THE EYE. 165 



The * cornea is tlie transparent, apparently 

 black, cap of uncalcified cuticle covering the end 

 of the eye-stalk, and bounded by an oval outline. 

 Its surface is divided into quadrangular facets by 

 lines crossing each other nearly at right angles, 

 each facet corresponding to one of the elements 

 of which the compound eye consists. 



b. Structure of the eye as seen in a longitudinal section. 



Carefully divide the eye-stalk and eye longitudinally into 

 two halves. The calcified portion of the cuticle is best cut with 

 scissors, the remainder with a sharp knife. Examine one of 

 the cut surfaces with a strong lens. Better sections are obtained 

 by fixing the eye of a crayfish just after changing its skin : 

 imbedding in paraffin or celloidin, cutting and staining. An 

 allied crustacean, Squilla, is still more suitable. 



The eye consists of a number of conical bodies 

 arranged in a radiate fashion, their bases being 

 turned outwards and formed by the corneal 

 facets, and their apices being directed inwards 

 and connected with the optic ganglion. Each 

 of these component elements is isolated optically 

 from its neighbours by pigment. 



i. The cornea is thin, and the strips of cuticle 

 separating its facets scarcely differ from the 

 facets themselves. 



ii. The crystalline pyramids are transparent conical 

 bodies arranged radially and abutting upon 

 the cornea, each pyramid corresponding to one 

 corneal facet. They are separated from each 

 other by movable layers of ' iris ' pigment. 



iii. The inner ends of the pyramids are imbedded in 

 a movable layer of retinal pigment, in which 

 lies a series of radially arranged retinulae, 

 each of the latter being surrounded by a 



