No. 1.] EYES OF ARTHROPODS. 121 
(Fig. 90). The posterior wall, however, is strongly convex, and 
the retina on that side is composed of a broad layer of nearly 
upright cells, whose short rods decrease in length from the fur- 
row toward the posterior periphery of the retina. This remark- 
able asymmetry is correlated with the position and inclination 
of the lens, which looks upwards, inwards, and forwards, so that 
its optic axis falls about in the middle of this posterior layer 
of horizontal rods, and not, as one might expect, upon the 
retinal furrow. In eye I. the horizontal retina is also asymmet- 
rical, but to a less degree, and in an exactly reversed manner, 
for the greatest expanse of horizontal rods is on the anterior 
side of the furrow instead of the posterior (Fig. 91). There 
is a similar asymmetry in the retinas of eyes II. and IV. 
The gigantic retinal cells, arranged with great regularity in 
two rows, one on either side of the retinal furrow, are very 
broad and thin at their freeends. At their opposite extremities, 
there is a slight swelling containing a large oval nucleus. 
These cells are similar in nature and arrangement to those 
at the bottom of eyes II, IV., and V. (Figs. 70-78), except 
that they are much larger, and, owing to the way in which 
they receive their nerve supply, somewhat differently shaped. 
Those on the side of the furrow with the greatest number 
of upright rods are longer than those in the opposite part 
of the retina. The important point is, that, except in size 
and shape, they do not differ from the remaining retinal 
cells. They must be regarded as horizontal retinal cells with 
very wide and short rods; compare the isolated cells d, e, and 2, 
Fig. 58, Pl. X. When these cells first become distinctly out- 
lined (Fig. 85), there is only a small space between the pri- 
mary nucleus and the rod, while the outer edge of the cell 
shelves steeply inwards to the level of the almost horizontal inner 
edge. They are also much darker than the smaller cells, and 
their free ends being concave, a space is formed between the 
two rows of rods. But the latter are soon brought more closely 
together, their terminal edges become as straight and ridged as 
though crystallized, and the narrow space is filled with a mass 
of densely pigmented nerve fibres. 
In order to economize space without disturbing the arrange- 
ment of the rod-bearing portion, the thick nuclear ends of the 
cells are turned alternately toward and away from the median 
