The Collaborative International Dictionary
Hydrothecas
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
hydrothecas
n. (plural of hydrotheca English)