Since mammals descended from terrestrial forms and the overwhelming majority of them remained terrestrial, it is not surprising that the mammalian ear is a terrestrial one. The middle ear is therefore a device to match the acoustic impedances of air and the liquids in the inner ear. To achieve this a membrane acts as the receiving area for airborne sound. Coupled to this membrane is a mechanical system, the auditory ossicles, which amplifies the pressure in part at the expense of the displacement. The last element is the output area of the middle ear, the footplate of the stapes, which is identical with the input area of the cochlea. A primarily aquatic ear does not need such a middle ear, because the acoustic impedance of sea water is practically identical to that of the perilymph and endolymph within the inner ear. Mammals adapted to an aquatic life have to adjust a basically terrestrial ear to the requirements of hearing under water. Despite the fact that considerable morphologic changes are often involved, it is nevertheless always apparent that such aquatic ears are modified terrestrial ones.