Underwater the human eye loses most of its refractive power and the lens cannot compensate for that. In the dolphin eye, the refractive power of the lens has become greatly increased, because the lens is located further forward and is completely spherical. The dolphin eye looks a lot like a fish eye. These narrow slits may give the dolphin more depth of vision above water and therefor better vision. Special adaptations in the edges of the lens may also improve above water vision.
Dolphins have tiny external ear openings, which are barely visible, just behind the eyes.
These openings probably have no or only a limited function in hearing. The acoustic faculty in dolphins is well developed. The auditory systems in the brain are highly developed and much larger than for instance in humans. This strong development of the auditory systems of the brain is at least in part an explanation for the large brains in dolphins. The auditory nerve has double the amount of nerve fibres compared to the human auditory nerve.