REACTIONS TO LIGHT IN PLANARIA MACULATA 69 
are of the inverted type and, hence, the arrangement of the 
three regions in relation to the entrance of the stimulating light 
is the same. 
The eyes of Planaria maculata appear as two small conspicuous 
dark spots on the anterior, dorsal surface of the animal near the 
median line. Each eye lies directly over, and near the anterior 
margin of a dorsal ganglion. The pigmented regions constitute 
the pigment-cups. The opening of each pigment-cup is directed 
laterally, anteriorly, and slightly dorsally. It is formed by a 
single layer of pigment or accessory cells (fig. 2, 4). By referring 
to figure 2, it may be seen that the pigment of the pigment-cup 
is in the form of granules and that these are represented as being 
concentrated at the inner surface of the accessory cells. It has, 
however, been repeatedly observed that these granules of pig- 
ment are concentrated at the inner surface of the pigment cell 
only after the animal has been exposed to long-continued illum- 
ination and that in darkness they tend to become scattered 
through the entire cell. 
The cavity of the pigment-cup is filled with the distal end of 
numerous retinulae (fig. 2, #). The number of these retinulae 
in any given eye has not been precisely ascertained, but it is 
approximately two hundred. Each retinula consists of three 
regions—a rhabdome, a central highly refractive region, and a 
nucleus-bearing portion (fig. 3, C). The rhabdome and central 
region constitute the portion of the retinula surrounded by the 
pigment cup. The nucleus-bearing portion is in reality a nerve- 
like fiber which extends from the central region of the rhabdome 
through the opening of the pigment cup to the ‘brain.’ Some 
distance from the cup there is an enlargement in the fiber which 
contains the nucleus. 
The three regions of the retinula are differentiated very dis- 
tinctly in material stained with Mallory’s connective-tissue 
stain. The rhabdome, which is situated next to the wall of the 
pigment cup, shows numerous radiating striae (fig. 2, 8S). The 
striae all radiate from the central region, which is a clear, more 
or less spherical body. This region (fig. 2,m) stains bright orange, 
while the rest of the retinulae, with the exception of the nucleus, 
