As you venture through Kraid's domain, you're attacked by the acid worm Mua.

Defeat Mua

Whenever Mua goes down into the acid, watch for it to rapidly move its teeth. This means that it's about to come up out of the acid and try to grab you, so you need to jump up to grab the ceiling rail before it can catch you. If you time it correctly, Mua will get stuck on one of the side blocks. When the rail drops you off, turn around and shoot at Mua's glowing body segment.

After you hit Mua several times, the acid will temporarily rise. Stand on one of the higher blocks until it recedes, then repeat the above strategy.

After Mua is defeated, you can continue to explore Kraid (Part 2).