SIGHT GENERAL 25 



the answers ; in fact, a field worker is more likely to provide 

 the clues in the first instance leaving the question of anatomi- 

 cal proof to the laboratory worker. 



Much more will be said about snakes in the chapters deal- 

 ing with scent and hearing; but there is no doubt that they 

 are assisted by their eyesight when feeding, especially those 

 kinds that have to catch swiftly moving prey. 



Lizards are very dependent upon their efficient eyes and 

 few of them, with the exception of a minority of vegetarian 

 species, take any notice of prey which is not moving. This 

 does not mean that they have no powers of scent; but it does 

 mean that eyesight is the dominant sense as will be demon- 

 strated later when dealing with field observations. Lizards 

 also have a good sense of hearing. 



Crocodiles, too, have fine vision both on land and water ; 

 but their keen sense of smell must not be ignored when study- 

 ing their behaviour. 



Although Part II of this book will be devoted to the 

 anatomy and physiology of the sense organs, it seems wise 

 to include a brief reference here to the eye-coverings of 

 reptiles, since these vary. It is a distinguishing feature as 

 between snakes and lizards that all snakes are devoid of 

 eyelids, their place being taken by a scaly window covering 

 the true eye and which is incapable of movement. Most 

 lizards, on the other hand, have eyelids which are easily seen 

 in a tame specimen ; there is a third eyelid as well. Turtles, 

 crocodiles and their close relatives also have a "third" eye- 

 lid, more properly called the nictitating membrane. This is a 

 transparent covering which moves across the eye and, in 

 addition to protecting the eyeball when under water, also 

 permits full vision under the same circumstances. This nicti- 

 tating membrane is present in the Amphibia, too, where it 

 serves the same purpose. 



In studying the senses of Fishes, a little field observation 

 will soon demonstrate the important part played by sight in 

 the majority of groups. If the student is a fisherman — par- 

 ticularly a fly-fisherman — he will quickly learn that the eyes 

 of the fish he hopes to catch are most efficient. Even though 

 the eyes are flattish and are norm.ally placed at right angles 

 to an imaginary line drawn from the snout down the back, 



