Polarized light micrograph of a Stentor sp. freshwater ciliate protozoan with algal symbionts (green), called zoochlorella. The zoochlorella photosynthesize inside the protozoan, providing it with sugars and oxygen, while deriving protection from their host. Some Stentor species can grow to several millimeters in length, making them among the largest single-celled organisms. Magnification: x330 when printed 10 centimeters wide.