Short answers, pulled from the story.
Kinorhyncha are a phylum of small marine invertebrates, commonly called mud dragons, that live in mud and sand on the ocean floor at all depths. Modern species are 1 mm or less in length, and they are part of the meiobenthos, the community of tiny animals living between sediment particles.
The common name mud dragon reflects both their habitat, the muddy sediment of the seafloor, and their formidable appearance, with multiple circles of spines around the head and additional spines along the body. The scientific name Kinorhyncha derives from the Greek for "moving snout," referring to the retractable, spine-covered head the animals use for locomotion.
Kinorhynchs are entirely limbless and move by anchoring the spines of their retractable head into the sediment, then drawing the body forward. The head can withdraw completely behind a set of neck plates called placids, and the spines on the body also grip the substrate during movement.
Adult kinorhynchs have a trunk of eleven segments, plus a head and neck. Juveniles start with eight or nine segments depending on genus, and the final two or three segments are added during growth. The Cambrian species Eokinorhynchus rarus had roughly twice as many segments as modern forms.
As of 2022, more than 300 species of Kinorhyncha have been formally described, distributed across 31 genera and 11 families. The genus Echinoderes alone holds more than 100 species and is the most diverse and abundant of all kinorhynch genera.
The oldest known kinorhynch fossil is Eokinorhynchus, from the Fortunian stage of the Cambrian period, found in China and formally described by Zhang and colleagues in 2015. Cambrian kinorhynch forms could reach 4 cm in length, far larger than the 1 mm maximum of modern species.