THE EYE OF THE CRAYFISH. 119 
to side nearly at right angles with one another. A longi- 
tudinal section shows that both the horizontal and the 
vertical contours of the cornea are very nearly semicir- 
cular, and that the lines which mark off the facets merely 
arise from a slight modification of its substance between 
the facets, The outer contour of each facet forms part 
Fie. 28.—Astacus fluviatilis.—A, a vertical section of the eye-stalk 
(x 6); B, a small portion of the same, showing the visual ap- 
paratus more highly magnified ; a, cornea ; 6, outer dark zone ; ¢c, 
outer white zone; d, middle dark zone; ¢, inner white zone ; 
J, inner dark zone; cv, crystalline cones ; g, optic ganglion ; op, 
optic nerve ; sp, striated spindles. 
of the general curvature of the outer face of the cornea ; 
the inner contour sometimes exhibits a slight deviation 
