no LIGHT AND THE BEHAVIOR OF ORGANISMS 



3. Summary 



(i) Some species of Euglena exist in three states, — • free- 

 swimming, crawling and encysted. 



(2) While in the crawHng state they push themselves 

 along at the rate of about 0.3 mm. per minute by alter- 

 nately curving and straightening the body very slightly as 

 they rotate on the long axis. There is no evidence of loco- 

 motion by means of amoeboid movement. 



(3) In this state they orient fairly accurately in light. 

 They may be either positive or negative. 



(4) When exposed to light from two sources they may 

 move toward or from a point located anywhere between the 

 two sources. The location of this point depends upon 

 the relation in amount of light from the two sources. If 

 the light from one source is stronger than that from the 

 other, this point will lie nearer the source from which the 

 stronger light comes. 



The orientation of sixteen other species in light from two 

 sources was ascertained. All of those without image-form- 

 ing eyes, fifteen in number, oriented just like Euglena. 

 The one with image-forming eyes always moved directly 

 toward one or the other of the sources of light, never 

 toward a point between them. It is therefore evident that 

 Loeb's statement regarding this point will not hold for 

 any of these organisms. 



(5) If the intensity is decreased without any change in 

 the direction of the rays, positive Euglenae in the crawling 

 state always respond by bending the anterior end toward 

 the ventral surface. This may be termed a shock-move- 

 ment, or avoiding reaction, or a bending reaction. 



(6) A change from a position in which the ventral sur- 

 face faces the source of light to one in which the dorsal, i.e., 

 the surface containing the eye-spot, faces it, induces the 

 bending reaction. Such a change in position therefore pro- 

 duces the same result as does a reduction in the light inten- 

 sity. These reactions can consequently be induced either 



