Cavalier-Smith 1998 The Cercozoa are a group of protists, including most amoebae 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.

The best-known Cercozoa are the euglyphids , filose amoebae that produce shells, which are common in soils, in nutrient-rich waters, and on aquatic plants. The shells are composed of siliceous scales or plates in a more or less regular arrangement, formed within the cell and then assembled on its surface. A few other filose amoebae produce shells, such as Gromia, and were once grouped with the euglyphids as the Testacealobosea. This does not appear to be a monophyletic group, but all studied members fall within the Cercozoa.

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.

Flagellates placed here include the cercomonads, spongomonads, thaumatomonads, and cryothecomonads. Further, the parasitic plasmodiophorids and haplosporids are either cercozoans or close relatives. Two groups with actinopods, the desmothoracids and Phaeodarea, also belong here. Their exact composition and relationships are still being worked out. They are placed closely to the Foraminifera and Radiolaria, amoeboids which usually have complex shells, together forming a group called the Rhizaria.