AMPHIBIOUS VISION IN TELEOSTS 435 



vatures of the lens which are used in looking through the lower pupil, 

 into the water, are thus sharper than those aligned with the inferior retina 

 and the superior pupil. But the inferior retina looks up through the optic- 

 ally effective corneal surface which is exposed to the air. Aerial and 

 aquatic objects are thus focused simultaneously on separate regions of 

 the retina. It is perhaps significant, in view of the impossibility of effective 

 osmosis over a half-submerged cornea, that Anableps is one of the two 

 or three teleosts known to have ciliary folds. At any rate, the eye of the 

 Cuatro Ojos is one of the most remarkable of vertebrate eyes. The reader 

 has probably by now given up trying to select the most remarkable! 



Apart from the species of Anableps, there are other teleosts which 

 never leave the water except in an occasional leap, but give a great deal 

 of attention to out-of -water objects. The trout, for example, certainly sees 

 flies before they hit the water and does not always wait for them to do 

 so. The wise dry-fly angler arouses the trout's interest by making 'false 

 casts', in which the fly is not allowed to touch the surface. But the trout 

 is a piker compared with a certain very famous looker-out-of -water. This 

 is the archer-fish, Toxotes jaculator, which spits a slender stream of water 

 at an overhead insect with excellent aim, knocking it down to the sur- 

 face-film of the water, from which it cannot escape. Toxotes is not 

 described as ever putting its eyes out of water, and should therefore 

 exhibit no adaptations for amphibious vision. But if the hydraulic artil- 

 lery of the archer-fish is dependent upon what he can see through the 

 surface from below, he must have a truly remarkable trigonometric range- 

 finder in his brain to cope with the ever-varying distortion of angles, 

 sizes, and distances (see pp. 377-9). Moreover, Toxotes does not, like 

 Anableps, have the benefit of the glassy calm of freshwater lagoons; but 

 the species does live in fairly calm brackish estuaries. 



One of the Indian mullets, Mtigil corsula, presents an interesting habit. 

 This fish swims in small schools, in quiet waters. The protruding eyes 

 are set high upon the sides of the head and are very mobile — especially 

 antero-posteriorly. They are sometimes converged forward. The mouth 

 is ventral, and the fish feeds upon filamentous alga and upon caddis- 

 flies trapped in the surface film. As the fish cruises along with the gape 

 at the surface, the eyes are well out of water. The vision in air appears 

 to be excellent, and the eyes, with a lens diameter-thickness ratio of 1.17 

 (compare Periophthalmiis, 1.14) are definitely adapted for aerial vision. 

 It has been suggested that this use of the eyes has been 'caused' (allowed, 

 rather!) by the underslung mouth — present in the mud-skippers too, and 



