Bacteria and Protozoa 



19 



that insects are often important factors in spread- 

 ing some of the most dreaded of the bacterial dis- 

 eases. 



THE PROTOZOA 



The Protozoa, or one-celled animals, belonged to 

 an unknown world before the invention of the 

 microscope. The first of these instruments enabled 

 the early observers to see some of the larger and 

 more conspicuous members of the group and each 

 improvement of the microscope has enabled us to 

 see more and more of them and to study in detail 

 not only the structure but to follow the life-history 

 of many of them. 



The Amoeba. With some, as the common amoeba 

 (Fig. 8), a minute little form that is to be found in 

 the slime at the bottom of almost any body of 

 water, the life-history is extremely simple. The 

 organism itself consists of a minute particle of 

 protoplasm, a single cell with no definite shape or 

 body-wall and no specialized organs or apparatus 

 for carrying on the life-functions. It lives in the 

 slime or ooze in fresh or salt water, takes its food 

 by simply flowing over the particle that is to be 

 ingested, grows to a certain limit of size, then di- 

 vides into two more or less equal parts, each part 

 becoming a new animal that goes on with its de- 



