Semicassis pyrum , which has a large aperture and a pronounced parietal callus
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 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 the Fissurellidae where the central smaller opening at the apex of the shell is called an orifice, and in the Haliotidae where the respiratory openings are also called orifices.
In gastropods
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 shell folds, which help make the soft parts of the animal less vulnerable to predation.
For convenience of reference, the apertural edge or margin of a gastropod shell is divided into three areas or walls:
The parietal wall: the area next to the penultimate whorl of the shell
The columellar wall: the wall next to the columella
The palatal wall: the outer free wall of the final whorl of the shell
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