222 
SYDNEY J. HICKSON. 
In Nepa the optic fibres leave the epi-opticon, and without 
decussating, at once form a very complicated anastomosis in 
which there are a number of small nerve-cells. Before entering 
the ommateum the anastomosis passes through the pores of a 
membrane situated a little distance behind the basilar mem- 
brane. A similar perforated membane in this position has 
been described by Berger (2) and Ciaccio (4) in Musca, but I 
have not been able to detect it. In that part of the anasto- 
mosis which corresponds in position with the peri-opticon of 
Musca, some of the principal fibrils have the appearance of 
broadening or flattening out, and when they are examined with 
a high power these flattened portions are seen to be made up 
of a number of minute fibrillse. Between the principal fibrils 
run ordinary anastomosing branches, transverse connecting 
fibrils and fibrils which seem to turn round and run back again 
to the epi-opticon, and which have a looped appearance in the 
sections. These iooped fibrils are also found in the optic tract 
of the developing bee. 
In Musca, as I have described above, the optic fibrils on 
leaving the epi-opticon decussate and then break up into small 
cylindrical masses of neurospongium, which together form 
what I have called the peri-opticon. 
In Agrion bifurcatum (figs. 20, 21), one of our common 
English dragon-flies, the opticon is small compared with the 
enormously large epi-opticon (fig. 20, e. op.) the fibrils passing 
between the two do not decussate, but after undergoing a 
considerable anastomosis with one another pass straight across. 
The decussation of the fibrils between the epi-opticon and the 
peri-opticon is complete (fig. 20, nf.). The peri-opticon is 
composed (figs. 20, 21, p. op.) of a number of long, slender 
cylindrical elements, each composed of a delicate neuro- 
spongium. Between the elements there are a few connecting 
fibrillse, and a number of nerve-cells. The terminal anasto- 
mosis is much more complicated here than in Musca, and takes 
up very much more room, so that the peri-opticon is, compara- 
tively speaking, situated some distance behind the basilar 
membrane (fig. 20, ta.). 
