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Both the sense of balance and that of hearing depend on the action of mechanoreceptors, hair cells that detect the movement of solid or liquid particles.
Most animals maintain a definite orientation to Earth's gravity; when taken from their typical position, they try to return to it. This happens thanks to the presence of special receptors, the statoreceptors, that continually inform the nervous system about the position of the body.
Some celenterates (jellyfish), several mollusks, several annelids and most arthropods have balancing organs, called statocysts, whose function is realize the sense in which the force of gravity is acting on the body. A statocyst is basically a internal pouch lined by statoreceptor cells, endowed with sensory hair.
Inside this bag is a structure called a statolith, formed by one or more pebbles stuck together. The statolith, attracted by the force of gravity, presses and stimulates the sensory cells that come into contact with it.
When the animal changes its position, the cells in contact with the statolith change, it is this change in the pattern of stimulation that enables the nervous system to recognize the change in position of the animal.
In vertebrates, balance and the sensation of movement result from the action of mechanoreceptors located in three types of structures present in the inner ear: saccule, O utricol and the semicircular canals.
The saccule and the utricle are two fluid-filled sacs whose inner walls have clusters of hair cells. The semicircular canals are three curved tubes, also filled with fluid, located above the utricle. At the base of each semicircular canal is a dilation called the ampoule, where there is a cluster of hair cells, encased in a jelly-like mass.