Amoeboids are single-celled life-forms characterized by an irregular shape.
"Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to the specific genus Amoeba. The genus Amoeba and amoeboids in general both derive their names from the ancient Greek word for change.
Amoeboids move using pseudopodia, which are bulges of cytoplasm.
Amoebas breathe using their entire cell membrane that is constantly immersed in water. Excess water can cross into the cytosol. Amoebas have a contractile vacuole to expel excess water.
Food sources vary in rhizopoda. They may consume bacteria or other protists. Some are detritivores and eat dead organic material. They extend a pair of pseudopodia around food. They fuse to make a food vacuole which then fuses with a lysosome to add digestive chemicals. Undigested food is expelled at the cell membrane.
Amoebas use pseudopodia to move and feed. They are powered by flexible microfilaments near the membrane. Microfilaments are at least 50% of the cytoskeleton. The other parts are more stiff and are composed of intermediate filaments and macrotubules. These are not used in amoeboid movement, but are stiff skeletons on which organelles are supported or can move on.
The shells of amoebas are often composed of calcium. The proteins or materials are synthesised in the cell and exported just outside the cell membrane.
Amoebas seem to have connections with two phyla of the lineage fungus-like protists. The two phyla are myxomycota (plasmodial slime molds), and acrasiomycota (cellular slime molds). These two phyla use amoeboid movement in their feeding stage. One is basically a giant multinucleate amoeba, while the other lives solitary until food runs out; in which a colony of these functions as a unit. Myxomycotes use amoeboid gametes, as well.
They have appeared in a number of different groups. Some cells in multicellular animals may be amoeboid, for instance human white blood cells, which consume pathogens. Many protists also 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 amoeba is variously used to describe its close relatives, other organisms similar to it, or the amoeboids in general.
As amoebas themselves are polyphyletic and subject to some imprecision in definition, the term "amoeboid" does not provide identification of an organism, and is better understood as description of locomotion.
When used in the broader sense, the term can include many different groups. One source includes 97 different genera. Others include far fewer.
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