28 JOURNAL OF THE [JuIY) 



The tangential sections will show the ommatidia, as these might 

 be seen, by looking directly into the compound eye from its outer 

 surface, and consequently, and as a general rule, especially in 

 the central region of each slice, they will give more or less 

 exact transverse sections of these ommatidia. 



The radial sections, on the other hand, because they lie in 

 the planes of the above-mentioned radii, and cut them longitudi- 

 nally, will show the various portions of the ommatidia in their 

 natural superimposed connection. 



The outer surface of the compound eye is formed by a skin of 

 chitine, which may be easily peeled off in continuity, and which 

 is as transparent as glass. This surface is divided into a great 

 number of small areas — "cornea-facets," — each one of which is 

 the distal end of an ommatidium, and isitself outwardly convex. 

 These facets have their junctures strengthened by a rim of 

 chitine, and their contour is that of a more or less regular hexa- 

 gon, with rounded corners. But this contour is not nearly so 

 constant and regular, over the whole compound eye, as it was 

 usually thought to be. At the juncture of three or four facets 

 the rim often supports stiff hairs or bristles. 



From the periphery of the transparent cornea the chitine skin 

 lined within by a pigmented layer, resembling a chorioid coat, 

 encloses the whole remaining globular mass of the compound 

 eye, embedded in the head, and appears to correspond with the 

 sclera, or sclerotic of the ball of the eye of vertebrates. 



Next below the cornea follow in a layer the lenses, or " crys- 

 talline cones " of the ommatidia. These cones are embedded in 

 pigmentous cells, which separate them respectively from their 

 neighbors. The evolution of these crystalline cones from four 

 cells is readily to be understood. We see in their transverse 

 section four nuclei, with distinct septa in the form of a cross. 



Below the crystalline cones lies a thick layer, comprising the 

 greater part of the bulk of the globular mass, and consisting of 

 the " nerve-rods." These nerve-rods occupy the greater portion 

 of the length of the ommatidia, and are respectively composed 

 of a central fibre, which has been named the " rhabdom," sur- 

 rounded by five to seven other delicate fibres, which are the 

 retinulse. 



The rhabdom is of extreme tenuity in proportion to its length. 

 It extends through the entire nerve-rod, from the crystalline 



