The Phylum Mollusca is made up of over 150,000 diverse species appearing very different but sharing certain basic characteristics.
All mollusks have a muscular foot used for locomotion, as well as a mantle, an outgrowth that covers the animal. Many mollusks have an external calcium carbonate shell, hanging over the mantle, that is actually produced by the mantle. The mantle is also used in respiration, waste disposal, and sensory reception. It often creates a mantle cavity, where the gill is housed. 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 composed mostly of chitin, in the mouth. The radula allows the animal to scrape food from surfaces, especially the ocean floor, by sliding back and forth.
Mollusks have a coelom, which is develops from solid cell masses. This makes all species in this phylum protosomes. All organs are suspended in mesentary tissue within this mesodermic coelom, between the outer covering (ectoderm) and the digestive tube (endoderm) of the animal. Interestingly, the phylum Mollusca is one of only two major coelomate phyla that do not have body segmentation (the other being Echinodermata, as well as other lophophorate phyla). There are seven main classes of mollusks.