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The Collaborative International Dictionary
Hydrorhiza

Hydrorhiza \Hy`dro*rhi"za\, n.; pl. L. Hydrorhiz[ae], E. Hydrorhizas. [NL., fr. E. hydra + Gr. ? a root.] (Zo["o]l.) The rootstock or decumbent stem by which a hydroid is attached to other objects. See Illust. under Hydroidea.

Wiktionary
hydrorhiza

n. (context zoology English) The rootstock or decumbent stem by which a hydroid is attached to other objects.