Anneliden

Anneliden is the term used in several languages to refer to the phylum Annelida, the segmented worms that include earthworms, leeches, and many marine species. In English, the corresponding term is annelid, and both reflect the characteristic body plan of these animals: a series of repeated segments.

Annelids are characterized by body segmentation (metamerism) with repeated organs and a closed circulatory system. They possess a true coelom, a ventral nerve cord, and a body wall made of an epidermis and muscle layers. Most have bristles or setae on segments, especially the marine polychaetes, which often bear prominent parapodia with chaetae.

The phylum comprises two major groups: Polychaeta (polychaetes) and Clitellata (earthworms and leeches). Polychaetes are primarily marine and often possess fleshy parapodia with chaetae for locomotion and gas exchange. Clitellata includes oligochaetes (earthworms) and hirudineans (leeches); they are generally terrestrial or freshwater and are united by having a clitellum, a glandular band involved in reproduction.

Ecology and life cycles vary widely. Annelids inhabit oceans, rivers, soils, and moist terrestrial environments, contributing to sediment turnover and soil structure. Reproduction is typically sexual, with many species being hermaphroditic; development ranges from direct to larval forms resembling trochophore-like stages in some groups.

Physiological and ecological significance is broad. Earthworms promote soil fertility and aeration; marine annelids play key roles in benthic ecosystems; leeches have historical and contemporary medical and research uses. Annelids are often studied as model organisms for understanding segmentation, nervous systems, and regeneration.