772 
ROBERT PAYNE BIGELOW 
SUMMARY 
1. The eight-armed ephyra of Chrysaora has between the lap- 
pets at the end of each arm a rhopalium not enclosed in a sensory 
niche. The rhopalium is covered by a columnar epithelium, over- 
lying a layer of nerve fibers. Within it contains at its distal end 
a mass of otoliths enclosed in endodermal cells that are continu- 
ous with the columnar epithelium lining the rhopalial canal, which 
is continuous with the gastric pouch of the arm. From the sides 
of the gastric pouch an endodermal lamella extends through the 
mesoglcea to the ectoderm. 
2. In the Pelagia stage of Chrysaora, 6 mm. in diameter, the 
rhopalium is enclosed in a sensory niche, covered by a hood, and 
is attached to the underside of the hood. From the point of attach- 
ment a rhopalial ridge runs centrally to the proximal wall of the 
niche. On each side of the rhopalial ridge there is an invaginated 
fold of epithelium consisting of small, ciliated, cuboidal cells and 
covering a layer of nerve fibers that is continuous with the layer 
of nerve fibers upon the rhopalium. This epithelium contains 
shallow secondary pits, and at some point beneath it the layer of 
nerve fibers comes into contact with the endodermal lamella. 
3. At the beginning of the Chrysaora-stage of Chrysaora when 
the animal has reached a diameter of 10 mm. the dorsal sensory 
groove makes its appearance as a shallow depression in the exum- 
brella over the base of the rhopalium, but is still lined by flat 
epithelium like that of the general surface. 
4. In the adult Chrysaora the dorsal sensory groove has become 
a deep conical depression and is lined by columnar, ciliated epi- 
thelium with an underlying stratum of nerve fibers. The topog- 
raphy of the sensory niche is heightened and the cuboidal pitted 
epithelium of the sides of the rhopalial ridge extends on each side 
into a deep lateral pocket that projects centrally beyond the open- 
ing of the rhopalial canal so that its fundus occupies a ridge on 
the roof of the gastric pouch. In this area the layer of nerve fibers 
comes into close contact with the endodermal epithelium as it 
goes along the whole extent of the rhopalial canal. These pockets 
