236 A STUDY OF VERTEBRATES 



secrete a digestive fluid. The intestine ends at the vent, which is usually 

 located on the under side of the fish, immediately in front of the anal fin. 

 Swim Bladder. An organ of unusual significance, called the swim 

 bladder, occupies the region just dorsal to the food tube. In young fishes 

 of many species this is connected by a tube with the anterior end of the 

 digestive tract. In some forms this tube persists throughout life, but in 

 other fishes it becomes closed, a thin, fibrous cord taking its place. The 

 swim bladder aids in giving the fish nearly the same weight as the water 



A fish opened to show H, the heart ; G, the gills ; L, the liver ; S, the stomach ; 

 I, the intestine ; O, the ovary ; K , the kidney, and B, the air bladder. 



it displaces, thus buoying it up. The walls of the organ are richly sup 

 plied with blood vessels, and it thus undoubtedly serves as an organ for 

 supplying oxygen to the blood when all other sources fail. In some 

 fishes (the dipnoi, page 187) it has come to be used as a lung. 



Circulation of the Blood. In the vertebrate animals the blood is 

 said to circulate in the body, because it passes through a more or less closed 

 system of tubes in its course around the body. In the fishes the heart is 

 a two-chambered muscular organ, a thin-walled auricle, the receiving 

 chamber, leading into a thick-walled muscular ventricle from which the 

 blood is forced out. The blood is pumped from the heart to the gills; 

 there it loses some of its carbon dioxide ; it then passes on to other parts 

 of the body, eventually breaking up into very tiny tubes called capillaries. 

 From the capillaries the blood returns, in tubes of gradually increasing 

 diameter, toward the heart again. The body cells lie between the smallest 

 branches of the capillaries. Thus they get from the blood food and oxy 

 gen and return to the blood the wastes resulting from oxidation within 

 the cell body. During its course some of the blood passes through the 

 kidneys and is there relieved of part of its nitrogenous waste. Circulation 



