A jellyfish was an animal lifeform native to Earth.
The blastoneurons, being no more than amorphous, colorless, gelatinous masses, looked as if somebody had dumped jellyfish out of a bucket. (TOS - Star Trek 2 novelization: Operation -- Annihilate!)
Korob and Sylvia's true forms resembled tiny blobs of jelly, their bodies veined like those of jellyfish. (TOS - Star Trek 8 novelization: Catspaw)