They are called echinoderms because bodies are covered with spines and sprites. All animals of this phylum are exclusively marine. All animals have internal skeleton of calcium carbonate in the form of plates or small pieces. These also have a water vascular system. They have dermal gills. These animals are considered to be closed to the chordates from evolutionary point of view.

Examples are sea star (also known as star fish), brittle star, sea urchin and sea cucumber etc. Star fish is not actually a fish but its body is star shape. It has five arms but some species of it have more than five arms. Central part of the body is disc like from which arms originate and mouth is on ventral side of disc. On the ventral side of every arm are many tube feet with the help of which it attaches itself to the rocks and are also used in the locomotion. It has a very strong power of regeneration. This sea star lay millions of eggs fro their reproduction.

Sea urchin has a rounded body with ling spines. Its body has skeleton made up of many calcium plates, pointing towards center. Mouth is present on the ventral side of the body and has five teeth; it lives at the bottom of sea.