Mollusca is a phylum, a taxonomic category, that covers about 85,000 existing species of molluscs. Molluscs are invertebrates that have mantles and radula. Examples of molluscs include snails, slugs, clams, mussels, squid and octopi.
In addition to the presence of a mantle (a shell or a protective body wall) and radula (a ribbon used for feeding), mollusca have a unique nervous system structure. There is a great diversity of molluscs, from the relatively simple mussels and oysters to very neurologically advanced invertebrates such as the cuttlefish and the octopus. There is great diversity in size as well, from the clam to the 1,000 pound colossal squid.