112 PROTOZOOLOGY 



Reaction to chemical stimuli. When methylgreen, methylene 

 blue, or sodium chloride is brought in contact with an advancing 

 amoeba, the latter organism reacts negatively (Jennings). Jen- 

 nings further observed various reactions of Paramecium against 

 chemical stimulation. This ciliate shows positive reaction to weak 

 solutions of many acids and negative reactions above certain con- 

 centrations. For example, Paramecium enters and stays within 

 the area of a drop of 0.02 per cent acetic acid introduced to the 

 preparation (Fig. 47, a); and if stronger acid is used, the organ- 

 isms collect about its periphery where the acid is diluted by the 

 surrounding water (Fig. 47, h). The reaction to chemical stimuli 

 is probably of the greatest importance for the existence of Proto- 

 zoa, since it leads them to proper food substances, the ingestion 

 of which is the foundation of metabolic activities. In the case of 

 parasitic Protozoa, possibly the reaction to chemical stimuli re- 

 sults in their finding specific host animals and their distribution 

 in different organs and tissues within the host body. 



Reaction to light stimuli. Most Protozoa seem to be indifferent 

 to the ordinary fight, but when the fight intensity is suddenly in- 

 creased, there is usually a negative reaction. Verworn saw the 

 direction of movements of an amoeba reversed when its anterior 

 end was given a sudden ilfiimination ; Rhumbler observed that an 

 amoeba, which was in the act of feeding, stopped feeding when it 

 was subjected to strong light. According to Mast, Amoeba -pro- 

 teus ceases to move when suddenly strongly illuminated, but con- 

 tinues to move if the increase in intensity is gradual and if the 

 illumination remains constant, the amoeba begins to move. Ac- 

 cording to Jennings, Stentor coeruleus reacts negatively against 

 fight. 



The positive reaction to light is most clearly shown in stigma- 

 bearing Mastigophora, as is well demonstrated by a jar contain- 

 ing Euglena, Phacus, etc., in which the organisms collect at the 

 place where the strongest light reaches. If the light is excluded 

 completely, the organisms become scattered throughout the con- 

 tainer, inactive and sometimes encysted, although the mixotroph- 

 ic forms would continue activities by saprozoic methods. The 

 positive reaction to light by chromatophore-bearing forms en- 

 ables them to find places in the water where photosynthesis can 

 be carried on to the maximum degree. 



All Protozoa seem to be more sensitive to ultraviolet rays. In- 



