Margaret Atwood's novel, Alias Grace tells the story of a murder from the perspective of one of the accused murderers. Grace Marks has been institutionalized because she has no recollection of participating in the murder. A young doctor interviews her in the hopes of discovering her innocence, and we learn the story through her words and memories.
The images you mention, those of jellyfish and birdcages, occur in the first chapter of the book. Grace is describing the ladies that come to visit the home of the Governor. She is in charge of cleaning up the parlor after the visiting ladies have left, a job that she was given because of her good behavior at the institution. Grace describes the ladies in their billowing dresses, saying:
They are like swans, drifting along on unseen feet; or else like the jellyfish in the waters of the rocky harbour near our house.
She first thinks of jellyfish because its shape is like that of the skirts of the upper-class ladies. But she extends the metaphor:
They were bell-shaped and ruffled, gracefully waving and lovely under the sea; but if they washed up on the beach and dried out in the sun there was nothing left of them. And that is what the ladies are like: mostly water.
The ladies, like jellyfish, are delicate and only able to survive in a very contained environment. The metaphor of the birdcage is similar. Grace begins by using the birdcage to describe the ladies' crinolines and underclothes, then extends the metaphor again:
They are like birdcages; but what is being caged in? Legs, the legs of ladies; legs penned in so they cannot get out and go rubbing up against the gentlemen's trousers.
Again, the image of the birdcage is used to emphasize the ladies confinement, both physically in the hooped skirts, and metaphorically into a certain mode of behavior.