84 ELEMENTARY ZOOLOGY. 



red blood corpuscles; or they may have a variety of 

 special functions, as in the white cells of blood and lymph, 

 which seem to secure the general health of the body by 

 protecting it against infections from the outside. 



89. Exercises for the Library. What part do the pigments (red, 

 for example) play in the work of the blood? What duties are the 

 white corpuscles supposed to do? What are the evidences for this? 

 Is the composition of blood the same in different parts of the circu- 

 lation? Illustrate. Is there anything in the one-celled animals 

 that at all corresponds to circulation? Restate just why it is a 

 necessity in the larger and more complex animals? What is the 

 difference between an artery and a vein? Describe briefly the 

 complexity of circulation in the human body. 



90. Excretory Organs. After the food and oxygen 

 have been carried by the blood to the tissues, they are there 

 taken up by the cells and used by them in building up 

 the material and in furnishing the energy of life. Here 

 is the really vital process. When the oxygen unites with 

 the carbon or the hydrogen of the complex materials in 

 the cells, it breaks these materials down into simpler 

 compounds, some at least of which are no longer of use 

 to the cell are indeed hurtful and must be eliminated. 

 This elimination is called excretion. One of these waste 

 products is carbon dioxide. We have seen that it is eli- 

 minated through the lungs and gills, where oxygen is 

 taken in. Another type of waste, always present and 

 more difficult to be rid of, contains nitrogen (urea and 

 kindred compounds). Most animals have specialized 

 organs by which these are excreted (Fig. 13). They 

 always consist essentially of tubes connecting the tissues, 

 or the body cavity, with the outside world. This connec- 

 tion may be direct, as in many worms (Fig. 13, f) ; or 

 very indirect through the blood, as in vertebrates. The 

 organs are known as kidneys in the vertebrates. The 



