• (n.) One of the auditory organs of certain medusae; -- called also auditory tentacle.
Ten·tac'u·lo·cyst noun [ Tentaculum + cyst .] (Zoology) One of the auditory organs of certain medusæ; -- called also auditory tentacle . 
tentaculocyst, tentaculocysts 1. One of the vesicular or cystic tentacles of a hydrozoan, representing a reduced and modified tentacle. 2. Modified tentacles that act as auditory organs and often have eye-spots on them. 3. Combined visual and auditory organs in the form of modified tentacles.