Echinoderms are a group of marine invertebrates that includes starfish, sea urchins, sand dollars, sea cucumbers, and brittle stars. The phylum Echinodermata, home to more than 7,000 living species, is found exclusively in the ocean, with no freshwater or land-dwelling members. The name comes from Ancient Greek words meaning “hedgehog skin,” a nod to the spiny outer texture many of these animals share.
What Makes Echinoderms Unique
Three features set echinoderms apart from every other animal group: five-part radial symmetry, an internal skeleton made of limestone, and a water-powered movement system found nowhere else in the animal kingdom.
Most animals are bilaterally symmetrical, meaning their left and right halves mirror each other. Adult echinoderms, by contrast, are built around a central point with five equal sections radiating outward, like the arms of a starfish or the petal-shaped pattern on top of a sand dollar. What makes this even more unusual is that echinoderm larvae start out bilateral, just like other animals. During development, the larva first becomes asymmetrical, then reorganizes into five-fold symmetry. Their ancestors were bilateral animals that evolved this radial body plan as they adapted to life on the ocean floor.
Their skeleton is internal, not external like a crab’s shell. It’s made of calcium carbonate plates called ossicles, embedded in the skin and derived from the same tissue layer that produces human bone. In sea urchins, these ossicles are fused into a rigid, globe-shaped structure called a test. In starfish and brittle stars, they form jointed segments that allow flexible arm movement. In sea cucumbers, the skeleton is reduced to tiny, scattered plates that simply stiffen the body wall. Short, blunt spines project outward from the ossicles in many species, giving the group its characteristic prickly texture. Sea urchins take this to an extreme, with long, movable spines attached to the test by ball-and-socket joints.
How the Water Vascular System Works
The most distinctive feature of echinoderms is their hydraulic plumbing. A network of internal canals filled with seawater connects to hundreds of tiny, flexible projections called tube feet that line the underside of each arm or body section. Water enters through a small pore on the animal’s surface and flows through the canal system. When internal muscles contract, they push water into individual tube feet, extending them outward. When the tube feet press against a surface, a muscular pull creates suction at the tip, allowing the animal to grip tightly.
This system handles far more than locomotion. Tube feet also function in feeding (pulling open clam shells, for instance), sensing the environment, and exchanging oxygen with the surrounding water. The result is a surprisingly effective all-purpose tool powered entirely by water pressure.
The Five Major Classes
Starfish (Asteroidea)
Starfish typically have five broad, fleshy arms radiating from a central disk. They use their tube feet to grip surfaces and pry open prey like mussels. Their feeding strategy is remarkable: a starfish can push its stomach out through its mouth, drape it over prey, and begin digesting tissue externally before pulling everything back inside to finish the job. This lets them consume animals larger than their own mouths.
Brittle Stars (Ophiuroidea)
Brittle stars look superficially like starfish but have long, thin, whip-like arms that are sharply distinct from a small central disk. Rather than creeping along on tube feet, they move by lashing their arms or wrapping them around objects and pulling themselves forward. They tend toward the strongest five-part symmetry of any echinoderm group. Some species can even reproduce by splitting their disk in half, with each piece regenerating the missing parts.
Sea Urchins and Sand Dollars (Echinoidea)
These echinoderms have no arms at all. Sea urchins are roughly spherical, covered in movable spines, with five rows of tube feet poking through their rigid test. Sand dollars are flattened relatives adapted for burrowing through sand, with a domed top, a flat underside, and reduced spines. Sea urchins feed using a complex chewing structure called Aristotle’s lantern, a cage of calcite plates, muscles, and ligaments surrounding five sharp teeth that can scrape algae off rock.
Sea Cucumbers (Holothuroidea)
Sea cucumbers break the echinoderm mold visually. They lie on their sides, stretched into a sausage shape, which gives them a “functional” bilateral symmetry even though their internal anatomy still follows the five-part plan. Their skeleton is reduced to microscopic ossicles scattered through the body wall. Sea cucumbers are capable of regenerating a wide range of organs, including their digestive tract, respiratory structures, muscles, and even parts of their nervous system.
Sea Lilies and Feather Stars (Crinoidea)
Crinoids are the most ancient-looking echinoderms. Sea lilies are anchored to the seafloor by a stalk, while feather stars are free-swimming. Both are suspension feeders, catching drifting food particles with feathery arms surrounding an upward-facing mouth. Their gut is U-shaped, looping back so the anus sits right next to the mouth. Although the five-part body plan is recognizable, most crinoids have far more than five arms.
Regeneration Abilities
Echinoderms are among the most regenerative animals on Earth. Starfish can regrow entire arms. Brittle stars can split their bodies and rebuild from the halves. Sea cucumbers can regenerate internal organs wholesale. This isn’t simple wound healing. It involves a complex cascade of gene activation, starting with the formation of a blastema, a mound of undifferentiated cells at the wound site similar to what happens when a salamander regrows a limb.
Studies on brittle star arm regeneration have identified over 4,000 genes that change their activity levels during regrowth. The earliest stage, when the blastema forms, shows the highest surge in gene activity, with cells ramping up energy production, protein building, and transcription. In sea cucumbers, a well-studied signaling pathway controls intestinal regeneration by coordinating cell behavior across the wound site. The nervous system plays a direct role too. A gene called Myc drives the initial response in nerve cord repair: when its activity is experimentally reduced, the process of converting existing cells back into a more flexible, stem-cell-like state slows dramatically.
Ecological Roles
Several echinoderms function as keystone species, meaning their presence or absence reshapes entire ecosystems. In temperate coastal waters, sea urchin grazing determines whether a habitat remains a lush kelp forest or shifts to a barren, algae-scraped rock surface. The balance tips depending on how many urchins are present and how many predators keep them in check.
On the Pacific coast of North America, the ochre sea star is a textbook keystone predator. By feeding on mussels, it prevents them from monopolizing space on rocky shorelines, which allows dozens of other species to coexist. In tropical systems, the long-spined sea urchin in the Caribbean and the crown-of-thorns starfish on the Great Barrier Reef both exert outsized influence on coral reef structure through their grazing and predation. When populations of these species crash or boom, the effects ripple across the entire reef community.