Muscle comes from the Latin musculus, a little mouse. Someone watched a flexed arm, the lump sliding about under the skin, and saw a small animal running beneath a blanket; the Greeks had seen it too, and their word mys meant both. The shellfish got the same name, perhaps for its shape, which is why muscle and mussel are one word spelled two ways. So the man on the beach raising his arm for a photograph is showing off his mouse, and the café behind him has it on the menu with a different spelling.