The shell may be highly ornamented and vividly colored. Some shells are composed of numerous whorls while others have a few to a single whorl. Each whorl increases in size from the peak or apex of the shell to the body whorl (largest and most recently formed one).
Check out the numerous pictures of Gastropod shells collected at this site, Hardy’s Internet Guide to Marine Gastropods.
Picture drawn by Melissa Jackson
Although most slugs and the rest of the nudibranchs do not construct shells, the typical gastropod produces a calcareous shell that is wrapped around a central axis called the columella. This shell is attached to the snail's body by either one columellar muscle or a series of muscles. As the shell grows and coils, each successive whorl is larger than the previous one. At the last, most recently created whorl, is the aperture (or the shell's opening). The mantle secretes the shell along the outer lip of this aperture. Modification and ornamentation of the shell are highly diverse and reflective of the gastropod's environment. In swift currents, for instance, the shell often takes the form of a non-spiral cap or cup shape. Also, spiral coiling can sometimes be difficult to detect, like in the abalones who’s last whorl can make up as much as 90 percent of the total shell size, which makes the previous whorls feel insignificant in comparison. The most important function of the gastropod shell is to provide protection and support to the visceral mass. A defense mechanism for many gastropods is to detract their head and body inside the shell and to seal up the opening with the operculum.
Phenotypic Plasticity in gastropod shells
An interesting factor that influences the thickness of snail shells is that their thickness varies within members of the same species. An experiment conducted by Geoffrey Trussell determined that shell thickness is influenced by the presence of predators during the snails development. On beaches where crabs are present, snail populations have thicker shells that populations of the same species that develop on beaches not inhabited by crabs.