Clitellata

Clitellata is a clade within the phylum Annelida that includes earthworms, their relatives, and leeches. A defining feature is the clitellum, a thickened glandular band that forms during sexual maturity and secretes a mucous cocoon in which fertilized eggs develop.

The group encompasses two major lineages: Oligochaeta (earthworms and their terrestrial and freshwater relatives) and Hirudinea (leeches). Oligochaetes usually have chaetae (bristles) on each segment, while leeches have reduced or absent chaetae and a typically flattened body.

Clitellata are hermaphroditic and typically engage in cross-fertilization. The clitellum secretes a mucous tube that slides forward to receive sperm and later forms a cocoon that contains the eggs and developing embryos. Development is generally direct, with juvenile worms emerging from cocoons rather than free-swimming larvae.

Ecology and life history: Earthworms are important soil engineers, contributing to soil structure, aeration, and nutrient cycling. Leeches inhabit freshwater and moist terrestrial habitats and include both predatory and hematophagous species. The ecological roles of Clitellata are diverse and often closely tied to their reproductive biology and habitat preferences.

Taxonomy and evolution: Clitellata is considered a monophyletic group within Annelida, defined by reproductive anatomy and the presence of a clitellum. Together with their sister annelid groups, clitellates illustrate the diversification of segmented worms from polychaete ancestors.