We present an automatic animation system for jellyfish that is based on a physical simulation. We model the thrust of an adult jellyfish, and the organism's morphology in its most active mode of locomotion. We reduce our model by considering only species that are axially symmetric so that we can approximate the full 3D geometry of a jellyfish with a 2D simulation. We simulate the organism's elastic volume with a spring-mass system, and the surrounding sea water using the semi-Lagrangian method. We couple the two representations with the immersed boundary method. We propose a simple open-loop controller to contract the swimming muscles of the jellyfish. A 3D rendering model is extrapolated from our 2D simulation. We add variation to the extrapolated 3D geometry, which is inspired by empirical observations of real jellyfish. The resulting animation system is efficient with an acceptable compromise in physical accuracy.