88 INTRODUCTION TO ZOOLOGY. 



SO well protected against drying out as are the gills of 

 a crawfish, so a fish cannot usually live so long out of 

 water as a crawfish can. 



From the gills the blood is gathered into the dorsal 

 aorta, through which it is distributed to the body gen- 

 erally. It returns from the body through veins to the 

 auricle. Since no part of the blood, after leaving the 

 heart, can return to the place from which it started 

 without passing both through the breathing organs 

 and through some part of the system, the circulation 

 is said to be complete. 



The air bladder is homologous to a lung, although 

 in our common fishes it is not a breathing organ. It 

 is filled with gases secreted from the blood, and serves 

 to make the body of the fish have the same specific 

 gravity as the water in which it lives. If the muscles 

 along the sides of the body contract, the air bladder 

 is compressed, thus occupying less space, which action 

 increases the specific gravity of the fish, enabling it to 

 sink more readily. In the perch, the air bladder is 

 attached to the walls of the body cavity. In the suck- 

 ers, and in many other fishes, it lies free in the body 

 cavity and is divided into two portions by a constric- 

 tion around the middle. In the garpike, and in the 

 ganoids generally, the air bladder contains many blood 

 vessels and communicates with the oesophagus by a 

 short tube through which air can enter it. In these 

 fishes, it serves as a partial lung and is of use in 

 breathing. In the Dipnoi, or lung fishes, it is so 

 efficient that, when necessary, the use of gills can be 

 dispensed with, and the fishes are thus enabled to 

 withstand for some time a complete drying up of the 

 ponds in which they live. 



In the perch, and in most bony fishes, the air blad- 

 der represents an organ that is in a state of retro- 

 gression. Instead of becoming more lung-like, it is 

 becoming less so. 



