THE FISHERIES. 49 



The water in which the fish hves will always 

 keep the outer tunics moist, — consequently, na- 

 ture has not lost labor by bestowing a lachrymal 

 sac ; — no tears are required for lubrication. 



Not being designed for facial expression, the 

 oblique muscles are merely cords in embryo, as it 

 were, showing the chain which establishes a rela- 

 tionship between all races of animals. The pig- 

 mentum nigrum covers only a portion of the pas- 

 terior surface of the retina, — enabling them to see 

 in the day, but the metallic lustre of the remaining 

 surface, becomes a concave mirror in the night, 

 which gives them vision, as distinct, probably, as 

 at noon day. Some fishes are only taken by a 

 hook in the night : those are the owls, that cannot 

 see by day. Others, the voracious kinds, dart at 

 all times, both night and day, at the bait, with 

 precision. Sharks follow vessels, hundreds of 

 miles, and never fail to catch every bone which 

 is thrown overboard, at all times. 



But there is another contrivance in the consti- 

 tution of the fish's eye, very wonderful, inas- 

 much as it shows most convincingly, a provision 

 against the pressure of the incumbent column of 

 water. Thus, in the voracious species, principally 

 the sharks, the sclerotic, or outside coat, is perfect- 

 ly hard bone, except on the back, where there is 

 an opening for the entrance of the optic nerve. In 

 4 



