What is that bump on earthworms? You know, the bump somewhat near the center of an earthworm that kind of looks like a segment of a thick rubber band? This may not seem like the most exciting question, but more people than you would imagine are curious about the bumps on earthworms. As a consequence, we decided to write about these earthworm bumps, which are called clitella. (“Clitellum” is the singular form, so plenty of people mistakenly think the plural form is “clitellums.”) What is an earthworm clitellum, and what does the clitellum do?