vn] 



THE SENSE ORGANS 



139 



it is separated from the ordinary cuticle of the head by a shallow 

 groove or depression (gr) of a deep red colour. The central part 

 of the lens is much swollen and sunk inwards. Its component 

 layers are very closely fused together, so as only to be distinguished 

 with great difficulty. They shade from a transparent pink colour 

 above to a deep clear ruby red below. 



(ii) A layer of very elongated corneagen cells (cng). These are 

 continuous with the hypoderm cells of the head, but differ from 

 them in being quite colourless and transparent. They are the 



cnl 



A. 



ocn. 



nc ocn 



B. 



Fig. 61. Ocelli of Austrolestes leda Selys. A. Oblique section through median ocellus 

 ( x 154). B. The same through lateral ocellus ( x 154). c. T.S. through 

 rhabdome near base ( x 460). D. The same near distal end ( x 460). cng 

 corneagen cells; cnl corneal lens; gr marginal groove; nc nerve-cells; ocn 

 ocellar nerve; pg pigment-layer; rh rhabdome; vd distal, vp proximal 

 visual cells. Original. 



cells which secrete the corneal lens, and support it on all sides, as 

 shewn in the figure. These cells, together with the lens, form the 

 dioptric portion of the ocellus. 



(iii) A retina formed by two series of visual cells or retinulae, 

 viz. the distal retinulae (vd) and the proximal retinulae (vp), and 

 surrounded by a layer of black pigment (pg). In the median 

 ocellus the retina forms a thick half-ring, with the two ends 

 directed forwards, so that the anterior portion is seen in section 

 as two symmetrical parts, with two separate and diverging nerve- 

 supplies. This arrangement is strong evidence in favour of the 



