Life of Plants and Animals 219 



processes and must be eliminated. The elimination 

 of waste, whether in plant or in animal, is called 

 excretion. Excretion is performed by the outer cover- 

 ing or cuticle in lower forms, like protozoa, ccelenter- 

 ates, jellyfish, corals, etc., in connection with special 

 excretory organs, as the segmental organs or nephridia 

 of worms and similar organs in insects. In vertebrates 

 kidneys are present. These are a collection of tubes 

 packed together into a compact organ, the tubes 

 having some resemblance to the nephridia of worms. 

 Besides the skin and the kidneys, the gills or lungs, also, 

 serve as excretory organs, inasmuch as they carry off 

 gaseous impurities. 



Respiration. Besides food taken into the alimentary 

 canal as either liquid or solid, the animal body is in 

 constant need of a gas called oxygen. It exists in a 

 free state mixed with nitrogen in the air or else dis- 

 solved in water. Oxygen is necessary to all animals, 

 because it is by the union of oxygen with the tissues 

 that energy is set free to produce heat, motion, etc. 

 The plant, too, so far as it does work, needs oxygen; 

 but for building up its tissues it uses carbon dioxide, 

 the very substance which, among other things, animals 

 throw off as waste. 



In protozoa and other low marine forms of life, 

 like sponges, corals, etc., the oxygen is obtained from 

 the water through the outer cuticle, there being no 

 special respiratory organs. Many worms, like the 

 planarians and earthworms, breathe wholly through 

 the cuticle or skin. The only provision for respiration, 

 in such cases, is an increase in the vascularity or the 

 multiplication of capillaries under the skin. 



The mechanism of respiration in all animals is per- 

 formed by bringing blood as nearly in contact with 

 pure air or oxygen as possible. The primitive breath- 

 ing organ is merely a thin layer of tissue separating 

 the blood from the air, the blood being spread out so 



