The phylum Echinodermata includes familiar marine invertebrates such as sea stars, sea urchins, sand dollars, and sea cucumbers. These organisms are exclusively found in marine environments, inhabiting ocean floors from the intertidal zone to the abyssal depths. Biological symmetry describes how an animal’s body parts are arranged around a central axis or plane. Many animals, including humans, exhibit bilateral symmetry, meaning the body can be divided into two mirror-image halves along a single plane. Adult echinoderms, however, exhibit radial symmetry.
The Defining Body Plan of Echinoderms
Radial symmetry is a body plan where multiple planes passing through the central axis can divide the organism into roughly similar halves, like a pie. This arrangement contrasts sharply with bilateral symmetry, where the body has distinct left and right sides. Echinoderms are characterized by a specific type of radial symmetry known as pentaradial symmetry.
Pentaradial symmetry means the body is organized around a central disk in five equal sections, or multiples of five. This five-fold structure is evident in the five arms radiating from the center of most sea stars and brittle stars. Even in sea urchins, which lack distinct arms, this five-part pattern is visible in the arrangement of the skeletal plates (ossicles) and the five rows of feeding and locomotory structures. These five sections contain corresponding sets of internal organs, including the radial canals of the water vascular system and nerves extending outward.
The external expression of this five-fold arrangement is often conspicuous, though the symmetry may appear modified in certain classes. Sea cucumbers, for instance, have an elongated body that superficially suggests bilateral symmetry, but their internal structure retains the foundational five-part organization. This body plan is a defining characteristic of the phylum, which is built upon an internal endoskeleton made of calcium carbonate plates.
Developmental History: From Bilateral Larva to Radial Adult
The radial symmetry of adult echinoderms represents a complexity in their developmental and evolutionary history. While mature forms display a radial body plan, the larval stages of all echinoderms are free-swimming and exhibit bilateral symmetry. This larval form, such as the bipinnaria larva of a sea star, can be divided into two mirror-image halves along a single plane.
This shift from a bilateral larva to a radial adult is achieved through metamorphosis. During this transformation, the original bilateral body of the larva is reabsorbed and reorganized. A new, small, radial body develops from the left side of the larva, while the right side is suppressed or absorbed into the developing adult structure.
The existence of a bilateral larval stage offers evidence regarding the evolutionary origins of the phylum. Scientists suggest that echinoderms evolved from bilaterally symmetrical ancestors, with the radial body plan developing secondarily as an adaptation to their lifestyle. The retention of the bilateral larval form is an echo of this ancestral condition, highlighting the unique evolutionary path taken by the echinoderm lineage.
Ecological Significance of Radial Symmetry
The radial body plan provides a functional advantage for the adult echinoderm’s lifestyle. Because most adult echinoderms are slow-moving or sessile bottom-dwellers, a body plan without a distinct head or tail is beneficial. Radial symmetry allows the animal to interact with its environment equally from all sides, offering 360-degree awareness.
This all-around organization is useful for sensing predators, gathering food particles, or filtering water from any direction. The mouth is typically located on the underside (oral surface), while sensory and water intake structures are distributed around the periphery. This arrangement ensures the animal is prepared to respond to stimuli arriving from any point on the sea floor.
The radial symmetry is linked to the unique water vascular system, a hydraulic network that operates the numerous tube feet. This system consists of a central ring canal and five radial canals extending outward along each of the five segments. The tube feet aid in locomotion, feeding, and gas exchange, and are distributed radially. This allows the animal to move or anchor itself in any direction without needing to reorient its entire body.