Source : Webster's Revised Unabridged Dictionary (1913)

Hydrocaulus \Hy`dro*cau"lus\, n.; pl. {Hydrocauli}. [NL., fr.
   Gr. "y`dwr water + ? a stalk.] (Zo["o]l.)
   The hollow stem of a hydroid, either simple or branched. See
   Illust. of {Gymnoblastea} and {Hydroidea}.