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Why is it that snakes do not have eyelids?

Is it due to that fact that they are not as evolved as we humans and other organisms that have eye lids, or is there any other significance behind it if any?

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    $\begingroup$ "the fact that they are not as evolved as we humans" - there is no such fact. Evolution is not a scale going from "less" to "more". All organisms are equally evolved. So, it can't be due to that "fact" by definition. $\endgroup$
    – rumtscho
    Oct 30, 2015 at 17:58
  • $\begingroup$ @Aneesh For more discussion on the concept of "more/less evolved", have a look at the post Are we “more evolved” than present-day bacteria? $\endgroup$
    – Remi.b
    Mar 31, 2018 at 5:47

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Snakes have no eyelids. Each eye is covered with a single, transparent scale. These eye scales protect the eyes from mechanical damage, and prevent the eyes from drying out, just as an eye lid would do. In effect, snakes developed another solution to protect their eyes.

As a result, snakes cannot blink and they sleep with their eyes open.

A snake’s eye scales are part of its skin. This means that when a snake sheds its skin, it must also shed its eye scales. When it is time for shedding, the eye scale turns opaque, reducing vision (Fig. 1).

opaque snake eye
Fig. 1. Opaque snake eye before skin shedding. Figure source: Snakes are long blogspot

Source: Museum of Victoria

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  • $\begingroup$ Good answer +1. It would be great (although that would probably take some work) to have some mapping of this trait on a phylogenetics tree (or just some information about their snakes and mammals sister species). $\endgroup$
    – Remi.b
    Mar 31, 2018 at 5:49
  • $\begingroup$ @Remi.b - thanks heaps. But the phylogenetics I leave up to you my friend. I'm in sensory systems you see :) $\endgroup$
    – AliceD
    Mar 31, 2018 at 9:47

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