Definition of siphuncle in US English:



  • (in shelled cephalopods such as nautiloids and ammonoids) a calcareous tube containing living tissue running through all the shell chambers, serving to pump fluid out of vacant chambers in order to adjust buoyancy.

    • ‘The higher the value, the nearer the siphuncle is toward the concave side of the conch.’
    • ‘The dorsal siphuncles of the Clymeniida tend to be larger than the ventral siphuncles of other ammonoid clades.’
    • ‘As shown above, a dorsal siphuncle is about 50% smaller than a ventral siphuncle of the same diameter.’
    • ‘Given that the shell that covers the siphuncle is its only permeable surface, the interchange of fluids can take place only through this structure.’
    • ‘Outwardly indistinguishable from other ammonoids, clymeniids are distinguished internally by their dorsal siphuncle, contrasted to a ventral siphuncle in almost all other ammonoids.’


Mid 18th century: from Latin siphunculus ‘small tube’.