A swelling of the epidermis of some earthworms and leeches, that secretes a fluid to form a cocoon around their eggs.
I came across this word while reading up on jumping worms here:
"Jumping worms...are relatively easy to identify if you take a look at their clitellum (the band around the body of a worm). The clitellum on a jumping worm is milky white to gray-colored, smooth and completely encircles the body of the worm. In contrast, the clitellum of European earthworms does not wrap entirely around the worm. Also, on a European species it is raised above the body of the worm, not smooth."
(About now is when adult jumping worms would become noticeable here in Wisconsin. Concerns about spreading them caused at least one plant sale to be cancelled this spring.)
Etymology: Latin clitellae, packsaddle.