Rotifer, algae and protists. Polarised light micrograph of a Collotheca sp. rotifer (centre) with a Synura sp. algae colony (round, lower left) and a Phacus sp. flagellate protist (upper right). Rotifers are microscopic aquatic animals that are related to roundworms. Synurids are a small group of golden-brown algae, found mostly in freshwater. They are covered in silicate scales and spines and occur as spherical colonies (as here). Phacus sp. protists are common in freshwater pools. Magnification: x200 when printed 10 centimetres wide.