Unlock the the following bonuses by performing the corresponding tasks:

BONUS - HOW TO UNLOCK
Boss On Parade mode: - Complete the game to unlock Boss On Parade mode.
All levels: - Complete Boss On Parade mode to unlock all levels.
God mode: - Complete level 10 on Hard difficulty to unlock the God difficulty setting.
Sound test: - To unlock the sound test menu, get a high score in Normal, 2-Minute, and 5-Minute modes with all three ships.

On level 8, attack the Crab Boss until it transforms into a ship but do not destroy it. Avoid its attacks until it leaves to unlock the fourth ship.