flagellate - Form and behavior, Groups of flagellates

A microscopic, single-celled organism that possesses 1, 2, 4, or more thread-like organelles (flagella), typically used for swimming; many are parasites of animal hosts; others live in aquatic or even terrestrial habitats. (Phylum: Mastigophora.)

Flagellates are cells with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the sperm cells 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.

Form and behavior

Eukaryotic flagella are supported by microtubules in a characteristic arrangement, with nine fused pairs surrounding two central singlets.

In protists and microscopic animals, flagella are generally used for propulsion. In most eukaryotes, one or more flagella are located at or near the anterior of the cell. Among animals, fungi, and Choanozoa, which make up a group called the opisthokonts, there is a single posterior flagellum.

Groups of flagellates

Originally the flagellate protozoa were treated as a single class of phylum, the Mastigophora.

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.

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. The excavates generally show similarities in the structure of their flagella and typically have a cytostome.

Other notable groups including flagellates are the Choanozoa, Cercozoa, alveolates (including dinoflagellates), ebriids, and Apusozoa.