Like the brittle stars, sea cucumbers perform a valuable function by constantly removing detritus from the substrate. 

Just as radial symmetry is most noticeable in sea stars, sea stars are also the most well known of all echinoderms. Sea stars live in all oceans and in virtually all zones within the reef. 

There are over 800 different species of sea urchins varying greatly in appearance, ranging in size from less than an inch to over a foot across in an assortment of shapes and colors. 

Although they look similar to the sea stars, there are some major morphological differences between these two groups.