PROTOZOA 



183 



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 in turn each divide. This is a kind of asexual repro- 

 duction. 



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. 



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

 malaria and by comparison with the amcebae which Hve in our ponds and 

 swamps, it seems Hkely that every amoeba has a compHcated hfe history 

 during which it passes through a sexual stage of existence. Such a stage is 

 seen in the conjugation of the paramoecium. 



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

 find the single cell performing all the 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 func- 

 tions; .but any part of the cell can 

 take in food, can abs-orb oxygen, 

 can change the food into protoplasm 

 and excrete the waste material. The 

 single cell is, in fact, an organism. 



One-celled Plants and Animals 

 Compared. — In our consideration 

 of the alg2e we found that the sim- 

 plest of all plants consists of a single 

 cell. This cell might be fixed in one 

 place, as the common form of pleurococcus, or it might move 

 about by means of cilia (as seen in the motile stage of pleurococcus 

 and in many other single cells considered to be plants). While 

 single-celled animals are usually free-swimming, nevertheless some 

 (especially parasitic protozoa) do not move about. So the power 



Skeleton of Radiolariaii. Highly 

 magnified. From model at Amer- 

 ican Museum of Natural History. 



