The Collaborative International Dictionary of English v.0.48:

Hydrophyllium \Hy`dro*phyl"li*um\, n.; pl. L. Hydrophyllia, E.
   Hydrophylliums. [NL., fr. Gr. "y`dwr water + ? a leaf.]
   (Zool.)
   One of the flat, leaflike, protective zooids, covering other
   zooids of certain Siphonophora.
   [1913 Webster]