What they are
Squids, Octopus, Cuttle Fish, Nautiloids, Ammonoids, Belemnites
Morphology
All cephalopods are types of Mollusks. Cephalopods are also the most advanced of all mollusks, having the most intelligence of any mollusk. They also have the largest brains of all the invertebrates.
Cephalopod shells can be a variety of shapes. They can be long and straight, looking something like a cigar. They can also be spiraled, usually remaining in the same plane of symmetry. Rarely though, a few species are characterized with a spiral upwards along a central axis. Their shells can easily be mistaken for gastropods, but cephalopod shells are chambered, having many internal partitions inside of their shells, while gastropod shells are unchambered.
The name cephalopod, in Greek, means "head foot." This name was applied because the foot of the organism is around the head. But cephalopods lack a traditional foot, instead having between eight and ten tentacles attached to their heads. The tentacles, which are lined with many suction cups or hooks, are used for capturing prey.
Most modern Cephalopods keep their shell inside of their bodies, like the squid and cuttle fish. The only modern cephalopod to have an external shell is the Nautilus.
Ecology
The cephalopods are carnivorous marine organisms. All cephalopods are pelagic organisms, which means they live their lives as swimmers, not crawling about the sea floor.
They are found at nearly all depths of the ocean. Some would swim just above the sea floor and hunt prey among the mud and rocks. Some would stalk their prey, using quick bursts of speed to catch fish. Still, others cephalopods were more passive hunters and would float through the water with their tentacles hanging out, grasping at anything that got too close.