Common octopus (Brian Gratwicke / CC BY)
The octopus has eight arms (four pairs). Its body is bilaterally symmetrical and lacks an internal or external skeleton. A chitin beak (similar to a parrot’s beak) is the only hard part of the body. Octopuses have highly developed nervous systems that make them the most intelligent of all invertebrates. Potentially dangerous to divers are blue-ringed octopuses. Their venom is potent enough to kill a human.