CHAPTER VII. 



PROTOZOA— THE SIMPLEST ANIMALS. 



Chief Classes — (i) Rhizopods; (2) Sporozoa; 

 (3) Infusoria. 



The Protozoa are the simplest animals, and they are of 

 peculiar interest on this account. They throw light upon 

 the beginnings of organic structure and vital activity, and 

 they give us hints as to the nature of the first forms of life, 

 of which we can know nothing directly. Almost all the 

 Protozoa are single cells, unit masses of living matter ; and 

 in virtue of their simplicity, they are in some measure 

 exempt from natural death, which is " the price paid for a 

 body." In their variety they exhibit, as it were, a natural 

 analysis of the higher animals, which are built up of many 

 diverse cells. 



General Characters. 



2he Protozoa, the simplest and most primitive animals, 

 are usually very small single cells. Most of them feed on 

 small plants or on other Protozoa, or on debris, and not a 

 few are parasitic. Most of them live in water, but many can 

 endure dryness for some time. In one set (Rhizopods) the 

 living matter is without any rind, and flows out in more 

 or less changeful threads and lobes, by the movements of which 

 the animals engulf their food and glide along. The others 

 have a definite rind, which in a large number ( Infusorians ) 

 bears motile cilia or flagella, but iti the others ( Sporozoa) 

 is without any obvious locomotor structures. But these three 

 phases may occur in the life of one form ; in fact, each of the 



