Cook the pasta shells according to packet instructions so they are al dente. Drain and set aside on a baking sheet.
Meanwhile, warm the oil in a medium skillet over a medium heat. Add the garlic and cook a minute or two until fragrant. Add the pork and cook a minute to soften. Add the arugula and cook, folding it in so it all wilts down. Set aside to cool a minute.
Mix together ½ of the mozzarella (ie ¼ cup) with the ricotta, add the pork mixture and mix.
Put approximately 1 ¼ cups of the marinara sauce in the bottom of a baking dish.
Fill the pasta shells with the pork-cheese mixture and set in the marinara in rows. (You might not use quite all of them, but a couple often break in boiling.)
Top with the remaining marinara sauce, the rest of the mozzarella and the parmesan.
Cover and bake for 30-40 minutes, uncovering for the last 5-10 minutes.