Starfish, properly known as sea stars, are marine invertebrates belonging to the Phylum Echinodermata. The absence of a vertebral column places them firmly outside the vertebrate classification, a large group that includes fishes, mammals, and birds. Instead of a bony spine, the sea star relies on a unique internal skeletal structure and a hydraulic system for support and movement.
The Difference Between Vertebrates and Invertebrates
The animal kingdom is broadly divided into two major groups based on the vertebral column. Animals that possess this internal spine, composed of articulating vertebrae, are classified as vertebrates. This bony structure provides a rigid central axis for the body, facilitating larger size and faster movement.
The vast majority of animal species fall into the other group, the invertebrates. These animals are defined by the complete absence of a backbone. Invertebrate body support varies widely, ranging from soft tissues to hard external shells, or exoskeletons, seen in insects and crabs.
Sea stars belong to this large invertebrate group, alongside animals like jellyfish, worms, and mollusks. Their taxonomic classification within the Echinodermata phylum means they share the trait of being spineless with relatives such as sea urchins and sea cucumbers. This lack of a central vertebral column necessitates a different strategy for structural support and locomotion.
The Starfish Internal Support Structure
The sea star possesses an internal skeleton, or endoskeleton, embedded just beneath its skin. This internal framework is made up of numerous small, interlocking calcareous plates known as ossicles. These plates are composed primarily of calcium carbonate crystals arranged in a porous, lattice-like structure.
The ossicles vary in form across the body, ranging from flat plates to granules and specialized spines on the upper surface. They are bound together by connective tissue, which can be rigid or flexible, allowing the sea star to slowly change its shape. This arrangement provides structural integrity and protects its internal organs without the need for a heavy, centralized spine.
This endoskeleton is a defining characteristic of echinoderms, providing a robust, lightweight support system. Specialized ossicles include the madreporite, a sieve-like plate that acts as the entry point for the water vascular system. The entire skeletal system is covered by a thin layer of epidermis, meaning the hard parts are internal, unlike the external shells of many other invertebrates.
How Starfish Achieve Movement
The sea star’s movement is powered by a unique hydraulic mechanism called the water vascular system, not by muscle attachments to a spine. This intricate system is a network of fluid-filled canals that uses water pressure to facilitate locomotion, feeding, and gas exchange. Water enters this system through the madreporite, a structure located on the animal’s upper surface.
From the madreporite, water travels through a stone canal to a ring canal in the central disc. This ring canal then branches into a radial canal running down the length of each arm. Thousands of tiny, suction-tipped appendages called tube feet line the underside of these radial canals.
Each tube foot is connected to a bulb-like muscular sac, known as an ampulla, located inside the body. Movement occurs when muscles in the ampullae contract, forcing the water within them into the attached tube feet.
This hydraulic pressure extends the tube feet, allowing them to push against the substrate or adhere to a surface via their suckers. By coordinating the extension and retraction of these thousands of tube feet, the sea star can slowly crawl along the ocean floor or pry open the shells of prey like clams and oysters.