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?
First, we’ve been speaking only of earthworms so far (because this is the creature we are currently concerned with), but we should note at the start that leeches also have clitella. So, although most people seem to be curious about earthworm clitella – hence all the “what is the bump on the earthworm” questions – it is important to mention that the clitellum is a body part that is not exclusive to earthworms.
Indeed, a clitellum is part of the reproductive systems of all clitellates, a subgroup of the Annelid phylum (also called “segmented worms” or “ringed worms”) that contains both earthworms (oligochaetes) and leeches (hirudineans). Clitella do not look the same on every creature, however, and its appearance further depends on the age of the earthworm or leech. It is hard to see the clitellum on immature clitellates (because young clitellates aren’t sexually mature), and the clitellum can only be visually located on leeches seasonally. No matter the season, it is generally harder to see the clitellum of a leech than it is to see the clitellum of an earthworm. While the clitella of both creatures is usually a little lighter than the rest of their bodies, the contrast is especially noticeable on earthworms.
Regardless of the creature it is found on, the clitellum is a thickened segment of the body that contains the gland cells. It looks a bit like a saddle that wraps around part of the creature’s body. The clitellum secretes a fluid that forms a cocoon in which eggs are placed. It is usually found near the anterior end (the mouth end) of an earthworm, around a quarter or a third of the way down the worm’s body. The positioning of the clitellum has to do with the mating habits of worms, which involves two worms lining up parallel, but facing opposite directions, to exchange sperm. This sperm is then used to fertilize each worm’s eggs. (Earthworms are hermaphroditic, so they have male and female sex organs, which explains why they produce both eggs and sperm.)
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So, to answer the question we originally posed: the bump on the earthworm is called the clitellum, and it is a part of the reproductive systems of clitellates, a group that not only includes earthworms, but also leeches.