Euglena gracilis is a species of single-celled Eukaryote algae in the genus Euglena. They have a secondary chloroplast and can feed by photosynthesis, heterotrophy or phagocytosis. They have a highly flexible cell surface, allowing them to change shape from thin cells up to 100 µm long to spheres of approximately 20 µm. They have two flagella, only one of which emerges from the posterior flagella pocket and can move by swimming, or by so-called "euglenoid" movement across surfaces. Euglena gracilis has been used extensively in the laboratory as a model organism, particularly for studying cell biology and biochemistry.