Externally, snails are characterized spiral shell flattened foot, and anterior head with prominent tentacies. When disturbed, the snail can withdraw into its shell, and in some cases the shell opening can be closed by a horny, or calcareous, plate called an operculum. Locomotion in snails is usually achieved by means of waves of contraction along the bottom of the muscular foot from rear to front.
All snails undergo body torsion during development Snail begin development as bilaterally symmetrical animals. During the larval stage the visceral mass is twisted 180 degres, so that posterior structures such as the anus are thought to accompany body torsion. Several advantages are thought to accompany body torsion. First the shell opening is rotated anteriorally, and consequently the snail withdraws into the shell headfirst, and providing added protection for that end, which contains concentrated sensory structures and the major part of the central nervous system. In addition, the gills used for respiration (in most species) are rotated anteriorally, and the snail can utilize the undisturbed water ahead of its own path of movement for inhalant respiratory currents.
