Euglena

Euglenas (singular, Euglena) are common Euglenophytes protist, typical of the euglenids, and commonly found in nutrient-rich freshwater, with a few marine species. It is considered an Euglenophyte, and not a flagellate, because of the presence of chloroplasts in most species. The cells vary in length from around 20 to 300 μm, and are typically cylindrical, oval, or spindle-shaped with a single emergent flagellum for movement. There are usually many bright green chloroplasts, although some species are colorless. If sunlight is not available, it can absorb nutrients from decayed organic material. Euglena is also found in sewage systems. Euglena reproduces through mitosis, a process in which one cell is divided into two. Euglena has a contractile vacuole which prevents the cell from bursting.

The name comes from the Greek words  (eu) and  (glēnē), meaning good eyeball, referring to Euglena's stigma (also known as eyespot). Until the discovery of the paraflagellar body it was wrongly believed that this was the organelle responsible for the sensing of light. Instead it is just a small part of Euglena's light receptive system and its role is to shield light from the actual light sensitive paraflagellar body (PFB) or so-called photoreceptor. Orientation of the organism towards light is called Phototaxis.

Over 1000 species of Euglena have been described. Marin et al. (2003) revised the genus so that it forms a monophyletic group, moving several species with rigid pellicles to the genus Lepocinclis and including several species without chloroplasts, formerly classified as Astasia and Khawkinea.