The group includes 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. Cercozoa are closely related to Foraminifera and Radiolaria, amoeboids that usually have complex shells, and together with them form a supergroup called the Rhizaria.
They are sometimes grouped by whether they are "filose" or "reticulose".
Filose (subphylum Filosa)
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. Other notable filose cercozoans include the cercomonads, which are common soil flagellates.
Reticulose (subphylum Endomyxa)
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.
Chlorarachniophytes are sometimes considered Filosa, rather than Endomyxa, while groups such as Gromia are considered Endomyxa. Filosa is apparently a monophyletic group, but Endomyxa is paraphyletic.
In addition, three groups that are traditionally considered heliozoans belong here: the Heliomonadida, Desmothoracida, and Gymnosphaerida, which were recently grouped into the new class of Granofilosea.
The exact composition and classification of the Cercozoa are still being worked out. A general scheme is:
|Class Proteomyxidea||Gymnophryida, Heliomonadida, Desmothoracida, Gymnosphaerida, etc.|
|Class Imbricatea / Silicofilosea||Euglyphida and Thaumatomonadida|
|Class Thecofilosea||Tectofilosida and Cryomonadida|
Some other small groups of protozoans are considered Cercozoa but are of uncertain placement, and it is likely many obscure genera will turn out to be cercozoans with further study.
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