Pentameral symmetry is a form of radial symmetry where the body is organized into five equal parts around a central point. This five-part arrangement is a recurring pattern in nature, particularly within a specific group of marine animals. Imagine a pie cut into five identical slices for a simple visual of this body plan. Unlike animals with bilateral symmetry, which have a distinct left and right side and a head region, organisms with this five-fold structure are arranged around a central axis.
The Echinoderms: A Five-Point Body Plan
The phylum Echinodermata is composed of animals that display pentameral symmetry in their adult form. This group is found exclusively in marine environments. Sea stars are the most recognizable example, with their classic five-armed body radiating from a central disc. While some species can have arms in multiples of five, the underlying structure is based on this five-part division.
This body plan is also present in sea urchins and sand dollars, though it is less obvious. In these animals, the internal skeleton, made of fused calcium carbonate plates, is organized into a five-part system. On the external surface of a sand dollar, a five-petaled flower pattern is visible, which reflects the arrangement of respiratory pores. Sea urchins also show this five-part symmetry in the patterning of their spherical, spiny shells.
Even in echinoderms without a star shape, the five-fold pattern persists internally. Sea cucumbers have elongated bodies, but their internal anatomy is pentamerally structured. This is evident in the arrangement of their feeding tentacles and the longitudinal muscles running along the body wall. Similarly, sea lilies and feather stars have feeding arms that branch out in multiples of five to filter food from the water.
Functional Benefits of a Radial Body
The five-part radial body plan offers advantages for the slow-moving or sessile lifestyles of echinoderms. Lacking a defined front or back, these animals can sense food or detect predators from any angle without needing to turn their entire body. Their sensory and nervous systems are diffuse, with networks capable of coordinating responses across their radial structure.
This body arrangement also contributes to the regenerative capabilities of many echinoderms. Because the body is modular, with repeating sets of organs and limbs, the loss of an arm is less damaging than for an animal with a centralized anatomy. A sea star can regrow a lost arm, and sometimes a single severed arm with a portion of the central disc can regenerate into a complete new animal.
The water vascular system, a network of water-filled canals unique to echinoderms, operates hydraulically to control thousands of tiny tube feet used for locomotion and feeding. This system radiates through each of the five body segments, providing coordinated movement and strong adhesion to surfaces. The skeletal plates, known as ossicles, form a supportive endoskeleton that is both robust and flexible.
From Bilateral Larva to Radial Adult
While adult echinoderms are defined by their pentameral symmetry, they do not start life that way. All echinoderms begin as bilaterally symmetrical larvae that swim freely in the water column. This larval form has a clear left and right side, a gut, and other features common to the early developmental stages of more complex animals.
This larval stage is temporary, leading to metamorphosis. The larva settles onto the seafloor and undergoes a complete reorganization of its body plan. During this transition, bilateral features are lost as the new radial body of the juvenile echinoderm develops, with one side of the larva becoming the oral (bottom) surface and the other the aboral (top) surface.
This two-part life cycle demonstrates that the pentameral symmetry of modern echinoderms is a secondary adaptation. Their ancestors were bilaterally symmetrical, and the evidence for this is retained in their larval stage. The evolution of a radial adult form from a bilateral larva allowed this group of animals to thrive in their specific marine niches.