VISUAL SYSTEM: STATE OF THE ART 



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Figure 1 Negaprion brevirostris with its nictitating membrane partially retracted. The 

 nictitating membrane is most highly developed in Negaprion and the other carcharhinid 

 sharks. (Photo by E. Fisher.) 



surface is covered with dermal denticles. Gilbert (1963) reported that 31% of a 

 representative 42 shark genera possess mobile third eyelids. Actually, nicti- 

 tating membranes are known from only four shark families. They are espe- 

 cially well developed in the Carcharhinidae (Gilbert 1963). 



