Examples of cephalopods include octopuses, nautiluses, squid, spirulas and cuttlefish. Cephalopods are found in all of Earth's oceans, either on the seabed or in open water.
Octopuses range in size from the tiny California octopus to the giant octopus, which can weigh 400 pounds and has arms up to 23 feet long. Octopuses are venomous, have eight arms, rounded bodies, good eyesight and are surprisingly intelligent.
Squid have eight arms plus two appendages that help them capture prey. Schools of squid are found in the cold waters off Antarctica, where they eat krill and are in turn eaten by whales, seals and humans. The 46-foot-long giant squid is one of the largest cephalopods on Earth.
The chambered nautilus' shell is full of gas-filled compartments that allow the animal to remain buoyant. In the meantime, the animal's body is contained in the last compartment. The nautilus may have as many as 90 tentacles, though unlike the squid and octopus, these tentacles have no suckers.
The spirula is a squid-like creature that lives in a spiral shell that resembles a ram's horn. It's seldom encountered because it lives deep in the ocean.
The cuttlefish, which resembles a shell-less nautilus, is famous for its spectacular ability to change not only the color of its skin, but also its texture. They do so to hypnotize prey, challenge rivals and impress potential mates.