522 WM. A. KEPNER AND A. M. FOSHEE 



of the contents of the enteron and then bring its anterior end 

 to lie beneath this ejected food mass. Care was taken, then, 

 to remove such material whenever it was present, but it is possible 

 that a small piece of shading material might have escaped our 

 attention in this particular specimen. Unless in this or some 

 other manner this exceptional eye had been shaded we have no 

 way to explain its departure from the type of a light adapted 

 eye. This one exceptional case, however, constitutes a low 

 coefficient of variation when we consider that twenty animals or 

 forty eyes of this series of light adapted eyes were studied. 



On the other hand, all dark exposed pigment cells have shown 

 that the lamellae of their cytoplasm move apart with, the result 

 that the cell itself becomes greatly expanded (figs. 2 to 8). The 

 section adjacent to and belonging to the section shown in fig. 1 

 also shows a greatly expanded pigment cell. The reaction of the 

 pigment cell of the eye of Prorhynchus, therefore, stands in sharp 

 contrast to that of the eye of a vertebrate, in that light causes a 

 contraction and dark an expansion of it, while the reverse is the 

 reaction of the pigment cell of a vertebrate eye. The response 

 of the pigment cell of Prorhyncus's eye to light is similar to 

 that of the photoreceptor's pigment cell of Amphioxus, as 

 Crozier ('17) describes. 



In the retinula or visual cells of some vertebrates the following 

 cytoplasmic details have been observed: (1), an inner segment, 

 the myoid, with which the nucleus is directly associated; (2), a 

 middle region, designated the ellipsoid, which is to be found in 

 the eye of hsh, amphibia, reptiles and birds; and (3), the outer 

 segment, the visual rod or rhabdome (text-fig 1). Kepner and 

 Tahaferro ('16) found three cytoplasmic regions in the retinula 

 of Prorhynchus; (1) the inner one most closely associated with 

 the nucleus; (2) the middle segment, a lens-shaped region, 

 that in a freshly dissected eye was highly refractive, thus strongly 

 resembling the ellipsoid of a vertebrate \isual element; and 

 (3) an outer segment, the rhabdome. These authors recognized, 

 at the time their paper had appeared, that they had carried the 

 knowledge of the histological details of a turbellarian eye a step 

 beyond where Hesse ('97) had taken it. However, it was not 



