114 W. H. TALIAFERRO 
20. It is possible to explain this localization of photic stimu- 
lation in the individual retinula: 1) by supposing that the highly 
refractive middle region of the retinula acts as a crude lens to 
concentrate the rays of light on the rhabdome; or 2) by assuming 
a certain structure of the rhabdomes coupled with the shading 
action of the pigment-cup. This last assumption ascribes a 
limited localizing function to the pigment, but it does not ascribe 
to the pigment the entire function of the localization of photic 
stimulation as is done by Hesse (page 106). 
21. Correlated with the assumption that light must penetrate 
a given rhabdome parallel with its longitudinal axis in order 
to cause stimulation, we find that light in entering the pigment- 
cup from any given direction illuminates the rhabdomes confined 
to a definite area and that a large proportion of the rhabdomes 
in such areas always have their longitudinal axes directed parallel 
to the stimulating rays of light and, in the case of light from 
certain directions, all of the rhabdomes which are illuminated 
have their longitudinal axes so directed. 
22. Once an animal is oriented in a horizontal beam of light, 
it receives no orienting stimulation until it leaves the path or 
axis of orientation. 
23. Orientation to light is not necessarily dependent upon the 
symmetrical arrangement of the photoreceptors. 
BIBLIOGRAPHY 
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BarDEEN, C. R. 1901 On the physiology of the Planaria maculata with especial 
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Buiaauw, A. H. 1908 The intensity of light and the length of illumination in 
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