THE FISHES 171 



The eyes of most fishes are highly developed, and are of 

 the greatest use at all times. Exceptions to the rule are 

 found in certain species which live in caves or in the dark 

 abysses of the ocean. In some of these the eyes have dis- 

 appeared almost completely, and the sense of touch be- 

 comes correspondingly more acute ; in other deep-sea forms 

 they have grown to a large size, enabling them to distin- 

 guish objects in the gloom, like the owls and other noc- 

 turnal animals. Embedded in the skin of some of these 

 deep-sea fishes, and certain nocturnal ones, are peculiar 

 spots, composed of a glandular substance, which produces 

 a bright glow like that of the fireflies. These may be located 

 on the head or arranged in patterns over various parts of 

 the body, and may serve to light the fish on its way and 

 enable it to see its food to better advantage, or it may act 

 as a lure to many fishes that become victims to their own 

 curiosity. In those fishes which are active most of the 

 time the eyes are located on the sides of the head, and in 

 those which remain at or near the bottom they are turned 

 toward the top ; in every case where they can be used to 

 the best advantage. 



164. Breeding habits. Among fishes the egg-laying time 

 usually comes with the spring, when the males of several 

 species become more resplendent, and sometimes engage 

 in struggles for their respective mates. In others this 

 ceremony is performed without show of hostility. Some 

 make nests, while others lay their eggs loosely in the water. 



In all the salmon family the young fishes are born in 

 the colder fresh-water rivers, and later make their way into 

 the sea, where they spend the greater part of their lives. 

 When the time comes for them to lay their eggs they 

 migrate in great companies, and make their way hundreds, 

 perhaps thousands, of miles to .the rivers in which they 

 spent their youth. Up these streams they rush in crowds, 

 leaping waterfalls and rapids, and, dashed and battered on 

 the rocks, many, and in some species all, die from injuries 



