statocyst In certain aquatic invertebrates, a vesicle containing mineral grains that stimulate sensory cells as they move in response to the movement of the animal. The statocyst is primarily a gravity receptor, acting as an organ of equilibrium that allows a swimming organism to maintain a horizontal attitude. One or more mineral grains (statoliths), often of calcium carbonate, rest in sensory hairs; any displacement relative to gravity causes the statolith(s) to stimulate the sensory hairs which send information to the nervous system.

statocyst (otocyst) A balancing organ found in many invertebrates. It consists of a fluid-filled sac lined with sensory hairs and contains granules of calcium carbonate, sand, etc. (statoliths). As the animal moves the statoliths stimulate different hairs, giving a sense of the position of the body or part of it. The semicircular canals in the ears of vertebrates act on the same principle and have a similar function.