Jellyfish or sea jellies are amazing creatures, not having a use for specialized eating or breathing organs...they don't even have a brain ! Their movements are limited, but they can be self propulsing to a certain extent by contracting and pulating their water-filled bodies.

These strange sea creatures swim around by opening and closing their bell. When a jellyfish tightens it's muscle, the bell closes and pushes the water out, propelling the jellyfish forward. Water refills the bell as the jellyfish relaxes it's muscle.