<P> Land snails move by gliding along on their muscular foot, which is lubricated with mucus and covered with epithelial cilia . This motion is powered by succeeding waves of muscular contractions that move down the ventral of the foot . This muscular action is clearly visible when a snail is crawling on the glass of a window or aquarium . Snails move at a proverbially low speed (1 mm / s is a typical speed for adult Helix lucorum). Snails secrete mucus externally to keep their soft bodies from drying out . They also secrete mucus from the foot to aid in locomotion by reducing friction, and to help reduce the risk of mechanical injury from sharp objects, meaning they can crawl over a sharp edge like a straight razor and not be injured . The mucus that land snails secrete with the foot leaves a slime trail behind them, which is often visible for some hours afterwards as a shiny "path" on the surface over which they have crawled . </P> <P> Snails (like all molluscs) also have a mantle, a specialized layer of tissue which covers all of the internal organs as they are grouped together in the visceral mass . The mantle also extends outward in flaps which reach to the edge of the shell and in some cases can cover the shell, and which are partially retractable . The mantle is attached to the shell, and creates the shell and makes shell growth possible by secretion . </P> <P> Most molluscs, including land snails, have a shell which is part of their anatomy since the larval stage, and which grows with them in size by the process of secreting calcium carbonate along the open edge and on the inner side for extra strength . Although some land snails create shells that are almost entirely formed from the protein conchiolin, most land snails need a good supply of calcium in their diet and environment to produce a strong shell . A lack of calcium, or low pH in their surroundings, can result in 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 . When retracted into their shells, many snails with gills (including some terrestrial species) are able to protect themselves with a door - like anatomical structure called an operculum . </P> <P> Land snails range greatly in size . The largest living species is the Giant African Snail or Ghana Tiger Snail (Achatina achatina; Family Achatinidae), which can measure up to 30 cm . The largest land snails of non-tropical Eurasia are endemic Caucasian snails Helix buchi and Helix goderdziana from the south - eastern Black Sea area in Georgia and Turkey; diameter of the shell of the latter may exceed 6 cm </P>

Where does a snail's shell come from
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