324 



THE EYE IN EVOLUTION 



Opisthoproctiis 



forwards and uj^wards {Stylophorus) (Fig. 378) ; in others, upwards 

 {Argyro'pelecus, OpistJwiiroctus), in whicli case the sclera on the 

 dorsal aspect becomes transparent and the ventral part of the retina 

 assumes the function of the " principal retina", so that the optic nerve 

 emerges from its edge instead of from its centre (Fig. 380). 



The intimate structure of such an eye is seen in Fig. 380. The principal 

 retina is well formed, the accessory retina atrophied, while the optic nerve 



emerges laterally between the two. To move 

 the immense lens there is a lens pad con- 

 trolled to some extent by muscles which 

 enable the eye to be focused on a distant 

 object. On the whole, however, such eyes 

 are myopic and specifically adapted for the 

 perception of the small amount of light avail- 

 able, although it is possible that a sufficiently 

 adequate image of prey may be appreciated 

 to allow its capture when it approaches so 

 closely that it can be snapped at. 



Such an eye is found in several species in 

 addition to Oiganturus, Stylophorus and 

 Argyropelecus — some relatives of the deep- 

 sea salmonids, Dolichopteryx and Winteria, 

 and some of the deep-sea lantern fishes 

 (Myctophidte), such as some species of Ever- 

 manella and Scopelarchus. 



It is interesting that a " deformed " 

 tubular eye of this ty^ie can be produced by artificial selection in breeding, as is 

 seen, for example, in the " telescope-eyed " goldfish (Fig. 381). 



Fig. 381. — The " Telescope 



EYED " Goldfish (Zool. Soc. 

 London). 



THE AMPHIBIOUS EYE 



Fishes which require to see both under water and in air are 

 presented with the difficulty of combining two very different optical 

 requirements. In many cases there seems to be little structural 

 adaptation to the comparative myopia of aerial and the hypermetropia 

 of aquatic vision unless the accommodative range is unusually great. 

 Very interesting modifications, however, occur in at least one 

 species — AnabJei^s teirophthalmus. the " Cuatro ojos " wliich swims 

 sedately in quiet waters of South and Central America in such a way 

 that the water-line cuts across the middle of the prominently raised 

 eyes (Figs. 382 to 384). This extraordinarily interesting eye has 

 received a considerable amount of study from the time of Artedi (1758) 

 and Soemmerring (1818) (Schneider and v. Orelli, 1908 ; Arruga, 1941). 

 It is provided with two distinct optical systems, the upper for aerial, 

 the lower for aquatic vision. The cornea is divided into two segments 

 b}'- a densely pigmented horizontal raphe, and the iris is similarly 

 divided so that two pupillary apertures are j)resent ; the lens is fusiform 

 in -nape, its short axis refracting rays onto the lower part of the retina 



