Turtles (Order Testudines)
Turtles have a true nictitating membrane which moves obliquely across the eye from the anterior canthus (corner). This is shown in an elongated turtle (Indotestudo elongata), Eastern snake-necked turtle (Chelodina longicollis) and Murray River turtle (Emydura macquarii).
Lower lid and nictitating membrane blink in an elongated turtle.
Video 1. Blink in an elongated turtle video-ed at 25fps, played back at 30% speed.
At 80ms, the eyeball has retracted and the lower eyelid has elevated, covering the eye. By 400ms, the lower eyelid has descended revealing a nictitating membrane which half covers the eye. Its free edge is at right angles to the palpebral fissure.
Again, retraction of the eyeball plays a major role in the blink, together with elevation of the lower lid. The only movement of the upper lid is passive as it sinks when the eyeball retracts.
Blink in an Eastern snake-necked turtle. Played at 30% speed.
At 80ms, the right lower eyelid has elevated and the eyeballs have retracted (best seen in profile in the left eye)
Upward eye movement and lower lid blink with globe retraction in a Murray River tortoise played back at 20% speed
According to Walls (1963), the nictitating membrane in turtles is drawn across the eye by the pyramidalis muscle (Figure). As the pyramidalis muscle pulls on both the nictitating membrane tendon and lower lid tendon, the nictitating membrane and lower lid cannot operate independently.
Medial view of a turtle eyeball showing the pyramidalis muscle (Pyr) which sends tendons to the nictitating membrane (n) and lower eyelid (p.i). Also seen is the retractor bulbi muscle (R.b). ‘Retraction of the globe will also pull on these tendons causing the lower lid to rise and the nictitating membrane to cross, though the latter seems to vary with different species’ (Walls, 1963).
The palpebral fissure is canted at an angle as the freshwater and marine turtles float in a slanted position at the water surface.
Angle of the palpebral fissure in a western painted turtle (Chrysemys picta). The line is drawn through the canthi.
Ecker, A (Translated by Haslam, G). 1889. The anatomy of the frog. Oxford: OUP.
Walls, GL. 1963. The vertebrate eye and its adaptive radiation. New York and London: Hafner.