Flagellates are cells with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Higher plants and fungi do not produce flagellate cells, but the closely related green algae and chytrids do. Many protists take the form of single-celled flagellates.
Flagellates are protozoans (animal-like protists). Eukaryotic flagella are supported by microtubules in a characteristic arrangement, with nine fused pairs surrounding two central singlets. These arise from a basal body or kinetosome, with microtubule roots that are an important part of the cell's brain. In some, for instance, they support a cytostome or mouth, where food is ingested. The flagella often support hairs, called mastigonemes, or contain rods. Their ultrastructure plays an important role in classifying eukaryotes.
In protists and microscopic animals, flagella are generally used for propulsion. They may also be used to create a current that brings in food. In most things, one or more flagella are located at or near the anterior of the cell eg Euglena. Often there is one directed forwards and one trailing behind. Among animals, fungi, and Choanozoa, which make up a group called the opisthokonts, there is a single posterior flagellum. They are from the phylum Mastigophora. They can cause diseases and they can make their own food. For example, Trypanosome which causes the African sleeping sickness.
This scheme has generally been abandoned or is retained only for convenience. However, the relationships among the flagellates are still mostly unknown, and their higher classification is confused. Some argue that the Linnaean ranks are not appropriate for such a diverse set of organisms.
Phytoflagellates are found in most groups of algae. Both the green algae and heterokonts include a variety of flagellates in addition to non-motile and multicellular forms. The dinoflagellates, cryptomonads, haptophytes, and euglenids are almost entirely single-celled flagellates.
Many of the other flagellates make up what are called the excavate taxa. These include the euglenids and a number of important parasites, such as trypanosomes and Giardia. The excavates generally show similarities in the structure of their flagella and typically have a cytostome. However, they may be a paraphyletic group, and in particular may have been ancestral to most or all other eukaryotes.