THE PARIETAL EYE OF THE LIMULUS. 



135 



An unpaired tubercle, or a more transparent spot in the chiten, usually marks 

 its location from the exterior. (Fig. 201.) 



The thick inner wall of the vesicle, en.p.e., now consists of irregular elongated , 

 cells with small nuclei. The cells may show an obscure arrangement into large 

 groups, and are completely filled with minute granules, which are snow-white 

 by reflected, and greenish-black by transmitted light. 



The outer wall consists of a few prominent sensory cells, whose pointed 

 outer ends terminate in nerve fibers. They are devoid of either white, or colored 

 pigment, or of visual rods, rt'. 



' This eye reaches the height of its development in the young animals from four 

 to six inches long, and from that stage on it appears to undergo a slow histological 

 degeneration, but without perceptible diminution in size. 



Fig. 94, — The three chambered parietal eye vesicle of Limulus. A, from above; B, in cross-section; C, in long- 

 itudinal section. Semi-diagrammatic. 



In the older animals, the distinction between the inner and outer walls dis- 

 appears, and the entire eye then consists of a solid mass of large vesicular cells, 

 with minute nuclei, crowded with "white pigment." 



After the early larval stages, all traces of the epithelial walls to the primitive 

 eye sacs have disappeared; the eyes appear to be separate organs, except in so 

 far as they are innervated by separate branches of a common nerve. 



The development of the eyes has shown us that the epithelium of the eye 

 tube merely represents the tract of ectoderm that separated the ocellar placodes 

 from the brain, before they were enclosed in the brain chamber; along the inner 

 surface of this tract the nerve fibers passed from one to the other. When the pla- 

 codes were infolded, the connecting paths of ectoderm were infolded also, forming 

 the walls of the eye tube or epiphysis. When the fibers of the optic nerve grew 

 from the eye to the brain, or from the brain to the eye, they were compelled to 



