crawling of a worm
蠕 centers on slow, wriggling movement like that of a worm or similar creature. The given meaning 'crawling of a worm' captures this idea of creeping, squirming motion.
蠕 combines 虫 (insect/worm) with 需, which likely contributes the sound. The character is used for wriggling or crawling movement, especially of worms.
The left side 虫 is a worm, and the right side 需 looks like a wriggling shape with a rain hat 雨 over a soft body 而. Picture a worm squirming under a leaf after rain.
For ゼン, imagine a zen garden where a single worm crawls slowly across the raked sand: zen -> ゼン, and the worm's creeping motion fits the calm scene.