The operculum (from Latin operculum 'cover, covering'; pl. opercula or operculums) is a corneous or calcareous 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 structure is found in some marine and freshwater gastropods, and in a minority of terrestrial gastropods, including the families Helicinidae, Cyclophoridae, Aciculidae, Maizaniidae, Pomatiidae, etc.
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.
Many families have opercula that are reduced in size, and which are not capable of closing the shell aperture. Opercula have sometimes been modified: in the Strombidae the operculum is claw-shaped and is used to push into the substrate in a leaping form of locomotion.
Virtually all pulmonate snails are inoperculate, i.e. they do not have an operculum, with the exception of the Amphiboloidea. However, some terrestrial pulmonate species are capable of secreting an epiphragm, a temporary structure that can in some cases serve some of the same functions as an operculum. The epiphragm may be distinguished from the true operculum by its homogeneity and want of growth marks.
In ammonites (extinct shelled cephalopods), a calcareous structure known as the aptychus (plural aptychi) existed. When these were first described they were thought to be valves of a bivalve species, then for many years after that they were considered to be a form of paired or single operculum-like structures belonging to ammonites. More recently the aptychus or paired aptychi have been hypothesized to be a jaw apparatus of ammonites.
Perhaps the most essential function of the operculum in gastropods 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 freshwater and land snails to survive periods of drought and dry weather.[ citation needed ]
In those marine species where the operculum completely seals the shell, it can also serve as a protection against predators when the snail body is retracted.[ citation needed ]
In life, the operculum is attached at the ending of the columellar muscle with an opercular disc dorsally to the upper surface of the posterior part of the foot. However, in Buccinum a layer of long cylindrical epithelial cells, with mostly distinct nuclei, and long divided processes enter between the muscular fibres. The operculum, a cuticular development of these cells, is composed, as may be seen in the corneous opercula of Murex , Purpura , Triton , of very thin superimposed layers. The cylindrical cells are attached with their head to the lowermost layer. 
The operculum 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.[ citation needed ]
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 a large number of families it has been eliminated completely.[ citation needed ]
In species of conchs, the operculum is elongated and sickle-shaped, and is used to dig into the sand to enable the conch to perform a leaping type of locomotion.[ citation needed ]
The structure of the operculum can be described as follows: 
In 1998 Checa and Jimeneze proposed three types of opercula: 
There are two basic types of opercula in terms of their material composition:
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.[ citation needed ]
The oldest known human depiction of an operculum together with the shell of Charonia tritonis is a seal made by the Minoan civilization. 
Opercula of certain gastropods, especially varieties from the Red Sea, have 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.[ citation needed ]
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. Incense producers in these countries use the operculum of many conches and other marine snails, including those found in Southeast Asia, South America, and East Africa. The opercula are 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 technique similar to that used in perfumes with certain plant resins.[ citation needed ]
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.[ citation needed ]
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").[ citation needed ]
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 a dark green area.[ citation needed ]
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.[ citation needed ]
Friedrich Ratzel in The History of Mankind  reported in 1896 that, in Oceania, chains and girdles composed of the coloured opercula of certain shells were considered valuable ornaments.[ citation needed ]
The largest opercula of Turbo marmoratus have been used as paperweights. 
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids than they are to shelled nautiloids such as the living Nautilus species. The earliest ammonites appeared during the Devonian, and the last species either vanished in the Cretaceous–Paleogene extinction event, or shortly after during the Danian epoch of the Paleocene.
Prosobranchia was a large taxonomic subclass of sea snails, land snails and freshwater snails. This taxon of gastropods dates back to the 1920s. It has however been proven to be polyphyletic. Generally speaking in biology taxonomy is required to reflect phylogeny, in other words the classification of a group must reflect its evolutionary descent, as far as that is known, so the taxon Prosobranchia is no longer considered suitable to be used.
Muricidae is a large and varied taxonomic family of small to large predatory sea snails, marine gastropod mollusks, commonly known as murex snails or rock snails. With about 1,600 living species, the Muricidae represent almost 10% of the Neogastropoda. Additionally, 1,200 fossil species have been recognized. Numerous subfamilies are recognized, although experts disagree about the subfamily divisions and the definitions of the genera. Many muricids have unusual shells which are considered attractive by shell collectors and by interior designers.
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where the head-foot part of the body of the animal emerges for locomotion, feeding, etc.
An aptychus is a type of marine fossil. It is a hard anatomical structure, a sort of curved shelly plate, now understood to be part of the body of an ammonite. Paired aptychi have, on rare occasions, been found at or within the aperture of ammonite shells. The aptychus was usually composed of calcite, whereas the ammonite shell was aragonite.
Sculpture is a feature of many of the shells of mollusks. It is three-dimensional ornamentation on the outer surface of the shell, as distinct from either the basic shape of the shell itself or the pattern of colouration, if any. Sculpture is a feature found in the shells of gastropods, bivalves, and scaphopods. The word "sculpture" is also applied to surface features of the aptychus of ammonites, and to the outer surface of some calcareous opercula of marine gastropods such as some species in the family Trochidae.
The gastropod shell is part of the body of a gastropod or snail, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium storage. Some gastropods appear shell-less (slugs) but may have a remnant within the mantle, or the shell is reduced such that the body cannot be retracted within (semi-slug). Some snails also possess an operculum that seals the opening of the shell, known as the aperture, which provides further protection. The study of mollusc shells is known as conchology. The biological study of gastropods, and other molluscs in general, is malacology. Shell morphology terms vary by species group.
Pleurocera is a genus of freshwater snails with an operculum, aquatic gastropod mollusks in the family Pleuroceridae.
The columella or pillar is a central anatomical feature of a coiled snail shell, a gastropod shell. The columella is often only clearly visible as a structure when the shell is broken, sliced in half vertically, or viewed as an X-ray image.
Liotiidae is a family of small sea snails, marine gastropod mollusks in the clade Vetigastropoda.
Modelia granosa, common name the southern cat's eye, is a medium to large species of sea snail which has a shell with a pearly interior and a calcareous operculum. It is a marine gastropod mollusc in the family Turbinidae, the turban snails.
Corneous is a biological and medical term meaning horny, in other words made out of a substance similar to that of horns and hooves in some mammals.
Bolma rugosa is a species of large sea snail with a calcareous operculum, a marine gastropod mollusc in the family Turbinidae, the turban snails.
Anadema macandrewii is a species of sea snail, a marine gastropod mollusk in the family Colloniidae,
Turbo cornutus, common name the horned turban, is a species of sea snail, marine gastropod mollusk in the family Turbinidae.
The Trochina is a taxon that is used by paleontologists. It is a suborder of primitive sea snails, marine gastropod mollusks.
Argalista fluctuata is a species of small sea snail with calcareous opercula, a marine gastropod mollusk in the family Colloniidae.
Homalopoma luridum, with the common name: Dall's dwarf turban, is a species of small sea snail with calcareous opercula, a marine gastropod mollusk in the family Colloniidae.
Eulithidium rubrilineatum, common name the red line pheasant, is an extremely small species of sea snail with a calcareous opercula, a marine gastropod mollusk in the family Phasianellidae, the pheasant snails.
Tricolia variabilis, common name the minute pheasant, is a species of small sea snail with calcareous opercula, a marine gastropod mollusk in the family Phasianellidae, the pheasant snails.
|Wikimedia Commons has media related to Operculum (gastropods) .|