earthworm
蟺 centers on the earthworm, a creature associated with soil and burrowing.
蟺 combines 虫 (insect/worm) with 亶, which likely contributes the sound. The exact historical development of 亶 is uncertain, but the compound points to a worm-like creature.
The left side 虫 is the insect/worm radical, and the right side 亶 looks like a complex burrow with layers. Picture an earthworm wriggling through the intricate tunnels of 亶.
For セン, imagine a thousand (千, せん) earthworms wriggling together in the soil: 千 -> セン, and the worm radical 虫 confirms the creature.