Bivalve shells carry out a variety of functions including support for soft tissues, protection from predators, locomotion (in scallops) and boring tunnels through hard substrates (shipworms). The shell is made of three layers: the nacreous layer, an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbonate that is continuously secreted by the mantle, the prismatic layer, a middle layer of chalky white crystals of calcium carbonate in a protein matrix and the periostracum, an outer pigmented layer composed of a protein called conchin that protects the prismatic layer from abrasion and dissolution by acids (especially important in freshwater forms where decay of leaf materials produce acids).