Hy`dro·the'ca noun ; plural Latin Hydrothecæ , English Hydrothecas . [ New Latin , from English hydra + Greek ... a box.] (Zoology) One of the calicles which, in some Hydroidea (Thecaphora), protect the hydrants. See Illust. of Hydroidea , and... Found on http://www.encyclo.co.uk/webster/H/78