From ZooTerms (Dictionary of Invertebrate Zoology)
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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.