224 
•SYDNEY J. HICKSON. 
In Aeschna grandis, one of the Libellulidse, owing to the 
very large size of the eyes, the peri-opticon can be easily seen, on 
making a simple dissection, to be a large flat ganglion under- 
lying the basilar membrane. On making a fine section through 
it it will be seen to differ from that of the forms hitherto 
described, in that it can no longer be divided into a number of 
cylindrical elements well marked off from one another, and 
easily separated by teasing (fig. 7, p. op.). In other words, 
the elements of which the peri-opticon is composed in Musca 
are here fused into a single mass of neurospongium, which 
differs from the neurospongium of the epi-opticon and opticon 
itself only in the fact that it contains a number of irregular 
spaces (s.), which correspond, perhaps, with the spaces between 
the elements in Musca. 
The nerve-cells ( n . c.), which in the forms hitherto de- 
scribed are situated between the elements of the peri-opticon, 
are here found to be scattered irregularly through its sub- 
stance. 
The terminal anastomosis of Aeschna is not so clearly divi- 
sible into four regions as it is in Agrion, but still the fibrils, on 
leaving the peri-opticon, may be seen to form a loose plexus 
with the numerous nerve-cells found in this region (fig. 7, 1), 
then to collect together into a number of bundles separated 
from one another by considerable spaces, (2) then to form a 
second plexus (3) before finally breaking up into the individual 
bundles (4) which run through the basilar membrane to supply 
the retinulse. 
In Eristalis lupinus the opticon and epi-opticon are very 
similar to those of Musca, but the peri-opticon and the neigh- 
bouring parts present some interesting features (fig. 23). The 
nerve-fibrils leaving the epi-opticon completely decussate, and 
run a very long course before entering the peri-opticon. In 
the middle of the decussation there may be usually found one 
or more large nerve-cells, and a few others close up to the 
peri-opticon (fig. 23, n.c.). The peri-opticon is in Eristalis a 
continuous ganglion, and cannot be said to be composed of 
numerous separate elements. 
