758 
ROBERT PAYNE BIGELOW 
Around the base of the rhopalium this very much thickened 
ectoderm passes into the general flat epitheUum of the body, 
except at two points. These are at sides of the rhopahal ridge, 
fig. 30, and here the rhopalial ectoderm is continuous with two 
folds of thickened ectoderm which run on each side of the ridge 
centrally to the proximal wall of the niche, fig. 31. 
The sensory epithelium of the rhopalium, figs. 26 and 28, is 
of the same character as that in the ephyra and the cells are pro- 
vided with comparatively long cilia. In this stage the layer of 
nerve fibers is very well marked; in the deepest part it is as thick 
as the cellular layer. This thickened portion of the nerve fiber 
layer describes a U-shaped figure. The loop of the U lies on the 
upper side of the rhopalium in the most distal part of the nerve 
layer, fig. 28, and the limbs of the U extend along the upper part 
of each side of the rhopalium and each one is continuous with a 
thin layer of nerve fibers underlying the before-mentioned lat- 
eral thickenings of the rhopalial ridge. The ectoderm in these 
latter areas is composed of ver}^ small cells with very long cilia. 
In section the layer of cells seems to be composed of a mass of 
crowded nuclei. It is probably a single layer of cuboidal or slightly 
elongated cells and it is folded so that there is an invaginated 
groove at each side of the rhopalial ridge which extends to the 
proximal end of this thickened ectoderm on the wall of the niche. 
The groove runs along the upper edge of the layer and in addi- 
tion to it there are a number of shallow secondary pits. On the 
proximal wall of the niche the lateral folds lie in the plane of the 
endodermal lamella and are apparently in contact with it, fig. 
32. All along the rhopalial ridge there is a line opposite the bot- 
tom of the principal fold and continuous with the endodermal 
lamella in which the endoderm and ectoderm are in contact. 
This lamella not only connects the endoderm of adjacent gastric 
pouches but also connects the endoderm with the ectoderm all 
around the margin of the umbrella, except where the jelly is 
too thin to permit it, and it is remarkable that in all forms that 
I have studied there is some point where this lamella comes into 
contact with the nerve-fiber layer. 
