290 GENERAL ZOOLOGY 



to the presence of food in all the stages of digestion and as- 

 similation, and waste materials that are to be expelled from 

 the body. There are two fairly conspicuous bodies within 

 the endoplasm, the nucleus, which controls most of the acts 

 of the cell, and the contractile vacuole, in which the liquid 

 wastes are accumulated until the vacuole reaches its capac- 

 ity. Then the vacuole bursts through the ectoplasm, throw- 

 ing the liquid waste out of the body. In its functioning 

 this contractile vacuole goes through regular cycles of 

 gradual increase in size, followed by sudden disappearance. 



In the life functions, oxygen dissolved in the surrounding 

 water is absorbed directly by the protoplasm in the process 

 of respiration. At the same time the carbon dioxide pro- 

 duced by the protoplasm is taken up by the surrounding 

 water. 



Microscopic plants and animals and parts of larger or- 

 ganisms serve the amoeba as food. When a moving amoeba 

 approaches a food particle, pseudopodia extend on each 

 side of the particle. As the animal flows along, the pseudo- 

 podia completely surround the object being taken as food, 

 which becomes recognizable as a food vacuole within the 

 endoplasm. Though a food vacuole lies in the protoplasm 

 it is not part of the living amoeba. The food must first 

 undergo digestion under the influence of the surrounding 

 protoplasm and become assimilated in a manner exactly 

 similar to the processes that take place in the higher animals. 



As an amoeba continues to feed, it grows, but soon reaches 

 a limit in size. It then undergoes reproduction. The nucleus 

 divides into two equal parts, and the cytoplasm becomes 

 separated into two masses. Each mass of cytoplasm sur- 

 rounds a nucleus and becomes a new individual. 



There are many species of Amoeba and amoeba-like forms, 

 mostly living in stagnant water and having relatively little 

 economic importance. In contrast with these some others 



