The amoeboids comprise those Protista that form pseudopods, temporary cytoplasmic projections that are involved in locomotion and ingesting food. Originally they were grouped together as the Sarcodina, but the different sorts of amoeboid forms have each evolved a number of times. These come in four main groups: those with lobose, filose, and reticulose pseudopods, which are unsupported, and those with axopods, which are rigid projections around microtubules.
Lobose pseudopods are blunt and filose pseudopods are tapering, occasionally branching, but both arise from the same sort of underlying pressure system. These are the kinds of pseudopods familiar to us from Amoeba and its kin. The main groups of such protists are:
- Ramicristates - most lobose and filose forms, including most slime mold groups
- Pelobionts - giant amoebae & kin
- Entamoebae - mostly parasitic forms
- Heterolobosea - amoeboflagellates, including acrasid slime molds
Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Granuloreticulosa, which includes primarily the Foraminifera - marine amoeboids with multichambered shells. Other forms include the Gymnophrea, Komokiacea, and Biomyxa.
Protists with axopods are traditionally divided into the Heliozoa and the Radiolaria. The former are usually sphere-shaped forms covered in axopods that catch food, and in motile forms, let them roll along the substrate. These are typically freshwater, and include the following:
The latter are primarily marine with intricate mineral skeletons, and include three separate groups: the Acantharea, the Polycystinea, and the Phaeodarea. They are distinguished from the Heliozoa by the presence of a central capsule separating the inner cytoplasm (endoplasm) from the outer (ectoplasm). Along with the Foraminifera, these are the primary protozoa found in the fossil record.
There is also a strange group of giant marine protists, the Xenophyophorea, that are clearly amoeboid but do not fit nicely into any of these categories. Amoeboid cells also occur in a few multicellular organisms, e.g. white blood cells have a lobose amoeboid form.