i 2 The Life of the Fish 



The Air-bladder. In the front part of the sunfish, just above 

 the stomach, is a closed sac, filled with air. This is called the 

 air-bladder, or swim-bladder. It helps the fish to maintain its 

 place in the water. In bottom fishes it is almost always small, 

 while fishes that rise and fall in the current generally have a 

 large swim-bladder. The gas inside it is secreted from the 

 blood, for the sunfish has no way of getting any air into it from 

 the outside. 



But the primal purpose of the air-bladder was not to serve 

 as a float. In very old-fashioned fishes it has a tube connecting 

 it with the throat, and instead of being an empty sac it is a true 

 lung made up of many lobes and parts and lined with little blood- 

 vessels. Such fishes as the garpike and the bowfin have lung- 

 like air-bladders and gulp air from the surface of the water. 



In the very little sunfish, when he is just hatched, the air- 

 bladder has an air-duct, which, however, is soon lost, leaving 

 only a closed sac. From all this we know that the air-bladder 

 is the remains of what was once a lung, or additional arrange- 

 ment for breathing. As the gills furnish oxygen enough, the 

 lung of the common fish has fallen into disuse and thrifty Nature 

 has used the parts and the space for another and a very different 

 purpose. This will serve to help us to understand the swim- 

 bladder and the way the fish came to acquire it as a substitute 

 for a lung. 



The Brain of the Fish. The movements of the fish, like those 

 of every other complex animal, are directed by a central ner- 

 vous system, of which the principal part is in the head and is 

 known as the brain. From the. eye of the fish a large nerve 

 goes to the brain to report what is in sight. Other nerves go 

 from the nostrils, the ears, the skin, and every part which has 

 any sort of capacity for feeling. These nerves carry their mes- 

 sages inward, and when they reach the brain they may be trans- 

 formed into movement. The brain sends back messages to the 

 muscles, directing them to contract. Their contraction moves 

 the fins, and the fish is shoved along through the water. To 

 scare the fish or to attract it to its food or to its mate is about 

 the whole range of the effect that sight or touch has on the 

 animal. These sensations changed into movement constitute 

 what is called reflex action, performance without thinking of 



