A small flap called an operculum protects the gill opening. Each gill filament is richly supplied with blood vessels. Some primitive fish species still have an organ called a swim bladder. This may have originally been used to allow fish to breathe in deep waters. However, it's main function today is to help keep fish afloat.
Fish have extremely small brains .
The cerebrum, which is the part of the brain used for thought and reason, is almost totally lacking in fish. Fish also have poorly developed nerves. 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.