THE SIMPLE EYE 



137 



cells associated with j)igmented cells capped by a " lens " formed by a 

 localized thickening of the cuticle (Fig. 96) (Hertwig, 1878 ; Jourdan. 

 1889). 



THE CUPULATE EYE 



The crpuLATE or cup-shaped eye {cupula, a cup) forms a distinct 

 functional advance, for the invagination of the light-sensitive epithelium 

 allows the development of a primitive directional sense (Patten, 1886). 

 Its development may be seen in three stages. The first is a simple 



Figs. 97-100. — Typical Cupulate Eyes of the Simplest Type. 



Fig. 97. — The ocellus of the limpet, 

 Patella. 



Ep, epithelium ; S, secretory sub- 

 stance covering visual cells ; N, nerve 

 (after Hesse). 



Fig. 98. — The ocelkis of the ear-shell, 

 Haliotis. 



The cup-shaped depression is deep 

 with a narrow neck and is filled with 

 secretion formed by the epithelial cells 

 (after Hesse). 



Fig. 99. — The visual organ of the larva 

 of the house-fly, Musca. 



There is a small cavity in the 

 cephalo-phar\Tigeal skeleton wherein 

 lie light-sensitive cells, C, from which 

 issues the optic nerve, N (after 

 Bolwig). 



Fig. 100. — The ocellus of the mollusc. 

 Nautilus, with its pin-hole opening 

 (after Hesse). 



