FOOD AND AIR TURNED INTO FLESH AND ENERGY 93 



concerned with the taking in of food as well as of oxygen 

 is immensely increased in the higher animals. 



We have now to consider that process which has to do 

 with carrying oxygen and food from the respiratory and 

 digestive surfaces to ail parts of the body. This process 

 is the circulation, and the organs for performing it com- 

 pose the circulatory system. 



How the blood circulates. — It has already been shown 

 that increase of size and activity in animals necessitates 

 blood and a means of circulating it through the body. 

 The uses of the circulation are : to bring oxygen from 

 the respiratory surface to every cell, to take carbon 

 dioxide from every cell to the respiratory surface, to 

 carry digested food substances from the absorbing surface 

 of the alimentary canal to every cell, and, further, to 

 remove from every cell the injurious and waste substances 

 formed by its activity to where they may be either ex- 

 creted from the body or otherwise disposed of. Circula- 

 tion is accomplished by the moving of a liquid through 

 a system of tubes and spaces channeling the whole body. 



In the very smallest and most sluggish of animals there 

 is no circulatory system. In those which are of compara- 

 tively large size and very active, and which therefore need 

 a great amount of energy, much oxygen and food must 

 be supplied and a large amount of waste substance is 

 produced which must be removed. Here the circulatory 

 system is found to be highly developed and to work with 

 great efficiency. 



In Amoeba, because of its small size and the constant 

 flowing of the body-substance there is no circulatory sys- 

 tem. In some Protozoa the contents of the body-cell 

 seem to have a definite movement, but there are no such 

 organs as heart and blood-vessels. In most animals we 

 find blood and a system of tubes and spaces for it to circu- 

 late in. In some, as the insects (fig. 58), only part of tlie 



