Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, about 8 to 10 minutes. Drain pasta.
In a large bowl, mix together ricotta, 3/4 cup Parmesan, egg yolk, basil, crab, salt, and pepper.
Preheat oven to 350 degrees F.
Fill the cannelloni with the crab mixture and place in a buttered baking dish. Top the filled cannelloni with the Bechamel Sauce and sprinkle with the remaining Parmesan cheese. Bake until bubbly and the top is golden brown, about 15 to 20 minutes. Serve immediately.
In a medium saucepan, melt the butter over medium heat. Add the flour and whisk until smooth, about 2 minutes. Gradually add the warm milk, whisking constantly, until the sauce is thick, smooth, and creamy, about 10 minutes (do not allow the sauce to boil). Remove from the heat and stir in the salt, pepper, and nutmeg. (The sauce can be made up to 3 days ahead. Cool and then cover and refrigerate.)