No. 383.] BASIS FOR A THEORY OF COLOR VISION. 839 
same animal, as in Acilius and in Lycosa (Figs. 4 and 5), the 
retinidial fibrils in one pair are nearly at right angles to those 
in two other pairs, so that the animal may be said to have its 
Fic. 2. — Vertical section through ocellus III of Acilius. 
retinidial fibrils arranged in the three planes of space, like the 
hairs in the semicircular canals of the vertebrate ear. But it 
is a curious fact that in Acilius the pair of ocelli marked No. VI 
—and in Lycosa the anterior median pair —do not have their 
retinidial fibrils arranged in parallel lines. The arrangement of 
the ocelli on the right side of the head of Acilius, as seen from 
