Foraminiferan tests (shells), macrophotograph. Species seen here include Bulimina gibba, Elphidium crispum, E. articulatum, Lagena sulcata, and Miliolinella circularis. Foraminifera are marine single-celled marine protozoa that construct and inhabit a calcium carbonate shell 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. Also seen are copepod tests and fragments of sea coal (black). Magnification: x36 at 10 centimetres wide.