The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass (statolith) and numerous innervated sensory hairs (setae). The statolith's inertia causes it to push against the setae when the animal accelerates. 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.
In other words, the statolith shifts as the animal moves. Any movement large enough to throw the organism off balance causes the statolith to brush against tiny bristles which in turn send a message to the brain to correct its balance.
Statocysts in echinoderms
Because many echinoderms of this group have only simple nervous systems without a controlling "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.