THE FLAGELLATES 171 



the interior of the cell. A bright red pigment spot, frequently 

 found in each cell, is regarded as a structure sensitive to light, 

 for these organisms generally move towards the source of bright 

 illumination. The flagellates are believed to be related to the 

 lowest green plants, the algae, and some groups of algae are 

 thought to have been derived from them. 



Euglena (Fig. 172, A) is a common flagellate found in stag- 

 nant pools. The cells are generally green, but some of the spe- 

 cies and related forms are colorless, having adopted the habit 

 of living exclusively on organic food substances in the drain- 

 age water which they frequent. Euglena gracilis becomes quite 

 colorless when cultivated in solutions of sugar away from the 

 light, thus suggesting the way in which colorless plants, such 

 as the fungi, may have arisen from chlorophyll-bearing ances- 

 tors under an environment which supplied an abundance of 

 organic food. 



Uroglena (Fig. 172, B) is a colonial flagellate which frequently 

 appears during the summer months in reservoirs and gives a 

 fishy, oily taste and odor to the water, making it unfit for use. 

 The taste and odor are caused by globules of oil that are set 

 free by the rupture of the delicate cells when the water is 

 carried through pipes. This is one of the organisms which can 

 easily be destroyed by treating reservoirs with copper sulphate. 1 



1 See papers by Moore and Kellerman, United States Department of 

 Agriculture, Bureau of Plant Industry, Bulletin 64, 1904 ;.also Bulletin 76, 

 1905. 



