Testate amoeba and desmid. Polarised light micrograph of an Arcella sp. testate (shelled) amoeba (round, top) with a Netrium sp. desmid (green). This protozoan is found in freshwater ponds and puddles. It feeds on diatoms, unicellular green algae and other protozoa. The circular shell is formed from the polysaccharide chitin. Magnification: x400 when printed at 10 centimetres wide.