The mouth is located under the center of the starfish disc.  This side is generally referred to as the "oral" side of a starfish.  When turned over, the starfish has a different look.  You find the bristles, of course, but you also can find a row of small filament type threads lining the center of each arm.  These filaments carry the food from the end of the arm to the mouth of the starfish.  I love to watch as my brittle stars reach out for a chunk of the food as it falls in the tank, and curl around it in an octopus - like fashion, curling it's arm in towards the mouth.    Given their shape, they could easily be called a spiny octopus.