esterly: eucalanus. 27 



opening in the basal plate {la. ha.) and one nerve fibre {fhr.) passing 

 through it. This fibre belongs to the anterior cell of the ventral eye 

 and is shown again in Figures 44 and 48. It may be said here that 

 this particular fibre is of constant occurrence and distribution, and 

 more easily followed than any other one. In Figure 44 some other 

 fibres of the ventral eye are shown, passing through the basal plate, 

 though it is not possible to trace them from individual cells. 



The fibres of the dorsal group shown in Figure 6 (Plate 1) pass 

 through the pigment cell to the anterior border (Plate 5, Figs. 44, 

 48), where they diverge to the right and left. The cells to w^iich 

 these fibres belong are the anterior cells of the lateral ocelli. 



It is difficult to follow the nerve fibres in frontal sections. • There are 

 a few cases, however, which agree with the conditions as described 

 from sagittal and cross sections. In Figure 53 (Plate 5) are show^n 

 several fibres (///'".) passing from the lateral eye of the right side. It 

 is plain enough that they leave by the basal or inner ends of the cells, 

 as already shown in many other cases. 



Whether one believes that the central cell (as I have called it) or the 

 basal plates contain the pigment of the eye, is immaterial in consider- 

 ing the nature of the innervation of the retinal cells, viz., whether the 

 eye is of the iuA^rted type or not. In any case it cannot be claimed that 

 the nerves spread over the outer faces of the cells and there gain en- 

 trance to the cells themselves for distribution toward their basal ends. 

 The contention of previous investigators with regard to the inverted 

 character of the median eye has been based upon their observations 

 that the nerve fibres leave from the outer sides or ends of the cells. 

 But in the eye of Eucalanus there is not an instance where an axis- 

 cylinder fibre leaves from any part of a visual cell except that which 

 must be regarded as inner, or proximal. It might seem that the axis- 

 cylinder which belongs to the anterior cell of the ventral eye (Plate 5, 

 Figs. 44, 48) is an exception to the above statement, but even in this 

 case the fibre passes through the basal plate and instead of leaving 

 that particular cell at the distal end, really does so from the side which 

 is directed toward the centre of the eye. The case of this cell certainly 

 differs in some respects from any other that shows the relation between 

 cell and fibre, but I believe it is a difference in degree and not in kind. 



The only way by which the retinal cells in Eucalanus could possibly 

 be regarded as "inverted," is to assume that the axis cylinders, which 

 leave the cells at their basal ends, pass toward the distal ends without 

 dividing into fibrillae, and that at the distal ends of the cells they turn 

 back. In that event the nerve-endings would be directed toward the 



