The octopus has a pouch-shaped body and eight powerful arms, with rows of suckers on each. If one arm is lost, another will eventually grow to replace it. There are many species of octopus and the biggest, the giant octopus, can grow to more than 10m. in diameter. The smallest octopus is less than one inch in diameter. All octopus have a beak, which they will use for injecting a nerve poison into their prey, while they hold it with their arms. Octopus have blue blood.

The squid is closely related to the octopus but it has two long tentacles as well as eight arms with suckers. Both the octopus and the squid can change colour to blend in with their environment, in order to protect themselves. Squid have a very large head (and brain), as well as a strong beak, which they will also use to protect themselves against predators.