An organ used to determine orientation relative to gravity. The statocyst is found in organisms ranging from jellyfish to vertebrates.

The statocyst is a chamber lined with mechanoreceptor cells. In this chamber is a mineral particle known as the statolith. The statolith may either be sand found in the environment or some mineralized concretion that is secreted inside of the organ itself. The specific gravity of the statolith must be significantly greater than that of the fluid inside the statocyst in order for the particle to fall in the direction of gravity. When it collides with the side of the statocyst, it causes a tonic (constant) nerve impulse which lets the organism know which direction is up.

In an experiment testing this organ in lobsters, the statolith was replaced with iron filings. As a result, researchers were able to predictively manipulate the posture of the lobster by changing the direction of a magnetic field imposed on the tank. Some scientists conjecture that small amounts of magnetite found in the brains of birds and other organisms may actually function in this fashion, allowing for direct perception of the direction and magnitude of the earth's magnetic field. Migratory birds may use this sense to help them map their long journies.