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The octopus is an invertebrate in the class Mollusca (the molluscs), which also includes snails, clams, and squid. Octopi are cephalopod molluscs which are generally considered to be the most advanced members of the class. There are about 220 species of octopus. Octopi are found in every ocean of the world, ranging in size from a tiny Philippine species barely an inch across to giant specimens that measure as much as 13 ft (4 m) in length and weigh 165 lb (75 kg). All octopi are predators.

The octopus has no hard, protective shell; instead, its boneless body is covered by the soft mantle. The body of the octopus is rounded, like a head, and positioned, apparently, "above" the octopus's eyes, which makes it look even more head like. The eyes are one of the octo pus's most striking features, and are comparable in complexity and design to human eyes.

The octopus has eight legs, lined with double rows of suction cups, that encircle its parrotlike beak. These cups are powerful; it requires 6 oz (170 g) of force to remove a single attached cup (of typical size), so the combined suction power of dozens of suckers makes a very secure grip. The octopus attaches the suction cups by placing them on the surface it wishes to cling to, and then tightening the tiny muscles at the top of each sucker, producing a vacuum effect.

Each of the octopus's skin cells contains a packet of pigments (red, yellow, blue, brown, and black) surrounded A gulf longarm octopus off the coast of western Mexico. JLM Visuals. Reproduced by permission. by muscles that, when contracted, can balloon the packet to many times its original size. When this happens, the entire octopus changes color—a trick it can perform faster than any other color-changing animal. These colors changes often seem to be associated with moods: a frightened octopus will turn stark white, an angry one, fiery red. A contented octopus usually is the color that will camouflage it with its surroundings. The skin can also change texture, becoming smooth, spiny, or lumpy as the octopus wishes. A few years ago a remarkable species of "mimic" octopus has been discovered that combines changes of shape with color alterations to make itself look like more dangerous creatures, such as banded sea snakes or poisonous flatfish. What makes the mimic octopus so remarkable is that it can mimic radically different-looking creatures, and does so by changing its own shape and coloration dramatically. While mimicry is common in nature, no other known species can alter itself so drastically.

The octopus distracts attackers by squirting out a jet of sepia, or ink, through its siphon. The resulting ink cloud is similar in size to the octopus, which immediately turns pale as it shoots out the ink. The octopus quickly flees, swimming backward via powerful jets of water sprayed through its siphon. Predators of the octopus include orcas, dolphins, sharks, groupers, moray eels, seals, and the Atlantic halibut.

Although the octopus has a dangerous reputation, it is, in fact, a shy creature that prefers to be left alone, even by other octopuses. Attacks on human swimmers rarely, if ever, happen except when the octopus has been tormented and bites its attacker. The hard beak can inflict deep wounds, and the blue-ringed octopus of Australian waters injects potentially fatal venom with its bite. The octopus's beak is used normally to subdue prey, such as fish, other molluscs, and crabs. When an octopus catches a fish, the octopus kills it quickly by biting the fish's backbone just behind the head. Single-shelled molluscs cannot be pulled apart by the octopus's strong suckers, so the octopus drills a hole in the shell with its radula, or rasp-covered tongue, a tactic typical of predatory molluscs. Once the mollusc shell is breached, the octopus injects venom that kills the snail and makes it semiliquid.

Octopi are the most intelligent molluscs, and their nervous systems are of interest because they are organized along quite different lines than mammalian nervous systems, which are highly centralized. The octopus's nervous system has a central component that is often said to be comparable to a bird's, plus an additional, distributed component spread throughout its arms and body in a network of nerve centers or ganglia. In particular, the motions of each arm are governed by an embedded system of some 50 million neurons that encodes the movements necessary for executing complex, coordinated movements and so relieves the brain of this work.

Octopuses prefer to live alone and come together only during the mating season. Copulation consists of the male slipping the tip of one of its arms into the female's mantle; this arm has a groove running along its length down which pass packets of sperm. In some species, the sperm are contained in the tip of an arm, which breaks off inside the female. After mating, the female octopus retires to a small cave, where she lays several thousand eggs. She weaves them into strings, which she attaches to the roof of the cave. As the eggs develop, she keeps them clean by blowing jets of water on them and running her arms through them. Hatchling octopuses are tiny replicas of their parents.

Every octopus has two optical glands (so named because they sit upon the optical nerves) which shut off the octopus's desire to eat once it has mated. This means that once a male or female octopus has reproduced, it will soon die, whether in the wild or in captivity. Most octopi live for about two years.

F. C. Nicholson

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