Short answers, pulled from the story.
The phylum Echinodermata includes starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers. These creatures inhabit every ocean on Earth from shallow intertidal zones to depths exceeding 10,000 meters in the abyssal zone.
Scientists estimate there are approximately 7,600 living species within this phylum. This count makes it the second-largest group of deuterostomes after chordates and the largest marine-only phylum known to science.
The first definitive echinoderms appeared near the start of the Cambrian period. The oldest potential fossil Arkarua dates back approximately 555 million years ago during the late Ediacaran period but lacks conclusive evidence of a water vascular system.
This network of fluid-filled canals functions in gas exchange, feeding, sensory reception, and locomotion. It opens to the exterior through a sieve-like madreporite located on the aboral surface of the animal.
Echinoderms serve as grazers, sediment processors, and suspension feeders that disturb large volumes of substrate. They sequester about 0.1 gigatonnes of carbon dioxide per year as calcium carbonate making them significant contributors to the global carbon cycle.