46 THE BEHAVIOR OF LOWER ORGANISMS. 



Engelmann (1882, a) on the light reactions of certain green ciliates 

 (Paramecium bursaria, Stentor viridis, etc.) were made before the 

 typical motor reaction the turning toward a certain structurally de- 

 fined side had been observed in any of the infusoria. Engelmann, 

 therefore, paid no attention to this point. Yet there is much in his 

 account of the reactions to light in these organisms to suggest that 

 it takes place in a way similar to that which I have described above 

 for Stentor cceruleus. Indeed, Engelmann's account, so far as it 

 goes, fits precisely into the reaction method which I have described 

 above. He found, as I have, that the organisms react either when only 

 the anterior end is affected, or when the entire organism is flooded with 

 light from beneath. The reaction consists in a sudden turn to one side, 

 or a sudden start backward, just as in Stentor cceruleus. The only 

 point which is lacking in Engelmann's account is the observation as 



to which side the organism 

 turned ; to this point he did 

 * not direct his attention. 



It is interesting to note that 

 in the account given by Ver- 

 worn (1889, Nachschrift) of 

 the reaction to light in Pleu- 

 ronema chrysalis there is 

 nothing tending to support 

 the theory of an orienting tro- 

 pism. According to Verworn the reaction of Pleuronema to light is by 

 a sudden leap (" Sprungbewegung"), which is lepeated several times if 

 the light continues. This sudden leap seems identical with the "motor 

 reflex " which I have described as the typical reaction to stimuli in 

 many ciliates, and which consists usually in a leap backward, followed 

 by a turning toward a structurally defined side. It is in this manner, 

 as we have seen, that Stentor cceruleus reacts to light and the reac- 

 tion, as in Pleuronema, is often repeated many times. 



Thus the other carefully studied accounts of reaction to light in the 

 Ciliata, while incomplete, agree so far as they go with that which I 

 have given for Stentor, and contain nothing to suggest the idea of an 

 orienting tropism dependent upon unequal stimulation of the motor 

 organs on the opposite sides of the animal. 



// 



FIG. 18.* 



*FiG. 18. Diagram of the reaction of Stentor to light, after Holt & Lee. 

 Stentors are confined in a vessel behind a wedge-shaped prism containing a 

 substance which partly cuts off the light, so that one end of the vessel is darker 

 than the other. The usual course of a Stentor near the lighter end is shown by 

 the broken line. 



