Cephalopods are molluscs whose foot is modified into a head and tentacles.  Many have no skeleton beyond some mouthparts ("beaks") that occasionally fossilize (octopi and most squids).  Others have an internal support structure called a "rostrum", such as the spongy aragonite structure of the extant cuttlefish (upper two photos below) and the denser one of the extinct belemnites (two photos below those).  The cuttlebone is from Sepia sp. from Nuweiba, Egypt (Carter Coll., Gi1) and the belemnite is Belimnitella americana from the Cretaceous of New Jersey (GSW Tax. Coll. 65002).  The end view of the belemnite (on right -- same specimen as on left) shows that the rostrum was partly hollow.  In fact, there was an anterior elongate dorsal projection of the rostrum called the "proostracum" (which is usually missing), and some septa and a siphuncle (see below under structure) in the cavity, suggesting that the belemnite, though internal, also had soft tissue inside, as in the more common shelled cephalopods.



Some Recent cephalopods make a very fragile egg case that is similar in coiling form (though lacking septa) to a coiled nautiloid or ammonoid.  A couple of pictures of the recent Argonauta hians are provided below (Carter Coll. RGi3).


The more common cephalopods, of course, are the nautiloid and ammonoid cephalopods that you learn about even in Geology 2.  The only living genus is Nautilus, which includes several species in the Indian and western Pacific Oceans.

The photos below show the basic external (on left -- Carter Coll, RGi2) and internal (on right -- GSW Tax. Coll. 65029) structure of Nautilus pompilius with some of the important parts labeled.  Notice that the shell is planispirally coiled, as are most coiled cephalopods, with the center of coiling being the umbilicusThere are numerous internal partitions (septa) separating the interior into chambers (camerae).  The oldest chamber is the protoconch, the chambered portion of the shell is the phragmocone, and the open part of the shell, containing the viscera, mantle, and foot (when retracted) is the body chamber.  More detail on the morphology is on the next page.