A jellyfish moves by filling its umbrella with water and then:

1. Jet propulsion is momentum conservation made visible. 2. A jellyfish fills its umbrella section with water and then pushes the water out. 3. That results in motion in the opposite direction to that of the jet of water. 4. Squids propel themselves in a similar manner but can aim their nozzle forward or backward to control direction. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 8 'Linear Momentum and Collisions', sections 8.1-8.7_