Gastropod shell

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A large land snail shell with parts broken off to show the interior structure.
1 - umbilicus
2 - parietal callus
3 - aperture
4 - columella
5 - suture
6 - whorl
7 - apex

The gastropod shell is an seashell which is part of the body of a gastropod or snail. It is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage. In land snails the shell is an essential protection against the sun, and against drying out.

The gastropod shell has several layers, and is typically made of calcium carbonate precipitated out into an organic matrix. It is secreted by a part of the molluscan body known as the mantle.

Not all gastropods have a shell, but the majority do. The shell is in one piece, and is typically spirally coiled, although some groups, such as the various different families and genera of limpets, have simple cone-shaped shells as adults.

1 Chirality in gastropods
2 Morphology
3 Parts of the gastropod shell
- 3.1 A selection of different kinds of gastropod shells
4 See also
5 External links

[edit] Chirality in gastropods

Because coiled shells are asymmetrical, they possess a quality called chirality, the "handedness" of an asymmetrical structure.

By far the majority of gastropod shells are dextral (right-handed) in their coiling, but a small minority of species and genera are virtually always sinistral (left-handed), and a very few species show an even mixture of dextral and sinistral individuals.

In species that are almost always dextral, very rarely a sinistral specimen will be produced, and these oddities are avidly sought after by some shell collectors.

If you hold a coiled gastropod shell with the aperture down and the spire pointing upwards, a dextral shell will have the aperture on the right hand side, and a sinistral shell will have the aperture on the left hand side.

This chirality of gastropods is often overlooked when photographs of coiled gastropods are "flipped" by a non-expert prior to being used in a publication. This image "flipping" results in a normal dextral gastropod appearing to be a rare and abnormal sinistral one.

[edit] Morphology

The shell of Zonitoides nitidus, a land snail, has dextral coiling.
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

Ideal gastropod shell morphology is usually quite constant among individuals of a species, and with exceptions, fairly constant among species within each family of gastropoda. Controlling variables are:

The rate of growth per revolution around the coiling axis. High rates give wide-mouthed forms such as the abalone, low rates give highly coiled forms such as turritella or some of the planorbidae.
The shape of the generating curve, roughly equivalent to the shape of the aperture. It may be round, for instance in the turban shell, elongate as in the cone shell or have an irregular shape with a siphonal canal extension as in the murex.
The rate of translation of the generating curve along the axis of coiling, controlling how high-spired the resulting shell becomes. This may range from zero, a flat planispiral shell, to nearly the diameter of the aperture.
Irregularities or "sculpturing" such as ribs, spines, knobs, and varices made by the snail regularly changing the shape of the generating curve during the course of growth, for instance in the many species of murex.
Ontologic growth changes as the animal reaches adulthood. Good examples are the flaring lip of the adult conch and the inward-coiled lip of the cowry.

Some of these factors can be modeled 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 adapted by any actual gastropod.

Certain shell shapes are found in certain environments, though there are many exceptions. Wave-washed high-energy environments are inhabited by snails with a wide aperture, a relatively low surface area, and a high growth rate per revolution. High-spired and highly sculpturd forms become more common in quiet water environments. Burrowing forms such as the olive and terebra shells are smooth and lack sculpture to decrease resistance to moving in sand. A few gastropods, for instance the vermetidae do not grow a coiled shell to cary as they move about, but instead cement themselves to and grow along the surface of a rock.

[edit] Parts of the gastropod shell

Periostracum: a thin layer of organic "skin" which forms the outer layer of the shell of many species
Protoconch: the larval shell, often remains in position even on an adult shell
apex: the smallest few whorls of the shell
spire: the part of the shell that protrudes above the body whorl
whorl: each one of the complete rotations of the shell spiral
Body whorl: the largest whorl in which the main part of the viseral mass of the mollusk is found
aperture: the opening of the shell
Peristome: the part of the shell that is right around the aperture
columella: the "little column" at the axis of revolution of the shell
umbilicus: in shells where the whorls move apart as they grow, on the underside of the shell there is a deep depression reaching up towards the spire; this is the umbilicus
lira: one kind of shell sculpture
plait: another kind of shell sculpture
varix: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices
operculum: the "trapdoor" of the shell
Siphonal canal: an extension of the aperture in certain gastropods
Parietal callus: a ridge on the inner lip of the aperture in certain gastropods
Suture: The junction between whorls of most gastropods