534 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The Eye* (Figs. 33 and 34). 



The optic peduncle, at the end of which the eye is 

 situated, is a two-jointed structure. The basal joint is 

 short and thick, and it bears on its inner side a spearhead 

 shaped " squame " ; the distal joint is long and cylin- 

 drical, but narrower in the middle than at either end. 

 The two parts are loosely joined by membrane and the 

 outer moves freely on the inner. The cornea takes up 

 all the anterior end of the second joint. It is circular in 

 outline except for a small invasion of the calcified portion 

 of the stalk on its dorsal and inner side. 



The eye is compound — that is to say, it is composed 

 of many separate, similar parts or elements, each of 

 which is called an ommatidium. 



The cornea is facetted, and each such area is 

 hexagonal in shape. A fine line bisects each facet 

 diagonally from angle to angle. These corneal facets 

 are the outer ends of the ommatidia, which, owing to the 

 convexity of the cornea, converge inwards radially. 



An ommatidium (fig. 33) is a definite arrangement 

 of five kinds of cells in a cluster about a central axis. 

 The cells comprising each ommatidium may be enumer- 

 ated as follows : — (1) Corneal hypodermis (secretes the 

 facet of the cornea immediately above it) ; (2) Cone cells ; 

 (3) Distal retinular cells (Iris cells, Hesse) ; these three 

 kinds constitute the dioptric portion of the eye ; (4) 

 Proximal retinular cells, which, together with the 

 rhabdome, constitute the receptive portion of the eye; 

 (5) Accessory cells (Tapetum cells, Hesse). 



(1) According to G. H. Parker there are two. corneal 

 cells (c. hy.), and the fine line bisecting the facet of the 

 cornea is the line of their division, but Schneider states 

 * See also Note, on p. 567. 



