Preheat oven to 375 degrees. Rinse shrimp. Using a knife, slice up back of shrimp about 1/2 inch and pull out black vein. Remove 3/4 inch of clear shell. Rinse again. Arrange in flat baking pan. In saucepan, cook celery and onion in a little water for 5 minutes, or until softened. Drain. Transfer to large bowl. Pour half of butter over celery mixture. Add cracker crumbs. Add garlic salt and scallops. Mix well with hands. (Mixture should be somewhat moist.)
Using a tablespoon, spoon mixture on top of shrimp. Bend tails over stuffing. Drizzle with remaining butter. Bake for 20 minutes, or until done. Garnish with lemon slices, if desired.