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 plaits, which help make the soft parts of the animal less vulnerable to predation.
The growth of the shell is provided for by non-continuous addition of minute layers to the aperture margin (also called peristome) from the mantle border, the principal agent in the secretion of the shell.
Read more about this topic: Aperture (mollusc)