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 individual amoeboid cells, or take such a form at some point in their life-cycle. The most famous such organism is Amoeba proteus; the name amoebae is variously used to describe its close relatives, other organisms similar to it, or the amoeboids in general.
Amoeboids may be divided into several morphological categories based on the form and structure of the pseudopods. Those where the pseudopods are supported by regular arrays of microtubules are called actinopods, and forms where they are not are called rhizopods, further divided into lobse, filose, and reticulose amoebae. There is also a strange group of giant marine amoeboids, the xenophyophores, that do not fall into any of these categories.
- Lobose pseudopods are blunt, and there may be one or several on a cell, which is usually divided into a layer of clear ectoplasm surrounding more granular endoplasm. Most, including Amoeba itself, move by the body mass flowing into an anterior pseudopod. The vast majority form a monophyletic group called the Amoebozoa, which also includes most slime moulds. A second group, the Percolozoa, includes protists that can transform between amoeboid and flagellate forms.
- Filose pseudopods are narrow and tapering. The vast majority of filose amoebae, including all those that produce shells, are placed within the Cercozoa together with various flagellates that tend to have amoeboid forms. The naked filose amoebae comprise two other groups, the vampyrellids and nucleariids. The latter appear to be close relatives of animals and fungi.
- Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Foraminifera, a large group of marine protists that generally produce multi-chambered shells. There are only a few sorts of naked reticulose amoeboids, and their relationships are not certain, though it appears most belong among the Cercozoa.
- Actinopods are divided into the radiolaria and heliozoa. The radiolaria are mostly marine protists with complex internal skeletons, including central capsules that divide the cells into granular endoplasm and frothy ectoplasm that keeps them buoyant. The heliozoa include both freshwater and marine forms that use their axopods to capture small prey, and only have simple scales or spines for skeletal elements. Both groups appear to be polyphyletic.
Traditionally the amoeboid protozoa are grouped together as the Sarcodina, variously ranked from class to phylum, with each of the above categories as a formal subtaxon. However, since they are all based on form rather than phylogeny, newer systems generally separate some out or abandon them entirely. Most amoeboids are now included in two major supergroups - the Amoebozoa, including most lobose amoebae and slime moulds, and the Rhizaria, including the Cercozoa, Foraminifera, radiolarian classes and certain heliozoa. However, amoeboids have appeared separately in many other groups, including various different lines of algae not listed above.