NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 187 



A curious modification of the eyes is found in the Four-eyed 

 Fishes {Anableps) of the rivers of Central and South America. 

 Each eye projects well above the top of the head, and is divided 

 into two equal parts by a dark horizontal band : each of these 

 sections is of a different structure, the upper being adapted for 

 vision in the air, the lower for vision under water (Fig. 740). 

 These fishes swim about in small shoals at the surface of the 



Fig. 74. E\'ES. 



A. Head of Hammer-headed Shark {Sphyrna zygaena),X^^ \ B. Gigantura 

 chuni,A \ ; c. Stylophthalmus paradoxus, X ^ ', D. Four-eyed Fish {Anableps 



tetrophthalmus) , X |. 



water, and the level of the water reaches as far as the bar 

 dividing the eye. They are thus enabled to detect, not only 

 insects skimming over the surface or actually flying in the air, 

 but also any swimming below the surface. 



Some oceanic fishes {e.g. Giganturidae) are provided with 

 curious telescopic eyes, and these generally take the form of short, 

 protruding cylinders, each ending in a very rounded cornea 

 (Fig. 74B) . They may be directed either upwards or forwards, 



