762 
ROBERT PAYNE BIGELOW 
the bases of the marginal lappets, distally as far as the extremity 
of the hood, and proximally to the edge of the subumbral mus- 
cle layer, a very small part of which comes within the hollow of 
the niche. The thickness of this layer of cells varies somewhat. 
The ectoderm lay er is thickest on the proximal wall of the niche 
and two thickened areas extend outward along the sides of the 
marginal lappets midway between the rhopalial ridge and the 
free edges of the lappets, gradually thinning out at a point beyond 
the margin of the hood. At the edge of the muscle layer this 
epithelium of the niche passes rather abruptly into the cells cov- 
ering the muscles, and the columnar epithelium extends some dis- 
tance outward between the edge of the muscle layer and the endo- 
dernial lamella on the subumbral wall of the marginal lappets, 
fig. 3. The free edges of the marginal lobes are covered by a sim- 
ple slightly flattened epithelium. The sensory epithelium of the 
niche grades gradually into this. Surrounding the base of the 
rhppalium on its distal and lateral sides, the epithelium is only 
about half as thick as it is in the deepest part of the niche. 
Where the sensory epithelium of the niche is deepest the cells 
are of about the same proportion as in the dorsal sensory groove, 
perhaps somewhat longer. These cells as seen in sections are 
long and columnar, each with a nucleus in the lower third. Below 
the nucleus the protoplasm is clear, above it, it is granular, and 
in the upper third of the cell there is often the characteristic struc- 
ture of a mucous cell. 
The mucous cells, which are here very abundant, are also 
found in the general ectoderm and endoderm, and particularly 
in the dorsal sensory groove (as already stated) and in the endo- 
derm of the rhopalial canal. In each place the goblet cell is 
of the same length as the adjoining cells and everywhere, except 
in the dorsal groove and the sensory niche, the globule of mucus 
nearly fills the whole cell. These cells seem, however, to be absent 
from the epithelium of the under side of the rhopalial ridge and 
from that lining the niche immediately proximal to it, so that in 
a longitudinal section through the rhopalium they are not seen, 
fig. 12. Scattered nettle cells are found also in the epithelium of 
