The term "echinoderm" means spiny skinned and is a very appropriate name for invertebrates in this group. Sand dollars, sea cucumbers, sea stars (sometimes called starfish) and sea urchins are all echinoderms. There are an estimated 70,000 species that belong to this group and echinoderms are only found in the marine environment. Unique to the bodies of all echinoderms is the water-vascular system. This hydraulic system -- a water-filled ring with canals radiating from it -- is used to provide the echinoderms with movement. Each canal is connected to many tube feet, each of which has the ability to grab onto an object through the use of suction. Working together, the tube feet can move the animal along, or can move food towards the animal’s mouth.