The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage. In land snails, in some freshwater snails and in intertidal marine snails, the shell is also an essential protection against the sun, and against drying out. Most gastropod shells are spirally coiled. The coiling is usually right-handed, but in some taxa the coiling is left-handed and in a very few species there can be both right-handed and left-handed individuals. The gastropod shell has several layers, and is typically made of calcium carbonate precipitated out into an organic matrix known as conchiolin. The shell is secreted by a part of the molluscan body known as the mantle. Some gastropods are shell-less (slugs), but the majority of gastropods do have a shell. In almost every case the shell consists of one piece, and is typically spirally coiled, although some groups, such as the various families and genera of limpets, have simple cone-shaped shells as adults. The study of mollusc shells, including gastropod shells, is called conchology.