Latin Octōpoda, from Ancient Greek ὀκτώ "eight" and πούς "foot".

Octopods (Octopoda) are a subdivision of the class Cephalopoda. The body is sac-shaped, oval. The head has 8 long tentacles. Body length up to 6 m, weight up to 100 kg. About 200 species. Widespread. They live in marine and fresh waters. Lead a benthic, less often pelagic way of life. Octopuses are objects of commercial fishing.

The body of octopuses is short, soft, and oval at the back. The mouth opening is located where its tentacles converge, and the anus opens under the mantle. The mantle resembles a wrinkled leather bag. The mouth of the octopus is equipped with two powerful jaws, similar to the beak of a parrot. The throat has a grater (radula) that grinds food. The head carries eight long tentacles - "arms". "Hands" are connected by a thin webbing and have one to three rows of suction cups. On all eight tentacles of an adult octopus, they are about 2000, each of which has a holding power of about 100 g, and the suction cups in the octopus require effort in holding, not sucking, that is, they are held only by muscular effort. The octopus has three hearts: one (the main one) drives blue blood throughout the body, and the other two - gill - push blood through the gills. Some species of octopus are poisonous. Hapalochlaena, which live off the western shores of the Pacific Ocean, are among the world's most venomous animals. Octopuses have an unusual ability - due to their lack of a skeleton, they can change shape. For example, some octopuses during the hunt spread out on the bottom, disguised as a flounder. Octopuses are known to be able to penetrate through surprisingly small holes. A bite from the venomous Hapalochlaena lunulata can be fatal to humans.

The octopus brain is highly developed (one of the most developed among invertebrates), has a rudimentary cortex. Encompasses a ring of esophagus. Eyes are large, with a lens similar to the human (but the retina, unlike the human, is non-inverted: photoreceptors are directed toward light). The pupil is rectangular. Octopuses are able to perceive sound, including infrasound. Each "arm" has up to ten thousand taste buds, which determine whether an object is edible or inedible.

The common octopus has the ability to change coloration, adapting to the environment. This is explained by the presence in its skin cells with different pigments, capable of stretching or shrinking under the influence of impulses from the central nervous system depending on the perception of the senses. The usual color is brown. If an octopus is frightened, it turns white, if angry, it turns red.

The length of adults ranges from 1 centimeter (in males of the species Argonauta argo) to 4 meters (in Haliphron atlanticus). There are reports that the Dauphleina octopus can reach a length of 960 cm and a mass of 270 kg, but usually the mass of octopuses does not exceed a few tens of kg.

Octopus life expectancy rarely exceeds 5 years, averaging 1-3 years.

They inhabit all tropical and subtropical seas and oceans, from shallow waters to depths of 100-150 meters. Most species of octopus lead a benthic lifestyle, dwelling among rocks, stones and algae. During the day octopuses are less active than at night, so they are considered nocturnal animals. On hard surfaces (including steep surfaces) octopus moves by crawling, using tentacles with suction cups. Can also swim with tentacles backward, driving itself in a kind of hydroplane propulsion - gaining water in the cavity in which the gills are located, and forcefully pushing it out in the direction opposite to the movement through the funnel, playing the role of nozzle. The direction of movement changes by turning the funnel. Both ways of movement octopus rather slow: swimming, he is inferior in speed to fish. Therefore, the octopus prefers to hunt from ambush, mimicry under the environment, and from the pursuers try to hide. 

Thanks to the soft elastic body octopus can penetrate through the holes and crevices, much smaller than the usual size of their body, which allows them to hide in all sorts of shelters. They even settle in crates, cans, car tires and rubber boots. They prefer shelters with a narrow entrance and a spacious room. They keep their dwelling clean: they sweep it with a jet of water from a funnel, put leftovers outside in a garbage heap. When approaching enemies (including divers or scuba divers) flee, hiding in the crevices of rocks and under rocks. Fleeing, octopuses of many species release streams of ink - a dark liquid produced by special glands. This liquid hangs in the water in the form of shapeless translucent spots and for some time is kept compact, until it is not washed away by water. Zoologists have not yet reached a consensus on the purpose of this behavior. Octopuses have a way of defense - autotomy: grabbed by the enemy tentacle can tear off due to a strong contraction of muscles, which in this case themselves tear. Torn off the tentacle for a certain time continues to move and respond to tactile stimuli, which serves as an additional distraction for the predator, chasing the octopus. Many species overwinter in deeper waters and move to shallow waters in summer.

Octopuses are predators. They eat mollusks, crustaceans, fish. Prey common octopus captures all eight tentacles. Octopus with its beak bites the victim, holding it suction cups. The venom of the salivary glands from its throat enters the victim's wound. Strongly expressed individual preferences in food and in the way of its extraction.

The octopus nest is a hole in the ground covered with a rampart of stones and shells. Eggs are spherical, connected in groups of 8-20 eggs. After fertilization, the female makes a nest in a burrow or cave in shallow water, where she lays up to 80 thousand eggs. The female always takes care of the eggs: she constantly ventilates them, washing them with a stream of water from a siphon. Tentacles she removes foreign objects and dirt. During the entire period of egg development, the female remains at the nest without food and often dies after hatching young. Female deep-sea octopuses spend several years near the clutch, as the eggs take a particularly long time to develop due to the low water temperature.

Octopuses are trainable, have good memory, distinguish geometric figures - they distinguish a small square from a larger one; a rectangle placed vertically from a rectangle placed horizontally; a circle from a square, a rhombus from a triangle. Recognize people, get used to those who feed them. If you spend enough time with the octopus, he becomes tame. Perfectly trainable. However, the level of intelligence of octopuses continues to be debated. Despite their intelligence, little is known about how they perceive pain, especially psychological pain. Octopuses are solitary and territorial. They often settle near octopuses of the same size.