient’s fæces, nor was it found in the urine on later occasions when taken aseptically. [Illustration: FIG. 25.--_Prowazekia urinaria_. Flagellate emerging from cyst. (After Sinton.)] In cultures _Prowazekia urinaria_ was always found in association with bacteria. The cultures died at a temperature of 37° C., but grew well at 20° C. Various media were useful at the lower temperature, such as urine, salt agar, nutrient agar, serum agar, blood agar, peptone salt solution, and diluted blood serum. The flagellate was, then, considered to be an accidental contamination and not a true parasite of human urine. *Prowazekia asiatica*, Castellani and Chalmers, 1910. The flagellate was found by the discoverers in the stools of patients suffering from ankylostomiasis and diarrhœa in Ceylon. It was referred by them to the genus _Bodo_, but in 1911 Whitmore[49] further studied it and placed it in the genus _Prowazekia_. In the stools the flagellate is found either as a long, slender form measuring 10 µ to 16 µ by 5 µ to 8 µ or as a rounded form 8 µ to 10 µ in diameter. Its cytoplasm is alveolar. A rhizoplast connects the basal granules to the kinetic nucleus. There is multiplication and cyst formation as before. The organism is easily cultivated, especially in the condensation water of nutrose agar and maltose agar. The pathogenicity is stated to be nil. [49] _Arch. f. Protistenk._ xxii, p. 370. *Prowazekia javanensis*, Flu, 1912.