Eyes of Molluscs and Arthropods. 5S7 



the text, but, from Ins figure 95, PI. XXI, one sees that he regarded the 

 inner ganglionic cells as nuclei. Carrière, also mistook them for the 

 nuclei of the retiuophorae while, the real nuclei, he regarded as those 

 of a circular gangliou, to which he attached considerable importance. 

 The observatious of Hickson are so poor, that it is impossible to deter- 

 mine exactly what he did thiuk. 



Besides the retinophorae, two other groups of cells, the inner, 

 and outer, ganglionic layers, are found, adding to the compli- 

 cation of this minute organ. The outer layer, which in its median, 

 thickest part is two, three, or even four cells deep, and toward the peri- 

 phery, gradually diminishes in thickness to a single celi layer, entirely 

 fills the saucer-shaped cavity formed by the retinophorae. 



Although the cells show ali variations in size and shape, three 

 types may be distinguished. 



(1) The larger, outer ones, whose broad ends terminate in many 

 fibres which penetrate the septal membrane and unite with the superior 

 brauch of the optic nerve. Their blunt inner ends, containiug large, ovai 

 nuclei, are drawn out into severa! fibres, which, with many others from 

 the deeper cells, extend, radiatingly, towards the inner ends of the re- 

 tinophorae, after crossing which, at right angles, they are continued 

 along the walls of the cells, to the rods (PI. 32, fig. 140 gc^.]. 



(2) The second kind are large, irregularly shaped cells, drawn out 

 into several fibres, a single one, directed toward the lens, being the 

 largest, and dividing. just before entering the fibrous layer, into several 

 small branches, which are continued onwards to the ganglionic brauch 

 of the optic nerve. The outer arms of these cells may be as fine as any 

 nerve fibre, or much thicker, resembling an elongated celi ; in this case^ 

 it terminates abruptly at the beginning of the fibrous layer, where it 

 breaks up into a number of fine fibres. The inner ends of the cells, 

 which contain large and nearly round nuclei, are drawn out into several 

 fibres, like those of the first type, extending towards the inner side of 

 the retina (PI. 29, fig. 33 ò, and PI. 32, fig. 140 ffcK). 



(3) The cells of the third form are small and provided with numerous 

 nervous prolongations, one of which passes through the fibrous layer 

 into the outer brauch of the optic nerve, while others extend inwards 

 towards the rods (PI. 32, fig. 140 <7c^.). Ali these kinds of cells, which 

 show no very definite arrangement except that the first generally con- 

 stitute the outer row, are evidently modified forms of the same Cle- 

 ments. 



At the periphery of the retina, the outer ganglionic cells are re- 



