460 GENERAL CONSIDERATIONS 



built into vegetable proteins. These vegetable proteins may be used 

 by the animal organism which returns its nitrogenous wastes more or 

 less directly to the soil and water, to be again utilized by plants. In 

 contrast to carbon, nitrogen is one of the most inert of elements while 

 in the animal organism. 



Animals secure oxygen either directly from the air or from water in 

 which it is in solution. After it is used in oxidation processes the organ- 

 ism returns the oxygen to the environment, largely in combination with 

 carbon as carbon dioxide and with hydrogen as water. Carbon dioxide 

 and water are in turn taken in by green plants which use the carbon and 

 hydrogen and free the oxygen. 



Phosphorus is an important element in animal cells, particularly 

 in the nerve cells of higher forms. It occurs in the soil and water in the 

 form of phosphoric oxides. Taken up by plants and built into certain 

 proteins it is utilized by the animal, to be later returned either to the 

 soil or to the water. 



507. Water. — The constant need of every animal for water requires no 

 demonstration. Water is important because of its various roles in the 

 metabolic cycle. It gives to living matter the proper consistency. It is 

 the vehicle by which foodstuffs are brought to the cells of the body and 

 by which wastes are removed; no substances enter or leave the cell 

 except in the form of aqueous solutions. It enters into digestive proc- 

 esses involving hydration, such as the change of starch to sugar. These 

 processes, however, are very limited when compared with the oxidations 

 which occur in metabolism. Protoplasm contains a large amount of 

 water, which makes up two-thirds of the human body by weight. The 

 amount varies in different animals and in such an extreme case as jelly- 

 fishes reaches 96 per cent. 



For all aquatic organisms the water which bathes them is their 

 environment. All natural waters contain salts in solution and the 

 exact composition changes with a variety of factors, such as evaporation, 

 rainfall, decomposition processes, and so on. Not only must an aquatic 

 animal be adapted by the chemical composition of its body to the water in 

 which it lives, but it must be capable of continual adjustment to these 

 changes in composition. If an animal cannot so adjust itself, its only 

 recourse is to encyst and await the return of favorable conditions or 

 produce eggs or spores which can withstand the adverse conditions. 

 Similar phenomena occur in response to extreme temperatures. To the 

 cells in the metazoan body the fluids of the body are a comparable 

 environment. 



508. Digestion and Absorption. — Digestion in protozoans is intra- 

 cellular and occurs in food vacuoles. In the ameboid forms these food 

 vacuoles may be formed anywhere at the surface, but in more complex 

 types a gullet admits the food to the body and the vacuoles are formed 



