SENSE-ORGANS IN MEDUSAE 773 
are probably similar to structures found by Eimer and Claus in 
Cyanea and Aurelia. 
5. The adult Pelagia cyanella possesses a shallow dorsal sen- 
sory groove. The rhopalium is similar in all important respects 
to that of Chrysaora and is attached to a short and low rhopalial 
ridge which is covered by a columnar epithelium overlying a layer 
of nerve fibers, among which are large bipolar ganglion cells. In 
the nerve-fiber layer of the rhopalium are found nuclei probably 
belonging to small ganglion cells. There is no pitted epithelium 
at the sides of the rhopalial canal nor lateral pockets, and there 
is no thickening of the epithelium of the niche. The endodermal 
lamella may be traced in contract with the ectoderm of the side 
of the rhopalial ridge to the base of the rhopalium. 
6. A comparison of the Pelagia-stage of Chrysaora with the 
adult Pelagia shows that both possess in each of the eight principal 
radii a rhopalium lying in a well developed sensory niche, which is 
lined for the most part by undifferentiated epithelium. Both have 
a thickening of the epithelium on the surface of the rhopalial 
ridge. They differ in that Pelagia has no pitted epithelium while 
in this stage Chrysaora has not yet acquired a dorsal sensory 
groove. If Hesse be right in saying that Pelagia likewise has no 
true dorsal sensory groove, the difference between the two forms 
becomes just so much less. But if it be not sensory, it is difficult 
to understand the presence of this persistent dimple in two species 
of Pelagia. 
7. The adult Dactylometra presents in its sensory apparatus 
all the characters of Chrysaora in an exaggerated degree. The 
pitted epithelium covers a greater surface and is more highly 
developed. It fills the whole of the lateral pockets with closely 
packed branching tubules. 
8. Of the larval forms of Dactylometra, the ephyra is essen- 
tially like the same stage in Chrysaora. The Pelagia stage has 
not been studied. A larva in the Chrysaora-stage, 18 mm. broad 
with the secondary tentacles 3 mm. long, shows the rudiment of 
a dorsal sensory groove consisting of a flat disc of columnar epi- 
thelium. The rhopalium is like that of the adult Chrysaora. But 
the rhopalial ridge is very short and completely covered by the 
