The primary function of the clitellum is to create a cocoon for the developing young of the annelids that possess them. The most well-known species which have a clitellum are earthworms and leeches, on which the clitellum appears as a pale saddle-like structure a few segments down from their mouths. While mating, the worms build the cocoon around them, leaving the fertilizing gametes behind as they withdraw and seal it.
According to Encyclopaedia Brittanica, the clitellum is solely a feature of adult worms, and it develops only as the worms reach sexual maturity. Each species of worm has a somewhat different clitellum, varying by relative size, shape and color. In all species, the structure tends to be paler than the rest of the worm. They can be saddle-shaped or more flattened, and either flared or non-flared.
The clitellum is very obvious on earthworms, overlaying their otherwise very evident segments. In earthworms, the clitellum sits about a third of the way down their bodies from their mouths, and this is an easy way of identifying which side is the mouth side. This can be helpful since earthworms don't have distinct heads, and they lack eyes, ears and noses. Many earthworms do have a sort of tongue, however, and find their way through their underground environment via a chemical sensor and touch.