subumbrella

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The concave underside of the medusa of a jellyfish or other cnidarian.

from The Century Dictionary.

• noun The internal ventral or oral disk of a hydrozoan, as a jellyfish; the muscular layer beneath the umbrella or swimming-bell of a hydromedusan, continuous with the velum.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The integument of the under surface of the bell, or disk-shaped body, of a jellyfish.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology The integument of the undersurface of the bell, or disk-shaped body, of a jellyfish.