The Collaborative International Dictionary
Hydrotheca
Hydrotheca \Hy`dro*the"ca\, n.; pl. L. Hydrothec[ae], E. Hydrothecas. [NL., fr. E. hydra + Gr. ? a box.] (Zo["o]l.) One of the calicles which, in some Hydroidea (Thecaphora), protect the hydrants. See Illust. of Hydroidea, and Campanularian.
Wiktionary
hydrotheca
n. (context zoology English) One of the calicles that protect the hydrants in certain Hydroidea (Thecaphora).