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a genus of microscopic motile single-celled organisms of the division Euglenophyta. The cell is spindle- or ribbon-shaped, with a pellicle in front, a single external flagellum, and a stigma next to the gullet. The ocellus is red. The chloroplasts are green and frequently complex in structure.
About 60 species of euglenas are known. They mainly inhabit shallow bodies of water, where they swim at any depth. Some species, such as Euglena viridis, can discard the flagellum and creep or, after taking on a spherical form, live immobile on the surface of the water, silt, or soil. Under unfavorable conditions they become transformed into spores. Nutrition is usually myxotrophic, or if there is a loss of chlorophyll, saprophytic. The organisms are distributed widely and sometimes cause the coloration of waters. Euglena sanguinea, which is rich in carotene, makes water red. Euglenas are used in laboratory experiments.