768 
ROBERT PAYNE BIGELOW 
The rhopalium is of about the same shape and size as in Chry- 
saora. It is clothed with the same kind of sensory epithelium 
overlying a layer of nerve fibers as thick as the layer of cells or 
thicker in the U-shaped areas. The supporting membrane is 
thickened under this area and the rhopalial canal extends into 
the mass of cells that contain the concretions. In short, the 
rhopalia in the two species are alike in every particular. (It is 
only inPelagia that I find any trace of ganglion cells in the nerve- 
fiber layer of the rhopalium.) The parts surrounding the rhopa- 
lium are, however, much larger than in Chrysaora. The meso- 
gloea of the hood is very much thicker and the dorsal sensory 
groove is proportionally deeper. It is a little longer than in 
Chrysaora but no wider, figs. 17 and 18. The sensory niche is also 
deeper, the lateral pockets are larger, and the rhopalial ridge is 
more prominent. 
It is in the latter that we find the most characteristic differences 
between Chrysaora and Dactylometra. Immediately at the base 
of the rhopalium the ridge is covered by a shallow pitted epithe- 
lium with its layer of nerve fibers, as in Chrysaora, fig. 18. Prox- 
imally this soon becomes confined to the sides of the ridge while 
the a^ea between is clothed with a single layer of small cuboidal 
cells. At about half the length of the ridge two small elevations 
appear, one on each side of this simple epithelium, fig. 19. The 
concave area between them gradually widens until it joins the 
proximal wall of the niche. The pitted epithelium covers a much 
greater part of the surface of the ridge and is much more highly 
developed than in Chrysaora. It gradually spreads out on to the 
roof of the niche and its pits become deeper and more numerous 
as it recedes from the rhopalium. At its outer edges the change 
into the ordinary epithelium of the niche is quite abrupt. A com- 
parison of figs. 7 and 19 will show these characteristic differences 
in the two species. With the increase in size of the lateral pockets 
there is an increase in the depth and number of the epithelial 
pits. These are branched and closely crowded so that they fill 
the whole pocket, obliterating its lumen, and the orifices of the 
pits open directly to the exterior at its mouth; not well shown 
in fig. 20. 
