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Bilaterus the alien with both heads.
Bilaterus
The seventh alien met in Tank 4-2. You have to destroy both heads to beat down Bilaterus. Any part of Bilaterus will eat the fish, even if it isn't the head. If you're too slow, the heads will swap places. Unlike some aliens, Bilaterus' signature is shown as II instead of B. He is probably one of the most annoying aliens to fight.