THE EYE OF THE CRAYFISH. 



119 



to side nearly at right angles witli 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 



Fig. 28. — Astacns flimiatilis. — A, a vertical section of the eye-stalk 

 ( X 6) ; B, a small portion of tlie same, showing the ■#sual ap- 

 paratus more highly magnified ; o, cornea ; J, outer dark zone ; e, 

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

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

 optic nerve ; sj>, striated spindles. 



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

 the inner contour sometimes exhibits a slight deviation 



