Eye8 ot Mollnscs and Arthropods. 583 



pletely enclosed within a membranous sac, which we sliall call the o ra - 

 mateal sac. The thick, anterior, concave wall, or septal mem- 

 brane (PL 29, fig. 19 Ò'.), serves at once to protect the ends of the 

 retinal cells, and as au elastic cushion, upon which the lens rests. The 

 inner wall is still thlcker, and constitutes the tough, double-layered 

 sclerotica ^fig. 19 sc). At the confi iieuce of these two membranes, 

 the wall of the sac is much thiuner, and perforated by innumerable pas- 

 sages for the entrance of nerve fibres from the axial branch of the optic 

 nerve. Within the ommateal sac, the cells constitute a closed vesicle, 

 whose anterior and posterior walls are so closely approximated as to 

 touch each other, thus obliterating the central cavity. The wall of the 

 vesicle, for theoretical reasons to be hereafter enumerated, we shall con- 

 sider as composed of a single layer of cells, although this simple 

 arrangement is obscured by mauy changes, resulting in the division of 

 both anterior and posterior walls into several secondary layers. The 

 posterior wall of the ommateal vesicle, theu, consists of four layers: 

 au outer vitreous network; a double layered argentea, and the red 

 tapetum. The anterior wall is likewise composed of four layers : an 

 outer gangliouic layer; an inner ganglionic layer; the retino- 

 phorae, and the rods containing the retinidia. 



We shall first consider the retinophorae, which constitute the 

 largest and most important part of the retiöJi. Their attenuated outer 

 ends, which become insensibly transformed into single nerve fibres, 

 are attached to the periphery of the retina; whence they are directed in- 

 wards towards the optic axis (fig. 19 n. rf.]. The most superficial of 

 these cells, after describing a long curve, bend suddenly at their ex- 

 panded inner ends almost at right angles, and terminate in the centre of 

 the retina. As the remaining cells end nearer the periphery, their ex- 

 panded inner extremities become relatively larger, longer, and less 

 sharply beut, until, finally, the shortest and peripheral cells describe a 

 nearly perfect semicircle (PI. 29, fig. 19). A saucer-shaped layer of 

 cells is thus produced, whose edges are formed by the curved, fibrous 

 ends of the retinophorae, the large, oval nuclei of which (containing 

 a nucleolus) are crowded together at the periphery of the retina. Beyond 

 the nuclei, the cells are continued as slender stalks which, before 

 reaching the ommateal sac, through which they pass as a single nerve 

 fibre, become expanded into a delicate, oblong vesicle, containing a 

 second, faintly stained, and often invisible nucleus (PI. 29, figs. 34. 35 

 and 36 h.). The expanded, inner extremities of the cells, filled with 

 fine, granular protoplasm, and containing a very faint and minute 



