With the spline still selected, choose the Shape > Lathe tool and accept its defaults. Then Subdivide the model by 2.

Open the object info palette and call the lathed object "Jellyfish_bell". To aide our next step, uncheck the visibility to hide the "Jellyfish_bell". You can also delete the original spline, since it will no longer be needed. Turn off snap to grid.

Collapse the front view and look at the camera window. The object appears a little large. No problem, simply zoom out by clicking and dragging on the magnifier tool in the title bar of that view.