466 WM. A. KEPNER AND JOHN S. LAWRE NCE 



Bohmig was the first to describe what may be called a com- 

 pound pigment cup in a Turbellarian eye. Of the eye of the 

 rhabdocoele, Vorticeros auriculatum, he says: 



Der Pigmentbecher jedes Auges wird durch eine mittlere Pig- 

 mentscheidewand in eine vordern und hintern Kammer gelegt. Dei- 

 Pigment besteht aus kleinen rotlichen Kornchen. Einen Plasmasaum 

 mit Kernen den Pigmentbecher habe ich nicht auffinden konnen, doch 

 soli damit nicht gesagt sein, das er in der That fehle. Durch die Pig- 

 mentscheidewand ist natiirlich bedingt, dass der Pigmentbecher zwei 

 Offnungen besitz, von denen jede durch eine einige vor ihr liegende 

 Zelle von linsenformigen Gesalt mit deutlichen Kern und Kernkor- 

 perchen geschlossen wird. (p. 486). 



Thus it appears that there are two kinds of pigment cups 

 associated with the visual elements of rhabdocoeles : (a) simple, 

 in which there is but a single lumen and, (b) compound, in 

 which the lumen of the cup is subdivided to give two secondary 

 lumina. 



In the eye of the flatworm there may be one or more visual 

 elements which send part of their cell bodies into the lumen 

 of the cups formed by the pigment cell or cells. In all cases 

 the nuclei of the visual cells or retinulae lie outside of the lumen 

 of the pigment cup. Bendy (see Benham '01) shows in his 

 figure of the eye of Geoplana a single visual cell whose nucleus 

 lies in the fundus of the pigment cup. This condition is so 

 strikingly exceptional that we feel that Dendy may have mis- 

 taken another structure for the nucleus of the cell. There has 

 recently been described a highly refractive body forming a part 

 of the retinula of Prorhynchus applanatus Kennel (Kepner and 

 Taliaferro, '16). The paper by Kepner and Taliaferro pre- 

 sents a description of the retinula of a rhabodocoele in which 

 there is a refractive body within the cytoplasm, between the 

 nucleus bearing portion of the cell and the end-organ or rhabdome 

 of the cell. Kepner and Foshee ('17) showed that a striking com- 

 parison could be made between the visual element of Prorhyn- 

 chus applanatus and the retinula of a vertebrate. 



The pigment cup, when seen in the living specimen, is sphe- 

 roidal in contour and intensely black both by reflected and trans- 

 mitted light. The average diameter of the cup is twenty-five 



