Amoeboid organisms, in common usage called amoebae, are protists that move and feed by means of cytoplasmic streaming or by means of pseudopodia, temporary cytoplasmic extensions. This unique method of locomotion is called amoeboid movement. Amoebae are also characterized by an irregular shape and the lack of a constant body form. The name ‘amoeba’ is derived from an ancient Greek word, which means ‘changing’. Cilia, if present, are usually restricted to developmental stages.
Amoeboid protists comprise a large and very diverse group with over 15,000 described species. Most are free-living, ubiquitous organisms that are widespread in nature and can be found in nearly any moist or aquatic habitat, aerobic or anaerobic. Some are endosymbiotic, including pathogenic forms and parasites. Writing about 'amoebae' is of no more scientific value than writing about 'flying animals' or 'four-legged animals', because there is a huge diversity of amoebae. Many protists move and feed by amoeboid behavior, but many of them aren't closely related.
Traditionally amoebae are divided into shelled (Testacea) and naked amoeboids (Gymnamoebae). This distinction is somewhat blurred by amoebae like Cochliopodiidae, which have a flexible coating of micro-scales. The largest number of amoeboid organisms can be found with the phyla Amoebozoa and Cercozoa. They have by far the greatest biomass of any protist taxon in soil and are also abundant in every marine habitat. They prey on bacteria and other protists.
Cercozoa is a diverse, increasingly well characterized phylum of protists founded mainly on the basis of 18S rDNA phylogenies that revealed a phylogenetic relationship between a variety of gliding zooflagellates and filose amoebae and the plasmodiophorid plant parasites. This diversity of cercozoan body forms includes:
• filose testate amoebae,
• amoeboflagellate algae,
• zooflagellates with pseudopodia and siliceous scales,
• diverse parasites,
• naked reticulose amoebae,
• testate reticulose amoebae,
• colonial flagellates,
• planktonic swimming flagellates,
• numerous small gliding flagellates,
• soft-bodied amoeboflagellates,
• thecate or scaly amoeboflagellates with a marked groove or cleft,
• and even protists with axopodia.