Parasitic excavate (Giardia lamblia)

Green algae (Chlamydomonas)

Flagellates are cells with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the sperm 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. They are found in most lines of eukaryotes, and it is likely that all surviving eukaryotes evolved from them.

Form and behavior

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 structure. 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 eukaryotes, one or more flagella are located at or near the anterior of the cell. 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.

Groups of flagellates

Originally the flagellate protozoa were treated as a single class of phylum, the Mastigophora. This was divided into the Phytomastigina or phytoflagellates, which have chloroplasts or are closely related to such forms, and the Zoomastigina or zooflagellates, which do not. Most phytoflagellates were given a separate classification by botanists, treating them in several divisions of algae.

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.

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