Cephalopods are a remarkable group of animals, with a huge array of adaptations. They can change colour faster than a chameleon plus change texture and body shape. They have three hearts pumping blue blood, are jet powered (water is forced through their siphon by contracting their mantel to provide quick thrust) and have representatives in all oceans of the world - from the tropics to the poles - the intertidal to the abyss. And have even inspired legends and stories since recorded history began.
             Cephalopods first appeared some time in the late Cambrian, several million years before the first primitive fish began swimming in the ocean. They are believed to be the ancestors of modern cephalopods (Subclass Coleoidea) diverged from the primitive externally shelled Nautiloidea (Nautilus) very early - possibly in the Ordovician, some 438 million years ago. -This is before the first mammals appeared, before vertebrates invaded land and even before there were fish in the ocean and upright plants on land! Thus, nautilus is very different from modern cephalopods in terms of morphology and life history. They were once one of the most dominant life forms in the world's oceans; today there are only 650 or so living species. (Levinton 2001)
             Octopuses, squids, cuttlefish and the chambered nautilus belong to class Cephalopoda ('head-foot') and the phylum Mollusca. They are nearly always carnivorous, and are characterised by complex behaviour, a well-organised nervous system and a circle of grasping arms, and a powerful beak. Cephalopods are therefore related to bivalves (scallops, oysters, and clams), gastropods (snails and slugs), scaphopoda (tusk shells), and polyplacophorans (chitons). Some of these molluscs, like the bivalves, don't even have a head, much less something large enough to be called a brain! Yet cephalopods have well-developed eyes and other senses that rival our own, large brains, and even show learning and memory skills.
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