The phylum Echinodermata encompasses a diverse group of over 7,000 living species, including sea stars, sea urchins, and sea cucumbers. These organisms are found exclusively in marine environments, inhabiting every ocean depth from the intertidal zone to the abyssal plain. A defining characteristic widely associated with this group is its unique five-fold body plan, known as pentaradial symmetry. This radial arrangement contrasts sharply with the bilateral symmetry observed in the vast majority of complex animal life, raising the question of whether this specific type of symmetry is universal across all echinoderms.
What is Pentaradial Symmetry?
Symmetry describes how an animal’s body parts are arranged in relation to a central axis or plane. Unlike bilateral symmetry, which allows division into two mirror-image halves along only one plane, radial symmetry allows division into identical parts along multiple planes radiating from the center. Pentaradial symmetry is a specific form of radial symmetry where the body is structured around five equal parts, or rays, extending outward from the central mouth.
A sea star provides a clear visual example, with its five arms illustrating the five identical segments. This five-part division is a derived trait within the phylum, thought to be an adaptation for a slow-moving or sessile lifestyle. This arrangement allows the animal to sense and react to its environment equally well from all directions.
The General Rule for Adult Echinoderms
The adult form of most living species exhibits pentaradial symmetry, even if it is not immediately obvious externally. This five-part organization is deeply integrated into the animal’s internal anatomy, providing a structural blueprint for its organ systems. The most definitive expression of this symmetry is found in the water vascular system, a hydraulic network unique to echinoderms that is responsible for locomotion, feeding, and gas exchange.
This system features a central ring canal surrounding the mouth, from which five radial canals extend outward. These canals run the length of each arm or ambulacral area, feeding into the hundreds of tube feet used for movement. The internal skeletal structure, composed of calcareous plates called ossicles, is also arranged in a five-part pattern to support the water vascular system.
Classes like Asteroidea (sea stars) and Echinoidea (sea urchins) clearly follow this rule. In sea urchins, the symmetry is less visible externally than in a sea star, but the five rows of tube feet and the five-part feeding apparatus, known as Aristotle’s lantern, confirm the underlying pentaradial structure.
The Bilateral Larval Stage
Despite the pentaradial symmetry of the adult form, all extant echinoderms begin life as free-swimming larvae that are distinctly bilaterally symmetric, possessing clear left and right sides. These larval forms, such as the bipinnaria of sea stars or the echinopluteus of sea urchins, are ciliated and use their bilateral shape for a planktonic existence.
The transition from this mobile, bilateral larval form to the sedentary or slow-moving pentaradial adult involves a complex metamorphosis. During this process, the majority of the larval body is reabsorbed or reorganized. Specifically, the adult’s pentaradial structure develops from the left side of the larva, while the right side is absorbed or degenerates.
This developmental shift indicates the phylum’s evolutionary history, suggesting that echinoderm ancestors were bilaterally symmetric animals. The adult radial form is considered a secondary adaptation, which evolved to suit their ecological niche on the ocean floor. The persistence of the bilateral larval stage is a biological signature of this ancestral lineage.
Variations and Secondary Bilateralism
While the five-part internal design is a defining trait of the phylum, some adult echinoderms display modifications that alter their external appearance, leading to what is called secondary bilateralism. Sea cucumbers (Holothuroidea), for instance, exhibit a dramatically elongated, worm-like body shape with the mouth and anus on opposite ends. This orientation gives them a superficial bilateral symmetry, making them appear to have a defined head and tail.
Even in sea cucumbers, however, the pentaradial template remains embedded within their anatomy, typically represented by five internal longitudinal bands of muscles and five rows of tube feet. Another notable variation occurs in the irregular echinoids, which include sand dollars and heart urchins.
These species, adapted for burrowing in soft sediment, have re-evolved a form of bilateral symmetry superimposed on their pentaradial structure, allowing for directional movement. The arrangement of their ambulacral areas, which are the zones containing the tube feet, shows a clear two-sided organization rather than a perfectly uniform five-part arrangement, demonstrating how lifestyle can modify even the most fundamental body plan.