A genus of free-living, ovoid, or slightly pyriform protozoa with two flagella, one projecting anteriorly and the other posteriorly; may be ingested as encysted forms in food or drink, or possibly deposited in feces or urine after excretion; in either instance, cysts frequently develop into trophozoites if the specimen is permitted to remain at room temperature for a few hours before examination; the organisms are not pathogenic in humans.