456 PHOTOREACTIONS OF WHIP-TAIL SCORPIONS 



the whip-tail scorpion as well as to animals with a single pair of re- 

 ceptors. The evidence may be summarized as follows : 



In normal scorpions bilateral equilibrium in excitation results in- 

 variably in bilaterally balanced muscular responses and a straight 

 path of locomotion. 



When the receptive mechanism is reduced but left functionally 

 symmetrical, the locomotor responses remain balanced. Animals in 

 five different symmetrical conditions of reduced sensitiveness showed 

 no unbalance in reaction although the rate of attaining orientation 

 was reduced in proportion to the extent of the interference with the 

 photoreceptive mechanism. 



When the receptive system is subjected to eliminations which leave 

 it in an asymmetrical condition, unbalanced locomotor responses in- 

 variably follow, resulting in a curved path of locomotion with the less 

 sensitive side of the animal on the inside of the curve. In a series 

 of ten different conditions of asymmetrical sensitiveness the degree of 

 unbalance in locomotion was proportional to the extent of asymmetry 

 which had been produced in the receptive apparatus. 



In their effect on orientation, the three pairs of receptors are com- 

 pletely coordinated, the excitation of the organs on the same side of 

 the head being summated in transmission to the associated muscles 

 of locomotion. 



Balanced muscular reactions with a persistently straight path of 

 locomotion depend on bringing the excitation of the receptive mech- 

 anism functional at the time into bilateral equiHbrium. 



SUMMARY. 



The experiments dealt with in this paper were devised to ascertain 

 (1) the relative effectiveness as photoreceptors of the whip-tail scor- 

 pion's median eyes, lateral eye groups, and cutaneous sensitive areas, 

 and (2) the effect on orientation produced by symmetrical and by 

 asymmetrical interference with the photoreceptive mechanism. 



Each of the receptors was eliminated unilaterally and bilaterally, 

 singly and in combinations with other receptors. In all, sixteen dif- 

 ferent abnormal conditions of the photoreceptive apparatus were 

 produced. 



