No. 1.] EVES OF ARTHROPODS. ISI 
adult ; the outer, into a layer of cells which, during the earlier 
stages, form, around the outer edge of the median lobe, a semi- 
circular band, conspicuous on account of the peculiar character 
of its cells (Fig. 2, wood-cut). The inner lobe, which likewise 
disappears during the later stages, forms a characteristically 
shaped layer of dark cells on the proximal side of the median 
one. It is over-arched by the cells of the brain in such a way 
that it appears to form the floor of an oblong cavity. Each lobe, 
except the outer one, contains a great medullary core. 
FU, 2. 
Figure 2.— Semi-diagrammatic views of the optic ganglion of: 
(A) Vespa, towards the close of the larval stage, (2) Astacus, (C) Cecropia, be- 
ginning of pupal stage; (D) Acilius, close of larval stage, — showing the position of 
the retinal ganglion and the three lobes of the imaginal optic ganglion. 
E. Eye I. of Acilius; c. e. ventral half, c. e.1 dorsal half of convex eye; g1® lobes 
of optic ganglion; 1-3 medulle of same; 7. g. retinal ganglion. 
The retinal ganglion arises at a comparatively late period, not 
by invagination, but by the formation of a layer of ganglion- 
cells among the fibres connecting the optic ganglion with the 
eye. 
In Astacus (Fig. 2, B.), the optic ganglion, after invagination, 
breaks up into three lobes. The outer one is formed of a folded 
