THE FISH. 85 



Make a drawing showing this view of the brain and 

 eyes ; open one of the eyes and remove the spherical 

 crystalline lens. 



The air-bladder (sometimes called the swim-bladder) is 

 believed to be of use to the fish in keeping its place in the 

 water. In many fishes the air-bladder is connected with 

 the oesophagus by a tube, and in many others there is a 

 circulation of blood in part of the wall of the air-bladder. 

 The air-bladder really corresponds to the lungs of the 

 higher animals. 



Parts of animals having essentially the same structure, 

 or corresponding in origin, are said to be homologous. 

 Thus the air-bladder of the fish and the lung of the snake 

 are homologous. The pectoral fins are homologous with 

 the fore limbs of the higher animals, and the hind limbs 

 of the higher animals are the homologues of the ventral 

 fins of the fish. Parts which are alike in use are said to 

 be analogous. Thus the gill of a fish and the lung of a 

 man are analogous ; both serve in getting oxygen into the 

 blood. 



Analogy is correspondence in use or function. 



Homology is correspondence in structure or origin. 



In some fishes the air-bladder is actually used as a lung. 

 These fishes are in consequence called lung-fishes. Some 

 of our fishes which breathe by gills have the power of 

 living out of water for some time, and are said to crawl 

 from one body of water to another when the former 

 dries up. 



Fishes are the lowest of the vertebrates ; they are also 

 the oldest of vertebrates, in the order of their appearance 

 on the earth. 



