Monsters of the Deep Cephalopods belong to a branch of the animal kingdom known as Molluscs.
Biologists can discern many similar features between cephalopods and slugs and snails.
The only cephalopod with an outside shell is the Nautilus. It's considered to be the most 'primitive' of the group. It's a slow swimmer, but strikingly beautiful. The rest of the group has only a remnant of the shell. In cuttlefish it's still quite large and strengthens the animal inside, rather like a backbone.