octopus

Soft-bodied sea animal with a round or oval body and eight slender arms (tentacles) in a ring surrounding its mouth. They are solitary creatures, living alone in rocky dens. They feed on crabs and other small animals. There are about 50 different species of octopus living in all the oceans of the world. Some are small, having bodies only 8 cm/3 in long, but the largest deep-sea species can grow to lengths of 20 m/64 ft.

Behaviour
Octopuses can change colour to blend in with their surroundings and can swim using their arms or by a form of jet propulsion by squirting out water from their bodies. The octopus has rows of suckers along the length of each arm (or tentacle) which, as well as helping it swim and crawl around the ocean floor, allows it to search in cracks and crevices and grab prey. The octopus is a carnivore (flesh-eater), usually feeding on crabs, shrimps, and mussels, but the larger species of octopus have been known to hunt small sharks and dogfish. They trap the prey in their arms and drag it towards their powerful beaklike jaws. Once it has bitten its prey, the octopus injects it with a poisonous saliva to kill it. Sometimes, when frightened or to avoid enemies, they squirt out a black ink from their bodies which hides them and allows them to escape. If they lose an arm, they can grow another in its place. Octopuses are highly intelligent with two well developed eyes, similar to those of vertebrates (animals with backbones). They breathe using gills as fish do, but are unique in that they have three hearts.

Classification
Octopuses belong to the phylum Mollusca (molluscs), class Cephalopoda (cephalopods), subclass Coleoidea. They belong to the genus Octopus and there are about 50 known species including the common octopus (O. vulgaris), which may reach 2 m/6 ft in length; the Australian blue-ringed octopus (genus Hapalochlaena) that can kill a human being in 15 minutes as a result of its venomous bite; and the giant deep-sea octopus (Architeuthis dux) that can grow to 20 m/64 ft.