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The mechanical energy generated by the contraction of the subumbrellar swimming muscles to power the jet cycle in the hydromedusan jellyfish Polyorchis penicillatus (Eschscholtz, 1829) was measured. This energy was experimentally partitioned into three components during the contraction. The sum of these components was taken to be the mechanical energy generated by the muscles during the jet cycle and was between 8.9×10−5 and 1.4×10−4J per contraction. Energy from one of these components is stored as strain energy in the mesoglea and powers the refilling phase. The mesoglea can clearly act as an effective elastic structure to antagonize the contraction of the swimming muscles completely, and it may be designed to function at some optimum. The mechanical significance of elastic energy storage systems in jet-propelled animals is discussed, and this significance is clearly displayed in Polyorchis. The unusually long-duration action potential of the swimming muscles may be an important component of the swimming mechanism, allowing the muscles to store energy in an elastic structure at the end of the contraction phase when little hydrodynamic thrust is developed. It is suggested that the action potential of vertebrate cardiac muscle may have a similar mechanical function.

Mechanics of Jet Propulsion in the Hydromedusan Jellyfish, Polyorchis Penicillatus:II. Energetics of the Jet Cycle