Shelled amoeba. Darkfield illuminated micrograph of an Arcella sp. testate (shelled) amoeba (round) on sphagnum sp. moss. The circular shell is formed form the polysaccharide chitin. This amoeba is found in freshwater ponds and puddles. It can develop vacuoles of carbon dioxide in its body to float up to the water's surface. Magnification: x200 when printed at 10 centimetres wide.