Snail shell

The gastropod shell is part of the body of a gastropod or snail, a kind of mollusk or mollusc. The shell is an external skeleton or exoskeleton, which may serve for muscle attachment, but also for protection from predators, mechanical damage, dehydration, 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.

The gastropod shell has three major layers secreted by the mantle. The calcareous central layer, the ostracum, is typically made of calcium carbonate precipitated into an organic matrix known as conchiolin. The outermost layer is the periostracum which is resistant to abrasion and provides most shell coloration. The body of the snail contacts the innermost smooth layer that may be composed of mother-of-pearl or shell nacre, a dense horizontally packed form of conchiolin, which is layered upon the periostracum as the snail grows.

Upper image: Dorsal view, showing whorls and apex
Central image: Lateral view showing the profile of the shell
Lower image: Basal view showing umbilicus in the centre.

Gastropod shell morphology is usually quite constant among individuals of a species. Controlling variables are:

Some of these factors can be modelled mathematically and programs exist to generate extremely realistic images. Early work by David Raup on the analog computer also revealed many possible combinations that were never adopted by any actual gastropod.

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