Foraminiferan, coloured scanning electron micrograph (SEM). Foraminifera are single-celled protists (protozoa, algae, and lower fungi) with shells. Their shells are also referred to as tests because in some forms the protoplasm covers the exterior of the shell. The shells are commonly divided into chambers which are added during growth, though the simplest forms are open tubes or hollow spheres. Depending on the species, the shell may be made of organic compounds, sand grains and other particles cemented together, or crystalline calcite. Magnification: x7000 when printed at 10 centimetres wide.