is a well-known genus of flagellate
protozoa, typical of the euglenids
, and commonly found in nutrient-rich freshwater, with a few marine species. The cells vary in length fom around 20 to 300 μm, and are typically cylindrical, oval, or spindle-shaped, with many bright green chloroplasts
and a single emergent flagellum
was first named by Ehrenberg
in 1830, and since then over 100 species have been described. The name comes from the Greek words eu
, or good eye
, referring to the light-sensitive eyespot.
Genetic studies suggest that the genus Euglena is paraphyletic to various other uniflagellate euglenids. Most species fall into two major lineages: those which exhibit metaboly, including colourless species normally treated as Astasia and Khawkinea, and those with more rigid pellicles, including some normally included in Lepocinclis. As such, it is likely that the genus will be redefined in future classifications.