288 



ESSENTIALS OF ZOOLOGY 



food vacuoles of Amoeba to perform this function, since enzymes 

 serve this purpose in larger animals where exact study can be made 

 on the process. A circulatory system is not necessary since the 

 vacuoles with the food in the process of digestion circulate so widely 

 in the endoplasm that all parts of the cell may receive nourishment 

 by direct absorption. 



Egestion. — Indigestible material or debris that has been ingested 

 with the food is carried to the surface of the cell and cast out or 

 egested by simply being left behind as the animal moves away. 



Assimilation. — This is the process of transforming the digested 

 food material into protoplasm. In Amoeba the digested food mate- 

 rial is absorbed directly from the food vacuoles by the surrounding 



liio'^ 



Eosjfion 



'etion 



1. Excretion 



Fig. 95. — Diagram showing the phases of the metabolic process as it occurs in 

 amoeba. (Redrawn by permission from Wolcott, Animal Biology, McGraw-Hill 

 Book Company, Inc.) 



cytoplasm. Since the vacuoles move rather generally through the 

 endosarc, most of the protoplasm of the cell is in rather close con- 

 tact with the dissolved food. 



Respiration. — This is a process whereby the gas, carbon dioxide 

 (CO2), leaving the protoplasm, is exchanged for oxygen (O2) en- 

 tering it. Such a process is essential to all living protoplasm. In 

 Amoeba this exchange is carried on primarily through the general 

 body surface. The water in which the animal lives must contain 

 dissolved oxygen in order that this diffusion may go on. Amoebae, 

 however, are able to and do live in rather foul water where the 

 oxygen content is rather low and the carbon dioxide high because 



