Octopuses (along with their closest cephalopod kin, nautiluses, cuttlefish and squids) are the most dynamic members of phylum Mollusca, which also includes snails and oysters.
There are thought to be about 200 species of octopuses. The most common species is Octopus vulgaris, which is found in every ocean and gets to be about three feet long.
They come in a wide range of sizes. The smallest species is the Californian Octopus (Octopus micropyrsus) which as adults may only be a half-inch long. The largest species is the North Pacific Octopus (Octopus dofleini) which can be over 30 feet in length and well over 100 pounds in weight.
The largest confirmed octopus ever caught was 33 feet long and weighed 600 pounds. However, there are rumors of some North Pacific octopuses living in deep waters off the coast of Canada that get to truly gigantic size. According to a perhaps-apocryphal story, a fisherman once found a rotted lump of cephalopod that weighed in at close to a ton; tissue samples later identified the flesh as belonging to an octopus rather than a squid.
Anatomically, octopuses are quite interesting. Instead of a penis, male octopuses have a specialized tentacle (usually the third tentacle on their right side) that they use for sex.
These cephalopods also have three hearts. They have a heart at the end of each of their gills; these hearts pump the blood through the gills. Their third heart pumps blood through the rest of the octopus' body.
Octopus blood itself is interesting because it uses a copper-based molecule to carry oxygen instead of the more familiar iron-using hemoglobin molecules found in vertebrate animals. Because of the copper, octopus blood appears blue instead of red.
Octopus eyes are quite sophisticated, and octopuses have very good vision. However, their eyes function differently than human eyes; instead of changing the shape of their lens to focus on different object, the muscles of their eyes move the lens back and forth within the eyeball.
But even without their vision, octopuses can get around quite well due to the extreme sensitivity of the suckers in their tentacles. In laboratory tests, blindfolded octopuses could tell different objects apart as well as visually-unimpeded octopuses.
Octopuses also have the most complex brains of any invertebrates; they quickly learn new tasks by trial-and-error, and they seem to form the same short-term and long-term memories as vertebrate animals.
Octopuses: Master Escape Artists
Being such highly intelligent creatures, octopuses are master escape artists, and can be hard to keep in an aquarium if they want out. I've known a couple of people who've gotten a small octopod for their home saltwater aquarium, only to find the creature dead and dried-up on the floor after it pried off the tank lid and crawled out in the night. One acquaintance of mine found his little octopod dead of electrocution after it escaped from its tank and made the mistake of probing a socket on a nearby power strip with one of its damp tentacles.
My marine biology professor from my undergrad college once had a laboratory job where they often kept octopuses. He learned a simple technique for convincing the new octopus that, yes, it really wanted to stay put in the aquarium.
He'd put the new octopus in the tank, do some odds and ends in the lab for a few minutes, then leave the room and turn out the lights. He'd wait outside until he heard the telltale, sodden slap that mean the octopus had staged a jailbreak and had hit the floor. He'd wait one minute, then go back into the lab and put the octopus back in the tank.
He'd repeat the process, the next time waiting three minutes. And on the third time, he waited a whole five minutes before rescuing the miserable, sticky, dust-bunny-covered octopus from the lab floor, rinsing it off, and placing it back in the aquarium.
After that third time, he told me, an octopus wouldn't try to escape again. In fact, he sometimes had to work hard to get it out if it needed to be examined or transferred to a new tank so the old one could be cleaned.