Anatomical and scanning electron microscopic investigations of the eyelids of Egyptian agama
The eye of the Egyptian agama, Trapelus mutabilis , has three eyelids: mobile upper and lower eyelids and a small, immobile nictitating membrane. The upper eyelid is shorter than the lower eyelid. The skin of the head above the upper eyelid extends laterally to form a superior extension. This extension bears large, plate-like scales that contain skin sense organs at their terminal points (Figs. 1a, b, and c). Highly variable polygonal scales are observed on the external surface of both eyelids, with approximately ten scales at the edges of the upper and lower eyelids, clearly observed by SEM (Figs. 1 and 2). The orientation of scales on the upper eyelid is similar to that on the lower eyelid; these scales are arranged in an imbricate order, and their surface features micro-ornamentation and Oberhäutchen with honeycomb-like structures. These honeycomb-like structures consist of numerous tiny honeycombs. Additionally, it was observed that the tips of these scales possess skin sense organs (Figs. 1d and 2b)\RL.
The anatomical investigation of the nictitating membrane of T. mutabilis revealed it to be a concave and non-mobile membrane. It is reduced and appears as a small projection connecting with the lacrimal gland posteriorly in the anterior canthus of the eye (Fig. 5a). SEM investigation of the nictitating membrane revealed that its surface has many folds marked by crypt-like structures (Fig. 5b).