CHAPTER VII. 



PROTOZOA THE SIMPLEST ANIMALS. 



Chief classes : (i) Rhizopods ; (2) Gregarinida, or Sporozoa ; 

 (3) Infusoria. 



THE study of Protozoa is a study of beginnings. For while 

 we know nothing directly about the beginnings of animal life, 

 the Protozoa give us hints of the original relative simplicity. 

 They have remained, almost all of them, 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." Moreover, 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. The Protozoa, the simplest and 

 most primitive animals, are usually very small unit masses 

 of living matter or single cells, and differ from plants 

 in their way of feeding. Most of them feed on small 

 plants or 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 ivhich 

 the animals engulf their food and glide along. The others 

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

 bears motile cilia orflagella, but in a minority ( Gregarines) is 

 without any obvious loco mo tor structures. But these three 

 states may occur in the life of one form', in fact, each of the 

 three great classes is marked by the predominant, and not by 

 the exclusive occurrence of the Rhizopod-like, or the Infusorian- 

 like, or the Gregarine-like phase of cell life. Ma ny have a skeleta I 



