THE SIMPLE EYE 



141 



"vitreous" mass (Fig. 102) (Berger, 1898; Berger and Conant, 

 1898-99). The lens may thus be aceUular and cuticular, or cellular ; 

 the vitreous cellular or gelatinous, formed either a3 a secretion of the 

 retinal cells or by their degeneration and coalescence. 



An interesting modification is seen in the stemmata or simple eyes 

 of the larval and pupal forms of some insects such as sawflies (Ten- 

 thredinidse) and many beetles (Coleoptera) as well as in the ocelli of 

 most adult insects, in the lateral eyes of the scorpion (Figs. 108-9), and 

 the median eyes of the king-crab, Limulus (Fig. 142) ; in these the 

 visual cells are arranged in loose groups of two or three around a rod- 

 like structure secreted by the visual cells — the rhabdome (/ia^Sajyiia, 

 a rod). Such an arrangement does not alter the essential simplicity 

 of the eye. 



It has been suggested that some accommodative adjustment of a 

 static type may be provided in these eyes by the existence of differences 

 in the distance between the sensory cells and the lens (some flies, as 

 Helojjhilus) (Fig. 106) (Hesse, 1908). 



THE VESICULAR ETE 



The final stage in the evolution of the simple eye is the closure of 

 the invaginated epithelium to form an enclosed vesicle divorced entirely 

 from the surface ectoderm and usually separated from it by mesen- 

 chyme. In its simplest form such an eye is merely a spherical vesicle 

 lined with ectodermal cells ; the cells of the proximal (deep) part of 



Helophilus 



Fig. 110. — The Vesicular Eye. 



The ocellus of the edible snail. 



Ep, epithelium ; vs, visual cell ; pc, pigment cell ; n, nerve (after 

 Hesse). 



