96 W. H. TALIAFERRO 
case, whenever the light is so placed that the pigment-cup shades 
all of the rhabdomes, no reaction follows (fig. 11, B, a-c). 
Let us now consider precisely which rhabdomes are illuminated 
under the various conditions mentioned above. It will be re- 
membered that when Planaria is illuminated from directly in 
front, or directly above, or to the left of these points, it turns 
away from the side containing the eye, i.e., to the left. Now, 
Fig. 12 Diagrams made from camera-lucida drawings representing the rhab- 
domes which are illuminated when light enters the eye from various directions. 
A, eye seen in frontal section; B, eye seen in transverse section; A-P, longitudinal 
axis of animal; D-V dorsoventral line; ac, accessory cells forming pigment-cup; 
p, pigment-cup; r, rhabdomes. Light entering the eye from any point between 
a-b illuminates only the rhabdomes in the area which is cross-hatched. Light 
entering the eye from any point between c-d illuminates only certain rhabdomes 
of the unshaded regions. Light from any point between b-c may illuminate rhab- 
domes of both the shaded and unshaded regions. 
when light enters the right eye from a point directly in front of 
the eye, or slightly to the left of this point, the only rhabdomes 
which are illuminated are those lying along the outer, posterior 
edge (fig. 12, A, a-b). The remaining rhabdomes are shaded 
by the anterior edge of the pigment-cup. In the same manner, 
when light enters the eye from a point directly above the eye, or 
slightly to the left of this point, the only rhabdomes illuminated 
are those placed along the outer ventral edge of the pigment- 
