1. Keep the eyeball surface moist when blinking.
2. When there is a sandstorm, close your eyes to prevent foreign objects from entering your eyes.
3. When sleeping, keep the optic nerve from being stimulated by light.
The living environment of fish is very different from that of terrestrial animals.
1. Because they live in water, they don't need to blink to keep the eyeball surface moist.
2. There is no sandstorm underwater, and basically no foreign body can enter the eyeball. Even if other foreign objects arrive near the eyes, they can easily get rid of them because they are in the water.
3. Compared with mammals, fish is a lower organism and has not evolved eyelids.
4. Some fish sleep alternately through two hemispheres. Usually, one and a half brains are asleep and the other is awake, which changes rhythmically every ten minutes. On the surface, it never seems to sleep.
Therefore, for most fish, eyelids have no practical function, and in the process of evolution, fish without eyelids have become the dominant species.
Hope to adopt! thank you