SENSE-ORGANS IN MEDUSAE 
757 
ginal lappets. This then may be regarded as in the Pelagia 
stage. 
The rhopaUum at this stage is twice as large as in the preceding 
one and the distal horizontal part is longer in proportion to the 
proximal vertical part, fig. 26. It lies in a small cavity generally 
known as the sensory niche (Sinnesnische) s. n., figs. 26 and 28, 
which is not at all developed in the ephyra. This is bounded above 
by the hood, figs. 26, 27, and 28, h, laterally, by the sides of the 
marginal lappets; and proximally, by that part of the subum- 
bral wall that rises to pass into the hood. The niche becomes 
gradually shallower proximally; distally and below, the niche is 
open, except that the free edges of the marginal lappets are folded 
together so as to convert the part of the niche immediately sur- 
rounding the rhopalium into a tube such as is figured by the Hert- 
wigs. There is a low, flat ridge, figs. 26 and 30, r.r, extending along 
the slanting roof of the niche from the proximal wall outward 
about halfway to the edge of the hood. The rhopalium hangs 
from this ridge and its base covers more than half of it. The 
ridge contains the rhopalial canal which sends a little conical 
diverticulum into a small part of the ridge that extends beyond 
the base of the rhopalium, fig. 26, r.c. The canal penetrates the 
rhopalium for about half of its length, considerably farther than 
in the ephyra. 
The endoderm at the mouth of the rhopalial canal begins to 
thicken and farther inward becomes columnar. These columnar 
cells distally fill the whole lumen of the tube and they grade off 
into the larger otolith cells of the extremity. These are about 
the same size as in the ephyra, but are more numerous, and 
as in the ephyra the endoderm is separated from the ectoderm 
throughout the rhopalium by only a thin supporting membrane. 
The general ectoderm of the body, including that lining the 
sensory niche, is flat as in the ephyra, but the thickened spots 
(6) are more developed and the nettle cells are clearly differen- 
tiated in them. The ectoderm on the rhopalium is twice as thick 
as in the ephyra and grades distally, as in that stage, into a thin 
membrane covering the outer end. The gradation is more 
abrupt on the upper than on the lower side. 
