140 THE WEALTH OF THE SEA 



Many fishes have a transparent or translucent 

 body in the abdominal cavity, close to the back. This 

 body, which is inflated with gas, is called the air- 

 bladder. It is in a measure comparable with the lungs 

 of air-breathing vertebrates and occupies a similar 

 position. When rudimentary lungs are developed in 

 fishes, as for example in the fresh-water gar, they 

 consist of a modified air-bladder. 



The air-bladder probably has no connection, when 

 normally developed, with breathing. It seems to be 

 chiefly an organ of equilibrium, aiding the animal 

 in rising and sinking in the water; and in some 

 species, as in the croakers, it is associated with the 

 voice. The sound made by the croaker, which prob- 

 ably denotes distress, is so loud that it can be heard 

 at a considerable distance. 



It has already been indicated that the food of 

 fishes is varied. Obviously various digestive systems 

 are required. In general, a short alimentary canal 

 is associated with an animal diet and a long one with 

 a vegetable diet. A few fishes, living largely on food 

 extracted from mud, like the mullets, have a gizzard 

 something like a chicken; the sharks and skates, as 

 well as a few fresh-water forms, have "leaves" in 

 the intestines, forming what is known as the spiral 

 valve. Many fish also have at the pyloric end of the 

 stomach blind sacs which are sometimes quite long 

 and numerous. They correspond somewhat to the 

 troublesome appendix in men. 



The color in fishes varies as greatly as it does in 

 birds, ranging from a somber gray to snow white, 

 metallic blue, green, purple, and crimson. Nearly 



