SENSE-ORGANS IN MEDUSAE 
767 
along each side of the rhopahal canal nearly to the tip of the diver- 
ticulum into the hood, fig. 14. Aside from the rhopalium and the 
band of peculiar epithelium and nerve fibers there is no other 
sensory apparatus in the niche. 
The sense-organs in Pelagia are then much simpler than in the 
adult Chrysaora, but they have many points of resemblance to 
the Pelagia-stage in the young Chrysaora. In both the sensory 
niche, while well developed, is not so prominent as in the adult 
Chrysaora and the greater part of it is lined with the ordinary 
surface epithelium. It is probable that in the larval as well as 
the adult form there is a thickening and a differentiation of the 
epithelium upon the lower surface of the rhopalial ridge. The 
latter in both is of about the same proportion, being short and but 
slightly raised, and in both the rhopalial canal extends only to 
the mass of concretions and does not penetrate it. The shape 
of the rhopalium in Pelagia differs from its shape in the young 
Chrysaora, as well as in the adult, in that the part which contains 
the concretions has a considerably smaller circumference than the 
part covered by the sensory epithelium, and the concretions them- 
selves are longer in Pelagia in proportion to their width. But the 
important points in which Pelagia differs from the corresponding 
stage in the development of Chrysaora are two: the presence of 
the dorsal sensory groove, and the absence of any fold of the ecto- 
derm at the side of the rhopalial ridge. Compare figs. 15 and 31. 
It will be noticed, however, that the line along which the endo- 
dermal lamella touches the e<;toderm at the sides of the rhopalial 
ridge in Pelagia (fig. 14, e.l.) coincides exactly with the position 
of the deepest part of the lateral folds in the Pelagia-like larva 
of Chrysaora. 
THE ADULT DACTYLOMETRA 
The sense organs of Dactylometra quinquecirrha differ from 
those in Chrysaora, as we would expect, in the opposite direction 
from Pelagia. While that genus lacks some of the features of 
Chrysaora and has a simpler sensory apparatus, the adult Dac- 
tylometra possesses all the characteristics of Chrysaora in an 
exaggerated degree, figs. 17 to 20. 
