Noun
The integument of the under surface of the bell, or disk-shaped body, of a jellyfish.
Source: Webster's dictionaryMost have tentacles around the outer margin of the bowl-shaped bell, and long, oral arms around the mouth in the center of the subumbrella. Source: Internet
The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). Source: Internet