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Sponges are an assembly of cells that filter feed. An outer pinacoderm layer contains small pore cells (porocytes) that allow water to enter the "body" of the sponge. This water is drawn in by the concerted beating of numerous ciliated cells called collar cells, choanocytes.These choanocytes line the interior of the sponge and also filter food particles out of the water as it passes through. Water then moves into a central space called an osculum and exits the sponge. Different sponges create different chamber arrangements of increasing complexity. They are supported by spicules of calcium carbonate, silica or protein and are policed within the sponge tissues by amoeboid archeocytes which may act as a primitive immune system and which retain the ability to turn into any other cell that is needed by the sponge.