It's true, although they can sleep, fish really don't close their eyes-there is no eyelid structure (only some sharks have instantaneous membranes), and the eyelids are made by land animals.
For fish, they always live in water and their eyes are always moist. Keeping your eyes moist is as simple as swimming. This situation has lasted for hundreds of millions of years. The situation changed after the animals landed. When our aquatic ancestors first came to the earth, they found that their eyesight was keen, because the light on land was much richer than that in water. Because of this, in the process of evolution, terrestrial animals know how to use light better, and their vision and color vision have been greatly developed. However, the eyes developed from water begin to face great risks, and the air will soon blow it dry, causing infection or blindness.
For this reason, land animals have a set of equipment to keep their eyes moist and clean in a world full of air and dust, such as Larimar system, eyelids, blink film and glasses, among which glasses are a protective transparent film, which has different origins in different vertebrates and the situation is more complicated. Close your eyes mainly by looking at your eyelids and blinking.
The lacrimal gland produces tears instead of seawater to infiltrate the eyes. In order to maintain the stability of internal and external osmotic pressure, tears contain a certain amount of salt, so tears are also salty. Tears contain sugar and protein nutrition, which is the nutrient source of cornea, and also contains bactericidal substances to maintain the health of eyes. Under normal circumstances, a person's lacrimal gland can produce 1 cubic centimeter of tears a day. When foreign bodies enter the eyes, the secretion of tears will increase to wash away these foreign bodies. Interestingly, people are the only animals that shed tears because of emotional and psychological changes, which seems to have nothing to do with physiological reasons, and the origin of their evolution is still a mystery. Tears are secreted and stored in the lacrimal sac in the inner corner of the eye and communicate with the nasal cavity, which is why you "cry" …
Most aquatic or major aquatic animals have no eyelids. Eyelids seem to distribute tears evenly over the eyes. They first appeared in semi-aquatic animals, that is, the ancestors of amphibians such as salamanders and frogs. When animals evolved into land creatures, some eyelids began to become opaque to protect their eyes from strong light. Of course, the result of closing your eyes is that there is no light, and to keep your eyes moist, you must close your eyes or blink constantly. However, in a world full of predators, even losing light for a while can be fatal.
Therefore, in order to keep the light, many animals keep another kind of transparent film-instant film, and some even give up the eyelids, of course, both. Blink membrane is a kind of translucent "eyelid", which has appeared since the shark era. Its original function is to prevent foreign bodies from scratching eyes, and now it is used by land animals to scrape tears. The human blink membrane has degenerated, but there are some things about cats. Oh, it's inside the eyelid. You can easily find it in the eyes of birds such as chickens. Are you interested? Look for it. ?
Transient membranes of different animals
Eagle in combat mode
The author of this article is Han Yue, an outstanding researcher, director of species website and popular science writer.