• (n.) The integument of the under surface of the bell, or disk-shaped body, of a jellyfish.
Sub`um·brel'la noun (Zoology) The integument of the under surface of the bell, or disk-shaped body, of a jellyfish. 
subumbrella The concave oral surface of the umbrella of a medusa (jelly fish); the concave inner surface of the medusoid bell.
the concave undersurface of a coelenterate medusa, as a jellyfish.