Eyes of MoUuscs and Arthropods. 639 



one ou the other. The bases of all seven retinulae rest upon the thicken- 

 ings of the circular membrane fìlling the inner corners of the euclosed 

 Squares {fig. 110). It now remains to indicate the position oecupied 

 by the inner ends of the four bacilli belongiug to the cells constituting 

 the pigmented collar of the calyx. We have seen that, on a level with 

 the pedicel, they formed two pairs arranged on either side of its broad 

 face, between the black retinulae on the one band , and the brown on 

 the other (tigs. 89 — 93). After passing the pedicels, the bacilli come 

 to lie outside of the cell group formed by the narrowed, inner ends of the 

 ommatidia (tigs. 95 — 100 , and still nearer the basal membrane they 

 approach each other untiltinally, they end, in the characteristic manner, 

 on the stronger inner fibres of the diagonal bundles, in the angle oppo- 

 site that oecupied by the thre.e brown retinulae i, 2, 5(tìgs. 106 and 1 10). 



No traces of the inner ends of the basal cells have been found. A 

 diagram, representiug these conditions, and lettered to correspond with 

 those of the sections at other niveaux, will make this rather confusing 

 series of changes more comprehensible. Thus we see that each connec- 

 tive tissue cross of the basal membrane furuishes the support for a single 

 ommatidium, and that both these structures correspond in number. 

 Moreover. that the distributiou of the various cireles of cells around 

 their respective centres is determiued by certain planes of division, the 

 resulting arrangement beiug identical in all the units and in all the 

 parts of the eye. 



A knowledge of the basal membrane and of the arrangement of the 

 cells upon it has prepared the way for a compreheusiou of the distri- 

 bution of nerve fibres to the cells of the ommatidia. In longitudinal sec- 

 tions, one can easily see that a single, large, pigmented bündle of nerve 

 fibres passes to each of the openiugs leading into the Square Spaces en- 

 closedby the neighboring crosses; just before reaching the latter, it breaks 

 up into four, smaller branches going to the four corners of the Square. 

 Each of these branches divides again into still smaller oneS; which 

 ascend along the inner ends of the cells ; the number of bundles for 

 each corner corresponds with the number of cells there. Thus each of 

 the eleven cells, whose ends rest in the corners of the Squares, is 

 snpplied with a small bündle of pigmented nerve fibres, all of which 

 have arisen from a single branch. But it will be seen, by Consulting 

 the diagram, that the eleven nerve bundles of each Square belong 

 to four diiferent ommatidia. It is also evident, that the eleven pig- 

 mented cells of each ommatidium rest in the corners of four differeut 

 Squares, and therefore must receive their nerve fibres from four 



