Echinoderms, a diverse group of marine animals including sea stars, sea urchins, and sea cucumbers, are often questioned about their internal structure. The answer is clear: they do not possess a backbone. They are classified as invertebrates, a category of animals that lack this distinguishing feature. Instead of relying on a centralized spinal column for support, echinoderms utilize a unique internal framework to thrive in marine environments. This article explores their specialized support structure and the biological differences separating them from animals with backbones.
The Defining Difference: Chordates Versus Echinoderms
The presence of a backbone, or vertebral column, defines vertebrates, which belong to the phylum Chordata. Chordates are identified by four specific anatomical features that appear during development: a stiff, flexible rod called a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.
In vertebrates, the notochord is typically replaced by the segmented vertebral column, providing the main axis of skeletal support. Echinoderms do not develop any of these chordate structures at any point in their life cycle. This lack of a notochord or spinal column is the biological reason they are placed in a separate phylum.
While echinoderms share an evolutionary link with chordates, their adult body plan is fundamentally different. Their lack of a centralized support structure is directly related to their unique radial body arrangement, representing a successful, alternative evolutionary path for structural support.
The Echinoderm Endoskeleton
Echinoderms possess a sophisticated internal support structure, or endoskeleton, that provides both rigidity and protection. This skeleton is composed of numerous small, interlocking plates made primarily of calcium carbonate. These skeletal elements are called ossicles, and they are formed within the dermal layer of the body wall, beneath the skin.
The ossicles feature a porous, mesh-like microstructure known as stereom, making the skeleton lightweight yet strong. The arrangement of these ossicles varies greatly among different classes of echinoderms, reflecting their diverse body shapes.
In sea urchins, the ossicles are tightly fused to create a rigid, hollow structure called a test, which acts like a protective shell. Sea stars have loosely connected ossicles that allow their arms to flex and bend. In sea cucumbers, the ossicles are microscopic pieces embedded in the leathery body wall, resulting in a soft, flexible body form. This endoskeleton serves the same supportive function as a vertebral column but uses a decentralized network of mineralized plates.
Key Anatomical Features Beyond the Skeleton
Beyond their unique endoskeleton, echinoderms are characterized by specialized anatomical systems that further distinguish them from vertebrates. The most notable is the water vascular system, a hydraulic network unique to this phylum. This system is a complex set of fluid-filled canals that powers the animal’s locomotion, feeding, and gas exchange.
Seawater enters the system through a porous plate called the madreporite and is circulated through a ring canal and five radial canals that extend throughout the body. The system operates by controlling water pressure within bulb-like sacs called ampullae, which connect to thousands of external projections known as tube feet. When the ampullae contract, water is forced into the tube feet, causing them to extend and adhere to surfaces, allowing the animal to move slowly but powerfully.
Adult echinoderms also display pentaradial symmetry, meaning their body parts are arranged in multiples of five around a central axis. This is clearly seen in the five arms of a sea star or the five rows of tube feet on a sea cucumber. This radial body plan contrasts sharply with the bilateral symmetry—where the body can be divided into two mirror-image halves—common in vertebrates.