278 



THE VERTEBRATE ANIMALS 



Digestive System. The gullet leads directly into a baglike stomach. 

 There are no salivary glands in the fishes. There is, however, a large 

 liver, which appears to be used as a digestive gland. This organ, because 

 of the oil it contains, is in some fishes, as the cod, of considerable economic 

 importance. Many fishes have outgrowths like a series of pockets from 

 the intestine. These structures, called the pyloric cceca, are believed to 

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

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



Anatomy of the carp : br, branchiae, or gills ; c, heart ; /, liver ; vn, swimming 

 bladder; ci, intestine. 



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

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

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

 fish (the dipnoi, p. 284) 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 



