Skeletal System:
Sea stars have a thick outer skin that protects their inner organs. This outer layer (endoskeleton) serves as the protector of the their organs and what keeps their shape. Their endoskeleton is made out of calcium carbonate plates called ossicles.
Muscular System:
To move their plates, they use their water vascular system where they use a hydraulic system to suction themselves to an object and move their tube feet and release from the object.
A Study In Biology 