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 chamber...
• (n.) The tube which runs through the partitions of chambered cephalopod shells.
(from the article `seashell`) ...operculum (trapdoor) to seal off the aperture. In the cephalopods Nautilus and Spirula, the planospirally coiled shell consists of multiple ...
Si'phun`cle noun [ Latin siphunculus , sipunculus , dim. of sipho . See Siphon .] (Zoology) The tube which runs through the partitions of chambered cephalopod shells. 
siphuncle A small cord of tissue that secretes gas into the buoyancy chambers of the external shell of a nautilus or similar mollusk.
Cornicle - tubular projections from the abdomen of an aphid.