242 



HISTOLOGY 



and most efficient eyes known, and far better organized than any 

 worm's. 



The forms of Nereis present a visual organ which well represents 

 the average type of worm eye. Figure 215 represents the eye of a small 

 pelagic Nereis of unknown species. This eye is plainly an invagination 

 of the epidermal layer, and has been subsequently constricted off from 

 the hypodermis at the point of invagination. The epidermal layer, 

 after the constriction closes over, becomes the cornea with its cuticle. 



FIG. 215. A, eye of a pelagic Nereis, cu., cuticle; hyp., hypodermis (a simple epithelium 

 with very long cells) ; ret., retina, developed by the specialization of an invajfinated area of 

 hypodermis; /., lens; U.S., blood sinus. X 200. B, small portion of retina much enlarged. 

 vis.c., visual cells; pg.c., pigment cells which surround the lower two thirds of the visual 

 cells; vis.r., visual rods, x 870. C, slightly oblique horizontal section across the retina 

 near level of tops of pigment cells. X 870. 



The sac-like eye changes its alternate, epithelial lining cells into visual 

 cells and pigment cells. This takes place in a greater degree on the 

 posterior, inside surface of the sac, and the visual specialization becomes 

 less and less toward the anterior side, where for a short space the cells 

 are clear and transparent to permit of the entrance of light-rays. The 

 pigment cells of this eye form a sheet when seen from the surface (Fig. 

 215, C), and spaces in this wall denote the presence of the visual cells 

 with their rhabdomes or visual cell-organs. This rhabdome consists 

 of a heavy cylindrical bar of some length, in the center of which is an 

 axial filament. 



