At the apex of the rhopalium the endoderm is greatly thickened and consists of concrementcells secreting otoliths (Con).
The more proximal portion of the rhopalium usually bears one or more ocelli (oc).
The rhopalia are lodged in the notches be tween the marginal lobes of the umbrella, and each rhopalium is covered over by a little protecting flap or lappet.
Each rhopalium is a centre round which, as already stated, nervous tissue is concentrated.
In Aurelia there are two on each rhopalium, a simple ocellus on the exumbral side, and a cupped ocellus on the subumbral side (not present in young individuals).