Acanthocystis. Coloured scanning electron micrograph (SEM) of a centrohelid heliozoan with tangential plate-scales and radial spines. This genus Acanthocystis contains unicellular, free-floating planktonic or benthonic living organisms which are common in freshwater but also found more rarely in marine habitats. The evolutionary position of the centrohelids is not clear. Magnification: x 2500, when printed 10 centimetres wide.