The term trypanosomiasis is applied to-day to a group of diseases affecting vertebrates and caused by parasitic Protozoa belonging to the family Trypanosomidae. These parasites occur chiefly in the blood plasma and the typical form possesses an elongated body, an undulating membrane, a single nucleus, a blepharoplast, and a chromatic filament running along its length from near the blepharoplast, along the margin of the undulating membrane, to terminate freely at one end of the body. The latter has a somewhat spiral form, the protoplasm being alveolar and at times showing a granular structure. An axial filament has been described as occurring in some forms. One species, Trypanosoma equiperdum, occurs largely in the lymph, and another, T. gambiense, may invade the cerebrospinal fluid. They are actively motile organisms, multiplying usually by longitudinal or multiple division. Numerous species of trypanosomes have been successfully cultivated in vitro in the presence of haemoglobin, and by this means many trypanosomes have been discovered in animals in which these parasites occur in such scanty numbers that their presence cannot be detected microscopically. Thus, a large percentage of birds and cattle in different parts of the world have been found to harbour trypanosomes in their blood.