516 STANLEY C. BALL 



left eye are viewed from the dorsal surface. The nerve fibers 

 show very plainly passing from the 'Punktsubstance' region of 

 the brain outward and between the ganglion cells. The course 

 of the nerve is then directed laterally around the posterior side 

 of the pigment cell to its external surface. 



At this point the eye cell, whose origin and growth have al- 

 ready been described, undergoes an invagination. This is 

 apparently effected by the pressure of a nerve cell which is seen 

 lying in the concavity. This undoubtedly will form the 'sight- 

 cell' or end organ which is stimulated by light rays. A second 

 nerve cell nucleus is observed just posteriorly. 



It can now be suggested that the two cells lying close outside 

 the pigment cell in figure 32 arose in this position and have sent 

 fibers backward to meet the anterior extension of the brain. Two 

 facts support this view. First, undifferentiated cells during 

 development lie in this region between the ectoderm and the 

 pigment cell. Secondly, it is well known that in higher forms 

 the cell body of the receptor is located in the retina, while the 

 axone extends into the brain. It is obvious, however, that one 

 must not attempt to homologise too closely the course of develop- 

 ment of a turbellarian with that of a vertebrate. In the litter 

 the retinal cells are first carried out by the optic cup from the 

 central nervous system, while in the Turbellaria the boundaries 

 of cell groups in the cephalic region are notably indistinct. 



It has proved impossible to discern in my material the struc- 

 ture of the cell or cells which occupy the external invagination 

 of the pigment cell in the adult worm. Since the histology of 

 the eye in related Turbellaria has been determined, it is to be 

 expected that the receptor of P. gemelhpara is similar to that 

 observed, for instance, in Phaenocora and Mesostomum as 

 shown in Bronn's Klassen und Ordnungen des Thier-Reichs. 



Figure 33 of this paper was drawn from an adult eye of P. 

 gemellipara, and shows that the optic nerve, as in the embryo 

 and in the species to which we have just referred, passes to the 

 lateral invaginated surface of the pigment eye-cell. A nucleus 

 rests close outside, apparently in the nerve. 



