Clitellum

The clitellum is a thickened glandular and non-segmented section of the body wall near the head in earthworms and leeches that secretes a viscid sac in which eggs are stored.

[2] In microdrile earthworms, the clitellum has only one layer, resulting in a smaller quantity of eggs than that of the megadrile earthworms, which have larger multi-layered clitellum that have special cells that secrete albumin into the worms' egg sac.

[3] The clitellum is part of the reproductive system of clitellates, a subgroup of annelids which contains oligochaetes (earthworms) and hirudineans (leeches).

The clitellum is a thick, saddle-like ring found in the epidermis (skin) of the worm, usually with a light-colored pigment.

[2] In leeches, the clitellum appears during mating season, where it is used for both sexual reproduction and the secretion of a cocoon for the eggs.

Earthworm with clitellum lablelled.
Close-up of the clitellum of a Lumbricus rubellus
Copulatory complex of Philobdella floridana from North Carolina, South Carolina and Georgia. A, Lake Phelps, NC (Phg-2). Note distinct clitellum from annulus XIc2 to annulus XIIIb5, as indicated. B, Santee Swamp, SC (Psa-1). Specimen slightly distorted during preservation, showing deep saddle-shaped depression. Note prehensile upper ‘lip’ covering mouth. C, Okefenokee Swamp, GA (Pfl-2). Single star, male gonopore; double star, female gonopore, three annuli posterior to male. Scale bars, 1.0 mm.