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 
