gastropod, member of the class Gastropoda, the largest and most successful class of mollusks (phylum Mollusca), containing over 35,000 living species and 15,000 fossil forms. The shell of gastropods is of one piece (called univalve) and usually coiled or spiraled as in snails, periwinkles, conches, whelks, limpets, and abalones; however, in some forms, as in slugs and sea slugs, it is reduced or completely absent. There is usually a definite head, bearing one or two sensory tentacles and a mouth that is often equipped with a rasplike tongue called a radula. The lower surface of the animal is modified into a large, flattened foot, used by bottom-dwelling forms for creeping about. The foot and other soft parts of the body can usually be completely withdrawn into the shell and the opening covered by a permanent plate called the operculum. Ancient gastropods were probably bilaterally symmetrical, but living species undergo a process known as torsion in which most of the body behind the head rotates 180° so that the anal and urinary openings are relocated behind the head, and the digestive tract and nervous system become U-shaped. Most gastropod species are marine but many groups, notably the pulmonate (lung-bearing) snails, have successfully invaded freshwater and moist terrestrial habitats.

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.

In its most typical condition, an operculum serves to close the opening or aperture of the shell when the soft parts of the animal are withdrawn into the shell.

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.

Functions

Perhaps the most essential function of the operculum is to allow snails to resist drying out, or desiccation. This is very important in intertidal marine snails during low tide, and this also enables operculate pond and land snails to survive periods of drought and periods of dry weather.

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.

Anatomy

In life, the operculum is attached dorsally to the upper surface of the posterior part of the foot, where it grows in size as the shell grows, such that the operculum remains in proportion to the apertural size. In many species, when the animal is active and crawling, part of the underside of the shell rests on the outer surface of the operculum.

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.

In species of conches, the operculum is elongated and claw-shaped, and is used to dig into the sand to enable the conch to perform a leaping type of locomotion.

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:

• The most common kind of operculum is composed of a thin to rather thick corneous protein material, which is yellow to brownish in color and is usually somewhat translucent. This matter is supple when in its natural state but may become brittle when it is dried out. The operculum varies in shape, depending on the family of snails and the shape of the aperture of their shells.
• The other kind of operculum is restricted to a few families of gastropods including the Turbinidae. This operculum structure has a corneous base with a heavy calcareous overlay. The calcareous surface in some genera has color or ornamentation of various kinds including, for example, pustules and incised grooves.

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 shelled land pulmonate snails

Many species of shelled land pulmonates can create an epiphragm, which is a temporary structure used to seal off the aperture of the shell during estivation or hibernation. This is not an operculum, but it can serve some of the same functions.

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.

Human use

As incense material

Operculum of certain gastropods, especially varieties from the Red Sea, has long served as an incense material in ancient Jewish tradition, as well as Arabian cultures. The operculum of conch species Strombus tricornis and Lambis truncata sebae are most commonly used in regions near the middle east. Opercula from these conches may be the "Onycha" incense material which is described in the Book of Exodus.

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.

When burnt on its own, high quality operculum reportedly smells of castoreum or other animal musks, while that of lower quality is reminiscent of burnt hair.

As a gemstone

The operculum of certain species of Turbinidae is sometimes used as 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, 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.

References

• Burnie, D. (2001). Animal: The Definitive Visual Guide to the World's Wildlife. DK Adult Publishing. pg. 24.

External links

• Pictures of diverse gastropod opercula This site also provides fairly extensive material on the growth, development, and physiology of the gastropod operculum as well as dozens of pictures of both the operculum and its host shell.
• Human uses of operculum