2O6 



AN INTRODUCTION TO ZOOLOGY 



at the end of movable stalks which extend out, one from each side 

 of the rostrum (Fig. 101, 28). The external convex surface of 



the eye is covered by a modified 

 portion of the transparent cuticle, 

 called the cornea. This cornea is 

 divided by a large number of fine 

 lines into four-sided areas, termed 

 facets. Each facet is but the ex- 

 ternal part of a long visual rod 

 known as an ommatidium (Fig. 103). 

 Sections show the compound eye 

 to be made up of similar omma- 

 tidia lying side by side, but sepa- 

 rated from one another by a layer 

 of dark pigment cells. The average 

 number of ommatidia in a single eye 

 is 2500 (180). 



Two ommatidia are shown in Fig- 

 ure 103. Beginning at the outer 

 surface, each ommatidium consists of 

 the following parts : (a) a corneal 

 facet (i), (b) two corneagen cells 

 (2} which secrete the cornea, (c) a 

 crystalline cone (5) formed by four 

 cone cells, or vitrella (j), (d) two 

 retinular cells surrounding the cone 

 (not shown in Fig. 103), (e) seven 



FIG. 103. Longitudinal sections of two om- 

 matidia of the crayfish ; A, pigment 

 arranged as influenced by light ; B, 

 pigment arranged as influenced by 

 darkness ; i, cornea ; 2, nucleus of 

 corneagen cells ; j, nucleus of vitrella ; 



4, nucleus of pigment cell ; 5, crystalline cone ; 6, tapetum cell ; 



9, basement membrane; 10, retinal 



B 



7, rhabdom ; 8, retinal cell ; 



nerve fiber. (From Sedgwick after Parker.) 



