This group includes fast moving, many-armed hunters, quite different from the more typical snails, slugs and clams that we think of as molluscs.

No Shells: Aside from the nautilus, other cephalopods have no external shells. Octopuses have no shells at all. Squids and cuttlefishes retain a small internal shell which helps to stiffen their bodies and to control bouyancy.

Hunter's Tools: All cephalopods are carnivorous hunters. There are no vegetarian cephalopods. As predators, most cephalopods have a well-developed brain and keen eyesight. Most have eyes similar in structure to vertebrates like us. Instead of a single broad foot, this has become adapted into many long and flexible arms covered with suckers and sensitive touch and taste sensors. A nautilus may have 90 such arms, while octopus have 8 and squids and cuttlefishes 8 to 10. The mouth lies in the middle of the arms; thus their arms can be considered 'super-lips'. Most cephalopods retain a radula, and all have a sharp two-part beak, like a bird's. These are used to tear their prey into smaller pieces. Some squids and octopuses can inject a toxin with their bites. Their digestive systems are adapted for rapid digestion. They also have a well-developed blood circulatory system to support their more active life-style.

Jet-setting Molluscs: Cephalopods can zoom about quickly with jet-propulsion. They forcefully squeeze water out of their mantle cavity (a cavity in their body) through a flexible funnel and shoot off in the opposite direction. The funnel can be pointed in different directions to control their movement. Squids and cuttlefishes also have fins along their bodies to control swimming movement.

Disappearing Ink: Many octopuses, squid and cuttlefish can squirt ink that distracts predators and clouds up the water. The ink might be combined with mucous to form a decoy to distract the predator while the animal makes its escape. The ink may also contain substances that affect the senses of other sea creatures. The ink contains melanin, the same pigment that colours our skin. It is produced in a special gland embedded in the liver. Another organ produces mucus to mix with the pigment. Jets of water from the funnel direct and shape the pigment and mucus into a shape that suits the cephalopod's purposes.

Colourful Cephalopods: Cephalopods have complex skin that can instantly change colour. Octopuses and cuttlefishes can also change the texture of their skin, creating bumps, flaps, fingers and other projections to match their surroundings. Many squids are also bioluminescent. They do this to hide from predators and prey, as well as to communicate with each other.

Cephalopod Society: Cephalopods have relatively well-developed behaviour for interacting with each other. This involves colours changes and other methods of communication during courtship, mating and territorial disputes.

Cephalopod Babies: Cephalopods have separate genders and practice internal fertilisation. The male usually has some sort of modified arm to insert his sperm packet into the female. She uses the sperm to fertlise her eggs which are usually laid in capsules or cases. These do not hatch into free-swimming larvae but into miniatures of the adults. However, some of these tiny juveniles might drift with the plankton for a while. Most cephalopods don't live long; about 1-2 years, dying soon after they reproduce.

Human uses: People everywhere enjoy eating cephalopods. They are also much studied for a better understanding of human anatomy.