198 BULLETIN OF THE 



the eye, the evidence against the presence of a lentigen is apparently 

 complete. 



The composition of the retina in the lateral eyes is much more diffi- 

 cult to study than in the median eyes. This is due in part to the small 

 size of the lateral retinas, and in part to their almost complete chitinous 

 investment. To- make isolation preparations is wellnigh impossible, by 

 far the best results being obtained from the study of sections. 



Graber ('79, PI. V. fig. 5), believing that the composition of the 

 median and lateral retinas was essentially the same, has figured in the lat- 

 eral eyes of Scorpio retinal elements with three nuclei. Moreover, the 

 retinal elements are grouped, as in the the median eyes, in fives (PI. V. 

 fig. 8). 



Lankester and Bourne ('83, pp. 181-187) claim that the retina con- 

 sists of unicellular elements, or nerve-end cells, as they call them, and of 

 indifferent cells. The indiiferent cells occur both between the nerve-end or 

 retinal cells, as "interneural cells," and around the edge of the retina, as 

 "perineural cells." The indifferent cells all contain pigment, the retinal 

 cells, in their opinion, are probably pigmented on their peripheries. 



In Centrurus the nuclei (PI. III. fig. 18, nl. r. and nL pi n.), as in 

 the median retinas, are limited to the deeper portion and to the periphery 

 of the eye, and Graber's anterior and median nuclei are not present. The 

 nuclei {nl. r.) belonging to the deep portion of the retina are slightly 

 larger than those (nl. pi n.) on the periphery, and very uniform in size. 

 The fact that in this part of the retina there is onlj' one form of nucleus 

 leads to the conclusion that the retina in Centrurus is composed of only 

 one kind of cells, and that here the interneural cells described by Lan- 

 kester and Bourne do not exist. 



Sections perpendicular to the axis of this retina show immediately 

 under the lens the sharp outlines of cells which deeper in the retina have 

 their walls thickened into rhabdomeres. No additional cells, like those 

 in the median eyes, appear in the outermost sections of the retina, end 

 therefore the interneural cells, if present, must be limited to the deeper 

 portion of the retina. The fact that there is no difference in the nuclei 

 of this region leads me to believe that interneural cells are entirely 

 wanting. In Centrurus the retinal cells (PI. III. fig. 19) show no 

 tendency to be arranged in groups of five, and the rhabdomeric thicken- 

 ing {rhb m.) takes place on all sides of the cell. This is particularly 

 noticeable in examining the region nearest the lens. In the outermost 

 sections the cells are sharply outlined and their walls are very thin. 

 In the second or third section from the lens, the walls suddenly become 



