The development of a coiled shell and the process of torsion, or the systematic twisting of the body over time, led gastropods to evolve from bilateral to assymetrical body structures. This diagram shows the generalized internal anatomy of a gastropod.

Some groups of snails have shells that coil to the left (left-handed), while others have shells coiling to the right (right-handed). In some groups both right- and left-handed members are present. The shell affords protection from predators and the rigors of the environment. When threatened, the snail pulls its entire body inside. A small, circular structure called the operculum closes securely behind the snail to form a tight seal that keeps out predators and keeps in moisture.