Hydrocaulus \Hy`dro*cau"lus\, n.; pl. {Hydrocauli}. [NL., fr. Gr. "y`dwr water + ? a stalk.] (Zool.) The hollow stem of a hydroid, either simple or branched. See Illust. of {Gymnoblastea} and {Hydroidea}. [1913 Webster]