Eyes of MoUuscs and Arthropods. 631 



four of which are nearly black , while the remaining three are filled 

 with light brown pigment. They attain their greatest size at the base 

 of the style, where the nearly spherical nuclei are situateci. The inner 

 ends are redueed to narrow bands, closely applied to, and surrounding, 

 in irregulär order, the pedicel. The nuclei, and consequently the most 

 expanded parts of the cells, are situated at different levels so that, in a 

 series of cross sections. one first sees, in the outer ones, the nuclei of 

 the three brown pigment cells, all placed on one side of the style, the 

 nuclei of the large median cell being the first to appear (figs. 74. 89 

 and 90). These three cells /, 2, 3) then become greatly redueed in size, 

 while the outer ends of the two black ones (*5, 7) gain the ascendency ; 

 their axial walls are compressed to thin edges resting upon the style, 

 while, near the expanded abaxial faces, are situated the nuclei (figs. 91 

 and 92). In the next sections, cells (^> and 7) have nearly disappeared and 

 the nuclei of the similarly shaped ones (5 and 7] appear (fig. 92). The re- 

 lation of these cells is represented in a somewhat diagrammatic way in 

 fig. 74. 



In the following sections, the uniformity in size and pigmentation 

 of the cells becomes gradually established, but they may still be easily 

 recognized by their arrangement. In nearly all the figures,consecutive sec- 

 tions of the ommatidia, as well as of the same cells, are placed in the 

 same direction, and uniformly lettered. In the largest portion of the pedi- 

 cels (which are much more closely arranged than is represented in 

 fig. 93), one may occasionally notice that the three cells of one half of 

 the pedicel are thinner and contain less pigment, than the four of the 

 other half. The arrangement shown in fig. 93 is very Constant and 

 characteristic, deviations from it being rare and of little importance. 

 In following the sections still farther towards the basal membrane, the 

 pedicel is seen to be less oblong and finally nearly round, while the re- 

 tinulae, which have become proportionally larger, still alford the means 

 of determining, by their shape and arrangement, the direction of the 

 secondary axes of the pedicel (figs. 93 — 96). The latter is finally re- 

 dueed to an extremely fine, and almost indistinguishable, central chord, 

 completely enveloped in the seven retinulae, which, in the following 

 sections, combine to form a butterfly-shaped figure, where the four 

 black cells form the wings, and the other three, the body and tail (figs. 

 99 — 100) . The median one (figs. 97 — 100, 1), at this poiut, much exceeds 

 its two neighboring cells in size, a fact which, later on, furnishes us 

 with the means of determining their arrangement upon the basal mem- 

 brane. 



