In Finding Nemo, the jellyfish surround Marlin and Dory when "Squishy" calls for them after Dory annoys him. Marlin and Dory are forced to bounce on the jellyfish to get out of the field of "stingers." Dory gets severely stung by one and passes out, forcing Marlin to retrieve her while also getting stung himself (although he is used to these kinds of stings because he lives on the reef). After Marlin and Dory get out of the jellyfish forest Marlin passes out then he and Dory are rescued by Crush and his gang of sea turtles.
Squishy, as nicknamed by Dory, is a tiny jellyfish Dory encounters after Marlin decides that he and Dory swim over the trench (rather than through it, as previously told by the school of Moonfish). Dory tries to coax "Squishy" into being her "pet" by baby-talking, but it stings her. This makes Dory scold the tiny jellyfish by saying, "Bad Squishy! Bad Squishy!" before Marlin shoos it away with his fin.
The Jellyfish from one of Finding Nemo books "Nemo's World".
Jellyfish Forest also in the same page of the books.
Jellyfish page on the paperback of the Finding Nemo storybook.