A mature earthworm shows a prominent circular band-like swelling in the skin near the anterior end is called as Clitellum. It covers over a few segments which are fused together. It can be flared, non-flared, saddle-shaped or annular.

Fig: Various forms of clitellum in: A. Glyphidrilus annandalei (flared Clitellum); B. Polypheretima elongata (non flared Clitellum);
C. Pontoscolex corethrurus (saddle-shaped Clitellum); D. Megascolex insignis (annular Clitellum)