XIPHOSURA. 



791 



downwards into numbers of conical processes, each forming a 

 lens. Beneath the processes the hypodermal cells are arranged 

 in globular ommatidia, which resemble taste-bulbs. They 

 consist of cells (retinulae), each with a highly refracting inner 

 border (rhabdom), grouped in varying numbers, commonly 

 from 9 to 11, about a central ganglion cell. The retinulae and 

 the ganglion cell are produced into nerve processes, which unite 

 in a plexus below the hypodermis, and the former, as well as the 

 hypodermic cells about them are highly pigmented. Between 

 the ommatidia the hypodermic cells retain their simple columnar 

 character (Watase). 



The lateral eyes present a very simple form of compound eye, 

 the elements of which are largely distinct. In the other Arach- 

 nids the lateral eyes of Limulus are represented by groups of 

 eyes (Lankester and Bourne). In these the ommatidia are much 

 reduced in subordination to the function of the eye as a whole. 



FIG. 513. Three ommatidia of the lateral eye of Limulus. B and C show the ommatidia m 

 surface view, A in longitudinal section, c Central ganglion cell ; ch chitinous cuticle ; 

 hyp hypodermis ; I lenticular cone ; mes mesodermal tissue ; n nerve ; rh rhabdom 

 rt retinula (from Korschelt and Heider after Watase). 



Both median and lateral eyes arise on the cephalic lobes of the embryo 

 {Kishinouye), though they soon shift to a more posterior position. The 

 median eyes arise, as in Scorpions and Spiders (p. 336), from an invagin- 

 ation of the epiblast immediately in front of the thickening for the brain. 

 The invaginated cells grow forward as a cord (single or double ?) whose end 



