Shelled amoeba. Darkfield illuminated polarised light micrograph of an Arcella sp. testate (shelled) amoeba (top, round), showing Oedogonium sp. algae (orange) that it has ingested. The circular shell is formed from 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: x400, when printed at 10 centimetres wide.