The group of animals called gastropods is a very ancient line, fossils having been found in rocks dated at 500 million years old. There are thought to be about 60,000 species on the Earth today. A large percentage of the gastropods are land animals but all require a damp environment. The snail's body is usually able to retract into the shell but in some species, such as the sandbowl snail, the body cannot be fully retracted. The large 'foot' of the snail secretes mucus, and muscular contractions in this foot propel the snail forward. The mucus also serves to prevent the snail drying out. The snail's internal organs are permanently within the shell, including its digestive system, and the mantle. The mantle secretes the shell and follows the shape of its internal structure. The mouth of the snail is at the front of the foot, and contains the 'radula' or tongue. This is covered in minute horny 'teeth' that the snail uses to rasp food into its mouth.
With a few exceptions, snails are hermaphrodites, which means individuals are both male and female. When mating, each snail draws along-side the other and a small dart composed of calcium is thrust from one snail into the other. This is thought to act as stimulation and as the two animals draw closer together, the snails exchange sperm. The eggs, up to 100 in some species, are laid in a cool, damp place under cover, usually in the soil, or under rocks or rotten wood. The eggs are usually laid in summer and autumn and the young snails look like miniature versions of the adults. They reach maturity in about a year and grow by adding more whorls to their shells. The survival rate is low; snails do not care for either their eggs or their young, and most species die after breeding. Occasionally, snails survive into a second year.
Most snails, such as the sandbowl snail, are herbivorous and feed on a variety of vegetable matter. They can digest cellulose, unlike many other herbivores, and sometimes eat damp paper and cardboard. A few snails are meat eaters, feeding on carrion or attacking other snails and slugs. Because their shells are made from calcium, snails are often found in greater numbers and varieties in areas with limestone or chalk underlying the soil. They do live in acid soils but their shells are usually thinner. They also tend to prefer a damp, humid climate.