Snail

The name snail applies to most members of the molluscan class Gastropoda that have coiled shells. Snails are found in freshwater, marine, and terrestrial environments. Most are of herbivorous nature, though a few land species and many marine species may be omnivores or carnivores. Other gastropods, which lack a conspicuous shell, are commonly called slugs, and are scattered throughout groups that primarily include snails. Although people more often come into contact with terrestrial snails, they form the minority, with aquatic snails being more common. Snails with lungs belong to the group Pulmonata, while those with gills form a paraphyletic group.

Snails move by alternating body contractions with stretching, with a proverbially low speed (1 mm/s is a typical speed for adult Helix lucorum). They produce mucus in order to aid locomotion by reducing friction. The mucus also reduces the snail's risk of injury. Snails also have a mantle that covers the internal organ which is called a foot.

When retracted into their shells, some snails protect themselves with a door-like anatomical structure called an operculum. The operculum of some snails has a pleasant scent when burned, so it is sometimes used as an ingredient in incense.

Snails range greatly in size. The largest land snail is the Giant African Snail or Ghana Tiger Snail (Achatina achatina; Family Achatinidae), which can measure up to 30 cm. Pomacea maculata (Family Ampullariidae), or Giant Apple Snail is the largest freshwater snail, with a diameter of up to 15 cm and a mass of over 600 g. The biggest of all snails is Syrinx aruanus, an Australian marine species which can grow up to 77.2 cm (30 inches) in length, and 18 kg (40lbs) in weight.

The proportions of snail and nautilus shells are an example of the appearance of the golden ratio in nature. Patterns on shells of certain sea snails (Conus, Cymbiola) are similar to those formed by cellular automata.

As the snail grows, so does its calcium carbonate shell. A snail's shell forms a logarithmic spiral; most are right-handed, meaning that the whorl is on the right hand side of the shell. At some point, the snail builds a lip around the opening of the shell, stops growing, and begins reproducing.

The shells of snails and other molluscs, and snail egg casings, are primarily made up of calcium carbonate. Because of this, they need calcium in their diet and a watery environment to produce a strong shell. A lack of calcium, or low pH in their surroundings, can cause thin, cracked, or perforated shells. Usually a snail can repair damage to its shell over time if its living conditions improve, but severe damage can be fatal.

Most snails bear one or two pairs of tentacles on their heads. In most land snails the eyes are carried on the tips of the first (upper) set of tentacles (called ommatophores or more informally 'eye stalks') which are usually roughly 75% of the width of the eyes. The second (lower) set of tentacles act as olfactory organs. Both sets of tentacles are retractable in land snails. The eyes of most marine and freshwater snails are found at the base of the first set of tentacles

A snail breaks up its food using the radula, which is a chitinous structure containing microscopic hooks called cuticulae. With this the snail scrapes at food, which is then transferred to the digestive tract. This is why snails are often heard 'crunching' their food: the radula is tearing away at what they are eating.

The cerebral ganglia of the snail form a primitive brain divided into four sections. This structure is much simpler than the brains of mammals, reptiles and birds, but snails are capable of associative learning.

Some snails hibernate during the winter (typically October through April in the Northern Hemisphere). They may also estivate in the summer in drought conditions. To stay moist during hibernation, a snail seals its shell opening with a dry layer of mucus called an epiphragm. Some apple snails have a "door" to close the shell when they withdraw, for protection from predators as well as to avoid desiccation.

All land snails are hermaphrodites, producing both spermatozoa and ova. Some aquatic snails, such as Apple Snails, are either male or female. Prior to reproduction, most snails will perform a ritual courtship before mating. This may last anywhere between two and twelve hours. Prolific breeders, snails inseminate each other in pairs to internally fertilize their ova. Each brood may consist of up to 100 eggs.

Snails have small slits on their necks where fertilization occurs and the eggs develop.

Garden snails bury their eggs in shallow topsoil primarily while the weather is warm and damp, usually 5 to 10 cm down, digging with their 'foot'- the back of their 'tail'. Egg sizes differ between species, from a 3 mm diameter in the grove snail to a 6 cm diameter in the Giant African Land Snail. After 2 to 4 weeks of favorable weather, these eggs hatch and the young emerge. Snails may lay eggs as often as once a month.

The snail's shell develops while it is still an embryo; it is, however, very weak, and they need an immediate supply of calcium. Newly hatched snails obtain this by eating the egg they hatched out of. Baby snails cannibalizing other eggs, even unhatched ones, has been recorded. Promptly after they are finished ingesting their egg casings, they crawl upwards through the small tunnel left from their parent digging their nest. At this stage, the young are almost completely transparent. Their shell is usually slightly smaller than the egg they hatched from, but their body length when out of their shell is slightly greater than the egg diameter. After a few weeks, the snails will begin to gain their first tingeing, usually slightly blue before they turn their adult color. In roughly three months after they have hatched, they will look like miniature versions of their mature kin. They will continue to grow, usually for two to three years until they reach adult size, although there have been confirmed recordings of snails growing amazingly fast - even bigger than their parents in little more than a month. Irrespective of their rate of growth, it will still take 2 to 6 years before they are sexually mature.

There have been hybridizations of snails; although these do not occur commonly in the wild, in captivity they can be coaxed into doing so.

Parthenogenesis has also been noted in certain species.

Pond snails do not lay their eggs in the ground but carry them around until they hatch.
 

Snail Video

The lifespan of snails varies from species to species. In the wild, Achatinidae snails live around 5 to 7 years and Helix snails live about 2 to 3 years. Aquatic Apple Snails live only a year or so. Most deaths are due to predators or parasites. In captivity, their lifespan is much longer, ranging from ten to fifteen years for most species. On occasions, snails have lived beyond this lifespan, up to 30 years or more.

In the wild, snails eat a variety of different foods, including leafy vegetation, fruits, manure and carrion. They can cause damage to agricultural crops and garden plants, and are therefore often regarded as pests. When kept as pets, snails will eat nearly anything, and snail owners should make sure which diets are appropriate to the species involved.