Euglenoids

Euglenoids, also Euglenoid flagellates, common name for microscopic, unicellular organisms such as the genus Euglena, common in freshwater habitats but sometimes found also in marine environments. Some euglenoids are colonial, some are parasitic. They constitute the order Euglenida in the class Phytomastigophorea, subphylum Mastigophora, kingdom Protista. Traditionally, euglenoids have been treated as algae, or simple plants, because they frequently are photosynthetic. Zoologists, however, have often considered them simple animals because they can swim actively and because some feed like animals. Some photosynthetic species can also obtain nutrition by alternate means.

A typical euglenoid has a pair of flagella, or whiplike appendages used in swimming, at the front end. It also executes a kind of crawling movement by changing the shape of its body. An eyespot enables it to move toward or away from light. Photosynthetic euglenoids contain several organelles, called chloroplasts, that give them a greenish color. Some euglenoids feed by absorbing dissolved substances, and many can ingest larger materials such as other euglenoids. The animals reproduce asexually by fission, or dividing in two, and little evidence of sexual reproduction exists. Therefore the 800 named “species” of euglenoids probably are not equivalent to the interbreeding populations that represent species in sexual organisms. See Species and Speciation. See also Flagellates.