Gastropod shell

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 in some cases the shell is reduced such that the body cannot be retracted within it (semi-slug).

The calcareous central layer, ostracum, is typically made of calcium carbonate (CaCO3) precipitated into an organic matrix known as conchiolin.

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.

Wave-washed high-energy environments, such as the rocky intertidal zone, are usually inhabited by snails whose shells have a wide aperture, a relatively low surface area, and a high growth rate per revolution.

The shell of burrowing forms, such as the olive and Terebra, are smooth, elongated, and lack elaborate sculpture, in order to decrease resistance when moving through sand.

[8] Populations or species with normally mixed coiling are much rarer, and, so far as is known, are confined, with one exception, to a few genera of arboreal tropical snails.

[8] Several sets of species differ only in the direction of coiling, but the evidence is inconclusive as to whether left- and right-handed shells live together.

372–385)[full citation needed] summarized previous discussions of the problem and concluded that the right- and left-handed populations were distinct species.

[8] The genetics of reverse coiling in a rare dextral mutant of another clausiliid, Alinda biplicata (Montagu), has been studied by Degner (1952).

Secondary spirals may appear by intercalation between the primary ones, and generally are absent in the young shell, except in some highly accelerated types.

When a row of spines is formed at the edge or outer lip of the shell during a resting period, this feature sometimes remains behind as a varix as in (Murex) and many of the Ranellidae.

An upper or posterior notch is present in certain taxa, and this may result in the formation of a ridge or shelf next to the suture (Clavilithes).

An outer (lateral) emargination or notch, sometimes prolonged into a slit occurs in certain types (Pleurotomidae, Pleurotomaridae, Bellerophontidae, etc.

On the inside of the outer lip, various ridges or plications called lirae are sometimes found, and these occasionally may be strong and tooth-like (Nerinea).

Similar ridges or columellar plicae or folds are more often found on the inner lip, next to the columella or central spiral twist.

The umbilicus varies greatly in size, and may be wholly or in part covered by an expansion or callus of the inner lip (Natica).

For example, three groups can be distinguished based on the height – width ratio:[10] The following are the principal modifications of form in the gastropod shell.

Among proposed roles invoked for the variability of shells during evolution include mechanical stability,[15] defense against predators[16] and climatic selection.

[19] In large enough quantities, gastropod shells can have enough of an impact on environmental conditions to affect the ability of organic remains in the local environment to fossilize.

[20] Eggshell fragments are present in only two microfossil sites, both of which are predominated by the preserved shells of invertebrate life, including gastropods.