Acid Worm

The Metroid Zero Mission Official Site, as translated by Metroid Database is a miniboss Samus fights in Metroid: Zero Mission.
In the Japanese website of Zero Mission, this creature is given the title . However, the Acid Worm is never associated with lava in the game itself.
==Battle==
The Acid Worm resides in a considerably large pool of acid in the second door down from Kraid's main shaft. The Acid Worm cannot be battled unless Samus arrives after activating the ancient Chozo machine in Kraid's Lair. If she arrives without doing this, the room will be flooded with acid and several Reos will be present instead. Once Samus has activated the rail system in Kraid and attempts to move onwards using the Zip line, she will encounter this creature.
Upon its arrival, the Acid Worm immediately crashes through the tall center block as it plows directly into the zip line. As soon as Samus lands on a block platform, within a second, the Acid Worms bunkers underneath the acid and the battle ensues. The Acid Worm attacks by lunging out of the acid at Samus, noted by the flick of its teeth in and out just before it lunges at Samus. Its lunging angle varies by where Samus is currently stationed at and will always attempt to charge at her most current position:
*If Samus is standing on one of the higher ledges, it will perform a simple lunge and retreat. If Samus retreats to one of the locked doors during this lunge, the Acid Worm will halt its head atop the high block platform. In doing so, it briefly pauses before opening its maw and spitting out three purple globs of some sort of substance (presumably more acid). These are difficult to dodge but can be evaded with a well placed jump. These globs can also be shot at, which contain Energy Capsules or Missile Ammo when destroyed, allowing Samus to replenish her strength and ammo supply during the battle. 
*If Samus stands on one of the lower blocks, the Acid Worm will lunge at her directly. To avoid this attack angle, Samus must either quickly jump to the adjacent higher platform or jump directly above her and catch the zip line before the Acid Worm reaches her. If successful, the Acid Worm will ram head first into the side of a higher block hard enough to accidentally lodge its teeth in the block, making it unable to move momentarily. If Samus used the zip line to avoid this attack, at the drop off to the opposite lower platform, she will be in a perfect position to inflict damage to the creature's weak point. 
Anytime the Worm successfully hits Samus with a lunge, it immediately heads back into the acid. Most of the time, its attacks drag Samus into the acid itself. The Acid Worm is only vulnerable during one of its lunge attacks when its head leaves the acid. As the Acid Worm takes more damage, it will strike more quickly, forcing Samus to act faster. When low on health, it will also performs a new attack where after almost every lunge attack, it will open its maw whilst in the center of the acid and cause the entire acid level to rise (not present in Easy Mode). Samus must stand on one of the higher blocks to avoid taking any damage from the rising Acid level until it lowers back to normal level. If Samus has performed a Sequence Break to obtain the Varia Suit early, the rising Acid poses no threat. Once the acid levels return to normal, the Acid Worm will immediately pursue Samus with a lunge when in position.
To cause injury, Samus must fire at what appear to be the creature's eyes when it exposes itself anytime out of the Acid. Samus can inflict minor damage at the start of the battle when the Acid Worm first appears. Samus's main opportunity to inflict damage is primarily after the creature has stuck itself in a platform after lunging, after Samus uses the zip line to escape its lower lunge. The most effective weapon to use is a Missile, but Samus will have to resort to normal Beam fire if she has run out of Missiles, which is most likely to happen on both Hard Mode and Low Percent Runs. If Samus happens to have the Wave Beam early or does not have the Long Beam, she won't be able to cause any damage with her Beam, either because her shots are too large or because the shots won't reach respectively. In such events where Missile fire is her only salvation, she must use the creature's glob projectiles to replenish her Missile ammo.
As the battle progresses, the Acid Worm's body changes color to a darker shade, then eventually deep red. Its eye color also darkens with damage taken. Once the beast has been defeated, the acid will drain, and the creature flashes and explodes from the head downward piece by piece. Soon, a part of the center floor opens up, allowing Samus to access the lowest floor of the room. A path leading to a Missile Expansion lies under the spot where the worm was resting, hidden behind Bomb Blocks.
==Development notes==
In the Event Viewer list for Metroid: Zero Mission's prototype build, a flag named 28 WORMSHI ﾎﾞｽ ﾜｰﾑ ｼ relates to the Acid Worm. When active, it prevents the creature from respawning and loads the post-Acid Worm version of the Acid Worm's room. The event flag 46 HANG must be active to trigger the boss fight, with the WORMSHI flag off as well. The WORMSHI flag suggests that the Acid Worm is internally named . The portion of the flag's name refers to the Japanese word for , referencing its defeat that activates this flag normally.
==Trivia==
*The Acid Worm bears a great deal of similarity to a Yapping Maw in appearance, habitat and behavior.
*The battle with the Acid Worm is somewhat similar to that of Serris, as both battles involve some sort of locomotion on the ceiling to avoid the enemy's attacks.
*It is possible to keep the Acid Worm from raising the acid. This can be made possible if Samus jumps onto one of the higher blocks just before the Acid Worm goes underneath the acid.
**It is even possible to prevent the Acid Worm from appearing by getting off of the zip line before passing over the platform the creature emerges through. Doing so stops all background music in play. However, the creature must still be killed to unlock the hatches.
*When the Acid Worm lunges, the acid level lowers slightly, as is the case when an object is moved within a pool of liquid.
*As seen via Debug Menu or Sequence Breaking, the Acid Worm receives minor damage from Power Bombs. The Plasma Beam is also capable of piercing through its entire armored body, damaging its eyes with relative ease.
==Gallery==
Image:Acid Worm1.png|Samus avoiding the Acid Worm's ramming attack.
Image:Acid Worm2.png|The Acid Worm raising the level of the acid.
Image:Acid Worm3.png|The room after Samus defeats the Acid Worm.
MZM Site Kraid.png|The Acid Worm on the Metroid Zero Mission Official Site
==References==
es:Gran Gusano (Kraid)
ru:Муа
Category:Species
Category:Bosses
Category:Zebes
Category:Kraid's Lair