THE EYE AND OPTIC TRACT OF INSECTS. 
225 
In longitudinal sections ( v . fig. 23) through the peri-opticon 
there is certainly an appearance somewhat similar to that of 
Musca ( v . fig. 2, p. op.), but this is not due in Eristalis to its 
being composed of a number of cylindrical elements, but to 
the fact that it is perforated by bundles of thin-walled tracheal 
vessels (v. fig. 23, t. v.), which seem in such a section to divide 
it into elements. "When a section is examined which is made in 
a plane at right angles to the direction of these vessels it is seen 
that the peri-opticon forms a continuous ganglion (PI. XVII, fig. 
24, p. op.), and that these vessels run through it in bundles ( t . v.) 
situated at fairly even distances from one another. In fact 
we have in Eristalis a further stage in the complexity of the 
ganglion than we have in Aeschna, for the elements which in 
the latter have only partially fused together are in this form 
completely fused. In Eristalis, however, a system of tracheal 
vessels perforates this structure, a system which seems to be 
absent in Aeschna and the other forms we have so far 
considered. 
In the silkworm moth (Bombyx), however, there is a solid 
homogeneous peri-opticon devoid of tracheal vessels. 
Passing from the Hexapoda I will describe briefly the optic 
tract of Carcinus moenas as a type of Crustacean. A lon- 
gitudinal section through the ophthalmic peduncle reveals the 
same three optic ganglia I have described in insects ; the 
opticon (fig. 14, op.) is situated at the end of the optic nerve 
(o. n.), and this is connected by nerve-fibrils, which do not 
appear to decussate with the epi-opticon (fig. 14, e. op.), and 
this again by nerve-fibrils ( N.f .), which completely decussate 
with the peri-opticon (p. op.). 
The peri-opticon in Crustacea (Carcinus, Astacus, Homarus, 
Squilla) is a solid ganglion, similar to the epi-opticon, and 
cannot be separated into elements. The terminal anastomosis 
(fig. 14, ta.) is more complicated than it is in most insects 
(ta.), but the four regions I previously described may be 
recognised (1, 2, 3, 4). 
I shall not continue here my description of the optic tract of 
Crustacea, as I hope soon to be able to collect my observations 
