The aperture is an opening in certain kinds of mollusc 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...

s: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc.

The term aperture is used for the main opening in gastropod shell

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

s, scaphopod shells, and also for Nautilus

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...

 and 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...

 shells.

The word is not used to describe bivalve 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

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...

 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

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 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

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...

 of the shell.

The aperture of a snail shell can have many other forms: semicircular, trilobate or auriculate. In some gastropods, the aperture is narrowed by protruding plaits

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....

, 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

Epitonium is a genus of small predatory sea snails, marine gastropod mollusks. Epitonium is the type genus of the family Epitoniidae, the wentletraps....

), or becomes continuous in the adult (Caracolus

Caracolus

Caracolus is a genus of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Pleurodontidae.- Species :Species within the genus Caracolus include:* Caracolus albilabris Lamarck* Caracolus caracolla Oken...

); very frequently it is interrupted, the left side of the aperture being formed only by the body-whorl.

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
  Siphonal notch
  A siphonal notch is a feature of the shell anatomy in some sea snails, marine gastropod mollusks.In these particular groups of sea snails the animal has 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...
  
   is situated at the top.
• The columellar wall with the columellar lip (labium): the wall next to the columella
  Columella
  Lucius Junius Moderatus Columella is the most important writer on agriculture of the Roman empire. Little is known of his life. He was probably born in Gades , possibly of Roman parents. After a career in the army , he took up farming...
  
  . The 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...
  
   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

Helix

A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...

). Sometimes it departs from contact with the preceding whorl (as frequently in Cylindrella).

Shape

The shape of the aperture can be:

• auriform, ear-shaped, as in Auriculella
  Auriculella
  Auriculella is a genus of air-breathing tropical land snails, terrestrial pulmonate gastropod mollusks in the family Achatinellidae.Auriculella is the type genus of the subfamily Auriculellinae.-Species:...
  
  * bean - for example: Aerotrochus perdepressa
• circular, rotundate, orbicular.
• claw-shaped aperture - Some Melongenidae
  Melongenidae
  Melongenidae, the crown conchs, busycon whelks and their relatives, are a taxonomic family of large to very large marine gastropods in the clade Neogastropoda.- Taxonomy :Genera Busycon and Busycotypus were placed in Melongenidae...
  
   have a claw-shaped aperture.
• crescent - examples: Hyalosagda arboreoides
  Hyalosagda arboreoides
  Hyalosagda arboreoides is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Sagdidae....
  
  , Dialeuca conspersula
• distorted aperture - Personidae
  Personidae
  Personidae is a family of sea snails, marine gastropod molluscs in the clade Littorinimorpha.- Taxonomy :* Genus Distorsio Röding, 1798** Distorsio anus Linnaeus, 1758** Distorsio constricta Broderip, 1833...
  
   has a distorted aperture.
• linear, narrow. 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...
  
  , 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 ...
  
  
• longitudinal, when its greatest diameter is parallel with the axis of volution.
• lunate, semilunar, semicircular or half-moon . Nerita
  Nerita
  Nerita is a genus of medium-sized sea snails with a gill and an operculum, marine gastropod molluscs in the family Neritidae, the nerites.This is the type genus of the family Neritidae.-Distribution and habitat:...
  
  .
• transverse, reverse of longitudinal.
• oblique, greatest diameter oblique to the axis.
• oblong, much longer than wide, rounded above and below.
• oval or teardrop - examples: Geomelania minor, Urocoptis ambigua
• ovate, egg-shaped.
• patulous when dilated and compressed when diminished at its entrance.
• quadrate. Architectonica
  Architectonica
  Architectonica is a genus of sea snails, marine gastropod mollusks in the family Architectonicidae, the staircase shells or sundials.-Species:...
  
  
• rounded, the circle slightly interrupted; example: Valvata sincera
  Valvata sincera
  Valvata sincera, common name the mossy valvata or boreal turret snail, is a species of small freshwater snail with a gill and an operculum, an aquatic gastropod mollusk in the family Valvatidae, the valve snails.-Distribution:...
  
  . Consequently there is no 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...
  
   but simply an opening for respiration.
• semicircular
• with top angle acute - examples: Leptopeas micrum, Varicella griffithii
• triangular. Janthina
  Janthina
  Janthina is a genus of small to medium-sized pelagic or planktonic sea snails, marine gastropod molluscs in the family Janthinidae, commonly known as the "purple snails" or the "violet shells". -Description:...
  
  .
• without aperture

With teeth

Juvenile shells retain a sharp aperture for a variable period, but finally acquire adult characters, consisting of a thickened, reflected, inflected or lipped
aperture—which is sometimes more or less contracted by inflected calcareous projections called teeth. These may be outer lip teeth, columellar teeth or parietal treeth.

Folds or plicae are named as follows:

Folds or plicae

1. suprapalatal


2. upper palatal


3. interpalatal


4. lower palatal


5. infrapalatal


6. basal

Lamellae are named as follows:

7. infracolumellar


8. columellar


9. supracolumellar


10. infraparietal


11. parietal


12. angular and twin


13. paraller