Trichomonas vaginalis was discovered by Donne in 1837, and for it he established the genus Trichomonas. It is, therefore, the type species of the genus. The organism is a flagellate protozoon. Measurements vary according to the age of the organism, from 18 x 6 microns to 26 x 16 microns. usually there are four anterior flagella and an axostyle protruding from the posterior end. An undulating membrane attached to the anterior end reaches back approximately one-half the langth of the organism. Recent authorities who have carefully studied the trichomonads parasitic in man, believe that T. buccalis and T. hominis are identical with this species.