The Collaborative International Dictionary of English v.0.48:

Hydrotheca \Hy`dro*the"ca\, n.; pl. L. Hydrothec[ae], E.
   Hydrothecas. [NL., fr. E. hydra + Gr. ? a box.] (Zool.)
   One of the calicles which, in some Hydroidea (Thecaphora),
   protect the hydrants. See Illust. of Hydroidea, and
   Campanularian.
   [1913 Webster]