In the shell of a gastropod mollusk (a snail shell), the margin of 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....

 (the opening) is called the lip or the peristome. In other words, this is a term used to describe part of 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...

 anatomy

Anatomy

Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

.

In dextral (right-handed) shells (most snail shells are right-handed), the right side or outer side of the aperture is known as the outer lip (labrum). The left side of the aperture is known as the inner lip or columellar lip (labium) if there is a pronounced lip there. In those species where there is no pronounced lip, the part 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...

 that adjoins the aperture is known as the parietal wall.

The outer lip is usually thin and sharp in immature shells, and in some adults (e.g. the land snails Helicella

Helicella

Helicella is a genus of small to medium-sized, air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Hygromiidae, the hairy snails and their allies.-Species:Species within the genus Helicella include:...

and Bulimulus

Bulimulus

Bulimulus is a genus of small to medium-sized tropical or sub-tropical, air-breathing land snails, pulmonate gastropod mollusks in the subfamily Bulimulinae.-Shell description:...

). However in some other land snails and in many marine species the outer lip is thickened, or reflected (turned outwards). In some other marine species it is curled inwards (inflected), as in the cowries such as 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...

. It can also be expanded, as in Strombus

Strombus

Strombus, common name the true conchs, is a genus of medium to large sea snails with an operculum, marine gastropod mollusks in the family Strombidae, the true conchs and their immediate relatives....

; it can have finger-like processes (digitate) or it can be fringed with spines (foliated), 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"...

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The lip is called emarginate when it is incised or slit, as in Pleurotomaria

Pleurotomaria

Pleurotomaria is a genus of sea snails, marine gastropod mollusks in the family Pleurotomariidae.-Species:Species within the genus Pleurotomaria include:Species brought into synonymy :...

. It is described as effuse when the basal or anterior extremity is slightly produced, depressed or reflected, as in Thiara

Thiara (gastropod)

Thiara is a genus of freshwater snails, aquatic gastropod mollusks in the family Thiaridae.-Species:Species with accepted names within the genus Thiara include:* Thiara rodericensis...

. When the lip is bent into an "s" curve it is called sinuous, as is the case in the sigmoidal margin ofJanthina exigua

Janthina exigua

Janthina exigua, also known as the dwarf janthina, is a species of small holoplanktonic sea snail, a marine gastropod mollusk in the family Janthinidae, the violet snails or purple storm snails.- Distribution :...

.

Interiorly when the lip has teeth it is dentate as in 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:...

. When the teeth are rib-like it is described as plicate or lamellate. When it has numerous large plications, nodules or teeth it is known as ringent, as in Pythia

Pythia (gastropod)

Pythia is a genus of small air-breathing salt marsh snails, pulmonate gastropod mollusks in the family Ellobiidae.Pythia is the type genus of the subfamily Pythiinae.- Species :Species in the genus Pythia include:* Pythia cecillei...

. The lip is called sulcate, when it is grooved within. It is known as labiate or marginate, when it is callously thickened near the margin.