The Collaborative International Dictionary of English v.0.48:

Siphonal \Si"phon*al\, a.
   Of or pertaining to a siphon; resembling a siphon.
   [1913 Webster]

   Siphonal stomach (Zool.), a stomach which is tubular and
      bent back upon itself, like a siphon, as in the salmon.
      [1913 Webster]