Answer:
Because its bodily functions only takes place in water.
Step-by-step explanation:
Like many other organisms they do not move using any mascular system. Echinoderms has a unique type of water vascular system.They move feed and also breathe using this system.The system consists of hundreds of water filled tubes.And through this tubes the food is absorbed from the water and also their oxygen.Due to this they also move very slowly.Thats why they ar exclusively marine animals.