Sponges (spun-ge) do not have nervous, digestive, or circulatory systems.  Instead most reley on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes, and the shapes of their bodies are adapted to maximize the efficiency of the water flow.  Sponges are broken into three groups-calcareous sponges, glass sponges, and demosponges.  These sponges have a skeleton made of calcium carbonate.  This is a subtance that clams use to break shells.  It is very brittle and hard.