Most mollusks are protected by a shell and many of them are not very mobile. Although the ancient nautilus has an external shell, the trend in cephalopods is to internalize and reduce the shell. The shell in cuttlefish, when present, is internal. The cuttlebone from cuttlefish 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 all together.
Cephalopods 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 some time in the late Cambrian several million years before the first primitive fish began swimming in the ocean. Thus, nautilus is very different from modern cephalopods in terms of morphology and life history.