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SYDNEY J. HICKSON. 
ov three main fibrillae on entering the cylindrical elements of 
the peri-opticon (PL XYI, figs. 16 and 17, A/), and these 
fibrillae again subdivide and form the ultimate fibrillar mesh- 
work of the neurospongium. 
Each little cylindrical mass of the peri-opticon is not entirely 
separated from its neighbours, but is connected with them, 
either directly by fine fibrillae, or indirectly by the intermedia- 
tion of nerve-cells (figs. 16 and 17, n.c.). I have observed also 
that some fibrils do not break up to form any neui’ospongium, 
but pass straight through without undergoing anv subdivision 
(fig- 16,/.). 
Sometimes the structure of the elements of the peri-opticon 
is complicated by the deposition of pigment. This deposition 
of pigment may take the form either of small cylindrical rods 
or of a fluted hollow cylinder, or of a smooth hollow cylinder 
(figs. 8, a, b, c, and 9). The presence of pigment in this region 
is, however, very variable. The figs. 8 and 9 were drawn from 
a permanent preparation I have in my possession now, all the 
varieties ( a,b,c ) being found in one eye, whilst the other eye 
has no pigment at all in this region. 
The number of the elements of the peri-opticon does not seem 
to bear any relation to the number of ommatidia; in some of 
my preparations they seem to be of about the same number; 
in others they are considerably less, and in one of my sections 
I could only count half as many. 
Externally a number of fibrils leave the elements of the 
peri-opticon, and at once form a complicated anastomosis with 
the numerous nerve-cells found in this region, which in its turn 
furnishes the fibrils which pass through the basilar membrane 
to supply the retinulae. 
In this “terminal anastomosis” are found a number of 
branching tracheae — distinguished by their spirally-marked 
walls — which run more or less parallel with the basilar mem- 
brane (fig. 2, t.). They spring from two main tracheal trunks 
(T.t.), situated at the sides of the head behind the eyes. The 
tracheae of the terminal optic anastomosis supply the tracheal 
vesicles which are found between the ommatidia. 
