Pisces. 155 



How the Fish Floats. Before taking up the question of 

 locomotion in fishes, let us first consider how it is that the 

 fish can keep its place in the water without effort, neither 

 rising nor sinking. A freshly killed fish usually sinks, 

 showing that its body is slightly heavier than water. 

 Almost every one knows that after a fish has been dead 

 a short time it usually floats (commonly with the ventral 

 surface upward). This is due to the development of gases 

 in the intestines. 



Most fishes have air bladders (or swim bladders), by 

 means of which they can regulate their position in water. 

 By shortening the muscle fibers in the walls of the air 

 bladder, or in the walls of the abdomen, the air bladder is 

 made smaller and the fish sinks. By relaxing the muscles 

 the air expands, the fish as a whole is relatively lighter, 

 and consequently rises. 



Most fishes have swim bladders, and stay in midwater, 

 that is, do not rest most of the time on the bottom. On 

 the other hand, many fishes that rest most of the time on 

 the bottom are without a swim bladder. In some fishes, as 

 the perch, the air bladder is attached to the walls of the 

 abdomen. In others, for example, suckers, the air bladder 

 is free from the walls of the abdomen and is readily 

 removed, and in dressing the fish the air bladder is taken 

 out with the other internal organs. 



Locomotion of Fishes. Most fishes have the body com- 

 pressed ; that is, flattened from side to side. The thickest 

 part of the body is in front of the middle. The longer 

 taper is toward the tail, and this gives greater flexibility 

 and freedom of motion to this part. When the fish wishes 

 to swim, it makes a sideways and backward stroke of the 

 tail. This sends the body ahead and sideways ; that is, if 

 the tail is struck back and to the right it pushes the fish 



