Source : Webster's Revised Unabridged Dictionary (1913)

Hydrophyllium \Hy`dro*phyl"li*um\, n.; pl. L. {Hydrophyllia}, E.
   {Hydrophylliums}. [NL., fr. Gr. "y`dwr water + ? a leaf.]
   (Zo["o]l.)
   One of the flat, leaflike, protective zooids, covering other
   zooids of certain Siphonophora.