TYPE CRUSTACEA. 



431 



As already noticed, a pair of simple eyes are borne upon the 

 dorsal surface of the carapace, one on each side of the median 

 line, while a pair of larger compound eyes are situated lat- 

 erally. The structure of these compound eyes is peculiar 

 (Fig. 197). Over their surface the cuticle is considerably 

 thickened and shows upon the outer surface no indication of 

 corneal facets, but its inner surface is prolonged into a num- 

 ber of papillae (I) each one of which projects into a depression 

 of the ectoderm. At the bottom of each depression is a bulb- 



Tt 



opn 



FIG. 197. COMPOUND EYE OF Limulus polyphemus, Two OMMATIDIA (after 



WATASE). 



c = central cell. ms - mesoderm. 



I = leus. opn -- optic nerve. 



rt = retiuula. 



like structure composed of a number of cells arranged in a 

 circle and constituting a retinula (rt), the lower ends of the 

 cells being continued inwards to form part of the optic nerves 

 (opn). Upon the face which is turned towards its fellow each 

 retinular cell secretes a layer of chitin, and these various chit- 

 inous rods being in contact there is formed a structure com 

 parable to the rhabdorn of the Crustacean eye. In the centre 

 of the retiuular cells and below the rhabdorn is a single clear 

 cell (c) whose lower end is also prolonged into a nerve-fibre. 

 Each depression with its retiuula and the chitinous papilla 

 which fits into it and represents its cornea is an ommatidium, 

 and the development shows that the ommatidia arise as 



