Flagellate protozoa, coloured scanning electron micrograph (SEM). These are fresh water protozoa that have flagellae (thin tail-like structures). The protozoa wave their flagellae to move through the surrounding liquid. Protozoa are single-celled (unicellular) micro-organisms that are eukaryotes, meaning they have a cell nucleus. They generally have animal-like behaviour, such as being mobile. There are over 50, 000 protozoan species, of which about a fifth are parasitic, causing disease in humans and animals. Protozoa range in size from 5 to 500 micrometres (millionths of a metre). Magnification: x2500 when printed at 10 centimetres wide.