statocyst

thumb|right|alt=|Drawing of the statocyst system thumb|Statocysts (ss) and statolith (sl) inside the head of sea snail Gigantopelta chessoia

The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophores, echinoderms, cephalopods, crustaceans, Proseriata and Catenulida (both are taxonomic groups of flatworms), and gastropods. 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.