What is any aquatic animal whose external covering consists of a shell, either testaceous, as in oysters, clams, and other mollusks, or crustaceous, as in lobsters and crabs called?

shellfish — Any aquatic animal whose external covering consists of a shell, either testaceous, as in oysters, clams, and other mollusks, or crustaceous, as in lobsters and crabs. (Webster's 1913.)

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