136 



THE EYE IN EVOLUTION 



is gained when the opening of the depression is narrowed so that a dark 

 chamber with a pin-hole opening is formed. The last step in the 

 differentiation of the simple eye is marked by the closure of the opening 

 leading into the depression by a circular in-folding of the surrounding 

 epithelium ; thus is formed the vesicular eye, the highest differentia- 

 tion of which is reached in the eye of Cephalopods wherein the vesicle 

 is associated with a secondary invagination of the ectodermal layer 

 which, in addition to providing a protective covering, helps to constitute 

 a dioptric mechanism. The scheme of the development of the simple 

 eye from its primitive beginning as a single cell to this highly complex 

 structure is seen in Fig. 94. 



The simiDlicity of these eyes is seen in their capacity for regeneration, a 

 potentiality first demonstrated by Bonnet (1781). If the tentacle with the eye 

 is removed from the edible snail or the grey slug, another regenerates, occasion- 

 ally equipped with two eyes, a process which has been known to occur twenty 

 times in succession (Galati-Mosella, 1915-17). ExiDerimenting similarly on the 

 gastropod, Murex, Carriere (1889) found that the regenerating eye initially took 

 the form of a simple depression, which gradually closed leaving only a pore-like 

 opening and eventually developed into a closed vesicle. 



THE FLAT EYE 



This is the most primitive association of light-sensitive cells and 

 usually consists of 5 or 6 epithelial cells lying upon the surface, 

 differentiated by being a little larger than their unspecialized neigh- 

 bours. Such an ocellus is seen in the aquatic worm, Stylaria lacustris 



Figs. 9.5 and 96 — Flat Eyes. 



Dendrocarhcm 



Fig. 95. — The ocellus of the aquatic 

 amielid worm, Stylaria lacustris (after 

 Hesse). 



Fig. 96. — The ocellus of the hydro- 

 medusan, Lizzia, the epithelial sen- 

 sory cells being capped by a lens-like 

 thickening of the cuticle (Hertwig 

 and Jourdan). 



(Fig. 95) (Hesse, 1908), in certain unsegmented planarian worms such 

 as Dendrocoslum and some leeches, while in the larvae of some insects 

 the eyes consist merely of a pair of visual cells and two overlying 

 pigTi^ent cells (Hesse, 1908 ; Imms, 1935). Occasionally a simple 

 cuti. ;lar refringent apparatus is added to collect the light as well as 

 pignic ' t to absorb it ; thus in the hydromedusan, Lizzia, the eye, 

 situa at the base of the tentacle, is composed of a number of sensory 



