230 



HISTOLOGY 



B 



pg. c.- 



B.C.' 



FIG. 202. 



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 



Eye of Charybdea marsupialis. A, general C y es O n One and the Same 



view; B, greater magnification of four cells to show f . , , rpi 



the alternation of sensory cells (s.c.) with pigment P art Ol lts Doa y- 



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 

 the sensory cells, 

 this function hav- 

 ing been left to al- 

 ternate visual cells 

 which have devel- mea 

 oped a sight rod 

 for the purpose. en '~ 

 These visual cells 

 are differentiated 

 out of the same 

 primitive epithe- 

 lium from which 



the pigment cells ... x .. ,. ,-;....-..-..,..... 



were derived, and ':/'. /!'; .;'. J '*'*"' 



tWO Ot them are p IG 203 Section of the double eye of Aurelia aurita. s.c., visual 

 pictured in the fig- cells; pg.c., pigmented cells; ec., ectoderm ; en., endoderm; mes., 

 i i i mesoelcea; x 760. (After SCHEWIAKOFF). 



ure, much enlarged 



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



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



