Notes: Three strains of Hypotrichomonas acosta were isolated in axenic culture. Attempts to develop a defined medium directly from a defined medium suitable for Tetrahymena pyriformis were unsuccessful. Development of partly defined media by substitution for undefined materials in Diamond's medium were more successful. Horse serum was replaced by 1 mg. % TEM-4T (a diacetyl tartaric acid ester of monoglycerides from tallow) and 0.5 mg.% cholesterol. Yeast extract was replaced by a mixture of ribonucleotides. Inclusion of several additional components permitted reducing the Trypticase concentration from 2% to 0.25%.

Notes: Hypotrichomonas gen nov., with a feeble undulating membrane but lacking a costa, is placed in the family Monocercomonadidae Kirby of the order Trichomonadida Kirby. H. acosta (Moskowitz) from squamate reptiles is designated as the type species. It has 3 unequal anterior flagella. The proximal part of its recurrent flagellum is attached to the surface of the body along an accessory filament. A low membrane with typically a few undulations extends between the accessory filament and the body surface. The membrane, which varies greatly in length in relation to the length of the body among strains of the species, is not supported by a costa. The recurrent flagellum continues as a free posterior flagellum beyond the end of the undulating membrane and terminates in an acroneme. The capitulum of the axostyle, spatulate or scoop-shaped, is closely applied to the anteriorly located nucleus. The capitulum extends anteriorly into a crescent-shaped pelta. The relatively stout trunk of the axostyle is straight or twisted. It projects for some distance from the posterior body surface, and the terminal segment of the projecting part tapers gradually to a point. The parabasal apparatus consists of a rod- or shaped body and of a filament or filaments.In division, the full complement of flagella in each of the mastigonts is restored relatively late during division but before cytokinesis. One of the daughter individuals retains the parental undulating membrane; the other develops a new organelle. The development of the new membrane is first marked by the appearance of an acroneme, presumably recurrent flagellum, early in division. The proximal part of this flagellum becomes progressively attached to the body surface. The process of attachment is paralleled by differentiation of the accessory filament and is followed by the appearance of the undulating membrane. The parental axostyle is resorbed during division, and each daughter individual develops this organelle de wove. In the very early stages of division the parabasal consisted of two arms and in all more advanced stages of a single rod-shaped body applied to a single filament.The significance of Hypotrichonionas in the evolution of trichomonads is discussed. Its morphology and morphogenesis suggest that this genus may represent an important step in the evolution of Trichomonadidae from Monocercomonadidae.