Eyes of Mollusca and Arthropods. 629 



formed by the tour rctiuophorae, is tinally rcduced to a slender, tube- 

 like stalk, or style, st.: during this process, the granular thicken- 

 ings become again reduced to thin, structureless walls, which, 

 farther on, are supplied with four, minute and refractive, inner ridges 

 (figs. 81 — 84 , As we progress still farther towards the basal mem- 

 brane, the tube becomes square , the four ridges having developed into 

 four rectaugular thickenings, whieh nearly fili the central canal (fig. 84). 

 Still the changes are not completed, for, farther on, the Square tube 

 becomes transfonned into an oval one; two of the ridges at the ends of 

 the small diameter constantly decrease in thickness, until that part of 

 the wall is reduced to a thin membrane, while the remainder, surround- 

 ing the uow circular opening, retains its original thickness (figs. 85 — 

 88). After a slight, final change, by which the tube with its central canal 

 is increased in diameter, we bave reached that expanded solid portion, 

 or pedicel, usually spoken of as the rhabdom (figs. 93 and 72 /></.). 

 The axial walls, in the middle of the four retinophorae, have dis- 

 appeared: while the abaxial ones, having so completely fused with 

 each other as to leave no trace of a former division, form a slender 

 tube, the style. 



In the pedicel, the abaxial walls have become so thick as to entirely 

 obliterate the central canal, while the divisions between the four 

 componeut segments are again visible (figs. 93 — 95). The base of the 

 style expands suddenly, but gracefully, into the large, pyramidal, outer 

 end of the pedicel, which , continued inward as a gradually narrowing 

 oblong column, contracts, shortly before reaching the basal membrane, 

 into the smaller, inverted pyramid, whose apex is drawn out into an 

 extremely slender stalk (fig. 72 si.pcl). Near the basal membrane, 

 the latter diverges into three legs composed of the attenuated, inner ends 

 of the four retinophorae, two of which have united with each other, 

 Each leg of the stalk is divided at its inner end into several 

 fibres by which it is united to the basal membrane (figs. 72 and 

 108). This fact is of great importance, for it proves that the segments 

 of the so-called rhabdom of Grenacher are not secretions of 

 the retinulae, but the inner ends of the retinophorae (or crys- 

 talline cone cells of Grenacher), which terminate in the same root- 

 like fibres, seen in nearly all hypodermic cells. 



The complicated structure of the pe die eis oulybecame intelligible 

 to me after the study of wax figures , which I was obliged to construct, 

 provided with lines similar to those of the pedicels, and which were 



