Foraminifera. Interphase contrast light micrograph of a selection of different foraminifera. 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: x28 when printed 10 centimetres wide.