Urchins are made up of hundreds of spines and tubefeet.  These are used for protection and movement.  They may attach loose rocks or shells to hide themselves from possible predators. And they may also make holes into rocks for an even better hiding spot.  They can do this because of their spines and teeth.  They have five specialized teeth that can grind into almost anything.  This is known as “Aristotle’s Lantern”.  If one or more of these teeth happen to fall or break off, it can grow back.  Urchins also have a hard exoskeleton that is known as a test, which the spines and tube feet are attached to.  Purple urchins can grow up to 4” in diameter having short purple spines.  The juveniles have greenish spines. Red urchins can grow up to 10” in diameter having sharp spines about 1/2 as long as the diameter of the test.  Red urchins can vary in color from dark purple to an almost black and the purple urchins can vary in color from greenish (juveniles) to a pink to dark purple.  The way to tell them apart is by the size of the spines verses the test, because the color can vary form one to the next.