SENSE-ORGANS IN MEDUSAE 
761 
If the animal now be looked at from the under side, the most 
conspicuous part of the sensory apparatus is the rhopalium, figs. 
1, 2, and 10, r., which has the same shape that it had in the pre- 
vious stage, that of a thick bent finger, and it has not increased 
in size in proportion to the surrounding parts. The sensory niche 
in which the rhopalium lies is ver}^ much deeper than in the pre- 
vious stage and the free edges of the marginal lobes are approxi- 
mated so as to form a tube which extends from about opposite 
the base of the rhopalium outward to the edge of the hood. The 
rhopalium is attached as before to a ridge which extends centrally 
along the arched roof of the niche from a point just distal to the 
base of the rhopalium to the proximal wall of the niche. But now 
the base of the rhopalium covers but a very small part of the 
ridge near its distal extremity, and the proximal part is relatively, 
as well as actually, very much larger than before, figs. 1, 2, and 
10. The ridge gradually becomes wider and thicker as it recedes 
from the rhopalium, figs. 1 and 2, and 5 to 8. Its under surface 
is convex and it is grooved on its sides. These lateral grooves are 
continued into two pocket-like cavities which lie on each side of 
the ridge in the upper proximal wall of the niche, figs. 1, 2, and 
8. The rhopalial canal becomes gradually narrower from its 
mouth outward to the base of the rhopalium, it is slightly dilated 
in the proximal part of the latter and extends some distance into 
the distal part where it ends as a narrow pocket. In a section 
of the mass of concretions it appears as a narrow vertical slit, 
figs. 1 and 3. The canal sends off a very small cone-shaped 
diverticulum into the hood above the rhopalium. 
With this heightening of the general topography comes a cor- 
responding increase in the importance of the various histological 
features. The thickening of the ectoderm, which in the last stage 
extended a short distance around the base of the rhopalium, has 
now spread so as to cover the whole surface of the sensory niche, 
including all but a small part at the base of the rhopalium of the 
convex under surface of the rhopalial ridge, and has come to 
form apparently a sensory epithelium. It extends in all direc- 
tions from the base of the rhopalium, laterally nearly as far as 
