Snails and mussels represent a familiar group of invertebrate animals called ‘mollusks’. Most mollusks have shells and a fleshy mantle that encloses the animal’s body. Oysters are mollusks that form pearls. Some type of oysters called edible oysters are cooked or eaten raw by human. They do not make pearls. The other type called pearl oysters is not edible. While edible ones live in muddy temperate water, pearl oysters live in the clean sandy bottoms of tropical seas. They form pearls in response to the irritation as a defense against the invasion of their body by a grain of sand or parasite. It secretes a material called ‘nacre’ (also called ‘mother of pearl’) around the foreign object in thin, regular layers. The oyster takes several years to build up a smooth and lustrous pearl.
Oysters are not the only mollusks that form the article. Fresh water mussels produce pinkish or red pearls, which were formerly in great demand for jewelry. The inner shell of fresh water mussels is used as a source of mother-of-pearl for buttons. A large button industry grew up along the banks of the Mississippi but dwindled after cheaper, mass produced plastic buttons replaced the real article.
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