What is the lighter colored flattened part of the earthworm?

The lighter colored flattened part of the earthworm is called the clitellum. It is a thickened, glandular region of the body that plays a crucial role in reproduction, as it secretes mucus during mating and forms a cocoon for fertilized eggs. The clitellum is typically more prominent in mature earthworms and can be seen as a band around the body.