RPG plans, designs and ideas
I don't think you need to add arrows, the legend shows increasing HD left to right, so it's obvious at least to me. Adding arrows would be an aesthetic judgement, rather than clarifying. Besides, why couldn't healing or other spells cause devolution as a side effect?
I don't mean arrows to show which direction the hit dice go... I mean arrows to show that rotting undead must become skeletal undead unless they become vampires before then. And so on.