Water vascular system of star fish acts as circulatory system,& is used for locomotion & capture of prey.They can use even sea water in the system. In case of prawns,having bilateral symmetry,organs being compact,it has a system of arteries,& blood has ability to clot,unlike in starfish.The heart in case of prawns pulsates,which is very mild in case of star fish.Because star fish can regenerate from a single arm, which is not so in prawns,the regenerated fish survives,unlike in prawns.
Agreed that prawn have advanced circulatory system,but to survive in nature,a star fish is better placed than a prawn,& hence in spite of it,the circulatory system of a starfish is better placed than that of prawn.