206 PEOFESSOE. E. RAY LANKESTER AND A. G. BOURNE. 



omtnateum (exochromic). This is precisely the character which 

 is revealed by our sections. In fig. 21 an approximately 

 median vertical section of one of the two central eyes of 

 Limulus is represented. The lens is coloured yellow, and the 

 intrusive connective tissue pink. It is at once clear from the 

 figure that we have an anterior layer of cells (vitreous body) 

 (o), separated by firm membrane from a posterior retinal body. 

 The vitreous body is peculiar in the fact that its cells are not 

 specially elongated, but are small like those of the adjacent 

 hypodermis. The retinal body is highly remarkable. I'hough 

 identical in plan of structure with that of Scorpions, it differs 

 in the exaggerated development of the intrusive connective 

 tissue, which forms an abundant growth both in front of (s s) 

 and around (s x) the retinulse (A). The individual nerve-end 

 cells are very large, exactly corresponding to those which form 

 the well-defined retinulse of the lateral eyes. Large groups of 

 nerve-fibres (m), surrounded by connective tissue, are seen at 

 the base of the section passing away from the retinulse to form 

 the optic nerve. We could not define an ommateal capsule, 

 though there is a perceptible difference between the tissue of 

 the ommateum and the loose reticulate connective tissue (e), 

 which is abundantly developed around the eye and in all 

 other parts of the King Crab's body. 



The retinulae are much less definitely constituted in tlie 

 central eyes of Limulus than are those of the lateral eyes. 

 The nerve-end cells are sometimes grouped quite irregularly, but 

 here and there a definite arrangement of five around a com- 

 mon axis can be observed (fig. S2, Ret. 2). Those towards the 

 periphery are less defined, those nearer the centre of the omma- 

 teum more clearly differentiated. In teasing a fresh ommateum 

 of the central eye a retinula was isolated, which is diagram- 

 matically represented in fig. 28. It appeared to be built up 

 of seven nerve-end cells, and was only partially disengaged from 

 the adherent connective tissue. In fig. 33. a drawing is given 

 showing nerveend cells, and surrounding pigmeutiferous con- 

 nective tissue. The action of nitric acid has dissolved some of the 

 pigment contained in the nerve-end cellsj and stained the proto- 



