Hermittacks are minor enemies that crawl slowly along any surface (including walls and ceilings). The red and purple ones retreat into their shells when hit with a weapon, then can be knocked around the screen. They deal half a damage point, along with their metal variant.
Metal Hermittack
Metal Hermittacks are enemies that, along with regular Hermittacks, can only be found in the Iron Whale. They are immune to most normal attacks. They can be temporarily knocked out if hit by the shell of a regular Hermittack or if the surface they're crawling on is destroyed. However, after a few seconds of being stunned they will just poke their heads out again and continue walking. To prevent this, hit their shell once more to send them ricocheting around, temporarily knocking them out again. They unfortunately, are rarely placed in rooms with regular Hermittacks alongside them.
This type is not immortal, however. Specter Knight can destroy them with a single swing of Dread Talon.