siphuncle
From ZooTerms (Dictionary of Invertebrate Zoology)
siphuncle (noun; Greek diminutive siphon, tube): 1. (Arthropoda: Insecta) see cornicle.
2. (Mollusca: Cephalopoda) A tubular vascular extension borne at the apex of the visceral hump, that runs through the outer chambers of the nautiloid shell and secretes gas into them for buoyancy.
