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.
