Frequently, we humans look down on invertebrates as inferior forms of life. But at least one invertebrate, the octopus, may possess enough brainpower to alter this biological prejudice.

Octopuses are mollusks, like snails, clams and oysters, but they are smarter, nimbler, more curious and more resourceful than any oyster. They have to be: Like their fellow cephalopods, squid and cuttlefish, they lost their external shells millions of years ago, but what they lack in armor, experts say, they make up for in brains. The central nervous system of the octopus is among the largest and most complex in the invertebrate world, rivaling that of many vertebrates, including birds and fish. How intelligent that nervous system makes the octopus is still a matter of scientific debate, however.

Over the years, scientists have tested octopus intelligence by teaching captive specimens to slither through simple mazes and to tell squares from crosses. Octopuses even learn to unscrew lids to get at food.

The most dramatic evidence for octopus intelligence came in 1992. A pair of researchers in Naples, Italy, Graziano Fiorito and Pietro Scotto, used conventional means--food as a carrot, mild electric shock as the stick--to train a group of captive common octopuses to grab a red ball instead of a white one. The scientists then let untrained animals watch from adjoining tanks as their experienced confreres reached for red balls over and over. Thereafter, Fiorito and Scotto reported, most of the watchers, when offered a choice, pounced on red balls. In fact, they learned to do so more quickly than had the original group. The octopuses, according to the researchers, were doing something invertebrate had never been known to do before: learning by watching.

Or so it seemed. Critics since then have weighed in with a list of complaints about the experiment. Controls were sloppy: Fiorito and Scotto themselves concede that untrained octopuses at the outset already preferred red balls by more than three to one. Gerald Biederman of the University of Toronto's Learning Laboratory wrote that octopuses typically "are reluctant to attack novel stimuli." Having watched trained octopuses repeatedly snatch the red ball, the untrained animals may simply have gotten used to watching that ball and so were more apt to pounce on it themselves.

What perplexed scientists most about Fiorito and Scotto's paper, however, was the assumption that the animals would do something in captivity that they would never do in nature. An ability to learn by watching makes sense, in evolutionary terms, only for animals that live in social groups. But octopuses do not.

Indeed, an octopus leads a remarkably solitary life. It never knows its parents. In most species, the mother stops eating while brooding her eggs and dies almost as soon as they hatch. Newborn common octopuses, flealike creatures the size of rice grains, spend their first weeks as ocean plankton, drifting at the surface. After gaining weight, they drop to the bottom, where they spend most of their lives hiding watchfully in dens, which can be rocky crevices, abandoned shells, holes scooped in the sand, even the odd oil drum or mayonnaise jar.

From 150 to 200 species of octopus inhabit the world's oceans. The common octopus, the species best known to scientists, thrives in warm rocky shallows off the coasts of the southeastern United States, western Central America and Japan, as well as in the Mediterranean and the Caribbean. It can weigh up to 50 pounds and have a 10-foot arm span. The species that comes closest in size to the monsters of science fiction is the giant Pacific octopus, found off the western coast of North America and across the northern Pacific to Japan. The biggest ever captured weighed more than 600 pounds and measured 31 feet from arm tip to arm tip. Despite their impressive growth, Pacific giants rarely live longer than three years. At the other extreme, adults of some pygmy species weigh less than a penny, grow to an arm span of less than 2 inches and are lucky to live six months.

"Out of the water, an octopus feels very loose and slimy," says Roland Anderson, a biologist at the Seattle Aquarium. "It's almost like holding a jellyfish. Underwater, though, their arms feel quite muscular, and they're very, very strong." At the aquarium one night, he reports, a 40-pound octopus smashed the sealed, quarter-inch-thick Plexiglas lid of its tank and crawled out. "A night biologist came in and found it slithering around on the floor," he says.

Octopus literature is filled with tales of naturalists briefly leaving animals in open tanks and returning to find them scaling a bookcase, hiding in a teapot or expired on the carpet. Astonishingly compressible, an octopus can ooze through an opening no bigger than one of its eyeballs. Its yen to get loose is probably linked to an instinctive urge to change dens every week or two. But on dry land, an octopus is doomed: Within half an hour, it will die from lack of oxygen.

An octopus in the open sea might seem easy pickings for predators such as moray eels, sea lions and bigger octopuses, but the octopus can marshal a dazzling array of defenses. Like a squid, it can disorient a pursuer with a burst of purplish-black ink. Should it lose an arm to the jaws of a predator, it can grow a new limb. More impressive still, an octopus can change color in less than a second.

"When it comes to camouflage, it's the most capable organism on the planet without question," says Roger Hanlon, a cephalopod-behavior expert at the Marine Biological Laboratory in Woods Hole, Massachusetts. "Chameleons are just dead-boring compared to octopuses." The octopus' secret is cells in its skin called chromatophores, which are under muscular control: Different pigments come into view as the cell walls are stretched or squeezed.

Octopuses are predators as well as prey, and their camouflage helps them ambush their favorite meals: crabs, snails, shrimp and other small mollusks and crustaceans. In its den, an octopus will often simply lie hidden, its arms coiled, before unrolling one to snag a passerby with the suckers at the tip. While swimming, its preferred attack posture is to parachute gently down with all eight arms outstretched and envelop its prey in the web connecting the arms. Having wrapped up its victim, an octopus holds it against its underside and bites it with a retractable, parrotlike beak. Octopuses also crawl over reefs, probing with their arms for hidden prey.

The salivary glands of all octopuses secrete a chemical that helps disable its prey and breakdown its muscle tissue. In at least one species, Australia's blue-ringed octopus, the secretion contains a neurotoxin that constitutes the deadliest venom known in nature, capable of killing an adult human in minutes. However, these octopuses do not bite humans unless handled or disturbed. The poison is used as a coup de grace on the blue ring's prey, allowing the octopus to eat at its leisure.

Octopuses as a group act like leisurely, almost lazy animals. The reason is chemical: Octopus blood is a poor carrier of oxygen. As a result, an octopus tires easily. To stay alive, it relies on a system involving three hearts and permanently high blood pressure.

This helps explain why even sex is a sluggish activity in many octopus species. With little or no foreplay, without even raising its pulse, a male will extend a specialized arm and insert it into the female's mantle cavity. A sperm packet then slides slowly down a narrow groove in the arm and enters the female's oviduct.

"Yes, it can be pretty blasé," says John Forsythe, a research scientist at the National Resource Center for Cephalopods in Galveston, Texas. "Sometimes the female will continue foraging."

Although scientists have learned a great deal about octopus biology, no one yet knows what and how these animals think. Roger Hanlon of the Woods Hole marine lab points out that explanations other than higher intelligence can account for much of the size of an octopus brain. To camouflage itself, for instance, the octopus must gauge its surroundings and transform its body shape, pattern, color, texture, and brightness in a fraction of a second. "It takes a lot of brain tissue to coordinate all that," says Hanlon.

Then there are all those suckers, which not only grip things but taste them. Each sucker may have 10,000 neurons to handle both taste and touch, and an octopus has thousands of suckers. "You've got to have a big brain to handle all that, too." And the animal needs brainpower to coordinate the movements of its eight long arms. "That's not trivial," says Hanlon. "They use their arms to walk, crawl, burrow, dig, swim, eat, mate--those arms do everything."

Jean Geary Boal is a researcher at the cephalopod center in Galveston who spent a year at Graziano Fiorito's lab in Naples. Having reviewed in depth the evidence for learning in octopuses, she sides with the skeptics. "Hard and fast data about the intelligence of octopuses are not very good," she says. Still, she understands what drives people to establish a sense of kinship with these creatures. "It's extremely easy to anthropomorphize octopuses. They make eye contact with you. They respond to you. They reach toward you. There's just something mesmerizing for people about octopuses."

In researching this article, Massachusetts writer Doug Stewart learned that "octopi" is not the plural that most scientists prefer.