ARTHROPODA. 



405 



facet corresponds to a small chitinous lens (the number of which 

 varies, in different species, between a dozen and several thousand), 



1 23 4 



FIG. 408. Section of compound eye of Forficula. (After Carriere, from Hatschek.) 

 7, cuticula, producing the cornea of many lenses over the eye; 2, epidermis, 

 which in the eye forms the ommatidia; 3, basal membrane; U, reentrant chitin- 

 ous fold ('sclerotic'); 5, rudimentary larval eye. 



and bounds the eye externally, whence this layer is called the 



cornea (fig. 408). The part of the eye be- ^ ^^ 



neath the cornea consists of radially arranged IT L r / 



prismatic 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 con- 

 necting with fibres of the optic nerve which 

 go to the brain. Each ommatidium (fig. 

 409) has essentially the structure of an 

 ocellus: (1) the lens (/) with its epi- 

 thelium; (2) the vitreous body (kz)\ (3) the 

 retinula (rz). The vitreous body is usually 

 composed of four cells which in the so-called 

 euconous eyes surround a transparent body, 

 the crystalline cone (&), secreted by these cells. 

 The retinular cells are almost always seven in 

 number, each bearing on its inner surface a 

 rhabdome (r), the seven rhabdomes frequently 

 fusing into a common mass. Each omma- FIG. 409. A single om- 



. . , . . ill i , i matidium (with sec- 



tidium is surrounded by a pigment sheath, tions) of a compound 



. , , . ., , . ii r ., -n eye. fc, crystalline 



isolating it optically irom its lellows. 



cone; fcz, cone cells: /, 

 lens with hypodermis; 

 r. rhabdomes; rz, re- 

 tinular cell. 



From this it appears that the compound 

 eye may be regarded as a complex of ocelli. 

 This anatomical conception must not, however, obscure the physio- 



