The cuttlefish cephalopod, including some fossil ones

The modern cuttlefish, Sepia officials (Linnaeus, 1758), is not a fish at all. It is actually a small (about "hand size") coleoid cephalopod called a sepiid, whose closest relatives are spiraled colloids (see the August 8, 2019 post, which precedes this present one), squids, and octopods. The internal shell of S. officialis is the familiar "cuttlebone," which is used commercially as the source of lime for canaries and parakeets.

                      Rubber replica of a modern sepiid (8 cm length).

The animal is somewhat squid-like with with two long tentacles and eight short "arms" (note: only five are "visible" in the diagram above). The animal can be as large as 49 cm in length. Sepia officinalis is primarily a bottom dweller in sands and muds, and it swims rapidly (just like a squid) by jetting water. When pursued, it shoots forth a brownish ink like fluid called "sepia." Specimens  especially like to live in the Mediterranean Sea.

Internally, the gut of Sepia is long and mostly straight, except for an abrupt turn near its end, where it connects to the gills.

The cuttlebone, which is a specialized structure, called a  phragmocone, is an internal plate that overlies and shields the vital organs of the Sepia animal. The cuttlebone is located dorsally on the animal, thus allowing for neutral buoyancy in the horizontal position. The underside of the cuttlebone is very soft (easily scratched with a fingernail) chalky material (non-nacreous aragonite) that is like styrofoam, thus providing some degree of buoyancy for the animal.

Lower surface of a cuttlebone. This specimen is nearly 15 cm long and is 4.5 cm wide. Notice the pointed rostrum (very sharp) at one end of the cuttlebone.

                                        Upper surface of same cuttlebone.

Side view of same cuttlebone, 1.5 cm maximum thickness.