Calcareous sponge
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 "Calcispongiae" from Ernst Haeckel's Kunstformen der Natur, 1904
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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 most have three points, the spicule of a calcareous sponge may have two to four points.
All sponges in this class are strictly marine dwellers. All three sponge body plans are represented within class Calcarea : asconoid, syconoid, and leuconoid. 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.
Species of Calcarea vary from radially symmetrical vase-shaped body types to colonies made up of a reticulum of thin tubes or irregular massive forms. Calcareous sponges are generally sedentary filter feeders, lacking any real tissues. Calcareous sponges are most commonly found in shallow tropical waters. Generally, Calcarea are small sponges. The different shapes of calcareous sponges are relatively simple, either purse, vase, pear or cylinder-shaped. The skeleton has either a mesh or honeycomb structure. Unlike most other sponges, calcareous sponges lack hollow canals, which makes the sponge stronger.
Calcarean sponges first appeared during the Cambrian and their diversity was greatest during the Cretaceous Period. Recent molecular analysis suggests that the class Calcarea should be delegated as a phylum, in particular the first to have diverged in the Kingdom Animalia; the other sponges belong to the phylum Silicarea.
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