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Euglena A genus of unicellular, green, photosynthetic protists, sometimes regarded as algae (division Euglenophyta), sometimes as protozoa (class Phytomastigophora). The cell body is typically spindle-shaped but is capable of broadening and narrowing to some extent; it is motile with a single flagellum. Euglena is common in all fresh-water habitats, especially when these are slightly polluted with organic matter.