Euglenas are unusual in that they are both heterotrophic, meaning they must consume food, and autotrophic, meaning they can make their own food. These unicellular organisms usually live in calm ponds or even puddles.
Euglenas create their own food through photosynthesis, the process of absorbing sunlight to synthesize foods from carbon dioxide and water. An eyespot at the front end of the euglena detects light, and its chloroplasts (structures that contain chlorophyll) trap the sunlight, allowing photosynthesis to occur.
But sunlight is not always present, so euglenas cannot remain autotrophic continuously. When light is not available, the organisms become heterotrophic, taking in foods from outside by absorbing nutrients across their cell membranes.
The euglena has a long, whip-like structure called a flagellum on its front end that twirls like a miniature motor on a boat to propel it through water. Also outside the cell membrane, a stiff pellicle (thin skin or film) helps the euglena retain its oblong shape. The pellicle is flexible, however, allowing the euglena to scrunch up and then elongate, so as to move like an inch-worm.
Euglenas are typically green in appearance due to the presence of chloroplasts, but some species appear red in color because they contain a large amount of carotenoids, the same pigment that gives reddish color to ripe tomatoes and autumn leaves. When large populations of colorful euglenas congregate on brackish ponds or lakes, the effect creates red or green "blooms" across the water.