ARTHROPODA 



thickening sharply marked off from the rest of the epithelium, 

 eyes, like those of vertebrates, form inverted images. 



These 



Spiders have two kinds of ocelli (fig. 371), the chief eyes and the accessory. 

 In the first (A) the rhabdomes are next the vitreous body and in front of the 

 nuclei of the cells. In the other (B) the relations are reversed, the structure 

 recalling the inverted eyes of vertebrates. Apparently the first are for distant 

 vision, the others for near objects. 



The compound 

 eyes to the fact that 



FIG. 373. A single 

 om.matid.him (with 

 sections) of a com- 

 pound eye. k, crys- 

 talline cone; kz, cone 

 cells; /, lens with 

 hypodermis; r, rhab- 

 domes; rz, retinular 

 cell. 



eyes are much larger. They owe their name faceted 

 the cuticle over them is divided into polygonal (usually 

 hexagonal) areas or facets. Each facet corresponds 

 to a small chitinous lens (the number of which varies, 

 in different species, between a dozen and several 

 thousand), and bounds the eye externally, whence 

 tliis layer is called the cornea (fig. 372). The part 

 of the eye beneath the cornea consists of radially 

 arranged pyramidal parts or ommatidia which corre- 

 spond in number and position to the facets, their 

 broader ends being placed beneath the facets, their 

 narrower internal ends connecting with fibres of the 

 optic nerve which go to the brain. Each omma- 

 tidium (fig. 373) has essentially the structure of an 

 ocellus: (i) the lens (/) with its epithelium; (2) the 

 virtreous body (kz); (3) the retinula (rz). The 

 vitreous body is usually composed of four cells 

 which, in the so-called enconous eyes, surround a 

 transparent body, the crystalline cone (k), secreted 

 by these cells. The retinular cells are almost 

 always seven in number, each bearing on its inner 

 surface a rhabdome (;), the seven rhabdomes fre- 

 quently fusing into a common mass. Each omma- 

 tidium is surrounded by a pigment sheath, isolating 

 it optically from its fellows. 



There are really two pigment zones, an anterior which corresponds function- 

 ally to the vertebrate iris, and a posterior comparable to tapetum and chorioid. 

 The iris pigment lies in special cells and is most abundant at the line between 

 the vitreous body and retinula. The posterior pigment is in the retinular cells. 



From this it appears that the compound eye may be regarded as a 

 complex of ocelli. This anatomical conception must not, however, ob- 

 scure the physiological. The whole compound eye forms but a single 

 erect picture composed of separate images of small area formed by the 



