Foraminiferans are members of a phylum or class of protists characterized by streaming, granular ectoplasm that is used for catching food. Typically formainiferans generate a shell commonly made of calcite (CaCO3) or agglutinated sediment. Over 10,000 living species are recognized as well as about 40,000 fossil forms. They are usually less than 1 mm in diameter with the largest reaching up to 20 cm. The majority are marine and live on the sea floor within the bottom sediment while a smaller variety are floaters in the water column at various depths. A few are known from fresh or brackish water environments. Foraminifera are largely identified based on their shells (or tests). Fossil foraminifera have been found as far back in the geologic record as the Cambrian. Many marine sediments are composed primarily of foraminifera shells. For example, the limestone that makes up the pyramids of Egypt is composed almost entirely of benthic foraminifera shells. Foraminiferas have been found in the deepest part of the oceansuch as the Mariana Trenchincluding the Challenger Deep, the deepest part of the trench. At these depts., calcite is water soluble. As a result, the shells are composed of organic material. Dying planktonic Foraminifera rain down to the sea floor in vast numbers. Because of their diversity, abundance, and complex morphology, fossil foraminifera are useful for biostratigraphy and can accurately give relative dates to sedimentary rocks.