LIFE PROCESSES IN ANIMALS 279 



In animals living in the water, the respiratory organs 

 consist of parts outside of the body known as gills. 

 In other animals large sacs (lungs) or tubes (in insects) 

 connect with the outside and serve as stations for the 



Gills 



Fig. 135. The head of a fish with the operculum removed to show 

 the breathing organs, the gills. 



taking in of oxygen and the giving off of carbon 

 dioxide. 



Circulation. In a very small animal, such as an 

 ameba, the food and the oxygen are absorbed any place 

 on the surface of the body and pass from there 

 into all parts of the organism. In all the larger ani- 

 mals, however, food and oxygen are taken into the 

 body in special regions from which they must be car- 

 ried to all parts of the body. For this purpose fluids 

 are necessary. Generally these fluids, blood and lymph, 

 are circulated through a series of vessels. Somewhere 

 along the route a device is needed for forcing the 

 blood through these vessels. The portion set aside for 

 this work constitutes the heart. In some animals this 

 is merely a thickened portion of a blood vessel, which 

 as it contracts forces the blood along in the tube. To 

 .prevent the fluid from flowing in both directions from 

 the heart, valves are present. In the higher animals 



