EUGLENA

genus of single-celled organisms with both plant and animal characteristics. It is considered a member of the protozoan order Euglenida or a member of the algal division Euglenophyta. The genus is characterized by an elongated cell (15 to 500 micrometres, or 0.0006 to 0.02 inch) with one nucleus, pigment-containing chloroplasts (some species are colourless), a contractile vacuole, an eyespot (stigma), and flagella. Certain species (e.g., E. rubra) appear red in sunlight because they contain a large amount of pigment. Some species, which lack a rigid cellulose wall, have a flexible pellicle (envelope) that allows changes in shape. Food, absorbed directly through the cell surface or produced by photosynthesis, is stored as a complex carbohydrate (paramylum). Reproduction is asexual, by longitudinal cell division; sexual reproduction is unknown. Euglena live in fresh and brackish water rich in organic matter. Some species develop tremendous populations as green or red blooms in ponds or lakes. Several species produce resting cysts that can withstand drying. The colourless species are used to study cell growth and metabolism at high temperatures, in the dark, in ultraviolet light, or with certain chemicals.