Desmids and shelled amoeba. Darkfield illuminated polarised light micrograph of Micrasterias apiculata (round, spiky) and Hyalotheca sp. (long, thin) desmids with an Arcella sp. testate (shelled) amoeba (smooth, round, bottom). Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Arcella amoebas are found in freshwater ponds and puddles. Their circular shell is formed from the polysaccharide chitin. Magnification: x100 when printed 10 centimetres wide.