Some types of amoeba have an external shell covering the cell. The Foraminifera secrete a shell of protein coated with calcium carbonate; their shells are covered with pores through which their long, filamentous pseudopodia project. Some foraminiferans are zooplankton, microscopic organisms living at the surface of the sea, while others are bottom-dwellers. It is the discarded shells of countless long-dead creatures such as these that make up deposits of limestone many hundreds of metres in depth. Many thousands of shells are needed to form just 1 g of such a deposit! The White Cliffs of Dover are an example of a limestone deposit made up largely of foraminiferan shells.
The outer surface of radiolarians is composed of silica, which again is perforated to allow the passage of many very fine pseudopodia.
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