TYPE TRACHEATA. 



473 



ends arid in continuation with tlie optic nerve, have developed 

 distinct rods (r) at their outer ends. 



In the Insecta and occasionally in the Myriapotla (Scuti- 

 gera) there are in addition to these simple ocelli compound 

 eyes, situated at the sides of the head and similar in structure 

 to the compound eyes of the Crustacea. Each of the oni- 

 matidia of which the eye is composed, and there may be 



FIG. 216. SECTIONS THROUGH AN OCELLUS OF A LARVA OF AciMus IN (A) A 

 VERY YOUNG AND IN (B) AN OLDER SPECIMEN (after PATTEN). 

 ir inverted rods. n = nerve. 



I = cornea. pg = pigment, 



mgc = median giunt cells. r = rods. 



vb = vitreous body. 



several thousand of them in each eye, consists of an external 

 cornea (Fig. 217, co), usually more or less hexagonal in outline, 

 giving the eye a faceted appearance. Beneath the cornea 

 are two cells which secrete it and form the corueal hypoder- 

 mis, and below these again come four cells, the crystalline- 

 cone cells, which may (euconous eyes) or may not (aconous 

 eyes) manufacture a crystalline cone (c), and finally beneath 

 these is a circle of seven retinular cells (four in Lepisma), 

 each one of which is pigmented and manufactures a portion 

 of the chitinlike rhabdoni (rh) which they surround ; these 

 cells are probably continuous at their inner ends with the 

 optic nerve. Additional pigment-cells (pg) separate the 



