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The Collaborative International Dictionary
Tentaculocyst

Tentaculocyst \Ten*tac"u*lo*cyst\, n. [Tentaculum + cyst.] (Zo["o]l.) One of the auditory organs of certain medus[ae]; -- called also auditory tentacle.

Wiktionary
tentaculocyst

n. (context zoology English) One of the auditory organs of certain medusae.