Skeletal System
Chironex fleckeri despite being as deadly as it is, is one of the simplest organisms. It has a "bi-radially symmetrical body, well developed tissues, and specialized cells." The Jelly fish has an organic exoskeleton, hydrostatic skeleton, and it is an encephalized organism with a non-cellular mesoglea separating the two layers.
http://eol.org/pages/200766/details
http://eol.org/pages/200766/details