Introduction
The ocular adnexa are the protective and supporting structures of the eye, including the eyelids and their parts, the third eyelid (nictitating membrane), orbital glands, and oculomotor muscles (Guerra-Fuentes et al., 2014). Peripheral to the housing of the orbit, the eye receives immediate protection and support from the eyelids (Franz-Odendaal and Vikaryous, 2006). All reptiles have external eyelids. Both upper and lower eyelids are present in most lizards, all turtles, tuataras, and crocodilians. Eyelids are modified in several species, resulting in partial fusion, as in chameleons, leaving a circular opening equivalent to the diameter of the cornea, or complete fusion with transparency, as in many geckos and snakes (Schwab, 2012). In snakes, the eyelids fuse during development to form the spectacle. Some gecko and skink species possess a secondarily derived spectacle. Additionally, some skinks, lacertid lizards, and iguanine lizards have a transparent lower eyelid formed by clear scales (Lawton, 2006; Schwab, 2012)\RL.
The nictitating membrane is a prominent semilunar fold of the conjunctiva situated at the anterior canthus of the eye, between the eyelids and the cornea (Schlegel et al., 2003). It is highly developed in crocodilians and turtles but is absent in snakes and many lizards (Lawton, 2006)\RL. Hiller (1995, 1998) studied the ultrastructure of the conjunctiva and nictitating membrane of the eyelids in agamid lizards and other reptilian species through scanning electron microscopy. Starostová et al. (2009, 2013) examined cell size and its effects on the scaling of metabolic rate in eyelid geckos, finding that ontogeny of metabolic rate and red blood cell size in these species follow different paths. Guerra-Fuentes et al. (2014) investigated the spectacle in gymnophthalmid lizards through the looking glass.
The study of extraocular muscles has received little attention from reptilian anatomists, as they consider these muscles’ function and innervation patterns to be standard among vertebrates. Vertebrate extraocular muscles consist of six muscles that typically control the movement of the eyeball (Nomina Anatomica Veterinaria, 2012). These muscles are named according to their insertion points and form three pairs (antagonistic and synergistic) during eyeball movement (Cunha et al., 2016)\RL.
Ocular movement varies among species, ranging from more than 270 degrees of rotation in chameleons to a few degrees in snakes (El Hassni et al., 2000). Chameleons’ highly independent eye movements, which differ from those of other reptiles, have garnered significant interest from many researchers. Pettigrew et al. (1999) studied the convergence of specialized behavior, eye movements, and visual optics in the sand lance (Teleostei) and the chameleon (Reptilia). El Hassni et al. (2000) investigated the localization of motor neurons innervating the extraocular muscles in the chameleon (Chamaeleo chameleon ). Ott (2001) found that chameleons have independent eye movements but synchronize both eyes during saccadic prey tracking\RL.
Reptilian anatomists have focused more on studying the eye structure of certain reptilian species, such as chameleons and snakes, in detail, but some species, like the Egyptian agama, remain neglected. The Egyptian agama, Trapelus mutabilis , is a small diurnal lizard found in North Sinai, Egypt (Baha el Din, 2006; Wagner et al., 2011). The hard life of Egyptian agama has attracted the attention of the present authors, and how these animals maintain their visual performance under such these difficult living conditions. Studying the structure of the eyelids helps to understand some information about the visual performance of these animals. This study presents the anatomical, scanning electron microscopic and histological investigations of the ocular eyelids and muscles of the Egyptian agama, Trapelus mutabilis . Additionally, the motion of the agama eye was monitored in the laboratory to observe the synchronization between eyelid and eyeball movement\RL.