Preheat the broiler (or grill as we call it in the UK).
Mash the avocado in a large bowl.
Add the crab meat, basil, red pepper, chopped chili (if using), celery salt and pepper.
Toast the slices of bread in a toaster and then place them on a broiler pan or baking sheet.
Spread the bread with the crab mixture.
Top each slice of toast with 4 or 5 of the large shrimp and 1 slice of cheese.
Broil or grill until the cheese begins to bubble and turn golden brown.
Sprinkle with parsley and cut each slice in halves or quarters to serve.