230 



HISTOLOGY 



nerve fibers from their proximal ends, and these fibers unite to form 

 the same nerve tract that was to be seen in Astropecten. 



It should be noticed that 

 these eyes are slight ad- 

 vances on the eye of Solen, 

 because of the development 

 of a visible rhabdome or 

 visual rod on the sensory 

 cell. In both of the above 

 visual organs the light strikes 

 the perceptory cell directly, 

 and from a distal position. 

 One more of the extremely 

 simple eyes should be studied 

 in a medusa, Charybdea mar- 

 supialis. This animal has 

 two very different kinds of 



FIG. 202. Eye of Charybdea marsupialis. 4, general eves O n One and the Same 



view; B, greater magnification of four cells to show ' r v U 1 TU 



the alternation of sensory cells (s.c.) with pigment P art of lts bocl y- - 1 he & } m ~ 



cells (pg.c.); sur., outer surface; A X 760. (After plest is shown, in a Vertical 



section, by Figure 202. Here 



we again find the pigment cells having very much the same appearance 

 as they had in Solen. But the point to be noticed is that these pig- 

 ment cells are not 



sensory cells, 

 function hav- 



the 

 this 



ing been left to al- 

 ternate visual cells 

 which have devel- 

 oped a sight rod 

 for the purpose. 

 These visual cells 

 are differentiated 

 out of the same 

 primitive epithe- 

 lium from which 

 the pigment cells 

 were derived, and 

 two of them are 

 pictured in the fig- 

 ure, much enlarged 



FIG. 203. Section of the double eye of Aurelia aurita. s.c., visual 

 cells; pg.c., pigmented cells; ec., ectoderm; en., endoderm; mes., 

 mesoglcea; x 760. (After SCHEWIAKOFF). 



and almost in their natural relations to the pigment cells, a slight 

 space being left for the sake of clearness. The complex eye of 



