24 MANUAL OF ANIMAL BIOLOGY 



for the construction of additional protoplasm. In practically all 

 kinds of food a certain amount of indigestible material is taken 

 into the body, and this must be eliminated. Such material has 

 never been a part of the organism : it is only a temporary inclusion. 

 The process of elimination of such material is known as egestion 

 and is to be distinguished from the process of excretion. The 

 latter, as we have seen in the previous chapter, is concerned with 

 getting rid of waste materials from substances that have actually 

 played a part in the life of the organism. In the Amoeba there is 

 no specialized structure for egesting the refuse from digestion. 

 These wastes leave the animal through a temporary opening in the 

 ectoplasm which forms at the spot which happens to be nearest to 

 the gastric vacuole at the time digestion is completed. 



2. Respiration 



Respiratory activities — involving the intake of oxygen which is 

 necessary for the metabolic processes, and the outgo of carbon 

 dioxide which has been formed as a result of the destructive meta- 

 bolic processes — are present in all organisms. Respiration may 

 be regarded as a double process ; on the one hand, the supplying of 

 oxygen, which is a phase of nutrition, and on the other hand, the 

 elimination of carbon dioxide, which is a phase of excretion. In the 

 Amoeba, the entire surface of the animal serves as a medium 

 through which this essential interchange of gases takes place. 

 (W. f. 13.) 



3. Excretion 



The process of nutrition in Amoeba, considered above, results in 

 supplying food materials to the animal, which are utilized either as 

 a source of immediate energy, for repair, or for the growth of the 

 animal by intussusception. So long as there is life, the protoplasm 

 is continually being torn down, and, if life is to exist, it must be 

 replaced just as continually by the assimilation of food material. 

 The waste products, resulting from the destruction of the complex 

 compounds in the protoplasm, consist chiefly of carbon dioxide, 

 water with various inorganic salts in solution, and urea ; the 

 latter containing nitrogenous materials. 



The nitrogenous wastes of Amoeba are excreted through the sur- 

 face ectoplasm to some extent and also by means of a specialized 

 structure, the contractile vacuole, into which the wastes from the 



