|"Calcispongiae" from Ernst Haeckel's Kunstformen der Natur, 1904|
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.
Typically, the Calcarea are very small, measuring about 7-10 cm (3-4 inches) in height. Of the 15,000 or so species of Porifera that exist, only 400 of those are Calcareans.