(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 dorsal siphuncles of the Clymeniida tend to be larger than the ventral siphuncles of other ammonoid clades.
- Outwardly indistinguishable from other ammonoids, clymeniids are distinguished internally by their dorsal siphuncle, contrasted to a ventral siphuncle in almost all other ammonoids.
- Given that the shell that covers the siphuncle is its only permeable surface, the interchange of fluids can take place only through this structure.
Mid 18th century: from Latin siphunculus 'small tube'.
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