Octopus is a soft eight armed mollusk of octopus. About 300 species have been identified and classified as cephalopods along with squid, squid and Nautilus. Like other cephalopods, the octopus is symmetrical on both sides, with two eyes and a beak whose mouth is at the center of the arm (sometimes referred to as the "tentacle"). The soft body can quickly change its shape so that the octopus can squeeze through the small gap. When they swim, they drag their eight arms behind them. By draining the water, the siphon can be used for both breathing and movement. Octopus, with its complex nervous system and excellent eyesight, is one of the most intelligent and diverse animals among all invertebrates.

Octopus live in various areas of the ocean, including coral reefs, upper middle water and seabed; some live in intertidal zones, some in abysses. Most species are fast growing, early maturing and short-lived. During reproduction, the male uses a specially adapted arm to deliver a bunch of sperm directly to the female's mantle cavity, after which he becomes old and dies. The female places the fertilized eggs in the nest and takes care of them until they hatch, after which she also dies.

Strategies for defending natural enemies include expelling ink, using camouflage and threat displays, the ability to spray quickly in the water, and the ability to hide. All octopus are poisonous, but only those with blue rings are deadly to humans.

In mythology, octopus appeared as sea monsters, such as the sea demon of Norway and akkorokamui of Ainu, as well as G Shi of ancient Greece. In Victor Hugo's "toils of the sea", the fight against octopus has inspired other works, such as Ian Fleming's "octopus". Octopus appears in Japanese erotic art. They are eaten by humans in many parts of the world, especially in the Mediterranean and Asian oceans.