60 W. H. TALIAFERRO 
INTRODUCTION 
In the extensive literature on the Turbellaria one finds rather 
detailed and careful studies concerning both the structure of the 
eyes and the reactions to light in various forms. In very little 
of this work, however, is there any attempt to codrdinate the 
known histological details of the eye with the reactions to light. 
The chief object of the present paper is to ascertain in how “ar 
the function of the eyes in the reactions to light in Planaria 
maculata can be correlated with the histology of these organs. 
One of the most important contributions to our knowledge 
of the turbellarian eye is Hesse’s (’97) study of the structure of 
these organs. Hesse, working for the most part with certain 
triclads, found that all such eyes consist of two parts: first, the 
visual cells or retinulae, each of which possesses a typical nucleus 
and is extended distally to form a bulb-like rhabdome, and 
proximally to form a nerve process which enters the ‘brain;’ 
second, the pigment or accessory cells which form a more or less 
cup-shaped structure partially enclosing the rhabdomes or 
sensory area of the eye. He states that the number of retinulae, 
as well as of pigment cells, varies in different species from one to 
a large number. The triclads studied by Hesse were negative 
to light, and hence moved away from the source of stimulation. 
He maintains that this reaction is due to the fact that when an 
animal moves away from the light the sensory cells or rhabdomes 
are shaded by the pigment cup; whereas, when the specimen 
moves in any other direction the pigment cup does not shade 
all of the rhabdomes. When certain of the rhabdomes are 
illuminated, as in the latter case, the animal turns so as to bring 
the sensory region of the eye again into the shadow of the pig- 
ment cup. Hesse maintains that localization of the photic 
stimulus is the specific function of the pigment cup which enables 
the animal to direct its course away from the source of light. 
In working on the eye of the rhabdocoele, Prorhynchus ap- 
planatus, Kepner and Taliaferro (16) found that it consisted 
of one retinula associated with a unicellular pigment cup or 
accessory cell. Although the relation of these two elements is 
