A land snail is any of the numerous species of snail that live on land, as opposed to the sea snails and freshwater snails. Land snail is the common name for terrestrial gastropod mollusks that have shells (those without shells are known as slugs). However, it is not always easy to say which species are terrestrial, because some are more or less amphibious between land and fresh water, and others are relatively amphibious between land and salt water. Land snails are a polyphyletic group comprising at least ten independent evolutionary transitions to terrestrial life (the last common ancestor of all gastropods was marine). The majority of land snails are pulmonates that have a lung and breathe air. Most of the non-pulmonate land snails belong to lineages in the Caenogastropoda, and tend to have a gill and an operculum. The largest clade of non-pulmonate land snails is the Cyclophoroidea, with more than 7,000 species. Many of these operculate land snails live in habitats or microhabitats that are sometimes (or often) damp or wet, such as in moss. Land snails have a strong muscular foot; they use mucus to enable them to crawl over rough surfaces and to keep their soft bodies from drying out. Like other mollusks, land snails have a mantle, and they have one or two pairs of tentacles on their head. Their internal anatomy includes a radula and a primitive brain. In terms of reproduction, many caenogastropod land snails (e.g., diplommatinids) are dioecious, but pulmonate land snails are hermaphrodites (they have a full set of organs of both sexes) and most lay clutches of eggs in the soil. Tiny snails hatch out of the egg with a small shell in place, and the shell grows spirally as the soft parts gradually increase in size. Most land snails have shells that are right-handed in their coiling. A wide range of different vertebrate and invertebrate animals prey on land snails. They are used as food by humans in various cultures worldwide, and are raised on farms in some areas for use as food.
Biology
Physical characteristics
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. 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. Most molluscs, including land snails, have a shell which is part of their anatomy since the larval stage. When they are active, the organs such as the lung, heart, kidney, and intestines remain inside the shell; only the head and foot emerge. The shell 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. 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. At the other end of the size spectrum is Angustopila psammion, a species with shell diameter of 0.60-0.68 mm. Most land snails bear one or two pairs of tentacles on their heads. In most land snails the eyes are carried on 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.
Digestion and nervous system
A snail breaks up its food using the radula inside its mouth. The radula is a chitinous ribbon-like structure containing rows of microscopic teeth. With this the snail scrapes at food, which is then transferred to the digestive tract. In a very quiet setting, a large land snail can be heard 'crunching' its food: the radula is tearing away at the surface of the food that the snail is eating. The cerebral ganglia of the snail form a primitive brain which is divided into four sections. This structure is very much simpler than the brains of mammals, reptiles and birds, but nonetheless, snails are capable of associative learning.
Respiration Oxygen is carried by the blood pigment hemocyanin. Both oxygen and carbon dioxide diffuse in and out of blood through the capillaries. A muscular valve regulates the process of opening and closing the entrance of the lung. When the valve opens, the air can either enter or leave the lung. The valve plays an important role in reducing water loss and preventing drowning.
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