The Collaborative International Dictionary
Hydrophyllium
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.
Wiktionary
hydrophyllium
n. (context zoology English) One of the flat, leaf-like, protective zooids covering other zooids of certain Siphonophora.