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THE AMERICAN NATURALIST [Vol. LI 



of clear spring water in the midst of an impure pool may 

 furnish an abundance of Monas and Cercomonas, and 

 because of the presence of these, also the larger carniv- 

 orous forms such as Dileptus or Onychodromus- Or the 

 growth of myriads of bacteria at another point may induce 

 the increase in the numbers of Vorticellce, Amoebce, or 

 HolophrycD. A complete study of the multitudinous 

 minute variations in the environments of Protozoa, and 

 their complex relationships with the associated species is 

 still far away. But I believe that we are now ready to 

 undertake a preliminary survey of a number of typical en- 

 vironments and to ascertain what genera and species may 

 be normally expected to occur in each. 



Of the many factors responsible for the localization of 

 species, the following are perhaps first in importance: 

 light, character of food, temperature of water, chemical 

 content of water, presence or absence of enemies. 



Light exerts a powerful influence in determining the 

 distribution of species within a given body of water. Thus 

 one will find certain species in sunlit areas and certain 

 others in shaded ones. Not all of the Protozoa respond to 

 ordinary light stimuli, as has been shown experimentally 

 by Jennings.^ He has found that comparatively few of 

 the ciliate infusoria react to light stimuli. The flagellates, 

 on the other hand, show a definite reaction, congregating 

 in that region where the light is ordinarily strongest. The 

 reactions of Euglena viridis and Cryptomonas ovata are 

 the forms which have been most carefully studied in this 

 regard. 



Jennings^ further showed that Paramcecia collect in 

 those regions of the water which contain a trace of any 

 weak acid, and this I have noted to be true of certain other 

 ciliates. This may be a dominant influence in localizing 

 forms in pools and streams in marshes where organic 

 acids are usually present. The presence of supe 

 amounts of oxygen, carbon dioxide, calcium ca 

 iron, and other dissolved substances may also 

 distribution of species. 



H. S. Jennings, W^ashington, 1904. ^ 



