The Life of the Fish II 
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- 
biadder 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 
