The experiments dealt with in this paper were devised to ascertain (1) the relative effectiveness as photoreceptors of the whip-tail scorpion'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 different abnormal conditions of the photoreceptive apparatus were produced.

The reactions of animals thus partially blinded were measured in terms of angular deflection from an initial path of locomotion. Measurements obtained under anterior, lateral, and bilaterally balanced illumination were compared with measurements made on normal animals under the same conditions of illumination. The change from the normal reaction induced by covering a photoreceptor was taken as an index of the effectiveness of the receptor prevented from functioning.

By comparing the values of the changes from normal reactions produced by the elimination of the several receptors, their relative effectiveness is approximated as median eyes : lateral eyes : cutaneous sensitive areas :: 1:1.6:2.2.

All animals in which the receptive mechanism was rendered functionally asymmetrical exhibited, when subJected to bilaterally balanced illumination, deflections toward the side which had been made less sensitive. In a series of measurements made on animals in ten different conditions of asymmetry the amplitudes of the deflections were proportional to the degree of unbalance which had been produced in the photosensitive mechanism.

Animals in which the receptive mechanism was reduced but left in a symmetrical condition maintained an undisturbed balance of reaction when subjected to equal, opposed lights. Under lateral or anterior illumination the rate of attaining a new direction of orientation was reduced in proportion to the extent of the interference with the receptive mechanism.

The reactions of symmetrically and asymmetrically blinded scorpions indicate that orientation is attained and maintained by a transmission of impulses to the muscles of locomotion which is proportional bilaterally to the excitation of the symmetrically located photoreceptors.

In their effect on orientation the three pairs of receptors are completely coordinated. Orientation depends upon bringing the excitation of the receptive mechanism as a whole into bilateral equilibrium.

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