MOTOR RESPONSES 283 



in an increase in illumination of photosensitive substance. The photoposi- 

 tive specimens consequently turn until they face the light, whereas photo- 

 negative specimens turn until they face in the opposite direction. When 

 the stimulus which induces turning ceases, the organisms continue either 

 directly toward or from the light. 



Mast (1911) made a very intensive study of the process of orientation 

 in a species of Euglena which crawls on the substratum but continuously 

 rotates on the longitudinal axis as it proceeds. This Euglena orients very 

 precisely in light, it has a well-developed eyespot, and it moves so slowly 

 that the different phases of its responses can readily be followed in 

 detail. It is therefore very favorable for the study of the process of 

 orientation. 



If the intensity of the light is rapidly decreased in a beam in which 

 specimens are proceeding toward the source of light, they stop suddenly 

 and bend in the middle toward the abeyespot surface until the two halves 

 form nearly a right angle; then they begin again to rotate on the longi- 

 tudinal axis; and, while rotating, they gradually straighten and proceed 

 once more toward the light source. If the intensity of the light is in- 

 creased, or if it is slowly decreased, there is no perceptible response. The 

 cessation of movement and the bending are therefore dependent upon 

 the rate of decrease in the intensity of the light in the field, i.e., it is a 

 shock reaction. The decrease in the intensity of light in the field neces- 

 sarily results in decrease in intensity of light on all the substance in the 

 field; it therefore must cause decrease in the illumination of the photo- 

 sensitive substance. The response, then, is dependent upon the rate of 

 decrease in the light on the photosensitive substance. 



If the direction of the beam of light is changed through 90^ without 

 alteration in intensity, the specimens oriented in it are illuminated 

 laterally. Those in which the eyespot surface faces the light after the 

 direction of the rays has been changed, stop at once. They bend in the 

 middle toward the abeyespot surface, then rotate, and gradually 

 straighten to resume their crawling movements. Those in which the eye- 

 spot surface does not face the light after the direction of the rays has been 

 changed, do not respond to the changed direction of the rays, until, in 

 the process of rotation on the longitudinal axis, this surface faces the 

 light; then they also stop, bend, rotate, straighten, and proceed. Thus 

 they continue until, in the process of rotation, the eyespot surface again 



