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A shell is a hard, rigid outer layer, which has evolved in a very wide variety of different animals, including mollusks, sea urchins, crustaceans, turtles and tortoises, armadillos, etc. Scientific names for this type of structure include exoskeleton, armour, test, carapace, and peltidium.
The shells that are perhaps most familiar and most commonly encountered, both in the wild and for sale as decorative objects, are seashells, or more precisely, the external shells of marine mollusks (also spelled molluscs). These are usually primarily composed of calcium carbonate, which can take different crystalline forms, one being nacre otherwise known as mother of pearl.
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