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Phylum Mollusca Largest Classes Class Gastropoda - snails Class Bivalvia - clams, mussels, oysters, scallops. Class Cephalopoda - squid, nautilus, octopus. General Characteristics i. Second largest phylum of animals and second most successful land animals, next to insects. There are more terrestrial mollusks than terrestrial vertebrates. ii. Includes snails, clams, octopus. Blueringed Octopus Caribbean Octopus Squid Scallop Mollusks have 3 body regions: a head/mantle, a visceral mass, and a "foot." Nudibranch Mollusks have well developed body organs (nervous system, circulatory system, respiratory system, etc.) but lack body segmentation. iii. Body cavity (coelom) is completely surrounded by mesoderm. iv. Coelom allows space for more complex internal organs. * Digestive tract can be longer than the animal. * Allows larger reproductive system. Nutrition The longer digestive tract allows better digestion Circulation A large body cavity requires the development of an efficient circulatory system. A network of vessels carries fluid (blood) to all parts of the body. Blood carries nutrients and oxygen to cells and carries wastes and carbon dioxide away. Circulation is accomplished by the contraction of a muscular heart (or hearts).