A class of the phylum Porifera, including exclusively marine sponges with a skeleton composed of spicules of calcium carbonate to which can be added a solid basal calcitic skeleton. Calcarea vary from radially symmetrical vase-shaped species, to aggregates made up of a reticulum of thin tubes, to irregular massive forms. Calcareous sponges are mostly small and inhabit the shallow waters of all seas, from tidal areas to depths of 660 ft (200 m), with a few species extending down to at least 13,200 ft (4000 m).