Anatomical structure like a trapdoor that exists in many (but not all) groups of sea snails and freshwater snails, and also in a few groups of land snails. The operculum is attached to the upper surface of the foot and in its most complete state, it serves as a sort of "trapdoor" to close the aperture of the shell when the soft parts of the animal are retracted. The shape of the operculum varies greatly from one family of gastropods to another. It is fairly often circular, or more or less oval in shape. In species where the operculum fits snugly, its outline corresponds exactly to the shape of the aperture of the shell and it serves to seal the entrance of the shell.

Rapana venosa with operculum (Wikimedia Commons George Chernilevsky)

Opercula of certain gastropods, especially varieties from the Red Sea (Strombus tricornis and Lambis truncata sebae), have long served as an incense material in ancient Jewish tradition, as well as Arabian cultures. Operculum powder is also an important ingredient in Chinese and Japanese incense making. Here it is called "bèi xiāng" (Chinese: 貝香; lit. sea shell fragrance) or "kai kou" (Japanese: 甲香, lit. shell/armour fragrance) respectively. The operculum of certain species of Turbinidae is sometimes used as a very inexpensive organic "gemstone" in rings, bracelets, amulets etc. These opercula are commonly known as "cats eye" (or more recently "Shiva's eye"). The turban snail Turbo petholatus is the species whose operculum is most widely used
Opercula are often used in Northwest Coast art of North America, as detailing in masks, panels and paddles. These objects are used to represent the teeth in masks, and are used as a decorative element in many of the objects which play an important part in the art and culture of Northwest Coast peoples, including dishes for food, bentwood boxes, and rattles.

Opercula of shellfish of the genus Turbo from New Guinea (Wikimedia Commons / Tojob41)