A yellow coat on the gut
Earthworms are the wiggly worms you find in garden soil after it rains. Their body is a long tube, and inside it is another tube: the gut, where food goes.
The outside of the gut is covered with chloragogen cells. They are yellowish or greenish-brown and full of little grains. They also sit along some of the worm's blood vessels.
Next time you see an earthworm after rain, look closely (without hurting it). Its body is made of many rings. The gut runs all the way through those rings from front to back.