XXII. RESPIRATION IN LOWER 



ANIMALS.^ 



The activity of tissue cells demands that they receive a 

 constant supply of oxygen; while the result of oxidation 

 in them is the production of quantities of carbon dioxide 

 which must be as constantly removed to permit of their con- 

 tinued activity. In other words, respiration is a property 

 of protoplasm, and wherever there is active protoplasm 

 there must be respiration. Since plants and animals are 

 all built up of cells containing active protoplasm, respira- 

 tion becomes a property of all living matter. The rate 

 of action of the cells depends upon the completeness and 

 speed with Avhich this exchange of oxygen and carbon 

 dioxide takes place. 



In all the lower organisms the mere contact of the cells 

 with a medium ^hich contains oxygen — such as air or 

 water — is sufficient for the exchange to take place. 

 ^Yhen, however, the organisms increase in size and the 

 number of cells, many cells are necessarily forced into 

 places where such contact is impossible. To these cells 

 the oxygen must be brought in some way, and so in most 

 plants and animals there are found special structures 

 whose purpose is the distribution of oxygen to the tissue 

 cells and the collection of carbon dioxide from them. 

 Such apparatus wherever found, whether in plant or 

 animal, is called a respiratory system. \\e shall consider 

 ' See Footnote, p. 146, Chapter X. 

 340 



