As soon as Atlas rushed into the hole he created, the sand flooded in and clogged the gap. In normal cases, one would expect the sand to follow him into the worm's body, but it did not.
The sand's contained heat seeped into the worm's flesh and blood, and the mass filling the cavity was turned into a patch that actually served to heal the wound.
As the sand's energy was sapped away, the material itself was expelled through the new flesh that was forming in its place. When the flesh formed, a portion of it separated away and thickened into cartilage through a transmutation-like process.
Atlas definitely caused the beast more pain than it had ever felt in its life, but he didn't actually manage to do any substantial damage.
But that was not a problem. In his current position, it was far easier for him to make an impact.
Holding his breath, he scanned the area around him.