THE PARIETAL EYE OF LIMULUS. 



133 



procephalic lobes, close to the margin of the palial fold, but, unlike the median 

 ocelh, they are not swept into the infolding, and hence onto the brain roof. They 

 develop into typical external eye-pits, which permanently remain in their original 

 position as regards the procephahc lobes. But in the adult, after the forebrain 

 has been folded back onto the heemal surface, they lie on the anterior lateral 

 margins of the cephalo-thoracic shield, on the heemal surface of the body, instead 

 of the neural. (Fig. 17.) 



The Parietal Eye of Limulus. 



In Limulus, the cephahc lobes, at first sight, bear no resemblance to those of 

 the scorpion, or of Acilius, but a more careful examination will show that the 

 essential features are the same in all of them. 



Fig. 92. — A, Ocellus of Lycosa (middle one of the three lateral ocelli); B, retinal portion of the same, more 

 highly magnified, showing the retinal cells, each with a large outer nucleus, «, and a smaller inner one, n^; the 

 lateral rods, rd, are in parallel rows, fenced off by vertical walls of dense pigment; concave reflecting membranes 

 underlie each double row of rods. 



Development. — The cephalic lobes at first form two wing-like expansions of 

 the neural plate, with the stomodaeum on the extreme anterior margin and the 

 chelicerae on the posterior one. (Fig. 140.) No division into segments is visible 

 at this stage. 



A little later (Fig. 141), one may recognize the various parts that belong to 

 these segments, viz., two large infoldings in the olfactory lobes representing the 

 first segment, ol.l.\ two pairs of minute pores representing the marginal infoldings 

 for the median ocelli on the second segment, p.e.; a large olfactory placode on the 



