In a medium saucepan, combine the ditalini with 1/2 cup of the stock and bring to a simmer. Cook over moderate heat, stirring constantly, until the stock is absorbed. Continue adding stock 1/2 cup at a time, allowing it to be absorbed before adding more. The ditalini is done when it is al dente and suspended in a thick, rich sauce, about 25 minutes. Stir in the garlic, mussels and clams and cook 1 minute longer. Transfer the pasta to bowls. Sprinkle with the parsley, drizzle with olive oil and serve.