THE PHYLUM PROTOZOA 



153 



pigment spot facing outward. Each stroke is not a simple backward lash, 

 however, but is directed obliquely to the long axis of the organism so 

 that the body not only turns to one side but also rotates a little (Fig. 

 8.4). Successive lashes thus produce a spiral path in which the organism 

 moves forward with the pigment spot continually facing the outside of 

 the spiral. 



This swimming pattern makes optimal use of the visual organelle. 

 As Euglena swims forward the pigment spot shades the photoreceptor 

 from behind and from one side. W^hen Euglena is swimming at right 

 angles to the direction of light the photoreceptor is shaded once during 

 each spiral loop. If the organism is seeking light it turns to one side more 

 than usual at the moment that the photoreceptor is shaded, gradually 

 turning the spiral path toward the light until the photoreceptor is never 

 shaded. If it is avoiding light, it turns sideways more than usual during 

 that part of the spiral in which the photoreceptor is illuminated, grad- 

 ually turning the spiral path away from the light until the photore- 



Flagellw 



igme^nt- spot — ■ 



l:3/>-— Phot oreceptor 



-Contractile 



J/( y^ ' vaciule 



Paramylum 

 bodies 



Chloroplasts^ 



Nuclexi^ 





Figure 8.4. pAigleua. A. A lateral \icw of Euglena viridi<i. B. A diagram showing 

 successive positions in the spiral swimming pattern of Euglena. The position of the pig- 

 ment spot shows the rotation that occurs. 



