No. 1.] EVES OF ARTHROPODS. 155 
any reason for me to doubt the accuracy of that part of Grena- 
cher’s account. 
When I became convinced that most of the rods of Acilius 
were horizontal, it seemed at first as though the above-mentioned 
view concerning the direction of the retinidial fibrillae would have 
to be abandoned, for here were undoubtedly highly specialized 
eyes in which as many retinidial fibrillae were parallel with the 
rays of light as at right angles to them, provided they radiated 
in all directions from an axial nerve. In Pecten, e.g., (Fig. 5, 
A.), we have cylindrical vertical rods containing axial nerves, 
from which radiate nerve fibrillz in all directions. All the 
fibrilla are consequently at right angles to the rays of light. 
But put the rod in a horizontal position, and just as many 
fibrillze will be vertical as horizontal. In order to bring these 
horizontal rods into harmony with my theory of nerve end- 
ings, it is requisite that the rods should be flattened, and of 
course so that the long diameter is vertical. Then if the 
axial fibres, instead of forming a single bundle, as in the 
cylindrical rods, should form a transverse row (Fig. 5, B.), 
fibrillae could arise from them, nearly all of which would be at 
right angles to the rays of light. If the rod were broad and 
very much flattened, and the upper and lower sides disap- 
peared, then all the fibrilla would be at right angles to the 
rays of light (Fig. 5, C.). In Aczlius all the horizontal rods 
fulfil the above requirements; they have exactly the structure 
they ought to have, and the only one they could have, to bring 
their retinidial fibrille at right angles to the rays of light; 
therefore, instead of horizontal rods being fatal to my theory 
of nerve endings, they afford by thetr arrangement convine- 
ing evidence in favor of it. For those who believe that the 
nerve fibres described by me are the product of coagulation, or, 
perhaps of the imagination, may not be so ready to push lightly 
aside the evidence which the structure of the horizontal rods 
affords. It certainly is remarkable that in Acilius, whose eyes 
show as wide differences in structure as we could expect to find 
between any larval ocelli of Insects, the horizontal rods should 
invariably assume the only shape that would permit all the reti- 
nidial fibrillze to be at right angles to the rays of light. 
A vertical section of a pair of broad horizontal rods would 
give an outline exactly like that formed by a similar section of 
