Octopus, squid and cuttlefish - Class Cephalopoda
Cephalopod literally means 'head foot' and members of this group, including octopuses, cuttlefishes, squids and nautiluses, have their foot or tentacles connected to their head, not their body.
Cephalopods have the most advanced nervous system of all invertebrate animals and are active hunters. They are carnivorous and use their strong beak to bite into their prey of fishes, crabs and other molluscs, occasionally injecting venom. They have excellent eyesight and can register shapes, textures and colour. To escape from predators such as seals, dolphins and sharks, cephalopods may release an inky screen.
The immediate and most obvious difference between cephalopods and other molluscs is cephalopods' apparent lack of a shell. Octopuses have no shell at all, while cuttlefishes have an internal shell and squids have the horny remains of a shell.