CHAPTER XV 



THE PARASITIC PROTOZOA IN RELATION TO 

 THEIR ENVIRONMENT 



IN the introductory chapter, mention was made of 

 the types of Protozoa grouped according to their 

 habits. They were either free-living or dependent in 

 part on dead organic matter, or else they were para- 

 sitic. Examples of each group have been briefly 

 noted in the succeeding chapters. It now remains 

 to deal briefly with Protozoa in their relations with 

 their surroundings. 



The medium in which any organism lives has an 

 enormous influence upon it. The degree of light to 

 which it is exposed may alter the character of its 

 nutrition, hinder or accelerate its movements, or may 

 even cause its death. Heat is so closely bound up 

 with light that the one may insensibly merge into the 

 other. A change from one host to another, which 

 is nearly allied (e.g., two kinds of fish), may be 

 fatal to the parasite. But the enormously greater 

 change of a blood - inhabiting parasite from a 

 " warm-blooded " host, such as a human being, to the 

 gut of a " cold-blooded " animal, such as a tsetse fly, 

 can be tolerated, and, more, is essential for the con- 



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