These Protozoa live in the sea, floating near the surface or crawling on the bottom. Like Difflugia they form a hard shell, sometimes of sand, more often of calcium carbonate, which they secrete from the sea water. The shells are always perforated b'y minuto round apertures. The protoplasm lies partly within the shell and partly around it, and is continuous through the apertures.
The mature shell is generally composed of several chambers. At the commencement of its life, the young Protozoon forms for itself a single chamber resembling that of Difflugia, except that it is perforated ; when it becomes too large for its shell it forms another, which is fixed to the first. Many others are formed in succession as the animal grows. The several chambers are arranged in different ways among the different kinds in a straight line, in a spiral, or in other ways.
The Foramenifera are of importance as builders of the earth's surface. They are very numerous in the sea ; by their united power of secretion they convert vast quantities of dissolved calcium salts into solid calcium carbonate, which is added to the crust of the earth; for when a Foraraeniferan dies, its shell sinks to the bottom of the ocean and remains there. In much of its area, the mud of the ocean is almost entirely composed of shells of Foramemfera, and it is also known that some of the rocks which constitute the earth's surface are also largely composed of the minute shells of Foramenifera which lived in bygone ages (Chap. IX.). The Foramenifera differ from the Lobosa in their method of reproduction.
The Badiolaria are another large group of Protozoa which also are found in the sea ; they mostly secrete perforated shells of silica, which sink to the floor of the ocean and remain there.