Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula. In the case of the cuttlefish, the siphuncle is indistinct and connects all the small chambers of that animal's highly modified shell; in the other cephalopods it is thread-like and passes through small openings in the walls dividing the chambers.

The siphuncle is used primarily in emptying water from new chambers as the shell grows. Essentially what happens is the cephalopod increases the saltiness of the blood in the siphuncle, and the water moves from the more dilute chamber into the blood through osmosis. At the same time gas, mostly nitrogen, oxygen, and carbon dioxide, diffuses from the blood in the siphuncle into the emptying chamber. Note that the cephalopod does not pump up the shell; the gas moving into the chamber is a passive process, instead the energy is used in absorbing the water from the chamber.