170 SINGLE-CELLED ANIMALS AS ORGANISMS 



portant organ is difficult to see, except in cells that have been 



stained. 



Although but a single cell, still the amoeba appears to be aware 

 of the existence of food when it is near at hand. Food may be 

 taken into the body at any point, the semifluid protoplasm simply 

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

 as in the paramoecium, 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 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 osmosis 

 through its delicate mem- 

 brane, giving up carbon dioxide 

 return. Thus the cell 



m 



" 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 the contractile 

 vacuole. 



The amoeba, like other one- 

 celled organisms, reproduces 

 by the process of fission. A 

 single cell divides by splitting 

 into two others, each of which 

 resembles the parent cell, except that they are of less bulk. 

 When these become the size of the parent amoeba, they each in 

 turn divide. This is a kind 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 



Amoeba, showing the changes which take 

 place during division of the cell. The 

 dark body in each figure is the nu- 

 cleus ; the transparent circle, the con- 

 tractile vacuole ; the large granular 

 masses, the food vacuoles. Much 

 magnified. 



