In octopus the soft head and body are fused. Octopus have 8 long arms covered in suckers They have an advanced nervous system, good vision and acute senses of taste and touch, and can exhibit complex behaviours. They have nervous control of their vascular system. Blood flow to a severed limb can be constricted so as to decrease bleeding. Octopuses are usually found hiding under rocks.

Squid live in mid-water. They have a thin, lightweight transparent horny structure supports the tissue. They hunt fast moving fish. Squid can emit clouds of black ink when stressed

Cuttlefish have an oval body supported by a "cuttlefish bone" which is used to regulate buoyancy by pumping liquid into the tiny chambers within itThere are narrow fins arround the edge of the body. They have a relativley large head, which is partly fused with the body, 8 short arms plus two long tentacles
Cuttlefish swim close to the bottom in search of prey.

Octopus, squid and cuttlefish can all change colour in response to aggression, fear, sexual arousal or the need for camouflage.
Octopus and cuttlefish can also change skin texture. In cephalopods the genders are separate and reproduction is sexual. In the males, sperm is produced in packets known as spermatophores.These pass along a groove in a specialised arm with reduced number of suckers. The arm places the spermatophore into the female mantle cavity where it explodes. The sperm then swim to the ovary. Eggs are laid with a high yolk content, so the young are well formed before hatching. More about Octopus mating and Octopus netting prey