Fact 1: Echinoderms have a unique water vascular system with tube feet.

Fact 2: Echinoderms have a hydraulic system of locomotion called the water vascular system.

Given the two facts above, answer the question "How do echinoderms use their feet to locomote themselves?" with the following options: 
- one at a time
 - to move people
 - running and lifting
 - protein channels
 - a hydraulic system
 - it needs them
 - undulations
 - It helps them survive


 

     a hydraulic system    