684 



DEPTHS OF THE OCEAN 



an Argyropelecus seen from above, and we see that the eyes 

 point upwards, which is probably the case in most fishes 

 possessing telescopic eyes, even if exceptions occur. 



Two interesting facts go to explain this peculiar adaptation. 

 Firstly, these telescopic eyes occur only in fishes which are 

 very bad swimmers, fishes which practically only float in the 

 water-layers. Secondly, the light-measurements in the Sargasso 

 Sea showed that the light-rays acted more strongly on the 

 top plate than on the side plates ; for fishes possessing small 

 swimming capacity the telescopic eyes seem to be most 

 perfectly adapted to receive the faint rays 

 of light which penetrate to these dusky 

 depths. 



Among eyes built on the general 

 principle the difference in size first com- 

 mands attention when the vertical pene- 

 tration of light and the vertical distribution 

 of each species come to be investigated. 

 As regards the upper layers, an interest- 

 ing subject will also be found in the 

 detailed study of the anatomy of dif- 

 ferent eyes. In the retina of the human 

 eye two special kinds of sensory cells 

 are known to occur, viz. "rods" and 

 "cones." These cells occur also in the 

 eyes of fish from the surface layers. 

 From Brauer's investigations we know 

 that in all deep-sea fishes, as well as in 

 silvery fishes from about 300 metres, only 

 the " rods " are found in the retina of the 

 eye. According to an old maxim of Max 

 Schultze, nocturnal animals possess only 

 "rods" while diurnal animals have both 

 It has therefore been generally believed that the "rods" alone 

 possess the faculty of observing light-intensity, light and shade, 

 while only the "cones" perceive colours, quality of light. 



Further, an interesting difference has been found in the 

 colour-substance or pigment of the retina by day and by night. 

 Brauer has also found that these conditions in the eyes of deep- 

 sea fishes signify that their eyes are constantly adapted to 

 nocturnal conditions. The deep-sea fishes are "nocturnal 

 animals" and "day-blind." But the gradual development of 

 these peculiarities from the surface to the bottom, from the 



Fig. 500. 

 A rgyropelec u s hemigymnus, 

 Cocco. Head seen from 

 above, enlarged. 



" rods " and "cones." 



