What is "calcareous sponge"

n. (context biology English) Many sponges, of the class ''Calcarea'', having skeletons composed of spicules of calcium carbonate

The calcareous sponges of class Calcarea are members of the animal phylum Porifera, the cellular sponges. They are characterized by spicules made out of calcium carbonate in the form of calcite or aragonite. While the spicules in most species have three points, in some species they have either two or four points.