Testate amoeba and desmid. Polarised light micrograph of Arcella sp., a testate (shelled) amoeba (top), and the filamentous desmid Desmidium swartzii (bottom). Arcella species are found in freshwater ponds and puddles. The circular shell is formed from the polysaccharide chitin. Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Magnification: x400 when printed at 10 centimetres wide.