*Prowazekia cruzi*, Hartmann and Chagas, 1910.
Found in a culture from human fæces on an agar plate in Brazil, and considered to be a free-living form.[51] The organism is oval or pear-shaped, 8 µ to 12 µ long and 5 µ to 6 µ broad. In human stools at Tsingtau, China, a _Prowazekia_ has been found by Martini which he thinks is the same as _Prowazekia cruzi_. He considers it to be a cause of human diarrhœa and intestinal catarrh.
*Prowazekia weinbergi*, Mathis and Léger, 1910.
This species was found in the fæces of men, both healthy and diarrhœic, in Tonkin.[52] It is pear-shaped, 8 µ to 15 µ long by 4 µ to 6·5 µ broad. The flagella occur at the broad end.
[52] _Bull. Soc. Med. Chir. Indo-Chine_, i, p. 471.
The discoverers think that _Prowazekia weinbergi_ is an intestinal inhabitant, but non-pathogenic, since it was found to occur in the fæces even when obtained with aseptic precautions.
A free-living form found in the slime on the stones at the biological station at Lunz. Another _Prowazekia_ was found in 1914 in tap-water in Calcutta.
The Trypanosomidæ, broadly considered, are uniflagellate organisms, the flagellum being at the anterior end. The flagellum arises near the blepharoplast (kinetic nucleus), which lies anterior, near or posterior to the nucleus.
The following genera will be considered:--
_Trypanosoma_--with an undulating membrane along the length of the body.
_Crithidia_--with a less well-developed undulating membrane anteriorly (see fig. 49).
_Herpetomonas_--including the so-called _Leptomonas_, with anterior free flagellum only, and no undulating membrane.
_Leishmania_--non-flagellate forms in mammalian blood, flagellate herpetomonad stages in culture, probably occurring naturally in Arthropods.
The members of the genus possess a single flagellum, which arises posteriorly, adjacent to a blepharoplast or kinetic nucleus. The flagellum forms a margin to an undulating membrane, and may or may not be continued beyond the body as a free flagellum. Many species are parasitic in vertebrate blood and in the digestive tracts of insects.
The history of blood flagellates goes back to the year 1841, in which Valentin discovered in the blood of a brook-trout (_Salmo fario_ L.) minute bodies, from 7 µ to 13 µ in length, with active movements and presenting marked changes in form. Valentin considered the parasite a new species of the old genus _Proteus_ or _Amœba_, Ehrbg. This announcement led Gluge (1842) to publish a similar discovery he had made in frog’s blood. The latter forms were called by Mayer (1843) _Amœba rotatoria_, _Paramœcium loricatum_ and _P. costatum_, while Gruby (1843) called them _Trypanosoma sanguinis_.[53] Later it was discovered that similar organisms occurred also in the blood of birds (Wedl (1850), Danilewsky) and of mammals. Gros (1845) found them in the mouse and mole, Chaussat (1850) in the house rat, Lewis (1879) in the Indian rat, Wittich (1881) in the hamster. Danilewsky (1886–89) and Chalachnikow (1888) investigated the structure and division of trypanosomes.