Molluscs are found in marine, freshwater and terrestrial environments and in terms of numbers of species are second only to the arthropods. Body form varies widely within the phylum but the basic molluscan plan shows a head, often well developed and bearing sensory structures, a muscular foot and a visceral hump. The delicate tissue covering the visceral hump is known as the mantle (or pallium). This projects beyond the edge of the visceral mass to enclose a cavity known as the mantle (pallial) cavity where gills and other structures are found. The mantle secretes a calcareous shell consisting of a number of layers. There is usually an outer proteinaceous layer known as the periostracum, beneath which are layers of calcium carbonate. In some molluscs an inner mother-of-pearl or nacreous layer is often exposed on the surface of the shell by the wearing away of the outer layers. Although described as one of the most characteristic features of the phylum, it is important to note that in some groups the shell is internal, while in others it is absent. The characteristic feeding organ is a ribbon-like, chitinous structure, the radula (Plate 16d), which comprises rows of teeth. In many species these have a rasp-like action and remove adhering microorganisms from the rock surface. In some gastropods, the radula is modified to enable it to bore into prey. The bivalves lack a radula and are deposit or suspension feeders using the gill to filter out fine particles of food.