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THE AMERICAN NATURALIST [Vol. LII 



Stentor polymorphous Ehrenb. (= S. viridis aut.) and 

 Phacus longicauda Ehrenb. rotate clockwise in the north- 

 ern hemisphere. Inasmuch as the phototactic relation of 

 the ciliates in general is negative, where a reaction exists, 

 it seems probable that the inclusion of the minute sym- 

 biotic forms of algae, Chlorella vulgaris Beyer, which 

 gives the species its characteristic green apperance, has 

 induced a change from a negatively phototactic to a 

 positively phototactic condition, while the organism re- 

 tained its original counterclockwise rotation. Small 

 forms like Chlorella which contain chloroplasts, are gen- 

 t-rally positively phototactic so far as their responses to 

 normal conditions are known. 



Phacus longicauda (Ehrenb.) is an euglenoid about 

 100m in length, with comparatively flat wing-like expan- 

 sions. The stria- covering the body are longitudinal. In 

 swimming, however, many of the forms have the anterior 

 part of the right expansion turned slightly down, while 

 the left expansion is turned up in a similar manner. This 

 gives their progressive movement a counterclockwise 

 rotation. 



In the southern hemisphere direct observation of the 

 characteristic rotation has not been made, but inasmuch 

 as the direction of the striae indicates the direction of the 

 rotation, certain evidence is available. Cunha (1913) in 

 his studies of Protozoa from Brazil has figured several 

 forms showing distinctly the arrangement of the striae 

 in the excellent plates drawn by himself. While it is not 

 impossible that a careless investigator might focus on the 

 lower part of the specimen, thus showing the reverse 

 position of the striae, the careful work of Cunha scarcely 

 permits one to suggest such a criticism. It may further- 

 more be noted that the apparently counterclockwise ro- 

 tating forms described by him are species quite different 

 from the typical northern forms, while the forms which 

 evidently rotate clockwise are closely allied to species 

 from the northern hemisphere, and may have been intro- 

 duced comparatively recently so far as evolutionary time 



