Finely chop the garlic and parsley. Cut the squid tubes into ¼-inch wide rings and cut the tentacles in half.
Put 4tablespoons of the olive oil, the garlic, crushed red pepper flakes, and parsley in a deep Sauté pan that can accommodate the clams and mussels later and place over medium high heat. Add the white wine and let it cook for about a minute to let the alcohol evaporate. Add the tomatoes and season with salt. Once the tomatoes begin bubbling, turn the heat down to low. Cover the pan with the lid slightly askew and cook until the squid is very tender, about 45 minutes. If all the liquid has evaporated before the squid is done add a little water. When the squid is tender remove from the heat and set aside. The sauce may be prepared ahead of time up to this point and refrigerated.
When you are ready to cook the pasta, cut the scallops into ½-inch pieces. Peel the shrimp. Devein if necessary, and cut them into ½-inch pieces as well. Ffill a pot for the pasta with about 6quarts of water, cover, and place over high heat.
Return the sauce to a medium high heat. Once it starts bubbling, add the clams and mussels. When they begin to open, add the scallops and the shrimp. Season with salt and continue cooking until the scallops and shrimp are cooked through then remove from the heat.
When the water boils, add 2tablespoons salt and add the spaghetti. Stir until all the strands are submerged and cook until al dente.
When the pasta is done, drain well, and toss with the sauce adding the remaining tablespoon of olive oil. Serve at once.