Light micrograph of a shell (test) of a Baculogypsina spaerulat foraminifera, or sand star. Foraminifera are marine single-celled protozoa that construct and inhabit shells composed of several chambers. These are usually penetrated by pores through which the cellular content is extruded. The shells form an important component of chalk. In past geological ages, foraminifera were so numerous that their shells, largely composed of calcium carbonate, have formed immense fossil deposits, seen today as limestone. Magnification: 10x when printed at 10 centimetres across.