The picture is not very clear. It looks like Nereis, also known as sea worms and sea centipedes. It belongs to Annelida, polychaeta, Nematoda and Nereistidae. Its body is obviously segmented, and its two sides protrude into warty feet with bristles for action. Nereis mostly inhabit the sediment in the freshwater estuaries, and can swim in the breeding season or when they go out for food at night, which is prolific along the Yellow Sea and Bohai Sea in China. Nereis protein is rich in content, and its taste is fishy and fresh. It is mostly used for sea fishing and is called "universal bait". Nereis can also live in fresh water, but why it appears in well water should be analyzed in combination with the actual situation.