Short answers, pulled from the story.
Annelids are segmented worms that make up the phylum Annelida, containing over 22,000 known living species. They include ragworms, earthworms, and leeches, and live in environments ranging from hydrothermal vents and tidal zones to freshwater and moist terrestrial soils.
The largest known annelid is Microchaetus rappi, which can reach 6.7 meters (22 feet) in length. The Australian giant Gippsland earthworm and Amynthas mekongianus can both grow up to 3 meters long.
In 1997, Damhnait McHugh used molecular phylogenetics to show that clitellates are an offshoot of the polychaete family tree, not a separate major group. The same research placed pogonophorans and echiurans, previously treated as independent phyla, on branches within the polychaete tree.
Accounts of leeches used for bloodletting date to China around 30 AD, India around 200 AD, and ancient Rome around 50 AD. By the 19th century demand was so high that some regions exhausted local leech populations, and Hirudo medicinalis is now listed as endangered by both the IUCN and CITES. More recently, leeches have been used in microsurgery to manage blood congestion when severed appendages are reattached.
In 1881, Darwin published The Formation of Vegetable Mould Through the Action of Worms, the first scientific analysis of earthworms' contributions to soil fertility. The book documented how earthworms loosen soil, mix organic and mineral matter, and convert nutrients into forms accessible to plants.
The fossil Phragmochaeta from Sirius Passet, reported by Simon Conway Morris and John Peel, was identified as the oldest known annelid at the time of its description. An even earlier candidate, Wenghuiia, was discovered in 2010 from the Ediacaran Doushantuo Formation at around 555 million years ago, suggesting annelid origins extend further back than previously confirmed.