Most animal-like protists are heterotrophs that move about capturing and consuming prey. Animal-like protists are sometimes called protozoa. Protozoa are single-celled protists that can move independently without cilia or flagella. Biologists group more than 40,000 species of protozoa in the phylum Protozoa, which includes the sub-phyla Sarcodina (SAHR-kuh-DIE-nuh) and Mycetozoa (mie-SEET-uh-zoH-uh). Of these species, about three-quarters are identified only by fossil remains. Figure 25-4 shows two examples of protozoans.
A key characteristic of most protozoa is the formation of pseudopodia (SOO-doh-POH-dee-uh). Pseudopodia are large, rounded cytoplasmic extensions that function both in movement and feeding. A pseudopodium forms when the cytoplasm flows forward to create a blunt, armlike extension. Simultaneously, other pseudopo-dia retract, and the cytoplasm flows in the direction of the new pseudopodium, causing the cell to move. This type of locomotion is called amoeboid movement. Amoeboid movement is a form of cytoplasmic streaming, the internal flowing of a cell's cytoplasm.
Protozoa actively prey on smaller cells, such as bacteria and smaller protists, and food particles. A sarcodine feeds by surrounding the food with pseudopodia and trapping the food in a vesicle. The sarcodine releases enzymes to digest the food trapped inside the vesicle. Figure 25-4a shows a sarcodine, an amoeba, using pseudopodia to capture food.
Sarcodines include hundreds of species that inhabit freshwater environments, marine environments, and soil. The cell membranes of some sarcodines, such as amoebas, are exposed directly to the environment. Other sarcodines are covered with a protective test, or shell. For example, foraminifera (fuh-RAM-uh-NIF-uhr-uh) are found primarily in oceans and are covered with intricate tests of calcium carbonate. The shells of ancient foraminifera accumulated at the ocean bottom where, after millions of years, they became limestone. The tests of a group of mycetozoans, the radiolarians (RAY-dee-oh-LER-ee-uhnz), contain silicon dioxide. Radiolarians often have radially arranged spines. Both foraminiferans and radiolari-ans have slender pseudopodia that extend through tiny openings in the test.
Although most amoebas live freely, some species live in human intestines and may cause disease. One such amoeba, Entamoeba histolytica, enters the body through contaminated food and water. It releases enzymes that attack the lining of the large intestine and cause ulcers. A sometimes fatal disease called amebiasis may result, causing acute abdominal pain.
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