THE COMPOUND EVE OF THE BLOW-FLY. 533 
creased tension of this membrane must increase the length of 
the great rods, hence I regard it as a muscle of accommodation 
(Pl. XXXVIII., Fig. 2). I shall hereafter explain that its 
probable action differs from that of the ciliary muscle in 
Vertebrates, although I have spoken of it as ‘accommodation’ 
for want of a distinctive term. 
b. The Retina and Optic Nerve. 
The Retina is termed the periopticon by Hickson. It consists 
of true bacilli or retinal rods, which extend from its inner to its 
outer surface. The inner extremities of these rods rest upon a 
fine membrane, the membrana limitans interna, and are con- 
tinuous with the fibrils of the optic nerve. Their outer ends 
are contiguous with the membrana basilaris of the dioptron. 
These rods are arranged in fasciculi, retinulee, and each retinula 
corresponds with an ommateum, and consists of a central rod 
surrounded by six or seven peripheral rods. Each retinula is 
surrounded by a delicate membranous sheath, which extends 
from the basilar membrane to the membrana limitans in- 
terna. 
The bacilli or rods are formed of two segments, like the 
bacilli of the retina of the simple eye, an outer cylindrical 
transparent segment and an inner granular protoplasmic seg- 
ment. The inner segments differ from the inner segments of 
the rods of the simple eye in not having any nucleus; in this 
respect they resemble the inner segments of the cones of the 
vertebrate eye. 
The principal difference between these elements and the 
rods and cones of the vertebrate retina is, that the transparent, 
rod-like outer segments correspond with the outer segments of 
the cones and rods of the Vertebrate; the rods are not reversed, 
but are turned towards the light, not from it, as they are in 
vertebrate retina. In all other respects—size, form and struc- 
ture—the two are exceedingly similar. 
The outer ends of the retinule are closely surrounded by 
numerous small round cells, the chaplet cells of Viallanes ; the 
existence of these cells renders the observation of the outer 
