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The octopus, which is also called devilfish, is a predatory mollusc with a pouch-shaped body and eight powerful arms with two rows of suction discs on each. It also has an ink sac, which it uses to darken the water when it senses danger. Octopuses are related to squids, cuttlefish, nautiloid, and other molluscs such as snails, mussels and clams. They can creep over hard surfaces using their arms; when they travel through water they move by jet propulsion, taking in water and ejecting it forcibly through a funnel as their muscular mantle contracts. They vary in size from a few centimetres to the giant octopus of the Pacific Ocean which can grow to over 10 m.