In the shell of

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s (a snail shell), the lip is the free margin of the peristome (synonym: peritreme) or

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

(the opening) of the

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 ...

. 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 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, the b ...

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'' is a genus of small to medium-sized, air-breathing land snails, terrestrial pulmonate gastropod mollusks in the subfamily Helicellinae of the family Geomitridae, the hairy snails and their allies. MolluscaBase eds. (2021). Mollusc ...

'' and ''

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 within the family Bulimulidae.Breure A. S. H. & Romero P. (2012). "Support and su ...

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

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 ficoides'' (Hutton, 1873) * '' Cypr ...

''. It can also be expanded, as in ''

Strombus ''Strombus'' is a genus of medium to large sea snails, marine gastropod molluscs in the family Strombidae, which comprises the true conchs and their immediate relatives. The genus ''Strombus'' was named by Swedish Naturalist Carl Linnaeus in 175 ...

''; it can have finger-like processes (''digitate'') or it can be fringed with spines (''foliated''), as in ''

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 called "murexes" or "rock snails".Houart, R.; Gofas, S. (2010). Murex Linnaeus, 1758 ...

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

Pleurotomaria ''Pleurotomaria'' is an extinct genus of sea snails, marine gastropod molluscs in the family Pleurotomariidae. Species With current taxonomic changes the genus ''Pleurotomaria'' has been reserved exclusively for fossil species (denoted with † ...

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

Thiara ''Thiara'' is a genus of freshwater snails, aquatic gastropod mollusks in the subfamily Thiarinae of the family Thiaridae. Species Species with accepted names within the genus ''Thiara'' include: * † '' Thiara aldrichi'' Palmer, 1944 * ' ...

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

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 Epitoniidae, the violet snails or purple storm snails.MolluscaBase (2019). MolluscaBase. Janthina exig ...

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

Nerita ''Nerita'' is a genus of medium-sized to small sea snails with a gill and an operculum, marine gastropod molluscs in the subfamily Neritinae of the family Neritidae, the nerites.MolluscaBase eds. (2021). MolluscaBase. Nerita Linnaeus, 1758. A ...

''. 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 (; grc, Πυθία ) was the name of the high priestess of the Temple of Apollo (Delphi), Temple of Apollo at Delphi. She specifically served as its oracle and was known as the Oracle of Delphi. Her title was also historically glossed i ...

''. 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.

References

George Washington Tryon George Washington Tryon Jr. (20 May 1838 – 5 February 1888) was an American malacologist who worked at the Academy of Natural Sciences in Philadelphia. Biography George Washington Tryon was the son of Edward K. Tryon and Adeline Savidt. ...

, Structural and systematic conchology, 1882, p. 46 File:Neobernaya spadicea 001.jpg, Inflected lip of ''Neobernaya spadicea'' File:MurexPectenGlobal.jpg, Foliated (fringed with spines) lip of ''Murex pecten'' File:Mikadotrochus hirasei 01.jpg, Slit lip of ''Mikadotrochus hirasei'' File:Thiara winteri 002.jpg, Depressed lip of ''Thiara winteri'' File:Pythia scarabaeus shell.jpg, Ringent lip of ''Pythia scarabaeus'' {{Gastropod anatomy Mollusc shells