The correct option is D Ambulacral system
Echinoderms are characterized by a unique coelomic water vascular system, called as ambulacral system.
This is a hydraulically controlled system consisting of a circumoral ring around the esophagus with connecting radial canals each leading to an ambulacrum. The radial canals in crinoids run along each arm into an ambulacral groove and tube feet. Thus, the correct answer is option (D).