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 



*P 



FIG. 28.A*t<tcu* ////,/ fin*. 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 ; b, outer dark zone ; c, 

 outer white zone ; d, middle dark zone ; e, inner white zone ; 

 /, inner dark zone ; c>; crystalline cones ; g, optic ganglion ; op, 

 optic nerve ; xp. striated spindles. 



of the general curvature of the outer face of the cornea ; 

 the inner contour sometimes exhibits a slight deviation 



