Foraminifera are a group of protozoans characterised by a test of one to many chambers composed of secreted calcite or agglutinated grains. These tests show a great variety of form and chamber arrangement: unilocular through multilocular, planispiral, trochospiral (high and low spired) and coiling in several planes. There may be many different kind of surface ornament. Test sizes are generally ranging from 50 to 1 000µm. In life, foraminifers are either planktonic, living in the top of the water column (surface to 300m), or benthonic (bottom dwelling).

Most of the carbonate tests of the planktonic forms make a major contribution to calcareous deep-sea oozes. Remains of benthonic forms may be widespread but only form a minor constituent of deep-sea sediments. They are more common in terrigenous sediments on the continental shelf. Foraminifera are very good stratigraphic markers due to their abundance, high diversity and occurrence in all marine environments. Their first ancestors appeared during the Cambrian, the planktonic forms appearing in the Late Jurassic (nearly 150 MA ago).
Today the specific composition of their assemblages is used to reconstruct palaeoenvironmental changes in oceanic and continental shelf areas.
The picture features a planktonic form from the Indonesian platform (Globigerinidae, approx. 250µm).