278 A YEAR IN SCIENCE 



to secrete juices for digesting the food. After digestion 

 the food diffuses into the body fluids. This process, 

 known as absorption, takes place principally in the 

 intestines. 



Esophagus 



Mouth Pharynx Cr P Gizzard 



Fig. 134. Section of an earth worm Showing the parts of the 

 alimentary canal. 



Respiration. The different cells in the body of an 

 animal not only need food, but also oxygen. This 

 is taken into the body from the medium in which 

 the animal lives, either air or water. By means of 

 the body fluids it is then transferred to all parts of 

 the body. The oxygen combines with some of the 

 tissues of the body, and as a result of this process of 

 oxidation, heat and energy are produced. At the same 

 time certain w r aste products are formed, chief of which 

 is carbon dioxide. This must be carried to some organ 

 that can eliminate it from the body. This whole proc- 

 ess, from the taking in of oxygen to the giving off of 

 carbon dioxide, is known as respiration. 



The body structure necessary for this process may 

 be very simple. A thin, moist membrane filled with ves- 

 sels containing blood or some other fluid, and in contact 

 with air (or water) is all that is essential. Such a 

 simple respiratory system is present in the earthworm, 

 in which the skin is the only organ used in breathing. 



