THE FISHES. 183 



nosntn } and that it has some influence in accommodating 

 the form of the eye to distances or to the density of the 

 surrounding medium. 



The presence of this choroid gland, a " rete mirabile" 

 says Gunther, is always combined with that of a pseudo 

 branchia. Bony fishes, in which this latter is absent, have 

 no choroid gland. 



The pupil of the eye in fishes is always very large, so as 

 to take in as much light as possible ; but it is generally 

 without motion and unable to contract. Mr. Forster, 

 however (" Scientific Angler"), states that he has seen the 

 pupil contract. Six muscles serve to turn the eye in 

 various directions four recti and two oblique as in man ; 

 and although, as Gunther says, in the range of their vision 

 and acuteness of sight, fishes are very inferior to the higher 

 classes of vertebrates, yet, at the same time, it is evident 

 that they perceive their prey or approaching danger from a 

 considerable distance, and some are able to discriminate 

 with clearness and precision one colour from another at 

 least, we may suppose so when we see how often the salmon 

 family reject, in the artificial flies placed above them, one 

 colour and instantly take another. 



It would be an interesting study to determine at what 

 angle fish perceive objects behind them or directly in front 

 of them. It would appear in many that the vision is 

 chiefly directed upwards and laterally, but how far their 

 vision, particularly in fish which get their food chiefly on 

 the surface of the water, extends backwards and forwards 

 is not yet determined. An object placed laterally or above 

 will almost immediately attract or scare. We have many 

 times been able to get close to a trout by approaching 

 directly from behind, when the slightest deviation laterally 

 would send him away. In approaching fish, not sufficient 

 consideration is given as to the powers of refraction and 

 the medium through which a fish sees ; and it must be 

 remembered that fish do not see objects as we see 

 them. 



Mr. George M. Kelson, who has, perhaps, more than any 

 one, studied the habits of the salmon, says : " I am fully 



