Lip (gastropod)

{{Short description|Mollusc shell}} {{No footnotes|date=March 2026}} thumb|A shell of ''Helix pomatia'' with part of shell removed; the outer lip and the inner lip are clearly visible In the shell of gastropod mollusks (a snail shell), the '''lip''' is the free margin of the '''peristome''' (synonym: peritreme) or aperture (the opening) of the gastropod shell.

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 that adjoins the aperture is known as the '''parietal wall'''.

The outer lip is the edge of the aperture, which can be thin (acute), thickened, or flared. This outer lip is the actual edge of the aperture that is actively deposited by the mantle margin during growth. The outer lip is usually thin and sharp in immature shells, and in some adults (e.g. the land snails ''Helicella'' and ''Bulimulus''). However, in some other land snails and in many marine species the outer lip is ''thickened'' (also called ''callused''), or ''reflected'' (turned outwards). This gives a varix: the thickened, flared (as in Cypraea or Bursa, or reflected (as in some Strombus) final margin that is often laid down when the snail reaches its adult size and stops growing. This varix is therefore an important indicator of the shell's developmental stage.

In some other marine species it is curled inwards (''inflected''), as in the cowries such as ''Cypraea''. It can also be expanded, as in ''Strombus''; it can have finger-like processes (''digitate'') or it can be fringed with spines (''foliated''), as in ''Murex''.

The lip is called ''emarginate'' when it is incised or slit, as in ''Pleurotomaria''. It is described as ''effuse'' when the basal or anterior extremity is slightly produced, depressed or reflected, as in Thiara. When the lip is bent into an "s" curve it is called ''sinuous'', as is the case in the sigmoidal margin of ''Janthina exigua''.

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

The varices, denticles (teeth), or severe narrowing of the lip defend the animal against shell-peeling predators like crabs. A thick, narrow lip also restricts access to the snail's soft tissues when it withdraws into the shell.

==References== * George Washington Tryon, Structural and systematic conchology, 1882, p. 46 * Chatzinikolaou, E., Keklikoglou, K., & Grigoriou, P. (2021). Morphological Properties of Gastropod Shells in a Warmer and More Acidic Future Ocean Using 3D Micro-Computed Tomography. Frontiers in Marine Science, 8. * Checa, A. G., & Jiménez-Jiménez, A. P. (1998). Constructional morphology, origin, and evolution of the gastropod operculum. Paleobiology, 24, 109–132. * McNair, C. G., Kier, W. M., LaCroix, P. D., & Linsley, R. M. (1981). The functional significance of aperture form in gastropods. Lethaia, 14, 63–70. * Sälgeback, J., & Savazzi, E. (2006). Constructional morphology of cerithiform gastropods. Paleontological Research, 10, 233–259. * Vermeij, G. J. (2017). Shell features associated with the sand-burying habit in gastropods. Journal of Molluscan Studies, 83, 153–160.

<gallery> 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'' </gallery>

Category:Mollusc shells