Octopus are molluscs in the class Cephalopoda, order Octopoda. Other molluscs include chiton (Polyplacophora), snails and slugs (Gastropoda), mussels, clams and their relatives (Bivalvia), and tooth-shells (Scaphopoda). Cephalopods include three main groups: nautilus, squids, and octopus.

For more information on cephalopods, visit The Cephalopod Page, courtesy of James Wood; the Tree of Life Cephalopoda page; or the National Museum of Natural History's Cephalopod Research page.