BRADLEY M. PATTEN 453 



Efect on Orientation Produced by Symmetrical and by Asymmetrical 

 Interference with the Photoreceptive Mechanism. 



Small changes in the tonus of the muscles of locomotion are fre- 

 quently difficult to detect and always difficult to handle quantitatively 

 in an animal which is not moving. An animal's locomotion, however, 

 is a direct resultant of muscular activities and any difference in the 

 tonus or in the vigor of contraction of the muscles on opposite sides 

 of the body is at once evidenced by a deflection in the path of advance. 

 Small differences in contraction are summated in continued locomo- 

 tion. The measurement of deflections in the path of travel is, there- 

 fore, a convenient and accurate method of deahng quantitatively 

 with the activities of locomotor muscles. 



Unbalanced muscular reactions as indicated by a curved path of 

 locomotion have been observed to follow unilateral bhnding in both 

 positively and negatively phototropic animals. The earlier observa- 

 tions were qualitative only, more recently the reactions have been 

 quantitatively handled. In all the cases the evidence is in accord 

 with Loeb's muscle tension theory of orientation, positively photo- 

 tropic forms moving in curves with their uninjured side inward, and 

 negative forms in curves with their blackened side inward (4). 



The reaction measurements made in terms of angular deflection on 

 partially bHnded whip-tail scorpions fall entirely in Hne with observa- 

 tions on other animals. But because the scorpion has three pairs of 

 photoreceptors, each of which can be ehminated separately, the series 

 of measurements which can be made on it is peculiarly extensive and 

 interesting. It is possible to eHminate on one side of the body the 

 median eye, the lateral eye, or the cutaneous sensitive areas; or any 

 two receptors; or all three receptors. By making eliminations on both 

 sides of the body, the series of cases may be still further extended. 

 In these experiments measurements were made, under bilaterally 

 balanced illumination, on animals in ten different conditions of asym- 

 metry. The results are collected in Table III. The graphical sum- 

 mary (Fig. 3) shows clearly the unerring consistency with which all 

 the animals were deflected toward their less sensitive side. All the 

 deflections are to be regarded as having the same significance as cir- 

 cus movements. In most of the deflections the radius of the curved 



