Cercozoa

Cavalier-Smith 1998

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The Cercozoa are a group of protists, including most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in many other protozoa. They show a variety of forms and have proven difficult to define in terms of structural characteristics, although their unity is strongly supported by genetic studies.

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The best-known Cercozoa are the euglyphids, filose amoebae with shells of siliceous scales or plates, which are commonly found in soils, nutrient-rich waters, and on aquatic plants. Some other filose amoebae produce organic shells, including the tectofilosids and Gromia. They were formerly classified with the euglyphids as the Testaceafilosia. This group is not monophyletic, but nearly all studied members fall in or near the Cercozoa, related to similarly shelled flagellates.

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Another important group placed here are the chlorarachniophytes, strange amoebae that form a reticulating net. They are set apart by the presence of chloroplasts, which apparently developed from an ingested green alga. They are bound by four membranes and still possess a vestigial nucleus, called a nucleomorph. As such, they have been of great interest to researchers studying the endosymbiotic origins of organelles.

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Other notable cercozoans include the cercomonads, which are common soil flagellates. Two groups traditionally considered heliozoa, the dimorphids and desmothoracids, belong here. Recently the marine Phaeodarea have also been included. The Cercozoa are closely related to the Foraminifera and Radiolaria, amoeboids that usually have complex shells, and together with them form a supergroup called the Rhizaria. Their exact composition and relationships are still being worked out. The main groups of core Cercozoa are the following:

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• Class Chlorarachnea (chlorarachniophytes)
• Class Spongomonadea
• Class Proteomyxidea - desmothoracids, dimorphids, gymnophryids, etc.
• Class Sarcomonadea - cercomonads
• Class Imbricatea - euglyphids and thaumatomonads
• Class Thecofilosea - tectofilosids and cryomonads
• Class Phaeodarea

Some other smaller groups are of uncertain placement. In addition two groups of parasites, the Phytomyxea and Ascetosporea, and the shelled amoeba Gromia may also be classified as Cercozoa, although some trees place them closer to the Foraminifera.

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Cercozoa: References ►



 

Amoeboid: Amoeboids are cells that move or feed by means of temporary projections, called pseudopods (false feet). They have appeared in a number of different groups. Some cells in multicellular animals may be amoeboid, for instance our white blood cells, which consume pathogens. Many protists exist as ind...

Flagellate: PROTISTA KINGDOM...

Cytostome: A cytostome or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuoles. Only certain groups of protozoa, such as the ciliates and excavates, have cytostomes. In othe...