clitellates

Clitellata is a major group of segmented worms within the phylum Annelida that includes the earthworms (oligochaetes) and leeches (hirudineans). Members are united by the presence of a clitellum, a thickened, glandular band that forms during sexual maturity and secretes a mucus cocoon in which eggs and sperm are temporarily housed for development.

Anatomy and characteristics of clitellates differ from other annelids in several ways. They typically lack parapodia, and their chaetae are reduced or absent in many leeches while present in earthworms. The clitellum is a distinctive feature that appears as a saddle-like ring of glandular tissue around the body. Most clitellates are hermaphroditic, possessing both male and female reproductive organs, and they usually reproduce through cross-fertilization after exchanging sperm with another individual.

Reproduction involves two clitellate worms aligning and exchanging sperm. The clitellum then secretes the mucous sheath that, along with secretions from the reproductive tracts, forms a protective cocoon. The cocoon slides off the worm and attaches to the soil or water, where fertilization and development occur, producing juvenile worms that emerge to continue the life cycle.

Ecology and life history vary by lineage. Oligochaetes, particularly earthworms, are mostly soil-dwelling detritivores that play a crucial role in soil structure and nutrient cycling. Leeches occupy freshwater, and some moist terrestrial habitats, where many are predators or ectoparasites. Clitellates are widely distributed and adapt to a range of environments, making them important for both ecosystem function and studies in development and evolution.