Calcareous sponges have a skeleton formed of spicules of calcium carbonate laid down as calcite. These spicules may be free in the sponge tissue, or may be in part fused together by secondary deposits of calcite and in part free, or may occur as free elements together with a reticulate or massive skeleton of crystalline calcite. The spicules are monactinal, diactinal, triactinal and tetractinal and do not in general occur in categories marked by distinct size differences as is true of the megascleres and microscleres of demosponges and hexactinellids.