The study of the species hitherto known has been carried on partly by the above mentioned authors and in part by others, _e.g._, Rabinowitsch and Kempner, Laveran and Mesnil, Wasiliewski, Senn. It was greatly advanced by the method of double staining (with alkaline methylene blue and eosin) introduced by Romanowsky (1891) and elaborated by Ziemann, Leishman, Giemsa and others. By this means the presence of a terminal flagellum and of an undulating membrane at the side of the flattened and extended body was demonstrated. Laveran and Mesnil (1901) discovered allied flagellates in the blood of the fish, _Scardinius erythrophthalmus_. These flagellates, now placed in the genus _Trypanoplasma_, had a second free flagellum in addition to the one bordering the undulating membrane. Trypanoplasms have since been found in both freshwater and marine fishes. The transmission of trypanoplasms of freshwater fishes is effected by leeches. _Trypanoplasma varium_ from _Cobitis_ is transmitted by _Hemiclepsis marginata_ according to Léger, while the Trypanoplasmata of _Cyprinus carpio_ and _Abramis brama_ reach new hosts by the agency of _Piscicola_ according to Keysselitz. Another ally of the Trypanosomidæ, _Trypanophis_, lives in the cœlenteric cavity of Siphonophores. It has also an extra terminal flagellum (Poche, Keysselitz). [_Trypanoplasma_ and _Trypanophis_ belong to the _Bodonidæ_, see p. 63]. Finally it was shown that Trypanosomes occurred in human beings. Although Nepveu’s early report of trypanosomes in the blood of malarial patients may be doubtful, subsequent researches by Forde and Dutton demonstrated trypanosomes (fig. 28) in the blood of a European, apparently suffering from malaria, living in the Gambia. Dutton (1902) called the human trypanosome, _T. gambiense_. The expedition despatched by the Liverpool School of Tropical Medicine (1902) to Senegambia found trypanosome infections in six cases among a thousand inhabitants examined.