<P> Euglena is a genus of single - celled flagellate eukaryotes . It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 800 species . Species of Euglena are found in fresh and salt waters . They are often abundant in quiet inland waters where they may bloom in numbers sufficient to color the surface of ponds and ditches green (E. viridis) or red (E. sanguinea). </P> <P> The species Euglena gracilis has been used extensively in the laboratory as a model organism . </P> <P> Most species of Euglena have photosynthesizing chloroplasts within the body of the cell, which enable them to feed by autotrophy, like plants . However, they can also take nourishment heterotrophically, like animals . Since Euglena have features of both animals and plants, early taxonomists, working within the Linnaean three - kingdom system of biological classification, found them difficult to classify . It was the question of where to put such "unclassifiable" creatures that prompted Ernst Haeckel to add a third living kingdom (a fourth kingdom in toto) to the Animale, Vegetabile (and Lapideum meaning Mineral) of Linnaeus: the Kingdom Protista . </P> <P> When feeding as a heterotroph, Euglena takes in nutrients by osmotrophy, and can survive without light on a diet of organic matter, such as beef extract, peptone, acetate, ethanol or carbohydrates . When there is sufficient sunlight for it to feed by phototrophy, it uses chloroplasts containing the pigments chlorophyll a and chlorophyll b to produce sugars by photosynthesis . Euglena's chloroplasts are surrounded by three membranes, while those of plants and the green algae (among which earlier taxonomists often placed Euglena) have only two membranes . This fact has been taken as morphological evidence that Euglena's chloroplasts evolved from a eukaryotic green alga . Thus, the intriguing similarities between Euglena and the plants would have arisen not because of kinship but because of a secondary endosymbiosis . Molecular phylogenetic analysis has lent support to this hypothesis, and it is now generally accepted . </P>

How is euglena considered a plant or an animal
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