REACTIONS TO LIGHT IN PLANARIA MACULATA 105 
carried by the knife and spread over the posterior surface, and 
that this might make an effective light screen just as in the case 
of the normal animal. Histological examination of such speci- 
mens showed this idea to be erroneous. A few granules are 
always displaced in making an incision, but these are very much 
scattered and obstruct the passage of light very little, if at all. 
Again, the body of the animal might act as a shading mechanism. 
This, however, was shown not to be true as the same results can 
be obtained when the source of illumination is lifted slightly 
above the horizontal plane, in which case the light does not pass 
through any more of the tissue of the animal’s body than in 
normal lateral illumination. We are consequently forced to 
accept the conclusion that light entering the eye from behind 
does not stimulate the rhabdomes as it does when it enters from 
the side. That is, that light striking certain rhabdomes from 
the direction indicated by the arrow a (fig. 17) is followed by a 
definite turning of the animal, whereas no such turning results 
when light strikes the same rhabdomes from the direction indi- 
cated by the arrow b. None of the experimental work has offered 
any explanation of this phenomenon. The structure of the 
retinula and its relation to the pigment-cup, however, offer two 
possible explanations. 
In the living condition in Prorhynchus applanatus, according 
to Kepner and Taliaferro (16), the middle region of the retinula 
or the region which corresponds to the ellipsoid of the vertebrates 
is the most refractive portion of the retinula. The same holds 
true for Planaria maculata after fixation. This region, because 
of its position, contour, and refractive index, must have some 
effect on the rays of light as they pass down the longitudinal 
axis of the retinula to strike the rhabdome (fig. 17, a). It occurs 
to one that possibly this region serves as a crude lens to concen- 
trate the rays of light upon the sensitive rhabdome and that 
photic stimulation depends upon this. If this is true, photic 
stimulation could not be set up in a given rhabdome unless the 
light struck the rhabdome approximately parallel to its longi- 
tudinal axis. Not only would the light have to strike the rhab- 
dome approximately parallel with its longitudinal axis, but it 
