Eyes of Molluscs and Arthropods. 685 



The outer circle of collar cells has the same shape as the inner, but its 

 arrangement is difterent, siuce here the bases and thickest parts of the 

 triangles are turned outwards instead of inwards (fig. 69). The four 

 cells of the collar are so arranged that the triangulär Spaces, left be- 

 tween the two inner cells, are exactly filled by the two outer ones, thus 

 forming a continuous. pigmented band around the calyx. The outer 

 edges of the cells eontain. instead of pigment, a mass of refractive gran- 

 ules, which in transmitted light appear nearly colorless, but in re- 

 flected light are of a faint yellowish-white color, and perfectly opaque. 

 The granules produce the same effect as those to be described in the 

 cells at the bases of the ommatidia. 



The collar cells are continued, as four delicate fibres, outwards to 

 the surface of the ommatidium where they produce four minute impres- 

 sions at each corner of a corneal facet, or, in case the ends of the retin- 

 ulae have remained undisturbed, they are seen as four fibres in the 

 centre of each tube (fig. 76). It may be well to call attention here to 

 the uncertainty of obtaining these impressions, for in many instances, 

 I have seen no trace of them; this is due, I think, to the fact that the 

 inner chitinous layer of which the cornea is composed may be removed, 

 and, consequently, all trace of the delicate impressions be lost. In cross 

 section through any part of the calyx, the bacilli of the collar cells, 

 of which those belonging to the inner row are the largest, may con- 

 stantly be seen at their respective corners (fig. 79 — 8ibc.) , an arrange- 

 ment maintained as far as the oiiter ends of the style, when the smaller 

 bacilli gradually change their position until each is united with one of 

 the larger ones, which have meantime retained the same direction, in 

 Order to form two pairs, one on each side of the style (figs. 81 and 82). 

 On the level of the calyces, the bacilli are situated within the mem- 

 branous tubes formed by the ends of the retinulae (fig. 71 — 8t). In 

 passing between the pedicels, they are continued along the median line 

 of their broader sides, between the paired lateral cells {2, 4 and 5, 5). 



Just before the basal membrane is reached, the two pairs of bacilli 

 approach each other at that side of the pedicel opposite the median, 

 scalloped cell , and there become fastened to the diagonal bar of the 

 basal membrane Squares, by means of the characteristic, root-like fibres 

 (figs. 99, 101 and 11 Je' and hc^) . A much better idea of the bacilli, or 

 cell stalks (which only differ in the amount of development from those 

 of the outer row of cover cells in Area, or the inner ends of the pigment 

 cells in the retina ofi?a/eo^e5Ì, may beobtainedby studying macerate dpre- 

 parations, in which the entire bacilli are isolated. In such cases they are 



