Snails also have a mantle that covers the internal organ that is called a foot. The mucus also reduces the snail's risk of injury. They produce mucus in order to aid locomotion by reducing friction. Snails move by alternating body contractions with stretching, with a proverbially low speed (1 millimeter per second is a typical speed for adult Helix lucorum) (Pavlova 2001). This structure is very simple compared to the brains of mammals, reptiles, and birds, but snails are capable of associative learning (Sahley et al. The cerebral ganglia of the snail form a primitive sort of brain, divided into four sections. The eyes of most marine and freshwater snails are found at the base of the first set of tentacles. Both sets of tentacles are retractable in land snails. The second set of tentacles act as olfactory organs. In most land snails, the eyes are carried on the tips of the first set of tentacles (called ommatophores or more informally 'eye stalks') which are usually roughly 75 percent of the width of the eyes. Most snails bear one or two pairs of tentacles on their heads. This is why snails are often heard 'crunching' their food: the radula is tearing away at what they are eating. With this the snail scrapes at food, which is then transferred to the digestive tract. The radula is a chitinous structure, containing microscopic hooks, called cuticulae. A snail breaks up its food using the radula. Most snails are of herbivorous nature, though a few land species and many marine species may be omnivores or carnivores. The biggest of all snails is Syrinx aruanus, an Australian marine species which can grow up to 77.2 centimeters (30 inches) in length (COA 2007), and 18 kg (40lbs) in weight. Pomacea maculata (Family Ampullariidae), or Giant Apple Snail is the largest freshwater snail, with a diameter of up to 15 centimeters and a mass of over 600 g. The largest land snail is the Giant African Snail or Ghana Tiger Snail ( Achatina achatina Family Achatinidae), which can measure up to 30 centimeters. 'Slugs,' which are gastropods that lack a conspicuous shell, are scattered throughout groups that primarily include 'snails.'

The term snail is not a taxonomic unit but is variously defined to include all members of Gastropoda, all members of the subclass Orthogastropoda, all members of Orthogastropoda with a high coiled shell, or a group of gastropods with shells that do not include limpets, abalones, cowries, whelks, and conches. Gastropods are typically placed into two subclasses: Orthogastropoda ('true snails') and Eogastropoda ('true limpets') (Ponder and Lindberg 1997). But there are also members that lack shells, such as slugs and sea slugs ( nudibranches), and some that have shells with two halves. Most have a single shell that is characteristically coiled, such as snails, limpets, abalones, cowries, whelks, and conches. Gastropoda, the largest and most diverse class of the phylum Mollusca, has about 75,000 known living species. They are integral to food chains and, in addition to a number of species being consumed by humans as delicacies, snails also touch upon the inner nature of people, adding aesthetic value and contributing to the human fascination with nature, particularly with their unique shells and behaviors. Snails are found in freshwater, marine, and terrestrial environments. Snail is the common name applied to most members of the mollusk class Gastropoda that have coiled shells.

Biggest snail in the world