The statocyst is a balance organ present in some aquatic invertebrates (Cnidarians, Ctenophores, Bilaterians). It consists of a sac-like structure containing a mineralised mass (statolith) and numerous innervated sensory hairs (setae). The statholith possesses inertia, causing the mass to move when accelerated. Deflection of seta by the statolith in response to gravity activates neurons, providing feedback to the animal on change in orientation and allowing balance to be maintained.
See: Accelerometer, Inertial guidance
