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ADAPTATIONS TO MEDIA AND SUBSTRATES 



to the greatly augmented illumination of the eye when it is in the air. 

 Since the lens prevents the pupil from closing, the effective aperture of 

 the eye has had to be cut down, and this has been done without sacrific- 

 ing periscopy in the all-important horizontal plane. The total area of the 

 two corneal windows is no greater than that which a single, central win- 

 dow would have if its diameter were equal to 1.414 (^V^ ) times that 

 of one of the two little ones. But such a window would limit the visual 

 field disastrously, particularly considering the bothersome absence of a 

 neck. If this interpretation is correct, we must suppose that Dialommus 

 does not have as insensitive (cone-rich) a retina as Periophthalmus, else 

 it would not need its blacked-out cornea; but no well-preserved material, 

 in which the retina could be studied, has become available. 



Fig. 147 — Anableps anableps, the 'four-eyed fish'. From Walls. 



a, schematic vertical section of eye. After Piitter. S,S- plane of water surface; A- line of 

 sight upward into air; W- line of sight downward into water, b, pupil of 35mm. larva, 

 with division commencing, c, pupil of adult, completely divided. After Schneider and 

 von Orelli. /-iris; p- pupil; y- sclera. (See frontispiece). 



Another fish — a cyprinodont this time — really does have two pupils 

 (Fig. 147). This is the famous 'Cuatro Ojos' or four-eyed fish of north- 

 ern South America and western Central America, Anableps (see frontis- 

 piece) . The eyes are similar in the three species of this genus. The upper 

 pupil is the larger of the two and is normally out of water; for the animal 

 is a top-minnow, and swims sedately at the surface in quiet waters. The 

 eye is elevated just enough in the head so that the water-line cuts it neatly 

 in two. There are no devices to guard the upper half of the cornea against 

 drying, so the fish periodically 'dunks' it. 



Internally, the Anableps eye combines an aquatic optical system har- 

 moniously with an aerial one, in a perfectly static situation (Fig. 147a). 

 The lens is pyriform, and an imaginary extension of its long axis would 

 pass through the superior retina and through the inferior pupil. The cur- 



