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This is a scanning electron microscope image of a shelled amoeba or "Thecamoebian" from the Macoun Marsh in central Ottawa. These single-celled microorganisms live in freshwater lakes and ponds. They can secrete shells or make them from grains of sand and other foreign material. As this photo shows, some shelled amoeba form rectangular plates for protection. Scientists use these organisms as environmental indicators to monitor pollutants in lakes and ponds or to interpret their levels in ancient environments, since various species are affected by contaminants. (Magnification: about 2000x)