What is hearing organ of fish?

"Fish Hearing Studies Physiological work on the octavolateralis system shows that fish can acquire information from a sound source using two systems, the inner ear and the lateral line (Parker, 1909; Popper and Platt, 1993), though sensitivity to sound pressure requires an additional connection between the ear and swim bladder or other air reservoir (Popper and Fay, 1993; Yan et al., 2000). For most fish, the lower frequency range between 10 and 30 Hz is perceptible through the lateral line mechanoreceptors, which can be responsive to frequencies as high as 300 Hz (Munz, 1989; Popper and Fay, 1993; Coombs and Montgomery, 1999)."

Source: http://www.ariamarine.com/Fish_%20hearing/fish_hearing.htm