246 



HOW DO WE CLASSIFY 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. The liver contains 

 a good deal of oil and there- 

 fore is in some fishes, as the 

 cod, of considerable eco- 

 nomic importance. Many 

 fishes have outgrowths like 

 a series of pockets from the 

 intestine. These structures, 

 called the pyloric caeca (pi- 

 lor'ik se'kd), are believed 

 to secrete a digestive fluid. 

 The intestine ends at thei 

 vent, or anus, which is usu- 

 ally located on the ventral 

 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. 

 The size of the swim bladder 

 can be changed by contrac- 

 tion or expansion of its 

 walls. The fish uses this 

 organ to make changes in 

 position so that the water 

 displaced will equal its own 

 weight. In some fishes it is: 

 used as a lung. 



Circulation of the blood. In fishes the heart is a muscular^ 

 organ, with two connecting chambers : a thin-walled auricle, or 

 receiving chamber, and a thick- walled, muscular ventricle from; 

 which the blood is forced out. The blood is pumped from the; 



.filament 



^^...arterx 



^^ : --giW arch 





Explain, by careful study of the diagram, how 

 the blood receives oxygen and how it gets rid of 

 carbon dioxide in the gill. 



