334 



PHYLUM ARTHROPOD A. 



The lateral eyes of Limulus offer an interesting intermediate 

 condition between the compound eye and a group of simple 

 eyes. They are raised reniform areas on the sides of the cephalo- 

 thorax, the cuticle over them being transparent. They consist 

 of a number of pits in the hypodermis (Fig. 234) the bottoms of 

 which are occupied by bulb-like retinulae consisting of some 

 10-15 cells grouped about a central ganglion cell. The inner 

 margin of each retinular cell is highly refracting. The group 

 of these refracting bands in a retinula evidently corresponds 



t-t 



FIG. 234. Three ommatidia of the lateral eye of Limulus (after Watase). lu^la retinula is 

 divided longitudinally ; in B and C whole retinulae are represented, c central ganglion 

 cell ; ch cornea ; hyp hypodermis ; I lens ; met mesoderm ; n nerve ; rh segment < f the 

 rhabdom ; rt retinula. 



with the rhabdom of an ommatidium of the compound eye. 

 The cells are continued below the basement membrane, as is 

 the central ganglion cell, into nerve fibres. The columnar cells 

 of the hypodermis surrounding the retinulae and forming the 

 walls of the pit are bordered with pigment. The cavity of the 

 pit is filled by a rounded process of the under surface of the 

 cuticle, which apparently acts as a lens, but each is continuous 

 above with the common investment of cuticle forming the 

 cornea. 



The lateral eyes of Scorpions (Fig. 235) form a group on either 



