Any of various unicellular protist organisms of the genus Euglena that live in fresh water, have a cylindrical or sausage-like shape, and move by means of a flagellum. Euglenas contain chloroplasts and can produce their own food by photosynthesis. They can also absorb nutrients directly into the cell from the environment. Euglenas have no rigid covering or cell wall, such as the cellulose cell walls of green algae or plants, over the membrane enclosing the plasma of their cells. They also have a reddish, light-sensitive eyespot which helps them navigate in relation to light sources. In warm weather, euglenas multiply rapidly and form scum on the surfaces of bodies of water.