EYES OF SCORPIONS. 



335 



side of the cephalothorax consisting of 2-7 distinct eyes, the 

 number varying according to the species. The whole group 

 however passes through a stage in development which strikingly 

 resembles the permanent condition in Limulus (Fig. 234). The 

 hypodermis is thickened and pigmented over a considerable 

 area, and the surface becomes invaginated to form pits corre- 

 sponding in number to the separate eyes of the adult. The cells 

 lining the floors 

 of the pits be- 

 come differenti- 

 ated into inter- 

 neural cells (in) 

 and retinal cells 

 (sz), which are 

 continued below 

 into nerve fibres, 

 and form rhab- 

 doms at their 

 adjacent borders, 

 though they do 

 not become 

 grouped together 

 to form retinu- 

 lae. Pigmented 

 perineural cells 

 line the sides of 

 the cup and a 

 convex cuticular 

 lens (I) is secreted 

 above it. As 



11US. 



.1. 



I r> o n f FIG - 235. Sections through 

 stages in the development. 



proc e e d s the 



the lateral eyes of Scorpio in two 

 . A earlier, B a single element at a 



later stage ; somewha't diagrammatic (from Korschelt and Heider, 

 after Paiker and Laurie); II-V optic invaginations; h \iy\>- 



li \mnrl f^rmie derails ; in interneural cells; / lens; mes mesodermal tissue; 



1 vl J ^ n optic nerve; pn perineural cells; r retina ; rh , rliabilmn ; sz 



tervening be- retinal cells - 



tween the pits assumes the ordinary character. 



According to Kishinouye the posterior median (Fig. 237, B) and 

 the lateral eyes of Spiders likewise arise as simple ectodermal 

 depressions, in which case they would belong to the same category 

 as the lateral eyes of Limulus and Scorpions. 



The paired arthropod eyes hitherto considered belong to a 



