NO. 3.) EYES OF ARTHROPODS. I59 
we compare eyes ITI. and IV. of Acilius with those of Iulus or of 
Heterostoma. 
The retinal pigment is in both cases most abundant at the 
outer ends of the retinophore, and is confined to the outside of 
the cell. At least on p. 437 Grenacher states that in the Scolo- 
pendriden, if we may regard the condition in Cormocephalus 
as normal, the pigment appears “in der Mantelflache der Zellen 
abgelagert zu sein, und die innern Theile derselben freizulassen.” 
My sections show just such figures as he represents in Fig. 6, 
except that in my judgment the pigment in Acilius and Dy- 
tiscus is outside the cell wall instead of inside. 
A peculiar feature of Acilius is that in the younger stages all 
the eyes are elongated in the direction of the retinal furrow, and 
this condition is more or less conspicuous in the larvee of Dytis- 
cus, Hydrophilus, Colymbetes, Psephenus, and Gyrinnus. There 
is a similar elongation of the retina in Iulus. 
One of the most striking points in the less specialized eyes of 
Acilius is the great notch at the bottom of the eye containing 
the broad rods of the gigantic cells. Grenacher has shown in 
Scolopendra a similar notch, and there are strong reasons for 
believing that it contains a double row of large cells. I think it 
probable that in Scolopendra, as in the first four eyes of Acil- 
ius, all the originally horizontal retinal cells, except the large 
ones at the bottom, have gradually withdrawn from the median 
plane of the eye to form a concave layer of nearly upright 
rods. 
When it is urged that Grenacher saw no such gigantic cells, 
and that there is no indication of them in his drawings, we shall 
answer that the same thing might be said of his description of 
the eyes of Acilius, and yet a casual examination of depigmented 
sections will show at once that such cells are there. 
When we compare the retina of Heterostoma (Fig. 4) with 
that of eyes IT. and IV., we cannot withhold the suspicion that 
in the retina of this Myriapod there is also a double row of large 
cells, and that their gigantic rods were provided with horizontal 
external nerve fibres which had been mistaken for the dividing 
lines between many small superimposed rods.! If this be so, 
then Grenacher’s “ Haargellen”’ are probably ordinary retinal 
1 At one time I also regarded these markings in Acilius as the divisions between 
small rods, 
