The hidden plumbing and 3D puzzles of our closest invertebrate relatives

Nature · Published 29 October 2026 Swipe more like this →

In short: Echinoderms are more than just beachside curiosities; they are the invertebrates most closely related to humans. From deep-sea predators to towering prehistoric lilies, these animals utilize a unique water vascular system and complex internal skeletons to navigate life in every ocean on Earth.

The name Echinodermata translates from Greek as "hedgehog skin," reflecting their characteristic hard, spiny coverings. While they appear diverse—ranging from the worm-like sea cucumber to the radiating starfish—they share a fundamental body plan known as pentameral symmetry. Most species feature arms in increments of five, though exceptions like six or seven arms occur. This structural unity is supported by an internal skeleton (endoskeleton) made of thousands to millions of calcium carbonate components called ossicles. These pieces fit together like a 3D puzzle, infused with tissue and covered by skin.

Unlike humans, who rely on blood vessels, echinoderms use a water vascular system. This network of tubes uses seawater to transport nutrients and gases while providing the hydraulic pressure necessary for movement and body shape. Water enters through tube foot grooves or specialized plates called madreporites, though sea lilies (crinoids) possess many pores across their bodies instead. These animals occupy every ocean, from shallow beach environments where some echinoids burrow into sediment to depths exceeding 5,000 meters (over three miles).

The evolutionary history of these creatures is vast. They have been recognized since the Cambrian Period, roughly 542 million years ago. While modern crinoids are less common, they once dominated benthic ecosystems and reached staggering sizes; some extinct species had stems exceeding 20 meters (66 feet) in length. Today, researchers continue to debate the phylogeny of starfish and brittle stars, using fossils from the Silurian, Devonian, and Mesozoic eras to map how these complex organisms evolved from their ancient ancestors.

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