Snakes and geckos
Snakes and some lizards have no moveable eyelids, and cannot blink. It is supposed that they evolved from burrowing ancestors. In these creatures, the eyes are protected from injury by a transparent scale, or spectacle (brille, German). This has a keratin layer and contains blood vessels which constrict during fixation, lessening the barrier they pose to light passing through to the eye beneath. The cornea is separated from the fixed spectacle by a fluid filled space, making limited movement of the eye beneath possible. The spectacle is shed during moulting.
During sleep, light is more or less prevented from reaching the retina by the closure of the slit-like pupil. Below are some examples:
Vertically aligned slit-shaped pupils, almost closed, in a sleeping green tree python (Morelia viridis)
Eye movement behind the spectacle is not easy to see:
Three examples of eye movement in a king cobra (Ophiophagus hannah) played at 30% speed
Eye movement in a coastal taipan (Oxyuranus scutellatus)
Closure of the vertically aligned pupil in a sleeping New Caledonian giant gecko (Rhacodactylus leachianus). The slit of the pupil is punctuated by larger circular openings.
Anatomical considerations
According to Walls (Walls...), animals such as burrowing lizards and fish, in which the eye is placed in intimate relation to a potentially injurious medium such as sand, mud or water, have evolved fixed protective spectacles. The eyes are free, more or less, to move beneath the spectacle. In snakes and some lizards, the upper and lower eyelids have fused to form a hard, horny, transparent scale. In Figure 1 (van Doorn 2012), the spectacle (C) is seen to be separated from the eye by an oily fluid-filled subspectacle space. The fluid is secreted by the Harderian gland.
Figure 1
The spectacle, unlike the cornea, has blood vessels coursing through it.
Figure 2
The anatomy of the snake eye is shown in more detail in Figure 3 (Walls).
Figure 3
References
Walls, GL. 1963. The vertebrate eye and its adaptive radiation. New York and London: Hafner.
van Doorn, K (2012). Investigations of the reptilian spectacle. PhD thesis University of Waterloo, Canada