As long as we're talking about cephalopods...

My last post Angry squids and octopi was about artwork involving squids and octopi. Both are of the class cephalopoda which belong to the phylum Mollusca. Cephalopods are an ancient type of invertebrate that date back to the Cambrian period. In fact, they predate fish.

If you're interested in more information about them visit the website The Cephalopod Page maintained by James B. Wood Ph.D. Below is an excerpt from his introduction to the site.

Cephalopods, the class of mollusks which scientists classify octopuses, squid, cuttlefish and nautiluses, can change color faster than a chameleon. They can also change texture and body shape, and, and if those camouflage techniques don't work, they can still "disappear" in a cloud of ink, which they use as a smoke-screen or decoy. Cephalopods are also fascinating because they have three hearts that pump blue blood, they're jet powered, and they're found in all oceans of the world, from the tropics to the poles, the intertidal to the abyss. Cephalopods have inspired legends and stories throughout history and are thought to be the most intelligent of invertebrates. Some can squeeze through the tiniest of cracks. They have eyes and other senses that rival those of humans.

The class Cephalopoda, which means "head foot", are mollusks and therefore related to bivalves (scallops, oysters, clams), gastropods (snails and slugs), scaphopoda (tusk shells), and polyplacophorans (chitons). Some mollusks, such as bivalves, don't even have a head, much less something large enough to be called a brain! Yet cephalopods have well-developed senses and large brains. Most mollusks are protected by a hard external shell and many of them are not very mobile. Although nautilus has an external shell, the trend in cephalopods is to internalize and reduce the shell. The shell in cuttlefish, is internal and is called the cuttlebone, which is sold in many pet shops to supply calcium to birds. Squid also have a reduced internal shell called a pen. Octopuses lack a shell altogether.

Nautilus pompiliusOctopus cyaneaCephalopods are found in all of the world's oceans, from the warm water of the tropics to the near freezing water at the poles. They are found from the wave swept intertidal region to the dark, cold abyss. All species are marine, and with a few exceptions, they do not tolerate brackish water.

Cephalopods are an ancient group that appeared in the late Cambrian period several million years before the first primitive fish began swimming in the ocean. Scientists believe that the ancestors of modern cephalopods (Subclass Coleoidea: octopus, squid, and cuttlefish) diverged from the primitive externally-shelled Nautiloidea (Nautilus) very early - perhaps in the Ordovician, some 438 million years ago. How long ago was this? To put this into perspective, 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!