154 PATTEN. (VoL. II. 
glion of the convex eye of Arthropods ts derived from a three-seg- 
mented, larval ganglion, each segment of which belongs toa pair 
of larval ocell. 
III. SumMMAry AND COMPARISON. 
The great nucleus that figures so prominently in the early 
stages is not the least remarkable feature of the eyes of Acilius. 
So far as my knowledge goes, it is without parallel and inex- 
plicable. 
In eyes II., III., IV., and V., there is a single nucleus in the 
middle of the retina, between the two rows of gigantic cells, 
that remains unaltered through life. In eye I. there are at least 
two of these nuclei, one in the horizontal, and one in the vertical 
furrow. In eye VI. the nucleus is only present a short time 
during the earliest stages. In all the eyes the large nucleus 
seems to be situated in the centre of a group of four sensory 
pits. 
Kleinenberg has described in Asterope a gigantic, flask-shaped 
cell that secretes the vitreous substance filling the cavity of the 
optic vesicle. But there is probably only a superficial resem- 
blance between this cell and that in the eye of Acilius. 
The reader has no doubt been impressed by the presence of 
horizontal rods in the eyes of Acilius, as well as by the fact that 
the rod mosaic varies tn a constant manner, according to the inclt- 
nation of the retinal cells. All the horizontal rods, large or 
small, are flat, and arranged in long, parallel rows. The semi- 
upright ones form, in cross sections, zigzag lines, and the up- 
right rods are bent so as to form the sides of regular hexagonal 
figures. Why are the horizontal rods invariably arranged in 
parallel lines? As the rods are mere frames for the support of 
nerve fibrilla, it must be, no doubt, because in this way the 
fibrilla are most advantageously arranged for the reception of 
light stimuli. In my paper upon the “Eyes of Molluscs and 
Arthropods,” page 652, the statement was made that the perfec- 
tion of a visual organ was dependent, among other things, upon 
the degree to which the retinidial fibrille have become perpen- 
dicular to the rays of light. At that time I did not think it 
probable that there were horizontal rods, as stated by Grenacher, 
in the eyes of Myriapods. It must be evident now from my 
description of horizontal rods in Acilius that there is no longer 
