Instead of a mouths they have tiny pores ( ostia) in their outer walls through which water is drawn. Sponges have a unique feeding system among animals. They rely on keeping up a constant water flow through their bodies to obtain food and oxygen and to remove wastes. Sponges do not have a nervous, digestive or circulatory system. Spongia officinalis, the bath sponge, have no spicules at all. Some sponges also secrete exoskeletons that lie completely outside their organic components whilst others, e.g. Spicules vary in shape from simple rods to three-dimensional "stars" with up to six rays.
Many also have a skeleton made up of spicules of calcium carbonate or silica. The body of a sponge consists of jelly-like material ( mesohyl) made mainly of collagen and reinforced by a dense network of fibres also made of collagen.sandwiched between two thin layers of cells.
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