otocyst

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The structure formed by invagination of the embryonic ectodermal tissue that develops into the inner ear.
• noun A statocyst.

from The Century Dictionary.

• noun In zoology, an auditory vesicle; any cavity or cyst which contains the essential parts of an organ of hearing; especially, the auditory vesicle or capsule of some of the Invertebrata, often containing otoliths, and subservient to the function of audition.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl. & Anat.) An auditory cyst or vesicle; one of the simple auditory organs of many invertebrates, containing a fluid and otoliths; also, the embryonic vesicle from which the parts of the internal ear of vertebrates are developed.

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

• noun An organ involved in balance and orientation.
• noun The precursor of the inner ear.