DISPOSITION OF THE LENSES. 390 
general cornea is peculiarly adapted to the uses 
of an animal destined to live at the bottom of the 
water: to look downwards was as much impos- 
sible as it was unnecessary to a creature living 
at the bottom ; but foi' horizontal vision in every 
direction the contiivance is complete.* The 
form of each eye is nearly that of the frustum 
of a cone (see Pi. 45, Figs. 9 and 10.), incom- 
plete on that side only which is directly opposite 
to the corresponding side of the other eye, and 
in which if facets were present, their chief range 
would be towards each other across the head, 
where no vision was required. The exterior of 
each eye, like a circular bastion, ranges nearly 
round three-fourths of a circle, each commanding 
so much of the horizon, that where the distinct 
vision of one eye ceases, that of the other eye 
be2:ins, so that in the horizontal direction the 
combined range of both eyes was panoramic. 
If we compare this disposition of the eyes with 
that in the three cognate Crustaceans, by which 
we have been illustrating the general structure of 
the Trilobites, we shall find the same mechanism 
pervading them all, modified by peculiar adap- 
tations to the state and habits of each ; thus in 
the Branchipus (PI. 45, Fig. 3, b, b), which 
moves with rapidity in all directions through the 
* The facetted eyes of Bees are disposed most favourably for 
horizontal vision, and for looking downwards. — Lib. Ent. Knowl. 
V. xii. p. 130. 
