The octopus belongs to a group of animals called “cephalopods.” The name means “head-footed” because the foot is divided into long armlike tentacles that grow out around the head. The octopus has eight such tentacles.

Even though the octopus belongs to that part of the animal kingdom known as mollusks, it is quite different from clams and oysters, which are mollusks, too. It is more closely related to the squids.

None of these has shells. They have only a soft mantle to enclose the body. The tentacles are long and flexible with rows of suckers on the underside. These enable the octopus to grab and hold very tightly to anything it catches.

In the back part of the body of the octopus is a funnel-siphon. Water comes into this siphon and the octopus extracts oxygen from it the way a fish does. The siphon is also the way it manages to move swiftly. The octopus can shoot a stream of water from this siphon with such force that it propels itself backward very rapidly. That is the way it can get away from an enemy that comes too quickly to allow it the chance to crawl over the rocks or into crevices by means of its eight tentacles.

When an octopus lies quietly, the tentacles rest spread out over the floor of the shallow pool. Should an enemy approach, it will either escape or grab the enemy tightly. If things grow too serious, it can throw up a “smoke screen” and escape. From a sac in the lower back part of its body it can throw out a black inky fluid that clouds the surrounding water.

Also, an octopus can change color to match its surroundings. It can go from red to gray, yellow, brown, or blue-green.