order of one-celled amoebalike marine organisms of the phylum (or superclass) Sarcodina in the kingdom PROTISTA, (q.v.). Foraminifera, also called foraminifers or forams, extrude shells, called tests, that may be either wholly organic, mixed with sand grains, or composed of a thin organic inner layer and a thick calcareous outer layer. Although many tests are solid, the most common type is calcareous and porous. Ranging from simple tubes to many-chambered spirals, the shells may reach a diameter of 8 cm (3 in) but average only 0.05 cm (0.02 in).

Foraminifera move about with slender pseudopodia, or extensions of cytoplasm, the living matter of the cell, which stream through an opening in the test known as the aperture; in porous tests, the pseudopodia also emerge through the pores. The cytoplasm contains dark, fine granules of mitochondria, which show a characteristic streaming as the creature moves. Larger granules found in some species are algae. Reproduction is sexual or asexual; all the cytoplasm is used in forming the young, and the parent dies in the process of reproduction.

More than 30,000 species of foraminifera, both living and extinct, have been cataloged. The living species occur on the bottom of shallow seas or float as plankton in the upper levels of the oceans. Their food consists mainly of bacteria and diatoms. When the planktonic species die, their tests sink to the bottom, forming a thick deposit known as the Globigerina ooze, which is named after the abundant family Globigerinidae. In past ages, chalk rocks were formed by the compression of similar foraminiferal oozes, and the pyramids of Egypt were built of foraminiferal limestone capped with granite. Geologists study deposits of foraminifera shells for clues to the location of petroleum.