All sponges are sessile aquatic animals, meaning that they attach to an underwater surface and remain fixed in place (i.e., do not travel). Many sponges have internal skeletons of spongin and/or spicules (skeletal-like fragments) of calcium carbonate or silicon dioxide. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where the water deposits nutrients and then leaves through a hole called the osculum. Some of them are radially symmetrical, but most are asymmetrical. Unlike other animals, they lack true tissues and organs. Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. Included are the yellow tube sponge, Aplysina fistularis, the purple vase sponge, Niphates digitalis, the red encrusting sponge, Spirastrella coccinea, and the gray rope sponge, Callyspongia sp. Sponge biodiversity and morphotypes at the lip of a wall site in 60 feet (20 m) of water.
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