As we noted above (p. 40), the study of the Flagellates has been largely in the hands of botanists. After the work of Bütschli in Bronn's _Thier-Reich_, Klebs[126] took up their study; and the principal monographs during the last decade have appeared in Engler and Prantl's _Pflanzenfamilien_, where Senn[127] treats the Flagellates generally, Wille[128] the Volvocaceae, and Schütt the "Peridiniales" or Dinoflagellata;[129] while only the Cystoflagellata, with but two genera, have been left to the undisputed sway of the zoologists.[130] Among this group the majority are saprophytes, found in water containing putrefying matter or bacteria. The forms so carefully studied by Dallinger and Drysdale belong to the genera _Bodo_, _Cercomonas_, _Tetramitus_, _Monas_, and _Dallingeria_. Many others are parasites in the blood or internal cavities of higher animals, some apparently harmless, such as _Trichomonas vaginalis_, parasitic in man, others of singular malignity. _Costia necatrix_, infesting the epithelial scales of fresh-water fish, often devastates hatcheries. The genus _Trypanosoma_, Gruby, contributes a number of parasites, giving rise to deadly disease in man and beast.[131] _T. lewisii_ is common in Rodents, but is relatively harmless. _T. evansii_ is the cause of the Surra disease of Ruminants in India, and is apparently communicated by the bites of "large brown flies" (almost certainly Breeze Flies or Tabanidae, Vol. VI. p. 481). _T. brucei_, transferred to cattle by the Tsetse Fly, _Glossina morsitans_ (see Vol. VI. Fig. 244, p. 513) in Equatorial Africa, is the cause of the deadly Nagana disease, which renders whole tracts of country impassable to ox or horse. Other Trypanosomic diseases of animals are, in Algeria and the Punjab, "dourine," infecting horses and dogs; in South America, Mal de Caderas (falling-sickness), an epidemic paralysis of cattle. During the printing of this book, much additional knowledge has been gained on this genus and the diseases it engenders. The Trypanosomic {120}fever recently recognised on the West Coast has been found to be the early stage of the sleeping-sickness, that well-known and most deadly epidemic of Tropical Africa. Through the researches of Castellani, Nabarro, and especially Colonel and Mrs. Bruce, we know now that the parasite _T. gambiense_ is transferred by an intermediate host, a kind of Tsetse Fly (_Glossina palpalis_). Schaudinn's full study of a parasite of the blood corpuscles of the Owl has shown that while in its intracorpuscular state it resembles closely the malarial parasites in behaviour, and in its schizogenic multiplication, so that it was considered an Acystosporidian, under the name of _Halteridium_, it is really a _Trypanosoma_;[132] for the accomplishment of successful sexual reproduction it requires transference to the gut of a gnat (_Culex_). The germs may infect the ovary, and give the offspring of the insect the innate power of infecting Owls.