Echinoidea

You simply can't stop big jaws from crushing you.  But you can make it so unpleasant that these newly evolved predators will choose someone else.  The flat plates you already have become lumpy, and bumpy, and unpleasant to put in your mouth.  As time passes, those lumps and bumps get pointier and pointier, until you look like an explosion in a needle factory.

You are a sea urchin - a round-ish body covered in pointy spines.  You roam around the ocean from the tidal zone down to around three miles down, and very few creatures will eat you.  Those that do - crabs and lobsters, triggerfish, eels, humans -  avoid the need to bite down with soft mouthparts on spines by having special adaptations such as strong claws or bony mouths.  Or hammers.