Foraminifera

Foraminifera are ocean dwelling amoeba-like protists that build shells known as tests. The shell is referred to as a test because some of the protoplasm of the unicellular organism covers the outside of the test. Tests have chambers that are added as the cell grows. Shells or tests can be made of sand and bits of discarded shells that adhere to an exuded sticky substance, a hard keratin-like material, or calcite (calcium carbonate).  Foraminifera can be planktonic or benthic. Many tropical beaches are composed of sands made primarily from the skeletons of benthic foraminifera. Much of the ocean floor that is less than 4.000 m deep is covered by calcareous ooze composed of microfossil shells made of calcite. Foraminifera that secrete tests of calcite are not typically found below this depth because their skeletons dissolve. The calcite compensation depth (CCD) is the depth at which calcite dissolves and is typically 4,000 to 5,000 meters.  Below this depth forms that construct their tests from cemented grains of sand or other foreign particles are more common.