CALCAREOUS SPONGE

any of a class (Calcispongiae, or Calcarea) of sponges characterized by skeletons composed of calcium carbonate spicules (needlelike structures). Calcareous sponges occur mainly on the rocky bottoms of the continental shelves in temperate, shallow waters; they are usually dull in colour. Most are small, seldom exceeding 15 cm (6 inches).