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 statolith possesses inertia, causing the mass to move when accelerated. Deflection of setae by the statolith in response to gravity activates neurons, providing feedback to the animal on change in orientation and allowing balance to be maintained. Because many echinoderms of this group have no “brain,” they are limited in their actions and responses to stimuli. The statocyst is therefore useful for telling the animal whether it is upside down or not. An upside-down echinoderm is in danger since its belly is not protected by its spiny skin.
