hydrœcium

from The Century Dictionary.

• noun A sac attached to the swimming-bell or nectocalyx of certain oceanic hydrozoans, as calycophorans, into which the cœnosarc may be retracted. See cut under Diphyidæ.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A sac attached onto a nectocalyx or swimming bell of specific types of oceanic hydrozoans.