Echinoderms are a diverse group of marine invertebrates that includes familiar creatures like sea stars, sea urchins, sea cucumbers, and sand dollars. The name Echinodermata means “spiny skin,” referencing the often-rough texture of their exterior. The phylum contains approximately 7,000 living species found exclusively in saltwater environments, inhabiting every ocean depth from intertidal zones to the deep sea floor. Despite their varied appearance, all members are defined by three unique biological characteristics.
Pentaradial Symmetry
The most recognizable feature of adult echinoderms is their distinctive body plan, known as pentaradial symmetry. This means their body parts are arranged in five equal sections around a central axis, much like a star with five arms. This arrangement is easily observed in sea stars and brittle stars, whose five arms radiate outward from a central disc.
For other echinoderms, such as sea urchins and sea cucumbers, the five-part symmetry may be less obvious externally but is still evident in their internal anatomy. This radial organization is an adaptation that suits their slow-moving or sessile lifestyle, allowing them to interact with their environment from all directions. The development of this radial structure is a profound change from their larval stage, which begins life with the bilateral symmetry found in most complex animals.
The Water Vascular System
A complex network of fluid-filled canals, known as the water vascular system, is a defining trait unique to echinoderms. This hydraulic system is essential for locomotion, feeding, gas exchange, and waste transport throughout the organism. Water enters the system through a porous, sieve-like plate called the madreporite, which is typically located on the aboral (top) surface of the animal.
From the madreporite, water flows down a short tube called the stone canal, connecting to a central ring canal that encircles the mouth. Five radial canals branch out from the ring canal, extending into each of the five segments of the body. The radial canals give rise to lateral canals, each of which connects to a tube foot.
Tube feet are small, muscular appendages that project outside the body and are often tipped with a sucker. The internal end of each tube foot has a muscular bulb called an ampulla. When the ampulla contracts, it forces water into the tube foot, causing it to extend and adhere to a surface via suction. The coordinated extension and retraction of thousands of these tube feet allow the echinoderm to move slowly but powerfully across the sea floor, climb vertical surfaces, and manipulate prey.
Internal Skeleton Construction
All echinoderms possess an internal skeleton, or endoskeleton, that provides structure and protection. This skeleton is composed of numerous small, calcium carbonate plates called ossicles, which are embedded within the dermis layer of the body wall. The ossicles are unique because they have a mesh-like, three-dimensional latticework structure known as a stereom.
This honeycomb stereom structure provides a combination of strength and lightness to the skeleton. In some echinoderms, like sea urchins, the ossicles are tightly fused to form a rigid shell called a test, while in others, such as sea stars, they are loosely connected.
The ossicles are held together by mutable collagenous tissue (MCT), a specialized connective tissue distinct to the phylum. MCT allows the animal to rapidly and reversibly alter the rigidity of its body. Under nervous control, the echinoderm can instantly change this tissue from a flexible state to a locked, stiff state, enabling it to hold a fixed posture without continuous muscle effort. This allows a sea star, for example, to lock its arms into position to hold tightly onto a surface or pry open a clam.