The mantle also secretes the shell in those forms that possess one. Internally, molluscs have a complicated digestive system, with a mouth in the head and the anus emptying into the mantle cavity. A rasping organ used in feeding, the radula, is present in all groups except bivalves and Aplacophora. The radula is usually toothed and its structure may be very complex. It is adapted to a wide variety of feeding styles in different species, including scraping, tearing, stabbing, and cutting.
Gas exchange is via gills, lungs, or the body surface.
Excretion takes place by means of kidneys that, like the digestive tract, pump waste into the mantle cavity. Some molluscs have complex, extremely sensitive eyes. The majority of molluscs have a shell of some kind. This calcareous structure is secreted by the mantle, and in living snails it is covered by a horny layer called a periostracum. Its shape, size, and color are widely used by taxonomists, and they has also made molluscs a popular pursuit of collectors.