CHAPTER VIII. 



PROTOZOA— THE SIMPLEST ANIMALS. 



Most of the simplest animals or Protozoa are very small unit 

 masses of living matter, or single cells, which usually differ 

 from plants in their way of feeding. Most of them feed on 

 small plants or other Protozoa, or on the debris of larger 

 living things, 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 bulgings, by 

 the movements of which the animals engulf their food and 

 glide along. The others have a definite rind, which in a 

 very large number (Infusorians) bears rapidly motile lashes 

 or cilia, but in a minority (Gregarines) is without any obvious 

 locomotor 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. Many have an external 

 framework of lime, flint, or other material, while within the 

 cell there is a special kernel or nucleus, or there may be 

 several. There are also other less constant structures. The 

 Protozoa multiply by dividing into two daughter units, or 

 into a large number ; and two individuals often unite 

 temporarily, or permanently, in conjugation, which is ana- 

 logous to the union of ovum and spermatozoon in higher 

 animals. 



Let us revise these facts in greater detail. 



Protozoa and Metazoa. — According to the cell theory 

 or doctrine, first clearly stated by Schwann and Schleiden 



