Short answers, pulled from the story.
Echinoderms are marine animals of the phylum Echinodermata, which includes starfish, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies. The phylum contains about 7,600 living species and roughly 13,000 known extinct species, making it the largest marine-only phylum on Earth.
Adult echinoderms develop pentaradial symmetry during metamorphosis: the left side of the larval body grows at the expense of the right, which is absorbed, and the remaining tissue reorganises around a central axis in five parts. Larvae are bilaterally symmetrical before this process begins.
Echinoderms move using a water vascular system, a network of fluid-filled canals connected to external tube feet. Fluid pushed into a tube foot extends it; withdrawing the fluid contracts it. Thousands of tube feet operating in coordinated waves propel the animal across the seabed. There is no central brain; nerves radiate from rings around the mouth into each arm.
Most echinoderms have strong regenerative abilities. Sea cucumbers can expel and then regrow internal organs over several months. Sea urchins continuously replace lost spines. A few starfish species can regenerate a complete individual from a single severed arm, though most require at least part of the central disc.
Echinoderms sequester about 0.1 gigatonnes of carbon dioxide per year as calcium carbonate. Sea urchins are primary herbivores on coral reefs; in 1983 the mass mortality of Diadema antillarum in the Caribbean shifted reefs from coral-dominated to alga-dominated systems. Sea cucumbers process large volumes of seabed sediment as deposit feeders.
In 2019, global echinoderm harvests totalled 129,052 tonnes, primarily sea urchins (66,341 tonnes) and sea cucumbers (59,262 tonnes), consumed as food and in traditional Chinese medicine. In research, sea urchins such as Strongylocentrotus purpuratus and Arbacia punctulata serve as model organisms in developmental biology. Brittle star arm regeneration is studied for its potential relevance to human neurodegenerative diseases.