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.
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 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. An excellent source for terminology of the gastropod shell is "How to Know the Eastern Land Snails" by John B. Burch now freely available at the Hathi Trust Digital Library.
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 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.
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). 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 .
Helicella is a genus of small to medium-sized, air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Geomitridae, the hairy snails and their allies.
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. Bulimulus is the type genus of the subfamily Bulimulinae. The shells are high and conical, and are medium-sized, ranging from about 20 mm to about 50 mm long.
Cypraea is a genus of medium-sized to large sea snails or cowries, marine gastropod mollusks in the family Cypraeidae, the cowries.
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 .
Pleurotomaria is an extinct genus of sea snails, marine gastropod mollusks in the family Pleurotomariidae.
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.
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.
Nerita is a genus of medium-sized to small sea snails with a gill and an operculum, marine gastropod molluscs in the family Neritidae, the nerites.
Pythia is a genus of small air-breathing salt marsh snails, pulmonate gastropod mollusks in the family Ellobiidae.
