Snails, slugs, oysters, and clams are protected by a hard shell made of calcium carbonate secreted by the mantle, a heavy fold of tissue that surrounds the mollusc's internal organs. Spiders, insects, lobsters, and shrimp, have bodies covered by an external skeleton, the exoskeleton, which is strong, impermeable, and allows some arthropods to live on land. The exoskeleton is composed of layers of protein and a tough polysaccharide called chitin, and can be a thick hard armor or a flexible paper-thin covering. Arthropods grow by shedding their exoskeletons and secreting a larger one in a process called molting.
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