THE PROTOZOA 



141 



"We may sum up the external factors that produce or determine 

 reactions as follows : 



1. The organisms may react to a change even though neither ben- 

 eficial nor injurious. 



2. Anything that tends to interfere with the normal current of life 

 activities produces reactions of a certain sort (negative). 



3. Any change that tends to restore or save the normal life pro- 

 cesses may produce reactions of a different sort (positive). 



4. Changes that in themselves neither interfere with nor assist the 

 normal stream of life processes may produce negative or positive reac- 

 tions, according as they are usually followed by changes that are injuri- 

 ous or beneficial. 



5. Whether a given change shall produce a reaction or not often 

 depends upon the completeness or incompleteness of the performance 

 of the metabolic processes of the organism under the existing condi- 

 tions. This makes the behavior fundamentally regulatory." 



When one organism causes disease in another it is said to be patho- 

 genic to the organism affected. For example, Amoebae bucallis are 

 found in pyorrhea, a disease of the teeth. The drug emetine kills 

 Amoebae bucallis and when these are killed the diseased condition im- 

 proves. From these facts it has been concluded that this particular 

 protozoan is the cause of pyorrhea, although this is not strictly true. 



While, as we shall shortly see. most of the pathogenic organisms 

 belong to the plant kingdom, still the following animal organisms which 

 cause disease in man, are rather important factors in the study of 

 biology : 



Fig. 54. 



Entamoeba histolytica from a case of amoebic dysentery in man. Ectp., ecto- 

 plasm ; Endp., endoplasm ; V, vacuoles ; A 7 , nucleus, cy, encysted amoebae. (After 

 Rivas). 



