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The Rhizaria are a diverse and species-rich clade of mostly unicellular . Except for the chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthetic, but many Foraminifera and Radiolaria have a symbiotic relationship with unicellular algae. A multicellular form, Guttulinopsis vulgaris, a cellular slime mold, has been described. This group was used by Cavalier-Smith in 2002, although the term "Rhizaria" had been long used for clades within the currently recognized taxon.
Being described mainly from Ribosomal DNA sequences, they vary considerably in form, having no clear morphological distinctive characters (synapomorphies), but for the most part they are with , , or microtubule-supported . In the absence of an apomorphy, the group is ill-defined, and its composition has been very fluid. Some Rhizaria possess mineral exoskeletons (thecae or loricas), which are in different clades within Rhizaria made out of opal (), celestite (), or calcite ().
Certain species can attain sizes of more than a centimeter with some species being able to form cylindrical colonies approximately 1 cm in diameter and greater than 1 m in length. They feed by capturing and engulfing prey with the extensions of their pseudopodia; forms that are symbiotic with unicellular algae contribute significantly to the total primary production of the ocean.
A few other groups may be included in the Cercozoa, but some trees appear closer to the Foraminifera. These are the Phytomyxea and Ascetosporea, parasites of plants and animals, respectively, and the peculiar amoeba Gromia. The different groups of Rhizaria are considered close relatives based mainly on genetic similarities, and have been regarded as an extension of the Cercozoa. The name Rhizaria for the expanded group was introduced by Cavalier-Smith in 2002, who also included the and Apusozoa.
A noteworthy order that belongs to Ascetosporea is the Mikrocytida. These are parasites of . This includes the causative agent of Denman Island Disease, Mikrocytos mackini a small (2−3 μm diameter) amitochondriate protistan.
Historically, many rhizarians were considered because of their motility and heterotrophy. However, when a simple animal-plant dichotomy was superseded by a recognition of additional kingdoms, taxonomists generally placed amoebae in the kingdom Protista. When scientists began examining the evolutionary relationships among eukaryotes in the 1970s, it became clear that the kingdom Protista was paraphyletic. Rhizaria appear to share a common ancestor with Stramenopiles and Alveolates forming part of the SAR super assemblage. Rhizaria has been supported by molecular phylogenetic studies as a monophyletic group. Biosynthesis of 24-isopropyl cholestane precursors in various rhizaria suggests a relevant ecological role already during the Ediacaran.
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