Peel off shrimp shells and place them in a large saucepan with the fish bones and water. Simmer for 1 minute, skimming thoroughly. Add ¼ cup sherry, celery, ginger, and optional lemon grass; simmer, partly covered, for 25 minutes. Meanwhile, devein shrimp and cut them in thin diagonal slices; sprinkle with the lemon rind and cayenne.