100 



THE SIMPLEST ORGANISMS 



ply rolling over and engulfing the food material. Within the body, 

 as in the Paramecium, the food becomes inclosed within a fluid 

 space or vacuole. The protoplasm has the power to take out such 

 material as it can use to form new protoplasm or to give energy. 

 Circulation of food material is accomplished by the constant 

 streaming of the protoplasm within the cell. 



The cell absorbs oxygen from the water by diffusion through its 

 delicate membrane, giving up carbon dioxide in return. Thus the 

 cell '^ breathes " through any part of its body covering. 



Waste nitrogenous products formed within the cell when work is 

 done are passed out by means of a contractile vacuole. 



The amoeba, like other one-celled organisms, reproduces by the 

 process of fission. A single cell divides by splitting into two others, 



Pseudopodium 



'^-'Profoplssm 



AmcBba, showing the changes which take place during division of a cell. 



each of which resembles the parent cell, except that they are 

 smaller. When these become the size of the parent amoeba, they 

 in turn divide. This is an example of asexual reproduction. 



When conditions unfavorable for life come, the amoeba, like 

 some one-celled plants, encysts itself within a membranous wall. 

 In this condition it may become dried and be blown through the 

 air. Upon return to a favorable environment, it begins life again, 

 as before. In this respect it resembles the spore of a plant. 



The Cell as a Unit. — In the daily life of a one-celled animal we 

 find the single cell performing all the vital activities which we 

 shall later find the many-celled animal is able to perform. In the 

 amoeba no definite parts of the cell appear to be set off to perform 

 certain functions ; but any part of the cell can take in food, absorb 

 oxygen, change the food into protoplasm, and excrete the waste 

 material. The single cell is, in fact, an organism able to carry on 



