REPRODUCTION AND DEVELOPMENT OF ANIMALS 303 



it divides again and again into minute tubes. At the end of 

 every tube is a little air sac. The walls of the air sacs are 

 very thin and on their surface lie blood vessels of minute 

 size. Oxygen from the air passes by osmosis into the blood 

 vessels from the air sacs. Carbon dioxide and water vapor 

 pass out of the blood vessels into the air sacs. In this 

 manner, these waste products are excreted from the body 

 by the same organs which receive the oxygen. 



353. The Need of Digestion. As in the case of plants, 

 so in animals, it is necessary that the food be changed into 

 soluble substances and then dissolved, in order that it may be 

 distributed to all parts of the body. Digestion is carried on 

 in a tube called the alimentary canal, which extends through 

 the body. It has two openings: one, the mouth, for the re- 

 ception of food; the other, the anus, for the expulsion of any 

 indigestible residue. The walls of the alimentary canal con- 

 tain circular involuntary muscles, which by contracting 

 slowly push the food along. There is no opening from this 

 tube into the other parts of the body. The substances that 

 enter the tissues of the body from this tube, must pass 

 as liquids through its walls, which are non-porous. Hence, 

 food must be digested, that is, made soluble and capable of 

 osmosis. 



354. The Parts of the Alimentary Canal. Behind the 

 mouth, the gullet of a fish and of other vertebrates broadens 

 into a baglike stomach, an important part of the alimentary 

 canal, where the food is thoroughly mixed with digestive 

 fluids formed in the walls of the stomach. These liquids 

 produce chemical changes in the food whereby some of it 

 becomes soluble. It is then dissolved and in part soaks 

 through the walls of the stomach into blood vessels and 

 is carried by the blood currents to the different organs 

 that require it. Food not digested in the stomach passes 

 into the small intestine, the next portion of the tube. 

 Here the food is mixed with other liquids formed by 



