siphuncle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tubelike structure in the body of a shelled cephalopod, such as a nautilus, extending through each chamber of the shell.

from The Century Dictionary.

• noun In zoöl:
• noun A siphon; especially, the siphon or funnel of tetrabranchiate cephalopods, between the chambers of the shell which it connects. See cut under Tetrabranchiata.
• noun In entomology, same as nectary, 2. Also called cornicle, honey-tube, siphonet, and siphunculus.
• noun In the cephalopods, the calcareous tubular wail of the siphon.
• noun The protrusible portion of the mouth-parts of a true louse, supposed to be modifications of the epipharynx and hypopharynx.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The tube which runs through the partitions of chambered cephalopod shells.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A strand of tissue passing longitudinally through the shell of a cephalopod mollusk, used primarily in emptying water from new chambers as the shell grows.