Many species of protozoa become inanimate, enclosing themselves in protective shell_like coverings, often sphere_shaped, called cysts, when conditions are not favorable. Some protozoa may stay encysted for months, even years, and still remain viable, returning to 磍ife?when conditions improve. This cyst, about 100 microns in diameter, is that of Spirostomum, the same species you see swimming about.