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

gastropod shell The gastropod shell is part of the body of a Gastropoda, gastropod or snail, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium s ...

s, scaphopod shells, and also for '' Nautilus'' and

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

shells. The word is not used to describe

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

shells, where a natural opening between the two shell valves in the closed position is usually called a '' gape''. Scaphopod shells are tubular, and thus they have two openings: a main anterior aperture and a smaller posterior aperture. As well as the aperture, some gastropod shells have additional openings in their shells for respiration; this is the case in some Fissurellidae (keyhole limpets) where the central smaller opening at the apex of the shell is called an orifice, and in the Haliotidae (abalones) where the row of respiratory openings in the shell are also called orifices.

In gastropods

In some prosobranch gastropods, the aperture of the shell can be closed, and even completely sealed, with a sort of door or operculum. The aperture of many snail shells is more or less round, rounded, elliptical or oval. This shape usually corresponds roughly to the cross-section of the

body whorl The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk. In gastropods In gastropods, th ...

of the shell. The aperture of a snail shell can have many other forms: semicircular, trilobate or

auriculate The following is a list of terms which are used to describe leaf morphology in the description and taxonomy of plants. Leaves may be simple (a single leaf blade or lamina) or compound (with several leaflets). The edge of the leaf may be regular o ...

. In some gastropods, the aperture is narrowed by protruding plaits, which help make the soft parts of the animal less vulnerable to predation. The growth of the shell is provided for by non-continuous addition of minute layers to the aperture margin (also called ''peristome'') from the mantle border, the principal agent in the secretion of the shell.

Terminology

The margin of the aperture is sometimes continuous or entire (''

Epitonium ''Epitonium'' is a genus of small predatory sea snails, marine gastropod mollusks. ''Epitonium'' is the type genus of the family Epitoniidae, the wentletraps.Bouchet, P.; Gofas, S. (2010). Epitonium Röding, 1798. In: Bouchet, P.; Gofas, S.; Ro ...

''), or becomes continuous in the adult (''

Caracolus ''Caracolus'' is a genus of air-breathing land snails, terrestrial pulmonate gastropod mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks ...

''); very frequently it is interrupted, the left side of the aperture being formed only by the

. When the aperture is called holostomatous, this means that the aperture is rounded or entire, uninterrupted by the

siphonal canal The siphonal canal is an anatomical feature of the shells of certain groups of sea snails within the clade Neogastropoda. Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is ...

, notch, or by any other extension For convenience of reference, the margin of a gastropod aperture is divided into three areas: * The parietal wall with the outer lip (''labrum''): the area next to the penultimate whorl of the shell. The siphonal notch is situated at the top. * The columellar wall with the columellar lip (''labium''): the wall next to the columella. The

is situated at its base. * The palatal wall: the outer free wall of the final whorl of the shell. The aperture is descending or deflected, when it does not follow the spiral of the shell, but turns downwards (such as in '' Helix''). Sometimes it departs from contact with the preceding whorl (as frequently in '' Cylindrella''.

Shape

The shape of the aperture in a gastropod shell can be: * auriform, ear-shaped, as in '' Auriculella''* bean - for example: '' Aerotrochus perdepressa'' * circular, rotundate, orbicular * claw-shaped aperture - Some Melongenidae have a claw-shaped aperture * crescent-shaped - examples: '' Hyalosagda arboreoides'', '' Dialeuca conspersula'' * distorted aperture -

Personidae The Personidae are a family of sea snails, marine gastropod molluscs in the order Littorinimorpha Littorinimorpha is a large order of snails, gastropods, consisting primarily of sea snails ( marine species), but also including some freshwater ...

has a distorted aperture * linear, narrow. '' Cypraea'', ''

Conus ''Conus'' is a genus of predatory sea snails, or cone snails, marine gastropod mollusks in the family Conidae.Bouchet, P.; Gofas, S. (2015). Conus Linnaeus, 1758. In: MolluscaBase (2015). Accessed through: World Register of Marine Species at ...

'' * longitudinal, when its greatest diameter is parallel with the axis of volution * lunate, semilunar, semicircular or half-moon '' Nerita'' * transverse, when its greatest diameter is at a right angle to the axis of volution * oblique, when the greatest diameter is oblique to the axis of volution * oblong, much longer than it is wide, rounded above and below * oval or teardrop - examples: '' Geomelania minor'', ''

Urocoptis ambigua ''Urocoptis'' is a genus of air-breathing land snails, terrestrial pulmonate gastropod mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). ...

'' * ovate, egg-shaped * patulous, when dilated and then compressed, when diminished at its entrance * quadrate, roughly square or rectangular, as in ''

Architectonica ''Architectonica'' is a genus of sea snails in the family Architectonicidae. Species According to the World Register of Marine Species, the following species are included in the genus Architectonica: * '' Architectonica arcana'' Bieler, 1993 ...

'' * rounded, the circle slightly interrupted; example: '' Valvata sincera''. Consequently, there is no

but simply an opening for respiration. * semicircular * with the top angle acute - examples: '' Leptopeas micrum'', '' Varicella griffithii''"Aperture shape Taller than wide, top angle acute"
accessed 3 January 2011. * triangular, as in '' Janthina'' * without an aperture File:Conus-betulinus-001.jpg, ''

Conus betulinus ''Conus betulinus'', common name the betuline cone, is a species of sea snail, a marine gastropod mollusk in the family Conidae, the cone snails and their allies. These snails are predatory and venomous. They are capable of "stinging" human ...

'' has a long and narrow aperture. File:Indrella ampulla shell.jpg, ''

Indrella ampulla ''Indrella'' is a monotypic genus containing the single species ''Indrella ampulla'', a tropical terrestrial air-breathing gastropod mollusk in the family Ariophantidae. It is endemic to the Western Ghats of India.2007''Tropical Land Snail Div ...

'' has an oval aperture. File:Valvata sincera shell lateral.jpg, '' Valvata sincera'' has a rounded aperture. File:Valvata piscinalis drawing apertural view.jpg, Rounded aperture of '' Valvata piscinalis'' can be covered by an operculum. File:Theodoxus danubialis.jpg, ''

Theodoxus danubialis ''Theodoxus danubialis'' is a species of small freshwater snail with an operculum, an aquatic gastropod mollusk in the family Neritidae, the nerites. The species is considered as endangered in Germany, Austria and in the Czech Republic. ...

'' has a semicircular aperture. File:Patella vulgata Napfschnecke.jpg, Some shells, like this '' Patella vulgata'', have no aperture.

With teeth

The shells of juveniles in some species (especially some families of

land snails A land snail is any of the numerous species of snail that live on land, as opposed to the sea snails and freshwater snails. ''Land snail'' is the common name for terrestrial gastropod mollusks that have shells (those without shells are known ...

) have a simple aperture with a sharp edge, but after reaching adult size the aperture of the shell finally acquires adult characters, consisting of a thickened, reflected, inflected or lipped edge, which is sometimes more or less contracted by inflected calcareous projections known as teeth. These teeth may be outer lip teeth, columellar teeth or parietal teeth. Aperture teeth

Folds or plicae are named by their position in the aperture, as follows. The numbers refer to those in the diagram shown opposite: Folds or plicae Lamellae are named as follows:

References

{{Gastropod anatomy Mollusc anatomy Cephalopod zootomy Gastropod anatomy

In gastropods

Terminology

Shape

With teeth

See also

References