Mollusca is one of the most diverse groups of animals on the planet, with at least 50,000 living species (and more likely around 200,000). It includes such familiar organisms as snails, octopuses, squid, clams, scallops, oysters, and chitons. They are the Largest marine phylum. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion the presence of a radula and the structure of the nervous system.