Yellow tube sponges have the shape of a long, hollow tube and can be as long as 3 feet. In shallow water they are bright yellow, while deeper-water tube sponges are darker in color. When a yellow tube sponge is exposed to air, it begins to turn purple and then black. A single sponge is actually made up of several tubes that are connected at the base, where they attach themselves to the ocean floor.
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Tube sponges breath oxygen and get that oxygen from the seawater. The seawater flows down the tube and through the mesohyl. The mesohyl is the inner tissue of the sponge. The mesohyl functions as an endoskeleton. It is the only skeleton in soft sponges that encrust hard surfaces. The choanocyte cells within the mesohyl tissue absorbs the oxygen that is present in the water.