LATEEAL AND CENTRAL EYES OF SCORPIO AND LIMULUS. 203 



coloured pink to distinguish them from the perineur^il cells of 

 hypodermic origin, and are marked with the letter r. In trans- 

 verse sections, ovving to the delicacy of the structures and the 

 destructive action of the acid used to remove the obscuring 

 pigment, great difficulty is found in tracing the perineural cells 

 and these intrusive connective-tissue cells. In fig. 26 the 

 letters y and r indicate what appear to belong to these two sets 

 of cells respectively. 



The whole question of the distribution of pigment granules 

 in the three sets of cells, viz. nerve-end (retinular), perineural, 

 and intrusive, is bound up with the proper distinction of the 

 distribution of the last-named cells. 



The nerve-end cells undoubtedly contain some pigment 

 granules both in the lateral and central eyes of Limulus. The 

 perineural cells are intensely pigmentiferous. But it is probable 

 that the chief clothing of pigment to each nerve-end cell — for 

 example, that which is seen in the right-hand retinula of the 

 section fig. 26 — is furnished by the intrusive connective-tissue 

 cells disposed between neighbouring retinula-cells. 



Connected with the fact that the ommateum of the lateral 

 eye of Limulus is invaded by intrusive connective tissue, is the 

 incomplete character of the ommateal capsule. Whilst well 

 marked in every other region (figs. 19, 25, 26 d), the capsule 

 is deficient immediately below the retinula where the group of 

 optic nerve filaments passes out of or into the capsule, and it 

 is here that the intrusive connective tissue (r) is seen to be 

 continuous with the extra-capsular connective tissue (e), as 

 shown in fig. 19. 



Comparison of Lateral Eye of Limulus with the 

 Lateral Eyes of Scorpion. 



The lateral eye of Limulus is shown above to be monosti- 

 chous, polymeniscous, exochromic(i.e. not purely autochromic), 

 and seems, therefore, at first sight, to differ largely from a lateral 

 eye of a Scorpion. 



But, as was stated at the commencement of this memoir, 



