THE SIJVIPLE EYE 



135 



epithelium and the elongated visual cells curve away from the surface 

 to crowd into a cellular cup of densely pigmented cells. Such an organ 

 in addition to being light-sensitive can appreciate the direction of 

 incident light, and forms a primitive type of directional eye. A still 

 more complicated organ of this type is seen in the paired eyes of 

 Chaetognaths, such as the marine arrow-worm, Spadella (Hesse, 1902), 

 and in the median eyes of certain Crustaceans.^ 



It is of interest that in this subepithehal type of eye the sensory pole of the 

 cell is usually directed away from the incident light which has to traverse the 

 cell -body in order to reach it ; technically, therefore, these are examples of an 

 inverted retina.^ 



Chsetognath 



The Epithelial Invaginated Eye 



A much more common arrangement, however, is an association of 

 a number of contiguous cells in the epithelial layer, which as evolution 

 progresses eventually invaginate into the underlying tissues. In such a 

 development the first stage is the specialization of a number of con- 



mii^ 



b 



Fig. 94. — Scheme of Development of the Simple Epithelial Eye of 



Invertebrates. 



(a) Single epithelial light-cell. 



(6) A group of light-cells forming a flat eye (Fig. 95). 



(c) The cupulate eye (Fig. 97). 



(d) The formation of a dark chamber (Fig. 100). 



(e) The vesicular eye (Fig. 110). 



(/) The eye of Cephalopods (Fig. 113). 



tiguous surface cells to form a plaque on the surface — the flat eye 

 (Fig. 946) ; the second stage is evident when the epithelium becomes 

 invaginated so that the sentient cells line a simple depression on the 

 surface^ — the cupulate eye ; thus, while to some degree protected, 

 their functional utility is increased by the crowding together of more 

 units into the same space, and by an arrangement whereby they can 

 orientate more accurately the incident light. A further improvement 



1 p. 152. - p. 146. 



