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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