Euglena

Euglena is a genus of single-celled, flagellate eukaryotes and the best-known member of the class Euglenophyceae, comprising more than 200 described species. Most species carry photosynthesizing chloroplasts that let them behave like plants, but they can also feed on organic matter directly like animals, a dual nature that historically confounded classification and helped prompt Ernst Haeckel to establish the kingdom Protista. A Euglena cell has no rigid wall, only a flexible protein-and-microtubule pellicle, and it senses light through a red eyespot while swimming with a whip-like flagellum. Euglena lives in both fresh and salt water and can bloom densely enough in calm ponds to tint the surface green or red.