AnnelidsEdit

Annelids are a diverse lineage of segmented worms found in nearly every habitat, from forest floors and freshwater streams to the deepest oceans. Members of the phylum Annelida share a body plan defined by metamerism—the serial repetition of similar units along the body—paired with a closed circulatory system, a coelom, and a hydrostatic skeleton that enables efficient movement through a fluid-filled body cavity. The combination of segmentation, specialized organs in different segments, and a relatively simple yet highly adaptable nervous system has allowed annelids to occupy a wide array of ecological niches. They play a key role in soil health, nutrient cycling, and marine ecosystems, and they have long been a focus of study for understanding body plan evolution and development. Annelida include familiar groups such as earthworms, leeches, and marine polychaetes, each with distinctive reproductive strategies, life cycles, and ecologies. Earthworms are among the best known for their contributions to soil structure, while Leech have a storied medical history and remain of interest for physiology and ecology. Polychaeta dominate many marine communities and exhibit a remarkable range of forms and behaviors.

Over the long arc of life, annelids provide a useful lens on evolution, development, and the interactions between organisms and their environments. Their fossil record extends back to the early Cambrian, illustrating a lineage that has endured dramatic earth-history changes by diversifying into lineages with different feeding modes, reproductive schemes, and habitat preferences. The modern view recognizes major subdivisions such as the clade Clitellata (which includes earthworms and leeches) and the diverse marine Polychaetes, underscoring both the unity of segmentation and the diversity of body plans within the phylum. Cambrian fossils and subsequent advances in comparative anatomy and molecular data together illuminate how metamerism, sensory structures, and reproductive strategies evolved in these worms.