Mollusc shell - AbsoluteAstronomy.com

The mollusc shell is typically a calcareous exoskeleton

Exoskeleton

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

which encloses, supports and protects the soft parts of an animal in the phylum

Phylum

In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

Mollusca

The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

, which includes snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, clam

Clam

The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

s, tusk shell

Tusk shell

The tusk shells or scaphopods are a class of shelled marine mollusks. The scientific name of this class is Scaphopoda, meaning "shovel-footed". Shells of species within this class range from about 0.5 to 15 cm in length...

s, and several other classes. Not all shelled molluscs live in the sea, many live on the land and in freshwater.

The ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid, octopus, and some smaller groups such as the caudofoveata

Caudofoveata

Caudofoveata is a small class within the phylum Mollusca, also known as Chaetodermomorpha. The class is often combined with Solenogastres and termed Aplacophora. However, such a grouping is not monophyletic; molecular data suggests that the Caudofoveata are a sister group to the cephalopods.-...

and solenogastres

Neomeniomorpha

The Solenogastres , common name solenogasters, are a subclass of worm-like, small, shell-less molluscs , the other subclass being the Caudofoveata .-Morphology:...

, and the highly derived Xenoturbella
Xenoturbella
Xenoturbella is a genus of bilaterian animals; it contains two marine worm-like species. The first known species was discovered in 1915 by Sixten Bock but the first published description was only in 1949 by Einar Westblad...

. Today, over 100,000 living species bear a shell; there is some dispute as to whether these shell-bearing molluscs form a monophyletic group (conchifera) or whether shell-less molluscs are interleaved into their family tree.

Malacology

Malacology

Malacology is the branch of invertebrate zoology which deals with the study of the Mollusca , the second-largest phylum of animals in terms of described species after the arthropods. Mollusks include snails and slugs, clams, octopus and squid, and numerous other kinds, many of which have shells...

, the scientific study of molluscs as living organisms, has a branch devoted to the study of shells, and this is called conchology

Conchology

Conchology is the scientific or amateur study of mollusc shells. Conchology is one aspect of malacology, the study of molluscs, however malacology studies molluscs as whole organisms, not just their shells. Conchology pre-dated malacology as a field of study. It includes the study of land and...

- although these terms used to be, and to a minor extent still are, used interchangeably, even by scientists (this is more common in Europe).

Within some species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

of molluscs there is often a surprising degree of variation in the exact shape, pattern, ornamentation, and color of the shell.

Formation

A mollusc shell is formed, repaired and maintained by a part of the anatomy called the mantle

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

. Any injuries to or abnormal conditions of the mantle are usually reflected in the shape and form and even color of the shell. When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle often ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" is produced.

The mantle edge secretes a shell which has two components. The organic constituent is mainly made up of polysaccharides and glycoproteins; its composition may vary widely: some molluscs employ a wide range of chitin-control genes to create their matrix, whereas others just express one, suggesting that the role of chitin in the shell framework is highly variable; it may even be absent in monoplacophora. This organic framework controls the formation of calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

crystals, (never phosphate with the questionable exception of Cobcrephora
Cobcrephora
Cobcrephora is a genus of chiton known from the Silurian of Gotland.Its mollusc shell is unique, because Cobcrephora was described on the basis of isolated phosphatic sclerites. Its overlapping sclerites are arched and small, comprise two shell layers, and have lamellar projections....

), and dictates when and where crystals start and stop growing, and how fast they expand; it even controls the polymorph of the crystal deposited, controlling positioning and elongation of crystals and preventing their growth where appropriate.

The shell formation requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, which is sealed from the environment by the periostracum

Periostracum

The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...

, a leathery outer layer around the rim of the shell, where growth occurs. This caps of the extrapallial space, which is bounded on its other surfaces by the existing shell and the mantle. The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but also, in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur. The accumulation of ions is driven by ion pumps packed within the calcifying epithelium. Calcium ions are obtained from the organism's environment through the gills, gut and epithelium, transported by the haemolymph ("blood") to the calcifying epithelium, and stored as granules within or in-between cells ready to be dissolved and pumped into the extrapallial space when they are required. The organic matrix forms the scaffold that directs crystallization, and the deposition and rate of crystals is also controlled by hormones produced by the mollusc. Because the extrapallial space is supersaturated, the matrix could be thought of as impeding, rather than encouraging, carbonate deposition; although it does act as a nucleating point for the crystals and controls their shape, orientation and polymorph, it also terminates their growth once they reach the necessary size. Nucleation is endoepithelial in Neopilina
Neopilina
Neopilina is a highly derived genus of modern monoplacophoran.However, molecular methods show that they fall within the polyplacophoran clade, and fossil and morphological data show that they are rather derived and bear very little resemblance to an 'ancestral mollusc'.- Anatomy :Its anatomy led...

and Nautilus
Nautilus (genus)
Nautilus is a genus of cephalopods in the family Nautilidae. Species in this genus differ significantly in terms of morphology from those placed in the sister taxon Allonautilus. The oldest fossils of the genus are known from the Late Eocene Hoko River Formation, in Washington State and from...

, but exoepithelial in the bivalves and gastropods.

The formation of the shell involves a number of genes and transcription factors. On the whole, the transcription factors and signalling genes are deeply conserved, but the proteins in the secretome are highly derived and rapidly evolving. engrailed

Engrailed (gene)

engrailed is a homeodomain transcription factor involved in many aspects of multicellular development. First known for its role in arthropod embryological development, working in consort with the Hox genes, engrailed has been found to be important in other areas of development...

serves to demark the edge of the shell field; dpp controls the shape of the shell, and Hox1 and Hox4 have been implicated in the onset of mineralization. In gastropod embryos, Hox1 is expressed where the shell is being accreted; however no association has been observed between Hox genes and cephalopod shell formation. Perlucin increases the rate at which calcium carbonate precipitates to form a shell when in saturated seawater; this protein is from the same group of proteins (C-type lectin

C-type lectin

A C-type lectin is a type of carbohydrate-binding protein domain known as a lectin. The C-type designation is from their requirement for calcium for binding. Proteins that contain C-type lectin domains have a diverse range of functions including cell-cell adhesion, immune response to pathogens and...

s) as those responsible for the formation of eggshell and pancreatic stone crystals, but the role of C-type lectins in mineralization is unclear. Perlucin operates in association with Perlustrin, a smaller relative of lustrin A

Lustrin A

Lustrin A is an insoluble protein used in the production of a nacreous layer in bivalve molluscs. It contributes to the properties of the nacreous layer, imparting resistance to cracking and elasticity. This is accomplished by its structure; it consists of many spring-like units which can expand...

, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking. Lustrin A bears remarkable structural similarity to the proteins involved in mineralization in diatoms

Diatom

Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...

– even though diatoms use silica, not calcite, to form their tests!

Development

The shell-secreting area is differentiated very early in embryonic development. An area of the ectoderm thickens, then invaginates to become a "shell gland". The shape of this gland is tied to the form of the adult shell; in gastropods, it is a simple pit, whereas in bivalves, it forms a groove which will eventually become the hinge line between the two shells, where they are connected by a ligament. The gland subsequently evaginates in molluscs that produce an external shell. Whilst invaginated, a periostracum - which will form a scaffold for the developing shell - is formed around the opening of the invagination, allowing the deposition of the shell when the gland is everted. A wide range of enzymes are expressed during the formation of the shell, including carbonic anhydrase, alkaline phosphatase, and DOPA-oxidase (tyrosinase)/peroxidase.

The form of the molluscan shell is constrained by the organism's ecology. In molluscs whose ecology changes from the larval to adult form, the morphology of the shell also undergoes a pronounced modification at metamorphosis. The larval shell may have a completely different mineralogy to the adult conch, perhaps formed from amorphous calcite as opposed to an aragonite adult conch.

In those shelled molluscs that have indeterminate growth

Indeterminate growth

In biology and especially botany, indeterminate growth refers to growth that is not terminated in contrast to determinate growth that stops once a genetically pre-determined structure has completely formed. Thus, a plant that grows and produces flowers and fruit until killed by frost or some other...

, the shell grows steadily over the lifetime of the mollusc by the addition of calcium carbonate to the leading edge or opening. Thus the shell gradually becomes longer and wider, in an increasing spiral shape, to better accommodate the growing animal inside. The shell thickens as it grows, so that it stays proportionately strong for its size.

Secondary loss

The loss of a shell in the adult form of some gastropods is achieved by the discarding of the larval shell; in other gastropods and in cephalopods, the shell is lost or demineralized by the resorption of its carbonate component by the mantle tissue.

Shell proteins

Hundreds of soluble and insoluble proteins, control shell formation. They are secreted into the extrapallial space by the mantle, which also secretes the glycoproteins, proteoglycans, polysaccharides and chitin that make up the organic shell matrix. Insoluble proteins tend to be thought of as playing a more important/major role in crystallization control. The organic matrix of shells tends to consist of β-chitin and silk fibroin. Perlucin

Conchiolin

Conchiolin and perlucin are complex proteins which are secreted by a mollusc's outer epithelium ....

encourages carbonate deposition, and is found at the interface of the chitinous and aragonitic layer in some shells. An acidic shell matrix appears to be essential to shell formation, in the cephalopods at least; the matrix in the non-mineralized squid gladius is basic.

In oysters and potentially most molluscs, the nacreous layer has an organic framework of the protein MSI60, which has a structure a little like spider silk and forms sheets; the prismatic layer uses MSI31 to construct its framework. This too forms beta-pleated sheets. Since acidic amino acids, such as aspartic acid and glutamic acid, are important mediators of biomineralization, shell proteins tend to be rich in these amino acids. Aspartic acid, which can make up up to 50% of shell framework proteins, is most abundant in calcitic layers, and also heavily present in aragonitic layers. Proteins with high proportions of glutamic acid are usually associated with amorphous calcium carbonate.

The soluble component of the shell matrix acts to inhibit crystallization when in its soluble form, but when it attaches to an insoluble substrate, it permits the nucleation of crystals. By switching from a dissolved to an attached form and back again, the proteins can produce 'bursts' of growth, producing the brick-wall structure of the shell.

Chemistry

The formation of a shell in molluscs appears to be related to the secretion of ammonia, which originates from urea. The presence of an ammonium ion raises the pH of the extrapallial fluid, favouring the deposition of calcium carbonate. This mechanism has been proposed not only for molluscs, but also for other unrelated mineralizing lineages.

Structure

The calcium carbonate layers in a shell are generally of two types: an outer, chalk-like prismatic layer and an inner pearly, lamellar or nacreous layer. The layers usually incorporate a substance called conchiolin

Conchiolin

Conchiolin and perlucin are complex proteins which are secreted by a mollusc's outer epithelium ....

, often in order to help bind the calcium carbonate crystals together. Conchiolin is composed largely of quinone

Quinone

A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...

-tanned protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s.

The periostracum and prismatic layer are secreted by a marginal band of cells, so that the shell grows at its outer edge. Conversely, the nacreous layer is derived from the main surface of the mantle.

Some shells contain pigments which are incorporated into the structure. This is what accounts for the striking colors and patterns that can be seen in some species of seashell

Seashell

A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers...

s, and the shells of some tropical land snails. These shell pigments sometimes include compounds such as pyrrole

Pyrrole

Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...

s and porphyrin

Porphyrin

Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...

s.

Shells are almost always composed of polymorphs of calcium carbonate

Calcium carbonate

- either calcite or aragonite. In many cases, such as the shells of many of the marine gastropods, different layers of the shell are composed of calcite and aragonite. In a few species which dwell near hydrothermal vents, iron sulfide

Iron sulfide

Iron sulfide or Iron sulphide refers to a chemical compound of iron and sulfur with a wide range of stoechiometric formulae and different crystalline structures.-Natural minerals:By increasing order of stability:...

is used to construct the shell. Phosphate is never utilised by molluscs, with the exception of Cobcrephora
Cobcrephora
Cobcrephora is a genus of chiton known from the Silurian of Gotland.Its mollusc shell is unique, because Cobcrephora was described on the basis of isolated phosphatic sclerites. Its overlapping sclerites are arched and small, comprise two shell layers, and have lamellar projections....

, whose molluscan affinity is uncertain.

Shells are composite materials of calcium carbonate (found either as calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

or aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

) and organic macromolecules (mainly proteins and polysaccharides.) Shells can have numerous ultrastructural motifs, the most common being crossed-lamellar (aragonite), prismatic (aragonite or calcite), homogeneous (aragonite), foliated (aragonite) and nacre

Nacre

Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....

(aragonite). Although not the most common, nacre

Nacre

is the most studied type of layer. Shells of the class Polyplacophora

Chiton

Chitons are small to large, primitive marine molluscs in the class Polyplacophora.There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura....

are made of aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

Size

In most shelled molluscs, the shell is large enough for all of the soft parts to be retracted inside when necessary, for protection from predation or from desiccation. However there are many species of gastropod mollusc in which the shell is somewhat reduced or considerably reduced, such that it offers some degree of protection only to the visceral mass, but is not large enough to allow the retraction of the other soft parts. This is particularly common in the opisthobranchs

Opisthobranchia

Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

and in some of the pulmonate

Pulmonata

The Pulmonata, or "pulmonates", are an informal group of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills...

s, for example in the semi-slug

Semi-slug

Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but the shell is not quite vestigial...

s.

Some gastropods have no shell at all, or only an internal shell or internal calcareous granules, and these species are often known as slug

Slug

Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

s. Semislugs are pulmonate slugs with a greatly reduced external shell which is in some cases partly covered by the mantle.

Shape

The shape of the molluscan shell is controlled both by transcription factors (such as engrailed

Engrailed (gene)

and decapentaplegic

Decapentaplegic

Decapentaplegic is a key morphogen involved in the development of the fruit fly Drosophila melanogaster. It is known to be necessary for the correct patterning of the fifteen imaginal discs, which are tissues that will become limbs and other organs and structures in the adult fly. It has also been...

) and by developmental rate. The simplification of a shall form is thought to be relatively easily evolved, and many gastropod lineages have independently lost the complex coiled shape. However, re-gaining the coiling requires many morphological modifications and is much rarer. Despite this, it can still be accomplished; it is known from one lineage that was uncoiled for at least 20 million years, before modifying its developmental timing to restore the coiled morphology.

In bivalves at least, the shape does change through growth, but the pattern of growth is constant. At each point around the aperture of the shell, the rate of growth remains constant. This results in different areas growing at different rates, and thus a coiling of the shell and a change in its shape - its convexity, and the shape of the opening - in a predictable and consistent fashion.

The shape of the shell has an environmental as well as a genetic component; clones of gastropods can exert different shell morphologies. Indeed intra-species variation can be many times larger than inter-species variation.

A number of terms are used to describe molluscan shell shape; in the univalved molluscs, endogastric shells coil backwards (away from the head), whereas exogastric shells coil forwards; the equivalent terms in bivalved molluscs are opisthogyrate and prosogyrate respectively.

Evolution

The fossil record shows that all molluscan classes evolved from a shelled ancestor looking something like a modern monoplacophoran, and that modifications of the shell form ultimately led to the formation of new classes and lifestyles. However, a growing body of molecular and biological data indicate that at least certain shell features have evolved many times, independently. The nacreous layer of shells is a complex structure, but rather than being difficult to evolve, it has in fact arisen many times convergently. The genes used to control its formation vary greatly between taxa: under 10% of the (non-housekeeping) genes expressed in the shells that produce gastropod nacre are also found in the equivalent shells of bivalves: and most of these shared genes are also found in mineralizing organs in the deuterostome lineage. The independent origins of this trait are further supported by crystallographic differences between clades: the orientation of the axes of the deposited aragonite 'bricks' that make up the nacreous layer is different in each of the monoplacophora, gastropods and bivalves.

Mollusc shells (especially those formed by marine species) are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time (sometimes thousands of years even without being fossilized). Most shells of marine molluscs fossilize rather easily, and fossil mollusc shells date all the way back to the Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

period. Large amounts of shells sometimes form sediment, and over a geological time span can become compressed into limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

deposits.

Most of the fossil record of molluscs consists of their shells, since the shell is often the only mineralised part of a mollusc (however also see Aptychus

Aptychus

An aptychus is a type of marine fossil, a hard anatomical structure like a curved shelly plate, which was part of the body of an ammonite. Paired aptychi have, on rare occasions, been found at or within the aperture of ammonite shells....

and operculum

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

). The shells are usually preserved as calcium carbonate – usually any aragonite is pseudomorphed with calcite. Aragonite can be protected from recrystalization if water is kept away by carbonaceous material, but this did not accumulate in sufficient quantity until the Carboniferous; consequently aragonite older than the Carboniferous is practically unknown: but the original crystal structure can sometimes be deduced in fortunate circumstances, such as if an alga closely encrusts the surface of a shell, or if a phosphatic mould quickly forms during diagenesis.

The shell-less aplacophora have a chitinous cuticle that has been likened to the shell framework; it has been suggested that tanning of this cuticle, in conjunction with the expression of additional proteins, could have set the evolutionary stage for the secretion of a calcareous shell in an aplacophoran-like ancestral mollusc.

The molluscan shell has been internalized in a number of lineages, including the coleoid cephalopods and many gastropod lineages. Detorsion of gastropods results in an internal shell, and can be triggered by relatively minor developmental modifications such as those induced by exposure to high platinum concentrations.

Nacre

, commonly known as mother of pearl, forms the inner layer of the shell structure in some groups of gastropod and bivalve molluscs, mostly in the more ancient families such as top snails (Trochidae

Trochidae

The Trochidae, common name top snails, are a taxonomic family of very small to large sea snails, marine gastropod molluscs in the clade Vetigastropoda ....

), and pearl oysters (Pteriidae

Pteriidae

Pteriidae is a family of medium-sized to large saltwater clams. They are pearl oysters, marine bivalve mollusks in the order Pterioida.Some of the species in this family are important economically as the source of saltwater pearls....

). Like the other calcareous layers of the shell, the nacre is created by the epithelial cells (formed by the germ layer ectoderm

Ectoderm

The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...

) of the mantle tissue.

Monoplacophora

The nacreous layer of monoplacophoran shells appears to have undergone some modification. Whilst normal nacre, and indeed part of the nacreous layer of one monoplacophoran species (Veleropilina zografi), consists of "brick-like" crystals of aragonite, in monoplacophora these bricks are more like layered sheets. The c-axis is perpendicular to the shell wall, and the a-axis parallel to the growth direction. This foliated aragonite is presumed to have evolved from the nacreous layer, with which it has historically been confused, but represents a novelty within the molluscs.

Chitons

Shells of chitons are made up of eight overlapping calcareous

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

valve

Valve (chiton)

Valve is an anatomical term applied to the shell of molluscs. Although in theory any mollusc shell can be termed a "valve", the word is now most commonly applied to members of two classes of molluscs: the Bivalvia and the Polyplacophora , in other words, to those molluscs whose shells are normally...

s, surrounded by a girdle.

Gastropods

In some marine genera, during the course of normal growth the animal undergoes periodic resting stages where the shell does not increase in overall size, but a greatly thickened and strengthened lip is produced instead. When these structures are formed repeatedly with normal growth between the stages, evidence of this pattern of growth is visible on the outside of the shell, and these unusual thickened vertical areas are called varices, singular "varix

Varix (mollusc)

A varix is an anatomical feature of the shell of certain sea snails, marine gastropod molluscs.It is a thickened axial ridge in the shell of some families of gastropods. A varix is located at intervals around the whorl, and is formed by considerable thickening of the outer lip at a resting stage...

". Varices are typical in some marine gastropod families, including the Bursidae

Bursidae

Bursidae, common name "frog snails" or "frog shells" is a rather small taxonomic family of large sea snails, marine gastropod predatory snails in the clade Littorinimorpha.-Distribution:...

, Muricidae

Muricidae

Muricidae, common name murex snails or rock snails, is a large and varied taxonomic family of small to large predatory sea snails. With approximately 1,600 living species the Muricidae represent almost 10% of the Neogastropoda. Additionally, 1,200 fossil species have been recognised...

, and Ranellidae

Ranellidae

Ranellidae, common name the triton shells or tritons, is a taxonomic family of small to very large predatory sea snails, marine gastropod mollusks in the clade Littorinimorpha.-Subfamilies:...

.

Finally, gastropods with a determinate growth pattern may create a single and terminal lip structure when approaching maturity, after which growth ceases. These include the cowries (Cypraeidae

Cypraeidae

Cypraeidae, common name the cowries , is a taxonomic family of small to large sea snails. These are marine gastropod mollusks in the superfamily Cypraeoidea, the cowries and cowry allies.-Shell description:...

) and helmet shells (Cassidae), both with in-turned lips, the true conchs (Strombidae

Strombidae

Strombidae, commonly known as the true conchs, is a taxonomic family of medium-sized to very large sea snails, marine gastropod mollusks in the superfamily Stromboidea....

) that develop flaring lips, and many land snails that develop tooth structures or constricted apertures upon reaching full size.

Cephalopods

Nautilus

Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...

es are the only extant cephalopod

Cephalopod

A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s which have an external shell. (For information on a very large extinct subclass of shelled cephalopods, please see Ammonite

Ammonite

Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...

s.) Cuttlefish

Cuttlefish

Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda . Despite their name, cuttlefish are not fish but molluscs....

, squid

Squid

Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...

, spirula, and cirrate octopus

Octopus

The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...

es have small internal shells. Females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they partially reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal.

Bivalves

The shell of the Bivalvia is composed of two parts, two valves which are hinged together and joined by a ligament.

Scaphopods

The shell of many of the scaphopods ("tusk shells") resembles a miniature elephant

Elephant

Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...

's tusk in overall shape, except that it is hollow, and is open at both ends.

Damage to shells in collections

As a structure made primarily of calcium carbonate, mollusc shells are vulnerable to attack by acidic fumes. This can become a problem when shells are in storage or on display and are in the proximity of non-archival materials, see Byne's disease

Byne's disease

Byne's disease, more accurately known as Bynesian decay, is a peculiar and permanently damaging condition which often attacks mollusk shells that are in storage or on display for long periods of time....

External links

Conchologists of America
Shells of freshwater species
From the Wolfram Demonstrations Project
Wolfram Demonstrations Project
The Wolfram Demonstrations Project is hosted by Wolfram Research, whose stated goal is to bring computational exploration to the widest possible audience. It consists of an organized, open-source collection of small interactive programs called Demonstrations, which are meant to visually and...

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The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Formation

Development

Secondary loss

Shell proteins

Chemistry

Structure

Size

Shape

Evolution

Nacre

Monoplacophora

Chitons

Gastropods

Cephalopods

Bivalves

Scaphopods

Damage to shells in collections

See also

Further reading

External links