REACTIONS TO LIGHT IN PLANARIA MACULATA 61 
exactly that found by Hesse (’97), they found that the retinula 
consists of three regions: 1) a lateral nucleus-bearing region 
which is closely applied to the ‘brain’ and extends as a nerve 
fiber into this organ; 2) a middle region which is lens-shaped and 
in both living and fixed material presents the structure of a 
highly refractive homogeneous body, and, 3) a mesial region or 
the true rhabdome which fills the pigment cup. 
Kepner and Foshee (717), in a further study of Prorhynchus 
applanatus, point out an interesting comparison between the 
three regions of the retinula in Prorhynchus and the retinula 
(rods and cones) of vertebrates. According to these authors, 
the nucleus-bearing region, the highly refractive region, and the 
rhabdome of Prorhynchus are analogous, if not indeed homo- 
logous with the myoid, ellipsoid, and rhabdome, respectively, 
in the rods and cones of the vertebrates. This comparison is 
especially interesting in that the retinula of both flatworms and 
vertebrates are of the inverted type. 
Jinichen (’97) was probably the first to describe the middle 
region in the retinula of the turbellarian eye as a definite structure. 
He referred to it in Planaria gonocephala as the ‘Zwischenstiick,’ 
but placed no emphasis upon it. Béhmig (90) T. xxi, fig. 12) 
figures much the same type of structure in the rhabdocoele 
Monoophorum striatum, but does not mention it in his descrip- 
tion. Also, in the eye of the polyclad Pelmatoplana as figured 
by Schmidt (’02) there is a body very much like the one under 
consideration, but it is considered by that author to be a nucleus. 
No attempt will be made to review in detail the literature of 
the reactions of the Turbellaria to light. Such a review may be 
found in Walter’s (’07) paper. We shall, therefore, confine 
ourselves to certain papers dealing with the functions of the 
eye. The first paper treating this specific subject is that of Loeb 
(94), who maintains that decapitated specimens of Planaria torva 
react to light in precisely the same manner as normal specimens, 
but that the reactions are slower. He found, however, that the 
removal of the ‘brain’ and eyes of the polyclad Thysanozoén 
brochii, unlike Planaria torva, caused the animals to lose their 
responsiveness to light. 
