Mollusca is the largest phylum of invertebrate animals. Around 85,000 extant species of molluscs are recognized. The three most universal features defining modern molluscs are a mantle with a large cavity used for breathing and excretion, a radula - ribbon of tissue that contains teeth - and the structure of the nervous system. They're important links in the food chains and provide food for humans and higher-level predators. The shells of dead molluscs provide habitat to other animals and can cause commercial damage as well as being commensal.