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 shells, scaphopod shells, and also for Nautilus and ammonite 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 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 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 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, 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.
The margin of the aperture is sometimes continuous or entire ( Epitonium ), or becomes continuous in the adult ( Caracolus ); very frequently it is interrupted, the left side of the aperture being formed only by the body whorl. When the aperture is called holostomatous, this means that the aperture is rounded or entire, uninterrupted by the siphonal canal, notch, or by any other extension
For convenience of reference, the margin of a gastropod aperture is divided into three areas:
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 .[ citation needed ]
The shape of the aperture in a gastropod shell can be:
The shells of juveniles in some species (especially some families of land snails) 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.
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:
The gastropod shell is part of the body of a 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 storage. Some gastropods appear shell-less (slugs) but may have a remnant within the mantle, or the shell is reduced such that the body cannot be retracted within (semi-slug). Some snails also possess an operculum that seals the opening of the shell, known as the aperture, which provides further protection. The study of mollusc shells is known as conchology. The biological study of gastropods, and other molluscs in general, is malacology. Shell morphology terms vary by species group.
The columella or pillar 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, sliced in half vertically, or viewed as an X-ray image.
Strobilopsidae is a family of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the superfamily Pupilloidea.
Anostoma depressum is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusc in the family Odontostomidae.
The following is a glossary of common English language and scientific terms used in the description of gastropods.
Ericusa is a small taxonomic genus of medium-sized predatory marine gastropod molluscs in the family Volutidae, the volutes.
Benthomangelia trophonoidea is a species of sea snail, a marine gastropod mollusk in the family Mangeliidae.
Aforia trilix is a species of sea snail, a marine gastropod mollusk in the family Cochlespiridae.
Aoteadrillia wanganuiensis is a species of sea snail, a marine gastropod mollusk in the family Horaiclavidae.
Crassopleura maravignae is a species of sea snail, a marine gastropod mollusk in the family Drilliidae. It is the only known species within the genus Crassopleura.
Tritia denticulata is a species of sea snail, a marine gastropod mollusk in the family Nassariidae, the Nassa mud snails or dog whelks.
Halgyrineum louisae is a species of predatory sea snail, a marine gastropod mollusk in the family Cymatiidae.
Atractophaedusa smithi is a species of air-breathing land snail, a terrestrial gastropod mollusk in the family Clausiliidae, the door snails.
In the shell of gastropod mollusks, the lip is the free margin of the peristome or aperture of the gastropod shell.
Gastrocopta sp. from the Viernheim borehole is an undescribed species of a very small fossil land snail, a terrestrial pulmonate gastropod mollusk in the family Vertiginidae, the whorl snails.
Halongella schlumbergeri is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Plectopylidae.
Antiguraleus mundus is a species of sea snail, a marine gastropod mollusc in the family Mangeliidae.
Mangelia gouetensis is an extinct species of sea snail, a marine gastropod mollusk in the family Mangeliidae.
Perrottetia aquilonaris is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Streptaxidae.
Zospeum zaldivarae is a cave-dwelling species of air-breathing land snail.