PHYSIOLOGY 133 



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

 tion (Fig. 51, a); and if stronger acid is used, the organisms collect 

 about its periphery where the acid is diluted by the surrounding 

 water (b) . The reaction to chemical stimuli is probably of the great- 

 est importance for the existence of Protozoa, 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 results in their finding specific host ani- 

 mals and their distribution in different organs and tissues within the 

 host body. Recent investigations tend to indicate that chemotaxis 

 plays an important role in the sexual reproduction in Protozoa. 

 Chemotaxis in Peranema (Chen, 1950). 



Reaction to light stimuli. Most Protozoa seem to be indifferent 

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

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

 rection of movements of an amoeba reversed when its anterior end 

 was subjected to a sudden illumination; 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 il- 

 lumination remains constant, the amoeba begins to move. Pelomyxa 

 carolinensis reacts negatively to light (Kudo, 1946). 



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

 bearing Mastigophora, as is well observable in a jar containing 

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

 where the light is strongest. If the light is excluded completely, 

 the organisms become scattered throughout the container, inac- 

 tive and sometimes encyst, although the mixotrophic forms would 

 continue activities by saprozoic method. The positive reaction to 

 light by chromatophore-bearing forms enables 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. Inman 

 found that amoeba shows a greater reaction to the rays than others 

 and Hertel observed that Paramecium which was indifferent to an 

 ordinary light, showed an immediate response (negative reaction) to 

 the rays. MacDougall brought about mutations in Chilodonella by 

 means of these rays (p. 229). Horvath (1950) exposed Kahlia sim- 

 plex to ultraviolet rays and destroyed the micronucleus. The emi- 

 cronucleate individuals lived and showed a greater vitality than nor- 

 mal individuals, as judged by the division rate at 34°C. Mazia and 



