Amoeboids are cellss that move or feed by means of temporary projections, called pseudopods (false feet). They may also be called amoebae, but that term is frequently restricted to the amoeboid genus Amoeba and creatures very similar to it. Amoeboid cells 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 have such a form as part of their life-cycle. These are generally larger cells, some reaching several millimetres in size, and are common in all sorts of habitats.
Amoeboid protozoa may be organized into several broad categories based on what types of pseudopods they form. Those which produce axopods supported by microtubule bundles are called actinopods, and are traditionally divided into the radiolaria and heliozoa. Radiolaria are primarily marine plankton, and produce intricate mineral skeletons that are often found in the fossil record. They are distinguished by the presence of a central capsule that separates the inner portion of the cell, or endoplasm, from the frothy outer portion, or ectoplasm. Heliozoa only produce simple scales and spines, if they have any skeletal elements at all, and are found in both freshwater and marine environments.
Amoeboids that lack axopods are called rhizopods, and are traditionally divided into those with lobose, filose, and reticulose pseudopods. Lobose pseudopods are blunt, and there may be one or several on a cell, which usually has a ditinct layer of clear ectoplasm surrounding granular endoplasm. Most such amoebae move by flowing into an anterior pseudopod, but some crawl using relatively permanent pseudopods as limbs, and others roll, the ectoplasm sliding around the outside of the cell like a tank tread. Most, including Amoeba itself, appear to form a monophyletic group called the Amoebozoa, which also includes most slime molds, and may or may not include the entamoebae and pelobionts. The Heterolobosea also have lobose forms.
Filose pseudopods arise from the same sort of underlying pressure system as lobose ones, but are tapering and frequently extended. Most such amoeboids are now placed among the Cercozoa, including the chlorarachniophytes, which have acquired chloroplasts. The vampyrellids and nucleariids belong elsewhere, however, and the latter have been shown to be close relatives of animals and fungi.
Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found almost entirely among the Foraminifera, marine benthos that produce multichambered shells. Together with the radiolaria, these are the dominant protozoa in the fossil record. Another important part of the benthos are the xenophyophores, strange rhizopods that do not fit into any of the above categories, and may reach up to 20 cm in diameter.
Traditionally the amoeboid protozoa are grouped together as the Sarcodina, ranked variously from class to phylum. Each of the above categories would then be treated as a formal taxon. However, since these categories are all based on form rather than phylogeny, newer systems often separate out some amoeboid groups or abandon the Sarcodina altogether. The higher classification of amoeboids, like most protists, remains unstable.
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