MOTOR RESPONSES IN INVERTEBRATES 591 



they get into light of a certain intensity, where they come to rest and thus 

 aggregate in this intensity. Others orient fairly precisely; some of them 

 are photopositive, others photonegative (Loeb, 126; Walter, 211; Mast, 

 140). 



If the light intensity is increased after a specimen has come to rest, 

 it remains at rest for some time, then slowly becomes active. This 

 activity gradually increases to a maximum and then remains fairly 

 constant (Mast, 140). The maximum depends somewhat upon the 

 intensity (Walter, 211). This relation should be more thoroughly 

 investigated. This response does not directly depend upon the rate of 

 change of intensity. It may be called a kinetic response. 



Taliaferro (203) made a thorough study of the process of orientation in 

 Planaria maculata under accurately controlled conditions and came to the 

 following conclusions: If Planaria maculata is illuminated from one side 

 with light of moderate intensity, the opposite side contracts, resulting 

 in gradual turning until it is oriented, after which it proceeds fairly 

 directly from the light. If the intensity is higher, it turns more rapidly, 

 and if it is very high, it may first swing the anterior end from side to side 

 several times and then turn from the light. The nature of the response 

 is therefore dependent upon the intensity of the light. 



If the two eyes are removed, it does not orient. Orientation is 

 consequently due to stimuli received through the eyes. Specimens with 

 one eye removed orient nearly as precisely as normal specimens. If they 

 are illuminated from the normal side, they turn directly from the light 

 until they are oriented and then continue fairly directly from the light. 

 If they are illuminated from the blind side, they continue without any 

 response until, owing to random wandering or swinging or twisting of the 

 anterior end, the light enters the intact eye. If it enters the posterior 

 or the ventral rhabdomes in the eye, they turn toward the normal side, 

 i.e., from the light, and soon become oriented. If it enters the anterior 

 or the dorsal rhabdomes, they turn from the normal side, i.e., toward the 

 light. This turning may continue until they are directed from the light, 

 or there may be a series of successive responses which finally result in 

 orientation. The direction of turning, i.e., the nature of the orienting 

 response, therefore, depends upon the location of the stimulus in the eye 

 as well as upon the intensity of the light. These responses are rapid 

 and of short duration. They are probably dependent upon rate of change 

 in intensity, i.e., they are probably shock-reactions. Orientation in 

 one-eyed specimens is, therefore, due to one or more responses, consist- 

 ing of rapid muscular contraction on one side. It obviously is not 

 due to "balanced" stimulation of photoreceptors on opposite sides, 

 resulting in "balanced" tonus of muscles on opposite sides, in accord 

 with Loeb's tonus theory of orientation, and there appears to be no 

 reason for assuming that it is essentially different in normal specimens. 



