IPA: /sʌb.ʌmˈbrɛl.ə/
KK: /sʌbʌmˈbrɛlə/
The curved, inner surface located beneath the body of a jellyfish.
The subumbrella of the jellyfish helps it to move through the water.
Subumbrella is formed from "sub-" (meaning under) and "umbra" (meaning shadow). The word describes something that is under a shadow or shade.
Think of being 'under' ('sub-') a 'shadow' ('umbra') — that's what subumbrella means.