no Journal of Comparative Neurology and Psychology. 



to get the organism away from the stimulus. In the one case 

 it is accompHshed by a direct reflex, without more ado; in the 

 other only after a considerable waste of energy in inconsequen- 

 tial vermiculations. In orientation according to the usual theo- 

 ry of tropisms errors are made in abundance ; but they are cor- 

 rected in a more direct and efficient way than in the more or 

 less haphazard method so frequently followed. 



By a careful analysis of the phototaxis of Stentor, Euglena, 

 and some other protozoans Jennings has conduced that the ori- 

 tation of these forms to light takes place according to the trial 

 and error method, and not by the method of simple forced re- 

 flexes. The reactions of Euglena are of especial interest since 

 this form apparently shows a combination of both direct and indi- 

 rect methods of orientation to the direction of the rays. Eii- 

 glena may react to a strong or sudden stimulus from the light 

 by backing off and starting ahead in anew direction. Several tri- 

 als of this kind may be made until finally the creature becomes 

 oriented when it swims to or from the light according to the in- 

 tensity of the stimulus. Euglena is also capable of orienting it- 

 self by gradually bending its course until it comes to be approx- 

 imately parallel with the rays. Ordinarily this form swims in a 

 straight spiral path. Should light shine on the body from one 

 side the sensitive anterior end would be stimulated differently in 

 different parts of its spiral course. According to Jennings, it 

 is the diminution of light as the animal turns the anterior end 

 away from the stimulus that causes the motor response. 

 When the Euglena turns so that the anterior end is less illumin- 

 ated, it is stimulated to swerve back further towards the light, 

 and, by a succession of such responses, it finally becomes ori- 

 ented to the direction of the rays. Swimming through that 

 portion of its spiral course that causes the diminution of light at 

 the anterior end is that part of the creatures activities that must 

 be looked upon as error, if we go so far as to regard the passing 

 through different sections of a continuous spiral course as trials. 

 But to view the matter in this way is to go far towards obliter- 

 ating the distinction between orientation through trial and error 

 and orientation by the direct method. In the mode of photo- 



