AY Honors/Shells - Advanced/Answer Key

Semicassis pyrum , which has a large aperture.

The aperture is the main (or only) opening in the shell of a gastropod or snail. It is where the soft parts of the animal emerge. In some prosobranch gastropods, the aperture is able to be closed, and even completely sealed, with a sort of door or operculum.

The aperture of many snail shells is round, rounded, elliptical or oval. This shape 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 or "bacon", 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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