No. I.] EYES OF ARTHROPODS. 115 
In eye V., the two groups of inverted cells arise from the 
dorsal and ventral sense organs, 5 and 6, in the same way that 
the tongue of cells in eye VI. is derived from its ventral sense 
organ, 6. 
In the full-grown larva, the group of inverted cells is propor- 
tionally as well developed and conspicuous as in the younger 
stages. It is not shown in Fig. 75, since the section is a semi- 
transverse one passing through the optic nerve z VI. (Fig. 
toa). The direction of the cells in the inverted sense organ is 
at right angles to that of the optic nerve, consequently it is im- 
possible to cut a longitudinal section of both structures at once. 
THE CORNEAGEN forms a thick cap to the eye, above which a 
lens is formed, as in eye V., from a striated layer of non-refrac- 
tive cuticula. 
In Fig. 73 there is a temporary indentation in the corneagen 
which seems to mark the place where the lips of the optic cup 
came together. 
Tue Retina. — Just before the closure of the cup, the reti- 
nal nuclei suddenly appear to decrease in number, owing to the 
formation, in the manner already explained, of the retinophore, 
which soon arrange themselves in a single layer, with all their 
primary nuclei at about the same level. The retinophore are 
the sanie as those in eye V., except that they are a little 
shorter, and the primary and secondary nuclei are often situated 
so close together as to appear like one nucleus. 
In the full-grown larva the retina is almost a hollow hemi- 
sphere, and consequently the rods on the periphery are very 
nearly at right angles with those in the centre. 
Tue Rops are remarkably uniform in structure. On the pos- 
terior edce of the retina they are not quite so long as elsewhere. 
Each pair forms a thin-walled, hexagonal tube, which when iso- 
lated shows distinctly its two component rods. Cross sections 
show a mosaic of closely packed, hexagonal figures without the 
regularly arranged thin places so conspicuous in the rod-mosaic 
of eye V. (Fig. 450). 
Eye III. 
Eye III. appears very early as an oval, dark area in the middle 
of the second segment of the optic plate. It is soon surrounded 
by a circular clear space, and then both parts become consider- 
