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. Further, shells decay slowly and fossilize well, and the fossil record of molluscs is excellent and ancient. Most molluscs are dioecious. Many pass through free-swimming larval stages, called trochophore and veliger larvae.
Molluscs are an extremely diverse group of organisms.
Over 50,000 living species have been named, making Mollusca second only to the Phylum Arthropoda in number of species known. The majority of molluscs are marine, but large numbers of species occupy freshwater and terrestrial habitats. Molluscs are also extremely diverse in their food habits, ranging from species that graze on microscopic algae, to those that eat the leaves of terrestrial plants, to predators that capture fish and other molluscs. Many species of molluscs are important to humans. A large number of bivalves and some snails are important sources of protein.