Euglena viridis is the type species for the genus Euglena, a group of tiny one-celled organisms that swims using flagella and creates energy using chloroplasts. Euglena viridis is spindle shaped and lives in water. It has between one and 15 chloroplasts and a flagellum as long as its body.
Flagella are long, whip-like structures that move quickly and violently behind cells to produce movement. Human sperm cells, for example, have flagella. The characteristics of Euglena viridis are considered definitive for describing a species as a euglena. The combined eating and photosynthesizing habits of Euglena viridis led to this classification. They remained classified as animals or plants until biologists eventually agreed that they belonged to the kingdom Protista.