Euglena gracilis is a tiny, single-celled organism with a long, oval body, tail-like flagella and chloroplasts for producing food energy from sunlight. Although they photosynthesize like plants, they are able to swim using their flagella and eat other organisms. They are classified as protists rather than as animals or plants.
Euglenia gracilis is just one of many similar creatures in the Euglenia genus. Most of them are aquatic species; Euglenia gracilis lives in fresh water. It can also survive in marine environments and in moist soil. Another notable feature of Euglena gracilis is that it possesses a pigmented eyespot. It uses the eyespot, which is red, to find sunlight to grow. When Euglenia gracilis is placed under dark conditions, it reduces its number of chloroplasts. It is capable of increasing its number of chloroplasts again when conditions change.