The chitons are in the phylum Mollusca. All mollusks have a muscular foot used for moving and a mantle, an outgrowth that covers the animal. Many mollusks have an external calcium carbonate shell that is produced by the mantle. The gill of a mollusk extracts oxygen from the water and disposes of waste. All species of the phylum Mollusca have a complete digestive tract, spanning from the mouth to the anus. Many also have a radula, a unique organ, mostly composed of chitin, in the mouth that allows the animal to scrape food from surfaces, especially the ocean floor, by sliding back and forth. Mollusks have a coelom, but the coelom is made from cell masses, making all species in this phylum protosomes. All organs are suspended in this coelom, between the outer covering and the digestive tube of the animal. However, animals in this phylum are unique to coelomates in that they lack body segmentation. There are seven classes in this phylum.

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