280 A YEAR IN SCIENCE 



the heart becomes a complex organ divided into sepa- 

 rate compartments for receiving the blood, for sending 

 it out, and for keeping the blood filled with oxygen 

 separated from that filled with carbon dioxide. 



Excretion. By means of the blood, food and oxygen 

 are carried to the body tissues. There the food is 

 used for building up the broken cells, or for adding 

 new ones; the oxygen unites with the tissues to 

 produce energy. As a result of this process of oxidation 

 waste products are formed, chief of which are carbon 

 dioxide, water, and a substance called urea, containing 

 compounds of nitrogen. These substances, if allowed 

 to accumulate, seriously interfere with the action of the 

 cells. It is necessary, therefore, that they be carried 

 directly to organs which can eliminate them from 

 the body. This removal of waste products from the 

 body is called excretion. In all animals the carbon 

 dioxide is removed by the respiratory organs. For the 

 elimination of the other waste products special organs 

 are present. In the lower animals these are often sim- 

 ple tubes opening to the outside of the body, but in 

 all of the higher animals a complex organ, the kidney, 

 is used for this purpose. 



Motion. Motion is necessary for the life of an animal. 

 In the ameba we found that this ability to move was 

 in the protoplasm itself. In the higher animals this 

 power of motion is characteristic of certain cells form- 

 ing muscle tissue. This tissue is present wherever there 

 is motion. In most animals special appendages, such 



