Early observations 's
Animalcula Infusoria. 1786 Species of
Euglena were among the first protists to be seen under the microscope. In 1674, in a letter to the Royal Society, the Dutch pioneer of microscopy
Antonie van Leeuwenhoek wrote that he had collected water samples from an inland lake, in which he found "animalcules" that were "green in the middle, and before and behind white." Clifford Dobell regards it as "almost certain" that these were
Euglena viridis, whose "peculiar arrangement of chromatophores...gives the flagellate this appearance at low magnification." Twenty-two years later,
John Harris published a brief series of "Microscopical Observations" reporting that he had examined "a small Drop of the Green Surface of some Puddle-Water" and found it to be "altogether composed of Animals of several Shapes and Magnitudes." Among them, were "oval creatures whose middle part was of a Grass Green, but each end Clear and Transparent," which "would contract and dilate themselves, tumble over and over many times together, and then shoot away like Fish." In 1786,
O.F. Müller gave a more complete description of the organism, which he named
Cercaria viridis, noting its distinctive color and changeable body shape. Müller also provided a series of illustrations, accurately depicting the undulating, contractile movements (
metaboly) of the cell body. In 1830,
C. G. Ehrenberg renamed Müller's
Cercaria Euglena viridis, and placed it, in keeping with the short-lived system of classification he invented, among the Polygastrica in the family Astasiaea: multi-stomached creatures with no alimentary canal, variable body shape but no pseudopods or lorica. By making use of the newly invented achromatic microscope, Ehrenberg was able to see
Euglena's eyespot, which he correctly identified as a "rudimentary eye" (although he reasoned, wrongly, that this meant the creature also had a nervous system). This feature was incorporated into Ehrenberg's name for the new genus, constructed from the Greek roots "eu-" (well, good) and glēnē (eyeball, socket of joint). 's
Histoire Naturelle des Zoophytes, 1841 Ehrenberg did not notice
Euglenas flagella, however. The first to publish a record of this feature was
Félix Dujardin, who added "filament flagelliforme" to the descriptive criteria of the genus in 1841. Subsequently, the class Flagellata (Cohn, 1853) was created for creatures, like
Euglena, possessing one or more flagella. While "Flagellata" has fallen from use as a taxon, the notion of using flagella as a phylogenetic criterion remains vigorous.
Recent classification In 1881,
Georg Klebs made a primary taxonomic distinction between green and colorless flagellate organisms, separating photosynthetic from heterotrophic euglenoids. The latter (largely colorless, shape-changing uniflagellates) were divided among the Astasiaceae and the
Peranemaceae, while flexible green euglenoids were generally assigned to the genus
Euglena. As early as 1935, it was recognized that this was an artificial grouping, however convenient. In 1948, Pringsheim affirmed that the distinction between green and colorless flagellates had little taxonomic justification, although he acknowledged its practical appeal. He proposed something of a compromise, placing colorless,
saprotrophic euglenoids in the genus
Astasia, while allowing some colorless euglenoids to share a genus with their photosynthesizing cousins, provided they had structural features that proved common ancestry. Among the green euglenoids themselves, Pringsheim recognized the close kinship of some species of
Phacus and
Lepocinclis with some species of
Euglena. In the 1970s, it was hypothesized that photosynthetic euglenoids derived their chloroplasts by engulfing an algal cell and took its photosynthetic machinery. This
secondary endosymbiosis hypothesis was later confirmed through molecular evidence, and it appears that the photosynthetic euglenoids are grouped into one clade. However, genetic analysis of the non-photosynthesizing euglenoid
Astasia longa confirmed that this organism retains sequences of DNA inherited from an ancestor that must have had functioning chloroplasts; therefore, some once-photosynthetic lineages must have later lost the chloroplasts. Recognizing the non-monophyletic nature of the genus
Euglena, Marin et al. (2003) have revised it to include certain members traditionally placed in
Astasia and
Khawkinea. To revise this, taxonomists have transferred species out of
Euglena and into other genera, including
Lepocinclis, and the newly proposed genera
Discoplastis,
Euglenaria, and
Euglenaformis. ==Form and function==