DISPOSITION OF THE LENSES. 399 
S^neral cornea is peculiarly adapted to the uses 
an animal destined to live at the bottom of the 
^ater : to look downwards was as much impos- 
®*ble as it was unnecessary to a creature living at 
bottom; but for horizontal vision in every 
•direction the contrivance is complete.! The 
form of each eye is nearly that of the frustum 
a cone (see PI. 45, Figs. 9 and 10.), incom- 
plete on that side only which is directly opposite 
the corresponding side of the other eye, and 
which if facets were present, their chief range 
^ould be towards each other across the head, 
^liere no vision was required. The exterior of 
each eye, like a circular bastion, ranges nearly 
^eund three-fourths of a circle, each commanding 
®o much of the horizon, that wliere the distinct 
''ision of one eye ceases, that of the other eye 
f^egins, so that in the horizontal direction the 
eombined range of both eyes was panoramic. 
If we compare this disposition of the eyes with 
fliat in the three cognate Crustaceans, by which 
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 
*^oves with rapidity in all directions through the 
f The facetted eyes of Bees are disposed most favourably for 
®rizontal vision, and for looking downwards. — Lib. Ent. Knowl. 
’‘ii- p. 130. 
