subumbrella

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The concave underside of the body of a jellyfish.

Century Dictionary and Cyclopedia

1. n. 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. If such an acaleph is likened to a woman's parasol, lined, then the lining is the subumbrella, the covering being the umbrella. Compare cut under Discophora.

Wiktionary

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

GNU Webster's 1913

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