The shell begins to form in a very early stage of development: first from the secretion of the "gland of the shell", then from the secretions of glands placed at the edges of the mantle. The latter is the outer layer of the snail's body and delimits a cavity used to breathe, like a lung. The shell, consisting mainly of crystals and calcium carbonate sheets closely connected together and immersed in a protein substance (conchiolina), consists of three layers. The shell grows in annual circles that start from the outer edge of the mantle, and little by little it wraps itself in a helix around an axis called columella. When the snail reaches its maximum size, growth stops.