Starfish, commonly known as sea stars, are familiar inhabitants of the ocean floor, recognized by their distinct, often five-armed shape. Starfish are invertebrates, meaning they lack a vertebral column or spinal structure. This fundamental difference in body plan requires a unique approach to support, protection, and movement compared to animals like fish, mammals, or birds. Their structural makeup relies on an internal skeleton and a complex hydraulic system that defines this entire group of sea creatures.
Starfish Classification: Defining Invertebrates
The classification of starfish begins with their placement in the vast category of invertebrates, animals that do not develop a bony spine or vertebrae. Starfish belong to the Phylum Echinodermata, a group whose name literally translates to “spiny skin”. This phylum includes other marine animals such as sea urchins, sea cucumbers, and sand dollars, all sharing a similar structural organization. A defining characteristic of adult echinoderms is their radial symmetry, typically arranged in five parts around a central axis. This contrasts sharply with the bilateral symmetry found in vertebrates. The radial body plan of a starfish means that its organs and internal structures are distributed around a central disk, rather than being concentrated along a central nervous cord protected by a backbone.
The Starfish Skeleton: Ossicles and Endoskeleton
Instead of a backbone, the starfish possesses an internal skeleton, or endoskeleton, which provides support and structure. This endoskeleton is composed of thousands of small, calcareous plates embedded within the dermal layer of the body wall. These plates are called ossicles, and they are formed from calcium carbonate microcrystals arranged in a porous, lattice-like structure known as stereom. These ossicles vary in shape, ranging from flat plates to granules and even specialized spines that cover the animal’s upper surface. The individual plates are linked together by a tough connective tissue, creating a flexible yet rigid framework. This meshwork provides protection against predators and gives the sea star its characteristic shape, but it does not form a connected, central column like the vertebrae of a fish.
Unique Function: How the Water Vascular System Works
The water vascular system (WVS) provides an alternative method for movement. This unique hydraulic network is a series of fluid-filled canals that powers the animal’s locomotion, feeding, and gas exchange. Water enters the system through a sieve-like plate on the top surface of the starfish called the madreporite.
From the madreporite, water flows through a stone canal to a ring canal encircling the mouth, which then branches into five radial canals, one extending down each arm. Attached to these radial canals are hundreds of small, bulb-like structures called ampullae, each connected to a tube foot. The starfish moves by contracting the muscular ampullae, which forces water into the attached tube feet, causing them to extend and adhere to the substrate. This coordinated hydraulic pressure system allows the starfish to move slowly and adhere firmly to surfaces, enabling it to exert the necessary force to open the shells of prey like clams and mussels.