OUR BEST FEATURES 



ectoderm of the cup-shaped body. In some cases 

 (Fig. 91 A) each ocellus consists only of a slightly 

 raised patch of larger pigment-bearing epithelial 

 cells alternating with smaller "light cells." The 

 patch grades into the ordinary epithelial cells 

 around it. In other cases (Fig. 91B) the patch 



31 



Fig. 91. The Beginnings of Etes. (From Plate, after Linko.) 



A. Section of an ocellus, or eye spot, at the base of a tentacle of 

 a jellyfish. B. Section of a "goblet eye" of a jellyfish. 



(From Allgem. Zool., Gustav Fischer.) 

 For details, see p. xxxv. 



sinks below the surface, forming a pouch lined 

 with pigment. Between the large deeply pig- 

 mented cells on the inside of the pouch are small 

 *'rods" at one end of the "light cells." Such an 

 alternation of two kinds of cells foreshadows the 

 alternation of the "rods" and "cones" of more 

 advanced types of eyes, in which the "rods" are 



believed to detect light and darkness, form and 



175 



