PHYLUM ARTHROPODA — CLASS CRUSTACEA 



277 



the possibility of reflection within the ommatidium is reduced. In 

 dim light the pigment is concentrated partly toward the outer and 

 partly toward the basal portion of the ommatidia which allows more 

 refraction of rays by the crystalline cones and a combination of 

 images in several adjacent units. In brighter light only the ray from 



%: 



Cornea ~ 



- Corneagen cells 



. Crystalline cone" 



Distal retinal 

 pigment cells 



^^^ 



-f1 



Proximal retinal 

 pigment cells 



Rhabdome-- — 



Basement 

 •membrane- 



-Mm 



■' Nerve fibers ■ 



a 



Fig. 157. — Longitudinal section of ommatidia from eye of crayfish, a, position 

 of pigment when light is present ; b, position of pigment when in the dark. Notice 

 in the latter the distal pigment is in the outward position and the proximal pig- 

 ment is concentrated inwardly. (From Hegner, College Zoology j published by The 

 Macmillan Company, after Bernhards. ) 



directly in front of the cornea will reach the retinula and stimulate 

 the nerve cells there. These cells are connected internally with 

 the optic nerve. The type of vision produced in the compound eye 



