VERTEBRATES 



469 



fish remains small. In order to increase the supply, laws have 

 been enacted for the protection of fish ; bass and trout, for 

 example, cannot be caught during the breeding season, and 

 salmon cannot be caught except on definite days of the week ; 

 and certain other fish cannot be caught unless they are large 

 enough to have had a chance to spawn. 



Vertebrates. Farm animals and fish have a jointed and 

 flexible backbone, or vertebral column, and hence they are called 

 vertebrates. The vertebral column serves as a place of attach-; 

 ment for the larger bones of the body, and also serves as a sup- 

 port or skeleton for the 

 soft parts of the body, 

 such as heart, kidneys, 

 and stomach. To the 

 backboned or vertebrated 

 animals belong frogs (Fig. 

 339)> toads, and reptiles, 

 but these animals are not 

 directly important to man 

 for food or clothing and 

 are not studied here. 

 With the exception of 



fish, most vertebrates live on land, and get oxygen directly from 

 the air by means of lungs. Fish live in the water, and secure 

 the oxygen they need from the air which is in solution in the 

 water. Fish take oxygen from the water by means of gills 

 which are feathery, red structures on each side of the head. 

 Most of the water which enters the mouth is not swallowed, 

 but is passed over the gills, and is then expelled through open- 

 ings in the side of the head. As the water flows over the gills, 

 the oxygen dissolved in it passes through the thin gill walls, 

 and becomes part of the blood. In this way fish secure a con- 

 stant supply of oxygen. 



FIG. 339. A frog. 



