Testate amoebae. Darkfield illuminated polarised light micrograph of two Arcella sp. testate (shelled) amoebae. Arcella species are found in freshwater ponds and puddles. They can develop internal vacuoles of carbon dioxide to allow them to float up to the water's surface. The circular shell is formed from the polysaccharide chitin. Magnification: x200 when printed at 10 centimetres wide.