Complete all stages in Arcade Mode to unlock Endless Mode, where you can: Play each stage successively without boss battles.
Extra Mode
Upon completing the arcade mode and the credits, you'll be informed that you've unlocked Extra Mode. When you start a new game and choose Arcade, you'll now be given the option between playing Normal and Extra Modes.
Stage Mode
Complete the first stage to unlock stage mode. Allows you to replay already played stages.