The eye of Prorhynchus applanatus Kennel consists of but two cells. One of these cells is the pigment cell and the other is the visual cell or retinula (figs. and). The earliest investigators in this field recognized that a pigment-cup was associated with the visual elements of turbellarian eyes. In 1864 .Leydig described in part the contents of this cup; but it was not until Hesse's ('97) work that we get a satisfactory account of the structure of the visual cells and pigment-cup and the relation between them. In one respect the work of Kepner and Taliaferro ('16) has carried the knowledge of the cytological details of the eye of this rhabdocoele beyond Hesse's observations, as will be later indicated.
The material and methods of fixation of the specimens for this work were on the whole similar to those used by Kepner and Taliaferro. The specimens have been collected by bringing into the laboratory pads of Vaucheria and other plant masses from the face of a dam over which water continually flows. These masses of algae and higher plants were placed in large glass vessels in tap water. The animals within forty-eight hours made their way to the surface in such aquaria where they were readily collected and transferred to embryological watch glasses that contained spring water. One part of each collection was placed during the day (from 9.00 a.m. until 5.00 p.m.) in the northern light of the laboratory from time to time during the months of April and May. At 5.00 p.m. each day a 40 Watt, 110 volt, Fostoria Mazda light lamp was turned on as it hung with its glass wall 21 cm. above the surface of the water