Columbia Encyclopedia World Factbook American Heritage Coll. Amebas were previously classified as members of the animal kingdom. A. proteus averages 0.25 mm in length. Although simple in form, amebas are very successful organisms and are found abundantly in a variety of habitats all over the world. Amebas live in freshwater, the oceans, and in the upper layers of the soil, and many have adapted to a parasitic life on the body surface of aquatic animals or in the internal organs of both aquatic and terrestrial animals.
Few animals escape invasion by some type of ameba.
In a process known as phagocytosis, amebas engulf their prey, or particles of appropriate size, with their pseudopodia, forming food vacuoles. Digestive enzymes, manufactured and secreted by the organism, are then poured into these vacuoles, and the particles are digested. Useful compounds are subsequently absorbed into the ameba's body. Freshwater amebas take up water constantly through the process of osmosis , and water content is regulated with a pulsating contractile vacuole. Marine amebas lack a contractile vacuole.