The Collaborative International Dictionary
Hydrocaulus
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.
Wiktionary
hydrocaulus
n. (context zoology English) The hollow stem of a hydroid, either simple or branched.