cornea,^ 



.ens 



NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 185 



believed to feed more at night than in the day-time, and may 

 rely largely on its olfactory sense. Mr. Gregg Wilson has shown 

 that the Dab {Limanda) is normally a sight-feeder, but under 

 experimental conditions, if a number of worms were placed 

 in a small wooden box with minute apertures to allow the 

 water to pass in and out considerable excitement was immedi- 

 ately produced, and the fish hunted eagerly in every direction. 

 "When water in which many worms had lain for some time 

 was simply poured into the tank through a tube that had been 

 in position for several days, and by a person who was out of 

 sight of the dabs, the result was most marked. . . . Yet there 

 was nothing visible to stimulate the quest." 



From the above and other sources of evidence it may be 



concluded that the ^. 



sense 01 smell plays 

 a fairly important 

 part in the daily life 

 of a fish, and although 

 as a general rule this 

 is not the only sense 

 upon which it relies 

 to obtain a meal, if 

 the eyes or ears should 

 in any way fail to 

 function it could 

 probably be induced 

 to search for its food 

 by smell alone. 



In its general form the eye of a fish is not unlike our own, 

 but it is necessarily somewhat modified for vision under water. 

 The eye, as is well known, acts after the manner of a photo- 

 graphic camera, the two essential parts being the sensitive 

 screen or retina at the back, and the lens at the front, which 

 projects an image of the outside world on the screen (Fig. 73). 

 The lens of a land vertebrate is somewhat flat and convex on 

 both sides, but in the fish it is a globular body, the extreme 

 convexity being a necessity under water because the substance 

 of the lens is not very much denser than the fluid medium in 

 which the fish lives. The space between lens and retina is 

 filled with a transparent jelly-like substance, the vitreous 

 humour. The transparent outer wall of the eye, the cornea, is 

 somewhat flatter in fishes, and the space between this and the 

 lens is filled by the watery aqueous humour. In land verte- 



iris 



optic nerve 



Fig. 73- 



Vertical section through the eye of a Trout (Salmo 



trutta). Semi-diagrammatic. (After Parker and 



Haswell.) 



