The genus Trichomonas is a common parasite in the digestive system of many animals. Trichomonas cells are pear-shaped and may have four flagella anteriorly and a fifth bordering the undulating membrane. A mouth and a basal rod (costa) are found along the membrane; an axostyle, a stiff rod of cytoplasm used for support, often protrudes posteriorly. Three species occur in humans: T. hominis in the intestine, T. vaginalis in the vagina, and T. buccalis in the mouth. Tritrichomonas foetus is a pathogenic form in cattle. See trichomoniasis.