REACTIONS TO LIGHT IN PLANARIA MACULATA 71 
stains blue. In fixed material this region is optically the densest 
portion of the retinula. No special structure can be distinguished 
in the third or nucleus-bearing region except the nucleus. This 
is large and approximately spherical (fig. 3,C,N). Its general 
appearance, as well as its staining reactions, indicates that it is 
a modified nerve element. 
All of the fibers of the nucleus-bearing region issue from the 
pigment-cup at the anteriolateral margin of the opening. The 
position of these fibers was later found of great importance as 
any injury to the anterior part of the eye destroys the function 
of the entire organ by destroying them. 
From this description it is evident that the retinula in Planaria 
maculata is very much like the retinula in Prorhynchus applana- 
tus and the regions found in it are similar to the rhabdome, ellip- 
soid, and myoid of the vertebrate rods and cones, just as Kepner 
and Foshee (’17) maintain for Prorhynchus applanatus (fig. 3). 
A similar differentiation has been described by Kepner and 
Lawrence (18) in the retinula of Polycystis goettel. 
The discovery of this analogy or, possibly, homology of the 
parts of the retinula of the rhabdocoele eye with the parts of 
the vertebrate eye is very significant, for it strongly supports 
the contention of some observers that the simple chordate eyes 
of Branchiostoma are homologous with the flatworm eyes. It 
is, however, questionable whether organs of two groups of animals 
as far removed as are the Platyhelminthes and chordates can be 
homologized. 
Fig. 2 Camera-lucida drawing of a transverse section of the eye of Planaria 
maculata. X 1500. A, accessory cells; An, nucleus of accessory cell; P, pig- 
ment-cup containing visual cells or retinulae, some of which have been omitted; 
R, retinulae; r, rhabdome; s, striae in the rhabdome; m, middle region of the re- 
tinula; n, portion of the nucleus-bearing region of the retinula (the nucleus as 
well as the greater part of this region lie outside of the pigment-cup and are not 
shown in the drawing). 
Fig. 3 Diagram representing analogous structures in the retinulae of ver- 
tebrates and flatworms. A, retinula (rod) of frog (after Kepner and Foshee, ’17, 
from Arey, ’16); B, retinula of Prorynchus applanatus (after Kepner and Foshee, 
17); C, retinula of Planaria maculata. Line 1 indicates the rhabdomes of the 
three retinulae; line 2 connects the ellipsoid of the frog with the analogous 
parts of the retinulae of Prorynchus and Planaria; line 3, the myoid; N, nuclei. 
