very diverse group includes octopus, squid, nautilus, and cuttlefish; a shell is sometimes present, and if present, it is divided into chambers; and the animal occupies the last-formed chamber with a filament of tissue extending to the oldest chamber; Cephalopods have the most complex brain of any invertebrate; most species also have excellent eyes that are astonishingly similar to those of vertebrates; most species have a single pair of gills, a mostly closed circulatory system, and accessory or branchial hearts.