Gastropod shell - AbsoluteAstronomy.com

The gastropod shell is a shell

Mollusc shell

The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...

which is part of the body of a gastropod

Gastropoda

The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to quite large...

or 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...

, one kind of mollusc. The gastropod shell is an external skeleton or 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 serves not only for muscle attachment, but also for protection from predators and from mechanical damage. In land snails, in some freshwater snails and in intertidal marine snails, the shell is also an essential protection against the sun, and against drying out.

Most gastropod shells are spirally coiled. The coiling is usually right-handed, but in some taxa the coiling is left-handed and in a very few species there can be both right-handed and left-handed individuals.

The gastropod shell has several layers, and is typically made 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,...

precipitated out into an organic matrix known as conchiolin

Conchiolin

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

. The shell is secreted by a part of the molluscan body known as 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...

.

Not all gastropods have a shell, but the majority do. In almost every case the shell consists of one piece, and is typically spiral

Spiral

In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...

ly coiled, although some groups, such as the various families and genera of limpet

Limpet

Limpet is a common name for a number of different kinds of saltwater and freshwater snails ; it is applied to those snails that have a simple shell which is more or less conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snails.The name limpet is most...

s, have simple cone-shaped shells as adults.

The study of mollusc shell

Mollusc shell

s, including gastropod shells, 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...

Chirality in gastropods

Because coiled shells are asymmetrical, they possess a quality called chirality

Chirality

Chirality is a property of asymmetry important in several branches of science. It may refer to:* Chirality , a property of molecules having a non-superimposable mirror image...

, the "handedness" of an asymmetrical structure.

By far the majority (over 90 %) of gastropod species have dextral (right-handed) shells in their coiling, but a small minority of species and genera are virtually always sinistral (left-handed), and a very few species (for example Amphidromus perversus
Amphidromus perversus
Amphidromus perversus is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Camaenidae.Amphidromus perversus is the type species of the genus Amphidromus, by the subsequent designation of Eduard von Martens .- Subspecies :There are six subspecies of...

) show an even mixture of dextral and sinistral individuals.

In species that are almost always dextral, very rarely a sinistral specimen will be produced, and these oddities are avidly sought after by some shell collector

Conchology

s.

If a coiled gastropod shell is held with the spire pointing upwards and the aperture more or less facing the observer, a dextral shell will have the aperture on the right-hand side, and a sinistral shell will have the aperture on the left-hand side.

This chirality of gastropods is often overlooked when photographs of coiled gastropods are "flipped" by a non-expert prior to being used in a publication. This image "flipping" results in a normal dextral gastropod appearing to be a rare and abnormal sinistral one.

The chirality in gastropods appears in early cleavage (spiral cleavage) and the gene NODAL

NODAL

Nodal is a protein that in humans is encoded by the NODAL gene. It is a member of the TGF beta superfamily. Like many other members of this superfamily, it is involved in cell differentiation...

is involved.

Mixed coiling populations

In few cases, both left- and right-handed coiling are found in the same population. Sinistral mutants of normally dextral species and dextral mutants of normally sinistral species are rare but well documented occurrences among land snails in general. Populations or species with normally mixed coiling are much rarer, and, so far as is known, are confined, with one exception, to a few genera of arboreal tropical snails. Besides Amphidromus
Amphidromus
Amphidromus is a genus of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Camaenidae.Amphidromus is a genus of arboreal pulmonate land snails...

, the Cuban Liguus vittatus
Liguus vittatus
Liguus vittatus is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Orthalicidae....

(Swainson), Haiti

Haiti

Haiti , officially the Republic of Haiti , is a Caribbean country. It occupies the western, smaller portion of the island of Hispaniola, in the Greater Antillean archipelago, which it shares with the Dominican Republic. Ayiti was the indigenous Taíno or Amerindian name for the island...

an Liguus virgineus
Liguus virgineus
Liguus virgineus is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Orthalicidae.Liguus virgineus is a type species of the genus Liguus....

(Linnaeus) (family Orthalicidae

Orthalicidae

Orthalicidae are a family of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks are classified in the informal group Sigmurethra of the clade Stylommatophora...

), some Hawaii

Hawaii

Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

an Partulina
Partulina
Partulina is a genus of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Achatinellidae.-Species:Species within the genus Partulina include:* Partulina confusa* Partulina crassa...

and many Hawaiian Achatinella (family Achatinellidae

Achatinellidae

Achatinellidae is a family of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks in the superfamily Achatinelloidea.- Taxonomy :...

), as well as several species of Pacific Island Partula (family Partulidae

Partulidae

Partulidae is a family of air-breathing land snails, terrestrial gastropod mollusks in the infraorder Stylommatophora, endemic to Pacific islands.- Genera :Partulidae is divided into three genera:* Eua has four species, confined to Tonga and Samoa....

), are known to have mixed dextral-sinistral populations. The independent appearance of this variation in unrelated groups is probably the result of a simple mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

, whose primary import is with physiological adaptations to arboreal life and not with the direction of coiling. In Partula both dextral and sinistral embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

s have been recovered from the same brood pouch, although normally all embryos coil in the same direction. In Amphidromus there is no information on the heredity of this character.

A possible exception may concern some of the European clausiliids

Clausiliidae

Clausiliidae, common name door snails, are a taxonomic family of small, very elongate, mostly left-handed, air-breathing land snails, sinistral terrestrial pulmonate gastropod mollusks....

of the subfamily Alopiinae. They are obligatory calciphiles living in isolated colonies on limestone outcrops. Several sets of species differ only in the direction of coiling, but the evidence is inconclusive as to whether left- and right-handed shells live together. Soos (1928, pp. 372–385) summarized previous discussions of the problem and concluded that the right- and left-handed populations were distinct species. Others have stated that these populations were not distinct, and the question is far from settled. The Peruvian clausiliid, Nenia callistoglypta Pilsbry (1949, pp. 216–217), also has been described as being an amphidromine species.

The genetics of reverse coiling in a rare dextral mutant of another clausiliid, Alinda biplicata
Alinda biplicata
Alinda biplicata, also known as Balea biplicata, common name the Two lipped door snail, is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Clausiliidae.-Distribution:...

(Montagu), has been studied by Degner (1952). The mechanism is the same as in Radix peregra
Radix peregra
Radix peregra, previously known as Lymnaea peregra or Lymnaea pereger, is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod in the family Lymnaeidae, the pond snails....

(Müller), with the direction of coiling determined by a simple Mendelian recessive. Any change in direction caused by cross-fertilization is delayed one generation by an unknown mechanism.

Morphology

Gastropod shell morphology is usually quite constant among individuals of a species, and with exceptions, fairly constant among species within each family of gastropoda. Controlling variables are:

The rate of growth per revolution around the coiling axis. High rates give wide-mouthed forms such as the abalone
Abalone
Abalone , from aulón, are small to very large-sized edible sea snails, marine gastropod molluscs in the family Haliotidae and the genus Haliotis...

, low rates give highly coiled forms such as Turritella
Turritella
Turritella is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae. They have tightly coiled shells, whose overall shape is basically that of an elongated cone....

or some of the Planorbidae
Planorbidae
Planorbidae, common name the ramshorn snails or ram's horn snails, is a family of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks....

.
The shape of the generating curve, roughly equivalent to the shape of the aperture. It may be round, for instance in the turban shell, elongate as in the cone shell or have an irregular shape with a siphonal canal extension, as in the Murex
Murex
Murex is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly calle "murexes" or "rock snails"...

.
The rate of translation of the generating curve along the axis of coiling, controlling how high-spired the resulting shell becomes. This may range from zero, a flat planispiral shell, to nearly the diameter of the aperture.
Irregularities or "sculpturing" such as ribs, spines, knobs, and varices made by the snail regularly changing the shape of the generating curve during the course of growth, for instance in the many species of Murex
Murex
Murex is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly calle "murexes" or "rock snails"...

.
Ontologic growth changes as the animal reaches adulthood. Good examples are the flaring lip of the adult conch
Conch
A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....

and the inward-coiled lip of the cowry
Cowry
Cowry, also sometimes spelled cowrie, plural cowries, is the common name for a group of small to large sea snails, marine gastropod molluscs in the family Cypraeidae, the cowries...

.

Some of these factors can be modelled mathematically and programs exist to generate extremely realistic images. Early work by David Raup on the analog computer also revealed many possible combinations that were never adapted by any actual gastropod.

Some shell shapes are found more often in certain environments, though there are many exceptions. Wave-washed high-energy environments, such as the rocky intertidal zone, are usually inhabited by snails whose shells have a wide aperture, a relatively low surface area, and a high growth rate per revolution. High-spired and highly sculptured forms become more common in quiet water environments. The shell of burrowing forms, such as the olive

Olive

The olive , Olea europaea), is a species of a small tree in the family Oleaceae, native to the coastal areas of the eastern Mediterranean Basin as well as northern Iran at the south end of the Caspian Sea.Its fruit, also called the olive, is of major agricultural importance in the...

and Terebra
Terebra
Terebra is a genus of medium-sized, predatory sea snails, marine gastropod mollusks in the family Terebridae, the auger snails. Members of this genus do not have a radula.-Species:Species in the genus Terebra include:...

, are smooth, elongated, and lack elaborate sculpture, in order to decrease resistance when moving through sand.

A few gastropods, for instance the Vermetidae

Vermetidae

Vermetidae, common name the worm snails or worm shells, is a taxonomic family of small to medium-sized sea snails, marine gastropod molluscs in the clade Littorinimorpha....

, cement the shell to, and grow along, solid surfaces such as rocks, or other shells.

Standard ways of viewing a shell

In photographs or illustrations, a gastropod shell can be shown oriented in a number of standard ways:

apertural view: this is the most common viewing angle. The shell is shown in its full length with its aperture to the viewer and the apex on top. When the aperture is on the right side, then the shell is called "right-handed"; if the aperture is on the left side, the shell is called "left-handed".
abapertural view: the shell is shown in its full length with its aperture 180° away from the viewer and with the apex on top.
apical view (or dorsal view): the shell is seen in its full width with the apex on top
basal view (or umbilical view): the shell is shown in its full width with the apex below. In most cases, the umbilicus is in clear view.

Description

The shell begins with the minute embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

nic whorls of the protoconch

Protoconch

A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

, which is often quite distinct from the rest of the
shell. From the protoconch, which forms the apex of the spire

Spire (mollusc)

A spire is a descriptive term for part of the coiled shell of mollusks. The word is a convenient aid in describing shells, but it does not refer to a very precise part of shell anatomy: the spire consists of all of the whorls except for the body whorl...

, the coils or whorls

Whorl (mollusc)

A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...

of the shell gradually increase in size. Normally the whorls are circular or elliptical in section, but from compression and other causes a variety of forms can result. The spire can be high or low, broad or slender according to the way the coils of the shell are arranged, and the apical angle of the shell varies accordingly. The whorls sometimes rest loosely upon one another (as in Epitonium scalare
Epitonium scalare
Epitonium scalare, common name the precious wentletrap, is a predatory or ectoparasitic species of marine gastropod with an operculum, in the family Epitoniidae, the wentletraps....

). They also can overlap the earlier whorls such that they may be largely or wholly covered by the later ones. When an angulation occurs, the space between it and the suture

Suture (anatomical)

In anatomy, a suture is a fairly rigid joint between two or more hard elements of an animal, with or without significant overlap of the elements....

above it constitutes the area known as the "shoulder" of the shell. The shoulder angle may be simple or keeled, and may sometimes have nodes or spines.

The most primitive sculpture

Sculpture (mollusc)

The sculpture of a mollusc shell is the three-dimensional ornamentation on the outer surface, as distinct from the basic shape of the shell itself or colouration. Sculpture may be concave as well as convex. Sometimes it has microscopic detail. Sculpture refers to the calcareous outer layer, not the...

of the gastropod shell consists of revolving ridges or spirals, and of transverse folds or ribs. Primary spirals appear in regular succession on either side of the first primary, which generally becomes the shoulder angle if angulation occurs. Secondary spirals appear by intercalation between the primary ones, and generally are absent in the young shell, except in some highly accelerated types. Tertiary spirals are intercalated between the preceding groups in more specialized species. Ribs are regular transverse foldings of the shell, which generally extend from the suture to suture. They are usually spaced uniformly and crossed by the spirals. In specialized types, when a shoulder angle is formed, they become concentrated as nodes upon this angle, disappearing from the shoulder above and the body below. Spines may replace the nodes in later stages. They form as notches in the margin of the shell and are subsequently abandoned, often remaining open in front. Irregular spines may also arise on various parts of the surface of the shell (see Platyceras
Platyceras
Platyceras is a genus of extinct Paleozoic gastropods in the family Platyceratidae known from the Ordovician to the Permian periods. It is the type genus of the family Platyceratidae.- Description :...

). When a row of spines is formed at the edge or outer lip of the shell this sometimes remains behind as a 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...

as in (Murex
Murex
Murex is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly calle "murexes" or "rock snails"...

) and many of the 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:...

. Varices may also be formed by simple expansion of the outer lip, and a subsequent resumption of growth from the base of the expansion. These simple varices may project from the shell (Epitonium
Epitonium
Epitonium is a genus of small predatory sea snails, marine gastropod mollusks. Epitonium is the type genus of the family Epitoniidae, the wentletraps....

) or be reflected backwards (Harpa
Harpa
Harpa, common name the "harp snails", is a genus of large predatory sea snails, marine gastropod mollusks in the family Harpidae.Harpa is the type genus of the family Harpidae.-Species:Species in the genus Harpa include:...

). Periodic enlargements of ribs are not considered as varices.

The aperture

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

or peristome of the shell may be simple or variously modified. An outer and an inner (columellar) lip are generally recognized. These may be continuous with each other, or may be divided below by an anterior notch. This, in some types (Fusinus
Fusinus
Fusinus is a genus of small to large sea snails, marine gastropod mollusks in the family Fasciolariidae, the spindle snails and tulip snaills.-Species:Species in the genus Fusinus include:...

, etc.) it is drawn out into an anterior siphonal canal

Siphonal canal

Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is drawn into the mantle cavity and over the gill and which serves as a chemoreceptor to locate food. In many carnivorous snails, where the siphon is particularly long, the structure...

, of greater or lesser length.

An upper or posterior notch is present in certain taxa, and this may result in the formation of a ridge or shelf next to the suture (Clavilithes
Clavilithes
Clavilithes is an extinct genus of fossil sea snails, marine gastropod mollusks in the family Fasciolariidae, the tulip snails and spindle snails.This genus lived from the Paleocene to Pliocene, in Africa, Asia, Europe, North America, and South America....

). An outer (lateral) emargination or notch, sometimes prolonged into a slit occurs in certain types (Pleurotomidae, Pleurotomaridae, Bellerophontidae, etc.), and the progressive closing of this slit may give rise to a definitely marked slit band. In some cases the slit is abandoned and left as a hole (Fissurellidae), or by periodic renewal as a succession of holes (Haliotis). The outer emargination is often only indicated by the reflected course of the lines of growth on the shell.

On the inside of the outer lip, various ridges or plications called lirae are sometimes found, and these occasionally may be strong and tooth-like (Nerinea
Nerinea
†Nerinea is an extinct genus of fossil sea snails, marine gastropod molluscs in the clade Heterobranchia.The genus is known from the Jurassic to the Cretaceous periods.- Species :Species in the genus Nerinea include:...

). Similar ridges or columellar plicae or folds are more often found on the inner lip, next to the columella or central spiral twist. These may be oblique or normal to the axis of coiling (horizontal), few or numerous, readily seen, or far within the shell so as to be invisible except in broken shells. When the axis of coiling is hollow (perforate spire) the opening at the base constitutes the umbilicus. The umbilicus varies greatly in size, and may be wholly or in part covered by an expansion or callus of the inner lip (Natica
Natica
Natica is a genus of small to medium-sized predatory sea snails, marine gastropods in the family Naticidae, the moon snails. The genus is known from the Cretaceous to the Recent periods.- Species :...

).

Many Recent shells, when the animal is alive or the shell is freshly empty, have an uppermost shell layer of horny, smooth, or hairy epidermis or 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 proteinaceous layer which sometimes is thick enough to hide the color markings of the surface of the shell. The periostracum, as well as the coloration, is only rarely preserved in fossil shells.

The apertural end of the gastropod shell is the anterior end, nearest to the head of the animal; the apex of the spire is often the posterior end or at least is the dorsal side. Most authors figure the shells with the apex of the spire uppermost.
In life, when the soft parts of these snail are retracted, in some groups the aperture of the shell is closed by using a horny or calcareous 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...

, a door-like structure which is secreted by, and attached to, the upper surface of the posterior part of the foot. The operculum is of very variable form in the different groups of snails that possess one.

Parts of the shell

The terminology used to describe the shells of gastropods includes:

Aperture
Aperture (mollusc)
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

: the opening of the shell
Lip
Lip (gastropod)
In the shell of a gastropod mollusk , the margin of the aperture is called the lip or the peristome. In other words, this is a term used to describe part of gastropod shell anatomy....

= peristome: the margin of the aperture
Apex
Apex (mollusc)
Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...

: the smallest few whorls of the shell
Body whorl
Body whorl
Body whorl is part of the morphology of a coiled gastropod mollusk.- In gastropods :In gastropods, the body whorl, or last whorl, is the most recently-formed and largest whorl of a spiral or helical shell, terminating in the aperture...

: the largest whorl in which the main part of the visceral mass of the mollusk is found
Columella
Columella (mollusk)
The columella , 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, or sliced in half vertically, or viewed as an X-ray image....

: the "little column" at the axis of revolution of the shell
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 "trapdoor" of the shell
Parietal callus
Parietal callus
A parietal callus is a feature of the shell anatomy of some groups of snails, i.e. gastropods. It is a thickened calcareous deposit which may be present on the parietal wall of the aperture of the adult shell. The parietal wall is the margin of the aperture and part of the wall of the body whorl...

: a ridge on the inner lip of the aperture in certain gastropods
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 thin layer of organic "skin" which forms the outer layer of the shell of many species
Peristome
Peristome
The word peristome is derived from the Greek peri, meaning 'around' or 'about', and stoma, 'mouth'. It is a term used to describe various anatomical features that surround an opening to an organ or structure. The term is used in plants and invertebrate animals, such as in describing the shells of...

: the part of the shell that is right around the aperture
Plait
Plait (gastropod)
A plait is an anatomical feature which is present the shells of some snails, or gastropods. This sculpture occurs often in the shells of marine gastropod mollusks in the clade Neogastropoda, but it is also found in some pulmonate land snails....

: folds on the columella.
Protoconch
Protoconch
A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

: the nuclear whorls; the larval shell, often remains in position even on an adult shell
Sculpture
Sculpture (mollusc)
The sculpture of a mollusc shell is the three-dimensional ornamentation on the outer surface, as distinct from the basic shape of the shell itself or colouration. Sculpture may be concave as well as convex. Sometimes it has microscopic detail. Sculpture refers to the calcareous outer layer, not the...

: ornamentation on the outer surface of a shell
- Lira: one kind of shell sculpture
Siphonal canal
Siphonal canal
Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is drawn into the mantle cavity and over the gill and which serves as a chemoreceptor to locate food. In many carnivorous snails, where the siphon is particularly long, the structure...

: an extension of the aperture in certain gastropods
Spire
Spire (mollusc)
A spire is a descriptive term for part of the coiled shell of mollusks. The word is a convenient aid in describing shells, but it does not refer to a very precise part of shell anatomy: the spire consists of all of the whorls except for the body whorl...

: the part of the shell above the body whorl.
Suture: The junction between whorl
Whorl (mollusc)
A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...

s of most gastropods
Teleoconch : the entire shell without the protoconch; the postnuclear whorls.
Umbilicus
Umbilicus (mollusk)
The umbilicus of a shell is the axially aligned, hollow cone-shaped space within the whorls of a coiled mollusc shell. The term umbilicus is often used in descriptions of gastropod shells, i.e...

: in shells where the whorls move apart as they grow, on the underside of the shell there is a deep depression reaching up towards the spire; this is the umbilicus
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...

: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices
Whorl
Whorl (mollusc)
A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...

: each one of the complete rotations of the shell spiral

Shape of the shell

The distinction of the shape of the shell may vary, based on the purpose. For example distinguishing into three groups can be based on the height - width ratio:

oblong - the height is much bigger than the width
globose or conical shell - the height and the width of the shell are approximatelly the same
depressed - the width is much bigger than the height

The following are the principal modifications of form in the gastropod shell.

Regularly spiral:
- Bulloid : bubble-shaped Bulla ampula
- Coeloconoid : a slightly concave shell in which the incremental angle increases steadily during growth
- Cone-shaped, obconic. Conus
  Conus
  Conus is a large genus of small to large predatory sea snails, marine gastropod molluscs, with the common names of cone snails, cone shells or cones. This genus is placed in the subfamily Coninae within the family Conidae. Geologically speaking, the genus is known from the Eocene to the Recent ...
- Contabulate, short, with shouldered whorls
- Convolute ; aperture as long as the shell, nearly or quite concealing the spire. Cypraea
  Cypraea
  Cypraea is a genus of medium-sized to large sea snails or cowries, marine gastropod mollusks in the family Cypraeidae, the cowries.-Species:Species within the genus Cypraea include:* Cypraea pantherina Lightfoot, 1786...
- Cylindrical, pupiform. Lioplax
  Lioplax
  Lioplax is a genus of freshwater snail with a gill and an operculum, an aquatic gastropod mollusk in the family Viviparidae.-Species:Species within the genus Lioplax include:...
  
  , Pupa
  Pupa (gastropod)
  Pupa is a genus of small sea snail, a marine gastropod mollusc in the family Acteonidae.The genus is named Pupa because the shell of these snails resemble an insect pupa in overall shape....
- Depressed, lenticular. Ethalia carneolata
- Discoidal. Elachorbis
  Elachorbis
  Elachorbis is a genus of minute sea snails or micromolluscs, marine gastropod molluscs in the family Tornidae.- Taxonomy :The exact placement of this genus has long been contested....
- Ear-shaped. Haliotis
- Elongated, subulate, elevated. Terebra
  Terebra
  Terebra is a genus of medium-sized, predatory sea snails, marine gastropod mollusks in the family Terebridae, the auger snails. Members of this genus do not have a radula.-Species:Species in the genus Terebra include:...
- Few-whorled. Helix pomatia
  Helix pomatia
  Helix pomatia, common names the Burgundy snail, Roman snail, edible snail or escargot, is a species of large, edible, air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Helicidae...
  
  .
- Fusiform, spindle-shaped. Fusinus
  Fusinus
  Fusinus is a genus of small to large sea snails, marine gastropod mollusks in the family Fasciolariidae, the spindle snails and tulip snaills.-Species:Species in the genus Fusinus include:...
- Gibbous. Whorls swelled beyond the normal contour of increase (usually on the aperture side ). Streptaxis
  Streptaxis
  Streptaxis is a genus of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Streptaxidae.- Distribution :The distribution of the genus includes:* Brazil* Colombia* Venezuela-Species:...
  
  .
- Globular. Natica
  Natica
  Natica is a genus of small to medium-sized predatory sea snails, marine gastropods in the family Naticidae, the moon snails. The genus is known from the Cretaceous to the Recent periods.- Species :...
- Many-whorled. Millerelix peregrina
  Strange many-whorled land snail
  The strange many-whorled land snail, scientific name Millerelix peregrina, is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Polygyridae. This snail is sometimes known by its earlier name Polygyra peregrina. It is endemic to the United States. Its...
  
  .
- Short, bucciniform. Buccinum
  Buccinum
  Buccinum is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Buccinidae, the true whelks.Snails in this genus are commonly called whelks, a name that is however shared with several related and unrelated species...
- Trochiform, pyramidal, conical with a flat base. Trochus
  Trochus
  Trochus is a genus of medium-sized to very large sea snails. They are marine gastropod molluscs in the family Trochidae, the top snails....
- Turbinated ; conical, with rounded base. Turbo
- Turrited, turriculated, babylonic ; an elongated shell with the whorls angulated or shouldered on their upper part. Turritella
  Turritella
  Turritella is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae. They have tightly coiled shells, whose overall shape is basically that of an elongated cone....
Scalariform, whorls not impinging. Epitonium
Epitonium
Epitonium is a genus of small predatory sea snails, marine gastropod mollusks. Epitonium is the type genus of the family Epitoniidae, the wentletraps....
Irregularly spiral, evolute. Siliquaria
Siliquaria
Siliquaria is a genus of sea snails. These are marine gastropod molluscs in the family Siliquariidae, common name the slit worm snails.Siliquaria is the type genus of the family Siliquariidae....

, Vermetus
Vermetus
Vermetus is a genus of sea snails, marine gastropod mollusks in the family Vermetidae, the worm snails or worm shells.-Species:Species within the genus Vermetus include:* Vermetus afer * Vermetus alii Hadfield & Kay, 1972...
Tubular. Dentalium
Dentalium (genus)
Dentalium is a large genus of tooth shells or tusk shells, marine scaphopod molluscs in the family Dentaliidae. It has fifty described species.-Native Americans:...

, or tooth-shell.
Shield-shaped. Umbraculum
Boat-shaped, slipper-shaped. Crepidula
Crepidula
Crepidula, common name the "slipper limpets" or "slipper shells", is a genus of sea snails, marine gastropod mollusks in the family Calyptraeidae, the slipper snails and cup-and-saucer snails....
Conical or limpet-shaped. Patella
Biconic : two conical shapes touching their bases and tapering at both ends : Fasciolaria tulipa
Fasciolaria tulipa
Fasciolaria tulipa, common name the true tulip, is a species of large sea snail, a marine gastropod mollusk in the family Fasciolariidae.-Distribution:...
Multivalve and imbricated. Chiton
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....

Detailed distinction of the shape can be:
Pear shape means both shapes: ovate-conic and conic.

Dimensions

The most frequently used measurements of a gastropod shell are: the height of the shell, the width of the shell, the height of the aperture and the width of the aperture. The number of whorls is also often used.

In this context, the height (or the length) of a shell is its maximum measurement along the central axis. The width (or breadth, or diameter) is the maximum measurement of the shell at right angles to the central axis. Both terms are only related to the description of the shell and not to the orientation of the shell on the living animal.

The largest height of any shell is found in the marine snail species Syrinx aruanus
Syrinx aruanus
Syrinx aruanus, common name the Australian trumpet or false trumpet, is a species of extremely large sea snail measuring up to 91 cm long and weighing up to 18 kg...

, which can be up to 91 cm.

The central axis is an imaginary axis along the length of a shell, around which, in a coiled shell, the whorls spiral. The central axis passes through the columella, the central pillar of the shell.

Evolutionary changes

Among proposed roles invoked the variability of shells during evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

include mechanical stability, defense against predators (evolution of shells against external attacks has been proved by nanoindentation

Nanoindentation

Nanoindentation is a variety of indentation hardness tests applied to small volumes. Indentation is perhaps the most commonly applied means of testing the mechanical properties of materials...

), sexual selection

Sexual selection

Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

and climatic selection.

The shell of some gastropods have been reduced or partly reduced during the evolution

Evolution

. This reduction can be seen in all 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, in semi-slugs and in various other marine and non-marine gastropods. Sometimes the reduction of the shell is associated with predatory way of feeding.

Some taxa even lost the coiling of their shell during evolution. According to Dollo's law

Dollo's law

Dollo's law of irreversibility is a hypothesis proposed in 1893 by French-born Belgian paleontologist Louis Dollo which states that evolution is not reversible...

, it is not possible to regain the coiling of the shell after it is lost. Despite that, there are few genera in the family Calyptraeidae

Calyptraeidae

Calyptraeidae, common name the slipper snails or slipper limpets, cup-and-saucer snails, and Chinese hat snails are a family of small to medium-sized marine prosobranch gastropods...

that changed their developmental timing (heterochrony

Heterochrony

In biology, heterochrony is defined as a developmental change in the timing of events, leading to changes in size and shape. There are two main components, namely the onset and offset of a particular process, and the rate at which the process operates...

) and gained back (re-evolution

Re-Evolution

Re-Evolution is a webcomic series by Gus Higuera and Juan Felipe Salcedo published on DC Comics' Zuda Comics site. The strip placed second in the monthly Zuda competition in April 2008. In July 2008 the strip was invited back to compete in the first annual Zuda Invitational, winning first place...

) a coiled shell from the previous condition of an uncoiled limpet

Limpet

-like shell.

Taphonomic implications

In large enough quantities, gastropod shells can have enough of an impact on environmental conditions to effect the ability of organic remains in the local environment to fossilize. For example, in the Dinosaur Park Formation

Dinosaur Park Formation

The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...

, fossil hadrosaur eggshell is rare. This is because the breakdown of tannins from local coniferous vegetation would have caused the ancient waters to become acidic. Eggshell fragments are present in only two microfossil sites, both of which are predominated by the preserved shells of invertebrate life, including gastropods. It was the slow dissolution of these shells releasing calcium carbonate

Calcium carbonate

into the water that raised the water's pH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

high enough to prevent the eggshell fragments from dissolving before they could be fossilized.

External links

[ftp://ftp.fao.org/docrep/fao/009/y4160e/y4160e08.pdf Gastropods by J. H. Leal] - Information on some gastropods of the tropical Western Atlantic, specifically the Caribbean Sea, with relevance to the fisheries in that region
Radiocarbon Dating of Gastropod Shells
Nair K. K. & Muthe P. T. (18 November 1961) "Effect of Ribonuclease on Shell Regeneration in Ariophanta sp.". Nature 192: 674-675. . Antonio Ruiz Ruiz, Ángel Cárcaba Pozo, Ana I. Porras Crevillen & José R. Arrébola Burgos Caracoles Terrestres de Andalúcia. Guía y manual de identificación. 303 pp., ISBN 84-935194-2-1. (from website)

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