Cassiopeia, the jellyfish's differently spelled namesake, was a queen in Greek mythology. After bragging that she was even more beautiful than sea nymphs called the Nereiads, the queen was punished by having her constellation placed where it sometimes appears upside-down as it rotates in the sky.
The jellyfish are passive but have a mild sting. Some crabs hoist Cassiopea jellyfish onto their backs for self-protection, carrying them around gardens and all.
Image: Cassiopea jellyfish at Shedd Aquarium, taken by me. If you like seeing bizarre animals up close (or adults with their faces pressed up to glass like kids), go see this exhibit.