Therefore, their ability to experience sensations such as pain would be diminished compared to other vertebrates. The structure of a fish's eye is similar to that of other vertebrates. The eyes are smooth and generally don't extend beyond the contours of the body. However, fish need the ability to see under water and so the outer wall of the eye is flatter in fish than in land dwelling vertebrates. The lens is also much more rounded in fish.
Fish have ears, which act as an organ of balance as well as hearing.
Their hearing consists of little more than the detection of vibrations in the water. In many fish the hearing is connected to the swim bladder by a tube like growth or a series of small bones. It is thought this intensifies the impulses from vibrations in the water.