Short answers, pulled from the story.
The Australian giant Gippsland earthworm, Microchaetus rappi, is the largest annelid on Earth and can grow to an astonishing 6.7 meters. This creature forms the phylum Annelida along with millions of other segmented worms that have adapted to nearly every environment on the planet.
Molecular research published in 1997 shattered the long-held view that segmentation separated annelids from other invertebrates. Greg Rouse and Kristian Fauchald, along with Damhnait McHugh, used genetic analysis to reveal that leeches were actually a subgroup of oligochaetes.
Annelids feature a closed circulatory system where blood flows entirely within vessels often pumped by muscular hearts located near the front of the body. The blood itself is a complex fluid sometimes containing red hemoglobin or green chlorocruorin to transport oxygen.
Ragworms possess jaws that are strong yet remarkably light composed of unusual proteins that bind strongly to zinc. These jaws offer an exceptional combination of durability and weight that has attracted the attention of engineers who study them to develop new materials for lightweight high-strength applications.
Leeches are unique among annelids in their inability to regenerate after severe damage despite most polychaetes being able to regenerate lost body parts. They possess a body structure that is significantly different from other annelids lacking septa and featuring a thick connective tissue layer that occupies much of the body.
The earliest indisputable fossils of oligochaetes appear in the Paleogene period which began 66 million years ago. The oldest known annelid fossils date back to the early Cambrian period approximately 540 million years ago with the Burgess Shale yielding significant discoveries including the polychaetes Canadia and Burgessochaeta.