NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 189 



at or near the surface of the sea have normal eyes, and in 

 regions from about one hundred and fifty to five hundred metres 

 below the surface the eyes tend to become large (Figs. 63A; 9 id), 

 a modification designed to make use of every available ray of 

 light which penetrates thus far. It is now known that light 

 penetrates to far greater depths than was formerly supposed, 

 and photographic plates have been acted upon at a depth of 

 five hundred metres, while at one thousand metres traces of 

 light are still perceptible. At greater depths, say from five 

 hundred to two thousand metres below the surface, the fishes 

 tend to possess small or imperfectly developed eyes, and, as a 

 general rule, the luminous organs also exhibit a decrease in 

 size (Figs. 31, 78, 91). Finally, in many fishes living on or 

 near to the sea-floor in the abyssal depths, of which the 

 Grenadiers or Rat Tails {Macruridae) will serve as examples, the 

 eyes are comparatively large and well developed, but luminous 

 organs are absent or but feebly developed (Fig. 62 a). This 

 curious fact can only be explained on the assumption that these 

 oceanic abysses are not completely dark, and it is probable 

 that the invertebrate bottom animals, which are known to be 

 luminous, emit light of sufficient strength to make objects on 

 the bottom visible to these fishes. 



Just as a man who has lost his sight tends to develop a 

 remarkably delicate sense of touch or acute sense of hearing, 

 fishes with eyes vestigial or absent tend to have one or other 

 of the remaining senses accentuated in order to compensate 

 for the loss of vision. Thus, most of the Cat-fishes [Siluroidea) 

 have long, sensitive barbels or feelers, some of the blind cave- 

 fishes have the lateral line system highly developed and covering 

 the greater part of the head {cf. p. 232), and several deep-sea 

 fishes have the rays of the paired fins prolonged to form sensitive 

 filaments. 



The position of the eyes departs from the normal in some 

 fishes, and in bottom-living forms, such as the Rays, Anglers, 

 and Star-gazers, instead of being placed on either side of the 

 head, the two eyes lie close together on its upper surface. 

 The Flat-fishes [Heterosomata) are unique in having both the 

 eyes on the same side of the head (Figs. 8b; 40A-G). In the 

 Mud Skipper (Periophthalmus) , which is in the habit of leaving 

 the water and walking about on the sand or mud, the eyes 

 are placed on the end of more or less prominent protuberances, 

 and can be turned in all directions, a modification of obvious 

 advantage (Fig. 34f). 



