194 THE PROTOZOA 



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

 of the existence of food when food 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 is 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. It will then 

 rid itself of any material that it cannot use. Thus it has the power 

 of selective absorption, a character found in the protoplasm of 

 plants previously studied. Circulation of food material is accom- 

 plished by the constant streaming of the protoplasm within the 

 cell. 



The cell absorbs oxygen from the water by osmosis through 

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

 the cell " breathes " through any part of its body covering. 



Waste products formed from the oxidations which take place 

 in the cell 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 sphtting 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 in turn each 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 begins life again, 

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



From the study of the amcebalike organisms which are known to cause 

 malaria, and by comparison with the amoeba which live in our ponds 

 and swamps, it seems likely that every amoeba has a comphoated life 

 history during which it passes through a sexual stage of existence. 



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

 find the single cell performing all the general 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, can 

 absorb oxjrgen, can change the food into protoplasm and excrete 



