Cut a starfish open and you won’t find bones, blood vessels, or a brain. Instead, you’ll see a surprisingly alien arrangement: a skeleton made of tiny crystal plates embedded in the body wall, two stomachs stacked in the central disc, a network of fluid-filled canals that power hundreds of tiny hydraulic feet, and nerves that run down each arm without ever connecting to a central brain. Every major organ system in a starfish works differently from what you’d expect in most animals.
A Skeleton Built Into the Skin
Starfish have an internal skeleton, but it looks nothing like bone. The body wall is filled with small plates called ossicles, each one a single crystal of magnesium-rich calcite (a mineral related to limestone). These plates aren’t fused together. Instead, they’re linked by collagen fibers and tiny muscles, creating a flexible mesh rather than a rigid frame.
The collagen fibers form loop-shaped straps that wrap around the edges of each ossicle, and muscle fibers thread through the same spaces. This arrangement lets a starfish do something remarkable: it can shift between being stiff and being flexible. When the collagen tightens, the body wall becomes rigid enough to hold a posture. When muscles contract, the starfish can bend and twist its arms. Under a microscope, the ossicles themselves are porous, riddled with tiny holes like a sponge, which keeps them lightweight while still providing structural support.
Two Stomachs for Inside-Out Eating
The most striking organ in the central disc is the stomach, which is actually two chambers stacked on top of each other. The lower one, called the cardiac stomach, is the one starfish push out through their mouth and over their prey. When a starfish pries open a mussel, it slides this stomach through the narrow gap between the shells and begins digesting the soft tissue externally. This is why starfish can eat animals larger than their own mouth.
Once the prey’s tissue has been partially broken down outside the body, the liquefied nutrients are pulled inward to the upper chamber, the pyloric stomach. From there, material flows into long, branching digestive glands that extend into each arm. These glands are where absorption actually happens. When feeding is complete, the cardiac stomach retracts back into the central disc. If you were to open a starfish that hadn’t recently eaten, you’d see the cardiac stomach folded up inside like a deflated bag, with the pyloric glands fanning out into the arms above it.
A Hydraulic System Powers the Feet
Running along the underside of each arm are rows of tube feet, the small, flexible projections starfish use to walk, grip surfaces, and pry open shellfish. What you see on the outside is only half the system. Inside each arm, every tube foot connects to a small muscular sac called an ampulla. Together, the tube foot and its ampulla work like a tiny hydraulic piston.
The entire system is filled with a fluid similar to seawater but with a higher concentration of potassium. When the ampulla contracts, it squeezes fluid into the tube foot, causing it to extend outward and press against whatever surface the starfish is walking on. A flattened disc at the tip of each foot creates temporary suction. When the starfish needs to release and take a step, muscles in the foot’s stem contract, pushing fluid back into the ampulla, and the foot shortens. Hundreds of these tiny pistons firing in coordinated waves is what lets a starfish move, slowly but with surprising grip strength.
The ampullae connect to a larger ring canal that circles the central disc and branches into each arm through radial canals. Water enters the system through the madreporite, a small, sieve-like plate visible on the starfish’s upper surface. This whole network, called the water vascular system, is one of the most distinctive features of starfish anatomy and has no equivalent in vertebrates.
No Brain, but Nerves in Every Arm
Starfish have no brain at all. Their nervous system consists of a nerve ring that circles the mouth in the central disc, with a radial nerve cord running down the length of each arm. Each nerve cord has two layers: an outer layer of sensory neurons near the skin surface and an inner layer containing motor neurons that control movement.
This decentralized design means each arm has a degree of independence. A single arm can sense food and initiate movement on its own, with the nerve ring coordinating the other arms to follow along. The nerve cords also connect to the tube feet, controlling the precise timing of extension and retraction that makes coordinated locomotion possible.
Breathing Through the Skin
Starfish have no lungs or gills in the traditional sense. Instead, scattered across the upper body surface are tiny, thin-walled projections called papulae, essentially small bumps where the body cavity pokes outward through gaps between ossicles. These papulae are lined inside with ciliated cells that keep fluid circulating, and their walls are thin enough for oxygen and carbon dioxide to pass through by simple diffusion.
The body cavity itself, called the coelom, is filled with fluid that bathes the internal organs. The cilia inside the papulae help move this fluid, creating a slow internal circulation that distributes oxygen from the skin inward. It’s a passive, low-energy system that works because starfish have relatively low metabolic demands.
Nutrient Transport Without Real Blood
Starfish don’t have a circulatory system with a heart and blood vessels. Instead, they have a simple network called the hemal system: thin strands of tissue that run alongside the digestive glands in each arm, connecting to small tufts of absorptive tissue near the stomach and a ring channel in the central disc.
The cells lining these channels have features that suggest they absorb nutrients directly from the digestive glands and release them into the surrounding coelomic fluid. Some of these cells produce bulbs of material at their surface that appear to secrete nutrients into the body cavity. But the hemal strands themselves are small and limited in distribution, so most nutrient movement likely happens through the coelomic fluid sloshing around the body cavity rather than through any dedicated transport network. It’s a simple system, but it works for an animal that digests slowly and doesn’t need rapid fuel delivery to muscles.
What the Cross-Section Looks Like
If you sliced through a starfish arm from top to bottom, you’d see distinct layers. The outermost layer is the body wall: skin, ossicles, and the collagen-muscle mesh holding them together. Just inside that are the papulae openings and the coelomic cavity filled with fluid. Running along the top of the arm are the two pyloric glands, large branching organs that take up much of the interior space. Along the bottom center is the radial canal of the water vascular system, with ampullae branching off to each side like rows of tiny balloons. Below the radial canal sits the radial nerve cord, and beneath that, the groove where tube feet emerge.
The central disc is more crowded. Both stomach chambers sit here, along with the nerve ring, the water vascular ring canal, and the hemal ring. The madreporite connects from the top surface down to the ring canal through a vertical channel called the stone canal. In species that reproduce sexually, gonads are also tucked into the base of each arm near the central disc, sometimes filling much of the available space during breeding season.