Octopus, squid, cuttlefish and nautilus are all molluscs of the class cephalopod. Cephalopods are exclusively marine animals, with a prominent head, a set of tentacles and a common ability to squirt ink in times of stress.
There are around 800 species of cephalopods, which are widely regarded as the most intelligent of the invertebrates, with all species having well developed senses and large brains.
Cephalopods can change their colors, patterns and textures in milliseconds, whether for signalling (both within the species and for warning) or active camouflage.
With the exception of the Nautilidae, cephalopods have an ink sac which can be used to expel a cloud of dark ink to confuse predators. This sac is a muscular bag which lies beneath the gut and opens into the anus, into which its contents - almost pure melanin - can be squirted.
Most cephalopods can move by jet propulsion which provides extremely rapid movement, however this is very energy intensive and all species have alternative methods of propulsion, e.g. squid and cuttlefish also employ fin motion.
All cephalopods have a two-part beak. They feed by capturing prey with their tentacles, drawing it into their mouth and taking bites from it.