THE COMPOUND EYE OE INSECTS AND CEUSTACEA. 
21 
15 and 16 show a thin cornea with both faces slightly curved 
(double convex lens). Fig. 12 shows a thin cornea with flat 
front facet and half-round inner facet. Fig. 1 1 shows a thin 
cornea with flat outer and inner facets. All these varieties 
stand in close connection with the particular disposition of the 
parts lying under the cornea. Sometimes (but rarely) the two 
facets form a meniscus, the front facet curving outwards, and the 
back facet having a slighter curve directed the same way (i.e. it 
appears concave when seen in section). 
Before entering into the details of the underlying parts we 
must direct attention to their general disposition and relation. 
Fig. 1 is a section through the central plane of the dragon-fly’s 
eye. The double outline of the cornea sweeps in a regular 
curve continuous with the chitin skin (the figure is not suf- 
ficiently magnified to show the small facets). Immediately 
behind the cornea (already fully described) a dark shade repre- 
sents a mass of pigment, which causes the peculiar blackness of 
the eye when seen in front. No pupillary opening can be seen, 
as in the vertebrate eye, until a piece of cornea is placed under 
the microscope (with a high power objective). Then a clear 
opening in the very centre of each facet is observed, and the 
pigment around it corresponds with the iris pigment of the 
vertebrate eye. The colour of this pigment often corresponds 
with that of the insect’s skin : sometimes white or yellowish white, 
grey, yellow grey, and so on to the deepest purple or black; 
and it may even possess the same metallic brilliancy observed 
in the iris colours of the fish, amphibian and reptilian, eyes. 
Between the cornea and optic ganglion a series of radial lines 
(5) indicates what we have called the bacillar stratum, as we con- 
sider it the equivalent of that portion of the vertebrate retina 
known under the same name. A detailed description of this 
will be given below. The radial lines in our figure extend out- 
wardly to the cornea, inwardly to the peripheral surface of the 
optic ganglion (d). This latter occupies the centre and bottom 
of the eye, and is composed of nerve fibres and associated ele- 
ments corresponding with the retinal ganglionic layers of the 
vertebrate eye, though less perfectly developed. 
Sclerotic coat . — Continuous with the border of the cornea and 
where it joins the chitin skin, a membrane (indicated in the 
figure by a dark line marking the posterior boundary of the eye- 
ball) is seen, which completes the outer tunic. Even in the 
simple eye, small as it is, the neurilem or sheath of the optic 
nerve covers the optic ganglion, and is continued forwards as a 
delicate membrane which loses itself in the cornea. But in the 
compound eye of Libellula (see fig. 1) it is a stiff chitinised 
membrane on which muscles rest which are attached to its outer 
surface. At the equator oculi it conserves the globular form of 
