The organ of balance and hearing: the inner ear or labyrinth organ. It consists of an otosaccus and the semicircular canal-system. The canals are arranged in the three planes of space and are the site of the rotary sense, which by perceiving the angular acceleration of the endolymph of the tubes informs the animal about any rotary motions, independent whether it itself turns or is showed around by the external medium. The otosaccus comprises the dorsally located utriculus, which forms the base of the semicircular canals and in all animals, except for the notopteroidei, opens more or less widely into the sacculus. Both are the site of the true organ of balance because they harbour the otoliths, in bony fishes calcareous bodies, also arranged in the three planes of space. The utriculus contains the largest one, the utricololith or lapillus, the sacculus contains the sacculolith or sagitta and the lagena, in fishes still a minor annex to the sacculus, the smallest of the otoliths, the lagenolith or asteriscus. When a vertebrate changes its position in its surrounding, either by tself or forced to by the medium, the otoliths or statoliths will stimulate sensory cells on which they lie by the shearing force they exert on the sensory cilia.

Alternative forms for stato-acoustic organ : Saccukus, Utriculus.