Medium Sized Gut Echinoderms

Your gut fills up all the space inside you, but you don't grow to create space for a really really large gut.  You can efficiently move around, find food, eat it, and digest it.  But while you've been evolving, so has everything else:  The Mesozoic Marine Revolution has arrived...

Elsewhere, some fairly close relatives of yours have just developed something called a 'jaw'.  It is a large, leveraged, crushing device, which means that even relatively tough creatures like yourself are vulnerable.  These new-shaped creatures roam the oceans, spotting slow moving, armoured individuals of your species, pick them up and crush them to bits.

You have a thick, armoured skin, but it's simply not enough to protect you against the big creatures who are evolving now.  You can't really get tougher fast enough.  In this arms race, increased toughness will lose to a bigger and bigger jaw, and more and more armour, until you look like a concrete block and you can't move at all.  Some other approach may work, or you can just run away.

What would make you really unpleasant to eat?

Spikes.  All over.
Save the weight and effort of growing spines, and just run away!