Protozoa. 297 



two equal parts, each of which is very soon a complete 

 individual, ready again to divide in the same manner. 

 Their great number is due to their rapid multiplication, 

 to their small size, which makes it easy for them to find 

 hiding places, and, further, to their complete adaptation 

 to the conditions in which they live. Multiplication does 

 not depend wholly on division. There is occasionally 

 what is called "conjugation"; that is, two individuals 

 come together and more or less completely fuse. At any 

 rate, it seems to be proved that the species could not con- 

 tinue to live indefinitely without the occasional occurrence 

 of conjugation. And this is supposed to be true of most 

 of the Protozoa. 



4. They are the oldest of animals ; that is, they are 

 supposed to have been on ,the earth longer than any other 

 kind of animals. We find their remains in very early 

 geologic formations. Some of them are supposed to have 

 changed but little as time passed, the conditions of their 

 surroundings being comparatively stable, while other 

 groups of animals have been greatly modified by changes 

 in their surroundings. 



5. They are the most independent of animals. The 

 conditions of their lives are such that they could live with- 

 out the larger animals, while many of the latter could not 

 live without them. 



Kinds of Protozoans. There are many kinds of pro- 

 tozoans, some of them widely different from any of the 

 three forms we have studied. Some are parasitic. One 

 of the parasitic forms causes malaria when introduced into 

 the blood by the proboscis of a mosquito. Some proto- 

 zoans, instead of having cilia, possess a few longer vibratile 

 projections called flagella. In the one-celled forms there 

 is usually one flagellum,-or, at most, two. But the colonial 



