Gastropod shell

{{Short description|Part of the body of a gastropod or snail}} [[File:Epitonium scalare (Linnaeus, 1758) - Wikimedia Commons image.jpg|thumb|300x300px|The shell of [[Epitonium scalare|''Epitonium scalare'']].]] {{Redirect|Snail shell|the cave in Tennessee|Snail Shell Cave|the They Might Be Giants song|John Henry (album)}} {{also|Mollusc shell}} {{biomineralization sidebar|exoskeletons}} The [[gastropod]] shell is part of the body of many gastropods, including snails, 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 ([[slug]]s) but may have a remnant within the mantle, or in some cases the shell is reduced such that the body cannot be retracted within it ([[semi-slug]]).

Some snails also possess an [[Operculum (gastropod)|operculum]] that seals the opening of the shell, known as the [[Aperture (mollusc)|aperture]], which provides further protection. The study of [[mollusc shell]]s is known as [[conchology]]. The biological study of gastropods, and other molluscs in general, is [[malacology]]. Shell morphology terms vary by species group.<ref name="Burch 1962"/>

== Shell layers == {{Main|mantle (mollusc)|mollusc shell|nacre}} The gastropod shell has three major layers secreted by the [[Mantle (mollusc)|mantle]]. The calcareous central layer, ostracum, is typically made of [[calcium carbonate]] (CaCO3) precipitated into an organic matrix known as [[conchiolin]]. The outermost layer is the [[periostracum]] which is resistant to abrasion and provides most shell coloration. The body of the snail contacts the innermost smooth layer that may be composed of mother-of-pearl or shell nacre, a dense horizontally packed form of conchiolin, which is layered upon the periostracum as the snail grows.{{citation needed|date=May 2023}}

== Morphology ==

{| align=center cellspacing="5" cellpadding="25" style="font-size: 90%;" |- align=center |[[File:Zonitoides nitidus drawing en.svg|left|500px]] |style="background: black"| |[[File:Zonitoides nitidus shell.jpg|180px]] |-valign=top |Morphology of typical spirally coiled shell. The shell of ''[[Zonitoides nitidus]]'', a land snail, has dextral coiling. Upper image: Dorsal view, showing whorls and apex Central image: Lateral view showing the profile of the shell Lower image: Basal view showing umbilicus in the centre. |Photo of the shell of ''Zonitoides nitidus'' with an apical view, an apertural view and a basal view |}

Gastropod shell morphology is usually quite constant among individuals of a species. Controlling variables are: * The rate of growth per revolution around the coiling axis. High rates give wide-mouthed forms such as the [[abalone]], low rates give highly coiled forms such as ''[[Turritella]]'' or some of the [[Planorbidae]]. * 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]]''. * 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]]''. * Ontologic growth changes as the animal reaches adulthood. Good examples are the flaring lip of the adult [[conch]] and the inward-coiled lip of the [[cowry]].

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 adopted 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 snail|olive]] and ''[[Terebra]]'', are smooth, elongated, and lack elaborate sculpture, in order to decrease resistance when moving through sand. On land, high-spired forms are often associated with vertical surfaces, whereas flat-shelled snails tend to live on the ground.

A few gastropods, for instance the [[Vermetidae]], cement the shell to, and grow along, solid surfaces such as rocks, or other shells.

==Chirality== [[File:Neptunea - links&rechts gewonden.jpg|thumb|Shells of two different species of sea snail: on the left is the normally sinistral (left-handed) shell of ''[[Neptunea angulata]]'', on the right is the normally dextral (right-handed) shell of ''[[Neptunea despecta]]'']]  Most gastropod shells are spirally coiled. The majority (over 90%)<ref>Schilthuizen M. & Davison A. (2005). "The convoluted evolution of snail chirality". ''[[Naturwissenschaften]]'' '''92'''(11): 504–515. {{PMID|16217668}}. {{doi|10.1007/s00114-005-0045-2}}.</ref> of gastropod species have '''dextral''' (right-handed) shells, but a small minority of species and genera are virtually always '''sinistral''' (left-handed), and a very few species (for example ''[[Amphidromus perversus]]''<ref>{{cite web|url=http://www.jaxshells.org/052ddr.htm|title=Amphidromus perversus (Linnaeus, 1758)|work=jaxshells.org}}</ref>) show a mixture of dextral and sinistral individuals. There occur also aberrantly sinistral forms of dextral species and some of these are highly sought by shell collectors.{{Citation needed|date=October 2020}}

Dextral shells coil clockwise as viewed from the [[Apex (mollusc)|apex]] down; sinistral shells coil counter-clockwise. 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 definition can be ambiguous for shells with a very low spire or an unusual axis.

This chirality of gastropods is sometimes 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 or abnormal sinistral one.

Sinistrality arose independently 19 times among marine gastropods since the start of the [[Cenozoic]].<ref>Geerat Vermeij (2002). [https://www.researchgate.net/publication/51225964_The_Geography_of_Evolutionary_Opportunity_Hypothesis_and_Two_Cases_in_Gastropods "The Geography of Evolutionary Opportunity: Hypothesis and Two Cases in Gastropods".] ''Integrative and Comparative Biology'' 42(5): 9359–40. {{doi|10.1093/icb/42.5.935}}.</ref> This left-handedness seems to be more common in freshwater and land pulmonates. But still the dextral living species in gastropods seem to account for 99% of the total number.<ref>Asami, T. (1993) "Genetic variation of coiling chirality in snails". ''Forma'', 8, 263–276</ref>

The chirality in gastropods appears in early cleavage ([[spiral cleavage]]) and the gene [[NODAL]] is involved.<ref>Myers P. Z. (13 April 2009) [http://pandasthumb.org/archives/2009/04/snails-have-nod.html "Snails have nodal!"]. ''The Panda's Thumb'', accessed 3 May 2009.</ref> A more recent study (2013) correlates the asymmetric coiling of the shell by the left-right asymmetric expression of the decapentaplegic gene in the mantle.<ref>Keisuke Shimizu, Minoru Iijima, Davin HE SetiamargaIsao Sarashina, Tetsuhiro Kudoh, Takahiro Asami, Edi Gittenberger, Kazuyoshi Endo; [http://www.evodevojournal.com/content/4/1/15/abstract "Left-right asymmetric expression of ''dpp'' in the mantle of gastropods correlates with asymmetric shell coiling."] ''EvoDevo'' 2013, 4:15. {{doi|10.1186/2041-9139-4-15|doi-access=free}}.</ref>

=== Mixed coiling populations === In a few cases, both left- and right-handed coiling are found in the same population.<ref name="Laidlaw 1961" /> Sinistral mutants of normally dextral species and dextral mutants of normally sinistral species are rare but well documented occurrences among land snails in general.<ref name="Laidlaw 1961" /> 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.<ref name="Laidlaw 1961" /> Besides ''[[Amphidromus]]'', the Cuban ''[[Liguus vittatus]]'' (Swainson), [[Haiti]]an ''[[Liguus virgineus]]'' (Linnaeus) (family [[Orthalicidae]]), some [[Hawaii]]an ''[[Partulina]]'' and many Hawaiian ''[[Achatinella]]'' (family [[Achatinellidae]]), as well as several species of Pacific islands ''[[Partula (gastropod)|Partula]]'' (family [[Partulidae]]), are known to have mixed dextral-sinistral populations.<ref name="Laidlaw 1961" />

A possible exception may concern some of the European [[Clausiliidae|clausiliids]] of the subfamily [[Alopiinae]].<ref name="Laidlaw 1961" /> They are obligatory [[calciphile]]s living in isolated colonies on limestone outcrops.<ref name="Laidlaw 1961" /> 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.<ref name="Laidlaw 1961" /> Soos (1928, pp. 372–385){{full citation needed|date=December 2013}} summarized previous discussions of the problem and concluded that the right- and left-handed populations were distinct species.<ref name="Laidlaw 1961" /> Others have stated that these populations were not distinct, and the question is far from settled.<ref name="Laidlaw 1961" /> The Peruvian clausiliid, ''[[Nenia callistoglypta]]'' Pilsbry (1949, pp. 216–217),{{full citation needed|date=December 2013}} also has been described as being an amphidromine species.<ref name="Laidlaw 1961" />

The genetics of reverse coiling in a rare dextral mutant of another clausiliid, ''[[Alinda biplicata]]'' (Montagu), has been studied by Degner (1952).<ref name="Laidlaw 1961" /> The mechanism is the same as in ''[[Radix peregra]]'' (Müller), with the direction of coiling determined by a simple [[Mendelian]] recessive.<ref name="Laidlaw 1961" />

== Standard ways of viewing a shell == [[File:Arianta arbustorum - Braunau-1968.jpg|thumb|Four views of a shell of ''[[Arianta arbustorum]]'': Apertural view (top left), lateral view (top right), apical view (bottom left), and umbilical view (bottom right)]] 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 entirety, with its aperture facing the viewer, and the apex at the top. If the aperture is on the right side when viewed like this, then the shell-coiling is "right-handed" or dextral; if the aperture is on the left side when viewed like this, the shell has "left-handed" or sinistral shell-coiling. * '''abapertural view''' (or '''dorsal view'''): the shell is shown with its aperture 180° away from the viewer, and with the apex at the top. * '''umbilical view''' (or '''basal view'''): the shell is shown viewed directly from below. In most cases where there is an umbilicus, this is in clear view. * '''apical view''': the shell is shown looking down directly onto the apex.

== Description == [[File:Atlanta lesueurii 2.png|left|thumb|Detail of apical view of the shell of ''[[Atlanta lesueurii]]'' showing clearly visible protoconch, that has 2¼ [[whorl (mollusc)|whorls]]]] The shell begins with the larval shell, the (usually) minute [[embryo]]nic whorls known as the [[protoconch]], which is often quite distinct from the rest of the shell and has no growth lines. From the protoconch, which forms the apex of the [[spire (mollusc)|spire]], the coils or [[whorl (mollusc)|whorls]] of the shell gradually increase in size. Normally the whorls are circular or elliptical in section. 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]]''). They also can overlap the earlier whorls such that the earlier whorls may be largely or wholly covered by the later ones. When an angulation occurs, the space between it and the [[suture (anatomical)|suture]] above it constitutes the area known as the "shoulder" of the shell. The shoulder angle may be smooth or keeled, and may sometimes have nodes or spines.

The most simple form of [[sculpture (mollusc)|sculpture]] of the gastropod shell consists of longitudinal ridges, and/or transverse ridges. Primary spirals may appear in regular succession on either side of the first primary, which generally becomes the shoulder angle if angulation occurs. Secondary spirals may 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]]'').

When a row of spines is formed at the edge or outer lip of the shell during a resting period, this feature sometimes remains behind as a [[varix (mollusc)|varix]] as in (''[[Murex]]'') and many of the [[Ranellidae]]. Varices may also be formed by simple expansion of the outer lip, and a subsequent resumption of growth from the base of the expansion.

The [[aperture (mollusc)|aperture]] 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 by an anterior notch. This, in some types (''[[Fusinus]]'', etc.) it is drawn out into an anterior [[siphonal canal]], 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]]''). 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 [[lira (mollusc)|lirae]] are sometimes found, and these occasionally may be strong and tooth-like (''[[Nerinea]]''). 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 (mollusc)|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]]'').

Many [[Holocene|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]], 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 (gastropod)|operculum]], 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.<ref name="Grabau" />

== Parts of the shell == [[File:Czescimuszli shells.jpg|thumb|The shell of a large land snail (probably ''[[Helix pomatia]]'') with parts broken off to show the interior structure. 1 – [[Umbilicus (mollusk)|umbilicus]] 2 – [[plait (gastropod)|columellar plait]] 3 – [[Aperture (mollusc)|aperture]] 4 – [[Columella (gastropod)|columella]] 5 – [[Suture (gastropod)|suture]] 6 – [[body whorl]] 7 – [[Apex (mollusc)|apex]] ]]

The terminology used to describe the shells of gastropods includes: * [[Aperture (mollusc)|Aperture]]: the opening of the shell * [[Lip (gastropod)|Lip]]: ''peristome'': the margin of the aperture * [[Apex (mollusc)|Apex]]: the smallest few whorls of the shell * [[Body whorl]] (or last whorl): the largest whorl in which the main part of the visceral mass of the mollusk is found * [[Columella (mollusk)|Columella]]: the "little column" at the axis of revolution of the shell * [[Operculum (gastropod)|Operculum]]: the "trapdoor" of the shell * [[Parietal callus]]: a ridge on the inner lip of the aperture in certain gastropods * [[Periostracum]]: a thin layer of organic "skin" which forms the outer layer of the shell of many species * [[Peristome]]: the part of the shell that is right around the aperture, also known as the lip * [[Plait (gastropod)|Plait]]: folds on the columella. * [[Protoconch]]: the nuclear or embryonic whorl; the larval shell, often remains in position even on an adult shell * [[Sculpture (mollusc)|Sculpture]]: ornamentation on the outer surface of a shell ** [[Lira (mollusc)|Lira]]: raised lines or ridges on the surface of the shell * [[Siphonal canal]]: an extension of the aperture in certain gastropods * [[Spire (mollusc)|Spire]]: the part of the shell above the body whorl. * [[Suture (gastropod)|Suture]]: The junction between [[whorl (mollusc)|whorls]] of most gastropods * [[Teleoconch]] : the entire shell without the protoconch; the postnuclear whorls. * [[Umbilicus (mollusk)|Umbilicus]]: 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 (mollusc)|Varix]]: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices * [[Whorl (mollusc)|Whorl]]: each one of the complete rotations of the shell spiral

== Shape of the shell == The overall shape of the shell varies. For example, three groups can be distinguished based on the height – width ratio:<ref name="Falkner">Falkner G., Obrdlík P., Castella E. & Speight M. C. D. (2001). ''Shelled Gastropoda of Western Europe''. München: Friedrich-Held-Gesellschaft, 267 pp.</ref> * oblong – the height is much bigger than the width * globose or conical shell – the height and the width of the shell are approximately the same * depressed – the width is much bigger than the height <gallery> File:Bulgarica denticulata shell.png|oblong shell of ''[[Bulgarica denticulata]]'' File:Sphincterochila candidissima.jpg|globose shell of ''[[Sphincterochila candidissima]]'' File:Elona quimperiana shell 2.jpg|depressed shell of [[Escargot de Quimper]] </gallery>

The following are the principal modifications of form in the gastropod shell.<ref name="Tryon">[[George Washington Tryon]], ''Structural and systematic conchology'', 1882, pp. 43–44</ref> * Regularly spiral: ** Bulloid: bubble-shaped ''[[Bulla (gastropod)|Bulla]]'' ** Coeloconoid a slightly concave conical shell in which the incremental angle increases steadily during growth (see: ''[[Calliostoma]]'') ** Cone-shaped, obconic. ''[[Conus]] '' ** Contabulate, short, with shouldered whorls ** Convolute: aperture as long as the shell, nearly or quite concealing the spire. ''[[Cypraea]]'' ** Cylindrical, pupiform. ''[[Lioplax]]'', ''[[Pupa (gastropod)|Pupa]]'' ** Cyrtoconoid: approaching a conical shape but with convex sides (see: ''[[Gibbula]]'') ** Depressed, lenticular. ''[[Ethalia carneolata]]'' ** Discoidal. ''[[Elachorbis]]'' ** Ear-shaped. ''[[Haliotis]]'' ** Elongated, subulate, elevated. ''[[Terebra]]'' ** Few-whorled. ''[[Helix pomatia]]''. ** Fusiform, spindle-shaped; thick in the middle, the ends tapering ''[[Fusinus]]'' ** Gibbous. Whorls swelled beyond the normal contour of increase (usually on the aperture side ). ''[[Streptaxis]]''. ** Globular. ''[[Natica]]'' ** Imperforate: without any hollow or hole ** Inflected: bent inwards ** Irregular: or of no regular, equal or determinate form ** Laciniated: as if irregularly cut or lacerated ** Linear: very narrow and of equal breath ** Lobe: a notched division ** Many-whorled. ''[[Strange many-whorled land snail|Millerelix peregrina]]''. ** Opaque: not transparent ** Orbicular: nearly round ** Reflected: bent outwards ** Reticular: resembling network ** Short, bucciniform. ''[[Buccinum]]'' ** Sinuated: taking a bend inwards ** Spatulate: spoon-shaped ** Striated: marked by raised or indented lines ** Symmetrical: proportionate, the sides or parts equal ** Transverse: cross-ways ** Trochiform, pyramidal, conical with a flat base. ''[[Trochus]]'' ** Tubular: pipe or tube-like: hollow within ** Truncate: ending abruptly as if cut off ** Tumid: swelled or bulged ** Turbinated: conical, with rounded base. ''[[Turbo (gastropod)|Turbo]]'' ** Turreted, turriculate, babylonic; an elongated shell with the whorls angulated or shouldered on their upper part. ''[[Turritella]]'' ** Scalariform, whorls not impinging. ''[[Epitonium scalare]]'' * Irregularly spiral, evolute. ''[[Siliquaria]]'', ''[[Vermetus]]'' * Tubular. * Shield-shaped. ''[[Umbraculum (gastropod)|Umbraculum]]'' * Boat-shaped, slipper-shaped. ''[[Crepidula]]'' * Conical or limpet-shaped. ''[[Patella (gastropod)|Patella]]'' * Biconic: shaped like two conical shapes that are touching their bases, and tapering at both ends: ''[[Fasciolaria tulipa]]'' * Pear-shaped: a combination of two shapes: ovate-conic and conic. ''[[Ficus (gastropod)|Ficus]]''

Detailed distinction of the shape can be:<ref>Hershler R. & Ponder W. F.(1998). "A Review of Morphological Characters of Hydrobioid Snails". ''[[Smithsonian Contributions to Zoology]]'' '''600''': 1–55. {{hdl|10088/5530}}.</ref><ref>Dance P. S. ''Shells''.{{page needed|date=October 2020}}</ref> <gallery> File:Cellana stellifera shell 2.jpg|cap shape File:Haliotis asinina.jpg|ear shape File:Theodoxus danubialis.jpg|neritiform File:Valvata cristata drawing.jpg|planispiral File:Valvata macrostoma.png|depressed trochiform or valvatiform File:Valvata piscinalis shell.jpg|trochiform File:Hydrobia ventrosa shell.jpg|ovate-conic File:Pyrgula annulata shell.jpg|conic File:VisTachetéI.png|elongate-conic or turriform or cockscrew shape File:Entemnotrochus rumphii.jpg|top shape File:Syrinx aruanus shell.jpg|spindle shape – the sea snail ''[[Syrinx aruanus]]'' has the largest shell of any living gastropod. File:Murex pecten shell 3.jpg|club shape - [[Venus comb murex|Venus Comb Murex]] File:Bullata guerrinii.jpg|egg shape File:Lambis scorpius shell.jpg|irregular shape </gallery>

[[File:Gastropod shell measuring.png|thumb|Schematic representation of the apical, apertural and basal view of a shell, showing 14 different commonly used measurements. Dotted lines represent the orientation axes (except of lower image).]]

== 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]]'', which can be up to 91 cm.<ref name="Wells">Wells F. E., Walker D. I. & Jones D. S. (eds.) (2003) "[http://www.museum.wa.gov.au/dampier/documents/pdf/taylor-glovergiants.pdf Food of giants – field observations on the diet of ''Syrinx aruanus'' (Linnaeus, 1758) (Turbinellidae) the largest living gastropod] {{Webarchive|url=https://web.archive.org/web/20091003083723/http://www.museum.wa.gov.au/dampier/documents/pdf/taylor-glovergiants.pdf |date=2009-10-03 }}". The Marine Flora and Fauna of Dampier, Western Australia. Western Australian Museum, Perth.</ref>

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 for the variability of shells during [[evolution]] include mechanical stability,<ref>Britton J. C (1995) "The relationship between position on shore and shell ornamentation in 2 size-dependent morphotypes of ''Littorina striata'', with an estimate of evaporative water-loss in these morphotypes and in ''Melarhaphe neritoides''". ''Hydrobiologia'' '''309''': 129–142. {{doi|10.1007/BF00014480}}.</ref> defense against predators<ref>Wilson A. B., Glaubrecht M. & Meyer A. (March 2004) "Ancient lakes as evolutionary reservoirs: evidence from the thalassoid gastropods of Lake Tanganyika". ''Proceedings of the Royal Society London Series B – Biological Sciences'' '''271''': 529–536. {{doi|10.1098/rspb.2003.2624}}.</ref> and climatic selection.<ref>Goodfriend G. A. (1986) "Variation in land-snail shell form and size and its causes – a Review". ''Systematic Zoology'' '''35''': 204–223.</ref><ref name="Pfenninger 2005" />

The shells of some gastropods have been reduced or partly reduced during their [[evolution]]. This reduction can be seen in all [[slug]]s, in semi-slugs and in various other marine and non-marine gastropods. Sometimes the reduction of the shell is associated with a predatory way of feeding.{{cn|date=February 2026}}

Some taxa lost the coiling of their shell during evolution.<ref name="Collin" /> According to [[Dollo's law]], 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]] that changed their developmental timing ([[heterochrony]]) and gained back ([[re-evolution]]) a coiled shell from the previous condition of an uncoiled [[limpet]]-like shell.<ref name="Collin">Collin R. & Cipriani R. (22 December 2003) "Dollo's law and the re-evolution of shell coiling". ''[[Proceedings of the Royal Society B]]'' '''270'''(1533): 2551–2555. {{doi|10.1098/rspb.2003.2517}} {{PMID|14728776}}.</ref>

== Taphonomic implications == {{See also|Taphonomy}}

In large enough quantities, gastropod shells can have enough of an impact on environmental conditions to affect the ability of organic remains in the local environment to fossilize.<ref name="tanke-brett-surman-2001" /> For example, in the [[Dinosaur Park Formation]], fossil hadrosaur eggshell is rare.<ref name="tanke-brett-surman-2001" /> This is because the breakdown of [[tannins]] from local [[coniferous]] vegetation would have caused the ancient waters to become acidic.<ref name="tanke-brett-surman-2001" /> Eggshell fragments are present in only two [[microfossil]] sites, both of which are predominated by the preserved shells of invertebrate life, including gastropods.<ref name="tanke-brett-surman-2001" /> It was the slow dissolution of these shells releasing [[calcium carbonate]] into the water that raised the water's [[pH]] high enough to prevent the eggshell fragments from dissolving before they could be fossilized.<ref name="tanke-brett-surman-2001">Tanke, D.H. and Brett-Surman, M.K. 2001. Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206–218. In: Mesozoic Vertebrate Life – New Research Inspired by the Paleontology of Philip J. Currie. Edited by D.H. Tanke and K. Carpenter. Indiana University Press: Bloomington. xviii + 577 pp.</ref>

== Variety of forms == <gallery> File:Turritella communis fossiel.jpg|''[[Turritella communis]]'', many-whorled shell of tower snail File:5-Turritele.jpg|X-ray image of ''[[Turritella]]'' File:Cypraea nebrites.jpg|Shell of marine cowry snail – ''[[Cypraea nebrites]]'' File:Helix pomatia MHNT.ZOO.2002.0.33.jpg|''[[Helix pomatia]]'' File:Naturalis Biodiversity Center - ZMA.MOLL.347109 - Epitonium scalare.jpg|''[[Epitonium scalare]]'' spiral shell File:Ovula ovum,.jpg|Ovula ovum </gallery>

<gallery> File:3-porcelaine-cyprea.jpg|X-ray image of ''[[Cypraea]]'' File:2-Dolium geant.jpg|[[X-ray]] image of the shell of ''[[Tonna galea]]'' File:4-Triton.jpg|''[[Charonia]]'' File:6-Venus.jpg|''[[Murex pecten]]'' File:Gastropod thin section PP.jpg|Thin section in plane-polarized light of microscopic gastropod shell, from [[Holocene]] lagoonal sediment of Rice Bay, [[San Salvador Island]], Bahamas. Scale bar 500 μm. File:Mauritia arabica,.jpg|Mauritia arabica </gallery>

== References == This article incorporates public domain text from references,<ref name="Burch 1962">John. B. Burch (1962). [http://catalog.hathitrust.org/Record/002140007 "How to Know the eastern land snails; pictured-key for determining the land snails of the United States occurring east of the Rocky Mountain Divide"] WM. C. Brown Company Publishers, Dubuque, Iowa. 214 pages.</ref><ref name="Laidlaw 1961">[[File:PD-icon.svg|15x15px]] [[Frank Fortescue Laidlaw|Laidlaw F. F.]] & [[Alan Solem|Solem A.]] (1961). [[iarchive:landsnailgenusam414laid|"The land snail genus ''Amphidromus'': a synoptic catalogue"]]. ''Fieldiana Zoology'' '''41'''(4): 505–720.</ref><ref name="Grabau">[[Amadeus William Grabau|Grabau A. W.]] & [[Hervey Woodburn Shimer|Shimer H. W.]] (1909) ''North American Index Fossils'' [[iarchive:northamericanind017749mbp|''Invertebrates. Volume I.'']]. A. G. Seiler & Company, New York. pages [https://archive.org/stream/northamericanind017749mbp#page/n593/mode/2up page 582]–584.</ref> and CC-BY-2.0 text from reference.<ref name="Pfenninger 2005">Pfenninger M., Hrabáková M., Steinke D. & Dèpraz A. (4 November 2005) "Why do snails have hairs? A Bayesian inference of character evolution". ''BMC Evolutionary Biology'' '''5''': 59. {{doi|10.1186/1471-2148-5-59|doi-access=free}}</ref> {{Reflist|30em}}

== Further reading == {{sea shell topics}} ;About chirality * van Batenburg1 F. H. D. & Gittenberger E. (1996). "Ease of fixation of a change in coiling: computer experiments on chirality in snails". ''[[Heredity (journal)|Heredity]]'' '''76''': 278–286. {{doi|10.1038/hdy.1996.41}}. * Wandelt J. & Nagy L. M. (24 August 2004) "Left-Right Asymmetry: More Than One Way to Coil a Shell". ''[[Current Biology]]'' '''14'''(16): R654–R656. {{doi|10.1016/j.cub.2004.08.010}}

== External links == {{Commons category|Gastropoda shells}} * [ftp://ftp.fao.org/docrep/fao/009/y4160e/y4160e08.pdf Gastropods by J. H. Leal]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}} – Information on some gastropods of the tropical Western Atlantic, specifically the Caribbean Sea, with relevance to the fisheries in that region * [https://web.archive.org/web/20081013202349/http://wwwpaztcn.wr.usgs.gov/rsch_highlight/articles/200401.html Radiocarbon Dating of Gastropod Shells] * Nair K. K. & Muthe P. T. (18 November 1961) [http://www.nature.com/nature/journal/v192/n4803/abs/192674b0.html "Effect of Ribonuclease on Shell Regeneration in ''Ariophanta'' sp."]. ''Nature'' '''192''': 674–675. {{doi|10.1038/192674b0}}. * {{in lang|es}} Antonio Ruiz Ruiz, Ángel Cárcaba Pozo, Ana I. Porras Crevillen & José R. Arrébola Burgos [http://www.juntadeandalucia.es/medioambiente/web/1_consejeria_de_medio_ambiente/dg_gestion_medio_natural/biodiversidad/static_files/fauna/caracoles_terrestres/guia_caracoles.pdf ''Caracoles Terrestres de Andalúcia. Guía y manual de identificación'']. 303 pp., {{ISBN|84-935194-2-1}}. ([http://www.juntadeandalucia.es/medioambiente/site/web/menuitem.a5664a214f73c3df81d8899661525ea0/?vgnextoid=21999bd4dd10a110VgnVCM1000000624e50aRCRD&vgnextchannel=3259b19c7acf2010VgnVCM1000001625e50aRCRD&lr=lang_es from website])

[[Category:Gastropod anatomy]] [[Category:Mollusc shells]]