Any member of the class Gastropoda, the largest group of mollusks, including about 65,000 species. Gastropods, which include the snails, conchs, whelks, limpets, periwinkles, abalones, slugs, and sea slugs (see nudibranch), are found worldwide, in marine, freshwater, and terrestrial environments. Gastropods typically have a large foot with a flat sole for crawling, a single coiled shell that covers the soft body, and a head that bears a pair of eyes and tentacles. However, they are so diverse that some forms lack shells, while animals in one genus have shells with two halves, like bivalves. Most feed by using a radula, a ribbon of small horny teeth that tear food into pieces. They may be herbivores, carnivores, predators, parasites, or filter feeders of plankton and detritus.
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The operculum, meaning little lid, (plural: opercula or operculums) is a corneous or calcareous structure which exists in some groups of marine, freshwater, and land snails or gastropods. It is present in most but not all gastropods that have shells and gills. Pulmonate snails do not have have opercula, but some terrestrial species are capable of secreting an epiphragm, see below.
An operculum can vary in shape greatly from one family of gastropods to another, but an operculum 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.
In those species where the operculum completely seals the shell, it can also serve as a protection against predators when the snail body is retracted.
In many species of marine shelled snails which live subtidally, the operculum is greatly reduced in size, and no longer serves to seal the shell entrance. In some families it has been eliminated.
The operculum has a concentric structure. The nucleus is central in some genera, and in other the nucleus is near the parietal margin of the shell.
There are two basic types of opercula in terms of their material composition:
Opercula may be described as multispiral (having many closely spaced spirals), paucispiral (with few spirals), and the different shapes of opercula can include ungulate (hooflike), claw-like, or ovate. The type and shape of the operculum is used to help identify and classify related groups (genera) of land operculates, and likewise some marine operculates.
The epiphragm in numerous species is a simple membrane composed of dried mucus, but in a few others such as Helix pomatia, it is a sturdy flat calcareous structure in which there is a small perforation, to allow for oxygen exchange.
Operculum powder is also an important ingredient in Chinese and Japanese incense making. Here it is called "beì xiāng" (Chinese: 貝香; lit. sea shell fragrance) or "kai kou" (Japanese: 甲香, lit. shell/armour fragrance) respectively. Incense producers in these countries use the operculum of many conches and other maine snails, including those found Southeast Asia, South America, and East Africa. Operculum is traditionally treated with vinegar, alcohol and water in order to remove any fishy smell. The clean opercula are then ground to a powder and used as a scent fixative, in a similar technique to that used in perfumes with certain plant resins.
The turban snail Turbo petholatus is the species whose operculum is most widely used, although the operculum of other species of Turbo are sometimes used. The operculum is almost always given a solid setting, because it has one unattractive flat and corneous side where it was attached to the animal, and one roughly hemispherical glossy side, which in the case of T. petholatus has an attractive dark green area.
The shell of the operculum is often used in Northwest Coast Native art, as a detail in masks, panels and paddles. Operculum is often used to deliniate the teeth of masks, and is used as a decorative element in many of the objects which take an important part in the art and culture of Northwest Coast people; including dishes for food, bentwood boxes and rattles.